Johannes Schmitz

Tunable Laser Diode System for Noninvasive Blood Glucose Measurements

Optical sensing of glucose would allow more frequent monitoring and tighter glucose control for people with diabetes. The key to a successful optical noninvasive measurement of glucose is the collection of an optical spectrum with a very high signal-to-noise ratio in a spectral region with significant glucose absorption. Unfortunately, the optical throughput of skin is low due to absorption and scattering. To overcome these difficulties, we have developed a high-brightness tunable laser system for measurements in the 2.0–2.5 μm wavelength range. The system is based on a 2.3 μm wavelength, strained quantum-well laser diode incorporating GaInAsSb wells and AlGaAsSb barrier and cladding layers. Wavelength control is provided by coupling the laser diode to an external cavity that includes an acousto-optic tunable filter. Tuning ranges of greater than 110 nm have been obtained. Because the tunable filter has no moving parts, scans can be completed very quickly, typically in less than 10 ms. We describe the performance of the present laser system and avenues for extending the tuning range beyond 400 nm.

Third generation focal plane array IR detection modules and applications

The 3rd generation of infrared (IR) detection modules is expected to provide advanced features like higher resolution 1024x1024 or 1280x720 pixels and/or new functionalities like multicolor or multi band capability, higher frame rates and better thermal resolution. This paper is intended to present the current status at AIM on the Mercury Cadmium Telluride (MCT), quantum well (QWIP) and antimonide superlattices (SL) detection modules for ground and airborne applications in the high performance range. For high resolution a 1280x720 MCT device in the 3-5μm range (MWIR) is presently under development. For spectral selective detection, a QWIP detector combining MWIR and 8-10μm (LWIR) detection in each pixel has been developed in a 384x288x2 format with 40 μm pitch, NETD < 35mK @ F/2, 6,8 ms for both peak wavelengths (4.8 μm and 8.0 μm). The device provides synchronous integration of both bands for temporal and spatial coincidence of the events observed. QWIP dual band or dual color detectors provide good resolution as long as integration times in the order of 5-10ms can be tolerated. This is acceptable for all applications where no fast motions of the platform or the targets are to be expected. For rapidly changing scenes - like e.g. in case of missile warning applications for airborne platforms - a material system with higher quantum efficiency is required to limit integration times to typically 1ms. For this case, several companies work on molecular beam epitaxy (MBE) of MCT to have access to double or multi layer structures. AIM and IAF selected antimonide based type II superlattices (SL) for such kind of applications. The SL technology provides -- similar to QWIPs -- an accurate engineering of sensitive layers by MBE with very good homogeneity and yield. While promising results on single SL pixels have been reported since many years, so far no SL based detection module could be realized. Just recently, IAF and AIM managed to realize first most promising SL based detectors. Fully integrated IDCAs with a MWIR SL device with 256 x 256 pixels in 40 μm pitch have been integrated and tested. The modules exhibit excellent thermal resolution of NETD > 12 mk @ F/2 and 5 ms. The next step will now be to stabilize the technology and to start the development of a dual color MWIR device based on SL technology and the existing 384 x 288 read out circuit (ROIC) used in the dual band QWIP device.

Bispectral thermal imaging with quantum well infrared photodetectors and InAs/GaSb type-II superlattices

We report on bispectral imaging systems based on quantum-well infrared photodetectors (QWIPs) and InAs/GaSb type-II superlattices (SLs) for the mid-wavelength infrared spectral range between 3-5 μm (MW) and the longwavelength infrared regime at 8-12 μm (LW). A dual-band MW/LW QWIP imager and a dual-color MW/MW InAs/GaSb SL camera are demonstrated. The two systems offer a spatial resolution of 288×384 pixels and a simultaneous detection of both channels on each pixel. Both technologies achieve an excellent noise equivalent temperature difference below 30 mK in each channel with F#/2.0 optics.

InAs/(GaIn)Sb short-period superlattices for focal plane arrays

An infrared camera based on a 256x256 focal plane array for the Mid-IR spectral range (3-5 μm) has been realized for the first time with InAs/GaSb short-period superlattices. The detector shows a cut-off wavelength of 5.4 μm and reveals a quantum efficiency of 30%. The noise equivalent temperature difference (NETD) reaches 9.4 mK at 73 K with F/2 optics and 6.5 ms integration time. Excellent thermal images with low NETD values and a very good modulation transfer function are presented. Furthermore, a new method to passivate InAs/GaInSb superlattice photodiodes for the 8-10 μm regime is demonstrated. The approach is based on the epitaxial overgrowth of wet-etched mesa diodes using lattice matched AlGaAsSb. A complete suppression of surface leakage currents in small sized test diodes with 70 μm diameter is observed.

Status of mid-infrared superlattice technology in Germany

In Germany, InAs/GaSb superlattice detector technology for the mid-wavelength infrared spectral range has been intensively developed in recent years. Mid-IR InAs/GaSb superlattice photodiodes achieve a very high quantum efficiency. The worlds first high-performance infrared imagers based on InAs/GaSb superlattices were realized offering high spatial and excellent thermal resolution at short integration times. Additionally, the technology for dual-color superlattice detectors featuring simultaneous, pixel-registered detection of two separate spectral regimes in the mid-IR has been developed. Due to the ability to detect signatures of hot carbon dioxide, dual-color superlattice detectors are ideally suited for missile alerting sensors. The capability for small volume production of InAs/GaSb superlattice detectors has been established.

Third generation focal plane array IR detection modules and applications (Invited Paper)

The 3rd generation of infrared (IR) detection modules is expected to provide advanced features like higher resolution 1024x1024 or 1280x720 pixels and/or new functions like multicolor or multi band capability, higher frame rates and better thermal resolution. This paper is intended to present the current status at AIM on quantum well (QWIP) and antimonide superlattices (SL) detection modules for ground and airborne applications in the high performance range. For spectral selective detection, a QWIP detector combining 3-5μm (MWIR) and 8-10μm (LWIR) detection in each pixel with coincident integration has been developed in a 384x288x2 format with 40 μm pitch. Excellent thermal resolution with NETD < 30mK @ F/2, 6.8 ms for both peak wavelengths (4.8 μm and 8.0 μm) has been achieved. Thanks to the well established QWIP technology, the pixel outage rates even in these complex structures are below 0.5% in both bands. QWIP dual band or dual color detectors provide good resolution as long as integration times in the order of 5-10ms can be tolerated. This is acceptable for all applications where no fast motions of the platform or the targets are to be expected. For rapidly changing scenes-like e.g. in case of missile warning applications for airborne platforms-a material system with higher quantum efficiency is required to limit integration times to typically 1ms. AIM and IAF selected antimonide based type II superlattices (SL) for such kind of applications. The SL technology provides-similar to QWIPs-an accurate engineering of sensitive layers by MBE with very good homogeneity and yield. While promising results on single SL pixels have been reported since many years, so far no SL based detection module could be realized. IAF and AIM last year managed to realize first most promising SL based detectors. Fully integrated IDCAs with a MWIR SL device with 256x256 pixels in 40µm pitch have been integrated and tested. The modules exhibit excellent thermal resolution of NETD<10mk @ F/2 and 5ms. Product improvement meanwhile allowed to reduce pixel outage rates below 1% i.e. down to a level as required for the military use of such detectors. Presently under development is therefore a dual color MWIR device based on SL technology and the existing 384x288 read out circuit (ROIC) used in the dual band QWIP device. This detector is primarily intended for the use in missile approach warning systems where the dual color capability significantly improves suppression of false alarms. Details of the modules and results of the electrooptical performance will be presented for the different items mentioned above.

GaSb-based 1.9- to 2.4-μm quantum well diode lasers with low-beam divergence

We present results on low beam divergence, low threshold current GaSb-based quantum-well diode lasers emitting in the 1.9 - 2.4 μm wavelength range. By carefully designing the active quantum-well region, low threshold current densities in the range of 148 to 190 A/cm2 could be achieved in the entire wavelength range. A novel structure for the epitaxial waveguide was designed and realized experimentally, leading to a reduced beam divergence in the fast axis of 44° full width at half maximum (FWHM), compared to 67° FWHM of a conventional broadened waveguide design. This improvement was achieved without any sacrifice in the laser performance, i.e. the novel laser structure showed the same threshold current and differential quantum effciency as the standard one. Ridge-waveguide lasers employing the new waveguide design and emitting at 2.3 μm were operated in an external cavity configuration. Due to the improved coupling effciency of the laser beam into the collimating optic, a wide tuning range of 130nm could be achieved, limited only by the gain bandwidth of the active material.

Band-edge aligned quaternary carrier barriers in InGaAs-AlGaAs high-power diode lasers for improved high-temperature operation

A new type of band-edge aligned carrier barrier is introduced into InGaAs-AlGaAs single quantum-well (SQW) high-power diode laser structures in order to prevent thermionic emission and the overflow of carriers at elevated operating temperatures. These barriers, which are located in the direct vicinity of the active zone of the laser, are undoped to avoid free-carrier absorption. An InGaAs-AlGaAs SQW laser structure with a 10-nm-thick AlGaAsSb electron-blocking layer on the p-side of an In/sub 0.2/Ga/sub 0.8/As quantum well was realized. The composition of this layer was adjusted so that its valence-band edge matches that of the adjacent AlGaAs waveguide layer. This is to prevent any additional voltage drop or series resistance due to the injection of holes into the quantum well through the electron blocking layer. These lasers show a high characteristic temperature T/sub 0/ of about 225 K for 1500-/spl mu/m-long as-cleaved devices, which is about 60 K higher than the same laser structure without the blocking layer. Simultaneously low internal losses (/spl alpha//sub i//spl ap/1.5 cm/sup -1/ at 20/spl deg/C) and high internal quantum efficiencies (/spl eta//sub i//spl ap/93% at 20/spl deg/C) are achieved. No additional voltage drop or series resistance was measured. The higher temperature stability is mainly attributed to the suppression of carrier leakage and a reduced free-carrier absorption at elevated temperatures.

Antimony-based superlattices for high-performance infrared imagers

InAs/GaSb short-period superlattices (SL) for the fabrication of mono- and bispectral thermal imaging systems in the mid-wavelength infrared region (MWIR) have been optimized in order to increase the spectral response of the imaging systems. The responsivity in monospectral InAs/GaSb short-period superlattices increases with the number of periods in the intrinsic region of the diode and does not show a diffusion limited behavior for detector structures with up to 1000 periods. This allows the fabrication of InAs/GaSb SL camera systems with high responsivity. Dual-color MWIR/MWIR InAs/GaSb SL camera systems with high quantum efficiency for missile approach warning systems with simultaneous and spatially coincident detection in both spectral channels have been realized.

High-power diode laser arrays emitting at 2 μm with reduced far-field angle

GaSb based diode laser arrays emitting at wavelengths around 2 μm have a significant potential for a variety of applications including material processing, such as welding of transparent plastic materials, and optical pumping of mid-infrared solid state lasers. Even though high output power broad area single emitters and laser arrays have already been demonstrated, they all suffer from a large fast axis beam divergence of typically 67° FWHM due to the broadened waveguide design employed. Here we will present results on (AlGaIn) (AsSb) quantum-well diode laser single emitters and linear arrays consisting of 19 emitters on a 1 cm long bar emitting at around 1.9 μm. To improve on the poor fast axis beam divergence we abandoned the broadened waveguide concept and changed over to a novel waveguide design which features a rather narrow waveguide core. This results in a remarkable reduction in fast axis beam divergence to 44° FWHM for the new waveguide design. For single emitters a cw output power of more than 1.9 W have been observed. 16.9 W in continuous-wave mode at a heat sink temperature of 20 °C have been achieved for arrays. The maximum wall-plug efficiency amounts to 26% both for the single emitters and the laser arrays. These efficiencies are among the highest values reported so far for GaSb based diode lasers, and allow us to use passively cooled and thus less expensive heat sinking technologies.