Coherent Signal Beam Research Articles

Investigation of influence incoherent background illumination on the nonlinear response of a lithium niobate crystal sample at low light intensity

The total compensation of nonlinear diffraction of coherent laser beams of He-Ne laser (wavelengths 633 nm) with diameters on full width of half maximum near to 15 - 20 μm due of assistance of incoherent background have been experimentally demonstrated. Incoherent background had a shorter wavelengths (455 – 465) nm and much lower intensity compared with coherent signal beam with wavelengths 633 nm. Obtained the dependences of the refractive index change in the LiNbO3:Fe crystal under the conditions of such incoherent background.

Misalignment Effects in Laser-Induced Grating Experiments.

Laser-induced grating spectroscopy (LIGS) is an experimental method in which two pulsed laser beams and a continuous-wave laser beam have to be superimposed under well-defined angles to generate a coherent signal beam. In this Note, the possible effects of different forms of misalignment are examined. This includes the overlap of the pump lasers as well as the influence of the probe laser alignment on the temporal profile of the signal.

Nonlinear Optical Stokes Ellipsometry. 2. Experimental Demonstration

Second harmonic generation (SHG) has developed into a powerful tool for characterizing oriented thin films, surfaces, and interfaces. Furthermore, the nonlinear optical nature of wave-mixing processes typically results in the generation of a coherent signal beam with a well-defined polarization state. This coherence offers unique opportunities for the extraction of detailed molecular and surface properties from polarization analysis. In previous studies, nonlinear optical ellipsometry (NOE) has been developed as a means to retain sign and phase information between the different nonzero χ(2) tensor elements present in a given sample. However, those previous methods and related approaches for polarization analysis have all relied on the physical movement of optical elements to perform the analysis. The time required to physically move the appropriate optical elements ultimately dictates the fastest analysis time possible in a given technique. Such long acquisition times have limited NOE analyses to systems exhibiting excellent photostability and have precluded the feasibility of NOE imaging. Development of nonlinear optical Stokes ellipsometry (NOSE) was shown to address many of these problems. By increasing the repetition rate of the laser system and replacing previously slow rotating polarization optics with a rapid photoelastic modulator, the acquisition time with full polarization analysis was reduced from several hours to less than a second. This technique was validated against established NOE techniques using z-cut quartz as a reference sample and then demonstrated on a dye thin film. Additionally, an orientation analysis of the thin film was performed. These studies resulted in an order of magnitude improvement in precision relative to previous NOE techniques, while simultaneously accompanied by a reduction in acquisition time of more than four orders of magnitude.

Cognitive ability process at the molecular level

In the standard SRG formation in azo-dye containing photoactive polymers, the photoactive molecules are excited by a coherent illumination pattern. The highly reactive molecules move in a non-uniform way, inducing a mass transport from the bright regions to the neighbouring dark regions. The maximum heights of the light induced SRG correspond to light intensity minima. We show that a well-defined surface relief grating is induced in an azo-polymer film by the combination of one low power coherent laser beam with another high power incoherent and unpolarised beam. The information brought by the coherent signal beam has been transmitted to peripheral incoherent regions by the molecular self-assembling process: i.e., the organised molecules communicate non-local information about photo-induced structural organisation to the non organised neighbouring ones. They communicate by exchanging light through surface relief variations. In this way, we see that a totally incoherent beam can provide the movement which is necessary to induce a well-defined SRG. We find in this way one of the simplest systems allowing to figure out the minimal requirements to organise disordered materials into well organised structures. We verify experimentally that random motion plus information exchange lead to self-organisation. Our experiment shows that complex behaviour can be experimented using simple systems: weak coherent light can serve as a seed to create information into a polymer film in such a way that molecules powered by incoherent light will build and transmit well defined complex structures.

Using multiple mutually incoherent fiber lasers to pump a coherent signal beam in an optical parametric oscillator

We use three-dimensional numerical modeling to show that multiple mutually incoherent lasers can be used to pump a single resonated coherent signal beam in an optical parametric oscillator (OPO). We consider as an example an OPO based on periodically poled lithium niobate pumped noncollinearly by CW Yb-doped fiber lasers. Our modeling, which neglects thermal lensing, is appropriate when the average power is kept low by chopping or pulsing the pump beams at a low duty cycle. We find that a signal beam can be produced with near-diffraction-limited beam quality and power considerably exceeding that of a single pump laser.

Circular Dichroism Spectroscopy by Four-Wave Mixing Using Polarization Grating-Induced Thermal Gratings

A novel four-wave mixing technique for the measurement of circular dichroism in optically active liquid samples is demonstrated. When two cross-polarized continuous-wave laser beams are crossed at a small angle in a circular dichroic liquid, a weak thermal grating is produced with a phase depending on the sign of the circular dichroism. The polarization of one of the beams can be modified to allow coherent interference with an intensity grating-induced thermal grating. A probe beam scattering from the composite grating results in a coherent signal beam that reveals the sign and the magnitude of analyte circular dichroism. The use of this technique to optimize the signal-to-noise ratio in the presence of scattered light and laser noise is discussed.

Detection of atomic hydrogen by two-color laser-induced grating spectroscopy

We report a novel multiphoton scheme for the detection of atomic hydrogen. Interference of two laser beams near 243 nm crossed at a small angle induces a spatially modulated two-photon excitation (i.e., a grating of excited atoms), which diffracts a third laser beam tuned to either 486 or 656 nm to generate a coherent signal beam. We demonstrate the technique by making H-atom concentration measurements that compare quantitatively with those made using laser-induced fluorescence in low-pressure H(2)/O(2) flames.

Coherent signal beam amplification in two-wave mixing experiments with photorefractive Bi 12SiO 20 crystals

The amplification of the input signal beam in two-wave mixing experiments with photorefractive Bi 12SiO 20 crystals is achieved when an additional phase shift is established between the photoinduced index modulation (phase volume hologram) and the incident fringe pattern. This stationary phase shift is introduced by either moving the crystal or the interference fringes at a constant speed. The transferred intensity is measured versus the applied electric field, fringe spacing and crystal velocity. The crystallographic orientation and the relative displacement with respect to the applied electric field polarity determine the amplitude of the energy transfer. For the first time in this crystal, signal beam amplification is reached for an applied field E 0 > 8 kV cm −1 and a crystal or fringe displacement speed around 5 μm s −1 at the green line ( λ = 514 nm) of an argon laser.