Z. X. Chao

Formulation and implementation of a phase-resolved fluorescence technique for latent-fingerprint imaging: theoretical and experimental analysis

A theoretical and experimental study of the imaging of latent fingerprints by a phase-resolved fluorescence technique along with associated signal-processing analysis is described. The system configuration is optimized by incorporation of a novel approach of homodyne-assisted even-step phase shifting in a signal-processing concept. The excitation laser source and gain of the detection device, which are modulated at megahertz frequency followed by sensitive signal-processing concepts, are employed to separate the fingerprint fluorescence from background fluorescence. Experiments are carried out with fingerprints deposited upon different types of substrate surfaces. Later, a quantitative image-quality assessment is carried out, which confirms the improvement in the quality of the phase-resolved fingerprint image. Imaging of older fingerprints with better contrast is also carried out with the proposed novel technique.

Homodyne and heterodyne signal processing assisted phase resolved optical technique for latent fingerprint imaging: a theoretical study

A general theory for phase-resolved fingerprint imaging and background suppression based on laser-induced fluorescence is developed and presented in this paper. Novel approaches of incorporating an even-step phase shifting method with a homodyne-assisted phase-resolved method as well as the camera exposure control approach for the heterodyne-assisted phase-resolved method are proposed, theoretically formulated and discussed. Theoretical results imply that the fluorescence from latent fingerprints can be extracted effectively, irrespective of whether its lifetime is longer than that of the background or not. Furthermore, it is shown that there exists an optimum modulation frequency, which is dependent on the fluorescence lifetimes of both the background and the fingerprint, to obtain a fingerprint image with better contrast.

Electronically tunable external-cavity laser diode using a liquid crystal deflector

A novel external-cavity wavelength tunable laser based on a liquid crystal (LC) deflector is proposed. This device consists of a gain chip, collimating lens, LC deflector, and diffraction grating. By controlling the voltage applied to the LC deflector, a maximum wavelength variation of 12 nm and a sidemode suppression ratio higher than 30 dB are obtained

Subnanosecond-resolution phase-resolved fluorescence imaging technique for biomedical applications

Characterization of fluorescence emissions from cells often leads to conclusive results in the early detection of cellular abnormalities. Cellular abnormalities can be characterized by their difference in the fluorescence lifetime, which may be less than nanoseconds. A sensitive frequency domain technique, also called a phase-resolved fluorescence imaging technique, is proposed in which fluorescence emissions at the same wavelengths can more effectively be separated with subnanosecond resolution in their lifetime difference. The system configuration is optimized by incorporating even-step phase shifting in the homodyne-assisted signal-processing concept along with the phase-resolved fluorescence technique to eliminate the dc offsets of emission. Experiments are carried out with simulated samples composed of two fluorescence emissions of the same wavelength but with different lifetime values. Suppression of either of the fluorescence emissions by selective imaging of the other validates the superiority of the proposed technique. Hence, this technique can potentially be applied in the early detection of cellular abnormalities.

Homodyne assisted multistep phase shifting in phase-resolved optical technique for latent fingerprint imaging

A novel homodyne signal processing assisted phase-resolved optical system for the imaging of latent fin- gerprints is proposed. A multistep phase shifting method is introduced to remove the DC part of the fluorescence sig- nals. Experiments are carried out with simulated fingerprint samples where their lifetime and emission wavelength are in close range to that of background fluorescence. The pro- posed phase-resolved technique can suppress the back- ground fluorescence provided there is lifetime difference be- tween two emissions.

Innovative design of a tunable laser using liquid crystal tuning elements

An innovative non-mechanical and low power consumption tunable external cavity laser (ECL) using liquid crystal tuning elements is proposed. This contains a gain chip, a collimating lens, tuning elements and a partial-reflection mirror. The proposed tunable ECL can achieve both coarse tuning and fine tuning, and it is designed to lase at wavelength matching the International Telecommunication Union (ITU) channels, which is one of the important requirements in optical communication. The tuning elements include an ITU etalon, a liquid crystal Fabry–Pérot interferometer (LC-FPI) and a fine tuner. Only two parameters are required to tune the wavelength over the whole C-band, namely the voltage over the LC-FPI and the fine tuner. This high reliability cost-effective design proposes a theoretical tuning range of about 80 nm. The LC tuning elements including LC-FPI and fine tuner has been fabricated and tested.

C-band external-cavity wavelength-tunable laser based on a liquid-crystal deflector

A novel C-band external-cavity wavelength-tunable laser is proposed. The laser consists of a semiconductor gain chip, a collimating lens, a fixed etalon, a liquid-crystal deflector and a diffraction grating in a Littrow configuration. The lasing wavelength of this tunable external-cavity laser can be tuned to 19 wavelength channels of 100 GHz spacing. All channels are within 2.5 GHz of the ITU grids with a side-mode suppression ratio of approximately 35 dB over the whole range.

Sensitivity improvement for phase-resolved fingerprint imaging using even-step π-shift methods

The π-shift method has been used to increase the sensitivity of lifetime-selective fluorescence imaging for different fluorophores. In this paper, to improve the sensitivity of a homodyne assisted phase-resolved fingerprint imaging system, an approach using an even-step-shift method to ease the application of the π-shift method is proposed and presented. By employing an even-step-shift method, the two phases used for the π-shift method can be determined directly by distinguishing the positions when the background fluorescence goes to zero. A detailed theoretical analysis of the proposed approach is presented. Experiments are carried out using fingerprint samples deposited on green fluorescent paper and black tape. The experimental results demonstrate the effectiveness and significance of the proposed approach.

Phase-resolved fluorescence technique with heterodyne signal processing for imaging of latent fingerprints

This paper describes the formulation and implementation of signal processing and experimental details of a system incorporating the heterodyne concept with a phase-resolved fluorescence technique for the detection and separation of latent fingerprint fluorescence emission from background fluorescence. The excitation laser source and gain of the detection device, which are modulated at megahertz frequency, are employed together with a heterodyne signal processing technique to separate the fingerprint fluorescence from the background fluorescence. The method provides improved contrast of fingerprint ridges. Experiments have been carried out on fingerprints deposited on various surfaces.