Coherent | Nufern has a long history of high power Yb-doped fiber amplifier and component development. This ranges from amplifiers in the >1.5kW regime up to demonstrations of >3kW output power. In this paper, we discuss our latest advancements in the performance of high power, narrow linewidth, kW-class, monolithic Yb-doped fiber amplifiers, as well as the packaging of our two newest amplifier systems. Our lightest amplifier offers 1.6kW output power at <4.5kg/kW. This is an OEM version which requires external power, but offers a significant (nearly 1.5x) decrease in overall mass compared to our previous generation of amplifiers. Our 2.1kW amplifier is our smallest and highest power amplifier. It contains integrated electronics to offer a complete stand-alone amplifier, requiring only DC voltage input, external cooling, and software control. This has nearly a 2x reduction in volume compared to our previous generation. We discuss the performance and specifications of these two amplifier packages. This includes power scaling with narrow linewidth, as well as a significant and simultaneous reduction in volume and mass compared to our previous generations. We discuss the packaging challenges with these architectures, as well as the diode technologies which have enabled such a reduction in our packaging volume and mass. We also discuss experimental results in the power scaling of these fiber amplifiers.
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.
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.
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.
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.
Fiber lasers are an ideal pump source for nonlinear frequency conversion because they have the capability to generate
short pulses with high peak-powers and excellent beam quality. Thulium-doped silica fibers allow for pulse generation
and amplification in the 2-micron spectral band. This opens the door to a variety of nonlinear crystals, such as ZnGeP<sub>2</sub>
(ZGP) and orientation patterned GaAs (OPGaAs), which cannot be pumped by Yb- or Er-doped fiber laser directly due
to high losses in the near-IR band. These crystals combine low losses with high nonlinearities and transparency for
efficient nonlinear mid-IR converters. Using such nonlinear crystals and a pulsed Tm-doped master oscillator fiber
amplifier (MOFA), we have demonstrated efficient mid-IR generation with watts of output power in the 3-5μm region.
The Tm-doped MOFA is capable of generating from 10 to 100W of average output power at a variety of repetition rates
(10kHz - >500kHz) and pulse widths (10ns - >100ns). Total mid-IR power is only limited by thermal effects in the
nonlinear materials. The use of Tm-doped fiber-pumped OPOs shows the path toward compact, efficient, and
lightweight mid-IR laser systems.
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
ZnGeP<sub>2</sub> (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.
Converting near infrared signals in a nonlinear medium is an attractive way to generate terahertz radiation due to the
availability of near-IR lasers and nonlinear materials. However, these terahertz generation schemes are typically
inefficient and are often cumbersome, which may limit their use in certain applications. We have developed and
demonstrated a compact, fiber pumped optical terahertz source based difference frequency mixing (DFM) of nanosecond
pulses in zinc germanium phosphide (ZGP). With this setup, we have successfully generated 2mW of average power
terahertz radiation at 2.45THz. This has enabled us to perform active, real-time terahertz imaging experiments using an
uncooled microbolometer array. In performing these experiments, we have also developed a theoretical model for
terahertz generation based on DFM of IR pump signals. In this paper, we discuss our compact fiber pumped terahertz
source technology, imaging system, model, and how we intend to overcome some of the common issues associated with
optical terahertz generation.