Page 407 - Fiber Bragg Gratings

384 Chapter 8 Fiber Grating Lasers and Amplifiers

If a fiber is placed between mirrors and pumped, the Stokes field
sees a roundtrip gain of

where g R is the Raman gain coefficient, 7 0 is the intensity of the pump,
L effis the effective length of the fiber ~l/a pMTOp (the fiber loss at the pump
wavelength), and the factor of 2 is for the double pass. The pump intensity
at which the Stokes field overcomes the cavity losses for a given set of
mirror reflectivities is called the threshold for oscillation. For 10 m of
fiber, a CW threshold can be reduced to ~1 W [108,109]. As the pump
power is increased, the Stokes field Si increases until the threshold for
the second-order Stokes S 2 is reached, at which point energy is transferred
to S 2- At this point, a signal at the third Stokes frequency will experience
gain, and so on. This is also the principle of the resonant Raman amplifier.
This type of a multi-Stokes oscillator has been demonstrated by Stolen
et al. [110,111], who generated five Stokes orders of independently tunable
radiation.
Perhaps one of the most elegant components that is the direct result
of the high transparency of Bragg gratings outside the band stop is the
Raman fiber grating laser, the RFGL. This laser has opened many opportu-
nities in communications, by allowing amplification in any part of the
communication spectrum by appropriate choice of pump lasers and fiber
Bragg gratings.
The general cavity configuration for a resonant Raman laser [112] is
shown in Fig. 8.24.
This laser has been shown to produce 1.5 W at 1485 nm when pumped
+3
+3
by a diode-pumped Yb double-clad laser. The Yb pump at 1117 nm

Figure 8.24: Cascaded fiber grating resonant Raman 1480-nm pump laser
for pumping erbium amplifiers [113].