Bijoy Krishna Das

Erbium-Doped Lithium Niobate Waveguide Lasers

The recent progress in the field of Ti:Er:LiNbO 3 waveguide lasers with emission wavelengths in the range 1530 nm < λ < 1603 nm is reviewed. After a short discussion of the relevant fabrication methods concepts and properties of different types of lasers with grating resonator, acoustooptically tunable Fabry Perot type lasers and new ring laser structures are presented.

Fabrication and characterization of straight and compact S -bend optical waveguides on a silicon-on-insulator platform

Straight and S-bend rib waveguide structures with a novel design have been fabricated and characterized on a silicon-on-insulator (SOI) platform. For a typical straight rib waveguide, the single-mode waveguide at lambda approximately 1550 nm has been verified by measuring the near-field output with an IR camera, and a nearly polarization-independent mode size is found to be approximately 10 microm x 4.5 microm. The waveguide loss has been estimated from low-finesse Fabry-Perot transmission characteristics, and a typical value of approximately 0.5 dB/cm is obtained. It is also shown experimentally that the bending radius of an asymmetrically etched S-bend waveguide can be ten times smaller than that of conventional symmetrically etched S-bend waveguides for similar optical losses. These bend waveguides (bending radii of approximately 1500 microm) are found to be low loss (<2.5 dB) and nearly polarization independent.

Dispersion-Free SOI Interleaver for DWDM Applications

Integrated optical asymmetric Mach-Zehnder interferometer-based dense wavelength division multiplexing (DWDM) channel interleaver structures (2 × 2) have been theoretically studied using various single-mode waveguide geometries in silicon-on-insulator platform. Subsequently, a dispersion-free interleaver design has been proposed, fabricated, and characterized. It was designed to separate alternate DWDM channels with 100 GHz spacings. Some of the fabricated prototype devices have been fiber pigtailed and packaged in a suitably designed metal housing. The transmitted spectra (1520 nm <; λ <; 1600 nm) at each of the output ports reveal that alternate DWDM channel wavelengths are separated with a uniform inter-channel extinction of ~ 18 dB. Each of the channel passbands shows their 3-dB bandwidth of ~ 40 GHz.

Dispersion Enhanced Critically Coupled Ring Resonator for Wide Range Refractive Index Sensing

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.

Compact integrated optical directional coupler with large cross section silicon waveguides

Compact integrated optical directional couplers with symmetrically- and asymmetrically etched S-bend waveguides on SOI platform have been designed, fabricated and characterized. We have found that the directional couplers with asymmetrically etched waveguide structures can increase the device compactness to about 4 to 5 times that of the conventional symmetrically etched bend waveguide structures without compromising much on the optical loss budgets. The over all waveguide loss and loss per S-bend have been measured to be ~ 0.5 dB/cm and ~ 1 dB, respectively. The directional couplers are found to be polarization dependent (up to ~ 2 dB) and nearly wavelength independent (1510 nm < λ < 1600 nm).

Design of Low-Loss Compact 90° Bend Optical Waveguide for Photonic Circuit Applications in SOI Platform

The modal transition loss between straight and 90deg 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 90deg bend waveguide is possible with a bending radius as low as 800 mum keeping the loss budget to ~1 dB.

Modeling and experimental investigation of an integrated optical microheater in silicon-on-insulator

A linear piecewise model has been formulated to analyze the performance of a metallic microheater integrated with single-mode waveguides (λ∼1550  nm) in silicon-on-insulator (SOI). The model has been used to evaluate integrated optical microheaters fabricated in a SOI substrate with 2 µm device layer thickness. The Fabry-Perot modulation technique has been used to extract the effective thermo-optic phase shift and response time. The effective thermal power budget of Peff,π∼500  µW (out of actually consumed power Pπ=1.1  mW) for a π phase shift and a switching time of τ∼9  µs, have been recorded for a typical Ti heater stripe of length LH=50  µm, width WH=2  µm, and thickness tH∼150  nm, integrated with a Fabry-Perot waveguide cavity of length ∼20  mm. It has been shown that the performance of a heater improves (in terms of power budget) as the length of a microheater decreases. However, smaller heater size requires higher joule heating to obtain a desired phase shift, which is again found to be dependent on polarization of the guided mode because of thermal stress.

Polarization-independent and dispersion-free integrated optical MZI in SOI substrate for DWDM applications

Polarization dependencies and dispersions are the two major bottlenecks in waveguide based silicon photonic devices for various applications – especially in DWDM systems. In this paper, we present the design and experimental demonstration of a 2×2 integrated optical MZI that shows both polarization-independent and dispersion-free response over a wide wavelength range (C+L optical band) in SOI platform - for the first time to our knowledge. The entire device footprint (W×L) is ~ 0.8 mm × 5.2 mm; which is comprised of optimally designed single-mode waveguides (for input/output and interferometer arms) and a pair of MMI based 3-dB power splitters. To monitor the wavelength dependent performance, unbalanced arm lengths (L ~ 3037 μm, L+ΔL ~ 3450 μm) were introduced to construct the MZI. The differential arm length (ΔL ~ 412 μm) has been specifically chosen to provide alternate ITU channel transmission peaks at both the output ports alternatively. Accordingly, the fabricated device separates 100 GHz DWDM channel wavelengths alternatively into two output ports and is nearly insensitive to the polarization of the guided light. We have observed a uniform channel extinction of ~ 10 dB (~ 6 dB) at both ports with a 3-dB bandwidth of ~ 110 GHz (~120 GHz) for TM (TE) polarization over the wavelength range of 1520 nm to 1600 nm. The lower extinction for TE polarization is due to its relatively higher bending loss in the longer arm of the MZI. This can be adjusted by introducing identical bends in both the arms.

Highly efficient DBR in silicon waveguides with eleventh order diffraction

The distributed Bragg reflector (DBR) plays a major role in integrated optics. Because of the recent advances of silicon photonics and CMOS electronics in SOI platform, various types of DBR structures are being investigated for integrated optical couplers, filters, (de-)multiplexers, interleavers, Fabry-Perot micro-cavities, laser sources, etc. The first order diffraction gratings in SOI waveguide requires surface relief gratings of period Λ = 225 nm for a DBR response at Λ = 1550 nm. Fabrication of such sub-micron gratings with a uniform period over a length of several mm is really a challenging issue. Here we report the realization of an eleventh order Bragg grating (Λ = 2.6 μm, λB ~ 1564 nm, L = 2.62 mm) on the surface of single-mode rib waveguides. The DBRs and waveguide structures were defined by conventional photolithography and subsequent reactive ion etching processes. The waveguide end-facets were polished suitably before they were taken for characterizations using a tunable laser source (tunability 10 pm) in our free-space waveguide coupling set-up. The characterization results of the fabricated DBRs showed a reflectivity R = 88.32% and FWHM = 2.43 nm. Higher reflectivity and narrower grating response can be achieved by further increasing the grating length. The waveguide loss has been increased (~ 0.5 dB/mm) because of the enhanced surface roughness during the RIE process for grating fabrication. It can be reduced if conventional RIE is replaced by ICP RIE.

Demonstration of Integrated Optical 2D Photonic Crystal Waveguides in SOI for Sensing Applications

We have experimentally demonstrated an integrated optical 2D photonic crystal slab waveguide fabricated in SOI platform for sensing applications. The fabrication process flow has been optimized and the transmission characteristics have been analysed.