Optical amplifiers are an enabling technology for free space laser communications. Transmission of four multiplexed 2.5 Gbps channels at 1550 nm over a 4.4 km terrestrial link is described and modeled.

We present new performance results for a multi-gigabit terrestrial free-space laser communications system. The measured laser communication link performance parameters include: link bit-error-rate, received optical power, scintillation characteristics and atmospheric visibility.

LDSC is developing the POCIT^TM (Portable Optical Communication Integrated Transceiver) family of products which now includes VideoBeam^TM and the latest addition, EtherBeam^TM. Each is a full duplex portable laser communicator: VideoBeam^TM providing near-broadcast- quality analog video and stereo audio, and EtherBeam^TM providing standard Ethernet connectivity. Each POCIT^TM transceiver consists of a 3.5-pound unit with a binocular- type form factor, which can be manually pointed, tripod- mounted or gyro-stabilized. Both units have an operational range of over two miles (clear air) with excellent jam- resistance and low probability of interception characteristics. The transmission wavelength of 1550 nm enables Class I eyesafe operation (ANSI, IEC). The POCIT^TM units are ideally suited for numerous miliary scenarios, surveillance/espionage, industrial precious mineral exploration, and campus video teleconferencing applications.

An optical link over several kilometers between buildings or remote sites will have significantly reduced bandwidth in the presence of turbulent air, such as may be caused by convection from a heated earth surface. Bandwidth is decreased by amplitude and phase fluctuations due to the turbulence. This paper investigates the performance improvement in bandwidth by using a phase conjugate reflected beam for signal transmission. Weak fluctuations, consistent with the Rytov transformation, are assumed. A computer simulation is developed that uses a statistical model of the turbulent channel to assess the effectiveness of this approach.

In this paper, we discuss the results of two experiments for monitoring the performance of optical wireless links. The first experiment is the measurement of the 10-second Bit Error Rate using a commercial transceiver system; the second experiment, which involves hardware assembled at our laboratory, is the measurement of optical power at the receiver at a 1 kHz sampling rate and the subsequent computation of relevant statistics. Both experiments were performed in an urban environment but over different paths with data being collected for several months. The results are compared with visibility measurements taken at local airports and with measurements of the path averaged atmospheric structure constant, C_n².

An experiment comparing atmospheric scintillation for 785 nm and 1550 nm laser beam transmission is presented. Fluctuations in received optical power were recorded for both wavelengths at terrestrial ranges of 1.2 km and 2.2 km. The number of transmit apertures was also varied. The results indicated that scintillation fades are more of a problem at 1550 nm compared to 785 nm. This will require more scintillation fade margin built into the design of free-space laser communication systems operating at 1550 nm. As well, any advantage in decreased atmospheric attenuation margin at 1550 nm could be lost because of the need for greater fade margin. The overall reduction in scintillation with an increased number of transmit apertures was verified. A possible physical explanation will explain why more scintillation was observed at 1550 nm as compared to 785 nm.

A carrier-based modem for an IR Wireless LAN (IR-WLAN) design is presented. It is based on GMSK modulation schemes. This modem is capable of operating at 2 Mb/s, but it can easily modify to work up to 4.8 Mb/s. GMSK Schemes are filtered modulation schemes with higher spectral efficiency and robustness against jitter than basic FSK. The emitter uses a pulse-conformation stage for full-digital gaussian pulse conformation. Transitions are codified and the resulting waveform is similar to the output of a gaussian filter. The main applications of the proposed GMSK modem are in the area of a full-duplex IR link. And can be applied either in point-to-point links or into a local area network. It can work without interference wither with IrDA systems (even with the proposed PPM 4.4 Mb/s link) or with IEEE 802.11-baseband link (1 or 2 Mb/s). It is also more spectral efficient and jitter resistant than PPM. Carrier-based systems can be used either in point-to-point links or in diffuse systems. For diffuse systems, this modem assumes a high level of use of the available IR spectrum, supporting at least 4 channels in a 30 MHz bandwidth. We also compare the performances of GMSK with other schemes (as can be OQPSK or FQPSK-2).

Multi-Spot Diffusing Configuration combines a multi-beam transmitter and an angle diversity receiver. One of the main system design issues is the joint optimization of the transmitter and the receiver. Such factors as signal power distribution, received ambient light, eye-safety regulations, receiver complexity, etc., should be taken into account when determining the design parameters. For a bit rate of 100 MB/s in the presence of intense ambient light, the utilization of an array of 10 X 10 diffusing spots and a seven-branch receiver with a total FOV of 34.5 deg, insures signal-to-shot-noise ratio as high as 23.4 dB when the best branch is selected and 26.5 dB when maximal-ratio combining is employed. Furthermore, no shadowing of the receiver occurs when a person crosses the line between the transmitter and the receiver.

This paper proposes a high integrated optoelectronic phase locked loop for the use in optical data transmission as well as in optical ranging systems. The so called PMD-PLL receiver module is based on a novel electro-optical modulator, called the Photonic Mixer Device (PMD). The sensor is a semiconductor device, which combines fast optical sensing and modulation of incoherent light signals in one component part by its unique and powerful principle of operation. Simulations and experimental results have already verified the operation principle of PMD structures, in CMOS-technology so far. But also other technologies may be suitable for the PMD realization.

Advanced optical design methods using the keys of nonimaging optics lead to some ultra compact designs which combine the concentrating (or collimating) capabilities of conventional long focal length systems with a high collection efficiency. One of those designs is the so-called RXI. Its aspect ratio (thickness/aperture diameter) is less than 1/3. Used as a receiver, i.e. placing a photodiode at the proper position, it gets an irradiance concentration of the 95% of the theoretical thermodynamic limit (this means for example, a concentration of 1600 times with an acceptance angle of +/- 2.14 degrees). When used as an emitter (replacing the aforementioned photodiode by an LED, for instance), similar intensity gains may be obtained within an angle cone almost as wide as the 95% of the thermodynamic limit. In a real device these irradiance(and intensity)gains are reduced by the optical efficiency. This combination of high concentration factors, relatively wide angles, simplicity and compactness make the optical device almost unique. This work will show the results of the measurements done with several RXI prototypes of 40-mm aperture diameter, all of them made of PMMA (by injection process).

A low complexity system of optical links using indoor unguided infrared channels and direct-sequence code division multiple access is presented. Direct sequence spread spectrum techniques improve performance of optical unguided links with background illumination noise, and multipath propagation. It also allows several users to use code division multiple access, to share the same infrared channel. In the system designed, optical infrared carrier is intensity modulation by a direct-sequence spread spectrum electrical signal, driving the optical emitter. Infrared radiation is directly detected by receiver photodiode.

In this paper, a modified Monte Carlo algorithm for the calculation of the impulse response on infrared wireless indoor channels is presented. This work follows a guideline of studies about the infrared wireless diffuse data communications systems. As is well known, the characteristics of the room where the IR diffuse channel is implemented determine some problems in the communication as can be multipath penalty over the maximum band rate or hidden station situations. Classical algorithms require high computational effort to calculate the impulse response in a regular size room. Monte Carlo offers the possibility of validating the assumptions made for these classic algorithms (basically, the lambertian nature of all reflections) with a computational complexity that is decided by the accuracy desired by the user. It is also an structure that can be easily assumed by a parallel computer architecture. In the other hand, its main drawback is that, for a regular sized room, we need to send much more rays than the components that we receive. This is due to the fact that usually rays are not intercepted by the receiver. We have developed a mixed Monte Carlo-Deterministic algorithm which assures that each ray contributes to the final channel response function each time it rebounds with an obstacle. It increases dramatically the number of contributions and reduces, in the same way, the time required for an accurate simulation. Extensive simulation results are presented. They are compared both with other simulation methods and with measured values. We will demonstrate that the method presented here is much faster than Monte Carlo classical simulation schemes. It can be used like a method of simulation itself or as a validation algorithm for other comparative studies of pulse broadening.

The huge and increasing need of information in the industrial world demands an enormous potential of bandwidth in telecommunication systems. Optical communication provides all participants with the whole spectrum of digital services like videophone, cable TV, video conferencing and online services. Especially fast and low cost opto-electrical receivers are badly needed in order to expand fiber networks to every home (FTTH--fiber to the home or FTTD--fiber to the desk, respectively). This paper proposes a new receiver structure which is designed to receiver optical data which are encoded by code division multiple access techniques (CDMA). For data recovery in such CDMA networks phase locked loops (PLL) are needed, which synchronize the local oscillator with the incoming clock. In optical code division multiple access networks these PLLs could be realized either with an electrical PLL after opto-electrical converting or directly in the optical path with a pure optical PLL.