This is a continuation from the previous tutorial - Polymer optical fibers

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The potential of single-crystal fibers was recognized 50 years ago, when the inherent strength and perfection of the crystalline matrix of the fibers were determined. Despite this, sustained research into the production of such fibers was delayed until the s.

Until then, control over the growth characteristics of the fibers was poor, and there were no well-defined areas of utilization. The first application that drove the production of single-crystal sapphire fibers was use as a reinforcing member for metal-matrix composites.

Sapphire, whose chemical formula is \(\text{Al}_2\text{O}_3\), was thought to be an attractive candidate to employ for this purpose because it possesses a high melting temperature of \(^\circ\text{C}\), has a low solubility in water, is resistant to chemically hostile environments, and has many attractive physical and mechanical properties.

However, fiber-reinforcing applications require fibers to possess good mechanical characteristics, but make no restriction on the optical qualities of the fibers. 

Interest in developing optical-quality sapphire fibers emerged after the commercial effort to manufacture structural-grade sapphire fibers began.

Sapphire optical fibers are not reasonable candidates for long-distance telecommunications lines because of the difficulties involved in manufacturing low-cost fibers having perfect crystalline structure and surfaces as smooth as conventional silica-based glass fibers.

Researchers at Stanford University, realizing that sapphire fibers were not likely to replace glass-based fibers for telecommunications applications, embarked on a program to investigate the potential applications and manufacture of optical-quality sapphire optical fibers.

Their investigation indicated that fibers grown using the laser-heated pedestal growth \(\text{(LHPG)}\) method were optically superior to those grown using the edge-defined film-fed growth \(\text{(EFG)}\) method, which was commonly used in the manufacture of structural-grade sapphire fibers for reinforcing applications. Other researchers concurred, and with the possible exception of Saint-Gobain Saphikon, the best optical-quality sapphire fibers have been grown by researchers using the \(\text{LHPG}\) method.

Optical-quality sapphire fibers have increased in popularity over the years and, now, are used in various fields and applications such as laser-power delivery probes and tips for surgical and treatment lasers; rugged high-temperature thermometers for industrial applications; visible and infrared \(\text{(IR)}\) transmitting waveguides for spectroscopy applications; and high-temperature&#; resistant sensing elements in optical fiber Fabry&#;Perot extrinsic sensors and devices.

This chapter begins by describing the two most popular methods used to grow single-crystal sapphire fiber: \(\text{LHPG}\) and the \(\text{EFG}\). The positive and negative aspects of each method are reviewed.

Next, the optical and mechanical properties of sapphire fibers are described. The chapter continues with a discussion of the possibilities that exist for cladding and overcoating the bare sapphire fiber, and the difficulties associated with this procedure are addressed.

The fourth section of the chapter provides a brief overview of sapphire fiber applications. Finally, the chapter concludes with a listing of relevant physical properties for \(\text{Al}_2\text{O}_3\).

 

 

1.  THE GROWTH OF SAPPHIRE FIBER

Single crystals of synthetic corundum, also known as \(a\)-\(\text{Al}_2\text{O}_3\) or &#;&#;white sapphire,&#;&#; may be grown according to a variety of methods.

The most common of these methods are: crystallization from a melt, solution growth, and condensation from the vapor phase.

The second of these methods has attracted considerable interest because it permits the continuous and controlled growth of single-crystal fibers of uniform diameter.

This method also permits the fibers to be grown along a specific crystallographic axis.

However, sapphire fiber, which is a birefringent waveguide, is customarily grown so that the \(c\) plane [] coincides with the optical axis of the fiber. The two most popular implementations of this technique are the \(\text{EFG}\) and the \(\text{LHPG}\) methods. 

The \(\text{LHPG}\) process, favored by university-based researchers, uses a float-zone technique to grow optical-quality sapphire fibers. Researchers at Stanford University, Rutgers University, and the University of South Florida are the most notable to use this method.

This method has not been commercially embraced because of cost considerations: the \(\text{LHPG}\) process allows only a single fiber to be grown at a time.

This method has been widely acknowledged to produce better optical-quality sapphire fibers than those made via the \(\text{EFG}\) process, although commercial product data claim that this may no longer be the case.

Use of the \(\text{LHPG}\) method, schematically represented in Fig.1 and likened to the float-zone method of crystal growth, results in a single-crystal fiber being

 

 

Figure 1.  The laser-heated pedestal growth process.

 

grown from the molten tip of a source crystal. The tip of an \(\text{Al}_2\text{O}_3\) source rod is liquefied when the beam of a \(\text{CO}_2\) laser is split and focused onto it. Inserting a seed crystal into the liquefied bead creates a molten zone extending from the source rod to the seed crystal, with the shape of the molten zone determined by surface tension. 

The fiber is grown by carefully controlling the speed at which the seed crystal is moved away from the source rod and the speed at which the source rod is advanced into the molten zone.

The longest length of fiber that can be grown using this method is determined by the dimensions of the source rod. The purity of the fiber should be determined by the purity of the source rod; the heating method is clean, and the molten zone never comes in contact with a crucible or a die.

Under optimal growth conditions, the fiber has a diameter of approximately one-third of the source rod. Because mass is conserved during the growth process, the ratio

\[\tag{1}\frac{d_s}{d_f}=\sqrt{\frac{v_f}{v_s}}\]

where \(d_s\) is the diameter of the source rod, \(d_f\) is the diameter of the fiber, \(v_s\) is the velocity of the source rod, and \(v_f\) is the velocity of the fiber, is valid.

Although this description of \(\text{LHPG}\) is straightforward, it omits mention of the efforts exerted to produce a fiber of uniform diameter and good optical quality. Surface irregularities in the fiber, including diameter fluctuations, contribute substantially to scattering losses, and the fiber diameter is directly dependent on the shape of the molten zone.

The shape of the molten zone is affected by fluctuations in the diameter of the source rod, variations in the power of the \(\text{CO}_2\) laser, and the velocities of the source rod and fiber. In addition, the melt possesses a very low viscosity, and the molten zone, stabilized only through surface tension, is easily perturbed by air currents and vibrations.

The detrimental effects of air currents are minimized by sealing the source rod and growing fiber end in an atmosphere-controlled chamber. This precaution also permits the growth of the sapphire fibers in an inert atmosphere; it is believed that losses arise when \(\text{OH}^{-}\) and other gaseous species are incorporated into the sapphire fiber while it is growing.

Researchers at Rutgers University used a computer-controlled feedback system to stabilize the laser output and to monitor and control the fiber diameter. With this system, the authors of the paper were successful in producing sapphire fibers possessing a \(\pm0.5\)% diameter variation.

Without the use of feedback, the diameter variations were greater by a factor of 10. With this stable growth process and tight diameter control, the researchers produced fibers with losses of 0.3 dB/m at a wavelength of 2.94 \(\mu m\).

This is very close to the theoretical minimum of 0.13 dB/m at the same wavelength.

The \(\text{EFG}\) process&#;patented by Saint-Gobain Saphikon&#;unlike \(\text{LHPG}\), permits the growth of many sapphire fibers simultaneously, making the commercial production of sapphire fiber more economically viable.

This method of sapphire fiber growth was developed to meet the demand for sapphire fiber structural reinforcements for metal-matrix composites.

The mechanical strength, as opposed to the optical quality, of the fiber was the overriding consideration; the high loss in the early sapphire fibers produced by this method attenuates nearly all injected light over a centimeter of propagation length.

When the demand for structural filaments lagged, Saint-Gobain Saphikon investigated the possibility of producing optical-grade sapphire fiber for use in high-temperature&#;resistant fiber optics.

Saint-Gobain Saphikon supplies sapphire fiber for this purpose and for medical and spectroscopy applications.

The \(\text{EFG}\) method uses a reservoir containing molten source material to continuously supply the growth of a single-crystal sapphire structure (Fig. 2). Continuous growth of lengths more than 300 m has been demonstrated.

Unlike the \(\text{LHPG}\) method, the melt is contained in a molybdenum crucible, and the cross-section of the sapphire crystal is determined by the surface configuration of the molybdenum die anchored in the melt.

During growth, the melt wicks into the die and the fiber growth, initiated by a seed crystal, occurs at the top of the die. The use of an anchored die in the melt presents a stable platform from which to grow the crystal.

Because the top of the die extends above the level of the melt in the crucible, thermal variations present in the melt are damped as the melt travels the length of the tube. This helps

 

 

Figure 2.  The edge-defined film-fed growth \(\text{(EFG)}\) process.

ensure a stable melt temperature at the solidification interface.

Molybdenum is considered to have good chemical compatibility with molten alumina; sapphire crystals grown using molybdenum crucibles and dies contain only 5 ppm of molybdenum, and molten alumina is able to wet the molybdenum. The liquid phase of one material wets the solid phase of another when the contact angle is less than 90 degrees.

As in the \(\text{LHPG}\) method, production of optical-quality sapphire fibers requires isolating the growing crystal from vibrations and atmospheric currents; ensuring a stable temperature at the growth interface; surrounding the growing crystal with an inert atmosphere; and carefully controlling the growth rate of the crystal. 

The edges of the die are used to control the shape of the cross section of the sapphire crystal. When the die is shaped as a hollow capillary tube and the top of the die makes an angle of 75 degrees or greater with the inner edge of the die, the melt wicks up through the inside of the die and spreads over the top surface of the die.

A fiber grown under these conditions will have a diameter equal to the outer diameter of the cylinder.

Similarly, a die consisting of three capillary tubes fastened together can be used to grow three-bore tubing. In this case, the melt wicks up, through the approximately triangular interstitial region created by the mating of the three tubes, and spreads to cover the ringed surface formed by the ends of the three capillaries.

This results in three-bore tubing as the melt never extends over the inner diameter of any of the three conjoined tubes. The flexibility imparted to the growth process results in the ability to grow a sapphire crystal of nearly any cross-section, including ribbons, single filaments, single-bore tubing, and multiple-bore tubing.

 

 

2.  OPTICAL AND MECHANICAL CHARACTERISTICS OF SINGLE-CRYSTAL SAPPHIRE FIBER

The appeal of using sapphire fiber as an optical waveguide is driven by its capability to transmit light radiation at longer wavelengths into the IR than glassbased fibers (up to \(\thicksim 3.5\mu m\)) coupled with some of its superior physical and mechanical properties (melting point in excess of \(^\circ\text{C}\), chemical inertness, high laser-power thresholds, etc.).

Sapphire fibers are crystalline in nature and not amorphous glasses. There are two types of crystalline fibers: single-crystal \(\text{(SC)}\) and polycrystalline \(\text{(PC)}\).

Meter-long lengths of \(\text{SC}\) fibers have been made only from a handful of materials out of a possible pool of more than 80 \(\text{IR}\) transmissive crystalline materials. In this context, sapphire is the most popular \(\text{SC}\) fiber. 

Exploration of the optical uses of sapphire fiber began and is sustained because it is capable of operation in chemically harsh and high-temperature environments, it has a large transparency window, and it is a strong and hard material.

Sapphire fiber has not been adopted for a wide variety of uses because growth processes, which are difficult to perfect, result in sapphire fibers being lossier than their glass counterparts, and because sapphire fibers will always be more expensive.

Additional complications arise from present-day sapphire fibers being unclad and highly multimoded. Sapphire has also been observed to decrease in strength and flexibility at high temperatures.

Sapphire \(\text{(Al}_2\text{O}_3)\) is an insoluble, uniaxial crystal (trigonal structure) material with a melting point more than 8C. It is hard and noncorrosive to organic solvents and acids. It has a refractive index of 1.75 at \(3.2\mu m\) and, compared to silica glass, it has a thermal expansion 10 times greater and a Young modulus approximately seven times greater.

However, because of this last property, sapphire fibers are very stiff and fragile compared to silica fibers. Sapphire fiber is most transparent over the 240- to -nm range of the spectrum and has an unrealized theoretical loss minimum of 0.13 dB/m at a wavelength of 2.94\(\mu m\).

The measured transmission spectrum for bulk sapphire fiber (Fig. 3) as published by Innocenzi et al. , and plotted as discrete squares, differs from the theoretical loss spectrum.

The intrinsic loss spectrum is composed of contributions from Urbach, Brillouin, and multiphonon absorption processes. The Urbach process causes an increase in the value of the absorption coefficient as the frequency increases toward the band-gap energy of sapphire.

Brillouin scattering occurs when incident light, consisting of photons, is scattered by a mode of vibration of the crystal lattice, composed of acoustic phonons. &#;&#;Multiphonon&#;&#; refers to multiphonon absorption processes that occur in the sapphire crystal. 

Unlike experimental data taken for fused silica, which typically coincide well with theoretical loss curves, loss measurements of crystalline materials are

 

 

Figure 3.  Measured and theoretical attenuation in bulk sapphire (used, with permission).

 

typically higher than those predicted by theory. These measurements show a minimum measured loss at 1.78 \(\mu m\) that is orders of magnitude larger than the \(3.4\times10^{-6} dB/m\) predicted by theory.

The greater losses arise from the influence of crystal defects in the material, which result from imperfect growth conditions. Experimental data for wavelengths less than \(1.5\mu m\) can be fitted by a power law that varies as \(\lambda^{-4}\), which implies the presence of Rayleigh scattering caused by inhomogeneities in the crystal.

Rayleigh scattering occurs from the presence of particles or material inhomogeneities that are small compared to the wavelength of light. Hence, the optical properties of as-grown sapphire fibers are typically inferior to those of the bulk material.

This degradation is particularly evident in the visible region of the spectrum and is the result of color-center defects formed during the drawing process. However, said defects can be greatly reduced by post-annealing of the drawn fibers at \(^\circ\text{C}\) in an air or oxygen atmosphere. 

The loss spectrum of low-loss sapphire fibers grown at Rutgers using the \(\text{LHPG}\) method, compared with those made by \(\text{EFG}\), is shown in Fig. 4.

In general, the quality of optical-grade sapphire fibers has improved considerably over the past few years. Fibers with losses less than 0.5 dB/m at 2.94 \(\mu m\) and approximately 3.0 dB/m at 0.633 \(\mu m\) are grown routinely with maximum lengths less than 3 m and with diameters of 150, 250, 325, and 425 \(\mu m\).

Extrinsic loss in sapphire fibers, which should be minimized for optical applications, is largely due to scattering and absorption mechanisms.

Impurities and color centers exacerbate extrinsic absorption, and voids, inhomogeneities, and surface perturbations contribute to scattering losses.

Impurities in sapphire

 

 

Figure 4.  Loss transmission for \(\text{LHPG}\) versus \(\text{EFG}\) sapphire fibers.

 

fibers may derive from the source material or gaseous species such as \(H_2\text{O}\).

In fibers grown using the \(\text{EFG}\) method, the crucible and die may also be contributors.

Measurements suggest that fibers possess \(\text{OH}^{-}\)&#;impurities and color centers, specifically \(V\)-type (hole) centers that contribute to absorption losses around the wavelengths of 410 and nm.

These defects are likely incorporated during growth of the crystal. \(V\)-centers occur when a hole is trapped in the vacancy of an ionic crystal. The absorption at 410 nmis attributed to an \(\text{O}^{2-}\)&#; ion trapping a hole pair in the vicinity of an \(\text{Al}^{3+}\) vacancy. There is also the indication of \(\text{OH}^{-}\) impurities partially compensating \(V\)-centers.

The presence of \(\text{OH}\) impurities is supported because the absorption corresponding to the stretching band for the \(\text{OH}^{-}\) impurity, occurring around nm, is observed.

Also occurring around nm are three peaks that indicate the presence of transition metal impurities. The magnitude of the absorption in the bands around both 410 and nm&#;as mentioned before&#;can be reduced by annealing. 

The majority of extrinsic scattering losses in sapphire fibers are attributed to voids internal to the fiber, inhomogeneities, and surface variations of the fiber. The number and type of scattering sites determine the attenuation of each fiber, and these are dependent on the quality of the growth process.

It is well understood that any variations in the growth conditions of the fiber, including the temperature at the growth interface, changes in the speed of the growth, and perturbations of the shape and size of the meniscus, which feeds the growth of the crystal, cause variations in the diameter of the fiber.

Unstable growth conditions are also responsible for the incorporation of inclusions into the fiber. It is noted in particular that some sapphire fibers grown using the \(\text{EFG}\) technique possess bubbles that resulted from a growth rate that was too fast.

Annealing does not appear to reduce these losses, but it is believed that refinements in the mechanics of the growth process can greatly reduce, if not eliminate, these sources of loss.

Sapphire fibers generally do not suffer from twinning and dislocation defects, unlike bulk sapphire. This is because the diameter of sapphire fibers is small; unless a defect propagates parallel to the axis of the fiber, it will eventually grow out of the fiber. Dislocations in sapphire fibers will occur, but they will be of a lesser density than in bulk sapphire.

Sapphire fibers are not clad, which results in a number of undesirable characteristics. Single-crystal sapphire fibers cannot be grown as a core&#;clad structure from rod-in-tube source rods, as convective currents in the molten zone quickly act to destroy the geometrical structure of the source rod.

Other techniques, discussed in Section 3, have been proposed to clad sapphire fibers, but none represents a universally acceptable solution. In general, the cladding of a fiber exists to mechanically support the core, act as a barrier separating the core from environmental contaminants, and reduce scattering losses by lessening the difference in dielectric constants at the surface of the core.

Scattering can result in power from the scattered mode being coupled into other modes or ejected from the fiber as radiation losses. Contaminants on the surface of the sapphire fiber attenuate the intensity of the guided light through a few mechanisms.

Any substance having an index of refraction higher than sapphire will, by defeating total internal reflection, cause loss at all points of contact with the sapphire fiber.

A material with an index of refraction that is both lower than that of sapphire and different than that of the surrounding atmosphere will also cause loss if it is in sporadic contact with the surface of the sapphire fiber; scattering occurs at the points of discontinuity between dielectric constants.

Sapphire fibers have also incorporated impurities into their structure, a behavior observed particularly at elevated temperatures, which causes losses through scattering. 

Sapphire fibers are not single moded for operation in the 250- to -nm window of transparency and are not likely to be in the future. Without a physical cladding, the entire volume of the sapphire fiber may be considered to be the core, and the surrounding atmosphere the cladding.

Sapphire fibers typically have diameters between 100 and 300 \(\mu m\). For single-mode operation, the diameter of such a fiber is given by

\[\tag{2}d=\frac{\lambda(2.405)}{\pi n_s},\]

where \(d\) is the diameter of the fiber, \(n_s\) is the index of refraction of sapphire, and l is the wavelength of operation. This specifies fiber diameters ranging from approximately 1 to 0.1 microns for single-mode operation over the transparency window for sapphire.

It is claimed that 40-\(\mu m\) diameter fibers can be grown without modification to existing \(\text{LHPG}\) machinery. With modified apparatus, 5 \(\mu m\) diameter fibers could potentially be grown, but the cost is projected to be prohibitive.

Successfully handling such a fiber without breaking it would also be extremely difficult.

Sapphire is a brittle material, and the flexibility and strength of sapphire fiber decreases at high temperatures. Sapphire is among the hardest of natural minerals and is rated as a 9 on the Mohs scale.

Despite this property, because of the brittle nature of sapphire, fiber handling results in surface damage and consequent reduction in strength. Fibers tumbled against one another have suffered a tensile strength reduction of 30%.

Saint-Gobain Saphikon has combated this problem by presenting the option of coating the surface of each fiber with an organic polymer to protect the fiber before shipping.

While this protection is recognized as necessary for strength preservation, the process used to remove the sizing can both cause damage to and fail to remove all of the impurities on the fiber&#;s surface.

Sapphire also suffers a severe degradation in strength as the operating temperature is increased, as is shown in Fig. 5. This property affects the minimum diameter bend that the fibers can survive at any given temperature. The bend survival strength \(\sigma_{bs}\), also shown in Fig. 5, is determined by

\[\tag{3}\sigma_{bs}=\frac{d_fE}{2R_s},\]

where \(d_f\) is the fiber diameter, \(E\) is the elastic modulus, and Rs is the bend survival radius. Fibers fail when plastic deformation permits the growth of fractures in the fibers.

Bend tests conducted at temperatures higher than \(^\circ\text{C}\) result in visible plastic deformation of the fibers. This occurs because of a dislocation (pyramidal plane) slip that is activated at temperatures exceeding \(^\circ\text{C}\).

The bend behavior is reportedly dependent on surface flaws and the presence of inclusions in the fiber. 

 

 

3.  CLADDING AND COATING OF SAPPHIRE FIBERS

Identifying suitable materials to act as coating and cladding materials for optical-grade sapphire fibers is challenging because of the number of requirements they must meet.

Ideal claddings should enhance the optical characteristics, without limiting the useful operation, of the sapphire fibers. Ideal coatings should act as a barrier between chemically harsh environments and sapphire, and the coatings should be capable of operation over at least the same temperature range as sapphire alone.

Obstacles to this goal include the need to match the 

 

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Figure 5.  One-hour bend survival strengths, fast-fracture tensile strengths, and bend strengths versus temperature.

coefficient of thermal expansion of the materials and sapphire, the need for a good bond between the fiber and the material, the (so far) insurmountable problem of growing a core&#;clad sapphire fiber in one process, and the inability to identify materials suitable for a wide range of applications.

The benefits of cladding and coating sapphire optical fibers were addressed in Section 2. 

Claddings will potentially permit single-mode operation, improve the waveguiding properties of the sapphire fiber core, prevent radiation losses, and provide mechanical support to the core of the fiber.

The cladding must have a lower index of refraction than sapphire, possess a coefficient of thermal expansion compatible with that of sapphire, form a layer with low intrinsic stress, and be able to survive large fluctuations in temperature.

Because sapphire fibers cannot be clad as they are being grown, claddings must be applied after the growth of the fiber. Sapphire fibers can be clad using either ion exchange techniques or the direct deposition of materials.

Few reports exist in the literature that describe the effects dopants have on the index of refraction of sapphire fiber; it is not a method that is being actively investigated. Direct deposition is a more viable method.

The direct deposition of a cladding on the surface of sapphire fiber can be effected by metal-organic chemical vapor deposition \(\text{(MOCVD)}, physical vapor deposition, or solution-based methods.

During physical deposition methods, such as evaporation and sputtering, the fiber must be rotated to ensure a uniform coating.

Solution-based methods, such as sol-gel, frequently require an additional process to densify the porous coating. This densification can result in considerable stress between the cladding and the sapphire fiber. \(\text{MOCVD}\) is presented as a superior technique because the deposited layer is typically dense, homogenous, and pure.

A wide variety of materials may be deposited using this method, and the composition and thickness of the deposited layer may be controlled to within tight tolerances. Suggested cladding materials to investigate for cladding deposition include \(\text{SiO}_\text{x}\text{N}_\text{y}\), \(\text{Mg}_\text{x}\text{SiO}_\text{y}\), and \(\text{Ti}_\text{x}\text{Si}_\text{y}\text{O}\), whose \(\text{(x/y})\) ratio may be varied to give a range of refractive indices.

In general, adequate cladding materials that are both transparent and capable of surviving temperatures higher than \(^\circ\text{C}\) are considered so rare as to not exist. There has been some success with polycrystalline alumina \(\text{(Al}_2\text{O}_3)\), but these coatings are considered prohibitively expensive.

Coatings isolate the fiber from environmental contaminants and protect the surface from abrasion. This protection will enable wider use of sapphire fiber by extending the useful lifetime of the fiber and increasing the number of potential operating environments.

There is no one candidate material that can serve as a sapphire fiber coating in any environment; the coating must be chosen, in part, according to the application. It is also desirable that elements do not interdiffuse between the coating and the fiber materials.

Commonly considered materials include polycrystalline alumina \(\text{(Al}_2\text{O}_3)\), metal niobium, silicon carbide \(\text{(SiC)}\), and zirconia \(\text{(ZrO}_2)\).

Coating techniques include chemical coating, vapor deposition, plasma spraying, and electrochemical plating.

These proposed sapphire fiber coatings have different strengths and weaknesses. Alumina and sapphire share the same chemical composition, and the only stable phase of alumina above \(^\circ\text{C}\) is \(a\)-\(\text{Al}_2\text{O}_3\); single crystal \(a\)-\(\text{Al}_2\text{O}_3\) is sapphire. \(a\)-Alumina has a coefficient of thermal expansion \(\text{(CTE)}\) of \(8.1\times10^{-6}/^\circ\text{C}\) at \(^\circ\text{C}\), and a melting temperature of \(^\circ\text{C}\).

Other properties of polycrystalline alumina, such as the tensile strength and hardness, are dependent on the grain size, porosity, and purity of the alumina. Alumina possesses high chemical stability, but in a sodium atmosphere above \(^\circ\text{C}\), sodium diffusion will occur. This leads directly to chemical reactions and microcracking.

Silicon carbide has a \(\text{CTE}\) of \(5.8\times10^{-6}/^{\circ}\text{C}\) at \(^\circ\text{C}\) and a melting temperature of \(^\circ\text{C}\). It is strong, dense, and thermally shock resistant.

While it readily oxidizes to form an outer layer of silica \(\text{(SiO}_2)\), this layer will protect the material from further oxidation. Silicon carbide is used to best effect in a reducing atmosphere.

Zirconia is a less attractive option, because pure, or unstabilized, zirconia undergoes a change in structure, from monoclinic to tetragonal, at \(950^\circ\text{C}\).

This results in approximately a 9% increase in volume. Sapphire fibers coated with unstabilized zirconia experience large strains when the zirconia changes structure leading to coating microcracks.

It is recommended that zirconia be stabilized with yttria \(\text{(Y}_2\text{O}_3)\) to force a stable cubic structure.

The monoclinic form of zirconia has a \(\text{CTE}\) of \(6.5\times 10^{-6}/^circ\text{C}\) at \(^\circ\text{C}\) and the stabilized cubic form \(10.5\times10^{-6}/\circ\text{C}\) at \(^\circ\text{C}\).

When zirconia is stabilized, the strain is reduced, but residual strain results from the difference in \(\text{CTEs}\) between the sapphire and stabilized zirconia; this will likely result in strength degradation. Zirconia will react with hydrochloric and nitric acids, but it is stable in oxidizing and moderately reducing atmospheres.

At \(^\circ\text{C}\), zirconia will react with refractory carbides and stabilized zirconia will begin to vaporize.

Niobium has a \(\text{CTE}\) of \(7.1\times10^{-6}/^{\circ}\text{C}\), which makes it an attractive option for a sapphire coating. It resists reactions with sodium, mercury, and other substances at elevated temperatures, but a high sodium vapor atmosphere at temperatures exceeding \(^{\circ}\text{C}\) results in reactions between the niobium and alumina.

A research group at Drexel University has clad sapphire fiber with alumina and overcoated the clad fiber with silicon carbonate. The fibers are intended for use as sensors embedded in ceramic matrix composites. The authors of the paper use a novel chemical deposition technique to ensure good adhesion between the sapphire and the polycrystalline alumina.

The technique involves combining alumina particles and a polymerizable monomer carrier. The monomer is allowed to polymerize on the sapphire surface, the binder is removed, and the layer is sintered. This process is repeated several times to achieve a cladding layer 20-\(\mu m\) thick.

A 0.25-\(\mu m\) layer of silicon carbide is applied, via plasma enhanced CVD, as an outer coating to protect the embedded fiber from the surrounding environment. After thermal cycling tests, performed at 900 and \(^\circ\text{C}\), evidence of microcracks, debonding, or damage to the fiber was observed.

 

 

4. APPLICATIONS OF SAPPHIRE FIBERS

 

Optical Fiber Sensors

Silica-based optical fiber sensors are generally restricted for use in environments below \(700^\circ\text{C}\), because the integrity of the glass fiber is adversely affected at higher temperatures.

Specifically, at \(^\circ\text{C}\) the migration of the dopants from the fiber core becomes significant.

In addition, at temperatures exceeding \(900^\circ\text{C}\), the combination of strain and elevated temperatures will also induce creep and plastic deformation in the silica optical fiber.

Most silica-based sensor systems are specified for use at less than \(300^\circ\text{C}\), which is intended to provide an adequate margin of safety given the upper operating temperature limit of available fiber coatings.

For instance, acrylate coatings are rated for \(85^\circ\text{C}\) and degrade at temperatures higher than \(150^\circ\text{C}\), while polyimide coatings deteriorate between 400 and \(500^\circ\text{C}\).

Some metal coatings are capable of surviving temperatures at which the silica softens.

In general, there is interest in the development of sensors that can operate in high temperature and chemically harsh environments for which most conventional sensors are not suited.

Such high-temperature sensors would find use in the control of high-temperature combustion and industrial processes and in the development of advanced high-temperature materials. Platinum gauges are used to perform some measurements, but their applicability is limited.

Optical sensors made of sapphire are able to withstand environments in which few other sensors can function. Sapphire-based sensors used for the spectroscopic analysis of liquids and gases have enjoyed the greatest commercial success.

Researchers have also constructed interferometric and polarimetric sensors from sapphire fiber, although these have been less of a research and development focus.

 

High-Temperature Sensors

The first proposed design for a sapphire-based temperature sensor relies on the principles of blackbody radiation, and it is the basis of both commercial products and the majority of next-generation sapphire-based temperature sensors under development.

In the first sapphire-based sensor design, developed by R. R. Dils and shown in Fig. 6, a blackbody cavity is sputtered onto the end of a sapphire rod 0.25&#;1.25 mm in diameter and 0.05&#;0.30 m in length.

The nonmetalized end of the sapphire fiber is butt-coupled to a standard glass optical fiber, and the output of the glass fiber is collected by a detector. The radiance emitted by the blackbody cavity is used to determine the temperature of the environment of the sensor; as the temperature of the environment increases, the spectrum emitted by the blackbody predictably shifts to shorter wavelengths according to the Planck radiation law.

Although the sapphire rod employed in this sensor is flame-polished to smooth the surface, most of the scattering losses arise from the remaining surface imperfections. At temperatures exceeding \(^\circ\text{C}\), scattering, absorption, and reemission at internal defects and surface imperfections also occur. Dils observes that because these sources of loss will likely

 

 

Figure 6.  Schematic of a blackbody cavity high-temperature sensor.

 

challenge the accuracy of the sensor, only high-quality crystals should be used. Commercial temperature sensors based on this design claim operating temperatures ranging between 300 and \(^\circ\text{C}\), an accuracy of 0.10% at \(^\circ\text{C}\). 

This type of sensor has been used to monitor the temperature of internal combustion engines, aircraft turbines, and high-velocity combustion flows. While platinum and iridium are commonly sputtered on the end of the sapphire fiber sensor to create a blackbody, these films can deteriorate at temperatures exceeding \(^\circ\text{C}\).

Success in doping the sapphire fiber end with \(\text{Cr}_2\text{O}_3\), as an alternative to coating the end of the fiber in a comparatively fragile metallic film, has been reported.

The low-temperature measurement range of this sensor is limited to more than \(300^\circ\text{C}\) because the low-absorption window of sapphire occurs between 0.25 and 6 \(\mu m\). Use of a hollow sapphire tube, which has a low-absorption window extending from 9.6 to 17.2 \(\mu m\), has been shown to enable temperature measurements between 45 and \(900^\circ\text{C}\).

Other adaptations of the sensor design aim to improve the accuracy of the measurement; system errors coupled with changing transmission and emission losses can result in inaccurate temperature measurements. One solution is to base the temperature measurement on the ratio of the optical powers detected in two wavelength bands.

 

Spectroscopy and Chemical Sensing 

Another popular use of sapphire fiber is in fiber optic attenuated total reflectance \(\text{(FO/ATR)}\) spectroscopy. The analysis of the transmitted \(\text{IR}\) spectra of a substance can be used to determine its chemical makeup. When a material absorbs infrared radiation too strongly to enable this measurement, \(\text{FO/ATR}\) can be an alternative. 

In \(\text{FO/ATR}\), as illustrated in Fig. 7, a section of unclad and uncoated fiber is submerged inside a sample of the substance under test. For total internal reflection to confine the source light to the fiber, the fiber must be of a higher index then the sample.

When this is the case, an evanescent field extends into the sample and is partially absorbed by it. Spectral analysis of the light exiting the sensor determines the chemical composition of the sample.

Several types of optical fibers have been employed in \(\text{FO/ATR}\) spectroscopy, including chalcogenide, silver halide, and heavy metal glass fibers, but only

 

 

Figure 7.  Illustration of the principle of a \(\text{FO/ATR}\) spectroscopic sensor.

 

sapphire fiber possesses the combination of mechanical strength, chemical resistance, and high-temperature survivability required for a number of applications.

Sapphire-based \(\text{FO/ATR}\) spectroscopy systems have been used to determine the \(C_2\) content of ethylene/propylene co-polymers; to monitor the thermal stability of jet fuel; to monitor coal liquefaction processes; for the on-line analysis of chlorinated hydrocarbons; to measure gaseous hydrocarbons at elevated temperatures; and as a cure-state monitoring device. 

 

Physical Parameter Sensors

Work towards developing sapphire-based strain, displacement, acoustic wave detection, and other physical measurand sensors has also been performed. The first such reported design describes an extrinsic intensity-based strain sensor, depicted in Fig. 8.

The intensity of light captured and guided by the second fiber decreases with the length of the gap; the gap length can be determined by the optical power incident on the detector. The resulting curve of detected power versus gap length is highly nonlinear and is, thus, not easily interpreted. 

Although intensity sensors are simple to implement and use, the use of more complex interferometers permits greater measurement accuracy. The report of the first sapphire-based extrinsic interferometric sensor was made by Murphy et al. The sensor, illustrated in Fig. 9, consists of two sapphire fiber rods inserted end-to-end into a sapphire tube.

The second rod has a reflective coating (metallic or dielectric). The tube protects the air gap and end-faces from undesired particulate accumulation. This sensing head configuration forms a low-finesse Fabry&#;Perot cavity, where two light beams produce optical interference. One beam is reflected back from the mirror-coated end-face of one of the rods, while the second beam is the back-reflected light arising from the Fresnel reflection from the uncoated rod&#;s end-face.

The interference of these two beams at the detector causes a sinusoidal variation of the detected intensity that is a function of the gap length. Measurements of temperature, pressure, strain, elongation, and other physical parameters can be performed by correlating the changes in the gap separation of this Fabry&#;Perot cavity, to the changes and shift in the interference fringes. 

 

 

Figure 8.  Intensity-based sensor schematic.

 

 

Figure 9.  Sapphire extrinsic Fabry&#;Perot interferometer.

 

Medical Applications 

Sapphire fibers have proven to be effective delivery waveguides for high-energy \(\text{Er/YAG}\) and \(\text{Er/YSGG}\) lasers. One of the most popular applications has been for hard-tissue dental procedures. Measurements conducted by Nubling and Harrington established the laser damage threshold on sapphire fibers at approximately \(\;J/\text{cm}^2\).

Laboratory experiments have also proven the possibility to deliver up to 150,000 laser pulses at 275 mJ/pulse. 

A variety of commercial sapphire fiber laser delivery probes and tips (Fig. 10) have been developed that help define a specific output radiation shape for various medical applications such as in ophthalmology, dentistry, endoscopic surgery, and orthopedics.

 

 

Figure 10.  Sapphire fiber laser delivery tips (photo courtesy Photran LLC).

 

 

 

5.  APPENDIX: MATERIAL PROPERTIES OF \(\text{Al}\;_{2}\text{O}_{3}\) 

The data in Table 1. are taken from the Handbook of Optical Materials by Marvin J. Weber, except where indicated.

Aluminum Oxide, \(\text{Al}_2\text{O}_3\), is an insulator known variously as sapphire, corundum, and alumina. This uniaxial crystal belongs to the trigonal crystal system and space group \(\text{R}\)-\(\text{3c}\).

The extraordinary ray, \(\text{e}\), is polarized parallel to the c-axis, the axis of anisotropy, and the ordinary ray, \(\text{o}\), is polarized perpendicular to the \(\text{c}\)-axis.

 

Table 1. Material properties of \(\text{Al}_2\text{O}_3\)

 

\[\tag{4}n^2_0=1+\frac{1.\lambda^2}{[\lambda^2-(0.)^2]}+\frac{0.\lambda^2}{[\lambda^2-(0.)^2]}+\frac{5.\lambda^2}{[\lambda^2-(18.)^2]}.\] 

\[\tag{5}n^2_e=1+\frac{1.\lambda^2}{[\lambda^2-(0.)^2]}+\frac{0.\lambda^2}{[\lambda^2-(0.)^2]}+\frac{6.\lambda^2}{[\lambda^2-(20.)^2]}.\]

d. From reference.

e. The Vickers hardness has been evaluated to be kg/\(\text{mm}^2\) using an applied load of kg.

f. From reference.

 

 

 

 

 

 

 

Sapphire Tubes

Sapphire tubes are made of synthetic sapphire possess a unique combination of excellent optical, physical, and chemical properties.

Sapphire Is The Hardest Material Next To Diamond. It's Almost Impossible To Make A Diamond Tube Or Rod. Sapphire Is The 1st Choice Because:
-Sapphire tube is the best high-pressure vessel and transportation pipeline in many high-pressure systems. Sapphire tubes can handle extreme pressures while allowing safe observation and spectral analysis of fluids and gases.
-Sapphire tubes can be used to accommodate sensitive sensors in harsh environments to protect sensors from pressure and mechanical abuse.
Sapphire rods have advantages in rod bearing and confrontation applications involving high wear and tear while maintaining dimensional stability.

Features of Sapphire Tubes  
1. High transparent to wavelengths of light between 0.3 μm (UV) and 5 μm (IR)
2. Extremely hard and scratch resistance
3. Extremely high melting temperature (°C)
4. Excellent thermal conductivity and heat resistance 
5. Exceptional chemical stability and corrosion resistance 
6. Stepanov growth Method provides the ability to produce various shapes sapphire tubes with minimal or no grinding.
7. Ideal material for the applications of Lamps, instruments, and semiconductor processing.

Specification for Sapphire Tubes

Inner Diameter:

10mm ~ 180mm

Outer Diameter:

20mm ~ 200mm

Diameter Tolerance :

0.004mm ~ 0.06mm

Length:

10mm ~ 250mm

Length tolerance:

±0.01mm

Surface quality:

40/20

Custom shape:

open ends tube, plugged tube, bonded end caps tube, custom shape tubes  with angles, slots or holes

Application of sapphire tube
Sapphire tube is an ideal material for a wide range of industrial applications, such as semiconductor processing, testing and analysis, lamps, and lamp housings. Typical applications of testing and analysis include biological and chemical sample analysis, nuclear magnetic resonance spectroscopy, mass spectrometry, blocked tube, Thermo optical temperature measurement, and online petroleum processing analysis. Sapphire tubes can also be used as plasma containment tubes, process gas injectors, thermocouple protection, water treatment and high-intensity lamps.
Sapphire tube has high mechanical strength. The sapphire tube with thick wall can be used to accommodate the sensor or laser and protect the sensor or laser from extreme pressure. Its outer diameter and inner diameter can be polished to realize laser emission and reflected light measurement. Sapphire tubes are ideal for high-pressure systems and vacuum applications.
Sapphire tube is the preferred material for plasma protection because of their chemical inertia and corrosion resistance. Chemical resistance enables sapphire tubes to withstand extreme plasma without generating particles.
Sapphire tube maintains its high strength and shape under high temperature and high pressure. Sapphire lamp tube has high thermal stability, good thermal conductivity, and excellent transparency. It is an ideal lamp material to support ultraviolet, visible, and infrared applications in harsh environments.

WDQ OPTICS provides finely ground and polished sapphire tubes for a variety of applications. Custom shapes include angles, slots, holes, and adhesive shapes to meet customer accuracy specifications. The specification of the sapphire tube we can realize is: the inner diameter starts from 0.40mm, the wall thickness reaches 20mm, the length reaches 600mm, and the surface quality reaches 40 / 20.

Want more information on Custom Sapphire Tubes? Feel free to contact us.