IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 3, FEBRUARY 1, 2006 469 All-Optical In-Band OSNR Monitoring at 40 Gb/s Using a Nonlinear Optical Loop Mirror Rhys Adams, Martin Rochette, Trina T. Ng, and Benjamin J. Eggleton Abstract—We present an all-optical in-band optical signal-to- noise ratio (OSNR) monitor using a nonlinear optical loop mirror. Monitoring is enabled from the nonlinear power transfer function of the loop mirror. Experimental results are provided at 40 Gb/s for three modulation formats: nonreturn-to-zero, carrier-suppressed return-to-zero, and return-to-zero. The monitor discriminates the various OSNR levels over a dynamic range of more than 25 dB with every modulation format. Index Terms—In-band optical signal-to-noise ratio (OSNR) monitoring, nonlinear optics, optical performance monitoring, ultrafast optics. I. INTRODUCTION A S OPTICAL communication data rates increase from 40 to 160 Gb/s and beyond, the pulse widths, as well as the separation between pulses of the encoded data decrease. Such a data rate increase does reduce the system tolerance to trans- mission impairments such as amplified spontaneous emission (ASE) noise, chromatic dispersion, and polarization-mode dis- persion. Among these impairments, ASE noise interferes with the propagating signal, thereby producing intensity noise and er- rors on the detected signal. In this context, a device to monitor the in-band optical signal-to-noise ratio (OSNR) is desirable to assess the impact of the ASE noise and/or to provide fault anal- ysis, as depicted in Fig. 1 [1], [2]. Traditionally, electronic components were responsible for the modulation, detection, and signal processing of the optical data flow. Currently, these components are facing important techno- logical issues at data rates beyond 40 Gb/s. All-optical solutions are now proposed as an alternative to the electronic devices. All-optical techniques for in-band OSNR monitoring have been proposed by using polarization-nulling [3], a semiconductor op- tical amplifier [4], a highly birefringent fiber [5], and an optical parametric amplifier [6]. However, desirable features of a mon- itor include the ability to monitor multiple impairments simulta- neously, transparency to different modulation formats and signal power, and scalability to high data rates. In this letter, we propose and demonstrate an all-optical de- vice for in-band OSNR monitoring that encompasses the afore- mentioned features. The device consists of a nonlinear optical loop-mirror (NOLM) and a power meter. The key mechanism that enables OSNR monitoring with the NOLM is the profile of its power transfer function (PTF). The PTF of the NOLM non- linearly maps input powers to output powers via the optical Kerr Manuscript received August 19, 2005; revised October 31, 2005. The authors are with the Centre for Ultrahigh Bandwidth Devices for Optical Systems (CUDOS), University of Sydney, Sydney, NSW 2006, Australia (e-mail: rochette@physics.usyd.edu.au). Digital Object Identifier 10.1109/LPT.2005.863641 Fig. 1. OSNR monitoring between cascaded optical amplifiers. SMF: Single-mode fiber. Fig. 2. PDF of (a) NRZ, (b) CSRZ, and (c) RZ signals at OSNR , and dB. The peak power of the noise-free signal has been chosen arbitrarily to 1 mW. Insets show corresponding noise-free temporal waveforms for “1011” bit transmission. effect. We demonstrate the flexibility of our device at 40 Gb/s by providing results with nonreturn-to-zero (NRZ), 66% duty cycle carrier-suppressed return-to-zero (CSRZ), and 33% duty cycle return-to-zero (RZ) modulation formats. The PTF can be ad- justed to maximize performance for various input signal power levels. Furthermore, the proposed device can simultaneously monitor chromatic dispersion and is inherently scalable to data rates beyond (and below) 40 Gb/s. II. POWER DISTRIBUTION OF THE NOISY SIGNALS One way to understand the impact of OSNR reduction on a signal is by considering probability distribution functions (pdfs) [7]. A pdf represents the distribution of the power levels of a signal. Fig. 2 illustrates the pdfs for NRZ, CSRZ, and RZ signals assuming a noise-free peak signal power of 1 mW. PDFs are given in each case for OSNR values of 30, 15, and 0 dB. The average power is kept constant among the OSNR values. We first consider the NRZ signal. For a relatively low-noise signal with OSNR dB, the temporal waveform essentially consists of two well-defined signal levels, one level representing 1041-1135/$20.00 © 2006 IEEE