Cunguang Zhu

High-sensitive measurement of water vapor: shot-noise level performance via a noise canceller

Taking advantages of distributed feedback laser diode a technique is described to achieve high-sensitive measurement for water vapor concentration. This technique, with a modified balanced ratio metric detection system, has improved the accuracy of measured absorption spectrum by two main aspects. Improvement by matching equivalent conductivity of signal or reference photo detector (PD) is presented, and with the additional matched resistance suppression for the power variation in the signal-beam has been improved from 53 to 88 dB. The importance of integrating amplifier bandwidth design from the circuit to the measured absorption spectrum has been demonstrated in our experiment. For a scan rate of 32 Hz with an optimal corresponding bandwidth of 15.9 kHz, the absorption spectrum is well described by Voigt profile, with a difference of 1% at an atmosphere pressure of 1 atm and a room temperature of 296 K. With the application of averaging and filtering, absorption sensitivity of 1.093×10(-6) for water vapor at 1368.597 nm has been demonstrated, and the corresponding concentration is 71.8 ppb in just a 10 cm path length.

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

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 PDs 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.

Acquisition of phase-shift fiber grating spectra with 23.5 femtometer spectral resolution using DFB-LD

A novel method based on distributed-feedback laser diode (DFB-LD) continuous wavelength-scanning for acquiring precise spectra of phase-shift fiber gratings is presented. Compared to the traditional method, the spectral resolution retrieved by this method is only limited by the optical line-width of the light source, which can reach up to the order of femtometer and is much higher than that of high-resolution optical spectrum analyzer (generally on the order of picometer). In addition, a Signal-to-Noise Ratio (SNR) advantage can be provided owing to a much higher spectral density of DFB-LD than amplified spontaneous emission (ASE) source. Precise spectra of three phase-shift fiber grating samples have been obtained at a resolution of 23.5 femtometer.

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

Noise from optical and electronic components of a fiber optic gas sensor system using wavelength modulation of the DFB laser diode in either transmission or reflection mode were investigated. Our experimental results indicate that reflective type cells give poorer performance due to interference effects from connectors and joints within the fiber system compare to transmissive type cell. Intensity noise from optical coupler, collimator, was measured and its affection to sensing system was discussed. In order to raise the signal to noise ratio (SNR), signal processing methods, such as data average, low pass filter were used and compared. The results would be a useful engineering tool to design high SNR optical gas sensing system.

Continuously Wavelength-Tunable Light Source With Constant-Power Output for Elimination of Residual Amplitude Modulation

A continuously wavelength-tunable light source with constant-power output is presented. The unwanted optical power modulation companying with the wavelength modulation of the distributed feedback laser diode by the injection current is successfully offset by the external amplitude modulation of electro-optic modulator. The optical power stability has been experimentally demonstrated better than 1.2% in a wavelength-tunable range of 0.876 cm-1. The background baseline appearing in the direct absorption spectrum can be removed with the new light source. Not only that, the spectral distortion of the second harmonic signal caused by residual-amplitude modulation can also be largely eliminated. Experimental results have shown that the new light source is more suitable to be served as a pure wavelength-tuning reference source for different modes of spectroscopic measurement.

The relative intensity noise and relaxation oscillation characteristics of a distributed-feedback fiber laser

The relative intensity noise (RIN) and relaxation oscillation characteristics of distributed-feedback fiber lasers (DFB-FLs) have been investigated theoretically and experimentally. The effects of Er3+ doping concentration and phase-shift grating parameters such as the coupling coefficient on the RIN and relaxation oscillation characteristics of DFB-FLs are presented by an analytical model. Also, four DFB-FLs with dissimilar coupling coefficients are fabricated and tested to confirm the theoretical results. In addition, the influence of the external environment and the contribution of the pump power fluctuations are analyzed in detail by experiments.

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

A demodulation algorithm based on the head-tail technique is proposed for single-beam water vapor detection under rough environmental conditions, which is immune to fluctuations of light power. In the head-tail technique, collected data are processed by adding the head and tail data together and gradually approaching the center. The majority of photocurrent attenuation caused by optical loss can be effectively compensated by combining an optical intensity normalization coefficient in the method. The experiment indicates that, when the light power attenuates 4%, the deviation in a single-beam system is 1.29%, which is obviously superior to a dual-beam subtraction system whose deviation is 8.45%. The connection and advantages compared to a previous single-beam detection system have been discussed. The whole arrangement is simply designed without a beam splitter, of which the reliability and validity are fully verified by the experimental results.

Measurement methods in trace water vapor concentration detection system based on direct absorption spectroscopy

In trace water vapor direct absorption spectroscopy, the absorption signal is buried in noise and up and downs of the light intensity, an effective signal extraction method is vital. In the basis of double-beam differential absorption, division method in voltage and an approach based on balanced ratiometer detection (BRD) were studied. Voltage division has an excellent stability to temperature variation, mechanical extrusion and fiber bend loss. As to the BRD method, it has an outstanding self-adjusting capability and it can also avoid an excess phase difference caused by current-to-voltage converting circuit, thus this method has a high sensitivity. Furthermore, a so called dual-peak method based on the differential value of two adjacent absorption lines is introduced, the differential value proved has a linear relation with water vapor concentration, and this method provides a way to measure the concentration at high pressure. In addition, the influence of water vapor inside the optical components has been discussed.

Applications of asymmetric distributed-feedback fiber laser in sensor array system

A 4-element wavelength division multiplexed linear array of asymmetric distributed feedback fiber lasers (DFB-FL), pumped at 1480nm is reported. A very desirable feature of asymmetric DFB-FL is unidirectionality, and relevant principle is presented. Larger output powers are obtained from shorter ends of all the four asymmetric DFB-FLs which are fabricated in our laboratory by phase mask moving method. This obvious advantage has important applications to design sensor array. And furthermore, many experiments are completed to confirm it. Output flatness of the sensor array system presents a good performance with the applications of asymmetric DFB-FLs.

Water vapor absorption spectrum measurements and its application in concentration measurement

Wavelength tunable distributed feedback laser diode (DFB-LD) were utilized to measure line 1368.597nm and line 1367.862 nm absorption character of water vapor, based on it, water vapor concentration can be measured by peak absorption rate according to Beer-Lambert law. Besides, we observe that the overlap between the line 1368.597nm and line 1367.862 nm appears and become serious with the increase of gas pressure, this agrees well with the theoretical prediction, and the overlap cause difficulty to determine the absorption peak value, a scheme is presented to cope with the difficulty, 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.