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Bo Cao, Kyohei Hayama, Shun Suezawa, Mamoru Hisamitsu, Katsuhiko Tokuda, [Sunao Kurimura](https://orcid.org/0000-0001-5220-1873), Ryo Okamoto, Shigeki Takeuchi

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[Non-collinear generation of ultra-broadband parametric fluorescence photon pairs using chirped quasi-phase matching slab waveguides](https://mdr.nims.go.jp/datasets/f82e037e-eddd-4bff-8e70-dc9e2f877cb7)

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Non-collinear generation of ultra-broadband parametric fluorescence photon pairs using chirped quasi-phase matching slab waveguidesResearch Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23551Non-collinear generation of ultra-broadbandparametric fluorescence photon pairs usingchirped quasi-phase matching slab waveguidesBO CAO,1 KYOHEI HAYAMA,1 SHUN SUEZAWA,1MAMORU HISAMITSU,2 KATSUHIKO TOKUDA,2 SUNAO KURIMURA,3RYO OKAMOTO,1,4 AND SHIGEKI TAKEUCHI1,*1Department of Electronic Science and Engineering, Kyoto University, Kyotodaigakukatsura, Nishikyo-ku,Kyoto 615-8510, Japan2Shimadzu Corporation, 3-9-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan3National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan4Japan Science and Technology Agency, PRESTO, Gobancho, Chiyoda-ku, Tokyo 102-0076, Japan*takeuchi@kuee.kyoto-u.ac.jpAbstract: Many optical quantum applications rely on broadband frequency correlated photonpair sources. We previously reported a scheme for collinear emission of high-efficiency andultra-broadband photon pairs using chirped quasi-phase matching (QPM) periodically poledstoichiometric lithium tantalate (PPSLT) ridge waveguides. However, collinearly emitted photonpairs cannot be directly adopted for applications that are based on two-photon interference,such as quantum optical coherence tomography (QOCT). In this work, we developed a chirpedQPM device with a slab waveguide structure. This device was designed to produce spatiallyseparable (photon pair non-collinear emission) parametric fluorescence photon pairs with anultra-broadband bandwidth in an extremely efficient manner. Using a non-chirped QPM slabwaveguide, we observed a photon pair spectrum with a full-width-at-half-maximum (FWHM)bandwidth of 26 nm. When using a 3% chirped QPM slab waveguide, the FWHM bandwidth ofthe spectrum increased to 190 nm, and the base-to-base width is 308 nm. We also confirmed ageneration efficiency of 2.4×106 pairs/(µW·s) using the non-chirped device, and a efficiency of8×105 pairs/(µW·s) using the 3% chirped device under non-collinear emission conditions aftersingle-mode fiber coupling. This is, to the best of our knowledge, the first report of frequencycorrelated photon pairs generation using slab waveguide device as a source. In addition, usingslab waveguides as photon pair sources, we performed two-photon interference experimentswith the non-chirped device and obtained a Hong–Ou–Mandel (HOM) dip with a FWHM of 7.7µm and visibility of 98%. When using the 3% chirped device as photon pair source, the HOMmeasurement gave a 2 µm FWHM dip and 74% visibility.© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement1. IntroductionBroadband parametric fluorescence photon pairs play an essential role in many quantum opticalapplications, including quantum information processing [1–3], quantum communication [4,5], andquantum sensing employing frequency correlated photons, which are being intensively developed,including for instance spectroscopy using frequency entangled photon pairs [6], quantum infraredspectroscopy [7–9], and quantum optical coherence tomography with dispersion tolerance[10–13]. In addition, broadband frequency correlated photon pairs are being used for efficiencyenhancement of two-photon absorption [14,15].Numerous methods to generate broadband frequency correlated photon pairs have beendocumented, including generation of frequency correlated photon pairs with a spectral bandwidthof 80 nm using a thin χ(2) bulk crystal [10,16] and broadband parametric fluorescence photon#488978 https://doi.org/10.1364/OE.488978Journal © 2023 Received 2 Mar 2023; revised 19 May 2023; accepted 20 May 2023; published 29 Jun 2023https://orcid.org/0000-0001-8451-8699https://orcid.org/0000-0002-7172-6533https://orcid.org/0000-0001-7731-003Xhttps://doi.org/10.1364/OA_License_v2#VOR-OAhttps://crossmark.crossref.org/dialog/?doi=10.1364/OE.488978&domain=pdf&date_stamp=2023-06-29Research Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23552pair generation with a spectral bandwidth of 160 nm using the superposition of phase matchingof spontaneous parametric down conversion (SPDC) processes in two beta barium borate (BBO)crystals [17]. A scheme that simultaneously uses two SPDC processes in a single QPM deviceis being developed to generate broadband visible-infrared frequency correlated photon pairs[18]. There are reports on the generation of bi-photons with bandwidths exceeding 100 THz viautilizing the group velocity matching in KTiOPO4 (KTP) based devices [19,20]. In addition,using nonlinear fibers, the bi-photons have been generated with a bandwidth of more than 55 THz[21]. Also of note is a method of observing large variable wavelength ranges of photons by tuningthe angle of a χ(2) nonlinear crystal [22]. Recently, in addition to SPDC-based devices, photonpairs with ultra-broadband frequency correlation and comb-like spectra have been generatedusing silicon-based ring resonator cavities [23,24].To the best of our knowledge, frequency correlated photon pairs with the largest spectralbandwidth in frequency (194 THz) are generated using chirped QPM devices [25]. In contrastto commonly used non-chirped QPM devices that have a poling period with a fixed lengththroughout the device, chirped QPM devices have multiple poling periods with varying lengths,which leads to the occurrence of multiple SPDC processes and the emission of photons withvarious colors. Since this scheme was proposed [26,27], the development of devices based onchirped QPM has rapidly picked up, and many studies have been published on the generation ofultra-broadband (bandwidth >100 nm) frequency correlated photon pairs [25,28,29].Recently, to improve the brightness of chirped QPM photon pair sources while maintainingultra-broadband spectral bandwidth generation, a scheme using chirped QPM periodically poledstoichiometric lithium tantalate (PPSLT) with a ridge waveguide structure [30] was developed.Using such a device, the authors managed to observe 320 nm ultra-broadband photon pairgeneration with a hundred times higher generation efficiency compared to bulk chirped QPMdevices.Despite the promising results obtained, chirped QPM ridge waveguides have an issue comparedto chirped QPM bulk crystals in that when employing ridge waveguide devices as photon pairsources, the frequency correlated photon pairs are collinearly emitted into the same spatial mode,i.e., the guided mode of the ridge waveguide. A scheme using time-reversed Hong-Ou-Mandel(HOM) interference has been proposed and demonstrated to separate collinearly emitted twofrequency degenerated photons that also share the same polarization [31,32]. However, using thisscheme, the efficiency of photon pairs separation is dependent on the performance of quantuminterference, which is technically difficult and will be more challenging when photons have alarger bandwidth. This makes it difficult to adopt ridge waveguide-based photon pair sourcesdirectly into existing two-photon interference applications. That is to say, conventional chirpedQPM bulk crystals can emit photon pairs into different spatial modes but are not bright enoughto be used as photon pair sources, whereas chirped QPM ridge waveguide devices are very brightbut only emit photon pairs in the same spatial mode. In this work, we propose a chirped QPMPPSLT device that has a slab waveguide structure to serve as an ultra-bright and ultra-broadbandparametric fluorescence photon pair source that emits photons into separated spatial modes(non-collinear emission). To the best of our knowledge, this is the first report of frequencycorrelated photon pairs generation using slab waveguides as sources. In experiments, we observeda spectrum with a bandwidth of 26 nm using a non-chirped QPM slab waveguide, and a bandwidthof 308 nm (base-to-base width) using a 3% chirped QPM waveguide. We also have carried outexperiments to assess the photon pair generation efficiency under non-collinear emission aftersingle-mode fiber coupling. Using the non-chirped device, the generation efficiency is 2.4×106pairs/(µW·s), while using the 3% chirped device it is 8×105 pairs/(µW·s). Additionally, weperformed a two-photon interference experiment using slab waveguides as photon pair sourcesand obtained a Hong–Ou–Mandel (HOM) dip with a full width half maximum (FWHM) ofResearch Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 235537.7 µm and a visibility of 98% using the non-chirped device, and a 2 µm FWHM dip and 74%visibility using the 3% chirped device.This paper is organized as follows. In the section 2, we introduce our slab waveguides indetail, as well as the method for theoretically predicting SPDC spectra using non-chirp andchirped QPM devices. The section 3 presents a second harmonic generation (SHG) experimentand the results using a non-chirped slab waveguide. Then, under non-collinear emissionphase-matching conditions, we carry out two experiments to evaluate photon pair generation.The experiments and results of measuring the photon spectra are shown in the subsection 4.1.The experiments and results of the evaluation of photon generation efficiency are described insubsection 4.2. Additionally, utilizing slab waveguides as photon pair sources, we perform atwo-photon interference experiment. The experiment and its results are given in section 5.2. DeviceFigure 1(a) depicts a schematic view of our 20-mm-long QPM slab waveguide. Two resincladding layers sandwich a Mg-doped SLT (Mg:SLT) layer with a thickness of ∼ 3.1 µm thatforms the slab waveguide core. The non-chirped QPM slab waveguide has a fixed poling periodof 3.19 µm along the device. We also developed a chirped QPM slab waveguide with a chirp rateof r = 3%. This chirped device is composed of 10 equal-length segments, each with a differentpoling-period length Λm. The length of the poling period Λm for the m-th segment is given by1Λm=1Λ0(︃1 −m − 11 + rrn − 1)︃, (1)where Λ0 = 3.19 µm is the minimum poling period of the chirped QPM device, r = 3% is thechirp rate, m denotes the m-th segment, and n = 10 is the total number of segments in this chirpedQPM device. The phase matching function for a QPM device, as is well known, can be expressedas the Fourier transform of the nonlinearity spatial profile [33]Φ(λs, λi) ∝∫ L0χ(2)Exp (−i∆k(λs, λi)z) dz, (2)where L is the length of the QPM device, χ(2) is the 2nd order nonlinearity, λs and λi are thewavelengths of signal and idler photons, respectively, and ∆k is the phase mismatch.For a device with a poling-period spatial profile Λ(z), the quasi-phase matching function is[25]:∆k(λ, T , θ) =βp(λp, T) − βs(λs, T)√︄1 −(︃sin (θ)n(λs, T))︃2− βi(λi, T)√︄1 −(︃λiλs)︃2 (︃ sin (θ)n(λi, T))︃2−2πΛ(z),(3)where λp, λs, and λi are the wavelengths of pump, signal, and idler photons, respectively, T is thedevice temperature, and θ is the SPDC light emission angle. The effective refractive index insidethe slab waveguide is n(λ), and the wave vectors for the pump, signal, and idler light are βi (i = p,s, i). For a non-chirped device, Λ(z) is constant throughout the device length L. In contrast, in alinearly chirped QPM device, Λ(z) is a step function.In the calculation of the effective refractive index of the slab waveguide, the cladding layerswere assumed to have a constant refractive index of nclad = 1.5, and the refractive index for theMg-SLT core layer was calculated using the Sellmeier equation [34]. The effective refractiveindex for the fundamental mode in the slab waveguide was estimated using a finite-differencetime-domain (FDTD) simulation (COMSOL). In the calculation of SPDC spectra using slabResearch Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23554xzyCladdingCore~ 3 μm20 mm(a)(b) (c)Fig. 1. (a) Schematic view of QPM PPSLT slab waveguide consisting of two cladding resinlayers and a Mg-SLT layer with a height of 3.1 µm as the waveguide core. The domains witharrows represent the periodically poled structure. The length of this device is about 20 mm.(b) Theoretically calculated spectra with FWHM bandwidth of 11.3 nm. This spectrum wascalculated assuming a non-chirped QPM slab waveguide under collinear emission conditionsand pumped by 405 nm continuous wave (CW) light. (c) Predicted spectrum for 3% chirpedQPM slab waveguide with ∼ 330 nm FWHM bandwidth.waveguides, we did not incorporate any slab waveguide properties other than the effectiverefractive index. The device’s thermal expansion impact was also not included. We assumeda continuous wave (CW) pump with a wavelength of 405 nm, applied the effective refractiveindex to Eq. (3), and then calculated the spectra of parametric fluorescence generated under acollinear emission condition using a non-chirped QPM slab waveguide (Fig. 1(b)) and a 3%chirped waveguide (Fig. 1(c)). With a non-chirped device, the calculated spectrum has a sinc-likeshape with a FWHM bandwidth of 13.4 nm. We also obtained an extended spectrum with aFWHM width of 329 nm using a 3% chirped waveguide. The comb-like spectrum of the chirpeddevice is caused by interference of numerous phase matching functions with discrete periodicallypoled lengths. A flatter spectrum can be obtained by increasing the number of segments in thechirped QPM device [11,25].3. Second harmonic generation using non-chirped QPM slab waveguideTo test this slab waveguide, we carried out a second harmonic generation (SHG) measurementusing a non-chirped QPM slab waveguide. In this experiment, we use 810 nm continuous wave(CW) light (Coherent, MBR110) as pump light. For coupling the pump beam into the slabwaveguide, a pair of cylindrical lenses with focal lengths of 80 mm and 3.9 mm are used tocondense the pump beam in the horizontal and vertical directions, respectively (Fig. 2(a)). Thecoupling efficiency of 810 nm pump light from air to waveguide is approximately 20%. Duringthis measurement, the device temperature is kept at 59.2 ◦C. The upconverted light is measuredby a power meter (Thorlabs, S130C) after eliminating the pump light using a shortpass filter.Research Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23555CW pump laser@810 nmCylindrical lens (H)Temperature controllerPower meterCylindrical lens (V)PPSLT slab waveguide0 1 2 3 40246810Pump power [mW]SHG power [µW](b)(a)Fig. 2. (a) Experimental setup for SHG measurement. The 810 nm pump light beam iscondensed horizontally by an f = 80 mm cylindrical lens and vertically by an f = 3.9 mmlens. The device temperature is set to 59.2 °C. The upconverted light is measured by a powermeter. (b) Measured upconverted light power as function of pump light power.Figure 2(b) shows the results for the measured upconverted light power I2ω while altering thepump power Iω . The red solid curve in Fig. 2 is the fitting result for I2ω = ηI2ω , where η = 76%/W is the normalized conversion efficiency. This result is about 7 times smaller than what wehave obtained with a ridge waveguide device [30], as the slab waveguide only confines the pumpbeam in one dimension.4. Non-collinear emission of photon pairs using slab waveguidesIn contrast to the commonly used ridge waveguides, slab waveguide devices can be used fornon-collinear photon pair emission in addition to collinear emission. This is an essential featurefor applications like QOCT. We use the setup shown in Fig. 3(a) to experimentally observe thenon-collinear emission of parametric light. In this experiment, a 405 nm CW laser (Coherent,MBR110, and MBD200) is used as the pump light, and the device temperature is controlled by atemperature controller (Cell system, TDB1700) with an accuracy of ± 0.02 °C. Following thewaveguide, to filter out the pump light, an 810 ± 10 nm bandpass filter is used. The light beamis then focused on an intensified CCD (ICCD) camera (Hamamatsu Photonics, C5909-08) toobserve the beam spatial profile. The spatial images are observed while the device temperature isbeing adjusted. The results are shown in Fig. 3(c). The emission angle changed from collinear tonon-collinear as the device temperature decreased from 59.4 °C to 59.0 °C. When the temperaturewas further decreased to 58 °C, the spatial separation also increased.4.1. Ultra-broadband photon-pair generation using chirped QPM slab waveguideWe observed the spectra of parametric fluorescence emitted from our slab waveguides usinga spectrometer (Princeton, Acton SP2300) connected to single-mode fiber A or B, as shownResearch Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23556(a)(b)CW pump laser@405 nmCylindrical lens (H)Temperature controllerBandpass filterlensICCD cameraCylindrical lens (V)PPSLT slab waveguideT = 58.0 ˚C T = 59.0 ˚C T = 59.4 ˚CFig. 3. (a) Experimental setup used to observe spatial profile of parametric fluorescencewhile tuning device temperature. (b) Intensified CCD (ICCD) camera images of SPDC lightemitted using non-chirped slab waveguide while tuning the device temperature from 58.0 °C(non-collinear emission condition) to 59.4 °C (collinear emission condition).in Fig. 4. In this experiment, 404.9 nm pump light (Coherent, MBR110, and MBD200) iscondensed horizontally and vertically using a pair of cylindrical lenses, a 200 mm cylindricallens (Newport, CKX200) for condensing the beam horizontally and a 3.9 mm cylindrical lens(Thorlabs, LJ1598L1-A) for condensing the beam vertically, and coupling the pump light intothe waveguides. The typical pump light coupling efficiency (from air to waveguide) is about20%. Then, the parametric fluorescence is collimated by another pair of cylindrical lenses withfocal lengths of 3.9 mm (Thorlabs, LJ1598L1-B) and 80 mm (Thorlabs, LJ1105L2-B). A prismmirror is used to separate the non-collinearly emitted photon pairs into two independent opticalpaths. After eliminating the pump light with two longpass filters (Thorlabs, FELH0550), wecouple the parametric downconverted photons into two single-mode fibers A and B (Thorlabs,P1-780PM-FC-2), then feed the photons to single-photon detectors or to the experimental setupshown in Fig. 7(a) for further measurements. In the following experiments, we set the devicetemperature to 57.5 °C with an accuracy of ± 0.02 °C.For the non-chirped QPM waveguide, the measured spectra of signal and idler photons showsingle peaks at 810 nm with a bandwidth of 26 nm FWHM (Fig. 5(a)). The bandwidth for themeasured spectra is larger than that for the simulation results. One possible reason for this couldbe the larger number of possible wave vectors involved in the SPDC process in the non-collinearcondition.We also observed expanded spectra of signal and idler photons when using a 3% chirpedQPM slab waveguide. The FWHM bandwidth was 190 nm and the base-to-base width was 308nm (682 to 990 nm). This results in Fig. 5(b) show an absence of frequency components atlonger wavelengths, compared with the simulated results shown in Fig. 1(c), which may be dueto chromatic aberration associated with the cylindrical lenses used to collimate the parametricResearch Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23557CW pump laser@405 nmCylindrical lens (H)Temperature controllerCylindrical lens (H)MirrorPrism mirrorLongpass filterSingle-mode fiber ATime analyzerSingle photon detectorsSingle-mode fiber BPPSLT slab waveguideCylindrical lens (V)MirrorFig. 4. Experimental setup for non-collinear emission of parametric fluorescence photonpairs. A 404.9 nm pump light is condensed by a pair of cylindrical lenses with focal lengthsof 200 mm (horizontal direction) and 3.9 mm (vertical direction). The downconverted lightbeams are collimated by another pair of cylindrical lenses with focal lengths of 80 mm(horizontal direction) and 3.9 mm (vertical direction). A prism mirror is used to reflect thenon-collinearly emitted photon pairs into two separate optical paths. A pair of longpassfilters is used to eliminate the pump light. Photon pairs are coupled into two single-modefibers A and B for detection. The device temperature is set to 57.5 °C.FWHM(a) (b)FWHMBase-to-basewidthFig. 5. (a) Measured parametric fluorescence photon pair spectra for non-chirped QPMslab waveguide. The FWHM is 26 nm. Signal and idler are photons emitted into differentspatial modes. (b) Spectra measured with 3% chirped QPM slab waveguide. The FWHMbandwidth is 190 nm and the base-to-base width is 308 nm.fluorescence light and the lens used for single-mode fiber coupling. In addition, the CCD camerain our spectrometer has a reduced quantum efficiency at long wavelengths. On the other hand,the unexpected frequency components with shorter wavelength could be explained by emissionunder the phase matching conditions of higher order guided modes. Similar to the simulatedresults, comb-like spectra are observed in the experimental results.4.2. Highly efficient photon pair generation using slab waveguidesTo evaluate the generation efficiency of our slab waveguides, we carried out photon pair correlationmeasurements. Photon pairs are generated and collected into single-mode fibers using the setupshown in Fig. 4. The photons are then fed into the detection unit where two single-photon countingmodules (SPCMs) (Excilitas, AQHR-14-FC) are used for single-count event measurements.The coincidence count events are measured by a time analyzer (ID Quantique, id800) with acoincidence window width of 4 ns.As shown in Figs. 6(a) and (b) for the non-chirped and 3% chirped slab waveguides, respectively,the measured single and coincidence count rates are proportional to the pump light power.Background counts and accidental coincidence counts are subtracted from these results. The totalgeneration rate N0 can be simply estimated as N0 = Na×Nb/Ncc, where Na and Nb are the singlecount rates obtained with detectors a and b, and Ncc is the coincidence count rate. The results ofResearch Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23558linear fitting indicate that the single-count rates using the non-chirped slab waveguide are Na= 1.6×105 counts/(µW·s) and Nb = 1.6×105 counts/(µW·s). The coincidence count rate Ncc is1.1×104 counts/(µW·s). Therefore, the generation efficiency of our non-chirped slab waveguideis N0 = 2.4×106 pairs/(µW·s). For the 3% chirped slab waveguide, the single and coincidencecount rates are Na = 4.0×104 counts/(µW·s), Nb = 3.7×104 counts/(µW·s), and Ncc = 1.8×103counts/(µW·s). The generation efficiency of the 3% chirped device is 8.2×105 pairs/(µW·s). Thecoupling efficiency of photons from waveguide to single mode fibers can be simply estimated byηcp = 2 Ncc/(ηde(Na + Nb)), where ηde is the detection efficiency, which is assumed to be 50%. Inthe case of using the non-chirped device, the ηcp = 13%, and while using a 3% chirped device, theηcp = 9%. The primary culprit of loss is due to two factors. The first one is chromatic aberrationin the lenses used to collect photons into single mode fibers. And the second is the difficulty incollecting photon pairs emitted from multiple segments with variable emission angles. Note thatwe believe that the propagation loss inside the PPSLT slab waveguide of both the pump light(405 nm) and the SPDC light (682nm to 990nm) are insignificant because the SLT crystal has atransparent range of 270 nm to 5500 nm and the waveguide core is 3 µm in size (height), whichis much greater than the longest wavelength component of the generated SPDC photons.(a) (b)Fig. 6. (a) Single and coincidence counting events measured using non-chirped device. Theblack and red dots are the measured single-count events against pump light power, and theblue dots are the coincidence count events. The black, red, and blue lines are linear fitsto the experiment data. (b) Single and coincidence counting events measured with a 3%chirped slab waveguide.Table 1, compares these results to those for our previously reported QPM devices. Thegeneration efficiency for our 20 mm long non-chirped slab waveguide under non-collinearemission conditions is comparable to that for a 10 mm non-chirped ridge waveguide deviceunder collinear emission conditions [30]. However, the generation efficiency for the 3% chirpedQPM slab waveguide is 70% less than that for a 3% chirped ridge waveguide. This reductionin efficiency may be caused by two reasons. One is the effect of interference of multiple phasematching functions with larger phase deviations in a longer device. The other reason is that slabwaveguides have a lower pump power density than ridge waveguides, since slab waveguides onlyconfine the pump light beam in one dimension. The generation efficiency for the chirped QPMslab waveguide is over 400 times higher than that for a 20 mm long 6.7% chirped QPM PPSLTbulk [11]. We believe that slab waveguides have advantages for both waveguide devices and bulkdevices, namely, slab waveguides generate photon pairs with an efficiency close to that for ridgewaveguides and can emit photon pairs into separate spatial modes like bulk crystals. This iscritical for applications such as QOCT.Research Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23559Table 1. Comparison of PPSLT chirped QPM devices.Device Chirp rate Device length(mm)Segments BandwidthFWHM (nm)Generationefficiency(pairs/(µW·s))Slab 0% 20 10 26 2.4 × 106waveguide 3% 20 10 190 8.2 × 105Ridge [30] 0% 10 10 16 2.7 × 106waveguide 3% 10 10 229 2.7 × 106Bulk [11] 6.7% 20 100 380 2.0 × 1035. Two-photon interference measurements using slab waveguidesSlab waveguides can be directly used as parametric fluorescence sources for applications likeQOCT without any post-selecting process. Therefore, we also carried out HOM measurementsusing the setup shown in Fig. 7(a). Non-collinearly emitted signal and idler photons arecoupled into single-mode fibers A and B (in Fig. 4) and then connected to inputs 1 and 2 of theinterferometer separately. Two pairs of quarter waveplates (QWPs) and polarization beamsplitters(PBSs) are used in the sample and reference arms as a pair of polarization circulators to improvethe total transmission efficiency of the interferometer [16]. In the following HOM experiments,we placed a mirror in the sample arm as the measurement sample, and installed another mirroron a motorized stage to adjust the optical path length in the reference arm. After interferenceat the non-polarized beam splitter (BS), photons are coupled into single-mode fibers again atoutputs 1 and 2, then sent to the detection unit (Fig. 7(b)) for coincidence measurements.(a) (b)Input 1Sample (mirror)Reference mirrorBSOutput 1 Output 2Input 2PBSPBSMirrorSingle-mode fibersSingle photon detector ASingle photon detector BQWPQWPTime analyzerFig. 7. (a) HOM measurement setup. We use quarter waveplates (QWPs) and polarizationbeamsplitters (PBSs) to improve the transmission efficiency of our interferometer. In thereference arm, a mirror mounted on a motorized stage is used for adjusting the optical pathlength. (b) Event-counting detection unit. Single photons are fed into detectors A and Busing single-mode fibers. Single-count events are measured by two single-photon detectors,and coincidence events are recorded by a time analyzer.In this experiment, we first pump the non-chirped QPM slab waveguide, using laser light(Coherent, MBD200) with a power of 50 mW and a wavelength of 404.9 nm. Then, non-collinearly emitted photon pairs are fed into a HOM interferometer as shown in Fig. 4(b) andphotons are detected using two SPCMs. The coincidence count rates are recorded using a timeanalyzer (ID Quantique, id800) with a coincidence window width of 4 ns. As shown by thered dots in Fig. 8(a), we observed a HOM dip with a FWHM of 7.7±0.2 µm , estimated usingGaussian fitting. The black curve in the figure is the theoretical calculation result using themeasured spectrum, which has a width of 7.2 µm and matches the experimental result. The HOMinterference visibility V = (Nmax - Nmin)/Nmax of this dip is 98% after subtracting accidentalResearch Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23560counts, where Nmax and Nmin are the average base count rate and dip count rate for the HOMresults.Exp. dataCal. curve(a)-30 -10 10 300200400600Delay [µm]Coincidence count rates [/0.1s](b)Exp. dataCal. curve-30 -10 10 300400800Delay [μm]Coincidence count rates [/0.1s]Fig. 8. HOM dips measured with slab waveguides after subtracting accidental coincidencecounts. (a) HOM interference measured using non-chirped slab waveguide as photon pairsource (red dots). The width of this HOM dip is 7.7±0.2 µm, and the visibility is 98%. Theblack curve is calculated using the measured spectra. (b) Results using 3% chirped device.The measured HOM dip width is 2±0.1 µm and the visibility is 74%.In the experiment using the 3% chirped device, a superconducting single-photon detector(SSPDs) (Scontel, FCOPRS-CCR-SW60-LW60) and a SPCM are used as photon detectors.Coincidence counts are measured with a coincidence window of 1 ns using a time analyzer (IDQuantique, id900). We observed a HOM dip with a FWHM of 2.0±0.1 µm, as shown by thered dots in Fig. 8(b). In our theoretical calculations after considering the loss from single-modefiber coupling, the FWHM of the HOM dip was 1.4 µm. This decrease in the HOM dip widthmay be caused by a lower detection efficiency when detecting photons with longer wavelengths.The visibility of this HOM dip after subtracting the accidental coincidence counts was 74%.The degradation in visibility measured with the 3% chirped device compared to that with thenon-chirped device could be explained by optical system loss and the uneven splitting ratio of the50:50 beamsplitter over the wide range of wavelengths used in the HOM interferometer.6. ConclusionBroadband photon pair sources using conversational QPM bulk crystals have insufficientbrightness, whereas QPM ridge waveguide devices are highly efficient but only emit photon pairsin the same spatial mode. To develop an efficient photon pair source that can be directly usedin applications based on two photon interference, we propose a chirped QPM PPSLT devicethat combines a chirped QPM and a slab waveguide structure. This device is developed forultra-broadband and highly efficient generation of spatially separable parametric fluorescencephoton pairs. In experiments, we observed a spectrum with a FWHM bandwidth of 26 nm usinga non-chirped slab waveguide and a FWHM bandwidth of 190 nm (base-to-base width of 308nm) using a 3% chirped QPM slab waveguide. Moreover, we observed a generation efficiency of2.4×106 pairs/(µW·s) using the non-chirped device and 8×105 pairs/(µW·s) using the 3% chirpeddevice under non-collinear emission conditions after single-mode fiber coupling. Additionally,we performed two-photon interference experiments using slab waveguides as photon sourcesand obtained a HOM dip with a FWHM of 7.7µm and a visibility of 98% using the non-chirpeddevice. When using the 3% chirped device, the HOM measurement results show a dip witha FWHM of 2 µm and a visibility of 74%. We believe that, when employed as a photon pairsource, slab waveguide QPM devices have the benefits of ridge waveguides (high brightness) andResearch Article Vol. 31, No. 14 / 3 Jul 2023 / Optics Express 23561bulk crystals (non-collinear emission capability), and are promising for a wide range of opticalapplications.Funding. Core Research for Evolutional Science and Technology (Grant No. JPMJCR1674); Ministry of Education,Culture, Sports, Science and Technology, Quantum Leap Flagship Program (Grant No. JPMXS0118067634); CabinetOffice, Government of Japan PRISM; Japan Society for the Promotion of Science (Grant No. 21H04444); MEXT WISEProgram.Acknowledgments. We wish to thank Zhenghao Yin for help in the simulation of the effective refractive index.Disclosures. The authors declare that there are no conflicts of interest related to this article.Data availability. 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