Dual comb generation from a mode-locked fiber laser with orthogonally polarized interlaced pulses

All-normal dispersion widely tunable dual-wavelength mode-locked fiber laser based on NALM

We experimentally and numerically demonstrate the all-normal dispersion (ANDi) ytterbium (Yb)-doped fiber laser based on nonlinear amplifying loop mirror (NALM) mode-locked, which allows tunable single-wavelength and dual-wavelength outputs. The pulses tuning ranges of the dual-wavelength are from 1032.24 nm to 1053.13 nm and from 1047.94 nm to 1069.05 nm, and the repetition frequency difference varies from 1766Hz to 1834Hz. To our knowledge, this is the widest dual-wavelength tuning range of Yb-doped fiber lasers based on NALM mode-locked. We test for 90 minutes and have high stability in both single-wavelength and dual-wavelength. In addition, the pulsed collision dynamics between two solitons at different wavelengths are numerically studied. Numerical results show that during the pulse collision, the two solitons pass through each other and maintain their properties, which also confirms the particle nature of the isolated wave. Our research contributes to the dynamics of dual-wavelength solitons collision in NALM mode-locked fiber laser and provides what we believe to be is a new idea for tunable Yb-doped dual-comb sources.

Jitter correction for asynchronous optical sampling terahertz spectroscopy using free-running pulsed lasers

We demonstrate a jitter correction method for asynchronous optical sampling (ASOPS) terahertz (THz) time-domain spectroscopy using two free-running oscillators. This method simultaneously records the THz waveform and a harmonic of the laser repetition rate difference, Δ f _r, to monitor the jitter information for software jitter correction. By suppressing the residual jitter below 0.1 ps, the accumulation of the THz waveform is achieved without losing the measurement bandwidth. Our measurement of water vapor successfully resolves the absorption linewidths below 1 GHz, demonstrating a robust ASOPS with a flexible, simple, and compact setup without any feedback control or additional continuous-wave THz source.

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.

Polarization-multiplexed, single-cavity dual-comb fiber laser based on a birefringent crystal and a saturable absorber

We introduce a calcium carbonate birefringent crystal into an Er-fiber laser mode-locked by a saturable absorber, where dual-comb ultrashort pulses with orthogonal polarization have been obtained. The two ultrashort pulse trains from the laser exhibit polarization contrast ratios of 30 dB and 20 dB, indicating that the dual-comb mode-locking is due to the polarization-multiplexing mechanism. The dual-comb ultrashort pulses have central wavelengths of 1564.41 nm and 1564.51 nm, and pulse durations of 825 fs and 805 fs respectively. The optical spectra and pulse durations of the asynchronous ultrashort pulses are nearly identical, so that the output of the laser could be directly used for dual-comb applications. Besides, the repetition-rate difference of the two mode-locked pulses is 673 Hz, while its drift is only 0.093 Hz within 2 hours' time. The low drift of the repetition-rate difference manifests the single-cavity dual-comb Er-fiber laser has a high stability and high common-mode noise suppression. At last, we have tested the dual-comb fiber laser in a ranging experiment, where clear interferogram signal can be observed. The experimental results prove that this single-cavity dual-comb Er-fiber laser based on the birefringent crystal and saturable absorber can be a potential source for spectroscopy, optical imaging, absolute distance measurement and other dual-comb applications.

Soliton phenomena in normal and anomalous dispersion regions in Er-doped mode-locked fiber lasers based on Cr2Si2Te6 saturable absorbers

Investigations of optical solitons have always been a hot topic due to their important scientific research value. In recent years, ultrafast lasers based on two-dimensional materials such as saturable absorbers (SAs) have become the focus of optical soliton research. In this work, various soliton operations are demonstrated in Er-doped fiber lasers (EDFLs) based on ${{\rm Cr}_2}{{\rm Si}_2}{{\rm Te}_6}$ SAs. First, a low-threshold passively mode-locked EDFL with traditional soliton output is constructed, and the pump threshold is as low as 10.1 mW. Second, by adjusting the net dispersion of the cavity, stable dissipative soliton operation can also be obtained. Traditional soliton mode-locked operation with controllable Kelly sidebands from first order to fourth order is realized by adjusting the pump power in a double-ended pumped structure, and the SNR is as high as 55 dB. All results prove that ${{\rm Cr}_2}{{\rm Si}_2}{{\rm Te}_6}$ used as SA material has great potential and wide application prospects in investigating optical soliton operations in mode-locked fiber lasers with both normal and anomalous dispersion.

Dual-dispersion-regime dual-comb mode-locked laser

We report on the first, to the best of our knowledge, solid-state dual-comb mode-locked laser that simultaneously operates in different dispersion regimes. Due to the intrinsic polarization multiplexing in a birefringent Yb:Ca₃NbGa₃Si₂O₁₄ (Yb:CNGS) gain medium, the laser emits two cross-polarized pulse trains with a repetition rate offset of ∼ 4.8 kHz from a single cavity. We obtain dual pulse generation with a 20-fold difference in duration by setting the net cavity group delay dispersion to cross zero across the emission band of the employed gain medium. While the duration of the soliton-like pulses experiencing anomalous dispersion amounts to 117 fs, the second laser output, which is spectrally located in the normal dispersion region, is strongly chirped with a pulse duration of 2360 fs.

Polarization dynamics of ultrafast solitons

We study the polarization dynamics of ultrafast solitons in mode-locked fiber lasers. We find that when a stable soliton is generated, its state of polarization shifts toward a stable state, and when the soliton is generated with excess power levels it experiences relaxation oscillations in its intensity and timing. On the other hand, when a soliton is generated in an unstable state of polarization, it either decays in intensity until it disappears, or its temporal width decreases until it explodes into several solitons, and then it disappears. We also found that when two solitons are simultaneously generated close to each other, they attract each other until they collide and merge into a single soliton. Although these two solitons are generated with different states-of-polarization, they shift their state of polarization closer to each other until the polarization coincides when they collide. We support our findings by numerical calculations of a non-Lagrangian approach by simulating the Ginzburg-Landau equation governing the dynamics of solitons in a laser cavity. Our model also predicts the relaxation oscillations of stable solitons and the two types of unstable solitons observed in the experimental measurements.

Computationally image-corrected dual-comb microscopy with a free-running single-cavity dual-comb fiber laser

Dual-comb microscopy (DCM), an interesting imaging modality based on the optical-frequency-comb (OFC) mode and image pixel one-to-one correspondence, benefits from scan-less full-field imaging and simultaneous confocal amplitude and phase imaging. However, the two fully frequency-stabilized OFC sources requirement hampers DCM practicality due to the complexity and costs. Here, a bidirectional single-cavity dual-comb fiber laser (SCDCFL) is adopted as a DCM low-complexity OFC source. Although the residual timing jitter in the SCDCFL blurs the image of a static object acquired by DCM, computational image correction significantly suppresses the image blur. Nanometer-order step surface profilometry with a 14.0 nm uncertainty highlights the computationally image-corrected DCM effectiveness. We further discuss a possibility to expand the computational image correction to a dynamic object and demonstrate its preliminary experiment. The proposed method enhances the DCM generality and practicality due to low-complexity OFC source.

A Multidimensional Multiplexing Mode-Locked Laser Based on a Dual-Ring Integrative Structure for Tri-Comb Generation

The tri-comb-based multi-heterodyne detection technique has been proven to be a powerful tool for precision metrology, e.g., laser ranging and spectroscopy. However, in existing tri-comb generation methods, it is difficult to provide a large and variable difference in tri-comb repetition rates. In this paper; we propose a multidimensional multiplexing mode-locked laser based on a dual-ring integrative structure. Combining the dimensions of sub-ring multiplexing and wavelength multiplexing, two modes of tri-comb generation can be achieved with the dual-ring single cavity laser. The generated combs are identified based on the relative intensity of the pulse trains and optical spectrum, and the repetition rates of dual-combs from the same sub-ring are distinguished based on dispersion analysis. With repetition rates of approximately 47 MHz and 49.6 MHz, the minimum and maximum repetition rate difference of the generated tri-comb can be changed from 2.38 kHz and 2.59526 MHz to 2.74 kHz and 2.59720 MHz merely by switching the operation mode of the dual-ring integrated mode-locked laser. The obtained results indicate that our method can offer a powerful scheme for future multi-comb generation and its application in multi-heterodyne detection-based laser ranging and spectroscopy.

Self-starting VCSEL-based optical frequency comb generator

In this paper, we present the simulation and experimental results of the first closed-loop system based on a directly modulated VCSEL in a gain-switching condition to generate optical frequency combs (OFC). In order to simulate the self-starting VCSEL-based optical frequency comb generator (SVOFC), we applied an intrinsic parameter extraction process to a C-band VCSEL using laser rate equations, static and dynamic measurements, and equivalent circuit models. The widest (62 GHz) and flattest (0.8) simulated OFC is obtained when the repetition frequency f₀ is 2.5 GHz. Implementation of the C-band SVOFC also shows that under constant electrical conditions, flatness higher than 0.85 and spectral widths of 50 GHz are obtained when f₀ = 2.5 GHz. The lowest phase noise at 10 kHz from the extracted electrical carrier is -127 dBc/Hz and is obtained when the optical fiber length is 5 km and f₀ = 1.25 GHz.

Ultrafast Fiber Lasers: An Expanding Versatile Toolbox

Ultrafast fiber lasers have gained rapid advances in last decades for their intrinsic merits such as potential of all-fiber format, excellent beam quality, superior power scalability, and high single-pass gain, which opened widespread applications in high-field science, laser machining, precision metrology, optical communication, microscopy and spectroscopy, and modern ophthalmology, to name a few. Performance of an ultrafast fiber laser is well defined by the laser parameters including repetition rate, spectral bandwidth, pulse duration, pulse energy, wavelength tuning range, and average power. During past years, these parameters have been pushed to an unprecedented level. In this paper, we review these enabling technologies and explicitly show that the nonlinear interaction between ultrafast pulses and optical fibers plays the essential role. As a result of rapid development in both active and passive fibers, the toolbox of ultrafast fiber lasers will continue to expand and provide solutions to scientific and industrial problems.

Promotion of pulse peak power by halving the repetition rate based on a vector soliton

We report on an all-fiber mode-locked repetition-rate-switch pulse operation in a Yb-doped fiber laser based on a polarization rotation vector soliton. The polarization controller (PC) in a fiber loop and a polarization-dependent isolator at the output port are incorporated into the laser resonator at the switch of the repetition rate. By adjusting the PC in the cavity, the mode locking can be switched between the fundamental repetition rate and half of it with a tiny pulse width change. Also, the halved pulse exhibits unique properties: a huge promotion in energy and peak power. To the best of our knowledge, this is the first all-fiber seed source with a passive switch of the repetition rate based on a vector soliton.

Tunable dual-comb from an all-polarization-maintaining single-cavity dual-color Yb:fiber laser

We demonstrate dual-comb generation from an all-polarization-maintaining dual-color ytterbium (Yb) fiber laser. Two pulse trains with center wavelengths at 1030 nm and 1060 nm respectively are generated within the same laser cavity with a repetition rate around 77 MHz. Dual-color operation is induced using a tunable mechanical spectral filter, which cuts the gain spectrum into two spectral regions that can be independently mode-locked. Spectral overlap of the two pulse trains is achieved outside the laser cavity by amplifying the 1030-nm pulses and broadening them in a nonlinear fiber. Spatially overlapping the two arms on a simple photodiode then generates a down-converted radio frequency comb. The difference in repetition rates between the two pulse trains and hence the line spacing of the down-converted comb can easily be tuned in this setup. This feature allows for a flexible adjustment of the tradeoff between non-aliasing bandwidth vs. measurement time in spectroscopy applications. Furthermore, we show that by fine-tuning the center-wavelengths of the two pulse trains, we are able to shift the down-converted frequency comb along the radio-frequency axis. The usability of this dual-comb setup is demonstrated by measuring the transmission of two different etalons while the laser is completely free-running.

All-polarization-maintaining Er-doped dual comb fiber laser using single-wall carbon nanotubes

We demonstrated a bi-directional, Er-doped dual comb fiber laser consisting of all-polarization-maintaining fiber devices. Polyimide films in which single-wall carbon nanotubes (SWNTs) were dispersed were used as the in-line saturable absorber. In order to avoid synchronization of the two combs and associated damage to the SWNT film, a two-branch configuration with two SWNT films was employed. Soliton pulses with almost the same optical spectra were generated stably in each direction, and dual comb beats were observed simply by overlapping the two outputs. The repetition frequency was 28 MHz, and the frequency difference was 105-140 Hz. Thanks to the small frequency difference, dual comb beats corresponding to the whole optical spectrum were observed without any overlapping. Fourier transform spectroscopy using the developed dual comb source was examined, and the characteristics of an optical filter were successfully obtained.

Tunable dual-color operation of Yb:fiber laser via mechanical spectral subdivision

We present a versatile method to generate a dual-color laser from a single fiber laser cavity by spectral subdivision using a tunable mechanical filter. As a proof-of-principle, we implement the concept in a nonlinear polarization evolution (NPE)-mode-locked ytterbium (Yb)-fiber laser. The division into two independent spectral regions is achieved by inserting a narrow blade-shaped beam block into the free-space grating compressor section of the cavity, where the spectrum is spatially dispersed. By mode-locking both spectral regions, two pulse trains, with different repetition rates around 23 MHz, are generated. Each pulse train has a FWHM of ~10 nm. The method presented here enables tuning of the difference in repetition rate as well as the spectral separation of the two independent pulse trains. The difference in repetition rates originates from intracavity dispersion and can be tuned over a large range (650 Hz - 3 kHz in this setup) by changing the length of the grating compressor. By changing the effective width of the beam block the spectral separation can be dynamically adjusted. This approach's simplicity holds great promises for the development of single-cavity dual-comb lasers featuring tunable sampling rates.

All-polarization-maintaining dual-wavelength mode-locked fiber laser based on Sagnac loop filter

We demonstrate an all polarization-maintaining (PM) fiber based dual-wavelength mode-locked Er-fiber laser. A nonlinear amplifying loop mirror (NALM) with an intracavity nonreciprocal phase shifter is used for self-started mode-locking. A short segment of PM fiber is angle-spliced to the NALM, functioning as a PM Sagnac loop filter, thus enabling dual-wavelength mode-locking. The wavelength separation is solely determined by the angle-spliced PM fiber length. Stable dual-wavelength mode-locking operation is switchable between 1570/1581 nm and 1581/1594 nm. The two-color pulse trains oscillating in the same cavity have an inherent offset repetition rate of ~1 kHz owing to cavity dispersion, allowing future high precision dual-comb applications with a simple and robust configuration.

Self-triggered Asynchronous Optical Sampling Terahertz Spectroscopy using a Bidirectional Mode-locked Fiber Laser

We report a self-triggered asynchronous optical sampling terahertz spectroscopy system based on a single bidirectional mode-locked fiber laser and plasmonics-enhanced photoconductive nanoantennas. The fiber laser generates two optical mutually coherent pulse trains with a stable repetition rate difference, enabling time-domain terahertz spectroscopy without using any mechanical delay line, stabilization electronics, or external trigger. The resolved terahertz spectra over a 0.1-2 THz frequency range and a 30-second measurement time show more than a 70-dB dynamic range, revealing water absorption lines matching the HITRAN database, through a light-weight and compact spectroscopy setup.

Route towards extreme optical pulsation in linear cavity ultrafast fibre lasers

Pathways towards the generation of extreme optical pulsation in a chaotic transition regime in a linear fibre laser cavity configuration are presented. In a thulium mode-locked fibre laser, extreme events that can be controllably induced by manipulating the cavity birefringence for pulse energies exceeding the single soliton pulse operating regime are studied in detail for the first time. While a solitonic pulsation structure at the fundamental repetition rate is maintained, additional energy is shed in a chaotic manner, leading to broader spectral generation and shorter pulse durations whose behaviour deviates significantly from a classical statistical distribution. These pulses display markedly different characteristics from any previously reported extreme events in fibre lasers associated with multiple solitons and pulse bunching, thus presenting a novel observation of extreme pulsation. Detailed noise studies indicate that significant enhancement of relaxation oscillations, modulation instability and the interplay with reabsorption mechanisms contribute in this transient chaotic regime. The extreme pulsation generated in a compact fibre laser without any additional nonlinear attractors can provide an attractive platform to accelerate the exploration of the underlying physics of the chaos observed in mode-locked laser systems and can lead to novel fibre laser cavity designs.

Bidirectional mode-locked thulium-doped fiber laser

We report a bidirectional mode-locked thulium-doped fiber laser. Mode-locking is enabled by the combination of semiconductor saturable absorption and nonlinear polarization rotation. Two stable mode-locked picosecond pulse trains in opposite directions are generated with a fundamental repetition rate of ∼16.57  MHz. Output wavelengths are tunable over 35 nm.

Dual terahertz comb spectroscopy with a single free-running fibre laser

Dual terahertz (THz) comb spectroscopy enables high spectral resolution, high spectral accuracy, and broad spectral coverage; however, the requirement for dual stabilized femtosecond lasers hampers its versatility. We here report the first demonstration of dual THz comb spectroscopy using a single free-running fibre laser. By tuning the cavity-loss-dependent gain profile with an intracavity Lyot filter together with precise management of the cavity length and dispersion, dual-wavelength comb light beams with slightly detuned repetition frequencies are generated in a single laser cavity. Due to sharing of the same cavity, such comb light beams suffer from common-mode fluctuation of the repetition frequency, and hence the corresponding frequency difference between them is passively stable around a few hundred hertz within millihertz fluctuation. While greatly reducing the size, complexity, and cost of the laser source by use of a single free-running fibre laser, the dual THz comb spectroscopy system maintains a spectral bandwidth and dynamic range of spectral power comparable to a system equipped with dual stabilized fibre lasers, and can be effectively applied to high-precision spectroscopy of acetonitrile gas at atmospheric pressure. The demonstrated results indicate that this system is an attractive solution for practical applications of THz spectroscopy and other applications.

Experimental study on polarization evolution locking in a stretched-pulse fiber laser

Polarization evolution locking (PEL) of a stretched-pulse fiber laser is experimentally investigated with a simple pulse selection method based on a fast electrooptic modulator, capable of revealing the temporal and spectrum evolution of the PEL pulses. Moreover, the wavelength dependence of PEL is observed by spectrally filtering the pulses and is further investigated for individual fiber laser comb lines through beat note measurements with narrow-linewidth cw lasers.

Dual-comb spectroscopy with a single free-running thulium-doped fiber laser

We demonstrate dual-comb spectroscopy in the vicinity of 2 µm wavelength based on a single dual-wavelength dual-comb Thulium-doped fiber laser. The shared laser cavity ensures passively maintained mutual coherence between the two combs due to common mode environmental noise rejection. In a proof-of-principle experiment, the absorption characteristics caused by the water in the optical path that composes the dual-comb spectrometer are measured. The retrieved spectral positions of the water absorption dips match with the HITRAN database.