We report the development of fused-fiber pump and signal combiners. These combiners are enabling components of a ytterbium fiber-laser emitting 4 kW of 1080-nm radiation. The fiber-laser system consists of seven fiber laser modules and a 7:1 signal combiner. The laser modules are end-pumped by 90 915-nm JDSU L4 diode-lasers, yielding a nominal pump power of 900 W. The diode laser radiation is coupled into the laser fiber through a 91:1 fused-fiber pump combiner. The input fibers of this pump combiner are standard 105/125-um multimode fibers with an NA of 0.22. These fibers form a hexagonally packed fused-fiber bundle, which is tapered to match the cladding diameter of the laser fiber. Eighty-six percent of the light exiting the pump-combiner is emitted within an NA of 0.32, and all measurable power is emitted within an NA of 0.45. The typical insertion loss of the pump combiners is <1%. The high-brightness radiation of seven laser modules is combined into a single output fiber using a 7:1 fused-fiber signal combiner providing a total power of >4 kW in the single output beam. The beam parameter product of the combined output was 2.5 mm-mrad. The low insertion loss of < 2% indicates that the signal combiner is suitable to handle even higher laser powers.
We have developed a commercial 4-kW fiber laser consisting of seven, 600-W modules whose outputs are combined
with a fused-fiber combiner. The system architecture has several practical advantages, including pumping with reliable
single-emitter diodes, monolithic fused-fiber construction (no free-space beams), and end pumping using a 91:1 pump
combiner (eliminating the need for complex pump/signal combiners). Typical results at 4-kW output power are a beamparameter
product of 2.6 mm-mrad, 8-hr power stability of < 0.5% rms, central wavelength of 1080 nm, and linewidth of
1.2 nm FWHM. These lasers have been incorporated into Amada machines used for cutting metal sheet and plate and
have been used to cut aluminum, mild steel, stainless steel, brass, titanium, and copper with a thickness up to 19 mm. A
world-record cutting speed of 62 m/min has been demonstrated for 1-mm aluminum sheet metal.
We describe a passively mode-locked, diode-pumped Nd:YAG laser that is used for frequency-conversion applications. The laser is based on a Direct-coupled Pump gain element and saturable Bragg reflector. The laser produces a 20-ps pulse with a 100-MHz repetition rate in a compact commercial package. It has typically <0.2% amplitude noise and diffraction-limited output beam. The average power is typically 7-8 W, and peak power is 4 kW which makes it well-suited for efficient frequency conversion. Using 2 stages of LBO for cascaded second-harmonic and sum-frequency generation, we have obtained >1 W at 355 nm. In addition, we have generated super-continuum output in the visible and infrared from micro-structured nonlinear fiber with pumping both at 1064 nm and 532 nm. Current applications for this laser, primarily in the ultraviolet, include flow cytometry, stereolithography, and semiconductor inspection.
In this paper, we describe a red OPO device that is synchronously pumped by a mode-locked laser. The pump source is a diode pumped mode-locked Nd:YAG laser. The motivation for this approach is that the high peak power of the mode- locked pulses allows the use of shorter PPLN crystals that reduces the impact of refractive index perturbations. We expect that eh mode-locked implementation will not exhibit the power instabilities that limited the previous continuous-wave version, while the high repetition rate of the mode-locked format will be comparable to true cw in most applications.
The effects of reduction, oxidation, Li-enrichment and impurity on LiTaO3 crystals were studied. It is demonstrated that the best LiTaO3 crystals show less absorption than LiNbO3, less photorefraction and no green-light-induced infrared absorption.
We investigate the phenomenology and modeling for the development of an active multispectral laser radar (LADAR) sensor to image and identify ground targets in the 1 to 5 micrometers wavelength region. This sensor will be especially useful in high clutter situations or situations where the target is partially concealed. A continuously tunable optical parametric oscillator using a periodically poled lithium niobate (PPLN) nonlinear optical crystal is investigated as a candidate light source for the sensor. A 1 micrometers Nd:YAG laser was frequency shifted in PPLN to produce continuously tunable output between 1.35 to 5 micrometers wavelengths and signal output energy of up to 3.3 mJ in a 3 ns pulse. A tunable monostatic reflectometer system is fabricate for the measurement of the bidirectional reflectance distribution function of the LADAR target materials A method or band selection is formulated and tested using library reflectance spectra. Results of this work will be used for tower based imaging of different targets in cluttered backgrounds at ranges out to 3 km.
We review progress on quasi-phasematched optical parametric oscillators (OPOs) in bulk periodically poled LiNbO3. We have extended the electric-field poling process so that we now reliably fabricate crystals over 60-mm long in full 3-inch-diameter, 0.5-mm-thick wafers. Periodically poled material retains the low loss and bulk power handling properties of single domain LiNbO3, and QPM allows noncritical phasematching with the highest value of the nonlinear coefficient. OPOs pumped by 1.064-micrometers Nd:YAG lasers have been operated over the wavelength range 1.36 micrometers to 4.9 micrometers with tuning by temperature or QPM period. We have shown oscillation threshold as low as 0.006 mJ with a Q-switched pump laser, and pumping over 25 times threshold without damage. We have also demonstrated a doubly resonant OPO pumped directly with a commercial cw diode laser at 978 nm, and a 1.064-micrometers -pumped cw singly-resonant OPO with threshold < 3 W.
We review progress of quasi-phasematched (QPM) optical parametric oscillators (OPOs) in bulk periodically poled LiNbO3. Using the electric field poling process, we can reliably fabricate 0.5-mm thick crystals with uniform domain structures over 15-mm long. Periodically poled material retains the low loss and bulk power handling properties of single domain LiNbO3, and QPM allows noncritical phasematching with the highest value of the nonlinear coefficient. OPOs pumped by 1.064-micrometers pulsed Nd:YAG lasers have been operated over the wavelength range 1.45 micrometers to 4.0 micrometers with tuning by temperature or QPM period. We have shown oscillation threshold as low as 0.020 mJ with a Q-switched pump laser, and pumping over two times threshold without damage. We have also demonstrated a doubly resonant oscillator near 1.96 micrometers pumped directly with a cw diode laser at 978 nm.
A low average power, pulsed, solid-state, 1.06-micron coherent laser radar (CLR) for range and velocity measurements of atmospheric and hard targets has been developed. The system has been operating at a field test site near Boulder, CO since September, 1988. Measurements have been taken on moving targets such as atmospheric aerosol particles, belt sanders, spinning disks, and various stationary targets. The field measurements have shown that this system exhibits excellent velocity measurement performance. A fast-tuning CW Nd:YAG oscillator has also been developed which has a frequency tuning range of greater than 30 GHz (which spans a target radial velocity range of over 16 km/s) and a tuning speed greater than 30 GHz/ms.