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. Qualifying fibers for use in space is both time consuming and expensive, and manufacturers of satellites and their payloads have started to ask for radiation performance data from optical fiber vendors. Over time, Nufern has developed fiber designs, compositions and processes to make radiation hard fibers. Radiation performance data of a variety of fibers that find application in space radiation environment are presented.
Beam delivery fibers have been used widely for transporting the optical beams from the laser to the subject of irradiation in a variety of markets including industrial, medical and defense applications. Standard beam delivery fibers range from 50 to 1500 μm core diameter and are used to guide CW or pulsed laser light, generated by solid state, fiber or diode lasers. Here, we introduce a novel fiber technology capable of simultaneously controlling the beam profile and the angular divergence of single-mode (SM) and multi-mode (MM) beams using a single-optical fiber. Results of beam transformation from a SM to a MM beam with flat-top intensity profile are presented in the case of a controlled BPP at 3.8 mm*mrad. The scaling capabilities of this flat-top fiber design to achieve a range of BPP values while ensuring a flat-top beam profile are discussed. In addition, we demonstrate, for the first time to the best of our knowledge, the homogenizer capabilities of this novel technology, able to transform random MM beams into uniform flat-top beam profiles with very limited impact on the beam brightness. This study is concluded with a discussion on the scalability of this fiber technology to fit from 50 up to 1500 μm core fibers and its potential for a broader range of applications.
With the rapid acceptance of fiber lasers and amplifiers for various materials processing and defense applications the long term optical and mechanical reliability of the fiber laser, and therefore the components that make up the laser, is of significant interest to the industrial and defense communities. The double clad fiber used in a fiber laser is a key component whose lifetime in typical deployment conditions needs to be understood. The optical reliability of double clad fiber has recently been studied and a predictive model of fiber lifetime has been published. In contrast, a rigorous model for the mechanical reliability of the fiber and an analysis of the variables affecting the lifetime of the fiber in typical deployment conditions has not been studied. This paper uses the COST-218 model which is widely used for analyzing the mechanical lifetime of fiber used in the telecom industry. The factors affecting lifetime are analyzed to make the reader aware of the design choices a laser manufacturer can make, and the information they must seek from fiber suppliers, to ensure excellent lifetime for double clad fiber and consequently for the fiber laser. It is shown that the fiber’s stress corrosion susceptibility, its proof strength, the coil diameter and the length of fiber coiled to achieve good beam quality all have important implications on fiber lifetime.
Single-mode (SM) kW-class fiber lasers are the tools of choice for material processing applications such as sheet metal cutting and welding. However, application requirements include a flat-top intensity profile and specific beam parameter product (BPP). Here, Nufern introduces a novel specialty fiber technology capable of converting a SM laser beam into a flat-top beam suited for these applications. The performances are demonstrated using a specialty fiber with 100 μm pure silica core, 0.22 NA surrounded by a 120 μm fluorine-doped layer and a 360 μm pure silica cladding, which was designed to match the conventional beam delivery fibers. A SM fiber laser operating at a wavelength of 1.07 μm and terminated with a large-mode area (LMA) fiber with 20 μm core and 0.06 NA was directly coupled in the core of the flat-top specialty fiber using conventional splicing technique. The output beam profile and BPP were characterized first with a low-power source and confirmed using a 2 kW laser and we report a beam transformation from a SM beam into a flat-top intensity profile beam with a 3.8 mm*mrad BPP. This is, to the best of our knowledge, the first successful beam transformation from SM to MM flat-top with controlled BPP in a single fiber integrated in a multi-kW all-fiber system architecture.
With the use of fiber lasers pervading diverse applications and environmental conditions, the long-term reliability of low index (LI) polymer coated double-clad (DC) fibers used for this purpose is significant. Mechanical reliability standards for 125um fibers are well established by Telcordia GR-20-CORE requirements and some work1 has been published investigating optical reliability of DC fibers with specially engineered coatings with respect to accelerated temperature and humidity aging. While these are helpful in providing a figure of merit to the optical reliability of LI fluoroacrylate coatings, it becomes important to decouple the effects of temperature and humidity in order to understand the underlying degradation mechanisms of LI polymers in various storage and operating environments. This paper identifies the effects of temperature and humidity on the spectral attenuation of DC fibers and presents a reliability model capable of predicting lifetimes under prolonged exposure to typical temperature and humidity conditions experienced during storage and operation of fiber lasers.
A large number of power delivery applications for optical fibers require beams with very specific output intensity profiles; in particular applications that require a focused high intensity beam typically image the near field (NF) intensity distribution at the exit surface of an optical fiber. In this work we discuss optical fiber designs that shape the output beam profile to more closely correspond to what is required in many real world industrial applications. Specifically we present results demonstrating the ability to transform Gaussian beams to shapes required for industrial applications and how that relates to system parameters such as beam product parameter (BPP) values. We report on the how different waveguide structures perform in the NF and show results on how to achieve flat-top with circular outputs.
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.
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. Among other things these satellites are expected
to carry out missions in the general areas of science and technology, remote sensing, national defense and
telecommunications. Except for critical missions, constraints of time and money practically mandate the use of
commercial-off-the-shelf (COTS) components as the only viable option. The near earth space environment (~50-50000
miles) is relatively hostile and among other things components/devices/systems are exposed to ionizing radiation.
Photonic devices/systems are and will continue to be an integral part of satellites and their payloads. The ability of such
devices/systems to withstand ionizing radiation is of extreme importance. Qualification of such devices/systems is time
consuming and very expensive. As a result, manufacturers of satellites and their payloads have started to ask for
radiation performance data on components from the individual vendors. As an independent manufacturer of both
passive and active specialty silica optical fibers, Nufern is beginning to address this issue. Over the years, Nufern has
developed fiber designs, compositions and processes to make radiation hard fibers. Radiation performance data (both
gamma and proton) of a variety of singlemode (SM), multimode (MM), polarization maintaining (PM) and rare-earth
doped (RED) fibers that find applications in space environment are presented.
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. However the fiber laser format has a number of intrinsic advantages over lamp and diode pumped YAG lasers including size, reliability, wavelength selectivity, heat dissipation, wallplug efficiency and operational cost; and with kiloWatt output powers now possible fiber lasers are beginning to replace lamp and diode pumped YAG lasers in many industrial applications. In this paper we review the recent and ongoing advances in fiber design that have facilitated this revolution.
The advent of double clad fiber technology has made high power lasers and amplifiers possible. However, the scalability of output powers can be limited by amplified spontaneous emission and nonlinear processes such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). These limitations can be overcome by using low numerical apertures (NAs), large-mode areas (LMAs), novel index profiles and high dopant concentrations. This paper discusses advances made in design and fabrication of highly efficient, large-mode area double clad fibers. Experimental and modeling results pertaining to changes in mode area, resultant power density and nonlinear threshold with changing core size are discussed. In addition, the mechanical reliability of the LMA fibers is discussed.
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. In general, requirements include small form-factor, good bend performance, tight tolerances on fiber geometry and ability to maintain a single polarization state. Currently, bow-tie or elliptical clad type high birefringence fibers are used in such sensors. This paper deals with the development and characterization of small form-factor (80 μm) PANDA style high birefringence fibers for sensing applications at different wavelengths of interest. The rationale and advantages of the new design are discussed along with geometrical and optical characteristics of one new fiber. Performance data of the fiber in terms of cross-talk variation in the -55 to + 85°C temperature range are presented.
Fibers for high-power laser and amplifier applications require large claddings with high numerical apertures for efficiently coupling pump energy. In addition, such fibers should have high rare-earth dopant concentrations in relatively large cores, with low numerical apertures, to reduce non-linearities. Furthermore, polarization maintaining double-clad fibers (PM-DCF) are needed for coherently combining the outputs of several lasers/amplifiers to achieve output powers in excess of 100 kW for military and industrial laser applications. In this paper, we report the progress made towards fabricating PM double-clad fibers, with a variety of fiber characteristics, to facilitate development and production of high-power lasers and amplifiers. In particular, a Panda-type PM-DCF with a 0.06 NA, 30 micron diameter, Yb-doped core is reported. We also discuss various criteria that are critical for designing these PM double clad fibers.