**1. Introduction**

124 Selected Topics on Optical Amplifiers in Present Scenario

Yoshida, H.; Yamashita, Y.; Kuwabara, M. & Kan, H. (2008b). Demonstration of an

93, No. 24, (December, 2008), 241106, ISSN 0003-6951

ultraviolet 336 nm AlGaN multiple-quantum-well laser diode. *Appl. Phys. Lett*., Vol.

Significant effort has been made in recent years to improve the Doped Fiber amplifier gain and noise figure. Extend the optical bandwidth of doped fiber amplifiers beyond the traditional 1550nm band, making the excellent EDFA characteristics available in a wider spectral region also was the main effort in optical amplifier fields. Several techniques have been developed to improve gain and shift the gain to the shorter wavelength region. In this chapter, the effects of external perturbation such as macro-bending and fiber length and internal perturbation such as transversal distribution profile and doped concentration on doped fiber performance have been demonstrated (S.D.Emami et al., 2010).

A macro-bending approach is demonstrated to increase a gain and noise figure at a shorter wavelength region of EDFA. The conventional double-pass configuration is used for the EDFA to obtain a higher gain with a shorter length and lower pump power. The macrobending suppresses the ASE at longer wavelength to achieve a higher population inversion at shorter wavelengths. Without the bending, the peak ASE at 1530nm, which is a few times higher than the ASE at the shorter wavelength, would deplete the population inversion and suppresses the gain in this region.

Macro-bending is introduced as a new method to increase gain flatness and bandwidth of EDFA in C-band region. Varying the bending radius and doped fiber length leads to the optimized condition with flatter and broader gain profile. Under the optimized condition, gain at shorter wavelengths is increased due to increment of population inversion which results in gain reduction in the longer wavelength regions. The balance of these two effects in the optimized condition has a significant result in achieving a flattened and broadened gain profile.

This technique is also capable to compensate the fluctuation in operating temperatures due to proportional temperature sensitivity of absorption cross section and bending loss of the aluminosilicate EDF . This new approach can be used to design a temperature insensitive EDFA for application in a real optical communication system which operates at different environments but still maintaining the gain characteristic regardless of temperature variations. The effect of macro-bending on high concentration EDFA using optimized

Doped Fiber Amplifier Characteristic Under Internal and External Perturbation 127

a higher attainable gain at a shorter wavelength region. This result shows that the distributed ASE filtering can be achieved by macro-bending of the fiber at an optimally chosen radius. This characteristic can be used in research of S-band EDFA and fiber lasers

Fig. 1. EDF bending loss profile (dB/m) against wavelength (nm) for different bending

**with macro-bending without macro-bending**

Fig. 2. Gain (solid symbols) and noise figure (hollow symbols) spectra with and without the macro-bending effect. The input signal and pump power is fixed at -30dBm and 100mW,

**1480 1490 1500 1510 1520 1530 1540 1550 1560 Signal Wavelength (nm)**

**Noise figure (dB)**

Fig. 2 shows the variation of gain and noise figure across the input signal wavelength for the double-pass EDFA with and without the macro-bending. The input signal and 980nm pump powers is fixed at -30 dBm and 100 mW respectively. The bending radius is set at 4 mm in case of the amplifier with the macro-bending. As shown in the figure, the gain enhancements of about 12 ~ 14 dB are obtained with macro-bending at wavelength region between 1480 nm and 1530 nm. This enhancement is attributed to macro-bending effect

(Daud, et al. 2008).

radius (3.5 mm, 4 mm and 5 mm).

**-40 -30 -20 -10 0 10 20 30 40 50**

**Gain (dB)**

respectively.

bending radius and length of the doped fiber is demonstrated. This gain increment compensates the gain reduction of the EDF before applying macro-bending and result in a flat and broad gain spectrum.

One of the many EDFA optimization parameters reported includes the Erbium Transversal Distribution Profile (TDP). The Erbium TDP is essential in determining the overlap factor, which affects the absorption and emission dynamics of the EDFA. At the end of this chapter, numerical models of different Erbium TDP is demonstrated and later verified by experiment. The model considers the overlap factor and absorption/ emission dynamics for different Erbium TDP. Results indicate a high performance EDFA is achievable with an optimized and yet realistic Erbium TDP.
