A new flexible pump source, the optically-pumped semiconductor disk laser (SDL), for the Cr2+:ZnSe laser is reported. The SDL provides up to 6W output power at a free running central wavelength of 1.98μm. The Cr2+:ZnSe laser operated at an output power of 1.8W and a slope efficiency of ~50% with respect to absorbed pump power whilst maintaining a low output intensity noise figure of <0.14% RMS. The system required no optical isolation even under the situation of significant optical feedback.
©2009 Optical Society of America
Since the first room temperature demonstration of zinc-based chromium-doped chalcogenide lasers in 1996 by DeLoach et al. , Cr2+-chalcogenide lasers have attracted considerable attention within the laser community. Chromium-doped zinc selenide (Cr2+:ZnSe) especially, has shown great potential due to its broad emission bandwidth between 1.8 and 3.1μm , its ability to be directly diode laser pumped  and the demonstration of mode-locked embodiments with pulses as short as ~80fs . These results make this type of laser a promising source for mid-infrared applications such as spectroscopy, process monitoring and free-space communication. Despite this, however, chromium-doped chalcogenide lasers have found limited commercial uptake due, in part, to their previous lack of practicality. The availability of low-cost, high-quality gain crystals has been addressed (e.g. by Photonics Innovations Inc. Birmingham, AL), however, the additional requirement for a practical and compact pump source persists. So far, various pump laser systems, such as rare-earth-doped solid-state lasers, diode lasers, optical parametric oscillators and Co2+:MgF2 have been used [2, 5–8]. In this paper we present an alternative practical pump laser for Cr2+:ZnSe lasers - the optically-pumped (AlGaIn)(AsSb)-based semiconductor disk laser (SDL) . In contrast to the majority of solid-state laser gain media, the SDL format generally permits engineering of the emission wavelength over a wide range in the visible and infrared [10, 11]. Specifically, when used as a pump source, the pump wavelength of the SDL may be tailored to particular system requirements, e.g. tuning precisely to the absorption maximum to increase efficiency or by trading the pump absorption against quantum defect in order to reduce parasitic thermal issues.
In addition to potential low quantum defect pumping, the notable low-noise and high-brightness operation of the SDL format also plays an advantageous and important role. While the latter offers the prospect of efficient pumping and flexibility in pump geometry design (e.g. multi-pass pumping), the low-noise operation is marked , resulting in negligible pump feedback instabilities and a very quiet Cr2+-laser output. Finally, the potentially compact and low-cost nature of the SDL format allows the prospect of a truly practical and flexible pump laser for these materials. Such sources will then provide cost effective and high performance lasers to address many prospective applications in the short-wave (2-3μm) mid-IR spectral region.
In this paper we present a compact Cr2+:ZnSe laser pumped with a 2µm SDL. The maximum output power obtained was 1.8W and was tuneable over a range of 300nm, limited by the available optics. The RMS noise figure of the system was measured to be <0.14%.
2. The semiconductor disk laser
The semiconductor disk laser used in this work  is based on the (AlGaIn)(AsSb) material system and consists of a distributed Bragg reflector (DBR) on which an active region containing ten compressively-strained GaInSb quantum wells and pump-absorbing AlGaAsSb barrier regions is grown. An AlGaAsSb confinement window and a GaSb cap layer complete the device structure. To aid in thermal management, a diamond heat-spreader was attached to the semiconductor chip using liquid assisted capillary bonding [14, 15]. This assembly was then affixed to a cooled brass mount.
To pump the SDL, two 11mm focal length lenses were used to focus the radiation of a 45W, fibre coupled diode laser, radiating at 980nm, onto the chip. An external resonator was then set up using a 100mm radius of curvature mirror and an R=91% output coupler (see Fig. 1 ). With a separation between curved mirror and chip off 60mm and an arm length of ~250mm, this created a cavity waist on the chip of 53μm. The cavity and pump mode overlap at the SDL chip could be adjusted by simple defocusing of the pump by translating the pump optics f1 & f2. This was possible because of the quasi-2-dimensional absorption profile of the SDL chip due to its relatively thin active region (2μm) compared to the pump and cavity spot sizes.
In this configuration the system delivered output powers as high as 3.3W and 6W at cooling temperatures of 21°C and −15°C respectively. The threshold and slope efficiency at −15°C were measured to be 2.4W and 25% (Fig. 2 ). The reason that low temperature was required for maximum performance is mainly associated with the limitation of the overall heat extraction from the chip, and the offset between the micro-cavity resonance and the peak gain wavelength. While the latter can be addressed by the design of the SDL chip, the overall heat extraction can be improved in various ways, e.g. substrate removal, thinning of the distributed Bragg reflector, and upside-down growth of the chip . However, in the mid-infrared region these methods are not as efficient as the intracavity heatspreader scheme as the thermal conductivity of the DBR is high compared to the counterparts in the near infrared and visible . A more efficient route to thermal management and power scaling is therefore the division of the pump induced heating onto multiple SDL chips such as demonstrated in .
The brightness (M2 parameter) of the SDL was measured using a scanning-slit beam profiler (DataRay Beamscope P8). In the configuration above, the highest power at room temperature (3.3W) resulted in an M2 value of ~5.8. By reduction of the resonator arm length to ~55mm, and thereby improving the mode overlap of the cavity and resonator mode , the M2 value was improved to <1.3 with a reduction in output power of only 6% (3.17W).
For tuning purposes a 2mm thick quartz birefringence filter (BRF) plate oriented at Brewster’s angle was inserted into the SDL cavity. By rotating the BRF, tuning of the central oscillation wavelength was achieved over a range of 80nm between 1.94 and 2.02μm. The maximum output power was obtained at a wavelength of 1.98μm, as shown in the inset to Fig. 2.
With the emission around 2μm, the SDL introduces a low quantum defect of ~20% when used to pump Cr2+:chalcogenides, hence mitigating problems associated with the strong thermal lensing of Cr2+-doped lasers. As less heat is introduced into the system, higher output powers can in principle be achieved with less stringent thermal management. However, moving away from the absorption peak near 1.8μm reduces the overall absorption for a given crystal length which may be countered by increasing the length of the gain medium or by employing a multi-pass pumping geometry. Alternatively, by redesigning the SDL chip itself, the emission wavelength can be shifted towards the absorption peak of Cr2+:ZnSe at the expense of quantum defect.
3. The SDL pumped Cr2+:ZnSe laser
To investigate the performance of the SDL as a pump source for chromium-doped chalcogenide lasers, a Cr2+:ZnSe laser having a 3-mirror resonator was configured (Fig. 3 ). The uncoated, 3mm long, Cr2+:ZnSe crystal was held in place by an uncooled brass mount and oriented normal to the resonator axis as defined by the cavity mirrors. The mirrors were coated to provide high reflectivity for the wavelength range 2.3-2.5μm.
The 100mm radius of curvature folding mirror and the 50mm radius of curvature end mirror were sited 50mm away from the crystal. The long arm length was approximately 260mm. This resulted in a calculated cavity waist within the crystal of ~60μm. An antireflection-coated f=70mm plano-convex lens was used to focus the 2μm SDL beam onto the Cr2+:ZnSe crystal. The single-pass pump absorption was measured to be 69% (i.e. α=3.9cm−1). Therefore the pump absorption with respect to incident power (i.e. taking Fresnel reflections into account) was ~60%. By way of a secondary pump reflector (Mp in Fig. 3 - a 100mm ROC mirror coated for 1.94μm) the unabsorbed pump was coupled back into the crystal, hence increasing the pump absorption to approximately 72%.
With the SDL mount held at room temperature (21°C), oscillation of the Cr2+:ZnSe laser was readily achieved using various output coupling mirrors with reflectivity values between R=61-99% (see Fig. 4 ). The lowest threshold of 40mW absorbed pump power was obtained using a 99% reflectivity mirror. Using an output coupler having a reflectivity of 71%, the laser generated a maximum output power and slope efficiency with respect to absorbed pump power of 1W and 56% respectively.
When the SDL mount was cooled to 15°C (increasing the available pump power to 6W) the Cr2+:ZnSe laser produced up to 1.8W of output power which was limited by the available pump power. The threshold and slope efficiency were found to be 125mW and 48% respectively with respect to the absorbed pump power (see Fig. 5 ).
Based on the power transfer characteristics, Findlay-Clay  and Caird  analyses were undertaken to estimate the passive losses inside the cavity. The resultant estimated roundtrip loss was ~3.5% - largely attributed to crystal surface preparation. Further improvement in the efficiency of the system would therefore be expected through improvements in the crystal quality.
To investigate the tuning behaviour of the SDL pumped Cr2+:ZnSe laser, the cavity was re-configured into a 4-mirror resonator that used a reduced output coupling of T=2%. Furthermore, a Brewster-angled ZnSe prism was inserted into the second cavity arm which by varying the angle of the high reflecting (HR) plane end mirror allowed tuning of the output wavelength over a spectral range of 300nm between 2.25µm and 2.55μm (see inset to Fig. 5). This tuning range was limited by the cavity mirror coatings, however, by using mirrors with broader HR coatings should enable full tuning over the range of 2-3μm. However, at the 2μm end of this range, re-absorption processes will dominate, and so, reduce the maximum output powers of the Cr2+:ZnSe laser.
A distinct advantage of using the SDL as a pump source is its low noise operation – a consequence of its short upper-state lifetime (on the order of nanoseconds). This also makes the SDL highly resistant to residual feedback from the pumped laser - a serious problem in more conventional systems  resulting in the need for stringent optical isolation. As the polarisation selectivity of a 3-mirror SDL is weak, feedback can result in polarisation switching leading to increased intensity noise. However, enhancing the polarisation selectivity by introducing a tuning element (e.g. prism or BRF) or a simple angled plate is sufficient to eliminate this problem, hence completely removing the need for optical isolation. In the context of this work, a 6mm thick, Brewster-angled, fused silica plate was placed within the resonator to define the polarisation. For this configuration, the output intensity noise of the Cr2+:ZnSe laser was measured using a fast photodiode and a digital oscilloscope in the frequency range of 0.1-100kHz and was found to be below the resolution limit of the measurement system – i.e. <0.14% RMS. No attempt was made to minimise the retro-reflected pump back to the SDL (N.B. the uncoated Cr2+:ZnSe was pumped at normal incidence giving a Fresnel reflection of ~17.5%). Furthermore, when the pump retro-reflecting arrangement was used the situation was more extreme; however, in both cases no pump laser instability was induced.
Efficient, low intensity noise operation of a Cr2+:ZnSe laser pumped with an optically-pumped semiconductor disk laser has been demonstrated. The laser exhibited a maximum output power of 1.8W with a threshold of 125mW and a slope efficiency of 48%. The intensity noise of the Cr2+:ZnSe laser system was measured to be less than 0.14% RMS. The (AlGaIn)(AsSb)-based SDL, emitting at ~2μm, shows great promise for use as a high-brightness, low-noise pump source for mid-infrared chalcogenide lasers. The SDL format permits efficient wavelength engineerable pumping while having the potential to be very compact and of low cost, and so, shows great potential for the development of practical mid-IR solid-state lasers. Future work will concentrate on the improvements of both the pump and the doped chalcogenide crystal quality, as well as the investigation of alternative pumping geometries, to further extend the system performance.
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