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Microphotonics-based frequency combs for habitable exoplanet detection

Periodic Reporting for period 2 - STARCHIP (Microphotonics-based frequency combs for habitable exoplanet detection)

Reporting period: 2021-11-01 to 2023-04-30

Integrated photonic microresonators are a key emerging photonic technology. They are expected to enable ground-breaking science and disruptive technological advances from astronomical searches for life beyond Earth to high-capacity optical communication.
Within STARCHIP, based on integrated photonic microresonator technology, a novel type of laser frequency comb for astronomical spectrograph calibration is developed, potentially contributing to the challenging search for habitable exoplanets. Generally, laser frequency combs provide a large set of well-defined, narrow laser lines that are equally spaced in optical frequency. Such frequency comb sources are well established and key to optical precision measurements. However, for astronomy, the individual comb lines need to be widely separated in optical frequency in order to be resolvable by the astronomical spectrograph. This is challenging to achieve with conventional laser systems.

Developing novel types of scientific instrumentation is key to making discoveries and advancing our knowledge about the world. This included in particular the fascinating question on whether (and/or where) there are other habitable planets similar to Earth. At the same time, the technology developed in this project, although geared towards a fundamental research application, can directly be transferred to other areas of application that are more ‘down to earth’; these include optical spectroscopy for environmental monitoring or for medical diagnostics.

While widely spaced frequency combs (‘astrocombs’) are challenging to achieve in conventional laser systems, this is naturally the case in photonic microresonator-based sources. The aim of STARCHIP is to establish a new class of photonic-chip microresonator-based comb sources of broadband spectra of resolvable lines, potentially from ultraviolet to mid-infrared wavelengths, that can overcome key challenges in astrocomb generation.
Our work addresses several key challenges of microresonator technology that need to be solved before they can be applied for either for astronomy or their many other applications. Specifically, these challenges include:
• Identifying ways to extend the operating regime towards longer rand shorter wavelength.
• Achieving robust operation on similar to current commercial mode-locked laser systems.

To address these challenges, we develop novel microresonator geometries, based on novel nano-structured geometries; importantly we use fabrication processes that can be readily scaled to large volume, which is critical for a high impact technology.

Our key results include
- the first soliton pulses in a chip-integrated Fabry-Perot microresonator, demonstrating photonic-crystal reflector-based dispersion engineering as a viable approach for ultra-short on chip pulse generation, beyond the usual near-infrared wavelength
- the first demonstration of synthetic reflection as a novel approach to ensure low-noise frequency comb generation with the desired characteristics in a deterministic way and in every sample by design of a nano-structured photonic crystal structure.
- the first demonstration of up-conversion of microresonator frequency combs from the infrared to visible and ultraviolet wavelength.
- the first demonstration of an astronomical spectrograph in the ultraviolet wavelength regime.
Key to this result was the interplay between large scale numeric simulations on DESY's Maxwell computing cluster and GHz-mode-locked laser frequency comb-based broadband dispersion measurements to calibrate and validate the designs with high precision. Surprisingly, we found that through careful process calibration the nano-structuring can be achieved in a scalable ultra-violet lithographic process, indicating that this new approach can also become relevant for large-scale applications in sensing, data transfer, and generally resonant integrated photonics.
These results provide a clear pathway for extending the spectral operating range of microresonator combs with a level of complexity that is compatible with operation outside a specialized photonics lab at an astronomical observatory.
We expect to have demonstrated novel microresonator comb generators that can provide widely spaced frequency comb lines in spectral regions that are currently not accessible by this technology. With astronomical spectrograph calibration as a challenging application, we will further have explored the potential of these novel light sources for high-precision optical metrology. Importantly, we will also work towards easier schemes of operation, an important requirement for this technology to impact out-of-lab applications. These results and the generated insights will also be transferrable to other potential applications ranging from optical spectroscopy to optical channel generation for broadband optical communication.
Fully-integrated microcomb system providing deterministically low-noise frequency combs without tedi
Microphotonic chip-based visible and ultraviolet frequency comb generation with ultra-high pulse rep