A temperature-tunable etalon for optical telecommunication wavelength

Members

Shi Yicheng (A0054800R), Du Jinyi (A0227185B), Zhang Qian(A0228752Y)

Rationale

A Fabry-Perot interferometer (or an Etalon), being probably the simplest form of all interferometers, is found useful in a variety of optical applications such as spectral filtering or frequency referencing.

An etalon is typically constructed out of the two parallel reflecting surfaces of a transparent plate. The plate needs to have low absorption loss for the desired working wavelengths to ensure a relatively high finesse of the etalon. The material choice for visible wavelengths is usually fused silica with an absorption coefficient of [bla] and a thermal expansion coefficient of [bla].

For optical telecommunication wavelengths, which range from about 1260nm to 1625nm, pure silicon becomes a more practical choice with an absorption coefficient of [bla] and thermal expansion coefficient of [bla].

Characteristic Parameters of an Etalon

How does an etalon work?

Design

A few photos for now.

Performance

The 100um-silicon wafer act as the etalon in this design. The small piece wafer is stick on a copper block and the light can transmit through the thin hole on the copper block. There is a Partier pad on the bottom of the copper so that we can stabilize and change temperature. The whole setup is put in a transparent plastic box for minimizing the influence of the environmental temperature change.

Bare Silicon Wafer

Temperature Tuning of silicon etalon

A small piece of the silicon wafer is mounted on a copper block with a 4mm diameter hole through hole. The copper serves as a thermally conductive mount for the silicon etalon and is placed on top of a Peltier stage. The temperature of this entire stack is adjusted and stabilized with a TEC controller. A few different temperatures were tried. Just to quickly note that the thermal expansion coefficient of silicon is $\frac{1}{l}\frac{dl}{dT}\approx 2.6\times 10^{-6}{\text{K}}^{-1}$ ([1]).

Besides thermal expansion of the etalon, the refractive index of silicon also changes with varying temperature. One report of the thermal-optic coefficient $\frac{dn}{dT}$ of silicon can be found here([2]), which states a coefficient $\frac{dn}{dT}\approx 2\times 10^{-4}{\text{K}}^{-1}$. Do note that the thermal-optical coefficient is two orders of magnitude larger than the thermal expansion coefficient, which suggest that the change in refractive index is the main contribution towards wavelength tuning of this etalon.

The shift of transmission center wavelength of a fringe can be expressed as: $\frac{d\lambda }{dT}=\lambda (\frac{1}{n}\frac{dn}{dT}+\frac{1}{l}\frac{dl}{dT})\approx 0.079\text{nm/K}$ for 1310nm.

HR Coated Silicon Wafer

We wrote some MATLAB codes to simulate the performance of high reflection coating. Here, we are using a six-layer coating. The first layer from the air is the Si3N4, 163nm for thickness. After that, is the SiO2 layer, 226nm for thickness. Then the Si3N4 (163nm), SiO2 (226nm), Si3N4 (163nm), SiO2 (226nm).

The reflective index of the coating should be as below:

When the thickness of all the layers is correct, the reflection spectrum should have a peak at a wavelength around 1310nm. The reflection peak should reach 90%. If so, our etalon would have high finesse and can act as a very useful filter for high bandwidth lasers.

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