JP2014098830A - Optical frequency com generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate a stable optical frequency com that keeps an appropriate operation condition.SOLUTION: Two branch waveguides propagate light coming from a light source. Two optical modulation sections modulate the lights propagating in the two branch waveguides on the basis of modulation signals, respectively. A phase control section controls, in accordance with a bias, the phase difference between the lights propagating in the two branch waveguides. A light intensity detection section detects the intensity of output lights from the two branch waveguides. A bias control section controls the bias so that the intensity of the output lights detected by the light intensity detection section is a predetermined value.

Description

  The present invention relates to an optical frequency comb generator.

Conventionally, a technique for generating an optical frequency comb by modulating light generated by a light source has been proposed. The optical frequency comb is also called an optical comb, and is light including a number of higher-order frequency components. As a result, an optical pulse is generated. The optical frequency comb is, for example, a wavelength division multiplexing (WDM), a high-density wavelength division multiplexing (DWDM), an orthogonal frequency division multiplexing (OFDM) or orthogonal frequency multiplexing (OFDM). It is applied to optical communication as a light source for division multiplexing and a light source for quantum information communication. In addition, it has been applied to measurements such as frequency standard and distance measurement (ranging) and medical treatment such as optical coherence tomography (OCT).
As a device for generating an optical frequency comb, for example, there is an optical modulator using an electro-optic crystal such as lithium niobate (LN: LiNbium Niobate (LiNbO ₃ )). An optical modulator using LN is called an LN modulator.

  For example, the LN modulator described in Patent Document 1 transmits a light wave propagating through two branched waveguides, a substrate having an electro-optic effect, a Mach-Zehnder optical waveguide formed on the substrate, and a Mach-Zehnder optical waveguide. Two optical modulation units that modulate independently and a phase control unit that controls the phase difference between the light waves propagating through the two branching waveguides. Continuous light is input to the Mach-Zehnder optical waveguide, and the two lights This is an optical frequency comb generator that applies an RF (Radio Frequency) signal to a modulation unit and outputs output light as an optical frequency comb from the Mach-Zehnder type waveguide.

  The LN modulator is further adjusted by an amplitude adjusting unit that adjusts the voltage amplitude of the RF signal applied to one of the optical modulators, a monitor unit that monitors the intensity of output light from the LN modulator, and a phase control unit. The change of the output light corresponding to the change of the phase difference is detected from the output signal of the monitoring means, and based on the detection result, the phase control unit is configured so that the width of the intensity change of the output light becomes a predetermined value. And a bias control circuit for adjusting the phase difference by controlling.

This LN modulator generates an optical frequency comb having a flat power spectrum as the optimum frequency distribution intensity. For this purpose, the relationship shown in Expression (1) is satisfied between the modulation degree difference ΔA of the RF signal between the two optical modulation units and the phase difference Δθ between the two branch waveguides.
ΔA + Δθ = π (1)
The modulation degree difference ΔA is a difference in modulation degree between RF signals applied to the respective optical modulation units. The phase difference Δθ is a phase difference due to a path length and a bias in each branch waveguide. The modulation degree difference ΔA and the phase difference Δθ are each expressed as a phase amount. At this time, the average optical output power P _out of the optical frequency comb signal is given by Expression (2).
P _out = P _in [1 + cos (Δθ) J ₀ (ΔA)] / 2 (2)
Here, P _in denotes the average value of the power of the input light (average optical input power). J ₀ (...) Represents a 0th-order Bessel function of. For example, when ΔA = 0.45π and Δθ = 0.55π, the average optical output power P _out is P _in [1 + cos (0.55π) J ₀ (0.45π)] / 2.

Further, when ΔA = Δθ = π / 2, an optimum operating state is reached in which the spectrum of the optical frequency comb signal is flat. At this time, P _out = P _in / 2, that is, the average optical output power P _out is half the average optical input power of the input light input to the LN modulator.
Even if the modulation degree difference ΔA is changed, cos (Δθ) = 0, so that the average optical output power P _out does not change. For this reason, the average optical output power P _out does not change even if the voltage amplitude is given by superimposing a low-frequency dither signal (frequency f is 1 kHz, for example) on the RF signal and the modulation degree difference ΔA is modulated. That is, the average optical output power P _out, when the modulation component corresponding to the dither signal does not occur, the phase difference Δθ = π / 2 is satisfied. Thus, in the bias control circuit constitutes a feedback loop for adjusting the phase difference Δθ, as modulated component of the average optical output power P _out becomes 0 to control the bias.

Japanese Patent No. 4949496

  However, in the LN modulator described in Patent Document 1, if an error occurs in the RF signal applied to the optical modulation unit, for example, a waveform different from a sine wave, etc., an operating condition represented by Equation (1), for example, ΔA Even if it satisfies = Δθ = π / 2, an optical frequency comb having a flat power spectrum cannot be generated. In other words, when such an error occurs, the relationship between the modulation degree difference ΔA and the phase difference Δθ when generating an optical output having a flat power spectrum is the operating condition shown in the equation (1), for example, ΔA = Δθ = π / 2 may be different. Under this different relationship, when the bias is controlled as described above, the optimum operating condition may be maintained and a stable optical frequency comb may not be generated.

  The present invention has been made in view of the above points, and provides an optical frequency comb generator that generates a stable optical frequency comb.

(1) The present invention has been made to solve the above-described problems. One embodiment of the present invention includes two branch waveguides for propagating light input from a light source, and the two branch waveguides. Two optical modulators for modulating propagating light based on modulation signals, a phase control unit for controlling a phase difference between each of the lights propagating through the two branch waveguides according to a bias, and A light intensity detector that detects the intensity of the output light that is combined and output from the two branch waveguides, and the bias is controlled so that the intensity of the output light detected by the light intensity detector is a predetermined value. And an optical frequency comb generator.

(2) According to another aspect of the present invention, in the optical frequency comb generator described above, the predetermined value is a value of a modulation signal between the two optical modulation units so that a spectrum of the output light has a predetermined shape. The intensity of the output light detected by the light intensity detector when the difference in modulation degree and the phase difference between the light propagating through the two branched waveguides are determined.

  According to the present invention, a stable optical frequency comb can be generated.

1 is a schematic block diagram of an optical signal generator according to an embodiment of the present invention. An example of the spectral envelope of the optical output power when distortion occurs in the output light is shown. The example of the spectrum envelope of the optical output power which concerns on the corrected output light is shown. The schematic block diagram of the optical signal generator which concerns on one modification in this embodiment is shown. This embodiment shows a schematic block diagram of an optical signal generator according to another modification.

The optical frequency comb generator (optical frequency comb generator 20) according to the present embodiment includes a substrate having an electro-optic effect, a Mach-Zehnder optical waveguide formed on the substrate (in this embodiment, an input waveguide 221 and a branch). Waveguides 222A and 222B, output waveguide 223), and two optical modulations that independently modulate light waves propagating through two branch waveguides of the Mach-Zehnder optical waveguide (in this embodiment, branch waveguides 222A and 222B) Section (modulation electrodes 224A and 224B in this embodiment), a phase control section (bias electrode 225 in this embodiment) for controlling the phase difference between the light waves propagating through the two branch waveguides, and the Mach-Zehnder optical waveguide A continuous light is input to the light source 10 (in this embodiment, the light source 10), an RF signal is applied to the two light modulators (in this embodiment, the RF signal source 60), and the Machzen An optical frequency comb generator for outputting an output light from the chromatography waveguide as an optical frequency comb.
The optical frequency comb generator according to the present embodiment further includes a monitor unit (in this embodiment, the monitor PD 27) for monitoring the intensity of the output light, and the phase control unit based on the output signal of the monitor unit. This is an optical frequency comb generator equipped with a bias control circuit (in this embodiment, an integration control circuit 29) for adjusting the phase difference so that the intensity of the output light becomes a preset value.

Hereinafter, the present embodiment will be described with reference to the drawings.
FIG. 1 is a schematic block diagram of an optical signal generator 1 according to this embodiment.
The optical signal generator 1 includes a light source 10, an optical frequency comb generator 20, a dispersion compensator 30, an optical intensity modulator 40, an optical pulse compressor 50, and an RF signal source (modulation signal generator) 60. .
The optical frequency comb generator 20 includes a polarization controller 21, an optical modulator 22, a distributor 23, a phase adjuster 24, an amplitude adjuster 25, a tap coupler (branch unit) 26, a monitor PD (Photodiode, photodiode; light intensity) A detection unit) 27, a reference voltage source 28, and an integration control circuit (bias control unit) 29. The optical frequency comb generator 20 may be integrated with other components such as the light source 10 or may be a separate body.

The light source 10 is, for example, a laser light source that generates continuous light having a predetermined wavelength. The light source 10 is, for example, a DFB (Distributed Feedback) laser having a wavelength of 1.55 μm. An output end of the light source 10 is connected to an incident end of the polarization controller 21 through an optical fiber, and outputs a light wave generated by the light source 10 to the polarization controller 21.
The polarization controller 21 is connected to the emission end of the light source 10 through an optical fiber, and a light wave is input from the light source 10. The polarization controller 21 adjusts the polarization plane of the input light wave to a polarization plane suitable for incidence on the optical modulator 22. The output end of the polarization controller 21 is connected to an input waveguide 221 (described later) provided in the optical modulator 22 through an optical fiber, and outputs a light wave whose polarization plane is adjusted to the optical modulator 22. As a result, the polarization characteristic of the polarization plane of the light wave input to the optical modulator 22 is adjusted to the polarization characteristic that can be input to the optical modulator 22.

The optical modulator 22 includes an input waveguide 221, two branched waveguides 222A and 222B, an output waveguide 223, two modulation electrodes (light modulation units) 224A and 224B, and a bias electrode (phase control unit) 225. Composed.
Each of the two branching waveguides 222A and 222B has one end connected to the input waveguide 221 and the other end connected to the output waveguide 223. The output waveguide 223 is connected to the incident end of the dispersion compensator 30 via an optical fiber via the tap coupler 26. The input waveguide 221, the two branch waveguides 222A and 222B, and the output waveguide 223 constitute a Mach-Zehnder interferometer.

The modulation electrodes 224A and 224B are formed on the branch waveguides 222A and 222B, respectively. The bias electrode 225 is formed on the branch waveguide 222A or 222B, or both. In the example shown in FIG. 1, the bias electrode 225 is formed only on the branching waveguide 222B.
The optical modulator 22 is, for example, an LN modulator in which the above-described waveguides and electrodes are formed on a Z-cut LN substrate. A Z-cut LN substrate is a substrate made of LN, which applies an electric field in the Z direction of the crystal optical axis of LN. Optical modulation is most effectively performed by applying an electric field in the Z direction. With this configuration, the optical modulator 22 can independently control the phase of light propagating through the two branch waveguides 222A and 222B.
The optical modulator 22 is not limited to a modulator configured using LN, but may be an optical modulator configured using another material having an electro-optic effect.
In the present embodiment, for example, the following materials can be used as materials having an electro-optic effect; ferroelectric crystals such as lithium tantalate (LT) (LitaO ₃ ), lanthanum zirconate titanate, and the like. Lead (PLZT) is a ceramic such as lead lanthanum zirconate titanate (Pb, La) (Zr, Ti) O ₃ ), silicon (Si) semiconductor, indium phosphide (InP) and its derivatives. Indium gallium phosphide (InGaP), Indium gallium aluminum phosphide (InGaAlP) , Indium gallium arsenide (Indium Gallium Arsenide (InGaAs)), arsenide indium gallium phosphide (Indium Gallium Arsenide Phosphide (InGaAsP)), III-V group compound semiconductor and the like, such as.

  The input terminal of the distributor 23 is connected to the output terminal of the RF signal source 60, and the modulation signal SIG <b> 1 is input from the RF signal source 60. The output terminal of the distributor 23 is connected to the input terminal of the phase adjuster 24 and the modulation electrode 224A, and applies the input modulation signal SIG1 to the phase adjuster 24 and the modulation electrode 224A. Thus, the modulation signal SIG1 applied to the modulation electrode 224A is referred to as a modulation signal SIG1a.

  The input terminal of the phase adjuster 24 is connected to the output terminal of the distributor 23, and the modulation signal SIG <b> 1 is input from the distributor 23. The phase adjuster 24 adjusts the phase of the modulation signal SIG1 by giving a predetermined phase to the input modulation signal SIG1. Thereby, for example, the phase of the modulation signal SIG1a may coincide with the phase of the modulation signal SIG1b (described later). When the phase difference between the phase of the modulation signal SIG1a and the phase of the modulation signal SIG1b is allowed, the allowed phase difference is branched into the branched waveguides 222A or 222B according to the bias voltage applied to the bias electrode 225. The phase imparted to the light wave propagating through may be offset. The output terminal of the phase adjuster 24 is connected to the input terminal of the amplitude adjuster 25, and outputs the modulated signal SIG <b> 1 whose phase is adjusted to the amplitude adjuster 25.

  The input end of the amplitude adjuster 25 is connected to the output end of the phase adjuster 24, and the modulation signal SIG <b> 1 whose phase is adjusted is input from the phase adjuster 24. The amplitude adjuster 25 amplifies the amplitude of the input modulation signal SIG1 with a predetermined amplification factor. The output terminal of the amplitude adjuster 25 is connected to the modulation electrode 224B, and applies the amplified modulation signal SIG1 to the modulation electrode 224B. The modulation signal SIG1 applied to the modulation electrode 224B is referred to as a modulation signal SIG1b.

  The tap coupler 26 branches a part (for example, 5%) of the light wave input from the output waveguide 223, outputs the branched light wave to the monitor PD 27, and passes the remaining most (for example, 95%) of the input light wave. And output to the dispersion compensator 30. By making the power of the branched light wave smaller than the power of the passed light wave, power loss due to branching is reduced.

The input terminal of the monitor PD 27 (intensity detection unit) is connected to the output terminal of the tap coupler 26, and the light wave input from the tap coupler 26 is input. The monitor PD 27 monitors the power of the input light wave, generates a detection signal indicating a voltage value proportional to the power, and indicates a time average value (detection voltage value) V _pd of the voltage value indicated by the generated detection signal. Generate a monitor signal. That is, the detected voltage value V _pd is proportional to the time average (average optical output) of the power of the light wave (optical frequency comb) output from the output waveguide 223.
The monitor PD 27 is a diode made of a semiconductor such as germanium or gallium, for example. The time width (average time) when taking the time average value is, for example, about 1 μs. The output terminal of the monitor PD 27 is connected to one of the two input terminals of the integration control circuit 29 and outputs the generated monitor signal to the integration control circuit 29.
The reference voltage source 28 generates a reference voltage signal having a predetermined constant reference voltage value V _set . The output terminal of the reference voltage source 28 is connected to the other of the two input terminals of the integration control circuit 29 and outputs the generated reference voltage signal to the integration control circuit 29.

The integration control circuit 29 has two input terminals, one of which is connected to the output terminal of the monitor PD 27 and the other is connected to the output terminal of the reference voltage source 28. The integration control circuit 29 subtracts the reference voltage value V _set indicated by the reference voltage signal input from the reference voltage source 28 from the detection voltage value V _pd indicated by the monitor signal input from the monitor PD 27 (V _pd −V _set ) is integrated over a predetermined period (integration time). The integration control circuit 29 determines a value obtained by multiplying the integral value obtained by the integration by a predetermined coefficient k _i, and a predetermined coefficient k _p that is different from the coefficient k _i on the difference value (V _pd −V _set ). A bias signal having a value obtained by synthesizing a value obtained by multiplying by a voltage value (bias voltage) is generated. The generated bias signal is also called a DC (Direct Current) bias or a DC bias.

Here, the coefficients k _i and k _p are determined in advance so as to have a negative feedback characteristic with respect to the bias voltage. That is, the negative feedback characteristic is a characteristic in which the difference value from the reference value of the voltage value (bias voltage) of the bias signal takes a negative value when the state where the difference value (V _pd −V _set ) is larger than 0 continues. It is. In other words, when the state where the difference value (V _pd −V _set ) is smaller than 0 continues, the difference value from the reference value of the bias voltage takes a positive value. The reference value of the bias voltage is a voltage value that gives a predetermined phase difference Δθ, for example, an initial value (described later) of the phase difference Δθ.
The output terminal of the integration control circuit 29 is connected to the bias electrode 225 and applies the generated bias signal to the bias electrode 225. The integration control circuit 29 may be configured to include, for example, an integration circuit having an operational amplifier (operational amplifier), a capacitor, and a resistance element. In such a case, the time constants that are coefficients of the coefficients k _i and k _p and the integration time are given based on circuit constants such as the capacitance of the capacitor and the resistance value of the resistance element.

Therefore, the bias electrode 225, the monitor PD 27, the reference voltage source 28, and the integration control circuit 29 observe the detection voltage value V _pd based on the power of the light wave output from the output waveguide 223, and bias the reference voltage value V _set as a target. A PI (Proportional Integral) control system for controlling the voltage is configured. As a result, the bias voltage is adjusted to be constant so that the detection voltage value V _pd becomes equal to the constant reference voltage value V _set, and thus the bias voltage is controlled to take a constant phase difference Δθ.

  The dispersion compensator 30 controls the phase and intensity of each frequency component of the light wave input from the output waveguide 223 to narrow the waveform of the input light wave and output it to the light intensity modulator 40. The dispersion compensator 30 is, for example, an optical fiber having a predetermined dispersion characteristic or another optical element. Here, the predetermined dispersion characteristic means that the group velocity for a long wavelength (low frequency) light component (the first half portion of the optical pulse) is larger and the short wavelength (high frequency) light component (the second half portion of the optical pulse). Dispersion characteristics with small group velocity. As an optical fiber having such a dispersion characteristic, for example, there is a 1.3 μm band single mode fiber for communication in which chromatic dispersion is zero in the wavelength 1.3 μm band and is proportional to the wavelength.

The light intensity modulator 40 includes an incident end and an input end. The incident end is connected to the output end of the dispersion compensator 30, and the input end is connected to the output end of the RF signal source 60. In the light intensity modulator 40, the light wave input from the dispersion compensator 30 is subjected to intensity modulation based on the modulation signal SIG 2 input from the RF signal source 60.
The light intensity modulator 40 includes a Mach-Zehnder light modulator 41, for example. The Mach-Zehnder optical modulator 41 outputs only a specific part of the optical pulses based on the signal pattern of the modulation signal SIG2 input from the RF signal source 60 in the optical pulse train forming the input optical wave. . That is, the Mach-Zehnder optical modulator 41 transmits an optical pulse having a specific time series pattern based on the modulation signal SIG2, and blocks other optical pulses. Thus, an optical wave composed of an optical pulse train in which a part of the optical pulse is thinned based on the modulation signal SIG2 is generated. The output end of the light intensity modulator 40 is connected to the input end of the optical pulse compressor 50, and the generated light wave is output to the optical pulse compressor 50.

The optical pulse compressor 50 amplifies the light wave input from the light intensity modulator 40 using, for example, (EDFA: Erbium Doped Fiber Amplifier).
The optical pulse compressor 50 adiabatically compresses (soliton compression) the optical pulse by increasing the peak power while narrowing the pulse width of the optical pulse constituting the amplified optical wave. In order to perform adiabatic compression, the optical pulse compressor 50 may use, for example, a fiber module configured by alternately connecting highly nonlinear fibers and highly dispersed fibers.
The optical pulse compressor 50 amplifies a light wave composed of adiabatic-compressed light pulses and emits the amplified light wave. As a result, an optical pulse train having the required output power can be obtained. Since the pulse width is compressed by adiabatic compression (for example, 0.2 ps per pulse), the spectrum bandwidth is expanded to about 40 nm. Therefore, it is desirable to use an amplifier capable of amplifying input light in a wide band.
Such a pulse that is very short in time is called an ultrashort pulse. The ultrashort pulse is applied to optical communication as a light source such as an optical time division multiplexing (OTDM). The ultrashort pulse is used as a light source for ranging, terahertz (THz) spectroscopy, a light source for a nonlinear microscope, and other applications.

  The RF signal source 60 generates a modulation signal SIG1 (for example, a sine wave) having a predetermined frequency, and outputs the generated modulation signal SIG1 to the distributor 23. The output end from which the RF signal source 60 outputs the modulation signal SIG 1 is connected to the input end of the distributor 23. The RF signal source 60 generates a modulation signal SIG2 synchronized with the modulation signal SIG1, and outputs the generated modulation signal SIG2 to the light intensity modulator 40. The output end from which the RF signal source 60 outputs the modulation signal SIG2 is connected to the input end of the light intensity modulator 40.

The fact that the modulation signal SIG1 and the modulation signal SIG2 are synchronized means that the frequency of the modulation signal SIG2 is equal to the frequency of the modulation signal SIG1 or is an integral fraction of the frequency of the modulation signal SIG1. For example, the RF signal source 60 can generate the modulation signal SIG1 and the modulation signal SIG2 by generating a common master clock and multiplying the generated common master clock by the same or different integer magnification.
In the RF signal source 60, the signal generator that generates the modulation signal SIG1 and the signal generator that generates the modulation signal SIG2 may be separate. Further, the frequency of the modulation signal SIG2 may be constant over a plurality of periods or may be different for each period.

  As described above, the modulation signal SIG2 is used in the light intensity modulator 40 to thin out the optical pulse from the optical pulse train. For example, when the modulation signal SIG2 is a signal obtained by dividing the modulation signal SIG1 by 2 (that is, a signal having a frequency of 1/2), every other optical pulse constituting the optical pulse train input to the optical intensity modulator 40 Is thinned out. Therefore, the repetition frequency of the light pulse output from the light intensity modulator 40 can be changed by changing the frequency of the modulation signal SIG2. As described above, since the modulation signal SIG2 can take an arbitrary waveform pattern as long as it is synchronized with the modulation signal SIG1, the light intensity modulator 40 thins out an optical pulse in accordance with the pattern. Thereby, an optical pulse train having an arbitrary pattern is generated.

(Operation example)
Next, an operation example of the optical signal generating apparatus 1 configured as described above, in particular, the optical frequency comb generator 20 will be described.
The light generated by the light source 10 is adjusted in the direction of the plane of polarization by the polarization controller 21 and input to the input waveguide 221 of the optical modulator 22. The light incident on the input waveguide 221 is branched into the branch waveguides 222A and 222B and propagates through the branch waveguides 222A and 222B, respectively. The propagating light propagating through the branch waveguides 222A and 222B undergoes a phase change according to the electric fields from the modulation electrodes 224A and 224B and the bias electrode 225. The propagating light that has undergone the phase changes is again synthesized by the output waveguide 223, and the synthesized light is output from the optical modulator 22 as output light.

Here, it is assumed that the amplitude and angular frequency of the light incident on the optical modulator 22 are E ₀ and ω ₀ , respectively, and the phase changes received by the propagation light in the branching waveguides 222A and 222B are θ ₁ and θ ₂ , respectively. The time variation E (t) of the amplitude of the output light from the optical modulator 22 is expressed by Expression (3).
E (t) = E ₀ [sin (ω ₀ t + θ ₁ ) + sin (ω ₀ t + θ ₂ )] / 2 (3)
However, θ ₁ and θ ₂ are expressed by the equations (4) and (5).
θ ₁ = A ₁ sin (ω _m t) (4)
θ ₂ = A ₂ sin (ω _m t) + B ₀ + B ₂ (5)

In equations (4) and (5), A ₁ and A ₂ are the amplitude of modulation (modulation degree) given to the propagation light of the branching waveguide 222A by the modulation electrode 224A, respectively, and the propagation of the branching waveguide 222B by the modulation electrode 224B. The amplitude of the modulation applied to the light. B ₀ is a phase difference that has originally between the branch waveguides 222A and the branch waveguide 222B, _{B 2} is a phase imparted to the propagating light branch waveguides 222B the bias electrode 225. The inherent phase difference is caused by the shape of the branching waveguide, such as a phase difference that occurs when no bias voltage is applied, that is, a difference in the path length of the branching waveguide 222B with respect to the path length of the branching waveguide 222A. Is the phase difference. omega _m is the angular frequency given by multiplying 2π to the modulation frequency _{f m} of the modulation signal SIG1 that RF signal source 60 is generated.

From Expression (3), the power P (t) and the frequency ω (t) of the output light from the optical modulator 22 are expressed by Expressions (6) and (7), respectively.
P (t) = P _in [1 + cos {ΔAsin (ω _m t) + Δθ}] / 2 (6)
ω (t) = ω ₀ + ω _m Asin (ω _m t) (7)
Here, ΔA is the above-described modulation degree difference, and is given by ΔA = A ₁ −A ₂ . Δθ is the above-described phase difference and is given by Δθ = −B ₀ −B ₂ . Further, A = (A ₁ + A ₂ ) / 2. P _in is the average optical input power to the optical modulator 22.
By taking the time average of the power P (t) of the output light shown in Expression (6), the average light output power P _out shown in Expression (2) is obtained.

The optical modulator 22 uses a constant value adjusted in advance as the modulation degree difference ΔA. Since the modulation degree difference ΔA does not vary from the initial value adjusted in advance, the average optical output power P _out is kept constant by controlling the optical modulator 22 to make the phase difference Δθ constant. In the LN modulator, there may be a phenomenon in which the phase difference B ₀ that the branching waveguide 222A and the branching waveguide 222B have in the state of no bias voltage fluctuate as time passes. As described above, the control for setting the phase difference Δθ to a constant value is performed by controlling the bias voltage applied to the bias electrode 225 output from the integration control circuit 29 so that the average optical output power P _out is kept constant. realizable.

As the initial values of the modulation degree difference ΔA and the phase difference Δθ, the modulation degree difference ΔA and the phase difference Δθ, which have a desired shape (for example, the flattest shape) of the spectrum of the optical output power (spectrum envelope), are set in advance. Set it up. For example, the value of the phase difference Δθ is kept constant, the modulation degree difference ΔA is changed by a minute amount, and the modulation degree difference ΔA in which the spectral envelope of the optical output power becomes the most desired shape and the average optical output power P at that time _out is determined. Thereafter, the phase difference Δθ is changed by a minute amount based on the set modulation degree difference ΔA, and the phase difference Δθ in which the spectral envelope of the optical output power has the most desired shape is determined. Thereafter, the phase difference Δθ is controlled by the above-described control so that the average optical output power P _out becomes constant under the set modulation degree difference ΔA. This is because the average light output power P _out is proportional to the detection voltage value V _pd output from the monitor PD 27. Since the average optical input power _{P in} to the optical modulator 22 takes a constant value, also constant output power ratio _P out _{/ P in} by the control. Further, the detection voltage value V _pd obtained when the initial values of the modulation degree difference ΔA and the phase difference Δθ are determined is determined as a reference voltage value V _set . The reference voltage value V _set is a detection voltage value V _pd corresponding to the average optical output power P _out when these initial values are determined.

The process for determining the initial values of the modulation degree difference ΔA and the phase difference Δθ described above may be an operation in which the operator manually changes the modulation degree difference ΔA or the phase difference Δθ while visually observing the spectral envelope of the optical output power. It may be performed by digital processing.
The digital processing includes, for example, the following steps.
(I) Data indicating the spectral envelope of the desired optical output power is stored in advance as the target spectral envelope.
(Ii) The waveform of the light wave input to the monitor PD 27 is acquired for each of a plurality of combinations of the modulation degree difference ΔA and the phase difference Δθ.
(Iii) For the acquired waveform, for example, FFT (Fast Fourier Transform) is performed to convert from the time domain to the frequency domain to calculate the spectrum of the optical output power.
(Iv) For the spectrum envelope of the calculated spectrum of optical output power, an index value (for example, a square error) indicating the magnitude of the error from the target spectrum envelope is calculated.
(V) For each combination of the modulation degree difference ΔA and the phase difference Δθ, the modulation degree difference ΔA and the position when the index value calculated by performing the steps (ii)-(iv) indicates the smallest error. The combination of the phase difference Δθ is defined as the initial value of the modulation degree difference ΔA and the phase difference Δθ.

In the digital processing described above, for one predetermined phase difference Δθ, first, steps (ii)-(iv) are performed for each of a plurality of modulation degree differences ΔA, and calculated in step (v). The modulation degree difference ΔA when the index value indicates the smallest error may be determined as the initial value of the modulation degree difference ΔA. Thereafter, the steps (ii)-(iv) are performed for each of the plurality of phase differences Δθ, and the phase difference Δθ when the index value calculated in the step (v) indicates that the error is the smallest is calculated as the phase difference. The initial value of Δθ may be determined.
In the present embodiment, the digital processing described above may be performed by a processing unit (not shown) integrated with the optical frequency comb generator 20, or a processing unit separate from the optical frequency comb generator 20 may be used. You may make it perform.

In the conventional optical modulator, even if the modulation degree difference ΔA and the phase difference Δθ are adjusted so as to satisfy Equation (1), each device constituting the optical frequency comb generator 20 operates in an ideal state. Is not limited. The factors include, for example, the distortion of the waveform of the modulation signal, various noises, the shape, arrangement, composition, and individual differences of each device. Therefore, distortion occurs in the output light as shown in FIG.
FIG. 2 shows an example of the spectral envelope of the optical output power when the output light is distorted.
In FIG. 2, the horizontal axis indicates the wavelength (Wavelength), and the vertical axis indicates the optical output power (Optical Power). In the example shown in FIG. 2, the optical output power is obtained when the passband wavelength is 1552.6 to 1554.4 nm, and the modulation degree difference ΔA and the phase difference Δθ are both 0.5π.
The optical output power shown in FIG. 2 is distributed between −58 and −25 [dB] in the pass band, and the maximum value and the minimum value are irregularly distributed.

On the other hand, in the present embodiment, the distortion to the output light is corrected as shown in FIG. 3 by adjusting the modulation degree difference ΔA or the phase difference Δθ without being constrained by the relationship shown in the equation (1). Is done.
FIG. 3 shows another example of the spectral envelope of the optical output power related to the corrected output light.
The relationship between the horizontal axis and the vertical axis in FIG. 3 is the same as the relationship between the horizontal axis and the vertical axis in FIG.
In the example shown in FIG. 3, the optical output power is obtained when the modulation degree difference ΔA is 0.5π and the phase difference Δθ is 0.58π.
The optical output power shown in FIG. 3 is distributed between −63 to −35 [dB] in the pass band, and the local maximum value is about −37 dB and the local minimum value is about −60 dB regularly. Distributed. That is, according to the present embodiment, the output light is corrected so that the spectral envelope of the optical output power becomes flat by controlling the phase difference Δθ to a constant value. Here, even if a phenomenon in which the bias voltage is displaced (DC drift) occurs in the LN modulator (optical modulator 22), the phase difference of the bias that is the control point always follows a constant value (see FIG. In the example shown in FIG. 3, the voltage is controlled to be Δθ = 0.58π.

  In other words, in this embodiment, even if the relationship between the modulation degree difference ΔA and the phase difference Δθ where the spectral envelope of the optical output power is flat varies from the relationship represented by the equation (1), A stable optical frequency comb can be generated by correcting the distortion. In this embodiment, when generating a bias signal for adjusting the phase difference Δθ, it is not necessary to use a signal that affects the phase difference Δθ, such as a dither signal. Therefore, it is avoided that noise due to the use of such a signal is included in the optical frequency comb that is output. As a result, a high-quality and stable optical frequency comb light source that can be used for analog measurement such as distance measurement can be realized.

(Modification)
Next, a modification of the above-described embodiment will be described.
In the above description, the case where the integration control circuit 29 configures a PI control system including an integration circuit has been described as an example. However, the present embodiment is not limited to this. In the present embodiment, any control system may be employed as long as a feedback control system that controls the bias signal using the reference voltage value V _set and the detected voltage value V _pd as the target signal and the observation signal is configured. .
In the present embodiment, for example, a PID (Proportional Differential Differential) control system may be configured by providing a differentiation circuit in parallel with the integration circuit included in the integration control circuit 29. That is, the integration control circuit 29 obtains a value obtained by multiplying the integral value obtained by the integration by a predetermined coefficient k _i , a value obtained by multiplying the difference value (V _pd −V _set ) by a predetermined coefficient k _p , and Further, a bias signal having a value obtained by combining a differential value obtained by differentiating the difference value (V _pd −V _set ) with a value obtained by multiplying a predetermined coefficient k _d as a bias voltage is generated. This differentiation circuit is also configured to include an operational amplifier, a capacitor, and a resistance element, and given by the integration circuit and the circuit constants of the differentiation circuit are coefficients k _i , k _p, k _d, and a time constant that is a measure of integration time.
Furthermore, a control system in which a plurality of these PI control systems or PID control systems are connected in series may be configured.

In the above description, the case where the integration control circuit 29 is configured by an analog circuit and performs analog control has been described as an example. However, the integration control circuit 29 is biased based on the above-described reference voltage value V _set and detection voltage value V _pd by digital calculation. You may perform the process which calculates the voltage value of a signal. That is, the integration control circuit 29 may be configured to realize the processing by a computer, record a program for realizing the control function on a computer-readable recording medium, and record the program recorded on the recording medium. May be realized by causing a computer system to read and execute the program. Further, the integration control circuit 29 may be realized as an integrated circuit such as an LSI (Large Scale Integration).

  Further, the processing unit used to determine the initial values of the modulation degree difference ΔA and the phase difference Δθ described above may be realized by a computer, and a program for realizing this control function is stored on a computer-readable recording medium. The program may be recorded, recorded on this recording medium, read into a computer system, and executed. The integration control circuit 29 may be realized as an integrated circuit such as an LSI.

In the above description, the case where the bias voltage by the integration control circuit 29 is applied to the branch waveguide 222B has been described as an example, but the phase difference Δθ between the branch waveguides 222A and 222B is controlled to a constant value based on the bias voltage. If you can, it is not limited to this.
For example, in one modification of the present embodiment shown in FIG. 4, the bias electrode 225 is formed on the branch waveguide 222A instead of the branch waveguide 222B. A bias voltage controlled to be constant by the integration control circuit 29 is applied to the formed bias electrode 225. Here, when the phase imparted to the propagating light branch waveguides 222A by biasing electrodes 225 and _{B 1,} the phase difference Δθ is equal to _-B 0 + _{B 1.} Therefore, the phase difference Δθ is also controlled to be constant by controlling the phase −B ₀ + B ₁ to be constant.

In another modification of the present embodiment shown in FIG. 5, a bias electrode 225A is formed on the branch waveguide 222A, and a bias electrode 225B is formed on the branch waveguide 222B. A bias voltage from the integration control circuit 29 is applied to the formed bias electrodes 225A and 225B. The bias electrodes 225A, when the _{B 2} phase given to propagation light branch waveguides 222B by 225B, the phase difference Δθ _is given as B 1 _-B 0 _{-B 2.} Therefore, the phase difference Δθ is also controlled to be constant by controlling the phase difference B ₁ −B ₀ −B ₂ to be constant. For example, the integration control circuit 29 controls the bias voltage applied to the bias electrodes 225A and 225B to a constant value, whereby the phase difference Δθ is controlled to be constant.

In the above description, the case where the phase adjuster 24 and the amplitude adjuster 25 are connected in series between the distributor 23 and the modulation electrode 224B has been described as an example. However, the phase difference and amplitude between the modulation signal SIG1a and the modulation signal SIG1b are described. The present embodiment is not limited to this as long as the difference can be adjusted.
For example, one or both of the phase adjuster 24 and the amplitude adjuster 25 may be connected between the distributor 23 and the modulation electrode 224A.
When the phase adjuster 24 and the amplitude adjuster 25 are connected in series between the distributor 23 and the modulation electrode 224B or between the distributor 23 and the modulation electrode 224A, the phase adjuster 24 and the amplitude adjuster 25 are The order of connection may be the reverse of the above-described example.
In addition, a phase adjuster and an amplitude adjuster may be provided between the distributor 23 and the modulation electrode 224B and between the distributor 23 and the modulation electrode 224A, respectively.

In the above description, the case where the dispersion compensator 30 is connected immediately after the optical frequency comb generator 20 has been described as an example, but the present embodiment is not limited thereto. The dispersion compensator 30 only needs to be provided at a later stage than the optical frequency comb generator 20. For example, the optical intensity modulator 40 may be connected immediately after the optical frequency comb generator 20, and the dispersion compensator 30 may be connected immediately thereafter.
In the above description, the case where the polarization controller 21 is integrated with the optical frequency comb generator 20 is taken as an example. However, the polarization controller 21 may be separate from the optical frequency comb generator 20, It may be omitted.

As described above, in this embodiment, the light input from the light source is propagated using the two branch waveguides formed on the substrate having the electro-optic effect, and the light propagating through the two branch waveguides is applied respectively. Modulation is performed based on the modulated signal. In the present embodiment, the phase difference between each of the light propagating through the two branch waveguides is controlled using an applied bias, and the bias is controlled so that the intensity of the output light becomes a predetermined value.
Thereby, even when the modulation degree difference ΔA and the phase difference Δθ of the light propagating through the two branch waveguides fluctuate from predetermined operating conditions, the distortion of the output light can be corrected. That is, it is possible to generate a stable optical frequency comb while maintaining optimum operating conditions.

  The embodiment of the present invention has been described above with reference to the drawings. However, the specific configuration is not limited to the above-described configuration, and various design changes and the like can be made without departing from the gist of the present invention. Is possible.

DESCRIPTION OF SYMBOLS 1 ... Optical signal generator, 10 ... Light source,
20 ... optical frequency comb generator, 21 ... polarization controller, 22 ... optical modulator,
221 ... Input waveguide, 222A, 222B ... Branch waveguide, 223 ... Output waveguide,
224A, 224B ... modulation electrode, 225 (225A, 225B) ... bias electrode,
23 ... distributor, 24 ... phase adjuster, 25 ... amplitude adjuster, 26 ... tap coupler,
27 ... Monitor PD, 28 ... Reference voltage source, 29 ... Integration control circuit,
30 ... Dispersion compensator, 40 ... Light intensity modulator, 41 ... Mach-Zehnder type optical modulator,
50: Optical pulse compressor, 60: RF signal source

Claims (2)

Two branching waveguides for propagating light input from the light source;
Two light modulators for modulating light propagating through the two branch waveguides based on modulation signals, respectively;
A phase control unit that controls a phase difference between each of the light propagating through the two branch waveguides according to a bias; and
A light intensity detector that detects the intensity of output light from the two branch waveguides;
A bias controller that controls the bias so that the intensity of the output light detected by the light intensity detector becomes a predetermined value;
An optical frequency comb generator comprising:   The predetermined value is a difference in modulation degree of the modulation signal between the two optical modulators and a phase difference between the light propagating through the two branch waveguides so that the spectrum of the output light has a predetermined shape. 2. The optical frequency comb generator according to claim 1, wherein the intensity of the output light detected by the light intensity detection unit is determined.

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