Bijoy Krishna Das

Over the last decade, silicon nitride (SiN) based integrated photonics has become popular due to its CMOS com-patibility, low loss, wide spectral operation band and tolerance to high optical power. It has great potential in different application areas like quantum information processing, microwave photonics, spectroscopy, etc. However, the growth of an optical grade oxide layer (for bottom cladding) and subsequent deposition of SiN device layer on the surface of a handle silicon wafer are the most important aspects for the realization of large- scale photonic integrated circuits with an acceptable waveguide losses and fabrication yields. In this paper, we report the in- house technology development of SiN waveguide devices starting from a 4- inch silicon wafer. Both the thermally grown buried oxide (BOX) and the LPCVD deposited SiN device layer show excellent uniformities in terms of their thickness $(1900\pm 20nm, 400\pm 4nm$, respectively) as well as refractive indices $(n_{SiO_{2}}= 1.45\pm 0.001, n_{SiN}=2.024\pm 0.003$ at $\lambda\sim 1.55\mu m)$ across the full wafer. We have discussed the design and characterization of the grating coupler, single mode waveguides operating at $\lambda\sim 1.55\mu m$ and their fabrication flow as well. The fabricated single mode waveguides (TE polarized) with input/output grating couplers exhibit a 3-dB bandwidth of $\sim 50nm$ with a peak transmission efficiency at $\lambda\sim 1570nm$. The SiN waveguides fabricated out of in-house processed SiN wafer exhibit reasonable third order non-linearity; confirmed by stimulated four wave mixing experiment.

The coupling coefficients of CROW structure have been modeled with the apodized physical separations between successive directional couplers. A fabrication friendly six-ring CROW design for 8-bit delay-line with 10 GHz spectral bandwidth has been presented.Optics and Photonics

A filter device with a flat-top passband of Δλpb > 40 nm is demonstrated using multi-mode SOI waveguide with a side-wall grating. The passband is bounded by highly extinguished sidebands of Δλsb > 10 nm.

Theoretical simulation shows that APE:LiNbO3 ridge waveguide can support a more tightly confined fundamental mode than that of conventional APE or Ti-indiffused channel waveguides. Successful demonstration of such waveguides on the surface of LiNbO3 crystal will have potential impact in the area of nonlinear integrated optics. However, the fabrication process of ridge structures has appeared to be a challenging task. The ridge structures on the surface of X-cut LiNbO3 crystal were fabricated by wet/dry etching process using various recipes and corresponding surface morphologies have been closely investigated. © 2011 SPIE.

A lossless flat-top h igher o rder r esonance fi lter (F WHM∼2nm) ha s been demonstrated using DBR structures in a silicon waveguide. It exhibits out-of-band rejection of > 40-dB and 3-dB/10-dB bandwidth shape factor of 0.9.

A detailed theoretical and experimental study of metal-microheater integrated silicon waveguide phase-shifters has been carried out. It has been shown that the effective thermal conductance gw and the effective heat capacitance hw evaluated per unit length of the waveguide are two useful parameters contributing to the overall performance of a thermo-optic phase-shifter. Calculated values of temperature sensitivity, SH = 1/gw and thermal response time, τth = hw/gw of the phase-shifter are found to be consistent with the experimental results. Thus, a new parameter FH = SH/τth = 1/hw has been introduced to capture the overall figure of merit of a thermo-optic phase-shifter. A folded waveguide phase-shifter design integrated in one of the arms of a balanced MZI switch is shown to be superior to that of a straight waveguide phase-shifter of the same waveguide cross-sectional geometry. The MZI switches were designed to operate in TE-polarization over a broad wavelength range (λ ∼ 1550 nm).

The modal transition loss between straight and 90° bend rib waveguide structures on Silicon-on-Insulator platform have been optimized with the simulated results using Finite Element Method (FEM) and Beam Propagation Method (BPM). We have studied different asymmetric configurations to minimize the radius of curvature of bend waveguide. The simulation results shows that a 90° bend waveguide is possible with a bending radius as low as 800 μm keeping the loss budget to ∼ 1 dB. © 2008 IEEE.

Photonic wire waveguide based DBR structure with a razor-edge-filter characteristics and microring resonator with a resonance-comb-envelope characteristics have been demonstrated in SOI platform; they are shown to be useful for sensing applications.

It is shown that the dispersion in transmission characteristics of a ring resonator designed with silicon-on-insulator waveguides in all-pass configuration can be enhanced significantly by increasing interaction length of the directional coupler. This in turn helps to single-out highly extinct resonance(s) at and around the critically coupled wavelength. Such a device is found to be useful for a wide range of refractive index sensing for the cladding materials/analytes (1.0 < nc ≤ 2.0). As a proof of concept, the sensor devices were fabricated and characterization results are shown to be consistent with theoretical prediction. The fabricated devices have been also used successfully to determine unknown refractive index of a given analyte (Newport F-IMF-150) with an error limit of δn ∼ 1.67 × 10-2 RIU. Analyzing experimental results, it is shown that the limit of detection can be further reduced (≤ 10-3 RIU), if the perimeter of the ring is increased without compromising the round-trip waveguide loss. The superiority of such a sensor device lies in its simpler design rule, easier operation, wider range, and nearly accurate detection mechanism.

A semi-analytical model has been developed to analyze fabrication limited phase errors and the output spectrum of AWGs in Silicon-On-Insulator platform. This model has been compared with conventional numerical methods and validated with published experimental results.