Faraday rotator for an optical isolator

A Faraday rotator includes two magnet sub-assemblies assemblies spaced apart and aligned with each other with a gap therebetween. Each magnet sub-assembly includes a central magnet magnetized in direction parallel to the gap. The central magnet is sandwiched between two end magnets magnetized in a direction perpendicular to the gap. A magneto-optic crystal is located in the gap between the central magnets.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to optical isolators (optical diodes) providing non-reciprocal transmission of optical radiation. The invention relates in particular to Faraday rotators for use in such elements.

DISCUSSION OF BACKGROUND ART

Optical isolators have various uses in optical devices. By way of example they are used to provide unidirectional circulation of radiation in a ring laser-resonator (traveling-wave resonator) and to prevent feedback between stages of an optical amplifier. An optical-diode includes a crystal of a magneto-optic material. The magneto-optic material is used as a unidirectional polarization rotator, in conjunction with polarization-selective elements to provide the non-reciprocal transmission. The polarization rotation of the magneto-optical material is achieved by applying a magnetic field to the magneto-optic material, longitudinal in the direction of light propagation in the magneto-optic material.

It is common to have a non-reciprocal polarization rotation of 45° in one forward pass and accordingly 90° for back-reflected light. The polarization-selective elements are oriented in 45° with respect to each other, resulting in optical isolation of the back-reflected light. A deviation of the rotation angle has direct impact on the optical isolation performance

Optical-isolators are most effective in a wavelength range between about 400 nanometers (nm) and 1100 nm. The effectiveness of an optical-isolator depends on a so-called “Verdet” constant of the magneto-optic material. This constant defines a degree of polarization-rotation, per unit length of the material, per unit applied magnetic field. The most widely used magneto-optic material for optical isolators is terbium gallium garnet (TGG) which has a relatively high Verdet constant compared with that of other magneto-optic materials. Polarization rotation provided by TGG is particularly temperature sensitive. Because of this, an optical isolator including TGG usually requires some form of temperature control to optimize optical isolation even under high power irradiation and under environmental changes.

A TGG crystal for use in an optical isolator is relatively expensive and contributes significantly to the cost of an optical isolator. Further, TGG, while nominally transparent to radiation in the above-referenced wavelength range, has a finite absorption for that radiation. The absorption can result in significant heating of the crystal in a case where high-power radiation is being transmitted by the crystal.

The higher the magnetic field that can be applied to a TGG crystal the smaller (shorter) the crystal needs to be to provide a required polarization rotation. The smaller the crystal, the less expensive the crystal will be, and the less the absorption of radiation will be.

One particularly effective arrangement for providing a high magnetic field in a crystal of a magneto-optic material is described in U.S. Pat. No. 7,206,166. Here, the magnetic field is provided by an effectively cylindrical arrangement of permanent magnets. The effectively cylindrical arrangement includes a central magnet which is an actual cylinder which is axially magnetized. The magnetic field of the cylinder extends within the cylinder, approximately parallel to the axis of symmetry of the cylinder, in only one direction from the north-pole to the south-pole. A roller-shaped magneto-optic crystal is arranged within the cylinder.

Terminal magnets are attached to each of the two end faces of the central magnet in a plane perpendicular to the axis of symmetry. Each of the terminal magnets is configured as a hollow cylinder and has a through-aperture in the extension of the axis of symmetry. Each terminal magnet is largely radially magnetized with regard to the axis of symmetry. One of the two terminal magnets is magnetized radially from interior to exterior and the other terminal magnet is magnetized radially from exterior to interior. Each of the terminal magnets is formed from a plurality of wedge-shaped magnets for effecting the radial magnetization of the terminal magnets.

While the arrangement of the '116 patent may be highly effective in providing a concentrated magnetic field, the arrangement has significant shortcomings. The cylindrical center magnet and the wedge-shaped magnets forming the terminal magnets will be expensive to produce compared with simple bar-magnets. The cylindrical assembly of magnets restricts direct thermal access to the magneto-optic crystal. Accordingly, thermal control of the magneto-optic crystal must be provided by placing the entire magnet assembly, with the magneto-optic crystal therein, inside a thermally controlled enclosure.

Such an enclosure would be relatively expensive and would require a relatively large power supply. Cost aside, however, control by such a large enclosure would have a very slow response time due to the large thermal mass of the magnet assembly, which could be over one-hundred times greater than the thermal mass of the magneto-optic crystal. There is a need for a magnet assembly capable of providing a magnetic field comparable to that of the '116 patent but which provides direct thermal access to the magneto-optic crystal, allowing the crystal temperature to be controlled independent of the magnets and with relatively fast response. Preferably the magnet assembly should be formed from simple bar-magnets for economy of construction.

SUMMARY OF THE INVENTION

In one aspect, a Faraday rotator in accordance with the present invention comprises first and second planar magnet-subassemblies spaced apart and parallel to each other forming a gap therebetween with a propagation-axis of the isolator extending through the gap. Each of the magnet subassemblies includes a first bar-magnet magnetized in a direction parallel to the propagation axis, the first bar-magnet being sandwiched between second and third bar-magnets magnetized in a direction perpendicular to the propagation axis. The first bar-magnets of each subassembly assembly create a dipole magnetic field in the gap, and the second and third bar magnets of each subassembly creating a quadrupole magnetic field reinforcing the dipole magnetic field in in the gap. The reinforced magnetic field provides magnetic lines of force in the gap parallel to propagation axis between the first magnets of the subassemblies. A magneto-optic crystal is located in the gap in the parallel magnetic lines of force.

In a preferred embodiment of the invention, a temperature control element is in thermal communication with the magneto-optic crystal. Thermal communication is provided by a thermal conductor extending into the gap.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like components are designated by like reference numerals,FIG. 1schematically illustrates a preferred embodiment10of a Faraday rotator in accordance with the present invention for use in an optical isolator. Faraday rotator10comprises first and second planar magnet sub-assemblies12A and12B, vertically aligned with each other but vertically spaced apart from each other and parallel to each other. A light-propagation axis20(generally corresponding to a horizontal axis of symmetry of the Faraday rotator) extends through a gap22between sub-assemblies12A and12B. Here, it should be noted that the terms “vertical” and “horizontal” are used relatively herein, merely for convenience of description, and do not imply any particular spatial orientation of the inventive Faraday rotator in actual use.

Each magnet sub-assembly comprises a central bar-magnet14magnetized in a direction parallel to light-propagation axis20. The magnetization direction of magnets14and other magnets described and depicted herein is indicated by an arrow between letters N and S indicating respectively North and South poles of the magnets. The direction of magnetization of magnets14is the same in each of the sub-assemblies. Because of this, magnets14can be considered together as forming a magnetic dipole unit.

In each magnet sub-assembly, central magnet14is sandwiched between end magnets16and18. Each of the end magnets is magnetized in a direction perpendicular to axis20, perpendicular to (the plane of) gap22. In magnets16, the direction of magnetization is toward gap22. In magnets18the direction of magnetization is away from gap22. Magnets16and18can be considered, together, as forming a magnetic quadrupole unit. The dipole unit magnetic field of magnets14is reinforced by the quadrupole field of magnets16and18. This can result in a magnetic field force in excess of 1 Tesla (T).

Regarding dimensions of magnets14,16, and18, magnets16and18each have a length “a” and magnets14have a length “b”. The length of the magnets is defined in the propagation-axis direction. All magnets have a width “c”. All magnets have a height “b” equal to the length of magnets14The gap width is “d”. In relative terms, “b” is preferably greater than “a”, and “c” is preferably greater than “b” and “a”. It was found that if the height dimensions of the magnets was not the same a weaker magnetic field would be obtained.

FIG. 2schematically depicts computed lines of magnetic force in an example of the Faraday rotator ofFIG. 1. InFIG. 2the components and axes ofFIG. 1are depicted together with a magneto-optic crystal24which is not visible inFIG. 1because of the dimensions of the crystal relative to the dimensions of the magnets. Components, reference numerals, and lead-lines are depicted in bold to avoid confusion with lines of force, which are depicted by fine, solid lines.

For computing the lines of force it is assumed that dimensions “a” and “b” are 15 millimeters (mm) and 20 mm respectively. Gap20is assumed to have height of 3.54 mm and dimension “c” is assumed to be indefinitely extended. The magnets are assumed to be made of a neodymium, iron, and boron (NdFeB) alloy having a remnant magnetization of about 1.2 T. It can be seen that in gap22between central magnets14, the lines of force are parallel to the gap (parallel to the light-propagation axis) and homogeneously distributed.

By making the width of the magnets14greater than the length of the magnets this homogeneous magnetic field extends laterally sufficiently to fill crystal24. This provides a field comparable to that produced by the above-discussed cylindrical arrangement of the '116 patent at the expense of some loss of compactness, but with a much simpler and less expensive construction. The length of magnets16and18is selected to be only sufficient to achieve the parallel lines of force in essentially the entire gap between magnets14while minimizing the overall length of the sub-assemblies. It has been found that it is possible to reduce the requirement on precise length control of the TGG crystal and precise magnetic field strength of the magnets even further by slightly tuning the width of gap22between the magnets. In any event Gap22provides for direct-heating access to crystal24, a description of one arrangement for such access is set forth below with reference toFIG. 3

FIG. 3is an end-elevation view of the Faraday rotator ofFIG. 2schematically illustrating a heating element26located adjacent the Faraday rotator and in thermal communication with the magneto-optic crystal via a thermal conductor28. One suitable heating element is a power resistor. One suitable element capable of heating and cooling is a Peltier element. Thermal conductor28is preferably made from a material having good heat conductivity and a thermal expansion coefficient comparable to the thermal expansion coefficient of TGG (around 7×10−6/K). Such a material can be a copper-tungsten (CuW) alloy or an aluminum based ceramic. This arrangement is much more convenient and significantly less expensive than heating the entire Faraday rotator, including the magnets. Thermal gradients can be kept to a practical minimum.

FIG. 4is a side elevation view schematically illustrating another embodiment30of a Faraday rotator in accordance with the present invention. This embodiment is similar to the embodiment ofFIGS. 1 and 2but includes extended sub-assemblies32A and32B vertically aligned with each other but vertically spaced apart from each other and parallel to each other. Light-propagation axis20(generally corresponding to a horizontal axis of symmetry of the Faraday rotator) extends through a gap22between the sub-assemblies as in the embodiment ofFIGS. 1 and 2. The sub-assemblies are extended as follows.

Two magnets34, each thereof magnetized in a direction parallel to light-propagation axis20, are added to form another magnetic dipole unit. Two magnets36, each thereof magnetized in a direction perpendicular to gap are added to form another magnetic quadrupole unit.

Faraday rotator30includes two magneto-optic crystals24A and24B, with crystal24A located between dipole magnets14, and with crystal24located between dipole magnets34. InFIG. 4, the inventive Faraday rotator is depicted as being configured as an optical isolator with crystal24A between a polarizer38A and a polarizer38B; and with crystal24B between polarizer38B and a polarizer38C. Each magneto-optic crystal rotates the polarization-plane of light transmitted therethrough by 45°. The polarization-plane of polarizer38B is rotated 45° with respect to that of polarizer38A; and the polarization-plane of polarizer38C is rotated 45° with respect to that of polarizer38B.

In terms used above for describing sub-assemblies12A and12B each sub-assembly24includes a central magnet18magnetized in a direction perpendicular to the light-propagation axis20, i.e., perpendicular to gap22. Central magnet18is sandwiched between first and intermediate magnets14and34with the direction of magnetization of the first and second end magnets parallel to the light-propagation axis, i.e., parallel to the gap. The central and intermediate magnets are sandwiched between first and second end magnets. The direction of magnetization of the end magnets is perpendicular to the light-propagation axis, i.e., perpendicular to gap22. In magnets16and magnets36, the direction of magnetization is toward the gap. In magnets18the direction of magnetization is away from the gap. The direction of magnetization of magnets14is opposite that of magnets34.

It should be noted here that the term “magnet,” as used in this description and the appended claims applies to a single magnet, such as described above for magnets in the sub-assemblies thereof, or a magnet assembled from a plurality of components providing a functionally equivalent polarity and direction of magnetization. Further, the functionality of the permanent magnets depicted in the drawings may also be embodied in a suitably designed electromagnets. Those skilled in the art will also recognize that the inventive Faraday rotator ofFIGS. 1 and 2can also be configured as an optical isolator by locating crystal24between polarizers in the manner of crystals24A or24B inFIG. 4.

In summary, the present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, by embodiments described and depicted herein. Rather the invention is limited only by the claims appended hereto.