Optical: systems and elements – Light interference
Reexamination Certificate
1999-11-01
2001-10-16
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Light interference
C359S629000, C359S637000, C359S839000, C359S861000
Reexamination Certificate
active
06304382
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a virtually imaged phased array (VIPA), or “wavelength splitter”, which receives a wavelength division multiplexed light comprising a plurality of carriers, and splits the wavelength division multiplexed light into a plurality of luminous fluxes which correspond, respectively, to the plurality of carriers and are spatially distinguishable from each other.
2. Description of the Related Art
Wavelength division multiplexing is used in fiber optic communication systems to transfer a relatively large amount of data at a high speed. More specifically, a plurality of carriers, each modulated with information, is combined into a wavelength division multiplexed light. The wavelength division multiplexed light is then transmitted through a single optical fiber to a receiver. The receiver splits the wavelength division multiplexed light into the individual carriers, so that the individual carriers can be detected. In this manner, a communication system can transfer a relatively large amount of data over an optical fiber.
Therefore, the ability of the receiver to accurately split the wavelength division multiplexed light will greatly effect the performance of the communication system. For example, even if a large number of carriers can be combined into a wavelength division multiplexed light, such a wavelength division multiplexed light should not be transmitted if the receiver cannot accurately split the wavelength division multiplexed light. Accordingly, it is desirable for a receiver to include a high-precision wavelength splitter.
FIG. 1
is a diagram illustrating a conventional filter using a multiple-layer interference film, for use as a wavelength splitter. Referring now to
FIG. 1
, a multiple-layer interference film
20
is formed on a transparent substrate
22
. Light
24
, which must be parallel light, is incident on film
20
and then repeatedly reflected in film
20
. Optical conditions determined by the characteristics of film
20
allow only a light
26
having wavelength &lgr;2 to pass therethrough. A light
28
, which includes all light not meeting the optical conditions, does not pass through the film
20
and is reflected. Thus, a filter as illustrated in
FIG. 1
is useful for splitting a wavelength division multiplexed light which includes only two carriers at different wavelengths, &lgr;1 and &lgr;2. Unfortunately, such a filter, by itself, cannot separate a wavelength division multiplexed light having more than two carriers.
FIG. 2
is a diagram illustrating a conventional Fabry-Perot interferometer for use as a wavelength splitter. Referring now to
FIG. 2
, high-reflectance reflecting films
30
and
32
are parallel to each other. Light
34
, which must be parallel light, is incident on reflecting film
30
and reflected many times between reflecting films
30
and
32
. Light
36
of wavelength &lgr;2 that meets passage conditions determined by the characteristics of the Fabry-Perot interferometer passes through reflecting film
32
. Light
38
of wavelength &lgr;1, which does not meet the passage conditions, is reflected. In this manner, light having two different wavelengths can be split into two different lights corresponding, respectively, to the two different wavelengths. Thus, as with the filter illustrated in
FIG. 1
, a conventional Fabry-Perot interferometer is useful for splitting a wavelength division multiplexed light which includes only two carriers at different wavelengths, &lgr;1 and &lgr;2. Unfortunately, such a Fabry-Perot interferometer cannot separate a wavelength division multiplexed light having more than two carriers.
FIG. 3
is a diagram illustrating a conventional Michelson interferometer for use as a wavelength splitter. Referring now to
FIG. 3
, parallel light
40
is incident on a half mirror
42
and split into a first light
44
and a second light
46
perpendicular to each other. A reflecting mirror
48
reflects first light
44
and a reflecting mirror
50
reflects second light
46
. The distance between half mirror
42
and reflecting mirror
48
, and the distance between half mirror
42
and reflecting mirror
50
indicate an optical path difference. Light reflected by reflecting mirror
48
is returned to half mirror
42
and interferes with light reflected by reflecting mirror
50
and returned to half mirror
42
. As a result, lights
52
and
54
having wavelengths &lgr;1 and &lgr;2, respectively, are separated from each other. As with the filter illustrated in FIG.
1
and the Fabry-Perot interferometer illustrated in
FIG. 2
, the Michelson interferometer illustrated in
FIG. 3
is useful for splitting a wavelength division multiplexed light which includes only two carriers at different wavelengths, &lgr;1 and &lgr;2. Unfortunately, such a Michelson interferometer cannot separate a wavelength division multiplexed light having more than two carriers.
It is possible to combine several filters, Fabry-Perot interferometers or Michelson interferometers into a giant array so that additional wavelength carriers can be split from a single wavelength division multiplexed light. However, such an array is expensive, inefficient and creates an undesirably large receiver.
A diffraction grating or an array waveguide grating is often used to split a wavelength division multiplexed light comprising two or more different wavelength carriers.
FIG. 4
is a diagram illustrating a conventional diffraction grating for splitting a wavelength division multiplexed light. Referring now to
FIG. 4
, a diffraction grating
56
has a concavo-convex surface
58
. Parallel light
60
having a plurality of different wavelength carriers is incident on concavo-convex surface
58
. Each wavelength carrier is reflected and interferes among the reflected lights from different steps of the grating. As a result, carriers
62
,
64
and
66
having different wavelengths are output from diffraction grating
56
at different angles, and are therefore separated from each other.
Unfortunately, a diffraction grating outputs the different wavelength carriers with relatively small difference of angle. Therefore, the angular dispersion produced by the diffraction grating will be extremely small. As a result, it is difficult for a receiver to accurately receive the various carrier signals split by the diffraction grating. This problem is especially severe with a diffraction grating which splits a wavelength division multiplexed light having a large number of carriers with relatively close wavelengths.
In addition, a diffraction grating is influenced by the optical polarization of the incident light. Therefore, the polarization of the incident light can affect the performance of the diffraction grating. Also, the concavo-convex surface of a diffraction grating requires complex manufacturing processes to produce an accurate diffraction grating.
FIG. 5
is a diagram illustrating a conventional array waveguide grating for splitting a wavelength division multiplexed light. Referring now to
FIG. 5
, light comprising a plurality of different wavelength carriers is received through an entrance
68
and is divided through a number of waveguides
70
. An optical exit
72
is at the end of each waveguide
70
, so that an output light
74
is produced. Waveguides
70
are different in length from each other, and therefore provide optical paths of different lengths. Therefore, lights passing through waveguides
70
have different path lengths from each other and thereby interfere with each other through exit
72
to form output
74
in different directions for different wavelengths.
In an array waveguide grating, the angular dispersion can be adjusted to some extent by properly configuring the waveguides. However, an array waveguide grating is influenced by temperature changes and other environmental factors. Therefore, temperature changes and environmental factors make it difficult to properly adjust the performance.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention
Cao Simon
Shirasaki Masataka
Jr. John Juba
Spyrou Cassandra
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