Development of pedestal-free large mode area fibers withTm3+ doped silica nanostructured core.

We present the pedestal-free thulium doped silica fiber with a large nanostructured core optimized for fiber lasers. The fiber is composed of over 6 thousand thulium doped silica nanorods with a diameter of 71 nm each which form a nanostructured step-index core. We study the influence of non-continuous distribution in nanoscale active areas on gain, beam quality, and fiber laser performance. The proof-of-concept fiber is effectively single mode for wavelength above 1.8 µm. We demonstrate the performance of the fiber in a laser setup pumped at 792 nm. Single mode laser emission with a slope efficiency of 29% at quasi-continuous output power of 4 W with M2 = 1.3 at the emission spectrum 1880-1925 nm is achieved.

Heat treatment and fiber drawing effect on the luminescence properties of RE-doped optical fibers (RE = Yb, Tm, Ho).

We investigate the influence of various optical fiber fabrication processes on the fluorescence decay of RE ions commonly used in fiber lasers and amplifiers, i.e. Yb3+, Tm3+ and Ho3+. Optical fiber preforms were prepared using the MCVD method combined with Al2O3 nanoparticle doping and subjected to subsequent heat treatment processes such as preform elongation and fiber drawing. The fluorescence decay of RE ions was measured in multiple stages of optical fiber preparation: in an original preform, in an elongated preform (cane), in a standard fiber, and in an overcladded fiber. It was found that heat treatment processing of the preforms generally leads to a faster fluorescence decay, which can be explained by the diffusion of dopants and clustering of RE ions. The fiber drawing exhibited a greater effect compared to preform elongation, which was ascribed to a faster cooling rate of the process. In general, the heat treatment of RE-doped silica glass preforms leads to the decline of fluorescence decay.

Ex-Vivo Measurement of the pH in Aqueous Humor Samples by a Tapered Fiber-Optic Sensor.

A practical demonstration of pH measurement in real biological samples with an in-house developed fiber-optic pH sensor system is presented. The sensor uses 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) fluorescent dye as the opto-chemical transducer. The dye is immobilized in a hybrid sol-gel matrix at the tip of a tapered optical fiber. We used 405 nm and 450 nm laser diodes for the dye excitation and a photomultiplier tube as a detector. The sensor was used for the measurement of pH in human aqueous humor samples during cataract surgery. Two groups of patients were tested, one underwent conventional phacoemulsification removal of the lens while the other was subjected to femtosecond laser assisted cataract surgery (FLACS). The precision of the measurement was ±0.04 pH units. The average pH of the aqueous humor of patients subjected to FLACS and those subjected to phacoemulsification were 7.24 ± 0.17 and 7.31 ± 0.20 respectively.

Femtosecond Laser Plane-by-Plane Inscribed Cavity Mirrors for Monolithic Fiber Lasers in Thulium-Doped Fiber.

A monolithic fiber laser operating in the short wavelength infrared that is suitable for CO2 gas sensing applications is proposed and presented. The current study reports a laser design based on the direct inscription of a monolithic Fabry-Perot (FP) cavity in a thulium-doped optical fiber using the femtosecond laser (FsL) plane-by-plane inscription method to produce the cavity mirrors. The FP cavity was inscribed directly into the active fiber using two wavelength-identical fiber Bragg gratings (FBGs), one with high and one with low reflectivity. Initially the effective length of the fiber was defined using a single high reflectivity FBG and subsequently a very weak FBG was inscribed at the other end of the fiber in order to demonstrate a fully monolithic fiber laser. All fiber lasers were designed for continuous wave operation at 1950 nm and characterized with respect to the power output, slope efficiency, stability, and effective resonator length. The performance of the presented monolithic laser cavities was evaluated using the same active fiber as a reference fiber spliced to FBGs inscribed in passive fiber; an improvement exceeding 12% slope efficiency is reported for the presented monolithic laser.

Active Optical Fibers and Components for Fiber Lasers Emitting in the 2-µm Spectral Range.

Laser sources emitting in the infrared range at around 2 µm are attracting great interest for a variety of applications like processing of transparent thermoplastic polymers in industry as well as plenty of applications in medicine, spectroscopy, gas sensing, nonlinear frequency conversion to the mid-infrared, to mention a few. Of late, fiber lasers compared to other kinds of lasers benefit from their all-fiber design, leading to a compact, robust, and well thermally manageable device. Particularly, thulium- and holmium-doped fiber lasers are the first choice in fiber lasers emitting light around 2 µm. In this paper, we give an overview of our recent results in the research on thulium- and holmium-doped optical fibers, fiber lasers, and related research topics in the 2-µm spectral range. In particular, we present, to our knowledge, the first results of improvement of pump absorption in double-clad fibers thanks to the fiber twist frozen during drawing. Finally, a brief demonstration of material processing by thulium all-fiber laser operating at 2 µm is presented.

Impact of shaping optical fiber preforms based on grinding and a CO2 laser on the inner-cladding losses of shaped double-clad fibers.

We experimentally compared for the first time, two techniques of optical fiber preform shaping based on the mechanical grinding and thermal CO2 laser processing from the point of the inner-cladding losses. The shaped preforms were fabricated of coreless pure silica technical rods as well as high purity silica Heraeus F300 rods and drawn them into coreless multimode fibers with various inner-cladding geometries coated with a low index fluorinated polymers. The background losses of the fibers were measured via the cut-back method and compared to the losses of the unshaped fibers with a circular cross-section. Results show that both preform-shaping techniques would induce additional losses in the inner-cladding. High surface scattering losses were observed in the mechanically-grinded fibers. On the other hand, the mechanical grinding retains the advantage of a significant reduction of attenuation peaks attributed to OH-groups that penetrated into the preform surface during the preform collapse. On the contrary, CO2 laser thermal-shaping provides the advantage of quick, fully automated shaping with smooth surface finish and induces much lower scattering losses, but it is not so effective in removing water penetrated surface layer of the preform so that OH-groups diffuse deeper towards the preform center. Additionally, laser thermal-shaping allows processing the preform to complex shapes which are more effective in scrambling cladding modes. Some of the absorption peaks of OH-groups and fluorinated polymers may be rather close to common pumping wavelengths and this should be considered in the design of the double-clad fibers and selection of proper shaping technology.

In vivo testing of a bioresorbable phosphate-based optical fiber.

Optical fibers have recently attracted a noticeable interest for biomedical applications because they provide a minimally invasive method for in vivo sensing, imaging techniques, deep-tissue photodynamic therapy or optogenetics. The silica optical fibers are the most commonly used because they offer excellent optical properties, and they are readily available at a reasonable price. The fused silica is a biocompatible material, but it is not bioresorbable so it does not decompose in the body and the fibers must be ex-planted after in vivo use and their fragments can present a considerable risk to the patient when the fiber breaks. In contrast, optical fibers made of phosphate glasses can bring many benefits because such glasses exhibit good transparency in ultraviolet-visible and near-infrared regions, and their solubility in water can be tailored by changing the chemical composition. The bioresorbability and toxicity of phosphate glass-based optical fibers were tested in vivo on male laboratory rats for the first time. The fiber was spliced together with a standard graded-index multi-mode fiber pigtail and an optical probe for in vitro pH measurement was prepared by the immobilization of a fluorescent dye on the fiber tip by a sol-gel method to demonstrate applicability and compatibility of the fiber with common fiber optics.

Self-swept holmium fiber laser near 2100 nm.

Self-sweeping of laser wavelength corresponding to holmium emission near 2100 nm is reported. The sweeping occurred in ~4 nm interval with rate ~0.7 nm/s from longer towards shorter wavelengths. Origins of the selection of the sweeping direction are discussed. The laser wavelength drift with time was registered by Fourier transform infrared spectrometer. To our knowledge it is the first observation of self-swept fiber laser beyond 2000 nm.

All-fiber Ho-doped mode-locked oscillator based on a graphene saturable absorber.

In this Letter, we demonstrate a graphene mode-locked, all-fiber Ho-doped fiber laser generating 1.3 nJ energy pulses directly from the oscillator. The graphene used as a saturable absorber was obtained via chemical vapor deposition on copper substrate and immersed in a poly(methyl methacrylate) support. The laser generated ultrashort soliton pulses at 2080 nm with bandwidth up to 6.1 nm. The influence of the output coupling ratio and the SA modulation depth on the mode-locking performance was also investigated.

Enhanced pump absorption efficiency in coiled and twisted double-clad thulium-doped fibers.

Results of the first experimental demonstration of the recently proposed technique for improvement of the pump absorption in double-clad fibers by their simultaneous coiling and twisting are reported. The peak absorption (14 dB) of 3-m long hexagonal thulium-doped fiber was increased by 8 dB by its simultaneous coiling and twisting. Explanation of the effect is given by numerical modelling of the pump absorption in hexagonal and panda-type double-clad fibers. Improvement of fiber laser performance was also proved. The slope efficiency increased from 19.6% of the straight fiber to 23.9% of the coiled only fiber and 29.4% of the simultaneously coiled and twisted fiber.

Wideband thulium-holmium-doped fiber source with combined forward and backward amplified spontaneous emission at 1600-2300 nm spectral band.

We have experimentally demonstrated two extremely wideband amplified spontaneous emission (ASE) sources. High bandwidth is achieved by combining the backward and forward ASEs generated in thulium-holmium-doped fiber using appropriate wideband couplers. The ASE source optimized for flat spectral power density covers a spectral range from 1527 to 2171 nm at a -10 dB level. The ASE source optimized for spectroscopy features an enhancement with respect to single-mode fiber (SMF) coupled halogen lamps within the spectral range from 1540 nm to more than 2340 nm covering the 800 nm bandwidth.

In vivo optical detection of pH in microscopic tissue samples of Arabidopsis thaliana.

Minimally invasive in vivo measurement of pH in microscopic biological samples of µm or µl size, e.g. plant cells, tissues and saps, may help to explain complex biological processes. Consequently, techniques to achieve such measurements are a focus of interest for botanists. This paper describes a technique for the in vivo measurement of pH in the range pH5.0 to pH7.8 in microscopic plant tissue samples of Arabidopsis thaliana based on a ratiometric fluorescence method using low-loss robust tapered fiber probes. For this purpose tapered fiber probes were prepared and coated with a detection layer containing ion-paired fluorescent pH-transducer 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (c-HPTS). A fluorescence ratiometric approach was employed based on excitation at 415 nm and 450 nm and on the comparison of the fluorescence response at 515 nm. The suitability of tapered fiber probes for local detection of pH between 5.0 and 7.8 was demonstrated. A pH sensitivity of 0.15 pH units was achieved within the pH ranges 5.0-5.9 and 7.1-7.8, and this was improved to 0.04 pH units within the pH range 5.9-7.1. Spatial resolution of the probes was better than 20 µm and a time response within 15-20s was achieved. Despite the minute dimensions of the tapered fiber probes the setup developed was relatively robust and compact in construction and performed reliably. It has been successfully employed for the in vivo local determination of pH of mechanically resistant plant tissues of A. thaliana of microscopic scale. The detection of momentary pH gradients across the intact plant seems to be a good tool for the determination of changes in pH in response to experimental treatments affecting for example enzyme activities, availability of mineral nutrients, hormonal control of plant development and plant responses to environmental cues.

Theoretical modeling of fiber laser at 810 nm based on thulium-doped silica fibers with enhanced 3H4 level lifetime.

A compact upconversion fiber laser operating around 810 nm is proposed using thulium-doped silica-based fiber with locally modified thulium environment by high alumina codoping. Using a comprehensive numerical model of thulium doped fiber we investigate performance of the proposed laser. Comparison with two other thulium hosts, fluoride glass and standard silica, is presented. Efficient lasing can be expected even for silica based fiber for specific ranges of the fiber and laser cavity parameters, especially when 3H4 lifetime is enhanced. With moderate pump power of 5 W at wavelength of 1064 nm, the predicted output power of the upconversion laser is about 2 W at 810 nm.

Experimental demonstration of novel end-pumping method for double-clad fiber devices.

We present experimental demonstration of an end-pumping scheme based on splicing the multimode pump and single-mode signal fibers directly to a double-clad fiber with a tailored cross section. The method is used to pump a double-clad, erbium- and ytterbium-doped, fiber ring laser. The efficiency of the end-pumping method is tested by determining the slope efficiencies of the fiber ring laser and the fiber laser in a Fabry-Perot configuration. Comparable slope efficiencies are found when both laser configurations have similar output coupler ratios. The developed pumping scheme and double-clad fiber can find applications in cost-effective power fiber amplifiers and lasers.