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Optical Fiber Techniques for Medical Applications 319

there were defects in the materials and at the interface. well only in the spectral range λ> 450 nm, and their laser
Nowadays, exceptionally pure materials are prepared by power handling is limited.
deposition in a vacuum system, and special efforts are
taken to reduce the number of defects. In the case of III. LASERS FOR FIBEROPTIC
optical communications, the fibers are coated with a plas- MEDICAL SYSTEMS
ticprimarycoatingastheyarebeingdrawn,inordertopro-
tect them from moisture or scratches, among other things.
A. Medical Lasers
Good-quality optical fibers have transmission losses on
the order of 1 dB/km. Such quality is normally not needed There are many regular light sources that are used in
for medical applications. medicine, such as incandescent lamps (e.g., halogen),
In optics, wavelength λ is normally specified in nano- high-pressure lamps (e.g., Xe), and light-emitting diodes
−9
meters (1 nm = 10 m), or in micrometers (1 µm = (i.e., LEDs ). The light emitted from these sources consists
10 −6 m). The optical spectrum is broadly divided into of many wavelengths; it is emitted from a relatively broad
three spectral regions: ultraviolet (UV) for λ< 400 nm, area, and is spread in many directions. Laser light has
visible (VIS) λ = 400–700 nm, and infrared (lR) for characteristics that make it especially useful in medicine.
λ> 700 nm. The UV region at wavelengths λ< 250 These are:
nm is sometimes called “deep UV.” The infrared spec-
trum may be further divided to near infrared (NIR) λ = Monochromatic—laser light consists of a narrow band
0.7–3.0 µm and middle infrared (MIR) λ = 3.0–30 µm. of wavelengths (i.e., “one wavelength”).
Optical fibers that are based on some mixture of SiO 2 , Collimated beam—the laser light is emitted in a
B 2 O 3 , and Na 2 O, are called silica fibers and they transmit parallel beam.
well in the UV, visible, and NIR regions of the optical High intensity—practically all the laser energy is
spectra. concentrated in a narrow pencil.
Coherent—the laser emission is “ordered” both in

space and in time.
2. Special Optical Fibers
Standard silica fibers transmit only in the visible and do The coherence helps to focus the beam to a very small
not transmit in the deep UV or in the middle infrared. Nor point (of the order of the wavelength of light). The en-
could these glass fibers transmit visible or NIR beams ergy density (energy divided by area) at the focal spot is
of high intensity. For the transmission of deep UV, one extremely high. Such energy density cannot be obtained
could use pure silica as the core material. Doped silica with standard (noncoherent) light sources.
(of lower index of refraction) serves as a cladding layer. All lasers are based on some active medium in which
These fibers (sometime called “quartz”fibers) also serve “optical amplification” occurs. This medium can be a solid
as power fibers for the transmission of laser power. For crystal (e.g., Nd:YAG or Er:YAG crystals), a liquid (e.g.,
transmission in the middle infrared, one may use nonox- dye in a solution), gas (e.g., Ar, CO 2 , or HeNe), or a semi-
ide glasses such as chalcogenides (e.g., As 2 S 3 )or fluo- conductor (e.g., GaAs). The medium has to be excited
rides (e.g., ZrF 4 –BaF 2 –LaF 3 ). Crystals of silver halides (pumped) in order to start the lasing process, and elec-
or thallium halide can be extruded through dies. The re- tric current often provides excitation. Also, in order to get
sulting fibers were found to be transparent in the middle laser emission the laser should contain two mirrors, which
infrared. An alternative is to use hollow plastic or glass reflect the light back and forth through the lasing medium.
tubes, coated on the inside with thin metallic coatings. One of these mirrors is partially transparent, and the laser
The hollow tubes are transparent in the mid IR and can beam emerges from this mirror.
deliver high laser power. These are not actually fibers, but There are many lasers that emit light in the UV, visi-
they are thin and fairly flexible and are sometimes referred ble, and IR. Some lasers emit continuously light of wave-
to as hollow fibers. length λ, and the total power emitted is measured in
Optical fibers made of plastic materials have been watts. These are called CW (continuous wave) lasers.
used in the past for the visible range. For example, poly- Other lasers emit short bursts of light, in which case the
methylmethacrylate (PMMA) could be drawn to form lasers are called pulsed lasers. Some of the lasers used in
unclad fibers. Alternatively, polystyrene may serve as a medicine emit long pulses, measured in msec (10 −3 sec),
core in these fibers, and PMMA as cladding. Such fibers others emit shorter pulses, measured in µsec (10 −6 sec)
are flexible and less expensive to fabricate, but their or nsec(10 −9 sec), and some emit very short pulses, mea-
properties are still inferior to those of glass. In particular, sured in psec (10 −12 sec) or fsec (10 −15 sec). The energy
plastic fibers are relatively thick (1 mm), they transmit emitted from these lasers can be specified by the pulse