We developed and fabricated an all-fiber add–drop filter by recording a Bragg grating in the waist of an asymmetric mode converter–coupler formed by adiabatic tapering and fusing of two locally dissimilar, single-mode optical fibers. The insertion loss of the device was ~0.1 dB. A narrow spectral bandwidth (90%) were also demonstrated. In addition, the filter was polarization independent

Kewitsch, Anthony S.

Rakuljic, George A.

Willems, Phil A.

Yariv, Amnon

Caltech Authors - Main

106 OPTICS LETTERS / Vol. 23, No. 2 / January 15, 1998All-fiber zero-insertion-loss add–drop filter for
wavelength-division multiplexing
Anthony S. Kewitsch, George A. Rakuljic, Phil A. Willems, and Amnon Yariv*
Arroyo Optics, Inc., Santa Monica, California 90404Received June 18, 1997
We developed and fabricated an all-fiber add–drop filter by recording a Bragg grating in the waist of an
asymmetric mode converter–coupler formed by adiabatic tapering and fusing of two locally dissimilar, single-
mode optical fibers. The insertion loss of the device was ,0.1 dB. A narrow spectral bandwidth s,1 nmd
and a large add–drop efficiency s.90%d were also demonstrated. In addition, the filter was polarization
independent.  1998 Optical Society of America
OCIS codes: 060.0060, 060.4230, 060.2310, 060.2340, 120.2440.Numerous optical f ilter concepts have been proposed
for dense wavelength-division multiplexing (WDM) ap-
plications.1 – 5 For the lowest insertion loss and most
reliably packaged device, an all-f iber solution is pre-
ferred. All-f iber add–drop filters have been demon-
strated by combination of fiber Bragg gratings with a
Mach–Zehnder interferometer6 or a polarization split-
ter,7 for example. A grating-assisted directional cou-
pler8 can also function as an add–drop filter, and an
all-f iber device based on evanescent wave coupling and
fiber Bragg gratings has been proposed.9
The present filter is based on grating-assisted mode
conversion and backward coupling in the merged re-
gion of two dissimilar waveguides formed by adiabatic
tapering and strong fusing of two single-mode optical
fibers. From the theory of adiabatic following,10 the
modes of the input fibers will evolve on a one-to-one ba-
sis into the first two eigenmodes of the merged region.
These eigenmodes will propagate through the waist in-
dependently of each other, transform back into their
original modes, and exit the device through their re-
spective output f ibers. This adiabatic mode evolution
is lossless and free of optical cross talk.
Figure 1(a) illustrates the one-to-one relationship
between input modes 1 and 2 in waveguides I and II,
and the eigenmodes into which they are transformed.
By recording a transversely asymmetric Bragg grating
in the waist of the device, one can convert eigenmode 1
and backward couple it into eigenmode 2, which will
then exit the merged region as mode 2 via wave-
guide II, as shown in Fig. 1(b). This transformation
will occur only at a specific wavelength l that satisfies
the phase-matching or Bragg condition
b1sld 1 b2sld ­ 2pyLg , (1)
where Lg is the period of the grating and b1, 2 are
the respective modal propagation constants. Because
the eigenmodes and the grating fully overlap in the
merged region, the coupling coeff icient of this device
is much larger than that of a grating-assisted di-
rectional coupler based on evanescent wave coupling.0146-9592/98/020106-03$10.00/0If b1 1 b2 Þ 2pyLg, input mode 1 will simply pass
through the merged region, unaffected by the grat-
ing, and exit waveguide I unattenuated, as shown in
Fig. 1(c). Conversely, if light that satisfies the Bragg
condition is launched into waveguide II it will be added
onto waveguide I by the grating. Thus this device
functions as an efficient, narrow-bandwidth four-port
all-f iber optical add–drop filter.
The structure of the filter that was fabricated is
shown in Fig. 2. The diameter of the merged waist re-
gion was 7 mm. Inasmuch as identical starting f ibers
were used, we introduced the necessary waveguide
dissimilarity by locally pretapering one of the two
Fig. 1. Operating principle of the grating-assisted mode
converter–coupler: (a) adiabatic mode evolution without a
grating, (b) mode transformation when b1 1 b2 ­ 2pyLg,
(c) mode transformation when b1 1 b2 Þ 2pyLg. 1998 Optical Society of America
January 15, 1998 / Vol. 23, No. 2 / OPTICS LETTERS 107Fig. 2. The structure of the add–drop filter that was
fabricated.
fibers before fusion. To record a strong grating in the
waist of the device, we used photosensitive single-mode
optical f iber. Eff iciently coupling the eigenmodes in
the merged region and preventing backref lection into
the input fiber at a different wavelength l0 given
by b1sl0d ­ pyLg required a transversely asymmet-
ric grating. To satisfy this requirement we recorded
a tilted Bragg grating in the waist of the device, using
an excimer laser. The tilt angle was 4–.
The out-of-band insertion loss of the f ilter measured
in throughput was ,0.1 dB, including the losses that
were due to splicing, nonadiabaticity of the tapered
and merged regions, and the UV-induced background
loss. The splicing losses were negligible. Figure 3
shows that ,0.1-dB excess loss was incurred during
the fabrication of the device before the UV exposure.
The UV-induced background loss was ,0.1 dB. We
can reduce this loss by exchanging the OH2 ions in
the photosensitive optical f iber with OD2 ions. With
OD2-exchanged fiber, a zero-insertion-loss add–drop
filter is expected.
A narrow-bandwidth s,0.7-nmd grating was re-
corded in the waist of the device. The spectral
response of the f ilter is shown in Fig. 4. A peak
add–drop eff iciency of ,98% centered at 1547 nm was
measured. We used an unpolarized erbium-doped
fiber amplified stimulated emission source and an
HP 71450B optical spectrum analyzer to obtain the
data. The response of the f ilter was also examined
with polarized input light, but no polarization depen-
dence was observed.
A 98% add–drop filter eff iciency is generally not suf-
ficient to provide adequate channel rejection for WDM
applications. To get 230 dB of rejection, for example,
requires a grating efficiency of 99.9%. Because the
ref lectivity of a standard fiber Bragg grating can
easily surpass this value,11 much greater add–drop
efficiency is expected with further development. With
improved channel rejection the device can simultane-
ously function as both an add and a drop filter. Thus
bidirectional operation is possible. One can also make
a wavelength-selective optical switch or router by com-
bining a high-efficiency four-port add–drop filter with
a standard optical switch, as shown in Fig. 5. Depend-
ing on the electrical input to this device, a particu-
lar WDM channel can be routed to either one of two
output f ibers without disturbing the other wavelength
channels.
In summary, we have developed and fabricated
an all-f iber four-port add–drop filter by recording
a Bragg grating in the merged region of two adia-batically tapered and fused optical fibers. A device
with low insertion loss, large add–drop eff iciency,
no polarization dependence, and a narrow spectral
bandwidth was demonstrated. As the operation of
the filter does not depend on optical interference or
directional coupling, strict optical path length control
is not required. With increased add–drop eff iciency,
the filter is expected to apply to numerous dense
WDM applications, including wavelength-selective
routing and optical switching.
This research was supported, in part, by the U.S.
Army Missile Command under contracts DAAH01-96-
C-R152 and DAAH01-97-C-R043 with funding from the
Defense Advanced Research Projects Agency and by the
Fig. 3. Throughput of the device during fabrication. Ini-
tially the two fibers were separate. After tapering and fu-
sion, but before the UV exposure, ,0.1-dB excess loss was
incurred.
Fig. 4. Spectral response of the add–drop f ilter.
Fig. 5. Wavelength-selective optical switch.
108 OPTICS LETTERS / Vol. 23, No. 2 / January 15, 1998Naval Surface Warfare Center under contract N00178-
96-C-3058 with funding from the Ballistic Missile
Defense Organization.
*Permanent address, California Institute of Technol-
ogy, Pasadena, California 91125.
References
1. C. Dragone, IEEE Photon. Technol. Lett. 3, 812 (1991).
2. F. Heismann, L. L. Buhl, B. I. Miller, M. Newkirk, U.
Koren, M. G. Young, and R. C. Alferness, Appl. Phys.
Lett. 64, 2335 (1994).
3. T. Komukai, Y. Miyajima, and M. Nakazawa, Jpn. J.
Appl. Phys. 34, L230 (1995).
4. J.-L. Archambault, P. St. J. Russell, S. Barcelos, P.
Hua, and L. Reekie, Opt. Lett. 19, 180 (1994).5. I. Baumann, J. Seifert, W. Nowak, and M. Sauer, IEEE
Photon. Technol. Lett. 8, 1331 (1996).
6. F. Bilodeau, D. C. Johnson, S. Theriault, B. Malo, J.
Albert, and K. O. Hill, IEEE Photon. Technol. Lett. 7,
388 (1995).
7. M. J. Guy, S. V. Chernikov, J. R. Taylor, and R.
Kashyap, Electron. Lett. 30, 1512 (1994).
8. D. Marcuse, Theory of Dielectric Optical Waveguides
(Academic, Boston, Mass., 1991), Chap. 7.
9. E. Snitzer, ‘‘Add–drop device for a wavelength division
multiple, f iber optic transmission system,’’ U.S. patent
5,457,758 (October 10, 1995).
10. L. I. Schiff, Quantum Mechanics (McGraw-Hill, New
York, 1968), pp. 213–216.
11. R. Kashyap, P. F. McKee, D. J. Armes, M. Shabeer, and
D. Cotter, Electron. Lett. 31, 1282 (1995).


All-fiber zero-insertion-loss add-drop filter for wavelength-division multiplexing

https://authors.library.caltech.edu/5572/1/KEWol98.pdf

All-fiber zero-insertion-loss add-drop filter for wavelength-division multiplexing

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main