IR Fiber Optics- An Introduction

Satish Chandel

Department of Physics, Govt. College Bilaspur (H.P.)-174001

*Corresponding Author’s Email: satishchandel@gmail.com

ABSTRACT:

During the past many years of the development of IR fibers there has been a great deal of fundamental research designed to produce a fiber with optical and mechanical properties close to that of silica. We can see that today we are still far from that Holy Grail but some viable IR fibers have emerged which as a class can be used to address some of the needs for a fiber which can transmit greater than 2 µm. Yet we are still limited with the current IR fiber technology by high loss and low strength. Nevertheless, more applications are being found for IR fibers as users become aware of their limitations and, more importantly, how to design around their properties. There are two near-term applications of IR fibers; laser power delivery and sensors. An important future application for these fibers, however, may be more in active fiber systems like the Er and Pr doped fluoride fibers and emerging doped chalcogenide fibers. As power delivery fibers, the best choice seems to be hollow waveguides for CO₂ lasers and either SC sapphire, germanate glass, or HGWs for Er: YAG laser delivery. Chemical, temperature, and imaging bundles make use mostly of solid -core fibers. Evanescent wave spectroscopy (EWS) using chalcogenide and fluoride fibers are quite successful. A distinct advantage of an IR fiber EWS sensor is that the signature of the analyte is often very strong in the infrared or fingerprint region of the spectrum. Temperature sensing generally involves the transmission of blackbody radiation. IR fibers can be very advantageous at low temperatures especially near room temperature where the peak in the blackbody radiation is near 10 m. Finally, there is an emerging interest in IR imaging using coherent bundles of IR fibers. Several thousand chalcogenide fibers have been bundled by Amorphous Materials (Garland, TX) to make an image bundle for the 3 to 10 m region.

INTRODUCTION:

Recently, in communication systems, fiber optics has become a most interesting development tool as a transmission tool. Infrared (IR) optical fibers may be defined as fiber optics transmitting radiation with wavelengths greater than approximately 2 µm. The first IR fibers were fabricated in the mid-1960’s from chalcogenide glasses such as arsenic trisulphide with losses in excess of 10 dB/m. During the mid-1970’s, the interest in developing an efficientand reliable IR fiber for short -haul applications increased partly in response to the need for a fiber to link broadband, long wavelength radiation to remote photodetectors in military sensor applications. In addition, there was an ever-increasing need for a flexible fiber delivery system for transmitting CO₂ laser radiation in surgical applications. Around 1975, a variety of IR materials and fibers were developed to meet these needs. These included the heavy metal fluoride glass (HMFG) and polycrystalline fibers as well as hollow rectangular waveguides.

IR fiber optics may logically be divided into three broad categories: glass, crystalline, and hollow waveguides. These categories may be further subdivided based on either the fiber material or structure or both as shown in Table 1. Over the past 25 years many novel IR fibers have been made in an effort to fabricate a fiber optic withproperties as close to silica as possible, but only a relatively small number have survived. In general, both the optical and mechanical properties of IR fibers remain inferior to silica fibers and, therefore, the use of IR fibers is still limited primarily to non-telecommunication, short -haul applications requiring only tens of meters of fiber rather than kilometer lengths common to telecommunication applications.

Table 1 Categories of IR fibers with a common example to illustrate each subcategory.

Main	Subcategory	Examples
Glass	Heavy metal fluoride-HMFG	ZBLAN - (ZrF₄-BaF₂-LaF₃-AlF₃-NaF)
	Germanate	GeO₂-PbO
	Chalcogenide	As₂S₃ and AsGeTeSe
Crystal	Polycrystalline –PC	AgBrCl
	Single crystal – SC	Sapphire
Hollow waveguide	Metal/dielectric film	Hollow glass waveguide
	Refractive index < 1	Hollow sapphire at 10.6 m

1. Non-oxide and heavy-metal oxide glass IR fibers

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

Table 2 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.

Property	Fluorozirconate	Fluoroaluminate
	ZBLAN	AlF₃-ZrF₄-BaF₂-CaF₂-YF₃
Glass transition temperature, ^oC	265	400
Durability	Medium	Excellent
Loss at 2.94 m, dB/m	0.01	0.1
Er: YAG laser peak output energy, mJ	300	850
	300-µm core	500-µm core

Chalcogenide fibers

Chalcogenide glass fibers were drawn into essentially the first IR fiber in the mid 1960s. Chalcogenide fibers fall into three categories: sulfide, selenide, and telluride. 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. 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. 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.

PC fibers

There are many halide crystals which have excellent IR transmission but only a few have been fabricated into fiber optics. The technique used to make PC fibers is hot extrusion. As a result, only the silver and thallium halides have the requisite physical properties such as ductility, low melting point, and independent slip systems to be successfully extruded into fiber. In the hot extrusion process, a single -crystal billet or preform is placed in a heated chamber and the fiber extruded to net shape through a diamond or tungsten carbide die at a temperature equal to about ½ the melting point. The final PC fibers are usually from 500 to 900 m in diameter with no buffer jacket. The polycrystalline structure of the fiber consists of grains on the order of 10 microns or larger in size. The billet may be clad using the rod-in-tube method. In this method, a mixed silver halide such as AgBrCl is used as the core and then a lower index tube is formed using a Cl^- rich AgBrCl crystal. The extrusion of a core/clad fiber is not as easy to achieve as it is in glass drawing, but Artjushenko and his colleagues at the GPI in Moscow have achieved clad Ag-halide fibers with losses nearly as low as the core -only Ag-halide fiber.

2. Hollow waveguides

The first optical frequency hollow waveguides were similar in design to microwave guides. Garmire, et al. made a simple rectangular waveguide using aluminum strips spaced 0.5 mm apart by bronze shim stock. Even when the aluminum was not well polished, these guides worked surprisingly well. Losses at 10.6 m were well below 1 dB/m and Garmire early demonstrated the high power handling capability of an air-core guide by delivering over 1 kW of CO₂ laser power through this simple structure. These rectangular waveguides, however, never gained much popularity primarily because their overall dimensions (about 0.5 x 10 mm) were quite large in comparison tocircular cross section guides and also because the rectangular guides cannot be bent uniformly in any direction. As a result, hollow circular waveguides with diameters of 1 mm or less fabricated using either metal, glass, or plastic tubing are the most common guide today. In general, hollow waveguides are an attractive alternative to conventional solid -core IR fibers for laser power delivery because of the inherent advantage of their air core.Hollow waveguides not only enjoy the advantage of high laser power thresholds but also low insertion loss, no end reflection, ruggedness, and small beam divergence. A disadvantage, however, is a loss on bending which varies as 1/R where R is the bending radius. In addition, the losses for these guides vary as 1/a³wherea is the radius of thebore and, therefore, the loss can be arbitrarily small for a sufficiently large core. The bore size and bending radius dependence of all hollow waveguides is a characteristic of these guides not shared by solid -core fibers. Initially these waveguides were developed for medical and industrial applications involving the delivery of CO₂ laserradiation, but more recently they have been used to transmit incoherent light for broadband spectroscopic and radiometric applications.

3. SUMMARY AND CONCLUSIONS:

During the past 25 years of the development of IR fibers there has been a great deal of fundamental research designed to produce a fiber with optical and mechanical properties close to that of silica. Yet we are still limited with the current IR fiber technology by high loss and low strength. Nevertheless, more applications are being found for IR fibers as users become aware of their limitations and, more importantly, how to design around their properties. IR fibers can be very advantageous at low temperatures especially near room temperature where the peak in the blackbody radiation is near 10 m. Finally, there is an emerging interest in IR imaging using coherent bundles of IR fibers. Several thousand chalcogenide fibers have been bundled by Amorphous Materials to make an image bundle for the 3 to 10 m region.

4. REFERENCES:

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Received on 02.01.2015 Accepted on 08.01.2015

Research J. Engineering and Tech. 6(1): Jan.-Mar. 2015 page 110-112

DOI: 10.5958/2321-581X.2015.00016.1