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Description  |
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FIELD OF THE INVENTION
This invention relates to the field of optical instruments such as
interferometers and fiber optic gyros, and more particularly to the field
of optical components such as integrated optical couplers for use in
integrated optical circuits.
RELATED ART
Fiber optic gyros typically use an optical coupler for the purpose of
coupling light beams into a fiber optic coil or for directing light to the
coil while splitting off and directing interference pattern signals to a
detector. Conventional optical couplers are made in accordance with
processes such as those characterized in U.S. Pat. No. 4,738,511 issuing
to John Fling on Apr. 9, 1988 for a Molecular Bonded Fiber Optic Coupler &
Method of Fabrication having a common assignee with the present invention.
Couplers of that type produce a predictable and reliable component from
individual fibers by using a multi-step labor intensive process.
U.S. Application Ser. No. 07/257,749 filed Oct. 14, 1988 now. U.S. Pat. No.
4,938,594 for an Anti-Symmetric Mode Filter having a common inventor and
assignee is related to this invention. The central waveguide segment does
not show or suggest a means for decorrelating symmetric and antisymmetric
mode light passing through the central waveguide segment.
U.S. Pat. No. 4,468,085 issued to Michel Papuchon et al on Aug. 28, 1984
for a "Hybrid Optical Junction and Its Use In A Loop Interferometer". FIG.
2 of the Papuchon reference shows a pair of integrated optical junctions
formed in a substrate. The waveguide segments are formed in a substrate of
feroelectric material such as lithium niobate by masking the surface of
the substrate and by depositing a material in the substrate such as
titanium to form the waveguides. The integrated coupler of Papuchon is
reproducible in large numbers without the labor intensive steps necessary
for the coupler of Fling.
The Papuchon reference is excited by launching light from a light source,
such as a light emitting diode, into a single optical input. The light
entering the input excites equal amounts of light in the symmetric mode
and in the anti-symmetric mode within the coupled modes of adjacent
waveguides. Antisymmetric mode light is radiated into the substrate in
which the waveguide segments are formed at the first junction. Small
portions of the anti-symmetric mode light radiated into the substrate,
pass through the substrate and recouple back into the wave guide segments
after the second junction to cause very large bias errors in symmetric
mode light passing through the device.
In a paper by T. R. Ranganath titled "Ti-Diffused LiNbO.sub.3
Branched-Waveguide Modulators: Performance and Design" published in the
IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-13, NO. 4, pages 290-295
dated APRIL 1977, at page 294, the author discusses the importance of a
single mode waveguides and the combining functions of a fork. The author
comments at page 294 that "It has been proved from very general symmetry
considerations that no 3-port junction can be exactly matched. The
importance of this result is in identifying that our dielectric forks have
3 ports which can propagate bound modes and one extra port connected with
the unbound radiation modes. Therefore, the presence of the fourth port
accounts for the scattering losses, which can be reduced by careful
design, but can never be eliminated".
In the above referenced article by Ranganath, the author comments that any
single-mode device making use of horn regions to feed light into the
structure will encounter scattering losses and advises that "ways have to
be found to take care of this scattered light". The paper does not show
how to prevent the scattered losses from introducing bias errors by
influencing optical signals elsewhere in the substrate.
U. S. Pat. No. 4,549,806 for A Method and Apparatus Measuring Absolute
Rotation, filed Sept. 30, 1982 and issuing Oct. 29, 1985 to P. Martin and
K. Petermann discusses the problem of decorrelating two polarization modes
but does not teach or suggest the use of a decorrelator waveguide segment
within an integrated optical coupler.
SUMMARY OF THE INVENTION
It is a first object of the invention to provide an integrated optics
decorrelator device that provides the function of an integrated optical
coupler capable of being manufactured in high volume with substantially
identical properties.
A second object of the invention is to receive light at a waveguide input
of the integrated optics decorrelator device. The waveguide input and
associated waveguides segments that comprise the device are formed on a
substrate. The light that is received excites both symmetrical and
asymmetrical modes of light in the input waveguide. The symmetrical mode
light passes through the input waveguide and splits into substantially
equal parts, each equal part being coupled to an output wave guide
segment. The invention integrated optics decorrelator device decorrelates
the anti-symmetric mode with respect to the symmetrical mode light which
recouples into output waveguide segments through which the symmetrical
mode light is passing.
These objects are realized in a preferred embodiment of the invention
integrated optics decorrelator device having a first integrated optics
junction having two input paths and at least one output path formed as a
Y-shaped assembly of two monomodal optical waveguides joined at a node
with a central multimodal optical waveguide segment having an output end.
A second integrated optics junction has two output paths and at least one
input path formed as a Y-shaped assembly of two output mono-modal optical
waveguides joined at a node with a central mono-modal optical waveguide
segment. The central mono-modal optical waveguide segment has an input
end.
The central multi-modal optical waveguide segment output end is coupled to
the central mono-modal optical waveguide segment input end. The first
integrated optics junction and the second optics junction are formed
inside a refractive medium having a lower refractive index than the
refractive index of the waveguides and are separated by an tapered
adiabatic segment interposed between the central multi-modal optical
waveguide segment output end and the central mono-modal optical waveguide
segment input end.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of an optical circuit using a PRIOR ART optical
coupler to supply light to a fiber optic ring;
FIG. 2 is a schematic graphical representation of electric intensity as a
function of distance measured across the cross section of an optical
waveguide for symmetrical mode optical energy moving through the wave
guide;
FIG. 3 is a schematic graphical representation of electric intensity as a
function of distance measured across the cross section of an optical
waveguide for anti-symmetrical mode optical energy moving through the wave
guide;
FIG. 4 is a schematic of an optical circuit using a the invention
integrated optical decorrelator to supply light to a fiber optic ring;
FIG. 5 is a schematic plan view of the substrate of the invention
integrated optics decorrelator showing the diffusion pattern of the
waveguide;
FIG. 6 is a schematic perspective view of the substrate of the invention
integrated optics decorrelator showing the diffusion regions of the
waveguide within the substrate.
FIG. 7 is a schematic graphical representation of electric field intensity
as a function of distance measured across the cross section of an optical
waveguide for single mode optical energy moving through the wave guide;
FIG. 8 is a schematic graphical representation of electric field intensity
as a function of distance measured across the cross section of an optical
waveguide for dual mode optical energy moving through the wave guide;
PREFERRED EMBODIMENT
The paper by T. R. Ranganath titled "Ti-Diffused LiNbO.sub.3
Branched-Waveguide Modulators: Performance and Design" published in the
IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-13, NO. 4, pages 290-295
dated APRIL 1977 is incorporated herein by reference for its information
on single and multi-mode waveguide design and the design of symmetric fork
guide structures in optical integrated circuits.
FIG. 1 is a schematic representation of a PRIOR ART optical circuit using
an integrated optics coupler device 10 formed on a substrate 20. The
integrated optics coupler device 10 has a first integrated optics junction
22 with two input paths 24, 26 and at least one output path formed as a
Y-shaped assembly of two mono-modal optical waveguides joined at a first
integrated optics junction 22 with a central mono-modal optical waveguide
segment 28.
The central mono-modal optical waveguide segment 28 has an output end at a
second integrated optics junction 30. The second integrated optics
junction 30 has first and second output paths 32, 34 formed as a Y-shaped
assembly of two output monomodal optical waveguides joined at the output
end of the central mono-modal optical waveguide segment 28.
The optical coupler 10 is coupled to receive light from a source such as a
superluminescent diode (SLD) 36 through waveguide segment 24. The light
from the SLD source 36 passes through waveguide 24, through the mono-modal
waveguide 28, to second integrated optics junction 30 of waveguides 32 and
34. The light is split at junction 30 to travel through waveguides 32 and
34 to supply clockwise (CW) and counterclockwise (CCW) light beams having
nominally equal intensities, into each end of the multi-turn fiber-optic
coil 38 at entrance ports 40 and 42 respectively.
Light received and carried by waveguide segment 24 excites equal amounts of
symmetric mode and anti-symmetric mode light as coupled modes within
waveguides 24 and 26. FIG. 2 schematically depicts the peak electric field
intensity at points across a cross-section of the two mono-mode wave
guides 24, 26 for symmetric mode light. FIG. 3 schematically depicts the
intensity of anti-symmetric mode across a cross-section of the same
two-mode waveguide.
As the light in waveguide 24 reaches the first integrated optics junction
22, light in the symmetric mode propagates though mono-modal waveguide 28
while light in the anti-symmetric mode is radiated into the substrate 20.
As the symmetric mode light in mono-modal waveguide 28 reaches the second
integrated optics junction 30, the energy splits into separate beams which
move through waveguides 32 and 34.
The problem PRIOR ART coupler 10 permits some of the anti-symmetric mode
light that passes into the substrate 20 to recouple back into waveguides
32 and 34. The anti-symmetric mode light that recouples into waveguides 32
and 34 is generally out of phase with the symmetric mode light that moved
directly through mono-modal waveguide 28. The light that moves through the
substrate 20 takes a longer path down into the substrate 20 that is a
multi-path route. An arbitrary environmentally dependent phase shift
results between the symmetric mode light reaching waveguides 32 and 34 and
the anti-symmetric mode light that depends on the exact details of the
path traveled as the signal re-enters wave guides 32 and 34 to add to the
original symmetric mode signal. The resulting composite CW and CCW signals
causes a bias error in the sensed interference signal at detector 44.
FIG. 4 is a schematic diagram of the optical circuit of FIG. 1 in which the
optical coupler 10 is replaced by the invention integrated optics
decorrelator 50. The invention integrated optics decorrelator 50 differs
from the optical coupler of FIG. 1 by the substitution of a central
multi-modal optical waveguide 52 for at least a portion of the mono-modal
wave guide 28. The central multi-modal optical waveguide 52 represents a
means for decorrelating symmetric mode light with respect to
anti-symmetric mode light as the respective beams pass from the first
integrated optics junction 22 through the central multi-modal optical
waveguide 52 to the second integrated optics junction 30. As shown in
FIGS. 4 and 5, the central multimodal optical waveguide 52 has a
multi-mode segment 54 with an input end 56 and an output end 62. A
mono-modal waveguide 60 has an input end at 62 and an output end at 64. In
the preferred embodiment, a tapered segment 66 is interposed between the
multimodal waveguide output end 58 and a mono-modal waveguide input end
62.
The multi-modal waveguide input end 56 is connected to the first integrated
optics junction 22 and the mono-modal waveguide output end 64 is connected
to the second integrated optics junction 30. As anti-symmetric mode energy
moves through the tapered segment 58, it is forced to radiate into the
substrate 58. Symmetric mode light passes through the tapered segment
without radiation. The length of the tapered segment is adjusted to be
adiabatic. The length is typically in excess of 20 times the width and can
be increased to lengths approaching 5 mm.
FIG. 5 is a plan view of the substrate 52 schematically showing the areas
on the substrate occupied by the exposed surfaces of imbedded optical
waveguide regions. In the preferred embodiment, the substrate is typically
formed from Y-cut cut LiNbO.sub.3, a refractive medium having a lower
refractive index than the refractive index of the waveguides. The first
integrated optics junction 22 and the second integrated optics junction 30
are formed inside the substrate 51.
The invention integrated optics decorrelator 50 is fabricated using
conventional processing steps, such as the steps characterized in the
above referenced paper by T. R. Ranganath titled "Ti-Diffused LiNbO.sub.3
Branched-Waveguide Modulators: Performance and Design". A typical series
of steps would begin with coating a poled and polished Y-cut LiNbO.sub.3
crystal having dimensions 1, 4, 32 mm (along Y, Z, X axes) with a positive
photoresist, such as (AZ 1350J) and exposing the resist through an
appropriate mask. The width of the guides is determined by the wavelength
of the light source and the number of modes to be supported. The
multi-modal guides are approximately double the width of the mono-modal
guides. After development of the photoresist, a thin layer of Titanium
approximately 10 nm, is sputtered onto the exposed surfaces of the
substrate. The photoresist layer is then stripped in acetone, leaving the
desired Ti pattern. Diffusion of the Ti into the substrate is then carried
out at an oven temperature of approximately 980.degree. C. for 4.5 h in an
argon environment. The process results in an integrated optics
decorrelator 50 with transverse mode guides in the depth direction.
Alternate embodiments of the process can use dopants such as Ti, H, He, Zn
for the fabrication of waveguides. The dopant raises the index of
refraction of the channel and forms a waveguide with high transmissibility
in comparison to the substrate. Ti and Zn are implanted by thermal
diffusion and Hydrogen H is implanted by proton exchange in an acid bath.
Helium He is implanted with an accelerator. The dopants are implanted
using conventional masking techniques, such as that characterized above,
to define the perimeters of the waveguide segments. Temperature,
concentration and time are adjusted to control the concentration of the
implants.
The function performed by the central multi-modal optical waveguide 52 is
similar to that encountered in the field of spread spectrum
communications. If a signal is transmitted from an antenna, and if the
signal is split and one portion of the radiated signal goes directly to
the receiver and a second portion travels to a mountain and then returns
to the receiver, and if the spectrum is broad enough and if the delay
between the reflected path and the direct path is large enough, then the
correlation of the first signal with respect to the second signal at the
receiver will approach zero. As the two signals become correlated, as in
the case of reflected television signals, a ghost signal is perceived in
the video image produced by the received signal.
In the invention optical decorrelator, the light source is broad band light
or spread spectrum light which insures a broad spectrum. Two paths are
traveled. The first path is traveled by the symmetrical mode energy that
remains within the waveguide and the second path is traveled by the
anti-symmetric mode energy that is launched into the substrate.
The central multi-modal optical waveguide 52 is designed to maximize the
transit time difference between the symmetric and anti-symmetric modal
light moving through the central multi-modal optical waveguide. The
maximization of the time difference is achieved by designing the geometry
of the waveguide to achieve the maximum velocity difference. Each of the
starting signals, the symmetric mode signal within the central multi-modal
optical waveguide and the anti-symmetric mode signal within the waveguide,
are spread spectrum signals that are correlated at the beginning. As the
signals are launched at node 22, they move down the multi-mode segment 54
and one will be delayed with respect to the other as they continue to move
along the segment. The multi-mode segment 54 is designed to have a length
that is long enough to introduce enough delay time to allow the slower
beam to be delayed by an interval that is sufficient to exceed the
decorrelation time of the light modes moving therein. As the symmetric and
anti-symmetric light move to the multi-mode output end 58, the modes will
not interfere with each other. For the best mode of operation, the length
of the multi-mode segment 54 or decorrelation segment should be at least
two decorrelation times in length.
The length of the multi-mode segment is determined by referring to the
following considerations. Using the letter L or lambda to represent the
center wavelength, and dL to represent the full width at half maximum of
the light spectrum, the coherence length (Lcoh) for light is usually
defined as:
Lcoh=(L) 2/(.pi.*dL)
The factor .pi. in the denominator is used to determine the line shape. A
factor of pi is used for a Lorentzian shaped bandwidth, or line shape, and
a different factor is used for a Gaussian line shape. The dL term is the
full width or bandwidth at half maximum power and L is the center
frequency wavelength. The equation is similar to the expression used to
establish the Q of an oscillator. The wavelength of the light L is in the
range of 800 to 1500 nanometers with the more typical applications being
at 1500 nanometers. The delta lambda or dL term is typically in the range
of 10 to 40 nanometers. Using the ranges thus described, the coherence
length Lcoh is typically in the range of 25 to 150 microns. This value is
the minimum theoretical length needed to decorrelate the symmetric and
antisymmetric modes. In practice a design coherence length of 200 to 250
nanometers is used.
As the wavefronts propagate along segment 6, the difference in the distance
delta increases as a function of the difference of the velocities of the
two wavefronts. The distance delta is calculated as a function of the
speed of light c, and the index of refraction of the media n as in
equation 2 below:
delta=((v-v2)*D)/(c/n)
The index of refraction for lithium niobate is about 2.2. The speed of
light is about 300,000 meters per second. The difference in the velocities
is typically about 1 percent. D is the length of the multi-mode segment
54.
FIG. 5 depicts an expanded view of multi-mode segment 54 followed by the
tapered segment 66. The combination is of a segment of uniform cross
section followed by a tapered segment 66 that is an adiabatic taper to
match the large cross section of the multi-mode waveguide 54 to the
smaller cross section of mono-mode segment 60. An alternate, and less
efficient embodiment is formed by omitting the tapered segment and
allowing the cross section of segment 7 to extend to and terminate
abruptly at the input end of segment 60.
The taper is not critical to the decorrelation process but operates to
reduce the loss of light into the substrate. An abrupt termination might
produce a loss of 3 db or more. A properly designed tapered segment 66 can
reduce the loss of light to about a 0.1 db. The taper length is typically
1 to 2 millimeters. The tapered angle of the surfaces are typically less
than 5 degrees. The length of multi-mode segment 54 is typically 10
millimeters. In a typical design, segment 60 would be 3 microns wide and
segment 54 would be 4 to 5 microns wide.
The symmetric mode continues to propagate from segment 54 to segment 66 to
waveguide 60. The anti-symmetric mode can not propagate beyond the tapered
segment 66. It therefore passes into the substrate. As the symmetric mode
light reaches node 30, some of the anti-symmetric mode light couples back
into the signal again through an undefined path or multipath. However,
since the anti-symmetric mode was decorrelated as it reached the output
end 58 of multi-mode segment 54, it is still decorrelated as it couples
back into the optical circuit at node 30 and into segments 32 and 34. The
anti-symmetric mode light thus coupled does not induce a phase error at
these locations because there is no correlation and that greatly reduces
the bias error in the output signal of the optical circuit at detector 44.
Optical couplers of the type characterized in connection with the circuit
and comments relating to FIG. 1 have been fabricated and tested in a fiber
optic gyroscope optical circuit. The optical coupler tested produced bias
errors in the order of 500 to 1000 degrees per hour as a result of
correlated energy from the anti-symmetric mode coupling back into segments
32 and 34. Coupling back can be reduced by increasing the length of
segment 28 with a corresponding reduction in bias error; however, the chip
would have to be made impractically long to achieve the low coupling
required for a navigational gyro.
By incorporating the multi-mode segment 54 or decorrelation segment, with a
length sufficient to achieve 2 or three decorrelation lengths, the
symmetric and anti-symmetric modes are decorrelated by more than 99%
resulting a reduction in bias error by a factor of at least four orders of
magnitude. An added advantage is that the present invention couples the
function of an optical coupler with that of a decorrelator with all of the
components on one chip increasing repeatability and reducing cost.
FIG. 8 schematically shows the integrated optics decorrelator chip in a
perspective view. The regions that are diffused with Titanium are shown
bordered in phantom.
The spatial mode decorrelator of FIGS. 4 and 5 is alternately characterized
as having a substrate 51 with first, second, third, fourth, fifth and
sixth waveguide segments (24, 26, 55, 60, 32, 34), each waveguide segment
having a first and second end. The first waveguide segment has a first-end
coupled to the optical input to receive broadband light from the broadband
light source 36. The second and third optical wave guide segments (26, 55)
have respective first-ends coupled to the first optical waveguide
second-end. The third optical waveguide segment second-end 62 is being
coupled to the fourth optical waveguide segment first-end. The fourth
optical waveguide segment second-end 64 is coupled to the fifth and sixth
waveguide segment 32, 34, first-ends. The fifth and sixth optical
waveguide segment second-ends are respectively coupled to the first and
second inputs 40, 42, of the closed optical path 38. The third optical
waveguide segment is dimensioned to propagate symmetric and anti-symmetric
mode beams of the broadband laser light. Each beam is characterized to
move with a different velocity. The third optical waveguide having a
length sufficient to introduce a delay time between the beams sufficient
to exceed the decorrelation time of the respective symmetric and
anti-symmetric beams.
The mono-modal waveguide segment 28 In FIG. 1 and the mono-modal waveguide
segment 60 in FIG. 4 are functionally identical since they are both
designed to support the transmission of light in a single transverse mode.
FIG. 7 is a schematic depiction of lines of constant electric field
intensity as they might exist within the cross section of a waveguide. The
lines represent the H-Field of a single transverse mode characterized in
FIG. 2 as a Gaussian centered field, in the cross section of a waveguide
having an X and Y axis normal to the optical axis, or Z axis of the
waveguide.
The dot at the center of the mode characterization of FIG. 7 is at the
physical center of the waveguide crosssection. The field intensity decays
away as the distance is increased away from the optical axis toward the
walls of the guide. Within the waveguide, the light wave moves in the
direction of the optical axis, the Z direction down the waveguide. The
wave looks the same coming or going from any reference point in the guide.
The fact that the wave is transverse means that the E-field of the wave is
oscillating in a plane that is transverse or normal to the Z axis as the
plane moves along the Z axis at the speed of light or at a speed
compatible with the media of the waveguides.
The multi-mode segment 54 of FIGS. 4, 5 and 6 is physically dimensioned to
be two-moded in accordance with the mode characterized in FIG. 8. The
multi-mode segment 54 supports the symmetric mode. The width of the
multi-mode segment 54 is enlarged to support multiple modes including the
antisymmetric mode as shown in FIG. 8. The multi-mode segment 54 is
typically of uniform cross section for the majority of its length.
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