**3.1 Influence of the optical confinement**

The optical gain for the optical confinements of Γ~20% and Γ~80% are compared depending on the input electrical current and optical power in Figure 5. The gain increases with the bias current as modelled in previous section and starts to saturate at high electrical injection. The low confinement factor (Γ~20%) devices show higher gain than the high confinement factor (Γ~80%) devices. This result is counter-intuitive as the net gain should increase with higher optical confinement and therefore the single pass gain. High Γ means more ASE and more saturation. Thus a low confinement factor induces lower spontaneous emission power by reducing the effect of the ASE inside the device (Brenot et al., 2005). As the RSOA is less saturated, the single pass gain is also increasing with the reduced confinement factor (because the LSHB is reduced).We demonstrated that RSOA devices have a non-uniform carrier density along the active zone. This interpretation can be confirmed by the simulations of the carrier density spatial distribution (section 2) and SE measurements (section 3.2).

Increasing the input power, the gain drops quickly due to the saturation effect. That is, the increase of optical input power at a constant current consumes many carriers for the stimulated emission therefore decreases the carrier density and increases the saturation effect. This transition corresponds to the frontier between the linear and the saturated regime. In this regime, the noise factor increases due to gain saturation. A common and useful figure of merit is the dependence of the optical gain on the output power. From this curve, we obtained the saturation power (Psat) when the gain drops by 3 dB. Figure 5 (b) shows the optical gain versus the output power.

Most of SOA devices show saturation power around 10 dBm and optimized SOA can reach 20 dBm (Tanaka et al., 2006). However optimizing for maximum saturation power induces low gain (<15 dB) and large energy consumption (I > 500mA). In RSOA devices, high gain is obtained as well as reasonable saturation power.

Fig. 5. Confinement effect on 700 µm long RSOA depending on the current (a) and the output power (b)

### **3.2 Saturation effect in long RSOA**

8 Selected Topics on Optical Amplifiers in Present Scenario

meter are used in order to determine the static performances of the device, such as optical gain, gain peak, bandwidth and ripple, noise figure and output saturation power. The impacts of these several parameters (Γ and L) are experimentally studied in the next sub-

*OSA RSOA*

The optical gain for the optical confinements of Γ~20% and Γ~80% are compared depending on the input electrical current and optical power in Figure 5. The gain increases with the bias current as modelled in previous section and starts to saturate at high electrical injection. The low confinement factor (Γ~20%) devices show higher gain than the high confinement factor (Γ~80%) devices. This result is counter-intuitive as the net gain should increase with higher optical confinement and therefore the single pass gain. High Γ means more ASE and more saturation. Thus a low confinement factor induces lower spontaneous emission power by reducing the effect of the ASE inside the device (Brenot et al., 2005). As the RSOA is less saturated, the single pass gain is also increasing with the reduced confinement factor (because the LSHB is reduced).We demonstrated that RSOA devices have a non-uniform carrier density along the active zone. This interpretation can be confirmed by the simulations of the carrier density spatial distribution (section 2) and SE measurements

*ECL*

*Variable attenuation*

*Lensed fibre*

*Bias current*

Increasing the input power, the gain drops quickly due to the saturation effect. That is, the increase of optical input power at a constant current consumes many carriers for the stimulated emission therefore decreases the carrier density and increases the saturation effect. This transition corresponds to the frontier between the linear and the saturated regime. In this regime, the noise factor increases due to gain saturation. A common and useful figure of merit is the dependence of the optical gain on the output power. From this curve, we obtained the saturation power (Psat) when the gain drops by 3 dB. Figure 5 (b)

Most of SOA devices show saturation power around 10 dBm and optimized SOA can reach 20 dBm (Tanaka et al., 2006). However optimizing for maximum saturation power induces low gain (<15 dB) and large energy consumption (I > 500mA). In RSOA devices, high gain is

sections.

Fig. 4. Static experimental setup

(section 3.2).

**3.1 Influence of the optical confinement** 

*Opticalspectrum analyzer*

shows the optical gain versus the output power.

obtained as well as reasonable saturation power.

Two optical confinement values have been studied and low optical confinement (Г~20 %) enables the fabrication of high gain devices. It was the consequence of the LSHB reduction inside the active material which leads to an overall higher gain. However, the length (L) also affects the single pass gain (Gs). Again two effects are in competition inside the active zone: the exponential growth of Gs with the length and the non-homogenous carrier density distribution (which leads to strong saturation effect). Therefore a trade-off needs to be found in order to balance these two effects. By increasing the length, the forward and backward amplifications are also increased up to an optimum point. Devices that are too long induce high saturation and reduce the optical gain. Figure 6 (a) shows the optical gain versus the current density in different RSOA cavity lengths. The current density (J) is more relevant from a device point of view in order to compare similar operating conditions.

Fig. 6. Length effect on 20% optical confinement RSOA depending on the current density (a) and on the output power (b) for J = 10 kA/cm2

Next Generation of Optical Access Network Based on Reflective-SOA 11

explained by two effects that are not present in RSOA devices. The first effect is gain clamping. The carrier density stays low even at high electrical input current while the photon density is increasing. This produces a shorter carrier lifetime particularly advantageous for high speed modulation. The second effect is the electron to photon resonance due to the presence of a cavity. The resonance appears in the modulation

The absence of cavity in RSOAs limits the modulation speed of this device. The modulation response behaves as a low pass filter with a characteristic cut-off frequency (when the link gain drops by 3dB). One limitation is due to carrier density spatial distribution. High carrier density combined with low photon density induces long carrier lifetime. Furthermore the carrier and photon densities strongly depend on the position z along the device. Therefore a

The objective is to obtain a first order approximation of the carrier lifetime for the steady

The carrier lifetime is inversely proportional to the recombination rate. The recombination rate can be described using two different terms: one directly proportional to the spontaneous emission and non-radiative recombination (due to the defect or Auger process as described in section 2.2) and the second one depending on the stimulated

Fig. 8. Carrier lifetime simulation along 700 µm RSOA device at (a) low (Pin = -40 dBm) and

Simulations of the carrier lifetime have been carried out along the active region. Figure 8 represents the results with the bias current as parameter at Pin = -40 dBm (Figure 8 (a)) and Pin = 0 dBm (Figure 8 (b)). Obviously, in both cases, carrier lifetime decreases by increasing

ൌ ܣ ܤǤ ݊ ܥǤ ݊<sup>ଶ</sup> ߁ൈܽൈܵൈݒ) 13 (

డே, Γ is the optical confinement factor and S is

state condition. We can demonstrate that the carrier lifetime can be approximated by:

response increasing the effective -3dB E/O bandwidth.

ଵ ఛ

the total photon density including the signal and the ASE.

non-homogeneous carrier lifetime is obtained.

Where the differential gain is defined by ܽ ൌ డ

(b) high (Pin = 0 dBm) optical injection

**4.1 Carrier lifetime analysis** 

recombination.

At first, the increase of the cavity length induces higher optical gain (from 300 µm to 700µm) however when it reaches 850 µm, the gain drops back. Therefore a maximum gain is obtained for 700 µm long devices. The optical gain versus the output power is presented in Fig. 6. (b) at the current density J = 10 kA/cm2. We can notice that increasing the gain leads to higher saturation power. It can be explained by the fact that we are at a constant current density therefore the electrical bias current increases with the length of the device leading to an improvement of the saturation power. For one specific optical confinement (Γ = 20%), an optimal length can be found in order to obtain the best static performances (high optical gain). At first, the optical gain increases linearly with the length. In fact, the forward and backward amplifications control the single pass gain. Figure 7. (a) represents the SE measurements where an optical fibre is placed along the active zone at the input/output, centre and mirror region. Then SE measurements as a function of the injected current are measured. SE measurements are performed in 700 µm long RSOA in order to confirm the presence of the saturation effect.

Fig. 7. (a) SE schematic and measurements; LSHB effect on (b) the optical gain in RSOA device

At low input bias current, no difference is observed due to the flat carrier density. The saturation effect starts to appear above 50 mA when the carrier density spatial distribution becomes non-homogeneous. Low SE power is collected at the input region due to the saturation effect which means low carrier density in the region. However the mirror region emits more SE power due to the high carrier density value. This demonstrates the presence of a strong saturation effect in the device.

In longer RSOAs, the depletion becomes stronger which induces a lower overall carrier density and a larger absolute difference in the carrier density between input and mirror facet. When varying the length of the RSOA, those several effects account for the existence of an optimum length where the optical gain is maximised. The optical gain versus the length of the device is plotted on Figure 7. (b) for two current densities.
