High-efficiency radiation-balanced Yb-doped silica fiber laser with 200-mW output.

The focus of this study was the development of a second generation of fiber lasers internally cooled by anti-Stokes fluorescence. The laser consisted of a length of a single-mode fiber spliced to fiber Bragg gratings to form the optical resonator. The fiber was single-moded at the pump (1040 nm) and signal (1064 nm) wavelengths. Its core was heavily doped with Yb, in the initial form of CaF2 nanoparticles, and co-doped with Al to reduce quenching and improve the cooling efficiency. After optimizing the fiber length (4.1 m) and output-coupler reflectivity (3.3%), the fiber laser exhibited a threshold of 160 mW, an optical efficiency of 56.8%, and a radiation-balanced output power (no net heat generation) of 192 mW. On all three metrics, this performance is significantly better than the only previously reported radiation-balanced fiber laser, which is even more meaningful given that the small size of the single-mode fiber core (7.8-µm diameter). At the maximum output power (∼2 W), the average fiber temperature was still barely above room temperature (428 mK). This work demonstrates that with anti-Stokes pumping, it is possible to induce significant gain and energy storage in a small-core Yb-doped fiber while keeping the fiber cool.

The photonics and optics workforce: unleashing the potential for greater industry growth-introduction to the feature issue.

This feature issue highlights specific photonics and optics workforce challenges, opportunities for industry support, and state-of-the-art-training methods.

Versatile luminescence thermometry via intense green defect emission from an infrared-pumped fluorosilicate optical fiber.

An all-glass optical fiber capable of two distinct methods of optical thermometry is described. Specifically, a silica-clad, barium fluorosilicate glass core fiber, when pumped in the infrared, exhibits visibly intense green defect luminescence whose intensity and upper-state lifetime are strong functions of temperature. Intensity-based optical thermometry over the range from 25°C to 130°C is demonstrated, while a lifetime-based temperature sensitivity is shown from 25°C to 100°C. Time-domain measurements yield a relative sensitivity of 2.85% K -1 at 373 K (100°C). A proof-of-concept distributed sensor system using a commercial digital single-lens reflex camera is presented, resulting in a measured maximum relative sensitivity of 1.13% K -1 at 368 K (95°C). The sensing system described herein stands as a new blueprint for defect-based luminescence thermometry that takes advantage of pre-existing and relatively inexpensive optical components, and allows for the use of standard cameras or simply direct human observation.

Power scaling limits of diffraction-limited fiber amplifiers considering transverse mode instability.

An empirical TMI threshold formula is derived based on a recently developed model and used to analyze the power-scaling performance of ytterbium-doped silica glass and YAG (Y3Al5O12) and lutetia (Lu2O3) single-crystalline fiber amplifiers. Overall, the single-crystalline fiber lasers are found to scale potentially to higher average powers due to their higher thermal conductivities compared to silica glass. This work serves as a useful extension to earlier works and shines significant new light on optimal fiber and amplifier designs for maximum average output power with TMI considered.

Virtual reality enabled asynchronous learning modules for fiber optic preform manufacturing education.

The demand for skilled workers and novel manufacturing training solutions has increased with the growing demand for fiber optic cables. Web-based simulations can be used for training, and this paper presents an approach for developing a fiber preform manufacturing browser-based VR simulation. Subsequently, a study was conducted to evaluate the effectiveness of the simulation based on learning gains and learner perception of ease of use, usefulness, intention of use, learning outcomes, and workload. A mixed-methods between-subjects study with 63 participants found that the combination of lecture and simulation was significantly better for perceived usefulness and learning outcomes compared to lecture-only or lecture-and-video conditions.

Anti-Stokes fluorescence cooling of nanoparticle-doped silica fibers.

The first observation of cooling by anti-Stokes pumping in nanoparticle-doped silica fibers is reported. Four Yb-doped fibers fabricated using conventional modified chemical vapor deposition (MCVD) techniques were evaluated, namely, an aluminosilicate fiber and three fibers in which the Yb ions were encapsulated in CaF2, SrF2, or BaF2 nanoparticles. The nanoparticles, which oxidize during preform processing, provide a modified chemical environment for the Yb3+ ions that is beneficial to cooling. When pumped at the near-optimum cooling wavelength of 1040 nm at atmospheric pressure, the fibers experienced a maximum measured temperature drop of 20.5 mK (aluminosilicate fiber), 26.2 mK (CaF2 fiber), and 16.7 mK (SrF2 fiber). The BaF2 fiber did not cool but warmed slightly. The three fibers that cooled had a cooling efficiency comparable to that of the best previously reported Yb-doped silica fiber that cooled. Data analysis shows that this efficiency is explained by the fibers' high critical quenching concentration and low residual absorptive loss (linked to sub-ppm OH contamination). This study demonstrates the large untapped potential of nanoparticle doping in the current search for silicate compositions that produce optimum anti-Stokes cooling.

Raman enhanced four-wave mixing in silicon core fibers.

A strong Raman enhancement to the four-wave mixing (FWM) conversion efficiency is obtained in a silicon core fiber (SCF) when pumped with a continuous-wave (CW) source in the telecom band. By tapering the SCFs to alter the core diameter and length, the role of phase-matching on the conversion enhancement is investigated, with a maximum Raman enhancement of ∼15 dB obtained for an SCF with a zero dispersion wavelength close to the pump. Simulations show that by optimizing the tapered waist diameter to overlap the FWM phase-matching with the peak Raman gain, it is possible to obtain large Raman enhanced FWM conversion efficiencies of up to ∼2 dB using modest CW pump powers over wavelengths covering the extended telecom bands.

All-optical high-speed modulation of THz transmission through silicon core optical fibers.

High speed optical modulation of THz radiation is of interest for information processing and communications applications. In this paper infrared femtosecond pulses are used to generate free carriers that reduce the THz transmission of silicon based waveguides over a broad spectral range. Up to 96% modulation is observed from 0.5 to 7 THz in an optical fiber with a 210 µm diameter gold-doped silicon core. The observed carrier recombination time of 2.0 ± 0.2 ns makes this material suitable for high speed all-optical signal processing. These results show both enhanced modulation depth and reduced carrier lifetime when compared to the performance of a high resistivity float zone silicon rectangular guide with comparable cross sectional area.

Experimental comparison of silica fibers for laser cooling.

Laser cooling in silica has recently been demonstrated, but there is still a lack of understanding on how fiber composition, core size, and OH- contamination influence cooling performance. In this work, six Yb-doped silica fibers were studied to illuminate the influence of these parameters. The best fiber cooled by -70mK with only 170 mW/m of absorbed pump power at 1040 nm, which corresponds to twice as much heat extracted per unit length compared to the first reported laser cooling in silica. This new fiber has an extremely low OH- loss and a higher Al concentration (2.0 wt.% Al), permitting a high Yb concentration (2.52 wt.% Yb) without incurring significant quenching. Strong correlations were found between the absorptive loss responsible for heating and the loss measured at 1380 nm due to absorption by OH-.

Observation of optical nonlinearities in an all-solid transverse Anderson localizing optical fiber.

An all-solid transverse Anderson localizing optical fiber (TALOF) was fabricated using a novel combination of the stack-and-draw and molten core methods. Strong Anderson localization is observed in multiple regions of the fiber cross section associated with the higher index strontium aluminosilicate phases randomly arranged within a pure silica matrix. Further, to the best of our knowledge, nonlinear four-wave mixing is reported for the first time in a TALOF.

Laser cooling in a silica optical fiber at atmospheric pressure.

For the first time, to the best of our knowledge, laser cooling is reported in a silica optical fiber. The fiber has a 21-µm diameter core doped with 2.06 wt.% ${{\rm Yb}^{3 + }}$Yb3+ and co-doped with ${{\rm Al}_2}{{\rm O}_3}$Al2O3 and ${{\rm F}^ - }$F- to increase the critical quenching concentration by a factor of 16 over the largest reported values for the Yb-doped silica. Using a custom slow-light fiber Bragg grating sensor, temperature changes up to $ - {50}\;{\rm mK}$-50mK were measured with 0.33 W/m of absorbed pump power per unit length at 1040 nm. The measured dependencies of the temperature change on the pump power and the pump wavelength are in excellent agreement with predictions from an existing model, and they reflect the fiber's groundbreaking quality for the radiation-balanced fiber lasers.

Tapered silicon core fibers with nano-spikes for optical coupling via spliced silica fibers.

Reported here is the fabrication of tapered silicon core fibers possessing a nano-spike input that facilitates their seamless splicing to conventional single mode fibers. A proof-of-concept 30 µm cladding diameter fiber-based device is demonstrated with nano-spike coupling and propagation losses below 4 dB and 2 dB/cm, respectively. Finite-element-method-based simulations show that the nano-spike coupling losses could be reduced to below 1 dB by decreasing the cladding diameters down to 10 µm. Such efficient and robust integration of the silicon core fibers with standard fiber devices will help to overcome significant barriers for all-fiber nonlinear photonics and optoelectronics.

Nanoparticle doping for high power fiber lasers at eye-safer wavelengths.

A nanoparticle (NP) doping technique was developed for fabricating erbium (Er)- and holmium (Ho)-doped silica-based optical fibers for high energy lasers. Slope efficiencies in excess of 74% were realized for Er NP doping in a single mode fiber based master oscillator power amplifier (MOPA) and 53% with multi-Watt-level output in a resonantly cladding-pumped power oscillator laser configuration based on a double-clad fiber. Cores comprising Ho doped LaF3 and Lu2O3 nanoparticles exhibited slope efficiencies as high as 85% at 2.09 µm in a laser configuration. To the best of the authors' knowledge, this is the first report of a holmium nanoparticle doped fiber laser as well as the highest efficiency and power output reported from an erbium nanoparticle doped fiber laser.

Tapered polysilicon core fibers for nonlinear photonics: erratum.

We correct an error of the nonlinear refractive index used in our original paper.

Optical gain in capillary light guides filled with NaYF4: Yb3+, Er3+ nanocolloids.

A capillary light guide optical amplifier using nanocolloids of Yb3+-Er3+ co-doped NaYF4 as a filler was successfully demonstrated. A 7-cm-long and 150-micron-inner-diameter capillary light guide was capable to amplify a pulsed optical signal at 1550 nm with a gain coefficient of 0.15 cm-1 at a pump power of 4 mW (980-nm wavelength). The nanocolloid gain medium was prepared by pulverizing the phosphor powder with a high-speed planetary ball mill. Ball milling of the powder in water produced nanoparticles with a size of approximately 130 nm that after drying were transferred to a liquid with high refractive index (1.551 at 1550 nm) required to maintain light confinement within the fused silica capillary light guide. The results show that RE-doped colloids of nanocrystals can be potentially used as liquid gain media fillers in capillary light guide lasers and amplifiers with high photostability and low toxicity.

Tapered polysilicon core fibers for nonlinear photonics.

We propose and demonstrate a novel approach to obtaining small-core polysilicon waveguides from the silicon fiber platform. The fibers were fabricated via a conventional drawing tower method and, subsequently, tapered down to achieve silicon core diameters of ∼1 µm, the smallest optical cores for this class of fiber to date. Characterization of the material properties have shown that the taper process helps to improve the local crystallinity of the silicon core, resulting in a significant reduction in the material loss. By exploiting the combination of small cores and low losses, these tapered fibers have enabled the first observation of nonlinear transmission within a polycrystalline silicon waveguide of any type. As the fiber drawing method is highly scalable, it opens a route for the development of low-cost and flexible nonlinear silicon photonic systems.

Light trapping in horizontally aligned silicon microwire solar cells.

In this study, we demonstrate a solar cell design based on horizontally aligned microwires fabricated from 99.98% pure silicon via the molten core fiber drawing method. A similar structure consisting of 50 µm diameter close packed wires (≈ 0.97 packing density) on a Lambertian white back-reflector showed 86 % absorption for incident light of wavelengths up to 850 nm. An array with a packing fraction of 0.35 showed an absorption of 58 % over the same range, demonstrating the potential for effective light trapping. Prototype solar cells were fabricated to demonstrate the concept. Horizontal wire cells offer several advantages as they can be flexible, and partially transparent, and absorb light efficiently over a wide range of incident angles.

Raman amplification at 2.2 µm in silicon core fibers with prospects for extended mid-infrared source generation.

Raman scattering provides a convenient mechanism to generate or amplify light at wavelengths where gain is not otherwise available. When combined with recent advancements in high-power fiber lasers that operate at wavelengths ~2 µm, great opportunities exist for Raman systems that extend operation further into the mid-infrared regime for applications such as gas sensing, spectroscopy, and biomedical analyses. Here, a thulium-doped fiber laser is used to demonstrate Raman emission and amplification from a highly nonlinear silicon core fiber (SCF) platform at wavelengths beyond 2 µm. The SCF has been tapered to obtain a micrometer-sized core diameter (~1.6 µm) over a length of 6 cm, with losses as low as 0.2 dB cm-1. A maximum on-off peak gain of 30.4 dB was obtained using 10 W of peak pump power at 1.99 µm, with simulations indicating that the gain could be increased to up to ~50 dB by extending the SCF length. Simulations also show that by exploiting the large Raman gain and extended mid-infrared transparency of the SCF, cascaded Raman processes could yield tunable systems with practical output powers across the 2-5 µm range.

Germanium microsphere high-Q resonator.

In this Letter, the fabrication and characterization of a microsphere resonator from the semiconductor germanium is demonstrated. Whispering gallery modes are excited in a 46 µm diameter germanium microsphere resonator using evanescent coupling from a tapered silica optical fiber with a waist diameter of 2 µm. Resonances with Q factors as high as 3.8×10(4) at wavelengths near 2 µm are observed. Because of their ultrahigh optical nonlinearities and extremely broad transparency window, germanium microsphere resonators offer the potential for optical processing devices, in particular at long wavelengths, such as around 2 µm.

Low-temperature growth of multiple-stack high-density ZnO nanoflowers/nanorods on plastic substrates.

Reported here is the low-temperature growth of multiple-stack high-density ZnO nanoflower/nanorod structures on polyethylene naphthalate (PEN) substrates derived from the surface modification of ZnO seed layers using an atmospheric-pressure plasma jet (APPJ) treatment. The plasma treatment could provide several advantages to the growth of multiple-stack ZnO nanoflower/nanorod structures: (i) the surface wettability of the seed layers changes from hydrophobic to hydrophilic, resulting in higher surface energies for the growth of high-density ZnO nanoflowers, (ii) the nucleation sites increase due to the increased surface roughness caused by the plasma etching, and (iii) there is no thermal damage to the plastic substrate from the plasma treatment due to its low-temperature weakly ionized discharge. It was also confirmed that multiple stacks of ZnO nanoflowers were obtained without degradation of the crystal quality or modification to the crystal shape or phase. The ZnO nanoflower/nanorod structures grew by lengths up to 4 µm due to an increased surface roughness of 10% and surface energy 5.5 times that of the seed layers. As shown, the APPJ is a very good method to obtain high-density ZnO nanostructures on plastic substrates below 150 °C, as is critical for flexible electronics.