Electro-optic Frequency Comb Research Articles

Single-modulator, direct frequency comb spectroscopy via serrodyne modulation.

Traditional electro-optic frequency comb spectrometers rely upon the use of an acousto-optic modulator (AOM) to provide a differential frequency shift between probe and local oscillator (LO) legs of the interferometer. Here we show that these modulators can be replaced by an electro-optic phase modulator (EOM) which is driven by a sawtooth waveform to induce serrodyne modulation. This approach enables direct frequency comb spectroscopy to be performed with a single dual-drive Mach-Zehnder modulator (DD-MZM), allowing for lower differential phase noise. Further, this method allows for simpler production of integrated photonic comb spectrometers on the chip scale.

A power-efficient integrated lithium niobate electro-optic comb generator

Integrated electro-optic (EO) frequency combs are essential components for future applications in optical communications, light detection and ranging, optical computation, sensing and spectroscopy. To date, broadband on-chip EO combs are typically generated in high-quality-factor micro-resonators, while the more straightforward and flexible non-resonant method, usually using single or cascaded EO phase modulators, often requires high driving power to realize a reasonably strong modulation index. Here, we show that the phase modulation efficiency of an integrated lithium niobate modulator could be enhanced by passing optical signals through the modulation electrodes for a total of 4 round trips, via multiple low-loss mode multiplexers and a waveguide crossing, reducing electrical power consumption by an experimentally measured factor of 15. Using devices fabricated from a wafer-scale stepper lithography process, we demonstrate a broadband optical frequency comb featuring 47 comb lines at a 25-GHz repetition rate, using a moderate radio frequency (RF) driving power of 28 dBm (0.63 W). Leveraging the tunability in repetition rate and operation wavelength, our power-efficient EO comb generator could serve as a compact low-cost solution for future high-speed data transmission, sensing and spectroscopy, as well as classical and quantum optical computation systems.

Non-linear amplification to 200 W of an electro-optic frequency comb with GHz tunable repetition rates

We present a monolithic Yb-doped fiber laser system delivering 200 W average power of femtosecond pulses at tunable GHz repetition rates. The system is based on a GHz electro-optic (EO) frequency comb operating in the nonlinear regime. The EO comb pulses at 1 µm wavelength are initially pre-compressed to sub-2 ps, amplified to 2.5 W, and finally boosted to 200 W in a newly designed large-mode-area, Yb-doped photonic crystal fiber. Continuously tunable across 1-18 GHz, the picosecond pulses experience nonlinear propagation in the booster amplifier, leading to output pulses compressible down to several hundreds of femtoseconds. To push our system deeper into the nonlinear amplification regime, the pulse repetition rate is further reduced to 2 GHz, enabling significant spectral broadening at 200 W. Characterization reveals sub-200 fs duration after compression. The present EO-comb seeded nonlinear amplification system opens a new route to the development of high-power, tunable GHz-repetition-rate, femtosecond fiber lasers.

Multi-GHz repetition rate femtosecond electro-optic frequency comb based on one single phase modulator and non-linear processes

We report on a multi-GHz repetition rate, femtosecond fiber laser operating in the burst mode, achieved by nonlinearly shaping and amplifying a phase-only modulated electro-optic comb at 1.03 μm. The system delivers an average power of 1.2 W with pulses compressible down to sub 100 fs.

Broadband coherently synthesized two-color electro-optic frequency combs with phase noise suppression

A coherently synthesized 1550/1560 nm two-color EO comb is generated by modulating two continuous-wave lasers with common EO modulators and phase-locking their beat note. Compared with a single-color EO comb, the phase noise of comb modes that accumulated from the RF synthesizer is effectively suppressed in our coherently synthesized two-color EO comb. Besides, a broader spectrum is demonstrated in a coherently synthesized 1535/1560 nm two-color EO comb, with a 60-nm total spectrum span. The coherently synthesized two-color EO combs are expected to be applied in coherent communication, spectroscopy, absolute distance measurement, etc.

Comb-mode resolved spectral domain interferometer enabled by a broadband electro-optic frequency comb

The spectral domain interferometer (SDI) has been widely used in dimensional metrology. Depending on the nature of the SDI, both wider spectral bandwidth and narrower linewidth of the light source are paradoxically required to achieve better resolution and longer measurable distances. From this perspective, a broadband frequency comb with a repetition rate high enough to be spectrally resolved can be an ideal light source for SDIs. In this paper, we propose and implement a broadband electro-optic frequency comb to realize a comb-mode resolved SDI. The proposed electro-optic frequency comb was designed with an optically recirculating loop to provide a broadband spectrum, which has a repetition rate of 17.5 GHz and a spectral range of 35 nm. In a preliminary test, we demonstrated absolute distance measurements with sub-100 nm repeatability. Because of these advantages, we believe this electro-optic frequency comb can open up new possibilities for SDIs.

High-Q Thin-Film Lithium Niobate Microrings Fabricated with Wet Etching.

Thin-film lithium niobate (TFLN) has been widely used in electro-optic modulators, acoustic--optic modulators, electro-optic frequency combs, and nonlinear wavelength converters owing to the excellent optical properties of lithium niobate. The performance of these devices is highly dependent on the fabrication quality of TFLN. Although state-of-the-art TFLN microrings with an intrinsic quality factor (Q-factor) exceeding 1 × 107 have been realized by inductively coupled plasma-reactive ion etching (ICP-RIE) and chemical mechanical polishing (CMP), ICP-RIE has moderate throughput, moderate reproducibility, and high cost in etching TFLN, while CMP features moderate throughput and low cost in etching TFLN. Here, a wet etching method for TFLN, leading to the fabrication of a micro-racetrack with an intrinsic Q-factor of over 9.27 × 106 is developed. The suitability of this method to fabricate a narrow coupling gap between the bus waveguide and microring enables the coupling conditions of the microring to be customized. This method features a high throughput, a high reproducibility, and a low cost in etching TFLN, showing the potential to boost the mass production of integrated LN photonic devices with high fidelity and affordabilitydramatically.

Phase Noise Analysis of Resonator-Enhanced Electro-Optic Comb-Based Analog Coherent Receivers

We present an architecture for wavelength-division-multiplexed coherent data center links employing resonator-enhanced electro-optic frequency comb generators as both transmitter and local oscillator (LO) sources. We show that in each frequency comb, the phase noise in all comb lines is determined completely by the optical phase noise of the seed laser and the phase noise of the microwave oscillator. We propose an analog coherent receiver that performs carrier recovery on all wavelength channels using only two phase-locked loops. We use an independent cascade of phase shifters driven by marker tone detection in each channel for polarization recovery and compensation of a static optical phase shift. We study design examples for quadrature phase-shift keying and 16-ary quadrature amplitude modulation (16-QAM), determining requirements on the transmitter and LO seed laser linewidths and the LO frequency modulation (FM) response. We find that for a dual-polarization (DP) 16-QAM link with 17 wavelength channels, each operating at 56 GBaud, if the combined linewidth of the transmitter and LO seed lasers is 100 kHz, the LO seed laser requires an FM response that is flat from d.c. to several hundred MHz to maintain a phase-error penalty below 1 dB.

Nonlinear spectral broadening of a dual-carrier electro-optic frequency comb in a graphene oxide cladded silicon-rich nitride hybrid waveguide.

We demonstrate a detailed theoretical analysis describing the generation of an electro-optic comb (EOC) in the near-IR range through discrete phase and amplitude modulation driven by radio frequency (RF) signal generators. Furthermore, the generated EOC spectra suffer nonlinear spectral broadening while propagating through a hybrid Si-rich nitride (SRN) waveguide structure integrated with two-dimensional (2D) layered graphene oxide (GO) films. We perform a detailed analysis to investigate the influence of GO layers, pump wavelength detuning, and other waveguide parameters on the evolution of comb spectra propagating through the hybrid waveguide structure. Owing to the strong modal overlapping between the SRN waveguides and the highly nonlinear GO films, the nonlinearity of the system is enhanced effectively, and broadband comb spectra have been achieved in the near-IR range. Furthermore, we investigate the spectral coherence of the generated comb spectra under different input conditions. The results exhibit strong potential to generate a tunable frequency comb with high spectral coherence in the near-IR range by employing the hybrid waveguide structure.

High-efficiency and broadband on-chip electro-optic frequency comb generators

Developments in integrated photonics have led to stable, compact, and broadband comb generators that support a wide range of applications. Current on-chip comb generators, however, are still limited by low optical pump-to-comb conversion efficiencies. Here, we demonstrate an integrated electro-optic frequency comb with a conversion efficiency of 30% and an optical bandwidth of 132 nm, featuring a 100-times higher conversion efficiency and 2.2-times broader optical bandwidth compared with previous state-of-the-art integrated electro-optic combs. We further show that, enabled by the high efficiency, the device acts as an on-chip femtosecond pulse source (336 fs pulse duration), which is important for applications in nonlinear optics, sensing, and computing. As an example, in the ultra-fast and high-power regime, we demonstrate the observation of a combined EO-\chi^(3) nonlinear frequency comb. Our device paves the way for practical optical frequency comb generators enabling energy-efficient computing, communication, and metrology, and provides a platform to investigate new regimes of optical physics that simultaneously involve multiple nonlinearities.

Air-gap Fabry-Pérot cavity filtered 30 nm broadband electro-optic frequency combs for high-order coherent communications.

Broadband electro-optic (EO) frequency combs, which have flexible and high repetition frequencies, are prospective light sources for dense-wavelength-division-multiplexed coherent optical communications. In most cases, nonlinear spectral broadening and amplification procedures are needed to achieve broadband and high-power EO frequency combs. This leads to a low optical carrier-to-noise ratio (OCNR) for comb lines, limiting the transmission capacity. Here, we propose to use an air-gap Fabry-Pérot (FP) cavity to improve the OCNR for all the comb lines covering a 30 nm broadband spectrum. A 12 dB OCNR (0.1 nm bandwidth) improvement is obtained experimentally via using an FP cavity with ∼790 MHz bandwidth. We apply a 150-channel filtered EO comb with 25 GHz channel spacing and load 20 GBaud signals on each comb line to demonstrate the effect of OCNR improvement. The 137/150 channels have a bit error rate below the threshold of soft-decision forward error correction when using the 128 quadrature amplitude modulation (QAM) format. However, none of these channels can support this modulation format without cavity filtering. We also investigate dispersion tolerance and the long-term stability when using an air-gap FP cavity, highlighting its advantages. Our results show a practical solution to boost the transmission capacity when applying broadband EO combs in optical communications.

Multi-Band Operation of Flat-Top Supercontinuum Laser Sources With Programmable Repetition Rate up to 50 GHz

We present ultra-broadband flat-topped supercontinuum laser sources with programmable repetition rate, spectral bandwidth, and wavelength regime. Supercontinuum sources with multiband coverage can be achieved by merging high-repetition-rate electro-optic frequency comb techniques with optical line-by-line pulse shaping. Because nonlinear broadening in the optical wave-breaking regime does not provide the required flatness and spectral bandwidth to the comb-based sources, we iteratively manipulate the spectral amplitude and phase of the optical pulse shaper. We implemented here a smooth flat-topped supercontinuum with a 97 nm bandwidth at 10 dB and programmable repetition rate up to 50 GHz by applying a bandwidth-limited quasi-super-Gaussian apodised pulse train to a highly nonlinear medium with programmable super-Gaussian and phase coefficients. This work opens new possibilities for generating robust and coherent multiple optical carriers with further C-bands in optical networks. We believe that the supercontinuum sources based on electro-optic frequency comb and pulse shaping techniques enable the provision of coherent flat-topped optical sources, at least from the L- to S-band with a programmable repetition rate and spectral bandwidth.

Electro-optic frequency comb based IPDA lidar: assessment of speckle issues.

We present a theoretical, numerical and experimental assessment of the impact of speckle on a dual electro-optic frequency comb (EOFC) based system for integrated path differential absorption (IPDA) measurements. The principle of gas concentration measurements in a dual EOFC configuration in the absence of speckle is first briefly reviewed and experimentally illustrated using a C2H2 gas cell. A numerical simulation of the system performance in the presence of speckle is then outlined. The speckle-related error in the concentration estimate is found to be an increasing function of the product between the roughness of the backscattering surface and the EOFC line-spacing. As this product increases, the speckle-induced power fluctuations in the comb lines are no longer correlated to each other. To confirm this, concentration measurements are conducted using backscattered light from two different surfaces. Experiment results are in very good agreement with numerical simulations. Though detrimental for IPDA measurements, it is finally shown that decorrelation of speckle noise can be advantageously exploited for surface characterization in a dual EOFC configuration.

Enhanced frequency-modulated continuous-wave generation by injection-locking period-one oscillation in a semiconductor laser with an intensity modulated comb.

We report on an enhanced photonic generation of frequency-modulated continuous-wave (FMCW) signals by injection-locking a semiconductor laser operating in period-one (P1) nonlinear dynamic with an intensity modulated electro-optic frequency comb. When the cavity mode is injection-locked with respect to any of the comb modes, through linearly sweeping the frequency of the injected comb mode while synchronously modulating the injected intensity, the center wavelength of the cavity mode can be tuned following the injected comb mode. This way, it allows maintaining the phase-locking between the cavity mode and comb mode even if beyond the original locking bandwidth of the cavity mode, since it is tuned accordingly. It thus leads to the generation of FMCW signal with efficient phase noise suppression and improved achievable sweep range compared with the limited original injection-locking bandwidth. Such injection enhanced phase-locking is investigated and a demonstration with the injection of -4th order comb mode has realized photonic FMCW generation with enhanced sweep range and suppressed phase noise. Thanks to the flexibility in sweep parameters, this method can also be readily applied for the generation of arbitrary waveforms.

High-power nonlinear amplification of an ultrafast electro-optic frequency comb with flexible GHz repetition rate.

We report on an all-fiber 200 W widely tunable GHz electro-optic (EO) frequency comb operating in the nonlinear regime. The EO comb pulses at 1030 nm are initially pre-compressed to sub-2 ps, then power amplified to 2.5 W, and finally boosted to 200 W in a newly designed large-mode-area, Yb-doped photonic crystal fiber. Continuously tunable across 12-18 GHz, the picosecond pulses experience nonlinear propagation in the last amplifier, leading to output pulses compressible down to several hundreds of femtoseconds. To push our system deeper into nonlinear amplification regime, the pulse repetition rate is further reduced to 2 GHz, enabling significant spectral broadening at 200 W. Characterization reveals sub-200 fs duration after compression. The present EO-comb seeded nonlinear amplification system opens a new route to the development of high-power, tunable GHz-repetition-rate, femtosecond fiber lasers.

Flat Optical Frequency Comb Generator Based on Integrated Lithium Niobate Modulators

Chip-scale electro-optic (EO) frequency combs are expected to play an essential role in future high-capacity optical communications systems and next-generation mobile communications. The application requires integrated EO frequency comb generators featuring good spectral flatness, high modulation bandwidth, low driving voltage and low insertion loss simultaneously. Here, we demonstrate a flat-top optical flat comb (OFC) generator based on high-performance lithium niobate on insulator modulators. The OFC generator shows a low on-chip loss (2.1 dB), a low driving voltage over a broad frequency range, and a large 3-dB EO bandwidth. Moreover, with consuming power of less than 2W, the presented device produces 13 lines with a power variation of less than 1.2 dB and a line spacing of 31 GHz which can further be extended to 67 GHz.

Sub-femtometer-resolution absolute spectroscopy with sweeping electro-optic combs

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 kHz 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 × 10⁸ 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 dB 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.

Progress in integrated electro-optic frequency combs (Invited)

Progress in integrated electro-optic frequency combs (Invited)

Dynamics and timing-jitter of regenerative RF feedback assisted resonant electro-optic frequency comb

Optical frequency combs have impacted a wide range of applications. The optical cavity assisted resonant electro-optic (EO) comb features broad spectral bandwidth and deterministic repetition-rate. However, the optical pulse timing-jitter is limited by the phase noise of the external RF drive source. To overcome this limitation at high repetition-rate, a regenerative RF feedback assisted resonant EO comb has been experimentally developed, which shows low phase noise of the regenerative RF drive signal. However, yet to date, the nonlinear dynamics and timing-jitter of the resonant EO comb coupled with a regenerative RF feedback loop remain unexplored. Here, for the first time, we quantitatively investigate the dynamics and timing-jitter of the resonant EO comb generation coupled with an optoelectronic oscillator (OEO). Our model indicates the detuning of the seed laser and EO comb cavity resonance determines the nonlinear feedback RF gain of OEO. Numerical simulations reveal the emergence of the bifurcation behaviors of the regenerative RF signal and the distinct EO comb formation dynamics with the increasing feedback RF gain of OEO, which provide the guidelines to achieve stable regenerative resonant EO comb. Moreover, in the presence of the noisy nonlinear feedback RF gain induced by the laser-cavity detuning fluctuations, our timing-jitter model predicts the seed laser frequency noise will be converted to the regenerative RF phase noise and optical pulse timing-jitter under the stable oscillation state of OEO. We highlight the mechanism of the transduction of laser frequency noise to the RF phase noise is differ from the Mach-Zehnder modulator based OEO. Experimentally, we attain a 10 GHz resonant EO comb generation with phase noise of −130 dBc/Hz at 10 kHz offset for the regenerative 10 GHz RF signal and optical pulse timing-jitter of 52.8 fs with an integral range from 100 Hz to 1 MHz. The experimental results agree well with the theoretical models. Our findings offer the solid guidelines for the design of the regenerative RF feedback assisted stable resonant EO comb with an optimized timing-jitter performance.

Flexible Ultra-Wide Electro-Optic Frequency Combs for a High-Capacity Tunable 5G+ Millimeter-Wave Frequency Synthesizer

This paper presents a new scheme of a cost-effective tunable millimeter-wave (MMW) frequency synthesizer based on an ultra-wideband electro-optic frequency comb. The architecture for the quasi-tunable millimeter-wave frequency synthesizer mainly consists of a compact ultra-wide flat electro-optic frequency comb and a multi-tone frequency generator, which only includes a quantum dot mode-locked laser, a LiNbO3 dual-driving Mach–Zehnder modulator (DD-MZM) and Uni-traveling-carrier photodiode (UTC-PD). MMW signals generated with a quasi-tunable frequency are experimentally demonstrated. The difference in power is obtained for the different frequencies. The linewidth of the quasi-tunable frequency signals is less than 273 Hz. In addition, the single side band (SSB) phase noise of the 25, 37.5, 50 and 75 GHz is measured as −115, −106, −102 and −95 dBc/Hz at an offset of 1 kHz, respectively. The proposed frequency synthesizer has ultra-low phase noise, quasi-tunable frequency and simple structure. The research results of the frequency synthesizer are applied for 5G+ transmission with radio wave working at K-band and V-band. The flexible, compact and robust MMW frequency synthesizer is suitable for the future of ultra-high capacity 5G+ communication.