SINGLE AND MULTI-STAGE HIGH POWER OPTICAL ISOLATORS USING A SINGLE POLARIZING ELEMENT

An optical isolator for generally collimated laser radiation includes a single polarizing element, at least one Faraday optical element, at least one reciprocal polarization altering optical element disposed at the single polarizing element, at least one reflective optical element for reflecting radiation to provide an even number of passes through the at least one Faraday optical element, and a magnetic structure. The magnetic structure is capable of generating a magnetic field within the at least one Faraday optical element that is generally aligned with the even number of passes along a beam propagation axis. The optical isolator is configured to receive generally collimated laser radiation, which passes through the single polarizing element and the at least one reciprocal polarization altering optical element and which makes at least two passes through the at least one Faraday optical element, whereby generally collimated laser radiation is output from the optical isolator.

FIELD OF THE INVENTION

The present invention relates to high power optical isolators and more particularly to high power single and multi-stage polarization maintaining [“PM”] and polarization insensitive [“PI”] optical isolators.

BACKGROUND OF THE INVENTION

Optical isolators are routinely used to decouple a laser oscillator from downstream laser amplifier noise radiation and/or target reflections. Optical isolators are typically comprised of a Faraday rotator surrounded by polarizers that are aligned with the input and output linear polarization states. A Faraday rotator is typically comprised of a non-reciprocal, optical element in a strong magnetic field that is co-axially aligned with the laser radiation so that the plane of polarization is rotated by 45 degrees. In an optical isolator, the non-reciprocal nature of the Faraday effect causes the plane of linear polarization in the backward propagating direction to be rotated an additional 45 degrees resulting in a polarization state which is 90 degrees to the transmission axis of the input polarizer. This results in reverse propagating radiation to experience high transmission losses while allowing forward propagating radiation to experience low transmission losses. Optical isolators suitable for randomly polarized light are also common and are termed polarization insensitive [“PI”] isolators such as disclosed in U.S. Pat. No. 4,178,073.

The sensitivity of distributed feedback diode lasers and other components in telecom systems to feedback from backward propagating radiation has prompted the development of multi-stage PI isolators to increase isolation to 60 dB. An example of such a multi-stage isolator is a two stage device disclosed in U.S. Pat. No. 5,237,445. To address the bulk and expense of high power Faraday rotators in the near infrared, the amount of Faraday rotation as given by the following equation has been examined:

where:λ(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); andLF: The optical path length within the Faraday element.

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(T), or the Faraday element length LF. In order to make an optical isolator as small and inexpensive as possible, multi-pass Faraday rotators have been disclosed, such as in U.S. Pat. Nos. 4,909,612; 5,715,080 and 7,057,791, which are hereby incorporated by reference in their entireties. The reduced gap between the magnets significantly improves the magnetic efficiency and uniformity of the magnetic assembly. As disclosed in U.S. Publication No. 2015/0124318 (which is hereby incorporated herein by reference in its entirety), these improvements increase the effective magnetic field H(r,T) and allow for reductions in the optical path length LF, magnetic material volume, and Faraday optic volume. The reduction of the optical path length LF, is additionally advantageous in high power applications for reductions in beam degradation due to absorption.

A fiber to fiber optical isolator for low power laser radiation is disclosed in U.S. Pat. No. 5,499,132. This optical isolator is applicable to only fiber to fiber devices and requires an internal focusing lens for proper operation. In addition, optical damage due to high fluence levels of the small beam diameters at the fibers as well as in the birefringent crystal plate prevents scaling of this device to powers above 10W.

SUMMARY OF THE INVENTION

The present invention provides PM and PI multi-pass isolator forms that are simple to align, have few optical parts to assemble and work well with scalable beam diameters large enough to prevent optical damage in the optical elements of the isolator when used with high peak and average power laser sources. The present invention relates to high peak and average power optical isolators and more particularly to high power single and multi-stage polarization maintaining [“PM”] and polarization insensitive [“PI”] optical isolators with improved size, improved alignment simplicity, reduced parts count, and reduced cost.

According to an aspect of the present invention, an optical isolator for generally collimated laser radiation is provisioned with one or more isolation stages using a single polarizing element, a multi-pass Faraday rotator through which light passes an even number of times and one 45 degree reciprocal polarization rotation element per isolation stage. The single polarizing element enables simple alignment and reduced parts count in an optical isolator that is scalable in beam diameter for high power operation.

In a preferred embodiment, the multi-pass Faraday rotator comprises a Faraday optic with a highly reflective coating on one optical face and an anti-reflective coating on the opposite optical face nearest to the single polarizing element. A magnetic field generally aligned to the beam path in the multi-pass Faraday rotator causes 45 degree non-reciprocal polarization rotation in the Faraday optic for each isolation stage.

The 45 degree reciprocal polarization rotation element may comprise a quartz wave plate and may be bonded, such as by adhesive free optical contact for high power applications, to the surface of the single polarizing element at only a portion of the single polarizing element such that the wave plate is located in only one pass of the beam path and is aligned for the opposite sense rotation that is opposite to the Faraday non-reciprocal rotation in the transmission direction.

Optionally, a high reflection region may be coated onto the single polarizing element's optical face that is adjacent to the anti-reflection coated surface of the Faraday optic, whereby increased beam overlap and reduced overall size can be realized for an even number of passes greater than two. If the single polarizing element is a fused silica polarizing beam splitter, a polarization maintaining or PM isolator suitable for high power with only two separate optical components is realized. Similarly, if the single polarizer element is a fused silica polarization splitting beam displacer, a polarization insensitive or PI isolator with only two separate optical components is realized with no critical alignment required. Both forms of isolators have readily scalable beam diameters for high power operation. High power beam quality is limited only by the thermal optic properties of the Faraday rotation optical element.

The present invention thus is an improvement over the systems and devices described in U.S. Pat. Nos. 4,909,612; 5,715,080 and 7,057,791, where two or more polarizing elements are used and high reflection faces are placed upon opposing optical faces of multi-pass Faraday optics to promote more than two passes through the Faraday optic. In contrast to the systems disclosed in these patents, the system of the present invention uses a single polarizing element to simplify construction while also reducing size and cost. In the multi-stage isolator form, the system of the present invention can be constructed with only two optical components and has the further benefit that the isolator is self-aligning with respect to polarization throughout the multi-stage isolator. In addition, the use of high reflection coating region(s) on the optical face of the polarizing element closest to the Faraday optic improves the magnetic efficiency by increasing the number of beam passes through the Faraday optic while maintaining beams that are generally parallel to the magnetic field, thereby reducing the thickness of the Faraday optic and minimizing the required magnetic structure. These distinctions can all be made while increasing the beam size, as required, to prevent optical damage to optical elements within the isolator to scale the power as desired up to the limit imposed by the thermal optic properties of the Faraday optic material used.

In accordance with another aspect of the present invention, multi-stage isolators can be realized by adding a high reflection coating region on the single polarizing element's optical surface furthest from the Faraday optic between duplicate isolation stages such as described above for the preferred embodiment. In the case of a PI isolator, this high reflection coating is best applied to the non-bonded external surface of a quartz quarter-wave plate to flip polarizations between pairs of isolation stages to make path-lengths identical for both polarizations. The quarter wave-plate may be first bonded, such as by optical contact, with its optic axis aligned 45 degrees to each polarization axis of the fused silica polarization splitting beam displacer. Again, simple multiple stage isolators with only two separate optical components that are self-aligning with respect to polarization for easy assembly are possible.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and the illustrative embodiments depicted therein,FIG. 1Ais a perspective view of a PM single stage, dual pass isolator for generally collimated laser radiation that is comprised of only two separate optical components in accordance with the invention. Collimated laser radiation source1100with substantially linear p polarization is incident upon single polarization element1101comprised of quartz half-waveplate1103bonded, such as by adhesive free optical contact, to fused silica PBS1102. The optic axis of half-waveplate1103is rotated by an angle of 22.5 degrees with respect to the original linear p polarization axis (shown as vertical inFIG. 1A), such that highly polarized p polarization transmitted through polarizing coating1108is rotated by +45 degrees of reciprocal rotation in half-waveplate1103as shown by the circled polarization state arrow1109.

Laser radiation at1109is then incident upon Faraday optic1104(such as a Terbium Gallium Garnet (TGG) or other suitable Faraday optic element) which is immersed in a magnetic field that is generally aligned to the beam path, where the beam receives −22.5 degrees of non-reciprocal Faraday polarization rotation for each pass of Faraday optic1104. Reflection coating1105on Faraday optic1104facilitates the dual pass beam path in Faraday optic1104for −45 degrees of total non-reciprocal polarization rotation which restores the original p polarization state in beam1106′ which is then re-incident upon single polarization element1101at point1110(where the re-incident location is at a location that does not have the half wave plate1103at the polarization element). Radiation from point1110is highly p polarized output radiation1106upon final pass through the single polarization element1101. In view of the angle β between collimated input radiation1100and output radiation1106as well as the separation L1between single polarization element1101and Faraday optic1104, half-waveplate1103is dimensioned and positioned to not clip input1100or output1106radiation.

The single stage, dual pass isolator ofFIG. 1Aalso limits or precludes light traveling in the reverse direction from reaching the source of light beam1100inFIG. 1A. As shown inFIG. 1B, light1120received at single polarizing element1101at the side or region of the element that does not have the half wave plate1103, passes through the single polarizing element and through the faraday optic1104, which, after it reflects off reflector1105so as to make a second pass through the optic1104, is rotated −45 degrees (beam1121), which then passes through the half wave plate1103, which rotates the beam −45 degrees, such that the beam1122is an s polarized beam1122. Such an s polarized beam is reflected by the polarizing coating1108away from the source of the light beam1100.

Optionally, if more than two passes are desired through the PM single stage isolator, a reflector may be added at the single polarizing element such that light (after two passes through the faraday optic) reflects back toward the faraday optic for a third and fourth pass (such a single stage, quad pass isolator may be suitable for small magnets or a low Verdet constant faraday optic). Such a configuration is shown inFIG. 1C, where the single polarizing element1201has a half wave plate1203and a polarizing coating1208similar to single polarizing element1101, discussed above, and the isolator includes a Faraday optic1204and reflector1205similar to the optic1104and reflector1105discussed above. Single polarizing element1201includes a reflector1211, such as at a center region between the half wave plate1203and the point or region1210where the light beam passes back through the single polarizing element1201. In this configuration, the Faraday optic1204comprises a thinner optic or less powered magnetic field, such that each pass through the optic only rotates the polarized light 11.25 degrees (half the rotation achieved by Faraday optic1104, discussed above), whereby after four passes through the Faraday optic1204, the light is rotated +45 degrees to counter the −45 degrees rotation achieved by the half wave plate1203. Thus, the light output from the Faraday optic (after the fourth pass) is “p” polarized light. Optionally, single polarizing element1201, Faraday optic1204, and reflector1211could be increased in size to support additional passes of the Faraday optic1204; further reducing the thickness of the Faraday optic or size of the magnetic structure (such as the magnetic structure1215ofFIG. 1C, which may have two separate magnetic elements1215a,1215b,and/or which may be adjustable, as discussed below).

Optionally, a dual stage, dual pass isolator may include two half wave plates, one for the light at an input region of the single isolator and another at an output region of the single isolator. For example, and such as shown inFIG. 1D, a single polarizing element1301has a half wave plate1303aand a polarizing coating1308similar to single polarizing element1101, discussed above, and the isolator includes a Faraday optic1304and reflector1305similar to the optic1104and reflector1105discussed above. Single polarizing element1301further includes a reflector1311, such as at a center region between the half wave plate1303aand the point or region1310where the light beam passes back through the single polarizing element1301. The single polarizing element1301also includes a second half wave plate1303bat region1310, such that the light from the Faraday optic1304(after its fourth pass through the Faraday optic) is rotated as it passes through the second half wave plate1303bso that the exiting beam is a p polarized beam. This is because a p polarized input beam will rotate −45 degrees when passing through first half wave plate1303a,and then will rotate back +45 degrees when making two passes through the Faraday optic1304, and then will rotate an additional +45 degrees when making another two passes through the Faraday optic (after reflecting off of reflector1311). The beam then is rotated back −45 degrees by the second half wave plate1303bto be a p polarized beam as it exits the single polarizing element1301.

The isolator of the present invention thus provides multiple passes through a Faraday optic. If only two passes are made through the Faraday optic, then more magnetic power may be needed at the optic, which may result in a larger package. By providing for four or more (even number of) passes through the Faraday optic, a smaller magnet package may be used at the Faraday optic.

Referring toFIG. 2A, a perspective view of a single stage, dual pass PI isolator1400is shown in accordance with the present invention. The isolator includes a polarizing element1401and a Faraday optic1404, with a half wave plate1403disposed at part of the polarizing element and a reflector1405disposed at the Faraday optic1404. In the illustrated embodiment, the polarizing element1401has a first polarizing coating1414and a second polarizing coating1415at the diagonal surfaces of parallelepiped1412that are similarly bonded, such as through optical contact, to fused silica prisms1413. Randomly polarized radiation1410from a source in the forward direction is resolved into “p” and “s” polarized beams1416and1417, respectively, at the first polarizing coating1414, with the s polarized beam being reflected upward by polarizing coating1414. The “s” polarized beam1417is further reflected at the second polarizing coating1415and then transmitted out of the output AR coated surface of parallelepiped1412precisely parallel to and displaced from the “p” polarized beam1416. As shown inFIG. 2A, the beams pass through the wave plate1403where they are rotated −45 degrees, and then the rotated p and s beams pass through the Faraday optic1404and reflect off of the reflector1405, so as to make two passes through the optic1404(where they are rotated back +45 degrees) and return towards the polarizing element1401. The beams enter the polarizing element at a location devoid of the half wave plate1403, whereby the s beam is reflected downward by the polarizing coating1415and further reflected by the polarizing coating1414so as to exit the polarizing element1401with the p beam.

For light traveling in the reverse direction (FIG. 2B), the 90 degree rotation (+45 degrees by the Faraday +45 degrees by the wave plate) in the reverse direction causes the upper beam (the beam reflected by the polarizing coatings1414,1415) to pass through the upper diagonal (not reflected by the polarizing coating1415) and the lower beam to be reflected by the lower diagonal polarizing coating1414. Thus, reverse propagation of light is precluded from exiting the isolator in the direction of the source of light beam1410ofFIG. 2A. Therefore, reverse propagating light is unable to couple back into the source.

A two stage PI isolator is increasingly desired to manage back reflections from PI laser systems generating over 100W of average power. Leakage from traditional single stage isolators can be sufficient to be amplified to levels which are harmful to internal laser components. Referring toFIG. 3A, a perspective view of a dual stage, dual pass PI isolator1500is shown in accordance with the present invention. The isolator includes a polarizing element1501with half wave plates1503aand1503band quarter wave plate1511disposed at part of the polarizing element and a reflector1505disposed at the Faraday optic1504. Collimated laser radiation1510is incident upon a single fused silica polarizing beam displacer1501(PBD) where it is resolved into forward propagating “s” polarization beam1516(solid line) and “p” polarization beam1517(dashed line). Both “p” and “s” polarization beams are transmitted through first isolation stage quartz half waveplate1503awhich may be bonded to beam displacer1501with optic axis +22.5 degrees such that the “p” and “s” polarized beams experience a +45 degree polarization rotation about the forward propagation axis to be +45 and +135 degree polarized beams, respectively.

The beams1516,1517then make two passes through the Faraday optic1504(via reflection off of reflector1505) and return to the PBD1501, where the p beam passes through the PBD and through a ¼ wave plate1511and reflects off a reflector at the back side of the wave plate1511so as to again propagate through the PBD1501. The s beam also passes through the PBD and reflects off of the upper reflector coating and again off of the lower reflector coating so as to pass through the ¼ wave plate1511and reflects off a reflector at the back side of the wave plate1511so as to again propagate through the PBD1501.

After the first pass of the ¼ wave plate1511, the light is circularly polarized. The reflection off of the backside of the ¼ wave plate causes a 180 degree phase shift thereby reversing the circularity. The return pass through the ¼ wave plate converts the light back to being planar polarized, but with the light then being rotated 90 degrees, such that the s beam becomes a p beam and the p beam becomes an s beam. This allows the two beams to flip planes and travel the same path length. In other words, the now s beam1517′ (formerly the p beam) now reflects off of the coatings in the PBD, while the now p beam1516′ (formerly the s beam) now passes directly through the PBD. Thus, by the time the two beams have again passed through the Faraday optic and again passed through the PBD so as to exit the PBD as beam1510′, the s beams and p beams have traveled the same path length. For collimated laser light, this is very important and allows very high beam quality to be maintained. If a particular application does not require high beam quality, ¼ wave plate1511could be removed and replaced with a high reflection coated region.

Thus, and as shown inFIG. 3A, the two beams will be reflected between Faraday optic high reflector1505(at the rear or opposite end of the Faraday optic1504) and single fused silica PBD first stage high reflector1511N times such that 2N passes are made through Faraday optic1504before the two beams are incident upon single fused silica AR coated region of the beam displacer1501.

Reverse propagating radiation1520(FIG. 3B), will follow the identical ray path and polarization states until after the rays have been transmitted through the second stage half-waveplate where the −45 degrees of second stage Faraday rotation is added to −45 degrees of reciprocal second stage half-waveplate rotation to rotate the polarizations of both beams by −90 degrees such that they are rejected away from the forward beam propagation axis as shown by rejected ray lines inFIG. 3B. As shown inFIG. 3B, one beam of reverse propagating beam1520passes through the PBD1501and wave plate1503band reflects back from the Faraday optic1504, whereby it is rotated such that it reflects downward at the lower reflector coating of the PBD1501and exits as a first stage rejected beam1521a.The other polarized beam of the initial reverse propagating beam1520reflects upward in the PBD1501and further reflects to the Faraday optic and back, where it is rotated such that it passes through the PBD1501and exits the PBD as first stage rejected beam1521b.

After the residual radiation is reduced in power by typically 30 dB relative to the original back-reflected power in the second stage isolator, the polarizations are again flipped by quarter waveplate before repeating the process in the first isolation stage, where again −45 degrees of first stage Faraday rotation is added to −45 degrees of reciprocal first stage half-waveplate rotation to again rotate the polarizations of both beams by −90 degrees such that they are once again rejected away from the original forward beam propagation axis as shown by the lines1522a,1522binFIG. 3B. With the first stage 30 dB isolation similar to the second stage 30 dB isolation, the residual radiation from the original back-reflected power level is reduced by approximately 60 dB, thereby ensuring that none of this back-reflected light can damage or disrupt the laser system.

Referring now toFIG. 4, an isolator1600comprises a single polarized beam displacer1601having three reflective layers or coatings1614a,1614b,1614c.The isolator1600operates in a similar manner to isolator1500, discussed above, for forward propagating beams. However, by adding a third layer to the displacer, all four rejected beams1621a,1621b,1622a,1622bcan be directed in the same direction as they exit the isolator. This simplifies the beam dump and thermal management of the high power reverse power.

The reciprocal polarization rotators need not be half-waveplates, they could also be (quartz) optical rotators, for example, or other suitable reciprocal polarization rotators. All of the above quartz waveplates need not be bonded to the single fused silica PBD, however aligning their optical axis and then bonding such waveplates to fused silica optical components such as polarizing beam-splitter cubes by optical contact is desired. Bonding these quartz waveplates directly to the single fused silica PBD to form a single optical part during final assembly has the advantage of greatly reducing the overall cost, parts count and assembly time for the optical isolator of this invention. Thus, the present invention provides a high performance PI isolator that is scalable in power with beam diameter that can be fabricated with only two separate optical components.

Although specifics above were given for a TGG Faraday optic, any Faraday optic material may be used in accordance with the present invention, such as, for example, ferromagnetic, paramagnetic, semiconductor and diamagnetic materials and/or the like. In particular, diamagnetic materials which typically have a very low Verdet constant but often have extremely low absorption can function well as temperature insensitive optical isolators in accordance with the invention because their Verdet constant is only very weakly related to temperature. The specific signs of reciprocal and non-reciprocal rotation need not be limited to those described above—they can be mutually reversed by reversing the sign of the applied magnetic field to the Faraday optic and rotating the direction of the reciprocal polarization rotators accordingly.

High reflection coatings should impart a pure 180 degree phase shift upon reflection and need not be limited to thin films as they can also be made from metal coatings.

The Faraday optics high reflection coated surface may have a protective overlayer, such as SiO2or the like, and then a metallization layer, such as gold or the like, so that the Faraday optic may be soldered directly to a heat sinking housing with, for example, a gold-tin solder layer for enhanced conduction of heat out through the high reflection coated surface. Heat flow substantially parallel to the beam path minimizes any radial heat flow across the beam cross section that can result in thermal lens focal shifts and thermal birefringence.

Optionally, it is another aspect of the present invention that the Faraday optic may comprise a layered structure with a transparent heat conductive layer bonded to one or both optical faces of a diamagnetic, paramagnetic or ferromagnetic Faraday rotating material. Such transparent heat conductive layers, in conjunction with sufficient multi-passes to ensure that the Faraday optic is thin relative to the beam diameter, ensures that heat flow is substantially parallel to the beam path within the Faraday optic. The function of the transparent heat conductive layer is described in detail in U.S. Publication No. 2014/0218795, which is hereby incorporated herein by reference in its entirety. Heat flow parallel to the beam path eliminates radial temperature gradients responsible for thermal lens focal shift and thermal birefringence.

Another aspect of the present invention is that the multi-pass Faraday rotator may use an adjustable magnetic structure that is capable of modifying the magnetic field strength generally aligned to the beam path with the Faraday optical element(s) used in the multi-pass Faraday rotator. In the case of multi-stage optical isolators, such magnetic field adjustability can be independent or different for each stage for improving the temperature and/or wavelength bandwidth performance of the optical isolator. The adjustable magnetic structure is adjustable relative to the optical elements via any suitable electrical or mechanical or electromechanical means that may adjust the space or gap between the magnetic structure and the optical element to provide the desired performance of the optical isolator. For example, and such as shown inFIG. 1C, an adjustable magnetic structure1215may include two magnetic structures1215aand1215b,which can be moved independently or in tandem (relative to the Faraday optic1204) to achieve different faraday rotations per stage.

Therefore, the present invention provides an optical isolator having one or more isolation stages using a single polarizing element in conjunction with a multi-pass Faraday rotator and one 45 degree reciprocal polarization rotation element per isolation stage for improved alignment simplicity, reduced parts count and lower cost. The multi-pass Faraday rotator optionally and desirably has an even number of multi-passes and may comprise a Faraday optic with a highly reflective coating on one optical face and an anti-reflective coating on the opposite optical face nearest to the single polarizing element. A magnetic field generally aligned to the beam path in the multi-pass Faraday rotator causes 45 degree non-reciprocal polarization rotation in the Faraday optic for each isolation stage. The 45 degree reciprocal polarization rotation element may comprise a quartz waveplate that is bonded, such as by adhesive free optical contact for high power applications, to a surface of the single polarizing element in the optical path of only one pass of the beam and aligned for the opposite sense rotation to the Faraday non-reciprocal rotation.