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Abstract
Laser glass is a highly engineered optical material that enables the amplification of light in laser systems. It is known for being the heart of the largest laser facilities ever built, where thousands of neodymium-doped meter-sized slabs of laser glass create intense beams of near-infrared laser radiation used to study fusion reactions. Or if doped with ytterbium and erbium, laser glass can be made into millimeter-sized components empowering range-finders and dermatological lasers. Decades of development have poised laser glass for the next challenge on the horizon: amplifying light for sustainable inertial fusion energy power plants.
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1. Introduction
On May 18, 2018, the United Nations General Assembly declared 2022 as a United Nations International Year of Glass, celebrating the scientific, economic, and cultural roles that glass has played in our lives [1]. Glass is a key material in a wide variety of industries, from guiding and amplifying light to providing inert vials for pharmaceuticals, to glass-ceramic cooktops, and mirror substrates for space telescopes.
Glass is often used as an optical element to transmit light in applications such as lens, windows, or fiber optics. However, with the right chemical composition, quality, format, and in the right application, glass can also be used to generate photons in a coherent beam of light – a laser beam. This special type of glass, laser glass, as well as its history and future uses, is the focus of this review article.
1.1 Solid state lasers
To start with, a simple laser can be described as follows: a resonant optical cavity in which a photon-generating medium is placed between two mirrors, one highly reflecting and one partially reflecting. The material which generates photons can be a solid, liquid, gas, or plasma, and is referred to as the gain medium. Energy is imparted to the gain medium through ‘pumping’, which is often in the form of light (flashlamps or diodes). Most commonly, atoms/ions or molecules in the gain medium absorb the pump light and their electrons (e.g., f-orbital electrons in rare earth cations) are excited to higher energy levels. In order to function as a laser, a sufficient fraction of the gain medium must be excited; this is called population inversion because the population of excited states must be greater than the population of lower energy or ground states, the opposite of normal conditions. When electromagnetic radiation of appropriate wavelength is incident on the gain medium, the excited states are stimulated to emit photons as they return to their ground state. The wavelength of the stimulated emission (i.e., the laser radiation) is determined by the transition between the excited and ground states; the bigger the difference in energy, the shorter the wavelength of the laser radiation. “Laser” is in fact an acronym for this process: light amplification by stimulated emission of radiation [2]. The energy in the resonant cavity can be output continuously in a steady-state manner (continuous wave or CW operation), or the laser can be operated in a pulsed fashion where the energy is ‘dumped’ over a very short time scale (ms to fs). Pulsed lasers can be cycled anywhere from kHz to one shot / day.
In the case where the gain medium is a solid, the laser is termed a solid state laser. The gain medium must be highly transparent at the lasing wavelength; common solid state gain media include crystals, ceramics, and glasses [2]. Which brings us to the focus of this article: laser glass. When glass of certain composition, excellent optical quality, and sufficient chemical purity is doped with certain laser-active rare earth (lanthanide) cations, the glass becomes a laser glass. The first laser glass was a Nd-doped barium crown glass in 1961 [3].
Laser glass has the following general advantages over other solid state gain media:
- 1. More stored energy, meaning more photons can be produced from a given material;
- 2. Efficient extraction of that stored energy, meaning a majority of those excited states can be stimulated to release photons and increase the intensity of the laser radiation in one pulse;
- 3. Excellent laser damage resistance, meaning the high intensity laser beam will not damage the laser glass itself; and
- 4. The manufacturability of glass, referring to the existing technologies which can produce glass at the rate of 10s of meter-class slabs/day at high optical quality [4].
1.2 Lasing ions
One of the main active lasing ions in glass is neodymium, which lases near 1.05 µm. Nd:glass finds its main applications in high energy high power (HEHP) scientific facilities where 10s to 100s of laser beams are combined to produce kilo-to-megajoules of energy that are delivered to a target over a period of nanoseconds, resulting in up to hundreds of terawatts (or even petawatts) of power in those few instants [5,6]. Nd:glass is used in the lasers that power inertial confinement fusion (ICF) experiments [5,7–10] or ultrashort lasers with peak power in the petawatt range [6]. Significant development efforts were undertaken at SCHOTT in order to develop a Nd-doped phosphate laser glass for large scale ICF facilities such as the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) [5] and Laser MegaJoule (LMJ) in France [9]. ICF is further discussed in Section 5.
When in a glass matrix, Nd3+ lases at different wavelengths depending on the host glass. At the extremes, the peak emission wavelength is between 1046 nm (fluoroberyllate) and 1067-1077 nm (aluminate), with an additional outlier at 1088 nm (nearly pure silica) [11,12]. Typical peak emission wavelengths for glasses produced at the commercial scale are between 1052.7 (phosphate, e.g., LG-770) and 1061 nm (silicate, e.g., LG-680) [11,13].
The absorption bands of Nd in the visible spectrum provide a purple color to the glass. As shown in Fig. 1, the flashlamp or diode pump light excites 4f electrons in the Nd3+ cation to higher energy states. In the case of flashlamp pumping (white light), the electrons then experience a series of non-radiative (non-photon-producing and instead phonon/heat-producing) transitions which bring them to the 4F3/2 state where the upper-state lifetime is sufficiently long for population inversion to build up. Diode pumping is typically targeted at the 805-810 nm absorption band using GaAlAs laser diodes, though the absorption band at 872 nm can also be pumped, which has the advantage of pumping directly to the upper lasing level of Nd3+, 4F3/2, which yields a higher quantum efficiency [2,14,15].
Fig. 1. Nd3+ absorption curve and energy levels. The laser transition from 4F3/2 to 4I11/2 is indicated, as well as the two diode pumping bands at 808 and 872 nm.
There is large energy gap between this level and the next lowest energy levels, 4I, so electrons are able to accumulate at the 4F3/2 level in sufficient numbers to create the population inversion required for lasing. When photons of appropriate wavelength pass through the laser glass gain medium, electrons in the upper lasing level 4F3/2 are stimulated to transition to the lower lasing level 4I11/2 and thus emit another photon of the same wavelength (∼1.05 µm), amplifying the signal. In this manner, a weak laser pulse can increase in energy through the thickness of the laser gain medium. Finally, electrons transition from the lower lasing level 4I11/2 to the Nd3+ ground state 4I9/2.
Another active lasing ion used in laser glass is erbium (Er3+), which lases near 1.5 µm [2,16]. While Er-doped yttrium aluminum garnet (Er:YAG) can lase at 2.9 µm, the only laser transition available in Er-doped oxide glasses is the 4I13/2-4I15/2 transition (1.5 µm); the others are quenched by non-radiative transitions [17]. This wavelength of near-infrared (IR) light is absorbed by water and yields two commercially interesting application spaces for Er:glass lasers, among others. The first is in ‘eye-safe’ range-finding, where laser pulses are emitted by a range-finding device and the time-of-flight for the reflections to return to the device are measured, allowing for the measurement of distances up to several kilometers. This mode of operation using Er:glass lasers is considered to be safe for human eyes because the laser radiation of ∼1.5 µm is strongly absorbed by liquid water in transparent parts of the eye (e.g., cornea, vitreous humor) before it can damage the retina [18]. On the other hand, there is very little absorption in humid air, which allows transmission through air for range finding [18].
The second application arising from strong absorption of laser radiation at ∼1.5 µm by water is in the field of medical dermatology, such as scar and wrinkle reduction, and treatment of fungal nail infections [19–21]. An advantage of the wavelength of light produced by Er:glass lasers is that it is not strongly absorbed by melanin in the skin, thus reducing damage to the surface skin layers and instead promoting the rebuilding of collagen deeper inside the skin (100-400 µm below the surface) through heating of the water naturally present in skin [19].
In glass, Er3+ is a 3 level system with relatively poor efficiency. In order to increase the efficiency, Yb3+ is commonly used as a sensitizer to absorb and transfer energy to Er3+. Yb3+ can strongly absorb pump light at ∼970 nm and electrons in Yb3+’s ground state, 2F7/2, are excited to 2F5/2. From there, energy can be resonantly transferred to electrons residing in Er3+’s ground state, 4I15/2, which are excited to Er3+’s upper state, 4I11/2. This energy transfer is from Yb3+’s 2F5/2 level to Er3+’s 4I11/2 is 10 times faster than the relaxation back to Yb3+’s ground state [2]. Excited electrons there quickly (<10 µs) relax to the upper laser level in Er3+, 4I13/2. When electrons in that upper laser level are stimulated, they return to Er3+’s ground state, 4I15/2, resulting in laser radiation at 1.54 µm [17,22]. If the laser system employs flashlamp pumping, Cr3+ can be added to the glass. Cr3+ absorbs flashlamp light mainly in two bands at 450 and 640 nm and broadly emits light at 760 nm, which allows the transfer of that energy to the 2F5/2 level in Yb3+ and the 4I9/2 and 4I11/2 levels in Er3+ [2,22]. Figure 2 shows the energy levels of Yb3+ and Er3+.
Other rare earth ions are known to have laser action in glass, as listed in Table 1, but are not used commercially to the extent that Nd3+ and Er3+/Yb3+ are.
Table 1. Lasing ions and wavelengths with possible sensitizing ions, adapted from [23].
2. Glass matrices
2.1 Silicate glass
Silicate glass was the first glass matrix used for laser glass [3]. As an optical material, silicates are a well-established technology, however, they have several disadvantages when it comes to laser glass technology.
The first and most significant is related to the fact that trace amounts of the vessel within which the glass is melted become incorporated into the molten glass during manufacturing. Platinum metal is commonly used as the melting vessel for high quality and homogeneous laser glass; microscopic Pt metal particles can get into the glass during the melting, refining, and forming processes [4]. While these metallic particles are very small, just microns in size, they are large enough to cause damage sites when the glass is exposed to high fluence laser irradiation. The fluence through the optical elements can be in the range of 18-20 J/cm2 and the peak irradiance can be up to 5.0 GW/cm2 [4]. This level of fluence and peak power causes inclusion damage sites at the location of the Pt particles. In order to avoid the presence of metallic Pt particles in the laser glass, laser glasses must have sufficiently high solubility for platinum. Higher solubility means that, under the right conditions, metallic particles can be dissolved into ionic Pt4+ species, which then act as ordinary cations in the glass. Platinum solubility increases based on composition in the following sequence: silicate, fluorophosphate, silica-phosphate, phosphate [4]. This topic is discussed further in section 3.1
Despite the fact that silicate laser glasses contain platinum particles, they still find use in certain applications. Their thermomechanical properties are better than most phosphate glasses (e.g., lower thermal expansion, higher thermal conductivity, improved fracture toughness), meaning they can be used in high repetition rate operation. They also lase at a slightly longer wavelength than phosphate glasses, as described in section 1.2, which can be leveraged to create a laser system with larger bandwidth. The Texas Petawatt Laser utilizes a mixed phosphate-silicate glass system in order to achieve the bandwidth required to achieve 100 fs pulses [6].
2.1 Phosphate glass
The compositional landscape for laser glasses fundamentally changed in the early 1980’s due to an extensive survey of glass types and compositions coordinated by Marv Weber and Stan Stokowski at LLNL [11]. The glasses investigated included both commercial compositions as well as highly experimental glass samples from various universities, companies, and national laboratories, spanning the range from silicates, phosphates, fluorophosphates, borosilicates, borates, germanates, and aluminates, to more exotic glasses such as tellurites, niobates, tantalates, sulphates, fluoroberyllates, and chlorides [11]. The experimental compositions were prone to manufacturing issues such as poor glass stability, expensive raw materials, and unfavorable viscosity characteristics. However, as a result of this study, phosphate glasses were clearly identified as offering the best tradeoff between laser properties and ease of manufacturability. Since the early 1980s, phosphate glasses have been the material of choice for HEHP laser facilities. Figure 3 shows the evolution of laser glass composition and size for lasers built by LLNL, culminating in meter-class scale slabs of phosphate laser glass.
Fig. 3. The size and composition of laser glass slabs in lasers built by LLNL over time. The composition changed from silicate to phosphate and the melting technology improved resulting in larger glass slabs. Image credit LLNL.
There are a variety of properties that make phosphate glass a better choice. To further the understanding of Nd:phosphate glass and optimize its manufacturability, multiple composition developments were performed over the years leading up to the glass selection for the NIF. In particular, a series of compositional developments explored the relationships between glass composition, thermal, optical, and laser properties. The base glass composition is typically a metaphosphate with O/P ratio of ∼3, regardless of cation modifiers. In one such study, described here as an illustrative example of the composition-property relationship in phosphate laser glasses, the proportions of alkali and alkaline earth oxides were varied and the concept of the compositionally averaged cation field strength was introduced as a useful parameter relating composition and properties, and is defined as [24]:
where Ei is the field strength of the cation, Ci is the mol% of the ith oxide, and Si is the cation/oxygen stoichiometric ratio (e.g., 1 for BaO and 2 for Na2O) [24]. The field strength comes from the Coulombic attractive force (F, in dynes) between cations and anions, and is defined as: where qO and qC are the valances of the oxygen anion and cation, e is the fundamental charge, and rO and rC are the oxygen and cation radii in cm.The cation field strength is the same as Eq. (2) without the qO term:
and has the units of esu/cm2.The framework of average cation field strength allows for glasses of different compositions, and especially with different amounts of various alkali and alkaline earth cations to easily be compared.
With increasing cation field strength, it was found that thermal expansion decreases, thermal conductivity increases, Young’s modulus increases, the Nd3+ emission cross section is largely unchanged, the bandwidth of the laser emission increases, and the fluorescence lifetime decreases [24]. The thermomechanical figure of merit (FOMTM) is defined as
Where k is thermal conductivity, ν is Poisson’s ratio, E is Young’s modulus, and α is thermal expansion [24], and the units are Wm/N.Figure 4 shows, as a function of compositionally averaged cation field strength, the trends of FOMTM and effective bandwidth of the laser emission (Δλeff), defined as [7]:
Fig. 4. Thermomechanical figure-of-merit (top) and effective emission bandwidth (bottom) versus compositionally averaged cation field strength. Data taken from Ref. [24].
Both the FOMTM and Δλeff increase as the cation field strength increases. In this study the amounts of glass- and network-formers were fixed. By interchanging the amounts of the other cations, they were able to change the field strength by a factor of ∼2, though it only resulted in a ∼6% change in the Δλeff [24]. It should be noted that the Δλeff is related to other spectroscopic properties including the radiative lifetime and peak emission cross-section, so the change in Δλeff is complicated by the relationship between average cation field strength and the two properties as well [7].
The range in FOMTM is significant, from a minimum of 0.6 to a maximum of 1.2. Assuming constant fracture toughness and surface flaw size (∼ 50 µm), doubling the FOMTM would double the maximum thermal load that a cooled laser slab could withstand without fracture [25].
3. Process improvements for manufacturing phosphate laser glass
In order for neodymium-doped phosphate laser glasses to be suitable for, e.g., megawatt lasers for ICF systems, the glass must meet a myriad of requirements [26]. Briefly, the glass must have:
- 1. Set composition (within a given tolerance) to achieve the desired properties
- 2. Transmission loss at the laser wavelength (∼1054 nm) < 0.0015 cm−1
- 3. Residual stress induced birefringence <5 nm/cm
- 4. Refractive index homogeneity <±2 × 10−6
- 5. Pt particle concentration < 0.1 /L and particle size < 5 µm
- 6. Hydroxyl (OH) content < 100 ppmw
Items 1 and 2 are addressed by the handling and selection of raw materials. The transmission losses at ∼1054 nm are primarily due to transition metal impurities [27]. Cu has the strongest absorption at that wavelength, though Fe is also problematic because many raw materials are processed using iron or steel based equipment. Therefore, limits are set on the impurities (e.g., Fe2O3 in raw materials and can be as low as 1-100 parts per million by weight (ppmw), depending on the raw material.
Item 3, stress induced birefringence, is taken care of by careful annealing of the laser glass parts. After melting and forming, the glass is sufficiently coarse-annealed to lower residual stresses so the glass can be inspected for quality. Coarse-annealing is challenging for phosphate glasses because they have a high coefficient of thermal expansion compared to other optical glasses (e.g., BK-7), and a lower fracture toughness. After the glass passes inspection it is fine-annealed at a temperature near the glass transition temperature and cooled at a slow rate (over several weeks) in order to remove residual stresses [26].
Item 4, the refractive index homogeneity, is also affected by the residual stresses, but more so by thorough mixing and homogenization of the glass during melting and refining. The process of casting the glass also affects the homogeneity of the final glass part; the viscosity is adjusted to give the right flow properties to avoid striae (variations in refraction index) and yield the final casting geometry.
Item 5, the removal of platinum particles, is critical for producing glass suitable for high power laser systems and is discussed in more detail in section 3.1.
Item 6, low hydroxyl (OH) content, is important for laser glass because OH or H2O in the glass reduces the radiative lifetime of the Nd3+ lasing ions. In other words, the presence of OH increase the rate of non-photon-producing decay from the excited state of Nd3+ back to the ground state, which negatively affects the performance of the laser.
3.1. Platinum-particle free laser glass
For HEHP laser systems it is essential to ensure that the concentration of platinum particles in the glass left over from the melting process is < 0.1 /L and any particles are < 5 µm in size, because of the damage that they could cause to the laser glass. The damage occurs when a metallic platinum particle experiences laser irradiation, causing a thin layer of Pt on the front of the inclusion to vaporize [28]. The ablated Pt causes a shock wave within the glass (a brittle material), which then fractures [28]. The damage sites progressively grow with each laser shot, potentially growing to centimeter size and making the glass unusable [4].
There are essentially just two options for avoiding Pt inclusions in laser glass. The metallic Pt particles can be dissolved into the glass (ionic Pt4+ poses no threat of laser damage, and does not otherwise adversely affect the glass as long as the concentration is not too high, e.g., above the 10-100 ppm level) or Pt can be avoided entirely during manufacturing [29]. It has been attempted to melt laser glass in refractory ceramic melters, however, that method instead resulted in ceramic inclusions in the glass, which have the advantage of having a slightly higher laser damage threshold, but also exist at a significantly higher concentration [29].
Different base glasses have varying solubility for ionic platinum, even without taking any other measures to enhance the dissolution. The trend for platinum solubility is phosphate $> $ silica-phosphate ${\gg} $ fluorophosphate ${\approx} $ borate $> $ silicate [26,30,31]. The ability of phosphate glass to dissolve metallic platinum particles more than other host glass compositions was one of the main reasons for phosphate glass being the material of choice for large scale laser facilities instead of silicates or fluorophosphates [29].
There are two ways to aid the dissolution of Pt: increasing the temperature of the melt and creating oxidizing conditions in the melt [29]. As shown in Fig. 5 (top), the level of metallic Pt decreases as the melt temperature increases from 1100 to 1500 °C. Concurrently, the level of ionic Pt4+ increases, though stays within tolerable limits. The gas environment for these melts were all the same: 2 h of N2 cover gas followed by 2 h of O2 cover gas (pressure of approximately 1 bar) [29]. While higher temperatures better dissolve metallic Pt into ionic Pt4+, the upper limit is capped by the usable temperature range of Pt (<1700 °C).
Fig. 5. Metallic and ionic platinum levels (ppm) in phosphate laser glass as a function of melt temperature (top) and as a function of oxygen partial pressure during the refining step (bottom). Data take from Ref. [29]
The other important factor is the oxidation state of the melt. Oxidizing conditions can be enhanced by bubbling gases through the glass melt. The rate of Pt dissolution as a function of different bubbling gases follows the following trend [26,28,29]:
Figure 5 (bottom) shows the trend of O2 partial pressure with the ionic and metallic Pt concentrations. Therefore, additions of O2 and chlorine containing gases are utilized in the melting of phosphate laser glasses, and results in a concentration of Pt particles < 0.1 per liter of glass [29]. For example, of the laser glass slabs (14 L each) produced for Beamlet at LLNL, 50% had no Pt inclusions at all [10].
One additional factor that affects the rate of Pt dissolution is Al2O3 content. Al2O3 is added to phosphate glasses to increase the thermomechanical properties and durability of the glass, however, lower Al2O3 content allows for faster Pt dissolution [27].
The addition of oxidizing gasses (O2 and/or Cl2) have the added advantage of enhancing the removal of hydroxyl (-OH) from the glass. Oxygen gas alone is somewhat effective at dehydroxylation, but not as effective as Cl2, which can react with H2O to form HCl via the reaction [26]:
3.2 Continuous melting
In order to meet the demand for the thousands of slabs of laser glass required for the NIF and LMJ, SCHOTT developed continuous melting technology for phosphate laser glass. The prior state-of-the-art, was a two-step melting process wherein the raw materials are first melted in an inert refractory vessel, and then the resulting glass is remelted in a Pt-lined vessel in order to dissolve metallic Pt inclusions and produce high-homogeneity glass, as described in Ref. [26]. This discontinuous melting process could produce a meter-class slab of laser glass every 1-2 days, and had the disadvantage that every melt could vary slightly in properties and quality. In contrast, once a continuous melter reaches its steady-state operating conditions, there is almost no measurable variation in glass from one slab to the next [26].
Though the details are proprietary, a schematic of the continuous melter is shown in Fig. 6. Each section has a specific purpose, which are briefly described here.
Fig. 6. Schematic of the laser glass continuous melter used at SCHOTT to manufacture slabs of LG-770 for the NIF and LMJ, adapted from Ref. [26].
The melter converts powdered raw batch materials into molten glass, which is mixed on a large scale by convection currents throughout the volume of melted glass. The melter is made of a refractory material such as fused silica, fused alumina, or zircon [26]. After the melter, the remainder of the system is lined with platinum. The purpose of the conditioner is to set the oxidation state of the melt in order to dissolve Pt particles into ionic Pt4+ cations as discussed in Section 3.1, and ‘dry’ the glass by removing hydroxyl (OH) groups. The refiner is next, and here, bubbles are removed from the glass. It has a slightly elevated temperature in order to reduce the viscosity of the glass and enable bubbles to rise to the surface faster. The homogenizer thoroughly stirs the glass in order to ensure the glass is as homogenous as possible. The temperature is lowered slightly and the viscosity increases so that the flow is appropriate for the forming operation (down pipe and mold). Lastly, the cast strip of the glass traverses the annealing oven (lehr) where it cools at a sufficiently slow rate to avoid unacceptable thermal stresses and can be cut and inspected at the end of the lehr [26].
4. Advanced thermal lensing analyses
Consider a laser glass rod under continuous wave (CW) lasing conditions: optically pumped from the side and air- or water-cooled along its length. The rod has a higher temperature at the center due to absorbed pump energy and a lower temperature at the surfaces are due to continuous cooling; the temperature profile within the laser glass rod has a parabolic profile [32]. The temperature gradient causes spatial variation in the refractive index of the material due to two effects: the thermo-optic effect (dn/dT) and the stress-optic effect. These two effects both contribute to the thermal lensing effect, but can have different magnitudes, and even different signs. One additional effect also contributes to the overall thermal lensing: bulging at the ends of the rod [32].
Recent work utilizing finite-element simulations of a laser glass rod confirmed that the original analytical expressions by Koechner [33] and Foster & Osterink [34] are accurate [32]. The authors then applied the analytical expressions to the portfolio of SCHOTT laser glasses and calculated the effective dioptric power (1/focal length) for each contribution to thermal lensing for each glass type. Negative dioptric power indicates a diverging lens and positive indicates a converging lens. As shown in Fig. 7(a), LG-750, LG-760, and LG-770 have large negative thermo-optic contributions, but positive contributions from the stress-optic effect. The thermo-optic effect is small for APG-1 and APG-760. In all cases the end-bulge effect is small. The large negative thermo-optic effect for LG-750, LG-760, and LG-770 is actually an advantage for those glass types, as it ‘cancel out’ the positive stress-optic effect. The total radial and tangential contributions to thermal lensing in dioptric power are shown in Fig. 7(b). LG-750 in particular has a remarkably small total thermal lensing effect – at least in the case of the rod geometry considered [32]. Note that this analysis used conventional stress-optic measurements performed at 632.8 nm using a HeNe laser.
Fig. 7. (a) Individual thermal lensing contributions and (b) total radial and tangential thermal lensing for Nd:glasses in the SCHOTT portfolio estimated using the Foster & Osterink /Koechner expressions; data from Ref. [32]. (c) Individual thermal lensing contributions and (d) total radial and tangential thermal lensing for SCHOTT Er/Yb:glasses; data from Ref. [35].
The analysis of thermal lensing contributions in SCHOTT laser glasses was later extended and the stress-optic effect was measured near the lasing wavelengths for the entire SCHOTT portfolio of laser glasses, including both Nd- and Er/Yb-doped glasses [35]. For the Nd-doped glasses (APG and LG-7XX series) measurements were performed at 1064 nm and for the Er/Yb-doped glasses (LG-9XX series), measurements used a 1550 nm laser. As before, the original Foster & Osterink/Koechner analytical expressions were used. The individual contributions to thermal lensing for the Er/Yb-doped glasses are shown in Fig. 7(c) and the total radial and tangential thermal lensing is shown in Fig. 7(d). Similar to the Nd-doped glasses, those with a larger negative thermal-optic effect have overall smaller thermal lensing due to compensation between negative thermal-optic effects and positive stress-optic effects.
5. Inertial confinement fusion
5.1 Progress in ICF
In inertial confinement fusion (ICF), a pellet of deuterium-tritium (DT) fuel (the target) is irradiated. The outer layers of the fuel are ablated away, causing an inward implosion of the fuel. If the fuel becomes hot and dense enough, confined by its own inertia, fusion reactions – like those that occur in our sun and other stars – can occur. If the output energy, typically in the form of neutrons and heat, exceeds the input energy, the excess energy can be harvested for electricity generation. In the context of a fusion energy plant, this is referred to as inertial fusion energy (IFE). The concept of using lasers as a driver for a fusion power plant is known as laser inertial fusion energy (LIFE) [14,15,36].
There are two main modes of operation for ICF. In both cases, the laser beams are frequency-tripled laser radiation from Nd:glass laser gain medium (near-infrared 1053 nm light is converted to UV 351 nm light). The first mode, indirect drive, utilizes a heavy metal (e.g., gold) capsule called a holraum. The UV lasers strikes the inside of the holraum through a hole in its side and produces X-rays, which then strike the DT fuel pellet [37]. In direct drive operation, the UV laser light directly irradiates the DT fuel target [38,39].
The National Ignition Facility (NIF) is the largest laser ever built, and its purpose is to produce fusion ignition via laser energy, as well as support the USA’s stockpile stewardship program and enable high-energy-density experiments related to astrophysics [5]. Since NIF was completed in 2009, progress has been made over time in achieving ever greater fusion yields. In recent years, this progress has accelerated. In February 2021, at least two shots (N210207 and N210220) achieved the ‘burning plasma’ regime with yields near 0.17 MJ [40]. While an order of magnitude smaller than the input energy, ∼1.9 MJ, the yields are 70-80% of the energy absorbed by the capsule. At the time, it was a significant achievement to have entered a regime where self-heating surpassed the energy losses [40].
Not long thereafter in August 2021, another experiment produced approximately 1.35 MJ of fusion yield, breaking all prior records [40]. The fusion yields on NIF over time are shown in Fig. 8. Shot N210808, the tall bar on the right hand side of the plot, stands out far above the rest. The publication detailing this remarkable result was published during the review process of this article [41], as well as another publication detailing the experimental evidence of the signatures for ignition and burn propagation [42].
5.2 Towards the future of inertial fusion energy
The results of N210808 demonstrate a significant jump in progress towards self-sustaining fusion energy and have ignited excitement in the IFE communities [41]. Since at least 2008, designs for lasers systems and power plants have been proposed for laser IFE [14,15,36]. In order to make laser IFE an efficient source of energy, there are a few key advancements related to laser glass required.
The use of laser diodes instead of flashlamps for pumping the laser glass will be key to increasing the efficiency of IFE power plants [14]. Flashlamps emit significant visible and IR light that cannot be used by Nd3+ to excite electrons to the appropriate energy levels, and is absorbed only as heat. As shown in Fig. 1., SCHOTT’s Nd-doped laser glasses have absorption bands that are suitable for diode pumping: either the absorption band at ∼808 nm or ∼872 nm.
For IFE power plants to efficiently produce energy, they will have to ignite fusion reactions frequently: not once per day, or once per hour, but many times per second. Proposed designs use repetition rates of 10-20 Hz [14,15]. One proposed laser system design used the parameters for the glass already in NIF (LG-770) and operated at 13.3 Hz [15]. Another proposal for an IFE laser driver utilized the parameters for SCHOTT’s APG-1 with an operating repetition rate of 16 Hz.
While existing laser glass can work in a laser IFE system, development of a tailored glass with improved thermomechanical properties would be advantageous. Operation at 10-20 Hz necessitates active cooling of the laser glass, meaning there will be a thermal gradient between the hotter interior and cooler surfaces, as discussed in Section 4. A laser glass’s ability to withstand the stresses caused by these thermal gradients is essential for high repetition rate applications, such as laser IFE.
SCHOTT has previously developed a glass with exceptional thermomechanical properties, referred to as APG-t [25] and later as APG-2 [13]. It was found to be able to withstand 2.3 times greater thermal loading without fracturing compared to APG-1 based on its thermomechanical properties and, more impressively, by thermally-induced fracture experiments using an Ar-ion laser to heat the samples. During the test, only one sample out of 9 fractured when subjected to 100 kW/cm3, the other APG-2 samples glowed red-hot and did not fracture [25]. All the samples of the other two compositions fractured.
Another desired feature for IFE power plants is broader bandwidth, achieved through one or more of the following methods: nonlinear optics, laser glass compositions with broader bandwidth, and/or the combination of gain media with different peak wavelengths. APG-2 has a broader emission bandwidth (λeff) (24% broader than LG-770), which is beneficial for laser IFE because broader bandwidth laser irradiation can mitigate and even suppress laser plasma instabilities and improve the coupling efficiency for laser direct drive IFE [43,44].
6. Conclusions and outlook
Laser glass is a solid state multicomponent oxide glass, which enables light amplification by stimulated emission of radiation. This review covered many of the salient features of laser glass, starting with lasing ions, host glasses, and composition-property relationships. Different rare earth lanthanide cations can be added to the glass matrix and enable lasing at different wavelengths. The most common is Nd3+, which lases near 1.05 µm and is used to generate HEHP laser radiation for ICF and other high energy physics experiments from stockpile stewardship to astrophysics. Er3+, sensitized with Yb3+, lases at ∼1.5 µm in phosphate glass and is used for eye-safe range-finding and dermatological applications.
While the first laser glass utilized a silicate host [3], the choice for most applications now is a phosphate host glass. Phosphate glasses can be compositionally modified to have a variety of optical, physical, and laser properties, but their most important feature is their ability to dissolve platinum inclusions that occur during the melting process. This review also covered the development of continuous melting of platinum-particle-free phosphate laser glass, an advancement which was essential for generating glass which can survive the high power and fluence experienced in petawatt/exawatt lasers and IFC/IFE research facilities
This review discussed advancements in thermal lensing analysis done at SCHOTT which confirmed prior analytical expressions and elucidated the contributions to thermal lensing in the portfolio of SCHOTT glasses (Nd3+ and Er3+/Yb3+ doped).
Finally, exciting new progress in ICF at NIF was discussed. The August 8th, 2021 shot with megajoule fusion yield is an incredibly encouraging result for the prospect of sustainable laser IFE. There has been great progress towards achieving clean fusion energy, but there is still work to do.
Acknowledgments
The author is deeply indebted to Joseph S. Hayden, PhD, for innumerable valuable and engaging discussions on all aspects of laser glass, as well as for guidance on this review article. The author is grateful for the knowledge shared by other colleagues at SCHOTT–Duryea, PA and thanks the staff at LLNL for supporting laser glass development at SCHOTT over the decades. Some of the results presented here were made possible with funding by the US Department of Energy by Lawrence Livermore National Laboratory (LLNL) under Contract No. W-7405-Eng-48, the French Commissariat à l’ Energie Atomique (CEA), and the US High Energy Laser – Joint Technology Office (HELJTO) under Contract No. FA9451-11-C-0274.
In memoriam of our friend and colleague, Mark J. Davis, PhD (1960-2021).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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