 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

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.

General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

The differential modal gain (DMG) in a few-mode erbium-doped fibre amplifier (FM-EDFA) is successfully improved by employing a ringcore FM erbium-doped fibre (RC-EDF). The DMGs between LP 01 and LP 11 mode signals in the FM-EDFA with the RC-EDF are 1.8 and 1.6 dB for LP 01 and LP 11 mode pumping, respectively, whereas those in a FM-EDFA with a conventional step-index FM EDF are 9.2 and 6.1 dB, indicating 7.4 and 4.5 dB DMG improvement for LP 01 and LP 11 mode pumping, respectively.Introduction: Long-haul transmission experiments employing a few-mode fibre (FMF) and a FM erbium-doped fibre amplifier (FM-EDFA) have been reported to overcome the capacity limit of transmission systems that employ a single-core and single-mode fibre (SMF) [1,2]. One of the issues with FM-EDFAs is the differential modal gain (DMG). It is important to reduce the difference between two overlap integrals, namely, that for the excited erbium ion area and the intensity profile of the LP 01 signal and that for the excited erbium ion area and the intensity profile of the LP 11 signal to reduce the DMG in FM-EDFAs. For this purpose, the doping of erbium ions with a ring profile and the use of a reconfigurable pump mode have been reported [3][4][5][6]. In this Letter, we adopt another approach, which involves employing a ring-core erbium-doped fibre (RC-EDF). Since an RC fibre supports similar intensity profiles for the LP 01 and LP 11 modes [7,8], the RC-EDF reduces the difference between the overlap integrals for LP 01 and LP 11 mode signals. We have fabricated an RC-EDF and demonstrated an improvement in the DMG between LP 01 and LP 11 mode signals by employing the RC-EDF in a FM-EDFA.

No abstract