Yae Okuno

High-power 1320-nm wafer-bonded VCSELs with tunnel junctions

A new long-wavelength vertical-cavity surface-emitting laser structure is described that utilizes AlGaAs-GaAs mirrors bonded to AlInGaAs-InP quantum wells with an intracavity buried tunnel junction. This structure offers complete wavelength flexibility in the 1250-1650 nm fiber communication bands and reduces the high free-carrier losses and bonded junction voltage drops in previous devices. The intracavity contacts electrically bypass the bonded junctions to reduce threshold voltage. N-type current spreading layers and undoped AlGaAs mirrors minimize optical losses. This has enabled 134/spl deg/C maximum continuous-wave lasing temperature, 2-mW room-temperature continuous-wave single-mode power, and 1-mW single-mode power at 80/spl deg/C, in various devices in the 1310-1340 nm wavelength range.

Vertical-cavity surface-emitting laser active regions for enhanced performance with optical pumping

We have developed an improved active region design for optically pumped vertical-cavity surface-emitting lasers. The design makes use of carrier-blocking layers to segment the absorber and promote uniform carrier populations in the quantum wells with pump efficiencies near 75%. A model to calculate the carrier distribution in the active region and a design methodology are presented along with a metric to describe the carrier uniformity in the quantum wells. Experimental verification of the performance improvements shows an over 50% reduction in device thresholds and an increase of 20/spl deg/C in maximum operating temperatures.

3-D photonic circuit technology

Vertically coupled, wafer-bonded III-V semiconductor waveguide devices provide a means to obtain more powerful, compact photonic integrated circuits and allow for the combination of different materials onto a single chip. Various switching, filtering, multiplexing, and beam splitting devices in the InP-InGaAsP and GaAs-AlGaAs systems for signals in the 1550-nm range have been realized. An investigation of optimal optical add-drop multiplexer waveguide layout shapes has been performed through integration of the coupled-mode Riccati equation, providing potential sidelobe levels of less than -32 dB and filter bandwidths over 20% narrower than those of previous devices. Effects of nonideal processing conditions on filter performance are analyzed as well.

1.3 μm wavelength vertical cavity surface emitting laser fabricated by orientation-mismatched wafer bonding: A prospect for polarization control

We propose and demonstrate a long-wavelength vertical cavity surface emitting laser (VCSEL) which consists of a (311)B InP-based active region and (100) GaAs-based distributed Bragg reflectors (DBRs), with an aim to control the in-plane polarization of output power. Crystal growth on (311)B InP substrates was performed under low-migration conditions to achieve good crystalline quality. The VCSEL was fabricated by wafer bonding, which enables us to combine different materials regardless of their lattice and orientation mismatch without degrading their quality. The VCSEL was polarized with a power extinction ratio of 31 dB.

Long-wavelength two-dimensional WDM vertical cavity surface-emitting laser arrays fabricated by nonplanar wafer bonding

We demonstrate the first long-wavelength two-dimensional wavelength-division-multiplexed vertical cavity surface-emitting laser array. The eight-channel single-mode array covers the C-band from 1532 to 1565 nm. The devices are fabricated using two separate active regions laterally integrated using nonplanar wafer bonding. We achieved single-mode powers up to 0.8 mW, 2-dB output power uniformity across the array, and sidemode suppression ratios in excess of 43 dB. This fabrication technique can be used to maintain the gain-peak and cavity-mode alignment across wide-band arrays and, with the use of nontraditional mirrors, can be extended to the fabrication of arrays covering the entire C-, S-, and L-bands as well as the 1310-nm transmission band.

InP-InGaAsP wafer-bonded vertically coupled X-crossing multiple channel optical add-drop multiplexer

A vertically coupled InP-InGaAsP crossed waveguide optical add-drop multiplexer has been realized through the use of wafer bonding. Designed for signals in the 1550-nm range, this novel device requires only a single epitaxial growth and illustrates the use of vertical optical interconnects for the three dimensional routing of optical signals. To our knowledge, it is also one of the first optical vertically coupled devices with no horizontally coupled counterpart.

Experimental Characterization and Modeling of InP-based Microcoolers

We present experimental and theoretical characterization of InP-based heterostructure integrated thermionic (HIT) coolers. In particular, the effect of doping on overall device performance is characterized. Several thin-film cooler devices have been fabricated and analyzed. The coolers consist of a 1µm thick superlattice structure composed of 25 periods of InGaAs well and InGaAsP (λgap ≈ 1.3µm) barrier layers 10 and 30nm thick, respectively. The superlattice is surrounded by highly-doped InGaAs layers that serve as the cathode and anode. All layers are lattice-matched to the n-type InP substrate. N-type doping of the well layers varies from 1.5×10 18 cm -3 to 8×10 18 cm -3 between devices, while the barrier layers are undoped. Device cooling performance was measured at room-temperature. Device current-versus-voltage relationships were measured from 45K to room-temperature. Detailed models of electron transport in superlattice structures were used to simulate device performance. Experimental results indicate that low-temperature electron transport is a strong function of well layer doping and that maximum cooling will decrease as this doping is increased. Theoretical models of both I-V curves and maximum cooling agree well with experimental results. The findings indicate that low-temperature electron transport is useful to characterize potential barriers and energy filtering in HIT coolers.

Wafer-fused thin film cooler semiconductor laser structures

We examine the cooling requirements and temperature stabilization needs of semiconductor lasers with emphasis on vertical cavity surface emitting laser (VCSEL) arrays. Semiconductor lasers in both in-plane and vertical cavity geometries are capable of generating large heat power densities of the order of kW/cm/sup 2/ over areas as small as 100 /spl mu/m/sup 2/. When cooling of the laser is needed, the cooler should be able to provide similar amounts of heat pumping. For these large amounts of heat pumping, a thin-film cooler structure is needed, especially if individual devices of an array must have precise temperature stabilization. Integration of the laser and thin film cooler by Au-Au wafer fusion is proposed. The fusion of the two interfaces is accomplished by mass transport in the deposited Au-films, which can be achieved under pressure at elevated temperatures. The quality of the fused interface is studied and preliminary experimental results are presented.

Integrated-cavity surface-emitting lasers

A novel integrated cavity surface emitting laser (ICSEL) is proposed in this paper. An ICSEL integrates two different laser structures. One is an in-plane laser with a short lasing wavelength, and the other is a vertical cavity laser structure with a long lasing wavelength. The in-plane laser is used as the light source for optical pumping the VCSEL. By monolithic integration of both the laser structures, some problems related to electrically pumped VCSELs, especially long wavelength VCSELs, and externally optically pumped VCSELs can be solved.

An InP/InGaAs tunnel junction fabricated on (311)B InP substrate by MOCVD

We investigated the doping characteristics of InP/InGaAs on the (311)B plane by metalorganic chemical vapour deposition (MOCVD) using metalorganic group-V regents. For both n-type Si-doping and p-type Zn-doping, we found that dopant incorporation is higher on the (311)B plane than the (100) plane, and that we can dope both n-type and p-type layers more than 10/sup 19/ cm/sup -3/ at the same growth condition. Applying this result, we grew a tunnel junction on (311)B InP substrates at a constant growth temperature. The as-grown junction showed good current-voltage characteristics and is promising for device applications.