In optical circuits where o/e conversion of data is to be reduced or avoided, optical labeling of data is an attractive alternative in combination with optical switching. Depending on the network architecture labeling of data packets or data channels may be required. In this section we will discuss systems for labeling of high speed data packets as well as labeling to identify individual high-speed time division multiplexed (TDM) data channels.
2.1. Principle of in-Band Optical Packet Labeling
There are several attractive features of in-band optical labeling of high speed data packets. In this context in-band labeling describes the situation where the label is transmitted within the optical bandwidth of the data payload. The ability to route packets with imbedded labels in a single operation is attractive for keeping network complexity low. Imbedding the labels into the optical packet serves the purpose of reducing the bandwidth used for network signaling and eliminating the risk of label and data packet becoming de-correlated. In-band optical labeling in the schemes outlined here relies on modifying the spectrum of the optical data packet to accommodate the optical labels within the bandwidth otherwise occupied by the signal. As such, this kind of labeling is best suited for high-speed packet data where the perturbation of the optical signal spectrum caused by labeling will be small relative to the total extent of the signal spectrum. The minimum width of the labels (L1–L4 in
Figure 1a) is limited by practical filtering schemes.
Figure 1.
(a) Labeling of high speed data signals by spectral carving. L1–L4 indicate labeling positions where the presence or absence of power is interpreted as a digital label; (b) Results of labeling and switching 640 Gbit/s data packets.
Figure 1.
(a) Labeling of high speed data signals by spectral carving. L1–L4 indicate labeling positions where the presence or absence of power is interpreted as a digital label; (b) Results of labeling and switching 640 Gbit/s data packets.
A simple scheme for in-band optical labeling is illustrated in
Figure 1a. The labeling relies on frequency sub-bands inside the signal spectrum, where optical power can be suppressed to indicate a label “mark” or passed to indicate a “space”. In practice this is achieved by applying optical notch filters to shape the spectrum. As the labels encoded in this way rely on detecting small fractions of the actual signal spectrum, appropriate data scrambling is required to fulfill a mark-space ratio close to 50% in the data packet. This scheme has the clear advantage of keeping system complexity low since the labels are encoded by simple notch filtering and no extra signals are transmitted for labeling. In [
3] it was verified that the scheme can be used to encode more than 60 logical labels while keeping the eye opening penalty less than 1 dB compared to the un-labeled data packet. The scheme has been shown in [
4] to work for 640 Gbit/s data packets, where a single label was used to control a switch, separating labeled and unlabeled packets as shown in
Figure 1b. As the scheme relies on detecting a fraction of the signal spectrum as a logical label, some regeneration of the amplitude of the detected label is required to extract a control signal suitable to drive an optical packet switch. This has been implemented using a simple electronic comparator circuit which regenerates the amplitude of the detected label and adds a fixed length to the label duration to ensure that stable and high-quality switching can be performed for every data bit in the packet. This fixed label padding is a few ns in this demonstration but can in principle be chosen freely according to the requirements of the label regenerator and switch. A packet guard interval of ~20 ns was used. In this demonstration the whole scheme could be implemented with a penalty of only 4 dB in receiver sensitivity of the labeled and switched data packet. Except for requiring slightly higher power into the receiver the signal could be detected with similar distribution in the performance of the 64 OTDM (optical TDM) channels compared with the un-labeled and un-switched signal.
It is expected that this labeling scheme could be particularly relevant in environments with high network complexity and relatively short transmission distances as is the case in e.g., data center networks.
A variation of the scheme mentioned above is demonstrated in [
5] where narrow spectral bands in the broad data spectrum are allocated for transmission of optical labels. In this scheme optical labels are inserted into the assigned portions of the signal spectrum as low rate binary signals. Additionally, this scheme also allows for the insertion of a clock pilot signal which can be used for synchronizing the receiver to the incoming data packet relaxing the requirements for high-speed clock recovery circuits. High quality clock transmission does, however, require the optical spectrum of the data payload to be suppressed in the narrow spectral range where the clock is inserted. This scheme offers more freedom to adjust the duration and power of the optical labels. In this way the use of a regenerating circuit for the received labels can be avoided and the quality of the transmitted labels does not depend on scrambling of the data payload. This scheme does however require an optical transmitter for each optical label to be added. Consequently, complexity is moved from the receiver to the transmitter. One critical point in this labeling scheme is the relative stabilization of the filters used to extract the optical labels and the optical transmitters generating the labels. The wavelength stability of the label transmitters has to be carefully controlled.
This scheme has been demonstrated at 640 Gbit/s packet data rate with 50 km transmission to verify the performance of the transmitted pilot clock. Packet labeling and packet clock transmission was implemented in this demonstration. Only marginal phase noise was added by transmitting the clock as an in-band pilot clock increasing the total timing jitter (integrating the phase noise from 100 Hz to 10 MHz) from 80 fs to 90 fs. The quality of the transmitted labels allowed for switching the data packets with a penalty of ~1.5 dB in receiver sensitivity for achieving BER of 10−9, compared to the performance of unlabeled data packets.
Both packet labeling schemes mentioned above employ similar simple electronic circuits for detecting and processing the labels, imposing only a few ns of latency (<10 ns) when combined with the packet switching. Regarding dynamic allocation of labels to individual packets, both schemes require fast optical switching, similar to what is used for switching packets. This is used either to generate labels to be added to the signal in the case of the latter scheme or to switch the notch filters which the packet is subjected to in the case of the first scheme. In both cases the latency caused by label creation is expected to be even less than for label detection and processing.
2.2. Optical Labeling of TDM Data Channels
In very high speed TDM transmission it may be attractive to label the TDM channels in such a way that an optical receiver will be able to selectively receive only specific channels without having to have the capacity to receive the entire TDM data stream. In [
6] a scheme for labeling of a 650 Gbit/s OTDM data stream is proposed and demonstrated. In this scheme the optical spectrum of one of the OTDM tributary channels is slightly altered compared to the remaining channels, allowing a receiver to identify that channel and subsequently enumerate the remaining channels based on this reference channel. Also, detecting a single channel allows for a low rate (in this case 10 GHz) clock recovery circuit to be used to extract the base rate clock, which is required for extracting the tributary channels. This should be seen in contrast to the pre-scaled clock recovery discussed in
Section 4. In this scheme no dedicated clock signal is transmitted alongside the data signal, eliminating the potential signal or clock degradation due to cross talk between several co-propagating signals. The clock signal, which is also used for channel identification, is extracted simply by tuning a band pass filter to one edge of the optical data spectrum as illustrated in
Figure 2a.
Figure 2.
(a) Principle of channel labeling by spectral shaping; (b) 650 Gbit/s data signal after 80 km transmission with marked channel indicated.
Figure 2.
(a) Principle of channel labeling by spectral shaping; (b) 650 Gbit/s data signal after 80 km transmission with marked channel indicated.
The scheme is shown to work in an 80 km transmission demonstration of 650 Gbit/s data incorporating a fully polarization insensitive receiver. The transmitted 650 Gbit/s signal is shown in
Figure 2b where the marked channel is also indicated. A slight penalty in receiver sensitivity of ~2 dB for achieving a BER of 10
−9 is observed for the labeled channel compared to the remaining channels in back-to-back configuration. After 80 km transmission the labeled channel has similar performance to the remaining channels as
Figure 2b would also indicate.
A main attraction of this labeling scheme is the low complexity of the setup in both generating and detecting the labels. Shaping the spectrum does alter the transmission properties of the channel slightly but it has been shown that this effect can be kept small while still achieving a detectable spectral change to the labeled channel.
2.4. Optical Packet Switching by Wavelength Conversion in a Si Waveguide
Optical packet switching can be combined with wavelength conversion in the scheme demonstrated in [
7]. In this scheme cross phase modulation (XPM) is used to transfer amplitude modulation between original data packets and a control signal. The presence of this control signal determines which data packets are switched to the wavelength of the control signal. It is important to note that the data packet is not erased at the original wavelength in this scheme. This adds a multicasting capability to the scheme, which may be desirable.
Figure 3a shows the working principle of the packet switching by wavelength conversion. A control signal indicated at λ
2 is selectively launched into a nonlinear device, in this case a silicon (Si) waveguide, to temporally overlap with the packet to be switched. The packet temporally aligned with the control signal is then wavelength converted by XPM and thus extracted from the original stream of packets as shown in
Figure 3b. λ
1 is the data packet stream before switching—acting as pump for the XPM. λ
2 is the control signal used to select the packet to be switched—acting as probe for the XPM. λ
2 + Δ is the filtered output copy of the selected data packet.
Figure 3.
(a) Operating principle of packet switching by wavelength conversion and offset filtering; (b) Results of packet switching implementation in an 8 mm long Si nanowire.
Figure 3.
(a) Operating principle of packet switching by wavelength conversion and offset filtering; (b) Results of packet switching implementation in an 8 mm long Si nanowire.
This scheme for optical packet switching has been shown to introduce ~2.5 dB of power penalty in the receiver sensitivity for a BER of 10−9 compared to the un-switched packets. The target wavelength of the switched packet can be chosen with some flexibility depending on the properties of the specific chosen nonlinear switching material.
Packet switching in this scheme is highly flexible. The nonlinear switching process is nearly instantaneous (based on Kerr nonlinearity) and the switching speed and the temporal resolution are limited only by the ability to generate appropriate optical control signals. One limitation of the packet switching scheme using wavelength conversion is the inability to remove the switched packet from the original packet flow. This prevents reusing time slots in the packet flow and may cause unnecessary traffic to be transmitted in parts of the network.
2.5. Optical Packet Switching in an Electro-Optic Switch
An alternate scheme for packet switching which has also been investigated and demonstrated, is switching of packets in an electro-optic switch [
4]. In this scheme the packets remain at the same wavelength and switching is performed by transmitting or attenuating packets in the switch according to an applied electrical control signal. This scheme allows for reutilization of the cleared packet slots and is thus similar to packet routing based on o-e-o conversion. As electro-optic switches typically have limited modulation bandwidth, this scheme will require careful control of the guard interval between data packets and the electrical signal controlling the switch, as discussed below.
In [
4], electro-optic packet switching is demonstrated in combination with the optical packet labeling scheme discussed in
Section 2.1. The optical labels are detected for individual packets and used as control signal for the switch, which in this demonstration is a Lithium-Niobate (LN) Mach-Zehnder modulator (MZM) used as an on/off switch. The availability of high performance space switches would significantly improve the scaling potential of such optical switching schemes. Semiconductor-based circuits as e.g., [
8] offer promising progress in this area in terms of switching speed and port count. In the current demonstration [
4], scaling the scheme to a large number of outputs would incur a prohibitively large loss from splitting the signal to one switch for each output path. It is, however, clearly demonstrated that optical labels imbedded within data packets can be detected and used as control signals for an electro-optical switch achieving high-quality switching of the entire packet. This approach may offer attractive possibilities of combining control and transport layer infrastructure in optical networks.