JP7370544B2 - Optical frequency measurement device - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act [Fact of disclosure] 1. Website publication date: March 5, 2018 2. Website address: 2018 65th Japan Society of Applied Physics Spring Academic Conference website ・WEB program https://confit. atlas. jp/guide/event/jsap2018s/top ・iTunes (App Store) https://goo. gl/sdAU4Q ・Google Play https://goo. gl/rsXFT9 3. Publisher: Kazuo Hara, Jun Ishizawa, Kenichi Nittatsu, Tadashi Nishikawa, Tetsuomi Samukawa, Hideki Goto

The present invention relates to an optical frequency measurement technique for measuring the optical frequency of a laser to be measured.

Until 2000, a method called a frequency chain was used to precisely measure the optical frequency of the laser to be measured, in which the frequency was sequentially multiplied from the 9 GHz frequency standard to the laser to be measured (for example, see Non-Patent Document 1). ). However, this method requires a large space because multiple large lasers are used as a reference, and it is not possible to easily measure the frequency of the laser to be measured. On the other hand, in 1999, optical combs appeared, making it possible to measure the laser to be measured at microwave frequencies, but an optical comb light source was required.

Y. Miki et al., "Frequency Chain to 3.39μm CH4-Stabilized He-Ne Laser Using Josephson Point Contact as Harmonic Mixer", Jpn. J. Appl. Phys. Vol. 33 pp. 1655-1658, 1994 H. Inaba et al., "Frequency Measurement Capability of a Fiber-Based Frequency Comb at 633nm", IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, Vol. 58(4) pp.1234-1239, 2009

In recent years, several optical frequency measuring devices have been commercially available, and the main optical frequency measuring devices include (1) an optical wavelength meter and (2) an optical frequency comb. These optical frequency measurement methods require the use of some kind of reference light source. Hereinafter, (1) an optical wavelength meter and (2) an optical frequency comb will be explained.

(1) Optical wavelength meter Optical wavelength meters are used as wavelength standards for adjusting the wavelength of laser diodes for optical communications and for calibrating spectrometers and optical spectrum analyzers. High-precision products achieve an accuracy of ±2 MHz. The basic optical configuration is as shown in FIG. The optical system of the Michelson interferometer has a movable cube corner. With this configuration, changes in interference fringes due to changes in the optical path length difference between the reference laser beam path and the measured laser beam LX path are measured for the reference laser beam and the measured laser beam LX, which serve as standards, and the optical wavelength is determined based on the measurement results. This is what we seek.

(2) Optical frequency measurement method using an optical frequency comb In the late 1990s, a method for stabilizing the frequency of an optical frequency comb was realized. A passive mode-locked laser equipped with an optical resonator device is used as the laser light source. Passively mode-locked lasers can oscillate pulsed lasers. A laser typically has a gain medium that amplifies light and a resonator with a 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). If the interval between the resonant frequencies of the resonator is narrower than the spectrum width of the laser light emitted by the laser, the resonator resonates in a plurality of modes (frequencies). At this time, by aligning the phases of each mode (mode locking), the laser beam is intensified at a repetition frequency frep=c/(2L). As a result, pulsed laser light with a repetition frequency frep is generated.

The pulsed laser beams emitted by this mode-locked laser are arranged at equal time intervals T on the time axis. On the other hand, on the frequency axis, it becomes a collection of many modes arranged in a comb shape with equal frequency intervals frep. As shown in FIG. 6, a collection of laser beams arranged in a comb shape on the frequency axis is called an "optical frequency comb." The spectral frequency fn of a certain mode of the optical frequency comb can be expressed as fn=N×frep+fceo (N is an integer greater than or equal to 0). Here, fceo is called the carrier envelope offset frequency.

Using the spectrum of a mode-locked laser oscillator, for example, using the self-phase modulation effect produced by a photonic crystal fiber, white light with a band of one octave or more is generated. When the second harmonic of the long wavelength component (N×frep+fceo) is generated, its frequency becomes 2×(N×frep+fceo). Further, the short wavelength component of white light can be expressed as 2×N×frep+fceo. By interfering these two lights, the second harmonic of the long wavelength component and the short wavelength component, and detecting the beat signal using, for example, a photodiode, it is possible to measure the value of the frequency difference between the two lights. The measured value is compared with an external microwave reference frequency, and an electronic circuit feeds back the magnitude of the nonlinear dispersion inside the resonator based on the magnitude of the deviation, thereby determining the carrier envelope offset frequency fceo. It can be stabilized.

On the other hand, the repetition frequency of the laser can be fixed within a certain finite range by feeding back the repetition frequency signal from the photodiode to the resonator length of the laser. In this way, an optical frequency comb stabilized light source with constant offset frequency and mode frequency spacing has been developed based on a mode-locked laser with a resonator device.

When measuring the unknown optical frequency fx, the interference signal with the optical frequency comb is measured. The frequency fbeat of the interference signal is fbeat=fx−(N×frep+fceo). To obtain the mode order N of the optical frequency comb, a reference light source is separately prepared and calculated. The accuracy of optical frequency measurement using this optical frequency comb depends on the external reference signal used to stabilize the optical frequency comb. For example, when using a GPS-synchronized external reference signal source, the accuracy is about 12 digits, so the optical frequency accuracy is about 200 Hz, which is more than 3 orders of magnitude higher than the optical wavelength meter's accuracy of ±2 MHz (for example, , Non-Patent Document 2, etc.).

However, according to such conventional techniques, there is a problem in that each method requires a separate highly accurate reference light source. For example, (1) in the case of a wavelength meter, a reference light source such as a wavelength stabilized helium neon laser is required. In addition, (2) in the case of an optical frequency comb, a reference light source such as a mode-locked laser is required. For this reason, the scale of equipment required for optical frequency measurement becomes large, and the frequency of the laser to be measured cannot be easily measured. Furthermore, since the laser light from a CW semiconductor laser has large fluctuations (frequency fluctuations), it cannot be accurately measured using an optical frequency comb generated by a mode-locked laser.

The present invention is intended to solve these problems, and aims to provide an optical frequency measurement technology that can measure the optical frequency of a laser to be measured with high accuracy without requiring a high-precision reference light source. It is said that

In order to achieve such an object, the optical frequency measuring device according to the present invention converts the seed light source laser beam output from the seed light source into a reference signal indicating the repetition frequency frep output from the first signal generator. an optical pulse train generating section that generates a first optical pulse train forming an optical frequency comb at the repetition frequency frep by performing phase modulation based on the repetition frequency frep; and a dispersion compensator compressing the pulse width of the first optical pulse train. a pulse width compressor that outputs a shortened third optical pulse train; and a fourth optical pulse train composed of SC light by expanding the optical spectral band of the third optical pulse train using a nonlinear optical medium. a band expansion section; an offset frequency detection section that inputs the fourth optical pulse train into a self-reference interferometer and detects an offset frequency fceo of a carrier envelope from the obtained interference signal; a first optical frequency specifying unit that specifies the optical frequency of the seed light source based on the output repetition frequency frep or the offset frequency fceo; By measuring the optical frequency, the mode order of the laser beam to be measured is specified, and the interference light frequency regarding the interference light between the laser beam to be measured and the branched light of the first optical pulse train is measured, and the and a second optical frequency identifying section that identifies the optical frequency of the laser beam to be measured based on the optical frequency of the light source, the mode order of the laser beam to be measured, and the interference optical frequency.

Further, another optical frequency measuring device according to the present invention phase-modulates the seed light source laser beam output from the seed light source based on a reference signal indicating the repetition frequency frep output from the first signal generator. an optical pulse train generator that generates a first optical pulse train forming an optical frequency comb at the repetition frequency frep; a pulse width compression section that outputs a fourth optical pulse train consisting of SC light by expanding the optical spectral band of the third optical pulse train using a nonlinear optical medium; an offset frequency detection unit that inputs a fourth optical pulse train into a self-reference interferometer and detects an offset frequency fceo of a carrier envelope from the obtained interference signal; and the repetition frequency output from the first signal generator. frep or a first optical frequency specifying unit that specifies the optical frequency of the seed light source based on the offset frequency fceo, and measuring a provisional optical frequency of the measured laser light output from the measured laser light source. specifies the mode order of the laser beam to be measured, and measures the interference light frequency regarding the interference light between the laser beam to be measured and the branched light of the fourth optical pulse train, and determines the optical frequency of the seed light source, the and a second optical frequency specifying section that specifies the optical frequency of the laser beam to be measured based on the mode order of the laser beam to be measured and the interference light frequency.

Further, in one configuration example of the optical frequency measuring device according to the present invention, the repetition frequency frep/D is reduced by reducing the duty ratio of the first optical pulse train to 1/D (D is an integer of 2 or more). The pulse width compressor further includes a duty ratio reducing unit that outputs a second optical pulse train having a pulse width of the second optical pulse train, and the pulse width compressing unit compresses the pulse width of the second optical pulse train by a dispersion compensating means to shorten the pulse width of the third optical pulse train. It is designed to output a pulse train.

Further, in one configuration example of the optical frequency measuring device according to the present invention, the phase of the setting signal output from the second signal generator and the offset signal indicating the offset frequency fceo is compared, and the obtained comparison result is The apparatus further includes a feedback control section that stabilizes the repetition frequency frep and stabilizes the frequency of the seed light source by feedback control based on the above.

According to the present invention, an excellent effect can be obtained in that the optical frequency of the laser to be measured can be measured with high accuracy without requiring a reference light source.

FIG. 1 is a block diagram showing the configuration of an optical frequency measuring device according to a first embodiment. FIG. 2 is a block diagram showing the configuration of an optical frequency measuring device according to a second embodiment. FIG. 3 is a block diagram showing the configuration of an optical frequency measuring device according to a third embodiment. It is a block diagram showing the composition of the optical frequency measuring device concerning a 4th embodiment. FIG. 2 is an explanatory diagram of a general optical wavelength meter. FIG. 2 is an explanatory diagram showing optical frequency measurement using an optical frequency comb as a reference light source.

Next, embodiments of the present invention will be described with reference to the drawings.
[First embodiment]
First, with reference to FIG. 1, an optical frequency measuring device 1 according to a first embodiment of the present invention will be described. FIG. 1 is a block diagram showing the configuration of an optical frequency measuring device according to a first embodiment.

This optical frequency measuring device 1 is a device that measures the optical frequency fx of a laser beam LX to be measured outputted from an arbitrary laser light source X to be measured, such as a CW laser light source, without requiring a highly accurate reference light source. It is.
As shown in FIG. 1, the optical frequency measurement device 1 includes an optical pulse train generation section 10, a duty ratio reduction section 20, a pulse width compression section 30, a band expansion section 40, an offset frequency detection section 50, and a feedback control section. 60, and an optical frequency identification section 70.

The optical pulse train generator 10 phase-modulates LX from the laser light source to be measured at the repetition frequency frep to expand the spectral band, and then forms an optical frequency comb at the repetition frequency frep by imparting wavelength dispersion. It has a function of generating a light pulse train (first light pulse train) PX. The optical pulse train generator 10 mainly includes a signal generator 11 (first signal generator), a phase modulator 12, a phase shifter 13, an intensity modulator 14, and a wavelength dispersion imparting means 15.

The duty ratio reducing unit 20 reduces the repetition frequency by reducing the duty ratio of the optical pulse train PX output from the optical pulse train generating unit 10 to 1/D based on the frequency division number D (D is an integer of 2 or more). It has a function of outputting an optical pulse train PN (second optical pulse train) in which frep is reduced to frep/D. N (N is an integer greater than or equal to 0) corresponds to the order of the optical frequency comb mode, and is an integer greater than or equal to 0. The duty ratio reduction unit 20 mainly includes a frequency divider 21 and an optical gate 22. Note that even if this duty ratio is not reduced, if the offset frequency can be detected below, the duty ratio reduction section 20 is not necessary, and the value of D below is 1.

The pulse width compression section 30 amplifies the optical pulse train PN having a repetition frequency frep/D output from the duty ratio reduction section 20, and then compresses the pulse width of the PN to shorten it by expanding the spectral bandwidth. It has a function of outputting a pulsed optical pulse train PS (third optical pulse train) having a repetition frequency frep/D. The pulse width compressor 30 mainly includes an optical amplifier 31 and a dispersion compensator 32.

The band expanding unit 40 inputs the optical pulse train PS output from the pulse width compressing unit 30 into a highly nonlinear medium to generate an optical pulse train PSC having a repetition frequency frep/D and consisting of SC light with an expanded optical spectral band. (fourth optical pulse train). The band expansion section 40 includes a highly nonlinear medium 41 as a main component.

The offset frequency detection unit 50 interferes the optical pulse train PSC output from the band expansion unit 40 by self-reference interferometry, and generates an offset signal Sceo, which is an electrical signal indicating the offset frequency fceo of the carrier envelope, from the obtained interference signal. It has a detection function. The offset frequency detection section 50 mainly includes a self-reference interferometer 51 and a photodetector 52.

The feedback control section 60 compares the phase of the offset signal Sceo indicating the offset frequency fceo detected by the offset frequency detection section 50 with the setting signal SS having the reference setting frequency f0, and changes the phase according to the obtained comparison result. By performing feedback control of the optical pulse train generator 10 based on the voltage control signal SV, the phase noise of the optical pulse train generator 10 is suppressed, the repetition frequency frep is stabilized with high accuracy, and the phase noise is extremely reduced. It has a function. The feedback control section 60 mainly includes a filter 61, a signal generator (second signal generator) 62, and a feedback circuit 63.

The optical frequency specifying section 70 down-converts the repetition frequency frep output from the optical pulse train generating section 10, and then measures the frequency to specify the mode order N of the optical frequency comb, and based on the obtained N. It has a function of calculating the optical frequency fx of the laser beam LX to be measured. The optical frequency identification section 70 mainly includes an external reference signal generator 71, a frequency mixer 72, a frequency counter 73, and a calculation section 74.

[Operation according to the first embodiment]
Next, with reference to FIG. 1, the operation of the optical frequency measuring device 1 according to this embodiment will be described.

[Optical pulse train generator]
It is assumed that the laser beam LX to be measured measured by the optical frequency measuring device 1 is a CW laser beam. In the optical pulse train generation section 10, the measured laser beam LX is phase modulated in the phase modulator 12 at the repetition frequency frep of the reference signal SR output from the signal generator 11 for driving the optical modulator to expand the spectral band. Intensity modulators 14 are arranged in series after (or before) the phase modulator 12, and one of the down-chirped and up-chirped modulators is selected by the phase modulator 12 to transmit light. This removes the remaining DC floor component in the time domain.

In order to adjust the time of modulation by the phase modulator 12 and the intensity modulator 14, a phase shifter 13 is used which shifts the phase of the repetition frequency frep and outputs the same to the intensity modulator 14.
Thereafter, the wavelength dispersion imparting means 15 imparts appropriate dispersion to the output light from the intensity modulator 14, thereby generating an optical pulse train PX having a repetition frequency frep. At this time, if a wavelength filter is used, wing components remaining in the optical pulse train PX can be suppressed, making it possible to generate clean pulses.

Note that the optical pulse train generator 10 may be configured to apply deep frequency modulation to CW light by installing an electro-optic modulator in a Fabry-Perot resonator to generate FM sideband waves. In the method of generating a pulse train using these phase modulators, it is possible to generate an optical pulse train having a repetition frequency of several tens of GHz because it is not limited by the resonator length.

[Duty ratio reduction section]
Since the repetition frequency frep of the optical pulse train PX is high, if sufficient pulse energy cannot be obtained to generate the optical pulse train PSC consisting of broadband light after optical amplification, the wavelength dispersion imparting means 15 of the optical pulse train generator 10 and , a duty ratio reducing section 20 is provided between the pulse width compressing section 30 and the optical amplifier 31.

In the duty ratio reducing section 20, an optical gate 22 is provided between the wavelength dispersion imparting means 15 of the optical pulse train generating section 10 and the optical amplifier 31 of the pulse width compressing section 30, and the optical pulse train outputted from the optical pulse train generating section 10 is An optical pulse train PN with the repetition frequency frep (duty ratio) of PX reduced to frep/D is output. Thereby, it becomes possible to increase the peak power after optical amplification, and it becomes possible to generate a wide spectrum band optical pulse train PSC necessary for the self-reference interferometer 51 of the offset frequency detection section 50.

In this case, the duty ratio reduction unit 20 divides the repetition frequency frep of the reference signal SR output from the signal generator 11 into 1/D by the frequency divider 21, and combines the resulting electric signal with the frequency frep/D. By driving the optical gates 22 in synchronization, the duty ratio of the optical pulse train PX is reduced, and an optical pulse train PN having a repetition frequency frep/D is output.

In this way, by using the optical frequency comb stabilized light source using the optical gate 22, the duty ratio is pulse width x frep at the repetition frequency frep (for example, several GHz to several tens of GHz) before the optical gate 22. After the optical gate 22, the optical waveform (optical pulse train PX) becomes an optical waveform (optical pulse train PN) with a repetition frequency frep/D (for example, several MHz to several 100 MHz) and a duty ratio of pulse width x frep/D.

If the peak power of the optical pulse train PX is insufficient for the subsequent wide-spectrum band light generation, a filter is installed between the wavelength dispersion imparting means 15 of the optical pulse train generator 10 and the optical gate 22 to increase the peak power of the optical pulse train PX. , one more stage of optical amplifier and optical bandpass filter may be added. Moreover, since broadband light is more likely to be generated as the incident optical pulse width becomes narrower, it is also possible to add a nonlinear medium between the wavelength dispersion imparting means 15 and the optical gate 22 to reduce the optical pulse width. good.

[Pulse width compression section]
In the pulse width compression section 30 , an optical amplifier 31 optically amplifies the optical pulse train PN having a repetition frequency frep/D, which is output from the optical gate 22 of the duty ratio reduction section 20 . At this time, a method may be used in which the spectral bandwidth is expanded without causing optical splitting by a method of stepwise optical amplification using a nonlinear effect such as a progressive self-phase modulation effect within the optical amplifier 31. The dispersion compensation means 32 compresses the pulse width to make the pulses shorter, and outputs an optical pulse train PS having a repetition frequency frep/D.

[Bandwidth expansion section]
In the band expanding section 40, the optical pulse train PS generated by the pulse width compressing section 30 is input to a highly nonlinear medium (nonlinear optical medium) 41, and is converted into an optical pulse train with a repetition frequency frep/D consisting of wide spectrum band light (SC light). Generate PSC. If a material with high nonlinear susceptibility is used in this highly nonlinear medium 41, it is possible to generate the desired wide spectrum band light with high efficiency, and it is possible to suppress the minimum threshold of the energy supplied from the laser beam LX to be measured to a low value. It is.

An optical frequency comb of mode order k (k=0, ±1, ±2, ±3, . . . ) of broadband light can be expressed by the following equation (1). In equation (1), Eo represents the optical electric field amplitude, and ψ0(t) and ψ1(t) represent the phase noise of the laser beam LX to be measured and the signal generator 11 for driving the phase modulator, respectively. .

[Offset frequency detection section]
In the offset frequency detection section 50, the optical pulse train PSC obtained in the band expansion section 40 is inputted into a self-reference type interferometer 51, the carrier envelope is detected using a photodetector 52, and an electrical signal indicating the offset frequency fceo is detected. A certain offset signal Sceo is output. The type of self-reference interferometer 51 can be selected depending on the bandwidth of the optical pulse train PSC.

For example, if the SC optical bandwidth is one octave or more, an f-2f self-reference interferometer is used, which detects the 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. Bye. Further, if the bandwidth of the SC light is a 2/3 octave band, the offset frequency fceo 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 2f-3f self-referencing interferometer may be used.

[Feedback control section]
The feedback control section 60 compares the phase of the offset signal Sceo indicating the offset frequency fceo detected by the offset frequency detection section 50 with the setting signal SS having the reference setting frequency f0, and changes the phase according to the obtained comparison result. By performing feedback control on the optical pulse train generating section 10 based on the voltage control signal SV, the phase noise of the optical pulse train generating section 10 is suppressed, and the repetition frequency frep is stabilized with high accuracy.

In this embodiment, as will be described later, the set frequency f0 is changed to two different shift frequencies f0a and f0b in the feedback control unit 60, and the repetition frequency frep is changed according to the variation of the repetition frequency frep. By measuring the variation amount, the order N of the optical frequency comb mode is specified, and the optical frequency fx of the laser beam LX to be measured is calculated from the obtained N.

[Optical frequency identification section]
The optical frequency fx of the measured laser beam LX can be expressed as fx=N×frep/D+f0. f0 is the setting frequency of the setting signal SS output from the signal generator 62 of the feedback control section 60. The repetition frequency frep is the frequency of the signal generator 11 when stabilized by the feedback control section 60. The set frequency f0 of the signal generator 62 is changed to two different shift frequencies f0a and f0b, the amount of variation in the repetition frequency frep that fluctuates thereby is measured, and the order N of the optical frequency comb mode is determined based on the obtained measurement results. can be identified.

The number of digits of N changes depending on the value of the repetition frequency frep/D of the optical pulse train PN. Therefore, the measurement accuracy required of the external reference frequency fex of the external reference signal generator 71 and the frequency counter 73 in the optical frequency specifying section 70 changes depending on the number of digits of N. In an experimental system in which a large frequency difference of 100 KHz or more is observed, the repetition frequency frep can be directly measured without using the following method with a configuration that excludes the external reference signal generator 71 and frequency mixer 72.
On the other hand, if the frequency difference is small, the external reference signal generator 71 and frequency mixer 72 are used to down-convert the frequency. Thereby, only the difference between the repetition frequency frep and the external reference frequency fex, that is, the difference frequency Δf=frep−fex, is measured by the frequency counter 73, so that measurement accuracy can be utilized to the maximum.

Furthermore, in order to improve accuracy, the frequency difference between the shift frequencies f0a and f0b of the signal generator 62 is increased. In the demonstration experiment, f0a was set to +130 MHz, f0b was set to -120 MHz, and the repetition frequency frep of the signal generator 11 was stabilized and controlled by the voltage control signal SV. Furthermore, since the repetition frequency frep after the optical gate 22 is around 25 GHz, the external reference frequency fex was set to 24.99998 GHz. The frequency counter 73 measures the difference frequency Δf between frep and fex.

Here, when the set frequency f0 is changed from f0a to f0b, the repetition frequency frep of the signal generator 11 changes, and the repetition frequency frep changes from frepa=fex+Δfa to frepb=fex+Δfb. Therefore, since the optical frequency fx of the laser beam LX to be measured when using these f0a and f0b is expressed by the following equation (2), it is necessary to measure frepa and frepb or Δfa and Δfb with the frequency counter 73. For example, the mode order N and further fx can be specified by the calculation unit 74 made up of a CPU, a PC, or the like.

In the demonstration experiment, for example, when D=20, when f0a was set to +130 MHz, the differential frequency Δfa was 19.40059938236 kHz, and when f0b was set to -120 MHz, the differential frequency Δfb was 51.5596872566 kHz. Therefore, the mode order N is obtained from the equation on the third line of equation (2) as N=155477, and the optical frequency fx of the laser beam to be measured LX is obtained first from the equation on the first line of equation (2). Using the value of N, it was possible to identify fx=194.346375340(1) THz. Note that the repetition frequency frep may be measured using the frequency-divided signal after the frequency divider 21.

Since the accuracy of the external reference signal this time is 12 digits, the optical frequency can be accurately determined down to 1 kHz. The accuracy of the optical frequency measurement depends on the external reference signal used to stabilize the offset frequency fceo. For example, when a GPS-synchronized external reference signal source is used, the accuracy is about 12 digits, so the optical frequency accuracy is about 200 Hz. (1) Accuracy is three orders of magnitude higher than the accuracy of an optical wavelength meter (±2 MHz). (2) The optical frequency measurement accuracy is comparable to that of an optical frequency comb using a mode-locked laser.

[Second embodiment]
Next, with reference to FIG. 2, an optical frequency measuring device 1 according to a second embodiment of the present invention will be described. FIG. 2 is a block diagram showing the configuration of an optical frequency measuring device according to a second embodiment.
In the first embodiment, the case where the mode order N of the optical frequency comb is specified based on the repetition frequency frep of the signal generator 11 has been described as an example. The feature of this embodiment is that the mode order N is specified based on the offset frequency fceo of the offset signal Sceo output from the feedback control section 60. In this embodiment, instead of the voltage control signal SV, the feedback control section 60 outputs an offset signal Sceo indicating the offset frequency fceo detected by the offset frequency detection section 50 to the optical frequency identification section 70. The other configurations of this embodiment are the same as those of the first embodiment.

[Optical frequency identification section]
In this embodiment, the repetition frequency frep of the signal generator 11 is changed to two different frequencies frepa and frepb, and the offset frequencies fceoa and fceob obtained at that time are used, or in addition to these, the external reference frequency of the external reference signal generator 71 is The mode order N is calculated by measuring the differential frequency Δfceoa (=fceoa-fex) and Δfceob (=fceob-fex) with respect to fex.

At this time, since the optical frequency fx of the laser beam LX to be measured when using these frepa and frepb is expressed by the following equation (3), fceoa, fceob or Δfceoa, Δfceob are measured by the frequency counter 73. Then, the calculation unit 74 including a CPU, a PC, etc. can specify the mode order N and further fx based on the following equation (3).

In this case as well, similarly to the first embodiment, the frequency counter 73 is You can take full advantage of the accuracy of Furthermore, measurement accuracy can be improved by increasing the difference between the two frequencies frepa and frepb.

[Third embodiment]
Next, with reference to FIG. 3, an optical frequency measuring device 1 according to a third embodiment of the present invention will be described. FIG. 3 is a block diagram showing the configuration of an optical frequency measuring device according to a third embodiment.
In this embodiment, a case will be described in which the frequency of the laser beam LX to be measured is measured using an optical frequency comb having a wide mode spacing as a reference light source. The feature of this embodiment is that a seed light source Y having a highly stable wavelength, such as a semiconductor laser, is used in place of the laser light source The optical frequency specifying section 80 measures the optical frequency fx of the laser beam LX to be measured from the laser light source X to be measured based on the branch light of the optical pulse train PX from 10.

Further, in this embodiment, the optical frequency specifying section (first optical frequency specifying section) 70 specifies the optical frequency comb, that is, the mode order N of the seed light source laser beam LY, and further specifies the seed light source optical frequency fy. In this embodiment, a case where fy is specified based on the configuration described in the first embodiment will be described as an example, but the present invention is not limited to this, and the configuration described in the second embodiment will be described. fy may be specified based on. The other configurations of this embodiment are the same as those of the first and second embodiments.

As shown in FIG. 3, in this embodiment, an optical pulse train generating section 10 generates a mode-interval optical comb using a set frequency of a signal generator 11. At this time, after specifying the mode order N of the seed light source laser beam LY, in the feedback control unit 60, only the offset frequency fceo is extracted by the filter 61, and the offset frequency fceo is extracted from the result of phase comparison with the set frequency f0 of the signal generator 62. The frequency of the seed light source laser beam LY may be stabilized by controlling the current value, temperature, etc. in the seed light source Y using the voltage control signal SY thus obtained.
This makes it possible to realize a frequency-stabilized optical comb with a wide mode spacing (10 GHz or more), which cannot be realized with the mode spacing (100 MHz) of a general mode-locked laser.

Consider the case of a 25 GHz mode spacing optical comb. Using an optical spectrum analyzer, the provisional optical frequency fx of the laser beam LX to be measured is roughly measured with an accuracy of 25 GHz. The wider the mode interval, the lower the measurement accuracy of fx may be. By calculating fx/(25GHz), the mode order M (M is an integer) of fx can be specified. Furthermore, if fx varies more than frep, the mode order M can be determined with high accuracy by measuring fx with a wavelength meter or the like that has higher measurement accuracy than an optical spectrum analyzer.

Next, an optical demultiplexer SP is inserted between the wavelength dispersion imparting means 15 of the optical pulse train generating section 10 and the optical gate 22 of the duty ratio reducing section 20. In the optical frequency specifying section (second optical frequency specifying section) 80, an optical multiplexer 81 combines the optical pulse train PX, which is one branch of the optical demultiplexer SP, and the measured laser beam LX from the measured laser light source X. The interference light frequency fe of the obtained interference light is measured by a photodetector 82 and a frequency counter 83.

Here, the mode order of fx is M, the interference light frequency of the interference signal between LX and LY is fe, and fx can be expressed by the following equation (4). Thereby, the calculation unit 84 can specify fx with high precision. At this time, the polarity of fe can be specified based on the increase or decrease in fe when the frequency of frep or fceo is varied.

From the above, the optical frequency of the laser beam LX to be measured can be measured with high precision. The third embodiment has the advantage that the frequency can be easily observed as long as the optical frequency fx is temporarily measured using a measuring instrument with low measurement accuracy and the mode order M is specified.

[Fourth embodiment]
Next, with reference to FIG. 4, an optical frequency measuring device 1 according to a fourth embodiment of the present invention will be described. FIG. 4 is a block diagram showing the configuration of an optical frequency measuring device according to the fourth embodiment.
In this embodiment, a case will be described in which optical frequency is measured using a broadband optical frequency comb as a reference light source. The feature of this embodiment is that a seed light source Y having a highly stable wavelength such as a semiconductor laser is used instead of the laser light source to be measured X as the seed light source connected to the optical pulse train generating section 10. The optical frequency specifying section 80 measures the optical frequency fx of the laser beam LX to be measured from the laser light source X to be measured based on the branched light of .

Further, in this embodiment, the optical frequency specifying section (first optical frequency specifying section) 70 specifies the optical frequency comb, that is, the mode order N of the seed light source laser beam LY, and further specifies the seed light source optical frequency fy. In this embodiment, a case where fy is specified based on the configuration described in the first embodiment will be described as an example, but the present invention is not limited to this, and the configuration described in the second embodiment will be described. fy may be specified based on. The other configurations of this embodiment are the same as those of the first and second embodiments.

The third embodiment described above can be used when the frequency of the laser beam LX to be measured is in the frequency band in which the phase modulator 12, the intensity modulator 14, and the optical amplifier 31 operate. This embodiment is used when the laser beam LX to be measured is outside the device operating frequency band.

As shown in FIG. 4, in this embodiment, an optical demultiplexer SP is inserted between the highly nonlinear medium 41 of the band expansion section 40 and the self-reference interferometer 51 of the offset frequency detection section 50. In the optical frequency specifying section (second optical frequency specifying section) 80, an optical multiplexer 81 combines the optical pulse train PSC, which is one branch of the optical demultiplexer SP, and the laser beam LX to be measured from the laser light source X to be measured. Then, the interference light frequency fe of the obtained interference light is measured by the photodetector 82 and the frequency counter 83. The optical frequency calculation method according to this embodiment is the same as that in the third embodiment.

The mode order M of the broadband frep/D mode interval optical comb interfering with the measured laser light LX from the measured laser light source X and the optical frequency fx of the measured laser light source X are specified. At this time, similarly to the third embodiment, the mode order M can be specified with a wavelength meter having a measurement accuracy of frep/D. The interference light frequency fe is determined by combining the optical pulse train PSC and the measured laser beam LX from the measured laser light source The counter 83 may be used to measure it. Therefore, fx can be specified based on the following equation (5) in the same manner as in the third embodiment.

As described above, the optical frequency measuring device of the present invention described in the first and second embodiments does not require a high-precision reference light source as in the conventional method, and does not require the use of a conventional optical wavelength meter. Since the optical frequency can be measured more accurately than in the case of the conventional method, it contributes to the development of the field of optical frequency measurement more than ever before, and it can be seen that the present invention is superior to the conventional technology.

[Expansion of embodiment]
Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. The configuration and details of the present invention may be modified in various ways within the scope of the present invention by those skilled in the art. Moreover, each embodiment can be implemented in any combination within the range not contradictory.

DESCRIPTION OF SYMBOLS 1... Optical frequency measuring device, 10... Optical pulse train generation part, 11... Signal generator, 12... Phase modulator, 13... Phase shifter, 14... Intensity modulator, 15... Wavelength dispersion imparting means, 20... Duty ratio reduction part , 21... Frequency divider, 22... Optical gate, 30... Pulse width compression section, 31... Optical amplifier, 32... Dispersion compensation means, 40... Bandwidth expansion section, 41... Highly nonlinear medium, 50... Offset frequency detection section, 51 ... Self-reference interferometer, 52 ... Photodetector, 60 ... Feedback control section, 61 ... Filter, 62 ... Signal generator, 63 ... Feedback circuit, 70 ... Optical frequency identification section, 71 ... External reference signal generator, 72 ...Frequency mixer, 73...Frequency counter, 74...Calculation unit, 80...Optical frequency identification unit, 81...Optical multiplexer, 82...Photodetector, 83...Frequency counter, 84...Calculation unit, X...Measurement laser light source , Y...Seed light source, SP...Optical demultiplexer, LX...Measurement laser beam, LY...Seed light source laser beam, SR...Reference signal, PX, PN, PS, PSC...Optical pulse train, Sceo...Offset signal, SS... Setting signal, SV, SY... Voltage control signal, frep, frepa, frepb... Repetition frequency, fceo, fceoa, fceob... Offset frequency, f0... Setting frequency, f0a, f0b... Shift frequency, fex... External reference frequency, Δf, Δfa , Δfb, Δfceoa, Δfceob...difference frequency, N, M...mode order, D...frequency division number, fx...optical frequency, fy...seed light source optical frequency, fe...interference optical frequency.

Claims (4)

A first step that forms an optical frequency comb at the repetition frequency frep by phase modulating the seed light source laser beam output from the seed light source based on a reference signal indicating the repetition frequency frep output from the first signal generator. an optical pulse train generating section that generates one optical pulse train;
a pulse width compression unit that outputs a third optical pulse train obtained by compressing the pulse width of the first optical pulse train by a dispersion compensating means and making the pulse width shorter;
a band expansion unit that outputs a fourth optical pulse train composed of SC light by expanding the optical spectral band of the third optical pulse train using a nonlinear optical medium;
an offset frequency detection unit that inputs the fourth optical pulse train into a self-reference interferometer and detects an offset frequency fceo of a carrier envelope from the obtained interference signal;
a first optical frequency identifying unit that identifies the optical frequency of the seed light source based on the repetition frequency frep or the offset frequency fceo output from the first signal generator;
By measuring the provisional optical frequency of the laser beam to be measured outputted from the laser light source to be measured, the mode order of the laser beam to be measured is specified, and the branched light of the laser beam to be measured and the first optical pulse train is determined. A second light that measures the interference light frequency of the interference light with the seed light source, and specifies the optical frequency of the measured laser light based on the optical frequency of the seed light source, the mode order of the measured laser light, and the interference light frequency. An optical frequency measuring device comprising: a frequency specifying section. A first step that forms an optical frequency comb at the repetition frequency frep by phase modulating the seed light source laser beam output from the seed light source based on a reference signal indicating the repetition frequency frep output from the first signal generator. an optical pulse train generating section that generates one optical pulse train;
a pulse width compression unit that outputs a third optical pulse train obtained by compressing the pulse width of the first optical pulse train by a dispersion compensating means and making the pulse width shorter;
a band expansion unit that outputs a fourth optical pulse train composed of SC light by expanding the optical spectral band of the third optical pulse train using a nonlinear optical medium;
an offset frequency detection unit that inputs the fourth optical pulse train into a self-reference interferometer and detects an offset frequency fceo of a carrier envelope from the obtained interference signal;
a first optical frequency identifying unit that identifies the optical frequency of the seed light source based on the repetition frequency frep or the offset frequency fceo output from the first signal generator;
By measuring the provisional optical frequency of the laser beam to be measured outputted from the laser light source to be measured, the mode order of the laser beam to be measured is specified, and the branch light of the laser beam to be measured and the fourth optical pulse train is determined. A second light that measures the interference light frequency of the interference light with the seed light source, and specifies the optical frequency of the measured laser light based on the optical frequency of the seed light source, the mode order of the measured laser light, and the interference light frequency. An optical frequency measuring device comprising: a frequency specifying section. The optical frequency measuring device according to claim 1 or 2 ,
further comprising a duty ratio reduction unit that outputs a second optical pulse train having a repetition frequency frep/D by reducing the duty ratio of the first optical pulse train to 1/D (D is an integer of 2 or more),
The optical frequency measuring device is characterized in that the pulse width compression section compresses the pulse width of the second optical pulse train using a dispersion compensating means and outputs the third optical pulse train, which is made into a short pulse. The optical frequency measuring device according to claim 1 or 2 ,
The setting signal output from the second signal generator and the offset signal indicating the offset frequency fceo are phase-compared, and the repetition frequency frep is stabilized by feedback control based on the comparison result obtained. An optical frequency measuring device further comprising a feedback control unit that stabilizes the frequency of a seed light source.

Images