Aminul I. Talukder

Propagation of arbitrarily shaped femtosecond laser pulses through a photonic crystal fiber

We produce femtosecond optical pulses of different shapes to investigate the linear pulse propagation through a photonic crystal fiber in a regime where the conventional group velocity has no meaning and show that the temporal positions of centroid of energy are the same for both arbitrary pulses and transform-limited coherent pulses. By the concept of net group delay we explain the propagation delay for both the cases despite the fact that transmitted pulses suffer a severe distortion in the fiber.

Propagation of the centroid of arbitrary pulses through angularly dispersive systems

We conduct the experimental investigation of arbitrarily shaped ultrashort pulse propagation through angularly dispersive systems, a pair of gratings, and a pair of prisms, in the linear regime. The propagation time has been explained by the net group delay in the context of centroid of energy arrival as the definition of the pulse propagation time. The temporal positions of the centroid of energy are apparently the same for both transform-limited coherent and arbitrary pulses, despite the fact that the pulses suffer severe distortion owing to strong group-velocity dispersion during propagation through the dispersive systems.

Propagation velocity of optical pulses in dispersive media

We experimentally demonstrate the direct measurement of net group and reshaping delays for arbitrary optical pulses in dispersive media, clearly verifying the earlier prediction of Peatross et al. [Phys. Rev. Lett. 84, 2370 (2000)]. The velocity of wave-packet in a material may be highly subluminal, superluminal or even negative as a result of coherent interaction between light and material. Garret and McCumber show that pulse peak for a small optical thickness of a resonantly absorber can actually propagate with the conventional group velocity even it is superluminal or negative [1]. Superluminal pulse propagation has been experimentally observed through resonantly absorber, Raman type gain doublet, 2D photonic crystals, etc. [2-3]. The key point is that the conventional group velocity can explain pulse propagation only in limited conditions; that is for small optical thickness of the medium, narrow input and/or broad absorption line [4]. In this article, we present our experimental investigation of the propagation of different shaped femtosecond optical pulses through various dispersive media hence the measurement of propagation delay. In order to describe the propagation of complicated pulses, one must define the pulse position clearly. It has been proposed to describe the arrival time of a pulse by the expectation integral, ( ) ( )dt t r S dt t r S t t r , / , ρ ρ ρ = , and pointed out that the total time interval between the pulses arrival at two points through the medium is given by, ∆t = Gr + Rr0 [5]. First term is the average of group delay of individual frequencies and called net group delay. Second term is the difference between the pulse arrival time at the initial point r0 evaluated without and with the spectral amplitude that is attenuated during propagation and called reshaping delay. Since wave propagation is a universal phenomenon in physics, experimental verification of the new concept of the group velocity is important. Our experimental results show that the propagation velocity, in terms of net group and reshaping delays, is always significant for any arbitrary conditions of the pulse or medium. The measurement system is based on a cross-correlation technique. In our setup, we have the option to modify and shape the incident pulses into incoherent and chirping pulses that we can do the propagation experiments for coherent, incoherent and chirping pulses. Firstly, we examine the propagation of incoherent optical pulses. In this case, absorption-less dispersive media, ZnSe of different lengths, are used as sample. Figure 1 displays one of our results. In order to verify the significance of net group delay, we also did the same experiments for coherent pulses. It can be seen that the center of mass of incoherent pulse coincides with that of coherent pulse with an accuracy of ±2 fs, as indicated by the downward arrows. So the propagation delays, described at pulse’s center of mass, are exactly same both for coherent and incoherent pulses, Fig.1. Observed cross-correlations of incoherent pulses (dotted curve is for incident coherent pulse) at 800 nm, transmitted trhough ZnSe of (a) 0.0 and (b) 30.0 mm. Center of mass for coherent pulses transmitted through 0.0 and 30.0 mm of ZnSe are at 0 and 173 ps, respectively, while that for incoherent pulses are indicated by downward arrows. Open circles, solid circles and upward triangles are the eye guides to three major peaks of cross correlations. The cross correlation is seen to widen and deformed significantly at 30 mm of ZnSe, evolving an additional peak shown by the downward triangle in (b). QTuL1-6

Propagation of Incoherent and Chirped Optical Pulses in Dispersive Media

We verify that the propagation velocity in the context of net group and reshaping delays is significant for arbitrarily shaped optical pulses in dispersive media. The so-called super-/sub-luminal velocity is modified by the reshaping delay.

Two beam spatial correlation method in amplifying scattering medium

We measured the size of the gain volume in a solution of kiton red with colloidal suspensions of polystyrene spheres using a new technique of two-beam spatial correlation method.

Superluminal pulse propagation and group velocity in anomalous dispersion region of an absorbing medium

We have observed a transition from the superluminal to subluminal velocity for optical pulses in the anomalous dispersion region of a resonantly absorbing medium. The pulse velocity is described by a new expression of group velocity using the saddle point method.