Vinod M. Menon

Photonic integration using asymmetric twin-waveguide (ATG) technology: part I-concepts and theory

In this first of two papers (hereinafter called Paper I), we present a universal approach for simply realizing monolithic photonic integration based on asymmetric twin-waveguide (ATG) technology. The concepts and important developments leading to ATG integration technology will be reviewed. The ATG structure consists of active and/or passive devices formed in separate, vertically displaced waveguides. Light is transferred between the waveguides via very low loss, lateral, adiabatic tapered mode transformers, allowing different optical functions to be realized in the different waveguides. The design of the adiabatic tapered mode transformer uses an algorithm based on perturbation theory. We show that the same designs can also be deduced from coupled local mode theory. Using the perturbation algorithm to design the taper coupler in an ATG based high bandwidth photodiode, a transfer efficiency of greater than 90% from the fiber waveguide to the coupling waveguide is achieved while the taper length can be reduced by 35% compared to conventional two-section linear taper couplers. The taper design algorithm is further optimized to make the adiabatic taper couplers tolerant to variations in incident light polarization, operation wavelength, and dimensional control during fabrication. Finally, we propose and design a taper that adiabatically couples light from the fundamental mode to the first-order mode. Such a taper coupler is useful in an integrated semiconductor optical amplifier/p-i-n detector circuit.

Control of quality factor and critical coupling in microring resonators through integration of a semiconductor optical amplifier

We demonstrate the integration of ring resonators with semiconductor optical amplifiers (SOAs) using a vertically stacked asymmetric twin waveguide structure. The ring resonator and the bus waveguides are fabricated on a dilute waveguide structure while the active material for the optical amplifier is comprised of compressively strained In/sub 0.8/Ga/sub 0.2/As/sub 0.75/P/sub 0.25/ multiple quantum wells. The racetrack-shaped ring resonators with optical amplifiers integrated at the input showed /spl sim/10-dB contrast, a quality factor of 2.2/spl times/10/sup 4/, and a free spectral range of 0.25 nm. The main aspect in the present work is the control of the quality factor and critical coupling in microring resonators by altering the internal loss in the ring by means of an integrated SOA.

Monolithic integration of a semiconductor optical amplifier and a high bandwidth p-i-n photodiode using asymmetric twin-waveguide technology

An optical mode transformer, a semiconductor optical amplifier (SOA), and a high-bandwidth waveguide-coupled photodiode are monolithically integrated using separately optimized materials based on asymmetric twin-waveguide (ATG) technology. Incident light is collected by a diluted, large fiber guide followed by transfer to an SOA. After amplification, light is coupled into the uppermost In/sub 0.53/Ga/sub 0.47/As light absorption layer by two consecutive taper couplers. The device shows a peak responsivity of 11 A/W (/spl sim/12.5-dB SOA-to-detector gain) and a 3-dB electrical bandwidth of 36 GHz, corresponding to a gain-bandwidth product of 640 GHz. In this SOA/p-i-n chip, separation of optical functions (light guiding, amplification, and detection) into different waveguides allows for optimization of materials for each function without material regrowth. Generalized photonic integrated circuits containing complex combinations of these three optical functions can be realized using the integration scheme demonstrated here.

Photonic integration using asymmetric twin-waveguide (ATG) technology: part II-devices

Asymmetric twin-waveguide (ATG) technology has been shown to be a versatile and high-performance platform for the integration of a wide range of photonic integrated circuits. Following the conceptual and theoretical description in the preceding paper, here we discuss several archetype ATG-based photonic integrated devices and circuits. We present results on the three basic device classes that span the entire range of photonic component needs: light emitters, detectors, and light transporting devices (e.g., waveguides) that have been realized using the twin waveguide platform. Specifically, we discuss Fabry-Pe/spl acute/rot and wavelength-tunable lasers, high-bandwidth p-i-n photodiodes, a polarization-insensitive arrayed waveguide grating receiver with integrated p-i-n photodiodes, as well as combinations of functions including integrated electroabsorption-modulated lasers, and a semiconductor optical amplifier integrated with a p-i-n photodiode. Finally, we address potential integration challenges and novel applications of the ATG scheme, and avenues for improvement.

All-optical wavelength conversion using a regrowth-free monolithically integrated Sagnac interferometer

All-optical wavelength conversion at 10 Gb/s is demonstrated using a regrowth-free monolithically integrated Sagnac interferometer based on asymmetric twin waveguide technology. A single semiconductor optical amplifier (SOA) offset from the center of the loop and integrated with the waveguide, acts as the nonlinear element in the interferometer. The device demonstrated uniform performance over a 30 nm (1530 nm-1560 nm) wide optical window. Results of theoretical simulations performed to explain the wavelength converted pulse shape are also presented.

An asymmetric twin waveguide eight-channel polarization-independent arrayed waveguide grating with an integrated photodiode array

We demonstrate the regrowth-free integration of an InP-based arrayed waveguide grating (AWG) with a 1 /spl times/ 8 In/sub 0.53/Ga/sub 0.47/As p-i-n photodiode array. Low-loss polarization-independent adiabatic tapers transfer light from a passive waveguide optimized for external fiber coupling into a higher index waveguide for absorption in the In/sub 0.53/Ga/sub 0.47/As layer. The AWG and individual photodiodes have an external responsivity of 0.46 A/W and a polarization-dependent loss of /spl les/0.76 dB. The integrated device shows a worst-case crosstalk of -12.9 dB and a best-case crosstalk of -16.2 dB between channels, 9-dB total loss (including coupling losses), and -1.5-dB channel nonuniformity. The photodiode 3-dB electrical bandwidth is (25 /spl plusmn/ 5) GHz.

Reduction of absorption loss in asymmetric twin waveguide laser tapers using argon plasma-enhanced quantum-well intermixing

Lasers based on asymmetric twin waveguide integration technology are limited by the necessity of pumping the tapers to avoid absorption losses within this section of the active region. Here, we demonstrate that the threshold current is reduced by argon plasma-enhanced quantum-well intermixing in the taper. Intermixing induces a (57/spl plusmn/5) nm wavelength blue shift in the emission peak, accompanied by a <12-nm linewidth broadening of the photoluminescence spectrum, indicating minimal material degradation. The threshold current of a 0.6-mm-long laser is reduced by (18/spl plusmn/1)% to (27/spl plusmn/1) mA using an intermixed taper as compared to a nonintermixed structure.

Light-induced symmetry breaking and related giant enhancement of nonlinear properties in CdZnTe:V crystals

We report on enormous light-induced reversible strain effects in CdZnTe:V crystals, which lead to a remarkable enhancement of their nonlinear properties, such as electrostriction and electro-optic effects. Using both high resolution x-ray diffraction and optical interferometry we measure light-induced relative deformation of the initial crystalline lattice (changes in d-spacings) up to 0.15%. The experimental findings are attributed to light-induced breaking of the initial cubic crystalline symmetry. Our results point to a family of inorganic materials whose nonlinear properties can be remarkably enhanced by light, offering new possibilities for nonlinear frequency conversion, generation of Terahertz radiation, electro-optic modulation, and self-deflection of optical beams.

Monolithic integration of a semiconductor optical amplifier (SOA) and a high-speed waveguide p-i-n photodiode using asymmetric twin-waveguide technology

An optical mode transformer, semiconductor optical amplifier, and waveguide photodiode are monolithically integrated without regrowth using separately optimized materials based on asymmetric twin-waveguide technology. The device shows a peak responsivity of 11 A/W and a 3 dB electrical bandwidth of 36 GHz.

Light-induced ionic displacement in CdZnTe:V crystals giving rise to crystalline symmetry breaking and giant nonlinearities

We report on direct experimental evidence that light-induced ionic displacement, driven by charge separation, is responsible for the enormous enhancement of the nonlinear properties of CdZnTe:V crystals.