Precise determination of energy transfer upconversion coefficient for the erbium and ytterbium codoped laser crystals.

Energy transfer upconversion (ETU) coefficient plays a crucial role in investigating complex laser systems as it greatly influences both the laser output behavior and heat generation. For some quasi-three-energy-level lasers based on Er3+ doped, Ho3+ doped and codoped gain media, the available theoretical studies relied on some unreasonable approximations due to the lack of spectroscopic data, notably the ETU coefficient. We put forward what we believe is a novel approach to overcome the difficulties caused by wavelength jump occurred in aforementioned laser systems. Based on net gain cross-section analysis and rate equations modelling, the functional relationship between the ETU coefficient, the laser power and pump power at the jumping wavelength are established. ETU coefficients and their temperature dependences of Er,Yb:YAB crystals with different crystal doped concentrations are experimentally determined for the first time. The results reveal that the ETU process in Er,Yb:YAB laser system is 5∼35 times stronger than that in Er3+ and Yb3+ codoped phosphate glass. The determination of these spectroscopic data paves the way for precise modelling of laser system based on Er,Yb:YAB or similar gain media.

56-fs diode-pumped SESAM mode-locked Yb:YAl3(BO3)4 laser.

We report on the first sub-60 fs pulse generated from a diode-pumped SESAM mode-locked Yb-laser based on a non-centrosymmetric Yb:YAl3(BO3)4 crystal as a gain medium. In the continuous-wave regime, pumping with a spatially single-mode, fiber-coupled 976 nm InGaAs laser diode, the Yb:YAl3(BO3)4 laser generated 391 mW at 1041.7 nm with a slope efficiency as high as 65.1%, and a wavelength tuning across 59 nm (1019 to 1078 nm) was achieved. By implementing a commercial SESAM to initiate and sustain the soliton type mode-locking, and using only a 1 mm-thick laser crystal, the Yb:YAl3(BO3)4 laser delivered pulses as short as 56 fs at a central wavelength of 1044.6 nm with an average output power of 76 mW at a pulse repetition rate of ∼67.55 MHz. To the best of our knowledge, this result represents the shortest pulses ever achieved from Yb:YAB crystal.

Continuous-wave and passively Q-switched pulsed 1.5†µm Er:Yb:Ba3Gd(PO4)3 lasers.

An eye-safe 1567 nm continuous-wave laser with a maximum output power of 50 mW and a slope efficiency of 21.1% was demonstrated in an Er:Yb:Ba3Gd(PO4)3 crystal. By using a Co2+:MgAl2O4 crystal with an initial transmission of 95% as a saturable absorber, a stable passively Q-switched pulsed laser was also realized in the crystal. The effects of the output coupler transmission and cavity length on pulsed performance were investigated. At an absorbed pump power of 350 mW, a 1541 nm Er:Yb:Ba3Gd(PO4)3 pulsed laser with a repetition frequency of 0.86 kHz, duration of 38 ns, energy of 21.2 µJ, and peak output power of 0.56 kW was obtained.

Clustering-based fusion for medical information retrieval.

Medicine is a fast-moving field, and the number of medical publications has increased rapidly over recent years. How to find relevant information from this vast pool of research effectively and efficiently has therefore become highly challenges. Previous studies have demonstrated that data fusion can improve search performance if properly utilized. However, in most cases effectiveness is the only concern and efficiency is not considered. A fusion-based system is by nature more complicated and expensive computationally than other retrieval models such as BM25, because many component retrieval systems and an extra layer of fusion are required. The number of component retrieval systems involved is an important indicator of complexity of the fusion-based system. We aim to select the optimal k-subset of component retrieval systems for any given number k, to optimize both fusion performance and reduce the cost of data fusion. A clustering-based approach is proposed. First all the candidates are divided into clusters by the Chameleon clustering algorithm, then representatives from every cluster are chosen by Sequential Forward Selection for fusion. Evaluated with two datasets from TREC, the proposed method performs more effectively than the other baseline methods including the state-of-the-art subset selection method significantly. When either of the two typical fusion methods is used, an improvement rate of over 10% is observed for both measures Mean Average Precision and Recall-level Precision, and an improvement rate of over 5% is observed for both measures Precision at 10 document level and Mean Reciprocal Rank.

25.4†kW, 1.9†ns passively Q-switched 1522†nm Er:Yb:LuAl3(BO3)4 pulse microlaser at 100†Hz.

Passively Q-switched Er:Yb:LuAl3(BO3)4 pulse microlasers were investigated at a low repetition frequency of 10-200 Hz. End-pumped by a 975.6 nm quasi-continuous-wave laser diode with pump pulse width of 0.5 ms and period of 10 ms, a stable 1522 nm pulse microlaser with single pulse energy of 48.3 µJ, duration of 1.9 ns, repetition frequency of 100 Hz, peak output power of 25.4 kW and beam quality factor less than 1.2 was realized at a pump beam waist diameter of 260 µm. This eye-safe passively Q-switched pulse microlaser with high peak output power and narrow duration can be used in the portable laser rangefinder.

Continuous-wave and SESAM mode-locked operation of the Yb:Bi4Si3O12 laser.

We demonstrate a comprehensive characterization of the diode-pumped Yb:Bi4Si3O12 laser operating in the continuous-wave and soliton mode-locked regimes. Pumping with a 650 mW, single-transverse mode, fiber-coupled laser diode, a maximum continuous-wave output power amounted to 213 mW with a slope efficiency up to 57.6%. A broadband wavelength tuning range of more than 70 nm was achieved in CW regime with a fused silica prism. Applying a SESAM as mode locker, nearly transform-limited pulses as short as 113 fs were generated for a maximum average power of 53 mW and a pulse repetition rate of ∼106 MHz. To the best of our knowledge, this is the first report on passively mode-locked operation with the Yb:Bi4Si3O12 crystal.

All-optical image identification with programmable matrix transformation.

An optical neural network is proposed and demonstrated with programmable matrix transformation and nonlinear activation function of photodetection (square-law detection). Based on discrete phase-coherent spatial modes, the dimensionality of programmable optical matrix operations is 30∼37, which is implemented by spatial light modulators. With this architecture, all-optical classification tasks of handwritten digits, objects and depth images are performed. The accuracy values of 85.0% and 81.0% are experimentally evaluated for MNIST (Modified National Institute of Standards and Technology) digit and MNIST fashion tasks, respectively. Due to the parallel nature of matrix multiplication, the processing speed of our proposed architecture is potentially as high as 7.4∼74 T FLOPs per second (with 10∼100 GHz detector).

The growth of a large GdPO4 crystal guided by theoretical simulation and the study of its phonon properties.

It has remained a big challenge to grow large crystals of compounds at very anisotropic crystal growth rates. GdPO4 is such an example, which usually grows into a needle-like shape. This work solved this problem by using Li+ ions, predicted by first-principles calculations, to adjust the crystal morphology of GdPO4. Based on the calculated surface energies and the polar properties of the surfaces, we identified two crystal planes, i.e. (-102) and (-111), which can be used to adjust the morphology of the GdPO4 crystal by using extrinsic cations in the flux system. With a predicted Li containing flux system, Li2O-MoO3-B2O3, a large crystal with a record size, 10.0 × 9.0 × 18.2 mm3, was grown by using the top-seeded solution growth (TSSG) method. Based on the grown large crystal and the polarized Raman measurements, four new Raman bands missing in early studies and other bands were unambiguously assigned. By introducing a new formula, the phonon-phonon interactions were shown to be weak in the temperature range from 83 to 803 K. Our study indicates that the crystal of GdPO4 can be used as a promising laser host material working under relatively high temperature conditions.

Single longitudinal-mode passively Q-switched 1537†nm Er:Yb:Lu2Si2O7 pulse microchip laser.

A single longitudinal-mode passively Q-switched 1537 nm pulse microchip laser was realized in an Er:Yb:Lu2Si2O7 crystal. The effects of the pump beam diameter and output mirror transmission on pulse characteristics of the Er:Yb:Lu2Si2O7 microchip laser were investigated, when a Co2+:MgAl2O4 saturable absorber with an initial transmission of 95% was used. At an absorbed pump power of 3.4 W, a 1537 nm single-longitudinal-mode pulse laser with energy of 25.8 µJ, repetition frequency of 0.89 kHz, duration of 4.3 ns, and peak output power of 6.0 kW was obtained, when the pump beam diameter and output mirror transmission were 420 µm and 3.0%, respectively. The beam quality factor of output laser with TEM00 mode was less than 1.3.

Cavity-dumped Q-switched Er:Yb:YAl3(BO3)4 pulse laser.

We reported an electro-optically cavity-dumped Q-switched Er:Yb:YAl3(BO3)4 pulse laser for the first time. A 1531.1 nm pulse laser with an average output power of 521 mW, energy of 10 µJ, and a duration of 3.1 ns was achieved at a repetition rate of 100 kHz under the quasi-continuous-wave pumping. The pulse characteristics of the laser were investigated in detail. The result shows that the depolarization effects and the length of high voltage (HV) time applied to the EO-switch had significant impacts on the pulse characteristics.

Continuously diode-pumped passively $Q$Q-switched eye-safe 1537 nm Er:Yb:Lu2Si2O7 pulse laser.

It is still a technical challenge to obtain a 1.55 µm passively $Q$Q-switched ${{\rm Er}^{3 + }}/{{\rm Yb}^{3 + }}$Er3+/Yb3+ pulse laser with high energy and high repetition frequency, especially when it is end-pumped by a continuous-wave 0.98 µm diode laser. Benefitting from its long fluorescence lifetime of the $^4{{\rm I}_{13/2}}$4I13/2 upper laser level and high thermal conductivity, ${\rm Er}:{\rm Yb}:{{\rm Lu}_2}{{\rm Si}_2}{{\rm O}_7}$Er:Yb:Lu2Si2O7 crystal may be an excellent gain medium for realizing a high energy and high repetition frequency 1.55 µm pulse laser. At a continuous-wave absorbed pump power of 6.1 W, a 1537 nm pulse laser with energy of 45.5 µJ, repetition frequency of 1.32 kHz, and duration of 25 ns was first, to the best of our knowledge, realized in an ${\rm Er}:{\rm Yb}:{{\rm Lu}_2}{{\rm Si}_2}{{\rm O}_7}$Er:Yb:Lu2Si2O7 crystal.

Single-longitudinal-mode 1521 nm passively q-switched Er:Yb:YAl3(BO3)4 pulse microchip laser.

A single-longitudinal-mode 1521 nm pulse microchip laser Q-switched by a Co2+:MgAl2O4 saturable absorber was demonstrated in an Er:Yb:YAl3(BO3)4 crystal. The influence of the waist radius of pump beam at 976 nm on the laser performance was investigated. At an incident pump power of 6.54 W and pump beam waist radius of 60 µm, a 1521.4 nm single-longitudinal-mode pulse laser with average output power of 434 mW, energy of 16.5 µJ, repetition frequency of 26.3 kHz and width of 2.9 ns was obtained. The result shows that caused by the mode selection of the saturable absorber and large cavity losses, a single-longitudinal-mode 1.55 µm pulse microchip laser can also be realized in the Er:Yb:YAl3(BO3)4 crystal.

Spectra and diode-pumped continuous-wave 1.55†µm laser of Er:Yb:Ca3NbGa3Si2O14 crystal.

The polarized spectra associated with 1.55 µm lasing in an Er:Yb:Ca3NbGa3Si2O14 crystal were investigated at room temperature. The crystal had a large absorption cross section of 3.60 × 10-20 cm2 at 978 nm, long fluorescence lifetime of 5.81 ms for the 4I13/2 multiplet of Er3+, and broad emission band at 1.55 µm with full width at half-maximum of 60 nm. End-pumped by a 975 nm diode laser, a 1555 nm continuous-wave microlaser with a maximum output power of 0.4 W and a slope efficiency of 10.6% was demonstrated in a c-cut, 2.5-mm-thick crystal. Combining with natures of the long fluorescence lifetime and the broad emission band, the Er:Yb:Ca3NbGa3Si2O14 crystal with a good thermal performance may be a promising gain medium for high-energy pulse and broadly tunable lasers around 1.55 µm.

Polarization-controllably launching localized cosine-Gauss beam with spatially varied metallic nano-apertures.

Two properly designed columns of orthogonally tilted and spatially varied nano-apertures have been demonstrated to achieve polarization-controlled launching of the localized cosine-Gauss beam, which is a kind of highly localized surface plasmonic wave without diffraction. According to the experimental results, the generated unidirectional propagating plasmonic beam could propagate with a non-diffracting length of up to 57µm as well as the extinction ratio of more than 150. We believe that our proposed device would play an important role in highly compact photonic circuit on-chip with plasmonic wave.

Polarization switching realized in the continuous-wave and acousto-optic Q-switched pulse Er:Yb:LaMgB5O10 lasers at 1556 and 1568 nm.

By using the natural birefringence of crystalline quartz, the switching of a 1568 nm laser with polarization parallel to the Z optical indicatrix axis to a 1556 nm laser with polarization parallel to the Y optical indicatrix axis was observed in both continuous-wave and acousto-optic Q-switched pulse lasers based on an X-cut Er:Yb:LaMgB5O10 crystal, when the alignment of the output mirror in a 976 nm diode-end-pumped plano-concave resonator was precisely tilted. For the continuous-wave regime, a 1568 nm laser with a maximum output power of 500 mW and slope efficiency of 16%, as well as a 1556 nm laser with a maximum output power of 400 mW and slope efficiency of 13.5% were realized, respectively. For the Q-switched regime, a 1568 nm pulse laser with an energy of 144 µJ and width of 300 ns, as well as a 1556 nm pulse laser with an energy of 168 µJ and width of 270 ns, were obtained respectively by precisely tilting the alignment of the output mirror, when the absorbed pump power was 4.0 W and pulse repetition frequency was 0.5 kHz.

Efficient continuous-wave and passively Q-switched pulse laser operations in a diffusion-bonded sapphire/Er:Yb:YAl3(BO3)4/sapphire composite crystal around 1.55 µm.

A composite crystal consisting of a 1.5-mm-thick Er:Yb:YAl3(BO3)4 crystal between two 1.2-mm-thick sapphire crystals was fabricated by the thermal diffusion bonding technique. Compared with a lone Er:Yb:YAl3(BO3)4 crystal measured under the identical experimental conditions, higher laser performances were demonstrated in the sapphire/Er:Yb:YAl3(BO3)4/sapphire composite crystal due to the reduction of the thermal effects. End-pumped by a 976 nm laser diode in a hemispherical cavity, a 1.55 µm continuous-wave laser with a maximum output power of 1.75 W and a slope efficiency of 36% was obtained in the composite crystal when the incident pump power was 6.54 W. Passively Q-switched by a Co2+:MgAl2O4 crystal, a 1.52 µm pulse laser with energy of 10 µJ and repetition frequency of 105 kHz was also realized in the composite crystal. Pulse width was 315 ns. The results show that the sapphire/Er:Yb:YAl3(BO3)4/sapphire composite crystal is an excellent active element for 1.55 µm laser.

Long-distance thermal temporal ghost imaging over optical fibers.

A thermal ghost imaging scheme between two distant parties is proposed and experimentally demonstrated over long-distance optical fibers. In the scheme, the weak thermal light is split into two paths. Photons in one path are spatially diffused according to their frequencies by a spatial dispersion component, then illuminate the object and record its spatial transmission information. Photons in the other path are temporally diffused by a temporal dispersion component. By the coincidence measurement between photons of two paths, the object can be imaged in a way of ghost imaging, based on the frequency correlation between photons in the two paths. In the experiment, the weak thermal light source is prepared by the spontaneous four-wave mixing in a silicon waveguide. The temporal dispersion is introduced by single-mode fibers of 50 km, which also could be looked at as a fiber link. Experimental results show that this scheme can be realized over long-distance optical fibers.

940 mW 1564 nm multi-longitudinal-mode and 440 mW 1537 nm single-longitudinal-mode continuous-wave Er:Yb:Lu2Si2O7 microchip lasers.

An Er:Yb:Lu2Si2O7 microchip laser was constructed by placing a 1.2 mm thick, Y-cut Er:Yb:Lu2Si2O7 microchip between two 1.2 mm thick sapphire crystals, in which input and output mirrors were directly deposited onto one face of each crystal. End-pumped by a continuous-wave 975.4 nm diode laser, a 1564 nm multi-longitudinal-mode laser with a maximum output power of 940 mW and slope efficiency of 20% was realized at an absorbed pump power of 5.5 W when the transmission of output mirror was 2.2%. When the transmission of the output mirror was increased to 6%, a 1537 nm single-longitudinal-mode laser with a maximum output power of 440 mW and slope efficiency of 12% was realized at an absorbed pump power of 4.3 W. The results indicate that the Er:Yb:Lu2Si2O7 crystal is a promising microchip gain medium to realize a single-longitudinal-mode laser.

Measuring the orbital angular momentum spectrum with a single point detector.

It is tough work to measure the orbital angular momentum (OAM) spectrum due to the requirement of a reference light or photodetector arrays, while measuring the spin angular momentum (SAM) is readily convenient and matured. In this work, the so-called OAM-SAM nonseparable states are employed, and the OAM spectrum can be obtained only by measuring the Stokes parameters of an arbitrary light beam with low loss noncascading structure and a single point detector. According to the experimental results, the fidelity of the measured OAM spectrum can be as high as 95.2% and 94.3% for OAM eigenstates and superposed states, respectively.

Fabrication and diode-pumped 1.55 µm continuous-wave laser performance of a diffusion-bonded Er:Yb:YAl3(BO3)4/YAl3(BO3)4 composite crystal.

A composite crystal consisting of a 0.4-mm-thick (1.1 at.%)Er:(25 at.%)Yb:YAl3(BO3)4 crystal and a pure 1.0-mm-thick YAl3(BO3)4 crystal was successfully fabricated by the thermal diffusion bonding method. When the pure YAl3(BO3)4 crystal was used as an effective heat sink, thermal effects in the Er:Yb:YAl3(BO3)4/YAl3(BO3)4 composite crystal can be reduced, and the maximum output power of 1.55 µm laser was increased from 350 mW in a lone 0.4-mm-thick Er:Yb:YAl3(BO3)4 crystal to 780 mW in the composite crystal under identical experimental conditions. The results show that using such a diffusion-bonded Er:Yb:YAl3(BO3)4/YAl3(BO3)4 composite crystal as a gain medium can effectively enhance the performance of a 1.55 µm laser.