Research on Amplified Spontaneous Emission and Gain of L-Band EDFA

Abstract :　Gain characteristics of small signal in L-band Erbium-doped fiber amplifier (EDFA)are analyzed with the numerical simulation using Giles model .The results show that the optimal fiber length varies with input signal wavelength , in general , shorter wavelength demands shorter fiber length .7 m and 9 m L-band EDF are employed to construct L-band EDFAs respectively .Evaluation of EDFAs is made by experimental comparision of their amplified spontaneous emission (ASE) spectr , gains and noise figures (NF), and the results show that the fiber length is crucial for proper gain characteristic .On the basis of these numerical , experimental study and also considering their C-band characteristics , optimal ASE spectrum related to the best behavior of L-band signal amplification is presented .

1  Introduction

With the advent of the network era, people's demand for information is increasing day by day, and fiber-optic communication has also flourished along the direction of ultra-large capacity, ultra-high speed, and ultra-long distance. In order to meet the requirements of capacity expansion, people have expanded their eyes from the C-band to the L-band. Therefore, the long-band erbium-doped fiber amplifier (L-band EDFA) has become one of the hotspots in recent years.

Directly using a 980 nm unidirectional pumped structure is the simplest and most basic structure for studying C-band erbium-doped fiber amplifiers or L-band erbium-doped fiber amplifiers. In many reports at home and abroad in recent years, traditional C-band erbium fiber of tens of meters or even hundreds of meters is generally used as a gain medium to form an L-band erbium-doped fiber amplifier. However, if the length of the fiber is too long, it will bring many disadvantages to the system. Influence: On the basis of numerical calculation, the spontaneous emission spectrum, gain and noise figure of high-doped L-band erbium fiber of 7 m and 9 m were measured, and the optimal spontaneous emission spectrum suitable for amplifying L-band signal was proposed.

2 Theoretical calculation

The scheme shown in Figure 1 is used, where the tunable light source (TLS) is a tunable external cavity laser source from Danish Photonetics with a wavelength range of 1525 to 1625 nm; since the laboratory does not currently have an isolator for the L-band (ISO) The two C-band isolators from E-TEK of the United States are used instead, so the loss is relatively large; the wavelength division multiplexer (WDM) is WDM-1 × 2-980/ 1590- of Xiamen Ante 0; erbium fiber is Canada's Coractive high-doped fiber: EDF-L1500, doping concentration is 6.4 × 1025 / m3; spectrum analyzer (OSA) is Japan's ANDO's AQ-6315A. Combined with laboratory conditions, the output power of the 980 nm pump is 90 mW. When the input signal wavelengths are 1570 nm, 1590 nm, and 1610 nm, the relationship between the fiber length calculated by the Giles model [10] and the signal gain is shown in Fig. 2.

The inflection point of each curve in the figure is the optimum chirp length at this wavelength when the input signal power is -15 dBm. As can be seen from the figure, the gain becomes smaller as the wavelength is longer, and the optimum length of the desired fiber becomes longer, about 6 m at 1570 nm, about 9 m at 1590 nm, and about 12 m at 1610 nm. This is determined by the absorption and emission spectra of the erbium fiber. The longer the wavelength, the longer the fiber is needed to fully absorb the pumping light to provide more inverted particles. It also shows that for an erbium-doped fiber amplifier, the optimum erbium fiber length is not a fixed value, it varies with the wavelength of the input signal. In addition, it is also related to input signal power, pump power, doping concentration and other factors. Therefore, in order to determine the length of the erbium fiber in the amplifier, it can be analyzed from its spontaneous emission spectrum.

3 Experimental research and analysis

First, compare the differences between the spontaneous emission spectra of the L-band and C-band amplifiers, as shown in Figure 3. The germanium fiber used in Figure 3(a) is a highly doped L-band germanium fiber: EDF-L1500; and the gain medium in Figure 3(b) is Lucent's C-band fiber, EDF-MP980. It can be seen that at the wavelength greater than 1570 nm, the spontaneous emission spectrum of the L-band erbium-doped fiber amplifier is convex, which is advantageous for amplifying the signal of the L-band, and it is also compatible with the C-band erbium-doped fiber amplifier [Figure 3 (b The difference between the self-emissive spectrum of the C-band erbium-doped fiber amplifier in Figure 3(b) is concave, and the number of inverted particles is almost used to provide the gain of the C-band signal, which is not conducive to amplification L. The signal of the band. The difference in the spontaneous emission spectra of the two band amplifiers reflects the difference in their absorption cross sections and emission cross sections.

The L-band fiber used in Figure 3(a) has a length of 7 m and a pumping power of 100 mW. The gain and noise figure measured by the point-by-point method are shown in Figure 4, in the wavelength range of 1568 nm to 1590 nm. The gain can be close to 15 dB or more, and the gain at 1570 nm is greater than 21 dB, but only when the wavelength is greater than 1600 nm, the amplitude of the drop is large.

Similarly, when the pumping power is 100 mW, the 9 m L-band erbium fiber is used as the gain medium of the amplifier, and the spontaneous emission spectrum and gain and noise figure under different pumping powers are measured as shown in Fig. 5. Compared with Fig. 3(a), the amplitude of the convexity at the tail of the spontaneous emission spectrum is larger than that at the peak of 1560 nm, and the peak at 1530 nm is significantly lower than that at 1560 nm, while in Fig. 3(a), two The peak is flat, indicating that the degree of particle inversion shown in Fig. 5(a) is lower than that in Fig. 3(a). In Figure 5(b), although the measured maximum gain is 18.5 dB, the gain is much flat compared to Figure 4. And the noise figure is also low, fluctuating around 5 dB.

From the above experimental results, the influence of the length of the L-band erbium fiber on the spontaneous emission spectrum of the amplifier can be obtained. When the pumping power is constant, the longer the length of the erbium fiber, the higher the peak at 1560 nm, the lower the peak at 1530 nm, and the larger the amplitude of the spontaneous emission spectrum when the wavelength is greater than 1570 nm. An amplifier consisting of the same length of C-band erbium fiber (such as several meters or more than ten meters), under the same pumping conditions, generally, at wavelengths greater than 1570 nm, the spontaneous emission spectrum will not appear convex shape unless its length It is 4 to 5 times (tens of meters or even hundreds of meters) of the required L-band erbium fiber [9], and at this time, the L-band erbium-doped fiber amplifier is composed of a relatively long C-band erbium fiber.

4 Conclusion

 In summary, the change in the shape of the spontaneous emission spectrum reflects the change in the average particle reversal in the erbium fiber. To amplify the L-band signal, the average particle reversal degree must be in an unsaturated state, and the lower the average particle reversal degree, the more favorable the signal at the longer wavelength, the flatter the gain, but at the expense of the gain reduction.