Potassium Titanyl Phosphate Crystal Research Articles

Pulsed source of spectrally uncorrelated and indistinguishable photons at telecom wavelengths.

We report on the generation of indistinguishable photon pairs at telecom wavelengths based on a type-II parametric down conversion process in a periodically poled potassium titanyl phosphate (PPKTP) crystal. The phase matching, pump laser characteristics and coupling geometry are optimised to obtain spectrally uncorrelated photons with high coupling efficiencies. Four photons are generated by a counter-propagating pump in the same crystal and anlysed via two photon interference experiments between photons from each pair source as well as joint spectral and g((2)) measurements. We obtain a spectral purity of 0.91 and coupling efficiencies around 90% for all four photons without any filtering. These pure indistinguishable photon sources at telecom wavelengths are perfectly adapted for quantum network demonstrations and other multi-photon protocols.

Catastrophic nanosecond laser induced damage in the bulk of potassium titanyl phosphate crystals

Due to its high effective nonlinearity and the possibility to produce periodically poled crystals, potassium titanyl phosphate (KTiOPO4, KTP) is still one of the economically important nonlinear optical materials. In this overview article, we present a large study on catastrophic nanosecond laser induced damage in this material and the very similar RbTiOPO4 (RTP). Several different systematic studies are included: multiple pulse laser damage, multi-wavelength laser damage in KTP, damage resistance anisotropy, and variations of the laser damage thresholds for RTP crystals of different qualities. All measurements were carried out in comparable experimental conditions using a 1064 nm Q-switched laser and some were repeated at 532 nm. After summarizing the experimental results, we detail the proposed model for laser damage in this material and discuss the experimental results in this context. According to the model, nanosecond laser damage is caused by light-induced generation of transient laser-damage precursors which subsequently provide free electrons that are heated by the same nanosecond pulse. We also present a stimulated Raman scattering measurement and confront slightly different models to the experimental data. Finally, the physical nature of the transient damage precursors is discussed and similarities and differences to laser damage in other crystals are pointed out.

Weak-signal conversion from 1550 to 532 nm with 84% efficiency.

We report on the experimental frequency conversion of a dim, coherent continuous-wave light field from 1550 to 532nm with an external photon-number conversion efficiency of (84.4±1.5)%. In contrast to previous works, our conversion efficiency value incorporates all losses before the photoelectric detection, including those introduced by frequency filters. We used sum-frequency generation, which was realized in a standing-wave cavity built around a periodically poled type I potassium titanyl phosphate (PPKTP) crystal, pumped by an intense field at 810nm. Our result is in full agreement with a numerical model. For optimized cavity coupler reflectivities, it predicts a conversion efficiency of up to 93% using the same PPKTP crystal.

Analysis of crystalline perfection of pure and Mo-doped KTP crystals on different growth planes by high-resolution X-ray diffraction

Single crystals of pure and molybdenum (Mo)-doped potassium titanyl phosphate (KTP) crystals were grown by the high-temperature solution growth technique. The presence of dopant ions in the grown crystal was confirmed by energy-dispersive X-ray analysis. Grown crystals, cut along various growth planes such as (100), (011) or (201), were analysed for crystalline perfection using high-resolution X-ray diffraction (HRXRD). Although the HRXRD study showed that the crystalline perfection of most of the crystals was quite good without any structural defects, structural grain boundaries were observed in some of the crystals chosen for study. The observed structural defects are probably due to mechanical or thermal fluctuations occurring during the growth process.

Temperature-dependent optical rotatory power in the presence of birefringence of KTA and KTP crystals by the high-accuracy universal polarimeter method at 632.8 nm wavelength

Temperature-dependent simultaneous measurement of birefringence and optical rotatory power (ORP) for orthorhombic crystals of potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) has been achieved by using the high-accuracy universal polarimeter method at 632.8 nm wavelength. The birefringence and ORP changes along the (110) planes for KTA and KTP crystals were found to have a nearly parabolic form for the temperature range 297–493 K. The thermal variation coefficients were found to be 0.9 (5) × 10−5 K−1and 5.6 (3) × 10−8 K−2for KTA and 0.9 (5) × 10−5K−1and 5.7 (3) × 10−8 K−2for KTP. The ORPs at 297 K were found to be 20.0 (20) and 20.5 (15)° mm−1for KTA and KTP, respectively. The thermal variation coefficients of the ORP were found to be 1.8 (2) × 10−3° mm−1 K−1and 1.1 (1) × 10–5° mm−1 K−2for KTA and 1.9 (2) × 10−3° mm−1 K−1and 1.2 (1) × 10−5° mm−1 K−2for KTP.

Cavity-enhanced bright photon pairs at telecom wavelengths with a triple-resonance configuration

We report on the generation of a bright photon source at telecom wavelengths using spontaneous parametric down-conversion in a type II periodically poled potassium titanyl phosphate crystal. Simultaneous resonances of the pump and the two down-converted fields are achieved with a properly designed optical cavity. This triple-resonance optical parametric oscillator operates far below threshold, generating photon pairs at 1560 nm wavelength with a bandwidth of 8 MHz. Time correlation measurements produce an estimate of 27.7 ns for the coherence time, and production rate of 134±25  s−1 MHz−1 mW−1 for photon pairs. As photon pairs in the telecom regime are suitable for long-distance transmission, this source may find application in quantum communications.

Simultaneous broadband Raman cascading and parametric conversion in potassium titanyl phosphate

We generated a broad spectrum of light between 1064 and 1300nm in the infrared by cascading stimulated Raman scattering in a potassium titanyl phosphate crystal while broadband conversion of the infrared Raman cascade was simultaneously achieved in the visible through second-harmonic generation (SHG) and sum-frequency mixing. We observed that odd- and even-order cascaded Stokes components were spatially addressed at different angles of propagation in the crystal. The efficiency of SHG and sum-frequency mixing is discussed as a function of the pump polarization. We also report on significant spatial distortions of the output Stokes beams.

Electrical and nonlinear optical studies of flux grown Mo and Fe doped KTP crystals

The molybdenum (Mo) and ferric (Fe) doped potassium titanyl phosphate (KTP) crystals were grown by high temperature solution growth (HTSG) technique. The concentration of Mo and Fe in grown crystals was measured by EDX analysis. The SHG efficiencies of the Mo and Fe doped KTP crystals were measured and it was found to be 1.77 and 1.38 times respectively higher than that of pure KTP crystal. The frequency dependence of dielectric constant, loss and ac conductivity was studied at room temperature. The phase matching measurements were made using a Q-switched Nd:YAG laser operating at 1064nm and the measured phase matching angles are 44.2° and 87.88° for Mo and Fe doped KTP crystals respectively.

Generation of self-oscillations from a singly resonant periodically poled potassium titanyl phosphate crystal frequency doubler

We report on the generation of self-oscillations from a continuously pumped singly resonant frequency doubler based on a periodically poled potassium titanyl phosphate crystal (PPKTP). The sustained square-wave and staircase curve of self-oscillations are obtained when the incident pump powers are below and above the threshold of subharmonic-pumped parametric oscillation (SPO), respectively. The self-oscillations can be explained by the competition between the phase shifts induced by cascading nonlinearity and thermal effect, and the influence of fundamental nonlinear phase shift by the generation of SPO. The simulation results are in good agreement with the experiment data.

High-bandwidth squeezed light at 1550 nm from a compact monolithic PPKTP cavity

We report the generation of squeezed vacuum states of light at 1550 nm with a broadband quantum noise reduction of up to 4.8 dB ranging from 5 MHz to 1.2 GHz sideband frequency. We used a custom-designed 2.6 mm long biconvex periodically-poled potassium titanyl phosphate (PPKTP) crystal. It featured reflectively coated end surfaces, 2.26 GHz of linewidth and generated the squeezing via optical parametric amplification. Two homodyne detectors with different quantum efficiencies and bandwidths were used to characterize the non-classical noise suppression. We measured squeezing values of up to 4.8 dB from 5 to 100 MHz and up to 3 dB from 100 MHz to 1.2 GHz. The squeezed vacuum measurements were limited by detection loss. We propose an improved detection scheme to measure up to 10 dB squeezing over 1 GHz. Our results of GHz bandwidth squeezed light generation provide new prospects for high-speed quantum key distribution.

Single tunable laser interrogation of slab-coupled optical sensors through resonance tuning

This paper describes a method for tuning the resonant wavelengths of slab-coupled optical fiber sensors (SCOSs). This method allows multiple sensors to be interrogated simultaneously with a single tunable laser. The resonances are tuned by rotating a biaxial slab waveguide relative to an optical D-fiber. As the slab waveguide rotates, its effective index of refraction changes causing the coupling wavelengths of the slab waveguide and D-fiber to shift. A SCOS fabricated with potassium titanyl phosphate crystal as the slab waveguide is shown to have resonance tuning ranges of 6.67 and 22.24 nm, respectively, for TM and TE polarized modes.

Spectral engineering by Gaussian phase-matching for quantum photonics

We demonstrate Gaussian-shaped phase matching of a periodically-poled potassium titanyl phosphate (PPKTP) crystal by imposing a custom duty-cycle pattern on its grating structure while keeping the grating period fixed. The PPKTP's phase-matching characteristics are verified through optical difference-frequency generation measurements, showing good agreement with expected values based on our design parameters. Our theoretical analysis predicts that under extended phase-matching conditions the custom-poled PPKTP crystal is capable of generating heralded single photons with a spectral purity of 97%, and can reach as high as 99.5% with gentle spectral filtering, something that is highly desirable for photonic quantum information processing applications.

Pseudosymmetric Features and Nonlinear Optical Properties of Potassium Titanyl Phosphate Crystals

A number of publications containing structural data, characteristics of nonlinear optical properties of pure and doped crystals of potassium titanyl phosphate (KTP) family have been reviewed to analyze the structural and symmetry conditionality of nonlinear optical properties of these crystals. The pseudosymmetric features of KTP-type crystals with respect to inversion are investigated. Specifically, pseudoinversion distribution maps are calculated; pseudoinversion extrema and coordinates of pseudoinversion centres are found; and the distributions of pure and doped KTP-type structures and their individual atomic sublattices over the degree of pseudoinversion are analyzed. A correlation between the characteristics of nonlinear optical properties of a number of crystals belonging to the KTP family and the degree of pseudoinversion of their atomic structures is demonstrated.

Generation of femtosecond pulsed quadrature phase squeezed light

We demonstrate experimentally the short pulse amplitude squeezed light from a singly resonant synchronously pumped optical parametric oscillator (SPOPO). The SPOPO operates in the frequency degenerate case and below threshold, with using a periodically poled potassium titanyl phosphate crystal. The pump laser is the second harmonic of a ultrashort mode-locked femtosecond pulse at 850 nm. The reduction of quantum noise of 2.58 dB is obtained experimentally. The squeezing extent is deduced to be 4.48 dB.

KTP crystal growth from the starting materials reacted in solution

AbstractUsing the solution‐reacted materials, potassium titanyl phosphate (KTiOPO4, KTP) crystal was grown by the top‐seeded solution growth (TSSG) method. The solution‐reacted precursor was characterized by scanning electron microscope, and the solubility of KTP in K8P6O19 was measured. The crystals were investigated by synchrotron radiation X‐ray topography, scattering centers measurement, weak absorption test and damage threshold test. The results showed that dispersion was better and solubility was higher than those by solid‐reacted method. Compared with the conventional crystal, KTP crystal grown from the solution‐reacted precursor had fewer defects, fewer scattering centers, lower weak absorption and higher damage threshold.

Continuous-wave nonclassical light with gigahertz squeezing bandwidth

Squeezed states can be employed for entanglement-based continuous-variable quantum key distribution, where the secure key rate is proportional to the bandwidth of the squeezing. We produced a nonclassical cw laser field at the telecommunication wavelength of 1550 nm, which showed squeezing over a bandwidth of more than 2 GHz. The experimental setup used parametric downconversion via a periodically poled potassium titanyl phosphate crystal. We did not use any resonant enhancement for the fundamental wavelength, which should in principle allow a production of squeezed light over the full phase-matching bandwidth of several nanometers. We measured the squeezing to be up to 0.3 dB below the vacuum noise from 50 MHz to 2 GHz limited by the measuring bandwidth of the homodyne detector. The squeezing strength was possibly limited by thermal lensing inside the nonlinear crystal.

A high-brightness source of polarization-entangled photons optimized for applications in free space

We present a simple but highly efficient source of polarization-entangled photons based on spontaneous parametric down-conversion (SPDC) in bulk periodically poled potassium titanyl phosphate crystals (PPKTP) pumped by a 405 nm laser diode. Utilizing one of the highest available nonlinear coefficients in a non-degenerate, collinear type-0 phase-matching configuration, we generate polarization entanglement via the crossed-crystal scheme and detect 0.64 million photon pair events/s/mW, while maintaining an overlap fidelity with the ideal Bell state of 0.98 at a pump power of 0.025 mW.

Study on the weak absorption of potassium titanyl phosphate crystals

Potassium titanyl phosphate (KTP) is mostly used in all-solid- state lasers as a good nonlinear optical crystal. Knowledge of variations in the weak absorption of different types of KTP crystals is important for the application. In this study, the flux-grown KTP crystals with titanium from TiO2, TiCl4, and TiðOC4H9Þ4, respectively, and the hydrothermal- grown KTP crystal were investigated. The weak absorption values of the KTP crystals were measured at 1064 nm and 532 nm by photother- mal common-path interferometer. The weak absorption values of four samples were 100 ppm · cm −1 , 800 ppm · cm −1 , 45 ppm · cm −1 , 1064 ppm · cm −1 at 1064 nm, and 2.6 × 10 4 ppm · cm −1 , 1.6× 10 5 ppm · cm −1 , 1.2 × 10 4 ppm · cm −1 , 2.25 × 10 2 ppm · cm −1 at 532 nm, respectively. The results showed that the weak absorption values of the flux-grown KTP with titanium from TiðOC4H9Þ 4 and the hydrothermal- grown KTP were low values at 1064 nm. In addition, the weak absorption value of the hydrothermal-grown KTP was at least two orders of magnitude lower than that of the flux-grown KTP at 532 nm. From the results, we con- cluded that the KTP grown by titanium from TiðOC4H9Þ4 represented a better quality than the other two flux-grown KTP crystals. The quality of the hydrothermal-grown KTP was obviously better than the flux-grown KTP. © 2012 Society of Photo-Optical Instrumentation Engineers (SPIE). (DOI: 10.1117/ 1.OE.51.3.039001)

Growth of potassium titanyl phosphate crystals using polyphosphate solvents with WO3 additions

The content of WO3 additions to polyphosphate fluxes has been optimized, and crystals have been grown from KTiOPO4-K6P4O13/WO3 and KTiOPO4-K4P2O7/WO3 high-temperature solutions. The crystals have been analyzed for W, and the tungsten distribution coefficients have been determined.

Achieving strong doubling power by optical phase-locked Ti:sapphire laser and MOPA system

We show two external cavity-enhanced second-harmonic generations of 922 nm with periodically poled potassium titanyl phosphate crystal, whose doubling cavities are locked separately with Hansch-Couillaud and intra-modulation methods. The outputs of second-harmonic generation reach 310 mW, 54.8% of the conversion efficiency from the Ti:sapphire laser with the crystal length of 10 mm, and 208 mW, 59% of the conversion efficiency from the MOPA system with the crystal length of 30 mm. It consists of heterodyning the Ti:sapphire laser and the MOPA system, and compares the phase of the beat frequency signal with the phase of a reference RF local oscillator. The resulting phase error is used as a feedback signal and fed back to the reference cavity of the Ti:sapphire laser to lock the two lasers in phase. A stable blue power of 520 mW is obtained, which supplies enough power for the cooling and trapping step of the strontium (Sr) optical lattice clock. Four stable isotopes of Sr, 84Sr, 86Sr, 87Sr, and 88Sr, are detected by probing the laser during a strong 460.7-nm cycling transition (5s21S0-5s5p1P1).