Abstract:
Free-space optical transmission of analog information is facilitated by transmitting constant-amplitude pilot information with the other information. The amount of attenuation of the pilot information at the receiver is detected and used to control the amount by which received information is amplified. In this way the deleterious effects of free-space optical attenuation are substantially eliminated. The pilot information may be transmitted either via its own separate light frequency or wavelength, or as a distinguishable part of a larger quantity of information that is used to modulate one light frequency or wavelength.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to free-space optical telecommunications, and more particularly to automatic gain control for free-space optical telecommunications links. 
     Free-space optical telecommunications offers an attractive alternative to hard-wired or radio communication in certain situations. For example, a telecommunications services provider who wants to enter a new geographical area may have little or no hard-wired plant in that area and may wish to avoid the cost and complexity of installing such plant to serve the new area. Similarly, radio communications resources are limited and regulated, and a new telecommunications services provider may not have sufficient rights to use those resources in a new geographical area. 
     Free-space optical telecommunication is therefore attractive because it avoids the need for hard-wired plant and because, unlike radio telecommunication, it is essentially unregulated. Optical telecommunication also has the advantage of very large information capacity. Thus optical telecommunications links can support a wide range of telecommunications services such as telephone, video, audio, and computer data transmission. 
     A possible problem with free-space optical telecommunication is that it is subject to time-varying attenuation through the atmosphere. For example, infrared or other light may scintillate at frequencies up to about 200 Hz as it passes through the atmosphere. Digital modulation of the light is one way to render free-space optical information transmission more immune from these atmospheric effects. However, digital modulation tends to increase transmission cost for at least some types of information, especially information which is initially in analog form and which is ultimately used in analog form. This is true, for example, for most telephone information and much video (television) information. 
     In view of the foregoing, it is an object of this invention to improve free-space optical telecommunication. 
     It is a more particular object of this invention to reduce the deleterious effects on analog, free-space, optical telecommunication of atmospheric disturbances such as scintillation. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are accomplished in accordance with the principles of the invention by providing analog free-space optical telecommunications apparatus in which a so-called pilot signal of known amplitude is sent along with the information signal via the free-space optical link. At the receiving end of the link, the pilot signal is separated from the information signal. The amplitude of the received pilot signal is compared to a reference, and the amount of deviation from the reference is used to control an amount by which the information signal is amplified. The pilot signal may be sent using a separate light wavelength which is preferably close to the light wavelength(s) used for the information signal. Alternatively, the pilot signal may be one of the frequencies used to modulate the light frequency that also carries the information signal. The comparison of the received pilot signal to a reference, and the automatic amplification variation of the received information signal are preferably done electronically at the receiver. 
     Further features of the invention, its nature and various advantages, will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic block diagram of a first illustrative embodiment of free-space optical communications apparatus constructed in accordance with the invention. 
     FIG. 2 is a simplified schematic block diagram of a second illustrative embodiment of free-space optical communications apparatus constructed in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the first illustrative embodiment shown in FIG. 1 the pilot signal is sent via a wavelength which is different from the wavelength used for the information signal. Information signal source 20 produces an output electrical signal which is analog-modulated with the information to be transmitted. This information signal is applied to light source 30 (e.g., a laser diode) to cause that light source to output light having the same analog modulation as the applied electrical signal. The frequency or wavelength of the light produced by light source 30 is different from, but preferably relatively close to, the frequency or wavelength of the light produced by light source 50 (described below). The light from light source 30 is applied to one input of combiner 60 via optical fiber 32. 
     Pilot signal source 40 produces an output signal of constant magnitude. The output signal of source 40 is applied to light source 50 (e.g., another laser diode) to cause that light source to produce light of constant intensity. The light produced by light source 50 is applied to a second input of combiner 60 via optical fiber 52. 
     Combiner 60 combines the light from optical fibers 32 and 52 and produces a single light output on optical fiber 62. Thus the light carried by optical fiber 62 is the information signal on one light frequency or wavelength, and the pilot signal on another light frequency or wavelength. 
     Optical fiber 62 is connected to light transmitter 70, which launches the light it receives via optical fiber 62 into free space as shown at 72. For example, transmitter 70 may be a lens system for appropriately focusing the light for free-space transmission 72. 
     After traveling a desired distance through free space (e.g., the earth&#39;s atmosphere), light 72 is received by receiver 80. Receiver 80 may be another lens system for focusing the received light toward light splitter 90. Splitter 90 allows a portion of the light it receives to pass through to filter 130. Splitter 90 deflects the remainder of the light it receives to mirror 100. Mirror 100 deflects the light it receives to filter 110. 
     Filter 110 passes only light having the frequency or wavelength produced by light source 30. The light passed by filter 110 is applied to photodetector 120, which accordingly produces an output electrical signal containing the information from source 30 as received at receiver 80. This electrical signal is applied to the main input of automatic gain control circuit 150. 
     Filter 130 passes only light having the frequency or wavelength produced by light source 50. The light passed by filter 130 is applied to photodetector 140, which therefore produces an electrical output signal indicative of the received pilot signal. The output signal of photodetector 140 is applied to amplitude detector 160, which produces an output signal indicative of the amplitude of the received pilot signal. 
     The output signal of amplitude detector 160 is applied to one input of differential or operational amplifier 170. The other input to amplifier 170 is a constant reference signal from reference signal source 180. Amplifier 170 produces an output signal which is indicative of the amount by which the output of amplitude detector 160 differs from the reference signal from source 180. The output signal of amplifier 170 is applied to the control input of automatic gain control circuit 150. 
     Automatic gain control circuit 150 amplifies the signal applied to its main input (i.e., from photodetector 120) by an amount proportional to the magnitude of the signal applied to its control input (i.e., from amplifier 170). Because both the information signal light and the pilot signal light travel along the same free-space optical path and have frequencies that are fairly close to one another, both of these lights experience approximately the same attenuations as they pass through the free space. For example, both the information light and the pilot light scintillate approximately similarly as they pass through the atmosphere between transmitter 70 and receiver 80. The pilot light, however, is known to start out with constant intensity. Therefore, the amount by which the output signal of amplitude detector 160 deviates from the constant reference signal from source 180 at any instant of time is a good indicator of the concurrent attenuation of the received information light. Automatic gain control circuit 150 automatically compensates for this attenuation by amplifying the received information signal by the amount required to substantially eliminate the effects of the time-varying attenuation of light through the free space between transmitter 70 and receive 80. 
     In the alternative embodiment shown in FIG. 2 the pilot signal is used along with the information signal to modulate a single light frequency or wavelength, rather than separate light frequencies or wavelengths being used for the information and pilot signals as in FIG. 1. As shown in FIG. 2, several information signals are used to analog-modulate several different electrical signal frequencies f2-fN. A pilot signal produced by oscillator 210 has another frequency f1. The amplitude of this pilot signal is preferably constant. 
     All of signals f1-fN are applied to combiner 220, which combines all of the applied signals into one frequency-division-multiplexed signal that is applied to amplifier/driver 230. The output signal of amplifier/driver 230 is applied to light source 240 (e.g., a laser diode). The light produced by light source 240 is applied to transmitter 250 (e.g., a lens system) for appropriately launching the light from source 240 into free space as indicated at 252. 
     After traveling the desired distance through free space, the light 252 from transmitter 250 is received by receiver 260 (e.g., another lens system) for focusing the received light on photodetector 270. Photodetector 270 produces an output electrical signal indicative of the received light. This output signal is applied to the main input of variable gain amplifier 280. The output signal of amplifier 280 is applied to splitter 290, which produces several replicas of the applied signal. One of these replicas is applied to band-pass filter 300, which passes substantially only frequency f1, the frequency used for the pilot signal. The output signal of band-pass filter 300 is applied to amplitude detector 310, which therefore produces an output signal indicative of the strength of the pilot signal from filter 300. The output signal of amplitude detector 310 is applied to summation amplifier 320. The other input to amplifier 320 is a constant reference signal 330. The output signal of amplifier 320 is indicative of the amount by which the pilot signal amplitude from band-pass filter 300 deviates from reference 330. The output signal of amplifier 320 is applied to the control input of amplifier 280 to control the amount by which amplifier 280 amplifies the signal from detector 270. Any weakening of the received pilot signal increases the gain produced by amplifier 280. 
     Another signal replica produced by splitter 290 is applied to receiver 340, which may be any suitable device or devices for recovering and utilizing information signals f2-fN. 
     It will be apparent that the effect of the apparatus shown in FIG. 2 is similar to the effect of the apparatus shown in FIG. 1. In particular, the constant amplitude pilot signal is used to detect time-varying attenuation of the light transmitted through free space from transmitter 250 to receiver 260. Amplification of the received signal is automatically varied to compensate for this attenuation and thereby effectively eliminate it. 
     It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. For example, in the embodiment shown in FIG. 1 several different information light frequencies or wavelengths can be sent with pilot light frequency from transmitter 70 to receiver 80.