Mode-locked Operation Research Articles

Passively harmonic dissipative soliton generation in normal dispersion erbium-doped fiber laser using SMS fiber as artificial saturable absorber

Passively harmonic mode-locking has been experimentally demonstrated in an erbium-doped fiber laser with large normal dispersion using single-multi-single mode structure as artificial saturable absorber. By increasing the pump power under the same polarization setting, the mode-locking operation can switch from fundamental mode-locked to 5th order harmonic mode-locked. Highest repetition rate of 4.26 MHz (5th order harmonic) is observed, with pulse width and pulse energy ascertained at 290 fs and 3.0 nJ, respectively. Excellent signal-to-noise ratio (SNR) of above 50 dB is observed for all harmonic orders. The findings validated that SMS structure can be used to generate stable and switchable high order of harmonic mode-locked. The low-cost SMS fiber for harmonic mode-locked generation technique could lay the groundwork for future sustainable industrial growth.

Polyacrylonitrile enabled soliton mode-locked fiber laser

Exploring novel saturable absorber (SA) materials with superior performance to enable mode-locking operation remains a forefront in ultrafast laser research. In this study, we unveil the potential of polyacrylonitrile (PAN) based SA for ultrafast fiber laser applications. By probing the nonlinear absorption characteristics of PAN SA within the telecommunication band using the balanced twin-detector method, we reveal its substantial modulation depth of 10.7 %. Incorporating PAN SA into the Erbium-doped fiber laser (EDFL) cavity facilitated stable mode-locking operation. The resultant EDFL system produces a conventional soliton pulse train centered at 1568.4 nm wavelength, with mode-locking sustained over a pump power range from 87.2 to 254.1 mW. Moreover, the laser exhibits a pulse duration of 3.64 ps and achieves a maximum pulse energy of 2.04 nJ. These results highlight the efficacy of PAN-based SA, demonstrating properties comparable to established 2D SAs. The implications of this research are pivotal in advancing the application of novel organic polymers in ultrafast lasers.

Nonlinear optical absorption of TiTe2 nanosheets and its application for pulse laser generation.

Two-dimensional TiTe2 materials have excellent optical and electrical properties due to their unique semi-metallic properties and are widely used in detectors, photocatalysis, and field effect tubes. However, their potential in field of nonlinear optics (NLO) and ultrafast photonics has not been explored. In this paper, TiTe2 nanosheets are prepared by liquid-phase exfoliation, and their excellent nonlinear optical properties are being demonstrated. Z-scan measurements show that their nonlinear absorption coefficients are -6.83 ± 0.52 and -15.10 ± 0.82 cm GW-1 at 1560 and 2000nm, respectively, exhibiting saturable absorption properties. Two saturable absorbers (SA) based on TiTe2 nanosheets are prepared, one by encapsulating TiTe2 nanosheets in a polyvinyl alcohol (PVA) film and the other by depositing them on D-shape fiber (DSF). Finally, a Q-switched operation is achieved using TiTe2-PVA SA, with the maximum output power of 14.2 mW, corresponding to a pulse energy of up to 212.9 nJ. Stable mode-locked operation is achieved using TiTe2-DSF SA with a signal-to-noise ratio and pulse duration of 70 dB and 788 fs, respectively. Notably, this is the first time that TiTe2 nanosheets have been used as SA. The results suggest that TiTe2 nanosheets are a high-performance nonlinear material that provides a new impetus for the development of NLO and ultrafast photonics.

Towards high-power MOPA architectures in single-mode fibers for mid-IR mode-locked operation.

We report on a master oscillator power amplifier (MOPA) system at 2.8 µm to achieve an average power of 10.28 W with a pulse energy and peak power of 403 nJ and 5.76 kW, respectively. The seed, a semiconductor saturable mirror (SESAM) based mode-locked linear cavity source, delivers approximately 70 ps pulses at 28.16 MHz. Two amplifier stages are utilized in counter-pumping and dual-pumping configurations to enable high extraction efficiency and signal gain. To the best of our knowledge, this result is the highest average power and pulse energy obtained from a SESAM-based 2.8 µm mode-locked laser employing ZBLAN fiber MOPA architecture. It paves the way for achieving high average power in-fiber nonlinear amplification in single-mode fibers in the mid-infrared wavelength region.

Cerium oxide/polydimethylsiloxane polymer deposited onto tapered fiber for 1.97 µm mode-locked thulium-doped fiber laser

This work presents the use of cerium oxide (CeO2) as saturable absorber (SA) in the mode-locked pulse fiber laser generation. The CeO2 materials were prepared through sonication which then incorporated into a polydimethylsiloxane (PDMS) polymer to be coated on a tapered fiber. The CeO2/PDMS-SA exhibited saturation intensity of 44 MW/cm2 and modulation depth of 3.4%, confirming its nonlinear intensity-dependent transmission characteristics. Integrated into a thulium-doped fiber laser cavity, the CeO2/PDMS-SA enabled mode-locking at a pump power of 420 mW, leading to a fundamental mode-locking operation up to 1484 mW within the net anomalous dispersion regime. The laser emitted pulses at wavelength of 1973 nm with signal-to-noise ratio of 54 dB, repetition rate of 9.04 MHz and pulse width of 1.49 ps. The CeO2/PDMS-SA mode-locked fiber laser demonstrated stability of an average 3 dB bandwidth of 2.89 ± 0.08 nm, average central wavelength of 1973.05 ± 0.14 nm and average pulse width of 1.5 ± 0.01 ps for the duration of 120 min. Notably, the CeO2/PDMS-SA demonstrated a high pulse energy of 10.62 nJ and average output power of 96 mW. This investigation suggests their viability for use in future studies. The results also demonstrated the potential of CeO2 for use in various photonics applications in the “eye-safe” region.

Dual-wavelength synchronously mode-locked Cr:LiSAF laser with a tunable beating frequency and a central wavelength.

We demonstrate a versatile dual-wavelength synchronous mode-locking of a diode-pumped Cr:LiSAF laser for the first time, to our knowledge. A two-color mode-locked operation is achieved by using intracavity birefringent filters (BRFs) or etalons as frequency-selective elements. Using filters with different thicknesses and hence different free spectral ranges (FSRs), wavelength separation in two-color mode-locking could be selected between 1 and 9 nm, with corresponding beating frequencies in the 0.4-3.5 THz range. Moreover, the central wavelength of the two-color output could be tuned smoothly between 840 and 875 nm, only limited by the bandwidth of the semiconductor saturable absorber mirror (SESAM) used for mode-locking. The method, which enables easy adjustment of the central wavelength and beating frequency of a dual-wavelength operation, is suitable for use in other laser gain media as well.

All-PM Yb-doped mode-locked fiber laser with high single pulse energy and high repetition frequency

We demonstrate an all-polarization-maintaining (PM) ytterbium (Yb)-doped fiber laser with a figure-of-9 structure to generate mode-locked pulses with high single pulse energy and high repetition frequency. By exploiting the nonlinear amplifying loop mirror, a stably self-started mode-locking is achieved with a spectrum bandwidth of 13 nm and a pulse duration of 4.53 ps. The fundamental frequency is 97.966 MHz at the maximum output power of 143 mW in single pulse mode-locked operation, corresponding to the single pulse energy is 1.46 nJ. The output pulses maintain both high repetition frequency and high single-pulse energy. This laser oscillator can be an ideal seed source for applications such as high-energy amplifiers.

Mode-Locked Operation of High-Order Transverse Modes in a Vertical-External-Cavity Surface-Emitting Laser.

Understanding the mechanism of mode-locking in a laser with high-order transverse mode is important for achieving an ultrashort pulses train under more complicated conditions. So far, mode-locking with high-order transverse mode has not been reported in other lasers except the multimode fiber laser. This paper demonstrates robust mode-locking with high-order transverse mode in a Kerr-lens mode-locked vertical-external-cavity surface-emitting laser for the first time, to the best of our knowledge. While the longitudinal modes are locked, continuous mode-locking accompanied by high-order transverse mode up to TEM40 is observed. The threshold of the mode-locking is only a little bigger than that of the lasing. After the laser oscillation is built up, the mode-locked pulse train can be obtained almost immediately and maintained until the thermal rollover of the laser. Output powers of 717 mW under fundamental mode and 666 mW under high-order transverse mode are achieved with a 4.3 ps pulse duration and 1.1 GHz pulses repetition rate, and some phenomenological explanations to the related characteristics of the mode-locked operation of high-order transverse mode in the vertical-external-cavity surface-emitting laser are proposed.

Soliton mode-locked pulse generation with Polyaniline saturable absorber

We demonstrate the successful mode-locking of an Erbium-doped fiber laser (EDFL) utilizing a Polyaniline (PANI) saturable absorber (SA), resulting in the production of picosecond pulses at a frequency of 1.8 MHz within an elongated ring cavity. The SA is engineered by embedding PANI into Polyvinyl Alcohol (PVA). This conducting polymer-based SA exhibits favorable optical nonlinear properties, boasting a modulation depth of 2.3 %, a saturation intensity of 7.8 MW/cm2, and a non-saturable loss of 5.6 %. The laser system generates a conventional soliton pulse train at a central wavelength of 1572.62 nm. The mode-locking operation is sustained within a pump power range from 139.4 to 254.12 mW. The laser also achieves a pulse duration of 3.0 ps and a maximum pulse energy of 0.4 nJ. These findings underscore the effectiveness of the proposed PANI-based SAs, which exhibit comparable properties to currently utilized 2D SAs. The successful mode-locking operation and the promising optical characteristics of the PANI-based SA provide a solid foundation for their potential applications in the realm of ultrafast photonics.

Passively mode-locked fiber laser based on ethylene glycol

A passively mode-locked Er-doped fiber laser employing ethylene glycol as a saturable absorber (SA) has been proposed and experimentally validated. The ethylene glycol-SA is fabricated by filling the space between the end facets of two optical patch cords inserted into a cannula with ethylene glycol. Utilizing such an SA with a modulation depth of 9.6 %, stable mode-locking operation are manifested in the laser cavity, including the production of bright pulse and bright-dark pulse pair. The generated bright pulse has a center wavelength at 1562.28 nm, a 3 dB bandwidth of 0.21 nm, a repetition frequency of 20.24 MHz, and a signal-to-noise ratio (SNR) of 57 dB. Through precisely manipulating the polarization state and the pump power in the laser cavity, the bright-dark pulse pairs can be successfully attained at the same repetition frequency. This is the first demonstration of a passively mode-locked fiber laser utilizing ethylene glycol as the saturable absorber. Due to their superior optical absorption characteristics, low cost, simplicity in fabrication and non-volatile nature, ethylene glycol-SAs have enormous potential in the field of nonlinear optics and ultrafast photonics.

2.8-µm polarization-maintaining Er fiber laser mode-locked by a GaSb-based SESAM

A GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM) enables continuous-wave picosecond mode-locked operation with excellent stability of a polarization-maintaining mid-infrared Er:ZBLAN fiber laser. The GaSb-based SESAM mode-locked fiber laser delivers an average output power of 190 mW at 2.76 µm at a repetition rate of 32.07 MHz (corresponding to a pulse energy of ∼6 nJ) and exhibits a high signal-to-noise ratio of ∼80 dB. The polarization extinction ratio is more than 23 dB. By employing an intracavity diffraction grating, the laser wavelength is continuously tunable across 2.706–2.816 µm. Passively Q-switched operation of this laser is also demonstrated.

Fundamental frequency and second harmonic mode-locked operations in an erbium-doped fiber laser based on Cr2Sn2Te6 saturable absorbers

With the rapid development of ultrafast photonics, the need for optical modulators is expanding. In particular, the optical modulator with a high damage threshold and the ability to generate multiple soliton states. This has prompted an ongoing search for optical modulators based on two-dimensional (2D) materials. Therefore, in this work, a novel 2D material Cr2Sn2Te6 (CSnT) with modulation depth of 6.17% and saturation intensity of 15.97 MW/cm2 is prepared as saturable absorber (SA). The nonlinear optical absorption characteristics and pulse modulation applications of CSnT are showed in the Erbium-doped fiber laser (EDFL). The fundamental frequency mode-locked operation and the second harmonic mode-locked operation are realized simultaneously in the ring cavity. Moreover, CSnT SA has a high damage threshold and high stability, the damage threshold reaches 467 mW and the signal-to-noise ratio (SNR) is higher than 70 dB. The experimental findings demonstrate that the CSnT-based saturable absorber possesses remarkable nonlinear optical properties, indicating its considerable potential in the realm of ultrafast photonics.

Diode-pumped 88 fs SESAM mode-locked Tm,Ho:CLNGG laser at 2090 nm

We report on a passively mode-locked Tm,Ho:CLNGG laser operating at 2090 nm, pumped by a multimode fiber-coupled 793 nm laser diode, representing the first demonstration of a diode-pumped sub-100-fs mode-locked laser at 2.1 μm wavelength. Due to the disordered nature of the gain material that exhibits a broad and smooth gain profile, pulses as short as 88 fs were achieved with an average output power of 120 mW at a repetition rate of 70.3 MHz. Stable self-starting mode-locked operation was obtained using a semiconductor saturable absorber mirror based on the GaSb material system.

Kerr-Lens Mode-Locked Yb:BaF2 Laser

We present sub-50 fs soliton pulse generation from a diode-pumped Kerr-lens mode-locked laser based on an Yb3+-doped BaF2 crystal. Utilizing a spatially single-mode, fiber-coupled InGaAs laser diode at 976 nm as a pump source, the Yb:BaF2 laser generates pulses as short as 46 fs at 1060.1 nm with an average output power of 45 mW at a pulse repetition rate of ~65.6 MHz via soft-aperture Kerr-lens mode locking. To the best of our knowledge, this represents the first demonstration of Kerr-lens mode-locked operation of the Yb:BaF2 crystal, as well as the shortest pulse duration ever achieved from any diode-pumped mode-locked laser based on an Yb3+-doped alkaline-earth fluoride crystal.

Passively harmonic mode-locked erbium-doped fiber laser with a gold nanofilm saturable absorber

We demonstrate a 1.5 GHz harmonic mode-locked erbium-doped fiber laser by incorporating gold nanofilm as a saturable absorber (SA). The high-quality gold nanofilm SA fabricated by the physical vapor deposition method possesses a high modulation depth of 12.9% and a low saturation intensity of 1.69 MW/cm2 at 1.56 µm, facilitating the generation of harmonic mode-locking. The fundamental mode-locked operation was obtained at 1564.7 nm, with a pulse duration of 586 fs and a repetition rate of 34.235 MHz. At the pump power of 610 mW, 44th-order harmonic mode-locking with a repetition rate of 1.506 GHz was achieved, which is the highest yet reported in mode-locked fiber lasers using gold nanomaterials as SAs. Moreover, the gold nanofilm-based harmonic mode-locked fiber laser shows relatively high signal-to-noise ratios, high output power, and good stability. These results highlight the advantage of the gold nanofilm-based SA in realizing high repetition rate laser sources.

Switchable dual-wavelength mode-locked cylindrical vector beam fiber laser based on unpumped EDF saturable absorber and nonlinear polarization rotation

A switchable single- and dual-wavelength mode-locked pulse laser with cylindrical vector beam (CVB) based on an unpumped EDF saturable absorber and a nonlinear polarization rotation (NPR) hybrid mode-locked mechanism has been proposed and experimentally demonstrated. An unpumped EDF saturable absorber is utilized to generate a mode-locked operation that generates high-order laser pulses. Utilizing a hybrid mode-locked mechanism, the signal-to-noise ratio (SNR) and operating stability of the laser are improved compared to the mode-locked operation with a single unpumped EDF saturable absorber. Through adjusting the polarization controller, single- and dual-wavelength stable mode-locked pulses can be easily realized. The single-wavelength mode-locked pulse operates with central wavelengths of 1550.16 nm and 1549.80 nm, repetition frequencies of 6.33 MHz and 6.35 MHz, corresponding to pulse intervals of 158 ns and 157 ns. Further, the long-period grating (LPFG) is used as a mode converter to achieve pulse output with a purity exceeding 95 % in the TM01 and TE01 modes at different operating wavelengths. The proposed fiber laser with a simple structure has provided a flexible pulsed CVB laser source for laser processing, time division, and mode division multiplexing applications.

Preparation of amorphous silicon-doped Y2O3 aerogel enabling nonlinear optical features for ultrafast photonics

Abstract Amorphous aerogels with the microscopic nanoscale three-dimensional meshes provide superb platforms for investigating unique physicochemical properties. In order to enhance the physical, thermal and mechanical performances, one efficient and common approach is integrating diverse functional materials. Herein, we report a simple strategy to fabricate the amorphous silicon doped Y2O3 aerogels with the post-gelation method under the N2/EtOH supercritical atmosphere. The impact of Si concentration on the nonlinear optical properties is investigated for the first time. The maximum modulation depth is 1.65 % with a saturation intensity of 0.78 MW cm−2 with the 1-ps laser excitation at 1590 nm. Finally, we incorporated the silicon-doped Y2O3 aerogel based saturable absorber (SA) into an erbium-doped fiber laser (EDFL) and achieved various mode-locking operations at different wavelengths in the super C band, in terms of the conventional soliton, harmonic soliton molecules pulses, and dual-wavelength soliton mode-locking. Overall, this work confirms that amorphous silicon-doped Y2O3 aerogels are good nonlinear optical materials and pave a way for the ultrafast photonic and nonlinear optical applications with amorphous materials in near future.

Generation of stable femtosecond pulses using MoS2−xSex as the saturable absorber in Er-doped fiber laser

A new transition-metal dichalcogenide (Janus TMDs), molybdenum selenide sulfide (MoS[Formula: see text]Se[Formula: see text]), has attracted wide attention due to its unique and highly stable alloy structure, high carrier mobility and the ability to modulate the band gap based on composition. In this work, MoS[Formula: see text]Se[Formula: see text] nanosheets were prepared through liquid phase exfoliation and further prepared as a saturable absorber (SA). The nonlinear optical response was investigated by the homemade balanced twin-detector measurement system, revealing the potential of MoS[Formula: see text]Se[Formula: see text]-SA in ultrafast photonics. The SA devices exhibited good saturation absorption characteristics. By applying it to the Er-doped fiber laser, a stable mode-locked fiber laser based on MoS[Formula: see text]Se[Formula: see text]-SA was successfully achieved. The pulse width was measured to be 509 fs, with a spectral width of 6.33 nm and a maximum output power of 7.44 mW in the mode-locked state. This is the first reported instance of achieving mode-locking operation in the Er-doped fiber laser using MoS[Formula: see text]Se[Formula: see text]-SA, indicating its great potential for applications in ultrafast photonics.

Germanene: demonstration of the conversion from mode-locked to Q-switched mode-locked in Er-doped fiber laser

Some two-dimensional layered mono-elemental materials have been reported as saturable absorbers (SAs) for the generation of various soliton phenomena, and many excellent results have been achieved. In our experiment, we made thin films of germanene-polyvinyl alcohol (Ge-PVA) and applied them as SA in Er-doped fiber lasers, investigated Ge’s characteristics, and demonstrated the conversion from mode-locked to Q-switched mode-locked (QML). The conventional mode-locked operation with a repetition rate of 9.63 MHz and a central wavelength of 1559.7 nm was stably triggered when the pump power exceeded the threshold of 40 mW. QML pulse with a central wavelength of 1530.26 nm and the maximum pulse energy of 86 nJ can be obtained by changing the pump power and polarization state of the light in the cavity. This work reveals the excellent optical properties of Ge SA in ultrafast fiber lasers and provide a new approach for the generation of QML pulses.

Generation of Q-switched and mode-locked pulses in erbium-doped fiber ring laser using saturable absorption of Ti2SnC

MAX phase materials have garnered significant attention due to their corrosion resistance, impressive antioxidation and high electrical and optical conductivity. One such material, Titanium Tin Carbide (Ti2SnC), has demonstrated notable nonlinear optical properties. The saturable absorption of Ti2SnC, embedded into a polyvinyl (PVA) film, was measured to assess its potential. The Ti2SnC -PVA film exhibited a calculated modulation depth of 7.8%. In the context of practical applications, the Ti2SnC-PVA film was employed as a saturable absorber (SA) in an Erbium-doped fiber laser (EDFL) cavity. This configuration enabled the generation of stable Q-switched and mode-locked pulse trains. Q-switched operation occurred in a pump power range of 51.67 mW–83.83 mW, reaching a peak pulse energy of 110.94 nJ. Transitioning to mode-locking operation, the laser cavity demonstrated efficiency in the pump power range of 105.27 mW–196.39 mW. At a pump power of 196.39 mW, the fiber laser cavity achieved its highest pulse energy of 5.42 nJ, characterized by a pulse width of 2.32 ps and a pulse repetition rate of 1.825 MHz. These results highlight the promising potential of Ti2SnC-PVA thin film as a cutting-edge material for applications in pulsed laser systems. The material's excellent nonlinear optical properties make it a compelling candidate for advancing the field of pulsed laser technology.