Optical waveguides – With optical coupler – Switch
Reexamination Certificate
2000-06-08
2002-10-22
Spyrou, Cassandra (Department: 2872)
Optical waveguides
With optical coupler
Switch
C385S016000, C385S019000, C385S050000, C359S199200
Reexamination Certificate
active
06470109
ABSTRACT:
TECHNICAL FIELD
The invention relates generally to efficiently coupling optical waveguides and more particularly to determining the positions and angles of waveguides relative to a plane of total internal reflection.
BACKGROUND ART
Increasingly, signal transfers within a telecommunications or data communications environment are being carried out using optical networks. Information can be exchanged in the form of modulations of laser-produced light. The equipment for efficiently generating optical signals and the cables for transmitting the optical signals over extended distances are readily available. However, there are still some concerns with regard to enabling localized manipulation of signals without a significant sacrifice of signal strength. The localized manipulation may be a steady-state signal transfer arrangement between two waveguides or may be a switching arrangement in which an optical signal along one waveguide can be transferred to any one of a number of output waveguides.
One technique for redirecting an optical signal from one waveguide to another waveguide is to use a mirror. The mirror may be stationary or may be used in a switching arrangement by connecting the mirror to a micromachine device. An alternative to using the mirror is to provide a plane of total internal reflection (TIR). As is known in the art, TIR occurs when a ray of light travels toward an interface between a region having a high refractive index and a region of low refractive index, with the ray of light approaching from the high index side of the interface. A switching arrangement that utilizes the phenomenon of TIR is described in U.S. Pat. No. 5,699,462 to Fouquet et al., which is assigned to the assignee of the present invention. An isolated switching unit
10
is shown in FIG.
1
. The switching unit includes planar waveguides that are formed by layers on a substrate. The waveguide layers include a lower cladding layer
14
, an optical core
16
, and an upper cladding layer, not shown. The optical core may be primarily silicon dioxide, but with doping materials that achieve a desired index of refraction. The cladding layers are formed of a material having a refractive index that is significantly different than that of the core material, so that the optical signals are guided along the core. The effective phase index of the waveguide is determined by the refractive indices of the core material and the material of the cladding layers, as is well known in the art. The layer of core material is patterned into waveguide segments that define a first input waveguide
20
and a first output waveguide
26
of a first optical path. The patterning also defines a second input waveguide
24
and a second output waveguide
22
of a second linear path. The upper cladding layer is then deposited over the patterned core material. A gap is formed by etching a trench
28
through the core material, the upper cladding layer, and at least a portion of the lower cladding layer
14
.
The first input waveguide
20
and the second output waveguide
22
have axes that intersect a sidewall of the trench
28
at an angle of incidence that results in TIR diverting light from the input waveguide
20
to the output waveguide
22
when the junction
30
of the waveguides is filled with vapor or gas. However, when the junction
30
is filled with a fluid that has an index of refraction substantially matching that of the effective phase index of the waveguides, light from the input waveguide
20
will travel through the index-matching fluid to the linearly aligned first output waveguide
26
.
The patent to Fouquet et al. describes a number of alternative embodiments to switching the optical switching unit
10
between a transmitting state and reflecting state. In the transmitting state, the two input waveguides
20
and
24
are optically coupled to their linearly aligned output waveguides
26
and
22
, respectively. In the reflecting state, the first input waveguide
20
is optically coupled to the second output waveguide
22
, but the second input waveguide
24
is not in communication with either of the output waveguides
22
and
26
. One approach to switching between the two states is illustrated in FIG.
1
. The switching unit
10
includes a microheater
38
that controls formation of a bubble within the fluid-containing trench. When the heater is brought to a temperature that is sufficiently high to form a bubble in the index-matching fluid, the bubble is positioned at the junction
30
of the four waveguides. Consequently, light propagating along the waveguide
20
encounters a refractive index mismatch upon reaching the sidewall of the trench
28
, causing TIR, so that the waveguides
20
and
22
are optically coupled. However, when the heater
38
is deactivated, the index-matching fluid will again reside within the junction between the four waveguides.
The principles described with reference to
FIG. 1
also apply to a steady-state reflecting arrangement. That is, if the index-matching fluid is removed from the trench
28
, the waveguides
20
and
22
will be continuously coupled by TIR at the wall of the trench. In this steady-state embodiment, the waveguides
24
and
26
would not be included.
While the phenomenon of TIR has been used successfully in the redirection of optical signals from one waveguide to another waveguide, further improvements are desired. Light that impinges an interface between a high refractive index region and a low refractive index region will vary between having a transverse electric (TE) polarization and having a transverse magnetic (TM) polarization. The light will react differently at the interface, depending upon its polarization. Consequently, there are polarization dependent losses (PDLs) due to imperfect coupling to the waveguide modes. Since light impinging the interface will randomly vary between polarizations, the variations in PDL are random.
What is needed is an optical coupling arrangement of waveguides in which polarization dependent losses are neutralized or rendered predictable. What is also needed is a method of determining efficient layouts for positioning waveguides relative to a TIR interface.
SUMMARY OF THE INVENTION
Optical efficiency in the coupling of two waveguides that intersect an interface between high and low refractive index regions is enhanced by determining compensation for the Goos-Hänchen effect along the interface. In one embodiment, the lateral shift of reflected light along the interface is predicted in order to determine a distance at which the axes of the two waveguides should be spaced apart along the interface. In another embodiment, the incidence angles of the two waveguides are selected so as to equalize the lateral shifts for the polarizations, since light collection by the second waveguide can be increased by selection of the proper angle, even if the spacing between the two waveguide axes remains fixed. In the preferred embodiment, both the distance between the axes and the incidence angles are selected to provide compensation for the Goos-Hänchen effect.
As previously noted, TIR occurs when a ray of light impinges the interface between the high and low refractive index regions from the high index side. However, the Goos-Hänchen effect causes the reflected light to emerge from the interface a short distance away from the point at which the incident light intersects the interface. By tailoring the positions and/or the angles of waveguides to maximize the collection of the reflected light, the reliability of signal processing can be improved. The optimal positioning is polarization dependent. That is, the lateral shift (z
TM
) of light having a TM polarization is different than the lateral shift (z
TE
) of light having a TE polarization. Reflection can be optimized for either the TM polarization or the TE polarization. Alternatively, the distance between the axes of the waveguides along the interface can be selected to equalize the loss for the two polarizations. By selecting the distance to be one-half of the difference between th
Agilent Technologie,s Inc.
Boutsikaris Leo
Spyrou Cassandra
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