Optical comb control device and optical comb control method

Abstract

An optical comb control device (100) comprises: an optical comb generation unit (1) that generates an optical comb; an optical multiplexing unit (4) that multiplexes the optical comb and reference light having a prescribed reference wavelength; a light detection unit (5) that detects light multiplexed by the optical multiplexing unit (4) to detect a beat frequency between the optical comb and the reference light and a repetition frequency of the optical comb; a first control unit (6) that inputs the beat frequency detected by the light detection unit (5) and controls the repetition frequency such that the beat frequency is stabilized to a constant value; and a second control unit (7) that inputs the repetition frequency detected by the light detection unit (5) and controls an offset frequency of the optical comb such that the repetition frequency is stabilized to a constant value.

Description

Optical comb control device and optical comb control method

[0001] The present disclosure relates to an optical comb control device and an optical comb control method.

[0002] An optical comb (optical frequency comb) is an ultrashort pulse laser having multiple frequency modes (longitudinal modes) arranged at equal intervals like a comb on the frequency axis. ｒｅｐ and offset frequency f ＣＥＯ Specifically, the nth mode frequency f ｎ is "f ｎ = n × f ｒｅｐ +f ＣＥＯ The optical comb is used, for example, to evaluate the absolute frequency and stability of an optical atomic clock (see, for example, Non-Patent Document 1).

[0003] In the method disclosed in Non-Patent Document 1, an optical comb for evaluating an optical atomic clock (i.e., a reference optical atomic clock of the same type as the optical atomic clock to be evaluated) is set to a frequency (f ｒｅｐ and f ＣＥＯ To obtain a stabilized optical frequency comb (f ), the following process is performed: First, the two parameters of the optical frequency comb (f ｒｅｐ、 f ＣＥＯ ) is measured, and the offset frequency f ＣＥＯ Then, the beat frequency f between the reference light output from the reference optical atomic clock and the optical comb is stabilized at a constant value. ｂｅａｔ The repetition frequency f ｒｅｐ This controls the repetition frequency f of the optical comb. ｒｅｐ is controlled by the stability of the reference light, and an optical comb that can be used to evaluate similar optical atomic clocks is obtained.

[0004] Yoshiaki Nakajima, Hajime Inaba, Kazumoto Hosaka, Kaoru Minoshima, Atsushi Onae, Masami Yasuda, Takuya Kohno, Sakae Kawato, Takao Kobayashi, Toshio Katsuyama, and Feng-Lei Hong, “A multi-branch, fiber-based frequency comb with millihertz-level relative linewidths using an intra-cavity electro-optic modulator”, Optics Express Vol. 18, Issue 2, pp. 1667-1676 (2010).

[0005] According to the conventional method described above, an optical comb can be obtained that can output microwaves controlled by the stability of the reference light. However, the above method requires a large offset frequency f ＣＥＯ Based on the measurement results, the offset frequency f ＣＥＯ The offset frequency f of the optical comb is stabilized to a constant value. ＣＥＯ In order to measure this, it is generally necessary to construct a 1f-2f interferometer with a complex optical configuration. Furthermore, constructing a 1f-2f interferometer requires rare components such as highly nonlinear fibers and wavelength conversion crystals. Highly nonlinear fibers, in particular, are custom-made and manufactured by only a few fiber manufacturers, making them difficult to obtain.

[0006] Therefore, an object of the present disclosure is to provide an optical comb control device and an optical comb control method that can obtain an optical comb that outputs microwaves controlled by the stability of reference light with a simple and inexpensive configuration.

[0007] The present disclosure includes the following optical comb control devices [1] to [7] and optical comb control methods [8].

[0008] [1] An optical comb control device comprising: an optical comb generator that generates an optical comb having a plurality of frequency modes arranged in a comb shape on a frequency axis; an optical combiner that combines the optical comb with reference light having a predetermined reference wavelength; an optical detector that detects a beat frequency between the optical comb and the reference light and a repetition frequency of the optical comb by detecting the light combined by the optical combiner; a first controller that inputs the beat frequency detected by the optical detector and controls the repetition frequency so that the beat frequency is stabilized at a constant value; and a second controller that inputs the repetition frequency detected by the optical detector and controls an offset frequency of the optical comb so that the repetition frequency is stabilized at a constant value.

[0009] The optical comb control device of [1] above includes a first control unit that controls the repetition frequency so that the beat frequency is stabilized at a constant value, and a second control unit that controls the offset frequency so that the repetition frequency is stabilized at a constant value. By controlling the repetition frequency with the first control unit and the offset frequency with the second control unit, it is possible to obtain a frequency (repetition frequency and offset frequency) stabilized to a constant value based on the reference light. Furthermore, with this control, it is not necessary to stabilize the offset frequency to a constant value in advance, so there is no need to construct a 1f-2f interferometer to measure the offset frequency. Therefore, with the optical comb control device, it is possible to obtain an optical comb that outputs microwaves controlled to the stability of the reference light with a simple and inexpensive configuration.

[0010] [2] The optical comb control device of [1], wherein the control of the offset frequency by the second control unit is executed after the control of the repetition frequency by the first control unit is completed.

[0011] According to the configuration [2] above, the first control unit stabilizes the beat frequency to a constant value, and then the second control unit controls the offset frequency so that the repetition frequency remains constant. This allows for smoother stabilization of the optical comb parameters (repetition frequency and offset frequency). More specifically, by first stabilizing the beat frequency using the first control unit, the main cause of fluctuations in the repetition frequency can be limited to the offset frequency. By executing control using the second control unit in this state, the repetition frequency can be stabilized to a constant value, and the offset frequency can ultimately be stabilized to a constant value. As a result, an optical comb whose repetition frequency and offset frequency are stabilized based on the reference light can be efficiently obtained.

[0012] [3] The optical comb control device of [1] or [2], further comprising a wavelength conversion unit disposed between the optical comb generation unit and the optical multiplexing unit, for changing the wavelength of the optical comb.

[0013] According to the configuration [3], it is possible to execute the control of the first and second control units described above for reference light of various reference wavelengths. That is, when the reference wavelength of the reference light is not included in the wavelength range of the optical comb output from the optical comb generator, the wavelength of the optical comb is changed by the wavelength converter to shift the wavelength range of the optical comb, thereby causing interference between the optical comb and the reference light and enabling the appropriate generation of a beat frequency.

[0014] [4] The optical comb control device according to any one of [1] to [3], wherein the optical comb generation unit has a pumping laser that outputs pumping light and a laser resonator that receives the pumping light from the pumping laser and generates the optical comb, and the first control unit controls the repetition frequency by controlling the resonator length of the laser resonator.

[0015] According to the configuration [4] above, by adjusting the resonator length of the laser resonator included in the optical comb generator, the control of the first controller (control of the repetition frequency) can be easily performed.

[0016] [5] The optical comb control device of [4], wherein the first control unit feedback controls the resonator length of the laser resonator so that a difference signal value corresponding to the frequency difference between the beat frequency detected by the optical detection unit and a first reference wave signal having a predetermined reference frequency becomes zero.

[0017] According to the configuration [5] above, the control of the first control unit (control of the repetition frequency) can be easily and reliably performed by feedback control based on the beat frequency and the first reference wave signal.

[0018] [6] The optical comb control device according to any one of [1] to [5], wherein the optical comb generating unit has a pumping laser that outputs pumping light, and a laser resonator that receives the pumping light from the pumping laser and generates the optical comb, and the second control unit controls the offset frequency by controlling the pumping light intensity of the pumping laser.

[0019] According to the configuration [6] above, the control of the second control unit (control of the offset frequency) can be easily performed by adjusting the excitation light intensity of the excitation laser included in the optical comb generator.

[0020] [7] The optical comb control device of [6], wherein the second control unit feedback controls the excitation light intensity of the excitation laser so that a difference signal value corresponding to the frequency difference between the repetition frequency detected by the optical detection unit and a second reference wave signal having a predetermined reference frequency becomes zero.

[0021] According to the configuration [7] above, the control of the second control unit (control of the offset frequency) can be easily and reliably performed by feedback control based on the repetition frequency and the second reference wave signal.

[0022] [8] An optical comb control method including: a first step of generating an optical comb having a plurality of frequency modes arranged in a comb shape on a frequency axis; a second step of multiplexing the optical comb with reference light having a predetermined reference wavelength; a third step of detecting a beat frequency between the optical comb and the reference light and a repetition frequency of the optical comb by detecting the light multiplexed in the second step; a fourth step of inputting the beat frequency detected in the third step and controlling the repetition frequency so that the beat frequency is stabilized at a constant value; and a fifth step of inputting the repetition frequency detected in the third step and controlling an offset frequency of the optical comb so that the repetition frequency is stabilized at a constant value.

[0023] The optical comb control method [8] above includes a fourth step of controlling the repetition frequency so that the beat frequency is stabilized at a constant value, and a fifth step of controlling the offset frequency so that the repetition frequency is stabilized at a constant value. By controlling the repetition frequency and the offset frequency in this manner, it is possible to obtain a frequency (repetition frequency and offset frequency) stabilized to a constant value based on the reference light. Furthermore, with this control, it is not necessary to stabilize the offset frequency to a constant value in advance, so it is not necessary to construct a 1f-2f interferometer to measure the offset frequency. Therefore, with the optical comb control method, it is possible to obtain an optical comb that outputs microwaves controlled by the stability of the reference light with a simple and inexpensive configuration.

[0024] According to the present disclosure, it is possible to provide an optical comb control device and an optical comb control method that can obtain an optical comb that outputs microwaves controlled by the stability of reference light with a simple and inexpensive configuration.

[0025] Fig. 1 is a block diagram showing an example of the system configuration of an optical comb control device. Fig. 2 is a diagram for explaining an optical comb. Fig. 3 shows a configuration of a frequency f of a reference light. ｏｐｔ , frequency f ｏｐｔ The nth mode frequency f of the optical comb closest to ｎ , and the beat frequency f ｂｅａｔ4 is a flowchart showing an example of the operation of the optical comb control device (optical comb control method).

[0026] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and duplicated description will be omitted.

[0027] 1, the optical comb control device 100 according to this embodiment includes an optical comb generator 1, an optical atomic clock 2, a wavelength converter 3, an optical multiplexer 4, an optical detector 5, a first controller 6, a second controller 7, a laser power supply 8, and a third controller 9. The optical comb control device 100 generates a microwave (repetition frequency f ) controlled by the stability (frequency stability) of the reference light L2 output from the optical atomic clock 2. ｒｅｐ ) can be output. Such an optical comb can be used to evaluate the optical atomic clock (evaluate its stability).

[0028] An optical comb is a frequency-controlled ultrashort pulse laser (mode-locked laser). As shown in Figure 2, when viewed on the time axis (time domain), an optical comb is represented as an ultrashort pulse train (upper part of Figure 2). By performing a Fourier transform on the ultrashort pulse train that constitutes an optical comb, an optical spectrum (lower part of Figure 2) with equally spaced frequency modes (longitudinal modes) is obtained. In other words, when viewed on the frequency axis (frequency domain), an optical comb is represented as a comb-like optical spectrum.

[0029] The optical comb has a repetition frequency of f ｒｅｐ and offset frequency f ＣＥＯ More specifically, the m-th mode frequency f of the optical comb is ｍ is expressed by the following formula (1).

[0030] f ｍ = m × f ｒｅｐ +f ＣＥＯ …(1)

[0031] As described above, the optical comb generator 1 generates an optical comb L1 having a plurality of frequency modes arranged in a comb-like pattern on the frequency axis. The optical comb generator 1 includes, for example, a pump laser 11 that outputs pump light, and a laser resonator 12 that receives the pump light from the pump laser 11 and generates an optical comb.

[0032] The pump laser 11 is, for example, a semiconductor laser with a wavelength of 980 nm. The pump laser 11 receives a drive current from the laser power supply 8 and outputs pump light. The intensity of the pump light from the pump laser 11 depends on the magnitude of the drive current supplied from the laser power supply 8. In other words, the intensity of the pump light from the pump laser 11 can be controlled by adjusting the magnitude of the drive current from the laser power supply 8.

[0033] The laser resonator 12 may be configured, for example, by an fs laser resonator that outputs femtosecond pulses. The laser resonator 12 may be configured, for example, by a mode-locked fiber laser. Examples of gain media for mode-locked fiber lasers include erbium (Er) with a center wavelength of 1550 nm, ytterbium (Yb) with a center wavelength of 1030 nm, thulium (Tm) with a center wavelength of 1900 nm, and niodymium (Nd) with a center wavelength of 1030 nm. However, the laser resonator 12 is not limited to the above-described mode-locked fiber laser. For example, the laser resonator 12 may be a titanium sapphire solid-state laser (Ti:S) with a center wavelength of 800 nm. The laser resonator 12 oscillates upon receiving pumping light from the pump laser 11.

[0034] The laser resonator 12 includes a resonator length adjuster 13 for changing the resonator length. The resonator length adjuster 13 may be configured, for example, by a voltage-driven piezoelectric element, an electro-optic modulator (EOM), or the like. Preferably, the resonator length adjuster 13 is configured by an EOM that can be controlled at higher speeds. The resonator length adjuster 13 is configured to change the resonator length of the laser resonator 12 based on an electrical signal (control signal) output from a signal control unit 63 of the first control unit 6, which will be described later.

[0035] The optical atomic clock 2 outputs a reference light L2 having a reference wavelength. As an example, the optical atomic clock 2 is a laser that outputs the reference light L2 with a wavelength of 780 nm stabilized in Rb gas. Other examples of the optical atomic clock 2 include optical lattice clocks using strontium (Sr) and ytterbium (Yb), and ion trap optical clocks using mercury ions.

[0036] The wavelength converter 3 is disposed between the optical comb generator 1 and the optical multiplexer 4 and converts the wavelength of the optical comb L1 output from the optical comb generator 1 (i.e., the fs pulse output from the laser resonator 12). The wavelength converter 3 is composed of, for example, a BBO crystal, a periodically poled niobate (PPLN) crystal, or the like. Preferably, the wavelength converter 3 is composed of a PPLN crystal, which has relatively high wavelength conversion efficiency. The wavelength converter 3 shifts the wavelength range of the optical comb L1 so that the reference wavelength of the reference light L2 (780 nm in this embodiment) is included in the wavelength range of the optical comb L1. Note that if the original wavelength range of the optical comb L1 output from the optical comb generator 1 includes the reference wavelength of the reference light L2, the wavelength converter 3 may be omitted.

[0037] The optical multiplexer 4 multiplexes the optical comb L1 and the reference light L2. The optical multiplexer 4 is composed of, for example, a beam splitter, a fiber coupler, etc. The optical comb L1 is optically mixed with the reference light L2 output from the optical atomic clock 2 via the optical multiplexer 4.

[0038] The optical detector 5 detects the light (optical mixing output) combined by the optical combiner 4. The optical detector 5 is configured with, for example, a photodetector. By detecting the optical mixing output from the optical combiner 4, the optical detector 5 detects the beat frequency f between the optical comb L1 and the reference light L2. ｂｅａｔ and the repetition frequency f of the optical comb L1 ｒｅｐ The beat frequency f detected by the light detection unit 5 is ｂｅａｔ is the repetition frequency f ｒｅｐ The repetition frequency f detected by the light detection unit 5 is output to the first control unit 6 for feedback control. ｒｅｐ is taken out and used, and the offset frequency f ＣＥＯThe signal is output to the second control unit 7 for feedback control.

[0039] Beat frequency f ｂｅａｔ is the beat frequency generated by the interference of two waves (the optical comb L1 and the reference light L2) with slightly different frequencies. ｏｐｔ The n-th mode frequency f of the optical comb L1 (in this embodiment, the optical comb L1 after wavelength conversion by the wavelength conversion unit 3) ｎ That is, in the example of FIG. 3, the n-th mode frequency f ｎ is the reference frequency f ｏｐｔ The reference frequency f ｏｐｔ and beat frequency f ｂｅａｔ The relationship of the following formula (2) holds between the reference frequency f ｏｐｔ is the mode frequency f ｎ Since it is slightly smaller than f in the following formula (2), ｂｅａｔ The sign of is negative, but the reference frequency f ｏｐｔ is the mode frequency f ｎ If the value is slightly larger than f in the following formula (2), ｂｅａｔ In this embodiment, as an example, the reference frequency f ｏｐｔ and beat frequency f ｂｅａｔ The control is performed so that the relationship shown in FIG. 3 and equation (2) holds between them.

[0040] f ｏｐｔ = n × f ｒｅｐ +f ＣＥＯ -f ｂｅａｔ …(2)

[0041] The first control unit 6 detects the beat frequency f ｂｅａｔ and input the beat frequency f ｂｅａｔ (That is, the beat frequency f ｂｅａｔ The repetition frequency f of the optical comb L1 is set so that the detected value of ｒｅｐ Control.

[0042] In this embodiment, the first control unit 6 controls the resonator length of the laser resonator 12 to adjust the repetition frequency f ｒｅｐ As an example, the first control unit 6 controls the beat frequency f detected by the light detection unit 5. ｂｅａｔ and a first reference wave signal S having a predetermined reference frequency f1. ｆ１ The resonator length of the laser resonator 12 is feedback-controlled so that a difference signal value VD1 corresponding to the frequency difference between the reference oscillator 61 and the reference oscillator 12 becomes 0. The first control unit 6 includes a reference oscillator 61, a frequency comparison unit 62, and a signal control unit 63 as elements for performing the feedback control.

[0043] The reference oscillator 61 generates the first reference wave signal S ｆ１ For example, the reference oscillator 61 uses a frequency reference oscillator (for example, a hydrogen maser, a Rb microwave oscillator, or the like) to generate a first reference wave signal S from an arbitrary waveform generator (for example, a function generator, or the like). ｆ１ In this embodiment, the beat frequency f ｂｅａｔ In this case, the first reference wave signal S fluctuates around 10 MHz before stabilization. ｆ１ The reference frequency f1 is set to, for example, 10 MHz.

[0044] The frequency comparison unit 62 compares the beat signal (beat frequency f ｂｅａｔ ) and the first reference wave signal S output from the reference oscillator 61 ｆ１ and the beat signal and the first reference wave signal S ｆ１ The frequency difference (f ｂｅａｔ The frequency comparing unit 62 is configured by, for example, a double balanced mixer.

[0045] The signal control unit 63 outputs a control signal for controlling the cavity length of the laser cavity 12 so that the value of the difference signal (difference signal value VD1) output from the frequency comparison unit 62 becomes 0. The signal control unit 63 is configured by, for example, a PID controller. In this embodiment, the signal control unit 63 outputs a control signal to the cavity length adjustment unit 13. As a result, the cavity length adjustment unit 13 executes a process of changing the cavity length of the laser cavity 12 based on the control signal.

[0046] When the cavity length of the laser cavity 12 changes, the repetition frequency f of the optical comb L1 changes accordingly. ｒｅｐ Therefore, by performing the feedback control of the first control unit 6 for a certain period of time as described above, the beat frequency f ｂｅａｔ to the reference frequency f (i.e., the beat frequency f ｂｅａｔ and the reference frequency f1 can be kept within a certain range from 0.

[0047] The second control unit 7 detects the repetition frequency f ｒｅｐ and the repetition frequency f ｒｅｐ (That is, the repetition frequency f ｒｅｐ The offset frequency f of the optical comb L1 is set so that the detected value of ＣＥＯ Control.

[0048] In this embodiment, the second control unit 7 controls the excitation light intensity of the excitation laser 11 to adjust the offset frequency f ＣＥＯ As an example, the second control unit 7 controls the repetition frequency f detected by the light detection unit 5. ｒｅｐ and a second reference wave signal S having a predetermined reference frequency f2. ｆ２ The second control unit 7 feedback-controls the excitation light intensity of the excitation laser 11 so that a difference signal value VD2 corresponding to the frequency difference between the reference oscillator 71 and the frequency comparator 72 becomes 0. The second control unit 7 includes a reference oscillator 71, a frequency comparator 72, and a signal control unit 73 as elements for executing the feedback control.

[0049] The reference oscillator 71 generates the second reference wave signal S ｆ２For example, similar to the reference oscillator 61, the reference oscillator 71 uses a frequency reference oscillator (for example, a hydrogen maser, a Rb microwave oscillator, or the like) to generate a second reference wave signal S from an arbitrary waveform generator (for example, a function generator, or the like). ｆ２ In this embodiment, the repetition frequency f ｒｅｐ In this case, the second reference wave signal S fluctuates around 50 MHz before stabilization. ｆ２ The reference frequency f2 is set to, for example, 50 MHz.

[0050] The frequency comparison unit 72 compares the repetition frequency detected by the light detection unit 5 ｆｒｅｐ and the second reference wave signal S output from the reference oscillator 71 ｆ２ and enter the repetition rate ｆｒｅｐ and the second reference wave signal S ｆ２ The frequency difference (f ｒｅｐ The frequency comparing unit 72 is configured by, for example, a double balanced mixer.

[0051] The signal control unit 73 outputs a control signal for controlling the excitation light intensity of the excitation laser 11 so that the value of the difference signal (difference signal value VD2) output from the frequency comparison unit 72 becomes 0. The signal control unit 73 is configured by, for example, a PID controller. In this embodiment, the signal control unit 73 outputs a control signal to the laser power supply 8 that supplies a drive current to the excitation laser 11. As a result, based on the control signal, the magnitude of the drive current supplied from the laser power supply 8 to the excitation laser 11 changes, and the excitation light intensity of the excitation laser 11 changes.

[0052] When the excitation light intensity of the excitation laser 11 changes, the offset frequency f of the optical comb L1 changes accordingly. ＣＥＯ Therefore, by performing the feedback control of the second control unit 7 for a certain period of time as described above, the repetition frequency f ｒｅｐ to the reference frequency f2 (i.e., the repetition frequency f ｒｅｐ and the reference frequency f2 can be kept within a certain range from 0.

[0053] The third control unit 9 controls the first control unit 6 and the second control unit 7. Specifically, the third control unit 9 controls the repetition frequency f ｒｅｐ Control of beat frequency f ｂｅａｔ After the feedback control for stabilization of the offset frequency f ＣＥＯ Control of repetition frequency f ｒｅｐ The third control unit 9 controls the first control unit 6 and the second control unit 7 so that feedback control for stabilization of the vehicle speed is performed. The third control unit 9 may, for example, monitor the difference signal value VD1 detected by the first control unit 6 (signal control unit 63) and determine that the feedback control of the first control unit 6 is completed when the difference signal value VD1 remains within a certain range (within a predetermined error) from 0 for a predetermined period or longer. After determining that the feedback control of the first control unit 6 is completed, the third control unit 9 may control the operation of the second control unit 7 (signal control unit 73) so that feedback control of the second control unit 7 is executed (started). In this case, the third control unit 9 may be configured by, for example, a computer device electrically connected to the first control unit 6 (signal control unit 63) and the second control unit 7 (signal control unit 73).

[0054] According to the control of the third control unit 9 described above (i.e., the control of the execution order of the feedback control of the first control unit 6 and the second control unit 7), the repetition frequency f ｒｅｐ and offset frequency f ＣＥＯ The reason for this will be explained below.

[0055] By modifying the above formula (2), the following formulas (3) and (4) are obtained.

[0056] f ｂｅａｔ = n × f ｒｅｐ +f ＣＥＯ -f ｏｐｔ …(3) f ｒｅｐ = (f ｏｐｔ -f ＣＥＯ +f ｂｅａｔ ) / n ... (4)

[0057] Here, the reference frequency f ｏｐｔis a constant value. Therefore, as shown in the above formula (3), the beat frequency f ｂｅａｔ is the repetition frequency f ｒｅｐ and offset frequency f ＣＥＯ where the repetition frequency f ｒｅｐ is multiplied by a coefficient n, so the beat frequency f ｂｅａｔ The influence of the offset frequency f ＣＥＯ than the repetition frequency f ｒｅｐ Therefore, by executing the feedback control by the first control unit 6, the beat frequency f ｂｅａｔ The fluctuation of the offset frequency f ＣＥＯ The beat frequency f ｂｅａｔ can be stabilized.

[0058] Furthermore, as can be seen from the above formula (4), the repetition frequency f ｒｅｐ is the offset frequency f ＣＥＯ and beat frequency f ｂｅａｔ As described above, after the first control unit 6 executes the feedback control, the beat frequency f ｂｅａｔ is the offset frequency f ＣＥＯ Therefore, after the first control unit 6 executes the feedback control, the repetition frequency f ｒｅｐ The fluctuation of the offset frequency f ＣＥＯ That is, after the first control unit 6 executes the feedback control, the repetition frequency f ｒｅｐ Stabilizing to a constant value effectively stabilizes the offset frequency f ＣＥＯ Therefore, after the first control unit 6 executes the feedback control, the second control unit 7 executes the feedback control to stabilize the repetition frequency f ｒｅｐ By stabilizing the repetition frequency f ｒｅｐ and offset frequency f ＣＥＯ can be stabilized at the same time.

[0059] An example of the operation of the optical comb control device 100 (optical comb control method) will be described with reference to FIG.

[0060] In step S1 (first step), the optical comb generator 1 generates an optical comb L1 having a plurality of frequency modes arranged in a comb-like pattern on the frequency axis.

[0061] In step S2 (second step), the optical multiplexer 4 multiplexes the optical comb L1 (in this embodiment, the optical comb output after the wavelength has been converted by the wavelength converter 3) with the reference light L2 output from the optical atomic clock 2.

[0062] In step S3 (third step), the optical detector 5 detects the light combined in step S2, thereby detecting the beat frequency f between the optical comb L1 and the reference light L2. ｂｅａｔ and the repetition frequency f of the optical comb L1 ｒｅｐ and detect.

[0063] The above-described processing of steps S1 to S3 is continuously executed until the processing of the flowchart in Fig. 4 is completed. Furthermore, while the processing of steps S1 to S3 is continuously executed, a first feedback control (step S4) and a second feedback control (step S5) described below are executed in this order.

[0064] In step S4 (fourth step), the first control unit 6 calculates the beat frequency f detected in step S3. ｂｅａｔ The repetition frequency f is set to a constant value so that ｒｅｐ In this embodiment, as an example, step S4 includes the following steps S41 to S43.

[0065] In step S41, the frequency comparison unit 62 compares the beat signal (beat frequency f ｂｅａｔ ) and the first reference wave signal S output from the reference oscillator 61 ｆ１ The frequency difference (f ｂｅａｔ -f1) and outputs a difference signal according to the difference signal.

[0066] In step S42, the signal control unit 63 feedback-controls the resonator length of the laser resonator 12 so that the value of the difference signal (difference signal value VD1) output from the frequency comparison unit 62 becomes 0. In this embodiment, the signal control unit 63 outputs a control signal for the above control to the resonator length adjustment unit 13, thereby changing the resonator length of the laser resonator 12 and adjusting the repetition frequency f of the optical comb L1 so that the difference signal value VD1 approaches 0. ｂｅａｔ changes.

[0067] The processes of steps S41 and S42 are repeatedly executed until a predetermined condition is satisfied. For example, the processes of steps S41 and S42 are executed to determine the beat frequency f ｂｅａｔ is stabilized (step S43). In this embodiment, the above determination is made by the third control unit 9. As an example, the third control unit 9 determines that the beat frequency f is stabilized when the difference signal value VD1 detected by the first control unit 6 (signal control unit 63) remains within a certain range (within a predetermined error) from 0 for a predetermined period or more. ｂｅａｔ has been stabilized (in other words, the feedback control of the first control unit 6 has been completed).

[0068] In step S5 (fifth step), the second control unit 7 calculates the repetition frequency f detected in step S3. ｒｅｐ The offset frequency f is set to a constant value so that ＣＥＯ In this embodiment, as an example, step S5 includes the following steps S51 to S53.

[0069] In step S51, the frequency comparison unit 72 compares the repetition frequency f detected by the light detection unit 5 with the repetition frequency f ｒｅｐ and the second reference wave signal S output from the reference oscillator 71 ｆ２ The frequency difference (f ｒｅｐ -f2) and outputs a difference signal according to the difference signal.

[0070] In step S52, the signal control unit 73 feedback-controls the excitation light intensity of the excitation laser 11 so that the value of the difference signal (difference signal value VD2) output from the frequency comparison unit 72 becomes 0. In this embodiment, the signal control unit 73 outputs a control signal for the above control to the laser power supply 8, thereby changing the excitation light intensity of the excitation laser 11 and adjusting the offset frequency f of the optical comb L1 so that the difference signal value VD2 approaches 0. ＣＥＯ changes.

[0071] The processes of steps S51 and S52 described above are repeatedly executed until a predetermined condition is satisfied. For example, the processes of steps S51 and S52 are executed by adjusting the repetition frequency f ｒｅｐ is determined to be stabilized (step S53). In this embodiment, the above determination is made by the third control unit 9. As an example, the third control unit 9 determines whether the repetition frequency f is stabilized when the difference signal value VD2 detected by the second control unit 7 (signal control unit 73) remains within a certain range (within a predetermined error) from 0 for a predetermined period or more. ｒｅｐ has been stabilized (in other words, the feedback control by the second control unit 7 has been completed).

[0072] As described above, the first and second feedback controls (steps S4 and S5) are performed, whereby the repetition frequency f ｒｅｐ and offset frequency f ＣＥＯ That is, an optical comb output (optical comb L1) having a microwave (repetition frequency f ｒｅｐ ) is obtained.

[0073] In this embodiment, the reference frequency f ｏｐｔ is 377,107,407.299 MHz (= 794.978,969,380 nm) corresponding to the Rb absorption line (D1 line), and the reference frequency f ｏｐｔThe mode number n of the optical comb L1 closest to is adjusted to 7,542,148. The reference frequency f1 used in the first control unit 6 is set to 10 MHz, and the reference frequency f2 used in the second control unit 7 is set to 50 MHz. In this case, by executing the first and second feedback controls described above, the beat frequency f ｂｅａｔ is stabilized at 10 MHz and the repetition frequency f ｒｅｐ is stabilized to 50 MHz, resulting in an offset frequency f ＣＥＯ is also stabilized to a constant value (17.299 MHz).

[0074] The optical comb control device 100 described above uses a beat frequency f ｂｅａｔ The repetition frequency f is stabilized to a constant value. ｒｅｐ and a first control unit 6 for controlling the repetition frequency f ｒｅｐ The offset frequency f is stabilized to a constant value. ＣＥＯ The first control unit 6 controls the repetition frequency f ｒｅｐ (in this embodiment, the first feedback control in step S4) and the second control unit 7 controls the offset frequency f ＣＥＯ (in this embodiment, the second feedback control in step S5) is performed, the frequency (repetition frequency f ｒｅｐ and offset frequency f ＣＥＯ ) can be obtained. In addition, according to the above control, the offset frequency f ＣＥＯ Since there is no need to stabilize the offset frequency f ＣＥＯ Therefore, according to the optical comb control device 100, the microwave (repetition frequency f ｒｅｐ ) can be obtained with a simple and inexpensive configuration.

[0075] The optical comb control method (see FIG. 4) includes the above-mentioned steps S1 to S5. In particular, the optical comb control method includes the steps of: ｂｅａｔThe repetition frequency f is stabilized to a constant value. ｒｅｐ Step S4 (fourth step) controls the repetition frequency f ｒｅｐ The offset frequency f is stabilized to a constant value. ＣＥＯ The fifth step includes step S5 (fifth step) of controlling the repetition frequency f ｒｅｐ and also controls the offset frequency f ＣＥＯ By controlling the frequency (repetition frequency f ｒｅｐ and offset frequency f ＣＥＯ ) can be obtained. In addition, according to the above control, the offset frequency f ＣＥＯ Since there is no need to stabilize the offset frequency f ＣＥＯ Therefore, according to the optical comb control method, it is not necessary to construct a 1f-2f interferometer for measuring the repetition frequency f ｒｅｐ ) can be obtained with a simple and inexpensive configuration.

[0076] The effects of the optical comb control device 100 and the optical comb control method described above will be supplemented. ｒｅｐ and offset frequency f ＣＥＯ ) to obtain the optical comb output, the offset frequency f ＣＥＯ It is common to measure (actually measure) the value of the repetition frequency f ｒｅｐ together with the offset frequency f ＣＥＯ Measure the offset frequency f ＣＥＯ and repetition frequency f ｒｅｐ More specifically, in the conventional method, the offset frequency f is controlled to be stabilized at a constant value based on the measurement results. ＣＥＯ After stabilizing to a constant value, the repetition frequency f ｒｅｐ In this way, in the conventional method, the measurement results (especially the offset frequency f ＣＥＯIn contrast, the optical comb control device 100 of this embodiment requires a complex and expensive configuration such as a 1f-2f interferometer, because the offset frequency f ＣＥＯ Without measuring the value of ｒｅｐ By performing feedback control (feedback control of the second control unit 7) to stabilize the repetition frequency f ｒｅｐ Stabilization of offset frequency f ＣＥＯ This makes it possible to obtain an optical comb L1 controlled by the stability of the reference light L2 (i.e., an optical comb that can evaluate the stability of an optical atomic clock of the same type as the optical atomic clock 2) without using a complex and expensive optical system such as a 1f-2f interferometer.

[0077] The optical comb control device 100 controls the repetition frequency f ｒｅｐ After the control of the offset frequency f ＣＥＯ In this embodiment, the third control unit 9 controls the execution order as described above. According to the above configuration, the first control unit 6 controls the beat frequency f ｂｅａｔ After stabilizing the repetition frequency f ｒｅｐ By controlling the offset frequency so that is constant, the parameter of the optical comb (repetition frequency f ｒｅｐ and offset frequency f ＣＥＯ More specifically, as described above using the equations (3) and (4), the first control unit 6 first controls the beat frequency f ｒｅｐ By stabilizing the repetition frequency f ｒｅｐ The main fluctuation factor (factor of fluctuation) is the offset frequency f ＣＥＯ In this state, the second control unit 7 executes control to limit the repetition frequency f ｒｅｐ is stabilized to a constant value, resulting in an offset frequency f ＣＥＯ As a result, the repetition frequency f can be stabilized to a constant value based on the reference light L2. ｒｅｐ and offset frequency fＣＥＯ Therefore, the optical comb L1 with a stabilized frequency can be efficiently obtained.

[0078] The optical comb control device 100 is disposed between the optical comb generator 1 and the optical multiplexer 4 and includes a wavelength converter 3 that converts the wavelength of the optical comb L1. This configuration allows the first and second controllers 6 and 7 to control the reference light L2 at various reference wavelengths. That is, when the wavelength range of the optical comb L1 output from the optical comb generator 1 does not include the reference wavelength of the reference light L2 (780 nm in this embodiment), the wavelength converter 3 changes the wavelength of the optical comb L1 to shift the wavelength range of the optical comb L1, causing interference between the optical comb L1 and the reference light L2 and resulting in a beat frequency f ｂｅａｔ As a result, the above-described controls by the first control unit 6 and the second control unit 7 can be executed appropriately.

[0079] The first control unit 6 controls the resonator length of the laser resonator 12 to adjust the repetition frequency f ｒｅｐ According to the above configuration, the resonator length of the laser resonator 12 included in the optical comb generator 1 is adjusted, thereby controlling the first controller 6 (repetition frequency f ｒｅｐ (control of the above) can be easily performed.

[0080] The first control unit 6 detects the beat frequency f ｂｅａｔ and a first reference wave signal S having a predetermined reference frequency f1. ｆ１ The resonator length of the laser resonator 12 is feedback-controlled so that the difference signal value VD1 corresponding to the frequency difference between the beat frequency f ｂｅａｔ and the first reference wave signal S ｆ１ The control of the first control unit 6 (repetition frequency f ｒｅｐ The control of the above can be easily and reliably performed.

[0081] The second control unit 7 controls the excitation light intensity of the excitation laser 11 to adjust the offset frequency f ＣＥＯAccording to the above configuration, the control of the second control unit 7 (offset frequency f ＣＥＯ (control of the above) can be easily performed.

[0082] The second control unit 7 detects the repetition frequency f ｒｅｐ and a second reference wave signal S having a predetermined reference frequency f2. ｆ２ The pumping light intensity of the pumping laser 11 is feedback-controlled so that the difference signal value VD2 corresponding to the frequency difference between the repetition frequency f ｒｅｐ and the second reference wave signal S ｆ２ The control of the second control unit 7 (offset frequency f ＣＥＯ The control of the above can be easily and reliably performed.

[0083] Although one embodiment of the present disclosure has been described above, the present disclosure is not limited to the above-described embodiment. The materials and shapes of each component are not limited to the specific materials and shapes described above, and various materials and shapes other than those described above can be used. Furthermore, some components included in the above embodiment may be omitted or modified as appropriate, and can be arbitrarily combined with other additional components.

[0084] For example, in the above embodiment, the feedback control of the first control unit 6 is executed first, followed by the feedback control of the second control unit 7. However, the order of the controls is not limited to this example. For example, there may be a period during which the feedback control of the first control unit 6 and the feedback control of the second control unit 7 are executed simultaneously in parallel.

[0085] Furthermore, in the above embodiment, a third control unit 9 separate from the first control unit 6 and the second control unit 7 is provided as a configuration for controlling the order of the above-described controls. However, the third control unit 9 may be omitted. For example, the first control unit 6 (e.g., a computer device that performs overall control of the first control unit 6) and the second control unit 7 (e.g., a computer device that performs overall control of the second control unit 7) may be configured to be able to communicate with each other. In this case, the first control unit 6 may output a control signal to the second control unit 7 instructing the start of the second feedback control (step S5 in FIG. 4 ) after the first feedback control (step S4 in FIG. 4 ) is completed (i.e., when the determination in step S43 in FIG. 4 is "YES"). Then, the second control unit 7 may start the second feedback control in response to receiving the control signal from the first control unit 6.

[0086] In the above embodiment, the optical comb generator 1 is configured by combining the pump laser 11 and the laser resonator 12. However, the optical comb generator 1 may have a configuration other than the above as a mechanism for generating the optical comb L1. In this case, the control contents by the first control unit 6 (signal control unit 63) and the second control unit 7 (signal control unit 73) may be changed depending on the mechanism provided in the optical comb generator 1.

[0087] REFERENCE SIGNS LIST 1...optical comb generator, 3...wavelength converter, 4...optical multiplexer, 5...optical detector, 6...first controller, 7...second controller, 9...third controller, 11...excitation laser, 12...laser resonator, 100...optical comb controller, f ｂｅａｔ ...beat frequency, f ＣＥＯ ...offset frequency, f ｒｅｐ ...repetition frequency, L1...optical comb, L2...reference light, VD1, VD2...difference signal value.

Claims

1. An optical comb control device comprising: an optical comb generator that generates an optical comb having a plurality of frequency modes arranged in a comb shape on a frequency axis; an optical combiner that combines the optical comb with reference light having a predetermined reference wavelength; an optical detector that detects the beat frequency between the optical comb and the reference light and the repetition frequency of the optical comb by detecting the light combined by the optical combiner; a first controller that inputs the beat frequency detected by the optical detector and controls the repetition frequency so that the beat frequency is stabilized at a constant value; and a second controller that inputs the repetition frequency detected by the optical detector and controls the offset frequency of the optical comb so that the repetition frequency is stabilized at a constant value.

2. The optical comb control device according to claim 1, wherein the second control unit controls the offset frequency after the first control unit has completed the control of the repetition frequency.

3. The optical comb control device according to claim 1 or 2, further comprising a wavelength conversion unit disposed between the optical comb generation unit and the optical multiplexing unit, for changing the wavelength of the optical comb.

4. An optical comb control device according to any one of claims 1 to 3, wherein the optical comb generation unit has a pumping laser that outputs pumping light and a laser resonator that receives the pumping light from the pumping laser and generates the optical comb, and the first control unit controls the repetition frequency by controlling the resonator length of the laser resonator.

5. An optical comb control device as described in claim 4, wherein the first control unit feedback controls the resonator length of the laser resonator so that a difference signal value corresponding to the frequency difference between the beat frequency detected by the optical detection unit and a first reference wave signal having a predetermined reference frequency becomes zero.

6. An optical comb control device according to any one of claims 1 to 5, wherein the optical comb generating unit has a pumping laser that outputs pumping light and a laser resonator that receives the pumping light from the pumping laser and generates the optical comb, and the second control unit controls the offset frequency by controlling the pumping light intensity of the pumping laser.

7. The optical comb control device described in claim 6, wherein the second control unit feedback controls the excitation light intensity of the excitation laser so that a difference signal value corresponding to the frequency difference between the repetition frequency detected by the optical detection unit and a second reference wave signal having a predetermined reference frequency becomes zero.

8. An optical comb control method comprising: a first step of generating an optical comb having a plurality of frequency modes arranged in a comb shape on a frequency axis; a second step of multiplexing the optical comb with reference light having a predetermined reference wavelength; a third step of detecting the beat frequency between the optical comb and the reference light and the repetition frequency of the optical comb by detecting the light multiplexed in the second step; a fourth step of inputting the beat frequency detected in the third step and controlling the repetition frequency so that the beat frequency is stabilized at a constant value; and a fifth step of inputting the repetition frequency detected in the third step and controlling the offset frequency of the optical comb so that the repetition frequency is stabilized at a constant value.

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