Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
1999-09-28
2002-04-09
Leung, Quyen (Department: 2881)
Coherent light generators
Particular beam control device
Tuning
C372S013000, C359S308000
Reexamination Certificate
active
06370167
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 lasers that are constructed using the double-passage optical device.
It is known to use optical fibers to send optical information-carrying signals for long-distance communication.
Optical telecommunication systems are known that use wavelength division multiplexing (WDM) transmission. In these systems several channels, i.e. a number of independent transmission signals, are sent over the same line by means of wavelength division multiplexing. The transmitted channels may be either digital or analog and are distinguishable because each of them is associated with a specific wavelength.
The Applicant has observed that known WDM communication systems are limited as concerns the number of channels, i.e. the independent wavelengths that can be used for transmission within the wavelength band available for signal transmission and amplification.
In order to combine and separate signals with different wavelengths—to combine the signals at the transmission station, for example, to drop some toward receivers located at intermediate nodes of the line or to introduce others at intermediate nodes or to send them to separate receivers at the receiving station—adjacent channels (in wavelength terms) must be separated by more than a minimum predetermined value.
Said minimum value depends on the characteristics of the components employed in the system, such as the spectral characteristics of the wavelength selective components (e.g. bandwidth, center-band attenuation, figure of merit) and the wavelength stability (thermal and temporal) of the selective components themselves and of optical signal sources.
In particular, the Applicant has observed that the spectral selectivity of currently available wavelength selective components may greatly limit the possibility of adding and dropping signals in multichannel transmission systems, particularly when there are signals with closely spaced wavelengths, e.g. separated by less than 2 nm.
In optical filtering, either in optical communication systems or for other purposes, 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 sound waves propagated on the surface of the substrate. The sound waves are generated by suitable transducers and are initially supplied by radio frequency signals.
The interaction between a polarized optical signal and a sound wave produces a 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 sound 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 sound 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 sound wave with respect to the optical wave.
In addition to their use as pure filters, acousto-optic devices have also been used as tuning devices for lasers. To Applicants' knowledge, however, conventional acousto-optic devices used as filters or tuners for lasers have generally employed either a single-stage acousto-optic substrate using a single pass of light through the device or have employed multiple stages of cascaded acousto-optic filters that are constructed on separate physical substrates.
For example, EP Application 97113188.3 describes an acousto-optic 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.
EP 0814364 A1 describes a double-stage acousto-optical waveguide device.
FIG. 12
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 waveguard
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. 5,611,004 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
. 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.
U.S. Pat. No. 5,452,314 describes an acousto-optic 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-optic 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-optic 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-optic filter as a tuning element, polarization-independent converters, and wavelength-division-multiplexing routing switches. “Acoustically tuned erbium-doped fiber ring laser” by D. A. Smith et al.,
Optics Letters,
vol. 16, no. 6, Mar. 15, 1991, describes a continuously tunable laser that uses an acousto-optic filter to achieve a broad tuning range. “Acoustically tunable Ti:Er:LiNbO-Waveguide Laser” by I. Baumann et al., ECOC '94, vol. 4, pp. 99-102 describes a waveguide laser comprising a double-stage wavelength filter utilized as a narrow-band tunable optical amplifier.
WO 98/11463 discloses, with reference to its
FIG. 7
, an embodiment that can alleviate the polarization sensitivity of a null coupler acousto-optic tunable filter by simultaneously applying two acoustic waves. The null coupler is made from two optical fibers with diameters that are mismatched to the extent that the resultant coupler gives an extremely small passive coupling efficiency. The coupler is made by pre-tapering one of the two identical single-mode fibers along a short length before both fibers are fused and elongated together to form the coupler.
In the device of WO 98/11463, input light in the fiber that was not pretapered excites only the fundamental mode in the narrow waist of the coupler. Light in the other fiber excites only the second order mode. When the acoustic wavelength matche
Donati Stefano
Morasca Salvatore
Schmid Steffen
Corning O.T.I. s.P.a.
Leung Quyen
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