Biophotonic applications of optical communication

Biophotonic applications of optical communication devices Gerd Keiser*a and Fu-Jen Kaob Dept of Electronic Engineering, National Taiwan University of Science and Technology, 43 Keelung Rd., Sec. 4, Taipei 106 Taiwan b Institute of Biophotonics, National Yang Ming University, 155 Li-Nong St., Sec. 2, Taipei 112 Taiwan

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ABSTRACT Recent advances in technology have spawned a rapidly growing use of photonic systems for life sciences related clinical and research applications. Many of these biomedical applications are using selections of passive and active optical components that were developed for optical fiber communication systems over the past two decades. This paper describes how the unique physical characteristics and light-transmission properties of various passive optical components developed for telecommunications address some of the basic challenges of photonic applications in the life sciences. Key words: biophotonics, optical components, passive devices, fiber optics

1. INTRODUCTION The extensive activities involved with developing optical fiber telecommunication systems over the past two decades has resulted in a large portfolio of passive and active optical components. The functions of these devices include combining, separating, distributing, isolating, and amplifying various optical power levels across spectral bandwidths ranging from 800 to 1600 nm. Many of these passive components can be applied in biophotonic and biomedical systems for healthcare diagnosis, therapy, and imaging, and for life sciences research. The challenges in developing biophotonic instrumentation for life sciences related clinical and research applications include collecting low light levels emitted or reflected from biological specimens, delivering a wide range of optical power levels to a localized tissue area or section during different categories of therapeutic healthcare sessions, and accessing in the least invasive manner a diagnostic or treatment area within a living being with an optical probe or a radiant energy source. The unique physical and lighttransmission properties of optical fiber-based photonic components enable the resolution of such implementation issues. Among the device applications are spectroscopic analyses of biological tissues and fluids, endoscopy, optical coherence tomography, photodynamic therapy, laser surgery, and fluorescent techniques. This paper presents a tutorial overview of the characteristics and functions of a variety of passive optical components that are widely used in telecommunication networks and illustrates how these devices can be applied in biophotonic systems. First, Sec. 2 discusses the spectral windows that are of interest to biomedical-related diagnosis, therapy, and imaging processes, and for life sciences research. Section 3 then gives the physical and operating characteristics of current and potentially applicable photonic components derived from the telecommunication field. Among the components being considered are arrayed waveguide gratings, planar and fiber Bragg gratings, thin-film filters, beam splitters, optical circulators, and optical couplers. Since many spectral regions of interest in biophotonic systems are in the UV and visible regions, whereas the operational spectral band for telecommunications is concentrated in the near infrared, some of the devices will require material or configuration modifications for efficient biophotonic uses. We will point out the advantages and limitations of current passive photonic devices when applied to different wavelength bands ranging from about 190 nm in the ultraviolet to around 10 µm in the near infrared. In addition, we will describe ongoing research for extending the spectral operating range beyond that used in telecommunications for selected photonic devices. *[email protected]; phone +886-2-2737-6409; fax +886-2-2737-6424

Biophotonics: Photonic Solutions for Better Health Care, edited by Jürgen Popp, Wolfgang Drexler, Valery V. Tuchin, Dennis L. Matthews, Proc. of SPIE Vol. 6991, 69911E, (2008) · 1605-7422/08/$18 · doi: 10.1117/12.779945 Proc. of SPIE Vol. 6991 69911E-1 2008 SPIE Digital Library -- Subscriber Archive Copy

2. BIOPHOTONIC SPECTRAL WINDOWS This section describes why specific lightwave windows are needed to carry out most therapeutic and diagnostic biophotonic processes. Having this knowledge allows the specification and selection of an optical fiber that meets the transmission criteria for carrying out a biological process in a selected optical wavelength band. The interaction of light with biological tissues and fluids is a complex process because the constituent materials are optically inhomogeneous1-3. (Note that for simplicity in the rest of this paper we will use the generic word “tissue” to refer to both biological tissues and fluids.) Since the diverse and intermingled tissue components have different indices of refraction, the effective refractive index of a tissue can be continuously varying or undergo abrupt changes at the boundaries between different material components. This spatial index variation gives rise to scattering, reflection, and refraction effects in the tissue. 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. Light absorption is another important factor in the interaction of light with tissue, since the degree of absorption determines how far light can penetrate into a biological material. Figure 1 shows the absorption coefficients for several major tissue components. These components include water (about 75 percent of the body), whole blood, melanosomes (skin pigments), the epidermis (outer layer of the skin), and blood vessels. The wavelengths of interest span the spectral range from about 190 nm in the ultraviolet (UV) to around 10 µm in the infrared (IR). Most tissues absorb light weakly in the spectral range that extends from 600 to about 1500 nm, 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, since it enables the viewing or treating of tissue regions within a living body by optical means. Light absorption characteristics of tissue for the UV and IR regions outside the therapeutic window are important for implementing processes such as ablation of tissue and dental enamel, UV and IR absorption spectroscopy, tissue welding, and laser surgery.

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3. FUNCTIONS OF PASSIVE OPTICAL COMPONENTS A key feature of a high-speed optical fiber link used for telecomm applications is the ability to combine and separate a large number of independent optical signals.4 Each of these signals operates at different peak wavelength and occupies a unique narrow spectral band around that wavelength. The spectral bands are selected so that they do not interfere with neighboring bands. Figure 2 illustrates the wavelength combining process, which is known as wavelength division multiplexing (WDM). Most WDM devices operate reciprocally, that is, they can be used either to combine wavelength bands onto a fiber or to separate an aggregate of such bands into individual channels. Several types of passive optical devices are being widely used by the telecom industry to combine or separate the independent wavelength channels. The technology used to realize these devices can be adapted to create components that will combine or separate narrow lightwave spectral bands of interest to biological and healthcare research. This paper describes several such WDM devices and discusses their application to biophotonic systems.

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3.1 Thin-film filters A dielectric thin-film filter (TFF) is used as an optical bandpass filter.5 This means that it allows a particular very narrow wavelength band to pass straight through it and reflects all others. The basis of these devices is a classical Fabry-Perot filter structure, which is a dielectric cavity formed by two parallel highly reflective mirror surfaces. This structure is called a Fabry-Perot interferometer, an etalon, or a thin-film resonant cavity filter. The power transmission function of an etalon is periodic in wavelength. The periodic spacing is called the passband. In order for a single wavelength to be selected by the filter from a particular spectral range, all the wavelengths must lie between two successive passbands of the filter transfer function. If some wavelengths lie outside this range, then the filter would transmit several wavelengths. The distance between adjacent peaks is called the free spectral range or FSR. Thin-film filters are available in a wide range of passbands varying from 50 to 800 GHz, or equivalently, from 0.4 to 6.4 nm in the 1550-nm wavelength region. One proposed application is to use an array of sixteen optical thin-film filters to measure the concentrations of the constituent substances in various types of biological fluids.6 A popular TFF device is a dichroic mirror which is used in two-wavelength systems to pass one wavelength and reflect the other. These components are used in applications such as optical coherence tomography, confocal microscopes, laser beam delivery mechanisms, flow cytometry, and laser traps. 3.2 Arrayed waveguide gratings An arrayed waveguide grating (AWG) is a popular compact WDM device used for combining and separating closely spaced wavelength information channels.7,8 Traditionally they have been applied in the 1300 to 1600-nm spectral band, but recent investigations have been done to extend operation to the visible spectral region. Figure 3 shows the


operational concept of an AWG. Starting from the left, the N input slab waveguides in region 1 are connected to planar star coupler (region 2) which acts as a lens. If a wide spectral band of multi-wavelength light enters one of the input waveguides, this lens distributes the entering optical power among the waveguides in the grating array in region 3. Since adjacent waveguides of the grating array in region 3 differ in path length by a precise length ∆L, all input wavelengths emerge at point 4 with different relative phase delays given by ∆L/λ. The second lens in region 5 refocuses the light onto the output slab waveguide array in region 6, so that each wavelength emerges from a different output waveguide in region 6.

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These operational characteristics enable an AWG to be used as a spectrometer or optical spectrum analyzer. A straightforward application is in the near-infrared region using a standard AWG that operates with a central wavelength in the C-band (1530 to 1565 nm). Eighty spectral channels with a spacing of 0.4 nm are achievable with such a device. However, the design of an AWG for operation in the visible region becomes more complex and difficult, since the core width and thickness of the individual waveguides becomes smaller with decreasing wavelength. To date, an eightchannel AWG has been realized for operation in the visible region. Further investigations related to material and waveguide layout issues are being pursued to extend the capabilities of such devices for spectroscopic applications in the biomedical field.9 3.3 Fiber Bragg gratings and planar gratings A grating is an important element in telecom systems for combining and separating individual wavelengths. Basically a grating is a periodic structure or perturbation in a material.10-12 This variation in the material has the property of reflecting or transmitting light in a certain direction depending on the wavelength. Through a variety of photo-imprinting processes, a permanent periodic variation in the refractive index can be produced inside the core of an optical fiber. When a multi-wavelength signal encounters the grating, those wavelengths that are phase-matched to the Bragg reflection condition of the grating are reflected. All other wavelengths will pass through the device. Such a device is called a fiber Bragg grating or FBG. Figure 4 shows an example of the optical bandpass characteristic of a 25-GHz FBG that has a steep spectral profile.


Applications of fiber Bragg gratings to biophotonics include sensors for studying the setting expansion of dental materials, in vivo temperature profiling in the human body, and as medical sensors to check for the presence of antibodies in blood or other biological samples.13,14

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Fig. 4. Example of the reflection band and spectral profile of a 25-GHz FBG. Planar reflection gratings are fine ruled or etched parallel lines on some type of reflective surface. With these gratings, light will bounce off the grating at an angle. The angle at which the light leaves the grating depends on its wavelength, so the reflected light fans out in a spectrum. If the lines are spaced equally, each individual wavelength will be reflected at a slightly different angle, as shown in Fig. 5. There can be a reception fiber at each of the positions where the reflected light gets focused. Thus, individual wavelengths will be directed to separate fibers. The reflective diffraction grating works reciprocally, that is, if different wavelengths come into the device on the individual input fibers, all of the wavelengths will be focused back into one fiber after traveling through the device. One also could have a photodiode array in place of the receiving fibers for functions such power-per-wavelength monitoring. A widely used application of reflection gratings is in optical spectrum analyzers. In this case, instead of using a receiving fiber there is a fixed location for a receiving photodetector. By rotating the grating, the intensity distributions of different wavelengths can be measured with the detector.

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3.4 Optical couplers The concept of an optical coupler encompasses a variety of functions, including splitting a light signal into two or more streams, combining two or more light streams, tapping off a small portion of optical power for monitoring purposes, or transferring a selective range of optical power from one fiber to another. When discussing couplers it is customary to designate couplers in terms of the number of input ports and output ports on the device. For example, a coupler with two inputs and two outputs would be called a 2 × 2 coupler. In general, an N × M coupler has N ≥ 2 input ports and M ≥ 2 output ports. The coupling devices can be fabricated either from optical fibers or by means of planar optical waveguides using material such as lithium niobate (LiNbO3) or InP. A commonly available 2 × 2 coupler is the fused-fiber coupler illustrated in Fig. 6. This is fabricated by twisting together, melting, and pulling two single-mode fibers so they get fused together over a uniform section of length W. Each input and output fiber has a long tapered section of length L, since the transverse dimensions are reduced gradually down to that of the coupling region when the fibers are pulled during the fusion process. Here P0 is the device input power on the top fiber, P1 is the throughput power to the first output fiber, and P2 is the power coupled into the second output fiber. The parameters P3 and P4 are extremely low optical signal levels that result from backward reflections and scattering due to packaging effects and bending in the device.

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