Dense Wavelength Division Multiplexing (DWDM)
With the increasing demand for high-speed data capacity and the need to support a variety of applications, system designers and installers are turning to dense wavelength division multiplexing (DWDM) technology. This technology has been proven to increase bandwidth over existing fiber networks and will continue to be utilized for years to come.
DWDM
DWDM is an optical technology that allows transmission of multiple wavelengths over a single fiber. It combines different signal wavelengths into a single channel and enables higher data rates and increased capacity than traditional CWDM systems. DWDM can be deployed in many different networks including high-bandwidth, long-haul applications such as OC-48 or OC-192.
A key component of a DWDM system is an OADM, which is used to add and drop individual wavelengths. Typically, the OADM is located at the hub of the network and is connected to the spoke nodes using a hubbed ring topology.
Another important component is the laser that provides the individual channels with their narrow and stable wavelengths. This laser is usually either an externally modulated or integrated one. Optical amplifiers are also essential to the performance of a DWDM system. These amplifiers can increase the total output power by several orders of magnitude.
Several test instruments are available to evaluate the performance of a DWDM network. These include osas, mwms, and optical spectrum analyzers (OSA).
OSAs divide an optical signal into its constituent wavelengths and measure the power of each wavelength. The result is displayed graphically, with wavelength on the horizontal axis and power on the vertical axis.
For DWDM applications, says Hiroshi Goto of Anritsu (Richardson, TX), a grating-based OSA with wavelength measurements to within +-0.02 nm and power-level accuracy to within +-0.5 dB is best. These are especially important for a DWDM network with its tight grid spacing, says Goto.
As a result, DWDM tests should be performed by highly trained personnel using a test instrument that is accurate and reliable. In addition to the quality of the osa, says Goto, a DWDM system tester should ensure that the osa has high-speed, low-noise amplifiers and a large dynamic range.
The most common DWDM system tests are to verify that each channel is within its specified wavelength and power limits. A dwdm system test also looks at the same channel characteristics over time to assess their stability and predict their behavior.
Several osas on the market are designed to perform these tests and meet the required specifications, including wavelength, power, and OSNR. They are available in various sizes and feature a variety of features. For example, the RXT-4500 osa has a full-band option that quickly determines the presence/absence of each of 16 wavelengths and checks their power levels accurately. Its 65 dB dynamic range and 20 dB monitoring tap on the OADM make it an ideal solution for non-intrusive channel analysis.
Optical Spectrum Analyzers
Optical spectrum analyzers (OSAs) are an important tool in the telecom industry. They allow telecom providers to modulate their signals dynamically and monitor the intensity of their outputs to ensure network reliability. Telecom equipment manufacturers also rely on OSAs for wavelength characterization during manufacturing.
These devices have a wide variety of features, including the ability to operate across multiple wavelength bands. They also have an interface to a computer, so they can be controlled and displayed remotely.
Some OSAs use scanning FPI interferometers to change the length of a Fabry-Perot cavity, which allows them to achieve high osa dwdm spectral resolution. However, they also have a limited wavelength range. In addition, they have a higher cost than other types of OSAs, making them primarily suitable for lab environments.
Another common type of OSA is grating-based. It uses a rotating filter or grating inside the instrument to determine the power at specific wavelengths. This grating is driven by a motorized opto-mechanical device that varies the rotation of the grating in a very precise manner.
The grating is often able to accommodate a broad spectral range, but the resolution and dynamic range are limited by the quality of the monochromator. The quality of the grating is further affected by the slit width and the number of lines on it, which can be modified to increase the wavelength resolution.
For increased sensitivity, one can also use a grating with an extremely narrow slit width or a very thin grating with a large line spacing. These options may be especially useful for testing low-loss, long-wavelength optical fibers or for measuring the diffraction pattern of a fiber.
These devices can be used to measure the OSNR (optical signal noise ratio) of individual light pulses, which is a critical parameter in cellular communications and other wireless networks. They are also useful for measuring the spectral purity of a fiber signal or the transmissivity of an optic component.
This type of optical spectrum analyzer is ideal for monitoring the performance of WDM transmission systems in telecommunication networks, such as CATV HFC/DAA, 5G x-haul, and hyperscale data center interconnects. In addition, they can be used to test other optical components such as lasers and amplifiers.
Multiwavelength Meters
Multiwavelength meters (MWM) are instruments that divide an optical signal into its constituent wavelengths and measure the power of each one. The measurement results are then displayed graphically, with the wavelength on the horizontal axis and the power on the vertical axis.
Wavelengths can be measured in different units, including kilometers, millimeters, micrometers and even nanometers and picometers. The shorter wavelengths on the electromagnetic spectrum, such as ultraviolet and infrared radiation, are typically measured using nanometers, while X-rays and gamma rays are usually measured with femtometers.
The multiwavelength meter is a sophisticated piece of equipment that offers a variety of features and applications. It is a high-performance, accurate laser-based instrument that measures the wavelength and optical power of multiple lines of CW or pulsed lasers in a specified wavelength range with fast, automated wavelength adjustment procedures.
DWDM networks are very narrow in spectral region, and therefore test equipment that is highly sensitive to wavelength is essential. Optical spectrum analyzers and multiwavelength meters are two key types of equipment that meet the challenge.
For DWDM test applications, the Keysight 438 Multi-Wavelength Meter provides a complete solution for performing wavelength and optical power measurements from 700 nm to 1700 nm. The meter has a compact, easy-to-use design with a touch screen that lets users control the measurement process and display data in a variety of formats.
Another type of MWM is the osa, which is a laser-based instrument that can isolate individual wavelengths within a spectrum osa dwdm and measure the power at each. The osa can be either single-pass or double-pass, with the latter having better wavelength accuracy and resolution than the former.
In a double-pass design, the input signal is diffracted by two diffraction gratings. The output signal is then reflected off of a second diffraction grating to separate the input into its component parts. The result is a much better orr than a single-pass osa, improving wavelength accuracy and resolution while increasing the accuracy of the power measurement.
In the osa, diffraction gratings are used to break the spectrum into its constituent wavelengths, and then to isolate each individual wavelength for power measurement. The osa is also capable of measuring the power at several wavelengths simultaneously to obtain the full range of output. This is a valuable feature when working with a large number of laser lines or DMX-controlled signals.
Applications
DWDM is an optical technology that utilizes multiplexing and de-multiplexing to create a single signal that can carry many data streams through a fibre-optic cable. This enables a single fiber-optic connection to transfer very large amounts of information over long distances, as well as enabling the use of dedicated optical fibre links. The resulting networks are able to deliver more bandwidth and lower cost than traditional fibre-optic networks.
Optical spectrum analyzers (osas) are a key test instrument for measuring spectral characteristics in CWDM and DWDM networks, which have become the preferred network building blocks for Next Generation high-speed network backbones. Optical spectrum analyzers offer accurate measurement and powerful processing capabilities for DWDM applications, including power measurements and optical signal loss (OSNR) analysis.
Most OSAs incorporate a rotatable grating or mobile detector that changes the spectrum-detector alignment in a continuous manner. This allows the osa to measure the power of all wavelengths in a light signal and generate characteristic graphs that are critical for DWDM testing.
The resolution bandwidth (RBW) of an OSA is critical for spectral analysis in DWDM systems, as it is the capacity of the instrument to separate closely spaced signals. A smaller RBW will enable the osa to clearly separate close channels, while a larger RBW will ensure sufficient noise measurements that are essential for a quality spectral measurement.
Some osas also incorporate gain tilt control to prevent the output spectrum from changing when gain ripple occurs. This is particularly true with OPT-BST and OPT-PRE amplifier cards, which are designed to work with gain setpoints based on a specific gain value, Gdesign.
A fl at-top rectangular slit is ideal for ensuring that the osa separates signals from each other, as shown in fi gure 3. However, if the slit is too small, it will let through too much noise and make it hard to obtain accurate spectral measurements.
Alternatively, the slit could be too wide and thereby cause crosstalk problems, as is the case with an osa with a fixed detector that only uses a single pass over the spectrum. This is especially the case in dwdm systems that use a combination of CWDM and DWDM channels, which can have different amplitudes, frequencies, and wavelengths.