Abstract
In this paper, the propagation of Temporal Coherence Grating (TCG) pulse trains in a dispersive medium with a chirp is investigated for the first time. The two-time mutual coherence function of the TCG pulse trains propagating through extended dispersive medium specified by temporal ABCD matrix is derived and the evolution of their mean intensity and temporal degree of coherence (DOC) is explored. It is shown that the distribution of the mean intensity can be modulated freely by the number of grating lobes N, grating constant a, pulse duration T0, power distributions vn, group-velocity dispersion coefficient β2 and the medium chirper s. Upon dispersive-medium propagation, the single pulse splits into N+1 subpulses with the same or different peak intensities which depend on power distributions vn. What’s more, during the propagation the pulse self-focusing occurs being the chirp-induced non-linear phenomenon. And the distribution of temporal DOC will degenerate into Gaussian form from initial periodic coherence distribution with increasing propagation distance z or adjusting incident pulse parameters and medium dispersion. The physical explanation and numerical illustrations relating to the pulse behavior are included.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As we all know, the coherence state of a light source plays an important role in the spatio-temporal intensity distribution of the radiated optical field [1,2]. Hence, the modulation of source coherence is an efficient tool for controlling the far-field light intensity distribution [3–7]. After Gori and Santarsiero introduced a simple mathematical procedure for devising genuine spatial correlation functions of stochastic optical sources in 2007 [8] modeling the novel classes of partially coherent beams became a broad research subfield of statistical optics. Structuring of the source coherence state was shown to result in a variety of extraordinary evolution scenarios for the beam average intensity, including self-splitting, self-focusing, self-steering, and flat-topping; as well as formation as rectangular, ring-shaped, dark-hollow, gridded, or cusped profiles in the far field [9–16]. Recently, the outcomes of phase structuring of the complex coherence states have also been thoroughly investigated and were shown to break Cartesian symmetry in the propagating beam’s average intensity and control its tilt and radial acceleration [17–20].
On the other hand stochastic optical pulses with partial spectral or temporal coherence that can be radiated by a variety of currently available sources, such as free-electron lasers, excimer lasers, random lasers and supercontinuum light generated in microstructured fibers, have recently attracted widespread attention because of their possible applications in optical telecommunication, imaging, and fiber optics [21–36]. In the majority of these studies concerning the coherence properties of pulses, the traditional Gaussian Schell-model form is chosen to characterize the correlations between different frequency components or time instants. In 2013 Lajunen and Saastamoinen introduced the correlation functions of the stochastic pulse trains having non-Gaussian correlation Schell-model form and found that the partially coherent pulsed source with non-uniform correlation may lead to the temporal self-focusing phenomenon upon dispersive-medium propagation [37]. Our own recent investigations indicated that in the dispersive fibers the average pulse intensity profiles can be finely controlled by assigning various source temporal coherence profiles [38–42]. Just like in spatial domain, average intensity of stochastic pulses can self-split, form flat-top profiles, form multi-level signals with prescribed order, etc. In addition, Tang et al. pointed out that partially coherent pulsed source with the sinc Schell-model correlation could give rise to a double-layer flat-top intensity distribution and flat-top temporal degree of polarization distribution [43]. Moreover, if the partially coherent pulsed sources are described by frequency-dependent transverse scale parameters, the interesting spatiotemporal coupling is revealed in few-cycle or sub-cycle regimes [44]. A possible experimental procedure for generation of partially coherent pulses has been proposed in [45].
In this paper we will investigate the propagation properties of Temporal Coherence Grating (TCG) pulse trains introduced by Ma and Ponomarenko [46]. A TCG is statistically stationary and has a time-periodic DOC with a given characteristic period, being non-Gaussian correlation Schell-model. We first derive the analytical expression of the mean intensity distribution of the TCG pulse train by means of a temporal ABCD matrix approach. Then we explore the evolution of the mean intensity distribution of the TCG pulse train in dispersive medium with a chirper, where the influence of pulse parameters and medium parameters on is emphasized.
2. Theory
We begin by considering a partially coherent pulsed train source with a two-time Mutual Coherence Function (MCF) having the form of a temporal coherence grating:
In this paper, we consider the paraxial propagation of the MCF of the TCG pulse train through an ABCD optical system. According to extended Huygens-Fresnel integral [22,48], the MCF at the z plane from the source can be expressed as
3. Numerical calculation results and analysis
In this section, we will illustrate the evolution of the temporal intensity profile and the temporal DOC of TCG pulse train in a linearly dispersive medium according to the analytical expressions given in Eqs. (11) and (13). The parameters used in numerical calculations are chosen to be T0=20ps, a=0.25, s=−0.8 × 10−2ps−2, β2=50ps2/km and vn=1 unless different values are specified.
Figure 2 presents the color-coded plot of normalized intensity distribution I(t, z) as a function of propagation distance z and time t for different numbers of grating lobes N with a=0.25. It is shown that the temporal coherence distribution of the incident pulse, characterized by the number of grating lobes N, has an important impact on the propagation properties of the TCG pulse train, especially, on its far-field temporal intensity profile. On the condition of uniform power distribution, i.e., vn=const., as shown in Figs. 2(a)–2(d), for N=1 [Fig. 2(b)], the single pulse begins to split into double pulses with the same peak intensity upon dispersive-medium propagation at propagation distance z=1km. And the single pulse starts to split into three pulses and four pulses for N=2 and N=3, respectively. More generally, the single pulse splits into K+1 subpulses for N = K. For the case of the non-uniform power distribution, i.e., vn=αn/n! with α=3, the peak intensities of the subpulses are different from each other [see Figs. 2(f)–2(h)], strongly depending on the value of parameter α. For larger values, e.g., α=5, the intensity distribution becomes less uniform and some subpulses even disappear near the optical axis [see Figs. 2(j)–2(l)].
In addition, the self-focusing phenomenon occurs during the dispersive-medium propagation. We can give a physical interpretation of this as follows. It is evident from Eq. (12) that, the pulse duration T(z) at z plane can be rewritten as
Figures 4(a)–4(c) shows the color-coded plot of normalized intensity distribution I(t, z) of the TCG pulse train as a function of grating constant a and time t for different N with vn=const., and the corresponding 3D plot is given in Figs. 4(d)–4(f). Here the calculation parameters are z=2 km and T0=20ps. It is shown that, at certain propagation distances (e.g., z=2 km), many subpulses merge into a single pulse possessing larger pulse width with increasing grating constant a. Figures 5(a)–5(c) show the color-coded plot of normalized intensity distribution I(t, z) of the TCG pulse train as a function of pulse duration T0 and time t for different N, and corresponding 3D plot is given in Figs. 5(d)–5(f). The calculation parameters are z=2 km and a=0.25. As can be seen, the influence of pulse duration T0 on intensity distribution I(t, z) has a similar behavior to that of grating constant a, i.e., many subpulses become a single pulse with increasing pulse duration T0. Compared with the effect of grating constant a on intensity distribution I(t, z), the single pulse has much wider pulse width, which approximates a flat-top intensity profile. Hence, Figs. 4 and 5 imply that one can obtain much wider pulse width by adjusting grating constant a or pulse duration T0 of incident pulse.
Figure 6 shows the effect of second-order dispersion coefficient β2 on the normalized intensity distribution I(t, z) of the TCG pulse train with vn=const. As can be seen, the position of the focal spots shifts toward the source plane with increasing β2. And the distances among different focal spots broaden much larger with the increase of β2. These results can be explained according to Eqs. (11) and (15). It follows from Eq. (15) that the position of focal spots is zmin, whose value will decrease with increasing β2. It is then implied by Eq. (11) that the center position of subpulses is tc=πβ2z/aT0, which is a monotonic increasing function of β2 and z. That is why these subpulses diverge so fast during the dispersive-medium propagation.
Figure 7 gives the absolute value of the temporal DOC γ (t1, t2, z) of a TCG pulse train as a function of time interval td=t2-t1, upper limit parameter N and propagation distance z. It is shown that for large td the peak values of the temporal DOC decrease gradually with increasing propagation distance z. Here, term Exp[-(t2-t1)2/4T2(z)] in Eq. (8) plays an important role for the magnitude of the DOC. Hence, this term results in the decrease of the peak values of temporal DOC at large time lags td. However, for short td the minima of the temporal DOC increase gradually with increasing z. Thus, the distribution of the temporal DOC changes from initial periodic comb-form distribution to Gaussian form, with increasing propagation distance z. Figure 8 shows the influence of incident pulse duration T0, grating constant a and second-order dispersion coefficient β2 on the distribution of temporal DOC γ (t1, t2, z) upon dispersive-medium propagation. The calculation parameters are N=2 and z=0.2km. Compared with Fig. 7(b2), we can see from Fig. 8 that the distribution of temporal DOC will degenerate into Gaussian form with decreasing incident pulse duration T0 and grating constant a, or increasing second-order dispersion coefficient β2.
4. Conclusion
In this paper, we have investigated the propagation of the TCG pulse train in dispersive medium with a chirper. The analytic expression for the two-time MCF of the TCG pulse train through an ABCD temporal matrix is derived, and used to explore the evolution of the mean temporal intensity and temporal DOC. We found that the number of grating lobes N plays an important role on the evolution of the mean temporal intensity. Upon dispersive-medium propagation, the single pulse splits into N+1 subpulses with the same or different peak intensities for the power distributions vn=const. or vn=αn/n!, respectively. In addition, the self-focusing effect occurs during the propagation, which is chirp-induced non-linear phenomenon. The physical mechanism is different from the self-focusing phenomenon revealed in [37] (self-focusing being induced by the non-uniform temporal coherence distribution). The chirp of the medium is equivalent to a converging lens, which leads to the compression of the incident pulse duration. The distribution of the temporal DOC will degenerate into Gaussian form with increasing propagation distance or suitable choice of the incident pulse parameters and medium dispersion strength. The results obtained in this study would have potential application in pulse shaping or laser micromachining [49,50]. For example, if we would like to obtain a stochastic pulse with profile with a wide average pulse width, we can modulate the grating constant a or incident pulse duration T0 to find desirable parameters. On the contrary, we can acquire a compressed pulse profile with short pulse duration by introducing a pulse chirper to realize a focusing function. And in the material micromachining, by the suitable choice of grating lobes N, the peak intensities of self-focusing spots can be modulated to ablate the appropriate material and improve fabrication resolution.
Funding
National Natural Science Foundation of China (NSFC) (61575091, 61675094, 11874321); Excellent Team of Spectrum Technology and Application of Henan province (18024123007).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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