Various applications, specifically those with a medical focus, have a great need for laser sources operating from 1.7 to 2.1 micrometers. For example, significant works with lasers were done in dermatology, cardiology, gynecology, urology and nephrology [1–3] and conclusion from this field of application shows that for most applications better results were achieved by a laser in a near 2 μm wavelength, because they had penetration depth lower than 0.5 mm, in comparison with 1 μm lasers which had a penetration depth up to 2 mm . A pilot study of 15 patients who underwent zero-ischemia LPN using laser emitting at 2011 nm showed minimal blood loss, negative tumour margins, and preservation of renal function . Thulium-doped fibre lasers (TDFLs) are compact continuous or pulsed solid state lasers that typically emit a wavelength of 1900–2040 nm and penetrates tissue to the depth of 0.5 mm . Apart from medicine, the TDFLs at about 2 micrometers are becoming important tools also for many other applications ranging from chemical sensing, material processing, optical fibre component manufacturing, to defence and telecommunications. The TDFLs are the efficient source used to pump of holmium-doped active media [7, 8] and praseodymium thin disk lasing media . TDFLs offer high efficiency and high output power while reducing the risk of damage to the retina, as 2 μm radiation is highly absorbed in water compared to 1 μm radiation of Yb fibre lasers. Despite the fact that TDFLs require novel components to be designed and developed [10–12] and the thermal management issues are more serious than in ytterbium doped fibre lasers, the TDFLs are gradually becoming a mature type of the fibre laser [13–17].
Wideband light sources at wavelengths around 2 micrometers are of increasing interest in many applications like in TDFLs component fabrication , skin melanoma diagnostics , and ophthalmology . There are a number of important chemical substances, such as carbon dioxide or water vapour, that have significant absorption bands around 2 micrometers and this can be used as fingerprints for the detection of these substances and for the determination of their concentrations, as well. Namely, high brightness, stable and broadband or tuneable sources in this spectral region are desired for such sensors of chemical substances. Apart from sensor systems, wideband sources could be used in manufacturing or testing components for a 2-micrometer spectral range. The first broadband source based on an amplified spontaneous emission (ASE) in a thulium-doped fibre (TDF) was reported by Oh et al. in multicomponent silicate fibre with 1 mW output power and 77 nm bandwidth centred at 1991 nm . Relatively high slope efficiency of 15% of the TDF ASE sources has been soon after demonstrated experimentally in a low-phonon energy host material of the fluoride-based TDF . Ultra-broad emission spectra were demonstrated in doubly-doped fibres, e.g., thulium- and ytterbium co-doped fibre with about 140 nm full-width at half-maximum (FWHM)  and thulium- and bismuth doped fibre with 167 nm wide spectrum  where energy transfer mechanisms between the dopants may extend the output spectrum and/or the pump wavelength range [24, 25]. However, the total output power of these experimental results was very low, less than 100 μW. The record bandwidth achieved so far in a TDF ASE source was achieved in a thulium- and holmium-doped fibre with the so called 10-dB bandwidth of 645 nm . The emission from the thulium ions was promoted at one of the fibre ends while the emission from holmium ions prevailed at the other end. The two outputs are combined together by an appropriate wavelength division multiplexer. In fact, this device acts similarly to a dual stage ASE sources with combined outputs [27–29]. The pumping schemes of TDF ASE sources include both core-pumping and cladding pumping. Recently, in-band core pumping was demonstrated in broadband sources with about 70 nm FWHM and 20 mW  and 40 mW  output powers. The cladding pumping arrangement with pump at 790 nm resulted in the output power of 120 mW and FWHM of 40 nm . Remarkable progress has been achieved in the high-power double-clad TDF superfluorescent sources, where the output power is optimized, rather than the spectral width. Output power of 11 W, slope efficiency of 38% and FWHM of 35 nm was achieved in a configuration with bulk-optics pumping  and in an all-fibre master-oscillator power-amplifier configuration the output power of 25 W, slope efficiency of 49 % and FWHM of 22 nm was achieved .
In this paper we report on methods for optimization of broadband sources based on ASE in TDF. In the following section of this paper we present an analysis of the spectrally flattened TDF ASE source. Numerical analysis of possible spectral flattening of TDF ASE sources has not been published yet, to our knowledge. Theoretical analysis of the TDF ASE source appeared in two papers [35, 36] where the numerical model was used to reveal the physical understanding of the power evolution in ASE sources based on TDFs rather than for optimization issues. The first paper contains a brief analysis that resulted in preference of the forward (in terms of the pump radiation propagation) ASE that should carry the most of the optical power. On the contrary, the latter analysis leads to the opposite findings that it is the backward ASE that carries the highest power in the most typical configurations. In the third section of this paper we present preliminary experimental results of TDF ASE source using TDF developed in house. TDF characteristics including its performance in ring-laser setup are included.
2 Numerical modelling of a Tm-doped fibre ASE source
For predicting the performance of various thulium doped fibre devices and their optimization, we developed a comprehensive, spectrally and spatially resolved numerical model, which is described in detail elsewhere [37–39]. For numerical simulations we set thulium ion concentration not higher than 1000 ppm mol. The pair-induced quenching processes among neighbouring thulium ions can still be assumed negligible at this concentration level provided that the thulium ions are homogeneously distributed in the core and not in clusters . The emission and absorption spectra are taken from Ref. 37. The other parameters of the fibre used in the simulations are summarized in Table 1. The configurations of the broadband source for the numerical modelling are shown in Fig. 1. The pump source at 1611 nm is assumed because this wavelength is the closest wavelength to the Tm peak absorption at around 1630 nm and in the same time it falls within the amplification range of commercially available L-band EDFAs. In the case of in-band pumping at 1611 nm and neglecting the cooperative up-conversion processes thanks to the limited concentration of thulium, the numerical model of TDF with a rich energy level structure [37, 38] is simplified to a system with two energy levels.
Evolution of pump and ASE optical powers and relative population of the metastable level along the fibre is shown in Fig. 2 for the configurations with and without a mirror. The pump power level was set to 1 W. The pump is almost fully absorbed within the first meter of the fibre and the level population is close to zero beyond z = 1 m. For the configuration without reflection mirror (dashed lines in Fig. 2), the blue-edge of the spectrum of the forward ASE (FASE) is reabsorbed where a level population is low and the spectrally integrated FASE power is stagnating. The backward ASE (BASE) grows steadily towards the pump input end, benefiting from increasing pump power towards this fibre end. The BASE power may become strong enough to saturate its gain and lower the inversion population, as it was the case for the pump power level of 1 W shown in Fig. 2. Attachment of a mirror with spectrally flat reflection R(λ) = 1 at the z = L forms a seed for the BASE. Such an ASE seed promotes amplification of the BASE waves that deplete even more the inversion population at the beginning of the fibre, thus lowering the FASE power. The ASE output power vs. pump power for the two configurations and spectra corresponding to the pump power level of 1 W are shown in Fig. 3. As expected, the configuration with the mirror provides higher output power of the broadband source in one fibre pigtail than the mirror-less configuration. The higher output power is at the expense of narrower spectra. Notably, the blue edge of the ASE spectrum is suppressed due to reabsorption in the depleted part of the fibre close to the mirror end.
It should be noted out that the threshold pump power of the linear increase of ASE power depends on the fluorescence lifetime of the metastable level. The dependences of the ASE output power vs. pump power for different host materials are shown in Fig. 4. The other parameters were intentionally left the same in order to point out the effect of the host material. The longer fluorescence lifetime significantly decreases the pump power threshold while having almost no effect on the slope. These trends are similar to the case of two-level fibre lasers where analytical expressions for the slope and threshold can be derived analytically . The materials with low-phonon energy, like in the case of fluoride glass ZBLAN, are known for an excellent quantum conversion efficiency of almost 100% of the radiative transitions form the level. But fluoride fibres suffer from hygroscopicity, high cost and it is difficult to splice them with conventional silica fibres. On the other hand, silica-host materials pose higher phonon energy. Therefore, the quantum conversion efficiency is significantly lower, e.g., about 10% for the pure silica host. Figure 4 points out the importance of hosts with longer metastable levels of thulium for low- and medium- power applications like the broadband sources studied in this paper. TDFs with enhanced life times of 400–700 μs have been reported in modified silica glasses . A promising way to enhance the fluorescence lifetime is to modify locally thulium environment by the ceramic nanoparticle doping and MCVD methods [41–44].
The broadband sources are not being optimized only in terms of an output power but also in terms of the ASE output spectrum. Typically, the flat or Gaussian shapes are required for component characterization and optical coherent tomography systems; or one may require such a spectrum that balances the responsivity of the detector used [26, 29]. In the following we present numerical optimization of the spectral shape of a band-stop filter in the setup in Fig. 1 in order to get the widest flat spectrum of the ASE source. We adopted an approach similar to the one presented earlier by Paschotta et al.  where the double-pass ytterbium-doped fibre ASE source is equipped at one end of the ytterbium-doped fibre with a bulk-grating-based filter and a mirror. In this method, the FASE output was collimated and sent to the mirror through a pair of gratings to disperse spatially the FASE spectrum. The dispersed spectrum traversed a mechanical comb made by a set of screws that protrudes the beam. The filter spectral shape can be finely tuned by a screwdriver; the higher protrusion of the screw into the beam, the higher the attenuation at the wavelength corresponding to that screw. On the contrary in our study, we use all-fibre components for spectral filtering so that the filter would be made by a combination of specially designed wavelength division multiplexer  and a mirror; by cascade of long-period fibre gratings ; or by a thin-film dichroic filter deposited on the perpendicularly cleaved fibre end, see Fig. 1. We tested the applicability of this approach in a TDF ASE source by means of a numerical model. The optimal spectral shape of the filter was searched iteratively. Starting shape mimics that of the emission spectrum and it is gradually modified unless the change of the full-width at half maximum of the output spectrum is less than a pre-set value. Examples of iterative steps are shown in Fig. 5, as well as the corresponding FWHM and output power values. The output spectra of the
ASE source with and without filter are compared in Fig. 5(b). The FWHM can be broadened from 55 nm for the case without filter to more than 140 nm for the optimized filter shape. It means that the FWHM can be more than doubled at the expense of only little decrease of the output power.
3 Tm-doped fibre characteristics and performance in a ring-laser broadband ASE source
The thulium-doped preform was fabricated in house by using the MCVD (Modified Chemical Vapour Deposition) and solution doping methods. The fibre of 125 μm outer diameter was drawn from the preform. The fibre core had approximately step-index profile with a diameter of 7 μm and a numerical aperture of 0.17. The concentration profile of thulium followed that of the core refractive-index. Spectral shape of the level ground state absorption measured by cut-back method is shown in Fig. 6(a). The background losses were less than 0.1 dB/m. The attenuation due to absorption of Tm3+ ions is given by the product 4.34 Γσa(λ) NTm where Γ is the overlap factor accounting for the overlap of the guided optical mode with thulium ions. Overlap factor Γ can be calculated using the known refractive index and concentration profiles  and is of 0.95 at 1650 nm. Using the cross section at the peak ground state absorption of the level σa (1650 nm) = 4.4×10–25 m2  we determined that the thulium concentration was 6.3×1025 m–3. It is about 1500 ppm mol Tm3+ assuming the density of the alumina-silicate core of 2.2 g/cm3 . The lifetime of 480 μs was determined from the fluorescence decay measurements. The normalized emission of the thulium-doped fibre is also shown in Fig. 6(a). The emission was measured in backward direction with respect to the pump at 1620 nm and using the WDM 1.6/2.0 μm. Note that the increased noise in the blue edge of the emission spectrum is due to the correction to the WDM 1600/2000 nm spectral transmission used in the measurement setup. Minimum WDM transmission was at 1610 nm. The spectral ripples in the emission spectrum in the interval of 1810–1930 nm were caused by water vapour absorption within the spectrum analyser, i.e., these ripples are not inherent to the TDF emission.
Fibre performance was tested in a ring laser cavity shown in the inset of Fig. 6(b). Length of the fibre was 1.8 m. In the setup we used the fused fibre components, i.e., the WDMs and the output coupler that were developed within a project EYESAFE2u sponsored by the Ministry of Industry and Trade of the Czech Republic (project No. FR-TI4/734). The fused fibre components fabrication and characteristics are described in detail elsewhere . The laser output characteristics we measured for 90% and 10% output coupling, see
Fig. 6(b). The laser threshold of 170 and 290 mW and the slope efficiency of 32 and 4.4 % were found for the 90% and 10% output coupling, respectively. The higher slope efficiency was found for the case of higher output coupling as expected. Nevertheless, the slope efficiency is lower than the value of about 80% imposed by the quantum defect that is given by the ratio of the pump and the laser signal wavelength. This is mainly attributed to the insertion losses of the components used in the setup. Indeed, the overall insertion losses in the setup were estimated as high as 2 dB. Despite higher cavity losses and resulting lower slope efficiency, the ring-laser setup has an advantage of better stability thank to unidirectional propagation of the laser signal. In addition, the Tm3+ concentration is high enough to promote the cooperative upconversion to a 3H4 level that would lead also to decrease of the laser slope efficiency.
The broadband ASE source was tested in the setup shown in the inset of Fig. 7(a). The fibre length was 3 m. The output power of the ASE source vs. the input pump power at 1550 nm is shown in Fig. 7(a). The output spectra for the pump powers of 0.73 and 1 W are shown in Fig. 7(b), as well as spectra of the backward and forward ASE when the mirror is removed from the setup. For the pump power level of 1 W, the peak wavelength, the 3-dB and 10-dB spectral widths of the double-pass arrangement were 1915 nm, 66 nm and 157 nm; while for the single-pass arrangement the corresponding values were 1842 nm, 60 nm, and 129 nm, respectively. Although no special spectral filter was used, the ASE spectrum was wider compared to the setup without mirror. It should be noted that most typical case is that double-pass ASE source built by the attachment of the mirror at one of the fibre ends has opposite effect. The double-pass configuration leads to higher output power, but a narrower spectrum. It means that by proper selection of the pump power and fibre length one can optimize the width of the broadband source. Note that in our case the mirror promoted the longer-wavelength edge peak defined by the reflected FASE seed. The spectral shape of the FASE is shifted towards shorter wavelengths than the corresponding part of the ASE spectrum with attached mirror. This is an evidence of further reabsorption of the ASE seed (reflected by the mirror) in the depleted part of the thulium-doped fibre. The depleted section of the fibre and its length shaped the output ASE spectrum. The effect of ASE spectrum widening by combination of two distinct ASE peaks is somewhat similar to the widening of the ASE spectrum of ytterbium-doped double-clad fibres . In contrast to the case described in our paper, the ytterbium ASE source was a single pass broadband source, i.e., without mirror, and the two distinct spectral peaks corresponded to typical ytterbium fibre laser wavelength regions around 1030 nm and 1070 nm.
We have described two methods of increasing the bandwidth of ASE source based on TDFs. Firstly, we have shown by means of the comprehensive numerical model that the FWHM of the ASE source can be more than doubled by using specially tailored spectral filter placed in front of the mirror in a double-pass configuration of ASE source. To our knowledge, it is the first report on numerical optimization of TDF ASE sources. Secondly, we reported initial results of experimental TDF ASE source, including fibre characteristics and performance of the TDF in fibre ring laser. We observed a counter-intuitive effect in the experimental ASE source that the spectrum in a double-pass configuration is broader than the spectrum of a single-pass ASE source. The broadening can be explained by the combination of shorter wavelength spectral peak of BASE and longer wavelength peak of the reflected FASE seed. By the proper combination of pump power and fibre length the two peaks may be set to similar optical powers and, in result, the output spectrum is broadened. The experimental and numerical optimization of the second approach is beyond the scope of this paper and it is a prospect of future work. It should be noted that numerical optimization of such device will require accurate spectroscopic characterization of the actual TDF, including absorption and emission cross-section spectra.
The TDF ASE sources offer higher stability and lower complexity and costs compared to other broadband sources, namely the supercontinuum sources that are based on ultrafast, quasi-continuous lasers with spectrum broadened in highly nonlinear fibre. Thus, the TDF ASE sources can substitute the SC sources in applications around 2 μm.
The authors thank Simon Hutchinson for careful reading of the manuscripts and his helpful comments. The authors acknowledge the company SQS Fibre Optics, Czech Republic, for cooperation in the development of fused fibre components for the spectral region around 2 μm. The research was supported by the Agency for Healthcare Research of the Czech Republic, under project No. 15-33459A.
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