Optical waveguides – Directional optical modulation within an optical waveguide – Acousto-optic
Reexamination Certificate
2002-04-15
2004-06-22
Healy, Brian (Department: 2874)
Optical waveguides
Directional optical modulation within an optical waveguide
Acousto-optic
C385S004000, C385S011000, C385S014000, C385S018000, C359S285000, C359S305000, C381S337000, C372S013000
Reexamination Certificate
active
06754403
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a double-passage acousto-optical device, that is, an acousto-optical device through which light passes twice, and specifically to various optical filters, wavelength add/drop devices, and optical cross-connects that are constructed using the double-passage optical device and methods for using them.
In optical filtering, double-stage optical filters are advantageous, because their filtering performance is increased compared to a single-stage filter having the same characteristics. Acousto-optical filters are known that provide for the interaction between light signals, propagated in waveguides formed on a substrate of birefringent and photoelastic material, and acoustic waves propagated on the surface of the substrate. The acoustic waves are generated by suitable transducers and are initially supplied by radio frequency signals.
The resonant (phase-matched) interaction between a polarized optical signal and an acoustic wave produces a wavelength-selective polarization conversion of the signal, in other words, a change of the polarization from its transverse electric TE component to its transverse magnetic TM component, which are orthogonal to each other, and vice versa. Following this interaction with the acoustic wave, the polarization components undergo not only the conversion to the corresponding orthogonal components, but also a frequency shift whose absolute value is equal to the frequency of the interacting acoustic wave (and therefore equal to that of the applied radio frequency signal). The sign of the frequency shift is a function of the state of polarization and of the direction of propagation of the acoustic wave with respect to the optical wave.
In such acousto-optical devices, by controlling the frequency of the optical and acoustic waves it is possible to tune the spectral response curve of the devices, which makes them suitable for being used as switches and as optical filters of the signals in optical telecommunications networks with wavelength-division multiplexing. These tunable switches and filters allow the selection of the signals to be changed and, thus, to reconfigure a network, without altering the cabling of the components.
These acousto-optical devices also allow the switching and simultaneous selection of different signals or channels, if the acoustic wave propagating at the surface of the substrate is the superimposition of different acoustic waves. In fact, the switches execute the combined switching of the signals at the wavelengths corresponding to the simultaneously applied frequencies and the filters have a pass band corresponding to the set of different wavelength intervals, determined by the frequencies of the acoustic waves.
As an example, EP 768555A1 in its
FIG. 1
illustrates a 2×2 acousto-optical switch with a polarization independent response. The switch comprises a substrate
1
in a birefringent and photoelastic material, consisting of Lithium Niobate (LiNbO
3
). Substrate
1
includes two polarization selective elements
2
and
3
and a conversion stage
4
. The two polarization selective elements
2
and
3
are formed by polarization splitters in an optical waveguide, each comprising respective central optical waveguide portions
5
and
6
and optical waveguide input and output branches
7
,
8
,
9
,
10
for splitter
2
and
11
,
12
,
13
,
14
for splitter
3
, respectively. The input branches
7
and
8
of splitter
2
are connected to input ports
71
and
81
of the switch through respective connecting optical waveguides
70
and
80
. The output branches
13
and
14
of splitter
3
are connected to output ports
131
and
141
of the switch through respective connecting optical waveguides
130
and
140
.
In practice these acousto-optical filters according to the description in EP 768555A1 comprise a waveguide chip of a length of about 65 mm, with optical guides
71
,
81
and
131
,
141
spaced apart by about 250 &mgr;m. Although
FIG. 1
of EP 0768555 is not drawn to scale, optical guides
15
and
16
are typically spaced apart by about 270 &mgr;m. This distance includes polarization beam splitters/combiners (PBS)
5
,
6
. The end-faces are usually slant-polished (about 6°) to avoid any back-reflection from the end-faces. The wafer on which the device is realized has a 3-inch diameter, this dimension fixing the maximum length of the substrate 1 to about 60-65 mm.
EP 814364A1 describes a double-stage acousto-optical waveguide device.
FIG. 12
in EP 814364A1 shows a switch, or add/drop node comprising, in addition to a third polarization conversion stage
303
, a fourth polarization conversion stage
403
. The fourth polarization conversion stage
403
is connected to an input polarization splitter
404
and to an output polarization splitter
405
. In turn the splitter
405
is connected to the polarization splitter
204
by means of the connecting branch
210
and to the lateral waveguide
255
. The ports
19
,
20
,
21
and
22
are connected to the line. The polarization splitter
404
is connected to input ports
25
and
26
through which the signals to be added or subtracted are introduced and signals to be added or subtracted are also introduced through the ports
23
and
24
.
U.S. Pat. No. 5,452,314 describes an acousto-optical tunable filter with a pair of electrodes on opposite sides of the waveguide. The patent discloses the use of a voltage source in which an applied electric field controls the birefringence of the filter, and a tunable laser incorporating such an acousto-optical tunable filter. Suitably adjusting the potential applied by the voltage source results in suppression of sidelobes, correction of asymmetric sidelobes, and compensation for physical variations in the waveguide.
U.S. Pat. No. 5,002,349 and EP 805372 describe single converter acousto-optical tunable filters. The '349 patent discloses an acousto-optical converter that allows multiple stages of such converters so as to provide for two-stage zero-frequency shifted converters and filters, lasers using an acousto-optical filter as a tuning element, polarization-independent converters, and wavelength-division-multiplexing routing switches.
U.S. Pat. No. 5,611,004 discloses a polarization independent acousto-optical tunable filter (AOTF). The patent describes in its
FIG. 6
an embodiment where two stages of signal filtering are realized with only one transducer
43
on the substrate
31
. Additionally, polarizer beam splitters
40
and
41
and a Faraday rotator
65
are used in the optical chain. Two stages of filtering are realized by passing the incoming beam of light through the AOTF a first time, reflecting the beam of light off of a mirror
67
and then passing the beam of light through the same AOTF a second time. A band pass filtered representation of the original beam of light is obtained at a circulator output
71
of an optical circulator
69
located at the input of the embodiment.
EP Application 98118377.5 describes a double passage acousto-optical device including an acousto-optical filter having a first converter coupled between first and second optical ports, a second converter coupled between third and fourth optical ports, and an optical combination coupled between the second port and the third port and including an optical isolating element.
EP Application 97113188.3 describes an acousto-optical device including a substrate of a material capable of propagating a surface acoustic wave along a portion of the surface of the substrate, a transducer for generating the surface acoustic wave, an optical waveguide formed in a substrate, and an acoustic absorber surrounding the portion of the substrate.
U.S. Pat. No. 5,712,932 describes optical cross connects for routing optical traffic between transmission paths in a wavelength-division-multiplexed optical communication system. The cross-connect switches in the '932 patent use Bragg grating filters.
Applicants have discovered that conventional double-stage acousto-optical devices requ
Bean Gregory V.
Corning Incorporated
Healy Brian
Petkovsek Daniel
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