Received September 17, 1987; accepted November 10, 1987. It is shown that wavelength-selective optical-fiber couplers can be made by choosing materials ...
158
OPTICS LETTERS / Vol. 13, No. 2 / February 1988
Wavelength-selective optical-fiber directional couplers using dispersive materials Katsumi Morishita Department
of Precision Engineering, Osaka Electro-Communication
University, Neyagawa, Osaka 572 Japan
Received September 17, 1987; accepted November 10, 1987 It is shown that wavelength-selective optical-fiber couplers can be made by choosing materials having appropriate The wavelength-selective effect is caused by differential material dispersion of the dispersive characteristics. are optical fibers that compose the coupler. The 1.30-1.57-jum wavelength-division multiplexers/demultiplexers designed by the use of optical-fiber couplers fabricated from dispersive materials.
Wavelength-selective
optical-fiber
couplers will be
important in-line components in wavelength-multiplexed single-mode optical-fiber communication
sys-
tems, light amplification using optical fibers, and spectral measurements. Several types of structure have been proposed to provide wavelength-selective coupling, including evanescent wave couplers with differential
waveguide dispersion
in dissimilar
wave-
guides,1- 5 the natural wavelength dependence of the coupling in evanescent wave couplers made of identical fibers, 6- 8 concatenated directional couplers, 9' 10 and
Bragg-reflection filters in single-mode-fiber directional couplers. 11
However, the design of wavelength-se-
lective couplers consisting of optical fibers fabricated from suitable dispersive materials has not yet been proposed. Optical fibers made from dispersive materials are applied to optical-fiber length-insensitive couplers.13
filters 1 2 and wave-
In this Letter we describe the use of differential material dispersion to provide wavelength-selective coupling that has a nonperiodic filter response gener-
core diameters and index profiles. If two single-mode fibers are designed in such a way that chromatic refractive-index curves of the core materials intersect at some wavelength, then the fundamental modes of the fibers with the same core diameters and index profiles can synchronize at that wavelength. Consider the specific example of optical fibers in which the core materials are made of PCD5 and BaCED1 glasses and the cladding materials are of ADF10 glass. The variations of refractive index with wavelength are calculated from the dispersion formula given in the Hoya Optical Glass Technical Data' 4 and are shown in Fig. 1. The refractive index is extrapolated in the wavelength regions above 1.014 ,gm. It is seen that the core (PCD5) index in the PCD5/ADF10 fiber coincides with the core (BaCED1) index in the BaCED1/ADF1O fiber at 1.566-gm wavelength.
Syn-
chronism of the fundamental modes of the fibers with can be obtained only over
the same core diameters
narrow wavelength ranges near 1.566,im.
By using coupled-mode analysis,15 the power trans-
ated by core using materials whose indices change with changes in wavelength. By choosing suitable core glasses, one can make the propagation constants of the
fundamental modes in two optical fibers unequal at
1.61
most wavelengths, reaching synchronism only over a
relatively narrow wavelength range. Complete power transfer between two fibers takes place among synchronous modes. Therefore we can get multiplexers/
demultiplexers and filters by using dispersive fiber
x
w
w
1.80
couplers in which significant power transfer is possible only over a synchronous wavelength range.
In conventional fibers the core and cladding glasses have similar spectral characteristics, so the index difference between them is almost constant relative to wavelength. The fibers that make up the coupler must have different core diameters and different corecladding index differences'-4 and/or different index profiles5 to produce synchronism of the fundamental modes over. only narrow wavelength ranges. However, it is possible to gain synchronism by selecting core
materials having different variations of refractive index with wavelength even if two fibers have the same 0146-9592/88/020158-03$02.00/0
cr 1.59 LU w
0
1.58
L 0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.8
1.7
WAVELENGTH (gm) Fig. 1.
Chromatic refractive indices of PCD5, BaCED1,
and ADF10 calculated from Hoya Optical Glass Technical
Data.14
© 1988, Optical Society of America
February 1988 / Vol. 13, No. 2 / OPTICS LETTERS
two wavelengths
is adjustable
by changing the dis-
tance between the core centers.
V a:
159
Isolation of more
than 20 and 30 dB can be provided by distances of
0
-10
0
so°
j//////BaCED1/ADF10
about 10.5 and 12.0 gm, respectively. The coupled power between the PCD5/ADF10 and fibers is calculated by using Eq. (2) and is shown in Fig. 3. Chromatic index curves intersect at the 1.566-gm wavelength, as shown in Fig. 1,
Ucr
a:
and synchronism of the fundamental modes occurs near there. The power is completely transferred into the second fiber at that wavelength. However, there is little coupled power at the 1.3-gm wavelength, e.g., -23 dB for d = 10.5 gimand less than -30 dB for d = 12.0 gum. We can obtain the desired coupling ratio between two wavelengths without using the natural
U-
co, -20
z
a1 = 2.9 gm
a:
a22.9 gm //
-30
1.30
1.35
1.40
1.45
1.50
1.55
wavelength dependenceofthe coupling, whichisdiffi-
1.60
WAVELENGTH (pm) Fig. 2. The epower-transfer factor between the PCD5/ ADF10 and I3aCED1/ADF1Ofibers with the same core radii of 2.9 Am.
fer between the fundamental mode powers P and of fibers 1 a nd 2, respectively, is governed by
Pl(z) = 1 -
F sin2 fiZ,
P2 (z) = F sin22bZ,
(1) (2)
2 F == 1/1 + (0d/C) ,
ide c
C=
= id+, i012
tersect at some wavelength. Synchronism of the fundamental modes appears near that wavelength. The synchronism point can be shifted to other wavelengths by choosing various the core glasses of two fibers and ters. The core-cladding-index difference in these fibers increases with wavelength, as shown in Fig. 1, and
the change of spot sizes of the propagation modes is reduced. The core diameters of both fibers can be almost equal because of their differential material dispersion. Therefore couplers using dispersive materials are advantageous for connecting and splicing. The desired fibers for couplers can be fabricated without precise control of the refractive-index difference'-3
and the profiles.4' 5
052
4~6_ 162 UJU2
a-a2 (VJV2
Letter is caused by differential material dispersion. Chromatic refractive-index curves of core glasses in-
can be adjusted by using slightly different core diame-
where
Ob
cult to control.6-8 The wavelength-selective effect described in this
)31 2
K0 (Wld/al) K,(WI)K(W 2 )
(3
is the coup]ling coefficient between the fundamental modes of the two fibers,' 6
In summary, a technique for realizing wavelengthselective optical-fiber couplers has been proposed. The wavelength-selective effect is caused by differential material dispersion. The useful wavelength can be varied by choosing various core glasses and adjust-
ed by using slightly different core diameters. Disper-
bi= 1 - (n.ciIci) 2 , Vij = ai2 ko 2 (nci2 - nC12)
Ui' = ai2 (k 0 2 nCi2 -i2), Wi 2 = a 2 (fi2
-
0-%
are the refractive indices of the core and cladding of fiber i, respectively, and d is the distance between the core centers.
Figure 2 shows the transfer factor F between the PCD5/ADF10 and BaCED1/ADF10 fibers with the same core radii of 2.9 gim. The transfer factor is the maximum fraction of power transferred, and it reaches its maximum value of 1.0 at synchronous wavelength gim.
I
d=12.0 gm
I
ko2 nc12 ),
ai is the core radius of fiber i, Ai is the propagation constant of the fundamental mode of fiber i, nci and nil
1.566
…---
We therefore can get the 1.30/1.57-gm
wavelength-division multiplexers/demultiplexers by using that pair of dispersive fibers. Isolation between
n-10 -2
:
0
1
a .0
1 0
.
-3
.5
c
.5
6
1.55
1.60
ri
U~~~~~~~
-30
1.30
1.35
1.40
1.45
WAVELENGTH
1.50
(gjm)
Fig. 3. The coupled power between the PCD5/ADF1O and BaCED1/ADF1O fibers with the same core radii of 2.9 gum.
160
OPTICS LETTERS / Vol. 13, No. 2 / February 1988
sive-fiber couplers are advantageous for connecting and splicing because of the reduced change in spot sizes with wavelength and similar core diameters. Dispersive fibers are produced from dispersive materials, possibly by the rod-in-tube technique. References 1. H. F. Taylor, Opt. Commun. 8, 421 (1973).
2. 0. Parriaux, F. Bernoux, and G. Chartier, J. Opt. Commun. 2, 105 (1981).
3. K. Kitayama and Y. Ishida, J. Opt. Soc. Am. A 2, 90 (1985). 4. M. S. Whalen and K. L. Walker, Electron. Lett. 21, 724 (1985). 5. D. Marcuse, Electron. Lett. 21, 726 (1985). 6. M. Digonnet and H. J. Shaw, Appl. Opt. 22, 484 (1983). 7. C. M. Ragdale, D. N. Payne, F. De Fornel, and R. J.
Mears, in Digest of the First International Conference
on Optical Fiber Sensors (Optical Society of America, Washington, D.C., 1983), pp. 75-78.
8. C. M. Lawson, P. M. Kopera, T. Y. Hsu, and V. J. Tekippe, Electron. Lett. 20, 963 (1984).
9. M. S. Yataki, and D. N. Payne, and M. P. Varnham, Electron. Lett. 21, 248 (1985). 10. K. Okamoto and J. Noda, Electron. Lett. 22, 211 (1986). 11. M. S. Whalen, M. D. Divino, and R. C. Alferness, Electron. Lett. 22, 681 (1986). 12. K. Morishita, M. S. Yataki, and W. A. Gambling, Electron. Lett. 23, 319 (1987). 13. K. Morishita, M. S. Yataki, and W. A. Gambling, Opt. Lett. 12, 534 (1987).
14. Hoya Optical Glass Technical Data (Hoya Corporation, Tokyo, 1985).
15. W. H. Louisell, Coupled Mode and Parametric Electronics (Wiley, New York, 1960), Chap. 1. 16. P. D. McIntyre and A. W. Snyder, J. Opt. Soc. Am. 63, 1518 (1973).
