JP2012078413A - Optical frequency comb generation device, optical pulse generation device and method for controlling optical pulse generation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable the generation of an optical pulse with reduced distortion.SOLUTION: The optical frequency comb generation device which includes an optical modulator for branching light into two, modulating each of the two branched light, and then combining the two modulated light and outputting the combined light, and a modulation signal generation section for generating each of modulation signals for modulating each of the branched light, generates an optical frequency comb by setting a difference ΔA between one amplitude and the other amplitude of each of the modulation signals at 0.2π≤ΔA<0.5π.

Description

  The present invention relates to an optical frequency comb generator, an optical pulse generator, and an optical pulse generator control method.

  2. Description of the Related Art Conventionally, as a technique for generating an optical pulse, a method of pulsing light by modulating light to generate an optical frequency comb composed of a number of higher-order frequency components is known. Patent Document 1 discloses a method for improving the flatness of the optical frequency comb and optimizing the generation efficiency of the optical pulse (power of the optical pulse).

JP 2007-248660 A

  However, in Patent Document 1, the range of driving conditions for obtaining an optimal optical pulse is very narrow, it is difficult to adjust the driving conditions to a desired state, and the generation efficiency of optical pulses is optimized. As a result, the distortion of the waveform remains in the optical pulse, and the cause of optical pulse degradation assumed in practical use, such as the phase shift of the modulation signal and the effects of harmonics, has not been studied. The problems such as were unresolved.

  The present invention has been made in view of the above points, and an object thereof is to enable generation of an optical pulse with reduced distortion.

  The present invention has been made to solve the above problems, and the optical frequency comb generator of the present invention splits light into two, modulates each of the branched lights, and then multiplexes and outputs them again. An optical modulator, and a modulation signal generation unit that generates each modulation signal for modulating each of the branched lights, and a difference ΔA between one amplitude and the other amplitude of each modulation signal is 0.2π An optical frequency comb is generated by setting ≦ ΔA <0.5π.

  The optical pulse generator of the present invention compensates for the chirp imparted by the optical frequency comb generator to the light output from the optical frequency comb generator and the optical frequency comb generator. As described above, the optical pulse compression unit is configured to compress the pulse by applying a predetermined amount of chirp opposite to the chirp.

Moreover, the optical pulse generator of the present invention is the above optical pulse generator, wherein the optical pulse compression unit is a 1.3 μm single mode fiber, and the optical fiber length L of the 1.3 μm single mode fiber is When the modulation frequency of the modulation signal is f (GHz), L = (40 / π) (Af) ⁻¹ .

  The optical pulse generator of the present invention is the optical pulse generator described above, wherein the modulation signal generator includes a phase adjustment unit that adjusts a phase shift between the modulation signals in the middle of the supply line of the modulation signal. And the phase adjustment unit adjusts the phase shift φ to a range of −0.02π ≦ φ ≦ 0.02π.

  In the optical pulse generator of the present invention, in the optical pulse generator described above, the modulation signal generation unit includes a filter that blocks harmonics of the modulation signal, and the optical modulator blocks the harmonics. The branched light is modulated by the modulated signal.

The optical pulse generator of the present invention is characterized in that, in the optical pulse generator, the filter suppresses a harmonic of the modulation signal by 40 dB or more with respect to a fundamental wave of the modulation signal.
The optical pulse generator of the present invention is characterized in that, in the optical pulse generator, the optical modulator has a termination resistor for terminating the modulated signal at a position physically separated from the optical modulator. And
The optical pulse generator of the present invention is the above optical pulse generator, wherein the optical modulator is an LN modulator in which a Mach-Zehnder type optical waveguide and a modulation electrode are formed on an LN substrate. And

Also, the control method of the optical pulse generator of the present invention includes an optical modulator that divides light into two, modulates each of the branched lights, and then multiplexes and outputs the modulated light, and modulates each of the branched lights A modulation signal generation unit for generating each modulation signal for the optical signal, and a predetermined amount of a sign opposite to the chirp so that the light output from the optical modulator compensates for the chirp given by the optical modulator to the light In the control method of the optical pulse generator provided with the optical pulse compression unit that compresses the pulse by applying the chirp of the above, a difference ΔA between one amplitude and the other amplitude of each modulation signal is 0.2π ≦ 0.2. ΔA <0.5π.
The optical pulse generator control method of the present invention is the optical pulse generator control method described above, in which the spectrum of the output light of the optical pulse generator is observed using an optical spectrum analyzer, and The difference ΔA is controlled based on the ratio of the sideband peak power.

The optical pulse generator control method of the present invention adjusts the phase shift φ between the modulated signals in the range of −0.02π ≦ φ ≦ 0.02π in the above-described optical pulse generator control method. It is characterized by that.
The optical pulse generator control method of the present invention is the above optical pulse generator control method, wherein the spectrum of the output light of the optical pulse generator is observed using an optical spectrum analyzer, and the + nth order of the spectrum is observed. The phase shift φ is controlled based on the peak power of the −nth order sideband.

  The optical pulse generator control method of the present invention is such that the optical power between pulses of the optical pulse train output from the optical pulse generator is minimized in the above-described optical pulse generator control method. And adjusting the bias of each modulation signal.

  According to the present invention, it is possible to generate an optical pulse with reduced distortion.

It is a figure which shows the structure of the optical pulse generator by embodiment of this invention. It is a figure which shows the time waveform of output optical power P (t) of the optical frequency comb generator in case the conditions of Formula (10) are satisfy | filled. It is a figure which shows a mode that the flatness of an optical frequency comb changes with respect to the change of (DELTA) A and (DELTA) (theta). It is a figure which shows the relationship between the optimal optical fiber length L and the modulation index A. It is a figure which shows the relationship between S / N ratio of the optical pulse pulse-compressed by the chirp (dispersion) compensator, and modulation parameters ΔA and Δθ. It is a figure for demonstrating the definition of the S / N ratio of an optical pulse. It is a figure which shows the relationship between phase shift (phi) of a modulation signal, and S / N ratio of the optical pulse after pulse compression. It is the figure which showed the influence which the harmonic of a modulation signal has on an optical pulse.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an optical pulse generator 1 according to an embodiment of the present invention.

The optical pulse generator 1 includes an optical frequency comb generator 20, a chirp (dispersion) compensator 30, an optical spectrum analyzer 101, a photoelectric converter 102, and a sampling oscilloscope 103.
The optical frequency comb generator 20 includes a Mach-Zehnder optical modulator 21, a modulation signal generator 22, a distributor 23, variable phase shifters 24A and 24B, variable attenuators 25A and 25B, amplifiers 26A and 26B, a bandpass filter. 27A and 27B, a bias voltage supply unit 28, a light source 10, and a polarization controller 11.

  The light source 10 is, for example, a laser light source, and generates continuous light having a predetermined wavelength. The exit end of the light source 10 is connected to the entrance end of the polarization controller 11 by an optical fiber. The polarization controller 11 controls the polarization state of incident light to a predetermined state. The output end of the polarization controller 11 is connected to the input waveguide 211 of the Mach-Zehnder optical modulator 21 by an optical fiber.

  The Mach-Zehnder optical modulator 21 includes an input waveguide 211, two branching waveguides 212 A and 212 B, an output waveguide 213, modulation electrodes 214 A and 214 B, and a bias electrode 215. The branching waveguides 212A and 212B are connected to the input waveguide 211 and the output waveguide 213, respectively. The input waveguide 211, the branch waveguides 212A and 212B, and the output waveguide 213 constitute a Mach-Zehnder interferometer. The modulation electrode 214A is formed on the branch waveguide 212A, and the modulation electrode 214B is formed on the branch waveguide 212B. The bias electrode 215 is formed on either or both of the branch waveguide 212A and 212B. The modulation electrodes 214A and 214B and the bias electrode 215 may be formed as the same electrode. As the Mach-Zehnder optical modulator 21, for example, an LN modulator in which each waveguide and each electrode are formed on a Z-cut LN substrate can be used. With this configuration, the Mach-Zehnder optical modulator 21 can independently control the phase received by the light propagating through the two branch waveguides 212A and 212B.

  The modulation signal generator 22 generates a modulation signal (for example, a sine wave) having a predetermined frequency. The output part of the modulation signal generator 22 is connected to the input part of the distributor 23. The distributor 23 can be a Wilkinson type or a 90 ° hybrid type, but is not limited thereto. One output section of the distributor 23 is connected to the variable phase shifter 24A, and the other output section is connected to the variable phase shifter 24B.

  The variable phase shifters 24A and 24B each shift the phase of the input signal by a predetermined amount. The output part of the variable phase shifter 24A is connected to the variable attenuator 25A, and the output part of the variable phase shifter 24B is connected to the variable attenuator 25B.

  The variable attenuators 25A and 25B each attenuate the input signal power by a predetermined amount. The output section of the variable attenuator 25A is connected to the amplifier 26A, and the output section of the variable attenuator 25B is connected to the amplifier 26B.

  The amplifiers 26A and 26B each amplify the input signal with a predetermined amplification factor. The output section of the amplifier 26A is connected to the bandpass filter 27A, and the output section of the amplifier 26B is connected to the bandpass filter 27B. A variable amplifier having a variable amplification factor may be used instead of the combination of the variable attenuators 25A and 25B and the amplifiers 26A and 26B.

  Each of the bandpass filters 27A and 27B allows a predetermined frequency band of the input signal to pass therethrough and blocks other frequency bands. The output part of the bandpass filter 27A is connected to one end of the modulation electrode 214A of the Mach-Zehnder optical modulator 21, and the output part of the bandpass filter 27B is connected to one end of the modulation electrode 214B of the Mach-Zehnder optical modulator 21. Yes. Note that a low-pass filter having a cutoff frequency higher than the predetermined frequency band may be used instead of the band-pass filter.

  The other ends of the modulation electrodes 214A and 214B of the Mach-Zehnder optical modulator 21 are connected to termination resistors 29A and 29B provided outside the Mach-Zehnder optical modulator 21, respectively. Since the termination resistors 29A and 29B are physically provided outside the Mach-Zehnder optical modulator 21, the characteristics of the termination resistors 29A and 29B when the Mach-Zehnder optical modulator 21 is driven under the driving conditions described later. It can suppress that it changes with the influence of the heat_generation | fever of 29A, 29B.

  The bias voltage supply unit 28 supplies a bias voltage to the Mach-Zehnder optical modulator 21. The output unit of the bias voltage supply unit 28 is connected to the bias electrode 215 of the Mach-Zehnder optical modulator 21.

  The chirp (dispersion) compensator 30 has a predetermined (described later) dispersion characteristic, and compresses and outputs an input optical pulse. The input part of the chirp compensator 30 is connected to the output waveguide 213 of the Mach-Zehnder optical modulator 21. As the chirp compensator 30, an optical fiber having a predetermined dispersion characteristic is used. However, optical elements other than optical fibers having equivalent characteristics may be used.

  The optical spectrum analyzer 101 is a device for analyzing the spectrum of input light, and its input unit is a branching unit 104 (optical coupler or the like) provided at the subsequent stage (output side) of the Mach-Zehnder optical modulator 21. It is connected to the.

  The photoelectric converter 102 is connected to a branching unit 105 (an optical coupler or the like) provided at the subsequent stage (output side) of the chirp compensator 30 and converts the light branched from the branching unit 105 into an electrical signal.

  The sampling oscilloscope 103 is a device for observing the time waveform of the input signal, and its input unit is connected to the photoelectric converter 102.

Next, the operation of the optical pulse generator 1 configured as described above will be described.
Light generated by the light source 10 is input to the Mach-Zehnder optical modulator 21 via the polarization controller 11. In the Mach-Zehnder optical modulator 21, this light is branched from the input waveguide 211 to the branch waveguides 212A and 212B, and propagates through the branch waveguides. At this time, while propagating through the branching waveguides 212A and 212B, the propagating light undergoes a phase change according to the electric fields from the modulation electrodes 214A and 214B and the bias electrode 215. Then, the light that has undergone this phase change is multiplexed again in the output waveguide 213.

Here, assuming that the amplitude and frequency of the input light to the Mach-Zehnder type optical modulator 21 are E ₀ and ω ₀ respectively, and the phase changes received by the propagating light in the branching waveguides 212A and 212B are θ ₁ and θ ₂ respectively. The time change E (t) of the amplitude of the light combined in the output waveguide 213, that is, the output light of the optical frequency comb generator 20 (the output light of the Mach-Zehnder optical modulator 21) is expressed by the following equation (1). It is expressed as follows.
E (t) = E ₀ [sin (ω ₀ t + θ ₁ ) + sin (ω ₀ t + θ ₂ )] / 2 (1)

However,
θ ₁ = A ₁ sin (ω _m t) + B ₁ (2)
θ ₂ = A ₂ sin (ω _m t) + B ₂ (3)
ω _m = 2πf _m (4)
A ₁ : Amplitude of modulation given to propagation light of branching waveguide 212 A by modulation electrode 214 A A ₂ : Amplitude of modulation given to propagation light of branching waveguide 212 B by modulation electrode 214 B B ₁ : Branching waveguide by bias electrode 215 Bias phase B ₂ given to the propagating light of 212 A B ₂ : Bias phase given to the propagating light of the branching waveguide 212 B by the bias electrode 215 f _m : Modulation frequency of the modulation signal generated by the modulation signal generator 22. In the expressions (2) and (3), the modulation signal input to the modulation electrode 214A via the path of the variable phase shifter 24A to the band pass filter 27A and the path of the variable phase shifter 24B to the band pass filter 27B are used. Thus, it is assumed that there is no phase shift (skew) between the modulation signals input to the modulation electrode 214B.

From the above equation (1), the power P (t) and the frequency ω (t) of the output light of the optical frequency comb generator 20 can be expressed as the following equations (5) and (6), respectively.
P (t) = P ₀ [1 + cos {ΔAsin (ω _m t) + Δθ}] / 2 (5)
ω (t) = ω ₀ + ω _{m A} cos (ω _m t) (6)
However,
ΔA = A ₁ −A ₂ (7)
Δθ = B ₁ −B ₂ (8)
A = (A ₁ + A ₂ ) / 2 (9)
It is. P ₀ is the power of the input light to the Mach-Zehnder optical modulator 21.

Now, let us consider a condition in which the optical power P (t) becomes 0 when the frequency shift (the second term in Expression (6)) becomes 0. From equation (6), the frequency shift becomes zero when ω _m t = ± π / 2. Substituting ω _m t = ± π / 2 into equation (5), the following equation (10) is obtained as a condition for the optical power P (t) to be zero.
± ΔA + Δθ = π (10)

  FIG. 2 shows a time waveform of the output optical power P (t) of the optical frequency comb generator 20 with ΔA as a parameter when the condition of the above equation (10) is satisfied. As can be seen from the figure, if the condition of equation (10) is satisfied, the optical power is suppressed to 0 between pulses of the optical pulse train even if ΔA changes, and the base of the optical pulse is distorted. It does not occur. However, when ΔA is larger than π / 2, the top portion of the light pulse is recessed and distortion occurs. According to Patent Document 1, the above equation (10) is also a condition for flattening the optical frequency comb (the output light of the optical frequency comb generator 20).

Next, attention is paid to the flatness of the optical frequency comb in order to determine the range of values that ΔA should satisfy. From the above equation (5), the n-th order sideband peak power P _n obtained by Fourier transforming the optical power P (t) is expressed by the following equation (11). Here, J _n is an nth-order Bessel function.
P _n = P ₀ [J _n ² (A) + J _n ² (A + ΔA) +2 J _n (A) J _n (A + ΔA) cos (Δθ)] / 4 (11)

FIG. 3 shows how the flatness of the optical frequency comb changes with respect to changes in ΔA and Δθ. However, in this case, from -Nth order to Nth order (when A = 2π (FIG. 3A)), N = 6, and when A = 4π (FIG. 3B), N = 12, and A = 5π. The difference between the maximum value and the minimum value of the peak power _Pn of each sideband up to (N = 15 in FIG. 3C) is defined as the flatness of the optical frequency comb. As can be seen from the figure, the flatness becomes the smallest (the optical frequency comb becomes the flattest) in the vicinity of ΔA = 0.2π for any value of A. Further, when ΔA is smaller than 0.2π, the range of Δθ where the flatness is minimized becomes very narrow, and when ΔA is larger than 0.5π, the flatness is deteriorated. From these facts, the following equation (12) is obtained as a condition of ΔA for obtaining an optical frequency comb with good flatness.
0.2π ≦ ΔA <0.5π (12)

  In general, in the Mach-Zehnder type optical modulator, the two branch waveguides have a slight asymmetry, and a drift phenomenon with time may occur. Therefore, the value of Δθ is uniquely determined by design. That is not realistic. Therefore, it is preferable to use a method in which only ΔA is determined as an optimum value as shown in the above equation (12), and the bias voltage is adjusted while observing an actual optical pulse as described later.

  As described above, by applying the equation (12) as the modulation driving condition of the Mach-Zehnder optical modulator 21, the output light from the optical frequency comb generator 20 is converted into an optical frequency comb (frequency domain) with good flatness. ). Further, by further applying Equation (10) as the modulation driving condition of the Mach-Zehnder optical modulator 21, the output light from the optical frequency comb generator 20 is converted into an optical pulse (time domain) having no distortion at the base of the pulse. ).

  The optical pulse (optical frequency comb) output from the optical frequency comb generator 20 is input to the chirp compensator 30 and subjected to pulse compression. According to the above equations (5) and (6), the optical pulse from the optical frequency comb generator 20 has a positive or negative chirp. Since the chirp amount is proportional to the degree of modulation A (see formula (9)) from the equation (6), the optical fiber of the chirp compensator 30 necessary for compensating the chirp of the optical pulse and performing the optimum pulse compression. The length is inversely proportional to the modulation degree A.

In FIG. 4, when the modulation frequency of the modulation signal generator 22 is set to f _m = 10 GHz and a 1.3 μm single mode fiber (dispersion value 17 ps / nm / km) is used as the chirp compensator 30, the optical pulse is compressed. Then, the relationship between the optical fiber length L and the modulation degree A that can make the pulse width (full width at half maximum) the narrowest is shown. From the figure, it can be seen that the optical fiber length L required for optimal pulse compression can be determined from the modulation degree A by the following equation (13).
L = (4 / π) A ⁻¹ (13)

Further, since the amount of chirp of the optical pulse is proportional to the modulation frequency f _m from equation (6), if the modulation frequency f of the (GHz), the above equation (13) is generalized as the following equation (14) be able to.
L = (40 / π) (Af) ⁻¹ (14)

FIG. 5 shows an S / N ratio of an optical pulse pulse-compressed by a chirp compensator 30 (single-mode fiber for 1.3 μm (G.652)) having an optical fiber length L defined by Expression (13). The relationship between the modulation parameters ΔA and Δθ of the Mach-Zehnder optical modulator 21 is shown. However, as shown in FIG. 6, the S / N ratio of the light pulse is S _{S as} the light amount of the main lobe portion and S _{N as} the light amount of the side lobe (pedestal) portion in the time waveform of the light pulse. In this case, the S / N ratio is defined as S _S / S _N.

  In FIG. 5, the conditions of ΔA = π / 2 and Δθ = π / 2, which are the optimum driving conditions of the optical modulator in Patent Document 1, are indicated by ●. According to FIG. 5, if the above equation (12) is applied as the modulation driving condition of the Mach-Zehnder optical modulator 21, the S / N ratio can be improved further than the optimum condition (● mark) of Patent Document 1. It is understood that is possible.

Next, two modulation signals input to the Mach-Zehnder optical modulator 21, that is, a modulation signal input to the modulation electrode 214A via the path of the variable phase shifter 24A to the band pass filter 27A, and the variable phase shifter 24B Consider the influence of a phase shift (skew) that may occur between the modulation signal input to the modulation electrode 214B via the path of the band-pass filter 27B. When the phase shift between the two modulation signals is φ, the n-th order sideband peak power P _n output from the optical frequency comb generator 20 is expressed by the following equation (15).
P _n = P ₀ [J _n ² (A) + J _n ² (A + ΔA) +2 J _n (A) J _n (A + ΔA) cos (nφ + Δθ)] / 4 (15)

According to the above equation (15), it is understood that the + n-order peak power P _{+ n} and the −n-order peak power P _−n are different because the term resulting from the phase shift φ is added to the cos argument. That is, the spectrum of the optical pulse output from the optical frequency comb generator 20 is asymmetrical on the long wavelength (+ order) side and the short wavelength (−order) side of the zeroth order peak, and thus the inverse Fourier of the spectrum. The time waveform of the optical pulse that is the conversion is also asymmetric before and after the peak of the pulse. As a result, the S / N ratio of the optical pulse after the pulse compression by the chirp compensator 30 is degraded as compared with the case where there is no phase shift in the modulation signal.

  FIG. 7 shows the relationship between the phase shift φ of the modulation signal and the S / N ratio of the optical pulse after pulse compression. From the figure, it can be seen that, for example, in order to suppress the deterioration of the S / N ratio to within 10%, it is necessary to suppress the phase shift of the modulation signal within the range of −0.02π ≦ φ ≦ 0.02π. In order to match the phase of the modulation signal with high accuracy in this way, in the optical pulse generator 1 of the present embodiment, as shown in FIG. The path from the variable phase shifter 24A to the band pass filter 27A and the path from the variable phase shifter 24B to the band pass filter 27B) are symmetrical. In particular, a variable phase shifter is provided in both of the two paths. This is because when the variable phase shifter is used in only one of the paths, minute dispersion occurs due to asymmetry, and the modulation frequency is changed. When the temperature changes, it may cause skew, and when the phase of the modulation signal is changed with the variable phase shifter, power fluctuations occur, so the variable phase shifter is used only on one path This is because it is difficult to adjust both the phase and intensity of the modulation signal to a desired state.

Next, the influence of the distortion of the modulation signal input to the Mach-Zehnder optical modulator 21 will be examined. As shown in the respective drawings described above, the modulation degree A of the modulation signal used in the optical pulse generator 1 of the present embodiment is about 2π to 5π, and is a normal communication optical modulator. It is twice or more larger than the modulation degree of the modulation signal to be used. Therefore, when amplifying such a modulated signal amplifier 26A, at 26B, the harmonic to the fundamental modulation frequency f _m is markedly occurs, the harmonics will be adversely affected to the light pulse. Therefore, in the optical pulse generator 1 of the present embodiment, bandpass filters 27A and 27B are provided in the subsequent stage of the amplifiers 26A and 26B in order to remove harmonics. Specifically, since the amplitude of the harmonic is about 10 dB with respect to the amplitude of the fundamental wave, the bandpass filters 27A and 27B are not filters having a gentle characteristic such as a Bessel filter but a Chebyshev filter. It is preferable to apply a filter having a sharp characteristic. In addition, it is necessary to design the filter so that there is no ripple at a frequency that matches the fundamental wave or harmonics.

  FIG. 8 shows optical pulse spectra and time waveforms for the case where the harmonic suppression ratio (ratio of the double harmonic amplitude to the amplitude of the fundamental wave) is 25 dB and 50 dB or more, respectively. From the figure, it can be seen that when the suppression ratio of harmonics is 50 dB or more, the flatness of the optical frequency comb is better and the pulse width of the optical pulse can be narrowed than when the suppression ratio is 25 dB. If the harmonic suppression ratio is 40 dB or more, it is considered that substantially the same effect can be obtained.

  Next, a method for adjusting and controlling the drive conditions (A, ΔA, Δθ, φ) of the optical pulse generator 1 to a desired state will be described.

First, A and ΔA are adjusted to desired values by a method described in, for example, Japanese Patent Application Laid-Open No. 2009-222926. Specifically, first, a modulation signal (amplitude A ₁ ) is input to the Mach-Zehnder optical modulator 21 only from the signal path of the variable phase shifter 24A to the band-pass filter 27A, and the Mach-Zehnder is used using the optical spectrum analyzer 101. The spectrum of the output light from the mold optical modulator 21 is observed. At this time, since the amplitude A _{1 of the} modulation signal can be obtained from the ratio of the peak powers of two predetermined sidebands, the output of the modulation signal generator 22 can be varied so that the value of the amplitude A ₁ becomes a predetermined value. The attenuation amount of the attenuator 25A is adjusted. Then, similarly, the modulation signal (amplitude A ₂ ) is input from only the signal path of the variable phase shifter 24B to the band pass filter 27B to the Mach-Zehnder optical modulator 21, and the attenuation amount of the variable attenuator 25B is adjusted. it is, a and ΔA is set to be a desired value by varying the amplitude a _2. Thereby, the drive condition of the above-mentioned formula (12) is realized. Since the Bessel function is a pseudo-periodic function, the solution of the amplitudes (A ₁ and A ₂ ) cannot be uniquely determined when two sidebands are arbitrarily selected. Therefore, for example, two sets of two sidebands may be selected, and a solution that is obtained from each of the two sets may be determined as an amplitude value to be obtained.

  Next, φ is adjusted to a desired value. Specifically, the spectrum of the output light from the Mach-Zehnder optical modulator 21 is observed using the optical spectrum analyzer 101, and the peak power of the + n-order sideband is equal to the peak power of the -n-order sideband. As described above, the phase shift amounts of the variable phase shifters 24A and 24B are controlled. This realizes a state in which the phase shift of the modulation signal falls within the range of −0.02π ≦ φ ≦ 0.02π.

  Finally, Δθ is adjusted to a desired value. Specifically, the optical pulse after pulse compression is observed using the sampling oscilloscope 103, and the bias voltage supply unit 28 is configured so that the optical power between the pulses of the optical pulse train becomes 0 (or the minimum value). Control the bias voltage. Thereby, the drive condition of the above-mentioned formula (10) is realized.

  As described above, the embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to the above, and various design changes and the like can be made without departing from the scope of the present invention. It is possible to

  DESCRIPTION OF SYMBOLS 1 ... Optical pulse generator 10 ... Light source 11 ... Polarization controller 20 ... Optical frequency comb generator 21 ... Mach-Zehnder type optical modulator 22 ... Modulation signal generator 23 ... Divider 24 ... Variable phase shifter 25 ... Variable attenuator DESCRIPTION OF SYMBOLS 26 ... Amplifier 27 ... Band pass filter 28 ... Bias voltage supply part 29 ... Termination resistor 30 ... Chirp (dispersion) compensator 101 ... Optical spectrum analyzer 102 ... Photoelectric converter 103 ... Sampling oscilloscope 211 ... Input waveguide 212 ... Branch waveguide 213: Output waveguide 214: Modulation electrode 215: Bias electrode

Claims (13)

An optical modulator that divides the light into two, modulates each of the branched light, and then combines and outputs the light again;
A modulation signal generator for generating each modulation signal for modulating each of the branched lights;
With
An optical frequency comb generator that generates an optical frequency comb by setting a difference ΔA between one amplitude and the other amplitude of each modulation signal to 0.2π ≦ ΔA <0.5π. An optical frequency comb generator according to claim 1;
Pulse compression is applied to the light output from the optical frequency comb generator by applying a predetermined amount of chirp of the opposite sign to the chirp so that the light compensates for the chirp provided by the optical frequency comb generator. An optical pulse compression unit to perform,
An optical pulse generator characterized by comprising: The optical pulse compression unit is a 1.3 μm single mode fiber,
The optical fiber length L of the 1.3 μm single mode fiber is L = (40 / π) (Af) ^−1, where the modulation frequency of the modulation signal is f (GHz). Item 3. The optical pulse generator according to Item 2. The modulation signal generation unit includes a phase adjustment unit that adjusts a phase shift between the modulation signals in the middle of the supply line of the modulation signal,
4. The optical pulse generator according to claim 2, wherein the phase adjustment unit adjusts the phase shift φ to a range of −0.02π ≦ φ ≦ 0.02π. The modulation signal generation unit includes a filter that blocks harmonics of the modulation signal;
The optical pulse generator according to any one of claims 2 to 4, wherein the optical modulator modulates the branched light by a modulation signal in which the harmonics are cut off.   6. The optical pulse generator according to claim 5, wherein the filter suppresses harmonics of the modulation signal by 40 dB or more with respect to a fundamental wave of the modulation signal.   The optical pulse according to any one of claims 2 to 6, wherein the optical modulator has a termination resistor for terminating the modulation signal at a position physically separated from the optical modulator. Generator.   The optical pulse according to any one of claims 2 to 7, wherein the optical modulator is an LN modulator in which a Mach-Zehnder type optical waveguide and a modulation electrode are formed on an LN substrate. Generator. An optical modulator that divides the light into two, modulates each of the branched light, and then combines and outputs the light again;
A modulation signal generator for generating each modulation signal for modulating each of the branched lights;
Optical pulse compression for compressing pulses by applying a predetermined amount of chirp having a sign opposite to that of the chirp so that the light is compensated for the chirp applied by the optical modulator to the light output from the optical modulator. And
In the control method of the optical pulse generator comprising:
The method of controlling an optical pulse generator, wherein a difference ΔA between one amplitude and the other amplitude of each modulation signal is 0.2π ≦ ΔA <0.5π.   10. The spectrum of the output light of the optical pulse generator is observed using an optical spectrum analyzer, and the difference ΔA is controlled based on a ratio of peak powers of two sidebands of the spectrum. The control method of the optical pulse generator as described.   11. The method of controlling an optical pulse generator according to claim 9, wherein a phase shift φ between the modulation signals is adjusted to a range of −0.02π ≦ φ ≦ 0.02π.   A spectrum of output light of the optical pulse generator is observed using an optical spectrum analyzer, and the phase shift φ is controlled based on peak powers of + n-order and −n-order sidebands of the spectrum. The control method of the optical pulse generator of Claim 11.   The bias of each modulation signal is adjusted so that the optical power between the pulses of the optical pulse train output from the optical pulse generator is minimized. 2. A method for controlling an optical pulse generator according to item 1.

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