The retardation change versus temperature was measured for deep UV multiorder crystal quartz wave plates. The influence of temperature on the wave-plate performance is even more significant for the deep UV region due to the shorter wavelength and increased number of wave periods in the retarder for the given thickness and sharp changes of refractive indices versus temperature and wavelength? Precise control of temperature is needed for accurate analysis of the wave-plate optical parameters. The wave plate should be designed for a certain temperature and used within a tightly controlled temperature range; otherwise its performance will be degraded, affecting the entire optical system. A simple laser-based method was exploited and developed for wave-plate performance evaluation versus temperature for the half and quarter wave retardation. All this information needs to be taken into account for deep UV wave-plate design to ensure the best possible performance of the retarder.
We present an overview of research, conducted and published by the author and colleagues during the preceding decade, with self-oscillating dynamic systems. Special attention has been addressed to sensor type applications that allow one to design a new type of sensors of different physical parameters as well as using system for chemical and biosensors. Many detection methods exploit self-oscillating systems, such as lasers and RF or microwave oscillators, and use changes introduced into a feedback mechanism (for instance laser inter-cavity spectroscopy) for evaluation of different physical parameters such as refractive indices or absorption coefficients. Typically, that approach is very efficient, is easy to implement, and gives high sensitivity. We have demonstrated that a similar method can be used in the case of an RF optoelectronic self-oscillating system (OSOS) with a fiber-optic feedback line. Using fiber as an element of a positive feedback line allows one to design a new family of fiber-optic sensors each of which can be integrated into a fiber-optic feedback line. Changes introduced into the feedback line of an OSOS typically cause an RF frequency shift that can be measured very precisely with an RF frequency counter or spectrum analyzer. For some types of sensors an OSOS can easily incorporate and utilize advantages of well-developed modern inexpensive light sources (VCSELs, LEDs) and opto-electronic components that have been designed for communication purposes. A single closed loop OSOS can be easily duplicated for sensor array measurement via the use of parallel fiber-optics (for example VCSEL arrays and fiber ribbon cables) that have been well developed for telecommunication systems.
We present a new detection instrument for sensor measurements based on excited-state fluorescence lifetimes. This system consists of a primary optoelectronic loop containing a resonance-type rf amplifier, a modulatable fluorescence-excitation light source, a fiber optic feedback loop (with a gap for a fluorescent sensor), and a photomultiplier tube. A secondary, phase-feedback optoelectronic circuit consists of a long-wavelength-pass optical filter, a second photomultiplier tube, a photodiode, an electronic phase detector, a dc amplifier, and an electronic phase shifter (inserted into the main loop). This phase-feedback circuit is new with respect to our previous work. Under the appropriate conditions, the main loop exhibits self-oscillations, manifesting themselves as sinusoidal rf modulation of light intensity. The phase-feedback circuit detects the modulation phase shift resulting from the finite excited-state lifetimes of a fluorophore. As the excited state lifetime changes, the phase shift from the electronic phase shifter also changes, which results in a shift in self-oscillation frequency. The detection system uses self-oscillation frequency as the detection parameter and has excellent resolution with respect to changes in excited-state lifetime (~1 ps).
We present a new detection instrument for chemical/biological fluorescence lifetime-based sensors. The instrument comprises a primary, closed loop with a secondary loop controlling a variable phase delay within the primary loop. The primary loop consists of a fluorescence excitation light source, a fiber-optic delay line (with a gap for placement of a fluorescent sensor), an electronic phase shifter, a photo-detector, and a resonance-type RF amplifier. The secondary loop consists of a long-wavelength-pass optical filter, multimode fiber, a PMT, and an electronic phase detector (which is connected to the phase shifter of the primary loop). The system exhibits self-oscillations in the form of RF sinusoidal intensity modulation with frequency dependent on the fluorescence lifetime. Since the primary loop does not contain an optical filter, it is easier to obtain self-oscillations (compared to single loop systems). The feedback also improves the stability of the detection platform. The detection system is simple, inexpensive, and scalable for sensor array purposes. We demonstrate the use of a cost-effective, multi-channel, computer-algorithm-based frequency counter with this new system. We illustrate the detection capabilities of this detection system with the pH-sensitive, fluorescent probe carboxy seminaphthofluorescein (SNAFL-2) and an immunosensor based on fluorescence resonance energy transfer.
We present a new approach to excited state fluorescence lifetime-based measurements which is inexpensive and highly sensitive. The detection system consists of a closed-loop optoelectronic arrangement containing an intermediate frequency resonance amplifier, a fluorescence excitation light source (for example, a light emitting diode or a semiconductor laser), a fiber optic delay line, and a photodetector. The system exhibits self-oscillations in the vicinity of the frequency (Omega) approximately 1/(tau) (where (tau) is the excited state lifetime) which manifest themselves as modulation of the light. Changes in the excited state lifetime alter the phase delay of the loop, which in turn causes a frequency shift in the modulation signal. The frequency shift can be measured very precisely with a frequency counter. With appropriate averaging, this technique can yield sub-picosecond resolution of shifts in lifetime. This technique is suited for chemical/biological sensing applications, and can be easily duplicated for chemical/biological sensor arrays.
In many medical, biological, chemical, and environmental applications it is desirable not only to monitor one specific chemical or biological species, but several simultaneously. Thus, we have focused our efforts on development of a detection system for multi-analyte sensor arrays that is able to monitor the changes in fluorophore lifetimes (via the detection of phase shifts) corresponding to the presence of many analytes of interest in near-real time. We describe a phase-sensitive electronic detection system employing a multianode photomultiplier tube. This system utilizes the frequency-domain method of time-resolved spectroscopy and is also suitable for lifetime-based imaging.
Fluorescence lifetime-based sensors are well-suited for chemical and biological applications since they are relatively insensitive to background light intensity, fluctuations and bleaching of fluorophores. Examples of applications include biosensing strategies where binding or a target analyte to an immobilized biological receptor molecule results in a change in the fluorescence lifetime of a fluorescent reporter group. It is desirable in many instances to have a sensor array to monitor the simultaneous binding of several analytes. We have designed a multichannel system using LEDs (or laser diodes) to excite fluorescence, multiple photodetectors, and a multichannel computer algorithm-based phase meter. The multichannel phase meter utilizes a PC and multichannel digital acquisition board. The resolution of each channel in the multichannel phase meter has been estimated at approximately 0.05 degrees. The eigen-phase fluctuations for each channel of the system are approximately 0.15 degrees, which allow us to estimate the lifetime resolution as better than 10 ps. We estimate the processing time of phase measurements for each channel as less than 200 ms. The usefulness of the system has been demonstrated in several operational examples, including a multichannnel pH meter and a fluoresphore competitive immunoassay-based chemical sensor.
Emmanuil Rabinovich, Michael O'Brien, Balaji Srinivasan, Steve Elliott, Xiang-Cun Long, Ravinder Jain, Victor Perez-Luna, Yuqing Zhou, Leonard Tender, Gabriel Lopez
This paper describes a design for an inexpensive phase fluorimeter based on blue or green light emitting diodes for use with chemical and biological sensors with fluorescence and phosphorescence lifetime-based transduction. The phase fluorimeter is based on a personal computer, two frequency synthesizers and off-the-shelf optical and electronic components and has an estimated lifetime resolution better than 10 ps. The phase fluorimeter is especially well-suited for implementation with arrays of chemically sensitive elements. The data acquisition system allows rapid monitoring of light emission from fluors or phosphors immobilized in the chemically sensitive array elements, each of which can be designed to be responsive to a particular chemical analyte. This paper describes chemically sensitive elements based on ultrathin films of porous polymers and on self-assembled monolayers. The paper focuses on methods for detection of low levels of luminescence emission from micropatterned array elements that comprise sensor arrays. Of particular importance is the detection of low levels of fluorescence and phosphorescence from SAMs of alkanethiolates on thin gold and silver films. Methods for enhancing the luminescence yield from these SAMs include optimization of the dielectric environment of the luminescent dyes and surface plasmon resonance enhanced excitation.
CO2 lasers are very sensitive to optical feedback and can be driven into a large variety of dynamical states by an external modulation of the feedback parameters. We report on the experimental results concerning instabilities of CO2 lasers induced by either an additional passive resonator of periodically varying geometrical length or an refractive index modulation in the external cavity by an electro- optical modulator. Experimental results show the presence of stable periodic orbits of different periods, bistability of periodic orbits and chaotic attractors with complex transition scenarios.
The microjet printing method of micro-optical element fabrication is being used to make arrays of high-performance hemi-elliptical and hemi-cylindrical microlenses for potential use in applications such as collimation of edge-emitting diode laser array beams. The printing method enables both the fabrication of very fast (e.g., f/0.75) microlenses and the potential for reducing costs and increasing flexibility in micro-optics manufacture. The process for fabricating anamorphic microlenses, including those of square or rectangular shape, involves the dispensing and placing of precisely sized microdroplets of optical material onto optical substrates, and then controlling their coalescence and solidification. By varying the number, diameter and spacing of adjacent microdroplets of optical materials deposited at elevated temperatures onto heated substrate, both the dimensional aspect ratios and the ratio of `fast'- to-`slow' focal lengths of a printed hemi-elliptical microlens may be varied over a very wide range. Arrays of hemi-elliptical and hemi-cylindrical microlenses on the order of 100 - 300 micrometers in width and 150 micrometers to 20 mm long, with focal length ratios (fast/slow) from 1 (circular) to 0 (cylindrical), have been printed. A model for predicting printed hemi-elliptical microlens focal lengths from printed lenslet geometry is illustrated, along with an interferometric method of detecting lenslet defects and aberrations.
Carbon-dioxide lasers are well-known example of optical systems, which can exhibit complex transitions from the CW to the chaotic regime in the intensity caused by the modulation of various systems parameters. They can demonstrate classical bifurcation scenarios with period doubling, bistability, and chaotic attractors. We report on a new method of observing chaos in carbon-dioxide lasers where the simple technique of modulation of the optical feedback parameters is applied. In this approach the parameters of the carbon-dioxide laser itself remain unchanged and modulation is carried out externally. This leads to a system which can be easily assembled and studied in a graduate laboratory. Experimental results show the presence of stable periodic orbits of different periods, bistability of periodic orbits and chaotic attractors with complex transition scenarios, providing a wealth of examples for the student to investigate.
The authors designed a number of sensors for different physical value measurements, based on a laser autogenerator with fiber-optical line of delay. Some influence upon the sensitive element joined into the gap of FOLD results in corresponding change of the generating frequency. This autogenerator property is used for measurements of such values as: displacement, vibrations, temperature, pressure, and refractive index. Contact and some remote sensors are possible.
In the paper diffraction of focused laser beam in thin blood-vessels
(TBV) is discussed. Theoretical description of this process is
given, a number of model experiments are carried out and TBV scattering in biological objects in vivo is analysed. Relations, establishing connection between scattered field and differential characteristics of blood flow in vessels, diameter of which are about or smaller than those of erythrocytes, are obtained. Analysis of theoretical model correctness is carried out. The paper also contains description of two modifications of a sensor for analysis of diff use
scattering biological objects vibrations. The modifications are
based on modified homodyne Michelson speckle-interferometer and
laser self-excited oscillator with fiber-optical delay line.
This paper discusses diffraction of focused laser beams in thin blood-vessels (TBV). A theoretical description of this process is given, model experiments are conducted, and TBV scattering in biological objects in vivo is analyzed. Relations, establishing connection between scattered field and differential characteristics of blood flow in vessels, diameter of which is equal or smaller than those of erythrocytes, are obtained. Analysis of theoretical model correctness is carried out. The paper also describes two modifications of a sensor for analysis of diffuse-scattering biological objects vibrations. The modifications are based on modified homodyne Michelson speckle-interferometer and laser self-excited oscillator with fiber-optical delay line.
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