High efficiency and high power operation of highly-doped 2 at.% crystalline Nd:YAG is demonstrated in a diode-side pumped bounce amplifier configuration. A linearly-polarized output power of 46.1W is obtained with 101W diode pumping representing the highest power achieved to date, to our knowledge, in a highly doped crystalline Nd:YAG laser. In a system operating at 19.1W output power, the slope efficiency was as high as 60%. With quasi-continuous wave diode pumping 11mJ pulses at 100Hz repetition rate were achieved and passive Q-switching with Cr4+:YAG produced pulses with 12ns duration.
© 2006 Optical Society of America
Diode-pumped solid-state (DPSS) lasers have gained high prominence as compact, high quality infra-red light sources. Neodymium-doped YAG (Nd:YAG) has been the predominant solid-state laser material used, although in recent years other materials such as Nd:YVO4 and Yb:YAG have attracted much interest. In particular, Nd:YVO4 has challenged Nd:YAG as a preferred solid-state lasing material due to its stronger absorption for diode pump radiation at 808nm and higher stimulated emission cross-section at 1064nm. These improved optical properties of Nd:YVO4 lead to higher efficiency in end-pumped DPSS laser systems compared to Nd:YAG and particularly in microchip lasers where crystal thickness may be less than 1mm.
However, Nd:YAG has better thermal and mechanical properties than Nd:YVO4 and is therefore better suited for high power laser operation. It also has a longer upper state lifetime, and hence higher energy storage capacity, making it suitable for high pulse energy using quasi-continuous wave (QCW) diode-pumping and Q-switched operation.
A key difficulty for Nd:YAG has been its low doping limitation. Standard Nd doping is typically 1 at.% and such crystals can be grown with excellent optical quality. One of the best-known growth techniques is the Czochralski (CZ) method but until recently, it was not possible to produce Nd:YAG with doping concentration larger than 1.4 at.%. In recent years, however, it has become possible to grow high optical quality crystals this way with doping concentration of up to 2.5 at.% in relatively large sizes .
Several other techniques to grow Nd:YAG to higher concentrations are now also available. The flux growth method can produce crystals with up to 4.3 at.% doping, but the growth is very slow and produces only small samples [2, 3]. Another technique to produce highly doped Nd:YAG in large samples is the temperature gradient technique (TGT), which has achieved doping concentrations of up to 3% [4–6]. Finally, polycrystalline ceramic Nd:YAG can be doped with up to 8.2 at.% Nd concentration in large samples . Table 1 compiles a number of laser performance results for highly doped Nd:YAG together with some comparative results for Czochralski grown samples with standard 1.0 and 1.1 at.% Nd doping.
These recent results testify to the improvement in the efficiency of the DPSS Nd:YAG laser with the use of highly Nd doped crystals. High doping of 2 at.% Nd in both Czochralski and TGT samples has been shown to produce higher efficiency than standard 1at.% Czochralski-grown Nd:YAG in the end-pumped configuration. Up to 12.3W output power was obtained in Czochralski 2 at.% Nd:YAG with 56.6% slope efficiency (with respect to absorbed pump power) compared to 53.8% for 1 at.% Nd . Slope efficiency of 46.7% was obtained for 2 at.% Nd:YAG grown by the TGT method compared to 39% for a 1.1 at.% Nd:YAG sample by Czochralski growth .
At the highest output power of 12.3W in crystalline 2 at.% Nd:YAG  the crystal is calculated to be operating at 39% of its surface stress fracture limit, so much further power scaling is severely restricted. The bounce amplifier configuration, involving diode side-pumping, has been demonstrated to be a viable design for producing higher power operation than end-pumping whilst also achieving high efficiency single mode laser output [9–12]. The bounce design requires a high absorption coefficient for the pump wavelength and has so far been demonstrated predominantly with Nd:YVO4 and Nd:GdVO4, which possess high absorption at standard 1.1 at.% doping concentrations. The bounce geometry becomes an attractive option with the enhanced absorption of highly doped Nd:YAG. This has been verified by high power laser operation with highly doped ceramic Nd:YAG, demonstrated by this group and co-workers .
In this paper we report high power continuous wave (CW) laser operation in Czochralski grown 2 at.% Nd:YAG at 1064nm by use of the bounce geometry, for the first time. We show very high slope efficiency of 60% and record power level of 46.1W with linear polarization in a highly doped Nd:YAG crystal. We also demonstrate efficient quasi-CW (QCW) operation with 11mJ pulse energy at 100Hz and passive Q-switching with 12ns pulse duration.
2. Experimental work
2.1 Cavity design
The bounce amplifier laser geometry under investigation is depicted in Fig. 1. The laser crystal used was a Nd:YAG slab with 2 at.% Nd doping grown by the Czochralski method . The crystal dimensions were 30mm x 5mm x 2mm, and the two 5mm x 2mm end faces were angled to allow the laser cavity beam to be incident at Brewster’s angle to these faces and to experience total internal reflection from the diode pump face. This allows laser operation in a horizontally polarized state. The slab was diode pumped on the 30mm x 2mm face, which was antireflection (AR) coated for the 808nm pump wavelength. Heat generated by the pumping process was removed by conduction cooling of the 30mm x 5mm faces of the slab.
In the experiments, a nominally 40W diode bar and a 100W diode bar, both with fast axis collimation and emitting at 808nm, were used for diode pumping. The pump radiation was focussed onto the crystal with a 12.7mm focal length cylindrical lens (VCLD) producing a line focus on the pump face. The temperature of the pump diodes was optimised to give maximum laser output power at maximum diode pumping.
The Nd:YAG crystal was arranged in a grazing-incidence bounce geometry. The cavity was formed by a high reflectivity (HR@1064nm) back mirror and a partially reflective output coupler. Two vertical cylindrical lenses (VCL) with focal lengths f=60mm and AR coated for 1064nm were used to match the cavity laser mode size to the small gain region in the vertical.
2.2 CW operation
Figure 2 shows the result of output power of the 2 at.% Nd:YAG bounce laser versus input pump power for pumping with the 40W diode bar. The oscillator was a compact symmetric cavity with total length 15.6cm and a 50% reflectivity output coupler. Maximum output power of 19.1W was produced for 41W of diode pump power corresponding to an optical-to-optical conversion efficiency of 47% and a slope efficiency of 60%. Spatial quality of the output beam was single mode in the vertical and multimode in the horizontal.
To increase the output power, the crystal was pumped with a 100W diode bar. The results are shown in Fig. 3. A maximum output power of 46.1W for 101W of diode pump power was obtained which corresponds to an optical conversion efficiency of 46% and a slope efficiency of 49%. The beam was spatially multimode in the horizontal but single mode in the vertical. This is the highest power achieved, to our knowledge, from a highly doped (>1.1 at.%) Nd:YAG laser.
A single mode TEM00 cavity was constructed with a back mirror to crystal center distance of 11.6cm and output coupler to crystal center distance of 20.5cm. This asymmetric cavity configuration has two stability regions due to the pump power dependence of the thermal lens in the crystal . The cavity has an unstable region of operation between pump powers ~ 45W and 90W. A maximum output of 39.0W was produced at 101W diode pumping with a TEM00 mode with weak subsidiary horizontal wings. When these are spatially filtered, the M2 was measured to be < 1.2 in both horizontal and vertical directions.
2.3 QCW operation
An interesting advantage of Nd:YAG is its longer upper-state lifetime than vanadate crystals leading to improved energy storage capability. The upper state lifetime of 2 at.% Nd:YAG has been measured to be 175μs  which compares to a lifetime of 90 µs in the vanadate crystals. To investigate this capability we used quasi-CW pumping of the Nd:YAG laser cavity. The Nd:YAG crystal was diode pumped with 200μs long pulses at a repetition rate of 100Hz.
Figure 4 shows the output pulse energy as a function of pump pulse energy for a compact cavity, as described earlier. Maximum output pulse energy of 11mJ for 28mJ of diode pumping has been obtained at a conversion efficiency of 39% and a slope efficiency of 45%. The pumping threshold was 1.0mJ. To our knowledge, this is the first demonstration of QCW pumping of 2 at.% Nd:YAG.
2.4 QCW operation and passive Q-switching
A passively Q-switched laser cavity using the 2 at.% YAG crystal in a bounce amplifier configuration is shown in Fig. 5. A 4.4mm long, 0.3% doped Cr4+:YAG crystal with optical density of 0.42 was used as a saturable absorber. To control the Q-switch time relative to the pump pulse, the pump radiation size was controlled in the vertical by varying the distance of a f=50mm cylindrical lens (VCLD) from the crystal pump face. The larger pump region made the intracavity lenses unnecessary. The back mirror was positioned at 3cm from the crystal center, and a 70% reflectivity output coupler was used, positioned 12cm from the crystal center.
In this configuration without the Cr:YAG passive Q-switch, a maximum pulse energy of 8.1mJ was obtained for a pump pulse energy of 28mJ, with a slope efficiency of 40% (see Fig. 6(a)). The threshold energy was 5.9mJ.
The QCW pumped cavity was then passively Q-switched with the Cr4+:YAG saturable absorber. The distance of the pump focusing lens from the crystal was optimized to obtain a single giant output pulse from the cavity at the end of the pump pulse, thus optimizing the output pulse energy. Maximum output pulse energy of 2.3mJ was produced in a 12ns pulse (see Fig. 6(b)) for 28mJ of diode pump power. Spatially the output was TEM00, as depicted in Fig. 7, with M2 of less than 1.3 and 1.1 in the horizontal and vertical, respectively.
In conclusion, we have demonstrated the highest output power to date in a highly doped crystalline 2 at.% Nd:YAG laser, producing 46.1W of multimode output and 37.8W of TEM00 mode output at 1064nm in a bounce amplifier geometry. Slope efficiency of 60% was produced in a system producing output power of 19.1W. Using QCW diode pumping, output pulses with 11mJ energy at 100Hz repetition rate were demonstrated. Passive Q-switching of a QCW diode-pumped laser produced pulses with 2.3mJ energy and pulse duration of 12ns.
The authors acknowledge support from the Engineering and Physical Sciences Research Council (UK) under grant number GR/T08555/01.
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