Subwavelength grating structures have been studied in the last two decades for a wide range of integrated optoelectronic applications that include narrow-band filters, lasers, couplers and broadband high reflectors. In particular, using high-contrast refractive indices, broadband reflectivities exceeding 99% have been demonstrated in such grating structures. In this study, we investigate an active mirror structure by embedding a gain medium. We call these devices gain-embedded meta-mirrors (GEMMs). Our optical wave propagation analysis uses a RCWA software to identify optimum GEMM structures that provide highest active reflectivity. An RCWA design procedure that can be interpreted in terms of elementary grating diffraction theory is presented. The procedure optimizes high reflectivity as well as manufacturing tolerance. A key advantage of DBR-free membrane VECSELs has been their potential for efficient heat removal. GEMM devices equally offer this advantage with the added benefit that the optical modes are only evanescent in the thermal substrate (diamond, SiC or sapphire). We will show that this property will allow us to perform more efficient heat-sinking of the thermal substrate. We will also present our preliminary experimental results on fabricating a GEMM device using e-beam lithography and AlGaAs /GaAs/AlGaAs double heterostructures as the gain medium.
With ultrathin gain media, traditional vertical-external-cavity surface-emitting lasers (VECSELs) in theory allow for ideal power scaling with mode area given the one-dimensional heat flow from the active material and into the underlying heatsinking structure. In experiments, however, the integrated semiconductor distributed Bragg reflector (DBR) and its large thermal resistance hamper that scalability. DBR-free semiconductor disk lasers (SDLs), using gain membranes without DBRs and taking advantage of direct bonding, allow for heat dissipation from both sides of the gain medium. Our previous numerical thermal analysis has shown potential advantages in thermal management for this dual-heatspreader configuration, or membrane external-cavity surface-emitting laser (MECSEL) structure, over traditional VECSELs. In this paper, we present both theoretical and experimental performance comparisons between DBR-free SDLs in both single- and dual-heatspreader configurations. Under similar cavity and pumping conditions, the dual-heatspreader configuration has a comparable slope efficiency but experiences thermal roll-over at twice the incident pump power when compared to the single-heatspreader configuration. After optimization of the output coupling efficiency, a maximum output power of 16 W near 1040 nm is collected with the dual-SiC-heatspreader configuration at a coolant temperature of 10 ͦC. With the availability of wafer-scale SiC heatspreaders, we show the potential for mass production of SDLs employing a dual-heatspreader configuration.
We present the experimental progress in the investigation of radiation-balanced lasers in Yb:YAG and Yb:YLF thin discs. Due to low absorption of the pump beam, we explored pump absorption schemes including VECSEL intracavity pumping as well as multipass approach. The results are compared with theory and predictions are made for prospects of these lasers for high power (multi kW) operation.
Using vibronic transitions in rare-earth doped crystals as a case-study, we present detailed analysis for the optimum operation of radiation-balanced lasers. In particular, conditions for achieving highest output power and highest optical-to-optical efficiency in Yb:YAG and Yb:YLF thin disc RBLs are given. Finally, we extend our analysis to two-tone RBL systems involving Ho-Tm doped crystals.
Laser cooling in Tm:YLF and Tm:BYF crystals has recently been reported. We investigate high power laser cooling of Tm doped crystals under high vacuum using multiple-pass Herriott cell configuration. We also model potential mid-IR Radiation Balanced Lasers (RBLs) in available Tm:YLF and Tm:BYF crystals. Our experiments and modelling shows that our 1% wt. Tm:BYF sample is a promising 2 µm RBL candidate, since it has high gain and high external quantum efficiency as well as good room temperature cooling efficiency. We will attempt to demonstrate the first mid-IR RBL experimentally in Tm:BYF crystal as well.
Despite achievements of extremely high external quantum efficiency (EQE), 99.5%, the net cooling of GaAs|GaInP double heterostructures (DHS) has been elusive. This is primarily due to the parasitic absorption, which originates from the GaInP passivation layers at long wavelengths. In samples with thin GaInP passivation layers, we report an EQE of 99%, approaching theoretical requirement for being heat neutral. Additionally, we investigate the EQE of MBE-grown GaAs|AlGaAs DHS versus temperature; the results compare well with that of GaAs|GaInP at and below 150 K. Also, initial measurements of parasitic absorption at shorter wavelengths is presented.
Employing large surface-area-to-volume ratio gain, thin-disk lasers have shown great potential in power scaling. But thermal management for these devices is still challenging. One possible approach is to balance the heat load generated by the lasing process with cooling power from the anti-Stokes cooling process, forming radiation balanced lasers (RBLs). Compared to bulk RBLs, thin-disk RBLs can be better thermally balanced with reduced thermal gradients, promising higher output power and better beam quality. In this paper, we analyze and investigate radiation balanced disk lasers with Yb:YAG and Yb:YLF crystals in different pumping configurations.
Semiconductor disk lasers (SDLs) are attractive for applications requiring good beam quality, wavelength versatility, and high output powers. Typical SDLs utilize the active mirror geometry, where a semiconductor DBR is integrated with the active region by growth or post-growth bonding. This imposes restrictions for the SDL design, like material system choice, thermal management, and effective gain bandwidth. In DBR-free geometry, these restrictions can be alleviated. An integrated gain model predicts DBR-free geometry with twice the gain bandwidth of typical SDLs, which has been experimentally verified with active regions near 1 μm and 1.15 μm. The lift-off and bonding technique enables the integration of semiconductor active regions with arbitrary high quality substrates, allowing novel monolithic geometries. Bonding an active region onto a straight side of a commercial fused silica right angle prism, and attaching a high reflectivity mirror onto the hypotenuse side, with quasi CW pumping at 780 nm, lasing operation was achieved at 1037 nm with 0.2 mW average power at 1.6 mW average pump power. Laser dynamics show that thermal lens generation in the active region bottlenecks the laser efficiency. Investigations on total internal reflection based monolithic ring cavities are ongoing. These geometries would allow the intracavity integration of 2D materials or other passive absorbers, which could be relevant for stable mode locking. Unlike typical monolithic microchip SDLs, with the evanescent wave coupling technique, these monolithic geometries allow variable coupling efficiency.
Despite achievements of extremely high external quantum efficiency (EQE), 99.5%, the net cooling of GaInP|GaAs double heterostructures (DHS) has never been realized. This is due to an unknown source of parasitic absorption. Prior studies have ruled out the possibility of the bulk absorption from the GaAs layer. Thus it is thought to be either at the air- GaInP interface, through the presence of dangling bonds, or in bulk GaInP through impurities. Using two-color thermallens calorimetry (based on the Z-scan technique), this study indicates that that the parasitic absorption likely originates from the GaInP bulk layers.
The work of Pique showed that multiple guidestars emitting at 1140 nm and 589 nm simultaneously could be utilized to correct for Tip and Tilt aberrations [1]. Such a guidestar is hence known as a PLGS (Polychromatic Laser Guidestar). However, no current high power (> 5W) narrow bandwidth (< 1GHz) exist for 1140 nm emission. A Vertical External Cavity Surface Emitting Laser (VECSEL) is shown with high power > 12W and narrow bandwidth emission which has been successfully used to pump the sodium 3P3/2 to 4S1/2 sodium transition as a testbed for the development of a CW PLGS system.
To date, three types of laser sources have been used to excite mesospheric sodium atoms to use as a sodium guidestar for adaptive optics (AO). All these sources have inherent challenges and a possible fourth source is to utilize a frequencydoubled Vertical External Cavity Surface Emitting Laser (VECSEL). Such a VECSEL presents output efficiency above 20% with power in excess of 20 W. Modelling is also presented to validate the efficacy of developing VECSEL guidestar systems for use with current guidestar systems or as a stand-alone guidestar. The model agrees with the data collected with the 3.5 m telescope and narrowband laser guidestar at Starfire Optical Range.
We report a DBR-free semiconductor disk lasers centered at 1160 nm with a tuning range of 78 nm, and ongoing effort on our DBR-free SDL centered at 1040 nm. Compared with conventional semiconductor disk lasers, DBR-free SDLs have a broader effective gain bandwidth. In CW operation, 2.5 W output power at 1160 nm and 6 W at 1055 nm were collected from the two lasers without thermal-rollover. Intracavity loss mitigation, currently underway, should improve power scaling and efficiency in these systems.
Optically pumped semiconductor disk lasers (SDLs) provide high beam quality with high average-power power at designer wavelengths. However, material choices are limited by the need for a distributed Bragg reflector (DBR), usually monolithically integrated with the active region. We demonstrate DBR-free SDL active regions, which have been lifted off and bonded to various transparent substrates. For an InGaAs multi-quantum well sample bonded to a diamond window heat spreader, we achieved CW lasing with an output power of 2 W at 1150 nm with good beam quality.
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