September 1986 / Vol. 11, No. 9 / OPTICS LETTERS
581
Highly selective evanescent modal filter for two-mode optical
fibers
W. V. Sorin,* B. Y. Kim, and H. J. Shaw
Edward L. Ginzton Laboratory, W. W. Hansen Laboratoriesof Physics, Stanford University, Stanford, California 94305
Received April 24, 1986; accepted June 24, 1986
Efficient and selective coupling between a single-mode fiber and the LP 11 mode of a double-mode fiber is demonstrated in an evanescent directional coupler. With greater than 90%coupling to the LP 11 mode, suppression of the
coupled power to the lower-order LP 01 mode was measured to be at least 24 dB. This device has application as a
modal filter for the construction of in-line all-fiber optical components.
The use of optical fibers in the low-mode-number regime has many potential applications. One possible
use consists of modal multiplexing, in which each spatial mode comprises an individual information channel.' Another application is for the development of
various in-line fiber devices. For example, an all-fiber
frequency shifter has recently been demonstrated that
uses a guided copropagating acoustic wave to couple
between the LP0 1 and LP,1 modes in a double-mode
fiber.2 Static coupling between the LP01 and LP11
modes has also been reported using periodic stresses or
microbending.3 - 5 The feasibility of developing the
potential uses of low-mode-number fibers depends on
being able to couple efficiently and selectively into and
out of the individual modes. Although a LPo1 mode
selector is readily available (e.g., a tightly wound fiber
coil), an efficient modal selector for the LP1, mode has
not yet been reported.
In this Letter a device is demonstrated that uses
phase-matched evanescent coupling to transfer energy
efficiently between a single-mode fiber and the LP1,
mode of a double-mode fiber. Coupling between fibers is accomplished by employing a grinding and polishing technique first used to make tunable singlemode directional couplers.6 This arrangement provides separately guided output channels for the two
modes of the double-mode fiber. Coupling between
low-mode-number fibers has previously been reported,1' 7' 8 but little attention has been paid to achieving
both large coupling efficiency and high modal selectivity. In a tunable directional coupler, evanescent coupling between a single-mode and a multimode fiber
has been demonstrated,9 but coupling was not restricted between only two modes.
In referring to low-mode-number fibers it is assumed that the index difference between core and
cladding is small so that the linearly polarized (LP)
mode approximation can be used.1 0 The operation of
the modal filter is based on phase-matched evanescent
coupling between two dissimilar fibers. Proper index
profiles for the two fibers must be chosen to ensure
phase matching and therefore complete power transfer.
Development of an efficient modal filter requires
0146-9592/86/090581-03$2.00/0
that the device maximize power transfer between the
two desired modes while suppressing any coupling to
the unwanted modes. In the device demonstrated
here, transfer of power to the unwanted mode is minimized by making use of both the phase mismatch and
the smaller modal overlap between the two lowestorder modes. To obtain an estimate on the upper
bound for the coupling between the two lowest-order
modes, a result from coupled-mode theory is used.
The power coupled between two modes is given by1 '
sin [K2 + (A132) 2 ] 2L
(
C
=
2
~1+(A#//2K)
(1
where Pc is the coupled power, Po is the initial power in
the input mode, K is the coupling coefficient per unit
length, L is the coupling length, and A# is the phase
mismatch between the two modes. This equation is
not strictly valid for the case of the modal filter since
not only two but three modes are being coupled. Its
use is intended only to give a rough estimate for the
modal extinctions that are possible.
Using Eq. (1) we can obtain an estimate for the
coupling selectivity between the two modes of the double-mode fiber [LP(2) and LP(21)]
when coupled to the
lowest-order mode of the single-mode fiber [LPgl)].
Assume that the LP11 mode is phase matched to the
LP(1)mode and 100%coupling occurs in a length of 1
mm (i.e., A: = 0 and
K =
7r/2 mm-').
Owing to the
difference in the extent of their evanescent fields the
LP(21)mode can have a coupling coefficient to the single-mode fiber that is more than 10 times larger than
that for the LP(21)mode. With this smaller coupling
coefficient and a beat length (LB) of 300 Am,the maximum coupling between the LP(21)and LP(1) modes is
lessthan -35 dB (i.e.,K =
,r/20
mm-' and A\ = 27r/LB
21 mm-'). When a proper coupling length is chosen to take advantage of the sinusoidal coupling dependence between these two lowest-order modes I[K2 +
(A13/2) 2 ]1/2 L
= Nr, N = integer), isolations may be as
large as -50 to -60 dB. This isolation will determine
the extinction ratio between the LP(2l)and LP(2l)modes
when the above arrangement is used as a modal filter.
In order to demonstrate efficient modal coupling
experimentally it was necessary to obtain two differ© 1986, Optical Society of America
OPTICS LETTERS / Vol. 11, No. 9 / September 1986
582
INDEX
4
DMF
SMF
LP0 1
LPI I
___
I.....~-
Fig. 1.
.
_
____
X
Relative index profiles for single-mode fiber (SMF)
and double-mode fiber (DMF) showing phase-velocity
matching between LP 0 , and LP11 modes. Dashed lines indicate values of effective indices for the guided modes.
the two modes as they traveled along the fiber. The
outputs of the directional modal coupler were then
projected onto a screen where their far-field patterns
could be observed.
Figure 3 shows the far-field output patterns from
the single-mode fiber (SMF) and double-mode fiber
(DMF) with the coupling adjusted to approximately
50%. When the distance between the parallel fibers in
the direction coupler was varied by translating one
coupler half with respect to the other, the output from
the double-mode fiber rotated and changed shape (i.e.,
became doughnutlike in shape), but the central portion of the pattern remained dark, indicating very
little coupling to the lower-order LP(2) mode. By introducing a relatively large angle between the two coupling fibers it was possible to excite the LP(') mode,
but owing to the decreased coupling length the power
transferred to this mode was very small.
The maximum coupling to the LP(') mode was measured to be 97%. This coupling efficiency was calcu-
ent fibers with the correct index profiles. Two almost
compatible fibers were selected from several commercially available fibers by using a technique that permitted measurement of individual modal phase velocities. This phase-velocity measurement technique
consisted of placing a prism on top of a polished fiber
to couple out energy radiatively from the guided
modes.'2 By measuring the angle of the radiated
light, modal phase velocities could be determined to
an accuracy of approximately 0.01%.
Figure 1 illustrates the relative index profiles of the
two fibers that were used to demonstrate the evanescent modal coupler experimentally. Both fibers had
silica cores but differed in their core diameters and the
amount of doping in their claddings. The doublemode fiber had a core diameter of about 5.6 ,Amwith a
numerical aperture approximately equal to 0.08. The
lated by dividing the coupled power by the sum of the
coupled and uncoupled powers. The large coupling
efficiency implies good phase matching between the
two modes. At the expense of decreasing the coupling
length the phase-matching condition could be tuned
by angling the fibers with respect to each other in the
directional coupler.
Figure 4 shows outputs from the double-mode fiber
when coupling efficiencies were greater than 90%.
The mode patterns shown in photos (a) through (c)
were consecutively generated by slowly translating the
two parallel fibers with respect to each other through
their maximum coupling position. The different intensity patterns of Fig. 4 can be obtained with little
change in total coupled power. Similar changes in
single-mode fiber had a core diameter of 3.8 Am and a
numerical aperture of 0.12. Both fibers had cutoff
wavelengths for the LP11mode of about 580 nm. To
obtain double-mode operation the visible lines of an
argon-ion laser were used. Owing to the constraints
imposed by the available fibers, the fiber that was to
act as a single-mode fiber allowed two modes to propagate. To ensure only single-mode operation a modal
stripper was employed to remove any of the unwanted
LV') mode.
MODE
STRIPPER PC
SMF
LPO
-
INPUT
DMF
SCREEN
Fig. 2. Experimental arrangement used in measuring parameters of the evanescent modal filter. Input LP0 1mode is
coupled to output LP11 mode.
Figure 2 shows the experimental arrangement used
to measure the coupling characteristics between the
two dissimilar fibers. The fibers had a radius of curvature of 25 cm, which resulted in an effective coupling
length of about 1 mm. An oil having an index slightly
lower than the cladding indices was placed between
the two coupler halves. An argon-ion laser provided
input signals at wavelengths of 457.9, 488.0, and 514.5
nm. The top fiber in Fig. 2 acted as a single-mode
input to the modal coupler. The mode stripper was
LP~11mode.
used to eliminate any of the unwanted
An in-line polarization controller'3 (PC) was placed
after the mode stripper so that the polarization of the
incoming LP(1)mode could be adjusted. The bottom
fiber in Fig. 2 was double moded, supporting both the
LP 01)and LP11 modes. The output section of this
fiber was kept straight to prevent coupling between
SMF
DMF
Fig. 3. Coupled (DMF) and uncoupled (SMF) outputs
from the modal coupler. Coupling is adjusted to approximately 50%; wavelength is 488.0 nm.
September 1986 / Vol. 11, No; 9 / OPTICS LETTERS
u
~~~~~(a)
4 |/ (b)
(C)
Fig. 4.
Double-mode fiber outputs for coupling efficiencies
greater than 90%. Mode shapes (a) through (c) resulted as
coupling fibers were translated with respect to each other.
intensity patterns could be obtained by varying the
input polarization while the fiber positions in the directional coupler remained constant. In its present
form, the mode pattern sensitivity to coupling and
polarization may limit the efficiency of the device as a
modal filter since coupling coefficients will be affected
by lobe positions. One method for avoiding this
would be to use a double-mode fiber with an elliptical
core to increase the separation between the propagation constants of the LP11 mode patterns.
Insertion losses were relatively high, with the best
measured result being a 5% loss at a coupling of 88%.
Scattering due to the short wavelength and any residual second-order mode not removed by the LP(1!)mode
stripper could contribute to these measured losses.
Losses for conventional polished single-mode couplers
can be well under 1%, and similar losses should be
possible with the evanescent modal coupler.
Accurate measurements of the extinction ratio for
the powers coupled to the LP(2) and LP(2)modes were
difficult to obtain. Owing to the W-index profile of
the double-mode fiber it was difficult to strip away the
LP(2) mode by fiber bending without inducing significant coupling to the lowest-order LP(') mode. The
best measurement for the modal extinction ratio was
obtained by placing an apertured detector (area 1
mm2 ) at the center location of the far-field LP11mode
pattern (fiber-detector distance _ 1 m). Assuming
583
that the major contribution to the power reaching the
detector is from the LP01 mode, this signal was calibrated to determine the total power in this mode.
The detector location was determined by finding the
minimum power reading while adjusting the polarization controller to rotate the LP(2l)mode pattern. Using this method, a ratio of -24 dB was measured when
coupling to the LP(') mode was greater than 90%. The
value of -24 dB is poorer than theoretically expected.
Possible reasons for this could be the scattering in the
coupling region or limitations in the measurement
scheme. With more carefully chosen fibers and longer
wavelengths the extinction of the LP(2l) mode is ex-
pected to be much better.
Although coupling was demonstrated with a doublemode fiber, this technique can be extended to allow
coupling to fibers that possess several modes. For this
case a different single-mode fiber will be needed to
phase match to each individual mode. Such devices
could act as modal multiplexers/demultiplexers to enhance the information capacity of optical fibers.
In summary, a highly selective modal coupler has
been demonstrated that evanescently couples between
the LP01 and LP11modes in two different fibers. Coupling efficiencies as high as 97% were obtained.
Cou-
pling purity of the excited LP11mode was measured to
be at least 24 dB. Theoretically this purity can be
much better, and the above measurement may have
been limited by experimental difficulties. This modal
filter is currently being used in the development of a
new fiber-optic single-sideband frequency shifter.2
Other applications of such a device could be for modal
multiplexing/demultiplexing and the development of
in-line optical-fiber devices.
This work was supported by Litton Systems Inc.
* Present address, Hewlett Packard Laboratories,
1501 Page Mill Road, Palo Alto, California 94304.
References
1. R. G. Lamont, K. 0. Hill, and D. C. Johnson, Opt. Lett.
10,46 (1985).
2. B. Y. Kim, J. N. Blake, H. E. Engan, and H. J. Shaw,
Opt. Lett. 11, 389 (1986).
3. R. C. Youngquist, J. L. Brooks, and H. J. Shaw, Opt.
Lett. 9, 177 (1984).
4. H. F. Taylor, IEEE J. Lightwave Technol. LT-2, 617
(1984).
5. J. N. Blake, B. Y. Kim, and H. J. Shaw, Opt. Lett. 11, 177
(1986).
6. R. A. Bergh, G. Kotler, and H. J. Shaw, Electron. Lett.
16, 260 (1980).
7. K. 0. Hill, B. S. Kawasaki, and D. C. Johnson, Appl.
Phys. Lett. 31, 740 (1977).
8. P. Bassi, G. Falciasecca, and M. Zoboli, "Experimental
results on coupling between optical fibers operating in
low multimode regime," presented at the Fifth National
Reunion of Applied Electromagnetism, St. Vincent, Ita-
ly, 1984.
9. M. S. Whalen and T. H. Wood, Electron. Lett. 21, 175
(1985).
10. D. Gloge, Appl. Opt. 10, 2252 (1971).
11. A. Yariv, IEEE J. Quantum Electron. QE-9, 919 (1973).
12. W. V. Sorin, B. Y. Kim, and H. J. Shaw, Opt. Lett. 11,
106 (1986).
13. H. C. Lefevre, Electron. Lett. 16, 778 (1980).