Light guides are fiber-optic components which transport light between different locations in a very flexible way. Besides a 1:1 transfer light guides can also be used to distribute and collect light, to reshape the light cross-section.
Light guides are made out of optical fibers or rods and cones (more or less thicker fibers). A fiber consists of a core as the main area of transportation, a cladding to realize the total reflection required to guide light, and a coating, applied to make the thin fibers less brittle.
The art of designing the optimum light guide for a certain job is to put together all the individual variables: fiber material & diameter, NA, length, fiber arrangement, protection, termination.
Fiber material: Glass, plastic, quartz
In spectroscopy only quartz fibers play a significant role. Two main qualities are available which differ by OH content:
Low OH for VIS-NIR range: ca. 350 to 2200 nm
Higher OH content for UV-VIS range: ca. 190 to 950 nm
Solarization effects limits the transmission below 240 nm. A specially treated higher OH content fiber can overcome this, it shows a stable transmission after an initial decrease at first
are possible, and coatings up to 390°C (polyimide).
NA = sin α
Standard value is 0.22, (or 25° full acceptance angle). 0.11 also available, higher NA by glass fibers
Diameter: In spectroscopy only so-called multi-mode fibers play a role, the thickness ranging from about 50µm to 800µm, although the latter quality is quite stiff already. Thicker elements are called rods, but one main advantage of light guides, the flexibility, is lost. In principle all diameters are producible, this is controlled by the pulling process, but certain diameters are a kind of standard: In principle all diameters are producible, this is a parameter of the pulling process, certain diameters are a kind of standard:
69 / 80 / 150 / 200 / 300 / 400 / 600 / 800 (Core diameters in µm)
The cladding thickness is set to 5% of the core for UV-VIS fibers and 10% for the longer wavelength fibers, while the coating is only a few µm. The minimum bending radius r scales with the diameter D of the fiber:
rmin = 200 x DFiber
The thinner a fiber is the more flexible, similarly a bundle made of thin fibers. To transfer as much light as possible, a thick fiber has an advantage over a bundle, for there are no voids. These so-called fill factor losses are at least 30%. Back to top ▲
Numerical Aperture: A fiber guides light within a certain acceptance cone, all rays within this angle a see the conditions of total reflection. Any ray outside this cone are not conducted. This acceptance cone described as Numerical Aperture is given by the index difference between core and cladding. Back to top ▲
Length: from few centimeters (or few mm if cemented) to 10th of meters are possible. If absorption / damping is high within selected spectral range the fibers should be as short as possible. Back to top ▲
Protection: A multitude of sleeve materials is available, the sturdier the clumsier; from thin Bowden cables to heavy-duty process environment ready. Special, all-metal sleeves are used for vacuum. Pressures down to about 10-7 bar are doable. Back to top ▲
Termination: Common: SMA, FC, ferrules. FC have a tighter tolerance in respect to centricity, while SMA is very common. Ferrules have the advantage that the z-axis (the optical axis) can be manipulated, e.g. focal positions can be adjusted; counterparts are also easy to make, and relatively big connectors can be realized, which are sturdier than the tiny SMA or FC. Last but not least, coding of rotary position is easier to realize. Back to top ▲
Specialty: bend terminations with pre-bend fibers, radii as small as 25 mm can be realized
Arrangement: By playing around with the arrangement of the individual fibers a big variety of fiber-optic devices can be created. E.g. by using a 8-arm fiber bundle, combined in 1 common arm, the emission of a light source can easily be split into 8 channels. Similarly, the light of 8 different spots can all be collected and send to one detector. Back to top ▲
By arranging the fibers in specific geometric pattern allows to generate optimized cross-sections, e.g. lines for spectrometer inputs.