Photonic integration using asymmetric twin-waveguides

A novel approach to fabrication of monolithic photonic integrated circuits based on the asymmetric twin-waveguide (ATG) structure is proposed and demonstrated. In contrast to the conventional integration methods relying on semiconductor regrowth, the ATG approach requires only one epitaxy step, while the integrated devices are defined by post-growth patterning. The ATG structure contains two evanescently coupled waveguide layers separated by a cladding layer. The upper layer provides optical gain for the active devices such as lasers and semiconductor optical amplifiers. The transparent lower layer is used to make waveguides and optical interconnects on the chip. Thus the ATG represents a versatile integration platform for cost-effective fabrication of photonic integrated circuits, similar in some respects to the electronic CMOS platform.; Light propagation and coupling in the ATG structure are analyzed using the beam propagation method to optimize the layer design. It is shown that the asymmetric refractive index profile eliminates undesirable optical coupling between the waveguide layers. At the interfaces between the active and passive devices, lateral tapers are used to induce vertical coupling of light with a coupling loss of typically <1 dB. Therefore various integrated devices can be separately optimized to achieve performance close to that of the conventional discrete components. The design of taper couplers is described in detail, and their performance is experimentally verified.; Using the ATG approach, several integrated devices were fabricated in the InGaAsP/InP material system for λ = 1.55 μm wavelength operation. Lasers and semiconductor optical amplifiers with integrated waveguides were characterized to test the integration approach. Single-frequency, distributed Bragg reflector (DBR) lasers achieved output power of 11 mW with a 40 dB side-mode suppression ratio. A DBR laser with integrated electroabsorption modulator had a 24 dB extinction ratio between 0V and −2V bias. Finally, an ATG-based monolithic Mach-Zehnder Terahertz Optical Asymmetric Demultiplexer (MZ-TOAD) was successfully fabricated and tested. Used as an ultrafast all-optical switch, it had a 28 ps switching window for the optical control pulse energies from 0.7 to 2.8 pJ. A version of the MZ-TOAD was also operated as a wavelength converter with a 12 dB dc extinction ratio.