Laser processing is a technique based on the interaction between a laser and the substance for cutting, drilling, cleaning, welding, and other operations on metallic or non-metallic materials. It is widely used in some important fields of the national economy such as automobiles, microelectronics, electrical appliances, aviation, metallurgy, medical treatment, and machinery manufacturing. In the process of high-powered laser processing, a large amount of plasma will be generated and there will be the obvious inverse Bremsstrahlung absorption (IBA) near the plasma plume. The effect of laser processing will be significantly deteriorated due to the absorption of laser photons and changes in light intensity distribution. Besides, laser-induced plasma is produced during the interaction between a high-powered laser and materials. Also, it has the very important value in the research of analyzing the high-powered laser processing. To fully understand the laserinduced plasma, this paper uses the Hilbert procedure to numerically investigate the plasma generated in the laser processing. The method firstly acquires the images corresponding to the fringes of a Mach–Zehnder interferometer by using the detection after a probe laser beam passing through the plasma plume. Then, a series of operations such as the spectrum shift, unwrap, and Abel inverse transformation are performed after a fast Fourier transform (FFT). Finally, the density distribution of plasma can be calculated. This methodology provides a new algorithm for the research of laserinduced plasma, and it also valuable for the understanding the high-powered laser processing process.
Laser processing plays a key role in treating a lot of materials. The mechanism of laser stealth dicing (SD) is based on irradiation of a laser beam which is focused inside the brittle material. The laser beam scans along the predetermined path, so that the characteristics of the interior brittle material can be changed, the stress layer can be therefore formed. Finally, an external force is applied to separate the brittle material. Since only the limited interior region of a wafer is processed by the laser irradiation, the damages and debris contaminants can be avoided during the SD process. SD has the advantages of a high speed for thinner wafers without any chipping, the smooth section without dust and slag, and completely dry process, which has been widely used in large scale integrated circuits and microelectronic manufacturing systems. However, further studies on the simulation analyze and parameter optimization have kept to be rear for SD so far. In this study, an approach named as constrained interpolation profile (CIP) was adopted, which has the advantages of compactness, stability, and low dissipation in computational fluid dynamics compared with other simulation procedures. We have finished a theoretical simulation to obtain the physical features of the temperature, pressure, density of the silicon substrate at different focal depth where a nanosecond pulsed laser is irradiated, then we found a suitable focal depth with a good dicing quality by analyzing these physical features.
We report the experimental observation of the squeezing effect in a graphene mechanical resonator due to an optical actuation force. We fabricate a circular suspended graphene mechanical resonator and measure two quadrature components of the mechanical mode via a phase-locked demodulation technique. By analyzing the correlated distribution of the two components, we find a squeezing effect when increasing the actuation power. We also observe singularity phenomena right at the cut-off frequency, which might be related to the nonlinear effect. Further study is needed to fully understand these phenomena. Our results might find new applications in the fields of sensing and mechanical information processing.
We present an optically assisted frequency modulation (FM) demodulation method to characterize the resonance mode of a graphene resonator. The intensity of a laser at 795 nm is FM demodulated to actuate the graphene resonator, where the carrier frequency is approximately around the resonant frequency of the resonator and the FM deviation is set by the reference signal from the lock-in amplifier. A continuous laser at 633 nm is directed in the sample to perform the optical interferometry technique and the resonance mode of the graphene resonator is extracted through the lock-in amplifier. In this way, resonance modes at high frequencies much larger than the bandwidth of the lock-in amplifier could be detected within a high accuracy. In our configuration, we have obtained the graphene resonance mode at frequencies around 10 MHz with a 100 kHz bandwidth lock-in amplifier.
Laser processing plays a key role in treating a lot of materials. The visible nanosecond laser processing based on a tripartite-interaction system has been proved to be an effective method of processing materials with high optical transparency, which has the advantages of low cost, high efficiency, and simplicity over the direct processing by using a femtosecond laser. However, further studies on the theoretical mechanism and parameter optimization keep to be rear for the hybrid tripartite-interaction laser processing. In this study, we have carried out the confirmatory experiment and numerical simulation of laser processing with a tripartite-interaction system, which includes a visible nanosecond laser (19 ns@532 nm), a piece of transparent glass, and a copper foil. The experiment indicates that drilled holes can be obtained on the glass sheets by using the visible nanosecond laser. The numerical results, which have been obtained by an approach named as constrained interpolation profile, reveal that the processing mechanism is based on the heat conduction, generation of stress and ablation between the glass and the copper foil. Our results could to be useful for the development of visible nanosecond laser processing in industrial applications.
It has always been difficult to process a metal film with high reflectivity in the field of manufacture, industry, medicine, and military, etc. Since much of the laser energy can be reflected especially when the reflectance of the target film surface is high, it is hard to process such a metallic film by laser radiation as the energy absorbed by the film material is very little. In this paper, we used a nanosecond pulsed laser to scribe some patterns on a smooth titanium (Ti) film, and investigated the surface morphology of a Ti film ablated by different laser spot sizes and laser energy. In our experiments, it has been found that the Ti film can be efficiently processed although the surface reflectance of the Ti film is about 57% at the wavelength of 532 nm. We also see that the processing range of the Ti film will decrease when the diameter of a laser beam increases. The experimental results show that the ablated status of the surface of a Ti film for a just-focus beam is much better than that for a defocus beam under the same laser power. Furthermore, the higher the laser power, the larger the processed area. By using the optimal parameters we obtained, we also produced some hole matrices and line patterns on a glass-based Ti film by employing a short pulsed laser. The processed samples were observed with a reflecting microscope and a transmitting microscope, respectively. Our research results can play an important role in the selection of laser parameters for laser processing of some materials with a high reflectivity.
In this work, we propose and demonstrate the frequency-bin entangled two-photon state via cascaded second harmonic generation and spontaneous parametric down conversion (SHG-SPDC) processes in a single piece of PPLN waveguide. Our scheme is based on all fiber-pigtailed components at 1.5 μm telecom band. Two frequency-bins at 1531.34 nm and 1548.91 nm with bandwidth of 1.04 nm are prepared to be entangled. The frequency entanglement property of our generated two-photon states is measured by spatial two-photon quantum beating. A two-photon beating curve with a visibility of 87.92±0.47% is obtained, showing a good property of frequency-bin entanglement.
Transmission properties of transverse magnetic light through periodic sub-wavelength slit apertures on a metallic film, behind which is another planar metallic film, are studied by finite-difference-time-domain method with constant periodicity and slit width. The result shows that the transmitted energy is strongly correlated to both the thickness of the metallic grating and the distance between such two films at a specific wavelength. The thickness of the grating acts as a filter that allows specific wavelengths to go through the slits, while the distance of dual metallic film dominantly determines a constructive or destructive interference between the transmitted light through the slits and the reflected wave from the back film. Besides, a strong vibration in the transmission spectrum as a function of the grating thickness is interestingly observed, which can be interpreted by the resonance of the surface plasmons of the front and the back metallic films.
In this study, we propose and experimentally demonstrate a picosecond pulse laser at 850 nm. To generate picosecond laser pulse, we operate a vertical cavity surface emitting laser under a gain-switched pulsed mode, which is realized by driving it with our home-made drive circuit based on field programmable gate array and radio frequency devices. The obtained laser pulses are with the pulse width of less than 675 ps, and with repetition rate from single shot to megahertz. On the other hand, based on our gain-switched pulsed laser, we design and realize a cost-effective optical time domain reflectometry prototype equipment with photon counting technology for monitoring the healthy condition of aeronautical fiber. Our prototype equipment achieves a spatial resolution of less than 9 cm, and a dynamic range of around 18 dB above the noise floor. Such prototype equipment has already been employed to monitor an optical cable with 32 fiber channels on plane.
It is very difficult to detect a moving object then estimate its speed with optical remote sensing images. In this paper, we propose a novel method that moving object detection and velocity measurement using only single multispectral image. Slight chromatic aberration among channels of the multispectral image is applied to detect the motional object by fames difference, and improve the accuracy of velocity measurement using pixel unmixing. Finally, experiments are carried out on satellite multispectral images from Google Earth and clearly show the comparative effectiveness of the proposed method with respect to traditional methods.
Single photons are essential resource for quantum communication and quantum information processing, which can carry
quantum information to distant locations. A promising scheme for single photon generation is the heralded single photon
source (HSPS), which is based on the generation of correlated photon pairs (CPPs). Utilizing the quantum correlation
property of the CPPs, one photon of the CPP is detected providing an electrical signal to herald the other photon as a
single photon output. Recently, 1.5 μm CPP generation through spontaneously four wave-mixing (SFWM) in fiber has
focused much attention, which provides a practical way to realize 1.5 μm fiber-based HSPS. The quality of a HSPS is
described by the preparation efficiency and g(2)(0). In the fiber-based HSPS, the preparation efficiency is determined by
the loss of the filtering and splitting system and the noise photons generated by spontaneously Raman scattering (SpRS).
Considering the impact of the SpRS can be reduced by cooling the fiber and optimizing the frequency detuning of
filtering and splitting system, the loss of the filtering and splitting system may give a theoretical up-limit of the
preparation efficiency. In this paper, using commercial fiber components, we realize a high quality HSPS based on
cooled fiber with a preparation efficiency of 80% under a g(2)(0) of0.06, showing its great potential in the application of
quantum information technology.
This paper introduces our recent works on quantum light sources based on third-order nonlinear waveguides. Based on
the spontaneous four wave mixing (SFWM) effect in optical fibers, we realized high quality correlated photon pair
generation. A fiber based heralded single photon source (HSPS) with a preparation efficiency of ~80% under a g2(0) of
0.06 was realized based on it. On the other hand, we demonstrated that the vector scattering processes of the SFWM can
be suppressed effectively by the polarization walk off effect in the polarization maintaining fibers (PMFs), by which
polarization entangled photon pair generation was realized in a piece of PMF experimentally. We also realized correlated
photon pair generation in silicon wire waveguides fabricated by ourselves, demonstrating its low noise performance.
These works shows that SFWM in third-order nonlinear waveguides provides a promising way to realize practical
quantum light sources compatible with today’s engineering technologies.
Single photon sources (SPSs) play important roles in quantum communication and quantum information processing.
Spontaneous four wave mixing (SFWM) in optical fibers provides a promising way to realize practical heralded single
photon sources (HSPSs), since it is compatible with current techniques of optical communications. In this paper, two
independent HSPSs at 1.5μm band are realized in one polarization maintaining dispersion shifted fiber (PM-DSF)
utilizing its large birefringence. When pulsed pump light passes through an optical fiber, two kinds of SFWM will take
place simultaneously. One is scalar processes, in which two annihilated pump photons and generated photon pair are all
polarized along the same fiber polarization axis. The other is vector processes, in which two annihilated pump photons
are polarized along different fiber polarization axes, either for the two photons of the generated pair. In the PM-DSF, the
large birefringence generates obvious walk-off effect on the two pump polarization components, which leads to an
effective suppression of the vector processes. Hence, by proper pump polarization, correlated photon pairs (CPPs) with
different polarization directions can be generated independently by the two scalar processes, which can be used to realize
two independent HSPSs. The indistinguishability of the heralded photons generated by the two independent sources is
demonstrated by an experiment of Hong-Ou-Mandel (HOM) interference. Using a fiber coupler as the beam splitter, a
visibility of HOM dip of 76% is achieved, showing their potential on quantum information.
Spontaneous four-wave mixing (SFWM) in optical fibers is an important way to generate correlated/entangled photon pairs. When the pulsed pump light passes through the optical fiber, two kinds of SFWM will take place simultaneously. One is scalar scattering processes, in which two annihilated pump photons and generated photon pair are all polarized along the same fiber polarization axis. The other is vector scattering processes, in which two annihilated pump photons are polarized along different fiber polarization axes, either to the two photons of the generated pair. If the fiber has large group birefringence, the intensity of vector scattering processes will be suppressed at the phase matching frequencies of the scalar scattering processes. On the other hand, the walk-off effect of the pump pulse components polarized along the two fiber polarization axes also suppresses the vector scattering processes. Hence, by proper pump polarization and signal/idle frequency selection, photon pairs can be generated only by the two independent scalar scattering processes in optical fibers with birefringence, which provide a simple way to realize polarization entangled photon pair generation. In this paper, related experiments based on the high nonlinearity microstructure fiber (HN-MSF) with group birefringence and polarization maintained dispersion shifted fiber (PM-DSF) are introduced, showing their potential on developing practical quantum light sources.
In this paper, correlated photon pairs at 1.55μm are generated in a silicon wire waveguidewith a length of 1.6 mm. The
ratio between coincidence count rate and accidental coincidence count rate under room temperature is 19, showing the
property of low Raman noise and strong quantum correlation. Moreover, the experiment shows that photon pair
generation isstrongly dependent on pump polarization direction. Using quantum perturbation theory, we analyze the
contribution of scalar and vectorspontaneous four-wave mixing processes to the generated photon pairs. Due tohigh
nonlinear coefficient and high coupling efficiency, the generation rate in quasi-TE mode is much larger than that in
quasi-TM mode.The combination of calculated photon pair generation rates through scalar and vector spontaneous four-wave
mixing processes agrees well with the experimental result.
We realize the correlated photon pair generation at 1.5μm by spontaneous four-wave mixing in high nonlinear
microstructure fibre with a length of 25m, showing that high nonlinear microstructure fibres have great potential in
bright, high efficiency and compact sources of correlated photon pairs at 1.5μm.
An optimized 1.66μm Q-switched fiber laser is demonstrated for Raman-based distributed temperature sensor. In a ring
type Q-switched erbium-doped fiber laser, 850m TrueWave fiber is introduced to serve as both delay line fiber and
Raman gain medium so that in addition to the wavelength shifted to 1.6μm, the pulse duration and the buildup time can
be relatively extended. By properly controlling the fall edge of the acousto-optic switch, the pulse duration of 30-74ns
with peak power of 1.3-3.3W is achieved. Based on this light source, a 2.4km Raman-based distributed temperature
sensor is obtained with temperature resolution of 1°C and spatial resolution of 8m.
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