Solid-State optical wavelength switches

Details Solid-State optical wavelength switches Solid-State optical wavelength switches

2001-12-18

2004-04-06

Healy, Brian (Department: 2874)

Optical waveguides

With optical coupler

Switch

C385S011000, C385S015000, C385S022000, C385S036000, C385S047000, C398S045000, C398S065000, C398S086000, C398S152000, C359S494010, C359S490020, C359S490020

Reexamination Certificate

active

06718082

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical device. More particularly, the invention relates to a non-mechanical optical wavelength selective switch.
2. Description of Related Art
Fiberoptic wavelength division multiplexing (WDM) has emerged as the dominant platform for telecommunications, providing a major leap in capacity by enabling a single fiberoptic cable to transmit multiple waves of light at once thereby multiply increasing communication bandwidth. WDM systems transmit information by employing optical signals including a number of different wavelengths, known as carrier signals or channels. Each carrier signal is modulated by one or more information signals. For further bandwidth expansion, intelligent optical networks become critical in which optical channels are dynamically routed/switched in the optical layer. Therefore, wavelength selective optical routers/switches are a key component in next-generation optical networks. Such devices are analogous to electrical switches in electrical networks. Optical wavelength selective switches can be used to perform basic WDM functionalities, such as optical signal routing, channel add/drop, and dynamic multiplexing/demultiplexing. However, optical wavelength selective switching has not been widely adopted because of the lack of commercially available components of needed reliability.
In an optical switch, a light signal must accurately enter into an optical fiber or much of the signal strength is lost. The alignment requirements of micro-optic devices are particularly stringent, as fiber core diameters are typically as small as 2 to 10 micrometers and their acceptance angle is fairly narrow. Furthermore, insertion losses reduce the amplitude of the optical signal. Therefore, optical switches that accept light from an input optical fiber and selectively couple that light to any of a number of output optical fibers must transfer that light precisely and within a small acceptance angle for the light to efficiently enter the fiber. Currently, optical wavelength selective switching is achieved by coupling optical filters with mechanical optical switches. Consequently, such devices have many drawbacks including slow switching speed, low reliability, and bulky size. One such mechanical wavelength selective switch is described by Lee in U.S. Pat. No. 6,192,174 issued on Feb. 20, 2001. It is therefore greatly desirable to have integrated optical wavelength selective switches that direct light beams according to their wavelength without moving parts, a feature generally associated with high reliability and high speed.
A non-mechanical optical wavelength selective switch is described and claimed by Wu et al. in U.S. Pat. No. 5,694,233 issued on Dec. 2, 1997.
FIG. 1
depicts the optical wavelength switch
999
from Wu et al., herein incorporated by reference. A WDM signal
500
containing two different channels
501
,
502
enters the optical wavelength switch
999
at an input port. A first birefringent element
30
spatially separates the WDM signal
500
into horizontal and vertically polarized signals
101
and
102
via a horizontal walk-off element. Signals
101
and
102
are coupled to a two-aperture polarization rotator
40
. The polarization rotator
40
selectively rotates the polarization state of either signal
101
or
102
by a predefined amount to render their polarization parallel. The polarization rotator
40
consists of two sub-element rotators that form a complementary state so that when one aperture turns ON the other turns OFF. By way of example, one signal
102
in
FIG. 1
is rotated by 90° so that signals
103
,
104
exiting the polarization rotator
40
are both horizontally polarized when they enter a wavelength filter
61
.
A waveplate wavelength filter
61
selectively rotates the polarization of wavelengths in either the first or second channel to produce filtered signals
105
and
106
. For example, the wavelength filter
61
may rotate wavelengths in the first channel
501
by 90° but not wavelengths in the second channel
502
. The filtered signals
105
and
106
then enter a second birefringent element
50
that vertically walks off the first channel into beams
107
and
108
and the second channel into beams
109
and
110
. A second wavelength filter
62
then selectively rotates the polarization of signals
107
and
108
but not signals
109
and
110
thereby producing signals
111
,
112
,
113
and
114
having polarizations that are parallel to each other. A second polarization rotator
41
then rotates the polarizations of signals
111
and
113
, but not
112
and
114
. The resulting signals
115
,
116
,
117
, and
118
then enter a third birefringent element
70
. This birefringent element
70
combines signals
115
and
116
, into the first channel, which is coupled to one output port and also combines signals
117
and
118
into the second channel, which is coupled into another output port.
As described above, by suitably controlling the polarization rotation induced by the polarization rotators
40
and
41
, the optical wavelength switch
999
operates as a wavelength selective device. Furthermore, the optical wavelength switch
999
can also operate as a passive interleaver multiplexer or de-multiplexer via a fixed set of polarization rotators in
40
and
41
.
The optical wavelength switch
999
has major drawbacks. First, it is disadvantageously based on a large spatial separation between two fibers located on the same side. The configuration requires individual imaging lens for each fiber port and consequently requires large and long-length crystals to deflect the beams. The use of three separated collimators to couple the signals into and out of optical fibers adds size, complexity, and cost. Moreover, the long couple distance increases signal loss. The bulky size also leads to instability, since operational stability is inversely related to the mass of birefringent materials. As a result, the optical wavelength switch
999
typically has high loss, excessively large size, and is expensive to produce and less stable in operation. Second, the electrically controllable polarization rotators
40
and
41
are based on a two-part aperture design that rotates the optical beams separately in a complementary manner, i.e. when one turns ON the other turns OFF. Such a design is primarily for the incorporation of organic liquid crystal device (LCD) based polarization rotators. The LCD usually employs surface electrodes in the light path to apply an electrical field. Consequently, two individually controllable rotators can be easily fabricated on the same element via electrode patterns. However, the use of liquid crystal materials leads to undesirable properties of slow speed and large temperature dependence, which are objectionable for optical network applications. Recent progress in inorganic magneto-optic and electro-optic materials has opened new opportunities to produce solid-state optical switches of faster speed and high stability. However, the two-part separately controlled polarization rotator
40
,
41
in the optical wavelength switch
999
is unsuitable for incorporating inorganic crystals. This is so because it is very difficult and impractical to apply two opposite fields with reasonable uniformity to two adjacent Faraday crystals or electro-optic crystals, due to the strong field interference across the small spatial separation.
An optical interleaver described by Li in U.S. Pat. No. 6,212,313 issued on Apr. 3, 2001 represents some improvement by using dual fiber sharing a single imaging lens to reduce the size of the optical device. However, wavelength selective devices based on Li are primarily designed for passive interleaver applications. Li is not amenable to active wavelength selective switches, because it too is based on the same two-part aperture polarization rotator design described by Wu. For reasons described above, the Li invention is unsuitable for wavelength switching/routing applications using solid-sta

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