Patent Description:
The present technology relates to Faraday rotators, and more specifically, relates to techniques which may dramatically improve the average power handling capability of Faraday rotators. The present technology involves a gas cooled high average power faraday rotator.

Faraday rotators are used in laser systems for polarization switching, isolation of laser amplifier components against back reflection, and depolarization correction. For high energy laser systems where apertures are large and wavefront is critical, these devices typically use crystals or glasses that have high Verdet constant, which sets the rotation for a given pathlength and fixed magnetic field. It is desirable to minimize optical pathlength both for wavefront reasons as well as b-integral and thermal effects. It is also desirable to use fixed magnets to create compact devices. This has generally converged the industry around Terbium doped optical media which display a relatively high Verdet constant and can be doped into a variety of materials including Terbium Gallium Garnet (TGG), Terbium doped glasses, and Potassium Terbium Fluoride (KTF). These materials have a small but nevertheless measurable absorption coefficient which results in thermal loading of these materials under average power. Thermal loading of an optical material results in changes in refractive index with temperature (dn/ dT), thermal expansion, and stress birefringence which is a result of this expansion. The stress birefringence modifies the incoming polarization and scatters a small amount into other polarization states, which is otherwise known as depolarization. Since these devices are utilized for polarization switching and isolation, the purity with which the polarization is handled is critical.

Depolarization of the Faraday elements effectively spoils the capability and defeats the purpose of utilizing the Faraday elements in the first place. Various methods have been utilized in the past to compensate for thermal effects including using two slabs of the material with a rotator between to thermal birefringence compensate as well as actively cooling through water or cold plate one face of a rotator material (active mirror concept) to minimize birefringence and create a longitudinal thermal gradient. As apertures and average powers increase it becomes harder and harder to maintain birefringence compensation. Similarly, the active mirror concept is also limiting as either the thickness must decrease (and therefore magnetic field increase) or the number of devices increase to handle high power. The current state of the art is incapable of scaling to be within the range of <NUM> kW - <NUM> MW.

<CIT> discloses transparent heat-conductive layers of significant thickness bonded or adhered to opposing optical faces of a Faraday optic to form a Faraday optic structure that can be used with beam-folding mirrors and an external magnetic field to form a multi-pass Faraday rotator with minimal thermal gradient across the beam within the Faraday optic. The transparent heat conductive layers conduct heat through the Faraday optic substantially parallel to the beam propagation axis for each pass through the Faraday optic structure and thereby reduce thermal gradients across the beam cross section that would otherwise contribute to thermal lens focal shifts and thermal birefringence in the Faraday optic structure. The multi-pass Faraday rotator of this invention is suitable for use with any device based upon the Faraday effect such as optical isolators, optical circulators and Faraday mirrors that are scalable with beam size to power levels in excess of <NUM> kW.

<CIT> discloses an electro-optic device that includes an electro-optic crystal having a predetermined thickness, a first face and a second face. The electro-optic device also includes a first electrode substrate disposed opposing the first face. The first electrode substrate includes a first substrate material having a first thickness and a first electrode coating coupled to the first substrate material. The electro-optic device further includes a second electrode substrate disposed opposing the second face. The second electrode substrate includes a second substrate material having a second thickness and a second electrode coating coupled to the second substrate material. The electro-optic device additionally includes a voltage source electrically coupled to the first electrode coating and the second electrode coating.

<CIT> discloses A compact narrow band imaging system includes a vapor cell having a gas that receives and transmits light in accordance with the Faraday effect. A magnetic source is provided for applying a magnetic field to the vapor cell. Crossed polarizers are disposed before and after the vapor cell creating a Faraday optical filter. The only light that passes through the filter is light within a narrow band near the absorption peaks of the vapor. Other optical elements of the imaging system including filters, image detectors, electron multipliers, signal digitizers, and heat filters are co-located within the imaging system's common thermal isolation container to provide improved performance. The compact system is suitable for wide area surveillance, including daylight surveillance for combustion sources such as forest fires and missile exhaust.

The present invetion is defined by the enclosed claims. The present technology enables scaling of average power handling from current sub-kW demonstrations to <NUM> kW - <NUM> MW capabilities. Applying the present technology to a Faraday rotator's optical components enables several orders of magnitude increase in average power and energy handling capability of these devices. Faraday rotators are used in laser systems for polarization switching, isolation of laser amplifier components against back reflection and depolarization correction. Currently these devices are limited in average power handling capability.

High speed gas cooling efficiently removes thermal loading from Faraday optic faces while minimizing the thermal wavefront and thermal birefringence by creating a longitudinal thermal gradient. A recirculating gas cooling manifold accelerates the gas over the surface of the slab to create a turbulent flow condition which maximizes the surface cooling rate. The present technology further provides a spatially uniform thermal profile on the Faraday slabs. One embodiment includes a uniformly heated slab that is insulated around the perimeter.

This device is useful in many applications, including in the high average power laser drivers for inertial fusion energy, drivers for laser accelerator systems, defense applications, laser peening, welding, cutting and additive manufacture.

The accompanying drawing, which is incorporated into and forma a part of the disclosure, illustrate an embodiment of the invention and, together with the description, serves to explain the principles of the invention.

The present technology may enable several orders of magnitude increase in average power and energy handling capability of Faraday rotator optical components. The optical aperture of a Faraday rotator for a particular laser system is set nominally by the laser damage threshold of the Faraday material. The thickness of the material is set by the magnet design required for uniformity and desired rotation at this aperture. With these two values known along with the absorption coefficient of the particular Faraday material chosen (see Table <NUM> of typical materials), one can calculate the absorbed power at the use location in the laser system.

High speed helium gas cooling efficiently removes several W/cm<NUM> thermal loading from amplifier faces while minimizing the thermal wavefront and thermal birefringence by creating a longitudinal thermal gradient. A recirculating helium cooling manifold accelerates the gas over the surface of the slab to create a turbulent flow condition which maximizes the surface cooling rate. An example of such a system sufficient for a <NUM> kW class rotator would be a system pressurized with helium to ~ <NUM> psi and flowing at a rate of ~ Mach <NUM> (turbulent flow conditions) over the surface of the Faraday material slab surface. This technology allows the heat removal to be accomplished in transmission, so the problem can be attacked by changing aperture size and/or by splitting the material into thinner and thinner slabs to achieve desired results. For the Faraday material case, the thermal loading is much lower than the amplifiers in the same system and comparable to current low power thermal loads on existing amplifiers. Consider a <NUM> kW laser system that must double pass a Faraday rotator made of TGG with an aperture of <NUM> x <NUM><NUM> and a thickness of <NUM>. The total thermal power absorbed in the crystal is <NUM> W. If this crystal is split into <NUM> slabs where each slab is <NUM> thick, then the thermal loading that must be removed from each surface is <NUM> W/cm<NUM> on each surface, which is well within the capability of high speed gas cooling. As with the amplifier systems, helium is the preferred gas due to it low refractive index and low dn/dT, minimizing turbulence and scattering effects on the transmitted beam.

Understanding and accommodating the thermal load is the first step in the design of this system. The second step is creating the most spatially uniform thermal profile on the Faraday slabs. To accomplish this, it is desirable to have a uniformly heated slab that is insulated around the perimeter (so no cooling can occur in that direction) and with the required heat transfer from the front and back slab faces (via the high-speed gas cooling). The techniques used in the present invention are not possible on an amplifier due to the much lower magnitude thermal load and the lack of requirement for edge cladding of a Faraday rotator material. Since it is not possible to completely fill the optical aperture with the beam (since this would incur huge diffractive loss and beam quality degradation on the transmitted beam), the present invention uses edge heaters to accomplish the thermal balance. There are several options which are functionally equivalent but offer pros and cons mechanically. One option is to attach heaters (e.g., thin film sheet heaters) to the edges of the slab. These also could be shaped to create a spatially dependent heat load across the edge of the slab to improve uniformity. This enables each unit to be independent and separately tunable for flat thermal profile. The optical mode-fill, slab thickness and heater power are all variables that can be tuned to achieve uniformity. This concept is depicted in <FIG>, which is a cross-sectional side view showing the Faraday material <NUM> in a series of slabs, Helium gas cooling <NUM>, gas cooling vanes including portions <NUM> and <NUM>, insulating material (e.g., insulating potting compound) <NUM>, heaters <NUM>, pressure vessel windows <NUM> and <NUM> and magnetic array housing <NUM> needed to achieve a high average power Faraday rotator. The figure shows laser beam <NUM> passing through the Faraday material <NUM>. The outer periphery of the slabs can be round or square or any shape desired. It is desirable to make each slab to be uniformly heated such that there is no cool boundary around the beam. The heater for each slab may be in contact with the slab all the way around the slab. The side of the heater that is opposite to the slab is in contact with the insulating material and the insulating material is in contact with the magnetic housing. The gas flows between each slab. The magnetic array housing includes openings between the cooling vanes to allow gas to flow between each Faraday slab. Also, the gas cooling vanes can be formed of magnetic material. In this embodiment, a portion <NUM> of the cooling vane is formed of aluminum and a portion <NUM> is formed of magnetic material. The refractive index of helium, and thus dn/dT, are extremely low (~10X lower than any other material besides vacuum), making any disturbance of the laser beam wavefront by the gas flow minimized. It is turbulent flow, so the wavefront distortion is analogous to white noise and has been demonstrated as a cooling method to provide wavefront errors which are below measurement capability. Generally, a plurality of slabs is used because it is desirable to provide as close to a longitudinal thermal gradient as possible. Large temperature rises are undesirable (the thicker the material the temperature goes up as t<NUM> set by thermal conductivity of the material. However, for lower power systems one slab could work. One embodiment uses only two slabs of the material with a rotator between.

A second method of heating, illustrated in <FIG>, is to project heater beams <NUM> onto the periphery of each slab (Faraday optic) <NUM>. This can be accomplished locally with IR emitters or remotely by imaging optical beams generated by diodes or another laser source. The figure also shows a cooling vane <NUM> and laser beam <NUM>. These techniques can be combined with the current technique of a birefringence compensated design. Such combination adds complication but would extend average power capability even further. Using this methodology, the Faraday technology can be scaled from the current scale ~ < <NUM> kW to <NUM> kW and potentially extend to ><NUM> MW class laser systems. This scaling will enable these lasers to function at this level and thereby enable accelerator, secondary radiation source, and defense applications. This technology combines current Faraday techniques with advanced amplifier cooling methods as well as a methodology for managing the stress birefringence as the thermal loading increases beyond current conventional levels < kW. <FIG> shows a birefringence compensated design having a Quartz rotator optic <NUM> between two sources of birefringence. Rotator optic <NUM> is cooled with cooling vane <NUM>. The requirement is that the two sources be well matched/symmetric about the quartz rotator. One source comprises two Faraday optic slabs <NUM> and <NUM>, including respective cooling vanes <NUM> and <NUM>. The other source comprises two Faraday optic slabs <NUM> and <NUM>, including respective cooling vanes <NUM> and <NUM>, The figure also shows laser beam <NUM> and the heater beams <NUM> and <NUM>. <FIG> shows a design similar to <FIG>, but including multiple slabs. The figure shows Faraday optic slabs <NUM>-<NUM>, with respective cooling vanes <NUM>-<NUM>. the figure also shows laser beam <NUM> and heater beams <NUM> and <NUM>. Another option would be absorbing glass mounted as a frame around the optic. Light could then be directed at the glass for purposes of heating - so this method is optically addressed and could add spatial sculpting to achieve better uniformity performance.

All elements, parts and steps described herein are preferably included. It is to be understood that any of these elements, parts and steps may be replaced by other elements, parts and steps or deleted altogether as will be obvious to those skilled in the art.

Claim 1:
An apparatus, comprising:
at least one Faraday optic (<NUM>; <NUM>; <NUM>, <NUM>, <NUM>, <NUM>; <NUM>, <NUM>, <NUM>, <NUM>) having opposing optical faces through which there is a beam propagation axis that is orthogonal to said opposing optical faces;
means (<NUM>; <NUM> ;<NUM>, <NUM>; <NUM>, <NUM>) for heating a portion of said at least one Faraday optic (<NUM>; <NUM>; <NUM>, <NUM>, <NUM>, <NUM>; <NUM>, <NUM>, <NUM>, <NUM>);
a gas cooling system (<NUM>, <NUM>, <NUM>; <NUM>; <NUM>, <NUM>, <NUM>, <NUM>; <NUM>, <NUM>, <NUM>, <NUM>) configured to provide gas to cool each face of said opposing faces; and
a magnetic field source configured to induce a desired Faraday rotation of a laser beam (<NUM>; <NUM>; <NUM>; <NUM>) propagating on said beam propagation axis through said at least one Faraday optic (<NUM>; <NUM>; <NUM>, <NUM>, <NUM>, <NUM>; <NUM>, <NUM>, <NUM>, <NUM>).