# . *<sup>I</sup> I J p p p*

**Coupler CW port PM port ISOA1 ISOA2 Formulation Value** K1 #I #A, #B, #C 200 mA 0 mA Eq. (1) 52% K2 #J #B, #C, #D 0 mA 200 mA Eq. (2) 42% K3 #I #B, #C 200 mA 0 mA Eq. (3) 52% K4 #A #I, #J 200 mA 0 mA Eq. (4) 60%

From the measurement results presented in **Table 1**, we conclude that couplers are not symmetrical, as ideally would be expected on this device. Furthermore, splitting factors are all different. This asymmetry induces an uneven power distribution along the interferometer arms that in any operational point will cause side effects, such as different saturation levels of the

a

following expression,

172 Optical Interferometry

CW VOA

SOAs from each arm.

The next stage of the interferometer characterization is the analysis of SOA output power dependence on input power and SOA bias current. These measurements were realized, considering the two arms individually, first with a light path from input #B to output #J and SOA1 unbiased; then, with a light path from input #A to output #I and SOA2 unbiased, as depicted, respectively, in **Figures 7** and **8**. It is important to emphasize that this analysis refers to the whole light path where the optical wave propagates, so it includes all power losses and any other type of asymmetry of the interferometer chip.

**Figure 7.** Setup for the characterization of SOA1.

**Figure 8.** Setup for the characterization of SOA2.

For the evaluation of the SOA dependence from bias current, the injected power of CW power is set previously at a low value and the bias current is swept from 150 to 400 mA. For each current step, the optical power value on the output port is first measured and then saved. Then, input power is gradually increased, respecting the maximum permissible power given by the device specifications, and the previous process and measurements are repeated. **Figure 9** shows the results of the dependence of the SOA output power as a function of the SOA bias current.

To characterize the influence of the input power on the SOA gain, the power of a CW laser is gradually increased from 1 to 10 mW, while the SOA bias current is kept constant. The output power is then recorded for each measurement. From **Figure 10**, we observe the SOA output power dependence on the input power, for both SOAs and several bias currents. From the resulting curves, we obtain the necessary setting points (input power and SOA bias) when the SOA gain begin to saturate.

**Figure 9.** MZI-SOA output power at port #I (full line) and #J (dashed line), as a function of SOA1 and SOA2 bias current, respectively. Input power equal to −8, −2 and 10 dBm (asterisks, circles and squares, respectively). The lines are guides for the eyes.

**Figure 10.** MZI-SOA output power at port #J (dashed line) and port #I (full line), vs. input CW power at ports #D and #A, respectively. SOA bias currents are adjusted to 100, 200 and 300 mA (asterisks, circles and squares, respectively). The lines are guides for the eyes.

### *2.1.3. Extinction ratio characterization*

resulting curves, we obtain the necessary setting points (input power and SOA bias) when the

150 200 250 300 350 400

SOA bias current (mA)

0 2 4 6 8 10

Input Power (mW)

**Figure 10.** MZI-SOA output power at port #J (dashed line) and port #I (full line), vs. input CW power at ports #D and #A, respectively. SOA bias currents are adjusted to 100, 200 and 300 mA (asterisks, circles and squares, respectively).

**Figure 9.** MZI-SOA output power at port #I (full line) and #J (dashed line), as a function of SOA1 and SOA2 bias current, respectively. Input power equal to −8, −2 and 10 dBm (asterisks, circles and squares, respectively). The lines are

SOA gain begin to saturate.

174 Optical Interferometry

2

The lines are guides for the eyes.

Output Power (mW)

guides for the eyes.

3

4

5

6

Output Power (mW)

7

8

9

10

The power at each output ports of the MZI-SOAs under test (ports #I and #J) is a result of an interference process taking place in coupler K4. The intensity and phase of the optical signal of coupler K4 inputs will set the conditions for the measured outputs. In particular, when used as an intensity modulator, one key factor to take into account is the ER between the output signals. By definition, the ER is the ratio of the optical power levels measured at output ports #J and #I,

$$ER = 10\log\_{10}\left(\frac{P\_{\sharp I}}{P\_{\sharp J}}\right). \tag{5}$$

The ER value is given in dB and power P#I and P#J are given on a linear scale.

As an example, the power distribution on the MZI-SOA internal waveguides can be varied even more, through SOAs current variation, leading to the subsequent change of the interference settings on coupler K4. To observe the effect of bias currents on the ER, we employ the setup from **Figure 11**. Port #B receives a CW signal, and both PSs of the MZI-SOA remains unbiased (voltage set to 0 V).

**Figure 11.** Setup for the characterization of ER dependence on bias current.

In **Figure 12**, we observe the dynamic of both the constructive and destructive interference, respectively, at the output port #J and #I, by gradually increasing SOA1 bias current, with SOA2 current constant at a reference value (200 mA).

This process is repeated with SOA1 bias current constant at 200 mA, while SOA2 current is swept, using a CW laser beam connected at input port #B. With this methodology and after measuring the power at the output port #I and #J, we observe in **Figure 11** that there is a misalignment between #J and #I maximum and minimum power levels. This is caused by the gain variation in the SOA with the varying bias current, together with the asymmetrical coupling factors of the couplers and the phase shift of the electromagnetic field. The best operational point, without phase shifters corrections, is established searching for SOA1 bias current for the maximum ER.

**Figure 12.** MZI-SOA output power at port #I (squares) and port #J (circles), as a function of SOA1 (dashed line) and SOA2 (full line) bias current. Input power is injected at port #B. The lines are guides for the eyes.

### **2.2. Dynamic characterization**

After the static characterization, this section investigates the dynamic properties of the MZI-SOA. This study is focused on the properties of the MZI-SOA as an optical gate.

#### *2.2.1. Experimental setup*

**Figure 13** shows the proposed setup to characterize the dynamic properties of the MZI-SOA. The setup follows the wavelength conversion design presented in Ref. [14], in a co-propagation design. This setup implements an all-optical exclusive OR (XOR) logical gate using a single MZI-SOA chip. From the MZI arms, both optical signals are launched into the SOAs, where their carrier densities and thereby the refractive index are modulated. This result in a phase modulation of the CW probe signal propagating through the SOAs by cross-phase modulation (XPM), according to the intensity variations of the input control signals. By carefully setting the input optical powers and controlling the SOA bias current, the control signal from the two SOAs interferes either destructively or constructively at the output of the MZI in order to provide the logical XOR operation of the two data sequences on the optical probe signal. From the XOR truth table, when both data signals injected at ports #D and #A are time synchronized, no pulses are observed on the probe signal at the MZI-SOA output (port #I). On the other hand, as the data signals give up time overlapping, some pulses with increasing intensity will appear on the probe signal, at the same MZI-SOA output.

The experimental setup consists of an external cavity laser peaking at 1549.32 nm (1), followed by a polarization controller (PC) and an external Mach-Zehnder modulator (MZM). The nonreturn-to-zero (NRZ) data signal generated by a serial bit error rate tester (ref. Agilent N4901B) is then optically amplified by an erbium-doped fiber amplifier (ref. IPG-EAD-500-C3-W) and divided into two identical signals using a 50:50 coupler (COUPLER1) with symmetrical outputs. Both optical signals coming from COUPLER1 are synchronized using optical delay lines. Polarization controllers are inserted on the light path to ports #D and #A of the MZI-SOA (ref. CIP 40G-2R2-ORP), to optimize the destructive interference at port #I. The probe signal, a CW light beam with 0 dBm and lasing at 1546.12 nm (2), is launched into port #B of the MZI-

SOA in a co-propagating direction with the data control signals. Different data patterns may be obtained by delaying signals at port #A and port #D. Finally, the probe signal is recovered at port #J, using a filter with a 40 GHz bandwidth (X-tract Net Test). The setup uses two instruments to analyse and measure the optical output signal: an optical complex spectrum analyser (OCSA) (APEX AP 2441A) to gather power and phase information of the output signal for time domain characterization and a sampling oscilloscope (Agilent 86100A) connected through a photodiode (HP-11982A).

**Figure 13.** All-optical XOR gate setup, based on a MZI-SOA, in a co-propagation scheme.

150 200 250 300 350 400

SOA current (mA)

**Figure 12.** MZI-SOA output power at port #I (squares) and port #J (circles), as a function of SOA1 (dashed line) and

After the static characterization, this section investigates the dynamic properties of the MZI-

**Figure 13** shows the proposed setup to characterize the dynamic properties of the MZI-SOA. The setup follows the wavelength conversion design presented in Ref. [14], in a co-propagation design. This setup implements an all-optical exclusive OR (XOR) logical gate using a single MZI-SOA chip. From the MZI arms, both optical signals are launched into the SOAs, where their carrier densities and thereby the refractive index are modulated. This result in a phase modulation of the CW probe signal propagating through the SOAs by cross-phase modulation (XPM), according to the intensity variations of the input control signals. By carefully setting the input optical powers and controlling the SOA bias current, the control signal from the two SOAs interferes either destructively or constructively at the output of the MZI in order to provide the logical XOR operation of the two data sequences on the optical probe signal. From the XOR truth table, when both data signals injected at ports #D and #A are time synchronized, no pulses are observed on the probe signal at the MZI-SOA output (port #I). On the other hand, as the data signals give up time overlapping, some pulses with increasing intensity will appear

The experimental setup consists of an external cavity laser peaking at 1549.32 nm (1), followed by a polarization controller (PC) and an external Mach-Zehnder modulator (MZM). The nonreturn-to-zero (NRZ) data signal generated by a serial bit error rate tester (ref. Agilent N4901B) is then optically amplified by an erbium-doped fiber amplifier (ref. IPG-EAD-500-C3-W) and divided into two identical signals using a 50:50 coupler (COUPLER1) with symmetrical

SOA2 (full line) bias current. Input power is injected at port #B. The lines are guides for the eyes.

SOA. This study is focused on the properties of the MZI-SOA as an optical gate.

0

on the probe signal, at the same MZI-SOA output.

**2.2. Dynamic characterization**

*2.2.1. Experimental setup*

5

10

15

Output Power (mW)

20

25

176 Optical Interferometry

**Figure 14.** Wavelength and format conversion setup in counter propagation scheme.

The setup for the counter-propagation scheme uses the same probe signal, but now the signal is injected into port #I. As shown in **Figure 14**, the output signal is recovered at the constructive interference output (port #B). An isolator is placed at ports #I, #D and #A to protect all laser sources from back propagation signals.

### *2.2.2. Experimental results and discussion*

**Figure 15** shows the data input signals (1) injected into the arms #A and #D of the MZI-SOA, each with 2 dBm mean power, and the corresponding XOR gate output (2), at 10 Gbps at port #J, in a co-propagating scheme. In this experimental scenario, the results are in conformity with the truth table of an XOR gate: the output presents a logical zero (0) if both the operands have identical value and a logical one (1) otherwise.

**Figure 15.** Optical sequences at MZI-SOA input ports #D and #A (first two signals from top) and resulting XOR output at port #J (bottom sequence). Horizontal scale: 500 ps/div. Vertical scale is arbitrary.

**Figure 16.** Experimental measurements of the ER of the output signal, as a function of the input power for co-propagation (+ sign) and counter-propagation (× sign) schemes.

**Figure 16** shows the dependence of the ER at the output signal as a function of the power of the NRZ input signals, varying from 0 to 4 dBm. We set the CW probe signal power at 0 dBm. For both co- and counter-propagation proposals, we observe that changes on the input power do not affect the overall performance of the all-optical logical XOR gate, since the power variation of the two control signals involved in the assessment process is equivalent. But when compared with the co-propagation scheme, the counter-propagation scheme present better results, increasing the ER from 0.72 to 1.64 dB and improving the performance of the MZI-SOA as an optical gate. These results are in line with other experimental studies [15].
