B. G. BOONE ET AL. Optical Communications Development for Spacecraft Applications Bradley G. Boone, Jonathan R. Bruzzi, Bernard E. Kluga, Wesley P. Millard, Karl B. Fielhauer, Donald D. Duncan, Daniel V. Hahn, Christian W. Drabenstadt, Donald E. Maurer, and Robert S. Bokulic Free-space optical communications systems for deep space and near-terrestrial space environments are now poised for deployment aboard spacecraft. Although many funda- mental technical problems have been solved, detailed engineering development is still needed to make space-worthy optical communications terminals. Very high bandwidths (10 Gbps or higher) and fi ne (1- to 10-␮rad accuracy) pointing systems are needed for near- terrestrial space. For deep space applications, optical links at competitive bandwidths rela- tive to RF systems (e.g., 30 Mbps) with very fi ne pointing accuracies (<1 ␮rad) are planned. Deep space applications also require very sensitive receivers and large ground receiving apertures, while minimizing mass and prime power penalties for the added capability, espe- cially for smaller spacecraft. Recent efforts at APL focusing on these issues are summarized in this article. OVERVIEW Free-space optical communications systems for result of several factors: ever-increasing requirements satellite-to-ground and deep space communications for high data rate (hundreds to thousands of Mbps) have been proposed, studied, and even implemented communications, signifi cant investments by NASA in laboratory demonstration systems for more than 30 and DoD, and signifi cant advances in the telecom- years. Nevertheless, few of these systems have actually munications technology for fi ber-optic communication been deployed aboard spacecraft. Even though most of components, including fi ber amplifi ers, fi ber lasers, and the technical problems associated with optical commu- sensitive receivers. These components may be appli- nications systems have been solved, advances in micro- cable, in some form, to free-space optical communica- wave sources and high-speed electronics have main- tions systems for use in space.1 Compact beamsteering tained traditional RF communications systems as the technology; very fi ne pointing, tracking, and stabiliza- technology of choice for space-based communications tion control; and ultra-lightweight antennas are also system designers. This situation is now changing as a critical technologies. 306 JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 25, NUMBER 4 (2004) OPTICAL COMMUNICATIONS DEVELOPMENT FOR SPACECRAFT The motivation for transitioning to optical com- SYSTEM REQUIREMENTS AND munications for deep space applications, apart from FUNCTIONAL ELEMENTS pragmatic considerations such as the increasing need APL business areas that have supported recent devel- to acquire more scientifi c data and real-time imagery, opments include civilian space (for deep space), military is fundamentally dependent on the wavelength of space (for near-terrestrial space), and exceptional sci- light relative to RF bands currently used. As will be ence and technology (for specifi c component technolo- described in the section on the link equation, which gies). Deep space applications being pursued by NASA governs all communications links (both RF and opti- include the transmission of live video (up to 30 Mbps) cal), there are three terms that explicitly depend on from Mars for the Mars Laser Communication Dem- wavelength: the transmitter antenna gain, the space onstrator planned for a 2009 launch.2 GEO (≈36,000 loss, and the receiver antenna gain. When these fac- km) and NEO (≈300–1000 km) optical communica- tors are combined, for equal antenna sizes, the advan- tions terminals are also being planned or developed for tages of shorter (optical) wavelengths become obvious: NASA and DoD. Deep space applications entail a single the received signal goes inversely as the square of the dedicated channel having a very high pointing accu- wavelength. This is mitigated by the fact that optical racy (≈300–400 nrad) but only a modest data rate (≈1– antennas are not always as big as RF antennas, and they 30 Mbps, depending on range and background levels). require greater precision to make and point because of Near-terrestrial space (GEO and NEO) applications will the shortness of the wavelength and the narrowness of likely involve networked links that will be multichan- the optical beam. nel and multi-access, require ≈1- to 10-␮rad pointing On the other hand, having smaller antennas (as accuracies, and entail very high data rates (as much as well as other components) can be a weight advan- 10 Gbps or higher). Similar requirements are envisioned tage for optics. Optical modulation bandwidths are for commercial applications. Thus the desired data rates also wider because for similar modulation electron- and pointing accuracy requirements are driven by two ics, which are roughly equally limited in their rela- distinct environments differing mainly in link range. tive (normalized) bandwidths between RF and optical Of course, link margin decreases with range, mainly modulators, higher (optical) frequencies directly imply because laser source power limits and receiver sensitivity wider bandwidths. This becomes compromised by the constrain the achievable data rate for a desired bit error fact that, for wider bandwidths (as in optics), in-band rate (BER). This is summarized in Fig. 1. noise energy is necessarily greater, thus potentially As illustrated in Fig. 2, there are several essential reducing the signal-to-noise ratio (SNR) for optical ingredients in a complete end-to-end optical com- receivers. munications system. Besides the communications Complicating the situation, however, are the conse- function, pointing, acquisition, tracking, and stabili- quences of a trade between good design practice versus zation are critical, as are the telescope optics and the the dominance of photon shot noise in optics and ther- mal noise in RF systems. Good design in optics drives down the thermal noise so that only the signal-induced Earth Mars Pluto 1000 AU (or sometimes the background-induced) shot noise subtense @: really matters. The real cost of implementing optics on spacecraft over RF systems, however, includes the 1 Kbps additional burden of pointing the optical system much more precisely than the RF system. The data rate gain 10 Kbps has to offset the additional cost of fi ne pointing. Most technologists and system developers believe that this 100 Kbps trade is now worth it. APL is developing optical communications system 1 Mbps architectures by initially using state-of-the-art com- 10 Mbps mercial off-the-shelf (COTS) components and then Achievable data rate Achievable replacing them with more advanced and/or innovative 100 Mbps technology components as they become available for insertion. The key requirement for optical communi- 1 Gbps cations development for space applications is to sup- port science mission data retrieval at higher rates than 10 Gbps 1 mrad 100 μrad 10 μrad 1 μrad 100 nrad heretofore possible with RF systems for space missions rms pointing requirement as far out as interstellar space and all the way in to Figure 1. Communications (data rate) and pointing and track- near-Earth orbit (NEO) or geosynchronous Earth orbit ing (rms) accuracy requirements for optical communications to (GEO) distances. increase link range for a fi xed transmitter power. JOHNS HOPKINS APL TECHNICAL DIGEST, VOLUME 25, NUMBER 4 (2004) 307 B. G. BOONE ET AL. impact on receiver performance, especially for links predominantly in the atmosphere. The link equation also includes other losses that account for imper- fect optical components, as denoted ␩ ␩ by T and R. These imperfections include losses caused by beam shape, truncation, obscuration, defocusing, obliquity, off-axis positioning (or Figure 2. Generic system architecture for an optical communications system. misalignment), near-fi eld effects, and beamsplitting losses between environmental effects of the atmosphere, back- the various optical elements in the transmitter and/or ground fl ux, and platform motion. We will describe receiver. Often for simplicity the incident irradiance is recent progress at APL addressing most of the tech- assumed to be due to a diffraction-limited plane wave, ␪ ␪ 2 nology components and some of the environmental which forms a jinc-squared [2J1(kr )/kr ] intensity pat- factors. tern on the detector plane for a single-mode beam. (The jinc-squared shape is characteristic of a light spot pro- The Link Equation: Key to System Design and duced by a circular aperture, and is defi ned mathemati- Component Selection cally as the ratio of the Bessel function of the fi rst kind of order 1 to its argument, where its argument is pro- The key relationship between received optical power, portional to displacement from the center of the spot. PR and transmitter optical power PT is given by the link This shape looks like a sombrero but takes its name jinc- equation (in which all the basic terms are included for squared by analogy with a similar function for a square an incoherent system): aperture called the sinc-squared function, where the sinc function is the ratio sin x/x.) Typically, the lack of ␩ ␩ PR = PTGT TLPTLFSGR RLPRLA , perfect diffraction is refl ected in a wavefront loss factor, exp[–(4␲␴/␭)2], or Strehl ratio, where the root-mean- where square (rms) wavefront error ␴ should be kept lower ␲ ␭ 2 than a fraction of a wavelength (at least no greater GT = ( DT/ ) = the gain of the transmitting aperture, than ␭/4). Ϫ ␪2 ␪ 2 Figure 3 shows how the link equation enters the LPT = exp( 8 / div) = the pointing loss of the transmitter (assuming a Gaussian-shaped overall link performance, which includes the detection single-mode beam), scheme with its associated noise model and the modu- ␭ ␲ 2 lation scheme with its BER. The BER required for the LFS = ( /4 R) = the free-space propagation loss, link depends essentially on link distance, desired bit ␲ ␭ 2 rate, available laser transmitter power, and an estimate GR = ( DR/ ) = the gain of the receiving aperture, of the average number of detected photons per bit at the receiver required for a specifi ed bit error probability.