Classifications

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  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/06—Construction or shape of active medium
  • H01S3/063—Waveguide lasers, i.e. whereby the dimensions of the waveguide are of the order of the light wavelength
  • H01S3/0632—Thin film lasers in which light propagates in the plane of the thin film
• • H—ELECTRICITY
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  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/02—Constructional details
  • H01S3/025—Constructional details of solid state lasers, e.g. housings or mountings
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  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/02—Constructional details
  • H01S3/04—Arrangements for thermal management
  • H01S3/0407—Liquid cooling, e.g. by water
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  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/02—Constructional details
  • H01S3/04—Arrangements for thermal management
  • H01S3/042—Arrangements for thermal management for solid state lasers
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  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
  • H01S3/08059—Constructional details of the reflector, e.g. shape
  • H01S3/08063—Graded reflectivity, e.g. variable reflectivity mirror
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  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
  • H01S3/08—Construction or shape of optical resonators or components thereof
  • H01S3/08081—Unstable resonators
• • H—ELECTRICITY
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  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
  • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
  • H01S3/094084—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light with pump light recycling, i.e. with reinjection of the unused pump light, e.g. by reflectors or circulators
• • H—ELECTRICITY
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  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/09—Processes or apparatus for excitation, e.g. pumping
  • H01S3/091—Processes or apparatus for excitation, e.g. pumping using optical pumping
  • H01S3/094—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light
  • H01S3/0941—Processes or apparatus for excitation, e.g. pumping using optical pumping by coherent light of a laser diode
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
  • H01S3/1123—Q-switching
  • H01S3/115—Q-switching using intracavity electro-optic devices
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
  • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
  • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
  • H01S3/1123—Q-switching
  • H01S3/117—Q-switching using intracavity acousto-optic devices

Definitions

• This invention relates to diode-pumped solid state lasers and more particularly to power scalable diode-pumped slab lasers compatible with high beam quality and high brightness outputs.
• the optical resonator contains active material configured as an elongated rod with either round or rectangular cross-sectional dimensions on the order of 1-10 mm in either direction perpendicular to the optical axis.
• These axially-symmetric "rods" may be side- or end-pumped by diode lasers, fiber coupled diode lasers, or diode laser bars.
• the state of the art for diode pumped lasers based on rod configurations ranges from 50 W for Q-switched DPSSL 's with near- diffraction-limited (NDL) beam quality to just over 200 W for highly multi- mode outputs using multiple gain media and complex pumping arrangements. It is well known that axially-symmetric rod based laser configurations exhibit a fundamental limitation in regards to both output power and beam quality. For typical crystalline laser rods, such as YAG, fracture occurs when the output power exceeds about 60 W per cm of length. The fracture limit is still lower for other commonly used materials such as YNO 4 and YLF.
• the output power is further limited due to beam size and mode matching considerations.
• the TEM 00 power is limited to less than about 30 W per rod, if the resonator is required to be stable over a wide pump power range.
• the power limit for stable TEM 0 o operation reduces to less than about 20 W.
• Higher TEMQ 0 mode output powers from rod geometries can be achieved only by limiting the pump power range over which the resonator is stable. Even use of direct pumping into the upper laser level may extend the aforementioned limits only by about 30%. Consequently, the axially symmetric rod geometry fundamentally limits the attainable output power for high brightness beams to 100 W at the most.
• a more favorable geometry for high power operation is provided by rectangularly shaped slabs, which are not constrained by axial symmetry considerations.
• the fracture limit of slab lasers is known to be higher as compared to a rod by half the aspect ratio w/t where w is the width of the slab and t is its thickness. This is the result of larger surface to volume ratio and a smaller temperature difference across the thinner dimension, which sets up a near one-dimensional temperature gradient.
• the larger the aspect ratios the more favorable the heat dissipation profiles.
• a thinner slab is particularly effective in minimizing the effects of thermally-induced distortions and stress birefringence, allowing thermal lensing to be compensated through various means known in the art of resonator design across a full operational power range.
• thin slabs presented certain difficulties to high pumping efficiencies due to unfavorable design trade-offs between efficiency, power and output beam brightness.
• the prior art recognizes many different designs employing zigzag compensation schemes in high power optical oscillators and amplifiers.
• the laser beam is made to travel along a zigzag path within slabs of relatively small aspect ratios by way of total internal reflection (TIR) at the faces of the slab.
• TIR total internal reflection
• the key premise behind all methods based on the zigzag approach is that as long as the temperature gradient is along the same plane as the direction of beam propagation, residual thermally induced variations of the index of refraction are substantially averaged out as the zigzag path moves across different temperature regions, at least to first order.
• Such an edge-pumped configuration has the advantage of separating the pumped and cooled surfaces while allowing for simultaneous optimization of both pump power and abso ⁇ tion as was described, for example, in US Patent No. 6,134,258.
• Still another alternative to zigzag slab lasers utilizes end-pumping in which the pump light is aligned with the laser beam resulting in high absorption efficiencies.
• An example of this configuration was taught in US Patent No. 6,268,956.
• the zigzag slab is known to be susceptible to edge effects and warping, a problem common to all pumping schemes.
• the relatively large slab shaped active materials described in the art also require a high degree of parallelism to support the TIR path, which render them expensive to fabricate and manufacture.
• the TEM 00 output power is fundamentally limited, due to a mismatch between the mode and the slab with its typically have relatively large cross-section.
• This approach provides a thin gain cross section with a high enough aspect ratio to allow the desired quasi-one-dimensional heat conduction.
• scaling from this approach may be limited, due to increasing complexity of the beam shaping optics and unfavorable trade-offs between pump absorption, pumped versus unpumped volume ratios and the gain-length product, the latter being especially critical for Q-switched operation.
• Sensitivity to doping and pump inhomogeneities may impose further restrictions on the power and beam quality attainable, limitations that are common to most end-pumped architectures.
• a waveguide slab CO 2 laser is generally configured with electrode separation small enough to cause waveguiding of the laser beam along only one dimension of the discharge volume, while propagating freely in the wider dimension.
• US Patent No. 4,719,639 issued to Tulip discloses, for the first time a CO 2 slab waveguide laser comprising an unstable resonator structure in the unconfined direction but a stable waveguide resonator in the guided direction.
• the unstable resonator described by Tulip includes one concave and one convex mirror and is known in the art as a positive branch unstable resonator.
• Another slab waveguide resonator structure was described in US Patent No. 4,939,738 issued to Opower which was also provided with a positive branch unstable resonator in the non-waveguide direction.
• US Patents No. 5,335,242 issued, for example, to Hobart et al and US Patent No.
• waveguide lasers have also been demonstrated as an efficient means to generate high brightness output beam from solid state media.
• composite configurations wherein the waveguide slab is sandwiched between one or more matching stacks of dielectric materials of lower indices of refraction than the active laser material were used to confine either or both pump and signal light.
• the signal beam is guided along the thin direction if the Fresnel number — defined as a 2 / ⁇ L - is much smaller than unity.
• the required thickness for a waveguide slab geometry is smaller by about an order of magnitude than the 1-2 mm typically utilized for 10 ⁇ m CO 2 lasers of similar length.
• an 8 ⁇ m single mode active core was found capable of providing 12 W output in a single fundamental mode from an Yb:YAG waveguide using a composite double-clad diffusion-bonded structure, constructed according to principles described in US Patent No. 6,160,824 to Meissner. It is however recognized that, whereas such thin waveguide constructions may be advantageous for high threshold and/or low gain systems, such as the quasi- three level Yb:YAG, (due to lower thresholds and improved overlap between pump and signal), they are not conducive to power scaling to the >100 W levels of interest herein.
• power scaling from such cladding-pumped thin waveguide structures may be gain limited, especially if short pulse operation is desired.
• efficient single mode laser oscillation from a, 8-10 ⁇ m thin waveguide cannot be readily sustained at pump power inputs in excess of 20 W input due to parasitic oscillations and amplified stimulated emission (ASE) effects. Losses attributed to these effects represent even more of an issue for pulsed operation, where overly high gains may prevent Q-switch hold-off.
• waveguides with small cross-sectional areas may be subject to optical coatings' damage due to high intra-resonator peak powers.
• an object of the present invention is to provide diode pumped solid state laser systems, and their methods of use, with high output power from a single active laser component with minimal restrictions on the useable pump power range.
• Another object of the present invention is to provide diode pumped solid state laser systems, and their methods of use, that provide improved beam brightness at scaled-up power levels by minimizing the effects of thermally-induced aberrations and stress birefringence.
• a further object of the present invention is to provide diode pumped solid state laser systems, and their methods of use, that provide improved beam brightness at scaled-up power levels utilizing high aspect ratio planar gain element geometries.
• a further object of the present invention is to provide diode pumped solid state laser systems, and their methods of use, that provide improved beam brightness at scaled-up power levels consistent with one dimensional heat flow perpendicular to a beam propagation direction across an entire width of the active region.
• Yet another object of the present invention is to provide diode-pumped solid state laser systems, and their methods of use, that select mutually orthogonal directions for pumping, cooling and beam propagation.
• Another object of the present invention is to provide diode-pumped solid state laser systems, and their methods of use, that use mutually orthogonal directions for pumping, cooling and beam propagation by use of a planar, thin slab gain medium
• a further object of the present invention is to provide diode-pumped solid state laser systems, and their methods of use, using a thin slab laser that is edge-pumped and has cooling along the two largest opposing faces which are orthogonal to the pump direction and to the beam propagation direction
• an object of the present invention is to provide an optical system that has a high reflector and an output coupler which define a resonator cavity and an optical axis.
• a slab gain medium is positioned in the resonator cavity.
• the slab gain medium is configured to provide propagation of an optical laser beam along the optical axis through the slab medium.
• a first diode pump source produces a first pump beam incident on the slab gain medium in a direction perpendicular to the optical axis.
• a cooling member is coupled to the slab gain medium and provides cooling in a direction perpendicular to the optical axis and to the direction of the first pump beam.
• a laser structure in another embodiment, includes a high reflector and an output coupler that define a resonator cavity with an optical axis.
• a slab gain medium is positioned in the resonator cavity and has an aspect ratio greater than 5.
• the slab medium is configured to provide propagation of an optical laser beam along the optical axis through the slab medium.
• a cooling member is coupled to the slab gain medium.
• a first diode pump source produces a first pump beam incident on the slab gain medium in a direction perpendicular to the optical axis.
• a laser structure in another embodiment, includes a high reflector and an output coupler that define a resonator cavity with an optical axis.
• a slab gain medium is positioned in the resonator cavity.
• the slab gain medium includes top and bottom surfaces, first and second side surfaces and first and second end faces.
• a cooling member is coupled to the top and bottom surfaces.
• a first diode pump source produces a first pump beam incident on a full face of at least one of the first and second side surfaces.
• An optical beam propagates in the slab gain medium in a plane that is parallel to at least one of the top and bottom surfaces.
• an optical system in another embodiment, includes a high reflector and an output coupler that define a resonator cavity with an optical axis.
• a slab gain medium is positioned in the resonator cavity and has an aspect ratio less than 50.
• the slab medium is configured to provide propagation of an optical laser beam along the optical axis through the slab medium.
• a cooling member is coupled to the slab gain medium.
• a first diode pump source produces a first pump beam incident on the slab gain medium in a direction perpendicular to the optical axis.
• an optical system includes a slab gain medium positioned along an optical axis and has an aspect ratio greater than 5. The slab gain medium is configured to provide propagation of an optical laser beam along the optical axis through the slab medium.
• a first diode pump source produces a first pump beam incident on the slab gain medium in a direction perpendicular to the optical axis.
• a cooling member is coupled to the slab gain medium and provides cooling in a direction perpendicular to the optical axis and to the direction of the first pump beam.
• a method for producing a high quality beam from a diode pumped solid state laser at high power.
• the high quality beam propagates an optical beam through a slab gain medium.
• An optical system is provided that is coupled to the slab gain medium that provides pumping, cooling and extraction of an optical beam along axes that are mutually orthogonal.
• An output beam is produced with a power of at least 80 W.
• a method for producing a high quality beam from a diode pumped solid state laser at high power.
• An optical system is provided with a slab gain medium that has a depth, length and a width, The width is selected to maximize absorption from a pumping radiation and the depth is selected to provide a one-dimensional thermal profile.
• the optical beam propagates through the slab gain medium.
• a beam is produced with a power of at least 80 W
• an optical system in another embodiment, includes a slab gain medium positioned in the resonator cavity.
• the slab gain medium is configured to provide propagation of an optical laser beam along the optical axis through the slab medium.
• a first diode pump source produces a first pump beam incident on the slab gain medium in a direction perpendicular to the optical axis.
• a cooling member is coupled to the slab gain medium and providing cooling in a direction perpendicular to the optical axis and to the direction of the first pump beam.
• Figure 1 depicts the thin slab geometry with mutually orthogonal cooling, pumping and beam propagation directions.
• Figure 2 is a schematic diagram of one embodiment of a laser oscillator of the present invention that includes a diode-pumped thin slab
• Figure 3 is a cross-sectional view of one embodiment of an edge- pumped, face-cooled thin slab laser of the present invention.
• Figure 4 is a three-dimensional representation of a slab mechanical mounting structure that can be utilized with the present invention.
• Figure 5 illustrates in greater detail the Figure 4 mechanical support structure.
• Figure 6 is a close-up view of a face-coated thin slab that can be utilized with the present invention.
• Figure 7 is a illustrates of one embodiment of a composite slab, with the active material sandwiched between two other slab-shaped stacks made of a different material, that can be utilized with the present invention.
• Figure 8 is a diagram that illustrates a more complex, 5-layer slab composite that can be utilized with the present invention.
• Figure 9 is a schematic diagram of one embodiment of a stable resonator that inco ⁇ orates a slab that can be utilized with the present invention.
• Figure 10 shows a plot of the multimode power output of a 0.7 mm thick 0.8% Nd:YAG slab
• Fig 11 shows a plot of the multimode performance of 1.0 mm thick, 0.8% Nd.YAG slab
• Figure 12 illustrates one embodiment of a hybrid resonator with a thin slab of the present invention.
• Figure 13 shows the output power performance of a 0.7 mm thick slab in the Figure 12 hybrid resonator.
• Figure 14 shows the output power from the 0.7 mm slab of Figure 13 as a function of hybrid cavity length
• Figure 15 is a plot of a projected beam propagation parameter, of one embodiment of the present invention, as a function of pump power for an optimized hybrid resonator design.
• Figure 16 is a schematic diagram that illustrates two types of off-axis hybrid resonators including a slab-shaped laser material of the present invention.
• Figure 17 illustrates the Q-switched output from a Q-switched hybrid resonator embodiment of the present invention with a 1mm thick slab.
• the present invention provides an active gain medium configured as a thin slab laser edge-pumped by radiation from diode arrays with cooling provided along the two largest opposing faces, which are orthogonal to the pump direction and also to the beam propagation direction
• Figure 1 illustrates the basic thin slab geometry configured according to the principles of the invention as a slab 1 of width w, thickness t and length 1, with the pumping, cooling and beam propagation directions indicated as all mutually orthogonal along Cartesian coordinates x, y and z, respectively.
• the slab's cross-section is defined by a pair of opposing end-faces 11 and 11A through which the laser beam propagates.
• the slab possess a high aspect ratio (defined as w/t) as well as thickness t small enough to allow efficient heat dissipation along the y direction through the top and bottom lateral faces 13 and 13 A, preferably by contact with solid cooling blocks made of material of high thermal conductivity.
• the pump radiation is preferably provided by diode arrays, which may be constructed as stacks of bars or be fiber coupled directly to the slab.
• the slab may be placed in a resonator or it may serve as an amplifier for a signal beam.
• the optical axis of the system coincides with the beam propagation direction z shown in Figure 1.
• the different dimensions of the slab can be optimized separately to provide efficient, power scalable, high brightness performance.
• the width w of the slab may be selected to maximize pump abso ⁇ tion, while the thickness t is chosen to provide optimal aspect ratio w/t consistent with gain constraints.
• gain for a slab configuration is an especially important consideration for short pulse operation because of the increased potential for parasitics and ASE losses.
• the laser designer is then free to select the length 1 of the slab to provide the desired power level by stacking diode bars (which may or may not be fiber-coupled) along this dimension. Power therefore scales with the slab length 1 for given slab aspect ratio and pump abso ⁇ tion parameters.
• the active slab material consists of a gain medium, such as Nd:YAG, which may be coated, bonded or brazed to different materials along its larger faces, prior to contact with the cooling blocks.
• a gain medium such as Nd:YAG
• Embodiments addressed in the present invention include straight-through thin slabs of high aspect ratio or, in the case of low gain materials, weakly-guiding multimode slab structures.
• thin slabs with aspect ratio greater than 5 are found to be best suited for maintaining uniform mechanical stress, birefringence and thermal lensing properties of the active element.
• the selection of the slab thickness is motivated by the need, on the one hand, to make it sufficiently small for efficient, one-dimensional heat transfer to the surrounding cooling structure and on the other hand, sufficiently large to provide efficient coupling to the pump and/or limiting the gain to thereby avoid undesirable ASE and parasitic losses.
• slabs with aspect ratios that are greater than 5 but smaller than about 20 are most beneficially utilized for higher gain, high conducting media such as Nd:YAG.
• the slab may be uncoated or it may be coated or sandwiched between suitably matched dielectric materials to provide some reflection of the pump light.
• FIG 2 illustrates schematically the diode-pumped slab laser resonator 11 formed in accordance with concepts of the subject invention.
• the resonator is defined by at least a high reflector 5 and an output coupler 6.
• a modulator 8 may further be inco ⁇ orated within the resonator, which may be a Q-switch or mode locker.
• Other optics such as polarizers, apertures etc. may be included within the cavity as required and are generically represented as optical element 9.
• Optical beam shaping elements, collectively indicated as composite 4 may be placed outside the resonator.
• the gain medium 10 includes one or more slab sections including optically active and inactive solid state materials all configured in the shape of an elongated rectangular slab as was shown in Figure 1.
• the longitudinal or optical axis 15 of the resonator 11 is parallel to the plane of the laser radiation 16 formed between the oscillator mirrors 5 and 6, upon excitation of the active gain material comprising the slab.
• the laser beam 16, defined by the resonator is generally rectangular in shape with an aspect ratio approximating that of the slab cross-section.
• Special beam transformation optics 110 may be utilized external to the resonator to symmetrize the beam, converting it to near- circularly shaped output beam 18.
• Pump radiation from an emission line of semiconductor diode laser arrays is allowed to enter the slab through the slab's edge faces indicated as 12 and 12 A, respectively, in Figure 1.
• the pump is arrayed as a plurality of stacked diode bars, located in close proximity to the slab's edges.
• Each array comprises multiple diode lasers. In the arrangement depicted in Figure 2, six stacks are shown on each side, but more or fewer stacks may be used depending on output power requirements.
• the diode stacks may be supplied by a commercial vendor such as Spectra-Physics Semiconductor lasers (SPSL) and CW power outputs of 50 W per bar are now readily available with emission wavelengths centered anywhere between 802 and 810 nm bands commonly utilized for Neodymium-doped materials such as Nd:YAG.
• SPSL Spectra-Physics Semiconductor lasers
• the selection of the center pumping wavelength is critical to establishing a uniform gain profile for a given width of the slab, as will be described further below.
• quasi-CW diode sources may be used, depending on the gain material excitation band parameters.
• pumping from two sides, using two sets of diode array stacks arrayed along the length of the slab is utilized, as illustrated in the embodiment of Figure 2.
• pumping from only one side may be employed with or without a reflective coating deposited on the opposite edge of the slab for pump light back reflection.
• Such one sided pumping may be well adapted to strongly absorbing laser materials, to slabs with shorter widths and/or to lower power applications.
• Figure 3 illustrates a cross-sectional view of the preferred embodiment of the edge-pumped slab laser.
• the diode laser 41 is shown mounted on bar 42.
• Embodiments using either lensed or unlensed bars i.e., with and without lenses 44) fall within the scope of the present invention, depending on the slab structure and desired operational parameters.
• the diode light is collimated along the fast direction using cylindrical microlenses, collectively indicated as 44.
• Unlensed bars are known to provide highly divergent light - typically over 10 x 60 degrees at the 85% intensity points.
• Lensed arrays may be provided by semiconductor laser vendors as a common option to standard products.
• microlenses generally reduce the divergence of the fast axis of the bars to less than approximately 2 degrees while the slow axis retains a full angle divergence on the order of about 10 degrees (all at the 85% intensity points).
• High coupling efficiency is achieved for pump light traversing straight through the slab, as long as the active slab thickness is greater than the corresponding spatial extent of the collimated diode light.
• No guiding of pump radiation is required in this case and slabs of the active material with only frosted or polished faces are sufficient for efficient operation of the laser, without any particular cladding, making this embodiment a readily manufacturable, cost effective option.
• radiation from unlensed diode bars is utilized to pump the slab.
• Such structures generally possess higher numerical aperture than the bare active slab with its relatively small thickness may provide, and may further partially or completely guide the pump radiation, thereby increasing the efficiency of coupling of divergent pump radiation to the active material.
• Such a composite slab is shown in Figure 3, where active material 50 of thickness t is contacted to slabs 51 and 51 A comprising undoped material of lower index of refraction, for a total slab thickness t c .
• Alternative embodiments of slab 10 may consist of uncoated, coated, or any other kind of composite slab, all of which share the property of high aspect ratio, and high pump light coupling efficiency for the active material portion of the slab.
• the optical fiber bundle may consist of a linear fiber array termination at each end of the fiber.
• the fibers in the linear array would have a lateral spacing corresponding to the laser diode emitter spacing, thereby allowing each emitter to be directly coupled into its corresponding fiber.
• lateral fiber spacings may be selected depending on pump light distribution requirements, and these may or may not differ from the fiber spacings at the input end of the fiber bundle.
• the slab 10 (which may be coated or multi-sectioned composite as alluded to above) is thermally controlled by contacting its top and bottom surfaces to cooling blocks 20 and 20A using thin interface layers 22 and 22A shown in Figure 3.
• the cooling blocks act as heat sinks, cooling the slab by drawing heat away from the faces according to known principles of direct conduction cooling. Efficient heat transfer from the pumped medium to the heat sinks is critical for establishing the desirable one dimensional temperature gradient within the lasing medium.
• the thermal interface layers placed between the slab surface (which may or may not be coated) and the heat sink help minimize thermal resistance at the interface, and also eliminate complications due to potential contamination of optical surfaces due to, for example, outgassing.
• the interface layers to provide an efficient thermal contact between the slab surface and the cooling blocks generally requires that the layers be thin and be able to conduct heat efficiently. It is further preferred that the thermal contact layers 22 and 22A be relatively soft compared to the cooling blocks so as to allow them to conform to any irregularities in the mounting heat sinks or the slab's surfaces.
• thermal contact layers 22 and 22A to act as flexible buffer layers, to help absorb thermal stress between the slab and the heat sink.
• Suitable materials for thermal contact layers include gold, indium and copper. These materials are available as thin foils, are sufficiently compliant and have thermal conductance that can compensate for variations in thermal conductivities between the slab (or the material that comprises the top and bottom surfaces of a slab composite) and the cooling block. Gold may be preferred material in embodiments where the thermal contact layer is also required to be especially thin and/or provide high pump light reflection, since gold has the added feature of efficient reflection at almost any wavelength.
• indium has several other advantages including a lower melting temperature (about 157 degrees compared to over 1000 degrees for gold) and is sufficiently soft to act as an excellent buffer layer.
• Indium may be used both as a cold contact layer or it may be used as a solder for bonding the slab to the heat sinks, a process usually carried out during assembly, wherein the cooling block/indium/slab assembly is held under pressure at elevated temperatures to flow the indium and eliminate contact resistance.
• Such bonding or "brazing” process is known in the art as an effective means for compensating for thermal expansion differences between crystalline or glass laser material and the material it is soldered to.
• More complex composite structures may alternatively be implemented to further reduce the stresses caused by thermal expansion differentials between a long and thin slab and the metallic cooling blocks.
• an thin alumina strip of the same surface dimensions as the slab may be sandwiched between two thin layers of indium, used to as buffer thermal contact to the slab on one side and the cooling thermal block on the opposing side. Since alumina and slabs made of crystalline materials (such as Nd:YAG) have comparable thermal expansion coefficients, there is minimal stress build-up along this critical interface. Still other alternatives, such as other types of ceramics or copper mesh filled with transition metal material may be considered, all of which fall within the scope of the present invention.
• cooling blocks 20 and 20A are fabricated of a metal with high thermal conductivity such as copper or aluminum alloy, and are generally of identical construction to help maintain symmetrical heat distribution.
• the cooling blocks are mounted on the broad surface of the slab and are of sufficient width to control the heat flow from the active area.
• Thermal modeling shows that to achieve a one dimensional heat flow across the thickness of the slab, the width of the cold plates should be equal to the width of the slab.
• coolant flow channels 25 and 25A are provided within the cold plates structure to allow water or other fluid to be pumped through. At least one such flow channels may be provided per each coolant block.
• FIG. 4 A three dimensional representation of the mechanical mounting structure for the slab is shown in Figure 4. Indicated are clamps 30, support structure 32 and base 33 as well as cooling blocks 20 and 20 A which are shown here as extending some distance beyond the length of slab 10. Annular water inlets 26 and 26A and outlets 27 and 27A provide the conduit to water flow channels 25 and 25A.
• FIG. 5 shows further detail of the mechanical support structure along with the mounted diode stacks 40 and 40 A, diode stack support structures 46 and 46A and the diode array cooling inlets 48 and 48A.
• FIG. 5 shows an example of a cooling scheme based on counter- flow using two inlets per block.
• the flow direction indicated in Figure 5 is in series but parallel flow is also feasible.
• Such additional cooling paths have the advantage of providing better temperature averaging across the block and may be especially useful at high powers, by affording better, more symmetric cooling and preventing the slab from flexing.
• microchannel interfaces may be inco ⁇ orated for still superior thermal cooling uniformity.
• an edge pumped thin slab laser that is pumped and cooled according to the principles of present invention, has a number of key advantages over many prior art slab lasers.
• an efficient passive cooling system can be readily designed separately from the pumping system.
• Passive conductive cooling comprising two cooled solid heat sinks as described above is easy to engineer at an acceptable cost.
• the present invention differs in a number of significant aspects from this prior art.
• the present invention provides for straight through propagation of the laser beam through the slab and does not rely on zigzag path.
• the thin, high aspect ratio slab laser displays far less sensitivity to cooling nonuniformities, distributes mechanical stress better and is less susceptible to wa ⁇ ing as compared with the device of the above referenced patent as well as other slabs of the prior art.
• FIG 6 shows an embodiment 110 of slab 10 wherein the top faces 13 and 13 A are coated with suitable reflecting layers 18 and 18A such that the pump light may be guided inside the slab through periodic reflections off these coated faces.
• Pump input faces 12 and 12A are anti-reflection (AR) coated at the pump wavelength, while end- faces 11 and 11 A are AR-coated for the lasing wavelength, as is customary in the art.
• the pairs of faces 11 and 11 A, 12 and 12 A and 13 and 13A are generally parallel but may include a slight wedge to suppress undesirable parasitics.
• only the end faces of the slab must be polished to high optical grade (typically, about ⁇ /10).
• the generic slab shown in Figure 6 may consist of any one of known solid state gain materials, including but not limited to garnets, fluoride and oxide crystals doped with rare-earth ions such as Nd, Tm, Er, Ho, Pr and Tm.
• Preparation of said coated slab proceeds through the steps of polishing the large upper and lower sides of the slab and then coating them with a material (dielectric or metallic) that is highly reflective at the pump wavelength.
• the coatings may be applied by standard techniques, such as ion sputtering, and coating material may be selected without regard to its reflection properties at the lasing wavelengths. This is because the slab thickness, although small compared with other slabs of the prior art, generally still exceeds the dimensions required for guiding the- signal.
• the coatings 18 and 18A are identical with the thermal contact layers 22 and 22 A shown in Figure 3. In this case, it is only necessary to polish slab surfaces 13 and 13 A to standard 20/10 optical grade, or they may be frosted to allow better adhesion of selected coatings.
• the minimum thickness dimension of the slab may be selected to match the numerical aperture and lateral dimension of the pump beam such that the pump light remains spatially confined inside the gain material with minimal spreading upon passage through the entire abso ⁇ tion length.
• the unguided configuration may impose some constraints on the aspect ratio that need to be taken into account.
• Homogeneous pump abso ⁇ tion may therefore require a trade-off against thermal considerations, which dictate a minimum aspect ratio for a given desired power level.
• the minimum aspect ratio as well as the slab thickness can generally be derived according to known scaling laws, which govern thermal dissipation in solid media.
• aspect ratios greater than about 5 are consistent with a near-one dimensional thermal gradient, for media such as Nd: YAG, which has high thermal conductivity.
• Thermal modeling indicates that for solid state gain materials with thermal conductivities and expansion coefficients similar to those of YAG, as long as the aspect ratio of the slab is greater than about 5, the temperature across the slab's thickness increases by only a few degrees Celsius.
• larger aspect ratios may be required (i.e., thinner and wider slabs) and/or more cooling channels have to be provided to accelerate the thermal transfer rate into the heat sinks.
• Uniformity of the pump abso ⁇ tion profile is another important consideration for optimal operation of a laser containing the thin slab of the present invention, h particular, the pump wavelength and the doping concentration of the active material must be selected so as to avoid over- inversion at the edge of the slab and under-inversion at the center. Therefore, under certain conditions, the optimal pump wavelength may be selected at an off-set from the center of the gain medium abso ⁇ tion peak, hi this case, the width of the slab may need to be increased as well, to assure complete abso ⁇ tion of the pump light. The resulting increase of the aspect ratio is not, however, expected to be detrimental to the overall operation of the slab laser, as heat removal properties will only be enhanced.
• composite slab 120 is constructed by placing active slab material 50 between two dielectric slabs 51 possessing lower index of refraction than doped active material 50, as illustrated schematically in Fig. 7. This is similar to the clad structure shown as an example in Figure 3 above.
• One suitable material for the outer two dielectric slabs is sapphire which has the additional beneficial property of high thermal conductivity and may therefore serve also as an intermediate heat sink for the slab.
• Outer slabs 51 may be joined to the active slab 50 using an adhesive, a thermal contact layer such as indium, or it may be optically bonded without adhesive.
• a particularly successful application of the later method that was demonstrated in a wide variety of solid state materials involves the approach of adhesive-free bonding, (AFB) as disclosed by Meissner in US Patent No. 5,846,638. This technology was successfully used to demonstrate numerous composite structures of doped and undoped solid state media. Slabs of different bonded materials, prepared according to this method are commercially available from Onyx, hie.
• Nd:YAG as the active material can be bonded to sapphire as the outer slabs, using this method.
• This provides a numerical aperture of greater than 0.45, which is sufficient to intercept the diverging pump light from unlensed diodes with reasonable coupling efficiency.
• the three-slab sandwich can then be efficiently edge-pumped by lensed or unlensed diode bars as long as the slab thickness is sufficiently large to intercept most of the diode light.
• long abso ⁇ tion paths and high abso ⁇ tion efficiency may be are achieved since the pump light is guided through total internal reflection from the outer slab interfaces.
• efficient pump coupling may require placement of the unlensed diode stacks in close proximity to the slab, which may not always be mechanically feasible.
• lensed bars may be used to advantage with this configuration, with slab thickness selected to closely match the incident pump light spatial dimension. Requirements on materials and interfaces with the outer slabs may be relaxed in this case, although it is recognized that composite slabs may still be instrumental in providing more homogeneous distribution of the pump light - a desirable property for assuring a high beam quality ou ⁇ ut.
• the active slab material 60 is placed between two stacks, each of which is comprised of two slabs of different dielectric materials as shown in Figure 8.
• the composite slab 130 comprises the active material 60 bonded or interfaced with inner slabs 61, which may comprise, for example, dielectric materials with a lower refractive index compared to the index of the active slab 60, while the outer slabs 62 have a lower index of refraction relative to the inner slabs at the pump wavelength.
• This "double-clad" configuration has the advantage of reducing the sensitivity to position variations of pump light from the diode stacks, h addition, the index differences between the active material and the first stack may be selected to guide the signal while the second stack will guide the pump beam.
• the material of the two slabs that are in contact with the center slab are again of the same material as the center slab, but have a different doping concentration or are undoped.
• Composite slabs of multiple different materials prepared in a "double clad" configuration according to the method of Adhesive-free Bonding are commercially available from Onyx, Inc. These structures were already used successfully to provide laser output from slabs configured as single mode waveguides, as taught by Meissner in U.S. Patent No. 6,160,824.
• active slabs of the present invention are not dimensioned for single mode operation, as the slab thickness generally exceeds the required single mode dimension (typically only 10-20 ⁇ m for Nd and Yb- doped crystals) by more than an order-of-magnitude.
• slab thickness selected according to principles of the present invention may range from several 10's of microns to over 1000 ⁇ m, depending on specific material figure-of-merit parameter, incident pump power and required output powers and mode of operation.
• thinner slabs may be used with or without outer slab claddings or bonded stacks, but the thickness in all cases still exceeds the single mode dimension.
• coated waveguides may be especially advantageous for active media emitting at longer wavelengths.
• a single transverse mode may be extracted from waveguides that are not overly thin, and are therefore readily manufacturable.
• a 500 - 700 ⁇ m thick waveguide slab may provide near single mode performance equivalent to that obtained from well-established 1.5 mm thick CO 2 waveguide slab lasers, using similar hybrid resonator constructions.
• This cross section should improve the performance from many low gain Erbium (Er) or holmium (Ho) doped materials, yet it is large enough to allow application of suitable metal or dielectric coatings with standard techniques.
• Er Erbium
• Ho holmium
• coated waveguides 300 - 400 ⁇ m thick should be thin enough to promote lower order mode operation, again by analogy with CO 2 waveguide slab lasers.
• the selection of thickness for such waveguide slabs will depend on trade-offs between the gain (which limits Q-switched operation) and desired spatial mode properties.
• the active laser component is placed inside a resonator, said resonator inco ⁇ orating at least two mirrors.
• the laser may be operated in a CW mode, or alternatively, in a pulsed mode using a modulating device, such as an AO or EO Q-switch.
• a modulating device such as an AO or EO Q-switch.
• Different resonators can be designed to provide either a spatially multimode output beam with M 2 values between 1.5 and 50 or a near diffraction limited output beam with M ⁇ 1.5.
• the active slab can be disposed within a hybrid resonator so as to reduce the beam divergence in the unstable direction while operating in a low order mode in the stable direction.
• hybrid resonators are known in the art of CO 2 slab waveguide laser designs and have recently been successfully implemented with solid state lasers as well. Therefore, such hybrid resonator constructions comprising an unstable resonator in the wider dimension and guided, stable or unstable resonator in the orthogonal, thin direction as are available in the art are all inco ⁇ orated by reference herein.
• the high power, diode-pumped lasers constructed with the thin, edge- pumped slab configurations of the invention preferably provide output powers in excess of 100 W in near-single transverse mode and over 200 W for multimode operation in either CW or Q-switched mode, all with high degree of stability for extended periods of time.
• the operational mode is selected by placing the appropriately dimensioned thin-slab shaped gain material along with the appropriate optical devices and elements inside the laser resonator.
• the laser beam emerging from the slab resonator generally has rectangular shaped cross section which is highly astigmatic. It is, however, well known in the art of optical design that with specially designed optics, astigmatic laser beams can be converted into rotationally symmetric beams.
• the element 6 indicated in Figure 1 external to the resonator consists of a bifocal telescope in conjunction with a mode converter. The telescope serves to equalize the Rayleigh lengths and waist positions in the x- and y- directions of the original astigmatic laser beam. Symmetrization is achieved by passing the beam through a transformation optics, consisting in a preferred embodiment of several cylindrical lenses, selected according to known principles of optics design.
• the symmetrized beam output shown as beam 18 in Figure 1, has equal beam radii, far-field divergences and waist positions in the x- and y- directions.
• a hybrid resonators may be implemented in conjunction with optical techniques for beam symmetrization or circularization to provide for spatially round output beam that has high beam quality in both the stable and unstable directions.
• a stable resonator consisting of the active slab, a convex high reflectivity (HR) mirror 55 and convex outcoupler 56.
• HR convex high reflectivity
• Several types of slab structures were used in experiments designed to test the effectiveness of different pump coupling techniques and slab fabrication methods in the simple resonator of Figure 9. hi the first example, a composite slab structurally similar to the configuration shown in Figure 7 was utilized.
• the slab laser was contact bonded to a sapphire slab on one side and soldered with indium to another sapphire block on the other side.
• Both sapphire blocks were 1 mm thick with widths and lengths specified to match the Nd:YAG.
• the slab was specified All the slabs were 10 mm wide and 90 mm long.
• Thin indium layer was used to contact the outer faces of the sapphire blocks to the copper blocks used as heat sinks according to the embodiments shown in Figures 3-4.
• the active material was pumped by two stacks six 50 W diode bars from each side as shown in figure 9.
• the dimensions of the composite slab provided a numerical aperture of 0.46, which was sufficient to couple a substantial portion of the pump radiation from the diode bars, h the first set of experiments, the diode bars were lensed, providing a collimated light with a diameter of 0.8 mm - 20% larger than the thickness of the slab.
• Figure 10 shows the output versus absorbed input power for 4m curvature HR mirror and 2 m curvature, 50% output coupler. The cavity length for these experiments was set at 135 mm. As Figure 10 shows, over 60% slope efficiencies are obtained from this cavity, indicating the robustness of the basic pumping and cooling approach used.
• a composite slab structure with the same 1mm slab was also constructed using a ceramic intralayer and indium solder to contact with the slab and copper cooling blocks. With this ""brazed" structure, multi-mode output power of over 350W was achieved for 660 W input power input. The increased power is a result of uniform stress provided by the composite slab structure.
• the slab gain medium is placed within a hybrid resonator, consisting of an unstable resonator along the two larger slab sides that are pe ⁇ endicular to the optical axis and a stable resonator along the two smaller slab sides.
• a hybrid resonator consisting of an unstable resonator along the two larger slab sides that are pe ⁇ endicular to the optical axis and a stable resonator along the two smaller slab sides.
• cylindrical resonator mirrors may be used.
• An output coupler with a graded reflectivity profile may further be used to improve the beam quality.
• a stable or flat-flat resonator may be sufficient to achieve good beam quality provided the thickness t of the medium is selected so as to generate a low Fresnel number, typically less than about 5.
• the Gaussian beam diameter in the slab, 2a is preferably adjusted relative to the thickness of the slab according to the relation t/2 ⁇ 2a ⁇ 31/2.
• the mirror separation, proximity to the waveguide and radii of curvature are selected based on desired output coupling, overall beam quality and required stability and physical size constraints, using customary resonator design selection criteria [1,2].
• Either positive branch or negative branch resonator may be implemented, depending on gain material and resonator parameters.
• the output coupler defines a variable reflectivity mirror (NRM) known from the art of unstable resonators design.
• NRM variable reflectivity mirror
• a NRM exhibits a supergaussian reflectivity profile conventionally expressed as:
• R 0 is the center reflectivity
• w is the profile radius
• n is the super-gaussian index
• x is the coordinate along the wide slab dimension.
• the resonator comprises a convex NRM output coupler (OC) mirror and a concave or flat high reflecting (HR) mirror, which may be selected to compensate for thermal lensing according to standard principles of laser design.
• the optics are cylindrical so as to accommodate the asymmetric properties of the hybrid resonator.
• the mirrors have long radii of curvature defining a stable resonator.
• the curvatures and the distances of the mirrors from the slab are selected according to known principles of Gaussian beam mode matching, and including the effect of thermal lens of the slab, such that only low order mode will couple efficiently into the slab.
• a resonator length of 14 cm and mirror curvatures of 2 m and 1.5 m for the HR and the OC mirrors respectively were found to provide good mode discrimination against higher order modes.
• results obtained for the uncoated, unclad, 1mm thick slab using lensed bar pumping also showed the improvement in the beam quality that can be obtained by implementing a hybrid resonator, with M 2 values of 1.9 x 2.2 were demonstrated in this case, even at output power levels as high as 190 W. Cavity length of 138 mm and 50% output coupling were used in these last experiments. It is projected that, using the edge-pumped, thin slab approach, even better beam quality can be obtained with a more optimized resonator as indicated in Figure 15, which shows the projected variation in M 2 as a function of the pump power. Ideally, with little or no aberrations due to thermal degradation M 2 would increase very slightly, even for powers levels exceeding 400 W.
• Figure 16 illustrates an off-axis hybrid resonator configurations that may be implemented as part of the present invention.
• Q-switched and mode- locked operation are provided where modulator 8 shown in Figure 1 is selected from a class of electro-optic or acousto-optic switches.
• modulator 8 shown in Figure 1 is selected from a class of electro-optic or acousto-optic switches.
• Q-switched powers in excess of 200 W will be obtained at repetition rates of 40 kHz for input powers of 600 W.
• h preliminary experiments using an AO Q-Switch in a 20 cm long hybrid cavity nearly 100 W Q-switched pulses were obtained at repetition rates of up to 50 kHz. Pulses were less than 30 nm long at 10 kHz.
• diode-pumped slab laser resonator 1 can be operation at 3 ⁇ m, typically from Er and Ho doped materials. Since these are known to have relatively low gains and high thresholds, thin slab constructions with a very small dimension are advantageously utilized.
• an Er:YAG slab with a thickness that is less than about 0.6 mm is constructed as a metallic or ceramic coated rectangular slab. At this wavelength, multimode guiding of the signal is achieved along the thin dimension. Single mode operation can however be obtained by exploiting mode discrimination properties using stable resonator design properties similar to those previously implemented for CO 2 waveguide lasers.

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