**3. Application of Macro-Bending Effect on Gain-Flattened EDFA**

The configuration of the single pass Macro Bent EDFA used in this research is shown in Fig. 4, which consists of a piece of EDF, a wavelength division multiplexing (WDM) coupler, and a pump laser. An Aluminosilicate host EDF with 1100 ppm erbium ion concentration is used in the setup. Alumina in this fiber is to overcome the quenching effect for high ion concentration. A WDM coupler is used to combine the pump and input signal. Optical isolators are used to ensure unidirectional operation of the optical amplifier. Laser pump power at 980nm is used for providing sufficient pumping power. The EDF is spooled on a rod of 6.5 mm radius to achieve consistent macro-bending effect. The rod has equally spaced threads (8 threads per cm) where each thread houses one turn of EDF to achieve consistency in the desired bending radius. Tunable laser source (TLS) is used to characterize the amplifier in conjunction with the optical spectrum analyzer (OSA) (Hajireza, et al. 2010).

Fig. 4. Configuration of the single-pass EDFA

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

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

**0 20 40 60 80 100 120 140 Pump power (mW)**

with macro-bending without macro-bending

**8**

**13**

**18**

**23**

**Noise figure (dB)**

**28**

**33**

**38**

The configuration of the single pass Macro Bent EDFA used in this research is shown in Fig. 4, which consists of a piece of EDF, a wavelength division multiplexing (WDM) coupler, and a pump laser. An Aluminosilicate host EDF with 1100 ppm erbium ion concentration is used in the setup. Alumina in this fiber is to overcome the quenching effect for high ion concentration. A WDM coupler is used to combine the pump and input signal. Optical isolators are used to ensure unidirectional operation of the optical amplifier. Laser pump power at 980nm is used for providing sufficient pumping power. The EDF is spooled on a rod of 6.5 mm radius to achieve consistent macro-bending effect. The rod has equally spaced threads (8 threads per cm) where each thread houses one turn of EDF to achieve consistency in the desired bending radius. Tunable laser source (TLS) is used to characterize the amplifier in conjunction with the optical spectrum analyzer (OSA) (Hajireza, et al. 2010).

**3. Application of Macro-Bending Effect on Gain-Flattened EDFA** 

EDFAs with and without the macro-bending effect.

**-30 -25 -20 -15 -10 -5 0 5**

**Gain (dB)**

shown in Fig. 2.

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 and broad gain profile across the C-band region.

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 (Hajireza, et al. 2010).

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

Doped Fiber Amplifier Characteristic Under Internal and External Perturbation 131

Fig. 7. Gain for un-spooled EDF in different length with -30dBm input signal

Fig. 8. Gain for 6.5mm spooled radius in different length with -30dBm input signal

used for S-band EDFA (Wysocki et al., 1998).

To achieve a flatten gain spectrum, the EDFA must operate with insufficient 980nm pump, where the shorter wavelength ASE is absorbed by the un-pumped EDF to emit at the longer wavelength. This will shift the peak gain wavelength from 1530nm to around 1560nm. The macro-bending induces a wavelength dependent bending loss that results in higher loss at longer wavelength compared to the shorter wavelength as shown in Fig. 5. In relation to the EDFA, the macro-bending also suppresses the population inversion in C-band and thus reduces the gain saturation effect in C-band. With this reduced gain saturation, the C-band gain will increase. On the other hand, the L-band gain will reduce due to the suppression of L-band stimulated emission induced by macro-bending. The net effect of both phenomena will result in a flattened gain profile as shown in figure 8. Thus, the level of population inversion is dependent on different parameters such as length of fiber, bending radius and erbium ion concentration of the EDF. The same mechanism of distributed ASE filtering is

Fig. 5. Bending loss (dB/m) vs. wavelength (nm) for different bending radius

changed. Finally after trying different radius, 6.5 mm was the optimized radius for this amplifier. As shown in Fig. 5, bending loss at radius of 6.5 mm is low especially at wavelengths shorter than 1560nm and therefore the gain spectrum maintains the original shape of the gain spectrum for un-spooled EDF (Hajireza, et al. 2010)..

Fig. 6. Efficient length of EDF (3m) in different bending radius for -30dBm input signal]

Fig. 7 shows the gain spectra for the single pass EDFA with unspooled EDF at various EDF lengths. The input signal and 980nm pump powers are fixed at -30 dBm and 200 mW. As the length of the EDF increases, the gain spectrum moves to a longer wavelength region. The Cband photons are absorbed to emit photons at longer wavelength. The overall gain drops at the maximum length of 11m due to the insufficient pump power. Fig. 5 shows the gain spectra for the EDFA with the optimum spooling radius of 6.5 at various EDF lengths.

Fig. 5. Bending loss (dB/m) vs. wavelength (nm) for different bending radius

shape of the gain spectrum for un-spooled EDF (Hajireza, et al. 2010)..

changed. Finally after trying different radius, 6.5 mm was the optimized radius for this amplifier. As shown in Fig. 5, bending loss at radius of 6.5 mm is low especially at wavelengths shorter than 1560nm and therefore the gain spectrum maintains the original

Fig. 6. Efficient length of EDF (3m) in different bending radius for -30dBm input signal]

Fig. 7 shows the gain spectra for the single pass EDFA with unspooled EDF at various EDF lengths. The input signal and 980nm pump powers are fixed at -30 dBm and 200 mW. As the length of the EDF increases, the gain spectrum moves to a longer wavelength region. The Cband photons are absorbed to emit photons at longer wavelength. The overall gain drops at the maximum length of 11m due to the insufficient pump power. Fig. 5 shows the gain spectra for the EDFA with the optimum spooling radius of 6.5 at various EDF lengths.

Fig. 7. Gain for un-spooled EDF in different length with -30dBm input signal

To achieve a flatten gain spectrum, the EDFA must operate with insufficient 980nm pump, where the shorter wavelength ASE is absorbed by the un-pumped EDF to emit at the longer wavelength. This will shift the peak gain wavelength from 1530nm to around 1560nm. The macro-bending induces a wavelength dependent bending loss that results in higher loss at longer wavelength compared to the shorter wavelength as shown in Fig. 5. In relation to the EDFA, the macro-bending also suppresses the population inversion in C-band and thus reduces the gain saturation effect in C-band. With this reduced gain saturation, the C-band gain will increase. On the other hand, the L-band gain will reduce due to the suppression of L-band stimulated emission induced by macro-bending. The net effect of both phenomena will result in a flattened gain profile as shown in figure 8. Thus, the level of population inversion is dependent on different parameters such as length of fiber, bending radius and erbium ion concentration of the EDF. The same mechanism of distributed ASE filtering is used for S-band EDFA (Wysocki et al., 1998).

Fig. 8. Gain for 6.5mm spooled radius in different length with -30dBm input signal

Doped Fiber Amplifier Characteristic Under Internal and External Perturbation 133

Fig. 10. EDFS noise figure spectra; with and without macro bending at various input signal

Macro-bending is defined as a smooth bend of fiber with a bending radius much larger than the fiber radius (Marcuse, 1982). Macro-bending modifies the field distribution in optical fibers and thus changes the spectrum of the wavelength dependent loss. Various mathematical models have been suggested to calculate the bending effects in optical waveguide. Earlier references for bending loss in single mode fibers with step index profiles was developed by Marcuse. According to Marcuse, the total loss of a macro bent fiber includes the pure bending loss and transition loss caused by mismatch between the quasimode of the bending fiber and the fundamental mode of the straight fiber (Marcuse, 1976). The analytical expression for a single meter fiber bend loss α can be expressed as follows

where eν=2 , a is the radius of fiber core, R is the bending radius, βg is the propagation constant of the fundamental mode, K(υ-1)(γα) and K(υ+1)(γα) are the modified Bessel functions and V is the well-known normalized frequency, which is defined as (Agrawal,

3 2 exp

*k R*

 

<sup>1</sup> <sup>1</sup> <sup>3</sup> <sup>2</sup> <sup>2</sup>

*aKaKRVe*

*v v v*

 

<sup>3</sup> <sup>2</sup>

)()(

2

 

 

*g*

(2)

*NAa <sup>V</sup>* .2

The values of k and γ can be defined as follows (Gred & Keiser, 2000):

)(

(1)

power.

(Marcuse, 1982):

1997):

**4. Modelling of the macro-bent EDFA** 

Fig. 8 shows a better flattening approach for 9m EDF, where the flattened gain profile is obtained by incremental gain enhancement of about 3 dB and 20 dB at wavelengths of 1550 nm and 1530 nm respectively. This enhancement is attributed to macro-bending effect in the EDF. This incremental gain compensates the incremental gain reduction of the EDFA before applying macro-bending resulting in a flat and broad gain spectrum.

The gain variation to gain ratio ΔG/G is generally used to characterize the gain variation, where ΔG and G are the gain excursion and the average gain value, respectively (Wysocki et al., 1997). In order to define the gain flatness of EDFAs, the ΔG/G for the EDFA with and without macro bending is compared between 1530 and 1555 nm under the same condition. The gain variation ΔG/G for this macro-bent EDFA was 0.10 (2.8 dB / 27.84 dB), which is a 50% improvement compared to earlier reports (Uh-Chan et al., 2002). Besides this, we also observe a gain variation within ±1 dB over 25 nm bandwidth in C-band region (S.D.Emami et al., 2009).

Fig. 9 compares the gain spectrum of the EDFA with and without macro bending EDF at various input signal power. The input signal power is varied for -10 dBm and -30 dBm. The input pump power is fixed at 200 mW. The EDF length and bending radius is fixed at 9m and 6.5 mm respectively. As shown in the figure, increasing the input signal power decreases the gain but improves the gain flatness. The macro bending also reduces the noise figure of EDFA at wavelength shorter than 1550 as shown in Fig. 10 (Hajireza, et al. 2010).

Fig. 9. EDFA gain spectra; with and without acro bending at various input signal power.

Fig. 8 shows a better flattening approach for 9m EDF, where the flattened gain profile is obtained by incremental gain enhancement of about 3 dB and 20 dB at wavelengths of 1550 nm and 1530 nm respectively. This enhancement is attributed to macro-bending effect in the EDF. This incremental gain compensates the incremental gain reduction of the EDFA before

The gain variation to gain ratio ΔG/G is generally used to characterize the gain variation, where ΔG and G are the gain excursion and the average gain value, respectively (Wysocki et al., 1997). In order to define the gain flatness of EDFAs, the ΔG/G for the EDFA with and without macro bending is compared between 1530 and 1555 nm under the same condition. The gain variation ΔG/G for this macro-bent EDFA was 0.10 (2.8 dB / 27.84 dB), which is a 50% improvement compared to earlier reports (Uh-Chan et al., 2002). Besides this, we also observe a gain variation within ±1 dB over 25 nm bandwidth in C-band region (S.D.Emami

Fig. 9 compares the gain spectrum of the EDFA with and without macro bending EDF at various input signal power. The input signal power is varied for -10 dBm and -30 dBm. The input pump power is fixed at 200 mW. The EDF length and bending radius is fixed at 9m and 6.5 mm respectively. As shown in the figure, increasing the input signal power decreases the gain but improves the gain flatness. The macro bending also reduces the noise figure of EDFA at wavelength shorter than 1550 as shown in Fig. 10 (Hajireza, et al. 2010).

Fig. 9. EDFA gain spectra; with and without acro bending at various input signal power.

applying macro-bending resulting in a flat and broad gain spectrum.

et al., 2009).

Fig. 10. EDFS noise figure spectra; with and without macro bending at various input signal power.
