Second harmonic generation efficiency of a pulsed Er fiber laser with periodically poled lithium niobate (PPLN) is optimized by varying input pulsewidth. The Er-doped fiber amplifier was a 3-stage, 1550 nm amplifier with 1-10 ns variable pulsewidth, 180 kHz pulse repetition frequency, and 5.5 W of output power. The laser was focused into a single, 10 mm long piece of PPLN to convert to 775 nm through second harmonic generation. The pulsewidth was varied and we observed a correlation between pulsewidth and conversion efficiency. At the minimum pulsewidth, τ = 2 ns, we achieved 31% conversion efficiency, and as we increased the pulsewidth we saw an increase in second harmonic generation conversion efficiency. At τ = 10 ns, our maximum pulsewidth, we saw a conversion efficiency of 68%, which was the highest conversion efficiency achieved in this experiment. The increase of efficiency with reduced pump intensity is attributed to the decrease in spectral width of the laser at longer pulsewidths. Measuring the spectrum of the laser verified the presence of self-phase modulation at the shorter pulsewidths.
We have developed a packaged fiber amplifier configuration that allows for nearly two orders of magnitude of pulse width adjustment from 1ns to >800ns. This has been developed for both the 1-micron and 1.55-micron spectral regions. Our 1.55-micron fiber laser is packaged into a 6.63 x 8.65 x 3.47 in3 box, while our 1-micron fiber laser is packaged into a 13.68 x 8.68 x 3.56 in3 box, with the larger package a result of larger fiber components. These lasers offer a wide range of adjustable operating points, with total output ultimately limited by available pump power. For 1ns pulses, our 1.55-micron system generates up to 6μJ of pulse energy (>6kW peak) with transform-limited spectral output. Higher energies and output powers are achievable (up to 33μJ at 25kW peak), but the spectral output broadens slightly due to nonlinearities with <5ns pulse durations. For 1ns pulses at 1-micron, the system can generate 10uJ pulse energy (>10kW peak) with high spectral purity. At >10ns pulse durations, the same laser can generate up to 40μJ pulse energy (pump limited). A unique aspect of our design is that a single fiber laser package can be electrically adjusted to produce the full range of pulse widths at repetition rates ranging from 100kHz to <1MHz with well-behaved output pulse shapes and no rising-edge pulse distortions typically seen in high gain amplifiers. In this paper, we discuss our laser architecture, performance, packaging layout, packaging limitations, and a path toward more compact designs using standard fiber components.
We measure changes in the 2um absorption and emission spectra of thulium-doped silica fiber lasers operating from 80 K – 373 K. Reduction of the long wavelength tail of the 3H6-3H4 absorption feature under cryogenic cooling allows for efficient lasing in the 1800nm region. Greater than 17 W of output power was generated at 1850 nm by 793 nm diode-pumping a free-running single-mode thulium oscillator under cryogenic cooling conditions.
We have recently demonstrated an Er:Yb fiber amplifier pumped off-peak at 940 nm which achieved 50.5% slope efficiency compared to 40.2% slope efficiency when pumped on-peak at 976 nm, the typical pumping wavelength for Er:Yb fiber. To further understand these results we implemented a model that predicts the behavior of high power Er:Yb-co-doped fiber amplifiers with strong correlation to data. Through this modeling effort, we were able to estimate the forward and backward energy transfer coefficients for the Er:Yb silicophosphate fiber used. We also conducted a theoretical cutback experiment to find the optimal amplifier fiber length for each pump wavelength.
High power continuous and pulsed fiber lasers and amplifiers have become more prevalent in laser systems over the last ten years. In fielding such systems, strong environmental and operational factors drive the packaging of the components. These include large operational temperature ranges, non-standard wavelengths of operation, strong vibration, and lack of water cooling. Typical commercial fiber components are not designed to survive these types of environments. Based on these constraints, we have had to develop and test a wide range of customized fiber-based components and systems to survive in these conditions. In this paper, we discuss some of those designs and detail the testing performed on those systems and components. This includes the use of commercial off-the-shelf (COTS) components, modified to survive extended temperature ranges, as well as customized components designed specifically for performance in harsh environments. Some of these custom components include: ruggedized/monolithic fiber spools; detachable and repeatable fiber collimators; low loss fiber-to-fiber coupling schemes; and high power fiber-coupled isolators.
Compact, high power blue light in the 470-490nm region is difficult to generate due to the lack of laser sources which are easily convertible (through parametric processes) to those wavelengths. By using a pulsed Tm-doped fiber laser as a pump source for a 2-stage second harmonic generation (SHG) scheme, we have generated ~2W of 486.5nm light at 500kHz pulse repetition frequency (PRF). To our knowledge, this is the highest PRF and output power achieved in the blue region based on a frequency converted, monolithic fiber laser. This pump laser is a pulsed Tm-doped fiber laser/amplifier which generates 12.8W of 1946nm power at 500kHz PRF with diffraction-limited output from a purely single-mode fiber. The output from this laser is converted to 973nm through second harmonic generation (SHG). The 973nm is then converted to 486.5nm via another SHG stage. This architecture operates with very low peak power, which can be challenging from a nonlinear conversion standpoint. However, the low peak power enables the use of a single-mode monolithic fiber amplifier without undergoing nonlinear effects in the fiber. This also eliminates the need for novel fiber designs, large-mode area fiber, or free-space coupling to rod-type amplifiers, improving reliability and robustness of the laser source. Higher power and conversion efficiency are possible through the addition of Tm-doped fiber amplification stages as well as optimization of the nonlinear conversion process and nonlinear materials. In this paper, we discuss the laser layout, results, and challenges with generating blue light using a low peak power approach.
We compare large mode area (LMA) and single-mode (SM) double-clad fiber geometries for use in high power 1908nm fiber lasers. With a simple end-pumped architecture, we have generated 100W of 1908nm power with LMA fiber at 40% optical efficiency and 117W at 52.2% optical efficiency with single-mode fiber. We show the LMA fiber is capable of generating >200W and the SM fiber is capable of >300W at 1908nm. In all cases, the fiber lasers are monolithic power-oscillators with no free-space coupling.
High power fiber lasers/amplifiers in the 1550nm spectral region have not scaled as rapidly as Yb-, Tm-, or Ho-doped fibers. This is primarily due to the low gain of the erbium ion. To overcome the low pump absorption, Yb is typically added as a sensitizer. Although this helps the pump absorption, it also creates a problem with parasitic lasing of the Yb ions under strong pumping conditions, which generally limits output power. Other pump schemes have shown high efficiency through resonant pumping of erbium only without the need for Yb as a sensitizer [1-2]. Although this can enable higher power scaling due to a decrease in the thermal loading, resonant pumping methods require long fiber lengths due to pump bleaching, which may limit the power scaling which can be achieved for single frequency output. By using an Er:Yb fiber and pumping in the minima of the Yb pump absorption at 940nm, we have been able to simultaneously generate high power, single frequency output at 1560nm while suppressing the 1-micron ASE and enabling higher efficiency compared to pumping at the absorption peak at 976nm. We have demonstrated single frequency amplification (540Hz linewidth) to 207W average output power with 49.3% optical efficiency (50.5% slope efficiency) in an LMA Er:Yb fiber. We believe this is the highest reported efficiency from a high power 9XXnm pumped Er:Yb-doped fiber amplifier. This is significantly more efficient that the best-reported efficiency for high power Er:Yb doped fibers, which, to-date, has been limited to ~41% slope efficiency .
We have demonstrated efficient lasing of a Tm-doped fiber when pumped with another Tm-doped fiber. In these experiments, we use a 1908 nm Tm-doped fiber laser as a pump source for another Tm-doped fiber laser, operating at a slightly longer wavelength (~2000 nm). Pumping in the 1900 nm region allows for very high optical efficiencies, low heat generation, and significant power scaling potential due to the use of fiber laser pumping. The trade-off is that the ground-state pump absorption at 1908 nm is ~37 times lower than at 795nm. However, the absorption cross-section is still sufficiently high enough to achieve effective pump absorption without exceedingly long fiber lengths. This may also be advantageous for distributing the thermal load in higher power applications.
Quasi-phase-matched (QPM) materials such as periodically poled lithium niobate (PPLN) and tantalate (PPLT) have led
to extremely efficient frequency-shifted laser sources in the visible and near-infrared, and QPM semiconductors promise
to extend this performance beyond 4um. Orientation patterned semiconductors are not only transparent far deeper into
the mid-IR but also offer higher nonlinear coefficients, higher thermal conductivity, higher purity levels, and very low
losses when grown from the vapor phase. We compare the properties, processing, and performance of orientationpatterned
GaAs (OPGaAs) with candidate compound semiconductors being for development as the next generation
QPM nonlinear optical materials in the mid-infrared, and identify gallium phosphide as the most promising material for
We report on the design and characterization of a cryogenically cooled, resonantly pumped Er:YAG slab laser operating
at 1645 nm. The Er:YAG slab is conductively cooled by liquid nitrogen and face-pumped by a 1.4 kW diode array
operating at 1452 nm. The slab is transversely extracted in a highly multi-mode oscillator, producing 386 W of cw
output power and 420 W of quasi-cw power at 50% duty cycle. We have measured 45% slope efficiency and 39%
optical conversion efficiency (relative to incident pump). The laser has also been configured to operate with low-order
multi-mode output, producing over 250 W of quasi-cw power at 25% duty cycle, and has been Q-switched at repetition
rates as low as 10 kHz.
Fiber lasers are advancing rapidly due to their ability to generate stable, efficient, and diffraction-limited beams with
significant peak and average powers. This is of particular interest as fibers provide an ideal pump source for driving
parametric processes. Most nonlinear optical crystals which provide phase-matching to the mid-IR at commercially
available fiber pump wavelengths suffer from high absorption above 4μm, resulting in low conversion efficiencies in the
4-5μm spectral region. The nonlinear optical crystals which combine low absorption in this same spectral region with
high nonlinear gain require pumping at longer wavelengths (typically >1.9μm). In this paper, we report a novel mid-IR
OPO pumped by a pulsed thulium-doped fiber laser operating at
2-microns. The eyesafe thulium-fiber pump laser
generates >3W of average power at >30kHz repetition rate with
15-30ns pulses in a near diffraction-limited beam. The
ZnGeP2 (ZGP) OPO produces tunable mid-IR output power in the
3.4-3.99μm (signal) and the 4.0-4.7μm (idler) spectral
regions in both singly resonant (SRO) and doubly resonant (DRO) formats. The highest mid-IR output power achieved
from this system was 800mW with 20% conversion efficiency at 40kHz. In a separate experiment, the 3W of 2-micron
light was further amplified to the 20W level. This amplified output was also used to pump a ZGP OPO, resulting in 2W
of output power in the mid-IR. To our knowledge, these are the first demonstrations of a fiber-pumped ZGP OPO.
The laser damage threshold (LDT) of single crystal zinc germanium phosphide (ZGP), ZnGeP2, was measured to be 2 J/cm2 by the S-on-1 method. This LDT was double the previously measured value of 1 J/cm2 and was achieved by improving the polishing technique for ZGP OPO crystals. ZGP is the nonlinear optical crystal of choice for laser frequency conversion in the 2-8 μm spectral range due to properties including its high non-linear coefficient (d14=75 pm/V) and thermal conductivity (0.35 W/cm K). The surface preparation of ZGP parts was determined to be of great importance because laser-induced damage has been observed to always initiate at the surface rather than in the bulk of the material. In this study, the surfaces of ZGP parts fabricated in the same manner apart from the polishing stage were quantitatively examined. Two different polishing techniques were examined, and both uncoated and anti-reflection coated parts were examined for each polishing technique. Surfaces were characterized using scanning white light interferometry (SWLI) in order to determine RMS surface roughness and sample flatness. The photon backscatter technique (PBS) was used to determine the degree of surface and subsurface damage in the sample induced through the fabrication process. Statistical LDT was measured using a high-average-power, repetitively Q-switched Tm,Ho:YLF 2.05-μm pump laser. Laser induced damage was observed after each exposure by examining the site where the laser beam entered the ZGP sample using optical microscopy. On average, lower surface roughness and photon backscatter measurements were a good indicator of ZGP parts exhibiting higher LDT.
We have demonstrated efficient operation of the eyesafe laser transition (4I13/2 -> 4I15/2) in Er:YAG by resonantly pumping with 1470nm diodes. Quasi-cw powers in excess of 30W have been achieved at 10% duty cycle with 47% slope efficiency, 26% conversion efficiency, and beam quality of M2=1.4 x 2.2. In energy storage mode, we have generated near-diffraction-limited 41mJ / 58ns pulses, more than 700kW of peak power, at 10Hz. Storage lifetimes in the range of 5 to 7msec have been measured, and pulses as short as 25ns have been obtained at reduced energy. We believe this to be the first-ever demonstration of a resonantly diode pumped (bulk) erbium laser.
Challenges typically associated with efficient operation of bulk solid-state erbium lasers operating on the eyesafe (4I13/2 → 4I15/2) transition have been effectively overcome using a high-brightness fiber laser as a pump source. We report exceptional performance in a variety of laser hosts resonantly pumped by a cw, high-average-power erbium fiber laser. Herein we present results from resonantly pumped erbium-doped YAG, LuAG, YLF, YAlO3, and YVO4, in cw and repetitively Q-switched operation. Most notable results include 9.5 W of near-diffraction-limited output from an Er:YAG laser at 1.645 μm, Q-switched at 10 kHz with 40 ns pulsewidth (25 kW peak power) as well as 3.4 μJ/pulse with 21 ns pulses at 1.1 kHz (160 kW peak). We have achieved overall optical conversion efficiency greater than 50% with incident slope efficiency >60%. This is, to our knowledge, the highest performance (average power and conversion efficiency) obtained from a bulk solid-state Q-switched erbium laser. To demonstrate the utility of such sources, we also present results from second harmonic generation to 822nm, as well as a periodically poled lithium niobate optical parametric oscillator generating tunable 2.5 - 3.8 micron output.
Zinc germanium diphosphide (ZnGeP2) has proven to be an important nonlinear optical crystal for the generation of midwave infrared radiation, especially in an optical parametric oscillator configuration pumped by a Ho:YAG laser operating at 2.09 micrometers. Future applications will require higher intensity levels of the laser pump beam which are limited by the crystal's laser damage threshold and nonlinear absorption. These two quantities were measured for silver-doped ZnGeP2 samples of which was uncoated, two had conventional anti-reflection coatings, and one had an AR coating with a quintic refractive index profile. Prominent nonlinear absorption was observed in some of the crystals; the nonlinear absorption coefficient was found to be anisotropic and a weak correlation between the values of the linear and the nonlinear absorption coefficient was observed. The values of the nonlinear optical coefficients measured in these crystals for ordinary and extraordinary polarizations of incident light are reported along with the measured values of the laser damage threshold of these newly grown crystals at 2.09 micrometers, both in the AR coated and uncoated forms.