O. Sjolund

Epitaxially-stacked multiple-active-region 1.55μm lasers for increased differential efficiency

Semiconductor lasers emitting at 1.55 μm with external differential efficiencies >1 have been created by monolithically connecting several active regions in series within a single optical waveguide. This is accomplished by epitaxially stacking a number of p–i–n multiquantum well active regions with intermediate n++–p++ back diodes, which enable the entire terminal current to flow through each active region stages in series. Such lasers should also improve the impedance match as well as provide for low-noise, high-efficiency microwave links.

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...

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.

Flip-chip bonded, back-emitting, microlensed arrays of monolithic 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 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. The VCLs exhibit maximum small-signal modulation bandwidths, which are limited by parasitics, of /spl sim/9.5 GHz at 20/spl deg/C and /spl sim/8.4 GHz at 70/spl deg/C. The VCLs have the lowest reported bias currents required to reach bandwidths of up to /spl sim/8 GHz. A free-space optical link is demonstrated with these flip-chip bonded arrays of microlensed, monolithically integrated VCLs and detectors. The link is found to be tolerant to temperature differences of /spl plusmn/75/spl deg/C between the VCL and detector, with error free (BER<10/sup -12/) data transmission demonstrated in each case.

Individually optimized bottom-emitting vertical-cavity lasers and bottom-illuminated resonant photodetectors sharing the same epitaxial structure

We demonstrate a novel design that allows fabrication of substrate input/output photodetectors and lasers on the same substrate by selective oxidation of the bottom mirror.

Room-temperature, electrically-pumped, multiple-active-region VCSELs with high differential efficiency at 1.55 /spl mu/m

We have demonstrated fully-epitaxial, room-temperature, electrically-pumped VCSELs based on III-As compounds with low threshold current density, very high differential efficiency and low threshold voltage by employing multiple-active-regions in a single cavity. Even higher differential efficiencies should be possible with optimized cavity designs. The high differential efficiency and high output powers makes these VCSELs good candidates for low-noise, high-efficiency microwave links. Also, when integrated with a series connected detector, analog repeaters, amplifying wavelength converters, and lossless signal tapping are possible.

Epitaxially-stacked multiple-active-regions for high differential efficiency monolithic 1.55 /spl mu/m VCSELs

The development of monolithic 1.55 /spl mu/m VCSELs has been hampered by the lack of readily available highly reflective DBRs. However, it is possible to achieve lasing with less reflective DBRs by epitaxially stacking multiple active regions and increasing the gain. For example, 30.5-period AlGaInAs/AlInAs DBRs can be mated to a three-stage active region to achieve lasing. Epitaxy of such VCSEL can be short enough to maintain source stability in an MBE system. The other benefit is greater differential efficiency, which can actually exceed unity, for greater signal to noise ratio. We have investigated the properties of such active regions. We have created edge-emitters with external differential efficiencies >1 by monolithically connecting several active regions in series within a single optical waveguide. In contrast to previous reports, we have accomplished this task at the technologically important 1.55 /spl mu/m wavelength.

Design considerations in electrically-pumped, single-epitaxial VCSELs at 1.55 /spl mu/m with Sb-based mirrors

The development of vertical-cavity surface-emitting lasers (VCSELs) at the telecommunications important wavelength of 1.55 /spl mu/m has been hindered by the absence of a substrate that is suitable for both technologically-developed distributed Bragg reflectors (DBRs) and quantum well active regions. AlGaAsSb-based DBRs, however, which can be lattice-matched to InP and have a refractive index-contrast that is similar to AlGaAs-based DBRs at 1.3 /spl mu/m-1.55 /spl mu/m, offer the opportunity to combine the mature InP-based active region technology with high reflectivity mirrors in a single growth. We have recently reported, in fact, electrically-pumped, Sb-based vertical-cavity lasers operating at 1.55 /spl mu/m and produced in a single epitaxial growth. These lasers, which employ two n-type, lattice-matched AlGaAsSb mirrors and an AlInGaAs-based active region, had room temperature threshold current densities of 1.4 kA/cm/sup 2/ and an external quantum efficiency of /spl sim/18%. The VCSELs, unfortunately, suffered from a very high operating voltage. We present design changes that have significantly reduced this voltage and discuss future improvements that could lead to further reductions.

Epitaxially-stacked multiple-active-region 1.55 /spl mu/m CW lasers for increased differential efficiency

Semiconductor CW lasers with external differential efficiencies >1 have been created by monolithically connecting several active regions in series within a single optical waveguide. This task has been accomplished at the technologically important 1.55/spl mu/m wavelength. This is achieved by epitaxially stacking a number of p-i-n multi-quantum well active regions with intermediate n/sup ++/-p/sup ++/ back-diodes, which enable the entire terminal current to flow through each active region stage in series. Such lasers should also improve impedance match as well as provide for low-noise, high-efficiency microwave links.

Novel technique for monolithic integration of microlensed resonant detectors and vertical cavity lasers

Summary form only given. We demonstrate a novel design that allows the fabrication of photodetectors and vertical-cavity lasers with integrated microlenses from a single epitaxial growth by selective oxidation of the bottom mirror.