Infra-red fibers: Properties and applications
Leah Boehm
Israel Aircraft Industries
Increasing interest in high speed data transmission, shifted the attention from metallic to
optical telecommunication devices, especially optical fibers and wave-guides (passive
and active). Now, telephone conversations, computer data and television images as well
as laser power for several medical applications, are transmitted through strands of highly
transparent, silica based glasses.
While silica based fibers exhibit excellent optical properties out to about 2.m, other
materials are required for transmission to longer wavelength in the infrared (IR) region.
These materials can be glassy, single crystalline and polycrystalline. Example include
fluoride (ZrF4 based glasses), chalcogenide glasses (e.g.: Ge-Se-Te), single crystalline
sapphire, polycrystalline silver halides (AgCIBr, KCI, CsBr) and hollow wave-guides.
The last family, posses hollow core and are based on hollow tubes with internal metallic
coating and with or without a dielectric coating. These wave-guides may be good
candidates for transmitting infrared radiation,. primary for CO2 laser power transmission.
Depending on their composition these materials can transmit beyond 20m. The
theoretical attenuation of these materials is given in Fig 1.
Consequently, optical fibers made from these materials enables numerous practical
applications in the fields of:

IR detection.

Laser power transmission

IR Spectroscopy.
In particular, IR transmitting fibers can be used in medical applications such as for laser
surgery and in industrial application such as metal cutting and machining using high
power IR laser sources (e.g. Er: YAG, CO2 lasers, Free electron lasers, OPO's).
More recently, there is an interest in using these fibers as chemical sensors systems for
environmental pollution monitoring using absorption, evanescent, or diffused reflectance
spectroscopy since all molecules posses characteristic vibration bands in the IR region.
Potential low attenuation
Fused silica
1.E+02
Loss [db /km ]
Loss (db /km )
1.E+03
1.E+02
1.E+01
1.E+00
1.E-01
1.E+00
1.E-02
1.E-04
0
0.5
1
1.5
2
2.5
3
0
W
avelength (micron)
2
4
6
b
W
avelength [micron]
Silver halide crystal
Chalcogenide glass
1.E+04
Loss [db /km ]
1.E+03
Loss [db /km ]
CO2 Laser Catheter
Fluoride glass
1.E+01
1.E-01
1.E-03
1.E+02
1.E+00
1.E-02
1.E-04
0
2
4
6
8
W
avelength[m
icron]
0
5
10
15
W
avelength[micron]
7
39
\
Fig. 1: Theoretical attenuation of IR
Fig. 2: Schematic setup of IR fiber bundle for
Materials.
endoscope use with CO2 laser.
Optical fibers and wave-guides are basically just a mean to transmit radiation from one
point to the other. The input can be any source at the optical spectrum and the target
can be any interaction subject. In the field of biomedical optics we have two main ways
of using of using this delivery device. Either deliver energy from a laser source to the
human body to cause interactions (tissue removal, heating, cutting, etc.) or it can deliver
signals from the human tissue (heat, fluorescence) and the target will be the appropriate
detector.
Laser delivery via fibers enables easy manipulation of the beam to the operating table. It
becomes crucial when minimal invasive surgery is carried out. (Fig. 2)
In this case fiber is inserted through the working channel of an endoscope and an
operation can take place within body cavities, through existing openings of the body or
through minor cuts made through the skin.
The opposite way is receiving signals from the body to a detector. This enables the use
of fiber as a vehicle to transmit signals for diagnostic purposes. These signals enable us
to study tissue structure, tumor detection or tissue temperature. Such a system can be
seen in Fig. 3, which shows possibility of sensing using a IR fiber bundle in human body.
Things could have been very simple if one kind of material or fiber could serve all
purposes but this is not the case. The optical spectrum is very wide. Useful wavelengths
can vary from very short at the X-ray side, to the mid and far infrared on the other side.
Signals can very from nano-jouls to joules and not every material can handle it.
Remote IR Fiberoptic System
Tunable
IR
Source
core/clad IR
fiber
Detector
IR cable
Thick
plastic
jacket
Lens
Sample
EM
field
Thin
protective
coating
unclad
IR fiber
50
Fig. 3: Schematic setup of remote sensing in the IR using silver halides fibers.
Fibers need to be bent to very small diameters (as is the case in endoscope) and each
material is brittle to some extend. Pulses might be broaden while transmitted and this
should be taken into consideration for time of flight measurements. Short pulses can
have very high peak power. Materials do have damage thresholds. Transmission is not
linear and can change with wavelength. Transmission can also change with bending.
Input beam shape will not necessarily be kept the same after being transmitted in bent
trajectories.
Biocompatibility is also an important issue. For sure no hazardous material can be
inserted into the human body no matter how efficient they are in delivering laser
radiation.
With all the above in mind, the fibers and wave-guides issue became a very big field of
research. There is more than one solution and each solution is partially covering the
requirements from the ultimate fiber. Different materials and ways of fabrication were
investigated intensively in order to produce commercial lR fibers for different
applications.
Fluoride glasses based on ZrF4 material as the major component (these compositions
contain also BaF2 and LaF3 which stabilize the glass), are predicted to have minimum
optical loss of less
than 0.01 dB/krn, which is more than an order of magnitude lower
than the 0.12 dB/krn predicted and practically realized for silica fibers. This phenomenon
is related to fact that these are low phonon frequency glasses and, hence, the multiphonon energy is shifted to longer wavelengths. In addition, fluoride glasses possess
low non-linear refractive indices and these glasses are excellent hosts for rare earth
elements. As a result, there are many applications for optical fibers, such as distance
telecommunications, fiber lasers and amplifiers, as well as infrared laser power delivery.
Fluoride fibers are prepared by the classical method of redrawing performs into fibers or
drawing fibers from crucible. The majority of fluoride fibers are multimode or step index
fibers. Attenuation of 0.45 dB/km at 2.3m was achieved with a 60m fiber. In Fig. 4
method of fluoride fibers fabrication is shown.
AgClBr Crystals &
Fibers
Fabrication methods
(b)
Schematic representation of fiber fabrication using
( a ) rod- in- tube technique;
( b ) double crucible process ( DCP ) .
35
15
Fig. 4: Glass fiber fabrication.
Fig. 5: Silver halides samples.
Chalcogenide glasses such as As2S3 or more complicated compositions (Ge-Se-Te) are
prepared in Quartz ampoules under vacuum at high temperatures. Fibers are drawn by
the preform drawing for core fibers and by the double crucible method for core/clad
fibers. No graded index fibers were reported in this case. Minimum loss of 23dB/km at
2.3m was achieved with fiber drawn from sulfide glass . At 10.6m the lowest
attenuation measured with telluride glass fiber, was 1.5dB/km.
Polycrystalline optical fibers have been fabricated from thallium halides, alkali halides
and silver halides which were obtained by the extrusion of crystal perform through a die.
Unclad fibers of composition AgClo.5 Bro.5 and core-clad fibers with AgClo.4Bro.6 core and
AgClo.9Bro.I cladding are the main compositions fabricated and used. Different samples
obtained from the above compositions are presented in Fig. 5. Optical loss of 59dB/km
at 10m and less than 0.5 dB/km at 3m were reported with AgClBrI unclad fibers.
Core/clad fibers are fabricated by the extrusion of a “rod in tube” preform.
Hollow glass waveguides were constructed from a silica glass tube with diameter
between O.5mm, and coated a layer of silver iodide AgI as dielectric coating. By this
process, the glass tubing substrate is deposited with a thin flexible metallic layer, then,
innermost part of this metallic coating is converted into a dielectric thin film. Other
groups reported hollow plastic wave-guides with attenuation on 1.8dB/m at different IR
wavelength for 1 mm hollow wave-guide.
As already mentioned, the variety of applications of these IR materials and fibers, their
future possibilities, will encourage more research and investment in the field. Both in the
development of new materials, new processes of fabrications and also in finding more
ways to utilize them.