Fixed-pointed Free Space Optics (FSO) systems must use beamwidths that are broadened sufficiently to account for building motion, wind loading, and other pointing instabilities. Actively pointed systems can potentially use beamwidths more than an order of magnitude narrower for 20 dB or more additional link margin. Pointing requirements and general tradeoffs with regard to active pointing are discussed, and the specific approach selected for fSONA's
actively pointed FSO design is described.
This paper discusses test methods and results of fSONA Communications Corporation's SONAbeam 155-M, SONAbeam 622-M and SONAbeam155-S systems - free space-optical data communication systems that use 1550 nm lasers. Presented are results of environmental qualification tests and field performance tests over link ranges of 450 meters and 5 kilometers. The SONAbeam 155-M, SONAbeam 622-M and 155-S are three representatives of a family of products that work in the range of 34 - 1250 Mbps. Very robust performance is the emphasis in the design of these systems.
The test methods and results are discussed for fSONA Communications SONAbeam 155-M product - a free-space-optics system that operates at 1550 nm. Results include environmental qualification testing and field testing of system performance over test ranges of 450 and 5000 meters. The SONAbeam 155-M is representative of a family of products that range from 34 to 1244 Mbps. This product line emphasizes very robust performance.
We report here on measurements made on a lasercom crosslink hardware testbed built on internal MITRE funds. Laser diodes rated at 150 mwatts were characterized to be flat to 100 MHz, rolling off 5 dB at 500 MHz. A microstrip laser driver with equalizer was implemented to provide flat, highly linear frequency response for analog modulation out to 550 MHz. Microstrip and hybrid versions of APD receivers were also fabricated with flat, linear response to 700 MHz (typically -3 dB at 850 - 1000 MHz), suitable for datarates up to 1.25 Gbps. The optical crosslink testbed with equalized driver, laser, and APD receiver exhibits +/- 0.25 dB flatness, +/- 2.5 degrees phase linearity deviation, and +/- 0.25 (eta) sec group delay variation over the full bandwidth for 650 Mbps. This testbed was evaluated with two modulation approaches: analog laser intensity modulation using an OQPSK subcarrier scheme, and baseband digital NRZ OOK laser intensity modulation, at datarates from 266 to 1244 Mbps. The QPSK subcarrier hardware characterization includes the high speed modem/demodulator and a pair of frequency converters. The digital NRZ hardware characterization includes the clock recovery and amplitude/timing decision circuit. Both the subcarrier QPSK and the digital OOK tests used the same laser and equalized laser driver, the same modulated laser power with 95% intensity modulatino depth, and the same APD receiver for these tests. As such, the comparison between these modulation schemes is an even-handed comparison of the end-to-end performance. Results and conclusions are presented.
We summarize a compact lasercom terminal implementation based on a previously described system concept, and report on measurements made on a prototype optical system built on internal MITRE funds. This paper discusses the fabrication and test of an innovative hardware proof-of-concept for an advanced satellite lasercom terminal with a ten-fold size, weight, and production cost reduction over current practice. We have built and tested a proof-of-concept of the optics portion of a full duplex `monolithic glass block' (MGB) lasercom terminal. The complete MGB optical system is only 6' X 4' X 0.5' and weighs less than a pound. A complete terminal which includes a 3 - 4 inch telescope and gimbal could be implemented for as little as 15 - 30 pounds. The optical system test results are reported. Our approach uses emerging technologies and a highly integrated system design, based on representative system requirements for satellite crosslinks. Technology evaluation and system trades led to a novel optics design for a lasercom terminal, based on thin film coatings and half-inch glass cubes. The emerging photonic technologies employed include liquid crystals for solid state switching, automatic gain control, and microradian alignment; multi-layer dielectric films for optical bandpass filters, dichroic separation, and polarization control; semiconductor lasers with microlens optics; and an original design where all the optics are realized in planar thin films incorporated on small glass substrates, nominally one-half inch in size. These glass cubes are permanently bonded together to form a monolithic ensemble. Hence, we have coined this implementation the monolithic glass block (MGB) approach. Fused silica is used throughout for reasons of superb radiation resistance and thermal stability. The thin film filters, switches, and polarizers perform all the necessary functions in collimated light. This approach is feasible because the optical paths have been dramatically reduced to eliminate the need for relay optics, and the design has been refined such that the entire optical assembly is implemented with a single lens for each laser transmitter for beamforming, and a single lens for each receiver for imaging. The collimated optics design and MGB approach results in greatly relaxed fabrication, alignment, and assembly requirements, which are dominant schedule and cost drivers in the implementation of current lasercom terminal designs.
An atmospheric lasercom link over a 4 km path has been implemented between The MITRE Corporation and Lahey Clinic, Bedford, MA. This testbed employs emerging 1550 nm laser and receiver technology for greatly enhanced covertness and eye safety compared to current 820 nm technology. Supporting testbed equipment provides automated monitoring of link performance, correlated with automated data acquisition of the local visibility and precipitation. This report describes the laser link and supporting testbed equipment under IBM PS/2 computer control in which continuous monitoring of the atmospheric conditions, link bit error rate, pointing jitter, and beam wander are accomplished. The bit error rate drives an automated adaptive data rate modem at one of three data rates, depending on weather conditions. Criteria for operation at 1550 nm, such as trade-offs with regard to atmospheric absorption, covertness and eye safety are presented. Characteristics of our custom designed laser transmitter (laser diode, beam parameters, modulation, etc.), and receiver (optics, detector electronics, bit synchronizer) are described. System alignment procedures and transmission performance as a function of temperature, visibility, and precipitation are presented.
This paper summarizes the design for a laboratory testbed currently under development to evaluate the performance of intersatellite laser crosslinks that directly impose a modulated waveform as an analog signal on the laser carrier. In our analog scheme we use a classical 'bent pipe' satellite implementation, where the RF uplink signal spectrum is frequency translated to directly modulate the laser as an analog waveform, and the received signal at the other end of the optical crosslink is frequency translated to directly modulate the RF downlink. This system design and hardware proof-of-concept addresses the optimum approach to such an analog optical crosslink to facilitate evaluation of its performance.
Nematic liquid crystals (NLCs) have electronically tunable birefringence and can be used as voltage-controlled switches for channel or redundancy beam switching in a lasercom terminal. To assess the performance of such a switch, we have evaluated the operating voltage, transmission efficiency, polarization crosstalk, and long-term operational reliability of an NLC device at four laser diode wavelengths. Preliminary results are encouraging. Transmission is around 95 percent, and polarization rotation purity is comparable to that obtained with a quartz or polymer waveplate, making this solid-state, voltage-controlled waveplate competitive with a conventional implementation that uses a mechanically inserted or rotated half-wave plate.
The generic system level requirements for lasercom satellite crosslinks are considered. The system and subsystem design approaches and technologies that can be pursued to achieve a lasercom terminal with minimum size, weight, and cost are examined. A preliminary lasercom terminal design is described which represents a major reduction in size and weight, resulting from a minimum cost design philosophy.