The generation of correlated photon pairs plays a central role in several quantum mechanics applications, such as quantum information, and cryptography. Correlated photon pairs can be generated inside an optical fiber through the parametric process of degenerated four-wave mixing (FWM). In this paper the authors report the results of two different experiments where wavelength shift of FWM bands in normal dispersive PCFs is investigated.
The generation of correlated photon pairs plays a central role in several quantum mechanics applications, such as quantum information, and cryptography. Correlated photon pairs can be generated inside an optical fiber through the parametric process of degenerated four-wave mixing (FWM). In this paper the authors report the results of two different experiments where wavelength shift of FWM bands in normal dispersive PCFs is investigated.
A fiber-based photon-pair source in the telecom C-band is suitable for quantum information science including quantum communications.Spontaneous four-wave mixing effects are known to create photon pairs that are slightly detuned from the pump wavelength only in the anomalous groupvelocity-dispersion (GVD) regime.Here, we achieve high-quality photon-pair generation slightly detuned from the pump wavelength in the normal GVD regime through a dispersion shifted fiber, for the first time.The photon pairs in C-band exhibit strong temporal correlation with each other and excellent heralded anti-bunching property.This photon-pair generation scheme can be exploited as telecom-band quantum light sources for quantum information applications.A non-classical light source is one of the key elements of quantum information processing systems, and several types of these sources have been investigated, including molecules [1], trapped atoms [2], quantum dots [3], diamond vacancy centers [4], spontaneous parametric down-conversion (SPDC) [5], and spontaneous fourwave mixing (SFWM) [6]. Among these processes, the SPDC and the SFWM processes create quantumcorrelated photon pairs from one and two pump photons, respectively, and both processes require satisfying the energy conservation as well as phase matching [7].Typically, the nonlinear coefficient of SPDC in second-order nonlinear crystals is larger than that of SFWM in amorphous single mode optical fibers as SFWM is a third-order nonlinear process.In the case of several hundred meters long single-mode fibers, however, the SFWM conversion efficiency from pump beam to photon pairs can be similar with that of the SPDC process in nonlinear crystals.In addition, the fiber based photon-pair sources are able to be directly coupled to fiber systems with negligible coupling loss, but waveguide-to-fiber coupling efficiency of a periodically poled SPDC crystal is reported about 50% [8].The SFWM process has been studied intensively for the fiber-based photon generation [6, 9-20] for quantum information applications including quantum key distribution.Photon-pair generation via SFWM is active near the zero group-velocity-dispersion (GVD) wavelength, λ ZGVD , in the anomalous GVD regime [6, 9-15] or normal GVD regime [16][17][18][19][20].In the normal GVD regime, the wavelength of the generated photon pairs is significantly detuned from the pump wavelength.In this case, Raman noise can be avoided, but the generated photon pair do not exhibit telecom-band wavelength together.On the other hand, in the anomalous GVD regime, the wavelength of the generated photon pair is slightly detuned from the pump in the telecom C-band and is strongly power dependent [16].Since the wavelength of the generated pair is sensitive to the λ ZGVD of the generation medium, several types of commercial fibers which have λ ZGVD in the telecom C-band have been investigated as the telecom C-band pair sources.Even if commercial optical fibers have small tolerances in their physical properties, it is common to have sizeable λ ZGVD variance along an optical fiber [21,22].Therefore, the precise measurement of the GVD spectrum is required, but it is often tricky with short length optical fibers.One way to overcome this problem is engineering the dispersion of a commercial fiber by heating and pulling the fiber, but this method is an ambitious research field [19,20,23].On the other hand, if a slightly detuned pair can be generated in the normal GVD regime, the constraint for the λ ZGVD of medium can be weakened, but it has not been reported to the best of our knowledge because phases are mis-matched in this regime due to the Kerr effects of the media.
Summary form only given. Generation of correlated photon pairs by spontaneous parametric four-wave mixing (FWM) in photonic crystal fibres (PCF) opens the way for many applications, particularly for fibre-based quantum communication devices. In degenerate FWM, two pump photons at frequency ωρare annihilated to produce simultaneously a signal photon at ωsand an idler photon at ωi, such as ωs+ ωi= 2ωp(energy conservation). This process occurs with high efficiency only if the phase-matching condition (i.e. momentum conservation) is satisfied [1].The group-velocity dispersion (GVD) of the silica-core PCF used in our experiments (fig.1.a, inset) has been experimentally determined by a time-of-flight measurement for wavelengths from 740 nm to 930 nm. The result is consistent with the analytical calculation provided by Saitoh and Koshiba [2]. Particularly, we estimate the zero-dispersion wavelength of the PCF: λZDW = 834 nm. From this GVD measurement, we deduce the phasematching curve given in fig.1.a, on which we note that there is a vertical asymptote at λp - 832 nm. Thus, FWM should not be observed if the pump wavelength λp is lower than this limit value. Our experimental observations are very different from this prediction. Indeed, by injecting inside the fundamental mode of a 1.3-m-long PCF hyperbolic secant picosecond pulses (with an intensity-FWHM duration of about 2 ps) at λinj = 815 nm with a 80-MHz repetition rate, we actually observe a spontaneous parametric FWM phenomenon (with sidebands generated around 925 nm and 745 nm), and moreover with a clear frequency dissymmetry with respect to the injected pulse frequency (ωinj = 2ππc/λinj) (fig.1.b): generated signal and idler frequencies ωs and ωi do not satisfy ωs + ωi = 2ωinj.A systematic study of this frequency-dissymmetric phenomenon shows that it occurs only when the injected pulse peak power (Pinj) is higher than a threshold value Pth, and that Pth increases when λinj decreases from λZDW. This variation of Pth as a function of λinj can be understood by considering the key role played by self-phase modulation (SPM) in the observed phenomenon. Indeed, the frequency-dissymmetric signal and idler waves are generated only when the spectral broadening (due to SPM) of the injected pulses is sufficient to reach a threshold wavelength λth (i.e. a threshold frequency ωth) of the order of 832 nm. This value corresponds to the position of the vertical asymptote in the phase-matching curve. Finally, we observe that ωs + ωi - 2ωth when Pinj Pth. All these observations highlight the fact that, when the injected pulse peak power Pinj is high enough, spectral broadening by SPM is sufficient to get a part of the injected pulse spectrum higher than the threshold value λth, and thus this part of the spectrum can play the role of pump in an efficient FWM process (i.e. with the phasematching condition satisfied). It is important to note that this phenomenon advantageously presents an extension of the spectral range on which an efficient FWM process can occur: instead of being restricted to injected pulse wavelengths higher than the zero-dispersion wavelength λZDW of the fibre (or at least, higher than a threshold wavelength λth of the order of λZDW), experimenters can work with lower wavelengths, providing the availability of a high enough peak power and/or interaction length to sufficiently broaden (by SPM) the injected pulse spectrum.
Photonic crystal fibers (PCFs) offer a high degree of design flexibility, particularly in terms of dispersion control, making them ideal for nonlinear optical processes. These fibers have found applications in various photonic fields, including fiber lasers [1] and single-photon generation [2]. Dispersion control allows for efficient four-wave mixing at desired wavelengths, which can be used in quantum optical sources. These processes require or are enhanced by the use of fiber Bragg gratings (FBGs) in the fiber. For example, in PCF-based FWM heralded single photon sources, FBGs can be used to filter out unwanted noise contributions, such as Raman noise. However, current filtering methods are lossy [3], making it desirable to integrate filters into the fibers themselves. Additionally, specially designed PCFs with FBGs can be used for in-fiber OPOs to generate squeezed light.
We present a study of the spectral properties of photon pairs generated through the process of spontaneous four-wave mixing (SFWM) in single mode fiber. Our analysis assumes narrowband pumps, which are allowed to be frequency degenerate or nondegenerate. Based on this analysis, we derive conditions on the pump frequencies and on the fiber dispersion parameters which guarantee the generation of ultrabroadband photon pairs. Such photon pairs are characterized by (i) a very large degree of entanglement, and (ii) a very high degree of temporal synchronization between the signal and idler photons. Through a numerical exercise, we find that the use of photonic crystal fiber (PCF) facilitates the fulfilment of the conditions for ultrabroadband photon pair generation; in particular, the spectral region in which emission occurs can be adjusted to particular needs through an appropriate choice of the PCF parameters. In addition, we present a quantum interference effect, resulting from indistinguishable pathways to the same outcome, which can occur when pumping a SFWM source with multiple spectral lines.
The interplay between chromatic dispersion and nonlinear effects is patent. Once a photonic crystal fiber (PCF) preform has been prepared, changing the parameters that control the fabrication process it is possible to adjust the dispersion properties of the fiber. In addition, it is very useful to develop postprocessing techniques that enable a fine adjustment of the dispersion along a section of PCF. The tapering of PCF, using a fusion and pulling technique, has been established as a rather useful technique to engineer the dispersion properties along tens of centimeters. Some of our recent experiments demonstrate that the use of erbium doped PCF enables an optical control of the dispersion through optical pumping of the rare earth ions. Our interest is focused on the exploitation of nonlinear PCF for the development of fiber light sources. Thus, we will present several experiments on supercontinuum and photon pairs generation, through degenerated four-wave mixing (FWM), in order to illustrate the control that one can achieve on the optical spectra of these light sources as a function of different dispersion control approaches.
The paper reviews a new source of time-correlated photon pairs based on phase matched four-wave mixing in an optical fiber. This source has the potential to replace sources based on a non-linear birefringent crystals and waveguides. It generates linear polarised photon pairs in a narrow bandwidth, single circular mode which can be efficiently coupled into single mode fibres. We generate the parametric gain by pumping with a laser wavelength slightly below the zero dispersion wavelength, in the normal dispersion regime.