Application of wavelength scanning for measuring water vapour concentration by distributed laser diode

Abstract

A technique which takes advantage of distributed feedback laser diode (DFB-LD) wavelength scanning to measure water vapor concentration is presented. Concentration is gotten by peak absorption rate according to Beer-Lambert law and absorption coefficient of water vapor in HITRAN database. Theoretical work on the pressure affection to light intensity absorption rate has been done, a scheme is presented to cope with the affection of overlap of two adjacent lines, it takes advantage of the peak absorption difference between 1368.597nm and 1367.862 nm, and the difference value is used to calculate the water-vapor concentration.

Noise investigation and signal processing in fiber optic gas sensor with wavelength modulation of the DFB laser diode

“…Measurements of the water vapor absorption spectrum involve recording the entire absorption line profile as the diode laser is tuned across an absorption line [1]. The single mode DFB lasers used in the present work possess a line width of 3 MHz, essentially monochromatic compared to the Doppler-and collisionbroadened absorption line shape.…”

“…Absorption spectrum optical fiber gas sensor based on wavelength tuning of distributed feedback laser diode (DFB-LD) is intensively researched recently [1,2], because the DFB-LD is feasible to have wavelength tuning either by injection current modulation or temperature modulation, and relatively low-cost devices in rugged, fiber-coupled packages that are widely available from a number of vendors [3], consequently it is easy to form compact, portable devices, besides it has the advantages of optical fiber sensors: small-size, light-weight, EMI immunity, electrically passive operation, and multiplexing capabilities [4]. However, the signal of spectrum absorption is very weak, especially at low gas concentration, belonging to weak signal [5], the valid signal with gas concentration information would easily be covered if noise is too much, so reducing noise and choosing appropriate signal processing method have to be used to reach high sensitivity.…”

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

“…Water vapor concentration measurement is of great importance in many fields, such as environmental analysis [1], fabrication of polymers [2], and maintenance of high-voltage equipment [3]. Traditional methods of water vapor concentration measurement include the gravimetric method, electrolysis, dew point meters [4], etc.…”

“…With distributed feedback laser diode (DFB-LD) processing technology maturing, a tunable diode laser absorption spectroscopy (TDLAS) is widely used in gas detection. The implementation of the TDLAS is generally based on the dual-beam optical structure of one signal beam and one reference beam [3][4][5]. In those methods, the output of the laser diode is split into signal and reference beams.…”

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

“…Trace detection for water vapor is of great importance in various applications, such as in the fabrication of polymers [1] and high-voltage electrical equipment (such as gas insulated switch gear) [2], and environmental analysis also requires realtime measurement of water vapor concentration distribution [3]. Methods of water vapor concentration measurement include microelectrolysis, dew point meters, and resistive-capacitive moisture meters [4].…”