Kentaro Ichii

Optical Fiber Amplifier Employing a Bundle of Reduced Cladding Erbium-Doped Fibers

We propose a novel optical amplifier that employs a bundle of reduced cladding erbium-doped fibers (EDFs) for multicore fiber transmission and successfully demonstrate the amplification of seven independent signals by employing a bundle of 60-μm cladding EDFs and a planar lightwave circuit integration technique. Average gains of 23 dB with low noise figures of less than 5.1 dB in the C-band were achieved for seven individual EDFs. We also confirmed that low crosstalk characteristics can be achieved for a multicore optical fiber amplifier by using bundled EDF.

Over 10 W Output Linearly-Polarized Single-Stage Fiber Laser Oscillating Above 1160 nm Using Yb-Doped Polarization-Maintaining Solid Photonic Bandgap Fiber

We propose an all-fiber linearly-polarized single-stage fiber laser oscillating above 1160 nm employing a low-loss Yb-doped polarization-maintaining solid photonic bandgap fiber (Yb-PM-SPGF) as a laser source for a yellow-orange frequency-doubling laser. We also present a new fabrication method for Yb-PM-SPBGF to reduce fiber attenuation in its photonic bandgap for increasing laser output power and improving slope efficiency. The attenuation of a fabricated Yb-PM-SPBGF is below 20 dB/km at 1180 nm, which is as low as that of a conventional Yb-doped fiber. All-fiber linearly-polarized single-stage fiber laser oscillating at 1180 nm using the fabricated Yb-PM-SPBGF is demonstrated. ASE and parasitic lasing in the wavelength range from 1030 nm to 1130 nm are suppressed and eliminated thanks to the filtering effect of the fiber. A 10.8 W output power is successfully achieved with a slope efficiency of 56% and a conversion efficiency of 50% by high-power 976 nm pumping. A spectral width of less than 0.05 nm, a polarization extinction ratio of more than 20 dB and nearly diffraction limited beam quality in output light are achieved. These results indicate that our proposed fiber laser can realize a compact and high-power yellow-orange frequency-doubling laser.

Gamma-radiation-induced photodarkening in ytterbium-doped silica glasses

Photodarkening phenomenon in ytterbium (Yb)-doped silica glasses was experimentally investigated by measurements such as electron spin resonance (ESR), X-ray absorption fine-structure (XAFS), and optical transmittance. A predominant increase of Al-oxygen hole center (OHC) was observed for Al-Yb co-doped silica glasses both by the incidence of pump light and by gamma-ray irradiation. It was also recognized that the optical transmission loss similarly increased in both cases. These results indicate that the formation of Al-OHC is the prime cause of the excess loss induced by photodarkening. XAFS measurement indicated that the second-nearest-neighbor atoms around Yb may be related to the mechanism of photodarkening phenomenon.

Defect analysis of photodarkened and gamma-ray irradiated ytterbium-doped silica glasses

Defects in photodarkened and gamma-ray irradiated ytterbium (Yb)-doped silica glasses has been experimentally investigated. The results indicate that the formation of Al-oxygen hole center (Al-OHC) is the prime cause of the photodarkening loss.

2-LP mode few-mode fiber amplifier employing ring-core erbium-doped fiber

A fiber amplifier supporting 2 LP modes that employs a ring-core erbium-doped fiber (RC-EDF) is investigated to reduce differential modal gain (DMG). The inner and outer radii of the ring-core of the RC-EDF are clarified for 2-LP mode operation of the amplifier, and are optimized to reduce the DMG. It is shown that using the overlap integral between the erbium-doped core area and the signal power mode distribution is a good way to optimize the inner and outer radii of the ring-core of the RC-EDF and thus minimize the DMG. A fabricated RC-EDF and a constructed 2-LP mode EDFA are described and a small DMG of around 1 dB is realized for LP01, LP11 and LP21 pumping.

Remotely pumped multicore erbium-doped fiber amplifier system with high pumping efficiency

We have experimentally clarified the optical amplification characteristics of a remotely pumped multicore erbium-doped fiber amplifier system. The pumping efficiencies of the system are ~4 times higher than those of the distributed Raman amplification system.

1180 nm linearly-polarized fiber laser with high slope efficiency employing low-loss ytterbium-doped polarization maintaining solid photonic bandgap fiber

A linearly-polarized fiber laser operating at 1180 nm was demonstrated using a low-loss ytterbium-doped polarization maintaining solid photonic bandgap fiber. A slope efficiency of 30% was achieved.

Amplification Technology for Multi-Core Fiber Transmission

Amplification technologies are categorized for a multi-core erbium-doped fiber amplifier (MC-EDFA) including a pump scheme and an active fiber. MC-EDFAs are described that employ discretely pumped single-cladding MC-EDF and cladding-pumped double-cladding MC-EDF.

Optical fiber amplifier employing a bundle of reduced cladding erbium-doped fibers for multi-core fiber transmission

We propose a novel optical amplifier that employs a bundle of erbium-doped fibers for multi-core fiber transmission. We demonstrate the amplification of seven independent signals by using a bundle of EDFs with 60 μm cladding.

Method for estimating gain coefficient spectrum of erbium-doped fiber based on net gain–loss measurement employing 4 I 13/2 level pumping

The method for estimating the gain coefficient spectrum of an erbium-doped fiber (EDF) is proposed and demonstrated experimentally. The method employs a net gain-loss measurement that uses the 4I(13/2) level pumping of the EDF and the McCumber relation. A formula used to obtain the quotient of the gain and loss coefficient from the net gain-loss measurement is derived to determine the parameter in the McCumber relation. It is confirmed that the gain coefficient spectrum of an EDF estimated with the method coincides with the value estimated using a conventional net gain-loss measurement that employs the 4I(11/2) level pumping. The method is successfully applied to an erbium/ytterbium-doped fiber, for which it is impossible to perform a conventional net gain-loss measurement with pumping at the 4I(11/2) level of erbium ions because of the absorption transition of ytterbium ions from 2F(7/2) to 2F(5/2) levels.