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.

I. Introduction

Optical multi-core fiber transmission is very attractive with a view to achieving an ultra-high exabit/s class capacity by using space division multiplexing (SDM) technologies [1], [2]. Several multi-core transmission fibers (MCFs) have been developed [3]–[6] and their feasibility has been confirmed by recent studies [7]–[9]. Multi-core optical fiber amplifiers (MC-OFAs) have been studied to make it possible to construct a practical long-haul multi-core fiber transmission system, and various MC-OFA configurations have been assumed for such applications as optical repeaters and optical nodes [10]–[14]. Several approaches have been considered for realizing an MC-OFA, and one approach employs a discrete core amplification configuration in which optical signals that pass through different cores of an MCF are amplified after being separated into individual signals. One issue as regards developing a discrete core amplification configuration is the integration of single-core doped fibers (EDFs) and the optical components for the MC-OFA. The bundled EDF, which consists of a bundle of identical single-core EDFs, is one candidate amplification medium for the MC-OFA [13]. We propose that bundled EDF has the following advantages over multi-core EDF. 1)

Mature fiber fabrication technology can be used: Other than the need for an additional fabrication process, namely bundling, bundled EDF can be prepared with a high yield ratio.

2)

Amplifier design flexibility: Various configurations are available by combining EDFs with different lengths and/or dopant concentrations. Then the amplification of different wavelength bands such as the C- and L-bands can be achieved. The combination of single- and multi-mode EDFs in a bundle is also possible.

3)

Lower crosstalk characteristics: This is an important issue regarding the MC-OFA [11], [13], [14], and there is the possibility of reducing the crosstalk by changing the environmental material surrounding the EDFs.

4)

Possibility of compact size: As mentioned above, the MC-OFA amplification medium requires low crosstalk and a certain distance between the doped cores of the multi-core EDF [11], [14], which is another candidate amplification medium for the MC-OFA, and this induced the cladding diameter increase. To allow us to construct a practical compact OFA unit, we store the fiber in the unit by coiling it on a bobbin with a certain bending diameter that achieves a low failure probability. We may be able to achieve compact storage with a bundled EDF consisting of reduced cladding EDFs by coiling them together on one bobbin.

5)

Efficient EDF control technology can be used: A practical MC-OFA unit for WDM transmission systems requires complicated EDF controls such as ALC or AGC, and gain tilt correction. High-speed control may also be required. An OFA unit constructed with bundled EDF can realize the benefit of current control technologies, which have been developed for a unit with single-core EDF.

6)

Eliminating fan-out devices: Each EDF can be directly connected to a signal and pump multiplexer.