Fiber Endface Reflectometer
The operation principle of Fiber Endface Reflectometer consists in careful measuring of the intensity of the light reflected by the cleaved end of a singlemode fiber (Fresnel reflection). Reflection coefficient R depends on the effective refractive index of the fundamental mode of the fiber neff and on the refractive index of the medium next in which the fiber endface is located. As it is well known R can be expressed by the formula R = [(neff - next)/(neff + next)]2.
Broadband light emitting diode (LED) with a central wavelength of 1.3 or 1.55 mm is used as a light source. Normalization of the measured signal on the output power of the LED as well as an application of a low-noise photodiode ensure high quality performance of the reflectometer and carrying out careful long-term measurements.
It should be noted that the reflectometer is easily connected to the PC through any type of analog-digital converters (for instance, using a prevalent computer component - sound card, for which we have developed convenient measuring software).
Reflectometer can be effectively used for precise measuring of:
- refractive index of liquids and gases in the infrared spectral range and its changes with time, temperature, pressure, etc;
- concentration of components in solutions;
- phase transitions (consolidation, polymerization, fusion, etc);
- dneff/dT of single mode fibers. This derivative defines the temperature sensitivity of fiber gratings (both Bragg and long-period) and can not be calculated precisely;
- optical losses induced in the fiber guiding region due to various reasons (bending, gamma- or UV-irradiation, etc);
- quality of the cleaved or polished fiber endface, fiber connections, splices, etc;
- time dependence of the intensity of the light reflected by a fiber Bragg grating during the grating fabrication process.
In addition, reflectometer can be used in the different schemes of fiber-based sensors (temperature, strain, etc).
|Dynamic range, dB||>50|
|Long-term stability, %||0.2|
Examples of measurements
Response of the reflectometer on the temperature
induced change of the water refractive index
During the experiment water was heated from +20 C (t = 70 sec) to +100 C (t = 340 sec). The value of dnwater/dT = -1.40·10-4 K-1 was found from this experiment (dneff/dT of the probe fiber SMF-28 was taken into account in the calculations).
Measuring refractive indexes of liquids:
1 - noise level, 2 - air, 3 - C2H5OH, 4 - H2O
Marked noise level (1) gives a dynamic range of measurements about 50 dB. At the next stages a fiber endface was successively placed from air (2) to ethanol C2H5OH (3) and then to water H2O (4). Taking into account the value of neff = 1.450 at 1.3 mkm for the probe fiber SMF-28, appropriate refractive indices are:
nwater = 1.322 +/- 0.001
nethanol = 1.350 +/- 0.001
Draw attention at the regions, marked by red ovals: they shows dynamics of reflection from the endface, where an appropriate liquid is evaporated.