Implementation of an ultra-fast all-optical
switch exploiting non-linear effects in fiber
Claudia Cantini
20 July, 2004
Preface
Our every day life increasingly depends on communication networks for in-formation exchange, data transfer, commerce and many other services. The growing Internet traffic demand calls for high capacity links, and optical networks remain one of the most promising ways of meeting these needs.
Today the long haul networks are based on WDM systems (Wavelength Division Multiplexing). The capacity of this system is, however, not fully exploited. Efforts are being made to transfer as many functions as possible in the optical layer in order to increase network capacity and efficiency. A WDM system, made up of 80 channels at 10 Gb/s, can transfer data at the speed of 0.8 Tb/s. Increasing the transmission rate of each channel and the number of available channels, it would be possible to transmit at more than 3 Tb/s [7]. Now the problem is how to manage this enormous data flow which electronics cannot process. Electronics is expected to increase its speed in the near future up to 80 Gb/s. Beyond this point, the only solution lies in optical data processing, reducing the need for optical-electrical conversion. Many researchers are directing their studies to this end. First, they developed hybrid networks, such as lightpaths or optical burst switching networks [13]. Then, they focused their attention on all-optical networks such as the Optical Packet Switching networks, OPS.
The OPS consists of a packet switching network which is capable of for-warding packets directly in the optical layer without resorting to optical-electrical-optical conversion. Packets, coming from different networks, are encapsulated with a short optical label, with few bits, and injected in the
ii
OPS-network. The nodes, of this network, process the label without convert-ing the packet and so increase the processconvert-ing speed. There are two different ways of processing the label: one, all-optical, the other, by converting it electrically. However, the OPS is, at the moment, in an experimental phase. Two aspects should be considered: the first, from a technological point of view, is concerned with the label encoding and its consequent elaboration, the other, from a network point of view, deals with the network forwarding protocol determination. Since today’s optical technology does not permit complex packet processing, the photonic nodes can only satisfy a few, fun-damental, functions, such as packet forwarding, switching and regeneration. The network protocol must allow the OPS network to interface with different types of networks and create the labels from the packet headers. It must also provide routing tables to each node to forward the packets in accordance with the label content.
This work shows the realization of an all-optical core node1which per-forms some of the node functions. The experiment was developed at a CNIT laboratory, a telecommunications center of excellence. In this work, we con-sider a photonic node which forwards and switches the packets. This 1×2 node receives a packet at 10 Gb/s, but it is scalable for higher bit-rates, reads the associated label, one bit of the same duration as the whole payload, and switches it to one of the two output ports in accordance with the label value. All this happens in the optical domain. Chapter 1 provides a detailed in-troduction to OPS-networks and the technologies required to realize them. The first part of Chapter 2 deals with the transmitted signal format, the label encoding and the transmission experimental setup. The second part of the chapter analyzes the receiver block of the optical node. This block must recover the label and send it to the control block. The experimental setup of the simple control block used to generate the switch control signal is also described. Chapter 3 deals with the node key component: the switch. An ultra-fast switch, based on non-linear fiber effects, is considered. The way
iii
it functions and its performance are analyzed in this chapter. The perfor-mance of the whole system and the modifications made, in order to optimize the device, are examined in Chapter 4. Finally, Chapter 5 recapitulates the findings of this research project and briefly presents the future perspectives resulting from this experiment.
Contents
1 An Overview of Optical Label Switching 1
1.1 Optical Label Switching Router Architecture . . . 3
2 Transmission and Reception 9 2.1 Transmission . . . 9
2.1.1 The Labview Program . . . 10
2.1.2 Signal Generation . . . 12
2.2 Reception . . . 13
2.2.1 Label Recovery and Switch Control Signal Generation. 14 2.3 Wavelength Conversion . . . 16
2.3.1 Nolm-Based Wavelength Conversion. . . 16
2.3.2 FWM-Based Wavelength Conversion . . . 18
3 Implementation of an All-Optical Switch 22 3.1 Principle of Operation . . . 22
3.2 Optical Kerr Effect . . . 23
3.3 Experiments on Optical Kerr Switching using HNF Fiber . . . 25
3.4 The Switching Characteristics . . . 28
3.4.1 The First Measurements . . . 29
3.4.2 The Second Set of Measurements . . . 31
3.4.3 Switch Optimization . . . 37
4 Implementation of a Photonic Node 1×2 41 4.1 Photonic Node: Preliminary Results and Q Measurements . . 42
CONTENTS v
4.1.1 Configuration with FWM-Based Wavelength Converter: Q Measurements . . . 44
4.2 Experimental Setup for Q-Factor Optimization . . . 47
5 Conclusions 49
5.1 Achievements . . . 49
5.2 Future Work . . . 50
A Cross-Phase Modulation 52
A.1 Fiber Non-Linear Effects . . . 52
A.2 Coupling between Waves of Different Frequencies . . . 53
A.3 Coupling between Orthogonally Polarized Component . . . 57
Bibliography 59
List of Figures
1.1 The next-generation Internet: an optical label switching core
network interfaces with various types of client networks.. . . . 3
1.2 A Generic Optical Node. . . 4
1.3 Label processing module . . . 5
1.4 All-optical 3R regenerator block. . . 7
2.1 LabView program: Control Panel. . . 11
2.2 Transmission setup. . . 13
2.3 Transmitted signal. . . 14
2.4 Label and payload recovery. . . 15
2.5 NOLM-based wavelength converter. . . 16
2.6 FWM-based wavelength converter. . . 18
3.1 Kerr device. . . 23
3.2 Experimental Setup. . . 26
3.3 First Measure: Extinction ratio with variable wavelength. . . . 30
3.4 First measure: Switching Ratio with variable Pcontr and δ = 50%. . . 31
3.5 First measure: Extinction Ratio with variable Pcontr and δ = 50%. . . 32
3.6 Second Measure: Switching Ratio with variable Pcontrand δ = 20%. . . 33
3.7 Second Measure: Extinction Ratio with variable Pcontr and δ = 20%. . . 34
LIST OF FIGURES vii
3.8 Second Measure: Switching Ratio with variable Pcontrand δ =
8%. . . 35
3.9 Second Measure: Extinction Ratio with variable Pcontr and δ = 8%. . . 36
3.10 Switch optimization: Extinction Ratio with variable control wavelength. . . 38
3.11 Switch optimization: Switching Ratio with variable Pcontr. . . 39
3.12 Switch Optimization: Extinction Ratio with variable Pcontr.. . 40
3.13 Optical Kerr Modulation Profile versus control power. . . 40
4.1 Experimental setup and principles of operation. . . 42
4.2 Signal at the output port “1”: Gaussian probability density of the sampled value. . . 45
4.3 Signals present at both PBS outputs. . . 46
4.4 New experimental setup. . . 47
4.5 Q-Factor Optimization: output signal. . . 48
Chapter 1
An Overview of Optical Label
Switching
Internet traffic has continued to grow exponentially since the early 1990s. The demand for high-bandwidth networking for data communication has led to the increasing use of optical internet. This explosion of optical network link capacity continues to widen the gap between the bandwidth that can be transmitted on fibers and the bandwidth that can be handled by all-electronic routers. This situation and the rapid advance of optical technologies are pro-moting the evolution of internet architecture. The next-generation internet is envisioned to involve two main functional layers: the IP layer and the optical layer. In such an IP-over-optical architecture, the optical Internet, will combine high-performance IP routers with WDM1switching and
trans-mission systems to provide a global networking infrastructure for both legacy and emerging IP services. Efforts are being made to find the most effective way to fill the gap between the electrical IP layer and the optical WDM layer. Current IP-over-WDM networking adopts wavelength-routed circuit switching in the optical transport layer, mainly based on optical crosscon-nects (OXC) and a Generalized Multi-Protocol Label Switching (GMPLS) control plane [4]. In this architecture, the minimum granularity of all-optical
1Wavelength Division Multiplexing.
2
connections is often a single wavelength. To reach a finer switching gran-ularity, an Optical-Electrical-Optical (OEO) conversion must be used. The realization of sub-wavelength granularity usually requires a heavy utilization of electronic IP routers. However, there is a growing concern about the scal-ability of electronic IP routers to match the massive transmission capacity of WDM fibers.
Recently, Optical Packet Switching (OPS), which is capable of forwarding packets directly in the optical layer without resorting to OEO conversion, has emerged as a promising technology for the long-term evolution of optical In-ternet. This approach would bypass the electronic switching bottleneck and provide a packet-based optical switching solution that matches WDM trans-mission capabilities. OPS, however, requires the development of a number of component/system technologies that are still in the experimental stage. Despite this, the OPS form that seems to be more promising, is the Optical Label Switching, OLS. The common label switching concept implies a natu-ral compatibility of OLS networking, based on a WDM optical platform, with the MPLS (Multi-Protocol Label Switching) architecture and its extension GMPLS, enabling OLS networking to benefit from the progress made in the GMPLS control plane architecture. The OLS networks utilize a short optical label attached to the data payload, which allows the optical label switching routers (OLSRs) to forward the packets thanks to its content. As long as the optical label is used, the data payload can be of any protocol, format, and length; thus, OLS-based packet switching, burst switching, and circuit switching are possible in the optical layer. The packets, coming from different networks, are encapsulated, provided with the optical label and sent through the optical network. The OLS-routers only need to look at the smaller label to make the routing decisions. Figure 1.1 shows a possible architecture for next-generation Internet, where an OLS core network interfaces with client networks through OLS edge routers. The edge router is responsible for op-tical label encoding at the boundary of the OLS core network. The client networks can be legacy networks (IP, ATM, SONET) as well as emerging 3G
1.1 Optical Label Switching Router Architecture 3
Figure 1.1: The next-generation Internet: an optical label switching core network interfaces with various types of client networks.
wireless networks and sensor networks. The OLS core network is capable of supporting multiple transport modes with full interoperability between cir-cuit, burst, and packet switching, thus providing a multiservice platform to interface with various client networks.
The next section discusses key features of the OLS technology utilized to design high-capacity OLSR architecture exploiting current optical technolo-gies.
1.1
Optical Label Switching Router
Archi-tecture
A generic optical core router architecture, illustrated in Figure 1.2, shows that interface, buffering, switching and control functions are required. The
1.1 Optical Label Switching Router Architecture 4
Figure 1.2: A Generic Optical Node.
function of each element would differ depending on whether synchronous or asynchronous switching is employed. In fact, the switching process in OPS can take one of two main forms. It can be synchronous (time slotted) with fixed packet length or asynchronous (nonslotted) with variable packet length. Both these forms are explored by researchers. Usually, the input in-terface would perform the 3R2regeneration, the packet delineation, the label
recovery and the conversion of the external wavelength (WDM transmission wavelength) of the packet to an internal one, if needed. The control unit pro-cesses the label information and issues all necessary commands to configure the switch fabric according to. In order to do so, it consults with forward-ing tables that are maintained at each node. The control unit also performs header update (or label swapping) and forwards the new label to the output interface. The new label identifies, among other things, the next node in the packet path. The switch fabric performs the switching operation of the payloads in accordance with the commands of the control unit. Finally, the output interface may forward the packet after regenerating it, converting its wavelength, if needed, and attaching the new label. The key elements of this architecture are the label processing module and the switching module.
The label processing module is shown in Figure 1.3. A small percentage
1.1 Optical Label Switching Router Architecture 5
Figure 1.3: Label processing module
of light is removed at the switch input while the remaining part is sent to the switch, undergoing a small fixed delay which accounts for the process-ing time. An optical label extractor is used to recover the label: this block strictly depends on the adopted label encoding method (e.g. serial encoding, orthogonal polarization). Then the label enters the label processor, which is composed of two blocks: an optical label recognizer, and an optical switch controller. The optical label recognizer is typically composed of optical logic devices (no more complicated than boolean ports): based on the incoming label match/mismatch different pulses are generated, corresponding to dif-ferent actions that the switch has to perform. These pulses anyway are not suitable to control an optical switch: for this they are sent to the optical switch controller, which sets its output for the whole duration of the packet, depending on the input pulse. This block is actually an optical flip-flop, which changes its state based on the optical input pulses. This all-optical label processing is an active area of research, but is still to a rudimentary stage. Then the output of this device is used to control the switch operation. The other key component of the core router is a high-speed switch fabric with nanosecond switching time. Optical switch will operate in three differ-ent ways: spatial switch only, or wavelength switch, or both of them. From
1.1 Optical Label Switching Router Architecture 6
the network point of view this feature could increase node flexibility in term of wavelength granularity and contention resolution. From the technologi-cal point of view, the most common switching techniques generally include wavelength conversion. Today, very few optical technologies are capable of switching speeds in the requested range. SOAs, electro-optic lithium niobate (LiNbO3) and non-linear fiber effect-based switches are the most promising candidates. SOAs have switching speed on the order of few nanoseconds and may be integrated on a relatively large scale. They have the advan-tage of compensating for power loss because of their inherent amplification. One of their limitations is due to the noise they add to the signal. Electro-optic LiNbO3 switches offer sub-nanosecond switching times. Only medium-scale integration is possible with LiNbO3 due to relatively high insertion loss (about 8 dB/unit), which imposes limitations on scalability of the technol-ogy. The non-linear effect based switches have response times of less than 1 ns, but they are unintegrable. These devices, however, are not affected by pattern effects and have low insertion losses [6] [10].
It can happen that two or more packets compete for the same output wavelength on the same output path at the same time. This situation is called contention. There are three main contention resolution tools: optical buffering, using fiber delay lines (FDLs), wavelength conversion, and deflec-tion routing. A combinadeflec-tion of two of these techniques, or all three, can be used to optimize system design and performance. To prevent an optical packet from being repeatedly deflected in the network or recirculated through the FDLs, the OLS network adopts a mechanism called optical-time-to-live (OTTL), which takes into account physical impairments induced by optical devices when a packet travels through its all-optical data plane. The OTTL the remaining lifetime of a packet. An optical packet will be discarded once its OTTL expires [21].
The OLS-node must also perform data 3R-regeneration and packet syn-chronization. Optical signals are susceptible to impairments caused by at-tenuation, noise, dispersion, crosstalk, jitter, and nonlinear effects. As the
1.1 Optical Label Switching Router Architecture 7
IEEE Communications Magazine • September 2002
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• Destination label identifying the egress edge node address
• Packet-type field to identify traffic nature/ priorities
• Packet sequence number to reorder data if out-of-sequence arrival of packets occurs • An operation, administration, and
mainte-nance (OAM) field
• A header error correction field
Contention can take place in the control path or data path. Contention in the control unit is of particular importance since it may lead to head-er loss or excessive delay to the extent that a payload precedes its header. In both cases, the payload has to be discarded. However, for as long as control is performed electronically, there are a number of established techniques for con-tention resolution. Proper electrical buffer sizing and efficient buffer management is important.
On the other hand, contention can happen in a data path when two distinct packets at two dif-ferent input ports are destined to the same output port at the same time. A couple of packets may also contend over an internal switch path or a network path. Three main contention resolution tools have been proposed. These are “virtual” optical buffering, using fiber delay lines (FDLs), wavelength conversion, and deflection routing. A combination of two of these techniques, or all three, can be used to optimize system design and performance. In the next section we discuss opti-cal buffering and wavelength conversion among other OPS enabling technologies. The subject of deflection routing is beyond the scope of this arti-cle. We only describe it briefly as follows. Con-tending packets are deflected to a number of alternative routes. Low-priority packets can have longer paths to their destinations compared to higher-priority packets. This approach has two main shortcomings. First, deflected packets can cause network congestion, especially at high traf-fic loads. A time-to-live field in the headers is usually proposed to prevent packets from roam-ing around for extended periods of time. Second, packets can arrive to their destinations out of sequence. Therefore, headers should carry sequence information.
S
TATUS OF
S
OME
OPS E
NABLING
T
ECHNOLOGIES
3R R
EGENERATIONOptical signals are susceptible to impairments caused by attenuation, noise, dispersion, crosstalk, jitter, and nonlinear effects. As the transmission span, number of wavelength chan-nels per fiber, and bit rate per channel increase,
transmission impairments become more serious and lead to significant amplitude loss, pulse-shape distortion, and timing drifts. It is often required to recover the original signal shape and clean it up from impairments for further trans-mission and switching in the network. As stated above, this process is known as 3R regeneration. Optical amplification boosts the amplitude of the signals but does not correct distorted pulse shape. Dispersion compensation can counterbal-ance the spread in pulse width caused by chro-matic dispersion and thereby reduce the pulse-shaping problem. Retiming is accom-plished by clock extraction and synchronization, packet-level synchronization for synchronous networks (bit-level synchronization for asyn-chronous networks). To retime the signal, data rate and format have to be known to the regen-erator and the latter must be capable of bit rate flexible operation.
The most widely used approach for 3R regen-eration involves optical-to-electrical (O/E) con-version of the data. In this case, regeneration can be carried out electrically. All-optical 3R regener-ation is regarded as an important technology to enable, and simplify, OPS implementation. Some regeneration operations, such as retiming, cannot easily be carried out all optically today. Attempts to realize all-optical 3R regeneration are there-fore limited to experimentation. Figure 3 is a sim-plified functional block diagram of an all-optical 3R regenerator. It is composed of three main blocks: an amplifier, a clock recovery system, and a threshold detection unit. The clock is extracted from the amplified signal. It is then combined with the signal to produce time-realigned pulses. The regenerated signal appears at the output of the threshold detector.
Most of optical 3R regeneration efforts are based on semiconductor optical amplifier (SOA) based Mach-Zehnder interferometers (MZIs). They offer high speed and low switching energy, and their fabrication techniques are mature enough to obtain almost polarization-insensitive operation. Another method uses the concept of soliton transmission combined with in-line syn-chronous intensity/phase modulation and optical narrowband filtering. Due to the separation of reshaping and retiming, this method has the potential for simultaneous regeneration of several WDM channels [5]. The clock recovery system must be capable of very fast locking to incoming optical signals and production of flexible repeti-tion rates. These requirements were satisfied in laboratory using self-pulsating distributed feed-back (DFB) lasers as optical oscillators. By adjust-ing the direct current (DC) applied to the laser, clock recovery was demonstrated with continuous tuning range from 6 to 46 GHz (leading to possi-bility of operation at OC-192 and OC-768 rates) and 1-ns locking time [6]. Despite these valuable demonstrations, all-optical 3R regeneration is still in the experimental phase and is not available today for commercial deployment.
P
ACKETD
ELINEATION ANDS
YNCHRONIZATIONPackets arrive at an OPS node from different origins, through various fiber paths and over dif-ferent wavelengths. It is natural that they experi-ence various propagation delays due to ■Figure 3. A functional block diagram of an all-optical 3R regenerator.
Clock recovery
Threshold AMP
Figure 1.4: All-optical 3R regenerator block.
transmission span, number of wavelength channels per fiber, and bit rate per channel increase, transmission impairments become more serious and lead to significant amplitude loss, pulse-shape distortion, and timing drifts. It is often required to recover the original signal shape and clean it up from impairments for further transmission and switching in the network. This process is known as 3R regeneration. Optical amplification boosts the am-plitude of the signals but does not correct distorted pulse shape. Dispersion compensation can counterbalance the spread in pulse width caused by chro-matic dispersion and thereby reduce the pulse-shaping problem. Re-timing is accomplished by clock extraction and synchronization. To re-time the signal, data rate and format have to be known to the regenerator and the latter must be capable of bit rate flexible operation. The most widely used approach for 3R regeneration involves optical-to-electrical (OE) conversion of the data. In this case, regeneration can be carried out electrically. All-optical 3R regeneration is regarded as an important technology to enable, and simplify, OPS implementation. Some regeneration operations, such as re-timing, cannot easily be carried out all optically today. Figure 1.4 is a simplified functional block diagram of an all-optical 3R regenerator. It is composed of three main blocks: an amplifier, a clock recovery system, and a threshold detection unit. The clock is extracted from the amplified signal. It is then combined with the signal to produce time-realigned pulses. The regenerated signal appears at the output of the threshold detector. Most of optical 3R regeneration efforts are based on semiconductor optical
am-1.1 Optical Label Switching Router Architecture 8
plifier (SOA) based Mach-Zehnder interferometers (MZIs). They offer high speed and low switching energy, and their fabrication techniques are mature enough to obtain almost polarization-insensitive operation. Another method uses the concept of soliton transmission combined with in-line synchronous intensity/phase modulation and optical narrow-band filtering. Due to the separation of reshaping and re-timing, this method has the potential for si-multaneous regeneration of several WDM channels [11].
In this work, we consider only the switching function and a simple label processing block. According to us, it is important to concentrate the research on the implementation of the building block of the OPS node to make this technology available in the near future.
Chapter 2
Transmission and Reception
2.1
Transmission
For the network to be able to carry information, a routing mechanism is needed. A generic IP packet is mainly composed by a header, which contains routing information, and a payload, which contains the transmitted data. All-optical processing is still limited to few bits, thus at the present it is not possible to directly process the IP header. The most widespread solution is to encapsulate the IP packet into an optical packet and labelling it with a short optical header called label. This label is generally short (few bits), and is modulated at a low bit-rate. This particular approach makes the label easy to be processed. In this way, the optical network is truly transparent to the packets and it may accept data at any bit-rate and with any protocol format [14]. It is the edge-routers task to build up the packet and to translate the routing information into the optical labels, as we have seen in Chapter1. Transparency also allows the scaling of the bit-rate.
Now the question is how to encode the label. Even though no optimal coding for the label has already been found, some alternative solutions have been proposed, [6] [19] [5]. An important constraint is that the label and the payload should be easily separated optically. Many researchers consider the
2.1 Transmission 10
Sub Carrier Multiplexing (SCM) the best label format for a WDM system1.
In SCM, the label is superimposed to the payload at a different frequency. A sub-carried label has many advantages, such as ease of transmission and recovery, but potentially suffers from dispersion induced fading [6]. Other researchers propose in-band label format such as a serial label, or a Phase Shift Key (PSK) label, or a Frequency Shift Key (FSK) label for the payload carrier modulation. The serial label, however, presents many timing prob-lems, while the PSK, or FSK, label is subject to dispersion [19]. There is another solution commonly adopted by researchers, namely, to transmit a label orthogonally polarized with respect to the payload [5]. This approach defeats chromatic dispersion, even if it generally suffers from polarization mode dispersion (PMD).
In our experiments we chose the last transmission format. We transmit-ted an intensity-modulatransmit-ted payload at 10 Gb/s and an intensity-modulatransmit-ted orthogonally polarized label at the same wavelength. This label consisted of one bit of the same time duration of the whole payload. To realize this transmission, we first serially generated the label and the payload, then we superimposed one signal on the other with orthogonal polarization.
A Labview2program was developed in order to generate the necessary bit
string. Its is shown in the next section. The transmitter setup is described in the third section while the last section deals with the wavelength converters utilized in the experiments.
2.1.1
The Labview Program
LabView is a powerful programming language which not only allows one to “write” a routine, but also to communicate, by the transmission standards
1Wavelength Division Multiplexing, which means more channels coupled onto the same
fiber with different wavelengths.
2.1 Transmission 11
Figure 2.1: LabView program: Control Panel.
GPIB3or RS 2324, with the laboratory instruments. We developed a pro-gram which enabled us to generate the bit-string and to “send” that to the Pulse Pattern Generator (PPG) by GPIB, thus increasing the system flexi-bility. The PPG we used, is manufactured by ANRITZU and permits to set sequences of 8388608 bits with a maximum bit-rate of 12.5 GHz.
In our experiment, we transmitted a payload made up of 640 bits at 10 GHz, but being an all-optical system, it is scalable for high bit-rates. Payload length, and consequently label length, were limited by the need of keeping the duty cycle of the signal low in order to obtain the high peak powers nec-essary for the non-linear Kerr effect, as will be seen below. The developed
3General Purpose Interface Bus, which is a bus system especially designed for
connect-ing computers and instruments (Standard IEEE488.1).
4Short for Recommended Standard-232C, a standard interface approved by the
2.1 Transmission 12
program generated a sequence made up of a first block of space level bits, the pre-guard, the payload, a second guard, the label and a post-guard. To give more flexibility to the system, the payload can be generated by Labview or red from an external file. The payload is produced, by Labview, by using a random number generator. This generator uses a random generator, already present in the program of the National Instruments, comparing the output with a 0.5 threshold. If the number is bigger than 0.5, the bit is set at one, otherwise at zero.
The different blocks of the sequence, can be varied from a minimum value of 128 bits length to the maximum sequence recorded by the PPG. The mini-mum value, 128 bits, is given by the memorization format of the PPG. As we can see from the control panel of the program, Figure2.1, the only inputs are: the length of the different blocks, expressed as a multiple of 128, the label value and the GPIB address of the PPG. If the string is red from an external file, the program opens a dialog window and accepts a generic text file with the bits expressed in hexadecimal form. After set the sequence, the bit-rate and the output voltage value, the PPG was connected to a Mach-Zender modulator5.
2.1.2
Signal Generation
The Mach-Zender modulator received, as an input, the RF-data from the PPG and a continuous wave from a laser. We used a DFB-Laser (Distributed Feedback laser) at 1557 nm wavelength. Both the label and payload were serially modulated on the same optical carrier. This modulated signal had to be split on two arms with orthogonal polarization and then coupled again with the label and payload superimposed, but with orthogonal polarization. Figure 2.2 shows the transmission setup. The modulated signal was first
5Electro-optic modulator, the Mach-Zender is an interferometric device that makes use
of two interfering paths. Since the refractive index of an electro-optic materials can be changed by an applied electric field. By changing the refractive index of one path, they can interfere in constructive or destructive way according to the electric field applied [8].
2.2 Reception 13
Figure 2.2: Transmission setup.
amplified, then sent to a polarizer which transmits just the linear component of the signal. Afterward, the signal was split on the two arms of a polarization beam splitter. One arm, the one relative to the label, was delayed by using 30 m of Polarization Maintaining fiber (PM-fiber). In this way, the two signals were superimposed. At the output, we used a polarization beam combiner (PBC) which couples together the signals at the input arms, with orthogonal polarization. The PBC is more efficient if the input signals are linearly polarized in the direction of its axis. The output signal presented the superimposed label and payload and two irrelevant copies on the sides, as the oscilloscope image shows (Figure 2.3).
2.2
Reception
To receive and switch the signal, the label and the payload had to be sepa-rated and the switch control signal had to be genesepa-rated in accordance with the label information. The payload and the label can be separated using a po-larization beam splitter. To switch the payload, we used an all-optical switch based on the non-linear optical Kerr effect, which permitted the switching of the packet thanks to a high power control signal with the appropriate wave-length and state of polarization, as will be explained in greater detail in the following chapter.
2.2 Reception 14
Figure 2.3: Transmitted signal.
2.2.1
Label Recovery and Switch Control Signal
Gen-eration
After the transmission, the signal is sent to a polarization beam splitter, to separate the data from the label. By adjusting the input state of polarization (SOP) of the received signal, the label and the payload are sent to the two output path of the PBS, as Figure 2.4 shows.
Every switch needs a control signal to switch the information accordingly. The switch control signal has to be of the same duration of the payload and at different wavelength. The label has the same duration of the payload and its value gives the information to switch the payload, our experiment consid-ers a 1×2 switch. Converting label wavelength to an appropriate value, we obtain the switching control signal. If the label is zero, we have no control signal and the payload reaches the output port “0”. On the contrary, if the label has high-value, there is a switching control signal and the payload is switched on the other output port, port number “1”. Our experiment utilizes a switch based on non-linear effects inside the fiber, these effects are effective only when the control signal is high, otherwise the payload is transmitted
2.2 Reception 15
Figure 2.4: Label and payload recovery.
unmodified. The switch functioning is described in the next chapter. As explained above, the label wavelength is converted in order to obtain the switch control signal. First, we used a non-linear loop mirror (NOLM), as a wavelength convert, [9] [15]. The converted signal, however, was not really stable. So, the wavelength converter was changed with a four-wave mixing based converter. Next section analyzes the wavelength converters utilized in greater detail. After conversion, a switching control signal, with the same label value, is obtained. The control signal wavelength is of 1555.75 nm, this value is imposed by the selective optical filtering requested after the conver-sion. The filtering was made up by a WDM multiplexer, that is a narrow band filter, but its wavelengths are fixed. Meanwhile the payload is delayed by a tunable delay line which maintained its state of polarization, SOP. The payload and the control signal must enter the switch at the same time. The payload had a linear state of polarization while the state of polarization of the control signal was controlled by a polarization controller. The SOP of the switch control signal must have a 45◦ angle with respect to the payload SOP in order to maximize the optical Kerr effect, as will be seen in Chapter 3. Now we have both the payload and the switch control signal, we have only to switch the packet.
2.3 Wavelength Conversion 16
Figure 2.5: NOLM-based wavelength converter.
2.3
Wavelength Conversion
2.3.1
Nolm-Based Wavelength Conversion
The NOLM is an interferometric structure made up by fiber loop and a 2×2 coupler. As can be seen in Figure2.5, the optical input signal is split into two counter-propagating fields, which recombine at the coupler. The optical path length is precisely the same for both propagating fields, since they follow the same path but in opposite directions. In this way the two fields are completely reflected onto the input port6. If a pump is injected in the loop too, it induces
a differences between the phases of the two counter-propagating fields. This phase difference is caused by the cross-phase modulation (XPM) which is a direct consequence of the intensity dependence of the fiber refractive index, see Appendix A.
6The 3 dB coupler give a π/2 phase shift to the signal which cross from one arm to
the other of the coupler. The counter-propagating signal crosses twice the coupler. In this way, it presents a π phase shift with respect to the coo-propagating field.
2.3 Wavelength Conversion 17
In a NOLM-based wavelength converter, a Continuous Wave (CW) is gated by a signal pulse, the pump, through XPM in the fiber loop. If the signal power is Psand the CW power is Pc, the power of the converted signal
is given as:
Po =
1
2Pc[1 − cos 2γPsLl] (2.1)
Where Lldenotes the loop length, and γ the fiber non-linear coefficient. The
condition for the NOLM, that maximize the output power, is Ll =
π
2Lnl (2.2)
where the nonlinearity length is given as: Lnl=
1 γPs
(2.3) We consider a NOLM made up by 2 Km of Dispersion Shifted fiber (DS-fiber). Let the signal, the pump, wavelength λs, the CW wavelength λc and
the zero-dispersion wavelength λ0. In the wavelength allocation suitable for
ultra-fast wavelength conversion, the three wavelengths must satisfy
∆λ ≡ λs− λ0 = λ0− λc (2.4)
In such a case, the amount of wavelength translation is given as 2∆λ. Under this condition, group velocities at λs and λc are the same and the walk-off is
minimized [15].
At NOLM output, an optical bandpass filter extracts λc. The loop
transmit-tance T at λc is given by:
T = sin2 ∆φ 2
(2.5) and depends on the net phase shift difference ∆φ between the counter-propagating signals inside the loop. This phase shift has the form:
∆φ(t) = 4π n2 λsAef f
Z Ll
0
Ps(t − W × z)dz (2.6)
where n2 is the non linear Kerr coefficient (see Chapter 3), P (t) is the time
2.3 Wavelength Conversion 18
Figure 2.6: FWM-based wavelength converter.
walk-off parameter. In Equation (2.6), the polarization of the two signal are assumed oriented parallel.
By controlling the pump peak power, the NOLM can be turned ON (∆φ = π) or OFF (∆φ = π) corresponding to bit 1 and bit 0, respectively, thereby effectively transferring coded information from λs to λc. This behavior gives
the NOLM the wavelength conversion property. For a specific case of zero walk-off (W = 0) the signal peak power corresponding to a π phase shift is given by [9] Pπ = λsAef f 4n2 1 Ll (2.7) In our experiment, first we used this device to convert the wavelength, but the output signal was not stable. So we substituted the NOLM-based wave-length converter with a FWM-based one. The following section describes the latter in detail.
2.3.2
FWM-Based Wavelength Conversion
Four-Wave Mixing, FWM, is a non-linear effect observable in an optical fiber when two or more fields propagate simultaneously. These fields interact each
2.3 Wavelength Conversion 19
with the other and generate contributions at new frequencies. These con-tributions respect the principle of conservation of the energy. FWM occurs when two photons are annihilated and new photons are created at different frequencies such that the net energy and momentum are conserved during the interaction. The relationship between the frequencies is ωi+ωj = ωk+ωf,
where ωi, ωj 6= ωk, ωf, otherwise phenomena of SPM and XPM are generated.
As the case of cross-phase modulation, see Chapter A, the starting point to calculate the FWM field, is the wave Equation (A.6) for the total electric field E(r, t) with PN L given in Equation (A.8). Consider four optical waves
oscillating at frequencies ω1, ω2, ω3 and ω4 and linearly polarized along the
same axis x. The total electric field can be written as: E = 1 2xˆ 4 X j=1 Ejexp [i(kjz − ωjt)] + c.c. (2.8)
Expressing PN L in the same form of the electric field, we obtain:
PN L = 1 2xˆ 4 X j=1 Pjexp [i(kjz − ωjt)] + c.c. (2.9)
Where the propagation constant kj = njωj/c, nj is the refractive index, and
all four waves are assumed to be propagating in the same direction.
We substitute Equation (2.8) and Equation (2.9) in Equation (A.6), together with a similar expression for the linear part of polarization, and neglect the time dependence of the field components Ej (j = 1 to 4) assuming
quasi-CW conditions. Their spatial dependence is, however, included using Equa-tion (A.20). Evolution of the amplitude Aj(z), inside a single mode fiber,
can be calculated assuming the pump waves to be much intense compared with the new-waves and to remain undepleted during the interaction. The amplitudes of the pump fields are given by equations:
A1(z) = p P1exp [iγ(P1+ 2P2)z] (2.10) A2(z) = p P2exp [iγ(P2+ 2P1)z] (2.11)
2.3 Wavelength Conversion 20
Where Pj = |Aj(0)|2, P1 and P2 are the incident pump powers at z = 0, and
γ is given by Equation (A.23). These equations show that, in undepleted-pump approximation, the undepleted-pump waves only acquire a phase shift occurring as a result of SPM and XPM.
The two linear coupled equation, for the idler and signal fields, are: dA3 dz = 2iγ[(P1+ P2)A3+ p P1P2e−iθA∗4] (2.12) dA∗4 dz = 2iγ[(P1+ P2)A ∗ 4+ p P1P2eiθA3] (2.13) Where θ = [∆k − 3γ(P1+ P2)]z (2.14)
To solve this equation, we introduce
Bj = A − j exp [−2iγ(P1+ P2)z] (j = 3, 4) (2.15)
Using Equation (2.12) and (2.13), we obtain: dB3 dz = 2iγ p P1P2exp (−iκz)B4∗ (2.16) dB4∗ dz = 2iγ p P1P2exp (iκz)B3 (2.17)
Where the net phase mismatch is given by
κ = ∆k + γ(P1+ P2) (2.18)
Equation (2.16) and (2.17) govern growth of the signal and idler waves oc-curring as a result of FWM. Their general solution is of the form:
B3 = (a3egz + b3egz) exp (−iκz/2) (2.19)
B4∗ = (a4egz + b4egz) exp (−iκz/2) (2.20)
Where a3, b3, a4 and b4 are determined from the boundary conditions. The
parametric gain g depends on the pump power and is defined as
2.3 Wavelength Conversion 21
Where r = 2(P1P2)1/2/P0 and P0 = P1+ P2.
The solutions given by Equation (2.19) and (2.20) are valid only when the conversion efficiency of the FWM process is relatively small so that the pump waves remain largely undepleted.
If a weak signal at ωsis launched into the fiber together with a pump at ωp,
a new wave is generated at the frequency 2ωp− ωs7. If a CW pump is injected
together with an amplitude-modulated signal, the idler wave is generated through FWM only when the pump and the signal are present simultaneously. As a result, FWM transfers the signal data to the new waves with perfect fidelity. It can even improve the signal quality reducing intensity noise [3]. Figure 2.6 shows the scheme of the wavelength converter used. To obtain the converted signal, a tunable laser is used. Its frequency was optimized at 1558.25 nm, in order to have a maximum first-order FWM contribution. The continuous wave from the tunable laser and the recovered label are first amplified, then injected in 10 Km of DS-fiber. The wavelengths are chosen near the zero-dispersion wavelength to increase the FWM efficiency. At the end of the fiber there is a selective optical filter which allows only the new frequency to pass. Thus a switch control signal, at 1555.75 nm, with the same label value, is obtained.
7The partially degenerate FWM was originally called three-wave mixing as only three
Chapter 3
Implementation of an
All-Optical Switch
3.1
Principle of Operation
The basic idea, in order to realize an all-optical switch, is to utilize the optical Kerr effect, caused by the non-linear effect of Cross Phase modulation, XPM. This effect allows a controlled rotation of the signal state of polarization. If the two different States Of Polarization, SOP, the signal SOP and the rotated one, are orthogonal, they are simply discriminated by a polarization beam splitter. To obtain this polarization rotation we use an highly non-linear fiber, HNF, in order to have a high non-linear coefficient, and a control signal with appropriate wavelength and state of polarization, to maximize the XPM effect [12]. The two optical signals, the real signal and the control signal, are injected in the non-linear fiber with a 45◦ angle between their directions of polarization. By inserting a Polarization Beam Splitter (PBS) the signal is switched between the two PBS ports. Without the control signal, all the information may be obtained from the output port number 0, line port. The presence of the control signal induces a polarization rotation in the SOP of the signal. A change in the non-linear refractive index, as a function of intensity, induces this rotation. An appropriate control signal intensity induces a π
3.2 Optical Kerr Effect 23
Figure 3.1: Kerr device.
rotation in the signal SOP. At this point all the information is available from the output port number 1 of the PBS. In this way we obtain an all-optical switch in which both the control signal and the the information signal are at optical frequencies [20] [18] [17]. This device is intrinsically fast. In the second section the optical Kerr effect is analyzed in greater detail, while in the last section the device is characterized.
3.2
Optical Kerr Effect
In the optical Kerr effect, the nonlinear phase shift induced by an intense, high-power, control beam is used to change the transmission of a weak infor-mation signal through a nonlinear medium. This effect can be used to make different devices with picosecond response times.
The operating principle of a Kerr device can be understood from Fig-ure 3.1. The signal and control beams are linearly polarized at the fiber input with a 45◦ angle between their directions of polarization. When the control signal is turned on, the refractive indices for the parallel and per-pendicular components of the signal (with respect to the direction of control polarization) become slightly different because of the control induced bire-fringence. The phase difference between the two component at fiber output manifests as a change in the signal polarization. The rotation angle depends
3.2 Optical Kerr Effect 24
on the control intensity and can be controlled simply by changing it. If the control and the signal have different wavelengths, this effect can be used to realize an optical modulator or an optical switch.
To calculate the phase difference between the x and y components of the signal we must consider both the unequal control and signal states of polarization, both the unequal wavelengths. At the moment we neglect fiber losses, we can include them later by replacing L with Lef f. The relative
phase difference, between the signal components, at the output of a fiber of length L can always be written as:
∆φ = (2π/λ)(˜nx− ˜ny)L (3.1)
Where λ is the signal wavelength and ˜
nx = nx+ ∆nx, n˜y = ny+ ∆ny (3.2)
The linear parts nx and ny of the refractive indices are different because of
modal birefringence. The nonlinear parts ∆nxand ∆ny are different because
the control induced birefringence.
Consider the case of a control signal polarized linearly along the x axis. The x component of the signal is polarized parallel to the control but its wave-length is different. The corresponding index change ∆nx must be obtained
by using Equation (A.18). If the self phase modulation, SPM, contribution is neglected:
∆nx = 2n2|E|2 (3.3)
Where |E|2 is the control intensity.
When the signal and the control are orthogonally polarized, only the first term in Equation (A.29) contributes to ∆ny because of different wavelengths
of the signal and the control beams. Neglecting the SPM again: ∆ny = 2n2b|E|2, b = χ(3)xxyy/χ
(3)
xxxx (3.4)
If the origin of χ(3) is purely electronic, b = 1
3. Combining Equations (3.3)
and (3.4), the phase difference becomes:
3.3 Experiments on Optical Kerr Switching using HNF Fiber 25
Where ∆nL = nx− ny accounts for linear birefringence, and the Kerr
coeffi-cient n2B is given by:
n2B = 2n2(1 − b) (3.6)
The constant phase shift ∆φLresulting from linear birefringence can be
com-pensate by using a quarter-wave plate, if it needs. Under ideal condition, the response time of a Kerr device would be limited only by the response time of Kerr nonlinearity (< 10 fs for optical fibers). In practice, however, fiber dispersion limits the response time to values in the range of 1 ps e 1 ns, depending on the operating parameters. A major limiting factor is the group velocity mismatch between the signal and the control. The fundamental limit on the response time is then set by GVD, group velocity dispersion, that broadens the control pulse during its propagation inside the fiber. The fiber we used, has a flat dispersion profile and a low dispersion value for a large range of wavelength.
To obtain a signal state of polarization orthogonal to the input one, the phase difference between the two component of the signal must be π. The control intensity required for this phase shift can be estimated by setting ∆φ = π and ∆φL= 0 in Equation (3.5). It results:
PC = |E|2Aef f = λAef f/(2n2BL) (3.7)
The parameters of the Highly Nonlinear Fiber (HNF) we used are summa-rized in Table 3.1. With these parameters, the control power to obtain a π phase shift is PC = 27 dBm [3] [16].
3.3
Experiments on Optical Kerr Switching
using HNF Fiber
To study the feasibility of this device, we used a continuous wave, as a signal, thereby producing a modulator. We analyzed the modulator behavior as a function of control wavelength and intensity. Figure 3.2 shows the experi-mental setup we devised to make this first characterization. We should now
3.3 Experiments on Optical Kerr Switching using HNF Fiber 26
3.3 Experiments on Optical Kerr Switching using HNF Fiber 27 Cladding diameter 125 nm coating diameter 200 nm Attenuation 0.74 dB/Km Dispersion at 1550 nm 0.07 ps/(nm Km) Dispersion slope at 1550 nm 0.018 ps/(nm2Km)
Zero dispersion wavelength 1546 nm
Dispersion slope at ZDW nm 0.018 ps/(nm2Km)
PMD 0.071 ps/(√Km)
Mode field field diameter at 1550 nm
Inner end 3.91 µm
Outer end 3.91 µm
Cut-off wavelength
Inner end 1180 nm
Outer end 1180 nm
Table 3.1: HNF fiber data sheet.
consider the scheme in greater detail. We used two tunable EC-LASERs1to
optimize the wavelengths of the signals. While the signal, i.e. the continuous wave, is amplified by an Erbium Doped Fiber Amplifier (EDFA), the control signal is first modulated by an acoustic-optic modulator driven by a square wave at 1 MHz. After this, the control signal is amplified by an EDFA in order to give it the required power to allow the π phase shift, as described above. The two signal are sent to two polarization controllers, PC 1 and PC 2, to determine their state. After coupling by a 50 : 50 coupler, both the signal and control are injected into 0.4 Km of HNF fiber. The HNF parame-ters are summarized in Table 3.1. At the end of the fiber there is a selective optical filter, consisting of a cascade of three filters, OBPF in Figure3.2. Fi-nally, a polarization controller, PC 3, orientates the signal polarization along the polarization beam splitter axis. Typically it was adjusted for maximum
3.4 The Switching Characteristics 28
power at port “0” (line port) in the absence of control pulse. The PBS dis-tinguishes the two possible states of polarization of the signal and realizes the real spatial switch. For the characterization measures, the outputs are sent to an oscilloscope by two photo-diodes.
The signal and the control should be polarized linearly and with a 45◦ an-gle between their polarization directions, at the fiber input. However, there is no well-defined axis in which the signals may be coupled in order to maintain the linear polarization. So the input states of polarization were varied until a maximum switching ratio was found. However, we defined a procedure which did not assure the above-mentioned 45◦ linear polarizations, but it led to satisfactory results verified under different operational conditions. We first sent the information signal into the device and we set the polarization controller 3, PC 3, so that the whole signal went out from the output number 0. Then we set PC 2, sending only the control signal, so that we had two equal signals in the two output ports, which means a control polarization of 45◦ with respect to the PBS axis. Finally, sending both the signals, we set PC 1 to have the best extinction ratio for the output signal. We repeated this procedure before each new measurement.
3.4
The Switching Characteristics
The testing of the switch was performed under different operational condi-tions. For each condition, we measured the extinction ratio, ER, and the normalized Switching Ratio, SR.
The extinction ratio is defined as:
ER = 10 · log PH PL
Where PH and PL are, respectively, the high power and the low power
re-ceived from the photo-detector.
The switching operation, the signal being a continuous wave, is an on-off modulation for the signal. So the high and low power received are the power
3.4 The Switching Characteristics 29
received after a “one” transmission and after a “zero” transmission. We cal-culate the ER with this meaning. Of course the meaning of low and high power are exchanged between the two channels.
The Switching Ratio is defined as:
SR = |P1− P0| PD
Where P1 e P0 are the power received by the photo-detector after the
trans-mission of a “1” and a “0” respectively. PD is the signal power received from
the output port 0 when the transmitted label is “0”, i.e. control signal equal to zero. This signal value is measured in dynamic condition with a control signal power equal to the power calculated2.
In the following sections, we show the experimental results. We undertook different series of measurements by changing the operating conditions to obtain the best performance.
3.4.1
The First Measurements
The operating conditions for these first results are: λsign = 15548.882 nm
δ = 50% Where δ is the square-wave duty-cycle.
First of all, we varied the control wavelength to obtain the optimal value for our experiment. We measured the ER of the two outputs by varying the control wavelength and keeping the control power to the theoretical value of 24 dBm3. Figure 3.3 shows that the XPM phenomenon is more efficient, for both channels, when the control wavelength is λcontr = 1546 nm.
After that, we set the control signal wavelength to the optimal value and varied the control signal power. Operating conditions for these
measure-2The control signal power needed to obtain a π phase shift, calculated in the previous
section, is equal to 27 dBm of peak power.
3.4 The Switching Characteristics 30 0 0,5 1 1,5 2 2,5 3 3,5 1540 1542 1544 1546 1548 Wavelength nm Extinction Ratio dB ER 1 ER 2
3.4 The Switching Characteristics 31 0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 15 17 19 21 23 25 27 Control Power dBm Switching Ratio SR 1 SR 2
Figure 3.4: First measure: Switching Ratio with variable Pcontrand δ = 50%.
ments4:
λcontr = 1546 nm
Pwsign = 10 dBm
Graphs3.4and3.5 described the ER and the SR as a function of the control signal power. They show an increasing trend with increasing power, therefor we reduced the control duty cycle in order to obtain higher peak power for the control signal.
3.4.2
The Second Set of Measurements
As mentioned above, we reduced the control duty cycle from 50% to 20%, then 8%. In this way, we increased the peak power, the useful power for cross phase modulation.
3.4 The Switching Characteristics 32 0 0,5 1 1,5 2 2,5 3 3,5 15 17 19 21 23 25 27 Control Power dBm Extinction Ratio dB ER 1 ER 2
3.4 The Switching Characteristics 33 0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 21 22 23 24 25 26 27 28 Control Power dBm Switching Ratio SR 1 SR 2
Figure 3.6: Second Measure: Switching Ratio with variable Pcontr and δ =
20%.
Operating conditions for these measurements: λsign = 15548.882 nm
λcontr = 15546 nm
δ = 20% Pwsign = 6 dBm
The average power of the information signal was reduced to avoid self phase modulation and cross phase modulation of this signal on the control signal.
Both the parameters5show quite low values, as can be seen in Figures 3.6
and 3.7, so we increased the peak power again. Now, in Figures 3.8 and
3.9, the control duty cycle is δ = 8%. It is evident, in these figures, that
5P
Dis measured with an average power of the control signal of 24 dBm yet, to consider
3.4 The Switching Characteristics 34 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Control Power dBm Extinction Ratio dB ER 1dB ER 2dB
Figure 3.7: Second Measure: Extinction Ratio with variable Pcontr and δ =
3.4 The Switching Characteristics 35 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 15 20 25 30 Control Power dBm Switching Ratio SR 1 SR 2
Figure 3.8: Second Measure: Switching Ratio with variable Pcontr and δ =
3.4 The Switching Characteristics 36 0 2 4 6 8 10 12 14 16 15 20 25 30 Control Power dBm Extinction Ratio dB ER 1 ER 2
Figure 3.9: Second Measure: Extinction Ratio with variable Pcontr and δ =
3.4 The Switching Characteristics 37
the trend of the ER and the SR is function of the control power. Increasing the latter, the polarization rotation increases until it becomes more than π. So we have an elliptical polarization that is split on both the axis of the polarization beam splitter reducing the switching capacity.
3.4.3
Switch Optimization
The two channels behave differently, as can be seen in the above measure-ments. The channel with the rotated state of polarization is more efficient than the other. Observing the past-filters signal spectrum, we noticed a high ASE6noise due to the EDFA amplifier. To reduce the presence of noise we
introduced another narrow-band filter and we changed the signal wavelength to λsign = 1555 nm in order to work within the amplifier band.
First of all, we repeated the measurement of the extinction ratio by vary-ing the control wavelength. The wavelength which optimizes the Ratio is λcontr = 1550 nm, as can be seen in Figure 3.10. Repeating the
measure-ments of the switching parameters by varying the control power, we obtained the same results as in the previous experiments with less disparity between the two channels. The operating conditions are summarized below and Fig-ures3.11 and3.12show the trend of the extinction ratio and of the switching ratio for the less efficient channel:
λsign = 1555 nm
λcontr = 1550 nm
δ = 8% Pwsign = 6 dBm
By increasing the control power, the induced phase shift increases. Fig-ure 3.13 shows the modulation profile as the control power was varied. The peak phase shift changes from π of Figure 3.13(a) to 2π of Figure 3.13(d). Notice the presence of two peaks for the 2π rotation.
3.4 The Switching Characteristics 38 0 2 4 6 8 10 12 14 16 1544 1546 1548 1550 1552 1554 Wavelength nm Extinction Ratio dB ER1 ER2
Figure 3.10: Switch optimization: Extinction Ratio with variable control wavelength.
3.4 The Switching Characteristics 39 0 0.1 0.2 0.3 0.4 0.5 0.6 18 20 22 24 26 28 Control Power dBm Switching Ratio Switching Ratio
3.4 The Switching Characteristics 40 0 0.5 1 1.5 2 2.5 3 3.5 4 18 20 22 24 26 28 Control Power dBm Extinction Ratio dB ER
Figure 3.12: Switch Optimization: Extinction Ratio with variable Pcontr.
0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 Time 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 Time 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 Time 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 Time
Chapter 4
Implementation of a Photonic
Node 1×2
In the preceding chapters, we described the type of transmission used, the control switch signal generation and the switch performances. Now we will consider the whole system and its performance.
The building of the system presented many synchronization problems. The label and the payload, as seen before, are generated serially and then they are superimposed with orthogonal polarization. After transmission, they are separated, by a polarization beam splitter, and the label wavelength is converted. The payload and the control signal must enter the switch simul-taneously. So the payload must be delayed while the label is converted. A tunable delay-line aligns the payload and the control signal. To facilitate this alignment, we acted on the guard time of the modulating string. By doing that, the required synchronization was achieved. The following section deals with the results obtained, while the last section presents a new experimental setup, which was realized in order to improve the quality-factor, Q, of the device.
4.1 Photonic Node: Preliminary Results and Q Measurements 42
Figure 4.1: Experimental setup and principles of operation.
4.1
Photonic Node: Preliminary Results and
Q Measurements
The transmitted signal, the label and the payload modulated on the same optical carrier with orthogonal states of polarization (SOP) was received by a polarization beam splitter which separated the label and the payload by sending them to different arms, as seen in Chapter 2. The arm with the payload was delayed, while the arm with the label was sent to a wavelength converter. Figure 4.1 summarizes the experimental setup after the signal reception. Figure 4.1(a) and Figure 4.1(b) show what happens when the label is, respectively, at marker and space levels. The wavelength converter is the critical component of this setup. Since the control signal generated must be amplified to 25–26 dBm, the conversion must introduce as few noise
4.1 Photonic Node: Preliminary Results and Q Measurements 43
as possible. To convert the label, we first used a NOLM, then an FWM-based converter. After the conversion, the control signal was injected with the payload in the HNF fiber. Two polarization controllers set the SOPs of the two signals in order to maximize the optical Kerr effect. At the output of the HNF fiber, after an optical filtering which allows only the signal to pass, a PBS discriminates the two possible states of polarization of the signal, see Chapter 3. To evaluate the effectiveness of the device, we measured the Q-factor of the output port of the rotated polarization signal. The absence of control signal does not influence the transmission of the payload except for an increase in the noise due to the EDFA-amplifier.
The sample value I fluctuates from bit to bit around the average value I1
or I0, depending on whether the bit corresponds to 1 or 0 in the bit stream.
The sampled value has a probability density function p(I) which depends on the statistics of noise sources responsible for current fluctuation. Both thermal and shot noise are approximately Gaussian for a p-i-n receiver. The resulting statistic is Gaussian with variance σ2 = σshot2 + σ2thermal. However both the average and the variance are different for 1 and 0 bits. The Q factor is defined as: Q = I1− ID σ1 = ID− I0 σ0 (4.1) Where Ii is the average value (i = 1, 0) and ID is the decision threshold. In
practice, ID is optimized to minimize the BER1. The minimum occur when
ID satisfy Equation(4.1). An explicit expression for ID is
ID =
σ0I1+ σ1I0
σ1+ σ0
(4.2) By using Equations(4.1) and (4.2), Q is obtained as:
Q = I1 − I0 σ1+ σ0
(4.3) The BER decreases as Q increase, so Q is a measure of the error probabil-ity [1].
1Bit Error Rate: error probability defined as BER = p(1)P (0/1) + p(0)P (1/0), where
p(1) and p(0) are the probability of receiving bits 1 and 0, P (0/1)is the probability of deciding 0 when 1 is received and P (1/0) is the probability of deciding 1 when 0 is received.
4.1 Photonic Node: Preliminary Results and Q Measurements 44
As mentioned above, the Q-measurements refers to the output port num-ber “1”, the port of the rotated SOP.
Figure 4.2 shows the PBS output, i.e. the signal with the rotated state of polarization2. The Q-factor obtained is still low, being:
Q = 2.21
The presence of a large amount of noise on the high level, decreases the efficiency of the device. By observing the wavelength converter output signal, the switch control signal was found to be affected by the presence of some noise on the high level and it was not stable. So we substituted the NOLM-based wavelength converter with a FWM-NOLM-based one.
4.1.1
Configuration with FWM-Based Wavelength
Con-verter: Q Measurements
The converted signal, by using a FWM-based wavelength converter, proved to be stable and less noisy than in the previous experiment.
As can be seen from Figure 4.3, the output presents a lower amount of noise than in the previous experiment, but the Q-factor is still low. Fig-ures 4.3(a) and 4.3(b) show the signal present at the output port “1”, while Figure 4.3(c) shows the output port “0”, with the almost total absence of the signal. The Q-factor for this experiment is equal to
Q = 3.08
In order to improve the Q value, we proposed another experimental setup in which the label and the payload are transmitted separately. This setup allows us to optimize the Q-factor of the switch, avoiding the signal generation and recovery.
2Being a burst transmission, we can not utilize the eye-diagram. We used the normal
oscilloscope mode by triggering it with the pattern synchronization, the output Pattern Sync of the pulse pattern generator.
4.1 Photonic Node: Preliminary Results and Q Measurements 45
(a) Gaussian probability density of 1 bit.
(b) Gaussian probability density of 0 bit.
Figure 4.2: Signal at the output port “1”: Gaussian probability density of the sampled value.
4.1 Photonic Node: Preliminary Results and Q Measurements 46
(a) Outport number “1”: Gaussian probability density of 1 bit.
(b) Outport number “1”: Gaussian probability density of 0 bit.
(c) Outport number “0”.
4.2 Experimental Setup for Q-Factor Optimization 47
Figure 4.4: New experimental setup.
4.2
Experimental Setup for Q-Factor
Opti-mization
The Q measurements, obtained with the previous setup, were not satisfactory. The output was affected by a large band of noise on the high level, which was caused by the amplification of the switch control signal. To reduce it, both the extinction ratio and the average power of the converted signal had be in-creased. The only way to do that was to pre-amplify the label. The previous setup did not allow a label amplification before the transmission, as explained in Chapter2. It was only possible to amplify the label before the wavelength conversion. In order to improve the Q-factor and to characterize the device as well as possible, we changed the experimental setup concentrating only on the switch optimization. Figure 4.4 shows the new experimental setup. A DFB-laser at λs = 1557 nm was split onto two arms by a 50 : 50 splitter.
The continuous wave was modulated by a 27 Pseudo-Random Bit Sequence
(PRBS) at 10 Gb/s in one arm, while a square-wave at 3 MHz modulated the optical beam on the other arm of the splitter. We used two Mach-Zender modulators. The square-wave was synchronized with the pulse pattern and it had a low duty-cycle, 4%, to obtain a high value of peak power, as seen in Chapter 3. Both the arms were amplified. The arm, with the modulated label, entered an FWM-based wavelength converter. The wavelength con-verter output was amplified. Two polarization controllers on the two arms
4.2 Experimental Setup for Q-Factor Optimization 48
Figure 4.5: Q-Factor Optimization: output signal.
permitted the setting of the signal SOPs before they entered the HNF fiber. At the fiber output an optical band pass filter rejected the control signal. Then the payload was sent to a PBS which enabled the spatial switching to occur, as seen above. The presence of the polarization controllers on the two arms and the double label amplification led to a significant improvement in the Q-factor. By controlling the SOP of the converted label at fiber input and its peak power, we obtained a Quality factor of:
Q = 6.02
This value being double that of the previous experiment. Figure 4.5 shows the signal present at the PBS output “1”. The improvement could also be seen by observing the output qualitatively. The band of noise on the high level was significantly reduced.
