Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-09-07
2003-10-28
Tweel, Jr., John A. (Department: 2632)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06639701
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fiber optic transmission. In particular, the invention relates to a method of providing supervisory signals to optical amplifiers in an optical transmission line. This is of particular relevance to fiber optic transmission systems used for long distance transmission, for example, in undersea cables used for intercontinental signal transmission.
BACKGROUND OF THE INVENTION
An example of a fiber optic transmission system is illustrated schematically in FIG.
1
. The system comprises a transmission fiber (
5
) carrying signals at eight carrier wavelengths (
6
). The transmission fiber extends from a first main station, or send station, (
1
) to a second main station, or receive station, (
2
). Spaced along the transmission fiber (
5
) are add/drop multiplexers (
4
). At each add/drop multiplexer (
4
) the signal at a selected one of the eight carrier wavelengths is dropped to a minor station (
3
), and replaced with a new signal at the same carrier wavelength from the minor station (
3
).
To maintain signal strength on a long transmission line, it is necessary to amplify the signal at periodic intervals. It is possible to do this electrically, by conversion of the optical signal on the fiber to an electrical signal followed by amplification of the electrical signal and then conversion back to an optical signal. However, it is generally preferred to use optical amplification methods, which have the advantage that there is no need to convert the transmitted optical signal to an electrical signal until it needs to be processed at a receiving station. In the illustrative example depicted in
FIG. 1
, there are 200 optical amplifiers between the main stations (
1
), (
2
), the add/drop multiplexers being spaced at forty amplifier intervals.
The structure of a typical optical amplifier unit (or repeater) is illustrated in FIG.
2
. The input fiber (
5
a
) of the fiber transmission line (
5
) carries the input signal, which first enters a fiber optic coupler (
14
a
). Here the signal is split into two unequal parts (typically in a ratio of 1:20), the smaller part of the coupler output being branched off to a monitor diode (
17
a
). The main output of the coupler (
14
a
) passes through into an erbium doped fiber amplifier (EDFA) (
11
). The amplified output from the EDFA (
11
) passes into a first input (
19
a
) of a multiplexer (
16
), which is adapted such that substantially the entire signal at the carrier wavelength passes out through a first output (
19
b
).
In this example the carrier wavelength are selected to lie close to 1560 nm. An EDFA provides amplification for light at 1560 nm when it is pumped with light at 1480 nm. The pump light is provided by an appropriate pump laser (
12
), which may be, for example, an InGaAsP laser. Pumping light is provided through second port (
19
c
) of the multiplexer (
16
), the multiplexer being adapted such that substantially all the pumping light is transmitted to the EDFA through first input (
19
a
) (serving as an output in this direction).
The multiplexer (
16
) is adapted such that at the carrier wavelengths, substantially all the light is transmitted directly from first multiplexer input (
19
a
) to the first output (
19
b
) with no transfer across the coupler to the second port (
19
c
), whereas at the pumping wavelength, there is a substantially complete transfer of pumping light across the coupler from second port (
19
c
) to the first input (
19
a
). It is possible through alternative structures to pump the EDFA at the input, or in the middle, rather than at the output side. It is possible to pump, for example, at both input and output sides—this can be advantageous in the case of failure in one pumping connection.
As the EDFA amplifies in both directions, an isolator (
15
) is required in the output path of the carrier wavelength signal to prevent instability and interference, which could otherwise result from reflections at the output port of the repeater. The isolator (
15
) has very low attenuation in the forward direction (typically <1 dB) and very high attenuation in the reverse direction (typically >25 dB). The output from the isolator (
15
) is provided as input to a further coupler (
14
b
), from which the greater part of the signal is transmitted back out onto the transmission fiber (
5
) at output (
5
b
), a small part of the signal being split off to a further monitor diode (
17
b
).
Monitor diodes (
17
a
), (
17
b
) hence enable monitoring of the input and output power levels of the EDFA. The input power level monitor (
17
a
) monitors the light received from the previous amplifier via the cable. Any fault in the cable or in the previous amplifier will change the input power. As the previous amplifier has had its output power level monitored (by a monitor diode (
17
b
)), the fault can be located to the previous repeater or to the cable in between. Either monitor diode may also be used as a means of receiving supervisory signals for the supervisory section (
13
): advantageously, the input monitor diode (
17
a
) is used for this purpose, as if the repeater itself is faulty the output monitor diode (
17
b
) may not be able to detect the supervisory signals. Separate photodiodes (not shown) are provided so that the power of the pump laser (
12
) can be monitored at the supervisory section (
13
).
It is a necessary feature of such extended fiber optic systems that the repeater units can be subjected to external control: for example, to adjust the gain of the amplifiers in order to optimise a given signal or correct an imbalance. For such adjustment to be possible, it is also necessary for signals indicative of the status of the repeater to be fed back and/or forward to an external control point. This is also desirable in the event of a physical break or other fault in the cable: the position of the break or fault in terms of the repeaters on the cable can be determined by use of such feedback. Normally, such responses signals are sent back to the terminal that initiated the response. This can be achieved, for example, with a
FIG. 2
repeater by modulation of the pump laser power by supervisory section (
13
). This will result in modulation of the output power of the traffic signals output by the EDFA, this modulation being detectable at a receiving station.
An important criterion in design of a system to transmit supervisory information between external control point and repeaters is the simplicity of the resulting repeater structure, and another is the minimization of the effect (e.g. noise) on the traffic signals themselves. High levels of reliability are also important, especially where failure necessitates the repair of an undersea cable. As the nature of the external control point (at a terminal station, where signal generation is straightforward and sophisticated signal processing possible) and of the repeater (where simplicity and reliability are of particular importance) are very different, different solutions are required to satisfy the same criteria. It is therefore preferable to use different approaches to send information from the repeaters to the external control point from those used to send supervisory information or instructions from the external control point to the repeaters.
Provision of information from the repeater to the external control point is generally achieved by a passive loop-back system. An exemplary arrangement is described in Hirst et al, Electronics Letters 29 (3): 255-6, 1993. For the signal travelling in one direction along the trunk of a fiber optic cable system, an optical coupler is provided at the output side of the amplifier, and a small part of the entire signal amplified is split off. The split-off signal is then looped back by means of a further optical coupler on to the fiber carrying signals along the trunk in the other direction. Typically, each coupler will provide about 10 dB of loss for this loop-back path, with the result that approximately 1% of the signal amplified is looped back. The transmitted test signal is thus
Alcatel
Tweel, Jr. John A.
Ware Fressola Van Der Sluys & Adolphson LLP
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