Polymer-cladded athermal high-index-contrast waveguides

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Polymer-cladded athermal high-index-contrast waveguides

There has been an enormous amount of research on the suppression of temperature sensitivity in silica-based

arrayed-waveguide gratings (AWGs). A common approach to eliminate the temperature dependent wavelength

shift in a silica-based AWG is the use of external heaters or thermoelectric coolers for stabilizing on-chip temperature.

However, this method demands large device foot-print, high power budget and cost. An alternative

approach utilizes the thermal expansion of a metal plate to compensate for the thermal effect. 3, 4 Such method requires

stringent fabrication tolerance and precise alignment between the metal compensator and the waveguides.

Recently, research reported TiO2 doping of the silica core could lower the temperature sensitivity of the entire

waveguide system. 5 This technique only requires doping of the silica waveguide core; however, it is material- and

geometry-specific. Temperature-independent characteristics have also been realized by forming polymer-filled

trenches in a waveguide grating 6 or in the slab coupler section7 of an AWG. The basic idea behind this approach

is to use polymers whose TO coefficients have negative values to compensate for the positive TO coefficient of

thesilicawaveguides(TOSiO2 =1× 10−5 K−1 8, 9 ).

Inspired by the successful use of the polymer for silica-based devices, 6, 7 we explore the feasibility of polymer

cladding for thermal stabilization in silicon-based waveguides. The basic idea behind this approach is to use

polymers whose negative TO coefficients compensates for the positive TO coefficient of the waveguide core

region. The elimination of temperature dependent characteristics is expected to be more challenging for silicon

than for silica due to the major differences between the two material systems. First, the TO coefficient of silicon

is 18 times higher than that of silica. In addition, the refractive index of silicon is much larger than that of

silica, resulting in highly confined optical modes in the waveguide core region. In other words, the confinement

of the optical mode plays a significant role in the compensation of the thermo-optic effect. Recently, Lee et. al 10

experimentally demonstrated the reduction of temperature sensitivity in Si-based ring resonators using polymer

claddings. However, the demonstration was material- and geometry-specific over a 60-K temperature range. In

this paper, we present a comprehensive study of temperature-insensitive performance for general high-indexcontrast

channel waveguides. Since a typical microprocessor operates in between 273 to 398 K, we consider the

device temperature sensitivity over the 125-K temperature range.

2. POLYMER CLADDING MATERIALS

The thermo-optic effect is the temperature dependence of the refractive index of a material. 11 The TO coefficient

( dn

dn

) itself is a complex function which depends on wavelength and temperature. We assume that is constant

dT dT

over the temperature range considered in this study (∆T = 125 K). The sign and magnitude of the TO coefficient

are primarily determined by the material density and polarizability. 12 Density normally decreases with increasing

temperature, causing a decrease in the number of polarizable species per unit volume and hence a decrease in

therefractiveindex. 13 Polarizability of the individual species in a material, on the other hand, usually increases

with increasing temperature, resulting in an increase in the refractive index. 13 The polarizability factor of

inorganic materials such as silica or silicon is usually much more dominant than the density change; hence

theTOcoefficientsofthesematerialshavepositivevalues( dn

dT > 0). However, in the case of polymers, TO

coefficient is primarily determined by the density change. Consequently, most polymers have negative TO

< 0). Table 1 lists the TO coefficients of several relevant materials.

coefficients ( dn

dT

Material Type Si poly-Si Si02, SiON, Si3N4 PE

Commercial Polymers [6]

Sol-gel Silicone Urethane acrylate elastonmer

Index 1.55mm 3.476 3.6 1.4-2.2 1.2-1.8

dn/dT (X104 K1) 1.8 I 2.3 0.1 —1.1 -1.3 -2.1 -3.1 -4.2

Table 1. The thermo-optic (TO) coefficients of various materials.

The simplest method to achieve temperature insensitivity in the effective index of an HIC waveguide device

is to control the overall TO coefficient of the device by adjusting the cladding material. The waveguide itself is

designed such that a fraction of the optical mode is expanded into the cladding materials. The cladding induced

negative TO effect on the expanded optical mode compensates for the positive TO effect experienced by the

mode confined in the silicon core. When the two effects perfectly balance each other, an athermal operation is

Proc. of SPIE Vol. 6897 68970S-2

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