3.2.1 Laser power delivery

High-power CO and CO₂ lasers operating at 5.4 μm and 10.6 μm, respectively, are readily available and can be used for industrial welding and cutting. Transmitting the laser power through fibers enables remote operation. Small core diameter (< 200 μm) fibers have demonstrated tolerance to power densities of ∼125 kW cm^–2 at 5.4 μm and ∼54 kW cm^–2 at 10.6 μm without damage 〚18〛 (Fig. 5a and b). Telluride glass fiber losses at 10.6 μm lie in the range of 1.5 dB m^–1 to 3 dB m^–1, depending upon composition and purity 〚3, 19〛.

The arsenic sulfide fibers transmit in the atmospheric 2–5 μm region and can be used for transmission of laser power in this region 〚20〛. Pulsed laser power delivery has been demonstrated (Fig. 5c). The average power is about 2.69 W but the peak power is 26.9 kW, which corresponds to a peak power density of 1.07 GW cm^–2 without fiber damage for up to 1.5 × 10⁷ pulses 〚21〛. This remarkable threshold to damage is close to the predicted value of about 3.0 GW cm^–2 due to dielectric breakdown at the surface. This threshold to damage is obtained by control of fiber polishing, otherwise the fiber end face undergoes damage at relatively lower powers.

Recent efforts have considered delivery of energy from a medical free electron laser (MFEL) operating between 2 and 10 μm through chalcogenide fiber 〚22, 23〛. The MFEL can emit more than 10 MW of power in a femtosecond pulse that relates to an average power of greater than 10 W. While not all this power is needed, it has been shown 〚24〛 that in certain cases, surgery at 6.1 and 6.45 μm based on cleaving of protein bonds is more efficient and leads to less denatured tissue and scarring than with conventional Er:YAG lasers at 2.94 μm based on OH absorption 〚24〛. We have successfully demonstrated (Fig. 5d) almost 18-mJ output at 6.1 μm (43 mJ launched) and 12-mJ output at 6.45 μm (30 mJ launched) using large core diameter (800 μm) sulfide and selenide fibers, respectively 〚23〛. The peak power densities were 1.6 and 1.2 GW cm^–2, respectively. While these fibers were not AR coated, it is expected that future AR coated fibers will enable higher output energies. Ophthalmic surgery requires operation at 2.94 μm, with an energy requirement of 1–2 mJ. This has been demonstrated using chalcogenide fibers. Output energy from an Er:YAG laser operating at 2.94 μm has been transmitted through sulfide fiber. Up to 271 kW cm^–2 power density has been transmitted without damage to the fiber. It is conceivable that laser power can be used for machining numerous materials and biological samples by tuning the wavelength and transmitting the laser power through the chalcogenide fibers to remote areas.

The end face reflection losses decrease the overall throughput of laser power. For example, As₄₀S₆₀ and Ge₃₀As₁₀Se₃₀Te₃₀ glasses have refractive indices of about 2.4 and 2.8, for which reflection losses are about 17% and 22% per face, respectively. Consequently, AR coatings are being developed for both discrete wavelengths as well as broadband operation 〚19, 20〛. Reflection losses have been reduced to about 1% per face for sulfide fibers at some IR wavelengths 〚20〛.

3.2.2 Chemical sensing

Chalcogenide fibers are well suited for chemical sensing applications since most molecular species vibrate in the infrared region. The chalcogenide fibers can be used in fiber optic chemical sensor systems for quantitative remote detection and identification. Examples of different sensing techniques include evanescent/ATR (attenuated total reflectance) 〚25–27〛, diffuse reflectance and absorption spectroscopy 〚28–30〛. The diffuse reflectance and evanescent/ATR techniques are useful for samples that scatter or are opaque at the IR wavelengths. Numerous systems have been studied and many species have been detected including aqueous, non-aqueous and toxic liquids as well as solids 〚25–30〛. Examples include oil, freon, soap, paints, polymer curing reactions, glucose/water, benzene and derivatives, chlorinated hydrocarbons, alcohols, carboxylic acids, aqueous acids, perfumes and pharmaceutical products. Fig. 6a shows some representative spectra.

Condition based maintenance (CBM) is becoming increasingly important and uses the approach, “if it isn’t broken, don’t fix it”. For example, changes of oil in motor vehicles are routinely performed every 3000 miles or so, but are not necessarily needed and therefore the incurred costs can be quite significant. A fiber optic dipstick probe could potentially monitor the quality of the oil and consequently save large amounts of money in preventing unnecessary oil changes in the military and civil sector. For example, there is an additive package used to inhibit viscosity breakdown of engine oil. Unfortunately, environmental breakdown of the additive package occurs and leads to viscosity breakdown of the oil. A fiber optic dipstick can be used to monitor the by-products of degradation of the additive package (Fig. 6b). The alarm bells sound when these reach a pre-determined threshold value, signaling an oil change.

A fiber optic based reflectance probe has been used to detect contaminants in soil (Fig. 7) 〚31〛. The detection was accomplished with the probe deployed in a cone penetrometer and tested in the field. Detection limits of 130 ppm of marine diesel fuel in sea sand have been demonstrated using a 20-m length of cable 〚31〛.

Chalcogenide fibers with a glass cladding have been used for evanescent sensing of a few tens of ppm of benzene (and derivatives) and chlorinated hydrocarbons. The low detection limits were achieved by etching the cladding glass and re-covering with specific polymer coatings that preferentially enrich one phase in them near the core surface 〚32〛. Polydimethylsiloxane (PDMS) preferentially enriched benzene (and derivatives) and low-density polyethylene enriched the chlorinated hydrocarbons on the core surface while preventing water from penetrating the coating.

Recently, a chalcogenide fiber ATR probe has been used to show the spectral differences between various tissues and organs in bio-medical samples. Fig. 8a shows the IR spectra for various organs/tissues from a dead chicken, while Fig. 8b shows the IR spectrum recorded from the liver of an anaesthetized sheep 〚22〛. Similar IR spectra have been recorded from an anaesthetized mouse with a malignant (cancer) human breast tumor grown near the surface. While these data were recorded using chalcogenide glass in contact with tissue, alternative IR transmitting and biocompatible probes will most likely be needed for human testing. However, the chalcogenide fibers can be utilized to generate a bio-medical database for medical diagnostics such as tissue evaluation and early detection of cancer 〚33〛. Currently, large portions are cut out of the body, prepared into thin sections and desiccated before FTIR analysis is performed. Obviously, the flexible fiber optic approach is minimally invasive. While silica fibers are available they are inappropriate for infrared spectroscopy in the 2–10-μm region.

3.2.3 Temperature monitoring, thermal imaging and hyperspectral imaging

Ueda et al. 〚34〛 have used As–S fibers with a Teflon cladding to measure temperature increases of up to 200 °C on the surface layer of ceramic plates during grinding.

Kapany and Simms first suggested the use of chalcogenide fibers for thermal imaging 〚35〛. Saito et al. 〚36〛 recorded the image of an electric iron at 773 K, with some degree of coherency through a 1000-fiber bundle. Nishii et al. 〚37〛 fabricated a flexible fiber bundle containing 8400 Teflon coated fibers and recorded the thermal image of an operating integrated circuit in the 3–5.4 μm region using an InSb detector. Techniques are needed for the fabrication of coherent and registered fiber bundles containing small diameter fiber but with tight tolerances and high precision. Fig. 9 shows the cross-section of an imaging bundle fabricated at NRL using glass clad sulfide fiber that transmits to beyond 5 μm. The bundle diameter was 165 μm and the fiber core diameters were only 6 μm.

The area of hyperspectral imaging can be exploited by coupling coherent fiber bundles to imaging spectrometers with focal plane array (FPA) detectors based on InSb (2–5.4 μm) or MCT (3–11 μm). The focal plane array detectors are extremely sensitive and can be used for performing both spatial and spectral analysis in the infrared 〚38〛. In other words, one can derive an IR spectrum from each pixel thereby performing chemical spectroscopy at every pixel. FPA detectors have been used for obtaining spectral and spatial information about the environment for a number of years, but without fibers. The only report in the literature pertaining to IR fibers is a 10 × 10 fiber bundle of As₂S₃ fiber with a Teflon cladding that was reformatted to a 1 × 100 array and the output analyzed using a grating spectrometer 〚39〛. The intensity contour of a Xenon lamp was recorded. Coupling to FPAs requires high precision diameter and high quality fiber bundles so that each fiber in the bundle couples to only one pixel on the detector otherwise the images will be blurred. Alternatively, many small diameter fibers, as shown in Fig. 9, can be used to couple to a single pixel.

3.2.4 Near-field microscopy

Sub-diffraction limit resolution has been demonstrated using single mode silica fibers by pulling and/or etching the fiber ends from a diameter of 125 μm to below 100 nm at the fiber tip. Preliminary results were obtained using multimode chalcogenide fiber micro-tips with about 1-μm resolution 〚40〛. More recently, the authors have used high-quality single-mode and multimode chalcogenide fibers to demonstrate about 20 nm topographic resolution and about 200 nm spectral resolution for different samples such as polycrystalline diamond 〚41〛, pancreatic cells 〚42〛, biofilms and semiconductor samples. Fig. 10 shows the topographic and optical images for a polycrystalline diamond film and pancreatic cells, respectively. The dark regions highlighted in the optical images are due to absorption by specific molecular vibrations. In our opinion, this is a promising technique for imaging and performing spectroscopy of semiconductor and biological samples with sub-micron resolution.

3.2.5 Fiber multiplexing

Fused fiber couplers are important since they enable fiber multiplexing. For example, they might be used in IR-fiber optic chemical sensing systems and data transmission systems. A preliminary fused taper fiber coupler has been fabricated using a multimode arsenic sulfide fiber with a coupling ratio of 3:1 at 2.65 μm 〚43〛, but it should be possible to use single-mode fibers.

3.3.1 Rare-earth-doped fibers

Rare-earth ions possess characteristic electronic energy levels that are only slightly influenced by the host matrix due to the screening effect of the d electrons. When pumped with the appropriate energy, the electrons are excited into upper levels from which they can subsequently decay to lower levels. Certain transitions become increasingly more efficient in longer wavelength transmitting hosts such as the chalcogenide glasses due to less multiphonon quenching and IR fluorescence emissions beyond 2 μm are only seen in chalcogenide glasses and not in silica. Table 5 lists the IR emission wavelengths found in chalcogenide glasses 〚44–54〛. The only laser oscillation observed in chalcogenide fibers has been from Nd at 1.08 μm 〚55〛 while amplification has been demonstrated using Nd at 1.08 μm 〚56〛 and Pr at 1.34 μm 〚7〛. These results were obtained in sulfide glass hosts. The 1.3-μm wavelength is of interest for telecommunications 〚57〛. The gain coefficient obtained from the single-mode sulfide (Ga–Na–S) fiber at 1.34 μm was a remarkable 0.8 dB mW^–1 and the efficiency was about 30%. The fiber loss at 1.34 μm was about 1 dB m^–1 〚57〛.

Recent work has shown that Dy-doped selenide glasses are better candidates for 1.3-μm fiber amplifiers due to the lower phonon energy of the host glass, and the larger absorption and emission cross-sections for Dy 〚54〛. The quantum efficiency is expected to be about 90% and the gain coefficient approximately double the value for the best Pr-doped sulfide fiber 〚54〛. Modeling has shown that the Dy-doped selenide fiber can tolerate higher losses compared with Pr doped sulfide fibers. Therefore, device lengths will be shorter, such that a 45-cm length of doped selenide fiber with a loss of about 10 dB m^–1 can give ∼40 dB gain at 1.34 μm. Table 6 shows some of the properties. Preliminary multimode fibers have been drawn with losses of about 6 dB m^–1 at 1.3 μm and less than 3 dB m^–1 at about 6 μm 〚58〛. Unclad fibers have been fabricated with losses of about 3 dB m^–1 at 1.3 μm and less than 1 dB m^–1 at 6 μm. Preliminary single mode fibers have been drawn with minimum losses of about 3 dB m^–1 〚58〛.

Since the rare earth doped chalcogenide fibers emit in the 2–5-μm region 〚59〛, they are an attractive alternative to blackbody sources for many applications. For example, arrays of the fibers can be used for infrared scene simulation (IRSS) for characterization of focal plane array detectors (e.g., InSb) 〚60〛. Fig. 11 shows the emission spectrum of a Pr-doped selenide glass fiber, demonstrating broadband emission between about 3 and 5 μm. Prototype bundles have been fabricated and mid-IR emission measured from the pixels 〚60〛. Black body temperatures of up to 2400 K have been simulated in single pixel pumping indicating that these fibers are capable of providing bright sources in the IR.

It is possible to use these fiber sources in chemical sensor systems 〚22〛. For example, Fig. 12 shows the IR transmission spectrum of a thin film of toluene between two CaF₂ plates recorded using a Pr-doped selenide fiber (pumped at 1.064 μm) as a mid-IR source 〚61〛.

Longer wavelength emission from 7–9 μm has been demonstrated in Tb-doped GeAsGaSe and GeAsGaTe glasses. This represents the longest wavelength emission seen from any rare-earth-doped glass or crystal system 〚62〛. Fig. 13 shows the emission spectrum in the 4–5-μm and 7–9-μm regions. The measured lifetimes of the excited state were 12 and 80 μs for the selenide and telluride glasses, respectively.

The ability to write gratings in chalcogenide fibers will enable smart compact devices including lasers and dispersion compensators. Some work has been performed on writing gratings in chalcogenide glass fibers 〚63–65〛. Asobe et al. 〚63〛 have fabricated an As₂S₃-based fiber Bragg grating for 1.55-μm-wavelength operation by the transverse holographic method using a He–Ne laser. A reflectivity of > 90% was obtained with an estimated change in refractive index of about 10^–4.

3.3.2 Non-linear effects

Glasses possessing high third-order non-linearities (X³) are required for ultra-fast switching in time division multiplexing (TDM) telecommunications systems. It is well established that the values of X³ for chalcogenide glasses are about two orders of magnitude larger than silica 〚66, 67〛. To our knowledge, the number of papers reporting the use of this phenomenon in actual switching applications is not extensive. Asobe et al. 〚68〛 demonstrated efficient optical Kerr shutter (OKS) switching operation with picosecond response time using a 1-m length of elliptical core fiber and a 100-GHz signal operating at a wavelength of about 1.5 μm. More recently, glasses have been reported with non-linearities approaching 1000 times silica 〚69, 70〛. This opens up the potential for compact and integrated all optical switches.

Since glasses lack a center of inversion symmetry and thus have no second order nonlinear susceptibility (X²) they should not exhibit second harmonic generation (SHG) 〚71〛. However, undoped and Pr-doped GaLaS glasses have exhibited SHG 〚72〛 through optical pumping. This SHG may be due to crystallization or the effect of frozen-in electric fields. The latter arises from the relationship X² = E_dc X³, where E_dc is the frozen-in electric field 〚71〛. Electric poling has been successfully used to produce SHG in silica based fiber systems 〚73〛 as well as As–S glass film fabricated via evaporation 〚74〛. It is not unreasonable to expect similar results in chalcogenide fibers. Since X³ is about 2 to 3 orders of magnitude larger in chalcogenides compared with silica, we expect larger SHG efficiencies in electrically poled chalcogenide glasses. However, the question arises as to whether the electric fields can be frozen-in for chalcogenide glasses. We have observed preliminary second-harmonic generation using electrically poled arsenic sulfide glass. Fig. 14 shows the observed SHG at 780 nm when pumping a 1-mm thick arsenic sulfide glass disk at 1560 nm. The sample was electrically poled at 100 °C for 5 h under nitrogen gas atmosphere. At the present time, the magnitude appears comparable to silica glass, but the mechanism is unknown.

We have demonstrated Raman amplification in small core As–Se fiber 〚75〛. We observed over 20 dB of gain in a 1.1-m length of fiber pumped by a nanosecond pulse of ∼10.8 W peak power at 1.50 μm. The peak of the Raman gain was shifted by ∼230 cm^–1 to 1.56 μm. The Raman gain coefficient is estimated to be about 2.3 × 10^–11 m W^–1, over 300 times greater than that of silica. The large Raman gain coefficient coupled with the large IR transparency window of these fibers shows promise for development of As–Se Raman fiber lasers and amplifiers in the near, mid and long IR spectral regions.