Optical Fiber, Coil Winding, Lasers and Amplifiers

The reliability of low-index polymer coated double-clad (DC) fibers used in the manufacture of fiber lasers and amplifiers has not received adequate attention. This paper evaluates the mechanical reliability of fibers, using standard fiber optic test procedures, and compares the performance of the DC fibers to the GR-20-CORE standard adopted by the industry. An 85°C hot water soak test is proposed as an accelerated test to evaluate a low-index polymer coated DC fiber performance with prolonged exposure to temperature and humidity conditions experienced during storage and operation of fiber lasers. The test is used to evaluate DC fibers with three different coatings, including a specially engineered coating, and benchmark fibers from competitors. The data in this paper demonstrate that a dual acrylate coated DC fiber, using the specially engineered coating, has median failure stress values of over 700 kpsi and an average stress corrosion parameter of 21, well exceeding the recommended industry minimum values of 550 kpsi and 18, respectively. The accelerated temperature and humidity aging test clearly demonstrates that DC fibers with specially engineered coatings have 2 to 3 orders of magnitude better optical reliability. Such remarkable optical and mechanical performance significantly alleviates long term reliability concerns of fiber lasers and amplifiers.

The development of high-power continuouswave (CW) single-mode fibre lasers operating at wavelengths of 1 μm has been ongoing for most of the past decade, and great progress has indeed been made in scaling the output power of such devices. Notable milestones include the demonstration of 100 W of CW power in 1999, followed by 1 kW in 2004 and more recently the demonstration of a 10 kW single-mode fibre device by IPG Photonics.

One of the early factors that enabled such successful power scaling was the adoption of large-mode-area (LMA) fibres as an alternative to conventional high numerical aperture (NA), small mode-field diameter fibre designs inherited from the telecommunications industry. Increasing the size of the mode field by using fibres with larger cores and lower NA made it possible to overcome the nonlinear limitations associated with high intensities in conventional fibres.The challenge of scaling the power output of single-mode fibres towards 1 kW thus became the simpler task of coupling sufficient pump power.

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The adoption of high power fiber lasers as an industrial material processing tool has been ongoing for several years and is now expanding into other markets including the medical and scientific markets. One key enhancement enabling penetration into the scientific market has been the improvements in high finesse fiber amplifiers, which are now capable of delivering single frequency linewidths (<5KHz) together with excellent beam quality and stable linearly polarized output. Systems meeting these specifications, have steadily progressed in the last few years from a few Watts of output power, initially to the 10’s Watts and now into the 100’s Watts power level. This power scaling has been achieved with developments in the fiber technology, such as the adoption of Stimulated Brillouin Scattering (SBS) suppression/mitigation techniques within the fiber, along with improvements in the overall amplifier design. The latest generation of high finesse, high power fiber amplifiers now deliver power levels exceeding what is available from commercial solid-state single frequency sources and are opening up new scientific applications as a result of the higher power levels and stable optimized performance.

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Here we report a compact monolithic 2000nm pulsed laser with a single spatial mode output, ~1.5ns pulse duration, 8kW peak power and >200mW average power at 20 kHz repetition rate. The gain-switched laser, consisting of a pair of fiber Bragg gratings and 0.5m of thulium-doped single cladding fiber, was core pumped by a high peak power pulsed 1.5m laser. When the input pulse energy of the 20 kHz pump pulses was sufficient enough to saturate the Thulium doped fiber, a stable 20 kHz pulse train was observed with measured linewidth of 0.05nm which corresponds to the limit of resolution for the Optical Spectrum Analyzer. This compact, pulsed 2000 nm laser, to the authors’ limited knowledge, represents the first Tm-doped fiber laser with 8kW peak power and several ns pulse duration; which is less than the previously reported tens of ns pulsewidth previously reported from gain-switched Tm-doped fiber lasers.

Fiber lasers for industrial, military, scientific and medical applications are now ubiquitous. Various active and passive fibers are used in fiber lasers as gain media, for numerous fiber-based components and beam delivery. To date, much of the discussion on reliability of fiber lasers has been focused on diode and fiber reliability. In the regime of fiber reliability, attention has primarily been given to photo-darkening of the double clad fibers (DCFs) used as gain media. Low index polymer (LIP) coatings play a key role in guiding the pump light, and their mechanical and environmental reliability has not received due attention.

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The technology to deliver narrow linewidth, single mode optical power from a fiber amplifier at the 1kW power level has been demonstrated over the last 12 months. In this paper we present measurements which address the suitability of these fiber amplifiers for various beam combining schemes, including coherent beam combining. Data on SBS induced intensity noise, phase noise at kW level output powers, and SBS threshold as a function of input seed linewidth are presented. Examples of several delivered systems will also be presented along with a discussion on future development efforts in higher power fiber amplifiers.

Generation of 5 ps, 32 pJ pulses from a carbon nanotube modelocked thulium fiber oscillator and their amplification to 0.6 W average power, 2.6 kW peak power, 13 nJ pulses by an LMA thulium fiber amplifier is discussed.

The capability of Tm-doped silica fibers pumped at 790nm to efficiently produce high power emission in the 1.9~2.1μm region has been well documented to date but little has been presented on the reliability of this technology.

We demonstrate a high efficiency PM fiber amplifier delivering 20W output power and operating in conjunction with an external single frequency, seed source at 2037nm (linewidth ~100kHz). The system is not limited by SBS.

We demonstrate a robust, highly efficient pulsed Tm-doped fiber laser systems operating at 1908nm and producing 6W average power. Using PPLN crystal we demonstrate 60% conversion efficiency to 954nm.

Tm-doped 790-nm-pumped silica fiber lasers are excellent candidates for producing emission at <1:95 μm, but achieving efficient operation at these wavelengths requires careful attention to fiber design because of the characteristic three-level reabsorption effects. We present a discussion of methods for mitigation of these effects and two high-efficiency systems that are capable of producing up to 70Wat <1:92 μm.

We report here a high power Er/Yb co-doped CW and pulse fiber amplifier with high slope efficiency (~40%) and high threshold of 1.0μm ASE. Under the master-oscillator-power-amplifier configuration, the fiber amplifier yielded over 42W with a CW seed source and more than 35W average power with a 5 nanosecond pulsed seed source at 1MHz repetition rate.

The first fiber laser is demonstrated at 1064-nm using a novel index-guiding-core singlemode fiber (Chirally-Coupled-Core) with V>>2.405. Robust single-mode operation achieved at all power levels (up-to ~40W) independently of beam excitation and fiber coiling conditions.

Spectroscopic properties of thulium-doped germanate, silica, and phosphate glasses were measured and compared since such glasses are of interest as materials for fiber lasers in the eye-safe wavelength region. ³F₄ excited state fluorescence decay dynamics was investigated at temperatures from 8 to 300 K and the results revealed a strong dependence of the ³F₄ lifetime on the host matrix. The temperature-dependent stimulated emission cross section was obtained by using the Fuchtbauer–Ladenburg technique. In phosphate glass the fluorescent lifetime is short, making this material difficult to use for 2 μm laser purposes. Tm³⁺-doped germanate glass shows a longer lifetime than silica, a comparable value of stimulated emission cross section and some interesting temperature-independent properties.

We demonstrate a novel large core diameter fiber design which facilitates the use of conventional mode selection techniques to achieve high output power, robustly single-mode laser operation in the 1.55 and 2.0μm eyesafe spectral regions. We also present recent progress in scaling the output power of narrow spectral linewidth all-fiber amplifiers into the kW domain.

We describe fundamental measurements of the properties of thulium (Tm)-doped silica, and power-scaling studies of fiber lasers based on the material. Data on the highlying Tm:silica energy levels, the first taken to our knowledge, indicates that pumping at 790-nm is unlikely to lead to fiber darkening via multi-photon excitation. Measurement of the crossrelaxation dynamics produce an estimate that, at the doping levels used, as much as 80% of the decay of the pumped Tm level pumped is due to cross relaxation. Using a fiber having a 25-μmdiameter, 0.08 NA core, we observed fiber-laser efficiencies as high as 64.5% and output powers of 300 W (around 2040 nm) for 500 W of launched pump power, with a nearly diffraction-limited beam. At these efficiencies, the cross-relaxation process was producing 1.8 laser photons per pump photon. We generated 885 W from a multimode laser using a 35-μm, 0.2 NA core fiber, and set a new record for Tm-doped fiber-laser cw power.

Improved power scaling of fiber lasers is being enabled by advances in high-brightness pump laser diodes—specifically, higher efficiency of the pump semiconductor material itself allowing more power per emitter, and innovations in coupling the diode energy into the target delivery fiber.

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1908nm is a key wavelength used as a low quantum defect pump source for Ho:YAG lasers. The long storage time of Ho:YAG enables the generation of high energy pulses at 2.1μm. Possible direct applications of pulsed Ho:YAG lasers include ranging and LIDAR, but more significantly, it is well suited for nonlinear frequency conversion into the mid and far-IR, key for DIRCM and remote sensing.

With output power levels now approaching the kW level, Tm-doped fiber lasers are beginning to emerge as the latest revolution in high-power fiber laser technology. Operating at 1.9–2.1 µm, this technology falls into the "eye-safer" category of lasers, giving it potential advantages over 1 µm lasers for industrial and military directed energy applications. The greater potential for pulse scaling is also beginning to be realized with peak powers now approaching 100 kW without requiring excessively complicated fiber designs or sacrificing beam quality.

Advanced light sources with optical efficiencies approaching 65% and kilowatt output powers have practical potential in areas ranging from defense to medicine.

Much of the recent improvement in high power fiber laser performance has been obtained by increasing the mode field area of the core guiding region beyond that found in standard telecom fiber designs. This simple scaling concept has delivered highly efficient kWatt level fiber lasers and amplifiers operating at 1µm, based on Yb-doping and more recently the extension to 2µm using highly efficient Tm-doped fibers.

We  report >20W of average output power at 1.995μm  from a pulsed Tm-doped  fiber amplifier system operating at 100kHz. Pulse energies of >325μJ have been generated at 50kHz with 13ns pulses in the same amplifier.

An evolution in fiber design and glass composition has spawned a renewed and rapidly growing interest in fiber laser technology.

In prior work, we have reported generation of 300 W of cw power from a Tm:silica fiber laser at 2050 nm, with >60% conversion of pump power to laser output power. The beam quality was close to the diffraction limit. We have also noted that we plan to scale the system to the 1-kW power level. To date, we have received and tested two 1-kW, fiber coupled, diode pump lasers, and have shown the ability to couple power efficiently from the diode lasers into the largercladding fibers we plan to use for the 1-kW fiber laser. In the work to be presented here, we will report on our results with this new effort in Tm:fiber-laser power scaling.

Current monolithic single-mode kW fiber laser systems predominantly rely on ~20μm-25μm core conventional LMA fibers. Further power scaling requires increase in fiber core size. However, conventional LMA approach can not provide with a robust monolithic performance of fiber systems with core sizes larger than ~25μm.