Page 381 - Fiber Bragg Gratings

358 Chapter 8 Fiber Grating Lasers and Amplifiers

lasers in WDM systems, where stocks may need to be maintained for each
laser. The concept of retaining a fiber grating as part of the transmitter
card, while replacing only the active gain medium when faulty, may have
a cost advantage by reducing stocks. Finally, for telecommunications and
sensor applications, lasers need to be pigtailed after fabrication. If the
fiber grating is allowed to define the lasing wavelength, the two functions
of operating wavelength and pigtailing may further reduce fabrication/
packaging costs. These advantages are considered worthwhile to overcome
the difficulties involved in optimizing laser chip-to-fiber coupling.
The first semiconductor laser to use an external cavity of an etched
in-fiber Bragg grating was reported by Brinkmeyer et al. [12]. This laser
used a standard uncoated Fabry-Perot fiber pigtailed laser chip operating
at 1.3 microns, which was fusion spliced to an etched fiber grating, forming
2-meter-long cavity. With the twin coupled cavities (laser chip and laser
chip with fiber grating), single-frequency operation of the laser was dem-
onstrated with a linewidth of ~50 kHz. The narrow bandwidth of the
grating (26 GHz) was much less than the FP mode spacing (140 GHz),
although the long cavity longitudinal mode spacing was only 50 MHz.
The reflectivity of the front facet of the FP laser p FP *=* 32%, while the
reflectivity p^ of the grating is very low, —0.01%; however, the combined
coupled cavities results in a constructive interference with a contrast ratio
of

so that when the cavity length is tuned for constructive interference, the
modes of the long fiber cavity have a differential gain that is higher than
the FP laser modes. This demonstration showed that the selectivity and
additional reflectivity of the external cavity grating have a beneficial
effect even in such a simple configuration. Etched gratings in fibers as
external cavity mirrors have also been used with 1500-nm diodes [13].
Morey et al, [14] showed the use of a photoinduced fiber Bragg grating
with a semiconductor laser to operate at the grating wavelength of 820
nm. A schematic of the FGSL device is shown in Fig. 8.1. An antireflection-
coated (~0.5% reflectivity) FP GalnAsP/InP buried-heterostructure 1.5-
/u,m laser chip is coupled with a lensed fiber with a narrowband reflection
grating spliced to it to form the external resonator [15]. The lensed fiber
is aligned to maximize the output coupling and welded in place. This laser
demonstrates a low chirp, which is restricted by the limited bandwidth