GB2393598A

GB2393598A - Bypass path for optical loss measurement signals

Info

Publication number: GB2393598A
Application number: GB0222230A
Authority: GB
Inventors: Derek C Willetts
Original assignee: Alcatel CIT SA
Current assignee: Alcatel CIT SA
Priority date: 2002-09-25
Filing date: 2002-09-25
Publication date: 2004-03-31
Also published as: GB0222230D0

Abstract

System and method for remote supervision in an optical communications system. A bypass path (7) is provided for bypassing optical signals from an optical amplifier (1). Said bypass path (7) is adapted for carrying an optical signal the wavelength of which is substantially far away from the wavelength spectrum of the optical signal travelling through the optical amplifier. The optical signal propagating through the bypass path (7) is used for loss measurement purposes.

Description

/ - 1 System and Method for Remote Supervision of Fault in or beyond

Remote Optical Amplifiers The present invention relates to optical communications. More particularly 5 the invention is related to the detection of loss in the operation of an optical amplifier as well as the detection of damage such as a break or an optical fault in a remote point beyond an optical amplifier used in a transmission line within the optical communication system.

10 BACKGROUND OF THE INVENTION

Optical amplifiers are widely used in optical transmission systems in order to amplify, as much and as many times as desired, the optical signal travelling through a transmission line of the optical system. One known and very commonly used type of optical amplifier is the erbium doped fiber amplifier 15 the structure and operation of which is well known in the related art.

A problem associated with an erbium doped amplifier is that due to its internal design and requirements, it is not possible to accurately assess final optical fiber splice loss values achieved in manufacture. Splice losses are calculated by measurement of optical power before and after the splice, 20 where a non-linear element such as erbium is in the measurement path, it is difficult to measure the optical power accurately and hence calculate the splice loss accurately. Splices generally have to be made blind where erbium fiber exists in the measurement path. A splice is said to be made "blind" where a measurement of optical power is not used to assess the splice loss, 25 instead the splicing machine is directly trusted to make a low loss splice.

Blind splices can be of significantly higher loss than measured splices and the extra loss can give rise to changes in the overall response of the transmission which in turn could result in loss of time and extra cost due to

- 2 the extra work required to eliminate the loss problem. Furthermore if the extra loss is not detected during manufacture, the loss of performance could result in far higher unforeseen operational costs and delays. A through measurement is not possible on the path containing the erbium fiber as this 5 loss is not predictable enough and is a function of input level.

A monitor could be included in the form of a standard TAP coupler. A TAP coupler is a split ratio coupler with a low loss for the signal and a high loss for the monitor path, but the TAP coupler signal path loss is still high compared to the typical value of a splice loss, thus this would induce extra losses too 10 high to be acceptable.

Another problem associated with the use of optical amplifiers is that breaks or other damages of the optical cable occurring beyond the remote pumped amplifier cannot be easily or accurately detected when in service.

An optical time domain reflectometer (OTDR) is an apparatus that is known 15 in the related art, which is used to determine location and nature of faults or breaks of fibers in an optical cable, by analyzing variations of the backscattered power level. However for a damage occurring beyond the remote optical amplifier, OTDRs cannot be used, as there is usually an optical isolator in the amplifier path which will block OTDR signals.

20 A known partial solution is to use power feed information to locate the fault.

However, this solution has the drawback that it is only effective when there is a complete cable break and thus it does not provide information on optical only faults other than complete breaks. An example of such fault is that damage can occur to a cable that only breaks the optical path, such damage 25 can be caused by trawlers and ship's anchors. It has even been known that a submarine has rested on a cable causing damage.

It is therefore desired that remote supervision of cable breaks or other faults therein, as well as remote detection of loss due to manufacture of the optical amplifier are made possible in an efficient and cost effective manner.

- 3 DESCRIPTION OF THE INVENTION

The above objective is reached by using the solution proposed by the present invention according to which a bypass path for the OTDR signal is 5 provided, said signal being transmitted at a wavelength outside of the normal operating wavelength range.

Accordingly it is an object of the present invention to provide a remote supervision arrangement for an optical communications system, said system comprising an optical transmission line for transmitting optical signals at a 10 predetermined wavelength spectrum and at least one optical amplifier adapted for amplifying said optical signal, characterized in that a bypass path is provided for bypassing said optical amplifier, said bypass path carrying an optical signal having a wavelength substantially far away from said predetermined wavelength spectrum.

15 According to an aspect of the present invention, said bypass path is coupled at each end to the optical transmission line using a wavelength division multiplexing -WDM- coupler.

According to another aspect of the present invention, the wavelength of the optical signal travailing through said bypass path is about or substantially 20 equal to 161 Onm.

According to still another aspect of the present invention, said bypass path, is adapted for further bypassing an isolator included in said optical transmission line.

Another object of the present invention is that of providing a method for 25 remote supervision for an optical communications system, said system comprising an optical transmission line for transmitting optical signal at a predetermined wavelength spectrum and at least one optical amplifier for amplifying said optical signal, characterized in that an optical signal having a

wavelength substantially far away from said predetermined wavelength spectrum bypasses said optical amplifier through a bypass path.

These and further advantages of the present invention are explained in more detail in the following description as well as in the claims with the aid of the

5 accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation of an optical bi-directional communication arrangement comprising an optical amplifier wherein for the 10 sake of simplification only the relevant parts for understanding the invention are shown.

Figure 2 is the schematic representation of figure 1 further including a bypass path incorporated in a transmission line according to the solution of the present invention.

EXAMPLES OF PREFERRED EMBODIMENTS

As shown in figure 1, a conventional optical amplifier is disclosed comprising a first optical path A for a first signal direction and a second optical path B for a second signal direction thus representing a bi-directional communication 20 arrangement between a transmitter terminal and receiver terminal (not shown) as known in the related art. An optical amplifier 1 is incorporated in an optical fiber path B. said optical amplifier 1 being formed by a specific length of erbium doped fiber for amplification of the optical signal travailing therethrough. In the example shown in figure 1, the direction of the incident 25 signal inputted to the optical amplifier 1 is shown by arrow 3, that is from the right to the left of the figure. An optical isolator 2 is interposed on the optical path B so as to avoid or reduce reflection of scattered light in a known way.

An energy source comprising, for example, a laser pump (not shown) generates excitation light which is incident onto the optical amplifier on the

- 5 same fiber (or path) B as the output thereof and in a direction 4 opposite to the direction of the signal 3 so as to contribute to the amplification of the optical signal. In a typical optical amplifier as disclosed in figure 1, the wavelength of the signal propagating through the transmission line is in the 5 range of 1558 nm.

The internal topology of the optical amplifier requires that the input and output losses are minimized so as to enhance performance, both in terms of pump power delivery and signal performance. However, as mentioned above, an accurate measurement of input and output losses including the 10 losses of all splices is difficult since there is no internal monitor.

The solution to this problem is achieved by incorporating fused fiber WDM couplers as illustrated in figure 2. These couplers 5 and 6 are located at respective ends of an optical path 7 and provide multiplexing of the optical signal optical travailing through the optical path B and the optical amplifier 1 15 with the optical path 7. The WDM couplers 5 and 6 are located at appropriate points at the input and the output of the optical amplifier 1 respectively thus providing bypassing of a specific light wavelength through the bypass path 7. In this arrangement, the losses of the path 7 can be easily measured before the optical amplifier unit is assembled into its final casing, 20 thus providing a through path reference for the combined input and output losses of the amplifier path.

The wavelength of the loss measurement path, i.e. the bypass path 7, is chosen so as to be sufficiently far away from the wavelength of the optical signal propagating through the optical signal path B. such that a good 25 isolation is obtained at the bypass path 7 when in service, and a low loss is obtained in the bypass path 7 and the signal path B. Preferably the wavelength of the signal travelling through the bypass path 7 is 1610nm.

- 6 Typical losses for the WDM multiplexers 5 and 6 are in the order of 0. 05 dB compared to that of 0.25 dB for a TAP coupler component., The arrangement of figure 2 may also be used in order to solve the problem associated with the OTDR as stated above. In fact, in a conventional optical 5 amplifier as shown in figure 1,0TDR signals cannot pass through the erbium doped fiber 1 and the isolator 2 because the back-scattered signal used for performing the fault detection are prevented from travelling through the isolator 2. However the OTDR signals can travel through the bypass path 7 and provide fault detection beyond the remote optical amplifier 1. Here again 10 the wavelength for transmitting the OTDR signal is chosen to be sufficiently far away from the wavelength of the optical signal such that a good isolation is obtained.

Preferably, the wavelength of the signal travelling through the bypass path is 1610nm. 15 It is to be noted that this operation, namely the propagation of an isolated signal through the bypass path 7 is an out-ofservice operation and it does not affect the traffic along the transmission line.

The choice of an appropriate wavelength for transmission through the bypass path 7 is made preferably in accordance to the following criteria: - that the transmission loss of the system fiber is low, hence maximizing the OTDR range; - that the sensitivity to detecting internal product fiber bend loss is maximized; 25 - that readily available WDM couplers are as much as possible usable in L band; - that readily available Laser sources are as much as possible usable in OTDR equipment.

Having all the above criteria in mind, it is noted that a wavelength of 1610 nm 30 is preferable as it encompasses all the advantages mentioned above.

Nevertheless the solution herein proposed is not to be understood to be

/ - 7 limited to the use of this wavelength and other wavelengths considered by those skilled in the art of being capable of producing the same results are understood to fall within the scope of the present invention.

Claims

- 8 CLAIMS

1- A system for remote supervision for an optical communications system, said system comprising an optical transmission line for transmitting 5 optical signal at a predetermined wavelength spectrum and at least one optical amplifier (1) adapted for amplifying said optical signal, characterized in that a bypass path (7) is provided for bypassing optical signal from said optical amplifier (1), said bypass path (7) carrying an I optical signal having a wavelength substantially far away from said 10 predetermined wavelength spectrum.

2- A system according to claim 1, wherein said bypass path (7) is coupled at each end to the optical transmission line using a wavelength division multiplexing -WDM- coupler (5; 6).

3- A system according to any one of the previous claims, wherein the 15 wavelength of the optical signal travailing through said bypass path (7) is about or substantially equal to 161 Onm.

4- A system according to any one of the previous claims, wherein, said bypass path (7), is adapted for further bypassing an isolator (2) included in said optical transmission line.

20 5- A method for remote supervision for an optical communications system, said system comprising an optical transmission line for transmitting optical signal at a predetermined wavelength spectrum and at least one optical amplifier (1) for amplifying said optical signal, characterized in that an optical signal having a wavelength substantially far away from said 25 predetermined wavelength spectrum bypasses said optical amplifier (1) through a bypass path (7).