Low loss, single mode rib waveguides, based on PECVD deposited multi-layer amorphous silicon are fabricated. These waveguide are refractive index and mode-matched to III/V laser waveguides. Methods for monolithic integration of these passive amorphous silicon waveguides with InGaAsP/InP gain sections are demonstrated. Results of a multi-wavelength laser based on an amorphous silicon arrayed waveguide grating integrated on a single chip with InGaAsP gain sections are presented.
High wall-plug efficiency and a wide range of available wavelengths make laser diode arrays preferable for many high-power applications, including optical pumping of solid state lasers. Recently, we designed and fabricated InGaAsP/InP arrays operating at 1.5-μm and In(Al)GaAsSb/GaSb arrays operating at 2.3-μm. We have demonstrated a high continuous-wave (CW) output power of 25 W from a one dimensional laser array and a quasi-CW (q-CW) output power of 110 W from a two dimensional laser array both operating near 1.5-μm. We have obtained a CW output power of 10 W from the 2.3-μm laser array. The 1.5-μm arrays are suitable for resonant pumping of erbium doped solid-state lasers, which require high power optical sources emitting in the narrow erbium absorption bands. Long current-injection pulses produce a considerable temperature increase within the diode laser structure which induces a red-shift of the output wavelength. This thermal drift of the laser array emission spectrum can lead to misalignment with the erbium absorption bands, which decreases pumping efficiency. We have developed an experimental technique to measure the time dependence of the laser emission spectrum during a single current pulse. From the red-shift of the laser emission, we determine the temperature of the laser active region as a function of time.
The spacing between the individual laser emitters has an effect on the array heating. In steady state operation, this spacing is a contributing factor in the non-uniformity of the thermal field within the bar, and thus to the overall thermal resistance of the laser bar. Under pulse operation, the transient heating process can be divided into three time periods; each with its own heat transport condition. It was shown that in the initial period of time the heat propagates within the laser bar structure and the laser bar design (fill factor) strongly affects the active region temperature rise. In the later periods the temperature kinetics is insensitive to the fill factor. This analysis has been verified in experimental studies using the 1.5-μm laser arrays.
Laser sources operating in spectral region 2 - 4 μm are in demand for ultra-sensitive laser spectroscopy, medical diagnostics, home security, industrial process monitoring, infrared countermeasures, optical wireless communications, etc. Currently, solid state lasers and optical parametric oscillators and amplifiers are used as coherent light sources in this spectral region. Solid state and parametric sources are being optically pumped by near infrared diode lasers. This intermediate energy transfer step from near infrared pumping diode to mid infrared emitting device reduces power-conversion system efficiency. Development of the highly efficient semiconductor diode lasers operating in 2 - 4 μm spectral region will significantly improve the performance of the many existing systems and enable new applications. In this work we will describe major breakthrough in the development of the high power room temperature operated mid-IR semiconductor lasers. The performance limitations of the devices based on type-I and type-II quantum well (QW) active region design will be analyzed. Future directions in device performance optimization and enhancement of the wavelength for high power room temperature operation will be discussed.
We have fabricated and characterized 2.7- and 2.8-μm wavelength In(Al)GaAsSb/GaSb two-quantum-well diode lasers. The material was grown using molecular-beam epitaxy. All lasers have 2-mm cavity lengths and 100 μm apertures. Continuous wave operation up to 500 mW was recorded at 16 °C from 2.7-μm lasers, while 160 mW was obtained from 2.8-μm lasers. Threshold current densities as low as 350 A/cm2 were recorded from 2.7-μm lasers with external quantum efficiencies of 0.26 photon/electrons. The maximum wall-plug efficiency was 9.2 % at a current of 2.4 A. A peak power of 2.5 W was recorded in the pulsed-current mode operation at 20 °C at 2.7 μm and 2 W at 2.8 μm. Characteristic temperatures of T0 = 71 K and T1 = 86 K were measured from the 2.7-μm devices. T0 = 59 K and T1 = 72 K for the 2.8-μm lasers. The devices have differential series resistances of about 0.18 Ω with estimated thermal resistances of about 5 K/W. Measurements of gain, losses, threshold current, device efficiency and spontaneous emission of the lasers show that it is the hole leakage from QWs into the waveguide, and not Auger recombination that limits CW room temperature output power of long wavelength GaSb-based type-I QW lasers at least up to wavelengths of 2.8 μm. A design approach to extend the operating wavelength of high power In(Al)GaAsSb/GaSb lasers to more than 3 μm is discussed.
Mid-infrared “W” quantum-well diode lasers with reduced turn-on voltages are reported. Devices with coated facets operated in continuous-wave mode up to 195 K, where the emission wavelength was 3.56 microns. At 78 K the threshold current density was 67 A/cm2, the maximum output power was 198 mW, and the maximum slope efficiency was 106 mW/A. One of these lasers was used to detect methane, by exploiting the absorption band in the vicinity of 3.3 microns. Preliminary measurements demonstrated detection of methane at partial pressures down to 7 x 10-7 atm. in a nitrogen atmosphere.