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Journal of the European Optical Society Rapid Publications

The influence of temperature for the single-cycle biphotons generated by type-II SPDC in chirp QPM PPLN

Haibo Lin, Jinbao Wang

Published Jul 15, 2026• 0 citations
Publisher

Abstract

We study the temperature-dependent type II spontaneous parametric down-conversion (SPDC) in the linearly chirped quasi-phase-matching (QPM) periodically poled lithium niobate (PPLN) crystal. The SPDC process can generate the biphotons with the single-cycle level. It shows that the chirp rate of the poled grating periods is the main influence factor for the phase matching function, frequency response range, Fedorov parameter and the count rate of the biphotons. Meanwhile, the temperature plays an important role for the type of $e\rightarrow o+e$, the higher temperature makes the frequency response range be wider, Fedorov parameter and count rate be higher. However, the appropriate chirp rate and temperature can help the type II SPDC generate the single cycle biphotons in chirp QPM PPLN.

DOI
10.1051/jeos/2026056

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We study the temperature-dependent type II spontaneous parametric down-conversion (SPDC) in the linearly chirped quasi-phase-matching (QPM) periodically poled lithium niobate (PPLN) crystal. The SPDC process can generate the biphotons with the single-cycle level. It shows that the chirp rate of the poled grating periods is the main influence factor for the phase matching function, frequency response range, Fedorov parameter and the count rate of the biphotons. Meanwhile, the temperature plays an important role for the type of e→o+ee\rightarrow o+ee→o+e, the higher temperature makes the frequency response range be wider, Fedorov parameter and count rate be higher. However, the appropriate chirp rate and temperature can help the type II SPDC generate the single cycle biphotons in chirp QPM PPLN.
The influence of temperature on generating single cycle biphotons by noncollinear spontaneous parametric down-conversion (SPDC) in chirped quasi-phase-matching (QPM) periodically poled lithium niobate (PPLN) crystal have been studied in this paper. Numerical simulation shows that characteristics of the biphotons are mainly determined by the phase matching function which is dependent on the angle of the noncollinear SPDC and the temperature of the nonlinear crystal. Moreover, the angle plays a more significant role in noncollinear SPDC. The smaller angle and lower temperature can produce better phase matching effect while the angle and temperature changes within an appropriate range. The count rate of the biphotons will decrease, the bandwidth of biphotons will be compressed and the temporal width is widened for a certain appropriate angle in noncollinear SPDC while temperature is increased. However, as long as the angle and temperature are suitable, the biphotons generated by SPDC in the chirped QPM PPLN crystal are ultrashort pulses with single cycles.
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.
In this paper, we have studied the process of monochromatic pump light in linearly chirped Periodically Poled [Formula: see text] (PPKTP) nonlinear crystal to generate single-cycle biphotons under Quasi-Phase-Matching (QPM) conditions through a Spontaneous Parametric Down-Conversion (SPDC). A detailed analysis of the phase mismatching function reveals that in PPKTP crystals, for the proper frequency range, the phase mismatching [Formula: see text] is stable and relatively close to 0 due to the relatively stable refractive index, and therefore has a good phase matching effect. Even when the chirp rate becomes small, it is able to produce single-cycle biphotons due to the near-perfect phase matching. Of course, as the chirp rate increases, the frequency response range becomes wider, the temporal width is narrower, and the number of cycles is smaller.
The wavelength management method was theoretically investigated for the polarization-entangled twin photons in telecommunication band generated by type-II quasi-phase-matched (QPM) spontaneous parametric down-conversion (SPDC) in periodically poled lithium niobate (PPLN) crystals. The wavelength of single-color twin photons from degenerate SPDC can be controlled freely through adjusting the QPM grating period or the working temperature. Especially, adjusting the temperature is more convenient since the QPM grating period is hardly changed once the PPLN structure is fabricated. For the two-color twin photons created by nondegenerate SPDC, strict wavelength exchange of the o-polarization and the e-polarization photons can be realized easily only by tuning the working temperature without pump or structure change.
The characteristics of quantum entangled light sources directly influence the application scenarios and overall performance of quantum systems. By modeling the process of generating time-energy entangled photons via spontaneous parametric down-conversion (SPDC) in a periodically poled lithium niobate (PPLN) crystal, with energy entanglement constituting an essential dimension of this source, the properties of energy entanglement have been systematically investigated in Type-I (e→o + o), Type-II (o→o + e), and Type-0 (e→e + e) quasi-phase matching (QPM) configurations. Through establishing a multivariate correlation model, we quantitatively analyzed the effects of pump parameters (pump linewidth and central wavelength deviation) and crystal parameters (crystal length and temperature) on entanglement metrics including single-photon spectral characteristics, Schmidt number K, measurable parameter R, and frequency overlap O for entangled sources generated by three QPM configurations. The results reveal the strong robustness of Type-I crystals to the pump central wavelength variation and Type-0 systems to the temperature variation. The conclusions might provide theoretical foundations and regulation paradigms for designing time-energy entangled sources tailored to different applications.
Biphoton generation with spontaneous parametric down conversion (SPDC) process has applications in quantum imaging and sensing. Simulations have been done on periodically poled lithium niobate (PPLN) crystals with varying polled period. The poled period has been varied from 20.6μm\mu \mathbf{m}μmto 23.3μm\mu \mathbf{m}μm. The pump wavelengths have been selected in the range of 750 to 850 nm. The variations in temperature and the poled grating facilitate the generation of tunable entangled photon pairs from 1100 nm to 3200 nm.