A long uniform taper applied to an all-fiber Tm<Superscript>3+</Superscript> doped double-clad fiber laser

A long uniform taper fabricated on a large mode area Tm³⁺-doped double-clad fiber laser, which was clad-pumped by a laser diode (LD), was found to be an effective wavelength filter while improving the output beam quality and narrowing the line-width significantly. The long uniform taper was fabricated directly on the multi-mode Tm³⁺-doped fiber by heating and stretching method, and located several centimeters before the output fiber end. By slightly bending the taper section, the output laser spectrum was left with only one peak with a line-width less than 0.5 nm, compared to the multi-peak spectrum with a 5 nm line-width before tapering, indicating that the multi-mode fiber could produce quasi-single wavelength output with a long uniform taper. The beam quality factor M ² declined from 6.6 to 2.6 compared. Only a slight decrease in slope efficiency, from 19.2 to 17.5%, was observed. The main output wavelength had a blue shift of 8 nm.     

High-power diode-cladding-pumped Tm<Superscript>3+</Superscript>, Ho<Superscript>3+</Superscript>-doped silica fibre laser

The fundamental characteristics of a continuous-wave high-power diode-pumped Tm³⁺, Ho³⁺-doped double-clad silica fibre laser are presented. A maximum output power of 5.2 W was measured and was generated at a slope efficiency of 42 (44)% with respect to the launched (absorbed) pump power. At the optimum length of 7 m (_effL=2.9, where _eff is the effective absorption coefficient of the fibre and L is the fibre length), the fibre laser output was measured to have a centre wavelength of 2105 nm and a line width of 20 nm. The centre wavelength of the emission was tunable over a 32-nm extent when 0.68<_effL<3.28 or for a 6.2-m change in L. PACS 42.55.Wd; 42.55.Xi; 42.60.Lh; 42.60.Pk     

All-fiber ultra-narrow linewidth 50 pm Tm<Superscript>3+</Superscript>-doped double-clad fiber laser at 1948 nm

A high stability all-fiber LD-clad-pumped Tm³⁺-doped fiber laser was reported. The fiber laser had the ultra-narrow linewidth 50 pm at 1.948 μm with the maximal output power of 12.8 W. The slope efficiency was 28.9%, and threshold was 5.7 W. The double-clad Tm³⁺-doped fiber core was multi-mode, which had a demission of 25/250 μm with the core NA of 0.1 and inner-clad NA of 0.46. The high reflectivity coupler FBG was directly written into the single-mode passive photosensitive optical fiber core, which had a core diameter of 15 μm and NA of 0.1. The cavity was build-up by the high reflectivity FBG and the output fiber end Fresnel reflectivity.     

Comparative study of continuous wave Tm<Superscript>3+</Superscript>-doped silica and fluoride fiber lasers

We compare Tm³⁺-doped silica and fluoride fiber lasers in continuous wave operation using the same experimental setup. For incident pump powers of up to 50 W, the fluoride fiber is shown to have higher output power and efficiency compared to silica and reaches a maximum output power of 20 W at 1.94 μm. A slope efficiency of 49% (41%) and a maximum optical-to-optical efficiency of 45% (35%) at an incident pump power of 25 W (37 W) were determined for the fluoride (silica) fibers, respectively. These optical-to-optical efficiencies are the highest reported for directly diode-pumped Tm³⁺-based fiber lasers. We establish that in free-running mode, fiber lasers using fluoride glass are temporally more stable than fiber lasers using a silica host. PACS 42.55.Wd; 42.55.Xi; 42.60.Mi; 42.70.Hj     

Linear polarization Yb<Superscript>3+</Superscript>-doped fiber laser with novel innerclad structures

Results on high radiance Yb³⁺-doped fiber lasers with novel double innerclad structures (double-D clad and four hole) and polarized output at ≈1090 nm are presented. We have demonstrated >40% of the total output power being polarized, making the fiber laser suitable for LIDAR and second-harmonic generation (SHG) applications. It also showed a 10-nm tuning range with low (less than 10 mW) average power variations. The narrow linewidth source was pumped with a low cost, low brightness laser diode, and exhibited a relatively low slope efficiency, which gives room for improvement by using a 976-nm pump source where Yb³⁺ has a narrower linewidth and at least five times higher absorption.     

The comparative analysis and optimization of the free-running Tm<Superscript>3+</Superscript>:YAP and Tm<Superscript>3+</Superscript>:YAG microlasers

The thulium microlasers based on yttrium-aluminium perovskite and yttrium-aluminium garnet crystals are considered as coherent light sources at the wavelength near 2 μm. The comparative analysis of these lasers is carried out by means of the lasers’ parameters optimization for each value of the pumping power taking into consideration the cross-relaxation and up-conversion processes as well as heating of the active medium. The optimal values of the active medium length, activator concentration and the output mirror reflectivity are determined both for Tm:YAP- and Tm:YAG-laser. The pulse generation of Tm:YAP- and Tm:YAG-lasers is also considered, the parameters of this regime are determined. The comparison shows the suitability of Tm:YAP laser crystal both for cw and pulse generation. The up-conversion influence on the laser characteristics is also analyzed. PACS 42.60.Gd     

Nonlinear optical properties of Cr<Superscript>3+</Superscript>-doped laser crystals

The nonlinear optical properties in six Cr³⁺-doped laser crystals LiCaAlF₆, LiSrGaAlF₆, Gd₃Ga₅O₁₂, Gd₃Sc₂Ga₃O₁₂, LaMgAl₁₁O₁₉ and Alexandrite are investigated with the help of the Z-Scan technique at λ = 532 nm in the CW regime. The data reported here include particularly the excited state absorption cross section and the third-order nonlinear susceptibilities. It is found that the three first systems only exhibit both refractive and absorptive nonlinear effects, whereas the three others have only absorptive effects. Gd₃Ga₅O₁₂ shows the best nonlinear potentialities. The excited state absorption cross section corresponding mainly to the ⁴T₂ →   ⁴T₁ transition is found to be ranging between 8.9 × 10⁻²² cm² in LiSGaF and 3.1 × 10⁻²⁰ cm² in LaMgAl₁₁O₁₉. The calculated ratio of the third order nonlinear susceptibility to the ground state absorption coefficient is found to be largest in GGG with a value of 146 × 10⁻⁶ esu.cm and smallest in Alexandrite (0.6 × 10⁻⁶ esu.cm).     

Wavelength tunable femtosecond laser based on short Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript> codoped phosphate fiber

A wavelength tunable laser system mode-locked by nonlinear polarization evolution based on a 3 cm-long homemade Er³⁺/Yb³⁺ codoped phosphate fiber has been reported. By simply adjusting the polarization controllers, the central wavelength of the mode-locked spectrum can be tuned over 1537.1–1563.3 nm continuously. Moreover, 264-fs pulse with 3-dB spectral width of 39.6 nm and peak power of 7.8 kW at a 7.55 MHz repetition rate is generated directly from the all-fiber ring cavity.     

Green,blue, red,near-infrared up-conversion luminescence of Yb<Superscript>3+</Superscript>/Er<Superscript>3+</Superscript>/Tm<Superscript>3+</Superscript> co-doped YVO<Subscript>4</Subscript> nanoparticles

YVO₄:Yb³⁺,Er³⁺; YVO₄:Yb³⁺,Tm³⁺; and YVO₄:Yb³⁺,Er³⁺,Tm³⁺ were all synthesized via sol-gel method with a subsequent thermal treatment. Specifically, YVO₄:Yb³⁺,Er³⁺,Tm³⁺ phosphors were prepared with different annealing temperatures to study the influence of temperature. The transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffractometer (XRD), and photoluminescent (PL) spectrofluorometer were used to investigate the morphology, crystal structure, and up-conversion luminescent properties of all samples. In summary, all samples were granular-like nanoparticles and well crystallized with the same tetragonal phase as YVO₄. Under the irradiation at 980 nm, YVO₄:Yb³⁺,Er³⁺ phosphors can generate green emission at 525 and 553 nm and red emission at 657 nm, while YVO₄:Yb³⁺,Tm³⁺ phosphors can generate blue emission at 476 nm, red emission at 648 nm, and near-infrared emission at 800 nm. Notably, YVO₄:Yb³⁺,Er³⁺,Tm³⁺ samples can exhibit green emission, blue emission, red emission, and near-infrared emission at the same time, which might endow the as-prepared samples with potential applications in many fields, such as luminous paint, infrared detection, and biological label.     

Femtosecond Er<Superscript>3+</Superscript> fiber laser for application in an optical clock

The main elements needed for the realization of a compact femtosecond methane optical clock are developed and studied. A femtosecond laser system on an Er³⁺ fiber (λ = 1.55 μm) contains an oscillator, an amplifier, and a fiber with a relatively high nonlinearity in which the supercontinuum radiation is generated in the range 1–2 μm. In the supercontinuum spectrum, the fragments separated by an interval that is close to the methane-optical reference frequency (λ = 3.39 μm) exhibit an increase in intensity. The supercontinuum radiation is converted into the difference frequency in a nonlinear crystal to the range of the methane-reference frequency (λ = 3.3–3.5 μm), so that the frequency components of the transformed spectrum have sufficient intensities for the subsequent frequency-phase stabilization with respect to the methane reference. A system that stabilizes the pulse repetition rate of the femtosecond Er³⁺ laser is also employed. Thus, the repetition rate of the ultrashort pulses of the femtosecond fiber laser is locked to the methane reference. The pulse repetition rate is compared with the standard second. Thus, the scheme of an optical clock is realized.     

73-nm tuning of a double-clad Yb<Superscript>3+</Superscript>-doped fiber laser based on a hybrid array

We report on a wide wavelength tuning in a double-clad ytterbium-doped fiber laser. The laser cavity consists of an array of broadband high-reflection fiber Bragg gratings and a bulk grating as the output coupler and wavelength selection element. The proposed fiber laser configuration combines a low intracavity loss of the fiber Bragg grating mirrors with a wide wavelength tuning of the bulk gratings. We demonstrate a >70-nm wavelength tuning range, limited only by the available fiber Bragg gratings.     

Wavelength tunable linear cavity cladding pump Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript>co-doped fiber laser operating in L-band

We have demonstrated a novel tunable linear cavity Er³⁺/Yb³⁺ co-doped fiber laser, which utilizes amplified spontaneous emission as a secondary pump source so that it can operate in L-band. The tuning wavelength range can be up to 34 nm, from 1588.6 to 1622.6 nm, and the output power excursion of the laser at different wavelengths can be less than 0.4 dB by using a two sections of high-birefringence fiber loop mirror as the wavelength filter. The high output power of 200 mW is realized by using the cladding-pump.     

Mid-Infrared Emission Characteristic and Energy Transfer of Ho<Superscript>3+</Superscript>-Doped Tellurite Glass Sensitized by Tm<Superscript>3+</Superscript>

We report on 2.0-μm emission characteristic and energy transfer of Ho³⁺-doped tellurite glass sensitized by Tm³⁺ upon excitation of a conventional 808 nm laser diode. The Judd-Ofelt strength parameters, spontaneous radiative transition probabilities and radiative lifetime of Ho³⁺ have been calculated from the absorption spectra by using the Judd-Ofelt theory. Significant enhancement of 2.0-μm emission of Ho³⁺ has been observed with increasing Tm³⁺ doping up to 0.7 mol%. The energy transfer coefficient of the forward Tm³⁺→Ho³⁺ is approximately 17 times larger than that of the backward Tm³⁺←Ho³⁺ energy transfer. Our result indicates that the maximum gain of 2.0-μm emission, assigned to the transition of ⁵I₇→⁵I₈ of Ho³⁺, might be achieved from the tellurite glass at the concentration of 0.5 mol% of Tm₂O₃ and 0.15 mol% of Ho₂O₃. The high gain coefficient and quantum efficiency (1.16) along with the large value of the product of the stimulated emission cross-section and the measured radiative lifetime (4.12×10⁻²⁷ m²s) of the Ho³⁺/Tm³⁺-codoped tellurite glasses might find potential applications in efficient 2.0-μm laser.     

Spectroscopy of NaLa(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Tm<Superscript>3+</Superscript> and NaGd(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Tm<Superscript>3+</Superscript> crystals as advanced laser materials

Single crystals of double sodium-containing lanthanum and gadolinium molybdates doped with Tm³⁺ ions were synthesized by the Czochralski method. The spectroscopic properties of these crystals were investigated from the viewpoint of their use as active media in diode-pumped lasers. The polarized spectra of absorption on the ³ H ₄ and ³ F ₄ levels and the polarized spectra of luminescence due to the ³ F ₄-³ H ₆ laser transition were recorded, and the lifetimes of the ³ H ₄ and ³ F ₄ excited states of the Tm³⁺ ions were determined. The luminescence cross sections were calculated using the Füchtbauer-Ladenburg formula. The simulation of the decay curve of the ³ H ₄ excited state according to the Golubov-Konobeev-Sakun method revealed that, in the crystals under investigation, the interaction between Tm³⁺ ions predominantly occurs through the dipole-dipole mechanism.     

Observation and interpretation of the Tm<Superscript> 3+</Superscript> free ion spectrum

The emission spectrum of thulium produced by a vacuum spark source was observed in the wavelength range from 700 to 2320 ? on the 10.7 m normal-incidence vacuum ultraviolet spectrograph at the Paris-Meudon observatory. In the unknown spectrum of Tm IV, more than 760 lines have been identified for the first time as transitions between 157 levels of 4f¹¹5d, 33 levels of 4f¹¹6p, 9 levels of 4f¹¹6s and 10 levels of the 4f¹² ground configuration. A parametric interpretation of the levels has been carried out using the Cowan codes. Configuration interaction effects are discussed, in particular with the core-excited configurations 5p⁵4f¹³ and 5p⁵4f¹²5d. Radial Slater parameters derived from 4f¹² levels are larger than those pertaining to trivalent Tm ions in compounds. A selection of 105 prominent lines is given.     

Enhanced Up-Conversion Emission in Al<Superscript>3+</Superscript> Co-Doped ZnGa<Subscript>2</Subscript>O<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript>,Tm<Superscript>3+</Superscript> Powder Phosphors

Yb³⁺-Tm³⁺ co-doped up-conversion powder phosphors using Zn(Al_xGa_1-x)₂O₄ (ZAGO) as the host materials were synthesized via solid-state reaction successfully. In addition, the morphology, structural characterization and up-conversion luminescent properties were all investigated by scanning electron microscope (SEM), x-ray diffraction (XRD) and fluorescence spectrophotometer (F-7000), respectively. Under the excitation of a 980 nm laser, all as-prepared powders can carry out blue emission at about 477 nm (corresponding to ¹G₄ → ³H₆ transition of Tm³⁺ ions), and red emission at about 691 nm (attributed to ³F₃ → ³H₆ transition of Tm³⁺ ions). Also, the influence of doping Al³⁺ ions were investigated. In brief, the doping of Al³⁺ ions has no effect on the position of emission peak. Howbeit the up-conversion efficiency and intensity of ZAGO:Yb,Tm phosphors are stronger than ZGO:Yb,Tm and ZAO:Yb,Tm phosphors, while the crystallinity is the opposite. More particularly, all as-prepared powder phosphors emit strong luminescence, which is observable by the naked eye, demonstrating the potential applications in luminous paint, luminescent dye, etc.     

Diode-pumped passively mode-locked yb<Superscript>3+</Superscript>-doped yttrium lanthanum oxide ceramic sub-picosecond laser

A diode-pumped sub-picosecond laser is realized with Yb³⁺-doped yttrium lanthanum oxide transparent ceramic Yb:(Y_0.9La_0.1)₂O₃ as the gain medium for the first time. By using Gires-Tournois interferometer mirrors and chirped mirror for dispersion compensation and semiconductor saturable absorber mirror for passive mode-locking, laser pulse as short as 730 fs was obtained at the central wavelength of 1033 nm and repetition rate of 65.5 MHz. Under the maximum pump power of 6 W at 976 nm, the output power is 92 mW.     

30-W Yb<Superscript>3+</Superscript>-pulsed fiber laser with wavelength tuning

We have investigated various pulsed operation regimes of a diode-pumped Yb³⁺-doped fiber laser with both an acoustooptic filter and a shutter inside the resonator. To imbed the polarization-sensitive acoustooptic-tunable spectral filter into the polarization-nonmaintaining resonator, based on an “isotropic” single-mode fiber without “polarization’ losses, we have used a CaCO₃ single-crystal nondispersive thermostable polarization splitter. Stable smooth bell-shaped laser pulses were obtained in the Q-switch generation regime across the entire wavelength tuning band. Their duration depended on the resonator travel time and their repetition rate was determined exclusively by the outer high-frequency generator controlling the acoustooptic shutter. A pulsed laser radiation tuning bandwidth of more than 20-nm at a repetition rate band of 10–100 kHz was observed in the amplification band of the Yb³⁺-doped fiber. A stable average power of 30 W of the pulsed 70-ns 100-kHz laser radiation in a near Gaussian beam was reached by means of the two-stage amplifier based on Yb³⁺-doped fibers with an enlarged mode field diameter (14 μm). The amplifier was pumped by λ = 975 nm CW multimode laser diodes with a maximum average power of 42 W.     

Localized desvitrifiation in Er<Superscript>3+</Superscript>-doped strontium barium niobate glass by laser irradiation

Localized desvitrifiation in strontium barium niobate glass doped with Er³⁺ under laser irradiation has been carried out. The samples of this study have been fabricated by the melt quenching method and doped with 5% mol of Er³⁺. A 1.5-W cw Ar laser was focused on the sample to obtain desvitrifiation of the glass. Evidence of the changes induced by the Ar laser has been observed through the analysis of the photoluminescence of the Er³⁺ ions. The transitions corresponding to ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2 have been studied to analyze structure changes. Microluminescence measurements have been carried out to spatially select positions inside and outside the irradiated area. We have observed changes in the emission bands corresponding to these transitions. The emission bands from Er³⁺ ions in the irradiated zone show a resolved structure while they are broadened outside that area. These changes in the optical properties of the Er³⁺ ions indicate that the Ar-laser irradiation has produced a change in the local structure of the material. These results show that a localized desvitrifiation has been produced after the laser action and the transition from glass to glass ceramic has been completed.     

Spectroscopic indications of the possible optical cooling effect in fluoride crystals activated by Yb<Superscript>3+</Superscript> and Tm<Superscript>3+</Superscript> ions

The results of spectral kinetic studies of several fluoride crystals activated by Yb³⁺ and Tm³⁺ ions are presented, and their possible application as cooling elements of optical refrigerators is discussed. Optimum excitation conditions for the laser cooling effect in these crystals are found. The cooling efficiency is established to depend on the degree of order in the active medium.