CN114361931A

CN114361931A - Ultra-low noise electro-optical frequency comb generating device

Info

Publication number: CN114361931A
Application number: CN202210008447.8A
Authority: CN
Inventors: 韩海年; 邵晓东; 马骏逸; 魏志义
Original assignee: Institute of Physics of CAS
Current assignee: Institute of Physics of CAS
Priority date: 2022-01-06
Filing date: 2022-01-06
Publication date: 2022-04-15
Anticipated expiration: 2042-01-06
Also published as: CN114361931B

Abstract

Provided is an ultra-low noise electro-optical frequency comb generating device, including: an optical frequency comb module for generating an optical frequency comb; the continuous laser module is used for outputting reference laser with stable frequency and continuous laser for electro-optic modulation; a feedback module for locking the optical frequency comb to the reference laser and for locking the continuous laser to the optical frequency comb; the microwave frequency generation module is used for receiving the optical frequency comb and converting the optical frequency comb into a microwave signal; and the electro-optical modulation module is used for receiving the microwave signal to electro-optically modulate the continuous laser so as to generate an electro-optical frequency comb.

Description

Ultra-low noise electro-optical frequency comb generating device

Technical Field

The invention relates to the technical field of laser, in particular to an ultralow-noise electro-optical frequency comb generating device.

Background

The optical frequency comb is used as an important tool for precise spectroscopy measurement and frequency scale measurement, and the characteristics of stable broadband frequency comb teeth and low phase noise output by the optical frequency comb play an important role in the fields of astronomical spectrometer calibration, double-optical comb spectroscopy, optical communication, microwave photonics and the like.

In recent years, due to the wide application of optical frequency comb technology, a variety of ways have been developed to produce optical frequency combs of different properties. Femtosecond lasers based on mode-locked technology are the most traditional and most common optical frequency combs. Common mode-locked laser-based optical frequency combs include titanium-sapphire optical combs, all-solid-state optical combs, fiber optical combs, and the like. After long-term research and development, the traditional optical frequency comb is mature in technical aspect, excellent in noise and frequency stability performance, and widely applied to various researches. In addition, driven by nonlinear optics, the spectrum of the optical frequency comb has been expanded to terahertz, mid-infrared, visible light, extreme ultraviolet and other bands by taking the traditional optical frequency comb as a seed source. However, there are still some limitations to conventional optical frequency combs based on mode-locked lasers: the repetition frequency is usually limited to 100MHz-1GHz, and is too low for astronomical spectrum calibration, microwave photonics and other applications, and the repetition frequency needs to be increased by adopting techniques such as filtering and amplification, so that the laser system is complex and inconvenient to integrate.

Another way to generate a frequency comb is to drive an electro-optic modulator with a microwave signal, where a continuous laser passing through the electro-optic modulator will generate a frequency sideband. By cascading multiple electro-optic modulators or placing the electro-optic modulators into a resonant cavity, etc., a series of frequency sidebands may be generated that are equally spaced in frequency, with the frequency spacing being equal to the frequency of the electro-optic modulator drive signal. The frequency combs generated in this way are called Electro-optic frequency combs (A. Parriaux, K. Hammann, G. Millot, Electro-optical frequency comb. Advances in Optics and Photonics 12,223-287 (2020)). The repetition frequency of the electro-optical frequency comb is generally over 10GHz, the directly output spectrum is narrow and only has a few THz, but an octave supercontinuum can be obtained through nonlinear broadening (K.Beha et al, Electronic synthesis of light. optical 4,406-411 (2017)). Compared with the traditional optical frequency comb, the electro-optical frequency comb has the advantages that the continuous optical frequency and the repetition frequency can be adjusted, the repetition frequency is higher, the structure is relatively simple, and the like. Due to these remarkable features, the electro-optic frequency comb shows unique advantages in some applications. For example, because of its high repetition frequency, the energy of a single longitudinal mode is also high, and its output can be directly applied to the rectification of an astronomical spectrometer (a.j. metcalf et al, Stellar spectroscopy in the near-isolated with a laser frequency comb. optica 6,233-239 (2019)). In addition, two optical frequency combs with different repetition frequencies based on the same continuous laser have inherent coherence, and are ideal light sources for double optical comb spectroscopy (V.Duran, S.Tainta, V.Torres-Company, ultra fast electro-optical dual-comb interference device. Optic. express 23,30557-30569 (2015)). In addition, the electro-optical frequency comb has irreplaceable effects in the fields of broadband optical communication, arbitrary wave generation, photoelectric signal processing and the like in the microwave photonics.

A disadvantage of electro-optical frequency combs is that noise and frequency stability are generally poor compared to conventional optical frequency combs based on mode locking. Low noise and high frequency stability are the most important features of an optical frequency comb and are also important bases for the use of an optical frequency comb for precision measurements. The noise performance of the electro-optic frequency comb limits its application in the field of precision measurements. Unlike the noise sources of mode-locked optical frequency combs, the phase noise and frequency stability of the longitudinal mode "comb" of electro-optic frequency combs depends primarily on the continuous laser and microwave drive source. If the frequency of the continuum laser is not precisely controlled, all longitudinal modes of the electro-optic frequency comb will dither along with the center frequency. Second, microwave-driven noise is introduced into a series of frequency sidebands, and the noise of the sidebands increases with distance from the center frequency. Therefore, the continuous laser and microwave drive signals are the main reasons limiting the frequency stability of the electro-optic frequency comb.

At present, the noise suppression technology of the advanced electro-optical frequency comb is developed by firstly obtaining an octave spectrum and detecting a carrier envelope phase shift signal f through an f-2f interferometer_ceoThen to f_ceoStabilization is performed to achieve suppression of electro-optical frequency comb noise (D.R. Carlson)et al, Ultrafast electro-optical light with subcycle control.science 361,1358-1362 (2018)). However, because the electro-optic frequency comb has a high repetition frequency, a narrow spectrum, and a wider pulse width, its single pulse energy is lower than that of the mode-locked optical frequency comb, which makes it very difficult to generate an octave spectrum. This solution therefore also increases the complexity of the device.

Disclosure of Invention

In accordance with the above-mentioned problems of the prior art, the present invention provides an ultra-low noise electro-optical frequency comb generating apparatus, comprising:

an optical frequency comb module for generating an optical frequency comb;

the continuous laser module is used for outputting reference laser with stable frequency and continuous laser for electro-optic modulation;

a feedback module for locking the optical frequency comb to the reference laser and for locking the continuous laser to the optical frequency comb;

the microwave frequency generation module is used for receiving the optical frequency comb and converting the optical frequency comb into a microwave signal; and

and the electro-optical modulation module is used for receiving the microwave signal to electro-optically modulate the continuous laser so as to generate an electro-optical frequency comb.

Preferably, the continuum laser module includes an ultrastable laser for generating the reference laser light, and a continuum laser for generating the continuum laser light.

Preferably, the continuum laser module comprises an ultrastable laser for generating the reference laser light and the continuum laser light.

Preferably, the ultra-low noise electro-optical frequency comb generating device further comprises:

the first photoelectric detector is used for detecting beat frequency signals of the optical frequency comb and the reference laser and converting the beat frequency signals into first electric signals; and

and the second photoelectric detector is used for detecting the beat frequency signals of the continuous laser and the optical frequency comb and converting the beat frequency signals into second electric signals.

Preferably, the feedback module comprises:

a first phase-locked loop circuit for generating a first control signal based on the first electrical signal and a preset first reference signal to control the optical frequency comb module such that the optical frequency comb is locked to the reference laser; and

and the second phase-locked loop circuit is used for generating a second control signal based on the second electric signal and a preset second reference signal so as to control the continuous laser module to lock the continuous laser to the optical frequency comb.

Preferably, the microwave frequency generation module comprises a high-speed photodetector for receiving the optical frequency comb and converting it into a microwave signal.

Preferably, the microwave frequency generation module further comprises a pulse repetition frequency doubling module, which is used for receiving the optical frequency comb, doubling the frequency and outputting the frequency to the high-speed photodetector.

Preferably, the electro-optical modulation module includes a filter for filtering the microwave signal to obtain a harmonic signal of a certain order of the repetition frequency of the optical frequency comb as the driving signal.

Preferably, the electro-optical modulation module further comprises:

the radio frequency power amplifier is used for receiving the driving signal and amplifying the power of the driving signal;

the phase shifter is connected to the radio frequency power amplifier and is used for shifting the phase of the amplified driving signal; and

and the electro-optical modulator is connected to the phase shifter and used for receiving the output of the phase shifter and electro-optically modulating the continuous laser to output the electro-optical frequency comb.

Preferably, the carrier envelope phase shift frequency of the optical frequency comb is locked to the radio frequency source.

In the ultra-low noise electro-optical frequency comb generating device, the incident continuous laser and the microwave driving signal of the electro-optical frequency comb are connected through the optical frequency comb and the reference laser, and the homology ensures that the incident continuous laser and the microwave driving signal have inherent good coherence. The invention does not require the spectrum of the electro-optical frequency comb with octave, and reduces the technical requirements on the power of the electro-optical frequency comb, the single pulse energy, the spectrum broadening and the like. The microwave driving signal and the continuous laser seed source used for generating the electro-optical frequency comb have a large tuning range, the frequency tuning range of the microwave driving signal is within the range of 10GHz-40GHz, and the incident continuous laser frequency can be any laser frequency within the optical frequency comb coverage spectrum range.

Because the continuous laser and the optical frequency comb which are locked with each other have important application in the frontier fields of high-resolution spectrum, gas molecule detection, optical frequency standard, optical communication and the like, the invention provides a novel high-precision high-stability research means for the multiple subjects, and has important academic applicability and certain economic benefit.

Drawings

FIG. 1 shows a schematic diagram of an ultra-low noise electro-optical frequency comb generation apparatus according to one embodiment of the present invention;

FIG. 2 is a schematic diagram of an ultra-low noise electro-optical frequency comb generation apparatus according to one embodiment of the present invention;

FIG. 3 is a schematic diagram of an ultra-low noise electro-optical frequency comb generation apparatus according to one embodiment of the present invention; and

fig. 4 is a schematic diagram of an ultra-low noise electro-optical frequency comb generating device according to another embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail by way of specific embodiments with reference to the accompanying drawings. It should be noted that the examples given herein are for illustration only and do not limit the scope of the invention.

Fig. 1 shows a schematic diagram of an ultra-low noise electro-optical frequency comb generation apparatus according to an embodiment of the present invention. The ultra-low noise electro-optical frequency comb generating device comprises an optical frequency comb module 101, a continuous laser module 102, a feedback module 103, a microwave frequency generating module 104 and an electro-optical modulating module 105. The optical frequency comb module 101 is used to generate an optical frequency comb that contains a number of different uniformly arranged frequency components in the frequency domain, each component representing a fixed frequency and which appears in the time domain in the form of a series of pulses. The continuous laser module 102 is used for outputting a frequency-stabilized reference laser and outputting a continuous laser for electro-optical modulation, wherein preferably the line width of the reference laser is in the order of sub-hertz and the frequency drift is in the order of hertz. For example, continuum laser module 102 may include an ultrastable laser for outputting a frequency stabilized reference laser and a continuum laser for outputting a continuum laser for electro-optical modulation. The continuum laser module 102 may also contain only an ultrastable laser, the output of which serves as both the reference laser and the continuum laser. The optical frequency comb output by the optical frequency comb module 101 is locked to the reference laser via the feedback module 103, and the frequency of the continuous laser is locked to the optical frequency comb via the feedback module 103. The microwave frequency generation module 104 is configured to receive the output from the optical frequency comb module 101 and perform photoelectric conversion thereon to generate a microwave signal. The microwave signal is used to drive the electro-optic modulation module 105 to electro-optically modulate the continuous laser to produce an electro-optic frequency comb.

Fig. 2 is a schematic diagram of an ultra-low noise electro-optical frequency comb generating device according to an embodiment of the present invention. In fig. 2, the continuum laser module includes an ultrastable laser 3 for outputting the reference laser and a continuum laser 1 for outputting the continuum laser, but the invention is not limited thereto, and the continuum laser module may include other types of lasers. As shown in fig. 2, the repetition frequency of the optical frequency comb generated by the optical frequency comb module 2 is typically on the order of hundreds of MHz (e.g., 100MHz-250 MHz). The output of the optical frequency comb module 2 is divided into three paths, the first path outputs a reference laser beat frequency generated by the ultrastable laser 3, and the beat frequency signal is converted into an electrical signal through a photodetector and processed through a feedback device (not shown in fig. 2) to lock the optical frequency comb to the reference laser. The second output of the optical frequency comb module 2 is beat-frequency with the continuous laser light generated by the continuous laser 1, which beat-frequency signal is converted into an electrical signal via a photodetector and processed via a feedback device (not shown in fig. 2) to lock the continuous laser light onto the optical frequency comb. The third output of the optical frequency comb module 2 produces a low noise microwave signal via the high speed photodetector 14. In this process, the low noise microwave signal generated by the optical frequency comb and the continuous laser locked to the optical frequency comb will obtain the same frequency stability as the reference laser. The electro-

optical modulators

23, 24, 25, 26 are then driven by low-noise microwave signals, modulating the continuous laser light generated by the continuous laser 1 to produce a low-noise electro-optical frequency comb having a repetition frequency of 10GHz-40 GHz.

The theoretical basis for realizing the invention is as follows:

repetition frequency f of optical frequency comb locked to reference laser_repCan be expressed as:

f_rep＝(f_N-f_ceo)/N＝(f_opt-f_b-f_ceo)/N (1)

wherein f is_bFrequency f of Nth longitudinal mode of optical frequency comb_NAnd the output frequency f of the hyperstable laser_optOf the beat signal, f_ceoThe carrier envelope phase shift frequency of the optical frequency comb. By detecting the optical frequency comb locked to the reference laser with the photodetector 14, the repetition frequency f can be obtained_repAnd repetition frequency f_repThe microwave signal of each order harmonic. According to the requirement, a proper filter is selected, and the repetition frequency f can be filtered out_repOf the nth harmonic of (a), with frequency f_n＝nf_rep. The optical frequency comb can transfer the frequency stability of the reference laser to the microwave frequency according to equation (1) with a transfer process satisfying

δf_opt/f_opt＝δf_rep/f_rep＝δf_n/f_n (2)

Wherein δ f_opt，δf_repAnd δ f_nRespectively representing the repetition frequencies f of the reference laser and the optical frequency comb_repAnd repetition frequency f_repThe nth harmonic of (a) is jittered. This equation shows that the microwave signal and reference generated via frequency division by the optical frequency combThe laser has the same frequency stability.

Another feature of microwave frequencies generated based on optical frequency combs is that the frequency division process is accompanied by a reduction in phase noise. For the repetition frequency f_repThe frequency division factor of the nth harmonic wave is recorded as M-N/N, and the phase noise of the microwave signal is attenuated by M compared with the phase noise of the reference laser²Double, i.e.

S_φ,n(f)＝S_φ,opt(f)/M² (3)

Wherein S is_φ,optAnd S_φ,nRespectively representing the reference laser light and the repetition frequency f_repThe phase noise of the microwave signal of the nth harmonic.

If a continuous laser (frequency f) locked to an optical frequency comb is used_c) As an incident source, the electro-optical frequency comb is generated by using a low-noise and high-frequency-stability microwave signal generated by the optical frequency comb as a drive, and then the generated electro-optical frequency comb is far away from the central frequency f_cThe frequency of the mth sideband of (a) can be expressed as

Wherein, nf_repThe microwave frequency, which is an nth harmonic of the repetition frequency, is the frequency at which the electro-optic modulator is driven. In the optical frequency range, M is an integer much smaller than or of the same order as M. Equation (4) thus shows that f is far away_nStill having a central frequency f_cApproximate frequency stability, and the phase noise is also similar to that of the reference laser.

The ultra-low noise electro-optical frequency comb generating device of the present invention will be described in detail with reference to the following embodiments.

Example 1:

fig. 3 is a schematic diagram of an ultra-low noise electro-optical frequency comb generation apparatus according to an embodiment of the present invention. For clarity, the optical paths are shown in solid lines and the electrical circuits in dashed lines in FIG. 3. As shown in fig. 3, the ultra-low noise electro-optical frequency comb generating device includes an optical frequency comb module 2; a continuous laser module consisting of an ultrastable laser 3 and a continuous laser 1; a feedback module consisting of a phase-locked loop circuit 8 and a phase-locked loop circuit 12; a microwave frequency generation module consisting of a pulse repetition frequency doubling module 13 and a photoelectric detector 14; and an electro-optical modulation module consisting of a filter 15, radio frequency power amplifiers 16, 17, 18, 19, phase shifters 20, 21, 22 and electro-

optical modulators

23, 24, 25, 26.

The first output of the optical frequency comb module 2 is reflected by a reflector 6 and then enters a beam splitter prism 5, the reflected light beam of the reference laser output by the ultrastable laser 3 after passing through the beam splitter prism 4 enters the beam splitter prism 5, the first output of the optical frequency comb module 2 and the output of the ultrastable laser 3 are combined at the beam splitter prism 5 to generate a beat frequency signal, and the beat frequency signal is detected by a photoelectric detector 7 and converted into a first electric signal. The first electrical signal is input to the phase-locked loop circuit 8 and compared with a preset first reference value to output a first driving signal, which is input to an actuator (e.g., piezo ceramic and electro-optic modulator actuator) within the optical frequency comb module 2 to control the frequency of the optical frequency comb such that the optical frequency comb locks onto the reference laser. Specifically, when the first electrical signal is equal to the first reference value, the first driving signal output by the phase-locked loop circuit 8 is zero. When the first electrical signal is not equal to the first reference value, the first driving signal output by the phase-locked loop circuit 8 is not zero, the first driving signal is input to the brake in the optical frequency comb module 2, the frequency of the output optical frequency comb is changed, and the first electrical signal is made to be equal to the first reference value, so that the optical frequency comb is locked to the reference laser output by the ultrastable laser 3 in real time.

The second output of the optical frequency comb module 2 is incident to the beam splitter prism 10, the reflected light beam of the output of the continuous laser 1 after passing through the beam splitter prism 9 is incident to the beam splitter prism 10, the outputs of the optical frequency comb module 2 and the continuous laser 1 are combined at the beam splitter prism 10 and generate a beat frequency signal, and the beat frequency signal is detected by the photoelectric detector 11 and converted into a second electric signal. The second electrical signal is input to the phase-locked loop circuit 12 and compared with a preset second reference value to output a second driving signal, and the second driving signal is input to the continuum laser 1 to control its pumping current or actuators such as piezoelectric ceramics on the intracavity grating, so as to control the frequency of the continuum laser generated by the continuum laser 1, so that the continuum laser generated by the continuum laser 1 is locked to the optical frequency comb. Specifically, when the second electrical signal is equal to the second reference value, the second driving signal output by the phase-locked loop circuit 12 is zero. When the second electrical signal is not equal to the second reference value, the second driving signal output by the phase-locked loop circuit 12 is not zero, the second driving signal is input to the actuator of the continuous laser 1, and the frequency of the output continuous laser is changed to make the second electrical signal equal to the second reference value, so that the continuous laser output by the continuous laser 1 is locked to the optical frequency comb in real time.

The pulse repetition frequency doubling module 13 and the high-speed photodetector 14 constitute a low-noise microwave frequency generation module. The pulse repetition frequency doubling module 13 is used for increasing the repetition frequency of the optical frequency comb, and the high-speed photodetector 14 is used for converting an optical signal into an electrical signal. The repetition frequency of the optical frequency comb is generally 100MHz-1GHz, and when the optical frequency comb is directly detected by a photodetector, due to the saturation effect of the detector, the main power is concentrated on the harmonic signal of the lower order, and the signal-to-noise ratio of the higher harmonic signal (for example, 10GHz or even higher frequency) of the repetition frequency is lower. In order to further improve the signal-to-noise ratio of the high-frequency signal (10GHz or higher) detected by the photodetector, the repetition frequency can be increased by the repetition frequency doubling module 13, and then the signal-to-noise ratio of the high-frequency signal can be improved when the photodetector is detected. The third output of the optical frequency comb module 2 passes through the repetition frequency doubling module 13 and is detected by the high-speed photoelectric detector 14, so as to obtain a low-noise microwave signal. It should be understood that in practical applications, the pulse repetition frequency doubling module 13 may also be omitted.

The low-noise microwave signal has a repetition frequency of the optical frequency comb after frequency doubling and a harmonic frequency thereof. The low-noise microwave signal is filtered by a filter 15 to obtain a harmonic signal of a certain order. The harmonic signal of a certain order is split into four paths, first amplified in power by radio frequency power amplifiers 16, 17, 18, 19, then phase shifted by phase shifters 20, 21, 22, and then respectively connected to modulation ports of phase modulators 23, 24, 25 and intensity modulator 26 for driving three phase modulators 23, 24, 25 and one intensity modulator 26. The frequency of the drive signal (i.e., the harmonic signal of a certain order) is within the bandwidth of the phase modulator and the intensity modulator, typically 10GHz-40 GHz. The transmitted beam of continuous laser light output by the continuous laser 1 after passing through the beam splitter prism 9 sequentially passes through three phase modulators 23, 24, 25 and an intensity modulator 26 to generate an electro-optic frequency comb. The spectrum of the electro-optic frequency comb can be monitored by a spectrum analyzer (not shown) and the spectral width and flatness of the electro-optic frequency comb can be optimized by adjusting the radio frequency power amplifiers 16, 17, 18, 19 and the phase shifters 20, 21, 22.

The output electro-optical frequency comb is subjected to power amplification sequentially through the optical fiber amplification module 27, the amplified electro-optical frequency comb compresses the pulse width through the

grating pair

29, 30, the light path is pressed down by the reflector 31, passes through the

grating pair

29, 30 again, and is output by the reflector 28. The reflected beam from mirror 28 is coupled into photonic crystal fiber 32 for spectral broadening to produce a supercontinuum. The supercontinuum is split into two by a beam splitting prism 33. The reflected beam of the beam splitter prism 33 is used for output, the transmitted beam is reflected by the beam splitter prism 34 to be combined with the reference laser output by the ultrastable laser 3, and a beat frequency signal generated by the combined beam is detected by a photoelectric detector 35 and then can be analyzed by a spectrum analyzer (not shown) and a frequency counter (not shown) for verifying the frequency stability of the electro-optical frequency comb.

According to one embodiment of the invention, the phase modulators 23, 24, 25 and the intensity modulator 26 may be made of lithium niobate of the waveguide type, with fiber coupling input and output, with a central wavelength of 1030nm and a bandwidth of 40 GHz. The high-speed photodetector 14 may be an indium gallium arsenide (InGaAs) detector with a detection wavelength of 750nm to 1650nm and a frequency response of DC-30 GHz. The filter 15, the rf power amplifiers 16, 17, 18, 19 and the phase shifters 20, 21, 22 are adapted to the same frequency as the low noise microwave signal, and may vary from 10GHz to 30GHz according to actual requirements. The optical fiber amplifying module 27 may be an ytterbium-doped optical fiber amplifier, and may include a primary single-mode fiber pre-amplification and a primary double-clad fiber main amplification therein, and may amplify the power up to 20W. The

grating pair

25, 26 is a 1000lines/mm transmission grating with 95% optical efficiency at 1030nm for the first order diffraction. The

reflectors

6, 28 and 31 are plated with 900nm-1100nm broadband reflection films. The

beam splitting prisms

4, 5, 9, 10, 33, 34 are adapted to have a wavelength of 900nm-1200 nm. The

photodetectors

7,11,14 are suitable for use in the wavelength range 850nm to 1650 nm.

The ultrastable laser 3 can be an external cavity semiconductor laser with the central wavelength of 972nm, the continuous laser is frequency stabilized to an F-P cavity with the fineness of 208000 by using a PDH technology, and the line width can reach the Hz magnitude theoretically. The continuous laser 1 may be an external cavity semiconductor laser with a central wavelength of 1030nm, a free-running linewidth of 100kHz, and an output power of 25mW via a polarization maintaining fiber. The optical frequency comb module 2 can be an ytterbium-doped optical fiber optical frequency comb with nonlinear polarization rotation mode locking, the repetition frequency is 200MHz, the supercontinuum obtained through the photonic crystal fiber 32 is 500nm-1300nm, and the wavelength 972nm of the supercontinuum covering the ultrastable laser.

In practical operation, the output lasers of the optical frequency comb module 2 and the ultrastable laser 3 are preferably collimated to the same height, so as to combine to generate a beat frequency signal; the output lasers of the continuous laser 1 and the optical frequency comb module 2 are collimated to the same height so as to be combined to generate a beat frequency signal.

Preferably, the devices 4-7, 9-11, 13-35 are all placed on a base plate and sealed with a housing, leaving a plurality of holes for light and electrical wiring. The continuous laser 1, the optical frequency comb module 2, the ultrastable laser 3 and the bottom plate are arranged on the same optical platform. The phase-locked loop circuits 8, 12 are placed in the vicinity of the optical path.

Preferably, the optical frequency comb module 2 comprises a radio frequency source on which the carrier envelope phase shift frequency of the optical frequency comb module 2 is frequency locked. The radio frequency source may be, for example, a 20MHz radio frequency source.

Preferably, the optical frequency comb is locked to the reference laser before the continuous laser is locked to the optical frequency comb.

Example 2:

fig. 4 is a schematic diagram of an ultra-low noise electro-optical frequency comb generating device according to another embodiment of the present invention. As shown in fig. 4, in contrast to embodiment 1, in this embodiment the continuum laser module contains only the ultrastable laser 3. The first output of the ultrastable laser 3 is reflected by the reflector 4 and then beat-frequency with the output of the optical frequency comb module 2, so that the optical frequency comb is locked to the reference laser output by the ultrastable laser 3. The second output of the ultrastable laser 3 is used as continuous laser for electro-optical modulation. In this embodiment, another ultrastable laser 38 may be selected and combed with the electro-optic frequency to measure its phase noise and frequency stability characteristics. The frequency of the ultrastable laser 38 is in the supercontinuum range and is different from the frequency of the ultrastable laser 3. The specific parameters and installation of other devices are the same as those in embodiment 1, and are not described herein again.

In the above embodiment of the present invention, three phase modulators 23, 24, 25 and one intensity modulator 26 are taken as an example for illustration, but the present invention is not limited thereto, and in practical application, any other number and types of phase modulator and intensity modulator combinations can be selected.

According to the above embodiments of the present invention, the feedback module includes, but is not limited to, a phase-locked loop circuit, and other feedback circuits capable of processing an electrical signal generated by the beat signal to generate a driving signal can be applied to the present invention, including a PID control circuit, a comparator circuit, etc.

The optical frequency comb acts as a frequency synthesizer and can transmit the frequency stability of the reference laser to other optical frequencies and microwave frequencies. A continuous laser is locked to a high-fineness F-P cavity, the line width of the continuous laser can reach the sub-Hz magnitude, and the frequency stability of the laser can reach 10^-16The order of/s can be used as reference laser. After further locking one longitudinal mode of the optical frequency comb to the reference laser, millions of longitudinal modes in the whole spectrum range of the optical frequency comb are stabilized, and the same frequency stability is obtained.

At this time, if another continuous laser is locked to a certain longitudinal mode of the optical frequency comb, the continuous laser will obtain the same frequency stability. The continuous laser can be completely used as an ultrastable seed source of the electro-optical frequency comb, and the stability of the center frequency of the electro-optical frequency comb is realized. Since the optical frequency comb has a very broad spectrum, the frequency stability of the reference laser is transferred over a very broad frequency range by means of the function of the synthesizer of the frequency comb.

In addition to connecting different optical frequencies, the optical frequency comb can also transfer the frequency stability of the continuous laser to microwave frequencies. If the output of the optical frequency comb is detected with a high-speed photodetector, the resulting repetition frequency and its harmonic signals will have the same frequency stability as the reference laser (F. quinlan et al, ultra low phase noise microwave generation with an Er: fiber-based optical frequency divider. Opt. Lett.36,3260-3262 (2011)). And the frequency division effect of the optical frequency comb can remarkably reduce the phase noise of the generated microwave signal. The phase noise of the microwave signal generated based on the optical frequency comb is lower than-100 dBc/Hz at 1Hz frequency offset and lower than-170 dBc/Hz at high frequency (>100kHz) (X.P. Xie et al, Photonic microwave signals with zeptosecond-level absolute timing noise. Nature Photonics 11,44-47 (2017)). Such high stability, low noise microwave signals can be used as a drive source for an electro-optic frequency comb, thereby producing an ultra-low noise electro-optic frequency comb.

The invention provides an ultralow-noise electrooptical frequency comb generating device aiming at the problems that an electrooptical frequency comb has high phase noise and the existing noise suppression technology is complex. The low-noise microwave frequency generated by the optical frequency comb is used as a drive, a continuous laser locked to the optical frequency comb is used as an incident light source, and the electro-optical frequency comb with ultra-low noise and high frequency stability is generated. The frequency stability of the reference laser can be transmitted to microwave frequency and other optical frequencies by utilizing the frequency of the optical frequency comb, and the overall coherence of the system is ensured. The resulting low-noise electro-optic frequency comb provides a reliable solution for a number of applications requiring high repetition frequency, high frequency stability, and low noise optical frequency combs.

The invention provides an ultra-low noise electro-optical frequency comb generating device, which adopts continuous laser locked to an ultra-stable optical frequency comb as an incident light source, and generates an electro-optical frequency comb by using a low noise microwave signal generated by the optical frequency comb as a driving source. The incident continuous laser and the driving signal are synchronized to the reference laser by the optical frequency comb, the whole system has very good coherence, and therefore the generated electro-optical frequency comb has extremely low phase noise and high frequency stability.

Although the present invention has been described by way of preferred embodiments, the present invention is not limited to the embodiments described herein, and various changes and modifications may be made without departing from the scope of the present invention.

Claims

1. An ultra-low noise electro-optic frequency comb generation apparatus, comprising:

an optical frequency comb module for generating an optical frequency comb;

2. The ultra-low noise electro-optic frequency comb generation apparatus of claim 1, wherein the continuum laser module comprises an ultrastable laser for generating the reference laser and a continuum laser for generating the continuum laser.

3. The ultra-low noise electro-optic frequency comb generation apparatus of claim 1, wherein the continuum laser module comprises an ultrastable laser for generating the reference laser and the continuum laser.

4. The ultra-low noise electro-optic frequency comb generation device of claim 2 or 3, wherein the ultra-low noise electro-optic frequency comb generation device further comprises:

5. The ultra-low noise electro-optic frequency comb generating device of claim 4, wherein the feedback module comprises:

6. An ultra-low noise electro-optic frequency comb generation apparatus as claimed in claim 2 or 3, wherein said microwave frequency generation module comprises a high speed photodetector for receiving said optical frequency comb and converting it into a microwave signal.

7. The ultra-low noise electro-optic frequency comb generation apparatus of claim 6, wherein the microwave frequency generation module further comprises a pulse repetition frequency doubling module for receiving the optical frequency comb and doubling it for output to the high-speed photodetector.

8. An ultra-low noise electro-optical frequency comb generation apparatus as claimed in claim 2 or 3, wherein said electro-optical modulation module comprises a filter for filtering said microwave signal to obtain a harmonic signal of an order of the repetition frequency of said optical frequency comb as a driving signal.

9. The ultra-low noise electro-optic frequency comb generation apparatus of claim 8, wherein the electro-optic modulation module further comprises:

10. The ultra-low noise electro-optic frequency comb generation apparatus of claim 1, wherein a carrier envelope phase shift of the optical frequency comb is frequency locked to a radio frequency source.