Measurement and analysis of water vapor inside optical components for optical fiber H₂O sensing system

Abstract

Water vapor existing inside internal end-face gaps of optical components of an optical fiber H2O sensing system makes it possible to influence the measurement accuracy and stability. The influence principle has been briefly analyzed based on the structure of three main optical components: a distributed feedback laser diode (DFB-LD), a collimator, and a photoelectric diode (PD). With application of a differential technique, the influence of water vapor inside the DFB-LD can be removed. With reasonable recombination of the collimator and the PD in a dual-beam detection system, the influence of water vapor inside the collimator and the PD's end-face gaps has been suppressed from more than 1.57×10(-3) to as low as -2.175×10(-5) in absorbance. After H2O isolation processing water vapor inside the end-face gaps of the DFB-LD, the collimator, and the PD can be utilized as a reference to design a simple but feasible H2O sensor. As a result, good linearity with an R2 of 0.9964 has been realized in a concentration range of 39-2110 ppm during an application test, and a long-term test of the designed H2O sensor against the S8000 with a difference of 10 ppm has been achieved.

Study of a distributed feedback diode laser based hygrometer combined Herriot-gas cell and waterless optical components

“…According to the analysis in [8], the water vapor inside the optical components can influence the measurement result. Combined with (5) in the WMS technique, the influenced expression of the water vapor concentration should be…”

“…To fully exhibit the influence of the water vapor inside the optical components especially the improvement of reliability and resolution, a comparison research was demonstrated experimentally between the normal transmissiontype gas cell with a 10-cm effective path length [8] and Herriot gas cell with a 3-m effective path length. Figure 3(a) shows the structure of the normal gas cell comprising two commercial fiber collimators used in communication.…”

“…Almost all of DFB-DL based hygrometers consist of several optical components, such as light source (DFB-DL), fiber collimators, and photoelectric detector. However, it was found in [8] that water vapor in these optical components seriously influenced the measurement result not only in resolution but also in reliability. To realize reliable and high-resolution measurement of water vapor, an approach, namely the photo detector (PD) matching, had been reported to achieve preliminary suppression for the influence of water vapor inside optical components (WVOC).…”

Demodulation algorithm used in single-beam system immune to light power drift

“…We presumed that the disproportion was due to background absorption of water vapor inside optical components in the system, and some previous work had been done to talk about the water vapor inside optical components. Further processing was done on the results of the experiment [15] to eliminate the effects of the water vapor inside the optical components as follows: Obviously, any concentration can be used as a reference to eliminate the effects of water vapor inside optical components in theory, but we are accustomed to taking the lowest concentration. So we chose 15 ppm concentration signal as the reference to eliminate the water vapor inside optical components as Eq.…”

“…Thus, we can conclude that a reference absorption should be required for any water vapor detection system for inducing errors. Taking the water vapor absorption inside the optical components as a reference to measure water concentration was a good idea and had been discussed [15].…”