Integrated Echelle Gratings as Compact Spectrometer for VIS and NIR Astronomy

Compact, high-performance optical components are much sought after in modern observational astronomy to bring down the system cost and maintenance complexity. Lightweight, miniature spectrometers are also essential components for instruments used in satellite and space observation. Integrated spectrometers based on planar light wave circuits are attractive as such compact device can provide high spectral resolution and simultaneously also provide great robustness and flexibility in spectrograph system design. Though arrayed waveguide gratings (AWGs) have been demonstrated for such purposes, the application is limited by their working principles: 1) The operation wavelengths are mostly constrained in the near infrared (NIR) region. Since the waveguide dimension scales with the wavelength, shorter wavelength require waveguide structures that proved too challenging to be fabricated accurately and consistently among the many waveguides in the array. 2) The free spectral range (FSR) of the AWG is relatively small as it works mostly at rather high grating orders for fine spectral resolution [1,2]. An integrated Echelle grating (IEG), as sketched in Fig. 1, relies on the optical path differences introduced by the reflective facets arranged on a Roland circle and requires virtually no waveguides [3,4]. The operation wavelength depends on the materials as well as the reflective characteristics of the facets. IEGs mostly work at lower grating orders compared to AWGs, thus, providing higher FSRs.