Due to the rapid growth of packet-based traffic and data exchange in networks and data centers, the demand for optical packet switching is higher and higher. Traditional electrical switching adopts the form of optical-electric-optical switching, which has serious problems of electronic bottlenecks and high-power consumption and thus can hardly meet the growth demand of switching networks. Optical label switching transfers information exchange from the electrical domain to the optical domain, which improves the exchange rate and boasts high speed, transparency, low costs, large bandwidth, and low power consumption. Optical label switching is an effective solution to optical packet switching. It separates the label from the payload so that only the label is read at intermediate nodes without the need to detect the payload. At each node, part of the power of the optical packet is first extracted to the low-speed receiver for label detection. If the packet has the same destination address as the node, the packet is sent to the payload receiver. Otherwise, the packet is forwarded directly without being handled over any other layers. Optical label switching minimizes the overhead of packets and simplifies network control and management. As a result, the efficiency, scalability, and throughput are improved, especially in networks with numerous intermediate nodes. It is expected that modified mark ratio modulation can be helpful for optical label switching.

Mark ratio modulation is proposed in the paper. Firstly, the payload data is coded a new 5b8b code with a low mark ratio. Then, the coded payload data is combined with low-speed label data by XOR operation. After that, eight-bit cells are of a high or low mark ratio when the label bit is "one" or "zero", respectively. The label data is superimposed by mark ratio modulation. Subsequently, the mark-ratio-modulated data is divided into two sequences for the following polarization division multiplexing. The first four bits in each eight-bit cell are assigned to one sequence, and the last four bits are assigned to the other one. The two sequences are amplitude shift keying (ASK) modulated onto two optical carriers as two optical signals, which are polarization division multiplexed as an optical packet and transmitted. When an optical packet arrives at a node, part of the power is drawn into a low-speed receiver for label information receiving. The polarization-division-multiplexed optical signals are directly converted to an electrical signal by a photodetector without polarization beam splitting. When the packet has the same address as the node, the packet is sent to the payload receiver. In the payload receiver, the two optical signals on two polarizations are separated by a polarization beam splitter. The two optical signals are detected and analog-to-digital converted. Finally, the converted two sets of data are combined and decoded to recover the original payload data.

After the low-pass filter, the waveforms of the mark ratio modulation show that the high-mark-ratio sections and low-mark-ratio sections are converted to high levels and low levels, respectively. The mark ratio difference is converted to the amplitude difference. The mark ratio information corresponding to the label data is converted to the ASK signal (Fig. 3). In the first row of tested eye diagrams, the recovered label signal shows better performance when the bit rate ratio is lower as it suffers from lower crosstalk from the coded payload signals. In the second and third rows, the performance of mark-ratio-modulated signals on two polarizations is almost the same under different bit rate ratios. The coded payload signals suffer from no crosstalk from the overlaid label signal (Fig. 4). In addition, the bit error rate (BER) curves of the mark-ratio-modulated signals are almost coincident with different rates on overlaid label signals. The label signal with a higher bit rate shows worse performance due to the higher crosstalk from the coded payload signals (Fig. 5). The receiving sensitivity of the label is improved compared with that of the 4PPM code before the improvement. The BER of the payload does not change much. The modified mark ratio modulation maintains the advantages of 4PPM mark ratio modulation (Fig. 6).

Modified mark ratio modulation is proposed for optical label switching. The code for payload data is changed to a novel 5b8b code, whose code efficiency is increased to 62.5%. The high-speed payload data is coded in 5b8b code and combined with low-speed label data by XOR operation. Then, the combined data is divided into two sequences. The two sequences are ASK modulated on two optical carriers that will be polarization-division-multiplexed. The overlaid label signal is recovered directly through a low-speed ASK receiver, which removes the high-speed coded payload signal. Label receiving requires no decoding or polarization separation, which has a low cost and low operation complexity. The modified mark ratio modulation maintains the advantages of mark ratio modulation while increasing the effective bit rate of the payload data to 125% of the transmission rate, 250% of the previous value. Optical label switching based on modified mark ratio modulation is demonstrated by simulation. The transmission bit rate is set to 40 Gbit/s so that the effective bit rate of the payload data reaches 50 Gbit/s. The label data is set to 10.00 Gbit/s, 5.00 Gbit/s, 2.50 Gbit/s, and 1.25 Gbit/s separately. The signals in the modified mark ratio modulation can all achieve error-free operation. The label signal of a higher bit rate suffers from higher crosstalk. The test results show that the optical packet with the bit rate of 40 Gbit/s (the effective bit rate of the payload is 50 Gbit/s) can travel almost 60 km even without in-line amplifying and pre-amplifying. The simulations verify the feasibility of the proposed modified mark ratio modulation-based optical label switching. An experimental demonstration will be given next to verify the proposal.