A. Abdolvand

Single-mode hollow-core photonic crystal fiber made from soft glass

We demonstrate the first soft-glass hollow core photonic crystal fiber. The fiber is made from a high-index lead-silicate glass (Schott SF6, refractive index 1.82 at 500 nm). Fabricated by the stack-and-draw technique, the fiber incorporates a 7-cell hollow core embedded in a highly uniform 6-layer cladding structure that resembles a kagomé-like lattice. Effective single mode guidance of light is observed from 750 to 1050 nm in a large mode area (core diameter ~30 µm) with a low loss of 0.74 dB/m. The underlying guidance mechanism of the fiber is investigated using finite element modeling. The fiber is promising for applications requiring single mode guidance in a large mode area, such as particle guidance, fluid and gas filled devices.

Generation of a phase-locked Raman frequency comb in gas-filled hollow-core photonic crystal fiber

In a relatively simple setup consisting of a microchip laser as pump source and two hydrogen-filled hollow-core photonic crystal fibers, a broad, phase-locked, purely rotational frequency comb is generated. This is achieved by producing a clean first Stokes seed pulse in a narrowband guiding photonic bandgap fiber via stimulated Raman scattering and then driving the same Raman transition resonantly with a pump and Stokes fields in a second broadband guiding kagomé-style fiber. Using a spectral interferometric technique based on sum frequency generation, we show that the comb components are phase locked.

Amplification of higher-order modes by stimulated Raman scattering in H 2 -filled hollow-core photonic crystal fiber

We report a method for amplifying higher-order guided modes, synthesized with a spatial light modulator, in a hydrogen-filled hollow-core photonic crystal fiber. The gain mechanism is intermodal stimulated Raman scattering, a pump laser source in the fundamental mode providing amplification for weak higher-order seed modes at the Stokes frequency. The gain for higher-order modes up to LP31 is calculated and verified experimentally.

Selective excitation of higher order modes in hollow-core PCF via prism-coupling.

Prism-coupling through the microstructured cladding is used to selectively excite individual higher order modes in hollow-core photonic crystal fibers (PCFs). Mode selection is achieved by varying the angle between the incoming beam and the fiber axis, in order to match the axial wavevector component to that of the desired mode. The technique allows accurate measurement of the effective indices and transmission losses of modes of arbitrary order, even those with highly complex transverse field distributions that would be extremely difficult to excite by conventional endfire coupling.

Efficient anti-Stokes generation via intermodal stimulated Raman scattering in gas-filled hollow-core PCF

A strong anti-Stokes Raman signal, from the vibrational Q(1) transition of hydrogen, is generated in gas-filled hollow-core photonic crystal fiber. To be efficient, this process requires phase-matching, which is not automatically provided since the group velocity dispersion is typically non-zero and--inside a fiber--cannot be compensated for using a crossed-beam geometry. Phase-matching can however be arranged by exploiting the different dispersion profiles of higher-order modes. We demonstrate the generation of first and second anti-Stokes signals in higher-order modes by pumping with an appropriate mixture of fundamental and a higher-order modes, synthesized using a spatial light modulator. Conversion efficiencies as high as 5.3% are achieved from the pump to the first anti-Stokes band.

Intermodal stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fiber

Stimulated Raman scattering is investigated in a slightly multimode gas-filled hollow-core photonic crystal fiber. Although, second-order Stokes light appears in the fundamental mode below a certain threshold energy, it is observed to switch to a two-lobed higher order mode above this threshold. Conversion to the higher order mode is made possible by the creation of a two-lobed moving coherence wave in the gas that provides both phase-matching and a strong intermodal pump-Stokes overlap. A theoretical model is developed, based on this physical interpretation that agrees quantitatively with the experimental results. The results suggest new opportunities for all-fiber gas-based nonlinear processes requiring phase-matching, such as coherent anti-Stokes Raman scattering, as well as providing a means (for example) of efficiently converting light from a higher order pump mode to a fundamental Stokes mode.

Hybrid photonic-crystal fiber

Optical fibers provide much more than a means to transport light between different locations. This article reviews how integration of functional fluid, solid, and gaseous materials in photonic crystal fibers enables control of their linear and nonlinear properties with applications in optoelectronics, sensing, and laser science.

Raman-induced temporal condensed matter physics in gas-filled photonic crystal fibers.

Raman effect in gases can generate an extremely long-living wave of coherence that can lead to the establishment of an almost perfect temporal periodic variation of the medium refractive index. We show theoretically and numerically that the equations, regulate the pulse propagation in hollow-core photonic crystal fibers filled by Raman-active gas, are exactly identical to a classical problem in quantum condensed matter physics - but with the role of space and time reversed - namely an electron in a periodic potential subject to a constant electric field. We are therefore able to infer the existence of Wannier-Stark ladders, Bloch oscillations, and Zener tunneling, phenomena that are normally associated with condensed matter physics, using purely optical means.

Manipulation of coherent Stokes light by transient stimulated Raman scattering in gas filled hollow-core PCF

Transient stimulated Raman scattering is investigated in methane-filled hollow-core photonic crystal fiber. Using frequency-chirped ps-pulses at 1.06 microm as pump and tunable CW-radiation as Stokes seed, the vibrational excitation of the CH(4) molecules can be controlled on the sub T(2) time-scale. In this way the generated Stokes pulse can be phase-locked to the pump pulse and its spectrum manipulated.

Supercontinuum up-conversion via molecular modulation in gas-filled hollow-core PCF.

We report on the efficient, tunable, and selective frequency up-conversion of a supercontinuum spectrum via molecular modulation in a hydrogen-filled hollow-core photonic crystal fiber. The vibrational Q(1) Raman transition of hydrogen is excited in the fiber by a pump pre-pulse, enabling the excitation of a synchronous, collective oscillation of the molecules. This coherence wave is then used to up-shift the frequency of an arbitrarily weak, delayed probe pulse. Perfect phase-matching for this process is achieved by using higher order fiber modes and adjusting the pressure of the filling gas. Conversion efficiencies of ~50% are obtained within a tuning range of 25 THz.