This patent document relates to lasers and materials for producing optical gain in lasers.
The past decade has seen significant advances in the development of high-energy laser (HEL) technologies, with improvements in pumping technology, cavity design, cooling methods, and improved gain media quality. The search for gain media with superior optical, thermal, and mechanical properties remains intense because improvements in the materials properties translate directly to increases in device performance. Advanced laser gain materials that provide access to different wavelengths, tunability, short pulses, etc. have paved the way for the study of light-matter interactions, break-through medical applications, and imaging/spectroscopy.
Alumina (Al2O3) used as an optical gain material has a higher fracture strength and thermal conductivity than current gain materials, which could lead to improved laser performance. Alumina also has uniaxial optical proprieties and the solubility of rare earth materials (REs) is two-to-three orders of magnitude lower than dopant concentrations in some RE-based gain media. The disclosed subject matter may be used to overcome these obstacles and demonstrate gain in a RE-doped alumina (Nd:Al2O3). The disclosed subject matter may be used to tailor the crystallite size to other length scales such as a wavelength of light and interatomic dopant distances, which minimizes the optical losses and allows for successful Nd doping.
In one aspect, a laser apparatus is disclosed. The apparatus includes a polycrystalline material configured to receive pumping light at a pump wavelength and to produce an optical gain for laser oscillation at a laser wavelength different from the pump wavelength. The polycrystalline material includes a ceramic material with a predetermined grain size. The polycrystalline material further includes a rare earth dopant with a predetermined concentration, wherein the predetermined grain size and the predetermined concentration cause the polycrystalline material to exhibit the optical gain at the laser wavelength.
In another aspect, a polycrystalline material is disclosed. The polycrystalline material includes a ceramic material with a predetermined grain size, and a rare earth dopant with a predetermined concentration, wherein the predetermined grain size and the predetermined concentration cause the polycrystalline material to exhibit optical gain at a predefined wavelength.
The following features can be included in various combinations. The predetermined grain size is less than the pump wavelength. A distribution of the rare earth dopant has a minimal segregation at grain boundaries. The pump wavelength of the pumping light is 806 nanometers. The laser wavelength and a predefined wavelength are 1064 nanometers. The laser wavelength and the predefined wavelength lie between 1000 nm and 2000 nm. The ceramic material is alumina (Al2O3). The rare earth dopant is neodymium (Nd). The rare earth dopant is one or more of neodymium (Nd), erbium (Er), thulium (Tm), holmium (Ho) or ytterbium (Yb).
Additional features are disclosed in the specification, figures, and the claims.
Section headings are used in the present document only for ease of understanding and do not limit scope of the embodiments to the section in which they are described.
Traditionally accepted design paradigms dictate that only optically isotropic (cubic) crystal structures with high equilibrium solubility of optically active ions are suitable for polycrystalline laser gain media. The restriction of symmetry is due to light scattering caused by randomly oriented anisotropic crystals, while the solubility arises from the need for sufficient active dopants in the media. These criteria can limit material choices and exclude materials that have superior thermo-mechanical properties over the state-of-the-art laser materials. As disclosed herein, Alumina (Al2O3) is an example; it has a higher fracture strength and thermal conductivity than current gain materials, which could lead to improved laser performance. Alumina also has uniaxial optical proprieties and the solubility of rare earth materials (REs) can be two-to-three orders of magnitude lower than dopant concentrations in typical RE-based gain media. The disclosed subject matter may be used to overcome these obstacles and demonstrate gain in a RE-doped alumina (Nd:Al2O3). The disclosed subject matter may be used to tailor the crystallite size to other length scales—wavelength of light and interatomic dopant distances, which minimizes optical losses and allows for successful Nd doping. The result is a laser gain medium with a thermo-mechanical figure of merit of Rs˜19,500 Wm−1, a 24 and 19,500 fold improvement over high-energy-lasers such as Nd:YAG (Rs˜800 Wm−1) and Nd:Glass (Rs˜1 Wm−1), respectively. Moreover, the emission bandwidth of Nd:Al2O3 is broad at ˜13 THz. The successful demonstration of gain and high bandwidth in a media with superior Rs leads to lasers with previously unobtainable high-peak powers, short-pulses, tunability, and high-duty-cycles. A polycrystalline laser gain media produces optical amplification.
Single-crystals and glasses dominate the gain media market, but polycrystalline ceramics have advantages such as improved mechanical properties and gradient doping. Ceramics also have the potential to improve thermal management of gain media. The power deliverable by a laser scales directly with the thermal conductivity k, and the fracture stress σF, places a limit of failure such that the figure of merit for a gain material is given by:
where E is the elastic modulus, a is the coefficient of thermal expansion and v is Poisson's ratio. The low thermal conductivities of leading gain media (˜1-2 Wm−1K−1 RE:Glass, and 7-14 Wm−1K−1 RE:YAG) continue to limit the power scaling of HELs.
Cubic materials such as RE-host media may have higher k than YAG. Cubic-symmetry materials such as garnets and RE-sesquioxides are transparent ceramics because grain growth need not be avoided to mitigate birefringence scattering and they readily accommodate RE dopants due to the similarity in ionic radii between dopant and cations. To supplant RE:Glass and/or RE:YAG, a gain material with substantially better thermo-mechanical properties is needed.
Sapphire/alumina may be a RE host because Al2O3 offers superior thermal conductivity (k˜30-35 Wm−1K−1) and a high-fracture toughness (3.5 MPam−1/2), the combination of which leads to a superior thermal shock resistance (Rs˜19,500 Wm−1) compared to Glass (Rs˜1 Wm−1) and YAG (Rs˜800 Wm−1). Moreover, sapphire has been used as a transition metal doped gain media. The addition of RE dopants at levels sufficient for gain could allow for efficient emission at other wavelengths, resulting in a laser gain medium with a combination of thermal, mechanical, and optical properties that will lead to more powerful lasers in scientific, medical, industrial, and mobile applications.
Two challenges to producing laser grade RE:Al2O3 ceramics include 1) the disparity in ionic radii between the RE3+ and Al3+, which leads to an equilibrium solubility ˜10−3%, lower than necessary for gain, and 2) the optical anisotropy arising from the hexagonal crystal structure of Al2O3 leads to birefringence scattering that must be mitigated to achieve high transparency.
Translucent alumina ceramics have been produced but no gain in RE:Al2O3 has not been demonstrated at least on part because RE:Al2O3 ceramics have not reached the necessary optical quality. The disclosed subject matter includes bulk polycrystalline Nd:Al2O3 ceramics that exhibit stimulated emission and optical gain. The disclosed gain can be achieved without single sight doping, i.e. with some Nd segregated to the grain boundaries. Using the disclosed subject matter, absorption bands in the transmission spectra are present thereby confirming the presence of optically active Nd3+ within the ceramic matrix. In some example embodiments, for the primary pumping band at 806 nm (4I9/2→4F5/2) the absorption cross-section is 1.36×10−20 cm2 and 1.69×10−20 cm2 for 0.25 at. % and 0.35 at. % Nd:Al2O3 ceramics, respectively.
In addition to improving thermal management, Nd:Al2O3 also addresses another challenge in HEL technologies—producing broadband emission in RE-doped media. Conventional gain media design aims for sharp single-site peaks resulting in lower lasing thresholds. The advantage of high bandwidth is wavelength tunability and allows the generation of short pulses (increased peak energy). In some example embodiments, when pumping at 806 nm, the ceramics show a 50 nm (FWHM), 13 THz peak at 1064 nm, (4F3/2→4I11/2). The fluorescence lifetime is ˜150 μs resulting in stimulated emission cross-sections as high as ˜9.8×10−21 cm2. The 13 THz gain bandwidth arising from multi-site doping of Nd in Al2O3 for Nd3+ gain media could lead to pulses as short as 8 fs. The measured gain coefficient, go, may be as high as 2.42 cm−1 for 0.35 at. % Nd3+:Al2O3 at 1064 nm. The combination of thermal, mechanical and optical properties offered by Nd3+:Al2O3 opens the door to producing HEL with superior performance. Moreover, the approach presented herein is applicable to other anisotropic material systems that are not readily considered for optical applications. In some example embodiments, a polycrystalline material exhibits gain in one ore more wavelength bands, or the entire wavelength band between 1000 nm and 2000 nm. In some example embodiments, the rare earth dopant is one or more of Neodymium, Erbium (Er), Thulium (Tm), Holmium (Ho) or Ytterbium (Yb). A laser apparatus may include a polycrystalline material as described herein.
The disclosed techniques and materials for obtaining gain in Nd:Al2O3 include a nano/microstructure design that includes: 1) Crystallite sizes below the wavelength of pump and emitted light, and 2) Dopant distribution in the grain volumes with minimal segregation at the grain boundaries.
There are length scale relationships for achieving gain in anisotropic ceramics.
In addition to low losses, RE dopant concentrations may be within a critical range; high enough to achieve a sufficient absorption cross-section and emission-cross-section, and low enough to prevent concentration quenching (energy relaxation through phonon rather than radiative photon processes) which occurs when ions are too closely spaced.
Traditional material processing can be employed in systems such as glasses and garnets where the RE solubility is high. However, in low solubility media, agglomeration occurs at grain boundaries (as shown in
In the disclosed subject matter, the fine crystallite sizes that allow for high transparency in anisotropic polycrystalline materials play a role in absorption/emission by providing a possibility for higher RE incorporation without luminescence quenching. By reducing the grain size, the grain boundary volume increases. When holding the global dopant concentration constant while decreasing the grain size, RE dopants can ‘spread out’ along the grain boundaries, increasing the average distance, {tilde over (l)} between RE ions (
To illustrate this scenario, an example deff is plotted as a function of grain size (Eq. 2) in
To obtain gain in an Nd:Al2O3 bulk polycrystalline material, processing techniques that will produce fully dense ceramics with fine average grain size (AGS) and/or that offer processing widows with increased rare-earth solubility are needed. Fortunately, alumina does have Nd solubility that can be increased using high heating and cooling rates (to be discussed below), easing the necessity for extremely fine grains. Using a solid-state powder processing technique along with a one-step simultaneous reaction/densification approach with current activated pressure assisted densification (CAPAD), an Nd3+ dopant concentration as high as 0.35 at. % (Nd:Al ratio) can be achieved, approximately 350 times greater than the equilibrium solubility limit.
At processing temperatures of 1200° C. (undoped) and 1260° C. (Nd-doped) the samples may have fine AGS of ˜250 nm, near theoretical density, and are phase pure. As such, they possess long-range transparency (
The CAPAD processing parameters were varied to optimize the microstructure and properties of various concentrations of Nd:Al2O3 (see methods below for details).
Reduced densification kinetics may occur that is caused by RE addition in reaction/densification of ceramics. This may be due to the presence of the RE oxide dopant powder along the particle/grain boundaries when the two phases are still separate reactants. In our previous work on alumina with Tb as a dopant, the decrease in density was lower compared to the present case of Nd at similar global concentrations. The difference in behavior between the Nd and Tb dopants can be attributed to the larger ionic radius of Nd3+ (0.983 Å) compared to Tb3+ (0.923 Å). A similar shift in the TOD with respect to RE ionic radius may occur for Nd3+, Eu3+, and Er3+ doped Al2O3 system (0.2 at. % RE:Al2O3 ratio, ˜0.04 at. % RE:Al) via free-sintering and hot-pressing.
By contrast, XRD of the ceramics processed using optimized CAPAD conditions reveal single phase α-Al2O3 with no signal from the starting Nd2O3 or from the ternary Nd4Al2O9 and NdAlO3 phases. This is in contrast to some previous reports that showed secondary phases in RE doped α-Al2O3 that have been produced at RE concentrations above the equilibrium solubility limit with other processing approaches. Moreover, the XRD spectra of the optimized Nd-doped samples reveal clear peak shifts to lower angles with increasing Nd concentration (Un-doped 2θ=35.18°, 2θ0.25 at. %=34.09° and 2θ0.35 at. %=34.98°). The dashed line in the inset on the right is the location of highest intensity peak from reference. This shift is evidence of stretching of the α-Al2O3 lattice from the doping of Nd-ions caused by CAPAD processing. The absence of the Nd2O3 reactant and ternary phases indicates a difference in the reaction kinetics associated with CAPAD processing in comparison to traditional processing approaches
Over-doping RE into Al2O3 to the high heating and cooling rates we employed in CAPAD processing that when optimized, produce a fine AGS and increase the RE-solubility may be due to increased reaction kinetics. The high heating rate ˜300° C. min−1 allows reaching the desired temperature quickly, minimizing unwanted grain growth while achieving a near theoretical relative density, pre-requisites for high optical transparency in Al2O3. An increase in reaction kinetics associated with high heating rates may occur in the Ce:YAG system. A ˜20-fold increase in reaction coefficients may occur in comparison to reaction/densification in free-sintering using much slower heating rates. Since the largest difference between optimized and un-optimized samples was the CR, this parameter also plays a role in RE incorporation. The Nd solubility increases at higher temperatures so that the high CR has the effect of “freezing in” Nd, minimizing segregation. There is a synergistic effect between a fine AGS and RE incorporation during CAPAD.
TEM may be used to further confirm incorporation of Nd into the alumina matrix. A high-angle annular dark-field (HAADF) TEM micrograph and corresponding energy-dispersive X-ray spectroscopy (EDS) distribution maps of a 0.35 at. % Nd:Al2O3 polycrystal (T=1260° C., HT=5 min, HR=300° C. min−1, CR=300° C. min−1) are shown in
An example of optical transparency of consolidated bulk Nd:Al2O3 polycrystals is shown in
A difference in the Nd:Al2O3 transmission spectra is the presence of the absorption bands centered at λ=583 nm (2.12 eV), 745 nm (1.85 eV), and 806 nm (1.54 eV), which correspond to the 4G5/2, 4F7/2, and 4F5/2 Stark transitions from the 4I9/2 manifold. The absorption bands associated with RE doping in Al2O3 and are strong evidence that the Nd3+ dopant is optically active within the ceramic matrix. The center of the Nd3+ absorption bands in Al2O3 are slightly blue shifted (˜2.5 nm), compared to Nd:YAG single crystals. The absorption bands are broadened in Nd:Al2O3 to AA-23 nm (FWHM) from ˜AA-˜2 nm compared to Nd:YAG, which is consistent with the Nd3+ being found on multiple doping sites within the alumina matrix. Moreover, the depth of the absorption bands increases with the dopant concentration, indicating more optical activity from the Nd3+ ions within the 0.35 at. % Nd:Al2O3 sample.
The absorption cross-sections, σabs for the region of interest are shown in the inset in
For the 4F5/2 transition, which is of interest for diode pumped lasers, the peak σabs are 1.36×10−20 cm2 and 1.69×10−20 cm2 for the 0.25 at. % and 0.35 at. % Nd:Al2O3. These cross-sections compare well with single-crystal 1.1 at. % Nd:YAG, (σabs˜7.7×10−20 cm2). The slightly lower σabs in Nd:Al2O3 may indicate the presence of Nd sites that are not optically active, or by the absorption band broadening, which also occurs in Nd:Glass and in Nd:YVO4.
The gain bandwidth (Gbw) can be approximated by measuring the full-width at half-maximum (FWHM) of the PL emission peaks. In some example embodiments, Gbw=0.6 nm (0.16 THz) for Nd3+:YAG and Gbw=20 nm (5.4 THz), for Nd3+:Glass which agree well with previous measurements. The Gbw˜49 nm (13 THz) of our Nd3+:Al2O3 may be the highest bandwidths measured for Nd3+ in any media. For bandwidth-limited pulses, the achievable pulse duration of a gain medium is determined by Gbw. The broader the emission spectra, the shorter the pulse and the pulse width can be estimated using, ΔτP=1/Gbw. Using Gbw measurements, we find ΔτP˜7.7 fs. The large bandwidth of Nd3+:Al2O3 may cause generation of high peak-power lasers by generation of ultra-short time pulses. These bandwidth-limited pulse widths represent a 2.5 fold increase in the single-shot peak power over Nd3+ glass and >80 fold increase over Nd3+:YAG (ΔτP=6.3 μs for Nd3+:YAG, ΔτP=18.5 fs for Nd3+:Glass), through pulse width compression. These estimated improvements are conservative since thermal shock resistance for Nd:Al2O3 (Rs˜19,500 Wm−1) is superior to Nd:YAG (Rs˜800 Wm−1) and Nd:Glass (Rs˜1 Wm−1), indicating the possibility to scale peak power extraction accordingly.
Given the absorption and PL characteristics, the radiative lifetimes were measured, τ, at 1064 nm for the Nd:Al2O3 ceramics for example optimized samples. The lifetimes are 152 μs and 141 μs for the 0.25 and 0.35 at. % Nd:Al2O3, respectively (
From the PL emission spectra, the emission cross-sections, σEm using the Fuchbauer-Landendurg relationship may be expressed as,
The σEm are large and adequate for lasing across the PL bandwidth; the peak σEm=7.5×10−21 cm2 for 0.25 at % and 9.8×10−21 cm2 for 0.35 at. % ceramics processed at CR=300° C. min−1. These σEm are consistent with σAbs derived from the measured transmission spectra. By contrast the σEm is 3.1×10−22 cm2 for the un-optimized sample. The substantially lower σEm proves that the presence of second phases deteriorates the optical activity for the Nd-dopant.
To ascertain the viability for lasing in Nd3+:Al2O3 their small signal gain coefficients may be measured using a single pass arrangement. The schematic for the optical arrangement is shown in
I
F=(z)=Io(z)e[g
where Io(z) and IF(z) are the intensities of the probe laser after having passed through the test specimen of thickness z, prior to and with pumping, respectively, and g0 is the small-signal gain coefficient, obtained here in a single-pass arrangement.
The demonstration of gain may be related to the nanostructure of the ceramics. The fine AGS results in an Al2O3 with a large grain boundary volume, which facilitates the accommodation of the RE without significant concentration quenching. In addition to microstructural control, high heating and cooling rates during CAPAD processing also affect the incorporation of Nd3+ into the grain and grain boundary regions without the formation of unwanted secondary phases that lead to poor optical activity.
In summary, a powder processing route in conjunction with single-step CAPAD reaction/densification is disclosed to produce transparent bulk polycrystalline Nd3+:Al2O3 with over-equilibrium Nd-doped (0.25 at. % and 0.35 at. %) concentrations. The ceramics have a high transmission at 1064 nm and display absorption bands at λ=585 nm, 748 nm, and 806 nm, corresponding to transitions from the 4I9/2 manifold of optically active Nd3+, resulting in high peak absorption cross-sections. The PL bandwidth of ˜13 THz centered at 1064 nm represents a new record for Nd3+ media, permitting the generation of ultrashort pulses. The radiative lifetimes are long and give a large emission cross-section, which result in optical gain that is suitable for amplification and lasing. Moreover, the significantly higher Rs˜19,500 W/m of Nd3+:Al2O3 promise a significantly higher duty-cycle and/or peak-power, making Nd3+:Al2O3, a potentially revolutionary gain material. Finally, the nano/microstructural strategies demonstrated here may be applicable to many other oxide and nitride gain systems that were not previously believed to be laser ceramics and thus represents a fundamentally new approach to producing gain media.
Relations Between Interionic Distance, Grain Size and Effective Length
A factor for gain is the average distance between dopant ions, {tilde over (l)}. Dopant concentrations, c are usually reported in [at. %] relative to cations. Interionic distances may be understood using volumetric concentration, cvol [ions/cm3] because {tilde over (l)} scales with total number of ions in a volume, V such that {tilde over (l)} ∝∛√{square root over (1/cvolV)}. While calculations or measurements of {tilde over (l)} can be complicated, it is easy to obtain a good estimate of {tilde over (l)} using a regular pattern of dopants such as a simple cubic cell with RE on each corner with I as a cell length. In this case {tilde over (l)}˜I=∛√{square root over (1/cvolV)}. Laser quality Nd:YAG used as an example, where the typical dopant concentration is 1-2 at. %. In the c=0.25 at. % case, cvol=7.53×1020 ions/cm3 so that {tilde over (l)}˜1.09 nm.
Alternate dopant distributions may be considered. Consider one crystallite of gain media approximated as a cube with global volumetric dopant concentration, cvol [ions/cm3]. The total number of ions, N in the volume of that cube is equal to cvold 3 where d is the cube edge length. If all the dopant ions in that cube are placed on the surface (i.e. grain boundary) rather than in the grain volume, one can calculate the effective length (edge length), deff necessary to accommodate all the dopants for a given arrangement on the surface of the cube. For simplicity, the random arrangement of ions can be approximated as a regular square unit cell with cell parameter 2r+I, where r is ionic radius and I is the distance between dopant ions. Since there are 6 sides to a cube, deff as a function of grain size (edge length), d is:
A value of r=1.15 Å for Nd ions and I=1 nm was used for calculations, since 1 nm is a good approximation of {tilde over (l)} as shown above.
Powder Preparation
α-Al2O3 (e.g., 99.99% purity) may be processed as received (un-doped) and doped with Nd2O3 (e.g., 99.99% purity). The powders may be mixed to achieve a doping level (Nd3+:Al3+) of 0.25 and 0.35 at. %. The powders may be mixed dry in an alumina mortar by hand for 20 min, followed by low-energy ball milling for 12 hrs with Ultra-High Purity (UHP, 99.99% purity) water as a dispersant. The slurries may be sieved and centrifuged for 15 min at 3400 RPM. The powders may be dried in a vacuum oven at 70° C. under a vacuum of 30 mm Hg for 12 hrs. Dried powders were subsequently planetary ball milled with UHP water at 150 RPM for 6 hrs. Finally, the powders may be sieved and dried in air at 120° C. for 12 hrs and kept dry until consolidation.
CAPAD Processing
The powders may be densified by CAPAD using a graphite die (19 mm outer and 10 mm inner diameter). This die and plunger set may be secured between two 19 mm punches and placed within a larger graphite die with a 19 mm inner diameter. The die and powder set may be placed into the CAPAD and a vacuum of 10−3 Torr established. The powders may be pre-pressed at 106 MPa for 20 minutes after which the load may be released. An ultimate pressure of 106 MPa with a pressure ramp of 35.33 MPa min−1 may be applied and held constant. In parallel with pressure application, the samples may be subjected to a heating rate of ˜300° C. min−1 and a maximum temperature ranging between 700-1300° C. with a hold time of 5 min. The temperature may be monitored with a dual wavelength optical pyrometer focused at the die midpoint.
Microstructural Characterization
Powders and densified ceramics may be characterized using X-Ray diffraction (XRD) using Cu Kα1 (λ=1.54058 Å) radiation using, for example, a PANalytical Empyrean Diffractometer (PANalytical, Almelo, The Netherlands) using a step size of 26=0.0050. Published standards may be used for comparison: Nd2O3 (ICSD #26867), and α-Al2O3(ICSD #:63647).
The average grain size (AGS) of the densified ceramics may be obtained from fracture surfaces by measuring >300 grains in multiple micrographs at random locations. The fractured surface may be sputter coated with a thin film of Pt/Pd before examination with a Phillips XL30 Field Emission Scanning Electron Microscope (FE-SEM). EDS mapping was performed using a Titan Themis 399 Scanning-TEM (STEM). The TEM specimen may be prepared using a gallium Focused Ion Beam (FIB) and attached to a copper TEM grid using a Pt FIB.
Transmission and Photoluminescence (PL) Measurements
The samples may be polished with diamond suspensions to 0.5 μm. The final specimen thickness was 0.8 mm±0.05 mm. Transmission spectra may be taken on, for example, a Varian Cary 500 UV-VIS-IR spectrometer from 300 nm to 2200 nm at normal incidence, in single-beam mode with a rectangular spot size of 2 mm by 9 mm, using a scan rate of 0.2 nms−1.
PL may be measured on, for example, a Horiba Spex Fluorolog 3 Spectrophotometer using an 806 nm laser diode as the excitation source with a 100 mW incident power and a spot size of 2 mm. Measurements may be taken in front face mode at 45° angle of incidence (AOI) on polished samples. Emission scans may be taken between λ=1000 nm and λ=1100 nm with an integration time of 1 snm−1.
Photoluminescence Lifetime Measurements
PL lifetimes (pump=806 nm) may be obtained using a pulsed tunable laser (Continuum Surelite with Optical Parametric Oscillator (OPO). For example, the pulse width was 6 ns, the spot size was 6 mm, and the incident energy was 3 mJ per pulse. The ceramics may be mounted within, for example, a Horiba Spex Fluorolog 3 Spectrophotometer, which may be coupled to a germanium photodiode and synchronized to a Tektronix TPS2024B oscilloscope. The monochromators may be adjusted to observe 1064 nm, with a spectral bandwidth of 1 nm. An optical notch filter centered at 1064 nm with 8 nm FWHM transmission band may be used to further isolate the pump source. Measurements may be taken in front face mode at 45° AOI. A double-exponential may be used to fit data and extract the lifetimes, where t, is defined as the time required for the intensity to decrease by 1/e.
Single-Pass Optical Gain
Optical gain may be measured using a single-pass arrangement shown schematically in
A continuous wave Nd:YAG laser, operating at the fundamental wavelength (λ=1064 nm) may be used as the probe laser. The collimated probe beam (˜1 mm diameter) may be focused onto the sample with a 100 mm focal length lens, resulting in a FWHM spot size of ˜220 μm. A fiber coupled Coherent FAP 35 W laser diode (λ=806 nm) and collimator may be the pumping source. The pump laser may be focused onto the sample collinear to, but counter-propagating with respect to the probe using a 35 mm focal length lens resulting in a spot size of ˜400 μm. The spot sizes may be determined by fitting a gaussian profile to the probe laser and a top-hat profile to the pump laser from CCD images of the focused beams. The pump beam waist was injected into the arrangement via a dichroic mirror (Thorlabs DMSP1000) with a reflective cut-on wavelength of 1000 nm at 45° AOI. In addition to the factory dielectric coatings, an additional anti-reflective coating for 806 nm was deposited onto the dichroic optics, which maximized the deliverable pump power onto the test specimens, while minimizing stray Fresnel reflections for the pump laser.
The focusing optics for the probe and pump beams may be mounted on 6-axis kinematic fixtures, allowing precise spatial alignment of the beams within a single sample interaction volume. The pump and probe beam power may be monitored with germanium photodetectors (for example, a Thorlabs PDA50B), PD1 and PD2, respectively, which may be optically isolated to the desired wavelengths with low and high-pass filters. The pump and probe lasers may be operated in quasi-continuous mode using an 8 Hz and 10 Hz boxcar waveform, respectively. The fluctuations in the pump and probe laser intensities may be recorded using a lock-in amplifier in parallel with an oscilloscope at their respective operating frequencies. This ensures that fluctuations in PD signals are isolated. The photodetectors may be calibrated against an optical power meter (for example, a Ophir Nova 2).
The disclosed technology can be embodied in the form of a laser apparatus that includes a polycrystalline material. The polycrystalline material may include a ceramic material and a rare earth dopant. The ceramic material may have a grain size and the rare earth dopant may have a predetermined concentration, which result in the polycrystalline material exhibiting an optical gain (e.g., greater than unity amplification) at a laser wavelength. The polycrystalline material may be positioned to receive pumping light at a pumping wavelength and produce the optical gain for laser oscillation at the laser wavelength that is different from the pumping wavelength.
The disclosed technology may be embodied in the form of a polycrystalline material that includes a ceramic material with a predetermined grain size and a rare earth dopant with a predetermined concentration, wherein the predetermined grain size and the predetermined concentration cause the polycrystalline material to exhibit an optical gain at a predefined wavelength.
In some implementations, wherein the predetermined grain size is less than the pump wavelength. In some implementations, a distribution of the rare earth dopant has a minimal segregation at grain boundaries. In some implementations the pump wavelength of the pumping light is 806 nanometers (nm) or within plus-minus one percent of this wavelength. In some implementations, the laser wavelength is 1064 nanometers (or within 1 percent of this value). In some implementations, the laser wavelength is between 1000 nm and 2000 nm. In some implementations, the ceramic material is alumina (Al2O3). In some implementations, the rare earth dopant is neodymium (Nd). In some implementations, the rare earth dopant is one or more of neodymium (Nd), erbium (Er), thulium (Tm), holmium (Ho) or ytterbium (Yb), providing a wider selection of laser wavelengths at the output.
In some example embodiments, a method of manufacturing a laser apparatus includes manufacturing a polycrystalline material configured to receive pumping light at a pump wavelength and to produce an optical gain for laser oscillation at a laser wavelength different from the pump wavelength. The polycrystalline material includes a ceramic material with a predetermined grain size, and a rare earth dopant with a predetermined concentration, wherein the predetermined grain size and the predetermined concentration cause the polycrystalline material to exhibit the optical gain at the laser wavelength. For example, as described in the present document, a particular grain size and/or concentration may be used to achieve a particular desired optical gain, or amplification, at the laser wavelength.
In some example embodiments, a method of manufacturing a polycrystalline material includes selecting a ceramic material with a predetermined grain size, and a rare earth dopant with a predetermined concentration, wherein the predetermined grain size and the predetermined concentration cause the polycrystalline material to exhibit the optical gain at the laser wavelength. For example, as described in the present document, to achieve a specific optical gain at a laser wavelength, a specific grain size and/or a specific concentration can be selected for the ceramic material and the rare earth dopant.
Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and/or variations may be provided in addition to those set forth herein. Moreover, the example embodiments described above may be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed above. In addition, the logic flow depicted in the accompanying figures and/or described herein does not require the particular order shown, or sequential order, to achieve desirable results. Other embodiments may be within the scope of the following claims.
Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various elements in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.
Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.
This patent document claims priority to and benefits of U.S. Provisional Patent Application No. 62/797,139, entitled “RARE-EARTH-DOPED ALUMINA-OXIDE LASER GAIN MEDIA,” filed on Jan. 25, 2019. The entire content of the above patent application is incorporated by reference as part of the disclosure of this patent document.
This invention was made with government support under W911NF-16-1-0571 awarded by the U.S. Army Research Office. The government has certain rights in the invention.
Filing Document | Filing Date | Country | Kind |
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PCT/US20/15094 | 1/24/2020 | WO | 00 |
Number | Date | Country | |
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62797139 | Jan 2019 | US |