Broadband thulium-doped fiber ASE source

Broadband thulium fiber amplifier for spectral region located beyond the L-band

We present the development of a pair of silica-based thulium-doped fiber amplifiers working together in a broad spectral range from 1.65 µm to 2.02 µm. For the one optimized for shorter wavelengths, we designed and prepared optical fiber with a depressed cladding. We show the performance of the amplifiers achieving small-signal gain of at least 10 dB over 350 nm range from 1670 nm to 2020 nm, maximum gain of 40.7 dB with a noise figure as low as 6.45 dB and an optical signal-to-noise ratio of up to 50 dB. To the best of our knowledge, it is the first time that thulium fiber amplifiers of straightforward design without using redundant spectral filters operating efficiently in such a wide spectral region are demonstrated.

CTH:YAG : from laser medium to luminescent concentrator

This work presents what we believe is a new way to use a CTH:YAG crystal for spontaneous emission instead of laser emission. The spontaneous emission is collected in one main direction thanks to a luminescent concentrator configuration. The CTH:YAG is indirectly LED-pumped by a Ce:YAG delivering 3.5 ms pulses at 10 Hz with an energy of 2 J in the visible (550-650 nm). In a configuration optimized for light extraction, the CTH:YAG luminescent concentrator provides a broadband emission between 1.8 µm and 2.1 µm with a unique combination of power (1 W) and brightness (21.2 W/cm2/sr) that could be useful for short-wave infrared (SWIR) lighting applications.

Broadband Profiled Eye-Safe Emission of LMA Silica Fiber Doped with Tm3+/Ho3+ Ions

LMA (Large Mode Area) optical fibers are presently under active investigation to explore their potential for generating laser action or broadband emission directly within the optical fiber structure. Additionally, a wide mode profile significantly reduces the power distribution density in the fiber cross-section, minimizing the power density, photodegradation, or thermal damage. Multi-stage deposition in the MCVD-CDT system was used to obtain the structural doping profile of the LMA fiber multi-ring core doped with Tm3+ and Tm3+/Ho3+ layer profiles. The low alumina content (Al2O3: 0.03wt%) results in low refractive index modification. The maximum concentrations of the lanthanide oxides were Tm2O3: 0.18wt % and Ho2O3: 0.15wt%. The double-clad construction of optical fiber with emission spectra in the eye-safe spectral range of (1.55-2.10 µm). The calculated LP01 Mode Field Diameter (MFD) was 69.7 µm (@ 2000 nm, and 1/e of maximum intensity), which confirms LMA fundamental mode guiding conditions. The FWHM and λmax vs. fiber length are presented and analyzed as a luminescence profile modification. The proposed structured optical fiber with a ring core can be used in new broadband optical radiation source designs.

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.

1.8 µm band broadband hybrid light source employing a combination of a super luminescent diode and thulium-doped fiber amplifier

We have proposed a broadband hybrid light source that combines a super luminescent diode (SLD) and thulium-doped fiber amplifier (TDFA) and operates in the 1.8 µm band. This light source can improve the characteristics of the output spectrum by amplifying the output light of the SLD with TDFA. In this study, we investigate the dependence of the output spectral characteristics of the hybrid broadband light source on the thulium-doped fiber (TDF) length used in the TDFA as well as the output spectra of three newly developed SLDs of 1660, 1690, and 1730 nm bands. In the evaluation of the output bandwidth, there are various definitions of output bandwidth, but we adopted the bandwidth with power density of over -40dBm/0.1nm. This is because it is possible to evaluate in this band with a dynamic range of "30 dB/0.1 nm" by using a general optical spectrum analyzer. The hybrid light source achieves the bandwidth of 332 nm, from 1579 to 1911 nm, and a high total output power of over 15 dBm. The maximum ripple was less than ∼0.1dB, which is similar to the maximum value of that of the SLD, without any deterioration in the ripple characteristics owing to the hybrid configuration of the SLD and TDFA.

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.