Frederick J. O'Donnell

Arrays of InP-based Avalanche Photodiodes for Photon Counting

Arrays of InP-based avalanche photodiodes (APDs) with InGaAsP absorber regions have been fabricated and characterized in the Geiger mode for photon-counting applications. Measurements of APDs with InGaAsP absorbers optimized for 1.06 mum wavelength show dark count rates (DCRs) <20 kHz for room-temperature operation with photon detection efficiency (PDE) up to 50% and a reset or dead time of 1s. APDs with InGaAs absorbers optimized for 1.55 μm wavelength and 240 K temperature have DCRs <20 kHz, PDE up to 45%, and a reset time of ~6 mus. Arrays for both wavelengths have been fabricated and packaged with GaP microlenses (of 100 and 50 μm pitch) and CMOS readout integrated circuits (ROICs). Comparisons are made between ROICs that operate in the framed-readout mode as well as those that operate in continuous-readout mode.

Microwave analog optical links using suboctave linearized modulators

We report experimental links at frequencies up to 4 GHz which use two integrated optical Mach-Zehnder interferometric modulators in series to reduce third-order intermodulation distortion. This design improves the intermodulation-free dynamic range (up to 88 dB with 1 MHz noise bandwidth is demonstrated) with no noise figure penalty relative to a link using a standard modulator. Detector current is low (<2 mA), which is necessary for the detector to provide adequate linearity.

High-Power, Low-Noise 1.5-μm Slab-Coupled Optical Waveguide (SCOW) Emitters: Physics, Devices, and Applications

We review the development of a new class of high-power, edge-emitting, semiconductor optical gain medium based on the slab-coupled optical waveguide (SCOW) concept. We restrict the scope to InP-based devices incorporating either InGaAsP or InGaAlAs quantum-well active regions and operating in the 1.5-μm-wavelength region. Key properties of the SCOW gain medium include large transverse optical mode dimensions (>;5 × 5 μm), ultralow optical confinement factor (Γ ~ 0.25-1%), and small internal loss coefficient (αi ~ 0.5 cm-1). These properties have enabled the realization of 1) packaged Watt-class semiconductor optical amplifiers (SOAs) having low-noise figure (4-5 dB), 2) monolithic passively mode-locked lasers generating 0.25-W average output power, 3) external-cavity fiber-ring actively mode-locked lasers exhibiting residual timing jitter of <;10 fs (1Hz to Nyquist), and 4) single-frequency external-cavity lasers producing 0.37-W output power with Gaussian (Lorentzian) linewidth of 35 kHz (1.75 kHz) and relative intensity noise (RIN) <; -160 dB/Hz from 200 kHz to 10 GHz. We provide an overview the SCOW design principles, describe simulation results that quantify the performance limitations due to confinement factor, linear optical loss mechanisms, and nonlinear two-photon absorption (TPA) loss, and review the SCOW devices that have been demonstrated and applications that these devices are expected to enable.

Packaged, High-Power, Narrow-Linewidth Slab-Coupled Optical Waveguide External Cavity Laser (SCOWECL)

We report the demonstration of an InGaAlAs/InP quantum-well, high-power, low-noise packaged semiconductor external cavity laser (ECL) operating at 1550 nm. The laser comprises a double-pass, curved-channel slab-coupled optical waveguide amplifier (SCOWA) coupled to a narrow-bandwidth (2.5 GHz) fiber Bragg grating passive cavity using a lensed-fiber. At a bias current of 4 A, the ECL produces 370 mW of fiber-coupled output power with a Voigt lineshape having Gaussian and Lorentzian linewidths of 35 and 1 kHz, respectively, and relative intensity noise <; -160 dB/Hz from 200 kHz to 10 GHz.

Single-mode optical waveguides and phase modulators in the InP material system

The characteristics of several different single-mode optical waveguides in the InP material system are discussed. Slab-coupled rib waveguides in GaInAsP ( \lambda_{gap} \approx 1 \mu m) epitaxial layers grown on InP have shown propagation losses as low as 1.7 cm-1at 1.3 μm and 2.7 cm-1at 1.15 μm. Oxide-confined InP rib guides fabricated using a lateral overgrowth technique have losses of about 1.5 cm-1at 1.15 μm. Three-guide couplers have been made by fabricating three parallel oxide-confined guides in close proximity. InP p+-n-n+ guides capable of modulating TE-polarized radiation have been fabricated using epitaxial techniques and Be-ion implantation. By measuring the phase difference between the TE-like and TM-like modes as a function of applied voltage, an estimate of the r 41 electrooptic coefficient in InP at 1.3 μm that is in good agreement with a previously reported value was obtained. Guides of this type should find use as the active components in InP switches and interferometers.

High-power 1.3-μm InGaAsP-InP amplifiers with tapered gain regions

Tapered structures fabricated in InGaAsP-InP 1.3-/spl mu/m quantum-well material have been evaluated as high-gain high-saturation-power amplifiers. The devices, which had a 1-mm-long ridge-waveguide input gain section followed by a 2-mm-long tapered section, demonstrated an unsaturated gain of 26 dB at 2.0 A and about 30 dB at 2.8 A. Saturated output power at 2.8 A was >750 mW. At 2.0-A drive current and /spl ap/10-mW input power, the relative intensity noise of the amplified signal was /spl les/-160 dB/Hz at frequencies /spl ges/2 GHz.

Packaged 1.5-

We report the demonstration of a lensed-fiber-pigtailed InGaAsP-InP quantum-well semiconductor optical amplifier based on the slab-coupled optical waveguide (SCOW) concept. At a 5-A bias current and a wavelength of 1540 nm, the packaged SCOW amplifier (SCOWA) exhibits a record 0.8-W saturation output power, 13.8-dB small-signal gain, 5.5-dB noise figure, and a maximum electrical-to-optical conversion efficiency of 11%. The estimated coupling efficiency between the large (5.6 times 7.5 mum), fundamental SCOWA mode and the lensed fibers (6.5-mum spot size) is 90%.

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We have evaluated photorefractive effects at 1320-nm and 1064-nm optical wavelengths in interferometric modulators built using Ti-indiffused waveguides in lithium niobate. The sensitivity to photorefractive damage is substantially increased by anneals in non-oxygen atmospheres at temperatures /spl ges/200/spl deg/C. The sensitivity can be reduced by an anneal in oxygen. Properly annealed modulators operated for 150 h with 400 mW at 1320 nm with no photorefractive effects other than a 3/spl deg/ change in bias point.<<ETX>>

m Quantum-Well SOA With 0.8-W Output Power and 5.5-dB Noise Figure

It has been previously shown that 3 dB is the lowest noise figure attainable for an amplifierless optical link with perfect lossless impedance matching to the RF source. In a prior experimental link with near-perfect impedance matching, dissipative loss in our input matching circuit prevented us from achieving a measured noise figure of less than 4 dB. Investigation of the effects of input impedance mismatch indicates that mismatch can actually lower the noise figure to below 3 dB even in the presence of some dissipative loss in the input circuit. We have verified this theory by using the mismatch effect to reduce the measured noise figure of our link to 2.5 dB at 130 MHz. We believe this is the first demonstration of amplifierless link noise figure of less than 3 dB. We confirmed the validity of our measurement technique by also measuring the noise figure of a 2.5 dB RF attenuator to be 2.5 dB.

Effect of annealing on photorefractive damage in titanium-indiffused LiNbO/sub 3/ modulators

We investigate the noise figure (NF) of high-power semiconductor InGaAsP optical amplifiers (SOAs) based on the slab-coupled optical waveguide (SCOW) concept having both ultralow optical confinement (Γ ~ 0.5%) and low optical loss (αi ~ 0.5 cm-1). At 1550 nm and 5-A current bias, the NF of SCOW amplifier (SCOWA) is 5.5 dB, and the small-signal gain and saturation output power are 13 dB and 0.8 W, respectively. A minimum NF of 4.5 dB is achieved at 2-A bias. These NF results represent the lowest reported for a packaged SOA. Using the measured NF, the population inversion factor (nsp) of the SCOWA was also estimated. The derived nsp values indicate that intervalence band absorption loss, carrier heating, and quasi-bound higher order modes may ultimately limit the noise performance of InGaAsP SOAs.