Optical Fiber, Coil Winding, Lasers and Amplifiers

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.

Ytterbium-doped large mode areas (LMA) fibers have become an established high power laser medium in recent years

In this paper we present advances made in the development and fabrication of highly efficient, large-mode area fibers for eye-safe wavelengths (1.55 mm, 2.0 mm).

A highly temperature-insensitive fiber is reported. Furthermore, a convenient method for selecting the temperature of minimum sensitivity, by controlling the B₂O₃ concentration in the core and monitoring it with 1550 nm attenuation, is presented.

Being the new frontier of science and technology, as the near earth space begins to attract attention, low cost and rapidly deployable earth observation satellites are becoming more important.

The use of optical fibers in low earth orbiting (LEO) satellites is a source of concern due to the radiation environment in which these satellites operate and the reliability of devices based on these fibers. Although radiation induced damage in optical fibers cannot be avoided, it can certainly be minimized by intelligent engineering.

Fibers for high-power laser and amplifier applications require large claddings with high numerical apertures for efficiently coupling pump energy.

This report presents a discussion of the engineering issues and results of high power 2μm Tm³⁺-doped fibre lasers pumped at 790nm.

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.

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.

Click here to view NuCOAT fibers

Solid state and gas lasers are routinely capable of producing cw multi-kW output powers and as such have become the mainstay of the laser cutting and welding industry.

CO₂ and YAG lasers are routinely capable of producing cw output powers in the range of sub-W to multi-kW and as such have become the mainstay of the laser cutting and welding industry.

The adoption of reduced cladding thickness fibers by the telecom industry has greatly increased over the last year. In fact, almost every specialty fiber manufacturer has launched products aimed at the small form factor (SFF) market. Despite appearances the use of 80µm OD fiber, rather than 125 µm, has actually been commonplace for years in certain non-telecom applications, fiber optic gyros for example. The general move toward SFF components within the optoelectronics industry -- specifically telecom and datacom applications -- is also well underway.

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.

Click here to view Double Clad (DC) Fibers.

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.

We demonstrate a large core diameter PM Er/Yb fiber incorporating a unique raised inner-cladding which facilitates the use of conventional LMA mode selection techniques to achieve robustly single-mode operation, making it ideally suited to high power applications.

High-power fiber-laser arrays can potentially produce tens or hundreds of kilowatts of output power, providing significant military capabilities. At the May Solid-State and Diode Laser Technology Review workshop, hosted by the Directed Energy Professional Society (DEPS) in Albuquerque, NM, researchers from Northrop Grumman Space Technology demonstrated significant progress toward a seven-element fiber-laser array based on ytterbium (Yb)-doped polarization-maintaining single-mode power amplifiers. The 155-W output from the single fiber laser was a record for this type of coherent-beam-combining design and should prove highly scalable.

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.

For intrinsic fiber optic sensors such as interferometric fiber optic gyroscopes that use polarization maintaining fibers, performance of the fibers that constitute the sensing coils is a key issue.

Fiber amplifier systems open up the possibility of highly flexible pulsed laser sources with characteristics that can be tailored exactly to the application requirements.  With a chain of fiber amplifiers, repetition rate and pulse duration can be varied over a wide range without affecting beam quality.

Click here to view Ytterbium (Yb) Fibers.

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.

Although fiber amplifiers have been employed in communications systems for many years, until very recently the fiber laser was little more than a scientific curiosity.

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.

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.