Nonlinear Fiber Optics - 4 ed. Agrawal

7.5. Applications of XPM 251
by higher-order solitons even though, strictly speaking, the signal pulse never forms
a soliton. With the use of dispersion-shifted fibers, the technique can be used even
when both pump and signal wavelengths are in the 1.55-μm region as long as the zerodispersion
wavelength of the fiber lies in the middle. In a 1993 experiment, 10.6-ps
signal pulses were compressed to 4.6 ps by using 12-ps pump pulses [69]. Pump and
signal pulses were obtained from mode-locked semiconductor lasers operating at 1.56
and 1.54 μm, respectively, with a 5-GHz repetition rate. Pump pulses were amplified to
an average power of 17 mW with the help of a fiber amplifier. This experiment demonstrated
that XPM-induced pulse compression can occur at power levels achievable with
semiconductor lasers.
7.5.2 XPM-Induced Optical Switching
The XPM-induced phase shift can also be used for optical switching [63]. Several
interferometric schemes have been used to take advantage of XPM for ultrafast optical
switching [71]–[83]. The physics behind XPM-induced switching can be understood
by considering a generic interferometer designed such that a weak signal pulse, divided
equally between its two arms, experiences identical phase shifts in each arm and is
transmitted through constructive interference. If a pump pulse at a different wavelength
is injected into one of the arms of the interferometer, it would change the signal phase
through XPM in that arm. If the XPM-induced phase shift is large enough (close to π),
the signal pulse will not be transmitted because of the destructive interference occurring
at the output. Thus, an intense pump pulse can switch the signal pulse through the
XPM-induced phase shift.
XPM-induced optical switching was demonstrated in 1990 using a fiber-loop mirror
acting as a Sagnac interferometer [73]. A dichroic fiber coupler, with 50:50 splitting
ratio at 1.53 μm and 100:0 splitting ratio at 1.3 μm, was used to allow for dualwavelength
operation. A 1.53-μm color-center laser provided a low-power (∼5 mW)
CW signal. As expected, the counterpropagating signal beams experienced identical
phase shifts, and the 500-m-long fiber loop acted as a perfect mirror, in the absence
of a pump beam. When 130-ps pump pulses, obtained from a 1.3-μm Nd:YAG laser,
were injected into the clockwise direction, the XPM interaction between the pump and
the signal introduced a phase difference between the counterpropagating signal beams.
Most of the signal power was transmitted when the peak power of the pump pulse was
large enough to introduce a π phase shift.
The XPM-induced phase shift depends not only on the width and the shape of the
pump pulse but also on the group-velocity mismatch. In the case in which both the
pump and signal beams are pulsed, the phase shift also depends on the initial relative
time delay between the pump and signal pulses. In fact, the magnitude and the duration
of the XPM-induced phase shift can be controlled through the initial delay (see Figure
7.3). The main point to note is that phase shift can be quite uniform over most of
the signal pulse when the two pulses are allowed to completely pass through each
other, resulting in complete switching of the signal pulse. The pump power required to
produce π phase shift is generally quite large because of the group-velocity mismatch.
The group-velocity mismatch can be reduced significantly if the pump and signal
pulses are orthogonally polarized but have the same wavelength. Moreover, even if