![]() ![]() Historically, the NRZ format used before the advent of lightwave technology was retained even for optical communication systems. In a different approach, a cascaded nonlinear process inside periodically poled LiNbO 3 waveguides (resulting in FWM) is employed.Īn important difference between the OTDM and WDM techniques should be apparent from the figure above: The OTDM technique requires the use of the RZ format. Such an integrated OTDM multiplexer was used in a 160-Gb/s experiment in which 16 channels, each operating at 10 Gb/s were multiplexed. This problem can be solved by integrating spot-size converters with the modulators. The main problem with this approach is the spot-size mismatch as the optical signal passes from Si to InP waveguide (and vice versa). A simple approach consists of inserting an InP chip containing an array of electroabsorption modulators in between the silica waveguides that are used for splitting, delaying and combining the multiple channels. However, the entire multiplexer cannot be built in the form of a planar lightwave circuit as modulators cannot be integrated with this technology. Such devices can be made polarization insensitive while providing a precise control of the delay lengths. Such precision is hard to realize using optical fibers.Īn alternative approach makes use of planar lightwave circuits fabricated using the silica-on-silicon technology. ![]() For a precision of 0.1 ps, typically required for a 40-Gb/s OTDM signal, the delay lengths should be controlled to within 20 μm. Note that the delay lines can be relatively large (10 cm or more) because only the length difference has to be matched precisely. ![]() As an example, a 1-mm length difference introduces a delay of about 5 ps. The optical delay lines can be implemented using fiber segments of controlled lengths. Splitting and recombining of signals in N branches can be accomplished with 1 x N fused fiber couplers. The entire OTDM multiplexer (except for modulators which require LiNbO 3 or semiconductor waveguides) can be built using single-mode fibers. Furthermore, N consecutive bits in each interval of during B -1 belong to N different channels, as required by the TDM scheme. It should be clear that the multiplexed bit stream produced using such a scheme has a bit slot corresponding to the bit rate NB. The output of all branches is then combined to form a composite signal. In this scheme, the bit stream in the nth branch is delayed by an amount (n-1)/(NB), where n = 1. Multiplexing of N bit streams is achieved by a delay technique that can be implemented optically in a simple manner. A modulator in each branch blocks the pulses representing 0 bits and creates N independent bit streams at the bit rate B. The laser output is split equally into N branches, after amplification if necessary. Moreover, the laser should produce pulses of width T p such that T p < T B = (NB) -1 to ensure that each pulse will fit within its allocated time slot T B. It requires a laser capable of generating a periodic pulse train at the repetition rate equal to the single-channel bit rate B. The figure below shows the design of an OTDM transmitter based on the delay-line technique. Several multiplexing techniques can be used for this purpose. In OTDM lightwave systems, several optical signals at a bit rate B share the same carrier frequency and are multiplexed optically to form a composite bit stream at the bit rate NB, where N is the number of channels. In this section we first discuss these new techniques and then focus on the design and performance issues related to OTDM lightwave systems. Its deployment requires new types of optical transmitters and receivers based on all-optical multiplexing and demultiplexing techniques. The OTDM technique was studied extensively during the 1990s, and further research has continued in recent years in the context of WDM systems with channel bit rates of 100 Gb/s or more. A solution is offered by the optical TDM (OTDM), a scheme that can increase the bit rate of a single optical carrier to values above 1 Tb/s. The electrical TDM becomes difficult to implement at bit rates above 40 Gb/s because of the limitations imposed by high-speed electronics. In this sense, even single-channel lightwave systems carry multiple TDM channels. TDM is commonly performed in the electrical domain to obtain digital hierarchies for telecommunication systems. ![]()
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