LSI photonics platform paves the way for highly integrated optical circuits

In electronics, the desire to reduce cost, improve power consumption, simplify packaging, and to increase functionality and yield, has always been the impetus for miniaturization and integration. The reasons to miniaturize and integrate are no less compelling for optical components. Indeed, healthy sustained growth for the industry may eventually demand it.

However, the intrinsic analogue nature of optical components, the lack of universal photonic building blocks, and the very demanding requirements of telecom applications have made progress towards VLSI photonics slow.

Momentum is picking up though, as versatile new materials, highly functional building blocks, and volume manufacturing know-how are being demonstrated. The commercial realization of VLSI photonics hinges on the conjunction of three platforms.

The first is a material system that has a high refractive index contrast, is low loss throughout the communications bands, and is processed using conventional IC industry tools. The second platform comprises of ultra-compact universal building blocks that can be architected into various optical signal processing functions. The third platform is the ability to leverage volume manufacturing tools and techniques from the IC industry.

Large-scale integration requires the miniaturization of today's photonic components. The minimum size of a photonic circuit is limited by the bending induced losses associated with the smallest radius of curvature. Bending losses decrease exponentially with increasing core-cladding refractive index contrast, making high index a fundamental requirement for VLSI Photonics.

On the other hand, arbitrarily large indexes are not necessarily desirable. As the index increases, fabrication tolerances decrease, single mode waveguide dimensions decrease making fiber pigtailing more difficult, and structural features such as coupling gaps become prohibitively small. A large index is only one of several requirements of a viable VLSIP materials platform. Other key material attributes include:

• low intrinsic losses,

• an index that is adjustable,

• long term stability,

• compatibility with IC industry processing tools and

• and low temperature processes which avoid annealing steps.

Several high-index material systems are being investigated for use as VLSIP platforms. These include Si/SiO2, InGaAsP, Ta2O5, SiO2, SiN, SiC, SiON, and several polymeric systems.

Of these Si, SiN, SiON and SiC are attractive for their long-term reliability and IC industry processing compatibility. Si and SiN have very high refractive indexes leading to extremely tight manufacturing tolerances, especially for wavelength selective applications. SiON has attracted interest because of its adjustable refractive index. In particular, its index can be made larger than that of conventional Ge-doped silica.

However, SiON exhibits a strong absorption peak that extends deep into the C-band, and whose magnitude is a function of refractive index. Annealing is a common approach used to partially reduce the absorption seen in material systems as well as to reflow cladding layers. The fate of high temperature anneals are well known: large material birefringences caused by frozen-in stresses, wafer bowing, and changes in refractive index and thickness affecting device performance.

Hydex, a photonics material breakthrough, was developed to overcome all the limitations of other high-index systems. It is a robust material that is deposited through conventional chemical vapor deposition (CVD) processes. It's refractive index contrast is adjustable from 0% to over 20%. It requires no anneal steps and its loss, as deposited, is low throughout in the S,C and L bands. Five hundred hours of 85/85 damp heat testing have revealed no index changes down to the measurement accuracy of 10-5.

Significant reductions

An immediate application for Hydex is the ability to reduce the dimensions of all conventional planar lightwave circuits (PLCs) by one to two orders of magnitude. This translates into a factor of ten to over a hundred more devices fabricated per wafer.

An example of such miniaturization is a 40-channel, 100 GHz micro-acoustic waveguide. The size of this device is approximately 5mm by 5mm and is limited solely by the pitch requirement of the 40 output fibers. The performance of these micro-PLC components is in no way compromised compared to conventional PLCs. The AWG filter noise floor for instance, which is indicative of statistical index and thickness variations in the process, is below -55 dB.

Indeed, the measured random phase errors in the AWG are below 8 nm, and these are repeatable from die to die across the wafer (i.e., they reside in the mask, not the process). These chips, which have compact integrated fiber spot size transformers as part of the process, exhibit fiber-to-fiber peak losses below 3 dB for Gaussian filter shapes. An additional benefit of size reduction is the reduced power consumption for temperature stabilizing the array.

Resonators have been used extensively in electronics for their frequency selective properties. In particular, dielectric ring and disk shaped cavities continue to be important for high frequency, high-Q microwave applications. For optical signal processing applications, dielectric micro-ring resonators have been demonstrated as demultiplexers, add/drop filters, dispersion compensators, modulators, lasers, and for four-wave mixing, to name just a few. Such devices have also been made as small as a few micrometers in size, and are thus ideal for large-scale integration.

In wavelength filter applications, filter shape is paramount. Like their thin film filter counterparts, ring resonators can be arranged in coupled-cavity configurations called higher order filters. Such configurations lead to box-like response with very high out of band signal rejection.

The three essential elements of VLSI photonics, a robust materials platform for miniaturization, ultra-compact universal building blocks for optical signal processing, and volume fabrication techniques borrowed from the IC industry, have been demonstrated. In the next several years, expect a new breed of highly integrated photonic circuits.

This article will be presented in full at the Optical Fiber Communications conference in a paper titled "A VLSI Photonics Platform."