We propose and demonstrate experimentally a fiber optic micro-device that is capable of tunably splitting, multiplexing, and demultiplexing optical signals using elliptical-core two-mode optical fiber. A crosstalk of 15 dB with an insertion loss of 1.2 dB was obtained

Claus, R. O.

Murphy, K. A.

Vengsarkar, A. M.

Wang, A.

Wang, G. Z.

English

NASA Technical Reports Server

Micro-Device for Coupling, Multiplexing and
Demultiplexing Using Elliptical-Core Two-Mode
A WANG, K A
/'
Fiber
MURPHY, G Z WANG, A M VENGSARKAR*, AND R O CLAUS
Fiber & Electro-Optics Research Center, Bradley Department of Electrical Engineering,
Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0111
* AT & T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974
ABSTRACT: In this paper we propose and demonstrate experimentally a fiber optic micro-
device that is capable of tunably splitting, multiplexing and demultiplexing.optical signals using
elliptical-core two-mode optical fiber. A crosstalk of 15 dB with an insertion loss of 1.2 dB
has been obtained.
1. INTRODUCTION
Passive fiber components such as couplers, multiplexers and demultiplexers are essential elements
in fiber optic telecommunication and sensing technology. Various kinds of devices have been
developed for these applications over the past twenty years. The fused biconical tapered fiber
coupler (Hill et al 1985) and the polished fiber coupler (Lefevre et al 1988) are the couplers most
frequently used today. The former has excess loss as little as 0.1 _IB and the latter has been shown
particularly suitable for polarization preserved applications. There are also two major classes of
wavelength division multiplexer (WDM) and demultiplexers. The first is constructed based on the
operation of the fused biconical taper fiber coupler. Since the coupling ratio of this kind of coupler
is strongly dependent on the operating wavelength (Marcuse 1972), it can be used as WDM for the
wavelength difference between two operating wavelengths is large (Winzer et al 1981). Another
class of WDM is designed based on a fiber optic micro-device which uses dispersive components
such as gratings or interference filters to separate the operating wavelengths (Allard 1990).
Recently, based on elliptical-core (e-core) two-mode fibers, an intermodal coupler, modal filter,
acoustic-optic frequency shifter, and fiber sensor have been demonstrated by Kim et al (1987),
Kim et al (1986) and Huang et al (1990). In this paper we propose and demonstrate experimentally
a fiber optic micro-device using the e-core two-mode fiber to multiplex and demultiplex two
optical signals. The objective of this paper is first to show the principle of operation of this new
fiber optic micro-device and then to describe the experiments that verify the concept.
2, PRINCIPLE OF OPERATION
Figure 1 illustrates the schematic of the two-mode fiber micro-device. Since the e-core fiber is
operated in its two-mode regime, only the LP01 and LPII even modes are excited. For an e-core
fiber the electric field of the LPIm mode can be written as
(NASA-CR-190490) MICRO-OEVICE FOR COUPLING,
MULTIPLEXING ANO _EMULTTPLEXING USING
_LLTPTICAL-CORE TWo-MODE gI_ER (Virginia
Polytechnic Inst. and Stato Univ.) 0 p
G3/76
N92-29331
Unclas
0105Z07
https://ntrs.nasa.gov/search.jsp?R=19920020088 2020-03-17T10:49:21+00:00Z
E(LPim)=Elm(X, y) exp(j_im z), (1)
where _lm is the propagation constant of the mode and Elm(X, y) gives a description of the
transverse electric field amplitude. The differential phase delay A_ between the LP01 and LP 11even
modes is given by
Aq_ = A]3 L, (2)
where A[_ is the difference in the propagation constants of the two modes and L is the length of the
fiber.
Lme.,_ly pol_.zed liner
E-Core Two-Mode Fil_" /[
/
Figure 1. Schematic of an elliptical-eore two-mode fiber micro-device.
Thc interference between the LP01 and LP 11even mode field gives a two-lobe spatial intensity
pattern at the fiber endface, and hence at the far-field from the end of the fiber, as shown in Figure
2. It is noted that a x/2 phase shift, known as the Gouy phase shift, between the patterns at the
fiber endface and in far field (Huang et al 1989), has been taken into account. The shape of the
spatial intensity pattern is determined solely by the differential phase delay A_. In Figure 2, it is
recognized that the left lobe is at its brightest and the right lobe is at its dimmest provided that Aq_ is
re/2. When A_ is _, the brightest lobe on the left becomes the dimmest and the dimmest on the
right changes to the brightest. Therefore, by controlling the differential phase delay A0 between the
kPOl and LP 11even modes, the optical power in the e-core tWo-mode fiber can be divided into two
channels.
One way to change A0, as suggested by Equation (2), is to alter the length of the fiber L. A
variation in the fiber length causes an oscillation of the intensity between the two lobes at the
output. If a detector with a spatial filter, such as a pin-hole detector, is used to pick up a fraction of
the power in only one lobe, the output of this detector is a sine function of the change in the fibcr
length. This modulation effect has been used in the past for strain measurement (Huang et al 1990
and Murphy et al 1990). In the process of fabricating the Y coupler as shown in Figure 1, we also
aridMurphyetal 1990).In theprocessof fabricatingtheY couplerasshown in Figure 1, we also
applied axial strain on the fiber to observe the toggling of the two-lobe spatial intensity.
It is further recognized that the two-lobe spatial intensity distribution can be changed by controlling
I
AtD_X
A
!
.....L ..................t
A_ = 3v.r2 I&
vl
Figure 2. View of the two lobe intensity distribution.
the wavelength. Because AI3 in Equation (2) is a function of the operating wavelength [8], a
continuous change of the wavelength of light launched into the fiber would also toggle the optical
power between the two lobes. This phenomenon was used in the past for demodulation in e-core
two-mode fiber sensing applications by Wang et al 1991. We simultaneously launch two laser
beams of different wavelengths _'1 and _'2 into the e-core two-mode fiber. These two wavelengths
are properly chosen in such a way that _1 corresponds to a maximum intensity in lobe 1 and _-2
corresponds to a maximum intensity in lobe 2, as indicated in Eigure 1. Therefore, two optical
signals, carried by X1 and _'2, respectively, are separated into two channels. It is realized that the
selection of _'1 and _2 is not unique since the intensity of either of these two lobes is a periodic
function of the operation wavelength.
3. EXPERIMENTS
To observe the two-lobe behavior, we launched a linearly polarized light of wavelength 633 nm
into the e-core fiber (1.25 x 2.5 I.tm core), manufactured by Andrew Corporation, which operated
in its two mode regime. It is customary to align the polarization orientation of the light with the
major axis of the elliptical core to obtain a clear spatial intensity pattern. A thin aluminum front
surface mirror was used to separate the two lobes as illustrated in Figure 1. For the convenience
of investigation, the optical power of each individual lobe was collected by two photodetectors.
Figure 3 shows the output of the two photodetectors, represented by channel 1 and channel 2 on
theoscilloscope,whenwegraduallyappliedanaxialstrainon thee-corefiber.FromFigure3,it is
,.,..,
O
....!llll
--:-s:--7 :-]
Figure 3. Oscilloscope trace of the two channel outputs.
Figure 3 shows the output of the two photodetectors, represented by channel 1 and channel 2 on
the oscilloscope, when we gradually applied an axial strain on the e-core fiber. From Figure 3, it is
noted that the device shown in Figure 1 can be used as a splitter with a tunable splitting ratio.
d,_ ---3r¢2
....
•_ = g/2
Shield Plate
1
i ....
Figure 4. View of the aperture positions.
To obtain experimentally the relation between the cross talk and insertion loss of this WDM device,
we used a opaque plate with a sharp edge to block a part of optical power from reaching the
photodetector as shown in Figure 4, and we subsequently changed the position of the plate. For
each position of the plate, maximum (Imax) and minimum (Imin) intensity values could be found by
axially stretching the fiber. The total input power was measured by cutting this e-core two-mode
fiber. The insertion loss, given by -10 log (ImaxBtotal), was derived from this experimental data.
Having these experimental results, we plotted the relation between the cross talk and insertion loss
as presented in Figure 5.
The dependence of the two-lobe spatial intensity pattern on operating wavelength was investigated
by first launching an Argon laser beam into the e-core two-mode fiber. The wavelength was tuned
within a certain range (590 nm to 650 nm) and the results are shown in Figure 6. This experimental
data plot demonstrates the wavelength dependence of the two-lobe spatial intensity distribution.
Since the Argon laser could produce only discrete wavelengths, we could not determine the exact
maximum or minimum power of a lobe. To explore this effect further, a semiconductor laser with
a central wavelength of 816 rim, manufactured by GALA, was employed. Since the wavelength of
a semiconductor laser is dependent on its current in general, it could be controlled within a certain
range by adjusting the current knob. Unlike the Argon laser, this semiconductor laser could
generate a continuous change of wavelength. The experimental result is presented in Figure 7. The
sigz_ificantchangesof thelaserwavelength.
en
"0
v
2
Figure 5.
16
15
14
13
12
11
10 I ' " I '' I
2 4 6
Insertion Loss (dB)
Crosstalk as a function of the insertion loss. The triangle dots are the
experimental data. The solid line is the calculation results based on
the theoretical model given in another our paper, entitled "Analyses of
Near- and Far-Field Two-Lobe Patterns Outside the Output Endface
of a Two-Mode Fiber", published in this proceeding.
o
'1o
N
O
Z
1.2
1.0
0.8
0.6"
0.4
0.2
0.0
45O
0 •
o
o
@
o
o
" I ' " i ' " I
470 490 510 530
Wavelength (nm)
o Channel 1
• Channel 2
Figure 6. Outputs of the two channels as functions of the operating wavelength.
To demonstrate the feasibility of the device in practical fiber systems, the output from the e-core
two-mode fiber was collimated by a quarter pitch GRIN lens and the former two photodetectors
were replaced by two single mode fibers with quarter pitch GRIN lenses attached. An insertion
loss of 1.2 dB with a crosstalk of 15 dB has been obtained using a HeNe laser source. The
insertion loss of this configuration could be reduced further by applying anti-reflection coatings on
those GRIN lenses.
!-
2
O
L
#'
¢
f
,/
t
iT"
.f
j_
Driving Current (Arbitrary Unit)
¢=
>,
"5
¢3.
O
t
!
r
!. j '7
_J
Driving Current (Arbitrary Unit)
(a) (b
Figure 7. One channel as a function of the laser diode current, a) The output of the lasc_ as a
function of the laser current, b) The output of the lobe intensity on the laser current.
CONCLUSIONS
A micro-device based on the two-lobe pattern obtained at the output of elliptical-core two-mode
fiber has been described. It has been demonstrated that this fiber device can be used as a fiber
splitter, a wavelength division multiplexer and a demuhiplexer. Crosstalk of 15 dB has been
achieved with an insertion loss of 1.2 dB. It is our hope that further research on this fibcr devicc
would lead us to develop low loss and highly accurate practical fiber optic components.
"/'his research was sponsored in part by Virginia Center for Innovative Technology. We would like
to thank Professor I. Jacobs for useful discussions.
REFERENCES
tlill K O et al 1985 Proceedings of the Society of Photo-Opticallnstrumentation Engineers 574 92
Lefevre H C et al 1988 Electronics Letters 24 1304
Marcuse D 1972 Light Transmission Optics (Van Nostrand, New York) Chap. 10
Winzer G et al 1981AppliedOptics 20 3128
Allard F C 1990 Fiber optics handbook for engineers and scientists (McGraw-Hill) Chapter 3-3.5
Kim B Y, Blake J N, Huang S Y, and Shaw H J 1987 0ptics Letters 12 p 729
Kim B Y, Blake J N, Engan H, and Shaw H J 19860pticsLetters 11 p 389
ttuang S Y, Blake J N, and Kim B Y 1990J. Lightwave Technology 8 p 23
ttuang S Y, Park H G, and Kim B Y 1989 14 p 1380
Murphy K A, Miller M S, Vengsarkar A M, and Claus R O 1990J. Lightwave Technology 8 p
1688
Wang A, Murphy K A, Vengsarkar A M, and Claus R O 1991ElectronicsLetters 27 p 1454


Micro-device for coupling, multiplexing and demultiplexing using elliptical-core two-mode fiber

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