Artificial Saturable Absorber Research Articles

Nanosecond pulse generation mode-locked by an artificial saturable absorber based on SMF-MMF-SMF structure

We present a noteworthy demonstration of nanosecond pulse generation utilizing an all-fiber artificial saturable absorber (SA) structure, composed of single-mode multimode single-mode fiber (SMF-MMF-SMF), integrated into a passively mode-locked erbium-doped fiber laser (EDFL). The SMS-based SA effectively phase-locked the longitudinal modes within the elongated EDFL cavity, resulting in the production of mode-locked pulses operating at 1567 nm. Mode-locking is achieved through the Kerr effect of nonlinear multimode interference. The pulse repetition rate is synchronized with the fundamental frequency at 1.28 MHz. At a pump power of 269.1 mW, the laser achieves a remarkable peak average pulse energy of 6.0 nJ. Demonstrating outstanding stability, the laser operation exhibits a peak-to-pedestal extinction ratio of 70 dB for the fundamental frequency. We are confident in the substantial potential of this SMS structure, positioning it as a compelling candidate for integration into ultrafast fiber lasers, with promising prospects in the field.

Performance enhancement via XPM suppression of a linear all-PM mode-locked fiber oscillator.

We demonstrate the strong performance enhancement of an all-polarization-maintaining mode-locked fiber oscillator using a linear self-stabilized fiber interferometer via the suppression of the cross-phase modulation (XPM). Numerical simulations reveal that XPM significantly affects the saturable absorber dynamics resulting in strong distortions of the mode-locked steady-states and output pulse quality. For experimental verification, we construct an oscillator with XPM suppression, employing an intra-cavity YVO4 crystal to obtain a differential walk-off effect and compare its characteristics with a reference oscillator in a standard configuration. It is shown, that the XPM suppression not only lowers the mode-locking threshold by more than 45% but further results in improved pulse quality at the output ports and reduced nonlinear loss in the artificial saturable absorber.

Domain-wall dark pulse generation with SMF-GIMF-SMF structure as artificial saturable absorber

We experimentally demonstrated the generation of domain-wall dark pulse in an Erbium-doped fiber laser using the combination of a 10 cm graded index multimode fiber sandwiched by single mode fibers as artificial saturable absorber. The interaction of phase difference in grade index multimode fiber allowed the stable dual-wavelength oscillation in the cavity. The dual-wavelength centered at 1567.2 nm and 1569.4 nm produces the topological defect in temporal domain and achieved a dark pulse formation with repetition rate of 21.5 MHz. The highest average pulse energy is calculated as 769.6 pJ with pulse width of 5 ns. Throughout the operating pump power range, the average pulse energy and output power increase linearly, with R2 of 0.9999 and achieved the laser efficiency of 9.33%. From the measurement in frequency domain, the signal-to-noise ratio is measured as 49 dB. As compared to reported DW dark pulse works, the proposed structure only required a short length of multimode fiber, which allowed the domain-wall dark pulse to achieve higher pulse repetition rate. The venture of domain wall dark pulse is potentially to pave the foundation toward sustainable industrial growth.

All-fibre mode-locked laser using SMF-GIMF-SMF structure as artificial saturable absorber

In this work, we proposed and experimentally demonstrated a combination of single mode fibre (SMF) with graded indexed multimode fibre (GIMF) to form an artificial saturable absorber. A SMF-GIMF-SMF structure is incorporated into Erbium-doped fibre laser (EDFL) system to induce mode-locking operation with very low threshold pump power of 41.6 mW. The highest pulse energy obtained is 406.65 pJ at pump power of 84.7 mW. With a relatively short GIMF used as artificial saturable absorber, high fundamental repetition rate with low threshold power is achieved. The proposed artificial saturable absorber is potentially to contribute to the development of sustainable industry, innovation, and infrastructure.

All-fiber Q-switched erbium-doped fiber laser generation with SMF-SIMF-SMF structure as artificial saturable absorber

A passively Q-switched Erbium-doped fiber laser (EDFL) was successfully demonstrated using SMF (single-mode fiber)-SIMF (step-index multimode fiber)-SMF or SMS structure as a saturable absorber (SA). The SMS structure operates on the Kerr-effect of multimode interference (MMI). To modulate the cavity loss, a 158 mm length of SIMF was fusion spliced into a ring cavity. By properly adjusting the in-line polarization controller (PC) that holds the SIMF, we were able to generate steady Q-switched pulses. As the input pump power increased from 115 to 343 mW, the repetition rate of the Q-switched laser increased from 87 to 158 kHz, while the corresponding pulse width decreased from 4.2 µs to 2.5 µs. The signal-to-noise ratio (SNR) of the fundamental mode for the pulsed laser was around 72 dB at a central wavelength of 1530 nm. Our results indicate that the SMS structure can be used as an SA to generate stable Q-switched EDFL with very simple cavity settings.

Wavelength-stabilized figure-of-9 thulium-doped all-fiber laser emitting 560 fs pulses

We demonstrate a figure-of-9 all-fiber thulium-doped laser (TDFL) that generates 560 fs long pulses at 1948 nm wavelength. In order to achieve self-starting passive mode-locking, we utilize an in-fiber Faraday rotator to induce a nonreciprocal phase shift. To the best of our knowledge, this is the first all-fiber TDFL that combines an artificial saturable absorber (SA) with a chirped fiber Bragg grating (CFBG) as a wavelength-selective reflector. This cavity design is an excellent candidate to pump nonlinear processes such as supercontinuum and frequency comb generation since it does not require any SA material that degrades over time for mode-locking and could be made wavelength-tuneable via the CFBG.

Amplitude noise suppression in Yb:doped NALM oscillators utilizing saturable absorber settings.

Optical frequency combs based on fiber lasers mode-locked (ML) with a nonlinear amplifying loop mirror (NALM) have become the backbone of many cutting-edge applications, ranging from precision spectroscopy to quantum physics. Being extremely precise measurement tools, understanding their passive stability and low-noise operation regimes is vital. While several influences on the laser noise have been studied, many parameters remain poorly understood. Here, we systematically analyze under which preconditions the artificial saturable absorber settings of the laser can be modified during operation without losing mode-locking and the effects on laser noise, the spectrum and the output power. Our results show that it is possible to decrease the amplitude noise (AM noise) of the laser by more than 50 % by simply rotating a wave plate within the laser cavity. Additionally, we discuss differences to a similar effect observed in a NALM-alike laser amplifier and of changing the output coupling. These findings deepen our understanding and capabilities of optimizing the noise performance of ML fiber lasers, enable us to investigate new parameter spaces, and can be used to further optimize the noise performance of the NALM laser design, making it an ideal light source for advanced setups both in research and industry.

L-band mode-locked fiber laser using all polarization-maintaining nonlinear polarization rotation.

For the first time, to the best of our knowledge, in the soliton regime, we demonstrate an L-band fiber laser mode-locked by all polarization-maintaining (all-PM) nonlinear polarization rotation (NPR). A numerical study suggests that lengthening the NPR section boosts modulation depth and lowers saturation power of the artificial saturable absorber (SA). With the longest NPR section to date (21 m), the laser emits 1.25-ps soliton pulses at 1584.2 nm and a 3.9-MHz repetition rate. Our laser provides a promising L-band seed source, exhibiting improved repeatability and stability compared with non-PM L-band pulse fiber lasers.

Three-step staircase-like pulses generation in a butterfly-shaped mode-locked fiber laser

We experimentally observed three-step staircase-like pulses (TSSLP) delivered from a butterfly-shaped mode-locked fiber laser An integrated twin nonlinear amplifying loop mirrors (NALM) incorporating highly nonlinear fibers (HNLF 65, 52 m) acts as a homogeneous hybrid-like artificial saturable absorber (HH-SA). With the most available pump power, the obtained sub-stairs in TSSLP with 10.06, 9.61, 12.59 ns durations and 0.33, 3.46 and 5.38 W peak intensities at 1.23 MHz, respectively. The second-peak can be tuned from 0.1 to 3.46 W while the third-duration ranging from 9.86 to 14.12 ns. The achievements reveal that the proposed HH-SA scheme creates effective intensity discrimination while enjoying more consistent properties. It provides more flexible transfer curve and pulse-shaping mechanism, with novel features to meet the requirements of various engineering customizations. The versatility and stability of the HH-SA device, make it highly attractive as a stable platform for studying new pulses together with nonlinear interaction dynamics in fundamental physics.

Large-mode-area soliton fiber oscillator mode-locked using NPE in an all-PM self-stabilized interferometer.

In this work, we investigate an approach to scale up the output pulse energy in an all-polarization-maintaining 17.3 MHz Yb-doped fiber oscillator via implementation of a 25 µm core-diameter large-mode-area fiber. The artificial saturable absorber is based on a Kerr-type linear self-stabilized fiber interferometer, enabling non-linear polarization rotation in polarization-maintaining fibers. Highly stable mode-locked steady states in the soliton-like operation regime are demonstrated with 170 mW average output power and a total output pulse energy of ∼10n J distributed between two output ports. An experimental parameter comparison with a reference oscillator constructed with 5.5 µm core-sized standard fiber components reveals an increase of pulse energy by a factor of 36 with simultaneously reduced intensity noise in the high-frequency range >100k H z.

Tunable mode-locked Tm-doped fiber laser based upon cross-phase modulation.

We demonstrate the generation of soliton and dissipative soliton in an ultrafast thulium (Tm) doped fiber laser based upon cross-phase modulation (XPM) induced mode-locking. The mode-locking is realized by periodically modulating the 2-µm signal through XPM that is activated by an injected 1.5-µm pulsed laser. Such a mechanism enables the laser to be mode-locked in various operation regimes without any real or artificial saturable absorbers. Thanks to the XPM pulling effect, the wavelength of the Tm-doped fiber laser can be tuned by adjusting the repetition frequency of the 1.5-µm pulsed laser. The maximum tuning ranges achieved in this work for the soliton and dissipative soliton regimes are respectively 11 nm and 15 nm. The outcomes of this work not only provide a continuously and controllably wavelength-tunable ultrafast laser but also offer a passively synchronized dual-color fiber laser system, which is promised for many important applications such as Raman spectroscopy, nonlinear frequency conversion systems, and multi-color pump-probe systems.

A Comparison for Saturable Absorbers: Carbon Nanotube Versus Graphene

Saturable absorbers (SAs) have been regarded as a low‐cost yet robust and compact solution to pulse generation in different fiber and solid‐state ultrafast lasers. Following the development of different artificial SAs and semiconductor SA mirrors, carbon nanotube (CNT) and graphene are discovered and employed as SAs for solid‐state as well as fiber lasers. In the past decade, a growing number of SAs based on new materials like MXenes and metal halide perovskites have emerged, while CNT and graphene are still irreplaceable by new materials due to their fascinating saturable absorption properties. Instead of searching for new SAs, the investigation of transient dynamics for mode‐locked pulse generation still relies on the use of CNT and graphene SAs in the fiber laser system due to their reproducible performance and facile fabrication. In this review, a comparison between CNT and graphene will be thoroughly studied in terms of optical nonlinearity, relaxation and recovery time, saturation intensity or fluence, modulation depth, non‐saturable loss, and damage threshold dynamics. In the end, this review will address the challenges in the selection of SAs and provides recommendations for CNT and graphene SAs used in different mode‐locked fiber lasers.

Self-starting Q-switched pulse generation in EDFL ring cavity based on Ta2AlC MAX-phase saturable absorber

A passive Q-switched erbium-doped fiber (EDF) laser is demonstrated using an MAX phase Ta2AlC saturable absorber (SA). By casting method, the Ta2AlC –PVA thin film is fabricated, and inserted into the laser cavity, which includes a 0.85 m segment of erbium doped fiber as the active medium pumped by a diode laser of 976 nm as the linear gain. The modulation depth, saturation intensity and non-saturable transmission of the MAX phase Ta2AlCSA are 7%, 15.2%, and 16.5 kW/cm2 respectively. The maximum pulse repetition rate (PRR) increase from 23.54 kHz to 111.4 kHz meanwhile, the pulse width gradually decreases from 8.175 μs to 1.589 µs at a range of a pump power from 30 mW to 150 mW and the highest pulse energy achieved is 68.2 nJ. Moreover, by adjusting the intra-cavity polarization controller state a tunable single–dual-triple-wavelength can be achieved with a tunable range of 5 nm. The experimental results demonstrated that the Ta2AlC MAX phase has a top placement as an artificial SA in Q-Switching fiber laser generation.

Graded-index multimode fiber in nonlinear absorbing loop mirror as saturable absorber for noise-like pulse generation in 2 μm region

This investigation explores the potential use of graded-index multimode fiber (GIMF) in a nonlinear absorbing loop mirror (NAbLM) as an artificial saturable absorber for ultrashort pulse generation. The saturable absorption mechanism of GIMF-NAbLM was based on a combination of nonlinear phase shift and multimodal interference that was satisfied by the asymmetric positioning of the GIMF in the single-mode fiber loop mirror. The proposed saturable absorber was integrated in a mode-locked thulium-doped fiber laser with figure-of-eight cavity. A noise-like pulse of 164 fs spike pulse duration at 1950 nm central wavelength was attained in the net anomalous dispersion with a 10 m long GIMF. The GIMF-NAbLM structure demonstrates immense prospects as an effective saturable absorption device for generating stable ultrashort pulse in the 2 µm region due to its simplicity.

Bound-State Soliton and Noise-like Pulse Generation in a Thulium-Doped Fiber Laser Based on a Nonlinear Optical Loop Mirror

We demonstrate a thulium-doped, mode-locked, all-fiber laser capable of operating in two generation regimes: dispersion-managed soliton and noise-like pulse (NLP). Employing a nonlinear optical loop mirror as an artificial saturable absorber, the oscillator generated optical pulses with a fundamental pulse repetition frequency of ~15.795 MHz. The total net dispersion of the laser cavity had a slightly anomalous group delay dispersion value of −0.016 ps2. After appropriate adjustment of a polarization controller, bound states of a dispersion-managed soliton composed of three pulses with fixed soliton separations were also observed. NLP generation, tunable over 35 nm from 1943.5 to 1978 nm, was also presented in the same laser setup. To our knowledge, this is the first report of the generation of tunable NLPs in a mode-locked thulium-doped fiber laser based on a nonlinear loop mirror saturable absorber.

Artificial saturable absorbers for ultrafast fibre lasers

A paper is presented of artificial saturable absorbers as applied in fibre lasers. Shown are the advantages and drawbacks of this saturable absorber type in comparison with material-based natural counterparts. The prospects of further development of components for passive (or self) mode locking in fibre-optical technology are discussed. Analysis demonstrates that artificial absorbers relying on comparatively short optical fibres hold higher potential and may become a real alternative to natural absorbers.

Different types of noise-like pulse in a nonlinear multimodal interference based mode-locked fiber laser

In this paper, we employ a symmetrical graded index multimode fiber-step index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF) structure as artificial saturable absorber to achieve mode-locked operation in an Erbium-doped fiber laser. After optimizing the cavity length, pump power and orientation of polarization controller, noise-like pulse (NLP) is built in anomalous dispersion region experimentally. It is worth noting that except for the traditional NLP, a special NLP with unique features is obtained whose temporal and spectral properties are different from the known case on NLP reported before. According to the experimental results, we also acquire and compare properties of subpulse within NLP envelope with the change of pump power. Last but not least, harmonic of the peculiar NLP is found under appropriate cavity conditions.

All-polarization-maintaining linear fiber laser mode-locked by nonlinear polarization evolution with phase bias

We report on a novel architecture for robust all polarization-maintaining (PM) femtosecond linear fiber lasers mode-locked by nonlinear polarization evolution (NPE) with phase bias. Due to a non-reciprocal phase shift, the reflectivity of the artificial saturable absorber has a steeper slope around zero difference in nonlinear phase shift between the orthogonal polarizations, offering the possibility to reduce mode locking threshold and reach high repetition rates. We demonstrate an environmentally stable self-starting 105 MHz repetition rate linear Yb:fiber laser based on NPE with phase bias mode locking. The laser delivers an average power of 26.3 mW and 332 fs pulse which is further compressed to 91 fs at a pump power of 425 mW. Our work provides a novel promising compact and robust femtosecond laser for ultrafast photonic applications.

Generation of dissipative soliton resonance in a fiber laser based on a nonlinear optical loop mirror

We experimentally demonstrate generation of mode-locked pulses in dissipative soliton resonance (DSR) regime. The DSR pulses are obtained from a passively mode-locked figure-8 Er:Yb doped fiber laser operating in large net anomalous dispersion regime. The modelocked laser emission is based on a polarization-imbalanced nonlinear optical loop mirror (NOLM), acting as artificial saturable absorber. Stable square DSR pulses, obtained by carefully adjusting the NOLM properties, increase their duration from 2.47 ns to 46.52 ns as the pump power is increased from 0.68 to 6.31 W. With the maximum pump power launched, average output power of 73.95 mW with pulse energy of 53.39 nJ is achieved.

Self-referenced frequency comb from a polarization-maintaining Er: Fiber laser based nonlinear polarization evolution

To the best of our knowledge, this paper presents the first self-referenced polarization-maintaining (PM) Er: nonlinear polarization evolution-based fiber frequency comb. A short PM fiber section segment acts as an artificial saturable absorber and compensates for the group velocity mismatch. A conventional f-2f interferometer allows detecting carrier-envelope offsetting frequency (fceo) after an octave-spanning spectrum generation. A complete description of the PM Er: fiber comb combining a noise study of the repetition rate (fr) and fceo is presented. Accordingly, the in-loop residual phase noise of fceo and fr are 468 mrad and 0.336 mrad from 1 Hz to 3 MHz, respectively. Further, we verify the long-term performance of the comb system by recording the fluctuation of fr and fceo for more than 10 h. We observed that combining fast actuators for phase noise suppression would potentially enable a compact and robust frequency comb system for a range of applications such as comb-based liars, optical clock, and gas sensing.