Length Of Nonlinear Crystal Research Articles

Quantum ghost imaging microscopy depth-of-field study

Quantum ghost imaging approaches have been proposed to enhance biological microscopy, for example, using 2D visible detectors to provide IR images or providing additional dimensions of spatial or spectral information. Toward the goal of making such imaging schemes practical, we compare image quality and depth-of-field between traditional images and ghost images at the same excitation levels. We measure how image quality and depth-of-field depend on the parameters of the entangled light produced using type-I spontaneous parametric down-conversion (SPDC). We use a pair of time-synchronized, photon-timing single-photon avalanche diode (SPAD) array detectors to capture two distinct microscope imaging paths simultaneously on a photon-pair-by-photon-pair basis: one in a traditional imaging pathway and the other a quantum ghost imaging pathway. We calculate the depth-of-field, resolution, contrast, and signal-to-noise ratio (SNR) through the parameter space of a β-Barium Borate (BBO) type-I bulk non-linear crystal length and angle. Our results provide a basis for choosing parameters for quantum ghost imaging with type-I SPDC sources.

Design Considerations for Continuous Wave Intracavity Backwards Optical Parametric Oscillators

In this paper, we report a theoretical systematic study of continuous wave intracavity backwards optical parametric oscillators based on periodically poled lithium niobate (PPLN) for mid-infrared (mid-IR) light generation. We study the effects of varying different cavity parameters including nonlinear crystal length, cavity size, pump laser diode spot size, output coupler radius, and cavity loss values on the output power and threshold of the proposed mid-IR laser. The effects of different physical phenomena are included in the model including pump depletion due to the nonlinear conversion process, the thermal lens effect, and mode overlap between the beams in the nonlinear crystal. We show that high output powers in the mid infrared (>500 mW at 3.2 μm) can be achieved with proper cavity design and that a laser threshold with a PPLN as short as 2 cm can be reached.

Intracavity frequency-doubled external-cavity surface-emitting blue laser with high conversion efficiency

Blue laser with high power and high beam quality has many applications such as in laser display, underwater communication and imaging, and non-ferrous metal processing. Optically pumped external-cavity surface-emitting laser combines the advantages of both surface-emitting semiconductor lasers and solid-state disk lasers, and can produce high output power and good beam quality simultaneously. Its high intracavity circulating power is more conducive to intracavity frequency doubling, achieving high-power and high beam quality blue light through fundamental laser in the near-infrared waveband. This paper reports an efficient intracavity frequency doubled 490 nm high power blue light by using a 980 nm fundamental laser in an external-cavity surface-emitting laser. The V-type resonant cavity is formed by the high reflectivity distributed Bragg reflector (DBR) at the bottom of gain chip, a folded flat concave mirror (high reflectivity coated for 980 nm and anti-reflectivity coated for 490 nm), and a flat concave end mirror (high reflectivity coated for 980 nm and 490 nm). By inserting a nonlinear crystal LBO into the cavity at the beam waist formed by the folded mirror and end mirror, and employing a birefringent filter (BRF) to polarize the fundamental laser and narrow the linewidth of the laser, a high power and high beam quality blue laser with high conversion efficiency is obtained. The effects of different factors including the length of nonlinear crystal, the linewidth of fundamental laser, and the compensation of walk off angle on the output power of the blue laser are studied experimentally. The length of the nonlinear crystal is optimized based on the size of the fundamental laser beam waist at the position of the crystal in the resonant cavity. Under the type-I phase matching condition of LBO, over 6 W output power at 491 nm wavelength is obtained when the crystal length is 5 mm and the BRF thickness is 1 mm. The beam quality <i>M</i><sup>2</sup> factor in the <i>x</i> direction and the <i>y</i> direction are both 1.08, and the conversion efficiency of frequency doubling is 63%. The experimental results also show that symmetrically placed nonlinear crystals can compensate for the walk-off angle during frequency doubling to a certain extent, thereby clearly improving the conversion efficiency of the frequency doubled blue laser.

SSA-BP network model based Hong-Ou-Mandel interference delay measurement and its application in quantum gyroscope

High sensitivity optical gyroscopes, as an important component of aerospace navigation system, have become a research hotspot. The sensitivity of the classical optical gyroscope is restricted by the shot-noise-limit owing to the vacuum zero energy fluctuation. Therefore, the classical optical gyroscope cannot meet the growing demand of navigation, sensing and communication. In this work, a measurement scheme of quantum gyroscope based on frequency entangled source and Hong-Ou-Mandel (HOM) interference is proposed. In order to realize high-precision delay measurement, the interference visibility and width of HOM interferogram are regulated by changing the bandwidth of pump laser and the length of nonlinear crystal. However, traditional experimental regulation method is inefficient and time consuming. On the basis of the above scheme, a delay measurement scheme of HOM interference based on SSA-BP network is established. The SSA-BP network is used to simulate different bandwidths of pump laser and the lengths of nonlinear crystal to predict the interference visibility and width of HOM interferogram. The verification results show that the mean square error (MSE), the mean absolute error (MAE) and the mean absolute percentage error (MAPE) predicted by SSA-BP network are smallest. Based on the above SSA-BP network model, the interference visibility and width of HOM interferogram are <inline-formula><tex-math id="M1">\begin{document}$\alpha = 1$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230283_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230283_M1.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M2">\begin{document}$\sigma = 5.9\;{\text{ ps}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230283_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20230283_M2.png"/></alternatives></inline-formula> respectively. Combined with quantum Fisher information, the maximum value of <i>F</i> is obtained to be 1.999. Meanwhile, according to the Cramer-Rao bound theory, the minimum ratio of the uncertainty of the delay to the shot-noise-limit can reach 0.707, indicating that the precision of delay measurement is increased by 2 orders of magnitude. According to the relationship between delay and rotational angular velocity, the measurement sensitivity of the rotational angular velocity is improved by 2 orders of magnitude compared with that of the classical optical gyroscope. These results prove that the above quantum gyroscope scheme can realize the measurement sensitivity of rotational angular velocity beyond the shot-noise-limit. Therefore, the SSA-BP network model can provide theoretical support for the subsequent experimental verification of quantum gyroscopes based on HOM interference delay measurement, and is the technical basis for the development of quantum navigation, quantum sensing and quantum communication.

Optimization of the nonlinear crystal length for high-power single-frequency intracavity frequency-doubling lasers.

We report a method of optimizing the nonlinear crystal length of the intracavity frequency-doubling laser, which is achieved by maximizing the output power of the frequency-doubling laser in the case of ensuring the single longitudinal mode (SLM) operation of the laser. The optimal length of the nonlinear crystal for an SLM oscillation of the intracavity frequency-doubling laser is firstly theoretically predicted by comparing the losses introduced by the nonlinear crystal with different lengths with that of ensuring the SLM operation of the laser. Then three nonlinear LiB3O5 (LBO) crystals with the length of 18, 20, and 22 mm are adopted to be the frequency-doubling components in the experiment. By recording the output power and monitoring the longitudinal mode structure of the laser, it is found that the nonlinear LBO crystal with the length of 20 mm is the best candidate, since the output power is higher than that of the LBO crystal with the length of 18 mm, and the SLM operation of the laser is readily achieved compared to that of the LBO crystal with the length of 22 mm. The experimental results well agree with the theoretical predictions. The current method can pave a good way to attain a single-frequency continuous-wave intracavity frequency-doubling laser.

A systematic study of PPLN length dependence in intracavity SHG

In this paper, a systematic study of the relationship between nonlinear crystal length and intracavity second-harmonic generation (SHG) using MgO-doped periodically-poled lithium niobate (MgO:PPLN) is presented. The experimental results demonstrate a relationship between the maximum SHG power generated and the full-width at half maximum (FWHM) of the crystal’s temperature tuning curve to the length of the nonlinear optical crystal. It was shown that maximum SHG power increases rapidly with the increase of MgO:PPLN length, reaching a saturation length (~ 2 mm), which is much shorter than that predicted by the single-pass SHG theory. This saturation length of the MgO:PPLN crystal is almost independent on 808 nm pump power for typical powers used in continuous wave intracavity SHG lasers. In addition to this saturation effect, a broadening effect was also observed, the FWHM of the temperature tuning curve was shown to have a larger FWHM than that predicted by the single-pass SHG theory for MgO:PPLN shorter than the saturation length. This work has the benefit of allowing engineers to optimize nonlinear crystal length when developing intracavity SHG based diode-pumped solid state (DPSS) lasers.

Cascaded Second Harmonic Generation with a Half-Order Periodical Orientation

We demonstrate theoretically cascaded fourth and second harmonics generation in a periodically oriented nonlinear crystal with a half-order of the periodical orientation. We consider a cascaded process in which four photons of the fundamental harmonic are initially converted into an intermediate photon of the fourth harmonic, which parametrically decays at the second stage into two photons of the second harmonic. We show that fractional phase matching can be achieved by using an asymmetric periodically poled grating. To describe the propagation and transformation of light inside a nonlinear crystal, the quantum spatial Heisenberg equations have been applied, using which the average numbers of photons of the fourth and second harmonics have been calculated as functions of the nonlinear crystal length.

A 355 nm ultraviolet femtosecond laser through second harmonic generation using K3B6O10Br nonlinear optical crystal

We demonstrate the 355 nm ultraviolet (UV) femtosecond (fs) laser through second harmonic generation using a type-I phase-matched nonlinear optical crystal of K3B6O10Br (KBOB). The effects of the length of nonlinear optical crystals and the focal length of lenses on the output power and conversion efficiency were investigated systematically. The maximum output power of 265 mW UV fs laser at 355 nm was obtained with a pump of 1.408 W fs laser at 710 nm with a repetition rate of 80 MHz. The corresponding optical conversion efficiency was 18.8%. The experimental results show that KBOB crystal is a promising crystal in generating high-power UV fs laser because of its excellent nonlinear optical properties.

Каскадная генерация второй гармоники с половинным порядком периодической ориентации

We theoretically demonstrate cascaded fourth and second harmonic generation in a periodically-poled nonlinear crystal with a half-order of phase-matching period. We consider a cascaded process in which four photons of the fundamental harmonic firstly convert into an intermediate photon of the fourth harmonic, which parametrically decays into two photons of the second harmonic at the second stage. Phase-matching for this cascaded nonlinear conversion is provided by using an asymmetric periodical poling. We use the quantum spatial Heisenberg equations to describe light propagation and conversion inside the nonlinear crystal. With this approach, we calculate second and fourth harmonic average number of photons as a function of the nonlinear crystal length.

A new approach to optimize the signal output power of the intracavity single resonator optical parametric oscillator

In this paper, the signal output power of an intracavity optical parametric oscillator is optimized so that the signal output power becomes maximum. The particle swarm optimization (PSO) algorithm is implemented to some practical setups and the results are compared with some experimental data in order to confirm our method. Finally, the PSO algorithm is performed on the rate equations to obtain and suggest the optimum values for gain medium length, nonlinear crystal length and output coupler reflectivity at signal wavelength simultaneously, in which the largest signal output power can be obtained. The numerical results show that at 25.9W for input power, the signal output power can be reached up to 4.55 W. In addition, the signal output power can be increased by 20%, by selecting appropriate values for the gain medium length, nonlinear crystal length and output coupler reflectivity at signal wavelength.

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.

Ultraviolet picosecond conversion efficiency improvement system at 355 nm based on fundamental frequency laser amplified

In recent years, picosecond laser in ultraviolet (UV) has manifested great importance for applications both in science and industry, such as biomedical research, micro machining, etc. Now, the well proven approach to generating ultra-short UV pulses is extra-cavity frequency conversion based on nonlinear optical (NLO) crystal, due to the lack of suitable laser sources directly generating UV laser. In this process of harmonic generation, the length of nonlinear crystal is an important factor affecting the conversion efficiency and beam-quality. The optimal length of the nonlinear crystal is influenced by incident laser parameters and crystal absorption coefficient. At present, for the UV 355 nm picosecond laser generated from extra-cavity sum frequency, published are few reports about detailed analysis and research on the influence of photon ratio of the incident laser beams and nonlinear crystal absorption on optimal length of sum frequency crystal. In this paper, the steady-state solutions with the highest conversion efficiency under different incident conditions are obtained by theoretical analysis and numerical simulation of the three waves coupling equations. The effects of different photon ratios and absorption effect of the sum frequency crystal on the optimum crystal length are analyzed. We propose a solution based on the fundamental frequency laser amplified to shorten crystal length and improve conversion efficiency. In this scheme, the 532 nm second harmonic laser with a high conversion efficiency over 65% can be achieved by LiB<sub>3</sub>O<sub>5</sub> crystal. After that, the 1064 nm fundamental frequency laser is separated from the second harmonic laser, and then it is amplified by the Nd:YVO<sub>4</sub> laser crystal pumped by an 808 nm laser diode. Finally, the ultraviolet 355 nm picosecond laser is obtained by combining the 1064 nm fundamental frequency laser with the 532 nm second harmonic laser in the LiB<sub>3</sub>O<sub>5</sub> crystal. The simulation results show that the incident photon ratio of the sum frequency reaction can be changed by amplifying the residual fundamental frequency laser, and the optimum length of the sum-frequency crystal corresponding to the highest conversion efficiency can be shortened. Meanwhile, the absorption and walk-away effect of the sum frequency crystal can be also reduced. The final 355 nm laser output power can be increased more than 40 percent compared with the traditional scheme of early reports. In consequence, the high sum frequency conversion efficiency of the UV 355 nm picosecond laser can be obtained by changing the photo ratio of the incident laser beams through amplifying the fundamental frequency laser.

Design and experimental demonstration of a high conversion efficiency OPCPA pre-amplifier for petawatt laser facility

We present the design and experiment of a broadband optical parametric chirped-pulse amplifier (OPCPA) which provides high conversion efficiency and good beam quality at 808 nm wavelength. Using a three-dimensional spatial and temporal numerical model, several design considerations necessary to achieve high conversion efficiency, good beam quality and good output stability are discussed. To improve the conversion efficiency and broaden the amplified signal bandwidth simultaneously, the nonlinear crystal length and OPCPA parameters are analyzed and optimized with the concept of dissipating amplified idler between optical parametric amplification (OPA) of two crystals configuration. In the experiment, an amplifier consisting of two OPCPA stages of ‘L’ type configuration was demonstrated by using the optimized parameters. An amplified signal energy of 160 mJ was achieved with a total pump-to-signal efficiency of 35% (43% efficiency for the OPCPA stage 2). The output bandwidth of signal pulse reached 80 nm and the signal pulse was compressed to 24 fs. The energy stability reached 1.67% RMS at 3% pump energy variation. The optimized OPCPA amplifier operates at a repetition rate of 1 Hz and is used as a front-end injection for the main amplifier of SG-II 5PW laser facility.

Investigation on the thermal characteristic of MgO:PPSLT crystal by transmission spectrum of a swept cavity.

A method of evaluating the thermal focal length of nonlinear crystal via transmission spectrum of a swept cavity (TSSC) is presented. By recording the resonant point offset of the TSSC, the thermal focal length can be successfully measured. Furtherly, by distinguishing the absorption of ultraviolet (UV) laser and UV laser induced infrared absorption (ULIIRA), it is clear that the ULIIRA is the important factor which induces the thermal lens effect compared to the absorption of UV laser for MgO-doped periodically poled stoichiometric lithium tantalate (MgO:PPSLT) crystal and it becomes serious with the increase of the generated UV laser. The ULIIRA coefficient measurement and thermal focal length evalution of MgO:PPSLT crystal can supply an useful reference for researchers to generate high quality UV laser and squeezed or entangled state of optical field by using MgO:PPSLT crystal. The presented method can also be used to precisely evaluate the thermal focal length of other nonlinear crystals.

Theoretical study on the characteristics of intracavity frequency doubling of a diode-pumped cesium vapor laser

Characteristics of intracavity frequency doubling of a diode-pump cesium vapor laser (Cs-DPAL) are studied in detail utilizing the rate equations theory. For intracavity frequency doubling of Cs-DPAL, very long nonlinear crystals such as LiB3O5 (LBO) and BiB3O6 (BIBO) with a relatively small effective nonlinear coefficient are required to get the maximum second harmonic (SH) power, and periodically poled crystals such as periodically poled KTiOPO4 (ppKTP) are promising. The reason for this phenomenon is that the simulated emission section is much higher, although it is favorable to reduce the laser threshold. For intracavity frequency doubling of Cs-DPAL in the quasi-two-level limit, the optimal length of nonlinear crystal is slightly shorter than the Cs-DPAL with the assumption of a very rapid transition from the 6P23/2 to 6P21/2 levels, and a qualitative interpretation is given. The SH power by intracavity frequency doubling of Cs-DPAL is also limited by the relatively slower collisional transfer rate from the 6P23/2 to 6P21/2 levels for higher pump power intensity.

Resolution of ghost imaging with entangled photons for different types of momentum correlation

We present an analytical analysis of the spatial resolution of quantum ghost imaging implemented by entangled photons from a general, spontaneously parametric, down-conversion process. We find that the resolution is affected by both the pump beam waist and the nonlinear crystal length. Hence, we determined a method to improve the resolution for a certain imaging setup. It should be noted that the resolution is not uniquely related to the degree of entanglement of the photon pair since the resolution can be optimized for a certain degree of entanglement. For certain types of Einstein-Podolsky-Rosen (EPR) states——namely the momentum-correlated or momentum-positively correlated states——the resolution exhibits a simpler relationship with the pump beam waist and crystal length. Further, a vivid numerical simulation of ghost imaging is presented for different types of EPR states, which supports our analysis. This work discusses applicable references to the applications of quantum ghost imaging.

Nonlinear absorption and phase shift in coupled optical cavities

The nonlinear absorption process and associated phase shift in coupled optical cavities are studied experimentally by observing optical resonance properties. In the coupled cavity configuration, two optical cavities, one for a fundamental beam and the other for a second-harmonic (SH) beam, are coupled by a nonlinear crystal for the second-harmonic generation (SHG). The cavity for the SH beam effectively extends the nonlinear crystal length so that the frequency-conversion efficiency of the SHG, which is proportional to the square of the crystal length, is enhanced. In the observation, power reduction of the fundamental beam at resonance is observed as a small dip in the resonance curve. According to a model calculation, it is found that this power reduction is caused by blue-induced infrared absorption.

Pulse compression effects of passive mode locker in ultrafast optical parametric oscillator

The pulse compression effects of passive mode locker, such as graphene and Kerr lens medium, which has been inserted into a synchronously-pumped optical parametric oscillator (SPOPO) pumped by pulse laser, and a continuous-wave (cw) pumped actively mode-locked OPO has been studied theoretically. By solving the nonlinear coupled-wave equations in the plane-wave approximation, we simulate the relations of the pulse width and peak power of the signal pulses to the normalized aperture radius, the nonlinear crystal length and the beam radius on the nonlinear crystal. The numerical results indicate that the Kerr lens medium and graphene are both favorable for the pulse compression in the SPOPO, and each of them has different advantages. Whereas in the actively mode locked OPO, the Kerr lens medium may be a better alternative for the pulse compression.

Characterization of frequency entanglement under extended phase-matching conditions

Frequency-entangled photon pairs generated by spontaneous parametric down-conversion process under extended phase-matching conditions are found to be prospective resources for quantum information technology. The spectral indistinguishability and the degree of frequency entanglement of the down-converted photon pairs are two key features determining their potential applications. In this paper, both figures of merits are investigated. It is shown that the broadening of the pulsed pump bandwidth, the chirping of the pulse duration, and the lengthening of the nonlinear crystal all degrade the spectral indistinguishability. Furthermore, by changing the above conditions, it is possible to transfer the entanglement type from frequency-correlated to frequency-anticorrelated. For frequency-correlated entanglement, the degree of entanglement grows linearly with the pump bandwidth and the nonlinear crystal length, and quadratically with the chirp parameter of the pulse, while for anticorrelated frequency entanglement, the degree of frequency entanglement turns to behave with an inverse dependence on the above parameters. Guided by such investigations, the generation of desired frequency-entangled source for various quantum information applications can be optimized.

Optimum design considerations for CW intra-cavity second harmonic generation in end-pumped solid-state lasers

An understanding of performance of a laser system in terms of design parameters is of the great importance to the engineer who is designing a laser system. This manuscript describes a theoretical report on the optimum design key parameters of an intracavity frequency doubled end-pumped solid-state laser based on the combination of space-dependent rate equations and nonlinear coupled-wave analysis of second-harmonic generation. The theoretical framework along with the results presenting the effect of various parameters such as pump beam characteristics, gain medium and nonlinear crystal properties to optimize the SHG efficiency has been investigated. By considering the optimized root mean square of pump beam radius as the effective value of pump spot size in the gain medium and perfect phase matching between fundamental and second harmonic waves,optimum nonlinear crystal's length, optimum mode size and second harmonic output power are derived as a function of pump-beam characteristics, and properties of the nonlinear and gain medium crystals. The results show that the optimum nonlinear crystal's length depends not only on the nonlinear coefficient and spot radius of laser beam but also on the pump beam properties and gain medium characteristic. In addition, the effect of pump and gain medium properties on the optimal fundamental beam by taking into account thermal aberration are investigated. The obtained results give useful guidelines for choosing the suitable active and nonlinear crystals for a given pump source and allow the straightforward evaluation of laser parameters for efficient intracavity doubling in end-pumped solid-state lasers.