1. Introduction

Thulium doped fiber laser (TDFL) [1–4] around 2 μm wavelength region attracts great attention due to its superior properties such as wide tunable range, good beam quality, simple thermal management and high nonlinear threshold. TDFL operates in the eye-safe region, and it has important applications in bio-medical treatment, remote sensing, spectroscopy and nonlinear frequency conversion. For example, many animal/human tissues have absorption bands between 1.94 μm and 2.05 μm [5, 6], so that lasers operated at these wavelengths could be used for tissue-welding efficiently. High power lasers at 1.95 μm could be used as high-efficient pump source for holmium doped lasers as they match well with the absorption peak of holmium-ion [7, 8]. Further, TDFLs also provide excellent overlap with the abundant molecular vibration absorption resonances of many gases such as water vapor and CO₂ [9] around 2 μm, so that they are very suitable for coherent Doppler wind LIDAR and spectroscopy. TDFLs are also used as outstanding pump sources instead of bulk lasers for mid-infrared optical parametric oscillators [10, 11] benefited from its favorable beam quality and high conversion efficiency.

In general, a high power TDFL with widely tunable output wavelength is required in many application areas [12]. Spectral controlling of a TDFL could be realized by taking advantages of fiber Bragg gratings (FBGs) [9, 13], volume Bragg gratings (VBGs) [12, 14], diffraction gratings [15] and Fabry-Perot tunable filter [16]. Since the demonstration of ultra-broad wavelength tunability in a cladding pumped TDFL from 1860 to 2090 nm in 2007 [17], many tunable TDFL sources around 2 μm are available. Shen et al. [18] studied the wavelength tunability of TDFLs in both the core pumped and cladding pumped regimes, with a recorded lasing wavelength tuning range from 1723 nm to 2061 nm. McComb et al. [12] reported an over 150 W wavelength-tunable TDFL based on a volume Bragg grating. However, the configuration of a single oscillator is not suitable for high-power operation limited by the weak power handling capability of optic components. Fortunately, there is the well-known master oscillator power amplifier (MOPA) configuration which offers a better way for further power scaling in fiber lasers [19–21]. In 2008, Pearson et al. [22] reported a high power wavelength-tunable MOPA system producing over 100 W of linearly-polarized output from 1820 nm to 2010 nm.

However, all the above demonstrations of wavelength tunability in TDFLs [12–15, 17, 18, 22] employed bulk optic components where gain fibers were pumped via fragile free-space coupling. It has been demonstrated that the great advantages of all-fiber configuration would offer ultra-high compactness and reliability of fiber lasers [16, 19, 23–26]. For all-fiber wavelength-tunable TDFLs, Geng et al. [16] demonstrated the first all-fiber wavelength-swept laser at 2 μm based on a fiber Fabry-Perot tunable filter with output wavelength tuned over 200 nm. Fang et al. obtained a wavelength-tunable mode-locked TDFL with over 50 nm tuning range by incorporating a fiber taper filter as the wavelength selector [26]. In 2013, Li et al. [24] reported a direct diode-pumped monolithic TDFL offering more than 250 nm continuous tuning range. Up to now, the output power of these all-fiber wavelength-tunable TDFL sources either continuous wave or pulsed operation are only on the miliwatt level. In order to meet more actual applications, it is essential to further scale up the output power of an all-fiber wavelength-tunable TDFL with the above mentioned MOPA configuration.

In this paper, an integrated high-power all-fiber TDFL with output wavelength tunable from 1940 nm to 2070 nm is reported. This TDFL adopts on a configuration of a MOPA system, with a tunable filter based TDFL as the master oscillator and a two-stage cladding-pumped thulium doped fiber amplifier (TDFA) for power scaling. The maximum output power of the MOPA system reaches over 100 W with superior spectral characteristics, for instance, the measured optical signal to noise ratio is better than 54 dB at 2050 nm. It is believed that this result represents the highest output power from an all-fiber wavelength-tunable TDFL reported to date.

2. Experimental setup

The experimental arrangement of the high-power all-fiber wavelength-tunable TDFL is depicted in Fig. 1.The MOPA system is comprised of a master oscillator details shown in Fig. 1(a), a pre-amplifier and a main-amplifier. The master oscillator has a ring cavity with a 3 m long double cladding thulium doped silica fiber (TDF-10/130) as the gain fiber, pumped by a 4.3 W multimode 793 nm laser diode (LD) through one pump port of a (2 + 1)$\times$1 fiber combiner. The TDF-10/130 has a core numerical aperture (NA) of 0.15 and a cladding absorption coefficient of ~3 dB/m at 793 nm.

 

Fig. 1 A schematic showing of the experimental setup. (a) The tunable TDFL. (b) The MOPA configuration.

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In order to tune the output wavelength of the master oscillator, a manual grating-based tunable filter is inserted into the ring cavity. The 3 dB bandwidth of the filter is measured to be ~2.8 nm with an amplified spontaneous emission (ASE) source. A polarization-independent optical isolator (ISO1) is spliced between the TDF-10/130 and the filter to ensure the unidirectional propagation of light. A 20:80 optical coupler (OC1) is used to extract the output light. Before the subsequent amplifier chain, another 20:80 optical coupler (OC2) is inserted to monitor the seed wavelength and power generated from the master oscillator.

The pre-amplifier is constructed with a piece of 6 m long TDF-10/130, forward pumped by a 10 W 793 nm LD through a (2 + 1)$\times$1 fiber combiner. Possible counter-propagating light from the main amplifier is blocked by an optical isolator (ISO2). The main amplifier is a large-mode-area (LMA) TDFA, in which six high-power 793 nm LDs, a (6 + 1)$\times$1 fiber combiner, a 4 m long of double clad TDF (TDF-25/250) served as the gain fiber and a home-made pump dumper are employed. The TDF-25/250 has a core/cladding diameter of 25/250 μm, an effective core/cladding NA of 0.11/0.46 and a cladding absorption coefficient of ~9.5 dB/m at 793 nm. Therefore, the total absorption at 793 nm is calculated to be as high as 38 dB which ensures efficient absorption of the pump light. The pump dumper can dump out not only the residual pump light, but also the signal light propagated in the fiber cladding. Goodno et al. [20] pointed out that due to the combination of high pump absorption, quantum defect and nonradiative losses, heavy heat would be loaded along the LMA gain fiber. So that during the experiment the TDF-25/250 is coiled onto an aluminum spool with a diameter of 15 cm and water-cooled to 15 °C to avoid possible thermal damage. The output fiber end of the pump dumper is angle cleaved at 7° to avoid optical feedback (~4% Fresnel reflection).

3. Results and discussion

Figure 2 shows the wavelength tunability of the master oscillator. The operating wavelength could be continuously tuned from 1920 nm to 2082 nm exceeding a range of 160 nm. Figure 2(a) depicts typical output spectra measured by an optical spectral analyzer (Yokogawa, AQ6375) with a spectral resolution of 0.05 nm. The optical signal to noise ratio for each spectrum is better than 45 dB. The measured maximum output power with respect to the operating wavelength is shown in Fig. 2(b). Up to 270.9 mW average power at 1960 nm is obtained from the master oscillator with a slope efficiency of 10.8%. The relatively low slope efficiency of the master oscillator originates from the high insertion loss of the tunable filter (~3.5 dB at 2000 nm), the loss of the combiner, and the insertion loss of the isolator. It is observed that the output power decreases at both longer and shorter operating wavelengths, depending on the net-gain profile of the gain fiber. The power profile is not symmetrical due to the strong re-absorption of the generated lasing at the short wavelength side [12]. It has to be noted that the output power and wavelength tunability may be further scaled up or extended by coupling more pump power into the oscillator or by optimizing the gain fiber length [13].

 

Fig. 2 Output characteristics of the master oscillator. (a) Output spectra at different wavelengths. (b) Output power profile.

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In the amplification experiment, seed lasers at several typical wavelengths are used. Detailed power and spectral characteristics of the MOPA system can be founded in Table 1.After the pre-amplifier, nearly 3 W average output power are obtained at these wavelengths. The highest slope efficiency of the output power with respect to pump power in the pre-amplifier is 46.1% at 1960 nm, corresponding to a highest output power of 3.61 W. The reasons for the decreasing of output power and slope efficiency at longer wavelength and shorter wavelength are different. Performance of the pre-amplifier is limited mainly by the lower thulium emission cross section at the longer wavelength side, and limited by the re-absorption of thulium-ion at the shorter wavelength side. This principle applies to the main-amplifier as well.

Table 1. Power and Spectral Characteristics of the MOPA System

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From Table 1, one can see that the slope efficiencies in the main-amplifier are much higher than those in the pre-amplifier. This can be attributed to the different concentrations of thulium-ions in TDF-25/250 and TDF-10/130 because that the well-known cross relaxation process will be enhanced with a higher thulium-ion concentration [27]. In the main-amplifier, maximum output power is 115 W at 1950 nm with 229 W pump power. As shown in Table 1, all the final output power exceed 100 W in the range from 1940 nm to 2050 nm. The output power at 2070 nm is a lower than 100 W because of its comparatively low slope efficiency.

The output power and spectral characteristics with seed lasers at 1950 nm, 2000 nm and 2070 nm are plotted in Fig. 3.The output power increases almost linearly with the increasing of the 793 nm pump power shown in Fig. 3(a). No decrease trend of the output power is observed during the amplification, meaning that output power could be further enhanced by coupling more pump power into the main-amplifier. Output spectra of the MOPA system at all these typical wavelengths are measured during the experiment. The output 3 dB spectral bandwidth is less than 200 pm across all the tuning range and listed in Table 1. For signal wavelength tuned between 1960 nm and 2050 nm, neither ASE noise nor any nonlinear spectral structure is measured. All measured optical signal to noise ratios in this range are better than 55 dB indicating a favorable performance of our MOPA system because that no particular noise reduction methods are used. For seed lasers at the wavelengths of 1940 nm, 1950 nm, and 2070 nm, slight ASE noise arise. Figure 3(b) plots the three output spectra where the seed wavelength is tuned at 1950 nm, 2000 nm and 2070 nm, respectively. It is observed that the measured signal to noise ratios after the main-amplifier are still better than 47 dB when the wavelengths of seed lasers are 1950 nm and 2070 nm. For seed laser at the wavelength of 1940 nm, the output ASE noise is enhanced compared to the one when the seed wavelength is 1950 nm. The optical signal to noise ratio decreases to only 33 dB when the seed wavelength is 1940 nm. This result could be attributed to the strong re-absorption process in the main-amplifier because that the length of the gain fiber is not optimized in the present experiment.

 

Fig. 3 Output characteristics of the main-amplifier at the wavelength of 1950 nm, 2000 nm and 2070 nm. (a) Output power versus the pump power. (b) Output spectra.

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Since the master oscillator could be tuned at much shorter and longer wavelengths, we also try to amplify seed lasers at these wavelengths. At the short wavelength side, seed laser with central wavelength of 1930 nm is used. The measured spectrum has a strong broadband ASE noise centered at 1980 nm and the slope efficiency in the main-amplifier is relatively low which is linearly fitted to be only 36% at low pump power. When the pump power is increased up to ~70 W, unstable lasing spikes [28] emerges around 1980 nm shown in Fig. 4(a) and the stability of the system is deteriorated. Considering that the back propagated ASE and lasing spikes are detrimental to the MOPA system, no more pump power are coupled into the main-amplifier. At the long wavelength side, seed laser at 2080 nm is used to make another trial. The average output power from the master oscillator is only 17 mW, and it increases to 1.78 W after the pre-amplifier. Similar lasing spikes are observed in the main-amplifier at a pump power of ~110 W as shown in Fig. 4(b). The obtained maximum output power before self-lasing is 29.1 W at a pump power of 91 W, and the corresponding slope efficiency is calculated to be 31.0% which is lower than 40.1% at 2070 nm. The relatively low slope efficiency at 1930 nm and 2080 nm, the low seed power and the lack of spectral narrowing components in the amplifier chain, are the main reasons for these lasing spikes. It should be noted that high power operations at 1930 nm and 2080 nm are also possible in an optimized MOPA system which will be our future work.

 

Fig. 4 Output spectra of the main-amplifier at the wavelengths of (a) 1930 nm and (b) 2080 nm.

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4. Conclusion

In conclusion, a high-power integrated TDFL operated with widely tunable wavelength is presented in this paper. More than 100 W average output power with excellent spectra tuning from 1940 nm to 2070 nm are experimentally demonstrated. The output power of the TDFL can be further enhanced with higher pump power. This is, to the best of our knowledge, the first all-fiber wavelength-tunable TDFL with output power exceeding 100 W. We believe that the obtained results are of great interest and may open up new prospects of many application areas.