Disk laser including an amplified spontaneous emission (ASE) suppression feature

A laser system may include a first portion of laser host material adapted for amplification of laser radiation and a second portion of laser host material surrounding the first portion which may be adapted for suppression of ASE. The first portion of laser host material and the second portion of laser host material may be respectively doped at a different predetermined concentration of laser ions. A heat exchanger may be provided to dissipate heat from the first portion and the second portion.

BACKGROUND OF THE INVENTION

The present invention relates to lasers and more particularly to a disk laser including an amplified spontaneous emission suppression (ASE) feature and a method for making the disk laser.

Amplified spontaneous emission (ASE) is a phenomenon wherein spontaneously emitted photons traverse a laser gain medium or laser host material and may be amplified (multiplied) before they exit the gain medium volume. A favorable condition for ASE is a combination of high gain and a long path for the spontaneously emitted photons. ASE may depopulate the upper energy level in an excited laser gain medium and may rob the laser of its power. Additionally, reflection of ASE photons at gain medium boundaries may provide feedback for parasitic oscillations that aggravate the loss of laser power. If unchecked, ASE may become large enough to deplete the upper level inversion in high-gain laser amplifiers. Furthermore, in certain disk lasers, such as ytterbium disk lasers and similar lasers, excessive ASE may lead to failure of the laser disk. Thus ineffective ASE suppression may require operating the laser disk at a substantially lower than design gain and may reduce the robustness and reliability of the laser system.

FIG. 1is an illustration of a prior art thin disk laser100. The disk laser100may include a laser host material of yttrium aluminum garnet (YAG) doped with laser ions, such as trivalent ytterbium (Yb3+) ions which are known to have a laser transition in the vicinity of 1029 nm. A back face106of the disk laser100may be bonded to a heat sink108.

A front face104of the Yb:YAG disk laser100may receive pump radiation102at about 941 nm which is absorbed by the Yb3+ions and excites them to a laser transition centered at about 1029 nm. The pump radiation102may be made to illuminate only a central portion110of the disk laser100, as illustrated by the broken or dash lines inFIG. 1. Yb:YAG being a quasi-3 level material normally exhibits absorption of light in the vicinity of its peak lasing wavelength of 1029 nm. To overcome such absorption, pump radiation102may be sufficiently intense to make the Yb:YAG material in the disk laser100transparent (non-absorbing) at 1029 nm. Hence, the central portion110of the disk100may exhibit a net laser gain which makes it suitable for amplification of laser radiation in the vicinity of 1029 nm. On the other hand, an annular edge portion112of the disk100does not receive any substantial pump radiation102. The disk100is monolithic and the central portion110and the edge portion112have the same doping and laser host material. As a result, the edge portion112not receiving any substantial pump radiation102may absorb radiation at 1029 nm. ASE radiation is emitted as the Yb3+laser ions in the central portion110spontaneously decay from their excited state. Some portion of the ASE radiation may be trapped between the front surface104and the back surface106of the disk100and may travel in a zigzag-like path from the central portion110to the edge portion112. If the amount of ASE radiation is rather small, the ASE radiation is effectively absorbed in the edge portion112. In this fashion, the possibility for an ASE photon being reflected from a disk edge114(e.g., by Fresnel reflection) and being re-amplified in the central portion110is very remote. However, with increasing ASE intensity, such as may be experienced because of increased pumping and/or a non-lasing condition in the central portion110, ASE photons entering the edge portion112may deplete the absorption property or capability of the edge portion112which is integral and homogeneous with the central portion110and has the same doping. Hence, the likelihood of ASE photons returning to and being re-amplified in the central portion110may be significantly increased. As a result, laser gain may be substantially depleted. If ASE intensity is further increased (e.g., due to increased pumping or due to a pause in lasing) parasitic lasing across the disk diameter may occur and the concomitant increase in thermal load may cause permanent damage to the laser disk100.

BRIEF SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, a laser system may include a first portion of laser host material adapted for amplification of laser radiation and a second portion of laser host material surrounding the first portion which may be adapted for suppression of ASE. The first portion of laser host material and the second portion of laser host material may be respectively doped or fabricated at a different predetermined concentration of laser ions. A heat exchanger may be provided to dissipate heat from the first portion and the second portion.

In accordance with another embodiment of the present invention, a thin disk laser may include a central portion of laser host material doped or fabricated with laser ions at a first predetermined concentration for amplification of laser radiation. The laser may also include an edge portion of laser host material surrounding the central portion and doped or fabricated with laser ions at a second predetermined concentration for suppression of ASE. The second predetermined concentration may be substantially higher than the first predetermined concentration.

In accordance with another embodiment of the present invention, a method for making a laser system may include forming a first plate of laser host material adapted for absorption of ASE. The method may also include forming a second plate of laser host material adapted for amplification of laser radiation. The method may further include forming an opening in the first plate to receive the second plate within the first plate to form a disk laser assembly.

In accordance with another embodiment of the present invention, a method for suppressing ASE in a laser system may include disposing a central portion of laser host material within a substantially annular edge portion of laser host material. The central portion of laser host material may include laser ion doping at a first predetermined concentration for amplification of laser radiation. The substantially annular edge portion of laser host material may include laser ion doping at a second predetermined concentration for absorption of ASE.

Other aspects and features of the present invention, as defined solely by the claims, will become apparent to those ordinarily skilled in the art upon review of the following non-limited detailed description of the invention in conjunction with the accompanying figures.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of embodiments refers to the accompanying drawings, which illustrate specific embodiments of the invention. Other embodiments having different structures and operations do not depart from the scope of the present invention.

As used herein, laser gain medium (LGM) may refer to an optical material having a host lattice doped with suitable ions, which may be pumped by an external source, such as a laser or other optical radiation to a laser transition. Examples of host lattice material that may be used in conjunction with the present invention may include yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG), gadolinium scandium gallium garnet (GSGG), lithium yttrium fluoride (YLF), yttrium vanadate, phosphate laser glass, silicate laser glass, sapphire or similar materials. The host material may be in a single crystal form or in a poly-crystalline (ceramic) form. Suitable dopants for such lasing mediums may include titanium (Ti), cobalt (Co), chromium (Cr), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), and ytterbium (Yb). Preferred dopants may be quasi-3 level ions such as holmium (Ho), erbium (Er), thulium (Tm), and ytterbium (Yb). Optical pump sources may be selected based on the absorption characteristics of the selected laser gain medium. For example, semiconductor diode lasers may be used for the optical pump source. The present invention is not intended to be limited to any specific lasing or laser gain material, or a specific pump source. Laser gain medium may also be referred to herein as laser gain material or laser host material or medium.

FIG. 2is an illustration of a laser system200including an ASE suppression feature202in accordance with an embodiment of the present invention. The laser system200may be a thin disk laser (TDL) laser system or similar system. The system200may include a thin disk laser gain assembly204. The thin disk gain assembly204may be attached to and in a good thermal communication with a heat sink205or other suitable heat exchanger. The laser gain assembly204may include a first portion or central portion206of laser host material adapted for receiving of pump radiation208from a source of pump radiation210and being pumped by the radiation208to a laser transition as described in more detail herein. The laser radiation source210may be a diode laser or other source for providing optical pumping.

The laser gain assembly204may also include a second portion or edge portion212of laser host material surrounding the first portion or central portion206. The edge portion212may be adapted to suppress or absorb ASE as described herein. The laser gain assembly204or laser host material may be selected from a group including yttrium aluminum garnet (YAG); gadolinium gallium garnet (GGG); gadolinium scandium gallium garnet (GSGG); potassium gadolinium tungstate (KGW); potassium yttrium tungstate (KYW); fluoroapatite (FAP), lithium yttrium fluoride (YLF); phosphate laser glass; silicate laser glass; sapphire; or other suitable host material for laser ions.

The first portion206or central portion of laser host material may include doping with laser ions at a first predetermined concentration for amplification of laser radiation. The first portion206may be doped with laser ions at a density or concentration such as may be required for laser operation under specific pumping conditions and characteristics of an out-coupling mirror, such as out-coupling mirror214. In one embodiment of the invention the first portion206may be doped with quasi-3 level laser ions, such as trivalent ytterbium (Yb3+); trivalent holmium (Ho3+), trivalent erbium (Er3+); trivalent thulium (Tm3+); or other suitable quasi-3 level ions. The out-coupling mirror214may couple an output laser beam216to other optical components for directing the laser beam216on an object or target.

The second portion or edge portion212may be generally annular in shape and may be doped with the same laser ions as the first portion206but at a second predetermined concentration. The doping concentration or density of the edge portion212may be substantially higher than the doping concentration or density in the central portion206or first portion to promote ASE absorption or suppression in the edge portion212. Preferably, the doping concentration of laser ions in the second portion may be at least 10% higher than in the first portion. Most preferably the doping concentration of laser ions in the second portion may more than 50% higher than in the first portion. In particular, it is known in the art that Yb can be doped into YAG lattice with up to 100% concentration. For example, in a Yb:YAG thin disk gain assembly204the first portion206may have a 10% atomic concentration of Yb and the second portion212may have a 15% atomic concentration of Yb. Accordingly, the central portion206may be adapted for amplification of laser radiation and the edge portion212may be adapted for suppression or absorption of ASE in accordance with their respective doping concentration or density of laser ions. One advantage of using the same type of ion in both the first portion206and the second portion212is that the two potions may be made to have substantially similar coefficient of thermal expansion and thus the two portions may be attached without inducing excessive thermal stresses at their joint. Furthermore, in case the laser beam216becomes misaligned and illuminates any part of the second portion212, the quasi-3 level nature of the ions reduces the absorption of laser light in that part and overheating of the material may thus be prevented.

In another embodiment of the invention, the edge portion212may be doped with a different kind of ion from that of the central portion206. In this case, the ions for doping into the first portion206may be any suitable lasing ions including but not limited to trivalent ytterbium (Yb3+); trivalent holmium (Ho3+); trivalent erbium (Er3+); trivalent thulium (Tm3+); trivalent neodymium (Nd3+); trivalent dysprosium (Dy3+); trivalent praseodymium (Pr3+); trivalent dysprosium (Dy3+); trivalent titanium (Ti3+); trivalent chromium (Cr3+); tetravalent chromium (Cr4+); and divalent cobalt (Co2+). The ions for doping into the second portion212may be selected to absorb spontaneous emission generated in the first portion206. Such suitable ions must be compatible with the host material and doped there into with appropriate valence.

As will be described in more detail with reference toFIG. 3, the central portion206may have a substantially cylindrical shape and the edge portion212may have a substantially annular shape. The central portion206may be adapted to fit within the edge portion as illustrated inFIG. 2to form an interface218between an outer cylindrical face220of the central portion206and an inner annular face222of the edge portion212.

The central portion206and the edge portion212may be co-joined to form an optically continuous and monolithic body224having substantially minimal variation in the index of refraction across the interface218. The thin disk gain assembly204including such a composite construction of two distinctly doped portions, central portion206and edge portion212, may be formed by a sintering process. For example, the two distinctly doped portions may be co-sintered during fabrication of polycrystalline YAG components or other laser host material. In another embodiment of the present invention, the two distinctly doped portions206and212can be diffusion bonded along a conical interface and sliced into disks as described in more detail with respect toFIG. 4or may be joined by any suitable method.

The thin disk gain assembly204may also include a front surface226coated with an anti-reflective (AR) coating228and a back surface230coated with a highly-reflective (HR) coating232. The front surface226and the back surface230may be machined to optical flatness and mutual parallelism.

The thin disk laser gain assembly204may be mounted on a heat sink205or other heat dissipation means for receiving and dissipating heat from the laser gain assembly204. The thin disk laser gain assembly204may be attached to the heat sink205by an adhesive, solder, or other suitable means. The disk laser gain assembly204may also be attached to the heat sink205by hydrostatic clamping similar to that described in U.S. Pat. No. 6,625,193, entitled “Side-Pumped Active Mirror Solid-State Laser for High-Average Power” by Jan Vetrovec, issued Sep. 23, 2003, assigned to the same assignee as the present invention and incorporated herein in its entirety by reference.

As described in more detail herein, a pump radiation reflector236may reflect unabsorbed pump radiation from the central portion206of the laser gain assembly204back to the central portion206for further absorption of the pump radiation208. An end mirror238may reflect the amplified laser radiation or laser beam216back to the central portion206. The amplified laser beam216′ may then be reflected from the highly-reflective coating232to the out-coupling mirror214.

In operation of the laser system200, the optical pump radiation source210may generate an optical pump beam or radiation208, which may be directed onto the laser gain material of the central portion206of the disk gain assembly204. The pump beam208passes through the AR coating228into the laser host material or laser gain material of the central portion206. The laser pump radiation208may be at least partially absorbed by the laser gain material of the central portion206of the laser gain assembly204. Any unabsorbed portion of the pump beam208may be reflected from the HR coating232and passes back through the laser gain material of the central portion206in a generally reverse direction relative to the first pass. The reflected, unabsorbed pump beam or radiation208′ may again be at least in partially absorbed as it passes back through the laser gain material of the central portion206. The unabsorbed portion of the pump beam208′ may exit the front surface226of the central portion206through the AR coating228and may be directed onto the pump radiation reflector236. The pump radiation reflector236may reflect the pump beam208′ back to the central portion206. The pump beam208′ may be reflected through the central portion206multiple times before a desired portion of the pump radiation208′ is absorbed in the laser gain or host material of the disk laser gain assembly204. The absorbed portion of the pump radiation208′ pumps laser ions in the central portion206to a laser transition. At least some of the excited laser ions may decay to a lower state by spontaneously emitting photons.

A substantial portion of such spontaneously emitted photons may be trapped between the front surface226and the back surface230of the laser gain assembly204. The trapped photons may propagate on generally zigzag-like trajectories toward the edge portion212while being amplified (multiplied) in the process. In accordance with an embodiment of the present invention, the edge portion212is not substantially illuminated by the pump radiation208and thus represents an efficient absorber of photons or ASE. Thus, the edge portion212serves as an absorber of ASE and defines an ASE suppression feature. Because the concentration of laser ions (Yb3+ions or other possible laser ions listed above) in the edge portion212is substantially higher than in the central portion206, the edge portion212is a more effective absorber than it would have been if the central portion206and edge portion212were doped at the same level as in the prior art system ofFIG. 1. In particular, the threshold for parasitic lasing is substantially increased and the possibility for damage of the disk laser gain assembly204is substantially reduced in the laser system200of the embodiment of the present invention illustrated inFIG. 2.

In accordance with an embodiment of the present invention, the laser system200may be mounted to a vehicle240or the vehicle may be part of the laser system200. The laser system200may be mounted to the vehicle by a mechanism242to permit the laser beam216from the system200to be directed or oriented onto an object or target. The mechanism242may permit adjustment of elevation and azimuth of the laser beam216relative to the vehicle240.

FIG. 3is a flow chart of an example of a method300for making a thin disk laser including an ASE suppression feature in accordance with an embodiment of the present invention. The method300may be used to form the disk gain assembly204ofFIG. 2. In block302, a first plate304of laser gain material or laser host material may be fabricated with a first predetermined concentration of laser ions306to adapt or modify the first plate304for absorption or suppression of ASE.

In block308, a second plate310of laser gain material or laser host material may be fabricated with a second predetermined concentration of laser ions312to adapt or modify the second plate310for amplification of laser radiation.

In block314, an opening316may be formed in the first plate306to receive the second plate310. In block318, the second plate310may be formed or shaped to be receivable into the opening316formed in the first plate304. The second plate310may be substantially cylindrically shaped and the opening316may be substantially cylindrically shaped to receive the second plate310.

In block320, the second plate310may be inserted into the opening316in the first plate304. In block322, the first plate304and the second plate310may be joined at their interface by any suitable technique. If first plate302and second plate310are fabricated from polycrystalline material, such as polycrystalline YAG, a sintering operation or process may be performed to join the first plate304and the second plate310to form an optically continuous and monolithic body324having substantially minimal variation in the index of refraction across the interface326. In another embodiment of the present invention, the first plate304and the second plate may be joined by diffusion bonding or any suitable means to form an optically continuous and monolithic body324with minimal variation in the index of refraction across the interface326.

In block328, the opposite faces or surfaces330and332may be machined to optical flatness and mutual parallelism. The disk324may have a thickness of about 100 to about 300 micrometers. The first plate or central portion310may have a diameter of about 1 to about 15 millimeters and the second plate or edge portion304may have a diameter of about 10 to about 30 millimeters. The outer diameter of the disk324may also be machined as may be needed for the particular system or implementation.

FIG. 4is a flow chart of an example of a method400for making a thin disk laser including an ASE suppression feature in accordance with another embodiment of the present invention. The method400may be used to form the disk gain assembly204ofFIG. 2. In block402, a first plate404of laser gain material or laser host material may be fabricated with a first predetermined concentration of laser ions406to adapt or modify the first plate404for absorption or suppression of ASE.

In block408, a second plate410of laser gain material or laser host material may be fabricated with a second predetermined concentration of laser ions412to adapt or modify the second plate410for amplification of laser radiation.

In block414, an opening416may be formed in the first plate406to receive the second plate410. The opening416may be substantially conically shaped. In block418, the second plate410may be formed or shaped to be receivable into the opening416formed in the first plate404. The second plate410may be formed to be substantially conically shaped for reception into the opening416of the first plate404.

In block420, the second conically shaped plate410may be inserted into the opening416in the first plate404. In block422, the first plate404and the second plate410may be joined at their interface by any suitable technique. If first plate404and second plate410are fabricated from polycrystalline material such as polycrystalline YAG, a sintering operation or process may be performed to join the first plate404and the second plate410to form an optically continuous and monolithic body424having substantially minimal variation in the index of refraction across the interface426. In another embodiment of the present invention, the first plate404and the second plate410may be joined by diffusion bonding.

In block428, the opposite faces or surfaces430and432may be machined to optical flatness and mutual parallelism. The monolithic body424may also be sliced into sections to form multiple disk assemblies similar to composite assembly434. The process or method400illustrated inFIG. 4may be more efficient requiring less extensive and costly machining than the process300illustrated inFIG. 3. The composite assembly434may be used for the laser gain assembly204ofFIG. 2. The outer diameter of the disk334may also be machined as may be needed for the particular system or implementation.