Page 379 - Fiber Bragg Gratings

356 Chapter 8 Fiber Grating Lasers and Amplifiers

lasers for a large number of applications [11. The linewidth may be further
narrowed by the use of a long external cavity. This technique has attracted
much attention over the years [2], Early work showed that while there
was a benefit in line-narrowing from the linewidth enhancement factor
of long external cavities, the instabilities were thought to be too difficult
to overcome. In particular, reflections from intracavity elements such as
the residual front facet reflections greatly affected the performance of the
laser, except in the case of strong coherent feedback [3]. Much of the
earlier work was limited to weak feedback due to in- and out-coupling
losses to the external cavity. Wyatt and Devlin demonstrated line-nar-
rowed (10 kHz) tunability over 55 nm in an InGaAsP 1.5-yttm laser, using
a bulk diffraction grating as the external cavity frequency selective mirror
[4]. A semiconductor laser integrated with an etched Bragg reflector in a
waveguide demonstrated the possibility of a compact configuration [5].
The fiber Bragg grating thus became the next obvious choice for an exter-
nal reflector owing to its narrow bandwidth, with the possible advantage
of defining the lasing wavelength and the potentially low coupling loss.
Destroying the front facet reflectivity of a semiconductor laser and replac-
ing it with a narrow-band fiber Bragg grating as an external cavity reflec-
tor is probably the worst possible configuration for making a high-quality
laser. First of all, the semiconductor chip with cleaved facet mirrors forms
an ultralow-loss cavity, since the mirrors are an integral part of the laser
gain medium. Secondly, the laser semiconductor chip alone forms the
shortest gain cavity, reducing the roundtrip time to a minimum and
thereby allowing direct high-speed modulation. Thirdly, the high reflecti-
vity of the output facet of the laser (~33%) is broadband; any external
reflections returning to the laser, other than at the lasing wavelength,
are attenuated equally, reducing the possibility of destabilizing laser oper-
ation. On the other hand, the fiber Bragg grating external-cavity semicon-
ductor laser has several major limitations. The output facet reflectivity
of the laser chip has to be reduced to low levels. The addition of a fiber
in the cavity requires good coupling and low residual subcavity reflections
in order to reduce intracavity losses and poses negligible return loss to
wavelengths other than within the bandwidth of the Bragg grating. The
cavity length increases correspondingly by the incorporation of a reflective
Bragg element at some point within the fiber, increasing the cavity round-
trip time. What has the FGSL (or FGL) to offer to make it worth pursuing?
First, the fiber Bragg grating has a temperature coefficient [6] of
wavelength change —6-8 times lower than that of a semiconductor laser,