Tm-doped Silica Fiber Research Articles

Hyperspectral absorption of water around 2 μm based on a boradband tunable, narrow linewidth Tm-doped fiber laser

The 1.8–2.0 μm waveband contains abundant absorption lines of water, which are much stronger than those in the traditional 1.3–1.5 μm waveband, exhibiting huge potentials for absorption spectrum applications of water. In the hyperspectral absorption spectrum, physical parameters of the target molecule can be derived from lots of absorption lines within a broadband scanning range, achieving the results more robust, accurate and versatile than the results from the conventional tunable diode laser absorption spectrum in which only one or two absorption lines are used. The key to hyperspectral absorption is the development of broadband tunable, narrow linewidth laser sources emitting in the wavelength range of interest. With a tunable fiber FP filter and a fiber saturable absorber, a Tm-doped fiber laser is established, featuring broadband tenability and narrow linewidth. Taking advantage of the re-absorption characteristics of Tm-doped silica fibers, a wavelength tuning range covering 60 nm from 1910–1970 nm is obtained through the appropriately designing of the active fiber length. The measured laser linewidth at steady state is smaller than 0.1 nm, which is suitable for water absorption spectrum. Hyperspectral absorption measurements of water in air and alcohol flame are conducted. In room-temperature air, more than 40 absorption lines are recognized within a tuning range of 1910–1965 nm, while, in alcohol flame, the number of detected lines reaches about 50. Comparison with the HITRAN2016 database gives a laser linewidth of about 0.06 nm which is very close to the static linewidth measured by an OSA. The temperature of the air is derived to be 298 K with a water mole fraction of about 2%, which is consistent with the measurement of the hygrothermograph. And the calculation indicates an alcohol flame temperature of about 1220 K, which is very close to the measurement result of the thermocouple.

Passively Q - Switched Linear Cavity IR Fibre Laser via Nonlinear Polarization Rotation

Abstract: Setup for self-starting passive Q-switch operation of IR fibre laser via nonlinear polarization rotation (NPR) was demonstrated for the first time for linear cavity. A Q-switched pulse duration of ~ 600 ns, a maximum peak power of ~16 W and an average power of 408 mW have been obtained at 223 kHz repetition rate for 5.1 W pump power from 1319 nm CW Nd: YAG laser launched to ~ 2.78 m unidirectional single-clad Tm-doped silica fibre linear cavity using only flat polarizer and feedback mirror to induce NPR. The dependence of the fibre laser output characteristics on the polarization angle of the polarizer is also reported. However, self induced passive Q-switching in linear fibre laser cavities is only observed in fibres with angled cleaved ends and with lengths that are around the optimum length for CW operation. Keywords: IR fibre laser, Passive Q-switching, Nonlinear polarization rotation, Linear cavity, Ring cavity. PACS: Fiber lasers, 42.55.Wd, Q-switching, 42.60.Gd.

Efficient multi-watt 1720 nm ring-cavity Tm-doped fiber laser.

Using commercial Tm-doped silica fiber and 1570-nm in-band pump source, we demonstrated an efficient 1720-nm all-fiber laser with ring-cavity configuration. The theoretical model based on rate equations was built up to analyze the laser performance of Tm-doped fiber, which exhibits strong absorption in the 1.7-μm region. The results show that efficient laser operation can be achieved through the optimization of output coupling and the length of Tm-doped fiber. An experimental investigation was performed and agreed with the calculation. By using homemade couplers, we experimentally achieved 2.36-W laser output at 1720 nm under a 6-W launched pump. The slope efficiency with respect to the absorbed pump power and optical efficiency were 50.2% and 39.3%, respectively. Due to the employment of a ring resonator, a narrow laser linewidth of ∼4 GHz at maximum output power was observed.

Experimental Performance of a Broadband Dual-Stage 1950 nm PM Single-Clad Tm-Doped Fiber Amplifier

We report the experimental performance of a packaged dual-stage all-single-clad broadband polarization maintaining (PM) Tm-doped silica fiber amplifier (TDFA) with a high signal output power of 2.2 W at 1909 and 1951 nm. Our novel design incorporates an uncooled 940 nm multimode semiconductor pump laser source. Combined high gain of >46 dB, wide operating bandwidth of 1875-2000 nm, low noise figure of 7 dB, high OSNR of >54 dB/0.1 nm, and a PER of >20 dB are achieved with our TDFA.

Bleaching of photodarkening in Tm-doped silica fiber with deuterium loading.

We demonstrate the rapid photodarkening (PD) phenomenon in Tm-doped fiber (TDF) core pumped by a laser at 1080 nm and the bleaching effect of deuterium (${{\rm D}_2}$D2) on PD TDF. By ${{\rm D}_2}$D2 loading for seven days, the PD-induced excess loss (PIEL) in the visible (VIS) and near-infrared (NIR) region have been largely eliminated, and no degradation was observed within 30 days. PD resistance of the ${{\rm D}_2}$D2 pretreated TDF has been investigated as well. The formation of color centers based on defects and precursors in the silica matrix and the mechanism of ${{\rm D}_2}$D2 bleaching are discussed.

Elimination of radiation damage in Tm-doped silica fibers based on the radical bleaching of deuterium loading

We present the deuterium-loading induced radical recovery of 2µm laser fiber and absorption efficiency in thulium (Tm)-doped silica fibers under gamma irradiation in the range of 185Gy-1000Gy. The optical-optical efficiency and fiber cladding absorption spectra of the Tm-doped silica fibers with multi-dose irradiation and deuterium treatment have been measured. It was found that the reduction of the optical-optical efficiency and the additional absorption, both induced by irradiation, were positively associated with the dose. Under the deuterium loading, the efficiencies of the multi-irradiated TDFs could be restored to above 91.9% of the pristine, and the radiation induced absorptions were almost completely vanished. The results indicate that deuterium-loading has an extensive prospect for the radiation hardness of Tm-doped silica fiber.

Broadband 2-W Output Power Tandem Thulium-Doped Single Clad Fiber Amplifier at 2 $\mu$ m

We report experimental and simulated performance of a tandem (dual-stage) Tm-doped silica fiber amplifier with a high signal output power of 2.6 W in the 2- $\mu \text{m}$ band. Combined high dynamic range, high gain, low noise figure, and high optical signal to noise ratios are achieved with our design.

Radical passive bleaching of Tm-doped silica fiber with deuterium.

We demonstrate the almost complete 2µm laser power recovery of the gamma-ray-irradiated thulium (Tm)-doped silica fiber under deuterium loading. The optical-optical slope efficiency and the cladding absorption spectra of the Tm-doped fiber with gamma-ray irradiation and deuterium treatment have been measured for comparison. It was found that the slope efficiency of the irradiated Tm-doped fiber could be recovered to 96.1% of the pristine after deuterium bleaching, which significantly degraded from 60.7% to 25.3% after irradiation. Meanwhile, the additional absorption attenuation of the irradiated Tm-doped with D2 treatment completely vanished. Based on the comprehensive comparison of cladding absorption spectra, the probable mechanism of the deuterium bleaching effect on irradiated Tm-doped fiber has also been discussed.

2 μm mode-locking laser performances of sol-gel-fabricated large-core Tm-doped silica fiber

High-power ultrafast fiber lasers operating at the 2 μm wavelength are extremely desirable for material processing, laser surgery, and nonlinear optics. Here we fabricated large-core (LC) double-cladding Tm-doped silica fiber via the sol-gel method. The sol-gel-fabricated Tm-doped silica (SGTS) fiber had a large core diameter of 30 μm with a high refractive index homogeneity (Δn=2×10−4). With the newly developed LC SGTS fiber as the gain fiber, high-power mode-locking was realized. By using a semiconductor saturable absorber mirror (SESAM) as a mode locker, the LC SGTS fiber oscillator generated mode-locked pulses with an average output power as high as 1.0 W and a pulse duration of 23.9 ps at the wavelength of 1955.0 nm. Our research results show that the self-developed LC Tm-doped silica fiber via the sol-gel method is a promising gain fiber for generating high-power ultrafast lasers in the 2 μm spectral region.

Tm-doped fiber laser resonantly diode-cladding-pumped at 1620 nm

We report the first demonstration of an efficient, high power, resonantly (in-band) diode-cladding-pumped Tm-doped fiber laser operating on the 3F4 ⇒ 3H6 transition of Tm3+ ion. The laser, pumped by a fiber coupled laser diode module at ~1620 nm, delivered ~15 W of power at 1930 nm with a slope efficiency of 67% versus the absorbed pump power. This presents, to the best of our knowledge, the highest slope efficiency and the highest output power reported so far for resonantly diode-cladding-pumped 2 µm fiber lasers based on double-clad Tm-doped silica fibers. These very preliminary results, obtained with commercial double-clad Tm-doped fibers, unoptimized for in-band pumping at the peak of resonant absorption, indicate a very high potential of resonantly diode-cladding-pumped Tm fiber lasers for major power scaling unaffected by photodarkening. Fiber optimization for resonant pumping at the maximum of the Tm3+ 3H6 ⇒ 3F4 absorption band in silica (1610–1710 nm) can lead to a new generation of Tm fiber lasers with power and wall-plug efficiency, competing with those of high power tandem-pumped Tm-doped fiber lasers, but potentially with a lighter weight and smaller dimensions.

Solid Tellurite Optical Fiber Based on Stack-and-Draw Method for Mid-Infrared Supercontinuum Generation

Broadband, high-power mid-infrared (mid-IR) sources are critical for many applications. Compared to alternatives such as fluorides and chalcogenides, tellurite fibers are more robust and can handle much higher power. Tellurite fibers also have high nonlinearity and a material zero dispersion close to 2 µm, making them ideal for nonlinear processes pumped by Tm-doped silica fiber lasers. In this work, we have demonstrated solid tellurite fibers fabricated by a stack-and-draw process and investigated their potential for broadband mid-IR supercontinuum generation. We have identified that fibers with low dispersion are beneficial and that low residual hydroxyl (OH) is critical for broadband mid-IR supercontinuum generation in tellurite fibers pumped at ~2 µm.

0.5-GHz Repetition Rate Fundamentally Tm-Doped Mode-Locked Fiber Laser

We demonstrate a passively mode-locked Tm-doped silica fiber laser with 535-MHz fundamental repetition rate by employing a semiconductor saturable absorber mirror. With in-band pumping at 1590 nm, the stable mode-locked fiber laser produces up to 50 mW at an incident pump power of 558 mW. The laser operating at 1938 nm has a spectral bandwidth of 0.72 nm. After amplification, the laser pulse width is measured to be $\sim 7.9$ ps, and the time-bandwidth product is $\sim 0.46$ .

High-power ultralong-wavelength Tm-doped silica fiber laser cladding-pumped with a random distributed feedback fiber laser.

We demonstrated a high-power ultralong-wavelength Tm-doped silica fiber laser operating at 2153 nm with the output power exceeding 18 W and the slope efficiency of 25.5%. A random distributed feedback fiber laser with the center wavelength of 1173 nm was employed as pump source of Tm-doped fiber laser for the first time. No amplified spontaneous emissions or parasitic oscillations were observed when the maximum output power reached, which indicates that employing 1173 nm random distributed feedback fiber laser as pump laser is a feasible and promising scheme to achieve high-power emission of long-wavelength Tm-doped fiber laser. The output power of this Tm-doped fiber laser could be further improved by optimizing the length of active fiber, reflectivity of FBGs, increasing optical efficiency of pump laser and using better temperature management. We also compared the operation of 2153 nm Tm-doped fiber lasers pumped with 793 nm laser diodes, and the maximum output powers were limited to ~2 W by strong amplified spontaneous emission and parasitic oscillation in the range of 1900–2000 nm.

Active radiation hardening of Tm-doped silica fiber based on pump bleaching.

Tm-doped fiber laser or amplifier can be applied in varied adverse environments. In this work, we demonstrate the pump bleaching of Tm-doped silica fiber with 793nm pump source under gamma-ray irradiation in the range 50Gy-675Gy. The recovery time, the fiber slope efficiency and the fiber cladding absorption spectra after irradiation and bleaching have been measured. It is found that the recovery time and radiation induce absorption are positively associated with doses, however, the fiber slope efficiency of irradiated TDF and bleached TDF are both negatively correlated with doses. Based on the simulation of the fiber core temperature, the probable mechanism of pump bleaching is also discussed.

400 mW narrow-linewidth Tm-doped silica fiber laser output near 1750 nm with volume Bragg grating.

A type of Tm-doped silica fiber laser with narrow-linewidth and output wavelength near 1750 nm was firstly presented, by using a 1550 nm Er-doped fiber laser pump source and a volume Bragg grating (VBG). By means of a 15 cm Tm-doped fiber, a 400 mW continuous wave (cw) at ~1750 nm with FWHW of ~54 pm was generated, corresponding to an average slope efficiency up to 23.5% with respect to absorbed pump power. The influence of gain fiber length on the spectrum wavelength has been investigated in detail. All experimental results demonstrate that this fiber laser will be an effective and promising pump source for mid-IR laser output.

Fabrication of Tm-Doped Fibers for High Power and 121 W Output All-Fiber Tm-Doped Fiber Laser**Supported by the National High-Technology Research and Development Program of China under Grant No 2013AA031501, and the National Natural Science Foundation of China for Director Fund of WNLO.

We fabricate the Tm-doped double cladding silica fiber by using the vapor-solution hybrid-doping method, then build up an all-fiber Tm-doped fiber laser which can provide the output power of up to 121 W, corresponding to a slope efficiency of 51% and an optical-optical efficiency of 48%. By using the domestic Tm-doped fiber, it is the first time a hundred-watt level output at 1915nm has been achieved, to the best of our knowledge. The thermal effect of Tm-doped fiber laser is also analyzed.

2 µm actively Q-switched all fiber laser based on stress-induced birefringence and commercial Tm-doped silica fiber

Abstract An actively Q-switched all-fiber laser operating at the wavelength of 1920 nm is experimentally demonstrated based on a readily accessible commercial Tm-doped silica fiber. The Q-switching is achieved by polarization modulation through the stress-induced birefringence using a piezoelectric transducer (PZT) as the Q-switcher. The pulsed fiber laser can be operated with the repetition rates ranging from tens of kilohertz to nearly two hundreds kilohertz with milliwatt average output power.

High-power single-stage thulium-doped superfluorescent fiber source

In this paper, we report a high-power thulium (Tm)-doped superfluorescent fiber source (SFS) in the 2-μm spectral region. The SFS is based on double angle-cleaved facet operation and uses a simple single-stage geometry. The copropagating amplified spontaneous emission (ASE) yields a maximum output of 20.7 W at a center wavelength of 1,960.7 nm, with a full width at half maximum (FWHM) of ~45 nm. The counterpropagating ASE yields a maximum output of 25.2 W at a center wavelength of 1,948.2 nm, with a FWHM of ~50 nm. The maximum combined output of the SFS is as much as 45.9 W, which corresponds to a slope efficiency of 38.9 %. In addition, a model of the ~2 μm SFS in Tm-doped silica fibers pumped at ~790 nm is developed, and the influence of fiber length and end-facet reflectivity on the ASE output performance and the parasitic lasing threshold are studied numerically.

A Versatile Model for Temperature-Dependent Effects in Tm-Doped Silica Fiber Lasers

We have derived a model to provide a unique insight into the temperature-dependent effects in Tm-doped silica fiber lasers with consideration of important energy transfer processes and full spectra of laser transition. By fitting experimental measurements, the dependence of emission and absorption cross sections on temperature was characterized. With careful thermal management to control the influence of thermomechanical and thermooptical effects, a good agreement was achieved between simulations and experiments, even in the kilowatt power domain. On the basis of the model, the investigation shows that the thermal distribution of ions over the Stark sublevels contributes a higher threshold and a lower laser power as the temperature increase. Fortunately, this degradation of laser performance can be largely weakened when higher pump intensity is employed. The simulations also show that lower output coupling and longer operational wavelength can mitigate the output power variation arising from the temperature. The model was further used to analyze the dependence of laser spectrum on the fiber length and pump power when temperature changes. In addition, the laser output power and temperature distribution in kW-level are also studied.

Rapid, electronically controllable transverse mode selection in a multimode fiber laser

A novel technique for the electronically-controllable generation and switching of transverse modes within a multi-mode fiber laser oscillator is presented. Preliminary results demonstrate individual transverse mode lasing and fast switching between modes with watt-level output powers. When applied to a core-pumped Tm-doped silica fiber laser with a multimode core the fundamental mode (LP₀₁), the next higher order mode (LP₁₁), or a donut-shaped LP₁₁ superposition were selectively excited with power levels in excess of 5 W. Fast switching between LP₀₁ and LP₁₁ modes at up to 20 kHz was also realized.