Xiao-Xi Jin, Xiong Wang, Pu Zhou, Hu Xiao, and Ze-Jin Liu

1. Introduction

Fiber lasers have attracted considerable attentions in the past few decades, attributed to remarkable advantages of great beam quality, high surface-area-to-volume ratio,robust operation, and scalable output power. Regarded as a promising candidate of powerful light sources with a range of applications, fiber lasers have been widely investigated[1]-[4]. Faced with many challenges for further power scaling, high-power fiber lasers have employed numerous techniques to overcome limitations, including nonlinear effects, thermal management, and modal instability, to achieve higher output power with good beam quality[5],[6]. Recent progress on tandem pumping, beam combining, and suppression techniques of nonlinear effects and thermal-induced modal instability in Yb-doped fiber lasers and amplifiers successfully break the bottlenecks of power scaling[7]-[10]. Single-mode output power of 20 kW and multi-mode output power of 100 kW level have been reported recently[11], which are the records of maximum output power produced from state-of-the-art fiber lasers at～1 μm.

Besides Yb3+, other rare-earth ions can also be doped in silica-based fibers to generate lasers at various wavelength bands. For the time being, Er3+, Tm3+, and Ho3+are relatively mature rare-earth ions in silica fibers to cover～1.5 μm, ～1.9 μm, and ～2.1 μm, respectively. Compared with fiber lasers at ～1 μm, fiber lasers operating at ～2 μm have advantages of higher nonlinear effects threshold,eye-safety, atmospheric transmission, and easier to be kept in single-mode state. Therefore, Tm- and Ho-doped fiber lasers operating at ～2 μm have great potential to produce higher power than Yb-doped fiber lasers. Gas sensing,optical communication, lidar, material processing, and medicine are included in the various applications with great demand of 2 μm powerful fiber sources[12]-[17]. Although the propagation loss of light in silica-based fiber lasers increases dramatically with the wavelength, which limits the wavelength bands of Tm- and Ho-doped fiber lasers in mid-infrared spectral region, it is still the silica-based fiber employed in 2 μm laser system to realize output power records[18],[19], which benefits from high Knoop hardness,fracture toughness, and laser damage threshold. Recently, a silica-based fiber Raman laser with the output power of～0.3 W at wavelength longer than 2.4 μm has been reported[20], which is thought to be the longest wavelength operation of silica fiber laser. Soft glass fibers doped with fluoride and chalcogenide have wide transparency window in infrared band. A representative candidate of soft glass fibers is ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fiber[21].However, with mechanically weakness and low power handling capacity, the power scaling of soft glass fiber-based lasers is hardly comparable to that of silica-based fiber lasers.

In this paper, history and recent progress of high-power 2 μm silica fiber sources in the past decades are reviewed.We review the state-of-the-art records and representative achievements of powerful continuous-wave (CW), pulsed fiber lasers/amplifiers, and superfluorescent sources in Section 2, Section 3, and Section 4, respectively. In Section 5, the challenges which confine the further power scaling of 2 μm silica fiber sources are discussed, including pump brightness and thermal and nonlinear limits. Prospects of powerful 2 μm silica fiber sources are also presented.

2. 2 μm High-Power CW Fiber Lasers/Amplifiers

After the report of the first Tm-doped silica fiber laser pumped by a dye laser at 797 nm in 1988[22]and the first Ho-doped silica fiber laser pumped by an argon laser at 457.9 nm in 1989[23]by D. C. Hanna et al., 2 μm silica fiber lasers have experienced a rapid development in recent decade[24],[25]. With the progress of high-power 0.79 μm laser diodes (LD) and 2 μm fiber components, the Tm-doped fiber laser has been regards as a competitive light source to achieve comparable or even beyond output power of Yb-doped fiber laser at 1 μm, which benefits from the cross-relaxation (CR) process when pumped by 0.79 μm LDs and the advantages of 2 μm fiber laser including higher nonlinear effect threshold. However, it is not easy to directly realize powerful lasing above 2.05 μm from Tm-doped silica fiber laser owing to the limited gain spectrum. Fortunately, Ho-doped silica fiber lasers have an emission spectrum with longer wavelengths, which helps to meet the special demands of powerful lasing above 2.1 μm.Ho-doped silica fiber with Tm3+as sensitized ions, which can be directly pumped by 0.79 μm LDs, seems to be an acceptable choice to realize high power lasing based on mature diode development. But the diode pumping of Tm/Ho sensitized fiber lasers are faced with up-conversion effect with thermal issues and complex dynamics introduced from Tm3+[26], which confines the power scalable capacity with the highest output power of 83 W achieved in 2007[27]. Therefore, pure Ho-doped silica fiber is preferred to realize high power laser emission above 2.05 μm. The absorption spectral peaks of Ho3+center at～1.15 μm and ～1.95 μm, which correspond to the emission band of LDs/long-wavelength Yb-doped fiber lasers/Raman Yb-doped fiber lasers and Tm-doped fiber lasers,respectively.

2.1 Tm-Doped Fiber Lasers/Amplifiers

After the first report of a 100 W-level clad-pumped Tm-doped fiber laser in 2005[28], the power scaling of CW Tm-doped fiber lasers and amplifiers has been widely investigated by many institutions in the next decade. A summary of representative high-power CW Tm-doped fiber lasers/amplifiers in free-space or monolithic architecture with various characteristics is depicted in Table 1. Different pumping schemes are included in the table to compare the feasibility. As shown in Table 1, 0.79 μm clad-pumping is the most mature pumping scheme, which has been employed to realize the power scaling records of 2 μm fiber lasers: a 1050 W single-mode all-fiber master oscillator power amplifier (MOPA) by Q-peak Inc.[18]and a 608 W single-frequency free-space MOPA by Northrop Grumman Aerospace Systems[29]. When 0.79 μm high-power LDs are adopted to pump Tm-doped fibers, cooling fibers are critical to realize high-efficiency operation and prevent thermal damage to produce high power in a quasi-three-level system.

Table 1: Progress of high-power CW Tm-doped silica fiber lasers and amplifiers

The configuration of Q-peak Inc. kW-level all-fiber MOPA system consists of a mater oscillator and two-stage amplifier. The output power of the master oscillator was 50 W at 2041 nm, which was experimentally verified to be amplified to ＞500 W pumped by six 150 W pump lasers in the first stage, and to ＞1000 W pumped by twelve 150 W pump lasers in the two-stage MOPA system[18]. Regretfully,no more public information on cooling technique details employed in the system were reported possibly due to commercial secret.

In the 608 W single-frequency Tm-doped MOPA system,a 3 mW and less than 5 MHz linewidth distributed feedback diode laser with wavelength of 2040 nm was employed as the seed source of MOPA system[29]. After a three-stage preamplifier chain, a 16 W single-frequency laser was obtained and coupled into active fiber in the final amplifier by freespace dichroic beam splitters. The output power reached 608 W, which is only limited by available pump power.The cooling technique of active fiber in the final stage is to embed in a low thermal-impedance conductive heatsink.

With rapid development of fiber components, the monolithic single-frequency MOPA has acquired high attentions in recent years. In 2013, the first 100 W-level single-frequency Tm-doped all-fiber MOPA was reported[35].Then the maximum output power reached 200 W in 2014[37].Recently, we reported a single-frequency Tm-doped all-fiber MOPA with the maximum output power exceeding 310 W[42]. A 40 mW and less than 100 kHz linewidth single-frequency fiber laser with ultra-short cavity and a two-stage pre-amplifier was employed in the MOPA system to produce 5.5 W single-frequency Tm-doped fiber laser to launch into a single-stage main-amplifier. Six 793 nm LDs with ＞550 W pump powers were used in the main-amplifier, combined by an all-fiber signal-pump combiner. The double-clad Tm-doped fiber and its two fusion spliced joints in the main-amplifier were placed on a water cooled heatsink with temperature of 14°C. The output power and spectrum of this single frequency Tm-doped MOPA are shown in Fig. 1 (a) and Fig. 1 (b). And the spectrum measured by a scanning Fabry-Perot interferometer to verify the single frequency operation is demonstrated in Fig. 1 (c).

Fig. 1. 310 W monolithic single-frequency Tm-doped MOPA[42]: (a) output power versus incident pump power, (b) spectrum (inset:spectrum at a zoomed-in scale), and (c) spectrum measured by a scanning Fabry-Perot interferometer (inset: spectrum at a zoomed-in scale).

Fig. 2. 100W-level Tm-doped fiber laser clad-pumped by two 1173 nm Raman fiber lasers[38]: (a) experimental setup (top: 1173 nm Raman fiber laser; bottom: Tm-doped fiber laser), (b) output power versus incident pump power, and (c) spectrum (inset: spectrum in a large scale).

Besides pumping with 0.79 μm high-power LDs,various pumping schemes have been explored and developed to achieve high-power Tm-doped fiber lasers/MOPAs. Powerful pump sources at wavelengths which correspond to other absorption peaks of Tm3+ions also show great potentials to realize high-power Tm-doped fiber lasers/MOPAs. These potential pumping schemes are also listed in Table 1, and the records of their power scaling are demonstrated as followed:

In 2007, IPG Inc. reported a 415 W Tm-doped all-fiber laser clad-pumped by eighteen 1.56 μm Er fiber lasers with the total pump power exceeding 720 W[31]. It is noted that the slope efficiency is ～60% when pumped with 1.56 μm pump sources, and no up-conversion process that degrades fiber due to photo-darkening happened, compared with 0.79 μm pumping scheme. However, the power scaling of Er-doped fiber lasers/amplifiers are still faced with many challenges which limit the maximum output power of Tm-doped fiber lasers pumped by 1.56 μm Er-doped fiber lasers.

In 2014, we reported the first 100W-level Tm-doped fiber laser pumped by 1173 Raman fiber lasers[38]. The system configuration is shown in Fig. 2 (a). The measured output power and spectrum are shown in Fig. 2 (b) and Fig.2 (c). The slope efficiency of the system reached 42%.Although up-conversion process was generated, the slope efficiency did not decrease much since the absorption cross-section of the excited-state absorption (ESA) effect was relatively low at 1173 nm. The power scaling of long-wavelength Yb-doped Raman fiber laser is limited,which means the maximum output power of Tm-doped fiber lasers/amplifiers pumped by ～1.2 μm Yb-doped Raman fiber lasers is similarly limited by the underdeveloped state of pump sources.

In 2014, BAE system Inc. reported a resonantly pumping scheme of Tm-doped fiber lasers/amplifiers[36]. A moderate-power core-pumped Tm-doped fiber laser at 2005 nm with the output power of 1.43 W and the slope efficiency of 90.2% was firstly demonstrated to verify the feasibility of resonantly pumping when a 1908 nm Tm-doped fiber laser was used as an in-band pump source.Afterwards, they reported a 41 W core-pumped Tm-doped fiber amplifier with a slope efficiency of 92.1% and a 123.1 W clad-pumped Tm-doped fiber amplifier with a slope efficiency of 91.6% when pumped by a 1908 nm Tm-doped fiber laser[40]. To eliminate 1908 nm pump light absorption in low-index acrylate coating in clad-pumped experiment, the pump light was coupled into the pedestal region of Tm-doped fiber to replace the role of inner cladding, which takes the place of outer cladding. Therefore,free-space components were used in these resonantly pumping amplifier systems to realize core-pumped or clad-pumped coupling. In 2015, a research group from Shanghai Jiao Tong University also reported a resonantly core-pumped Tm-doped fiber laser at 2020 nm with the maximum output power of ～35 W and the slope efficiency exceeding 90% when pumped by a 1942 Tm-doped fiber laser[41].

2.2 Ho-Doped Fiber Lasers/Amplifiers

The development of Ho-doped fiber laser was much slower than that of Tm-doped fiber laser, limited by the underdeveloped pump sources and drawbacks of longer wavelength in silica glass. As previous stated, diode pumping Ho-doped fibers sensitized with Tm3+has achieved 83 W output power in 2007[27]. However, due to side-effects introduced from Tm3+, researchers pay more attention on the development of high-power sensitizer-free Ho-doped silica fiber lasers. The absorption spectrum of pure Ho-doped fiber peaks at ～1.15 μm and ～1.95 μm.Different pumping schemes have been developed to realize high-power laser above 2.05 μm from Ho-doped silica fiber lasers. A summary of representative high-power CW Ho-doped silica fiber lasers/amplifiers in various architectures and pumping schemes is demonstrated in Table 2.

Table 2: Progress of high-power CW Ho-doped silica fiber lasers and amplifiers

For ～1.15 μm pumping scheme, LDs, long-wavelength Yb-doped fiber lasers, and Yb-doped Raman fiber lasers have played a role of pump source. Although the development of LDs operating at this wavelength band is limited, high-power Yb-doped fiber lasers and Yb-doped Raman fiber lasers make this pumping scheme feasible.The major challenge for ～1.15 μm pumping scheme is the large quantum defect, which results in a relatively low slope efficiency of ～40%. In 2010, A. S. Kurkov et al. from Russian Academy of Science reported an all-fiber 10 W Ho-doped fiber laser pumped by an Yb-doped fiber laser at 1147 nm with a power up to 35 W[47]. The slope efficiency was ～30% in the core-pumped architecture, corresponding to the quantum efficiency of 54%. In 2014, we reported an all-fiber 42 W Ho-doped fiber laser at ～2048 nm when core-pumped by a 1.15 μm Yb-doped Raman fiber laser[50].The system configuration, output power, and spectra are shown in Fig. 3. The maximum output power of Raman fiber laser reached 110 W, and the slope efficiency of this Ho-doped fiber laser was ～37%, corresponding to the quantum efficiency of 66%. Higher output power when pumped by ～1.15 μm can be achieved by increasing the power of pump sources and employing clad-pumping scheme.

Fig. 3. 42 W Ho-doped fiber laser core-pumped by a 1.15 μm Raman fiber laser[50]: (a) experimental setup, (b) output power of the Ho-doped fiber laser (inset: spectra at 4.2 W and 42.1 W), and (c) spectra at different output power levels.

For ～1.95 μm tandem pumping scheme, which was first demonstrated by S. D. Jackson from Optical Fiber Technology Centre in 2006 with a slope efficiency of 82%for core-pumping[44], high-power Tm-doped fiber lasers have been employed as pump sources. In 2009, J. W. Kim et al. from Optoelectronics Research Centre reported a 4.3 W core-pumped Ho-doped fiber laser with a slope efficiency of 64% when pumped by a 1980 nm Tm-doped fiber laser[26]. When it comes to clad-pumping,conventional double-clad fibers fail to launch more pump power into the inner cladding of gain fiber, due to the pump wavelength absorption in standard polymer coating of double-clad fibers. This problem can be fixed by a novel all-glass fiber, in which a glass cladding replaces standard polymer coatings in conventional double-clad fibers.Corresponding to all-glass signal-pump combiners, all-glass Ho-doped double-clad fibers and all-glass passive fibers have been fabricated to achieve high-power clad-pumped Ho-doped all-fiber lasers and amplifiers.

In 2013, A. Hemming et al. from Defense Science and Technology Organization of Australia reported a monolithic clad-pumped Ho-doped fiber laser with the maximum output power of 407 W[19]. An array of six single-mode Tm-doped fiber lasers operating at 1.95 μm with the total output power exceeding 1 kW is employed as the pump source of Ho-doped fiber laser. To eliminate the absorption of polymer coating at pump wavelength,all-glass double-clad fibers and pump combiner have been used to realize tandem clad-pumping of Ho-doped fiber lasers. According to the data acquired in [19], the slope efficiency is estimated as ～38%, which is much below the quantum limit of tandem pumping.

In 2014, A. Hemming et al. reported a 265 W Ho-doped all-fiber amplifier which was resonantly pumped by an array of high-power Tm-doped fiber lasers[49]. An all-fiber linearly-polarized core-pumped single-mode laser with the output power of 27 W at 2.11 μm was employed as the master oscillator. An all-glass large-mode-area double-clad Ho-doped fiber was used as the gain medium,which was pumped by nine high-power Tm-doped fiber lasers with the total output power of ～700 W. An all-glass pump-signal combiner with matched fibers was used to launch the pump light into the active fiber. The slope efficiency of this amplifier is ～40%, also much lower than the quantum limit.

Proper pump sources and successful fabrication of special components are significant for the tandem pumping of Ho-doped all-fiber sources. A. Hemming et al. found that Tm-doped fiber lasers are more appropriate than Tm-doped fiber amplifiers to be used as pump sources of Ho-doped fiber lasers[34]. And the fabrication of all-glass double-clad fibers and matched combiners are essential conditions to realize monolithic tandem clad-pumped Ho-doped fiber sources.

The next challenge to realize high-power, efficient tandem pumped Ho-doped fiber lasers/amplifiers is increasing the quantum efficiency to make the slope efficiency close to the quantum limit. The reasons why slope efficiencies of Ho-doped fiber lasers/amplifiers clad-pumped by high-power Tm-doped fiber lasers are much lower than quantum limit are still under research.Background infrared silica losses, OH-combination mode absorption, re-absorption and non-radiative decay processes of Ho3+in silica are the sources of loss that decrease the quantum efficiency, which results in the low slope efficiency of tandem clad-pumped Ho-doped fiber sources.

3. 2 μm High-Average-Power Pulsed Fiber Lasers/Amplifiers

As the development of 2 μm high-power CW fiber lasers and amplifiers, researchers have pay more and more attentions on 2 μm pulsed fiber lasers owing to their characteristics of high average power, large pulse energy,high peak power, and short pulse duration. Applications including material processing, pumping sources for mid-infrared optical parametric oscillator (OPO), and mid-infrared supercontinuum generation are of great demands of 2 μm pulsed fiber lasers with high average power and high pulse energy. Various techniques have been employed to achieve stable pulse generation, such as mode-locking, Q-switching, gain-switching, and chirped pulse amplification (CPA). Femtosecond-level ultra-short pulses in 2 μm region are usually generated via CPA technique[51],[52]. In 2014, Jena University reported a Tm-doped fiber CPA system with the record compressed average output power of 152 W and 4 MW peak power[53].In 2015, they reported a pulsed Tm-based fiber CPA system delivering a peak power higher than 200 MW with 24 W average power and 120 μJ pulse energy[54]. The results of the CPA system in 2 μm region are quite impressive.Whereas the CPA system consists of free-space optics,including large bulk grating pairs which are expensive and increase the system complexity. Besides CPA technique,the mode-locking, Q-switching, gain-switching, and direct intensity modulation techniques have been used to realize 2 μm high-average-power pulse generation with the help of multi-stage amplifiers. Gain-switching is an effective technique to produce high energy pulses in Tm-doped fiber lasers, whereas the generated spiky pulses may degrade the performance. Usually, the power of seed sources is moderate, thus, amplifiers in multi-stage configuration are required to realize power scaling, which also increase the nonlinearity. Efforts were spent on the suppression of nonlinear effects and amplified stimulated emission. For the time being, the average power of nanosecond or picosecond monolithic Tm- or Ho-doped MOPAs is with ～100 W-level.The representative reports of high-average-power 2 μm pulsed fiber lasers and amplifiers with different dopants in nanosecond or picosecond level are presented in Table 3.

In 2015, we reported a monolithic 105 W ultranarrowband nanosecond pulsed Tm-doped fiber MOPA[55].The experimental setup of seed source and main-amplifier is demonstrated in Fig. 4 (a). A phase modulator and an intensity modulator were inserted into the cavity to modulate a single frequency fiber laser. The pulsed seed laser was amplified by a monolithic Tm-doped MOPA.Initially, the performance of pulsed Tm-doped fiber MOPA in single frequency operation (without modulation) with the repetition rate of 1 MHz and pulse duration of 66 ns was tested. The maximum output power of this single-frequency pulsed fiber MOPA reached 37 W, corresponding to the peak power of 490 W and the pulse energy of 37 μJ, which was limited by the onset of Brillouin scattering (SBS).Phase modulation was used to broaden the linewidth of pulsed seed laser to increase the threshold of SBS. The linewidth data measured before and after phase modulation by a scanning Fabry-Perot interferometer were depicted in Fig 4. (c). The linewidth increased from ～24 MHz to ～307 MHz, which is still an ultra-narrowband linewidth which increases the SBS threshold. Thus, the maximum output power of MOPA increased linearly to 105 W, as shown in Fig. 4 (b), with the slope efficiency of 41%. The pulse duration was 66 ns and the repetition rate of pulse trains was 1 MHz, corresponding to the peak power of 1.59 kW and pulse energy of 105 μJ. Further power scaling can be achieved by increasing the SBS threshold.

Table 3: Progress of high-average-power silica fiber-based 2 μm pulsed fiber lasers and amplifiers in ns and ps levels

Fig. 4. 105 W ultra-narrowband nanosecond pulsed Tm-doped fiber MOPA[55]: (a) experimental setup (top: seed source, bottom:main-amplifier), (b) output average power of the ultra-narrowband pulsed MOPA, and (c) spectral linewidth data measured before and after phase modulation (PM) by a scanning Fabry-Perot interferometer.

Our institution reported a high-average-power monolithic nanosecond Tm-doped fiber MOPA in this year.A linearly polarized gain-switched fiber laser was employed as the seed source, pumped by a 1550 nm distributed-feedback (DFB) MOPA system. Two-stage amplifier was used to achieve power scaling to the average power of 40.5 W and pulse energy of 1 mJ, when the seed pulse repetition rate was 40 kHz[56].

Fig. 5. 238 W high-average-power pulsed monolithic Tm-doped MOPA: (a) experimental setup (top: seed source and two-stage pre-amplifier; bottom: main-amplifier), (b) output power with the repetition rate of 500 kHz, and (c) spectrum (inset: spectrum at a zoomed-in scale).

Recently, we also achieved a high-average-power monolithic nanosecond Tm-doped fiber MOPA with the record average power reaching 238 W. The maximum peak power could reach 12.1 kW and the pulse energy 0.749 mJ when the pulse train’s repetition rate is 200 kHz with a pulse width of 58.2 ns. The experimental setup and results of this high-average-power pulsed Tm-doped MOPA are shown in Fig. 5. The average power was limited to 150 W when the repetition rate was 200 kHz, due to the backward parasitic lasing in the main-amplifier. When the repetition rate increased to 500 kHz, 238 W, the average power was obtained. The manuscript on this work has been submitted to a journal.

High-power pulsed 2 μm fiber lasers could also be utilized to generate supercontinuum (SC) in mid-infrared region. 2 μm SC sources based on silica fiber have been investigated in recent years. In 2013, Yang et al.demonstrated an all-fiber mid-infrared SC generated from a Tm/Ho co-doped fiber amplifier[69], with a spectrum range of 1760 nm to 2600 nm, where the Tm/Ho co-doped fiber acted as both nonlinear and gain medium. Jacek Swiderski et al. reported a 2.3 W single-mode SC generated in a two-stage Tm-doped fiber amplifier, in which the 10 dB bandwidth of the SC was 570 nm (1950 nm to 2520 nm)[70].Vinay V. Alexander et al. presented a ＞25 W SC source covering 2000 nm to 2500 nm[71], which employed a highpower pulsed Tm-doped amplifier and a piece of silica fiber as the pump source and SC generation fiber, respectively.Liu et al. reported a 36 W SC source with the bandwidth from 1950 nm to 2400 nm based on a high-power pulsed Tm-doped all-fiber MOPA[60]. In 2015, Xue et al. reported a 2.32 W SC with a 6 dB spectral range from 1955 to 2505 nm generated from a Tm-doped fiber amplifier[72].

4. 2 μm Powerful Silica Fiber-Based Superfluorescent Sources

Superfluorescent sources (SFSs), also known as amplified spontaneous emission (ASE) sources, have been regarded as a novel candidate of powerful light sources with advantages: good temporal stability, high brightness,and without obvious spectral broadening in power scaling process. Silica fiber-based SFSs operating at 2 μm have been through a rapid development in recent years. Many applications, e.g., gas sensing, material processing, optical communication system, and light detection, are in great potential demand of this new 2 μm fiber sources. A summary of representative achievements of high-power 2 μm silica fiber-based SFSs is demonstrated in Table 4.After 10 Watt-level SFSs achieved in 2008[73], the highest output power of silica fiber-based Tm-doped SFSs kept increasing in these years and reached 46 W with the spectral full width at half maximum (FWHM) of 45 nm in an all-fiber single-stage source[74], and 316 W with the FWHM of 24 nm[75], 364 W with the FWHM of 1.9 nm in an all-fiber amplifier[76]. Whereas the progress of Ho silica fiber-based SFSs was slower than that of Tm. When in-band core-pumped by ～1.1 μm pump sources, Ho silica fiber-based SFSs achieved the maximum output power of 0.273 W with the FWHM of 54 nm[77]and 1.5 W with the FWHM of 30 nm[78]. Power scaling of Tm- and Ho- doped silica fiber-based SFSs is limited by pump brightness and available pumping scheme. Clad-pumping of Ho-doped SFSs has not been achieved up to now. It is predictable that further power scaling of Tm- and Ho-doped silica fiber based SFSs can be realized by utilizing mature pumping techniques, such as utilizing in-band pumping and all-glass double-clad fibers.

Table 4: Progress of high-power silica fiber-based superfluorescent sources

4.1 Tm-Doped Fiber-Based Superfluorescent Sources

After the first report on Tm-doped fiber-based SFS with the output power of ～1.2 mW and the FWHM of 77nm when pumped by a 810 nm Ti:sapphire laser[83], the output power and the slope efficiency were not comparable with those of Tm-doped fiber lasers in decade, due to the underdeveloped techniques to suppress laser oscillation and pumping schemes.

In 2008, Shen et al. from Optoelectronics Research Centre of University of Southampton reported a broadband Tm-doped SFS with the single-ended output power of 11 W,the FWHM of 36 nm, and the slope efficiency of 38%[73].790 nm pump light from diode bars were launched into the inner-cladding of Tm-doped fiber by lenses and dichroic mirrors. The single-ended output was realized by cleaving the ends of Tm-doped fiber at different angles, to simultaneously ensure the low feedback reflectivities of both ends and extremely large difference between them. In the experiment, the angles of 14° and 0° were chosen and two ends of the fiber were cooled in the heatsinks to achieve the single-ended output power of 11 W. The slope efficiency of 38% was comparable with that of conventional Tm-doped fiber laser oscillators.

In 2015, Hu et al. from Tsinghua University demonstrated a high-power single-stage all-fiber Tm-doped SFS with the total two-ended output power of ～60 W and the FWHM of ～50 nm[74]. The 790 nm pump light from LDs was launched into the Tm-doped fiber via all-fiber signalpump combiner. Effective suppression of the parasitic lasing was provided by double angle-cleaved facets. The active fiber and the combiner were cooled to ～10°C in a water-cooled heatsink to promote efficient cross-relaxation process, with a slope efficiency of 38.9% as the result.

The power scaling of the single-stage Tm-doped SFS was limited by the quality of angle-cleaved facet and available pump power. With decreasing reflectivity of fiber facets and optimal active fiber length, the multi-hundredwatt-level output power could be realized in a single-stage Tm-doped SFS by launching more pump power.

The multi-stage amplifier configuration has dramatically increased the power scaling capacity of Tm-doped SFS. Liu et al. from Beijing University of Technology reported a 25 W all-fiber Tm-doped SFS with the FWHM of 25 nm based on an amplifier configuration in 2013[79]. By employing more powerful pump sources, they achieved 122 W broadband Tm-doped SFS with the FWHM of 25 nm and 120 W narrowband Tm-doped SFS with the FWHM of 1.2 nm in 2014[80]. In 2015, we reported a 316 W broadband Tm-doped SFS with the FWHM of 24 nm[75], a 228 W narrowband Tm-doped SFS with the FWHM of 0.19 nm[81],and a exceeding 250 W widely tunable narrowband Tm-doped SFS with a tuning rage of ～35 nm (1966 nm to 2001 nm) and the FWHM of ～1.5 nm to 2.0 nm[84]. The output power and spectra of them are shown in Fig. 6. All the maximum output powers above are limited by pump power and could be further improved if higher pump power and slope efficiency are achieved. Liu et al. also reported a high-power narrow-linewidth Tm-doped SFS with wavelength tunable range from 1940 nm to 2010 nm in MOPA configuration, which can produce 364 W output at 1980 nm with the FWHM of 1.9 nm[76].

4.2 Ho-Doped Fiber-Based Superfluorescent Sources

Although Ho-doped fiber lasers and amplifiers are in rapid development, only few experimental results of Ho-doped SFS have been reported and the output power are quite low. In 2008, Antipov et al. from Russian Academy of Science demonstrated the first broadband Ho-doped silica fiber-based SFS with the maximum output power of 8 mW and the FWHM of 45 nm when pumped by a 1120 nm Yb-doped fiber laser[82]. The fiber of 1120 nm output Bragg grating was bent at a small radius to prevent parasitic lasing.By improving the quality and optimizing the length of Ho-doped silica fiber, they realized a 273 mW Ho-doped SFS with the FWHM of 54 nm at a central wavelength of 2.03 μm in 2013[77]. Due to the efficiency of Yb-doped fiber laser decreased dramatically with the increase of operating wavelength, Yb-doped fiber laser centered at 1125 nm was selected as pump source, nor 1150 nm which coincides with the absorption peak of Ho fiber, to provide sufficient pump power.

In 2015, we reported a 1.5 W Ho-doped all-fiber SFS with the FWHM of 30 nm that was core-pumped by 1150 nm Yb-doped Raman fiber laser[78]. The system configuration and experimental results are shown in Fig. 7.The powerful Yb-doped Raman fiber laser covers the absorption peak of Ho-doped silica fiber. The maximum output power was obtained when ～30 W Raman fiber laser launched into active fiber. Double peaks were observed in the optical spectra of SFS when power was beyond ～200 mW, which is in accordance with the luminescence spectrum of Ho3+ions.

Fig. 6. Output power and spectra of Tm-doped SFSs: (a) 316 W broadband Tm-doped SFS[75], (b) 228 W narrowband Tm-doped SFS[81],and (c) 250 W widely tunable narrowband Tm-doped SFS[84].

Fig. 7. 1.5 W monolithic Ho-doped fiber SFS core-pumped by a 1150 nm Yb-doped Raman fiber laser[78]: (a) experimental setup, (b)output power versus incident 1150 nm pump power, and (c) spectra in different output power levels.

5. Current Challenges and Future Prospects

Further power scaling of 2 μm fiber sources is currently limited by the brightness of pump sources and power handling capacity of fiber components. Although benefiting from the CR process when pumped by 0.79 μm LDs,Tm-doped fiber sources are still faced with challenges,including limited brightness of pump sources, slope efficiency, and large quantum defect compared with that of Yb-doped fiber sources. Also, CR process, which breaks the Stokes limit, challenges the cooling technique because high temperature decreases the CR effect. Lack of mature pump sources, Ho-doped fiber sources are also limited by pump brightness at ～1.15 nm and ～1.95 nm.

The next predictable challenge is thermal problem. For Tm-doped fiber sources, CR process increases the slope efficiency when 0.79 μm diode-pumped, but there are still～40% pump energy transfer to heat, which is a disaster for high-power Tm-doped fiber sources with the output power exceeding kW-level. P. Moulton from Q-peak Inc.estimated that the thermal damage limit of Tm-doped fiber laser is ～5 kW[85]when bi-directional pumped by power of 8.6 kW, if assuming 300 W/m maximum thermal load, 70%laser slope efficiency, and 10 m active fiber with 90%absorption. The slope efficiency of high-power Ho-doped fiber sources is even lower than that of Tm-doped fiber sources, therefore, a lower thermal damage limit for Ho-doped fiber laser is predictable. To acquire higher output power without reaching thermal limit, feasible roadmaps may be beam combining and tandem pumping,which avoid or increase the thermal limit, respectively, but also increase the system complexity. Also, a great amount of heat may introduce modal instability problem in high-power narrow-linewidth 2 μm fiber sources with near diffraction-limited beam quality. For thermal challenge in 2 μm silica fiber sources, to avoid massive heats amassed in a singular part of the system, a reasonable and promising solution is to decentralize those heats in different parts.Tandem pumping and beam combining belong to this kind of solution. Heat generated in singular part decreases with the price of increasing the complexity and the cost of system, therefore, the thermal endurance capacity of system is enhanced. As in 1 μm fiber source systems, tandem pumping brings advantages including low quantum defect,less thermal waste, and high modal instability threshold.The tandem pumping scheme of Tm-doped fiber sources has been achieved in a monolithic but core-pumped configuration, and a clad-pumped but free-spaced configuration. Higher power is possible to be realized in the monolithic tandem clad-pumped Tm-doped fiber sources if all-glass double-clad Tm-doped fiber and matched fiber combiner are fabricated and employed. However, such a promising technique is also faced with potential problems:due to bleaching effect and pump absorption, the maximum power produced from a tandem pumping Tm-doped fiber sources was estimated to be ＞2 kW-level[40].

The third challenge is nonlinearity. Limited pump brightness leads to multi-stage amplifiers to realize power scaling, but with the increase of nonlinearity. Although the nonlinearity threshold of 2 μm fiber sources is much higher than that of 1 μm fiber sources, the 2 μm CW fiber sources will finally meet it with continuous progress. Some techniques have been used to suppress the nonlinearity in high-power pulsed 2 μm fiber sources.

Apart from pump brightness, thermal problem, and nonlinearity limits, a Tm-doped silica fiber laser still has scalability limits of 36.2 kW in broad bandwidth case and 1.85 kW in the single-frequency case[86]. Coherent beam combining and spectral beam combining are promising methods to avoid the scalability limit and reach higher power via beam combination. Some institutions have realized coherent beam combining of high power Tm-doped fiber amplifiers[87]-[91], including the coherent beam combining of two monolithic CW single-frequency Tm-doped fiber MOPAs with the combined output power of 108 W[90]and two pulsed Tm-doped fiber amplifiers based on CPA with the combined output peak power of 25 MW peak power[91]. The power scaling of spectral beam combining at 2 μm also made a rapid progress[92],[93].Spectral beam combining of three linearly polarized Tm-doped MOPAs spectral combined by dielectric mirrors with the maximum combined output power of 253W was reported recently[93].

It is also noteworthy that 2 μm silica fiber sources have potentials of power scaling when they operate close to the edge of gain spectrum[20],[94]-[96], which is under-developed but significant in theory and practice. According to the previous experiment results, hybrid or Raman gain has the advantage on achieving high power lasing at edgewavelength[96]. Further power scaling at extreme short or long wavelengths within gain spectrum directly from silica based fiber sources is one of the prospects, although large background infrared silica losses caused by the nature of host material in silica fiber will introduce challenges.

6. Conclusions

In conclusion, we reviewed the history and recent progress of high-power 2 μm silica fiber sources. Some representative results are presented and discussed.Although the power scaling of high-power 2 μm silica fiber sources are confined by pump brightness now, the thermal limit, nonlinear limit, and scalability limit are also predictable. Tandem pumping and beam combining provide promising roadmaps to further power scaling. Other prospects like high-power operation in edge-wavelength of 2 μm silica fiber sources are also analyzed.

Acknowledgment

This work was supported by the National Natural Science Foundation of China under Grant No. 61322505 and Innovation Foundation for Graduates of National University of Defense Technology under Grant No.B130704).

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