The calibration method based on correlated photons can realize absolute calibration without standard transfer. The solar spectral irradiance meter using this calibration method needs a correlated photon source with a broad spectrum to achieve absolute calibration in the solar reflection band. In the early stage, non-collinear phase matching combined with multiple crystals was generally adopted to achieve wide spectral coverage. In this paper, a continuous wide spectral correlated photon source is obtained using pumping a BBO crystal by a 532nm laser and continuous angle tuning collinear phase matching. The collinear phase matching positions at different wavelengths are determined by the rotation crystal which shrinks the relevant photon rings into circular spots. The spectral range and correlation of the broad spectral correlated photon source are verified by the correlated photon ring image and the coincidence counting of four pairs of correlated photons of 800nm/1580nm, 840nm/1450nm, 900 nm/ 1300nm, and 950nm/1200nm.
The calibration method based on correlated photons can realize absolute calibration without standard transfer. The solar spectral irradiance meter using this calibration method needs a correlated photon source with a broad spectrum to achieve absolute calibration in the solar reflection band. In the early stage, non-collinear phase matching combined with multiple crystals was generally adopted to achieve wide spectral coverage. In this paper, a continuous wide spectral correlated photon source is obtained using pumping a BBO crystal by a 532nm laser and continuous angle tuning collinear phase matching. The collinear phase matching positions at different wavelengths are determined by the rotation crystal which shrinks the relevant photon rings into circular spots. The spectral range and correlation of the broad spectral correlated photon source are verified by the correlated photon ring image and the coincidence counting of four pairs of correlated photons of 800nm/1580nm, 840nm/1450nm, 900 nm/ 1300nm, and 950nm/1200nm.
The correlated photon technique makes it possible to achieve a radiation calibration processing without chain transferring. The study of the spectral radiation character and time correlation of the photon is of great significance for broad-band radiometric calibration. To absolutely calibrate quantum efficiency of photoelectric detector, it is necessary to extend calibration band from single band to more bands. According to the phase-matching condition of the spontaneous parametric down-conversion, non-collinear angle formula of correlated photon in the crystal is derived. The distribution of the photon spectrum is simulated. Meanwhile, the phase-matching angle of the crystal is optimized. The photon generated by the spontaneous parameters down-conversion has a broad spectral distribution and the correlated photon wavelength is corresponding to its emission angle. Based on the result of the simulation, the experimental measurement system of the spectral distribution and time-correlation of the correlated photon is established. The spectral distribution, the time-correlation and the polarization properties of the four pair photon were measured by using this system. The experimental results show that, (1) the spectral distribution of the measurements is 633~808 nm, where the maximum spectral distribution measurement bias is 1.51 mm. The experiment result is highly consistent with theoretical ones; (2) the correlation time of four pairs correlated photon is measured, during which “three coincidence-peaks” is observed and the minimum correlation time was 0.32 ns; (3) single photon count rate along with coincidence count rate of correlated photon vary with the polarization direction of the pump as sine function. The experimental result is shown that the correlated photon pairs are ranging from the visible to near infrared band distribution, and that the spectrum has time-correlation and polarization characteristic. The research result is firstly reported at home and abroad, and the results are promising for the application of the photoelectric detector in the multi-band radiometric calibration.
There are two general methods in radiometric calibration of detectors, one is based on radiation sources and the other based on detectors. Because the two methods need to establish a primary standard of high precision and a transfer chain, precision of the standard will be reduced by extension of the chain. A new calibration method of detectors can be realized by using correlated photons generated in spontaneous parametric down-conversion (SPDC) effect of nonlinear crystal, without needing transfer chain. Using 351.1-nm output of a tunable laser to pump 'beta'-barium borate (BBO) crystal, an absolute calibration experimental system of single photon detectors based on correlated photons is performed. The quantum efficiency of photomultiplier (PMT) at 702.2 nm is measured by the setup. Advantages of this method over traditional methods are also pointed out by comparison.
In this paper, the spatial properties of correlated photon in collinear phase-matching in the process of spontaneous parametric down conversion (SPDC) are researched. Based on the study of the phase-matching angle, non-collinear angle, and correlated photon wavelength, a theoretical model of non-collinear angular variation is derived, which can be used to estimate and predict the width of the correlated photon ring. The experimental measurement is carried out with CMOS camera, and the measurement results are consistent with the theoretical simulation results, which verifies the rationality of theoretical reasoning. Meanwhile, the change of the correlated photon divergence angle outside the crystal is studied, the closer the wavelength is to the degenerate, the smaller the measurement value of the divergence angle, which is agreement with the theoretical simulation. The results of the study play a reference role in the evaluation of the spatial properties of correlated photon and lay a foundation for the measurement of the correlated photon number rate and the calibration of a photodetector.
Entangled photon pairs generated via spontaneous parametric down-conversion (SPDC) in beta-barium borate (BBO) crystals are fundamental to the realization of anchor-free quantum metrology and spaceborne radiometric standards. High-precision calibration strictly necessitates rigorous spectral and spatial matching of these entangled photons. However, the spatial distribution of entangled photons exhibits a profound dependence on the phase-matching angle. Due to the combined effects of crystal fabrication tolerances, environmental perturbations, and the extremely low intensity of entangled signals (∼pW level), conventional mono-dimensional angle tuning struggles to balance the conflicting requirements of spectral bandwidth and spatial distribution matching. To address this bottleneck, a synergistic tuning strategy integrating angle, temperature, and stress is proposed in this work. A multi-channel, high-precision temperature control system based on series-connected thermoelectric coolers (TECs) was developed, achieving a control precision superior to 0.01 ℃ at a setpoint of 50 ℃. By combining theoretical derivations with thermo-structural coupled simulations, the tuning regularities of temperature (30–70 ℃) and stress on the photon output angle were quantitatively characterized. Experimental measurements on the multi-channel BBO crystal configuration (comprising eight crystals) achieved a maximum tuning range (Δ θ OA ) of 2.941° and identified a non-destructive stress threshold of 312.5 N. This work elucidates the intrinsic mechanisms of temperature and stress in BBO phase-matching, providing critical engineering methodologies and theoretical underpinnings for enhancing quantum calibration accuracy while offering a paradigm for the multi-dimensional tuning of nonlinear optical crystals.
A new calibration method of detectors can be realized by using correlated photons generated in spontaneous parametric down-conversion (SPDC) effect of nonlinear crystal.An absolute calibration system of detector quantum efficiency is performed.And its principle and experimental setup are introduced.A continuouswave (CW) ultraviolet (351 nm), diode-pumped, frequency-doubled, and solid-state laser is used to pump BBO crystal.The quantum efficiencies of the photomultiplier at 633, 702, and 789 nm are measured respectively.The coincidence peaks are observed using coincidence circuit.Some measurement factors including the filter bandwidth of trigger channel, the detector position alignment and polarization of the pump light are analyzed.The uncertainties of this calibration method are also analyzed, and the relative uncertainties of total calibration are less than 5.8%.The accuracy of this method could be improved in the future.
A small, portable, high-flux correlated photon-pair source has been designed and constructed from simple opto-mechanical parts. Unique to this device is its straightforward alignment process, together with the direct coupling of signal and idler photons into polarization-maintaining single-mode optical fibers. Spontaneous parametric down-conversion is used to produce photon pairs in β-barium borate (BBO) at a center wavelength of 810 nm. Owing to the applied type-I phase-matching, coincident photons have identical polarization. The estimated fiber-coupling efficiency is 51%, the measured photon and coincidence flux are 636 and 130 kHz/mW, respectively, normalized to pump power (44 mW). The source has an extremely wide wavelength spectrum of 202-nm FWHM, measured at the fiber output, which limits the actual heralding ratio to 20%.