Sub-100 fs all-fiber broadband electro-optic optical frequency comb at 1.5 µm

Highly precise timing alignment of multi-wavelength interleaved cavity-less pulse sources with FROG

Strictly uniform time interval between adjacent channels is a crucial requirement for the multi-wavelength interleaved (MWI) pulse sources, which difficult alignment can be easily solved by what we believe to be our novel method based on frequency-resolved optical gating (FROG). By utilizing highly precise measurements from FROG, which provide fully two-dimensional information of the pulses in time and frequency domain, we can intuitively identify the time mismatches between different channels in the MWI pulse sources. This enables us to directly align the timing of each channel with sub-picosecond resolution at the first time. MWI pulse sources with total repetition rate of 20.8 GHz (four wavelengths) and 41.6 GHz (eight wavelengths) are precisely aligned by the proposed method, this achievement will pave the way for advancements in photonic analog-digital converters (PADC), high-speed optical communications and so on.

Optical frequency comb assisted reconfigurable broadband spread spectrum signal generation

A photonic-assisted scheme for spread spectrum communication signals generation is proposed and demonstrated in this article. The spreading sequence and the baseband data codes are modulated on the photonic link by electro-optic modulators, and the spread spectrum process is completed through stream processing on the analog microwave photonic link. By combining optical frequency comb and injection locking technologies, the carrier frequency of the communication signals can be tuned over an ultra-broadband range of 3-39 GHz. In the proof-of-concept experiments, spread spectrum signals at 3 GHz and 6 GHz are obtained with a spread factor of 31. The analysis results indicate that the generated signals possess excellent reconfiguration, anti-interference, and anti-interception properties. Overall, our proposed scheme offers a flexible photonic architecture with significant potential in the application of ultra-broadband covert communication systems.

Massively parallel FMCW lidar with cm range resolution using an electro-optic frequency comb

Frequency-modulated continuous wave (FMCW) light detection and ranging (lidar) is a promising solution for three-dimensional (3D) imaging and autonomous driving. This technique maps range and velocity measurement to frequency counting via coherent detection. Compared with single-channel FMCW lidar, multi-channel FMCW lidar can greatly improve the measurement rate. A chip-scale soliton micro-comb is currently used in FMCW lidar to enable multi-channel parallel ranging and significantly increase the measurement rate. However, its range resolution is limited due to the soliton comb having only a few-GHz frequency sweep bandwidth. To overcome this limitation, we propose using a cascaded modulator electro-optic (EO) frequency comb for massively parallel FMCW lidar. We demonstrate a 31-channel FMCW lidar with a bulk EO frequency comb and a 19-channel FMCW lidar using an integrated thin-film lithium niobate (TFLN) EO frequency comb. Both systems have a sweep bandwidth of up to 15 GHz for each channel, corresponding to a 1-cm range resolution. We also analyze the limiting factors of the sweep bandwidth in 3D imaging and perform 3D imaging for a specific target. The measurement rate achieved is over 12 megapixels per second, which verifies its feasibility for massively parallel ranging. Our approach has the potential to greatly benefit 3D imaging in fields where high range resolution is required, such as in criminal investigation and precision machining.

Time reversal of broadband microwave signal based on frequency conversion of multiple subbands

Time reversal of broadband microwave signals based on frequency conversion of multiple subbands is proposed and experimentally demonstrated. The broadband input spectrum is cut into a number of narrowband subbands, and the center frequency of each subband is reassigned by multi-heterodyne measurement. The input spectrum is inversed, while the time reversal of the temporal waveform is also realized. The equivalence between time reversal and the spectral inversion of the proposed system is verified by mathematical derivation and numerical simulation. Meanwhile, spectral inversion and time reversal of a broadband signal with instantaneous bandwidth larger than 2 GHz are experimentally demonstrated. Our solution shows good potential for integration where no dispersion element is employed in the system. Moreover, this solution for an instantaneous bandwidth larger than 2 GHz is competitive in the processing of broadband microwave signals.

Narrow linewidth optical frequency comb based on a directly modulated microcavity laser with optical feedback

A narrow linewidth optical frequency comb (OFC) based on a directly modulated microcavity laser with external optical feedback is investigated numerically and demonstrated experimentally. Based on the numerical simulations with rate equations, the evolution of the optical and electrical spectra is presented for the direct-modulated microcavity laser with increased feedback strength, and the linewidth property is improved at suitable feedback conditions. The simulation results also show good robustness for the generated OFC in terms of feedback strength and phase. Moreover, the OFC generation experiment is performed by combining with the dual-loop feedback structure to suppress the side mode, and an OFC with a side-mode suppression ratio of 31 dB is realized. Thanks to the high electro-optical response of the microcavity laser, a 15-tone OFC with a frequency interval of 10 GHz is obtained. Finally, the linewidth of each comb tooth is measured to be around 7 kHz under the feedback power of 47 µW, which indicates an enormous compression of approximately 2000 times compared with the free-running continuous-wave microcavity laser.

Wideband multiwavelength Brillouin fiber laser with switchable channel spacing

A multiwavelength Brillouin fiber laser (MBFL) with a switchable channel spacing is demonstrated using a 1.55-µm single-mode AlGaInAs/InP hybrid square-rectangular laser as a seeding source. The scheme employs a highly nonlinear fiber loop with a feedback path to generate a 10-GHz-spacing MBFL. Then, assisted by a tunable optical bandpass filter, MBFLs with spacing from 20 GHz to 100 GHz at a step of 10 GHz are generated in another highly nonlinear fiber loop based on the cavity-enhanced four-wave mixing. More than 60 lasing lines with an optical signal-to-noise ratio over 10 dB are obtained successfully in all the switchable spacings. The total output power and the channel spacing of the MBFLs are proved to be stable.

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.

Design of Broadband Flat Optical Frequency Comb Based on Cascaded Sign-Alternated Dispersion Tellurite Microstructure Fiber

We designed a tellurite microstructure fiber (TMF) and proposed a broadband optical frequency comb generation scheme that was based on electro-optical modulation and cascaded sign-alternated dispersion TMF (CSAD-TMF). In addition, the influence of different nonlinear effects, the ultrashort pulse evolution in the CSAD-TMF with the anomalous dispersion (AD) zones and the normal dispersion (ND) zones were analyzed based on the generalized nonlinear Schrodinger equations (GNLSE) modelling. According to the simulations, when the input seed comb had a repetition rate of 20 GHz and had an input pulse peak power of 30 W, the generation scheme could generate optical frequency combs with a 6 dB spectral bandwidth spanning over 170 nm centered at 1550 nm. Furthermore, the generated combs showed good coherence in performance over the whole 6 dB spectral bandwidth. The highly coherent optical frequency combs can be used as high-repetition-rate, multi-wavelength light sources for various integrated microwave photonics and ultrafast optical signal processing applications.

Electro-optic comb pumped optical parametric oscillator with flexible repetition rate at GHz level

We present a gigahertz (GHz)-repetition-rate optical parametric oscillator (OPO) pumped by an electro-optic comb at 1.03 µm, delivering sub-picosecond signal pulses across 1.5-1.7 µm from a MgO-doped periodically poled LiNbO₃ crystal. Using a pump power of 5 W at 14.2 GHz repetition rate, 378 mW of signal power is obtained at 1.52 µm from a subharmonic cavity, corresponding to a signal extraction efficiency of 7.6%. By cascading a Mach-Zehnder modulator, the pump pulse repetition rate can be divided by any integer number from one to 14, allowing the OPO to operate with a flexible repetition rate from 1 to 14.2 GHz. A strategy leading to quasi-continuous repetition rate tunability of the OPO is also discussed.