Optical apparatus which uses a virtually imaged phased array...

Details Optical apparatus which uses a virtually imaged phased array... Optical apparatus which uses a virtually imaged phased array...

2002-09-24

2004-09-07

Sikder, Mohammad (Department: 2872)

Optical: systems and elements

Mirror

With mirror surface of varied radius

C359S869000, C359S577000, C359S578000

Reexamination Certificate

active

06786611

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus producing chromatic dispersion, and which can be used to compensate for chromatic dispersion accumulated in an optical fiber transmission line. More specifically, the present invention relates to an apparatus which uses a virtually imaged phased array to produce chromatic dispersion.
2. Description of the Related Art
FIG.
1
(A) is a diagram illustrating a conventional fiber optic communication system, for transmitting information via light. Referring now to FIG.
1
(A), a transmitter
30
transmits pulses
32
through an optical fiber
34
to a receiver
36
. Unfortunately, chromatic dispersion, also referred to as “wavelength dispersion”, of optical fiber
34
degrades the signal quality of the system.
More specifically, as a result of chromatic dispersion, the propagating speed of a signal in an optical fiber depends on the wavelength of the signal. For example, when a pulse with a longer wavelength (for example, a pulse with wavelengths representing a “red” color pulse) travels faster than a pulse with a shorter wavelength (for example, a pulse with wavelengths representing a “blue” color pulse), the dispersion is typically referred to as “normal” dispersion. By contrast, when a pulse with a shorter wavelength (such as a blue color pulse) is faster than a pulse with a longer wavelength (such as a red color pulse), the dispersion is typically referred to as “anomalous” dispersion.
Therefore, if pulse
32
consists of red and blue color pulses when emitted from transmitter
30
, pulse
32
will be split as it travels through optical fiber
34
so that a separate red color pulse
38
and a blue color pulse
40
are received by receiver
36
at different times. FIG.
1
(A) illustrates a case of “normal” dispersion, where a red color pulse travels faster than a blue color pulse.
As another example of pulse transmission, FIG.
1
(B) is a diagram illustrating a pulse
42
having wavelength components continuously from blue to red, and transmitted by transmitter
30
. FIG.
1
(C) is a diagram illustrating pulse
42
when arrived at receiver
36
. Since the red component and the blue component travel at different speeds, pulse
42
is broadened in optical fiber
34
and, as illustrated by FIG.
1
(C), is distorted by chromatic dispersion. Such chromatic dispersion is very common in fiber optic communication systems, since all pulses include a finite range of wavelengths.
Therefore, for a fiber optic communication system to provide a high transmission capacity, the fiber optic communication system must compensate for chromatic dispersion.
FIG. 2
is a diagram illustrating a fiber optic communication system having an opposite dispersion component to compensate for chromatic dispersion. Referring now to
FIG. 2
, generally, an opposite dispersion component
44
adds an “opposite” dispersion to a pulse to cancel dispersion caused by traveling through optical fiber
34
.
There are conventional devices which can be used as opposite dispersion component
44
. For example,
FIG. 3
is a diagram illustrating a fiber optic communication system having a dispersion compensation fiber which has a special cross-section index profile and thereby acts as an opposite dispersion component, to compensate for chromatic dispersion. Referring now to
FIG. 3
, a dispersion compensation fiber
46
provides an opposite dispersion to cancel dispersion caused by optical fiber
34
. However, a dispersion compensation fiber is expensive to manufacture, and must be relatively long to sufficiently compensate for chromatic dispersion. For example, if optical fiber
34
is 100 km in length, then dispersion compensation fiber
46
should be approximately 20 to 30 km in length.
FIG. 4
is a diagram illustrating a chirped grating for use as an opposite dispersion component, to compensate for chromatic dispersion. Referring now to
FIG. 4
, light traveling through an optical fiber and experiencing chromatic dispersion is provided to an input port
48
of an optical circulator
50
. Circulator
50
provides the light to chirped grating
52
. Chirped grating
52
reflects the light back towards circulator
50
, with different wavelength components reflected at different distances along chirped grating
52
so that different wavelength components travel different distances to thereby compensate for chromatic dispersion. For example, chirped grating
52
can be designed so that longer wavelength components are reflected at a farther distance along chirped grating
52
, and thereby travel a farther distance than shorter wavelength components. Circulator
50
then provides the light reflected from chirped grating
52
to an output port
54
. Therefore, chirped grating
52
can add opposite dispersion to a pulse.
Unfortunately, a chirped grating has a very narrow bandwidth for reflecting pulses, and therefore cannot provide a wavelength band sufficient to compensate for light including many wavelengths, such as a wavelength division multiplexed light. A number of chirped gratings may be cascaded for wavelength multiplexed signals, but this results in an expensive system. Instead, a chirped grating with a circulator, as in
FIG. 4
, is more suitable for use when a single channel is transmitted through a fiber optic communication system.
FIG. 5
is a diagram illustrating a conventional diffraction grating, which can be used in producing chromatic dispersion. Referring now to
FIG. 5
, a diffraction grating
56
has a grating surface
58
. Parallel lights
60
having different wavelengths are incident on grating surface
58
. Lights are reflected at each step of grating surface
58
and interfere with each other. As a result, lights
62
,
64
and
66
having different wavelengths are output from diffraction grating
56
at different angles. A diffraction grating can be used in a spatial grating pair arrangement, as discussed in more detail below, to compensate for chromatic dispersion.
More specifically, FIG.
6
(A) is a diagram illustrating a spatial grating pair arrangement for use as an opposite dispersion component, to compensate for chromatic dispersion. Referring now to FIG.
6
(A), light
67
is diffracted from a first diffraction grating
68
into a light
69
for shorter wavelength and a light
70
for longer wavelength. These lights
69
and
70
are then diffracted by a second diffraction grating
71
into lights propagating in the same direction. As can be seen from FIG.
6
(A), wavelength components having different wavelengths travel different distances, to add opposite dispersion and thereby compensate for chromatic dispersion. Since longer wavelengths (such as lights
70
) travel longer distance than shorter wavelengths (such as lights
69
), a spatial grating pair arrangement as illustrated in FIG.
6
(A) has anomalous dispersion.
FIG.
6
(B) is a diagram illustrating an additional spatial grating pair arrangement for use as an opposite dispersion component, to compensate for chromatic dispersion. As illustrated in FIG.
6
(B), lenses
72
and
74
are positioned between first and second diffraction gratings
68
and
71
so that they share one of the focal points. Since longer wavelengths (such as lights
70
) travel shorter distance than shorter wavelengths (such as lights
69
), a spatial grating pair arrangement as illustrated in FIG.
6
(B) has normal dispersion.
A spatial grating pair arrangement as illustrated in FIGS.
6
(A) and
6
(B) is typically used to control dispersion in a laser resonator. However, a practical spatial grating pair arrangement cannot provide a large enough dispersion to compensate for the relatively large amount of chromatic dispersion occurring in a fiber optic communication system. More specifically, the angular dispersion produced by a diffraction grating is usually extremely small, and is typically approximately 0.05 degrees
m. Therefore, to compensate for chromatic dispersion occurring in a fiber optic communication system, first and second gratings
68
and
71
would have

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