**5. System performances**

14 Selected Topics on Optical Amplifiers in Present Scenario

Fig. 11. Direct modulation measurements S21 in 700μm long RSOA device


RSOA modulation response (dB)

equivalent electrical circuit exhibits a cut-off frequency around 3 GHz.

 Simulation Experiment

We simulate the modulation bandwidth depending on the carrier lifetime based on the first order approximation. The carrier lifetime can be estimated along the RSOA but shows a non-homogenous spatial distribution. The first approach consists of considering an average carrier lifetime over the whole device. Simulation and experimental data are compared in Figure 12-(a) for a 700 µm long RSOA at 80 mA. The simulation results fit well with the measurements over a limited range (from 0 to 2GHz). The difference beyond can be explained by the addition of the buried ridge structure (BRS) limitation. In fact, the BRS

0 1 2 3 4 5 6 7 8 9 10

*I (from 40 to 200 mA)*

Frequency (GHz)

Fig. 12. RSOA (a) E/O modulation bandwidth versus frequency at I = 80 mA (b) -3 dB E/O

*a) b)* Bias current (mA)

0,0 0,2 0,4 0,6 0,8 1,0 1,2

E/O -3dB bandwidth (GHz)

1,4 Measured

 Model ( @ z = 0) Model (

average)

20 40 60 80 100

modulation bandwidth versus bias current for 700µm of AZ

0 1x109 2x109 3x109 4x109 5x109

Frequency (Hz)


RSOA modulation response (a. u.)

The role of a RSOA as an optical transmitter is to launch a modulated optical signal into an optical fiber communication network. Reflective semiconductor optical amplifier (RSOA) devices have been developed as remote modulators for optical access networks during the past few years and their large optical bandwidth (colorless operation) has placed them in a leading position for the next generation of transmitters in WDM systems. In RSOA devices, the wavelength is externally fixed. Various options have been studied such as using multiwavelength sources (such as tuneable lasers, External cavity laser (ECL) , Photonic Integrated Circuits (PIC) or a set of Directly Modulated Laser (DML) at selected wavelengths), creating a cavity with the active medium of the RSOA, or using filtered white source. Therefore, RSOA devices as colourless transmitters can be used in different configurations:


In the laser seeding approach, the multi-wavelength external laser source can be located at the CO (Chanclou et al., 2007) or at the remote node (de Valicourt et al., 2009). From the CO, the optical budget is limited to 25 dB and strong RBS impairments appear. These limits are overcome by locating the laser at the remote node. One laser per remote node is needed, thus raising deployment cost, control management and power consumption issues.

Another possible architecture is using spectrum-sliced EDFA seeding. An erbium-doped fibre amplifier (EDFA) is used as a broadband source of un-polarised amplified spontaneous emission and this broad spectrum is then sliced by the Arrayed Waveguide Grating (AWG) for each ONU (Healey et al., 2001).

Wavelength re-use has been developed by Korean and Japanese companies (Lee W. R. et al., 2005). The downstream source from the CO is re-modulated as an upstream signal at the

Next Generation of Optical Access Network Based on Reflective-SOA 17

demultiplexer is coupled into a 20 km long Single Mode Fibre (SMF) followed by a 12 dB optical attenuator used to simulate a passive splitter for 16 subscribers. The CW signal is then modulated by the RSOA, generating the upstream signal. The RSOA is driven by a 231- 1 pseudo-random bit sequence (PRBS) at 2.5 Gbit/s, with a DC bias of 90 mA. From the remote node, the upstream signal propagates on another 25 km long SMF which simulates the reach extension provided by the proposed network design. A variable optical attenuator is placed in front of the receiver in order to analyze the performance of the system as a function of the optical budget. This attenuator also accounts for the insertion-loss of the multiplexer at the CO (between 3 to 5 dB). Bit-error-rate (BER) measurements are done

Fig. 14. Experimental setup of WDM/TDM architecture using RSOA (de Valicourt et al.,

At low bit rate, the best trade-off between gain, modulation bandwidth and saturation power is obtained for a 700 µm long cavity RSOA, therefore we chose this device in the experimental setup. The RSOA is driven at 90 mA with a -10 dBm input power. Figure 15 displays BER measurements performed at 1554.1 nm and 2.5 Gbit/s as a function of the optical budget between the CO and the extended optical network unit (ONU). The inset shows the open eye diagram measured at the output of the RSOA. Sensitivities at 10-9 in back-to-back (BtB) configuration and after transmission are -32 dBm and -27 dBm respectively. These performances are mainly due to the large output power of the RSOA, which allows for an increased optical budget compared to standard RSOAs: a BER of 10-9 is thus measured with an optical budget of more than 36 dB. Whatever the OB, the input power in the RSOA is -10dBm, which ensures that the device operates in the saturated regime, with a reflection gain of 20 dB. Gain saturation leads to a low sensitivity of the RSOA to back-reflections, since the output power only slightly depends on the input power. In Figure 15 (b), the BER of 8 WDM channels (100GHz spacing), is shown, for a 40 dB optical budget ; in this case, the BER is 10-7, well below the forward error correction (FEC) limit. No penalty is observed due to the large bandwidth of the RSOA. Besides, this OB corresponds to two 12 dB (16\*16 subscribers) power splitters, taking into account mux/demux, propagation and circulator losses. A compromise between split ratio and range needs to be

2010a)

using an Avalanche Photo-Diode (APD) receiver and an error analyzer.

ONU using RSOA. Simple efficient ONU is obtained as no additional optical source is needed.

The final approach is using a RSOA-based self-seeding architecture. This recent concept has been proposed by Wong in 2007. This novel scheme uses at the remote node (RN) a reflective path to send back the ASE (sliced by the AWG) into the active medium. The selfseeding of the RSOA creates a several km long cavity between ONU and RN. The wavelength is determined by the connection at the RN. This technique is attractive because a self-seeded source is functionally equivalent to a tuneable laser. Recent progresses show 2.5 Gbit/s operation based on stable self-seeding of RSOA (Marazzi et al, 2011). Another way to obtain self-seeding configuration is using an external cavity laser based on a RSOA and Fiber-Bragg Grating (FBG). BER measurements show that the device can be used for upstream bit rates of 1.25 Gbit/s and 2.5 Gbit/s (Trung Le et al., 2011).

In this chapter, we focus on the laser seeding approach. We present the scheme of a laser seeding architecture based on RSOA on Figure 13. Actually, Figure 13 shows the up-stream part of the link using an RSOA, i.e. the information sent from the subscriber to operator/network. At the central office, a transmitter is used to send light (containing no information) to the subscriber through an optical circulator. Light propagates through several kilometres of optical fibre. The signal is then amplified and modulated by the RSOA in order to transmit the subscriber data for uplink transmission.

In this section, we demonstrate an extended reach hybrid PON, based on a very high gain RSOA operating at 2.5 Gbit/s. To reduce RBS impairments, we locate the continuous wave (CW) feeding light source in the remote node, and the large gain of the RSOA allows using moderate CW powers. Alternative devices such as remotely pumped erbium doped fiber amplifier (EDFA) can be used in order to avoid the deployment of active devices in a remote node; this approach could also reduce the RBS level owing to a lower seed power and the management cost of the system (Oh et al., 2007).

Fig. 13. Laser seeding Network architecture based on RSOA

Figure 14 shows the up-stream part of the proposed link. At the remote node, an external cavity laser (ECL) is used to launch an 8 dBm CW signal into the system through an optical circulator (OC). A wavelength demultiplexer is used to break a potential multi-wavelength signal back into individual signals. A given wavelength represents one of up to 8 sub-PON on a 100 GHz grid (from λ1 = 1553.3 nm to λ8 = 1558.9 nm). The output of the wavelength

ONU using RSOA. Simple efficient ONU is obtained as no additional optical source is

The final approach is using a RSOA-based self-seeding architecture. This recent concept has been proposed by Wong in 2007. This novel scheme uses at the remote node (RN) a reflective path to send back the ASE (sliced by the AWG) into the active medium. The selfseeding of the RSOA creates a several km long cavity between ONU and RN. The wavelength is determined by the connection at the RN. This technique is attractive because a self-seeded source is functionally equivalent to a tuneable laser. Recent progresses show 2.5 Gbit/s operation based on stable self-seeding of RSOA (Marazzi et al, 2011). Another way to obtain self-seeding configuration is using an external cavity laser based on a RSOA and Fiber-Bragg Grating (FBG). BER measurements show that the device can be used for

In this chapter, we focus on the laser seeding approach. We present the scheme of a laser seeding architecture based on RSOA on Figure 13. Actually, Figure 13 shows the up-stream part of the link using an RSOA, i.e. the information sent from the subscriber to operator/network. At the central office, a transmitter is used to send light (containing no information) to the subscriber through an optical circulator. Light propagates through several kilometres of optical fibre. The signal is then amplified and modulated by the RSOA

In this section, we demonstrate an extended reach hybrid PON, based on a very high gain RSOA operating at 2.5 Gbit/s. To reduce RBS impairments, we locate the continuous wave (CW) feeding light source in the remote node, and the large gain of the RSOA allows using moderate CW powers. Alternative devices such as remotely pumped erbium doped fiber amplifier (EDFA) can be used in order to avoid the deployment of active devices in a remote node; this approach could also reduce the RBS level owing to a lower seed power and the

Figure 14 shows the up-stream part of the proposed link. At the remote node, an external cavity laser (ECL) is used to launch an 8 dBm CW signal into the system through an optical circulator (OC). A wavelength demultiplexer is used to break a potential multi-wavelength signal back into individual signals. A given wavelength represents one of up to 8 sub-PON on a 100 GHz grid (from λ1 = 1553.3 nm to λ8 = 1558.9 nm). The output of the wavelength

upstream bit rates of 1.25 Gbit/s and 2.5 Gbit/s (Trung Le et al., 2011).

in order to transmit the subscriber data for uplink transmission.

management cost of the system (Oh et al., 2007).

Fig. 13. Laser seeding Network architecture based on RSOA

needed.

demultiplexer is coupled into a 20 km long Single Mode Fibre (SMF) followed by a 12 dB optical attenuator used to simulate a passive splitter for 16 subscribers. The CW signal is then modulated by the RSOA, generating the upstream signal. The RSOA is driven by a 231- 1 pseudo-random bit sequence (PRBS) at 2.5 Gbit/s, with a DC bias of 90 mA. From the remote node, the upstream signal propagates on another 25 km long SMF which simulates the reach extension provided by the proposed network design. A variable optical attenuator is placed in front of the receiver in order to analyze the performance of the system as a function of the optical budget. This attenuator also accounts for the insertion-loss of the multiplexer at the CO (between 3 to 5 dB). Bit-error-rate (BER) measurements are done using an Avalanche Photo-Diode (APD) receiver and an error analyzer.

Fig. 14. Experimental setup of WDM/TDM architecture using RSOA (de Valicourt et al., 2010a)

At low bit rate, the best trade-off between gain, modulation bandwidth and saturation power is obtained for a 700 µm long cavity RSOA, therefore we chose this device in the experimental setup. The RSOA is driven at 90 mA with a -10 dBm input power. Figure 15 displays BER measurements performed at 1554.1 nm and 2.5 Gbit/s as a function of the optical budget between the CO and the extended optical network unit (ONU). The inset shows the open eye diagram measured at the output of the RSOA. Sensitivities at 10-9 in back-to-back (BtB) configuration and after transmission are -32 dBm and -27 dBm respectively. These performances are mainly due to the large output power of the RSOA, which allows for an increased optical budget compared to standard RSOAs: a BER of 10-9 is thus measured with an optical budget of more than 36 dB. Whatever the OB, the input power in the RSOA is -10dBm, which ensures that the device operates in the saturated regime, with a reflection gain of 20 dB. Gain saturation leads to a low sensitivity of the RSOA to back-reflections, since the output power only slightly depends on the input power. In Figure 15 (b), the BER of 8 WDM channels (100GHz spacing), is shown, for a 40 dB optical budget ; in this case, the BER is 10-7, well below the forward error correction (FEC) limit. No penalty is observed due to the large bandwidth of the RSOA. Besides, this OB corresponds to two 12 dB (16\*16 subscribers) power splitters, taking into account mux/demux, propagation and circulator losses. A compromise between split ratio and range needs to be

Next Generation of Optical Access Network Based on Reflective-SOA 19

Another question about Hybrid PON is its property to be compatible with long reach network configuration. It was shown in the previous section that high gain RSOAs enable high optical budget, for instance, up to 36 dB and 45km transmission at 2.5 Gbit/s. A high optical budget is necessary to obtain a long reach PON (compensation of the fibre attenuation). The limitation imposed on the bit rate and distance by the fibre dispersion can dramatically increase depending on the spectral width of the source. This problem can be overcome by reducing the chirp produced by the RSOA device. Chirp reduction was demonstrated using a 2-section RSOA and how it can be used to reduce the transmission penalties (de Valicourt et al., 2010b). We propose an extended reach hybrid PON, taking advantages of a very high gain Reflective Semiconductor Optical Amplifier (RSOA) and the

Two RSOAs with a cavity length of 500 μm are used in the experimental setup, one with mono and the other with dual-electrode configuration. The dual-electrode RSOA was realized by proton implantation in order to separate both electrodes. The single electrode RSOA was driven at 60 mA and the dual-electrode at 20 mA on the input electrode and 115 mA at the mirror electrode. Both current values correspond to optimized conditions in order to obtain low transmission penalties. It is to be noted that in a dual-electrode RSOA, the AC current is applied to the input/output electrode. In both cases, the CW optical input power was –8.5 dBm. Fig. 16 displays BER measurements performed at 1554 nm and 2.5 Gbps as a function of the received power for one electrode and two-electrode RSOA. The penalties due to 100 km transmission with a single electrode RSOA do not enable to reach the FEC limit. From 25 km to 50 km (100 km), we obtain penalties of 1.2 (3.4) dB. One can see that a BER of 10-4 (FEC limit) has thus been measured with bi-electrode RSOA at a received optical power of -35 dBm over 100 km SMF. The penalties due to extended 25 and 50 km SMF are much lower than with single electrode RSOA (respectively 0.5 and 1.4 dB). These transmission

Fig. 16. Comparison of BER value as a function of the received power for mono-electrode and bi-electrode RSOA over 50, 75 and 100 km at 2.5 Gbps (de Valicourt et al., 2010c).

two-electrode configuration operating at 2.5 Gbit/s (de Valicourt et al., 2010c).

**6.1 Long reach PON using low chirp RSOA** 

considered. Thus, one of the two 12 dB budget increase can also allow reach extension between the CO and the remote node (including the 25 km reach extension). However, propagation effects such as RBS and dispersion in the fibre would limit this extension. A reduction in RBS level is also needed to improve the performance of this configuration. Different solutions have been studied to reduce the RBS level such as: frequency modulation of the laser source or applying bias dithering at the RSOA.

Fig. 15. (a) BER as a function of the optical budget. Inset: 2.5 Gbit/s eye-diagram at the output of the RSOA driven at 90 mA and with an input power of -10 dBm (b) BER values for different λ-channels for an optical budget of 40 dB, or a Rx input power of -30 dBm (de Valicourt et al., 2010a)

A cost effective hybrid WDM/TDM-PON which can potentially feed 2048 subscribers (16×16×8 = 2048 subscribers) at a data rate of 2.5 Gbit/s is presented in this section. The large gain and high output power of the RSOA have also allowed extending the link reach up to 45 km instead of the standard 20 km. However, these achievements are obtained at the expense of an increase in deployment and operation costs. We believe this solution is economically viable since these costs are shared between many users, and multi-wavelength sources are becoming cheaper with the advent of Photonic Integrated Circuits (PIC). This 2.5 Gbit/s upstream colourless result allows investigating this solution to achieve in the trunk line a wavelength multiplex of several next generation access solutions (10 Gbit/s downand 2.5 Gbit/s up-stream).
