Holographic laser mode converter inside a cavity

Abstract

The present invention is a broadband intracavity laser mode converter. It is a complex phase mask hologram inscribed inside a volume Bragg grating with a wide spectral width recorded in photothermorefractive (PTR) glass. This hologram is a broadband phase conversion monolithic device that can be used over a wide range of wavelengths at high instantaneous and average powers due to the small absorption coefficient and small nonlinear refractive index of PTR glass. Therefore, it can be used for broadband optical beam conversion and for conversion of modes in laser resonators.

Description

[Technical Field]

[0001] This disclosure relates to a phase beam conversion method. More specifically, this disclosure relates to a laser in which an in-cavity chromatic aberration holographic phase mask is provided configured to provide spatial transverse mode conversion. [Background technology]

[0002] Transverse mode conversion methods are used, for example, to provide mode conversion between a plane wavefront and a Gaussian transverse profile of intensity, and other more complex mode profiles required by various laser applications. These converted modes are not limited to TEM. mn , Lager-Gaussian LG nm There can be airy, bessel, and others. Mode shaping techniques include amplitude modulation, phase modulation, or a combination of both, and can be performed outside or inside the laser cavity, with the inside of the laser cavity being the most relevant here.

[0003] The mode phase profile transformation method involves phase correction applied to a local region of the mode wavefront. As a result, the beam propagation characteristics can be modified to provide the desired emission profile of the shaped mode in the far field. The phase delay that enables the desired mode shape transformation is determined by the optical path difference measured over a portion of the wavelength. The optical path difference is a byproduct of the thickness of the medium through which the beam travels and the refractive index of the medium. The elements that control the phase profile include phase masks, which are diffractive optical elements (DOEs) and spatial light modulators (SLMs), and are of particular interest here. Since specific phase shift effects can only be realized for specific wavelengths, all phase shaping elements exhibit a high degree of chromatic aberration, i.e., a phenomenon characterized by narrow bandwidths and relatively small output modes. Nevertheless, modes with high output and broad spectral lines are essential for various laser industrial applications.

[0004] A phase mask is an optical element in which the optical path of transmitted modes has a specific distribution across the aperture, and the term is used to define any optical element that excludes conventional lenses from which spatially dependent phase profiles are induced. Two methods are known for fabricating permanent phase masks with a given phase distribution. The first method involves creating a profile-formed surface, i.e., a surface phase mask, of an optically transparent and uniform material. This method can be performed by different techniques of selective etching or deposition. These methods create a desired profile of geometric thickness and, correspondingly, create a profile of the optical phase in the transmitted beam. The second method involves changing the local refractive index of the material across the aperture of the beam and in the volume of the medium. These changes create a desired profile of the optical path and, as a result, create an optical phase in the transmitted modes. Suitable materials for creating VBGs are photosensitive.

[0005] An example of a photosensitive material is photothermal refraction (PTR) glass, which provides a change in refractive index after thermal development following exposure to ultraviolet radiation. This sequence results in a permanent change in the refractive index of the material that cannot be bleached by laser radiation, and small absorption in the visible and near-infrared spectral regions. These characteristics enable the creation of VBGs, characterized by high efficiency and large tolerances to laser radiation, mechanical shock, and elevated temperatures. Specifically, VBGs are fabricated by exposing a homogeneous PTR glass plate to an interference pattern of two parallel ultraviolet beams focused at a certain angle. For these conditions, the interference pattern is a system of interference fringes in parallel planes. The focusing angle determines the duration of the interference pattern. After thermal development, a system of parallel plane layers with a changed refractive index is created in the volume of the PTR glass. The distance between the layers is equal to the duration of the interference pattern. The diffraction of light in VBGs follows Bragg's law.

[0006]

number

[0007] Here, λ is the wavelength, n is the average refractive index of the photosensitive medium, d is the distance between layers with uniform refractive indices, and θ is the angle between the direction of beam propagation and the plane of uniform refractive index. This formula shows that a VBG is a narrowband device with acceptance of a narrow spectrum and angle, and that a key feature of conventional VBGs is their tunability. Changing the angle of incidence will tune the VBG to a different resonant wavelength.

[0008] Volume Bragg masks (VPMs) are created by exposing PTR glass plates to ultraviolet radiation through amplitude masks produced using conventional or stochastic photolithography. This technique enables the creation of complex phase masks (PMs) between glass plates, each having a flat, polished surface that provides high resistance to laser radiation and surface contamination of the optical system.

[0009] The narrowband limitation has been overcome fairly recently with the relatively new development of holographic volume phase masks (HPMs). An HPM is an optical element created by incorporating a volume phase mask in a VBG. This method creates a desired profile in the beam diffracted by the VBG. Specifically, an HPM is created by embedding the desired phase information into a transmitted VBG and recording it holographically onto a thick medium of PTR glass. As a result, the HPM operates so that the phase profile of the diffracted modes is the same as the phase profile of the recorded ultraviolet beam used during the creation of the HPM, regardless of the diffracted wavelength. In use, the HPM thus created embeds its own phase profile into the diffracted modes. This is in contrast to conventional VBGs, which, when in use, diffract modes with the phase profile of the incident beam. This means that, unlike conventional surface or volume PMs, an HPM is a tunable device within a transparent window of a photosensitive medium. For PTR glass, this window is 350–2700 nm. The demonstration of converting Gaussian modes to different spatial modes, and vice versa, was created by the angle tuning of the HPM.

[0010] The beam shaping capabilities of HPMs have been utilized in optical configurations including HPMs, often in combination with two surface gratings. This configuration enables chromatic aberration mode conversion for broadband or multi-wavelength beams, such as femtosecond (fs) beams in a frequency range of at least 300 nm. Furthermore, multiple HPMs can be recorded into the same PTR volume, allowing amplification of several broadband beams propagating at different wavelengths in different spatial modes. Thus, HPMs can provide spatial conversion and spatial combination of multiple broadband laser beams.

[0011] Further developments in HPM technology are disclosed in U.S. Patent Application No. 62 / 970,001. This patent application teaches a method for HPM recording in a PTR volume when the parameters for recording an ultraviolet beam provide broad spectral acceptance. Within this spectral acceptance, it is found that the phase penetration for any spectral component is identical. Thus, the HPM produced by the disclosed method is monolithic and chromatic, and does not require a surface grating.

[0012] The HPM disclosed above is used in the context of a laser beam propagating in free space outside a laser cavity, a typical method of laser mode shaping, to obtain the desired spatial distribution and propagation characteristics of the modes. Nevertheless, shaping the laser modes within the resonator ensures that the gain is preferentially coupled to a given spatial mode distribution, thereby minimizing losses. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] U.S. Patent Application No. 62 / 970,001 [Overview of the project] [Problems that the invention aims to solve]

[0014] Therefore, what is needed is a broadband laser that provides an in-cavity HPM operating within a broadband laser resonator to provide mode conversion between various shaped modes. [Means for solving the problem]

[0015] This need is met by disclosed laser configurations that incorporate HPMs, which are chromatic aberration-tunable diffraction elements capable of transverse mode shaping of broadband beams. Several aspects of the present invention and their respective features are disclosed below, and are conceptually and structurally linked so that each of the specific features disclosed below can be combined with one or more other features.

[0016] According to the fundamental features of this disclosure, a resonator configured to generate radiation in a predetermined transverse mode includes an HPM tuned to a Bragg angle to diffract a portion of the predetermined transverse mode. While diffracting, the HPM encodes a phase profile in the diffracted transverse mode that differs from the phase profile of the predetermined mode. As a result, the HPM acts as an output coupler, guiding the diffracted transverse mode, which has a spectral width up to equal to the spectral width of the HPM, to the outside of the resonator.

[0017] A broadband resonator is configured to support the fundamental mode with a Gaussian phase profile. Typically, the laser source and its pump scheme provide the spatial profile of the most coherent optical beam with a Gaussian phase profile. Therefore, HPMs are complex TEMs. mn , Lager-Gaussian LG nm It is configured to provide transformations of Gaussian modes to the forms of Aery, Bessel, and other complex modes, and transformations back to Gaussian modes when necessary.

[0018] However, the disclosed resonator is not limited to supporting only Gaussian modes, but can be configured to support multiple higher-order modes. Such a laser is referred to as a multimode (MM) laser. Thus, in a further development of this embodiment, the HPM is configured with a spectral width of at least up to 300 nm to provide phase shifts between non-Gaussian modes or complex modes.

[0019] According to another feature, the broadband resonator is planar and defined between two spaced reflectors. The resonator further includes a gain medium that can be selected from various materials that provide amplification of a given mode at a desired wavelength. There are too many different gain media to list herein, but broadly, crystal growth YAG doped with various rare earth ions such as ytterbium, Yb:KGW, Yb:KYW, and other crystals used for pulse generation, including high-power pulses less than a nanosecond, have been successfully tested in the context of the present disclosure.

[0020] In a further feature, the disclosed reflectors are configured as respective planar high reflectors (HRs) that define a resonant cavity therebetween and retain all of the energy generated within the resonator therein. The cavity architecture provides a shape for a given transverse mode that travels between the HRs along the axis of the resonator. Spaced from the HRs inside the resonator, the HPM is provided to function as a bidirectional output coupler that provides two outputs in a diffraction plane transverse to the axial plane of the resonator. Said another way, the output direction of each of the two diffracted transverse modes is specific to the axial direction of propagation of a given mode. Thus, the HPM mode converter is configured such that a given transverse mode remains within the resonator while a diffracted transverse mode having the phase profile of the HPM is separated from the resonator.

[0021] Yet another structural feature includes an additional HR mirror provided in the diffraction plane along one of the opposing directions of the diffracted transverse mode. This feature enables the intended target of the resonator output to accept substantially higher output power than in an architecture with two outputs of the diffracted transverse mode.

[0022] Further structural changes to the features previously considered result from the fact that the diffracted lateral modes reflected from the additional HR mirror can have a different phase intrusion from the phase intrusion of the diffracted lateral modes propagating in the opposite diffraction direction. As a result, the diffracted lateral modes interfere destructively with each other and attenuate the output signal. To prevent this, the additional HR mirror is displaceable in the diffraction plane, thereby changing the path of the reflected diffracted lateral modes to adjust the interference pattern.

[0023] According to yet other features, the HPM does not function as an output coupler but only as an intra-cavity mode converter. The resonator configuration comprises end reflectors that define the cavity, with only one of the end reflectors being an HR mirror. The other output mirror (PR) only partially reflects the incident light. The HPM is configured with a high conversion efficiency and is provided to convert a predetermined lateral mode into a diffracted lateral mode incident on the PR at an angle of 90°. When a portion of the diffracted beam is reflected back into the cavity, the HPM converts the diffracted lateral mode back into the predetermined lateral mode. Thus, the resonator in this aspect has a single output, and its cavity supports each predetermined diffracted lateral mode and is divided into regions separated from each other by the HPM. Put another way, in contrast to the previous aspect, the laser in this aspect is configured to have different lateral modes propagating in the same cavity.

[0024] Further features include different configurations of the laser components. Specifically, the HPM and the PR mirror can be configured as two separated components similar to the structural features of the previous aspect. Alternatively, the HPM and the PR mirror can be configured as an integrated or monolithic component.

[0025] Other features relate to the monolithic laser in which the HPM is provided. Structurally, the resonator of this embodiment consists of a plate of PTR glass doped with one of the rare earth ions. Thus, in contrast to the structural features of each of the preceding embodiments in which the gain medium is only one of the individual cavity components, the entire plate is the gain medium representing a portable, monolithic solid-state laser. Two HR coatings are deposited on opposite sides of each of the gain elements to define a resonant cavity between them.

[0026] Similar to one of the previously disclosed features, the monolithic laser of the previously disclosed configuration has two outputs of diffracted modes. According to the features of this embodiment, the gain element includes an additional HR coating deposited on the side of the element located in one path of the output diffracted modes, which provides a single output to the structure.

[0027] Another feature of this disclosure is that it comprises several VBGs recorded on the same volume of photosensitive glass, each VBG with a respective HPM, which are optically independent while physically overlapping spatially with each other. By emitting beams with different wavelengths at different angles of incidence, diffraction by different HPMs is provided. Structurally, this embodiment comprises a tiltable glass that allows switching between different HPMs, and thus allows differently shaped output transverse modes.

[0028] The above and other structurally and conceptually complementary features will become clearer with reference to the attached figures, which are not drawn to a fixed scale. The figures, which provide illustration and a further understanding of various associated embodiments and schematics, constitute part of this specification but do not depict any specific schematic or embodiment to an extent. In the drawings, each identical or nearly identical component appearing in different figures is indicated by the same reference numeral. For the purpose of clarity, not all components may necessarily have the same reference numeral. [Brief explanation of the drawing]

[0029] [Figure 1] This is a diagram of an optical layout for a recording HPM operating according to the concept of the present invention. [Figure 2A] This is an illustrative optical schematic diagram providing measurement and optical beam conversion from Gauss's TEM00 mode to TEM11 mode. [Figure 2B] This is an illustrative optical schematic diagram providing measurement and optical beam conversion from Gauss's TEM00 mode to TEM11 mode. [Figure 3] This is an illustrative optical schematic diagram of one example of a broadband laser of the present invention, comprising an HPM cavity mode converter operating as a bidirectional output coupler. [Figure 4] This is a schematic diagram illustrating another example of the broadband laser of the present invention, with an HPM cavity mode converter operating as a single-output coupler. [Figure 5] This is a diagram illustrating another optical overview of the broadband laser of the present invention, which is equipped with an HPM mode converter. [Figure 6] This is a diagram of the laser of the present invention, which is equipped with an HPM cavity mode converter and configured as a monolithic optical generator. [Figure 7] Figure 3 is an optical overview of a prototype of the broadband laser of the present invention in operation. [Figure 8A] Figure 7 shows an overview of the process for capturing the broadband laser power beam that has been diffracted by the optical diagram. [Figure 8B] Figure 8A outlines the spatial modes of different laser powers captured in the far field of view of the diffracted transverse modes. [Figure 9] Figure 5 is a diagram illustrating the optical overview of a prototype of the broadband laser of the present invention in operation. [Figure 10A] Figure 9 shows a schematic diagram illustrating the capture of a broadband laser power beam that has been diffracted by the optical overview. [Figure 10B] Figure 10A outlines the different spatial modes of broadband laser power captured. [Figure 11] Figures 3, 4, and 5 illustrate the operation of a multiplex HPM that implements the concept of the present invention. [Modes for carrying out the invention]

[0030] This disclosure provides a laser having a resonant cavity shape that conditions a predetermined cavity transverse mode, and an HPM installed in the cavity to diffract a portion of the predetermined mode, while embedding a desired phase profile in the diffracted mode having a spectral width up to the spectral width of the HPM.

[0031] It is important to note that VBG is the simplest volume hologram, capable of diffracting different wavelengths without distorting its beam profile, as long as the initial beam profile satisfies the Bragg condition. In contrast to VBG, HPM alters the incident beam wavefront. This also means that HPM can be tested at different or the same wavelength as the recorded one.

[0032] Referring to Figures 1 and 2A-2B, the encoding of a desired phase profile to the propagation Bragg grating (TBG) 12 is performed by a holographic two-light recording system 10, which includes a standard binary phase mask (PM) 20 mounted on one of the arms, i.e., by a chromatic aberration ultraviolet beam 14. The PM 20 has a desired phase profile with respect to the hologram wavelength of the recording beam 14', not with respect to the regeneration wavelength. The recording beam 14', shaped by the PM 20 and no longer having the Gaussian shape of beam 14, and the ultraviolet chromatic aberration beam 16, which is split from beam 14 by a beam splitter, interfere at an angle with respect to the normal of the TBG 12 to create an interference fringe pattern. The HPM 22 created by the system 10 has a binary phase profile. Based on the above, when the HPM 22 in Figure 2B is in use, the phase profile of the diffracted beam 24, which propagates at any wavelength corresponding to the Bragg condition and is measured at a portion of the wavelength by the CCD 26 in Figure 2A, is the same as the phase profile with respect to the ultraviolet beam 14'.

[0033] Specifically, Figures 2A and 2B show Gauss TEM 00 Mode to Gauss TEM 11 This shows an illustrative beam conversion to a mode. The HPM22 of four regions is encoded in TBG12 in Figure 1. The spatial profiles of both the diffracted beam 24 and the transmitted beam 28 in Figure 2B are recorded in the far field via a Fourier lens. The spectral width of the diffracted beam may be the same as the spectral width of HPM22. HPM22 can be up to 300 nm or more, which allows the beam to be shaped according to the concept of the present invention, with a range between narrow linewidths up to 1 nm and broad spectral lines up to HPM22.

[0034] Returning to Figure 1, HPM22 is not limited to the transformation of an incident beam having Gaussian modes. As a beam shaping element, HPM22 can be created to transform any complex mode into a different complex mode. The recording overview in Figure 1 can be used to create HPM22 operating in complex modes by incorporating the phase profile of PM20 and an additional PM20' with a different phase profile and the Gaussian beam 16. The interference between the complex diffracted lateral recording modes 14' and 16' thus results in HPM22 having two complex modes recorded in its volume.

[0035] Referring to Figure 3, the HPM22 mounted in the resonant cavity 30 of the laser of the present invention acts as an output coupler. A planar resonator is shown here for simplicity. In practice, different types of resonators may be considered for the laser of the present invention. In addition to the gain element G36, the resonator 30 is composed of two high-reflectivity planar mirrors (HR) 32 and 34, respectively. Such a resonator provides a single transverse mode when the Fresnel number (F) is less than 1 in the following equation.

[0036]

number

[0037] Here, r is the radius of the fundamental mode, L is the length of the resonator, and λ is the wavelength. This mode has a Gaussian transverse profile of intensity. HR mirrors 32 and 34 each maintain all the generated power inside the resonator 30. Placing an HPM 22 tuned to a desired Bragg angle in the resonator 30 results in diffraction of a predetermined generated cavity mode 42 in opposite directions within the diffraction plane 35. As a result, the HPM 22 functions as a bidirectional output coupler. The second output is the result of back reflection of a predetermined cavity mode 42 from HR 34, which, when incident on the HPM 22, is in the same diffraction plane 36 as the initially diffracted transverse mode, but diffracted in the opposite direction. The coupling efficiency can be varied by the changing diffraction efficiency of the HPM 22, or by progressive detuning of the HPM from the Bragg angle, which can be achieved by rotating the HPM 22 around its axis with any suitable actuator 38. The efficiency of the HPM 22 is selected to meet customer-specified requirements. In the configuration of Figure 3 (and Figure 4), the efficiency of the mask does not need to be very high and may be limited to a range of 20-30%. A given cavity transverse mode 42 may be, for example, TMoo, and the desired mode may be a higher-order transverse mode TMmn. Placing the HPM22 in the resonator 30 introduces a phase intrusion into the diffracted beam 24. This means that while a given mode in the cavity between the HR mirrors 32, 34 is, for example, Gaussian, the two output transverse modes 24, determined by the HPM22, have identical profiles but are different from the Gaussian mode. The HPM can provide a nearly random wavefront for both mode conversion and aberration correction.

[0038] Figure 4 shows the same configuration as Figure 3, but with one additional structural feature. Specifically, the laser 50 in this figure corrects the drawback of Figure 3, which is bidirectional output emission resulting in diffracted modes with a 50% power loss as they may be incident on the target being laser-treated. Structurally, the resonator 30 is provided with an additional HR mirror 40 aligned with the HPM 22 in the diffraction plane, which reflects upward-diffracted modes 24' (relative to the plane of the paper) back into the cavity. The reflected transverse modes 24' travel through the HPM 22 and have the same phase profile as the downward-diffracted modes 24''.

[0039] The diffracted beams 24' and 24'' can each have different phase delays and therefore interfere constructively or destructively with each other on their paths out of the resonant cavity 30. To optimize output power, prevent destructive interference, or simply control it, the HR40 and cavity 30 are displaced relative to each other in the diffraction plane by actuators 38, for example, as shown in Figure 3. Such a back-reflected predetermined transverse beam 24' has the phase profile of HPM22. All output diffracted modes 24 are further referred to as having complex phase profiles or simple complex modes.

[0040] Figure 5 shows an overview of another functional aspect of the concept of the present invention. Here, the HPM22 is not configured as an output coupler, but solely as an in-cavity mode converter mounted in the cavity 30. Similar to the configurations shown in Figures 3 and 4 respectively, the HPM22 has a desired efficiency that varies over a wide range to satisfy the requirements of any given specification for the mask. In contrast to the configurations disclosed earlier, the efficiency of the HPM22 used in the configuration of Figure 5 is preferably above 90%. A further difference between the overview of Figure 5 and the overviews of Figures 3 and 4 is that the output mirror 44 is a partially reflective (PR) mirror with a desired reflection coefficient, and is not like the HR mirror in Figure 3.

[0041] The HPM22 is installed in such a manner that a diffracted beam 24 with a desired phase profile is emitted to the output coupler PR44 at a normal incidence angle. Similar to the previous configuration, a predetermined transverse mode 42 is generated with the help of the gain element G36. When the Fresnel number is less than 1, the predetermined mode 42 is Gaussian. The predetermined transverse mode 42 is converted by the HPM22 into the complex phase profile of the desired transverse mode 24. Thus, two different transverse modes, namely the predetermined one 42 to the left of the HPM22 and the desired one 24 to the right of the HPM, coexist in this resonator.

[0042] Figure 6 shows the laser of the present invention with an alternative configuration that includes a photosensitive gain medium PSG46, such as PTR glass, as an integrated resonator. PSG46 is doped with rare earth ions and possesses both high photosensitivity and high quantum yield of emission. The use of PTR enables the following design in which the monolithic solid-state laser 50 of the present invention emits radiation with a nearly random phase profile. To meet efficiency requirements, HPM22 is recorded on the PSG46 at a predetermined angle that varies over a wide angular range. In the example shown, HPM22 is recorded at 45°.

[0043] Three high-reflectivity coatings HR are deposited on opposite sides of the gain element 46, comprising two end HRs 48 and 52 that define a resonant cavity 56 between them, and a third HR 54 that coats the other side of the PSG 46 in the diffraction plane adjacent to the HPM 22. A predetermined mode 42 generated in the resonator 56 between ends HRs 48 and 52 is partially diffracted as it travels through the HPM 22 and propagates as desired transverse modes 60 and 60' having the same phase profile, which are embedded by the HPM 22. Thus, the HPM 22 functions as a bidirectional output coupler. To prevent two unwanted outputs of the desired transverse mode 60, the desired mode diffracted upward is reflected back by HR 54 in a manner similar to that of Figure 4. Thus, the desired mode 60' is partially diffracted in the propagation plane of the predetermined mode 42 as it is converted back to the predetermined mode 42. The propagated diffracted mode 60' interferes with the downward diffracted mode 60. The difference in phase penetration for the diffracted modes can be adjusted by changing the difference between the axis of a given mode 42 and the upper mirror HR54. Cancellation of the phase penetration difference is achieved either by positioning the laser 50 on the multi-axis stage 82, or by displacing a pump (not shown) that outputs a pump beam coupled to the resonator. The laser 50 shown in Figure 6 is compact and highly resistant to various environmental stresses.

[0044] Figure 7 shows an experimental device based on the laser 50 of the present invention shown in Figure 3. Specifically, a birefringent single crystal Yb 3+ :KYW, with a dopant concentration of 2% and a thickness of 3 mm, pIt is used as an active (doped) gain medium 36 cut along the axis. The known broad emission linewidth of the crystal with its maximum in the vicinity of 1040 nm enables the wavelength tunability of the laser 50. The gain medium 36 is optically pumped by a fiber-coupled continuous-wave (CW) laser diode (not shown) that outputs an average power of up to 40 W at a maximum of 981 nm. A set of two aspherical lenses, which is arranged in a 4f detection configuration and used to image a diode that is output to a spot size of approximately 250 μm, is positioned within the gain medium 36. A dichroic end mirror 32 optimized for an incident angle of 0° is arranged between the pump and the gain medium 36 with the gain medium 36 arranged adjacent to the mirror 32. The gain medium 36 has a high transmission efficiency at a wavelength of 981 nm and a high reflection at a wavelength of 1040 nm.

[0045] An aspherical lens 58 with a focal length f1 = 100 mm is inserted at a focal distance L1 from the end face of the Yb 3+ :KYW gain medium 36, and the other aspherical lens 66 is configured with a focal length of 250 mm corresponding to a length L3. The lens 58 makes parallel a predetermined transverse mode 42 generated by the gain element 36 and incident on the HPM22. The HPM22 is provided in the optical path of the predetermined mode 42 and is angularly adjusted to satisfy its Bragg condition. The predetermined transverse mode 42 transmitted through the HPM22 is emitted to the high-reflection mirror 32 that forms the HR mirror 34 and the cavity 30. The desired modes 24’ and 24” are diffracted by the HPM22 to form the desired mode 24” focused by the lens 66 on the beam profiler 68, the desired mode 24’ measured by the spectrometer or the photodiode 62, and the laser output. The reason for having two desired mode outputs 24’ and 24” for each round trip of the cavity is the same as that explained earlier in relation to Figure 3. The percentage of the combined energy of the output is determined by the diffraction efficiency of the HPM22. For an HPM with a diffraction efficiency of 5% at the lasing wavelength, the round-trip output coupling loss is 9.75% (1 - 0.9 2 )

[0046] Since the Bragg condition of the HPM is satisfied, the diffracted beam is encoded by the phase profile of HPM22. The output beam 24" is then converted to the desired spatial distribution on a target illuminated in the far field, which can be easily done by sending these beams through a Fourier lens 66 (f=250mm) in a 2f configuration and observing the beam profile in a CCD camera at the focal length of the lens (L~f=250mm). It should be emphasized that both diffracted beams receive the same phase profile. Thus, while this laser has a resonator limited by two high-reflectance mirrors, the HPM plays the role of a mode that converts the output coupler. The efficiency of the output coupling can be controlled by the diffraction efficiency of the HPM and the detuning of the HPM from the Bragg condition.

[0047] Referring to Figures 8A and 8B1-8B3, the HPM22 encodes information for the phase mask of four regions in its insertion into the laser cavity, which is used as a bidirectional output coupler in Figure 3. The laser output, which is the desired mode 24, is imaged to a CCD camera by a Fourier lens 70, and the CCD camera is spaced away from the lens 70 at a distance L equal to the focal length f to capture the far-field spatial distribution. Each of the different far-field spatial profiles in Figures 8B1-8B3 is recorded depending on the position of the HPM, which is displaceable relative to a given laser beam in the cavity by the actuator 38 in Figure 3.

[0048] Figure 9 shows the optical layout of another experimental laser 50 based on the configuration of Figure 5. The main difference between this device and the device in Figure 7 lies in the nature and role of the HPM22. In the laser of Figure 7, the diffraction efficiency of the HPM22 may be rather low to provide optimal output coupling, whereas here the HPM22 is configured to be very efficient. Therefore, the intensity of the transmitted beam is very small, and the main portion of the radiation is diffracted by the HPM22. The diffracted beam 24 is emitted to a retroreflector or PR end mirror 44 that acts as an output coupler. The reflected portion 24r of the diffracted beam is returned to the HPM22, converted to a predetermined mode 42, and directed back to the gain medium 36. A key feature of this resonator is that the predetermined mode 42 propagates to the left from the HPM22, while the desired mode 24, with the phase profile of the HPM22, propagates to the right from the HPM22. Thus, the HPM22 acts as an in-cavity mode converter.

[0049] Referring to Figures 10A and 10B1-10B3, a sample HPM22 encoding information for the phase masks of four regions is inserted into the laser cavity, which is used as an in-cavity mode converter. The laser output, which is the desired mode 24 in Figures 5 and 9, is imaged onto a CCD camera by a Fourier lens 70 to capture its far-field spatial distribution. Different far-field spatial profiles are recorded depending on the position of the HPM for a given mode, as shown in Figures 10B1-10B3.

[0050] Referring to Figure 11, one of the key features of VBGs in PTR glass is the possibility of recording several VBGs in the same PTR volume. Several VBGs are physically superimposed in space but optically independent. By emitting beams with different wavelengths at different angles of incidence, diffraction by different VBGs is provided. This concept was tested by placing a multiplexed HPM80 in the laser 50 of the present invention in the configuration disclosed earlier in Figures 3, 4, and 5. Thus, the multiplexed HPM80 consists of a single PTR volume hosting multiple HPMs to be recorded, each having a different phase profile from one another. The laser 50 with the multiplexed HPM80 was tested and demonstrated that the efficiency of the phase profile of each particular desired mode embedded by the corresponding HPMs that are part of the multiplexed HPM80 is the same as if the individual HPMs were being used. By tilting or rotating the multiplexed HPM80 around its axis A by any conventional actuator, the laser 50 can be made to switch its output between the desired modes that have been recorded.

[0051] The embodiments disclosed herein by the present invention are not limited in their applications to the structural and arrangement details of the components described below or shown in the accompanying drawings. Specifically, while broadband modes with linewidths up to 300-400 nm can be successfully formed in the disclosed configurations, narrowband modes with linewidths as small as 0.02 nm may also be formed. These embodiments can take other forms and can be implemented or carried out in various ways. For example, the disclosed HPM can be used to counteract thermal lensing formed in a resonator by a high-power broadband beam associated with, for example, an ultrashort pulse laser and a high-power CW laser having a broadband emission spectrum. In other commercial applications, the present invention is used to produce a near-diffraction-limited high-power laser beam with a broad spectrum when phase conversion is performed between different Gaussian modes.

[0052] Examples of specific embodiments are provided herein for illustrative purposes only and are not intended to be limiting. Specifically, actions, components, elements, and features considered in relation to any one or more embodiments are not intended to be excluded from similar roles in any other embodiments.

[0053] Furthermore, the wording and terminology used herein are for the purposes of description only and should not be construed as limiting. Singular or plural references are not intended to limit the systems or methods currently disclosed or their components, actions, or elements. In the event of any inconsistency in terminology between this document and any reference incorporated herein by reference, the terminology used in the incorporated reference is supplementary to the terminology used in this document, and the terminology used in this document shall prevail in the event of any irreconcilable inconsistency.

[0054] While several aspects of at least one example have been described in this manner, those skilled in the art will understand that various modifications, changes, and improvements are readily conceivable. Therefore, the foregoing description and drawings are provided for illustrative purposes only. [Explanation of symbols]

[0055] 10 Holographic dual-light recording system 12. Transfer Bragg grid, TBG 14 Chromatic aberration ultraviolet beam 14' recording beam, ultraviolet beam, horizontal recording mode 16. Ultraviolet chromatic aberration beam, Gaussian beam 16' Horizontal recording mode 20 Binary phase mask, PM 22 HPM 24 Diffracted beam, transverse mode, diffracted mode 24r Reflected portion 24' Upward diffracted mode, reflected transverse mode, diffracted beam, reflected transverse beam 24" downward diffracted mode, diffracted beam, output beam 26 CCD 28 Transmitted beam 30 Resonant cavity, resonator 32 Dichroic end mirrors, plane mirrors, HR, high reflectivity mirrors 34. Flat mirror, HR 35 Diffraction Plane 36 Diffraction plane, gain medium G36 gain factor 38 Actuators 40 Additional HR Mirrors 42 Cavity lateral mode, predetermined lateral mode 44 Output mirror, output coupler PR, PR end mirror 46 Gain Element, Photosensitive Gain Medium, PSG 48 Edge HR 50 lasers 52 Edge HR 54. Third home run 56 Resonant cavity, resonator 58 Aspherical lenses 60 Desired transverse mode, downward diffracted mode 60' Desired transverse mode, diffracted mode 62 Photodiodes 66 Aspherical lenses, Fourier lenses 68 Beam Profiler 70 Fourier Lens 80 Multiple HPM 82 Multi-axis stage

Claims

1. A resonant cavity configured to generate radiation in a predetermined transverse mode that vibrates on the propagation surface, A broadband holographic phase mask (HPM) is provided in the resonant cavity and is created with a phase profile different from the phase profile of a predetermined transverse mode, wherein the HPM is adjusted to a Bragg angle, thereby diffracting a portion of the generated radiation that propagates in a desired transverse mode having the phase profile of the HPM, with a spectral width up to the bandwidth of the HPM, on a diffraction plane extending across the propagation plane, Equipped with, The aforementioned resonant cavity is A plurality of reflectors, spaced apart, define the aforementioned resonant cavity, wherein at least one of the reflectors is a high reflectivity (HR) mirror. A gain element spaced inward from the reflector, Furthermore, The laser is characterized in that the plurality of reflectors arranged at the aforementioned intervals include two HR mirrors on either side of the HPM, the two HR mirrors acting as output couplers that diffract the desired transverse modes propagating on the diffraction plane outside the resonant cavity.

2. The laser according to claim 1, wherein the diffracted radiation propagating in the desired transverse mode is output from the resonant cavity in both directions on the diffraction plane, each depending on the direction of propagation of the predetermined transverse mode on the propagation plane between the two HR mirrors.

3. The laser according to claim 2, further comprising an additional HR mirror positioned at a distance from the HPM on the diffraction plane and provided for reflecting the output transverse modes propagating in one direction on either side of the diffraction plane, thereby separating both diffracted transverse modes from the resonant cavity in opposite directions on either side of the diffraction plane.

4. The additional HR mirror is displaceable on the diffraction plane to control the difference in phase penetration to the isolated desired transverse modes, thereby causing the desired transverse modes to interfere in a reinforcement manner while being output from the resonant cavity in the opposite direction, as in the laser according to claim 3.

5. The laser according to claim 3, wherein the HPM has a diffraction efficiency selected to enable optimal output coupling.

6. The laser according to claim 2, wherein one of the spaced-out reflectors is a partially reflective (PR) mirror spaced apart from the HR mirror, and the HPM is provided to diffract the desired transverse mode incident perpendicularly to the PR mirror, which is configured to reflect a portion of the desired transverse mode into the resonant cavity and separate the remaining portion of the desired transverse mode from the resonant cavity.

7. The laser according to claim 6, wherein the PR mirror is configured with a reflectance coefficient selected to provide a desired output to the diffracted radiation in the desired transverse mode.

8. The laser according to claim 6, wherein the PR mirror and the HPM are spaced apart from each other.

9. The laser according to claim 6, wherein the PR mirror and the HPM are configured as monolithic elements.

10. The laser according to claim 1, wherein the HPM is pivotably mounted around an axis extending perpendicular to the propagation plane of the predetermined transverse mode and the propagation plane of the predetermined transverse mode, in order to provide a controllable output coupling of the desired transverse mode.

11. The laser according to claim 1, wherein the HPM has a plurality of regions, and the HPM is controllably displaceable on the diffraction plane such that the predetermined modes are incident on different locations of the HPM that encode the respective phase profiles of the desired transverse modes which are different from each other.

12. The laser according to claim 1, wherein the HPM comprises a spectral width in the range of 0.02 nm to 300 nm.

13. The laser according to claim 1, wherein the gain element is a volume of PTR glass doped with one or a combination of rare earth ions, at least two HR reflectors are covered at each spaced-out location around the PTR glass so as to define the propagation plane of the predetermined transverse mode between them, the HPM is formed inside the gain element, and the gain element having the covered HR reflectors and the PTR glass is configured as a monolithic laser.

14. The laser according to claim 13, wherein the HPM is configured as a bidirectional output coupler that provides the desired transverse mode output in each opposing direction on the diffraction plane.

15. The laser according to claim 13, further comprising an additional HR coating aligned with the HPM on the diffraction plane and covering additional locations of the gain element, the additional HR coating restricting the output of the diffracted radiation in the desired transverse mode to a single output in the opposite direction.

16. The laser according to claim 15, further comprising a multi-axis stage for supporting and displacing the gain element at a desired distance on the diffraction plane to control the difference in phase penetration for the diffracted transverse modes so that, in the single output, the diffracted transverse modes provide constructive interference among them.

17. The laser according to claim 1, wherein multiple HPMs are formed in a single PTR glass, each having a different phase profile, and the PTR glass is mounted in the resonant cavity to rotate around an axis extending perpendicular to both the propagation plane and the diffraction plane to controllly modify the phase profile of the output desired transverse mode.

18. The laser according to claim 1, wherein the HPM is configured to cancel out the thermal lens formed in the resonant cavity by the generated radiation.

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