1 . 6 W continuous-wave Raman laser using low-loss synthetic diamond

Low-birefringence (Δn<2x10 6 ), low-loss (absorption coefficient <0.006cm 1 at 1064nm), single-crystal, synthetic diamond has been exploited in a CW Raman laser. The diamond Raman laser was intracavity pumped within a Nd:YVO4 laser. At the Raman laser wavelength of 1240nm, CW output powers of 1.6W and a slope efficiency with respect to the absorbed diode-laser pump power (at 808nm) of ~18% were measured. In quasi-CW operation, maximum on-time output powers of 2.8W (slope efficiency ~24%) were observed, resulting in an absorbed diode-laser pump power to the Raman laser output power conversion efficiency of 13%. ©2011 Optical Society of America OCIS codes: (140.3580) Laser, solid-state; (140.3550) Laser, Raman; (160.4670) Optical materials. References and links 1. P. Cerný, H. Jelinkova, P. G. Zverev, and T. T. Basiev, “Solid state lasers with Raman frequency conversion,” Prog. Quantum Electron. 28(2), 113–143 (2004). 2. J. Piper and H. Pask, “Crystalline Raman lasers,” IEEE J. Sel. 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Introduction
Crystalline Raman lasers enable wavelengths between 1.1 and 1.6 m to be reached using systems based on ubiquitous neodymium lasers [1,2]. Recently, the use of high optical quality, single-crystal, synthetic diamond as a Raman material in external cavity [3][4][5][6] and intracavity [7][8][9] configurations has been demonstratedmade possible by the progress in diamond chemical vapour deposition (CVD) [10,11]. Diamond has a large Raman gain (64cm/GW at = 532nm [12] and between 12.5cm/GW [13] and 16.6cm/GW [5] at = 1064nm), broad transparency (0.23-2.5µm and >7µm [14]) and a large Raman shift (1332cm 1 [15]). Its thermal conductivity (2000W/m.K [16]) is 2 to 3 orders of magnitude greater than typical crystalline Raman media such as KGd(WO 4 ) 2 and YVO 4reducing thermal lensing and hence increasing the power scaling potential of continuous-wave (CW) Raman lasers [2]. In this paper, the use of CVD-grown, low-loss, low-birefringence diamond as a Raman medium in a CW Nd:YVO 4 disk laser is reported. By improving the thermal management of the pump laser gain medium (Nd:YVO 4 ) using a diamond heat spreader [17], and using a lower loss diamond as the Raman laser medium, CW output powers of 1.6W at the Raman laser wavelength (1240nm) were achieved, eight times higher than previously reported [9]. The next section describes the diamond sample. Section 3 outlines the laser architecture; the results are outlined and discussed in sections 4 and 5.

Low-loss synthetic diamond
The Raman laser medium was a 4.1mm long, single-crystal, CVD diamond plate with a 3.1mm × 1.6mm cross-section. The 1.6, 3.1 and 4.1mm edges were parallel to <100>, <110>, and <110> directions respectively. The sample was grown homoepitaxially by microwaveplasma chemical vapour deposition on a {100} type Ib synthetic diamond substrate in a H 2 /CH 4 /N 2 gas atmosphere. The substrate was carefully prepared to minimize surface and sub-surface damage in order to reduce the dislocation density commensurate with low birefringence material [18]. After growth, the CVD diamond layer was removed from the substrate by laser sawing and polished to provide a high optical quality finish. Birefringence, n, along <110>the direction of light propagation in the Raman laser experimentswas measured to be between 9 × 10 7 and 2 × 10 6 using the Metripol technique [19]. Optical absorption in synthetic diamond is broadly determined by levels of nitrogen-containing defects (ignoring the 2.5-6 m multiphonon absorption band), the most important being single substitutional nitrogena nitrogen atom substituted for a carbon atom on the diamond lattice. This defect gives rise to absorption features in the UV, visible and infrared spectral ranges [20]. The level of nitrogen was estimated using electron paramagnetic resonance (EPR) ) only. However, this is the dominant nitrogen-containing impurity, and the density was estimated to be 20 ± 10 ppb. The absorption coefficient at 1064nm was measured to be <0.001cm 1 by Laser Zentrum Hannover e.V., using ISO-standard calorimetry [22]. Anti-reflection (AR) coatings (R<0.1% at = 1064nm and = 1240nm) were then applied to the front and back surfaces. The absorption coefficient was then measured to be <0.006cm 1 using a simplified calorimetry technique similar to that in [3], rather than the ISO approved methodology. The diamond sample was mounted in a thermo-electrically cooled brass mount. With a 1064nm beam passing through the sample, the current required to maintain a constant sample temperature was recorded and calibrated against a known heat load provided by a precision resistor. The higher upper bound given by this technique compared to the ISO approach results from its lower accuracy, the effect of the coatings (these had some visible damage as a result of the laser experiments which may have contributed to the measured absorption), and the effect of any scattered light absorbed in the mount. Even so, an absorption coefficient of <0.006cm 1 is a significant improvement on the sample used in [8,9] where an absorption coefficient of ~0.03cm 1 was measured using the same apparatus. However, laser calorimetry is not, in general, sensitive to scatter, which may be a significant fraction of the overall loss if the absorption is small [23]: a different measurement technique would be required to determine this and hence the total loss.

Intracavity diamond Raman laser
A 4-mirror intracavity-pumped Raman laser resonator ( Fig. 1) was built around an a-cut Nd:YVO 4 crystal (0.5mm thick, 1 atm.% doping). The rear face of the Nd:YVO 4 was highreflectivity (HR) coated for 1064 and 1240nm (reflectivity R>99.9%), and for 808nm (R>95%). A low birefringence ( n<3 × 10 6 ), 0.5mm-thick synthetic single-crystal diamond heat spreader (4mm diameter) was bonded to the uncoated face of the Nd:YVO 4 using liquidassisted optical contacting [24]. The other face of the diamond disk was AR coated for = 808 (R<10%), 1064 and 1240nm (R<0.1%). The Nd:YVO 4 / diamond structure was mounted on a water-cooled brass heat sink (water temperature = 20°C) using indium and was pumped with a 35W fibre-coupled diode-laser (100 m core diameter, 0.22 NA, = 808nm). The diode-laser pump spot radius in the Nd:YVO 4 crystal was 330 m. Both curved cavity mirrors were HR coated for 1064nm and 1240nm, while the output coupler was HR coated at 1064nm and had a transmission of 1% at 1240nm. The cuboid of diamond used as the Raman laser medium was orientated for light propagation along its 4.1mm length and with the 1.6 and 3.1mm edges at 45° to the horizontal. Both the 1064 and 1240nm fields were horizontally polarized. The fundamental transverse mode radii for the pump radiation ( = 1064nm) within the diamond and the Nd:YVO 4 were calculated to be ~60 m and ~260 m respectively.

Results
In this configuration, the slope efficiency of the Raman laser ( = 1240nm) was measured to be ~18% with respect to the absorbed diode-laser pump power (Fig. 2). The maximum output power was 1.6W for an absorbed diode-laser pump power of 15W: an absorbed pump power to Raman laser conversion efficiency of ~11%. Output power stability traces are shown in Fig. 3(a) and Fig. 3(b) and demonstrate an output power variation of less than 10% over 20s.
The beam propagation factors were M 2 x = 2.8 and M 2 y = 2.6 for the Raman laser (1240nm) and M 2 x = 3.4 and M 2 y = 3.5 for the pump laser (1064nm). Both lasers were horizontally polarized.    The laser was operated in quasi-CW mode to assess the performance unimpeded by thermal lensing in the Nd:YVO 4 . The diode-laser was chopped at a 50% duty cycle (2.5ms on/2.5ms off) giving a maximum on-time output power of 2.8W, an on-time slope efficiency of ~24% and an absorbed diode-laser pump power to Raman laser conversion efficiency of 13% (Fig. 4). Thermal effects begin to restrict the output power at the end of the pump pulse (Fig. 4, inset); however, the main limitation was the available diode-laser pump power.

Discussion
The output power of the CW diamond Raman laser reported here is 8 times greater than the first such laser reported in [9]. In addition, the diode-laser pump power range over which the laser oscillates is significantly increased and amplitude noise reduced. These improvements can mainly be explained by the use of a significantly lower loss diamond and improved thermal management in the laser gain medium. In the Raman laser described in [9], the large diamond-induced losses (~2% per round trip) meant that all other losses had to be minimized to enable Raman laser oscillation. A 2-mirror cavity was therefore used to reduce the component count and hence the loss. However, to ensure a small mode radius in the diamond, and hence a small Raman laser threshold, the fundamental mode radius of the pump laser ( = 1064nm) at the centre of the Nd:YVO 4 crystal was less than half the diode-laser pump mode radius. This configuration encouraged the oscillation of higher-order transverse modes and was close to a cavity stability edge and hence extremely sensitive to thermal lensingseverely limiting the performance of the Raman laser. In contrast, for the laser described here, a lower loss diamond sample enables the use of a 4-mirror resonator where the laser mode radii in the conventional and Raman laser gain materials can be independently controlled.
The use of a diamond heat spreader reduces the thermal lensing in the Nd:YVO 4 crystal: the other factor limiting the output power achieved in [9]. This is illustrated in Fig. 5 where a simplified finite element model has been used to simulate the temperature rise at the diodelaser pump power required for maximum Raman laser output power: for the conventional Nd:YVO 4 laser rod used in [9] (Fig. 5(a)) and for the Nd:YVO 4 / diamond composite used in this work (Fig. 5(b)). (To keep the required computing resources reasonable, averaged values have been used for some anisotropic materials parameters to permit an axisymmetric simulation.) The absorbed diode-laser pump power density is higher in the present work, due to the thin piece of Nd:YVO 4 used and the higher absorbed pump power (15W as opposed to 10W)despite a larger pump spot radius (330µm as opposed to 150µm). However, the predicted maximum temperature rise is similar in the two scenarios: 152K as opposed to 139K. As a result, the thermal lens caused by the change of refractive index with temperature is significantly weaker in the present case where a thin piece of gain material is used: a focal length of approximately 450mm as opposed to 60mm (estimated from a quadratic fit to the radial variation in the axially averaged temperature [25]). The radius of curvature of the most significant mechanical deformation was also calculated to be longer: 590mm for the HR coated surface of the Nd:YVO 4 / diamond composite in the current case as opposed to 210mm for the pumped end of the Nd:YVO 4 rod in [9]. Weaker thermal lensing means that higher diode-laser pump powers can be used without the risk of resonator instability. For the work described in this paper, roll-over of the Raman laser output power was observed for diodelaser pump powers above 15W for CW operation; such behavior was not observed in the quasi-CW experiment indicating that this effect results from thermal issues within the Nd:YVO 4 . Improved thermal management in the laser gain medium, for example through the use of ytterbium-doped materials should allow for further power scaling. Therefore, diamond has the potential to compete with more conventional Raman laser (e.g. KGd(WO 4 ) 2 and YVO 4 ) materials in the CW regime as already demonstrated in the pulsed regime [3,5].
The use of diamond also reduces the thermal lensing in the Raman material compared to a more conventional Raman laser material such as YVO 4 . This is illustrated in Fig. 6 where the thermal lens in a 4mm long diamond sample ( Fig. 6(a)) is compared, theoretically, to the use of a 25mm long YVO 4 sample ( Fig. 6(b)). A larger mode radius (160µm) is assumed in YVO 4 than the diamond (100µm) to reduce the thermal effects whilst maintaining a similar Raman laser threshold. Assuming the same Raman laser output power (1.6W as per the experimental result described in section 3), the temperature rise is six-fold higher in YVO 4 and the thermal lens focal length is 30 fold shorter (0.5m as opposed to 16m). This illustrates the potential of diamond for power-scaling of CW Raman lasers. Further, so-called "beam clean-up", leading to an improvement in the beam quality of the Raman laser over the pump laser [2], is observed in our laser indicating the potential for efficient, high brightness Raman lasers even when the beam quality of the pump laser is not perfect.

Conclusion
A CW diamond Raman laser has been operated above the Watt level. The maximum output power was 1.6W and the conversion efficiency was 11% from the diode-laser pump power. In part, this was enabled by the excellent optical properties of the single-crystal synthetic diamond sample: in particular, low birefringence and low loss. In addition, thermal effects within the conventional laser gain medium were significantly reduced using a diamond heat spreader. These improvements combined to give an eight-fold increase in output power over our previous report of a CW diamond Raman laser [9].