[Characteristics of anion and cation in rhizosphere soil of saline grassland in North China under different nitrogen addition levels]

We investigated the effects and mechanisms of nitrogen additions (0, 1, 2, 4, 8, 16, 24, 32 g N·m-2·a-1) on contents of anion and cation in rhizosphere soil, bulk soil, and mixed rhizosphere and bulk soil in the heavily salinized grassland in the agro-pastoral ecotone of North China. The results showed that pH of rhizosphere, mixed and bulk soils decreased significantly with the increases of nitrogen addition levels. Moreover, pH of three soil types under the 32 g N·m-2·a-1 treatment decreased by 1.2, 0.9, and 0.6, respectively, while pH of rhizosphere soil decreased by 0.44 compared with the bulk soil. Na+ content of rhizosphere, mixed and bulk soils significantly decreased, while the NO3- content significantly increased. The proportion of Na+ content in total soluble salt content in rhizosphere soil decreased by 14% and that in bulk soil decreased by 12% after the 32 g N·m-2·a-1 addition. NO3- content increased by 29% in rhizosphere soil and by 26% in bulk soil. There was significant negative correlation between pH and NO3- content, and significant positive correlation between pH and Na+ content. The total soluble salt content of rhizosphere soil under the 32 g N·m-2·a-1 treatment was significantly reduced by 31.5%. Collectedly, nitrogen deposition could reduce soil pH and total soluble salt content of rhizosphere soil and alleviate saline-alkali stress.

Analysis and compensation of the measurement error in a lock-in amplifier used for wavelength shift measurements in optical sensing application

The phase measurement error in an interferometric wavelength shift measurement scheme, such as that used in association with in-fiber Bragg grating sensors, has been investigated experimentally with appropriate underpinning analytical theory. It has been shown that when a lock-in amplifier is used to detect the phase shift generated by the Bragg wavelength shift, a pseudoperiodical measurement error can be introduced owing to the difference between the amplitude of the optical path difference ramp and the value of the Bragg wavelength. If the initial ramp deviation equals 20 nm, the measurement error may be as large as +/-2.5% of the total measurement range. With a double-phase lock-in amplifier approach to measure the ac strain, the measurement error can be decreased to 0.4% of the total measurement range. With the real-time measured period that corresponds to the Bragg wavelength with the distorted carrier signal of the interferometer as the reference period of a digital lock-in amplifier, the effect of the initial ramp deviation can be principally avoided, and the measurement error can be kept to an acceptably low level, about 0.1% of the total measurement range.

High-accuracy wavelength-change measurement system based on a Wollaston interferometer, incorporating a self-referencing scheme

A novel wavelength-difference measurement scheme with a Wollaston prism is presented. By using a suitable reference wavelength, a small variation in the signal wavelength can be converted into a relatively larger change in the modulated wavelength, as a result of the so-called fringe beating effect, resulting in enhanced measurement sensitivity by use of autocorrelation and Gaussian filtering techniques. From the results of a simulation carried out, we observed a wavelength variation of 0.01 nm over 15 nm or 0.1 nm over 60 nm for a typical pair of laser diodes with wavelengths of 785 and 810 nm, and wavelength variations of 0.5 nm over 40 nm or 1 nm over 110 nm for 671-and 785-nm wavelengths. These results were partially verified by the experimental results obtained for which a resolution of 0.01 nm over a range of 2.5 nm for the first pair and 0.5 nm over 4 nm for the second pair of laser diodes was seen. The results have applications to the determination of wavelength variations in a wavelength-division multiplexing system or measurement of the wavelength changes induced in a range of optical sensors.

Accurate mode characterization of graded-index multimode fibers for the application of mode-noise analysis

Guided modes in graded-index multimode optical fibers are accurately analyzed with the vector H-field finite-element method, aided by the use of the WKB method. As a result, exact mode-propagation constants and the corresponding modal eigenfield distributions are provided for the study of the modal noise that is due to the mode-coupling effect.

Measurement of up- and down-lead fiber sensitivity caused by the lead in a multimode fiber in an interferometric system

The results of a comparative experimental study on the effect of the modal noise induced by lead-in fibers in an extrinsic interferometric system, illuminated by high- and low-coherence light sources, respectively, are reported. When the up-lead fiber was subject to a perturbation, the sensitivity of the system was reduced by 20.9 dB through the use of a high-coherence source, and by 1.8 dB through the use of a low-coherence source. When the down-lead fiber was perturbed, the sensitivity dropped by 30.3 dB and 4.9 dB for high- and low-coherence sources, respectively. The results from the experimental analysis supported qualitatively by simple theory show that the use of a low-coherence light source can greatly suppress the modal noise induced in both the up- and down-lead fibers, if the coherence length of the light source used is less than the optical path difference between two adjacent fiber modes. This shows the practicality of the use of niultimode fibers in an interferometric system with a suitable lig t source.

Optimized multiwavelength combination sources for interferometric use

We present the use of multiwavelength combination sources in a direct method for improved central fringe identification in a white-light interferometric system. The optimum wavelength combinations of such sources can be obtained by the use of the results of a simple analysis. We find that this multiwavelength technique can greatly reduce the minimum signal-to-noise ratio required by the systemwhen used to identify the central fringe, and thus it offers an increased signal resolution. As a result, it is suitable for high-precision measurement purposes as well as for applications in coherence multiplexed interferometric sensor systems.

Relation between the coherence length and modal noise in a graded-index multimode fiber for white-light interferometric systems

The results of a comparative experimental study on the effect of modal noise induced by the modal coupling effect in a graded-index multimode fiber that is illuminated by a light source with a tunable coherence length are reported. It has been shown that, in the coherence-length region of 30-80 microm, the value of the signal-to-noise ratio in an interferometric system could be reduced by the perturbation-induced modal noise, and, as the coherence length increases, the more the signal-to-noise ratio decreases. However, when the value of the coherence length is in the region of 80 microm and upward, the value of the corresponding signal-to-noise ratio reduction is seen to vary in only a very small range (<2 dBV). Under this condition the modal noise induced in the fiber cannot be suppressed by the incoherent nature of the light source, thus showing the practical limitation of the use of multimode fibers in an interferometric system with a low-coherence light source.

Characteristics of synthesized light sources for white-light interferometric systems

Results of a study on the use of synthesized light sources in white-light interferometry are presented. The optimum wavelength combination with a pair of multimode laser diodes used to generate a synthetic wavelength source was simulated theoretically and verified experimentally. Using the best wavelength combination, we found that the lowest signal-to-noise ratio required by the system was 18.1 dB in theory and 22.1 dB from experiment. The relationships between the wavelengths of the two diodes used, their coherence lengths, and the signal-to-noise ratio required by the system are shown and discussed.

Faraday effect optical current clamp using a bulk-glass sensing element

A new topology for an openable Faraday current sensor is presented with a demonstrated resolution of 7.2 mA/ radicalHz over a measurement range from 1 to 1500 A. The sensitivity of the system is 2.21 x 10(-5) rad/A. Since this sensor is fabricated in an openable form, emulating a conventional current clamp, it can be used either in permanent installations or for short-term diagnostic studies.

Synthesized source for white-light sensing systems

We report a novel technique in which by combining the output from two multimode laser diodes a synthesized source with an extremely short coherence length is produced for use in white-light interferometric sensing systems. Experimentally and theoretically it is demonstrated that, by summing the autocorrelation function of two multimode laser diodes with a wavelength difference of 108 nm, a synthesized source with an equivalent coherence length of ~4 microm is generated, greatly reducing the signal-to-noise ratio required to identify the central fringe position.

Faraday current sensor that uses a triangular-shaped bulk-optic sensing element

A new triangular topology for a bulk-optic Faraday current sensor is presented with a demonstrated resolution of 20 mA/ radicalHz over a measurement range of 1 to 3000 A. The sensitivity of the system is 2.35 x 10(-5)rad/A. This sensor is relatively easy to fabricate and overcomes problems encountered with the use of current sensors based on bulk-optic square configurations and all-fiber systems.

Coherence length modulation of a multimode laser diode in a dual Michelson interferometer configuration

The use of a multimode (compact-disk type) laser diode in a dual Michelson interferometer arrangement is investigated, both theoretically and experimentally, by using the technique of coherence length modulation. A reproducible way of shifting the interference regions is considered for the potential use of the technique in optical sensors, for flow or distance measurement.

Miniature Faraday current sensor based on multiple critical angle reflections in a bulk-optic ring

A novel optical element for a bulk glass current sensor that utilizes the Faraday effect is presented with a demonstrated sensitivity of 1 amp-turn and a flat frequency response in the range of 10-10(4) Hz. The new sensing element overcomes the problems associated with birefringence in optical fiber current sensors and the requirement to make bulk-optic current sensors in complex three-dimensional topologies.