Heaters#
Thermal phase shifters are phase shifters that operate based on the thermo-optic effect. For most materials, as a material increases in temperature, the material expands as the molecules of the material shift away from each other. As the molecules of the material shift away from each other, the material’s index of refraction increases. At low temperatures, the relationship between temperature and refractive index is generally linear, but at higher temperatures, the relationship must be approximated using second order differential equations.
The phase shift of light through a thermal phase shifter is proportional to the index of refraction of the thermal phase shifter, which in turn is proportional to its temperature. Therefore, the phase shift of a thermal phase shifter can be adjusted by changing its temperature.
There are several ways of implementing a thermal phase shifter. Some thermal phase shifters comprise waveguides in proximity to metal resistors. As an electrical current runs through the resistor, the resistor dissipates power in the form of heat, changing the temperature of the waveguide. The resistor may be placed above the waveguide, within the waveguide itself, or beside the waveguide.
Advantages of Thermal Phase Shifters#
There are several advantages to thermal phase shifters, including:
Precision. The thermo-optic effect that governs the phase shift of a thermal phase shifter is mostly linear at low temperatures, so small changes in temperature correspond to small changes in phase shift.
Low loss. Unlike other phase shifters, like the PIN phase shifter, thermal phase shifters don’t introduce optical insertion loss. Because of this, thermal phase shifters can be used to shift the phase of a signal without affecting its amplitude.
Ease of manufacture. Thermal phase shifters, especially simple shifters where the heating element is a resistor, are relatively easy to implement using the most common fabrication processes.
Disadvantages of Thermal Phase Shifters#
Thermal phase shifters offer many advantages, but the tradeoff is speed. Because phase shifts are dependent on heat transferred between a heating element and a waveguide, the time it takes for the heat to transfer is dependent on the heat capacities of the element, the waveguide, and the material between the two. In tuning applciations, the time it takes for a thermal phase shifter to heat up may not be a problem, but in modulation applications where an index of refraction must constantly change, thermal phase shifters are not as useful as other types of phase shifters.
Tuning Efficiency of Thermal Phase Shifters#
The efficiency of a phase shifter is a measurement of the power supplied to the phase shifter in order to obtain a phase shift of 360 degrees (one complete period of the signal). This measurement is called the tuning efficiency of a phase shifter, and is typically in expressed in units of mW/FSR (Free Spectral Range - the spacing between two maxima in an optical frequency).
The tuning efficiency of a thermal phase shifter is independent of its length. Shorter thermal phase shifters will require a higher temperature of operation to achieve the phase shift of a longer thermal phase shifter, but both phase shifters will consume the same amount of power.
To increase the tuning efficiency of a thermal phase shifter, the heating element of a thermal phase shifter may be placed closer to the waveguide, or material between the waveguide and the heating element may be removed. A higher rate of heat transfer will increase a thermal phase shifter’s tuning efficiency.