**2.1. Static properties**

next five years, with an annual global Internet protocol (IP) traffic exceeding the tens of

High definition (HD) streaming video and peer-to-peer applications use the majority of bandwidth in most broadband networks today. When we add the bandwidth requirements of online social networks, Internet browsing or online gaming, broadband service providers have to supply an increasingly larger amount of bandwidth. Besides human-made Internet traffic, machine-to-machine (M2M) communications [2], fostered by smart city applications [3] and the opening of new radio channel opportunities [4] are spreading globally. The Internet of things (IOT) [5] will contribute even more to increase the quantity of data exchanged in communication networks, and especially on optical communication networks, used both by

Photonics technologies have largely contribute to the considerable development of telecommunication networks, since they appearance 30 years ago, and one can predict that they will serve as ground for most of the network revolutions still to come. A recent study [6] of the drivers of photonics suggests that its future development will be made along four main paths: to make optical networks faster, more transparent, more dynamic and greener. With current technology, it may be difficult to follow simultaneously all four path, which is a major constrain to implement a novel generation of optical networks. However, these difficulties may be relieved by recent developments in the field of wavelength conversion, all-optical signal processing and flexible techniques to generate enhanced modulation formats in optical signals.

In this context, this chapter presents several techniques on format conversion of modulated signals, using Mach-Zehnder interferometers with semiconductor optical amplifiers (MZI-SOA). The MZI-SOA show very attractive properties, and therefore, the goal is to investigate their potential as an optical node for the dynamic conversion and generation of optical signals. As such, two techniques to implement all-optical modulation format conversion are explored. These techniques, when applied in interconnection nodes between different optical networks with variable bit rates and modulation systems, allow a better efficiency and scalability of the

This section presents an experimental methodology to study the static and dynamic properties of the MZI-SOA. The device under test is a commercial hybrid-integrated device, manufactured by the centre for integrated photonics (CIP) [7], consisting of a passive, planar silica balanced MZI with nonlinear SOAs and phase shifters (PSs) assembled in each interferometer arm. Sections 2.1 and 2.2 describe the characterization of active and passive components of the MZI-SOA, in static operational conditions. The methodology uses simple optical power measurements to extract operative parameters. Then, section 2.3 investigates the dynamic properties of the MZI-SOA as an optical gate. Overall conclusions about this topic are pre-

zettabyte mark by the end of 2017.

168 Optical Interferometry

wireless and cable communications.

network.

**2. MZI-SOA features**

sented at the end.

MZI-SOAs are devices with a small footprint, but a huge potential for application in many optical domains. They can be used in optical gates [8], digital phase and amplitude modulation [9], wavelength conversion and switching [10], signal regeneration and all-optical processing [11], among others. In all these applications, the operational parameters of MZI-SOA active elements (SOA and PSs) must be previously calibrated, to set an optimal setting point in terms of power/phase variations and mean output power, according to the available inputs and the expected application. The majority of the biasing point procedures require extensive and complex initial setup stage, which changes from device to device, due to their intrinsic production yields.

**Figure 1** show a photograph of the MZI-SOA under test in this experimental study. The package contains two MZI-SOA structures on a single chip [12]. The MZI-SOA is hybridly incorporated with a silica substrate (motherboard) that contains the optical waveguides, such as Y junctions and couplers; the SOAs are built-in on an independent silicon board, which is passively assembled in the motherboard; the active elements, i.e. the PSs and the SOAs, are assembled in the daughterboard; a Vgroove to simplify fibre pigtailing of the motherboard [13]. MZI-SOA with hybrid integration result in more flexibility but increased yields, where each active and passive element has its own asymmetries and tolerances (e.g. asymmetric splitting ratio of the couplers). These issues have various implications in some MZI-SOA functionalities, for example, on the maximization of extinction ratio (ER) between the output ports. For experimental testing, the chip was installed on a prototype box that included SOA current and phase shifter voltage electronics together with temperature control. A temperature control system takes measurements from a thermistor and actuates on a Peltier cell to keep the chip temperature constant at the desired value. All measurements were made at a temperature of 25°C.

**Figure 1.** Twin MZI-SOA device used for experimental measurements. A strip of eight fibres enters the MZI-SOAs on the right (four fibres per MZI-SOA), and a strip of four fibre exits on the left (two fibres per MZI-SOA).

Due to the interferometric configuration of the MZI-SOA, the power distribution along the interferometer arms will affect the interference strength at the output and its eventual optimization. Thus, it is essential to characterize all passive parts. To execute the experimental analysis on the device asymmetry, we use the setup depicted in **Figure 2**. Each arm of the MZI incorporates one SOA and one PS. For each input-output path, two sets of switches are used, as seen in **Figure 2**. This configuration allows measurements of the gain of one of the arms when the other arm is blocked by a switched off SOA, and the SOA gain dependence on the biasing current.

**Figure 2.** Experimental setup used to characterize the MZI-SOA with power measurements.

A distributed feed-back (DFB) laser with wavelength 1546.12 nm is used as the input signal and will be kept fixed in all upcoming experimental tests (The wavelength was chosen according to the MZI-SOA manufacturer specifications). All power measurements were obtained through a power meter (PM).

### *2.1.1. Internal couplers*

In order to carry out the analysis of all MZI-SOA internal couplers, and their asymmetry properties, each input-output path is analysed independently, biasing one SOA at time accordingly to the path. For all coupling factor measurements, continuous wave (CW) laser input power is constant at 3 dBm. When simply biased, the SOAs current value is set to 200 mA. PS1 and PS2 are always switched off, with voltage source = 0 V, since a PS has little impact on couplers characterization, and only one SOA is biased at each measurement.

**Figure 3.** Setup for the characterization of K1 coupling factor.

Coupling factor of coupler K1 is characterized using the setup depicted in **Figure 3**. SOA1 is biased at 200 mA and SOA2 is switched off, i.e. with 0 mA current.

Port #I is used as the input, but port #J could also be used as well. Power measurements must be carried out at ports #A, #B and #C, with a PM. Coupling factor 1 is obtained as

$$\alpha\_1 = \frac{P\_{\#\_A}}{\left(P\_{\#\_A} + P\_{\#\_B} + P\_{\#\_C}\right)}\tag{1}$$

using a linear scale to express power values.

analysis on the device asymmetry, we use the setup depicted in **Figure 2**. Each arm of the MZI incorporates one SOA and one PS. For each input-output path, two sets of switches are used, as seen in **Figure 2**. This configuration allows measurements of the gain of one of the arms when the other arm is blocked by a switched off SOA, and the SOA gain dependence on the

DFB DFB PC VOA SM SM VOA PC

PM PM

MZI-SOA

PS1 PS2

K1

#C #J

A distributed feed-back (DFB) laser with wavelength 1546.12 nm is used as the input signal and will be kept fixed in all upcoming experimental tests (The wavelength was chosen according to the MZI-SOA manufacturer specifications). All power measurements were

In order to carry out the analysis of all MZI-SOA internal couplers, and their asymmetry properties, each input-output path is analysed independently, biasing one SOA at time accordingly to the path. For all coupling factor measurements, continuous wave (CW) laser input power is constant at 3 dBm. When simply biased, the SOAs current value is set to 200 mA. PS1 and PS2 are always switched off, with voltage source = 0 V, since a PS has little impact

> SOA1 SOA2

#I

PC

VOA CW

on couplers characterization, and only one SOA is biased at each measurement.

PS1 PS2

K1

#C #J

K2 K4

K2 K4

SOA1 SOA2

K1,2 - Combiner/splitter K3,4 - Directional Couplers SOA1,2 - Semiconductor Optical Amplifiers

#I

PS1,2 - Phase Shifters

biasing current.

170 Optical Interferometry

PM - Power Meter PC - Polarization Controller

*2.1.1. Internal couplers*

PM

PM

PM

VOA - Variable Optical Attenuator SM - Switch Matrix

obtained through a power meter (PM).

#A

#B

#D

**Figure 3.** Setup for the characterization of K1 coupling factor.

K3

DFB - Distribute Feedback Laser

#A

#D #B

K3

**Figure 2.** Experimental setup used to characterize the MZI-SOA with power measurements.

With SOA1 unbiased (0 mA) and SOA2 biased at 200 mA, K2 coupling factor is measured when light is injected through MZI-SOA port #J and optical power measurements are made at ports #B, #C and #D, as depicted in **Figure 4**.

**Figure 4.** Setup for the characterization of K2 coupling factor.

Coupling factor 2 is computed as,

$$\alpha\_2 = \frac{P\_{\#\_D}}{\left(P\_{\#\_B} + P\_{\#\_C} + P\_{\#\_D}\right)}.\tag{2}$$

**Figure 5.** Setup for the characterization of K3 coupling factor.

The coupling factor of coupler K3 is computed from two power measurements, at port #B and port #C, as shown in **Figure 5**. Either SOA1 or SOA2 could be biased at 200 mA, but one SOA must be switched off.

Input power can be injected from port #I or port #J. Coupling factor 3 is obtained through the following expression,

$$
\alpha\_3 = \frac{P\_{\#\_B}}{\left(P\_{\#\_B} + P\_{\#\_C}\right)}.\tag{3}
$$

At last, K4 coupling factor calculation follows the setup depicted in **Figure 6**. When the input optical power is injected on Port #D, then we should bias SOA2, and SOA1 should remain unbiased. If the selected input is port #A, then SOA1 should be biased and SOA2 switched off. If the input is either port #B or #C, then anyone of the two SOAs can be used.

**Figure 6.** Setup for the characterization of K4 coupling factor.

Power measurement is taken at ports #I and #J. Coupling factor 4 is given by,

$$\alpha\_4 = \frac{p\_{\#I}}{\left(p\_{\#I} + p\_{\#J}\right)}.\tag{4}$$

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 SOAs from each arm.


**Table 1.** Procedure for measurements and calculations.

From here, we can observe the coupling factor of all passive couplers inside the interferometric structure and compute the overall asymmetry on the power distribution along the interferometer arms.

### *2.1.2. SOA*

Input power can be injected from port #I or port #J. Coupling factor 3 is obtained through the

( ) #

<sup>=</sup> <sup>+</sup> (3)

SOA1 SOA2

<sup>=</sup> <sup>+</sup> (4)

#I

PS1 PS2

K1

#C #J

K2 K4

PM

PM
