This PDF file contains the front matter associated with SPIE Proceedings Volume 10981, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Volume Bragg Gratings (VBGs) in photo-thermo-refractive glass demonstrate outstanding properties in visible and near IR spectral region providing extremely high spectral and angular selectivity, and diffraction efficiency exceeding 99%. This paper reviews recent VBG technology improvements as well as various results on VBG applications that can lead to major improvements of fiber, solid-state, and diode laser system performance in 2 3 micron spectral range.

The development of novel crystals with low-phonon-energies continues to be an active area of research for laser applications in the mid-infrared (MIR) region [1-3]. In this work, the material purification, synthesis, crystal growth, and spectroscopic characterization of Pr3+ doped ternary lead halides including Pr: TlPb2Br5 (Pr: TPB), Pr: KPb2Br5 (Pr: KPB), and Pr: CsPbCl3 will be presented. The synthesis of the studied materials was based on the purification of commercial starting materials including directional freezing, multi-pass translation in a zone-melting system and halogenation. The growth of the purified materials was then carried out through vertical Bridgman technique using a two-zone furnace. The trivalent praseodymium ion (Pr3+) offers several intra-4f transitions of interest for laser applications. Using the output of a ~1.5 m fiber laser as excitation source, IR emissions centered at ~1.6 m (3F4/3F33H4) ~2.4 m (3F2/3H63H4), and ~4.6 μm (3H53H4) were observed from all samples at room-temperature. The decay-times of the two bromide crystals Pr: TPB and Pr: KPB were in the millisecond range, whereas Pr: CsPbCl3 exhibited a significantly shorter lifetime of only ~20 s at room-temperature. The short MIR emission lifetime of Pr: CsPbCl3 indicates strong emission quenching, which is most likely through a combination of multi-phonon-relaxation and impurity related quenching mechanism. Reference [1] S. R. Bowman, J. Ganem, B. J. Feldman, and A. W. Kueny, J. Ganem, "Infrared Laser Characteristics of Praseodymium-Doped Lanthum Trichloride", IEEE J. Quant. Electron. 30, (1994) 2925. [2] N. Ter-Gabrielyan, T. Sanamyan, and M. Dubinskii, “Resonantly pumped Pr3+-doped eye-safe laser at ~1.65µm" Opt. Lett. 34 (2009) 1949. [3] B. M. Walsh, U. Hommerich, A. Yoshikawa, A. "Toncelli, Mid-infrared spectroscopy of Pr-doped materials", Journal of Luminescence 197 (2018) 349.

Recent progress in development of Mid–IR lasers at ~2.8 µm and ~3.5 µm based on commercial Er:ZBLAN fibers has enabled variety of environmental sensing, defense and medical applications. This development faces a few major challenges, among which are relatively low laser efficiency (stemming from the naturally high quantum defect of laser operation) and power scaling limitation due to output self-pulsing (perceived to be coming from presence of clustering or ion pairs in a highly doped fiber, which act as saturable absorbers).We report on a study of the power scaling of a 976 nm diode-pumped double-clad Er:ZBLAN fiber laser at the ~2.8 µm, 4I11/2-4I13/2 transition. The passively cooled 7% Er-doped fluoride fiber laser was shown to achieve slope efficiency over 25% and 50 W with respect to launched pump power in both CW and Quasi-CW regimes of free-running operation. Laser power scaling was found to be limited by available 976 nm diode pump power.

The performance of mid-infrared fiber lasers operating on the 3.5 μm transition in erbium has improved significantly since the first demonstration that dual wavelength pumping allowed efficient operation. In this contribution, we will discuss the progress of fiber lasers that operate on this transition with an emphasis on advances towards short pulse generation and wavelength agility. Mode-locked operation using saturable absorption is a robust means of achieving ultra-short pulse operation in the near infrared but achieving this in the mid-infrared has been elusive. We will also describe our characterization of the mid-infrared performance of graphene, a material which has been very successfully applied to mode-locked pulse generation in the near infrared.

Progress towards the development of intrinsically low-nonlinearity and low-loss optical fibers for high-energy laser applications is outlined. Owing to the high optical intensities and relatively long interaction lengths in high-energy fiber lasers systems, a wide range of deleterious, power-limiting processes can be excited, all of which require careful management. Here, a materials-based approach is taken to enhance the power available from such systems. Specifically discussed in this context are: 1) decreasing the strength of Brillouin scattering, mainly through the reduction of glass photoelastic constant p12; 2) the reduction of the strength of Raman scattering by taking advantage of glass disorder and the judicious use low-Raman-gain additives; 3) elevation of the threshold for the onset of thermal mode instabilities through a reduction in the thermo-optic coefficient, dn/dT; and 4) all while not adversely impacting the relatively low value of n2 afforded by glasses comprised mainly of silica. The ultimate goal of this work is the development of active fibers with Brillouin gain coefficients reduced by 15 dB, Raman gain coefficients reduced by 3 dB, and dn/dT values reduced by 3 dB, with background losses well below 100 dB/km all in a single fiber. The current state of this effort will be discussed and the outlook for the development of such a fiber will be described.

Fiber lasers rely on clad pump architectures where double clad designs are used. Currently, polymers are used successfully as pump claddings on Yb-doped laser fibers. In order to transition to resonantly pumped fiber lasers at eye safer wavelengths, polymer pump claddings must have low absorption at those wavelengths. There is currently no suitable low index, low absorption, thermally stable, and high thermal conductivity polymer to act as a low-index coating. This work presents a new class of polymers, termed fluorinated polymer composites (FPCs), which possess improved thermal properties. The FPCs are fabricated by incorporating ceramic nanoparticles into fluorinated polymers. Ultimately, the FPCs could enable further use of fiber lasers at commonly used wavelengths, as well as at eye-safer wavelengths.

There have been two significant shifts in laser technology over the last 40 years, beginning with the CO₂ laser being displaced by the Nd:YAG laser, then the Nd:YAG laser being displaced by fiber lasers. Today, the blue laser technology being developed will be the next major shift in technology for both industrial and military applications. The high-power blue laser light offers many new capabilities that were unachievable without a high power visible light source. These applications range from welding copper to transmitting high power laser energy through water for underwater imaging and communications. This paper will introduce this new technology, summarize the tests performed to date on the industrial applications and review a number of military applications that could benefit from a high power visible light source.

Avalanche Photodiodes (APDs) that target a wavelength of 1550 nm, have several applications ranging from optical communications to imaging to single photon detection. The DE-JTO will use an array of APDs to image the wavefront of its 1550 nm laser. The distinctive feature of an APD is high sensitivity due to the gain achieved by impact ionization of carriers. Because impact ionization is a stochastic process, it introduces excess noise that limits the signal to noise ratio of an APD. However, the excess noise may be reduced by engineering the k (=β/α) value of the device, where β and α are the impact ionization coefficients of holes and electrons, respectively. k can be engineered by band diagram engineering ^[1], band structure engineering ^[2], or dead space effect ^[3,4]. Combinations of these are also used ^[5]. Band diagram engineering enables the implementation of Capasso’s channeling APD ^[1]. In this design, electrons and holes are spatially separated in different channels with distinct materials and bandgaps. These channel materials are designed to minimize the impact ionization of one carrier and promote the other, thereby optimizing the k and excess noise. The two limitations of the Capasso design are (1) the leakage current due to doping the channels and (2) excess noise due to dual carrier injection. Firstly, to spatially separate the carriers between narrow and wide bandgap materials, type I band alignment with doping is suggested by Capasso. However, type II band alignment, due to the valance band offset, may inherently provide the field required for the spatial separation of carriers. And type II alignment avoids the doping that could lead to leakage currents. Secondly, channeling APD is a planar configuration leading to dual carrier injection that increases the excess noise. Using a window injection layer defined by lithography, a channeling APD with single carrier injection is designed.

We describe the development of a number of eyesafe Er:YAG laser systems. Based on the Co Planar Folded Zig Zag geometry, these lasers were primarily developed for hard target ranging at distances greater than 20 km. We also present results where these lasers have been used to explore the profiles of cloud structures over Adelaide, South Australia.

At the system level, poor beam quality often leads to significant performance degradations. For instance, if a high-energy laser (HEL) source has poor beam quality, then the effectiveness of a beam-control system becomes compromised. This outcome is the result of non-common path phase errors in the HEL beam that are not sensed by the beam-control system. Therefore, phase-compensation performance becomes degraded, because if the phase errors that degrade performance cannot be sensed, then their effects cannot be corrected. To help quantity these system-level performance degradations, we need a reliable and accurate way to model the effects of poor beam quality. As such, this paper develops a statistical approach using scalar Schell-model sources. Applying the work of Santarsiero et al. [J. Opt. Soc. Am. A 16, 106 (1999)], we show that we can quantify the effects of poor beam quality statistically in terms of the M² parameter. Then, we develop Monte Carlo wave-optics simulations which specify the field correlation coefficient in terms of M² for modeling HEL phase errors. The results show that we obtain convergence to the desired M² parameter thus properly modeling the phase errors. In addition, we investigate the statistics of the "instantaneous" M² parameter. We find, quite interestingly, that the instantaneous M₂ is very nearly lognormal. This information will be useful in HEL beam-control system design.

This study advances the benefits of previously reported 4D Weather Cubes towards creation of high fidelity cloud free line of sight (CFLOS) beam propagation for realistic assessment of auto-tracked/dynamically routed free space optical communication datalink concepts. 4D Weather Cubes are the product of efficient processing of large, computationally intensive, National Oceanic and Atmospheric Administration (NOAA) gridded numerical weather prediction (NWP) data coupled with embedded physical relationships governing cloud, fog, and precipitation formation to render highly realistic 4D cloud free line of sight analytical environments. The Weather Cubes accrue parameterization of optical effects and custom atmospheric resolution through implementation of the verified and validated Laser Environmental Effects Definition and Reference (LEEDR) atmospheric characterization and radiative transfer code. 4D Weather Cube analyses have recently been expanded to accurately assess Directed Energy weapons and sensor performance (probabilistic climatologies and performance forecasts) at any wavelength/frequency or spectral band in the absence of field test and employment data. The 4D Weather Cubes initialize the High Energy Laser End to End Operational Simulation (HELEEOS) propagation code, which provides a means to dynamically point the communication link. HELEEOS’ calculation of irradiance at the detector as a function of transmission, optical turbulence, and noise sources such as path radiance was the basis for comparative percentile performance binning of FSO communication bit error rates as a function of wide-ranging azimuth/elevation, earth-to-space uplinks. The aggregated, comparative bit error rate binning analyses for different regions, times of day, and seasons using a full year of data provided numerous occasions of clouds, fogs, and precipitation events, thus demonstrating the relevance of 4D Weather Cubes for adroit management of CFLOS opportunities to enhance performance analyses of point-to-point as well as evolving multilayer wireless network concepts.