WO2005112309A1 - Radio frequency burst mode optical to electrical converter for passive optical networks - Google Patents

Abstract

An optical to electrical converter that detects RF burst from a terminal on a passive optical network and selectively activates a laser detection circuit and a laser based on the power level of the RF burst such that noise being transmitted from said terminal to a head-end will not activate said laser detection circuit. The RF burst is detected prior to activating the laser via a burst detection circuit which comprises a burst detector and a reference voltage generator.

Description

RADIO FREQUENCY BURST MODE OPTICAL TO ELECTRICAL CONVERTER FOR PASSIVE OPTICAL NETWORKS

FIELD OF INVENTION

[0001] The field of this invention concerns fiber-to-the-X (FTTX) deployments. In particular, the present invention relates to an optical to electrical converter (OEC) that includes a circuit to transmit radio frequency (RF) return path signals over a passive optical network (PON).

BACKGROUND OF THE INVENTION

[0002] The rise of the internet and other bandwidth intensive applications such as Video- On-Demand (VOD) has created a significant need for bandwidth in access networks. Hybrid fiber-coax (HFC) systems are one of the leading methods for delivering this bandwidth to residential subscribers.

[0003] An HFC system supports the bi-directional communication between the cable modem termination system (CMTS) and the set-top box controller located in the head-end and the cable modem and set-top box located in a subscriber's house. In the downstream direction, the broadcast video channels are combined with the control signals from set-top box controller and CMTS and transported to a node over fiber. In the node these signals are converted into RF and transported to the home over coaxial cable. [0004] In the upstream direction the signals from the cable modem and the set-top box carrying data and pay-per-view movie requests, travel to the node over coaxial cable, where they are modulated on a laser and transported to the head-end over fiber. In the head-end, a return path receiver detects these signals and feeds them to the CMTS and the set-top controller.

[0005] One problem with HFC networks is that they need power in the outside plant to power the nodes. This increases operating and maintenance costs of the network. Another problem is that in the upstream direction, the laser in the node is biased at all times, so in addition to sending data signals back to the head-end, ingress noise from un-terminated coaxial ports is also modulated on the laser and transported to the head-end. This noise degrades the Carrier-to-Noise ratio of the data signal, reduces the bandwidth efficiency of the system and reduce the reach of the system.

[0006] Furthermore, since the laser in the node is biased at all times, multiple node lasers cannot be optically combined into a single return path receiver because of the potential for an interference known as Optical Beat Interference (OBI). OBI occurs when two or more lasers whose wavelengths are close together are combined into a single optical receiver, and since the node lasers are on all the time, the chances that OBI occurs is high. [0007] The occurrence of OBI and the accumulation of ingress noise forces HFC systems to use one return path receiver for every node laser. This one-to-one pairing of a node laser to a return path receiver is economically feasible for HFC networks where several hundred homes are served from one node. However in deep fiber architectures, such as fϊber-to-the- curb and fiber-to-ώe-home, this one-to-one pairing is economically unfeasible. [0008] This invention describes an OEC that includes a bi-directional circuit that receives downstream broadcast video channels, set-top box control messages, and cable modem control messages and transmits RF return signals upstream over a PON from cable modem and set-top boxes to the PON head-end.

[0009] A unique feature of this OEC is that it detects the RF burst from PON terminals such as set-top boxes or cable modems in homes or businesses and selectively turns on the laser only when an RF burst is present. This prevents the ingress noise from being transmitted to the return path receiver in the head-end from the unterminated RF ports at the PON terminals. It also significantly reduces the probability of OBI since only two lasers, one transmitting the cable modem traffic and another fransmitting the set-top box traffic can be on at the same time. This improves the CNR and results in greater bandwidth availability and increased speed in the network.

[0010] The selective biasing of the laser only when an RF burst is present also enables the optical combining of multiple return path lasers on the PON into a single return path receiver. This makes the system cost effective. SUMMARY OF INVENTION [0011] The OEC is intended to overcome the aforementioned shortcomings. It is therefore an object of this invention to detect upstream RF burst from any terminal on a passive optical network that generates an RF signal including, but not limited to, a cable modem or set-top box. [0012] A further object of this invention is to selectively turn on the laser thereby preventing ingress noise from being transmitted to the head-end receiver. [0013] A further object of this invention is to selectively turn on the laser thereby reducing the probability of OBI.

[0014] A further object of this invention is to quickly stabilize the RF burst upstream using a reference voltage generator such that the output power of the laser is properly controlled.

[0015] Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] FIG. 1 is a general schematic of the invention in a network.

[0017] FIG. 2 is a block diagram of the OEC.

[0018] FIG. 3 is a block diagram of the return path, upstream circuit.

[0019] FIG. 4 is a block diagram of the power control loop used to stabilize upstream burst. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS [0020] hi the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, one skilled in the art would recognize that the invention may be practiced without these specific details. In other instances, well known methods, procedures, and/or components have not been described in detail so as not to unnecessarily obscure aspects of the invention.

[0021] FIG. 1 is a schematic overview of the placement of the invention in a network, particularly a passive optical network. A signal will travel downstream from the optical to electrical converter 105 to a location 100 which could be for example, a home or business, via a connection 130. In one embodiment of the invention, the connection 130 might be a HFC, FTTH, or FTTC. In the downstream direction, the OEC 105 will receive signal from the PON head-end 110 at the optical front end 115 and will transmit the signal to said location 100. In the upstream direction, RF bursts from 100 are received by the optical front end 115 and then transmitted to the PON head-end 110.

[0022] At said location 100, the PON terminals 150 and 155 will generate an RF signal or burst. In one embodiment of the invention the terminals 150 and 155 might include to a modem 150 or set-top box 155. Said modem 150 or set-top box 155 may be connected to a device such as a telephone 135, computer 140, or television 145. Said location 100 might also have a number of unterminated RF ports 160 that generate noise that travels upstream into the OEC 105.

[0023] The signal traveling upstream to the OEC 105 includes the RF burst from the modem 150 or set-top box 155 as well as any noise traveling from the unterminated RF ports 160. The RF burst and noise will travel to the OEC 105. The diplexer 125 functions to separate upstream and downstream signals at the OEC 105. If an upstream burst is detected from the any of the terminals 150 and 155 on the PON that generate an RF signal, the laser bias circuit 120 will be activated. The signal will then be transmitted via the optical front end 115 to the PON head-end 110.

[0024] If ingress noise below a certain threshold level is detected from an unterminated RF burst 160 at said location 100, the laser bias circuit 120 will not be activated, and thus the laser will not modulate the noise, and there will be no transmission to the PON head-end 110. As a result, there is little to no degradation in CNR of upstream signals other OEC devices on the PON. This translates to an increase in the bandwidth capacity and a longer reach of the PON.

[0025] As shown in FIG. 2 the present invention is an RF transceiver 200. A signal will travel downstream from the PON head-end 202 to the wavelength division multiplexer (WDM) 204. According to one embodiment of the invention, the signal may travel to the WDM 204 through a number of connections, including but not limited to, a SC/APC pigtail. In an embodiment of the invention the signal will travel at 1550 n but could travel at a variety of different wavelengths. The WDM 204 separates the signal and feeds the optical signal into a detector 206 which converts the downstream spectrum from the optical medium to RF. In one embodiment of the invention, the RF burst will fall into the range of about 50 to 870 MHz.

[0026] The RF signal is then transmitted to the high pass filter 208 of the RF diplexer 220. The RF diplexer 220 then combines this signal and transmits it to the PON terminal 210. In an embodiment of the invention the signal can be transmitted via various devices including but not limited to a coaxial cable or F Connector. The cable or other transmission device may be connected to a number of different PON terminals 210 including a set-top box or cable modem 210. The function of the RF diplexer 220 is to separate the upstream signal applied to the laser 216 from the downstream signal sent from the detector 206. [0027] As further shown in FIG. 2, from the set-top box 210 in the upstream direction, the low pass filter 212 of the RF diplexer 220 separates the band and transmits the signal into a carrier detection or laser bias circuit 214. In an embodiment of the invention, the frequency of the signal may of about 5-42 MHz. This circuit 214 is used to detect a burst from the set- top box or cable modem 210 and ensures that the laser 216 is biased only when there is an upstream burst on the coaxial cable above a certain threshold level. In a preferred embodiment of the invention, the laser 216 is a Fabry-Perot (FP) laser but there are a number of lasers that may be used and are available to those skilled in the art. The output of the laser 216 is then combined into the fiber for transport to the PON Interface 202 via the WDM 204.

[0028] Because the laser 216 is only selectively turned on when the OEC 200 detects an RF burst from the set-top box or cable modem 210, there is a substantial reduction of ingress noise as the data travels upstream to the head-end of the PON. There is also a substantial reduction in the level and probability of OBI occurring at the head-end return path receiver.

[0029] A detailed block diagram of the upstream return path circuit is shown in FIG. 3. The upstream signal from the cable modem or set-top box 320 goes through the diplexer 302 and then through the low pass filter 304 and is split into two paths 306 or 312. The frequency of the upstream signal may fall into the range of about 5-65 MHz. The burst detector 312 detects bursts over a certain threshold and turns on the laser driver 314. According to one embodiment of the invention, the delay line 306 compensates for the delay in the burst detector circuit 312. The signal then modulates the laser 310. As described above, this circuit keeps the laser off until it detects an incoming RF burst from the set-top box or cable modem 320. As such, any ingress noise from unterminated ports at the terminal will not activate or turn on the laser.

[0030] The downstream circuit 322 is also shown in FIG. 3, whereby the signal travels through the WDM 318 and then through the RF Receiver 316. In one embodiment of the invention, the signal travels at 1550 nm but the signal may travel at other frequencies. This signal then travels downstream through the diplexer 302 as discussed above. [0031] Another aspect of the invention is shown in FIG. 4. FIG. 4 shows a power control loop whereby upstream burst power can be stabilized quickly. When there is an upstream burst from the PON terminal, the power must be stabilized very quickly after the burst arrives. The PON terminals include all terminals that will release an RF burst, including set-top boxes or cable modems.

[0032] In this power control loop, the burst detector output 410 detects the burst from the PON terminal. When the burst arrives and is detected, the reference voltage generator 420 generates two reference voltages depending on the output of the burst detector 410. The comparator 440 compares the two voltages and produces a signal. The signal from the comparator 440 is a function of which input voltage was higher from the reference voltage generator 420. The ramp-up time between the two reference voltages is properly controlled to guarantee the stability of the power control loop with the integrator 450. The current driver 460 then interacts with the laser. As sμch, there is a fast stabilization of the laser during a burst.

[0033] While certain exemplary embodiments have been described and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not restrictive of the broad invention, and that this invention need not be limited to the specific constructions and arrangements shown and described, since various other changes, combinations, omissions, modifications and substitutions, in addition to those set forth in the above paragraphs, are possible. Those skilled in the art will appreciate that various adaptations and modifications of the just described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.

Claims

CLAIMSWe claim:

1. A passive optical network comprising: a head-end connected to at least one optical to electrical converter; a converter by which a signal is transmitted from said head end to at least one of said terminals; said converter comprising: an optical to electrical converter; a diplexer; and a laser bias circuit.

2. The network of Claim 1, wherein said terminal is a set-top box.

3. The network of Claim 1, wherein said terminal is a cable modem.

4. The network of Claim 1, wherein said signal comprises a burst of data.

5. The network of Claim 4, wherein said signal further comprises a burst of noise.

6. The network of Claim 4, wherein said laser bias circuit only biases a laser when said laser bias circuit detects said burst of data.

7. The network of Claim 5, wherein said burst of noise does not activate said laser bias circuit.

8. An optical to electrical converter wherein said optical to electrical converter receives a signal X from a head-end and transmits said signal X to at least one terminal and wherein said optical to electrical converter receives a signal Y from said terminal and transmits said signal Y to said head end; said optical to electrical converter comprising: a wavelength division multiplexer which is connected to said head-end and transmits said signal X from said head-end to a detector; said detector fransimtting said signal X to a diplexer; said diplexer transmitting said signal X from said detector to said terminal; said terminal ttansmitting said signal Y to a laser bias circuit; said laser bias circuit activating a laser when said signal Y is detected; said laser ttansmitting said signal Y to said multiplexer; said multiplexer ttansmitting said signal Y to said head-end.

9. The optical to electrical converter of Claim 8, wherein said terminal is a cable modem.

10. The optical to electrical converter of Claim 8, wherein said terminal is a set-top box.

11. The optical to electrical converter of Claim 8, wherein said connection between said multiplexer and said head-end is a SC/APC pigtail.

12. The optical to electrical converter of Claim 8, wherein said signal X received from said head-end and said multiplexer is about 1550 nanometers.

13. The optical to electrical converter of Claim 8, wherein said signal X transmitted by said detector to said diplexer wherein the frequency is about 50-870 MHz.

14. The optical to electrical converter of Claim 8, wherein said signal Y transmitted from said diplexer to said laser bias circuit wherein the frequency is about 5-42 MHz.

15. The optical to electrical converter of Claim 8, wherein said laser is a Fabry-Perot laser ttansmitting at a wavelength of about 1310 nanometers.

16. The optical to electrical converter of Claim 8, wherein said terminal may transmit a burst of noise.

17. The optical to electrical converter of Claim 8, wherein said noise does not activate said laser bias circuit.

18. The optical to electrical converter of Claim 8, wherein said diplexer comprises a high pass filter and a low pass filter whereby said high pass filter transmits signal X to said terminal and said low pass filter transmits signal Y from said terminal to said laser bias circuit.

19. The low pass filter of Claim 18, wherein said signal Y is split into a first path and a second path.

20. The low pass filter of Claim 19, whereby said second path has a delay line.

21. The low pass filter of Claim 19, whereby said first path detects said signal Y via a burst detector and turns on said laser.

22. The burst detector of Claim 21, whereby said burst detector stabilizes said signal Y using a reference voltage generator.

23. A method for transmitting a signal over a PON whereby said signal is transmitted from a terminal to a head-end via a laser bias circuit and a laser is activated when said signal is in a pre-determined power range.

24. The method of Claim 23, whereby said laser is not activated by a burst of noise signal from said PON terminal.

25. The method of Claim 23, whereby said signal is about 5-42 MHz.

26. The method of Claim 25, whereby said signal is detected by a burst detector in a burst detector circuit.

27. The burst detector circuit of Claim 26, whereby said signal is modulated on the laser and stabilized when said signal is detected by the burst detector and transmitted to a reference voltage generator whereby said reference voltage generator generates a voltage in order to activate said laser.

28. The method of Claim 23, whereby when said signal is released from said PON terminal to the optical to electrical converter, said signal is split into a first path and a second path.

29. The method of Claim 28, whereby said second path is a burst detector circuit and the first path is a delay line which compensates for the delay in said second path.

30. The burst detector circuit of claim 29, whereby said signal is detected by a burst detector and said signal is stabilized when said burst detector transmits said signal to a reference voltage generator whereby said reference voltage generator generates a voltage in order to activate said laser.