Page 389 - Fiber Bragg Gratings

366 Chapter 8 Fiber Grating Lasers and Amplifiers

part of the gain band to be used. A semiconductor laser was shown to lase
from 1480 to 1573 nm. The SMSR was measured to be -50 dB for such a
laser, at a power output of 1 mW in the fiber. Figure 8.10 shows the emission
spectrum. This laser was shown to operate single-frequency at all wave-
lengths. However, the temperature has to be controlled carefully to prevent
mode hopping and allow stable operation. A combination of the uncommit-
ted laser concept and the large-spot laser should result in a cheap solution
for wavelength selection.
Apart from applications that require the use of a modulation tech-
nique external to the laser, direct modulation of FGLs is desirable for
low-cost devices. A large number of factors, such as electrical capacitance
and inductance, cavity roundtrip, bandwidth of the external grating, and
the interplay between the sub- and supercavity reflections, influence the
dynamic behavior of FGLs. It has been shown that an external cavity
laser with strong feedback can operate with high stability [3].
Provided the laser is operated within mode hops, the modulated perfor-
mance has been shown to be excellent at transmission rates in excess of 2.5
Gb/sec [27-30]. Further, application of a FGL for millimetric-wave genera-
tion has been investigated [31]. In the latter demonstration, the modulation
frequency injection locks two otherwise unstable lasing modes within the
grating reflection spectrum to produce a high spectral purity RF beat signal
at -35 GHz.
With a short external cavity multi-quantum-well (MQW) FGL, modu-
lation at >10 Gb/sec is possible for optical fiber transmission [32]. This
design relies on the high-differential gain and low intrinsic chirp of a
strained MQW, AR-coated front facet FP laser and the high-speed chip
design using semi-insulating blocking layers. The linewidth of this laser
was measured to be 40 kHz (—13 GHz roundtrip resonance; 8-mm external
cavity), with -3 dB modulation bandwidth of —15 GHz. The frequency
response of the laser for different bias currents is shown in Fig. 8.11. The
external cavity resonance is clearly visible as the current is increased to
100 mA. Measurements have shown that the chirp of this laser remains
<10 MHz/mA compared with —250 MHz/mA for a standard DFB laser.
With a 2-mm-long external-cavity FGL, the frequency response is ex-
tended beyond 20 GHz [32].

8.2.1 Modeling of external cavity lasers
Figure 8.12 shows a schematic of a semiconductor laser coupled to an exter-
nal cavity. Also shown are the parameters that need to be taken into account