Ronald Millett

Fabrication-Tolerant Higher Order Laterally-Coupled Distributed Feedback Lasers

To avoid the commonly required regrowth steps in conventional distributed feedback laser fabrication, laterally-coupled distributed feedback (LC-DFB) lasers lithographically pattern the grating out of the ridge waveguide. Using higher order gratings increases the lithographic tolerances, resulting in lasers that are more amenable to mass-manufacturing techniques, such as stepper lithography. We have extended the modified coupled-mode theory to a two-dimensional cross-section, and thereby identified grating geometries that are both fabrication-tolerant and provide high performance.

O-band semiconductor optical amplifier design for CWDM applications

A broadband semiconductor optical amplifier (SOA) has been realized for the coarse WDM (CWDM) based systems operating over the O-band range. The SOA exhibits low polarization sensitivity, 23 dB gain and low noise based on the asymmetric multi-quantum well (AMQW) technique. Reflective SOAs (for modulating the upstream data in passive optical networks (PONs)) as well as in-line or booster amplifiers in CWDM systems are some of the applications of such a SOA.

The impact of laterally coupled grating microstructure on effective coupling coefficients

Lithographic fabrication may be used to define laterally coupled gratings of high refractive index contrast on waveguide ridges, eliminating the need for regrowth steps in such distributed feedback lasers. These may be made more amenable to fabrication by employing higher-order gratings. Reliable exploration of the laser design space requires that the radiating partial waves be accurately incorporated in numerical simulations. We modify the coupled-mode approach to fully consider the two-dimensional cross section, analyzing rectangular, sinusoidal, triangular and trapezoidal grating shapes. Effective coupling coefficients are determined for grating orders from first to third. We show that, by tailoring the grating microstructure, effective coupling coefficients up to double that of a 0.5 duty cycle rectangular grating can be achieved. The actual grating microstructure of an as-fabricated grating was analyzed and its effective coupling coefficient predicted as [Formula: see text]. This was found to be in excellent agreement with the value extracted from the amplified spontaneous emission spectrum, [Formula: see text].

Synthesis of Planar Reflective Gratings for Silicon Interconnects

The design and simulations of planar reflective gratings for building optical interconnects in silicon-on-insulator (SOI) were studied for a range of silicon core thicknesses of 0.1 to 10 𝜇m. The verticality of the grating facets has been shown to be the main contributing factor to the cumulative crosstalk in thick silicon cores. The dispersion property of the slab was found to limit the minimal thickness of the core for polarization-insensitive gratings. The effects of polarization-dependent confinement on optical crosstalk were studied. The findings were used to design and simulate a polarization-insensitive 18-channel coarse wavelength division demultiplexer (CWDM) with a free spectral range of over 600 nm. The CWDM demultiplexer uses a 1.7 𝜇m silicon core and combines a shallow-etch tapered rib structure and multimode silicon channels to produce box-like passbands for integrated receiver applications. The diffraction grating was constructed using double astigmatic point design with phase-corrected grating facets to reduce astigmatism. Optical properties of the planar gratings have been simulated using quasivectorial diffraction grating theory. The simulation results confirm that there is high diffraction efficiency and low optical crosstalk over the entire range of operation. Applications of planar silicon gratings to the synthesis of silicon interconnects are discussed.

Properties of laterally-coupled distributed feedback lasers with higher order gratings

We demonstrate the results of an analysis of laterally-coupled distributed feedback (LC-DFB) lasers with higher order gratings, including the effects of radiating partial waves. We determine grating geometries that result in lasers with improved performance and fabrication-tolerance. We also find that the addition of a lambda/4 phase-shift will generally diminish the performance of LC-DFB lasers with higher order gratings.

Optimal cavity lengths and duty cycles for second-order laterally-coupled distributed feedback lasers

Using laterally-coupled gratings in a DFB laser produces a more manufacturable design that can be easily monolithically integrated with a variety of other photonic devices. Higher order gratings further improve the manufacturing tolerances, at the cost of more optical loss compared to first-order gratings. We analyze the effect of duty cycle and cavity length on the threshold gain, Bragg detuning, normalized gain difference and optical field flatness of laterally-coupled DFB lasers with second-order gratings. A duty cycle of 0.7 and a cavity length of 300 mum was found to result in a high-performance laser design. The duty cycle can be used to trade off low threshold gain (for duty cycles ~0.8) and high mode discrimination (for duty cycles ~0.5) when designing these lasers.

Irrelevance of bending angle in simple Y-branch power splitter design

Early development work in the design of optical power splitters, likely influenced by similar construction in the microwave regime, placed heavy emphasis on Y-branch designs with the output waveguides immediately branching from the input waveguide at non-zero angle. This design approach, which is still prevalent, is fundamentally flawed from the perspective of both optical power flow and fabrication, as it leads to significant excess loss and/or a large statistical variance. If inherent broadband response is not a critical requirement, directional-coupler or multimode-interference splitters are usually chosen instead. We demonstrate, choosing a minimal function perspective where the optical design is sensitive to the smallest possible set of critical fabrication parameters, that robust and low-loss Y-branch designs are indeed possible. The minimum gap width between waveguides being the critical parameter, we reveal the dependence of the irreducibly simplest design on all elements of the parameter space, as they relate to the critical one. In so doing, we show that the concept of bending angle is irrelevant.

Monolithically integrated 4x4 SOA switch fabricated using quantum well intermixing

Monolithically-integrated semiconductor optical amplifiers (SOAs) have the potential for enabling high-speed and low-crosstalk optical switches in reconfigurable optical add-drop multiplexers (ROADMs). Using integrated 4x4 switches as the building blocks for large-scale ROADMs, instead of 2x2 switches, will reduce alignment issues and assembly steps during manufacturing. The switch is based on SOAs, quantum well intermixed (QWI) passive 1x4 MMI splitters/combiners, and total internal reflection mirrors. We present the results of the 4x4 switch design, for a switch of 5.3 mm x 3.5 mm in size, with estimated total excess on-chip losses of 23 dB.

Polarization compensators in silicon-on-insulator reflective interconnects

We report on our recent progress in polarization control and polarization compensator designs in SOI-based planar reflective gratings for a range of silicon core thicknesses of 0.1 μm to 10 μm. The dispersion property of the silicon slab, without a compensator region, was found to limit the applicability of SOI gratings for achieving the polarizationinsensitive performance of wavelength division multiplexing systems based on planar gratings. We have found that in coarse wavelength division multiplexing systems, the birefringence of the uncompensated slab becomes impractical at core thicknesses below 1.7 μm. Our findings clearly show that shallow etched polarization compensators can effectively eliminate polarization dependence only in thick-core gratings and only in applications requiring free spectral ranges (FSRs) of no more than 80 nm. In silicon cores with thicknesses of less than 1.0 μm, the significantly different value of linear dispersion strength for the two polarization states make traditional compensator designs ineffective, since only the central wavelength can be fully compensated. We used our findings to construct a procedure for building compensators with a flat polarization response over wide FSRs (>80 nm). The results of our study were applied to the design of a polarization compensator in an 18-channel multiplexer for use in coarse wavelength division multiplexing. Our simulation results show that a careful selection of the silicon core thicknesses in the slab and compensator regions is essential for achieving low-cross talk and low insertion loss devices. The application of thin core planar silicon gratings to building silicon interconnects is discussed.

Monolithically Integrated InGaAsP/InP 1x2 SOA Optical Switch

This paper describes a monolithically integrated 1x2 SOA-based switch in InGaAsP/InP. It can be fabricated in one epitaxial growth step, has a footprint of only 4.2mm x 0.35mm, operates on sub-ns time scales and is meant to be integrated with other passive and active waveguide devices on the same InP substrate. The design process optimized the device dimensions using a modified finite-element modal-overlap method. This method provides significant computational savings compared to full beam-propagation method (BPM) simulations. The device uses a single-mode vertical integration technique for a monolithic integration of active and passive waveguide components. To compensate for the polarization sensitivity, tensile-strained quantum well active regions are used. To switch a signal to an output waveguide, the SOA in that waveguide is forward-biased while the SOA in the other output waveguide is reverse-biased to provide a large attenuation (>30dB), resulting in minimal crosstalk. This switch has an estimated insertion loss of 4dB, with a polarization dependent loss of < 1dB.