Thermal control of long delay lines in a high-resolution astrophotonic spectrograph
arxiv(2024)
摘要
High-resolution astronomical spectroscopy carried out with a photonic Fourier
transform spectrograph (FTS) requires long asymmetrical optical delay lines
that can be dynamically tuned. For example, to achieve a spectral resolution of
R = 30,000, a delay line as long as 1.5 cm would be required. Such delays are
inherently prone to phase errors caused by temperature fluctuations. This is
due to the relatively large thermo-optic coefficient and long lengths of the
waveguides, in this case composed of SiN, resulting in thermally dependent
changes to the optical path length. To minimize phase error to the order of
0.05 radians, thermal stability of the order of 0.05 C is necessary. A
thermal control system capable of stability such as this would require a fast
thermal response and minimal overshoot/undershoot. With a PID temperature
control loop driven by a Peltier cooler and thermistor, we minimized
interference fringe phase error to +/- 0.025 radians and achieved temperature
stability on the order of 0.05 C. We present a practical system for
precision temperature control of a foundry-fabricated and packaged FTS device
on a SiN platform with delay lines ranging from 0.5 to 1.5 cm in length using
inexpensive off-the-shelf components, including design details, control loop
optimization, and considerations for thermal control of integrated photonics.
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