Type-II vertical-external-cavity surface-emitting laser with Watt level output powers at 1.2 μm

Many applications have a demand for lasers in the infrared regime. Prominent examples are telecommunication and optical data transfer where the wavelength of light sources has to be adapted to the available propagation media.¹ For instance, a minimum absorption in optical fibers is around 1.3 μm.² Other examples are frequency doubling for red emitters in digital projectors, where wavelengths above 1.2 μm are of particular interest, or medical applications.³ In the latter case, eye-safe emitters with wavelengths above 1.4 μm are desired where the corneal absorption can provide a natural protection of the retina.⁴ 

In the infrared, above 1.2 μm, conventional semiconductor lasers with gain media based on type-I quantum well heterostructures suffer from significant non-radiative Auger losses.⁵ In recent years, type-II band-aligned quantum wells have become a promising alternative. Especially, “W”-quantum well heterostructures have been considered which, despite the spatial separation of electrons and holes, still provide sufficient overlap between the electron and hole wave functions.^6–9 In comparison to type-I systems, a type-II band alignment enables a more flexible band structure engineering in order to reduce intrinsic losses while the emission wavelength can be kept constant. Thus, such type-II lasers are promising candidates to surpass conventional type-I lasers with respect to wavelength versatility and performance in the infrared regime. To date, first lasers based on type-II quantum well heterostructures have been reported based on the InP, GaSb, and GaAs material systems with emission wavelengths even in the mid infrared.^10–13 Particularly, at an emission wavelength of 1.2 μm, an edge emitter with a maximum output power of 50 mW at 20 °C was demonstrated.¹⁰ 

Recently, we have evaluated the potential of the GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs material system as gain medium for vertical-external-cavity surface-emitting lasers (VECSELs) with an emission wavelength of 1.2 μm.¹⁴ Photoluminescence (PL) spectra of such type-II multiple quantum well heterostructures (MQWHs) were measured and compared with the results from a fully microscopic theory. The involved excitonic transitions could be identified in an experiment–theory comparison for photomodulation reflectance spectroscopy measurements.¹⁵ Based on the good agreement between the experimental and theoretical results, also the absorption and gain properties were calculated using the semiconductor Bloch equations. Gain values as high as for type-I systems were predicted. Overall, it is expected that the previously investigated “W”-design will perform well as gain medium in optically pumped semiconductor lasers.

In this letter, we demonstrate a continuous wave pumped type-II semiconductor disk laser with an emission wavelength of 1.2 μm at room temperature. Therefore, we take on our previous approach and implement a GaAs/$(Ga0.8In0.2)As/Ga(As0.8Sb0.2)/(Ga0.8In0.2)As$/GaAs 10× MQWH as resonant periodic gain structure. The sample growth is carried out by metalorganic vapor phase epitaxy (MOVPE) using a commercial horizontal AIXTRON AIX 200 Gas Foil Rotation (GFR) system.¹⁴ In preparation for the growth of the laser structure, a type-I $(Ga0.8In0.2)As/GaAs$ MQWH and a $Ga(As0.8Sb0.2)/GaAs$ MQWH are grown, in order to determine the individual growth conditions which are used for the growth of the type-II MQWH. The laser structure is grown bottom-up onto (001) GaAs substrate, starting with a $(Ga0.52In0.48)P$ capping layer and followed by the resonant periodic gain containing the “W”-quantum wells separated by Ga(AsP) strain compensating layers. The optical layer thicknesses of the capping layer and the barriers are matched to λ/2 with respect to the lasing wavelength. Finally, a distributed Bragg reflector (DBR) is grown, with $22 1/2$ pairs of $(Al0.1Ga0.9)As/AlAs$⁠.

As in our previous work, the MQWHs are characterized using high resolution X-ray diffraction (HR-XRD). The determined In and Sb concentrations as well as the (GaIn)As and Ga(AsSb) layer thicknesses are summarized in Table I. Furthermore, we record the PL spectra of all three structures. Regular PL measurements are performed with the (GaIn)As/GaAs and the Ga(AsSb)/GaAs MQWHs, where excitation and detection are carried out perpendicular to the surface of the sample. However, for VECSELs, it is more meaningful to record the edge PL, where the filtering by the Fabry–Pérot resonances from the microcavity, which is formed by the capping layer and the DBR, is avoided.¹⁶ Therefore, we excite through the ternary DBR using an 808 nm diode laser and detect the PL under an angle of $90°$⁠, i.e., only the PL emitted from the edge of the sample, with an optical spectrum analyzer (Yokagawa AQ6370B). The normalized spectra are presented in Fig. 1. It is observed that the characteristic PL of the type-I transitions according to recombinations inside an individual layer in the “W”-structure is suppressed. Overall, the HR-XRD and edge PL measurement of the VECSEL show that a MQWH very similar to the previously investigated structures was realized. Hence, the observed luminescence can be related to type-II transitions for which gain is expected.^14,15

One important requirement for VECSELs is to match the material gain peak wavelength and the microcavity resonance. For type-I setups, it is known that the red shift of the material gain is stronger than the red shift of the microcavity. Thus, following the definition of Hader et al.,¹⁷ a negative detuning is usually introduced for type-I VECSELs in order to achieve an efficient laser operation. This detuning can be used as design parameter to achieve a low threshold or high output power.

Due to the importance of the microcavity, the reflectivity of a processed laser chip is measured in order to evaluate the longitudinal confinement factor, i.e., the average light field intensity at the quantum wells. For the reflection measurement, a sample from an adjacent wafer position of the edge PL sample is flip-chip bonded onto a 350 μm thick chemical vapor deposition (CVD) diamond heat spreader via solid liquid inter-diffusion bonding based on Au and In. Then, the substrate is removed by selective chemical wet etching onto the Ga(InP) capping layer. The processed VECSEL chip is mounted on a temperature controlled copper heat sink. To measure the reflectivity white light is focussed perpendicular to the processed chip and a beamsplitter is used to collect the reflected light in a spectrum analyzer.

The reflectivity measurement is shown as grey-shaded area in Fig. 2. An absorption dip within the DBR stop band is observed at 1168 nm. For the reflectivity simulation, the barrier and QW thicknesses are extracted from the HR-XRD results. The capping layer and the DBR layer thicknesses are varied in order to fit the experimental data by the transfer-matrix method.¹⁶ Consequently, the intensity distribution of the light field within the structure can be calculated, and the longitudinal confinement factor is determined. It is found that the drop of the measured reflectance at the short-wavelength side of the stop band is in agreement with the maximum of the longitudinal confinement factor. This also explains the deviation of the simulated reflectivity where the absorption of the QWs is not considered.

The comparison between the edge PL (cf. Fig. 1) and the reflectance as well as the simulation of the longitudinal confinement factor reveals that a large positive detuning of about 35 nm is present in this particular type-II VECSEL structure. This is in contrast to common type-I VECSELs where no laser operation should be achieved with such a detuning.¹⁷ However, for type-II quantum wells the exact carrier density and temperature dependent shift rates are not yet established. Still, it has been reported that a significant blue shift is obtained with increasing pump densities despite the simultaneous heating of the gain structure.^10,18 These observations are also consistent with our recent investigations concerning the GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs material system.¹⁴ 

An outcoupling mirror with −100 mm radius of curvature is used to arrange a linear laser cavity with a length of 64 mm. The VECSEL is driven by an 808 nm diode pump laser (JenOptik JOLD-400-CAXF-6P2) under a $30°$ angle of incidence. A CCD camera was utilized to record the pump spot profile, and a super-Gaussian fit is applied in order to estimate the exact pump intensity. The complete procedure is described in more detail by Heinen et al.¹⁹ At full width at half maximum (FWHM), the elliptical pump spot is of the size $750 μm×870 μm$⁠, which implies highly multi-transverse mode operation concerning the VECSEL. For comparison, the TEM₀₀ mode size at the VECSEL chip for this cavity configuration is $270 μm$ at FWHM. It is found that the pump spot exhibits a flat-top profile with a super-Gaussian order of m = 3.00. Hence, it is valid to estimate the pump intensity directly from the pump spot radius.¹⁹ 

Laser operation is achieved with output coupler transmissivities of 0.2%, 0.7%, 1.0%, and 1.5%. The best performance is observed with the 0.7% output coupler for which the laser power curves are recorded with different heat sink temperatures in $15 °C$ steps between $−15 °C$ and $30 °C$ (cf. Fig. 3). The net input power is denoted at the bottom x-axis, considering that measured 23.2% of the pump light is reflected. The net pump intensity is denoted at the top x-axis. At $−15 °C$ heat sink temperature, the maximum output power of 4 W is achieved, and the threshold pump intensity is $1.18 kW/cm2$⁠. With increasing temperature, the threshold is continuously increasing to $1.43 kW/cm2, 1.92 kW/cm2$⁠, and $2.54 kW/cm2$ while maximum output powers are decreasing to 2.8 W, 1.6 W, and 0.6 W, for $0 °C$⁠, $15 °C$⁠, and $30 °C$⁠, respectively.

In comparison to a type-I VECSEL in the same wavelength regime, the pump intensities at threshold are in the same order of magnitude. On the other hand, the slope efficiencies of less than 10% for the type-II structure are clearly smaller as in type-I systems with 31%–34% slope efficiency.^20,21

The optical spectrum analyzer is used to record the laser spectra while the power curve with $0 °C$ heat sink temperature is recorded. The measurement is shown as false color plot in Fig. 4. The wavelength at lasing threshold is 1173 nm. A continuous red shift of the laser spectrum is observed although a blue shift of the material gain must be present due to the compensation of the large positive detuning. At the peak output power, a maximum wavelength of 1182 nm is observed. This result indicates that the maximum of the modal gain is dominated by the longitudinal confinement factor. If a linear shift of 0.12 nm/K is assumed, which is a common value for the spectral shift of the reflectivity of such resonant periodic gain structures, temperatures of $60 °C$ at threshold and $135 °C$ at the maximum applied pump intensity are estimated. These values are similar to typical temperatures in type-I VECSELs.²² 

In conclusion, we have demonstrated the feasibility of a VECSEL based on a type-II resonant periodic gain at $1.2 μm$ emission wavelength. It emits up to 4 W at $−15 °C$ and 1.6 W at $15 °C$ heat sink temperature with an emission wavelength between 1173 nm and 1182 nm. Measurements confirm lasing at the type-II transitions. The detuning is estimated by a comparison of the edge PL and reflectivity measurement. In accordance to previous reports, we have observed a significant blue shift of the material gain because the positive detuning of 35 nm between the low density PL peak wavelengths and the microcavity resonance is compensated. However, the red shift of the laser wavelength indicates that the modal gain is dominated by the microcavity resonance. The observed characteristic of decreasing thresholds and increasing slope efficiencies for reduced heat sink temperatures cannot only be an indicator for an increasing material gain but also for an improved overlap of material gain with the microcavity resonance. As for type-I systems, it is expected that the performance of type-II VECSELs is critically depending on the detuning. Therefore, an optimized design concerning the detuning of a type-II setup can potentially lead to a significant improvement of the laser performance. The characteristic blue shift of type-II structures with increasing carrier densities implies that the optimization of type-II VECSELs is completely different from the well established type-I VECSEL.

The presented results will facilitate the realization of more efficient lasers based on the GaAs/(GaIn)As/Ga(AsSb)/(GaIn)As/GaAs material system. In order to exploit the full potential, optimizations can be performed in interplay of experiment and theoretical quantum design,²³ but also with respect to optimized growth conditions. Furthermore, the design based on this material system has the potential for emission wavelengths up to 1.4 μm and beyond.

The Marburg work was a project of the Sonderforschungsbereich 1083 funded by the Deutsche Forschungsgemeinschaft (DFG). The work at Nonlinear Control Strategies, Inc. is supported via STTR Phase II, Contract No. FA9550-13-C-0009. The work from J. Hader and J. V. Moloney was also supported by the U.S. Air Force Office of Scientific Research (AFOSR), contract FA9550-14-1-0062.