Abstract:
A multi-pass-type Faraday rotator useful in an optical isolator is provisioned with high-efficiency, high-field permanent magnets formed with minimal magnetic material. A high magnetic field is generated by two sets of magnets attached to outer pole plates that are mirror images of each other. Like-type poles of the magnets in each set are disposed against each other above and below the beam path plane of a multi-pass Faraday optic. Each set of magnets is formed of a central block of magnetic material with magnetization oriented substantially parallel to the multi-pass beam path on the Faraday optic, adjoined by adjacent blocks of magnetic material with magnetization oriented substantially perpendicular to the central magnet block and with like poles to the central magnet block where the magnets border the multi-pass Faraday optic.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims benefit under 35 U.S.C. 119(e) to provisional patent application Ser. No. 61/900,080 filed 5 Nov. 2013. 
     
    
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    Not Applicable 
       REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISK 
       [0003]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0004]    The present invention relates generally to Faraday rotators and Faraday rotators used in optical isolators, and, more particularly, to design of permanent magnet based efficient, uniform high fields for use in multi-pass Faraday rotators. 
         [0005]    Optical isolators are routinely used to decouple a laser oscillator from downstream laser amplifier noise radiation and/or target reflections. The key elements of an optical isolator are shown schematically in  FIG. 1 . Optical isolators use non-reciprocal magneto-optic polarization rotation in a Faraday rotator  6  comprised of a Faraday optic  4  in a strong magnetic field  5  co-axial with the laser radiation along axis  1  from laser source  2  to rotate the plane of polarization 45°. Surrounding the Faraday rotator  6  by polarizers  3 ,  7  aligned with the input and output linear polarization states respectively completes the optical isolator. Because Faraday effect rotation is non-reciprocal (i.e. the sense of rotation does not depend upon the direction of propagation), any backward propagating radiation will have the plane of linear polarization rotated a further 45° resulting in a polarization state which is 90° to the transmission axis of the input polarizer—where it will consequently experience high backward transmission loss as rejected beam  9 . This reverse radiation loss is typically on the order of 30 dB for single stage optical isolators. An optional 45° quartz rotator  7  with the same sense of rotation in the forward, transmission direction as the 45° of Faraday rotation in Faraday rotator  6  to flip the input and output polarization states by 90°. The rotation sense of reciprocal quartz rotator  7  is opposite that of nonreciprocal Faraday rotation in Faraday optic  4  in the reverse, isolation, direction such that dispersion of reciprocal and nonreciprocal rotations largely cancel to achieve broadband isolation. (For reference see P. A. Schulz “Wavelength independent Faraday isolator”,  Appl. Opt.  28, 4458-4464 (1989)). 
         [0006]    The amount of Faraday rotation is given by: 
         [0000]      θ(λ, T )= V (λ, T )× H ( r,T )× L   F    (1)
 
         [0000]    where:
 
θ(λ,T): The Faraday rotation angle (a function of wavelength, λ, and temperature, T);
   V(λ,T): A proportionality constant, termed the Verdet constant, of the Faraday element (a function of wavelength, λ, and temperature, T);   H(r,T): The strength of the magnetic field in the direction of light through the Faraday element (a function of radial position r across the beam and temperature, T); and   L F : The length of the Faraday element.   
 
         [0010]    In order to make an optical isolator as small and inexpensive as possible, the Faraday rotation is desired to be large. Equation 1 states that the Faraday rotation angle can be increased by either an increase in the Verdet constant V (λ,T), the magnetic field strength H(r,T), or the Faraday element length L F . Because the Faraday effect is enhanced near an absorption, it is often desirable to reduce, rather than increase, the Faraday element length L F  required to achieve the desired 45° of Faraday rotation in order to minimize undesirable heating due to absorbed power in a Faraday element. Especially when used with high power lasers, absorbed power in a Faraday element is known to cause a temperature gradient across the laser beam profile which results in deleterious thermal effects such as thermal birefringence and thermal lensing. Thermal birefringence can reduce the maximum isolation of an optical isolator well below the typical 30 dB level. Thermal lensing can significantly shift the position of a focus along the axis of beam propagation when the source laser power is varied and thereby change the desired results in a process or experiment which relies on stable laser beam focusing. For at least these reasons, high performance optical isolators for use with high power laser beams seek to minimize the length L F  of the Faraday element. 
         [0011]    As noted above, equation 1 also states that Faraday rotators using Faraday elements with the largest Verdet constant can achieve the desired 45° of polarization rotation with shorter Faraday elements L F , lower magnetic fields H(r,T), or both. Because they are ferromagnetic, Faraday elements used in high volume telecom isolators with wavelengths from 1.3 to 1.55 μm have extremely large Verdet constants&gt;1,500 degrees per (kGauss−cm) and can therefore be extremely small and inexpensive. However, Faraday elements used in optical isolators at common high power laser wavelengths near 1 μm cannot always use ferromagnetic materials due to high absorption from iron in the crystal structure and therefore usually use much lower Verdet paramagnetic or diamagnetic Faraday elements. Faraday rotators near 1 μm commonly use paramagnetic Faraday elements which typically contain significant amounts of terbium in a glass, crystalline or ceramic optical host. The most commonly used Faraday optic material near 1 μm has been terbium gallium garnet single crystal (“TGG”). Recently, a polycrystalline ceramic form of TGG (“cTGG”) has become available. Terbium glasses are typically used only for very large aperture Faraday rotators that require Faraday elements of larger dimension than are available in single crystal or ceramic form because they have even lower Verdet constants and greatly increased deleterious thermal effects due to their low thermal conductivity. The Verdet constant of paramagnetic terbium based single crystals and ceramics is currently limited to 2 to 3 degrees of polarization rotation per (kGauss−cm)—at least 500× less than ferromagnetic Faraday elements used in telecom optical isolators. For this reason, designers of 
         [0012]    Faraday rotators and optical isolators for use near 1 μm have sought to use magnetic designs which provide for the highest magnetic fields that are practically achievable with readily available permanent magnets. 
         [0013]    Because the Faraday effect requires magnetic fields to be co-axial with light propagation through a Faraday optic, conventional single pass “straight through” Faraday rotators have gap lengths (the distance between magnetic pole faces) that are similar to the length of the Faraday optic(s). A fundamental tenet of permanent magnet design is that it is easiest to produce high fields across short gaps. In order to achieve high fields across short gaps with a minimal amount of permanent magnet material, prior art efforts have used multi-pass Faraday rotators because the effective gap length is effectively reduced by the number of passes through the multi-pass Faraday optic. 
         [0014]    U.S. Pat. Nos. 4,909,612 and 5,715,080 describe multi-pass Faraday rotators wherein two pairs of oppositely poled adjacent block magnets with magnetization normal to the plane of a multi-pass beam path are serially disposed and on opposite sides of a multi-pass Faraday rotator slab with poles of like polarity being disposed in transverse registration on opposite sides of the beam path to produce an intense magnetic field substantially parallel to the beam path of a laser beam passing through the material. U.S. Pat. No. 5,715,080 teaches that adjacent magnets of each pair of magnets on opposite sides of a multi-pass Faraday rotator slab are spaced apart in order to greatly reduce magnetic field non-uniformity present in the magnet configuration shown in U.S. Pat. No. 4,909,612. However, high magnetic field magnets require use of rare and thus expensive materials, so there is a premium on material usage. 
         [0015]    What is needed is an efficient multi-pass Faraday rotator magnet configuration that maximizes magnetic field generation with minimal material. 
       SUMMARY OF THE INVENTION 
       [0016]    According to the invention, a multi-pass-type Faraday rotator useful in an optical isolator is provisioned with high-efficiency, high-field permanent magnets formed with minimal magnetic material. A high magnetic field is generated by two sets of magnets attached to outer pole plates that are mirror images of each other. Like-type poles of the magnets in each set are disposed against each other above and below the beam path plane of a multi-pass Faraday optic. 
         [0017]    Each set of magnets is formed of a central block of magnetic material with magnetization oriented substantially parallel to the multi-pass beam path on the Faraday optic, adjoined by adjacent blocks of magnetic material with magnetization oriented substantially perpendicular to the central magnet block and with like poles to the central magnet block where the magnets border the multi-pass Faraday optic. Highly uniform magnetic fields that are approximately two-fold stronger than prior art multi-pass Faraday rotator magnet configurations are realized. 
         [0018]    Internal pole pieces are shaped to further increase magnetic fields within the multi-pass Faraday optic. The central block magnet in one or both magnet sets may optionally have projections that surround the non-optical sides of the Faraday optic, approximating a single axial magnet with a central hole to further direct and increase magnetic field within the Faraday optic. 
         [0019]    An isotropic Faraday rotation material, with or without thermally conductive transparent windows bonded to it, can be used as the multi-pass Faraday optic. One or both magnet sets may be translated normal to the plane of the multi-pass beam path in order to tune the strength of the magnetic field. High refractive index first deposition layers are used for thin film reflective mirrors deposited directly on the multi-pass Faraday optic to maintain linear polarizations for reflected beams. Slab shaped multi-pass Faraday optics are passively heat sunk to the housing or actively temperature stabilized with a thermoelectric cooler or heater to maintain constant Faraday rotation with changing ambient temperature. 
         [0020]    The present invention is an improvement over U.S. Pat. Nos. 4,909,612 and 5,715,080. Unlike the prior art configuration, the present invention uses a third central block magnet between each pair of magnets with a magnetization that is substantially parallel to the multi-pass beam path through a Faraday optic. Within each magnet set, the central block magnet has pole faces that are the same polarity as adjacent block magnets where they border the multi-pass Faraday optic. Outer pole plates on each magnet set are used to reduce external leakage fields and direct them towards the multi-pass Faraday optic. Highly uniform magnetic fields are achieved that are approximately two-fold stronger than that of the apparatus disclosed in U.S. Pat. No. 5,715,080 for a similar total amount of permanent magnet material. This is commercially important when preferred rare-earth permanent magnets are used in view of recent disruptions in the availability of rare earths and corresponding rapid &gt;20× price fluctuations for dysprosium and neodymium, commonly used elements in rare earth magnets. Reduced demand for dysprosium and neodymium may result for use of a magnet configuration as herein described. 
         [0021]    An important benefit of the invention is that the stronger uniform magnetic fields produced by the present invention using a comparable amount of permanent magnetic material as that disclosed in U.S. Pat. No. 5,715,080 allows for an approximate two-fold reduction in total beam path length through the multi-pass Faraday optic. A shortened beam path length in the Faraday optic reduces deleterious thermal effects, such as thermal birefringence and thermal lens focal shifts in a Faraday rotator used with high average power lasers, or nonlinear refractive index phase shifts resulting from high beam intensities in short pulse lasers. 
         [0022]    One aspect of the present invention is that shaped internal pole pieces may be used to further concentrate uniform fields substantially along the multi-pass beam path within the Faraday optic. 
         [0023]    Another aspect of the invention is that one or both central magnet blocks have projections such that the central magnet blocks substantially surround the non-optical surfaces of said Faraday optic to further increase magnetic field strength in the region of the Faraday optic. 
         [0024]    Another aspect of the invention is that one or both sets of magnets may be translated normal to the plane of the multi-pass beam path in order to tune the strength of the magnetic fields and Faraday rotation within the Faraday optic. 
         [0025]    Another aspect of the invention is that high reflection thin film coatings applied directly to the Faraday optic to define a multi-pass beam path have a first high index deposition layer of higher refractive index than the Faraday optic refractive index in order to maintain a linear polarization of a reflected beam. 
         [0026]    Yet another aspect of the present invention is that it is suitable for use with any diamagnetic, paramagnetic, or semiconductor isotropic Faraday rotator material that may be either a glass, transparent polycrystalline ceramic or single crystal. 
         [0027]    In accordance with this aspect of the invention, all of these Faraday rotator materials may have transparent heat-conductive layers of thermally significant thickness bonded to their optical faces in order to minimize thermal gradients across the beam within the Faraday optic. 
         [0028]    A final aspect of the present invention is that slab shaped multi-pass Faraday optics may be readily heat sunk to the housing or actively temperature stabilized with a thermoelectric cooler or heater as desired to maintain substantially constant Faraday rotation. 
         [0029]    The invention will be better understood by reference to the following detailed description in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a schematic view of the essential elements of an optical isolator according to the prior art. 
           [0031]      FIG. 2  a cross section plan view of a multi-pass Faraday rotator of the present invention along the cross section line labeled  2 - 2  in  FIG. 3  suitable for use in a polarization independent optical isolator. 
           [0032]      FIG. 3  is the cross section side view along path  3 - 3  in  FIG. 2   
           [0033]      FIG. 4  is a cross section plan view of a multi-pass Faraday rotator of the present invention along cross section line  4 - 4  in  FIG. 5  suitable for use in a polarization maintaining optical isolator. 
           [0034]      FIG. 5  is a cross section side view along path  5 - 5  in  FIG. 4 . 
           [0035]      FIG. 6  is a cross section side view along path  6 - 6  in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0036]    In a first embodiment of the invention, a Faraday rotator using the magnet design of this invention is used with a beam that is reflected in a multi-pass Faraday optic having an optically transparent input face portion, at least one reflective coated opposite face portion and an optically transparent output face portion. In the case of a two-pass Faraday rotator, substantially all of one optical face of the Faraday optic is coated with a high reflection coating, and substantially all of the other opposite optical face is anti-reflection coated to serve as both the input and output transparent faces. 
         [0037]    In the case of a 3 or more pass Faraday rotator, each optical face of the Faraday optic is coated with both a transparent portion(s) and a reflective portion as shown in  FIG. 2  which is a cross section plan view along the line labeled  2 - 2  in  FIG. 3  of multi-pass Faraday rotator  13  used in a polarization insensitive optical isolator. Randomly polarized radiation such as from a pulsed fiber laser is propagated in an approximately 0.5 mm collimated beam  11  through aperture  12  of Faraday rotator  13 . Collimated beam  11  is directed through input Vanadate crystal polarization displacer  14 . Input Vanadate polarization displacer  14  resolves the randomly polarized collimated beam  11  into orthogonally polarized o-ray  15  and e-ray  16  shown in  FIG. 3  before transmitting both rays through 45° crystal quartz rotator  17  which is mounted in a channel of input inner pole  18 . Both beams are transmitted through a generally rectangular aperture in input pole  18  and are then incident on optically transparent input face portion  20  of slab shaped ceramic TGG multi-pass Faraday optic  19 . Faraday optic  19  is bonded and heat sunk to housing  28 . After transmission through transparent input face portion  20  the beams propagate through Faraday optic  19  until they are reflected by a first high reflection coating  21  onto a second high reflection coating  22  and then propagate out of Faraday optic  19  through transparent output face portion  23 . 
         [0038]    According to an aspect of the invention used in this embodiment, reflective coated portions  21 ,  22  are multi-layer high reflector thin film coatings at the wavelength range of interest, wherein the first deposition layer onto the Faraday optic  19  for each multi-layer stack of high/low refractive index layers comprising the thin film reflective coating  21 ,  22  is a high refractive index layer with higher refractive index than the Faraday optic  19  material. Such first high index layer eliminates the need for an additional waveplate when the Faraday rotator of this invention is used in an optical isolator to compensate for phase shifts that would otherwise occur for the non-normal reflections at the high reflective mirror coatings as described in U.S. Pat. Nos. 4,909,612 and 5,715,080. Transparent input and output face portions  20 ,  23  are typically anti-reflection coatings on the Faraday optic  19 . The two orthogonally polarized beams  15  and  16  are then propagated through a generally rectangular aperture in output inner pole  24  and then recombined (after 45° of Faraday rotation in multi-pass Faraday optic  19  and 45° quartz rotator  17 ) in output Vanadate crystal displacer  25  mounted in a channel in output inner pole  24  into a single randomly polarized output beam  26  which is transmitted through output aperture  27 . 
         [0039]    Referring to  FIG. 3 , the Faraday rotator  13  of this embodiment comprises a lower magnet set  31  and upper magnet set  32 . Each magnet set comprises three magnets. Magnets  33  and  35  have magnetization normal to the multi-pass beam path plane and, for example, polarity indicated by the arrows on each magnet. Magnets  34  have magnetization substantially parallel to the multi-pass beam path with magnetization and, for example, polarity indicated by the arrows on each magnet in  FIG. 3  The polarity of each magnet within a set, and between sets  31  and  32  in transverse registration to each other, is such that each magnet has like poles to adjacent magnets at their boundary edges nearest to the multi-pass Faraday optic  19 , for example as shown in  FIG. 3 . For reference and clarity, magnets  35 ,  34  and  33  of the lower magnet set  31  are shown as hidden dashed lines in  FIG. 2  because they are obscured by the housing  28  in the cross section plan view of  FIG. 2 . 
         [0040]    In  FIG. 3  internal pole pieces  18 ,  24  are used in accordance with the invention to enhance magnetic field strength and further improve uniformity in this embodiment of the invention. Transmission holes in internal poles  18 ,  24  further function as input and/or output apertures which define a single beam path of the correct number of passes through the Faraday rotator  13  to achieve the desired rotation. High permeability outer pole pieces  36  and  37  reduce external fields from upper magnet set  32  and lower magnet set  31  and thereby further increase the magnetic field in the region of Faraday optic  19 . Tuning screws  38  may be used to adjust gaps  39  to tune the magnetic field strength and Faraday rotation angle in Faraday optic  19  to the desired 45° at the fiber laser center frequency of 1064 nm. The 45° quartz rotator  17  is used with 45° of Faraday rotation in multi-pass Faraday optic  19  to conveniently flip the input and output polarization states in the transmission direction such that the displacement planes of input  14  and output  25  Vanadate displacers lie in the same plane. The rotation sense of reciprocal quartz rotator  17  is also opposite that of nonreciprocal Faraday rotation in multi-pass Faraday optic  19  in the reverse, isolation, direction such that dispersion of reciprocal and nonreciprocal rotations largely cancel to achieve broadband isolation. The high performance, small size, and low cost of the Faraday rotator of this embodiment is particularly useful with small beams such as used with polarization insensitive (“PI”) or polarization maintaining (“PM”) optical isolators for use with high power randomly polarized and linearly polarized fiber lasers respectively. 
         [0041]    In a second embodiment of the invention, a multi-pass Faraday rotator using the magnet design of this invention is constructed with at least one external mirror and substantially all of one or both optical faces of the Faraday optic being anti-reflection coated. If internal pole pieces are used to enhance magnetic field strength, multiple transmission holes and/or slots are used in the internal poles as appropriate to permit transmission of the input and output beams as well as reflection(s) from any external mirrors. This Faraday rotator embodiment of the invention is particularly well suited for use in optical isolators used with larger beam diameters and higher peak powers such as sub-nanosecond ultrafast laser sources and/or multi-kW average power lasers when anti-reflection coated transparent heat conductive windows are bonded to the multi-pass Faraday optic. 
         [0042]      FIG. 4  is a cross section plan view of a Faraday rotator along the line labeled  4 - 4  in  FIG. 5  for use in a polarization maintaining optical isolator for a CO 2  laser at 10.6 μm in accordance with this second embodiment. Polarized 10.6 μm radiation is directed along beam path  40  into input aperture  41  of the multi-pass Faraday rotator. 10.6 μm radiation along beam path  40  is transmitted through a slot in input inner pole  42  into Faraday optic  43 . Faraday optic  43  is comprised of an inner layer of InSb  44  that is bonded, such as by diffusion bonding, to transmissive heat conducting germanium windows  45  and  46 . Because both InSb and Ge have nearly identical refractive indices at 10.6 μm, transmissive Ge windows  45  and  46  are only anti-reflection coated on the two optical surfaces not bonded to InSb. After the first transmission pass through Faraday optic  43 , the beam path  40  passes through a slot in output inner pole  47  and is then reflected between first reflection mirror  48  and second reflection mirror  49  such that beam path  40  makes a total of 5 passes through Faraday optic  43  before the beam path exits the Faraday rotator through output aperture  50  Inner poles  42 ,  47  are secured to the housing  59  with screws  60 . A copper mount  58  provides a heat conduit to the housing  59  for absorbed power from the Faraday optic  43 . 
         [0043]      FIG. 5  is a cross section side view along the line labeled  5 - 5  in  FIG. 4 . The magnet geometry of this embodiment is similar to the first embodiment with an upper magnet set  51  and lower magnet set  52 . Magnets  53  and  55  have magnetization normal to the multi-pass beam path  40  plane and, for example, polarity indicated by the arrows on each magnet. Central magnets  54  have magnetization substantially parallel to the multi-pass beam path with magnetization and polarity as indicated in  FIG. 5 . Central magnets  54  can be slightly recessed as shown to provide additional space for heat sinking of the Faraday optic  43  or temperature controlling it with heaters or thermo-electric coolers as appropriate. In this embodiment, as shown in  FIG. 6  which is a cross section side view along line  6 - 6  of  FIG. 4 , central magnets  54  have protrusions such that the central magnets  54  generally surround the non-optical surfaces of the Faraday optic  43  in order to further increase the magnetic fields in the region of the Faraday optic  43 . Upper and lower outer poles  56  and  57  have the same function as outer poles in the first embodiment. Spacing the reflection mirrors  48  and  49  as appropriate allows the beam diameter and degree of beam overlap between multi-passes in the Faraday optic  43  shown in  FIG. 4  to be adjusted as desired enabling the Faraday rotator of this embodiment to be used with high peak power laser beams to prevent damage to optical elements in the device. In accordance with the prior art discussion and  FIG. 1 , a polarization maintaining optical isolator suitable for use with CO 2  lasers at 10.6 μm can be realized by surrounding the Faraday rotator of this embodiment with polarizers with transmission axis at 45° to each other (such as thin film Brewster angle ZnSe) at the input and output ends of the Faraday rotator. 
         [0044]    The invention has been described with reference to specific embodiments. Other embodiments will be evident to those of ordinary skill in the art. It is therefore not intended that the invention be limited, except as indicated by the appended claims.