Divide and conquer: an efficient solution to highly multimoded photonic lanterns from multicore fibres.

Photonic lanterns typically allow for single-mode action in a multimode fibre. Since their invention over a decade ago for applications in astrophotonics, they have found important uses in diverse fields of applied science. To date, large aperture highly-mulitmoded to single-mode lanterns have been difficult as fabrication techniques are not practical for mass replication. Here as a proof of concept, we demonstrate three different devices based on multicore fibre photonic lanterns with: 100µm core diameters; NAs = 0.16 and 0.15; and requiring 259 single-mode core system, specifically 7 multicore fibres each with 37 cores, instead of 259 individual single-mode fibres. The average insertion loss excluding coupling efficiencies is only 0.4dB (>91% transmission). This concept has numerous advantages, in particular, (i) it is a direct scaleable solution, (ii) eases imprinting of photonic functions, e.g. fibre Bragg gratings; and (iii) new approach for large-area optical fibre slicers for future large-aperture telescopes.

Writing Bragg Gratings in Multicore Fibers.

Fiber Bragg gratings in multicore fibers can be used as compact and robust filters in astronomical and other research and commercial applications. Strong suppression at a single wavelength requires that all cores have matching transmission profiles. These gratings cannot be inscribed using the same method as for single-core fibers because the curved surface of the cladding acts as a lens, focusing the incoming UV laser beam and causing variations in exposure between cores. Therefore we use an additional optical element to ensure that the beam shape does not change while passing through the cross-section of the multicore fiber. This consists of a glass capillary tube which has been polished flat on one side, which is then placed over the section of the fiber to be inscribed. The laser beam enters the fiber through the flat surface of the capillary tube and hence maintains its original dimensions. This paper demonstrates the improvements in core-to-core uniformity for a 7-core fiber using this method. The technique can be generalized to larger multicore fibers.

Demonstration of uniform multicore fiber Bragg gratings.

Fiber Bragg gratings in multicore fibers have significant potential as compact and robust filters for research and commercial applications. With the aid of an innovative, flat-fielded Mach-Zehnder interferometer, we demonstrate deep (>30 dB) narrow (100 pm at 3 dB; 90 pm at 10 dB) notches in the outer 6 cores of a 7-core fiber at a constant wavelength ( ± 15 pm). This is a crucial step in the development of FBGs operating within multimode fibers that carry an arbitrary number of spatial modes.

Spatial dispersion in three-dimensional drawn magnetic metamaterials.

We characterize spatial dispersion in longitudinally invariant drawn metamaterials with a magnetic response at terahertz frequencies, whereby a change in the angle of the incident field produces a shift in the resonant frequency. We present a simple analytical model to predict this shift. We also demonstrate that the spatial dispersion is eliminated by breaking the longitudinal invariance using laser ablation. The experimental results are in agreement with numerical simulations.

Semiconductor optical amplifier based high duty-cycle, self-starting figure-eight 1.7 GHz laser source.

The operation of a self-starting, passively harmonic modelocked, figure-eight laser is experimentally demonstrated. A stable pulse train with near half duty-cycle is produced at a repetition rate of 1.7 GHz at 1536 nm wavelength without any starting/triggering mechanism and stays modelocked as long as it is being pumped.

High-power figure-of-eight fiber laser with passive sub-ring loops for repetition rate control.

The operation of an erbium figure-of-eight all-fiber laser with a fundamental repetition rate of 5.68 MHz, peak power of 0.7 kW and pulse width of 420 fs is reported. Four times the fundamental repetition rate is achieved with two passive sub-ring loops at a repetition rate of 22.8 MHz. The sub-ring loop consists of only a single-mode fiber coupler with an input port and output port connected so as to make the time delay an odd multiple half-period of the input pulse period. This method is simple and inexpensive when compared with conventional methods employing sub-ring cavity or special fiber Bragg grating to control repetition rate.