1. Introduction

There is increasing interest in diamond as a very attractive gain medium for Raman lasers. This is due, amongst other reasons, to its broad optical transparency, high Raman gain coefficient (~15 cm/GW at 1 µm), large Stokes shift (1332 cm⁻¹), and a thermal conductivity (~2000 W/m·K) 2 to 3 orders of magnitude greater than other crystalline Raman media. The development of synthetic single-crystal diamond, produced via chemical vapor deposition (CVD), has now matured to the point that large (few mm³), high optical quality single-crystals, suitable for intracavity use, are becoming commercially available [1]. Following these developments, efficient diamond Raman lasers have been successfully demonstrated in a variety of configurations. In pulsed operation, and pumped in an external resonant cavity, optical conversion efficiency up to 63.5% [2], slope efficiency of 84% [3], and Stokes average output power of 24.5 W [4] have been reported. In the continuous-wave (cw) regime, synthetic diamond pumped within a Nd:YVO₄ laser achieved a conversion efficiency of 11% and output power up to 1.6 W [5], and more recently, >5 W when pumped within a Nd:YLF laser [6]. All previously reported diamond Raman lasers have, however, been pumped by rare-earth-doped solid-state lasers for operation at fixed wavelengths.

In this paper, we present a tunable (1217-1244 nm) cw diamond Raman laser, achieved by pumping diamond within a 1060-nm-wavelength InGaAs semiconductor disk laser (SDL). Maximum output power of 1.3 W at 1227 nm and optical conversion efficiency up to 14.4% has been obtained. This is, to our knowledge, the first tunable diamond Raman laser; and in addition also shows competitive efficiency compared with previously reported cw crystalline Raman lasers (e.g [7].). Importantly, the laser rivals the optical efficiency of SDLs designed for fundamental emission in the 1200-1300nm range [8].

2. Diamond Raman laser configuration

The Raman medium used for this work was a 6.5 x 3.0 x 1.5 mm³ single-crystal synthetic diamond provided by Element Six Ltd., Ascot UK. The diamond was cut for beam propagation along a <110> direction and orientated in the laser such that the <111> direction (~54.7° with respect to <100>) was horizontal. This gave access to the high gain orientation identified in ref [3]. Both end faces were broadband antireflection coated for 1040-1240 nm (R~0.15%). In previous work, the absorption coefficient of the diamond crystal was measured via calorimetry to be <0.004 cm⁻¹ [9]. No additional thermal management was implemented for the diamond crystal.

The Raman resonator was aligned within an all-high-reflector (R>99. 98%, 1000-1250 nm) 4-mirror SDL cavity [10, 11] with an intracavity mode waist at the SDL gain structure and a second mode waist in the center of the diamond Raman crystal (see Fig. 1 ). The SDL gain structure used was designed for operation around 1060 nm and contained 15 InGaAs quantum wells and an integral AlAs/AlGaAs distributed Bragg reflector (DBR). Further details on this structure are given in ref [11]. In common with our previous work, an uncoated, plane-parallel, 500-μm-thick, synthetic single-crystal diamond heatspreader was bonded to the intracavity surface of the gain structure for effective thermal management. Note that this was a separate diamond to that pumped to provide the Raman gain (see Fig. 1), although in principle both functions could be combined in the one diamond crystal. The composite SDL structure was mounted in a water-cooled brass holder (water temperature 7 °C), and optically-pumped with an 808 nm fibre-coupled diode laser (100-μm core diameter, 0.22 NA), focused to a beam waist radius of ~50 μm. A planar dichroic mirror with high transmission for the SDL wavelength range (R<1%, 1030-1080 nm) and high reflectivity for the Raman laser (R>99.98%, >1200 nm) was inserted (tilt angle ~2°) to separate the Raman laser intracavity beam and steer it to an output coupler (OC) external to the SDL cavity. Both resonators were co-aligned to produce a calculated ~20 μm fundamental mode waist radius in the diamond Raman crystal. A 4-mm-thick quartz birefringent filter (BRF), inserted at Brewster’s angle in the SDL sub-resonator, allowed broad tuning of the SDL oscillation wavelength with narrow linewidth.

 

Fig. 1 Schematic of the tunable diamond Raman laser pumped within an InGaAs SDL: HS, diamond heatspreader; HR, high reflectors; DM, dichroic mirror; OC, output coupler; and BRF, birefringent filter.

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3. Results and discussion

Figure 2 shows the power transfer characteristic of the diamond Raman laser obtained with output coupling of ~1.2% at 1225 nm. The ‘absorbed’ diode pump power refers to the input power to the SDL gain structure after pump reflection losses of 19.7% at the surface of the uncoated diamond heatspreader. It is important to note that – in contrast to most conventional diode-pumped solid-state lasers – all pump power entering an SDL structure is absorbed. The Raman laser achieved a maximum output power of 1.3 W at 1227 nm for an absorbed diode pump power of 9 W (11.2 W incident pump power), resulting in a calculated optical conversion efficiency of 14.4%. For higher input power, the SDL was affected by thermal rollover [10], leading to a corresponding rollover of the Raman laser output power. The slope efficiency of the Raman laser before rollover was 36% with respect to absorbed diode pump power. From the known reflectivity of the cavity mirrors we were able to estimate the SDL intracavity power by measuring the leakage signal. The Raman laser threshold was reached when the SDL intracavity power was around 83 ± 10 W, corresponding to an average optical power density of ~4.6 MW/cm² over the length of the diamond.

 

Fig. 2 Power transfer characteristic of the cw diamond Raman laser (red circles) using a 1.2% OC. Also plotted is the SDL intracavity power (open squares) measured via the calibrated signal leakage through a cavity folding mirror. The inset shows the far-field Raman laser beam profile, with M²~1.1, measured using a commercial beam profiler (Coherent BeamMaster).

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The 14.4% optical efficiency of the SDL-pumped diamond Raman laser is competitive with previously reported cw crystalline Raman lasers pumped by doped dielectric solid state lasers, despite the lower slope efficiencies of SDLs (typically ~40-50% for InGaAs SDLs). The highest optical conversion efficiency previously reported for a cw crystalline Raman laser is 13.2%, as demonstrated by Fan et al. using a 30-mm-long BaWO₄ crystal [7]. In this case the output coupling was only 0.2% and the slope efficiency was 15.3%; however, high optical efficiency was achieved by pumping several times above the Raman laser threshold. The cw diamond laser reported by Lubeigt et al. used a 4.1-mm-long diamond pumped within a Nd:YVO₄ disk laser and 1% output coupling to demonstrate up to 1.6 W output power with slope efficiency of 18% and optical conversion efficiency of 11%, using a diamond with an absorption coefficient of <0.006 cm⁻¹ [5]. The absorption loss for the diamond we used was measured to be <0.004 cm⁻¹ [9], corresponding to an estimated round-trip loss of ~0.5%. The AR-coatings on the diamond crystal contribute an additional round-trip loss of ~0.5%; however, the separate arm of the Raman laser cavity allows the optimization of the SDL pump beam and Raman beam overlap in the diamond. In addition, the high reflectivity dichroic mirror removes the losses associated with the conventional laser medium from the Raman laser cavity. These attributes, together with the slightly higher output coupling c.f [5]. of 1.2%, lead to higher slope efficiency of 36%.

While a Brewster-cut crystal could be used to reduce reflection losses, the enlargement of the intracavity beam would increase the Raman laser threshold. Higher output power and higher optical conversion efficiency is therefore expected to be achieved via SDL power scaling so that the diamond Raman laser may be pumped many more times above threshold. For example, an InGaAs SDL with up to 20 W output power (>2.8 kW intracavity power) in a single transverse mode has previously been demonstrated [12].

The beam propagation factors of the ~1055 nm output from the SDL were measured during Raman conversion to be M²_horizontal = 2.05 and M²_vertical = 1.82. Turning off the Raman laser via slight misalignment of the dichroic mirror led to improvement in the SDL beam quality: M²_horizontal = 1.5 and M²_vertical = 1.4. This is consistent with the losses associated with preferential Raman conversion of lower order transverse modes resulting in the oscillation of higher order transverse modes in the SDL. At maximum output power, the beam propagation factors of the Raman laser were M²_horizontal = 1.14 and M²_vertical = 1.05. Compared with the KGW Raman laser we reported earlier [11], the beam quality of the diamond Raman laser is clearly superior, despite tighter focusing in the Raman crystal. We attribute this to the very high thermal conductivity of diamond (~600 times greater than that of KGW), which is therefore much less susceptible to thermal aberration. Indeed, based on the approximations in [13], we estimate the magnitude of the thermal lens focal length to be greater than 0.5 m in diamond but less than 0.05 m in KGW. That is to say the thermal lens is at least an order of magnitude weaker in the diamond Raman laser.

The SDL beam was constrained by the Brewster surfaces of the BRF to be horizontally polarized, and therefore parallel to a <111> axis of the diamond crystal. The Raman laser, which had no such constraints (aside from minor cavity anisotropy) was also measured to be horizontally polarized, parallel to <111>. This is consistent with the polarized diamond Raman laser threshold measurements reported by Sabella et al. [3].

Rotation of the BRF allowed the tuning of the SDL and therefore of the Raman laser. For an absorbed diode pump power of 9 W and using ~1.2% OC, the Raman laser operated over the range 1217-1244 nm (SDL range 1047-1067 nm), with output power exceeding 1 W over a 10 nm range (see Fig. 3 ). The SDL in a similar configuration but without Raman conversion tunes between ~1040-1070nm. This would equate to potential tuning of the Raman laser between about 1207 and 1248nm. Whilst differences in set-up preclude a rigorous comparison, the smaller tuning of the Raman laser achieved experimentally suggests that the varying reflectivity of the Raman laser output coupler, which had increased transmission at shorter wavelengths (Fig. 3), played a role in limiting the tuning range.

 

Fig. 3 Tuning of the Raman laser via rotation of the intracavity BRF for an absorbed pump power of 9 W. The dashed line shows the variation in the output coupler transmission, as measured by the supplier.

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The laser emission linewidth was measured using an optical spectrum analyzer with 0.01 nm resolution, and a typical output spectrum thus observed is shown in Fig. 4 . The use of the BRF narrowed the SDL linewidth to ~0.25 nm full width at half maximum (FWHM), whereas the Raman linewidth was 0.22 nm.

 

Fig. 4 Typical emission spectrum of the Raman laser, taken using an optical spectrum analyzer with 0.01 nm resolution. Inset: plotted on a log scale.

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5. Conclusions

In conclusion, an SDL-pumped and broadly tunable cw diamond Raman laser has been demonstrated. The maximum output power was 1.3 W; the slope efficiency and optical conversion efficiency were 36% and 14.4% respectively, both with respect to absorbed pump power (29% and 11.6% with respect to incident pump power before pump reflection losses); the Raman laser output was tunable over 27 nm from 1217 nm to 1244 nm; and the beam quality was excellent (M²~1.1). Our previous work demonstrated the potential for cascaded cw Raman conversion in an SDL [11]. With the use of the larger Stokes shift of diamond together with appropriate mirrors, cascaded Raman conversion to >1.5 μm may be possible within an InGaAs SDL. We also note that the broad transparency of diamond, together with the exceptional spectral coverage of SDLs [14,15], offers prospects for such SDL-pumped diamond Raman lasers over a wide wavelength range.