KTP Crystal Research Articles

Unveiling quantum correlations of SPDC biphoton spatial modes using spatial light modulator

Abstract We experimentally demonstrate the quantum entanglement in various spatial modes of the correlated photons generated via spontaneous parametric down-conversion (SPDC) in a type-II phase-matched periodicallypoled KTP (ppKTP) crystal. The two-fold coincidence measurement reveals the spatial entanglement between the photon pairs by projecting one of the correlated photons onto a Gaussian mode while scanning the spatial mode of its partner photon through selective projection (Gaussian, Laguerre-Gaussian, Bessel-Gaussian, and Airy-Gaussian) using a spatial light modulator (SLM). We also show the spatially-resolved detection of the orbital angular momentum (OAM) non-conservation via modal decomposition of the correlated photon in Laguerre-Gaussian and Bessel-Gaussian bases.

Investigation of the capabilities of an electro-optic modulator based on a temperature-noncritical KTP crystal

Investigation of the capabilities of an electro-optic modulator based on a temperature-noncritical KTP crystal

Fabrication of hydrothermal PPKTP and its spontaneous parametric down-conversion characteristics

High-voltage electric field polarization, selective corrosion, current monitoring, and other methods are used to develop hydrothermal PPKTP. It is found that the reversal domain nucleation of hydrothermal-grown KTP crystals has four kinds of microscopic morphology, namely, strip shape, spindle shape, bullet shape and irregular shape, in the process of high voltage electric field polarization. Compared with the leakage current in the process of polarization of flux-grown KTP crystals, the hydrothermal-grown KTP crystals only exist when the applied electric field is much larger than the coercive field. The leakage current can be suppressed by multiple loading of short pulses, and the PPKTP with a poled period of 10 μm and 46 μm were fabricated, respectively. Subsequently, based on the fabricated PPKTP, the SPDC characteristics are studied. To the PPKTP with a poled period of 10 μm, the quantum entanglement source whose brightness exceeds 2.1 kHz @ 810 nm is prepared by the SPDC technique. Otherwise, the PPKTP with a poled period of 46 μm is used for SPDC, and the brightness of entangled photon pairs in channel 1 and channel 2 are 3.21 kHz @ 1560 nm and 5.31 kHz @ 1560 nm, respectively.

Compact 2-μm Hundred Hertz optical parametric oscillator Based on KTP crystal

A 100 Hz compact 2 μm optical parametric oscillator (OPO) based on a type II non-critically phase-matched KTiOPO4 crystal (KTP) was reported. The monolithic KTP crystal was pumped with a passively Q-switched 1064 nm Nd:YAG laser. At a repetition rate of 100 Hz and a single pulse energy of 5.8 mJ of pump light, a 2128-nm laser output was achieved with a maximum of 1.17 mJ of parametric light at the degenerate point, with a parametric light pulse width of approximately 10.5 ns, corresponding to a pump to parametric light conversion efficiency of 20.1 %.

Evaluation of the effectiveness of picosecond KTP and fractional thulium lasers on pigmented-type periorbital dark circles.

To determine the effectiveness of picosecond KTP in reducing peri-ocular dark circles caused mainly by excessive pigmentation and to compare Picosecond KTP with Thulium laser ability in reducing the intensity and extent of peri-ocular dark circles. This split-face prospective study included twelve women with periorbital dark circles (pigmented or mixed-pigmented type). The left lower eyelid was treated using the PicoHi machine (HIRONIC Ltd), a full beam Q-switched Nd-YAG provided by KTP crystal (523nm) at settings of 0.3J/cm2, 5mm, 5Hz, and 300 Ps. Whereas the right lower eyelid was treated using the Lavieen machine (WON TECH Co., Ltd), a fractional Thulium laser (1927nm) at setting 1320mJ/cm2, 30 × 15mm, 1500 microseconds. Patients received a series of 3 treatment sessions, given at 4-week intervals. The 532-nm full beam Q-switched KTP and fractional Thulium lasers were more likely to induce post-inflammatory hyperpigmentation rather than decrease the pigmentation. The risk is higher with a Q-switched KTP laser, which may be attributed to the skin tone of the participants. Nonetheless, some improvement in the pigmented type of PDCs, although not detected clinically, was documented by the VISIA software. No solid conclusion can be drawn from the results of the study. Picosecond KTP and Thulium lasers may have a role in reducing PDCs yet more studies should be performed in order to determine the exact impact these lasers have.

Self-injection-locked second-harmonic generation at 532 nm in high-Q Fabry-Perot micro-cavities

Based on the combination of DFB laser self-injection-locking (SIL) and external-cavity frequency-doubling, we have successfully demonstrated, for the first time to our knowledge, the self-injection-locked second-harmonic generation (SHG) at 532 nm by adopting the high-Q Fabry-Perot (FP) micro-cavities. With the DFB incident pump power of 79 mW at 1064 nm, the maximum output SHG power of 20.7 mW at 532 nm was achieved from the 11 mm long plane-concave FP external-cavity of 0.12 mL volume with the 10 mm long PPLN crystal. The corresponding output SHG efficiency of 26.2 % and the Q-factor of 56.5 M both exceeded all known SIL-SHG results reported, and the SIL SHG output power was one order of magnitude higher than the previous maximum of over 2 mW. The reliable SIL has been realized by utilizing the natural birefringence of the intracavity nonlinear crystal to provide laser feedback, and the intrinsic linewidth of the SIL pump laser at 1064 nm was about 30 Hz. Moreover, by replacing the PPLN crystal with the LN and KTP crystals, the output SHG power of 6.1 mW and 2.7 mW at 532 nm were respectively obtained with the Q-factors of 29.3 M and 27.6 M. Our new SIL-SHG design with the high-Q FP micro-cavity paves the way to a new type of the field-deployable ultra-narrow-linewidth visible and near-visible lasers of low size, weight, and power (SWaP).

Self-illuminated third-harmonic image upconversion.

We demonstrate third-harmonic upconversion imaging to visualize real time infrared images in the visible with standard CCD or CMOS silicon-based cameras operating at room temperature. Different from the usual sum-frequency mixing image upconversion, our third-harmonic image upconversion approach does not require an auxiliary IR source to illuminate a target. The upconversion system uses a passively Q-switched Nd3+:YVO4 laser operating at 1342 nm with two intracavity KTP crystals, obtaining a 447 nm upconverted image by a cascaded process where an upconverted image at 671 nm is also obtained as an intermediate step.

Double-pass second-harmonic generation of picosecond pulses with custom-poled KTP crystal

We present the double-pass second-harmonic generation (SHG) of picosecond pulses with a custom-poled potassium titanyl phosphate (KTP) nonlinear crystal. The average output power of 466 mW at central wavelength of 515.7 nm is obtained with the input of 1.2 W fundamental laser pulses. Compared to the highest conversion efficiency of 29.1% in the single-pass SHG, the conversion efficiency in the double-pass SHG is increased to 38.8%. Moreover, the average RMS stability of 0.67% in 2 hours and high beam quality (M2 < 1.10) of the second-harmonic pulses is observed. The presented results provide an efficient method to enhance the conversion efficiency of SHG for picosecond pulses.

Narrowband terahertz generation in a plane-parallel Rb:KTP crystal using a phase mask

A scheme for efficient generation of multi-cycle terahertz pulses by optical rectification in an artificial periodically poled rubidium (Rb)-doped potassium titanyl phosphate (Rb:PPKTP) structure is proposed. The structure consists of a multi-slit phase mask placed in front of a large-aperture Rb:KTP crystal. The developed theory predicts the THz pulse energy of 343 µJ and the efficiency of the pump-to-terahertz conversion of 0.44% for pump pulse peak intensity of 70GW/cm2 inside an aperture of 9.4×15mm2 at a crystal temperature of 85 K. The opportunity to control the THz spectral bandwidth (in the range of 1.8%∼85%) and frequency of the generation (from 0.3 to 0.8 THz) is shown. The results of the study indicate the high potential of the artificial Rb:PPKTP structure for tunable high-energy THz generation.

Efficient 10 ns-scale 2-μm optical parametric oscillator based on 1064 nm pump source

Lasers operating around 2-µm have been extensively researched for various applications, such as LiDAR, remote sensing, optical communication, and medicine. One of the approaches to generate 2-µm emission is to employ an optical parametric oscillator pumped by 1-µm laser; however, improving the 2-µm laser conversion efficiency of optical parametric oscillators is still a challenge. In this study, we exploit the high nonlinear coefficient of the KTP crystal at the 2-µm band to realize a high-efficiency optical parametric oscillator. The OPO is driven by a laser diode (LD) side-pumped, electro-optic Q-modulated Nd:YAG laser. The cavity was designed and optimized based on mode matching to increase the peak power density of the pump laser. Different powers were obtained for different pump laser focusing conditions, and the maximum power was obtained when the KTP crystal was placed in front of the pump laser focus. The output characteristics of the KTP at different phase-matching angles were also investigated. An output power of 2.44 W and an output pulse width of 9.04 ns were obtained at 2154.4 nm with an optical-to-optical conversion efficiency of 23.39% and a slope efficiency of 36.09%.

10-dB squeeze laser tuneable over half a nanometer around 1550 nm.

Lasers for generating monochromatic light beams with sideband spectra in strongly squeezed vacuum states are the basis for aspired optical continuous-variable quantum computers. We have developed a "squeeze laser" that produces 10 dB squeezed vacuum states at a wavelength of 1550 nm, the latter being tunable by 0.5 nm without losing the high squeeze factor. Several identical squeeze lasers can thus be combined to realise wavelength-division multiplexing. Our squeeze laser uses the mature technology of parametric down-conversion in a periodically poled KTP crystal placed in a cavity that resonates both the squeezed field and the second harmonic pump field. Unlike previous realisations, we achieve the double resonance and phase matching by individually optimising and controlling the temperatures of two sections of the crystal body. The wavelength range is currently limited by the tuneability of the 1550 nm master laser.

Properties of Rubidium-Doped Potassium Titanyl Phosphate (Rb: KTP) Grown by Hydrothermal Method

As an electro-optical crystal with excellent properties, potassium titanyl phosphate (KTP) has high conductivity limits electro-optical applications. In this experiment, Rb-doped KTP crystals were grown by the hydrothermal technique of micro-doping and sustained-release, the crystal structure was determined by XRD, the presence of Rb in the crystals was explored by EDS, and the role of Rb in the crystals was investigated by Raman technique. Frequency-dependent dielectric constants were studied at room temperature, which decreases sharply at low frequencies. The variation of the conductivity of the crystals at different temperatures was investigated, and it was found that the room temperature conductivity of the KTP crystals, which were micro-doping by large ionic, was able to drop to 10−13 S/cm, which is about two orders of magnitude lower than that of the pure KTP crystals.

The effect of co-substitution of heterovalent ions Ga3+and Sb5+on nonlinear optical properties of phosphate crystals

The nonlinear optical coefficient of the KTP crystal has been significantly improved after co doping with Ga3+and Sb5+ions (1.25 times that of the pure KTP crystal).

Vibrational contributions to the electro-optic effect of BBO, KTP and RTP crystals

Barium borate (Ba3(B3O6)2, low temperature phase, BBO), potassium titanyl phosphate (KTiOPO4, KTP) and rubidium titanyl phosphate (RbTiOPO4, RTP) are not only best performing nonlinear optical materials but also show preferable electro-optic (EO) Pockels effect that serve as practical EO modulators widely applied in laser technology. Disparity to the repeatedly measurements of their properties related to the EO effects, the mechanism leads to their large EO coefficients have never been fully studied theoretically from first-principles calculations. In this paper, using an ab initio program CRYSTAL based on a linear combination of atomic orbitals approach, the vibrational spectra of the title crystals were firstly obtained. Then the electronic and vibrational contributions to the EO effect were calculated through coupled perturbed Kohn–Sham approximation and variations of atomic displacements. The results show that the EO coefficients, as well as other properties of dielectric constants, SHG coefficients and IR and Raman spectra of the three crystals are in excellent agreement with the experimental values. It is also found that the EO coefficients of all the crystals studies are dominantly contributed from few vibrational modes at low frequencies and the vibrational contributions amount to about 60% in BBO, 90% in KTP and RTP. General rules of finding crystals with large EO effects are finally proposed based on present study and previous studies on LiNbO3 crystals.

Near‐Video Frame Rate Quantum Sensing Using Hong–Ou–Mandel Interferometry

AbstractHong–Ou–Mandel (HOM) interference, bunching of two indistinguishable photons on a balanced beam‐splitter, has emerged as a promising tool for quantum sensing. There is a need for wide spectral‐bandwidth photon pairs (for high‐resolution sensing) with high brightness (for fast sensing). Here, the generation of photon‐pairs with flexible spectral‐bandwidth even using single‐frequency, continuous‐wave diode laser enabling high‐precision, real‐time sensing is showed. Using 1‐mm‐long periodically‐poled KTP crystal, degenerate, photon‐pairs with spectral‐bandwidth of 163.42±1.68 nm are produced resulting in a HOM‐dip width of 4.01±0.04 m to measure a displacement of 60 nm, and sufficiently high brightness to enable the measurement of vibrations with amplitude of 205 ± 0.75 nm and frequency of 8 Hz. Fisher‐information (FI) and maximum likelihood estimation enables optical delay measurements as small as 4.97 nm with precision (Cramér–Rao bound) and accuracy of 0.89 and 0.54 nm, respectively, therefore showing HOM sensing capability for real‐time, precision‐augmented, in‐field quantum sensing applications.

High-energy parametric oscillator and amplifier pulsed light source at 2-µm.

A laser system generating high-energy pulses at 2-µm wavelength with pulse widths tunable from 10-24 ns is described. It comprises an optical parametric oscillator that generates mJ-level signal seed radiation and an optical parametric amplifier that boosts the output to 800 mJ of combined signal and idler when pumped with 2 J pulses of 1064-nm laser light. The system operated with KTP crystals and running at 10 Hz repetition rate is characterized in the spatial, temporal, and spectral domains. The effect of saturation leads to an output pulse approaching flat-top spatial and box-shaped temporal profiles, as desired in various applications. The amplified pulses can be imaged down to sub-100 µm diameters, making this laser system a suitable driver for plasma sources of extreme ultraviolet light.

High Average Power Green Laser Based on LED-Side-Pumped Q-Switched Nd:YAG Laser

In this paper, we demonstrated a watt-level acousto-optic (A-O) Q-switched intracavity frequency-doubled green laser with high average power and beam quality operation based on an efficient LED-pumped Nd:YAG laser module. For the quasi-continuous wave (QCW) running operation, the average output power of fundamental laser at 1064 nm was as high as 10.3 W at a repetition rate of 100 Hz, corresponding to the highest optical conversion efficiency of 25.4% and a slope efficiency of 31.3%, and the maximum energy of single pulse was up to 100mJ. By optimizing the incidence angle of the KTP crystal to meet the non-critical phase matching condition, greater than 3.8 W of 532 nm average power at a repetition rate of 10 kHz was generated with duration of 192 ns and a beam quality factor of M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> = 1.6 and M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> = 1.7, corresponding to a frequency doubling conversion efficiency of 45.5%. To the best of our ability, this is the highest average power and high beam quality of an LED-pumped A-O Q-switched intracavity frequency-doubled green laser with watt-level output reported so far.

Pulsed Parametric Oscillator Based on KTP Crystals with a Three-Mirror Ring Cavity: Numerical Simulation

Pulsed Parametric Oscillator Based on KTP Crystals with a Three-Mirror Ring Cavity: Numerical Simulation

Proton irradiation hardness of periodically poled Rb:KTP for spaceborne parametric frequency converters

Large aperture periodically-poled Rb:KTP crystals designed for optical parametric amplifiers in 2 µm LIDAR systems were radiation hardness tested by exposure of proton beams at 10 MeV and 60 MeV energies. An irradiation dose of 55 Gy was used to commensurate the crystals’ estimated exposure on board a mission in the low-Earth orbit. The irradiation effects were investigated by comparing optical transmission spectra and 2D effective nonlinearity mapping in a 2 µm OPO setup before and after irradiation. The results reveal that the periodically poled structure remained intact after irradiation, and the changes in the optical transmission and nonlinear properties were close to the measurement uncertainty. This investigation is essential for realizing efficient frequency converters for space applications, such as spaceborne active greenhouse gas monitoring LIDAR instruments or correlated photon-pair sources.

Multi-color laser generation in periodically poled KTP crystal with single period

Frequency conversion based on three-wave mixing is a critical nonlinear optic application, extending the frequency range of existing lasers and realizing frequency-transduced detectors in a wavelength range that lacks an effective detector. Phase matching is vital for effective frequency conversion. The advantages of quasi-phase matching (QPM) over birefringent phase matching are a lack of walk-off effect, a maximum nonlinear coefficient, and phase matching in the entire transparency window. Herein, using different types and orders of QPM, four kinds of effective frequency doubling processes are realized in a periodically poled potassium titanyl phosphate (KTP) crystal with a single period, and three kinds of frequency doubling processes are experimentally verified. We also show a feasible way to construct an RGB color generator based on two different QPM processes. This study significantly expands the feasible frequency conversion of existing lasers to different wavelengths, providing an effective method for multi-color laser generation based on periodically poled KTP crystals.