Optical switch

Details Optical switch Optical switch

2001-03-16

2002-12-10

Lester, Evelyn A (Department: 2873)

Optical: systems and elements

Polarization without modulation

Polarization using a time invariant electric, magnetic, or...

C359S256000, C359S320000, C359S301000, C359S199200, C359S199200, C385S016000, C385S017000

Reexamination Certificate

active

06493139

ABSTRACT:

BACKGROUND
1. Technical Field
The present invention relates to optical switches and, more particularly, to optical switches based upon rotation of the plane of polarization of components of the incident light and, most particularly, to switches based upon magneto-optical effects.
2. Description of Related Art
One of the key components in optical communication networks is the optical switch which controls the direction of one or more incoming optical signals among several output ports under the control of one or more control signals. An example is the “1×2” switch in which light entering the 1×2 switch via an input port (port
1
), is caused to exit from the switch via either of two output ports, port
2
or port
3
, under the control of an external control signal, typically an electrical control signal. Traditionally, optical switches are used for network protection and for multiplexing two light beams into a single beam (“add/drop”). With the advent of all-optical networks (that is, without interconversion between optical and electrical signals during transmission) optical switches are expected to play a more and more important role in communication networks. New applications based upon optical switching are continuously being introduced, including an all-optical cross-connect.
Optical switches are typically classified into categories. One category is mechanical-optical switches in which the mechanical motion of a micromirror or other component directs the light beam to one of many optical fibers or other output ports. While mechanical-optical switches have been used in network systems, the necessity for mechanical motion may limit the number of switching cycles, the switching speed as well as the size and the long-term reliability of the device.
Optical mirror switches have been proposed that make use of a “walk-off” device that causes a light beam to take an angular path with respect to a direct, “pass-through” beam causing the angular beam to walk-off laterally with respect to the pass-through beam. In a first state of this switch, the input and output ports are directly coupled in a pass-through state. In a second or “reflective state,” the input and input ports are decoupled so that an input signal is directly reflected and returned thorough the same input port. That is, an optical signal directed into a first port would be reflected and returned through the same port. Possibly an optical signal input via a second input port is reflected and returned to the second port. This type of switch cannot be used in multiple port switches.
Another category of optical switch is thermal-optical in which an optical path change is effected by a change in the ambient temperature. While thermal-optical switches have also been used in optical network systems, some drawbacks occur. Due to the need for precise temperature control, continuous power consumption is required. Thermal inertia may also limit the range of switching possible as well as limit the switching speed.
Acousto-optical switching makes use of piezoelectric materials in which the refractive index changes by a useful amount when pressure in the form of an acoustic wave is applied to the material. While microsecond switching speeds are obtainable with acousto-optical switches, continuous RF is typically needed to maintain the switching status, limiting the usefulness of such switches.
Electro-optical switches make use of the electro-optical effect in some materials in which the index of refraction changes under the influence of an applied electrical field. Such materials include lithium niobate, electro-optical ceramics, polymers and other nonlinear optical and semiconductor materials. Fast switching times can be obtained with electro-optical switches, on the order of nanoseconds (10
−9
sec.) for the case of lithium niobate. However, it is difficult to implement an electro-optical switch that has low insertion losses of light and whose performance is independent of the polarization of the input light. The high cost of typical electro-optical switches has hindered their wide application in networks.
Another type of proposed switch is the optical add/drop wavelength switch. This switch can change from a bridge state (in which output is identical to input) to an add/drop state. Polarization controllers and discriminators are used to selectively align or combine the add signal. This is typically a switch having two input ports and two output ports in which the light beam passes once through each polarization device within the switch. Thus, the crosstalk of the switch is basically determined by the contrast ratio of the polarization controllers used (i. e. the intensity ratio between on and off states), rendering it difficult to achieve very high isolation. Switches of this type typically achieve isolation less than about −50 dB.
The present invention relates to optical switches making use of rotation of the plane of the light's polarization, particularly by means of the magneto-optical effect in which application of a magnetic field to a magneto-optical material (a Faraday rotator) leads to switching. Particular embodiments of 1×2, 1×4, three-port optical circulator and optical add/drop switches are described. Some embodiments employ a dual forward and reverse traversal of the switch to accomplish the switching function. Such dual traverse optical switches reduce crosstalk without increasing the size of the switch.
SUMMARY
The present invention relates to optical switches and particularly to magneto-optical switches controlling the direction or port by which light emerges from the switch by externally-applied magnetic fields, in particular, Faraday rotation. The input light need not have any particular state of polarization, the switches of the present invention function for randomly polarized input light, producing similarly randomly polarized light at the appropriate output port. Specific embodiments relate to 1×2 and 1×4 switches in both single pass and dual pass embodiments. Single pass embodiments are switches in which light passes through the optical components in one direction emerging at the appropriate output port depending on the desired switching. Dual pass embodiments are switches in which the light makes a forward and reverse traverse of the switch with total reflection. Dual pass switches typically reduce crosstalk without increasing the size of the switch.
In the operation of a typical 1×2 switch, a beam of randomly polarized input light is split into 2 beams having orthogonal polarizations by the action of a first birefringent crystal functioning as a beam separator, causing the extraordinary beam to leave the first birefringent crystal displaced by a walk-off distance, L from the ordinary beam. The beams then pass through half-wave plates having configurations so as to rotate the plane of polarization by 45° in opposite directions. Both beams now pass through a Faraday rotator having a configuration and magnitude of applied magnetic field such that light passing through the rotator has its plane of polarization rotated by 45° in either clockwise or counterclockwise (positive or negative) directions as determined by the orientation of the applied magnetic field. Passage through another set of half-wave plates and a second birefringent crystal of appropriate configurations leads to the light beams being recombined at one of two output ports, determined by the sense (positive or negative) of Faraday rotation. A 1×4 switch is made by a combination and generalization of the techniques used in the 1×2 switch. Further sequential uses of Faraday rotators pursuant to the present invention produce a 1×N switch where N=2
k
, and k=the number of Faraday rotators.
Other embodiments of the 1×2 magneto-optical switch employ two Faraday rotators configured so as to provide reduced crosstalk (typically less than approximately −40 dB), in a single pass switch which is beneficial for applications in optical network systems. Ge

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