Tunable Comb Research Articles

Generation of Radiation with a Tunable Comb Spectrum in Ring Fiber Cavities Based on Active Photonic Crystal Structures

Generation of Radiation with a Tunable Comb Spectrum in Ring Fiber Cavities Based on Active Photonic Crystal Structures

Design and simulation of a tunable comb-drive actuator with a wide frequency tuning range and low pull-in voltage

The presented paper describes the design, simulation, and analysis of a novel tunable comb-drive actuator that aims to increase the range of tuning resonance frequency while reducing the pull-in voltage. The conventional comb resonator has a limited resonant frequency tuning range, and hence, modifying the spring stiffness of the structure is crucial to obtaining a tunable comb resonator. The proposed design includes eight flexible beams on each side that support a set of comb finger parts. The actuator achieves the goal of shifting the resonance frequency both downwards and upwards by utilizing Serpentine nested-folded beams. A triangular comb with non-uniform varied finger lengths combined with variable gap fingers is designed to adjust the frequency downwards, while a comb with constant finger lengths combined with variable gap fingers is designed to adjust the frequency upwards. To simulate and design the structure, the IntelliSuite software is utilized. The results show that the actuator has a primary resonant frequency of 3052 Hz, which can be lowered to 335 Hz by applying a tuning voltage of 74 V to the downward tuning part. Similarly, the resonant frequency can be increased to 3159 Hz by applying a tuning voltage of 60 V to the upward tuning part. The resonator achieves a maximum frequency tuning range of 90%, and the simulation results are in good agreement with the theoretical predictions. However, it is worth noting that the size of this resonator is relatively small, approximately 1077 × 328 μm2. This innovative design offers potential applications in various fields, including micro-electromechanical systems (MEMS), micro-optics, and microsensors. Overall, the presented work demonstrates the feasibility of achieving a tunable comb resonator with a significantly improved resonant frequency tuning range and reduced pull-in voltage.

Research on Optical Mutual Injection to Generate Tunable Microwave Frequency Combs

In this study, a scheme for generating tunable microwave frequency combs (MFCs) based on optical mutual injection is proposed and experimentally investigated. The scheme is based on the optical injection of lasers to generate MFCs, and constitutes a feedback loop by using dual-laser mutual injection to obtain MFCs with a large continuous bandwidth and tunable comb spacing. The experimental setup analyzes the effects of injected optical power, modulation frequency and amplitude, and wavelength detuning on the generated MFC signals. The experimental results indicate that when the single-frequency electrical signal is set to 2 GHz, flat MFCs with amplitude variations within 10 dB can be obtained by optimizing the injected power and the frequency detuning between the two semiconductor lasers. Furthermore, the comb spacing of the MFCs can be made tunable by varying the modulation frequency and selecting the matched operating parameters to adapt to different application scenarios.

Frequency combs induced by optical feedback and harmonic order tunability in quantum cascade lasers

This study investigates the interaction between frequency combs and optical feedback effects in Quantum Cascade Lasers (QCLs). The theoretical analysis reveals new phenomena arising from the interplay between comb generation and feedback. By considering the bias current corresponding to free-running single mode emission, the introduction of optical feedback can trigger the generation of frequency combs, including both fundamental and harmonic combs. This presents opportunities to extend the comb region and generate harmonic frequency combs with different orders through optimization of external cavity parameters, such as losses and length. Furthermore, this study demonstrates that optical feedback can selectively tune the harmonic order of a pre-existing free-running comb by adjusting the external cavity length, particularly for feedback ratios around 1%, which are readily achievable in experimental setups. Under strong feedback conditions (Acket parameter C > 4.6), mixed states emerge, displaying the features of both laser and external cavity dynamics. While this study is predominantly centered on terahertz QCLs, we have also confirmed that the described phenomena occur when utilizing mid-infrared QCL parameters. This work establishes a connection between comb technology and the utilization of optical feedback, providing new avenues for exploration and advancement in the field. In fact, the novel reported phenomena open a pathway toward new methodologies across various domains, such as the design of tunable comb sources, hyperspectral imaging, multi-mode coherent sensing, and multi-channel communication.

Wavelength-tunable soliton molecule in spatiotemporal mode-locked Yb-doped fiber laser

Spatiotemporal mode-locking (STML) fiber lasers could realize the synchronized locked of multiple transverse and longitudinal modes, which could be regarded as an ideal platform to investigate high-dimensional nonlinear dynamics. The observation of STML in the soliton molecule state could offer a unique opportunity to reveal the mechanism of molecular complexity. Due to the distinctive spatiotemporal characteristics, realizing the wavelength tunable of soliton molecule in STML fiber laser is still challenging. In this letter, we report wavelength-tunable soliton molecules in an STML Yb-doped fiber laser. By rotating the wave plates at a fixed pump power, both single dissipative soliton (SDS) and various soliton molecules, including loose bounded dissipative soliton pairs (BDSPs), tight BDSPs, soliton molecules of triplet pulses or soliton molecules of quadruplets pulses, have all realized wavelength tunable with a tuning range of ∼25 nm. The realization of the wavelength tunable and the formation of the soliton molecule can all be attributed to the tunable comb filter and saturable absorber induced by the nonlinear polarization rotation technique. The formation of SDS and BDSPs was further confirmed by the numerical simulations. And the numerical results contribute to further comprehending the nonlinear dynamics of the STML fiber laser.

On-chip gain switched frequency comb generation using a two sectioned single cavity laser without additional optical injection.

A tunable comb source is demonstrated on a monolithically integrated photonic integrated circuit (PIC). The PIC is a two section device designed to produce a single mode tunable spectrum, and the comb is generated by gain switching one section of the two sectioned laser. The laser produces a single mode spectra with a tunable range of 1543 - 1565 nm, and combs were generated with a frequency range of 1 - 10 GHz without requiring additional optical injection to maintain the phase coherence.

Tunable, coherent optical comb source via on-chip bidirectional coupling.

A tunable comb source is demonstrated through gain switching on a three-sectioned photonic integrated circuit (PIC). The PIC consists of two mutually coupled lasers connected by a passive waveguide. One of these is a tunable, two-section, single mode laser. The second laser is a simple Fabry-Perot cavity laser which can be phase-locked with the single mode laser via bidirectional coupling. Frequency combs are produced by gain switching the Fabry-Perot laser by applying a high-power radio frequency signal. Combs are generated with line spacings ranging from 3.5 to 8 GHz. The on-chip bidirectional coupling causes the comb to also be generated in the two-section device. Despite the lack of on-chip optical isolation between the lasers, the resulting combs are stable. Numerical simulations using a delay-differential model reproduce the results and reveal the important role played by the short delay times inherent to on-chip integration in this stability.

Optical Spectral Shaping Based on Reconfigurable Integrated Microring Resonator-Coupled Fabry–Perot Cavity

We demonstrate a multifunctional optical spectral shaper on a silicon photonic chip. The proposed structure is composed of a tunable microring resonator (MRR) and three tunable Sagnac loop mirrors (SLMs), which can be regarded as a large SLM incorporating an MRR-coupled Fabry–Perot (FP) cavity. To realize flexible tuning, thermo-optically tunable Mach-Zehnder interferometer (MZI) couplers are used to replace conventional directional couplers with fixed coupling coefficients. By controlling the tunable MZI couplers, the proposed structure can be reconfigured as various functional structures including an all-pass MRR with tunable coupling regime, a tunable FP cavity, an SLM incorporating FP cavity, and an MRR-coupled FP cavity. This flexible reconfiguration allows for multifunctional optical spectral shaping including tunable comb filtering, Fano and EIT-like resonances. In particular, the complementary flat-top comb filtering responses can be theoretically obtained in the two ports of this device with same free spectral range (FSR) value, which enable the (de)interleaver operation. The experimental results show that the implemented all-pass MRR has a maximum extinction ratio (ER) of 38.9 dB and the corresponding quality factor is 6.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> . The comb filter based on configured FP cavity can be switched from narrowband response (0.06 nm) to wideband flat-top response (0.23 nm), with ERs up to 10.6 dB and 22 dB, respectively. Moreover, a dynamic conversion between Lorentz, EIT, and Fano resonance line shapes based on the coupled resonant system can be realized. The demonstrated integrated device holds great potential for various applications such as filtering and switching in optical wavelength-division multiplexing (WDM) communication networks.

A Rational Harmonic Mode-Locking Optoelectronic Oscillator for Microwave Frequency Comb Generation

Recently, optoelectronic oscillators (OEOs) have shown good behavior in microwave signal generation. In this letter, we propose a rational harmonic mode-locking OEO (RHML-OEO) to generate microwave frequency combs (MFCs) with tunable comb intervals. By embedding an extra Mach–Zehnder modulator (MZM) into the OEO cavity, the intensity of the oscillating mode can be modulated by the signal applied on the MZM, and sidebands having fixed phase differences are produced in addition to the oscillating mode. A main feature of the RHML-OEO is that there is a frequency detuning between the modulation signal and longitudinal mode spacing of the OEO. Consequently, MFCs with large comb intervals can be generated even with a low-frequency modulation signal. In our experiment, third-order, fifth-order, and tenth-order RHML-OEOs are realized to generate MFCs with different frequency intervals of 1.128, 1.88, and 3.76 MHz when the modulation frequencies are 564, 940 kHz, and 1.253 MHz, respectively.

RFI Mitigation in Time Domain Wideband Autocorrelation Radiometry (WiBAR) Using a Comb Filter

This letter presents a new hardware setup to suppress radio frequency interference (RFI) for a recently developed microwave radiometer technique, known as wideband autocorrelation radiometry (WiBAR). WiBAR is a method that can directly measure the thickness of a low-loss layer like lake ice or dry snow on the ground by finding the lag time that corresponds to the transit time of layer in the autocorrelation function (ACF) of the received signal. However, RFI increases the noise floor of the ACF and results in a decreased signal-to-noise ratio (SNR) of the WiBAR delay peak in the ACF. We enhanced a WiBAR instrument with a tunable comb filter having a frequency response with many evenly spaced alternating pass and stop bands. We show the RFI mitigation performance of a WiBAR set-up with a comb filter in the laboratory with a simulation circuit that creates a spectrum polluted with RFI.

Self-Injection Locking of a Gain-Switched Laser Diode

We experimentally observed self-injection locking regime of the gain-switched laser to high-Q optical microresonator. We revealed that comb generated by the gain-switched laser experiences a dramatic reduce of comb teeth linewidths in this regime. We demonstrated the Lorentzian linewidth of the comb teeth of sub-kHz scale as narrow as for non-switched self-injection locked laser. Such setup allows generation of high-contrast electrically-tunable optical frequency combs with tunable comb line spacing in a wide range from 10 kHz up to 10 GHz. The characteristics of the generated combs were studied for various modulation parameters - modulation frequency and amplitude, and for parameters, defining the efficiency of the self-injection locking - locking phase, coupling efficiency, pump frequency detuning.

Continuously Tunable Comb Filter Based on a High-Birefringence Fiber Loop Mirror With a Polarization Controller

A spectrum tunable method is presented for a high-birefringence fiber loop mirror (HiBi-FLM) comb filter with a polarization controller (PC) in the loop, where not only the wavelength of the comb filter can be continuously tuned with peak transmission remaining at unity, but the peak amplitude can be also continuously tuned without wavelength shift. By using the Jones matrix, we theoretically analyze the comb filter. The analytical expressions of the reflectivity and transmissivity are derived in terms of the orientation angles of three waveplates of the PC. The optimum conditions for wavelength and peak amplitude tunable operations are obtained. Our theoretical results show that when the two quarter-waveplate (QWP) angles are optimally set, the PC could act as a phase shifter or a polarization rotator, which empowers the comb filter to be wavelength continuously tunable or to be peak amplitude variable from 0 to 1 by only rotating the half-waveplate (HWP) of the PC. Moreover, the wavelength shift has a linear relationship with the HWP angle. When the two QWP angles are not optimized, PC has a combined effect of a phase shifter and a polarization rotator, leading to a wavelength shift accompanied by a change of the peak amplitude when tuning the HWP. Our theoretical prediction has been verified by experimental results.

Efficient carrier-envelope phase tunable mid-infrared frequency combs based on CW-seeded optical parametric generation

We present an efficient method for generating frequency combs in the mid-infrared (MIR) spectral range in bulk lithium niobate crystals as well as in waveguides. Our approach is simple and robust, since it is based on single-pass configuration of femtosecond Optical Parametric Generation (OPG) seeded by a telecom continuous-wave laser. Precise and fast tuning of the seed laser allows to rapidly change the offset frequency of the generated MIR comb independent of its repetition rate. The MIR comb offset frequency is inherently known, so its direct detection is not required. An additional degree of freedom that further complements the versatility of our light source is a unique way of repetition rate or mode spacing adjustments. This is achieved by our new concept of coherent multi-seeding that depending on the configuration allows for pulse train modulation and mode spacing, or repetition rate division by an integer number.

Optical Frequency Comb Generation via Cascaded Intensity and Phase Photonic Crystal Modulators

Nanophotonics, driven by low processing power and high density integration, is appearing as the next logical step for microwave photonics. Optical frequency combs (OFCs), which play a vital role in integrated microwave photonics, have not yet generated using nano-structured devices. In this paper, we propose OFC generation based on all-optical modulation of nanophotonic structures. A theoretical model based on temporal coupled mode theory is developed to describe cascaded all-optical photonic crystal intensity and phase modulators. Using a modulation light with a sinusoidal waveform, a separate carrier light is modulated to generate an OFC. By manipulating the modulation power and device dimensions, a flat OFC that spans over a 600 GHz is delivered. The proposed system offers OFC generation with tunable comb line spacing using devices with high density integration capabilities.

A Novel MEMS Tunable Comb Resonator With Non-Uniform Varied Finger Lengths

We present the development of tunable comb resonators that use a triangular comb of non-uniform varied finger lengths to the frequency tuning of the micromechanical resonator. The frequency tuning comb structure, composed of a triangular comb of non-uniform varied finger lengths, generates a linear electrostatic tuning force from the translational motion of the DC-biased comb. Increasing the tuning range of resonant frequency with less displacement, one of the challenges is there. Also, the nested folded beam suspension is used to reduce mechanical spring and thus reduce voltage. The electrostatic tuning force, the modified spring, and the resonant frequency were analyzed and investigated. The tunable comb resonator consists of three parts: the driving, sensing, and tuning parts. The resonant frequency of the resonator can be changed upon applying a DC voltage to the tuning part. To validate the theoretical analysis, this structure is simulated by Intellisuite software. Simulation results illustrate that the resonator has a resonant frequency of about 3578 Hz and a driving voltage of 5 V; the resonant frequency reaches 1549 Hz when a tuning voltage of 78 V is applied. The resonator has a maximum frequency tuning range of 57%, compared with previous works, the resonant frequency tuning range has been increased. However, the area of this resonator is about $1120\times 530 \mu ^{2}$ .

Photonics-Assisted Multi-Band Microwave Receiver Based on Spectrum Analysis and Coherent Channelization

Due to the limitation of analog-to-digital/digital-to-analog converters, photonics-assisted channelized receivers are thought to be a promising approach to receiving wideband microwave signals. Herein, based on the spectrum analysis and the coherent channelization, we develop a photonics-assisted channelized receiver for multi-band microwave signals. In the proposed channelized receiver, the instantaneous spectral analysis is introduced to determine the frequencies and bandwidth of a dynamic wideband signal. The dynamic wideband signal is then received by a multi-band coherent channelizer. By exploiting transparency of the optoelectronic devices, we equivalently build a multiband coherent channelizer that can work for dynamic microwave signals, where a few optoelectronic devices are required. Compared with the existing coherent channelizers, the operative bandwidth of the proposed multi-band coherent channelizer is much larger (up to 28 GHz). The proposed photonics-assisted channelized receiver doesn't need tunable optical comb generators and RF sources, and it also doesn't require knowing the spectrum information in advance. Moreover, the designed example of the proposed photonics-assisted channelized receiver for a 4 GHz original signal in 2~30 GHz microwave bands is given and discussed. The spectrum information of the dynamic original signals is obtained by monitoring the optical power in each sub-channel. We verify the feasibility of the multi-band coherent channelizer by channelizing two 4 GHz linearly-chirped signals with center frequencies of 4 GHz and 28 GHz into four 1 GHz sub-channels, respectively.

Perfect Soliton Crystals on Demand

AbstractRecent advances in of soliton microcombs have shown great promise to revolutionize many important areas such as optical communication, spectroscopic sensing, optical clock, and frequency synthesis. A largely tunable comb line spacing is crucial for the practical application of soliton microcombs, which unfortunately is challenging to realize for an on‐chip monolithic microresonators. The recently discovered perfect soliton crystal (PSC) offers a convenient route to tune the comb line spacing. However, excitation of a PSC is generally stochastic by its nature and accessing a certain PSC state requires delicate tuning procedure. Here the on‐demand generation of PSCs in a lithium niobate microresonator is demonstrated. The unique device characteristics allow to produce a variety of PSCs and to switch between different PSC states, deterministically and repetitively. The device is utilized to show arbitrary dialing of the comb line spacing from 1 to 11 times of the free‐spectral range of the resonator. The demonstration of PSCs on demand may now open up a great avenue for flexibly controlling the repetition rate of soliton pulses, which would significantly enhance and extend the application potential of soliton microcombs for communication, signal processing, and sensing.

Generation of Tunable Microwave Frequency Comb Utilizing a Semiconductor Laser Subject to Optical Injection from an SFP Module Modulated by an Arbitrary Periodic Signal

In this paper, a microwave frequency comb (MFC) is generated from a semiconductor laser subject to optical injection from a commercial small form-factor pluggable (SFP) optical module which is modulated by an arbitrary periodic signal. A sinusoidal signal or square wave signal is employed as the arbitrary periodic signal instead of the electric pulse signal adopted in the former references. When the frequency of the modulated signal is 1 GHz, the MFC with a maximal bandwidth of 15 GHz can be obtained. In addition, taking a sinusoidal signal as an example, the influence of the injection optical power to the slave laser and the modulation frequency of the optical module on the generation of the MFC is analyzed in detail. Finally, the results of MFC generated with a square wave signal injection are presented. The experimental results of this paper provide an important reference for the practical applications of MFC.

基于耦合射频信号和单个马赫-曾德尔调制器的可调光学频率梳

基于耦合射频信号和单个马赫-曾德尔调制器的可调光学频率梳

Octave-wide phase-matched four-wave mixing in dispersion-engineered crystalline microresonators.

In this Letter, we report phase-matched four-wave mixing separated by over one octave in a dispersion-engineered crystalline microresonator. Experimental and numerical results presented here confirm that primary sidebands were generated with a frequency shift up to 140 THz, and that secondary sidebands formed a localized comb structure, known as a clustered comb in the vicinity of the primary sidebands. A theoretical analysis of the phase-matching condition validated our experimental observations, and our results agree well with numerical simulations. These results offer the potential to realize a frequency-tunable comb cluster generator operating from 1 μm to mid-infrared wavelengths with a single and compact device.