A double longitudinal mode He-Ne laser with frequency stabilization is proposed. Compared with general methods, such
as Lamb dip, Zeeman splitting and molecule saturation absorption method, this design has some advantages, such as no
piezocrystal or magnetic field, a short frequency-stabilized time, lower cost, and higher frequency stability and
reproducibility. The metal wire is uniformly wrapped on the discharge tube of the laser. When the metal wire is heated
up, the resonant cavity changes with the temperature field around the discharge tube to make the frequency of the laser
to be tuned. The polarizations of the two longitudinal modes from the laser must be orthogonal. The parallelly polarized
light and the vertically polarized light compete with each other, i. e., the parallelly polarized light generates a larger
output power, while, the vertically polarized light correspondingly generates a smaller one, but an equal value is found at
the reference frequencies by automatically adjusting the length of the resonant cavity, due to change of the temperature
in the discharge tube. Consequently the frequencies of the laser are stabilized. In my experiment, an intracavity He-Ne
laser whose length of the resonant cavity is larger than 50mm and smaller than 300mm is selected for the double
longitudinal-mode laser. Influence factors of frequency stability of this laser is only change of the length of the resonant
cavity. The laser includes three stages: mode hopping, transition stage, and modes stability from startup to laser stability.
When this laser is in modes stability, the waveform of heating metal wire is observed to a pulse whose duty is almost
50%, and thermal balances of the resonant cavity mainly rely on discharge tube.
The capillary electrophoresis (CE) with laser induced fluorescence detection (LIFD) system was founded according to confocal theory. The 3-D adjustment of the exciting and collecting optical paths was realized. The photomultiplier tube (PMT) is used and the signals are processed by a software designed by ourselves. Under computer control, high voltage is applied to appropriate reservoirs and to inject and separate DNA samples respectively. Two fluorescent dyes Thiazole Orange (TO) and SYBR Green I were contrasted. With both of the dyes, high signals-to-noise images were obtained with the CE-LIFD system. The single-bases can be distinguished from the electrophoretogram and high resolution of DNA sample separation was obtained.
General wavemeters based on Michelson interferometer only have a moving arm, which cann't more multiply optical
paths' differences, and is unable to avoid dispersion from a beamsplitter. Commonly, the moving mirror driven by a
direct current motor and a ball screw have some disadvantage, such as heavy weight, unstable motion. In the paper, a
better optical layout, and configuration and a driving method of moving mirrors are proposed. A newly optical paths
layout of a wavemeter based on Michelson Interferometer is present, including two moving mirrors for forming optical
paths' differences, a beamsplitter for splitting a light into a transmitted light and a reflected light, two reflectors, and a
reference laser. It has two moving arms and can eliminate dispersion from the beamsplitter. According to Doppler effect,
how to form the interference fringes in the photodiodes is analyzed and formulated. The Doppler effect appears with
motion of the moving mirrors. Consequently, alternately dark and bright interference fringes are generated, then received
and converted into the electronic signals by the photodiodes. It is concluded that the electronic signals involves the
wavelength of a light and the velocity of the moving mirror by investigating the Doppler effect. The structure of the
moving mirrors is clarified. The moving mirrors are made of the two pyramid prisms which are placed symmetrically on
the driving motor. A controlling system for keeping the moving mirrors in constant velocity is designed. In order to make
frequencies of electronic signals from interference fringes stable, the moving mirrors must move in a uniform speed. The
voice coil motor (VCM) drags the moving mirror to and fro. VCM in uniform motion is realized by an
optical-mechanical-electrical closed-loop feedback system. The Doppler frequency difference of the reference laser is the
standard of the system. The PID controller comprising parallel proportional-integral-differential operational circuit
regulates the velocity of VCM.
A wave-meter based on Michelson interferometer consists of a reference and a measurement channel. The voice-coiled
motor using PID means can realize to move in stable motion. The wavelength of a measurement laser can be obtained by
counting interference fringes of reference and measurement laser. Reference laser with frequency stabilization creates a
cosine interferogram signal whose frequency is proportional to velocity of the moving motor. The interferogram of the
reference laser is converted to pulse signal, and it is subdivided into 16 times. In order to get optical spectrum, the analog
signal of measurement channel should be collected. The
Analog-to-Digital Converter (ADC) for measurement channel is
triggered by the 16-times pulse signal of reference laser. So the sampling rate is constant only depending on frequency of
reference laser and irrelative to the motor velocity. This means the sampling rate of measurement channel signals is on a
uniform time-scale. The optical spectrum of measurement channel can be processed with Fast Fourier Transform (FFT)
method by DSP and displayed on LCD.
Laser-induced fluorescence (LIF) is widely used in biological detection system in characteristic of high sensitivity and selectivity, especially for microarray biochip readout and capillary electrophoresis detection. In these systems, fluorescence separation from background noise is necessary. In this paper, two methods of fluorescence separation were investigated. One adopts a total reflection mirror with a hole at the center; the other uses a dichroic mirror. For dichroic mirror system, fluorescence could transmit through the filter or be reflected by it. Signal to noise ratio depends on dichroic mirror transmitting spectra and reflecting spectra. For center hole mirror system, partial fluorescence loses during propagating through the center hole directly. Detected fluorescence is the part that reflected by the mirror outside the center hole. Size of the hole in the mirror must be changed in different systems. Performance of system with an f-theta lens as scanning lens for laser focus and fluorescence collecting was simulated. Collinear systems with above-mentioned two methods were set up and compared. Simulated results were verified by experiments.
Detection of fluorescent microarray slides can be divided into two categories: one is confocal scanning detection in which a photomultiplier tube is used as fluorescence sensor, and the other is flood illumination of the entire microarray slides and uses a Peltier cooled CCD as fluorescence sensor. CCD can afford high quantum efficiency, simultaneous illumination and detection of multiple pixels, and easy mechanical design, but its dynamic range and sensitivity are not as good as those of photomultiplier tube. At the same time, the Peltier cooled CCD camera is much more expensive than a high performance photomultiplier tube. Confocal scanning provides high dynamic range, good sensitivity and high signal-to-noise ratio, but the system design is difficult when considering rapid scanning speed, high resolution and large numerical aperture. There are three typical confocal scanning apparatuses which are mechanical scanning, optical scanning and optical-mechanical scanning. Their scanning mechanism, advantage and disadvantage are analyzed respectively. With the former understanding, a new optical-mechanical scanning apparatus is described in detail. It employs two lasers to excite the Cy3 and Cy5 fluorophores on the microarray slides. The emitted fluorescent signal is detected using a photomultiplier tube sequentially. One dimension scanning of the slides is performed by a telecentric f-θ objective with a moving coil optical scanner; the other dimension is scanned through a stepping motor driving a precision guidance. This apparatus is low-noise, economical and fast in scanning speed.
A wavemeter based on Michelson interferometer accurately measure static wavelength of a tunable laser. Its operation principle is formulated in details. Double longitudinal-mode He-Ne laser with frequency stabilization is used as the reference optical source of the wavemeter. Voice-coil motor using PID means can realize to move in uniform motion. Phase-locked loop circuit including NE564 and 74LS193 is used to enhance resolution of the wavemeter. Data processing is carried out by the counter unit including two 8254 programmable timer, a MCU, a LCD. The test shows that its measurement accuracy is 1×10<sup>-6</sup> and is higher than those of other wavemeters such as Fizeau interference and Fabry-Perot wavemeter.
A novel confocal microarray scanner was introduced, which employed a 532nm laser and a 635nm laser to excite Cy3 and Cy5 fluorophores respectively. The fluorescent signal was detected using a photomultiplier tube (PMT) sequentially. One dimension scanning of the microarray slide was performed by a telecentric objective with a moving coil optical scanner; the other dimension was scanned by a stepping motor driving the precise guidance. Experimental results show that scanning resolution of the presented microarray scanner can reach 5 microns, its dynamic range is near 4 orders of magnitude and the limit of detection is 1.12 about fluors per square micron. The cross-talk error is eliminated almost completely by its sequential scanning mode.