WO2017160720A1

WO2017160720A1 - Laser cladding material, apparatus, and methods for transverse oscillation suppression

Info

Publication number: WO2017160720A1
Application number: PCT/US2017/022102
Authority: WO
Inventors: Romain Gaume; Eric Cunningham; Shi Chen
Original assignee: University of Central Florida Research Foundation Inc
Current assignee: University of Central Florida Research Foundation Inc
Priority date: 2016-03-14
Filing date: 2017-03-13
Publication date: 2017-09-21
Anticipated expiration: 2018-09-14
Also published as: US20190020172A1; US10686289B2

Abstract

This invention suppresses transverse oscillations in solid-state, single-crystal or ceramic laser amplifiers by employing a native-material, solid-state, index-matched cladding containing appropriate absorbers.

Description

LASER CLADDING MATERIAL, APPARATUS, AND METHODS FOR TRANSVERSE

OSCILLATION SUPPRESSION

Related Application Data

The instant application claims priority to US provisional application serial number 62/307,858 filed 03/14/2016, the subject matter of which is incorporated by reference herein in its entirety.

Background

Ultra-high-intensity and ultra-fast laser pulses are essential requirements for high energy physics and National defense applications. Transverse parasitic lasing has however been up until now a bottleneck in Petawatt level Ti: sapphire laser systems with large aperture gain crystals. Larger pumping areas leads to higher transverse gain experienced by radially- emitted spontaneous emission; if this gain becomes higher than the Fresnel losses experienced at the edge of the crystal, parasitic oscillations inside this transverse laser cavity will arise, depleting the population inversion and preventing desired amplifying action of the longitudinally-incident seed pulse. Besides, thermal load from the higher pumping powers can degrade laser quality from index gradient or lead to catastrophic damage to the gain crystals. Thus, either power or repetition rate has to be limited, and special cryogenic cooling has to be applied. The conventional procedure against parasitic laser generation is to reduce the reflectivity of the side wall of the gain crystals coating them with an index-matched absorptive polymer layer or liquids. These approaches however have various issues; for example, the absorptive polymer generally only has matched index at one wavelength point and it has low thermal conductivity and different thermal expansion than the gain crystal.

The embodied invention provides a solution to these problems by employing an absorptive solid-state cladding that is of the same material as the laser-grade core (i.e., gain crystal) of the laser amplifier (e.g. an alumina/sapphire cladding for a Ti:sapphire laser, a YAG cladding for an Nd: YAG laser, etc.) but in a polycrystalline form. Therefore, the optical and thermal properties are naturally matched. This solution overcomes many problems of existing index- matching claddings, which 1) may only approximate the gain medium's refractive index (or perhaps exactly match only at a single wavelength), 2) may not possess the same thermal properties of the gain medium (e.g. : the thermal conductivity, important for heat removal; the temperature-dependent changes in the refractive index, which could change the quality of index-matching at different temperatures; the thermal expansion coefficient, which can cause strain, cracking, or delamination of the crystal and cladding; etc.), and 3) may not be cryo- or vacuum-compatible. Indeed, by using an index-matching cladding made of the same material as the core of the gain medium: 1) the refractive index of the cladding essentially matches that of the core for all wavelengths, which can be important for suppressing transverse oscillations across broad bandwidths, 2) the thermal properties of the cladding essentially match that of the core, which can be important for the functionality of transverse oscillation suppression in solid-state amplifiers regardless of the repetition rate, heat load, and operating temperature, and 3) the cladding itself is cryo- and vacuum- compatible.

Summary

An aspect of the invention is a cladding material for a solid-state, single-crystal, or ceramic laser amplifier. In an embodiment, the cladding material is the same material as a core material of the solid-state, single-crystal, or ceramic laser amplifier and includes a broadband absorber material. Various non-limiting, exemplary embodiments may include one or more of the following limitations, features, components, characteristics or steps in various combinations as one skilled in the art would understand: -wherein the core material is sapphire and the cladding material is alumina-doped sapphire; -wherein the core material is YAG and the cladding material is Sm3+ doped YAG;

-wherein the broadband absorber material is graphene;

-wherein the graphene is in an amount of equal to or less than 50 atomic layers;

-wherein the cladding material is in a polycrystalline form of the core material;

-wherein the broadband absorber material is a rare earth ion;

-wherein the rare earth ion is Sm3+;

-wherein the broadband absorber material is transition metal ion;

-wherein the transition metal ion is Cr4+;

-wherein the broadband absorber material is a p-block element;

-wherein the p-block element is Bi3+.

An aspect of the invention is a method for suppressing transverse oscillations in a solid-state, single-crystal, or ceramic laser amplifier. In an exemplary embodiment, the method includes applying a cladding material to an outer perimeter region of a core of the solid-state, single-crystal, or ceramic laser amplifier, wherein the cladding material is the same material as that of the core and includes a broadband absorber material. Various non- limiting, exemplary embodiments may include one or more of the following limitations, features, components, characteristics or steps in various combinations as one skilled in the art would understand:

-comprising pressing or depositing the absorber doped ceramic powder on the core material and sintering to full density under a heat-treatment;

-comprising in-diffusing the broadband absorber material at the periphery of the core through controlled thermal annealing in a reducing or an oxidizing atmosphere; -further comprising first coating the periphery of the core material with a pigment and then heat-treating to promote the diffusion of the pigment through the outer layer of the core material;

-further comprising in-diffusing the broadband absorber material through a controlled electrochemical process;

-further comprising depositing pairs of ion-blocking/electron-blocking electrodes and controlled potentiostatic electro-reduction or electro-oxidation of the periphery of the core material under a controlled atmosphere;

-wherein the broadband absorber material is equal to or less than 50 atomic layers of graphene.

Brief description of the Figures

Fig. 1 : Index-matching ceramic cladding deposited onto a laser gain medium.

Fig. 2 is a schematic of a graphene doped ceramic cladding on a single crystal core (left), the interface between the cladding and core (center), and graphene on the grain boundary (right), according to an exemplary embodiment of the invention.

Fig. 3 : Interface between a Ti: sapphire single-crystal and a graphene-doped alumina ceramic.

Detailed Description of Non-limiting, Exemplary Embodiments Exemplary embodiments of the invention include an absorptive, solid-state, index- matched cladding containing an appropriate absorber for use in high power, solid-state, single-crystal or ceramic laser amplifiers to suppress transverse oscillations, and associated methods. The embodied cladding material is of the same material as the gain crystal but in a polycrystalline form. The embodied invention provides four approaches:

(i) using a rare-earth ion (e.g., Sm3+), transition metal ion (e.g. Cr4+), or p-block element (e.g., Bi3+)-doped transparent/translucent ceramic for the cladding of a solid-state laser amplifier. Ceramic powders pressed or deposited on a gain-medium can be sintered to full density under appropriate heat-treatments and turned into a fully dense and optically absorptive cladding. By using a ceramic cladding of the same material as the single- crystalline core but doped with a selected absorptive dopant (such as Sm3+ in the case of a Nd3+: YAG gain medium), the ASE of the core can be suppressed.

According to an aspect, Fig. 2 schematically shows a ppm-graphene-doped fully dense polycrystalline ceramic cladding on a single crystal core (left), the interface between the cladding and core (center), and graphene on the grain boundary (right). Here, a mixture of graphene and ceramic powders were pressed or deposited on the gain-medium and sintered to full density under appropriate heat-treatments. Monolayer graphene, a truly 2D gapless semiconductor, is found to absorb a significant (πα = 2.3%) fraction of incident light. Trace amounts and less than 50 atomic layers of graphene can absorb all the incident spontaneous emission.

Fig. 2 is a schematic of a graphene doped ceramic cladding on a single crystal core (left), the interface between the cladding and core (center), and graphene on the grain boundary (right), according to an exemplary embodiment of the invention. Here, graphene or graphite nanoparticles (100 ppm to 0.1% in weight) are mixed with ceramic powders, then pressed or deposited on a gain-medium can be sintered to full density under appropriate heat- treatments and turned into a fully dense and optically absorptive cladding. Thickness of the cladding can be varied from 1 mm to 10 mm depending on the mixing ratio of graphene or graphite in the ceramic powder. Generally, graphene or graphite are mixed with the ceramic powder in a planetary ball-mill with alumina grinding media and ethanol, while casting slurry is made with water, dispersants and binders. The carbon containing ceramic powder compact is typically sintered between 1600 and 1800 degree Celsius under high vacuum (10^"7 torr) into a dense ceramic solid and diffuse bond this solid onto the gain media core. Fig. 2 shows the microstructure of the interface of ceramic cladding and single crystal gain media.

Imaging on the two sides of this interface, materials have the same properties like refractive index, thermal expansion and conductivity, and transverse lights from core side will be absorbed when they pass the interface into the ceramic side, which contains the broadband absorbing graphene dispersed at grain boundaries.

Fig. 3 shows the interface between a Ti: sapphire single-crystal and a graphene-doped alumina ceramic.

In an alternative aspect, one can in-diffuse dopants or point-defects that have appropriate absorption bands at the periphery of the gain-medium through controlled thermal annealing in reducing or oxidizing atmospheres.

Alternatively, the gain-medium can either be coated with a pigment and then heat- treated to promote the diffusion of the pigment through the outer layer of the gain-medium or simply heat-treated in a reducing or oxidizing environment of the wide absorption bandwidth of color centers created by in-diffusing dopants or point-defects that have appropriate absorption bands at the periphery of the gain- medium through controlled electrochemical processes.

An embodied approach generally includes the preparation of varied solid load slurry with dopant and laser gain material particles mixed by, e.g., planetar ball milling, the wet forming of cladding structure on laser media core through dip coating, slip casting or gel casting depending on the required thickness of the cladding, calcination of organic dispersant, consolidation or single crystalline conversion of the cladding in different atmospheres and elevated temperatures, post-annealing, and optical polishing. The electrochemical approach, also known as electrocoloration, involves the deposition of a pairs of ion-blocking/electron-blocking electrodes and careful potentiostatic electro-reduction or electro-oxidation of the periphery of the gain-medium under controlled atmosphere. Examples of electrode stacks include platinum electrodes in contact with nickel- nickel oxide (Ni-NiO) or titanium suboxides (including Ti₄0₇, T1₅O₉) ceramics on yttria- stabilized zirconia (YSZ). The experimental cell arrangements for the oxidation of

Ti: sapphire gain media can be schematically represented by:

(-) Pt|Ni-NiO|YSZ|Ti:Al₂0₃|Pt (+)

(-) Pt|Ti-Ti0₂|YSZ|Ti:Al₂0 |Pt (+)An external voltage is applied to the electrochemical cell, which when negative at the oxygen conducting electrode allows a current to pass and increase the concentration of oxygen ions in the outer layer of the gain medium (i.e. oxidize Ti ⁺ into Ti⁴⁺). Maximum optical absorption by charge transfer is obtained when the concentration of Ti ⁺ equates that of Ti⁴⁺. Ni-NiO is a mixed electronic-ionic conductor. Y₂0 stabilized zirconia is a well-known oxygen ion conductor and the two-layer electrode configuration blocks the electronic component of the current and allows only the flux of oxygen ions through alumina at temperatures of about 1000° C. The advantages of using this type of cladding are numerous. Because this cladding is the same material as the core, the index matching is naturally perfect across all wavelengths, (e.g., even if the crystalline orientations are random, the birefringence of sapphire is only -0.008 throughout the near infrared, which still provides index matching similar or superior to fluids and thermoplastics as known in the art). Since the cladding is the same material as the core, there are no worries about thermal expansion mismatching at the core-cladding interface, eliminating a major problem common to other solid-state index-matching solutions. The cladding material, alumina, has a higher thermal conductivity than other index-matching materials such as polymers, epoxies, and liquid-based solutions, making it easier to manage the thermal load of the gain medium. The cladding is stable, vacuum compatible, non-toxic, and easy to handle, unlike many thermoplastic and liquid-based index-matching options. The cladding is compatible with cryogenic cooling methods, unlike many thermoplastic and liquid-based index-matching options.

If liquid cooling is still desired, a clamp/seal can extend onto the face of the ceramic (where extra area is easy and cheap to grow) instead of onto the face of the single crystal (where extra area takes time and money). In liquid cooling, one must use single crystals larger than needed for amplification to provide necessary dead space around the edge of the face of the crystal for sealing the chilled liquid loop. This dead space could instead be part of the ceramic cladding, which would allow the use of a smaller (i.e., cheaper, higher-quality) crystal.

The ceramic nature of the embodied cladding provides several advantages of its own including freedom of doping, geometry, scaling, etc.

The small powder grain size and the baking/growth process result in excellent optical contact between the core and the cladding, which can be a problem with other solid-state index-matching solutions.

The dopant graphene acts as a uniform broadband absorber across the visible and near IR spectrum.

The embodied invention may be utilized as the cladding for high-energy or high- power solid-state, single-crystal laser systems where thermal management and/or parasitic lasing is a concern.

An exemplary commercial use of the embodied invention is in high-power

Ti: sapphire laser amplifiers. As an example, one manufacturer offers a PW-level Ti: sapphire laser amplifier with an anti-transverse-lasing solution but without the benefits of cryogenic cooling, while another offers a PW-level Ti: sapphire amplifier with cryogenic cooling but without the possibility of index matching. One could utilize the embodied invention as part of an amplifier capable of offering the benefits of both cryogenic cooling and anti-transverse lasing. Additionally, the embodied invention could be used to provide superior laser crystal products, or it can be applied to new and old crystals alike.

It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

We claim:

1. A cladding material for a solid-state, single-crystal, or ceramic laser amplifier, wherein the cladding material is the same material as a core material of the solid-state, single- crystal, or ceramic laser amplifier and includes a broadband absorber material.

2. The cladding material of claim 1, wherein the core material is sapphire and the cladding material is alumina-doped sapphire.

3. The cladding material of claim 1, wherein the core material is YAG and the cladding material is Sm3+ doped YAG.

4. The cladding material of claim 1, wherein the broadband absorber material is graphene.

5. The cladding material of claim 4, wherein the graphene is in an amount of equal to or less than 50 atomic layers.

6. The cladding material of claim 1, wherein the cladding material is in a polycrystalline form of the core material.

7. The cladding material of claim 1, wherein the broadband absorber material is a rare earth ion.

8. The cladding material of claim 7, wherein the rare earth ion is Sm3+.

9. The cladding material of claim 1, wherein the broadband absorber material is transition metal ion.

10. The cladding material of claim 9, wherein the transition metal ion is Cr4+.

11. The cladding material of claim 1, wherein the broadband absorber material is a p- block element.

12. The cladding material of claim 11, wherein the p-block element is Bi3+.

13. A method for suppressing transverse oscillations in a solid-state, single-crystal, or ceramic laser amplifier, comprising: applying a cladding material to an outer perimeter region of a core of the solid-state, single-crystal, or ceramic laser amplifier, wherein the cladding material is the same material as that of the core and includes a broadband absorber material.

14. The method of claim 13, comprising pressing or depositing the absorber doped ceramic powder on the core material and sintering to full density under a heat-treatment.

15. The method of claim 13, comprising in-diffusing the broadband absorber material at the periphery of the core through controlled thermal annealing in a reducing or an oxidizing atmosphere.

16. The method of claim 15, further comprising first coating the periphery of the core material with a pigment and then heat-treating to promote the diffusion of the pigment through the outer layer of the core material.

17. The method of claim 15, further comprising in-diffusing the broadband absorber material through a controlled electrochemical process.

18. The method of claim 17, further comprising depositing pairs of ion-blocking/electron- blocking electrodes and controlled potentiostatic electro-reduction or electro-oxidation of the periphery of the core material under a controlled atmosphere.

19. The method of claim 13, wherein the broadband absorber material is equal to or less than 50 atomic layers of graphene.