Stable Mode-locking Operation Research Articles

Figure-8 mode-locked Yb-doped fibre oscillator with a nonlinear amplifying loop mirror and power amplification

Abstract We report an all-polarization-maintaining Yb-doped fibre (YDF) femtosecond (fs) laser source at ∼1 µm comprising a figure-8 mode-locked oscillator and two YDF amplifier stages. By employing a nonlinear amplifying loop mirror, we could achieve dissipative soliton mode-locking with an output of 24.6 mW at a repetition rate of 6.2 MHz. In our experiments, a Raman signal of 16% in the pulse energy helps to obtain stable mode-locked operation in the figure-8 oscillator. The dissipative soliton pulse from the oscillator is stretched by a chirped fibre Bragg grating and is amplified by a two-stage YDF amplifier. After compression, the YDF master-oscillator power-amplifier system yields fs pulses with a pulse energy of 2.3 µJ and a pulse duration of 274.6 fs at a repetition rate of 0.52 MHz. The output pulse energy is limited by the third-order nonlinearity of the fibre amplifier chain. Prospects for further pulse energy scaling are discussed.

Integrated mid-infrared sensing and ultrashort lasers based on wafer-level Td-WTe2 Weyl semimetal

There is an urgent need for infrared (IR) detection systems with high-level miniaturization and room-temperature operation capability. The rising star of two-dimensional (2D) semimetals with extraordinary optoelectronic properties can fulfill these criteria. However, the formidable challenges with regard to large-scale patterning and substrate-selective requirements limit material deposition options for device fabrication. Here, we report a convenient and straightforward eutectic-tellurization transformation method for the wafer-level synthesis of 2D type-II Weyl semimetal WTe2. The non-cryogenic WTe2/Si Schottky junction device displays an ultrawide detection range covering 10.6 μm with a high detectivity of ∼109 Jones in the mid-infrared (MIR) region and a short response time of 1.3 μs. The detection performance has surpassed most reported IR sensors. On top of that, on-chip device arrays based on Schottky junction display an outstanding MIR imaging capability without cryogenic cooling, and 2D WTe2 Weyl semimetal can serve as a saturable absorber for stable Q-switched and mode-locked laser operation applications. Our work offers a viable route for wafer-scale vdW preparation of 2D semimetals, showcasing their intriguing potential in on-chip integrated MIR detection systems and ultrafast laser photonics.

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.

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.

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.

Mode-locked thulium-doped fiber laser using a stretched few-mode fiber as a saturable absorber

In this work, an all-fiber saturable absorber (SA) based on nonlinear multimode interference in a short few-mode fiber (FMF) was proposed. Saturable absorption in this all-fiber device was demonstrated with an absorption modulation depth and a saturation fluence of 14.3% and 55.6 μJ cm−2, respectively. A stable mode-locking operation was observed in a thulium-doped passively mode-locked fiber laser using the FMF-based FMF SA. Ultrafast pulses were generated at 1911 nm with a pulse width of 1.96 ps and a 15.69 MHz repetition rate, respectively. The output average power was 2.8 mW, corresponding to a pulse energy of 178 pJ, which was significantly improved compared to graded-index multimode fiber-based SA. The experimental results demonstrate that FMF SA has great potential in eye-safe ultrafast photonics.

Conversion from Q-switched to bright-dark soliton pair mode-locked operation in an InSb based erbium-doped fiber laser

In this experiment, stable Q-switched pulses and bright-dark soliton pairs were realized in an InSb based erbium-doped fiber laser. Firstly, when the pump power was between 70 and 115 mW, a stable passive Q-switched operation with a central wavelength of 1565.38 nm was achieved, and the signal-to-noise ratio (SNR) was about 37 dB. As the pump power increased, the repetition frequency of the Q-switched pulse can be increased from 4.0145 to 10.623 kHz. When the pump power was increased to the range of 185 to 450 mW, stable bright-dark soliton pairs mode-locked operation was realized, and the two peaks of the spectrum were located at 1568.67 and 1569.34 nm, respectively. The pulse repetition rate was 3.419 MHz, and the SNR was about 59 dB. This research will broaden the application of InSb in ultrafast optics and nonlinear optics.

A switchable multi-wavelength mode-locked fiber laser based on a multi-mode interference device

A switchable multi-wavelength, passively mode-locked fiber laser in the 1 μm regime is demonstrated in this paper. The key principles of working include the nonlinear polarization rotation (NPR) mechanism for the mode-locked pulse generation and a single mode-multimode-single mode (SMS) structure as the spectral filter. The SMS structure is coiled on the paddles of a polarization controller (PC). Stable mode-locking operation is achieved at the fundamental repetition rate of 7.97 MHz. The pulse duration is 429 ps at the pump level of 216 mW, with the dual-wavelength laser emitting at 1037.86 nm and 1045.72 nm. By properly tuning the paddles of PCs, multi-wavelength mode-locked laser spectra can be observed switching from triple- to sextuple-wavelength, with the emission bandwidth of 27.6 nm from 1035.6 nm to 1063.2 nm.

Generation of dissipative soliton resonance and soliton rain in passively mode-locked fiber laser based on GeTe saturable absorber

We report the generation of dissipative soliton resonance and soliton rain in an Er-doped fiber laser based on GeTe as saturable absorber. When the pump power is 60 mW, the fiber laser achieved the stable dissipative soliton resonance mode-locked operation. The pulse width of the trapezoidal envelope pulses increases from 21 to 35.5 ns with the increase of the pump power, while keeping the pulse profile and intensity invariable. By properly adjusting the polarization controller, a stable soliton rain pulse is obtained at the pump power of 70.4 mW. With the increase of pump power, its average output power increased from 0.52 to 1.56 mW, and its corresponding fundamental repetition rate, central wavelength and 3-dB bandwidth is 3.21 MHz, 1562.6 nm and 2.74 nm, respectively. The experimental results demonstrate that GeTe nanosheets could be developed as an effective saturable absorber for ultrashort pulse fiber lasers.

Vector dark–bright pulses from a ytterbium doped fiber laser mode-locked by nonlinear multimode interference

We report dark–bright pulse pair generation from the ytterbium doped fiber laser mode-locked by cascaded nonlinear multimode interference (MMI) effect under an all-normal dispersion configuration. The MMI is implemented in a step index single mode–multimode–single mode fiber structure. The combination of two MMI structures, acting like a saturable absorber, has been used for stable mode-locking operation in the resonator. By adjusting the polarization controller attached to one of the MMI structure, stable dark–bright pulses are observed at fundamental, second and third harmonic repetition rates. The optical spectrum of the dark–bright pulses is doubly peaked at 1036.6 and 1040.6 nm with a spectral width ∼0.7 nm. The width of the dark pulses varied from 72 to 50 ns and that of the bright pulses from 30 to 14 ns when the pump power is increased from 102 to 170 mW at the fundamental repetition rate mode-locking. The darkness of the dark and intensity of bright pulses increase with the pump power. Dark and bright pulses in the pair are resolved by placing a polarizing component confirming their vector nature.

Sub-100 fs watt-level Kerr-lens mode-locked Yb:CaYAlO4 laser with a gigahertz repetition rate

We report a 1.04 GHz high-power Kerr-lens mode-locked Yb:CaYAlO4 laser pumped by a single-mode fiber laser at 976 nm. Based on a bow-tie cavity, stable unidirectional mode-locked operation is obtained with an output coupler of 1.6%. The oscillator delivers pulses with an average power of 1.46 W and with the pulse duration of 99 fs, which, to the best of our knowledge, is the first gigahertz-level Kerr-lens mode-locked laser based on the Yb:CaYAlO4 gain medium. We believe that the watt-level solid-state femtosecond laser at GHz would be an excellent source for developing time-resolved broadband dual-comb spectroscopy.

Revealing the Evolution from Q-Switching to Mode-Locking in an Erbium-Doped Fiber Laser Using Tungsten Trioxide Saturable Absorber

Passively Q-switching and mode-locking technologies can generate short pulses with durations that differ by several orders of magnitude widely used in different applications. Recently, Q-switching and mode-locking realized in an identical laser cavity with saturable absorbers was reported. The analysis of pulse conversion is helpful for us to further understand the pulse dynamics of a laser. In this paper, the pulse evolution from Q-switching, Q-switched mode-locking to mode-locking, is demonstrated by using a tungsten trioxide saturable absorber in a ring-cavity erbium-doped fiber laser. Firstly, self-started Q-switching at 1563 nm is observed, the repetition rate continuously increases, and the duration decreases when the pump power increased. Then, with an adjusting intra-cavity state of polarization under a high pump power level, stable Q-switched mode-locking pulses evolved from Q-switching, are observed. The amplitude of the emerged pulse sequence with a period of 36.8 ns, determined by cavity length, is modulated by the Q-switched envelope with the period of 10.3 μs. By optimizing the intracavity polarization carefully, stable continuous wave mode-locking operation is achieved eventually. To the best of our knowledge, this is the first experimental demonstration of Q-switching and mode-locking, respectively, in an identical transition-metal-oxides-based pulsed fiber laser without modification of cavity structure.

Dispersion-Managed Tm-Ho Co-Doped Ultrashort Pulse Fiber Laser Using Single-Walled Carbon Nanotube and Spectral Filter

In this paper, we have demonstrated a dispersion-managed, high-power, Tm-Ho co-doped ultrashort pulse fiber laser using a single walled carbon nanotube (SWNT) dispersed in polyimide film. An in-line type spectral filter was developed to control the output pulse spectra. Two SWNT films with different modulation depths were examined as a mode-locker. Normal dispersion fiber was used in the fiber laser oscillator, and dependence on net cavity dispersion was investigated. Passive mode-locking was achieved in a wide dispersion range, from −0.319 to +0.101 ps2. Stable soliton mode-locking operation and dissipative soliton mode-locking operations were observed. The pumping efficiency was ~3 times higher than that in a Tm-doped fiber laser with a similar configuration. The developed fiber laser showed self-start and stable operations, and this laser is useful for practical applications.

Stable and high energy ytterbium-doped mode-locked fiber laser based on a franckeite-polymer film modulator

As a novel material with narrow band gap and natural van der Waals heterostructures (vdWH), francketie has potential applications in the field of optoelectronic fields. However, few studies have applied its nonlinear optical absorption properties to ultrafast fiber lasers. Here, we synthesized francketie nanosheets via the liquid-phase exfoliation (LPE) method. By incorporating the polyvinyl alcohol (PVA), a franckeite-PVA saturable absorber (SA) was fabricated to achieve a mode-locked Yb-doped fiber laser (YDFL) for the first time to the best of our knowledge. The saturation intensity and modulation depth of the SA were measured about 75 MW∕cm2 and 7%, respectively. The proposed franckeite-based YDFL demonstrates stable mode-locked operation with the maximum single energy of 5.35 nJ and the pulse duration of 1.57 ns. Our experimental results fully prove that franckeite may have wide potential for designing ultrafast photonics devices with low cost, high stability and excellent performance.

2D BP/InSe Heterostructures as a Nonlinear Optical Material for Ultrafast Photonics.

The BP/InSe heterojunction has attracted the attention of many fields in successful combined high hole mobility of black phosphorus (BP) and high electron mobility of indium selenide (InSe), and enhanced the environmental stability of BP. Nevertheless, photonics research on the BP/InSe heterostructure was insufficient, while both components are considered promising in the field. In this work, a two-dimensional (2D) BP/InSe heterostructure was fabricated using the liquid-phase exfoliation method. Its linear and non-linear optical (NLO) absorption was characterized by ultraviolet−visible−infrared and Open-aperture Z-scan technology. On account of the revealed superior NLO properties, an SA based on 2D BP/InSe was prepared and embedded into an erbium-doped fiber laser, traditional soliton pulses were observed at 1.5 μm with the pulse duration of 881 fs. Furthermore, harmonic mode locking of bound solitons and dark-bright soliton pairs were also obtained in the same laser cavity due to the cross-coupling effect. The stable mode-locked operation can be maintained for several days, which overcome the low air stability of BP. This contribution further proves the excellent optical properties of 2D BP/InSe heterostructure and provides new probability of developing nano-photonics devices for the applications of double pulses laser source and long-distance information transmission.

Near-infrared all-fiber mode-locked laser based on vanadium carbide nanosheets

Vanadium carbide (V2C), a newly developed MXene, has been theoretically predicted to be an excellent nonlinear optical materials. Here, few-layer V2CTx nanosheets were synthesized by liquid phase exfoliation (LPE) method. Z-scan measurement was performed to explore their third-order nonlinear optical properties, highlighting the strong nonlinearities of few-layer V2CTx nanosheets. And the effective nonlinear absorption coefficient was determined to be − 98.3 cm/MW, corresponding to the imaginary part of the third-order nonlinear effects was obtained to be − 1.106 × 10−8 esu. In addition, microfiber-based few-layer V2CTx saturable absorber (SA) was fabricated by optical deposition method and the saturable absorption characteristic was measured by I-scan technique. The Yb-doped and Er-doped fiber lasers (YDFLs and EDFLs) were performed by inserting microfiber-based few-layer V2CTx SA into the ring cavity. And the stable mode-locked operation was realized with super high signal- noise ratio (SNR). Our work systematically reveals that few-layer V2CTx nanosheets should be an excellent SA for ultrafast photonics in the near-infrared band.

High Power and Large-Energy Pulse Generation in an Erbium-Doped Fiber Laser by a Ferromagnetic Insulator-Cr2Si2Te6 Saturable Absorber.

Large-energy mode-locked fiber lasers are extensively studied due to their indispensable use in various fields and applications. Recently, ferromagnetic insulators have attracted tremendous research interest in ultra-fast photonics because of their unique ferromagnetic properties and typical layered structure. In our work, Cr2Si2Te6 nanosheets are prepared and utilized as a saturable absorber (SA) in a large-energy mode-locked erbium-doped fiber (EDF) laser. With a total cavity length of 240 m, a stable mode-locked operation characterized by maximum pulse energy as high as 244.76 nJ with a repetition rate of 847.64 kHz is achieved. When the cavity length is extended to 390 m, the output maximum pulse energy is successfully scaled up to 325.50 nJ. To our knowledge, this is the largest pulse energy and highest output power level to be achieved in mode-locked fiber lasers by two-dimensional (2D) material saturable absorbers (SAs) so far. This work not only makes a forward step to the investigation of the generation of large-energy pulses in mode-locked fiber lasers but also fully proves that the ferromagnetic insulator-Cr2Si2Te6 possesses an excellent nonlinear absorption property, antioxidant capacity in ambient conditions, as well as outstanding thermal stability, which enriches our insight into 2D materials.

Pulsed-laser-ablated gold-nanoparticles saturable absorber for mode-locked erbium-doped fiber lasers

We demonstrate the optical performance of passively mode-locked pulses in erbium-doped fiber laser incorporating gold-nanoparticles (Au-NPs) as a saturable absorber (SA). Au-NPs of diameters between 5 to 15 nm were synthesized using the pulsed laser ablation method and blended with polydimethylsiloxane polymer. The resulting nanocomposite was deposited on the tapered region of a microfiber with a spin-coating method. The proposed Au-NPs SA recorded a modulation depth of 0.4% and low saturation intensity of 0.1 MW/cm2 leading to stable mode-locking operation in the erbium-doped fiber laser cavity at a low threshold pump power of about 45.6 mW. A mode-locked pulse train with a duration of 933 fs, pulse repetition rate of 6.25 MHz, and peak-to-pedestal extinction ratio of 54.1 dB was achieved at a pump power of 168.1 mW. The obtained results demonstrate that the spherical Au-NPs synthesized by pulsed laser ablation is a feasible material for SA fabrication, validating its saturable absorption properties in a 1.55 μm wavelength region.

High-efficiency mode-locked fiber laser with a switchable oscillating transverse mode in an all few-mode fiber linear cavity.

In this paper, an oscillating transverse mode switchable mode-locked fiber laser with a few-mode fiber linear cavity is proposed and demonstrated. An artificial filter is used to realize the mode gain modulation of the laser. The stable mode-locked pulsed operation with switchable wavelength is easily achieved and the oscillating transverse mode can be flexibly switched between the fundamental mode and high-order mode by adjusting the polarization controller. The mode-locked fiber laser directly oscillates in the high-order mode stably with a slope efficiency of as high as 12%, and the corresponding operating wavelength, repetition rate as well as pulse duration are 1054.07 nm, 22.662 MHz, 31.5 ps, respectively. Besides, a cylindrical vector beam with a high mode purity of 98.6% is obtained by removing the degeneracy of the LP11 mode. This compact and high-efficiency mode-locked fiber laser operating in switchable transverse mode has the potential application for laser processing, particle trapping, bioimaging, and mode division multiplexing system.