1. Introduction

The 1650-1750 nm wavelength range is a region of rich spectroscopic features and one that, despite a growing number of promising applications, has remained elusive. Thulium-doped silica glass, when strongly pumped, yields fluorescence spanning the band from 1600 to 2200nm and hence is an attractive gain medium for power scalable laser sources operating in this eye-safe wavelength range. However, the increasing three-level nature of the transition at the short wavelength end of this emission band leads to significant challenges for short wavelength operation, and to date, operation of thulium fibre lasers at wavelengths below 1750 nm has made only modest progress in comparison to the more established longer wavelength operation. Within this wavelength regime a number of chemical resonances can be seen and in particular strong absorption features relating to C-H bond stretch resonance (1st overtone) around 1700 nm are present in many common materials containing hydrocarbons [1]. In the same wavelength range liquid water has a local minimum in absorption that allows preferential targeting of hydrocarbon rich materials. Several promising medical applications have been identified which take advantage of this fortuitous combination. At a wavelength of 1726 nm fat/lipid rich tissues in the human body show local absorption maxima (due to high C-H content). At this same wavelength, soft tissue shows a local absorption minimum (due to high water content). The result is a situation where lipid absorption is stronger than that of the surrounding soft tissue allowing the preferential targeting of lipid rich tissues, such as sebaceous glands. As described in [2,3], this effect can be employed in the treatment of skin conditions such as acne, allowing the preferential heating of these sebaceous glands whilst leaving the surrounding tissue relatively unharmed. For the same reasons mentioned above 172x nm sources also have the potential to be applied to a number of targeted medical applications where preferential heating of lipid rich tissues is desirable. In addition to the above medical applications, several strong absorption features can be present in polymers relating to O-H, C-O and C-H bond resonances. These can be seen across the entire thulium emission band, but reach a maximum strength around 1700 nm. At around 1720 nm C-H stretch overtones result in absorption features several times stronger than that at ~1 μm or ~1.5 μm absorption [4]. At longer wavelengths absorption features due to O-H bonds as well as a number of other resonance features are also seen. These prominent absorption features present the ability to allow more efficient laser welding and processing of plastics without the need for absorption enhancing additives [4,5]. It may also be possible to exploit the differing absorption features within this range to target specific plastics for preferential heating via wavelength selection and tuning. Thulium-doped fibre amplifiers (TDFAs) have generated much interest of late for their potential to help open up a new wavelength window for optical communications, sitting past the long wavelength end of Erbium and within a demonstrated low loss region of photonic band gap fibres [6]. To date, silica TDFAs have been demonstrated across 1650-2050 nm [7–9] with further extension of this short wavelength edge towards that of more established Erbium amplifiers of interest to the telecoms community. In addition to the direct application of sources in this wavelength range, short wavelength thulium can also be used as a high brightness pump source allowing further wavelength coverage into the mid-infrared. Quimby et al, have highlighted 1680 nm as a potential pump source for Dy:fluoride pumping. Here short wavelength thulium silica sources may be leveraged to enable fibre-based sources operating at 3.7 and 4.7 µm [10].

Whilst thulium florescence from the ³F₄ – ³H₆ transition can be seen across 1600-2200 nm in silica, it is in practice difficult to cover extreme ends of this emission band. The combination of varying quasi-three-level behaviour as a function of wavelength and gain saturation effects caused by amplified spontaneous emission from the centre of this emission band, require trade-offs to be made when attempting to access either the long or short wavelength ends of this transition. As a result, to access the shorter wavelengths of the ³F₄ – ³H₆ transition, where signal reabsorption is prominent, a high excitation fraction and a short cavity length are required. This is generally achieved with low thulium doping concentrations and high brightness (core) pumping. This is highlighted in Fig. 1 where the net gain cross section σ(λ_l) is plotted for a range of fractional inversion levels F as given by σ(λ_l) = Fσ_e(λ_l)–(1-F)σ_a(λ_l) where σ_a(λ_l) and σ_e(λ_l) are the absorption and emission cross sections of the laser transition at the wavelength λ_l.

 

Fig. 1 Calculated net gain cross sections of Tm:silica over a range of fractional inversion levels. Successive lines represent increasing fractional inversion levels F in 0.1 increments from 0 (lower curve) to 1 (upper curve). Absorption and emission data taken from [11,12].

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In published literature, the broad wavelength coverage of thulium fibre lasers has been demonstrated many times [13–18]. However to date demonstrations of power scalable short wavelength (sub 1750 nm) thulium fibre sources have been limited. In [14], 2 W of output power was shown at a wavelength of 1723 nm for 20 W of launched pump power (1565 nm) in a slightly multimode configuration. With the exception of this result all other demonstrations have been sub 100 mW level. The shortest operating wavelength achieved from thulium in a silica host is 1650 nm. This was achieved using a core-pumped configuration and a Ti:sapphire pump laser, but the maximum output power was only ~1 mW [19]. In a non-silica host (ZBLAN) thulium lasing and amplification has been demonstrated down to 1636 nm. Here another rare earth dopant (terbium) was added to either the cladding [20] or core [21] of the active fibre to provide preferential loss at wavelengths longer than ~1750 nm and thus reduced the effects of longer wavelength ASE emission. Whilst a very interesting approach, to date demonstrations have been limited to output power and efficiency of <200 µW and <0.2%, respectively [21], ASE filtering was also adopted in [7] and enabled an all fibre thulium silica amplifier which operated from 1650 – 1710 nm. This intracavity ASE filter relies on providing substantial loss at wavelengths > 1750 nm in order to prevent parasitic lasing at these wavelengths. Typically these filters also introduce a substantial loss at the shorter wavelengths <1710 nm and while suitable in small-signal amplifiers, this would drastically reduce the efficiency of a laser source.

The approach presented in this work does not rely on intracavity loss elements in order to access this wavelength region but rather highly wavelength selective Fibre Bragg gratings (FBGs) to enable a power scalable source within this wavelength range.

2. Short wavelength tunable thulium fibre laser

To investigate the short wavelength operation of Tm-doped silica fibre lasers on the ³F₄ – ³H₆ transition, a simple all-fibre cavity was constructed, as shown in Fig. 2. The fibre laser comprised a ~0.6m length of Tm-doped silica fibre with a core diameter of 8 μm and was surrounded by a circular cladding of 100 μm diameter. Using an optical fibre refractive index profiler (IFA-100) the step index equivalent NA for this fibre was measured to be approximately 0.13 - 0.14 and thus the fibre was single mode at both the signal and pump wavelengths. To achieve the high fractional inversion densities needed to operate at short wavelengths a low Tm dopant concentration (~0.2 wt%) was used in conjunction with core pumping by a commercial Er:Yb fibre laser at 1565 nm. The cladding of the fibre was overclad with a high refractive index polymer to remove any cladding light. The active fibre was mounted on a metal heat-sink to facilitate cooling. Pump light was coupled into the Tm fibre using a ruggedized filter type wavelength division multiplexor (WMD) spliced to one end of the Tm-doped fibre gain medium (see Fig. 2). This WDM provided low loss at both the pump wavelength (~0.45 dB at 1565 nm) and signal wavelength (<1 dB from 1650 – 2100 nm) and specified maximum power handling of 5 W. The splice loss between the active fibre and WDM was measured to be approximately 2%. The laser cavity was formed between a narrowband high reflectivity fibre Bragg grating (FBG) spliced to the WDM and, at the opposite end, by the ~3.6% Fresnel reflection from a perpendicularly-cleaved fibre facet serving as the output coupler. The high reflectivity FBG was mounted in a simple mechanical arrangement to allow tuning of the Bragg wavelength by compression [22]. The FBG had an uncompressed central wavelength of 1726 nm with a specified reflectivity of >99% at this wavelength and a 3 dB spectral bandwidth of 0.5 nm. Using this compression tuning scheme, previous reports have shown that an FBG with central wavelength of 1613 nm could be blue-shifted by over 100 nm with only minimal reduction in reflectivity over the majority of this range [23].

 

Fig. 2 Experimental layout for compression-tuned short wavelength thulium fibre laser. FBG: fibre Bragg grating, WDM: wavelength division multiplexor. TDF: thulium doped fibre. P₁ & P₂: thermal power meters 1 & 2. OSA: optical spectrum analyser. PD: photodiode.

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An FC/APC type angled patch-lead was spliced to the opposite end of the fibre containing the FBG (as shown in Fig. 2) in order to eliminate any residual broadband feedback and suppress parasitic lasing between the fibre end-facets. This all-fibre arrangement removes the challenges associated with maintaining a high efficiency fibre-to-free-space coupling ensuring repeatable low-loss wavelength selection in an “all-fibre” format. The output from the Tm-doped fibre laser was collimated onto a dichroic filter to separate the residual 1565 nm pump radiation and generated laser output. This filter was highly reflective at 1500-1630 nm and highly transmissive at 1660-2000 nm. Both residual pump and generated signal were monitored with the aid of thermal power meters. A glass wedge was inserted into the output beam between the dichroic filter and power meter to allow monitoring of the signal wavelength and any parasitic lasing or amplified spontaneous emission (ASE) with the aid of a spectrum analyser, as well as the temporal behaviour using a fast photodiode.

The Tm-doped fibre length (~0.6 m) was selected via a series of cut-back measurements to yield the maximum wavelength tuning without the onset of parasitic lasing when operating at the wavelength extreme, whilst still achieving efficient pump absorption. The fibre Bragg grating was compression tuned from an initial wavelength of 1720 nm down to 1660 nm in 5 nm increments. Over this wavelength range the maximum laser output power and pump power at laser threshold were recorded (see Fig. 3). At a maximum launched pump power of 4 W, the laser produced 1.5 W of signal output at a wavelength of 1720 nm. At this wavelength the residual (unabsorbed) pump power was 0.53 W. When tuning to shorter wavelengths laser threshold increased to 3.6 W of launched pump power (2.4 W absorbed) at a lasing wavelength of 1660 nm. Due to the relatively high threshold and limited power handling of the WDM, output power at this wavelength was limited to 65 mW, and tuning to shorter wavelengths was not possible.

 

Fig. 3 Pump threshold (left axis) and maximum output power (right axis) as a function of operating wavelength.

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The laser output power versus pump power (and corresponding slope efficiency) was measured at 20 nm increments from the uncompressed FBG wavelength at 1720 nm downwards. The results are shown in Fig. 4(a). The laser yielded slope efficiencies of 46% (55%), 40% (51%) and 26% (36%) with respect to launched (absorbed) pump power at wavelengths of 1720 nm, 1700 nm and 1679 nm respectively. At these wavelengths, when operating with maximum pump power, pump absorption was 87%, 79% and 70% of total launched power indicating that pump bleaching due to ground-state depletion becomes more pronounced at shorter wavelengths. This is due to the larger inversion required to achieve transparency at these wavelengths. Due to the high threshold pump power, the laser slope efficiency could not be reliably determined at 1660 nm.

 

Fig. 4 (a) Laser output power versus launched pump power for laser operation at 1720 nm (blue triangles), 1700 nm (red circles) and 1679 nm (black squares). (b) Laser output spectrum when tuned to 1669 nm demonstrating a >40 dB height above ASE background (measured with a 2 nm spectral resolution).

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At a wavelength of 1669 nm the total ASE present in the laser output was estimated to be 0.6% of output power based on the integrated spectra taken from 1650 – 2100 nm (Fig. 4(b)). The peak of the ASE output was seen at ~1830 nm. At wavelengths shorter than 1669 nm the close proximity to threshold resulted in a larger portion of ASE in the laser output.

3. High power operation at 1726 nm

In order to investigate laser performance at higher power levels, the low power handling WDM was removed and pump light was coupled into the Tm-doped fibre directly through a high reflectivity FBG mounted to a heat-sink (i.e. without the mechanical arrangement for compression tuning). In this case, the FBG employed had reflectivity of >99% centred at 1726.4 nm. A fused WDM was introduced to ensure that any thulium emission transmitted past the FBG did not damage the 1565 nm pump. The insertion loss of the WDM at 1565 nm was 0.2 dB and it provided >10 dB coupling for radiation at 1700 – 2000 nm. The active fibre length was increased to ~1.2 m to ensure complete pump absorption. The output coupler was a flat cleave at the output end of the fibre (as shown in Fig. 5).

 

Fig. 5 Experimental layout of monolithic all-fibre Tm laser. FBG: fibre Bragg grating. TDF: thulium doped fibre. P₁ & P₂: thermal power meters 1 & 2. OSA: optical spectrum analyser.

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Using this simplified lower loss cavity we observed an increase in laser slope efficiency (see Fig. 6(a)) to 63% with respect to launched pump (and 67% with respect to absorbed pump power) in comparison to 46% measured for the wavelength tunable cavity configuration. These values corresponded to an output power of 12.6 W at the maximum available pump power of 20.9 W (20 W absorbed). At this pump power the laser showed CW output behaviour. The measured spectral linewidth was 0.07 nm (FWHM) as shown in Fig. 6(b). At maximum output power the laser emission was >45 dB above the ASE background (as measured with a 2 nm spectral bandwidth).

 

Fig. 6 (a) Fixed wavelength laser performance at 1726.4 nm with slope efficiency of 63% with respect to launched pump power and 67% with respect to absorbed pump power. (b) Output spectrum of source with a central wavelength of 1726.4 nm and 0.07 nm (FWHM) spectral linewidth (measured with a 0.05 nm spectral resolution).

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4. Discussion

In this work we have demonstrated a 60 nm extension to the short wavelength operating range of thulium-doped fibre lasers in comparison to previous ~1.5-1.6 µm pumped results. Suppression of parasitic lasing and ASE at longer wavelengths was achieved by using a low Tm³⁺ dopant concentration active fibre in conjunction with high brightness core pumping. High efficiency wavelength selective feedback also facilitated the extension of this operating wavelength range. Strong three-level laser behaviour was observed as demonstrated by the saturation of pump absorption, particularly at the shorter wavelength end of the laser tuning curve. This is indicative of the high level of excitation density needed to reach transparency. This approach is power scalable from the multi-watt to tens-of-watt level as demonstrated by the 12.6 W of output power achieved at 1726 nm, limited only by the available pump power. The addition of a higher reflectivity wavelength selective output coupler (e.g. an FBG) in future device iterations should serve to lower pump threshold at the short wavelength end of the emission band and thus has the potential to allow still shorter wavelength operation.

For the practical use of these wavelengths in materials processing applications it is advantageous to further increase output power. Power scaling Er:Yb pump sources beyond the ~50 W level presents a number of challenges and sources above this output power are not currently commercially available. Thus, for power scaling of short-wavelength Tm-doped fibre lasers beyond the tens-of-watt level an alternative approach based on the use of multiple Er:Yb fibre lasers sources and a cladding-pumped fibre architecture with a relatively small inner-cladding-to-core area ratio will be needed.

5. Conclusion

We have demonstrated the power scalable operation of Tm-doped silica fibre lasers at the short wavelength end of the ³F₄ – ³H₆ transition. Using a compression-tuned FBG feedback element we have achieved continuously tunable laser operation from 1720 nm down to 1660 nm. In this configuration, the maximum output power of 1.5 W for a launched pump power of 4.05 W was limited by the power handling of the pump and signal combiner (WDM). Laser slope efficiencies were at wavelengths of 1720 nm, 1700 nm and 1680 nm were measured to be 55%, 51% and 36% respectively (with respect to absorbed pump power). To demonstrate the power scaling potential of this source, fixed wavelength operation at 1726 nm was also demonstrated. In this approach the WDM was removed and pump light was directly coupled into the active fibre through an FBG allowing higher pump power to be used. Using this configuration, the laser produced up to 12.6 W of laser output for 20.9 W (20 W) of launched (absorbed) pump light. In this arrangement, the laser slope efficiency was measured to be 67% with respect to absorbed pump power. These results represent the highest power operation for either fixed wavelength or tunable thulium sources operating at sub 1750 nm wavelengths reported in the published literature. The use of a higher power single mode pump source in the ~1.5-1.6 µm band should allow scaling to powers in the multi-tens of watt regime without any significant engineering challenges and should benefit a wide range of applications.