Page 385 - Fiber Bragg Gratings

362 Chapter 8 Fiber Grating Lasers and Amplifiers

allows good coupling to a cleaved standard fiber [24,25], which can be
covered in index matching gel to reduce end reflections to a very low level.
The angled facet ensures low back reflection, virtually eliminating the
formation of a subcavity No ripple due to intracavity reflections can be
seen in the amplified spontaneous emission (ASE) spectrum. The positive
effect of such a design is the lack of mode competition between subcavity
modes as the current is ramped. A large-bandwidth (0.3-nm) fiber Bragg
grating reflector allows the lasing mode to tune slightly in wavelength
as the laser is modulated, with the virtual absence of mode hops.
The alignment is simplified with the help of a mini silicon-optical
bench. A key on the bench allows easy positioning and soldering of the
laser chip. A silicon micro-machined vee-groove aligned to the laser com-
pletes the passive alignment of the assembly. A glass sliver is then used
to bond the fiber in place with epoxy. The gap between the laser and the
fiber-end is then filled with gel. The package may then be completed by
injection molding [25] or by hermetic sealing. A photograph of the pack-
aged laser is shown in Figure 8.4.

8.2 Static and dynamic properties of FGLs

The light-current characteristics are shown in Fig. 8.5. The kink-free
curve shows mode-hop-free operation as a function of bias current. The
extremely low hysteresis in the wavelength of the laser as the current is
altered, shows the potential of such a device. Typical of most FGLs is the
weak temperature dependence of the lasing wavelength when the grating

Figure 8.4: Photograph of packaged laser large spot laser FGL (photograph
courtesy C. Ford, BT Laboratories).