Milind R. Gokhale

An asymmetric twin-waveguide high-bandwidth photodiode using a lateral taper coupler

We describe a high-bandwidth high-responsivity low-polarization sensitivity p-i-n photodiode based on an integratable asymmetric twin-waveguide structure. Incident light is collected by a diluted large-fiber guide followed by transfer to a thin-coupling waveguide using a low-loss lateral-taper coupler. The light is finally absorbed by the uppermost In/sub 0.53/Ga/sub 0.47/As layer. The device has a responsivity of (0.75/spl plusmn/0.03) ampere per watt and a polarization sensitivity of /spl les/0.4 dB. The measured electrical 3 dB bandwidth is /spl ges/40 GHz. The responsivity is comparable with the best 40-GHz waveguide p-type-intrinsic-n-type photodiodes, while the twin-waveguide design provides a single-epitaxial growth step and a simple means of fabrication with possibility for monolithic integration of the photodiodes with other optical components, such as semiconductor optical amplifiers and in-plane waveguide filters.

Efficient coupling in integrated twin-waveguide lasers using waveguide tapers

We demonstrate a 1.55-/spl mu/m wavelength, InGaAsP-InP, twin-waveguide (TG) laser integrated with passive ridge waveguides using low-loss taper couplers. The lateral taper on the laser waveguide induces efficient resonant coupling of light between the active and passive layers. The device is fabricated using low-cost conventional photolithography and reactive ion etching of a TG structure grown by gas-source molecular beam epitaxy. This structure is suitable for integrating a variety of photonic devices without requiring epitaxial regrowth.

Growth and characterization of small band gap (∼0.6 eV) InGaAsN layers on InP

We demonstrate the growth of small band gap (Eg∼0.6 eV) strained and lattice matched single crystal InGaAsN alloys on InP substrates. InGaAsN layers with N concentrations varying from 0.6% to 3.25% were grown by gas source molecular beam epitaxy using a radio frequency plasma nitrogen source. Lattice-matched, 0.5-μm-thick InGaAsN layers with smooth surface morphologies and abrupt interfaces were achieved. Low temperature photoluminescence measurements reveal a band gap emission wavelength of 1.9 μm (at 20 K) for lattice matched InGaAsN (N∼2%). Tensile strained In0.53Ga0.47As/In0.53Ga0.47As0.994N0.006 multiple quantum wells emitting at 1.75 μm at 20 K are also reported.

Asymmetric twin-waveguide 1.55-μm wavelength laser with a distributed Bragg reflector

We demonstrate a single frequency, 1.55 /spl mu/m wavelength laser based on an asymmetric twin-waveguide structure using a single growth step and a simple fabrication process. The external Bragg grating is formed on the passive ridge waveguide, optically coupled to the twin-guide gain section using a low loss, tapered mode transformer. The grating is produced by near-field holographic printing using a phase mask. Output powers >11 mW in a small-spot waveguide with a side-mode suppression ratio >40 dB and a slope efficiency of 0.11 W/A are obtained under pulsed operation. These performance characteristics are comparable to conventional, nonintegrated, conventional discrete DBR lasers, although the twin-waveguide design is compatible with photonic integrated circuits such as monolithic transmitters and WDM coherent receivers.

Low-threshold current, high-efficiency 1.3-μm wavelength aluminum-free InGaAsN-based quantum-well lasers

We demonstrate high-performance Al-free InGaAsN-GaAs-InGaP-based long-wavelength quantum-well (QW) lasers grown on GaAs substrates by gas-source molecular beam epitaxy using a RF plasma nitrogen source. Continuous wave (CW) operation of InGaAsN-GaAs QW lasers is demonstrated at /spl lambda/=1.3 /spl mu/m at a threshold current density of only J/sub TH/=1.32 kA/cm/sup 2/. These narrow ridge (W=8.5 /spl mu/m) lasers also exhibit an internal loss of only 3.1 cm/sup -1/ and an internal efficiency of 60%. Also, a characteristic temperature of T/sub 0/=150 K from 10/spl deg/C to 60/spl deg/C was measured, representing a significant improvement over conventional /spl lambda/=1.3 /spl mu/m InGaAsP-InP lasers. Under pulsed operation, a record high maximum operating temperature of 125/spl deg/C and output powers greater than 300 mW (pulsed) and 120 mW (CW) were also achieved.

High-power high-efficiency 0.98-/spl mu/m wavelength InGaAs-(In)GaAs(P)-InGaP broadened waveguide lasers grown by gas-source molecular beam epitaxy

We describe the design and experimental results for high-power, high-efficiency, low threshold current, 0.98-/spl mu/m wavelength, broadened waveguide (BW) aluminum-free InGaAs-(In)GaAs(P)-InGaP lasers. The decrease in the internal losses with an increase in the width of the waveguide layer for a separate-confinement heterostructure multiple-quantum-well (SCW-MQW) structure is attributed to lower free-carrier absorption due to the reduced overlap of the optical mode with the highly doped cladding regions. The BW lasers grown with both InGaAsP and GaAs waveguides show lower internal losses and similar threshold currents than those designed for an optimum optical confinement factor within the QW region. We report a record-low internal loss of 1.8/spl plusmn/0.2 cm/sup -2/ for (In)GaAs(P)-InGaP lasers grown by gas-source molecular beam epitaxy (GSMBE). The temperature dependence of internal loss suggests that optical loss from free-carrier absorption in the waveguide dominates at T>40/spl deg/C, while near room temperature, the residual loss is attributed to scattering and free-carrier absorption in the QWs. The relative insensitivity of internal loss near room temperature has enabled the use of a simplified InGaAs-GaAs-InGaP BW structure to achieve very high CW and quasi-CW (QCW) power operation. We report the highest CW output power of 6.8 W for a GaAs-InGaP laser, and the highest quasi-continuous output power of 13.3 W measured for a single 100-/spl mu/m-wide aperture, 0.8-0.98-/spl mu/m wavelength Al-free laser diode grown by GSMBE.

Monolithic integration of an all-optical Mach-Zehnder demultiplexer using an asymmetric twin-waveguide structure

We demonstrate monolithic integration of a twin-waveguide Mach-Zehnder terahertz optical asymmetric demultiplexer (MZ-TOAD). The InGaAsP-InP device operates at 1.55 /spl mu/m wavelength and has two semiconductor optical amplifiers in the asymmetric MZ interferometer configuration with a single control input and a built-in switching delay. A 28-ps full-width at half-maximum (FWHM) switching window with high contrast ratio for a range of control powers is obtained using a counter-propagating optical signal and control pulses.

Monolithic integration of a quantum-well laser and an optical amplifier using an asymmetric twin-waveguide structure

We demonstrate the monolithic integration of a 1.55 /spl mu/m wavelength InGaAsP-InP multiple-quantum-well (MQW) laser and a traveling-wave optical amplifier using an asymmetric, vertical twin-waveguide structure. The laser and amplifier share the same strained InGaAsP MQW active layer grown by gas-source molecular beam epitaxy, while the underlying passive waveguide layer is used for on-chip optical interconnections between the active devices. The asymmetric twin-waveguide structure uses the difference in modal gains to discriminate between the even and odd modes.

Uncooled, 10 Gb/s 1310 nm electroabsorption modulated laser

We demonstrate the first 10 Gb/s, 1310 nm EML operating uncooled from 0-85 /spl deg/C. Transmission over 20 km with zero power penalty and a 26% SONET eye-mask margin after 10 km are reported.

Strain compensated In1−xGaxAs(x<0.47) quantum well photodiodes for extended wavelength operation

The use of highly strained (−2.0%) In0.83Ga0.17As quantum wells for the detection of light to a wavelength of λ∼2.0 μm is reported. Crystal quality for a 50 period multiple quantum well (MQW) detector grown on InP substrates is maintained through strain compensation using tensile strained In0.83Ga0.17P barriers. Transmission electron microscopy and double crystal x-ray diffraction reveal smooth interfaces and no observable defects for In0.83Ga0.17As layers with widths less than 80 A. Single-pass quantum efficiencies of 30% have been achieved at λ=1.95 μm, using a 75 μm diam MQW, strain-compensated, top illuminated, low dark current (∼250 pA at 20 V) p-i-n detector. The theoretical cutoff wavelength limit for diodes fabricated using this technique is calculated to be λ=2.15 μm.