Thomas Tanggaard Alkeskjold

All-optical modulation in dye-doped nematic liquid crystal photonic bandgap fibers

Photonic crystal fibers (PCFs) have attracted significant attention during the last years and much research has been devoted to develop fiber designs for various applications, hereunder tunable fiber devices. Recently, thermally and electrically tunable PCF devices based on liquid crystals (LCs) have been demonstrated. However, optical tuning of the LC PCF has until now not been demonstrated. Here we demonstrate an all-optical modulator, which utilizes a pulsed 532nm laser to modulate the spectral position of the bandgaps in a photonic crystal fiber infiltrated with a dye-doped nematic liquid crystal. We demonstrate a modulation frequency of 2kHz for a moderate pump power of 2-3mW and describe two pump pulse regimes in which there is an order of magnitude difference between the decay times.

Electrically tunable photonic bandgap guidance in a liquid-crystal-filled photonic crystal fiber

Tunable bandgap guidance is obtained by filling the holes of a solid core photonic crystal fiber with a nematic liquid crystal and applying an electric field. The response times are measured and found to be in the millisecond range.

Continuously tunable devices based on electrical control of dual-frequency liquid crystal filled photonic bandgap fibers.

We present an electrically controlled photonic bandgap fiber device obtained by infiltrating the air holes of a photonic crystal fiber (PCF) with a dual-frequency liquid crystal (LC) with pre-tilted molecules. Compared to previously demonstrated devices of this kind, the main new feature of this one is its continuous tunability due to the fact that the used LC does not exhibit reverse tilt domain defects and threshold effects. Furthermore, the dual-frequency features of the LC enables electrical control of the spectral position of the bandgaps towards both shorter and longer wavelengths in the same device. We investigate the dynamics of this device and demonstrate a birefringence controller based on this principle.

Electrically and mechanically induced long period gratings in liquid crystal photonic bandgap fibers

We demonstrate electrically and mechanically induced long period gratings (LPGs) in a photonic crystal fiber (PCF) filled with a high-index liquid crystal. The presence of the liquid crystal changes the guiding properties of the fiber from an index guiding fiber to a photonic bandgap guiding fiber - a so called liquid crystal photonic bandgap (LCPBG) fiber. Both the strength and resonance wavelength of the gratings are highly tunable. By adjusting the amplitude of the applied electric field, the grating strength can be tuned and by changing the temperature, the resonance wavelength can be tuned as well. Numerical calculations of the higher order modes of the fiber cladding are presented, allowing the resonance wavelengths to be calculated. A high polarization dependent loss of the induced gratings is also observed.

Theoretical analysis of mode instability in high-power fiber amplifiers

We present a simple theoretical model of transverse mode instability in high-power rare-earth doped fiber amplifiers. The model shows that efficient power transfer between the fundamental and higher-order modes of the fiber can be induced by a nonlinear interaction mediated through the thermo-optic effect, leading to transverse mode instability. The temporal and spectral characteristics of the instability dynamics are investigated, and it is shown that the instability can be seeded by both quantum noise and signal intensity noise, while pure phase noise of the signal does not induce instability. It is also shown that the presence of a small harmonic amplitude modulation of the signal can lead to generation of higher harmonics in the output intensity when operating near the instability threshold.

Electrically controlled broadband liquid crystal photonic bandgap fiber polarimeter

We demonstrate a liquid crystal photonic bandgap fiber based polarizer integrated in a double silicon v-groove assembly. The polarizer axis can be electrically controlled as well as switched on and off.

Distributed mode filtering rod fiber amplifier delivering 292W with improved mode stability.

We demonstrate a high power fiber (85 μm core) amplifier delivering up to 292 Watts of average output power using a mode-locked 30 ps source at 1032 nm. Utilizing a single mode distributed mode filter bandgap rod fiber, we demonstrate 44% power improvement before the threshold-like onset of mode instabilities by operating the rod fiber in a leaky waveguide regime. We investigate the guiding dynamics of the rod fiber and report a distinct bandgap blue-shifting as function of increased signal power level. Furthermore, we theoretically analyze the guiding dynamics of the DMF rod fiber and explain the bandgap blue-shifting with thermally induced refractive index change of the refractive index profile.

Thermally induced mode coupling in rare-earth doped fiber amplifiers.

We present a simple semianalytical model of thermally induced mode coupling in multimode rare-earth doped fiber amplifiers. The model predicts that power can be transferred from the fundamental mode to a higher-order mode when the operating power exceeds a certain threshold, and thus provides an explanation of recently reported mode instability in such fiber amplifiers under high average-power operation. We apply our model to a simple step-index fiber design, and investigate how the power threshold depends on various design parameters of the fiber.

Highly tunable large-core single-mode liquid-crystal photonic bandgap fiber

We demonstrate a highly tunable photonic bandgap fiber, which has a core diameter of 25 mm, and a bandgap tuning sensitivity of 27 nm/degC at room temperature. The insertion loss is estimated to be less than 0.5 dB.

Continuously tunable all-in-fiber devices based on thermal and electrical control of negative dielectric anisotropy liquid crystal photonic bandgap fibers

We infiltrate photonic crystal fibers with a negative dielectric anisotropy liquid crystal. A 396 nm bandgap shift is obtained in the temperature range of 22-80 degrees C, and a 67 nm shift of long-wavelength bandgap edge is achieved by applying a voltage of 200 Vrms. The polarization sensitivity and corresponding activation loss are measured using polarized light and a full broadband polarization control setup. The electrically induced phase shift on the Poincaré sphere and corresponding birefringence change are also measured. According to the results, tunable wave plates working in the wavelength range of 1520-1580 nm and a potential for realizing a polarimeter working at the 1310 nm region are experimentally demonstrated.