7.29 Optical Instruments
267
7.29.6 Fiber Optics
Optical light pipes include any transparent cylindrical rods used to conduct light.
Plastic and glass rods serve this purpose. Any light sent into a glass fiber at an angle
below the critical angle of the material of the rod will internally reflect and propagate
down the rod.
Optical light pipes clustered together as filaments create an optical fiber bundle.
Glass filaments engineered to have low loss of transmitted light are widely used
conductors of information over multiple kilometer distances. Such glass conduits of
light also have a variety of applications for medical remote viewing.
The intensity of light in one fiber is most often given in terms of the dB level,
defined analogously to that for sound, by β = 10 log 10 (I /I 0 ), where I 0 is a base
intensity. Typically, these intensities are in the micro- to milliwatts over an effective
area (such as 40 (μm) 2 ) of the glass core of an individual fiber, with light generated
and detected by semiconductors made from silicon and germanium. When I 0 is
taken as 1 mW for a given area fiber, then the decibel level β is appended with the
name ‘dBm’.
Information can be coded digitally on the light beam by pulsing the light on and
off. The light wave is then called the ‘carrier’ wave. Digital encoding has a distinct
advantage over analog encoding (which historically used amplitude modulation or
frequency modulation of the carrier) in that low-level noise and other unwanted
variations of amplitude do not spoil the decoding of digital signals. The frequencies
of light used in fiber optic systems range from ultraviolet to the near infrared, 400 nm
to 1600 nm. Wavelengths of 850 nm, 1300 nm, and 1550 nm are often used in fiber
optical systems. Losses along glass fibers can be as low as 0.2 dB per kilometer,
making it possible to lay hundreds of kilometers of fiber without repeaters.
The wavelengths used correspond to frequencies in the 10 14 –10 15 Hz range. In
principal, encoded pulses as rapid as 10 15 Hz (i.e. a petaHz) can be used, but loss
in the glass and the use of electronic digital circuitry makes upper practical limits
in the multiple terahertz range. This also gives an upper limit on the digital bit rate.
(Divide by (8 plus 2 redundancy check bits) to get an estimate of the bytes per
second rate.)
There are important uses of fiber optics in medicine. Here are a few:
• Transporting an image through a small-diameter conduit, from one local location,
such as inside a patient, to another local location, such as a viewing screen for
a physician. At one end, light is focused by a small lens onto the face of one
end of a collection of fibers, which we will call the ‘business’ end. If the fiber
is to make images in an initially dark region (e.g. inside a body cavity), then
illumination comes by sending light from the viewing end to the business end.
The fibers can be held fixed in position by epoxy and then cut flat and polished.
The image is divided into a pixelated set according to the number of individual
fibers, with a density sufficient to discern structures in the image. (Typically,
about 50,000 individual fibers are used to produce an image with 50,000 pixels,
267
7.29.6 Fiber Optics
Optical light pipes include any transparent cylindrical rods used to conduct light.
Plastic and glass rods serve this purpose. Any light sent into a glass fiber at an angle
below the critical angle of the material of the rod will internally reflect and propagate
down the rod.
Optical light pipes clustered together as filaments create an optical fiber bundle.
Glass filaments engineered to have low loss of transmitted light are widely used
conductors of information over multiple kilometer distances. Such glass conduits of
light also have a variety of applications for medical remote viewing.
The intensity of light in one fiber is most often given in terms of the dB level,
defined analogously to that for sound, by β = 10 log 10 (I /I 0 ), where I 0 is a base
intensity. Typically, these intensities are in the micro- to milliwatts over an effective
area (such as 40 (μm) 2 ) of the glass core of an individual fiber, with light generated
and detected by semiconductors made from silicon and germanium. When I 0 is
taken as 1 mW for a given area fiber, then the decibel level β is appended with the
name ‘dBm’.
Information can be coded digitally on the light beam by pulsing the light on and
off. The light wave is then called the ‘carrier’ wave. Digital encoding has a distinct
advantage over analog encoding (which historically used amplitude modulation or
frequency modulation of the carrier) in that low-level noise and other unwanted
variations of amplitude do not spoil the decoding of digital signals. The frequencies
of light used in fiber optic systems range from ultraviolet to the near infrared, 400 nm
to 1600 nm. Wavelengths of 850 nm, 1300 nm, and 1550 nm are often used in fiber
optical systems. Losses along glass fibers can be as low as 0.2 dB per kilometer,
making it possible to lay hundreds of kilometers of fiber without repeaters.
The wavelengths used correspond to frequencies in the 10 14 –10 15 Hz range. In
principal, encoded pulses as rapid as 10 15 Hz (i.e. a petaHz) can be used, but loss
in the glass and the use of electronic digital circuitry makes upper practical limits
in the multiple terahertz range. This also gives an upper limit on the digital bit rate.
(Divide by (8 plus 2 redundancy check bits) to get an estimate of the bytes per
second rate.)
There are important uses of fiber optics in medicine. Here are a few:
• Transporting an image through a small-diameter conduit, from one local location,
such as inside a patient, to another local location, such as a viewing screen for
a physician. At one end, light is focused by a small lens onto the face of one
end of a collection of fibers, which we will call the ‘business’ end. If the fiber
is to make images in an initially dark region (e.g. inside a body cavity), then
illumination comes by sending light from the viewing end to the business end.
The fibers can be held fixed in position by epoxy and then cut flat and polished.
The image is divided into a pixelated set according to the number of individual
fibers, with a density sufficient to discern structures in the image. (Typically,
about 50,000 individual fibers are used to produce an image with 50,000 pixels,