1490nm vs 1577nm

1490 nm and 1577 nm are the two downstream wavelengths used in passive optical networks. 1490 nm is the downstream wavelength of GPON, carrying data from the Optical Line Terminal (OLT) to the Optical Network Terminal (ONT). 1577 nm is the downstream wavelength of XGS-PON, carrying 10 Gbps downstream over the same optical distribution network.

Both wavelengths serve the same function—downstream transmission from the OLT to the ONT—but they belong to different PON generations. GPON uses 1490 nm downstream and 1310 nm upstream. XGS-PON uses 1577 nm downstream and 1270 nm upstream. The two wavelength pairs are separated by wavelength division multiplexing, allowing GPON and XGS-PON to coexist on the same fiber and enabling gradual migration from GPON to XGS-PON without replacing the optical distribution network.

The differences between 1490 nm and 1577 nm affect fiber attenuation, chromatic dispersion, laser technology, receiver design, power budget, and coexistence planning. Understanding these differences clarifies why XGS-PON chose 1577 nm rather than reusing 1490 nm, and how operators manage the transition from GPON to XGS-PON.

1. Wavelength Fundamentals

1490 nm is in the upper O-band, near the edge of the low-loss window. 1577 nm is in the lower C-band, close to the conventional DWDM band. Both wavelengths are transmitted over standard single-mode fiber (G.652) and are separated from the upstream wavelengths by wavelength division multiplexing.

2. PON Generation and Downstream Role

1490 nm and 1577 nm are both downstream wavelengths, but they belong to different PON generations and support different data rates.

2.1 1490 nm in GPON

GPON uses 1490 nm for downstream transmission at 2.5 Gbps. The OLT transmits continuously on 1490 nm, and the signal is broadcast to all ONTs on the PON tree through passive splitters. Each ONT receives the full downstream signal and filters the frames addressed to it based on the ONT identifier.

The 1490 nm wavelength was chosen for GPON because it offers lower fiber attenuation than 1310 nm, which helps the downstream signal reach the farthest ONT. It also coexists with the 1310 nm upstream and the 1550 nm RF video overlay, forming a three-wavelength plan that has been the foundation of PON access networks for nearly two decades.

2.2 1577 nm in XGS-PON

XGS-PON uses 1577 nm for downstream transmission at 10 Gbps. The OLT transmits continuously on 1577 nm, and the signal is broadcast to all ONTs on the PON tree. The 1577 nm wavelength is chosen to coexist with GPON's 1490 nm downstream, allowing XGS-PON and GPON to share the same fiber plant.

XGS-PON's higher data rate—10 Gbps versus GPON's 2.5 Gbps—requires a higher-quality laser and receiver. The 1577 nm wavelength benefits from the lower fiber attenuation of the C-band, which partially compensates for the higher dispersion at this wavelength.

3. Attenuation and Dispersion

Attenuation and dispersion determine the reach and power budget of each downstream wavelength.

3.1 Attenuation

Standard single-mode fiber attenuation is approximately 0.25 dB/km at 1490 nm and approximately 0.20 dB/km at 1577 nm. The lower attenuation at 1577 nm provides a small advantage for long-reach PON deployments. Over a 20 km link, the difference is approximately 1 dB, which is modest but meaningful in tight power budgets.

3.2 Chromatic Dispersion

Chromatic dispersion is approximately 8 ps/nm/km at 1490 nm and approximately 17 ps/nm/km at 1577 nm. The higher dispersion at 1577 nm is a trade-off for the lower attenuation. At 10 Gbps, the dispersion penalty at 1577 nm is manageable over the PON reach, but it requires careful transmitter design and may require dispersion compensation in some cases.

4. Laser Sources and Transmitter Design

The laser sources differ between the two wavelengths because of the different data rates and the different requirements of GPON and XGS-PON transmitters.

4.1 1490 nm Laser Sources

GPON OLT transmitters at 1490 nm use DFB or EML lasers operating at 2.5 Gbps. These lasers are mature, high-volume products with competitive pricing. The lower data rate allows the use of simpler and less expensive laser designs.

4.2 1577 nm Laser Sources

XGS-PON OLT transmitters at 1577 nm use EML or directly modulated lasers (DMLs) operating at 10 Gbps. The higher data rate requires a laser with higher modulation bandwidth and lower chirp. EMLs are preferred because they integrate the laser and the modulator on the same chip, providing high extinction ratio and low chirp, which is essential for 10 Gbps transmission over the PON reach.

5. Coexistence and Wavelength Plan

1490 nm and 1577 nm are designed to coexist on the same fiber plant. This coexistence is the foundation of the migration path from GPON to XGS-PON.

5.1 GPON Wavelength Plan

The GPON wavelength plan consists of 1490 nm downstream, 1310 nm upstream, and 1550 nm for RF video overlay. These three wavelengths are separated by wavelength division multiplexers at the OLT and ONT.

5.2 XGS-PON Wavelength Plan

The XGS-PON wavelength plan consists of 1577 nm downstream and 1270 nm upstream. These wavelengths are chosen to avoid overlap with the GPON wavelengths, allowing GPON and XGS-PON to coexist on the same fiber.

5.3 Coexistence Element

A coexistence element (CE) is a passive optical device that combines the GPON and XGS-PON wavelengths onto a single fiber and separates them at the subscriber end. The CE uses thin-film filters or arrayed waveguide gratings to route each wavelength to the appropriate PON interface. This allows an operator to deploy XGS-PON alongside GPON and migrate subscribers gradually.

6. Power Budget and Reach

The power budget of a PON is determined by the OLT transmitter output power, the splitter loss, the fiber attenuation, the connector losses, and the ONT receiver sensitivity. The two downstream wavelengths have different attenuation and dispersion characteristics, but the overall power budget is designed to support the same reach and split ratio for both GPON and XGS-PON.

For GPON with a 1:32 split, the power budget is typically 28 dB, supporting a reach of up to 20 km. XGS-PON has a slightly higher power budget of 29 to 33 dB, partly because the 1577 nm wavelength has lower fiber attenuation and partly because the higher data rate requires more transmitter power and receiver sensitivity. With a 1:32 split, XGS-PON can reach 20 km; with a 1:64 split, it can also reach 20 km; with a 1:128 split, the reach is typically 10 to 15 km.

7. Applications

1490 nm and 1577 nm serve the downstream direction of GPON and XGS-PON respectively. The application determines which wavelength is used.

7.1 1490 nm Applications

• GPON downstream: OLT to ONT at 2.5 Gbps.

• Residential broadband: FTTH services using GPON.

• Triple-play services: Data, voice, and video over GPON.

• Business broadband: GPON-based business services.

• Mobile backhaul: GPON-based backhaul for small cells.

7.2 1577 nm Applications

• XGS-PON downstream: OLT to ONT at 10 Gbps.

• 10G residential broadband: FTTH services using XGS-PON.

• Business services: Symmetrical 10G for enterprises.

• Mobile backhaul and fronthaul: XGS-PON for 5G transport.

• Wholesale access: XGS-PON for open access networks.

8. Comparison Summary

9. Selection Framework

10. Emerging Trends

10.1 50G-PON Wavelength Plan

50G-PON defines several wavelength options for downstream and upstream. One option uses 1342 nm for downstream and 1310 nm for upstream, maintaining compatibility with the GPON wavelength plan and allowing coexistence with GPON and XGS-PON. The 50G-PON standard supports coexistence with previous generations on the same fiber plant, and the coexistence elements will need to handle the additional wavelengths.

10.2 Higher-Speed XGS-PON

XGS-PON is being deployed at scale worldwide, and the 1577 nm downstream wavelength is becoming the mainstream for new FTTH builds. As XGS-PON volumes increase, the cost of 1577 nm EML lasers and receivers is declining, narrowing the cost gap with GPON.

10.3 Coexistence Element Integration

Coexistence elements that combine GPON, XGS-PON, and RF video onto the same fiber are becoming more integrated and lower cost. These elements use thin-film filters or arrayed waveguide gratings to separate and combine the wavelengths, allowing multiple PON generations to share the same optical distribution network.

10.4 Wavelength Plans for Future PON

Future PON generations beyond 50G-PON will require new wavelength plans. The ITU-T is studying higher-speed PON options that may use different wavelengths in the O-band, C-band, or L-band. The 1490 nm and 1577 nm wavelengths will remain in use for GPON and XGS-PON respectively, and coexistence with future generations will be a key requirement.

11. Summary

1490 nm and 1577 nm are the downstream wavelengths of GPON and XGS-PON respectively. 1490 nm carries 2.5 Gbps downstream from the OLT to the ONT in GPON, paired with 1310 nm upstream. 1577 nm carries 10 Gbps downstream from the OLT to the ONT in XGS-PON, paired with 1270 nm upstream.

The two wavelengths are separated by wavelength division multiplexing, allowing GPON and XGS-PON to coexist on the same fiber. This coexistence is the foundation of the migration path from GPON to XGS-PON, enabling operators to deploy XGS-PON alongside existing GPON equipment and migrate subscribers gradually without replacing the optical distribution network.

1490 nm has lower chromatic dispersion and slightly higher fiber attenuation than 1577 nm. 1577 nm has lower fiber attenuation and higher chromatic dispersion. The lower attenuation at 1577 nm provides a small advantage for long-reach PON deployments, while the lower dispersion at 1490 nm simplifies transmitter design for GPON.

For new FTTH deployments, 1577 nm (XGS-PON) is the mainstream choice, offering 10 Gbps downstream and symmetrical 10 Gbps upstream. For existing GPON deployments, 1490 nm remains in use, and the migration to 1577 nm XGS-PON is enabled by coexistence elements and dual-mode ONTs.

12. Q&A