With recent successes of laboratory, inatmosphere, and space demonstrations of free-space optical communications, there is no doubt that the technology is ready for operational deployment. While these successes have shown that there are no laws of physics against such systems, their estimated system costs are still much too high for serious considerations. Two types of development can reduce the cost dramatically. The first is via the improvement of physical-link communication efficiency by an order of magnitude using photon-counting receivers for vacuum channels, system complexity, weight, and power for space systems can be greatly reduced. The second is through the use of coherent systems in links where clear-air turbulence impairs communication efficiency, and in multiple access applications where coherent processing can reduce the level of interference, significant reduction in system costs can be realized

Free-Space Optical Communications

Information, transmission, modulation and noise

The effects of a 1/f frequency noise on self-heterodyne detection are described, and the results are applied to the problem of laser diode linewidth measurement. The self-heterodyne autocorrelation function and power spectrum are evaluated for both the white and the 1/f components of the frequency noise. From numerical analysis, the power spectrum resulting from the 1/f frequency noise is shown to be approximately Gaussian, and an empirical expression is given for its linewidth. These results are applied to the problem of self-heterodyne linewidth measurements for coherent optical communications, and the amount of broadening due to 1/f frequency noise is predicted. >

/pdf/1-f-frequency-noise-effects-on-self-heterodyne-linewidth-46plfa4a9y.pdf

1/f frequency noise effects on self-heterodyne linewidth measurements

Waves are treated that modulate by either on-off keying (OOK) or binary frequency-shift keying (FSK) and are further impaired by additive Gaussian noise. Heterodyne detection of such a waveform produces an electronic bandpass signal, which, to ease demodulation in the presence of phase noise, is noncoherently demodulated to extract the baseband pulse stream. The treatment goes beyond previous bit error rate (BER) analyses of optical heterodyne receivers for OOK and FSK. First, there is full adherence to the standard (Brownian motion) model of phase noise. Also, the receiver structure is formulated in such a way that the probability density function of the receiver output samples can be accurately determined. This permits calculations of the additive noise and phase noise tolerable when achieving bit error rates as small as 10/sup -9/. Finally, the study is comprehensive regarding the range of parameters explored. Filtering at an intermediate frequency (IF) alone, as well as IF filtering plus postdetection low-pass filtering, is considered. When the receiver parameters decision threshold (for OOK) and IF filter bandwidth are optimized, large amounts of phase noise can be accommodated with only minor increases in required signal-to-noise ratio. This is especially important when the bit rate is moderate compared to the laser linewidth. >

Noncoherent detection of coherent lightwave signals corrupted by phase noise

With high-speed space optical crosslink being a reality, the construction of an optical satellite network as part of a larger integrated space-terrestrial network is now feasible. This paper explores the architecture implications of the invention of such a radical technology building block. Not only can the satellite network performance and cost undergo quantum-leap improvements but also such a network can have profound transforming effects on space system architectures and data network user applications.

/pdf/optical-satellite-networks-y95jnfupy0.pdf

Optical satellite networks

We have demonstrated unsurpassed communication performance, only 2 dB from theory, by careful matching of the transmitted waveforms and receiver filtering at all bit-rates using an average power limited (APL) transmitter anti variable duty-cycle waveforms. While we have demonstrated duty cycle enhanced (DCE) multi-rate communications from 51 Mbps through 1.244 Gbps using binary-pulse position modulation (PPM), this approach can easily be extended to higher bit-rates, other modulation formats, and is extremely well suited for M-ary PPM.

High-sensitivity variable-rate transmit/receive architecture

Heterodyne lasercom systems using GaAs lasers for ISL applications

Atmospheric turbulence corrupts both the amplitude and phase of an optical field propagating from space to an earth-based receiver. While aperture averaging can mitigate amplitude scintillation effects, the performance of single spatial-mode receiver systems such as coherent detection or preamplified direction detection can be significantly degraded by the corrupted phase when the ratio of aperture diameter D to atmospheric coherence length r0 exceeds unity. Although adaptive optics may be employed to correct the wavefront, in practice the correction is imperfect and the residual phase errors induce a communications performance loss. That loss is quantified here by Monte Carlo simulation techniques. Single-mode-receiver fade statistics for imperfect phase correction are calculated in terms of the atmospheric Greenwood frequency fg, the adaptive optic servo loop cutoff frequency fc, and the ratio D/r0. From these statistics, link bit-error rate (BER) performance is calculated. The results reveal that conventional performance measures such as Strehl ratio or mean signal-to- noise ratio loss can significantly underestimate receiver BER losses. Only when the ratio fg/fc is 0.1 or less will communications losses be small (about 0.5 dB) over a wide range of D/r0.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Performance limits of high-rate space-to-ground optical communications through the turbulent atmospheric channel

Accurate characterization and understanding of laser diode frequency noise are particularly important for the design of heterodyne optical communication systems. The nature of the frequency noise will substantially impact the design of modulation/ demodulation and acquisition/tracking systems. Measurements of linewidth,1,2 line shape,2,3 and frequency noise spectrum over a limited range4,5 have been reported previously. By heterodyning pairs of laser diodes we have generated carefully calibrated measurements of frequency noise power spectral density over the widest frequency range reported to date and the first reported measurements of frequency noise statistics.

Characterization of GaAlAs laser diode frequency noise

This paper provides an overview of the opto-mechanical subsystem (OMS) for the Lincoln Laboratory Laser Intersatellite Transmission Experiment. The OMS contains the telescope, relay optics, and beam steering mechanisms. The optical, mechanical and thermal aspects of the OMS design are discussed and the predicted design performance is presented.

An Opto-Mechanical Subsystem For Space Based Coherent Optical Communication

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