Comparison of Several Multiplexing Technologies in Optical Fiber Networks
As communication applications become increasingly widespread, the role of multiplexing technologies is becoming more prominent. Multiplexing is a means of more effectively improving data utilization, and in optical fiber communications, multiplexing is also considered a primary method for expanding the capacity of existing optical fiber network projects. Multiplexing technologies mainly include Time Division Multiplexing (TDM), Space Division Multiplexing (SDM), Wavelength Division Multiplexing (WDM), and Frequency Division Multiplexing (FDM). However, since FDM and WDM are generally considered to have no essential difference, WDM can be regarded as "coarse division" while FDM is "fine division", thus grouping the two into one category. Below, Raytrans mainly discusses Time Division Multiplexing (TDM), Space Division Multiplexing (SDM), Wavelength Division Multiplexing (WDM), Coarse Wavelength Division Multiplexing (CWDM), and Optical Add/Drop Multiplexing (OADM).
- TDM Technology
Time Division Multiplexing (TDM) is a technology that transmits multiple digitized data, voice, and video signals simultaneously over the same communication medium through interleaved bit pulses in different channels or timeslots. However, in electronic communications, this technology is limited by electronic speed, capacity, and spatial compatibility, making it difficult for electronic TDM rates to be too high. For example, PDH signals only reach 0.5 Gbps. Although SDH signals using synchronous interleaved multiplexing have reached 10 Gbps (STM-64), reaching 20 Gbps is quite difficult. On the other hand, in optical fiber, attenuation, reflectance, chromatic dispersion, and polarization mode dispersion (PMD) all seriously affect the transmission of high-speed modulated signals. When signals reach STM-64 or higher rates, the pulse broadening effect of PMD can cause signal "blurring", leading to erroneous signal judgment at the receiver and thus generating bit errors. This is because polarized light in different modes produces slight time differences during propagation in the fiber, and therefore the PMD coefficient is generally required to be below 0.1 ps/km. In summary, the limitations of electrical TDM technology restrict the transmission rate of electronic communications to below 10–20 Gbps.
1.1 Optical Time Division Multiplexing (OTDM)
Optical Time Division Multiplexing is an expansion technology in which multiple telecommunication channel signals modulate different channels of the same optical frequency, and after multiplexing, are transmitted over the same optical fiber. OTDM technologies mainly include: generation and modulation of ultra-narrow optical pulses, all-optical multiplexing/demultiplexing, and optical clock extraction.
1.1.1 Generation of ultra-narrow optical pulses. OTDM requires the light source to provide ultra-narrow optical pulse output with a duty cycle of 5–20 GHz. Methods for achieving this include the gain switching method, LD mode-locking method, electro-absorption continuous-wave gating modulation method, fiber grating method, and SC (Supercontinuum) optical pulses. The gain switching method can generate optical pulses with a pulse width of 5–7 ps and a pulse repetition frequency adjustable around 10 GHz, with the advantage of easy synchronization with other signals. The gain switching method has been used for pulse source generation and optical measurement in various high-speed optical transmission experiments. SC optical pulses can have a pulse width greater than 1 ps, with the narrowest reaching 0.17 ps. In addition, by adjusting the dispersion value of a linearly modulated fiber grating to correct the pulse shape output by the electro-absorption modulator, 10 GHz optical pulses with a pulse width of 5.8 ps and a duty cycle of 6.3% can also be generated.
1.1.2 All-optical multiplexing/demultiplexing. All-optical TDM can be constructed using optical delay lines and 3 dB optical directional couplers. In ultra-high-speed systems, it is preferable to integrate the optical delay lines and 3 dB optical directional couplers on a planar silicon substrate to form a planar lightwave circuit (PLC) as the optical multiplexer. The all-optical demultiplexer demultiplexes the OTDM signal at the optical receiving end. Four types of devices have been developed as demultiplexers: optical Kerr switch matrix optical demultiplexers, cross-phase modulation frequency-shift optical demultiplexers, four-wave mixing switch optical demultiplexers, and nonlinear optical loop mirror (NOLM) optical demultiplexers. Regardless of the device used, reliable and stable operation, low control optical signal power, and polarization independence are required.
1.1.3 Optical clock extraction. Optical clock extraction requires ultra-high-speed operation, low phase noise, high sensitivity, and polarization independence. A phase-locked loop (PLL) using a high-speed microwave mixer as a phase detector has been developed, and an optical oscillation loop (FPT) using a Fabry-Perot interferometer optical path can also accomplish clock recovery.
SDM Technology
The general understanding of SDM is: multiplexing of multiple optical fibers, i.e., cable multiplexing. In some locations, existing optical fiber communication network ducts have spare space. Therefore, to increase capacity, more fibers can be pulled into the ducts, which is more convenient than electronic methods. Another understanding of SDM is achieving space division multiplexing within a single optical fiber, i.e., spatial division of the light beam in the fiber core region. Because the core diameter of a single-mode fiber is only 9–10 mm, and the phase of each point on the transmitted beam wavefront fluctuates, spatial division of such a wavefront is extremely difficult. Although a coherence-based theoretical division method has recently been proposed, there is still a long way to go before practical application.WDM Technology
Optical Wavelength Division Multiplexing is a technology in which electrical signals from multiple sources modulate their respective optical carriers, and after multiplexing, are transmitted over a single optical fiber. At the receiving end, channel selection can be achieved using coherent communication with heterodyne detection or conventional direct detection with tunable passive filters. WDM technology not only expands communication capacity but also brings significant economic benefits to communications. Therefore, research in this area has been flourishing in recent years. WDM technology carries multiple wavelengths (channels) on a single optical fiber, converting one fiber into multiple "virtual" fibers, each operating independently on a different wavelength. Each channel can run at speeds of 2.5–10 Gbps.
3.1 Dense Wavelength Division Multiplexing (DWDM)
Dense Wavelength Division Multiplexing (DWDM) technology, commonly referred to as DWDM, is an optical fiber data transmission technology that uses laser wavelengths to transmit data in the fiber in a bit-parallel or character-serial manner.
DWDM first assigns incoming optical signals to specified frequencies (wavelengths, lambda) within a specific frequency band, and then multiplexes the signals onto a single optical fiber. In this way, the bandwidth of existing installed cables can be greatly increased. Since incoming signals do not terminate at the optical layer, the interface rate and format can remain independent, allowing service providers to integrate DWDM technology with existing network equipment while gaining access to substantial unused bandwidth in existing installed cables.
DWDM can combine multiple optical signals for transmission. As a result, these optical signals can be grouped, amplified simultaneously, and transmitted through a single fiber, greatly increasing network bandwidth. Each carried signal can be set to different transmission rates (OC-3/12/24, etc.) and different formats (SONET, ATM, data, etc.). For example, a DWDM network can mix OC-48 (2.5 Gbps) and OC-192 (10 Gbps) SONET signals on the DWDM basis, achieving a huge bandwidth of up to 40 Gbps. Systems using DWDM can achieve the above goals while maintaining system performance, reliability, and robustness comparable to existing transmission systems. Future DWDM terminals can carry up to 80 wavelengths of OC-48 to achieve transmission rates of 200 Gbps, or up to 40 wavelengths of OC-192 to achieve 400 Gbps—bandwidth sufficient to transmit 90,000 volumes of an encyclopedia in one second!
3.2 FDM Technology
FDM divides the optical waves transmitted in an optical fiber into several optical frequency channels according to frequency, with each channel serving as an independent carrier of information. This enables multi-channel multiplexed transmission over a single fiber. FDM technology can be used in conjunction with WDM technology to multiply the number of multiplexed channels. That is, the optical wave channels are first coarsely divided by wavelength. If each channel width is Δλ, then within each channel of width Δλ, several sub-channels (f1, f2, …, fn) can be loaded, each independently carrying information. Since coherent optical communication provides excellent selectivity, combining FDM technology with it creates conditions for the practical application of FDM-based optical fiber networks. Optical FDM multiplexing equipment is complex and places high demands on optical component performance, so considerable effort is still needed before it enters the practical engineering stage.
3.3 Coarse Wavelength Division Multiplexing (CWDM)
In response to the demands of the communications market, CWDM (Coarse Wavelength Division Multiplexing) emerged. As the name implies, CWDM is a close relative of DWDM. The differences between them are mainly two:
The carrier channel spacing of CWDM is wider; therefore, only about 5 to 6 wavelengths can be multiplexed on the same fiber. The distinction between "coarse" and "dense" comes from this;
CWDM modulated lasers use uncooled lasers, while DWDM uses cooled lasers. Cooled lasers use temperature tuning, while uncooled lasers use electronic tuning. Since temperature distribution is highly uneven over a wide wavelength range, temperature tuning is difficult to implement and costly. CWDM avoids this difficulty, thereby significantly reducing costs—the total cost of a CWDM system is only 30% of that of DWDM.
CWDM provides high access bandwidth at very low cost and is suitable for various popular network structures such as point-to-point, Ethernet, and SONET rings. It is particularly suitable for short-distance, high-bandwidth, and densely populated access point communication applications, such as intra-building or inter-building network communications. Of particular note is the combination of CWDM with PON (Passive Optical Network). PON is an inexpensive, point-to-multipoint optical fiber communication method. By combining with CWDM, each individual wavelength channel can serve as a virtual optical link for PON, enabling broadband data transmission between a central node and multiple distributed nodes.
However, CWDM is a compromise between cost and performance and inevitably has some performance limitations. Industry experts point out that CWDM currently has the following three shortcomings:
(1) CWDM supports a relatively small number of multiplexed wavelengths on a single fiber, leading to higher future expansion costs;
(2) The cost of equipment such as multiplexers and multiplexer/demultiplexers should be further reduced; these devices should not be simple modifications of corresponding DWDM equipment;
(3) CWDM has not yet formed a standard.Optical Add/Drop Multiplexing (OADM)
In the field of WDM optical networks, interest is increasingly focusing on Optical Add/Drop Multiplexers. These devices perform in the optical wavelength domain functions similar to those of traditional SDH Add/Drop Multiplexers (SDH ADM) in the time domain. In particular, OADM can drop one channel from a WDM optical beam (drop function) and typically insert new information onto the optical carrier at the same wavelength (add function). OADM is selective: it can drop or add signals from the transmission equipment, or simply pass through a certain wavelength signal, without affecting the transmission of other wavelength channels. OADM accomplishes in the optical domain what SDH Add/Drop Multiplexers do in the time domain, and it is transparent, capable of handling signals of any format and rate. It improves network reliability, reduces node costs, and enhances network operating efficiency, making it an indispensable key device for building all-optical networks. For OADM, high isolation (>25 dB) must be maintained between the drop port and add port, as well as between the input port and output port, to minimize same-wavelength interference effects; otherwise, transmission performance will be seriously affected. Several technologies for implementing OADM have been proposed: WDM MUX/DEMUX; fiber gratings between optical circulators or in Mach-Zehnder structures; and cascaded Mach-Zehnder structures and interference filters implemented with integrated optics technology. The first two methods achieve the highest isolation but require expensive equipment. The Mach-Zehnder structure (using fiber gratings or optical integration technology) is still under development and requires further improvement to achieve the required isolation.