Periodically Poled Lithium Niobate Crystal Research Articles

Temperature dependence of THz generation efficiency, THz refractive index, and THz absorption in lithium-niobate around 275 GHz

In this study, we demonstrate the capabilities of the pulse train excitation approach in determining key material properties of nonlinear crystals, such as refractive index, thermo-optic coefficient, and absorption. The method provides reliable results even at relatively low THz frequencies, where other characterization methods, such as THz time-domain spectroscopy, have difficulties. To illustrate the capabilities of our approach, we used pulse trains with 800-fs long pulses and adjustable time delay to investigate the material properties of periodically poled lithium niobate (PPLN) crystal with a poling period of 400 µm. Via scanning the incident pulse-train frequency, we measured the frequency response of the crystal at different temperatures (78-350 K), which enabled us to determine the temperature dependence of the refractive index and thermo-optic coefficient of the PPLN crystal around 275 GHz with very high precision. We further studied the variation of THz generation efficiency with temperature in detail to understand the temperature dependence of THz absorption in PPLN material. The technique employed is quite general and could be applied to both other frequency ranges and nonlinear crystals.

Promoting Periodical Poling in Lithium Niobate Crystal Through Surface Acoustic Wave‐Induced Local Lattice Activation

AbstractAchieving precise construction of ferroelectric domain structure in lithium niobate (LN) crystals holds significance for expanding the applications of periodically poled lithium niobate (PPLN), especially in nonlinear frequency conversion. However, challenges exist during the application of electric field for poling, especially for small‐period PPLN crystals, where ferroelectric domains in LN crystals tend to unevenly expand laterally, making it hard to precisely control the domain structures. LN, as a piezoelectric crystal, has the capability to generate energy‐carrying surface acoustic waves (SAW). Herein, an method is presented to optimize the periodic poling process by integrating SAW and poling on a LN chip, which remarkably reduces the poling electric field by 10%, attributed to the enhanced local lattice vibrations during SAW propagation. The integrated SAW and poling chip achieves a 9 µm‐period PPLN crystal. Through second harmonic generation, a nonlinear light conversion from 1158 to 579 nm is realized, exhibiting a nonlinear optical efficiency up to 17.4%. This efficiency aligns with the theoretical prediction, validating the high‐quality domain structure. This integrated chip provides a unique opportunity for manipulating ferroelectric domain with high‐precision, benefiting a wide range of applications in nonlinear‐optics, ferroelectrics, and integrated optics.

Generation of high energy 780 nm femtosecond pulses by frequency doubling of an all-polarization-maintaining Er-doped fiber laser

We demonstrate a high-energy 780 nm wavelength band femtosecond laser by utilizing single-pass frequency doubling all-PM Er-doped fiber amplifier (EDFA) in a periodically-poled lithium niobate (PPLN) crystal. The high power 1560 nm laser is realized based on chirped pulse amplification (CPA) technology, which has obtained the average power of 3.02 W, repetition frequency of 1.03 MHz, and pulse width of 640 fs. Subsequently, by using a PPLN crystal as frequency doubling element, a maximum output power of 0.46 W with 620 fs pulse width at 780.29 nm is generated, the corresponding pulse energy is 0.44 μJ and the peak power is 0.71 MW. To the best of our knowledge, the 0.44 μJ laser output in 780 nm wavelength band is the highest pulse energy generated from a frequency doubling femtosecond Er-doped PM fiber laser.

Effective multiband synthetic four-wave mixing by cascading quadratic processes

Four-wave mixing (FWM) is an important technique for supercontinuum and frequency comb generation in the mid-infrared band. Here, we report simultaneous synthetic FWM in both the visible and mid-infrared bands by cascading quadratic nonlinear processes in a periodically poled lithium niobate (PPLN) crystal, which has a conversion efficiency that is 110 dB (at 3000 nm) higher than the FWM generated directly using third-order susceptibilities in bulk PPLN crystals. A general model of the proposed process is developed that shows full agreement with the experimental verification results. The frequency difference between the emerging frequency components can be tuned freely by varying the frequency difference between the dual pump lasers. Furthermore, by increasing the conversion bandwidth and the efficiency of the cascaded processes, it becomes feasible to generate frequency combs simultaneously in three bands, comprising the visible, near-infrared, and mid-infrared bands, via high-order cascaded processes. This work represents a route toward free-tuning multiband frequency comb generation with multi-octave frequency spanning that will have significant applications in fields, including mid-infrared gas sensing, lidar, and high-precision spectroscopy.

The single-cycle biphotons generated by noncollinear SPDC in the chirped QPM crystals

We analysis the noncollinear Spontaneous Parametric Down Conversion (SPDC) and compare the biphotons generated by the chirped Quasi-Phase-Matching (QPM) between the Periodically Poled Lithium Niobate (PPLN) and Periodically Poled KTiOPO4 (PPKTP) crystals. Due to the chirping of the crystals, the frequency response range of the biphotons would be greatly increased. For nonlinear SPDC, angular variation is limited (less than 0.06° in this paper), and the angle would narrow the frequency response range of the biphotons. We compare the effect of angle in PPLN crystals and PPKTP crystals for biphotons. Both the two crystals with chirped QPM, the single-cycle biphotons can be generated during noncollinear SPDC within a suitable angle range, which is favorable for wider applications in experiments.

Subnanosecond microlaser pumped fan-out grating design MgO:PPLN optical parametric generator continuously tunable from near- to mid-infrared

An in-depth analysis and description of, to the best of our knowledge, the first subnanosecond pulse duration optical parametric generator (OPG) based on a fan-out grating design MgO-doped periodically-poled lithium niobate (MgO:PPLN) crystal is presented. Pumped by high energy subnanosecond pulse duration Q-switched Nd:YAG microlaser pulses, our OPG provides high output energies ranging from tens to hundreds of μJ with up to 45% conversion efficiency and enables fast and precise wavelength tuning across the near-to-mid infrared spectral regions (1414–4303 nm) just by laterally displacing the fan-out crystal without the need for temperature tuning. Limits of operation of the device were found in terms of the laser-induced damage threshold (LIDT) of the fan-out MgO:PPLN crystal. The OPG was fully characterized in terms of spectral, energy, and spatial properties of the signal wave, and analyzed experimentally and theoretically for the feasibility of producing broadband infrared output through non-collinear quasi-phase-matching and conversion of the fan-out grating structure into a chirped one.

Analytical solution to incident angle quasi-phase-matching engineering for second harmonic generation in a periodic-poled lithium niobate crystal

Phase matching or quasi-phase matching (QPM) is of significant importance to the conversion efficiency of second harmonic generation (SHG) in artificial nonlinear crystals like lithium niobate (LN) crystal or microstructured nonlinear crystals like periodic-poled lithium niobate (PPLN) crystals. In this paper, we propose and show that the incident angle of pump laser light can be harnessed as an alternative versatile tool to engineer QPM for high-efficiency SHG in a PPLN crystal, in addition to conventional means of period adjusting or temperature tuning. A rigorous model is established and analytical solution of the nonlinear conversion efficiency under the small and large signal approximation theory is obtained at different incident angles. The variation of phase mismatching and walk-off length with incident angle or incident wavelength are also explored. Numerical simulations for a PPLN crystal with first order QPM structure are used to confirm our theoretical predictions based on the exact analytical solution of the general large-signal theory. The results show that the narrow-band tunable SHG output covers a range of 532 nm–552.8 nm at the ideal incident angle from 0° to 90°. This theoretical scheme, fully considering the reflection and transmission at the air-crystal interface, would offer an efficient theoretical system to evaluate the nonlinear frequency conversion and help to obtain the maximum SHG conversion efficiency by selecting an optimum incident wavelength and incident angle in a specially designed PPLN crystal, which would be very helpful for the design of tunable narrow-band pulse nanosecond, picosecond, and femtosecond laser devices via PPLN and other microstructured LN crystals.

Rainbow Cherenkov Second-Harmonic Radiation

Nonlinear Cherenkov radiation, an intriguing noncollinear optical parametric process with versatile longitudinal phase matching, is commonly realized in engineered nonlinear photonic structures. Here, we report on the observation of ultrabroadband rainbow Cherenkov second-harmonic generation (UBR CSHG) in a single periodically poled lithium niobate (PPLN) nonlinear crystal driven by a near-infrared high-peak-power ultrashort femtosecond pump laser. Multiple colored nonlinear radiation bands along the direction of Cherenkov conical angles are simultaneously exhibited, leading to a beautifully colorful rainbow picture never reported before. We systematically analyze the multicolor spectral characteristics of the UBR CSHG in the whole conical direction and explicitly clarify their phase-matching conditions. We show that this UBR CSHG interaction is subject to remarkable spectral broadening owing to both the third-order nonlinearity effect induced by the high-peak-power ultrashort pump laser and the reciprocal lattice vector of the PPLN crystal, which eventually enables the generation of three bright dominating colors---red, orange-yellow, and green bands---and their automatic dispersion in space. The experimental results suggest rich physics of nonlinear optical interactions and indicate the possibility to engineer broadband nonlinear frequency conversion for applications in compact colored laser sources and others.

Radiation source based on an optical parametric oscillator with MgO : PPLN crystal and volume Bragg grating, tunable in ranges of 2050 – 2117 and 2140 – 2208 nm

A tunable radiation source based on an optical parametric oscillator (OPO) with a periodically poled lithium niobate (PPLN), generating light with a wavelength of ∼2.1 μm in a near-degenerate regime, is developed. The use of a volume Bragg grating (VBG) as a selective element makes it possible to reduce significantly (by a factor of more than 180) the radiation linewidth. Wavelength tuning from 2050 to 2117 nm for the signal wave and from 2140 to 2208 nm for the idler wave is demonstrated. The maximum average OPO output power is found to be 617 mW (123 μJ), which corresponds to a conversion efficiency of 10 %. The use of a VBG provides a fundamental possibility of designing a pump source with a small linewidth (of no more than 1 nm) and smooth wavelength tuning in a desired range for frequency converters based on non-oxide nonlinear crystals generating mid-IR light (5 – 15 μm).

Widely tunable single-photon source with high spectral-purity from telecom wavelength to mid-infrared wavelength based on MgO:PPLN* *Project supported by the National Basic Research Program of China (Grant Nos. 2017YFA0303700 and 2019YFA0308700) and the National Natural Science Foundation of China (Grant Nos. 11627810 and 11690031).

By utilizing the extended phase-matching (EPM) method, we investigate the generation of single photons with high spectral-purity in a magnesium-doped periodically-poled lithium niobate (MgO:PPLN) crystal via the spontaneous parametric down-conversion (SPDC) process. By adjusting the temperature and pump wavelength, the wavelength of the single photons can be tuned from telecom to mid-infrared (MIR) wavelengths, for which the spectral-purity can be above 0.95 with high transmission filters. In experiments, we engineer a MgO:PPLN with poling period of 20.35 μm which emits the EPM photon pair centered at 1496.6 nm and 1644.0 nm and carry out the joint spectral intensity (JSI) and Glauber’s second-order self-correlation measurements to characterize the spectral purity. The results are in good agreement with the numerical simulations. Our work may provide a valuable approach for the generation of spectrally pure single photons at a wide range of wavelengths which is competent for various photonic quantum technologies.

Measurement of the absorption coefficient of a periodically poled LiNbO3 crystal at a radiation wavelength of 3 μm

Problem formulating. In the case of nonlinear-optical conversion of high-power laser radiation in nonlinear-optical crystals the optical absorption leads to a nonuniform heating of the crystals, which results in the violation of phase matching conditions, the formation of defects, and optical damage. Goal. Measurements of optical absorption coefficient of the periodically poled lithium niobate (PPLN) crystal at 3 μm radiation wavelength using the piezoelectric resonance laser calorimetry (PRLC). Result. The optical absorption coefficient of PPLN crystal at 3.045 μm radiation wavelength was measured using the piezoelectric resonance laser calorimetry. The value (1.15±0.15) 10⋅ -2cm-1 of absorption coefficient within the measurement error limits was independent of the initial temperature of PPLN and the incident radiation power in the ranges 293–343 K and 0.2–10 W respectively. Practical meaning. The interaction of mid-IR laser radiation with PPLN crystal was studied using the piezoelectric resonance laser calorimetry.

Design of simultaneous multicolor coherent light generation in a single MgO:PPLN bulk crystal

We report on simultaneous sum-frequency mixing and doubling of C-band and ∼ 980 n m laser sources using a single segmented periodically poled lithium niobate (PPLN) crystal in a single-pass configuration. Our theoretical analysis on the spectral outputs show that a proper design of a five-segment single PPLN crystal has potential to generate violet, blue, green, and orange wavelengths by simultaneous sum-frequency and second-harmonic generation processes. Such a design has promising potential for a compact, tunable multicolored laser that can find various applications such as in biomedicine, visible light communications, laser-based color displays, and sensing.

Accurate measurements of longitudinal temperature distribution of PPLN crystal in the process of second-harmonic generation.

Phase-matching temperature tuning curves and longitudinal temperature distribution of periodically poled lithium niobate (PPLN) crystal were measured in the process of second-harmonic generation (SHG) of 1070 nm pump wavelength using tiny transparent piezoelectric crystals as the temperature sensors. The temperatures of the crystal sensors placed along the PPLN length were determined directly by measuring the induced frequency shifts of their piezoelectric resonances, which were excited in a noncontact manner by a probe radio-frequency electric field. Such temperature sensors do not suffer additional heating conditioned by absorption of scattered radiation and also do not have any contact wires. Due to the high Q factors of piezoelectric resonances, the measurement accuracy of 0.05°C was achieved for a local determination of PPLN surface temperature during SHG. The self-consistent theoretical model that describes the SHG process in the presence of induced nonuniform temperature distribution of the nonlinear-optical crystal is introduced.

Two-Color Quantum Correlation between Down-Conversion Beams at 1.5 and 3.3 μm from a Singly Resonant Optical Parametric Oscillator

We demonstrated a two-color quantum correlation between the down-conversion beams with a telecommunication wavelength at 1.5 μm and mid-infrared wavelength at 3.3 μm generated by a singly resonant optical parametric oscillator (SRO) operated above the pump threshold with a magnesium-oxide doped periodically-poled lithium niobate crystal in the cavity. A maximum of 1.8 dB noise reduction of the intensity difference of the twin beams was measured at the analysis frequency of 5 MHz. Based on a theoretical model for the quantum correlation between the twin beams given by a semi-classical approach, the influence of the analysis frequency and pump parameter on the quantum correlation between the twin beams was discussed theoretically and experimentally. The quantum correlation between the twin beams was degraded at the analysis frequencies above 5 MHz due to the limitation of the bandwidth of SRO cavity and was degraded at the analysis frequencies below 5 MHz due to the excess intensity noise of the pump. The two-color quantum correlated twin beams at 1.5 and 3.3 μm have potential applications in high-precision measurements beyond the shot noise level.

Nonlinear optical vortex coronagraph.

A nonlinear optical vortex coronagraph (n-OVC) based on sum-frequency generation (SFG) in a periodically poled lithium niobate (PPLN) crystal is presented. We demonstrate an n-OVC by mixing the image of an on-axis point source ($ {\lambda _s} = 1.6\;{\unicode{x00B5}{\rm m}} $λs=1.6µm) inside the PPLN crystal with a pump beam (${\lambda _p} = 1064\;{\rm nm}$λp=1064nm) imprinted with a helical phase profile from a vector vortex mask (topological charge ${l} = 2$l=2). Due to quasi-phase matching and orbital angular momentum conservation, a coronagraphic image is produced at the SFG wavelength (${\lambda _{\rm up}}\sim 630\;{\rm nm}$λup∼630nm). We validate that the n-OVC is tunable to signal wavelength but only requires a vortex mask operating at the pump wavelength. The acceptance bandwidth of the SFG process provides the n-OVC a degree of achromaticity even with a monochromatic vortex mask. The n-OVC exhibits an inner working angle of $ \sim {\lambda _s}/{D}$∼λs/D and an experimental contrast of ${{10}^{ - 4}}$10-4 at $3{\lambda _s}/{D}$3λs/D.

RF intensity modulated mid-IR light source based on dual-frequency optical parametric oscillation.

Laser detection and ranging (Ladar)-radar uses intensity-modulated laser beam for ranging and remote sensing. It has the advantages of high spatial resolution from Ladar and immunity to atmospheric turbulence from radar, since the synthetic wavelength is in the order of meters. Intensity modulated mid-IR laser can extend the Ladar-radar concept to mid-IR spectrum. An intensity modulated mid-IR light source with tunable wavelength and modulation frequency is presented. A dual-frequency 1064 nm laser is used to pump an optical parametric oscillator with magnesium oxide doped periodically-poled lithium niobate crystal (MgO:PPLN) as the nonlinear medium. The beat note frequency of the dual-frequency pump laser can be tuned from 140 to 160 MHz. When the pump power is 13 W, the idler output power at mid-IR is 2.38 W, corresponding to a pump-idler conversion efficiency of 19.4%. The wavelength of the idler light is tuned from 3.1 to 3.8 μm by changing the temperature of the MgO:PPLN crystal. The modulation spectra of the mid-IR light are studied. The frequency stability of the beat note in mid-IR is compared with the one in the pump, which are 4.1 Hz and 3 Hz in 240 second measuring time, respectively.

Thermal noise in mid-infrared broadband upconversion detectors.

Low noise detection with state-of-the-art mid-infrared (MIR) detectors (e.g., PbS, PbSe, InSb, HgCdTe) is a primary challenge owing to the intrinsic thermal background radiation of the low bandgap detector material itself. However, researchers have employed frequency upconversion based detectors (UCD), operable at room temperature, as a promising alternative to traditional direct detection schemes. UCD allows for the use of a low noise silicon-CCD/camera to improve the SNR. Using UCD, the noise contributions from the nonlinear material itself should be evaluated in order to estimate the limits of the noise-equivalent power of an UCD system. In this article, we rigorously analyze the optical power generated by frequency upconversion of the intrinsic black-body radiation in the nonlinear material itself due to the crystals residual emissivity, i.e. absorption. The thermal radiation is particularly prominent at the optical absorption edge of the nonlinear material even at room temperature. We consider a conventional periodically poled lithium niobate (PPLN) based MIR-UCD for the investigation. The UCD is designed to cover a broad spectral range, overlapping with the entire absorption edge of the PPLN (3.5 - 5 µm). Finally, an upconverted thermal radiation power of ~30 pW at room temperature (~30°C) and a maximum of ~70 pW at 120°C of the PPLN crystal are measured for a CW mixing beam of power ~60 W, supporting a good quantitative agreement with the theory. The analysis can easily be extended to other popular nonlinear conversion processes including OPO, DFG, and SHG.

Transient absorption imaging of hemes with 2-color, independently tunable visible-wavelength ultrafast source.

Pump probe microscopy is a time-resolved multiphoton imaging technique capable of generating contrast between non-fluorescent pigments based on differences in excited-state lifetimes. Here we describe a fiber-based ultrafast system designed for imaging heme proteins with an independently-tunable pulse pair in the visible-wavelength regime. Starting with a 1060 nm fiber amplifier (1.3 W at 63 MHz, 140 fs pulses), visible pulses were produced in the vicinity of 488 nm and 532 nm by doubling the output of a short photonic crystal fiber with a pair of periodically-poled lithium niobate crystals, providing 5-20 mW power in each beam. This was sufficient for acquiring transient absorption images from unstained cryosectioned tissue.

Fiber-integrated frequency-doubling of a picosecond Raman laser to 560 nm.

We report the development of a fiber-integrated picosecond source at 560 nm by second harmonic generation of a Raman fiber laser. A picosecond ytterbium master oscillator power fiber amplifier is used to pulse-pump a Raman amplifier, which is seeded by a continuous wave distributed feedback laser diode operating at 1120 nm. The pulse train generated at 1120 nm is frequency-doubled in a fiber-coupled periodically-poled lithium niobate crystal module, producing 450 mW of average power at 560 nm with a pulse duration of 150 ps at a repetition rate of 47.5 MHz. The near diffraction-limited (M(2) = 1.02) collimated output beam is ideal for super-resolution microscopy applications.

Femtosecond optical parametric oscillators toward real-time dual-comb spectroscopy

We demonstrate mid-infrared dual-comb spectroscopy with an optical parametric oscillator (OPO) toward real-time field measurement. A singly resonant OPO based on a MgO-doped periodically poled lithium niobate (PPLN) crystal is demonstrated. Chirped mirrors are used to compensate the dispersion caused by the optical cavity and the crystal. A low threshold of 17 mW has been achieved. The OPO source generates a tunable idler frequency comb between 2.7 and 4.7 μm. Dual-comb spectroscopy is achieved by coupling two identical Yb-fiber mode-locked lasers to this OPO with slightly different repetition frequencies. A measured absorption spectrum of methane is presented with a spectral bandwidth of $$300\,\hbox {cm}^{-1}$$ , giving an instrumental resolution of $$0.4\,\hbox {cm}^{-1}$$ . In addition, a second OPO containing two MgO-doped PPLN crystals in a singly resonant ring cavity is demonstrated. As such, this OPO generates two idler combs (average power up to 220 mW), covering a wavelength range between 2.7 and 4.2 μm, from which a mid-infrared dual-comb Fourier transform spectrometer is constructed. By detecting the heterodyned signal between the two idler combs, broadband spectra of molecular gases can be observed over a spectral bandwidth of more than $$350\,\hbox {cm}^{-1}$$ . This special cavity design allows the spectral resolution to be improved to $$0.2\,\hbox {cm}^{-1}$$ without locking the OPO cavity, indicating that this OPO represents an ideal high-power broadband mid-infrared source for real-time gas sensing.