Stress-optic measurements of Nd- and Er-doped SCHOTT laser glasses were made at 1064 and 1550 nm, close to their respective lasing wavelengths. In addition, measurements of fused silica at 1064 nm serve as a useful benchmark owing to the existence of multiple studies which report on this material, including several from NBS/NIST. Owing to a currently unexplained discrepancy amongst our fused silica data using three different IR cameras, the results reported herein must be considered preliminary. Nevertheless, we believe the general trends reported herein for thermal lensing will hold, owing to a comparable increase in magnitude of the individual stress-optic parameters <i>K<sub>par</sub></i> and <i>K<sub>perp</sub></i> with increasing wavelength as previously observed for fused silica and as seen in our current data.
Retina-safe operation in open-air is of high interest to the next generation of lasers that are being utilized for many industrial, defense and medical applications. Those wavelengths that are considered to be the best for retina safe operations (also called eye-safe) fall in the range between 1400nm and1800nm. This wavelength region also coincides with the low loss window of fused silica fibers used for optical fiber communications , where the S and C bands near 1500nm are heavily utilized for long range communications due to the lowest attenuation losses possible in the fiber. The trivalent Er ion can produce direct emission into the 1540 nm wavelength, thus, it is the rare-earth emitter of choice for many eye-safe applications. In recent years, the need for high beam quality under passive operation in open air applications have renewed interest in Er-doped bulk glasses as the gain material of choice for solid-state eye-safe lasers.
The need for performance stability under a broad operating range from -400C to 1000C without active cooling is a challenge for amorphous gain materials. Moreover, there is very little known about how temperature may affect performance. In this study, we describe our first attempts to understand material behavior by systematically analyzing temperature driven variations exhibited in absorption and emission from the commercially available gain materials. As part of these investigations, we will also present our method for assessing quantum efficiency through measurements for critical evaluation from laser community at large.
This paper focuses on the three main effects that can induce wave-front distortion due to thermal lensing in laser gain media: 1) thermo-optic (<i>dn/dT</i>); 2) stress-optic; and 3) surface deformation (e.g., “end-bulging” of a laser rod). Considering the simple case of a side-pumped cylindrical rod which is air- or water-cooled along its length, the internal temperature distribution has long been known to assume a simple parabolic profile. Resulting from this are two induced refractive index variations due to thermo-optic and stress-optic effects that also assume a parabolic profile, but generally not of the same magnitude, nor even of the same sign. Finally, a small deformation on the rod ends can induce a small additional lensing contribution. We had two goals in this study: a) use finite-element simulations to verify the existing analytical expressions due to Koechner1 and Foster and Osterink; and b) apply them to glasses from the SCHOTT laser glass portfolio. The first goal was a reaction to more recent work by Chenais et al. who claimed Koechner made an error in his analysis with regard to thermal stress, throwing into doubt conclusions within studies since 1970 which made use of his equations. However, our re-analysis of their derivations, coupled with our FE modeling, confirmed that the Koechner and Foster and Osterink treatments are correct, and that Chenais et al. made mistakes in their derivation of the thermally-induced strain. Finally, for a nominal laser rod geometry, we compared the thermally-induced optical distortions in LG-680, LG-750, LG-760, LG-770, APG-1, and APG-2. While LG-750, -760, and -770 undergo considerable thermo-optic lensing, their stress-optic lensing is nearly of the same magnitude but of opposite sign, leading to a small total thermal lensing signature.
Laser design codes utilize laser properties provided by materials manufacturers for performance modeling. Large scale manufacturing of materials during compositional developments for a particular laser design is not economically feasible. Nevertheless, the laser properties derived from the available sample volumes must be reliable and reproducible. In recent years, as a result of the renewed interest in novel glasses for ultrafast laser applications, SCHOTT has developed improved measurements and methodologies for providing the most accurate information possible to laser scientists. Even though the J-O method is robust and time tested for the spectroscopic characterization of Nd3+, the accuracy of the results requires reliable measurements. This paper outlines the J-O approximation for manifold to manifold transitions, measurements needed, and some of the pitfalls to watch for during the collection of data for Nd-doped materials.
Laser performance measurements (quasi-CW) were made of various Nd-doped SCHOTT catalog laser glasses: LG-680,
LG-750, LG-760, LG-770, APG-1, and APG-2; all but the first, a silicate, are phosphate glasses. Nominal Nd<sup>3+</sup> doping
was approximately 3 × 10<sup>20</sup> ions/cm<sup>3</sup>. An end-pumped, laser diode geometry was used and input powers, pump pulse
length, and pump rep-rates were kept low to avoid thermal lensing (4 W, 1 msec, and 0.1 Hz, respectively). As
expected, the phosphate glasses performed better than the silicate glass. Slope efficiencies ranged from 25% for LG-680
up to nearly 33% for LG-760. APG-1, designed for high rep-rate, high-power systems, performed nearly the same in
this particular configuration as glasses designed for high-energy applications (e.g., LG-770).
Coefficient of thermal expansion (CTE) measurements using small Fabry-Perot etalons were conducted on high and low
thermal expansion materials differing in CTE by a factor of nearly 400. The smallest detectable change in length was
~10<sup>-12</sup> m. The sample consisted of a mm-sized Fabry-Perot etalon equipped with spherical mirrors; the material-under-test
served as the 2.5 mm-thick spacer between the mirrors. A heterodyne optical setup was used with one laser locked
to an ~780 nm hyperfine line of Rb gas and the other locked to a resonance of the sample etalon; changes in the beat
frequency between the two lasers as a function of temperature directly provided a CTE value. The measurement system
was tested using the high-CTE SCHOTT optical glass N-KF9 (CTE = 9.5 ppm/K at 23 °C). Measurements conducted
under reproducibility conditions using five identically-prepared N-KF9 etalons demonstrate a precision of 0.1 ppm/K;
absolute values (accuracy) are within 2-sigma errors with those made using mechanical dilatometers with 100-mm long
sample rods. Etalon-based CTE measurements were also made on a high-CTE (~10.5 ppm/K), proprietary glass-ceramic
used for high peak-pressure electrical feedthroughs and revealed statistically significant differences among parts made
under what were assumed to be identical conditions. Finally, CTE measurements were made on etalons constructed
from SCHOTT's ultra-low CTE Zerodur<sup>(R)</sup> glass-ceramic (CTE about -20 ppb/K at 50 °C for the material tested herein).
We review the development of a new glass formulation and manufacturing technology for a neodymium-doped phosphate based laser glass used in the LLNL National Ignition Facility (NIF) and the French Laser MegaJoule (LMJ). The glass development process built on both accumulated experience and the utilization of glass science principles, and the resultant new glass offers superior laser properties in combination with improvements in physical properties to enhance manufacturing yield. Essentially in parallel, a continuous melting production line was also conceived, designed and operated to meet both the schedule and cost targets of the NIF. Prior to 1997, phosphate laser glasses were manufactured by a discontinuous pot-melting process with limited production rate and associated high costs. The continuous melting process met several technical challenges, including producing glass with low residual water content and absence of inclusions which become damage sites when used in the NIF laser system.
Fluorescence techniques are known for their high sensitivity and are widely used as analytical tools and detection
methods for product and process control, material sciences, environmental and bio-technical analysis, molecular
genetics, cell biology, medical diagnostics and drug screening. According to DIN/ISO 17025 certified standards are
used for fluorescence diagnostics having the drawback of giving relative values for fluorescence intensities only.
Therefore reference materials for a quantitative characterization have to be related directly to the materials under
investigation. In order to evaluate these figures it is necessary to calculate absolute numbers like absorption/excitation
cross section and quantum yield. This can be done for different types of dopants in different materials like glass, glass
ceramics, crystals or nano crystalline material embedded in polymer matrices. Here we consider a special type of glass
ceramic with Ce doped YAG as the main crystalline phase. This material has been developed for the generation of white
light realized by a blue 460 nm semiconductor transition using a yellow phosphor or converter material respectively.
Our glass ceramic is a pure solid state solution for a yellow phosphor. For the production of such a kind of material a
well controlled thermal treatment is employed to transfer the original glass into a glass ceramic with a specific
crystalline phase. In our material Ce doped YAG crystallites of a size of several µm are embedded in a matrix of a
residual glass. We present chemical, structural and spectroscopic properties of our material. Based on this we will
discuss design options for white LED's with respect to heat management, scattering regime, reflection losses, chemical
durability and stability against blue and UV radiation, which evolve from our recently developed material. In this paper
we present first results on our approaches to evaluate quantum yield and light output. Used diagnostics are fluorescence
(steady state, decay time) and absorption (remission, absorption) spectroscopy working in different temperature regimes
(10 - 350 K) of the measured samples in order to get a microscopic view of the relevant physical processes and to prove
the correctness of the obtained data.
The use of fs lasers to directly write phonic structures <i>inside</i> a glass has great potential as a fabrication method for three-dimensional all-optical integrated components. The ability to use this technique with different glass compositions --specifically tailored for a specific photonics application -- is critical to its successful exploitation. Consequently, it is important to understand how glass composition effects waveguide fabrication with fs laser pulses and how different glasses are structurally modified after exposure to fs laser pulses. We have used confocal laser spectroscopy to monitor the changes in glass structure that are associated with waveguide fabrication. Using a low power continuous wave (cw) Ar laser as excitation source we have measured both Raman and fluorescence spectra of the modified regions. Raman spectroscopy provides us with information on the network structure, whereas fluorescence measurements reveal the presence of optically active point defects in the glass. In this paper we review our work on fs-laser fabrication and characterization of photonic structures in glass and discuss the effect of glass composition on processing parameters and structural modification.
We report on the frist experimental demonstration of a scalable fiber laser approach based on phase-locking multiple gain cores in an antiguided structure. A novel fabrication technology is used with soft glass components to construct the multie core fiber used in our experiments. The waveguide region is rectangular in shape and comprised of a periodic sequence of gain and no-gain segments having nearly uniform refractive index. The rectangular waveguide is itself embedded in a lower refractive index cladding region. Experimental resutls confirm taht our five-core Nd doped glass prototyep structure runs predominately in two spatial antiguided modes as predicted by our modeling.
Properties of a new rare-earth doped heavy metal oxide containing silicate glass are presented. The glass has potential for fabrication of ultra-short wideband fiber and planar waveguide amplifiers. We report specific results for a fiber amplifier geometry, discussing achieved improvements in device compactness (Giles gain g* = 210 dB/m allowing up to 100 times shorter fiber) and amplification bandwidth (50% more bandwidth in C-/L-band) compared to the conventional EDFA. We also access the potential of this material for fabrication of active planar integrated waveguide devices.
Hybrid glass parts composed of dissimilar glass sections are an attractive route to integrate multiple functions onto a single substrate and offer the potential to fabricate advanced laser sources, amplifiers, lossless splitters and other photonic devices such as Fabry-Perot etalons. We review the most promising bonding technologies, placing particular emphasis on techniques that do not require the use of high processing temperatures. In particular, we discuss in detail a recently developed low temperature bonding technology that relies on inorganic adhesives. Characterization of interfacial joints prepared with this inorganic technology indicate low insertion loss, high mechanical strength and chemical resistance to attack during the conventional lithographic and ion exchange steps employed to fabricate waveguide structures.
Phosphate glasses have become increasingly popular for planar waveguide devices owing in part to the development of a number of different commercial compositions with a wide range of optical, physical, chemical and laser properties. In addition, the recent development of low temperature bonding technology has made possible the fabrication of structures involving multiple glasses prepared as a single hybrid substrate. Combined, these new materials and technologies make possible the creation of devices with increasing integration and complexity. Here, we present passive characterization data collected on glass joints prepared with the low temperature bonding technology and active performance data of a hybrid DBR laser where the surface relief grating has been fabricated in the passive glass region of a hybrid substrate.
Rates of dehydroxylation of two Nd-doped metaphosphate laser glasses (LG-770 and LHG-8) are measured and modeled. Glass melts ranging in size from 100 g to 2.8 kg were bubbled with O<SUB>2</SUB> containing various H<SUB>2</SUB>O partial pressures (P<SUB>H(subscript 2</SUB>O)) and with O<SUB>2</SUB>/Cl<SUB>2</SUB> mixtures at temperatures ranging from 925 - 1300 degree(s)C. The OH content in the glass was measured by monitoring the OH absorption at 3.333 micrometers at various bubbling times. The OH removal by inert gas bubbling (e.g. O<SUB>2</SUB> bubbling) is governed by the transport (diffusion) of OH to the glass liquid/vapor interface and by the chemical equilibrium between OH at the surface and H<SUB>2</SUB>O in the gas phase. The equilibrium OH content in glass melts bubbled with O<SUB>2</SUB> containing different P<SUB>H(subscript 2</SUB>O) varies as P<SUB>H(subscript 2</SUB>O)<SUP>1/2</SUP>.
The ability to engineer glass properties through the selection and adjustment of chemical composition continues to make glass a leading material in both active and passive applications. The development of optimal glass compositions for integrated optical applications requires a number of considerations that are often at variance with one another. Of critical importance is that the glass offers compatibility with standard ion exchange technologies, allowing fabrication of guided wave structures. In addition, for application as an active material, the resultant structures must be characterized by absence of inclusions and low absorption at the lasing wavelength, putting demands on both the selection and identity of the raw materials used to prepare the glass. We report on the development of an optimized glass composition for integrated optic applications that combines good laser properties with good chemical durability allowing for a wide range of chemical processing steps to be employed without substrate deterioration. In addition, care was taken during the development of this glass to insure that the selected composition was consistent with manufacturing technology for producing high optical quality glass. We present the properties of the resultant glasses, including results of detailed chemical and laser properties, for use in the design and modeling of active waveguides prepared with these glasses.
The next generation of high energy laser systems for ICF research demands an unprecedented volume of laser glass to be produced over a limited manufacturing period while still meeting ambitious targets of internal quality and overall cost. To meet this challenge, Schott has conceived a continuous manufacturing unit capable of producing 5,000 meter class PH 4 slabs of platinum particle-free phosphate laser glass within a three-year time period. This manufacturing unit concept draws on years of prior production experience with phosphate laser glass and other high quality optical materials but still represents a significant departure from the proven discontinuous manufacturing technology successfully employed over the last ten years for platinum-free phosphate laser glass. In addition, Schott has developed a new phosphate laser glass that simultaneously offers improvements in properties that relate to both laser performance and to characteristics related to forming the glass into large, high quality slabs. In this paper we will describe the key technology issues addressed in the manufacturing development and present a brief description of the planned manufacturing method to be employed. Lastly, the status of the development will be reviewed including characterization of pilot production melts of the new laser glass and the schedule for completion of the development program.
Erbium and erbium/ytterbium co-doped silicate glass waveguide lasers have been fabricated by silver ion-exchange and their characteristics analyzed. We report on measurements and comparisons made in the lasing properties of these devices, including threshold, slope-efficiencies and pump tuning ranges. The results presented show that through proper choice of host glass, it is possible to make low-threshold lasers both in singly and co-doped devices.
The optical properties of the CaO-Al<SUB>2</SUB>O<SUB>3</SUB>-B<SUB>2</SUB>O<SUB>3</SUB> glasses are reported in this paper. The variation of refractive index, dispersion, ultra-violet absorption and density with composition is in agreement with the dielectric properties results previously published. Inflections in the physical property curves are discussed in terms of formation of the non- bridging oxygens. Infrared spectroscopy suggests the presence of the boroxol groups up to 40 mol% CaO. The tetraborate groups gradually diminish on increasing the CaO concentration higher than 25 mol%.
Waveguide lasers formed by ion exchange in rare-earth-doped glasses have emerged as an attractive new technology on the threshold of commercial insertion. These devices can be used as both laser oscillators and optical amplifiers. In this article, we review ion exchange and glass composition. We then discuss the performance of ion-exchanged waveguide lasers made in silicate and phosphate glasses.
The Nd-doped phosphate laser glass described herein can withstand 2.3 times greater thermal loading without fracture, compared to APG-1 (commercially available average-power glass from Schott Glass Technologies). The enhanced thermal loading capability is established on the basis of the intrinsic thermomechanical properties (expansion, conduction, fracture toughness, and Young's modulus), and by direct thermally induced fracture experiments using Ar-ion laser heating of the samples. This Nd-doped phosphate glass (referred to as APG-t) is found to be characterized by a 29% gain cross section and a 25% longer low-concentration emission lifetime.
We report recent results from our work on the fabrication of neodymium waveguide lasers. Several neodymium doped glasses. APG-1, LG-680, BK 7 and S 3 made by Schott Glass Technologies, Inc. were studied as candidates for use as waveguide lasers. It was found that S 3, a standard ophthalmic glass, had the best ion-exchange properties of any of the glasses studied. A waveguide laser was successfully made using the neodymium doped S 3 glass.
In this paper we describe a multiwatt Nd<SUP>3+</SUP> fiber laser pumped via a second cladding by the DIOMED 25 laser diode unit. This multiple diode array source is designed for coupling up to 25 Watts of diode power into a plastic-clad silica fiber of 400 micrometers diameter. The double-clad laser fiber is interchangeable with the normal PCS delivery fiber. The device operates at 1.058 micrometers with a slope efficiency > 50% and a 150 times brightness enhancement. This laser though useful in itself is also a key intermediate laser for generation of high powers at other wavelengths. Tandem pumping of Tm<SUP>3+</SUP> and Er<SUP>3+</SUP>/Yb<SUP>3+</SUP> fiber lasers at 1.058 micrometers enables efficient generation of 2.0 micrometers and 1.55 micrometers radiation respectively. In addition the Nd<SUP>3+</SUP> laser can be operated close to 1.3 micrometers and there are prospects for in-fiber frequency doubling of the 1.06 micrometers line to generate a high power source in the green.
The ability to vary glass properties by adjusting composition continues to make glass a leading material for application as both active and passive elements, in bulk as well as in guided-wave laser systems. We consider here how glass is engineered for specific intracavity and extracavity laser applications. Mention also is made of process and manufacturing techniques which result in glasses with improved or special properties critical for applications involving laser systems.
The range of optical and thermal properties offered by Nd-doped phosphate glasses allows these materials to service a variety of laser applications. For example, the high extraction efficiency, low non-linear index and superb optical quality render Nd:phosphate glasses suitable for large-scale fusion laser technology. In this case laser efficiency as well as cost and glass yield issues are critical. As a consequence of the very low repetition rate (< 1/hour), however, issues surrounding thermal fracture during laser operation are not important. On the other hand, high and moderate-average power laser systems require that the active element performs well under substantial heat loading. Here, the most important issues involve the thermal properties of the medium, and design considerations suggest that it is worthwhile to have somewhat lower extraction efficiency if the material can be engineered to endure significantly more heat loading without thermal fracture. It has been observed that the Nd:phosphate composition space spans thermal figure-of-merit and emission cross section values exceeding a factor of two. It turns out, unfortunately, that improved thermal properties tend to be roughly correlated with inferior laser attributes. In addition to the laser and thermal properties many other property issues need to be addressed including devitrification, durability, and platinum solubility. The manner in which the glass constituents affect glass properties and the nature of the trade-offs that are encountered in engineering practical media are discussed.
Hostile environments created by short wavelength electromagnetic radiation (UV, X-ray and gamma-radiation) or from particle fluxes (alpha-particles, beta-particles, protons, and neutrons), can produce discoloration within optical glasses. The associated loss in transmission is detrimental to the performance of any optical system and must be eliminated or reduced to a manageable level. For applications within these hostile environments, radiation-stabilized optical glasses have been developed. To optimize system performance, optical glasses which have been stabilized for applications within the particular radiation environment must be selected. If the environment is a mixture of radiation fields, compromises are called for.
Results are reported from a composition study of forty-one phosphate glasses that
are derivatives of a commercially available Nd-doped laser glass. Systematic variations
have been made in the quantities of alkali and alkaline earth components in the
glass. Correlations are developed relating the compositionally-averaged electric field
strength and certain thermal, mechanical, optical and laser properties of these
glasses. The compositionally-averaged field strength is found to be a good parameter
for predicting the effects of complex mixtures of alkali and alkaline earth components
in the glass. However, it is inadequate for explaining the effects of glass components
that are predominantly covalently bonded with oxygen. Some possible explanations are
given for the observed trends in glass properties with certain compositional changes.