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A thulium-doped fiber laser (TDFL) with a dual-active cavity and a directly linked three-coupler triple-ring filter is designed and demonstrated. Its operational principle is analyzed, and a corresponding experimental setup is built. Eleven single-wavelength laser outputs with a single-longitudinal-mode (SLM) output near 2 μm are obtained. The laser output covers a wavelength range from 1933.95 nm to 1971.76 nm, with a continuous switchable output range of 37.81 nm and a minimum center wavelength interval of 0.22 nm. The optical signal-to-noise ratio (OSNR) of the output laser within the tuning range is >48.53 dB, and its maximum OSNR is 70.24 dB. The minimum wavelength fluctuation is 0.03 nm, and the power fluctuation is between 0.15 and 2.61 dB. A single wavelength with a center wavelength of 1933.95 nm is monitored for 75 min, and the radio-frequency spectrum is scanned 27 times within the frequency range of 0 to 400 MHz. The results demonstrate that the TDFL can operate continuously and stably in an SLM state. The linewidth and linewidth fluctuation of the TDFL are measured, and the minimum linewidth, corresponding to a measurement time of 0.001 s, is 65.14 kHz. The experimental results show that the proposed TDFL has a high OSNR and excellent wavelength-switching ability, and its SLM operation is very stable. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we report a coherent beam combining (CBC) system that involves two thulium-doped all-polarization maintaining (PM) fiber chirped pulse amplifiers. Through phase-locking the two channels via a fiber stretcher by using the stochastic parallel gradient descent (SPGD) algorithm, a maximum average power of 265 W is obtained, with a CBC efficiency of 81% and a residual phase error of λ/17. After de-chirping by a pair of diffraction gratings, the duration of the combined laser pulse is compressed to 690 fs. Taking into account the compression efficiency of 90% and the main peak energy proportion of 91%, the corresponding peak power is calculated to be 4 MW. The laser noise characteristics before and after CBC are examined, and the results indicate that the CBC would degrade the low frequency relative intensity noise (RIN), of which the integration is 1.74% in [100 Hz, 2 MHz] at the maximum combined output power. In addition, the effects of the nonlinear spectrum broadening during chirped pulse amplification on the CBC efficiency are also investigated, showing that a higher extent of pulse stretching is effective in alleviating the spectrum broadening and realizing a higher output power with decent combining efficiency. Graphical Abstract

In this paper, we report a coherent beam combining (CBC) system that involves two thulium-doped all-polarization maintaining (PM) fiber chirped pulse amplifiers. Through phase-locking the two channels via a fiber stretcher by using the stochastic parallel gradient descent (SPGD) algorithm, a maximum average power of 265 W is obtained, with a CBC efficiency of 81% and a residual phase error of λ/17. After de-chirping by a pair of diffraction gratings, the duration of the combined laser pulse is compressed to 690 fs. Taking into account the compression efficiency of 90% and the main peak energy proportion of 91%, the corresponding peak power is calculated to be 4 MW. The laser noise characteristics before and after CBC are examined, and the results indicate that the CBC would degrade the low frequency relative intensity noise (RIN), of which the integration is 1.74% in [100 Hz, 2 MHz] at the maximum combined output power. In addition, the effects of the nonlinear spectrum broadening during chirped pulse amplification on the CBC efficiency are also investigated, showing that a higher extent of pulse stretching is effective in alleviating the spectrum broadening and realizing a higher output power with decent combining efficiency. [ABSTRACT FROM AUTHOR]

In this paper, we explored the different performances of the evolution of polarization control parameters under algorithm optimization in one‐dimensional, two‐dimensional, and three‐dimensional parameter space. We found that the genetic algorithm (GA) operation has a typical converging trend as the epoch increasing of groups in ultra‐fast pulse generation. The corresponding deviations were calculated to be 10.8%, 11.2%, and 5.0%, respectively, which is different from the traversal algorithm operation. The typical noise‐like pulse state was effectively automatically searched at different length cavities, with center wavelengths of 1973, 1988.7, and 2002.7 nm, repetition rate of 9.4, 10.4, and 5.4 MHz, and typical spike duration of ~224 fs. Our research shows the effectiveness of the GA in searching and controlling the ultrashort pulse output of a 2‐µm thulium‐doped fiber laser. [ABSTRACT FROM AUTHOR]

This paper presents experimental evidence regarding a novel switchable dual-wavelength thulium-doped fiber laser (TDFL). Wavelength switching is achieved by combining a polarization-maintaining fiber Bragg grating (PM-FBG) with a polarization controller (PC). The three-coupler double-ring compound cavity (TC-DRC) structure, acting as a mode-selection filter, is designed to select a single longitudinal mode (SLM) from the dense longitudinal modes. This paper introduces the design and fabrication method of the TC-DRC filter and analyzes, in detail, the mechanism for SLM selection. The experimental results demonstrate that the designed filter exhibits excellent performance. By adjusting the PC, the TDFL achieves stable SLM operation at the wavelengths of 1940.54 nm and 1941.06 nm, respectively. The optical signal-to-noise ratio (OSNR) is superior to 65 dB. When the TDFL is tested at room temperature, there is no significant wavelength drift, and power fluctuations are less than 1.5 dB. The operation of the SLM is verified through the self-heterodyne method, and the laser maintains stable SLM states for both wavelengths after continuous operation for an hour. Furthermore, based on the phase noise demodulation method, the linewidths of both wavelengths are measured to be less than 10 kHz at the integration time of 0.001 s. [ABSTRACT FROM AUTHOR]

In the context of the E-TEST Interreg project, Fraunhofer ILT develops high stability, narrow linewidth laser seed sources and amplifiers at a wavelength of approx. 2 μm for potential usage in a third-generation gravitational wave detector: the Einstein telescope. While for such an application, there are highest demands on laser parameters such as linewidth, spectral purity, and polarization, especially stability properties such as the relative intensity noise must be optimized. To achieve highest output power stabilities, we develop a multi-stage low-noise holmium-doped fiber amplifier at a wavelength of 2095 nm, which is core-pumped by low-noise thulium-doped fiber lasers at a wavelength of approx. 1950 nm. In this article, we present the concept of the full laser system, and the achieved results for the pre-amplifier section, which perfectly fulfills the E-TEST pre-amplifier requirements. Our pump source, a thulium-doped fiber amplifier, achieves approx. 2 W output power at a wavelength of 1950 nm with 5 MHz linewidth and a polarization extinction ratio of 28 dB. With our holmium-doped fiber amplifier, we demonstrate an output power of approx. 400 mW at a wavelength of approx. 2095 nm with a linewidth of 2 MHz and a polarization extinction ratio of 18 dB. For both currently free-running systems we analyze the relative intensity noise (RIN) and obtained for example RINs of approx. 10−6 Hz−0.5 at a frequency of 100 Hz, which shows the suitability of the concept to achieve highest stabilities. To evaluate our system, we perform a system performance analysis, where we present the influence of signal and pump wavelength, and signal and pump power on the achievable optical-optical efficiencies.

A thulium-doped fiber laser (TDFL) with a dual-active cavity and a directly linked three-coupler triple-ring filter is designed and demonstrated. Its operational principle is analyzed, and a corresponding experimental setup is built. Eleven single-wavelength laser outputs with a single-longitudinal-mode (SLM) output near 2 μm are obtained. The laser output covers a wavelength range from 1933.95 nm to 1971.76 nm, with a continuous switchable output range of 37.81 nm and a minimum center wavelength interval of 0.22 nm. The optical signal-to-noise ratio (OSNR) of the output laser within the tuning range is >48.53 dB, and its maximum OSNR is 70.24 dB. The minimum wavelength fluctuation is 0.03 nm, and the power fluctuation is between 0.15 and 2.61 dB. A single wavelength with a center wavelength of 1933.95 nm is monitored for 75 min, and the radio-frequency spectrum is scanned 27 times within the frequency range of 0 to 400 MHz. The results demonstrate that the TDFL can operate continuously and stably in an SLM state. The linewidth and linewidth fluctuation of the TDFL are measured, and the minimum linewidth, corresponding to a measurement time of 0.001 s, is 65.14 kHz. The experimental results show that the proposed TDFL has a high OSNR and excellent wavelength-switching ability, and its SLM operation is very stable.

We demonstrated a narrow‐linewidth Ho:yttrium aluminum garnet (YAG) injection‐locked laser by using a 1908 nm thulium‐doped fiber laser as a pumping source to generate 2.09 μm single‐frequency pulses. A compact single‐longitudinal‐mode nonplanar ring oscillator (NPRO) based on Ho:YAG material with a beam quality factor of 1.01 at the central wavelength of 2090.91 nm was considered as the seed source. In the experiment, the pulse energy of 3.6 mJ with a pulse width of 161.6 ns from single‐frequency Ho laser was achieved at a pulse repetition frequency (PRF) of 100 Hz. The linewidth of the single‐frequency pulse spectrum monitored through the heterodyne method was approximately 1.9 MHz at the lowest Ho laser energy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The proposed titanium aluminum carbide (Ti3AlC2) film‐based saturable absorber (SA) for Q‐switching in thulium‐doped fiber laser (TDFL) cavity offers a cost‐effective and practical alternative to conventional SA materials such as graphene or semiconductor. The Ti3AlC2 film was produced using a simple and low‐cost casting approach with polyvinyl alcohol as a binder. The SA was devised from the film using a sandwich‐structured fiber‐ferrule platform and integrated into a TDFL ring cavity. The generated Q‐switched pulses were stable and had a repetition rate of up to 20.82 kHz with a minimum pulse width of 3.167 µs and a maximum pulse energy of 58.12 nJ. The Q‐switched laser produced the bright‐dark pulse pair. These results demonstrate the potential of Ti3AlC2 as a promising SA material for practical applications in the field of fiber lasers. [ABSTRACT FROM AUTHOR]

This paper presents experimental evidence regarding a novel switchable dual-wavelength thulium-doped fiber laser (TDFL). Wavelength switching is achieved by combining a polarization-maintaining fiber Bragg grating (PM-FBG) with a polarization controller (PC). The three-coupler double-ring compound cavity (TC-DRC) structure, acting as a mode-selection filter, is designed to select a single longitudinal mode (SLM) from the dense longitudinal modes. This paper introduces the design and fabrication method of the TC-DRC filter and analyzes, in detail, the mechanism for SLM selection. The experimental results demonstrate that the designed filter exhibits excellent performance. By adjusting the PC, the TDFL achieves stable SLM operation at the wavelengths of 1940.54 nm and 1941.06 nm, respectively. The optical signal-to-noise ratio (OSNR) is superior to 65 dB. When the TDFL is tested at room temperature, there is no significant wavelength drift, and power fluctuations are less than 1.5 dB. The operation of the SLM is verified through the self-heterodyne method, and the laser maintains stable SLM states for both wavelengths after continuous operation for an hour. Furthermore, based on the phase noise demodulation method, the linewidths of both wavelengths are measured to be less than 10 kHz at the integration time of 0.001 s.

A single-longitudinal-mode (SLM) thulium-doped fiber laser, based on an eye-shaped passive dual-ring filter, is designed and constructed. The eye-shaped passive compound cavity consisting of four couplers is used to increase the longitudinal mode spacing, and its performance is numerically analyzed in detail. A homemade uniform fiber Bragg grating serves as a wavelength selection device and a saturable absorber is used to further suppress the intense longitudinal mode competition in the laser cavity, ensuring the single-longitudinal-mode output. The experimental results demonstrate a laser output with a center wavelength of 2,049.85 nm and an optical signal-to-noise ratio of 63 dB. Moreover, the power fluctuation is less than 0.6 dB, and the center wavelength fluctuation is less than 0.03 nm over a continuous measurement period of 60 min, demonstrating an excellent stability. The laser linewidth is measured using an unbalanced Michelson interferometer and β-separation line method, resulting in a linewidth of 11.22 kHz.

We propose and demonstrate a new Q-switched thulium-doped fiber laser (TDFL) by utilizing a passive saturable absorber (SA) based on Molybdenum–Aluminium boride (MoAlB). The MoAlB is a metal–ceramic-based material; it is embedded in polyvinyl-alcohol (PVA) material to operate as the SA. The laser successfully generates stable Q-switched pulses operating at 1974.7 nm with a maximum repetition rate of 57.7 kHz and a minimum pulse width of 1.79 μs. A maximum pulse energy of 90.2 nJ is recorded at 436 mW pump power. Our experiment is the first demonstration on the use of MoAlB for the pulse laser generation in the 2 μm wavelength region. [ABSTRACT FROM AUTHOR]

This paper proposes and demonstrates a single-longitudinal-mode thulium-doped fiber laser using a passive triple-coupler ring-based compound-cavity filter (TCR-CC) and a uniform fiber Bragg grating. For the first time, the TCR-CC filter is used to select a single mode from dense longitudinal modes. Experimental results show that laser in the wavelength of 1941.28 nm can maintain exceptional stability with an optical signal-to-noise ratio of 74.1 dB. The measured maximum wavelength drift and power fluctuation are 0.01 nm and 0.45 dB, respectively. Meanwhile, the measured linewidth of the laser is 910 Hz, and the relative intensity noise is below −125.82 dB/Hz above 2 MHz frequencies. [ABSTRACT FROM AUTHOR]

A high-power all polarization-maintaining (PM) chirped pulse amplification (CPA) system operating in the 2.0 µm range is experimentally demonstrated. Large mode area (LMA) thulium-doped fiber (TDF) with a core/cladding diameter of 25/400 µm is employed to construct the main amplifier. Through dedicated coiling and cooling of the LMA-TDF to manage the loss of the higher order mode and thermal effect, a maximum average power of 314Wwith a slope efficiency of 52% and polarization extinction ratio of 20 dB is realized. The pulse duration is compressed to 283 fs with a grating pair, corresponding to a calculated peak power of 10.8 MW, considering the compression efficiency of 88% and the estimated Strehl ratio of 89%. Moreover, through characterizing the noise properties of the laser, an integrated relative intensity noise of 0.11% at 100 Hz-1 MHz is obtained at the maximum output power, whereas the laser timing jitter is degraded by the final amplifier from 318 to 410 fs at an integration frequency of 5 kHz to 1 MHz, owing to the self-phase modulation effect-induced spectrum broadening. The root-mean-square of long-term power fluctuation is tested to be 0.6%, verifying the good stability of the laser operation. To the best of our knowledge, this is the highest average power of an ultrafast laser realized from an all-PM-fiber TDF-CPA system ever reported. [ABSTRACT FROM AUTHOR]

• A quasi-continuous wave thulium-doped fiber laser is constructed with output power reached 70 W and peak power at 700 W, and then applied in laser lithotripsy. • It is proved that the laser-induced bubble channel is a prerequisite for TFL noncontact effective lithotripsy, with bubble length and width markedly influencing stone ablation rates. • Thulium fiber laser with higher peak power performs higher ablation rate compared to previous research in laser lithotripsy. • With no restrictions on the stone, the maximum ablation rate obtained are 8.9 mg/s for uric acid stones and 7.5 mg/s for calcium oxalate monohydrate stones. Despite its widespread use in laser lithotripsy, the Ho:YAG solid-state laser suffers from limited parameter combinations, poor beam quality, and low photoelectric conversion efficiency. The development of 2.0 μm fiber laser technology has positioned the thulium-doped fiber laser (TFL) as a superior alternative to the Ho:YAG laser in lithotripsy. The TFL excels because of its high-water absorption coefficient, all-fiber structure and pump modulation flexibility. However, the peak power reported for TFL in lithotripsy remains at most 500 W. This study presented a quasi-continuous wave (QCW) TFL featuring a wide tunable range of pulse width and pulse repetition rates, and a peak power of 700 W to realize enhanced ablation efficiency. A high-speed camera was used to acquire the laser-induced cavitation bubble images for the analysis of the interaction mechanism of a high peak power QCW TFL pulse with water and stones. While the synergistic action of photothermal and photomechanical effects constituted the mechanism of TFL lithotripsy, the laser-induced bubble channel was a prerequisite for TFL noncontact effective lithotripsy, with bubble length and width markedly influencing stone ablation rates. Ablation experiments on uric acid and calcium oxalate monohydrate stones conducted with various parameter combinations at a peak power of 700 W yielded rates of 8.9 mg/s for uric acid stones and 7.5 mg/s for calcium oxalate monohydrate stones. A comparison with prior findings confirmed that a high peak power TFL could notably improve stone ablation rates. [ABSTRACT FROM AUTHOR]

• High-Stability Single-Longitudinal-Mode Thulium-Doped Fiber Laser was proposed. • Using thulium holmium co doped fibers as saturable absorbers (SA). • The maximum wavelength drift and power fluctuation are 0.02 nm and 0.45 dB, respectively, and the linewidth of the laser is about 7 kHz. • Using a Quad Coupler Triple Ring Cavity (QC-TRC) filter based on passive four-coupler and a Uniform Fiber Bragg Grating (UFBG). This paper reports an experimental demonstration of a stable single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL) using a quad-coupler triple-ring cavity (QC-TRC) filter based on a passive quad coupler and a uniform fiber Bragg grating (UFBG). The TDFL adopts a ring cavity structure and utilizes a QC-TRC filter with four couplers to select a single longitudinal mode from a dense mode spectrum. The design and fabrication thinking on the QC-TRC filter are explained, and the selection principles for SLM are analyzed in detail. Experimental results show the filter features excellent performance, with the laser at 2048.40 nm exhibiting high stability with an optical signal-to-noise ratio (OSNR) of 69 dB. No significant wavelength drift is observed during the repeating measurement, the maximum wavelength drift and power fluctuation being 0.02 nm and 0.45 dB, respectively. The linewidth of the laser is approximately 7 kHz, measured using the β -separation linewidth method with an imbalanced Michelson interferometer linewidth measurement device. [ABSTRACT FROM AUTHOR]

• A highly stable single-longitudinal-mode thulium-doped fiber laser. • A linear compound cavity fiber laser utilizing a cascaded dual-coupler double-ring cavity filter. • The peak power fluctuation and wavelength drift over 60 min were less than 0.11 dB and 0.02 nm, respectively. • The minimum linewidth at an integration time of 0.001 s was 8.88 kHz. A linear compound-ring cavity single longitudinal mode (SLM) thulium-doped fiber laser (TDFL) based on a cascaded dual-coupler double-ring cavity filter has been experimentally investigated and is proposed. The cascaded dual-coupler double-ring cavity filter was analyzed theoretically in detail by adjusting the effective free spectral range and transmission bandwidth, which ensures SLM operation and narrow linewidth. The experimental results indicate that the proposed TDFL can generate stable SLM lasing with a high optical signal-to-noise ratio of 69 dB, at 2048.39 nm for more than 60 min. The wavelength drift and peak power fluctuation in that observation time were less than 0.02 nm and 0.11 dB, respectively. The linewidth of the laser was 8.8 kHz at an integration time of 0.001 s. The relative intensity noise was less than −128.88 dB/Hz at frequencies greater than 1 MHz. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a stable single-longitudinal mode (SLM) thulium-doped fiber laser (TDFL) with a Fabry- Pérot (F-P) fiber Bragg grating (FBG) filter and a triple-coupler based double-ring cavity (TC-DRC) filter. For the first time, this structure of TC-DRC filter is used to select a single mode from dense longitudinal-modes in a ring cavity TDFL. The design and fabricate methods of TC-DRC filter are revealed and the principle of SLM selection is also analyzed in detail. The experimental results demonstrated the good performance of the proposed filter. The central wavelength of proposed TDFL is 2049.49 nm and its OSNR is higher than 35 dB. There is no obvious wavelength drift during the test, and the power fluctuation is less 0.5 dB. The SLM operation is verified through the self-homodyne method, this laser can be stably maintained in a SLM state after operating for one hour under laboratory condition. In addition, the linewidth is measured less than 10 kHz based on the phase noise demodulation method.

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.

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 λ11–λ13 and > 65 dB, >61 dB, >59 dB for λ21–λ23, 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.

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.

A thulium-doped fiber laser (TDFL) with switchable single-wavelength output was proposed and experimentally constructed, and its output characteristics were investigated. The central wavelengths were 1940.52 nm and 2048.04 nm, defined by a superimposed uniform fiber Bragg grating (SI-UFBG) with reflectivity larger than 92%. Switchable output was successfully achieved by using polarization-dependent loss, with the help of a drop-in polarization controller and an in-line polarizer. For both output wavelengths, optical signal-to-noise ratios larger than 80 dB were achieved. Maximum fluctuations of the central wavelengths of 1940.52 nm and 2048.02 nm under 60 min were, respectively, 0.04 nm and 0.01 nm. Maximum fluctuations of output power for the same wavelengths over the same time period were, respectively, 1.09 dB and 0.12 dB. Maximum output powers of 215.12 mW and 155.53 mW were achieved for the respective output wavelengths of 1940.52 nm and 2048.02 nm. Moreover, a tuning range of ∼2 nm was realized by enforcing a strain on the SI-UFBG. The proposed TDFL may be applied in laser medicine and free-space-related applications.

Harmonic mode-locking (HML) is realized in a thulium-doped fiber laser (TDFL) using indium sulfate (In2(SO4)3) as a saturable absorber (SA). The In2(SO4)3 has nonlinear saturable absorption properties with the nonlinear optical modulation depth of 13.3 % and the saturation power of 0.11 kW at 1930 nm. The fundamentally mode-locked pulses operate at 1964.44 nm have the 3-dB bandwidth of 4.70 nm, the pulse width of 1.266 ps, and the pulse repetition rate of 25.37 MHz. The maximum resonance frequency of 228 MHz is achieved, which corresponds to the 9th harmonic. The supermode suppression level (SMSL) up to 63 dB is realized. The spectral and temporal characteristics of the output pulses are investigated as a function of harmonic order. The experimental results demonstrated that In2(SO4)3 can be a promising mode-locker for generating harmonic ultrafast pulses in 2 μm waveband.

We report on the first demonstration of the Tm-doped all-PM mode-locked fiber laser with CNT used as a saturable absorber and two fiber Lyot filters embedded into the ring resonator. Using these two filters, we were able to adjust the central wavelength of laser generation to ~1855 nm as well as to obtain the required spectral width and necessary pulse duration that are required for the proposed applications. Possible issues with the method used are discussed. [ABSTRACT FROM AUTHOR]

PEEK is an excellent thermoplastic material for functional coatings due to its outstanding material properties (e.g. biocompatibility, temperature stability, corrosion and wear resistance). In order to reduce the heat input into the base material and to enable a high local selectivity of polymer deposition, a laser-based coating process is a promising approach. In this study, the consolidation of PEEK coatings on stainless steel substrates was investigated experimentally for the first time by using a thulium-doped fiber laser with a wavelength of 1.94 µm. As most polymers exhibit characteristic absorption bands at wavelengths above 1.5 µm, a thulium-doped fiber laser is more appropriate for consolidating thermoplastics in comparison to more common near-infrared laser beam sources with wavelengths of around 1 µm. The effect of admixing fumed silica particles on the flow behavior of the PEEK powder was analyzed by degree of coverage tests to improve the powder deposition process via doctor blade. Furthermore, the influence of an additional substrate heating on the resulting coating characteristics was investigated. Optical microscopy analyses were conducted to characterize the coating surfaces and cross-sections in terms of possible defects. As a result, the flow behavior and hence the deposition process were significantly improved by admixing 1.0 wt. % fumed silica. Optical microscopy analyses verify the basic feasibility of a laser-based coating process with a thulium-doped fiber laser for the consolidation of PEEK powder on stainless steel substrates. In this context, a substrate heating is an important prerequisite to enhance the wetting of the stainless steel substrate and to produce dense as well as adherent PEEK coatings. [ABSTRACT FROM AUTHOR]

A wavelength-tunable thulium-doped fiber laser (TDFL) based on a Machâ€"Zehnder (MZ) filter and Sagnac loop was proposed in this study. The dual-pass MZ fiber filter, and Sagnac loop were fabricated to operate as reflectors in a linear cavity. The laser operating threshold was set to 138 mW. A single-wavelength laser output with an optical signal-to-noise ratio (OSNR) of up to 38 dB was realized with a tuning range of 1854â€"1882 nm. The tuning range was obtained with a tuning interval of 6 nm by tuning the polarization controller (PC), which yielded a power shift of less than 0.42 dB. Stability of the single-wavelength center wavelength power fluctuation was less than 0.39 dB within 20 min. When the laser was operated in the dual-wavelength condition, the OSNR was more than 30.5 dB, and the power fluctuation was less than 0.89 dB within 20 min at room temperature. The tunable triple-wavelength laser emission could be realized experimentally by adjusting the PC, which yielded an OSNR of more than 30.8 dB. The proposed TDFL is suitable for fiber-sensing system applications. [ABSTRACT FROM AUTHOR]

The performance of high-power fiber lasers is limited by different thermal mechanisms such as a variation of spectroscopic quantities with temperature. In this paper, the effect of temperature dependent thulium cross sections on high-power thulium-doped fiber lasers and amplifiers pumped at 790 nm and operating above 1900 nm is investigated by means of numerical simulations and use of analytical models. The most significant self-heating effect was identified as an output signal power drop associated with increased amplified spontaneous emission caused by the temperature induced shift of the transparency wavelength of the single pass gain spectra. • High power thulium doped active fibers affected by thermal effects. • Temperature dependent thulium cross sections applied in amp/laser simulations. • Slope efficiency drops by 4% for temperature change from 23 to 200 °C. • Amplified spontaneous emission raises due to fiber self-heating. • Output power at around 1900 nm diminishes due to fiber self heating and ASE rise. [ABSTRACT FROM AUTHOR]

• A switchable and tunable dual-wavelength thulium-doped fiber laser is proposed. • The all-fiber Sagnac loop embedded with FBG and the FPI are used as the filter. • The tuning range is up to 5.84 nm and 99.86 nm for the two output wavelength. • The wavelength and power variation are less than 0.03 nm and 0.5 dB, respectively. A switchable and tunable dual-wavelength thulium-doped fiber laser (ST-DW-TDFL) based on the parallel filter is proposed and experimentally demonstrated in this study. In the proposed ST-DW-TDFL, there are two independent branches to form the parallel filter. One branch uses an all-fiber Sagnac interferometer embedded with a fiber Bragg grating (FBG-FSI) as the wavelength selection element, and the other branch uses a Fabry-Perot (FP) filter as the wavelength selection element. By adjusting two optical attenuators to change the light power of two branches, single and dual-wavelength output can be achieved and switched. On the basis of achieving dual-wavelength output, independent and continuous tuning of two wavelengths can be achieved by heating the FBG and adjusting the FP filter. The tuning range of one wavelength is up to 5.84 nm, while the tuning range of the other wavelength is up to 99.86 nm. The maximum wavelength variation and power fluctuation are less than 0.03 nm and 0.5 dB at room temperature. And the experimental results show that the proposed ST-DW-TDFL has flexible wavelength switching and tuning capabilities and good wavelength and power stabilities. [ABSTRACT FROM AUTHOR]

High-energy ultrashort pulses fiber lasers could be widely used in scientific research and industrial applications. Researchers often use two methods to boost the pulse energy, one is to achieve self-similar pulse (SSP) output in a single mode fiber (SMF) laser, and the other is to use multimode fiber (MMF) instead of SMF. Here we combine the above two methods to optimally design the output characteristics of SSP in thulium-doped MMF lasers. The influence of different parameters, including spectral filter bandwidth, fiber length, and gain saturation energy, on the output characteristics of SSP have been analyzed, and an optimal SSP with pulse energy of 63.48 nJ, spectrum width of 52.53 nm, dechirped pulse peak power of 283.34 kW, and dechirped pulse width of 219.74 fs has been obtained. • We first proposed and optimized the theoretical model of a thulium-doped multimode fiber laser. • We combine self-similar pulse with high energy characteristics, and multimode fiber to tolerate high-energy pulse transmission to optimally design the output characteristics of self-similar pulse in thulium-doped multimode fiber lasers to obtain higher pulse energy. • A self-similar pulse with 63.48-nJ energy has been obtained in the multimode fiber lasers. [ABSTRACT FROM AUTHOR]

• A high OSNR single-longitudinal-mode thulium-doped fiber laser based on a cascaded dual-ring cavity filter was proposed and demonstrated. • A CDRC filter was used as the mode-selection mechanism. • The proposed TDFL has highly stability and SLM laser output over 60 min, and the linewidth was 11.82 kHz with an integration time of 0.001 s. A high optical-signal-to-noise ratio (OSNR) single-longitudinal-mode (SLM) thulium-doped fiber laser (TDFL) is proposed and has been investigated. SLM lasing was realized by utilizing a uniform fiber Bragg grating (UFBG) and a cascaded dual-ring cavity (CDRC) filter. The design, simulation, and characteristic analysis of three compound-cavity filters based on the Vernier effect are presented in detail, and provided the basis for experiments. The single-wavelength SLM lasing with a high OSNR of ∼75.10 dB, operating at 2048.44 nm was pumped by a 793 nm laser diode, and the SLM lasing was continuously monitored for 60 minutes. The peak power fluctuation and wavelength drift were less than 0.685 dB and 0.01 nm, respectively, over this time span. During the test, the relative intensity noise was below −128.49 dB/Hz above 1 MHz and the linewidth was 11.82 kHz with an integration time of 0.001 s. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a multi-wavelength thulium-doped fiber laser using two cascaded single-mode-four-mode-single-mode (SFS) fiber interferometers and a 150-m-long highly nonlinear fiber. The transmission properties of SFS fiber interferometers with two different four-mode fiber (FMF) lengths are analyzed theoretically. The interferometer with a longer FMF is used to select specific wavelengths, and the one with a shorter FMF is used to stimulate more laser lines. By adjusting the polarization controller and increasing the pump power, the number of output laser channels and the wavelength spacings can be changed and switched. By selecting the proper length of the FMFs, 31 lasing lines are achieved with covering a wavelength range of ~22 nm. The power fluctuation and the wavelength drift of the 31-wavelength operation for the proposed fiber laser are within 0.706 dB and 0.02 nm, respectively.

A wavelength-switchable single-longitudinal-mode thulium-doped fiber laser using a sampled fiber Bragg grating is proposed in this study. The laser can be switched between three single-wavelength and three dual-wavelength modes of operation. For the single-wavelength mode of operation, the maximum power and wavelength fluctuations in 50 min were ±0.404 dB and ±0.01 nm, and the linewidths at three wavelengths were 0.39, 1.88, and 0.89 kHz, respectively. For the dual-wavelength mode of operation, the maximum power and wavelength fluctuations in 20 min were ±0.926 dB and ±0.03 nm, respectively. The proposed thulium-doped fiber laser will find useful application in optical communication and free-space optical detection and sensing.

A passively mode-locked thulium-doped fiber laser with square-wave pulse generation has been demonstrated using a figure-of-eight configuration with two gain mediums respectively located in the left and right loops. In this laser, mode-locking is achieved based on equivalent saturation absorption of a nonlinear amplifying loop mirror (NALM). The pulse amplitude and width of the square-wave pulse can be continuously extended through controlling pump power of gain mediums in the left and right loops. Moreover, the pulse dynamics and autocorrelation trace reveal the obtained square-wave pulse is a new kind of noise-like pulse, i.e., noise-like square-wave pulse, with capability of supporting higher pulse energy. This find enriches laser pulse dynamics and has important potential applications in laser detection, sensing and other fields.

In this article, a precisely controlled, switchable thulium-doped fiber laser (TDFL) with high optical signal-to-noise ratio (OSNR) is proposed and demonstrated. A home-made multimode fiber Bragg grating (MMFBG) with multireflection peaks was used as an optical filter to select the desired lasing channel. Using a manually controlled fusion splicer, the axial misalignment was applied between the single mode fiber and the MMFBG to switch the output wavelength. In the experiment, the number of switchable lasing lines was up to 12, and the fluctuation of the center wavelength and output power was less than 0.03 nm and 0.67 dB, respectively. A maximum slope efficiency of 1.3% was obtained when the axial misalignment was 0 μm, and the OSNR was higher than 65 dB. In addition, the proposed fiber laser was used as an optical fiber laser sensor to quantify the strain applied on the MMFBG, and a sensitivity of 1.16 pm/μϵ was obtained.

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. [ABSTRACT FROM AUTHOR]

This paper proposes and demonstrates a single-longitudinal-mode thulium-doped fiber laser using a passive triple-coupler ring-based compound-cavity filter (TCR-CC) and a uniform fiber Bragg grating. For the first time, the TCR-CC filter is used to select a single mode from dense longitudinal modes. Experimental results show that laser in the wavelength of 1941.28 nm can maintain exceptional stability with an optical signal-to-noise ratio of 74.1 dB. The measured maximum wavelength drift and power fluctuation are 0.01 nm and 0.45 dB, respectively. Meanwhile, the measured linewidth of the laser is 910 Hz, and the relative intensity noise is below −125.82 dB/Hz above 2 MHz frequencies.

We demonstrate the potential of nanosized titanium dioxide (TiO2) in forming ultrashort pulses using a ring cavity thulium-doped fiber laser. A TiO2 polymer composite was transferred onto a microfiber through a spin coating technique to act as a light-absorbing material, saturable absorber (SA). The nonlinear transmission characteristics of this TiO2-SA exhibited 3.8% modulation depth and 17 MW/cm2 saturation intensity at 1.92 µm wavelength. At 1587 mW pump power, the central wavelength, average output power, pulse duration, peak power, and pulse energy were 1925.6 nm, 126.8 mW, 1.5 ps, 6.45 kW, and 11 nJ, respectively. The fabricated TiO2-SA displayed a wide pump power range of fundamental mode-locking at 11.53 MHz without any sign of laser-induced damage threshold or pulse breaking effect. This work has shown the potential use of nanosized TiO2-SA for fabricating broadband nonlinear photonic devices.

A modified polarization-maintaining Sagnac loop mirror is proposed, and the simulated transmission spectrum of the proposed filter is in accordance with experimental transmission spectrum. Based on the proposed filter, a multi-wavelength thulium-doped fiber laser is constructed and experimentally demonstrated. The stable multi-wavelength oscillation within 60 minutes is achieved through the nonlinear polarization rotation effect enhanced by the highly nonlinear fiber. By fine tuning the polarization state, up to 35 lasing lines are obtained within a 10-dB bandwidth with wavelength separation of ∼1.38 nm at room temperature. The wavelength shift and the output power fluctuation are less than 0.04 nm and 1.69 dB, respectively. In addition, by changing the optical coupling ratio in the Sagnac loop mirror, different outputs with 7, 8, 16 lasing lines are also obtained. [ABSTRACT FROM AUTHOR]

A dual-wavelength dissipative soliton resonance thulium-doped fiber laser based on nonlinear optical loop mirror (NOLM) has been demonstrated. In this laser, mode-locking is achieved based on equivalent saturation absorption of the NOLM, while the dual-wavelength operation is attributed to the online birefringent fiber filter composed of a polarizer and a section of polarization-maintaining fiber (PMF). The wavelength spacing of dual-wavelength spectrum can be adjusted through changing the PMF length, and the central wavelengths can be tuned through rotating the polarization controllers. It is also found that at the two central wavelengths of the dual-wavelength pulse, switchable single wavelength mode-locked pulse can be generated. In addition, noise-like pulses with broadband spectra can be generated, when two kinds of mode-locking mechanisms conflict. The dual-wavelength dissipative soliton resonance thulium-doped fiber laser has important potential applications in the laser detection, sensing and other fields. Its rich dynamics also provides a system platform for basic research on laser physics.

In this paper, we demonstrate experimentally a thulium-doped passively mode-locked figure-of-eight fiber laser to generate dissipative-soliton-resonance (DSR) pulses in an all-anomalous-dispersion regime. And two different DSR pulses are obtained by simply adjusting the polarization controllers. At maximum pump power of 2.4 W, the average power and repetition rate of two DSR pulses is 113.5 mW and 2.532 MHz respectively, corresponding to a pulse energy of 45 nJ. Subsequently, the effect of NALM length on the two DSR pulses is investigated, and pulses with maximum pulse energy of 63 nJ near 2 μm are obtained.

A continuous wave, switchable multi-wavelength, thulium-doped fiber laser (TDFL) with a polarization-maintaining sampled fiber Bragg grating (PM-SFBG) is proposed and demonstrated for the first time. A length of 150 m highly-nonlinear fiber (HNLF) was used to introduce the four-wave mixing effect for the efficient suppression of the wavelength competition existing in the gain medium of thulium-doped fiber. By adjusting the state of polarization (SOP) of light in the laser cavity, two six-wavelength operations at two orthogonal SOPs and a ten-wavelength operation were obtained. When the TDFL ran at the six-wavelength operation mode with a pump power of 4.11 W, there were at least four lasing wavelengths with an optical signal-to-noise ratio (OSNR) higher than 30 dB, and when it ran at the ten-wavelength operation mode with a pump power of 4.60 W, there were nine lasing wavelengths with an OSNR higher than 30 dB. Regardless of the operation mode, the TDFL exhibited high stability.

We report on the first demonstration of the Tm-doped all-PM mode-locked fiber laser with CNT used as a saturable absorber and two fiber Lyot filters embedded into the ring resonator. Using these two filters, we were able to adjust the central wavelength of laser generation to ~1855 nm as well as to obtain the required spectral width and necessary pulse duration that are required for the proposed applications. Possible issues with the method used are discussed.

In this article, a precisely controlled, switchable thulium-doped fiber laser (TDFL) with high optical signal-to-noise ratio (OSNR) is proposed and demonstrated. A home-made multimode fiber Bragg grating (MMFBG) with multireflection peaks was used as an optical filter to select the desired lasing channel. Using a manually controlled fusion splicer, the axial misalignment was applied between the single mode fiber and the MMFBG to switch the output wavelength. In the experiment, the number of switchable lasing lines was up to 12, and the fluctuation of the center wavelength and output power was less than 0.03 nm and 0.67 dB, respectively. A maximum slope efficiency of 1.3% was obtained when the axial misalignment was 0 μm, and the OSNR was higher than 65 dB. In addition, the proposed fiber laser was used as an optical fiber laser sensor to quantify the strain applied on the MMFBG, and a sensitivity of 1.16 pm/μϵ was obtained. [ABSTRACT FROM AUTHOR]

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.

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• To the best of our knowledge, this is the first report of wavelength tunable NLP output in the all-PM thulium-doped fiber laser. • A temperature-controlled polarization-maintaining fiber filter is proposed, which can be realized not only by co-splicing the polarization-maintaining dispersion compensation fiber and normal polarization-maintaining fiber but also by simply splicing the normal polarization-maintaining fiber with a rotation angle. • The wavelength tunable range of 32.4 nm from 1910.4 nm to 1942.8 nm is realized in the noise-like pulse regime from an all-PM thulium-doped fiber laser. • The wavelength tunable range of 26.6 nm from 1937.7 nm to 1964.3 nm is realized in the continuous-wave regime from an all-PM thulium-doped fiber laser. In this paper, we present both wavelength tunable noise-like pulse (NLP) and continuous-wave (CW) generation in all-polarization-maintaining (all-PM) thulium-doped fiber lasers. By introducing a piece of polarization-maintaining fiber (PMF) whose birefringence changes with temperature in the all-PM thulium-doped fiber laser, a wavelength tunable range of 32.4 nm from 1910.4 nm to 1942.8 nm is realized in NLP regime, and a wavelength tunable range of 26.6 nm from 1937.7 nm to 1964.3 nm is realized in CW regime. The designed laser cavity is configured with all-PM fiber scheme and the special PMF is temperature-controlled, thus the laser owns high environmental stability and repeatability. To the best of our knowledge, this is the first report of wavelength tunable NLP output in the all-PM thulium-doped fiber laser. [ABSTRACT FROM AUTHOR]