Compact wavelength interleaver

Details Compact wavelength interleaver Compact wavelength interleaver

2000-10-24

2002-06-04

Schuberg, Darren (Department: 2872)

Optical: systems and elements

Polarization without modulation

By relatively adjustable superimposed or in series polarizers

C359S199200, C359S490020, C359S490020

Reexamination Certificate

active

06400508

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a signal transmission system implemented with optical fibers and related optical components. More particularly, this invention relates to an optical interleave device implemented in a dense wavelength division multiplexing (DWDM) system.
BACKGROUND OF THE INVENTION
As the optical wavelength division multiplexing (WDM) technology gradually becomes the standard backbone network for the fiber optic communication systems, a challenge is continuously faced by those of ordinary skill in the art to increase the transmission capacity due to the bandwidth limitations of the optical fiber signal transmission systems. Specifically, the bandwidth of the optical fiber amplifier, such as Erbium doped fiber amplifier (EDFA), is limited as more and more channels are inserted into the transmission band. The wavelength spacing between adjacent channels employed for carrying the optical signals becomes narrower when more channels are “squeezed in” the transmission band for the purpose of satisfying a requirement of increasing the transmitting capacity of the signal transmission system. However, the wavelength division multiplexing (WDM) technologies employing dielectric filters are confronted with a limitation due to the ability to separate one channel from adjacent ones when the channel spacing is further reduced with increased number of channels.
As the WDM technology now enables the utilization of substantially wider fiber bandwidth for signal transmission, a number of prior art patents disclosed methods and configurations deal with problems of multiplexing, demultiplexing, and routing optical signals such that these systems can become commercially viable. Particularly, these disclosures deal with problems arise from addition of the wavelength domain that increases the complexity for network management because processing now involves both filtering and routing. Multiplexing involves the process of coupling many wavelengths in the same fiber. Demultiplexing is the opposite process in which wavelengths coupled in the same fiber are separated. The individual channels are spatially separated and coupled to specific output ports, Routing differs from demultiplexing in that a router spatially separates the input optical channels into output ports and permutes these channels according to control signals to a desired coupling between an input channel and an output port.
Lyot Orman invented a first birefringent filter in 1933, and in 1953, Solc further disclosed a different type of birefringent filter. These two types of birefringent filters provide a special function of selecting desired wavelengths in certain polarization state and in the meantime, select the adjacent wavelength with a perpendicular polarization state. Lyot's birefringent filter is previously employed by a prior art Patent for a switchable wavelength router. Specifically, a conventional switchable router is disclosed by Wu et al. in U.S. Pat. No. 5,694,233 entitled “Switchable Wavelength Router” (issued on Dec. 2, 1997) and is now incorporated herein by reference for all purposes in the Patent Application. Wu et al. disclosed a switchable wavelength router as that shown in FIG.
1
. The switchable wavelength router
999
has a first birefringent element
30
that decomposes and spatially separates an incoming WDM optical signal into two orthogonally polarized beams
101
and
102
. A first polarization rotator
40
selectably rotates the polarization of one of the beams to match the polarization of other beam, based on an external control signal. A wavelength filter
61
(e.g., stacked waveplates) provides a polarization-dependent optical transmission function such that the first beam decomposes into third and fourth orthogonal beams, and the second beam decomposes into fifth and sixth orthogonal beams. The third and fifth beams carry a first spectral band at a first polarization and the fourth and sixth beams carry a second spectral band at an orthogonal polarization. A second birefringent element
50
spatially separates these four beams into four horizontally polarized and vertically polarized components. A second polarization rotator
41
rotates the polarizations of the beams so that the third and fifth beams and the fourth and sixth beams are orthogonally polarized. A third birefringent element
70
recombines the third and fifth beams (i.e., the first spectral band), and also recombines the fourth and sixth beams (i.e., the second spectral band) which are coupled to the two output ports based on the control state of the wavelength router. Wu et al. disclosed a switchable wavelength router having an input port for the incoming WDM signals and a two output ports. The router divides the received optical signals into divided optical signals comprising a subset of the channels and spatially positions the divided optical signals in response to a control signal applied to the router. Wu's router can divide a received WDM signals into two subsets that are either single channel or WDM signals.
The disclosure of Wu et al. as discussed above is useful for switchable wavelength routing. It is also useful for a wavelength selector if there is no controlled polarization rotator. However, the router as disclosed is not practical for component miniaturization. Particularly, the arrangement of the two separate output ports
14
and
15
requires two independent collimators separated in certain distance due to the physical shapes and sizes of these collimators. Consequently, as the beams are separated with a spatial distance, a large crystal
50
is required and that would increase the production cost of the router and make the device quite expensive. Long optical path according to Wu's configuration would also increase the insertion loss and increase the volume, size and weight of the router assembly.
In view of the existing technologies, besides the challenge of processing transmission signals carried by wavelengths narrower channel spacing, another challenge in constructing an optical transmission system is to provide miniaturized optical components with improved performance, compact size and lower cost. The task of miniaturization is often difficult to achieve. This is due to the particular reasons that the spacing between adjacent channels in the WDM system is around 0.8 nm and even lower and the device is very sensitive to the accuracy of the central wavelength of the pass band and pass band profile. Additionally, there are stringent requirements for other optical performance parameters such as polarization mode dispersion, polarization dependent loss and other thermal-dependent effects imposed on different operational parameters. Weight and size reductions to miniaturize an optical module such as an interleaver cannot be conveniently achieved taking into consideration of these complexities and design constraints.
Therefore, a need still exists in the art of manufacturing and designing the fiber optic interleaver to provide improved configurations that would reduce the required optical components. Specifically, novel and improved interleaver configurations utilizing less components and can reduce manufacture process to lower the manufacture cost are needed to resolve the difficulties and limitations encountered by the fiber optical industries such that compact fiber optical interleaver can be manufactured at a low production cost.
SUMMARY OF THE PRESENT INVENTION
It is therefore an object of the present invention to provide an improved design and configuration for manufacturing and assembling a fiber optic interleaver with reduced number of components to reduce the weight and volume and lower the production costs. With the improved design and configuration, the aforementioned difficulties and limitations in the prior art can be overcome.
Specifically, it is an object of the present invention to provide an interleaver implemented with improved configuration by employing an incline beam angle directing means for projecting the output beams with small incline

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