Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-03-14
2003-09-09
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S283000, C359S483010
Reexamination Certificate
active
06618182
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a magneto-optical switching element with a Faraday rotator formed from a magnetically uniaxial crystal, and can be used, for example, in optical switching systems, in optical telecommunications , and in data processing, for altering the optical path of a light beam, in optical shutters, optical attenuators, or in spatial light modulation systems, which are capable of altering the intensity of given parts of light beams.
Mechanical optical switches, shutters, and the like have the advantage that stable switching states without permanent energy consumption can be attained with relatively simple means. The disadvantage of such mechanical switching elements is their relatively low switching speed, which by nature excludes their use in many fields referred to in the preamble.
High switching speeds are provided, for example, by known electro-optical switching elements (see, for example, U.S. Pat. No. 5,712,935) or acoustic-optical switching elements (see, for example, U.S. Pat. No. 5,883,734), but they require a permanent supply of energy in order for stable switching states to be maintained. Exceptions to this are, for example, switching elements on the basis of the electro-optical effect with amorphous materials, which likewise feature stable states without permanent energy supply (see, for example, EP 0 500 402), whereby, however, their switching time is relatively high, since the transition times between the stable states lie in the ms-range.
The known magneto-optical switching elements referred to in the preamble offer a compromise solution. In comparison with mechanical elements they do not have any moving parts, and therefore have correspondingly low sensitivity to vibrations or shocks, and higher switching speeds, although associated with a wavelength-dependent operation. Compared with high-speed electro-optical and acoustic-optical switching elements they possess stationary states without external energy supply, although associated with somewhat higher switching times.
In this latter connection, reference may be made, for example, to the following works and publications: M. Shirasaki, H. Nakajima, T. Obokata, and K. Asama: Non-mechanical Optical Switch for Single-Mode Fibers, Appl. Opt. 21, 4229 (1982); M. Shirasaki, H. Takamatsu, and T. Obokata: Bistable Magnetooptic Switch for multimode Optic Fiber, Appl. Opt. 21, 1943 (1982); M. Shirasaki et al: Magnetooptical 2×2 switch for single-mode fibers, Appl. Opt. 23, 3271 (1984); M. Shirasaki: Faraday Rotator Assembly, U.S. Pat. No. 4,609,257 (1986); S. Takeda, Faraday rotator device and optical switch containing same, EP Patent 0 381 117 (1991); M. Shirasaki: Faraday rotator which generates a uniform magnetic field in a magnetic optical element, U.S. Pat. No. 5,812,304 (1998).
These feature bistable magneto-optical Faraday rotators, which do not require any permanent energy supply, but require energy solely for the process of switching between the stable states. The bistability is based in this case on the magnetic hysteresis, i.e. on the ability of certain magnetic materials to remain in a magnetised state after magnetising up to saturation. The magneto-optical (MO) materials used in this study represented compositions of iron garnets. These materials do not possess square-shaped hysteresis loops and accordingly remain demagnetised in the absence of an external field.
In order to create the bistability, the MO material is placed into the field of an electro-magnet with a core from semi-hard magnetic material (M. Shirasaki et al.: Magnetooptical 2×2 switch for single-mode fibers, Appl. Opt. 23, 3271 (1984). The core is magnetised by the electrical current pulse flowing through the windings with a specific polarity, until saturation. After the end of the pulse, both the core as well as the MO material remain magnetised and a rotation of the polarisation plane of the light passing through the MO material takes place. A change in the current polarity has the effect of a change in the direction of rotation of the polarisation plane. Both states are stable and the system accordingly remains without further energy consumption.
In order to prevent domain wall displacements in the rotator during the reversal of the magnetic field, which cause an irregular change of polarisation of the emergent light, an additional magnetic field was introduced (M. Shirasaki: Faraday Rotator Assembly, U.S. Pat. No. 4,609,257 (1986) and M. Shirasaki: Faraday rotator which generates a uniform magnetic field in a magnetic optical element, U.S. Pat. No. 5,812,304 (1998). This field is generated by a magnet and holds the rotator in the monodomain state. The magnetic field of the electromagnet accordingly only exerts magnetisation rotation and does not influence the domain structure.
One disadvantage in this context is the slow switch-over of the direction of magnetisation and in particular the switching of the direction of rotation of the polarisation. According to EP 0 381 117 (1991), “Faraday rotator device and optical switch containing the same”, in Example 2 the switching time was some 500 ms. In the article by M. Shirasaki et al.: “Non-mechanical Optical Switch for Single-Mode Fibers, Appl. Opt. 21, 4229 (1982)”, the switching time was about 10 &mgr;s. This relatively long switching time is associated with the high inductivity of the coil of the electromagnet; this amounts to some 7 mH. If permanent magnets and other configurations are used, for example in accordance with U.S. Pat. No. 5,812,304, the field of the electromagnet must have very much higher values, since for the magnetisation rotation the saturation field generated by the permanent magnets must be attained. This necessarily leads to much higher switching times than the 10 &mgr;s. referred to above. A further disadvantage of this known switching element is its size, which is determined by the dimensions of the core of the electromagnet; moreover, no multistable operation of the rotator is possible.
SUMMARY OF THE INVENTION
The aim of the present invention is to avoid the disadvantages mentioned of the described known magneto-optical switching elements of the type referred to in the preamble, and in particular to reduce the switching times, the switching energy required, and the overall dimensions of the switching element, as well as also making possible, in an advantageous manner, a multistable operation with several switching states.
This aim is achieved according to the present invention with a magneto-optical switching element of the type referred to in the preamble, in that the rotator, in each of its stable states without an external magnetic field imposed, features magnetic domains of both orientations, the walls of which are capable of being moved for switching over into another stable state by the imposition of an external magnetic field without the creation of additional domains. The switching element thus derived can in a simple manner possess different stable states, in which it can remain without time limitation and without energy consumption. The transition between these stable switching states takes place in the nano-second range, whereby only low optical losses occur in the infrared range, very little switchover energy is required, and, overall, very small dimensions can be maintained.
Magneto-optical materials are known with square-shaped hysteresis loops; that it to say, these materials remain magnetised in the absence of an external magnetic field. Orthoferrites are, for example, representatives of this material group. Orthoferrites are weak ferromagnets, which are characterised by a low resultant magnetisation and a very high uniaxial magnetic anisotropy. Orthoferrites are optically biaxial crystals. Large angles of the Faraday rotation can only be achieved in the absence of crystallographic birefringence, i.e. when the light. is propagated along the optical axes of the crystal. In this case, high magnetooptical figure of merit (FM), i.e. the ratio of the Faraday rotation to the absorption, takes place. In t
Dykema Gossett PLLC
Schwartz Jordan M.
Thompson Tim
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