Switchable, double wavelength generation is demonstrated from a single vertical external cavity surface-emitting laser chip. Power of ~0.5W for two wavelengths λ≈967nm and 1018nm i.e. within the spectral distance of 51nm were registered. In the semiconductor heterostructure a single set of nominally identical quantum wells was enclosed in a single, two-mode resonant microcavity. The wavelength switching was induced by the change of the pump power. The increase or decrease of the pump power changes the active region temperature and thus tunes spectrally the gain spectrum to the one of two modes.
© 2014 Optical Society of America
Optically Pumped Vertical External Cavity Surface-Emitting Lasers combine many advantages of the Thin Disc Lasers [1, 2] among them power scalability, ability to emit a pure TEMoo mode, and a wavelength emission flexibility inherent in semiconductor technology [3, 4]. Due to the flexibility of the external resonators and fabrication of semiconductor heterostructure by epitaxial techniques, VECSELs can be designed for unique applications based on generation of two wavelengths. Such lasers were successfully realized both by a special design of the active regions enclosed in the epitaxial heterostructure , by applying two different chips in co-linear T-cavity  or by using intracavity elements . In contrast to all these lasers emitting two wavelengths simultaneously we developed a laser which can be switched between two different wavelengths. A single source capable of co-axial emission of two wavelengths is of great interest in a number of applications as, for instance optical position meters or as two color source for RGB projectors if used with a nonlinear crystal and the second harmonic generation.
For proper operation VECSELs require an efficient heat extraction. This is realized in so called flip-chip technology  or by application of transparent, highly thermal conductive heat spreader [8–10]. In the case of the flip-chip technology the active region is deposited directly on the substrate and followed in the course of the process by deposition of the distributed Bragg reflectors (DBR). This permits to bond the DBR directly to the heat sink and then to remove the substrate by etching that gives direct optical access to the active region. In the case of low thermal resistivity of the GaAs/AlAs DBR this technology allowed to achieve record high emission power . An alternative way of the heat dissipation is application of a transparent heatspreader. The single crystal diamonds perfectly suit this application due to their very high thermal conductivity  and transparency in a broad spectral range. The very closed proximity of the heatspreader to the active region shortens the heat diffusion- length and time.
Any rise of the VECSEL active region temperature is unfavorable since it is the origin of the thermal rollover of the output characteristics which may totally switch off the laser at high power excitation. However, it can be also turned to good use and employed for the laser wavelength switching. In a standard VECSEL heterostructure, a large number of QWs are deposited in the layer which forms a microcavity. This microcavity arises due to high reflectivity coefficient of the DBR and the semiconductor–air interface. The spatial distribution of QWs follows the resonant periodic gain approach . This assures efficient optical pumping of all QWs and resonant enhancement of the emission at the designed wavelength corresponding to the microcavity resonance. The measure of this enhancement is a longitudinal confinement factor Γ [12,13]. For optimum operation the spectral emission of the QWs must be slightly blue shifted in respect to the microcavity resonant wavelength in order to accumulate the different emission thermal drift of the QWs (0.3 nm/K) and the microcavity resonance (0.1 nm/K)  at high pump power. Modification of a standard VECSEL heterostructure by enlarging the QW separation layers results in two mode resonant cavity. In such a case there are two wavelengths for which the emission is favorable. For the emission to happen the spectral position of the gain has to be tuned to the one of the resonant wavelengths. This takes place when the temperature of the device is varied accordingly. The switching time depends on delay in the heat dissipation. This delay is reduced when the heat dissipation is properly enhanced by the diamond heatspreader.
The VECSEL heterostructure was grown by molecular beam epitaxy. In the process first the AlAs/GaAs DBR was deposited on GaAs wafer and followed by deposition of the active region containing 12 InGaAs QWs enclosed by GaAs layers. In order to prevent the strain relaxation that results in so called black lines, strain compensating GaAs1-xPx, x≈0.04, 30 nm thick layers were added on both sides of each QW. The whole heterostructure was covered by a half-wavelength thick AlGaAs window layer and 10nm thick GaAs layer in order to protect the heterostructure against oxidation. The DBR resonant wavelength was designed to be 980nm, whereas the GaAs barrier layers separating the QWs were grown 10nm thicker than in the standard single wavelength heterostructure. A 3 × 3 mm2 piece of the wafer was pressed against a 300μm thick transparent synthetic (CVD) diamond and mounted on a copper sample holder. The temperature of the holder was stabilized by a thermo-electric cooler connected to a PID controller and supported by water flow. This laser holder temperature could be varied. The laser was excited using a high power semiconductor laser bar coupled to a 200 μm fiber and emitting at 808 nm in continuous wave mode. A 4f collimator was used to focus the pump beam to a 200 μm diameter spot. The laser linear resonator was enclosed by an external dielectric mirror with the reflectivity of 0.97 and curvature of 90 mm. The mirror distance was adjusted to match the mode size to the excitation spot diameter.
Prior to excitation of lasing, the heterostructure was subjected to the X-ray and optical characterization. The standard X-ray 2Θ−ω characterization was used to establish the thickness of the individual layers. In Fig. 1(a). measured and simulated reflectivity spectra are shown, respectively. The calculations were performed with aid of the transfer matrix method  in which the layer thickness obtained from the X-ray data and indexes of refraction according to  were employed. In the calculations a step-like absorption of the QWs was assumed. In the broad spectral range of the DBR’s stop-band resonances only those at the wavelength of 926 nm and 967 nm are strongly manifested. This is because of the high absorption in the QWs which takes place for the wavelengths in the low energy tail of emission line.
The third resonance occurring at 1018nm is only slightly pronounced due to the lack of the QWs absorption at this wavelength. The good agreement between the measured and simulated data allows calculation of the longitudinal mode confinement factor Γ . The in the Fig. 1(a) the calculations show that the Γ value in the case of the long-wavelength resonance is higher than the one for the short wavelength. This higher value of Γ compensates the gain drop when the peak gain is tuned to the long wavelength resonance at high temperatures of the active region. The optical characterization was completed by photoluminescence (PL) measurements taken at the edge and on the plane of the sample as it is shown in Fig. 1(b). Data of the first PL provides an information on the QWs emission unmodified by the microcavity. Here the QW edge emission at 957 nm is blueshifted by 10nm in respect to the resonance at 967 nm. The second PL spectrum is modified by the microcavity's confinement factor. The maximum of this second PL spectrum appears at 963 nm, i.e. at the wavelength 4 nm lower than microcavity resonance at 967 nm.
The maximum available power at each wavelength, when excited separately, exceeded 1W. However the optimum thermal conditions for each emission wavelength were different. At the holder temperature approaching 0°C only short wavelength emission could be exited due to the limited pumping power. On the contrary, at the high temperature of 45°C only the long wavelength emission was possible. The optimal conditions for the wavelength switching were found at the holder's temperature of 20°C. At this temperature the output power characteristic, shown in Fig. 2, exhibits two ranges of a linear dependency of the output power on the pump power. The first one characterized by the threshold at 3 W corresponds to the emission at the wavelength of 967 nm. At this wavelength, at the temperature of 20°C the output power reaches its maximum of 0.5 W for the 11.5 W of pump power. Further increase of the pump power results in the thermal rollover of the first wavelength emission and crossing over the second threshold at 15 W that leads to the emission at 1018 nm. The maximum power emission at this wavelength can be driven much above 1W. In Fig. 3 there is shown the spectrum registered at the intermediary pump power of 16 W when both wavelengths are emitted. Splitting of the emission into narrow spectral lines is due to the Fabry-Perot modes that originate because of the diamond heat spreader. The large spectral separation of 51 nm between the emitted wavelengths suggest high temperature rise in an active region of the device. For comparison, the 28 nm wavelength shift was observed in the record high power VECSEL emission of 106 W . However, the spectrally broad gain of the QW permits for the large tuning range of 30 nm even at constant temperature , thus only a fraction can be attributed to the thermal gain drift. The precise evaluation would require additional measurements [17, 18] which are beyond the scope of this paper.
The switching time of the described laser, shown in Fig. 4, was measured by changing the excitation power. Since the available high power laser pump was not suitable for the rapid re-adjustment of the emitted power, a mechanical shutter was constructed in order to switch between low/high excitation levels. The shutter was similar to the standard chopper blade. It was placed in-between two lenses of 4f collimator in such a way that it could partially shadow the high diameter beam. Consequently the low level excitation of the VECSEL heterostructure could be easily varied whereas the spot size remained constant. A beam splitter and same band edge filters were used to register separately the emitted wavelengths with two identical high-speed photo-detectors. The third photo-detector provided a reference signal of the pump radiation reflected from the heterostructure surface. Although the photo-detectors have not been calibrated, taking into account transmission of the band edge filters we estimate the emission suppression ratio to be equal to 13 dB and 20 dB for short and long wavelengths switching, respectively.
The time resolved data has proven that the emitted wavelength could be switched in 1ms. This short switching time, was limited in our experimental set-up only by a rotation speed of the shutter blade. In order to estimate the upper switching frequency limit resulting from the speed of thermal processes inside the active region, we performed time-dependent finite-element numerical simulations using our model [19, 20]. The preliminary results showed that, although there is no sharp limit, frequencies up about 10 kHz should be feasible. Such a short-switching time is sufficient for the application as a two-color emitter, after the frequency doubling.
The small increase of the short wavelength emission just after the change of the excitation power to the higher value can be due to a slightly too large spectral detuning of the QW emission with regard to the short wavelength resonance. A more precise laser tuning would likely eliminate this phenomena. It is expected that improvement of the bonding process would also rise the output power of this proof-of-concept device.
We presented a switchable double wavelength VECSEL. The laser was capable to emit two different wavelengths λ≈967 nm and 1018 nm with the output power exceeding 0.5 W. The switching time was in the range of 1ms. The short switching time makes this laser an attractive source for an application in position meters. Such lasers, optimized for bigger separation between the wavelengths, if combined with a nonlinear crystal could also be attractive sources of two colors in RGB projectors.
The authors would like to acknowledge the financial support of NCBiR under contract NR 02 0023 06 and of NCN under contract N N515 360 636.
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