| 摘要 |
<p>A chirped optical fibre grating formed by impressing a chirped substantially periodic refractive index variation on a polarisation-maintaining optical fibre. The polarisation state of a laser transmitter (10) is maintained through an external modulator (20) and a polarisation maintaining circulator (30) by the use of polarisation maintaining fibre pigtails (40) aligned to one of the principal axes of high birefringence optical fibre used to fabricate the dispersion compensating grating (50). On reflection, light is output from the third port (60) of the circulator. Since only one polarisation mode of the grating (50) is excited, the PMD in the grating is eliminated. The polarisation maintaining circulator (30) comprises input/output lenses (32) at each port, a polarisation beam splitter (34) and a Faraday rotator (36). Light entering the circulator at the input port is arranged to be in a polarisation that passes through the polarisation beam splitter (34) towards the second port of the circulator. The light is rotated by 45° in the rotator (36). The grating (50) and the fibre pigtail at the second port of the circulator are arranged with their principal axes rotated by 45° with respect to the axes of the input to the circulator. Light reflected from the grating is then further rotated by 45° by the rotator, into the polarisation which is deverted by the polarisation beam splitter (34). This light is therefore diverted to the third output (60) of the circulator. Another embodiment in which two chirped gratings (100, 110) are formed in high birefringence polarisation maintaining fibre with axes aligned, and a 45° Faraday rotator (120) is placed inbetween.</p> |
