In general, there are two common data formats for the transmission of high-speed digital data. Non-Return-to-Zero (NRZ) signal format is the more popular of the two formats due to its inherent simplicity. In this particular format, each “0” or “1” data bit is represented by a low or high signal level, respectively, lasting an entire clock period. However, with ever-increasing data rates, especially in optical transmission systems, Return-to-Zero (RZ) signal formats are becoming the transmission format of choice. In RZ modulation format, each data bit occupies only a portion of the clock period creating a distinct transition between adjacent bits and, thereby, producing a cleaner optical signal for the receiver to read. For high-rate (>10 Gbit/s) or ultra-long-haul (>1000 km) transmission, the RZ modulation technique is now coming into vogue as it affords certain efficiency gains such as higher signal-to-noise ratio (SNR) and lower crosstalk amongst adjacent bits. In this way, bit-error-rate (BER) may be improved. RZ encoding also offers better immunity to fiber nonlinear effects and the effects of polarization mode dispersion (PMD), factors which can limit long-haul or high-rate transmission severely. Optical transmission based on OTDM technology uses the RZ format primarily because of the relative ease it affords for multiplexing in the optical time domain.
The explosive growth of the Internet, and the corresponding demand for bandwidth has necessitated the introduction of optical time division multiplexing (OTDM) technology. The bandwidth of electrical components currently used for multiplexing and routing is rapidly being pushed to its fundamental physical limit. With the tremendous growth in data traffic predicted over the next few years, OTDM technology will be needed to avoid the potential electronic bottleneck that these multiplexers and routers will impose upon the next generation Internet.
In today's optical backbone network, dense wavelength division multiplexing (DWDM) enables high capacity transmission by combining multiple optical carriers on a single fiber. Each carrier or wavelength is modulated with a data channel having a rate up to, for example, 10 Gbps. In this way, the electronic bottleneck may be alleviated by shifting the electrical multiplexing to the optical spectral domain. However, the operation and management of DWDM systems is highly complicated and costly. Accordingly, single wavelength systems with higher data rates e.g. 40 or 80 Gbits/s are resurfacing as an alternative to DWDM systems in order to maintain the same total capacity. Single wavelength channels with higher rates could be multiplexed together to form a higher capacity DWDM system. Therefore, a key issue is to achieve higher rates per single optical channel which cannot be achieved by electrical time division multiplexing (ETDM) technology. OTDM technology is used for achieving higher rate per single channel in the optical domain.
In optical transmission systems, the bit rate is rising continuously. Target values for the future are on the order of 40 to 80 Gbit/s. With known transmitter combinations of semiconductor lasers with external modulators, such target values are attainable only with great difficulty. However, signal streams generated by individual transmitters may be combined in an optical time division multiplexer to make a signal stream with a higher bit rate.
The fundamental premise of OTDM technology is to solve the bandwidth bottleneck problem that ETDM technology is not currently able to address. ETDM technology is based on a traditional scheme of multiplexing individual lower-rate electronic signals into a high-speed serial electronic signal. The high-speed serial electrical signal may then be converted to an optical signal using a directly modulated laser or external modulator. These methods have worked well for data transmission rates up to 40 Gbit/s. However, electrical components for ETDM technology such as electrical multiplexers and demultiplexers (EMUXs and EDMUXs), that can achieve the high data rates (e.g. work beyond 40 Gbit/s) required today, are not expected to be available for the next several years.
On the other hand, OTDM technology makes the implementation of 80 Gbit/s, 160 Gbit/s or even higher capacity systems more achievable as compared to their electrical counterparts. In this lies the biggest advantage of OTDM technology. OTDM also opens the door for higher-rate nonlinear optical transmission such as, for example, soliton transmission which may make it possible to achieve transmission rates as high a 160 Gbit/s for very long distances without regeneration.
OTDM technology is based on a purely optical method for achieving very high data rate systems. For example, to generate an 80 Gbit/s RZ data stream, a 40 GHz clock pulse with, say, a pulsewidth of 6 picoseconds is first generated. Two sets of four 10 Gbit/s NRZ electrical signals may then be multiplexed via 4:1 EMUXs to form two 40 Gbit/s NRZ signal streams. The two 40 Gbit/s NRZ signal streams may then be applied to two corresponding external optical modulators to gate the incoming 40 GHz RZ clock pulses. In this way, the NRZ electrical signals are converted to RZ optical signals. One of the two 40 Gbit/s optical RZ signal streams may then be delayed by half a clock period to allow for interleaving in the time domain. The two 40 Gbit/s optical RZ signal streams may then be combined to form the final 80 Gbit/s optical RZ data stream.
The advantage of OTDM technology is that it alleviates the bandwidth bottleneck that ETDM technology is not able to currently address. Furthermore, the data received after transmission is of a better quality than that achieved through ETDM techniques. For example, generation of a 40 Gbit/s signal can easily be achieved through OTDM by applying two 20 Gbit/s NRZ electrical signal streams on an optical multiplexer (OMUX) e.g combination of two amplitude modulators and then delaying one optical data stream 25 ps relative to the other. Interleaving these two signal streams will then produce a 40 Gbit/s signal.
However, there is a downside to OTDM technology as well. Specifically, current OTDM implementations are relatively more complicated, expensive, and bulky than their counterpart ETDM solutions which normally just consist of integrated electronic chips. SO even though it's easier to get to 40 Gbit/s systems using OTDM technology today (because 40 Gbit/s electronics have not been fully developed), these are not very easy systems to build and manage. Even so, for data rates greater than 40 Gbit/s, OTDM technology is definitely the technology of choice in that it satisfies important transmission requirements e.g. minimal chromatic dispersion, reduced polarization mode dispersion and reduced impact of fiber non-lineararaties.
Today, 40 Gbit/s ETDM systems are, in fact, emerging. Compared to current OTDM implementations, ETDM is generally more compact and cost effective due to processing technology available for electronic chip integration. However, for higher rate (e.g. 80 Gbit/s) and long-haul transmission requiring minimal chromatic and polarization mode dispersion, the use of OTDM technology is more effective than ETDM.
Existing approaches for OTDM implementation are based on fiber optics and discreet electro-optic components. For example, the optical clock pulse described above may be divided amongst the two modulators via a fiber coupler and the delay required for interleaving may be realized by using fiber of differing lengths. The two RZ optical signal streams may then be combined, again using a fiber coupler, These kind of approaches are rather expensive and bulky and face stability and processing problems.
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