Nonlinear BBO Crystal Research Articles

Evaluating the efficacy of Nd:YAG fourth harmonic (266 nm) in comparison with ArF excimer (193 nm) in laser corneal reshaping: ex vivo pilot study

PurposeLaser corneal reshaping is a common eye surgery utilized to overcome many vision disorders. Different UV laser wavelengths can be effective in the treatment. However, the ArF excimer laser (193 nm) is the most commonly used due to its high absorption in the cornea. In the current study, we investigate the efficacy of applying a solid-state laser (Nd:YAG fourth harmonic at 266 nm) for the corneal reshaping procedure. MethodsThe utilized laser is generated using an optical setup based on a BBO nonlinear crystal which converts the Q-switched laser (532 nm) to its fourth harmonic (266 nm). Different pulse energies were applied with the same number of the shoots on ex vivo rabbit corneas, and the histological effect is studied. Moreover, the possible thermal damage on the treated corneal tissues was inspected via electron microscope. Additionally, the DNA damage on the corneal cells due to the application of the proposed laser was examined and compared with the existing technology (ArF Excimer laser at 193 nm) using the comet assay. ResultsThe histological examination revealed an appropriate ablation result with the minimum thermal effect at 1.5 mJ and 2.0 mJ. The overall results show that applying 50-shoots of the 1.5-mJ pulse energy using the proposed 266-nm solid-state laser produces the optimum ablation effect with the minimum thermal damage, and almost the same DNA damage occurred using the commercial 193-nm ArF excimer laser. ConclusionSolid-state laser at 266 nm could be a good alternative to the common 193-nm excimer laser for corneal reshaping procedures.

Measurement of entropy and quantum coherence properties of two type-I entangled photonic qubits

AbstractUsing the type-I SPDC process in BBO nonlinear crystal, we generate a polarization-entangled state near to the maximally-entangled Bell-state with high-visibility (high-brightness) 98.50 ± 1.33% (87.71 ± 4.45%) for HV (DA) basis. We calculate the CHSH version of the Bell inequality, as a nonlocal realism test, and find a strong violation from the classical physics or any hidden variable theory, S = 2.71 ± 0.10. Via measuring the coincidence count rate in the SPDC process, we obtain the quantum efficiency of single-photon detectors around (25.5 ± 3.4)%, which is in good agreement to their manufacturer company. As expected, we verify the linear dependency of the CC rate vs. pump power of input CW-laser, which may yield to find the effective second-order susceptibility crystal. Using the theory of the measurement of qubits, includes a tomographic reconstruction of quantum states due to the linear set of 16 polarization-measurement, together with a maximum-likelihood-technique, which is based on the numerical optimization, we calculate the physical non-negative definite density matrices, which implies on the non-separability and entanglement of prepared state. By having the maximum likelihood density operator, we calculate precisely the entanglement measures such as Concurrence, entanglement of formation, tangle, logarithmic negativity, and different entanglement entropies such as linear entropy, Von-Neumann entropy, and Renyi 2-entropy. Finally, this high-brightness and low-rate entangled photons source can be used for short-range quantum measurements in the Lab.

High-precision quantum transmittometry of DNA and methylene-blue using a frequency-entangled twin-photon beam in type-I SPDC

Using the coincidence-count (CC) measurement of the generated frequency-entangled twin-photon beam (TWB) via the process of type-I spontaneous parametric-down conversion (SPDC) in BBO nonlinear crystal (NLC), we have precisely measured the transmittance of very diluted rabbit- and human-DNA, methylene-blue (MB) as a disinfectant, and a thin-film multilayer at near IR wavelength 810nm with an accuracy in order of %0.01 due to the quantum correlation, while the accuracy of classical-like measurement, single-count (SC), is in order of %0.1 in our setup. Moreover, using quantum measurement of the transmittance, the different types of DNA with the same concentration, and also very diluted (in order of pg/μl) different concentrations of DNA and MB solutions, are distinguished and detected with high-reliability. Interestingly, in the case of human-DNA samples in contrast to our classical-like measurement, we could precisely detect and distinguish two very diluted concentrations 0.01ng/μl and 0.1ng/μl with high reliability while the commercial standard spectrometer device of our DNA-manufacturer could neither detect nor distinguish them. Surprisingly, measurement on the thin-film multilayer illustrates that the introduced method in this work might be performed to cancer/brain tissues or stem cells for cancer therapy and may hopefully open a pave and platform for non-invasive quantum diagnosis in the future.

Quality enhancement of a 355 nm sum-frequency light generated using a quality-deteriorated fundamental beam

This work provides a comprehensive study around the effect of a quality-deteriorated fundamental beam on the quality of a 355 nm wavelength beam experimentally generated using sum-frequency mixing of a second-harmonic generation (SHG) beam and its un-depleted fundamental inside a walk-off (WO) affected BBO nonlinear crystal. A slit-variable aperture is used inside the resonator of a utilized Nd:YAG resonator to generate a wide range of Hermite–Gaussian modes for the fundamental and subsequent SHG and sum-frequency generation (SFG) beams. The M2-factor of interacting beams is simulated based on the reconstruction of the recorded intensities using a recently reported technique known as few-frame decomposition method. It is found that in the worst case, due to the WO occurring in the X-direction, the M2-factor of the SFG beam is converged to 2.5 after 2 mm of crystal length and remained constant in the remaining length when the fundamental M2-factor is set for 4.5. Based on the simulation results, it is shown that the beam quality of SFG beam along the WO direction is independent of the fundamental M2-factor. It is further realized that by changing the focusing factor up to 2.84, again M2-factor of the UV-SFG beam is not exceeded from 2.5.

The Focal Length Effect on Energy Absorption and Terahertz Generation upon Focusing Two-Color Radiation in Air

Generation of terahertz radiation upon filamentation in air of two-color radiation in loose focusing conditions with focal lengths from 30 to 312 cm and a beam diameter of 0.8 cm was studied. Two-color radiation was generated in a nonlinear BBO crystal irradiated by a converging laser beam. Measurements show that the energy input into the medium increases dramatically with a decrease in the focal length to 50 cm or less; at F = 30 cm, the energy of a terahertz pulse linearly depends on the laser pulse energy and increases significantly at F = 50 cm. Terahertz radiation is still not observed at longer focal lengths, which is apparently due to a spatio—temporal mismatch of the radiation of the first and second harmonics.

Sensitivity enhancement by increasing the nonlinear crystal length in second-order autocorrelators for ultrashort laser pulses measurement

A theoretical model for interferometric autocorrelation with long nonlinear crystal (input depleted) has been developed and applied to the measurement of the duration of ultrashort pulses. The phase-matching condition is assumed throughout pulse spectrum. The interferometric autocorrelation trace of a mode-locked fibre laser (20 nJ energy, 100 kW peak power, centred at 1595 nm) has been measured by employing a fibre interferometer to avoid misalignment effects and a BBO nonlinear crystal as long as 2 mm, in order to generate higher second-harmonic power. BBO crystal was used because it can keep the phase-matching condition throughout a wide spectrum around 1600 nm. By fitting the experimental measurements and computing according to the theoretical model exposed, it has been demonstrated that the autocorrelator sensitivity is clearly enhanced by increasing the nonlinear crystal length. A temporal duration of 0.18 ps has been obtained by fitting theoretical and experimental values.

Diamond Raman laser emitting at 1194, 1419, and 597 nm

A Raman laser based on a synthetic diamond crystal pumped by nanosecond pulses of a 1030-nm Yb : YAG laser and emitting in the IR region at the first and second Stokes wavelengths of 1194 and 1419 nm, respectively, was developed. The conversion efficiency was 34 % with a slope efficiency of 50 % and an average power of 1.1 W at a wavelength of 1194 nm; the average power at 1419 nm was 0.52 W. Frequency doubling of the first Stokes component in a nonlinear BBO crystal resulted in orange (597.3 nm) radiation with a pulse energy of 0.15 mJ, an average power of 0.22 W, and a maximum efficiency of 20 %.

Frequency doubling of an InGaAs multiple quantum wells semiconductor disk laser

We demonstrate a good beam quality 483 nm blue coherent radiation from a frequency doubled InGaAs multiple quantum wells semiconductor disk laser. The gain chip is consisted of 6 repeats of strain uncompensated InGaAs/GaAs quantum wells and 25 pairs of GaAs/AlAs distributed Bragg reflector. A 4 × 4 × 7 mm3 type I phase-matched BBO nonlinear crystal is used in a V-shaped laser cavity for the second harmonic generation, and 210 mW blue output power is obtained when the absorbed pump power is 3.5 W. The M2 factors of the laser beam in x and y directions are about 1.04 and 1.01, respectively. The output power of the blue laser is limited by the relatively small number of the multiple quantum wells, and higher power can be expected by increasing the number of the multiple quantum wells and improving the heat management of the laser.

Tunable-wavelength picosecond vortex generation in fiber and its application in frequency-doubled vortex

We present a method for tunable-wavelength picosecond vortex pulse generation by using an acoustically-induced fiber grating (AIFG). The AIFG-driven mode conversion characteristic was activated via a shear-mode piezoelectric transducer that excels in excitation efficiency of acoustic flexural wave and mechanical stability. The linearly-polarized ±1-order picosecond vortex pulse was experimentally generated via AIFG with a uniform coupling efficiency of ∼98.4% from the fundamental mode to the ±1-order vortex mode within the wavelength range 1540 nm ∼ 1560 nm. The topological charge and the linearly-polarized characteristic of the picosecond vortex pulse were verified by examination of the off-axial interference and the polarization angle-dependent intensity, respectively. Furthermore, the picosecond vortex pulse with wavelength tunability was input to a nonlinear BBO crystal to generate a frequency-doubled ±2-order vortex in the wavelength range 770 nm ∼ 780 nm. This technology provides a convenient apparatus for generating a picosecond vortex pulse and the frequency-doubled vortex with wavelength tunability.

Nonlinear mirror mode-locked Pr:YAlO3 laser

Mode-locking of a praseodymium based solid-state laser by a nonlinear mirror is reported, for the first time to the best of our knowledge. Pr:YAlO3 crystal having 0.6 at.% of Pr3+ ions was used as an active medium. For mode-locking, a nonlinear BBO crystal together with a properly designed dichroic mirror were employed to form an output coupler with intensity dependent reflection. A continuous-wave mode-locked laser operation with a mean output power of 100 mW was achieved under 3.5 W InGaN laser diode pumping. The laser emitted 250 ps pulses with a repetition rate of 105 MHz at a center oscillation wavelength of 747 nm.

Parametric generation of broadband biphotons in a periodic sequence of thin crystals

Collinear parametric scattering of light under type-II phase matching is considered in a periodic sequence of nonlinear cells made of BBO crystals. The possibilities of reducing the duration of the wave packets of biphotons and increasing their intensity under monochromatic pumping are studied. The analysis is based on the account of dispersion of the refractive indices using Sellmeier equations. The obtained results allow one to conclude that at an appropriate choice of the crystal thicknesses and the number of cells, it is possible to significantly increase the flux of broadband biphotons and obtain extremely short quantum packets. It is shown that for sufficiently small thicknesses of individual crystals (layers), this system exhibits a fine structure in the spectrum of biphotons: with an increase in the number of cells (layers) the broadband spectrum is divided into a number of narrow bands and the number of these bands increases. When use is made of a nonlinear BBO crystal, this method allows one to obtain ultrashort packets of biphotons with duration . Compared with a different method of generation of broadband biphotons, which is based on the use of quasi-phase-matched periodically poled crystals, the method studied in this paper does not necessitate the need for the phase modulation of a biphoton to be converted into the amplitude modulation.

Polarization tomography of a narrow-band biphoton field

Narrow-band orthogonally polarized collinear frequency-degenerate biphotons are generated in the process of the spontaneous parametric down-conversion of light in a nonlinear BBO crystal placed in an optical resonator. Quantum polarization tomography and polarization transformation of the generated biphoton states are performed. The results from our experiment agree well with theoretical calculations.

Numerical simulation of ultra-broadband phase-sensitive optical parametric amplification of image

The ultra-broadband phase-sensitive optical parametric amplification and de-amplification of image in a BBO nonlinear crystal with type I phase-matching are theoretically investigated. Amplified images with a gain of 108 and a bandwidth as broad as 420 nm are realized using a 25 GW/cm−2 peak pump-power density with 1.125 ps chirped-pulse duration. The spatial resolution after optical parametric amplification is estimated to be q0 =200 rad/mm (i.e., 31.8 lines/mm), which corresponds to the size of the minimum resolvable feature of the amplified image ~5 μm.

High power tunable picosecond green laser pulse generation by frequency doubling of an Yb-doped fiber power amplifier seeded by a gain switch laser diode

A compact tunable high power picosecond green laser pulse source based on frequency doubling of an Yb-doped fiber amplifier seeded by a gain switch laser diode has been developed. The fiber amplifier generates the picosecond infrared pulses with average power of 10.3 W, repetition rate of 1 MHz, pulse duration of 150 ps, and tunable range of 20 nm around 1064 nm. For underwater use, the tunable output infrared pulses are frequency doubled into picosecond green laser pulses, which can be tuned from 527 to 537 nm with average power of more than 1.1 W, corresponding to an overall conversion efficiency of 10.7% by a BBO nonlinear crystal. This kind of laser source will have potential application for underwater optical communication.

Broadband lasing and nonlinear conversion of radiationfrom LiF : F2+ and LiF : F2- colour centre lasers

Broadband LiF : F2+ and LiF : F2- colour centre lasers are studied theoretically and experimentally. The cavitiesof these lasers ensure spatial wavelength selection. The lasersgenerate nanosecond pulses in the near-infrared range with aspectrum wider than 1300 cm-1. As a result of optimisationof the optical layout and selection of proper nonlinearcrystals, the broadband laser radiation with a spectral widthof more than 1300 cm-1 is efficiently and simultaneouslyconverted into the second harmonic in a nonlinear LiIO3crystal and into the fourth harmonic, in a nonlinear BBOcrystal.

Femtosecond forsterite Kerr-lens mode-locked laser pumped synchronously by an Nd3+:YAG laser

A femtosecond laser (pulse duration 30 fs, centre of the emission spectrum at λ = 1260 nm) with a chromium-doped forsterite crystal was developed. This laser operated on the basis of Kerr-lens mode locking. Synchronous pumping with an Nd3+:YAG picosecond laser resulted in forced generation of femtosecond pulses characterised by an enhanced stability of the output power, which was 5–7 times better than the stability of the pump laser. The output radiation of the forsterite laser was converted efficiently to the second harmonic (central wavelength 630 nm) in a thin nonlinear BBO crystal. The average power of the second-harmonic radiation was 15 mW.