Diffraction-limited Output Beam Research Articles

Sub-nanosecond Raman fiber amplifier based red-orange light source

We introduce a fully-integrated two-color sub-nanosecond fiber laser system that incorporates a backward-pumped polarization-maintaining (PM) Raman phosphosilicate fiber amplifier (RFA) followed by two fully-integrated fiber-coupled second harmonic generator (SHG) modules. The RFA is pumped by a continuous-wave (CW) Yb laser operating at 1116 nm. The pulsed signals are generated by gain-switched distributed feedback (DFB) laser diodes at 1178 nm and 1310 nm, respectively. The output pulsed DFB signals are independently or simultaneously amplified in the RFA. This amplification is achieved using both the broad SiO2 (∼13.2 THz) and relatively narrow P2O5 (39.9 THz) Stokes shifts. The laser system produces sub-nanosecond pulses at 589 and 655 nm, featuring repetition rates ranging from 40 to 100 MHz and an average power of up to 3 W (limited by the SHG crystal damage threshold) at each wavelength. The diffraction-limited output beams maintain an M2 value of < 1.05 across the entire range of output powers and repetition rates for both wavelengths.

10-µJ-level femtosecond pulse generation in the erbium CPA fiber source with microstructured hollow-core fiber assisted delivery and nonlinear frequency conversion.

We report on the development of a chirped pulse amplification (CPA) designed erbium fiber source with a hybrid high-power amplifier, which is composed of erbium-doped and erbium/ytterbium-co-doped double-clad large-mode-area fibers. Stretched pulses from the high-power amplifier with up to 21.9µJ energy and 198.5kHz repetition rate are dechirped in the transmission grating pair-based compressor with 73% efficiency, yielding as short as 742fs duration with 15.8µJ energy and ≈13M W peak power (maximum average power up to 3.14W) at the central wavelength of 1.56µm. Compressed pulses are coupled into microstructured negative-curvature hollow-core fibers with a single row capillary cladding and different core sizes of 34µm and 75µm in order to realize femtosecond pulse delivery with a diffraction-limited output beam (M 2≤1.09) and demonstrate ∼200n J Stokes pulse generation at 1712nm via rotational SRS in pressurized hydrogen (H 2). We believe that the developed system may be a prospect for high-precision material processing and other high-energy and high-peak-power laser applications.

Picosecond thin-disk laser platform PERLA for multi-beam micromachining

Multi-beam micro- and nano-machining of material surfaces has been getting more important because of its great potential to increase production speed of large size laser induced periodic surface structures (LIPSS). Fast and cheap production of engineered surfaces structures can bring unique properties of surfaces like tailored wettability, friction, antibacterial properties, etc., to mass-production with consequence in, for example, energy and costs savings. However, tailoring of long-term stable interference patterns from ultrashort laser pulses requires an extremely stable laser system with nearly diffraction-limited output beams. HiLASE Centre developed such a thin-disk-based Yb:YAG sub-picosecond laser platform, PERLA, providing average output power up to 0.5 kW with 2nd and 4th harmonic generation extensions and demonstrated its potential for direct laser interference patterning (DLIP). In this paper, we focus on details of the thin-disk PERLA laser.

Efficient continuous-wave Nd:YLF laser in-band diode-pumped at 908 nm and its thermal lensing

We report on what we believe is the first continuous-wave yttrium lithium fluoride laser with in-band diode pumping at 908 nm. Pumping at this wavelength reduced the quantum defect by more than 50% when compared to the conventional pumping at ~800 nm. The laser produced an output power of 850 mW at 1047 nm and its radiation was -polarized. It had a near diffraction limited output beam and a maximum slope efficiency of ~73.9% with respect to the absorbed pump power. The thermal lensing strength was experimentally measured and compared with the previous experimental data. It was found that the thermal lensing was significantly reduced when compared to the traditional ~800 nm pumping.

Nd:YVO4 disk laser with multipoint diode pumping and diffraction limited output

The output beam parameters of the disk laser were investigated at different pump powers of nine-beam spatially periodic pumping and different resonator lengths. It was realized that a super-mode lasing regime and the values of thermally-induced distortions of an active disk were measured. The diffraction-limited output beam parameters came from the total aperture of all the lasing spots. An explanation of the in-phase super-mode lasing conditions was proposed, and the limits of its operation were defined.

Improvement to beam quality of optically pumped terahertz gas lasers with hole-coupling resonators

The hole-coupling method, which is commonly used in the optically pumped THz gas laser, will reduce the beam quality due to the hard-edge diffraction. It is shown that by optimizing the parameters of the resonator a diffraction-limited output beam can be obtained. A method is proposed for the selection of a higher order waveguide mode, whose first zero point along the radial direction coincides with the edge of the output hole coupler and the effect of hard edge diffraction is depressed. Measurements of beam quality are taken on an experimental prototype whose optical setup is designed by this method. A quasi-Gaussian beam output is obtained, and the beam quality M2 factor is about 1.15

Thermo-optical and lasing characteristics of Cr^2+-doped CdSe single crystal as tunable coherent source in the mid-infrared

We report on a comprehensive characterization of Cr2+-doped CdSe single crystal as an efficient active material for tunable laser applications in the mid-infrared spectral region. Optical gain, thermo-optical behavior, power efficiency, scalability and wavelength tunability have been thoroughly investigated. Using an antireflection-coated crystal pumped by a Tm-fiber laser at at 1.94 μm, 1-W CW output power and 50% slope efficiency at 2.65 μm emission wavelength have been obtained in a diffraction-limited output beam. An output peak power of 2.5 W has been achieved without significant beam distortion in a quasi-CW regime. Exploiting an intra-cavity diffraction grating in a Littrow configuration, a maximum tuning range of 900 nm from 2.22 to 3.12 μm, limited by the finite bandwidth of resonator components, has been demonstrated with an uncoated crystal.

Highly-efficient Q-switched Yb:YLF laser at 995 nm with a second harmonic conversion

We report on a high-average power, pulsed source at 497.5 nm based on the frequency doubling of an acousto-optically Q-switched, cryogenically-cooled Yb:YLiF4 (Yb:YLF) laser. The Yb:YLF was resonantly pumped by a laser diode module into the 960 nm absorption band and generated a diffraction limited output beam at 995 nm. At this wavelength, the laser delivered up to 50 W of average Q-switched output power at a pulse repetition frequency (PRF) of 10 kHz with a pulse duration of 60 ns and a slope efficiency of 69%. At a PRF of 500 Hz, this laser yielded 14 ns long, 31 mJ pulses with the slope efficiency of ~40%. Frequency doubling was achieved using an LBO crystal with a conversion efficiency approaching 50% at both 500 and 1000 Hz PRF. The maximum achieved pulse energy at 497.5 nm was 14.3 mJ at 500 Hz PRF (average power over 7 W). To the best of our knowledge, this is the highest reported pulse energy and average power for an all-solid-state laser operating under 500 nm.

Narrow ridge GaSb‐based cascade diode lasers fabricated by methane–hydrogen reactive ion etching

GaSb-based type-I quantum wells cascade diode lasers with nearly diffraction limited output beam were fabricated using methane–hydrogen dry etching plasma process. Rapid thermal annealing was shown to be a necessary step to reverse the effect of hydrogen plasma on conductivity of AlGaAsSb cladding alloy. Annealed two-step narrow ridge lasers confined both optical field and current in lateral direction and demonstrated operating voltage, threshold current density and slope efficiency comparable to those of reference wide ridge devices.

Inkjet-printed vertically emitting solid-state organic lasers

In this paper, we show that Inkjet Printing can be successfully applied to external-cavity vertically emitting thin-film organic lasers and can be used to generate a diffraction-limited output beam with an output energy as high as 33.6 μJ with a slope efficiency S of 34%. Laser emission shows to be continuously tunable from 570 to 670 nm using an intracavity polymer-based Fabry-Perot etalon. High-optical quality films with several μm thicknesses are realized, thanks to ink-jet printing. We introduce a new optical material where EMD6415 commercial ink constitutes the optical host matrix and exhibits a refractive index of 1.5 and an absorption coefficient of 0.66 cm−1 at 550–680 nm. Standard laser dyes like Pyrromethene 597 and Rhodamine 640 are incorporated in solution to the EMD6415 ink. Such large size “printed pixels” of 50 mm2 present uniform and flat surfaces, with roughness measured as low as 1.5 nm in different locations of a 50 μm × 50 μm AFM scan. Finally, as the gain capsules fabricated by Inkjet printing are simple and do not incorporate any tuning or cavity element, they are simple to make, have a negligible fabrication cost, and can be used as fully disposable items. This work opens the way towards the fabrication of really low-cost tunable visible lasers with an affordable technology that has the potential to be widely disseminated.

In-band-pumped Ho:KLu(WO4)2 microchip laser with 84% slope efficiency.

We report on a continuous-wave Ho:KLu(WO4)2 (KLuW) microchip laser with a record slope efficiency of 84%, the highest value among the holmium inband-pumped lasers, delivering 201 mW output power at 2105 nm. The Ho laser operating at room temperature on the (5)I8→(5)I7 transition is in-band-pumped by a diode-pumped Tm:KLuW microchip laser at 1946 nm. Ho:KLuW laser operation at 2061 and 2079 nm is also demonstrated with a maximum slope efficiency of 79%. The microchip laser generates an almost diffraction-limited output beam with a Gaussian profile and a M2<1.1. The laser performance of the Ng-cut Ho:KLuW crystal is very similar for pump light polarizations ‖Nm and Np. The positive thermal lens plays a key role in the laser mode stabilization and proper mode-matching. The latter, together with the low quantum defect under in-band-pumping (∼0.08), is responsible for the extraordinary high slope efficiency.

High efficiency single-mode-multimode-single-mode fiber laser with diffraction-limited beam output.

We designed and tested an all-fiber, high efficiency Yb-doped laser operating at 1088nm with a single-mode-multimode-single-mode (SMS) structure. A larger-mode-area gain fiber of 1.5m length, with 20/130μm core/cladding diameters was used to increase the absorption, and a diffraction-limited Gaussian output beam was obtained from the single-mode output fiber. Using a 976nm laser diode as the pump source, the laser generated an output power up to 38.5W with a slope efficiency of 70%. The output beam qualities, with and without SMS structure, were compared and showed that the fiber laser with the SMS structure can achieve high gain, short fiber length, and excellent beam quality.

Highly efficient, kW peak power, 1.55 µm all-fiber MOPA system with a diffraction-limited laser output beam

We report a high-efficiency, single-mode, all-fiber pulsed laser system built in a master oscillator power amplifier format, operating in the eye-safe (λ ~1,549 nm) spectral range, providing over 1.5 W of average output power with up to 51 dB of total signal gain. It comprises the flexibility of smooth pulse duration control from single ns to 260 ns with independently tunable repetition rate ranging from 40 kHz to 1 MHz. Pulses as short as 4 ns with up to 20 μJ energy and corresponding peak power of 5 kW are demonstrated. The laser system delivered a nearly diffraction-limited beam with M 2 ~1.

Low Temperature Gold-to-Gold Bonded Semiconductor Disk Laser

We present a gold-to-gold bonding method that combines features of surface activated bonding and capillary bonding. The process is performed at a relatively low temperature of 150°C and therefore allows the integration of materials with highly mismatched coefficients of thermal expansion. In this letter, the potential of this technique is illustrated by assembling a high-power flip chip semiconductor disk laser utilizing a chemical vapor deposition diamond heat spreader. The laser produces up to 14 W of output power at 15°C gain element temperature with a nearly diffraction-limited output beam. Further scaling of bonding area to wafer-level could make this method useful in the packaging of various optoelectronic and microelectronic components.

Accurate Modeling and Design of Graded-Index Fiber Probes for Optical Coherence Tomography Using the Beam Propagation Method

Fiber-optic probes for sensing and biomedical imaging applications such as optical coherence tomography (OCT) frequently employ sections of graded-index (GRIN) fiber to re-focus the diverging light from the delivery fiber. Such GRIN fiber microlenses often possess aberrations that cause significant distortions of the focused output beam. Current design methods based on ABCD matrix transformations of Gaussian beams cannot model such effects and are therefore inadequate for the analysis and design of high-performance probes that require diffraction-limited output beams. We demonstrate use of the beam propagation method (BPM) to analyze beam distortion in GRIN-lensed fibers resulting from index profiles that exhibit a deviation from the ideal parabolic shape or artifacts such as ripples or a central dip. Furthermore, we demonstrate the power of this method for exploring novel probe designs that incorporate GRIN phase masks to generate wavefront-shaped output beams with extended depth-of-focus (DOF). We present results using our method that are in good agreement with experimental data. The BPM enables accurate simulation of fiber probes using non-ideal or custom-engineered GRIN fibers with arbitrary refractive index profiles, which is important in the design of high-performance fiber-based micro-imaging systems for biomedical applications.

Thermal Effects on the Single-Mode Regime of Distributed Modal Filtering Rod Fiber

Power scaling of fiber laser systems requires the development of innovative active fibers, capable of providing high pump absorption, ultralarge effective area, high-order mode suppression, and resilience to thermal effects. Thermally induced refractive index change has been recently appointed as one major limitation to the achievable power, causing degradation of the modal properties and preventing to obtain stable diffraction-limited output beam. In this paper, the effects of thermally induced refractive index change on the guiding properties of a double-cladding distributed modal filtering rod-type photonic crystal fiber, which exploits resonant coupling with high-index elements to suppress high-order modes, are thoroughly investigated. A computationally efficient model has been developed to calculate the refractive index change due to the thermo-optical effect, and it has been integrated into a full-vector modal solver based on the finite-element method to obtain the guided modes, considering different heating conditions. Results have shown that the single-mode regime of the distributed modal filtering fiber is less sensitive to thermal effects with respect to index-guiding fibers with the same effective area. In fact, as the pump power is increased, their single-mode regime is preserved, being only blue-shifted in wavelength.

High-power room-temperature continuous-wave mid-infrared interband cascade lasers

We demonstrate cw output powers >290 mW into a nearly diffraction-limited (M² ≈2.2) output beam from an interband cascade laser operating at λ = 3.6-3.7 μm at room temperature. The interband cascade laser was designed for nearly equal electron and hole populations in the active region with heavy electron-injector doping, and was processed into narrow ridges mounted epitaxial side down on a copper heat sink. A 15.7-μm-wide, 4-mm-long ridge with the back facet coated for high reflection (HR) and an anti-reflection-coated front facet produced 253 mW of cw output power at T = 25°C into a beam with M² ≈2.7. Furthermore, corrugating the sidewalls of the ridge leads to a 20% improvement in the brightness. A 15.7-μm-wide, 0.5-mm-long ridge with an HR-coated back facet and an uncoated front facet exhibited a maximum cw wall-plug efficiency of nearly 15% at room temperature.

Reflective Heterostructure Photonic Crystal Superprism Demultiplexer

A new design for a wideband and compact demultiplexer for coarse wavelength-division-multiplexing applications is proposed. This is based on the superprism effect, using diffraction compensation in a stratified hetero-lattice photonic crystal (PhC) operating in reflective mode. Modeling the variation of the beam envelope within the hetero-lattice, an optimum eight-channel design with 20-nm wavelength spacing is feasible within 42000 μm2 of PhC area. The design results in almost diffraction-limited output beams, and provides a size reduction of 2.6 to 7 times when compared to the PhC area required in the homo-lattice SP design.

Er-doped photonic crystal fiber amplifier with 70 W of output power

We report for the first time, to the best of our knowledge, on a large mode area Er-doped photonic crystal fiber laser system. The fiber core had a diameter of 40 μm and NA of <0.04 leading to a mode field diameter of about 31 μm around 1550 nm wavelength. More than 70 W of single-frequency output power at 1556 nm could be extracted using a master-oscillator power-amplifier scheme. Near diffraction limited output beam quality has been verified by measuring the TEM(00) content with a nonconfocal scanning ring cavity.

Femtosecond pulse generation and amplification in Yb-doped fibre oscillator-amplifier system

In recent times ytterbium (Yb) doped fibre-based mode-locked master oscillator and power amplifier have attracted a great deal of interest because of their inherent advantages like flexibility, reliability, compactness, high power handling capability and diffraction limited output beam quality as compared to the solid-state counterpart. But, to successfully develope of high-power femtosecond oscillator-amplifier system based on Ybdoped fibre, an appropriate choice of the mode-locking regime and the amplifier geometry are required. Development of an all-fibre integrated high-power Yb-doped fibre oscillator-amplifier system in which the advantages of a fibre-based system can be fully exploited remained a challenge as it requires the careful optimization of dispersion, nonlinearity, gain and ASE contribution.