Thulium-doped Fiber Laser Research Articles

Intracavity Tandemly-Pumped and Gain-Switched Tm-doped Fiber Laser at 1.7 μm

We propose and demonstrate a novel method to achieve pulsed operation of thulium-doped fiber lasers at 1.7 μm. In our design, the thulium-doped fiber laser cavity was embedded within the cavity of an erbium/ytterbium-codoped fiber laser operating at 1.5 μm. The thulium-doped fiber in the erbium/ytterbium-codoped fiber laser cavity served as a saturable absorber, forcing the 1.5 μm laser to operate in a passively Q-switched scheme. The 1.7 μm thulium-doped fiber laser was in-turn pumped by the Q-switched 1.5 μm laser and was gain-switched. This made the pulse duration of 1.7 μm emission much shorter than that of 1.5 μm emission. 1.02 W average output power at 1720 nm was obtained for 10 W of pump power at 976 nm. The pulse repetition frequency, pulse energy and pulse duration were 134 kHz, 7.6 μJ and 102 ns, respectively. A rate equation model was developed to examine the power- and temporal-behavior of the laser system. The calculated pulse duration and pulse repetition frequency matched well with the experimental results. Methods of further improving the laser performance are discussed based on insights gleaned from our modeling.

Switchable multi-wavelength thulium-doped fiber laser using a two-taper fiber filter

An individually switchable and tunable multiwavelength thulium-doped fiber laser (TDFL) is proposed and experimentally demonstrated. The key component of the laser is a comb filter composed of two fiber tapers separated by a short middle section. A nonlinear loop mirror (NOLM) is inserted into the laser cavity to suppress the intense gain competition. By properly adjusting the polarization controllers (PCs) in the cavity, the laser can operate in at most sextuple wavelength state. In addition to these laser output characteristics, the experiment result also shows that single wavelength tuning can be achieved, and the tuning range is 1952.06 nm to 1975.62 nm. This multi-wavelength TDFL may be useful in optical fiber sensing or other fields desiring flexible optical source.

Thulium-doped fiber laser with bidirectional output in a ring laser cavity

A thulium-doped fiber laser (TDFL) with bidirectional output was proposed and demonstrated herein. Clockwise (CW) and counter-clockwise (CCW) lasing output with different lasing wavelengths was realized by two uniform fiber Bragg gratings (UFBGs) and two optical circulators. The UFBGs located at 1941.238 and 2049.325 nm were incorporated in the laser cavity to act as the wavelength-selective element. Switchable operation was realized through tuning the gain and loss of the circulating light by applying curvature on a section of the single-mode fiber. When the TDFL was operated in the CW direction, single-wavelength output with a center wavelength, optical signal-to-noise ratio (OSNR), and maximal output power of 2049.335 nm, 33.175 dB, and 280.6 mW was obtained, and the fluctuation of the center wavelength and the output power were less than 0.041 nm and 1.400 dB within an observation time of 60 min. When the TDFL was operated in the CCW direction, single-wavelength output with a center wavelength, OSNR, and maximal output power of 1941.312 nm, 37.108 dB, and 486 mW could be obtained, and the maximal fluctuation of the center wavelength and the output power were 0.128 nm and 0.832 dB within an observation period of 60 min. In addition, the proposed TDFL can operate in both CC and CCW directions simultaneously with different output wavelengths. The potential application of a TDFL strain sensor with sensitivities of 1.33 and 1.22 pm/με in the CW and CCW directions was also demonstrated and the proposed TDFL may also well suited for use in medical and free-space optical communication systems.

Noise-Like Pulse Generated by All-Fiber Ultrafast Thulium-Doped Fiber Laser Based on Single-Wall Carbon Nanotubes

A noise-like pulse (NLP) thulium-doped fiber laser based on single-wall carbon nanotubes (SWCNTs) has been proposed and demonstrated in this paper, which the microfiber decorated with SWCNTs work as a saturable absorber to generate an ultrafast pulse. The length of the cavity is 12 m, corresponding to the fundamental frequency of 17.2 MHz. NLP with a duration of 643 fs can be obtained at 1965 nm under the signal-to-noise ratio of 56 dB. This work may assist us to better understand the dynamic characteristic of NLP with the help of materials and offer a fine laser source to the subsequent applications just like the generation of supercontinuum.

950 nm Femtosecond Laser by Directly Frequency-Doubling of a Thulium-Doped Fiber Laser

We demonstrate a compact and efficient femtosecond laser operating at ~950 nm by directly frequency-doubling of a mode-locked Tm-doped fiber laser. The laser is based on a highly compact architecture, with a dispersion-managed oscillator together with only one-step frequency conversion. The output pulse width is ~270 fs, with a repetition rate of 22.1 MHz and a pulse energy of 12.9 nJ. It is believed that the greatly simplified setup makes the current laser well suited for two-photon microscopy and other biomedical and clinical applications.

Selectable multiwavelength thulium-doped fiber laser based on parallel Lyot filter

This paper demonstrates a selectable wavelength spacing of thulium-doped fiber laser (TDFL) based on a parallel Lyot filter. An intensity-dependent loss (IDL) mechanism acts as an amplitude equalizer to flatten and stabilize the multiwavelength spectrum. The IDL mechanism is obtained at a specific polarization state from the combination of highly nonlinear fiber and polarizer. The wavelength spacing of the TDFL can be selected to either 0.9 nm or 1.8 nm when the polarization controllers (PCs) in each branch of the parallel Lyot filter are adjusted. The wavelength spacing is selected by adjusting PC2 and PC3 to suppress or transmit the light at each branch. Up to 4 and 9 lasing lines are generated based on wavelength spacing of 0.9 nm and 1.8 nm, respectively. Lower pump power has disappeared lasing lines, while different half-wave plate angle of PC has shown variation in the laser performances.

High-efficiency Thulium-doped fiber laser at 1.7 μm

An efficient Tm-doped fiber laser was demonstrated at the wavelength region of 1.7 μm, which wavelength was ever considered as an obstacle for efficient laser generation with Tm-doped fiber due to the serious re-absorption effect. In this work, bi-directional pump at 1570 nm was employed for sufficient pump absorption, and gain fiber length was optimized to decrease re-absorption loss of signal light and suppress amplified spontaneous emission in the 1.8–2 μm region. A maximum output power of 5.92 W at 1720 nm was achieved in experiment, corresponding to a slope efficiency with respect to the launched pump power of 64%. Numerical simulation was also implemented to investigate the further output power optimization. This work shows that a high 1.7 μm laser efficiency, which is comparable with that achieved with the special homemade Tm-doped fiber, can be realized by using the commercial silica Tm-doped fiber.

Thulium-doped fiber laser mode-locked by nonlinear polarization rotation in a chalcogenide tapered fiber.

We demonstrate a thulium-doped fiber laser that is mode-locked thanks to nonlinear polarization rotation (NPR) in a chalcogenide tapered fiber. The high nonlinearity of the tapered fiber leads to a combined reduction in mode-locking threshold power and cavity length compared to any all-silica NPR based mode-locked lasers. In the continuous wave mode-locking regime, the laser generates stable, tunable solitons pulses. In the Q-switched mode-locked regime, it allows single and multiwavelength pulses, tunable central wavelength and tunable multiwavelength separation.

Wavelength-stabilized tunable mode-locked thulium-doped fiber laser beyond 2 µm.

We report the development of a widely tunable mode-locked thulium-doped fiber laser based on a robust chirped fiber Bragg grating (CFBG). By applying mechanical tension and compression to the CFBG, an overall tunability of 20.1 nm, spanning from 2022.1 nm to 2042.2 nm, was achieved. The observed mode-locked pulse train from this fiber laser has a repetition rate of 9.4 MHz with an average power of 12.6 dBm and a pulse duration between 9.0 ps and 12.8 ps, depending on the central wavelength. To the best of our knowledge, this is the first demonstration of a tunable mode-locked thulium-doped fiber laser operating beyond 2 µm using a CFBG as a wavelength-selective element.

Generation of Mode-Locked Thulium-Doped Fiber Laser in 2.0-μm Wavelength Operation by Polymer-Coated Iron Phosphorus Trisulfide (FePS3)-Based Saturable Absorber

In this study, a mode-locked, Thulium-doped fiber (TDF) laser operating in the 2.0-<inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> wavelength region and using an iron phosphorus trisulfide (FePS₃) overlay with a polymer-coated on an arc-shaped fiber was successfully demonstrated. The mode-locked pulses were generated at a central wavelength of 1935.7 nm and a 3-dB bandwidth of 2.68 nm. Its soliton-like nature was validated by the presence of Kelly&#x2019;s sidebands in the observed optical spectrum. The maximum repetition rate of the TDF mode-locked laser was 10.95 MHz, with a pulse width of 1.47 ps. The generated mode-locked output has significant stability, with no observable shifts in the central wavelength or Kelly&#x2019;s sidebands over the measurement time due to the polymer&#x2019;s protective layer. The experimental results show that a polymer-coated FePS₃ arc-shaped fiber can be used as a SA device to produce stable and long-term mode-locked pulses in the TDF-based fiber laser.

Six-Wavelength-Switchable SLM Thulium-Doped Fiber Laser Enabled by Sampled FBGs and 3 × 3 Coupler Based Dual-Ring Compound Cavity Filter

A switchable and tunable six-wavelength single-longitudinal mode (SLM) thulium-doped fiber laser in the 2050 nm band is proposed and experimentally demonstrated. Two cascaded sampled fiber Bragg gratings (SFBGs), acted as an integrated six-channel reflector, are to define the lasing wavelength channels in a multi-wavelength switchable fiber laser for the first time. The switchability of the fiber laser was realized by polarization-dependent loss modulation. SLM lasing in each channel was guaranteed by a novel passive dual-ring compound cavity (DRCC), composed of two symmetric 3 × 3 optical couplers (OCs). Six single-wavelength operations are easily obtained and switched, with the high optical signal-to-noise ratios of >71 dB, >72 dB, >62 dB for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sub> and > 65 dB, >61 dB, >59 dB for <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">λ</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">23,</sub> respectively. The maximum power and wavelength fluctuations are as low as 0.56 dB and 0.01 nm, respectively. The measured laser linewidths ranged from 7.5 to 9.2 kHz, calculated by the β-separation line method based on the measured frequency noise spectra. Nine switchable dual-wavelength operations with different wavelength intervals and two triple-wavelength operations were also obtained. Our proposed fiber laser has great potential in free-space optical communication.

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.

Tunable thulium-doped mode-locked fiber laser with watt-level average power.

We demonstrate a wavelength-tunable, sub-200 fs, and watt-level thulium-doped ultrafast fiber oscillator with a fundamental frequency repetition rate of 509.7 MHz. The wavelength can be tuned between 1918.5 nm and 2031 nm by adjusting the intra-cavity waveplates. When the wavelength is tuned to below 2000 nm, the average output power exceeds 1 W. The oscillator provides a maximum average power of 1.314 W (corresponding to a pulse energy of 2.58 nJ) and a highest peak power of 12.5 kW at 1940 nm. Such a high-power, tunable 2-µm mode-locked fiber laser is an ideal light source candidate for a variety of applications, such as frequency metrology, molecular spectroscopy, and ultrafast pump-probe spectroscopy.

Modified two-mode fiber-based filter and its application in a switchable thulium-doped fiber laser

A few-mode fiber-based polarization controller (FMF-based PC) fabricated by coiling a two-mode fiber filter into the three plates of a single-mode fiber-based PC is proposed and characterized in this work. The output spectrum of the proposed FMF-based PC varies with the rotation angle of the three plates. By sequentially rotating the three plates, the maximum peak wavelength and extinction ratio variation is 1.399 nm and 4.371 dB, respectively. Based on the proposed FMF-based PC, a thulium-doped fiber laser (TDFL) with an optical signal-to-noise ratio of ∼ 40 dB and switchable single-wavelength output is constructed. Within the observation period of an hour, the maximum fluctuation of center wavelength and corresponding output optical power is 0.097 nm and 0.837 dB, respectively. The output performance of the TDFL regarding power scaling, slope efficiency, and threshold power is also investigated. The proposed fiber laser with compact structure and switchable output demonstrates advantages for application in optical sensors and communication systems.

Transmission performance of a 1.96 µm active mode-locked laser in smoke channel.

We demonstrate the data transmission characteristics of a 1.96 µm laser in an indoor simulated smoke channel. An active mode-locked thulium-doped fiber laser at 1961.63 nm can be generated, and the repetition rate is 2.11 GHz corresponding pulse duration of 12.47 ps. Therefore, the pulse can be modulated by a 2.11 Gb/s digital sequence to act as a carrier light source. The transmission characteristics of signal light under different visibility conditions of 0.5, 0.05, and 0.005 km are studied. Compared with the back-to-back conditions, the signal-to-noise ratios of the carrier can decrease after passing through the smoke channels, which are 3.93, 7.1, and 9.08 dB, respectively. At the same time, the received power jitter can increase from ±0.16 to ±1.14dB. The sensitivity can be -19.52dBm at the visibility of 0.005 km. The experimental results show that thulium-doped mode-locked laser has an excellent performance in a smoke channel.

Application of TiCN on passively harmonic mode-locked ultrashort pulse generation at 2 µm

TiCN could be a promising optical switch for the generation of ultrashort pulses with high repetition rates. Harmonic mode-locking (HML) is realized in a thulium-doped fiber laser (TDFL) using titanium carbonitride (TiCN) as a saturable absorber (SA). The linear absorption of TiCN is about 50% at 1950 nm, while the nonlinear modulation depth and saturation power are 6.76% and 1.83 kW, respectively. The fundamentally mode-locked pulses have the central wavelength of 1949.60 nm, 3-dB bandwidth of 5.20 nm, repetition rate of 27.73 MHz, and pulse width of 790 fs. The 2nd to the 16th harmonics are observed, corresponding to the repetition rates ranging from 55.44 to 444.10 MHz. To the best of our knowledge, this is the highest operating frequency in TiCN-based fiber laser. The central wavelength, 3-dB bandwidth, and pulse width are investigated as a function of harmonic order. This work would advise a new application area for TiCN.

Real-time observation of chaotic and periodic explosions in a mode-locked Tm-doped fiber laser.

We experimentally characterize the dynamics of soliton explosions in a transient chaotic state between a single and double pulsing state, as well as periodic explosions induced by soliton collisions in a dual wavelength soliton state. These explosions occurring in a thulium-doped linear fiber laser with net anomalous dispersion are characterized with real-time measurements based on a modified time-stretched dispersive Fourier transform method relying on second-harmonic generation.

Ultrashort pulse thulium-doped fiber laser with molybdenum trioxide on tapered fiber

This paper demonstrates ultrashort pulse generation using molybdenum trioxide (MoO3) saturable absorber (SA) integrated into a thulium-doped fiber laser. The average particle size of the MoO3 material was 9.54 µm. The MoO3/polydimethylsiloxane composite was prepared and spin-coated on a tapered fiber to fabricate the SA device. It had a modulation depth of 2.89% and demonstrated a high damage threshold, extending beyond 19.34 GW/cm2. From the evanescent light-matter interaction, stable self-starting mode-locked soliton pulses of 1.22 ps duration at 1941.4 nm central wavelength were obtained. The pulse repetition rate was 10.84 MHz with 3.3 nm spectral bandwidth. At 1.15 W pump power, the soliton pulse energy was 0.68 nJ corresponding to a peak power of 492 W. The work exhibited comparable pulse performances in the 2 µm wavelength range while utilizing simple SA fabrication procedures of micro-sized MoO3 materials.

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.

Five-Wavelength-Switchable Single-Longitudinal-Mode Thulium-Doped Fiber Laser Based on a Passive Cascaded Triple-Ring Cavity Filter

A five-wavelength-switchable thulium-doped fiber laser with a fiber Bragg grating (FBG)-based five-channel transmitting Fabry-Pérot (F-P) filter and a multi-coupler based triple-ring cavity (CTRC) filter, is experimentally proposed. For the first time, a multi-channel F-P filter and specially-designed coupler-based compound cavity filter were combined to realize multi-wavelength-switchable operation, with single-longitudinal-mode (SLM) lasing for each wavelength channel in a 2 µm band fiber laser. The theoretical analysis of the CTRC filter and the principle for achieving SLM operation by corporately using the F-P filter and CTRC filter were presented in detail. The experimental results demonstrated the good performance of the proposed filters. Five lasing wavelengths (1941.13 nm, 1941.22 nm, 1941.30 nm, 1941.39 nm, and 1941.47 nm) were obtained with good switchability, and each operated in a stable SLM state. The OSNR of each lasing wavelength was higher than 50 dB, and the linewidths of all obtained wavelengths were less than 8 kHz.