Enhanced polymer ablation rates using high-repetition-rateultraviolet lasers

Etch rates (μm/pulse) for glycol-modified polyethylene terephthalate (PETG) under pulsed UV (255 nm) laser processing are measured as a function of pulse repetition frequency in the range 0.7-15 kHz. Materials removal rates (μm/s) scale approximately linearly with pulse repetition frequency at a fluence of 0.59 J/cm², and there appears to be no attenuation of the ablating laser beam by the ejected material plume for pulse rates up to 15 kHz. The instantaneous etch rate for pulses in a sequence increases markedly (~40%) for long pulse sequences (>100 pulses) at high PRF (15 kHz), an effect which can be used to increase machining rates while operating at a moderate laser fluence     

Low-heat high-power scaling using InGaAs-diode-pumped Yb:YAG lasers

We report to our knowledge the highest to date quasi-CW output power, 600 W and pulse energy, >1 J, for an InGaAs diode-pumped Yb:YAG laser. In separate preliminary results, we have also obtained 225 W of average output power under true CW diode pumping. This performance was obtained using a laser head designed to be part of a master oscillator power amplifier (MOPA) operating at 3 kW. We summarize why the diode-pumped Yb:YAG crystal laser is ideal for scaling to high average powers and the different approaches being pursued. We also report our latest results for side-pumped rod devices     

High-power continuous-wave 3- and 2-μm cascadeHo3+:ZBLAN fiber laser and its medical applications

A new medical fiber laser oscillating at two useful wavelengths (3 and 2 μm) is reported. We have demonstrated highly efficient and high-power continuous-wave cascade oscillation at room temperature with a holmium ion-doped fluoride glass fiber laser pumped with a 1.15-μm fiber Raman laser. The simultaneous oscillation wavelengths were 3 and 2 μm, and their combined output power was 3.0 W with a slope efficiency of 65%. To our best knowledge, this is the first achievement of watt-level-output power in the mid-infrared region with ZrF₄-BaF₂-LaF₃-AlF₃-NaF (ZBLAN) glass fiber. In experiments to evaluate potential for medical applications, we tested the two wavelength beam as a laser surgical knife on soft rabbit tissues and demonstrated that it had strong cutting capability, and that the coagulation layer thickness could be controlled by varying the power ratio of the two-wavelength laser     

High magnification imaging with a laser-based hard X-ray source

The usefulness of laser-based X-ray sources for imaging is assessed. A very bright point source of 20-keV X-rays, with a diameter of 10 μm, has been created by the interaction of a high contrast femtosecond laser pulse with solid targets. This X-ray source has been used for a feasibility study of high magnification imaging. Images of mammography phantoms have been obtained and spatial resolution of 35 μm has been demonstrated with object magnification up to ten. A new laser technology recently introduced allows the construction of very intense, compact and cost effective short pulse laser systems and makes now the ultrafast laser based hard X-ray sources very attractive for medical application     

Active tracker laser (ATLAS)

A high brightness diode-pumped, Nd-YAG solid state laser has been designed, fabricated, and tested. This phase conjugated master oscillator/power amplifier (MOPA) device produces 20-ns Q-switched pulses at 2500 Hz at an average power of 690 W and a beam quality of 1.1×DL when the pump diodes are operated at 27.5% duty cycle. With an external KTP doubler, this device has produced 175 W of green average power at a beam quality of 1.5 × DL and a conversion efficiency of 45% over continuous operating times as long as one hour. This 1.06 μm result is believed to be the highest average power brightness achieved, and the 532-nm performance is both the highest average green power and the highest average brightness ever reported     

Integrated GaInAsP laser diodes with monitoring photodiodes throughsemiconductor/air Bragg reflector (SABAR)

A 1.3-μm GaInAsP laser diode (LD) is integrated with a monitoring photodiode (M-PD) through a semiconductor/air Bragg reflector (SABAR). Instead of conventional cleavage, the SABAR can provide not only Fabry-Perot resonance with high reflectivity, but also possibility of integration of laser with other functional devices. The design, fabrication, and some characteristics including threshold current, monitoring photocurrent, SABAR reflectivity as a function of the number of semiconductor/air pairs N are reported. The threshold current of ridge waveguide laser with SABAR (cavity length L=160 μm, ridge width W=7 μm, SABAR pairs N=3) is 20 mA. The threshold current is reduced by improving butt-coupled interface between active and passive waveguides employed in this laser and is expected 2 mA/μm. The monitoring photocurrent responds linearly with output power from the laser and 0.024 mA at laser output power of 5 mW. From the threshold characteristics, SABAR reflectivity is determined to >80%. The increase of photocurrent can be achieved by optimizing the number of SABAR pairs to N=1. We have obtained threshold current of 22 mA in the followed laser structure (L=270 μm, W=7 μm, N=1), and detector photocurrent of 1.13 mA (@5 mW). The experimental SABAR reflectivity is ~50%, which is estimated by threshold characteristics and efficiency of light output power. The laser has a mode field converter section, resulting in narrow beam divergence 11° along vertical axis. This integrated laser is very promising candidate for coming optical module in low-power consumption and low-cost access network systems     

Image quality analysis for dual energy subtraction imaging with afemtosecond laser-based hard X-ray source

We present a study of image quality for dual energy subtraction imaging using an iodinated contrast agent and a femtosecond laser-based hard X-ray source. The INRS CPA laser (400 fs pulse focused on solid targets in a 3 μm spot at 4 × 10¹⁸ W cm^-2) was used to create a bright hard X-ray source (conversion efficiency of 10^-5 in the characteristic K_∝ line emission, 12 μm X-ray source diameter). A model of image quality has been developed and been benchmarked with specific experiments using specially made angiography phantoms     

Fiber amplifiers for coherent space communication

We report on the application of double-clad doped fiber amplifiers for coherent space communication systems using a master oscillator power amplifier (MOPA) design at 1.06 μm. The master oscillator is either a single-frequency Nd:YAG solid-state laser or a distributed-feedback fiber laser. The power amplifier is a diode-laser-pumped double-clad Nd doped fiber with polarization control, 20 dB gain, and about 1.3 W output power. A dual stage configuration using a solid-state Nd:YAG amplifier as second stage is presented as well, increasing the output power to 3.5 W with 28 dB gain. We also report on the possibility to integrate a single-frequency fiber laser, an all-fiber phase modulator, and a fiber amplifier to build an all-fiber phase-modulated MOPA. Up to 1 W continuous-wave output phase-modulated with a bandwidth of 196 MHz has been achieved     

Development of a 2-kW XeCl laser with a surface coronapreionization scheme and a spiker-sustainer circuit

A maximum average power of 2.1 kW is demonstrated in a XeCl laser with an efficiency of 3.0% at a repetition rate of about 800 Hz. The 2-kW laser has a discharge region of 4 cm in gap length, 2.5 cm in discharge width, and an effective discharge length of 3 m with a resonator length of 5.2 m. A reverse voltage mode spiker-sustainer circuit makes it possible to apply a steep voltage rise of nearly 1000 kV/μs across the main discharge electrodes and to supply excitation energy effectively into the discharge region. The control of the preionization timing before the initiation of the spiker circuit is found to be indispensable in maintaining a homogeneous volume discharge under high repetition-rate operation. These techniques enable the laser to realize a long pulse excitation under the high repetition-rate operation conditions. The excitation process is analyzed by computer-model simulation. The application of a cascaded stable resonator has improved the average power by 6% compared with that of a conventional stable resonator     

Repetition rate scaling up to 100 kHz of a small-scale (50 W) kinetically enhanced copper vapor laser

We report on the pulse repetition frequency (PRF) scaling of a small-scale (25 mm bore and 0.61 m long) copper vapor laser (CVL). When operated as an elemental CVL, the laser had a stable output power of 15 W at 15 kHz PRF (9.6% efficiency). After the addition of small quantities of hydrogen and hydrogen chloride to the neon buffer gas, the maximum recorded stable output power increased to 41 W at 25 kHz PRF (1.4% efficiency). This represents a record stable specific output power of 0.14 W cm/sup -3/. Pulse repetition frequency scaling of the laser was demonstrated up to 100 kHz where the output power was 9.0 W. By operating the laser at elevated input powers, transient output powers of over 50 W were achieved between PRFs of 25-40 kHz. These results are the highest recorded specific output powers (0.17 W cm/sup -3/) for a CVL with this tube diameter.     

Self-starting 6.5-fs pulses from a Ti:sapphire laser using asemiconductor saturable absorber and double chirped mirrors

We demonstrate self-starting 6.5-fs pulses from a Kerr-lens-mode-locked Ti:sapphire laser with an average output power of 200 mW at a pulse repetition rate of 86 MHz. We have achieved a mode-locking buildup time of only 60 μs, using a broad-band semiconductor saturable absorber mirror to initiate the pulse formation. The dispersion has been compensated with a prism pair in combination with improved double-chirped mirrors. The prism pair allows for the flexible adjustment of both the duration and the center wavelength of the pulse. The double-chirped mirrors show a high reflectivity better than 99.8% over the full bandwidth of 300 nm and a controlled group delay over more than 250 nm. The choice of a proper output coupler turns out to be critical for ultrashort pulse generation directly from the laser     

High-power highly reliable Al-free 940-nm diode lasers

Al-free diode lasers emitting at 930 nm having a broadened step-index waveguide structure and a single active InGaAs quantum well have been realized by MOVPE. The impact of waveguide thickness on device performance has been studied. The highest wall plug efficiency of about 60% has been obtained with diode lasers having a 1-μm-thick waveguide. Increasing the waveguide thickness to 1.5 μm resulted in record low degradation rates below 10^-5 h^-1 for 3-W output power (100 μm stripe width). The same diode lasers showed a good long-term reliability even at an output power of 4 W. The best beam quality had diode lasers with a 2-μm-thick waveguide, at the expense of a reduced temperature stability     

Transmission properties of hollow glass waveguides for the deliveryof CO2 surgical laser power

Hollow glass waveguides are an attractive fiber delivery system for a broad range of infrared wavelengths, including the 3-μm Er:YAG and 10.6 μm CO₂ lasers. The losses for these waveguides are as low as 0.1 dB/m at the 10.6-μm wavelength for waveguides with a 700-μm bore. The guides are suitable for delivering laser powers well in excess of 100 W. Continuous power delivery for over 250 h is possible for powers less than 35 W. When stored under normal laboratory conditions, the loss is seen to change only slightly over a period up to two years     

Ce3+:LiCaAlF6 crystal for high-gain orhigh-peak-power amplification of ultraviolet femtosecond pulses and newpotential ultraviolet gain medium: Ce3+:LiSr0.8Ca0.2AlF6

To develop high-peak-power ultrashort pulse laser systems in the ultraviolet region, a large Ce³⁺:LiCaAlF₆ (Ce:LiCAF) crystal, a tunable ultraviolet laser medium with large saturation fluence and broad gain spectrum width, was grown successfully with a diameter of more than 70 mm. To demonstrate high small signal gain, a four-pass confocal amplifier with 60 dB gain and 54 μJ output energy was constructed. Chirped pulse amplification (CPA) in the ultraviolet region was demonstrated using Ce:LiCAF for higher energy extraction. A modified bow-tie-style four-pass amplifier pumped by 100-mJ 266-nm 10-Hz pulses from a Q-switched Nd:YAG laser had 370-times gain and delivered 6-mJ 290-nm pulses. After dispersion compensation, the output pulses can be compressed down to 115 fs. This is the first ultraviolet, all-solid-state high-peak-power CPA laser system using ultraviolet gain media, and this demonstration shows further scalability of the Ce:LiCAF CPA system. Additionally, a new gain medium, Ce³⁺ :LiSr_0.8Ca_0.2AlF₆, with longer fluorescence lifetime and sufficient gain spectrum width over 18 nm was grown to upgrade this system as a candidate for a final power amplifier gain module     

Widely tunable, near- to mid-infrared femtosecond and picosecondoptical parametric oscillators using periodically poledLiNbO3 and RbTiOAsO4

We describe the operation and characterization of Ti:sapphire laser-pumped femtosecond and picosecond optical parametric oscillators based the new quasi-phase-matched nonlinear materials of periodically poled LiNbO₃ and RbTiOAsO₄ with broad tunability in the near- to mid-infrared. We discuss the merits of the two materials for use in ultrafast optical parametric oscillators (OPOs) and compare and contrast their properties to the birefringent materials. We demonstrate an extended spectral coverage from <1 μm to >5 μm, pump power thresholds as low as 45 mW, average mid-infrared output powers in excess of 100 mW, and pulse durations of 100-200 fs and 1-2 ps at ~80 MHz repetition rate. We also report the efficient operation of Ti:sapphire-pumped femtosecond OPOs in all-solid-state configurations by utilizing diode-laser-based input pump sources     

High-power broad-area tapered amplifier with a monolithicallyintegrated output focusing lens at 0.98-μm wavelength

We have demonstrated a 0.98-μm wavelength tapered broad-area amplifier with a monolithically integrated aspherical waveguide lens. CW output exceeding 1 W from the amplifier-lens chip was measured with 10 mW input from a 0.98-μm diode laser. The integrated semiconductor waveguide lens focused the amplifier output to a 8 μm×3 μm spot, which was measured at output power up to about 0.5 W, corresponding to 2.5 times the diffraction limit The beam propagation method was used to model the integrated amplifier-lens chip, and the calculated focal distances agree with the experiment to within 5%. The integrated lens may be used for output coupling to a single mode fiber with the requirement that the focal point should be positioned on the output facet. Based on BPM simulation, however, the focal point position becomes uncritical if a single mode output waveguide is integrated. Our results indicate that the waveguiding lens is a useful component for the design of high-power photonic integrated circuits     

Advanced Ti:Er:LiNbO3 waveguide lasers

This paper reviews the latest developments of diode-pumped Ti,Er:LiNbO₃ waveguide lasers emitting at wavelengths around 1.5 μm. In particular, harmonically mode-locked lasers, Q-switched lasers, distributed Bragg reflector (DBR)-lasers, and self-frequency doubling lasers are discussed in detail. Supermode stabilized mode-locked lasers have been realized using a coupled cavity concept; a side mode suppression ratio of 55 dB has been achieved at 10-GHz pulse repetition rate with almost transform limited pulses. Q-switched lasers with a high extinction ratio (>25 dB) intracavity electrooptic switch emitted pulses with a peak power level up to 2.5 kW and a pulsewidth down to 2.1 ns at 1-kHz repetition frequency. Numerical simulations for both lasers are in a good, almost quantitative agreement with experimental results. A DBR-laser of narrow linewidth (≈3 GHz) with a permanent (fixed) photorefractive grating and 5 mW output power has been realized. Self frequency doubling lasers have been fabricated with a periodic microdomain structure inside an Er-doped laser cavity; simultaneous emission at the fundamental wavelength, 1531 nm, and at the second harmonic wavelength, 765 nm, has been obtained     

Research on efficient X-ray lasing at XingGuang II Laser Facility

Research on efficient X ray lasing at the XingGuang II Laser Facility has been reviewed, in which lasing at 18.9, 20.3, and 28.5 nm from nickel-like molybdenum, niobium and neon-like chromium ions has been observed by using two 200-ps laser pulses with a total energy of 50 J at 1.053 μm. This shows the possibility of extending nickel-like and neon-like X-ray lasing in low-Z elements and paves the may toward small scale X-ray lasers for applications at University laboratories. A comparison has been made of performance of the neon-like chromium soft X-ray lasing at 28.5 nm driven by a double 900-ps pulse at 6 TW·cm^-2, with that driven by a double 200-ps pulse at similar irradiance. The double 200-ps pulse has been found to be more efficient to drive the neon-like X ray lasing     

Diode-pumped neodymium lasers repetitively Q-switched by Cr4+:YAG solid-state saturable absorbers

We report on the experimental results of a continuously diode-laser pumped Nd:YAG laser, operating at 1064 nm and repetitively Q-switched by a Cr⁴⁺:YAG solid-state saturable absorber. End-pumping the Nd:YAG with a 10-W fiber-coupled diode-laser we could either optimize the energy or the average output power, depending on the choice of the saturable absorber and the output coupler. The maximum energy was ≈200 μJ in single TEM₀₀, 17 ns pulses at 6 kHz, whereas a maximum average power of ≈2 W with 32-ns pulses at 20 kHz was obtained. We also present preliminary results of a repetitively Q-switched Nd:YVO₄ laser at 1064 nm. The repetitive Q-switching operation is described by an improved model, which accounts for the behavior of both the active medium and the solid-state saturable absorber. The results of the model agree fairly well with the experimental data. Experimental results of second harmonic conversion are also reported and interpreted using a depleted pump model     

Ultralow threshold 1.3-μm InGaAsP-InP compressive-strainedmultiquantum-well monolithic laser array for parallel high-densityoptical interconnects

An ultralow-threshold 1.3-μm InGaAsP-InP 10-element monolithic laser array is achieved through careful optimization of a strained multiquantum-well active layer, especially the amount of strain, the well thickness, the barrier thickness, the number of wells, and the active laser width. This array has a record-low threshold current, highly uniform threshold current characteristics (1.3±0.09 mA and slope efficiency of 0.37±0.01 W/A), extremely low operating current of 14 mA under 5-mW output power, and long-term reliability. This array is suitable as light sources for a parallel high-density optical interconnection system. In addition, a record low CW threshold current of 0.58 mA at 20°C and 1.62 mA at 90°C, as a long-wavelength laser, is obtained by employing a short cavity (100 μm) uith high-reflection coatings