K. Senthilnathan

Efficient Pulse Compression Using Tapered Photonic Crystal Fiber at 850 nm

By using appropriate self-similar scaling analysis, we delineate the generation of linearly chirped solitary pulses in photonic crystal fiber (PCF) at 850 nm to obtain the short pulses with large compression factor and minimal pedestal energy when compared to adiabatic compression scheme. The dispersion and nonlinearity varying nonlinear Schrödinger equation aptly models the pulse propagation in such a PCF. The analytical results demand that the effective dispersion must decrease exponentially while the nonlinearity must increase exponentially in the PCF. Thus, based on the analytical results, we propose the new design of tapered PCF by varying the pitch and diameter of the air hole. We adopt the projection operator method to derive the pulse parameter equations which indeed very clearly describe the self-similar pulse compression process at different parts of the PCF structures. As we are interested in constructing compact compressor, we also introduce another designing of PCF by filling chloroform in the core region. The chloroform filled tapered PCF exhibits low dispersion length for efficient pulse compression with low input pulse energy over small propagation distances.

All-normal dispersion passively mode-locked Yb-doped fiber laser using MoS2–PVA saturable absorber

We demonstrate the generation of a dissipative soliton in an all-normal dispersion ytterbium (Yb)-doped fiber laser using few-layer molybdenum disulfide (MoS2) as a saturable absorber. The saturable absorber is prepared by mixing few-layer MoS2 solution with polyvinyl alcohol (PVA) to form a free-standing composite film. The modulation depth and saturation intensity of the MoS2–PVA film are 11 and 5.86 MW cm−2, respectively. By incorporating the MoS2 saturable absorber in the fiber laser cavity, the mode-locked pulses are generated with a pulse width of 1.55 ns and a 3 dB spectral bandwidth of 0.9 nm centered at 1037.5 nm. The fundamental repetition rate and the average power are measured as 15.43 MHz and 1.5 mW, respectively. These results reveal the feasibility of deploying liquid-phase exfoliated few-layer MoS2 nanosheets for dissipative soliton generation in the near-IR region.

Designing a Biosensor Using a Photonic Quasi-Crystal Fiber

Using finite-element method, we propose a photonic quasi-crystal fiber-based refractive index biosensor (PQF-RIBS), which works based on the surface plasmon polariton. We determine the loss spectra for two different variations of the refractive index of analyte, na. From the detailed numerical analysis, we find that the PQF-RIBS exhibits a maximum refractive index sensitivity of 6000 nm/RIU and a resolution of 1.6 × 10-6 RIU when na is increased from 1.45 to 1.46. Besides, this sensor does exhibit the negative refractive index sensitivity of -4000 nm/RIU and a resolution of 2.5 × 10-6 RIU for a sensing range from 1.52 to 1.53. Furthermore, we carry out selective filling of liquid in the selective holes of the proposed biosensor for a sensing wavelength range from 900 to 1200 nm. Finally, we also study the influence of the structural parameters, namely, diameter of the core and diameter of the air holes in the cladding over the loss spectra of a fundamental mode for a particular na of 1.47.

Designing photonic quasi-crystal fibers of various folds: onto optimization of efficiency and bandwidth of second harmonic generation

We design photonic quasi-crystal fibers (PQFs) of six-, eight-, ten-, and twelve-folds for determining the optimized efficiency as well as the bandwidth of second harmonic generation (SHG). We report a maximum SHG relative efficiency of 941.36% W⁻¹ cm⁻² for a twelve-fold PQF of 2 μm pitch. The detailed numerical results reveal that, while the relative efficiency increases appreciably, the phase-matching bandwidth increases marginally, as and when the number of folds increases. As the primary interest of this work is to enhance the relative efficiency, we focus our analysis with a twelve-fold PQF for which the efficiency turns a maximum. In line with the practical feasibility of poling, we keep the pitch at 7 μm and report an optimized relative efficiency and phase-matching bandwidth as 95.28% W⁻¹ cm⁻² and 50.51 nm.cm, respectively.

Generation of a Train of Ultrashort Pulses Near-Infrared Regime in a Tapered Photonic Crystal Fiber Using Raised-Cosine Pulses

We consider an optical pulse propagating in a tapered photonic crystal fiber (PCF) wherein dispersion as well as nonlinearity varies along the propagation direction. The generalized nonlinear Schrödinger equation aptly models the pulse propagation in such a PCF. The design of the tapered PCF is based on the analytical results, which demand that the dispersion decrease exponentially and the nonlinearity increase exponentially. In this paper, we adopt the generalized projection operator method for deriving the pulse-parameter equations of the Lagrangian variation method and the collective variable method. Besides, we consider another pulse profile called raised cosine (RC), which is aimed at replacing the conventional hyperbolic secant pulse. From the detailed results, we infer that the initial RC pulse evolves into a hyperbolic secant pulse. Further, in order to minimize the input power requirement, we employ the idea of replacing the solid core in the PCF with chloroform. In addition to the single pulse compression, we also investigate the possibility of multisoliton pulse compression. Here, we consider eight chirped hyperbolic secant pulses as input and generate a train of ultrashort pulses at 850 nm based on the chirped multisoliton pulse compression. In a similar way, we extend this pulse compression with eight RC pulses.

Realizing a robust optical pulse compressor operating at 850 nm using a photonic crystal fiber

This work stems from the motivation of a recent investigation on single- and pulse-train compression schemes using solid core as well as chloroform-filled photonic crystal fiber (PCF), with both hyperbolic secant and raised-cosine pulse profiles. To study the robustness of the proposed optical pulse compressor, we perturb the loss coefficient and study the deviations in the pedestal energy and compression factor for both pulse profiles. To this end, we perturb the loss coefficient of the solid core PCF over a range of % and, in a similar way, we carry out this study for the chloroform-filled PCF, but with the perturbation restricted to a lower level, i.e. over a range of % due to the higher value of the nonlinear growth rate. We compare the intensity profiles of the perturbed compressed pulses with that of an unperturbed pulse. Although the self-similar analysis essentially demands that the loss coefficient matches the nonlinear growth rate for the realization of ideal pulse compressors, based on the numerical results we confirm that the proposed compressor is capable of producing good quality short pulses with negligible amount of pedestals and high compression factor even if there is a deviation of admissible level from the above mentioned condition. Consequently, we corroborate that the proposed compressor is very robust against perturbations.

Tapering photonic crystal fibers for generating self-similar ultrashort pulses at 1550 nm

Abstract. The generation of high-quality self-similar ultrashort pulses at 1550 nm by tapering the photonic crystal fibers (PCFs) is numerically demonstrated. We taper the PCF to achieve the exponentially decreasing dispersion and exponentially increasing nonlinearity profiles, which turn out to be the fundamental requirements for generating the chirped self-similar pulses. Further, we find that the chirped solitons could also be generated with the other three possible exponential variations. Thus, for the first time, we attempt tapering the PCFs for bringing in these exponentially varying dispersion and nonlinear profiles. We carry out the detailed pulse compression studies for various decay rates of the dispersion profiles as the decay rates of dispersion depend on the initial chirp and hence on compression factor, too. The unique feature of this pulse compressor lies in the fact that the required length of the tapered PCF is about 20 times less than that of the previously reported pulse compressor operating at 850 nm.

Large dispersion and high nonlinearity in silicon nanowire embedded photonic crystal fiber

We design a photonic silicon nanowire embedded microstructured optical fiber which is a special class of waveguide whose core diameter is of subwavelength or nanometer size with the air holes in the cladding. We study the optical waveguiding properties, namely, waveguide dispersions and effective nonlinearity by varying the core diameter. The results reveal that the air-clad silicon subwavelength nanowire exhibits several interesting properties such as a large normal dispersion (82385 ps2/km) for 300 nm core diameter and a large anomalous dispersion (-6817.3 ps2/km) for 500 nm core diameter at 1.95 μm wavelength. The structure provides a large nonlinearity (3648 1/Wm) at 0.450 μm wavelength for 300 nm core diameter. These enhanced optical properties might find suitable for various nonlinear applications that include the generation of few cycle pulses, supercontinuum generation and optical processing.

Dynamics of 850 nm optical pulses upon compression in a tapered photonic crystal fiber

We consider the optical pulse propagation in a tapered photonic crystal fiber (PCF) wherein dispersion as well as nonlinearity varies along the propagation direction. The generalized nonlinear Schrödinger equation aptly models the pulse propagation in such a PCF. The design of the tapered PCF is based on the analytical results which demand that the dispersion decrease exponentially and the nonlinearity increase exponentially. By employing the self-similar scaling analysis, we have already proposed the efficient pulse compression scheme with the chirped soliton. In order to get more insight into the dynamics of the pulses (the variations in the amplitude, pulse width and chirp) while being compressed, we adopt the generalized projection operator method (POM) which, in turn, helps arrive at two different sets of pulse parameter equations of Lagrangian variation method (LVM) and collective variable method (CVM).

Pedestal-free pulse compression in nonlinear fiber Bragg gratings with exponentially varying dispersion

We demonstrate almost chirp-fee and pedestal-free optical pulse compression in nonlinear fiber Bragg gratings with exponentially decreasing dispersion. The exponential profile is well approximated by a number of gratings with constant dispersion.