1. Introduction

There is a requirement for higher power, compact, efficient laser sources in the mid-IR wavelength region (3-5 μm) for defence applications such as infrared countermeasures. To date the highest average power solid-state mid-IR sources demonstrated have been based on the frequency conversion of a pulsed 2.1 μm Ho:YAG laser [1, 2]. These systems have produced up to 22 W in the mid-IR from 40 W of q-switched Ho:YAG output [1]. Power scaling of these systems is limited by thermal effects leading to the degradation of beam quality at high average powers.

Fibre lasers provide a platform for achieving high average power operation with high beam quality. Thulium doped fibre (TDF) lasers operating at 2.05 μm have demonstrated 608 W from a single amplifier stage [3, 4]. The replacement of a Ho:YAG laser with a pulsed thulium fibre source thus offers an attractive path to the further power scaling of mid-IR sources. The challenge for the efficient frequency conversion of fibre lasers is achieving the required pulse energy and peak power while maintaining a narrow spectral width suitable for efficient frequency conversion.

The non-linear crystals used in the parametric oscillator of high power mid-IR lasers have typically been Type I critically phase matched Zinc Germanium Phosphide (ZGP) [1, 2] and more recently Orientation Patterned Gallium Arsenide (OP:GaAs) [5, 6]. For efficient operation, in practical terms, these materials require a linearly polarised pump source with pulse energies of 0.3-2 mJ and pulse durations of 20-100 ns.

Several groups have demonstrated sub-100 ns pulses with energies of 200-400 µJ from a thulium doped, double clad, polarisation maintaining (PM), large mode area (LMA) fibre amplifier with a 25 μm diameter core and a 400 μm cladding [7, 8]. At these energies, ~30 ns pulses are required to achieve the peak power necessary for efficient frequency conversion. The LMA amplifier can provide 10-13 dB of gain while operating efficiently; consequently 10-20 µJ seed pulses are required to extract the stored energy. Furthermore current commercially available high power isolators at 2 μm have around 2 dB insertion loss. Thus an ideal master oscillator for a mid-IR source power scaled using a fibre amplifier needs to produce linearly polarised pulses with ~30 μJ pulse energy and ~30 ns pulse length at a repetition rate of several 100 kHz.

Gain switching of thulium fibre lasers provides an efficient all-fibre method for realising a master oscillator for such an amplifier. Gain switched operation of a thulium fibre laser by core pumping with a pulsed 1.55 µm source has been demonstrated in several systems [8–10]. These systems were pump limited to <1 W average power, with pulse energies of <10 µJ and produced an unpolarised output. These solutions require an intermediate amplification stage which will potentially add more noise into the system and increases susceptibility to nonlinear effects.

Our approach is to scale the output power of the 1.55 μm pump source and consequently the master laser. We present a monolithic ‘all-fibre’ polarised source that either meets or exceeds these requirements, producing a polarised output of 30 ns pulses with 35 µJ pulse energy at repetition rates up to 300 kHz.

2. Experiment

The laser system consists of four distinct subsystems, a 1.55 μm seed diode, two stages of amplification at 1.55 μm and a thulium gain switched fibre laser operating at 2.044 µm. A schematic of the laser setup is shown in Fig. 1 .

 

Fig. 1 Schematic of 1550 nm pulsed pump source and gain-switched Thulium fibre laser. Also shown is the 90° splice to the strain tuned output coupler (OC) which enables single polarisation operation.

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The seed laser consists of a single mode 1.55 μm diode (QPC BrightLase) which is driven by an 8 A, 100 ns current pulse. This produces 100 ns pulses with peak powers of 1 W. The high peak power of this seed source enables efficient amplification directly in double clad Er:Yb doped fibre (EYDF). With a lower gain required, fewer amplification stages are needed removing the need for an erbium doped fibre preamplifier. This simplifies the architecture, reduces the fibre lengths and associated nonlinearities and reduces the potential loss of stored energy due to amplified spontaneous emission (ASE).

The first stage 1.55 μm amplifier consists of two dual stage isolators which protect the seed diode from back propagating pulses followed by 11 m of Er:Yb double clad fibre with a 6 μm core and a 125 μm cladding (Nufern). This is pumped in the counter propagating direction by two fibre coupled 10 W 940 nm single emitter diodes (JDSU) using a two port pump combiner with a signal feed through (Avensys).

The second stage 1.55 μm amplifier again consists of an isolator to reduce back propagating pulses and ASE followed by 4 m of Er:Yb double clad fibre with a 12 μm core and 130 μm cladding (Nufern). This stage is pumped in the co-propagating direction by two 25 W 940 nm fibre coupled single emitter diodes (Oclaro) using a two port combiner with a signal feed through (ITF).

The pulsed 1.55 μm output pumps and gain switches a thulium laser. All of the fibre in the thulium laser has a PANDA polarisation maintaining (PM) structure, a 10 μm diameter core and a 130 μm diameter cladding (Nufern). The laser consists of a high reflectivity fibre Bragg grating (FBG) written into PM passive photosensitive fibre which is spliced to ~20 cm of double clad PM thulium fibre. This in turn is spliced with a 90° rotational offset of the stress-rods to an output coupler grating, also written into passive photosensitive PM fibre. Output coupler reflectivities of 5%, 8% and 10% were used. A cavity is formed for only one polarisation mode at one wavelength as shown in Fig. 2 [11–14]. As a result the laser operated on a single spectral line and the output was robustly linearly polarised. A ~0.4 nm bandwidth on the output coupler allows spooling and handling of the fibre laser without any change in performance or any need for tension or temperature control of the grating. This greatly simplifies the mounting and heat-sinking of the laser.

 

Fig. 2 Shown are the grating spectra as measured by an unpolarised source with a spectrum analyser at 0.05 nm resolution. The 2 peaks correspond to the fast and slow axis of the highly birefringent fibre. The gratings were strain tuned during the writing process such that the out-coupler slow axis overlapped with the high reflector’s fast axis.

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3. Results

The first stage 1.55 μm amplifier provides 17 dB of gain to the 100 ns, 100 nJ seed pulses. This resulted in 100 ns, 5 µJ pulses with a slope efficiency of 20% relative to pump power launched from the combiner. This stage is not well saturated by the seed pulse, so care must be taken to ensure that the pump level is not so large as to cause significant ASE which would reduce the amount of energy able to be stored in latter amplification stages.

The second stage 1.55 μm amplifier produces pulse energies of 80 µJ before the onset of Stimulated Brillouin Scattering. This stage operates at 34% slope efficiency with respect to pump power launched from the combiner and provides up to 18 W of output power as shown in Fig. 3 . The system can be operated at a repetition rate of up to 600 kHz; however a pump pulse energy >50 µJ at 1.55 μm is required for stable operation of the thulium laser. For this reason the repetition rate was limited to 300 kHz corresponding to 60 µJ pump pulses. At pulse energies >30 µJ, four wave mixing was evident through spectral broadening to a bandwidth of 6 nm. This slight spectral broadening does not affect the performance of the gain switched thulium laser because of the extremely broad absorption band of thulium doped silica around 1.55 μm.

 

Fig. 3 The second stage of the 1.55 μm pump operates at a slope efficiency of 34% indicating that the stored energy is extracted efficiently. The power was measured at the output of a passive 10 µm, 0.15 NA core fibre which contained the high reflectivity grating of the Thulium laser.

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The thulium gain switched laser operated at a slope efficiency of >55% with respect to launched 1.55 μm pump power as shown in Fig. 4 . The slope efficiency of the laser was not observed to change with repetition rate for any rate >50 kHz. The quantum defect is 25% which indicates that there may be other energy transfer mechanisms that are decreasing the efficiency of the laser. This is further confirmed by purple fluorescence in the TDF associated with the population of the ¹G₄ level in the thulium ions.

 

Fig. 4 Thulium laser output power vs. 1.55 μm pump power for output coupler reflectivities of 5%, 8% and 10% at a repetition rate of 300 kHz. Inset: Average 2.044 µm output power at different repetition rates.

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The polarisation of the laser output was measured by propagating it through a polarising beam splitter cube. The laser was >97% linearly polarised at all operating conditions and all cavity configurations that were tested. Any depolarisation is attributed to the inaccuracy of the angular alignment of the stress rods in the splices of the polarisation maintaining fibres in the system.

The pump absorption was >90% in the 20 cm of TDF. At pump powers >14 W the polymer outer cladding began showing signs of thermal degradation at the splice between the passive fibre of the HR grating and the thulium doped fibre. The optical-optical efficiency of the laser is 50% which suggests that the heat load may be as large as ~100 W/m at this splice. At this heat load the temperature differential between the inside and outside of the polymer coating is expected to be around 50 °K which is consistent with the temperature at which we have observed damage in other lasers. As there is no light propagating in the cladding of the fibre laser, it is possible to use a more thermally stable recoating material which may have a higher refractive index.

The pulse duration decreased as the reflectivity of the output coupler was changed from 10% to 5% while the length of the fibre laser was kept constant as shown in Fig. 5 . This is attributed to a reduction in the cavity photon lifetime and thus a corresponding increase in the rate at which photons are able to escape the cavity [15]. 25 ns pulses were achieved at repetition rates of up to 300 kHz.

 

Fig. 5 Pulse duration as a function of thulium fibre output energy, Inset: Temporal trace of the pulses from the 1.55 μm seed diode, the 1.55 μm amplifier and the 2.044 µm thulium laser.

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4. Conclusion

We have to the best of our knowledge demonstrated the highest power gain-switched thulium fibre laser. Robust single polarisation operation was achieved in a monolithic all-fibre cavity. The laser produced 25 ns, 23 μJ pulses or 35 ns, 35 μJ pulses at repetition rates of up to 300 kHz. This laser is more than sufficient to meet all the requirements for seeding a high power thulium fibre amplifier suitable for a power scaled mid-IR source.