**3. All-optical format conversion techniques**

In the last two decades, the information volume flowing on communication networks increased exponentially, fostering the search for fast optical switching, gating and transmission techniques, along with equipment with integration facilities and low power consumption. Among those techniques, phase modulation of optical signals is an option that allows greater transmission distances in both analogue and digital transmission systems. Other method for more advanced modulation format conversion based on interferometric techniques will be also briefly described.

### **3.1. Format conversion from amplitude to phase modulation**

Phase modulation generates signals with logical value 0 or 1, by varying the phase of light, while allowing it to be in the ON position. As opposed to intensity modulation, phase modulation has superior bandwidth efficiency and is not easily affected by signal distortions caused by relay nodes and transmission fibres. Several techniques have already been proposed to implement optical phase modulators, based on frequency shifters [16], lithium niobate (LiNbO3) waveguide [17], gain transparent SOA [18] or using highly nonlinear fiber (HNLF) as the optical medium to phase modulate a CW laser [19].

Following the experimental findings with the XOR gate, this section characterizes the phase modulation properties of a MZI-SOA, using both interferometric arms, in co-propagation schemes. The setup is the same as the XOR gate of **Figure 13**, with MZI-SOA operational parameters (SOA bias current, input optical power) tuned to create a destructive output at port #I. When both control signals are synchronized, the optical CW coming out of the output of the two SOAs have opposite phases and interfere destructively when combined at COUPLER4. According to the XOR truth table in **Figure 17**, the resulting optical signal at port #I has no observable amplitude variation [20]. Though, the phase *φ* of the probe signal 2 will vary in accordance to the input pattern, as depicted in **Figure 17** [21].

#### *3.1.1. Experimental results and discussion*

To investigate the phase modulation performance of an MZI-SOA for different bit rates, the operational parameters and the input power were optimized according to the phase eye diagram opening, also called phase span in the subsequent paragraphs. BER measurements were not performed due to setup limitations imposed by the coherent receiver. The OCSA limits the size of the data sequence length to 4 bits at 2.5 Gbps or 16 bits at 10 Gbps [22], thus BER measurements are not feasible.

observable amplitude variation [20]. Though, the phase *φ* of the probe signal 2 will vary in

1 0 1 1

1 0 1 1

*-O*

*O O O*

To investigate the phase modulation performance of an MZI-SOA for different bit rates, the operational parameters and the input power were optimized according to the phase eye

time

time

accordance to the input pattern, as depicted in **Figure 17** [21].

Power

Phase

**Figure 17.** Principle of operation diagram of the conversion technique.

*3.1.1. Experimental results and discussion*

Port #A

180 Optical Interferometry

Port #D

Port #I

Port #I

In order to confirm the feasibility of the interferometer as a phase modulator, experiments were carried out at bitrates of 2.5 and 10 Gbps. Data signals are launched into ports #D and #A with 2.5 dBm mean power and an average ER of 11.3 dB. The bias currents of both SOAs (ISOA) were increased at the same time, from 150 to 300 mA at 2.5 Gbps and from 150 to 400 mA at 10 Gbps bit rate. For each bias value, the mean power of the control signal (PCW) was swept from −6 to 2 dBm. The voltage applied to the PS was set to maximize the destructive interference at output port #I.

**Figure 18** (left) presents a 4 bit pattern signal injected at ports #D and #A. **Figure 18** (right) shows the output signal at port #I, when control signal mean power is −4 dBm and SOA bias current is 250 mA. The phase variation associated to different logic levels is well pronounced but reversed when compared with the control signal intensity. Phase span and the mean power on the output signal are also proportional to the power of the CW probe signal and the bias current of the SOASs, as long as the SOAs are not in the saturated regime [9].

**Figure 18.** Left—Input sequence "0100"; Right—Phase and power output, with SOA input current ISOA equal to 250 mA and input laser power PCW equal to −4 dBm.

The proposed optical phase modulator was also characterized at 10 Gbps, and the tests were performed using data sequences 16 bits long [22]. **Figure 19** (left) shows the bit pattern launched at the input ports (#D and #A) of the interferometer. With SOA biased at 150 mA and control signal power launched at 0 dBm, the resulting output signal is depicted in **Figure 19** (right). Output power fluctuations are primarily due to noise. Similar to the previous experiments at 2.5 Gbps, phase shifts are inverted when compared with the logic levels of the data (control) signals. However, due to the carrier recovery time and the dynamics of the SOA, output phase levels are less pronounced at 10 Gbps when fast transitions occurs at the MZI-SOA input signals. Moreover, if the power of the probe signal is increased above 0 dBm, the SOAs saturates and the conversion process is less efficient, which reduces the output mean power and phase span [9].

**Figure 19.** Left—Input sequence "1110010010101100"; Right—Phase and power output, with SOA input current ISOA equal to 150 mA and input laser power PCW equal to 0 dBm.

### *3.1.2. Summary*

We have presented a method to perform optical phase modulation, from an all-optical XOR gate configuration. We measure the influence of input CW power and SOAs bias current on the signal phase at the MZI-SOA output port. We verify that an increase in the SOA bias current produces higher values of the mean power and the phase span of the output signal, but SOAs gain saturation has an inverse result on the same output signal. Overall, the experimental results show the viability of the MZI-SOA as device capable of all-optical modulation and format conversion.
