Passive Q-switched Dy:ZBLAN fibre laser at 3.1 µm

. The passive Q-switching performance of an in-band pumped Dy-doped fluorozirconate fibre laser emitting around 3 . 1 µ m is investigated. Passively Q-switched laser operation is demonstrated employing semi-conductor saturable absorber mirrors. Stable operation is achieved, with a minimum pulse duration of 464ns, a highest repetition frequency of 210kHz and peak pulse powers up to 3W.


Introduction
The mid-infrared spectral region, usually defined as the wavelength interval 2 ÷ 20 µm, is where most trace gas molecules have their fundamental rovibrational absorption bands.The 3 ÷ 5 µm wavelength range is of particular interest because it present relatively small water vapour absorption and contains the strongest absorption bands of molecules with CH, NH or OH stretching vibrations.
The aim of this work is to experimentally investigate a novel passively Q-switched fibre laser source with wide emission in the 3 µm spectral range.To this purpose, the Dy-doped fluorozirconate fibre is selected as active medium for its broad wavelength tuneability in the range from 2.9 µm to 3.2 µm [1,2].Passive Q-switching laser operation is achieved with commercially available semiconductor saturable absorber mirrors, demonstrating stable operation with a minimum pulse duration of 464 ns, a highest repetition frequency of 210 kHz and peak pulse powers up to 3 W.

Experiment and results
The Dy 3+ :ZBLAN active fibre is pumped in-band with a continuous wave diode-pumped Er 3+ :ZBLAN fibre laser emitting at 2825 nm.The pump power injected into the Dy-doped fluoride fibre is limited by thermal issues and the risk of damaging the fibre end-face.To increase the maximum injected pump power by up to a factor of two, the fibre is pumped from both sides.
Figure 1 shows the linear resonator design used to achieve pulsed operation.The 1.82 m-long Dy-doped ZBLAN fibre has a numerical aperture of 0.16 and a core diameter of 12.5 µm, and is doped with a 3.63 × 10 25 m −3 (2000 mol ppm) concentration of Dy 3+ ions.
⋆ e-mail: fedele.pisani@polimi.it⋆⋆ e-mail: gianluca.galzerano@polimi.itOn the left, the laser cavity is closed by a semiconductor saturable absorber mirror designed for operation at 3000 nm (see Table 1), while on the right different sapphire output couplers are tested.
Figure 2a shows the average output power as a function of the pump power, for different output couplers.In the case of the 30 % output coupler, two different positions of the SESAM are investigated, corresponding to two different Q-switching pulse repetition frequency regimes.Figure 2b shows, as a function of the pump power and for different output couplers, the pulse repetition frequency and pulse duration (full width at half maximum).For a given pump power, to a higher repetition frequency corresponds a shorter lifetime of the single pulse inside the cavity and, accordingly, a lower amplification.Because of the lower pulse power, the saturable absorption effect decreases, increasing the pulse width.However, the performance of the saturable absorber is limited by its modulation depth, therefore it does not change considerably once the pump power is high enough.Lastly, when the transmission of the output coupler is increased the pulses have a shorter full width at half maximum, since the increased losses make the radiation in the cavity discharge faster.
The same measurements are taken with another semiconductor saturable absorber, designed for operation at 2900 nm (see Table 1), obtaining a similar performance.An interesting regime was observed, for high enough pump powers, with the 50 % output coupler: we measured short pulses with simultaneously high average power and low repetition frequency, thus high peak powers, up to 3 W.
For both SESAMs, the electrical spectra of the recorded pulse trains present a strong peak at 42 MHz and a weaker one at double the frequency.Since the frequency of the main peak corresponds to the expected free spectral range, the two peaks represent mode beating between very close longitudinal modes of the cavity.This spectrum corresponds therefore to Q-switching with only a few modes oscillating in the cavity, as confirmed by the recorded optical spectrum.

Conclusions
In this work we have investigated the laser performance of an in-band pumped Dy-doped fluorozirconate fibre emitting around 3 µm, demonstrating stable passive Qswitching operation with a minimum pulse duration of 464 ns, a highest repetition frequency of 210 kHz and peak pulse powers up to 3 W. Since these results were obtained without taking special precautions on the tips, we expect the maximum coupled pump power to considerably increase once fibre protection techniques are put in place.
Our results are promising for the generation of ultrashort pulses in the 3 µm spectral region.Indeed, starting from the experimental results obtained in the Q-switching regime we have been able to identify some steps which could bring the laser in passive mode-locking regime.Namely, the gain of the laser cavity must be increased, by employing a longer fibre or by increasing the maximum injected power, and higher-performing saturable absorbers must be employed.These results will open the way for the synthesis of novel fibre frequency combs in the mid-infrared spectral region around 3 µm, which in recent years have been revolutionising the field of high-precision broadband spectroscopy.

Figure 2 :
Figure 2: Properties of the laser output as a function of the pump power, for output couplers with different transmission values.LF and HF identify the lower and higher frequency regimes, respectively.

Table 1 :
Nominal specifications of the two SESAMs.