1310nm vs 1490nm

1310 nm and 1490 nm are the two primary wavelengths in Passive Optical Network (PON) access systems. 1310 nm carries upstream traffic from the Optical Network Terminal (ONT) at the subscriber premises to the Optical Line Terminal (OLT) at the central office. 1490 nm carries downstream traffic from the OLT to the ONT. The two wavelengths travel in opposite directions on the same single-mode fiber, separated by wavelength division multiplexing.

The pairing of 1310 nm for upstream and 1490 nm for downstream is standardized in GPON (ITU-T G.984), XG-PON (G.987), and XGS-PON (G.9807.1). The choice of these wavelengths is driven by fiber attenuation, laser availability, chromatic dispersion, and the need to coexist with other services such as RF video overlay at 1550 nm.

Understanding the characteristics of each wavelength clarifies why the upstream and downstream directions use different wavelengths, how the OLT and ONT separate the two signals, and how PON systems evolve from GPON to XGS-PON to 50G-PON without replacing the fiber plant.

1. Wavelength Fundamentals

1310 nm is in the O-band, near the zero-dispersion wavelength of standard single-mode fiber. 1490 nm is also in the O-band but closer to the edge of the low-loss window. Both wavelengths are transmitted over standard single-mode fiber (G.652) using wavelength division multiplexing to separate the two directions.

2. Upstream and Downstream Direction

The most fundamental distinction between the two wavelengths is direction. 1310 nm carries upstream traffic; 1490 nm carries downstream traffic. This directional split is what allows a single fiber to carry bidirectional traffic in a PON.

2.1 1310 nm Upstream

In the upstream direction, each ONT transmits at 1310 nm. Because multiple ONTs share the same fiber and the same wavelength, the OLT schedules upstream transmission using time-division multiple access (TDMA). Each ONT is allocated time slots during which it can transmit, and the OLT receives bursts from different ONTs in sequence.

The 1310 nm wavelength is chosen for upstream because it has near-zero chromatic dispersion and because low-cost uncooled DFB and DML lasers are available at this wavelength. The burst-mode operation requires the OLT receiver to rapidly adjust its threshold and timing for each ONT's burst, which is a specialized capability not required in the downstream direction.

2.2 1490 nm Downstream

In the downstream direction, the OLT transmits continuously at 1490 nm. The signal is broadcast to all ONTs on the PON tree, and each ONT filters the traffic destined for it based on the ONT identifier in the frame header.

The 1490 nm wavelength is chosen for downstream because it has lower attenuation than 1310 nm and because it can coexist with the 1310 nm upstream and the 1550 nm RF video overlay. The continuous-mode operation allows the ONT receiver to use a simpler design than the burst-mode OLT receiver.

3. Attenuation and Dispersion

Attenuation and dispersion determine the reach and power budget of each direction.

3.1 Attenuation

Standard single-mode fiber has an attenuation of approximately 0.35 dB/km at 1310 nm and approximately 0.25 dB/km at 1490 nm. The lower attenuation at 1490 nm provides a slight advantage for the downstream direction, which is beneficial because the OLT transmitter serves all ONTs on the PON tree and the downstream signal must reach the farthest subscriber.

3.2 Chromatic Dispersion

Chromatic dispersion in standard single-mode fiber is near zero at 1310 nm and approximately 8 ps/nm/km at 1490 nm. The low dispersion at 1310 nm simplifies the upstream burst-mode receiver design, while the moderate dispersion at 1490 nm is manageable for the downstream data rates used in GPON and XGS-PON.

4. Laser Sources

The laser sources differ between the two wavelengths because of the different requirements of upstream burst-mode transmission and downstream continuous-mode transmission.

4.1 1310 nm Laser Sources

Upstream ONT transmitters use low-cost uncooled DFB or directly modulated lasers (DMLs) at 1310 nm. The lasers must support burst-mode operation, turning on and off rapidly and maintaining wavelength stability over the burst. Uncooled operation is possible because the 1310 nm wavelength has low dispersion and the PON power budget tolerates the wavelength drift.

4.2 1490 nm Laser Sources

Downstream OLT transmitters use DFB or EML lasers at 1490 nm. The lasers operate continuously and must deliver high output power to reach all ONTs on the PON tree. Temperature control may be used to maintain wavelength stability, although the requirements are less stringent than for DWDM systems.

5. PON Standards and Wavelength Plans

The 1310/1490 nm pairing is standardized across multiple PON generations. GPON, XG-PON, and XGS-PON all use 1310 nm for upstream and 1490 nm for downstream, with additional wavelengths for video overlay and for coexistence with other PON technologies.

XG-PON and XGS-PON use different wavelengths—1270 nm upstream and 1577 nm downstream—to allow coexistence with GPON on the same fiber. This wavelength plan allows an operator to deploy XGS-PON alongside GPON and migrate subscribers gradually without replacing the fiber plant.

6. Wavelength Division Multiplexing and Coexistence

In a PON, the OLT and ONT separate the upstream and downstream wavelengths using wavelength division multiplexers. The OLT has a WDM that combines the 1490 nm downstream transmitter and the 1310 nm upstream receiver onto a single fiber. The ONT has a WDM that separates the 1490 nm downstream from the 1310 nm upstream and routes each to the appropriate component.

Coexistence with other PON technologies requires additional WDM elements. A coexistence element at the OLT combines GPON (1490/1310 nm), XGS-PON (1577/1270 nm), and RF video (1550 nm) onto the same fiber. At the subscriber end, a dual-mode ONT or a coexistence module separates the wavelengths and routes them to the appropriate PON interface.

7. 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 wavelengths have different attenuation and different transmitter and receiver characteristics, but the overall power budget is designed to support the same reach and split ratio for both directions.

For GPON with a 1:32 split, the power budget is typically 28 dB, supporting a reach of up to 20 km. The 1490 nm downstream benefits from lower fiber attenuation, while the 1310 nm upstream benefits from lower dispersion. The OLT transmitter at 1490 nm must deliver enough power to reach the farthest ONT, and the ONT transmitter at 1310 nm must deliver enough power to reach the OLT through the same splitter and fiber.

8. Applications

The 1310/1490 nm pairing is used in all PON-based access networks. The two wavelengths serve the upstream and downstream directions of the same service.

8.1 1310 nm Applications

• GPON upstream: ONT to OLT at 1.25 Gbps.

• XG-PON upstream: ONT to OLT at 2.5 Gbps.

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

• 50G-PON upstream: ONT to OLT at 50 Gbps (option).

• Point-to-point upstream: Some dedicated access links.

8.2 1490 nm Applications

• GPON downstream: OLT to ONT at 2.5 Gbps.

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

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

• 50G-PON downstream: OLT to ONT at 50 Gbps (option).

• RF video overlay: 1550 nm is used for video, not 1490 nm.

9. Comparison Summary

10. Selection Framework

11. Emerging Trends

11.1 50G-PON Wavelength Options

50G-PON defines several wavelength options. 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.

11.2 Higher-Speed Upstream

As upstream demand grows—driven by video conferencing, cloud backup, and remote work—the upstream speed of PON systems is increasing. GPON provides 1.25 Gbps upstream; XGS-PON provides 10 Gbps upstream; 50G-PON will provide 50 Gbps upstream. The 1310 nm wavelength continues to be used for upstream in GPON and in one 50G-PON option, while XGS-PON uses 1270 nm.

11.3 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.

12. Summary

1310 nm and 1490 nm are the two primary wavelengths in GPON and other PON access systems. 1310 nm carries upstream traffic from the ONT to the OLT, using burst-mode transmission and low-cost uncooled DFB or DML lasers. 1490 nm carries downstream traffic from the OLT to the ONT, using continuous-mode transmission and higher-power DFB or EML lasers.

The two wavelengths are separated by wavelength division multiplexing at the OLT and ONT, allowing bidirectional transmission over a single fiber. The 1310 nm upstream benefits from near-zero chromatic dispersion, while the 1490 nm downstream benefits from lower fiber attenuation. Both wavelengths support reaches of up to 20 km with a 1:32 split ratio in GPON.

XGS-PON uses a different wavelength pair—1270 nm upstream and 1577 nm downstream—to allow coexistence with GPON on the same fiber. The 1310/1490 nm pair remains the standard for GPON, while XGS-PON uses the 1270/1577 nm pair. Both pairs can coexist on the same optical distribution network through wavelength division multiplexing.

The choice between 1310 nm and 1490 nm is not a choice at all in a PON—both are used, one for each direction. The wavelengths are selected to optimize the upstream and downstream transmission characteristics and to allow coexistence with other services and PON generations.

13. Q&A