Coherent light generators – Particular beam control device – Tuning
Patent
1997-04-22
1998-10-27
Healy, Brian
Coherent light generators
Particular beam control device
Tuning
372 18, 372 19, 372 26, 372 31, 372 92, 372102, 372 50, 359180, 359181, 359188, 359130, 385 37, 385 42, H01S 310, H04B 1004, G02B 634
Patent
active
058286810
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
In optical communications it can be desirable to have strict control over the emission wavelength of an optical transmitter. This is particularly likely to be the case in a wavelength division multiplexed (WDM) system. In such systems the precise control of wavelength allows a reduced wavelength spacing between adjacent channel, and thereby allows a greater number of channels to be accommodated within any particular waveband of, for instance, an erbium doped optical fibre amplifier.
For a reasonably high bandwidth WDM system, the current preferred technology for transmitters is a continuous wave externally modulated distributed feedback (DFB) semiconductor laser. A DFB laser provides significantly better stability than a simple Fabry-Perot semiconductor laser, but the emission wavelength typically varies significantly from laser chip to laser chip, even though they all be nominally identical. The reason for this is that though it is possible to obtain a relatively tight tolerance over the physical pitch of the grating of a DFB laser, the emission wavelength is determined by the product of this physical pitch with the effective refractive index, and value of this effective refractive index is not only fairly critically dependent upon thicknesses and compositions of the layer structure of the laser chip, but also upon carrier concentration, which in its turn is affected by drive current, temperature and ageing processes that reduce electro-optic conversion efficiency.
In a DFB laser, the wavelength selectivity is provided by a Bragg grating structure but, as discussed in the preceding paragraph, the wavelength control presented by such a grating is complicated by the fact that it is formed in a composite material whose effective refractive index is variable. For quite a number of years researchers have worked with semiconductor lasers which have a reflector external to the semiconductor material that defines one end of the optical cavity of the laser. The additional length of the optical cavity means that the longitudinal modes are at a correspondingly reduced wavelength spacing, thereby affording the prospect of very narrow bandwidth single mode operation. Advantageously such an external reflector is itself spectrally selective. It may be constituted for instance by a Bragg grating in a length of single mode optical fibre. Such an arrangement has been disclosed for instance in U.S. Pat. No. 4,786,132. In this instance, neglecting second order effects the effective refractive index `seen` by the Bragg grating is constant, and so the spectral reflection characteristic of such a reflector can be stable and well defined.
Semiconductor lasers with external Bragg grating reflectors have been employed in the laboratory to achieve narrow band single mode operation under controlled conditions providing the appropriate stability in the free-space equivalent length of the optical cavity. However, if the equivalent free-space length is allowed to drift, then the wavelength of that mode also drifts in order to keep the equivalent free-space length a constant half-integral number of wavelengths long. If this drifting is allowed to progress far enough, the wavelength change is sufficient to take this mode far enough away from the peak in the spectral reflectivity characteristic of the Bragg reflector for its place to be taken by the adjacent longitudinal mode moving into the wavelength range just vacated by the former mode. Continued drifting in the same direction will eventually cause this adjacent mode to be supplemented by the longitudinal mode adjacent to it, and so on.
The replacement of one longitudinal mode by its successor involves, at best, a sudden change of wavelength, and may also involve a period when the two modes co-exist and beat together. Such effects are most liable to cause large error bursts which would be unacceptable in practical transmission systems. In this context it may be observed that customers typically demand error rates (BER) of between 10.sup.-12 and 10.sup.-18
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Healy Brian
Northern Telecom Limited
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