laser

Abstract

Broadly speaking, embodiments of the present technology provide laser devices. In particular, the present technology provides a laser device comprising a laser cavity, a nonlinear crystal for generating radiation converted from fundamental frequency radiation, and a first optical element configured to separate the fundamental frequency radiation from the converted radiation. The present technology also provides a laser device comprising a laser cavity and a photodiode located within the laser cavity and configured to ensure that the fundamental frequency radiation resonates in a single direction around the laser cavity. Advantageously, the present technology provides means for in-cavity frequency conversion that minimizes damage to optical components within the laser cavity.

Description

[[Technical Field]]

[0001] The present technology relates to a laser device including, for example, an intracavity second harmonic generation (SHG) laser and / or an ultraviolet laser. [[Background Art]]

[0002] A laser includes an array of mirrors or other optical elements forming a cavity resonator for light waves. The cavity resonator may also be referred to as an optical cavity, a resonant cavity, or an optical resonator. An example of an intracavity SHG laser device is described in U.S. Pat. No. 8,498,316. This device includes a laser cavity formed by first and second laser cavity end reflectors. An active medium and at least one nonlinear crystal for second harmonic generation are provided within the laser cavity. At least one of the laser cavity end reflectors comprises an interference arrangement that provides spectrally selective reflection for radiation near the fundamental laser frequency, such that the laser cavity limits the spectrum of laser radiation to a single longitudinal mode. [[Summary of the Invention]] [[Problem to be Solved by the Invention]]

[0003] Intracavity frequency conversion for generating second harmonics can achieve higher efficiency and lower complexity than devices employing external frequency conversion. In such devices, two wavelengths typically circulate within the cavity. In the wavelength range below 380 nm (i.e., the ultraviolet range), laser radiation may damage optical coatings and optical materials. The present applicant recognizes the need to minimize the interaction between ultraviolet light and the coatings and materials within the laser cavity (i.e., inside the cavity).

[0004] An example of a cavity resonator is a ring resonator, such as a bowtie ring resonator, which has four mirrors that define a closed path through which light rays circulate. In a ring resonator, light can circulate in two different directions; therefore, a photodiode may be used within the cavity to ensure that light circulates in only one direction. Photodiodes are sometimes referred to as optical isolators. The applicant recognizes the need for in-cavity optical isolators for use in lasers that generate output wavelengths in the range of less than 650 nm, including, for example, lasers that operate in the ultraviolet spectrum below 400 nm. [Means for solving the problem]

[0005] The present invention provides the apparatus described in the appended claims. Other features of the present invention will become apparent from the dependent claims and the following description.

[0006] According to a first aspect of this technology, a laser apparatus for in-cavity frequency conversion is provided, which converts radiation having a fundamental laser frequency (ω) to converted radiation having a second harmonic frequency (2ω) of the fundamental frequency (ω). The apparatus (sometimes referred to as the laser) comprises a laser cavity formed by a plurality of reflectors that are reflective to radiation of the fundamental laser frequency; a nonlinear crystal located within the laser cavity, the nonlinear crystal being for transmitting radiation of the fundamental frequency and generating converted radiation having a second harmonic frequency, the wavelength of which is in the ultraviolet region; and a first optical element positioned to receive radiation of the fundamental frequency and converted radiation having a second harmonic frequency from the nonlinear crystal, and configured to separate the radiation of the fundamental frequency from the converted radiation having a second harmonic frequency. The fundamental frequency may be the frequency generated by the laser crystal and may sometimes be referred to as the first frequency.

[0007] As used herein, the terms “for” mean “suitable for” or “for” a functional limitation. For example, “a laser device for in-cavity frequency conversion” means “a laser device suitable for in-cavity frequency conversion.” Similarly, “a plurality of reflectors reflective to laser fundamental frequency radiation” means “a plurality of reflectors having the function of reflecting laser fundamental frequency radiation.”

[0008] As used herein, the term "ultraviolet" refers to wavelengths or frequencies in the ultraviolet range of the electromagnetic spectrum. Specifically, wavelengths typically range from 10 nm to 400 nm, and frequencies typically range from 750 THz to 30,000 THz.

[0009] As used herein, the term “optical element” means an optical component or optical element formed from any suitable optical material. For example, an optical element may be a crystal formed from fused silica, ultraviolet fused silica, calcium fluoride, or magnesium fluoride.

[0010] As used herein, the term “nonlinear crystal” means an optical component or optical element formed from any suitable optically nonlinear material. For example, a nonlinear crystal may be a crystal formed from lithium triborate, barium betaborate, or lithium cesium borate.

[0011] As used herein, the term “optical contact” means a contact between opposing surfaces of different optical components that creates an interface between them, minimizing or reducing (i.e., a seamless transition) any loss of radiation propagating across this interface due to reflection, scattering, and / or distortion. It will be understood that any combination of optical elements, nonlinear crystals, and / or any other optical components can be in optical contact. When two or more components are joined together via optical contact, a monolithic structure is formed. Optical contact can be achieved in several different ways, such as the example described in R. Paschotta, “Optical Contact - an encyclopedia article,” RP Photonics Encyclopaedia (2023), the entirety of which is incorporated herein by reference. For example, optical contact can be achieved by an adhesive-free process in which two closely matched surfaces are joined together by intermolecular forces, such as van der Waals forces, which arise when, for example, precisely polished surfaces are joined together. In alternative examples, optical contact can be achieved, for example, by adhesive-free chemical bonding, hydroxide catalysts, or bonding using adhesive supports, enabling seamless, reflection-free, and light transmission across the interface. When using hydroxide catalysts or similar techniques, a thin layer of silicate or bonding / adhesive material will exist between the adjacent pair of surfaces, potentially resulting in some reflection loss. However, this connection still allows for seamless transitions across the interface and still minimizes losses due to reflection / scattering / strain. It will be understood that optical contact can be achieved using any suitable technique.

[0012] Radiation in the ultraviolet spectrum has higher photon energies, and when this radiation passes through some materials, the materials can heat up as they absorb the radiation. This can damage materials, especially any coatings. By separating the fundamental frequency radiation from the converted radiation with second harmonic frequencies, it is possible to minimize the number of components through which the second harmonic radiation passes, thereby reducing damage to components within the laser. Second harmonic wavelengths are less than 400 nm, more specifically less than 380 nm and greater than 100 nm.

[0013] The second harmonic wavelength can be generated using any suitable technique. For example, second harmonic generation can be achieved using a type I critical phase matching, which means that the fundamental wavelength is orthogonally polarized with respect to the second harmonic wavelength. Alternatively, a noncritical type I phase matching can be used. However, noncritical type I usually requires the crystal to be heated, and for this reason it is more complex and less frequently used compared to type I critical phase matching.

[0014] The first optical element is spaced apart from the nonlinear crystal, with an air-filled gap between the two components. Alternatively, the first optical element may be in optical contact with the nonlinear crystal (i.e., form an optical contact bond) to form a monolithic component. Optical contact is typically achieved by a glueless process in which two closely matched surfaces are joined together by intermolecular forces, such as van der Waals forces, which occur when precisely polished surfaces are joined together, for example. Alternatively, optical contact can be achieved by adhesive-free chemical bonding, a hydroxide catalyst, or bonding using an adhesive support, for example, achieving seamless, reflection-free, light transmission at the interface. When using a hydroxide catalyst or similar technique, a thin layer of silicate may be present between the adjacent pair of surfaces, which may result in some reflection loss. However, this connection still allows for seamless transitions across the interface. In any configuration, the first optical element may have a first surface from which radiation from a nonlinear crystal is incident, and a second surface from which radiation of the fundamental frequency is emitted. In other words, the second surface may be considered to be an opposite or opposite surface of the first surface of the first optical element.

[0015] Both the first and second surfaces of the first optical element can be aligned at the Brewster angle with respect to the radiation of the fundamental frequency. In this case, it will be understood that the first and second surfaces can be parallel to each other. The Brewster angle, also called the polarization angle, is the angle of incidence at which light of a specific frequency with a specific polarization (in this case, p-polarization) is completely transmitted through the crystal without reflection. In other words, the p-polarized radiation of the fundamental frequency passes through the first surface of the optical element, through the optical element, and exits through the second surface without reflection. The Brewster angle will depend on the wavelength of the radiation and, furthermore, on the properties of the interface through which the radiation passes.

[0016] When there is a space between the first optical element and the nonlinear crystal, the first optical element may have a coating on its first surface, the first surface being the surface to which radiation from the nonlinear crystal is incident, and the coating is non-reflective to fundamental frequency radiation and reflective to converted radiation having second harmonic frequencies. The second surface may not be coated. In this way, further exposure of the coating to ultraviolet light can be further reduced. The shape of the first optical element may be generally planar. In this case, the first optical element may be called a pick-off mirror because it significantly reflects second harmonic light and transmits fundamental frequency light.

[0017] When the first optical element is in optical contact with a nonlinear crystal, all or part of the first surface of the optical element may be in contact with the nonlinear crystal to form an optical contact area. In either configuration, the degree of optical contact is such that the first optical element receives radiation at the fundamental frequency and converted radiation having a second harmonic frequency. When forming the optical contact area, adjacent surfaces (typically polished) are in good contact so that radiation can pass through and reflection loss at the optical contact area (i.e., the interface between the optical element and the nonlinear crystal) is minimized. The first optical element may have a coating on the second surface, which is non-reflective to radiation at the fundamental frequency and reflective to converted radiation having a second harmonic frequency.

[0018] As described above, the fundamental frequency radiation enters the optical element through its first surface, which can optically contact the nonlinear crystal, and exits the optical element through the second surface. The converted radiation having the second harmonic frequency also enters through the first surface, but is reflected by at least a portion of the second surface such that the converted radiation exits the optical element through a different location than or at a different point from where the fundamental frequency radiation exits the optical element. In other words, the fundamental frequency radiation and the converted radiation exit the optical element in non-coaxial directions. For example, the converted radiation may be reflected at the second surface such that it does not pass through the second surface when it exits the first optical element, but instead exits the first optical element through a different surface. Alternatively, the converted radiation may be reflected at the second surface and then reflected back towards the second surface by, for example, a portion of the first surface having a reflective coating, such that the converted radiation exits through a different location on the second surface than where the fundamental frequency radiation exits the optical element.

[0019] The converted radiation having the second harmonic frequency can be reflected so as to exit the first optical element through a side surface. This side surface can be aligned at a Brewster angle with respect to the radiation of the second harmonic frequency. Such a configuration may be particularly suitable when the entire first surface of the optical element is in optical contact with the nonlinear crystal, for the reason that otherwise the converted radiation having the second harmonic frequency would pass through the nonlinear crystal and return; however, it will be understood that the optical element may be used in other configurations. When the entire first surface of the optical element is in optical contact with the nonlinear crystal, the first optical element can be considered an end cap for the nonlinear crystal. The first optical element may also be wedge-shaped.

[0020] If only a portion of the first surface of the optical element optically contacts the nonlinear crystal, the converted radiation having a second harmonic frequency can be reflected and returned through the first surface of the optical element without entering the nonlinear crystal. In other words, a first portion of the first surface of the optical element may optically contact the nonlinear crystal, and a second portion of the first surface of the optical element may extend beyond the nonlinear crystal, and the converted radiation having a second harmonic frequency may be reflected so as to exit the first optical element through the second portion of the first surface. In other words, the converted radiation can exit the first optical element through a different portion of the same surface from which the converted radiation entered the first optical element. In this configuration, the first optical element may also be considered an end cap of the nonlinear crystal, but the second portion of the first optical element is not in contact with the nonlinear crystal. The first optical element may also be wedge-shaped.

[0021] A coating applied to either the first surface of the first optical element and / or the second surface of the first optical element may have high reflectivity to the second harmonic frequency, in other words, more than 98% of the radiation may be reflected. Similarly, an anti-reflective coating may be defined as a coating, meaning that less than 0.2% is reflected. The anti-reflectiveness of the coating may be more important because the fundamental frequency is amplified within the laser device, and therefore any decrease in laser power becomes significantly nonlinear with respect to the loss associated with the fundamental frequency. The radiation at the second harmonic frequency (i.e., ultraviolet radiation) is not amplified within the laser device, and therefore no loss at the second harmonic frequency is amplified.

[0022] The laser device may further include a second optical element, which is positioned to receive reflected radiation having a second harmonic frequency from a first optical element and is configured to reflect radiation having a second harmonic frequency toward the output section of the laser device. In this case, the second optical element may be considered a steering mirror. The shape of the second optical element may be generally planar.

[0023] The laser apparatus may include, within the laser cavity, any of the following standard components: an active medium (also called a gain medium), a tuning means for selecting the fundamental frequency within the cavity, such as an etalon, and an optical oven that can be used to efficiently control the temperature of optical elements such as the etalon. A feedback mechanism may also be present for adjusting the length of the optical path, for example, using a piezoelectric element or an electro-optic modulator.

[0024] Multiple reflectors can form a ring resonator, more specifically, a bowtie-ring resonator having four reflectors. The apparatus may further include a photodiode configured to ensure that radiation of the fundamental frequency resonates around the ring resonator with unidirectional oscillations (i.e., in a single direction). The photodiode may include a magneto-optical element having a first surface and a second surface located on the opposite side; a waveplate having a first surface and a second surface located on the opposite side, wherein the first surface of the waveplate faces the second surface of the magneto-optical element; and a magnet configured to generate a magnetic field within the magneto-optical element. The magneto-optical element is configured such that, when exposed to a magnetic field, it rotates the polarization direction of the fundamental frequency radiation incident on the first surface of the magneto-optical element by a first rotation, and rotates the polarization direction of the fundamental frequency radiation incident on the second surface of the magneto-optical element by a mirror rotation of the first rotation, wherein the mirror rotation of the first rotation is different from the first rotation. The waveplate is configured such that it rotates the polarization direction of the fundamental frequency radiation transmitted through the waveplate in either direction by a second rotation, wherein the second rotation is in the opposite direction to the first rotation and has the same magnitude. The mirror rotation of the first rotation is equal to the first rotation but in the opposite direction.

[0025] Therefore, when radiation of the fundamental frequency is incident on the first surface of the magneto-optical element, its polarization direction is rotated by a first rotation as it travels through the magneto-optical element and exits at the second surface of the magneto-optical element. In other words, the first rotation corresponds to the rotation of the polarization direction of the fundamental frequency radiation exiting the magneto-optical element at the opposite second surface, relative to the polarization direction of the fundamental frequency radiation incident on the first surface. Here, the first rotation can also be understood as a continuous rotation of the polarization direction occurring between the first surface and the opposite second surface within the magneto-optical element. The first surface of the waveplate faces and / or contacts the second surface of the magneto-optical element, and therefore, radiation exiting the magneto-optical element through the second surface is incident on the first surface of the waveplate, and its polarization direction is rotated by a second rotation equal to but opposite to the first rotation. Similar to the first rotation, the second rotation corresponds to a rotation of the polarization direction of the fundamental frequency radiation emitted from the waveplate on the opposite side of the second surface, relative to the polarization direction of the fundamental frequency radiation incident on the first surface. Here, the second rotation can also be understood as a continuous rotation of the polarization direction occurring between the first surface and the opposite side of the second surface within the waveplate. In other words, the rotation added by the waveplate cancels out the rotation added by the magneto-optical element.

[0026] Thus, there are two rotations applied to radiation: the first one (first rotation) applied by the magneto-optical element and the subsequent rotation (second rotation) applied by the wave plate. When radiation travels in the opposite direction, the radiation first enters the wave plate and is rotated by the second rotation. That is, in contrast to the magneto-optical element, the rotation of the polarization direction applied by the wave plate does not depend on the direction of transmission of radiation passing through the wave plate. In other words, the second rotation also corresponds to the rotation of the polarization direction of fundamental frequency radiation emitted from the wave plate at the first surface with respect to the polarization direction of fundamental frequency radiation incident on the second surface disposed on the opposite side. The rotated radiation then enters the second surface of the magneto-optical element and is subjected to the mirror-image rotation of the first rotation. Similar to the first rotation, the mirror-image rotation of the first rotation corresponds to the rotation of the polarization direction of fundamental frequency radiation emitted from the magneto-optical element at the first surface with respect to the polarization direction of fundamental frequency radiation incident on the second surface. Therefore, in this case, two rotations are still applied to the radiation, but in this configuration, the first rotation is applied by the wave plate, and the subsequent rotation is applied by the magneto-optical element.

[0027] The optical diode can be used independently of second harmonic generation, and therefore according to the second aspect, there is provided a laser device comprising: a laser cavity in the form of a ring resonator formed by four reflectors having reflectivity for radiation at the laser fundamental frequency; and an optical diode configured to ensure that radiation at the fundamental frequency resonates in a single direction around the ring resonator. The optical diode may have the features described above. Further details described below may be applied to the combination of the optical diode in an SHG laser device, or may be applied to separate cases.

[0028] Unidirectional oscillation is achieved by two rotations applied separately by a magneto-optical element and a wave plate. As explained above, radiation of the fundamental frequency with proper polarization that first passes through the magneto-optical element is rotated by approximately 4 to 5 degrees along its length, and then rotated by the same amount in the opposite direction by the wave plate, such that the rotation of the wave plate cancels out the rotation of the magneto-optical element. In effect, radiation traveling in this direction is not lost by the optical diode. In contrast, radiation that first passes through the wave plate is first rotated by the second rotation (i.e., 4 to 5 degrees), and then when passing through the magneto-optical element, it is further rotated by the mirror-image rotation of the first rotation (i.e., rotated by 4 to 5 degrees in the same direction as the second rotation). Since these rotations do not cancel each other out, radiation passing in the incorrect direction, that is, radiation that passes through the wave plate first and then the magneto-optical element, has a different polarization, resulting in reflection loss in use at each component in the laser, such as a reflector, a nonlinear crystal, and / or an optical element.

[0029] Each of the first rotation, the mirror-image rotation of the first rotation, and the second rotation may be a rotation of about 3 to 5 degrees, more specifically 4 to 5 degrees. Thus, the required amount of rotation is relatively small. The cumulative rotation when radiation travels in the incorrect direction may be 8 to 10 degrees, which may bring about reflection loss effective to prevent the laser from oscillating in this direction when making one round trip through the device.

[0030] A magneto-optical element is an optical element that exhibits birefringence properties when exposed to a magnetic field. A preferred example is a Faraday crystal. Suitable materials include potassium terbium fluoride (KTF) or terbium gallium garnet (TGG). The magnet may be a permanent magnet, or an electromagnet having a radial magnetic field distribution, that is, a ring magnet.

[0031] The wave plate may be made of crystalline quartz. Crystalline quartz has known birefringence and optical activity, which can be utilized separately in different configurations to achieve the desired rotation.

[0032] Magneto-optical elements may be in optical contact with or coupled to the waveplate, or they may be spaced apart from the waveplate. When magneto-optical elements are coupled to or in optical contact with the waveplate, monolithic optical components exist. As described above, optical contacts can be formed by precisely polishing the surfaces. As an alternative to or in addition to optical contacts, connections can be formed by using a bonding material applied to one or both adjacent surfaces. For example, by using hydroxide-catalyzed bonding or silicate solutions, it is possible to form a thin silicate layer (i.e., a true covalent bond) between components that connects the surfaces together.

[0033] Each of the first and second surfaces of the magneto-optical element may be aligned at a Brewster angle with respect to the fundamental frequency radiation (which is p-polarized as described above). Similar to the first and second surfaces of the first optical element described above, it will be understood that the first and second surfaces of the magneto-optical element may be parallel to each other when aligned at a Brewster angle. This ensures that the fundamental frequency radiation is transmitted through the magneto-optical element without reflection. Alternatively, each of the first and second surfaces may be planar; that is, each of the first and second surfaces of the magneto-optical element may be aligned perpendicular to the axis of the magneto-optical element. In either case, the axis of the magneto-optical element may be aligned with the intended direction of radiation propagation around the ring resonator. In the first configuration, the fundamental frequency radiation is refracted upon contact with the first and second surfaces. However, because the surfaces are aligned at a Brewster angle, the radiation is not reflected by the first and second surfaces. In the second configuration, each of the first and second surfaces is coated to ensure proper refraction through the magneto-optical element, in which case the refractive index of the coated magneto-optical element is such that radiation of a fundamental frequency parallel to the axis of the magneto-optical element is not refracted when passing through the magneto-optical element. The coating may also be non-reflective with respect to the fundamental frequency in order to promote transmission to the magneto-optical element. Thus, there are two possible options for the magneto-optical element: wedge-shaped surfaces without coating, or flat surfaces with coatings.

[0034] Similarly, there are two configurations for the waveplate. In one configuration, each of the first and second faces of the waveplate can be aligned perpendicular to the axis of the waveplate (i.e., with flat surfaces facing each other). In other words, the waveplate can be cut such that the optical axis (c-axis) lies in the plane of the waveplate surface. The waveplate can be supported in a mount, and a second rotation can be achieved using the birefringence properties of the waveplate material. In this case, the waveplate can be rotated in the mount to a fixed setting such that the rotation applied by the waveplate cancels out the rotation applied by the magneto-optical element when the radiation is traveling in the correct direction. In the second configuration, each face of the waveplate can be cut at the Brewster angle. The waveplate has a thickness to achieve the second rotation. The waveplate can be any suitable shape, e.g., circular or rectangular.

[0035] The waveplate may be coated with an anti-reflective coating, in which case the refractive index of the coated waveplate is such that radiation of fundamental frequencies orthogonal to each of the first and second surfaces is not refracted as it passes through the magneto-optical element. This coating is particularly applicable when using a waveplate that is cut such that the optical axis (c-axis) lies in the plane of the waveplate surface.

[0036] In all the configurations described above, the fundamental frequency may have a wavelength of 650 nm or less. The fundamental frequency can therefore be in the green, red, blue, or possibly ultraviolet region.

[0037] To better understand the present invention and to show how its embodiments can be realized, references to the following appended schematic drawings are made hereby, merely as examples. [Brief explanation of the drawing]

[0038] [Figure 1] This is a diagram showing the components of a laser and the beam path within the optical cavity. [Figure 2a] This figure shows further details of one of the second-harmonic generators used in the laser shown in Figure 1. [Figure 2b]This is a schematic diagram showing details of the alternative second-harmonic generator used in the laser in Figure 1. [Figure 2c] This is a schematic diagram showing details of another alternative second-harmonic generator used in the laser in Figure 1. [Figure 3] Two perspective views of an optical diode that may be used in the laser shown in Figure 1 to provide unidirectional oscillation in the direction of the dashed line. [Figure 4] Two perspective views of an optical diode that may be used in the laser shown in Figure 1 to provide unidirectional oscillation in the direction of the dashed line. [Figure 5a] Figure 3 shows a half-wave plate used in a photodiode. [Figure 5b] Figure 5a is a side view of the half-wave plate, showing how radiation passes through it. [Figure 6a] This figure shows the components of the mount used to support the half-wave plate in Figure 5a. [Figure 6b] This figure shows the components of the mount used to support the half-wave plate in Figure 5a. [Figure 7a] Figure 3 is a perspective view of a first Faraday crystal that may be used in a photodiode. [Figure 7b] Figure 7a is a perspective view of a mount for a Faraday crystal. [Figure 7c] This is a diagram of an alternative Faraday crystal that may be used. [Figure 7d] This is a diagram of an alternative Faraday crystal that may be used. [Figure 8a] This figure shows the beam paths through three different configurations of the Faraday crystal and waveplate used in the photodiode shown in Figure 3. [Figure 8b] This figure shows the beam paths through three different configurations of the Faraday crystal and waveplate used in the photodiode shown in Figure 3. [Figure 8c] This figure shows the beam paths through three different configurations of the Faraday crystal and waveplate used in the photodiode shown in Figure 3. [Figure 8d]This figure shows the beam paths through three different configurations of the Faraday crystal and waveplate used in the photodiode shown in Figure 3. [Figure 9a] This figure plots the refractive index against wavelength when incident on a KTF crystal for light that is p-plane polarized or s-plane polarized, respectively. [Figure 9b] This figure plots the reflectance against the angle of incidence for light that is p-plane polarized or s-plane polarized, respectively. [Modes for carrying out the invention]

[0039] Figure 1 shows a laser with a laser cavity in the form of a bowtie ring resonator. The ring resonator is formed by four reflectors 104a, 104b, 104c, and 104d, which have high reflectivity to radiation (or light; these terms are used interchangeably) at the laser fundamental frequency ω. Since the reflectivity of the mirrors in the cavity is nearly 100% near the laser fundamental frequency, the cavity resonates and increases the power of the circulating laser at frequency ω. Each reflector 104a, 104b, 104c, and 104d has a mirror held within a mount. The active (laser gain) medium 106 is held within a mount in the cavity. The active medium 106 can be any suitable solid laser material, e.g., a neodymium-doped laser crystal or a praseodymium-doped crystal. Laser light is output from the laser device through an output unit 150.

[0040] In this example, the laser is a cavity-type second harmonic generation (SHG) laser. In this case, the laser further comprises an SHG nonlinear crystal 140 that generates second harmonic light, i.e., light of frequency 2ω. As will be described in more detail below, some of the laser's fundamental frequency light is converted to second harmonic frequency light, and for this reason the radiation of the second harmonic frequency is sometimes referred to as converted radiation. The generated (i.e., converted) light is led to a first optical element 142 that separates the fundamental wavelength from the second harmonic wavelength by reflecting the second harmonic wavelength (ultraviolet light in this example). In other words, the radiation of the fundamental wavelength is spatially separated from the radiation of the second harmonic wavelength. Spatial separation is achieved by the reflection of the radiation of the second harmonic wavelength, and as a result of this reflection, the radiation of the fundamental wavelength and the radiation of the second harmonic wavelength follow spatially separated paths. As shown in Figure 1, the first optical element 142 is positioned within the laser cavity to receive light at the fundamental frequency of the laser. In other words, the first optical element 142 is positioned along the beam path of the laser cavity. The reflected light is directed to the second optical element 144, which then directs the second harmonic light to the output unit 150. In other words, power at the second harmonic frequency 2ω is output from the laser.

[0041] An etalon 130 may be present in the beam path of the ring resonator. The etalon 130 may be an optical cavity through which only light waves resonating with the optical cavity can pass. In other words, the etalon functions to tune the laser to a desired fundamental frequency. The temperature of the etalon 130 is carefully controlled, and for this reason, the etalon 130 is sometimes referred to as an etalon oven.

[0042] As illustrated by the dotted line, leakage 204 of radiation at the fundamental wavelength may exist. This leaked radiation 204 can be reflected to an optional feedback mechanism using a reflector 110, particularly when using an etalon 130 with a high-finesse broadband coating in which the fundamental frequency is locked using fringe locking. The leaked radiation 204 is then split using a beam splitter 136 to generate two separate signals, each of which is received separately by a photodiode 134. The first signal is an intensity reference signal, and the second signal is spectrally selected by a second etalon 132. These signals can be used to measure optical leakage. One of the reflectors, the first reflector 104a in this example, can be attached to a piezoelectric element 214, which can be used to change the optical path length in the laser cavity based on the measured optical leakage. As an alternative to the use of a piezoelectric element, a birefringent optical element (such as an electro-optic modulator) may be used.

[0043] Figure 1 shows the beam path within a ring resonator. The ring resonator has a traveling wave that requires unidirectional operation. Accordingly, the photodiode 120 is preferably placed within the cavity (i.e., intraracavity). The photodiode 120 prevents laser emission in one direction but allows laser emission in other directions. The photodiode will be described in more detail below.

[0044] Light from the active medium 106 enters the first reflector 104a, which reflects the incident light of fundamental frequency ω through the photodiode 120 toward the second reflector 104b. The second reflector reflects the light of fundamental frequency ω such that it passes through the SHG crystal 140 and generates a second harmonic wavelength. At this point, two wavelengths, namely the fundamental wavelength 200 shown in red and the second harmonic wavelength 202 shown in blue, are circulating within the cavity. The second harmonic wavelength 202 will be examined in more detail below. The fundamental wavelength 200 enters the third reflector 104c and is reflected toward the fourth reflector 104d. As the wave travels between the third and fourth reflectors, it passes through the etalon oven 130. The fourth reflector 104d then reflects the wave toward the first reflector 104a. As a result, the first, second, third, and fourth reflectors form a bowtie resonator, in which the beam path of light at the fundamental frequency resembles a bowtie.

[0045] It will be understood that a bowtie resonator can be used even without an SHG crystal. Similarly, it will be understood that this is only one of the appropriate configurations of the resonator. In this configuration, all components of the photodiode are placed in the diagonal arms of the bowtie resonator, and the SHG crystal is in one of the horizontal arms (horizontal as seen in Figure 1). However, the components of the photodiode may be divided into multiple different arms; for example, the Faraday crystal may be in the diagonal arms (as in Figure 1), but the waveplate may be behind a second reflector in the horizontal arm (for example, in the arm where the SHG crystal is shown in Figure 1). In general, the goal is to place the crystal near the center of the arm to minimize beam divergence through the crystal. When the components are divided into two different arms, only a reflector can exist between the waveplate and the Faraday crystal. In other words, other components of the laser device, such as the SHG crystal, should not be between the components of the photodiode.

[0046] Figure 2a shows further details of the components of one configuration for generating the second harmonic. The SHG crystal 140 is supported within the mount 340. The SHG crystal 140 is a generally wedge-shaped nonlinear crystal. The SHG crystal 140 can be formed from any suitable nonlinear optical material, for example, lithium triborate (LBO). Alternatively, the SHG crystal 140 may be formed from barium betaborate (BBO) or lithium cesium borate (CLBO). The front surface 352 of the crystal 140 is cut at the Brewster angle with respect to p-polarized fundamental frequency light. The Brewster angle, also called the polarization angle, is the angle of incidence at which light with a specific polarization (in this case, p-polarization) is completely transmitted through the crystal without reflection. Brewster angle θ B It is defined as follows:

number

[0047] SHG crystals also generate second-harmonic light (i.e., light with twice the frequency and half the wavelength). Second-harmonic generation is a nonlinear optical process in which two photons of the same frequency interact with a nonlinear material to generate a new photon with twice the energy. In one example, second-harmonic light is produced as a single type. In other words, two photons that are normally polarized relative to the crystal combine to form a single photon with twice the frequency and anomalous polarization. In other words, the second harmonic has s-polarization, while the fundamental wave has the opposite polarization.

[0048] As shown in Figure 2a, the fundamental wavelength 200, shown in red, and the second harmonic wavelength 202, shown in blue, both emanate from the SHG crystal 140. Both wavelengths are incident on a first optical element 142, which is in the form of a rectangular parallelepiped and supported by a support or mount 342. The first optical element 142 has a first face or front surface to which the fundamental and second harmonic wavelengths are incident. This first face is set to a Brewster angle with respect to the p-polarized fundamental frequency, so that the fundamental frequency light passes through the optical element 142 without reflection loss and emanates through a second face or rear surface. The first face has a coating that is highly reflective at the second harmonic wavelength so that the light of the second harmonic wavelength is reflected. The coating is non-reflective with respect to the fundamental wavelength, so that the fundamental wavelength can pass through the optical element. The second face does not have a coating, but the second face may also be set to a Brewster angle to minimize any reflection loss of the fundamental wavelength light as it travels through the optical element. Returning to Figure 1, the light traveling through the first optical element at its fundamental frequency is incident on one of the four reflectors of the ring resonator. The first optical element 142 is typically formed from any suitable optical material, such as fused silica or UV fused silica (UVFS). Alternatively, the first optical element may be formed from calcium fluoride (CaF2) or magnesium fluoride (MgF2). It will be understood that any suitable optical material can be used to form the first optical element.

[0049] As shown in Figure 2a, light of the second harmonic wavelength is reflected onto the second optical element 144. The second optical element 144 may have the same shape as the first optical element 142 and may be mounted within a similar support 344. The second optical element 144 reflects light of the second harmonic wavelength towards the output. The second optical element 144 may be referred to as a steering mirror for the second harmonic wavelength. Theoretically, there is no fundamental light at this point because the first optical element 142 has transmitted it all to the reflector. Similar to the first optical element 142 described above, the second optical element 144 may be formed from any suitable optical material such as UVFS, CaF2, or MgF2.

[0050] When the fundamental wavelength is in the red spectrum (i.e., between 630 nm and 700 nm) or at the lower end of the infrared spectrum (i.e., between 700 nm and 800 nm), the second harmonic wavelength is usually in the ultraviolet spectrum (i.e., less than 400 nm). It will be understood that if the fundamental wavelength is short, for example less than 650 nm, the wavelength of the second harmonic will also be short. Shorter wavelengths, such as those in the ultraviolet spectrum, have higher photon energy, and these wavelengths can heat some materials as they are absorbed by those materials when passing through them. In particular, this can destroy coatings. The second harmonic wavelength is separated from the fundamental wavelength using a first optical element. Both the first and second optical elements have only a single coating, thus minimizing heating of the coating within the laser. It should also be noted that when using one type of second harmonic generation, the fundamental wavelength and the second harmonic wavelength have orthogonal polarization, which helps in beam separation.

[0051] As an alternative to the use of the first and second optical elements, an output coupler may be used that is reflective to the fundamental wavelength and transmits the second harmonic wavelength. However, in this case, the second harmonic wavelength will pass through the output coupler and may be absorbed by the material of the output coupler, including the coating on one or both surfaces. Alternatively, this may lead to heating of the material and / or coating.

[0052] Figure 2b shows an alternative configuration for generating the second harmonic. In this configuration, the SHG crystal 240 and the first optical element 242 form a monolithic structure. As in the previous configuration, the SHG crystal 240 is a generally wedge-shaped nonlinear crystal. The SHG crystal 240 and the first optical element 242 can similarly be formed from any of the materials described above in relation to Figure 2a. The front surface 252 of the crystal 240 is cut at the Brewster angle with respect to p-polarized fundamental frequency light. The rear surface 254 of the crystal 240 is also cut at the Brewster angle to minimize any reflection loss of fundamental wavelength light. That is, the front surface 252 and the rear surface 254 can be parallel to each other when cut at the Brewster angle. In both cases, the Brewster angle is calculated with respect to the fundamental frequency. At the first surface 252, the interface is between air and the crystal material, and at the second surface 254, the interface is between the crystal material and the optical element material.

[0053] As shown in Figure 2b, the fundamental wavelength, shown in red, enters the SHG crystal 240 through the first surface 252 and exits the SHG crystal through the second surface 254. When light is incident on the first surface of the SHG crystal 240, it is refracted. It should be understood that refraction usually occurs when radiation passes from one component to another. When the refractive indices of the materials are similar, the angle is very small and can be approximated by a straight line. The SHG crystal has a higher refractive index than its adjacent components, which is why the angle of refraction is visible in Figure 2b. The first surface 252 is cut at the Brewster angle for p-polarized radiation of the fundamental wavelength, so no reflection or reflection loss occurs. The SHG crystal 240 also generates second harmonic wavelengths, which also exit the SHG crystal through the second surface 254. The second surface is also cut at the Brewster angle, so no reflection loss occurs there. The first optical element 242 has a first surface or front surface 244 to which the fundamental wavelength and second harmonic wavelength are incident.

[0054] In this example, the first surface 244 and the second surface 254 of the SHG crystal are in optical contact. Furthermore, the entire first surface 244 of the first optical element 242 is in optical contact with the nonlinear crystal 240, forming a monolithic component. A connection or bond is formed between the SHG crystal 240 and the first optical element 242 using any suitable mechanism. For example, this may be by glueless optical contact, such as intermolecular forces acting across the first surface 244 and the second surface 254, or by using any suitable bonding process, such as hydroxide catalyst technology, which enables seamless transmission of light across the first surface 244 and the second surface 254. The optical contact of the SHG crystal 240 with the first optical element 242 ensures that reflection loss at the interface between the SHG crystal 240 and the first optical element 242 is minimized. In other words, any light propagating across the optical interface between the crystal 240 and the optical element 242 is not affected by the interface.

[0055] When precisely polished surfaces are joined together, attractive van der Waals forces induce bonding between them. Precision may be defined by a standard specification. For example, this specification typically includes one or more of the following: surface quality greater than scratch / dig (S / D) = 10 / 5, surface flatness less than λ / 10 (633 nm / 10, i.e., less than 63 nm in surface rms), and surface roughness less than 5 angstroms. These forces are usually sufficient to maintain the connection between each of the adjacent surfaces. In addition to or as an alternative to optical contacts, the connection can be formed by using a bonding material applied to one or both of the adjacent surfaces. For example, by using hydroxide-catalyzed bonding or silicate solutions, it is possible to form a thin silicate layer (i.e., a true covalent bond) between the constituent surfaces that connects them.

[0056] The first surface 244 is set to a Brewster angle with respect to the p-polarized fundamental frequency, so that the fundamental frequency light is not reflected but passes through the optical element 242 and exits through a second surface or rear surface 246, which is also set to a Brewster angle. In this case, the first surface 244 and the second surface 246 can be parallel to each other when set to a Brewster angle (i.e., aligned). The Brewster angle of the first surface 244 is calculated for the interface between the nonlinear crystalline material and the optical element material, which may be, for example, fused silica. The Brewster angle of the second surface 246 is calculated for the interface between the optical element material and air.

[0057] Light at the second harmonic frequency also passes through the optical element. The second surface 246 of the optical element has a coating applied to its outer surface (i.e., the surface in contact with the air). The coating is non-reflective with respect to the fundamental wavelength, so that the fundamental wavelength passes through the second surface 246 without any change in direction. The coating is highly reflective at the second harmonic wavelength (i.e., more than 98% is reflected), so that the light at the second harmonic wavelength is reflected and does not exit the optical element through the second surface 246, but instead exits the optical element through a third surface or side surface 248. This side surface 248 is cut at a Brewster angle with respect to the second harmonic wavelength, based on the interface between the material of the optical element and the air.

[0058] Figure 2c shows another alternative configuration for generating the second harmonic. The features related to the SHG crystal 240 described in relation to Figure 2b above also apply to the example shown in Figure 2c, but are not repeated for brevity. In addition, light of the second harmonic frequency also passes through the optical element, and the second surface 246 of the optical element has a coating applied to its outer surface. However, as shown in Figure 2b, the entire first surface 244 of the first optical element 242 does not optically contact the nonlinear crystal 240, but only the first portion 244-1 of the first surface 244 of the optical element optically contacts the SHG crystal 240.

[0059] In this configuration, light of the second harmonic frequency is reflected and exits the optical element through a second portion 244-2 of the first surface that extends beyond the SHG crystal and does not contact the SHG crystal. In other words, the second harmonic wavelength exits the first optical element through a different portion of the same surface that it passed through when incident on the optical element. The first surface 244 and the second surface 246 are also set to their corresponding Brewster angles, as described above in relation to Figure 2b. The only difference from the configuration in Figure 2b is that the first surface 244 of the first optical element is not adjacent to the corresponding surface of the SHG crystal 240. While the configuration in Figure 2c allows light of the second harmonic frequency to be reflected back through the first surface of the optical element, it will be understood that this configuration can also be adapted to reflect the second harmonic frequency through the side, as described in Figure 2b. Furthermore, this configuration can be adapted to reflect the second harmonic frequency so that the light of the fundamental frequency returns through a different location on the second surface than where the fundamental frequency light is emitted from the optical element.

[0060] Returning to Figure 1, the fundamental frequency light traveling through the first optical element is incident on one of the four reflectors of the ring resonator. The second harmonic wavelength light can then be directed towards the exit, for example, using a second optical element or other suitable mechanism.

[0061] In this configuration, as in the first configuration, the second harmonic wavelengths are separated from the fundamental wavelength using the first optical element. In contrast, in this example, the second harmonic wavelengths pass through the first optical element. This can reduce the possibility of ultraviolet light interacting with air (particularly oxygen), which may catalyze some of the degradation processes.

[0062] As shown in Figure 1, a photodiode may be used in a ring resonator to ensure that the fundamental frequency light resonates in a single direction around the cavity. Figures 3 and 4 show further details of a photodiode comprising two main components: a Faraday crystal (also called a magneto-optical element, these terms can be used interchangeably) mounted in a first retainer 410 and a half-wave plate (also called a wave plate or plate rotor, these terms can be used interchangeably) mounted in a second retainer 420. As shown in Figure 1, the fundamental frequency light traveling in the correct direction around the cavity first passes through the Faraday crystal and then through the wave plate. The wave plate, sometimes called a phaser, is an optical device that changes the polarization state of the light wave passing through it. More specifically, the half-wave plate shifts the polarization direction of linearly polarized light.

[0063] Examples of half-wave plates 610 are shown in Figures 5a and 5b. Half-wave plates 610 are birefringent, and therefore their refractive index depends on the polarization and propagation direction of light incident on them. In other words, half-wave plates can be considered to be either optically anisotropic or birefringent. Waveplates 610 are also non-reciprocal optical elements. In other words, light propagating in both directions through waveplates 610 has the same polarization rotation. Waveplates may be made of a suitable material, such as crystalline quartz or calcite, and have a thickness t.

[0064] Waveplates can be designed using any of the following properties, which are described in detail for crystalline silica:

[0065] Plate rotor - optical activity: In this case, the quartz crystal is cut so that the beam propagation is parallel to the optical axis of the crystal. Optical activity occurs spontaneously in some materials such as quartz, where it is possible to define optical rotation (ρ) as described by Radhakrishnan in "The dispersion, birefringence and optical activity of quartz," published in January 1947. This specifies the rotation per millimeter of propagation through the material in degrees. For example, at a wavelength of 640 nm, this is approximately 18.251 degrees / mm, and therefore, to rotate by 4 degrees, the thickness t of the quartz should be 0.2191 mm.

number

[0066] Waveplate-birefringence properties: In this case, the quartz crystal is cut so that the beam propagation is parallel to the optical axis of the crystal. Light polarized along the optical axis (leading axis) travels faster than light polarized perpendicular to the optical axis (lagging axis), and thus the birefringence properties of quartz create a relative phase lag between the polarization components along the leading and lagging axes. The half-wave plate gives a phase lag of half the wavelength (or π lag), and therefore a linearly polarized input remains linearly polarized when it exits the wave plate. The thickness d of the wave plate is proportional to the birefringence at a particular wavelength λ, and M is an integer specifying the order.

number

[0067] Therefore, a fifth-order half-wave plate for 640 nm made of quartz has a thickness of 0.389 mm. It should be noted that in order to achieve a 4-degree rotation, the waveplate is rotated such that the incoming linearly polarized light is separated by 2 degrees (or half the required rotation) with respect to the phase-advancing axis of the waveplate, while the propagation direction remains perpendicular to the optical axis.

[0068] Returning to Figures 3 and 4, the half-wave plate is mounted in a generally circular mount 430, which fits within a corresponding generally circular opening in the support 432. As shown in Figures 6a and 6b, the mount comprises a central body 632 having an outer ring 630 and an outer rim 634 that fits within and is secured to the outer ring 630. The central body 632 also has a second portion 636 that protrudes from the outer rim 632. As shown in Figure 3, the outer ring 630 protrudes from the rear surface of the support 432. As shown in Figure 4, the second portion 636 of the central body fits within the opening in the support 432.

[0069] Mount 430 may be rotatable so as to be able to generate a phase difference using the birefringence properties of the waveplate material. Alternatively, the waveplate may be cut as a plate rotor, utilizing the concept of optical activity to impart rotation.

[0070] Faraday crystals are crystals that exhibit birefringence when exposed to a magnetic field. For this reason, Faraday crystals are also called magneto-optical elements, and these terms can be used interchangeably. Suitable materials for the crystal include potassium terbium fluoride (KTF) or terbium gallium garnet (TGG). Faraday crystals are reciprocal elements, and therefore any rotation applied to radiation passing through a Faraday crystal depends on the magnetic field and the direction of radiation propagation. Thus, radiation incident on a first face of a Faraday crystal and subsequently transmitted through it may undergo a first rotation, which is different from the second rotation applied to radiation incident on a second face of the Faraday crystal and subsequently transmitted through it.

[0071] In the first configuration shown in Figure 7a, the Faraday crystal 710 is cylindrical in shape and has planar end faces 712, 714 perpendicular to the major axis 720 of the crystal. The Faraday crystal is mounted in a crystal mount 750, which is shown in more detail in Figure 7b. The crystal mount 750 has a cylindrical (or rod-shaped) cavity for receiving the crystal and a circular flange 752 around which a ring magnet 450 is attached. The ring magnet 450 is used to expose the crystal to a magnetic field.

[0072] In the second configuration of Figure 7c, the Faraday crystal 730 is generally cylindrical in shape, but has angled end faces 732 and 734 that both take the Brewster angle, and the end faces 732 and 734 may be parallel to each other. In both Figure 7a and Figure 7b, the length of the crystal is close to the length of the diameter (e.g., a ratio of 4:3 or 3.5:3). In Figure 7d, the Faraday crystal 740 is an elongated version of the crystal in Figure 7c. Therefore, in Figure 7d, the end faces are also angled at the Brewster angle, but the length of the crystal is approximately twice the diameter (e.g., a ratio of 8:3).

[0073] Depending on the shape of the crystal and / or the orientation of the waveplate, as well as any coating applied to part or all of the surface of the crystal and / or waveplate, various configurations of photodiodes exist. A first configuration, as shown in Figure 8a, uses a crystal 810 such as the crystals shown in Figures 7c and 7d. In this case, the crystal 810 is cut or wedge-shaped so that the first surface takes a Brewster angle, and is not coated. The waveplate 820 is also not coated. The waveplate 820 is a generally flat disc of a certain thickness and is set at a Brewster angle. Light 812 of a frequency corresponding to the Brewster angle is not reflected by the crystal. As explained above, refraction occurs due to the different materials, and the angle of refraction is exaggerated to show that radiation travels through the crystal in a direction not parallel to the crystal axis. The direction of light 814 emanating from the crystal is parallel to the direction of light 812 entering the crystal. Similarly, light 814 incident on the waveplate 820 is refracted through the waveplate and emanates in a parallel direction 816. In this configuration, both the Faraday and waveplates are Brewster cut, and therefore the beam propagates without reflection loss, although the Brewster angles may differ due to the different materials. Air is present between the two components.

[0074] In Figure 8b, the same crystal 810 as in Figure 8a is used. However, in this configuration, the waveplate 830 is coated with an anti-reflective coating, and the birefringence properties of the waveplate 830 material are used. In this configuration, the waveplate 830 can be set perpendicular to the direction of the light 814 emanating from the crystal. If the optical path is perpendicular to the incident plane of the waveplate, the light passes through the waveplate without changing its direction 836. There is no refraction of light passing through the waveplate. In this configuration, the Faraday crystal is Brewster cut, and the waveplate is coated on both sides with an anti-reflective coating for the fundamental wavelength to minimize reflection loss.

[0075] In Figure 8c, the same waveplate 830 as in Figure 8b is used. However, in this configuration, the crystal 840 is also coated and has an end face perpendicular to its principal axis, as shown in Figure 7a. Light 812 with a frequency corresponding to the refractive index of the coated crystal, having an optical path perpendicular to the incident plane of crystal 830, passes through the crystal without changing its direction 844. This unchanging light then enters the waveplate and passes through the waveplate without changing its direction 836. There is no refraction of light passing through either the waveplate or the crystal. In this configuration, both the Faraday crystal and the waveplate have anti-reflective coatings on both surfaces.

[0076] As an alternative to the configuration in which the crystal and waveplate are spaced apart, as shown in Figure 8a, it is possible to create a monolithic photodiode by coupling the exit surface of the crystal 810 to the incident surface of the waveplate 820 (or vice versa, depending on the direction of propagation). This configuration is shown in Figure 8d. Each surface that is coupled together should not be coated. When the two optical surfaces are in optical contact in this way, they become one material, and therefore there is no reflection loss between them. Thus such a configuration can be beneficial. Light 812 with a frequency corresponding to the refractive index of the crystal-waveplate combination passes through without changing direction 846. In this configuration, both the Faraday crystal and the waveplate are wedge-shaped, and therefore any beam incident on any surface experiences Brewster reflection. This potentially requires that the waveplate has non-parallel surfaces, i.e., the refraction between the Faraday crystal and the waveplate is different from the refraction between the waveplate and air. Therefore, the second surface of the Faraday crystal is in optical contact with the first surface of the waveplate; in other words, refraction is gradual and there is no refractive loss.

[0077] The options shown in Figures 8a to 8d illustrate two different configurations of the waveplate. In the first configuration shown in Figures 8b and 8c, the waveplate is cut such that the optical axis (c-axis) lies in the plane of the waveplate surface. In this configuration, a roughly circular waveplate (as shown in Figure 5a) may be used, and the waveplate may be rotated to match the phase difference of the Faraday crystal. For this purpose, the waveplate is held in a circular mount as shown in Figure 6a. In this configuration, the birefringence properties of the crystal are used to achieve the desired rotation (or phase difference).

[0078] In the second configuration shown in Figures 8a and 8d, the waveplate can be called a Brewster-cut waveplate. In this case, the waveplate is roughly rectangular and cut at a certain angle, for example, as illustrated in Figure 5b. The thickness is calculated to achieve a rotation that matches the rotation imparted by the Faraday crystal. The use of a rectangular waveplate facilitates alignment, but it should be noted that the rectangular waveplate may be rotated as described above. Furthermore, as shown in the configuration of Figure 8d, the waveplate can be in optical contact with the Faraday crystal, with the sides parallel to each other. In this case, there is no rotation of the waveplate, and therefore the degrees of freedom / flexibility are lower, but the design is more compact. This configuration utilizes the optical activity properties of the waveplate, which are well known for quartz, as described in Radhakrishnan's "The dispersion, birefringence and optical activity of quartz," published in January 1947. Thus, the two different configurations can utilize two different properties of the waveplate.

[0079] In each option shown in Figures 8a to 8d, it is important to cut the crystal material in different ways. The crystal has different optical axes, sometimes referred to as a-cut and c-cut. Each configuration requires the refractive index of the material at a given wavelength. KTF crystals are known in the telecommunications industry but are typically used in the near-infrared and infrared ranges (e.g., 980–1550 nm). Similarly, TGG has so far been used in the range of 650 nm–1500 nm. Therefore, data below these wavelengths was not available for this crystal.

[0080] Figure 9a plots the refractive index of a crystal formed from KTF against wavelength. Laser light of three different wavelengths was passed through, and the refractive index was measured. With three points, it is possible to estimate the change in refractive index for all wavelengths. This change was plotted for both p-plane and s-plane polarization so that the changes in the normal and anomalous refractive indices across the visible wavelength spectrum could be understood.

[0081] Figure 9b plots the Fresnel curves of the selected material. The Fresnel curve plots the reflectance at various angles of incidence. The reflectance was measured at three different wavelengths, for both p-plane and s-plane polarization at each wavelength, resulting in a total of six plots. Each plot gives one refractive index estimate plotted in Figure 9b. The refractive index values ​​are calculated or estimated from line fitting.

[0082] The data from Figures 9a and 9b can be used to calculate the Brewster angle and further to design anti-reflective coatings. Anti-reflective coatings can have a reflectivity of less than 0.02%. If the reflectivity and incident intensity are known, the absorptivity of the crystal can be measured from the difference between the incident power and the reflection loss. For example, the absorptivity of a crystal fabricated from TGG was measured to be approximately 2% over 10 mm at a wavelength of 650 nm. The absorptivity of a crystal fabricated from KGF was measured to be less than 0.1% over 10 mm at a wavelength of 650 nm.

[0083] As mentioned above, crystals exhibit birefringence properties when exposed to a magnetic field. For example, as described in U.S. Patent No. 10,120,213, KTF or TGG crystals are known to be used optically as optical isolators or in conjunction with polarizers. A KTF crystal adds a 45-degree rotation to polarized light incident on it in the forward direction. In the reverse direction, light reflected from behind is polarized again and rotated by another 45 degrees as it passes through the crystal. In this way, a full 90-degree rotation is added by the Faraday rotator, and the light can be completely attenuated.

[0084] Considering the beam path shown in Figure 2, in this case, when an electric field is applied to the Faraday crystal, the polarized light incident on the crystal from the first reflector 104a is rotated by a first rotation of several degrees, for example, 3 to 5 degrees. The rotated light then enters the waveplate, which cancels out the rotation by adding a second rotation, and as a result the light passes through the photodiode without any rotation. If the light passes through the photodiode 120 in an incorrect direction, for example, from the second reflector 104b to the first reflector 104a, the light is rotated by a second rotation by the waveplate (for example, 3 to 5 degrees) and further rotated by a mirror rotation of the first rotation of several degrees (for example, 3 to 5 degrees) by the Faraday crystal. In other words, there is a total rotation of about 8 to 10 degrees. Within the cavity, there are several optical surfaces, including the reflector, specific coatings on the optical elements, and optical elements cut at or set at the Brewster angle. This relatively small rotation results in reflection losses that suppress the laser from oscillating in this direction during one revolution, because the laser is an optical amplifier, and therefore small reflection losses are amplified into enormous losses of output power. In other words, only small losses are sufficient to suppress bidirectional laser emission and for the photodiode to function as it generates unidirectional oscillation in the ring resonator.

[0085] While various combinations of optional features have been described in this specification, it should be understood that the described features can be combined in any suitable combination. In particular, features of any one exemplary embodiment may be combined with features of any other embodiment as appropriate, unless such combinations are mutually exclusive. Throughout this specification, the terms “comprising” or “comprise” mean including the specified components, but do not exclude the existence of other components.

[0086] Attention is drawn to all documents and literature filed in connection with this Application, either concurrently with or prior to this Specification, and made available to the public together with this Specification, and the contents of all such documents and literature are incorporated herein by reference.

[0087] All features disclosed herein (including all of the attached claims, abstract, and drawings), and / or all steps of any similarly disclosed methods or processes, may be combined in any combination except any combination in which at least some of such features and / or steps are mutually exclusive. Each feature disclosed herein (including all of the attached claims, abstract, and drawings) may be replaced by an alternative feature that serves the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each disclosed feature is merely an example of a comprehensive set of equivalent or similar features.

[0088] The present invention is not limited to the details of the embodiments described above. The present invention extends to any novel one or any novel combination of features disclosed herein (including all of the appended claims, abstract, and drawings), or to any novel one or any novel combination of any method or process step disclosed herein. While several preferred embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes and modifications can be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A laser apparatus for in-cavity frequency conversion from radiation having a laser fundamental frequency (ω) to converted radiation having the second harmonic frequency (2ω) of the fundamental frequency (ω), A laser cavity formed by a plurality of reflectors that are reflective to the radiation of the laser fundamental frequency, A nonlinear crystal located within the laser cavity, for transmitting the fundamental frequency of the laser and for generating the converted radiation having the second harmonic frequency, wherein the second harmonic wavelength of the nonlinear crystal is in the ultraviolet region. A laser device comprising: a first optical element arranged to receive the radiation of the fundamental frequency and the converted radiation having the second harmonic frequency from the nonlinear crystal, and configured to separate the radiation of the fundamental frequency from the converted radiation having the second harmonic frequency.

2. The laser apparatus according to claim 1, wherein the first optical element is arranged at a distance from the nonlinear crystal, the first optical element has a coating on a first surface, the first surface is the surface to which radiation from the nonlinear crystal is incident, the coating is non-reflective to the radiation of the fundamental frequency and reflective to the converted radiation having the second harmonic frequency.

3. The laser apparatus according to claim 1, wherein the first surface of the first optical element is in optical contact with the nonlinear crystal, and the first surface is the surface to which radiation from the nonlinear crystal is incident.

4. The laser apparatus according to any one of claims 1 to 3, wherein the first optical element has a second surface through which the radiation of the fundamental frequency passes when it is emitted from the first optical element, and the second surface is uncoated.

5. The laser apparatus according to claim 4, wherein the first optical element has a coating on at least a portion of the second surface, the coating being non-reflective to radiation of the fundamental frequency and reflective to the converted radiation having the second harmonic frequency.

6. The laser apparatus according to claim 5, wherein the converted radiation having the second harmonic frequency and the radiation of the fundamental frequency are emitted from the first optical element at different locations.

7. The laser apparatus according to claim 6, wherein the converted radiation having the second harmonic frequency is emitted from the first optical element through a side surface aligned at a Brewster angle with respect to the radiation of the second harmonic frequency.

8. The laser device according to claim 6, wherein a first portion of the first surface of the optical element is in optical contact with the nonlinear crystal, a second portion of the first surface of the optical element extends beyond the nonlinear crystal, and the converted radiation having the second harmonic frequency is emitted from the first optical element through the second portion of the first surface.

9. The laser apparatus according to any one of claims 4 to 8, wherein both the first surface and the second surface of the first optical element are aligned at a Brewster angle with respect to the radiation of the fundamental frequency.

10. The laser apparatus according to any one of claims 1 to 9, wherein the second harmonic wavelength is less than 380 nm.

11. The plurality of reflectors form a ring resonator, and the apparatus further comprises a photodiode configured to ensure that the radiation of the fundamental frequency resonates in a single direction around the ring resonator, the photodiode is A magneto-optical element having a first surface and a second surface located on the opposite side, A waveplate having a first surface and a second surface located on the opposite side, wherein the first surface of the waveplate faces the second surface of the magneto-optical element, The magneto-optical element comprises a magnet configured to generate a magnetic field within the magneto-optical element, and the magneto-optical element is The polarization direction of the radiation of the fundamental frequency incident on the first surface of the magneto-optical element is rotated by the first rotation. The magneto-optical element is configured to rotate the polarization direction of the radiation of the fundamental frequency incident on the second surface by a mirror rotation of the first rotation. The mirror image rotation of the first rotation is in a different direction from the first rotation. The laser apparatus according to any one of claims 1 to 10, wherein the waveplate is configured to rotate the polarization direction of the radiation of the fundamental frequency transmitted through the waveplate in any direction by a second rotation, the second rotation being in the opposite direction to the first rotation and having the same magnitude as the first rotation.

12. A laser cavity in the form of a ring resonator formed by four reflectors that are reflective to the radiation of the laser fundamental frequency, The laser cavity comprises a photodiode, which is located within the laser cavity and configured to ensure that the radiation of the fundamental frequency resonates in a single direction around the ring resonator, wherein the photodiode is A magneto-optical element having a first surface and a second surface located on the opposite side, A waveplate having a first surface and a second surface located on the opposite side, wherein the first surface of the waveplate faces the second surface of the magneto-optical element, The magneto-optical element comprises a magnet configured to generate a magnetic field within the magneto-optical element, and the magneto-optical element is The polarization direction of the radiation of the fundamental frequency incident on the first surface of the magneto-optical element is rotated by the first rotation. The magneto-optical element is configured to rotate the polarization direction of the radiation of the fundamental frequency incident on the second surface by a mirror rotation of the first rotation. The mirror image rotation of the first rotation is in a different direction from the first rotation. A laser device wherein the waveplate is configured to rotate the polarization direction of the radiation of the fundamental frequency transmitted through the waveplate in any direction by a second rotation, the second rotation being in the opposite direction to the first rotation and having the same magnitude as the first rotation.

13. The laser apparatus according to claim 11 or claim 12, wherein each of the first rotation, the mirror rotation of the first rotation, and the second rotation is a rotation of approximately 4 to 5 degrees.

14. The laser apparatus according to any one of claims 11 to 13, wherein the magneto-optical element is a Faraday crystal made from terbium potassium fluoride (KTF).

15. The laser apparatus according to any one of claims 11 to 14, wherein the waveplate is made from crystalline silica.

16. The laser apparatus according to any one of claims 11 to 15, wherein the magneto-optical element is coupled to the waveplate or is in optical contact with the waveplate.

17. The laser apparatus according to any one of claims 11 to 16, wherein each of the first surface and the second surface of the magneto-optical element is aligned at a Brewster angle with respect to the radiation of the fundamental frequency.

18. The laser apparatus according to any one of claims 11 to 16, wherein each of the first surface and the second surface of the magneto-optical element is aligned perpendicular to the axis of the magneto-optical element, each of the first surface and the second surface is coated with an anti-reflective coating, and the refractive index of the coated magneto-optical element is such that radiation of the fundamental frequency parallel to the axis of the magneto-optical element is not refracted when it passes through the magneto-optical element.

19. The laser apparatus according to claim 17 or 18, wherein each of the first and second surfaces of the waveplate is aligned perpendicular to the axis of the waveplate.

20. The laser apparatus according to claim 19, wherein the waveplate is rotatable within the mount, and the waveplate is configured to rotate the polarization direction of the radiation of the fundamental frequency by the second rotation.

21. The laser apparatus according to claim 19 or 20, wherein the waveplate is coated with an anti-reflective coating, and the refractive index of the coated waveplate is such that radiation of the fundamental frequency orthogonal to each of the first and second surfaces is not refracted when it passes through the magneto-optical element.

22. The laser apparatus according to claims 11 to 18, wherein each of the first and second surfaces of the waveplate is aligned at a Brewster angle with respect to the radiation of the fundamental frequency.

23. The laser apparatus according to any one of claims 1 to 22, wherein the fundamental frequency has a wavelength of 650 nm or less.

Images