All-optical intermodal switch using periodic coupling in a two-mode waveguide. H. G. Park,* S. Y. Huang, t and B. Y. Kim. Edward L. Ginzton Laboratory, W. W. ...
August 15, 1989 / Vol. 14, No. 16 / OPTICS LETTERS
877
All-optical intermodal switch using periodic coupling in a two-mode waveguide H. G. Park,* S. Y. Huang,
t
and B. Y. Kim
Edward L. Ginzton Laboratory, W. W. Hansen Laboratoriesof Physics, Stanford University, Stanford, California 94305 Received February 16, 1989; accepted May 18, 1989 An intermodal switch based on optically induced (through the Kerr effect) periodic coupling in a two-mode A high-power pump beam injected into the two modes of the waveguide waveguide is described and demonstrated.
produces a periodic modulation of the refractive-index profile with a period equal to the modal beat length, which causes coupling between the two modes of a simultaneously injected probe beam. The operating principle was successfully demonstrated in an elliptical-core two-mode fiber with counterpropagating pump and probe beams.
All-optical switching devices utilizing waveguides are of current interest for use in high-speed optical signal-
processing and telecommunication systems.'- 8 In this Letter we describe a new type of all-optical switch using optically induced periodic coupling between two waveguide eigenmodes. This device uses a two-mode waveguide whose inter-
direction. When the spatial frequency of the grating 27r/A is close to the difference in propagation constants of the two modes, AO= 01 - 02, and assuming that the probe beam enters in the fundamental mode at z = 0, it can be shown using conventional coupled-
mode theory' 2' 1 3 that
Al(z) = expQi 2 z)(cos nz -i
nal field pattern results from the superposition of two (symmetric and antisymmetric) modes. This results in the formation of a high-intensity
lobe in the cross
section of the waveguide whose position with regard to the waveguide center line oscillates periodically along the length of the waveguide with a period equal to the modal beat length, as indicated in Fig. 1(a). A num-
ber of devices using this two-mode interference have been described elsewhere.8 -10 In the present device a high-power beam (a pump beam) is launched into both
modes, and periodic intensity variations result,
through the optical Kerr effect, in corresponding periodic changes in refractive-index distribution over the waveguide cross section, which constitute a dynamic
axial index grating. A low-intensity probe beam fed simultaneously into the waveguide then experiences intermodal coupling if its beat length is similar to that of the pump. Let us consider a simplified model for the above
A2(Z) = -exp(-i
2 z) 1 sin 77z,
(1)
(2)
where Al and A 2 are the slowly varying amplitudes of
the fundamental and second-order modes, respectively;
7 =
[IyJ2 + (6/2)2]1/2; y
=
(wEOnavAn/7r)
SS'
+1,42*
dxdy, with if' representing integration over the upper half of the core cross section; EOis the vacuum dielectric constant; and 6 = (13,+ K11) - (12 + K22) - (2r/A),
2 dxdy (m = 1, 2). with Kmm = (uEOfnavAn/2) if' kt'mJ From this solution we see that the mode power oscillates with a spatial frequency of 2 7.
LP1 1
LP0 1
+
grating, as shown in Fig. 1(b), in which the refractive
indices of the upper and the lower halves of the core are alternately modulated with a constant increment An and a spatial period A that corresponds to the modal beat length of the pump beam in this waveguide. If we assume that the total pump power is
sin nz ,
6
(a)
0=
7C
2ir
qb>eio 3rr
47c
localized in one half of the core and that all fields are polarized in the same direction, An is given by'" An = 1927r2 xP/cn5 v2A, where X is the third-,rder suscepti-
bility in esu, P is the total pump power, c is the speed of light, naVis the average index, and A is the core area.
Consider now a probe beam propagating with a field distribution of i1(x, y) in the symmetric fundamental mode and 42(x, y) in the antisymmetric second-order mode, where f, and 02 are each normalized to carry unit power. The probe beam should be separable from the pump beam by its wavelength or propagation 0146-9592/89/160877-03$2.00/0
(b)
core
.
-A
-
Fig. 1. (1) Modal interference pattern at a transverse plane in a two-mode fiber as a function of the relative phase shift k between the two modes. (b) Simplifed model of the periodic nonlinear index grating. © 1989 Optical Society of America
878
OPTICS LETTERS / Vol. 14, No. 16 / August 15, 1989 variable attenuator l2 el iL /_2___1: { |beam ", j
-switched|
Nd:YAG laser
Y
' beam -..
splitter
'Al N
A.
Ssphuler
Ae \
peak pump power was increased to 100 W [Fig. 3(b)] lOx
polarizer II
oscilloscope lox
'z2ZO
LP11 mode stripper
Fig. 2. Diagram of the experimental setup. The detector was put at the center position of the fundamental-mode pattern of the probe-beam output.
The experimental setup used to demonstrate the
principle is shown in Fig. 2. The waveguide medium was a 7-m-long polarization-maintaining fiber (pro-
vided by Lightwave Technology Inc.) having a 4.5 ,rm X
12.5 Atm elliptical core and a second-mode cutoff
wavelength X,
peak pump powers, as well as near-field spatial mode patterns of the probe outputs taken with an infrared vidicon camera. The pump-free (Ppeak = 0) probe output pulse [Fig. 3(a)] had the same shape as the input pulse, and the mode pattern on the right corresponds to that of the fundamental mode. When the
1190nm. Both ends of the fiber were
polished at a 10° tilt angle. A Q-switched Nd:YAG laser (A = 1064 nm, 100-Hz repetition rate, 180 nsec
FWHM pulse width) was used as the source for both pump and probe signals, which were fed into the fiber in opposite directions to allow them to be separated at the output. The probe signal power was approximately 1% of the pump power. Here, input and output fiber ends are defined with respect to the probe signal. The pump beam passed through a variable attenuator and a polarizer and was focused onto the output end of the fiber core by a 1OXmicroscope objective.
the probe power in the time range near the peak of the pump-free pulse decreased, owing to conversion to the second-order mode. The corresponding mode pattern, which is integrated over the interaction time, clearly shows the second-order mode. As the peak pump power was increased to 273 W [Fig. 3(c)] the
probe power near the time of the peak of the pumpfree pulse was largely recovered, indicating that the second-mode power was coupled back into the fundamental mode, as confirmed by the mode pattern. The fundamental-mode power at the time of the peak of the pump-free pulse was measured as a function of the peak pump power and is shown in Fig. 4. As the pump power increases, the optical power in the fundamental mode oscillates as expected from the theory. Figure 4 covers the first cycle of oscillation. It is strong evidence of coherent periodic coupling. In Fig. 4 we can see that the rate of change of the coupled power decreased with increasing pump power. In Fig. 3 we observe that the maximum mode coupling occurred -30 nsec ahead of the peak of the probe pulse. These can be explained by the degradation of the index-grating contrast owing to self-phase modulation. With high pump power input, the pump pulse undergoes spectral broadening along the fiber owing to self-phase modulation,' 4 and the coherence between the two modes decreases because of their group-
(a) Ppeak = ° W
The polarizer was oriented along one of the eigenpo-
larization axes of the fiber. The pump beam was launched offset from the center of the fiber core in such a way that the two fiber modes were excited with approximately equal power. The probe beam, split from the laser output, was focused onto the input end of the fiber. The probe pulse overlapped with the counterpropagating pump pulse in the middle of the fiber. A short section of the fiber was coiled near the input end with a tight radius to strip off the light contained in the second mode so that the probe beam entered the interaction (overlap) region in only the fundamental mode. The probe output from the fiber passed through a polarizer so that only the same polarization component as that of the input pump was directed to the detector. To detect only the fundamental-mode component of the output of the probe beam, a silicon P-I-N detector was located at the center position of the fundamental-mode radiation pattern, where the second mode always has zero intensity. Figure 3 shows observed time waveforms of the probe output beam pulse at the detector for different
(b) Ppeak = 100 W
(c) Ppeak = 273 W
Fig. 3. Probe-beam output waveforms for different peak pump powers Ppeak. At the right are the corresponding near-field patterns of the probe-beam output.
August 15,1989 / Vol. 14, No. 16 / OPTICS LETTERS
0
1.
W CC
0cc CL
W
0
0
0.5 -
0 0
0
zW
0
0
_
0 z
0
0
D0
Fig. 4.
200 100 PEAKPUMPPOWER(W)
Measured fundamental-mode
300
output power versus
the peak pump power.
velocity difference. It followsthat the visibility in the two-mode interference, and thus the grating strength, decreases as the pump pulse propagates along the fiber. In this case efficient mode coupling occurs mainly in the output section of the fiber, resulting in maximum mode coupling at the rising part of the probe pulse that coincides with the peak of the counterpropagating pump pulse at the output section of the fiber as seen in Fig. 3.
Another configuration of the intermodal switch is possible using the same principle but with copropagating pump and probe beams. In this case the pump wavelength should be different from the probe wavelength, and the waveguide should have similar beat lengths at the two wavelengths. The probe output could then be separated from the pump by using a dispersive element. We should mention that the modal beat length can be insensitive to wavelength variation, especially near the wavelength where the group velocities of the two modes are matched.' 0 For example, the modal beat length of the fiber used in Ref. 10 was measured to vary less than 1% over the
wavelength range of approximately 450-550 nm, while the group-velocity-matched wavelength was -500 nm. If the pump wavelength is chosen to be close to the wavelength where the two-mode group velocities are equal, a grating with good contrast is obtainable over a long length of the fiber despite some spectral broadening caused by self-phase modulation. It is also possi-
879
ble to design a waveguide in which the two modes have nearly equal group velocities over a wider wavelength range. This would further reduce the pulse walk-off between the two modes and also between the pump and the probe beams. In this case, a long length of fiber could be used to lower the required pump power without losing switching speed. In conclusion, we have shown that all-optical intermodal switching is possible in a two-mode waveguide using periodic index modification through the optical Kerr effect. This research was supported by Litton Systems Inc. * Present address, Electronics and Telecommunications Research Institute, P.O. Box 8, Daedugdanji, Chungnam, Korea. t Present address, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai 200030, China.
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