[圖]The Best Solution to Increase Channel Isolation

The Best Solution to Increase Channel Isolation
（Patent pending）
Alex Mao. CTO of Lightel Technologies Inc.
Michael Luo, Manager of Instrument division 
Jim Lei,     Manager of Components division

With the exponential growth in bandwidth demand, WDM (Wavelength Division Multiplexed) technology has been widely used in fiber optic networks. Adding wavelengths in the same fiber effectively increases the bandwidth capacity of a fiber and thus avoid the need to install more fibers or increase the bit rate to higher level. For example, in the optical fiber of an FTTH/FTTP 1310/1490/1550 triplex service network, the 1490 nm wavelength is used for downstream voice and data signals, 1550 nm wavelength for downstream video signals, and 1310 nm wavelength for upstream voice and data signals. In such networks, thin-film-filter based 1310/1490/1550 MUX/DEMUX devices are used for adding or dropping individual wavelength channels. 

In WDM add/drop devices, the wavelength channel isolation via a single thin-film-filter is rather limited, especially on its reflection side (channel isolation <= 20dB). In order to further restrain the leaky portion of the transmission channel in the reflection light, another filter is sometime used in the reflective path to further block any off-band wavelengths. In past years, so-called high-isolation add/drop devices have been used in WDM systems for achieving higher signal quality.  But its higher price brings users to seeking for cost-effective alternatives. 

Lightel Technologies Inc has recently released two new filtering devices with low cost and high reliability. The first is an In-line Filter (see Fig. 1) - a fiber optic patchcord with an in-line filter in the middle.

Ferrule 2 is simply a section of connector ferrule with a section of bare optical fiber inside. One endface of this bare fiber section is coated with a channel filter to block all off-band wavelengths, while the other endface has no coating. As a stand-alone piece and having no fiber buffer around it, this piece of bare fiber section can be easily settled into the coating chamber and sustain under a high temperature for long hours.

In the In-line configuration shown in Fig.1, the Ferrule 2 is self-aligned to Ferrule 1 (on its left) and Ferrule 3 (on its right) by means of an alignment sleeve. In this configuration, Ferrule 2 is laid such that its coated endface is facing to Ferrule 3.  The junction between Ferrule 1 and Ferrule 2 is virtually identical to the junction of a pair of standard APC connectors. Ferrule 3 has AR-coating on its endface in order to minimize its interfacing loss to Ferrule 2. 

This device adopts commercial components that have been widely used in optical fiber connector/adapter industry, such as Zirconia ferrules and sleeves, thus allowing low cost and superior performance, including high-grade concentricity, easy alignment, and high alignment accuracy.

The second filtering device is a Male-Female Connector type filter. It is based on the same concept as the first device. As shown in Fig. 2, F1 is a special connector ferrule with one end inserted into a connector adapter and the other end (angled) coated with a channel filter.   F2 is also a special ferrule with one end (angled) having AR-coating and the other end polished as PC or APC.  Via the sleeve S2, F2 can be mated with foreign connectors. Again, all parts involved in this device are commercial standard components used for fiber optic connectors/adapters. 

圖2. Mail/Femail connector type Filter

In summary, Lightel has successfully developed In-Line type and Connector type devices of channel filtering. Their good performance and low cost have made them the best solution for channel isolation enhancement.