Over the past two decades, fiber laser technologies have matured to such an extent that they have captured a large portion of the commercial laser marketplace. Yet, there still is a seemingly unquenchable thirst for ever greater optical power to levels where certain deleterious light-matter interactions that limit continued power scaling become significant. In the past decade or so, the industry has focused mainly on waveguide engineering to overcome many of these hurdles. However, there is an emerging body of work emphasizing the enabling role of the material. In an effort to underpin these developments, this paper reviews the relevance of the material in high power fiber laser technologies. As the durable material-of-choice for the application, the discussion will mainly be limited to silicate host glasses. The discussion presented herein follows an outward path, starting with the trivalent rare earth ions and their spectroscopic properties. The ion then is placed into a host, whose impact on the spectroscopy is reviewed. Finally, adverse interactions between the laser lightwave and the host are discussed, and novel composition glass fiber design and fabrication methodologies are presented. With deference to the symbiosis required between material and waveguide engineering in active fiber development, this review will emphasize the former. Specifically, where appropriate, materials-based paths to the enhancement of laser performance will be underscored.

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Materials for optical fiber lasers: A review

The background to the development of polymer optical fibres (POFs) and the methods used for their manufacture are described. The optical properties of polymers are then discussed in the context of fibres, taking the most common POF materials, polymethylmethacrylate and polystyrene, as examples. The poor market penetration of POFs, due to inadequate performance in terms of attenuation, bandwidth and temperature resistance, is then explained. The article concludes with a discussion of techniques which show great promise for the development of a new generation of high-performance POFs which will satisfy market demands.

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Polymer optical fibres

Experimental results indicate that the stress-induced strain birefringence in borosilicate elliptically clad fibers arises from the expansion coefficient mismatch between the borosilicate elliptical cladding and the pure silica outer jacket. This strain birefringence cannot be annealed even after repeated thermal cycling. It is shown that, for a given dopant concentration, the birefringence depends primarily on the cladding ellipticity ∊ and increases linearly with increasing ellipticity and expansion coefficient mismatch.

Birefringence in elliptically clad borosilicate single-mode fibers.

A simple method is described for fabricating fibers with large strain birefringence starting with standard MCVD preforms. The method is illustrated using borosilicate fibers. The birefringence and polarization properties are measured in a long fiber and the birefringence is also measured using a fiber slice and polarizing microscope.

Single polarization optical fibers: Exposed cladding technique

: This interim technical report summarizes the first year's research efforts to fabricate optical communications fibers that are transmissive between 1 and 12 micrometers. The ultimate objective of this program is to prepare infrared transmitting fibers with losses less than 5 dB/km. In preparing infrared transparent fibers with these losses, we are emphasizing the extrusion into fiber of very pure KCl as this material has demonstrated bulk losses equal to or less than 5 dB/km in the 2- to 6-micrometers region. The primary approach used to meet the program goals, therefore, was the extrusion of reactive atmosphere process (RAP) grown KCl into 250- to 1000-micrometers-diameter optical fibers. After one year of effort, the best KCl fiber extruded had a loss of 4200 dB/km. This report also summarizes our efforts in fabricating a special infrared fiber prototype device for the detection of pulsed CO2 laser radiation. The prototype device, which is to be used in a battlefield identification friend or foe (BIFF) application, was successfully field tested in Germany.

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Infrared Fiber Optics

Easy-to-fabricate, low-loss optical fibers are described that consist of a pure fused silica core surrounded with a loosely fitting, extruded, FEP cladding tube. Because of their large numerical aperture (NA), these fibers are particularly well suited for the transmission of the incoherent light emitted by light-emitting diodes. Whereas the losses approached those of the core material for small angle excitation (i.e., 7.6 dB/km at 0.8 μm for a Suprasil 2 fiber of 230-m length), they increased to 14 dB/km for a steady-state NA of 0.3. The measured pulse dispersion of up to 30 nsec agrees well with the expected 24 nsec for a step-index fiber with an NA of 0.3.

Low-Loss FEP-Clad Silica Fibers

Design considerations, fabrication, and transmission characteristics of single-mode optical fibers made with different borosilicate compositions are discussed. Fibers with core sizes as large as 15 microm and refractive-index differences below 0.1% were made with the modified chemical vapor deposition (MCVD) process. Minimum losses achieved with fibers up to 1-km lengths were 1.3-1.4 dB/km in the 1.02-1.1-microm range. Microbending losses were avoided by jacketing the fibers with silicone resin. Single-mode operation is maintained in moderately overmoded fibers (V ~ 3.0) because of leakage loss through a finite cladding thickness and because of bends in the fiber axis.

Low-loss single-mode fibers with different B(2)O(3)-SiO(2) compositions.

Contamination originating in an electric resistance furnace was found to increase substantially the losses of unclad and plastic-clad silica fibers. In contrast, the losses of doped silica fibers with sufficient cladding thickness were unaffected by impure drawing conditions. Operating the furnace without muffle tube and protecting the preform with a pure, inert gas injected via a counter-flow resulted in practically contamination-free operation and unclad-fiber losses as low as 3 dB/km. The removal of the muffle tube significantly simplified the furnace operation and reduced the cycling time from many hours to a few minutes.

Contamination of furnace-drawn silica fibers

The loss spectra, refractive-index profiles, numerical apertures, and pulse dispersion characteristics of three multimode optical fibers made by Corning Glass Works have been measured at Bell Laboratories. The lowest transmission loss, 4.3–6.8 dB/km, was observed at 1.06-μm wavelength; in the 0.8–0.9-μm spectral region the losses ranged from 6.6 dB/km to 11.6 dB/km. The numerical apertures of the fibers, designated as Nos. 1, 2, and 3, were calculated from the measured refractive-index differences to be 0.133, 0.157, and 0.121, respectively. Pulse dispersion due to multimode effects in fiber No. 1 (1 km long) was 8.4 nsec at 0.9-μm wavelength; the values were 6.0 nsec for fiber No. 2 (0.363 km long) and 2.6 nsec for fiber No. 3 (0.290 km).

Transmission characteristics of three corning multimode optical fibers.

Recently we reported on a new type of low-loss, large-numerical- aperture optical fiber consisting of a pure fused silica core loosely surrounded by an extruded cladding tube of a low-index polymer. Teflon PEP (PEP).1 One of the difficulties encountered in that work was the presence of large diameter variations of the oxy-hydrogen-torch-drawn fibers, which resulted in the coupling of well-guided modes to lossy radiation modes. In order to avoid this difficulty, we used a CO2 laser for the drawing operation which has been demonstrated to produce contamination-free fibers with good diameter control.2

P. Kaiser

Papers

Low-Loss FEP-Clad Silica Fibers

Low-loss single-mode fibers with different B(2)O(3)-SiO(2) compositions.

Contamination of furnace-drawn silica fibers

Transmission characteristics of three corning multimode optical fibers.

Low-Loss, Polymer-Clad Silica Fibers Produced by Laser Drawing