Showing papers on "Comb generator published in 2022"

Flat Optical Frequency Comb Generator Based on Integrated Lithium Niobate Modulators

Abstract: 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.

Tunable microwave frequency comb generation using a Si3N4-MDR based actively mode-locked OEO.

Abstract: Microwave frequency combs (MFCs) have important applications in communication and sensing owing to their characteristics of large number of comb lines, wide frequency range, and high precision of comb spacing. In many applications, MFCs are required to emit signals with tunable center frequency and variable comb spacing to accommodate different operating frequency bands and accuracies. Here, we demonstrate a tunable MFC by injecting a low-frequency electrical signal into a tunable optoelectronic oscillator (OEO). Tuning of MFC's center frequency and comb spacing are realized, allowing a frequency tuning range from 1 to 22 GHz and 50 comb lines within a 5 MHz bandwidth obtained in the MFC generator. In addition, the introduction of the silicon nitride micro-disk resonator (Si3N4-MDR) in the system paves the way for the integration of MFC generator.

Gain-switched dual frequency comb at 2 µm

Abstract: This article shows a dual frequency comb in the 2 µm wavelength region using mutually injection locked gain-switched semiconductor lasers. Strained InGaAs multi-quantum-well discrete mode lasers and gain switching were used to generate two optical frequency combs with repetition rates of 2 GHz and 2.0001 GHz respectively, centred at 2.002 µm. Each optical comb spanned approximately 100 GHz. Through mutual injection locking to an edge comb line common in both combs, a phase locked dual frequency comb was demonstrated with 44 beating tones unique to single comb line pair interactions. This scheme allows for the comb information to be compressed into a 5 MHz detection bandwidth and captured with millisecond acquisition times, which could be of benefit to a number of sensing applications.

Quasi-real-time dual-comb spectroscopy with 750-MHz Yb:fiber combs.

Abstract: We present quasi-real-time dual-comb spectroscopy (DCS) using two Yb:fiber combs with ∼750 MHz repetition rates. A computational coherent averaging technique is employed to correct timing and phase fluctuations of the measured dual-comb interferogram (IGM). Quasi-real-time phase correction of 1-ms long acquisitions occurs every 1.5 seconds and is assisted by coarse radio frequency (RF) phase-locking of an isolated RF comb mode. After resampling and global offset phase correction, the RF comb linewidth is reduced from 200 kHz to ∼1 kHz, while the line-to-floor ratio increases 13 dB in power in 1 ms. Using simultaneous offset frequency correction in opposite phases, we correct the aliased RF spectrum spanning three Nyquist zones, which yields an optical coverage of ∼180 GHz around 1.035 µm probed on a sub-microsecond timescale. The absorption profile of gaseous acetylene is observed to validate the presented technique.

A tunable optical frequency comb source using cascaded frequency modulator and Mach–Zehnder modulators

Abstract: Abstract In this work, we demonstrate a tunable optical frequency comb (OFC) source based on a cascaded frequency modulator (FM) and two Mach–Zehnder modulators (MZMs). The setup includes one FM and two MZMs, and a sinusoidal RF signal source that directly drive all these modulators. A Flat OFC source with a high number of comb lines, and tunable frequency spacing and center wavelength is analytically modelled and simulated. The results reveal that 51 comb lines with a frequency spacing of 25 GHz are generated when only FM is used. Thirteen of these lines have power variations of 1 dB. Next, by cascading FM with two MZMs, 127 comb lines are obtained. In addition, 101 of these lines have power variations of 1 dB. An optical frequency comb, with tunable frequency spacing ranging from 10 to 40 GHz is successfully generated. Moreover, the center wavelength of the generated OFC can be tuned from 1310 to 1610 nm.

High-resolution wide-band optical frequency comb control using stimulated Brillouin scattering.

Abstract: We introduce a technique to manipulate an optical frequency comb on a line-by-line basis using stimulated Brillouin scattering (SBS). The narrow-linewidth SBS process has been used to address individual lines in optical frequency combs, but previous demonstrations required a dedicated laser to modulate each comb tooth, prohibiting complete comb control. Here, we use a pair of frequency shifting fiber optic loops to generate both an optical frequency comb and a train of frequency-locked pulses that can be used to manipulate the comb via SBS. This approach enables control of the entire frequency comb using a single seed laser without active frequency locking. To demonstrate the versatility of this technique, we generate and manipulate a comb consisting of 50 lines with 200 MHz spacing. By using polarization pulling assisted SBS, we achieve a modulation depth of 30 dB. This represents a scalable approach to control large numbers of comb teeth with high resolution using standard fiber-optic components.

Acousto-optic frequency shifter-based microwave photonic channelized receiver using a single optical frequency comb.

Abstract: A microwave photonic channelization receiver is a promising technology for broadband radio frequency (RF) signal monitoring and reception. In this Letter, by exploiting acousto-optic frequency shifters (AOFSs), a microwave photonic channelization receiver is proposed. The proposed microwave photonic channelized receiver can reduce dual coherent optical frequency comb (OFC) generators with detuning frequency spacing into a single OFC generator. To verify the feasibility of the proposed channelization scheme, a broadband RF signal with 3.2 GHz bandwidth is channelized into eight narrowband RF signals with 0.4 GHz bandwidth. Moreover, we investigate the effect of the tuning error imposed by AOFSs, where the error vector magnitude (EVM) of subchannels is obtained by channelizing the four 16-quadrature amplitude modulation (16-QAM) signals into four subchannels.

Measurement and analysis of instantaneous microwave frequency based on an optical frequency comb.

Abstract: A microwave photonics instantaneous frequency measurement scheme with 14 channels based on an optical frequency comb (OFC) is proposed. In this scheme, a 14-line flat OFC is generated by cascading a dual-parallel Mach-Zehnder modulator (DPMZM) with a Mach-Zehnder modulator (MZM). The intercepted microwave signal with multiple-frequency components can be measured by using DPMZM, Fabry-Perot filter (FPF), wavelength division multiplexer (WDM), and optical power detector array. This scheme can measure and analyze the frequency of microwave signals in the ranges of 0.5-13.5 GHz, 13.5-26.5 GHz, and 26.5-39.5 GHz with the measurement accuracy of ±0.5GHz. The reconfigurability of the system can be realized by adjusting the comb-line spacing of the OFC and the free spectral range (FSR) of the FPF.

Frequency comb-to-comb stabilization over a 1.3-km free-space atmospheric optical link

Abstract: Abstract Stabilizing a frequency comb to an ultra-stable optical frequency reference requires a multitude of optoelectronic peripherals that have to operate under strict ambient control. Meanwhile, the frequency comb-to-comb stabilization aims to synchronize a slave comb to a well-established master comb with a substantial saving in required equipment and efforts. Here, we report an utmost case of frequency comb-to-comb stabilization made through a 1.3 km free-space optical (FSO) link by coherent transfer of two separate comb lines along with a feedback suppression control of atmospheric phase noise. The FSO link offers a transfer stability of 1.7 × 10 –15 at 0.1 s averaging, while transporting the master comb’s stability of 1.2 × 10 –15 at 1.0 s over the entire spectrum of the slave comb. Our remote comb-to-comb stabilization is intended to expedite diverse long-distance ground-to-ground or ground-to-satellite applications; as demonstrated here for broadband molecular spectroscopy over a 6 THz bandwidth as well as ultra-stable microwaves generation with phase noise of -80 dBc Hz –1 at 1 Hz.

Proposed Scheme for Ultra-Flat Optical Frequency Comb Generation Based on Dual-Drive Mach–Zehnder Modulators and Bidirectional Recirculating Frequency Shifting in Single Loop

Abstract: Recirculating frequency shifting has attracted much attention for its advantages in the generation of the flexible and high-quality optical frequency comb. A new scheme of ultra-flat optical frequency comb generation system based on single-loop bidirectional recirculating frequency shift is proposed and studied in this paper. The generation system employs two pairs of dual-drive Mach–Zehnder modulators and several polarization devices. Compared with the method of single-loop unidirectional recirculation frequency shift, under the same cycles, the number of comb lines generated by the proposed method is doubled, and the generated optical frequency combs have less noise accumulation and better flatness. The theoretical model is established, and the proposed scheme is verified by software simulation. A 111-line optical frequency comb with the spacing of 12.5 GHz, the flatness of 0.76 dB, and the optical signal-to-noise ratio of 27.39 dB was obtained by adopting the proposed scheme.

Frequency comb-to-comb stabilization over a 1.3-km free-space atmospheric optical link

Abstract: Abstract Stabilizing a frequency comb to an ultra-stable optical frequency reference requires a multitude of optoelectronic peripherals that have to operate under strict ambient control. Meanwhile, the frequency comb-to-comb stabilization aims to synchronize a slave comb to a well-established master comb with a substantial saving in required equipment and efforts. Here, we report an utmost case of frequency comb-to-comb stabilization made through a 1.3 km free-space optical (FSO) link by coherent transfer of two separate comb lines along with a feedback suppression control of atmospheric phase noise. The FSO link offers a transfer stability of 1.7 × 10 –15 at 0.1 s averaging, while transporting the master comb’s stability of 1.2 × 10 –15 at 1.0 s over the entire spectrum of the slave comb. Our remote comb-to-comb stabilization is intended to expedite diverse long-distance ground-to-ground or ground-to-satellite applications; as demonstrated here for broadband molecular spectroscopy over a 6 THz bandwidth as well as ultra-stable microwaves generation with phase noise of -80 dBc Hz –1 at 1 Hz.

Coherently averaged dual-comb spectroscopy with a low-noise and high-power free-running gigahertz dual-comb laser

Abstract: We present a new type of dual optical frequency comb source capable of scaling applications to high measurement speeds while combining high average power, ultra-low noise operation, and a compact setup. Our approach is based on a diode-pumped solid-state laser cavity which includes an intracavity biprism operated at Brewster angle to generate two spatially-separated modes with highly correlated properties. The 15-cm-long cavity uses an Yb:CALGO crystal and a SESAM as an end mirror to generate more than 3 W average power per comb, below 80 fs pulse duration, a repetition rate of 1.03 GHz, and a continuously tunable repetition rate difference up to 27 kHz. We carefully investigate the coherence properties of the dual-comb by a series of heterodyne measurements, revealing several important features: (1) ultra-low jitter on the uncorrelated part of the timing noise; (2) the radio frequency comb lines of the interferograms are fully resolved in free-running operation; (3) we validate that through a simple measurement of the interferograms we can determine the fluctuations of the phase of all the radio frequency comb lines; (4) this phase information is used in a post-processing routine to perform coherently averaged dual-comb spectroscopy of acetylene (C2H2) over long timescales. Our results represent a powerful and general approach to dual-comb applications by combining low noise and high power operation directly from a highly compact laser oscillator.

Multiple microwave frequency measurement system based on a sawtooth-wave-modulated non-flat optical frequency comb.

Abstract: A photonic-assisted instantaneous microwave measurement system, capable of measuring multiple frequency signals, is demonstrated and analyzed. The principle lies in the combination of a channelizer and frequency-to-power mapping. An effective generation method of a non-flat optical frequency comb is proposed based on sawtooth wave modulation, which has more comb lines and adjustable comb spacing. Under this method, two low-speed post-processing devices are utilized to realize frequency measurements up to 32 GHz. The scheme is verified by simulation, and factors affecting system performance are also studied.

Frequency Stability Transfer in Passive Mode-Locked Quantum-Dash Laser Diode Using Optical Injection Locking

Abstract: In this paper, we present an experimental study of the metrological stabilization of a solid-state frequency comb for embedded metrology applications. The comb is a passively mode-locked laser diode based on InGaAs/InP Quantum-dash structure emitting optical lines into a 9 nm bandwidth centered at 1.55 $\mu$m with a repetition rate of 10.09 GHz. The frequency stabilization is achieved by optical injection locking of the comb with an external cavity laser diode referenced onto a metrological frequency standard. One observes the transfer of the spectral purity from the injection laser to the neighbouring modes of the injected one as well as the transfer of stability to the adjacent modes. The measurement of the long term stability highlights a frequency noise with random walk behavior specific of the passive mode locking process. Demonstration of sidebands of the injection laser at the repetition frequency of the comb also makes it possible to propose a transfer mechanism and to consider a complete stabilization of the frequency comb at a metrological stability level.

Dual electro-optic comb spectroscopy using a single pseudo-randomly driven modulator

Abstract: We present a dual-comb scheme based on a single intensity modulator driven by inexpensive board-level pseudo-random bit sequence generators. The result is a simplified architecture that exhibits a long mutual coherence time (up to 50 s) with no need of stabilization feedback loops or self-correction algorithms. Unlike approaches that employ ultrafast arbitrary waveform generators, our scheme makes it possible to produce long interferograms in the time domain, reducing the difference in the line spacing of the combs even below the hertz level. In order to check the system accuracy, we report two spectroscopic measurements with a frequency sampling of 140 MHz. All these results are analyzed and discussed to evaluate the potential of our scheme to implement a field-deployable dual-comb generator.

Synchronization of frequency combs by optical injection

Abstract: Optical frequency combs based on semiconductor lasers are a promising technology for monolithic integration of dual-comb spectrometers. However, the stabilization of offset frequency fceo remains a challenging feat due the lack of octave-spanning spectra. In a dual-comb configuration, the uncorrelated jitter of the offset frequencies leads to a non-periodic signal resulting in broadened beatnotes with a limited signal-to-noise ratio (SNR). Hence, expensive data acquisition schemes and complex signal processing are currently required. Here, we show that the offset frequencies of two frequency combs can be synchronized by optical injection locking, which allows full phase-stabilization when combined with electrical injection locking of both repetition frequencies frep. A single comb line isolated via an optical Vernier filter serves as Master oscillator for injection locking. The resulting dual-comb signal is periodic and stable over thousands of periods. This enables coherent averaging using analog electronics, which increases the SNR and reduces the data size by one and three orders of magnitude, respectively. The presented method will enable fully phase-stabilized dual-comb spectrometers by leveraging on integrated optical filters and provides access for comparing and stabilizing fceo to narrow-linewidth optical references.

Optical frequency comb in optoelectronic oscillator with delay line and microresonator

Abstract: We proposed and studied a new method which allows producing an optical frequency comb in an optoelectronic oscillator. In this method, comb parameters can be adjusted by controlling CW laser and microwave filter frequencies and amplitudes of modulating signals supplied to the arms of Mach-Zehnder modulator having two separate microwave inputs. Characteristics of the system were studied with numerical modeling for delay line and mictoresonator in the optoelectronic oscillator loop.

Dual-Comb Interferometry with Fiber-Based Comb Synthesizers

Abstract: Self-referenced frequency-comb generators based on ultrafast fiber lasers enable new approaches to interferometry, owning to their ultralow noise and extreme stability. Benefits for precision spectroscopy over broad spectral bandwidths and for digital holography are presented.

Designing an optical frequency comb generator for visible light communication applications

Abstract: <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>

Multi-branch Fiber Frequency Comb for Precision Frequency Measurement of Molecular Transitions

Abstract: SummaryPrecision spectrum measurement is one of the main applications of optical frequency combs. In this paper, we demonstrate a multi-branch optical frequency comb based on an erbium-doped-fiber femtosecond laser with a polarization-maintaining ‘9’ cavity. The comb includes five output ports with the target wavelengths including 1083 nm, 1380 nm, 1637 nm, 1064 nm and 1750 nm. The in-loop stabilities of the repetition rate and carrier-envelope-offset frequency are 2.9×10−13 and 1.7×10−18 at 1 second integration time respectively, which meets the sub-kilo hertz cavity ring-down spectroscopy (CRDS) measurements requirements.

Fast and high-resolution time-resolved spectroscopy assisted by microcomb-based hybrid asynchronous optical sampling

Abstract: Capability of characterizing arbitrary and non-repetitive emission spectra with a high resolution in real-time is of great merit in various research fields. Optical frequency combs provide precise and stable frequency grid for the measurement of single spectral line with high accuracy. Particularly, dual-comb spectroscopy enables spectral measurement with large bandwidth spanning tens of nanometers, but it is limited to measuring absorption spectra and has to trade-off spectral resolution versus acquisition frame rate set by the repetition rate. Here, to alleviate these restrictions, we propose and demonstrate a time-resolved spectroscopy for emission spectrum based on hybrid asynchronous optical sampling, which features a spectral resolution of 0.63 pm, a frame rate of 1 MHz, and a measurement bandwidth of 13.6 nm, simultaneously. A mode-locked fiber comb of repetition frequency f1 is harnessed to interrogate emission spectral features with high resolution via optical Fourier transform achieved using a time-lens. Subsequently, a soliton microcomb of repetition frequency f2s ≈ 1000 f1 serving as a probe pulse implement hybrid asynchronous optical sampling, thus significantly increasing the acquisition rate by nearly 3 orders of magnitude. As a proof-of-concept demonstration, the frequency trajectory of a rapidly scanning laser of a linear chirp of 6.2 THz/s is tracked. We believe that chip-scale microcombs will make the fast and high-resolution emission spectroscopy presented here a powerful tool for widespread applications.

InP integrated optical frequency comb generator using an amplified recirculating loop.

Abstract: A novel realisation of photonically integrated optical frequency comb generation is demonstrated on indium phosphide (InP) using a generic foundry platform. The architecture, based on the amplified recirculating loop technique, consists of cascaded electro-optic phase modulators embedded within a short waveguide loop. While an injected continuous wave laser signal is recirculated by the loop, the modulators are driven with a modulation frequency corresponding to the round-trip loop length frequency. This results in many phase coherent, evenly spaced optical comb lines being generated. The choice of InP as an integration platform allows immediate optical amplification of the modulated signal by embedded semiconductor optical amplifiers, enabling loop losses to be compensated and expanding the comb across broad optical bandwidths. This approach reduces the requirement for external, high-power optical amplifiers, improving the compactness and power efficiency of the full system. The system was modelled to identify off-resonance behaviour, outlining limits in matching both the modulation frequency and seed laser frequency to the round-trip loop frequency for optimal comb line generation to be achieved. The experimental device occupied a fraction of the 6 x 2 mm2 InP chip and operated at round-trip loop frequencies of 6.71 GHz to produce 59 comb lines within a 20 dB power envelope. All comb lines exhibited strong phase coherence as characterised by low composite phase noise measurements of -105 dBc/Hz at 100 kHz. A second device is also presented with a shorter loop length operating at ∼10 GHz which generated 57 comb lines. Both loop configurations included short waveguide phase shifters providing a degree of tunability of the free spectral range with a tuning range of 150 MHz for small injection currents of < 2.5 mA.

Generating an ultra-stable dual frequency comb with a microresonator

Abstract: Frequency combs are coherent light sources comprising multiple evenly spaced emission lines, allowing coherent sampling over a broad part of the optical spectrum. The addition of a second frequency comb results in a dual comb. Now, one comb can be used as a local oscillator frequency reference for the other, allowing spectroscopic measurement to take place at radio-frequencies. An important figure of merit is the relative frequency stability of the two combs. In this work we demonstrate efficient generation of ultrastable dual combs using an electro-optic whispering gallery mode resonator, with a relative comb line stability of order 1 mHz.

Self-sustained Optical Frequency Combs Generation with a Tunable Line Spacing Based on Coupled Optoelectronic Oscillators

Abstract: An optical frequency comb (OFC) generator based on a coupled optoelectronic oscillator (COEO) is demonstrated. It consists of a mode-locked ring laser and an optoelectronic feedback loop. The line spacing of the generated OFC is adjustable by only tuning the center frequency of the electrical bandpass filter in the optoelectronic loop. In the experiments, two OFCs with a spacing of 8 GHz and 10 GHz are respectively produced. The phase noise of the self-oscillating signal corresponding to the two spectra is -79.5dBc/Hz and - 75.8dBc/Hz @10KHz offset frequency, respectively.