Soliton propagation and polarization mode-locking in birefringent optical fibres

In recent years much work has been devoted to the development of erbium-doped fibre lasers [ 1-91. because a broad zain bandwidth centred nearly I.54 urn. cegative group velocitv dispersion and tight confinement of optical field allow the exploitation of nonlinear optical effects for very efficient generatlon of ultra-short optical pulses. In the last few years several schemes of fibre soliton lasers have been reported [Z-9]. Experimental study of such lasers has reveaied a strong influence of fibre lxrefringence on laser output CharacterIstxs. Polarization propenles of optical fibres manifest themselves more clearly in mode-locked svstems based on polarlzation switch [7-91. It is interestmg illat the desirable et’fecect an be reached in both linear [7] and ring configurations [8.9]. Thus the problem of soliton propagation in birefringent optical fibres becomes of considerable interest and various aspects of the problem have been studied in many papers :10-15]. 4s has been shown [ IO-121 a bound state of orthogonally polarized solitons can exist in fibres with arbitrary blrefringence provided there is compensation of polarization time dela! by the proup velocity iiisperston that causes spectral separation of orthogonal componenrs. Formally it means that. if input conditions for the coupled nonlinear ShrCdinger equations have a properly designed spectrum, the term describing birefiingence can be omitted. Another soliton behaviour occurs if an initial pulse has unshifted spectral components. In this case there will be a transient region, characterized by an oscillating behaviour of the ortho’gonal spectral components and depending upon the value of birefringence this process might result in formation either of a single soliton with coupled polarization components or two independent solitons with an orthogonal state of polarization. Thus the problem of soliton propagation in an optical fibre has in fact two aspects. The first question IS concerned with polarization evolution of the steady bound state of orthogonally polarized solitons. This problem is closely related to an effect of polarization intensity discrimination which plays a key role in the operational principle of a passive mode-locked laser based on polarization switch [8,9], and in the general case this problem