1041-1135 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LPT.2015.2431853, IEEE Photonics Technology Letters JOURNAL OF L A T E X CLASS FILES, VOL. 13, NO. 9, SEPTEMBER 2014 1 Time Resolved Chirp Measurement Based on a Polarization-Maintaining Fiber Mohamed Essghair Chaibi, Hoang Trung Nguyen, Christophe Gosset, Fr´ ed´ eric Grillot, and Didier Erasme Abstract—In high speed optical networks, quantifying both the intensity and the frequency behaviors of optical signals has become a crucial task to evaluate transmission performance. In this letter, we report on a new technique to characterize optical signals. It is based on low cost components and its operating mode is easy to provide accurate and fast measurements. The proposed technique is experimentally validated with signals whose characteristics are known. It is also tested with different optical transmitters. Index Terms—Frequency chirp, Interferometry, Polarization- Maintaining Fiber. I. I NTRODUCTION I N optical networks, the easiest way to convey data from one point to another is modulating the intensity of light at the emitter and detecting it at the receiver with a sim- ple photo-detector after propagation through an optical fiber. The intensity modulation (IM) of an optical field is always accompanied by the modulation of its phase (PM) and its frequency (FM), as the latter is the time derivative of the former. The way in which IM and FM modulations interact depends on several parameters such as the structure of the modulator and the modulation frequency. Although FM can not be detected at the receiver side, it has an important effect on the transmission performance. FM is usually seen as an unwanted effect: the spectral broadening that it generates inter- acts with the fiber chromatic dispersion and then causes inter- symbols interference (ISI) at the receiver. On the other hand, it was shown in [1], [2] that the FM of a DFB laser can be exploited to generate single sideband (SSB) signals suitable for transmission through an intensity modulation/direct detection (IM/DD) dispersive channel. Thus, quantifying both FM and IM behaviors of an optical field becomes crucial. This is referred to as time resolved chirp (TRC) measurement. Several techniques have been developed to characterize the TRC of optical transmitters. Among them, the frequency discriminator method has been extensively reported. It is based on an interferometer configuration used to convert the frequency deviation into an intensity variation detectable by a photo-detector. The implementations of this technique differ M.E. Chaibi, C. Gosset, F. Grillot and D. Erasme are with the Department of Communications and Electronics, Institute MINES-TELECOM, TELECOM ParisTech, CNRS LTCI, 46 rue Barrault, 75634 Paris Cedex 13, France (e-mail: chaibi@telecom-paristech.fr; christophe.gosset@telecom-paristech.fr; frederic.grillot@telecom-paristech.fr; didier.erasme@telecom-paristech.fr). H. T. Nguyen is with Apex Technologies, 9 bis rue Angiboust, 91460 Marcoussis, France (e-mail: hoang.trung.nguyen@apex-t.com). Copyright (c) 2015 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to pubs-permissions@ieee.org. from each other on the used interferometer: Mach-Zehnder [3] and Fabry-Perot [4] interferometers are the most popular. While the first is bulky and requires a tunable delay line, the second is known for its nonlinear transmission. In this work, we present a new implementation of the frequency discriminator method. The used interferometer is a polarization-maintaining fiber (PMF) sandwiched between a polarizer and a phase shifting system. The latter consists of a half-wave plate, a quarter-wave plate and a polarizer, all of them can be independently rotated about the optical axis. The birefringence property of the PMF leads to considering its rapid and slow axes as the two arms of the interferometer when two linearly polarized beams propagate along them. The rest of this paper is organized as follows: In section 2, we describe in details the proposed interferometer and the way to extract the IM and FM profiles of an optical field. In section 3, the experimental setup is described. In section 4, we present some results on different optical transmitters, including a DFB laser and a Dual-Electro-absorption Modulated Laser (D-EML). Finally, section 5 summarizes the paper. II. OPERATING PRINCIPLE A. The interferometer Let us consider an optical beam at the input of the proposed interferometer depicted in figure 1. It is generated by a continuous wave optical source. The optical field is expressed in complex notation as: E(t)=  I 0 e jw0t (1) Where I 0 is the average optical power and w 0 is the central angular frequency. The first polarizer of the interferometer is oriented at 45 o with respect to the PMF birefringent axes, such an arrangement splits the input signal into two equal power, in- phase, independent modes whose linear polarization is aligned with the rapid and the slow axes of the PMF. This polarizer can be replaced by a half-wave plate, thus avoiding a 3 dB loss, provided availability of such a plate at the wavelength of operation. After propagation with different phase velocity along the axes of the PMF, the orthogonal components emerge with a differential phase shift that can be expressed as: Δθ =2π ΔnL λ (2) Where Δn is the refractive index difference between the axes of the PMF whose length is L and λ is the emitted wavelength. The quantity ΔnL/c, c being the light velocity in vacuum, is referred to as the free spectral range (FSR) of the interferome- ter. Consequently, the FSR can be easily adjusted by changing