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Passive DWDM solutions
2021-04-07

Dense Wavelength Division Multiplexing (DWDM) is an optical, multiplexing technology used to increase bandwidth over existing fiber networks. It works by combining and transmitting multiple signals simultaneously at different wavelengths on a single fiber or dual fiber strand.

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DWDM is a method of increasing the link capacity with the largest functional and transmission possibilities. The basic requirements for DWDM transmission (such as number of channels, channel separation, nominal central channel frequencies, etc.) have been defined in the ITU-T G.694.1 recommendation. Today, most commonly used DWDM grids are 50 GHz and 100 GHz. Comparing to CWDM transmission, the channel isolation in the DWDM method is lower by two orders of magnitude and more channels are available – usually about 40 channels for the 100 GHz grid (and 96 Channel when using the 50 GHz grid). Almost always in DWDM transmission, the so-called C-band, or 1525 – 1560 nm wavelength range, is used. Comparing to the wavelength range used in the CWDM method (i.e. 1270-1610 nm), the spectral range of DWDM system operation is much narrower, which together with a larger number of channels means that the exploitation of the available spectrum is much more efficient.

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Due to transmitting the signal only around 1550 nm, losses in the optical fiber seen by each channel are practically identical (it can be different when using CWDM transmission). 

Like in CWDM systems, DWDM transport networks can be constructed as active or passive. The name “active DWDM solutions” means systems with dedicated 1st-layer transport platforms, or active devices (mainly transponders) responsible for signal transmission. Active DWDM solutions are applied in most extensive skeleton networks because they assure built-in functionalities of regeneration, monitoring, whole network management, provisioning, and signal demarcation. Corephy also offers active DWDM solutions, which can be found in the “Active devices” category.

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The biggest disadvantage of active DWDM solutions is of course their cost. The main cost component are the transponders. Their share in the cost of the whole system differs depending on the functionalities, but in simple networks it usually exceeds 70%. Therefore, many operators decide to build DWDM networks without transponders, or so-called “passive DWDM solutions”. This applies mostly to small and middle-sized operators, from whose point of view additional costs generated by the purchase of transponders are not countered by their advantages and functionality.


Passive filters and DWDM multiplexers, used in terminals and add/drop nodes for connecting (multiplexers), division (demultiplexers) and separation of selected DWDM channels from the optical multiplex (add/drop filters).

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Due to rigorous tolerances for DWDM filters and multiplexers, they must be measured in a proper way with high class measuring equipment. With such narrow passbands and channel separation, each shift of measurement light source by a few GHz may completely change the measurement results. Moreover, the DWDM system user must have it guaranteed that the values given in the test report are reliable and measured for worst case. All key parameters from the point of view of network reliability, such as insertion loss, channel isolation, attenuation uniformity, and passband ripple, must be measured with a high accuracy and their worst case values must be given in a test report. Only a measurement of the whole spectral profile gives full information about real values of attenuation and isolation. Moreover, only a measurement made by the tunable laser method is able to assure an adequate resolution (of the order of single GHz), even a measurement made by using a wideband source and an optical spectrum analyzer is usually insufficient due to an inadequate resolution of the analyzer. This is why when purchasing DWDM filters and multiplexers, one should always demand that they be delivered with test reports showing the whole spectral profile to assure their quality and reliability of the manufacturer.

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Passive DWDM Module offered by Corephy are fully specialized and may have a capacity of up to 96 DWDM channels with additional multiplexer functionalities (e.g. 1% power monitoring port, OTDR 1625/1650 nm monitoring port, grey 1310 nm port, Upgrade port). Similarly, solutions for bidirectional transmission in one fiber or dual fiber are offered. Hybrid DWDM/CWDM systems are of course also available.