The angular velocity of a vaterite microsphere spinning in the optical trap is measured using rotational Doppler effect. The perfectly spherical vaterite microspheres are synthesized via coprecipitation in the presence of silk fibroin nanospheres. When trapped by a circularly polarized beam, the vaterite microsphere is uniformly rotated in the trap center. The probe beams containing two Laguerre–Gaussian beams of opposite topological charge l = ± 7, l = ± 8, and l = ± 9 are illuminated on the spinning vaterite. By analyzing the backscattered light, a frequency shift is observed scaling with the rotation rate of the vaterite microsphere. The multiplicative enhancement of the frequency shift proportion to the topological charge has greatly improved the measurement precision. The reliability and practicability of this approach are verified through varying the topological charge of the probe beam and the trapping laser power. In consideration of the excellent measurement precision of the rotation frequency, this technique might be generally applicable in studying the torsional properties of micro-objects.
Optical trap has become a powerful tool of biology and physics, since it has some useful functions such as optical rotator, optical spanner and optical binding. We present the translational motions in the transverse plane of a 4.4μm-diameter vaterite particle which is optically trapped in low pressures utilizing the Monte-Carlo method. We find that the air pressure around the microparticle plays an important part in the determination of dynamics of the trapped particle. According to the energy equipartition theorem, the position fluctuations of the optically trapped particle satisfy Maxwell-Bolzmann distributions. We present the features of particles’ displacements and velocities changing with air pressures in detail, and find that the modulation of the trap stiffness makes a higher position variance. The mechanical quality factor Q larger than 10 induces a high peak of power spectral density. Our research presents a powerful tool towards further discovery of dynamical characteristics of optically trapped Brownian particles in low air pressures.
Optical traps have been widely used in a large variety of applications ranging from biophysics to nano-sciences. More than one microscopic object can be captured in an optical trap. In the practical application, it is always necessary to distinguish and control the number of captured objects in the optical trap. In this paper, a novel method has been presented to distinguish the number of trapped microspheres by measuring the intensity of back signal. Clear descent of the back signal has been observed when a microsphere is captured in the center of optical trap. The relative coupling efficiency of back signal decreases as the number of captured microspheres increases both in experiment and theory. This method contributes to miniaturization and integration of applied systems due to getting rid of the imaging system, and is generally applicable to the area of nanoparticle trapping.
Controllable rotation of the trapped microscopic objects has traditionally been thought of one of the most valuable optical manipulation techniques. The controllable rotation of a microsphere chain was achieved by the dual-beam fiber-optic trap with transverse offset. The experimental device was made up of a PDMS chip housing two counter-propagating fibers across a microfluidic flow channel. Each fiber was coupled with different laser diode source to avoid the generation of coherent interference, both operating at a wavelength of 980 nm. Each fiber was attached to a translation stage to adjust the transverse offset distance. The polystyrene microspheres with diameter of 10 μm were chosen as the trapped particles. The microfluidic flow channel of the device was flushed with the polystyrene microspheres solution by the mechanical fluid pump. At the beginning, the two fibers were strictly aligned to each other. Five microspheres were captured as a chain parallel to the axis of the fibers. When introducing a transverse offset to the counter-propagating fibers by adjusting the translation stages, the microsphere chain was observed to rotating in the trap center. When the offset distance was set as 9 μm, the rotation period is approximately 1.2s. A comprehensive analysis has been presented of the characteristics of the rotation. The functionality of rotated chain could be extended to applications requiring microfluidic mixing or to improving the reaction speed in a localized environment, and is generally applicable to biological and medical research.
We build numerical models of dual-waveguide trap with rough and tilted endfaces using both the finite element method. The optical field distribution of waveguide trapping house with rough and tilt endfaces is simulated and analyzed. The results shows that rough endfaces cause the incident beam scattered and the tilted endfaces make incident beam refracted. According to optical field distribution, axial and transversal optical trapping forces are calculated. When endfaces roughness increase, both the axial and transversal trapping forces decrease, meaning trapping depth decreased. The transversal equilibrium positions move around unpredictably, off center. The stiffness and width of optical trap change little. When endfaces tilt angles increase, both the axial and transversal trapping forces decrease, meaning trapping depth decreased. The transversal equilibrium positions move along minus transversal axis. It is no obvious change in stiffness and width of optical trap.
Design a chip for flexible multifunction optical micro-manipulation based on elastomeric materials-PDMS. We realized the different motion types of microspheres, including stably capture, spiral motion and orbital rotation, by adjusting the input voltage of piezoceramics designed in PDMS Chip. Compared to conventional techniques, this PDMS chip based method does not require special optical properties of the microspheres to be manipulated. In addition, the technique was convenient and precise for dynamical adjustment of motion types without external influences. From these results, we verify that this multifunctional optical micro-manipulation technique of PDMS elastomeric materials can find potential applications for optical manipulation, including cost-effective on-chip diagnostics, optical sorting and optical binding, etc.
A new method and its principle are presented for measuring the each component gas pressures in Rubidium (Rb) by the analysis of absorption spectral profile. The experiment system is set up to obtain Rb absorption spectra. And then each component gas pressures in atom vapor cell is estimated. First, the relationships between transmittance of probe light, atom density and absorption cross section are introduced, and the factors which influence the absorption spectral profile and methods to measure gas pressures are given. Second, the frequency-dependence curves of transmittance and the absorption spectra are obtained through tuning the laser frequency through the Rb D1 transition. Finally, the gas pressures of Rb, N2 and He are achieved, through fitting absorption spectral profile referring to half-width and minimum transmittance value of absorption spectra. The experiment results show that gas pressures in Rb atom vapor cell can be accurately measured by absorption spectrometric methods, which will be helpful for the following study of atom vapor cell. The gas pressures of N2 and He measured by the experiments are well matched with design values. The Rb gas pressure is 30%~50% less than the saturated vapor pressure and the suppression may be due to the adsorption of the cell surfaces coated with octadecyltrichlorosilane (OTS) film.
Although a sinusoidal bias method is introduced to avoid working in the death band for a majority of time, the mechanical dithered ring laser gyro (RLG) still encounters information loss when crossing the zero rate point. A novel lock-in error correction method is proposed which can pick up the lost information and remove the random walk error radically. The lost information at the zero rate crossing can be expressed by such parameters as the phase and phase acceleration of a beat frequency signal. These parameters are sampled every time the phase crosses the zero rate point, and two correction trains are formed. Using the minimum variance method, the correction trains are properly scaled to compensate the output of RLG. Experiment shows that even in a bad working condition, this method can achieve satisfactory results.
We present a compact parallel optical correlator (CPOC) based on joint transform correlation manipulation that can be used for various applications. Two innovative approaches are adopted to make
the whole system more compact with a volume of 17×5×3cm<sup>3</sup>. Two demo systems, i.e., guidance of guided missile and on-orbit rendezvous and docking have been established. The experimental result showed that CPOC system currently operates at approximately 300 Hz and can accurately determine the positioning information when a 1024×768 pixel spatial light modulator (SLM) is employed.
Optical correlation manipulation presents great potential in future machine vision systems, which
can be used in a large variety of application fields. Nevertheless, how to extract the tracking signal
effectively and fast for the feedback system is still an open question. In the present paper, we present
novel target recognition and tracking approach in optical correlator system. The tracking signal is
extracted by using genetic algorithm. In this way, a photoelectric detector with short response time
can be employed in the system instead of digital camera, the tracking signal can be extracted by
iteratively evaluating the signal detected by the photoelectric detector until converge. Numerical
simulations were performed to validate the validity of the approach presented in the manuscript.