Patent Description:
The present application relates to a laser system and method having a disc-like gain media formed with a non-flat shape, and more specifically, it relates to a means for reducing the generation of amplified spontaneous emission enabling scaling of the transverse modal area to scale the average output power.

This section provides background information related to the present disclosure. Thin disk lasers make use of a thin disc gain media which forms part of the lasing cavity. The thin disc gain media is typically provided with a planar (i.e., flat) shape. This shape, while being generally effective for the purpose of enabling the laser system to produce a uniform output beam with minimum distortions due to thermal gradients, does have certain limitations. One such limitation is the susceptibility of the flat thin disc media to transverse amplified spontaneous emission ("ASE"). Transverse ASE limits the spot size of the output laser beam that can be produced using the flat thin disc gain media, which in turn limits the power that can be delivered by the output laser beam. Another limitation is the susceptibility of the flat thin active disks to thermal induced lens effects which can also limit laser performance. To scale the average power of a thin disk laser, an anti-ASE cap (undoped index matched layer) is typically utilized to mitigate the effects of ASE thus enabling power scaling via modal enlargement. This approach works up to a point, but eventually deleterious effects of ASE limit further scaling. Accordingly, there exists a need for further improvements in the laser art for laser systems that are capable of producing beams with even larger spot sizes than previously developed laser systems. Utilization of a non-flat (curved) disk enables further power scaling over that supported by flat thin disk laser architectures.

An example of a laser system to reduce amplified spontaneous emission is disclosed in <CIT>. The system comprises a non-flat gain media disc and a pump source configured to generate a beam that pumps the non-flat gain media disc. A laser cavity is formed by an optical component, the non-flat gain media disc and an output coupler enabling the laser beam to exit the laser cavity. A radius of curvature of the non-flat gain media is selected to reduce the amplified spontaneous emission.

In one aspect the present disclosure relates to a laser system disclosed in claim <NUM>.

The laser system may comprise a first pump source configured to generate a first beam, a second pump source configured to generate a second beam, a first mirror for receiving the first beam, and a second mirror for receiving the second beam. The laser system may also comprise a non-flat, thin disc gain media optical component for receiving the first and second beams reflected from the first and second mirrors thus exciting the non-flat gain media disc. An output coupler may be included for forming a resonant laser cavity.

A highly reflective coating may be applied to a surface of the hemispherical shaped thin disc gain media, constituting an active mirror. The optical component may also include a cap layer consisting of a media refractive index and coefficient of thermal expansion matched to the gain media. Furthermore the cap layer may also include an anti-reflective coating or other wavelength dependent coating.

In another aspect a method for forming a laser oscillator system is disclosed. The method may comprise using at least one pump source configured to generate a pump beam incident upon the non flat gain mirror. The method may further comprise using a laser cavity formed by the non-flat gain mirror and at least one additional optical component forming a resonant cavity. The resonate cavity may include one or more non-flat gain mirrors. The method may further comprise using an output coupler to receive and direct the output beam toward an external component.

In another aspect the present disclosure relates to a method of forming a laser amplifier system according to claim <NUM>.

The description and specific examples in this summary are intended for purposes of illustrating various aspects of the present disclosure.

The drawings described herein are for illustrative purposes of selected embodiments and not all possible are shown. The accompanying drawings, which are incorporated into and constitute a part of the specification, illustrate specific embodiments of the apparatus, systems, and methods and, together with the general description given above, and the detailed description of specific embodiments serve to explain the principles of the apparatus, systems, and methods.

Referring to the drawings, to the following detailed description, and to incorporated materials, detailed information about the apparatus, systems, and methods is provided including the description of specific embodiments. The detailed description serves to explain the principles of the apparatus, systems, and methods described herein. The apparatus, systems, and methods described herein are susceptible to modifications and alternative forms. The application covers all modifications, and alternatives as long as they fall within scope as defined by the claims.

The present invention utilizes a gain and index tailored multi-layered (composite) non-flat (curved) disk to further reduce the deleterious effects of ASE by reducing the spontaneous emission interaction length within the gain media. This allows the laser to be substantially scaled in power output beyond what is achievable utilizing conventional disk laser architectures.

Referring to <FIG>, there is shown one example of a solid-state laser apparatus <NUM> in accordance with the present disclosure. The apparatus <NUM> in this example makes use of hemi-spherical thin disc gain media <NUM> that is illuminated by a pair of pump sources, shown in this example as diode pump lasers <NUM> and <NUM>. An output coupler <NUM>, along with the thin disc gain media <NUM>, forms the lasing cavity. A coolant <NUM> may be supplied to the thin disc gain media <NUM> to cool the thin disc gain media.

The curved thin disc gain media <NUM> is shown in high level form in <FIG> and may include a substrate portion <NUM> having an outer surface <NUM>, an inner surface <NUM>, a gain media <NUM>, and a high reflection ("HR") coating <NUM> covering the gain media coating <NUM>. The diode pump lasers <NUM> and <NUM> provide pump laser beams 14a and 16a that illuminate a spot through the outer surface <NUM> of the thin disc gain media <NUM> at a vertex <NUM> of the thin disc gain media creating an excited state capable of providing optical gain. A resonant laser cavity is formed between the excited laser gain media <NUM> at the vertex <NUM> and the output coupler <NUM> forming a laser beam that may exit the device via the output coupler <NUM> port. The substrate portion <NUM> and the gain media <NUM> are index of refraction and coefficient of thermal expansion matched to reduce reflection and stress between the layers. The curved configuration of the thin disc gain media <NUM> helps to reduce transverse ASE and may enable even larger spot sizes to be generated with any given laser system. The curved configuration of the thin disc gain media <NUM> may also enable better mode selection, and may also be intrinsically stronger and more robust, and thus be more resistant to thermal induced deformations.

<FIG> shows a laser system <NUM> in accordance with another embodiment of the present disclosure. In this example a hemi-spherical thin disc gain media <NUM> receives energy via a pump beam 54a from a first pump source, which in this example is shown as diode pump laser <NUM>, and lasing beam 56a from a second pump source, which is shown as diode pump laser <NUM>. The lasing beams 54a and 56a excite the thin disc gain media <NUM>. An output beam <NUM> is formed in the resonant laser cavity comprising the excited thin disc gain media region, the HR coating <NUM> and the output coupler <NUM>. Optical energy in the form of an output beam exits the cavity through the output coupler <NUM>. A coolant <NUM> may be supplied to the thin disc gain media <NUM> to cool it.

Referring to <FIG>, the thin disc gain media <NUM> in this example has a substrate <NUM> with an inner surface <NUM> and an outer surface <NUM>. A coating <NUM> comprising a gain media is placed on the inner surface <NUM> while an HR coating <NUM> is placed on the outer surface <NUM>. As will be discussed below, this configuration (i.e. direction of curvature of the disc relative to the geometry of the laser resonator) may not provide the benefit of reduced ASE below the flat disc case. However, this configuration may have other advantages, such as improved optical alignment for some specific laser configurations. The example is included in the range of examples of non-flat thin disc geometries but may not be the optimal choice of curvature for power and energy scaling.

Referring to <FIG>, a solid-state laser apparatus <NUM> is shown in accordance with the embodiment of <FIG>. In this example, the apparatus <NUM> comprises a non-flat active mirror <NUM> and an output coupling mirror <NUM> forming a laser resonator. The active hemispherical mirror <NUM> is optically pumped by an external pump source (<NUM>). The active hemispherical mirror <NUM> absorbs the pump energy and re-emits radiation into the laser resonator. The pump beam which is reflected by the active hemispherical mirror <NUM> is redirected back to the active hemispherical mirror by reflectors <NUM> to increase the pump absorption efficiency.

Referring to <FIG>, a solid-state laser apparatus <NUM> is illustrated which consists of two optical mirrors <NUM>,<NUM> separated by a distance, thus forming an optical resonator (e.g., stable, unstable, linear, ring, etc.). The mirror <NUM> comprises a non-flat active mirror and may consist of a plurality of layers including at least one optically active layer, optical coating layers and an anti-ASE layer. In one configuration, the layers are as specified in <FIG>, with an outside high reflective coating <NUM>, an optically active layer <NUM> capable of providing optical gain, an anti-ASE cap layer <NUM> and an anti-reflection coating <NUM>. Other configurations of the layers and number of layers are possible. The three dimensional shape of the non-flat optical mirror <NUM> containing the optically active media <NUM> may be in the form of a hemisphere or other three dimensional curved shape, for example an ellipse, a parabola, etc., applicable to a specific resonator design which may include ether a concave (-R) or convex (+R) orientation of the active mirror <NUM>. Referring to <FIG>, active mirrors with a +R or convex curvature are beneficial towards mitigating the detrimental effects of ASE, whereas concave mirrors (-R) may not be beneficial for ASE mitigation but may be beneficial towards other resonator configurations (i.e., stable geometrics not supported by a convex active mirror) and/ or parametric sensitivities, for example mirror misalignments, or thermal variations or other factors.

<FIG> shows a laser system <NUM> representing an unstable laser architecture formed by two three-dimensional active mirrors <NUM> and <NUM>. In this example, the three dimensional active mirrors <NUM> and <NUM> are each formed with a hemispherical shape, and each operates as both a mirror and as a gain media.

<FIG> shows a laser system <NUM> which forms a multi-active mirror linear optical resonator laser consisting of two three-dimensional active mirror elements <NUM> and <NUM>. Coolant <NUM> and <NUM> may be supplied to both of the active mirror elements <NUM> and <NUM>. Again, each one of the active mirror elements <NUM> and <NUM> operate as both a mirror and a gain media. Element <NUM> forms the output coupler of the laser resonator in this configuration.

The solid-state matrix used to make the three-dimensional active mirrors <NUM>, <NUM>, <NUM>, <NUM> or the thin disc gain media <NUM>, <NUM> or <NUM>, may each comprise a homogeneous gain loaded matrix or a multi-layer matrix consisting of distinct layers of varying Rare Earth dopant concentrations and refractive indices. The Rare Earth dopants may include, but are not limited to, one or more of Erbium (Er), Ytterbium (Yb), Neodymium (Nd);Thulium (Tm); Praseodymium (Pr);Cerium (Ce);Holmium (Ho);Yttrium (Y); Samarium (Sm); Europium (Eu);Gadolinium (Gd);Terbium (Tb);Dysprosium (Dy); and Lutetium (Lu). Transition metals such as Chromium (Cr) and Titanium (Ti) may also be incorporated.

<FIG> shows one example of a three dimensional active mirror <NUM> having HR/AR coatings <NUM> and <NUM> on its inside or outside surfaces, the optical coatings by be uniform or tailored for resonator performance, and three distinct layers <NUM>-<NUM> of gain loaded matrix consisting of at least one optically active layer and at least one index match anti-ASE cap layer. It will be appreciated that a greater or lesser number of the matrix layers may be incorporated, and the use of three matrix layers <NUM>-<NUM> shown in <FIG> is therefore merely one example of how the gain loaded matrix may be implemented. The dopant concentration may be tailored to reduce ASE and to tailor gain and modal confinement, while the thickness of the layers <NUM>-<NUM> and refractive index may be tailored for modal constituency, thermal dissipation, pump efficiency, structural rigidity, and other design or operational considerations.

Referring to <FIG>, a stability diagram <NUM> is shown to illustrate various beam patterns that may be produced for a laser resonator consisting of two spherical mirrors of radius R1 and R2 separated by a distance L. The stability criteria is defined as the product of g1*g2 falling between o and <NUM>, where g1=<NUM>-L/R1 and g2=<NUM>-L/R2. The present invention, relating to non-flat active mirrors of radius R1 when coupled to a second mirror of radius R2, is applicable to both stable and unstable laser resonator architectures featuring single or dual active mirrors. Mirror configurations falling within the shaded region of <FIG> pertain to stable laser resonator architectures. Those outside this region are considered unstable.

Referring to <FIG> and <FIG>, a comparison between a well-known flat gain media disc <NUM> (<FIG>) and a curved gain media disc <NUM> in accordance with one embodiment of the present disclosure is shown.

Referring to <FIG>, considerations for parameterizing ASE are illustrated. It will be appreciated that the ASE multiplier ("MASE") is the factor by which the power of a spontaneous emitted photon is amplified by the gain region of the non-flat disc, such as disc <NUM> in <FIG> or disc <NUM> in <FIG>. The MASE is calculated by launching a number of rays from random locations and in random directions and adding up the total power emerging from the gain media where upon each pass of a ray through the gain media the ASE power is increased by egL. A local MASE can also be calculated by finding the total net power leaving some subregion of the gain media. Spatial gain variations can be incorporated if needed. <FIG> shows a graph <NUM> which illustrates how the MASE for a flat disc (line <NUM>) which is shown as a constant line in the plot for the purpose of providing a simple comparison to the non-flat case. In the non-flat geometry (in this specific case hemispherical), the MASE is a function of the radius of curvature of the disc <NUM>. Graph <NUM> further illustrates (portion 704a) that an optimal non-flat gain media radius maximizes this reduction; further increase in bend radius reduces the effect (portion 704b).

<FIG> shows a graph <NUM> of the ASE multiplier vs. anti ASE cap thickness (in millimeters) for a flat gain media disc and a curved gain media disc of optimal radius of curvature, where for a given ASE cap thickness an optimal radius has been calculated and plotted as green triangles whose values may be determined by comparison with the right hand vertical axis. <FIG> also illustrates that increasing the ASE cap thickness for a curved gain media disc continues to minimizing the ASE multiplier (red squares whose value can be determined from the left hand vertical axis), whereas in the flat case the benefit of the ASE cap is not increasing with increasing ASE cap thickness (blue diamonds whose value can be determined from the left hand vertical axis). The graph <NUM> illustrates how a thicker cap for a curved given gain media disc can be used to significantly reduce the ASE compared to a flat disk with thicker anti-ASE cap.

<FIG> show graphical distributions for the power distribution over an area of a conventional flat gain media disc <NUM>, as shown in <FIG>, versus a curved gain media disc <NUM> shown in <FIG>. In this example, <FIG> shows that the ASE is uniform over about <NUM>%-<NUM>% of the surface of the curved gain media disc <NUM> when the gain media disc <NUM> has an optimal radius of <NUM> and cap thickness of <NUM> The uniformity of the ASE intensity for the curved disk is improved by a factor of <NUM>-<NUM> over that of the flat disk with comparable geometry. <FIG> shows a graph <NUM> illustrating that for a given curved gain media disc, the. ASE can be further reduced by reducing the radius of the disk for a given anti-ASE cap, constrained by the limits discussed above with regards to <FIG> and <FIG>.

<FIG> shows a pair of graphs <NUM> and <NUM> to illustrate how the curvature (both negative and positive) of a curved gain media disc may influence the MASE. Curve <NUM> represents the ASE multiplier for a cap of a curved gain media disc having a <NUM> thickness, and curve <NUM> represents the ASE multiplier for a cap of a curved gain media disc having a <NUM> thickness. <FIG> shows the benefit of the configuration only occurs for thin disc radius of curvatures greater than zero, where positive and negative curvature are as defined above.

<FIG> illustrates a prior art flat gain media disc <NUM> and a path length 1500a of one ray through the disc, while <FIG> illustrates a curved gain media disc <NUM> with a path length of one ray 1502a through the disc. In this example the path ASE multiplier is significantly greater for the curved gain media disc <NUM>.

<FIG> illustrates a graph <NUM> of a distribution of the ASE of a conventional flat gain media disc, relative to height, while <FIG> illustrates a graph <NUM> of the distribution of the ASE multiplier, relative to height, of a concave gain media disc.

<FIG> shows a graph <NUM> that illustrates a plurality of curves showing examples of the single-pass gain for the pumped area of a conventional flat gain media disc, for pumped areas of different diameters. <FIG> and <FIG> show graphs <NUM> and <NUM>, respectively, which illustrate examples of how a curved gain media disc allows for a larger pumped area.

A curved disc <NUM> is shown in <FIG> in accordance with another embodiment of the present disclosure. The curved disc <NUM> in this example includes an index and gain tailored multi-layer composite active mirror containing a substrate portion <NUM> and a layer forming a gain tailored portion <NUM>. The layer forming a gain tailored portion, i.e., gain tailored portion <NUM>, may be formed collectively by portions 2004a and 2004b. Optionally, three or more distinct portions may be provided. Optical coatings may also be used on interior <NUM> and exterior <NUM> surfaces, respectively. One or more of the portions 2004a and 2004b may contain an active media defined within a central portion of the curved shell that forms the curved disc <NUM>. The composite mirror layers <NUM>, 2004a and 2004b form layers which may be matched with respect to coefficient of thermal expansion and index of refraction to minimize optical reflections between layers and to mitigate stress related thermal effects. Restricting the active media (i.e., the layers of media capable of optical amplification actuated by optical pumping) to a central portion of the curved disc <NUM> enables mode tailoring by offsetting the confinement factors of higher order modes to favor fundamental mode (or a sub-set of modes) operation for power scaling.

Claim 1:
A laser system comprising:
a non-flat gain media disc having a gain region and an anti ASE cap layer, configured to reduce transverse ASE;
at least one pump source configured to generate a beam that pumps the non-flat gain media disc; and
a laser cavity formed by at least one optical component, the non-flat gain media disc and
an output coupler enabling the laser beam to exit the laser cavity,
wherein the non-flat gain media disc comprises a hemispherical curved shell-shaped gain media disc, and
wherein a radius R of curvature of the gain region is determined by <MAT> where
L = the diameter of a same gain medium flat disc having the gain region and the anti ASE cap of the diameter L, and
tc = thickness of the anti ASE cap layer.