Performance analysis of a compact auto-phoropter for accessible refractive assessment of the human eye.

We present the performance analysis and specifications of a portable auto-phoropter system that can be employed for fast refractive assessment of a large population. A customized Shack-Hartmann wavefront sensor is developed to accurately measure the defocus and astigmatism of the eye within ±10D and ±6D, respectively. Three fluidic lenses are designed to correct the vision in real time. A digital Snellen chart is integrated into the system to validate the accuracy of the measurement and the correction by means of achieving 20/20 vision. The refractive error of eight subjects (16 eyes) has been measured objectively (without patient's feedback) using the proposed system and the results are compared with their clinical prescription through the Bland-Altman method. It is shown that the auto-phoropter takes less than 8 s to measure and correct the eye refractive error with an accuracy of ±0.25D.

Parametric dog-bone-shaped tunable cylindrical fluidic lens.

Tunable spherical fluidic lenses are among the most essential components in adaptive optics. However, fabricating cylindrical tunable lenses has proven more challenging, mainly due to the difficulty in eliminating the defocus component. We demonstrate a parametric approach to minimize the defocus in cylindrical tunable fluidic lenses. We theoretically model and experimentally verify that a dog-bone-shaped tunable cylindrical fluidic lens exhibits almost pure cylindrical performance within the range of ${\pm{\rm 5D}}$ of astigmatism. We anticipate these results will facilitate the use of tunable cylindrical fluidic lenses in adaptive optics applications and particularly ophthalmic devices, where rapid and reliable wavefront correction is required.

Single-cavity dual-wavelength all-fiber femtosecond laser for multimodal multiphoton microscopy.

A single-cavity dual-wavelength all-fiber femtosecond laser is designed to generate 1030 nm wavelength for high resolution multiphoton imaging and 1700 nm wavelength for long penetration depth imaging. Considering two-photon and three-photon microscopy (2PM and 3PM), the proposed laser provides the single-photon wavelength equivalent to 343 nm, 515 nm, 566 nm and 850 nm, that can be employed to excite a wide variety of intrinsic fluorophores, dyes, and fluorescent proteins. Generating two excitation wavelengths from a single laser reduces the footprint and cost significantly compared to having two separate lasers. Furthermore, an all-reflective microscope is designed to eliminate the chromatic aberration while employing two excitation wavelengths. The compact all-fiber alignment-free laser design makes the overall size of the microscope appropriate for clinical applications.

Magneto-optical properties of highly Dy3+ doped multicomponent glasses.

Due to their large effective magnetic moment, Dy3+-doped materials have attracted much interest for magneto-optical applications. In this paper, we report highly Dy3+ doped multicomponent glasses with concentrations from 40 wt.% to 75 wt.% and their magneto-optical properties. A Verdet constant of -7.4 rad/T/m at 1950 nm was measured with the 75 wt.% Dy3+-doped glass. This is the highest reported Verdet constant around 2 µm for a paramagnetic glass. Our experimental results show that highly Dy3+-doped glasses are promising isotropic magneto-optical materials for applications in the 2 µm wavelength region.

High Verdet constant of Te20As30Se50 glass in the mid-infrared.

Magneto-optical properties of tellurium-arsenic-selenium glass (${{\rm Te}_{20}}{{\rm As}_{30}}{{\rm Se}_{50}}$Te20As30Se50) were measured and analyzed. A Verdet constant of 15.18 rad/T/m at 1950 nm with the figure of merit of more than 8.72 rad/T, which is the highest value reported in glass materials at this wavelength, was measured. Compared to other chalcogenide glasses, such as ${{\rm Ge}_{10}}{{\rm Se}_{90}}$Ge10Se90 and ${{\rm Ge}_{25}}{{\rm As}_{15}}{{\rm S}_{60}}$Ge25As15S60, ${{\rm Te}_{20}}{{\rm As}_{30}}{{\rm Se}_{50}}$Te20As30Se50 glass exhibits higher Verdet constants, broader mid-infrared transparency window, and longer infrared absorption edge, making it a very promising material to fabricate magneto-optical devices for mid-infrared applications.

Investigation of ion-ion interaction effects on Yb3+-doped fiber amplifiers.

Ytterbium (Yb3+)-doped materials have been widely used for high efficiency high energy laser sources at the 1 µm wavelength region because of their very low quantum defect and the unique simple energy level structure of Yb3+, resulting in no excited-state absorption and low occurrence probability of deleterious ion-ion interaction processes. It has been generally recognized that these ion-ion interaction processes have very little influence on the operation of Yb3+-doped fiber lasers at low and moderate power levels. However, our recent study shows that the performance of Yb3+-doped fiber amplifiers operating at low power levels is still influenced by the ion-ion interaction processes due to the large amount of population at the upper laser level 2F5/2. In this paper, experimental evidences of the ion-ion interaction effects in Yb3+-doped fiber amplifiers are presented and a new model including these effects is developed for the numerical simulation. Our experimental and numerical investigations on the 976 nm and 1030 nm Yb3+-doped silica and phosphate fiber amplifiers show that ion-ion interaction has non-negligible impact on the performance of Yb3+-doped fiber amplifiers indeed, and compared to Yb3+-doped silica fibers, Yb3+-doped phosphate fibers suffer much less from the ion-ion interaction effects due to the much less clustered ions.

A single shot coherent Ising machine based on a network of injection-locked multicore fiber lasers.

Combinatorial optimization problems over large and complex systems have many applications in social networks, image processing, artificial intelligence, computational biology and a variety of other areas. Finding the optimized solution for such problems in general are usually in non-deterministic polynomial time (NP)-hard complexity class. Some NP-hard problems can be easily mapped to minimizing an Ising energy function. Here, we present an analog all-optical implementation of a coherent Ising machine (CIM) based on a network of injection-locked multicore fiber (MCF) lasers. The Zeeman terms and the mutual couplings appearing in the Ising Hamiltonians are implemented using spatial light modulators (SLMs). As a proof-of-principle, we demonstrate the use of optics to solve several Ising Hamiltonians for up to thirteen nodes. Overall, the average accuracy of the CIM to find the ground state energy was ~90% for 120 trials. The fundamental bottlenecks for the scalability and programmability of the presented CIM are discussed as well.

Injection-Locked, Single Frequency, Multi-core Yb-doped Phosphate Fiber Laser.

For the first time, we demonstrate injection locking and single frequency operation of a multi-core Yb-doped phosphate fiber laser (MCF). The 19 MCF laser cores operated in CW mode at 1030 nm. Each laser core was locked to the frequency and polarization of the single-frequency master laser, and produced milliwatts of power with similar lasing thresholds. The pump beam was homogenized with a simple technique to increase uniform lasing behavior of the cores. This behavior was verified using a MCF laser model developed in-house. This unique MCF laser can be useful for applications of coherent, coupled oscillator networks, for example in an all-optical coherent Ising machine configuration.

Quasi-phase-matched third harmonic generation in organic multilayers.

We report the first realization of quasi-phase-matched (QPM) third harmonic generation in isotropic polymer films. Spin-coated thin films of ethyl-violet molecules dispersed in a polymer host (EV) were used as cubic nonlinear optical media because of their transparency at both the fundamental (1230 nm) and the third harmonic (410 nm) wavelengths. A passive layer of a UV-curable material was formed to compensate the phase shift between the two light waves after propagating through each EV layer. We fabricated a series of samples with 1~4 EV layers (0~3 alternatingly coated passive layers). The third harmonic output power showed a quadratic increase with the number of layers, providing a strong evidence for successful quasi-phase-matching. A conversion efficiency of 0.15% was observed with a 190 fs pulse input.

A 300 THz tabletop radar range system with sub-micron distance accuracy.

We are presenting a compact radar range system with a scale factor of 105. Replacing the radio frequency (RF) by optical wavelength (300 THz), the system easily fit on a tabletop. We used interferometric time-of-flight to reproduce radar ranging measurements. Sub-micron range accuracy was achieved with a 100 fs laser pulse, which correspond to 3 cm for a s-band (3 GHz) radar. We demonstrated the system potential on a simple target, and compared the results with radio frequency measurement using a vector network analyzer. We also present measurement with a more realistic model, a 3D printed reproduction of the USS Arizona battleship, for which a 3D model is extracted from the ranging data. Together with our previous demonstration of radar cross section measurement with a similar system, this report further validates our proposal to use optics to simulate radar properties of complex radio frequency systems.

Compact fiber-based multi-photon endoscope working at 1700 nm.

We present the design, implementation and performance analysis of a compact multi-photon endoscope based on a piezo electric scanning tube. A miniature objective lens with a long working distance and a high numerical aperture (≈ 0.5) is designed to provide a diffraction limited spot size. Furthermore, a 1700 nm wavelength femtosecond fiber laser is used as an excitation source to overcome the scattering of biological tissues and reduce water absorption. Therefore, the novel optical system along with the unique wavelength allows us to increase the imaging depth. We demonstrate that the endoscope is capable of performing third and second harmonic generation (THG/SHG) and three-photon excitation fluorescence (3PEF) imaging over a large field of view (> 400 µm) with high lateral resolution (2.2 µm). The compact and lightweight probe design makes it suitable for minimally-invasive in-vivo imaging as a potential alternative to surgical biopsies.

Nonlinear optical components for all-optical probabilistic graphical model.

The probabilistic graphical models (PGMs) are tools that are used to compute probability distributions over large and complex interacting variables. They have applications in social networks, speech recognition, artificial intelligence, machine learning, and many more areas. Here, we present an all-optical implementation of a PGM through the sum-product message passing algorithm (SPMPA) governed by a wavelength multiplexing architecture. As a proof-of-concept, we demonstrate the use of optics to solve a two node graphical model governed by SPMPA and successfully map the message passing algorithm onto photonics operations. The essential mathematical functions required for this algorithm, including multiplication and division, are implemented using nonlinear optics in thin film materials. The multiplication and division are demonstrated through a logarithm-summation-exponentiation operation and a pump-probe saturation process, respectively. The fundamental bottlenecks for the scalability of the presented scheme are discussed as well.

Single-frequency blue laser fiber amplifier.

An all-fiber amplifier for a single-frequency blue laser was demonstrated for the first time, to the best of our knowledge. Over 150 mW continuous-wave single-transverse-mode blue laser output was obtained with a 10 m 1000 ppm thulium-doped fluoride fiber pumped by a 1125 nm fiber laser at a power of 2 W. The output power was limited due to the onset of the competitive lasing at 783 nm. Photodarkening and photo-curing of the thulium-doped fiber amplifier were also studied and analyzed.

Power scalable 10 W 976 nm single-frequency linearly polarized laser source.

A 10 W level 976 nm single-frequency linearly polarized laser source was demonstrated with a two-stage all-fiber amplifier configuration. The continuous-wave output power of 10.1 W was obtained from the second stage amplifier by using a 20/130 µm single-mode, polarization maintaining, 1.5 wt. % ytterbium-doped phosphate double-clad fiber. This all-fiber single-frequency laser source is very promising for watt-level deep ultraviolet laser generation via frequency quadrupling.

A new low-cost, compact, auto-phoropter for refractive assessment in developing countries.

Using a phoropter to measure the refractive error is one of the most commonly used methods by ophthalmologists and optometrists. Here, we demonstrate design and fabrication of a portable automatic phoropter with no need for patient's feedback. The system is based on three tunable-focus fluidic lenses and thin-film holographic optical elements to perform automatic refractive error measurement and provide a diagnostic prescription without supervision. Three separate lenses are deployed to correct the defocus and astigmatism. The refractive error is measured using a Shack-Hartmann wavefront sensor that calculates the Zernike values of an infrared wavefront emerging from the eye. Holographic optical elements steer the emerging wavefront into the wavefront sensor, while simultaneously providing an unobstructed view for the subject. The power of each lens is controlled by pumping a liquid in and out of the lens chamber using servo motor actuated diaphragm pumps. Spherical and cylindrical correction range of -10 to +10 diopters with 0.1 diopter increments is achieved in less than 15 seconds using wavefront sensor feedback to the pumps. This system can be used in rapid screening of large patient populations especially in the developing countries that lack sufficient facilities and specialist doctors.

All-reflective multiphoton microscope.

We present the design, construction, and characterization of a multiphoton microscope that uses reflective elements for beam shaping and steering. This compact all reflective design removes the adverse effects of dispersion on laser pulse broadening as well as chromatic aberration in the focusing of broadband and multicolored laser sources. The design of this system is discussed in detail, including aberrations analysis via ray-tracing simulation and opto-mechanical design. The resolution of this mirror based all-reflective microscope is characterized using fluorescent microbeads. The performance of the system at multiple wavelengths is investigated along with some potential multiphoton imaging and writing applications.

Dual-wavelength fiber laser operating above 2 µm based on cascaded single-mode-multimode-single-mode fiber structures.

A stable dual-wavelength Tm3+:Ho3+ co-doped fiber laser operating above 2 µm based on cascaded single-mode-multimode-single-mode (SMS) fiber structures is proposed and experimentally demonstrated. Based on the theoretical analysis of the transmission properties of the SMS fiber structure, two cascaded SMS fiber devices with different multimode fiber (MMF) lengths were used in our laser system, where one acted as a long-pass filter to suppress the competitive laser below 2 µm, and the other worked as a band-pass filter to select the specific operating wavelengths of the laser. Dual-wavelength operation of the fiber laser at 2002.8 and 2016.1 nm has been achieved in the experiment with a signal to a noise ratio up to 50 dB.

Cascaded gain fibers for increasing output power and the stimulated Brillouin scattering threshold of narrow linewidth fiber Raman amplifiers.

We show both experimentally and theoretically a method to increase the stimulated Brillouin scattering (SBS) threshold and output power of narrow linewidth fiber Raman amplifiers. This method employs two or more fibers with varying concentrations of the Raman gain material dopant such as GeO2 or P2O5 in silicate-based glasses. These fibers are then cascaded to form an amplifier gain stage, disrupting the buildup of SBS that normally occurs in single continuous fibers. The numerical model shown is applicable to arbitrary amplifier systems for gain stage optimization and increased power scaling. We give experimental results for phosphosilicate fibers that agree well with simulation predictions that support the numerical model used.

Pyrolytic carbon coated black silicon.

Carbon is the most well-known black material in the history of man. Throughout the centuries, carbon has been used as a black material for paintings, camouflage, and optics. Although, the techniques to make other black surfaces have evolved and become more sophisticated with time, carbon still remains one of the best black materials. Another well-known black surface is black silicon, reflecting less than 0.5% of incident light in visible spectral range but becomes a highly reflecting surface in wavelengths above 1000 nm. On the other hand, carbon absorbs at those and longer wavelengths. Thus, it is possible to combine black silicon with carbon to create an artificial material with very low reflectivity over a wide spectral range. Here we report our results on coating conformally black silicon substrate with amorphous pyrolytic carbon. We present a superior black surface with reflectance of light less than 0.5% in the spectral range of 350 nm to 2000 nm.

Passive Q-switching of an all-fiber laser induced by the Kerr effect of multimode interference.

A novel passively Q-switched all-fiber laser using a single mode-multimode-single mode fiber device as the saturable absorber based on the Kerr effect of multimode interference is reported. Stable Q-switched operation of an Er(3+)/Yb(3+) co-doped fiber laser at 1559.5 nm was obtained at a pump power range of 190-510 mW with the repetition rate varying from 14.1 kHz to 35.2 kHz and the pulse duration ranging from 5.69 µs to 3.86 µs. A maximum pulse energy of 0.8 µJ at an average output power of 27.6 mW was achieved. This demonstrates a new modulation mechanism for realizing Q-switched all-fiber laser sources.