IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 3, MARCH 2003 485 Loss-Less Bridging in 1 1 Optical Protection Using a Modified Transmitter and Receiver David S. Levy, Associate Member, IEEE, Kevin Solomon, and Steven K. Korotky, Fellow, IEEE Abstract—Traditional 1 1 optical layer protection is imple- mented by creating two signals from one via a 3-dB splitter (with 3-dB insertion loss) and transmitting both signals along two dif- ferent path-diverse optical fibers. In this letter, a transmitter has been modified to provide two signals in such a manner that no excess insertion loss is encountered. Optical protection switching using this transmitter and a modified invertible receiver is demon- strated under 10-Gb/s nonreturn-to-zero transmission. A protec- tion time of 5 ms is obtained. Index Terms—Modulators, optical components, optical switches, protection. I. INTRODUCTION C ONVENTIONAL path-diverse 1 1 protection has been used to date to protect synchronous optical network (SONET) [or synchronous digital heirarchy (SDH)] optical networks [1]–[3]. However, it can also be used to protect non-SONET traffic and to perform equipment-level protection [e.g., of large-scale optical cross connects (OXCs)]. To date, such protection has been implemented by placing a 3-dB splitter following a single-output transmitter on the head end and using a selector switch on the tail end. Note that the losses in such a 1 1 architecture are typically at least 4 dB, primarily due to the loss of the 3-dB coupler, splices, connectors, etc. In an environment where there is limited optical power margin (e.g., where short-reach optics are used [4]), this loss can be prohibitive. In commonly deployed externally modulated lasers, a single input-port/single output-port Mach–Zhender (1 1 MZ) mod- ulator is used to create a data stream. However, other types of MZ modulators such as single input-port/dual output-port (1 2) MZ modulators exist that have comparable optical and elec- trical drive characteristics as those of the 1 1 MZ. The basic difference between these two particular devices is in the output section, in which the 1 1 MZ device has a 2 1 combiner and the 1 2 MZ device has a directional coupler. The inser- tion losses of each of these integrated circuits depends upon numerous waveguide design and processing parameters, but a comparison of commercially available devices shows that the 1 2 MZ device can have an insertion loss only a few tenths of a decibel greater than the 1 1 MZ device. This difference is believed to be due to the additional length of the directional coupler and its associated propagation loss. The 1 2 MZ de- vice, however, can serve the additional function of duplicating Manuscript received October 18, 2002; revised November 12, 2002. The authors are with Lucent Technologies Inc., Holmdel, NJ 07733 USA (e-mail: dslevy@lucent.com). Digital Object Identifier 10.1109/LPT.2002.807951 Fig. 1. (Bottom) The conventional form of 1 1-type optical protection utilizing a 1 1 Mach–Zhender modulator and a 3-dB optical power splitter. (Top) A1 2 Mach–Zhender modulator is used to produce two output signals of equal optical power. Each arm traverses a unique path to the destination receiver, and an optical selector switch is used to select the signal of the higher quality. Note that in the work presented here each arm is inverted in phase from the other at all times. the primary service signal. These devices have typically been used to date in low-speed analog cable television transmission and experimental optical time-division multiplexing, but to our knowledge they have not been used in high-speed digital optical networking. In this letter, a transmitter utilizing a dual output-port MZ modulator was used to produce two path-diverse signals with phase complementary intensity [9]. An optical switch was used to select between these signals and a logical inversion in the dig- ital domain was performed before retransmission of the signal to a transmitter. The demonstration of this protection technique is reported here, and the results are discussed. II. EXPERIMENT The conventional form of 1 1 optical protection is shown in Fig. 1 (bottom). As shown in the figure, electronic data is imparted upon an optical carrier via the use of a 1 1 MZ modulator. An exact copy of the data signal, similar in both amplitude and phase, is then bridged to the protection line by an optical power splitter. As in traditional 1 1 protection, the service and protection signals propagate around the network in a path-diverse manner and converge on an optical selector switch before being detected at a receiver [1]–[3]. The splitter can be an external device (e.g., fiber coupler) or it can integrated 1041-1135/03$17.00 © 2003 IEEE