We show for the first time that multi-ten Watt operation of an Alexandrite laser can be achieved with direct red diode-pumping and with high efficiency. An investigation of diode end-pumped Alexandrite rod lasers demonstrates continuous-wave output power in excess of 26W, more than an order of magnitude higher than previous diode end-pumping systems, and slope efficiency 49%, the highest reported for a diode-pumped Alexandrite laser. Wavelength tuning from 730 to 792nm is demonstrated using self-seeding feedback from an external grating. Q-switched laser operation based on polarization-switching to a lower gain axis of Alexandrite has produced ~mJ-pulse energy at 1kHz pulse rate in fundamental TEM00 mode.
© 2014 Optical Society of America
Laser sources with flexible wavelength and diverse pulse capability (ns/ps/fs) provide benefit to many applications (e.g. remote sensing, bio-photonics, multiphoton microscopy, biochemical detection) and scientific studies. Diode-pumped Nd:YAG/Nd:YVO4 lasers at 1064nm are well-established but offer no tunability, and together with their harmonics (532nm/355nm/266nm) they entirely miss whole wavelength regions. Optical parametric conversion can be used to generate other tunable wavelength regions but leads to high complexity, loss of efficiency and reliability, and increased cost that is unacceptable or undesirable in many cases.
Vibronic solid-state lasers provide an alternative approach due to their broad tunability and femtosecond pulse capabilities. Perhaps the most widely used vibronic laser is Ti-sapphire (Ti:S), providing the widest wavelength tuning range ~700–1100nm and direct generation of the shortest femtosecond pulse duration (~5fs) [1,2]. Attempts at diode-pumped operation of Ti:S has been very limited in power and efficiency [3,4], so optical pumping of Ti:S still requires complex pump sources, especially frequency-doubled Nd:YAG/Nd:YVO4 lasers, that add significantly to the cost and complexity, and lead to poor overall system efficiency. Large size, high cost and low efficiency limit widespread take-up of such technology into many applications.
An alternative vibronic laser is Alexandrite (Cr-doped chrysoberyl), occupying a similar part of the spectrum ~700–858nm [5–8]. A key advantage of Alexandrite is its capability for direct diode-pumping by red (AlGaInP) diodes. Although red diodes (630–690nm) are not at the absorption peak of Alexandrite (~590nm) , the absorption coefficient is suitably high (~6cm−1 at 639nm for 0.2 at.% Cr-doping) to provide sufficiently high absorption to deliver intensive pumping of the Alexandrite gain medium.
Alexandrite is amongst a broad class of Cr-doped lasers, including the Cr-doped colquiriites (Cr:LiSAF, Cr:LiCAF, Cr:LiSGaF ), that can be pumped by red diodes. However Alexandrite has a number of superior thermo-mechanical properties that make it particularly attractive as a laser gain medium for high power/energy operation. Its thermal conductivity (23Wm−1K−1)  is almost twice that of Nd:YAG and five-times that of the Cr-doped colquiriites. Additionally, its fracture resistance is five-times that of Nd:YAG . Alexandrite is strongly birefringent, giving highly linearly-polarized laser emission, and eliminating depolarization problems. Its long upper-state (room-temperature) lifetime ~260μs  allows for good energy storage potential, making it advantageous for Q-switched operation, in stark comparison to that of Ti:S whose upper-state lifetime is just ~3μs . The stimulated emission cross-section for Alexandrite is low (0.7x10−20cm2) requiring high laser fluence for efficient gain extraction, but this is offset by Alexandrite’s extraordinarily high optical damage threshold (>270Jcm−2) .
Despite its potential as a directly diode-pumped laser source to address high power, tunable and femtosecond applications in this key wavelength region, there have been only a small number of publications reporting diode-pumped Alexandrite laser operation, the majority of which operating at very modest sub-Watt power level. The first diode-pumped Alexandrite laser demonstration was by Scheps (1990)  who also (1993) obtained 25mW with 28% slope efficiency . Peng (2005) produced 1.3W (slope efficiency 24%) , and Beyatli (2013) 195mW (slope efficiency 34%) using high brightness tapered diodes . All these demonstrations were end-pumped. Our previous work, Damzen (2013), produced 6.4W in a diode side-pumped Alexandrite slab laser .
One purpose of this present paper is to experimentally demonstrate that it is possible to scale diode-pumped Alexandrite to multi-ten-Watt powers with end-pumping, whilst maintaining high efficiency. By employing an end-pumped rod laser configuration, 26.2W of continuous-wave (CW) output power is demonstrated from 64.5W pump power (at 639nm), with slope efficiency of 49%. This power is one to two orders of magnitude higher than previous diode end-pumped demonstrations. The slope efficiency is also the highest of diode-pumped Alexandrite lasers to date. In a TEM00 laser cavity design, wavelength tuning from 730 to 792nm is demonstrated, using a self-seeding technique from feedback provided by an external Littrow grating. Q-switched operation of the laser is presented, where an electro-optic modulator provides loss by switching the laser polarisation to the lower gain axis of Alexandrite. Preliminary results show ~mJ-level pulse energy at 1kHz pulse rate in fundamental TEM00 operation. This is the first Q-switched operation of a diode-pumped Alexandrite laser, to our knowledge.
2. Compact diode end-pumped Alexandrite laser cavity
Initial studies were performed on a compact rod laser cavity, to investigate the power and efficiency potential of Alexandrite laser operation with high power (multi-ten Watt) diode end-pumping. The laser configuration is shown in Fig. 1 with the end-pumped Alexandrite rod in a plane-plane mirror cavity. The cavity length was 15mm. The back mirror (BM) was highly-reflecting (R>99.9%) at laser wavelength (~755nm) and highly-transmitting (R<0.2%) for pump (~639nm). Several output couplers (OC) were tested with reflectivity R = 99%, 98%, 97%, and 95%.
Two Alexandrite rods were investigated in this work, both with length 10mm and diameter 4mm: one with 0.13 at.% and the other with 0.22 at.% Cr-doping concentration. The end faces of the rods were plane-parallel and anti-reflection coated at the Alexandrite wavelength (~755nm). Both rods were mounted in water-cooled copper heat-sinks with an indium foil interface for enhanced thermal contacting of the crystal to the copper. The absorption coefficient (α) for the two crystals, measured with a He-Ne at 633nm, was ~4cm−1 for the 0.13 at.% rod and ~6cm−1 for the 0.22 at.% rod for light polarized parallel to the crystal b-axis of Alexandrite.
Pumping was achieved with a red diode module, operating nominally at 639nm with bandwidth (FWHM) of 1.2nm and capable of providing high pump power exceeding 60W in continuous-wave mode. The highly-multimode spatial output of this module (fast axis Mf2 ~25; slow axis Ms2 ~200) was reshaped with cylindrical lenses, providing a near-circularized pump beam with spot size (FWHM) ~350μm located near the input face of the rod. The confocal parameter of the pump in the slow axis (~4mm) was sufficiently longer than the absorption depth of the crystals to allow good laser mode overlap with the pump. The polarisation of the module was near-linear (with polarisation purity ~95%) in the fast axis and was oriented parallel to the b-axis of the Alexandrite rod for maximum absorption. The ~5% polarisation component orthogonal to the b-axis is poorly absorbed by the crystal.
Figure 2 shows the results of the output power of the diode end-pumped laser against absorbed pump power, using the 0.13 at.% rod with the OC of reflectivity R = 99%.
The results for pumping with the near-circularized pump diameter (FWHM) ~350μm (red circles) show an output power of 21.3W with 57.8W of pump power, corresponding to an optical-to-optical conversion efficiency of 37%. The slope efficiency is 41%. The fast-axis size of the pump beam was decreased to ~210μm, whilst maintaining the slow-axis size, to demonstrate the effect of more intensive pumping. The results of this are also shown (blue squares) in Fig. 2. The slope efficiency of the laser increased to 45%, and an output power 26.1W was produced for pump power of 64.5W (optical-to-optical conversion efficiency 40.5%).
Figure 3 shows the peak lasing wavelength and lasing bandwidth (FWHM) against pump power for the circularized pump beam of ~350μm diameter. The laser shifts to longer wavelengths with increasing pump power, which may be associated with increasing crystal temperature . The wavelength near 60W pumping is ~759nm. The lasing bandwidth narrows from about 4 nm at 20W pumping to 2.3 nm near 60W.
Figure 4 shows the output power for the end-pumped Alexandrite laser, this time using the higher doped 0.22 at.% rod, for OC reflectivity R from 99% to 95%.
The highest slope efficient was 49% for the R = 99% output coupler with the reduced pump size (fast-axis size ~210μm) producing 26.2W at the pump level of 64.5W. It is noted that the slope efficiency at lower pumping level is even higher than 49%. Threshold is somewhat higher and there is more evidence of roll-over at high pump power than the 0.13 at.% rod, although some of this effect may be cavity alignment. The variation of output power with different OC’s using the 350μm circular pump size reveals a trend to lower slope efficiency with lower OC reflectivites: 43% (R = 99%); 39% (R = 98%); 33% (R = 97%); and 31% (R = 95%). The decreased slope efficiency is not consistent with simple 4-level laser theory, and has been noted previously . It may suggest the presence of energy transfer up-conversion in the Alexandrite crystal and would be worthy of further investigation.
The results for the two Alexandrite rod lasers show that the output power is limited only by the pump power. Just two pump distributions were attempted in this study and it is likely that with further pump optimization even higher efficiency can be achieved. It is informative to consider some efficiency factors involved in the Alexandrite laser. The photon quantum efficiency (ratio of laser to pump photon energy) = λp/λL = 84% where λp and λL are the pump and laser wavelength, respectively. The cavity loss factor = T/(T + L) ~71%, where T = 1% is the output coupling transmission and L ~0.4% is the roundtrip intracavity loss (due to the rod AR-coatings). The beam overlap efficiency (fill-factor) between the laser mode volume with the gain volume is estimated to be ~80–90% . The pump quantum yield is taken to be 100% . The product of these efficiency factors gives slope efficiency ~48–54%. Interestingly, Scheps  achieved 63.8% slope efficiency of an Alexandrite laser pumped by a red dye laser at 645nm, showing potential for higher efficiency with optimized cavity and diode pumping.
3. TEM00 laser operation with wavelength tuning and Q-switching
This section describes a first attempt at configuring the Alexandrite laser for tunable, Q-switched and TEM00 operation. A motivation for developing such a pulsed laser source is to address space-borne remote sensing applications, including atmospheric lidar and altimetry.
Figure 5 shows an extended end-pumped laser cavity design (cavity length ~125mm), with intracavity plano-convex lens (PCX, f = 100mm) and Pockels cell Q-switch. The intra-cavity lens design was chosen to optimize for TEM00 operation and also to achieve a larger mode size in the resonator, to minimize laser-induced damage to the Pockels cell. The OC with lower reflectivity (R = 95%) was chosen to reduce the intra-cavity flux, that also may cause laser-induced damage at high peak power Q-switched operation. For this part of the study, the laser rod with 0.22 at.% Cr-doping was used with circularized pump size ~350μm. An external feedback grating was used, just for the wavelength tuning demonstration.
3.1 Wavelength tuning
A brief investigation was undertaken to explore the impact of wavelength tuning on the performance of the end-pumped laser. A 1800 lines mm−1 holographic grating in Littrow configuration was placed external to the output of the laser cavity, as shown in Fig. 5. Tuning of the cavity was achieved by adjusting the angle of the grating, with the first diffraction order providing the seed to the cavity, and the zeroth order used as the output. The laser was tuned continuously between 730 and 792nm, with cavity in free-running (non-Q-switched) mode. Outside of this range, the cavity reverted to the central wavelength of Alexandrite, ~755nm. Tuning was limited by self-seeding strength but indicative that a much broader tuning range is possible. It is noted that at 792nm, the output power was 39% of the power at peak wavelength ~755nm.
3.2 Q-switching demonstration
For Q-switching demonstration, the pump module was operated in QCW mode with adjustable pump duration and repetition rate, as required. The Q-switching element was a BBO Pockels cell. The Pockels cell was operated at 2kV, near its quarter-wave voltage, and timed for voltage on during pumping to inhibit lasing, and removed at the end of the pump pulse to induce Q-switched output. To minimize complexity, Q-switched operation was based on a polarization switching technique [5,15], where the modulator achieves loss by switching the laser polarization to the lower gain axis of Alexandrite on alternate cavity transits. This requires no additional Brewster plate or polarizer, but the Pockels cell hold-off, in principle, is limited to twice threshold pumping of the cavity .
Figure 6 shows Q-switched output energy against pump energy, for two example cases. In case 1 (red squares), the laser cavity length was 125mm; the intra-cavity lens had focal length 100mm and was positioned to form a stable cavity configuration and to optimize TEM00 operation. Pulse energy 0.74mJ (pulse duration 92ns) was achieved at 1kHz repetition rate for a pump duration of 0.22ms, for circularized pump size ~350μm. In case 2 (blue circles), a shorter cavity of length 60mm was used, without the lens, with pump duration 0.2ms and fast-axis pump size reduced to 210μm. Pulse energy 0.7mJ (pulse duration 58ns) was achieved at 100Hz; the slope efficiency is 12.5%. Beam quality was TEM00 with M2 ~1.1 for all cases, except the final point for 1kHz pulse data (M2 ~1.6) where the mode size was adjusted to achieve higher output energy by increased mode-gain overlap.
The pulse energy and efficiency of the laser increased, in general, with repetition rate. The improvement is thought to be due to the increasing pump load and hence temperature of the crystal that enhances its emission cross-section . Higher temperature crystal operation may be a route to enhanced performance; this will be a later investigation as the cooling system used did not allow operation much above room temperature (20°C).
This work shows for the first time that highly efficient, multi-ten Watt operation of an Alexandrite laser can be achieved with direct high-power red diode-pumping, even with relatively low brightness diodes. A diode end-pumped Alexandrite rod laser has demonstrated output power 26.2W, more than an order of magnitude higher than previous diode end-pumped Alexandrite lasers. A slope efficiency of 49% was demonstrated, the highest reported for a diode-pumped Alexandrite laser to date.
Wavelength tuning between 730 and 792 nm was demonstrated, with a self-seeding technique using feedback from an external grating. Q-switched laser operation based on a polarization switching technique shows preliminary results with ~mJ-pulse energy at kHz pulse rate in TEM00 operation. Laser efficiency and pulse energy is found to improve at higher repetition rate. Notably, no intra-cavity optical damage occurred during Q-switching. Further pump optimization is expected to increase pulse energy and efficiency and reduce pulse duration.
This work shows promising capability for diode-pumped Alexandrite lasers, for high average power, and pulsed operation at high repetition rates. This work is unoptimized and there is considerable scope for improvements, including high temperature operation of Alexandrite, exploring different Cr-doping concentrations, and enhanced cavity design, with optimized pumping. Further development of the current end-pumped laser system will be towards multi-mJ and multi-kHz operation, as well as harmonic generation as a tunable UV laser source. Alongside we are developing side-pumped slab Alexandrite lasers  for higher pulse energy capability. It is our hope that efficient direct diode-pumped Alexandrite laser technology could address future remote sensing requirements as well as other applications, including biophotonics and femtosecond/attosecond science.
We gratefully acknowledge financial support for this work from the European Space Agency (ESA) under contract 4000107239/12/NL/PA.
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