Fabrication and characterisation of few mode DBR Brillouin-Raman lasers

Abstract

Stimulated Raman scattering (SRS) fiber lasers remain an area of interest as they provide a simple and compact method of creating lasers at wavelengths beyond the rare-earth ion bands. The authors and others have demonstrated that short π-phase shifted distributed-feedback (DFB) cavity structures can be used to create single-frequency lasers [1–3]. To reach longer wavelengths, cascading SRS cavities are typically used, which usually involve many-meter-long distributed Bragg reflector (DBR) Fabry–Pérot cavities with 100s or 1000s of longitudinal modes. However, to keep the laser system compact, these cavities must be as short as possible, which results in few longitudinal modes and a different behaviour. Siekiera et al. have reported such short DBRs which consist of two 50 mm gratings separated by 120 mm in a high germania fiber with an effective area of 7.3 μm² [4]. This produced a laser with a threshold of 4 W with a maximum output of 700 mW for 8 W of pump with the expected 607 MHz beat frequency between the few longitudinal modes and 20 dB side mode suppression.Our DBRs were fabricated in the same manner as the previous π-phase shifted DFB laser using UHNA3 fiber again for its high germania content and small effective area of 5.4 μm² [3]. The DBR’s 241 mm long cavity is formed from two 22 mm gratings at 1116 nm which are 99.9 % reflective. The cavity is pumped with a freespace linearly polarised 1064.4 nm pump setup as shown in Fig. 1a. Unlike Siekiera et al., the lack of an output coupler grating leads to a much higher finesse cavity which, along with the smaller effective area, leads to a lower threshold of 2.2 W, higher maximum output powers of 900 mW at 5 W pump power, and the presence of stimulated Brillouin scattering (SBS) as we saw in our DFB laser, as shown in Fig. 1b. We also see a much reduced broadening with a FWHM of 23 pm for the SRS and 18 pm for the SBS with 70 dB sidemode suppression. Given the similarities to our DFB we expected a low lasing threshold, which we believe is increased by larger loss in the gratings due to the higher fluence required (1 kJ compare to 0.03 kJ). As with the DFB, we see pulsing of the DBR laser, as shown in Fig. 1c. However, most of this is as expected for longitudinal mode beating (repetition rate matches the expected free spectral range) with occasional single-frequency SBS pulses and massive pulse bursts which always lead to pump depletion. It should also be noted that if these lasers are run near threshold they can enter stable CW single or at slightly higher powers quasi-CW dual mode lasing with no SBS. We will report on our latest findings into DBR lasers, optimisation of grating fabrication to improve lasing parameters, further investigations into pulsed behaviour, and potentially cascading using DFB cavities.

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Identifiers

ORCID for Alexander Iain Flint: orcid.org/0000-0001-5103-0419

ORCID for James C. Gates: orcid.org/0000-0001-8671-5987

ORCID for Corin Gawith: orcid.org/0000-0002-3502-3558

ORCID for Peter G.R. Smith: orcid.org/0000-0003-0319-718X

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