"Free-Space Optical Communications Analysis Software" (FOCAS) is the name of a spreadsheet computer program for analysis of direct-detection (as distinguished from heterodyne-detection) optical data-communication links. Implemented within the Microsoft Excel software system, FOCAS exploits the flexibility and power of the Excel spreadsheet features. An easy-to-use graphical user interface to the spreadsheet has been developed in Visual Basic for Applications to facilitate insertion of parameters of optical components and of other input data. Optionally, the user can select the automatic insertion of parameters from a data base of known commercial components. Thus, even nonexperts who lack detailed knowledge of optical components can perform analyses.
The following is a partial list of communication-link features accommodated in FOCAS.
- Modulation schemes: on/off keying and M-ary pulse-position modulation, including Manchester coding (binary PPM) as a special case of M-ary PPM.
- Coding: Reed-Solomon code with choice of code-word length and number of information symbols.
- Lasers: Energy-storage-based pulsed lasers or continuous-wave lasers with current modulation or external modulators.
- Transmitter and Receiver Optics: Gains, beam widths, and related parameters are calculated for Gaussian or near-diffraction-limited beam profiles. The effect of transmitting-telescope wavefront quality is incorporated in the Strehl ratio.
- Detectors: Quantum efficiencies of positive/intrinsic/negative (PIN) photodetectors, avalanche photodetectors (APDs) and photomultiplier tubes (PMTs) are represented by lookup tables. Gaussian statistics are used for PIN photodetectors and APDs; Poisson statistics are used for PMTs.
- Amplifiers: Noise, frequency response, and other characteristics of transimpedance and high-impedance preamplifiers can be obtained by use of mathematical models or by selection of amplifiers listed in the data base.
- Other: Wavelength- and zenith-angle-dependent atmospheric-transmission data and wavelength-dependent sky-radiance data are obtained from another program called "MODTRAN." By use of a Rician probability model, FOCAS calculates burst error probability due to pointing jitter and bias. Link ranges are determined automatically for given transmitter and receiver locations.
When FOCAS is opened in Excel, the program displays a block diagram of an optical communication link, including a transmitter, a free-space optical communication medium or channel containing noise sources, and a receiver (see figure). Each block in the diagram appears as a button that can be pressed to bring up a dialog box, through which the user can view and edit the parameters of the transmitting or receiving component, transmitting or receiving subsystem, channel, or link characteristic represented by the block. Other buttons below the block diagram enable the user to load or save parameters or view a link table, which is described below.
In a typical case, the user clicks on the buttons to enter the link parameters manually or, where desired, automatically by selection of a component from the data base. FOCAS automatically compares each parameter with minimum and maximum values to generate a warning and/or prevent the entry of a physically unrealistic value; for example, if the user attempts to enter a beam width less than the diffraction-limit minimum for a selected telescope diameter, then FOCAS displays an error message and does not allow the user to proceed further until the user enters a larger, realistic value.
Once all input parameters have been entered, the user can cause the program to display the link table by pressing the "Go to Link Table" button. The link table is a spreadsheet that shows both the input parameters and the values calculated by the program. The link table is divided into a link summary and a detailed link section. The link summary section contains parameters that characterize the overall link, including the overall link margin, link range, data rate, bit-error rate, transmitted power, received power, power required to achieve a specified overall link margin, transmitter gain, receiver gain, and terms that account for losses along the link path. The detailed link section follows the link summary section and consists of subsections (transmitter, channel, receiver, link margin, coding, calculation of required signal power, and background radiation), each of which provides information in depth on one aspect of the link.
Almost all of the cells in the link table are locked to prevent the user from inadvertently changing dependent variables calculated by the program. However, the user would find it inconvenient to go back to the dialog boxes to change the values of some common parameters in performing a series of related analyses. Therefore, FOCAS allows some of the parameters (for example, the link range, data rate, average laser power, width of the transmitted beam, and the diameter of the receiver aperture) to be changed in the link table. Following such changes, the auto-calculation feature of Excel updates the remainder of the link table; thus, FOCAS provides an interactive means of designing an optical communication link.
This work was done by Muthu Jeganathan, G. Stephen Mecherle, and James Lesh of Caltech for NASA's Jet Propulsion Laboratory. NPO-20412