On-chip quantum information network requires qubit transfer between different wavelengths while preserving quantum coherence and entanglement, which requires the availability of broadband upconversion. Herein, we demonstrate a mode-hybridization-based broadband nonlinear frequency conversion on X-cut thin film lithium niobate. With the spontaneous quasi-phase matching and quasi-group-velocity matching being simultaneously satisfied, broadband second-harmonic generation with a 3-dB bandwidth up to 13 nm has been achieved in a micro-racetrack resonator. The same mechanism can work on the frequency conversion of the ultrashort pulse in the bent waveguide structure. This work will be beneficial to on-chip tunable frequency conversion and quantum light source generation on integrated photonic platforms and further enable on-chip large-capacity multiplexing, multichannel optical information processing, and large quantum information networks.
Integrated optical amplifiers and light sources are of great significance for photonic integrated circuits (PICs) and have attracted many research interests. Doping rare-earth ions in materials as a solution to realize efficient optical amplifiers and lasing has been investigated a lot. We investigated the erbium-doped lithium niobate on insulator (LNOI). Here, we fabricated a 1-mol% erbium-doped LNOI microdisk with a high-quality factor. C-band laser emission at ∼1530 and ∼1560 nm (linewidth 0.12 nm) from the high-Q erbium-doped LNOI microdisk was demonstrated with 974- and 1460-nm pumping. What’s more, spiral waveguide amplifiers were also fabricated by the CMOS-compatible technique. A maximum internal net gain of 8.3 dB at 1530 nm indicating a net gain per unit length of 15.6 dB/cm with a compact spiral waveguide of 5.3 mm length and ~0.06 mm2 footprint was demonstrated.
Based on second-order nonlinearity, we present a tunable optical bandpass filter at c-band by introducing a back quasiphase-matching technique with a nanostructured named multiple resonator waveguide. Two injecting forward lights and one backward propagating light interact with difference frequency generation. At that juncture, the transmission of the forward signal can be modulated via changing the forward control power. As a result, a tunable optical bandpass filter with multiple flat-top transmit bands of the forward signal can be formed in the waveguide.
We theoretically report a new method to achieve variable-in, variable-out wavelength conversion in LiNbO3 with aperiodic optical superlattices (AOS) structure. The optimal structure of the LiNbO3 sample was obtained through simulated annealing (SA) method. About 3.4nm prescribe pump bandwidth at 1559nm is achieved to fulfill variable wavelength conversion among four selected ITU signal channels (C23-C20). As the result of large pump bandwidth, the reduced effective nonlinear coefficient of the AOS sample decreases to 0.21. Fluctuation of the operation temperature, variation of the incident angle and the errors in poling process show little influence on the performance of the device.
For many nonlinear optical process, it is desirable to maximize the product of the intensity and the interaction length. In this paper, very long subwavelength-diameter optical fibers with low optical loss about 0.1 dB cm-1 were fabricated with a new drawing process by heating the standard single mode fiber with a designed electric strip heater. Pumping the taper fibers with 532 nm mode- locked peco-second laser, we observed exclusive SRS phenomena appeared in these fibers with low pump power and fiber length.
An improved two-wavelength prism coupling method is used to measure the third-order nonlinear optical susceptibility χ(3) of polymer planar waveguide based on the m-line spectroscopy technique. Due to the optical Kerr effect, the index of the waveguide layer is modulated by the strong pump laser which is incident upon the upper base of the prism perpendicularly to guarantee that it is always coupled into the loading force point with the motorized rotary assembly rotate. Measurement of the third-order nonlinear optical susceptibility χ(3) of 1,4-diamino-2,3-diphenoxy-anthraquinone organic dye doped as guests into polymethyl-methacrylate (PMMA) waveguide films is first reported in this paper. A novel method to calculate the third-order nonlinear optical susceptibility χ(3) by the tiny change of the probe beam intensity at a certain synchronous angle of the guided mode is demonstrated and gave the value χ(3)= 3.34x10-11 esu.
We theoretically investigate the evolvement of dark spatial soliton with cascading quadratic nonlinearity in quasi-phase-matched second harmonic generation. It is shown that the dark solitary wave can propagate stably when background intensity is large enough, in which diffraction of beam can be balanced by the cascading quadratic nonlinearity. We also analyze the influence of phase-mismatch on the stability of dark soliton propagation.
We report a theoretical analysis of the generation of broadband and multiple-peak terahertz radiation in aperiodically poled lithium niobate, whose sequence of opposite domains is optimized by simulated annealing (SA) algorithm. The full width at half maximum (FWHM) of the broadband terahertz radiation in our simulation is 0.26 THz. Both of the center wavelength and FWHM can be easily tuned by choosing proper objective functions. Terahertz radiation with wider and flatter FWHM can be achieved by increasing the length of the lithium niobate crystal. The two-peak terahertz generation is also provided as an example of multiple-peak with the central wavelength at 1.68 and 1.80 THz, respectively
We present in this paper a method to recover the refractive index profile of graded index waveguide from the effective indices by cubic spline interpolation function. It is proved by numerical analysis of several typical index distributions that the refractive index profile can be reconstructed closely to its exact profile with the presented interpolation model. This method can reliably retrieve index profile of both more-mode (more than 4 guiding mode) and fewer-mode (2-4) waveguides.
The grating period of optimal all-optical converter based on periodically poled LiNbO3 (PPLN) waveguide dependent on the mode sizes and effective index have been theoretically investigated. Variational method of TM mode in an annealed, proton-exchanged waveguide is first discussed. The optimal grating periods are obtained, which are sensitive to the waveguide width and wavelength change of pump waves, and not to input signal waves. The grating periods in 1.5 um-band are consistent with previously published experimental results.
In this paper, we investigate the acceptance bandwidths due to different polarization of fundamental waves for type I quasi-phase-matched (QPM) second-harmonic-generation (SHG) in bulk periodically poled lithium niobate (PPLN) theoretically, which are determined by the departures from ideal quasi-phase matching in periodicity, wavelength and temperature. This analysis is useful for establishing fabrication tolerances for practical QPM bulk PPLN device. The numerical comparisons of acceptance bandwidths between type I QPM SHG Ez(omega )->Ez2(omega ) and Ey(omega )Ey(omega )->Ez2(omega )(oo->e) at 150 degree(s)C in bulk PPLN are first presented. The grating periods for two various polarization interactions are given. The result shows that the acceptance bandwidths can be significantly enhanced by the use of the perpendicular polarization. Furthermore, the limitation of input fundamental wavelength for Ey(omega )Ey(omega )->Ez2(omega )(oo->e) QPM SHG in bulk PPLN is discussed.
A first order quasi phase matched periodically poled LiNbO3 for SHG with 6.5 micrometers period, 10-mm length and 0.5-mm thickness has been successfully fabricated by applying an external electric field with an optimum high pulsec voltage generator. About 18-mW of cw 0.532 micrometers green light was obtained pumped by a 1.1 W of cw 1.064 μm Nd:YAG laser with end coupling, which corresponds to 1.6 percent power conversion efficiency. The SHG normalized conversion is about 1.5 percent cm-1W-1, which shows 79 percent of the ideal nonlinear coefficient.
Waveguide quasi-phase-matched second-harmonic generation devices are being studied as a promising candidate for realization of a compact short wavelength coherent light source. In this paper, we report quasi-phase-matched second harmonic generation (SHG) in proton-exchanged (PE) lithium niobate waveguide with the fundamental wavelength of 1300nm. The domain inversion was obtained by Ti-indiffusion method near Cuire temperature. The waveguide was fabricated by proton-exchange in the melting benzonic acid, followed by annealing in the air. SHG efficiency as high as 22 percent was observed in the 5mm long periodically pulsed PE lithium niobate waveguide. The SHG conversion efficiency as a function of the fundamental wavelength was measured which exhibits good agreement with the theoretical study. We also found the third harmonic generation from the QPM PE waveguide caused by the cascading nonlinear optical procedures including SHG and sum frequency generation.
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