Normal Cavity Dispersion Research Articles

Reliable intracavity reflection for self-injection locking lasers and microcomb generation

Self-injection locking has emerged as a crucial technique for coherent optical sources, spanning from narrow linewidth lasers to the generation of localized microcombs. This technique involves key components, namely a laser diode and a high-quality cavity that induces narrow-band reflection back into the laser diode. However, in prior studies, the reflection mainly relied on the random intracavity Rayleigh backscattering, rendering it unpredictable and unsuitable for large-scale production and wide-band operation. In this work, we present a simple approach to achieve reliable intracavity reflection for self-injection locking to address this challenge by introducing a Sagnac loop into the cavity. This method guarantees robust reflection for every resonance within a wide operational band without compromising the quality factor or adding complexity to the fabrication process. As a proof of concept, we showcase the robust generation of narrow linewidth lasers and localized microcombs locked to different resonances within a normal-dispersion microcavity. Furthermore, the existence and generation of localized patterns in a normal-dispersion cavity with broadband forward–backward field coupling is first proved, as far as we know, both in simulation and in experiment. Our research offers a transformative approach to self-injection locking and holds great potential for large-scale production.

Cnoidal waves and their soliton limits in single mode fiber lasers

Cnoidal waves are a type of nonlinear periodic wave solutions of the nonlinear dynamic equations. They are well known in fluid dynamics, but it is not the case in optics. In this paper we show both experimentally and numerically that cnoidal waves could be formed in a fiber laser either in the net normal or net anomalous cavity dispersion regime, especially because, as the pump power is increased, the formed cnoidal waves could eventually evolve into a train of bright (in the net anomalous cavity dispersion regime) or dark (in the net normal cavity dispersion regime) solitons. Numerical simulations of the laser operation based on the extended nonlinear Schrödinger equation (NLSE) have well reproduced the experimental observations. The result not only explains why solitons can still be formed in a fiber laser even without mode locking but also suggests a new effective way of automatic stable periodic pulse train generation in lasers with a nonlinear cavity.

Self-starting mode-locking in an all-PM Yb-doped fiber laser oscillator enabled by a 3x3 nonlinear optical lossy loop mirror

Ultrafast fiber oscillators based on loop mirror saturable absorbers often suffer from a problematic self-starting mode-locking operation. It is usually necessary to provide an additional phase shift in the loop, guaranteeing repeatable initiation of the pulsed regime. 3x3 even splitting ratio fiber couplers have recently introduced a required phase shift in an all-fiber loop mirror architecture. Until now, mode-locking induced by a 3x3 fiber coupler has been associated only with nonlinear amplifying loop mirrors. Here, we present a self-starting ultrafast dispersion-managed all-polarization-maintaining Yb-doped oscillator that utilizes a nonlinear optical lossy loop mirror instead. We show three ways of inserting asymmetric losses in a loop via a variable optical attenuator, a fiber coupler, and a very simple lossy splice. Complete characterization of all output ports of the oscillator proves significant spectral and temporal breathing of the pulse when circulating through the net normal dispersion cavity, which can deliver nJ-level pulse energy. The system guarantees excellent stability, low noise and performance comparable to nonlinear amplifying loop mirrors while being simpler, cheaper, and providing more usable output ports with different pulse characteristics.

Dynamic process of dissipative soliton build-up in mode-locked fiber laser

We experimentally studied the dynamic process of dissipative soliton build-up by dispersive Fourier transform in net normal dispersion cavity. The oscillation of spectral bandwidth and pulse energy before the stabilization of dissipative soliton was observed, the build-up and termination of the double dissipative solitons accompanied with soliton explosion and energy interaction were observed with higher pump power. Simulation of dissipative soliton build-up process was demonstrated through the extended nonlinear Schrödinger equation, negative feedback mechanism was summarized as the reason of the pulse oscillation during the build-up process.

Hysteresis of a passively mode-locked fiber laser: effects from cavity dispersion.

Hysteresis is a common phenomenon in passively mode-locked lasers and refers to the effect where the thresholds marking transitions between different pulsation states are not the same for an increasing or decreasing pump power. Despite wide presence in experimental observations, the general dynamics of hysteresis remains elusive, largely due to the challenge to acquire the full hysteresis dynamics of a given mode-locked laser. In this Letter, we overcome this technical bottleneck by fully characterizing an exemplar figure-9 fiber laser cavity, which exhibits well-defined mode-locking patterns in its parameter space or "primitive cell." We varied the net cavity dispersion and observed the salient change of hysteresis characteristics. Specifically, transiting from an anomalous to a normal cavity dispersion is found to consistently increase the likelihood of the single-pulse mode-locking regime. To the best of our knowledge, this is the first time that a laser's hysteresis dynamic is fully probed and related to fundamental cavity parameters.

Dark-bright soliton trapping in a fiber laser.

We report, to the best of our knowledge, the first experimental observation of coexistence of scalar dark and bright solitons in a fiber laser operating in the near zero-dispersion regime. We show that because of the incoherent cross coupling, under suitable conditions a bright soliton formed in the net anomalous cavity dispersion regime could bind with a dark soliton formed in the net normal cavity dispersion regime in a fiber laser. The properties of the dark and bright solitons, as well as their bound states, are experimentally investigated. The numerical simulations based on the coupled Ginzburg-Landau equations have reproduced the experimental results well.

Anti-dark solitons in a single mode fiber laser

We report on the experimental observation of temporal anti-dark solitons in optical fibers. Under suitable condition the average dynamics of light propagation in a unidirectional fiber ring laser is governed by the nonlinear Schrödinger equation (NLSE). Dark solitons were found in the fiber laser in the net normal cavity dispersion region. We further show experimentally that at net normal near zero cavity dispersion, where the third-order dispersion plays a role, either the dark solitons, or anti-dark solitons, or coexistence of both types of solitons could be obtained in the same fiber laser.

Coherently coupled vector black solitons in a quasi-isotropic cavity fiber laser.

We report the first experimental observation of coherently coupled black-black solitons in a quasi-isotropic cavity fiber laser with normal cavity dispersion. The properties of the vector solitons accord well with the theoretical predictions based on the coherently coupled nonlinear Schrödinger equations. Numerical studies and experimental results clearly confirmed the existence of the highly robust vector black solitons in the fiber laser system.

Towards CW modelocked laser on chip - a large mode area and NLI for stretched pulse mode locking.

Integrated modelocked lasers with high power are of utmost importance for next generation optical systems that can be field-deployable and mass produced. Here we study fully integrated modelocked laser designs that have the potential to generate ultrashort, high power, and high quality pulses. We explore a large mode area laser for high power pulse generation and study the various mode-locking regimes of dispersion managed soliton pulses in net anomalous and net normal dispersion cavities. Furthermore, we study numerically and experimentally general properties and tunability of a fast integrated saturable absorber based on low loss silicon nitride nonlinear interferometer. We believe this work guides the exploration of the future for integrated high power modelocked lasers.

Dark solitons embedded in a stable periodic pulse train emitted by a fiber ring laser

We report on the experimental observation of dark solitons embedded in a stable periodic pulse train emitted by a net normal dispersion cavity fiber laser. Our experimental result shows that in addition of forming in a continuous-wave background, a dark soliton could also be formed in a stable periodic pulse train background. Numerical simulations have well reproduced the experimental result. Moreover, numerical simulations also show that as a result of the periodic pulse train background the formed dark soliton could exhibit a dark pulse envelope that is much broader than its actual pulse width.

Observation of dark-bright vector solitons in fiber lasers.

We report the experimental observation of coherently coupled dark-bright vector solitons in single mode fiber lasers with either normal or anomalous cavity dispersion. The properties of these vector solitons were found to agree well with theoretical predictions based on the coherently coupled nonlinear Schrödinger equations. The experimental results clearly confirmed the existence of a fundamentally new form of optical solitons in coupled nonlinear systems.

Cavity-assisted modulation instability lasing of a fiber ring laser

We report on the experimental evidences of cavity-assisted modulation instability (CAMI) lasing of fiber lasers. We first reviewed the theory of CAMI. We then presented experimental evidences of CAMI lasing of fiber lasers with either an anomalous or a normal dispersion cavity. It is shown that even in a fiber laser without any mode-locking element in cavity, if the orientation of the intracavity polarization controller is changed, a CW emission could be suddenly converted into a stable periodic pulse train emission. Moreover, the repetition rate of the pulse train can be adjusted over a wide range from the fundamental cavity repetition frequency to hundreds of GHz by changing the cavity detuning. We point out that the CAMI lasing should be a general effect of lasers with a nonlinear cavity.

Ultra-short pulse fiber optical parametric oscillator.

This work explores the frequency conversion and generation of short pulses with an optical parametric oscillator based on micro-structured fibers. Depending on the operation regime, the optical cavity can either behave as a normal-dispersion cavity delivering linearly chirped pulses, which were externally compressed down to only 26fs, or as a dispersion-managed oscillator, which directly delivered compressed pulses with a pulse duration of only 39fs.

152 fs nanotube-mode-locked thulium-doped all-fiber laser.

Ultrafast fiber lasers with broad bandwidth and short pulse duration have a variety of applications, such as ultrafast time-resolved spectroscopy and supercontinuum generation. We report a simple and compact all-fiber thulium-doped femtosecond laser mode-locked by carbon nanotubes. The oscillator operates in slightly normal cavity dispersion at 0.055 ps2, and delivers 152 fs pulses with 52.8 nm bandwidth and 0.19 nJ pulse energy. This is the shortest pulse duration and the widest spectral width demonstrated from Tm-doped all-fiber lasers based on 1 or 2 dimensional nanomaterials, underscoring their growing potential as versatile saturable absorber materials.

Vector gain-guided dissipative solitons in a net normal dispersive fiber laser

We report on the experimental observation of spontaneous vector dissipative soliton train formation in a dispersion-managed cavity fiber laser with net normal cavity dispersion. The formation of the dissipative solitons is triggered by the nonlinearity-induced wave mixing and gain bandwidth limitation. So, they are named the gain-guided dissipative solitons to differentiate from those of the dissipative solitons formed in the mode-locked fiber lasers. The vector dissipative solitons are phase locked, and the coherent energy exchange between their two orthogonal polarization components is first revealed. Noise-like pulse emission and vector soliton rain effect are also experimentally observed.

Mode-Locked Thulium-Doped Fiber Lasers Based on Normal Dispersion Active Fiber

We report and compare three configurations of the semiconductor saturable absorber mirror mode-locked, linear cavity femtosecond all-fiber master oscillator power amplifier based on a highly Ge-doped thulium-doped normal dispersion fibers. We have studied the performance of the system and have obtained stable mode-locking in a wide cavity dispersion range $\sim 1.88~\mu \text{m}$ . In this letter, we focus our attention on three mode-locking regimes: laser operating in the anomalous, nearly zero and normal cavity dispersion regimes without the use of the additional dispersion compensating elements. For the nearly zero and normal cavity dispersion regimes the femtosecond pulses with several nanojoule energy could be obtained. The pulses were further compressed down to 630 fs using a simple fiber compressor consisting of a piece of the conventional telecommunication fiber, making the laser design particularly simple and cost effective.

Narrow-linewidth double-peak figure-eight passively mode-locked fiber laser

We report on the experimental observation of narrow-linewidth double-peak dissipative solitons in a figure-eight Yb-doped passively mode-locked fiber laser in the large net normal cavity dispersion regime. Without any physical bandpass filters in the cavity, stable mode-locking pulses with a duration of 416 ps and a 3 dB bandwidth of 0.3 nm can be directly generated from the laser cavity. The edge-to-edge bandwidth of the spectrum is about 5 nm. The repetition rate of the dissipative solitons is 3.9 MHz. With the increase of pump power, the central wavelength of the dissipative solitons shifts toward a short wavelength and edge-to-edge spectral widths are widened gradually. These phenomena are theoretically explained.

Fundamental Limits to Mode-Locked Lasers: Toward Terawatt Peak Powers

Most mode-locked solid-state lasers are based on soliton solutions of the cubic Ginzburg-Landau equation. Their performance is close to theoretical limits, and new design paradigms will be needed if solid-state systems are to continue to improve at their historically aggressive rates. Through comprehensive analysis of the interplay of group-velocity dispersion, nonlinear refraction, spectral filtering, and saturable absorption, we identify the factors that fundamentally limit the achievable peak power of solid-state lasers. Numerical calculations that accurately model existing solid-state lasers are used to assess new designs. In particular, we focus on normal-dispersion cavities, which support dissipative solitons. The calculations reveal that the gain bandwidth, modulation depth of the saturable absorber, and nonlinear phase accumulation present the primary limitations to the peak power. We assess these limitations in Ti:sapphire, Yb:KGW, and thin-disk oscillators. Guidelines for the optimum designs are presented, and we show that dissipative soliton pulse-shaping in cavities with net normal dispersion can fundamentally allow performance improvements of several orders of magnitude. High-performance oscillators should offer low-noise, low-cost alternatives to some current amplifier-based systems.

Reduction of timing jitter and intensity noise in normal-dispersion passively mode-locked fiber lasers by narrow band-pass filtering.

Fiber lasers mode-locked with normal cavity dispersion have recently attracted great attention due to large output pulse energy and femtosecond pulse duration. Here we accurately characterized the timing jitter of normal-dispersion fiber lasers using a balanced cross-correlation method. The timing jitter characterization experiments show that the timing jitter of normal-dispersion mode-locked fiber lasers can be significantly reduced by using narrow band-pass filtering (e.g., 7-nm bandwidth filtering in this work). We further identify that the timing jitter of the fiber laser is confined in a limited range, which is almost independent of cavity dispersion map due to the amplifier-similariton formation by insertion of the narrow bandpass filter. The lowest observed timing jitter reaches 0.57 fs (rms) integrated from 10 kHz to 10 MHz Fourier frequency. The rms relative intensity noise (RIN) is also reduced from 0.37% to 0.02% (integrated from 1 kHz to 5 MHz Fourier frequency) by the insertion of narrow band-pass filter.

Wavelength-spacing tuning of a dual-wavelength active-mode locking fiber laser.

We demonstrate a novel concept of wavelength-spacing tuning of a dual-wavelength active-mode locking fiber laser in the 1.3-μm wavelength region. A dual-cavity configuration is implemented using a chirped fiber Bragg grating pair for wavelength-spacing tuning. Both normal and anomalous dispersion cavities were schematically designed for wavelength-spacing tuning ranging from 3.52-33.54 nm. The side-mode suppression ratio was ∼30 dB for both wavelengths. Because of the filter-less tuning mechanism, a fast repletion rate of wavelength-spacing sweeping and switching was demonstrated for frequencies up to 50 kHz.