In recent years, free space optical (FSO) communication has gained significant importance owing to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power, and low mass requirements. FSO communication uses optical carrier in the near infrared band to establish either terrestrial links within the Earth’s atmosphere or inter-satellite/deep space links or ground-to-satellite/satellite-to-ground links. It also finds its applications in remote sensing, radio astronomy, military, disaster recovery, last mile access, backhaul for wireless cellular networks, and many more. However, despite of great potential of FSO communication, its performance is limited by the adverse effects (viz., absorption, scattering, and turbulence) of the atmospheric channel. Out of these three effects, the atmospheric turbulence is a major challenge that may lead to serious degradation in the bit error rate performance of the system and make the communication link infeasible. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of this paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite/satellite-to-ground and inter-satellite links. The latter part of this paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers (link, network, or transport layer) to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high capacity and low cost backhaul solutions.

/pdf/optical-communication-in-space-challenges-and-mitigation-41d8by09lh.pdf

Optical Communication in Space: Challenges and Mitigation Techniques

In recent years, free space optical communication has gained significant importance owing to its unique features: large bandwidth, license-free spectrum, high data rate, easy and quick deployability, less power and low mass requirements. FSO communication uses the optical carrier in the near infrared band to establish either terrestrial links within the Earth's atmosphere or inter-satellite or deep space links or ground-to-satellite or satellite-to-ground links. However, despite the great potential of FSO communication, its performance is limited by the adverse effects viz., absorption, scattering, and turbulence of the atmospheric channel. This paper presents a comprehensive survey on various challenges faced by FSO communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. It also provides details of various performance mitigation techniques in order to have high link availability and reliability. The first part of the paper will focus on various types of impairments that pose a serious challenge to the performance of optical communication system for ground-to-satellite or satellite-to-ground and inter-satellite links. The latter part of the paper will provide the reader with an exhaustive review of various techniques both at physical layer as well as at the other layers i.e., link, network or transport layer to combat the adverse effects of the atmosphere. It also uniquely presents a recently developed technique using orbital angular momentum for utilizing the high capacity advantage of the optical carrier in case of space-based and near-Earth optical communication links. This survey provides the reader with comprehensive details on the use of space-based optical backhaul links in order to provide high-capacity and low-cost backhaul solutions.

/pdf/optical-communication-in-space-challenges-and-mitigation-4wng3q8rfl.pdf

We present theoretical and experimental research on coherent beam combining of fiber amplifiers using stochastic parallel gradient descent (SPGD) algorithm. The feasibility of coherent beam combining using SPGD algorithm is detailed analytically. Numerical simulation is accomplished to explore the scaling potential to higher number of laser beams. Experimental investigation on coherent beam combining of two and three W-level fiber amplifiers is demonstrated. Several application fields, i.e., atmosphere distortion compensating, beam steering, and beam shaping based on coherent beam combining using SPGD algorithm are proposed.

Coherent Beam Combining of Fiber Amplifiers Using Stochastic Parallel Gradient Descent Algorithm and Its Application

An optical device includes a non-transparent substrate. The optical device further includes a first optical layer which is at least partially transmissive and at least partially reflective to incident light. The optical device further includes a second optical layer which is at least partially reflective to incident light. The second optical layer is spaced from the first optical layer. At least one of the first optical layer and the second optical layer is movable between a first position with a first distance between the first and second optical layers and a second position with a second distance between the first and second optical layers. Movement of the at least one of the first optical layer and the second optical layer between the first and second positions modulates the reflectivity of the device.

Photonic mems and structures

A multi-state light modulator comprises a first reflector. A first electrode is positioned at a distance from the first reflector. A second reflector is positioned between the first reflector and the first electrode. The second reflector is movable between an undriven position, a first driven position, and a second driven position, each having a corresponding distance from the first reflector. In one embodiment, the three positions correspond to reflecting white light, being non-reflective, and reflecting a selected color of light. Another embodiment is a method of making the light modulator. Another embodiment is a display including the light modulator.

Method and device for multistate interferometric light modulation

A deformable mirror includes a vertical comb actuator having a reflective surface attached thereto. The vertical comb drive includes stationary elements interspersed with moving elements. When a potential difference is provided between these elements, the moving elements are pulled downward, thereby deforming the reflective surface. The vertical comb drive typically includes a plurality of actuators, which are individually electrically addressed. Each actuator may be an array of interspersed elements or a cavity and corresponding tooth. Springs support the moving elements and bias the reflective surface in an original position. The vertical comb drive provides a large stroke and substantially linear voltage-versus-displacement curve throughout the stroke.

Vertical comb drive actuated deformable mirror device and method

The performance of adaptive systems that consist of microscale
on-chip elements [microelectromechanical mirror
(µ-mirror) arrays and a VLSI stochastic gradient descent
microelectronic control system] is analyzed. The
µ-mirror arrays with 5 × 5 and 6 × 6 actuators
were driven with a control system composed of two mixed-mode VLSI chips
implementing model-free beam-quality metric optimization by the
stochastic parallel perturbative gradient descent technique. The
adaptation rate achieved was near 6000 iterations/s. A secondary
(learning) feedback loop was used to control system parameters
during the adaptation process, further increasing the adaptation
rate.

/pdf/microscale-adaptive-optics-wave-front-control-with-a-m-51ksc5owzm.pdf

Microscale adaptive optics: wave-front control with a m-mirror array and a VLSI stochastic gradient descent controller

The performance of different MEMS mirrors from Boston University, MEMS Optical LLC, University of Colorado and OKO Technologies was studied in respect to an application in a model-free adaptive optics system. The frequency response characteristic was determined in a simple laser beam focusing set-up. Closed-loop adaptation experiments were performed using a VLSI controller system implementing a stochastic parallel gradient descent optimization algorithm. The system behavior using different MEMS mirror types, esp. adaptation speed, was compared.

Performance evaluation of micromachined mirror arrays for adaptive optics

Micro-scale Adaptive optics: Wavefront Control with μ-mirror Array and VLSI Stochastic Gradient Descent Controller

Using two micro lens arrays and a MEMS micro shutter array, an intensity modulating Spatial Light Modulator is being developed at MEMS Optical, Inc. (patent pending) for high speed printing applications. The micro lens arrays are used to focus incident light to a point and then expand it back to its original size. At the focus point, a Foucault micro shutter array is used to modulate the amount of light allowed to pass through the aperture. The purpose for this device is for exposure control for high-speed electronic printing applications. The drive mechanism is based on an electrostatic lateral comb interdigitated drive. Design analysis shows a rise time of 1 - 2 microseconds for high voltage systems. This array of shutters is being implemented in a CMOS compatible process, and is capable of being integrated with on chip circuitry for opening and closing the shutters. The apertures are made using deep RIE etching, and the shutters are released using plasma etching. The result is an electronically controlled method of exposing a photosensitive surface at high speeds for the printing industry, with or without lasers.

Jay A. Hammer

Papers

Vertical comb drive actuated deformable mirror device and method

Microscale adaptive optics: wave-front control with a m-mirror array and a VLSI stochastic gradient descent controller

Performance evaluation of micromachined mirror arrays for adaptive optics

Micro-scale Adaptive optics: Wavefront Control with μ-mirror Array and VLSI Stochastic Gradient Descent Controller

MEMS microshutter SLM for intensity modulation