Nonlinear Fiber Optics - 4 ed. Agrawal

8.2. Quasi-Continuous SRS 287
Figure 8.7: Schematic of a high-power all-fiber Raman laser. Two fiber Bragg gratings (FBG)
act as mirrors and form a Fabry–Perot cavity. A third grating is used to reflect residual pump
back into the laser cavity. The middle grating acts as the output coupler (OC) and is made weakly
reflecting. (After Ref. [68]; c○2004 IEEE.)
in tuning pulsed Raman lasers over a wide wavelength range. The tuning rate can be
obtained as follows. If the cavity length is changed by ΔL, the time delay Δt should be
exactly compensated by a wavelength change Δλ such that
Δt ≡ ΔL/c = |D(λ)|LΔλ, (8.2.2)
where L is the fiber length and D is the dispersion parameter introduced in Section
1.2.3. The tuning rate is therefore given by
Δλ
ΔL = 1
cL|D(λ)| = λ 2
2πc 2 L|β 2 | , (8.2.3)
where Eq. (1.2.11) was used to relate D to the GVD coefficient β 2 . The tuning rate
depends on the fiber length L and the wavelength λ, and is typically ∼1 nm/cm. In one
experiment, a tuning rate of 1.8 nm/cm with a tuning range of 24 nm was obtained for
L = 600 m and λ = 1.12 μm [56].
Synchronously pumped Raman lasers have attracted attention for generating ultrashort
optical pulses [61]. In general, it becomes necessary to take into account the
effects of GVD, group-velocity mismatch, SPM, and XPM when such lasers are pulsed
using pump pulses of widths below 100 ps. These effects are discussed in Section 8.3.
If the Raman pulse falls in the anomalous GVD regime of the fiber, the soliton effects
can create pulses of widths ∼100 fs or less. Such lasers are called Raman soliton lasers
and are covered in Section 8.4.
The development of Raman lasers advanced considerably during the 1990s. A new
feature was the integration of cavity mirrors within the fiber to make a compact device.
In an early approach, a ring-cavity configuration was used to make a low-threshold,
all-fiber Raman laser using a fiber loop and a fiber coupler [62]. With the advent
of fiber Bragg gratings, it has become possible to replace cavity mirrors with such
gratings [63]. Fused fiber couplers can also be used for this purpose. In an interesting
approach, three pairs of fiber gratings or couplers are arranged such that they form
three cavities for the three Raman lasers operating at wavelengths 1.117, 1.175, and
1.24 μm, corresponding to the first-, second-, and third-order Stokes lines of a 1.06-
μm pump [65]. The resulting 1.24-μm Raman laser is useful for amplifying 1.31-μm
signals [66].
The same approach can be used for making a 1.48-μm Raman laser if a phosphosilicate
fiber is used [67]. Such a fiber provides a Stokes shift of nearly 40 THz and