Theoretical background: Fiber Optics

Introduction Fiber-Optics

The use of fiber-optics as light guidance allows a great modularity and flexibility in the setup of an optical measurement system. Optical fibers can be made of many materials, such as plastic, glasses and silicates (SiO2). For high quality fiber-optics, as used in spectroscopic applications, synthetic fused silica (amorphous silicon dioxide) is used, that can be intentionally doped with trace elements to adjust the optical properties of the glass.

Basic principles

The basic principle of light transport through an optical fiber is total internal reflection. This means that the light within the numerical aperture of a fiber (NA = input acceptance cone) will be reflected and transported through the fiber. The size of the numerical aperture depends on the materials used for core and cladding.

Two basic types of silica fibers can be distinguished; single-mode and multi-mode fibers, depending on the propagation state of the light, traveling down the fiber. For most spectroscopic applications multi-mode fibers are used. Multi-mode fibers can be divided into 2 subcategories, step-index and graded-index. A relatively large core and high NA allow light to be easily coupled into the fiber, which allows the use of relatively inexpensive termination techniques. Step-index fibers are mainly used in spectroscopic applications.

Graded-index multimode fibers have a refractive index gradually decreasing from the core out through the cladding. Since the light travels faster in material with lower refractive index, the modal dispersion (amount of pulse-spreading) will be less.
These graded-index fibers are mainly used in telecommunication application, where dispersion at long distance (2-15 km) plays an important role.

Product codes

For example FC-20UV200-3-BX-SR
A product code is designed as follows:

 Type of product Total number of fibers Wavelength  Fiber core diameter Overall length Jacketing Other options

FC = standard fiber cable
FCB = bifurcated fiber
FCR = fiber reflection probe
FDP = fiber dip probe

Almost any number possible UV = 200-800 nm
IR = 350-2500 nm
UVIR = 250-2500 nm
8 μm*
50 μm**
100 μm
200 μm
400 μm
600 μm
800 μm**
1000 μm**
in meters BX =stainless steel
ME = chrome-plated brass
MS = metall silicone
SR = solarization resistant
HT= high temperature
HTX= extreme high temperature
PK= PEEK
HY= Hastelloy®

*Only for IR fibers
**Only for UV or IR fibers

Fiber-Optic Design

Core
Blz. 82 fiberoptic design
For spectroscopic applications, generally, multi-mode step index silica fibers are used. These range in core thickness from 50 to 1000 microns. The core is made out of pure silica. Other fiber cores with much higher absorption are made out of certain glass types or plastics. These are not offered in this catalog.
First a distinction is made between silica with high or low OH content. Silica fibers with high OH (600-1000 PPM) are used in the UV/VIS wavelength range because of the low absorption in the UV. They are referred to as UV/VIS fibers. For Deep-UV applications (below 230 nm) special solarization resistant fibers can be used.
The water content causes strong absorption peaks in the NIR wavelength range. In order to get good fibers for the NIR range, the “water” is removed from the silica. This results in low OH fibers (<2 PPM) with low absorption in the NIR. They are referred to as VIS/NIR fibers.
New in this catalog are the so-called broadband fibers, which can be used for the UV-NIR range (250-2500 nm), the product code for these fibers is UVIR.

Cladding
In order to get the light guiding effect the core is cladded with a lower index of refraction material. For the highest quality fibers with the lowest absorption this is a fluorine-doped silica, the so-called silica-silica or all-silica fibers with a numerical aperture (NA) of 0.22.

Buffers
blz. 83
Without further protection fibers would easily break, because of small scratches or other irregularities on the surface. Therefore a next layer, the buffer, is added. This buffer also determines under what circumstances the fiber can be used. Temperature range, radiation, vacuum, chemical environment and bending are factors to be considered.
Polyimide buffers offer a wide temperature range (-100 to 400°C) and superior solvent resistance. Also, this material is non-flammable. Drawbacks are sensitivity to micro bending and the difficulty to remove it.
 For extreme temperatures (-270 to 700°C) metal buffers are used. Metal buffers can withstand a continuous high temperature up to 500 °C and intermittent even up to 700°C. Low outgassing makes them also excellent for use in UHV environments.

Technical Data

Fiber Material Standard HTX
Temperature Range -190 °C to +400°C -270 °C to +700°C
Fiber type

Step index Mutimode

Core Numerical Aperture

0.22 ± 0.02

Buffer Polyimide Metal
Available Diameters 50/100/200/400/600/800/1000µm 200/400µm
Laser damage resistant core

1,3 kW/mm2 CW at 1060 nm, up to 10 J, pulsed

CW up to 100kW/cm2
For pulsed lasers (<1μs) 500kW/cm2

Bend radius

Momentary 100 x clad radius

Long term 600 x clad radius

Momentary 40 x clad radius

Long term 100 x clad radius

Transmission UV/VIS Fibers

uvfiber

Transmission VIS/NIR Fibers

irfiber

Transmission UV/VIS/NIR Fibers

pagina 101 bpi chart with line

Solarization Resistant Fibers for Deep UV applications

Most spectroscopic applications with fiber-optics have been restricted to wavelength ranges above 230 nm, because standard silica fibers with an undoped core and fluorine doped cladding are frequently damaged by exposure to deep-UV light (below 230 nm). This solarization effect is induced by the formation of “color centers” with an absorbance band of 214 nm. These color centers are formed when impurities (like Cl) exist in the core fiber material and form unbound electron pairs on the Si atom, which are affected by the deep-UV radiation.

Not long ago, solarization resistant fibers, which were hydrogen loaded, were developed (UVI). The disadvantage of these fibers is the limitation on smaller fiber diameters and limited lifetime, caused by the H2 outgassing from the fiber. Recently, with the availability of a modified core preform, a new fiber became available (UVM). This fiber provides long-term stability at 30-40% transmission (for 215 nm).

All UV/VIS fiber-optic probes, cables and bundles with core diameters of 100 µm, 200 µm, 400 µm, 600 µm, 800 µm and 1000 µm can be delivered with solarization resistant fibers. All assemblies, made by Avantes, are pre-solarized for an 8-hrs period, to have a constant transmission of 30-40% @ 215 nm.

 

Solarization normal UV400 fiber

solnormuv400

Solarization UV100-SR fiber

sol uvm100

Solarization 100 micron UVM fiber

sol100micron uvm0

Ordering Information

-SR

solarization resistant fiber for DUV applications

 

Fiber-Optic Jacketing

For different applications Avantes offers different jacketing material. Standard fiber-optic cables and bifurcated cables are protected by a Kevlar reinforced polypropylene inner tubing with PVC red outer jacket. All of our standard reflection probes are protected by a flexible stainless steel jacket with interlocking profile (BX) or a chrome-plated brass outer jacket, with hooked profile (ME) for optimal strain relief with silicon or PTFE inner tubing. For waterproof and some medical applications stainless steel spiral jacketing with glassilk and gray outer silicon rubber coating can be provided. Inside this jacket silicon or PTFE inner tubing is used as well. For heavy industrial environments we advise the metal stainless steel (-BX) jacketing. It features a tensile strength of 950N. Especially for small, flexible, endoscopic probes we use a PVC rubber jacketing. Some specifics on the jacketing can be found in the following technical information.Contact us if you have any special conditions requirements.

 

Technical Data

Sleeve material

Kevlar reinforced PVC

Chrome plated brass (ME)

Stainless steel (BX)

Silicon coated stainless steel (MS)

Stainless steel (FX)

Inner Tubing

Polypropylene

Silicon/PTFE

Silicon/PTFE

Silicon/PTFE

Steel/Copper

Outer dimensions

3.8 mm

5.0 mm

6.0 mm

5.8 mm

6.1 mm

Min. bending radius

18 mm

18 mm

35 mm

18 mm

35 mm

Temperature Range

-20°C to +65°C

-65°C to +250°C

-65°C to +250°C

-60°C to +180°C

-65°C to +250°C

Tensile Strength

150 N

350 N

950 N

70 N

n.a.

Application

Standard

Industrial

Heavy Industrial

Waterproof IP67

Industrial


Ordering Information

-MS Stainless steel spiral jacket with glassilk and gray outer silicon rubber coating
-ME Flexible chrome-plated brass outer jacket, with hooked profile
-BX Metal stainless steel jacket, with fully interlocking profile
-FX Stainless steel jacket for rigid/fixated set ups

 

 

 Standard

fiber-cables

 -MS
 -ME
 -BX
- FX

Fiber-Optic Probe Properties

All Avantes fiber-optic cables and probes can be modified to customers request. Most materials we use in our fiber-optic assemblies can be replaced with others to improve specific chemical or thermal resistance or to enhance vacuum or pressure properties. Please contact our fiber design engineers with your specific request.
In the following paragraphs some of the most essential technical parameters are listed for the materials we use.

Thermal resistance
The thermal resistance of a fiber-optic assembly depends on some of the materials used:

  1. Fiber, the standard fiber design has a polyimide buffer, covering a wide thermal range –190 to 400 °C. For higher temperatures metal clad coated (to 500°C) fibers are recommended.
  2. Jacketing, the standard jacketing is PVC based and has a small temperature range (-20°C to 65°C), for higher temperatures a flexible metal jacketing (-BX/ME) with silicone inner tubing is recommended (up to 250°C) or stainless steel tubing (not flexible, to 750°C).
  3. Probe ends, connectors and ferrules are standard made of metal and have a wide temperature range. For special plastics, like PVC, PEEK and Teflon a limited temperature range is applicable.
  4. Bonding epoxy, the standard epoxy used is a heat curing bonding epoxy with a temperature range of –60°C to 175°C. The curing temperature is standard 100 °C, for high temperature ranges (order code -HT), the curing temperature is 200°C. For the HTX (extreme high temperature) fibers and probes silver soldering is used, a process that can withstand temperatures up to 500°C.

 

Technical Data

Temperature range

Fiber

Sleeving

Probe end

Bonding

-20°C to +65°C

Standard Polyimide

Standard PVC

Standard metal/ PVC/PEEK/PTFE

Standard Epoxy

-30°C to +100°C

Standard Polyimide

Metal (-ME/-BX) or silicone (-MS)

Standard metal/ PEEK/PTFE

Standard Epoxy

-60°C to +200°C (HT)

Standard Polyimide

Metal (-ME/-BX) or silicone (-MS)

Standard metal/ PEEK/PTFE

High temperature curing epoxy

-100°C to +500°C (HTX)

Metal clad coated

BX/ME-jacket or none

metal

Silver soldering


Ordering Information

-HT High temperature version (up to 200°C)
-HTX Extreme high temperature version (up tp 500°C)


Chemical resistance

The chemical resistance of a fiber-optic assembly depends on some of the materials used:

  1. Fiber, the standard fiber design has a polyimide buffer, which normally will not be in contact with the sample; the quartz core provides good resistance against most solvents.
  2. Jacketing, the standard jacketing is PVC based and has a relative good chemical resistance. The –BX stainless steel and –ME chrome plated brass jacketing also have a good chemical resistance, but are not waterproof. The Silicone metal jacketing (-MS) is recommended for waterproof environment, biomedical applications, etc. The PEEK and PTFE jacketing have the best chemical resistance.
  3. Probe ends, connectors and ferrules are standard made of stainless steel (316) and are not very well suitable in corrosive environment. For most corrosive environments PEEK, PTFE or Hastelloy® C276 are recommended.
  4. Bonding, the standard heat-curing two- component epoxy used is resistant to water, inorganic acids and salts, alkalis and many aggressive organic solvents and most petrochemical products, and an extended range of organic and inorganic environments.


The table below gives a summary for the chemical resistance for most materials used. It has been drawn up on the basis of relevant sources in accordance with the state of the art; no claim to completeness. The data constitutes recommendations only, for which no liability can be accepted.

Please contact us if you have any doubt about the materials to use for your application.

Technical Data

Chemical environment

Fiber

Jacketing

Probe end

Epoxy

Acids weak

Standard Polyimide

±

-ME/-BX

-MS

-PEEK

-PVC

±

+

+

+

St. steel 316

PEEK

PTFE

Hastelloy® C276

-

+

+

+

+

Acids strong

Standard Polyimide

-

-ME/-BX

-MS

-PEEK

-PVC

-

±

+

±

St. steel 316

PEEK

PTFE

Hastelloy® C276

-

+

+

+

±

Bases weak

Standard Polyimide

±

-ME/-BX

-MS

-PEEK

-PVC

+

+

+

+

St. steel 316

PEEK

PTFE

Hastelloy® C276

+

+

+

+

+

Bases strong

Standard Polyimide

-

-ME/-BX

-MS

-PEEK

-PVC

+

+

+

+

St. steel 316

PEEK

PTFE

Hastelloy® C276

+

+

+

+

+

Aromatic carbons

Standard Polyimide

+

-ME/-BX

-MS

-PEEK

-PVC

+

+

+

+

St. steel 316

PEEK

PTFE

Hastelloy® C276

+

+

+

+

+

Alcohols

Standard Polyimide

±

-ME/-BX

-MS

-PEEK

-PVC

+

±

+

+

St. steel 316

PEEK

PTFE

Hastelloy® C276

+

+

+

+

+

Ketons/Ethers

Standard Polyimide

+

-ME/-BX

-MS

-PEEK

-PVC

+

-

+

-

St. steel 316

PEEK

PTFE

Hastelloy® C276

+

+

+

±

±

+ = good resistance
± = conditional resistant
- = not resistant

Ordering Information

-PK PEEK Probe material replaces Stainless Steel
-HY Hastelloy® C276 Probe material replaces Stainless Steel

Fiber-Optic Connectors

Standard SMA

 blz 88 SMA with ext fer

Standard SMA connector
We supply all of our standard fiber-optic cables, bundles and probes with SMA-905 connectors that easily fit into our complete range of spectrometers, light sources and accessories.
The SMA-905 connectors are screw-fitted and can be rotated over 360 degrees. The typical insertion loss for the connectors is 0.5 dB. The maximum filling diameter for bundles is 2.46 mm.

FC/PC connectors

blz 88 FCPC connector

FC/PC connectors
Optional FC/PC-connectors can be mounted to our fiber-optic products. The multimode FC/PC connectors have an extremely low insertion loss of < 0.2 dB. The FC/PC connector cannot rotate, always mounts into the same fixed position and therefore has a high reproducibility.

ST connectors

blz 88 ST-connector v4

ST connectors
Optional ST-connectors can be mounted to our fiber-optic products. ST-connectors easily mount with their bayonet type of fitting, and can therefore not rotate, i.e. they mount in a fixed position. The maximum filling diameter is 1.5 mm, typical insertion loss is 0.3 dB.

Ordering Information

-FC/PC FC/PC connector instead of standard SMA
-ST ST connector instead of standard SMA