Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2000-08-02
2002-07-09
Spyrou, Cassandra (Department: 2872)
Optical waveguides
With optical coupler
Plural
C385S034000, C385S037000, C385S051000, C385S052000, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06418250
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to optical fiber telecommunication systems and, in particular, to an apparatus and method of manufacturing a blockless optical multiplexing device employed in such telecommunication systems.
2. Technical Background
The high-cost of installing new fiber-optic cable in order to increase the transmission capacity of an existing fiber-optic telecommunication system has given rise to the widespread use of optical multiplexing devices. Such optical multiplexing devices increase transmission capacity of a single fiber-optic waveguide by employing optical multiplexing techniques, such as, wavelength division multiplexing (WDM). WDM allows multiple different wavelengths to be carried over a common fiber-optic waveguide. Presently preferred wavelength bands for fiber-optic transmission media include those centered at 1.3 micrometer and 1.55 micrometer. The latter, with a useful bandwidth of approximately 10 to 40 nm depending on the application, is especially preferred because of its minimal absorption and the commercial availability of erbium doped optical fiber amplifiers. WDM can separate/divide this bandwidth into multiple channels. One particular technique of WDM referred to as dense wavelength division multiplexing (DWDM) divides this bandwidth into multiple discreet channels, such as 4, 8, 16 or even 32 channels. By combining and transmitting multiple signals simultaneously at different wavelengths over a single optical fiber transmission line, DWDM in effect, transforms one optical fiber into multiple virtual optical fibers, thus, increasing bandwidth over existing fiber-optic networks and providing a relatively low cost method of substantially increasing telecommunication capacity. One key advantage of a DWDM-based network is that it can transmit different types of traffic/data at different speeds over an optical channel. Accordingly, DWDM-based networks provide an efficient and cheaper way to quickly respond to customers' bandwidth demands and protocol changes.
A prior art optical multiplexing device
200
is shown in FIG.
1
. An essential optical component employed in such optical multiplexing devices
200
is an optical bandpass filter
132
. As shown in
FIG. 1
, typically, two or more optical filters
132
are joined together to separate light of different wavelengths transmitted down a common optical waveguide. At a minimum, at least two optical filters are attached adjacent each other on an optical block
120
(as shown in
FIG. 1
) that has an optical slot
108
passing through the body of the optical block
120
, where a collimated beam of light
118
passes through the optical slot
108
of the optical block
120
to each of the optical filters
132
. Each of the optical filters
132
transmit light having a different predetermined wavelength and reflects light having other wavelengths. The optical block
120
is made of ceramic, metal (e.g., stainless steel, aluminum, etc.) or preferably, any other nontransparent material. Further, the optical filters
132
are arranged so that an optical beam is partially transmitted and partially reflected by each optical filter, in sequence, producing a cascading (zig-zag) light path.
One significant problem associated with the prior art optical multiplexing devices
200
(shown in
FIG. 1
) having optical blocks
120
is the expense associated with precisely machining a pair of opposite sides
112
and
114
of an optical block
120
, so that the optical filters
132
that are attached to the sides
112
and
114
of the optical block
120
can be mounted and aligned in nearly perfect parallelism to the optical block
120
. The prior art design of the optical block
120
had a relatively large area, namely, the sides
112
and
114
that required an optical (mirror) finish. The mirror finish was required in order to mount the filters
132
flat against the optical block
120
in order to maintain parallelism. The relatively large surface area
112
and
114
was required on the optical block
120
because this served as the gluing contact area for the filters
132
. It is difficult to maintain a large surface without any pits, scratches or dust to the point that a completely flat surface is maintained. Any surface anomaly tilted the filter out of parallel. To overcome this problem, a microsphere solution was applied to even out the mirror surface during the filter mounting process. Parallelism of the filters
132
to the mounting surface is critical to the optical performance of the optical multiplexing assembly, since it is presumed that every channel will match a specified center wavelength for its transmission bandpass at the same angle of incidence (AOI). The parallelism is measured, typically using an interferometer, and is kept to within 5 fringes at a wavelength of about 650 nm, that is, to within 0.03 degrees relative to the optical block surface. The effect of deviation from parallelism can accumulate as the beam of light travels from one filter to another. For instance, in an assembly consisting of five primary filters, the effect of deviation can result in an AOI error of approximately 0.2 degrees on the last filter in the assembly. Moreover, since the filters are quite small, generally being on the order of 1 to 5 mm in cubic size, difficulties in handling the filters and in precisely mounting the filters onto the optical block, can be time consuming and costly given the uncertainty as to the precise wavelength of a manufactured optical filter. Furthermore, improper mounting of the filters can significantly decrease the optical accuracy and thermal stability of the device.
A related problem of optical multiplexing devices
200
is the gluing of the filters
132
to the respective opposite sides
112
and
114
of the optical block
120
(shown in FIG.
1
), where a thermally cured epoxy is applied to the interface between each of the filters
132
and the sides
112
and
114
of the optical block
120
. Since the bandpass film in the filters
132
faces the sides
112
and
114
of the optical block
120
containing the optical slot
108
, the epoxy tends to interfere with the path of the optical signal, thus, resulting in system degradation. Another limitation of such an optical multiplexing device
200
design is the difficulty in cleaning the bandpass filter
132
surfaces after the filters
132
are attached to the sides
112
and
114
of the optical block
120
. Additionally, the precise mounting and gluing of the individual optical filters
132
to the optical block
120
tends to be a lengthy process. In particular, the filters
132
are first attached to one side
114
of the optical block
120
and then the optical block
120
is baked at 60 degrees Celsius for two hour, which secures the respective filters
132
to that one side
114
of the optical block
120
. The filters
132
are then inspected for parallelism and flatness to the optical block
120
, and any excess glue is cleaned off the filters
132
. Next, additional optical filters
132
are mounted onto an opposite side two
112
of the optical block
120
and then the optical block
120
is again baked at 60 degrees Celsius for two hours, which secures the respective filters
132
to that side two
112
of the optical block
120
. The filters
132
are then inspected for parallelism and flatness to the optical block
120
, and the filters
132
are also cleaned. The optical block
120
is then mounted onto the substrate
125
. The optical block
120
and the substrate
25
are baked at 120 degrees C. for another hour. Thus, the curing time can take about five hours of the manufacturing cycle time.
A problem with the design of optical multiplexing devices
200
is that such devices employ optical blocks that are all the same size and, thus, the optical blocks can only accommodate optical filters that are optimized at one tilt angle, for instance, about 5.0 degrees, whereas, the tilt angle at which the collimated light
118
enters the optical block
120
can
Corbosiero Daniel M.
Ehemann Karl D.
Boutsikaris Leo
Corning Incorporated
Spyrou Cassandra
Suggs James V.
LandOfFree
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