Full-locking femtosecond optical frequency comb device

Abstract

The invention discloses a full-locking femtosecond optical frequency comb device which comprises an optical system module, a first locking module and a second locking module, and the optical system module comprises a laser and is used for generating laser. The first locking module is used for receiving the first locking control signal, sending the first locking control signal to the optical system module and adjusting the repetition frequency of the first to-be-locked laser, the first to-be-locked laser becomes third to-be-locked laser, and the third to-be-locked laser enters the second locking module; and the second locking module is used for receiving the second locking passive feedback signal, sending the second locking passive feedback signal to the optical system module and then adjusting the carrier envelope phase shift frequency of the third to-be-locked laser, so that the third to-be-locked laser becomes laser with a stable target frequency. The device has the advantages of being simple in structure, stable and reliable, can effectively solve the tight locking problem from the solid-state optical comb to the optical frequency reference, and provides an optimal scheme for optical clock comparison.

Description

Technical Field

[0001] The invention relates to the field of solid-state laser technology, and in particular to a fully locked femtosecond optical frequency comb device. Background Art

[0002] At present, there are two main types of optical combs used for optical clock comparison: solid-state optical combs and fiber optical combs. Although fiber optical combs have been widely used due to their compact structure, the strong nonlinear effect makes it extremely difficult to control the phase noise of fiber optical combs because the transmission of fiber laser pulses is bounded in the micron-level fiber core. Solid-state optical combs have low nonlinear noise and are unique in obtaining ultra-low phase noise optical frequency combs. They are the preferred solution for ultra-high frequency stability optical clock comparison. In order to obtain a self-referenced solid-state optical frequency comb with full frequency locking, there are three main challenges: the first is the measurement and locking of the carrier envelope phase shift frequency (fceo), and the second is to control the repetition frequency (frep) by accurately referencing the optical comb to the optical frequency. For challenge one, the traditional method is to compare the fceo signal with the external RF reference signal through a phase-locked loop circuit, obtain the error signal and feed it back to the pump power of the laser. Because the bandwidth of the phase-locked loop is limited by the upper energy level lifetime of the gain medium, this method is often only tens of kHz, resulting in a large amount of high-frequency phase noise that cannot be suppressed, which brings difficulties to optical frequency comparison and measurement. For challenge 2, the resonant cavity of a solid-state optical comb generally controls the cavity length through piezoelectric ceramics to achieve repetition frequency locking. Due to the low response rate of piezoelectric ceramics, this process cannot achieve residual noise of the optical comb relative to the optical frequency. In addition, when fceo and frep are locked at the same time, the pump power and cavity length tuning will have a cross-effect, making it difficult to achieve full frequency locking. Summary of the invention

[0003] In order to solve the above technical problems, the present invention proposes a stable solid-state optical comb full locking scheme, which uses a passive feedback method to lock the fceo, avoiding modulation of the pump laser under the condition of ensuring a wide servo bandwidth. The method of inserting an electro-optic modulator and a piezoelectric ceramic dual servo bandwidth in the cavity is used to lock the frep, effectively improving the locking bandwidth while increasing the long-term locking stability. The scheme has the characteristics of simple structure, stability and reliability, and can effectively solve the problem of tight locking of the solid-state optical comb to the optical frequency reference, providing a preferred solution for optical clock comparison.

[0004] The embodiment of the present invention provides a fully locked femtosecond optical frequency comb device, comprising: an optical system module, a first locking module, and a second locking module.

[0005] The optical system module includes a laser for generating laser light, wherein the laser light outputs a first laser light to be locked after the repetition frequency is adjusted by the first locking module; further includes a first optical path auxiliary unit, wherein the first laser light to be locked outputs a second laser light to be locked after passing through the first optical path auxiliary unit; further includes a second optical path auxiliary unit, wherein the second laser light to be locked is converted into a first locking control signal and a second locking passive feedback signal after passing through the second optical path auxiliary unit;

[0006] The first locking module is used to receive the first locking control signal, send the first locking control signal to the optical system module, adjust the repetition frequency of the first laser to be locked, so that the first laser to be locked becomes the third laser to be locked, and the third laser to be locked enters the second locking module;

[0007] The second locking module is used to receive the second locking passive feedback signal, send the second locking passive feedback signal to the optical system module, and then adjust the carrier envelope phase shift frequency of the third laser to be locked, so that the third laser to be locked becomes a laser with a stable target frequency.

[0008] In some embodiments, the laser includes a pump laser, a pump focusing mirror, a laser crystal, and a laser resonant cavity including a piezoelectric ceramic drive mirror and an output coupling mirror. The pump laser emits laser light which is focused by the pump focusing mirror to the laser crystal and incident on the laser resonant cavity. The laser light is then divided into two laser beams. The first laser beam is incident on the piezoelectric ceramic drive mirror, and the second laser beam is output through the output coupling mirror and is called the first laser to be locked.

[0009] In some embodiments, the first optical path auxiliary unit includes a first reflector, a highly nonlinear optical fiber, and an acousto-optic modulator arranged along the optical path. The first laser to be locked is incident on the first reflector, passes through the highly nonlinear optical fiber and is incident on the acousto-optic modulator, and then outputs a second laser to be locked.

[0010] In some embodiments, the second optical path auxiliary unit includes an acousto-optic modulator, an f-2f interferometer, a beam splitter, an avalanche photodiode, a polarization beam splitter, an optical target frequency reference source and a photodetector arranged along the optical path. The second laser to be locked outputted from the acousto-optic modulator enters the beam splitter after passing through the f-2f interferometer. The beam splitter divides the second laser to be locked into a third laser beam and a fourth laser beam. After the third laser beam enters the polarization beam splitter, it beats with the optical signal of the optical target frequency reference source. After the beat light outputted from the polarization beam splitter enters the photodetector, a first locking control signal is outputted; the fourth laser beam enters the avalanche photodiode to output a second locking passive feedback signal.

[0011] In some embodiments, the first locking module includes a photodetector, a radio frequency reference source, an electronic control loop, a piezoelectric ceramic drive mirror and an electro-optic modulator, wherein the electro-optic modulator is inserted into the laser resonant cavity and arranged on the optical path before the output coupling mirror; the photodetector outputs a first locking control signal which is mixed with a signal of the radio frequency reference source, and issues an instruction to control the piezoelectric ceramic drive mirror through the electronic control loop to adjust the length of the laser resonant cavity; at the same time, an instruction to control the electro-optic modulator is also issued to adjust the refractive index of the first laser to be locked in the electro-optic modulator before it exits the laser resonant cavity; after adjusting the repetition frequency of the first laser to be locked by adjusting the length and refractive index of the laser resonant cavity, the third laser to be locked is output from the output coupling mirror.

[0012] In some embodiments, the second locking module includes an acousto-optic modulator and an avalanche photodiode, the acousto-optic modulator receives the second locking passive feedback signal output by the avalanche photodiode, and adjusts the carrier-envelope phase shift frequency of the third laser to be locked, so that a laser with a stable target frequency is finally output from the acousto-optic modulator.

[0013] In some embodiments, the second locking module further includes a bandpass filter, a local frequency synthesizer and a multi-stage RF amplification circuit. The second locked passive feedback signal output by the avalanche photodiode is filtered by a bandpass filter and mixed with an external reference signal emitted by the local frequency synthesizer to generate a mixed signal. The mixed signal is then amplified by a multi-stage RF amplification circuit to amplify the mixed signal containing carrier envelope phase shift frequency change information to the driving frequency requirement of the acousto-optic modulator, so as to drive the acousto-optic modulator to adjust the carrier envelope phase shift frequency of the third laser to be locked.

[0014] In some embodiments, the photodetector is a visible light band photodetector.

[0015] In some embodiments, the RF reference source is a hydrogen clock signal source.

[0016] In some embodiments, the AOM is a 1 micron band AOM.

[0017] The technical solution provided by the embodiments of the present invention brings at least the following beneficial effects:

[0018] Different from the previous scheme of using simultaneous feedback control of pump power and cavity length to achieve full locking, the passive feedback method is used to lock the fceo. Under the condition of ensuring a wide servo bandwidth, there is no need to modulate the pump laser, avoiding the cross-influence of the simultaneous action of pump power and cavity length tuning, effectively improving the locking bandwidth while increasing long-term locking stability; at the same time, the method of inserting an electro-optic modulator and piezoelectric ceramic dual servo bandwidth in the cavity is used to lock the frep, effectively improving the locking bandwidth while increasing long-term locking stability. This scheme has the characteristics of simple structure, stability and reliability, and can effectively solve the problem of tight locking of solid-state optical comb to optical frequency reference, providing a low phase noise optical frequency comb for high-performance comparison of optical clocks, and the fully locked femtosecond optical frequency comb device provided has the advantages of low noise, excellent frequency stability, and stable and reliable system device, which really provides an optimal solution for optical clock comparison.

[0019] It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a structural schematic diagram of the device of the present invention;

[0021] Figure 2 Schematic diagram of the photoelectric working principle of the device of the present invention.

[0022] Among them, 1-pump laser, 2-pump focusing mirror, 3-laser crystal, 4-first concave mirror, 5-second concave mirror, 6-second reflector, 7-piezoelectric ceramic drive mirror, 8-electro-optic modulator, 9-output coupling mirror, 10-first reflector, 11-high nonlinear optical fiber, 12-acoustic-optic modulator, 13-f-2f interferometer, 14-avalanche photodiode, 15-beam splitter, 16-polarization beam splitter, 17-optical target frequency reference source (i.e., optical clock), 18-photodetector, 19-radio frequency reference source, 20-electronic control loop, 21-laser with stable target frequency, 100-first locking module, 200-second locking module, 300-optical system module, 301-first optical path auxiliary unit, 302-second optical path auxiliary unit, 303-laser. Figure 2 In the figure, the solid line represents the optical signal line, and the short dashed line represents the RF signal line. DETAILED DESCRIPTION

[0023] The embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0024] This embodiment provides a fully locked femtosecond optical frequency comb device, such as Figure 1As shown, it includes: an optical system module, a first locking module, and a second locking module. Specifically, the first locking module is a bidirectional servo bandwidth to repetition frequency (frep) locking module, and the second locking module is a passive feedback to carrier envelope phase shift frequency (fceo) locking module; the optical system module includes a laser, which is used to generate laser, and the laser outputs a first laser to be locked after the repetition frequency is adjusted by the first locking module;

[0025] The laser includes a pump laser, a pump focusing mirror, a laser crystal, and a laser resonant cavity including a piezoelectric ceramic drive mirror and an output coupling mirror. Specifically, the first concave mirror, the second concave mirror, the second reflecting mirror, the piezoelectric ceramic drive mirror, and the output coupling mirror constitute the laser resonant cavity. After the pump laser emits laser light and enters the laser resonant cavity through the pump focusing mirror, it is divided into two laser beams. The first laser beam enters the piezoelectric ceramic drive mirror, and the second laser beam is output through the output coupling mirror and is called the first laser to be locked.

[0026] It also includes a first optical path auxiliary unit, and the first laser to be locked outputs a second laser to be locked after passing through the first optical path auxiliary unit; the first optical path auxiliary unit includes a first reflector, a highly nonlinear optical fiber and an acousto-optic modulator arranged along the optical path, and the highly nonlinear optical fiber is used to generate an octave spectrum for use by the f-2f interferometer, so as to detect fceo; the first laser to be locked is incident on the first reflector, and after passing through the highly nonlinear optical fiber and incident on the acousto-optic modulator, the second laser to be locked is output; the acousto-optic modulator receives a frequency signal (a second locked passive feedback signal) and modulates the laser frequency;

[0027] It also includes a second optical path auxiliary unit, and the second laser to be locked is converted into a first locking control signal and a second locking passive feedback signal after passing through the second optical path auxiliary unit; the second optical path auxiliary unit includes an acousto-optic modulator, an f-2f interferometer, a beam splitter, an avalanche photodiode, a polarization beam splitter, an optical target frequency reference source and a photodetector arranged along the optical path; the second laser to be locked output from the acousto-optic modulator is incident on the beam splitter after passing through the f-2f interferometer, and the beam splitter splits the second laser to be locked into a third laser beam and a fourth laser beam; the third laser beam enters the polarization beam splitter and beats with the optical signal of the optical target frequency reference source to generate a beat frequency optical signal; the optical signal emitted by the optical target frequency reference source is used as a locking reference for the beat frequency optical signal; the beat frequency optical signal output from the polarization beam splitter is incident on the photodetector to output the first locking control signal; the fourth laser beam enters the avalanche photodiode to output the second locking passive feedback signal.

[0028] The first locking module is used to receive the first locking control signal, send the first locking control signal to the optical system module, adjust the length of the laser resonant cavity and adjust the frequency of the electro-optic modulator inserted into the laser resonant cavity, and finally adjust the repetition frequency of the first laser to be locked, so that the first laser to be locked becomes the third laser to be locked; the third laser to be locked enters the second locking module; specifically, the first locking module includes a photodetector, a radio frequency reference source, an electronic control loop, a piezoelectric ceramic drive mirror and an electro-optic modulator, the electro-optic modulator is inserted into the laser resonant cavity and arranged on the optical path before the output coupling mirror; the photodetector outputs the first locking control signal and mixes it with the signal of the radio frequency reference source, and issues an instruction to control the piezoelectric ceramic drive mirror through the electronic control loop to adjust the length of the laser resonant cavity; at the same time, an instruction to control the electro-optic modulator is also issued to adjust the refractive index of the first laser to be locked in the electro-optic modulator before it is emitted from the laser resonant cavity; after adjusting the repetition frequency of the first laser to be locked by adjusting the length and refractive index of the laser resonant cavity, the third laser to be locked is output from the output coupling mirror;

[0029] The second locking module is used to receive the second locking passive feedback signal, send the second locking passive feedback signal to the optical system module, and then adjust the carrier envelope phase shift frequency of the third laser to be locked, so that the third laser to be locked becomes a laser with a stable target frequency; specifically, the second locking module includes an acousto-optic modulator and an avalanche photodiode. The acousto-optic modulator directly receives the second locking passive feedback signal output by the avalanche photodiode, which is simple and direct, and does not require the use of PID. The carrier envelope phase shift frequency of the third laser to be locked is adjusted, so that the laser with a stable target frequency is finally output from the acousto-optic modulator; the second locking module also includes a bandpass filter, a local frequency synthesizer and a multi-stage radio frequency amplification circuit. The second locking passive feedback signal output by the avalanche photodiode is filtered by a bandpass filter, mixed with an external reference signal emitted by the local frequency synthesizer to generate a mixed signal, and the mixed signal is then amplified by a multi-stage radio frequency amplification circuit to amplify the mixed signal containing the carrier envelope phase shift frequency change information to the driving frequency requirement of the acousto-optic modulator, so as to drive the acousto-optic modulator to adjust the carrier envelope phase shift frequency of the third laser to be locked.

[0030] In this embodiment, the photodetector is a visible light band photodetector.

[0031] In this embodiment, the radio frequency reference source is a hydrogen clock signal source.

[0032] In this embodiment, the AOM is a 1 micron band AOM.

[0033] Based on the fully locked femtosecond optical frequency comb device provided in this embodiment, its optoelectronic working principle is as follows: Figure 2As shown, the working process specifically includes: the pump laser source emits a pump laser, the pump laser is focused onto the laser crystal through a pump focusing mirror, and the optical laser resonant cavity composed of a second concave mirror, a second reflector, a piezoelectric ceramic drive mirror, and an output coupling mirror is used to realize femtosecond laser output. The output femtosecond laser enters the high nonlinear optical fiber through the first reflector to generate a supercontinuum spectrum. The supercontinuum spectrum generates 0th order diffraction light and 1st order diffraction light through an acousto-optic modulator. The 0th order diffraction spectrum is detected by the f-2f interferometer 13 and the avalanche photodiode 14 to realize the carrier envelope phase shift frequency fceo signal detection, and the signal is used to control the acousto-optic modulator through the second locking passive feedback signal to realize the carrier envelope phase shift frequency fceo signal locking. The 0th order diffraction light is reflected by the third laser beam of the beam splitter and the optical target frequency reference source to generate a beat frequency optical signal (i.e., the first locking control signal), and the beam is combined by a polarization beam splitter, and the generated combined beam optical signal is detected by a photodetector. The beat frequency optical signal and the signal emitted by the RF reference source are passed through an electronic control loop to realize error signal generation and feedback control, and the beat frequency signal is stabilized by regulating the piezoelectric ceramic drive mirror and the electro-optic modulator.

[0034] The principle of locking the FCEO using the passive feedback method is described in detail as follows:

[0035] The carrier-envelope phase-shift frequency signal obtained by the f-2f interferometer through the avalanche photodiode detection is filtered through a bandpass filter, mixed with the external reference signal sent by the local frequency synthesizer, and then amplified by a multi-stage RF amplifier circuit to the driving frequency requirement of the acousto-optic modulator. It is directly used to drive the acousto-optic frequency shifter (i.e., acousto-optic modulator). The acousto-optic frequency shifter works under the drive of the RF frequency. Through the spectrum therein, the acousto-optic effect occurs with the sound wave in the acousto-optic crystal. The supercontinuum spectrum diffracts into 0th order light and 1st order light. The 1st order light deviates from the 0th order optical path by a certain angle, and the optical frequency also shifts by a certain amount. The offset is exactly equal to the RF driving frequency. Therefore, the carrier-envelope phase-shift frequency of the 1st order laser will be stabilized near a certain frequency by subtracting the carrier-envelope phase-shift frequency of the 0th order light and its noise, so as to achieve the purpose of locking the carrier-envelope phase shift (fceo). The 1st order laser is Figure 2 The line with the downward arrow in the middle represents the stable target frequency of the laser.

[0036] The principle of locking the FREEP by inserting an electro-optic modulator into the cavity and the dual servo bandwidth of piezoelectric ceramics is also described in detail as follows:

[0037] The beat signal (i.e., the first locking control signal) and the signal emitted by the RF reference source are controlled by an electronic control loop to realize error signal generation and feedback control, and the beat signal is stabilized by regulating the piezoelectric ceramic drive mirror and the electro-optic modulator. It is well known in the art that the repetition frequency frep = c / 2nL, where c is the speed of light, L is the cavity length of the laser resonant cavity, and n is the refractive index of the electro-optic modulator device. From the formula, it can be seen that there are two factors n and L for adjusting the repetition frequency frep: ① The working principle of piezoelectric ceramics is to change the length of the piezoelectric ceramics by changing the driving voltage. The piezoelectric ceramic drive mirror composed of a reflector attached to the piezoelectric ceramic can realize the regulation of the cavity length L; ② By inserting an electro-optic modulator in the laser resonant cavity, the refractive index n of the electro-optic modulator device is adjusted by changing the voltage of the electro-optic modulator, thereby realizing the regulation and locking of the repetition frequency.

[0038] The beat signal is the beat signal between the optical clock signal and the optical comb, i.e., the output signal (the third laser signal to be locked) of a certain optical comb tooth (i.e., the acousto-optic modulator) of the device, and is expressed as fbeat=fopt-fn=fopt-nfrep-fceo, wherein fopt is the frequency of the optical clock, i.e., the optical target frequency reference source. The full phase locking of the optical comb and the optical clock can be achieved by regulating and locking the repetition frequency frep and the carrier envelope phase shift fceo.

[0039] The electronic control loop includes a phase detector, a PID circuit, and an electro-optic modulator. The specific locking process of the electronic control loop is as follows: the beat frequency signal (the signal detected by the photodetector) and the signal emitted by the external reference RF signal source are phase-detected by the phase detector to obtain an error signal, and the error signal is passed through the PID circuit to obtain a feedback control signal. The high-frequency part of the feedback control signal is applied to the driving voltage of the electro-optic modulator to achieve a control bandwidth of more than 1MHz. The low-frequency part of the feedback control signal is applied to the driving voltage of the piezoelectric ceramic driving mirror to achieve a wide range of cavity length control.

Claims

1. A fully locked femtosecond optical frequency comb device, characterized in that: include: The optical system module, the first locking module and the second locking module are characterized in that: The optical system module includes a laser for generating laser light, wherein the laser light outputs a first laser light to be locked after the repetition frequency is adjusted by the first locking module; further includes a first optical path auxiliary unit, wherein the first laser light to be locked outputs a second laser light to be locked after passing through the first optical path auxiliary unit; further includes a second optical path auxiliary unit, wherein the second laser light to be locked is converted into a first locking control signal and a second locking passive feedback signal after passing through the second optical path auxiliary unit; The first locking module is used to receive the first locking control signal, send the first locking control signal to the optical system module, adjust the repetition frequency of the first laser to be locked, the first laser to be locked becomes the third laser to be locked, and the third laser to be locked enters the second locking module; The second locking module is used to receive the second locking passive feedback signal, send the second locking passive feedback signal to the optical system module, and then adjust the carrier envelope phase shift frequency of the third laser to be locked, so that the third laser to be locked becomes a laser with a stable target frequency.

2. The fully locked femtosecond optical frequency comb device according to claim 1, characterized in that: The laser comprises a pump laser, a pump focusing mirror, a laser crystal, and a laser resonant cavity comprising a piezoelectric ceramic drive mirror and an output coupling mirror. The pump laser emits laser light which is focused by the pump focusing mirror to the laser crystal and incident on the laser resonant cavity, and then is divided into two laser beams. The first laser beam is incident on the piezoelectric ceramic drive mirror, and the second laser beam is output by the output coupling mirror and is called the first laser to be locked.

3. The fully locked femtosecond optical frequency comb device according to claim 2, characterized in that: The first optical path auxiliary unit includes a first reflector, a highly nonlinear optical fiber and an acousto-optic modulator arranged along the optical path. The first laser to be locked is incident on the first reflector, passes through the highly nonlinear optical fiber and enters the acousto-optic modulator, and then outputs a second laser to be locked.

4. The fully locked femtosecond optical frequency comb device according to claim 3, characterized in that: The second optical path auxiliary unit includes an acousto-optic modulator, an f-2f interferometer, a beam splitter, an avalanche photodiode, a polarization beam splitter, an optical target frequency reference source and a photodetector arranged along the optical path. The second laser to be locked outputted from the acousto-optic modulator enters the beam splitter after passing through the f-2f interferometer. The beam splitter divides the second laser to be locked into a third laser beam and a fourth laser beam. After entering the polarization beam splitter, the third laser beam beats with the optical signal of the optical target frequency reference source. After the beat light outputted from the polarization beam splitter enters the photodetector, a first locking control signal is outputted; the fourth laser beam enters the avalanche photodiode to output a second locking passive feedback signal.

5. The fully locked femtosecond optical frequency comb device according to claim 4, characterized in that: The first locking module includes a photodetector, a radio frequency reference source, an electronic control loop, a piezoelectric ceramic drive mirror and an electro-optic modulator. The electro-optic modulator is inserted into the laser resonant cavity and arranged on the optical path before the output coupling mirror. The photodetector outputs a first locking control signal that is mixed with a signal of the radio frequency reference source, and issues an instruction to control the piezoelectric ceramic drive mirror through the electronic control loop to adjust the length of the laser resonant cavity. At the same time, an instruction to control the electro-optic modulator is also issued to adjust the refractive index of the first laser to be locked before it exits the laser resonant cavity. After adjusting the repetition frequency of the first laser to be locked by adjusting the length and refractive index of the laser resonant cavity, the third laser to be locked is output from the output coupling mirror.

6. The fully locked femtosecond optical frequency comb device according to claim 5, characterized in that: The second locking module includes an acousto-optic modulator and an avalanche photodiode. The acousto-optic modulator receives a second locking passive feedback signal output by the avalanche photodiode and adjusts the carrier-envelope phase shift frequency of the third laser to be locked, so that a laser with a stable target frequency is finally output from the acousto-optic modulator.

7. The fully locked femtosecond optical frequency comb device according to claim 6, characterized in that: The second locking module also includes a bandpass filter, a local frequency synthesizer and a multi-stage radio frequency amplification circuit. The second locked passive feedback signal output by the avalanche photodiode is filtered by a bandpass filter and mixed with an external reference signal emitted by the local frequency synthesizer to generate a mixed signal. The mixed signal is then amplified by a multi-stage radio frequency amplification circuit to amplify the mixed signal containing carrier envelope phase shift frequency change information to the driving frequency requirement of the acousto-optic modulator, so as to drive the acousto-optic modulator to adjust the carrier envelope phase shift frequency of the third laser to be locked.

8. The fully locked femtosecond optical frequency comb device according to claim 5, characterized in that: The photodetector is a visible light band photodetector.

9. The fully locked femtosecond optical frequency comb device according to claim 5, characterized in that: The radio frequency reference source is a hydrogen clock signal source.

10. The fully locked femtosecond optical frequency comb device according to claim 6, characterized in that: The acousto-optic modulator is a 1 micron band acousto-optic modulator.

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