Fig. 1. Calculated poling period of the PPKTP crystal as a function of pump wavelength under different phase-matching conditions. The black horizontal line represents the 10 µm poling period. In this calculation, we assume the degenerate case of λ_s = λ_i = 2λ_p, and the temperature of the crystal is set at 25°C. As shown in (a), three phase-matching conditions are satisfied near λ_p = 405 nm. In (b), we show a zoomed-in plot for pump wavelengths between 400 and 410 nm. Here, these shadowed regions represent the temperature tuning range of each phase-matching condition. For each region, the upper bound and lower bound are at 25°C and 150°C, respectively.

Fig. 2. Experimental setup for observing controllable transitions among different SPDC processes. Here, we used two lasers at 405 and 315 nm. We control the power and polarization of the pump beam, and we also control the temperature of the PPKTP crystal. The properties of these SPDC processes are characterized using a time interval analyzer and a single-photon level spectrometer. HWP, half-wave plate; PBS, polarizing beam splitter; SPFs, short-pass filters; L, lens; DM, dichroic mirror; LPFs, long-pass filters; M, mirror; SMFC, single-mode-fiber coupler; SMF, single-mode fiber; FBS, fiber beam splitter; D, detector.

Fig. 3. Measured properties of our versatile biphoton source. (a)–(d) Spectra of the biphotons for different crystal temperatures; (e)–(h) tuning curves of the four observed SPDCs. Here, we report the center wavelengths of the signal and idler photons at different temperatures. (i)–(l) FWHMs of the signal and idler photons at different temperatures. We note that the values in (e)–(l) were obtained by fitting each spectrum to a Gaussian distribution. (m)–(p) Simulated JSIs of the biphotons.