Polarization-rotating device and faraday isolator

Abstract

The present disclosure provides a polarization rotation device (12) suitable for rotating a polarization direction of laser radiation (14). The polarization rotation device (12) comprises a Faraday medium (16) and a multi-pass optical arrangement (18), the multi-pass optical arrangement (18) consisting of or comprising a Herriott cell (30) and having the Faraday medium (16) at least partially arranged within the multi-pass optical arrangement (18), wherein the multi-pass optical arrangement (18) is adapted such that the laser radiation (14) when coupled into the multi-pass optical arrangement (18) performs multiple round trips in the multi-pass optical arrangement and at least ten passes through the Faraday medium (16). Furthermore, the polarization rotation device comprises a magnetic element (20) suitable for providing a magnetic field at a position of the Faraday medium (16) inside the multi-pass optical arrangement (18). The polarization rotation device is characterized in that the Faraday medium (16) has a thickness of 2 mm or more, through which thickness the laser radiation (14) propagates at each of the multiple passes.

Description

Technical Field

[0001] This disclosure provides a polarization rotation device and a Faraday isolator. Therefore, this disclosure relates to laser technology. Background Technology

[0002] Conventional optical isolators are typically key components of laser systems sensitive to back reflection of laser radiation. Optical isolation is usually achieved using Faraday isolators. In a conventional Faraday isolator, the polarization of the incident laser radiation is rotated using the Faraday effect within the Faraday medium between two linear polarizers, causing a 45° polarization deflection between the two polarizers. A magnetic field is applied to the Faraday medium, aligning it parallel to the propagation direction of the laser radiation through the medium. For laser radiation entering the Faraday isolator from one side, the resulting Faraday effect typically rotates the polarization by 45°, allowing essentially complete transmission through the second polarizer (polarizer 2). When the laser radiation leaving the Faraday isolator is finally reflected back into it, its polarization is also rotated by approximately 45°. However, for the corresponding wave vector, the direction of the Faraday rotation is reversed, resulting in essentially complete suppression at the corresponding second linear polarizer (polarizer 1). Viewed from a fixed laboratory frame, the polarization rotations of the two beams superimpose, producing a polarization oriented 90° towards polarizer 1. The rotation angle of laser radiation through the Faraday medium in a single pass is given by the following formula:

[0003]

[0004] The rotation angle is determined by the following: the Verdet constant V of the medium, and the component of the applied magnetic field parallel to the direction of laser radiation propagation B. z The propagation length *l* within the material. A magnetic field parallel to the direction of laser radiation propagation needs to change sign, thus reversing the direction of polarization rotation relative to the wave vector. However, when viewed from a fixed laboratory coordinate system, the Faraday rotator causes the electric field vector of the passing electromagnetic wave to always rotate in the same direction. Therefore, when the electromagnetic wave is reflected back into the Faraday medium by the reflecting surface and then passes through the medium a second time, the rotation angle of its electric field vector accumulates. It is worth noting that while the reflecting surface changes the propagation direction, it does not change the orientation of the electric field, a seemingly obvious but crucial point.

[0005] Terbium gallium garnet (TGG) is a commonly used Faraday medium in conventional Faraday isolators for near-infrared (NIR) lasers with high average power (see: KT Stevens et al.: “Promising Materials for High Power Laser Isolators,” LTJ 13, 18–21 (2016); and ILSnetkov et al.: “Review of Faraday Isolators for Kilowatt Average Power Lasers,” IEEE J. QuantumElectron. 50, 434–443 (2014)). TGG exhibits a high Wilderness constant of approximately 39 rad / (T·m) at a wavelength of 1 μm and an absorption coefficient of approximately 1 × 10⁻⁶. -3 cm -1Another emerging Faraday medium is potassium terbium fluoride (KTF), which has a similar Wilder constant but an order of magnitude lower absorption and thermo-optic coefficients, allowing for higher average power handling, but is more difficult to grow (see: KT Stevens, W. Schlichting, G. Foundos, A. Payne, and E. Rogers, “Promising Materials for High Power Laser Isolators,” LTJ 13, 18–21 (2016).). Typically, TGG rods a few millimeters in length are placed within a permanent magnet toroidal system providing an axial magnetic field on the order of 1 T. In practice, the average power of operation is limited to a few hundred watts (see: ILSnetkov, A.Voitovich, O.V. Palashov, and E.A. Hazanov, “Review of Faraday Isolators for Kilowatt Average Power Lasers,” IEEE J. Quantum Electron. 50, 434–443 (2014).). The limiting factor in such conventional Faraday isolators is the reduced isolation performance due to the temperature gradient caused by the absorption of optical radiation. Although thermal lensing and the temperature variation of the Wilder constant pose problems, the main limitation stems from birefringence caused by photoelastic effects (see: E.A. Khazanov et al.: "Investigation of self-induced depolarization of laser radiation in terbium gallium garnet," IEEE J. Quantum Electron. 35, 1116–1122 (1999)). Complex schemes for compensating for birefringence within the isolator allow for average power exceeding 1 kW while maintaining 30 dB of isolation performance (see: E.A. Khazanov, "Compensation of thermally induced polarization distortions in Faraday isolators," Quantum Electron. 29, 59–64 (1999)).Another conventional approach has been proposed to reduce the thermal gradient within the Faraday medium by using a cooling geometry known for its effective application in high-power sheet lasers (see: DE 10 2010028213A1; and A. Giesen et al.: “Scalable concept for diode-pumped high-power solid-state lasers,” Appl. Phys. B 58, 365–372 (1994)). Therefore, the Faraday medium is prepared as a sheet and mounted on a water-cooled heatsink from one side. Reducing the thickness of the sheet weakens the Faraday rotation, thus requiring compensation through multiple passes through the sheet. Besides the obvious cost and complexity, this approach requires producing a Faraday medium in the shape of a sheet with an antireflective coating, ideally with low thickness and a large aperture. Furthermore, a strong and uniform magnetic field must be applied to the aperture of the sheet to produce the necessary effects and avoid depolarization. This requires the magnetic system to be positioned beneath the sheet element, rather than surrounding it as is done in existing designs of other conventional Faraday isolators. Furthermore, this magnetic system is prone to localized interference with the water-cooling system.

[0006] US 4,909,612 describes an optical Faraday isolator comprising a Faraday rotator dielectric plate coated to define an input and an output surface, and an inner reflecting surface for causing a beam to travel in a zigzag path between the input and output surfaces. Summary of the Invention

[0007] Therefore, the problems solved by this disclosure relate to providing polarization rotating devices and Faraday isolators that overcome the limitations of conventional polarization rotating devices and Faraday isolators. More specifically, the problems solved by this disclosure may relate to providing a polarization rotating device that reduces the requirements for the available Faraday medium.

[0008] This problem is solved by a polarization rotation device and a Faraday isolator having the features of the respective independent claims. Optional embodiments are provided in the dependent claims and the specification.

[0009] In one aspect, a polarization rotating device is provided suitable for rotating the polarization direction of laser radiation. The polarization rotating device includes a Faraday medium and a multipass arrangement, which comprises or includes Herriott cells and has a Faraday medium at least partially disposed within the multipass arrangement. The multipass arrangement is adapted such that the laser radiation, when coupled into the multipass arrangement, performs multiple round trips within the arrangement and passes through the Faraday medium at least ten times. Furthermore, the polarization rotating device includes a magnetic element adapted to provide a magnetic field at the location of the Faraday medium within the multipass arrangement. The polarization rotating device is characterized in that the Faraday medium has a thickness of 2 mm or greater, through which the laser radiation propagates in each of the multiple passes.

[0010] On the other hand, a Faraday isolator is provided, which includes a polarization rotation device according to the present disclosure.

[0011] The polarization rotation device relates to an optical apparatus adapted to rotate the polarization direction of laser radiation propagating through it. The polarization rotation device can be adapted to achieve the highest polarization rotation efficiency at a predetermined design wavelength. This predetermined design wavelength may correspond to the center wavelength of the laser system, and the polarization rotation device is intended for use in conjunction with this laser system.

[0012] The term "laser radiation" is used throughout this disclosure for any kind of optical electromagnetic radiation, particularly for coherent electromagnetic radiation. Laser radiation can be provided in the form of laser pulses (i.e., pulsed laser radiation). Pulsed laser radiation can be provided as a pulse train or a single isolated pulse. Pulsed laser radiation can also be provided as a group of laser pulses. The pulse duration of a laser pulse can be, for example, 1 ps or less, and optionally as short as 100 fs or less. The laser oscillator system can be mode-locked to provide pulsed laser radiation. Unless otherwise specified, the values ​​for pulse duration provided throughout this disclosure are specified as the full width at half maximum (FWHM) assuming a Gaussian pulse shape. Laser radiation and pulsed laser radiation can be used synonymously throughout this disclosure.

[0013] A polarization rotator is an optical device that allows for the polarization rotation of laser radiation, particularly pulsed laser radiation, coupled into the polarization rotator. After the polarization of the laser radiation coupled into the polarization rotator is rotated, the laser radiation can be coupled out from the polarization rotator. Laser radiation with linear polarization is particularly suitable for polarization rotation based on the Faraday effect, or for applications requiring polarization rotation based on the Faraday effect.

[0014] A multi-pass arrangement is an arrangement of optical elements that deflect and couple laser radiation into the multi-pass arrangement, causing the laser radiation to propagate several times within the arrangement before being coupled out. Redirection of the laser radiation can optionally be achieved through reflection, thereby altering its propagation direction within the multi-pass arrangement. Unlike arrangements that guide laser radiation via total internal reflection using optical fibers, the propagation of laser radiation in a multi-pass arrangement can occur in free space, and the mode of laser radiation is not restricted by the optical fiber at any point along the optical path of the laser beam or pulse. The central axis of the multi-pass arrangement can be the central axis of the modulus of the multi-pass arrangement. This central axis can at least partially coincide with the optical axis of the first and / or second reflector of the multi-pass arrangement. This central axis does not necessarily have to be a straight line, but can include one or more bends, for example, when the multi-pass arrangement includes one or more intermediate reflectors for reflecting laser radiation propagating between the first and second reflectors. The distance between the first and second mirrors in a multi-channel arrangement can be the interval between the first and second mirrors, measured along a central axis, particularly between corresponding center points on the reflective surfaces of the mirrors. The multi-channel arrangement can be configured as or include Heriot-Trench, White, and / or Pfund cells. The multi-channel arrangement can have a central axis that can be deflected by one or more additional mirrors. The first and / or second mirrors in the multi-channel arrangement can have a flat or convex shape, while the other mirror has a concave shape. In other words, the multi-channel arrangement can have a convex-concave or concave-flat configuration. Optionally, the first and second mirrors can have concave shapes. According to an optional embodiment, the multi-channel arrangement can specifically consist of or include Heriot-Trench cells.

[0015] The round trip of laser radiation in a multi-pass arrangement can each have a very similar optical path through the arrangement. Specifically, in each or most round trip, the laser radiation can be deflected by the same optical elements of the multi-pass arrangement, particularly the first and second mirrors of the arrangement. For example, the first round trip may include laser radiation coupling into the multi-pass arrangement, and / or the last round trip may include laser radiation coupling out of the arrangement. Therefore, the first and / or last round trips may differ from the other round trips in terms of the optical elements that deflect the laser radiation. Optionally, the round trip does not completely return the optical path of the laser radiation propagating in the multi-pass arrangement to its initial state, but after completing a full round trip, the laser radiation may strike the corresponding optical elements (i.e., the first and / or second mirrors) at positions significantly different from their initial positions at the start of the round trip. In particular, the laser radiation can propagate on separate optical paths in each round trip within the multi-pass arrangement, where these separate optical paths may not overlap. The first and / or second reflectors may have apertures for coupling laser radiation into and / or out of the multiple light-passing arrangement, respectively.

[0016] A Faraday medium can be a medium that is at least partially optically transparent to laser radiation, in which the Faraday effect occurs when the laser radiation propagates through the Faraday medium. The Faraday effect is often also called the magneto-optical effect. The Faraday effect causes a polarization rotation that is proportional to the projection of the applied magnetic field along the direction of propagation of the laser radiation through the Faraday medium. The Faraday effect can typically be caused by left-handed and right-handed circularly polarized light waves propagating at slightly different speeds; this property is called circular birefringence, where linear polarization can be decomposed into a superposition of two circularly polarized components with equal amplitude, opposite chirality, and different phases. The relative phase shift caused by the Faraday effect rotates the direction of the wave's linear polarization.

[0017] A single passage of laser radiation through a Faraday medium represents one propagation of the laser radiation through the Faraday medium. In some embodiments, each round trip of the laser radiation in a multi-pass arrangement may include two passages through the Faraday medium, for example, one passage in a first direction and one passage in a second direction opposite to the first direction. In other embodiments, each round trip may include only a single passage through the Faraday medium. For example, the optical path of the round trip may be configured such that the laser radiation passes through the Faraday medium only in one direction, but not in the second direction. Each round trip may optionally include more than one, particularly two, passages through the Faraday medium. Optionally, some round trips may not include passage through the Faraday medium. For example, polarization rotation devices, and particularly multi-pass arrangements, may be arranged such that in some round trips, such as when the laser radiation is just coupled into the multi-pass arrangement and / or just before the laser radiation is about to be coupled out of the multi-pass arrangement, there is no passage through the Faraday medium. Each time the laser radiation passes through the Faraday medium, the laser radiation may enter the Faraday medium at a first surface and exit the Faraday medium at a second surface different from the first surface.

[0018] The magnetic element can be any suitable element adapted to provide a magnetic field at the location of the Faraday medium within the multi-pass arrangement in a manner sufficient to induce the desired Faraday effect and polarization rotation of the laser radiation. The magnetic element can include one or more permanent magnets and / or one or more electromagnets. The magnetic element can be adapted to provide a magnetic field at the location of the Faraday medium, particularly in a direction parallel or antiparallel to the propagation direction of the laser radiation, having a field strength of 0.1T or greater, optionally 0.2T or greater, optionally 0.5T or greater, optionally 0.75T or greater, and optionally 1T or greater. This ensures a large Faraday effect experienced by the laser radiation as it propagates through the Faraday medium. The thickness of the Faraday medium can refer to the spatial extent of the Faraday medium along the average propagation direction of the laser radiation through it. The term "average propagation direction" can refer to the direction typically followed by the laser radiation as it propagates through the Faraday medium, although the propagation direction on each path may differ slightly due to variations in the propagation paths within the multi-pass arrangement. "The thickness traversed by the laser radiation during each of its multiple passes" can refer to the effective thickness experienced by the laser radiation as it propagates through the Faraday medium. Alternatively, this thickness can be the average of the corresponding thicknesses experienced by the laser radiation during multiple passes through the Faraday medium.

[0019] The thickness of the Faraday medium can optionally be 20 mm or less, and optionally 10 mm or less. This allows for the provision of the Faraday medium in a compact manner, as well as optional multiple light transmission arrangements and optional polarization rotation devices.

[0020] The advantage provided by this disclosure is that, compared with conventional polarization rotation devices that pass through the Faraday medium only once or are limited to very thin Faraday media with a thickness of only 1 mm or less, a polarization rotation device with a significantly increased propagation length through the Faraday medium can be provided.

[0021] Therefore, a further advantage provided by this disclosure is that such materials, having a lower Wilder constant than many conventionally used materials (e.g., TGG), can be used as Faraday media. In particular, since the propagation length of laser radiation through a Faraday medium can be several orders of magnitude greater due to multiple passes than that of a conventional single-pass polarization rotation device, according to this disclosure, Faraday media having a Wilder constant several orders of magnitude lower than that of conventional Faraday media can be selected. This greatly increases the selection of possible materials for use as Faraday media and thus allows for the selection of materials with additional properties optimized for the intended purpose and design wavelength.

[0022] Furthermore, this disclosure offers the advantage of providing a polarization rotation device that can achieve a large number of passes through the Faraday medium while maintaining the limited spatial dimensions of the Faraday medium. Unlike the Faraday isolator described in US 4,909,612 (where the propagation length of the beam through the Faraday rotator medium depends primarily on the dimensions of the Faraday rotator medium), this disclosure offers the advantage that the number of passes through the Faraday medium can be limited by a multi-pass arrangement (i.e., Heriot-Telling units), and therefore can be set up independently of the Faraday medium. In other words, this disclosure provides the advantage of allowing optimization of the laser radiation's round-trip journey through the multi-pass arrangement and the Faraday medium, independent of the dimensions of the Faraday medium. This allows for greater freedom in the fabrication of the polarization rotation device and can reduce manufacturing costs because the requirements for the Faraday elements can be lower compared to conventional polarization rotation devices. Moreover, at least ten passes through the Faraday medium can result in a longer propagation length of the laser pulse through the Faraday medium. This can facilitate the use of materials with low Wilder constants as the Faraday medium, which can further help limit manufacturing costs.

[0023] In particular, this allows the use of materials with lower absorptivity but lower Wilder constants at the wavelength of laser radiation (which can correspond to the design wavelength) as Faraday media. For example, materials such as quartz and / or fused silica can be used as Faraday media, providing exceptionally low absorption coefficients at a wavelength of 1 μm, approximately two orders of magnitude lower than that of TGG. Furthermore, these materials exhibit lower thermo-optical coefficients compared to TGG, resulting in lower temperature gradients and thus reduced depolarization due to temperature gradients. Since the Faraday media is arranged in a multi-pass arrangement, the total thickness of the Faraday media can still be kept within a moderate range because the effective thickness (i.e., cumulative thickness) increases due to the large number of round trips.

[0024] Furthermore, the advantage provided by this disclosure is that polarization rotation devices can be provided for design wavelengths that are typically unattainable when using conventional Faraday media (such as TGG). Because TGG typically exhibits color centers and is subject to high absorption and thermal degradation in the visible and ultraviolet spectral ranges, it is generally unsuitable as a Faraday media in the ultraviolet and visible spectral ranges (KT Stevens, W. Schlichting, G. Foundos, A. Payne, and E. Rogers, “Promising Materials for High Power Laser Isolators,” LTJ 13, 18–21 (2016).). In particular, according to this disclosure, polarization rotation devices can be provided for design wavelengths in the ultraviolet spectral region (such as the wavelength range of 200 nm to 450 nm). For example, this disclosure may allow the use of materials such as cerium(III) fluoride (CeF3) and / or magnesium fluoride (MgF2) as Faraday media (see: MJ Weber, Handbook of Optical Materials (CRC Press, 2018); and DLSteinmetz et al.: “A polarizer for the vacuum ultraviolet,” Applied optics 6, 1001–1004 (1967)), which still have high transmittance at wavelengths down to 200 nm, but have significantly lower Wilder constants than TGG (E. Munin, CBPedroso, and A.B. Villaverde, “Magneto-optical constants of fluoride optical crystals and other AB2 and A2B type compounds,” Faraday Trans. 92, 2753 (1996)). Since the polarization rotation device according to this disclosure includes a multiple-pass arrangement that provides laser radiation to pass through the Faraday medium multiple times, the lower Wilder constant can be compensated by the larger effective propagation length obtained in the Faraday medium, thus allowing the use of the advantageous transmission characteristics of these materials.

[0025] Furthermore, the advantage provided by this disclosure is that the manufacturing cost of the polarization rotation device can be reduced because it can be manufactured without the need for high-cost materials for a Faraday medium. This allows for the integration of one or more polarization rotation devices in low-budget laser systems and applications.

[0026] Furthermore, this disclosure offers the advantage that the length or volume providing high magnetic flux can be kept small. In particular, since the propagation length of the laser radiation through the Faraday medium can be kept small for each individual pass, the length or volume of the magnetic field can be confined within a small size of the interaction volume between the laser radiation and the Faraday medium.

[0027] The polarization rotation device is adapted to cause the laser radiation to pass through the Faraday medium at least 10 times, and optionally at least 12 times. The polarization rotation device can be adapted to cause the laser radiation to pass through the Faraday medium at least 14 times, optionally at least 16 times, optionally at least 18 times, optionally at least 20 times, optionally at least 24 times, and optionally at least 30 times in a coupled-input multi-pass arrangement. This ensures a large effective propagation length of the laser radiation through the Faraday medium, and therefore allows for high Faraday rotations through accumulation even if the Faraday medium exhibits only a moderate Wilder constant.

[0028] The Faraday element can have a Wilder constant of 20 rad / (T·m) or less at the predetermined design wavelength of the polarization rotation device. This allows the use of a variety of materials with advantageous properties, such as high transmittance at the design wavelength, particularly in the visible and / or ultraviolet spectral range, and / or low coefficients of thermal expansion and / or low thermo-optical coefficients, resulting in lower depolarization due to thermal effects. Therefore, this allows for the provision of polarization rotation devices with high extinction ratios. Furthermore, the Faraday element can be selected to have a Wilder constant of 0.5 rad / (T·m) or greater, optionally 1 rad / (T·m) or greater, at the predetermined design wavelength of the polarization rotation device. This ensures effective polarization rotation of the laser radiation. The Faraday medium can be provided in the form of a separate medium comprising one or more solid Faraday elements and / or one or more gaseous Faraday media having predetermined dimensions and / or predetermined physical properties, particularly a predetermined Wilder constant at the design wavelength of the polarization rotation device.

[0029] Faraday media can include solid Faraday elements. This offers the advantage of keeping the technical complexity of the polarization rotation device relatively low. Furthermore, it provides the advantage that the Faraday medium can have precisely predetermined Wilder constants and / or precisely predetermined spatial dimensions.

[0030] The polarization rotation device can be adapted such that, each time the laser radiation passes through the solid Faraday element, it enters the Faraday medium at a surface different from the surface where the laser radiation leaves the solid Faraday element. In other words, with each pass, the laser radiation is transmitted through the Faraday medium without complete internal reflection. This provides the advantage that the Faraday medium can be used in a transmission manner, and therefore can be inserted into the optical path of the multiple-pass arrangement without reconfiguration. Furthermore, this provides the advantage that the thickness of the Faraday medium (i.e., its spatial extension in the direction of laser radiation propagation) can be chosen to be longer than its lateral extension in the direction perpendicular to the propagation direction without causing the laser beam to be truncated.

[0031] Solid Faraday elements may be composed of or include at least one of the following materials: fused silica, quartz, MgF2, CaF2, Al2O3 (sapphire), YAG, Gd3Al5O 12 KBr, ZnS, ZnSe, and ZnTe. These materials can exhibit lower Wilder constants than those commonly used as Faraday media (e.g., TGG), but can exhibit superior optical properties, such as: high transmittance over a wide spectral range, high transmittance in the visible and / or ultraviolet spectral ranges, high laser-induced damage threshold, good thermal properties that avoid or reduce undesirable thermal effects and temperature gradients, and good optical quality at a reasonable cost.

[0032] Alternatively or additionally, the Faraday medium may comprise or be composed of a gaseous Faraday medium. This provides the advantage of low absorption of laser radiation, which reduces the thermal effects of confinement properties within the Faraday medium. Since there is no temperature- or pressure-induced birefringence that can occur in solid materials, the amount of depolarization of the confinement properties can be reduced. Furthermore, the amplitude of polarization rotation can be adjusted by changing the pressure of the gaseous Faraday medium. Moreover, this provides the advantage of altering the properties of the Faraday medium by providing a mixture of different gases as the Faraday medium. The gaseous Faraday medium may optionally consist of or include one or more of the following gases: He, Ne, Ar, Kr, Xe, H2, D2, O2, N2, CO2, and CH4. The polarization rotation device may also include a pressure unit containing the gaseous Faraday medium, wherein the multiple-pass arrangement may be at least partially arranged within the pressure unit. Alternatively, the pressure unit may be arranged within the multiple-pass arrangement, and particularly within the laser beam path of the pressure unit. The pressure unit may include windows for coupling laser radiation into the pressure unit and windows for coupling laser radiation out of the pressure unit. The pressure of the gaseous Faraday medium inside the pressure unit may be in the range of 1 bar to 1,000 bar, particularly in the range of 1 bar to 100 bar.

[0033] A gaseous Faraday medium can possess a pressure-dependent Wilder constant. Furthermore, the pressure-dependent Wilder constant can be 1 mrad / (T·m bar) or greater. A gaseous Faraday medium can be supplied at an absolute pressure of 1 bar or higher. At a pressure of 1 bar, the density of a gaseous Faraday medium composed of pure gas can be 1 amagat = 44.6 mod / m³. 3 Optionally, the pressure of the gaseous Faraday medium can be 10 bar or higher. At a pressure of 10 bar, the density of a gaseous Faraday medium composed of pure gas can be 10 amagat = 446 mol / m³. 3 .

[0034] A Faraday medium comprises a mixture of multiple solid Faraday elements and / or different gases. Optionally, a Faraday medium may include at least one solid Faraday element and a gaseous Faraday medium. This allows for high flexibility in adjusting the optical properties and Wilder constant of the Faraday medium.

[0035] Magnetic elements may include or be composed of permanent magnets. This provides the following advantages: a magnetic field for inducing the Faraday effect can be provided without supplying electrical power to the magnetic element. This allows the polarization rotation device to be provided as a completely passive device without the need for electrical power supply. Therefore, this facilitates the implementation of the polarization rotation device in other devices, such as laser systems or other optical devices. Additionally, this provides the advantage of achieving a favorable ratio between the provided magnetic field strength and the spatial dimensions of the magnetic element. Furthermore, this reduces the cost of manufacturing and operating the polarization rotation device. The permanent magnet may be toroidal and adapted to provide a magnetic field along the central axis of the toroidal permanent magnet. The toroidal permanent magnet may at least partially surround the Faraday medium within a multi-channel arrangement. This allows for optimized use of the total magnetic flux provided by the magnetic element, and thus reduces the required performance, size, and / or cost of the magnetic element.

[0036] The polarization rotation device may further include a first linear polarizer disposed in front of the multiple light transmission arrangement and / or a second linear polarizer disposed after the multiple light transmission arrangement. This allows for the isolation of polarization components that have already undergone the intended polarization rotation in the polarization rotation device, while blocking other polarization components. In particular, this allows the polarization rotation device to be used as a Faraday isolator or in a Faraday isolator.

[0037] Faraday media can exhibit 80% or higher, optionally 90% or higher, and optionally 95% or higher optical transmittance across the entire spectral range of 150 nm to 550 nm. This allows for the provision of polarization rotation devices for designed wavelengths in the visible or ultraviolet spectral range. For example, a Faraday medium may comprise or consist of a solid Faraday element made of quartz or fused silica, which exhibits high transmittance at least in the spectral range of 150 nm to 550 nm and provides good optical quality at low cost.

[0038] The polarization rotation device can be adapted to rotate the polarization direction of laser radiation by an angle ranging from 30° to 60° between coupling laser radiation into a multi-pass arrangement and coupling laser radiation out of the multi-pass arrangement. In particular, the polarization rotation device can be adapted to rotate the polarization direction of laser radiation by an angle of 45°. This allows the polarization rotation device to be used as a Faraday isolator or in a Faraday isolator, and thus, depending on the design of the polarization rotation device, to block laser radiation propagating in the opposite direction to the intended direction.

[0039] As described above, a Faraday isolator is provided, which includes the polarization rotation device proposed above in this disclosure. All features and characteristics disclosed with respect to the polarization rotation device should be considered as disclosed also with respect to the Faraday isolator, and vice versa.

[0040] Faraday isolators can be adapted to rotate the polarization direction of laser radiation by an angle ranging from 30° to 60°, particularly by 45°. This allows for the effective blocking of laser radiation propagating in the opposite direction to its intended propagation, such as undesirable back reflections.

[0041] A Faraday isolator can include multiple polarization rotation devices. A Faraday isolator can also include a cascaded arrangement of multiple polarization rotation devices. This allows for the accumulation of a large number of polarization rotations while maintaining low complexity in the multiple-pass arrangement used.

[0042] Those skilled in the art will understand that the features described above, as well as those in the following description and drawings, are not only disclosed in the explicitly disclosed embodiments and combinations, but also other technically feasible combinations and individual features are included in this disclosure. Hereinafter, several alternative embodiments and specific examples are described with reference to the accompanying drawings to illustrate this disclosure, but not to limit this disclosure to the described embodiments. Attached Figure Description

[0043] Other optional embodiments will now be described with reference to the accompanying drawings. The drawings show:

[0044] Figure 1 This is a schematic diagram of a Faraday isolator according to an optional embodiment;

[0045] Figure 2 and Figure 3 It is a visualization of the magnetic field flux in different permanent magnets.

[0046] In the accompanying drawings, the same reference numerals are used for corresponding or similar features in different drawings. Detailed Implementation

[0047] Figure 1 A Faraday isolator 10 according to an optional embodiment is illustrated in schematic diagram, which includes a polarization rotation device 12 according to an optional embodiment.

[0048] The polarization rotation device 12 is adapted to rotate the polarization direction of laser radiation 14 by a rotation angle ranging from 30° to 60°, and optionally by a rotation angle of 45°. The polarization rotation device 12 includes a Faraday medium 16 and a multiple-pass arrangement 18 having the Faraday medium 16 at least partially disposed within it. The multiple-pass arrangement 18 is adapted such that the laser radiation 14, upon coupling into the multiple-pass arrangement 18, makes multiple round trips within the multiple-pass arrangement 18 and passes through the Faraday medium 16 multiple times. Furthermore, the polarization rotation device 12 includes a magnetic element 20 adapted to provide a magnetic field at the location of the Faraday medium 16 within the multiple-pass arrangement 18. The polarization rotation device is characterized in that the Faraday medium 16 has a thickness of 2 mm or greater, through which the laser radiation 14 propagates in each of the multiple passes. The thickness of the Faraday medium 16 may correspond to the spatial extension of the Faraday medium 16 along the central axis 100 of the multiple-pass arrangement 18. The thickness of the Faraday medium 16 can optionally be 10 mm.

[0049] Furthermore, the Faraday isolator 10 may include a first polarizer 22 and a second polarizer 24, respectively arranged in front of and after the multiple light transmission arrangement 18. The first polarizer 22 and the second polarizer 24 may be part of the polarization rotation device 12, or separate components of the Faraday isolator 10 that are not part of the polarization rotation device 12. Before coupling the laser radiation 14 into the polarization rotation device 12, the first polarizer 22 may provide a predetermined polarization to the laser radiation 14, specifically a linear polarization with a predetermined direction. The first polarizer 22 may be provided in the form of a Glan-Taylor polarizer. The second polarizer 24 may be adjusted to transmit only those portions of the laser radiation 14 that have already undergone a predetermined angle of polarization rotation in the polarization rotation device 12. Any other possible components that have not undergone a predetermined angle of polarization rotation are blocked by the second polarizer 24. According to the presented embodiment, the Faraday isolator 10 ensures that laser radiation traveling in the opposite direction (e.g., back reflection of laser radiation 14) is blocked, thereby preventing it from reaching the optical components arranged in front of the Faraday isolator 10.

[0050] Furthermore, the Faraday isolator 10 may include various optical components, such as a mirror 26, for guiding and possibly deflecting the laser radiation 14 beam, and for helping to couple the laser radiation 14 into and out of the multiple light transmission arrangement 18.

[0051] also, Figure 1A laser source 28 providing laser radiation 14 is shown. However, the laser source 28 does not necessarily form part of the Faraday isolator 10. The Faraday isolator 10 can be used with any type of laser radiation 14 provided by any type of source. The Faraday isolator 10 can be adapted to a design wavelength corresponding to the center wavelength of the laser radiation 14.

[0052] The multiple light transmission arrangement 18 may consist of or include Heriot-type elements 30, which include a first reflector 32 and a second reflector 34. Both the first reflector 32 and the second reflector 34 may be provided in the form of concave mirrors, or in the form of a combination of concave and convex mirrors, or in the form of a combination of concave and plane mirrors.

[0053] The polarization rotation device 12 can be adapted to allow the laser radiation 14 to pass through the Faraday medium 18 at least ten times.

[0054] The Faraday medium 16 may have a Wilder constant of 20 rad / (T·m) or less at a predetermined design wavelength of the polarization rotation device 12. The Faraday medium 16 may include a solid Faraday element, wherein the polarization rotation device 12 may be adapted such that, each time the laser radiation 14 passes through the solid Faraday element, it enters the Faraday element at a surface different from the surface where the laser radiation leaves the solid Faraday element.

[0055] Solid Faraday elements may be composed of or include at least one of the following materials: fused silica, quartz, MgF2, CaF2, Al2O3 (sapphire), YAG, Gd3Al5O 12 KBr, ZnS, ZnSe and ZnTe.

[0056] Alternatively or additionally, the Faraday medium 16 may include a gaseous Faraday medium 16. The gaseous Faraday medium 16 may consist of or include at least one of the following gases: He, Ne, Ar, Kr, Xe, H2, D2, O2, N2, CO2, and CH4.

[0057] The Faraday medium 16 may include a plurality of solid Faraday elements and / or a mixture of different gases. The Faraday medium may optionally include at least one solid Faraday element and a gaseous Faraday medium.

[0058] The polarization rotation device may also include a pressure unit 36 ​​containing a gaseous Faraday medium 16, wherein the multiple light transmission arrangement 18 is at least partially disposed within the pressure unit 36. Alternatively, the pressure unit 36 ​​may be integrated into the multiple light transmission arrangement 18. The Wilder constant of the gaseous Faraday medium 16, and thus the amount of Faraday effect experienced by the laser radiation propagating through the gaseous Faraday medium 16, can be adjusted by regulating the pressure of the gaseous Faraday medium in the pressure unit.

[0059] The magnetic element 20 may include or be composed of a permanent magnet, wherein the permanent magnet may be annular and adapted to provide a magnetic field along the central axis of the annular permanent magnet. The central axis of the annular permanent magnet may be parallel to and optionally coincide with the central axis 100 of the multiple light transmission arrangement 18. The annular permanent magnet may at least partially surround the Faraday medium 16 within the multiple light transmission arrangement 18.

[0060] The Faraday medium 16 can provide 80% or higher optical transmittance across the entire spectral range of 150 nm to 550 nm. The polarization rotation device 12 can be adapted to rotate the polarization direction of the laser radiation 14 by an angle ranging from 30° to 60° between coupling the laser radiation 14 into the multiple light transmission arrangement 18 and coupling the laser radiation 14 out of the multiple light transmission arrangement 18.

[0061] Therefore, the Faraday isolator 10 can be adapted to rotate the polarization direction of the laser radiation 14 coupled into the Faraday isolator 10 by an angle ranging from 30° to 60°, particularly by an angle of 45°.

[0062] The Faraday isolator 10 may include multiple polarization rotation devices 12 arranged in cascade. This allows for the division of the desired total polarization rotation angle.

[0063] In the following text, the basis will be as follows: Figure 1 Other optional features of the Faraday isolator 10 presented in the optional embodiments are described, but this disclosure and embodiments are not limited to the details described.

[0064] Generally speaking, Faraday media with low absorptivity, low coefficient of thermal expansion, and low thermo-optical coefficient are desirable to ensure minimal depolarization due to temperature gradients. At a wavelength of 1 μm, quartz or fused silica exhibits a low absorptivity, two orders of magnitude lower than TGG, and its thermo-optical coefficient is also slightly lower than TGG, as shown in Table 1.

[0065]

[0066]

[0067] aPolarization perpendicular to the optical axis, i.e., ordinary light (o.) or unpolarized light (un-pol.).

[0068] b Ordinary light (o) or unusual light (e)

[0069] The values ​​given in Table 1 are taken from the following publications:

[0070] KT Stevens, W. Schlichting, G. Foundos, A. Payne, and E. Rogers, “PromisingMaterials for High Power Laser Isolators,” LTJ 13, 18–21 (2016).

[0071] ILSnetkov, AVVoitovich, OVPalashov, and EAKhazanov, “Review of Faraday Isolators for Kilowatt Average Power Lasers,” IEEE J.QuantumElectron.50, 434–443 (2014).

[0072] MJWeber, Handbook of Optical Materials (CRC Press, 2018).

[0073] S. Asraf, Y. Sintov, and Z. Zalevsky, “Novel configuration for an enhanced and compact all-fiber Faraday rotator with matched birefringence,” Opticsexpress 25, 18643–18655 (2017).

[0074] JLCruz, MVAndres, and MAHernandez, “Faraday effect in standard optical fibers: dispersion of the effective Verdet constant,” Applied optics 35, 922–927 (1996).

[0075] E.Munin,J.A.Roversi,and A.B.Villaverde,“Faraday effect and energy gapin optical materials,”Journal of Physics D:Applied Physics 25,1635–1639(1992).

[0076] E.Munin,C.B.Pedroso,and A.B.Villaverde,“Magneto-optical constants offluoride optical crystals and other AB2 and A2B type compounds,”FaradayTrans.92,2753(1996).

[0077] T.C.Rich and D.A.Pinnow,“Total Optical Attenuation in Bulk FusedSilica,”Appl.Phys.Lett.20,264–266(1972).

[0078] G.A.Slack and D.W.Oliver,“Thermal Conductivity of Garnets and PhononScattering by Rare-Earth Ions,”Phys.Rev.B 4,592–609(1971).

[0079] R.Yasuhara,H.Nozawa,T.Yanagitani,S.Motokoshi,and J.Kawanaka,“Temperature dependence of thermo-optic effects of single-crystal and ceramicTGG,”Optics express 21,31443–31452(2013).

[0080] T.Toyoda and M.Yabe,“The temperature dependence of the refractiveindices of fused silica and crystal quartz,”Journal of Physics D:AppliedPhysics 16,L97-L100(1983).

[0081] D.L.Steinmetz,W.G.Phillips,M.Wirick,and F.F.Forbes,“A polarizer forthe vacuum ultraviolet,”Applied optics 6,1001–1004(1967).

[0082] Because quartz and fused silica have relatively moderate Wilderness constants compared to TGG, they are traditionally considered unsuitable for use in conventional Faraday isolators. However, the low absorption of quartz and fused silica at 1 μm wavelength, compared to TGG, can partially compensate for the increased propagation length in terms of power limitation due to depolarization. In the visible to ultraviolet (UV) spectral range, TGG is increasingly affected by absorption due to color centers, which in turn exacerbates thermal degradation. In these spectral ranges, alternative materials are needed as Faraday materials. Faraday isolators based on cerium(III) fluoride (CeF3) have been validated at wavelengths of 405 nm and 355 nm (see: EGVíllora, K. Shimamura, and GRPlaza, “Ultraviolet-visible optical isolators based on CeF3 Faraday rotator,” Journal of Applied Physics 117, 233101 (2015)). These Faraday isolators can achieve transmission at wavelengths as low as 300 nm (see: P. Molina, V. Vasyliev, EGVíllora, and K. Shimamura, “CeF3 and PrF3 as UV-visible Faraday rotators,” Optics express 19, 11786–11791 (2011)). For the UV range down to 200 nm, PrF3 has been proposed as a potential Faraday medium with a high Wilder constant (see: P. Molina, V. Vasyliev, E.G. Víllora, and K. Shimamura, “CeF3 and PrF3 as UV-visible Faraday rotators,” Optics express 19, 11786–11791 (2011)). On the other hand, materials like quartz and magnesium fluoride (MgF2) are readily available and exhibit similar transmission ranges down to approximately 200 nm. Due to their lower Wilder constants, the latter two materials can be used as Faraday media within Faraday isolators that employ multiple-pass media.In addition, MgF2 exhibits a thermo-optic coefficient that is an order of magnitude lower than that of TGG and quartz, which can reduce thermal effects (see: MJ Weber, Handbook of Optical Materials (CRC Press, 2018); and DLSteinmetz, WG Phillips, M. Wirick, and FF Forbes, “A polarizer for the vacuum ultraviolet,” Applied Optics 6, 1001–1004 (1967)).

[0083] In the following sections, we will discuss various aspects of using Faraday media with low Wilder constants and propose a method based on such materials having lower Wilder constants compared to TGG or other conventionally used Faraday media, enabling Faraday isolators to achieve sufficient polarization rotation angles. Furthermore, the applicability of this method to gases as Faraday media will be discussed, although gases typically exhibit even lower Wilder constants, this method may be beneficial when operating under higher average power conditions (see: L.R. Ingersoll and D.H. Liebenberg, “The Faraday Effect in Gases and Vapors I*,” J. Opt. Soc. Am. 44, 566 (1954); and L.R. Ingersoll and D.H. Liebenberg, “Faraday Effect in Gases and Vapors II*,” J. Opt. Soc. Am. 46, 538 (1956)).

[0084] To achieve a polarization rotation of approximately 45° in materials with lower Wilder constants compared to TGG (such as quartz and fused silica), a magnetic field with an exceptionally high flux density can be applied to the material, and / or the propagation length of the laser radiation in the Faraday medium can be significantly increased. The magnetic flux density provided by permanent neodymium iron boron magnets is typically around 1 T. Taking this as a practical limitation of the magnetic field, it is necessary to increase the propagation length to tens of centimeters or even several meters.

[0085] Using such a method presents the additional challenge of applying a magnetic field over such a long path. Optionally, the Faraday medium could be surrounded by a toroidal permanent magnet. Figure 2 As shown, simply increasing the length of the toroidal magnet to match the length of the medium will result in a decrease in the magnetic field inside the magnet, or even its near disappearance, due to the reversal of the magnetic field direction in the far field of the magnetic dipole.

[0086] Figure 2 The magnetic flux density B generated by the toroidal permanent magnet 300 is shown on the left. z A simulation of the qualitative distribution in the z-direction (i.e., along the central axis 302 of the permanent magnet 300), the annular permanent magnet 300 (shown as a cross-sectional view through a plane including the central axis) having a length of 20 mm along the central axis 302. The right side shows the magnetic flux density B in the z-direction generated by the annular permanent magnet 304 with a length of 80 mm. z The corresponding simulation.

[0087] To provide a suitable field strength for Faraday rotation, the diameter of a toroidal permanent magnet can be scaled up by an order of magnitude of its length. Permanent magnets of this size tend to be quite large, making them impractical for many optical devices. On the other hand, electromagnets (i.e., solenoids) can, in principle, be scaled up to the required length without sacrificing the internal magnetic field. However, a driving current of several amperes must be continuously supplied. Unless a sophisticated cooling mechanism is employed, the magnetic field strength is typically lower than that obtained using a permanent magnet. Furthermore, current fluctuations directly translate into fluctuations in the resulting polarization rotation. The polarization rotation mechanism required in Faraday rotators based on this conventional method converts these fluctuations into amplitude noise. Moreover, fluctuations in polarization rotation degrade the isolation performance of the Faraday isolator. This necessitates providing a sufficiently stable current when the device is sensitive to noise or small back reflections.

[0088] When considering fused silica as a medium, the required extended optical path length when using a Faraday medium with a low Wilder constant can be achieved using optical fiber. Within a multi-winded fused silica fiber, a magnetic field through a solenoid has been shown to produce several degrees of Faraday rotation (see: S. Asraf, Y. Sintov, and Z. Zalevsky, “Novel configuration for an enhanced and compact all-fiber Faraday rotator with matched birefringence,” Optics express 25, 18643–18655 (2017)). At this point, it is first necessary to compensate for birefringence caused by bending in the fiber to ensure that Faraday rotation still produces a constructive buildup after polarization reversal due to birefringence (see: GWDay, DNPayne, AJ Barlow, and J.J. Ramskov-Hansen, "Faraday rotation in coiled, monomode optical fibers: isolators, filters, and magnetic sensors," Optics Letters 7, 238 (1982)). Furthermore, a small amount of uncompensated birefringence often results in elliptical polarization, which limits the isolation performance of the Faraday isolator. However, the coupling into a single-mode fiber makes this method difficult to use with high-power free-space laser beams.

[0089] Therefore, this disclosure provides a method based on free-space optics for generating enhanced Faraday rotations in a medium having a low Wilder constant, potentially for applications such as... Figure 1The Faraday isolator 10 is shown. In this method, a Faraday medium 16 is placed within a multi-pass arrangement 18. By guiding the laser radiation 14 through the Faraday medium 18 several times, the effective optical path length of the laser radiation 14 within the Faraday medium 18 is increased to a desired amount. In the case of a solid Faraday medium, the length of the Faraday medium 16 itself is kept within a few centimeters or even millimeters. For a gas used as the Faraday medium 16, longer distances or more passes may be suitable. The desired number of passes can be achieved at a moderate level of complexity in the Herriott cell (see: Y. Wang, M. Nikodem, B. Brumfield, and G. Wysocki, “Compact multi-pass cell based Faraday rotation spectrometer for nitric oxide detection,” in 2012 Conference on Lasers and Electro-Optics (CLEO) (2012), pp. 1–2; H. Adams, D. Reinert, P. Kalkert, and W. Urban, “A differential detection scheme for Faraday rotation spectroscopy with a color center laser,” Appl. Phys. B 34, 179–185 (1984); and D. Herriott, H. Kogelnik, and R. Kompfner, “Off-Axis Paths in Spherical Mirror Interferometers,” Appl. Opt. 3, 523 (1964)). The multiple-pass method according to this disclosure not only reduces the length requirement of the Faraday medium 16, but also limits the area where a magnetic field must be applied in the same way. Therefore, a standard-sized permanent magnet ring system can be used, further simplifying the device. As a proof of the concept, we below demonstrate, using Heriot-Treant elements, that the Faraday isolator 10 can provide a polarization rotation of 22° within a 6.35 mm long fused silica plate (as an example of a low-Wilder constant Faraday medium 16). This verification was performed using a 532 nm collimated laser at the center band of the visible light range.

[0090] Faraday isolators can be like Figure 1The configuration is schematically presented as shown. A fused silica plate coated with an anti-reflective layer, 6.35 mm in length and 12.7 mm in diameter, serves as the Faraday medium 16 and is mounted inside a ring-shaped permanent magnet serving as a magnetic element 20, which provides a generally uniform magnetic field pointing in the direction normal to the plate surface. Special care is taken during installation to avoid any stress-induced birefringence within the fused silica plate. Even the slightest pressure on the plate can cause significant interference with the resulting polarization rotation. The magnet is an N52 type neodymium iron boron magnet. It has a length of 20 mm, an inner diameter of 20 mm, and an outer diameter of 40 mm. A magnetic flux density of approximately 0.3 T is measured inside the magnet. A multi-pass arrangement 18 of Heriot-type unitary optical transmission is constructed around the fused silica plate serving as the Faraday medium 16, allowing the laser radiation to propagate through the Faraday medium 16 42 times. This corresponds to an effective optical path length of 267 mm in the Faraday medium. The multiple-pass arrangement includes two 1-inch diameter concave spherical mirrors 32 and 34 with radii of curvature (ROC) of 150 mm and 100 mm, respectively. The first mirror 32 and the second mirror 34 are spaced approximately 250 mm apart along the central axis 100 of the multiple-pass arrangement 18 and are precisely adjusted to achieve collimation of the multiple-pass arrangement 18. Laser radiation 14 is coupled into the multiple-pass arrangement 18 via an aperture in the second mirror 34. The beam is focused into the multiple-pass arrangement 18 by another concave spherical mirror 26 with an ROC of 600 mm. Therefore, the ROC values ​​of mirrors 32, 34, and 26 are chosen to roughly match the eigenmode of the unit to the caustic characteristics of the incident laser radiation 14, which has a center wavelength of 532 nm and a mode radius of approximately 1.9 mm at its output. Pure linear polarization is ensured by placing a Glan-Taylor polarizer 22 in front of the multiple-pass arrangement. The laser power after the polarizer is approximately 0.6 mW. The laser radiation 14 exiting the multi-channel arrangement 18 is collimated by the reflector 26. The incident beam and the outgoing beam are separated by a gap of a few millimeters. The polarization of the outgoing laser radiation 14 is checked by an analytical polarizer 24, which can rotate around the propagation direction of the laser radiation 14. At this time, using a Wollaston polarizer as a second polarizer 34, or a secondary polarizer, makes it easy to obtain two beams with orthogonal polarization.

[0091] To measure the polarization produced after multiple passes through the beam arrangement 18, the analytical polarizer 24 was rotated around the beam axis to minimize the power of one of the beams separated by the polarizer 24. After 42 passes through the Faraday medium, a polarization rotation of 22° was observed. This polarization rotation can be definitively attributed to Faraday rotation when the magnetic field direction is reversed. As expected, the polarization rotates 22° in the opposite direction, thus ruling out other potential causes such as stress-induced birefringence. Furthermore, when the value of the Verdet constant of approximately 5 rad / (T·m) from the literature was used as the basis for calculation, the observed 22° rotation was very consistent with the expectation (see: J.L. Cruz, M.V. Andres, and M.A. Hernandez, “Faraday effect in standard optical fibers: dispersion of the effective Verdet constant,” Applied Optics 35, 922–927 (1996); and E. Munin, J.A. Oversi, and A.B. Villaverde, “Faraday effect and energy gap in optical materials,” Journal of Physics D: Applied Physics 25, 1635–1639 (1992)).

[0092] For high isolation performance of the Faraday isolator 10, the polarization of the rotated polarized laser radiation 14 must remain linear. Otherwise, the isolation performance may be degraded. We study the degree of linear polarization after the Faraday isolator 10 by measuring the power of the two orthogonally polarized beams of laser radiation 14 after the analytical polarizer 24. Therefore, the power in one of the beams is minimized again. Depolarization is characterized by the ratio γ of the depolarized power remaining in the minimized beam from the analytical polarizer to the total power of the two beams from the analytical polarizer 24:

[0093]

[0094] After a 22° polarization rotation, the depolarization ratio γ was measured to be between 1:90 and 1:40. When the magnet was removed, but the Faraday medium 16 remained in place, a depolarization ratio γ of less than 1:450 was measured, which was limited by the measurement sensitivity. These results indicate that the beam becomes elliptically polarized during the Faraday rotation, while maintaining perfect linear polarization when passing through a cell without a magnetic field.

[0095] This result clearly demonstrates how Faraday rotation within a Faraday medium 16 (such as fused silica) with a low Wilderness constant can be amplified within a multi-pass arrangement 18 (such as a Heriot-Lieutenant unit). Using a 6.35 mm thick fused silica plate with a simple AR coating as the solid Faraday medium 16, a rotation angle of 22° was produced at a wavelength of 532 nm. By doubling the thickness of the material, the 45° angle typically required for the Faraday isolator 10 can be easily achieved. The required thickness of the Faraday medium 16 can even be reduced when entering the ultraviolet spectral range, due to the increased Wilderness constant at shorter wavelengths for many materials used as the Faraday medium 16. In the UV-visible range, fused silica, quartz, and MgF2 are likely ideal candidates for the Faraday isolator 10 when used in combination with a simple Heriot-Lieutenant unit as a multi-pass arrangement 18. Further expansion may be advantageous when approaching wavelengths of 1 μm.Similar to existing Faraday isolators, using a stronger magnetic field via permanent magnets would provide a much stronger magnetic field of approximately 2 to 3 T (see: EAMironov, AV Voitovich, and OV Palashov, “Permanent-magnet Faradayisolator with the field intensity of more than 3 tesla,” Laser Phys. Lett. 17, 15001 (2020); I. Mukhin, A. Voitovich, O. Palashov, and E. Khazanov, “2.1 Teslap permanent-magnet Faraday isolator for subkilowatt average power lasers,” Optics Communications 282, 1969–1972 (2009); G. Trénec, W. Volondat, O. Cugat, and J. Vigué, “Permanent magnets for Faraday rotators inspired by the design of the magic sphere,” Applied Optics). 50, 4788–4797 (2011); and EAMironov, ILSnetkov, AVVoitovich, and OVPalashov, “Permanent-magnet Faraday isolator with the field intensity of 25kOe” Quantum Electron. 43, 740–743 (2013)), which should be sufficient to achieve the desired rotation at a wavelength of 1 μm in molten silica or quartz. Further expansion potential depends on the number of passes. For the Heriotte unit used as a multiple-pass arrangement 18, the number of passes is linearly related to the diameters of the mirrors 32 and 34 in the multiple-pass arrangement. Increasing the mirror diameters of the first mirror 32 and the second mirror 34 from 1 inch to 2 inches would imply another expansion factor of 2. In these ways, it seems promising to realize a Faraday isolator with crystalline quartz used as the Faraday medium 16 for high-power lasers at a wavelength of 1 μm. At high power, the thermal lensing effect occurring in the Faraday medium can be easily compensated by adjusting the distance between the first reflector 32 and the second reflector 34 in the multi-pass light arrangement 18. Extending further into the mid-infrared region, the Wilder constant decreases even more.The multiple-pass method using Heriott units can also be used to enable Faraday isolators at wavelengths of several micrometers, even with materials that have insufficient Wilder constants. For example, ZnSe, which transmits wavelengths up to 20 μm, has a Wilder constant of only about 8 rad / (T·m) at wavelengths of 2 μm (see: EAMironov, OV Palashov, ILSnetkov, and S.S. Balabanov, “ZnSe-based Faraday isolator for high-power mid-IR lasers,” Laser Phys. Lett. 17, 125801 (2020)). ZnSe-based Faraday isolators have proven technically challenging, involving extremely strong magnetic fields and the cascading of two Faraday rotators. The multiple-pass method using Heriott units reduces the need for high magnetic fields and the length of the ZnSe rod used as a solid Faraday element. Moreover, this method allows the use of ZnSe as a Faraday medium at longer wavelengths, even with even lower Wilder constants.

[0096] When using quartz as the Faraday medium, the birefringence of crystalline quartz must be carefully considered. However, c-cut quartz plates can be used, with the c-axis oriented parallel to the beam propagation direction. However, within Heriot-Tirius units, the angle of incidence on the plate always deviates from perpendicular incidence. This angle can be minimized by considering longer units to reduce the birefringence effect. Another approach is to use paired a-cut plates with orthogonal c-axises to compensate for birefringence. The same considerations apply to MgF2, as it exhibits similar birefringence.

[0097] As the average power of the laser radiation 14 passing through the Faraday isolator 10 increases, the eventual absorption and thermally induced birefringence may limit the isolator's isolation performance. Using a gas as the Faraday medium 16 could be a potential way to circumvent the typical limitations present in solids. Even though the very low Wilder constant (compared to TGG) becomes more challenging at this point, multiple passes within the multiple-pass arrangement 18 can help overcome this obstacle. As a gas, xenon still has a value of 18 x 10⁻⁶ at 500 nm. -3The relatively high Wilder constant is rad / (T·m·bar). To enhance the effect, the entire isolator can be constructed within the pressurized gas element (called the pressure element). Considering a pressure of 20 bar, this would result in a Wilder constant of approximately 0.35 rad / (T·m), still an order of magnitude lower than that of molten silica. This deficiency can be compensated for by utilizing a longer propagation length within the medium. Extending the length of the gas element makes it challenging to provide a magnetic field with sufficient flux density along the entire length of the pressure element. Results show that, as Figure 2 As shown, simply increasing the length of a given toroidal permanent magnet will weaken the magnetic field within the magnet. The root cause is likely the reversal of the magnetic field direction of the magnetic dipole, leading to a destructive field component inside the toroidal magnet. The magnetic field can be restored by increasing the outer diameter of the toroidal magnet. Figure 3 The image shows the magnetic field of a hypothetical magnet that is still within the actual dimensions. Figure 3 The left side shows a visualization of the simulated magnetic flux density in the z-direction of a magnet with a length of 200 mm. The right side shows the magnetic flux density in the z-coordinate and z-direction (i.e., along the central axis of the magnet) in a graph.

[0098] Here, the relevant parameter for Faraday rotation is the magnetic field integral along the beam path. For this magnet, the calculated value is 66 mT·m. With 20 bar xenon and 35 passes, a rotation angle of 45° is produced at a wavelength of 500 nm. In the UV range, even smaller magnets with weaker magnetic fields can be applied due to the increase in the Wilder constant. This clearly demonstrates the feasibility of the Faraday isolator 10 based on a gas as the Faraday medium 16.

[0099] In summary, we propose a method for using a material with a moderate Faraday effect (i.e., a low Wilder constant) as the Faraday medium 16 in the Faraday isolator 10. Due to the non-reciprocal nature of the Faraday effect, the multiple passages of incident laser radiation 14 through the Faraday medium 16 in a multi-pass arrangement 18 result in the accumulation of Faraday rotation angles for each passage. The required 45° rotation angle can, in principle, be achieved using materials unsuitable for conventional Faraday isolator schemes. Therefore, materials such as crystalline quartz, fused silica, or magnesium fluoride have potential applications in ultraviolet, mid-infrared, and high-power Faraday isolators 10. Even gases can be used, which may help avoid limiting effects typically found in solids, such as thermal lensing and / or stress-induced birefringence. In a proof-of-concept experiment, we demonstrated the applicability of this concept. With the aid of a Heriot-type multipass unit, the polarization of a 532 nm collimated laser was rotated by 22° in the fused silica plate. Further extensions can be made by increasing the number of passes, the magnetic field strength, or simply increasing the length of the Faraday medium 16.

[0100] List of reference numerals

[0101] 10 Faraday Isolators

[0102] 12 Polarization Rotation Device

[0103] 14. Laser radiation

[0104] 16 Faraday media

[0105] 18. Multiple light transmission arrangements

[0106] 20 Magnetic Components

[0107] 22 First Polarizer

[0108] 24 Second Polarizer

[0109] 26 Optical elements / mirrors

[0110] 28 laser sources

[0111] 30 Heriot Units

[0112] 32 First reflecting mirror

[0113] 34 Second reflecting mirror

[0114] 36 pressure units

[0115] The central axis of the arrangement with more than 100 light transmissions

[0116] 300 permanent magnet

[0117] 302 The central axis of the permanent magnet

[0118] 304 permanent magnet

Claims

1. A polarization rotation device (12) suitable for rotating the polarization direction of laser radiation (14), the polarization rotation device (12) comprising: Faraday medium (16); A multiple-pass arrangement (18) comprising or including Heriot-type elements (30) and having a Faraday medium (16) at least partially disposed within the multiple-pass arrangement (18), wherein the multiple-pass arrangement (18) is adapted such that the laser radiation (14), when coupled into the multiple-pass arrangement, performs multiple round trips within the multiple-pass arrangement and passes through the Faraday medium (16) at least ten times; A magnetic element (20) is adapted to provide a magnetic field at the location of the Faraday medium (16) within the multiple light transmission arrangement (18); The Faraday medium (16) has a thickness of 2 mm or greater, and the laser radiation (14) propagates through the thickness in each of the multiple passes.

2. The polarization rotation device (12) according to claim 1, wherein the Faraday medium (16) has a Wilder constant of 20 rad / (T·m) or less at the predetermined design wavelength of the polarization rotation device.

3. The polarization rotation device (12) according to any one of the preceding claims, wherein the Faraday medium (16) comprises a solid Faraday element (16).

4. The polarization rotation device (12) according to claim 3, wherein the polarization rotation device (12) is adapted such that, each time the laser radiation (14) passes through the solid Faraday element, it enters the Faraday medium at a surface of the solid Faraday element that is different from the surface where the laser radiation leaves the solid Faraday element.

5. The polarization rotation device (12) according to claim 3 or 4, wherein the solid Faraday element is composed of or includes at least one of the following materials: fused silica, quartz, MgF2, CaF2, Al2O3 (sapphire), YAG, Gd3Al5O 12 KBr, ZnS, ZnSe and ZnTe.

6. The polarization rotation device (12) according to any one of the preceding claims, wherein the Faraday medium (16) comprises a gaseous Faraday medium.

7. The polarization rotation device (12) according to claim 6, wherein the gaseous Faraday medium is composed of or includes one or more of the following gases: He, Ne, Ar, Kr, Xe, H2, D2, O2, N2, CO2 and CH4.

8. The polarization rotation device (12) according to claim 6 or 7 further includes a pressure unit (36) containing a gaseous Faraday medium, wherein the multiple light transmission arrangement (18) is at least partially arranged inside the pressure unit (36).

9. The polarization rotation device (12) according to any one of the preceding claims, wherein the magnetic element (20) comprises or is composed of a permanent magnet (300).

10. The polarization rotation device (12) according to claim 9, wherein the permanent magnet (300) is annular and adapted to provide a magnetic field along the central axis (302) of the annular permanent magnet (300), and wherein the annular permanent magnet (300) at least partially surrounds the Faraday medium (16) within the multiple light transmission arrangement (18).

11. The polarization rotation device (12) according to any one of the preceding claims further includes a first linear polarizer (22) arranged in front of the multiple light transmission arrangement (18) and / or a second linear polarizer (24) arranged after the multiple light transmission arrangement (18).

12. The polarization rotation device (12) according to any one of the preceding claims, wherein the Faraday medium (16) has 80% or higher optical transmission over the entire spectral range of 150 nm to 550 nm.

13. The polarization rotation device (12) according to any one of the preceding claims, wherein the polarization rotation device (12) is adapted to rotate the polarization direction of the laser radiation (14) by an angle in the range of 30° to 60° between the coupling input of the laser radiation (14) into the multiple light transmission arrangement (18) and the coupling output of the laser radiation (14) from the multiple light transmission arrangement (18).

14. The polarization rotation device (12) according to any one of the preceding claims, wherein the Faraday medium (16) comprises a plurality of solid Faraday elements and / or a mixture of different gases.

15. The polarization rotation device (12) according to any one of the preceding claims, wherein the Faraday medium (16) comprises at least one solid Faraday element and a gaseous Faraday medium (16).

16. A Faraday isolator (10) comprising a polarization rotation device (12) according to any one of the preceding claims.

17. The Faraday isolator (10) according to claim 16, wherein the Faraday isolator (10) is adapted to rotate the polarization direction of laser radiation by an angle in the range of 30° to 60°.

18. The Faraday isolator (10) according to claim 16, wherein the Faraday isolator (10) is adapted to rotate the polarization direction of the laser radiation by an angle of 45°.

19. The Faraday isolator (10) according to any one of claims 16 to 18, wherein the Faraday isolator (10) comprises a cascaded arrangement of a plurality of polarization rotation devices (12) according to any one of claims 1 to 15.

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