SIGNAL POWER CONTROL SYSTEM FOR FIBER OPTIC COMMUNICATION SYSTEMS

Abstract: A signal power control system for fiber optic communications systems is disclosed herein. The invention is adapted for use in a system including a transmitter (760), a receiver (440), an optic fiber (160) therebetween, and bobbin means (180) for dispensing the fiber. The invention includes means (480) for detecting the power of the signal delivered to the receiver (440) from the transmitter (760) and for producing a control signal which varies in response to the detected signal power level. Means for providing the control signal to power control means (740) are included. The power control means (740) is responsive to the control signal and operates to vary the output power of the transmitter (760) as necessary to accommodate the dynamic range of the receiver (440).

Description: SIGNAL POWER CONTROL SYSTEM FOR FIBER OPTIC COMMUNICATION SYSTEMS BACKGROUND OF THE INVENTION Field of the Invention; This invention relates to fiber optic communication systems. More specifically, this invention relates to feedback control systems for regulating signal power within such communication systems. While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof. Description of the Related Art: In fiber optic guidance systems, communication with a guided vehicle is facilitated via a fiber optic cable between a vehicle and a control station. The optical fiber is typically wound around a bobbin, or secured by other means capable of dispensing the fiber as the vehicle travels downrange. In a wound fiber, propagating light energy is attenuated as a result of bending induced optical radiation. This bending induced radiation loss may result in significant attenuation of the optical power delivered to a receiver mounted on the guided vehicle. Consequently, the unwinding and associated straightening of dispensed fiber during motion of the vehicle is accompanied by a decrease in signal attenuation and corresponding increase in power delivered to the receiver. This necessitates a receiver with a sufficiently wide dynamic range or suitable automatic gain control circuitry (AGC) . For example, U.S. Pat. No. 4,461,542 issued to Thomas D. Shovlin et al on March 24, 1987 disclosed a monopulse receiver having a wide dynamic range. In this system, two channels are employed, each being sensitive to a selected dynamic range to provide a composite system with the desired dynamic range. Circuitry is included for selected the appropriate channel depending on the amplitude of the input signal. Unfortunately, overall receiver dynamic range remains fundamentally limited by the summation of the dynamic ranges of each channel. That is, the design of such systems may be inadequate when shift in dynamic range occurs due for example to a change in the field in length of the optical fiber, that is, the range of the system. Thus there is a need in the art for a signal power control system for fiber optic guidance systems which regulates the power incident on a receiver coupled to a wound or folded fiber optic data link. SUMMARY OF THE INVENTION The present invention provides signal power control in fiber optic communication systems. The invention is adapted for use within a system including a transmitter, a receiver, an optic fiber therebetween, and bobbin means for dispensing the fiber. The invention includes means for detecting the power of the signal delivered to the receiver from the transmitter and for producing a control signal which varies in response to the detected signal power level. Means for providing the control signal to power control means are included. The power control means is responsive to the control signal and operates to vary the output power of the transmitter as necessary to accommodate the dynamic range of the receiver. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a typical conventional fiber optic communication system. Fig. 2 is a block diagram of a preferred embodiment of the signal power control system of the present invention. Fig. 3 shows the circuitry included in the power sensing electronics of the signal power control system of the present invention. Fig. 4 is a block diagram of the transmitter driver of the signal power control system of the present invention. DETAILED DESCRIPTION OF THE INVENTION As shown in Figure 1, a conventional electro-optical system 10 includes a vehicle subsystem 12, control station subsystem 14, and a fiber optic link 16 therebetween. A bobbin 18 is connected to the fiber optic cable 16 at one end thereof, and attached to the vehicle 12 by either an optical fiber connection or an electromechanical coupler 20. The electromechanical coupler 20 enables rotation of the bobbin 18, while simultaneously allowing passage of light energy between the bobbin 18 and the electro-optical transducer 22. The fiber optic cable 16 is operatively coupled to the control station 14 through a control station optical multiplexer 24. The optical multiplexers 22 and 24 couple, to the fiber optic cable 16, light energy of a first known frequency in one direction and light energy of a second known frequency in a second direction. The vehicle subsystem 12 includes a transmit section 26 and a receive section 40. The transmit section 26 includes a multiplexer 28 which accepts input from a multiple channel data bus 30 and outputs an electrical signal to a transmitter driver 32. Information carried on the data bus 30 may, for example, be composed of output signals from vehicle sensors or of status information from other electrical circuitry. As is known in the art, the multiplexer 28 interleaves inputs from the data bus 30 and produces a composite electrical signal for input to the transmitter driver 32. The transmitter driver 32 includes circuitry for biasing an injection laser diode or light emitting diode (LED) within the transmitter 34 and for relaying the composite signal from the multiplexer 28. These conditioned electrical signals are then sent to the transmitter 34 where they are amplified and subsequently converted to optical energy. The receive section 40 includes a demultiplexer 42, a receiver 44 and associated AGC circuitry 46. The receiver 44 accepts optical signals from the optical multiplexer 22. Typically, a photodiode within the receiver 44 produces an electrical signal of amplitude proportional to optical signal power. The AGC circuitry 46 compensates for fluctuations in the electrical signal power in the receiver 44 provided these fluctuations are within a predetermined dynamic range. As is well known, a signal having magnitude exceeding the dynamic range of the AGC circuitry 46 may saturate the receiver 44. Similarly, low level signals beneath the dynamic range of the AGC circuitry 46 may go undetected by the receiver 44. Hence, AGC circuitry alone is generally ineffective in signal power compensation outside a limited dynamic range. The receiver 44 may include demodulation or frequency conversion circuitry for processing the signal prior to sending it to the demultiplexer 42. The demultiplexer 42 performs the converse function of the multiplexer 28; namely, the demultiplexer 42 accepts a composite electrical signal and separates the signal into fundamental constituents. These component signals are then routed to their respective channels on the data bus 50. The control station 14 includes a receive section 60 and transmit section 70. The receive section 60 includes a demultiplexer 62 and a receiver 64 with associated AGC circuitry 66. The receiver 64 accepts optical signals from the optical multiplexer 24. The receiver 64 typically includes a photodiode which produces an electrical signal of amplitude proportional to optical power of the received signal. The AGC circuitry 66 compensates for fluctuations in electrical signal level in the receiver 64 provided these fluctuations are within a predetermined dynamic range. As discussed above, signals outside the dynamic range of the AGC circuitry 66 within the control station 14 pose difficulties as they do in the vehicle 12. Specifically, signals exceeding the dynamic range of the AGC circuitry 66 may saturate the receiver 64, while signals of insufficient magnitude may go undetected. Hence, AGC circuitry 66 is effective in signal power compensation only over a predetermined, limited dynamic range. The receiver 64 may also include demodulation or frequency conversion circuitry- for processing the signal prior to sending it to the demultiplexer 62. The transmit section includes a control station multiplexer 72 which combines excitations from a multi-channel data bus 90 into an electrical signal delivered to a control station transmitter driver 74. The transmitter driver 74 includes circuitry for biasing an injection laser diode or light emitting diode (LED) within a control station transmitter 76. The control station transmitter 76 converts this composite electrical signal into an optical signal and sends it to the optical multiplexer 24. The optical signal is then transmitted to the vehicle 12 through the fiber optic link 16. As shown in Fig. 2, the system 100 of the present invention substantially overcomes the signal power dynamic range constraints of the conventional system 10 of Fig. 1. - The system 100 includes a vehicle subsystem 120, control station subsystem 140, and a fiber optic link 160 therebetween. A bobbin 180 is connected to the fiber optic cable 160 at one end thereof, and attached to the vehicle subsystem 120 by an optical fiber connection or an electromechanical coupler 200. Again, the electromechanical coupler 200 enables rotation of the bobbin 180, while simultaneously allowing passage of light energy between the bobbin 180 and an optical multiplexer 220. The fiber optic cable 160 is operatively coupled to the control station 140 through a control station optical multiplexer 240. The optical multiplexers 220 and 240 couple light energy of first and second frequencies traveling in first and second directions, respectively, to the fiber optic cable 160. Disposed within the vehicle subsystem 120 are a transmit section 260 and a receive section 400. The receive section 400 includes a demultiplexer 420, receiver 440 with associated AGC circuitry 460 and power sensing electronics 480. The receiver 440 accepts optical signals from the optical multiplexer 220. A photodetector within the receiver 440 generates an electrical signal of amplitude proportional to the incident optical signal power. The signal from the photodetector appears at the output 410 of the receiver 440 is sent to the demultiplexer 420 and sampled by power sensing electronics 480. As shown in Fig. 3, the power sensing electronics include a low pass filter 485 which produces a DC signal proportional to the amplitude of the electrical signal output by the receiver 440. An analog-to-digital (A/D) converter 490 produces a digital power control signal at its output 540 in response to the DC signal from the low pass filter 485. Thus, power sensing electronics 480 generate a digital power control signal at the output 540 proportional to optical signal power incident on the receiver 440. Referring again to Fig. 2, the control signal from the power sensing electronics 480 is provided to the multiplexer 280 where it is interleaved with other signals from the data bus 300 to form a composite electrical signal on line 290 from the multiplexer 280 which is sent to transmitter driver electronics 320. As shown in Fig " . 4, information contained in the electrical signal from the multiplexer 280 on line 290 is converted to an optical signal 350 through modulation of the LED 346 bias current via the modulation network 330. As is known in the art, the design of the modulation network 330 will be dependent on the modulation scheme employed. The optical signal 350 is transmitted to the control station subsystem 140 via the optical multiplexer 220 and the fiber optic link 160. The link 160 provides means for providing said control signal to the control station transmitter 760. As is next described, the information regarding the intensity of optical energy received by the vehicle subsystem 120, included in the optical signal 350, will be extracted within the control station subsystem 140 and used to adjust the optical output power from the control station transmitter 760. The receiver 640 output signal on line 610 of Fig. 2 provides power control information generated by power sensing electronics 480 in response to the optical signal power incident on the receiver 440 within the vehicle subsystem 120. This information is synthesized into a separate digital power control signal on line 920 from the composite signal on line 610 within the demultiplexer 620 and routed to transmitter driver electronics 740. As shown in Fig. 4, the transmitter driver 740 includes a bias network 742 for biasing the transmitter 760, as well as a digital to analog converter 744 for producing a continuous time control signal from digital power control signal on line 920 from the demultiplexer 620. This signal is utilized by the bias network 742 to produce a bias control current. The bias control current regulates optical output power 770 from the transmitter 760 by adjusting the bias point of an LED 765. Thus, the present invention provides means for detecting the signal power delivered to a vehicle receiver 640 and for producing a control signal responsive thereto. Disposed within the control station subsystem 140 are a transmit section 700 and receive section 600. The receive section 600 includes a demultiplexer 620, receiver 640 with associated AGC circuitry 660 and power sensing electronics 680. The receiver 640 accepts optical signals from the optical multiplexer 240. A photodetector within the receiver 640 generates an electrical signal at the output of the receiver 610 of amplitude proportional to the incident optical signal power. The electrical signal 610 is sent to the demultiplexer 620 and sampled by power sensing electronics 680. As shown in Fig. 3, the power sensing electronics 680 include a low pass filter 685 which produces a DC signal proportional to the amplitude of the electrical signal at the output of the receiver 610. An analog to digital converter 690 produces a digital power control signal on line 940 in response to the DC signal from the low pass filter 685. Thus, power sensing electronics 680 generate a digital power control signal on line 940 proportional to optical signal power incident on the receiver 640. Referring again to Fig. 2, the control signal on line 940 is sent to the multiplexer 720 where it is interleaved with other signals from the data bus 900 to form a composite electrical signal appearing at the output of the multiplexer 730 and sent to the transmitter driver 740. As shown in Fig. 4, information contained in the electrical signal at the output of the multiplexer 730 is converted to an optical signal 770 through modulation of the photodiode 765 bias current via the modulation network 742. As is known in the art, the specific circuitry included in the modulation network 742 depends on the modulation scheme employed. This optical signal 770 is transmitted to the vehicle subsystem 120 via the optical multiplexer 240 and the fiber optic link 160. The information regarding the intensity of optical energy received by the control station subsystem 140, included in the optical signal 770, will be extracted within the vehicle subsystem 120 and used to adjust the optical output power from the vehicle transmitter 340. The signal at the output of the receiver 410 of Fig. 2 includes power control information generated by power sensing electronics 680 in response to the optical signal power incident on the receiver 640 within the control station subsystem 140. This information is synthesized into a separate digital power control signal on line 560 from the composite signal at the output of the receiver 410 within the demultiplexer 420 and routed to transmitter driver electronics 320. As shown in Fig. 4, the transmitter driver electronics 320 comprise a bias network 330 for biasing a transmitter 340, as well as a digital to analog converter 325 for producing a control signal from digital power control information on line 560. The control signal is utilized by the bias network 330 to produce a bias control current. This control current regulates optical output power 350 from the transmitter 340 by adjusting the bias point of the LED 346. Thus, the present invention provides means for detecting the signal power delivered to a control station receiver 640 and for producing a control signal responsive thereto. The invention further includes means for providing said control signal to the vehicle transmitter 340. Initially, the vehicle 120 is in close proximity to the control station 140 and the fiber optic cable 160 is wound around the bobbin 180. As the vehicle 120 moves away from the control station 140, the fiber optic cable 160 begins to unspool. As is well known, bending or winding fiber optic cable generally increases the attenuation of the light energy traveling therein. Hence, as more cable is dispensed from the vehicle 120, less remains in a wound state on the bobbin 180 and the optical signal loss through the fiber optic cable 160 decreases. Since the signal loss through the fiber optic cable 160 is decreasing as the vehicle 120 moves further from the control station 140, the optical signal power detected by power sensing electronics 480 and 680 increases during this motion. These power sensing circuits subsequently generate control signals on lines 540 and 940 indicative of the increase in received signal power. Automatic gain control circuits, alternative embodiments of which are know to those skilled in the art, are used to compensate for signal power fluctuations resulting from the time delay of propagation of control data through the system 100. For example, a finite amount of time elapses between the sensing of signal power by power sensing electronics 480 and receipt of associated control data by the control station transmitter driver electronics 740. If, for example, cumulative signal loss through the fiber optic data link 160 is decreasing during this time interval, input optical signal power to the vehicle receiver 440 will continue to be higher than desired. The AGC circuitry 460 associated with the vehicle receiver 440 attenuates the incident optical signal during this time period until the power output of the control station transmitter 760 can be adjusted by the control signal. The control signal can be updated as frequently as the data channel update rate. Hence, an advantage of the present invention over the related art is in the reduced dynamic range necessary for AGC circuits 460 & 660 as a result of the feedback control mechanism. The system of the present invention contemplates an external adjustment of the operating point of the control station transmitter 760, and vehicle transmitter 340, to accommodate changes in the length of the fiber optic data link 160. While the present invention has been described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. The teachings of this invention may be utilized by one having ordinary skill in the art to make modifications within the scope thereof. For example, the invention is not limited to particular electronics used to detect optical energy and provide a control signal indicative of the incident power. Further, the invention is not limited to a communications path comprising a fiber optic cable. Other suitable signal transmission media may be employed without departing from the scope of the invention. It is therefore contemplated by the appended claims to cover any and all such modifications. Accordingly, WHAT IS CLAIMED IS:

Claims: CLAIMS 1. In a fiber optic communication system having a first transmitter, a first receiver, an optic fiber therebetween, and bobbin means for dispensing said fiber, a signal power control system comprising: detection means for detecting signal power delivered to said first receiver from said first transmitter and for producing a control signal which varies in response to said detected signal power; power control means, responsive to said control signal, for varying the output power of said first transmitter; and, means for providing said control signal to said power control means. 2. The system of Claim 1 wherein said means for providing said control signal includes said optic fiber. 3. The system of Claim 1 wherein said means for providing said control signal further includes power sensing electronics coupled to said first receiver. 4. The system. * ,of Claim 3 wherein said means for providing said control signal further includes a second transmitter, coupled to the output of said power sensing electronics, for providing an output signal to said optic fiber. 5. The system of Claim 4 wherein said means for providing said control signal further includes a second receiver, coupled to said optic fiber, for receiving said output signal. 6. The system of Claim 5 wherein said means for providing said control signal further includes demultiplexing means, coupled between said second receiver and said power control means, for extracting said control signal from the output of said second receiver. 7. The system of Claim ^ wherein said detection means further includes power sensing electronics responsive to said detection signal. 8. The system of Claim 7 wherein said detection means includes a photodetector generating a detection signal having amplitude proportional to said signal power. 9. The system of Claim 8 wherein said power sensing electronics includes a low pass filter which generates a DC signal of magnitude proportional to the amplitude of said detection signal. 10. The system of Claim 9 wherein said power sensing electronics further includes an analog to digital converter connected to the output of said low pass filter. 11. The system of Claim 1 wherein said power control means includes transmitter driver electronics operatively coupled to said first transmitter. 12. The system of Claim 11 wherein said transmitter driver electronics includes a digital to analog converter for synthesizing a power control signal. 13. The system of Claim 12 wherein said transmitter driver electronics further includes a bias network which produces a bias control current in response to said power control signal. 14. The system of Claim 13 wherein said bias control current is applied to an optical signal source within said first transmitter. 15. In a fiber optic communications system having a first transmitter, a first receiver, an optic fiber therebetween, and bobbin means for dispensing said fiber, a signal power control system comprising: a photodetector mounted within said first receiver producing a detection signal having amplitude proportional to signal power delivered to said first receiver from said first transmitter; a low pass filter connected to said first receiver which generates a DC signal of magnitude proportional to the amplitude of said detection signal; an analog to digital converter connected to said low pass filter for generating a digital power control signal in response to said DC signal; a modulation network connected to said analog to digital converter for producing an optical power control signal by imposing said multiplexed digital power control signal on an optical carrier generated within a second transmitter; a first optical multiplexer fastened to said second transmitter for coupling said optical control signal to said optic fiber; a second optical multiplexer for decoupling said optical control signal from said optic fiber to a second receiver which generates an electrical control signal in response to said optical control signal; a demultiplexer connected to said second receiver for recovering said digital power control signal from said electrical control signal; a digital to analog converter coupled to said demultiplexer for synthesizing an analog power control signal in response to said digital power control signal; a bias network connected to said digital to analog converter for producing a bias control current in response to said analog power control signal; and an optical source, within said first transmitter, responsive to said bias control current. 16. In a fiber optic communications system having a transmitter, a receiver, an optic fiber therebetween, and bobbin means for dispensing said fiber, a method of regulating the signal power incident on said receiver comprising the steps of: a) detecting said signal power delivered to said receiver from said transmitter; b) producing a control signal which varies in response to said detected signal power; and, c) varying the signal power from said transmitter in response to said control signal.

Inventors: HSU HUI-PIN, HSU, HUI-PIN
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1989
Main classification: H04B10
IPC codes: H04B10