Optical: systems and elements – Optical amplifier – Raman or brillouin process
Reexamination Certificate
2001-01-18
2002-04-16
Hellner, Mark (Department: 3662)
Optical: systems and elements
Optical amplifier
Raman or brillouin process
C359S341440
Reexamination Certificate
active
06373621
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus providing safer operation of Raman amplifiers (also known as distributed Raman amplifiers), especially in communication systems.
BACKGROUND OF THE INVENTION
Distributed Raman amplifiers are become increasingly important features of today's high bandwidth optical transmission systems. Raman amplification operates by sending a pump wavelength down the system fibre, producing additional amplification of the optical signal using the silica of the fibre as a gain medium.
In order for this to be effective, substantial pump powers have to be used. Typical pump powers are up to 1 Watt optical in the 1420-1480 nm band, and when combined with the signal, the laser power could in principle be as high as 1.5 Watts. The system is preferably safe in normal operation, since no light leaks from the fibre/cable/connector assemblies, However, when open to view, the light is Laser Class 4 (dangerous by direct and scattered viewing), and to conform to international standards, requires a closed and interlocked environment, specially trained personnel, and equipment such as laser safety goggles, for handling.
In a real communications system, however, this radiation could potentially be exposed in the open by cable break, or within a telecom hut by untrained personnel pulling connectors. The danger, particularly to eyes, is enhanced since the radiation is entirely invisible but is partially transmitted by the eye to the retina.
Although a great deal of work has been undertaken on the theory and application of Raman amplifiers, little has been done to implement appropriate safety systems to guard against accidental exposure to pump radiation in real communication systems.
Traditionally, laser protection in communication systems has been achieved in two ways, either individually or in combination.
Firstly, the system can look for a back reflection from the fibre that indicates a fibre break, or broken connection, and turn the optical signal amplifier(s) off when this back reflection is seen. This system is used for example in Ebrium Doped Fibre Amplifiers (EDFAs) to bring high output power down to a safe level in the event of a fibre break.
A second mechanism is to look for a loss of signal into the pre-amplifier in the system and use this as an indication of failure.
Both of these systems have significant limitations when used in conjunction with distributed Raman amplifiers.
In a Raman amplifier, the level of amplified spontaneous back-scatter in the fibre from a high level pump signal is too high to allow a cable break to be detected in this way. This spontaneous backscatter results from reflections from imperfect system components, Rayleigh scattering, and stimulated Brillouin scattering and cannot be avoided.
Looking for loss of signal power at the amplified wavelengths is potentially attractive, but in the absence of input signal a Raman amplifier can still generate significant amplified spontaneous emission at signal wavelengths. For example, a single wavelength in a functioning transmission system may have an optical signal to noise ratio (OSNR) of 20 dB in the bandwidth of the signal. However, the noise can occupy a bandwidth 300 times greater than the signal, so the ratio of total noise power to signal power at the output of a Raman amplifier can be less than unity. As a result, a reliable indication of loss of signal by direct power measurement cannot be made.
Embodiments of the present invention therefore aim to provide methods and apparatus enabling safer (improved) operation of Raman amplifiers, especially in optical communications systems, i.e. methods and apparatus which address and overcome, at least partially, one or more of the problems associated with the previous safety systems. Thus, embodiments of the present invention aim to provide methods and apparatus which offer improved protection, e.g. in communication systems, from accidental exposure to Raman amplifier pump lasers.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is provided a method of operating Raman amplification pump lasers in an optical communication system, the method comprising the steps of:
pumping a length of optical fibre in a first direction with the output of a first Raman amplification pump laser, from a first end of the length of fibre towards a second end;
pumping the length of fibre in a second, opposite direction with the output of a second Raman amplification pump laser, from the second end towards the first end;
modulating the output of the second pump laser with a characteristic modulation;
detecting at the first end a predetermined parameter (i.e. characteristic or feature) of the characteristic modulation; and
reducing the pumping of the length of fibre in the first direction in response to ceasing to detect at the first end the predetermined parameter (i.e. in response to “losing” the characteristic signal from the second pump laser).
The characteristic modulation may take a wide variety of forms. For example, it may be a digital modulation, such as a pulse pattern or profile superimposed on the laser output. Such a pattern can be recognised (detected) remotely using signal processing techniques.
Preferably, however, the modulation takes the form of modulating the output power with a characteristic frequency, and detection involves simply detecting a signal at the appropriate characteristic frequency, received from the “other” pump laser.
Thus, as long as the signal from the second pump laser at the characteristic frequency (i.e. a unique identification frequency) is being received at the first end of the fibre, the fibre is known to be continuous (i.e. unbroken) from its first end to its second end.
More generally, as long as the parameter (i.e. characteristic) of the modulation applied to the second pump laser output can be detected (recognised) at the first end, the fibre link in between the two pumps is known to be intact. The method can therefore provide the advantage that the presence of the signal from the second pump laser, and therefore the continuity of the fibre, can be detected even when high levels of noise are being received at the first end as a result of back-scatter of the output from the first pump laser. Equally, when a break or some other kind of discontinuity in the fibre occurs, the loss of the signal at the characteristic frequency from the second pump laser can be detected at the first end even in the presence of strong reflection from the break of the output signal from the first pump laser.
Thus, the continuity of the length of optical fibre can be continuously monitored without the addition of significant hardware, complex control procedures, or specialised fibre cabling. The inventive method therefore provides a simple, cost effective way of safely operating Raman pump lasers in communication systems.
It will be appreciated that the terms “first end” and “second end” are not intended necessarily to mean that the length of fibre has any discernible “ends” as such. These terms are simply used to denote positions along the length of fibre to assist in specifying the direction of pumping from the first and second lasers. In practice, the pump lasers will typically be coupled to the length of fibre using appropriately arranged fibre optic couplers.
When the parameter can no longer be detected at the first end (e.g. when the system detects a loss of the signal from the second pump laser at the first end), the pumping of the fibre in the first direction is reduced.
Reducing the pumping may involve a partial reduction in pumping power, or alternatively a full radiation to zero (or substantially zero), i.e. the pumping may be ceased. Preferably, the reduction may be achieved by reducing the power output of the pump laser, or by re-routing the power output. Preferably, when the pumping is arranged to cease in response to the loss of the signal or parameter detection this cessation simply involves the switching off/powering down of the first pump laser. This can
Beckett Douglas James Stewart
Large Timothy A.
Hellner Mark
Lee Mann Smith McWilliams Sweeney & Ohlson
Nortel Networks Limited
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