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1. Focal-plane wavefront sensing with photonic lanterns II: numerical characterization and optimization

   https://doi.org/10.1364/JOSAB.502962  

   Lin, Jonathan; Fitzgerald, Michael P.; Xin, Yinzi; Kim, Yoo Jung; Guyon, Olivier; Leon-Saval, Sergio G.; Norris, Barnaby; Jovanovic, Nemanja (January 2023, Journal of the Optical Society of America B)

   We present numerical characterizations of the wavefront sensing performance for few-mode photonic lantern wavefront sensors (PLWFSs). These characterizations include calculations of the throughput, control space, sensor linearity, and an estimate of the maximum linear reconstruction range for standard and hybrid lanterns with between 3 and 19 ports, atλ=1550nm. We additionally consider the impact of beam-shaping optics and a charge-1 vortex mask placed in the pupil plane. The former is motivated by the application of PLs to high-resolution spectroscopy, which could enable efficient injection into the spectrometer along with simultaneous focal-plane wavefront sensing; similarly, the latter is motivated by the application of PLs to vortex fiber nulling (VFN), which can simultaneously enable wavefront sensing and the nulling of on-axis starlight. Overall, we find that the PLWFS setups tested in this work exhibit good linearity out to ∼0.25−0.5 radians of RMS wavefront error (WFE). Meanwhile, we estimate the maximum amount of WFE that can be handled by these sensors to be around ∼1−2 radians RMS before the sensor response becomes degenerate. In the future, we expect these limits can be pushed further by increasing the number of degrees of freedom, either by adopting higher mode-count lanterns, dispersing lantern outputs, or separating polarizations. Finally, we consider optimization strategies for the design of the PLWFS, which involve both modification of the lantern itself and the use of pre- and post-lantern optics like phase masks and interferometric beam recombiners. 

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2. Experimental and on-sky demonstration of spectrally dispersed wavefront sensing using a photonic lantern

   https://doi.org/10.1364/OL.551624  

   Lin, Jonathan; Fitzgerald, Michael P; Xin, Yinzi; Jung_Kim, Yoo; Guyon, Olivier; Norris, Barnaby; Betters, Christopher; Leon-Saval, Sergio; Ahn, Kyohoon; Deo, Vincent; et al (January 2025, Optics Letters)

   Adaptive optics (AO) systems are critical in any application where highly resolved imaging or beam control must be performed through a dynamic medium. Such applications include astronomy and free-space optical communications, where light propagates through the atmosphere, as well as medical microscopy and vision science, where light propagates through biological tissues. Recent works have demonstrated common-path wavefront sensors (WFSs) for adaptive optics using the photonic lantern (PL), a slowly varying waveguide that can efficiently couple multi-moded light into single-mode fibers (SMFs). We use the SCExAO astrophotonics platform at the 8 m Subaru Telescope to show that spectral dispersion of lantern outputs can improve correction fidelity, culminating with an on-sky demonstration of real-time wavefront control. This is the first, to the best of our knowledge, result for either a spectrally dispersed or a photonic lantern wavefront sensor. Combined with the benefits offered by lanterns in precision spectroscopy, our results suggest the future possibility of a unified wavefront sensing spectrograph using compact photonic devices. 

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3. Exoplanet detection with photonic lanterns for focal-plane wavefront sensing and control

   https://doi.org/10.1117/12.2630692  

   Lin, Jonathan; Xin, Yinzi; Norris, Barnaby R.; Kim, Yoo Jung; Sallum, Steph; Betters, Christopher; Leon-Saval, Sergio; Lozi, Julien; Vievard, Sebastian; Guyon, Olivier; et al (August 2022, Adaptive Optics Systems VIII)

   Schmidt, Dirk; Schreiber, Laura; Vernet, Elise (Ed.)

   Inner working angle is a key parameter for enabling scientific discovery in direct exoplanet imaging and characterization. Approaches to improving the inner working angle to reach the diffraction limit center on the sensing and control of wavefront errors, starlight suppression via coronagraphy, and differential techniques applied in post-processing. These approaches are ultimately limited by the shot noise of the residual starlight, placing a premium on the ability of the adaptive optics system to sense and control wavefront errors so that the coronagraph can effectively suppress starlight reaching the science focal plane. Photonic lanterns are attractive for use in the science focal plane because of their ability to spatially filter light using a finite basis of accepted modes and effectively couple the results to diffraction-limited spectrometers, providing a compact and cost-effective means to implement post-processing based on spectral diversity. We aim to characterize the ability of photonic lanterns to serve as focal-plane wavefront sensors, allowing the adaptive optics system to control aberrations affecting the science focal plane and reject additional stellar photon noise. By serving as focal-plane wavefront sensors, photonic lanterns can improve sensitivity to exoplanets through both direct and coronagraphic observations. We have studied the sensing capabilities of photonic lanterns in the linear and quadratic regimes with analytical and numerical treatments for different lantern geometries (including non-mode-selective, mode-selective, and hybrid geometries) as a function of port number. In this presentation we report on the sensitivity of such lanterns and comment on the relative suitability and sensitivity impacts of different lantern geometries for focal-plane wavefront sensing. 

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4. Demonstration of a photonic-lantern focal-plane wavefront sensor using fibre mode conversion and deep learning

   https://doi.org/10.1117/12.2629852  

   Norris, Barnaby R.; Wei, Jin; Betters, Christopher; Leon-Saval, Sergio; Xin, Yinzi; Lin, Jonathan; Kim, Yoo Jung; Sallum, Steph; Lozi, Julien; Vievard, Sébastien; et al (August 2022, Adaptive Optics Systems VIII)

   Schmidt, Dirk; Schreiber, Laura; Vernet, Elise (Ed.)

   A focal plane wavefront sensor offers major advantages to adaptive optics, including removal of non-commonpath error and providing sensitivity to blind modes (such as petalling). But simply using the observed point spread function (PSF) is not sufficient for wavefront correction, as only the intensity, not phase, is measured. Here we demonstrate the use of a multimode fiber mode converter (photonic lantern) to directly measure the wavefront phase and amplitude at the focal plane. Starlight is injected into a multimode fiber at the image plane, with the combination of modes excited within the fiber a function of the phase and amplitude of the incident wavefront. The fiber undergoes an adiabatic transition into a set of multiple, single-mode outputs, such that the distribution of intensities between them encodes the incident wavefront. The mapping (which may be strongly non-linear) between spatial modes in the PSF and the outputs is stable but must be learned. This is done by a deep neural network, trained by applying random combinations of spatial modes to the deformable mirror. Once trained, the neural network can instantaneously predict the incident wavefront for any set of output intensities. We demonstrate the successful reconstruction of wavefronts produced in the laboratory with low-wind-effect, and an on-sky demonstration of reconstruction of low-order modes consistent with those measured by the existing pyramid wavefront sensor, using SCExAO observations at the Subaru Telescope. 

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5. Single-ended characterization of the coherent transfer matrix of coupled multimode transmission channels

   https://doi.org/10.1364/PRJ.491967  

   Fardoost, Alireza; Vanani, Fatemeh Ghaedi; Chandrasekhar, Sethumadhavan; Li, Guifang (September 2023, Photonics Research)

   Light propagation in random media is a subject of interest to the optics community at large, with applications ranging from imaging to communication and sensing. However, real-time characterization of wavefront distortion in random media remains a major challenge. Compounding the difficulties, for many applications such as imaging (e.g., endoscopy) and focusing through random media, we only have single-ended access. In this work, we propose to represent wavefronts as superpositions of spatial modes. Within this framework, random media can be represented as a coupled multimode transmission channel. Once the distributed coherent transfer matrix of the channel is characterized, wavefront distortions along the path can be obtained. Fortunately, backreflections almost always accompany mode coupling and wavefront distortions. Therefore, we further propose to utilize backreflections to perform single-ended characterization of the coherent transfer matrix. We first develop the general framework for single-ended characterization of the coherent transfer matrix of coupled multimode transmission channels. Then, we apply this framework to the case of a two-mode channel, a single-mode fiber, which supports two randomly coupled polarization modes, to provide a proof-of-concept demonstration. Furthermore, as one of the main applications of coherent channel estimation, a polarization imaging system through single-mode fibers is implemented. We envision that the proposed method can be applied to both guided and free-space channels with a multitude of applications. 

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