In this work, we consider the design of a self-referencing interferometer for wavefront sensing. The design is put forward as a key element for adaptive optics systems implementing laser-based (free-space optical) communication through the atmosphere. The self-referencing interferometer is pursued given its ability for operation under weak through strong atmospheric turbulence conditions. This sets it apart from traditional wavefront sensing systems, which can falter under strong turbulence conditions. The self-referencing interferometer takes the form of a traditional (Michelson) interferometer with the input beam, having wavefront/phase distortion across its transverse profile, split into signal and reference arms. The signal beam is subjected to a linear tilt, while the reference beam undergoes spatial filtering/aperturing to give it a sufficiently flat wavefront/phase profile. The signal and reference beams are then overlapped at the output of the interferometer, and the output beam is imaged on a camera. The image is processed to extract a profile of the distorted wavefront/phase across the input beam, with the conjugate of this distorted wavefront/phase profile applied to a deformable mirror for its correction. In this work, we consider the key design parameters for such a system, operating at a wavelength of 1550 nm, with particular thought given to the levels of linear tilt on the signal beam and spatial filtering/aperturing on the reference beam. We illustrate the sensitivity of the output characteristics to these levels and provide recommendations for optimal functioning of self-referencing interferometers in future laser-based (free-space optical) communication systems.
In this work, we explore the manifestation of optical nonlinearities in silicon, given illumination by radiation with wavelengths in the optical communication (C-band) spectrum, near 1550 nm, and extreme intensities, spanning 100-1000 GW/cm2. We photoexcite a silicon photodiode with femtosecond-duration 1550-nm laser pulses and observe the resulting optical autocorrelations as a function of the peak pulse intensity. Such measurements in silicon reveal (i) negligible single-photon absorption, suggesting that there are few defect (trap) states in the bandgap that can assist below-bandgap photoexcitation, (ii) significant two-photon absorption at intensities above 100 GW/cm2, (iii) growing three-photon absorption at intensities rising above a threshold of 300 GW/cm2, and (iv) increasing saturation at intensities rising above a threshold of 650 GW/cm2. We attribute this saturation to the extremely high density of charge carriers brought about by three-photon absorption—as this depletes the available electrons in the valence band and the available states in the conduction band. We hope that this work will be a foundation for the future integration of telecom (C-band) technologies and silicon nanostructures.
In this work, we introduce the concept of a hemispherical retro-modulator for the realization of passive free-space optical communication links. The hemispherical retro-modulator is implemented with a high-refractive-index glass (S-LAH79) hemisphere on a semi-insulating-InP (SI-InP) layer, whose thickness dictates the effectiveness of both retroreflection and modulation. A voltage is applied across transparent indium tin oxide (ITO) and gold (Au) films on either side of the SIInP layer to bring about the desired modulation. The overall device is designed to enable low divergence on the retroreflected beam, as defined by a small divergence angle, and deep modulation on the retroreflected beam, as a result of electroabsorption in the SI-InP layer. To this end, the device is analysed with a ray-based model for retroflection and a unified Franz-Keldysh/Einstein model for modulation in the SI-InP layer. The theoretical results show strong agreement with the experimental results from our prototype. Moreover, the results show effective retroflection and deep modulation—with an applied electric field of 2.167 kV/cm yielding modulation depths of 13%, 34%, and 50% for our 980-nm photons and SI-InP layer thicknesses of 200, 600, and 1,000 μm, respectively. From this, we deem the SI-InP layer thickness of 600 μm to be optimal given its combined capabilities for retroflection and modulation. Ultimately, the introduced hemispherical retro-modulator is shown to be an effective element for future realizations of passive freespace optical communication links.
In this work, we explore the band edge absorption characteristics of semiconductors as applied to optoelectronic modulation—with careful consideration to the departures from ideality in the semiconductors. To this end, we develop a rigorous model of electroabsorption in semiconductors that characterizes the electric-field-induced constriction/narrowing of the bandgap and the resulting increase in absorption of photons, whose energies are slightly below the bandgap energy. The model unifies the Franz-Keldysh effect, characterizing the electric-field-induced tunneling of photoexcited electrons from valence band states to conduction band states, and the Einstein model, quantifying the encroachment of valence and conduction band states into the bandgap. Careful consideration is given here to the nonidealities in the semiconductor, which arise within the valence band as degenerate states, due to light and heavy holes, and within the bandgap, as encroaching Urbach tail states. We apply the model in characterizing optoelectronic modulation of 980-nm photons with semi-insulating indium phosphide (SI-InP), and we see strong agreement between our theoretical and experimental results over a wide range of electric fields and photon energies. Ultimately, the findings show that optoelectronic modulation can be had with large modulation depths over short propagation lengths through the semiconductor. This opens the door to highly effective implementations of optoelectronic modulators in emerging free-space optical communication systems—given that such modulators do not allow for prolonged (guided-wave) propagation and have thus exhibited small modulation depths.
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