Optical tapped delay line

Details Optical tapped delay line Optical tapped delay line

2000-10-13

2003-08-19

Juba, John (Department: 2872)

Optical: systems and elements

Light interference

C359S580000, C359S589000, C359S634000, C385S024000, C398S079000

Reexamination Certificate

active

06608721

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method and apparatus for communications data transmission using wavelength division multiplexing (WDM). More specifically, the present invention relates to a method and apparatus for enabling hyperfine wavelength division multiplexing (HWDM) by subchannelizing each channel of conventional dense wavelength division multiplexing (DWDM) into many subchannels.
BACKGROUND OF THE INVENTION
Fiber optic cable is widely used for data transmission and other telecommunication applications. However, the relatively high cost of installing new fiber optic cable presents a barrier to increased carrying capacity.
Wavelength division multiplexing (WDM) enables different wavelengths to be carried over a common fiber optic waveguide. WDM can separate the fiber bandwidth into multiple discrete channels through a technique referred to as dense wavelength division multiplexing (DWDM). This provides a relatively low cost method of substantially increasing long-haul telecommunication capacity over existing fiber optic transmission lines.
Techniques and devices are required, however, for multiplexing the different discrete carrier wavelengths. That is, the individual optical signals must be combined onto a common fiber optic waveguide and then later separated again into the individual signals or channels at the opposite end of the fiber optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength bands) from a broad spectral source is of growing importance to the fiber optic telecommunications field and other fields employing optical instruments.
Devices that assemble multiple tightly spaced carrier wavelengths within a single fiber are called multiplexers. Devices that separate the carrier wavelengths at the receiving end of a fiber are called demultiplexers or channelizers. The following types of known technologies can be used as WDM channelizers.
Fabry-Perot Interferometer
The Fabry-Perot interferometer is a known device for resolving light into its component frequencies, or equivalently, its component wavelengths.
FIG. 1
illustrates one example of a prior art Fabry-Perot interferometer. The illustrated device comprises two mirrors M
1
and M
2
. Each of the two mirrors M
1
and M
2
is a partially reflecting mirror. The mirrors M
1
and M
2
are separated by an air space. Alternatively, the Fabry-Perot interferometer device could be made by coating both sides of a transparent plate with a partially reflecting material.
Light from a spectrally broadband source is input at plane S
1
. Light rays at an angle &thgr; and a wavelength &lgr; undergo multiple reflections between mirrors M
1
and M
2
. The light rays interfere constructively along a circular locus P
2
in the output plane S
2
. The condition for constructive interference that relates a particular angle &thgr; and a particular wavelength &lgr; is given by
2
dcos&thgr;=m&lgr;
where d is the separation of the partially reflecting surfaces, and m is an integer known as the order parameter. The Fabry-Perot interferometer thereby separates the component frequencies of the input light by using multiple beam reflection and interference. It is apparent from the equation above that the output light pattern of the system, i.e., the interference fringes, in the case of a diverging input beam, is a set of concentric circular rings. One ring is present for each wavelength component of the input light for each integer m, with the diameter of each ring being proportional to the corresponding light frequency.
The Fabry-Perot interferometer is not well-suited for use as a WDM channelizer due to the difficulty in obtaining high optical throughput efficiency. If the input beam is divergent, e.g., the direct output of an optical fiber, then the output pattern for a given wavelength is a set of rings. Multiple wavelengths produce nested sets of concentric rings. It is difficult to collect this light efficiently and concentrate it at multiple detector points, or couple it to multiple output fibers, especially while maintaining the separation of wavelength components that the interferometer has produced. If the input beam is collimated, e.g., the collimated output of an optical fiber, then the beam can be fanned over a narrow range of angles to produce only a single-order output (e.g., m =+1) for each wavelength of interest. This fanning makes it easy to concentrate the output light at multiple detector points or fibers, but there is inherently high loss. The throughput efficiency can be no greater than 1/N, where N is the number of wavelength components to be separated.
Lummer-Gehrcke Interferometer
FIG. 2
illustrates an example of a Lummer-Gehrcke interferometer. The illustrated interferometer comprises an uncoated glass plate and a prism for coupling a beam of light into the plate. Internally, the plate is highly reflective at internal incidence angles that approach the critical angle. The internal incidence angle controls the reflectivity of the surfaces. The output of the illustrated Lummer-Gehrcke interferometer is a series of multiple reflected beams that have a frequency-dependent phase shift from beam to beam and that are focused at the output plane by a lens. The interference fringes that are formed at the output plane in the case of a diverging input beam and a particular wavelength &lgr; are a family of hyperbolae near the center of the output plane. Each wavelength component of the input beam gives rise to a unique set of hyperbolic fringes.
The Lummer-Gehrcke interferometer relies upon a glass plate that is uncoated. However, the absence of a surface coating means that it is not possible to tailor the fringe intensity profile. This makes the Lummer-Gehrcke interferometer impractical for use in WDM applications in which the fringe profile controls the channel filter shape.
The Lummer-Gehrcke interferometer also is not well-suited for use as a WDM channelizer due to the difficulty in obtaining high optical throughput efficiency. If the input beam is divergent, e.g., the direct output of an optical fiber, then the output pattern for a given wavelength is a family of hyperbolae. Multiple wavelengths produce nested sets of hyperbolae. It is difficult to collect this light efficiently and concentrate it at multiple detector points, or couple it to multiple output fibers, especially while maintaining the separation of wavelength components that the interferometer has produced. If the input beam is collimated, e.g., the collimated output of an optical fiber, then the output pattern for a given wavelength is a set of focused spots corresponding to multiple interference orders. Again, it is difficult to collect this light efficiently, and there is generally an inherent loss. The throughput efficiency can be no greater than 1/N, where N is the number of focused spots per wavelength.
Virtually Imaged Phased Array
FIG. 3
illustrates an example of a Virtually Imaged Phased Array (commonly referred to as a VIPA). The VIPA illustrated in
FIG. 3
includes a rectangular glass plate
10
that has a 100% reflective coating
12
on a first side and a partially reflective coating
14
on an opposing side. Light enters the plate
10
below the reflective coating
12
in the form of a focused line source
16
produced by cylinder lens
18
.
FIG. 4
illustrates an operational side view of the VIPA. Input light rays
20
and
22
represent the boundaries of the line-focused input beam. The lens
18
focuses the input rays at the point
24
, after which the rays diverge as the beam propagates. The focused input rays
20
and
22
are partially reflected by the coating
14
and then totally reflected by the coating
12
. This reflection produces a virtual image of point
24
at location
26
. The reflective process is continued, producing additional receding virtual images at locations
28
and
30
. This process produces virtual images of the line source that recede away from the input side of the glass plate (i.e., to the left of element
10
in

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