Optical Multiplexing

The most important feature of optical fiber is its large capacity, capable of transmitting high-speed digital signals. To further improve the utilization of optical fiber, various optical multiplexing methods have been adopted with reference to the relatively mature electrical multiplexing methods, such as wavelength division multiplexing, frequency division multiplexing, time division multiplexing, space division multiplexing, subcarrier multiplexing, and code division multiplexing. Among these, wavelength division multiplexing, frequency division multiplexing, and code division multiplexing are considered to have the greatest potential.

I. Wavelength Division Multiplexing (WDM)

Wavelength division multiplexing refers to the simultaneous transmission of multiple optical carriers with different wavelengths over a single optical fiber, rather than just one optical carrier. As a result, the original single optical channel that could only carry one optical carrier per fiber becomes an optical channel capable of carrying multiple optical carriers of different wavelengths, multiplying the transmission capacity of the fiber. It is also possible to achieve bidirectional transmission over a single fiber by using different wavelengths for transmission in different directions.

The working principles of WDM technology can generally be divided into passive WDM multiplexers and active WDM multiplexers, each of which can be further subdivided into several types. For example, passive WDM multiplexers (PWDM) can be classified into prism type, fused biconical taper type, grating type, and interference filter type, while active WDM multiplexers can be classified into wavelength-tunable filters, source directional couplers, wavelength-tunable lasers, and integrated optical waveguides. Currently, passive WDM multiplexers are more widely used in practice.

Optical WDM technology offers the following advantages:
(1) By utilizing optical WDM technology, the transmission capacity of an optical cable can be expanded by several times, dozens of times, or even hundreds of times without constructing new cable lines or modifying existing cables. This is of great significance given the large proportion of line investment in current projects.

(2) The WDM multiplexers currently in use are mainly passive devices, which feature simple structure, small size, high reliability, easy fiber coupling, and low cost.
(3) In optical WDM technology, the working systems of each wavelength are independent of one another. The modulation schemes and signal transmission rates used in each system can be different, and even analog and digital signals can occupy different wavelengths for transmission within the same fiber. This transparency of WDM transmission brings great convenience and flexibility in practical use.

(4) The same optical WDM multiplexer can be used for both multiplexing and demultiplexing, offering directional reversibility, thus enabling bidirectional transmission over the same fiber.

WDM multiplexes multiple wavelengths, and the number of wavelengths that can be multiplexed depends on the spacing between adjacent wavelengths — the smaller the spacing, the greater the number of multiplexed wavelengths. Generally, when the spacing between adjacent peak wavelengths is 50–100 nm, the system is referred to as a WDM system. When the spacing between adjacent peak wavelengths is 1–10 nm, it is referred to as a dense wavelength division multiplexing (DWDM) system.

DWDM is currently one of the hottest products on the market, with 40-wavelength DWDM already in commercial use. According to the latest report from a Boston-based consulting firm, the market growth rate for DWDM equipment is expected to reach 65% in 2000. With the emergence of "IP over WDM" and "IP over DWDM" technologies, WDM and DWDM have attracted even greater attention. WDM and DWDM will inevitably occupy an important position in the all-optical networks under construction.

II. Frequency Division Multiplexing (FDM)

When the spacing between adjacent peak wavelengths is less than 1 nm, the system is generally referred to as an optical frequency division multiplexing (FDM) system, which is essentially no different from WDM. Frequency represents the number of wave peaks per second, while wavelength represents the length from one wave peak to the adjacent wave peak of the electromagnetic wave; the two are reciprocally related. When the optical carrier spacing is relatively large, it is more convenient to measure by wavelength, and this is generally referred to as wavelength division multiplexing. When the optical carrier spacing is relatively small, measuring by wavelength becomes inconvenient, so systems with optical carrier spacing of less than 1 nm are conventionally referred to as frequency division multiplexing systems.

Since the optical carrier spacing in FDM is very dense, traditional WDM devices such as demultiplexers and multiplexers can hardly distinguish between optical carriers, requiring higher-resolution techniques to select individual optical carriers. The main approaches currently available include tunable optical filters and coherent optical communication technologies. FDM is generally applicable to high-capacity high-speed communication systems or distributed network systems such as CATV and broadcasting.

III. Optical Code Division Multiple Access (OCDMA)

CDMA technology is not a new technology. As a multiple access scheme, it has been successfully applied in satellite communications and cellular telephony, demonstrating many advantages over other technologies, such as effectively addressing anti-interference and anti-multipath fading in mobile communications, and offering significant advantages in improving system capacity. However, due to bandwidth limitations in satellite and mobile communications, CDMA technology has not yet fully realized its advantages. Optical fiber communications offer abundant bandwidth, which can well compensate for this shortcoming. In recent years, OCDMA has become a highly regarded hotspot technology.

OCDMA technology is similar in principle to electrical code division multiplexing. In an OCDMA communication system, each user is assigned a unique optical orthogonal code as their address code. At the transmitting end, the data to be transmitted is optically orthogonally encoded with the address code, followed by channel multiplexing; at the receiving end, optical orthogonal decoding is performed using the same address code as the transmitting end.

The advantages of OCDMA technology include: increased network capacity; improved signal-to-noise ratio and system performance; enhanced security; increased network flexibility; reduced synchronization requirements; and random access with channel sharing.

OCDMA achieves optical channel multiplexing and signal switching through direct optical encoding and decoding, offering tremendous application prospects in optical fiber communications. Of course, from the current perspective, due to technical reasons, OCDMA is not yet mature and still has a way to go before practical implementation.