KEYWORDS: Receivers, Sensors, Transmitters, Scintillation, Scanners, Modulation, Telecommunications, Transponders, Data acquisition, Global Positioning System
Laser based free-space-optical communication (FSOC) links are known to provide covert, secure, jam-proof and very
high bandwidth performances. For mobile platforms, precision pointing and tracking schemes are critical for continuous
guiding of a modulated laser beam to establish data link maintenance. In this paper, preliminary experiments of an
angle-discrimination based smart pointing and tracking scheme suitable for high-speed, closed-loop, FSOC is discussed.
A dual-axis, high-speed, galvo-mirror based scanner was utilized for conical scanning at 550 Hz. Greenwood frequency
in the presence of moderate atmospheric turbulence over a range of 1 km at 1.5 μm was measured. It is shown that
selection of a scan frequency much higher than the Greenwood frequency reduces scintillation effects on scan angle
measurements for track loop maintenance. The measured scan angle value of the receiver with respect to transmit beam
when fed back to the scanner through an optical transponder would allow pointing error estimation and correction.
Based on our initial phenomenology study, it is shown that the scan-angle modulation based pointing and tracking
scheme would provide data-link reliability for dynamic platforms traveling on rough terrains.
We evaluate a simple model for predicting and understanding the structural behavior of Cn2 for a specific location, date, time, and given environmental parameters. This model is compared with Cn2 data taken at the Chesapeake Bay Detachment of the Naval Research Laboratory in Chesapeake Beach, Maryland. This simplified model predicts and explains the fluctuation in Cn2 reasonably well, and also shows that Cn2 is a strong function of solar irradiation.
KEYWORDS: Receivers, Scintillation, Sensors, Signal to noise ratio, Signal detection, Turbulence, Free space optical communications, Transmitters, Refractive index, Atmospheric turbulence
A free-space, line-of-sight, ground-based optical link at 1.5 microns is attractive for tactical communications because it would provide eye-safety, covertness and jam-proof operation. However, the effects of atmospheric turbulence have to be appropriately mitigated for achieving acceptable bit-error-rate (BER) for reliable dissemination of information. Models to predict achievable BER at 1.5 microns for several beam propagation schemes that include beam scanning have been developed for various turbulence conditions. In this paper, we report performance characterization of free-space, high-data (>1Gb/s) rate beam propagation parameters at 1.5 microns for achieving BER reduction under the presence of turbulence. For standard free-space optical links, the mean SNR limits the achievable BER to lesser than 10-6 for Cn2 (structure constant of refractive index fluctuations) around 10-12 m-2/3. To validate these models, simultaneous measurements of structure constant of refractive index fluctuations, Cn2, and coherence diameter over tactical ranges have been carried out and analyzed. The effect of input beam conditioning to reduce BER levels have been explored. Furthermore, single and multiple transmit beams in conjunction with single and multiple detector arrangements have been examined. Based on these measurements, it is shown that the advantages of input beam conditioning coupled with modified receiver geometric characteristics would provide a path for BER reduction and hence, appreciable enhancements in data link reliability.
KEYWORDS: Telecommunications, Laser communications, Laser systems engineering, Data communications, Data modeling, Systems modeling, Performance modeling, Atmospheric modeling, Free space optical communications, Video
The Army's objective is to design, develop and demonstrate its 'ability to distribute information around the battlefield.' Future Army systems will be based on a survivable, adaptable network capable of integrating commercial services and securely utilizing bandwidth for voice, data, and video applications. However, microwave bandwidth allocation has been a serious problem (given crosstalk, interference and frequency management) for a mobile, adaptive communication network. Because of the inherent advantages of the high data rate, crosstalk independence, jam - resistance, covertness and quick system setup time, the Army is looking into optical wireless communication as a means to address this communications requirement. However, development of a fielded laser communication system requires the development of enabling technologies, the understanding of physical limits and performance, and concept of operations (CONOPS).
We generated a low phase noise 16 GHz electrical signal of 20 Hz linewidth from a single 1 MHz linewidth diode laser. At 36 Ghz, our frequency angle mm-wave generator produced an electrical signal with an instrument resolution limited linewidth of < 1 kHz. Our approach uses a relatively low frequency (4-8 GHz) phase modulator to generate a multiline sideband spectrum from a single mode laser, which is filtered to pass or select a pair of spectral lines. Interference between the lines on the active area of a photodetector produces an electrical signal that can be set between 4 and 60 Ghz. B y controlling the phase modulator frequency and amplitude we set the sideband frequency spacing between 4 and 8 Ghz, and efficiently couple optical power into sidebands up to the fourth order. Rapid transitioning between sideband spectra best suited for a particular mm-wave frequency could be made in a few milliseconds using a programmable synthesizer and RF amplifier. Two- sideband selection was obtained by splitting the signal between two legs of a filter network and using ultra-narrowband fiber Bragg grating (FBG) fiber filters to select single lines. We controlled the fiber filter strain to tune the filters rapidly, setting the tension using PZT actuators. The generator prototype was packaged as a pair of portable twin rack modules under PC control.
Conference Committee Involvement (5)
Artificial Intelligence and Machine Learning for Multi-Domain Operations Applications IV
4 April 2022 | Orlando, Florida, United States
Free-Space Laser Communications VI
15 August 2006 | San Diego, California, United States
Free-Space Laser Communications V
31 July 2005 | San Diego, California, United States
Free-Space Laser Communications IV
2 August 2004 | Denver, Colorado, United States
Free-Space Laser Communication and Active Laser Illumination III
4 August 2003 | San Diego, California, United States
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