Bi-functional active regions, capable of light generation and detection at the same wavelength, allow a
straightforward implementation of the mid-infrared quantum cascade technology for integrated photonics. Different parts of the chip can be used for laser and for photodetectors. Potential applications are on-chip integrated sensors or lasers with integrated power monitoring capabilities. In the first bi-functional designs, wavelength matching was achieved using thicker barriers and reduced energy splitings between the extraction levels, but with the drawback of a reduced laser performance. The following introduction of the horizontal-vertical extraction scheme was a significant step towards high performance laser operation.
In this work, we combine our design experience with optimized, laterally overgrown waveguides and refined bandstructure modelling. The device was designed for emission at 8 μm to show that wavelength matching can also be achieved at longer wavelengths, where it becomes increasingly difficult due to the smaller ratio between photon energy and LO-phonon energy. Graded interfaces were used in the bandstructure design to consider the behaviour of the MOVPE growth. In pulsed mode a threshold current density of 1.3 kA/cm 2 and a total wallplug efficiency of over 10% was achieved. In continuous-wave operation, the device emits 80mW output power in an episide-up configuration. A much higher performance can be expected after episide-down mounting on AlN substrates. In detector operation the device has a responsivity at the emission wavelength of about 20mA/W.
There are a number of military and commercial applications for high-power laser systems in the mid-to-long-infrared wavelength range. By virtue of their demonstrated watt-level performance and wavelength diversity, quantum cascade laser (QCL) and amplifier devices are an excellent choice of emitter for those applications. To realize the power levels of interest, beam combining of arrays of these emitters is required and as a result, array technology must be developed. With this in mind, packaging and thermal management strategies were developed to facilitate the demonstration of a monolithic QCL array operating under CW conditions. Thermal models were constructed and simulations performed to determine the effect of parameters such as array-element ridge width and pitch on gain region temperature rise. The results of the simulations were considered in determining an appropriate QCL array configuration. State-of-the-art micro-impingement cooling along with an electrical distribution scheme comprised of AlN multi-layer technology were integrated into the design. The design of the module allows for individual electrical addressability of the array elements, a method of phase control demonstrated previously for coherent beam combining of diode arrays, along with access to both front and rear facets. Hence, both laser and single-pass amplifier arrays can be accommodated. A module was realized containing a 5 mm cavity length monolithic QCL array comprised of 7 elements on 450 m pitch. An output power of 3.16 W was demonstrated under CW conditions at an emission wavelength of 9μm.
GaInAsSb/AlGaAsSb multiple-quantum-well diode lasers grown by organometallic vapor phase epitaxy are reported. The laser structure consists of n- and p-doped Al0.59Ga0.41As0.05Sb0.95 cladding layer, Al0.28Ga0.72As0.02Sb0.98 confining layers, and four 15-nm- thick Ga0.87In0.13As0.12Sb0.88 quantum wells with 20-nm-thick Al0.28Ga0.72As0.02Sb0.98 barrier layers. These lasers, emitting at 2.1 micrometers , have exhibited pulsed threshold current densities as low as 1.2 kA/cm2.
Semiconductor laser devices with tapered gain regions have recently generated much interest because they promise high output power with near-diffraction-limited spatial beam quality and good electrical to optical conversion efficiency. We report recent progress on two specific applications: a ring laser and a high- power erbium-doped fiber amplifier (EDFA). The ring laser operates unidirectionally in a single longitudinal mode with an output power of 170 mW and without a Faraday isolator. The high- power EDFA has an output power of 520 mW at 1.55 micrometers , the highest power reported to dates for an erbium-doped fiber amplifier using all semiconductor pump lasers. The common theme for both of these applications is the development of optical systems that produce high power in near-diffraction-limited collimated beams and efficient coupling into single mode optical fiber. We present an experimental procedure for quantitatively predicting the optical fiber power coupling efficiency. We have measured 64% power coupling efficiency measure fiber fact to power in the single-mode fiber, or 51% laser facet to power in the fiber, in good agreement with the predictions.
The recent development of semiconductor diode optical amplifiers and lasers having a laterally tapered gain region has changed the outlook for high-power semiconductor optical sources. For the first time, highbrightness, single-element, all-semiconductor sources which emit several watts of cw power in a nearly ideal, single-lobed, diffraction-limited beam have been demonstrated. As semiconductor sources these devices have the inherent advantages of high efficiency, small size, light weight, and reliability. The amplifier12 and all-semiconductor master-oscillator power-amplifier (MOPA) devices34 have gain regions linearly tapered from a few micrometers at the amplifier input to several hundred micrometers at the output. Device lengths are typically 2 mm or more. The angle of the taper is chosen to match the diffraction angle of the input beam which has its waist near the narrow end of the taper. Such a structure is shown schematically in Fig. 1 . The etched grooves have angled side walls and act as cavity spoilers, designed to prevent oscillation of the device as a broad-area laser. The devices are fabricated in single-quantum-well strained-layer InGaAs/AlGaAs graded-index separate-confinement heterostructure laser wafers grown by organometallic vapor phase epitaxy.5 The tapered devices also operate as laser oscillators6 by increasing the input facet reflectivity. For amplifiers, both the input and output facets are coated for low reflectivity (in Fig. 1 , Ri = R2 = 1%), but for oscillators, the input facet is left uncoated (R1 —30%). The oscillators also emit several watts of cw power into a nearly single-lobed, nearly diffraction-limited beam, though their beam quality is usually somewhat inferior to that obtained for amplifiers, particularly at the highest output powers. The lateral mode of the oscillator is similar to the modes described by Fox and Li7 for unstable resonators, except that the semiconductor medium has a significant effect on the self-consistent mode which oscillates. A beam propagation calculation has been carried out to model these effects, as described below. This paper includes a review of the properties of both tapered amplifiers and oscillators.
The OMVPE growth and performance of graded-index separate-confinement heterostructure strained quantum-well InGaAs-AlGaAs diode lasers are reviewed. Broad-stripe lasers have exhibited Jth as low as 60 A cm-2 for a cavity length L equals 1500 micrometers and differential quantum efficiency (eta) d as high as 90% for L equals 300 micrometers . Similar heterostructures have been used to fabricate traveling wave amplifiers with a laterally tapered gain region that emit over 1 W cw in a nearly diffraction-limited spatial lobe at 0.98 micrometers , linear arrays of 200-micrometers -long uncoated ridge-waveguide lasers with average threshold currents of 4 mA and (eta) d approximately 90%, and high-power broad-stripe lasers with power conversion efficiency exceeding 50% at 75 degree(s)C.
Performance trends in the development of monolithic two-dimensional, coherent grating surface emitting (GSE) laser arrays are presented. Such GSE arrays now operate continuously to more than 3 W/surface and pulsed to more than 30 W/surface. They have obtained cw threshold current densities of under 140 A/cm2 with cw differential quantum efficiencies of 20 to 30% per surface. Linewidths in the 50 MHz range have been obtained with output powers of up to 270 mW per surface. The arrays typically consist of 10 to 30 mutually injection coupled gain sections with 10 laterally coupled ridge-guided lasers in each gain section. A single GaInAs strained-layer quantum well with a graded index separate confinement heterostructure geometry allows junction down mounting with light emission through the transparent GaAs substrate. A surface relief grating is used for feedback and outcoupling.
We describe a new technique for performing femtosecond transient measurements of nonlinear
index and absorption in waveguide devices. Using a time division interferometry technique in
conjunction with a tunable femtosecond laser source we have performed the first measurement of
the wavelength dependent nonresonant nonlinear index in A1GaAs. Contributions to nonlinear
index arise from both virtual as well as real population mediated processes depending on the
wavelength detuning from resonance. Complementary pump-probe measurements of transient
absorption provide information on excited state population as well as two-photon induced absorption
processes. These measurements provide imformation on the mechanism and dynamics of
fundamental nonlinear optical processes below the band edge in semiconductors and are relevant
to possible all optical switching applications in waveguide devices.
Two-dimensional surface-emitting AlGaAs diode laser array modules, each containing two 1 sq cm hybrid arrays, have been fabricated and tested. For quasi-CW operation, peak output powers greater than 300 W/sq cm appear to be easily achievable at repetition rates up to 500 Hz. The measurements also indicate that CW output powers of 100-150 W/sq cm can be achieved from these arrays.