1. Introduction

Frequency conversion by means of stimulated Raman scattering (SRS) in crystalline media has been identified as an efficient method for extending the IR emission range of solid-state Nd based lasers, and when combined with second harmonic generation, can be used to generate a wide range of visible laser wavelengths. As well as output at single discrete wavelengths, multiwavelength outputs can be obtained, either simultaneously or selectably. A review of visible and UV Raman laser-based sources can be found in [1]. When SRS and frequency doubling are both performed within the resonator of a Nd laser, efficient conversion to the yellow-orange spectral region can occur, as reported for both pulsed [2] and more recently, continuous-wave (CW) regimes [3,4]. The use of Raman-active laser hosts enables very simple laser resonator designs since a single crystal generates both the fundamental and Raman shifted output. The first diode-pumped self-Raman laser was based on Nd:KGW [5]. Then in 2001, Kaminskii et al. predicted that Nd-doped vanadates would be promising self- Raman crystals [6], as was subsequently verified by Chen [7]. These self-Raman lasers have been successfully demonstrated in both pulsed (Q-switched) [5] and continuous-wave (CW) regimes [8].

The development of lasers operating in the yellow-orange wavelength range is of significant interest due to potential applications in the medical and bio-medical arena, as well as for laser display and remote sensing applications. Real world applications of these systems, for example in ophthalmology, require yellow output powers in the multi-Watt range, and a variety of approaches to achieving these power levels have been explored in recent years. These include frequency-doubled VCSELS [9], sum frequency generation of Nd lasers at 1.06 µm and 1.32 µm [10], and fibre lasers [11]. In our opinion, a Raman-based approach offers great simplicity compared to these alternatives. In fact the core components of the yellow laser described here: diode-pumped Nd:GdVO₄, LBO and dielectric mirrors, are essentially the same as those in many commercial 532 nm lasers.

Despite the physical simplicity of Raman laser sources, the design of the laser resonator is critical to achieving efficient and high power operation. To the best of our knowledge, the highest CW yellow power reported previously for a frequency-doubled, Raman laser was 704 mW [3]. Dekker et al. also demonstrated that by pumping with a 50 % duty cycle to reduce thermal lensing, higher yellow output powers of up to 1.88 W (instantaneous) could be achieved [4]. Thermal loading of the self-Raman crystal was the dominant factor limiting the CW yellow output in [3], and this arises from three main processes: the absorption of pump light, the inelastic nature of the Raman scattering process, and absorption of yellow light passing through the crystal. The relative contributions of these were analysed in detail in [4].

In this paper we report a CW, frequency doubled, Nd:GdVO₄ self-Raman laser and we demonstrate 2.51 W of CW yellow emission with a 12.2 % overall optical (diode-to-yellow) conversion efficiency. This level of output power is a result of a laser design which minimises thermal loading of the laser crystal, and maximises the collection efficiency of the yellow light generated in the doubling crystal. More specifically, the laser was pumped at 880 nm, as opposed to 808 nm, resulting in a reduced thermal load in the self-Raman laser crystal and enabled the use of higher pump powers. The reduced thermal lens in turn allowed for a longer (20 mm) Nd:GdVO₄ crystal to be used, resulting in higher Raman gain. For efficient collection of the yellow emission into a single output beam, an intracavity dichroic mirror was used with the additional benefit of preventing any thermal loading of the laser crystal caused by absorption of yellow light by the laser crystal.

2. Design strategies for power scaling

The laser reported here was pumped at 880 nm, with the goal of reducing the contribution to thermal loading of the self-Raman crystal due to pump absorption. It is well known [12] that pumping Nd laser media at 880 nm enables direct excitation of the upper ⁴F_3/2 laser level of the 1064 nm transition, bypassing the non-radiative ⁴F_5/2 to ⁴F_3/2 transition (which occurs during 808 nm pumping). This in-turn increases overall laser efficiency (due to reduced quantum-defect deficit) and reduces the effective thermal load and thermal lensing within the Nd host medium, which in turn allows for pumping with higher powers while maintaining resonator stability.

In this work we have investigated operation of the laser using Nd:GdVO₄ crystals with lengths 10 mm and 20 mm. Modelling performed by Spence et al.[13] showed that improvements in Stokes generation within CW Raman lasers can be obtained by increasing the Raman coupling parameter, a parameter which is directly proportional to the length of the Raman conversion medium. It was therefore anticipated that the 20 mm long crystal would be more efficient at generating yellow emission.

One factor limiting the yellow output power achieved in [4] was that only the yellow light generated in the direction of the output coupler was collected. It was anticipated that similar amounts of yellow light would be generated in the opposite direction, and calculations performed in [4] showed that a majority of this backward propagating emission would be absorbed by the laser crystal, thereby increasing the strength of the thermal lens within the laser crystal by ~30 %. We therefore included an intracavity mirror to aid collection of the yellow light and prevent absorption by the laser crystal.

3. Experimental arrangement

The experimental layout is shown in Fig. 1. The pump source was a 30 W, 880 nm, fibre-coupled laser diode (LIMO) with 200 µm diameter fibre core. The output from the diode was imaged with 1:2 magnification (to an effective diameter of 400 µm) onto the face of a 0.3 at.% doped a-cut Nd:GdVO4 laser crystal. It should be noted that the pump diode wavelength exhibited a substantial red-shift (876.6 nm to 880 nm) with applied pump current due to the increased temperature sensitivity of aluminium free laser diodes (as used in 880 nm packages). We measured a temperature induced wavelength shift of 0.35 nm/deg C in our 880 nm laser diode, compared to 0.25 nm/deg C for a similar 808 nm laser diode (AlGaAs). The temperature of the diode package used in this work was typically maintained at 14deg C, such that the output wavelength was 879.5 nm when the diode output was 20 W (corresponding to the maximum yellow output power achieved).

 

Fig. 1. Resonator configuration for intracavity frequency-doubling of self-Raman generated emission from Nd:GdVO₄. System performance was evaluated with and without the intracavity mirror.

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Two Nd:GdVO₄ laser crystals were evaluated, one with dimensions 4×4×20 mm and another with dimensions 4×4×10 mm, both coated AR for 808–880 nm and 1063–1173 nm. The crystals were mounted in copper blocks which were kept at a constant temperature of 17 °C by water cooling. The laser resonator was formed using a flat input mirror, and a concave output coupler, both coated R >99.994% @ 1063/1173 nm. A variety of output couplers with different radii of curvature were utilised, with the highest yellow output power obtained using an output coupler with a 5 cm radius of curvature (ROC). Intracavity frequency doubling of the Stokes-shifted IR emission (λ=1173.5 nm) was achieved using a 4×4×10 mm non-critically phase-matched (NCPM, θ=90°, φ=0°) lithium triborate (LBO) crystal coated AR for 1063–1173 nm and temperature tuned to ~45.5 °C for second harmonic generation of the first Stokes beam.

Figure 1 shows the inclusion of an intracavity mirror between the Nd:GdVO₄ crystal and the LBO. This intracavity mirror was flat, with the side facing the Nd:GdVO₄ coated antireflection (R<0.6%) at 1063 and 1173 nm, and the side facing the LBO coated anti-reflection (R<0.6%) at 1064 and 1173 nm, and high reflection at 586.5 nm (R=98.7%). The intracavity mirror was used to maximise the yellow output collected through the output coupler with the added benefit of preventing absorption of backward-propagating yellow emission in the Nd:GdVO₄ crystal. All of the resonator components were positioned as close to one another as possible to minimise the resonator length, which varied from 31 to 34 mm when using the 20 mm long laser crystal (according to whether or not the intracavity mirror was included). The performance of the yellow laser was evaluated both with and without the intracavity mirror.

Dichroic mirrors were used to distinguish the collinear yellow and IR emission (both fundamental and Stokes) from the laser resonator. The spectral properties of the emission were monitored using CCD spectrometers (Ocean Optics USB 2000/HR4000) and beam quality measurements were performed using an automated beam profiler (Gentec-EO Inc. BeamScope-P8).

4. Results

4.1 Pumping with an 880 nm laser diode

Laser resonator stability was assessed at the fundamental (1063 nm) wavelength in order to estimate the strength of the thermal lens in the Nd:GdVO₄ crystal due to pump absorption. The laser was set up as per Fig. 1 using a 20 mm long laser crystal with flat input and output mirrors and an output coupling of 5 % at 1063 nm. To determine the onset of resonator instability, the resonator length was incrementally increased (by moving the output coupler) and the output was monitored for an abrupt drop in power.

The resonator length could be extended to ~80 mm before the onset of instability, when pumping at 880 nm with 20 W incident pump power. This corresponds to a thermal lens of ~70 mm being generated within the laser crystal, as determined through ABCD resonator modelling. By comparison, Dekker et al. reported a thermal lens of ~60 mm [4], when pumping a similar resonator with an 808 nm laser diode. This result is consistent with the expected reduction in the thermal load when pumping at 880 nm vs. 808 nm.

The performance of the yellow laser system was first evaluated using the 10 mm long Nd:GdVO₄ crystal pumped at 880 nm, and without the use of the intracavity mirror. It should be noted that the output power and beam quality of the fundamental and Stokes emission (measured at λ=1173.5 nm) were difficult to measure due to the very high reflectivity of the output coupler. The output coupler was however, highly transmitting for the yellow emission. A maximum yellow (measured at λ=586.5 nm) output power of 972 mW was achieved with 17.4 W pump incident on the laser crystal. This is 300 mW more than that reported in [4] for pumping an almost identical system at 808 nm (they reported 672 mW yellow emission from 16.3 W pump incident on crystal). The longer pump wavelength of 880 nm in our experiments is undoubtedly the major factor responsible for the improvement in performance. A second reason for the improved performance may be the longer Rayleigh range of our pump focussing system. In [4], the pump diode was coupled to a 400 µm diameter fibre, with 1:1 imaging onto their laser crystal. So while the spot size incident onto the crystal was identical to our experiments, the Rayleigh range was effectively shorter and as a consequence, a greater proportion of the pump light diverged from the resonator mode, thereby reducing overall laser efficiency.

4.2 10 mm vs 20mm self-Raman crystal

The laser was setup as shown in Fig. 1 and evaluated with the two Nd:GdVO₄ crystals and without the intracavity mirror. Laser performance was investigated using output couplers with several radii of curvature, since this had an important effect on the resonator mode sizes in the two crystals, resonator stability, and therefore the laser output power and efficiency. For example, when the 20 mm long Nd:GdVO₄ crystal and a 30 cm concave RoC output coupler were used, the resonator became unstable for pump powers above 15 W (incident on crystal). However, when a 5 cm concave RoC output coupler was used, the resonator remained stable for pump powers up to 20 W.

For the 10 mm long crystal, the highest yellow output power was obtained using a 30 cm concave output coupler. The threshold for yellow output was 2.03 W (pump power incident on the laser crystal) and a maximum yellow output of 972 mW was achieved. For the 20 mm long crystal, the highest yellow output power was obtained when a 5 cm concave mirror was used. The threshold for yellow output was 0.72 W (pump power incident on crystal) and a maximum yellow output of 1140 mW was achieved. These results support improved Raman conversion efficiency, power generation and reduced lasing threshold as a result of increasing the length of the self-Raman laser crystal.

4.3 Use of an intracavity dichroic mirror to collect all yellow emission

In order to collect as much yellow emission from the laser resonator as possible, we have included an intracavity mirror between the Nd:GdVO₄ and LBO crystals (as shown in Fig. 1). The resonator was setup using the 20 mm length Nd:GdVO₄ crystal and a 5 cm RoC output coupler. CW yellow output power is shown in Fig. 2 as a function of pump power incident on the crystal both with and without the intracavity mirror.

 

Fig. 2. Yellow (586.5 nm) output power as a function of pump power incident on 20 mm laser crystal for resonators with and without the intracavity mirror.

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Without the inclusion of the intracavity mirror, the threshold for emission at 586.5 nm was achieved for 0.72 W incident pump power. A peak output power of 1.14 W at 586.5 nm was achieved for 18.2 W of pump power incident on the laser crystal (with an overall diode-to- yellow efficiency of 6.3 %). The yellow output power did not increase with pump power in a linear fashion. Because the focus of this work was on achieving the highest yellow output power, the diode temperature was optimised to give an output wavelength of 879.5 nm (corresponding with the peak absorption of Nd:GdVO₄ in the 880 nm absorption band), for a diode output power of 20 W. Due to the substantial shift in diode wavelength as a function of diode current (noted in section 3), the diode wavelength was not optimum for lower diode powers and a resultant decrease in conversion efficiency was observed. Of course the temperature of the diode can be adjusted to maximise the conversion efficiency at any desired pump power.

With the inclusion of the intracavity mirror, the laser exhibited a similar yellow emission threshold, along with a similar (not linear) dependence of yellow output power on pump power. However, a significant increase in output power was observed, a maximum yellow output power of 2.51 W was achieved for 20.5 W pump power incident on the laser crystal. This corresponded to an overall diode-to-yellow conversion efficiency of ~12.2 %.

The laser with the intracavity mirror had excellent long term stability and exhibited a peak-to-peak fluctuation in yellow output power of ~2.8 %, recorded over a period of 40 minutes at a pump power of 17 W. The beam quality of the yellow emission from the cavities both with and without the intracavity mirror was also recorded. Both systems exhibited beam quality factors of M²~2 just above the yellow emission threshold and rose to M²~6 at maximum pump power.

Our results demonstrate that significant improvement in power extraction can be obtained with the use of the intracavity mirror, with up to 120 % increase in output power achieved from the laser resonator. The results also indicate a reduction in thermal load, since despite the resonator with the intracavity mirror being 3 mm longer it is able to sustain 2 W more incident pump power before the onset of instability. This increased stability is significant in the context of the estimated thermal lens focal lengths being as short as 2 cm.

5. Discussion

When power scaling these CW self-Raman lasers, output power typically reaches a maximum for a given incident pump power and then rapidly decreases at higher pump powers. This rapid decrease in power is characteristic of resonator instability and is indicative of strong thermal lensing within the system. The fact that the yellow laser resonators become unstable for incident pump powers of >21 W suggests that a stronger thermal lens (with focal length ~2 cm) is being generated in these cavities in comparison to the thermal lens (with focal length ~7 cm) being generated in the resonator setup for the fundamental wavelength (1063 nm). Similar observations were reported in [4, 14] where it was suggested that additional thermal loading in the high-Q Raman laser resonator may be attributed to a combination of high intracavity power and trace impurity absorption and/or excited state absorption within the laser crystal (perhaps evidenced by blue emission from the self-Raman crystal when pumping above the Raman threshold). We have observed blue emission from Nd:GdVO₄, Nd:YVO₄ and Nd:KGW crystals, the physical origin of which is the subject of further investigation.

7. Conclusion

In conclusion, we have reported what we believe to be the highest-power, and most efficient CW yellow emission (at 586.5 nm) obtained from an intracavity-doubled, self-Raman laser. A combination of long (20 mm) self-Raman laser crystal, direct pumping at 880 nm and the use of an intracavity mirror have increased yellow power and efficiency, enabling a record yellow output power of 2.51 W with an overall diode-to-yellow conversion efficiency of 12.2 %. This result is of particular significance as it demonstrates that CW yellow emission with power levels (>2 W) suitable for real-world application can be achieved in a compact, intracavity-doubled self-Raman laser.