Thulium-doped Silica Fiber Research Articles

High Average Power Thulium-Doped Silica Fiber Lasers: Review of Systems and Concepts

Thulium-doped fiber lasers (TDFLs) have had the second highest growth in average output power next to ytterbium-doped fiber lasers. This has been enabled by access to high power, high brightness ∼790-nm pump diodes in conjunction with the cross-relaxation process that improves laser efficiency. While numerous high power TDFLs have been recently demonstrated, a 1-kW result from 2010 remains the highest output power system reported to date. This paper reviews these systems and the concepts behind high power TDFLs. The spectroscopic properties of Tm3+ -doped silica are first detailed, revealing complex processes and large variations among published measurements. Notable multi-100 W TDFLs are then summarized, with outputs ranging from 1908 to 2130 nm. Another route for power scaling is to in-band pump with another TDFL to enable >90% efficiencies. Both 790- and 1900-nm pumped TDFL architectures are theoretically modeled based on currently available systems. Hindered by high background losses and available pump sources, achieving >4 kW like ytterbium-fiber systems will be a substantial challenge.

Enhancement on the generation of amplified spontaneous emission in thulium-doped silica fiber at 2 [formula omitted

This paper investigates the generation of the amplified spontaneous emission (ASE) from thulium-doped silica fiber pumped at 1570 nm and 793 nm. The developed model provides the ASE spectral power as functions of the fiber length and the pump power under single-pass forward (SPF) and double-pass bi-directional (DPB) pumping schemes. A broadband ASE source generated by solving a set of rate and propagation equations for 1570 nm and 793 nm pumping transitions, and taking into account the influences of cross relaxation (CR) in both pump schemes. Our findings reveal that for 1570 nm pumping scheme, CR transition reduces the ASE generation. In addition, longer fluorescent lifetime increases the ASE power and reduces the pump power threshold. We numerically enhanced the generation of ASE broadband source from 1570 nm/1570 nm DPB pumping scheme. Our results show that 1570 nm/1570 nm DPB produces broadband ASE source with high slope efficiency and broader spectral bandwidth than that in SPF configuration. As a result, a 1570 nm/1570 nm DPB pump scheme is a suitable configuration to obtain higher power efficiency and a wider broadband source for the chosen thulium-doped silica fiber characteristics.

3 W narrow-linewidth ultra-short wavelength operation near 1707 nm in thulium-doped silica fiber laser with bidirectional pumping

We have developed an ultra-short wavelength operation of all-fiber Tm-doped laser system, in which the laser system was structured in a FBGs based linear cavity with FBG associated bidirectional pump mechanism at 1550 nm. The laser yielded 3.15 W of continuous-wave output at 1706.75 nm with a narrow-linewidth of ~50 pm. A high reflectivity FBG (HR-FBG) at 1550 nm was used to reflect the residual pump light and achieve the bidirectional pump mechanism. The maximum slope efficiency was improved from 23.2% without HR-FBG to 42.1% with HR-FBG at 1550 nm and the lowest pump threshold was reduced from 1.6 to 0.8 W accordingly. In the experiment, we have also investigated the dependencies of the slope efficiencies and pump threshold of the laser versus the length of active fiber and reflectivity of the output mirror (FBG) in detail. It is indisputable that the development of 1.7 μm silicate fiber lasers with Watt-level output power will open up a number of promising application windows.

Theoretical Model of a Thulium-Doped Fiber Amplifier Pumped at 1570 nm and 793 nm in the Presence of Cross Relaxation

The static behavior of a thulium-doped fiber amplifier (TDFA) operating around 2 μm region at different pump wavelengths is investigated in this paper. A theoretical model is built up by solving a set of rate and propagation equations with considering the effect of cross relaxation mechanism. The developed model provides the influences of the amplified spontaneous emission noise, seed wavelength, and the thulium-doped fiber length into the TDFA performance. Simulation results indicate that the TDFA performance with pump at 1570 nm is more efficient than pump at 793 nm for core pumped thulium-doped silica fiber. Our findings show that the maximum gain reaches up to 30 dB with a 27 dBm pump power when a -10 dBm seed wavelength at 1840 nm is used. In contrast to indirect pumping at 793 nm, only 22 dB maximum gain is achieved under the same conditions. The model is also validated with previous experimental work. Our simulations are consistent with the experimental findings with small variations.

High-peak-power single-oscillator actively Q-switched mode-locked Tm3+-doped fiber laser and its application for high-average output power mid-IR supercontinuum generation in a ZBLAN fiber.

A single-oscillator actively Q-switched mode-locked (QML) thulium-doped silica fiber laser is presented and used to pump a ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) fiber for mid-infrared (mid-IR) supercontinuum (SC) generation. The fiber laser provided high-peak-power levels directly from the oscillator delivering single mode-locked pulse energies up to 48 μJ, being 2-4 orders of magnitude higher than conventional continuous wave mode-locked lasers. By pumping a ZBLAN fiber specially designed for high-output-power SC generation, 7.8 W have been achieved in all spectral bands with a spectrum extending to 4.2 μm.

Microscopic analysis of laser-induced proximal fiber tip damage during holmium:YAG and thulium fiber laser lithotripsy

The thulium fiber laser (TFL) is being studied as an alternative to the standard holmium:YAG laser for lithotripsy. The TFL beam originates within an 18-μm-core thulium-doped silica fiber, and its near single mode, Gaussian beam profile enables transmission of higher laser power through smaller (e.g., 50- to 150-μm core) fibers than possible during holmium laser lithotripsy. This study examines whether the more uniform TFL beam profile also reduces proximal fiber tip damage compared with the holmium laser multimodal beam. Light and confocal microscopy images were taken of the proximal surface of each fiber to inspect for possible laser-induced damage. A TFL beam at a wavelength of 1908 nm was coupled into 105-μm-core silica fibers, with 35-mJ energy, and 500-μs pulse duration, and 100,000 pulses were delivered at each pulse rate setting of 50, 100, 200, 300, and 400 Hz. For comparison, single use, 270-μm-core fibers were collected after clinical holmium laser lithotripsy procedures performed with standard settings (600 mJ, 350 μs, 6 Hz). Total laser energy, number of laser pulses, and laser irradiation time were recorded, and fibers were rated for damage. For TFL studies, output pulse energy and average power were stable, and no proximal fiber damage was observed at settings up to 35 mJ, 400 Hz, and 14 W average power (n=5). In contrast, confocal microscopy images of fiber tips after holmium lithotripsy showed proximal fiber tip degradation, indicated by small ablation craters on the scale of several micrometers in all fibers (n=20). In summary, the proximal fiber tip of a 105-μm-core fiber transmitted up to 14 W of TFL power without degradation, compared to degradation of 270-μm-core fibers after transmission of 3.6 W of holmium laser power. The smaller and more uniform TFL beam profile may improve fiber lifetime, and potentially translate into lower costs for the surgical disposables as well.

Q‐switched thulium‐doped fibre ring lasers operating at 1.9 μm using multi‐walled carbon nanotubes in UV‐curing resin

The passively Q-switched operation of all-fibre thulium fibre lasers using UV-curing resin doped with multi-walled carbon nanotubes as a saturable absorber is demonstrated. A single-mode thulium-doped silica fibre is pumped by a laser diode emitting at 1.63 µm through a wavelength-division multiplexing coupler. Unidirectional laser output from the fibre ring resonator is attained by using the retroreflector at the counter-directional output port. The fibre laser exhibits 1.9 µm wavelength pulsed oscillation at a repetition rate of 7.2 kHz. When the pump power is increased to 161 mW, the output pulse power is measured by 150 mW with a pulse width of 3.2 µs.

Single-frequency fiber laser at 1950 nm based on thulium-doped silica fiber.

A single-frequency fiber laser operating at 1950 nm has been demonstrated in an all-fiber distributed Bragg reflection laser cavity by using a 1.9 cm commercially available thulium-doped silica fiber, for the first time, to the best of our knowledge. The laser was pumped by a 793 nm single-mode diode laser and had a threshold pump power of 75 mW. The maximum output power of the single longitudinal mode laser was 18 mW and the slope efficiency with respect to the launched pump power was 11%. Moreover, the linewidth and relative intensity noise at different pump power have been measured and analyzed.

Multiwavelength thulium-doped silica fiber laser incorporating an all-fiber phase modulator

A novel multiwavelength thulium-doped silica fiber laser incorporating an all-fiber phase modulator is first presented in 2μm band. In the laser cavity, a phase modulator with a modulation frequency is efficient and beneficial to overcome the mode competitions. A polarization maintaining fiber Sagnac loop mirror is used as the filter. Large number of lasing wavelengths can easily and conveniently obtained at a proper modulation frequency under room temperature. The fiber laser can achieve ten output channels within a 15dB bandwidth from 1982nm to 1998nm.

The thermoluminescence characteristics and the glow curves of Thulium doped silica fiber exposed to 10 MV photon and 21 MeV electron radiation

The thermoluminescence (TL) glow curves and kinetics parameters of Thulium (Tm) doped silica cylindrical fibers (CF) are presented. A linear accelerator (LINAC) was used to deliver high-energy radiation of 21MeV electrons and 10MV photons. The CFs were irradiated in the dose range of 0.2–10Gy. The experimental glow curve data was reconstructed by using WinREMS. The WinGCF software was used for the kinetic parameters evaluation. The TL sensitivity of Tm-doped silica CF is about 2 times higher as compared to pure silica CF. Tm-doped silica CF seems to be more sensitive to 21MeV electrons than to 10MV photons. Surprisingly, no supralinearity was displayed and a sub-linear response of Tm-doped silica CF was observed within the analyzed dose range for both 21MeV electrons and 10MV photons. The Tm-doped silica CF glow curve consists of 5 individual glow peaks. The Ea of peak 4 and peak 5 was highly dependent on dose when irradiated with photons. We also noticed that the electron radiation (21MeV) caused a shift of glow peak by 7–13°C to the higher temperature region compared with photons radiation (10MV). Our Tm-doped fibers seem to give high TL response after 21MeV electrons, which gives around 2 times higher peak integral as compared with 10MV photon radiation. We concluded that peak 4 is the first-order kinetic peak and can be used as the main dosimetric peak of Tm-doped silica CF.

Switchable and spacing-tunable dual-wavelength thulium-doped silica fiber laser based on a nonlinear amplifier loop mirror.

A kind of switchable and spacing-tunable dual-wavelength thulium-doped silica fiber laser based on a nonlinear amplifier loop mirror is presented and experimentally demonstrated. By adjusting the polarization controllers (PCs), stable dual-wavelength operation is obtained at the 2μm band. The optical signal-to-noise ratio (OSNR) is better than 56dB. The wavelength tuning is performed by applying static strain into the fiber Bragg grating. A tuning range from 0 to 5.14nm is achieved for the dual-wavelength spacing. By adjusting the PCs properly, the fiber laser can also operate in single-wavelength state with the OSNR for each wavelength more than 50dB.

Wavelength-swept Tm-doped fiber laser operating in the two-micron wavelength band.

A wavelength-swept thulium-doped silica fiber laser using an intracavity rotating slotted-disk wavelength scanning filter in combination with an intracavity solid etalon for passive control of temporal and spectral profiles is reported. The laser yielded a wavelength swept output in a step-wise fashion with each laser pulse separated from the previous pulse by a frequency interval equal to the free-spectral-range of the etalon and with an instantaneous linewidth of <0.05 nm. Scanning ranges from 1905 nm to 2049 nm for a cladding-pumping laser configuration, and from 1768 nm to 1956 nm for a core-pumping laser configuration were achieved at average output powers up to ~1 W.

A multiwavelength thulium-doped silica fiber laser incorporating a highly nonlinear fiber

A multiwavelength Tm3+-doped silica fiber laser incorporating a highly nonlinear fiber is presented. In the laser cavity, a nonlinear polarization rotation (NPR) based fiber loop mirror is used to overcome the mode competition and a polarization-maintaining fiber Sagnac loop mirror is used as the filter. The 150 m highly nonlinear fiber can improve the NPR effect in the laser cavity, which can increase the number of output channels. By adjusting the polarization state of the polarization controllers properly, stable multiwavelength operation at the 2 μm spectrum can be obtained at room temperature. It can achieve 15 output channels within a 10 dB bandwidth from 1990 to 2007.5 nm. The channel spacing of the multiwavelength Tm3+-doped silica fiber laser is 1.22 nm. Moreover, the performance of the laser is investigated for different pump powers and fibers.

A 1.97 μm multiwavelength thulium-doped silica fiber laser based on a nonlinear amplifier loop mirror

A 1.97 μm multiwavelength Tm-doped fiber laser based on a nonlinear amplifier loop mirror (NALM) has been experimentally demonstrated. The NALM with 500 m standard single mode fiber introduces intensity-dependent gain to alleviate mode competition caused by homogeneous gain broadening in Tm-doped fibers. At room temperature, up to 42 wavelengths with a wavelength spacing of 0.33 nm oscillate within a 10-dB bandwidth simultaneously. Repeat measurement of the laser spectrum shows the wavelength drift of a multiwavelength Tm-doped fiber laser is less than 0.05 nm in 40 min.

A high-Q low threshold thulium-doped silica microsphere laser in the 2 μm wavelength region designed for gas sensing applications

A high-Q and low threshold laser resonator, operating in the 2 μm wavelength region, has been demonstrated by coupling a thulium-doped silica microsphere to a tapered fibre. Microspheres with diameters ranging from fifty to a few hundred micrometres were carefully fabricated for this purpose by melting an etched-clad thulium-doped silica fibre tip using a focused beam from a CO2 laser, while the tapered fibre with waist diameter in the desired range of 2 μm was fabricated by using heating and stretching of standard single-mode telecommunication fibre. The tapered fibre served the dual purpose of transporting pump power into the sphere and allowing the extraction of the resulting laser emission. Under excitation at a wavelength of ∼1.6 μm, lasing occurred at wavelengths over the range from 1.9 to 2.0 μm. Single-mode laser operation was obtained by exciting the fundamental whispering gallery mode resonance of the microsphere, while multi-mode lasing occurred for non-fundamental mode excitation. The threshold power of the laser was measured to be about 50 μW delivered pump power, and a maximum laser power of 0.8 mW at around 1.94 μm was observed for a 6 mW pump power, operating at wavelengths around 1.6 μm. The laser was designed as a low threshold and compact source for miniaturized gas sensing devices operating over this important wavelength region.

Flatly broadened mid-infrared supercontinuum generation in a cascade of thulium-doped silica fiber amplifiers

Single-mode, flatly broadened supercontinuum (SC) generated in a thulium two-stage fiber amplifier spanning nearly the mid-infrared band is reported. The output average power and 10dB bandwidth of the obtained SC are over 2.3W and 570nm (from 1.95μm to 2.52μm), respectively. For wavelengths beyond 2.4μm the output power was 495mW constituting almost 21% of the total output power. Applying Tm-doped single-mode silica fibers as nonlinear and amplification media it was possible to extend the long wavelength cut-off to 2.7μm.

An all-fiber, resonantly pumped, gain-switched, 2 μm Tm-doped silica fiber laser

An all-fiber, gain-switched, single-mode, thulium-doped silica fiber laser operating at a wavelength of 1994.6 nm was developed. The laser was pumped by a 1.55 μm master oscillator power amplifier (MOPA) system and operated at a repetition rate ranging from 50 to 300 kHz. Different configurations of the laser cavity were tested. When operating at a 100 kHz repetition rate the laser delivered a maximum average power as high as 1.08 W with a slope efficiency of 49%. For a repetition rate of 50 kHz, stable 23 ns pulses with an energy of 15.3 μJ, corresponding to a peak power of 623 W, were achieved. We also report, to the best of our knowledge, the first demonstration of 2 μm gain-switched, mode-locked pulses with 100% modulation depth obtained in a Tm-doped fiber laser.

Model of the amplified spontaneous emission generation in thulium-doped silica fibers

A model of the ∼2 μm amplified spontaneous emission (ASE) generation in the thulium-doped silica fibers pumped at 1575 nm is presented. Both Al-codoped and Al/Ge-codoped fiber core compositions are studied. The results show that the composition affects the relative slope efficiency of 10% and the bandwidth of 19% of the output ASE. Our results predict that the backward ASE is more powerful and spectrally broader compared to the forward ASE, which is in agreement with previous experiments. Using an asymmetric cavity feedback, 98% of the total output power can be directed in the backward ASE, but with the consequence of losing ∼50% of the bandwidth. Such sources are expected to deliver single-mode output with more than 70% slope and 39% power conversion efficiency.

All-fiber Q-switched operation of thulium-doped silica fiber laser by piezoelectric microbending

We demonstrate an all-fiber Q-switched laser operation in the 2 µm region on the basis of a dynamic periodic microbend and pulsed-pump configuration. A single-mode thulium-doped silica fiber is pumped by 1.6 µm-band laser diodes, and the dynamic loss is introduced in the fiber ring resonator by the periodic microbend that is electrically controlled with a piezoelectric actuator. When the voltage-off period of the piezoelectric actuator is set at 20 µs for the pump power of 120 mW, the output pulse power is measured by 420 mW with a pulse width of 1.3 µs.

Theoretical modeling of fiber laser at 810 nm based on thulium-doped silica fibers with enhanced ^3H_4 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.