Patent Application
20130064545
The present invention pertains generally to optical communications systems and networks. More particularly, the present invention pertains to systems and networks that transmit radio frequency signals “f”, as optical signals “λ”, through optical fibers. The present invention is particularly, but not exclusively, useful as a point-to-multipoint network that effectively accommodates a plurality of signals within a confined bandwidth for simultaneous transmission on an essentially single wavelength, optical signal.
Many telecommunication networks typically function by transmitting signals that are carried on radio frequency (RF) waves. It is known, however, that such signals can also be optically transmitted over glass fibers. Consequently, depending on the particular application, and the economics that are involved, it may be desirable to incorporate an optical transmission capability within a communication network. When doing so, however, it is desirable to optimize the use of glass fibers or, stated differently, to use as few glass fibers as possible.
As a general consideration, in order to incorporate an optical communication capability into a communication network, it is first necessary to up-convert signals from a radio frequency carrier “f” to an optical wavelength “λ”. Next, it is necessary to introduce the optical signal into the upstream end of an optical fiber cable. The optical signal is then transmitted over the cable. At the downstream end of the cable, it is necessary to down-convert the signal from its optical wavelength “λ” back to the original radio frequency signal “f”. In general, this works fine for point-to-point communications (i.e. direct communication from one point to another point). It can become problematical, however, when more than two points at the same end of the optical cable are involved at the same time.
One solution for using a single optical fiber in a network, for the purpose of communicating between a single upstream point [modem], and numerous downstream points [modems], has been to employ a transmission protocol. Normally, such a protocol allows the different points to queue in a manner that gives them sequential access to the optical fiber. This obviously has its limitations, as queuing often delays transmissions. An option here is to incorporate more and different fiber optic cables. This, however, can be costly.
In light of the above, it is an object of the present invention to provide a point-to-multipoint optical communications network that allows a plurality of downstream points to simultaneously transmit optical signals to a single upstream point using a same confined bandwidth, on a same optical fiber. Another object of the present invention is to provide a system and method for simultaneously receiving a plurality of optical signals over a single optical fiber at a same point. Yet another object of the present invention is to provide a point-to-multipoint optical communications network that is easy to use, is simple to implement, and is comparatively cost effective.
A system for simultaneously transmitting a plurality of optical signals over a single optical fiber cable between a single upstream point and an “n” number of downstream points is provided. In particular, the present invention envisions there will be a single point connected to the upstream end of the optical fiber, and there will be an “n” number of points that are connected to a respective “n” number of downstream ends of the optical fiber. Typically, “n” will be an integer in the range from 1 to 10.
An “n” number of up-converters are required for use at the downstream end of the optical fiber cable. Specifically, each of these up-converters is connected to a downstream end of the optical fiber between the optical fiber and a respective downstream point. The purpose of these up-converters is to impress a radio frequency signal “fn” from the particular downstream point onto an optical signal of wavelength “λn”. The optical signal “λn” will then be transmitted over the optical fiber cable.
For the present invention, the radio frequency that is used for the signal “fn” at each particular downstream point [modem] will be specific for the particular point. Further, the difference between frequencies (Δf), where fn=f(n−1)+Δfn, may either be the same, or it may vary to make it unique. Typically, each Δf will be in a range between 50 MHz and 1 GHz. With this in mind, each radio frequency fn is up-converted to a respective optical signal λn. Along with the frequency differences for fn, the optical signals λn will also differ. Specifically, λn=λ(n−1)+Δλn. In this relationship, the difference in wavelength (Δλ) between an optical signal λn and λ(n+1) may either be the same, or it may vary to make it unique. Typically, each Δλ will be greater than approximately 0.5 nm. Despite their differences, all of the optical signals λn that are to be simultaneously transmitted upstream through the optical fiber will be within a relatively narrow bandwidth that extends between λLo and λHi (λLo<λn<λHi).
As indicated above, there are an “n” number of downstream points individually connected to the “n” number of downstream ends of the optical fiber. Although there is only one upstream point, there may be a plurality of modems. Consequently, it is necessary there be only one receiver with only a single photodiode connected to the upstream end of the optical fiber. Importantly, the photodiode at the upstream point needs to accommodate the narrow bandwidth between λLo and λHi (λLo<λn<λHi). For example, if n=10 and Δλ=0.5 nm, the photodiode will need to accommodate a 5 nm bandwidth. It will then be able to simultaneously receive all signals transmitted on wavelengths λn from the various “n” downstream points.
A plurality of RF down-converters are connected to the photodiode at the single upstream end of the optical fiber. Specifically, these down-converters are provided to convert the respective optical signal “λn” to their respective radio frequency signals “fn”, and to segregate the signals “fn” from each other for use at the upstream point, according to frequency.
In order to provide for downstream communications over the network, a single downstream transmitter is connected to the upstream end of the optical fiber. Thus, signals “fn” from the upstream point(s) are transmitted to the plurality of downstream points on a single optical signal of wavelength λ. The signals “fn” are then routed at the downstream end for further transmission to the particular downstream points. As an added feature, each modem at the upstream point and each downstream point may separately include a tuner for selectively receiving the radio frequency signal “fn” addressed to the particular downstream point.
The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawing, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which the FIGURE is a schematic presentation of the components required for a system in accordance with the present invention.
Referring to the FIGURE, a point-to-multipoint network in accordance with the present invention is shown schematically and is designated 10. As shown, the network 10 includes an “n” number of upstream modems 12, and a same “n” number of downstream points [modems] 14, of which the modems 12a, 12b, 12c, 14a, 14b and 14c are exemplary. For the present invention, the number “n” may vary, but it is anticipated to typically be an integer in the range from two to ten. Thus, the upstream modem 12c and the downstream modem 14c each sequentially represent the possibility for a respective third through tenth modem.
As shown in the FIGURE, the network 10 includes an optical fiber 16 having an upstream end 18 and a downstream end 20. Further, a wavelength-division multiplexer (WDM) 22 of a type well known in the pertinent art is shown connected to the optical fiber 16 at its upstream end 18. Along with the WDM 22, there is a transmit, electrical/optical up-converter 24 that includes a laser diode. And, there is a receive, optical/electrical down-converter 26 that includes a photodiode. Using the upstream modem 12a as an example, it will be seen that a radio frequency (RF) up-converter 27 and a transmitter 28 are connected with the modem 12a. Specifically, within this combination, when a signal leaves the modem 12a for transmission over the network 10, it leaves as an RF signal with a frequency “fn”. Thus, in the specific case of modem 12a, this signal will be passed from the modem 12a to the electrical/optical up-converter 24 as a downstream signal with the frequency “f1” (i.e. n=1). Still referring to the upstream modem 12a, it will be seen that this modem 12a also includes an RF down-converter 29 and a receiver 30 that are connected with the optical/electrical down-converter 26. Specifically, within this combination, the modem 12a is set to receive an upstream signal from the optical/electrical down-converter 26 that has an RF frequency “f1”. Recall from above, the modem 12a is only exemplary. As intended for the present invention, the modems 12n will all function substantially the same, and each will individually interact directly with the electrical/optical up-converter 24 and the optical/electrical down-converter 26. The only difference here will be that each modem 12n will be using a different respective frequency “fn”.
At its downstream end 20, the optical fiber 16 is connected with an optical distribution network 32 that, in turn, is separately connected to the various downstream points [modems] 14a-c. More specifically, the optical distribution network 32 is directly connected to WDMs 34a, 34b and 34c via respective optical fibers 35a, 35b and 35c. Further, each of the WDMs 34a, 34b and 34c is connected to a respective electrical/optical up-converter 36a, 36b and 36c, as well as a respective optical/electrical down-converter 38a, 38b and 38c. In turn, each of the electrical/optical up-converters 36a, 36b and 36c is respectively connected to a transmitter 40a, 40b and 40c with a respective RF up-converter 41a, 41b and 41c. And, each of the optical/electrical down-converters 38a, 38b and 38c is respectively connected to a receiver 42a, 42b and 42c with a respective RF down-converter 43a, 43b and 43c.
An important aspect of the present invention concerns the down-converter 26 and its photodiode that are connected with the upstream end 18 of the optical fiber 16. Specifically, this single photodiode (i.e. optical/electrical down-converter 26) is selected to simultaneously accommodate several optical wavelengths (λ) that are within a relatively narrow bandwidth. Preferably, this narrow bandwidth will be between a wavelength λLO and a wavelength λHI, and will be approximately 5 nm.
In an operation of the present invention, all of the downstream points [modems] 14a-c are able to simultaneously transmit to a respective upstream modem 12a-c over the optical fiber 16. By way of example, the modem 14c is chosen for disclosure here as it is considered to be representative of all downstream points [modems] 14. Accordingly, the designation “n” may be any number, and it is used to apply to a particular downstream point 14. With this in mind, consider a radio frequency (RF) signal “fn” that is created at the modem 14c. Once created, the signal “fn” is sent by the transmitter 40c to the electrical/optical up-converter 36c where it is converted from the RF signal “fn” into an optical signal “λn”. WDM 34c then passes the optical signal “λn” to the optical distribution network 32 for direct transmission over the optical fiber 16, without delay. Along with this transmission of the optical signal “λn”, also consider the possibility of a radio frequency (RF) signal “f2” being simultaneously created at the modem 14b. In this case, the signal “f2” is sent by the transmitter 40b to the electrical/optical up-converter 36b, where it is converted from the RF signal “f2” into an optical signal “λ2”. WDM 34b then passes the optical signal “λ2” via the optical fiber 35b to the optical distribution network 32 for direct transmission over the optical fiber 16, together with any other optical signal “λn”.
In the above example, the RF signal “fn” (e.g. f3) will be different from the signal “f2”. Specifically, as envisioned for the present invention, the difference between one radio frequency “fn” and any other radio frequency in the system (i.e. Δf=f(n+1)−fn) will be approximately 100 MHz. In a similar manner, there will also be a difference between one optical signal and another (Δλ). In this case a relationship is envisioned wherein λn=λn−1+Δλn, with each Δλ being predetermined. Specifically, each wavelength difference Δλ may be unique or it may be relatively constant. In any event, each Δλ is preferably greater than approximately 0.5 nm. Importantly, in each instance, the resultant λn will be within the bandwidth between λLO and λHI that is established by the photodiode of optical/electrical down-converter 26.
At the upstream end 18 of optical fiber 16, as the optical signals λn (e.g. λ2 and λ3) are received at WDM 22 they are sent to the down-converter 26 where they are converted back to their original radio frequency “fn”. The radio frequency signals “fn” are then sent, according to their RF frequency, to the appropriate receiver 30 of upstream point [modem] 12 where they are processed.
For a downstream transmission of signals from the upstream point [modem] 12 to the corresponding downstream point [modems] 14, the nature of the single upstream point 12 allows for the more traditional communication arrangement. In particular, only one electric/optical up-converter 24 is needed, and all of the radio frequency signals “fn” can be converted onto a same optical signal wavelength “λ”. This optical signal “λ” is then sent over the optical fiber 16, and is routed by the optical distribution network 32 to a particular downstream point [modem] 14, according to the frequency “fn”. Thus, at the optical/electrical down-converter 38a, radio frequency signal f1 will be routed to downstream point 14a. Similarly, radio frequency signal fn will be routed to downstream point 14c.
While the particular Point-to-Multipoint Simultaneous Optical Transmission System as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.
