EDFA equalization technology - GFF introduction

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2022-08-20

EDFA is the core functional device in the optical transmission system, which has the advantages of high gain, large bandwidth, low noise, gain characteristics insensitive to optical polarization state, transparent to data rate and format, and negligible channel crosstalk in multi-channel systems. . The combination of DWDM and EDFA can not only increase the capacity of the system, but also has high flexibility and economy. However, due to the dense multiplexing of channel wavelengths and the uniform broadening characteristics of EDFA, there is fierce competition between different channels, and the gain and spectral line shape of EDFA need to be kept stable, so the problem of EDFA gain control and flatness is introduced. At present, there are mainly technical means to achieve EDFA gain flatness, such as thin-film filtering, low-light sinusoidal filtering, and fiber grating filtering. Optical gain flattening filter GFF is based on mature thin-film filtering technology, which is used in EDFA to compensate the uneven gain amplification under different DWDM channels. Figure 1 shows the working principle of the GFF diaphragm.

The gain flat response curve after GFF mainly depends on the following important parameters:

1. Operating wavelength: λrange = λmax – λmin (minimum and maximum wavelength in target band)

2. Additional loss: EL (dB) = minimum(ILEF)

3. Error function: ILEF (dB) = IL– ILtgt

4. Peak-to-peak error function: ILEFrange (dB) = ILEFmax – ILEFmin

The peak-to-peak error function (ILEFrange) represents the maximum flatness of the transmitted signal by comparing the spectral performance of the manufactured filter with the target curve, which describes the quality of a GFF filter. In general, the determining factors for achieving the minimum ILEFrange are the maximum slope and smoothness of the target curve. Some applications require multiple flattened regions, or increased passbands outside the flattened regions; these can add complexity to the filter.

The gain spectrum lines of different EDFAs are not the same, so the transmittance curves of GFF filters are also not completely the same. We take this device as an example, and its parameter performance is shown in Table 1.

SPECIFICATIONS

Parameter		unit	spec
Center Wavelength		nm	1550
Operating Wavelength (GFF)		nm	1530-1565.5nm
EL	Max	dB	0.6
ILEFrange	Max	dB	0.4
PDL	Max	dB	0.10
TDL	Max	dB	0.15
PMDs	Max	ps	0.05
Fiber Type			SMF28 Ultra
Fiber Length	Min	m	1.0
Power Handling	Max	mw	500
Operating Temperature		℃	-5-75
Storage Temperature		℃	-40-85
Package Dimension		mm	3.8×33