JP5198833B2 - Optical frequency comb stabilized light source - Google Patents

Description

  The present invention relates to an optical frequency comb stabilized light source used for measuring the frequency of light.

  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 to 7). In this technology, an optical frequency comb is used for optical frequency measurement as a measure of optical frequency. In this optical frequency comb, a plurality of line spectra of frequencies appearing at constant frequency intervals of a stable optical pulse train in the frequency domain are regarded as combs. The interval between the combs is the repetition frequency of the optical pulse in the microwave band, and the optical frequency of each comb is obtained by adding the offset optical frequency to the integral multiple of the frequency between the adjacent combs (microwave frequency). Therefore, in the optical frequency comb, the optical frequency of each line spectrum can be determined by the frequency of the interval between the combs and the integer value.

  If such an optical frequency comb is used, the optical frequency can be measured based on an electrically measurable interval frequency. For example, using the optical beat observed when the light to be measured and the optical frequency comb are mixed (interfered), the optical frequency of each line spectrum of the optical frequency comb is known, so The optical frequency can be determined. The optical frequency comb that enables direct measurement of the optical frequency in this way is a frequency reference that connects the microwave frequency to the frequency region of light. Optical frequency combs are expected to be applied not only to high-precision frequency standards and related basic physics, but also to fields such as communications, precision measurement, and quantum information communications.

  As a light source of such an optical frequency comb, for example, a passive mode-locked laser including a resonator is used (see Non-Patent Documents 1 to 3 and 5 to 7). Passive mode-locked lasers can oscillate pulsed lasers. In addition, a method for stabilizing the frequency of an optical frequency comb using a passive mode-locked laser as a light source has been proposed (see Patent Document 1 and Non-Patent Documents 1 to 6).

Hereinafter, frequency stabilization of an optical frequency comb using a passive mode-locked laser as a light source in the prior art will be described. First, a laser generally includes a gain medium for amplifying light and a resonator having a predetermined resonator length (L). The resonance frequency of the longitudinal mode of this resonator is an integral multiple of c / 2L (c is the speed of light). Further, although the spectral width of the laser light oscillated by the laser is very narrow, if the interval between the resonance frequencies of the resonator is narrower than this spectral width, the resonator will resonate in a plurality of modes (frequencies). Thus, in the state of resonating in a plurality of modes, by aligning the phases of the modes (mode synchronization), the laser light oscillated at the repetition frequency f _rep = c / (2L) is strengthened. As described above, by _performing mode synchronization in a state of resonating in a plurality of modes, a pulse laser having a repetition frequency f _rep can be oscillated.

In such a pulsed laser beam oscillated by the passive mode-locked laser, pulses as indicated by “pulse envelope” in FIG. 6 are arranged at equal time intervals T on the time axis. In FIG. 6, a pulse due to a change in the optical carrier electric field is shown as an envelope of the optical carrier electric field, and one pulse is shown. On the other hand, on the frequency axis, as shown in FIG. 7, an aggregate of a large number of modes (line spectra) arranged in a comb shape at equal frequency intervals f _rep is formed, and an optical frequency comb is obtained. In general, the peak of the optical carrier electric field aligned on the time axis does not always coincide with the peak of the pulse of the pulse laser, but shifts with time, and this change is not constant. FIG. 6 shows a state where the peak of the optical carrier electric field and the peak of the pulse indicated by the envelope are shifted by φ. Further, as shown in FIG. 7, the optical frequency comb is also offset by the offset optical frequency δ, corresponding to the φ deviation Δφ between the repetitive pulses.

Incidentally, the time width T between the peaks of the pulses shown in the above-described envelope, between the frequency spacing f _rep is the distance of the optical frequency comb mode shown in FIG. 7, as "f _rep = 1 / T" There is a relationship, and the spectral frequency f _n of each mode (peak of optical frequency comb) can be expressed as “f _n = n × f _rep + δ (n is an integer)”. 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, then “δ = (Δφ / 2π) Xf _rep ".

Here, first, the frequency band of the spectrum of the passive mode-locked laser oscillator is widened to more than twice (band 1 octave or more) using the self-phase modulation effect generated by, for example, a photonic crystal fiber. generate. As a result, the number of modes serving as combs of the optical frequency comb can be increased. When the second harmonic of the long wavelength component f ₁ (= n × f _rep + δ) of the white light thus obtained is generated, this frequency is f ₁ ′ = 2 × (n × f _rep + δ). It becomes. Further, the short wavelength component f ₂ of the white light can be expressed as “f ₂ = 2 × n × f _rep + δ”. The light of the short wavelength component f ₂ interferes with the light of f ₁ ′, thereby By detecting the generated beat signal (optical beat) using, for example, a photodiode, the value of the frequency difference δ (= f ₁ ′ −f ₂ ) between f ₁ ′ and f ₂ can be measured.

  Based on the magnitude of these deviations obtained by comparing the frequency difference δ measured in this way with an external microwave reference frequency, the amount of nonlinear dispersion felt by the light in the resonator using an electronic circuit, etc. By applying feedback to (nonlinear refractive index), the optical frequency comb can be obtained from the passive mode-locked laser in a state in which the offset optical frequency δ (difference of the carrier envelope phase φ) of the optical frequency comb described above is stable.

The repetition frequency f _rep of the passive mode-locked laser is fed back to the resonator length of the passive mode-locked laser based on the repetition frequency signal from a photodetector such as a photodiode receiving the laser light. Thus, it can be fixed within a certain finite range.

  As described above, an optical frequency comb stabilized light source in which the offset optical frequency and the repetition frequency are constant has been developed based on a passive mode-locked laser including a resonator (see Non-Patent Documents 1 to 6).

  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 to 3 and 5 to 7). Therefore, in the conventional technique, 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 disposed 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. In addition, in 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 optical components arranged 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 77 to 83 MHz.

  On the other hand, as a method of obtaining a stable optical frequency comb, there is a method of generating an FM sideband by using a wavelength-stabilized CW light source that continuously emits a laser, subjecting this light source to deep frequency modulation with an electro-optic modulator. It has been proposed (see Non-Patent Document 7). In this method, the restriction on the repetition frequency is eliminated, but the offset optical frequency of the optical frequency comb is determined by the center wavelength of the original CW light, so that stabilization of the obtained optical frequency comb is realized. 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, and the wavelength of each mode of the optical frequency comb is fixed only by an external microwave reference frequency. Cannot be obtained, and the accuracy obtained is inferior.

US Pat. No. 6,785,303 JP 2006-209067 A 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. K. Sugiyama, wt al., "Frequency Control of a Chirped-Mirror-Dispersion-Controlled Mode-Locked Ti: Al2O3 Laser for Comparison between Microwave and Optical Frequencies", Proceedings of SPIE, Vol.4269, pp.95-104, 2001 T.R.Schibli, et al., "Frequency metrology with a turnkey all-fiber system", Optics Letters, Vol.29, No.21, pp.2467-2469, 2004. T.M.Foritier, et al., "Octave-spanning Ti: sapphire laser with a repetition rate> 1 GHz for optical frequency measurements and comparisons", Optics Letters, Vol.31, No.7, pp.1011-1013, 2006. I. Hartl, et al., "Integrated self-referenced frequency-comb laser based on a combination of fiber and waveguide technology", Optics Express, Vol. 13, No. 17, pp. 6490-6496, 2005. 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.

  As described above, in the conventional optical frequency comb stabilized light source using the passively mode-locked laser including the resonator, the variable range of the center optical frequency and the repetition frequency of the light source is limited, and the repetition of 1 GHz or more is performed. There was a problem that it was difficult to obtain the frequency.

  In addition, the method of generating FM sidebands by applying deep frequency modulation to a CW wavelength stabilized light source with an electro-optic modulator requires a wavelength stabilized light source that locks the wavelength of continuous wave light to an absorption line such as acetylene gas. In other words, the wavelength of each mode of the optical frequency comb cannot be fixed only by the microwave reference frequency, and the accuracy of the obtained frequency is low. For this reason, there is a problem that the apparatus becomes complicated and high accuracy cannot be obtained.

  The present invention has been made to solve the above-described problems, and an object thereof is to obtain an optical frequency comb having an arbitrary center optical frequency and repetition frequency with high accuracy.

  The optical frequency comb stabilized light source according to the present invention generates a light pulse having a pulse repetition frequency of the first frequency by phase-modulating the laser light generated from the light source that generates continuous laser light and the laser light generated from the light source at the first frequency. Optical pulse generating means, a nonlinear optical medium that expands the optical spectrum band of the optical pulse generated by the optical pulse generating means, and a long wavelength component obtained from the optical pulse whose optical spectrum band is expanded by the nonlinear optical medium Detecting means for photoelectrically converting an optical signal having a frequency difference between the nth harmonic (n is an integer of 2 or more) and an n-1th harmonic of a short wavelength component and outputting an electrical signal; and a predetermined second frequency A reference signal supplying means for supplying a reference signal, an electric signal output from the detecting means and a reference signal having a predetermined frequency are compared, and a feedback control method for controlling the light source based on the comparison state. Those comprising at least and. Therefore, the optical pulse obtained from the nonlinear optical crystal medium has the center optical frequency determined by the light source, and the pulse repetition frequency is determined by the phase modulation frequency (first frequency) in the optical pulse generating means.

  In the optical frequency comb stabilized light source, the optical pulse generation means includes a phase modulation function unit that phase-modulates the laser light at the first frequency, and wavelength dispersion to the laser light phase-modulated by the phase modulation function unit, You may make it provide the wavelength dispersion provision function part which produces | generates the optical pulse which has the repetition frequency of the pulse of a 1st frequency.

  In the optical frequency comb stabilized light source, the optical pulse generation means may include an electro-optic modulator disposed in a Fabry-Perot resonator, and the electro-optic modulator may modulate the laser light. Further, an optical gate may be provided that is disposed between the optical pulse generation unit and the nonlinear optical medium and reduces the repetition frequency of the optical pulse generated by the optical pulse generation unit. Moreover, you may make it provide the control means which controls the laser beam output of a light source based on the intensity | strength of the electrical signal output from a detection means.

  As described above, according to the present invention, a laser pulse generated from a light source that generates continuous laser light is phase-modulated at the first frequency, thereby generating an optical pulse having a pulse repetition frequency of the first frequency. The electrical spectrum band of the optical pulse is expanded, and the electrical signal corresponding to the optical signal having the frequency difference between the nth harmonic of the long wavelength component and the n-1th harmonic of the short wavelength component obtained from the optical pulse with the expanded band is obtained. Since the signal and the reference signal are compared and the light source is controlled based on the comparison state, an optical frequency comb having an arbitrary center optical frequency and repetition frequency can be obtained with high accuracy.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The optical frequency comb stabilized light source in the present embodiment described below is an example of the present invention, and the specific configuration is not limited to the following embodiment, and does not depart from the spirit of the present invention. Such design changes are included in the present invention.

FIG. 1 is a configuration diagram showing a configuration example of an optical frequency comb stabilized light source according to an embodiment of the present invention. The optical frequency comb stabilized light source in the present embodiment first has a laser light source 101 that generates continuous wave (CW) light, and a CW light emitted from the laser light source 101 with a repetition frequency f _rep (for example, several GHz to several tens GHz). ), A microwave reference frequency generator 103 that generates a repetitive reference signal having a repetitive frequency f _rep and supplies the repetitive reference signal to the phase modulator 102, and CW light phase-modulated by the phase modulator 102 _Is provided with a chromatic dispersion imparting unit 104 that converts the chromatic dispersion into an optical pulse train having a repetition frequency f _rep . The laser light source 101 is, for example, a wavelength variable CW light source, and the center wavelength can be arbitrarily selected. Further, the chromatic dispersion imparting unit 104 can be configured by, for example, an optical fiber, a fiber grating, and a planar lightwave circuit.

Further, the optical frequency comb stabilized light source in the present embodiment includes a beam splitter 105 for extracting optical frequency comb stabilized light with a high accuracy (accuracy) and a repetition frequency f _rep and an offset optical frequency, and a repetition frequency. an optical amplifier 106 for amplifying the light intensity of f _rep of the optical pulse train to the converted light (optical pulse train), an optical filter (optical bandpass to remove noise components such as ASE (amplified Spontaneous Emission) noise from the optical amplifier 106 Filter) 107 and a non-linear optical medium 108 that expands the spectral band of the optical pulse train converted into the optical pulse train having the repetition frequency f _rep .

  As the nonlinear optical medium 108, for example, a nonlinear light that exhibits a nonlinear optical effect, such as a photonic crystal fiber in which a large number of holes are provided around a core and this region is equivalently reduced in refractive index, is used. The nonlinear optical medium 108 can be configured by the fiber. The nonlinear optical medium 108 may be a nonlinear optical fiber in which chromatic dispersion decreases from anomalous dispersion to normal dispersion in the waveguide direction (longitudinal direction) (see Patent Document 2). In the case of a photonic crystal fiber, the chromatic dispersion can be reduced from anomalous dispersion to normal dispersion in the waveguide direction depending on the hole diameter and the state of arrangement of the holes in the clad region.

  Further, the optical frequency comb stabilized light source in the present embodiment includes an nth harmonic wave (n is an integer of 2 or more) obtained from an optical pulse having a wide spectral band expanded by the nonlinear optical medium 108, and a short wavelength component. A detection unit 109 that detects and photoelectrically converts an optical signal (optical beat) having a frequency difference of the n-1st harmonic, an electrical signal that is detected and photoelectrically converted by the detection unit 109, and a microwave input from the outside A feedback control unit 110 that feedback-controls the laser light source 101 by comparing and referring to the frequency, and a microwave reference signal supply unit that supplies a signal (reference signal) of a predetermined frequency (microwave) serving as a reference to the feedback control unit 110 111 and a control unit 112 for optimizing the measurement of the offset optical frequency in the detection unit 109.

The detection unit 109 includes a self-referencing interferometer and a photodetector. In the self-referencing interferometer, the second harmonic wave and the third harmonic wave are generated from the input optical pulse train, the second harmonic wave and the fundamental wave are interfered, and the third harmonic wave and the second harmonic wave are generated. Interfere. For example, a Mach-Zehnder interferometer can be used. In the photodetector, the optical beat generated by the interference by the self-referencing interferometer is photoelectrically converted and output as an electrical signal. The photodetector also detects the fundamental wave, the second harmonic, and the third harmonic and performs photoelectric conversion. The frequency of the electrical signal of the optical beat detected and photoelectrically converted by the detection unit 109 is the offset optical frequency of the optical pulse train (optical frequency comb) having the frequency f _rep output from the chromatic dispersion providing unit 104.

  The microwave reference frequency generator 103 and the microwave reference signal supply unit 111 receive, for example, a GPS signal transmitted from a GPS (Global Positioning System) satellite, and output the received GPS signal as a reference signal. is there. The GPS signal is an electrical signal having a known frequency, and this frequency maintains high accuracy.

  Further, as shown in FIG. 2, the control unit 112 first indicates the difference (ratio) between the peak height and the bottom in the optical beat spectrum from the electrical signal obtained by photoelectrically converting the optical beat detected by the detection unit 109. From the electric signal obtained by photoelectrically converting the fundamental wave, the second harmonic wave, or the third harmonic wave detected by the detection unit 109, an optical beat measurement value obtained by quantifying the effective peak light intensity (visibility) generated is generated. A numerical unit 117 is provided that generates a light intensity value obtained by quantifying each peak light intensity.

  Further, the control unit 112 calculates the maximum peak light intensity value (optical beat calculation value) of the theoretical optical beat from the light intensity value of the fundamental wave, the second harmonic wave, or the third harmonic wave generated by the digitizing unit 117. And the calculated optical beat calculated value and the optical beat measurement value generated by the digitizing unit 117 are compared to adjust the optical elements constituting the self-referencing interferometer of the detecting unit 109. The optical element control unit 118 that generates and outputs the set value is provided.

  Further, the control unit 112 compares the optical beat measurement value generated by the digitizing unit 117 with a predetermined value set in advance, and the light intensity of the measured optical beat is extracted by the beam splitter 105. A light source intensity control unit 119 is provided to control the output intensity of the laser light source 101 and the amplification state of the optical amplifier 106 so that the intensity is sufficient to stabilize the optical frequency comb.

Hereinafter, an operation example of the optical frequency comb stabilized light source in the present embodiment will be described. First, the CW light output from the laser light source 101 is phase-modulated by the phase modulation unit 102 with a repetitive reference signal having a frequency f _rep obtained from the microwave reference frequency generator 103. As a result, the chirped light whose instantaneous intensity changes sinusoidally while the light intensity remains constant and the CW light remains constant is obtained. On the optical frequency axis, the line spectrum (optical comb) of the CW light serving as a comb is obtained. ) _Are arranged in a comb shape at the repetition frequency f _rep . Therefore, even in this state, it can be used as an optical frequency comb. For example, the beam splitter 105 may be disposed between the phase modulation unit 102 and the chromatic dispersion providing unit 104.

Next, the phase-modulated light is given predetermined dispersion by the chromatic dispersion imparting unit 104, is given a different delay for each optical frequency component, and is made into an optical pulse train having a repetition frequency f _rep . By using such an optical pulse train having a repetition frequency f _rep , the spectral band can be _broadened as described later, and the number of optical combs arranged at the repetition frequency f _rep on the optical frequency axis can be increased. In order to use as an optical frequency comb, it is not always necessary to increase the number of optical combs in this way, but as described later, in order to obtain an offset optical frequency, it is necessary to increase the number of optical combs. Become.

  Next, the optical pulse train generated by the pulse light source generator is amplified by the optical amplifier 106, the noise component is removed by the optical filter 107, and input to the nonlinear optical medium 108. The optical pulse train input to the nonlinear optical medium 108 in this manner is wide spectral band light (SC light) in which the spectral band of each optical pulse is expanded. Due to the self-phase modulation effect caused by transmitting through the nonlinear optical medium 108, the phase velocity of the input optical pulse train is delayed near the peak of each pulse, the wavelength of the rising part of the pulse is lengthened, and the wavelength of the latter half is shortened. As a result, the spectrum of the pulse is expanded.

  As described above, by using the nonlinear optical medium 108 having a large nonlinear effect change due to a photonic crystal fiber or the like, a desired wide spectrum band light can be generated with high efficiency, which is necessary for stabilizing the optical frequency comb. The minimum threshold value of the light energy of the light source light supplied from the laser light source 101 can be suppressed low.

  Next, in the broad spectrum band light obtained by the nonlinear optical medium 108, the detection unit 109 generates the second harmonic from the component on the long wavelength side (any comb on the long wavelength side) by the self-reference interferometer. This is made to interfere with the fundamental wave (first harmonic) of the short wavelength side component (any comb on the short wavelength side), and the resulting optical beat is detected and photoelectrically converted. 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 and detected by a difference frequency generation method using the fundamental wave on the short wavelength side and the generated fundamental wave on the long wavelength side.

  On the other hand, when the bandwidth of the obtained broad 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 are caused to interfere with each other, and the resulting optical beat is detected. In other words, in the detection unit 109, the frequency difference between 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 obtained from the input optical pulse of the wide spectrum band light. The optical signal of is detected.

The electrical signal having the offset optical frequency generated and photoelectrically converted as described above is compared with the reference signal supplied from the microwave reference signal supply unit 111 in the feedback control unit 110. The feedback control unit 110 controls the center frequency of the laser output from the laser light source 101 by changing the resonance frequency, injection current, temperature, and the like of the laser light source 101 so that the comparison result is constant. As a result, the optical frequency comb output from the chromatic dispersion imparting unit 104 is in a state in which the offset optical frequency (carrier envelope phase difference) is stable, and high accuracy is obtained for the repetition frequency f _rep .

As described above, in this embodiment, the phase modulation unit 102, the microwave reference frequency generator 103, and the chromatic dispersion imparting unit 104 constitute an optical pulse generation unit. As described above, in this embodiment, since the optical pulse is generated from the CW light from the laser light source 101 by the optical pulse generation unit that generates a pulse train by phase modulation, the resonator length is not limited. It becomes feasible to set the repetition frequency f _rep (comb spacing) to several tens of GHz. In addition, the repetition frequency f _rep has high accuracy as described above.

Therefore, according to the optical frequency comb stabilized light source of the present embodiment, the beam splitter 105 outputs (takes out) the optical frequency comb stabilized light in which the repetition frequency f _rep and the offset optical frequency are maintained with high accuracy. Can do. The beam splitter 105 is not limited to being provided between the chromatic dispersion providing unit 104 and the optical amplifier 106, and may be disposed between the phase modulating unit 102 and the chromatic dispersion providing unit 104 as described above. The optical filter 107 may be disposed between the optical filter 107, the optical filter 107 and the nonlinear optical medium 108, and the nonlinear optical medium 108 and the detection unit 109. Among these, if the beam splitter 105 is disposed between the nonlinear optical medium 108 and the detection unit 109, optical frequency comb stabilized light in a wider band state can be obtained.

In addition, for example, a configuration in which a CW light is frequency-modulated using a modulation device in which an electro-optic modulator is disposed in a Fabry-Perot resonator may be used as a pulse light source generation unit that generates an optical pulse train of FM sidebands (non- (See Patent Document 7). Here, by applying deeper frequency modulation, the number of optical combs arranged at the repetition frequency f _rep on the optical frequency axis can be increased. As the electro-optic modulator, an electro-optic crystal such as LiNbO ₃ can be used. In this case, the electro-optic modulator disposed in the resonator serves as an optical pulse generation unit.

  By the way, the repetition frequency (period) of the optical pulse train in which the wide spectral band is expanded in the nonlinear optical medium 108 is high and has a large number of pulses per unit time. For this reason, in the optical amplifier 106, all the outputs are amplified and become high, but there are cases in which the pulse energy required for generating the wide spectral band by the nonlinear optical medium 108 is not amplified per pulse. . In such a case, for example, as illustrated in FIG. 3, the peak intensity of the optical pulse train may be increased by using an optical gate 306 that reduces the repetition frequency (period) of the optical pulse (decreases the duty ratio). .

FIG. 3 is a block diagram showing a configuration example of another optical frequency comb stabilized light source according to the embodiment of the present invention. The optical frequency comb stabilized light source includes a phase modulation unit 102 and a microwave reference frequency generator 103. , And the repetition frequency of the optical pulse generated from the optical pulse generating means constituted by the chromatic dispersion imparting unit 104 is reduced by the optical gate 306 and then introduced into the optical amplifier 106. The optical gate 306 is an optical gate that synchronizes with an electrical signal obtained by _reducing the repetition reference signal of the repetition frequency f _rep output from the microwave reference frequency generator 103 to 1 / n (n is a natural number) by the frequency divider 305.

  By using such an optical gate 306, at the point P1 before the optical gate 306, as shown in FIG. 4A, an optical pulse having a waveform with a repetition frequency of f and a duty ratio of pulse width × f. However, at the point P2 that has passed through the optical gate 306, as shown in FIG. 4B, an optical pulse having a waveform with a repetition frequency of f / n and a duty ratio of pulse width × (f / n) is obtained. As a result, the total output intensity before optical amplification is suppressed, the number of pulses per unit time is reduced, and as a result, the peak intensity (intensity for each pulse) of the optical pulse train can be increased.

  In this optical frequency comb stabilized light source, as shown in FIG. 3, an optical amplifier 302 and an optical filter 303 for removing ASE noise and the like are provided between the chromatic dispersion providing unit 104 and the optical gate 306, and an optical pulse is provided. The peak power is increased further. In addition, this optical frequency comb stabilized light source includes a non-linear optical medium 304 that reduces the optical pulse width in front of the optical gate 306, and the non-linear optical medium 108 is more likely to generate wide spectrum band light. In the nonlinear optical medium 108, the narrower the width of the incident light pulse, the easier it is to generate wide spectrum band light.

  Further, in the optical frequency comb stabilized light source shown in the configuration example in FIG. 3, the optical pulse that has passed through the optical gate 306 has a reduced duty ratio, and therefore is not suitable as an optical frequency comb. For this reason, the beam splitter 105 is preferably arranged in front of the optical gate 306 (on the phase modulation unit 102 side from the optical gate 306). Note that a nonlinear optical medium similar to the nonlinear optical medium 108 may be disposed in front of the optical gate 306. By doing in this way, the spectrum band of the optical pulse train incident on the optical gate 306 can be further expanded. In this case, the beam splitter 105 may be arranged before the newly added nonlinear optical medium, or may be arranged between the newly added nonlinear optical medium and the optical gate 306. By disposing the beam splitter 105 between the newly added nonlinear optical medium and the optical gate 306, an optical frequency comb with a wider spectral band can be obtained, which becomes more practical.

  Next, operation | movement of the control part 112 is demonstrated in detail using the flowchart of FIG. In the control unit 112, first, the digitizing unit 117 generates an optical beat measurement value in which the effective peak light intensity is digitized from an electrical signal obtained by photoelectrically converting the optical beat detected by the detecting unit 109 (step S501). . For example, when the bandwidth of the broad spectrum band light is 1 octave or more, the digitizing unit 117 photoelectrically converts the short wave side fundamental wave detected by the detection unit and the second harmonic wave on the long wavelength side. Each effective light peak intensity value is generated from an electrical signal obtained by photoelectrically converting a wave, and an optical beat generated when the fundamental wave and the second harmonic are caused to interfere from the generated two effective light peak intensity values. The maximum peak light intensity is calculated as the optical beat calculation value (step S502).

  Next, in the control unit 112, the optical element control unit 118 determines that the optical beat measurement value generated as described above matches the calculated optical beat calculation value (step S503). Note that it may be determined that the difference between the optical beat measurement value and the optical beat calculation value is equal to or less than a predetermined value (allowable error) set in advance. If this determination does not match or exceeds the specified value, the optical element control unit 118 controls (adjusts) the self-referencing interferometer based on the optical beat calculation value. (Step S504). For example, the optical element control unit 118 includes the rotation angle of the wave plate constituting the self-reference interferometer of the detection unit 109, the angle of the condensing system lens and the mirror (swing), and the target wavelength (second harmonic). A control value such as a wavelength to be extracted as a wave is generated, and this is notified to the detection unit 109. By this notification, the detection unit 109 changes (adjusts) the settings of each part of the self-referencing interferometer.

  When the above steps S501 to S504 are repeated and the determinations in step S503 match or are within the specified value range, in the control unit 112, the light source intensity control unit 119 indicates that the optical beat measurement value is It is determined that the intensity value is sufficient to stabilize the optical frequency comb (step S505). An intensity value sufficient for stabilizing the optical frequency comb is set in advance. Note that it may be determined that the optical beat calculation value exceeds an intensity value sufficient for stabilizing the optical frequency comb.

  If it is determined in step S505 that the optical beat measurement value does not exceed the intensity value sufficient to stabilize the optical frequency comb, the light source intensity control unit 119 amplifies the intensity of the laser light source 101, and the optical amplifier 106 Control (adjustment) such as increasing the amplification factor is performed (step S506). If it is determined in step S505 that the optical beat measurement value exceeds the intensity value sufficient for stabilizing the optical frequency comb, the control unit 112 ends the control operation.

  The control unit 112 is, for example, a computer device that includes a CPU, a main storage device, an external storage device, a network connection device, and the like, and is described above when the CPU is operated by a program developed in the main storage device. Each function is realized.

  As described above, the optical frequency comb stabilized light source in the present invention can be easily set to an arbitrary center optical frequency and frequency interval, and has a repetition frequency higher by one digit or more than the conventional one. In addition, an optical frequency comb having a wider band can be realized. For example, the optical frequency can be measured in a wider frequency range. The use of each mode of the optical comb by the optical frequency comb stabilized light source in the present invention greatly contributes to the development of the communication field and the spectroscopic field.

It is a block diagram which shows the structural example of the optical frequency comb stabilization light source in embodiment of this invention. It is a block diagram which shows the structural example of the control part 112 in this Embodiment. It is a block diagram which shows the structural example of the other optical frequency comb stabilization light source in embodiment of this invention. It is explanatory drawing for demonstrating the state of the optical waveform in the process of stabilization of the optical frequency comb stabilization light source in this Embodiment. 5 is a flowchart for explaining an operation example of a control unit 112. It is explanatory drawing about the phase of the optical carrier wave in an optical pulse. It is explanatory drawing which shows the state of the line spectrum on the frequency axis of an optical pulse.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 101 ... Laser light source, 102 ... Phase modulation part, 103 ... Microwave reference frequency generator, 104 ... Wavelength dispersion provision part, 105 ... Beam splitter, 106 ... Optical amplifier, 107 ... Optical filter, 108 ... Nonlinear optical medium, 109 ... Detection unit, 110... Feedback control unit, 111... Microwave reference signal supply unit, 112.

Claims (5)

A light source that generates continuous laser light;
Optical pulse generating means for phase-modulating laser light generated from the light source at a first frequency to generate an optical pulse having a pulse repetition frequency of the first frequency;
A non-linear optical medium that expands the optical spectrum band of the optical pulse generated by the optical pulse generating means;
An optical signal having a frequency difference between 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 obtained from the optical pulse whose optical spectrum band is expanded by the nonlinear optical medium. Detecting means for photoelectrically converting and outputting an electrical signal;
Reference signal supply means for supplying a reference signal of a predetermined second frequency;
An optical frequency comb stable comprising at least feedback control means for comparing the electrical signal output from the detection means with a reference signal of a predetermined frequency and controlling the light source based on the comparison state. Light source. The optical frequency comb stabilized light source according to claim 1,
The optical pulse generation means includes
A phase modulation function unit for phase-modulating the laser beam at the first frequency;
A chromatic dispersion providing function unit configured to generate an optical pulse having a repetition frequency of the pulse of the first frequency by giving chromatic dispersion to the laser light phase-modulated by the phase modulation function unit. Optical frequency comb stabilized light source. The optical frequency comb stabilized light source according to claim 1,
The optical pulse generation means includes
Including an electro-optic modulator disposed in a Fabry-Perot resonator;
The electro-optic modulator modulates the laser light. An optical frequency comb stabilized light source, wherein: In the optical frequency comb stabilized light source according to any one of claims 1 to 3,
Arranged between the optical pulse generating means and the nonlinear optical medium;
An optical frequency comb stabilized light source comprising an optical gate for reducing a repetition frequency of an optical pulse generated by the optical pulse generating means. In the optical frequency comb stabilized light source according to any one of claims 1 to 4,
An optical frequency comb stabilized light source comprising control means for controlling the laser light output of the light source based on the intensity of the electrical signal output from the detection means.

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