IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 21, NOVEMBER 1, 2011 1591 Dynamic Linewidth Measurement Method via an Optical Quadrature Front End Kai Shi, Regan Watts, Doug Reid, Tam N. Huynh, Colm Browning, Prince M. Anandarajah, Frank Smyth, and Liam P. Barry, Member, IEEE Abstract—This letter describes a dynamic linewidth character- ization method using an optical quadrature front end. The phase noise of the laser is recorded using a real-time oscilloscope in the time domain and the linewidth of the laser can be estimated statisti- cally offline. The major advantage of this technique compared with conventional linewidth measurements in the frequency domain, is that this method enables the dynamic phase noise characteriza- tion which is increasingly important for fast wavelength tunable and switched networks employing advanced modulation formats. The dynamic linewidth of an sampled grating distributed Bragg reflector (SG-DBR) laser is characterized by using this method. Index Terms—Dynamic linewidth, phase noise, sampled grating distributed Bragg reflector (SG-DBR), tunable laser. I. INTRODUCTION S AMPLED grating distributed Bragg reflector (SG-DBR) lasers have become a crucial component in optical packet switching networks, due to their wide tunability, high side-mode suppression ratio (SMSR) and short switching time [1]. When wavelength switching networks start to employ advanced mod- ulation formats to increase the spectral efficiency, the linewidth dynamics of the laser during the switching become increasingly important, especially for higher order modulation formats. The- oretical analysis has shown that a linewidth of less than 0.7% of the bit rate is required to achieve error rate in a differ- ential phase shift keying (DPSK) system employing an optical heterodyne receiver [2], and a much more stringent requirement ( 2 MHz) has been reported for no penalty in a 10 Gbaud differ- ential quadrature PSK system [3]. However the transformation of the phase noise from the time domain to the frequency do- main (required to facilitate the measurement of the full-width half-maximum of the spectrum) makes it complicated to extract the linewidth dynamics during wavelength switching [4]. In this letter, we propose a new linewidth characterization method. The phase noise of the laser can be derived by using an optical quadrature front end. As the instantaneous phase of Manuscript received March 04, 2011; revised June 24, 2011; accepted August 05, 2011. Date of publication August 15, 2011; date of current version October 12, 2011. This work was supported in part by the Science Foundation Ireland, Principal Investigate fund. K. Shi, R. Watts, T. N. Huynh, C. Browning, P. M. Anandarajah, F. Smyth, and L. P. Barry are with the Rince Institute, Dublin City University, Dublin 9, Ireland (e-mail: kaishi@eeng.dcu.ie). D. Reid was with the Rince Institute, Dublin City University, Dublin 9, Ire- land, and is now with Odenberg Engineering Ltd., Dublin 24, Ireland. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2011.2164784 Fig. 1. Setup of the linewidth characterization using a delayed self-homodyne technique with an optical quadrature front end. the laser is recorded in the time domain, the transient varia- tion of the linewidth during switching can be derived by di- viding the captured time domain signal into short gating win- dows at different times during the switching interval. Section II describes the theory of this method. The static linewidth char- acterization of an SG-DBR and a distributed feedback (DFB) laser using this method is then shown and compared with the delayed self-homodyne (DSH) method in Section III. Finally, the dynamic linewidth of the laser with two different switching combinations are investigated in Section IV. II. LINEWIDTH CHARACTERIZATION USING AN OPTICAL QUADRATURE FRONT END The setup diagram of the linewidth characterization method is shown in Fig. 1. In contrast with the conventional DSH tech- nique, the optical field and the delayed replica are not interfered directly on the photodetector [5]. The two optical fields defined as (1) (2) are superposed in a 2 4 90 hybrid whose output signals are detected by two balanced detectors (BD). and represent the continuous wave (CW) powers, and are the angular frequencies, and are the initial phases, and are the phase noise and and represent the polarization unit vectors of the two optical fields. A micro-optics-based dual po- larization coherent mixer is used in our experiment with two po- larization controllers (PC) to maximize . After detecting the output fields with the upper and lower BD, the in-phase and quadrature photocurrents are obtained with the overall shot-noise photocurrents in each arm respectively 1041-1135/$26.00 © 2011 IEEE