Yanong N. Ning

Fibre optic anemometry using an optical delay cavity technique

Limitations on the performance and capabilities of conventional fibre optic Doppler Anemometers (FODAs) are highlighted in the context of fluid flow measurement. In order to overcome these difficulties, a novel optical fibre signal processing scheme is outlined, based on a low coherence interferometric approach using an optical delay cavity matched for the position of the required measurement volume. Both a homodyne and a heterodyne system are discussed and experimental results are presented to verify the feasibility of the proposed method.

Novel wavelength measurement scheme using a stabilized interferometric system

A novel interferometric detection scheme for measuring wavelength shift in an optical sensing system is described. In this scheme, in addition to the source wavelength to be measured, a further reference wavelength is employed to stabilize the optical path difference of the interferometer. This is designed to make the wavelength detection system insensitive to environment disturbances such as temperature fluctuation and mechanical vibration, and using this scheme a signal-to-noise ratio improvement of 25 db has been demonstrated.

Novel white-light interferometer using an electronically scanned Mach-Zehnder interferometer

A theoretical analysis and experimental evaluation of a novel Mach-Zehnder interferometer (MZI), used as a recovery interferometer in an electronically-scanned white-light interferometer is reported. This modified version of a MZI has the advantages of being very simple and compact, with high stability and does not introduce any spatial `misoverlapping of the beams. In addition, since only one beamsplitter is employed in the interferometer (instead of the usual two used in a conventional MZI), the system is relatively easy to implement and at low cost. Furthermore, this configuration can be simply set to cover a wide range of optical path difference values by simply changing the angle and position of one of the mirrors. This gives the interferometer the advantage of being easily adapted for a range of different measurement applications.

Conditions for suppressing modal noise induced in a white light interferometer

When a low coherence light source is used in a multimode fibre linked interferometer, the level of thenmodal noise [1] induced by the environmental variations on the fibres may be dramatically suppressed, providednthe value of the coherence length, Lc, of the light source employed is less than that of the optical pathndifferences (OPD) between the guided modes at the far end of the fibre [2, 3].

General review of optical current sensors

In recent years, a considerable research effort has been expended on the field of optical current measurement in order to replace the conventional curient transformer (CT) used in power distribution 1 Although current measurement schemes based upon the use of an optical fibre as the sensing element were proposed as early as 1976, and a number of new methods which employ optical fibre, or bulk glass, or even electro-optic hybrid sensing elements have been explored and aviable commercial product has not yet emerged, despite the considerable level of research that has been devoted to the concept. An opticai current sensor consists of an optical current sensing element which measures the integral of the magnetic field along a closed (or nearly closed) optical loop around the current to be measured, an optical fibre link which connects the sensing element to an opto-eletronic control/processing unit, used to launch a light beam into the optical fibre and detect/demodulate the modulated optical signal collected from the fibre. According to the sensing mechanism employed and the sensing materials used, the optical current measurement devices may be categorized into three main groups: (i) optical current sensors (OCS) employing optical fibre as their sensing elements, (ii) the OCS type using bulk glass to sense the current, and (iii) the OCS type using electro-optic hybrid devices. In general, the principle ofthe operation ofthe first two types of sensors is based upon the so-called Faraday magneto-optic effect, in which the polarization azimuth, 1F a linearly polarized light beam propagating inside an optical material is rotated under the influence of a magnetic field, H, generated by the electrical current to be measured and is given by (Formula available in paper) (1) where V is the Verdet constant ofthe fibre core and 1 is the interaction length. Ifthe medium is subject to the magnetic field within a long solenoid this then becomes (Formula available in paper) (2) where N1 and NC the number of passes of the light through the solenoid and the number of turns of wire in the solenoid respectively, and I is the electric current. The electro-optic hybrid current sensors are often constructed from a conventional current Iransfonner interrogated with different electro-optic or fibre optic sensing schemes. Among these groups ofsensor, optical fibre current sensors have been discussed and reviewed by a number of scientists 1 3. Hence, in this paper, we will summarize the possible solutions for dealing with the stability problems associated with bendinduced linear birefringence in optical fibre current sensors and review the recent development in the area of current measurement using optic bulk glasses and electro-optic hybrid sensing techniques.

Characteristics of a multimode laser diode source in several types of dual-interferometer configuration

The characteristics of a multimode laser diode light source in several dual interferometers are investigated both theoretically and experimentally and a comparison is made. Their applications to. coherence modulation based on optical fibre sensors are discussed.

Scanning in a white light interferometer using a tunable laser

White light interferometry (WLI) has been proven to be an efficient technique to increase the unambiguousnrange and to perform absolute measurement1 . Abasic white light interferometric sensor system is generallyncomposed of a sensing interferometer and a receving interferometer.

Suppression of modal noise of a graded-index multimode fiber in a white-light interferometric system

A study on the effect of modal noise induced by the modal coupling effect in a graded index multimode fiber, illuminated by a light source with a tunable coherence length was carried out. It has been shown that the value of the signal-to-noise (S/N) ratio in an interferometric system could be reduced by the perturbation-induced modal noise if in the coherence length is in the region of 30 to 80 micrometers . As the coherence length increase, the S/N ratio will reduce correspondingly. However, when the value of the coherence length is in the region of 80 micrometers and upward, the value of the corresponding reduction in S/N ratio was seen to vary only over a very small range.

Detailed study of synthesized light sources for white-light interferometric systems

Results of a study on the use of synthesized ligth sources are presented. The favarable wavelength combinations for a pair of MLDs was simulated theroetically and verified experiementally. Under the condition of the best wavelength combination the lowest signal-to-noise ratio (SNR) required by the system was found to be 18. 1 dB in theory and 22. 1 db from experiment. The relationships between the wavelengths of two diodes their coherence lengths and the SNR required by the system are shown and discussed.

Use of multimode laser diodes in low-coherence coupled-cavity interferometry

Experimental and theoretical investigations of the use of multi-mode laser diodes in a coupled Michelson: Fabry-Perot interferometer and a novel Michelson: Bragg interferometer for fibre-linked interferometry are reported. These results show that the coherence regions of such diodes can be varied over a large range of optical path imbalance of the interferometers allowing the interference region in fibre optic sensors to be tailored to specific sensor applications.