CWDM Technology
The CWDM grid wavelengths are within the range of 1270 and 1610 nm. A nominal center wavelength gap of no less than 20 nm is required to use uncooled lasers and widebandpass filters.
To be compatible with current filtering technologies, an overall source wavelength variation of ±6–7 nm is expected.
There are two main factors contributing to this wavelength variation:
manufacturing tolerances
and uncooled lasers. Uncooled lasers
cause wavelength shift with temperature (within the laser's operating temperature range).
Conventional single-mode fibers exhibit significant attenuation around 1400 nm due to the water absorption peak. Consequently, the number of CWDM channels used is reduced to 12—three in the 1310–1350 nm range and nine in the 1450–1610 nm range. Typical (unamplified) transmission distances are in the range of 40 to 80 km, due to the power limitations of the "uncooled" laser sources (approximately 4 dBm maximum) used in CWDM systems and the increased losses caused by the introduction of more passive devices into the networks (multiplexer up/down, multiplexer/demultiplexer, and dispersion compensation modules), especially for data rates of 10-Gbit/s.
Sometimes, it is desirable to extend transmission distances beyond this range or to improve electrical system budgets. EDFA optical amplifiers, commonly used in DWDM transmission systems, are impractical because their limited bandwidth gain profile restricts their use to only a small portion of the CWDM spectrum, while DWDM covers only the C&L band.
Without the use of amplifiers, the investment required for a CWDM network is much lower, making the multiple access points advantageous. This ability to offer multiple access points (regenerated network) puts the CWDM network in an ideal position for regional or metropolitan networks, as it provides the flexibility to add and remove user traffic from multiple locations. A regenerated network eliminates the OSNR (Optical Signal-to-Noise Ratio) requirement and the complexity it causes, thus helping to reduce costs related to network planning and testing requirements.
Testing Methods for a CWDM Network
There are different ways to test a CWDM network, typically including:
• Power Meter (PM) in combination with a filter.
A standard power meter will measure the total light (all) at the detector within its sensor's detectable range, approximately 850 to 1650 nm. Since a CWDM system comprises many wavelengths, a single power meter cannot provide all the details, but when combined with a CWDM filter, the user can check the power of each channel. The power meter and filter combination is the least expensive of the methods described here.
The method does not allow testing the wavelength of a channel, so it is impossible to evaluate the actual channel wavelength or deviation according to standards.
• OCA (Optical Channel Analyzer).
An optical channel analyzer can best be described as having many optical power detectors housed in a single unit with fixed optical components sending the different CWDM channels to the detectors.
• OSA (Optical Spectrum Analyzer).
The traditional optical spectrum analyzer is a much more complex and specialized test instrument than the previous ones. An optical spectrum analyzer offers full DWDM and CWDM testing capabilities and is typically a specialized unit with a much higher cost and contains moving parts. The optical spectrum analyzer will provide the most accurate test results for a CWDM network, as well as being more customizable. This also has the disadvantage of being a more complicated and expensive solution to use.
Each method varies in complexity, potential margin of error, and cost.
Background Technology of the Optical Channel Analyzer:
The optical channel analyzer is based on a monochromator, with no moving parts. The monochromator principle relies on the fact that light is diffracted into its spectral components (light beams) by a grating. A diffraction grating is a mirror with slits on its surface. The directions of these light beams depend on the distance between the grating and the wavelength of the signal. Finally, the beams are received by the array detector.
The detection element allows for near-parallel measurement and processing. As the optical channel analyzer operates on optical power technology, it is possible to design it without moving parts at a reasonable price in a portable field unit. This concept and design provides faster CWDM network testing than any other optical spectrum analyzer and allows a single connection to measure power and wavelength across all 18 CWDM channels and monitor their drift over time.
The Optical Channel Analyzer and its Applications
Effect of Temperature on CWDM
For an evaluation of access to a CWDM network it is essential to control the parameters and values of the physical layer for point-to-point links or with a ring structure.
Because this type of laser passes through filters, which are not flat, a wavelength deviation would be seen as a power issue, not a wavelength problem. Therefore, it is crucial to be able to measure both power and wavelength to determine or prevent the cause of the problem.
As mentioned earlier, the effect of temperature on CDWM lasers is observable due to the wide wavelength tolerance and the possibility of using "uncooled" lasers that vary with temperature. Furthermore, the CDWM laser output will be affected by any temperature variation.
The channels most affected by wavelength deviation are channel 18 and channel 1, due to power output deviation. In other words, we can observe that different channels are affected differently by temperature changes.
This behavior can be easily monitored and displayed with the optical channel analyzer, either across all channels or an individual channel, see Figure 1 and Figure 2.
a) b)
c) d)
Figure 5. Alarm indicators on the different screens of the optical channel analyzer: a) Normal graph screen; b) Normal table screen; c) Deviation graph screen; d) Channel 1 deviation screen.
Network compliance testing according to ITU-T Recommendation G.695.
ITU-T Recommendation G.695 provides physical layer parameters for single- and multi-channel CDWM interfaces. The objective is to allow multiple compatible CWDM systems from various vendors to be configured in a point-to-point or ring topology. Only non-amplified multi-channel interfaces are considered by this recommendation.
To facilitate verification of CWDM network compliance with ITU-T Recommendation G.695n, Anritsu has developed a CWDM-specific test instrument called the Network Master Optical Channel Analyzer (OCA). The Network Master Optical Channel Analyzer is a modular field device designed to measure power and wavelength across all 18 CWDM channels and monitor their drift over time. In a small, lightweight, and rugged portable form factor, this user-friendly instrument is ideally suited to assist field technicians in the installation, maintenance, and troubleshooting of CWDM access networks.
Providing a comprehensive overview of all CWDM channels displayed in a graphic or table on the device's large screen, and complemented by various drift functions for long-term channel characterization, the Network Master Optical Channel Analyzer enables fast and reliable measurement of CWDM network parameters in all environments. The software interface has been carefully developed to facilitate the qualification of CWDM links, requiring almost no preparation to characterize a CWDM network.
Alarm thresholds for all configurations defined in ITU-T Recommendation G.695 are stored in the device, facilitating PASS/FAIL testing. Figure 3 shows the [Settings/Alarms] menu interface for activating the appropriate alarm thresholds at the CWDM network reference measurement point.
The application code of the network under test can be selected in the [Settings/Alarms] menu of the optical channel analyzer.
All alarm settings for the ITU-T G.695 c application code set can be viewed in the [Settings/Alarms] menu of the optical channel analyzer by selecting the “Power Alarms” or “Wavelength Alarms” button. The maximum and minimum tolerated values are displayed in a table format, as shown in Figure 4. These alarm limits can be adjusted to be more or less restrictive than the ITU-T standard. This can be done directly in the tables shown in Figure 4.
Based on this alarm menu, you can choose to compare the CWDM network performance with the target values defined in the ITU-T Recommendations, or define your own criteria, depending on your architecture and objectives.
Red indicators will then be displayed in each graph and table, according to the power and wavelength limits recommended by ITU-T G.695, as shown in Figure 5.
Previously, a channel was simulated outside the target values, according to application code B-1D2-C8S1, at the start of the test, with a power excess of 7 dBm at a center wavelength of 1544.0 nm, outside the permitted band for the 1551 nm CWDM channel. The detected channel power and wavelength values appear in red on both the graphic display and the table of the optical channel analyzer. They also appear in alarm status at the start of the deviation test, and we can see on the analyzer's deviation displays that the channel moves out of the alarm zones after a period, due, for example, to environmental changes. With these intuitive displays, the OCA analyzer can provide a complete picture of a CWDM network, both instantaneously and over time, to facilitate link commissioning and troubleshooting.
Conclusion:
As CWDM networks become more common, the data they carry is not only more important but also far more critical. Standards are designed to allow flexibility in critical network components, enabling more cost-effective network elements, which is essential in metropolitan and regional networks. Testing these networks has historically been avoided due to its cost and complexity using optical spectrum analyzer technology, leading many to adopt the power meter plus filter testing methodology. Anritsu has introduced the right tool for installing and maintaining CWDM networks, making it significantly cheaper than using an optical spectrum analyzer and eliminating the inherited complexities of the power meter filter method.
MT9090A Network Master
Optical Channel Analyzer from Anritsu (Figure 6) finally addresses this need, providing all the features and performance required for the installation and maintenance of CWDM networks in a compact, modular test suite. It offers unparalleled value and ease of use without compromising performance. The MT9090A provides a clear overview of the power levels and wavelengths of all 18 CWDM channels at a glance, with easy comparison of PASS/FAIL indicators. It also enables long-term characterization with various deviation functions.
The MT9090A with the MU909020A module is an essential tool for CWDM networks. Its ease of use, low price, measurement speed, robustness, and size make it the perfect product for CWDM field testing.
Key Features:
• Dedicated tool for the installation, commissioning, and troubleshooting of CWDM networks.
• Fast and accurate view of all CWDM channels and channel drift over time.
• Fully compliant with ITU-T G.695 and G.694.2 standards.
• Pre-stored or user-defined limit values for PASS/FAIL testing.
• Power measurement operation with total incident power measurement.
• Short start-up and warm-up times, and long battery life, for rapid field characterization of CWDM networks.
• High-resolution, wide-color display for easy reading indoors or outdoors.
• Rugged, sealed construction for years of service in the harshest environments.
• Modular platform ensures maximum return on investment.
• Compact and lightweight design for maximum portability in the field.
• High performance at a low cost.
Author: Juergen Rummelsberger (Anritsu)
