CTH:YAG : from laser medium to luminescent concentrator.

This work presents what we believe is a new way to use a CTH:YAG crystal for spontaneous emission instead of laser emission. The spontaneous emission is collected in one main direction thanks to a luminescent concentrator configuration. The CTH:YAG is indirectly LED-pumped by a Ce:YAG delivering 3.5 ms pulses at 10 Hz with an energy of 2 J in the visible (550-650 nm). In a configuration optimized for light extraction, the CTH:YAG luminescent concentrator provides a broadband emission between 1.8 µm and 2.1 µm with a unique combination of power (1 W) and brightness (21.2 W/cm2/sr) that could be useful for short-wave infrared (SWIR) lighting applications.

LED-pumped Er:Cr:YSGG light sources.

For, what we believe is, the first time, an Er:Cr:YSGG crystal is pumped by LEDs through a Ce:YAG luminescent concentrator. We demonstrate both laser emission at 2.79 µm and strong spontaneous emission at 1.6 µm. The luminescent concentrator delivers 1.5 ms pulses at 10 Hz in the visible (550-650 nm) to the Er:Cr:YSGG crystal, in a transverse pumping configuration. The Er:Cr:YSGG laser produces up to 6.8 mJ at 2.79 µm in a biconcave cavity. The Er:Cr:YSGG also stands out as a bright broadband incoherent source around 1.6 µm with a unique combination of peak power (351 mW) and brightness (1.4 W/sr/cm2).

Ce:LYSO, from scintillator to solid-state lighting as a blue luminescent concentrator.

Cerium-doped lutetium-yttrium oxyorthosilicate (Ce:LYSO) is a well-known single crystal scintillator used in medical imaging and security scanners. Recent development of high power UV LED, matching its absorption band, questions the possibility to use Ce:LYSO in a new way: as LED-pumped solid-state light source. Since Ce:LYSO is available in large size crystals, we investigate its potential as a luminescent concentrator. This paper reports an extensive study of the performance in close relation to the spectroscopic properties of this crystal. It gives the reasons why the Ce:LYSO crystal tested in this study is less efficient than Ce:YAG for luminescent concentration: limited quantum efficiency and high losses coming from self-absorption and from excited-state absorption are playing key roles. However, we demonstrate that a Ce:LYSO luminescent concentrator is an innovative source for solid-state lighting. Pumped by a peak power of 3400 W in quasi-continuous wave regime (40 µs, 10 Hz), a rectangular (1 × 22 × 105 mm3) Ce:LYSO crystal delivers a broadband spectrum (60 nm FWHM) centered at 430 nm. At full output aperture (20 × 1 mm2), it emits a peak power of 116 W. On a squared output surface (1 × 1 mm2) it emits 16 W corresponding to a brightness of 509 W cm-2 sr-1. This combination of spectrum power and brightness is higher than blue LEDs and opens perspectives for Ce:LYSO in the field of illumination namely for imaging.

LED-pumped Cr:LiSAF laser system operating at 100†Hz based on a multipass amplifier.

The LED-pumping technology is used for the first time, to the best of our knowledge, to develop a complete master oscillator power amplifier (MOPA) system including a multipass amplifier. A pumping head using an original slab architecture is developed integrating a Cr:LiSAF slab pumped by 2112 blue LEDs via a Ce:YAG luminescent concentrator. The slab configuration enables the reaching of a large number of passes-up to 22-together with access to efficient cooling, allowing for a repetition rate scale up. For 22 passes, the amplifier delivers pulses with energy up to 2.4 mJ at 10-Hz repetition rate with a gain of 4.36 at 825 nm. A complete study of the MOPA is described, concluding in nearly constant performances versus the repetition rate, up to 100 Hz.

Light recycling in LED-pumped Ce:YAG luminescent concentrators.

We report the development of a high-brightness, high-power Ce:YAG luminescent concentrator pumped by 2240 blue LEDs in quasi-continuous wave operation (10 µs, 10  Hz). Using light confinement and recycling in the three space dimensions, the parallelepiped (1mm×14×mm×200mm) Ce:YAG emits a power of 145 W from a square output surface (1 × 1mm2) corresponding to a brightness of 4.6 kW/cm2/sr. This broadband yellow source has a unique combination of luminous flux (7.6 104 lm) and brightness (2.4 104 cd/mm2) and overcomes many other visible incoherent sources by one order of magnitude. This paper also proposes a deep understanding of the performance drop compared to a linear behavior when the pump power increases. Despite excited state absorption was unexpected for this low doped Ce:YAG pumped at a low irradiance level, we demonstrated that it affects the performance by tripling the losses in the concentrator. This effect is particularly important for small output surfaces corresponding to strong light recycling in the concentrator and to average travel distances inside the medium reaching meters.

Efficient and high-throughput ablation of platinum using high-repetition rate radially and azimuthally polarized sub-picosecond laser pulses.

A highly productive ablation process of 100 nm thick platinum films with a processed area rate of up to 378 cm2/min is presented using radially and azimuthally polarized laser beams. This was achieved by developing a laser amplifier generating 757 fs long laser pulses at a maximum average power of 390 W and a repetition rate of 10.6 MHz with adjustable polarization states, i.e., linear, radial, and azimuthal polarization on the work piece. The pulse train emitted from the laser was synchronized to a custom-designed polygon scanner and directed into an application machine.

LED-pumped femtosecond Cr:LiSAF regenerative amplifier system.

We report on the first, to the best of our knowledge, LED-pumped femtosecond regenerative amplifier. It is based on a Cr:LiSAF crystal pumped by 2240 blue LEDs via a Ce:YAG luminescent concentrator. The amplifier was seeded by pulses from a Ti:sapphire oscillator at 835 nm temporally stretched from 90 fs to 100 ps. At the output of the regenerative amplifier, we obtain 1 mJ pulse energy at a 10 Hz repetition rate, given by the frequency of the LED-pumping module. After compression, we obtain 100 fs pulses with a spectral bandwidth of 10 nm at 835 nm.

3D luminescent concentrators.

A solution to develop high-brightness incoherent sources consists in luminescent concentration. Indeed, the absorption/emission process in a high index medium allows us to circumvent the brightness conservation law by the confinement of the light in 1 or 2 dimensions. In practice, Ce-doped luminescent concentrators pumped with InGaN LED exceed LED's brightness by one order of magnitude. This work shows how light confinement in 3 dimensions increases the brightness by an additional order of magnitude. Thanks to an analytical approach validated by experimental results, this concept gives new degrees of freedom for the design of luminescent concentrators and paves the way to a generation of incoherent sources among the brightest ever designed.

Tunable UV source based on an LED-pumped cavity-dumped Cr:LiSAF laser.

We developed a light-emitting diode (LED)-pumped Cr:LiSAF laser operating in Q-switched and cavity-dumped regimes. The laser produces 1.1 mJ pulses with a pulse duration of 8.5 ns at a repetition rate of 10 Hz on a broad spectrum centered at 840 nm with a full width at half maximum of 23 nm. After frequency tripling in two cascaded LBO crystals, we obtained 7 ns pulses with an energy of 13 µJ at 280 nm and with a spectral width of 0.5 nm, limited by the spectral acceptance of the phase matching process. By rotating both LBO crystals, UV emission is tuned from 276 nm to 284 nm taking advantage of the broad infrared spectrum of the Cr:LiSAF laser.

LED-pumped passively Q-switched Cr:LiSAF laser.

We report, to the best of our knowledge, the first light-emitting diode (LED)-pumped Cr:LiSAF laser, in both quasi-continuous-wave (QCW) and passively Q-switched operation. This Letter is based on the recent development of LED-pumped luminescent concentrators (LCs). Combining the capacity of high-density integration of blue LEDs with the excellent properties of Ce:YAG LCs, this new pump source can deliver high irradiance (7.3 kW/cm2) in the visible to pump Cr:LiSAF. The Cr:LiSAF laser demonstrates an energy of 8.4 mJ at 850 nm in QCW (250 µs pulses at 10 Hz). A small signal gain per roundtrip of 1.44 at 850 nm and a wavelength tunability between 810 and 960 nm have been performed. A passively Q-switched oscillator is also presented using a Cr:YAG saturable absorber. A peak power of 3.1 kW is obtained with a pulse energy of 130 µJ and duration of 41.6 ns.

New LED-based high-brightness incoherent light source in the SWIR.

The first LED-pumped luminescent concentrator (LC) emitting in the short-wave infrared (SWIR) is reported. Low cost LEDs (at 940 nm) are used to pump a Yb,Er:Glass LC emitting at 1550 nm. The optical conversion efficiency of the system is optimized and studied in detail for several optical configurations. A total of 128 LEDs having an emitting surface of 1 mm2 and an irradiance of 51.6 W/cm2, corresponding to a total pump power of 66 W, are used. Optimizing the output power out of a 100-mm-long LC in a continuous wave regime, a power of 850 mW is extracted from the 2.5 x 2 mm2 LC emitting surface area. The optical efficiency is then 1.29%. The performance of this luminescent concentrator is higher by one order of magnitude in term of radiance compared to an LED emitting at the same wavelength.

LED-pumped alexandrite laser oscillator and amplifier.

Taking advantage of light-emitting-diode (LED) performance breakthrough driven by the lighting market, we report, to the best of our knowledge, the first LED-pumped chromium-doped crystal laser oscillator and amplifier based on alexandrite crystals (Cr3+:BeAl2O4). We developed a Ce:YAG concentrator as the pumped source, illuminated by blue LEDs that can be easily power scaled. With 2200 LEDs (450 nm), the Ce:YAG concentrator can deliver to the gain medium up to 268 mJ at 10 Hz at 550 nm with a irradiance of 8.5 kW/cm2. We demonstrate, in oscillator configuration, an LED-pumped alexandrite laser delivering an energy of 2.9 mJ at 748 nm in free running operation. In the cavity, we measured a double-pass small signal gain of 1.28, which is in good agreement with numerical simulations. As an amplifier, the system demonstrated to boost a CW Ti:sapphire laser by a factor of 4 at 750 nm in eight passes with a large tuning range from 710 nm to 800 nm.

Yb:YAG single-crystal fiber amplifiers for picosecond lasers using the divided pulse amplification technique.

A two-stage master-oscillator power-amplifier (MOPA) system based on Yb:YAG single-crystal-fiber (SCF) technology and designed for high peak power is studied to significantly increase the pulse energy of a low-power picosecond laser. The first SCF amplifier has been designed for high gain. Using a gain medium optimized in terms of doping concentration and length, an optical gain of 32 dB has been demonstrated. The second amplifier stage designed for high energy using the divided pulse technique allows us to generate a recombined output pulse energy of 2 mJ at 12.5 kHz with a pulse duration of 6 ps corresponding to a peak power of 320 MW. Average powers ranging from 25 to 55 W with repetition rates varying from 12.5 to 500 kHz have been demonstrated.

Contradiction within wave optics and its solution within a particle picture: comment.

An error in the rationale presented in the paper "Contradiction within wave optics and its solution within a particle picture" by Altmann [Opt. Express 23, 3731 (2015)10.1364/OE.23.003731] is discussed.

High-power Yb:YAG single-crystal fiber amplifiers for femtosecond lasers in cylindrical polarization.

We demonstrate a three-stage diode-pumped Yb:YAG single-crystal-fiber amplifier to generate femtosecond pulses at high average powers with linear or cylindrical (i.e., radial or azimuthal) polarization. At a repetition rate of 20 MHz, 750-fs pulses were obtained at an average power of 85 W in cylindrical polarization and at 100 W in linear polarization. The report includes investigations on the use of Yb:YAG single-crystal fibers with different length/doping ratio and the zero-phonon pumping at a wavelength of 969 nm in order to optimize the performance.

Revisiting of LED pumped bulk laser: first demonstration of Nd:YVO4 LED pumped laser.

We describe here what is, to the best of our knowledge, the first LED pumped Nd:YVO4 laser. Near-IR LED arrays with a wavelength centered close to 850 nm were used to pump transversely the crystal. By pulsing LEDs, with a duration of the order of the laser transition lifetime, we obtained sufficient pump intensities to reach the laser threshold. At a frequency of 250 Hz, we obtained an output energy of 40 µJ at 1064 nm for an input pump energy of 7.4 mJ, which corresponds to an optical efficiency of 0.5%. Experimental results of small signal gain are compared with theoretical analysis.

Single YVO4:Eu nanoparticle emission spectra using direct Eu3+ ion excitation with a sum-frequency 465-nm solid-state laser.

We report emission spectrum measurements on single YxEu(1-x)VO4 nanoparticles. The inhomogeneous widths of the emission peaks are identical for single nanoparticles and for ensembles of nanoparticles, while being broader than those of the bulk material. This indicates that individual nanoparticles are identical in terms of the distribution of different local Eu3+ sites due to crystalline defects and confirms their usability as identical, single-particle oxidant biosensors. Moreover, we report a 465 nm solid-state laser based on sum-frequency mixing that provides a compact, efficient solution for direct Eu3+ excitation of these nanoparticles. Both these two aspects should broaden the scope of Eu-doped nanoparticle applications.

Mechanical phase matching of birefringent non-linear crystals.

Second-order nonlinear processes such as second harmonic generation or parametric amplification have found numerous applications in the scientific and industrial world, from micromachining to petawatt laser facilities. These nonlinear interactions are mostly carried out in birefringent crystals because of their low cost and the possibility to operate at high powers Phase-matching configurations in birefringent crystals are determined by their refractive indexes. Here, we show that an important mechanical stress can be used to significantly change the phase-matching properties of a birefringent crystal. As an example, we demonstrate the shift of second harmonic non-critical phase matching wavelength of LiB3O5 (LBO) crystal at room temperature from 1200 nm to 1120 nm by applying compressive forces up to 100 MPa. We believe that this mechanical phase matching can be used as an additional degree of freedom to optimize nonlinear optical frequency mixing geometries.

Deep-UV 236.5 nm laser by fourth-harmonic generation of a single-crystal fiber Nd:YAG oscillator.

We demonstrate a deep-UV laser at 236.5 nm based on extracavity fourth-harmonic generation of a Q-switched Nd:YAG single-crystal fiber laser at 946 nm. We first compare two nonlinear crystals available for second-harmonic generation: LBO and BiBO. The best results at 473 nm are obtained with a BiBO crystal, with an average output power of 3.4 W at 20 kHz, corresponding to a second-harmonic generation efficiency of 38%. This blue laser is frequency-converted to 236.5 nm in a BBO crystal with an overall fourth-harmonic generation yield of 6.5%, corresponding to an average output power of 600 mW at 20 kHz. This represents an order of magnitude increase in average power and energy compared to previously reported pulsed lasers at 236.5 nm. This work opens the possibility of LIDAR detection of dangerous compounds for military or civilian applications.

Direct amplification of a nanosecond laser diode in a high gain diode-pumped Nd:YVO4 amplifier.

We demonstrate that Nd:YVO4 can efficiently amplify a nanosecond laser diode in a very simple double-pass configuration. Based on longitudinal pumping with a high brightness fiber-coupled laser diode at 808 nm (60 W, 100 µm, 0.22 NA) and a low Nd-doped (0.2%) temperature controlled Nd:YVO4 we achieved an optical gain of 62 dB with very low (<2%) parasitic laser emission and an average output power of 10 W. At 15 kHz, we observed a strong gain saturation dynamic resulting in a pulse duration reduction from 100 to 3.5 ns. This effect enhances the peak power by a factor of 18 (130 kW) with an energy of 620 µJ.