Coherent light generators – Optical fiber laser
Patent
1992-11-18
1994-11-08
Scott, Jr., Leon
Coherent light generators
Optical fiber laser
372 18, 372 39, 372102, 372 64, H01S 330
Patent
active
053633865
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to optical waveguide lasers and in particular to soliton lasers.
2. Related Art
Ultra-fast light pulse sources will be key components in future high bit rate telecommunications systems and soliton pulse sources in particular are recognised as being valuable for long distance high bit rate systems. A current requirement is for the generation of solitons with picosecond optical pulses and sufficient peak powers for soliton propagation in the 1.55 .mu.m silica optical fibre transmission window.
Although soliton pulses are robust to quite large effects of both chirp and phase noise, the evolution of such pulses inevitably involves the shedding of radiation, leaving behind a less energetic soliton superposed on a continually spreading background component. This dispersive radiation can give rise to interaction between soliton bits and thereby limit the information capacity of the system. The ideal soliton source, therefore, is one providing picosecond duration pulses with temporal, spectral and power characteristics compatible with those of fundamental soliton pulses in the transmission fibre.
One method of generating picosecond pulses is discussed in an article entitled "Mode-locked erbium-doped fibre laser with soliton pulse shaping" by J. D. Kafka, T. Baer and D. W. Hall, Optics Letters 14, No. 22 (November 1989) pp 1269-1271. A mode-locked erbium doped fibre laser is in the form of a ring laser incorporating a 70 m long erbium-doped fibre as the gain fibre, an integrated modulator and a 2 km length of telecommunications fibre. The telecommunications fibre provides a degree of soliton pulse shaping to the pulse produced by the laser.
BRIEF SUMMARY OF THE INVENTION
According to the present invention a laser including a soliton supporting waveguide doped with a material capable of providing optical gain is operated such that the solitons propagating in the waveguide have a soliton period greater than the amplification period of the laser.
The pulse energy in the pulse has to be sufficient to allow the pulse to cancel out the effects of anomolous group delay dispersion (which may be positive or negative) through the non-linearity of the waveguide, i.e. the change in refractive index of the waveguide with optical intensity. This is the basic mechanism of soliton formation.
The applicants have determined that even through the pulses may undergo large excursions in peak power, in a laser with mirrors of reflectivity of 100% and 4%, for example, there is a stable solution of a soliton nature when the laser is operated according to the present invention.
Preferably the laser is operated such that there are at most five pulses propagating in the laser at any given time and with a pulse repetition rate such that the pulses are fundamentally mode locked.
The value of the pulse energy, which can be adjusted for a given optical waveguide laser by adjusting the pump power, is not critical. As will be discussed later a low level pedestal component is introduced if the energy is too high.
The laser is preferably arranged as a ring laser. Conveniently, the laser is a mode-locked laser to form initial pulses which then become true soliton pulses. The method of seeding the pulses is not an essential element of the present invention so other methods, including self seeding may be used.
The invention is applicable generally to optical waveguides, where by "optical" is meant that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable of being transmitted by dielectric optical waveguides such as optical fibres.
The invention is of particular application to long distance optical communications systems which generally use the 1.5 .mu.m transmission window of silica optical fibres. Preferably, therefore, the optical waveguide comprises an erbium doped optical fibre which has a highly efficient operation over
REFERENCES:
patent: 4635263 (1987-01-01), Mollenauer
patent: 4685107 (1987-08-01), Kafka et al.
patent: 4835778 (1989-05-01), Kafka et al.
patent: 5008887 (1991-04-01), Kafka et al.
Smith et al., "Erbium Fibre Soliton Laser"; Electronics Letters, 19 Jul. 1990, vol. 26, No. 15.
British Telecommunications public limited company
Jr. Leon Scott
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