IL146691A - Optical communication system - Google Patents

Description

OPTICAL COMMUNICATION SYSTEM Field of the Invention The present invention relates to multi-line, multi-wavelength transmitters and receivers for dense wavelength division and multiplexing (DWDM) optical communication. More particularly, the invention is directed toward providing dense transmission channels, providing users with wavelength communication lines at a desired bandwidth in two directions, namely, one downstream from a sender or a central or main office (referred to herein as "main office") to an end user/subscriber, and one upstream from an end user/subscriber to a central or main office.

Background of the Invention In optical communication systems, the portion closest to the end-user/subscriber, also known as the access network, is characterized by the need to bring a separate optical channel to each subscriber, allowing for the use of a wide band and for bi-directional optical communications capability. In order to efficiently utilize the existing, available wavelength range or bandwidth, a common fiber optical link or loop serves many subscribers, each subscriber picking up its own designated downstream wavelength. This arrangement calls for very dense line distribution, wherein the stability of the line spacing in varying environmental conditions is of major importance.

Another problem encountered when using closely packed spectral lines is the difficulty in separating the lines at the receiver end. The requested separation is beyond the present state of the art of existing thin film or grating technology. The introduction of ultra-dense lines, e.g., of 1-2 GHz separation or less, calls for new generation and separation methods.

General access communication in an optical fiber network requires at least 100 Mbps, and preferably up to 2 Gbps, in each direction, upstream and downstream of the optical link to the subscriber. The system may designate two wavelengths for each subscriber, one modulated with the downstream data and the second unmodulated, to be used for modulation at the subscriber's station and for sending the modulated data back through the fiber. The two downstream and upstream channels may be close to each other in their wavelengths. For example, both channels may be in the 1500 nm range or in the 1300 nm range, or spectrally separated, one in the 1500 nm range and the other in the 1300 nm range.

» Accurate wavelength control of many closely spaced communication channels is a key challenge in implementation of ultra-dense, multi-line optical communication systems. A few approaches have been proposed in the past for the generation of a closely spaced series of spectral lines having predetermined wavelengths and spacings between them, referred to as a "wavelength comb." These are described in Israel Patent Applications Nos. 141727 and 144498, and are incorporated herein by reference.

A dense multi-wavelength source, supplying a multitude (up to hundreds or even thousands) of different wavelengths, is placed in a central or main office. Each of the different wavelengths or channels is separated, spatially or angularly, so it can be propagated in a dedicated optical fiber. Depending on the data rate required by the specific end user, the main office designates a specific channel or a number of channels to that subscriber. Then, the data designated for each of the specific subscribers is modulated at the central office on the specific channels. Sometimes the same data is sent to a multitude of subscribers ("broadcasting"); in this case, a single modulator may be used for a few channels. Some of the channels may be left unmodulated in the central office, after being transmitted from the wavelengths source. These channels are dedicated for upstream communication, namely, for data sent by the subscribers to the main office. Furthermore, these channels may have wavelengths in the same wavelength band as that of the modulated channels, or they may be of different wavelength bands, e.g., one in the 1.3 micro-meter band, and the other in the 1.55 micro-meter band.

Both the modulated light, containing data from the main office to the subscribers, and the unmodulated light, dedicated for modulation by the subscribers for data to be sent from the subscribers to the main office, can be combined to propagate in the same waveguide. This is achieved by a multiplexing device, which combines the multitude of different wavelengths propagating in a plurality of waveguides into a single waveguide.

Some other unmodulated channels emerging from the wavelength source can be inserted into another, second, waveguide. These channels may have the same wavelengths as some of the modulated channels propagating towards the subscribers in the first waveguide. These channels are later used as part of the demultiplexing (or dropping) subsystem at the subscriber's end, or as parts of the all-optical, selective amplification of the transmitted data. These channels serve as reference wavelengths between the source/transmitter and the receiver, and only a single or few of these wavelengths that are transmitted can generate multiple combs for reference and selective amplification. These reference wavelengths are also called "pumps" for optical Brillouin amplifiers, as will be described in detail below.

All of the modulated wavelengths, as well as the unmodulated wavelengths, pass through one or more optical waveguides to the subscriber's region; either a loop or a single waveguide passes through each subscriber premises, where it delivers the dedicated wavelengths to the subscriber. As the state of the art today is about 10 GHz of a drop filter bandwidth, it cannot adequately handle the 1 GHz separation of channels, and the process is done in two steps. The first step is to drop a group of wavelengths having a total width of, e.g., 10 GHz (10 channels with a 1 GHz separation), using a grating or another selective filter, followed by a novel separation module where the separation is carried out by a specially designed set of, e.g., 10, parallel Brillouin amplifiers.

These Brillouin amplifiers each amplify a single wavelength. The Brillouin amplification occurs when a narrow band optical seed signal, having the same frequency as the back-scattered wave, is propagated in the opposite direction of the pump light in an optical waveguide. In an optical medium, the interaction between the seed or signal and the pump enhances the acoustic grating initially generated by the pump, and this increases the back-scattering of the pump into the seed signal, thereby amplifying the signal. This effect can be used to selectively amplify desired signals for further processing, detection or regeneration, where all other signals remain low and do not affect the system.

The central, amplified wavelength for each Brillouin amplifier is designed by selecting one wavelength from a wavelength comb as a pump for a group of Brillouin amplifiers, where the low spacing line is defined by material selection. The amplifiers are each made of a different material, selected so that each amplifier amplifies a channel (a single wavelength), although the pump wavelength is the same for all the amplifiers, matching the channels to be separated/amplified, each having an input end and an output end, the pump line fitting all the amplifiers. The pump wavelength is supplied from the main office, serving as the energy source for the amplification and as a reference wavelength. The pump line may be different for each set of amplifiers, and may be created in the main office as a comb. The width of the pump line can be made to fit the modulated line width data line to be amplified by the use of a few adjacent, very closely packed pump lines, creating a wider, combined line. These lines are created in parallel and are transmitted as one wide pump line.

When the dropped wavelengths reach the subscriber, the data-containing lines are detected by a light receiver/detector and, from this point, are treated as electronic data signals. The unmodulated channels are treated in an "all optical" way, namely, they are modulated by the subscriber's data through an optical modulator and sent back to the main office in either the incoming waveguide or additional waveguides.

Since the distance between the two stations, source/transmitter and receiver/detector, is in many cases large, and losses of the optical signals are high, all of the wavelengths, modulated, unmodulated and pump, may have optical (fiber or solid state) amplifiers along the lines and at their two ends.

The system can have either a single main office or multiple main offices carrying data in multiple directions. When multiple main offices are employed, the system is built of duplicates of the single main office unit, and all share the same pump lines which serve as pumps for the Bruillouin amplifiers and a reference wavelength for the light sources in all of the main offices. The light source and pump are generated only at one of the main offices, and are shared by all of them.

Disclosure of the Invention It is therefore an object of the present invention to provide an optical communication system having the ability to generate a very large number of lines, to stabilize the central wavelength, to generate combs with controllable, narrow spacing and to deliver the generated spectral lines out of the light source, each spectral line in a separate fiber and ready for individual modulation by its dedicated modulator.

It is a further object of the present invention to provide adequate separation of the narrowly spaced spectral lines at the receiver end.

Accordingly, the invention provides an optical communication system between a main office and a user/subscriber, comprising at least one source/transmitter for producing a plurality of optical radiation output signals, the output signals having predetermined wavelengths and being fed to output waveguides, the source/transmitter being capable of maintaining constant intra-wavelength spacings between the output signals; a plurality of modulators, each coupled to one of at least several of the output waveguides and leading to a multiplexer; two receiver/demultiplexer units, each followed by a detector unit, and at least one pump line for passing optical radiation from the source/transmitter to the receiver/demultiplexer units.

Brief Description of the Drawings The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.

With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

In the drawings: Fig. 1 is a schematic illustration of a first embodiment of a two-way communication system according to the present invention; Fig. 2 is a detailed illustration of the structure of a very dense demultiplexer and detector; Fig. 3 is a schematic illustration of a multi-user/subscriber system; Fig. 4 depicts a second embodiment of a two-way communication system according to the invention; Figs. 5 and 6 illustrate the principle of the generation of the spectrally widened source, and Fig. 7 shows the spectrum of the widened pump source pump needed for the separation of modulated lines.

Detailed Description Referring now to Fig. 1, there is shown an embodiment of a communication system according to the present invention, utilizing a single source/transmitter 10 and two dense receiver/demultiplexer units 12, 14, followed by electronic detector units 16, 18. Receiver/demultiplexer unit 14 and detector unit 18 are located at the subscriber's end and receiver/demultiplexer unit 12 and its detector unit 16 are located at the sender, or main office, end, either as part of, or separated from, source/transmitter 10. As explained above, source/transmitter 10 generates optical signals of wavelengths λι to λ,„ each being transmitted over a separate waveguide 201 to 20,„ such as, e.g., fibers. Each waveguide 20] to 20„ leads through a modulator 22j 7 146,691/2 to 22„ (Modi to Mod„) to a common multiplexer 24, where the outputs from the modulators are joined together, and from there leads, via a single waveguide common line 26, to the dense demultiplexer unit 14, acting as a receiver, and, in turn, to the detectors 18i to 18„ . Output lines 281 to 28„ exit from source/transmitter 10 and lead to the same common point in multiplexer 24. Lines 28] to 28„ are unmodulated output lines.

Similarly, the output from dense demultiplexer unit 14 is constituted by a first group of unmodulated lines 30i to 30„, leading through subscriber modulators 32i to 32„ and carrying the subscriber's data to a common point R, where they are joined together, and from there, via return line 34, to the dense demultiplexer unit 12 and its detector 16. Output lines 361 to 36„ carry modulated data at wavelengths λ ι to λ„, and corresponding data on waveguides 20] to 20„ are transmitted to optical detectors 181 to 18„ at the subscriber's end. A pump line 38, which is common to the entire system, serves as a reference and connects source/transmitter 10 with dense demultiplexer units 12 and 14. Optionally, the system may also include several amplifiers 40, depicted by broken lines at various locations along the optical transmission lines.

Turning now to Fig. 2, the structure of dense demultiplexer unit 14 is shown in detail. The common line 26, carrying all of the wavelengths, enters the demultiplexer unit 14 via a splitter or optical circulator 42 and feeds a plurality of Brillouin amplifiers 44i to 44„, connected in parallel. Each of input pump lines 461 to 46„ is commonly connected to pump line 38 of the system via a splitter 48 and passes, via a splitter 50, at the end opposite to their inputs from common line 26, to each of the amplifiers 44] to 44„, all having the same pump wavelength. The input pump lines 461 to 46„ are connected to the amplifier output lines 36 j to 36„ through circulators 51 i to 51„.

The Brillouin amplification occurs when a narrow band seed signal, having the same frequency as the back-scattered wave, is propagated in the opposite direction of the direction of propagation of the pump. In an optical medium, the interaction between the seed signal and the pump enhances the acoustic grating initially 8 146,691/2 generated by the pump, and this increases the back-scattering of the pump into the seed signal, thereby amplifying the signal. This effect can be used to selectively amplify desired signals for further processing, where all other signals remain low and do not affect the system. The central, received wavelength is designed by material selection. For example, different doping of germanium oxide in silica fibers changes the refraction index n, resulting in a preselected frequency difference vD = 2nVs/ p for each one of the amplifiers. Strain and temperature are used to fine-tune the frequency difference to the desired wavelength. A selective amplifier allows detection of the amplified signal in the presence of all the others as background noise. The line width is material- and pump-dependent, and is selected in a manner appropriate to the designed bandwidth of the communication channel. In cases where the single pump line width is not wide enough to contain the width of the data channel, a widened pump line is used, combined of an additional few closely packed lines, as will be described below with reference to Fig. 7. The amplified signals in each output line 36 [ to 36„ are eventually separated by Brillouin amplifiers of different materials, pumped by the system's internal pump line. Each of the multitude of further Brillouin amplifiers is dedicated to a different wavelength by the use of different materials, e.g., by different doping of germanium oxide in silica fibers so as to have a matching preselected frequency difference vD = 2nVsIXp at the common pump, serving all of the amplifiers. The doping strongly affects the index n. Strain and temperature are used for fine tuning, affecting both n and the velocity of sound.

Fig. 3 illustrates a multi-subscriber system fed from a line 26 of a common source and having a single pump line 38 and return line 34. Each of the demultiplexer units 14 is connected to lines 26 and 38 via splitters/optical circulators 42, 48 and to the return line 34 via modulators 32 to 32„ and directly to the subscriber's detectors 18i to 18„ through output lines 36) to 36„. Between two adjacent splitters/optical circulators 42 or 48 on lines 26 and 38, there are affixed reflection filters or gratings 52! to 52„ and 531 to 53„. The gratings may be constituted by per se known Bragg gratings, or by arrayed waveguide gratings (AWGs), each produced to provide a predetermined wavelength separation or 9 146,691/2 spacing, e.g., a separation of 10 GHz from each other, while the lines exiting modulators 32 j to 32„ are separated by, e.g., 1 GHz. The multi-subscriber system may contain a plurality of dense demultiplexer units 14 and 18, as shown, wherein each of the plurality of units is fed separately by the data wavelength and unmodulated wavelength via line 26 and obtains the needed pump via line 38. Line 38 may carry one or many pump wavelengths. In the case of a plurality of wavelengths, they can be generated as a comb having a spacing of, e.g., 10 GHz, as described in Israel Patent Application Nos. 141,727 and 144,498.

A further embodiment of a two-way communication system is illustrated in Fig. 4. Here, the system is duplicated with all its parts in two locations, but is served with a single, common optical pump line for both systems. In each system, there is a source/transmitter 10, 10' and a dense demultiplexer unit 12, 12', followed by a detector at each end of the communication network. The waveguides 20 j to 20„, 20 \' to 20„ ', of the source/transmitters 10, 10' are lead through modulators 22i to 22„, 22\' to 22„ ", to common multiplexers 24, 24' and to the dense demultiplexer units 12, 12', followed by a detector. A single pump line 38 interconnects each one of the source/ transmitters 10, 10' with one respective dense demultiplexer unit 12, 12', serving as the reference and pump of the two-way system.

The formation of a broad bandwidth line source for the pump, as broad as needed by the bandwidth of the transmitted modulated data line, is shown in Figs. 5, 6 and 7. Figs. 5 and 6 illustrate the layout of the line generation, where the pump line λ0 is injected via optical splitters 60 to a multitude of different materials 62, each having, for example, different doping of germanium oxide in silica fibers, resulting in a preselected frequency difference between the pump and the SBS (Simulated Brillouin Scattering) generated light for each one of the stages. The injection can be done in parallel, as depicted in Fig. 5, or in series, as depicted in Fig. 6. The SBS generators are each made of a different material, selected so that each gives the required wavelength for a wide source, as shown spectrally in Fig. 7. This Figure shows, in the spectral dimension, the SBS generation of a plurality of lines having different spacing from the λ0 pump line, using a different composition of materials for the SBS generating parts, The plurality of lines produces a widened line of n times the individual line.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (11)

11 146,691/2 WHAT IS CLAIMED IS:

1. An optical communication system between a main office and a user/subscriber, comprising: at least one source/transmitter for producing a plurality of optical radiation output signals, said output signals having wavelengths spaced-apart by equal distances and being fed to output waveguides, said source/transmitter being capable of maintaining constant intra-wavelength spacings between said output signals; a plurality of modulators, each coupled to one of at least several of said output waveguides and leading to a multiplexer; two receiver/demultiplexer units, each followed by a detector unit, and at least one pump line common to the entire system for passing optical radiation from said source/transmitter to said receiver/demultiplexer units.

2. The optical communication system as claimed in claim 1, wherein each of said receiver/demultiplexer units comprises a plurality of parallel, interconnected Brillouin amplifiers, each having an input end and an output end.

3. The optical communication system as claimed in claim 2, wherein said pump line is connected to the output ends of said amplifiers.

4. The optical communication system as claimed in claim 1, wherein at least one of said receiver/demultiplexer units is located at or adjacent to said source/transmitter.

5. The optical communication system as claimed in claim 4, further comprising a second source/transmitter, each of said source/transmitters being located adjacent to said source, at least some of said second modulators being located at or adjacent to one of said two receiver/demultiplexer units.

6. The optical communication system as claimed in claim 1, further comprising: a plurality of receiver/demultiplexer units, each connected to said single output waveguide via a first circulator and connected to said pump line via a second circulator; 12 146,691/2 a first plurality of reflection filters, each coupled between two adjacent first circulators, and a second plurality of reflection filters, each coupled between two adjacent second circulators.

7. The optical communication system as claimed in claim 6, wherein said first and second pluralities of reflection filters are optical gratings or thin films.

8. The optical communication system as claimed in claim 1, wherein at least one data line and at least one return line comprising a modulator is allocated to each user/subscriber.

9. The optical communication system as claimed in claim 2, wherein said Brillouin amplifiers are produced to amplify only one selected wavelength of the wavelengths passing therethrough, using one pump wavelength and different materials for the amplifiers.

10. The optical communication system as claimed in claim 1, further comprising at least one amplifier connected along said at least one waveguide.

11. An optical communication system as claimed in claim 1, substantially as herinbefore described and with reference to the accompanying drawings. For the Applicant WOLFF, BREGMAN AND GOLLER