Distribution Of Photon Density Research Articles

Optimization of diode-pumped doubly Q-switched mode-locked laser with electro-optic modulator and GaAs saturable absorber

In this paper, by considering Gaussian distribution of the intracavity photon density, the coupled rate equations for the diode-pumped doubly Q-switched and mode-locked (QML) laser with EO modulator and GaAs saturable absorber are presented. These equations are numerically solved, and the key parameters of an optimally coupled doubly QML laser have been determined. These key parameters include parameters of the gain medium, the small-signal transmission of saturable absorber as well as the modulation frequency of EO modulator, which can maximize the pulse energy of singly Q-switched envelope. A diode-pumped doubly QML Nd:Lu0.15Y0.85VO4 laser with EO modulator and GaAs saturable absorber is constructed, the experimental results have verified numerical simulation applicable.

Theoretical and experimental study of single mode-locking pulse generation with low repetition rate in a dual-loss-modulated QML YVO4/Nd:YVO4 laser with EO and GaAs

Using electro-optic (EO) modulator and GaAs saturable absorber, a diode-pumped doubly Q-switched and mode-locked (QML) YVO4/Nd:YVO4 laser at 1.06 μm is realized. The experimental results show that the number of the mode-locking pulses underneath the Q-switched envelope decreased with increasing pump power. With an output coupling of 6.5 %, the single mode-locking pulse underneath the Q-switched envelope with 1 kHz repetition rate was obtained when the pump power exceeded 4.65 W. At a pump power of 8.25 W for an output coupling of 10 %, a stable mode-locking pulse train at a repetition rate of 1 kHz was achieved with pulse energy as high as 582 μJ and pulse duration of about 580 ps, corresponding to a peak power of 1 MW. Using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled rate equations for diode-pumped doubly QML YVO4/Nd:YVO4 laser are given and the numerical solutions of the equations are basically in accordance with the experimental results.

High Performance Asymmetric Three Corrugation-Pitch-Modulated DFB Lasers Suitable for Stable Single Longitudinal Mode Operation

This paper presents an optimized asymmetric three corrugation-pitch-modulated DFB laser (3CPM-DFB) with extremely high mode selectivity(△αL= 0.97) and low flatness(F = 0.009), which are two key parameters to indicate the laser’s single longitudinal mode(SLM) performance. In threshold analysis, the optimization process based on transfer matrix method is demonstrated to maximize △αL and minimize F simultaneously. In the above-threshold regime, the evolutions of △αL and longitudinal distribution of photon density with injection current are evaluated. More importantly, nanoimprint lithography which was proved an efficient way to fabricate DFB gratings can provide completely same simple fabrication procedure for both 3CPM grating and conventional uniform grating. So the big practical value of 3CPM-DFB can be expected because of its advanced performance and easy manufacturability.

Passively Q-switched and mode-locked laser performance of the mixed crystals with GaAs saturable absorber

By using the mixed crystals Nd:Lu0.33Y0.37Gd0.3VO4, Nd:Lu0.5Y0.5VO4 and Nd:Lu0.15Y0.85VO4 as laser media, the LD pumped passively Q-switched and mode-locked (QML) lasers with GaAs saturable absorber are realized. The average output power, the pulse width and the repetition rates have been measured and compared. The experimental results show that the passively Q-switched and mode-locked Nd:Lu0.33Y0.37Gd0.3VO4 laser can generate shorter pulse width with higher peak power due to broader fluorescence line-width. By considering the Gaussian distribution of the intracavity photon density and the influence of the continuous pump rate, the coupled rate equations for QML lasers with GaAs saturable absorber are given. The theoretical solutions of the equations are in agreement with the experimental results.

Radiance detection of non-scattering inclusions in turbid media

Detection of non-scattering domains (voids) is an area of active research in biomedical optics. To avoid complexities of image reconstruction algorithms and requirements of a priori knowledge of void locations inherent to diffuse optical tomography (DOT), it would be useful to establish specific experimental signatures of voids that would help identify and detect them by other means. To address this, we present a radiance-based spectro-angular mapping approach that identifies void locations in the angular domain and establishes their spectral features. Using water-filled capillaries in scattering Intralipid as a test platform, we demonstrate perturbations in the directional photon density distribution produced by individual voids.

Single mode-locking pulse generation underneath the Q-switched envelope of the doubly QML green laser with EO and GaAs

By using electro-optic (EO) modulator and GaAs saturable absorber, a diode-pumped doubly Q-switched and mode-locked (QML) YVO4/NdYVO4/KTP laser at 0.53μm is realized. The experimental results show both the pulse width of the Q-switched envelope and the number of the mode-locking pulse underneath the Q-switched envelope decrease with increasing pump power. When the pump power exceeds 7.09W and the repetition rate of EO is 1kHz, a single mode-locking pulse underneath the Q-switched envelope can be generated. At the pump power of 8.76W, the pulse width of the single mode-locking pulse is evaluated to be sub-nanosecond 700ps and the corresponding peak power is 150kW. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped doubly QML YVO4/Nd:YVO4/KTP green laser are given and the numerical solutions of the equations are basically in accordance with the experimental results.

Numerical Estimations of Carrier Generation–Recombination Processes and the Photon Recycling Effect in HgCdTe Heterostructure Photodiodes

An enhanced computer program has been applied to explain in detail the photon recycling effect which drastically limits the influence of radiative recombination on the performance of p-on-n HgCdTe heterostructure photodiodes. The computer program is based on a solution of the carrier transport equations, as well as the photon transport equations for semiconductor heterostructures. We distinguish photons in two energy ranges according to p+ and n region with unequal band gaps. As a result, both the distribution of thermal carrier generation and recombination rates and spatial photon density distribution in photodiode structures have been obtained. The general conclusion, similar to our earlier work concerning 3-μm n-on-p HgCdTe heterostructure photodiodes, confirms the previous assertion by Humphreys that radiative recombination does not limit HgCdTe photodiode performance.

HiSCORE: A new detector for astroparticle and particle physics beyond 10 TeV

The new large-area (100km2) wide-angle (0.9sr) air Cherenkov detector HiSCORE (Hundred ⁎ i Square-km Cosmic ORigin Explorer) aims at the exploration of the cosmic ray and γ-ray sky (accelerator sky) in the so far poorly covered energy range from 10TeV to 1EeV.The main motivation for observations in this energy regime is to solve the origin of Galactic cosmic rays. Other questions of astroparticle and particle physics can be addressed in this energy regime. Furthermore, new physics questions might arise by opening the last remaining observation window of γ-ray astronomy (TeV/PeV).HiSCORE is based on non-imaging Cherenkov light-front sampling with sensitive large-area detector modules of the order of 0.5m2. Sampling the lateral photon density and arrival-time distribution allows the reconstruction of the direction, the energy and the type (mainly via the shower depth) of the primary particle.

The Second‐Order Born Approximation in Diffuse Optical Tomography

Diffuse optical tomography is used to find the optical parameters of a turbid medium with infrared red light. The problem is mathematically formulated as a nonlinear problem to find the solution for the diffusion operator mapping the optical coefficients to the photon density distribution on the boundary of the region of interest, which is also represented by the Born expansion with respect to the unperturbed photon densities and perturbed optical coefficients. We suggest a new method of finding the solution by using the second‐order Born approximation of the operator. The error analysis for the suggested method based on the second‐order Born approximation is presented and compared with the conventional linearized method based on the first‐order Born approximation. The suggested method has better convergence order than the linearized method, and this is verified in the numerical implementation.

The influence of thermal lens effect on pulse repetition rate in diode-pumped passively Q-switched Nd:GdVO 4/V:YAG laser

The influence of thermal lens effect on pulse repetition rate in diode-pumped Nd:GdVO 4 1.34 μm laser has been demonstrated. The repetition rate first increased, then reduced with augment of the pump power, for V:YAG with initial transmission ( T 0) of 96%. The maximum repetition rate was 70 kHz, obtained at the pump power of 6.78 W. The best output laser characteristics—the pulse width of 39.2 ns, the peak power of 2.5 kW and the single pulse energy of 102 μJ—were obtained by V:YAG of T 0=89%. With the Gaussian spatial distribution of intracavity photon density being considered, the rate equations were solved numerically and the theoretical calculations agreed with the experimental results.

Hanbury Brown and Twiss correlations of Anderson localized waves

When light waves propagate through disordered photonic lattices, they can eventually become localized due to multiple scattering effects. Here we show experimentally that while the evolution and localization of the photon density distribution is similar in the two cases of diagonal and off-diagonal disorder, the density-density correlation carries a distinct signature of the type of disorder. We show that these differences reflect a symmetry in the spectrum and eigenmodes that exists in off-diagonally disordered lattices but is absent in lattices with diagonal disorder.

Enhancement of stability and peak power in a diode-pumped doubly QML YVO 4/Nd:YVO 4 laser with EO and Cr 4+:YAG saturable absorber

A diode-pumped doubly Q-switched and mode-locked (QML) YVO 4/NdYVO 4 laser is realized with the electro-optic (EO) modulator and Cr 4+:YAG saturable absorber, in which the repetition rate of the Q-switched envelope is controlled by the active EO modulation while the mode-locked pulses inside the Q-switched envelope depend on both the actively modulated loss and the passive saturable absorption. The experimental results show that the doubly QML laser can generate more stable and shorter pulses with higher peak power when compared with the singly passively QML laser with Cr 4+:YAG. At the pump power of 20 W and the repetition rate 1 kHz, a 21 ns Q-switched pulse envelope with a average mode-locked peak power of 544 kW is obtained, which is the shortest Q-switched pulse envelope to my knowledge. In comparison to the singly passively QML laser with Cr 4+:YAG, the doubly QML laser has compressed the Q-switched envelope pulse width 70% and improved the mode-locked pulsed peak power 27 times. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML laser is given and the numerical solutions of the equations are in good agreement with the experimental results.

Numerical estimations of carrier generation-recombination processes and photon recycling effect in 3-<inline-formula><tex-math>${\bm \mu}$</tex-math><math altimg="m1.gif" overflow="scroll"><mrow><mi mathvariant="bold-italic">μ</mi></mrow></math></inline-formula>m n-on-p HgCdTe photodiodes

An enhanced original computer program is applied to explain in detail the influence of the photon recycling effect on carrier lifetime in a selected 3-μm n-on-p HgCdTe photodiode structure. The computer program is based on solution of carrier and photon transport equations for practical photodiode design. As a result, both distribution of thermal carrier generation and recombination rates and spatial photon density distribution in photodiode structures have been obtained. It is clearly shown that the photon recycling effect drastically limits the influence of radiative recombination on performance of HgCdTe photodiodes. A general conclusion confirms previous Humpreys' ascertainment that the radiative recombination, although of fundamental nature, does not limit the ultimate performance of HgCdTe photodiodes.

The effect of the infrared excess from the Be star on inverse Compton gamma-ray production in PSR B1259−63/SS 2883

The binary system PSR B1259-63 consists of a 48 ms pulsar in a 3.4 year orbit around a Be star and unpulsed TeV gamma-ray emission has been detected near previous periastron passages. A likely source of the gamma-rays is the inverse Compton up-scattering of target photons from the Be star by the ultra-relativistic electron/positron pulsar wind in the region of the plerion shock front. In this study the effect of the infrared emission from the Be star's circumstellar disc on inverse Compton gamma-ray production in PSR B1259-63 is investigated by considering an isotropic photon/electron approximation. The modified photon density distribution is determined by using a curve of growth method fitted to previous optical and infrared observations. The inverse Compton scattering rate is calculated using the modified photon distribution and the exact scattering equation. It is shown that including the infrared emission increase the GeV gamma-ray flux by a factor $\ga 2$.

Diode-pumped doubly Q-switched mode-locked YVO4/Nd:YVO4/KTP green laser with AO and GaAs saturable absorber

By using both acousto-optic modulator and GaAs saturable absorber, a diode-pumped Q-switched and mode-locked(QML) YVO4/Nd:YVO4/KTP green laser is presented. The experimental results show that the doubly QML YVO4/Nd:YVO4/KTP green laser has good stability and nearly 100% modulation depth. The average output power and the pulse width of the Q-switched envelope have been measured. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 137MHz and its pulse width is estimated to be about 280ps. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML green laser is given and the numerical solutions of the equations are in good agreement with the experimental results.

Numerical and experimental study of excited light and auto-fluorescence diffusion in teeth

Excited light and corresponding intrinsic fluorescence diffusion inside teeth tissue are an essential problem for light-based carious lesion detection. Based on finite element numerical analysis of diffusion equation, the photon density distribution of both excited light and autofluorescence of 2D premolar teeth model is obtained. The dependence of excited light and autofluorescence density distribution inside the teeth model on the scattering coefficient of enamel (5–25 mm−1) and dentine (100–140 mm−1) is numerically simulated and analyzed. The fitted results reveal that fluorescence intensity decreases exponentially. Optical penetration depth and fluorescence relative depth declined with the increment of scattering coefficient of enamel. And the dentine had the opposite effect. Finally, the experiment of measurement of fluorescence intensity on the teeth surface is conducted and the result is compared with the numerical computation.

Variation of photon density distribution with system component within Intralipid emulsion

In order to recover distributions of optical parameters of underlining tissues utilizing backreflection optic tomography or diffuse reflectance spectroscopy for diagnostic proposes, trajectories of photons should be correctly estimated within tissue. Therefore, we have simulated the soft tissues with 1% Intralipid emulsion and have determined photon density distribution as a function of depth inside the Intralipid emulsions using laser light of wavelength 808 nm and two different detectors for different source-to-detector (S–D) separations. In this study we have shown difference in path distribution of the detected photons with two different detectors with different sensitivity for the same S–D separations in the Intralipid emulsions. This study has also shown that, the maximum penetration depth (MPD) of the detected photons is not an increasing function of the S–D separations. After reaching a maximum depth, the penetration depth decreases as S–D separation increases.

Stable Q-switched and mode-locked Nd:GdVO_4/KTP green laser with dual-loss-modulation

A diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) Nd:GdVO(4)/KTP green laser with an acousto-optic modulator (AOM) and GaAs saturable absorber is presented. The experimental results show that the stability and the peak power of the dual-loss-modulated QML Nd:GdVO(4)/KTP green laser are significantly improved. The pulse width of the Q-switched pulse envelope in the dual-loss-modulation QML green laser has a compression of 50% compared with that in the singly passively QML green laser with GaAs. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML Nd:GdVO(4) green laser are given, and the numerical solutions of the equations are in good agreement with the experimental results.

Diode-pumped doubly Q-switched mode-locked YVO 4/Nd:YVO 4 laser with AO and GaAs saturable absorber

By using both acousto-optic (AO) modulator and GaAs saturable absorber, a diode-pumped doubly Q-switched and mode-locked (QML) YVO 4/Nd:YVO 4 laser is presented. The average output power and the pulse width of the Q-switched envelope have been measured. The Q-switch pulse energy of the doubly QML laser are higher than that only with GaAs. The stability of the QML laser with the dual-loss-modulation is significantly improved if compared to that only with GaAs.The experimental results show that the doubly QML YVO 4/Nd:YVO 4 laser has nearly 80% modulation depth and deeper than that of the singly passively QML pulse. The doubly Q-switched mode-locked pulse inside the Q-switched envelope has a repetition rate of 111 MHz and its pulse width is estimated to be about 700 ps. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML laser is given and the numerical solutions of the equations are in good agreement with the experimental results.

Two-dimensional measurement of focused hard X-ray beam profile using coherent X-ray diffraction of isolated nanoparticle

A method for evaluating the two-dimensional photon density distribution in focused hard X-ray beams is proposed and demonstrated in a synchrotron experiment at SPring-8. A synchrotron X-ray beam of 11.8 keV is focused to a ∼ 1 μ m spot by Kirkpatrick–Baez mirrors. The two-dimensional intensity distribution of the focused beam is derived by monitoring the forward diffracted intensity from an isolated silver nanocube with an edge length of ∼ 150 nm positioned in the beam waist, which is two-dimensionally scanned. Furthermore, the photon density of X-rays illuminated onto the nanocube is estimated by utilizing coherent X-ray diffraction microscopy. This method is useful for evaluating the photon density distribution of hard X-ray beams focused to a spot size of less than a few micrometers.