Monitoring Methods in DWDM Systems (Part 1)

    The basic principle of WDM (Wavelength-division multiplexing) technology is the ability to transmit multiple signals at different carrier wavelengths in one optical fiber. In Russian telecom, transmission systems created using WDM technology are called “compaction systems”.

    Currently, there are three types of WDM systems:
    1. CWDM (Coarse Wavelength-division multiplexing) —Systems with an optical carrier spacing of 20 nm (2500 GHz). The operating range is 1261-1611 nm, in which up to 18 simplex channels can be implemented. ITU Standard G.694.2.
    2. DWDM (Dense Wavelength-division multiplexing) - systems with an optical carrier spacing of 0.8 nm (100 GHz). There are two operating ranges - 1525-1565 nm and 1570-1610 nm, in which you can implement up to 44 simplex channels. ITU Standard G.694.1.
    3. HDWDM (High Dense Wavelength-division multiplexing) - systems with an optical carrier spacing of 0.4 nm (50 GHz) or less. It is possible to implement up to 80 simplex channels.

    This article (review) pays attention to the monitoring problem in DWDM compression systems, for more details about the various types of WDM systems, see the link - link .

    DWDM spectral multiplexing systems can use one of two carrier wavelength ranges: C-band - 1525-1565 nm (a conventional band or C-band may also occur) and L-band - 1570-1610 nm (a long wavelength band may also be L-band).

    The division into two ranges is justified by using different optical amplifiers with different working amplification ranges. The gain bandwidth for a traditional amplifier configuration is approximately 30 nm, 1530-1560 nm, which is the C-band. To amplify in the long-wavelength range (L-band), the configuration of the erbium amplifier is changed by lengthening the erbium fiber, which leads to a shift in the gain range in the wavelengths of 1560-1600 nm.

    At the moment, DWDM C-band equipment has received great recognition in Russian telecom. This is due to the abundance of various equipment that supports this range. It should be noted that equipment manufacturers are both venerable domestic companies and leading world brands, as well as numerous faceless Asian manufacturers.

    The main issue on any part of the compaction system (regardless of type) is the power level in the optical channel. First you need to understand what a DWDM compaction system usually consists of.

    DWDM system components:
    1) Transponder
    2) Multiplexer / demultiplexer
    3) Optical amplifier
    4) Chromatic dispersion compensator

    The transponder performs 3R regeneration (reshaping, re-amplifying, retiming — restoration of the shape, power and synchronization of the signal) of the incoming client optical signal. The transponder can also convert client traffic from one transmission protocol (often Ethernet) to another, more noise-resistant (for example, OTN using FEC) and transmit the signal to the linear port.

    In simpler systems, the OEO converter can act as a transponder, which performs 2R regeneration ("reshaping", "re-amplifying") and without changing the transmission protocol transmits the client signal to the linear port.

    The client port is often implemented as a slot for optical transceivers, into which a module is inserted to communicate with client equipment. The line port in the transponder can be made in the form of a slot for an optical transceiver or in the form of a simple optical adapter. The execution of the linear port depends on the design and purpose of the system as a whole. In an OEO converter, the line port is always designed as a slot for an optical transceiver.
    In many systems, the intermediate link - the transponder, is excluded in order to reduce the cost of the system or due to functional redundancy in a particular task.

    Optical multiplexers are designed for combining (mixing) individual WDM channels into a group signal for their simultaneous transmission over a single optical fiber. Optical demultiplexers are designed to separate the received group signal at the receiving side. In modern sealing systems, the functions of multiplexing and demultiplexing are performed by one device - a multiplexer / demultiplexer (MUX / DEMUX).

    The multiplexer / demultiplexer can conditionally be divided into a multiplexing unit and a demultiplexing unit.
    An optical amplifier based on an erbium-doped optical fiber (Erbium Doped Fiber Amplifier-EDFA) increases the power of a group (without preliminary demultiplexing) optical signal entering it without optoelectronic conversion. The EDFA amplifier consists of two active elements: an active fiber doped with Er3 + and a suitable pump.

    Depending on the type, EDFA can provide an output power of +16 to +26 dBm.
    There are several types of amplifiers, the application of which is determined by a specific task:
    Input optical power amplifiers (boosters) - are installed at the beginning of the route
    Optical preamplifiers - are installed at the end of the route in front of optical receivers
    • Linear optical amplifiers - mounted on intermediate amplification nodes to maintain the necessary optical power.

    Optical amplifiers are widely used on long data lines with DWDM spectral multiplexing systems.

    The chromatic dispersion compensator (Dispersion Compensation Module) is designed to correct the shape of the optical signals transmitted in the optical fiber, which, in turn, are distorted under the influence of chromatic dispersion.

    Chromatic dispersion is a physical phenomenon in an optical fiber, consisting in the fact that light signals with different wavelengths travel the same distance over a different period of time and, as a result, the transmitted optical pulse is broadened. Thus, chromatic dispersion is one of the main factors limiting the length of the relay section of the route. A standard fiber has a chromatic dispersion value of about 17 ps / nm.

    To increase the length of the relay section, chromatic dispersion compensators are installed on the transmission line. The installation of compensators often requires a transmission line with a speed of 10 Gbit / s or more.

    There are two main types of DCM:

    1. Chromatic dispersion compensation fiber - DCF(Dispersion Compensation Fiber). The main component of these passive devices is a fiber with a negative chromatic dispersion in the wavelength range of 1525-1565 nm.

    2. The compensation of chromatic dispersion based on the Bragg grating - DCM FBG (Dispersion Compensation Module Fiber Bragg Grating). Passive optical device consisting of chirped fiber and an optical circulator. Chirped fiber due to the structure creates a conditionally negative chromatic dispersion of the incoming signals in the wavelength range of 1525-1600 nm. The optical circulator in the device acts as a filtering device, directing the signals to the corresponding conclusions.

    Thus, the standard circuit consists of only two types of active components — a transponder and an amplifier, with which you can monitor the current power level of the transmitted signals. The transponders have the function of monitoring the status of linear ports either based on the built-in DDMI function in optical transceivers, or with the organization of their own monitoring. Using this function allows the operator to obtain relevant information about the status of a particular communication channel.

    Due to the fact that optical amplifiers are feedback amplifiers, they always have the function of monitoring the input group signal (the total optical power of all incoming signals) and the outgoing group signal. But this monitoring is inconvenient in the case of control of specific communication channels and can be used as evaluative (presence or absence of light). Thus, the only instrument for monitoring optical power in a data channel is a transponder.

    And since sealing systems consist not only of active, but also of passive elements, the organization of full-fledged monitoring in sealing systems is a very non-trivial and popular task.

    Monitoring organization options for WDM compaction systems will be discussed in the next article.

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