JPH10148596A - Light reflection measurement circuit - Google Patents

Abstract

[PROBLEMS] To measure a long optical fiber even with a light source having a low coherence or a light source having a large optical frequency interval, and removing the influence of a response delay of the light source, thereby enabling more accurate measurement. A light reflection measurement circuit is provided. SOLUTION: Reflectometry (OCDR) using synthesis of a light wave coherence function is used. The laser 11 is a light source that outputs an optical pulse, and the output pulse interval is Δ
T, the optical pulse width is Δt, and the optical frequency of the i-th optical pulse is fi. When the optical frequency is switched, control is performed by an optical switch or the like so that the measurement light is output after the laser output is stabilized, and a transitional portion of the optical frequency modulation of the laser 11 is masked. The output light of the laser 11 is split by the optical coupler 4 into reference light and measurement light. The measuring light is guided to the measured optical component 3 to be measured, reflected by the measured optical component 3, and then the reflected light is extracted by the optical coupler 4.
A beat signal between the reflected light and the reference light output from the optical coupler 4 is received by the optical receiver 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light reflection measuring circuit suitable for measuring light reflection characteristics of an optical component, an optical transmission line, and the like.

[0002]

2. Description of the Related Art In order to non-destructively monitor, diagnose, and maintain an optical communication transmission line, there is a need for a method of measuring a loss distribution in a longitudinal direction of an optical fiber and a reflection characteristic of a remote optical device with a high distance resolution. Have been. One such measurement technique is the literature "Reflectometry by combining lightwave coherence functions-sweeping coherence functions by phase modulation.
-, Takashi Saida, Kazuo Hotate, IEICE Technical Report, OPE94-2,
1994, "Reflectometry using synthesis of lightwave coherence function" (hereinafter referred to as OCDR). This method has features such as not requiring a mechanically movable part and not requiring numerical processing.

FIG. 29 shows a basic configuration of a light reflection measuring circuit using the OCDR technique. As shown in the figure, the light reflection measuring circuit includes a laser 1, an optical receiver 2, an optical component under test 3, and an optical coupler 4. The laser 1 is a light source whose optical frequency is variable, and switches so that the optical frequency fi of the laser 1 becomes fi = f0 + ai · Δf (1) at every time interval ΔT. Here, i = 1, 2,..., N (N: natural number), f0: optical frequency, ai: integer, Δf: optical frequency interval. Ai in the expression (1) is 0, ± 1, ± 2,
.., And the optical frequency “f (t) −f0” is, for example, 0 → Δf → −Δf → 2Δf as shown in FIG.
→ −2Δf → 3Δf →...

At this time, the delay time difference τ between the reference light and the reflected light
On the other hand, the light wave coherence function γ (τ) indicating the coherence of these two light waves is

(Equation 1) Given by The light wave coherence function γ (τ) has a shape of a delta function train having a peak (coherence peak) at the position of τ = m / Δf (m: integer) (3). FIG. 31 shows a lightwave coherence function γ (τ) in which these peaks are combined.

On the other hand, the optical receiver 2 receives a beat signal of the reference light and the reflected light from the optical component 3 to be measured. The measured optical component 3 is a measurement target whose light reflection characteristic is to be measured, and is, for example, an optical fiber. The optical coupler 4 guides the measurement light to the measured optical component 3, and also guides the reference light and the reflected light from the measured optical component 3 to the optical receiver 2. Therefore, after the measurement light is guided to the measured optical component 3, the reflected light from the measured optical component 3 is extracted by the optical coupler 4, and the reference light and the reflected light are received by the optical receiver 2. Will be.

In the OCDR, the reflected light power from the position τ where the coherence peak is combined can be selectively extracted and directly measured as the received signal power of the optical receiver 2. For this purpose, for example, the output signal of the optical receiver 2 may be square-detected to measure the power P of the beat signal of the reference light and the signal light. At this time, P∝∫R ² (z) γ ² (z) dz... (4)

In the expression (4), z is a position in the optical component 3 to be measured, and R (z) is a reflected light intensity distribution depending on the position z. Here, since γ (z) = 0 at positions other than the position τ where the coherence peak is synthesized, it is possible to select and measure only the reflected light power from the position τ. In addition, by changing the optical frequency interval Δf, the coherence peak interval is changed, and the position of the coherence peak equal to or higher than the primary peak is swept to measure the reflected light power distribution inside the optical component 3 to be measured. Can also.

Further, as shown in FIG.
9, the optical phase modulator 5 is added to the reference optical path 9, and the phase φi of the reference light is synchronized with the switching of the optical frequency fi.
Is also known to modulate φi = 2πk · ai · Δf (5). The coherence peak at this time is synthesized at the position of τ = k + m / Δf (6) as shown in FIG. Therefore, by changing the value of k and sweeping the position of the coherence peak while keeping the peak interval constant, the reflected light power distribution inside the measured optical component 3 can be measured. As described above, according to the OCDR, the measured optical component 3 can be used without any mechanically movable parts or numerical processing.
It is possible to observe the reflected light at a specific location inside and the reflected light distribution over the entire length.

[0009]

However, the conventional OC
The DR has various problems with respect to the measurement range as described below. (1) To measure a wide range, a light source having good coherence is required. In this measurement method, the reference light and the reflected light must be coherent. That is, when the coherence time of the output light of the laser 1 is τc, the delay time difference τ is τ <τ
must be c. Therefore, for example, when measuring the reflected light at the far end of the optical fiber, the delay time difference τ increases as the measured optical fiber length increases,
A light source with better coherence is required.

(2) The measurement range is limited by a value inversely proportional to the optical frequency interval Δf. Now, consider a case where the reflections A and B in the optical fiber whose delay times are τA and τB are measured as shown in FIG. in this case,
To measure the reflection A, set the coherence peak to τ = τ
What is necessary is just to combine at the position of A. However, since a coherence peak adjacent to the synthesized coherence peak also exists at the same time, when τB = τA + 1 / Δf (7), the sum of the reflected light powers of the reflection A and the reflection B is measured. The two cannot be separated.
For this reason, as the measurement range becomes wider, a light source capable of making the optical frequency interval Δf smaller becomes necessary.

(3) Measurement accuracy is degraded due to a response delay of the light source. Immediately after switching the optical frequency, intensity fluctuations and frequency fluctuations occur due to a delay in the response of the light source. At this time, the conventional optical reflection measurement circuit deteriorates the measurement accuracy of the reflected light power and the reflected position. was there. As described above, when OCDR is applied to a long optical fiber or the like used for a communication cable, there is a problem that the requirements for the characteristics of the light source become strict.

The present invention has been made in view of the above points, and has as its object to provide a light reflection measuring circuit having the following features. (1) Even when a light source having low coherence is used, the measurable distance is not limited to the coherent distance of the light source, and a long optical fiber can be measured. (2) Even when a light source having a large optical frequency interval Δf is used, long optical fibers can be measured. (3) Eliminating the influence of the response delay of the light source enables more accurate measurement.

[0013]

In order to solve the above-mentioned problems, the invention according to claim 1 comprises an optical frequency variable optical output means, and an output light from the optical frequency variable optical output means, which is used as a reference light and a measured light. First light branching means for branching into measurement light to an optical component, second light branching means for extracting reflected light from the measured optical component, and reflected light extracted by the second light branching means And an optical receiver for receiving a beat signal between two lightwaves of the reference light, wherein the optical frequency variable optical output means changes the optical frequency fi of the output light at a time interval ΔT by fi = f0 +
ai · Δf where i = 1, 2,..., N (integer), f
0: an optical frequency, ai: an integer, and Δf: an optical frequency interval, in an optical reflection measurement circuit that synthesizes a lightwave coherent function in a delta function sequence, wherein the optical frequency variable optical output means includes a time interval ΔT, an optical pulse An optical pulse having a width Δt and an optical frequency fi is output as the output light.

The invention according to claim 2 is characterized in that, in the invention according to claim 1, the optical pulse width Δt and the optical frequency interval Δf are set to Δt <2 / Δf. The invention according to claim 3 is the invention according to claim 1, wherein the phase φi of the reference light is φi = 2πk ·
an optical phase modulator that modulates the optical pulse width Δt and the optical frequency interval Δf from the first optical branching unit to the optical receiver is set to 2Δt <1/2.
It is characterized by being set to Δf.

The invention described in claim 4 is the first invention.
3. The invention according to claim 3, wherein a first optical delay element for giving a delay τd is added to a reference optical path from the first optical branching unit to the optical receiver, and the reference light and the light to be measured are added. The delay τd is set to a value that satisfies τ <τc between the delay time difference τ between two light waves of reflected light from a point of interest in the component and the coherence time τc of the output light of the optical frequency variable optical output means. It is characterized by:

Further, the invention described in claim 5 is the same as that in claim 1.
3. The invention according to claim 3, wherein a first frequency conversion ring circuit having an optical circuit for providing a constant delay and a constant frequency shift every time the light wave makes one round is provided by the first optical branching means. It is characterized in that it is added to a reference optical path leading to the optical receiver. According to a sixth aspect of the present invention, in the invention according to any one of the first to third aspects, the optical frequency variable optical output means includes a light source that outputs a light pulse having a constant optical frequency, and an output of the light source. It is characterized by having a second frequency conversion ring circuit having an optical circuit for inputting light and providing a constant delay and a constant frequency shift every time the light wave makes one round.

According to a seventh aspect of the present invention, in the fifth or the sixth aspect of the present invention, the round delay time ΔT ′ required to make one round of the optical loop circuit is defined with respect to the coherence time τc of the output light. It is characterized in that ΔT ′ <2τc is set. The invention according to claim 8 is the same as the invention according to claim 5.
7. The invention according to any one of items 7 to 7, wherein a circulating delay time ΔT ′ required to make one round of the optical circuit is set to ΔT ′ <2Δt with respect to an optical pulse width Δt of the output light. And The invention according to claim 9 is the invention according to claim 5, wherein the maximum value τM of the delay time difference between the two light waves of the reference light and the reflected light from the measured optical component, the time interval ΔT, the light The pulse width Δt is set so that ΔT> τM + Δt.

According to a tenth aspect of the present invention, there is provided the optical frequency variable optical output means, and the first light that branches the output light of the optical frequency variable optical output means into reference light and measurement light to the optical component to be measured.
Optical branching means, second optical branching means for extracting reflected light from the measured optical component, and a beat signal between the reflected light extracted by the second optical branching means and the two lightwaves of the reference light. An optical receiver for receiving, wherein the optical frequency variable optical output means changes the optical frequency fi of the output light at a time interval ΔT.
fi = f0 + ai · Δf where i = 1, 2,..., N
(Integer), f0: optical frequency, ai: integer, Δf: optical frequency interval, in a light reflection measurement circuit that synthesizes a lightwave coherent function in the form of a delta function sequence, with a constant delay every time the lightwave makes one round A first frequency conversion ring circuit having an optical circuit for providing a constant frequency shift,
It is characterized in that it is added to a reference optical path from the first optical branching means to the optical receiver.

The invention according to claim 11 is the invention according to claim 5
11. The invention according to any one of items 10 to 10, wherein each of the frequency conversion ring circuits includes: an optical ring circuit that circulates a light wave in one direction; an optical input unit that inputs the light wave to the optical ring circuit; A third optical branching means for extracting a light wave from a circuit, a second optical delay element for adjusting a circulating time of the optical ring circuit, a frequency shifter for applying a frequency shift to circulating light circulating the optical ring circuit, It is characterized by having an optical switch for intermittently circulating light and an optical amplifier for compensating for the loss of the circulating light.

According to a twelfth aspect of the present invention, there is provided the optical frequency variable optical output means, and the output light of the optical frequency variable optical output means is branched into reference light and measurement light to the optical component to be measured.
Optical branching means, second optical branching means for extracting reflected light from the measured optical component, and a beat signal between the reflected light extracted by the second optical branching means and the two lightwaves of the reference light. An optical receiver for receiving, wherein the optical frequency variable optical output means changes the optical frequency fi of the output light at a time interval ΔT.
fi = f0 + ai · Δf where i = 1, 2,..., N
(Integer), f0: optical frequency, ai: integer, Δf: optical frequency interval, a first optical delay element that gives a delay τd in an optical reflection measurement circuit that synthesizes a lightwave coherent function in a delta function sequence by switching A delay time difference τ between two light waves of the reference light and the reflected light from a point of interest in the optical component under test, which is added to a reference light path from the first light branching means to the optical receiver; The delay τd is set so that the coherence time τc of the output light of the frequency variable light output means satisfies τ <τc. The invention according to claim 13 is based on claims 1, 10, and
13. The invention according to claim 12, wherein an optical phase modulator for modulating the phase φi of the reference light to φi = 2πk · ai · Δf is provided on the reference light path from the first optical branching unit to the optical receiver. It is characterized by being added.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. [First Embodiment (Claims 1 and 2)] FIG. 1 is a block diagram showing a configuration of a light reflection measuring circuit according to the present embodiment. In FIG. 1, the same components as those in FIG. The description is omitted. The laser 11 shown in FIG. 1 is a light source that outputs an optical pulse. The pulse interval is ΔT, the optical pulse width is Δt, and the ith (i: natural number)
Has an optical frequency of fi.

In the present embodiment, the time τ
The beat signal of the reflected light of the optical frequency fi delayed by (> 0) and the beat signal of the reference light of the optical frequency fi are measured using the optical receiver 2. Note that beat signals of two light waves having frequencies different from each other, other than the beat signal, are removed by a filter (not shown) at the subsequent stage of the optical receiver 2 and are ignored in the following description. At this time, the reflected light power from the position where the delay time τ satisfies the following condition is measured.

The delay time τ is smaller than the optical pulse width Δt (τ <Δt). When this condition is satisfied, the reference light of the optical frequency fi and the reflected light of the optical frequency fi overlap on the time axis as shown in FIG. Hereinafter, the range of the delay time τ where the two light waves overlap on the time axis is referred to as a “pulse window”. The delay time τ is smaller than the coherence time τc of the laser 11 (τ <τc). A coherence peak is synthesized at the position of the delay time τ (τ = m / Δf).

In the present embodiment, the laser output light is converted into an optical pulse, whereby an undesired portion of the transient characteristic of laser frequency modulation can be masked. That is, for example, when the measurement light power fluctuates due to laser frequency switching, the measurement accuracy of the reflected light power deteriorates, but if only the portion where the laser output is stabilized is cut out with an external optical switch (not shown) or the like, measurement is performed. Accurate measurement of the reflected light power becomes possible.

Here, the optical pulse width Δt and the optical frequency interval Δ
If f is set to be Δt <2 / Δf, it is possible to remove the contribution of the coherence peak of the second or higher order. FIG. 3 shows the relationship between the pulse window and the coherence peak. As shown, if 1 / Δf <Δt <2 / Δf, τ = 1 / Δf without crosstalk of the coherence peak of the second order or higher. The reflection at the position can be measured.

[Second Embodiment (Claims 3 and 13)] FIG.
Is a block diagram showing the configuration of the light reflection measurement circuit according to the present embodiment. In the figure, the same components as those in FIG. 1 or FIG. 32 are denoted by the same reference numerals, and description thereof will be omitted. As shown, the present embodiment has a configuration in which an optical phase modulator 5 is added to the reference optical path of the configuration of FIG. Therefore, as described in the prior art, the phase φi
Is modulated by the above equation (5), and the coherence peak is synthesized at the position of the above equation (6). That is, φi = 2πk · ai · Δf, and τ = k + m / Δf.

In this embodiment, the reflected light power from the position where the delay time τ satisfies the following condition is measured. The delay time τ is within the pulse window (shown in FIG. 5) (| τ | <Δt). The delay time τ is smaller than the coherence time τc of the laser 11 (τ <τc). A coherence peak is synthesized at the position of the delay time τ (τ = k + m / Δf). Also in the present embodiment, the laser output light is converted into an optical pulse, and by masking an undesired portion of the transient characteristics of the laser frequency modulation, it becomes possible to measure the reflected light power with high accuracy. Here, FIG. 5 shows the relationship between the position of the coherence peak and the pulse window.
As shown, the coherence peaks are equally spaced 1 / Δf
The peak position is swept by changing the value of k.

As shown in the figure, the pulse window is in a range of a width “2Δt” centered on the delay time τ = 0.
Therefore, if the optical pulse width Δt is set so as to satisfy 2Δt <1 / Δf, the reflected light at the position of the m-th order coherence peak can be measured without crosstalk of other order coherence peaks. As described above, in the present embodiment, after the measurement range is roughly cut out using the pulse window set by the optical pulse, the coherence peak is swept, and the measurement position is specified more precisely. The reflected light distribution inside the measured optical component 3 can be measured without peak crosstalk.

[Third Embodiment (Claims 4, 12, 1)
3)] FIG. 6 is a block diagram showing the configuration of the light reflection measurement circuit according to the present embodiment. In FIG. 6, the same components as those in FIG. 29 are denoted by the same reference numerals, and description thereof will be omitted. Now, as shown in FIG. 6, in the present embodiment, FIG.
The configuration is such that a delay optical fiber 12 is added to the reference optical path 9. Then, in order to measure the reflection at a point of interest inside the optical component 3 to be measured, the delay optical fiber length is set so that the delay time difference τ between the reference light and the reflected light satisfies τ <τc. At this time, by switching the length of the delay optical fiber according to the point of interest, it is possible to measure the distribution of the reflected light at a distance. For this reason, for example, when measuring the reflection at the far end of a long optical fiber, a light source having good coherence was conventionally required, but in the present embodiment, the coherence of the light source may be low.

In the present embodiment, the effect of the above-described delay optical fiber can be similarly obtained even in a configuration in which an optical phase modulator is provided in the reference optical path or a configuration in which the light output from the light source is pulsed. As described above, according to the present embodiment, even when a light source having low coherence is used, the delay optical fiber 12 is provided in the reference optical path and the length thereof is set such that the delay time difference τ becomes τ <τc. By doing so, it is possible to measure a distant reflected light distribution without being limited by the coherence distance of the light source.

[Fourth Embodiment (Claims 5, 7 to 11, 1)
3)] FIG. 7 is a block diagram showing the configuration of the light reflection measuring circuit according to the present embodiment. In FIG. 7, the same reference numerals are given to the same components as those in FIG. 29, and the description thereof will be omitted. Now, as shown in FIG. 7, this embodiment has a configuration in which a frequency conversion ring circuit 13 is added to the reference optical path in FIG. 29, and a more detailed configuration example of this frequency conversion ring circuit 13 is shown in FIG.

As shown in FIG. 8, the frequency conversion ring circuit 13 is configured by connecting an optical coupler 14, an optical frequency shifter 15, an optical switch 16, and an optical amplifier 17 in a loop, and a light wave travels in one direction through this loop. (Counterclockwise). Here, the optical coupler 14 introduces the reference light into the ring circuit, extracts a part of the light circulating around the frequency conversion ring circuit 13, and guides the light to the optical receiver 2. The optical frequency shifter 15 gives a frequency shift by a shift amount fr to the circulating light. The optical switch 16 interrupts the circulating light, and the optical amplifier 17 compensates for the loss of the circulating light.

The light input to the frequency conversion ring circuit 13 has a constant delay Δ each time the circuit goes around the ring circuit.
T ′ (ring orbiting time ΔT ′) and a constant frequency shift fr. Therefore, the frequency conversion ring circuit 13
The input light frequency of input light to f0 is f0, and the input light pulse width is Δ
When t, the output light of the frequency conversion ring circuit 13 is an optical pulse train having a pulse width Δt and a pulse interval ΔT ′. Further, the optical frequency f′x of the optical pulse after x (x = 0, 1, 2,...) Rotation is as follows: f′x = f0 + x · fr (8)

Here, as shown in FIG. 9, the delay time τx with respect to the reference light not circulating in the frequency ring circuit 13
Τx = k + xΔT ′ (9) Attention is paid to a reflection (hereinafter, referred to as a reflection Ax). At this time, the reflected light of the frequency f0 overlaps with the reference light pulse of the frequency f′x that has made x rounds in the ring circuit, and the delay time difference τ between these two light waves is τ = k. Therefore, when the coherence peak is combined with τ = k, if k <τc, the reflection Ax will be observed. In addition, although there are a plurality of reflections Ax, since each frequency is x · fr, the reflections Ax are separated on the frequency axis and can be observed simultaneously without mutual crosstalk.

As described above, in the method using the frequency conversion ring circuit as in this embodiment, the delay time τx is
The range that satisfies τc can be measured. The method of the present embodiment is excellent in that it is not necessary to switch the delay optical fiber according to the point of interest as in the above-described third embodiment.

In the present embodiment, the effect of the above-described frequency conversion ring circuit can be similarly obtained even in a configuration in which an optical phase modulator is provided in the reference optical path or in a configuration in which the light source output light is pulsed. In the present embodiment, it is desirable to set the ring orbiting time ΔT ′ so that ΔT ′ <2τc. In this case, since there is always a reference light having a delay time difference τ <τc with respect to the reflected light, it is possible to perform measurement in which the delay time τ satisfies τ <τc over the entire length of the optical fiber to be measured.

Further, in this embodiment, when the light source output light is pulsed, the light pulse width Δt and the ring rotation time Δ
It is desirable to set T ′ such that ΔT ′ <2Δt. By doing so, a pulse window satisfying | τ | <Δt can be provided over the entire length of the measured optical fiber, and measurement over the entire length of the measured optical fiber can be performed.

In addition, in this embodiment, when the maximum value of the delay time difference between the two light waves of the reference light and the reflected light from the measured optical component 3 is set to τM, as shown in FIG. It is desirable to set ΔT and the optical pulse width Δt so that ΔT> τM + Δt. Therefore, the reason will be described in detail below.

Now, consider a case where the reflected light A (see FIG. 10) from the end of the measured optical fiber is measured. In this case, if xΔT ′ ≒ τM, the reflected light A has a frequency f
An optical pulse of 0 can be measured using a reference optical pulse P1 of frequency f′x obtained by rotating the frequency conversion ring circuit 13 x times. However, at this time, if the reflected light A overlaps the reference light pulse P2 of the frequency f1 on the time axis, for example,
Not only the reference light pulse P1 and the beat signal of the reflected light A (hereinafter referred to as beat signal B1), but also the reference light pulse P2 and the beat signal of the reflected light A (hereinafter referred to as beat signal B2) are generated. In some cases, an erroneous measurement may be made.

That is, when the frequency of the reference light pulse P1 and the frequency of the reference light pulse P2 are equal (ie, f'x = f1), the frequencies of the beat signal B1 and the beat signal B2 become the same, and the two are separated. The problem is that measurement becomes impossible. However, as described above, ΔT> τM + Δt
If the time interval ΔT is set so as to satisfy these conditions, these light waves do not overlap on the time axis t, and the beat signal B2 between the reflected light A at the frequency f0 and the reference light pulse P2 is not generated. Will be resolved.

Here, the frequency conversion ring circuit 13 is used so that the reflected light of the frequency f1 and the reference light pulse P1 do not overlap.
If the light circling the ring is turned off using the optical switch 16 inside, the generation of a beat signal between the reflected light of the frequency f1 and the reference light pulse P1 can be prevented. As described above, in the present embodiment, by using the configuration of FIGS. 7 and 8 for measuring the long optical fiber, the entire length of the optical fiber to be measured can be measured without switching the length of the delay optical fiber according to the point of interest. Measurement becomes possible.

[Fifth Embodiment (Claims 6 to 8)] FIG.
29 is a block diagram showing the configuration of the light reflection measurement circuit according to the present embodiment. In FIG. 29, the same components as those in FIG. 29 are denoted by the same reference numerals, and description thereof will be omitted. In FIG. 11, the laser 18 is a light source having a fixed optical frequency. On the other hand, the frequency conversion ring circuit 19 has a configuration similar to that of the above-described frequency conversion ring circuit 13 (see FIG. 7), the ring orbiting time is ΔT ″, and the shift amount of the optical frequency shifter in the ring circuit is fr. That is, FIG.
As shown in (2), the frequency conversion ring circuit 19 outputs an optical pulse train at time intervals ΔT ″, and the optical frequency of each optical pulse changes stepwise at equal intervals fr.

Here, consider the measurement of the reflected light whose difference in optical path length from the reference light not circulating through the frequency conversion ring circuit 19 is τ0 in terms of time as the reflected light. FIG. 12 shows the relationship between the reference light and the reflected light. In the figure, a flat portion having a width of the ring orbiting time ΔT ″ with respect to the time axis t is hereinafter referred to as a “step”. When the reflected light is observed, since the optical path length difference between the reference light and the reflected light is almost 0, the beat signal in the same step is measured for the reference light and the reflected light.
It does not show the case of 0 ≒ 0. When the coherence peak is synthesized with the delay time difference τ = k (k <ΔT ”and k <τc), the reflected light power from the position of τ0 = k is measured.

The delay time τ0 of the reflected light increases as the reflection point moves farther (the end side of the optical component 3 to be measured), and each time the delay time τ0 becomes an integral multiple of the ring orbiting time ΔT ″, the reflected light pulse Is overlapped with the reference light pulse of the next step, where the reference light of the next step circulates through the frequency conversion ring circuit 19 extra by the ring circling time ΔT ”as compared with the immediately preceding step. Because
Each time the step is switched, the delay time difference τ between the two lightwaves is also τ
≒ 0. Then, for example, as shown in FIG.
The reflected light power from the position of 0 = ΔT ″ + k is measured. As shown, the delay time difference τ = k between the reference light and the reflected light, and the beat frequency between the reference light and the reflected light is This is the shift amount fr of the frequency shifter in the frequency conversion ring circuit 19. Although there are a plurality of places where the delay time difference τ = k, the frequencies of the beat signals generated each time the step is switched are different from each other. It can be measured separately on the frequency axis.

As described above, according to the present embodiment, the power distribution of the reflected light from the long optical fiber can be measured without inserting a delay optical fiber or a frequency conversion ring circuit into the reference optical path. . Note that, in the present embodiment, the same effect can be obtained even when the optical phase modulator is provided on the reference optical path in FIG.

In this embodiment, the ring orbiting time ΔT ″ is set to ΔT ″ for the same reason as in the above-described fourth embodiment.
It is desirable to set so as to be <2τc. In this way, it is possible to measure the delay time difference τ to be τ <τc over the entire length of the optical fiber to be measured. Further, in the present embodiment, for the same reason as in the above-described fourth embodiment,
When the ring orbiting time ΔT ′ and the optical pulse width Δt are ΔT ″ <2
It is desirable to set so as to be Δt. This makes it possible to provide a pulse window satisfying τ <Δt over the entire length of the optical fiber to be measured, thereby enabling measurement over the entire length of the optical fiber to be measured.

[0047]

Embodiments of the present invention will be described below with reference to the drawings. [First Embodiment] FIG. 13 is a block diagram showing a configuration of a light reflection measuring circuit according to the present embodiment. In the figure, the semiconductor laser 21 has a coherence time τc of output light of τc = 2.
00 ns. The optical switch 22 pulsates the output light of the semiconductor laser 21. Optical coupler 23
Guides the measurement light to the measurement target, and also guides the reference light and the reflected light from the measurement target to a balanced optical receiver 26 described later. The optical frequency shifter 24 has a frequency shift amount of 100M
Hz.

The measured optical fiber 25 is a measured optical component to be measured. Here, the measuring light is first guided to the measured optical fiber 25 through the optical frequency shifter 24, and the reflected light from the measured optical fiber 25 is extracted by the optical coupler 23 through the optical frequency shifter 24. That is, since the measurement light passes through the optical frequency shifter 24 twice, the frequency difference between the reflected light and the reference light is 200 MHz.
The balanced optical receiver 26 is a receiver that receives beat signals of the reference light and the reflected light. The band pass filter (BPF) 27 has a center frequency of 200 MHz and removes a beat signal of two light waves whose frequency difference is not 200 MHz.

In the above configuration, the optical frequency of the semiconductor laser 21 is switched every time ΔT, and the optical switch 22 is synchronized with this to output a pulse having an optical pulse width Δt. Here, in this embodiment, the time (light pulse interval) Δ
T = 500 ns and optical pulse width Δt = 300 ns.
Then, the semiconductor laser 21 is set so that the i-th optical frequency fi is given by: fi = f0 + ai · Δf where ai = 0, 1, −1, 2, −2, 3, 3,. Is controlled.
That is, the optical frequency switching waveform is as shown in FIG.

In this embodiment, the reflection at the position where the delay time τ = 50 ns (hereinafter referred to as reflection A) is measured, and therefore τ <τc (= 200 ns) is satisfied. Further, in order to measure the reflection A, the optical frequency interval Δf =
20 MHz, so that 1 / Δf = 50 ns
Therefore, the zero-order coherence peak is synthesized at the position of the reflection A. FIG. 15 is a diagram showing the relationship between the coherence peak and the pulse window. Also,
In the present embodiment, as shown in FIG. 14, after the optical frequency is switched, the optical switch 22 is switched on only for the illustrated time width so that the measurement light is output after the laser output is stabilized. It is controlled to become. Thereby, the measurement can be performed while masking the transitional portion of the optical frequency modulation of the semiconductor laser 21.

[Second Embodiment] The configuration of this embodiment is basically the same as that of the first embodiment.
It is assumed that t = 80 ns. FIG. 15 shows the relationship between the coherence peak and the pulse window. In this embodiment, Δ
Since the relationship of t <2 / Δf (= 100 ns) holds,
The secondary peak crosstalk is eliminated by the pulse window. That is, τ <τc, the primary coherence peak is synthesized at the position of the reflection A, and the optical pulse width Δt is set so that Δt <2 / Δf.
The coherence peaks higher than the next are removed by the pulse window, and the optical power of the reflection A is measured.

As described above, according to the present embodiment, the optical switch 22 is controlled so that the pulse light is output some time after the switching of the optical frequency, and the semiconductor laser 21 is controlled.
Masks the transient part of the optical frequency modulation of
By narrowing the pulse width Δt to Δt <2 / Δf, the crosstalk of the coherence peak of the second or higher order can be removed, and the reflected light power from the position of the primary coherence peak can be measured.

[Third Embodiment] FIG. 16 is a block diagram showing a configuration of a light reflection measuring circuit according to this embodiment. In FIG. 16, the same components as those in FIG. Omitted. Also in this embodiment, the optical frequency is switched with the waveform of FIG. And
In this embodiment, as shown in FIG. 16, an optical phase modulator 28 is provided on the reference optical path. The optical phase modulator 28 modulates the phase φi of the i-th optical pulse so that φi = 2πk · ai · Δf + 2Nπ (10). At this time, N (integer) is adjusted so that | φi | <π, and the optical phase modulation waveform is shown in FIG.
7 is obtained.

In this embodiment, the light pulse interval ΔT = 500
ns, optical frequency interval Δf = 20 MHz, optical pulse width Δt
= 20 ns, and the relationship between the coherence peak and the pulse window is as shown in FIG. As shown, the pulse window is set to −20 ns <τ <20 ns. Since τ <τc (= 200 ns), the zero-order coherence peak position k is set to −20 ns.
<K <20 ns, -20 ns <τ
The reflected light power distribution from the <20 ns range is measured. Further, since Δt <1 / 2Δf (= 25 ns), peaks other than the zero-order are removed by the pulse window.

Thus, τ <τc and -20 ns
<K <20 ns and the optical pulse width Δt is set so as to satisfy Δt <ｆΔf. Therefore, coherence peaks other than the 0th order are removed by a pulse window, and the range of the delay time τ is −20 ns. The reflected light power distribution from the point where <τ <20 ns is measured. As described above, in the present embodiment, the zero-order coherence peak position k is defined as k <
Sweeping in the range of | Δt | and changing the optical pulse width Δt to Δt <1 /
By narrowing to 2Δf, crosstalk of non-zero-order coherence peaks is removed and delay time τ <| Δt
The reflected light power distribution in the range of | can be measured.

[Fourth Embodiment] FIG. 19 is a block diagram showing a configuration of a light reflection measuring circuit according to this embodiment. In FIG. 19, the same components as those in FIG. Omitted. In this embodiment, the optical frequency is
In addition to switching with the waveform shown in FIG.
Are switched by the waveform shown in FIG. In this embodiment, as shown in FIG. 19, a delay optical fiber 29 is provided in the reference optical path. In the present embodiment, the reflection near the end of the optical fiber 25 to be measured having a length of 100 m (hereinafter referred to as reflection B) is measured. Therefore, the length of the delay optical fiber 29 is set to about 200 m so that the delay time difference τ of the reflection B with respect to the reference light is τ <τc.

In this embodiment, the reflected light power near the end of the measured optical fiber 25 can be measured by using the same method as in the third embodiment. By changing the length of the fiber 29, the reflected light power distribution can be measured over the entire length of the measured optical fiber 25. Incidentally, in the case of the configuration of FIG. 16 which does not use the delay optical fiber 29, the delay time τ0 ≒ 1 μs of the reflection B with respect to the reference light, and τ0> τc (= 200n
s), it is difficult to measure the reflection B.

As described above, in this embodiment, the delay optical fiber 29 is inserted into the reference optical path, and τ <τ according to the position to be measured.
The length of the delay optical fiber 29 is switched so as to be c, so that the reflected light power distribution can be observed over the entire length of the long optical fiber. Further, similarly to the third embodiment, by reducing the optical pulse width Δt to Δt <１ ／ Δf, crosstalk of coherence peaks other than the 0th order is removed, and the reflected light power is measured.

[Fifth Embodiment] FIG. 20 is a block diagram showing the configuration of a light reflection measuring circuit according to this embodiment. In FIG. 20, the same components as those in FIG. Omitted. Also in this embodiment, the optical frequency is changed as shown in FIG.
4 and the reference light phase is changed as shown in FIG.
7 is switched. In this embodiment, the frequency conversion ring circuit 30 is provided in the reference optical path as shown in FIG.
Is configured so that its center frequency can be changed.

In this embodiment, in addition to the reflection near the end of the optical fiber 25 to be measured (the above-mentioned reflection B), the reflection near the middle point in the longitudinal direction of the optical fiber 25 to be measured (hereinafter referred to as reflection C). Shall be measured. Here, in the case of a configuration in which the frequency conversion ring circuit 30 does not exist as shown in FIG. 16, τ0> τc, and it is difficult to measure the reflection B.
In this embodiment, the circuit delay time ΔT ′ of the frequency conversion ring circuit 30 is equal to 500 ns, and the optical pulse width Δt is equal to 100.
ns, shift amount fr of the optical frequency shifter in the ring circuit =
-50 MHz.

Here, FIG. 21 shows the overlap of the reference light and the reflected light on the time axis in this embodiment. As shown in the figure, the reference light immediately before the balanced receiver 26 has a pulse interval ΔT ′ (= 500 ns) and a pulse width Δt.
(= 100 ns) and an optical pulse train with a frequency interval fr. Then, as described above, since the delay time of the reflection B with respect to the reference light is τ0 ≒ 1 μs, as shown in the figure, the reference light having made two rounds of the frequency conversion ring circuit 30 and the reflection B overlap on the time axis. At this time, the delay time difference τ between these two light waves
Since τ <τc, by combining the coherence peak at the position to be measured using the same method as in the third embodiment, the reflected light power near the end of the measured optical fiber 25 can be measured.

Further, according to the present embodiment, since the delay time difference τ ′ of the reflection C with respect to the reference light that has made one round of the frequency conversion ring circuit 30 also satisfies τ ′ <τc, the power of the reflection light near the reflection C is also reduced. It can be measured. At this time, reflection B
The frequency of the reference light that interferes with the reflection C differs from the frequency of the reference light that interferes with the reflection C by the shift amount fr (= −50 MHz). Therefore, if the center frequency of the bandpass filter 31 is set to 300 MHz when measuring the reflection B, and the center frequency is set to 250 MHz when measuring the reflection C, the respective reflected light powers can be measured without crosstalk. As described above, this embodiment is excellent in that it is not necessary to switch the delay optical fiber according to the position to be measured as in the fourth embodiment.

Sixth Embodiment The configuration of this embodiment is basically the same as that of the fifth embodiment. However, in this embodiment, as shown in FIG.
T ′ = 300 ns, and ΔT ′ <2τc (= 40
0 ns). FIG. 22 shows the overlap of the reference light and the reflected light on the time axis in this embodiment. According to this embodiment, since there is always a reference light whose relative time difference τ with respect to the reflected light is τ <τc, it is possible to measure τ <τc over the entire length of the optical fiber 25 to be measured.

[Seventh Embodiment] The configuration of this embodiment is basically the same as that of the fifth embodiment. However, in this embodiment, the circulation delay time ΔT '= 500 ns and the optical pulse width Δt = 400 ns. In this case, ΔT ′ <2Δt, and a pulse window is formed over the entire length of the measured optical fiber 25 in the longitudinal direction. FIG. 23 shows this state, in which a pulse window having a width 2Δt is formed at an interval ΔT ′. The horizontal axis of FIG.
5, a delay time τ0 corresponding to the reflection position in FIG.

[Eighth Embodiment] The configuration of this embodiment is basically the same as that of the fifth embodiment. However, in this embodiment, as shown in FIG. The interval ΔT = 2 μs and the light pulse width Δt = 100 ns. Here, the measured optical fiber 2 having the largest delay amount
Even at the reflection at the end of No. 5 (corresponding to the above-described reflection B), the delay time τM ≒ 1 μs, so that the reflected light of the frequency f0 is the reference light of the next frequency f1 generated from the optical switch 22 and the reference light further circulated Does not overlap on the time axis. By setting ΔT> τM + Δt in this way, measurement can be performed without crosstalk between adjacent pulses output from the optical switch 22.

As described above, Examples 5 to 8 were used.
According to the method, the reflected light power distribution can be observed over the entire length of the long optical fiber without switching the delay optical fiber.

Ninth Embodiment FIG. 25 is a block diagram showing the configuration of a light reflection measuring circuit according to the ninth embodiment. In FIG. 25, the same components as those in FIG. Omitted. Now, in FIG. 25, the semiconductor laser 32 outputs a constant frequency optical pulse having a coherence time τc = 200 ns, and the time interval ΔT = 1
0 ms and the optical pulse width Δt = 100 ns. On the other hand, the frequency conversion ring circuit 33 has the configuration shown in FIG. 8, in which the circulating time ΔT ″ = 500 ns and the frequency shift amount fr of the internal optical frequency shifter fr = 10 MHz. Switch with the waveform of
The reference light phase is switched by the waveform in FIG.

In this embodiment, the zero-order coherence peak is synthesized at the position of k = 20 ns, the first reflected light from the position of the delay time τ0 = k with respect to the reference light which does not circulate, and the delay time τ0 = ΔT The second reflected light from the position of “+ k” is measured simultaneously. Here, FIG. 26 shows how the reference light and the reflected light overlap in this embodiment. At this time, for example, the frequency difference between the optical frequency f0 and the optical frequency f1 is the frequency shift amount fr (= 10 MHz) of the optical frequency shifter inside the frequency conversion ring circuit 33. The time difference is ΔT ”(= 500 ns).

The delay time difference τ1 = k between the first reflected light and the reference light
And the beat frequency of these light waves is the light frequency shifter 2
4 is 200 MHz which is twice the shift amount. Also,
The delay time difference τ2 between the second reflected light and the reference light is τ2 = k, and the beat frequency is 210 MHz, which is further shifted from the previous 200 MHz by the frequency shift fr. Therefore, the first
The optical powers of the reflected light and the second reflected light can be measured simultaneously. Further, the N-th position at the position of τ0 = NΔT ″ + k
The reflected light has a delay time difference τN = k from the reference light, and its beat frequency is 200 MHz + Nfr, and all of these can be measured simultaneously. As described above, in this embodiment, it is possible to measure a long optical fiber without switching the delay optical fiber.

[Tenth Embodiment] The structure of this embodiment is basically the same as that of the ninth embodiment. However, in this embodiment, as shown in FIG. 27, the orbiting time .DELTA.T "= 300 ns.
T ″ <2τc (= 400 ns).
Shows a state in which the reference light and the reflected light overlap on the time axis. In this embodiment, the delay time difference τ from the reflected light is τ <τ.
Since there is always a reference light with τc, it is possible to perform measurement with τ <τc over the entire length of the optical fiber 25 to be measured.

[Eleventh Embodiment] The configuration of this embodiment is basically the same as that of the ninth embodiment, but in this embodiment, the orbiting time ΔT ″ = 100 ns, and ΔT ″ <2Δt (= 2
00 ns). In this case, as shown in FIG. 28, a pulse window is formed over the entire length of the measured optical fiber 25, so that measurement over the entire length of the measured optical fiber 25 is possible.

[0072]

As described above, according to the first aspect of the present invention, the output light of the optical frequency variable optical output means is converted into an optical pulse in the reflectometry utilizing the synthesis of the optical coherence function. For example, if the light source is a laser, undesired portions of the transient characteristics of laser frequency modulation can be masked, so cutting out only the stable portion of the laser output has the effect of enabling more accurate reflected light power measurement. Can be

According to the second aspect of the present invention, since the optical pulse width Δt and the optical frequency interval Δf are set to Δt <2 / Δf, the contribution of the coherence peak of the second or higher order can be removed, and the optical frequency interval Δt can be eliminated. Even if a light source having a large coherence peak is used, it is possible to measure a long optical fiber or the like without crosstalk of these coherence peaks.

According to the third aspect of the present invention, the phase φi of the reference light is modulated by φi = 2πk · ai · Δf, and the optical pulse width Δt and the optical frequency interval Δf are set to 2Δt <1 / Δf. So, after roughly cutting out the measurement range using the pulse window generated by the light pulse, the coherence peak can be swept and the measurement position can be specified more precisely.
Even if a light source having a large optical frequency interval is used, the reflected light at the position of the coherence peak of interest can be measured without crosstalk between coherence peaks of different orders.

According to the fourth or twelfth aspect of the present invention, the first optical delay element is added to the reference light path, and the delay time difference τ between the two light waves of the reference light and the reflected light and the optical frequency variable light output Since the delay τd of the first optical delay element is set so that the coherence time τc of the output light of the means satisfies τ <τc, if the delay τd is set in accordance with a point of interest inside the optical component to be measured, Without being limited by the coherence length of the light source,
The effect is obtained that the distribution of reflected light at the far end of the long optical fiber can be measured using a light source having low coherence.

According to the fifth or tenth aspect of the present invention, the first frequency conversion ring circuit for providing a constant delay and a constant frequency shift for each rotation is added to the reference light path, so that the light source having low coherence is provided. Can be used to measure the range in which the delay time difference τ exceeds τc without limiting the measurable distance to the coherent distance of the light source. The effect is obtained that the reflected light distribution can be measured without performing adjustment such as switching the delay time of the element.

According to the sixth aspect of the present invention, the optical frequency variable optical output means includes a light source for outputting an optical pulse having a constant optical frequency and a second frequency for providing a constant delay and a constant frequency shift for each rotation. Since it is constituted by the conversion ring circuit, it is possible to measure the reflected light power distribution of the measured optical component without inserting an optical delay element, a frequency conversion ring circuit, or the like in the reference optical path.

According to the seventh aspect of the present invention, the circulating delay time ΔT ′ required to make one round of the optical circuit and the coherence time τc of the output light are set to ΔT ′ <2τc. There is always a reference beam whose delay time difference τ satisfies τ <τc, and an effect is obtained that measurement can be performed with τ <τc over the entire length of the optical component to be measured. According to the eighth aspect of the present invention, since the circulating delay time ΔT ′ and the optical pulse width Δt required to make one round of the optical loop circuit are set to ΔT ′ <2Δt, the pulse width over the entire length of the optical component to be measured is set. An effect is obtained in which a window is formed and measurement of the distribution of reflected light over the entire length of the long optical fiber becomes possible.

According to the ninth aspect of the present invention, the maximum value τM, the time interval ΔT, and the optical pulse width Δt of the delay time difference between the two light waves of the reference light and the reflected light from the optical component to be measured are respectively set as ΔT> Since τM + Δt is set, it is possible to avoid the problem caused by the reflected light overlapping with a plurality of reference lights on the time axis, and to measure even a light source having a large optical frequency interval without crosstalk between adjacent pulses. The effect is obtained. According to the thirteenth aspect, since the phase φi of the reference light is modulated by φi = 2πk · ai · Δf, the output light of the optical frequency variable light output unit is optically pulsed, Even when an optical delay element or a frequency conversion ring circuit is inserted in the reference optical path, by sweeping the position of the coherence peak while keeping the peak interval constant, it is possible to measure the reflected light distribution inside the optical component to be measured. .

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a light reflection measurement circuit according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating overlap of pulses of reference light and reflected light in the embodiment.

FIG. 3 is a light wave coherence function γ in the embodiment.
FIG. 4 is a diagram illustrating a relationship between (τ) and a pulse window.

FIG. 4 is a block diagram illustrating a configuration of a light reflection measurement circuit according to a second embodiment of the present invention.

FIG. 5 is a light wave coherence function γ in the embodiment.
FIG. 4 is a diagram illustrating a relationship between (τ) and a pulse window.

FIG. 6 is a block diagram illustrating a configuration of a light reflection measurement circuit according to a third embodiment of the present invention.

FIG. 7 is a block diagram illustrating a configuration of a light reflection measurement circuit according to a fourth embodiment of the present invention.

FIG. 8 is a frequency conversion ring circuit 13 according to the first embodiment;
FIG. 3 is a block diagram showing the configuration of FIG.

FIG. 9 is a diagram illustrating a state of interference between reference light and reflected light in the embodiment.

FIG. 10 is a diagram illustrating overlap between reference light and reflected light in the embodiment.

FIG. 11 is a block diagram illustrating a configuration of a light reflection measurement circuit according to a fifth embodiment of the present invention.

FIG. 12 is a diagram illustrating a state of interference between reference light and reflected light in the embodiment.

FIG. 13 is a block diagram showing a configuration of a light reflection measuring circuit according to the first embodiment of the present invention.

FIG. 14 is a waveform chart showing an optical frequency switching waveform in the embodiment.

FIG. 15 shows a light wave coherence function γ in the embodiment.
FIG. 4 is a diagram illustrating a relationship between (τ) and a pulse window.

FIG. 16 is a block diagram illustrating a configuration of a light reflection measurement circuit according to a third embodiment of the present invention.

FIG. 17 is a waveform chart showing an optical phase modulation waveform in the example.

FIG. 18 shows a light wave coherence function γ in the embodiment.
FIG. 4 is a diagram illustrating a relationship between (τ) and a pulse window.

FIG. 19 is a block diagram showing a configuration of a light reflection measuring circuit according to a fourth embodiment of the present invention.

FIG. 20 is a block diagram showing a configuration of a light reflection measurement circuit according to a fifth embodiment of the present invention.

FIG. 21 is a diagram showing a relationship between reference light and reflected light in the example.

FIG. 22 is a diagram illustrating a relationship between reference light and reflected light according to a sixth embodiment of the present invention.

FIG. 23 shows a delay time τ0 according to a seventh embodiment of the present invention.
FIG. 3 is a diagram showing a relationship between the pulse window and the pulse window.

FIG. 24 is a diagram illustrating a relationship between reference light and reflected light according to an eighth embodiment of the present invention.

FIG. 25 is a block diagram showing a configuration of a light reflection measurement circuit according to a ninth embodiment of the present invention.

FIG. 26 is a diagram showing a relationship between reference light and reflected light in the example.

FIG. 27 is a diagram illustrating a relationship between reference light and reflected light according to a tenth embodiment of the present invention.

FIG. 28 illustrates a delay time τ according to an eleventh embodiment of the present invention.
It is a figure showing the relation between 0 and a pulse window.

FIG. 29 is a block diagram showing a configuration of a light reflection measurement circuit according to a conventional technique.

FIG. 30 is a waveform chart showing an example of optical frequency switching in the conventional technique.

FIG. 31 is a waveform chart showing a lightwave coherence function γ (τ) synthesized in the conventional technique.

FIG. 32 is a block diagram illustrating a configuration example of an apparatus that sweeps a coherence peak by phase modulation in the related art.

FIG. 33 is a waveform diagram showing a lightwave coherence function γ (τ) synthesized in the configuration of FIG. 32;

FIG. 34 is a waveform chart for explaining a limitation of a measurement range in the conventional technique.

[Explanation of symbols]

 1,11,18 laser 2 optical receiver 3 optical component under test 4,14,23 optical coupler 5,28 optical phase modulator 12,29 delay optical fiber 13,19,30,33 frequency conversion ring circuit 15,24 light Frequency shifter 16, 22 Optical switch 17 Optical amplifier 21, 32 Semiconductor laser 25 Optical fiber to be measured 26 Balanced optical receiver 27, 31 Bandpass filter

Continuing on the front page (72) Inventor Tsuneo Horiguchi Nippon Telegraph and Telephone Corporation 3-9-1-2 Nishi-Shinjuku, Shinjuku-ku, Tokyo (72) Inventor Kazuo Hotate 6-17-17 Fuda, Chofu-shi, Tokyo (72) Inventor Takashi Saida 2-36-19-210 Daisawa, Setagaya-ku, Tokyo

Claims (13)

[Claims] An optical frequency variable optical output unit; a first optical branching unit that branches output light from the optical frequency variable optical output unit into reference light and measurement light to an optical component to be measured; A second optical branching unit for extracting reflected light from the optical component; and an optical receiver for receiving a beat signal between the reflected light extracted by the second optical branching unit and the two light waves of the reference light. The optical frequency variable optical output means sets the optical frequency fi of the output light at a time interval ΔT as follows: fi = f0 + ai · Δf, where i = 1, 2,..., N (integer), f0: optical frequency, ai: integer .DELTA.f: an optical frequency interval, in an optical reflection measurement circuit for synthesizing a light wave coherent function in the form of a delta function sequence, wherein the optical frequency variable optical output means comprises a light having an optical frequency fi at a time interval .DELTA. Pulse as the output light A light reflection measurement circuit characterized by outputting. 2. The light reflection measuring circuit according to claim 1, wherein the light pulse width Δt and the light frequency interval Δf are set to Δt <2 / Δf. 3. The phase φi of the reference light is defined as φi = 2πk.
An optical phase modulator that modulates the optical pulse width Δt and the optical frequency interval Δf from the first optical branching means to the optical receiver is set to 2Δt <
The light reflection measurement circuit according to claim 1, wherein 1 / Δf is set. 4. A first optical delay element for providing a delay τd is added to a reference optical path from the first optical branching unit to the optical receiver, and a portion of interest in the reference light and the optical component to be measured. The delay time difference τ between the two light waves of the reflected light from the light source and the coherence time τc of the output light from the optical frequency variable light output means are τ.
4. The light reflection measurement circuit according to claim 1, wherein the delay [tau] d is set to a value satisfying <[tau] c. 5. A first frequency conversion ring circuit having an optical circuit for providing a constant delay and a constant frequency shift every time a light wave makes one round, from the first optical branching means to the optical receiver. 4. The light reflection measurement circuit according to claim 1, wherein the light reflection measurement circuit is added to a reference light path. 6. An optical frequency variable optical output means, comprising: a light source for outputting an optical pulse with a constant optical frequency; an output light from the light source as an input; and a constant delay and a constant frequency shift every time the light wave makes one round. 4. A light reflection measuring circuit according to claim 1, further comprising: a second frequency conversion ring circuit having an optical circuit that provides the following. 7. A circulating delay time ΔT ′ required to circulate the optical circuit once is defined as a coherence time τc of the output light.
7. The light reflection measuring circuit according to claim 5, wherein ΔT ′ <2τc is set with respect to. 8. The circuit according to claim 5, wherein a round-trip delay time ΔT ′ required to make one round of the optical circuit is set to ΔT ′ <2Δt with respect to an optical pulse width Δt of the output light.
8. The light reflection measurement circuit according to any one of items 7 to 7. 9. The maximum value τM of the delay time difference between the two light waves of the reference light and the reflected light from the measured optical component, the time interval ΔT, and the optical pulse width Δt are respectively expressed as ΔT> τM.
6. The light reflection measuring circuit according to claim 5, wherein the value is set to + Δt. 10. An optical frequency variable optical output means, first optical splitting means for splitting output light of the optical frequency variable optical output means into reference light and measurement light to an optical component to be measured, A second optical branching unit for extracting reflected light from the optical component; and an optical receiver for receiving a beat signal between the reflected light extracted by the second optical branching unit and the two light waves of the reference light. The optical frequency variable optical output means sets the optical frequency fi of the output light at a time interval ΔT as follows: fi = f0 + ai · Δf, where i = 1, 2,..., N (integer), f0: optical frequency, ai: integer , Δf: optical frequency interval, an optical reflection measurement circuit for synthesizing a lightwave coherent function in the form of a delta function train, comprising an optical circuit for providing a constant delay and a constant frequency shift every time the lightwave makes one round. 1 frequency conversion ring Road, said first optical reflection measurement circuit, characterized in that from the light branching means is added to the reference optical path to the optical receiver. 11. Each of the frequency conversion ring circuits includes: an optical ring circuit that circulates an optical wave in one direction; an optical input unit that inputs the optical wave to the optical ring circuit; and a third optical circuit that extracts an optical wave from the optical ring circuit. An optical branching means, a second optical delay element for adjusting the time required for circulating the optical ring circuit, a frequency shifter for applying a frequency shift to the circulated light circulating the optical ring circuit, and an optical switch for intermittently circulating the circulated light The light reflection measurement circuit according to claim 5, further comprising: an optical amplifier that compensates for the loss of the circulating light. 12. An optical frequency variable optical output means, first optical splitting means for splitting output light of the optical frequency variable optical output means into reference light and measurement light to an optical component to be measured, and A second optical branching unit for extracting reflected light from the optical component; and an optical receiver for receiving a beat signal between the reflected light extracted by the second optical branching unit and the two light waves of the reference light. The optical frequency variable optical output means sets the optical frequency fi of the output light at a time interval ΔT as follows: fi = f0 + ai · Δf, where i = 1, 2,..., N (integer), f0: optical frequency, ai: integer Δf: an optical frequency interval, in an optical reflection measurement circuit for synthesizing a lightwave coherent function in the form of a delta function sequence, wherein a first optical delay element for giving a delay τd is provided by the first optical branching means to the optical receiver. To the reference beam path leading to A delay time difference between two light waves of the light reflected from the target to position of the reference light and the light to be measured in parts tau, and the coherence time τc of the output light of the optical variable frequency optical output means tau
A light reflection measuring circuit, wherein the delay τd is set to a value satisfying <τc. 13. The phase φi of the reference light is defined as φi = 2π
13. The optical phase modulator for modulating k.ai..DELTA.f is added to a reference optical path from the first optical branching means to the optical receiver. Light reflection measurement circuit.