The first optical frequency hollow waveguides were similar in design to microwave guides. Garmire, et al.³⁰ 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 µm 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 to circular 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 theseguides vary as 1/a³ where a is the radius of the bore 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 laser radiation, but more recently they have been used to transmit incoherent light for broadband spectroscopic and radiometric applications.^31,32,33 They are today one of the best alternatives for power delivery in IR laser surgery and industrial laser delivery systems with losses as low as 0.1 dB/m and transmitted cw laser powers as high as 2.7 kW.³⁴

Hollow-core waveguides may be grouped into two categories: 1.) those whose inner core materials have refractive indices greater than one (leaky guides) and 2.) those whose inner wall material has a refractive index less than one (attenuated total reflectance, i.e. ATR, guides). Leaky or n>1 guides have metallic and dielectric films deposited on the inside of metallic,³⁵ plastic,³⁶ or glass tubing.³⁷ ATR guides are made from dielectric materials with refractive indices less than one in the wavelength region of interest.³⁸ Therefore, n<1 guides are fiber-like in that the core index (n » 1) is greater than the clad index. Hollow sapphire fibers operating at 10.6 µm (n = 0.67) are an example of this class of hollow guide.³⁹

Hollow metal and plastic waveguides

The earliest circular cross section hollow guides were formed using metallic and plastic tubing as the structural members. Miyagi and his group in Japan used sputtering methods to deposit Ge,⁴⁰ ZnSe, and ZnS³⁵ coatings on aluminum mandrels. Then a final layer of Ni was electroplated over these coatings before the aluminum mandrel was removed by chemical leaching. The final structure was then a flexible Ni tube with optically thick dielectric layers on the inner wall to enhance the reflectivity in the infrared. Croitoru and his group at Tel Aviv University applied Ag followed by AgI coatings on the inside of polyethylene and Teflon tubing to make a very flexible waveguide.⁴¹ Similar Ag and Ag-halide coatings were deposited inside Ag tubes by Morrow, et al.⁴²

Hollow glass waveguides

The most popular structure today is the hollow glass waveguide (HGW) developed initially at Rutgers University.⁴³ The advantage of glass tubing is that it is much smoother than either metal and plastic tubing and, therefore, the scattering losses are less. HGWs are fabricated using wet-chemistry methods to first deposit a Ag layer on the inside of silica glass tubing and then to form a dielectric layer of AgI over the metallic film by converting some of the Ag to AgI. The silica tubing used has a polymer coating of UV acrylate or polyimide on the outside surface to preserve the mechanical strength. The thickness of the AgI is optimized to give high reflectivity at a particular laser wavelength or range of wavelengths. Using these techniques, HGWs have been fabricated with lengths as long as 13 m and bore sizes ranging from 250 to 1,300 µm.

Figure 7 - Straight losses measured in hollow glass waveguides with Ag/AgI films. The
guide labeled CO₂ laser was designed for optimal transmission at 10.6 µm while
that labeled Er:YAG laser was designed for optimal transmission at 3 µm.
Note that the loss varies approximately as 1/a³.

The spectral loss for a 530-µm-bore HGW is given in Fig. 2. This HGW was designed for an optimal response at 10 µm. The peaks at about 3 and 5 µm are not absorption peaks but rather interference bands due to thin film optical effects. For broadband applications and shorter wavelength applications, a thinner AgI coating would be used to shift the interference peaks to shorter wavelengths. For such HGWs the optical response will be nearly flat without interference bands in the far IR fiber region of the spectrum. The data in Fig. 7 shows the straight loss measured using a CO₂ and Er:YAG laser for different bore sizes. An important feature of this data is the 1/a³ dependence of loss on bore size predicted by theory of Marcatili and Schmeltzer.⁴⁴ In general, the losses are less than 0.5 dB/m at 10 µm for bore sizes larger than ~400 µm. Furthermore, the data at 10.6 µm agrees well with the calculated values but at 3 µm the measured losses are somewhat above those predicted by Marcatili and Schmeltzer. This is a result of increased scattering at the shorter wavelengths from the metallic and dielectric films. The bending loss depends on many factors such as the quality of the films, the bore size, and the uniformity of the silica tubing. A typical bending loss curve for a 530-µm bore HGW measured with a CO₂ laser is given in Fig. 8. The losses are seen to increase linearly with increasing curvature as predicted. It is important to note that while there is an additional loss on bending for any hollow guide, it does not necessarily mean that this restricts their use in power delivery or sensor applications. Normally most fiber delivery systems have rather large bend radii and, therefore, a minimal amount of the guide is under tight bending conditions and the bending loss is low. From the data in Fig. 8 one can calculate the bending loss contribution for a HGW link by assuming some modest bends over a small section of guide length. An additional important feature of hollow waveguides is that they are nearly single mode. This is a result of the strong dependence of loss on the fiber mode parameter. That is, the loss of high order modes increases as the square of the mode parameter so even though the guides are very multimode, in practice only the lowest order modes propagate. This is particularly true for the small bore (<300 µm) guides in which virtually only the lowest order HE₁₁ mode is propagated.

Figure 8 - Additional loss on bending a 530-µm-bore HGW measured at 10.6 µm. The
loss is seen to increase as the curvature increases.

HGWs have been used quite successfully in IR laser power delivery and, more recently, in some sensor applications. Modest CO₂ and Er:YAG laser powers below about 80 W can be delivered without difficulty. At higher powers, water-cooling jackets have been placed around the guides to prevent laser damage. The highest CO₂ laser power delivered through a water-cooled, hollow metallic waveguide with a bore of 1,800 µm was 2,700 W and the highest power through a water-cooled, 700-µm-bore HGW was 1,040 W.⁴⁵ Sensor applications include gas and temperature measurements. A coiled HGW filled with gas can be used in place of a more complex and costly White cell to provide an effective means for gas analysis. Unlike evanescent wave spectroscopy in which light is coupled out of a solid-core-only fiber into media in contact with the core, all of the light is passing through the gas in the hollow guide cell making this a sensitive, quick response fiber sensor. Temperature measurements may be aided by using a HGW to transmit blackbody radiation from a remote site to an IR detector. Such an arrangement has been used to measure jet engine temperatures.

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

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