Peter G. Goetz

Free-space optical communications link at 1550 nm using multiple-quantum-well modulating retroreflectors in a marine environment

A 1550-nm eye-safe, free-space optical communications link is demonstrated at rates up to 5 Mbits/s over a distance of 2 km in the Chesapeake Bay, using quantum-well-based modulating retroreflectors. Tests are conducted under various atmospheric conditions over a time period of about a year. The experimental and theoretical link budgets are compared and statistical measurements of the effects of scintillation are collected.

High-Current Photodetectors as Efficient, Linear, and High-Power RF Output Stages

Recent progress in high-current photodiodes now makes it possible to efficiently generate over 26 dBm of RF power directly from the output of a photodiode. This paper describes two photodetector designs which demonstrate excellent large- and small-signal behavior. Maximum small-signal compression currents have increased to over 700 mA at 300 MHz. Output RF power amplifier stage efficiencies of over 45% (class AB operation) and 35% (class A) have been achieved from 0.3 to 6 GHz with RF power outputs over 26 dBm. The linearity figure of merit (LFOM) is also shown to be greater than 50, leading to the possibility of implementing very high linearity RF power amplifiers in the future.

A cat's eye multiple quantum-well modulating retro-reflector

A new kind of modulating retro-reflector using cats eye optics and a multiple quantum-well electro-absorption modulator array is described. The device exhibits retro-reflection over a 30/spl deg/ field of view and can support data rates of up to 50 Mb/s using 1-mm pixels. The use of the device in free-space optical communication is discussed.

45-Mbit/s cat’s-eye modulating retroreflectors

Modulating retroreflectors (MRRs) couple passive optical retroreflectors with electro-optic modulators to allow free-space optical communication with a laser and pointing-acquisition-tracking system required on only one end of the link. Recently, MRR using multiple quantum well (MQW) modulators have been demonstrated using a large-area MQW placed in front of the aperture of a corner cube. For a MQW modulator, the maximum modulation rate can range into the gigahertz, limited only by the RC time constant of the device. Most MRR systems have used corner-cube retroreflectors with apertures of about 1 cm, which require large, and hence high-capacitance, modulators. Thus data rates exceeding a few megabits per second are not possible. We describe a new kind of MQW MRR that uses a cats-eye retroreflector with the MQW in the focal plane of the cats-eye. This system decouples the size of the modulator from the size of the optical aperture and allows much higher data rates. A 45-Mbit/s free space link over a range of 7 km is demonstrated.

A surface-normal coupled-quantum-well modulator at 1.55 /spl mu/m

We demonstrate a surface-normal coupled-quantum-well InGaAs-InAlAs electroabsorption modulator that provides optical modulation with a contrast ratio in excess of 1.5 at only 6 V. The device operates at 1.55 /spl mu/m and is based on a novel strain-balanced layer structure. The operating voltage is about two times lower than that of a conventional square quantum-well modulator that achieves a comparable contrast ratio.

InGaAs Multiple Quantum Well Modulating Retro-reflector for Free Space Optical Communications"

Modulating retro-reflectors provide means for free space optical communication without the need for a laser, telescope or pointer tracker on one end of the link. These systems work by coupling a retro-reflector with an electro- optic shutter. The modulating retro-reflector is then interrogated by a cw laser beam from a conventional optical communications system and returns a modulated signal beam to the interrogator. Over the last few years the Naval Research Laboratory has developed modulating retro-reflector based on corner cubes and large area Transmissive InGaAs multiple quantum well modulators. These devices can allow optical links at speeds up to about 10 Mbps. We will discuss the critical performance characteristics of such systems including modulating rate, power consumption, optical contrast ratio and operating wavelength. In addition a new modulating retro-reflector architecture based upon cat s eye retroreflectors will be discussed. This architecture has the possibility for data rates of hundreds of megabits per second at power consumptions below 100 mW.

Free-space optical communications research and demonstrations at the U.S. Naval Research Laboratory.

Free-space optical communication can allow high-bandwidth data links that are hard to detect, intercept, or jam. This makes them attractive for many applications. However, these links also require very accurate pointing, and their availability is affected by weather. These challenges have limited the deployment of free-space optical systems. The U.S. Naval Research Laboratory has, for the last 15 years, engaged in research into atmospheric propagation and photonic components with a goal of characterizing and overcoming these limitations. In addition several demonstrations of free-space optical links in real-world Navy applications have been conducted. This paper reviews this work and the principles guiding it.

Free-space optical communication link across 16 kilometers over the Chesapeake Bay to a modulated retroreflector array

This paper presents the results of a successful bidirectional free-space optical link across 16 km to a modulated retroreflector array. The link was implemented at the Naval Research Laboratorys Chesapeake Bay Detachment laser test range. A 6-W cw 1550-nm class 1 M interrogation beam was used to illuminate an array of three modulated cats-eye retroreflectors located on a tower across the Chesapeake Bay on Tilghman Island. The modulated retroreflectors had a diameter of 16 mm and were arranged in a triangular pattern with a spacing of 30 cm. The interrogating terminal employed a 100-µrad divergence and a high-speed pointing and tracking system to maintain link alignment. Link testing occurred over 12 days in the months of September, October, and November of 2006. Topics presented in this paper include the link scenario for the 16-km free-space optical link, the link budget, and terminal designs, as well as link acquisition and performance. Link performance results presented include data transmission throughput, scintillation data, and pointing and tracking results.

Measurement of thermal-mechanical noise in microelectromechanical systems

We report absolute measurements of thermal-mechanical noise in microelectromechanical systems. The devices are studied with an optical microcavity technique that has a resolution on the order of tens of femtometers per root hertz. The measured noise spectrum agrees with the calculated noise level to within 25%, a discrepancy most likely due to uncertainty in the effective dynamic mass of the vibrating bridge. These measurements demonstrate that thermal-mechanical noise can be the dominant noise source in actuated microelectromechanical devices.

Demonstration of a microelectromechanical tunable asymmetric Fabry–Pérot quantum well modulator

Asymmetric Fabry–Perot multiple quantum well modulators can have much higher extinction ratios than are otherwise possible with surface normal devices. This performance comes at the price of very tight tolerances on the epitaxial growth and high sensitivity to temperature and angle of incidence. We demonstrate that a quantum well modulator with an integrated microelectromechanical mirror can remove these sensitivities.