1. Introduction

High power and high brightness broadband light sources operating in the 2 μm spectral region have many applications including spectroscopy, gas sensing [1], low coherence interferometry and medical imaging via optical coherence tomography [2]. One attractive way to generate broadband output is via the process of amplified spontaneous emission (ASE) in a rare-earth doped fiber. Fiber sources benefit from a geometry that allows a high degree of immunity from the detrimental effects of heat generation and good output beam quality owing to the waveguiding properties of the active-ion-doped core. Over the last few years there has been very rapid progress in scaling the output power from cladding-pumped Tm:fiber lasers in the two-micron regime [3–6], but rather less attention has been paid to broadband two-micron sources. The first reported Tm-doped fiber-based superfluorescent source employed a tunable Ti:sapphire laser operating at ~ 810 nm as the pump source and generated ~ 1.2 mW of output power at 1.91 μm with a bandwidth of 77 nm (FWHM) and a slope efficiency of ~ 0.3% [7]. Very recently, a ~ 120 mW broadband source, based on Tm-doped silica fiber cladding-pumped by a diode laser at 803 nm was reported with a slope efficiency of ~ 3% and a spectral bandwidth of ~ 30 nm centered at 1960nm [8]. Two-micron superfluorescent sources, in-band pumped at 1.6 μm, based a Tm,Ho-doped silica fiber and a Tm-doped fluoride fiber have also been demonstrated with output powers of 40 mW [9] and ~2.3 mW [10] and with slope efficiencies of ~7% and 15% respectively. However, in spite of recent progress the output powers and efficiencies achieved from Tm:fiber superfluorescent sources are well below those routinely achieved in conventional Tm:fiber laser oscillators. This is primarily due to the difficulty in achieving a single-ended output whilst at the same time suppressing laser oscillation. The standard method for achieving single-ended operation is to use a low feedback fiber end termination (e.g. angle-cleaved or angle-polished facet) at the output end of the fiber and employ a broadband high reflectivity mirror butted to the opposite end of the fiber [8]. This has the attraction over double-ended configurations (i.e. with low feedback terminations at both fiber ends) that all ASE power is extracted from only one end, but suffers from the disadvantage that the threshold for lasing is much lower and hence the maximum ASE power is generally quite low.

In this paper, we report a broadband two-micron Tm-doped fiber superfluorescent source with output power almost two orders-of-magnitude higher than has so far reported at this wavelength regime and with slope efficiency comparable to that routinely achieved in high-power Tm-doped fiber laser oscillators. Single-ended operation of the superfluorescent source was achieved using a simple all-fiber scheme and a maximum output power > 11W was obtained for a launched diode pump power of 40 W at ~ 790 nm. The corresponding slope efficiency with respect the launched pump power was 38%, and the bandwidth (FWHM) of the emission spectrum was ~ 280 nm at low output powers (< 8mW) and ~36 nm at the highest output power.

2. Experiments and results

The experimental arrangement used for the Tm fiber superfluorescent source is shown schematically in Fig.1. The Tm-doped fiber used in our experiments was fabricated using the standard modified chemical vapour deposition and solution doping technique. The resulting fiber had a 25 μm diameter (0.18 NA) Tm-doped alumino-silicate core with a Tm³⁺ concentration of ~1.5wt.% and an Al³⁺:Tm³⁺ concentration ratio of 10:1, surrounded by a 300 μm diameter D-shaped pure silica inner-cladding with a nominal NA of approximately 0.49. The latter was coated by a low refractive index polymer outer-cladding. The effective absorption coefficient for pump light launched into the inner-cladding at 790 nm was estimated, via a cut-back measurement, to be ~ 3.4 dB/m and hence a fiber length of ~ 5 m was used for efficient pump absorption. Pump light was provided by two beam-shaped diode-bars at 790 nm with ~ 28W and 30W output powers. The beam quality factors, M²_x and M²_y, were < ~70 for both diode-bar modules after beam shaping. Pump light was launched into inner-cladding of the fiber with the aid of anti-reflection coated lenses of 30 mm focal length and dichroic mirrors with high reflectivity (>98%) at 785–795 nm and high transmission (>97%) at 1600–2100 nm at 45° degrees to allow extraction of the two-micron ASE light (see Fig. 1). With this arrangement, approximately 87% of the incident pump light (i.e. ~58W) was launched into the fiber.

 

Fig. 1. Schematic diagram of the Tm-doped fiber ASE source cladding-pumped by beam-shaped 790 nm diode-bars.

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The Tm fiber was first tested in a simple laser oscillator configuration with feedback for lasing provided by a perpendicularly-cleaved facet, at one end (B) of the fiber, and, at the opposite end (A), by a simple external cavity comprising a plane mirror with high reflectivity at 2 μm (not shown), and an anti-reflection coated 40 mm focal length collimating lens. Both end sections of the fiber were carefully mounted in water-cooled V-groove heat sinks to prevent thermal damage to the fiber coating due to unlaunched pump power and by heat generated in the core due to quantum defect heating. At the maximum launched pump power of 50.3 W the fiber laser produced 18.2 W output with a corresponding slope efficiency of ~ 41%. In a laser oscillator, the ratio of the laser powers coupled out and/or lost from both ends of the resonator is given by

where R_A and R_B are the effective feedback reflectivities at ends A and B of the resonator respectively. Thus, achieving single-ended operation of in an oscillator is relatively straightforward. Equation (1) is only strictly valid for an oscillator, but also serves as a good guide for a superfluorescent source when operating at output power levels well above the saturation power. Thus, by using end terminations with very low feedback reflectivities at both ends of the fiber, but at the same time ensuring R_B≫R_A, it is possible to achieve a predominantly single-ended output from end A of the fiber. This can be achieved by using angle-polished (or cleaved) facets at both ends of the fiber, but with different angles selected to give different feedback reflectivities at the two ends. In our Tm fiber superfluorescent source this was achieved by simply using an angle-polished facet at 14° to achieve very low feedback reflectivity at end A and a perpendicularly-cleaved facet with a ~ 3.6% broadband Fresnel reflection at end B to dominate over the feedback provided by end A.

 

Fig. 2. ASE output power versus launched pump power for pumping at the perpendicular-cleaved and the angle-polished fiber end.

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To investigate the influence of pump direction on the performance, the Tm fiber was pumped through end A or B using the 30 W diode-bar. The ASE output powers from both ends of the fiber were recorded as a function of pump power and the results are shown in Fig.2. The output spectrum from the angle-polished end (A) was monitored with a monochromator. The output powers and spectral properties at the highest available pump power of 25 W (launched) are summarized in Table 1. It can be seen that the superfluorescent source had a predominantly singe-ended output from end A irrespective of the pumping direction. The ratio of the output powers from the two ends (i.e. P_A/P_B) was ~23 when pumped from end B (i.e. for a co-propagating pump) and ~ 42 when pumped from end A (i.e. for a counter-propagating pump) at the highest pump power. It is worth noting that the latter-configuration yields a higher output power of 4.9 W compared with the co-propagating arrangement which yields a maximum power of 3.8 W. In both cases, the ASE power was limited by the available pump power. The slightly poorer performance for the co-propagating sceheme can be attributed to increased ground-state re-absorption at the un-pumped end of the fiber [11]. In fact, for the same launched pump power, we measured an ASE output power of 2.3 W from the pump launching end and only 1 W from the opposite end of the fiber when both end facets were polished at the same angle. The ASE emission spectrum for the single-ended source was similar for both pumping directions with a centre wavelength of 1960 nm and a bandwidth (FWHM) ~36 – 39 nm at the highest pump power.

Table 1. ASE output power and spectral properties of the Tm fiber superfluorescent source for single-ended pumping with a 30 W beam-shaped diode bar at 790 nm

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To investigate operation at higher power levels, both beam-shaped diode bars were used to pump the Tm fiber at both ends as shown in Fig. 1. The ASE output powers from ends A an B as a function of launched pump power are shown in Fig. 3, and the ASE spectra from the angle-polished-end at various power levels are shown in Fig. 4. The maximum ASE output power generated (without any parasitic lasing) was 11 W at a launched pump power of 40.3W. The corresponding slope efficiency with respect to the launched pump power was ~ 38% and the beam propagation factor (M²) for the ASE output beam was measured to be less than 2.8. Improved beam quality and operation efficiency should be achievable by the use of a modified fiber design with a smaller V parameter and a larger Tm doping concentration to enhance the fortuitous ‘two-for-one’ cross-relaxation process. The centre wavelength of the emission spectrum was ~ 1958 nm and the bandwidth (FWHM) was ~ 36 nm (Fig. 4(b)). At higher pump powers the ASE spectrum developed sharp peaks indicating the onset of parasitic lasing due to residual feedback from the end facets and/or imperfections along the fiber. At the highest pump power (50.3 W launched) the Tm fiber source produced 15 W of output. From Fig. 4 it can be seen that although there is evidence of parasitic lasing the emission is still predominantly ASE. At low powers (< 20 mW) emission spectra was very broad spanning the wavelength range from ~ 1650 nm to ~ 2100 nm. Absorption lines due to water vapour in the wavelength range from ~ 1800 nm to ~ 1950 nm can be clearly seen in the spectra (see Fig. 4(a)). For ASE output powers of 8, 51 and 300mW, the emission bandwidths (FWHM) are 280, 80 and 42 nm, respectively. Thus, the emission bandwidths decrease with increasing output power as expected. For power levels above 300 mW the ASE spectra have a nearly-perfect Gaussian line shape (see Fig. 4) suggesting that Tm-doped fiber superfluorescent sources should be ideal for applications where a Gaussian distribution shape is desired, such as in optical coherence tomography [12]. At the highest available pump power, the output power emitted from the perpendicularly-cleaved facet was <280 mW, hence the corresponding power ratio, P_A/P_B, was >53. Using equation (1) we can estimate the effective feedback reflectivity of the angle-polished facet as ~1.3×10^-5.

 

Fig. 3. Output power versus launched pump power for the single-ended output Tm-doped fiber ASE source.

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We also investigated double-ended operation of the Tm:fiber superfluorescent source using the same set-up as shown in Fig. 1, but with both ends of the fiber angle-polished. At the maximum launched pump power of 50.3 W, the fiber yielded an ASE output power of 8.2 W and 6.8 W from the two ends of the fiber, and we did not observe any evidence of lasing. The maximum combined ASE output power of 15 W was achieved with slope efficiency with respect to launched pump power of 42.5% for pump powers above 10 W. These results suggest that further power scaling of the single-ended superfluorescent source should be possible by using a slightly angled facet instead of the perpendicularly-cleaved facet and by using a higher power pump source.

 

Fig. 4. Single-ended output ASE spectra at different output power levels.

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3. Conclusion

In conclusion, we have demonstrated an efficient high-power single-ended broadband Tm:fiber superfluorescent source at 2 μm with a simple, novel all-fiber geometry. 11 W of ASE output was obtained for a launched pump power of 40 W, corresponding to a slope efficiency of 38% with respect the launched pump power. Further scaling of the output power should be possible by further reducing the effective optical feedback from the fiber end facets and by employing a higher power diode pump source. The availability of high power broadband sources in 2 μm spectral region with good beam quality will benefit a range of applications.