In semiconductor laser amplifiers it is generally found that it is difficult to have identical confinement factors for the orthogonal polarisations of input light, which propagate within the active region or, more generally, within the waveguide region (of which the active region forms the core and the cladding includes additional layers, as the TE and TM modes. Usually there is a higher peak gain for the TE mode than for the TM mode. The result of this is that for laser amplifiers there is a variation in the gain of the amplifier dependent upon the polarisation of the input optical signal. In practical terms this polarisation sensitivity means that the amplifier can not be operated consistently at maximum gain and as the polarisation changes the amplifier bias has to be adjusted to increase or decrease the gain to a consistent operating level.
One solution to polarisation sensitivity is to utilise a polarisation scrambler on the light input to the laser amplifier, but this has the disadvantages of a reduction in gain (usually 3dB) and, more significantly, at high bit rates a degraded signal to noise ratio due to noise originating in the scrambler.
Within a waveguide the effective refractive index is a function of the refractive index of the material and also the dimensional ratio of the waveguide, so that an asymmetric waveguide or active region has an asymmetric effective refractive index. This asymmetric refractive index arises from a difference in the propagation constants for TE and TM. The difference in propagation constants may also lead to a difference in TE and TM confinement factors which will in turn result in different TE and TM gains. One way around the problem of different TE and TM gains would be to fabricate symmetrical waveguides, but in practice such a structure is difficult to fabricate and does not lend itself to efficient optical coupling to and from the amplifier. It has however been the trend, as far as possible, to fabricate waveguide regions with minimum asymmetry because this produces a more symmetrical spot and eliminates the need for anamorphic coupling lenses.
An alternative approach would be for a laser to be fabricated that has an asymmetric active region but nevertheless has similar confinement factors for TE and TM. Devices have been fabricated with confinement factor ratios as close as 0.9 (within this specification confinement factor ratios are given as TM/TE, and are therefore generally less than unity) in which the gain peaks for TE and TM have a gain difference of only 2.5dB (apparently enabling operation within the top 2.5dB gain) and thus this appears to offer an improvement over the 3dB loss and attendant noise of a polarisation scrambler. However, within the waveguide region of such devices although the confinement factors have been closely similar giving close gain envelope peaks, there are longitudinal mode ripples superimposed on the gain characteristic which result from residual cavity reflectivity. Due to the asymmetry of effective refractive index resulting from waveguide asymmetry the longitudinal mode ripples are of a slightly different frequency for the different polarisations and are not in general in phase so that although the peaks of the 3dB ripple envelopes of TE and TM gain may lie only 2.5dB apart the actual gain difference at a given wavelength is usually greater than this, for example about 5dB, because of the longitudinal mode ripples being out of phase.
The approach adopted in the present invention is to make the effective refractive index in the waveguide region for the different polarisations sufficiently different to provide frequent longitudinal mode ripple coincidence within the gain envelope peak.
Accordingly the present invention provides a laser amplifier in which the active region has an effective refractive index difference for light propagating in the TE and TM modes such that one of the TE and TM modes has a greater number of longitudinal modes oscillating within the 3dB gain envelope than the other, and such that at least four longitudinal modes oscillating in the TE mode substantially coincide in wavelength with longitudinal modes oscillating in the TM mode within said 3dB gain envelope.
According to another aspect of the invention there is a laser amplifier having a cavity in which there is an effective optical path length difference between TE and TM propagating polarisations of at least 2.5.times.10.sup.-4 meters.