Page 395 - Fiber Bragg Gratings

372 Chapter 8 Fiber Grating Lasers and Amplifiers

Figure 8.15: A schematic of the fiber Bragg grating rare-earth-doped fiber
laser. The simple configuration encloses a piece of rare-earth-doped fiber between
two Bragg matched gratings and is end-pumped.
The simple configuration allows a single-frequency laser [50,51] or
twin laser configuration to be pumped in series with a single pump [52,53],
or enables multiwavelength operation for sensor applications [54]. The
transparency of the gratings to all other wavelengths allows closely spaced
wavelengths to be emitted from one laser source, or the cascading of the
laser with an amplifier section in a master oscillator power amplifier
(MOPA) configuration [55]. A review of these lasers covers some of the
general aspects [56]. Some of these lasers are considered in the following
sections.
Spatial hole-burnt gratings may be used to stabilize semiconductor
lasers. In the case, a piece of unpumped erbium fiber is used with a Bragg
grating as the external cavity with an AR-coated semiconductor chip. The
standing wave formed in the erbium doped fiber forms a population grat-
ing and line-narrows the laser. The 3-m-long cavity with a 1535-nm grat-
ing reflector was measured to have a linewidth of ~1 kHz [57].
High-power, double-clad Yb: doped fiber lasers with integral gratings
in the cladding have achieved output powers in excess of 6.8 W in a single
longitudinal mode [58]. At 1090 nm, 9-W fiber lasers are available that can
be used as pumps for amplifiers and Raman oscillators (Section 8.7) [59].

8.4 Erbium-doped fiber lasers

The erbium ion in germanosilicate fiber has a three-level laser transition
4
4
( ^i3/2 —> ^i5/2X which may be modeled using the simplified three-level
energy diagram of Digonnet [60]. A simple model for single-mode operation
of an erbium fiber laser incorporating fiber gratings shows that the tran-
sient energy density per lasing mode is [61]