Optical fibers have been used extensively in biomedical imaging for both illumination and imaging, mainly because they are thin and flexible. Optical fibers can deliver light from bulky light sources to remote locations, such as internal organs that are accessible noninvasively through natural openings or minimally invasively through tissues. In some imaging systems, the optical fibers also collect an optical signal from the measurement region and deliver it to the detector. Applications of optical fibers include light therapy, surgical instruments, diagnostic devices, endoscopy, optical spectroscopy, x-ray imaging, dental handheld devices, and many others.
Four different types of waveguides, namely, solid-core fibers, liquid light guides, hollow waveguides, and photonic crystal fibers, have been developed and used for biomedical applications. Solid-core fibers and liquid light guides direct light by total internal reflection (TIR), whereas the light-guiding principle for hollow waveguides involves only reflection.
A hollow waveguide consists of a hollow tube coated internally with a thin metal layer and a dielectric layer. The light is guided by the reflection at the inner surfaces. Hollow waveguides are simple and low cost; they can transmit wavelengths over a large range and can transmit high laser power without any damage to the waveguide. The disadvantages of hollow waveguides include a large attenuation and lack of flexibility. Hollow waveguides are usually used in the mid-IR region for medical applications.
Liquid light guides are semiflexible hollow tubes filled with a liquid that is transparent in the wavelength range of interest. They are ideal for visible and UV light applications. Unlike optical fiber bundles, liquid light guides do not suffer from packing-fraction losses. Their advantage over solid-core fibers is that they are easier to manufacture. Liquid light guides have been used in some medical applications as laser- and light-beam delivery conduits.