WO1999035767A2

WO1999035767A2 - High power optical receiver

Info

Publication number: WO1999035767A2
Application number: PCT/IB1998/001974
Authority: WO
Inventors: Eric J. Schnettler; Kerry D. La Violette
Original assignee: Koninklijke Philips Electronics N.V.; Philips Ab
Priority date: 1997-12-31
Filing date: 1998-12-07
Publication date: 1999-07-15
Also published as: WO1999035767A3; JP2001515683A; EP0965228A2

Abstract

An optical receiver for converting an optical signal in a fiber optic cable into a high frequency electrical signal, includes an optical splitter for providing a portion of the signal into each of at least two photodiodes which convert the optical signal into a plurality of separate electrical signals. The phase of the electrical signals is adjusted or the length of the conductors for the separate signal is selected so that, when the separate signals combined they are in phase and the signal to noise ratio of the combined signal is higher than that in the separate signals, but the distortion in the combined signal is not substantially increased over that in the separate signals.

Description

High power optical receiver.

This invention is related to analog and digital signal conversion from an optical signal to an electronic or radio frequency signal. The invention is most closely related to optical receivers for high speed information distribution.

Currently in optical communications, an electrical signal regulates a source of light such as a laser or light emitting diode in order to convert an electrical signal into an optical signal. A fiber optic cable is positioned to transmit the optical signal through the cable to a receiver. In the receiver a photodiode converts the optical signal back into an electrical signal.

Optical systems can be divided into long distance systems and short distance system. In long distance systems, distortions and noise are primarily due to the length of the optical cable. Such systems include computer networks and multiplexed digital telephone. In short distance optical systems, distortion and noise are primarily due to the optical transmitter (which converts a high frequency electrical signal into an optical signal) and the optical receiver (which converts the optical signal back into an electrical signal), and the equipment connected to such systems. Such systems include cable television distribution systems such as analog television systems which transmit NTSC or PAL television signals and digital television systems which transmit (MPEG-2) signals. Currently, the transmission distance for most digital and analog signals through optical fiber cable is limited to about 50 miles due to distortions of the signal and reduction in signal to noise ratio. The distortion and noise are produced in the optical cable over long distances, and to a lesser extent in the transmitter (that converts the electrical signals to optical signals), and in the receiver (that converts the optical signals back into electrical signals). For digital signals, if longer distances for a given frequency (bit rate) or a higher frequency (bit rate) for a given distance is required then a digital repeater can be used to receive the signals from a first optical cable, remove the distortions and noise in the resulting electrical signal, and retransmit the digital signals through another optical cable. Such repeaters (optical amplifiers) are relatively costly and result in increased maintenance and increased energy use, so that energy and other resources could be conserved by increasing the distance that optical signals can be transmitted through fiber optic cables for a given frequency or bit rate without using a repeater or increasing the frequency or bit rate that can be transmitted through a fiber optic cable over a given distance without using a repeater. On the other hand, analog signals can not generally be improved by using repeaters because noise and distortions can not easily be separated from the signal.

In a cable television system, a head-end includes a laser transmitter which converts high frequency signals into optical signals to provide a multitude of television channels in an optical distribution network. The signals travel to a multitude (e.g. 20-2000) of distribution nodes in which a receiver converts the optical signals back into high frequency electrical signals. Generally, since different receivers in the system receive different signal strengths an optical attenuator must be provided in the input to the receiver. The electrical signal is amplified and transmitted through a coaxial cable distribution network to a large number of customers (e.g. 50-500) who are local to the distribution node. In a cable television system, there are stringent requirements for minimum signal strength, minimum signal to noise ratio, and maximum signal distortion at the customer connections. These requirements limit the distance that customers can be serviced through coaxial cable from a distribution node and also, the number of customers who can be serviced from each distribution node. Although amplifiers may be provided in the coaxial distribution network to improve signal strength they are expensive. Also, in analog television systems they do not improve and generally have some deleterious effects on both signal to noise ratio and signal distortion. In these systems the majority of the distortion and noise at the customer connection are due to the coaxial distribution network, however improvements in the optical transmission system can also contribute to improving signal quality in such systems and thus allow further extensions of the coaxial network from the distribution node. Thus, there is a need to improve the quality of signals transmitted through an optical cable system even over short distances.

Other alternatives and advantages of applicant's inventions will be disclosed or become obvious to those skilled in the art by studying the detailed description below with reference to the following drawings which illustrate the elements of the appended claims of the inventions. It is an object of the invention, to improve the quality of analog signals transmitted through an optical cable system over relatively short distances. It is another object of the invention to increase the distance that optical signals can be transmitted through fiber optic cables for a given frequency or bit rate. It is also an object of this invention to increase the frequency or bit rate that can be transmitted through a fiber optic cable over a given distance.

In the receiver of the invention an optical splitter divides the optical signal into two approximately equal signals, each optical signal is separately converted into a respective high frequency electrical signal, the phase between the electrical signals is adjusted to match, and then the electrical signals are combined. The signal to noise ratio is improved without increasing distortion.

In a cable television system of the invention, the signal output to the customer is improved, and the coaxial distribution system can be extended to serve new customers. In a digital communication system, a higher bit rate may be provided so that the throughput and reaction time of a computer network can be improved, the number of repeaters can be reduced, thus conserving energy and other resources.

Figure 1 is a block diagram illustrating a specific embodiment of the optical receiver of the invention in which the optical signal is split, received, and resulting electrical signals are phase adjusted and recombined.

Figure 2 is a block diagram depicting a specific embodiment of the analog cable television distribution system of the invention which uses the optical receivers of figure 1 to improve signal quality and otherwise improve performance of the network. Figure 3 is a block diagram showing a specific embodiment of the digital computer network of the invention, in which computer nodes use the optical receivers of figure 1 to improve throughput and response time.

In figure 1, a high power optical receiver (HPOR) 100 receives an optical signal through input 101. Attenuator 102 is selected or adjusted to provide the desired power level to minimize signal distortion and noise during conversion from optic to electrical signals. An optical splitter 103 provides two independent optical signals. The splitter may be, for example, a "Y" formed by melting an end of an optical fiber to another optical fiber as is well known in the art. The splitter itself attenuates the signal so that at least in some systems the requirement for an input attenuator may be eliminated. Each optical signal is independently received by respective optical to electrical converters 104 and 105. The converters may each include, for example, a photodiode. Such converters are well known in the art. Preferably, the splitter and the two photo-diodes are both in a single package, even more preferably, the splitter and the two photodiodes are on a single integrated circuit.

A phase adjuster 106 matches the phase of the electrical signals. Combiner 107 recombines the two electrical signals into a single signal, and the signal is output through conductor 108. The phases of the electrical signals are adjusted so that the signal strength is increased but the distortions are not increased. Thus, the signal to noise ratio of the combined signal is substantially greater (e.g. 1.3 db) than the signal to noise ratio of either of the separate signals. Preferably, the phase adjuster includes a circuit board 110 with a shielded conductor 111 on a surface with a gap 112 and a plug-in module 113 conductively interconnecting across the gap. The module may have a section of shielded wire 114 of selected length to provide the desired phase adjustment.

The phase of the two electrical signals may be matched using a trial and error approach in which different plug-in modules having different conductor lengths (e.g. in gradients of 1/4 inch) are tried until the phases are matched.

In figure 2, an analog cable television distribution system 120 of the invention includes a head-end 121 and an analog distribution node 122 interconnected by optical cables 123-126. In a typical system, each distribution node services a large number (e.g. 50-500) customers and such systems may have one head end and a multitude (e.g. 20-2000) of distribution nodes. At the head-end for each cable, a multitude of programs are modulated by modulators 130, 132, and 133 to provide all the programs provided to a modulator, in its own high frequency channel. That is, each modulator provides a different modulation frequency for each program in the set of programs provided to such modulator through the respective input cable 127, 128, 129 for the modulator. Each of the cables may include a large number of coaxial cables, for example, a separate cable for each program or small group of programs. The modulated signals are output onto optical cables 123, 124 and 125 by transmitters 133, 134 and 135 respectively. The transmitters simply convert the electrical signals into optical signals. In optical cable television systems, the transmitters typically include lasers which can be modulated at frequencies up to 750 MHz.

Inputs 140, 141, 142 of the analog distribution node are spliced to optic cables 123, 124 and 125 respectively within connector 143. Inputs 140, 141 and 142 are connected to respective high power optical receivers (HPOR) 150, 151 and 152 which are receivers of the invention as described above with relation to figure 1. The high frequency electrical output from each of HPOR's 150, 151 and 152 is conducted to respective preliminary amplifiers 153, 154 and 155. A circuit of high frequency signal splitters such as 160 and switches such as 161, directs the signals to apparatus for any respective coaxial cable output.

The signals provided to HPOR 151 is preferably the main broadcast signal with programs modulated at 70 to 560 MHz. The splitter switching circuit directs a copy of this signal to every coaxial cable output. The signals provided to HPORs 150 and 152 are Preferably, narrow cast signals with programs modulated at frequencies, for example, between 550 and 750 MHz. The switches such as 161 allow switching both narrow cast signals off for a particular coaxial cable output, or switching the signals from either HPOR 150 or 152 but not both, to be received at each particular coaxial cable output.

The output apparatus includes a post amplifier 161, 162, 163 and 164 for each respective coaxial cable output. Also, a diplexer 170, 172, 173 and 174 for each respective coaxial output 175, 176, 177 and 178 separates return signals provided by the customer from the signals provided by the cable system. The return signals from the customers are typically modulated at 50 to 70 MHz. The return signals are routed to combiner 180 which combines the customer signals. The combiner remodulates the customer signals to various frequencies and multiplexes the signals so that a large volumn of return signals from the customers can be transmitted. In such a scheme each distribution node in the system has its own return path so that there may be a large number of return paths back to the head end. Transmitter 181 converts the electrical signal received from combiner 180 into an optical signal and outputs the optical signal through optical cable 126.

Back at head-end 121, HPOR 183 converts the optical signal from cable 126 into electrical signals transmitted onto output 184. Again, receiver 183 is an HPOR of the invention as described above in relation to figure 1 , and its use improves the performance of the return path.

Other distribution nodes in the network have their own return path such as optical cable 190 back to their own HPOR such as HPOR 186 at the head-end, and their own output cable such as cable 187.

Figure 3 illustrates a digital computer network in which, optical fiber cables 201 and 202 interconnect nodes 203 and 204 of the network. Only two cables and two nodes are shown but typically such systems have hundreds or thousands of distribution nodes and large numbers of optical cables interconnecting the nodes. The layout of the cables depends on the type of communication network. Such a network could be an Eithernet, token ring network, ARCnet, Internet, telephone network, or digital television cable network. Since the nodes are illustrated as approximate mirror images, only node 204 will be discussed herein. Node 204 receives optical signals from optical cable 201 into receiver 205 which is a HPOR of the invention described above with relation to figure 1. The use of such HPOR, allows the digital network to provide higher throughput and faster reaction times and/or to provide greater distances between nodes for a given bit rate or to provide a higher bitrate for a given distance, and may eliminate the requirement for digital repeaters in the network thus conserving energy and other resources. The node also includes a microcomputer or microcontroller (CPU) 210 communicating with a memory 211 containing buffers 212 and 213 to store data received into or to be transmitted from the computer, and programs 214 and 215 which control the operation of the CPU and the input circuit 216, output circuit 217 and input and/or output circuits 218- 220 of the node. Program 214 controls CPU 210 and IC 216 to copy data from HPOR 205 into input buffer 212. Program 215 controls the CPU and OC 217 to copy data from output buffer 213 to output circuit 217 from which it is output by transmitter 221 onto optic cable 202. Transmitter 221 converts the electrical signals provided by output circuit 221 into optical signals transmitted through optic fiber cable 202 to node 203.

The invention has been described with reference to specific embodiments including the best mode for carrying out the invention, and with sufficient detail that anyone skilled in the art can make and use the invention. Those skilled in the art may modify these embodiments or provide other embodiments within the spirit of the invention, and thus, the description does not limit the present invention to the disclosed embodiments. The invention is limited only by the following appended claims.

Claims

CLAIMS:

1. An optical receiver system 100, comprising:

ΓÇó an optical input path 101 for an electromagnetic beam carrying an optical signal;

ΓÇó two or more optical sub-paths (115, 116);

ΓÇó beam splitting means 103 for splitting the beam received from the optical input path into two or more sub-beams directed through respective optical sub-paths (115, 116) and carrying approximately equivalent optical signals;

ΓÇó two or more high frequency electrical sub-paths 117, 118;

ΓÇó two or more respective optical to high frequency electrical converters (104, 105), each for receiving a respective sub-beam through one of the optical sub-paths (115, 116) and converting the sub-beam into a respective high frequency electrical sub-signal and transmitting the sub-signal through the respective high frequency electrical sub-path (117, 118);

ΓÇó a high frequency electrical output path 108; and

ΓÇó a signal combiner 107 for combining the respective sub-signals from the high frequency electrical sub-paths into one signal in the high frequency electrical output path.

2. The receiver of claim 1 in which:

ΓÇó the receiver includes an enclosure 119 with a single optical input path and a single high frequency electrical output path and the optical sub-paths (115, 116) include optical fibers within the enclosure through which the sub-beams are directed.

3. The receiver of claim 1 in which one or more of the high frequency electrical sub-paths 117, 118 include means 106 for tuning the phase of the respective sub-signal in one of the high frequency electrical sub-paths 117 to match the phase of another sub-signal in another of the high frequency electrical sub-paths 118 so that the signals of the sub-beams are cumulative when combined.

4. The receiver of claim 3 in which the phase tuning means 106 include means 113 for adjusting the length of the high frequency electrical sub-paths.

5. The receiver of claim 4 in which the length adjusting means include a component 113 that includes a section 114 of a high frequency electrical, path which component can be replaced with another component having a longer or shorter section of high frequency electrical path.

6. The receiver of claim 5 in which the components are selected from predetermined fixed length shielded electrical conductors on the surface of a circuit board that is plugged into a connecter 112 on another circuit board carrying the high frequency electrical path within the enclosure.

7. An analog cable television distribution system, comprising:

ΓÇó a head-end 121 including a optical transmitter (133, 134) that converts electrical signals into an electromagnetic beam carrying the signals; ΓÇó an optical cable 123-126 for transmitting the electromagnetic beam;

ΓÇó optical signal splitting means 143 connected to the cable to divert a portion of the beam from the optical cable;

ΓÇó a distribution node 122 including:

ΓÇó a coaxial cable output 175-178; ΓÇó an amplifier (163, 166) for amplifying an electrical signal provided to the coaxial cable output; and

ΓÇó a first optical receiver (150-152), including:

ΓÇó an optical input path (140-142) connected to the optical signal splitting means 143 to transmit the diverted portion of the beam; ΓÇó two or more optical sub-paths (115, 116);

ΓÇó beam splitting means 103 for splitting the beam received from the optical input path into two or more sub-beams directed through respective optical sub-paths and carrying approximately equivalent optical signal;

ΓÇó two or more high frequency electrical sub-paths (117, 118); ΓÇó two or more respective optical to high frequency electrical converters (104, 105), each for receiving a respective sub-beam through one of the optical sub-paths and converting the sub-beam into a respective high frequency electrical sub-signal and transmitting the sub- signal through the respective high frequency electrical sub-path; ΓÇó a high frequency electrical output path 108; and

8. The system of claim 7 in which:

ΓÇó the distribution node further includes:

ΓÇó a diplexer (170-174) for separating a return signal from the coaxial cable output;

ΓÇó a transmitter 181 for transmitting the return signal as an optical beam through a return output of the distribution node; ΓÇó the head-end further includes an optical receiver, including:

ΓÇó an optical input path 101 for receiving optical return signal;

ΓÇó two or more optical sub-paths (115, 116);

ΓÇó two or more high frequency electrical sub-paths (117, 118);

ΓÇó two or more respective optical to high frequency electrical converters (104, 105), each for receiving a respective sub-beam through one of the optical sub-paths and converting the sub-beam into a respective high frequency electrical sub-signal and transmitting the sub- signal through the respective high frequency electrical sub-path;

ΓÇó a high frequency electrical output path 108; and

ΓÇó a signal combiner 107 for combining the respective signals from the high frequency electrical sub-paths into one signal in the high frequency electrical output path;

9. The system of claim 7 in which one or more of the high frequency electrical sub-paths 117, 118 include means 106 for tuning the phase of the respective sub-signal in one of the high frequency electrical sub-paths 117 to match the phase of another sub-signal in another high frequency electrical sub-paths so that the signals of the sub-beams are cumulative when combined.

10. The system of claim 9 in which the phase tuning means 106 include means 113 for adjusting the length of the high frequency electrical sub-paths; ΓÇó the length adjusting means include a component 113 that includes a section 114 of a high frequency electrical path, which component can be replaced with another component having a longer or shorter section of high frequency electrical path.

11. The system of claim 10 in which the components are selected from components with predetermined fixed length shielded electrical conductors on the surface of a circuit board that is plugged into a connecter on another circuit board carrying the high frequency electrical path.

12. A computer network, comprising :

ΓÇó a fiber optic communications network 201 , 202;

ΓÇó a first node 204 having a first processor 210 communicating with a first memory 211 and a first high frequency electrical to optical converter 221 communicating with the first processor and connected with the communication network for converting a high frequency electrical signal from a buffer in the first memory into an optical signal transmitted onto the communication network;

ΓÇó a second node 203 having a second processor 230 communicating with a second memory 231 and a first optical to high frequency electrical converter 232 connected for receiving an optical signal from the communications network and converting the optical signal into a high frequency electrical signal stored into a buffer in the second memory, the first optical to high frequency electrical converter including:

ΓÇó an optical signal splitter 103 connected to the communication network to split the optical signal from the network into two or more approximately equal optical sub-signals;

ΓÇó a respective optical to high frequency electrical converter 104, 105 for each of the optical sub-signals for converting the optical signal to a respective high frequency electrical signal; and

ΓÇó a high frequency electrical signal combiner 107 communicating with each respective optical to high frequency electrical converter to combine the high frequency electrical signals and store the signals into a buffer 233 in the second memory.

13. The network of claim 12 in which:

ΓÇó the first node 204 further includes a second optical to high frequency electrical converter 205 connected for receiving an optical signal from the communications network and converting the optical signal into a high frequency electrical signal stored into a first output buffer 212 in the first memory, the second optical to high frequency electrical converter including:

ΓÇó an optical signal splitter 103 connected to the communication network to split the optical signal from the network into two or more approximately equal optical sub-signals; ΓÇó a respective optical to high frequency electrical converter (104, 105) for each of the optical sub-signals for converting the optical signal to a respective high frequency electrical signal; and

ΓÇó a high frequency electrical signal combiner 107 communicating with each respective optical to high frequency electrical converter to combine the high frequency electrical signals and store the signals into an input buffer 233 in the second memory; and

ΓÇó the second node 203 further includes a second high frequency electrical to optical converter 234 communicating with the second processor 230 and connected with the communication network 201, 202 for converting a high frequency electrical signal from a second output buffer 235 in the second memory into an optical signal transmitted onto the communication network;

14. The network of claim 13 which includes a digital cable television distribution network, the first node is a digital head-end of a cable television system, and the second node is a digital distribution node connected to several individual customer cables.

15. The method for producing an electrical signal, comprising the steps of:

ΓÇó splitting an optical signal into two or more optical sub-signals;

ΓÇó converting the sub-signals into respective electrical sub-signals;

ΓÇó combining two or more of the electrical sub-signals into a combined electrical signal.

16. The method of claim 7 further comprising the steps of:

ΓÇó tuning the phase of one or more of the sub-signals so that the combined signal has a higher signal to noise ratio than any of the sub-signals.

17. The method of claim 8 in which the tuning is performed by adjusting the length of one or more electrical signal paths of the electrical sub-signals.