# Zero-water-peak fiber extends metro CWDM reach

After being proposed to standards bodies only two years ago, coarse wavelength-division multiplexing (CWDM) is introducing low-cost, commercial multiwavelength optical systems into short- and medium-haul metro-edge networks. The availability of G.652C fibers, such as a zero-water-peak fiber (ZWPF), has increased the number of available CWDM channels throughout the optical spectrum compared to conventional single-mode fiber (CSMF).^{1} Further, ZWPFs lower the cost of deploying CWDM systems in metro networks.

In nonamplified CWDM, fiber attenuation, fiber-splice loss, connector loss, and mux/demux losses limit the attainable system reach. The system reach is also dependent on the number of channels and channel bit rate, which dictate the power budget of a transmitter/receiver pair. For example, a 16-channel 2.5-Gbit/s hubbed-ring network using optical add/drop filters at four nodes can be limited to approximately 40 km in circumference, depending on component insertion losses and power budgets. However, because a significant percentage of metro-edge rings exceed a perimeter of 40 km, it is beneficial to extend CWDM system reach and hence the number of applications for nonamplified CWDM technology.

A wavelength-assignment technique compensates for the attenuation slope in fiber and therefore increases the reach of metro-local or edge rings using nonamplified CWDM. Conventional SMF has a nominal attenuation of less than 0.4 dB in the 1300-nm window, approximately 0.3 dB in the 1550-nm window, and an attenuation peak around 1400 nm due to hydroxyl (OH-) absorption. Zero-water-peak fibers permanently eliminate the water peak, resulting in low loss at 1400 nm (see Fig. 1). Consequently, a ZWPF offers up to 33% more CWDM capacity than CSMF and supports all 16 ITU G.694.2 CWDM channels on a single fiber.

The 16 channels of the ITU G.694.2 CWDM wavelength grid extend from 1310 nm in the O-band to 1610 nm in the L-band with 20-nm channel spacing (see table) . In a ZWPF, maximum attenuation loss is at 1310 nm and decreases as 1/λ^{4} (due to decreasing Rayleigh scattering and elimination of the OH content) to a minimum around 1550 nm in the C-band. The loss then increases slightly between 1550 and 1610 nm in the L-band. Hence fiber transmission loss incurred by the CWDM wavelength channels around the ring is a maximum in the O-band and decreases to a minimum in the C- and L-bands.

On the other hand, mux/demux loss (including connector and splice losses) is a function of the number of mux/demux filter stages and the number of spans that a wavelength encounters on its path between transmission and reception. Therefore, CWDM wavelength channels should be assigned to the nodes to minimize the filter losses around the ring in the high-attenuation O-band and maximized in the low attenuation C- and L-bands. For such wavelength assignment, the mux/demux losses will compensate for fiber attenuation, minimizing the combined fiber and filter losses, thereby maximizing the ring perimeter.

A case study of a 16-channel 4-node CWDM hubbed-ring network employing ZWPF demonstrates a 25% extension of the ring perimeter applying an intelligent wavelength assignment. In this network, four local central offices (CO) transmit/receive signals from a tandem exchange hub, each on a different set of four wavelengths (see Fig. 2, left). This configuration is equivalent to a logical star network (see Fig. 2, right), except all 16 wavelengths can be physically supported on a single ZWPF. The hubbed-ring configuration allows for easy implementation of either unidirectional or bidirectional protection schemes. Here we will assume a unidirectional ring.

A four-CO system has four bands of 4-wavelength channels each (see table). All 16 channels are added/dropped from the network at the hub. Each of the central-office nodes will have three express/through bands (12 λs) and one add/drop band of CWDM channels (see Fig. 3). The design minimizes the number of filters encountered in add/drops between the hub and central office and back. For example, at CO 1, the drop channels encounter 4 filters (2 add/drop channel filters and 2 express-band filters) from the hub to CO 1, while the add channels encounter 10 filters from CO 1 to the hub. Thus, a higher filter insertion loss path is encountered going from the central office back to the hub.

In the worst-case assignment of channels, bands 1, 2, 3, and 4 traverse a total of 10, 8, 8, and 10 mux/demux filters from transmission to reception, respectively. Hence, bands 1 and 4 incur filter losses that are higher than the losses incurred by bands 2 and 3, since the latter traverse fewer nodes. Therefore, in a given loss budget, bands 1 and 4 have less power margin for fiber loss than bands 2 and 3. Because fiber attenuation is highest in band 1 (O-band), the maximum attainable ring perimeter is constrained by band-1 wavelengths.

An optimized wavelength assignment occurs when bands 1, 2, 3, and 4 traverse 8, 8, 10, and 10 mux/demux filters from transmission to reception, respectively. Therefore bands 3 and 4 incur higher filter losses than bands 1 and 2. Hence, for a given loss budget, the higher-attenuation band 1 and 2 (O- and E-bands) are given more power margin for fiber loss than the lower-attenuation bands 3 and 4 (S-, C- and L-Band). Therefore, higher fiber loss in the O- and E-bands is counterbalanced by lower filter losses to minimize the total ring loss and maximize the ring perimeter.

**RING PERIMETER**

The ring perimeter is calculated from:

where n = 4 is the total number of central offices in the hubbed ring, P = -1 dBm is the optical source power, S = -29 dBm is the avalanche photodiode (APD) receiver sensitivity of a typical 2.5-Gbit/s APD, F(k) is the worst-case mux/demux filter loss incurred by wavelength channel λ_{i} assigned to CO k, C(k) is the total connector loss incurred by wavelength channel λ_{i} assigned to CO k (assume a 1-dB connector loss per span), and A(λ_{i}) is the fiber attenuation at wavelength λ_{i}. Assuming a 1-dB system margin for splicing and so forth, the loss budget is 28 dB.

In the worst-case channel assignment, by including the connector loss and a 1-dB power margin for splicing, mux/demux filter loss for bands 1 and 4 is 15.5 dB. For the available budget of 28 dB, this leaves 12.5 dB for fiber loss. The attained ring perimeter is limited to 48 km by the O-band channel, λ_{i}, at 1310 nm (see Fig. 4).

In the optimized channel-assignment case, the filter loss for bands 1 and 4 are 12.5 and 15.5 dB, respectively, which leaves 15.5 dB for fiber loss in the O-band. The lower filter loss of 3 dB in the O-band compensates for the higher fiber attenuation in the O-band, consequently maximizing the ring perimeter. The attained ring perimeter reaches 60 km in channel λ_{i} = 1310 nm, a full 25% extension compared to the worst-case channel assignment.

**Charles Ufongene** is a systems engineer, metro optical networking, at OFS, 19 Schoolhouse Road, Somerset, NJ. **Raymond Boncek** is a systems engineering manager at OFS in Norcross, GA. Charles Ufongene can be reached at cufongene@ofsoptics.com.

- G. Keiser, Optical Fiber Communications., 93, McGraw-Hill 3rd Ed. (2000).