Laser-frequency-comb calibration for the Extremely Large Telescope: an OPO-based infrared astrocomb covering the H and J bands

The Extremely Large Telescope (ELT) will address an unprecedented optical wavelength range from 370 to 2400 nm, and its high-resolution spectrograph (HIRES) will require a laser frequency comb calibrator of comparable coverage. An architecture based around a Ti:sapphire master comb in principle enables wavelengths across this range to be obtained by a combination of second- and third-order nonlinear effects. In this scheme, near-infrared wavelength coverage can be addressed by downconversion of the comb to 1600 nm using an optical parametric oscillator (OPO), followed by broadband supercontinuum generation in highly nonlinear fiber. Here we present an example of this approach in the form of a 10 GHz astrocomb comprising a Fabry–Perot-filtered supercontinuum derived from a degenerate OPO and spanning 1.15–1.80 µm. We characterize the astrocomb using Fourier-transform spectroscopy, enabling the mode orders within the filtered comb to be identified.

极大望远镜（ELT）将覆盖从370到2400纳米的前所未有的光波长范围，其高分辨率光谱仪（HIRES）将需要一个具有相当覆盖范围的激光频率梳校准器。一种基于钛宝石主频率梳的架构原则上能够通过二阶和三阶非线性效应的组合来获得这个范围内的波长。在这个方案中，近红外波长覆盖范围可以通过使用光学参量振荡器（OPO）将频率梳下转换到1600纳米，然后在高非线性光纤中产生宽带超连续谱来实现。在这里，我们以一个10吉赫兹天文频率梳为例介绍这种方法，该天文频率梳包含一个由简并OPO产生的经法布里 - 珀罗滤波的超连续谱，其跨度为1.15 - 1.80微米。我们使用傅里叶变换光谱学对天文频率梳进行了表征，从而能够识别滤波后频率梳内的模式阶数。