Laser Oscillation Threshold Research Articles

Effective enhancement on mid-infrared fluorescence emission of Ho3+/Yb3+ doped tellurite glass introduced Ag nanoparticles

Abstract In this paper, Ho3+, Yb3+ and metal Ag nanoparticles (NPs) tri-doped tellurite glass were synthesized by melt quenching. Combining X-ray diffraction, Raman spectroscopy, and Differential thermal analysis tests, multiple structural units were found in the synthetic glass samples which provided the possibility to achieve high concentration and uniform doping of rare earth ions without degrading the glass forming ability and thermal stability. The fluorescence emission of glasses at 2 μm, 3.9 μm, and 4.1 μm were focused on, as a consequence, the Ho3+/Yb3+/AgNPs tri-doped tellurite glass achieved strong emission at 2 μm, 3.9 μm and 4.1 μm, and the full width at half maximum (FWHM) at 4.1 μm is 316 nm under the 980 nm LD excitation, which realized broadband fluorescence emission. In addition, the fluorescence lifetimes of the glass at 2 μm, 3.9 μm and 4.1 μm are 4.79 ms, 3.81 ms and 4.28 ms meaning the lower laser oscillation threshold the glass holds. It is particularly noteworthy that the emission intensity had been greatly improved about 30% at 2 μm and 27% at 3.9 μm and 4.1 μm with the introduction of AgCl into the matrix, respectively, which was mainly effected by local surface plasmon resonance (LSPR) of AgNPs and the energy transfer between Ag species and Ho3+, whereas the content of AgCl must be strictly controlled to prevent the concentration quenching to decrease the emission intensity. The obtained results demonstrate that Ho3+/Yb3+/AgNPs tri-doped tellurite glass possesses an ideal application prospect on mid-infrared fiber and laser materials.

A fully planar solar pumped laser based on a luminescent solar collector

A solar-pumped laser (SPL) that converts sunlight directly into a coherent and intense laser beam generally requires a large concentrating lens and precise solar tracking, thereby limiting its potential utility. Here, we demonstrate a fully-planar SPL without a lens or solar tracking. A Nd3+-doped silica fiber is coiled into a cylindrical chamber filled with a sensitizer solution, which acts as a luminescent solar collector. The body of the chamber is highly reflective while the top window is a dichroic mirror that transmits incoming sunlight and traps the fluorescence emitted by the sensitizer. The laser-oscillation threshold was reached at a natural sunlight illumination of 60% on the top window. Calculations indicated that a solar-to-laser power-conversion efficiency could eventually reach 8%. Such an SPL has potential applications in long-term renewable-energy storage or decentralised power supplies for electric vehicles and Internet-of-Things devices.

Effect of the thermally excited lower laser level in a neodymium-doped fiber

The neodymium-doped optical fiber operated at 1.1 μm is a very promising material for the solar-pumped laser without concentrator because of its strong absorption bands in the visible region and its extremely low optical losses. It is generally considered a true four-level system owing to the large energy gap of the lower level of the laser transition to the ground level. In this study, the exquisitely small thermally excited population in the I11/24 Stark level is shown to be primarily responsible for the absorption losses at the laser wavelength at room temperature. Thanks to its long geometry, the absorption cross section and linestrength of the laser transition could be directly measured, allowing easier estimation of the emission cross section than with usual methods relying on fluorescence decay time and quantum efficiency measurements, or a Judd–Ofelt analysis. Our measurements are corroborated by McCumber’s reciprocity principle. The small-signal gain spectrum measured in an amplifier experiment matches well with the emission cross section. Order-of-magnitude loss reduction is demonstrated by lowering the temperature to −34°C, implying substantial reduction of the laser oscillation threshold in cold solar-pumping environments.

Study on the power scalability of all-fiber superfluorescent source operating near 980 nm.

The high-power all-fiber superfluorescent source operating near 980 nm is studied experimentally and numerically. In experiment, an all-fiber superfluorescent source operating near 980 nm is fabricated with the distributed side-coupled cladding-pumping (DSCCP) Yb-doped fiber (YDF). By optimizing the active fiber and angle-cleaving of the output port, a recorded 17.1-W output power and 14.6% slope efficiency of 980-nm ASE are obtained. No parasitic laser oscillation is observed at the maximum output power. The power scalability of the source is also numerically investigated. A simple but effective method is present to numerically determine the threshold of parasitic laser oscillation. It is found that the output power can be scaled up to 50 W and 100 W with the optical feedback of each output port suppressed to 1.2 × 10-6 and 7 × 10-7, respectively. It is also revealed that coupling coefficient should be larger than 6 to realize more than 50% slope efficiency. These results provide significant guidance for understanding and designing the high-power superfluorescent Yb-doped source (SYFS) operating near 980 nm and other sorts of three-level fiber sources.

Influence of temperature on Ti:sapphire spectroscopic and laser characteristics

We report on the influence of temperature on the spectroscopic and lasing properties of Ti:sapphire (Ti3+ :Al2O3) crystal. The fluorescence lifetime, polarization-resolved absorption and emission spectra, and laser characteristics at ∼750 nm are described in detail within a temperature range of 78–300 K. The cooling of Ti:sapphire crystal down to cryogenic temperatures resulted in dramatic improvement in terms of its laser output power and oscillation threshold. More than two times better results were reached at 78 K when compared with room temperature operation. A continuous output power of 325 mW with a slope efficiency of 24% (related to absorbed pump power) was obtained using 98% output coupler reflectivity at a 750 nm wavelength at cryogenic temperatures. The corresponding oscillation threshold was 580 mW of absorbed pump power.

Lasing characteristics of refractive-index-matched composite Y3Al5O12 rods employing transparent ceramics for solar-pumped lasers

We have proposed a new configuration of solar-pumped lasers employing transparent ceramic rods. The laser rod has a composite structure consisting of a Nd/Cr:YAG gain domain surrounded by Gd:YAG with the same refractive index as that of Nd/Cr:YAG. The lasing mode is well controlled by the output coupler, and the parasitic oscillation is suppressed, owing to the refractive index matching. A high laser slope efficiency and a low laser oscillation threshold were achieved owing to the suppressed absorption outside the lasing mode, which was previously a serious issue for the end-pumping configuration using a high-NA focusing optics. The laser oscillation threshold of 136 mW and the slope efficiency of 25.3% were derived. Thus, we have resolved the issue of useless absorption associated with the high-NA end-pumping, and achieved significant improvements compared with the conventional structure of uniform Nd/Cr:YAG.

Organic-inorganic hybrid microdisk laser with dye and silica mixed doping prepared by ink-jet printing method

We developed an ink-jet printing method for fabricating inorganic microdisks at room temperature, which is much lower than the melting point of solid-state inorganic oxide, and have fabricated an organic-inorganic hybrid microdisk laser. Silica was used as the inorganic disk material, and microdisk-shaped aggregates were formed by the ink-jet printing method using a solution in which nanosilica particles were dispersed in propylene glycol monomethylether (PGME) solvent. Then, a microdisk capable of laser oscillation was also prepared by preliminarily adding the laser dye rhodamine 6G to the ink to form a mixed organic material. The structural evaluation of the printed microdisk was first conducted using an optical microscope, a scanning electron microscope (SEM), and an atomic force microscope (AFM). The results of laser oscillation evaluation by optical excitation showed that the printed microdisk sufficiently functions as an optical resonator with a low optical loss. In these evaluations, excellent values such as a surface roughness of 5.83 nm from root mean square (R. M. S.) which is one forth smaller than the particle diameter, and a laser oscillation threshold of 4.76 µJ/mm2 at a wavelength of 601.4 nm were obtained. To the best of our knowledge, this is the first time that an inorganic microdisk has been fabricated at room temperature to realize an organic-inorganic hybrid microdisk laser.

Luminescent properties of Er3+/Yb3+ co-doped oxyfluoride glass ceramic microsphere with the tapered fiber-microsphere coupling system

We prepare Er3+/Yb3+ co-doped oxyfluoride glass ceramic samples by high temperature calcination method (41.2SiO2-29.4Al2O3-17.6Na2CO3-11.8LaF3-0.5ErF3-2.5 YbF3), and also fabricate transparent microspheres each with a handle. We propose a new method of studying the luminescent properties of Er3+/Yb3+ co-doped oxyfluoride glass ceramic with the tapered fiber-microsphere coupling system. The method has characteristics such as low excitation optical power, easy preparation and testing. We couple the 976 nm laser into the microspheres with the coupler of optical tapered fiber, then the fluorescence and laser are connected to spectrum analyzer with the optical tapered fiber. The strong up-conversion fluorescences at 522, 545 and 657 nm are obtained, and the laser oscillation spectra generated by Er3+ ions at 1562 nm are also measured. The up-conversion luminescence mechanism of Er3+, the mechanism of high luminescence efficiency in Er3+/Yb3+ co-doped oxyfluoride glass ceramic microsphere, and the mechanism of higher laser oscillation threshold in the oxyfluoride glass ceramic microsphere than in the SiO2 matrix microsphere are all analyzed.

Study on the optical bistability in the laser oscillation of Yb doped vanadate crystals

We have studied the effect of optical bistability exhibited in the laser oscillation of Yb:LuVO4, Yb:YVO4, and Yb:GdVO4, the three Yb doped vanadate crystals. The bistability range can be in excess of 1 W in terms of the pump power absorbed by the crystal. At the high-power-end side of the bistability region, i.e., the upper threshold for laser oscillation, discontinuous growing in the output power takes place, along with large fluctuations in the laser intensity occurring on sub-millisecond time scale. It is found that the bistability effect is affected significantly by the output coupling of the resonator, the crystal thickness, and the thermal effect existing in the crystal.

First Approach to the Analysis of the Lasing Conditions in POLIPHEM© Structures

We report a study devoted to the creation of a distributed feedback device on a basis of waveguide with periodical composition polymer – liquid crystal. Due to the refractive indices difference between the components the Bragg diffraction grating is formed, which provides coupling of the forward and backward propagating waves. The influence of the gain and conditions of laser oscillation threshold are analyzed for gain-type reflection grating. The conclusion is done on the advantage of second-order Bragg diffraction exploitation. Numerical simulations demonstrated the stability of the waveguide mode.

Laser action in nanowires: Observation of the transition from amplified spontaneous emission to laser oscillation

Direct evidence of the transition from amplified spontaneous emission to laser action in optically pumped zinc oxide (ZnO) nanowires, at room temperature, is presented. The optical power evolves from a superlinear to a linear regime as the pump power exceeds threshold, concomitant with a transition to directional emission along the nanowire and the emergence of well defined cavity Fabry–Pérot modes around a wavelength of ≈385 nm, the intensity of which exceeds the spontaneous emission background by orders of magnitude. The laser oscillation threshold is found to be strongly dependent on nanowire diameter, with no laser oscillation observed for diameters smaller than ∼150 nm. Finally, we use an alternative “head on” detection geometry to measure the output power of a single nanowire laser.

Single polarized Tm3+ laser in Zn-diffused LiNbO3 channel waveguides

In this work, laser operation at 1.76μm in Tm3+:LiNbO3 Zn-diffused channel waveguides is reported. The laser emission is single polarized with the electric field parallel to the optic axis(π-polarization) and operates in continuous-wave regime, at room temperature. The threshold of laser oscillation is in the range of 40mW, the slope efficiency is around 1%, and both magnitudes are dependent on the channel width, in accordance with the mode overlap between the pump and signal modes within the waveguides.

A Nd doped Multimode-Fiber-Laser with D-Shape Large Clad under Quasi-Solar-Pumping

We observed laser oscillation for a multimode Nd-doped fiber laser with D-shaped cladding under quasi-solar-pumping simulated by an intense halogen lamp with a color temperature of 5,600 Kelvin. The fiber had a millimeter-scale diameter and was 78 m long, and the doping rate was 3000 ppm of Nd ions in silica core-glass. Results show that the output power obtained was 300 mW when the injected power of the lamplight was 1.1 W at an entrance to the fiber, and the optical-optical conversion efficiency was 27 %. The threshold of laser oscillation is 24 times for the solar constant (0.14 W/cm2), a value which is one order of magnitude lower than the case of crystal lasers reported under solar-pumping.

The mechanism research of the Cr 4+ Nd 3+ :YAG laser

The 946nm diode-pump microchip self-Q-switched laser of a chromium and neodymium codoped yttrium aluminum garnet crystal material (Cr4+Nd3+:YAG) is studied, especially about its physical mechanism of operation. The 4F3/2 → 4I9/2 transition of Nd3+ ion is beneficial to achieving laser oscillation in a quasi-three-level system based on coating the cavity mirrors of the microchip with films that suppress the 1064nm operation and enhance the 946nm laser. The Cr4+ ion is a saturable absorber. The initial loss Nt1 is high, which acts as the threshold for laser oscillation. The stable loss Nt2 is low because the Cr4+ ion is acceleratively bleached by the fast enhancement of the oscillating laser. The high Nt1, small Nt2 and fast progresses permit the oscillating laser of the Cr4+Nd3+:YAG to have a good self-Q-switched property whose full width at half maximum is about 4.2ns. Its highest laser power is about 5.7mW. Its peak power is about 150W. Its good fundamental transverse TEM00 mode results from the absorption bleaching established by both the pump and oscillating lasers, which suppress other transverse mode and allow the oscillation only in the fundamental transverse TEM00 mode.

Low-Threshold Photopumped Distributed Feedback Plastic Laser Made by Replica Molding

We describe low-threshold photopumped organic distributed feedback (DFB) lasers with a polymer grating fabricated by replica molding, which is a soft imprinting method. These simply fabricated lasers oscillate at a low threshold comparable to that of organic DFB lasers precisely constructed using photolithographic technology. The threshold of laser oscillation was 250 nJcm-2pulse-1, that is, a peak-power threshold of 500 Wcm-2pulse-1.

Organic Polymer DBR Laser by Soft Lithography II: Optimization of Distributed Bragg Reflector

We describe detailed analysis and optimization of the photopumped organic polymer distributed Bragg reflector laser having polymer grating fabricated using a replica molding method, which is an imprinting technology often called “soft lithography”, whose advantage is much less damage to materials. After the optimization, the laser oscillation threshold of only 0.6 μJ/cm2 was achieved. This threshold is comparable to that of organic DFB lasers with a quartz grating precisely fabricated using photolithography and dry etching. While glass was used as a substrate in this study, plastic substrates are also available. In near future, flexible low-threshold organic laser will be demonstrated.

Organic Polymer DBR laser by Softlithography.

We fabricated the distributed Bragg reflector (DBR) laser that consisted of the grating of 600nm in period on the surface of the charge transport polymer film doped with a low threshold laser dye by using a soft lithography technique. The 600-nm-period grating acted as the fourth order diffraction DBR for the laser oscillation wavelength. The laser oscillation of FWHM of 0.028nm and the Q-value of approximately 9200 were observed from the device by optical pumping. The laser oscillation threshold obtained from the input-output characteristics of the device was 37.6μJ/cm2.

From amplified spontaneous emission to laser oscillation:?dynamics in a short-cavity polymer laser

We present an experimental study of the input-output characteristics of an ultrashort pumped pi -conjugated polymer in solution. By comparing the results for configurations with zero, one, and two mirrors around the polymer, we show that the physics is driven by amplified spontaneous emission and not by cooperative emission. This finding is substantiated by picosecond-time-scale measurements of the evolution of the emission of the polymer. For the two-mirror configuration a sharply defined threshold for laser oscillation is found; the output of the laser exhibits strong pulsations.

Whispering-gallery-mode microring laser using a conjugated polymer

We observed laser emission in whispering gallery modes using a microring composed of a semiconducting polymer poly[2,5-bis-(2′-ethylhexyloxy)-p-phenylenevinylene coated on an etched fiber under transient and quasisteady-state pumping conditions. The threshold for laser oscillation was 1 mJ/cm2 (0.1 MW/cm2) and 30 μJ/cm2 (300 MW/cm2) for nanosecond and femtosecond excitation, respectively. The laser output showed superlinear dependence on the excitation energy above the threshold. The demonstration of lasing under quasisteady-state pumping shows the possibility to develop electrically pumped polymer lasers.

Nonlinear dynamics induced by external optical injection in semiconductor lasers

External optical injection of semiconductor lasers changes the resonant coupling characteristics between the circulating optical field in the laser cavity and the free carriers (gain medium). Changes in the characteristic resonance frequency and damping of the system can lead to dynamic instabilities and deterministic chaos and to enhancement of the modulation bandwidth. The changes induced depend on key dynamic parameters of the semiconductor laser such as the photon and carrier decay rates, gain characteristics and the linewidth enhancement factor, and the operating point of the laser determined by the injection current (pump) level, circulating optical power, the amplitude of the external optical injection and the frequency offset between the master and injected lasers. Here, we describe a mapping of the typical dynamics induced in a nearly single-mode semiconductor laser biased well above the threshold for laser oscillation as the amplitude and frequency offset of the master laser are changed. We also present results on a laser initially biased near threshold in a free-running condition where it displays a near-Gaussian optical lineshape. The external optical injection induces spectral holes and spikes, as well as spectral shifts. All features that we observe can be recovered in a standard coupled equation model of semiconductor laser operation.