1. Introduction

High-power thulium-doped fiber lasers at ∼2 µm wavelength regions have attracted increasing interest for numerous applications such as material processing, laser surgery, remote sensing, and nonlinear optics [1–5]. Generally, TDFLs offer great advantages in beam quality, thermal management, and power scaling capability over conventional ‘bulk’ solid-state lasers. Diode pumping Tm³⁺ ions at 793 nm allows TDFLs to obtain a theoretical quantum efficiency of ∼80% by exploiting the well-known “two for one” cross relaxation process [6], which shows the great potential for high-power laser operation. Following that approach, several studies of efficient TDFLs at 2 µm with versatile output have been conducted [7–13]. The first demonstration of 1 kW output from a Tm-doped fiber amplifier (TDFA) at 2045nm has been presented with a slope efficiency of 53.2% using a two-stage MOPA configuration [8]. Laser power of 567 W at 1970nm and 790 W at 1943nm was demonstrated from a monolithic thulium oscillator and beam combined laser system, respectively [9,10]. kW-class narrow linewidth and ultrafast TDFAs were demonstrated using single-frequency DFB seed laser phase-modulated for SBS suppression [11], and large mode area PCF in free-space configuration [12]. Regardless of these impressive developments, power scaling over the kilowatt level of the TDFLs remains challenging with primary constraints of thermal effect or transverse mode instability (TMI) [13,14].

High-power TDFLs with a wide spectrum tuning range are highly desirable to meet more practical applications [15], which is enabled by the ³F₄ → ³H₆ transition of Tm³⁺ ions in silica fiber that features a broad emission spectrum range from1600 to 2200 nm [16,17]. Since the demonstration of the wide wavelength tunability from 1860 to 2090nm for a watt-level Tm fiber laser [18], rapid progress of wavelength-tunable TDFLs has been made [15,19–24]. Using a diffraction grating to provide wavelength selective feedback, the ultra-broad tuning range of 338 nm (1723-2061nm) and 371 nm (1654-2025nm) was achieved using ∼1.55 µm core- and/or cladding-pumping configurations [19,24], and the output power was later scaled to be > 200 W over 170 nm tuning range from 1927 to 2097nm [15]. A widely tunable dual-wavelength operation of a high power Tm:fiber laser was demonstrated by means of a volume Bragg gratings, yielding an output power of more than 115 W for 40 nm tuning range [20]. In term of all-fiber integrated TDFLs offering ultra-high compactness and reliability, lase power of 250 W with a tuning range of ∼35 nm (1966-2001nm), and of > 270 W over a 140 nm tuning range spanning from 1910 to 2050nm have been realized using a spectral tunable filter for the selection of the operating wavelength [21,22]. Further power scaling of the wavelength-tunable TDFL should be essential for a great variety of actual applications.

In this paper, a high-power widely-tunable all-fiber integrated Tm-doped fiber laser is demonstrated. More than 1 kW laser output over the full wavelength tuning range from 1943nm to 2050nm is obtained with a slope efficiency of > 51% using a tunable bandpass filter in the seed oscillator followed by three amplification stages. The root mean square (RMS) of the laser output over ∼10 min is calculated to be ∼0.52%, showing a relatively stable laser operation of the TDFL. The beam quality factor M² is estimated to be ∼1.18 at an output power of 920 W. To our knowledge, the result represents the highest power level demonstrated to date from such a wavelength-tunable TDFL. Further power scaling should be possible with elevated pump power considering the absence of parasitic oscillation or self-pulsing effects even at the maximum laser power level.

2. Experimental setup

The experimental setup of kilowatt-class wavelength-tunable TDFL is based on an all-fiber master oscillator power amplifier scheme, as depicted in Fig. 1. The seed oscillator is designed as a ring-cavity including a tunable band-pass filter (TBF) for wavelength tunability in the range of 1940-2050nm. The double-clad TDF utilized has a length of 3 m with a core diameter of 10 µm (NA = 0.15) and a cladding absorption coefficient of 3.6 dB/m at ∼793 nm. The TDF is pumped by a 30 W fiber coupled laser diode (LD) operating at 793 nm through a (2 + 1) × 1 signal-pump combiner. The residual pump light and high-order signal modes in the fiber inner-cladding were dumped using an intra-cavity cladding pump stripper (CPS). An optical isolator (ISO) is inserted into the cavity to ensure the unidirectional propagation of the signal light, and the laser power is exacted from the 80% port of a 20:80 optical coupler (OC).

 

Fig. 1. Experimental setup of > kW wavelength-tunable TDFL.

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The signal laser from the master oscillator is then coupled into a preamplifier through an optical circulator (CIR) to prevent any feedback from the latter amplification stage and possible influence on the seed source. The idle port 3 of the CIR is angle-cleaved around 8° to monitor backward power and spectrum. The utilized LDs, gain fiber, and signal-pump combiner have the same parameters as that used in the seed oscillator. The main amplifier with two-stage pumping is constructed for further power scaling of the signal laser passing through the mode field adaptor (MFA). A piece of 7 m-long large-mode-area (LMA) TDF used in each-stage main amplifier has a core/cladding diameter of 25/400 µm with corresponding NA of 0.11/0.46 (CETC, China Electron. Technol. Group Corp.), and possess a rather low cladding absorption coefficient of ∼2 dB/m at 793 nm, which can effectively reduce the average heat load in the gain fiber while enabling the pump light absorbed sufficiently. Two groups of wavelength-stabilized 793 nm LDs with 200/220 µm, 0.22 NA pigtail fibers are utilized to pump the LMA-TDF through two (6 + 1) × 1 signal-pump combiner with a signal insertion loss of ∼0.12 dB and pump coupling efficiency of more than 97.5%, delivering a total launched pump power of ∼2000 W. A homemade mode-field matched CPS with a pump power handling of ∼200 W and pump light stripping efficiency of 98.6% is fusion spliced to the gain fiber to strip the residual pump light and undesirable cladding signal light. To ensure efficient heat dissipation and mode instability suppression, the gain fiber is coiled with a diameter of ∼10 cm and is directly water-cooled at 10 °C. A fiber endcap made of a piece of angle-cleaved coreless fiber (∼1.5 mm) with a diameter of 400 µm is fusion spliced to the output end to prevent the unwanted optical feedback and potential damage to the end facet. In the experiment, Laser output power was recorded using a thermal-sensor power meter (Ophir, L1500-BB-50) and laser spectrum was monitored by an optical spectrum analyzer (AQ6375B, Yokogawa) with a resolution of 0.05 nm. The temporal characteristic of the laser output was monitored using a high-speed photodetector (100 MHz bandwidth) and a digital phosphor oscilloscope (500 MHz bandwidth) and the beam quality factor M² was analyzed via a beam profiler (NanoScan, Photon Inc.).

3. Experimental results

The wavelength tunability of seed oscillator is depicted in Fig. 2(a). Through adjusting the TBF, the operating wavelength was continuously tuned from 1940nm to 2050nm with an optical signal to noise ratio (OSNR) of > 35 dB and a narrow linewidth of ∼0.15 nm. Figure 2(b) plots the measured output power of seed laser with respect to the wavelength at a launched pump power of 12.3 W. The maximum output power of 1.72 W at 1965nm was achieved with a slope efficiency of 15.6%. The low slope efficiency can be attributed to the high insertion loss of the TBF (∼4.5 dB) and ISO (∼1.5 dB). As can be seen, laser power decreases obviously at longer wavelengths owing to the reduced emission cross section of Tm³⁺ and increased background absorption loss in silica fiber. Wavelength tuning range and output power level of the seed was limited by the spectrum bandwidth and power handling capability of the TBF.

 

Fig. 2. (a) Output spectra of the seed laser. (b) Seed power and background loss in silica fiber as a function of wavelength [25,26].

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The seed laser was then injected into the preamplifier for further power boosting, amplification and output characteristics at selected wavelengths of 1943nm, 1980nm, 2015nm, and 2050nm are summarized in Table 1. At a launched pump power of 41.4 W, the seed power was scaled to 24.5 W, 25.3 W, 24.1 W, and 22.9 W with a high slope efficiency of 60.3%, 63.1%, 61.1% and 57.3% for operating wavelengths of 1943nm, 1980nm, 2015nm, and 2050nm, respectively. Again, reduced net-gain at 2050nm resulted in slightly lower slope efficiency than at other wavelengths. The 3 dB spectra bandwidth was 0.14∼0.16 nm. To avoid any damage to the optical components, further attempt in power scaling of the preamplifier is not carried out.

Table 1. Output characteristics of the preamplifier

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Two-stage main amplifiers (denoted as amplifier 1 and 2) were constructed to scale the laser power to > 1 kW with. Laser output of the amplifier 1 versus the launched pump power at different wavelength is shown in Fig. 3(a). The maximum output power of the four wavelengths were measured to be 475 W, 464 W, 451 W, and 429 W at a launched pump power of 851.5 W, respectively, with corresponding slope efficiency of 55.6%, 53.6%, 52.0% and 49.8%. Compared with the preamplifier, the slope efficiency reduction in amplifier 1 can be attributed to the difference in the cross-relaxation depending on the Tm³⁺ concentration doped in TDF-10/130 and TDF-25/400 [6,27]. Limited by the signal power handling capability of the (6 + 1) × 1 signal-pump combiner in the amplifier 2, the laser power of the amplifier 1 was not further scaled.

 

Fig. 3. (a) Amplifier 1 output power and (b) final output power versus the launched pump power at different wavelength. Inset: output power at 1980nm recorded by a power meter and the maximum output power measured at different wavelengths.

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Figure 3(b) shows the output power of final amplifier with respect to the total launched pump power. One can see that the output power increase linearly and reaches 1023 W, 1039 W, 1010 W, and 985 W for the preselected wavelength at a launched pump power of 1905 W. The output power at 2050nm is further scaled to 1028 W at an increased pump power of 1974 W. The corresponding slope efficiency for the four selected wavelength was 52.9%, 54.3%, 51.9% and 51.5%, respectively. The inset of Fig. 3(b) displays > 1 kW output of the MOPA over a tuning range of 107 nm from 1943 to 2050nm, further extension in operation wavelength, especially to the shorter wavelength side, should be possible with a seed of wider tuning range. It is noteworthy that the laser power obtained in this experiment is limited merely by the available pump power, further power scaling is possible with more powerful 793 nm laser diodes.

Output spectra of the MOPA at the maximum output power are depicted in Fig. 4 (a). As can be seen, the spectra have an OSNR of more than 30 dB without the onset of ASE content in a wide wavelength range. Evidently, two less pronounced spikes are sometimes observed symmetrically located by the two sides of the main peak, which could be attributed to the four-wave-mixing effect [28–30]. The typical evolution of laser spectra versus the output power at 1943nm is illustrated in Fig. 4(b), from which we can see that 3 dB spectral linewidth increased slightly from ∼0.15 nm (seed) to ∼0.54 nm at 1023 W, and the side band of the spectra broadened obviously. This spectra broadening is mainly caused by the nonlinear effects such as self-phase modulation [31].

 

Fig. 4. (a) Output spectra of the wavelength tunable MOPA at maximum power level. (b) Evolution of laser spectrum versus output power at 1943nm.

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Output power stability over the full wavelength-tuning range was characterized at an output power of ∼920 W with a relatively stable laser operation. Typically, the result monitored at 1943nm was shown in the Fig. 5(a). The time-domain RMS value over ∼10 min is calculated to be ∼0.52%. Power stability measurement over longer period of time and higher power was not tried since the LDs were already operated at their full powers. The inset in Fig. 5(a) gives the temporal characteristic of laser signal in ms-class as well as the calculated Fourier spectrum of the signal and noise floor. As can be seen, there is no typical discrete frequency peak in the Fourier spectra of the laser signal within 0-5 kHz, indicating the absence of the parasitic oscillation or self-pulsing effect. The beam propagation factor (M²) at ∼920 W of output power was measured to be ∼1.18 and ∼1.19 in the horizontal and vertical direction, which shows that near-diffraction-limited beam quality has been achieved with the TMI effect being suppressed effectively. The inset of Fig. 5(b) shows a near-field 2-D profile of the laser beam with a near Gaussian intensity distribution.

 

Fig. 5. (a) Signal in time-domain and the Fourier spectrum at 920 W. (b) Beam quality factors of laser output at 920 W and its typical 2-D beam profile.

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4. Conclusion

In conclusion, we report on a high-power widely tunable thulium doped fiber laser with a ∼107 nm tuning range. More than 1000 W output power with a slope efficiency of >51% over the entire wavelength tuning range from 1943 to 2050nm is presented using a monolithic MOPA configuration. The diffraction-limited beam quality factor M² is estimated to be ∼1.18 at an output power of ∼920 W. The temporal characteristics of the laser output confirm a relatively robust laser operation without the onset of the parasitic oscillation and self-pulsing effects, which indicates that further power scaling and tuning range broadening is possible with the combination of the elevated pump power levels and optimal BPF offering wider wavelength tunability. To our knowledge, this is the first report on a widely tunable TDFL with laser power reaching 1 kW, which should facilitate a range of applications.