Page 384 - Fiber Bragg Gratings

8.1 Fiber grating semiconductor lasers: The FGSL 361

Such a long cavity (as with the one described earlier [12]) works under
the regime of "coherence collapse" [22], where the coherence length of the
diode modes is much shorter than the laser cavity length.
An alternative method of controlling the wavelength of the laser is
to use the fiber grating at the rear facet of the chip [23]. The output
coupler is a standard fiber pigtail. This device was fabricated in a package
with all the in and out coupling with aspherical collimating lenses, as
well as an isolator in the output path. The advantage of this scheme is
the use of a highly reflective grating (>95%), potentially increasing the
output power. The grating had a bandwidth of <0.4 nm, which was burnt-
in at 350°C for 6 min. The fabrication technique is claimed to have a
reproducibility of ±0.05 nm, allowing devices to be fabricated to ITU-T
standards for dense WDM. With temperature control of the package, both
the grating and the laser chip are maintained at the same temperature.
Consequently, the stability of the wavelength was reported to be better
than ±0.005 nm, with an output power stability within ±0.02 dB. This
laser, as is common with well-designed FEGSLs had a measured line-
width of <30 kHz and a side-mode suppression ratio of >50 dB.
A combination of a reflective amplifier and a large spot laser has
resulted in a robust laser, which may be operated over a wide temperature
range without temperature control [24]. This type of laser has good wave-
length stability as a function of temperature and therefore is ideally suited
to applications in Access to a local area network, with the potential of
being a low-cost source. Figure 8.3 shows a schematic of this laser. The gain
medium is a semiconductor chip, which has a passive tapered waveguide to
allow the mode to expand so that it has a mode-field close to that of a
standard telecommunications fiber. Further, the waveguide is arranged
to terminate at an angle to the output facet. The large mode field size

Figure 8.3: The large spot-angled facet, cleaved standard fiber FGSL [24].