Typically, Ethernet signals and CATV signals are transported on separate fibers to the customers in point-to-point fiber-to-the-home (FTTH) networks. This is often not desirable, because it needs double the fiber in the field and, perhaps even more important, it requires double the fiber handling in the field and at the customer.

To reduce the required fiber and the fiber handling the Ethernet signal and the CATV signal can be combined onto a single fiber. The upstream data signal, the downstream data signal, and the CATV signal can be combined on a single fiber when these signals use different wavelengths. The most suitable wavelengths that can be used to combine the signals are 1310nm and 1490nm for the upstream and the downstream data signals and 1550nm for the CATV signal.

One problem is that both the 100Mb/s Ethernet signal as well as the CATV signal use 1550nm as a transmission wavelength¹. The downstream transmission window for Ethernet therefore has to be reduced to the subset of 1480-1500nm which is still compliant to the IEEE 802.3ah standard. This reduced transmission window requires the use of Distributed FeedBack (DFB) laser diodes which have a narrow emission spectrum. Note that the CATV distribution wavelength has to remain around 1550nm, because optical amplification is available for this transmission band only. For gigabit Ethernet, the downstream transmission window is already specified by the IEEE to fit in the 1480-1500nm window (IEEE 802.3ah 1000baseBX-10).

¹ The IEEE 802.3ah 100baseBX-10 standard specifies a downstream transmission window from 1480-1580nm and an upstream transmission window from 1260-1360nm. These wide transmission windows are defined historically, because FP laserdiodes are widely available in this window. FP laserdiodes can be manufactured at low-cost, which is an important requirement for FTTH networks. The drawback of this type of laserdiodes is the fact they show large variations on the actual emission wavelength.

Combining Ethernet and CATV signals

To realize a PtP network based on single fiber, the wavelength pair 1310/1490nm used for Ethernet communication has to be combined with the wavelength 1550nm for CATV distribution in the central office. In the CPE at the customer these signals have to be separated again.

Solution in the central office

Besides the 1490nm laser, additional equipment for combining Ethernet and CATV is required in the central office. Normally, an optical splitter is used for distributing CATV to a large number of customers. This splitter can be combined with wavelengths filter elements to combine the Ethernet and the CATV signals onto a single fiber. By combining the functionalities the central office footprint is not increased.

Solution at the customer

Separation of the Ethernet and CATV signals is done fully optically by using a wavelength filter. The incoming single fiber transports both the Ethernet and the CATV signal to the filter device which is mounted into the FTU of the CPE. The filter splits the incoming signal in an Ethernet signal which is forwarded to the Ethernet Gateway and in a CATV signal, which is forwarded to the CATV receiver.

Conclusion

A PtP network based on single fiber enables to offer customers Ethernet and CATV on a single fiber connection. This single fiber connection reduces the fiber handling significantly. Furthermore, the concept of single fiber is future-proof, because operators can move seamlessly towards gigabit Ethernet connections, without any re-design of the passive network. Taken all together, a single fiber network is considered to be very cost-effective and future-proof.