Microsphere Laser Research Articles

Bismuth-doped glass microsphere lasers

In this work, a hybrid structure consisting of a multicomponent germanate glass microsphere containing bismuth as a gain medium is proposed and presented. The bismuth-doped germanate glass microspheres were fabricated from a glass fiber tip with no precipitation of the bismuth metal. Coupling with a fiber taper, the bismuth-doped microsphere single-mode laser was observed to lase at around 1305.8 nm using 808 nm excitation. The low threshold of absorbed pump power at 215 μW makes this microlaser appealing for various applications, including tunable lasers for a range of purposes in telecommunication, biomedical, and optical information processing.

WHISPERING GALLERY MODE LASERS IN ERBIUM/YTTERBIUM CODOPED PHOSPHATE GLASSES

The study of whispering gallery mode laser in Er3+/Yb3+ co-doped phosphate glass microsphere has been reported. The coupling was carried out by fiber half taper technique. The microsphere lasers have been studied under pumping at 980 nm, to take full advantage of energy transfer effect from ion Ytterbium to ion Erbium. The observed lasing range extends from 1533 nm to 1612 nm.

Fabrication and characterization of Tm3+-Ho3+ co-doped tellurite glass microsphere lasers operating at ∼2.1 μm

We used a Tm3+-Ho3+ co-doped tellurite glass as the laser medium to build active microsphere laser resonators. A droplet method is implemented and hundreds of high quality microspheres can be fabricated simultaneously. Typical Quality factors (Q-factors) of microspheres fabricated in this work reach 106. Silica fiber tapers are used as the coupling mechanism and a commercial 808 nm laser diode is used as the pump source. Laser lines at ∼2.1 μm can be observed in the emission spectrum of these active microsphere resonators. Pump thresholds for generating single mode laser lines in a 59.52 μm diameter microsphere is measured to be 0.887 mW and as the pump power is increased to 1.413 mW, multi-mode laser lines can be generated. We also demonstrate microsphere lasers fabricated in this work can be thermally tuned with a temperature sensitivity of 32 pm/°C, implying these microspheres can be used as highly compact temperature sensors in various mid-infrared applications.

Real 3D microsphere lasers by femtosecond laser processing

Real 3D microspheres are fabricated using femtosecond laser processing and might be applicable for use as single-mode microlasers with controlled resonance wavelengths.

Optically-free-standing InGaN microdisks with metallic reflectors

The optical properties of free-standing thin-film microdisks with NiAg metallic reflectors are compared with those with an indium tin oxide (ITO) interfacial layer. The microdisks have been fabricated by a combination of microsphere lithography and laser lift-off processes. Optical-pumped lasing from the microdisk with NiAg reflector has been observed, with reduced threshold and higher quality factor compared those with ITO layers, attributed to improved optical confinement due to the reflectivity of the Ag coating. The results are supported by three-dimensional (3D) finite-difference-time-domain (FDTD) simulations.

Manipulative Dual-Microsphere Lasing Based on Nanosilica Waveguide Integrated in a Simplified Hollow-Core Microstructured Optical Fiber

Manipulative dual-microsphere lasing based on a nanosilica waveguide integrated in a simplified hollow-core microstructured optical fiber (SHMOF) is proposed and demonstrated. The SHMOF, which has a silica-branch structure, provides a nanoscale waveguide array and offers a versatile platform for selective lasing. Emissions at 600 and 640 nm are produced by excitation of different dye-coated microspheres, enabling a laser array with an ultralow nJ-level threshold. Because the transitive energy is bridged by the silica ring, the microsphere lasing could be controlled by another microsphere laser because of the interaction between and dependence of their thresholds. In addition, semidirectional lasing is also achieved by deliberately embedding a microsphere against the hexagonal silica wall.

Ultralow-threshold neodymium-doped microsphere lasers on a silicon chip

We demonstrate ultralow-threshold neodymium-doped silica microsphere lasers on a silicon chip with lasing wavelengths of ~900nm and ~1060nm. Neodymium-doped microsphere cavities are fabricated with a series of doping concentrations using silica sol–gel films. Experimentally, we observe single-mode lasing emissions from the high-Q microsphere cavities with a threshold of as low as 1.2µW.

Vertical coupling of laser glass microspheres to buried silicon nitride ellipses and waveguides

We demonstrate the integration of Nd3+ doped barium-titanium-silicate microsphere lasers with a silicon nitride photonic platform. Devices with two different geometrical configurations for extracting the laser light to buried waveguides have been fabricated and characterized. The first configuration relies on a standard coupling scheme, where the microspheres are placed over strip waveguides. The second is based on a buried elliptical geometry whose working principle is that of an elliptical mirror. In the latter case, the input of a strip waveguide is placed on one focus of the ellipse, while a lasing microsphere is placed on top of the other focus. The fabricated elliptical geometry (ellipticity = 0.9) presents a light collecting capacity that is 50% greater than that of the standard waveguide coupling configuration and could be further improved by increasing the ellipticity. Moreover, since the dimensions of the spheres are much smaller than those of the ellipses, surface planarization is not required. On the contrary, we show that the absence of a planarization step strongly damages the microsphere lasing performance in the standard configuration.

Ultralow-threshold laser in a Nd3+ doped silica microsphere

A neodymium-doped silica microsphere (NDSM) laser is described in this work, which was a pure silica sphere coated with Nd3+ doped sol–gel film and showed a rather simple production procedure, good characteristics, low cost and flexibility. To create the NDSM lasers, the neodymium-doped sol–gel film was applied to the surface of a silica microsphere, an optical tapered fiber (OTF) which was used to couple an 808nm continuous-wave diode laser into the inner surface of the NDSM, served as a pump light source. Multi-mode and single-mode laser oscillations around 1100nm were observed due to the 4F3/2–4I11/2 transitions of Nd3+ ions. Highly confined whispering gallery modes of pump laser made an ultralow threshold microlaser, we could observe the laser oscillations when the pump power was as low as 15.7µW. The multi-mode laser oscillation spectrum depends on the microsphere morphology characteristics. The free spectral range (FSR) was in good agreement with the calculated value which was based on the Mie scattering theory.

Quasi-single mode laser output from a terrace structure added on a Nd(3+)-doped tellurite-glass microsphere prepared using localized laser heating.

A Nd(3+)-doped tellurite-glass terrace microsphere was fabricated, and its laser characteristics using free-space pumping were investigated. A localized laser heating technique was used for preparing the 29-µm-diameter microsphere. The uncoated sphere exhibited many laser lines with 1.3-mW threshold. Fewer laser lines were observed after terrace formation. The terrace microsphere's lasing threshold was 0.6-2.4 mW depending on the pumping position in the terrace. These results indicate that the terrace structure can modify the modes of a microsphere laser and decrease the laser threshold due to an increase in the coupling efficiency between the cavity and free-space beam.

Low threshold fiber taper coupled rare earth ion-doped chalcogenide microsphere laser**Project supported by the National Natural Science Foundation of China (Grant Nos. 61177087 and 61435009), the National Key Basic Research Program of China (Grant No. 2012CB722703), the Program for Innovative Research Team of Ningbo City, China (Grant No. 2009B21007) , the K. C. Wong Magna Fund in Ningbo University, the

We report the applications of a low-cost and environmentally friendly chalcogenide glass, 75GeS2-15Ga2S3-10CsI, in building active microsphere laser oscillators. A silica fiber taper is used as the coupling mechanism. With an 808-nm laser diode as a pump source, we show that a high-Q (∼ 6×104) laser mode could be obtained from a 75-μm diameter microsphere that is coupled with a 1.77-μm waist-diameter fiber taper. The threshold of the incident pump power is 1.39 mW, which is considerably lower than those of previously reported free-space coupled chalcogenide microsphere lasers. We also note an apparent enhancement in laser power generated from this chalcogenide microsphere laser.

Polymer based whispering gallery mode laser for biosensing applications

Whispering gallery mode lasers are of interest for a wide range of applications and especially biological sensing, exploiting the dependence of the resonance wavelengths on the surrounding refractive index. Upon lasing, the Q factors of the resonances are greatly improved, enabling measurements of wavelength shifts with increased accuracy. A way forward to improve the performance of the refractive index sensing mechanism is to reduce the size of the optical resonator, as the refractive index sensitivity is inversely proportional to the resonator dimensions. However, as the lasing threshold is believed to depend on the Q factor among other parameters, and the reduction of the microresonator size results in lower Q, this poses additional challenges for reaching the lasing threshold. In this letter, we demonstrate lasing in 10 μm diameter dye doped polystyrene microspheres in aqueous solution, the smallest polystyrene microsphere lasers ever reported in these conditions. We also investigate the dependence of the lasing threshold on the Q factor by changing the refractive index surrounding the sphere, highlighting a much stronger dependency than initially reported.

Detachable microsphere scalpel tips for potential use in ophthalmic surgery with the erbium:YAG laser

Vitreoretinal surgery is performed using mechanical dissection that sometimes results in iatrogenic complications, including vitreous hemorrhage, retinal breaks, incomplete membrane delamination, retinal distortion, microscopic damage, etc. An ultraprecise laser probe would be an ideal tool for cutting away pathologic membranes; however, the depth of surgery should be precisely controlled to protect the sensitive underlying retina. The ultraprecise surgical microprobe formed by chains of dielectric spheres for use with the erbium:YAG laser source (λ=2940 nm), with extremely short optical penetration depth in tissue, was optimized. Numerical modeling demonstrated a potential advantage of five-sphere focusing chains of sapphire spheres with index n=1.71 for ablating the tissue with self-limited depth around 10 to 20 μm. Novel detachable microsphere scalpel tips formed by chains of 300 μm sapphire (or ruby) spheres were tested on ophthalmic tissues, ex vivo. Detachable scalpel tips could allow for reusability of expensive mid-infrared trunk fibers between procedures, and offer more surgical customization by interchanging various scalpel tip configurations. An innovative method for aiming beam integration into the microsphere scalpel to improve the illumination of the surgical site was also shown. Single Er:YAG pulses of 0.2 mJ and 75-μs duration produced ablation craters in cornea epithelium for one, three, and five sphere structures with the latter generating the smallest crater depth (10 μm) with the least amount of thermal damage depth (30 μm). Detachable microsphere laser scalpel tips may allow surgeons better precision and safety compared to mechanical scalpels when operating on delicate or sensitive areas like the retina.

Demonstration of an erbium-doped microsphere laser on a silicon chip

We demonstrate a low-threshold microsphere laser on a silicon chip, fabricated from erbium-doped silica sol–gel film. Single-longitudinal mode lasing emission is observed from a 37 μm diameter microsphere cavity with an erbium ion concentration of 2 × 1019 cm−3. The measured lasing threshold of the silica microsphere laser is as low as 4.8 μW.

Micro-sphere layered targets efficiency in laser driven proton acceleration

Proton acceleration from the interaction of high contrast, 25 fs laser pulses at >1019 W/cm2 intensity with plastic foils covered with a single layer of regularly packed micro-spheres has been investigated experimentally. The proton cut-off energy has been measured as a function of the micro-sphere size and laser incidence angle for different substrate thickness, and for both P and S polarization. The presence of micro-spheres with a size comparable to the laser wavelength allows to increase the proton cut-off energy for both polarizations at small angles of incidence (10∘). For large angles of incidence, however, proton energy enhancement with respect to flat targets is absent. Analysis of electron trajectories in particle-in-cell simulations highlights the role of the surface geometry in the heating of electrons.

Design of Mid-IR $\hbox{Er}^{3+}$-Doped Microsphere Laser

A mid-IR lasing system based on a tapered fiber coupled to an Er3+-doped microsphere has been modeled and numerically investigated. In order to design and optimize the device performance, a dedicated 3-D numerical code exploiting the coupled mode theory and the rate equations model has been developed. The main energy level transitions among the Er3+ ions, the most relevant secondary ion-ion interactions, the amplified spontaneous emission, and the fiber-microsphere coupling phenomena have been taken into account. In order to optimize the lasing performance, several parametric simulations have been carried out. The obtained numerical results show that a laser threshold of about 55 mW and an output power of about -17.8 dBm can be obtained by using small microspheres.

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.

Control of whispering-gallery-mode spectrum from erbium-doped silica microsphere lasers

High-power whispering-gallery modes (WGMs) emitted from microcavity lasers have attracted attention for many applications, such as optical signal processing, spectroscopy, optical sensors, and large-bandwidth optical communications. In this paper, we present a simple approach for controlling the output WGMs of erbium-doped silica microsphere lasers. With the presented scheme, accurate adjustment of the coupling gap between the collection fiber taper and the spherical surface allows us to select different single modes of the microsphere laser or different multimode configurations (also functions of the waist diameter of taper and the Er-doped concentration). The nonlinear frequency shift of the microsphere cavity as a function of intracavity power has also been studied. The high intensity and high side mode suppression ratio of the obtained single WGM are suitable for spectroscopy, optical sensors, and communications.

A novel laser gain medium: Micro-sphere array cooled by refractive index-matched coolant

A micro-sphere array laser gain medium cooled by refractive index-matched coolant was demonstrated for the first time to our knowledge. The new concept is possessed of potential to improve thermal management, to increase the thermal load capacity and to build large scale laser medium with small-sized solid laser materials. The feasibility was experimentally proved with an oscillator experiment in which reliable laser output was observed: the maximum output was 113.3mJ for the pump energy of 765.8mJ, the threshold was 131.7mJ and the slope efficiency was 19%. Promising improvement could be achieved by employing methods for maintaining accurate refractive index matching during the lasing process.

Ultralow-Threshold Two-Photon Pumped Amplified Spontaneous Emission and Lasing from Seeded CdSe/CdS Nanorod Heterostructures

Ultralow-threshold two-photon pumped amplified spontaneous emission (2ASE) and lasing in seeded CdSe/CdS nanodot/nanorod heterostructures is demonstrated for the first time. Such heterostructures allow the independent tunability of the two-photon absorption (2PA) cross-section (σ(2)) through varying the CdS rod size, and that of the emission wavelength through varying the CdSe dot size. With an enhanced σ(2), 2ASE in these heterostructures is achieved with an ultralow threshold fluence of ~1.5 mJ/cm(2), which is as much as one order less than that required for spherical semiconductor NCs. Importantly, by exploiting this unique property of the seeded nanorods exhibiting strong quantum confinement even at relatively large rod sizes, a near reciprocal relation between the 2ASE threshold and the 2PA action cross-section (σ(2)η) (where η is the quantum yield) was found and validated over a wide volume range for II-VI semiconductor nanostructures. Ultrafast optical spectroscopy verified that while the Auger processes in these heterostructures are indeed suppressed, ASE in these samples could also be strongly affected by a fast hole-trapping process to the NR surface states. Lastly, to exemplify the potential of these seeded CdSe/CdS nanodot/nanorod heterostructures as a viable gain media for achieving two-photon lasing, a highly photostable microsphere laser with an ultralow pump threshold is showcased.