Nonlinear Fiber Optics - 4 ed. Agrawal

9.3. Brillouin Fiber Amplifiers 343
Figure 9.8: Schematic illustration of a Brillouin amplifier; ECL, ISO, and PD stand for externalcavity
laser, isolator, and photodetector, respectively. Solid and dashed arrows show the path of
pump and probe lasers. (After Ref. [61]; c○1986 AIP.)
for G A in the range 20−30 dB, where P in is the incident power of the signal being
amplified. As discussed in Section 9.3.1, the saturation power of Brillouin amplifiers
is ∼1 mW. In spite of gain saturation, Brillouin amplifiers are capable of providing
30 dB gain at a pump power under 10 mW. However, because Δν B < 100 MHz, the
bandwidth of such amplifiers is also less than 100 MHz, in sharp contrast with Raman
amplifiers whose bandwidth exceeds 5 THz. In fact, the difference between the signal
and pump frequencies should be matched to the Brillouin shift ν B (about 11 GHz in
the 1.55-μm region) with an accuracy to better than 10 MHz. For this reason, Brillouin
amplifiers are not suitable for amplifying signals in fiber-optic communication systems.
Brillouin amplifiers may be useful in practice for applications requiring selective
amplification [62]–[64]. One such application consists of amplifying the carrier of a
modulated signal selectively, while leaving its modulation sidebands unamplified [75].
The underlying principle is similar to that of homodyne detection, except that the amplified
carrier acts as a reference signal. This feature eliminates the need of a local
oscillator that must be phase locked to the transmitter—a difficult task in general. In a
demonstration of this scheme, the carrier was amplified by 30 dB more than the modulation
sidebands even at a modulation frequency as low as 80 MHz [62]. With a proper
design, sensitivity improvements of up to 15 dB or more are possible at bit rates in
excess of 100 Mb/s. The limiting factor is the nonlinear phase shift induced by the
pump (a kind of cross-phase modulation), if the difference between the pump and carrier
frequencies does not match the Brillouin shift exactly. The calculations show [63]
that deviations from the Brillouin shift should be within 100 kHz for a phase stability
of 0.1 rad. Nonlinear phase shifts can also lead to undesirable amplitude modulation
of a frequency-modulated signal [66].
Another application of narrowband Brillouin amplifiers consists of using them as a
tunable narrowband optical filter for channel selection in a densely packed multichannel
communication system [64]. If channel spacing exceeds but the bit rate is smaller
than the bandwidth Δν B , the pump laser can be tuned to amplify a particular channel
selectively. This scheme was demonstrated in 1986 using a tunable color-center laser
as a pump [64]. Two 45-Mb/s channels were transmitted through a 10-km-long fiber.