Starlight coupling through atmospheric turbulence into few-mode fibers and photonic lanterns in the presence of partial adaptive optics correction
ABSTRACT
Starlight corrupted by atmospheric turbulence cannot couple efficiently into astronomical instruments based on integrated optics as they require light of high spatial coherence to couple into their single-mode waveguides. Low-order adaptive optics in combination with photonic lanterns offer a practical approach to achieve efficient coupling into multiplexed astrophotonic devices. We investigate, aided by simulations and an experimental testbed, the trade-off between the degrees of freedom of the adaptive optics system and those of the input waveguide of an integrated optic component leading to a cost-effective hybrid system that achieves a signal-to-noise ratio higher than a standalone device fed by a single-mode fiber.
1 INTRODUCTION
As telescope apertures increase in diameter, optical instruments at their foci such as spectrographs need to proportionally expand in size to make use of the additional flux without compromising performance, e.g. resolving power or sensitivity (Spanò et al. 2008, 2006). This results in costly instruments with large physical dimensions, making them more sensitive to vibrational and environmental changes. Photonic technologies offer an opportunity to avoid bulk optics, thus limiting the increase in size. Using integrated optics (IO) to manipulate starlight in astronomical instruments before detection– an emerging field known as astrophotonics– has the potential of reducing the footprint and mass of astronomical instruments, cutting costs owing to simpler vacuum and thermal control, enhancing performance, and enabling multiplexing (Minardi et al. 2020).
Photonic spectrographs (Blind et al. 2017), e.g. arrayed waveguide gratings (AWGs) (Bland-Hawthorn & Horton 2006), fiber Bragg gratings for OH suppression (Bland-Hawthorn et al. 2011; Rahman et al. 2020), and photonic beam combiners, e.g. GRAVITY (Eisenhauer et al. 2008) and discrete beam combiners (DBCs) (Minardi 2015, 2012), need to operate in the single mode regime in order to deliver their promised spectral resolution, filter characteristics and phase retrieval capabilities, respectively, while avoiding modal noise and focal ratio degradation. Coupling a seeing-limited point spread function (PSF) at the focus of a large telescope into a single mode waveguide is challenging and typically results in low efficiency. Two mitigation techniques can be applied to enable the use of a photonic instrument behind a ground-based telescope: On the one hand, an extreme adaptive optics (ExAO) system may be used to entirely correct for the atmospheric aberrations present in the received wavefronts, and in doing so convert the focal speckle pattern into a diffraction-limited spot that couples efficiently into a single-mode fiber (SMF). On the other hand, a photonic lantern can be employed to split the optical power coupled from the telescope into multiple SMFs (Leon-Saval et al. 2005). ExAO systems have more degrees of freedom and run faster than conventional AO systems to deliver high Strehl ratios (SR > 0.8 in NIR) but can only do so for bright objects that act as their own guide stars (Guyon 2018). As a result, they are more suited to highcontrast imaging of exoplanets and circumstellar disks. They also tend to be notoriously expensive for midsize (2 – 4 m) telescopes and can overwhelm the cost of the telescope itself.
Photonic lanterns, conversely, are mode converting devices that redistribute multimodal light into multiple single-mode beams. They do so by guiding the light through an adiabatic taper from a multimode core to several single-mode cores. If the transition is gradual and the number of SMFs is equal to or greater than the number of modes supported by the multimode port, the conversion is theoretically lossless. Degrees of freedom are therefore conserved and the second law of thermodynamics (brightness theorem) is not violated (McMahon 1975). A copy of the IO-based astrophotonic device, which tends to be inexpensive to replicate, is then needed at the output of every SMF in order to recover all the collected flux. The number of modes required to efficiently couple starlight at a telescope’s focus scales as the square of the aperture diameter. This results in ∼ 100s of modes being required and consequently ∼ 100s of single-mode channels at the output of the photonic lantern (almost 1000 modes for a 4 m telescope at median seeing). While such complex lanterns can, in theory, be fabricated, the total flux divided among too many channels will result in every SMF having a fractional share of the total optical power comparable to, or even less than, the noise floor of the detector. Accumulating these noisy signals in post-processing would result in a signal-to-noise ratio (SNR) smaller than that had all the flux been collected by a sole SMF directly from the focal plane to the instrument.
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