There are two IR transmitting glass fiber systems that are relatively similar to conventional silica-containing glass fibers. One is the HMFG and the other is heavy-metal germanate glass fibers based on GeO₂. The germanate glass fibers generally do not contain fluoride compounds; instead they contain heavy metal oxides to shift the IR absorption edge to longer wavelengths. The advantage of germanate fibers over HMFG fibers is that germanate glass has a higher glass transition temperature and, therefore, higher laser-damage thresholds. But the loss for the HMFG fibers is lower. Finally, chalcogenide glass fibers made from chalcogen elements such as As, Ge, S, and Te contain no oxides or halides. They are a good fiber for non-laser power delivery applications.

HMFG fibers

Poulain and Lucas discovered HMFGs or fluoride glasses accidentally in 1975 at the University of Rennes.⁸ In general, the typical fluoride glass has a glass transition temperature, T_g, four times less than silica; is considerably less stable; and has failure strains of only a few percent compared to silica's greater than 5%. While an enormous number of multicomponent fluoride glass compositions have been fabricated, comparably few have been drawn into fiber. This is because the temperature range for fiber drawing is normally too small in most HMFGs to permit fiberization of the glass. The most popular HMFGs for fabrication into fibers are the fluorozirconate and fluoroaluminate glasses of which the most common are ZBLAN (ZrF₄-BaF₂-LaF₃-AlF₃-NaF) and AlF₃-ZrF₄-BaF₂-CaF₂-YF₃, respectively. The key physical properties which contrast these glasses are summarized in Table 4. An important feature of the fluoroaluminate glass is its higher T_g which largely accounts for the higher laser damage threshold for the fluoroaluminate glasses compared to ZBLAN at the Er:YAG laser wavelength of 2.94 µm.

Table 4 - Comparison between fluorozirconate and fluoroaluminate glasses of some
key properties which relate to laser power transmission and durability of the
two HMFG fibers. Other physical properties are relatively similar.

The fabrication of HMFG fiber is similar to any glass-fiber drawing technology except that the preforms are made using some type of melt-forming method rather than by a vapor deposition process common with silica fibers. Specifically, a casting method based on first forming a clad glass tube and then adding the molten core glass is used to form either multimode or single-mode fluorozirconate-fiber preforms. The cladding tube is made either by a rotational casting technique in which the clad tube is spun in a metal mold or by merely inverting and pouring out most of the molten clad glass contained in a metal mold to form a tube.⁹ The clad tubing is then filled with a higher index core glass. Other preform fabrication techniques include rod-in-tube and crucible techniques. The fluoroaluminate-fiber preforms have been made using an unusual extrusion technique in which core and clad glass plates are extruded into a core/clad preform.¹⁰ All methods, however, involve fabrication from the melt glass rather than from the more pristine technique of vapor deposition used to form SiO₂-based fibers. This process creates inhrent problems such as the formation of bubbles, core-clad interface irregularities, and small preform sizes. Most HMFG fiber drawing is done using preforms rather than the crucible method. A ZBLAN preform is drawn at about 310 ^oC in a controlled atmosphere (to minimize contamination by moisture or oxygen impurities which significantly weaken the fiber) using a narrow heat zone compared to silica. Either UV acrylate or Teflon coatings are applied to the fiber. In the case of Teflon, heat shrink FEP fluoride is generally applied to the glass preform prior to the draw.

Figure 2 - Losses in the best BTRL¹² and typical (Infrared Fiber Systems, Silver Spring,
MD) ZBLAN fluoride glass fibers compared to fluoroaluminate glass fibers.¹⁰

The attenuation in HMFG fibers is predicted to be about 10 times less than that for silica fibers.¹¹ Based on extrapolations of the intrinsic losses resulting from Rayleigh scattering and multiphonon absorption, the minimum in the loss curves or V-curves is projected to be about 0.01 dB/km at 2.55 µm. Recent refinements of the scattering loss have modified this value slightly to be 0.24 dB/km or about 8 times less than that for silica fiber.^12, In practice, however, extrinsic loss mechanisms still dominate fiber loss. In Fig. 2 losses for two ZBLAN fibers are shown. The data from British Telecom (BTRL) represents state-of-the-art fiber 110 m in length.¹² The other curve is more typical of commercially available (Infrared Fiber Systems, Silver Spring, MD) ZBLAN fiber. The lowest measured loss for a BTRL, 60-m-long fiber is 0.45 dB/km at 2.3 µm. Some of the extrinsic absorption bands that contribute to the total loss shown in Fig. 2 for the BTRL fiber are; Ho³⁺ (0.64 and 1.95 µm), Nd³⁺ (0.74 and 0.81 µm), Cu²⁺ (0.97 µm), and OH^- (2.87 µm). Scattering centers such as crystals, oxides, and bubbles have also been found in the HMFG fibers. In their analysis of the data in Fig. 2, the BTRL group separated the total minimum attenuation coefficient (0.65 dB/km at 2.59 µm) into an absorptive loss component equal to 0.3 dB/km and a scattering loss component equal to 0.35 dB/km. The losses for the fluoroaluminate glass fibers are also shown for comparison in Fig. 2.¹⁰ Clearly the losses are not as low as for the BTRL-ZBLAN fiber, but the AlF₃-based fluoride fibers do have the advantage of higher glass transition temperatures and, therefore, are better candidates for laser power delivery.

The reliability of HMFG fibers depends on protecting the fiber from attack by moisture and on pretreatment of the preform to reduce surface crystallization. In general, the HMFGs are much less durable than oxide glasses. The leach rates for ZBLAN glass ranges between 10^-3 and 10^-2 g/cm2/day. This is about 5 orders of magnitude higher than the leach rate for Pyrex glass. The fluoroaluminate glasses are more durable with leach rates that are more than three times lower than the fluorozirconate glasses. The strength of HMFG fibers is less than that for silica fibers. From Table 2 we see that Young's modulus E for fluoride glass is 51 GPa compared to 73 GPa for silica glass. Taking the theoretical strength to be about 1/5 that of Young's modulus gives a theoretical value of strength of 11 GPa for fluoride glass. The largest bending strength measured has been about 1.4 Gpa, well below the theoretical value. To estimate the bending radius R we may use the approximate expression:

where s_max is the maximum fracture stress and r is the fiber radius.¹³

Germanate fibers

Heavy metal oxide glass fibers based on GeO₂ have recently shown great promise as an alternative to HMFG fibers for 3 µm laser power delivery.¹⁴ Today, GeO₂-based glass fibers are composed of GeO₂ (30-76%) - RO (15-43%) - XO (3-20%) where R represents an alkaline-earth metal and X represents an element of Group IIIA.¹⁵ In addition, small amounts of heavy metal fluorides may be added to the oxide mixture. The oxide-only germanate glasses have glass transition temperatures as high as 680 ^oC, excellent durability, and a relatively high refractive index of 1.84. In Fig. 3, loss data is given for a typical germanate glass fiber. While the losses are not as low as they are for the fluoride glasses shown in Fig. 2, these fibers have an exceptionally high damage threshold at 3 µm. Specifically, over 20 W (2J at 10 Hz) of Er:YAG laser power has been launched into these fibers.

Figure 3 - Germanate glass fiber manufactured by Infrared Fiber Systems, Silver Spring, MD.

Chalcogenide fibers

Chalcogenide glass fibers were drawn into essentially the first IR fiber in the mid 1960s.¹ Chalcogenide fibers fall into three categories: sulfide, selenide, and telluride.¹⁶ One or more chalcogen elements are mixed with one or more elements such as As, Ge, P, Sb, Ga, Al, Si, etc. to form a two or more component glass. From the data in Table 2 we see that the glasses have low softening temperatures more comparable to fluoride glass than the oxide glasses. They are very stable, durable, and insensitive to moisture. A distinctive difference between these glasses and the other IR fiber glasses is that they do not transmit well in the visible region and their refractive indices are quite high. Additionally, most of the chalcogenide glasses, except for As₂S₃, have a rather large value of dn/dT.¹⁷ This fact limits the laser power handling capability of the fibers. In general, chalcogenide glass fibers have proven to be an excellent candidate for evanescent wave fiber sensors and for IR fiber image bundles.¹⁸

Chalcogenide glass is made by combining highly purified (>6 nines purity) raw elements in an ampoule which is heated in a rocking furnace for about 10 hours. After melting and mixing, the glass is quenched and a glass preform fabricated using rod-in-tube or rotational casting methods. Preform fiber draws involve drawing a core/clad preform or a core-only preform. For the core-only preform draw either a soft chalcogenide cladding can be extruded over the fiber as it is drawn or the preform can be Teflon clad. Crucible drawing is also possible.

Figure 4 - Two common chalcogenide glass fibers: As₂S₃ and an AsGeSeTe fiber.¹⁷
Note the many impurity bands pervasive in these fiber systems.

The losses for the most important chalcogenide fibers are given in Fig. 4. Arsenic trisulfide (As₂S₃) fiber, one of the simplest and oldest chalcogenide fibers, has a transmission range from 0.7 to about 6 µm.¹⁶ This fiber is red in color and, therefore, transmits furthest into the visible region but cuts off in the long wavelength end well before the heavier chalcogenide fibers.¹⁷ Longer wavelengths are transmitted through the addition of heavier elements like Te, Ge, and Se as shown in the figure. A key feature of essentially all chalcogenide glasses is the strong extrinsic absorption resulting from contaminants such as hydrogen, H₂O, and OH^- bonding to the elemental cations. In particular, absorption peaks between 4.0 and 4.6 µm are due to S-H or Se-H bonds and those at 2.78 µm and 6.3 µm are due to OH^- (2.78 µm) and/or molecular water. The hydride impurities are often especially strong and can be deleterious when using these fibers in chemical sensing applications where the desired chemical signature falls in the region of extrinsic absorption. Another important feature of most of the chalcogenide fibers is that their losses are in general much higher than the fluoride glasses. In fact at the important CO₂ laser wavelength of 10.6 µm, the lowest loss is still above 1 dB/m for the Se-based fibers.¹⁶

IR Fibers | Introduction | Non-oxide & H-M oxide glass | Crystalline | Hollow | Conclusions | References
 

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