JP2019039972A - Signal generator and signal generation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate a signal such as a microwave or a millimeter wave in which phase noise is suppressed in a variable frequency state without separately requiring a stable optical frequency comb reference light source. SOLUTION: A self-reference interferometer 107 interferes with the nth harmonic of a long wavelength component and the n-1th harmonic of a short wavelength component obtained from an optical pulse whose optical spectrum band is expanded by a nonlinear optical medium 106. The optical signal of the frequency difference between the nth harmonic and the n-1th harmonic is generated, and the optical signal generated from the self-reference interferometer 107 is photoelectrically converted by the optical detector 108 to output an electric signal. , The feedback control unit 109 compares the electric signal output after photoelectric conversion by the optical detector 108 with the second signal output from the second signal generation unit 103, and based on the state of this comparison, the first The first signal generation unit 102 is controlled so that the phase noise of one signal is suppressed. [Selection diagram] Fig. 1

Description

  The present invention relates to a signal generator and a signal generation method for generating a reference signal source such as an optical frequency comb stabilized light source.

  In recent years, new technologies have been developed that directly measure the frequency of light based on a microwave reference frequency that is equal to or lower than the microwave that can be measured by an electrical method (see Non-Patent Documents 1 and 2). In this technology, an optical frequency comb is used for optical frequency measurement as a measure of optical frequency. For example, pulsed laser light oscillated by a mode-locked laser becomes an optical frequency comb.

In the pulse laser light oscillated by the mode-locked laser, pulses as shown by “optical carrier envelope” shown in FIG. 3A are arranged at equal time intervals T on the time axis. On the other hand, on the frequency axis, as shown in FIG. 3B, an aggregate of a large number of modes (line spectra) arranged in a comb shape at equal frequency intervals f _rep is obtained. In this way, an assembly of laser beams arranged in a comb shape on the frequency axis becomes an optical frequency comb.

Between the time width T between the peaks of the pulse indicated by the envelope and the frequency interval f _rep , which is the mode interval of the optical frequency comb shown in FIG. 3B, “f _rep = 1 / T”. There is a relationship, and the spectral frequency f _n of each mode of the optical frequency comb can be expressed as “f _n = n × f _rep + δ (n is an integer)”. Also, if the phase of the optical carrier electric field (carrier envelope phase) measured from the peak of the envelope is φ and the deviation between adjacent pulses (envelope) of φ is Δφ, the offset frequency δ and the laser repetition frequency are A relationship of “δ = (Δφ / 2π) × f _rep ” is established with a certain f _rep .

  Thus, if the laser repetition frequency and the carrier envelope phase difference can be fixed, the wavelength of each mode of the optical frequency comb can be stabilized, which becomes a frequency reference for connecting the microwave frequency to the light frequency region. The optical frequency comb in which the wavelength of each mode is accurately determined will be applied not only to high-precision frequency standards and related basic physics, but also to fields such as communication, precision measurement, and quantum information communication.

  In recent years, a technique for stabilizing the frequency of an optical frequency comb has been realized (see Non-Patent Documents 1 and 2). As this laser light source, a passive mode-locked laser including an optical resonator device is used (see Non-Patent Documents 1 and 2). Passive mode-locked lasers can oscillate pulsed lasers. A laser usually has a gain medium for amplifying light and a resonator having a resonator length of L. The resonance frequency of the longitudinal mode of this resonator is an integral multiple of c / 2L (c is the speed of light).

When the resonance frequency interval of the resonator is narrower than the spectrum width of the laser beam oscillated by the laser, the resonator resonates in a plurality of modes (frequency). By aligning the phase of each mode in this state (mode synchronization), the laser beam is intensified at the repetition frequency f _rep = c / (2L). As a result, pulsed laser light having a repetition frequency f _rep is generated.

As shown in FIG. 3, the pulse laser beams oscillated by the mode-locked laser are arranged at equal time intervals T on the time axis. On the other hand, on the frequency axis, it becomes an aggregate of a large number of modes arranged in a comb shape at equal frequency intervals f _rep . An assembly of laser beams arranged in a comb shape on the frequency axis is called an “optical frequency comb”. In general, the peak of the envelope and the peak of the optical carrier electric field do not always coincide with each other and shift with time, and this change is not constant.

When the time width between the peaks of the envelope is T, and the shift of the optical carrier electric field phase φ measured from the peak of the envelope of adjacent pulses is Δφ, the mode interval f _rep between the obtained optical frequency combs and T _Has a relationship of f _rep = 1 / T, and the spectrum frequency of each mode of the comb can be expressed as f _n = n × f _rep + δ (where n is an integer). Note that “δ = (Δφ / 2π) × f _rep ”.

The spectrum of the mode-locked laser oscillator is used to generate white light having a band of 1 octave or more by using a self-phase modulation effect generated by, for example, a photonic crystal fiber. When the second harmonic of the long wavelength component (n × f _rep + δ) is generated, the frequency becomes 2 × (n × f _rep + δ). Further, the short wavelength component of white light can be expressed as 2 × n × f _rep + δ. The value of the frequency difference δ of the two lights can be measured by causing the second light of the long wavelength component and the two lights of the short wavelength component to interfere with each other and detecting the beat signal with a photodiode, for example.

  The measured value is compared with an external microwave reference frequency, and feedback is made to the magnitude of the nonlinear dispersion in the resonator using an electronic circuit based on the magnitude of the deviation from the microwave reference frequency. The phase difference, that is, the comb offset frequency can be stabilized. On the other hand, the laser repetition frequency can be fixed within a certain finite range by feeding back to the laser resonator length based on the repetition frequency signal from the photodiode.

  An optical frequency comb stabilized light source having a constant offset frequency and mode frequency interval as described above has been developed based on a mode-locked laser including a resonator device (see Non-Patent Documents 1 and 2).

  By the way, in the optical frequency comb light source using the passive mode-locked laser described above, as a means for controlling the carrier envelope phase, the refractive index (dispersion amount) in the resonator is changed by changing the insertion amount of the glass wedge installed in the resonator. And the nonlinear refractive index of the gain medium (for example, titanium sapphire crystal) is changed by changing the intensity of the pump light (see Non-Patent Documents 1 and 2). Therefore, in the prior art, in order to obtain an optical frequency comb having a stable frequency, it is assumed that a passive mode-locked laser including a resonator is used.

  When a passively mode-locked laser is used, the center optical frequency of the obtained laser depends on the gain medium arranged in the resonator. For example, when a titanium sapphire crystal is used as the gain medium, the wavelength of the oscillatable laser is 650. Although it is in the range of ˜1100 nm, the wavelength that can oscillate most efficiently is 800 nm. Thus, in the passive mode-locked laser, the center optical frequency of the obtained laser is limited.

  In addition, as described above, the detection of the carrier envelope phase difference requires a laser output sufficient to obtain light having a broad spectrum using a photonic crystal fiber or a highly nonlinear fiber. For this reason, the kind of laser which can be used is limited. Also, with a laser that satisfies the conditions, the resonator length cannot be shortened due to restrictions such as the spatial arrangement of the gain medium and the optical components placed in the resonator, and the repetition frequency determined by the resonator length is About 1 GHz is the upper limit.

  Furthermore, adjustment (control) of the length of the resonator for changing the repetition frequency is usually performed by an actuator such as a piezo element fixed to the resonator end mirror. For this reason, the variable range of the repetition frequency is also small. For example, the variable range of a laser light source with a repetition frequency of 80 MHz is about 77 to 83 MHz.

  On the other hand, as a method for obtaining a stable optical frequency comb, a wavelength-stabilized continuous wave (CW) light source that continuously emits a laser is used, and this light source is subjected to deep frequency modulation by an electro-optic modulator, and FM. A method of generating sideband waves has been proposed (see Non-Patent Document 3). In this method, the restriction on the repetition frequency is removed. However, in this method, the offset optical frequency of the optical frequency comb is determined by the center wavelength of the original CW light. For this reason, in order to realize stabilization of the obtained optical frequency comb, for example, a CW light source such as a wavelength stabilized light source obtained by locking to an absorption line such as acetylene gas is required. The wavelength of each mode of the optical frequency comb cannot be fixed only by the reference frequency, and the obtained accuracy is inferior.

  In order to solve the problems described above, the following techniques have been proposed. First, the laser light generated from the CW light source is phase-modulated at the first frequency to generate an optical pulse whose pulse repetition frequency is the first frequency, and the optical spectrum band of the optical pulse is expanded. Next, a self-reference interferometer is used to detect an optical signal having a frequency difference (offset frequency) between the nth harmonic of the long wavelength component and the n-1th harmonic of the short wavelength component obtained from the optical pulse whose band has been expanded. To do. Next, an electrical signal corresponding to the detected optical signal is compared with an external microwave reference frequency, and the comparison wavelength is fed back to control and stabilize the center wavelength of the CW light source (see Patent Document 1). .

  The stabilized high repetition optical pulse train is branched by an optical coupler installed before amplification and output. With such a configuration, an optical frequency comb having a stable offset frequency can be obtained.

JP 2009-116242 A Japanese Patent No. 6133236

D. J. Jones, et al., "Carrier-Envelope Phase Control of Femtosecond Mode-Locked Lasers and Direct Optical Frequency Synthesis", Science, vol. 288, pp. 635-639, 2000. R. Holzwarth, et al., "Optical Frequency Synthesizer for Precision Spectroscopy", Phys. Rev. Lett., Vol. 85, no. 11, pp. 2264-2267, 2000. M. Kourogi, et al., "Limit of Optical-Frequency Comb Generation Due to Material Dispersion", IEEE Journal of Quantum Electronics, vol. 31, no. 12, pp. 2120-2126, 1995.

  However, the technique of Patent Document 1 has a restriction that the microwave signal generator that drives the phase modulator for phase modulation must have low time jitter. When the time jitter of the microwave signal generator increases, the optical spectrum is expanded, and phase noise increases when detecting an offset frequency using a self-reference interferometer, which makes signal detection difficult.

  Patent Document 2 solves the above problem. In Patent Document 2, based on an optical frequency comb based on a passive mode-locked laser, which is a reference light source, and a microwave reference signal, each mode line width has a repetition frequency that is one digit higher than that of the prior art. A narrow optical frequency comb can be realized. Further, in Patent Document 2, since the phase noise of the microwave signal generator can be reduced below the phase noise of the conventional crystal oscillator, an ultra-low phase noise microwave generator can be realized, and the optical frequency of the phase modulation laser can be realized. Com mode phase noise is reduced.

  However, the technique of Patent Document 2 has a problem that a stable optical frequency comb reference light source such as a passive mode-locked laser having a wavelength band different from that of the CW laser light source is required separately.

  The present invention has been made to solve the above-described problems, and does not require a separate stable optical frequency comb reference light source, and signals such as microwaves and millimeter waves with suppressed phase noise can be obtained. An object is to enable generation with a variable frequency.

  A signal generator according to the present invention includes a light source configured by a laser that generates continuous light, a first signal generator that outputs a first signal having a predetermined frequency, and a second signal that outputs a second signal having a predetermined frequency. A two-signal generator, a phase modulator for phase-modulating the continuous light output from the light source with the first signal, and an optical pulse having a pulse repetition frequency of the first frequency from the continuous light phase-modulated by the phase modulator An optical pulse generator that generates the optical signal, a nonlinear optical medium that expands an optical spectrum band of the optical pulse generated by the optical pulse generator, and a long wavelength component obtained from an optical pulse whose optical spectrum band is expanded by the nonlinear optical medium. Self-reference interference that generates an optical signal having a frequency difference between the nth harmonic and the n-1th harmonic by interfering with the nth harmonic (n is an integer of 2 or more) and the n-1th harmonic of the short wavelength component. Total and self A comparison is made between the photodetector that photoelectrically converts the optical signal generated from the illumination interferometer and outputs an electrical signal, the electrical signal that is photoelectrically converted by the photodetector and the second signal, and the state of this comparison And a feedback control unit that controls the first signal generation unit so that the phase noise of the first signal is suppressed.

  The signal generator may further include a dispersion compensator that compresses the pulse width of the optical pulse generated from the optical pulse generator.

  The signal generation method according to the present invention includes a first step of generating continuous light from a light source, and a phase modulator that performs phase modulation of the continuous light using a first signal of a predetermined frequency output from the first signal generator. 2 steps, a third step of generating an optical pulse having a pulse repetition frequency of the first frequency by the optical pulse generator from the phase-modulated continuous light, and an optical spectrum band of the optical pulse generated by the optical pulse generator A fourth step of expanding with a nonlinear optical medium, and an nth harmonic of a long wavelength component (n is an integer of 2 or more) obtained from an optical pulse whose optical spectrum band is expanded with a nonlinear optical medium by a self-reference interferometer; A fifth step of generating an optical signal having a frequency difference between the nth harmonic and the n-1th harmonic by interfering with the n-1th harmonic of the short wavelength component; and an optical signal generated from the self-reference interferometer The light A sixth step of outputting an electric signal by photoelectric conversion by the output unit; an electric signal output by photoelectric conversion by the photodetector by the feedback control unit; and a predetermined frequency output by the second signal generation unit. A seventh step of comparing the two signals and performing feedback control so that the phase noise of the first signal is suppressed based on the comparison state.

  The signal generation method may further include an eighth step of compressing the pulse width of the optical pulse generated from the optical pulse generator by the dispersion compensator.

  As described above, according to the present invention, a signal such as a microwave or a millimeter wave in which phase noise is suppressed can be generated with a variable frequency without separately requiring a stable optical frequency comb reference light source. An excellent effect is obtained.

FIG. 1 is a configuration diagram showing a configuration of a signal generator in an embodiment of the present invention. FIG. 2 is a characteristic diagram showing the measurement results of the phase noise accumulated value of each mode in the SC light using the SC light and the wavelength tunable laser obtained from the light source 101 of the signal generator in the embodiment. FIG. 3 is an explanatory diagram (a) showing the carrier envelope phase of the optical carrier in the optical pulse, and an explanatory diagram (b) showing the state of the line spectrum on the frequency axis of the optical pulse.

  Hereinafter, a signal generator according to an embodiment of the present invention will be described with reference to FIG. The signal generator includes a light source 101, a first signal generation unit 102, a second signal generation unit 103, a phase modulator 104, a dispersion applying unit (optical pulse generation unit) 105, a nonlinear optical medium 106, a self-reference interferometer 107, A photodetector 108 and a feedback control unit 109 are provided.

The light source 101 is composed of a laser that generates continuous light. For example, the light source 101 is a narrow line width CW laser using a ULE (Ultra Low Expansion) resonator, or a CW laser whose frequency is stabilized by a narrow line width optical frequency comb or an optical lattice clock. Continuous light is generated from the light source 101 (first step). The first signal generator 102 outputs a first signal (f _m ) having a predetermined frequency. The first signal generator 102 is, for example, a crystal oscillator-based microwave signal generator. The second signal generator 103 outputs a second signal having a predetermined frequency.

  The phase modulator 104 phase-modulates the continuous light output from the light source 101 with the first signal (second step). The dispersion providing unit 105 generates an optical pulse having a pulse repetition frequency of the first frequency from the continuous light phase-modulated by the phase modulator 104 (third step). The nonlinear optical medium 106 expands the optical spectrum band of the optical pulse generated by the dispersion providing unit 105 (fourth step).

Continuous light from the light source 101, the phase modulator 104, spectral band is phase modulated by a signal (first signal) of a frequency f _m which is transmitted (output) from the first signal generating unit 102 is expanded. In the embodiment, the intensity modulator 111 is arranged in tandem before or after the phase modulator 104. The intensity modulator 111 selects one of the down or up-chirped ones by the phase modulator 104 and transmits the light, and the DC base portion generated by the phase modulator 104 is removed. Thereby, it is possible to contribute to suppression of the DC component when the dispersion applying unit 105 compresses the pulse width of each pulse. In addition, a phase shifter 110 is used for time adjustment for modulation by the phase modulator 104 and the intensity modulator 111.

  In the embodiment, the intensity modulator 111 is arranged in tandem at the subsequent stage of the phase modulator 104, but the intensity modulator 111 may be arranged in tandem at the preceding stage of the phase modulator 104. In this case, the continuous light from the light source 101 is intensity-modulated by the intensity modulator 111 and then phase-modulated by the phase modulator 104.

  The self-reference interferometer 107 includes a long wavelength component nth harmonic (n is an integer of 2 or more) and a short wavelength component n-1th harmonic obtained from an optical pulse whose optical spectrum band is expanded by the nonlinear optical medium 106. An optical signal having a frequency difference between the nth harmonic and the n−1th harmonic is generated by causing the waves to interfere (fifth step).

  The self-referencing interferometer 107 generates a second harmonic from a component on the long wavelength side (any comb on the long wavelength side), and the basis of this and the component on the short wavelength side (any comb on the short wavelength side). The wave (first harmonic) is caused to interfere. This is a case where the bandwidth of the broad spectrum band light is 1 octave or more. Alternatively, an optical signal having an offset optical frequency may be generated by a difference frequency generation method using a fundamental wave on the short wavelength side and a fundamental wave on the long wavelength side that is generated. On the other hand, when the bandwidth of the obtained wide spectrum band light is less than one octave, the third harmonic on the long wavelength side and the second harmonic on the short wavelength side may be caused to interfere with each other.

  The photodetector 108 photoelectrically converts the optical signal generated from the self-reference interferometer 107 and outputs an electrical signal (sixth step). The feedback control unit 109 compares the electrical signal that is photoelectrically converted by the photodetector 108 and the second signal, and based on this comparison state, the first signal is suppressed so that the phase noise of the first signal is suppressed. 1 signal generator 102 is controlled (seventh step).

  In the embodiment, the dispersion applying unit 105 converts the continuous light output from the intensity modulator 111 into an optical pulse train having a predetermined repetition frequency by applying chromatic dispersion to the continuous light. A short optical pulse train having a repetition frequency can be generated by giving appropriate dispersion by the dispersion providing unit 105. Further, if the dispersion imparting unit 105 includes a wavelength filter, the wing component remaining in the converted optical pulse train can be suppressed, and clean pulses can be generated.

  In the embodiment, the optical amplifier 112 amplifies the light intensity of the light (optical pulse train) converted into the optical pulse train by the dispersion applying unit 105. For example, in the optical amplifier 112, a method of performing optical amplification in stages using a nonlinear effect such as a progressive self-phase modulation effect in the optical amplifier 112 is used, and the spectral bandwidth is expanded without causing optical splitting. In the embodiment, the dispersion compensation unit 113 at the next stage of the optical amplifier 112 compresses the pulse width to generate a short pulse to a desired pulse width (eighth step). The dispersion compensation unit 113 may be made of, for example, a nonlinear optical medium.

  In addition, the pulse light source portion including the light source 101, the phase modulator 104, the intensity modulator 111, and the dispersion imparting unit 105 is configured to apply deep frequency modulation to the CW light by installing an electro-optic modulator in the Fabry-Perot resonator. A method of generating sideband waves may also be used. In the method of generating a pulse train using these phase modulators, it is possible to generate a repeated pulse train of several tens of GHz because there is no restriction on the resonator length.

  The optical pulse train generated as described above is input to the nonlinear optical medium 106 and output as wide spectrum band light (SC light). If a material having a large nonlinear susceptibility is used in the nonlinear optical medium 106, desired wide spectrum band light can be generated with high efficiency, and the minimum threshold value of the energy supplied from the light source 101 necessary for stabilizing the optical frequency comb is lowered. It is possible to suppress. This point will be described with reference to FIG.

  FIG. 2 shows the measurement result of the accumulated phase noise value of each mode of the SC light using the SC light obtained as described above and a wavelength variable laser prepared separately. In FIG. 2, the horizontal axis represents the mode order, and the vertical axis represents the accumulated phase noise value. As shown in FIG. 2, it was found that the accumulated phase noise value of each mode of the SC light increases with the square of the mode order. At the same time, it was verified from experiments that the increment was the phase noise of the microwave generator for driving the optical modulator.

Therefore, the optical frequency comb of the mode order k (k = 0, ± 1, ± 2, ± 3,...) Of SC light is expressed as “E ₀ (t) = E ₀ exp [−i {2π (f ₀ + K × f _m ) t + ψ ₀ (t) + k × ψ ₁ (t)}] (1) ”. Here, E ₀ is the photoelectric field amplitude, and ψ ₀ (t) is the phase noise of the light source 101. ψ ₁ (t) is the phase noise of the first signal generator 102.

  The SC light having a wide spectral band by the nonlinear optical medium 106 as described above is input to the self-reference interferometer 107. As a result, interference light is generated, and the interference light is detected (photoelectric conversion) by the photodetector 108 and an electric signal (offset frequency) is output.

  The type of self-referencing interferometer 107 can be selected according to the bandwidth of the SC light. For example, if the SC optical bandwidth is 1 octave or more, an f-2f self-reference interferometer that detects an offset frequency from the interference signal of the high-frequency component of the fundamental wave and the second harmonic of the low-frequency component of the fundamental wave is used. Good. If the bandwidth of the SC light is 2/3 octave, the offset frequency is detected from the interference signal of the second harmonic of the high frequency component of the fundamental wave and the third harmonic of the low frequency component of the fundamental wave. A −3f self-referencing interferometer may be used.

  Next, the electrical signal of the offset frequency output from the photodetector 108 is filtered (cut off) at a predetermined frequency by the frequency filter 114 and input to the feedback control unit 109. Hereinafter, feedback control will be described in detail. Hereinafter, a case where the self-reference interferometer 107 is an f-2f self-reference interferometer will be described as an example.

The optical field E ₁ of the optical frequency comb of the high frequency component of the fundamental wave is expressed as “E ₁ (t) = E ₁ exp [−i {2π (f ₀ + k × f _m ) t + ψ ₀ ( t) + k × ψ ₁ (t)}] (2) ”. Here, E ₁ is the photoelectric field amplitude. Also, ψ ₀ (t) is phase noise of the light source 101. Also, ψ ₁ (t) is a phase noise of the first signal generator 102 for driving the phase modulator 104.

On the other hand, the second harmonic photoelectric field E ₂ generated from the optical frequency comb of the low frequency component of the fundamental wave is expressed as “E ₂ (t) = E ₂ exp [−i {4π ( f ₀ + m × f _m ) t + 2ψ ₀ (t) + 2m × ψ ₁ (t)}] (3) ”. Here, E ₂ is the photoelectric field amplitude. Also, ψ ₀ (t) is phase noise of the light source 101. Further, ψ ₁ (t) is phase noise of the first signal generation unit 102.

The optical signal E ₁ of the optical frequency comb of the high frequency component of the fundamental wave and the interference signal of the second harmonic photoelectric field E ₂ generated from the optical frequency comb of the low frequency component of the fundamental wave are detected by the photodetector 108. . This interference signal I _obs is expressed as “I _obs = | E ₁ (t) | ² + | E ₂ (t) | ² + 2E ₁ E ₂ cos {2π [f ₀ + (k−2m) × f _m ] t + [ ψ ₀ (t) + (k−2m) × ψ ₁ (t)]} (4) ”. _{Here, V obs = 2E 1 E 2} , Δf = f 0 + (k-2m) × f m, Δψ = ψ and _{0 (t) + (k-} 2m) × ψ 1 (t).

The interference signal output from the photodetector 108 passes through the frequency filter 114. The interference term “V _obs cos {2πΔft + Δψ} (5)” in the expression (3) is detected (taken out) by the frequency filter 114.

Here, the feedback control unit 109 uses the second signal generation unit 103 as a reference signal source having sufficiently smaller phase noise than the interference signal for comparison with the interference signal. When the phase of the second signal generation unit 103 is shifted by 90 degrees, “V _ref sin {2πf _n t + ψ ₂ (t)} (6)” is obtained. Comparing and referring to Expression (5) and Expression (6), “V _obs cos {2πΔft + Δψ} × V _ref sin {2πf _n t + ψ ₂ (t)} = (V _obs × V _ref / 2) {sin (2πΔft + Δψ + ψ ₂ ( t)) + sin (Δψ−ψ ₂ (t))} (7) ”.

Here, by using a low-pass filter included in the feedback control unit 109, the high-frequency term sin (4πΔft + Δψ + ψ ₂ (t)) in Expression (7) is cut off, and the low-frequency term sin (Δψ−ψ in Expression (7) is used. ₂ (t)) is detected. The detected voltage V (t) can be described as “V (t) = V ₀ sin {Δψ−ψ ₂ (t)} (8)”. Here, V ₀ = V _obs × V _ref / 2.

The feedback control unit 109 controls the voltage of the first signal generation unit 102 so that the phase difference Δψ−ψ ₂ (t) in Expression (8) becomes zero. The phase noise ψ ₁ (t) of the first signal generator 102 converges to {ψ ₂ (t) −ψ ₀ (t)} / (k−2m), and has a phase inversely proportional to the mode order k−2m. Noise can be reduced.

  Thus, according to the embodiment, high-accuracy stabilization of the optical pulse that is phase-modulated by the phase modulator 104 and is generated by the dispersion providing unit 105, and the first signal that is output from the first signal generating unit 102 The phase noise can be made very small at the same time.

  As described above, in the present invention, the optical pulse train is generated from the continuous light obtained from the light source by using the phase modulation and intensity modulation means based on the first signal from the first signal generation unit. Broadband light is generated using an optical pulse train and a non-linear optical medium, and a long-wavelength component and a short-wavelength component are caused to interfere from the broadband light by a self-reference interferometer to generate an optical signal having a frequency difference between them. A signal was obtained, and the first signal generator was feedback controlled by the feedback controller based on the interference signal. As a result, according to the present invention, a signal such as a microwave or a millimeter wave in which phase noise is suppressed can be generated with a variable frequency without separately requiring a stable optical frequency comb reference light source.

  According to the present invention, the phase noise of the first signal generation unit can be suppressed with a suppression ratio that is inversely proportional to the harmonic order in the self-reference meter, and an optical frequency comb light source that is stable at frequency intervals can be realized. In the present invention, another reference light source as in Patent Document 1 is not necessary, and an optical frequency comb having a repetition frequency higher by one digit or more and a wider frequency interval than that of the prior art can be realized. The present invention contributes to the development of the communication field and the spectroscopy field using optical comb modes more than before, and it can be seen that the present invention is superior to the prior art. Further, according to the present invention, the phase noise in the RF signal generator is updated by updating the limit value of the low phase noise intensity that can be realized by the crystal oscillator, so that a completely new RF signal generator can be provided. According to the present invention, it is considered that the present invention can greatly contribute to the development of many fields using microwave technology.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious.

  DESCRIPTION OF SYMBOLS 101 ... Light source, 102 ... 1st signal generation part, 103 ... 2nd signal generation part, 104 ... Phase modulator, 105 ... Dispersion provision part (light pulse generation part), 106 ... Nonlinear optical medium, 107 ... Self-reference interferometer DESCRIPTION OF SYMBOLS 108 ... Photodetector 109 ... Feedback control part 110 ... Phase shifter 111 ... Intensity modulator 112 ... Optical amplifier 113 ... Dispersion compensation part 114 ... Frequency filter.

Claims (4)

A light source composed of a laser that produces continuous light;
A first signal generator for outputting a first signal having a predetermined frequency;
A second signal generator for outputting a second signal of a predetermined frequency;
A phase modulator for phase-modulating continuous light output from the light source with the first signal;
An optical pulse generator that generates an optical pulse having a pulse repetition frequency of a first frequency from the continuous light phase-modulated by the phase modulator;
A non-linear optical medium that expands the optical spectrum band of the optical pulse generated by the optical pulse generator;
The nth harmonic of a long wavelength component (n is an integer of 2 or more) and the n−1th harmonic of a short wavelength component obtained from an optical pulse whose optical spectrum band is expanded by the nonlinear optical medium are caused to interfere with each other. a self-referencing interferometer that generates an optical signal having a frequency difference between an n harmonic and the n-1 th harmonic;
A photodetector that photoelectrically converts an optical signal generated from the self-reference interferometer and outputs an electrical signal;
The electrical signal photoelectrically converted by the photodetector is compared with the second signal, and the first signal is generated so that the phase noise of the first signal is suppressed based on the comparison state. And a feedback control unit for controlling the unit. The signal generator of claim 1.
The signal generator further comprising a dispersion compensation unit that compresses a pulse width of the optical pulse generated from the optical pulse generation unit. A first step of generating continuous light from a light source;
A second step of phase-modulating the continuous light with a phase modulator using a first signal of a predetermined frequency output from the first signal generator;
A third step of generating an optical pulse having a pulse repetition frequency of the first frequency in the optical pulse generator from the phase-modulated continuous light;
A fourth step of expanding the optical spectrum band of the optical pulse generated by the optical pulse generator with a nonlinear optical medium;
The self-reference interferometer uses the nonlinear optical medium to expand the optical spectrum band of the optical pulse to obtain the nth harmonic of the long wavelength component (n is an integer of 2 or more) and the n−1th harmonic of the short wavelength component. And generating an optical signal having a frequency difference between the nth harmonic and the n-1th harmonic,
A sixth step of photoelectrically converting an optical signal generated from the self-referencing interferometer with a photodetector and outputting an electrical signal;
In the feedback control unit, the electrical signal photoelectrically converted by the photodetector and output is compared with the second signal having a predetermined frequency output from the second signal generating unit, and based on the comparison state, And a seventh step of performing feedback control so that the phase noise of the first signal is suppressed. The signal generation method according to claim 3.
8. The signal generation method according to claim 8, further comprising an eighth step of compressing a pulse width of the optical pulse generated from the optical pulse generator by a dispersion compensator.

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