**2. Macrobending effects on doped fiber amplifier**

In the first part of this chapter, a macrobending approach is demonstrated to increase the 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 a longer wavelength to achieve a higher population inversion at shorter wavelengths. Without the bending, the peak ASE at 1530 nm, 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 (Harun et al., 2008).

The configuration of the EDFA is based on a standard double-pass configuration, where a circulator was used at the input and output ends of the EDF to couple light out of the amplifier and to allow the double propagation of light in the gain medium, respectively. The EDF is pumped by a 980-nm laser diode using a propagating pump scheme. The commercial EDF used is 15 m long with an erbium ion concentration of 440 ppm. A tunable laser source is used to characterize the amplifier in conjunction with an optical spectrum analyzer (OSA). The amplifier is characterized in the wavelength region between 1480 to 1560 nm in terms of the gain and noise figure under changes in the optical power. Before the amplifier experiment, the optical loss of the EDF was characterized for both cases with and without macrobending. The macrobending is obtained by winding the EDF in a bobbin with various radiuses between 0.35 and 0.50 mm (Daud, et al. 2008).

The optical losses of EDF were measured against wavelengths at various radius of macrobending and the result is compared to the straight EDF. Then the bending loss spectrum (dB/m) is obtained by taking the difference of the optical loss measurement between bent and straight EDF. Fig. 1 shows the bending loss spectrum at various bending radius between 0.35 to 0.50 mm. The experimental result is in agreement with the earlier reported theoretical prediction on bending loss in optical fiber (Thyagarajan & Kakkar, 2004), which uses a simple infinite cladding model. The theoretical result shows that the bending loss profile is almost exponential with respect to wavelength, with strong dependencies on fiber bending radius and refractive index profile. Bending of optical fiber, including EDF causes the propagating power of the guided modes to be transferred into cladding, which in turn resulted in loss of power and therefore the bending loss spectrum is obtained as shown in Fig. 2. The bending loss has a strong spectral variation because of the proportional changes of the mode field diameter with the signal wavelength. At bending radius of 0.40 mm, the experimental result shows that the bending loss is drastically increase (>10 dB/m) at the wavelengths above 1505 nm whereas the minimal loss is observed at the wavelengths below 1505 nm. This provides the ASE suppression of more than 270 dB at 1530 nm, which allows

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

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

In the first part of this chapter, a macrobending approach is demonstrated to increase the 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 a longer wavelength to achieve a higher population inversion at shorter wavelengths. Without the bending, the peak ASE at 1530 nm, which is a few times higher than the ASE at the shorter wavelength, would deplete

The configuration of the EDFA is based on a standard double-pass configuration, where a circulator was used at the input and output ends of the EDF to couple light out of the amplifier and to allow the double propagation of light in the gain medium, respectively. The EDF is pumped by a 980-nm laser diode using a propagating pump scheme. The commercial EDF used is 15 m long with an erbium ion concentration of 440 ppm. A tunable laser source is used to characterize the amplifier in conjunction with an optical spectrum analyzer (OSA). The amplifier is characterized in the wavelength region between 1480 to 1560 nm in terms of the gain and noise figure under changes in the optical power. Before the amplifier experiment, the optical loss of the EDF was characterized for both cases with and without macrobending. The macrobending is obtained by winding the EDF in a bobbin with various

The optical losses of EDF were measured against wavelengths at various radius of macrobending and the result is compared to the straight EDF. Then the bending loss spectrum (dB/m) is obtained by taking the difference of the optical loss measurement between bent and straight EDF. Fig. 1 shows the bending loss spectrum at various bending radius between 0.35 to 0.50 mm. The experimental result is in agreement with the earlier reported theoretical prediction on bending loss in optical fiber (Thyagarajan & Kakkar, 2004), which uses a simple infinite cladding model. The theoretical result shows that the bending loss profile is almost exponential with respect to wavelength, with strong dependencies on fiber bending radius and refractive index profile. Bending of optical fiber, including EDF causes the propagating power of the guided modes to be transferred into cladding, which in turn resulted in loss of power and therefore the bending loss spectrum is obtained as shown in Fig. 2. The bending loss has a strong spectral variation because of the proportional changes of the mode field diameter with the signal wavelength. At bending radius of 0.40 mm, the experimental result shows that the bending loss is drastically increase (>10 dB/m) at the wavelengths above 1505 nm whereas the minimal loss is observed at the wavelengths below 1505 nm. This provides the ASE suppression of more than 270 dB at 1530 nm, which allows

the population inversion and suppresses the gain in this region (Harun et al., 2008).

flat and broad gain spectrum.

optimized and yet realistic Erbium TDP.

**2. Macrobending effects on doped fiber amplifier** 

radiuses between 0.35 and 0.50 mm (Daud, et al. 2008).

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 (Daud, et al. 2008).

Fig. 1. EDF bending loss profile (dB/m) against wavelength (nm) for different bending radius (3.5 mm, 4 mm and 5 mm).

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, respectively.

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

Doped Fiber Amplifier Characteristic Under Internal and External Perturbation 129

Initially, the gain and noise figure of the single pass EDFA is characterized without any macro-bending at different EDF lengths. The input signal power is fixed at -30 dBm and the 980 nm pump power is fixed at 200 mW. The wavelength range is chosen between 1520 nm and 1570 nm which covering the entire C-band. It is important to note that using macrobending to achieve gain flatness depend on suppression of longer wavelength gains. The EDF length used must be slightly longer than the conventional C-band EDFA to allow an energy transfer from C-band to L-band taking place. This will reduce the gain peak at 1530nm and increases the gain at longer wavelengths. The macro-bending provides a higher loss at the longer wavelengths and thus flattening the gain spectrum of the proposed Cband EDFA. The combination of appropriate EDF length and bending radius, leads to flat

The bending loss spectrum of the EDF is measured across the wavelength region from 1530 nm to 1570 nm. Fig. 2 illustrates the bending loss profile at bending radius of 4.5 mm, 5.5 mm and 6.5 mm, which clearly show an exponential relationship between the bending loss and wavelength, with strong dependencies on the fiber bending radius. Bending the EDF causes the guided modes to partially couple into the cladding layer, which in turn results in losses as earlier reported. The bending loss has a strong spectral variation because of the proportional changes of the mode field diameter with signal wavelength (Giles et al., 1991) As shown in Fig. 5, the bending loss dramatically increases at wavelengths above 1550 nm. This result shows that the distributed ASE filtering can be achieved by macro bending the EDF at an optimally chosen radius. This provides high ASE or gain suppression around 1560 nm, which reduces the L-band gain. Besides this, lower level suppression of C-band population inversion reduces the effect of gain saturation, providing better C-band gain. Eventually, this characteristic is used to achieve C-band gain flattening in the EDFA

The gain spectrum of the EDFA is then investigated when a short length of high concentration EDF spooled in different radius. Fig. 6 shows the gain spectrum of the EDFA with 3m long EDF at different spooling radius. The result was also compared with straight EDF. The input signal power and pump power are fixed at -30dBm and 200 mW respectively in the experiment. As shown in the figure, the original shape of the gain spectrum is maintained in the whole C-band region with the gain decreases exponentially at wavelengths higher than 1560nm. Without bending, the peak gain of 28dB is obtained at 1530 nm which is the reference point to find the optimized length. When the EDF was spooled at a rod with 4.5mm and 5.5mm radius, the shape of gain spectra are totally

Fig. 4. Configuration of the single-pass EDFA

and broad gain profile across the C-band region.

(Hajireza, et al. 2010).

which suppresses the ASE at the longer wavelength. This resulted in an increase of population inversion at shorter wavelength, which in turn improves the EDFA's gain at the shorter wavelength as shown in Fig. 2. With the macro-bending, the positive gain is observed for input signal wavelength of 1516 nm and above. On the other hand, the macrobending also reduces the noise figure of the EDFA at wavelengths shorter than 1525nm as shown in Fig. 2.

Fig. 3 shows the gain and noise figure as a function of 980nm pump power with and without the macro-bending. In this experiment, the input signal power and wavelength is fixed at -30 and 1516nm, respectively. The bending radius is fixed at 4 mm. As shown in the figure, the macro-bending improves both gain and noise figure by approximately 6 dB and 3 dB, respectively. These improvements are due to the longer wavelength ASE suppression by the macro-bending effect in the EDF. With the macro-bending, the double-pass EDFA is able to achieve a positive gain with pump power of 90 mW and above. These results show that the bending effect can be used to increase the gain at a shorter wavelength, which has potential applications in S-band EDFA and fiber lasers. The operating wavelength of EDF fiber laser is expected can be tuned to a shorter wavelength region by the macro-bending.

Fig. 3. Gain (solid symbols) and noise figure (hollow symbols) against pump power for EDFAs with and without the macro-bending effect.
