Optical frequency combs,as powerful tools for precision spectroscopy and research into optical frequency standards,have driven continuous progress and significant breakthroughs in applications such as time-frequency t...Optical frequency combs,as powerful tools for precision spectroscopy and research into optical frequency standards,have driven continuous progress and significant breakthroughs in applications such as time-frequency transfer,measurement of fundamental physical constants,and high-precision ranging,achieving a series of milestone results in ground-based environments.With the continuous maturation and evolution of femtosecond lasers and related technologies,optical frequency combs are moving from ground-based applications to astronomical and space-based applications,playing an increasingly important role in atomic clocks,exoplanet observations,gravitational wave measurements,and other areas.This paper,focusing on astronomical and space-based applications,reviews research progress on astronomical frequency combs,optical clock time-frequency networks,gravitational waves,dark matter measurement,dual-comb large-scale absolute ranging,and high-resolution atmospheric spectroscopy.With enhanced performance and their gradual application in the field of space-based research,optical frequency combs will undoubtedly provide more powerful support for astronomical science and cosmic exploration in the future.展开更多
Frequency-comb emission via high-order harmonic generation(HHG)provides an alternative method for the coherent vacuum ultraviolet(VUV)and extreme ultraviolet(XUV)radiation at ultrahigh repetition rates.In particular,t...Frequency-comb emission via high-order harmonic generation(HHG)provides an alternative method for the coherent vacuum ultraviolet(VUV)and extreme ultraviolet(XUV)radiation at ultrahigh repetition rates.In particular,the temporal and spectral features of the HHG were shown to carry profound insight into frequency-comb emission dynamics.Here we present an ab initio investigation of the temporal and spectral coherence of the frequency comb emitted in HHG of He atom driven by few-cycle pulse trains.We find that the emission of frequency combs features a destructive and constructive coherences caused by the phase interference of HHG,leading to suppression and enhancement of frequency-comb emission.The results reveal intriguing and substantially different nonlinear optical response behaviors for frequency-comb emission via HHG.The dynamical origin of frequency-comb emission is clarified by analyzing the phase coherence in HHG processes in detail.Our results provide fresh insight into the experimental realization of selective enhancement of frequency comb in the VUV–XUV regimes.展开更多
This paper presents a novel approach to improve aliasing rejection in comb-based decimation filters. The method is established on certain palindromic polynomials with all zeros on the unit circle and the sharpening te...This paper presents a novel approach to improve aliasing rejection in comb-based decimation filters. The method is established on certain palindromic polynomials with all zeros on the unit circle and the sharpening technique. As a result, aliasing rejection and the passband characteristic are improved. The method is illustrated with various examples and compared with the methods from the literature. .展开更多
Optical frequency comb with evenly spaced lines over a broad bandwidth has revolutionized the fields of optical metrology and spectroscopy.Here,we propose a fast interleaved dual-comb spectroscopy with sub-femtometer-...Optical frequency comb with evenly spaced lines over a broad bandwidth has revolutionized the fields of optical metrology and spectroscopy.Here,we propose a fast interleaved dual-comb spectroscopy with sub-femtometer-resolution and absolute frequency,in which two electro-optic frequency combs are swept.Electrically-modulated stabilized laser enables ultrahigh resolution of 0.16 fm(or 20 k Hz in optical frequency)and single-shot measurement in 90 ms.Total 20 million points are recorded spanning 3.2 nm(or 400 GHz)bandwidth,corresponding to a spectral sampling rate of 2.5×10^(8)points/s under Nyquist-limitation.Besides,considering the trade-off between the measurement time and spectral resolution,a fast single-shot measurement is also realized in 1.6 ms with 8 fm(or 1 MHz)resolution.We demonstrate the 25-averaged result with 30.6 d B spectral measurement signal-to-noise ratio(SNR)by reducing the filter bandwidth in demodulation.The results show great prospect for precise measurement with flexibly fast refresh time,high spectral resolution,and high SNR.展开更多
Passively mode-locked fiber lasers emit femtosecond pulse trains with excellent short-term stability. The quantum-limited timing jitter of a free running femtosecond erbium-doped fiber laser working at room temperatur...Passively mode-locked fiber lasers emit femtosecond pulse trains with excellent short-term stability. The quantum-limited timing jitter of a free running femtosecond erbium-doped fiber laser working at room temperature is considerably below one femtosecond at high Fourier frequency. The ultrashort pulse train with ultralow timing jitter enables absolute time-of-flight measurements based on a dual-comb implementation, which is typically composed of a pair of optical frequency combs generated by femtosecond lasers. Dead-zone-free absolute distance measurement with sub-micrometer precision and kHz update rate has been routinely achieved with a dual-comb configuration, which is promising for a number of precision manufacturing applications, from large step-structure measurements prevalent in microelectronic profilometry to three coordinate measurements in large-scale aerospace manufacturing and shipbuilding. In this paper, we first review the sub-femtosecond precision timing jitter characterization methods and approaches for ultralow timing jitter mode-locked fiber laser design. Then, we provide an overview of the state-of-the-art dual-comb absolute ranging technology in terms of working principles, experimental implementations, and measurement precisions. Finally, we discuss the impact of quantum-limited timing jitter on the dual-comb ranging precision at a high update rate. The route to highprecision dual-comb range finder design based on ultralow jitter femtosecond fiber lasers is proposed.展开更多
The characteristics of wave transmission, reflection and energy dissipation of comb-type caisson breakwaters are studied through laboratory physical model tests. Regular and irregular waves, with a wide range of wave ...The characteristics of wave transmission, reflection and energy dissipation of comb-type caisson breakwaters are studied through laboratory physical model tests. Regular and irregular waves, with a wide range of wave heights and periods and a constant water depth, are considered. Different dimensions of each portion of the comb-type caisson breakwater are tested. Empirical formulae for calculating the reduction coefficient k, which is the ratio of horizontal wave force on unit length of the comb-type breakwater to that on unit length of the vertical wall breakwater, and for calculating the reflection coefficient of waves kr are obtained from the measurements. The comb-type caisson breakwater has been found to be very efficient in dissipating incident wave energy and in reducing wave reflection, and has already been used for the construction of an island breakwater in the Dayao Bay of Dalian Port, Liaoning Province, China. Compared with the cost of a common caisson breakwater, about 24.5% of the investment has been saved owing to the use of this comb-type breakwater.展开更多
Based on the nonlinear Schr?dinger equation(NLSE) with damping, detuning, and driving terms describing the evolution of signals in a Kerr microresonator, we apply periodic nonlinear Fourier transform(NFT) to the study...Based on the nonlinear Schr?dinger equation(NLSE) with damping, detuning, and driving terms describing the evolution of signals in a Kerr microresonator, we apply periodic nonlinear Fourier transform(NFT) to the study of signals during the generation of the Kerr optical frequency combs(OFCs). We find that the signals in different states, including the Turing pattern, the chaos, the single soliton state, and the multi-solitons state, can be distinguished according to different distributions of the eigenvalue spectrum. Specially, the eigenvalue spectrum of the single soliton pulse is composed of a pair of conjugate symmetric discrete eigenvalues and the quasi-continuous eigenvalue spectrum with eye-like structure.Moreover, we have successfully demonstrated that the number of discrete eigenvalue pairs in the eigenvalue spectrum corresponds to the number of solitons formed in a round-trip time inside the Kerr microresonator. This work shows that some characteristics of the time-domain signal can be well reflected in the nonlinear domain.展开更多
Femtosecond optical frequency combs correlate the microwave and optical frequencies accurately and coherently.Therefore,any optical frequency in visible to near-infrared region can be directly traced to a microwave fr...Femtosecond optical frequency combs correlate the microwave and optical frequencies accurately and coherently.Therefore,any optical frequency in visible to near-infrared region can be directly traced to a microwave frequency.As a result,the length unit“meter”is directly related to the time unit“second”.This paper validates the capability of the national wavelength standards based on a home-made Er-doped fiber femtosecond optical frequency comb to measure the laser frequencies ranging from visible to near-infrared region.Optical frequency conversion in the femtosecond optical frequency comb is achieved by combining spectral broadening in a highly nonlinear fiber with a single-point frequencydoubling scheme.The signal-to-noise ratio of the beat notes between the femtosecond optical frequency comb and the lasers at 633,698,729,780,1064,and 1542 nm is better than 30 d B.The frequency instability of the above lasers is evaluated by using a hydrogen clock signal with a instability of better than 1×10^(-13)at 1-s averaging time.The measurement is further validated by measuring the absolute optical frequency of an iodine-stabilized 532-nm laser and an acetylenestabilized 1542-nm laser.The results are within the uncertainty range of the international recommended values.Our results demonstrate the accurate optical frequency measurement of lasers at different frequencies using the femtosecond optical frequency comb,which is not only important for the precise and accurate traceability and calibration of the laser frequencies,but also provides technical support for establishing the national wavelength standards based on the femtosecond optical frequency comb.展开更多
We present a fully stabilized Yb-fiber frequency comb locked to a microwave standard and an optical reference separately. The carrier-envelope offset frequency is generated by a standard f–2f interferometer with 40 d...We present a fully stabilized Yb-fiber frequency comb locked to a microwave standard and an optical reference separately. The carrier-envelope offset frequency is generated by a standard f–2f interferometer with 40 dB signal-tonoise ratio. The offset frequency and the repetition rate are stabilized simultaneously to the radio frequency reference for more than 30 hours, and the fractional Allan deviation of the comb is the same as the microwave standard of 10^(-12) at 1 s.Alternatively, the comb is locked to an ultra-stable optical reference at 972 nm using an intracavity electro-optic modulator,exhibiting a residual integrated phase noise of 458 mrad(1 Hz–10 MHz) and an in-loop tracking stability of 1.77× 10^(-18) at 1 s, which is significantly raised by six orders comparing to the case locked to the microwave frequency standard.展开更多
This study develops an analytical solution for oblique wave interaction with a comb-type caisson breakwater based on linear potential theory. The fluid domain is divided into inner and outer regions according to the g...This study develops an analytical solution for oblique wave interaction with a comb-type caisson breakwater based on linear potential theory. The fluid domain is divided into inner and outer regions according to the geometrical shape of breakwater. By using periodic boundary condition and separation of variables, series solutions of velocity potentials in inner and outer regions are developed. Unknown expansion coefficients in series solutions are determined by matching velocity and pressure of continuous conditions on the interface between two regions. Then, hydrodynamic quantities involving reflection coefficients and wave forces acting on breakwater are estimated. Analytical solution is validated by a multi-domain boundary element method solution for the present problem. Diffusion reflection due to periodic variations in breakwater shape and corresponding surface elevations around the breakwater are analyzed. Numerical examples are also presented to examine effects of caisson parameters on total wave forces acting on caissons and total wave forces acting on side plates. Compared with a traditional vertical wall breakwater, the wave force acting on a suitably designed comb-type caisson breakwater can be significantly reduced. This study can give a better understanding of the hydrodynamic performance of comb-type caisson breakwaters.展开更多
The prediction of protein coding regions in DNA sequences is an important problem in computational biology. It is observed that nucleotides in the protein coding regions or exons of a DNA sequence show period-3 proper...The prediction of protein coding regions in DNA sequences is an important problem in computational biology. It is observed that nucleotides in the protein coding regions or exons of a DNA sequence show period-3 property. Hence identification of the period-3 regions helps in predicting the gene locations within the billions long DNA sequence of eukaryotic cells. The period-3 property exhibited in exons of eukaryotic gene sequences enables signal processing based time-domain and frequency domain methods to predict these regions efficiently. Several approaches based on signal processing tools have, therefore, been applied to this problem, to predict these regions effectively. This paper describes novel and efficient comb filter-based techniques for the prediction of protein coding region based on the period-3 behavior of codon sequences. The proposed method is then validated on Burset/Guigo1996, HMR195 and KEGG standard datasets using various prediction measures. It is shown that cascaded differentiator comb (CDC) filter can be used for prediction of protein coding region with better prediction efficiency, and involves less computational complexity compared with the other signal processing techniques based on period-3 property.展开更多
Optical frequency combs have emerged as an important tool enabling diverse applications from test-and-measurement,including spectroscopy,metrology,precision distance measurement,sensing,as well as optical and microwav...Optical frequency combs have emerged as an important tool enabling diverse applications from test-and-measurement,including spectroscopy,metrology,precision distance measurement,sensing,as well as optical and microwave waveform synthesis,signal processing,and communications.Several techniques exist to generate optical frequency combs,such as mode-locked lasers,Kerr micro-resonators,and electro-optic modulation.Important characteristics of optical frequency combs include the number of comb lines,their spacing,spectral shape and/or flatness,and intensity noise.While mode-locked lasers and Kerr micro-resonators can be used to obtain a large number of comb lines compared to electro-optic modulation,the latter provides increased flexibility in tuning the comb spacing.For some applications in optical communications and microwave photonics,a high degree of integration may be more desirable over a very large number of comb lines.In this paper,we review recent progress on integrated electro-optic frequency comb generators,including those based on indium phosphide,lithium niobate,and silicon photonics.展开更多
基金support of the National Natural Sci-ence Foundation of China(NSFC)(62305373)the Strategic Priority Research Program of the Chinese Academy of Sciences(XDA1502040404,XDB2101040004).
文摘Optical frequency combs,as powerful tools for precision spectroscopy and research into optical frequency standards,have driven continuous progress and significant breakthroughs in applications such as time-frequency transfer,measurement of fundamental physical constants,and high-precision ranging,achieving a series of milestone results in ground-based environments.With the continuous maturation and evolution of femtosecond lasers and related technologies,optical frequency combs are moving from ground-based applications to astronomical and space-based applications,playing an increasingly important role in atomic clocks,exoplanet observations,gravitational wave measurements,and other areas.This paper,focusing on astronomical and space-based applications,reviews research progress on astronomical frequency combs,optical clock time-frequency networks,gravitational waves,dark matter measurement,dual-comb large-scale absolute ranging,and high-resolution atmospheric spectroscopy.With enhanced performance and their gradual application in the field of space-based research,optical frequency combs will undoubtedly provide more powerful support for astronomical science and cosmic exploration in the future.
基金the National Natural Science Foundation of China(Grant Nos.12074239 and 91850209)the Natural Science Foundation of Guangdong Province,China(Grant Nos.2020A1515010927 and 2020ST084)+1 种基金the Fund from the Department of Education of Guangdong Province,China(Grant Nos.2019KTSCX038 and 2020KCXTD012)the Fund from Shantou University(Grant No.NTF18030).
文摘Frequency-comb emission via high-order harmonic generation(HHG)provides an alternative method for the coherent vacuum ultraviolet(VUV)and extreme ultraviolet(XUV)radiation at ultrahigh repetition rates.In particular,the temporal and spectral features of the HHG were shown to carry profound insight into frequency-comb emission dynamics.Here we present an ab initio investigation of the temporal and spectral coherence of the frequency comb emitted in HHG of He atom driven by few-cycle pulse trains.We find that the emission of frequency combs features a destructive and constructive coherences caused by the phase interference of HHG,leading to suppression and enhancement of frequency-comb emission.The results reveal intriguing and substantially different nonlinear optical response behaviors for frequency-comb emission via HHG.The dynamical origin of frequency-comb emission is clarified by analyzing the phase coherence in HHG processes in detail.Our results provide fresh insight into the experimental realization of selective enhancement of frequency comb in the VUV–XUV regimes.
文摘This paper presents a novel approach to improve aliasing rejection in comb-based decimation filters. The method is established on certain palindromic polynomials with all zeros on the unit circle and the sharpening technique. As a result, aliasing rejection and the passband characteristic are improved. The method is illustrated with various examples and compared with the methods from the literature. .
基金funding from National Natural Science Foundation of China(NSFC)under Grant Nos.61775132,61735015,61620106015supported by the Major Key Project of Peng Cheng Laboratory(PCL)。
文摘Optical frequency comb with evenly spaced lines over a broad bandwidth has revolutionized the fields of optical metrology and spectroscopy.Here,we propose a fast interleaved dual-comb spectroscopy with sub-femtometer-resolution and absolute frequency,in which two electro-optic frequency combs are swept.Electrically-modulated stabilized laser enables ultrahigh resolution of 0.16 fm(or 20 k Hz in optical frequency)and single-shot measurement in 90 ms.Total 20 million points are recorded spanning 3.2 nm(or 400 GHz)bandwidth,corresponding to a spectral sampling rate of 2.5×10^(8)points/s under Nyquist-limitation.Besides,considering the trade-off between the measurement time and spectral resolution,a fast single-shot measurement is also realized in 1.6 ms with 8 fm(or 1 MHz)resolution.We demonstrate the 25-averaged result with 30.6 d B spectral measurement signal-to-noise ratio(SNR)by reducing the filter bandwidth in demodulation.The results show great prospect for precise measurement with flexibly fast refresh time,high spectral resolution,and high SNR.
基金supported by National Natural Science Foundation of China (Grant Nos.61475162,61675150,and 61535009)Tianjin Natural Science Foundation (Grant No.18JCYBJC16900)Tianjin Research Program of Application Foundation and Advanced Technology (Grant No.17JCJQJC43500)
文摘Passively mode-locked fiber lasers emit femtosecond pulse trains with excellent short-term stability. The quantum-limited timing jitter of a free running femtosecond erbium-doped fiber laser working at room temperature is considerably below one femtosecond at high Fourier frequency. The ultrashort pulse train with ultralow timing jitter enables absolute time-of-flight measurements based on a dual-comb implementation, which is typically composed of a pair of optical frequency combs generated by femtosecond lasers. Dead-zone-free absolute distance measurement with sub-micrometer precision and kHz update rate has been routinely achieved with a dual-comb configuration, which is promising for a number of precision manufacturing applications, from large step-structure measurements prevalent in microelectronic profilometry to three coordinate measurements in large-scale aerospace manufacturing and shipbuilding. In this paper, we first review the sub-femtosecond precision timing jitter characterization methods and approaches for ultralow timing jitter mode-locked fiber laser design. Then, we provide an overview of the state-of-the-art dual-comb absolute ranging technology in terms of working principles, experimental implementations, and measurement precisions. Finally, we discuss the impact of quantum-limited timing jitter on the dual-comb ranging precision at a high update rate. The route to highprecision dual-comb range finder design based on ultralow jitter femtosecond fiber lasers is proposed.
基金This paper presents part of the achievement in the China National Key Project"Construction Techniques for Breakwa-ters in Deep Water"(96-415-02-03)
文摘The characteristics of wave transmission, reflection and energy dissipation of comb-type caisson breakwaters are studied through laboratory physical model tests. Regular and irregular waves, with a wide range of wave heights and periods and a constant water depth, are considered. Different dimensions of each portion of the comb-type caisson breakwater are tested. Empirical formulae for calculating the reduction coefficient k, which is the ratio of horizontal wave force on unit length of the comb-type breakwater to that on unit length of the vertical wall breakwater, and for calculating the reflection coefficient of waves kr are obtained from the measurements. The comb-type caisson breakwater has been found to be very efficient in dissipating incident wave energy and in reducing wave reflection, and has already been used for the construction of an island breakwater in the Dayao Bay of Dalian Port, Liaoning Province, China. Compared with the cost of a common caisson breakwater, about 24.5% of the investment has been saved owing to the use of this comb-type breakwater.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.61475099 and 61922040)Program of State Key Laboratory of Quantum Optics and Quantum Optics Devices,China(Grant No.KF201701)the Key R&D Program of Guangdong Province,China(Grant No.2018B030325002)。
文摘Based on the nonlinear Schr?dinger equation(NLSE) with damping, detuning, and driving terms describing the evolution of signals in a Kerr microresonator, we apply periodic nonlinear Fourier transform(NFT) to the study of signals during the generation of the Kerr optical frequency combs(OFCs). We find that the signals in different states, including the Turing pattern, the chaos, the single soliton state, and the multi-solitons state, can be distinguished according to different distributions of the eigenvalue spectrum. Specially, the eigenvalue spectrum of the single soliton pulse is composed of a pair of conjugate symmetric discrete eigenvalues and the quasi-continuous eigenvalue spectrum with eye-like structure.Moreover, we have successfully demonstrated that the number of discrete eigenvalue pairs in the eigenvalue spectrum corresponds to the number of solitons formed in a round-trip time inside the Kerr microresonator. This work shows that some characteristics of the time-domain signal can be well reflected in the nonlinear domain.
基金the National Key Research and Development Program of China(Grant No.2016YFF0200204)。
文摘Femtosecond optical frequency combs correlate the microwave and optical frequencies accurately and coherently.Therefore,any optical frequency in visible to near-infrared region can be directly traced to a microwave frequency.As a result,the length unit“meter”is directly related to the time unit“second”.This paper validates the capability of the national wavelength standards based on a home-made Er-doped fiber femtosecond optical frequency comb to measure the laser frequencies ranging from visible to near-infrared region.Optical frequency conversion in the femtosecond optical frequency comb is achieved by combining spectral broadening in a highly nonlinear fiber with a single-point frequencydoubling scheme.The signal-to-noise ratio of the beat notes between the femtosecond optical frequency comb and the lasers at 633,698,729,780,1064,and 1542 nm is better than 30 d B.The frequency instability of the above lasers is evaluated by using a hydrogen clock signal with a instability of better than 1×10^(-13)at 1-s averaging time.The measurement is further validated by measuring the absolute optical frequency of an iodine-stabilized 532-nm laser and an acetylenestabilized 1542-nm laser.The results are within the uncertainty range of the international recommended values.Our results demonstrate the accurate optical frequency measurement of lasers at different frequencies using the femtosecond optical frequency comb,which is not only important for the precise and accurate traceability and calibration of the laser frequencies,but also provides technical support for establishing the national wavelength standards based on the femtosecond optical frequency comb.
基金Project supported by the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant Nos.XDA1502040404 and XDB21010400)the National Natural Science Foundation of China(Grant Nos.91850209 and 11774234)。
文摘We present a fully stabilized Yb-fiber frequency comb locked to a microwave standard and an optical reference separately. The carrier-envelope offset frequency is generated by a standard f–2f interferometer with 40 dB signal-tonoise ratio. The offset frequency and the repetition rate are stabilized simultaneously to the radio frequency reference for more than 30 hours, and the fractional Allan deviation of the comb is the same as the microwave standard of 10^(-12) at 1 s.Alternatively, the comb is locked to an ultra-stable optical reference at 972 nm using an intracavity electro-optic modulator,exhibiting a residual integrated phase noise of 458 mrad(1 Hz–10 MHz) and an in-loop tracking stability of 1.77× 10^(-18) at 1 s, which is significantly raised by six orders comparing to the case locked to the microwave frequency standard.
基金supported by the National Natural Science Foundation of China(Nos.51490675,51322903 and 51279224)
文摘This study develops an analytical solution for oblique wave interaction with a comb-type caisson breakwater based on linear potential theory. The fluid domain is divided into inner and outer regions according to the geometrical shape of breakwater. By using periodic boundary condition and separation of variables, series solutions of velocity potentials in inner and outer regions are developed. Unknown expansion coefficients in series solutions are determined by matching velocity and pressure of continuous conditions on the interface between two regions. Then, hydrodynamic quantities involving reflection coefficients and wave forces acting on breakwater are estimated. Analytical solution is validated by a multi-domain boundary element method solution for the present problem. Diffusion reflection due to periodic variations in breakwater shape and corresponding surface elevations around the breakwater are analyzed. Numerical examples are also presented to examine effects of caisson parameters on total wave forces acting on caissons and total wave forces acting on side plates. Compared with a traditional vertical wall breakwater, the wave force acting on a suitably designed comb-type caisson breakwater can be significantly reduced. This study can give a better understanding of the hydrodynamic performance of comb-type caisson breakwaters.
文摘The prediction of protein coding regions in DNA sequences is an important problem in computational biology. It is observed that nucleotides in the protein coding regions or exons of a DNA sequence show period-3 property. Hence identification of the period-3 regions helps in predicting the gene locations within the billions long DNA sequence of eukaryotic cells. The period-3 property exhibited in exons of eukaryotic gene sequences enables signal processing based time-domain and frequency domain methods to predict these regions efficiently. Several approaches based on signal processing tools have, therefore, been applied to this problem, to predict these regions effectively. This paper describes novel and efficient comb filter-based techniques for the prediction of protein coding region based on the period-3 behavior of codon sequences. The proposed method is then validated on Burset/Guigo1996, HMR195 and KEGG standard datasets using various prediction measures. It is shown that cascaded differentiator comb (CDC) filter can be used for prediction of protein coding region with better prediction efficiency, and involves less computational complexity compared with the other signal processing techniques based on period-3 property.
基金This research was supported in part by the Natural Sciences and Engineering Research Council of Canada and the Fonds du Québec–Nature et Technologies.
文摘Optical frequency combs have emerged as an important tool enabling diverse applications from test-and-measurement,including spectroscopy,metrology,precision distance measurement,sensing,as well as optical and microwave waveform synthesis,signal processing,and communications.Several techniques exist to generate optical frequency combs,such as mode-locked lasers,Kerr micro-resonators,and electro-optic modulation.Important characteristics of optical frequency combs include the number of comb lines,their spacing,spectral shape and/or flatness,and intensity noise.While mode-locked lasers and Kerr micro-resonators can be used to obtain a large number of comb lines compared to electro-optic modulation,the latter provides increased flexibility in tuning the comb spacing.For some applications in optical communications and microwave photonics,a high degree of integration may be more desirable over a very large number of comb lines.In this paper,we review recent progress on integrated electro-optic frequency comb generators,including those based on indium phosphide,lithium niobate,and silicon photonics.
