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FIELD OF THE INVENTION
The invention relates to the field of optical devices formed in optical
waveguides for interacting with propagating light in the waveguide, and
more particularly to optical field intensity modulation and optical
switching in optical fibers or optical planar waveguides, and more
particularly to an optical modulator employing side electrode geometry for
modulating electromagnetic radiation propagating in an optical fiber
through nonresonant evanescent field interactive coupling using a
polymeric material as the active overlay medium.
BACKGROUND OF THE INVENTION
The side-polishing of waveguides, such as optical fibers, to form
side-polished couplers to access its radiation propagating field has been
discussed in some detail over the past two decades in the optical
waveguide art. In the case of optical fibers, a portion of the cladding of
the fiber is removed, such as by polishing away upper portion, region or
layers of the fiber waveguide, and optical transmission through the fiber
is interrupted or otherwise perturbed by placing an optical medium, i.e.,
an overlay medium, with a higher refractive index than the fiber core on
the side-polished portion. Examples are found in the articles of R. A.
Bergh et al. entitled, "Single Mode Fibre Optic Directional
Coupler",Electronic Letters, Vol. 16(7), pp. 260-261, Mar. 27, 1980; C D.
Hussey et al. entitled "Optical Fibre Polishing With a Motor Driven
Polishing Wheel", Electronics Letters, vol. 24(13), pp. 805-807, Jun. 23,
1988; and C. Y. Cryan et al., entitled "Overcoming the Effects of
Polishing Induced Stress When Fabricating Fused Polished Couplers",
Electronics Letters, Vol. 29(14), pp. 1243-1244, Jul. 8, 1993. Such access
to the field propagating in the core of a waveguide is accomplished
through evanescent field coupling. Changing the strength of this coupling
provides a way to modulate the transmission function of a given waveguide.
For the device to be useful, abrupt changes in the transmission, T, (or
coupling strength, .DELTA.k) must be possible.
One way to provide a fast and efficient change of coupling strength,
.DELTA.k, is to place an overlay medium in the side-polished region of the
waveguide. Examples of the use overlay mediums are disclosed in the
articles of R. Ulrich entitled, "Theory of the Prism-Film Coupler by
Plane-Wave Analysis", Journal of the Optical Society of America, Vol.
60(10), pp. 1337-1350, October, 1970; and A. Chandonnet et al. entitled,
"A Fibre Intensity Modulator for Q-Switching", SPIE Proceedings, Section
of Mode-Locked and Solid State Lasers, Amplifiers, and Applications, Vol.
2041, 282-290, Aug. 17-19, 1993, Quebec, Canada. In general, coupling of
radiation between normally guided mode or modes in the waveguide and
outside the waveguide is carried out by changing the refractive index of
the overlay medium in the side-polished region of the waveguide. Resonant
coupling between the field propagating in the waveguide and the overlay
medium results in an energy transfer from the waveguide to the overlay
medium. When the interaction length, L, and the coupling strength, k, of
the fields are carefully adjusted, essentially all the energy from the
waveguide can be transferred into the overlay medium. The extent to which
this transfer is allowed is a function of the effective propagation
constant n.sub.eff of the waveguide. Modulating n.sub.eff is, therefore,
the key to modulate the transmission, T, of a side-polished fiber and is
usually accomplished by changing the refractive index of the overlay
medium. The way in which the coupling strength, k, is changed for a given
variation, .DELTA.n, of the medium is largely dependent on its geometry
and its composition. The more sensitive it is to a given .DELTA.n, the
faster the resultant modulation. In other words, sharp resonances between
the optical mode or modes of the waveguide and the overlay medium are
desirable since it translates into a strong coupling variation, .DELTA.k,
for a relatively small .DELTA.n.
This is the principle of operation of plasmon polaritons fiber modulators
and polarizers. Examples of such devices are disclosed in the articles of
J. C. Quail et al. entitled, "Long Range Surface Plasmon Modes in Silver
and Aluminum Films", Optics Letters, Vol. 8(7), pp. 377-379, July, 1983;
J. S. Schildkraut entitled, "Long-Range Surface Plasmon Electrooptic
Modulator", Applied Optics, Vol. 27(21), pp. 4587-4590, Nov. 1, 1980; W.
Johnstone et al. entitled, "Surface Plasmon Polaritons in Thin Metal Films
and Their Role in Fiber Optic Polarizing Devices", Journal of Lightwave
Technology, Vol. 8(4), pp. 538-544, April, 1990; M. N. Zervas entitled,
"Surface Plasmon Polariton Fiber Optic Polarizers Using Thin Nickel
Films", IEEE Photonics Technology Letters, Vol. 2(4), pp. 253-256, April,
1990; K. Thyagarajan et al. entitled, "Thin Metal Clad Waveguide
Polarizers: Analysis and Comparison With Experiment", Optics Letters, Vol.
15(18), pp. 1041-1043, Sep. 15, 1990; S. Pilevar et al. entitled,
"Analysis of Dual Metal Coated In-Line Fiber Optic Polarizer", Journal of
Optical Communications, Vol. 12, pp. 22-25, (1991); and K. Welford
entitled "Surface Plasmon Polaritons and their Uses, Tutorial Review",
Section on "Properties of Surface Plasmon Polaritons", Journal of Optical
and Quantum Electronics, Vol. 23, pp. 427, (1991). In these devices, a
thin layer or layers of metal are sandwiched between the side-polished
fiber and an active overlay medium. Changing the refractive index of the
electro-optic overlay medium by applying an electric field through the
medium strongly modifies the long range plasmon resonances of the thin
metallic layer. Because the phase matching condition between the guided
mode of the fiber and the plasmon is very stringent, very sharp
transitions can, in principle, be achieved.
Another type of modulator relies on an overlay medium of polymeric
composition which is itself a thin planar electro-optically active
waveguide sandwiched between transparent electrodes, such as ITO. Examples
of such modulators are disclosed in the articles of D. A. Ender et al.
entitled "Polymeric and Organic Crystalline Films for Electro-Optic
Applications", SPIE Proceedings of Nonlinear Optical Properties of Organic
Materials, Vol. 971, pp. 144-153, (1988); R. Lytel et al. entitled,
"Organic Electro-Optic Waveguide Modulators and Switches", SPIE
Proceedings of Nonlinear Optical Properties of Organic Materials, Vol.
971, pp. 218-229, (1988); G. T. Boyd entitled, "Applications Requirements
for Nonlinear Optical Devices and the Status of Organic Materials",
Optical Society of America B, Vol. 6(4), pp. 685-692, April, 1989; G. H.
Cross et al. entitled, "Polymeric Integrated Electro-Optic Modulators",
SPIE Proceedings of Integrated Optics and Optoelectronics, Vol. 1177, pp.
79-91, (1989); K. D. Katz et al. entitled, "Second Order Nonlinear Optical
Devices in Poled Polymers", SPIE Proceedings of Nonlinear Optical
Properties of Organic Materials II, Vol. 1147, pp. 233-244, (1989); W.
Johnstone et al. entitled, "Fibre Optic Modulators Using Active Multimode
Waveguide Overlays", Electronics Letters, Vol. 27(11), pp. 894-896, May
23, 1991; M. Wilkinson et al. entitled, "Optical Fibre Modulator using
Active Electro-Optic Polymer Overlay", Electronics Letters, Vol. Vol.
27(11), pp. 979-981, May, 1991; Y. Shuto et al. entitled, "Electrooptic
Light Modulation and Second Harmonic Generation in Novel Diazo
Dye-Substituted Poled Polymers", IEEE Photonics Technology Letters, Vol.
3(11), pp. 1003-1006, November, 1991; and G. Fawcett et al. entitled,
"In-Line Fibre Optic Intensity Modulator Using Electro Optic Polymer",
Electronics Letters, Vol. 28(11), pp. 985-986, May 21, 1992. Since this
polymeric material is electro-optically active, applying an electric field
through the material changes its effective propagation constants.
Several patents also exemplify these modulator devices which are U.S. Pat.
No. 4,807,982 to Jaeger et al.; U.S. Pat. No. 4,925,269 to Scrivener; U.S.
Pat. No. 4,948,225 to Rider et al.; U.S. Pat. No. 4,971,426 to
Schhildkraut et al.; U.S. Pat. No. 5,007,695 to Chang; U.S. Pat. No.
5,060,307 to El-Sherif; U.S. Pat. No. 5,067,788 to Jannson et al.; U.S.
Pat. No. 5,133,037 to Yoon et al.; and U.S. Pat. No. 5,444,723 to
Chandonnet et al. All these modulator devices can be characterized by the
resonant nature of the coupling between the fields in both the fiber and
the overlay medium, i.e., energy transfer from one optical medium to the
other is allowed between guided modes of each medium.
In all the above approaches, the overlay medium can be of any class of
electro-optically active material, i.e., a material having an index of
refraction that is changed or varied by an applied electric field through
the material. One such class of particularly promising material is poled
polymers. These dye-doped polymers are organic compounds into which
chromophores are included either by direct incorporation in solution or by
backbone grafting or cross-linkage of the dye molecules into the polymer
chains. Poling this material is done by heating the polymer close to its
glass transition temperature, T.sub.g, while applying and maintaining a
strong electric field across the material while the temperature of the
material is returned to room temperature. This poling procedure serves the
purpose of macroscopically orienting the dye molecules in a preferential
direction to lock them permanently in that state so that the material
becomes non-centrosymmetric and exhibits a large permanent second order
susceptibility X.sup.(2). These materials have very important properties,
for example, a small dielectric constant (important for high speed
operation), large nonlinearities (important for low-voltage operation),
ease of processability and a relatively low-cost as compared to other
non-centrosymmetric inorganic nonlinear crystals, such as LiNbO.sub.3 and
LiTaO.sub.3. Their properties and fabrication process have been the object
of an extensive number of books and papers in the literature some of which
are fairly recent, for example, the books of P. N. Prasad et al. entitled,
"Introduction to Nonlinear Optical Effects in Molecules and Polymers",
John Wiley & Sons, New York, 307 pages (1991); H. Kuhn et al. entitled,
Nonlinear Optical Materials, CRC Press, Boca Raton, Fla., 335 pages
(1992); and G. Mohlmann (Ed.) entitled, "Nonlinear Optical properties of
Organic Materials VII, SPIE, The International Society for Optical
Engineering, Bellingham, Wash., 428 pages (1994). Dye-doped polymer with
nonlinearities as large as 100 pm/V and thermal stability at temperatures
as high as 300.degree. C. have been envisioned recently as disclosed in
the book entitled, "Organic Thin Films for Photonics Applications", Vol.
21, 1995 OSA Technical Digest Series (Optical Society of America,
Washington, D.C., 1995), 480 pages.
It is accordingly an object of the present invention to provide an improved
optical waveguide device functioning as a modulator or switching device
for interacting with propagating light in an optical waveguide employing
novel geometry.
It is another object of this invention to provide a waveguide intensity
modulator or optical switch having an electro-optic polymer medium with
edge-coupled or side electrode geometry.
SUMMARY OF THE INVENTION
According to this invention, an optical device for modulating, switching or
otherwise interacting with radiation guided and propagating along an
optical longitudinal axis of an optical waveguide, such as an optical
fiber or an optical planar waveguide, is based on a novel nonresonant
geometry, which may be referred to as an edge-coupled waveguide geometry,
and is capable of high speed switching with large on-off contrast ratios,
small insertion losses, low operation voltage, high damage threshold and
is suitable for low-cost large volume manufacturing. The edge-coupled
geometry allows a relatively smaller electrical field to be applied across
a relatively thin layer of polymer (compared to its optical coupling
depth) to achieve index modulation of the polymer as compared to a
conventional stacked-layer modulator which has a much thicker polymer
layer requiring a stronger applied electrical field to index modulate the
layer.
The edge-coupled geometry comprises an electro-optic polymer medium
sandwiched between a pair of substantially parallel conductive electrodes.
The gap, W, formed between the electrodes is small within which the
polymer medium is provided. The sandwich is arranged to have one of its
orthogonal planar extent extend transversely, e.g., radially, from the
radiation axis of propagation of the waveguide medium and the other
orthogonal planar extent extend parallel with the radiation axis (z axis)
of propagation of the waveguide medium. The reason for calling this
geometry, "edge-coupled" geometry is, therefore, because the spatially
disposed electrodes for applying an electric field through the
electro-optic medium are placed side by side in the longitudinal direction
of propagation along the z axis of the waveguide. The modulator waveguide
medium is considered "semi-infinite" since the edge-coupled geometry must
have a depth, D, that is considerably larger than its width, W, i.e., D
>>W, W being of very small value. The electro-optic polymer medium can be
considered as semi-infinite in the direction perpendicular to the
waveguide propagation axis z so that no resonance occurs between the
waveguide mode and the electro-optic polymer medium. Optical coupling from
one medium to the other is the result of a change in the refractive index,
.DELTA.n, of the electro-optic polymer medium that destroys the total
internal reflection condition of the waveguide mode at the
modulator/waveguide formed interface. The .DELTA.n of the electro-optic
overlay medium is accomplished through the application of an electric
field applied to the spatially disposed electrodes. The energy propagating
in the waveguide can, therefore, can be selectively coupled to radiative
modes of the electro-optic polymer medium and this energy is selectively
allowed to escape into the electro-optic polymer medium by modulation of
the edge-coupled modulator.
Because of this particular edge-coupled geometry, the use of a polymer as
the electro-optic medium is particularly useful in filling such a narrow
gap of width, W. The electrodes may be assembled on top of the prepared
waveguide and the gap formed between the electrodes is carefully adjusted.
The gap is very narrow so that filling with a chip of electro-optic
material would be impossible. However, capillary forces act on the liquid
polymer or polymer solution to completely fill the gap. Thus, the
employment of an additional step to fill this narrow gap, such as by
spinning or dip coating, is not necessary. This is an important eliminated
step in fabrication because its avoidance eliminates the delicate
procedure of spin coating the modulator device which includes long optical
fiber leads on either side of the modulator, i.e., the presence of these
leads would make the spinning coating operation very difficult.
The new geometry of this invention can be fabricated as an all-fiber
modulator as well as a planar modulator, and is unique from the
conventional approaches in that modulator operation does not rely on
resonant coupling between guided modes in the optical fiber and the
overlay medium. Rather, the onset of modulated switching is created by a
change in the refractive index, .DELTA.n, of the electro-optic polymer
medium. The switching transition of a particular mode propagating in the
optical waveguide is accomplished when the refractive index of the
electro-optic polymer medium increases to become larger than the effective
index of the particular mode. The energy in the waveguide mode is,
consequently, allowed to escape irreversibly by coupling to radiative
modes of the edge-coupled modulator structure. This is made possible
through the employment of side electrodes extending transversely of the
longitudinal axis of the waveguide, instead of the more conventional
approach of employing a stacked layer geometry which forbids the
employment of a very thick overlay structure which would be non-guiding
(semi-infinite regime) but would require a very high applied voltage to
operate.
The side electrode geometry of this invention also allows very long
interaction length, such as one or more centimeters, which is important in
creating large contrast ratios for relatively modest variations in the
index of refraction of the electro-optic medium. This feature is also an
essential prerequisite for achieving low voltage operation of the
modulator.
The unique geometry of this invention lends itself to large volume
fabrication processes, such as conventional photolithography techniques
employing a photoresist. Both the electrodes and their supporting
substrate may be made of materials, such as silicon, which can be easily
etched. Etching v-shaped grooves in silicon can be accomplished in a batch
mode at a very affordable cost per unit. Electrode deposition on glass or
silicon supports can be likewise accomplished in an automated step format.
The forming of fiducial marks may be formed on the parts to be assembled,
which marks may be etched regions for physical or optical discernability.
Optically discernible marks, such as comprising titanium, adhere well to
silicon. Waveguide polishing to form an optical interactive region between
the waveguide and the modulator can also be achieved very efficiently and
rapidly by automatization of the polishing steps. On-line optical signal
monitoring of the induced loss of a propagating optical signal in the
optical waveguide may be monitored to terminate the polishing procedure at
a predetermined point. This "dry" fabrication process is particularly
useful in the case of an optical fiber since it maintains the chemical and
mechanical integrity of the protecting jacket of the optical fiber on
either side of the formed modulator.
Other objects and attainments together with a fuller understanding of the
invention will become apparent and appreciated by referring to the
following description and claims taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of a conventional plasmon modulator
approach based on coupling to plasmon resonances.
FIG. 2 is a schematic illustration of a conventional waveguide modulator
approach based on coupling via refractive index variation.
FIG. 3 is a schematic illustration of the edge-coupled modulator approach
of this invention based on coupling via refractive index variation.
FIG. 4 is a cross-sectional view taken along the line 4--4 of FIG. 6 of a
first embodiment of the edge-coupled modulator of this invention
comprising an all-fiber modulator.
FIG. 5 is a side view of the edge-coupled modulator of the first embodiment
of this invention.
FIG. 6 is a plan view of the edge-coupled modulator of a first embodiment
of this invention.
FIG. 7 is a cross-sectional view of a second embodiment of the edge-coupled
modulator of this invention comprising a planar waveguide.
FIG. 8 is a cross-sectional view of a third embodiment of the edge-coupled
modulator of this invention comprising a planar waveguide and a
modification of the second embodiment.
FIG. 9 is a cross-sectional view of a fourth embodiment of the edge-coupled
modulator of this invention as applied relative to the core of a double
clad fiber.
FIG. 10 is a cross-sectional view of the edge-coupled modulator of a fifth
embodiment of this invention as applied relative to the inner cladding of
a double clad fiber.
FIG. 11 is a perspective view of a sixth embodiment of the edge-coupled
modulator of this invention integrated with a double clad amplifier or
laser.
FIG. 12 is a graphical illustration of the typical electric field poling
process for either corona or electrode field application.
FIG. 13 is a graphical illustration of the induced loss as a function of
the refractive index the electro-optic polymer for different edge-coupled
interactive depths relative to other parameters.
FIG. 14 is a graphical illustration of the induced loss as a function of
the refractive index the electro-optic polymer for different residual
cladding thickness relative to other parameters.
FIG. 15 is a graphical illustration of the induced loss as a function of
the refractive index the electro-optic polymer for edge-coupled
interactive depths of D=b 50 .mu.m and D=.infin. relative to other
parameters.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
As used herein, the term, "waveguide" includes optical fibers, surface
waveguides, nonlinear crystal waveguides, planar waveguides and other such
optical waveguide devices. Also, the figures employed to describe the
several embodiment are not drawn to scale in order to better illustrate
and describe detail structure of the invention.
According to the prior art, there are two approaches for forming a
waveguide coupler to provide electro-optic modulation of propagating
radiation in an optical waveguide. On approach is the plasmon approach
shown in FIG. 1 and the other approach is the waveguide approach shown in
FIG. 2. In FIG. 1, the plasmon coupler of the Kretschmann type is shown
coupled to an optical fiber 11 having a core 12 and cladding 13 wherein a
portion of cladding layer 13 has been removed to permit good exposure of
the coupler to the evanescent wave portions of radiation propagating in
fiber core 12. The plasmon coupler comprises a metal layer or electrode 4,
an electro-optic dielectric layer 5 comprised of electro-optic material
and another metal layer or electrode 6. Evanescent wave portions,
extending outside of core 12 into cladding 13, are coupled with surface
plasmon waves generated at the interface of metal layer 4 and
electro-optic layer 5 by applying an electric field across electrodes 4
and 6 of sufficient magnitude to cause a change in the refractive index of
electro-optic layer 5 bringing about its resonance. The surface plasmon
wave is an electromagnetic wave supported at the interface between metal
layer 4 and electro-optic layer 5 and the intensity of the radiation
propagating in waveguide 11 can be modulated by coupling the propagating
wave with the surface plasmon wave via evanescent coupling. In general,
the TM mode of the propagating radiation will be absorbed by the surface
plasmon wave at the resonant frequency of electro-optic layer 5. The
plasmon coupler is extremely sensitive to changes in the refractive index
of electro-optic layer 5 caused by the applied field so that the
refractive index of layer 5 must precise and very close to the effective
refractive index of waveguide 11.
In FIG. 2, the waveguide coupler comprises an electro-optic dielectric
layer 8, e.g., an nonlinear polymer having electro-optic properties,
sandwiched between two metal or conductive layers 7 and 9, with conductive
layer 7 being transparent, e.g., ITO, at the wavelength of radiation
propagating in waveguide 11. The amount of optical coupling between
waveguide 11 and the coupler can be changed by varying the refractive
index of polymer layer 8. The geometry of this coupler is similar to that
of the plasmon geometry of FIG. 1 except that the refractive index of
polymer layer 8 may have an arbitrary value as long as the effective
refractive index waveguide 8 can be changed through electro-optic
modulation to match the effective refractive index of waveguide 11.
For comparison purposes with FIGS. 1 and 2, the general geometry of the
coupler of this invention is shown in FIG. 3. The planar extent of the
coupler is positioned to be transverse to the longitudinal optical axis of
waveguide 11, in particular, radially disposed of or substantially
perpendicular to the waveguide optical axis, whereas, in the case of FIGS.
1 and 2, the planar extent of the coupler is parallel with longitudinal
optical axis of waveguide 11. The coupler comprises an electro-optic
dielectric layer, preferably comprising a nonlinear polymer medium 16P,
sandwiched between two electrodes 17A, 17B. The polymer medium 16P is
positioned radially along the optical axis of core 12 so that evanescent
wave portions extending outside of core 12 into cladding 13 are
longitudinally coupled into medium 16P. Because of this kind of
longitudinal coupling into nonlinear medium 16P, we refer to this coupler
as an "edge-coupled" modulator.
As in the case of the plasmon coupler of FIG. 1, polymer medium 16P must
have a refractive index in close proximity to the effective refractive
index of waveguide 11. In particular, the polymer refractive index has a
value just below the effective refractive index of waveguide 11 when no
electric field is applied across electrodes 17A, 17B so that no
significant losses are introduced by its presence. The polymer refractive
index must be increased to a value higher than the effective refractive
index of waveguide 11 when an electric field is applied across electrodes
17A, 17B so that radiative guiding conditions are lost in waveguide 11 and
significant losses are introduced by the higher refractive index of the
edge-coupled coupler. In a typical telecommunication optical fiber, the
core and cladding indices at 1.53 .mu.m | | |
