MULTI-STAGE FIBER AMPLIFIER WITH FLATTENED GAIN CURVES

Abstract: This invention provides noise figure reduction and gain flatness enhancement of a fiber amplifier using multi-stage equalizing filter. According to the invention, noise figure can be lowered by splitting equalizing filter into multiple stage filtering conditions.

Description: MULTI-STAGE FIBER AMPLIFIER WITH FLATTENED GAIN CURVES TECHNICAL FIELD The present invention relates to a fiber amplifier, more particularly to a multi-stage erbium doped fiber amplifier with flattened gain curves and reduced noise figure when incorporating a gain flatness filter for WDM applications. BACKGROUND ART It has long been recognized that fiber amplifiers are an attractive alternative for amplifying optical signals. For example, fiber amplifiers have been proposed for the use as a booster amplifier to increase the output power from a telecommunication optical transmitter or as a telecommunication repeater to amplify optical signals without converting the optical signals into electrical signals. Especially, the advent of the Erbium Doped Fiber Amplifier (EDFA) has had a profound impact on the design, manufacturing and performance of optical communication systems. Its low noise, high optical power, high gain, linearity, wide bandwidth, wavelength transparency, polarization independence and fiber compatibility have revolutionized optical communications by effectively removing the transmission and splitting loss barriers. As a result today, long haul terrestrial and transoceanic fiber communications, as well as local and wide area networks, have become realities. However, the flatness of the EDFA pass-band becomes a critical issue in Wavelength Division Multiplexing (WDM) systems with cascaded amplifiers. The amplifier gain is not exactly same at each wavelength. For example, in the case of 0.8nm channel- width, one EDFA can simultaneously amplify optical signals from approximately 50 WDM channels. However, if several EDFAs are connected in series for long- distance transmission, the gain curve flatness of overall amplification gets worse and show the problem of insufficient amplification at the edge of amplification peak. Even small variations in gain between channels in a stage can therefore cause large variations in the power difference between channels at the output of the chain, lowering the performance of the entire system. In general, the performance of an optical amplifier is determined by its design parameters such as gain, output power and noise figure. Noise figure is defined as an amplifier output signal to noise ratio, scaled by the signal to noise ratio at the input port of the optical amplifier. When optical amplifiers are used as a telecommunication repeater, high power and low noise become critical parameters in increasing the distance between adjacent repeaters. The importance of a wavelength-independent (flat) gain in EDFAs is evidently manifested by the large current research effort in the field since it is directly related to the transmission capacity of dense WDM systems. Fluoride based EDFAs and Tellurite based EDFAs have been demonstrated to achieve the wider flattened bandwidth. Such a research result can be found in M. Yamada et al. , "Low-noise and gain-flattened fluoride-based Er 3+ -doped fiber amplifier pumped by 0.97um laser diode," Electron. Lett. , vol.33, no.9, p.809, 1997, and in A. Mori et al. , " 1.5um broadband amplification by tellurite-based EDFAs," OFC'97, PD1-1, 1997. It was also shown that a long-period fiber-grating filter could be implemented to obtain the flattened bandwidth beyond 40nm. Even though, discreet and careful design of the EDFA is required because the filter insertion in the mid stage of the EDFA could produce a noise figure penalty, as is described in P.F. Wysocki et al., "Erbium-doped fiber amplifier flattened beyond 40nm using long-period grating," OFC'97, PD2-1 , 1997. The inventors therefore analytically derived the dependence of the noise figure on filter transmission through a numerical simulation. FIG. 1 schematically shows the configuration of an EDFA comprising a single-stage equalizing filter which was used for the simulation. Referring to FIG. 1, a single stage filter 10 is located at the 1/3 position of total fiber amplifier length L. A long-period fiber grating (LPFG) is used as the single stage filter 10. The amplifier was modeled as a homogeneously broadened three level system as was described in C. R. Giles et al. , "Propagation of signal and noise in concatenated EDFAs," J. Lightwave Technol., vol.9, no.2, p.147, 1991. The spectral range of 120nm between 1480nm and 1600nm was used with the spectral grid of 0.1 nm to cover the pump wavelength of 1480nm. The fiber amplifier was assumed to a silica based aluminum co-doped fiber and then the absorption cross section was taken from the value disclosed in N. Kagi et al. , "Temperature dependence of the gain in erbium-doped fibers," IEEE J. Lightwave Technol. , vol.9, no.2, p.261 , 1991. The emission cross section was calculated from McCumber theory. FIG. 2 shows the detailed spectral profile of the LPFGs in a single- stage equalizing filter of FIG. 1. FIG. 3 shows the output spectrum of the EDFA described in FIG. 1. 41 input signals of 0.8nm spacing between 1531nm and 1563nm were used to observe gain flatness and noise figure, at -20dBm input power for each channel. The achieved multi-channel gain flatness was within 0.4dB over 32nm spectral range, and signal gain per channel was around 23dB. Even though this configuration meets good gain flatness, the amplified spontaneous emission (ASE) in this amplifier is not regulated due to high noise figure in short wavelength regime, making the application of this amplifier to the noise sensitive application such as long distance link difficult. DISCLOSURE OF THE INVENTION Therefore, it is an object of the present invention to provide a high performance flat-gain fiber amplifier. It is another object of the present invention to provide a fiber amplifier with lower noise figure. In order to accomplish the aforementioned objects, the present invention provides a multi-stage optical fiber amplifier that achieves ultra- flat gain using multi-stage equalizing filters. The fiber amplifier of the present invention comprises at least three fiber amplifier stages, each of which includes a length of doped optical fiber. The amplifier stages are connected in series for the sequential amplification of optical signals provided to the first amplifier stage. One of the amplifier stages is provided with pumped radiation so as to support laser amplification therein. The fiber amplifier also includes at least two equalizing filters, each of which is located between each pair of said amplifier stages. Since the noise figure is inversely proportional to the first-stage equalizing filter transmission, noise figure can be lowered by splitting equalizing filter into multiple stage filtering conditions. Since the output power of the amplifier is inversely proportional to the end-stage equalizing filter transmission, careful adjustment of the transmission spectra of each filter is necessary. BRIEF DESCRIPTION OF THE DRAWINGS The detailed description particularly refers to the accompanying drawings in which: FIG. 1 schematically shows the configuration of an EDFA comprising a single-stage equalizing filter which was used for the simulation; FIG. 2 shows the detailed spectral profile of the LPFGs in a single- stage equalizing filter of FIG. 1 ; FIG. 3 shows the output spectrum of the EDFA described in FIG. l ; FIG. 4 schematically shows the configuration of the EDFA according to an embodiment of the present invention; FIG. 5 shows the detailed spectral profile of the LPFGs in double- stage equalizing filters of FIG. 4; FIG. 6 shows the output spectrum of the EDFA described in FIG. 4; FIG. 7 shows calculated noise figures of EDFAs with two kinds of equalizing filters. BEST MODE FOR CARRYING OUT THE INVENTION Referring to FIG. 4, two LPFGs 110 are located at the 1/3 and 2/3 positions of total fiber amplifier length L, respectively. For the double stage filter configuration comprising two LPFGs, each gain flattening filter is square rooted one of the total filter transmission. The fiber amplifier employed a silica-based aluminum co-doped EDF(erbium doped fiber). The EDF is pumped at 1480nm by a semiconductor laser that is used as an optical pump. Referring to FIG. 5, the LPFGs show a notch filter characteristic. The absorption peak positions are the same as those of FIG. 3. However, the depth profile is different from each other. Referring to FIG. 6 and FIG. 3, the output signal power and ASE power of the EDFA employing a single stage filter is stronger than those of the EDFA employing a double stage filter by ldB. However, the ASE power spectrum in a single stage filter is less regulated, compared with that in a double stage filter, implying higher noise figure penalty. Calculated noise figures of EDFAs with two kinds of equalizing filters are plotted in FIG. 7. Referring to FIG. 7, maximum noise figure difference of 0.7dB is found at the wavelength of 1535nm where filter transmission is a minimum. Not much difference in noise figure is observed at 1543nm where the filter transmission is maximum. From these results, it is concluded that the EDFA with a double stage equalizing filter is superior to that with a single stage filter in terms of noise figure, though using a multi-stage equalizing filter decreases the EDFA gain a little.

Claims: WHAT IS CLAIMED IS : 1. A multi-stage optical fiber amplifier, comprising: at least three fiber amplifier stages, each of which includes a length of doped optical fiber, the stages being connected in series for the sequential amplification of optical signals provided to the first amplifier stage; means for providing pumped radiation into said amplifier stages so as to support laser amplification therein; and at least two equalizing filters, each of which is located between each pair of said amplifier stages. 2. The multi-stage optical fiber amplifier of claim 1 , wherein each of said equalizing filters is a long-period fiber grating. 3. The multi-stage optical fiber amplifier of claim 1 , wherein said optical fiber is doped with rare earth element. 4. The multi-stage optical fiber amplifier of claim 3, wherein said rare earth element is erbium.

Inventors: LEE JU HAN, PARK NAM KYOO, LEE, JU, HAN, PARK, NAM, KYOO
Applicants: LEE JU HAN, PARK NAM KYOO, LEE, JU, HAN, PARK, NAM, KYOO
Application year: 1998
Main classification: G02F1
IPC codes: G02B6, H01S3, H04B10