Introduction
In the rapidly evolving world of optical interconnects, interoperability and standardization are crucial for enabling seamless integration and widespread adoption. The CW-WDM (Continuous Wavelength Division Multiplexing) MSA (Multi-Source Agreement) aims to address this need by defining a comprehensive set of technical specifications for wavelength grid assignments, optical parameters, and measurement methods. This tutorial will provide a detailed exploration of the CW-WDM MSA, guiding you through the key aspects of this important industry standard.
Editor — Matt Sysak, Ayar Labs Associate Editor — John Johnson, Broadcom Associate Editor — David Lewis, Lumentum Chair — Chris Cole, Il-VI Incorporated
Understanding the Scope and Purpose
The CW-WDM MSA defines a set of wavelength grids in the O-band, covering three different wavelength spans: 9 nm, 18 nm, and 36 nm. These grids are designed to support a wide range of optical interconnect applications, such as artificial intelligence (AI), optical computing, machine learning, and high-density co-packaged optics. By standardizing the technical details of these wavelength grids, the CW-WDM MSA aims to ensure interoperability across various optical interconnect systems and facilitate seamless integration.
Optical Source Configurations
The CW-WDM MSA describes two possible configurations for optical sources: modular optical sources and integrated optical sources.
Modular optical sources have each output port carrying a single wavelength from a grid set, while integrated optical sources have each output port carrying all the wavelengths of a grid set. Figure 1 in the specification illustrates these two configurations:
For modular optical sources, compliance is measured at the output fiber, designated as TP1. For integrated optical sources, compliance is measured at the output of the optical distribution network, designated as TP1a.
Wavelength Grid Assignments
The CW-WDM MSA defines eight wavelength grid sets, each with a different number of channels and channel spacing, as shown in Tables 1, 2, and 3:
Table 1 9 nm wavelength span lane assignments
8 + 1 wavelength grid | 16 + 1 avelength gridw | |||
Lane | Center wavelegth (nm) | Frequency (THz) | Center wavelength (nm) | Frequency (THz) |
L-16 | ||||
L-15 | ||||
L-14 | ||||
L-13 | ||||
L-12 | ||||
L-11 | ||||
L-10 | ||||
L-9 | ||||
L-8 | 1295.56a | 231.4a | ||
L-7 | 1296.12 | 231.3 | ||
L-6 | 1296.68 | 231.2 | ||
L-5 | 1297.24 | 231.1 | ||
L-4 | 1295.56a | 231.4a | 1297.80 | 231.0 |
L-3 | 1296.68 | 231.2 | 1298.36 | 230.9 |
L-2 | 1297.80 | 231.0 | 1298.93 | 230.8 |
L-1 | 1298.93 | 230.8 | 1299.49 | 230.7 |
L-0 | 1300.05 | 230.6 | 1300.05 | 230.6 |
L1 | 1301.18 | 230.4 | 1300.62 | 230.5 |
L2 | 1302.31 | 230.2 | 1301.18 | 230.4 |
L3 | 1303,45 | 230.0 | 1301.75 | 230_3 |
L4 | 1304.58 | 229.8 | 1302.31 | 230.2 |
L5 | 1302.88 | 230.1 | ||
L6 | 1303.45 | 230.0 | ||
L7 | 1304.01 | 229.9 | ||
L8 | 1304.58 | 229.8 | ||
L9 | ||||
L10 | ||||
L11 | ||||
L12 | ||||
L13 | ||||
L14 | ||||
L15 | ||||
L16 |
Table 2 18 nm wavelength span lane assignments
8 + 1 wavelength grid | 16 + 1 wavelength grid | 32 + 1 wavelength grid | ||||
Lane | Center wavelength (nm) | Frequency (THz) | Center wavelength (nm) | Frequency (THz) | Center wavelength (nm) | Frequency (THz) |
L-16 | 1291.1a | 232.2a | ||||
L-15 | 1291.65 | 232.1 | ||||
L-14 | 1292.21 | 232.0 | ||||
L-13 | 1292.77 | 231.9 | ||||
L-12 | 1293.32 | 231.8 | ||||
L-11 | 1293.88 | 231.7 | ||||
L-10 | 1294.44 | 231.6 | ||||
L-9 | 1295.00 | 231.5 | ||||
L-8 | 1291.1a | 232.2a | 1295.56 | 231.4 | ||
L-7 | 1292.21 | 232.0 | 1296.12 | 231.3 | ||
L-6 | 1293.32 | 231.8 | 1296.68 | 231.2 | ||
L-5 | 1294.44 | 231.6 | 1297.24 | 231.1 | ||
L-4 | 1291.1a | 232.2a | 1295.56 | 231.4 | 1297.80 | 231.0 |
L-3 | 1293.32 | 231.8 | 1296.68 | 231.2 | 1298.36 | 230.9 |
L-2 | 1295.56 | 231.4 | 1297.80 | 231.0 | 1298.93 | 230.8 |
L-1 | 1297.80 | 231.0 | 1298.93 | 230.8 | 1299.49 | 230.7 |
L0 | 1300.05 | 230.6 | 1300.05 | 230.6 | 1300.05 | 230.6 |
L1 | 130131 | 230.2 | 1301.18 | 230.4 | 1300.62 | 230.5 |
L2 | 1304.58 | 229.8 | 1302.31 | 230.2 | 1301.18 | 230.4 |
L3 | 1306.85 | 229.4 | 1303.45 | 230.0 | 1301.75 | 230.3 |
L4 | 1309.14 | 229.0 | 1304.58 | 229.8 | 1302.31 | 230.2 |
L5 | 1305.72 | 229.6 | 1302.88 | 230.1 | ||
L6 | 1306.85 | 229.4 | 1303.45 | 230.0 | ||
L7 | 1308,00 | 229.2 | 1304,01 | 229.9 | ||
L8 | 1309.14 | 229.0 | 1304.58 | 229.8 | ||
L9 | 1305.15 | 229.7 | ||||
L10 | 1305.72 | 229.6 | ||||
L11 | 1306.29 | 229.5 | ||||
L12 | 1306.85 | 229.4 | ||||
L13 | 1307.42 | 229.3 | ||||
L14 | 1308.00 | 229.2 | ||||
L15 | 1308.57 | 229.1 | ||||
L16 | 1309.14 | 229.0 |
Table 3 36 nm wavelength span lane assignments
8 + 1 wavelength grid | 16 + 1 wavelength grid | 32 + 1 wavelength grid | ||||
Lane | Center wavelength (nm) | Frequency (THz) | Center wavelength (nm) | Frequency (THz) | Center wavelength (nm) | Frequency (THz) |
L-16 | 1282.26a | 233.8a | ||||
L-15 | 1283.36 | 233.6 | ||||
L-14 | 1284.46 | 233.4 | ||||
L-13 | 1285.56 | 233.2 | ||||
L-12 | 1286.66 | 233.0 | ||||
L-11 | 1187.77 | 232.8 | ||||
L-10 | 1288.88 | 232.6 | ||||
L-9 | 1289.98 | 232.4 | ||||
L-8 | 1282.26a | 233.8a | 1291.10 | 232.2 | ||
L-7 | 1284.46 | 233.4 | 1292.21 | 232.0 | ||
L-6 | 1286.66 | 233.0 | 1293.32 | 231.8 | ||
L-5 | 1288.88 | 232.6 | 1194.44 | 231.6 | ||
L-4 | 1282.26a | 233.8a | 1291.10 | 232.2 | 1295.56 | 231.4 |
L-3 | 1286.66 | 233.0 | 1293.32 | 231.8 | 1296.68 | 231.2 |
L-2 | 1291.10 | 232.2 | 1295.56 | 231.4 | 1297.80 | 231.0 |
L-1 | 1295.56 | 231.4 | 1297.80 | 231.0 | 1198.93 | 230.8 |
L0 | 1300.05 | 230.6 | 1300.05 | 230.6 | 1300.05 | 230.6 |
L1 | 1304.58 | 229.8 | 1302.31 | 230.2 | 1301.18 | 230.4 |
L2 | 1309.14 | 229.0 | 1304.58 | 229.8 | 1302.31 | 230.2 |
L3 | 1313.73 | 228.2 | 1306.85 | 229.4 | 1303.45 | 230.0 |
L4 | 1318.35 | 227.4 | 1309.14 | 229.0 | 1304.58 | 229.8 |
L5 | 1311.43 | 228.6 | 1305.72 | 229.6 | ||
L6 | 1313.73 | 228.2 | 1306.85 | 229.4 | ||
L7 | 1316.03 | 227.8 | 1308.00 | 229.2 | ||
L8 | 1318.35 | 227.4 | 1309.14 | 229.0 | ||
L9 | 1310.28 | 228.8 | ||||
L10 | 131 1.43 | 228.6 | ||||
L11 | 1312.58 | 228.4 | ||||
L12 | 1313.03 | 228.2 | ||||
L13 | 1314.88 | 228.0 | ||||
L14 | 1316.03 | 277.8 | ||||
L15 | 1317.19 | 277.6 | ||||
L16 | 1318.35 | 227.4 |
These grid sets include 8+1, 16+1, and 32+1 wavelengths in 9 nm, 18 nm, and 36 nm wavelength spans, respectively. The shortest wavelength in each grid set is optional, as indicated by the "a" superscript in the tables.
The CW-WDM MSA also specifies the port assignments for both modular and integrated optical sources, as shown in Table 4:
Table 4 Output port configurations
Optical source type | Wavelengths | Number of Ports |
Modular | 8 + 1 | 8 + 1 |
16+1 | 16+1 | |
32+1 | 32+1 | |
Integrated | 8 + 1 | AS |
16+1 | ||
32+1 |
Optical Specifications
The CW-WDM MSA defines a comprehensive set of optical specifications that must be met by both modular and integrated optical sources. These specifications are outlined in Tables 5 and 6:
Table 5 Grid specifications
Description | Wavelength span (nm) | Number of channels | Channel spacing (GHz) | Channel bandwidth (GHz) |
Grid definition | 9 | 8+1 | 200 | 100 |
16+1 | 100 | 50 | ||
18 | 8+1 | 400 | 200 | |
16+1 | 200 | 100 | ||
32+1 | 100 | 50 | ||
36 | 8+1 | 800 | 400 | |
16+1 | 400 | 200 | ||
32+1 | 200 | 100 |
Table 6 Optical source specifications
Description | Value | Unit |
Nominal wavelength range, 9nm span | 1295.56 to 1304.58 | nm |
Nominal wavelength range, 18nm span a | 1291.1 to 1309.14 | nm |
Nominal wavelength range, 36nm span a | 1282.26 to 1318.35 | nm |
Nominal center wavelength a | 1300.05 | nm |
Center wavelength offset range a | AS | nm |
Center wavelength variation range b | AS | nm |
Side-mode suppression ratio (SMSR), (min) | AS | dB |
Optical return loss tolerance (max) c | AS | dB |
Relative intensity noise (per wavelength) | AS | dB/Hz |
Optical linewidth (max) | AS | MHz |
Optical port reflectance, each port (max) d | AS | dB |
Launch power variation, each port (max) | AS | dB |
The key optical parameters include:
Wavelength: The center wavelength shall be measured with all ports and all laser sources active, following the measurement methods described in IEC 61280-1-3.
Output Power: The optical output power shall be measured using the methods given in IEC 61280-1-1, without any patterns applied.
Relative Intensity Noise (RIN): The RIN measurement follows the general description provided in IEEE Std. 802.3-2018, with modifications to the electrical receiver bandwidth and the use of an optical filter to isolate individual wavelengths for integrated optical sources.
Side Mode Suppression Ratio (SMSR): SMSR is defined as the ratio of the average optical power in the dominant longitudinal mode to the optical power of the most significant side mode, in the presence of worst-case reflections. For integrated optical sources, an additional SMSR requirement refers to the power that exists outside the grid.
Linewidth: The linewidth is defined as the Lorentzian component of the optical noise spectrum, related to the white phase noise component of the optical field. It is measured using a self-heterodyne (3.5 μs delay) technique, as described in the OIF-ITLA-MSA-01.3 standard.
The CW-WDM MSA also defines three power classes (Type 1, Type 2, and Type 3) for the maximum optical power levels, as shown in Table 7:
Table 7 Maximum output power classes
Output power class a | 9 nm span Maximum optical power | 18 nm span Maximum optical power | 36 nm span Maximum optical power | Units |
Type 1 | 20 | 20 | 14 | dBm |
Type 2 | 14 | 14 | 8 | dBm |
Type 3 | 26 | 26 | 20 | dBm |
These power classes are intended to comply with the eye safety limits outlined in the IEC 60825-1 2014 standard.
Optical Parameter Measurement Methods
The CW-WDM MSA provides detailed guidelines for measuring the optical parameters, ensuring consistency and repeatability across different manufacturers and applications.
Wavelength Measurement: The center wavelength shall be measured with all ports and all laser sources active, following the methods outlined in TIA/EIA-455-127-A or IEC 61280-1-3.
Output Power Measurement: The optical output power shall be measured using the methods given in IEC 61280-1-1, without any patterns applied.
Relative Intensity Noise Measurement: The RIN measurement follows the general description in IEEE Std. 802.3-2018, with specific modifications for the electrical receiver bandwidth and the use of optical filters to isolate individual wavelengths for integrated optical sources.
Side Mode Suppression Ratio Measurement: SMSR is defined as the ratio of the average optical power in the dominant longitudinal mode to the optical power of the most significant side mode, in the presence of worst-case reflections.
Linewidth Measurement: The linewidth is measured using a self-heterodyne (3.5 μs delay) technique, as described in the OIF-ITLA-MSA-01.3 standard.
Grid Definitions: The CW-WDM MSA defines key parameters, such as grid spacing, channel bandwidth, and the allowable shift in the grid over environmental conditions, as illustrated in Figure 2.
Safety Requirements
The CW-WDM MSA requires optical sources to comply with the eye safety limits outlined in the IEC 60825-1 2014 standard. This ensures that the optical power levels do not pose a risk to user safety when accessing the optical sources.
Optical Fiber Cable Requirements
The CW-WDM MSA is compatible with both standard single-mode fiber and polarization-maintaining fiber, allowing for the use of standard APC and PC connections.
Identification and Color Coding
The CW-WDM MSA defines a color coding scheme to visually identify the type of optical source, as shown in Table 6-1:
Table 8 Color Coding
Color Code | Application |
Red | Modular optical |
Blue | Integrated optical |
Red is used for modular optical sources, while blue is used for integrated optical sources.
Example Applications
The CW-WDM MSA provides several informative example applications to illustrate the use of the defined specifications. These examples showcase different use cases and the corresponding optical source configurations and specifications.
Flexible Wavelength Links
The first example application allows for wide variation in center wavelength, including laser wavelength drift over temperature. Table 9 and Table 10 provide the example specifications for modular and integrated optical sources, respectively:
Table 9 Informative modular optical source specifications
Description | Value | Unit |
Grid spacing and channel bandwidth ( 18nm span) | 400 ± 10 (8+1 Set) 200 ± 50 (16+1 set) 100 ± 25 (32+1 set) | GHz |
Nominal wavelength range ( 18nm span) a | 1291.1 to 1309.14 | nm |
Launch NW-cr. cach wavclcngth Onax. Snrn span) b | 14 | dBm |
Nominal center a | 1300.05 | nm |
wavelength offset range a | ±5 | nm |
Launch power variation. each wavelength (max) | ±1 | dB |
Siderrnodc suppression ratio (SMSR). (min) | 30 | dB |
Relative intensity noise (per wavelength) c | -135 | dB/Hz |
Optical lincwidth (max) c | 20 | MHz |
Optical return loss tolerance (max) | -20 | dB |
wavelength variation range d | ±4 | nm |
Table 10 Informative output port configurations
Optical power supply type | Wavelengths | Number of Ports |
Modular | 8 | 8 |
16 | 16 | |
32 | 32 | |
Integrated | 8 | ≥8 |
16 | ≥16 | |
32 | ≥32 |
This example is designed to accommodate a wide range of operating conditions and applications that require flexibility in wavelength selection.
Fixed Wavelength Links
The second example application allows for narrow variation in center wavelength, including laser wavelength drift over temperature. Table 11 and Table 12 provide the example specifications for modular and integrated optical sources, respectively:
Table 11 Informative modular optical source specifications
Description | Value | Unit |
Grid spacing and channel bandwidth (18nm span) | 400 ± 100 (8+1 set) 200 ± 50 (16+ 1 set) 100 ± 25 (32+ 1 set) | GHz |
Nominal wavelength range (18nm span) a | 1291.1 to 1309.14 | nm |
Launch power, each wavelength (max, 18nm span) b | 14 | dBm |
Nominal center wavelength a | 1300.05 | nm |
Center wavelength offset range a | ± 0.5 | nm |
Launch power variation, each wavelength (max) | ± 1 | dB |
Side-mode suppression ratio (SMSR), (min) | 30 | dB |
Relative intensity noise (per wavelength) c | -135 | dB/Hz |
Optical linewidth (max) c | 20 | MHz |
Optical return loss tolerance (max) | -20 | dB |
Center wavelength variation range d | ±0.5 | nm |
Table 12 Informative output port configurations
Optical power supply type | Wavelengths | Number of Ports |
Modular | 8 | 8 |
16 | 16 | |
32 | 32 | |
Integrated | 8 | 28 |
16 | 216 | |
32 | 232 |
This example is suitable for applications that require more precise wavelength control, such as fixed-wavelength optical interconnects.
Dual ELS 8+8 Channel
The third example application describes a dual external laser source (ELS) configuration, where two physically separate 8-wavelength ELS units are combined to create a 16-wavelength source. Table 13 provides the example specifications for this dual ELS implementation:
Table 13 Informative Dual ELS 8+8 channel source specifications
This example highlights the flexibility of the CW-WDM MSA in accommodating more complex optical source configurations.
Informative Grid Definitions
In addition to the normative specifications, the CW-WDM MSA includes an informative 72-nm span O-band grid definition, as shown in Table 14:
Table 14 Informative 72 nm span CWDM lane assignments
Lane | Center Wavelength | Unit |
L0 | 1271 | nm |
L1 | 1281 | nm |
L2 | 1291 | nm |
L3 | 1301 | nm |
L4 | 1311 | nm |
L5 | 1321 | nm |
L6 | 1331 | nm |
L7 | 1341 | nm |
This grid, with a 10 nm channel spacing, is provided for consideration in the development of specifications for uncooled applications over a restricted temperature range.
Conclusion
The CW-WDM MSA is a comprehensive industry standard that aims to facilitate the adoption and interoperability of optical interconnect technologies. By defining detailed wavelength grid assignments, optical specifications, and measurement methods, the CW-WDM MSA provides a solid foundation for the development and integration of optical sources in various applications.
The flexibility offered by the example applications showcases the versatility of the CW-WDM MSA, catering to the diverse needs of the optical interconnect ecosystem. As the industry continues to evolve, the CW-WDM MSA will play a crucial role in enabling seamless integration and driving the widespread adoption of high-performance optical interconnect solutions.
Reference
[2] M. Sysak, J. Johnson, D. Lewis, and C. Cole, "CW-WDM MSA Technical Specifications Rev 1.0," June 4, 2021.
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