Saba N. Khan

Designing a two-octave spanning flat-top supercontinuum source by control of nonlinear dynamics through multi-order dispersion engineering in binary multi-clad microstructured fiber

A flat-top, coherent supercontinuum generation (SCG) spanning ∼1540  nm from the near-infrared to short-wave infrared (NIR–SWIR) band in a host lead–silicate-based binary multi-clad microstructure fiber (BMMF) is analyzed and reported. This ultra wide band (903–2443 nm) SCG with flatness <5  dB is theoretically achieved with a combination of a low input pump power source (peak power=25  kW, pulse width=75  fs) and a short fiber length (∼8  cm) by controlling the nonlinear dynamics of propagating ultrashort pulses accurately through multi-order dispersion engineering. Simulations reveal that by appropriately controlling the fourth-order dispersion coefficient, a great enhancement in the spectral flatness can be achieved when the device is operated close to the maximum dispersion wavelength in the all-normal dispersion regime.

Characteristics of transverse-stress-induced phase change through a distinct dual-mode fiber in a Sagnac loop

We report here the experimental realization of a fiber-optic transverse-stress sensor devised by a dual-mode optical-fiber segment in a standard Sagnac interferometer loop. The intermodal interference of the LP01 and LP02 modes of the dual-mode fiber (DMF) configuration is analyzed theoretically in the platform of polarization transmittance of the Sagnac loop in implementing the theoretical model. Several experimental measurements for various conditions of applied birefringence are studied at length and the results are compared with those estimated theoretically toward configuring a stress-measuring device. The study provides an understanding of the underlying physics of the working of DMF interference in a Sagnac configuration.

Single all-optical platform for measurement of twist and transverse stress using polarization modulation in distinct dual-mode fiber placed in a Sagnac loop.

We report here the experimental demonstration of measurement of both twist and transverse stress using polarization modulation in a single all-fiber circuit consisting of a single-mode fiber (SMF)/dual-mode fiber (DMF) in a Sagnac interferometer (SI) loop. The SMF-SI prototype setup is seen to be suitable for precise measurement of twist over a broad range of ±50° and transverse stress up to 5 N with a sensitivity as high as 2.85×10(6)  pW/° and 2.08×10(7)  pW/N, respectively. It is envisaged that nearly ideal operation for twist measurement can be achieved by appropriately selecting the operating domain (pretwisted Sagnac loop for practical realization of the device) and required magnitude of applied transverse stress (weight yielding maximum sensitivity). Unlike SMF-SI, a DMF assisted SI exhibits asymmetric transmittance yielding a peak shift (∼45°) in addition to falling/rising peak amplitude of effective power(∼20  μW). This key characteristic is further utilized for tunable measurement of torsion (unidirectional from -70° to 40°) while keeping the sensitivity fixed. This research problem is then analyzed on the avenue of theoretical consideration and using classical polarization optics; we have derived the Jones birefringence matrix that accurately describes the transmission behavior of the configured fiber circuit (SMF-SI and DMF-SI) for each of the three cases, namely, transverse stress, twist, and both twist and transverse stress. Series of experimental measurements for various conditions of induced birefringence (linear/circular) were performed at length, and the results were compared with those determined theoretically towards configuring a twist and stress measuring device. The study provides an understanding of the underlying physics of dual-mode interference in a Sagnac configuration experiencing linear and circular birefringence.

Polarization and propagation characteristics of switchable first-order azimuthally asymmetric beam generated in dual-mode fiber.

We report here the controlled generation of a linearly polarized first-order azimuthally asymmetric beam (F-AAB) in a dual-mode fiber (DMF) by appropriate superposition of selectively excited zeroth-order vector modes that are doughnut-shaped azimuthally symmetric beams (D-ASBs). We first demonstrate continually switching polarization mode structures having an identical two-lobe intensity profile (i.e., intra-F-AAB conversion). Then, under a distinct launching state, we generate mode structures progressively toggling between the doughnut-shaped profile and two-lobe pattern having dissimilar polarization orientations (i.e., F-AAB to D-ASB conversion). Interestingly, a decentralized elliptical Gaussian beam possessing homogenous spatial polarization is obtained by enhancing the contribution of the fundamental mode (HE11/LP01) in selectively excited F-AAB. A smoothly varying azimuth of the input beam in this situation resulted in redistribution of transverse energy procuring a unique and exciting unconventional two-grain T-polarized beam having mutually orthogonal state of polarization (SOP). All of the above three were achieved under a given set of launching conditions (tilt/offset) of a Gaussian mode (TEM00) devised with changing SOP of the input beam. A strong modulation in the output beam characteristics was also observed with the variation in propagation distance (for a fixed input SOP) owing to the large difference in propagation constants of the participating modes (LP01 and one of the F-AABs). Finally, this particular study led to a design for a low-cost highly sensitive strain measuring device based on tracking the centroid movement of the output intensity pattern. Each of our experimentally observed intensity/polarization distributions is theoretically mapped on a one-to-one basis considering a linear superposition of appropriately excited LP basis modes of the waveguide toward a complete understanding of the polarization and mode propagation in the dual-mode structure.

Selective excitation of higher-order modes in etched gelatin-coated few-mode fiber and demonstration of high relative humidity measurement

We report here our experimental studies on efficient excitation of higher-order modes in a few-mode fiber realized through selective chemical etching of single-mode fiber, depositing a dynamic refractive index profile coating (gelatin) and designing a well-performing high relative humidity (RH) (73%-99% RH) sensor with sensitivity as high as ∼1.2  dBm/% RH and fast response (∼125  ms). The design relies on fixing quasi-adiabatic transition of the single-mode-few-mode-single-mode configuration to launch light at angles corresponding to desired higher-order modes, pushing more power into the coating. Our systematic study using the devised hybrid theoretical ray- and wave-optic model uniquely interprets the variety of experimental results.

Binary Multi-clad Microstructured Fiber with All-normal Dispersion for Two-octave Spanning flat-top Coherent Supercontinuum Generation

We demonstrate the possibility of realizing two-octave spanning (990-2546 nm) flat-top coherent Supercontinuum Generation in a host lead-silicate binary multi-clad microstructured fiber by appropriate combination of designed fiber geometry and input pulse-parameter(s).

Switchable Hermite Gaussian Beam Generation in Dual-mode Fiber by Controlling Incident Polarization

We analyze theoretically and experimentally demonstrate the selective excitation in dual-mode-fiber with Gaussian beam generating switchable/asymmetric hybrid polarized Hermite-Gaussian beam with strong dependence of the output beam characteristics on the input beam polarization and fiber-length.

Fabrication and analysis of transmission characteristics of chemically etched 2×2 fiber coupler

We demonstrate the fabrication of chemically etched 2×2 fiber-coupler using a real-time transmission monitoring setup. The transmission-signature provides adequate feedback to control the power-dividing process in attaining targeted specification towards realizing various all-fiber branching components.

Smooth Supercontinuum generation in dispersion-flattened nonlinear High-Index-Core Bragg Fiber

An efficient Supercontinuum generation in High-Index-Core Bragg Fiber under all-normal group velocity dispersion regime is demonstrated. The Full-vector-mode formulation is utilized to obtain ultra-flattened dispersion profile; a stable and spectrum-width of 400 nm is achieved.

Exact mode field solutions and dispersion characteristics of N-layered high-index-core Bragg fiber

We report here the outcomes of our detailed studies on the propagation and dispersion characteristics of N-layered high-index-core Bragg fiber by the analytical evaluations of the mode effective indices and hence the mode-field distribution.