JPH06324204A - Optical member for forming optical path length difference - Google Patents

Abstract

(57) [Abstract] [Purpose] To stabilize the wavelength of the light output from the light source device even if the surrounding environment changes. An optical path difference forming optical member that provides a predetermined optical path length difference between a first path and a second path has a first member that forms a first path and a second member that forms a second path. An optical path length difference forming optical member is characterized in that both optical path length differences do not change even if the temperature changes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical member for forming an optical path length difference applicable to a wavelength stabilized light source device and others.

[0002]

2. Description of the Related Art Semiconductor lasers are widely used as coherent light sources in various fields. However, since the oscillation wavelength of the semiconductor laser is extremely sensitive to temperature fluctuations and injection current fluctuations, it is necessary to suppress these fluctuations as much as possible.

Therefore, conventionally, a semiconductor laser is provided with an electronic heating / cooling element such as a Peltier element and a temperature detecting element such as a thermistor for measuring the temperature of the semiconductor laser itself, and the temperature of the semiconductor laser is measured according to the output signal of the thermistor. The Peltier device is driven so that the value of P becomes constant. In addition, in a more accurate stabilization method, a wavelength reference such as an etalon plate or atomic / molecular absorption line is provided, and the error signal from it is fed back to the temperature and injection current of the semiconductor laser to stabilize the oscillation wavelength. A feedback method is adopted.

However, the wavelength reference as described above is expensive and many are large, resulting in a very large-scale system. Further, the accuracy of stabilization depends on the bandwidth of the detection characteristic at the time of error signal detection. And its peak value detection accuracy.

Therefore, the present applicant has filed Japanese Patent Application No. 4-15113.
No. 3 invention was proposed to realize a wavelength stabilized light source device that solves the above problems. This is because, for example, in a fringe scanning interferometer equipped with an optical means for giving a specific optical path difference, for example, by keeping the phase of two fringe scan interference signals having different optical path differences at a specific value, for example, the oscillation wavelength of a semiconductor laser. Is a wavelength stabilized light source device such as a wavelength stabilized semiconductor laser light source having a simple structure. Further, the oscillation wavelength of a light source device such as a plurality of pulse-driven alternately oscillating semiconductor lasers is stabilized at the same time. Furthermore, by modulating the injection current to a light source device such as a semiconductor laser,
Two different stabilized wavelengths are obtained from a light source device such as one semiconductor laser.

The invention of Japanese Patent Application No. 4-151133 will be described in more detail with reference to FIGS.

FIG. 1 shows the structure of a semiconductor laser light source device which implements the wavelength stabilized light source device of the invention of Japanese Patent Application No. 4-151133.

The light source section 1 comprises a semiconductor laser 11, a thermistor 12, a Peltier element 13 and a heat radiating plate 14. The light continuously emitted from the light source unit 1 is collimated by the collimator lens 3
Is collimated by the light source, passes through the isolator 33, and its light P
Part of the light P1 is reflected by the beam splitter 30. The other part of the light P passes through the beam splitter 30 and becomes the output light P2.

The light P1 is split by the beam splitter 31 into reflected light P3 and transmitted light P4. The reflected light P3 passes through the condenser lens 5 and is reflected by the scanning mirror 35.
Then, the reflected light P3 returns to the same optical path, enters the beam splitter 31 again, passes through the beam splitter 31, and goes to the condenser lens 6. On the other hand, the transmitted light P4 is reflected by the two first reflection surfaces A and B of the step mirror 34, returns through the same optical path, is reflected by the beam splitter 31, and overlaps with the reflection light P3 from the scanning mirror 35. It becomes interference light and is condensed by the condenser lens 6. The two first reflecting surfaces A and the second reflecting surface B of the step mirror 34 are stepped by the step amount L. The step mirror 34 is fixed. The step amount L is set along the directions of the light P1 and the transmitted light P4. Of this interference light, the interference light by the two first reflection surfaces A and B of the step mirror 34 is split by the prism 7 and detected by the detectors 41, 4 respectively.
Detected in 2.

The scanning mirror 35 is attached to a piezoelectric element 36 such as PZT, and the oscillator 3
8, pulse signal generator 39, and PZT driver 37
1 is swept in the direction of the arrow in FIG. 1 by several wavelengths of the oscillation wavelength of the semiconductor laser 11.

Now, assuming that the optical path length d2 from the beam splitter 31 to the scanning mirror 35 and the optical path length d1 to the first reflecting surface A of the step mirror 34 are substantially equal,
The output signal from the detector 41 is a fringe scan signal with an optical path difference of around 0, and the output signal from 42 is a fringe scan signal with an optical path difference of around 2L. The wavelength of the semiconductor laser is stabilized using these output signals. These output signals are phase-compared by the phase comparator 100 and 2
Light source drive circuit 1 so that the phases of the two signals are exactly the same
The command is fed back to the light emitting unit 1 via 0.

The above feedback system will be described in more detail. FIG. 2 shows an example of the configuration of the feedback system, and FIG. 3 shows signals of various parts of the feedback system of FIG. 2 (a phase delay on the circuit is ignored).

The signals obtained by extracting only the AC component from the outputs of the detectors 41 and 42 are as signals a and b in FIG. 3, respectively. As described above, the signal a is a fringe scan signal near the optical path length difference of 0, and the signal b is a fringe scan signal near the optical path length difference of 2L. By comparing the phases of these signals a and b and feeding them back to the light emitting unit 1 via the light source drive circuit 10 of FIG. 2 so that the phases of the two signals are completely matched, the semiconductor laser 11 of FIG. Stabilizes the oscillation wavelength of. In order to match the phases of the two signals, the oscillation wavelength λ may be changed by changing the temperature and injection current of the semiconductor laser 11.

The signal a from the detector 41 is converted into two rectangular signals c and d whose phases are inverted by the comparator 60 shown in FIG. The signal b from the detector 42 is sampled and held by the sample and hold 61 and 62 at the rising timing of the signals c and d, and the sampled and held output signals are e and f, respectively.

Further, the output signals e and f are sampled and held by sample and hold 63 and 64, and the hold portions are taken out and passed through the low pass filters 65 and 66 to obtain outputs g and h. A differential amplification of these outputs g and h by the differential amplifier 70 becomes a feedback signal corresponding to the phase difference between the signals a and b. The oscillation wavelength can be stabilized by feeding back this feedback signal to the temperature or the injection current.

The circuit configuration of FIG. 2 shows a case where feedback is made to both temperature and injection current. Here, the output of the differential amplifier 70 is passed through two low-pass filters 81 and 82 having different time constants and amplification degrees, and the feedback signal corresponds to the phase difference between the signals a and b.
The output of the low pass filter 81 is fed back to the light emitting section 1 via the temperature control circuit 10a, and the output of the low pass filter 82 is fed back to the light emitting section 1 via the current control circuit 10b and the drive circuit 10c. Of these, the one with temperature control
Since the response is slow, the time constant is long, and since the wavelength change rate is large, it is used as a rough adjustment. The current control has a short response because of its quick response, and is used as a fine adjustment because the rate of wavelength change is small.

The signal a is represented by the equation 1 and the signal b is represented by the equation 2
It is represented by.

[0018]

[Equation 1]

[0019]

[Equation 2] Here, α (t) represents the amount of movement of the scanning mirror 35 (the direction of the arrow in FIG. 1). In order to match the phases of the two signals a and b, it is necessary that 2L / λ = n (n is an integer), and the oscillation wavelength λ is fixed to λ = 2L / n. However, if the value of n is wrong, it is set to a different wavelength, so the temperature of the semiconductor laser 11 must always be controlled within a certain temperature range based on a certain set temperature (a temperature at which no mode hop is in the vicinity). is there. The temperature control range is determined by the temperature dependence of the oscillation wavelength of the semiconductor laser and the step amount L. The long-term stability can be improved, for example, by controlling the temperature of the step mirror 34 to suppress the variation of the step amount L.

With the above method, the phases of the signals a and b from the detectors 41 and 42 can be matched, and the oscillation wavelength λ of the semiconductor laser 11 is locked at λ = 2L / n. According to this method, the scanning mirror 35 only needs to scan the interference fringes, and the nonlinearity of the movement, amplitude fluctuation, and origin fluctuation do not pose any problem.
The influence of the movement of the scanning mirror 35 such as yawing can be eliminated by using the condenser lens 5 having an appropriate focal length in front of the scanning mirror 35 as shown in FIG.

In this way, the oscillation wavelength of the semiconductor laser can be stabilized by matching the phases of the two fringe scan signals a and b having different optical path differences in the two-beam interferometer. Furthermore, by combining with an APC (auto power control) circuit, a wavelength / output stabilized semiconductor laser light source can be realized.

The apparatus shown in FIGS. 1 to 3 is an example in which one semiconductor laser (light source section or light emitting section) is driven by continuous oscillation. In other words, this is an example in the case of DC lighting.

The wavelength-stabilized light source device of the invention of Japanese Patent Application No. 4-151133 can be applied not only when DC lighting is performed, but also when the semiconductor laser is pulse-driven. Figure 4
In the case of the semiconductor laser light source device, the plurality of semiconductor lasers (light source units or light emitting units) that alternately oscillate can be simultaneously stabilized.

For simplification, FIG. 4 shows a configuration in which two semiconductor lasers are used as an example. In the apparatus of FIG. 4, the same elements as those of the apparatus of FIG. 1 are designated by the same reference numerals.

In FIG. 4, reference numeral 2 denotes a second light emitting portion, 10
X and 20X are light source driving circuits for the first light emitting unit 1 and the second light emitting unit 2, respectively, and 40 are various pulse signal generators.
And 110 is a phase comparator.

The light emitted from the light emitting section 2 is collimated by the collimating lens 4 and is made to have the same optical axis as the light from the light emitting section 1 by the beam splitter 32. The semiconductor lasers 11 and 21 of the light emitting units 1 and 2 are respectively denoted by (1) and (2) in FIG.
When the pulse drive is performed by the signal of, the state of the wavelength change at that time is as shown in waveforms (3) and (4), and the wavelength of the pulsed laser light is always changing. Here, the wavelength is stabilized at a constant timing in pulse oscillation. Specifically, signals are taken out from the outputs of the detectors 41 and 42 at timings (5) and (6), and the wavelengths at these timings (5) and (6) are stabilized.
The sweep frequency of the scanning mirror 35 is sufficiently slower than the frequencies of the signals (1) and (2).

FIG. 6 shows the configuration of the feedback system. The same parts as those in FIG. 2 are designated by the same reference numerals. For simplicity, only the light emitting unit 1 is shown, but the configuration of the feedback system for the light emitting unit 2 is also the same.

The outputs of the detectors 41 and 42 are output to the timing signal (5) in the sample and hold 50 and 51, respectively.
The sample is held by. The outputs of these sample and hold 50 and 51 are a1 and b1 in FIG. 7, respectively. When these are passed through filters 52 and 53, they become a2 and b2, which are equivalent to the fringe scan signals (signals a and b in FIG. 3) when the DC lighting is performed as described in the first embodiment. Therefore, thereafter, the wavelength can be stabilized by performing the signal processing in the above-described first embodiment. The same applies to the light emitting unit 2. The sample-holds 50 and 51 and the filters 52 and 53 include detectors 41 and 42.
Constitutes a sampling unit for the output of.

When the semiconductor laser is pulse-driven as in the apparatus shown in FIG. 4, a feedback method for controlling the pulse width may be used. If the extinction timing of the sampling timing signals (5) and (6) and the driving pulses (1) and (2) in FIG. 5 is fixed, the driving pulse (1),
If the light emission timing of (2) is changed, the wavelength change waveforms (3) and (4) change, so the oscillation wavelength changes. Feedback can be applied using this relationship.

As shown in FIG. 6, the low-pass filter 8
As described above, the output of 1 is fed back to the light emitting section 1 via the temperature control circuit 10a, and the output of the low pass filter 82 is fed back to the light emitting section 1 via the pulse width control circuit 10d and the pulse drive circuit 10e. However, in this case, the pulse width has the maximum duty 50. Since this feedback method stabilizes the wavelength by controlling the temperature and the pulse width, the injection current can be controlled by the APC alone.

As described above, even in the pulse-driven semiconductor laser, the wavelength can be stabilized at a constant timing, and a plurality of wavelength-stabilized semiconductor laser light sources that alternately oscillate can be realized. By extracting the light at the same timing from the output light and processing the signal, it is possible to simultaneously obtain signals of a plurality of wavelengths.

When the feedback is applied, the oscillation wavelength λ of the semiconductor lasers 11 and 12 at a fixed timing.
1 and λ2 are fixed as shown in Expression 3 and Expression 4.

[0033]

[Equation 3]

[0034]

[Equation 4] Here, n1 and n2 are integers. When the combined wavelength is handled from the combination of these two wavelengths, the combined wavelength Λ is given by Equation 5, and Λ is also fixed to an integer fraction of 2L.

[0035]

[Equation 5] Therefore, by appropriately selecting the two semiconductor lasers, an arbitrary synthetic wavelength can be created.

Further, by applying this pulse driving case, it is possible to obtain two different wavelengths from one semiconductor laser. FIG. 8 shows the drive waveform. Waveform 1) is when the number of semiconductor lasers used is 1, and waveform 2)
Is more than one case. The oscillation wavelength at the timing A is stabilized by the method described above. By increasing (or decreasing) the injection current from there, different wavelengths can be obtained at the timing of B. Since the wavelength of the timing of B is slightly different from that of A, the stabilization is performed by increasing (or decreasing) the current so that the phase difference between the outputs a2 and b2 of the filters 52 and 53 in FIG. 6 becomes an appropriate value. You can control the value. Alternatively, it can be stabilized by simply suppressing the current value to be increased (or decreased) with high accuracy. In particular, this is sufficient when the combined wavelength is handled from the wavelengths of A and B timings. Assuming that the timing wavelengths of A and B are λA and λB, the combined wavelength is λA ·
λB / | λA−λB |, and the stability is λA−λB
You can say that is decided by. Therefore, if the current value is stable, the wavelength λB changes following the wavelength λA, and the change in the combined wavelength itself is small.

By using such a wavelength-stabilized light source device, by modulating the injection current of the semiconductor laser,
It is possible to obtain two different stabilized wavelengths from one semiconductor laser.

[0038]

However, in the above wavelength stabilized light source device, the optical path length defined by the step amount (L) of the step mirror changes due to changes in the surrounding environment. Specifically, due to the thermal expansion of glass and the change of refractive index of air, the difference in optical path length as a wavelength reference is (2
L) will change. Therefore, in order to maintain long-term stability of the wavelength, it is necessary to take measures such as controlling the temperature of the step mirror itself.

An object of the present invention is to provide an optical path length difference forming optical member which can solve the above problems and realize a wavelength stabilized light source device having excellent environment resistance.

[0040]

A first invention of the present application is based on the following optical path length difference forming optical member. That is, in the optical path length difference forming optical member that gives a predetermined optical path length difference between the first path and the second path, it has a first member that forms the first path and a second member that forms the second path. ,
An optical member for forming an optical path length difference satisfying the following formula so that the optical path length difference between the two does not change even if the temperature changes.

{ΔL2 (n2 + Δn2) −ΔL1 (n1 + Δn1)} + (Δn2L2−Δn1L1) = (n + Δn) × (h1 + ΔL2−ΔL1) −n × h1 .. Equation 6 Here, L1: the first Length of member, ΔL1: Change in length of first member due to temperature change L2: Length of second member, ΔL2: Change in length of second member due to temperature change n1: First Refractive index of member, Δn1: Change in refractive index of first member due to temperature change n2: Refractive index of second member, Δn2: Change in refractive index of second member due to temperature change n: First Refractive index of the medium that causes the difference in length between the member and the second member, Δn: Amount of change in the refractive index of the medium that causes the difference in length between the first member and the second member due to a change in temperature h1: Reference Difference in length between the first member and the second member at temperature The second invention of the present application is the optical path length difference forming optical member according to the first invention, which is the first part. When those set substantially the same the length of the second member.

The third invention of the present application further includes a first member and a second member.
The material is selected so that the expansion coefficient of the member is almost the same.

Preferably, in the first invention, the lengths (dimensions) of the glass itself of the first member and the glass of the second member are made substantially equal to each other, and the predetermined expansion coefficient and refractive index are satisfied so as to satisfy the above-mentioned formula (6). Select a glass with a temperature coefficient of.

Preferably, in the first invention, the lengths (dimensions) of the glass itself of the first member and the second member are made substantially equal to each other, and the expansion coefficients of the glass themselves of the first member and the second member. By setting α ₁ and α ₂ to be substantially equal to each other, a glass having a temperature coefficient of a refractive index that satisfies Expression 6 is selected. Then, even if the temperature changes, it is possible to prevent the air layer portion from entering only one optical path as much as possible, and it is possible to minimize the influence of the difference in the optical path lengths due to the change in the refractive index of air due to the change in atmospheric pressure.

With reference to FIG. 9, the principle of the present invention and the method of obtaining the above-mentioned formula 6 will be described below.

First, the optical path lengths of the first and second glass members at the reference temperature are La1 and La2.

(L1, L2 length of first and second members, n1
, N2: Refractive indices of the first and second members) La1 = n1 x L1 La2 = n2 x L2 The optical path length difference is as follows.

ΔLa = La2−La1−n × h1 = n2 × L2−n1 × L1−n × h1 Equation 7 Here, the optical path length difference means the longer one of the first and second members. It refers to the difference in optical path length. In the case of short glass, an air layer (n.times.h1) is included. n is the refractive index of air,
h1 is the difference between the lengths of the first and second members at the reference temperature.
Next, let Lb1 and Lb2 be the optical path lengths of the first member and the second member when the temperature changes by Δt. Due to this temperature change, α ₁ and α ₂ are the expansion coefficients of the first member and the second member, and β 1, β 2, and β are the first member, the second member, and
It is the temperature coefficient of the refractive index of air.

Lb1 = (n1 + β1Δt) × L1 × (1
+ Α ₁ Δt) = (n1 + Δn1) × (L1 + ΔL1) Lb2 = (n2 + β2 Δt) × L2 × (1 + α2 Δt)
= (N2 + Δn2) × ( L2 + ΔL2) However, Δn1 = β1 Δt, Δn2 = β2 Δt, ΔL1 = L1 α 1 Δt, ΔL2 = L2 α 2 Δt The optical path length difference ΔLb is determined as follows.

ΔLb = Lb2−Lb1− (n + βΔt) × h2 = Lb2−Lb1− (n + Δn) × h2 = Lb2−Lb1− (n + Δn) × (h1 + ΔL2−ΔL1) = (n2 + Δn2) × (L2 + ΔL2) − (N1 + Δn1) × (L1 + ΔL1) − (n + Δn) × (h1 + ΔL2−ΔL1) ... Equation 8 where h2 is the difference between the glass lengths of the first and second members after the temperature changes by Δt. And is as follows:

H2 = h1 + ΔL2−ΔL1 With respect to the optical path length difference ΔLa at the reference temperature, how much the optical path length difference ΔLb changes when the temperature changes by Δt, and the change amount ΔL are calculated as follows. Ask.

ΔL = ΔLb−ΔLa = (n2 + Δn2) × (L2 + ΔL2) − (n1 + Δn1) × (L1 + ΔL1) − (n + Δn) × (h1 + ΔL2-ΔL1)-{n2 × L2-n1 × L1 -N * h1} = (n2 [Delta] L2 + [Delta] n2 L2 + [Delta] n2 [Delta] L2)-(n1 [Delta] L1 + [Delta] n1 L1 + [Delta] n1 [Delta] L1) -n * ([Delta] L2- [Delta] L1)-[Delta] n * (h1 + [Delta] L2- [Delta] L2 (n1 [Delta] L2 (n1 [Delta] L2 + .DELTA.n1)} + (. DELTA.n2 L2-.DELTA.n1 L1)-(n + .DELTA.n) .times. (H1 + .DELTA.L2-.DELTA.L1) + n.times.h1 (9) When ΔL = 0, there is no change in the optical path length difference.

When ΔL = 0 is set as the ideal state and the transfer is performed, the above-mentioned formula 6 is obtained. Mathematical Expression 6 is explained. The first term on the left side is the amount of change in glass length of the first member and the second member after the temperature changes by Δt (ΔL2, ΔL1).
Respectively to the first member after the temperature has changed by Δt,
Refractive index of the glass of the second member (n2 + Δn2), (n1 +
Δn1) multiplied by the difference, and the second term on the left side shows that the temperature changes by Δt to the glass lengths (L1, L2) of the first member and the second member before the temperature changes by Δt, respectively. Is the difference of those multiplied by the amount of change in the refractive index (Δn1, Δn2,), and the right side is the optical path of the air layer due to the difference in the glass lengths of the first member and the second member after the temperature has changed by Δt. From the length (n + Δn) h2 to the first member before the temperature change, the second
Optical path length of air layer due to difference in glass length of members (n × h1
) Is subtracted.

[0054]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 10 shows the structure of a wavelength stabilized light source device (semiconductor laser light source device) which employs the optical path length difference forming light source member of the present invention. In the embodiment of FIG. 10, the same parts as those in FIGS. 1 to 8 are designated by the same reference numerals.

P1 which is a part of the light P emitted from the light source unit 1
Is split by a beam splitter 50 into two parallel beams P1A, P1B with a spacing of 2x. These beams are further split by a beam splitter 31 into reflected lights P3A and P3B and transmitted lights P4A and P4B.

The optical paths of these beams will be described in detail with reference to FIGS. 10 and 11. FIG. 11 is a diagram showing a part of the configuration of FIG. 10 and seen from another viewpoint.

P3A and P3B enter the condenser lens 5 from an optical axis height different from the optical axis of the condenser lens 5 by y, are reflected by the scanning mirror 35, and are only y from the optical axis of the condenser lens 5. The light exits the condenser lens 5 from different optical axis heights and heads for the beam splitter 31. The condenser lens 5 and the scanning mirror 35 configure a so-called cat's eye optical system.

The scanning mirror 35 is, for example, PZT.
Attached to the piezoelectric element 36 such as the above, and is swept in the direction of the arrow by several wavelengths of the oscillation wavelength of the semiconductor laser 11 by the ramp wave W from the oscillator 38, the pulse signal generator 39, and the PZT driver 37.

Alternatively, the condenser lens 5 and the scanning mirror 35 are integrated and attached to a piezoelectric element 36 such as PZT, and a ramp wave W from an oscillator 38, a pulse signal generator 39 and a PZT driver 37 is used. , The oscillation wavelength of the semiconductor laser 11 is swept in the direction of the arrow.

On the other hand, P4A and P4B enter the corner cube 53 from an optical axis height different from the optical axis of the corner cube 53 by y, and the optical axis of the corner cube 53 is y.
Exit the corner cube 53 from different optical axis heights.
Here, as shown in FIG. 12, x and y are set so that P4A and P4B do not collide with the ridgeline of the corner cube 53.
Has been decided. After this, P4A is a low refractive index glass 5
It goes toward the beam splitter 31 via 1. Also P4B
Goes through the high-refractive-index glass 52 and the beam splitter 31.
Head to.

P3A and P4A are beam splitters 31
The interference light P5A is detected by the detector 41. P3B and P4B are combined by the beam splitter 31 and interfere with each other, and the interference light P5B is detected by the detector 42.

Since the cat's eye optical system and the corner cube have the same function, the condenser lens 5 and the scanning mirror 35 may be replaced by a corner cube, and the corner cube 53 is replaced by the cat's eye optical system. May be replaced with

Further, in the present invention, since the optical axis is shifted by the cat's eye optical system and the corner cube, there is no return light to the light source unit 1, and the isolator 33 in the apparatus of FIGS. Becomes

Now, the glass length of the low refractive index glass 51 (corresponding to the first member in FIG. 9) is L1, the refractive index is n1, and the glass length of the high refractive index glass 52 (corresponding to the second member in FIG. 9). To L
2, where n 2 is the refractive index and n is the refractive index of air, the output signal from the detector 41 and the output signal from the detector 42 are two fringe scan signals that differ by the optical path length difference ΔLa. That is, .DELTA.La = n2.times.L2-n1.times.L1-n.times. (L2-L1), where L2> L1.

In the present invention, the optical path length difference ΔLa for wavelength stabilization is thus obtained by using two different types of glass.
To set. At this time, the material and the length of the glass are determined using the above-mentioned mathematical expressions 6 to 9 in consideration of the expansion coefficient of glass, the temperature coefficient of the refractive index, the temperature coefficient of the refractive index of air, and the like.

The wavelength stabilization of the semiconductor laser 11 is performed by using the two fringe scan signals having the optical path length differences thus set that are different by ΔLa. These signals are phase-compared by the phase comparator 100, and are passed through the light source drive circuit 10 so that the phases of the two signals are held at a specific phase difference, and the light source unit 1
Be fed back to. Since the feedback system is as described above, the description is omitted.

In the above optical system (FIG. 10), the beam splitter 50 is used in order to clearly represent the optical path of the beam. However, as shown in FIG. You may divide into two using. Here, the diaphragm 60
The position of is not limited to the position of FIG. 13, and may be, for example, a position in front of the detector.

Example 1 Temperature coefficient of refractive index ... Low refractive index glass> High refractive index glass Expansion coefficient ... Low refractive index glass to high refractive index glass For wavelength stabilization according to the present invention The stability of the optical path length difference ΔL will be described in detail with a specific example. Consider the semiconductor laser 11 as a 780 nm band semiconductor laser. Stability is
± 3 based on 23 degrees (° C) and 760 mmHg
Consider the temperature at ± 10 mmHg.

For example, the material of the low refractive index glass 51 is shown in Table 1.
The special light flint glass "LLF6" is used, and the material of the high refractive index glass 52 is lanthanum flint glass "LaF" in Table 1.
02 ”. The characteristics of each are as follows.

LLF6 LaF02 Refractive index (780 nm) 20 ° C. 1.524916 1.709736 23 ° C. 1.524909 1.709733 26 ° C. 1.524902 1.709730 (Temperature coefficient of refractive index (/ ° C.) 2.3 × 10 ^-6 1.0 × 10 ^-6 ) Expansion coefficient (/ ° C) 8.3 × 10 ^-6 8.0 × 10 ^-6

[0072]

[Table 1] Further, the refractive index of air at a wavelength of 780 nm is as follows. The effect of humidity change is small, so here 5
0%

Temperature / Atmosphere 750 mmHg 760 mmHg 770 mmHg 20 ° C. 1.000268 1.000271 1.000275 23 ° C. 1.000265 1.000268 1.000272 26 ° C. 1.000262 1.000265 1.000269 If the glass length is L1 = 19 mm and L2 = 19.03 mm, the optical path length difference ΔL at 23 ° and 760 mmHg is ΔL. It looks like this:

ΔL = 1.709733 × 19.03-1.524909 × 19-1.0
00268 × (19.03-19) = 3.53293995 mm When controlling so that the phases of the two signals are completely matched, the wavelength λ of the semiconductor laser 11 is stabilized to an integral fraction of ΔL as described above. It will be as follows.

N = (value of ΔL / 780 nm rounded off after the decimal point) = 4529 λ = ΔL / 4529 = 780.07064747 nm The integer N is the number of waves included in ΔL. By changing the set temperature of the semiconductor laser 11, the integer N can be set to another value and another wavelength can be obtained.

Taking the above into consideration, the stability of the optical path length difference ΔL at 23 ± 3 ° C. and 760 ± 10 mmHg is calculated as follows.

Temperature / Atmosphere 750mmHg 760mmHg 770mmHg 20 ° C 0.025 -0.0003 -0.034 23 ° C 0.025 ------- -0.034 26 ° C 0.025 -0.001 -0.035 Unit (ppm) Thus, in the above glass combination , ± 3 ℃
With respect to the temperature change and the atmospheric pressure change of ± 10 mmHg, the stability of the optical path length difference ΔL is about ± 0.04 ppm, and it is possible to realize a wavelength reference having no practical problem.

[0078]Example 2  Temperature coefficient of refractive index ・ ・ ・ Low refractive index glass <High refractive index
Index glass Expansion coefficient ・ ・ ・ ・ ・ ・ Low refractive index glass ＞ High refractive index
Index glass Stability of optical path length difference ΔL for wavelength stabilization according to the present invention
Will be described in detail by showing a concrete example. Semiconductor laser 11
Consider a semiconductor laser in the 780 nm band. Stability is
± 3 ° C, ± 1 based on 23 ° C and 760 mmHg
Consider 0 mmHg.

For example, the material of the low refractive index glass 51 is shown in Table 1.
The flint glass "F5" is used, and the material of the high refractive index glass 52 is the lanthanum crown glass "LaK18" shown in Table 1. The characteristics of each are as follows.

F5 LaK18 Refractive index (780 nm) 20 ° C. 1.593641 1.720540 23 ° C. 1.593649 1.720551 26 ° C. 1.593657 1.720562 (Temperature coefficient of refractive index (/ ° C.) 2.6 × 10 ⁻⁶ 3.7 × 10 ⁻⁶ ) Expansion coefficient (/ ° C.) 8.8 × 10 ^-6 5.9 × 10 ^-6 The refractive index of air at a wavelength of 780 nm is as follows. The effect of humidity change is small, so here 5
0%

Temperature / Atmosphere 750 mmHg 760 mmHg 770 mmHg 20 ° C. 1.000268 1.000271 1.000275 23 ° C. 1.000265 1.000268 1.000272 26 ° C. 1.000262 1.000265 1.000269 Glass length L 1 = 22.37 mm, L 2 = 22.3 mm
Then, the optical path length difference ΔL at 23 ° C. and 760 mmHg is as follows.

ΔL = 1.720551 × 22.3 + 1.000268 × (22.37
-22.3) -1.593649 × 22.37 = 2.778837793 mm When controlling so that the phases of the two signals are completely matched, the wavelength λ of the semiconductor laser 11 is stabilized to an integral fraction of ΔL as described above. It will be as follows.

N = (value of ΔL / 780 nm rounded to the nearest whole number) = 3575 λ = ΔL / 3575 = 779.96565845 nm The integer N is the number of waves included in ΔL. By changing the set temperature of the semiconductor laser 11, the integer N can be set to another value and another wavelength can be obtained.

In consideration of the above, the stability of the optical path length difference ΔL at 23 ± 3 ° C. and 760 ± 10 mmHg is calculated as follows.

Temperature / Atmosphere 750mmHg 760mmHg 770mmHg 20 ° C -0.076 -0.001 0.099 23 ° C -0.075 ------- 0.100 26 ° C -0.076 -0.00002 0.101 Unit (ppm) Thus, in the above glass combination , ± 3 ℃
The stability of the optical path length difference ΔL is about ± 0.1 ppm with respect to the temperature change and the atmospheric pressure change of ± 10 mmHg.
It is possible to realize a wavelength reference that has no practical problems.

Example 3 Temperature coefficient of refractive index ... Low refractive index glass> High refractive index glass Expansion coefficient ... Low refractive index glass <High refractive index glass For wavelength stabilization according to the present invention The stability of the optical path length difference ΔL will be described in detail with a specific example. Consider the semiconductor laser 11 as a 780 nm band semiconductor laser. Stability is
± 3 ° C, ± 1 based on 23 ° C and 760 mmHg
Consider 0 mmHg.

For example, the material of the low refractive index glass 51 is shown in Table 1.
The heavy crown glass “SK2” and the high refractive index 52 material are the heavy flint glass “SFL14” shown in Table 1. The characteristics of each are as follows.

SK2 SFL14 Refractive index (780 nm) 20 ° C. 1.600622 1.744507 23 ° C. 1.600633 1.744510 26 ° C. 1.600644 1.744513 (Temperature coefficient of refractive index (/ ° C.) 3.6 × 10 ⁻⁶ 0.9 × 10 ⁻⁶ ) Expansion coefficient (/ ° C.) 6.3 × 10 ^-6 8.7 × 10 ^-6 The refractive index of air at a wavelength of 780 nm is as follows. The effect of humidity change is small, so here 5
0%

Temperature / Atmosphere 750 mmHg 760 mmHg 770 mmHg 20 ° C. 1.000268 1.000271 1.000275 23 ° C. 1.000265 1.000268 1.000272 26 ° C. 1.000262 1.000265 1.000269 If the glass length is L1 = 26.08 mm and L2 = 26 mm, the optical path length difference ΔL at 23 ° C. and 760 mmHg is ΔL. It looks like this:

ΔL = 1.744510 × 26 + 1.000268 × (26.08-2
6) -1.600633 × 26.08 = 3.6927728 mm When controlling so that the phases of the two signals are completely matched, the wavelength λ of the semiconductor laser 11 is stabilized to an integral fraction of ΔL as described above. So it looks like this:

N = (value of ΔL / 780 nm rounded to the nearest whole number) = 4734 λ = ΔL / 4734 = 780.0534009 nm The integer N is the number of waves included in ΔL. By changing the set temperature of the semiconductor laser 11, the integer N can be set to another value and another wavelength can be obtained.

In consideration of the above, the stability of the optical path length difference ΔL at 23 ± 3 ° C. and 760 ± 10 mmHg is calculated as follows.

Temperature / Atmosphere 750mmHg 760mmHg 770mmHg 20 ° C -0.065 -0.0004 0.087 23 ° C -0.065 ------- 0.087 26 ° C -0.067 -0.002 0.084 Unit (ppm) Thus, in the above glass combination , ± 3 ℃
With respect to the temperature change and the atmospheric pressure change of ± 10 mmHg, the stability of the optical path length difference ΔL is about ± 0.09 ppm, and it is possible to realize a wavelength reference having no practical problem.

The wavelength standard with the same stability can be realized by combining arbitrary glass materials different from those described above.

[0095]

As described above, according to the present invention, it is possible to realize a wavelength reference having a simple structure and excellent environmental resistance without requiring a method of controlling the temperature of the wavelength reference itself. Therefore, it is possible to realize a wavelength-stabilized light source device having a simple structure and excellent environmental resistance.

Further, it is possible to realize a stable wavelength reference against environmental changes, and it is possible to realize a wavelength stabilized light source device having excellent long-term stability and a simple structure.

[Brief description of drawings]

FIG. 1 is a diagram showing a wavelength stabilized light source device of the invention of Japanese Patent Application No. 4-151133.

FIG. 2 is a diagram showing a feedback system of a light source unit in FIG.

FIG. 3 is a diagram showing various signal waveforms in the feedback system of FIG.

FIG. 4 is a view showing another wavelength-stabilized light source device of the invention of Japanese Patent Application No. 4-151133.

5 is a diagram showing a drive signal and the like of the light source unit in FIG.

6 is a diagram showing a feedback system of a light source unit in FIG.

FIG. 7 is a diagram showing a sampled and held signal and a waveform-shaped signal in FIG.

FIG. 8 is a diagram showing drive waveforms in another example of the invention of Japanese Patent Application No. 4-151133.

FIG. 9 is a diagram showing the principle of the present invention.

FIG. 10 is a view showing a wavelength stabilized light source device using an optical member for forming an optical path difference according to the present invention.

FIG. 11 is a view partially extracted from the apparatus of FIG. 10 and viewed from another viewpoint.

FIG. 12 is an explanatory diagram of a parallel beam.

13 is a modified example of the apparatus of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 light emission part (light source part) 3 collimating lens 5 condensing lens 10 light source drive circuit (wavelength stabilization part) 10X light source drive circuit (wavelength stabilization part) 20X light source drive circuit (wavelength stabilization part) 11 semiconductor laser 12 thermistor 13 Peltier element 14 Heat sink 30 Beam splitter 31 Beam splitter 33 Isolator 34 Step mirror (Reflecting part) A First reflecting surface B Second reflecting surface 35 Scanning mirror 38 Oscillator (driving part) 40 Pulse signal generator (driving part) 41 Detection Device (first light receiving part) 42 detector (second light receiving part) 50 sample hold part (sampling part) 51 sample hold part (sampling part) 52 filter (sampling part) 53 filter (sampling part) 100 phase comparator (phase Detector) 110 Phase comparator (Phase detector

Claims (3)

[Claims] 1. An optical path length difference forming optical member that gives a predetermined optical path length difference between a first path and a second path, a first member forming a first path, and a second member forming a second path. And an optical path length difference forming optical member satisfying the following mathematical expression so that both optical path length differences do not change even if the temperature changes. {ΔL2 (n2 + Δn2) −ΔL1 (n1 + Δn1)}
+ (Δn2 L2 −Δn1 L1) = (n + Δn) × (h1
+ ΔL2 −ΔL1) −n × h1 where L1: the length of the first member, ΔL1: the amount of change in the length of the first member due to the change in temperature L2: the length of the second member, ΔL2: the change in temperature Amount of change in length of the second member with respect to n1: Refractive index of the first member, Δn1: Amount of change in refractive index of the first member with a change in temperature n2: Refractive index of the second member, Δn2: Change in temperature Amount of change in refractive index of second member due to: n: Refractive index of medium that causes difference in length between first member and second member, Δn: Length between first member and second member due to temperature change The difference in the refractive index of the medium that makes the difference in height h1: The difference in length between the first member and the second member at the reference temperature 2. The optical path length difference forming optical member according to claim 1, wherein the first member and the second member are set to have substantially the same length. 3. The optical path length difference forming optical member according to claim 2, wherein a material is selected so that the expansion coefficients of the first member and the second member are substantially the same.