Mid-IR devices on the brink of widespread commercialization

Quantum cascade lasers

Quantum cascade lasers (QCLs) are the most mature coherent source for the 4.5-15 µm spectral region. In the QCL, each injected electron can emit multiple photons as it hops between conduction subbands arranged in a staircase-like geometry. State-of-the-art QCLs are capable of operating in CW mode at temperatures of up to 39 °C, and producing a few mW of optical power at ambient temperature.

Daniel Hofstetter of the University of Neuchâtel in Switzerland emphasized the importance of improvements in the QCL active-region design in lowering the threshold current density to 5 kA/cm2 at room temperature. Thermal management has also been improved through junction-side-down mounting of narrow-ridge devices on a diamond heat sink. Hofstetter also described single-mode DFB QCLs that are operated in CW mode up to 253 K, with a side-mode suppression ratio of 27 dB.

Although record-setting QCLs are based on the In0.53Ga0.47As/In0.52Al0.48As material system on InP substrates, Cyrille Becker of Thales discussed the progress of AlGaAs-based devices on GaAs substrates and InAs/ AlSb devices on GaSb substrates. The advantages of these combinations of materials include the considerable technological maturity of the AlGaAs/GaAs system and the poten-tial for reduced thresholds in the InAs/AlSb system on GaSb.

GaAs-based QCLs emitting in the vicinity of 10 µm currently operate at temperatures up to 95 °C under pulsed excitation, and produce 60 mW of CW power at 77 K. Although electroluminescence and possibly a lasing signature have been observed from InAs/AlSb active regions, Becker concluded by saying that problems associated with electrical damage and low index contrast in the available cladding layers remain to be overcome.

Sensing and communication

A testament to the continually improving performance of QCLs is the growing interest in incorporating these devices into practical systems. According to John Schultz of the Pacific Northwest National Laboratory, the use of cooled QCLs offers a promising route towards reducing the clutter and noise that currently dominate the spectral characteristics of remote chemical sensors (figure 1). Frequency-modulated differential absorption LIDAR (or FM DIAL) experiments at distances up to 2.5 km have been encouraging, said Schultz in his invited presentation. Miniaturized QCL transmitter modules with chalcogenide-glass waveguide connections are also under development.

Meanwhile, it is prohibitively expensive to install fiber-optic links that would connect all the users of high-speed networks to the backbone. One potential bypass of this last-mile bottleneck that does not sacrifice much of the bandwidth is free-space optical propagation. Rainer Martini of the Stevens Institute of Technology, NJ, noted that compared with the 1.55 µm standard, free-space transmission in one of the mid-infrared atmospheric windows (e.g. near 5 µm or close to 8 µm) greatly reduces losses due to weather-dependent Mie scattering, Rayleigh scattering and scintillation. Martini argued that these windows would more than compensate for the lower detector sensitivity and poorer quality optics at the longer wavelengths.

QCLs have already demonstrated wide-open eye diagrams when they are digitally modulated with pseudo-random bit patterns at 2.5 Gbit/s. Martini described a very foggy morning in August 2001 where a QCL mid-IR link spanning a 200 m open-air path stabilized nearly an hour earlier than a similar near-IR link, as the haze gradually dissipated.

Type-II interband lasers

At present, the built-in lattice strain in QCLs needs to be accommodated, which makes them increasingly less attractive for wavelengths shorter than about 5 µm. In the 3-5 µm spectral region, the primary contenders among compact coherent sources are narrow-gap antimonide interband lasers that use the type-II band alignment in the active region, where the difference in conduction or valence band energies of the two materials is greater than that in the bandgaps. Using type-II band alignment in the active region reduces non-radiative Auger recombination. Optically pumped lasers of this class have operated in CW mode up to room temperature.

George Turner of Lincoln Laboratory described the integrated absorber approach to improve the optical-to-optical conversion efficiency of type-II "W" lasers. W lasers are antimonide-based lasers that have a type-II band lineup. They contain a superlattice structure with four layers per period in the active region, in which two InAs electron wells sandwich a GaInSb hole well. The InAs/GaInSb/InAs layers are separated by electron-confining barriers such as AlSb or a GaInAsSb quaternary.

The essence of Turner s approach is the combination of strong pump absorption in the integrated absorber with a relatively small number of periods (typically 10) in the active regions, which leads to a low internal loss. Since the integrated absorber structure has a slightly higher refractive index than the GaSb substrate, the need for Al-based cladding layers is eliminated.

The US Air Force also has a strong interest in such lasers operating from 2 to 5 µm for tactical applications. As Andrew Ongstad of the Air Force Research Laboratory (AFRL) reported, type-II W InAs/InGaSb lasers with Al-free cladding layers exhibit reduced divergence angles in the far-field pattern both parallel and perpendicular to the axis of the slab waveguide. AFRL s Ron Kaspi described an Al-free integrated absorber laser (with an InGaAsSb/InAs/InGaSb/InAs active region) that produced 8 W of quasi-CW power at 77 K. The current challenge for integrated absorber lasers appears to be reaching the high operating temperatures already demonstrated by other antimonide-based optically pumped lasers. Kaspi said that this could be achieved by enhancing the hole confinement in the active region.

To produce low angular-divergence light from high-power laser stripes several hundred microns wide, it is necessary to enforce the spatial coherence across the stripe. One such approach to this is the photonic-crystal DFB laser, in which the light in the waveguide is diffracted by a two-dimensional grating that simultaneously diffracts along two axes formed by the rows and columns of the grating (figure 2). The result is strong coherence across a wide stripe with no filamentation. Simulations predict much higher single-mode power than previous approaches. Jerry Meyer of the Naval Research Laboratory presented two experimental photonic-crystal DFB lasers based on etched GaSb photonic crystals and antimonide active regions on GaSb substrates, in addition to a theoretical surface-emitting version of the device.

In a recent development related to electrically pumped type-II lasers, John Bradshaw from Maxion Technologies, MD, reported single-mode emission with greater than 30 dB side-mode suppression, from an interband cascade laser that uses the staircase geometry of the QCL in conjunction with interband transitions in antimonide material. The de-vice employed a DFB grating etched into a p-doped GaSb top contact layer.