Optical Frequency Comb Generation Research Articles

Generation of a 25-line flattened optical frequency comb and its coherent beating properties.

A 25-line flattened optical frequency comb (OFC) is generated by cascading Mach-Zehnder modulators, and its coherent beating properties are investigated. The effect of bias voltages and the power of radio frequency signals on the flatness of the OFC, and the phase relationships between each comb line are theoretically deduced. In the experiment, a 25-line OFC with a flatness of less than 1 dB and an optical sideband suppression ratio of 12 dB is successfully demonstrated, and the agreement between simulation and experimental results supports the validity of the model for analysis of the flatness. The measured amplitude of harmonic signals by coherent beating of the generated 25-line OFC matches well with the theoretical prediction. A phase noise of -105.31dBc/Hz at 10 kHz frequency offset for the 12th harmonic signal almost without phase noise degradation proves the high coherence of the generated OFC lines.

Efficient Kerr soliton comb generation in micro-resonator with interferometric back-coupling

Nonlinear Kerr micro-resonators have enabled fundamental breakthroughs in the understanding of dissipative solitons, as well as in their application to optical frequency comb generation. However, the conversion efficiency of the pump power into a soliton frequency comb typically remains below a few percent. We fabricate and characterize a hybrid Mach-Zehnder ring resonator geometry, consisting of a micro-ring resonator embedded in an additional cavity with twice the optical path length of the ring. The resulting interferometric back coupling enables to achieve an unprecedented control of the pump depletion: pump-to-frequency comb conversion efficiencies of up to 55% of the input pump power is experimentally demonstrated with a soliton crystal comb. We assess the robustness of the proposed on-chip geometry by generating a large variety of dissipative Kerr soliton combs, which require a lower amount of pump power to be accessed, when compared with an isolated micro-ring resonator with identical parameters. Micro-resonators with feedback enable accessing new regimes of coherent soliton comb generation, and are well suited for comb applications in astronomy, spectroscopy and telecommunications.

Development of frequency comb generation by spectral broadening of periodic optical pulses in semiconductor laser amplifiers

This paper reports an approach for generating an ultra-flat and ultra-wide optical frequency comb by exploiting gain modulation, experienced by a periodic third ordered Gaussian-shaped optical pulse propagating through the semiconductor optical amplifier (SOA). The gain experienced by optical signal propagating inside SOA is independent of its polarization state and phase, makes the technique more favorable and stable to the optical frequency comb generation. This paper reports a 94-line optical frequency comb with 5 dB maximum power deviation and a 190-line optical frequency comb with 10 dB maximum power deviation.

Designing an optical frequency comb generator for visible light communication applications

<p>The optical frequency comb generator (OFCG) is an efficient optoelectronic device that is included in many important applications over a various field such as microwave and optical communication. A novel scheme of OFCG presented in this work for visible light communication application based on amplitude modulation, radio frequency (RF) signal, phase shift and two Mach-Zehnder modulators (MZMs), our design features are simple with more efficient power and premium flatness of comb lines, the number of generating frequencies lines was 64 with a power stronger than -2 dBm over a 340 GHz bandwidth from a single continuous laser diode. Different chirping factor (α) of MZMs are implemented (3, 5, 7), as the results the best results related to α=5 with extra flatness, the system was designed and simulated by VPI design suite 9.8.</p>

2 μm optical frequency comb generation via optical parametric oscillation from a lithium niobate optical superlattice box resonator

Optical parametric oscillators (OPOs) can downconvert the pump laser to longer wavelengths with octave separation via χ ( 2 ) , which is widely used for laser wavelength extension including mid-infrared (MIR) generation. Such a process can be integrated in monolithic resonators, being compact and low in threshold. In this work, we show that the monolithic χ ( 2 ) mini-OPO can also be used for optical frequency comb generation around 2096 nm and enters the boundary of MIR range. A new geometry called an optical superlattice box resonator is developed for this realization with near-material-limited quality factor of 4.0 × 10 7 . Only a continuous-wave near-infrared pump laser is required, with OPO threshold of 80 mW and output power up to 340 mW. Revival temporal profiles are measured at a detectable repetition frequency of 1.426 GHz, and narrow beat note linewidth of less than 10 Hz shows high comb coherence. These results are in good agreement with our simulation for a stable comb generation. Such an OPO-based comb source is useful for carbon dioxide sensing or the mine prospect applications and can be generalized to longer MIR wavelengths for general gas spectroscopy.

Compensation of The Nonlinear Impairments in All-Optical OFDM Systems Based on The Optical Phase Conjugation (OPC) Module

This study presents a nonlinearity mitigation technique for achieving high performance in the All-Optical Orthogonal Frequency Division Multiplexing (AO-OFDM) transmission systems. Therefore, the Optical Phase Conjugation (OPC) technique has been employed for mitigating the Nonlinear Phase Noise (NLPN) in the AO-OFDM transmission systems. The NLPN has been mitigated in the AO-OFDM transmission system by utilizing the proposed OPC at the middle point of the transmission link. The proposed system is numerically simulated by Virtual Photonics Integrated (VPI) Transmission Maker 9.0 at a symbol rate of 25 Gsymbol/s. During the simulation, 29 subcarriers were generated by Optical Frequency Comb Generator (OFCG) and modulated by 4-array Quadrature Amplitude Modulator (4-QAM). The generated signals are transmitted over 580 km fiber link and received by the coherent receiver. The transmission link contains 4 spans before and 4 spans after OPC module. Each span comprises the Standard Single-Mode Fiber (SSMF) and an EDFA (noise figure = 6 dB) to compensate for the fiber loss. The length of each span is fixed at 70 km in the system simulation. In addition, a 20 km Dispersion Compensation Fiber (DCF) has been used just before the OPC module to compensate for accumulated fiber dispersion. The Signal-to-Noise Ratio (SNR) and Error Vector Magnitude (EVM) have been used to certify the feasibility of the proposed technique. The results reveal that by employing the proposed OPC module, the SNR is improved by ~3.4 dB, and the EVMs are substantially reduced.

Modulation instability of Kerr optical frequency combs in dual-coupled optical cavities

Kerr optical frequency combs generated in a coherently driven Kerr nonlinear resonator have the potential for a wide range of applications. However, in a single cavity which is a widely adopted configuration for Kerr optical frequency-comb generation, modulation instability is suppressed in the normal dispersion regime, and the pump-to-comb conversion efficiency is extremely low for a single dissipative Kerr soliton (DKS) in the anomalous dispersion regime. Dual-coupled cavities have been proposed to generate Kerr optical frequency combs in the normal dispersion regime, and have the potential to remarkably increase conversion efficiency for Kerr optical frequency combs. Here, we investigate modulation instability and Kerr optical frequency-comb formation in dual-coupled cavities. Based on solutions of the continuous-wave steady state, we obtain a quadric algebraic equation describing the modulation instability gain, and we find that it is intensely influenced by the scaled free spectral range mismatch between the two cavities. Our numerical simulations demonstrate that platicons can be generated via a pump scanning scheme for the case that both the two cavities possess normal dispersion, and a single DKS can be generated in the cavity with anomalous dispersion while the dispersion of the other cavity is normal. The conversion efficiency of a single DKS is determined by the pump power and detunings as well as intrinsic quality factors of two cavities. Our analysis of modulation instability provides a powerful tool for Kerr optical frequency-comb generation via pump modulation and cavity detuning tuning scheme in dual-coupled cavities.

Optical frequency comb generation from a 1.65 µm single-section quantum well laser.

Optical frequency combs (OFCs) in the 1.65 µm wavelength band are promising for methane sensing and extended high-capacity optical communications. In this work, a frequency-modulated (FM) OFC is generated from a 1.65 µm single-section quantum well laser. This is characterized by a 1 kHz-wide beatnote signal at ∼19.4 GHz. Typical FM optical spectra are shown and optical linewidth of the OFC narrows through the mutual injection locking process in the comb formation. No distinct pulse train is observed on oscilloscope, which conforms with the FM operation. Furthermore, to add further evidence that four-wave mixing (FWM) is the driving mechanism of the comb formation, FWM frequency conversion characterization is conducted on a semiconductor optical amplifier (SOA) fabricated together with the tested laser. An efficiency of ∼-30 dB confirms the capability of FM mode locking.

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.

Fabrication and optical frequency comb generation in high-quality factor silicon oxide microcavity (Invited)

Fabrication and optical frequency comb generation in high-quality factor silicon oxide microcavity (Invited)

Design of normal dispersion high nonlinear silica fiber and generation of flat optical frequency comb

<sec>The scheme of generating optical frequency comb mainly includes mode-locked laser, electro-optic modulation comb, nonlinear Kerr micro-resonator comb, and nonlinear supercontinuum comb. For the nonlinear supercontinuum comb scheme, the silica-based high nonlinear fiber with near-zero flattened normal dispersion is required. However the fiber dispersion varies along the fiber due to the fabrication inaccuracy. Furthermore, nonlinear supercontinuum comb generation based on the nonlinear fiber has not been systematically studied. In this paper, an optimal design of four-clad flat normal dispersion high nonlinear silica fiber with a triangular core refractive index distribution for the flat optical frequency comb generation is carried out. The effects of the fiber cladding width and refractive index on the fiber dispersion characteristics and cut-off wavelength are studied through using the finite element method mode solver. The optimally designed fiber can obtain relatively flat near-zero normal dispersion in a wavelength range of 1400–1700 nm, the dispersion range is –3–0 <inline-formula><tex-math id="M5">\begin{document}$ \rm{p}\rm{s}/(\rm{k}\rm{m}\cdot \rm{n}\rm{m}) $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M5.png"/></alternatives></inline-formula>, and the dispersion slope is close to 0 at nearly 1550 nm. The effective mode field area of the nonlinear silica fiber is about 11<inline-formula><tex-math id="M6">\begin{document}$ {\text{μm}}^{2} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M6.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M6.png"/></alternatives></inline-formula>, and the nonlinear coefficient can reach 12.8<inline-formula><tex-math id="M7">\begin{document}$ {\rm{W}}^{-1}{\cdot \rm{k}\rm{m}}^{-1} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M7.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="23-20221115_M7.png"/></alternatives></inline-formula>.</sec><sec>Based on the electro-optic modulation pulse pumping the flat normal dispersion high nonlinear silica fiber, the flat optical frequency comb generation is systematically simulated with the generalized nonlinear Schrödinger equation. The time-frequency evolutions of a hyperbolic secant pulse, a Gaussian pulse and a super Gaussian pulse are simulated by using the X-Frog technology. The time-frequency spectrograms connect the time domain and the frequency domain of the pulse, clearly showing the change of pulse chirp during the propagation. The effects of various parameters on the optical frequency comb are studied, such as the fiber length, second-order dispersion, third-order dispersion, pulse peak power, pulse half width, pulse initial chirp, and pulse shape. An optical frequency comb with 3-dB flatness and about 40-nm bandwidth can be achieved based on hyperbolic secant pulse or Gaussian pulse pumping. Compared with the hyperbolic secant pulse and Gaussian pulse, the super Gaussian pulse can produce a flatter optical frequency comb. An optical frequency comb with 2-dB flatness and about 92-nm bandwidth can be achieved based on the super Gaussian pulse pumping. Therefore, based on the proposed high nonlinear fiber with normal dispersion , it is possible to realize an optical frequency comb with a repetition rate above 10 GHz, power flatness within 3 dB, and spectral bandwidth of about 40–90 nm. The simulation results are beneficial to promoting the localization of normal dispersion high nonlinear silica fiber and its application in flat optical frequency comb.</sec>

Low FSR Mode-Locked Laser Based on InP-Si3N4 Hybrid Integration

Generation of optical frequency combs (OFC) using semiconductor laser sources has been an active research topic for years, since they offer good performances for many applications without the bulky and expensive aspects of ion doped fiber or crystal based laser sources. In this paper, we demonstrate the generation of low free spectral range mode-locked laser using hybrid III-V/Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> butt coupling. We start by describing the design and the fabrication process of the active R-SOA chip made at our laboratory, and the passive Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> chip having low losses measured to be around 6 dB/m. We then report the integration process and the results of passive mode locking. Our mode-locked laser has a repetition rate of 360 MHz, which is, to the best of our knowledge, the lowest repetition rate achieved for a hybrid integrated mode-locked laser. The frequency comb generated has a 30 dB bandwidth of 2.8 nm (349 GHz). Finally, we show the demonstration of active and harmonic mode locking up to the sixth harmonic.

Theoretical model of cascaded electro-optic intensity modulator-based 2.31-THz broad optical frequency comb generator

We report a theoretical model to design a stable and tunable optical frequency comb by controlling the periodic radiofrequency signal applied to the two cascaded lithium niobate Mach–Zehnder modulators. The spectrum flatness, frequency line spacing, and spectrum bandwidth depend on the periodic optical signal’s shape, periodicity, and duty cycle. So the shape and duty cycle of the optical signal are controlled by controlling the amplitude and phase of the periodic radiofrequency signal and realizing a 2.1% duty-cycled periodic signal, resulting in a 1.92-dB flat and 2.31-THz broad-spectrum optical frequency comb. The duty cycle of the optical signal is decreased by controlling the modulators in such a way that it will give the output signal for the transition period of the radiofrequency signal only. The line spacing is controlled by controlling the frequency of the radiofrequency signal and realizes a 30-GHz channel spaced frequency comb. The comb’s channel spacing and frequency can also be controlled by controlling the periodicity of radiofrequency signal and seed laser frequency, making the frequency comb tunable.

Cross-phase modulation based ultra-flat 90-line optical frequency comb generation

This paper proposed an approach to design an evenly spaced, 1.8 THz spectrally broad and 1.6 dB flat optical frequency comb (OFC) by exploiting the cross-phase modulation in highly nonlinear fiber. The OFC is realized by controlling the phase of the signals in two parallelly placed highly nonlinear fibers. The frequency and line spacing of the OFC can be tuned by simply varying the periodicity and central wavelength of input electrical and optical signal, respectively.

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.

Interleaved electro-optic dual comb generation to expand bandwidth and scan rate for molecular spectroscopy and dynamics studies near 1.6 µm.

A chirped-pulse interleaving method is reported for generation of dual optical frequency combs based on electro-optic phase modulators (EOM) in a free-running all-fiber based system. Methods are discussed to easily modify the linear scan rate and comb resolution by more than three orders of magnitude and to significantly increase the spectral bandwidth coverage. The agility of the technique is shown to both capture complex line shapes and to magnify rapid passage effects in spectroscopic and molecular dynamics studies of CO2. These methods are well-suited for applications in the areas of remote sensing of greenhouse gas emissions, molecular reaction dynamics, and sub-Doppler studies across the wide spectral regions accessible to EOMs.

Generation of a dual optical frequency comb by large signal modulation of a semiconductor laser.

In this paper, we present and theoretically investigate a simple and power efficient scheme for dual optical frequency comb generation residing on a single directly modulated semiconductor laser driven by two superimposed current waveforms. Our detailed model estimates dual combs comprising teeth pairs within 20 dB margin spanning up to 116 GHz with power per teeth pair up to 8.8 µW. In addition, we report dual combs with 40 GHz span comprising ultraflat teeth pairs, with flatness of the order of 1 dB.

Generation of Optical Frequency Comb via Giant Optomechanical Oscillation.

Optical frequency combs (OFCs) are essential in precision metrology, spectroscopy, distance measurement, and optical communications. Significant advances have been made recently in achieving micro-OFC devices based on parametric frequency conversion or electro-optic phase modulation. Here, we demonstrate a new kind of microcomb using a cavity optomechanical system with giant oscillation amplitude. We observe both optical and microwave frequency combs in a microtoroid resonator, which feature a flat OFC with 938 comb lines and a repetition rate as low as 50.22MHz, as well as a flat microwave frequency comb with 867 comb lines. To generate such giant oscillation amplitude, we excite an overcoupled optical mode with a large blue detuning that is assisted with the thermo-optic nonlinearity. A new type of nonlinear oscillation, induced by competition between the optomechanical oscillation and thermo-optic nonlinearity, is also observed.

Experimental Investigation of External Optical Injection and its Application in Gain-Switched Wavelength Tunable Optical Frequency Comb Generation

A detailed experimental investigation of the effects of external optical injection on a Fabry Perot laser when in continuous wave (CW) mode and in gain switched mode is presented. Such detailed characterisation can help in optimising operating condition of the transmitters to significantly improve the system performance for dedicated applications. In CW mode, we characterised various performance indicators of a laser such as side mode suppression ratio (SMSR), wavelength, peak power, modulation bandwidth, resonance frequency, optical linewidth, and phase noise under external optical injection with the use of tuning maps. We also present in brief the theoretical origin of these parameters using rate equations. We then investigated the performance of the externally injected Fabry Perot laser under gain switching for optical frequency comb (OFC) generation. We demonstrated the generation of an OFC with a free spectral range of 6.25 GHz to 25 GHz and a quasi-continuous wavelength tunability of 31 nm. The phase noise of the tones of the generated OFC is measured and is found to be similar to that of the externally injected light and the measured optical linewidth was ∼40 kHz. We also presented experimentally the trade-off that exists between bandwidth of the generated OFC and the phase noise of individual comb tones under external optical injection and their optimisation for the application at hand.

Optical Frequency Combs Generated in Silica Microspheres in the Telecommunication C-, U-, and E-Bands

Optical frequency combs (OFCs) generated in microresonators with whispering gallery modes are demanded for different applications including telecommunications. Extending operating spectral ranges is an important problem for wavelength-division multiplexing systems based on microresonators. We demonstrate experimentally three spectrally separated OFCs in the C-, U-, and E-bands in silica microspheres which, in principle, can be used for telecommunication applications. For qualitative explanation of the OFC generation in the sidebands, we calculated gain coefficients and gain bandwidths for degenerate four-wave mixing (FWM) processes. We also attained a regime when the pump frequency was in the normal dispersion range and only two OFCs were generated. The first OFC was near the pump frequency and the second Raman-assisted OFC with a soliton-like spectrum was in the U-band. Numerical simulation based on the Lugiato–Lefever equation was performed to support this result and demonstrate that the Raman-assisted OFC may be a soliton.