Review Papers

Review of diverse optical fibers used in biomedical research and clinical practice

[+] Author Affiliations
Gerd Keiser

Boston University, Department of Electrical and Computer Engineering, 8 Saint Mary’s Street, Boston, Massachusetts 02215, United States

Fei Xiong

City University London, Department of Electrical and Electronic Engineering, Northampton Square, London, EC1V 0HB, United Kingdom

Ying Cui

Nanyang Technological University, Photonics Centre of Excellence, School of Electrical and Electronic Engineering, 50 Nanyang Avenue, 639798, Singapore

CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, 50 Nanyang Drive, 637553, Singapore

Perry Ping Shum

Nanyang Technological University, Photonics Centre of Excellence, School of Electrical and Electronic Engineering, 50 Nanyang Avenue, 639798, Singapore

J. Biomed. Opt. 19(8), 080902 (Aug 28, 2014). doi:10.1117/1.JBO.19.8.080902
History: Received June 16, 2014; Revised August 4, 2014; Accepted August 5, 2014
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Abstract.  Optical fiber technology has significantly bolstered the growth of photonics applications in basic life sciences research and in biomedical diagnosis, therapy, monitoring, and surgery. The unique operational characteristics of diverse fibers have been exploited to realize advanced biomedical functions in areas such as illumination, imaging, minimally invasive surgery, tissue ablation, biological sensing, and tissue diagnosis. This review paper provides the necessary background to understand how optical fibers function, to describe the various categories of available fibers, and to illustrate how specific fibers are used for selected biomedical photonics applications. Research articles and vendor data sheets were consulted to describe the operational characteristics of conventional and specialty multimode and single-mode solid-core fibers, double-clad fibers, hard-clad silica fibers, conventional hollow-core fibers, photonic crystal fibers, polymer optical fibers, side-emitting and side-firing fibers, middle-infrared fibers, and optical fiber bundles. Representative applications from the recent literature illustrate how various fibers can be utilized in a wide range of biomedical disciplines. In addition to helping researchers refine current experimental setups, the material in this review paper will help conceptualize and develop emerging optical fiber-based diagnostic and analysis tools.

In recent years, there has been an extensive and rapidly growing use of photonics technology for basic life sciences research and for biomedical diagnosis, therapy, monitoring, and surgery.13 Among the numerous diverse applications are imaging, spectroscopy, endoscopy, tissue pathology, blood flow monitoring, light therapy, biosensing, biostimulation, laser surgery, dentistry, dermatology, and health status monitoring. Major challenges in biophotonics applications to life sciences include how to collect and transmit low-power (down to the nanowatt range) emitted light to a photodetector, how to deliver a wide range of optical power levels to a tissue area or section during different categories of therapeutic healthcare sessions, and how to access a diagnostic or treatment area within a living being with an optical detection probe or a radiant energy source in the least invasive manner. Depending on the application, all three of these factors may need to be addressed at the same time.

The unique physical and light-transmission properties of optical fibers enable them to help resolve such implementation issues. Consequently, various types of optical fibers are finding widespread use in biophotonics instrumentation for life sciences related clinical and research applications. Each optical fiber structure has certain advantages and limitations for specific uses in different spectral bands. Therefore, it is essential that biophotonics researchers and implementers know which type of fiber is best suited for a certain application. This paper provides the background that is necessary to understand how optical fibers function, explains various categories of fibers, and illustrates how certain fibers are used for specific biophotonics implementations.

To understand when and where to use specific optical fiber types, some background information is presented in Sec. 2 on why various lightwave bands in the ultraviolet, visible, and infrared regions are of interest for biomedical diagnostic and therapeutic implementations. Next, Sec. 3 discusses the fundamental principles for light guiding in conventional solid-core fibers. This discussion will be used as a basis for describing light guiding in other optical fiber structures. In addition, Sec. 3 also describes the optical fiber performance characteristics needed for specific spectral bands.

With this background information, Sec. 4 then presents several categories of optical fiber structures and materials that are appropriate for use at different wavelengths. This discussion includes conventional and specialty multimode and single-mode solid-core fibers, double-clad fibers (DCFs), hard-clad silica (HCS) fibers, conventional hollow-core fibers, photonic crystal fibers (PCFs), polymer optical fibers (POFs), side-emitting and side-firing fibers, middle-infrared fibers, and optical fiber bundles. Included in this discussion are the fiber materials that are appropriate for use at different wavelengths. Finally, Sec. 5 describes some examples of optical fiber applications to various biomedical disciplines.

This section describes the fundamental background as to why specific lightwave windows are needed to carry out various therapeutic and diagnostic biomedical photonics processes. Having this knowledge allows the selection of an optical fiber whose specification could meet the transmission criteria for carrying out a specific biomedical process.

The interaction of light with biological tissues and fluids is a complex process because the constituent materials are optically inhomogeneous. Because diverse biological tissue components have different indices of refraction, the refractive index along some path through a given tissue volume can vary continuously or undergo abrupt changes at material boundaries, such as at flesh and blood vessel or bone interfaces. This spatial index variation gives rise to scattering, reflection, and refraction effects in the tissue.412 Thus, although light can penetrate several centimeters into a tissue, strong scattering of light can prevent observers from getting a clear image of tissue abnormalities beyond a few millimeters in depth.

Light absorption is another important factor in the interaction of light with tissue, because the degree of absorption determines on how far light can penetrate into a specific tissue. Figure 1 shows the absorption coefficients for several major tissue components. These components include water (75% of the body), whole blood, melanin, epidermis, and blood vessels. The wavelengths of interest span the spectral range from 190nm in the ultraviolet (UV) to 10μm in the infrared (IR).

Graphic Jump LocationF1 :

Absorption coefficients of water, hemoglobin (HbO2), melanin, and skin as a function of wavelength.

Most tissues exhibit comparatively low absorption in the spectral range that extends from 500 to 1500nm, that is, from the orange region in the visible spectrum to the near-infrared (NIR). This wavelength band is popularly known as the therapeutic window or the diagnostic window because it enables viewing or treating tissue regions within a living body by optical means. Light absorption characteristics of tissue for regions outside the therapeutic window are important for implementing functions that depend on high optical power absorption, such as drilling, cutting, bonding, and ablation of tissue. A wide variety of optical sources can be used to carry out these functions.1,1316 For example, as indicated in Fig. 1, UV light from ArF or KrF lasers emitting at wavelengths of 193 and 249 nm, respectively, is strongly absorbed in the surface of a tissue and, thus, can be used for many surgical applications. As an example, in the IR region, the 2940-nm light from an Er:YAG laser is strongly absorbed by osseous minerals, which makes optical sawing and drilling in bones and teeth possible.

This section discusses the fundamental principles for light guiding in conventional optical fibers. Here the term conventional refers to the structure of optical fibers that are widely used in telecom networks. This discussion will set a basis for describing light guiding in other optical fiber structures that are presented in Sec. 4. In addition, Sec. 3 also describes the performance characteristics needed for specific spectral regions, for example, optical signal attenuation, bending loss sensitivity, mechanical properties, and optical power-handling capabilities.1723

Light Guiding Principles in Conventional Optical Fibers

An optical fiber is a dielectric waveguide that operates at optical frequencies. This fiber waveguide is normally cylindrical in form. It confines electromagnetic energy in the form of light within its surfaces and guides the light in a direction parallel to its axis. The propagation of light along a waveguide can be described in terms of a set of guided electromagnetic waves called the modes of the waveguide. Each guided mode is a pattern of electric and magnetic field distributions that is repeated along the fiber at equal intervals. Only a certain discrete number of modes are capable of propagating along the waveguide. These modes are those electromagnetic waves that satisfy the homogeneous wave equation in the fiber and the boundary condition at the waveguide surfaces.

Figure 2 shows a schematic of a conventional optical fiber, which consists of a cylindrical silica-based glass core surrounded by a glass cladding that has a slightly different composition.1719 The core of diameter 2a has a refractive index n1 and the cladding has a slightly lower refractive index n2. Surrounding these two layers is a polymer buffer coating that protects the fiber from mechanical and environmental effects. The refractive index of pure silica varies with wavelengths ranging from 1.453 at 850 nm to 1.445 at 1550 nm. By adding certain impurities, such as germanium dioxide (GeO2), to the silica during the fiber manufacturing process, the index can be slightly changed. This is done so that the refractive index n2 of the cladding is slightly smaller than the index of the core (i.e., n2<n1). This condition is required so that light traveling in the core is totally internally reflected at the boundary with the cladding, which is the physical mechanism that guides light signals along a fiber.

Graphic Jump LocationF2 :

Schematic of a conventional silica fiber structure.

The variations in material and size of the conventional solid-core fiber structure dictate how a light signal is transmitted along a fiber and also influence how the fiber performance responds to environmental perturbations, such as stress, bending, and temperature variations. Variations in the material composition of the core give rise to two commonly used fiber types, as shown in Fig. 3. In the first case, the refractive index of the core is uniform throughout and undergoes an abrupt change (or step) at the cladding boundary. This is called a step-index fiber. In the second case, the core refractive index varies as a function of the radial distance from the center of the fiber. This type is a graded-index fiber.

Graphic Jump LocationF3 :

Comparison of conventional single-mode and multimode step-index and graded-index optical fibers.

Both the step- and the graded-index fibers can be further divided into single-mode and multimode classes. As the name implies, a single-mode fiber (SMF) sustains only one mode of propagation, whereas a multimode fiber (MMF) contains many hundreds of modes. A few typical sizes of SMF and MMF are given in Fig. 3 to provide an idea of the dimensional scale. MMFs offer several advantages compared with SMFs. The larger core radii of MMFs make it easier to launch optical power into the fiber and to collect light emitted or reflected from a biological sample. SMFs are more advantageous when delivering a narrow light beam to a specific tissue area and also are needed for applications that deal with coherence effects between propagating light beams.

The remainder of Sec. 3.1 describes the operational characteristics of step-index fibers, and Sec. 3.2 describes graded-index fiber structures.

Ray optics concepts

To get an understanding of how light travels along a fiber, first consider the case when the core diameter is much larger than the wavelength of the light. For such a case, a simple geometric optics approach based on the concept of light rays can be used. Figure 4 shows a light ray entering the fiber core from a medium of refractive index n, which is less than the index n1 of the core. When the ray meets the fiber end face, it is refracted into the core and propagates at an angle θ, which is smaller than the entrance angle θ0 of that ray. Inside the core, the ray strikes the core-cladding interface at an angle ϕ relative to the normal to the surface. If the light ray strikes this interface at such an angle that it is totally internally reflected, the ray becomes confined to the core region and follows a zigzag path as it travels along the fiber.

Graphic Jump LocationF4 :

Ray optics representation of the propagation mechanism in an optical fiber.

From Snell’s law, the minimum or critical angle ϕc that supports total internal reflection is given by Display Formula

sinϕc=n2n1.(1)

Rays striking the core-cladding interface at angles less than ϕc will refract out of the core and be lost in the cladding as the dashed line shows. The condition of Eq. (1) can be related to the maximum entrance angle θ0,max, which is called the acceptance angle θA, through the relationship Display Formula

nsinθ0,max=nsinθA=n1sinθc=(n12n22)1/2,(2)
where θc=π/2ϕc. Thus, those rays having entrance angles θ0 less than θA will be totally internally reflected at the core-cladding interface. Thus, θA defines an acceptance cone for an optical fiber. Rays outside of the acceptance cone, such as the ray shown by the dashed line in Fig. 4, will refract out of the core and be lost in the cladding.

The critical angle also defines a parameter called the numerical aperture (NA), which is used to describe the light acceptance or gathering capability of fibers that have a core size much larger than a wavelength.5,17,18 This parameter defines the size of the acceptance cone shown in Fig. 4. NA is a dimensionless quantity that is less than unity, with values nominally ranging from 0.14 to 0.50. NA is given by Display Formula

NA=nsinθA=n1sinθc=(n12n22)1/2n12Δ.(3)

The parameter Δ is called the core-cladding index difference or simply the index difference. It is defined through the equation n2=n1(1Δ). Typical values of Δ range from 1 to 3 percent for MMF and from 0.2 to 1.0 percent for SMF. Thus, since Δ is much less than 1, the approximation on the right-hand side of Eq. (3) is valid. Because NA is related to the maximum acceptance angle, it is commonly used to describe the light acceptance or gathering capability of an MMF and to calculate the source-to-fiber optical power coupling efficiencies. The NA value is listed on vendor data sheets for fibers.

Modal concepts

Although the ray representation gives a general picture of light propagation along a fiber, mode theory is needed for a more detailed understanding of concepts such as mode coupling, dispersion, coherence or interference phenomena, and light propagation in single-mode and few-mode fibers. Figure 5 is a longitudinal cross-sectional view of an optical fiber that shows the field patterns of some of the lower-order transverse electric. The order of a mode is equal to the number of field zeroes across the guiding core. The plots show that the electric fields of the guided modes are not completely confined to the core but extend partially into the cladding. The fields vary harmonically in the core region of refractive index n1 and decay exponentially in the cladding of refractive index n2. For low-order modes, the fields are tightly concentrated near the axis of an optical fiber with little penetration into the cladding region. Higher-order mode fields are distributed more toward the edges of the core and penetrate farther into the cladding.

Graphic Jump LocationF5 :

Electric field distributions of lower-order guided modes in an optical fiber (longitudinal cross-sectional view).

As the core radius a shown in Fig. 2 is made progressively smaller, all modes except the fundamental mode shown in Fig. 5 will start getting cut off. A fiber in which only the fundamental mode can propagate is an SMF. An important parameter related to the cutoff condition is the V number defined by Display Formula

V=2πaλ(n12n22)1/2=2πaλNA2πaλn12Δ,(4)
where the approximation on the right-hand side comes from Eq. (3). This parameter is a dimensionless number that determines how many modes a fiber can support. Except for the lowest-order fundamental mode, each mode can exist only for values of V that exceed the limiting value V=2.405 (with each mode having a different V limit). The wavelength at which all higher-order modes are cut off is called the cutoff wavelength λc. For example, if a=8.0μm and Δ=0.01, then from Eq. (4), with V=2.40, the cutoff wavelength is λc=1481nm. That is, only the fundamental mode will propagate in the fiber for wavelengths >1481nm. The fundamental mode has no cutoff and ceases to exist only when the core diameter is zero. This is the principle on which SMFs are based.

The V number can be also used to express the number of modes M in a multimode step-index fiber when V is large. For this case, an estimate of the total number of modes supported in such a fiber is Display Formula

M=12(2πaλ)2(n12n22)=V22.(5)

Because the field of a guided mode extends partly into the cladding, as shown in Fig. 5, another quantity of interest for a step-index fiber is the fractional power flow in the core and cladding for a given mode. As the V number approaches cutoff for any particular mode, more of the power of that mode is in the cladding. At the cutoff point, all the optical power of the mode resides in the cladding. For large values of V far from cutoff, the fraction of the average optical power residing in the cladding can be estimated by Display Formula

PcladP43M,(6)
where P is the total optical power in the fiber.

In an SMF, the geometric distribution of light in the propagating mode is needed when predicting the performance characteristics of these fibers, such as splice loss, bending loss, cutoff wavelength, and waveguide dispersion. Thus, a fundamental parameter of an SMF is the mode-field diameter (MFD), which can be determined from the mode-field distribution of the fundamental fiber mode.17,18,21 MFD is a function of the optical source wavelength, the core radius, and the refractive index profile of the fiber. MFD is analogous to the core diameter in an MMF, except that in an SMF, not all the light that propagates through the fiber is carried in the core.

A standard technique to find MFD is to measure the far-field intensity distribution E2(r) and then calculate the MFD using the Petermann II equation.17,18Display Formula

MFD=2w0=2[20E2(r)r3dr0E2(r)rdr]1/2,(7)
where the parameter 2w0 (with w0 being called the spot size or the mode field radius) is the full width of the far-field distribution. For calculation simplicity, the exact field distribution can be fitted to a Gaussian function. Display Formula
E(r)=E0exp(r2/w02),(8)
where r is the radius and E0 is the field at zero radius, as shown in Fig. 6. Then MFD is given by the 1/e2 width of the optical power. The Gaussian pattern given in Eq. (8) is a good approximation for values of V lying between 1.8 and 2.4, which designates the operational region of practical SMFs. An approximation of the relative spot size w0/a, which, for a step-index fiber, has an accuracy better than 1% in the range 1.2<V<2.4, is given by Display Formula
w0a=0.65+1.619V3/2+2.879V6.(9)

Graphic Jump LocationF6 :

Distribution of light in a single-mode fiber (SMF) above its cutoff wavelength. For a Gaussian distribution, the mode-field diameter is given by the 1/e2 width of the optical power.

Manufacturers typically design an SMF to have V values >2.0 to prevent high cladding losses, but <2.4 to avoid the possibility of having more than one mode in the fiber.

Graded-Index Optical Fibers
Core index structure

In the graded-index fiber design, the core refractive index continuously decreases with an increasing radial distance r from the center of the fiber but is generally constant in the cladding. The most commonly used construction for the refractive-index variation in the core is the power law relationship. Display Formula

n(r)=n1[12Δ(ra)α]1/2for0ra=n1(12Δ)1/2n1(1Δ)=n2forra.(10)

Here, r is the radial distance from the fiber axis, a is the core radius, n1 is the refractive index at the core axis, n2 is the refractive index of the cladding, and the dimensionless parameter α defines the shape of the index profile. The index difference Δ for the graded-index fiber is given by Display Formula

Δ=n12n222n12n1n2n1.(11)

The approximation on the right-hand side reduces this expression for Δ to that of the step-index fiber. Thus, the same symbol is used in both cases. For α=, inside the core, Eq. (10) reduces to the step-index profile n(r)=nl.

Graded-index numerical aperture

Determining the NA for graded-index fibers is more complex than for step-index fibers because it is a function of position across the core end face. This is in contrast to the step-index fiber, where NA is constant across the core. Geometrical optics considerations show that light incident on the fiber core at position r will propagate as a guided mode only if it is within the local numerical aperture NA(r) at that point. The local NA is defined as Display Formula

NA(r)=[n2(r)n22]1/2NA(0)1(r/a)αforra=0forr>a,(12)
where the axial NA is defined as Display Formula
NA(0)=[n2(0)n22]1/2=(n12n22)1/2n12Δ.(13)

Thus, the NA of a graded-index fiber decreases from NA(0) to zero as r moves from the fiber axis to the core-cladding boundary. The number of bound modes Mg in a graded-index fiber is Display Formula

Mg=αα+2(2πaλ)2n12Δαα+2V22.(14)

Fiber manufacturers typically choose a parabolic refractive index profile given by α=2.0. In this case, Mg=V2/4, which is half the number of modes supported by a step-index fiber (for which α=) that has the same V value.

Cutoff condition in graded-index fibers

Similar to step-index fibers, in order to eliminate intermodal dispersion, graded-index fibers can be designed as an SMF in which only the fundamental mode is allowed to propagate at the desired operational wavelength. An empirical expression of the V parameter at which the second lowest-order mode is cut off for graded-index fibers has been shown to be17,18Display Formula

V=2.4051+2α.(15)

Equation (15) shows that, in general, for a graded-index fiber, the value of V decreases as the profile parameter α increases. It also shows that the critical value of V for the cutoff condition in parabolic graded-index fibers (α=2) is a factor of 2 larger than for a similar-sized step-index fiber. Furthermore, from the definition of V given by Eq. (4), the NA of a graded-index fiber is larger than that of a step-index fiber of comparable size.

Performance Characteristics of Generic Optical Fibers

When considering the type of fiber to use in a particular biophotonics system application, some performance characteristics that need to be taken into account are optical signal attenuation as a function of wavelength, optical power-handling capability, the degree of signal loss as the fiber is bent, and mechanical properties of the optical fiber.

Attenuation versus wavelength