Wide-Range Variable Gain Fiber Amplifier With Erbium-Doped Fiber Switching

A variable gain erbium-doped fiber amplifier (EDFA) with a wide gain range and a small noise figure (NF) variation is analyzed numerically and demonstrated experimentally. The EDFA can change the total erbium-doped fiber length by selecting six combinations of three EDFs with optical switches (SWs). The numerical calculation of the NF and the pump power using the analytical model of an EDFA indicates that such a variable gain EDFA has an advantage over a conventional multi-stage EDFA in terms of NF variation and required pump power. These characteristics are demonstrated by constructing a variable gain EDFA that employs SWs and variable optical attenuators (VOAs) integrated on a silica-based planar lightwave circuit (PLC). The variable gain EDFA exhibits a smaller NF variation of 1.2 dB for the 24-dB variable gain range than conventional two- and four-stage EDFAs, which have NF variations of 12.6 and 2.0 dB, respectively, for the same variable gain range. The power consumption of these EDFAs is also discussed by comparing the operating electric power of the SWs and VOAs with that of the pump laser.

I. Introduction

The erbium-doped fiber amplifier (EDFA) is a key component of wavelength-division-multiplexing (WDM) optical transmission systems. Since each span in a transmission system has a different attenuation, the EDFA gain must change according to the optical power level input into the amplifier. In such an EDFA, the population inversion level averaged along the erbium-doped fiber (EDF) is kept constant to maintain a flat gain condition. The average population inversion determines the EDF gain per unit length and EDFs with different lengths have different flattened gain spectra, which are the optimized gain spectra for WDM signal amplification. Therefore, a typical variable gain amplifier consists of two amplifier stages with a variable optical attenuator (VOA) inserted between them [1]–[3]. The total average inversion level of the two amplifier stages is kept constant and variable gain is achieved by changing the VOA attenuation. An amplifier with such a two-stage amplifier configuration suffers from an increased noise figure (NF) in the low gain region, which is caused by the additional attenuation of the VOA. The NF degradation limits the gain range and results in the need for a large number of amplifier inventories with different gain ranges, which increases the cost of optical transmission systems.