During the last decades, with the increasing demands of bandwidth and high speed, the technologies of optical communication have been growing rapidly and achieved significant performance. But due to the fiber attenuation, dispersion and nonlinearity, the achievable transmission capacity of conventional fiber-optic communication systems is still limited. Wavelength Division Multiplexing (WDM) and Optical Time Division Multiplexing (OTDM) are the technologies that can increase the transmission capacity of optical fiber at present. However, there are some defects of WDM, the appearance of fiber nonlinearities, or the unequal gain spectrum of the amplifiers. OTDM can overcome these defects of WDM based on its much more attractive features. It is considerd as a long-term network technology and develops constantly. Today, Fiberstore’s Blog will introduce the basic knowledge of OTDM, as well as the difference from WDM. In addition, TDM-PON and its difference from WDM-PON, as well as WDM/TDM-PON are also introduced in the paper.

OTDM, short for optical time division multiplexing, is a channel multiplexing technology which multiplexes signals in different bit slots in the time domain. In other words, it’s practical to combine a set of low-bit-rate streams, each with a fixed and pre-defined bit rate, into a single high-speed bit stream that can be transmitted over a single channel. In contrast to WDM, OTDM only uses one wavelength, intuitively speaking, only a “color” of light in a fiber. OTDM provides a user the full channel capacity but divides the channel usage into time slots. Maybe you are still confused with OTDM just via these boring description. Here is a simple example to help you understand OTDM intuitively. To suppose that a channel capable of transmitting 192 kbit/s from Los Angeles to New York. And there are three sources, all located in Los Angeles. So, each have 64 kbit/s of data that they want to transmit to individual users in New York. As shown in Figure 1, the high-bit-rate channel can be divided into a series of time slots, and the time slots can be alternately used by the three sources. The three sources are thus capable of transmitting all of their data across the single, shared channel. Clearly, at the other end of the channel (in this case, in New York), the process must be reversed. The system must divide the 192 kbit/sec multiplexed data stream back into the original three 64 kbit/sec data streams, which are then provided to three different users. This reverse process is called demultiplexing. OTDM makes the most of these advantages in the optical domain and is another important technique for the construction of photonic networks in addition to the development of highspeed signal processing.

You may find OTDM is similar to WDM if you just scan this picture quickly. Because there are many channels both in OTDM and WDM. In fact, they are not the same. For OTDM, in a single fiber, there is only one wavelength, and also called one bandwidth. Channels are called time slots as they are divided according to the time domain. Signals are multiplexed in different bit slots. While, in WDM, channels are called wavelengths and there are multiple wavelengths in a singal fiber. You will obviously find these difference between OTDM transmission and WDM transmission. In OTDM, the signal wavelength (color red) transmits throughout the whole process, while in WDM, there are several wavelengths (several colors) and each wavelength is divided into separate channels.

WDM technology has been widely used in today’s network as it is the mature and practical optical transmission technology for large capacity transmission at present. With the advantages of transparency, reconfigurable, network survivability, WDM will be developed in the direction of flexible optical networks which are based on optical wavelength switching and wavelength routing. With the features of faster network restoration and reconstruction of capacities, WDM will be the main direction of future optical transport network. However, there are some unavoidable limitations of WDM. Thus, we need TDM technology in optical transmission, called OTDM due to its much more attractive features. OTDM is a very effective method of optical multiplexing. It can make full use of the spectrum resource, and remove some restrictions of nonlinear effects in WDM system. However, although we have made great progress in recent years on research of OTDM, it is not mature enough, because some of the key technologies have still to be resolved. Actually, we believe that, with the deepening of research, WDM and OTDM technologies will be combined to complement for each other and widely used in future ultra high-speed transmission networks.