Photonics has become an essential technology for analyzing materials or the environment. Spectroscopy in particular is very effective for non-destructive testing and process control.
Optical fibers are a great help. They are used to place the instrumentation away from the measurement area, or to increase the number of reading channels without adding too much instrumentation. They can also transmit an excitation light creating fluorescence collected for the measure. In all cases fibers must be precisely selected to obtain non perturbated readings. Particularly to be adapted to the wavelength used.
On the spectrograph end the fiber bundle must be optimized to fit the instrument slit avoiding photon losses. Fibers are arranged in a line and fibers diameter chosen as close as possible of the slit thickness. Note that spectroscope’s resolution correlates to the width of the slit. The narrower the slit, the finer the resolution. Fibers can be side by side or separated by spacers to differentiate measures on the same detector array.
On the light collecting end the probe must be optimized to be positioned as close as possible of the photon source. Standard mechanical parts are metal cylinder with a 10mm diameter and 60mm length or can be SMA 905 standard ferrules (3.17mm). The centering tolerance of the fibers is + 5µm on X or Y axis.
The fiber bundles either arranged or random are made with silica / silica fibers optimized for UV+visible or Visible+IR. The numerical aperture is .22 with core diameters of 100, 200 or 400µm
Important to note that for UV applications solarization resistant fibers are to be preferred and even mandatory for deep UV.(see technical documentation)
Fluorescence spectroscopy is a type of spectroscopy that analyzes the composition of a sample. It involves the use of a light beam (normally in the ultraviolet) to excite the electrons of certain compounds’ molecules, forcing them to emit lower-energy light, generally in the visible range.
The probe is Y-shaped. One branch of the Y is connected to a source (like LED or laser), while another is connected to the spectrometer, consequently with fibers in slit to optimize coupling and spectrometer resolution. The common branch of the Y is equipped with a mechanical end as compact as possible such as a needle (for analyzing tissues and very small samples of materials). This needle most commonly includes an excitation fiber in the center, surrounded by a ring of collection fibers
Absorption spectrometry is a physical method of chemical analysis. It is mainly used on liquids. The color of a body during transmission (transparency) represents its ability to absorb certain wavelengths. This makes it possible to recognize the chemical properties of certain materials.
LIDAR (LIght Detection And Ranging) is an optical measurement or remote detection technology based on analyzing the properties of a laser light which returns to the emitter. The distance to an object or surface is given by measuring the time that elapses between the pulse and the detection of the reflected signal (telemetry).
The LIBS (Laser-Induced Breakdown Spectroscopy) technique consists of focusing a pulsed laser beam onto the surface of a sample to be analyzed. The light causes the point of impact to break down and turn into plasma. The electrons of the (ionized) excited atoms, by returning to their fundamental state, emit a photon whose energy is characteristic of the chemical element present.
An analysis method based on separating chemicals elements by a continuous electrical field in a capillary tube 50 to 100µm in diameter filled with an electrolyte solution. UV-Vis (absorption) or fluorescence detection is used. Fiber optics makes it possible to have many analysis channels simultaneously.