Optical waveguides – With optical coupler – Switch
Reexamination Certificate
1999-06-16
2002-03-19
Ullah, Akm E. (Department: 2874)
Optical waveguides
With optical coupler
Switch
C385S047000, C359S199200
Reexamination Certificate
active
06360037
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fiber-optic switching devices and, more particularly, to a fiber-optic (FO) switch using polarization switching for routing multiple light signals in optical networks such as wavelength division multiplexed (WDM) optical communications, distributed sensor networks, and photonic signal processing systems requiring optical routing and gain control.
BACKGROUND OF INVENTION
The FO switch is a basic building block for many optical applications such as routing in fiber communications networks, photonic signal processing, distributed optical sensing, and optical controls. The desired features for a FO switch include low optical loss (e.g., <1 dB), low interchannel crosstalk (<−30 dB), and simple to align low cost designs for large scale commercial production and deployment. Depending on the application, FO switching speeds can range from nanoseconds to several milliseconds. An attractive technology for making FO switches is via the use of liquid crystal (LC) technology for making light polarization rotators that when placed between polarization sensitive optics forms a switch.
Prior art LC FO switches have suffered from various limitations such as excessive crosstalk, difficult alignment, unbalanced loss between the switch states, and unequal time delays between the switched states. Unequal time delays can cause timing problems when cascading small (e.g., 2×2) switches to make larger N×N switches (e.g., N=50) in very wide bandwidth (e.g., 10 Gigabits/sec) fiber-optic communication systems. Crosstalk significantly depends on the quality of the polarization switches. For instance, a major problem with LC based FO switches is that as the LC cell quality goes down, the FO switch crosstalk gets worse. This problem is recognized in the industry and several patents directed to reducing crosstalk have been issued. However, it is not believed that prior patents have adequately resolved the problems arising in this area from fiber alignment. See, for example, A. P. Baker, “Liquid Crystal Optical Switching Device Having Reduced Crosstalk,” U.S. Pat. No. 4,720,171, Jan. 19, 1988; J. S. Patel and Y. Silberberg, “Frequency Selective Optical Switch Employing A Frequency Dispersive Element, Polarization Dispersive Element, and Polarization Modulating Elements,” U.S. Pat. No. 5,414,540, May 9, 1995; N. K. Shankar, J. A. Morris, C. R. Pollack, C. P. Yakymyshyn, and W. Whitehead, “Optical Switches Using Cholesteric or Chiral Nematic Liquid Crystals and Method of Using The Same,” U.S. Pat. No. 4,991,924, Feb. 12, 1991; M. Liebowitz, “Liquid Crystal Optical Switching Device,” U.S. Pat. No. 4,917,452, Apr. 17, 1990: R. A. Soref, “Optical Switches Using Ferroelectric Liquid Crystals ,” U.S. Pat. No. 4,948,229, Aug. 14, 1990; M. R. Meadows, “Electro-optical Switch With 4-port Modules With Electro-optic Polarization Rotators,” U.S. Pat. No. 5,381,250, Jan. 10, 1995; K. T. Yamamoto, Y. H. Itoh, T. Nagayami, “Optical Space Switch And Network For Such Optical Space Switches,” U.S. Pat. No. 5,162,944, Nov. 10, 1992; Y. Hakamata, T. Yoshizawa, and T. Kodaira, “A 1.3&mgr;m Single-Mode 2×2 Liquid Crystal Optical Switch,” IEICE Trans. Commun., Vol. E77-B, No. 10, 1994; R. A. Soref, “Optical Bypass Switch,” U.S. Pat. No. 4,478,494, Oct. 23, 1984. “R. E. Wagner and J. Cheng, “Electrically Controlled Optical Switch For Multimode Fiber Applications,” Applied Optics, Vol. 19, pp. 2921-2925, 1980.
Proper use of electronically active and passive polarization components appropriately placed in an optical structure such as a polarization switched optical delay line can greatly suppress structure noise, as suggested in N. A. Riza, “Liquid crystal-based optical controllers for phased array antennas,”
Optoelectronic Signal Processing for Phased Array Antennas IV Conference Proceedings of the
SPIE, Vol. 2155, pp. 169-180, Jan. 1994. Using this concept of proper positioning of polarization components, the patent by K. Y. Wu, “Fault tolerant optical routing switch,” U.S. Pat. No. 5,724,165, 1998, introduces a low crosstalk 2×2 FO switch. Nevertheless, this transmissive mode switch has high fiber alignment complexity along with an unequal time delay problem between the switch states. Specifically, the two input fibers of the switch have to be simultaneously aligned to the two output fibers through a complex switching structure, making a long duration and tedious alignment task during switch fabrication. Furthermore, it uses several large birefringent beam displacing optics that are costly, adding to the overall expense on building such a switch.
One approach that improves switch alignment is to use an optical structure interconnected with FO circulators. A commercial FO circulator such as available from JDS Fitel, Canada, has three fiber ports. One port is reversible with light flow in and out of the port. The other two ports are a unidirectional input port and a unidirectional output port. Recently, the circulator has been used in a number of liquid crystal-based optical switches where the LC structure for light beam flow control is interconnected with circulators. A Fabry-Perot LC filter is used in the tuning structure proposed in Kuang-Yi Wu, “Tunable Add-Drop Optical Filter,” U.S. Pat. No. 5,606,439, Feb. 25, 1997. Another similar design but using stacked waveplates is by Kuang-Yi Wu and Jian-Yu Liu, “Switchable Wavelength Router,” U.S. Pat. No. 5,694,233, Dec. 2, 1997.
SUMMARY OF THE INVENTION
The present invention addresses the crosstalk and alignment problems of a 2×2 optical switch. The present inventor disclosed an improved fiber-optic switch in a presentation entitled “Ferroelectric liquid crystal polarization switching-based high speed multiwavelength add/drop filters using fiber and array-waveguide gratings,”
OC
'98,
Optics in Computing European Optical Society Topical Meeting Digest Series
, pp. 335-338, Belgium, Jun. 17-20, 1998. The accompanying paper on LC switching describes structure using cube polarization beam splitters (PBSs) for a two circulator structure. It is believed that this is the first time that an LC polarization rotation-based switching structure using a low cost cube PBS was sandwiched between two circulators to form a 2×2 FO switch. Using this retroreflective two circulator approach, the switching structure has only two ports for alignment and not four like in the transmissive 2×2 switches mentioned earlier. Using two circulators makes the switch alignment a simpler task, thus making a more economical device. In addition, the use of low cost cube PBS's reduces overall switch cost. A further disclosure was made on Jun. 25, 1998 by the present inventor and a co-author in an article entitled “Low Optical Interchannel Crosstalk, Fast Switching Speed, Polarization Independent 2×2 Fiber Optic Switch using Ferroelectric Liquid Crystals,” Electronics Letters, Vol.34, No.13, Jun. 25, 1998 by N. A. Riza and Shifu Yuan.
Further improvement on the applicant's FO switch invention include by N. A. Riza described earlier in N. A. Riza, “Ferroelectric liquid crystal polarization switching-based high speed multiwavelength add/drop filters using fiber and array-waveguide gratings,”
OC
'98
, Optics in Computing European Optical Society Topical Meeting Digest Series
, pp. 335-338, Belgium, Jun. 17-20, 1998, and N. A. Riza and Shifu Yuan. A highly useful module for WDM add-drop routing applications using a unique LC switch structure based on low cost cube PBS's for sandwiching between two circulators to form 2×2 FO switch invention with low complexity alignment and zero-relative time delay between the switch settings. Additionally, a compact transmissive 2×2 FO switch structure is disclosed for general purpose 2×2 switch applications where the routing channel numbers are small (e.g., 5).
More particularly, the present invention includes a novel 2×2 FO switch structure based on a retro-reflection scheme coupled with FO-circulators, cube PBS&apos
Beusse Brownlee Bowdoin & Wolter PA
Connelly-Cushwa Michelle R.
James H. Beusse, Esq.
Nuonics, Inc.
Ullah Akm E.
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