Intracavity second harmonic generation for UV laser emission

Abstract

Generally speaking, embodiments of the present technology provide a laser device. Specifically, the present technology provides a laser device including a laser cavity, a nonlinear crystal for generating converted radiation from radiation on a fundamental frequency, and a first optical element configured to separate the radiation on the fundamental frequency from the converted radiation. The technology also provides a laser device including a laser cavity and an optical diode within the laser cavity and configured to ensure that radiation on a fundamental frequency resonates in a single direction around a ring resonator. Advantageously, the present technology provides a means of intra-cavity frequency conversion that minimizes damage to optical components within the laser cavity.

Description

Technical Field

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

[0002] A laser includes an arrangement of mirrors or other optical elements that forms a cavity resonator for light waves. A cavity resonator may also be referred to as an optical cavity, resonant cavity, or optical resonator. An example of an intracavity SHG laser device is described in US8498316. This device includes a laser cavity formed by a first and a second laser cavity-end reflector. An active medium for second harmonic generation and at least one nonlinear crystal are disposed within the laser cavity. At least one laser cavity-end reflector includes an interference arrangement that provides spectrally selective reflection of radiation near the laser fundamental frequency, such that the laser cavity confines the spectrum of laser radiation to a single longitudinal mode.

[0003] Intracavity frequency conversion to generate the second harmonic offers high efficiency and lower complexity than external frequency conversion. In such devices, two wavelengths typically cycle within the cavity. In the wavelength range below 380 nm (i.e., in the ultraviolet range), laser radiation can damage optical coatings and materials. The applicant has recognized the need to minimize the interaction between ultraviolet light and the coatings and materials within the laser cavity.

[0004] An example of a cavity resonator is a ring resonator, such as a bow-shaped ring resonator comprising four mirrors that define a closed path in which the light beam circulates. In a ring resonator, light can circulate in two different directions, so an optical diode can be used within the cavity to ensure that the light circulates in only one direction. An optical diode can also be referred to as an optical isolator. The applicant has recognized the need for intracavity optical isolators in lasers that produce output wavelengths in the range below 650 nm, including lasers operating in the ultraviolet spectrum (e.g., below 400 nm). Summary of the Invention

[0005] According to the present invention, an apparatus as set forth in the appended claims is provided. Other features of the invention will become apparent from the dependent claims and the following description.

[0006] According to a first aspect of the present technology, a laser device is provided for transmitting laser signals having a laser fundamental frequency (…). The frequency within the radiation cavity is converted to a fundamental frequency ( The second harmonic frequency (2) The device (which may also be referred to as a laser) comprises: a laser cavity formed by a plurality of reflectors that are reflective of radiation at the fundamental frequency of the laser; a nonlinear crystal in the laser cavity, wherein the nonlinear crystal is used to transmit radiation at the fundamental frequency and generate converted radiation having a second harmonic frequency, wherein the second harmonic wavelength is ultraviolet light; and a first optical element positioned to receive the radiation at the fundamental frequency and the converted radiation having the second harmonic frequency from the nonlinear crystal, and configured to separate the radiation at the fundamental frequency from the converted radiation having the second harmonic frequency. The fundamental frequency may be a frequency generated by the laser crystal and may also be referred to as a first frequency.

[0007] The term “for” as used herein means “suitable for” or is used as a word to specify a functional limitation. For example, “laser device for intracavity frequency conversion” means “laser device suitable for intracavity frequency conversion.” Similarly, “multiple reflectors that are reflective of radiation at the laser fundamental frequency” means “multiple reflectors whose function is to reflect radiation at the laser fundamental frequency.”

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

[0009] As used herein, the term "optical element" refers to an optical assembly or component formed from any suitable optical material. For example, an optical element can be a crystal formed from fused silica, ultraviolet fused silica, calcium fluoride, or magnesium fluoride.

[0010] As used herein, the term "nonlinear crystal" refers to any optical component or optical assembly form made of a suitable optical nonlinear material. For example, a nonlinear crystal can be a crystal formed from lithium triborate, barium β-borate, or lithium cesium borate.

[0011] As used herein, the term "optical contact" refers to the contact between mating surfaces of different optical components, such contact minimizing or mitigating losses of any radiation propagating through the interface due to reflection, scattering, and / or distortion (i.e., seamless transition). It will be understood that any combination of optical components, nonlinear crystals, and / or any other optical components can be an optical contact. When two or more components are joined together via an optical contact, a monolithic structure is formed. Optical contacts can be achieved in a variety of different ways, as illustrated in R. Paschotta's "Optical Contact - an encyclopedia article," RP Photonics Encyclopaedia (2023), the entire contents of which are incorporated herein by reference. For example, optical contacts can be achieved through a glue-free process, in which two closely conformal surfaces are joined together, for example, by intermolecular forces (e.g., van der Waals forces) generated when precisely polished surfaces come into contact. As an alternative example, optical contacts can be achieved using, for example, adhesive-free chemical bonding, hydroxide-catalyzed bonding, or adhesive-supported bonding, which provides seamless-reflection-free transmission of light at the mating interface. When using hydroxide catalysis or similar techniques, a thin layer of silicate or bonding / adhesive material will exist between adjacent surface pairs, potentially causing some reflection loss. However, the connection still enables a seamless transition across the interface and minimizes reflection / scattering / distortion losses. It will be understood that any suitable technique can be used to achieve optical contact.

[0012] Radiation in the ultraviolet spectrum has high photon energy, and when this radiation passes through certain materials, these materials can heat up as they absorb the radiation. This can damage the materials, especially any coating. By separating the radiation at the fundamental frequency from the converted radiation with the second harmonic frequency, the amount of radiation at the second harmonic frequency that transmits through the components can be minimized, thereby reducing damage to the components within the laser. The second harmonic wavelength can be below 400 nm, more specifically, below 380 nm and above 100 nm.

[0013] Second harmonic wavelengths can be generated using any suitable technique. For example, second harmonic generation can be achieved using Class 1 critical phase matching, which means that the fundamental wavelength is orthogonally polarized to the second harmonic wavelength. Alternatively, non-critical Class 1 phase matching can be used. However, non-critical Class 1 typically requires heating the crystal, and is therefore more complex and used at lower frequencies compared to Class 1 critical phase matching.

[0014] The first optical element can be separated from the nonlinear crystal, thereby creating an air-filled gap between the two elements. Alternatively, the first optical element can be optically contacted with the nonlinear crystal (i.e., forming an optical contact bond) to form a monolithic assembly. Optical contact is typically achieved through a glue-free process, in which two closely conformal surfaces are joined together, for example, by intermolecular forces (e.g., van der Waals forces) generated when precisely polished surfaces come into contact. Alternatively, optical contact can be achieved using, for example, adhesive-free chemical bonding, hydroxide catalysis, or adhesive-supported bonding, which provides seamless (reflection-free) transmission of light at the mating interface. When using hydroxide catalysis or similar techniques, a thin silicate layer will exist between adjacent surface pairs, potentially causing some reflection loss. However, the connection still enables a seamless transition across the interface. In both arrangements, the first optical element can have a first surface through which radiation from the nonlinear crystal enters, and a second surface through which radiation at the fundamental frequency exits. In other words, the second surface can be considered as a surface opposite or opposite to the first surface of the first optical element.

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

[0016] When a space exists between the first optical element and the nonlinear crystal, the first optical element may have a coating on a first surface, which is the surface on which radiation from the nonlinear crystal is incident. This coating is anti-reflective to radiation at the fundamental frequency and reflective to converted radiation at the second harmonic frequency. The second surface may be uncoated. In this way, the coating's exposure to ultraviolet radiation can be further reduced. The first optical element may be generally planar. Therefore, the first optical element can be referred to as a pickup mirror because it is highly reflective to second harmonic light and transmissive to the fundamental frequency light.

[0017] When the first optical element is in optical contact with the nonlinear crystal, all or a portion of the first surface of the optical element can contact the nonlinear crystal to form an optical contact. In both arrangements, the degree of optical contact is such that the first optical element receives radiation at the fundamental frequency and converted radiation at the second harmonic frequency. When an optical contact is formed, the adjacent surfaces (usually polished) are in good contact, allowing radiation to pass through and minimizing reflection loss at the optical contact (i.e., the interface between the optical element and the nonlinear crystal). The first optical element may have a coating on its second surface, wherein the coating is anti-reflective to radiation at the fundamental frequency and reflective to converted radiation at the second harmonic frequency.

[0018] As described above, radiation at the fundamental frequency enters the optical element through a first surface (which may be in contact with a nonlinear crystal optics) and exits through a second surface. Conversion radiation with a second harmonic frequency also enters through the first surface, but is reflected from at least a portion of the second surface, causing the conversion radiation to exit the optical element at a location different from or separate from the location where the fundamental frequency radiation exits the optical element. In other words, the fundamental frequency radiation and the conversion radiation exit the optical element in a non-coaxial direction. For example, the conversion radiation may be reflected at the second surface, causing it to exit the first optical element not through the second surface, but through a surface different from the second surface. Alternatively, the conversion radiation may be reflected at the second surface and then, for example, from a portion of the first surface with a reflective coating, back to the second surface, causing the conversion radiation to exit through the second surface at a location different from the location where the fundamental frequency radiation exits the optical element.

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

[0020] When only a portion of the first surface of the optical element is optically in contact with the nonlinear crystal, the converted radiation having the second harmonic frequency can be reflected back to 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 can be optically in contact with the nonlinear crystal, and a second portion of the first surface of the optical element can extend beyond the nonlinear crystal, and the converted radiation having the second harmonic frequency can be reflected 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 from a different portion of the same surface into which it enters. In this arrangement, the first optical element can also be considered as an end cap of the nonlinear crystal, but the second portion of the first optical element does not contact the nonlinear crystal. The first optical element can also be wedge-shaped.

[0021] The coating applied to the first surface and / or the second surface of the first optical element can be highly reflective at the second harmonic frequency; in other words, it can reflect more than 98% of radiation. Similarly, an antireflective coating can be defined as a coating with a reflectivity of less than 0.2%. The antireflective properties of the coating may be more important because the fundamental frequency is amplified within the laser device, so any decrease in laser power is highly nonlinearly related to any losses associated with the fundamental frequency. Radiation at the second harmonic frequency (i.e., ultraviolet radiation) is not amplified within the laser device, therefore any losses at the second harmonic frequency are not amplified.

[0022] The laser device may also include a second optical element positioned to receive reflected radiation having a second harmonic frequency from the first optical element and configured to reflect the radiation having the second harmonic frequency toward the output of the laser device. Therefore, the second optical element can be considered a steering mirror. The second optical element may be generally planar in shape.

[0023] Laser devices may include other standard components within the laser cavity, such as an active medium (also known as a gain medium), a tuning device (e.g., an etalon) for selecting the intracavity fundamental frequency, and an optical furnace for effectively controlling the temperature of optical elements (e.g., an etalon). Feedback mechanisms may also be present for adjusting the length of the optical path, for example, using piezoelectric elements or electro-optic modulators.

[0024] Multiple reflectors can form a ring resonator, more specifically, a bow-shaped ring resonator with four reflectors. The device may also include an optical diode configured to ensure that radiation at the fundamental frequency resonates unidirectionally (i.e., in a single direction) around the ring resonator. The optical diode may include: a magneto-optical element having a first surface and an opposing second surface; a waveplate having a first surface and an opposing second surface, 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. When exposed to the magnetic field, the magneto-optical element is configured to: rotate the polarization direction of radiation at the fundamental frequency incident on the first surface of the magneto-optical element by a first rotation, and rotate the polarization direction of radiation at the fundamental frequency incident on the second surface of the magneto-optical element by a mirror-image first rotation, wherein the mirror-image first rotation is different from the first rotation; and wherein the waveplate is configured to rotate the polarization direction of radiation at the fundamental frequency 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 is equal in magnitude to the first rotation. The first rotation of the mirror image is equal to the first rotation but opposite in direction to the first rotation.

[0025] Therefore, when radiation at the fundamental frequency is incident on the first surface of the magneto-optical element, its polarization direction will be rotated by a first rotation as it travels through the magneto-optical element and exits at the second surface of the element. In other words, the first rotation corresponds to the rotation of the polarization direction of the radiation at the fundamental frequency exiting the magneto-optical element at the opposite second surface relative to the polarization direction of the radiation at the fundamental frequency incident on the first surface. Here, the first rotation can also be understood as a continuous rotation of the polarization direction between the first surface and the opposite second surface within the magneto-optical element. The first surface of the waveplate faces and / or is in contact with the second surface of the magneto-optical element; therefore, radiation exiting the magneto-optical element through the second surface is incident on the first surface of the waveplate and will have its polarization direction rotated by a second rotation equal to and opposite to the first rotation. In a similar manner to the first rotation, the second rotation corresponds to the rotation of the polarization direction of the radiation at the fundamental frequency exiting the waveplate at the opposite second surface relative to the polarization direction of the radiation at the fundamental frequency incident on the first surface. Here, the second rotation can also be understood as a continuous rotation of the polarization direction between the first surface and the opposite second surface within the waveplate. In other words, the rotation applied by the waveplate cancels out the rotation applied by the magneto-optical element.

[0026] Therefore, there are two rotations applied to the radiation: a rotation first applied by the magneto-optical element (first rotation) and a subsequent rotation applied by the waveplate (second rotation). As the radiation travels in opposite directions, it first strikes the waveplate and undergoes the second rotation. That is, the rotation of the polarization direction applied by the waveplate, compared to the magneto-optical element, does not depend on the direction of radiation transmission through the waveplate. In other words, the second rotation also corresponds to the rotation of the polarization direction of the radiation exiting the fundamental frequency at the first surface relative to the polarization direction of the radiation incident on the opposing second surface. The rotated radiation then strikes the second surface of the magneto-optical element and undergoes a mirrored first rotation. In a similar manner to the first rotation, the mirrored first rotation corresponds to the rotation of the polarization direction of the radiation exiting the fundamental frequency at the first surface relative to the polarization direction of the radiation incident on the second surface. Thus, both rotations are applied to the radiation again, but in this arrangement, the first rotation is applied by the waveplate, and the subsequent rotation is applied by the magneto-optical element.

[0027] Optical diodes can be used independently of second harmonic generation; therefore, according to the second aspect, there exists a laser device comprising: a laser cavity in the form of a ring resonator formed by four reflectors that are reflective to radiation at the fundamental laser frequency; and an optical diode configured to ensure that radiation at the fundamental frequency resonates around the ring resonator in a single direction. The photodiode can possess the aforementioned characteristics. The following further details can be applied to combinations of optical diodes within an SHG laser device, or to the case of a single optical diode.

[0028] Unidirectional oscillation is achieved through two rotations applied separately by the magneto-optical element and the waveplate. As described above, the radiation at its fundamental frequency, which is correctly polarized and first passes through the magneto-optical element, rotates approximately 4 to 5 degrees along its length, and is then rotated by the same amount in the opposite direction by the waveplate, such that the waveplate rotation compensates for the rotation of the magneto-optical element. In effect, the optical diode does not alter the radiation in that direction. In contrast, the radiation that first passes through the waveplate will first rotate (i.e., 4 to 5 degrees) as it passes through the magneto-optical element, and then further rotate by a mirrored first rotation (i.e., 4 to 5 degrees in the same direction as the second rotation). These rotations do not cancel each other out; therefore, the radiation passing in the wrong direction (i.e., first through the waveplate and then through the magneto-optical element) has different polarizations, resulting in reflection losses at each component within the laser (e.g., reflectors, nonlinear crystals, and / or optical elements).

[0029] The first rotation, the first rotation of the mirror image, and the second rotation can be rotations of about 3 to 5 degrees (more specifically, 4 to 5 degrees). Therefore, a relatively small amount of rotation is required. When the radiation travels in the wrong direction, the cumulative rotation angle can be 8 to 10 degrees, and this can result in reflection losses during a round trip around the device, which effectively prevents oscillation of the laser in that direction.

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

[0031] Waveplates can be made of crystalline quartz. Crystalline quartz has known birefringence and optical rotation, and these two properties can be used separately in different arrangements to achieve the desired rotation.

[0032] Magneto-optical elements can be optically contacted or bonded to a waveplate, or they can be spaced apart from the waveplate. When a magneto-optical element is bonded to or optically contacted with a waveplate, a monolithic optical assembly exists. As described above, optical contact can be formed by precisely polishing the surface. As an alternative or supplement to optical contact, a connection can also be formed by using a bonding material applied to one or both of the adjacent surfaces. For example, by using hydroxide-catalyzed bonding or a silicate solution, a thin silicate layer (i.e., true covalent bonds) can be formed between the components, thereby connecting the surfaces.

[0033] Each of the first and second surfaces of the magneto-optical element can be aligned with Brewster's angle for radiation at the fundamental frequency (as described above, which is p-polarized). In the same manner as 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, when aligned with Brewster's angle, can be parallel to each other. This ensures that radiation at the fundamental frequency is transmitted through the magneto-optical element without reflection. Alternatively, each of the first and second surfaces can be planar, i.e., each of the first and second surfaces of the magneto-optical element can be aligned perpendicular to the axis of the magneto-optical element. In both cases, the axis of the magneto-optical element can be aligned with the intended direction of radiation travel around the ring resonator. In the first arrangement, radiation at the fundamental frequency is refracted when incident on the first and second surfaces. However, since the first and second surfaces are aligned with Brewster's angle, the radiation is not reflected at the first and second surfaces. In the second arrangement, to ensure proper refraction through the magneto-optical element, each of the first and second surfaces is coated such that the refractive index of the coated magneto-optical element is such that radiation at the fundamental frequency parallel to the axis of the magneto-optical element is not refracted as it passes through the magneto-optical element. The coating can also provide anti-reflectivity for the fundamental frequency to facilitate transmission through the magneto-optical element. Therefore, there are two possible options for the magneto-optical element: uncoated with a wedge-shaped surface; or flat-flat with a coating.

[0034] Similarly, waveplates exist in two arrangements. In one arrangement, each of the first and second faces of the waveplate can be perpendicular to the axis of the waveplate (i.e., flat-flat). 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 within a mounting base, and the birefringence property of the waveplate material can be used to achieve a second rotation. Thus, the waveplate can be rotated to a fixed position within the mounting base such that the rotation applied by the waveplate can cancel out the rotation applied by the magneto-optical element when the radiation is traveling in the correct direction. In the second arrangement, each face of the waveplate can be cut at a Brewster angle. The waveplate has a thickness sufficient to achieve the second rotation. The waveplate can be any suitable shape, such as circular or rectangular.

[0035] The waveplate can be coated with an anti-reflective coating, thereby increasing the refractive index of the coated waveplate so that radiation at the fundamental frequency orthogonal to each of the first and second surfaces does not refract as it passes through the magneto-optical element. This coating is particularly necessary when using waveplates cut such that the optical axis (c-axis) lies in the plane of the waveplate surface.

[0036] In all the above configurations, the baseband can have a wavelength of 650 nm or less. Therefore, the baseband can be in the green, red, blue, or possibly ultraviolet region. Attached Figure Description

[0037] To better understand the invention and to illustrate how embodiments of the invention can be practiced, reference will now be made to the accompanying drawings by way of example only, in which:

[0038] Figure 1 It is a diagram showing the components of the laser and the beam path within the optical cavity;

[0039] Figure 2a It shows Figure 1 More details about a second harmonic generator used in lasers;

[0040] Figure 2b It shows Figure 1 A schematic diagram showing details of the alternative second harmonic generator used in the laser;

[0041] Figure 2c It shows Figure 1 A schematic diagram showing details of another alternative second harmonic generator used in the laser;

[0042] Figure 3 and Figure 4 These are two perspective views of an optical diode, which can... Figure 1 In the laser, it is used to provide unidirectional oscillation in the direction of the dashed line;

[0043] Figure 5a yes Figure 3 The half-wave plate used in optical diodes;

[0044] Figure 5b yes Figure 5a A side view of a half-wave plate, showing radiation passing through the half-wave plate;

[0045] Figure 6a and Figure 6b It shows the support Figure 5a The mounting bracket assembly for the half-wave plate;

[0046] Figure 7a yes Figure 3 A perspective view of the first Faraday crystal that can be used in an optical diode;

[0047] Figure 7b yes Figure 7a A perspective view of the mounting base for a Faraday crystal;

[0048] Figure 7c and Figure 7d It is a usable alternative Faraday crystal;

[0049] Figures 8a to 8d It shows the way Figure 3 Three different beam paths using Faraday crystals and waveplates in optical diodes;

[0050] Figure 9a The refractive index versus wavelength of p-plane polarized light and s-plane polarized light incident on a KTF crystal were plotted respectively; and

[0051] Figure 9b The reflectivity of light with p-plane polarization and light with s-plane polarization relative to the angle of incidence were plotted respectively. Detailed Implementation

[0052] Figure 1 A laser is shown, including a laser cavity in the form of a bow-shaped ring resonator. The ring resonator is composed of components for the laser fundamental frequency. The radiation (or light, the two terms are used interchangeably) on the cavity is formed by four highly reflective reflectors 104a, 104b, 104c, and 104d. Since the cavity mirrors have near 100% reflectivity for the laser fundamental frequency, the cavity resonates and enhances the frequency. The circulating laser power is [specified]. Each reflector 104a, 104b, 104c, 104d includes a mirror held in a mounting. An active (laser gain) medium 106 is held in a mounting within the cavity. The active medium 106 can be any suitable solid-state laser material, such as a neodymium-doped laser crystal or a praseodymium-doped crystal. The laser is output from the laser device via output 150.

[0053] In this example, the laser is an intracavity second harmonic generation (SHG) laser. Therefore, the laser also includes the generation of second harmonic light (i.e., frequency 2). The SHG nonlinear crystal 140 (the light below) is used. As described in more detail below, some light at the laser's fundamental frequency is converted into light at the second harmonic frequency; therefore, the radiation at the second harmonic frequency can be referred to as the converted radiation. The generated (i.e., converted) light is directed to a first optical element 142, which separates the fundamental wavelength from the second harmonic wavelength by reflecting the second harmonic wavelength (ultraviolet light in this example). In other words, the fundamental wavelength radiation is spatially separated from the second harmonic wavelength radiation. Spatial separation is achieved by the reflection of the second harmonic wavelength radiation, wherein the fundamental wavelength radiation and the second harmonic wavelength radiation propagate along spatially separated paths due to reflection. Figure 1 As shown, the first optical element 142 is located within the laser cavity to receive light at the laser's fundamental frequency. In other words, the first optical element 142 is positioned along the beam path of the laser cavity. The reflected light is guided to the second optical element 144, which then guides the second harmonic light to the output 150. In other words, the second harmonic frequency 2 is output from the laser. The power on.

[0054] The etalon 130 can be located within the beam path of the ring resonator. The etalon 130 can be an optical cavity through which only light waves resonating with it can pass. In other words, the etalon's function is to tune the laser to the desired fundamental frequency. The temperature of the etalon 130 is strictly controlled; therefore, the etalon 130 can be referred to as an etalon furnace.

[0055] As shown by the dashed line, there may be leakage of radiation 204 at the fundamental wavelength. This leaking radiation 204 can be reflected to an optional feedback mechanism using reflector 110, particularly when using a high-precision, wide-coated etalon 130 that has already been locked to the fundamental frequency using fringe locking. The leaking radiation 204 is then split using beam splitter 136 to generate two independent signals, each of which is received separately at 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 (in this example, the first reflector 104a) can be attached to a piezoelectric element 214, which can be used to change the optical path length within the laser cavity based on the measured optical leakage. As an alternative to using a piezoelectric element, birefringent optical elements (e.g., electro-optic modulators) can also be used.

[0056] Figure 1 The beam path within the ring resonator is shown. The ring resonator has a traveling wave that requires unidirectional operation. Therefore, the optical diode 120 is preferably located within the cavity (i.e., inside the cavity). The optical diode 120 prevents the laser from emitting laser light in one direction but allows the laser to emit laser light in another direction. The optical diode is described in more detail below.

[0057] Light from the active medium 106 is incident on the first reflector 104a, which causes the fundamental frequency to be... The light from the upper part is reflected by the second reflector 104b and passes through the optical diode 120. The second reflector reflects the fundamental frequency... The light is directed through the SHG crystal 140 to generate a second harmonic wavelength. Two wavelengths are now circulating within the cavity: the fundamental wavelength 200, shown in red, and the second harmonic wavelength 202, shown in blue. The second harmonic wavelength 202 is discussed in more detail below. The fundamental wavelength 200 is incident on the third reflector 104c and reflected towards the fourth reflector 104d. As the wave travels between the third and fourth reflectors, it passes through the etalon furnace 130. The fourth reflector 104d then reflects the wave back to the first reflector 104a. Thus, the first, second, third, and fourth reflectors form a bowtie resonator, where the beam path of the light at the fundamental frequency resembles a bowtie.

[0058] It will be understood that a bowtie resonator can be used without an SHG crystal. Similarly, it will be understood that this is merely one suitable arrangement of the resonator. In this arrangement, the two components of the optical diode are placed in the diagonal arms of the bowtie resonator, and the SHG crystal is placed in the horizontal arm (as in...). Figure 1 In one of the horizontal arms (observed in the middle). However, the components of an optical diode can be separated into different arms; for example, a Faraday crystal can be in a diagonal arm (such as...). Figure 1 As shown), the waveplate can be placed after the second reflector in the horizontal arm (e.g., in...). Figure 1 (The image shows an arm containing an SHG crystal). Generally, we aim to position the crystal near the middle of the arm to minimize beam divergence through the crystal. When the assembly is separated into two different arms, only the reflector exists between the waveplate and the Faraday crystal. In other words, there are no other components of the laser device (e.g., SHG crystal) between the optical diode assembly.

[0059] Figure 2a Further details of the components arranged for generating the second harmonic are shown. An SHG crystal 140 is supported in a mounting base 340. The SHG crystal 140 is a generally wedge-shaped nonlinear crystal. The SHG crystal 140 can be formed of any suitable nonlinear optical material (e.g., lithium triborate (LBO)). Alternatively, the SHG crystal 140 can be formed of barium β-borate (BBO) or lithium cesium borate (CLBO). The front face 352 of the crystal 140 is cut at the Brewster angle for p-polarized fundamental frequency light. The Brewster angle, also known as the polarization angle, is the angle of incidence at which light with a specific polarization (in this case, p-polarized) is completely transmitted through the crystal without reflection. Defined as:

[0060]

[0061] Where n1 is the refractive index of the initial medium through which light propagates (i.e., the air inside the cavity), and n2 is the refractive index of the crystal. Since the refractive index of a given medium depends on the wavelength of light, the Brewster angle will also vary with wavelength. The back face of the crystal can also be cut at the Brewster angle to minimize any loss of light at the fundamental wavelength.

[0062] SHG crystals also generate second-harmonic generation (i.e., light with twice the frequency and half the wavelength). Second-harmonic generation is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material and produce a new photon with twice the energy. In one example, second-harmonic light is generated as class 1. In other words, two photons with ordinary polarization relative to the crystal will combine to form a single photon with twice the frequency and an unusual polarization. In other words, the second harmonic has s-polarization, while the fundamental frequency has the opposite polarization.

[0063] like Figure 2a As shown, the fundamental wavelength 200 (shown in red) and the second harmonic wavelength 202 (shown in blue) both exit the SHG crystal 140. Both wavelengths are incident on a first optical element 142, which is in the form of a cuboid and supported in a support or mounting base 342. The first optical element 142 has a first or front surface on which the fundamental wavelength and the second harmonic wavelength are incident. The first surface is set at a Brewster angle for the p-polarized fundamental frequency, such that light at the fundamental frequency passes through the optical element 142 without reflection loss and exits through a second or back surface. The first surface has a coating that is highly reflective at the second harmonic wavelength, such that light at the second harmonic wavelength is reflected. This coating is anti-reflective for the fundamental wavelength, allowing it to pass through the optical element. The second surface has no coating, but it can also be set at a Brewster angle to minimize any reflection loss of light at the fundamental wavelength as it travels through the optical element. Figure 1 Light traveling at the fundamental frequency of the first optical element is incident on one of the four reflectors of the ring resonator. The first optical element 142 is typically formed of any suitable optical material (e.g., fused silica or UV fused silica (UVFS)). Alternatively, the first optical element may be formed of 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.

[0064] like Figure 2a As shown, light at 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 in a similar support 344. The second optical element 144 causes light at the second harmonic wavelength to be reflected toward the output. The second optical element 144 may be referred to as a deflector for the second harmonic wavelength. In this case, theoretically, there is no fundamental frequency light, because the first optical element 142 has already transmitted it entirely to the reflector. Similar to the first optical element 142 described above, the second optical element 144 may be formed of any suitable optical material (e.g., UVFS, CaF2, or MgF2).

[0065] 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 typically in the ultraviolet spectrum (i.e., below 400 nm). It will be understood that when the fundamental wavelength is shortened (e.g., below 650 nm), the second harmonic wavelength is also shortened. Shorter wavelengths (e.g., wavelengths in the ultraviolet spectrum) have greater photon energy, and when they pass through certain materials, they can heat the material when absorbed. Specifically, this can damage the coating. A first optical element is used to separate the second harmonic wavelength from the fundamental wavelength. Both the first and second optical elements have only a single coating, thus minimizing heating of the coating within the laser. Also note that when using Class 1 second harmonic generation, the fundamental wavelength and the second harmonic wavelength are orthogonal in polarization, and this contributes to beam separation.

[0066] As an alternative to using the first and second optical elements, an output coupler that is reflective to the fundamental wavelength and transmissive to the second harmonic wavelength can also be used. 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 any coating on one or both of its surfaces). This may result in heating of the material and / or coating.

[0067] Figure 2b An alternative arrangement for generating the second harmonic is shown. In this arrangement, the SHG crystal 240 and the first optical element 242 form a monolithic structure. Similar to the previous arrangement, the SHG crystal 240 is a generally wedge-shaped nonlinear crystal. The SHG crystal 240 and the first optical element 242 can be similarly constructed from the above-described... Figure 2a The material described is formed as follows. The front side 252 of crystal 240 is cut at a Brewster angle for p-polarized fundamental frequency light. The back side 254 of crystal 240 is also cut at a Brewster angle to minimize any reflection loss of light at the fundamental wavelength. That is, when cut at a Brewster angle, the front side 252 and the back side 254 can be parallel to each other. In both cases, the Brewster angle is calculated for the fundamental frequency. For the first side 252, the interface is air and the crystal material, and for the second side 254, the interface is the crystal material and the optical element material.

[0068] like Figure 2b As shown, the fundamental wavelength, indicated in red, enters the SHG crystal 240 through the first surface 252 and exits through the second surface 254. Light is refracted when it enters the SHG crystal 240 at the first surface. It will be understood that refraction typically occurs when radiation travels from one component to another. When the refractive indices of the materials are similar, the angle of refraction is very small and can be approximated by a straight line. The SHG crystal has a higher refractive index than its adjacent components, therefore the angle of refraction is... Figure 2b The first optical element 242 is visible in the middle. Since the first surface 252 is cut at a Brewster angle for p-polarized radiation at the fundamental wavelength, no reflection or reflection loss occurs. The SHG crystal 240 also generates a second harmonic wavelength, which also exits the SHG crystal through the second surface 254. Since the second surface is also cut at a Brewster angle, no reflection loss occurs at the second surface. The first optical element 242 has a first surface or front surface 244 on which the fundamental wavelength and the second harmonic wavelength are incident.

[0069] In this example, the first facet 244 and the second facet 254 of the SHG crystal are optically contacted. Furthermore, the entire first facet (surface) 244 of the first optical element 242 is optically contacted with the nonlinear crystal 240 to form a monolithic assembly. A connection or bonding is formed between the SHG crystal 240 and the first optical element 242 using any suitable mechanism. For example, seamless light transmission across the first facet 244 and the second facet 254 is provided by adhesive-free optical contact (e.g., intermolecular forces acting on the first facet 244 and the second facet 254) or by using any suitable bonding process (e.g., hydroxide catalysis). Optically contacting 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 through the optical contact interface between the crystal 240 and the optical element 242 is unaffected by that interface.

[0070] When precisely polished surfaces come into contact, attractive van der Waals forces form bonds between them. Precision can be defined by standard specifications. For example, specifications typically include one or more of the following: surface quality greater than scratch / dimple (S / D) = 10 / 5, surface flatness less than lambda / 10 (633 nm / 10, i.e., the root mean square of the surface is less than 63 nm), and surface roughness less than 5 Armstrong. These forces are generally sufficient to maintain the connection between each of the adjacent surfaces. As an alternative or supplement to optical contact, connections can also be formed by using bonding materials applied to one or both of the adjacent surfaces. For example, by using hydroxide-catalyzed bonding or silicate solutions, a thin silicate layer (i.e., true covalent bonds) can be formed between components, thereby connecting the surfaces.

[0071] The first surface 244 is set at a Brewster angle for the p-polarized fundamental frequency, such that light at the fundamental frequency is not reflected and passes through the optical element 242, exiting through the second surface or back surface 246, which is also set at a Brewster angle. In this case, the first surfaces 244 and 246 can be parallel to each other when set at a Brewster angle (i.e., aligned). The Brewster angle of the first surface 244 is calculated for the interface between the material of the nonlinear crystal and the material of the optical element (which may be, for example, fused silica). The Brewster angle of the second surface 246 is calculated for the interface between the material of the optical element and air.

[0072] 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 air). This coating is anti-reflective to the fundamental wavelength, so it passes through the second surface 246 without any change in direction. The coating is highly reflective at the second harmonic wavelength (i.e., reflectivity exceeding 98%), causing light at that wavelength to be reflected and not exit the optical element through the second surface 246, but instead exiting through the third surface or side surface 248. Based on the interface between the material of the optical element and air, this side surface 248 is cut at a Brewster angle for the second harmonic wavelength.

[0073] Figure 2c Another alternative arrangement for generating the second harmonic is shown. (The above is about...) Figure 2b The characteristics described for SHG crystal 240 also apply to Figure 2c The example shown is for simplicity and will not be repeated here. Additionally, light at 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, not as... Figure 2b The entire first surface 244 of the first optical element 242 shown is optically in contact with the nonlinear crystal 240, but only a first portion 244-1 of the first surface 244 of the optical element is optically in contact with the SHG crystal 240.

[0074] In this arrangement, light at the second harmonic frequency is reflected and exits the optical element through a second portion 244-2 of the first surface, which extends beyond the SHG crystal but does not contact it. In other words, the second harmonic wavelength exits the optical element through a different portion of the same surface that enters the optical element. The first surface 244 and the second surface 246 are also respectively set at their respective Brewster angles, as described above regarding... Figure 2b As stated above. However, with Figure 2b The arrangement differs in that the first surface 244 of the first optical element does not coincide with the corresponding surface of the SHG crystal 240. This will be understood... Figure 2cThe arrangement allows light at the second harmonic frequency to be reflected back to the first surface of the optical element, but the arrangement can also be adjusted so that the second harmonic frequency is reflected from the sides, such as... Figure 2b As shown. This arrangement can also be adjusted so that the second harmonic frequency is reflected back to the second surface at a position different from where the light at the fundamental frequency leaves the optical element.

[0075] return Figure 1 Light traveling through the fundamental frequency of the first optical element is incident on one of the four reflectors of the ring resonator. Then, light at the second harmonic wavelength can be directed outwards, for example, using a second optical element or other suitable mechanism.

[0076] In this arrangement, similar to the first arrangement, a first optical element is used to separate the second harmonic wavelength from the fundamental wavelength. In contrast, in this example, the second harmonic wavelength passes through the first optical element. This reduces the likelihood of ultraviolet light interacting with air (specifically, oxygen) (potentially catalyzing certain degradation processes).

[0077] like Figure 1 As shown, an optical diode can be used in a ring resonator to ensure that the light at the fundamental frequency resonates around the cavity in a single direction. Figure 3 and Figure 4 Further details of the optical diode are shown, which includes two key components: a Faraday crystal (also referred to as a magneto-optical element, and the two terms are used interchangeably) mounted in a first holder 410; and a half-wave plate (also referred to as a wave plate or plate rotator, and the two terms are used interchangeably) mounted in a second holder 420. Figure 1 As shown, light at the fundamental frequency propagates in the correct direction around the cavity, first passing through the Faraday crystal and then through the waveplate. A waveplate, also known as a retarder, is an optical device that alters the polarization state of light waves traveling through it. More specifically, a half-wave plate changes the polarization direction of linearly polarized light.

[0078] Figure 5a and Figure 5b An example of a half-wave plate 610 is shown. The half-wave plate 610 is birefringent, therefore its refractive index depends on the polarization direction and propagation direction of the light incident on it. In other words, the half-wave plate can be considered an optically anisotropic material or possess birefringence. The wave plate 610 is also a non-reciprocal optical element. In other words, light propagating through the wave plate 610 in both directions has the same polarization rotation. The wave plate can be made of any suitable material (e.g., crystalline quartz or calcite) and has a thickness t.

[0079] Waveplates can be designed using any of the following properties, which are described in detail for crystalline quartz:

[0080] Plate rotators—optical rotation: In this case, the quartz crystal is cut such that the light beam propagates parallel to the crystal's optical axis. Optical rotation naturally exists in certain materials such as quartz, which can define the rotational force (…). This rotational force, described in Radhakrishnan's "The dispersion, birefringence and optical activity of quartz" published in January 1947, specifies the rotation angle per millimeter propagating through the material. As an example, for a wavelength of 640 nm, this rotation angle is approximately 18.251 deg / mm; therefore, for a 4-degree rotation, the quartz thickness t should be 0.2191 mm.

[0081]

[0082] Waveplate – Birefringence: In this case, the quartz crystal is cut such that the light beam propagates perpendicular to the crystal's optical axis. Light polarized along the optical axis (fast axis) travels faster than light polarized perpendicular to the optical axis (slow axis), and in this way, the birefringence of quartz creates a relative phase delay between the polarized components along the fast and slow axes. A half-waveplate provides a phase delay of half a wavelength (or π delay), so a linearly polarized input remains linear as it leaves the waveplate. The thickness d of the waveplate is proportional to the birefringence at a specific wavelength λ, and M is an integer specifying the order.

[0083]

[0084] Therefore, a fifth-order half-wave plate made of quartz at 640 nm has a thickness of 0.389 mm. To achieve a 4-degree rotation, the wave plate is rotated such that the incoming linear polarization is offset by 2 degrees (or half the desired rotation) relative to the fast axis of the wave plate, while keeping the propagation direction orthogonal to the optical axis.

[0085] Return to reference Figure 3 and Figure 4 The half-wave plate is mounted in a generally circular mounting base 430, which is located within a corresponding generally circular opening on the support member 432. For example... Figure 6a and Figure 6b As shown, the mounting base includes an outer ring 630 and a central body 632, the central body 632 having an outer edge 634 located within and fixed to the outer ring 630. The central body 632 also has a second portion 636 projecting from the outer edge 632. Figure 3 As shown, the outer ring 630 protrudes from the back of the support member 432. Figure 4 As shown, the second part 636 of the central body is located in the opening on the support member 432.

[0086] Mount 430 can be rotatable, allowing the birefringence of the waveplate material to generate a delay. Alternatively, the waveplate can be cut into a plate rotator that relies on the principle of optical rotation to achieve rotation.

[0087] A Faraday crystal is a crystal that exhibits birefringence when exposed to a magnetic field. Therefore, a Faraday crystal can also be called a magneto-optical element, and the two terms are used interchangeably. Examples of suitable materials for the crystal include potassium terbium fluoride (KTF) and terbium gallium garnet (TGG). A Faraday crystal is a reciprocal element; therefore, any rotation applied to radiation passing through it depends on the magnetic field and the direction of radiation propagation. Thus, radiation incident on a first facet of the Faraday crystal and then transmitted through it can undergo a first rotation, which differs from a second rotation applied to radiation incident on a second facet of the Faraday crystal and then transmitted through it.

[0088] exist Figure 7a In the first arrangement shown, the Faraday crystal 710 is cylindrical and has planar end faces 712 and 714 perpendicular to the major axis 720 of the crystal. The Faraday crystal is mounted on... Figure 7b The crystal mount 750 is shown in detail below. 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.

[0089] exist Figure 7c In the second arrangement, the Faraday crystal 730 is approximately cylindrical, but has inclined end faces 732 and 734, both tilted at a Brewster angle, and these end faces 732 and 734 can be parallel to each other. Figure 7a and Figure 7b In this context, the length of the crystal is similar to its diameter (e.g., a ratio of 4:3 or 3.5:3). Figure 7d In China, Faraday crystal 740 is... Figure 7c A slender version of the crystal. Therefore, in Figure 7d In the middle, the end face is also inclined at the Brewster angle, but the length of the crystal is about twice the diameter (e.g., an 8:3 ratio).

[0090] Depending on the shape of the crystal and / or the orientation of the waveplate, as well as any coatings applied to some or all of the surfaces of the crystal and / or the waveplate, there are various arrangements of photodiodes. Figure 8a The first arrangement is shown, in which a crystal 810 is used, for example... Figure 7c and Figure 7dThe crystal shown is cut or wedge-polished such that the first surface is at a Brewster angle and is uncoated. The waveplate 820 is also uncoated. The waveplate 820 is a generally flat disk of a specific thickness and is set at a Brewster angle. Light 812 at a frequency corresponding to the Brewster angle is not reflected by the crystal. As described above, refraction occurs due to the different materials, and the angle of refraction is magnified to show radiation traveling through the crystal in a direction not parallel to the crystal's axis. The direction of light 814 leaving 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 exits in a parallel direction 816. In this arrangement, both the Faraday crystal and the waveplate are cut at a Brewster angle so that the beam propagates without reflection loss; the Brewster angle can vary depending on the materials. Air exists between the two components.

[0091] exist Figure 8b In China, use and Figure 8a The crystal 810 is the same as the crystal in the original. However, in this arrangement, the waveplate 830 is coated with an anti-reflective coating, and the birefringence property of the material of the waveplate 830 is utilized. In this arrangement, the waveplate 830 can be positioned perpendicular to the direction of the light 814 leaving the crystal. When the light path is orthogonal to the incident plane of the waveplate, the light passes through the waveplate without changing its direction 836. No refraction occurs when the light passes through the waveplate. In this arrangement, the Faraday crystal is Brewster-cut, and the waveplate is coated with an anti-reflective coating at the fundamental wavelength on both sides to minimize reflection loss.

[0092] exist Figure 8c In China, use and Figure 8b The waveplate 830 is the same as the waveplate in the previous arrangement. However, in this arrangement, the crystal 840 is also coated and has an end face perpendicular to its main axis, as shown in the previous arrangement. Figure 7a As shown, light 812, at a frequency corresponding to the refractive index of the coated crystal and with a path orthogonal to the incident plane of crystal 830, passes through the crystal without any change in direction 844. This unchanged light then strikes the waveplate and passes through it without altering its direction 836. No refraction occurs when light passes through the waveplate or the crystal. In this arrangement, both the Faraday crystal and the waveplate are coated with antireflective coatings on both surfaces.

[0093] As such Figure 8a The alternative arrangement of the crystal and waveplate shown can also be used to manufacture a monolithic optical diode by bonding the exit surface of the crystal 810 to the entry surface of the waveplate 820 (or vice versa, depending on the direction of travel). Figure 8dThe arrangement is illustrated. Each surface joined together should be uncoated. When two optical surfaces are optically in contact in this manner, they become one material, and therefore there is no reflection loss between them. Thus, this arrangement can be advantageous. Light 812 at a frequency corresponding to the refractive index of the combined crystal and waveplate passes through, while the direction 846 remains unchanged. In this arrangement, both the Faraday crystal and the waveplate are wedge-shaped, such that a beam of light incident on any surface undergoes Brewster reflection. This may require the waveplate to have a non-parallel surface, 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 optically in contact with the first surface of the waveplate, i.e., the refraction is asymptotic and there is no refractive loss.

[0094] Figures 8a to 8d The options shown illustrate two different configurations of the waveplate. Figure 8b and Figure 8c In the first configuration shown, the waveplate is cut such that the optical axis (c-axis) lies in the plane of the waveplate surface. In this arrangement, a roughly circular waveplate (e.g., Figure 5a (as shown), and the waveplate can be rotated to match the retardation of the Faraday crystal. Therefore, the waveplate is held in a circular mounting, as... Figure 6a As shown. In this arrangement, the desired rotation (or retardation) is achieved by utilizing the birefringence property of the crystal.

[0095] exist Figure 8a and Figure 8d In the second configuration shown, the waveplate can be referred to as a Brewster-cut waveplate. In this case, the waveplate can be approximately rectangular and cut at a certain angle, for example, as shown. Figure 5b As shown. The thickness is calculated to achieve a rotation that matches the rotation applied by the Faraday crystal. Using a rectangular waveplate facilitates alignment, but note that the rectangular waveplate can be rotated as described above. Furthermore, as... Figure 8d As shown in the arrangement, the waveplate can optically contact the Faraday crystal, and the two sides are parallel. In this case, the waveplate cannot rotate, thus offering less freedom / flexibility, but allowing for a more compact design. This arrangement utilizes the optical rotation of the waveplate, and the optical rotation of quartz is well-known, as described by Radhakrishnan in "The dispersion, birefringence and optical activity of quartz" published in January 1947. Therefore, these two different arrangements can utilize two different properties of the waveplate.

[0096] for Figures 8a to 8dEach option shown, importantly, involves a different cut of the crystal material. Crystals have different optical axes, sometimes referred to as α-cut and β-cut. For each arrangement, the refractive index of the material at a given wavelength is required. KTF crystals are known in the telecommunications industry but are typically used in near-infrared and infrared ranges (e.g., from 980 to 1550 nm). Similarly, TGG has previously been used in the range covering 650 nm to 1500 nm. Therefore, below these wavelengths, no data is available regarding this crystal.

[0097] Figure 9a The refractive index of a KTF crystal was plotted against wavelength. Lasers at three different wavelengths were passed through the crystal, and the refractive index was measured. The refractive index variation across all wavelengths can be estimated using three points. The variations were plotted for both p-plane and s-plane polarization, allowing for an understanding of the changes in both ordinary and extraordinary refractive indices within the visible light spectrum.

[0098] Figure 9b Fresnel curves were plotted for the selected material. The Fresnel curves plotted reflections at different incident angles. For each wavelength, reflections were measured at three different wavelengths for both p-plane and s-plane polarization, resulting in a total of six plots (polts). Each plot provides... Figure 9b This is an estimated value of the refractive index plotted in the image. The refractive index value is calculated or estimated based on line fitting.

[0099] From Figure 9a and Figure 9b The data can be used to calculate the Brewster angle and also to design anti-reflective coatings. Anti-reflective coatings can have a reflectivity of less than 0.02%. Knowing the reflectivity and incident intensity, the absorptivity of the crystal can be measured based on the difference between the incident power and the reflection loss. For example, the absorptivity of a crystal made of TGG at a wavelength of 650 nm over a 10 mm range was measured to be approximately 2%. The absorptivity of a crystal made of KGF at a wavelength of 650 nm over a 10 mm range was measured to be less than 0.1%.

[0100] As described above, the crystal exhibits birefringence when exposed to a magnetic field. KTF or TGG crystals are known to be used as optical devices or optical isolators in conjunction with polarizers, as described, for example, in US10,120,213. The KTF crystal applies a 45-degree rotation to polarized light incident on the crystal in the forward direction. In the opposite direction, the reflected light is repolarized as it passes through the crystal and is again rotated by 45 degrees. In this way, the Faraday rotator applies a full 90-degree rotation, and the light can be completely attenuated.

[0101] Consider 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 small first rotation (e.g., 3 to 5 degrees). The rotated light then incident on a waveplate, which then counteracts this rotation by applying a second rotation, causing the light to pass through the optical diode without any rotation. If the light passes through the optical diode 120 in the wrong direction (e.g., from the second reflector 104b toward the first reflector 104a), the light is rotated by a small second rotation (e.g., 3 to 5 degrees) by the waveplate, and then by a small first rotation (e.g., 3 to 5 degrees) mirrored by the Faraday crystal. In other words, there is an overall rotation of about 8 to 10 degrees. Several optical surfaces are present within the cavity, including reflectors, specific coatings on optical elements, and optical elements cut or arranged at Brewster angles. This relatively small rotation results in a reflection loss for one round trip, which effectively limits the laser's oscillation in that direction. Since a laser is an optical amplifier, a small reflection loss is amplified into a huge loss in output power. In other words, a small loss is sufficient to suppress bidirectional lasers and allow the optical diode to perform its function of generating unidirectional oscillations in a ring resonator.

[0102] This document has described various combinations of optional features, and it will be understood that the described features can be combined in any suitable combination. Specifically, features of any example embodiment can be appropriately combined with features of any other embodiment, except for mutually exclusive combinations. Throughout this specification, the terms "comprising" or "including" mean including the specified component, but do not exclude the presence of other components.

[0103] Note that all documents and materials related to this application and submitted at the same time as or before this application, all documents and materials made publicly available together with this specification, and the contents of all such documents and materials are incorporated herein by reference.

[0104] All features disclosed in this specification (including any appended claims, abstract, and drawings) and / or all steps of any method or process so disclosed may be combined in any combination, except for at least some mutually exclusive combinations of these features and / or steps. Unless expressly stated otherwise, each feature disclosed in this specification (including any appended claims, abstract, and drawings) may be replaced by an alternative feature serving the same, equivalent, or similar purpose. Therefore, unless expressly stated otherwise, each disclosed feature is merely an example of a general series of equivalent or similar features.

[0105] This invention is not limited to the details of the foregoing embodiments. This invention covers any novel feature or any novel combination of features disclosed in this specification (including any appended claims, abstract, and drawings), or any novel step or any novel combination of steps in any method or process so disclosed. Although several preferred embodiments of the invention have been shown and described, those skilled in the art will understand 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 device for transmitting laser signals with a fundamental laser frequency ( The frequency within the radiation cavity is converted to a fundamental frequency ( The second harmonic frequency (2) The device includes: (The converted radiation is then described, but the context is unclear.) a laser cavity formed by a plurality of reflectors having reflectivity for radiation on the laser fundamental frequency; a nonlinear crystal within the laser cavity and for transmitting the laser fundamental frequency and for generating converted radiation having the second harmonic frequency, wherein the second harmonic wavelength is ultraviolet; and a first optical element positioned to receive radiation on the fundamental frequency and converted radiation having the second harmonic frequency from the nonlinear crystal and configured to separate the radiation on the fundamental frequency from the converted radiation having the second harmonic frequency.

2. The laser device according to claim 1, wherein, the first optical element is spaced apart from the nonlinear crystal and wherein the first optical element has a coating on a first surface, the first surface being the surface from which radiation from the nonlinear crystal is incident, wherein the coating is anti-reflective for radiation on the fundamental frequency and reflective for converted radiation having the second harmonic frequency.

3. The laser device of claim 1, wherein, a first surface of the first optical element optically contacts the nonlinear crystal, the first surface being the surface from which radiation from the nonlinear crystal is incident.

4. The laser device according to any one of the preceding claims, wherein, the first optical element has a second surface through which radiation on the fundamental frequency exits the first optical element and wherein the second surface is uncoated.

5. The laser device of claim 4, wherein, the first optical element has a coating on at least a portion of the second surface, wherein the coating is anti-reflective for radiation on the fundamental frequency and reflective for converted radiation having the second harmonic frequency.

6. The laser device of claim 5, wherein, converted radiation having the second harmonic frequency and radiation on the fundamental frequency exit the first optical element at different locations.

7. The laser device of claim 6, wherein, converted radiation having the second harmonic frequency exits the first optical element through a side surface, the side surface being aligned at a Brewster angle for radiation on the second harmonic frequency.

8. The laser device of claim 6, wherein, a first portion of the first surface of the optical element optically contacts the nonlinear crystal and a second portion of the first surface of the optical element extends beyond the nonlinear crystal and converted radiation having the second harmonic frequency exits the first optical element through the second portion of the first surface.

9. The laser device 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 for radiation on the fundamental frequency.

10. The laser device according to any one of the preceding claims, wherein, the second harmonic wavelength is below 380 nm.

11. The laser device according to any one of the preceding claims, wherein, the plurality of reflectors form a ring resonator and the device further comprises an optical diode configured to ensure that radiation on the fundamental frequency resonates in a single direction around the ring resonator, wherein the optical diode comprises: a magneto-optical element having a first surface and an opposite second surface, a waveplate having a first face and an opposite second face, wherein the first face 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, whereby the magneto-optical element is configured to, rotate a polarization direction of radiation on the fundamental frequency incident on the first surface of the magneto-optical element by a first rotation, and rotating a polarization direction of radiation on the fundamental frequency incident on the second surface of the magneto-optical element with a mirrored first rotation, wherein the mirrored first rotation is in a different direction than the first rotation; and wherein the waveplate is configured to rotate a polarization direction of radiation on the fundamental frequency transmitted through the waveplate in either direction with a second rotation, wherein the second rotation is in an opposite direction to the first rotation and equal in magnitude to the first rotation.

12. A laser device comprising: a laser cavity in the form of a ring resonator formed by four reflectors that are reflective for radiation on a laser fundamental frequency; and an optical diode within the laser cavity and configured to ensure that radiation on the fundamental frequency resonates in a single direction around the ring resonator, wherein the optical diode comprises: a magneto-optical element having a first surface and an opposite second surface, a waveplate having a first face and an opposite second face, wherein the first face 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, wherein the magneto-optical element is configured to: rotate a polarization direction of radiation on the fundamental frequency incident on the first surface of the magneto-optical element with a first rotation, and rotate a polarization direction of radiation on the fundamental frequency incident on the second surface of the magneto-optical element with a mirrored first rotation, wherein the mirrored first rotation is in a different direction than the first rotation; and wherein the waveplate is configured to rotate a polarization direction of radiation on the fundamental frequency transmitted through the waveplate in either direction with a second rotation, wherein the second rotation is in an opposite direction to the first rotation and equal in magnitude to the first rotation.

13. The laser device according to claim 11 or 12, wherein, Each of the first rotation, the mirrored first rotation, and the second rotation is a rotation of about 4 to 5 degrees.

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

15. The laser device according to any one of claims 11 to 14, wherein, The waveplate is made of crystalline quartz.

16. The laser device according to any one of claims 11 to 15, wherein, The magneto-optical element is bonded to or optically contacts the waveplate.

17. The laser device 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 for radiation on the fundamental frequency.

18. The laser device 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 an axis of the magneto-optical element, and each of the first surface and the second surface is coated with an anti-reflection coating, whereby a refractive index of the coated magneto-optical element is such that radiation on the fundamental frequency parallel to the axis of the magneto-optical element is not refracted when passing through the magneto-optical element.

19. The laser device according to claim 17 or 18, wherein, Each of the first face and the second face of the waveplate is aligned perpendicular to an axis of the waveplate.

20. The laser device of claim 19, wherein, The waveplate is rotatable within a mount, whereby the waveplate is configured to rotate a polarization direction of radiation on the fundamental frequency with the second rotation.

21. The laser device according to claim 19 or 20, wherein, The waveplate is coated with an anti-reflection coating, whereby a refractive index of the coated waveplate is such that radiation on the fundamental frequency orthogonal to each of the first face and the second face is not refracted when passing through the magneto-optical element.

22. The laser device according to claims 11 to 18, wherein, Each of the first and second faces of the magneto-optical element is aligned at the Brewster angle for radiation at the fundamental frequency.

23. The laser device of any of the preceding claims, wherein, The fundamental frequency has a wavelength of 650 nm or less.

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