JPH08264870A - Optical com generator - Google Patents

Abstract

PURPOSE: To provide an optical COM generator for stably generating an optical COM signal by removing the operating instability of the generator due to the variation in the resonator length of an optical resonator. CONSTITUTION: An optical COM generator has a phase modulator 2 driven by a microwave oscillator 3 and an optical resonator 1, and comprises resonator length varying means 8 (piezoelectric element (81). The emitted light branched by a branching filter 4 is received by a photoreceiver 5, the signal from the photoreceiver 5 with a modulation signal from a low frequency oscillator 7 for modulating the resonator length as a reference signal is synchronously detected by control means 6 having a synchronous detector 61 and an adder 62, and a resonator length control signal based on the detected signal is fed back together with the modulation signal to the means 8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical comb generator which generates an optical frequency reference over a wide band and an optical comb generator which is also used as a short optical pulse source, and more particularly to stably generate an optical comb. The present invention relates to an optical comb generator.

[0002]

2. Description of the Related Art An optical comb generator including an optical resonator 1 and a phase modulator 2 provided in the optical resonator as shown in FIG. 8 has been experimentally manufactured and tested. The optical comb generator modulates monochromatic light incident from the outside to generate a large number of sideband waves. The sidebands can be generated by the phase modulator alone, but with microwave power of about several W, high-order sidebands of several orders or more become weak enough not to be used practically. Therefore, the phase modulator 2 is arranged inside the optical resonator 1 so that the sidebands of several hundreds of orders can be generated with sufficient intensity so that the light traveling back and forth in the optical resonator 1 undergoes repeated modulation. The structure is adopted.

The optical resonator 1 is generally a Fabry-Perot resonator composed of opposing mirrors. Optical phase modulation changes the phase of light passing through a crystal by applying a microwave electric field to the crystal having an electro-optical effect. In order to efficiently apply a microwave electric field of a single frequency, the following two structures are frequently used in the phase modulator.
One is to arrange a crystal inside a microwave waveguide to operate as a microwave resonator, and here, it is referred to as a "bulk type" because of the shape of the crystal. The other is to provide an optical waveguide on a crystal substrate and apply a concentrated electric field from electrodes arranged near the optical waveguide.
Called. In particular, there is a type in which a microstrip line is used as an electrode without providing an optical waveguide, but it is included in the waveguide type here. Normally, the modulation signal for driving the phase modulator is a microwave of several GHz, and its frequency fluctuation is 10 −6 or less of the frequency fluctuation of general semiconductor laser light. Therefore, the interval between the sidebands generated by the optical comb generator is exactly equal to the frequency of the microwave, which is the modulation signal, and all sidebands can be used as the frequency reference if light with high frequency accuracy is incident. For this purpose, the frequencies of the incident light and the modulation signal are usually fixed and used in order to maintain high frequency accuracy.

In order to maximize the sideband generation range, the following two conditions are necessary for the resonator length of the optical resonator. One is to set the time for light to make a round trip in the optical resonator to be an integral multiple of the period of the modulation signal. The other is to adjust the resonator length so that the incident light causes optical resonance without performing phase modulation. The realistic allowable adjustment amount of the resonator length for these two conditions is about 10 μm for the optical round-trip time adjustment and 0.
It is about 1 μm, and the condition for adjusting the incident light resonance is stricter than the condition for adjusting the optical round trip time.

[0005]

However, the resonator length is determined by the mechanical change of the distance between the mirrors forming the resonator and the change of the refractive index of the crystal in the phase modulator due to the temperature change. It will change. Regarding the temperature, which has a great influence on the change in the resonator length, the power consumption of the phase modulator is about several W, and most of it is due to the dielectric loss of the crystal. For this reason, when temperature control is not performed, the temperature of the crystal rises from room temperature by 10 ° C or more. Usually, lithium niobate or lithium tantalate is used as the electro-optic crystal, and the temperature coefficient of the refractive index thereof is on the order of 10 −4 power per 1 ° C. This value causes a phase change of about one rotation with respect to a temperature change of 1 ° C. when light with a wavelength of about 1.5 μm passes through a crystal of several cm. Therefore, in order to stabilize the operation of the optical comb generator by only the temperature control, it is necessary to stabilize the temperature at about 0.01 ° C. However, in reality, it is difficult to realize temperature stabilization of about 0.01 ° C. over the whole because the heat generation of the crystal is not uniform and the length of the phase modulator is about several cm.

As described above, in the conventional optical comb generator, the resonator length of the optical resonator may change, and the resonance condition for incident light may not be satisfied. In such a case, the sideband generation band is narrowed, and further, the sideband may be hardly generated. An object of the present invention is to provide an optical comb generating device that eliminates operational instability of the optical comb generating device due to fluctuations in the cavity length of an optical resonator and stably generates an optical comb signal. is there.

[0007]

The average amount of transmitted light of an optical comb generator including an optical resonator and a phase modulator provided in the optical resonator is determined when a resonance condition for incident light is satisfied. It becomes extremely small, and becomes larger as it is detuned. On the contrary, the average amount of reflected light becomes maximum when the resonance condition is satisfied. In the present invention, by utilizing this property, the resonator length is controlled so that the average light amount of the transmitted light or the reflected light has an extreme value.

That is, the resonator length changing means for changing the resonator length of the optical resonator, and the modulation applied to the resonator length changing means for modulating the resonator length via the resonator length changing means. And an oscillator for generating a signal. And
Control for receiving the transmitted light or the reflected light of the optical resonator whose resonator length is modulated by a light receiver to convert the electric signal into an electric signal, and synchronously detecting the electric signal using the modulation signal from the oscillator as a reference signal. The resonator length changing means is controlled based on the synchronously detected signal via the means.

[0009]

First, the average amount of transmitted light or reflected light of the optical comb generator composed of the optical resonator and the phase modulator and the round-trip phase of the light in the optical resonator [wavenumber (twice the resonator length) = 2π / wavelength)]. The total amount of light transmitted and reflected from the optical resonator is equal to the amount of incident light if there is no loss, so the amount of transmitted light will be considered here.

4 to 6 schematically show the operating principle of the optical comb generator viewed from the time domain. 4 to 6, (a) is a diagram showing the relationship between the amount φ (the degree of detuning) in which the round trip phase of light deviates from an integer multiple of 2π and the transmittance of the optical resonator, and the horizontal axis represents The reciprocal phase shift amount φ (degree of detuning), and the vertical axis represents the transmittance. When the round-trip phase of light is an integral multiple of 2π, the optical resonator causes optical resonance and maximizes the transmittance. (B) is a diagram showing a temporal change of a reciprocal phase due to a modulation signal (microwave) for driving a phase modulator of the optical comb generator, the horizontal axis represents a reciprocal phase shift amount φ, and the vertical axis represents a time t. is there. (C) is a figure which shows the time change of the transmitted light amount, a horizontal axis is a transmitted light amount, and a vertical axis is time t. As shown in FIG. 4B, when the round-trip phase of the light that changes according to the modulation signal (microwave) to the phase modulator changes symmetrically around φ = 0, the transmitted light of the optical comb generator temporally changes. It becomes a pulse train at equal intervals (interval of 1/2 of the period T of the modulation signal) [FIG. 4 (c)]. At this time, the time width of each pulse becomes the minimum. The narrow pulse width means that the spectrum is wide.

Next, in FIG. 5B, the round-trip phase of light is φ = φ.
0 (0 <φ0 <m, where m is the round-trip phase modulation index of the modulation signal) indicates a detuned state that changes symmetrically with respect to the center. At this time, the transmitted light of the optical comb generator is not only unequal in time, but the time width of the pulse is wider than that in FIG. 4 (c) [FIG. 5 (c)]. The fact that the pulse time width is wide is nothing but the fact that the average amount of light for a time sufficiently longer than the period of the modulation signal is large. In addition, when considered in the frequency domain, the spread of the spectrum becomes narrow.

Further, in FIG. 6B, the round-trip phase of light is φ =
A detuned state that changes symmetrically around φ0 (m <φ0) is shown. In such a situation, transmitted light can no longer be obtained, and the optical comb generator no longer generates sidebands [FIG. 6 (c)]. From the above, the round trip phase of light is 2π
The average transmitted light amount of the optical comb generator for detuning from an integer multiple of is as shown in FIG. In FIG. 7, the horizontal axis represents the round trip phase of light, which is 2
The deviation amount φ from an integer multiple of π, the vertical axis corresponds to the average transmitted light amount, and (a), (b), and (c) in the figure correspond to the states of FIGS. From the relationship in FIG. 7, if control is performed so that the average transmitted light amount is minimized, the optical comb generator can be operated without detuning, and high-order sidebands can be stably generated. .

The control to the above-mentioned minimum point is 1 of the average transmitted light amount.
Stabilization of the second derivative to the zero-cross point is used. The resonator length of the optical resonator 1 is modulated by the modulation signal from the oscillator 7 in order to approximately obtain the first derivative. The light transmitted through the optical resonator 1 is detected by the light receiver 5, and the received light signal is controlled by the control means 6.
Then, the signal synchronously detected using the modulated signal from the oscillator 7 as a reference signal has an approximate first-order differential coefficient to be obtained.
By controlling the resonator length changing means 8 based on this signal, stabilization control of the first derivative to the zero cross point is performed.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a first embodiment of the present invention. In this embodiment, a bulk type optical comb generator is subjected to stabilization control of the resonator length. The optical comb generator includes an optical resonator 1 composed of mirrors 1a and 1b facing each other and the mirrors 1a and 1a.
1b and a phase modulator 2 arranged between the mirrors 1b, and the mirror 1b has a resonator length changing means 8 for minutely changing the resonator length.
Is provided as a piezoelectric element 81.

The stabilizing control system is a low frequency oscillator 7 as an oscillator for generating a modulation signal for modulating the resonator length.
And a demultiplexer 4 for branching the light emitted from the optical comb generator,
A light receiver 5 for receiving one of the lights demultiplexed by the demultiplexer 4,
A synchronous detector 61 whose input terminals are connected to the photodetector 5 and the low frequency oscillator 7, and an input terminal thereof is connected to the synchronous detector 61 and the low frequency oscillator 7, and its output terminal is And an adder 62 connected to the piezoelectric element 81. The synchronous detector 61 and the adder 62 correspond to the control means 6. The microwave oscillator 3 connected to the phase modulator 2 generates a modulation signal for driving the phase modulator 2 (hereinafter referred to as a drive modulation signal in order to distinguish it from the modulation signal from the low frequency oscillator 7). It is a thing.

Although the resonator length changing means 8 uses the piezoelectric element 81 in this embodiment, the mirror 1 constituting the optical resonator 1 is used.
It is sufficient that the distance between a and 1b can be changed by about several μm. The resonator length of the optical resonator 1 is modulated by the driving modulation signal (microwave) applied to the phase modulator 2, and the low frequency oscillator 7
Slow modulation is performed with the modulation signal from (hereinafter, referred to as a control modulation signal for distinguishing from the drive modulation signal). Usually, the frequency of this control modulation signal is 10 to 1
It is appropriate that the modulation depth is set to about 00 Hz and to about 1/10 to 1/2 of the round-trip phase modulation index m of the drive modulation signal (microwave) applied to the phase modulator 2. .

The optical resonator 1 is composed of the drive modulation signal from the microwave oscillator 3 and the control modulation signal from the low frequency oscillator 7.
A part of the light emitted from the optical comb generator whose resonator length is modulated is extracted by the demultiplexer 4. The remaining emitted light is used as an optical comb signal. The emitted light extracted by the demultiplexer 4 is detected by the light receiver 5. This received light signal is detected by the synchronous detector 6
At 1, synchronous detection is performed using the control modulation signal from the low frequency oscillator 7 as a reference signal. The feedback control signal obtained here is an approximate value of the primary differential coefficient with respect to the round-trip phase of the average transmitted light amount. This feedback control signal is fed back to the resonator length changing means 8 via the adder 62. As a result, stabilization control of the first derivative to the zero-cross point is performed, and the resonator length is stabilized mainly at the point where the resonator condition satisfies the resonance condition for the incident light.

When the reflected light is detected and controlled, the demultiplexer 4 and the light receiver 5 may be arranged on the reflected light side (that is, the incident light side) and the polarity of the feedback control signal may be inverted. . By doing so, it is possible to perform control so that the average reflected light amount becomes maximum. This example will be described below as a second embodiment.

FIG. 2 shows a second embodiment of the present invention. In this embodiment, a piezoelectric element is used as the resonator length changing means 8 and the optical resonator 1 is used instead of changing the distance between the mirrors 1a and 1b.
A control phase modulator 82 made of a crystal having an electro-optical effect is arranged in the inside, and the resonator length is changed by changing the refractive index of the crystal, and the average light quantity as a guide for control is This is different from the first embodiment in that the average light quantity of reflected light is used instead of the average light quantity of transmitted light.

The optical comb generator comprises mirrors 1a, 1 facing each other.
b, and the phase modulator 2 arranged between the mirrors 1a and 1b.
A control phase modulator 82 as the resonator length changing means 8 is further provided between a and 1b.

The stabilizing control system includes a low-frequency oscillator 7 as an oscillator for generating a control modulation signal for modulating the resonator length, a demultiplexer 4 for extracting the reflected light of the optical comb generator, Light receiver 5 for receiving the light extracted by the demultiplexer 4.
A synchronous detector 61 whose input terminals are connected to the photodetector 5 and the low-frequency oscillator 7, a polarity inverter 63 whose input terminal is connected to the synchronous detector 61, and a polarity inverter The input terminal of the low frequency oscillator 63 is connected to the low frequency oscillator 7, and the output terminal of the low frequency oscillator 7 is connected to the adder 62 connected to the control phase modulator 82. The synchronous detector 6
1, the polarity inverter 63, and the adder 62 correspond to the control means 6. Since the microwave oscillator 3 connected to the phase modulator (hereinafter referred to as a driving phase modulator for distinguishing from the controlling phase modulator 82) 2 generates a driving modulation signal for the driving phase modulator 2. belongs to.

The control phase modulator 82 as the resonator length changing means 8 is a crystal having an electro-optic effect, for example, lithium niobate, lithium tantalate, or the like provided with two electrodes, and a voltage is applied between the electrodes. Is applied to change the refractive index of the crystal and change the phase of the transmitted light.

The resonator length of the optical resonator 1 is controlled by the control phase modulator 82 in addition to the modulation by the drive modulation signal (microwave) applied to the drive phase modulator 2.
Slow modulation is applied by the control modulation signal from the low frequency oscillator 7. The demultiplexer 4 extracts the reflected light of the optical comb generator in which the resonator length of the optical resonator 1 is modulated by the drive modulation signal and the control modulation signal. The reflected light extracted by the demultiplexer 4 is detected by the light receiver 5. The received light signal is synchronously detected by the synchronous detector 61 using the control modulation signal from the low frequency oscillator 7 as a reference signal. The feedback control signal obtained here is 1 with respect to the round-trip phase of the average reflected light amount.
It is an approximate value of the next derivative. The polarity of this signal is inverted by the polarity inverter 63 and is fed back to the control phase modulator 82 via the adder 62. As a result, stabilization control of the first derivative to the zero-cross point is performed, and the resonator length is stabilized at a point that satisfies the resonance condition for incident light.

As described above, the average transmitted light quantity is used, or
The polarity of the control signal to be returned must be inverted depending on whether the average reflected light amount is used.
In addition to the method used in the second embodiment, for example, a method in which the subtractor is used instead of the adder 62 without inverting the polarity of the synchronously detected signal, and the piezoelectric element 81 is used as in the first embodiment. If it is used, the polarity of the synchronously detected signal is not inverted, and the piezoelectric element is provided inside the mirror while the adder 62 is also used.

FIG. 3 shows a third embodiment of the present invention. In this embodiment, a waveguide type optical comb generator is subjected to stabilization control of the resonator length. As the average light amount as a guide for control, the average light amount of transmitted light is used as in the first embodiment.
In the waveguide type optical comb generator, usually, both ends of the optical waveguide are coated with a reflection film having a high reflectance, and the reflection film serves as the mirrors 1a and 1b to form the optical resonator 1. Therefore, it is difficult to mechanically change the cavity length, but since the optical waveguide is provided on the substrate of the electro-optic crystal, it is necessary to apply a DC voltage or a low frequency voltage to the electrodes 2a and 2b. Thus, the cavity length can be changed. Therefore, the resonator length can be controlled by controlling the voltage applied to the electrodes 2a and 2b without providing the resonator length changing means 8 as in the first embodiment. That is, the phase modulator 2 also serves as the resonator length changing means 8.

The stabilization control system is almost the same as that of the first embodiment, but as described above, since the signal from the adder 62 is fed back to the electrodes 2a and 2b of the phase modulator 2,
A bias circuit 9 including a capacitor 91 and a coil 92 is provided to add the modulation signal for driving the microwave oscillator 3. The resistor 10 is a terminating resistor.

The resonator length of the optical resonator 1 has electrodes 2a and 2b.
In addition to the modulation by the driving modulation signal, the signal from the low-frequency oscillator 7 is used to slowly modulate the voltage applied to the signal. A part of the transmitted light of the optical resonator 1 is taken out by the demultiplexer 4 and detected by the light receiver 5. The received light signal is synchronously detected by the synchronous detector 61 using the control modulation signal from the low frequency oscillator 7 as a reference signal. The feedback control signal obtained here is an approximate value of the primary differential coefficient with respect to the round-trip phase of the average transmitted light amount. This signal is passed through the adder 62,
It returns to the electrodes 2a and 2b through the bias circuit 9.
As a result, stabilization control of the first derivative to the zero-cross point is performed, and the resonator length is stabilized mainly at the point where the resonator condition satisfies the resonance condition for the incident light.

According to the experiments conducted by the inventors, the optical comb generator of the first embodiment terminates the experiment as compared with the conventional optical comb generator which could sustain the generation of sidebands for only about several tens of seconds. The stable generation of sidebands continued for more than 4 hours. The low-frequency modulation used for stabilization control is used when the output light from the optical comb generator is used as a frequency reference having a frequency accuracy of about 100 MHz (about 1 pm in wavelength in the 1.5 μm band). Has also confirmed that it is harmless. This can be understood from the fact that the frequency displacement of the sideband due to the modulation of 10 Hz is at most 10 kHz even for the 1000th sideband.

[0029]

The resonator length is controlled so that the average amount of transmitted light or reflected light of the optical comb generator takes an extreme value, that is, the resonator length for changing the resonator length of the optical resonator. A changing means is provided, a control modulation signal from an oscillator is applied to the resonator length changing means to modulate the resonator length, the transmitted light or the reflected light is received by a light receiver, and the light receiving signal is received by the control means. The control modulation signal from the oscillator is synchronously detected as a reference signal, and the resonator length changing means is controlled based on the synchronously detected signal. Therefore, the resonator length of the optical resonator with respect to the incident light. It was possible to provide an optical comb generator that is stabilized around the point of satisfying the resonance condition and stably generates the optical comb signal.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a third exemplary embodiment of the present invention.

FIG. 4 is a diagram schematically showing the operation principle of the optical comb generator viewed from the time domain, particularly showing a state when the round-trip phase of light is not detuned, and FIG. Round trip phase is 2π
Is a diagram showing the relationship between the amount φ (the degree of detuning) deviated from an integer multiple of and the transmittance of the optical resonator, (b) is a modulation signal (microwave) that drives the phase modulator of the optical comb generator FIG. 6 is a diagram showing a temporal change in the reciprocating phase, and FIG. 7C is a diagram showing a temporal change in the amount of transmitted light.

FIG. 5 is a diagram schematically showing the operation principle of the optical comb generator viewed from the time domain, particularly when detuning is performed in a range in which the round-trip phase of light is smaller than the round-trip phase modulation index by the drive modulation signal. It is a figure which shows a state, (a) The round-trip phase of light is 2π.
Is a diagram showing the relationship between the amount φ (the degree of detuning) deviated from an integer multiple of and the transmittance of the optical resonator, (b) is a modulation signal (microwave) that drives the phase modulator of the optical comb generator FIG. 6 is a diagram showing a temporal change in the reciprocating phase, and FIG. 7C is a diagram showing a temporal change in the amount of transmitted light.

FIG. 6 is a diagram schematically showing the operating principle of the optical comb generator viewed from the time domain, particularly when detuning is performed in a range in which the round-trip phase of light is larger than the round-trip phase modulation index by the drive modulation signal. It is a figure which shows a state, (a) The round-trip phase of light is 2π.
Is a diagram showing the relationship between the amount φ (the degree of detuning) deviated from an integer multiple of and the transmittance of the optical resonator, (b) is a modulation signal (microwave) that drives the phase modulator of the optical comb generator FIG. 6 is a diagram showing a temporal change in the reciprocating phase, and FIG. 7C is a diagram showing a temporal change in the amount of transmitted light.

FIG. 7 is a diagram showing the relationship between the degree of detuning of the reciprocal phase of light and the average amount of transmitted light of the optical comb generator.

FIG. 8 is a diagram showing a configuration of a conventional optical comb generator.

[Explanation of symbols]

1 Optical Resonator 1a, 1b Mirror 2 Phase Modulator (Drive Phase Modulator) 2a, 2b Electrode 3 Microwave Oscillator 4 Demultiplexer 5 Light Receiver 6 Control Means 7 Low Frequency Oscillator as Oscillator 8 Resonator Length Changing Means 9 Bias Circuit 10 Resistance 61 Synchronous Detector 62 Adder 63 Polarity Inverter 81 Piezoelectric Element as Resonator Length Changing Means 82 Control Phase Modulator as Resonator Length Changing Means 91 Capacitor 92 Coil

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Claims (2)

[Claims] 1. An optical comb generator comprising an optical resonator (1) and a phase modulator (2) provided in the optical resonator, which receives incident light and generates an optical comb signal as emitted light. In the resonator length changing means (8) for changing the resonator length of the optical resonator, a light receiver (5) for receiving the emitted light and converting it into an electric signal, and a resonator length of the optical resonator. An oscillator (7) for generating a modulation signal for applying modulation, and a control means for synchronously detecting the electric signal with the modulation signal as a reference signal and controlling the resonator length changing means based on the synchronously detected signal. An optical comb generator including (6). 2. An optical comb generator comprising an optical resonator (1) and a phase modulator (2) provided in the optical resonator, which receives incident light and generates an optical comb signal as outgoing light. In the above, the resonator length changing means (8) for changing the resonator length of the optical resonator, the light receiver (5) for receiving the reflected light from the optical resonator and converting it into an electric signal, and the optical resonator. An oscillator (7) for generating a modulation signal for modulating the resonator length of, and the resonator length changing means based on the synchronously detected signal, the electric signal being synchronously detected with the modulation signal as a reference signal. An optical comb generator comprising a control means (6) for controlling the.