J. Ko

Technique for integration of vertical cavity lasers and resonant photodetectors

We demonstrate a design that allows fabrication of substrate input/output resonant-cavity photodetectors and vertical cavity lasers (VCLs) on the same substrate without regrowth. By selectively oxidizing a few layers in the bottom mirror the as-grown 80% reflectivity mirror, used as the input mirror for the detector, is converted to a 99.3% reflectivity mirror allowing fabrication of VCLs from the same epitaxial material. Since these two reflectivities are uncorrelated, the detectors and VCLs can be individually designed. Despite the change in refractive index from ∼3 to ∼1.6 in the oxidized layers, the structure can be designed to have nearly the same resonance wavelength for both the detectors and VCLs. Using this design strategy, we have successfully fabricated high-performance resonant photodetectors and VCLs from the same epitaxial material. The photodetectors have an absorption of 56% and an optical bandwidth of 5.9 nm, in good agreement with theory. Small diameter, single-mode VCLs have threshold c...

Generation of first-order terahertz optical sidebands in asymmetric coupled quantum wells

We have generated first-order tetrahertz (THz) optical sidebands on a near-infrared (NIR) probe beam by driving an excitonic intersubband resonance with THz electric fields. We use THz radiation polarized along the noncentrosymmetric axis of a quantum well system to generate a comb of sidebands ωsideband=ωNIR+nωTHz. The n=1 process offers an efficient means of modulating a NIR carrier beam at THz frequencies and yields new spectroscopic information on excitonic intersubband transitions.

Comparison of optical losses in dielectric-apertured vertical-cavity lasers

The performance of vertical-cavity lasers (VCLs) employing dielectric apertures formed by lateral oxidation or wet-etch undercutting of an AlAs layer on a common substrate are compared. Although both device types performed well, extracted optical losses from the wet-etch undercut structures exceed those of AlAs-oxide apertured lasers. The difference in performance is attributed to optical scattering losses specific to the aperture fabrication method, enhanced by loss from the larger index of refraction discontinuity of the air-gap aperture relative to the AlAs-oxide aperture.

Multimode WDM optical data links with monolithically integrated multiple-channel VCSEL and photodetector arrays

We report on the design and implementation of a novel multiple-wavelength optical data link for low-cost multimode wavelength-division multiplexing (WDM) local-area network applications. This link utilizes a monolithically integrated multiple-wavelength vertical-cavity laser array and a narrow-band resonant-cavity photodetector array to transmit multiple channels of information simultaneously via a single multimode fiber. A detailed analysis on wavelength tuning and threshold characteristics of different laser cavity designs is presented. Theoretical results are compared to our experimental data. On the receiver part, both Schottky and p-i-n photodetectors with a single- or coupled-cavity structure are discussed. A novel p-i-n resonant-cavity photodetector design with a partially oxidized front mirror for linewidth control is proposed. Moreover, we also demonstrate preliminary measurements on the optical link built with our multiple-wavelength vertical-cavity laser array and photodetector array. Finally, the feasibility of constructing a multiwavelength optical data link with a single dual-core multimode fiber and an integrated laser/detector array is evaluated.

Resonant-cavity InGaAs/InAlGaAs/InP photodetector arrays for wavelength demultiplexing applications

Resonant-cavity photodetector arrays are demonstrated in the InGaAs/InAlAs/InP material system grown by solid-source molecular beam epitaxy. To reduce crosstalk between channels, these devices are designed with a high-Q cavity to obtain narrow-band photoresponse. In addition, a novel double-absorber design has been proposed and implemented to avoid position sensitivity related to the cavity standing wave and eliminate the need for in situ cavity-mode adjustment. Both Schottky and P-i-N diode structures are studied and compared. Eight element arrays with linearly distributed resonant wavelengths spanning over 30 nm are fabricated by using a three-level anodic oxidation process. An experimental channel rejection ratio of 14.5 dB at 4 nm away from the resonant peak has been achieved.

Flip-chip bonded arrays of monolithically integrated, microlensed vertical-cavity lasers and resonant photodetectors

We present flip-chip bonded arrays of monolithically integrated vertical-cavity lasers (VCLs) and resonant photodetectors. The VCLs and photodetectors are integrated using a novel structure that allows through-the-substrate emission and detection without compromising device performance. Substrate-side microlenses have been integrated to take advantage of the through-the-substrate architecture. Flip-chip bonded VCLs exhibit threshold currents as low as 135 /spl mu/A with differential efficiencies of /spl sim/53%. The detectors have the same operating wavelength as the VCLs and responsivities of 0.48 A/W, corresponding to 60% absorption, with optical bandwidths of 7 nm.

Postgrowth tuning of semiconductor vertical cavities for multiple-wavelength laser arrays

Combined lateral-vertical oxidation of AlGaAs is investigated as a means of tuning the resonant wavelength of a semiconductor microcavity after the epitaxial growth. It is shown that this technique can provide arrays with a wavelength spread equal to the cavitys free spectral range with a single postgrowth processing step. Design issues for multiple-wavelength vertical-cavity laser arrays using this postgrowth tuning technique are discussed, comparing the performance of devices with all-semiconductor and partially or totally oxidized Bragg mirrors. Experimental results are presented on arrays with a 48-nm lasing span around 970 nm, using partially and totally oxidized mirrors.

Monolithic integration of substrate input/output resonant photodetectors and vertical-cavity lasers

We present theoretical and experimental results on monolithically integrated through-the-substrate input/output vertical-cavity lasers (VCLs) and resonant photodetectors that are compatible with substrate-side micro optics and flip-chip bonding. The required difference in bottom mirror reflectivity between the VCL and the detector is achieved by selective oxidation of a few high Al-content AlGaAs layers in the bottom mirror for the VCL. The modeling shows that using this approach makes it possible to individually design VCLs and resonant detectors from the same epitaxial structure without compromising performance of either device. Furthermore, since the oxidized layers are placed far enough from the active region, the VCL design is very robust with respect to uncertainties in the oxidized layers. For the detectors, we expect about 60% quantum efficiency, a 6-nm full-width at half-maximum optical bandwidth, and less than 1 nm difference in operating wavelength from the VCLs. Experimentally, VCLs and adjacent detectors with integrated microlenses have a difference of less than 0.5 nm in operating wavelength. The detectors have responsivities of 0.48 A/W, corresponding to 60% quantum efficiency and 7-nm optical bandwidths. Single-mode VCLs exhibit threshold currents as low as 135 /spl mu/A while maintaining differential efficiencies above 50%. Larger multimode VCLs have differential efficiencies exceeding 70% with threshold currents of 0.5 mA.

Low-temperature optimized vertical-cavity lasers with submilliamp threshold currents over the 77-370 K temperature range

We demonstrate an extended temperature range (77-370 K) of continuous wave (CW) operation for dielectrically-apertured double-intracavity-contacted vertical-cavity InGaAs strained QW lasers optimized for operation at cryogenic temperatures. Superior performance is achieved through the alignment of the cavity mode with the gain of the first and second quantized subbands at 77 K and room temperature, respectively. This design results in submilliamp threshold currents over a 77-370 K temperature range for 5.4-/spl mu/m diameter lasers. The threshold is 120 /spl mu/A and the output power is >8 mW at 77 K.

High-performance densely packed vertical-cavity photonic integrated emitter arrays for direct-coupled WDM applications

High-performance multiple-wavelength photonic integrated InGaAs-GaAs MQW DBR VCSEL laser emitter arrays with a manufacturable planar scheme are demonstrated for low-cost multimode wavelength-division-multiplexing (WDM) local-area networks. Each array consists of eight pie-shaped bottom-emitting vertical-cavity lasers (VCLs) enclosed within a 60-/spl mu/m-diameter circle for direct-coupling into a single multimode fiber. The arrays exhibit a wide 32.9-nm lasing wavelength span and relatively high 7.3-mW single-channel output. In particular, we have achieved a differential efficiency as high as 50.2% and a maximum wall-plug efficiency of 11.8%. These devices operate with multilateral modes and have a wide 3-dB spectral bandwidth of 1.7 nm in average, which makes them especially suitable for multimode fiber systems.