Reduction of differential modal gain in a two-mode amplifier using a void-inscribed EDF.

We propose a technique for reducing the differential modal gain (DMG) that occurs in a two-mode erbium-doped fiber (2M-EDF) by inscribing voids in the core center of a 2M-EDF with a femtosecond laser. We show that an empty void inscribed at the core center can attenuate the linearly polarized (LP01) mode selectively while suppressing excess loss for the LP11 mode. We also reveal that DMG can be controlled by means of void diameter. The longitudinal position dependence of the void in a 2M-EDF was also investigated considering its influence on gain and noise figure (NF) characteristics. Finally, we realize a sufficiently low DMG of less than 0.5 dB in the full C-band as well as a sufficient gain and NF by using the proposed technique.

Optimal design of 4LP-mode multicore fibers for high spatial multiplicity.

High spatial multiplicity fiber designs are presented for homogeneous and heterogeneous 4LP-mode multicore fibers (MCFs) that support six spatial modes per core. The high-spatial-density 4LP-mode MCF design methodology is explained in detail. The influence of the number of cores on the cladding diameter (Dcl) and relative core multiplicity factor (RCMF) is investigated. The optimal core designs and MCF layouts with square and triangular lattices maintain glass fiber reliability (maximum Dcl = 250 µm). For homogeneous 4LP-mode MCFs, a 19-core triangular-lattice fiber gives the highest RCMF of 61.7. For heterogeneous 4LP-mode MCFs, an RCMF of 65.4 is obtained for a 21-core square-lattice fiber.

Loss reduction in few-mode photonic crystal fiber by reducing inner surface imperfections in air holes.

We studied both theoretically and experimentally the additional loss in photonic crystal fiber (PCF) that results from inner surface imperfections such as contamination and the surface roughness of air holes. We estimated the modal loss dependence of these imperfections using a model with a "defective layer" for the first time. The theoretical studies suggest that higher order modes have a larger loss due to imperfections in the air holes. By minimizing the inner surface imperfections of the six innermost air holes, we can theoretically expect any additional loss to be reduced to a negligible level. Moreover, we examined our theoretical prediction experimentally. We fabricated few-mode PCFs by employing a suitable inner surface treatment for just the six innermost holes. As expected theoretically, the transmission loss was greatly reduced by employing these processes. The lowest transmission losses in the 1550 nm band were 0.31 dB/km for the LP01 mode and 0.43 dB/km for the LP11 mode. Our theoretical model will be useful with a view to realizing few-mode PCF with a loss comparable to that of conventional fibers.

Silica-based PLC with heterogeneously-integrated PDs for one-chip DP-QPSK receiver.

To realize a DP-QPSK receiver PLC, we heterogeneously integrated eight high-speed PDs on a silica-based PLC platform with a PBS, 90-degree optical hybrids and a VOA. The use of a 2.5%-Δ waveguide reduced the receiver PLC size to 11 mm x 11 mm. We successfully demonstrated 32 Gbaud DP-QPSK signal demodulation with the receiver PLC.

Transmission over large-core few-mode photonic crystal fiber using distance-independent modal dispersion compensation technique.

We propose a transmission distance-independent technique for modal dispersion compensation over few-mode fiber that uses a single-input multiple-output configuration and adaptive equalization. Our technique can compensate for the modal dispersion of a signal with 1-tap FIR filters regardless of the amount of modal delay difference, and enables us to utilize fiber with a large core and few modes as a long-haul transmission line. We also show numerically the advantage of few-mode photonic crystal fiber (PCF) for realizing a larger effective area (A(eff)), and finally we report a transmission over a large-core two-mode PCF with A(eff)>280 µm(2).

Demonstration of carrier envelope offset locking with low pulse energy.

We demonstrate a carrier-envelope-offset (CEO)- locked frequency comb with 230-pJ fiber coupling pulse energy by using a passively mode-locked Er-fiber amplifier laser. For the generation of an octave-bandwidth spectrum in a highly nonlinear fiber and the second harmonic in a self-referenced interferometer with the lower pulse energy, we use a tellurite photonic crystal fiber and a direct-bonded quasi-phasematched LiNbO3 ridge waveguide, respectively. Our method is feasible for locking the CEO with a lower pulse energy to obtain a low-noise and highaccuracy optical frequency comb at telecommunications wavelengths.