Eva M. Strzelecka

Parallel free-space optical interconnect based on arrays of vertical-cavity lasers and detectors with monolithic microlenses.

We investigate the use of low-threshold 980-nm vertical-cavity surface-emitting lasers for free-space optical interconnects. The vertical-cavity surface-emitting lasers and backilluminated detectors are monolithically integrated with microlenses on the back sides of the growth substrates to eliminate the necessity of external optics. With a channel pitch of 250 mum, an interconnect length between boards of the order of 5 to 10 mm with a ?50-mum lateral alignment tolerance can be achieved without external relay optics. The complete link is modeled to predict the systems efficiency and maximum bit rate. Data transmission at 500 Mbits/s per channel is demonstrated. The data rate was limited by parasitics, not the inherent bandwidth of the laser diodes.

Fabrication of refractive microlenses in semiconductors by mask shape transfer in reactive ion etching

Abstract Refractive microlenses are formed in semiconductor materials by transfer of a lens-like shape of reflowed erodable polyimide mask into the substrate by reactive ion etching. Simultaneous monitoring of the etch rates of the mask material and the semiconductor results in precise control of the lens radius of curvature. We demonstrate wafer-scale integration of GaAs microlenses with vertical-cavity laser diodes and InP microlenses with InGaAs photodetectors. These integrated components can be used directly in optical systems without additional external optics.

High-power high-brightness 980-nm lasers based on the extended cavity surface emitting lasers concept

We describe design and performance of novel, electrically pumped, vertical compound cavity semiconductor lasers emitting at 980 nm. The laser combines a vertical cavity semiconductor laser with a partially reflecting output coupler and an external cavity for mode control. The concept is scalable and has been demonstrated in monolithic low power (few miliwatts) devices all the way to high power extended cavity devices which generate over 950 mW CW multimode power and 0.5 W CW power in a TEM00 mode, the latter with 90% coupling efficiency into a single mode telecommunication fiber. The concept has been applied to the development of uncooled lasers, mounted in TO-56 cans, capable of producing 50 to 100 mW of fiber-coupled power. We have also demonstrated the extended cavity lasers at wavelengths of 920 nm and 1064 nm. We present reliability data for the chips used in the extended cavity lasers.

Monolithic integration of refractive lenses with vertical-cavity lasers and detectors for optical interconnections

We present a technique for monolithic integration of vertical cavity lasers and detectors with refractive microlenses etched on the back side of the semiconductor substrate in a wafer-scale process. This integration provides collimated or focused laser beam sources for applications in free-space interconnections or for coupling to optical fibers, and it improves the collection efficiency of detectors.

Novel 980-nm and 490-nm light sources using vertical cavity lasers with extended coupled cavities

We have developed novel electrically pumped, surface-emitting lasers emitting at 980 nm with an extended coupled cavity. The concept is scalable from monolithic low power devices all the way to high power extended cavity lasers. The latter have demonstrated 1W cw multi-mode and 0.5 W cw in a TEM00 mode and a single frequency, with 90% coupling efficiency into a single-mode fiber. By inserting a nonlinear optical medium in the external cavity, efficient and compact frequency doubling has been achieved with CW output powers 5-40 mW demonstrated at 490 nm. The latter devices are especially noteworthy due to their very low noise, sub 10 μrad beam pointing stability combined with small size, low power consumption and high efficiency.

Recent advances and important issues in vertical-cavity lasers

The rapid pace of advances in vertical-cavity surface- emitting lasers (VCSELs) has continued over the past couple of years. The widespread use of dielectric apertures formed primarily by lateral oxidation has provided much lower cavity losses, and this has enables a large decrease in device threshold as well as an increase in efficiency. The lowest optical losses have been obtained with thin or tapered oxide apertures. Within the past year, new strained- layer materials such as AlGaInAs have been incorporated to extend the benefits of strain to the 850 nm wavelength range. A record threshold of 290 (mu) A at 840 nm has been obtained. Devices have been designed for ultra-wide operating temperature ranges by using gain from different quantum levels at different temperatures. Submilliamp thresholds from 77 K to 373 K were demonstrated. The inclusion of low-loss dielectric apertures in wafer-bonded 1.55 micrometer InP/GaAs has yielded VCSELs with submilliamp thresholds for the first time. In addition, there has been considerable effort in making VCSEL arrays for parallel or free-space interconnect applications. Multiple wavelength arrays for even denser interconnects or wavelength addressing schemes have also been explored. In this paper we review some of this recent progress and point out issues still inhibiting further advances.

490-nm coherent emission by intracavity frequency doubling of extended cavity surface-emitting diode lasers

We describe a novel blue-green laser platform, based on the intracavity frequency doubling of Novalux Extended Cavity Surface Emitting Lasers. We have demonstrated 5 to 40mW of single-ended, 488nm, single-longitudinal mode emission with beam quality M2<1.2. The optical quality of these lasers matches that of gas lasers; their compactness and efficiency exceed ion, DPSS, and OPSL platforms. These unique properties are designed to serve diverse instrumentation markets such as bio-medical, semiconductor inspection, reprographics, imaging, etc., and to enable new applications. We also present data on the reliability of this novel laser platform and its extensions to different wavelengths (in particular, 460nm and 532nm) and to next-generation, highly compact, monolithic intracavity-doubled lasers.

Vertical-Cavity Lasers for Parallel Optical Interconnects

Continuing increases in efficiency, uniformity, and yield for low-threshold dielectrically Apertured Vertical-Cavity Surface-Emitting Lasers (VCSELs) suggest that this new generation of sources may be ready for insertion into practical parallel interconnect systems. This paper will review the recent evolution of these devices, pointing out key enabling advances and potential roadblocks yet to be addressed. Included will be advances that have led to record low optical losses as well as record high wall-plug efficiency at powers of a few hundred microwatts, desirable for massively parallel optical interconnects. The use of engineered oxide apertures is a key element in these cases. Experimental results will also include recent free-space and WDM fiber interconnects. Remaining issues to be addressed include some sort of lateral carrier confinement, such as a buried-heterostructure, to reduce carrier losses as devices are scaled to small lateral dimensions.

VCSELs in '98: what we have and what we can expect

Recent results from the authors group are summarized as a general indicator of the current state of the art in vertical-cavity surface-emitting lasers (VCSELs). These include results from engineered-aperture VCSELs with high wall-plug efficiencies at low powers, high-efficiency bottom-emitting cryo-VCSELs with wavelengths < 900 nm, low-threshold AlGaInAs VCSELs emitting at 850 nm, and arrays of VCSELs used in parallel free-space links as well as WDM arrays butt-coupled to multimode fiber. Analysis indicates that size-dependent losses limit the scaling of VCSELs below 5 micrometers in diameter unless special engineered apertures and/or short cavities are used. The analysis also shows that lateral carrier confinement is necessary to obtain efficient devices below 2 micrometers in diameter.

Monolithic integration of an array of multiple-wavelength vertical-cavity lasers with a refractive microlens for optical interconnections

Vertical-cavity surface-emitting lasers (VCSELs) are very promising for data transmission in fiber links and free-space optical interconnections. Recently, a new method for fabricating arrays of multiple-wavelength VCSELs has been demonstrated. In this paper, we present integration of a closely-spaced array of these multiple-wavelength VCSELs with a single refractive microlens. With a microlens designed for collimation the output beams had a far-field divergence half-angle of <0.85/spl deg/. We show that these devices are suitable for wavelength-multiplexed free-space interconnects as well as for coupling into multimode fibers.