Time-resolved chirp measurement for 100GBaud test systems using an ideal frequency discriminator Regan T. Watts ⁎, Kai Shi, Liam P. Barry abstract article info Article history: Received 12 September 2011 Accepted 14 December 2011 Available online 27 December 2011 Keywords: Optical frequency discriminator Ultrafast measurements Pulse shaping Constellation diagram Coherent optical communications In this paper we present multi-channel chirp measurements of wide-band sources, using a programmable Fourier-domain optical processor (FDOP) as a near-perfect linear frequency discriminator element followed by a fast photodiode and electrical sampling oscilloscope. The electric field of a 10.7 Gbit/s phase-encoded data source and a directly modulated laser diode are simultaneously interrogated with this measurement system. The constellation diagram of the phase-encoded data source is demonstrated, and a comparison with another phase-sensitive measurement technique is performed. Additionally, an extension to this technique is demonstrated in which the time-resolved chirp of a picosecond-duration mode-locked laser diode with a 260 GHz spectral bandwidth is characterised using the FDOP and a high-bandwidth optical sampling oscillo- scope. This measurement ensemble has sufficient temporal resolution to characterise random or repetitive data signals up to 100GBaud. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Current deployments of long-haul and metro access fiber optic communications networks employ bandwidth-efficient phase-shift keyed modulation format, be they simple differential phase-shift keyed (DPSK) modulation formats or advanced coherent modulation formats such as n-ary quadrature amplitude modulation (n-QAM) or modulation formats which combine m-number of amplitude-shift keyed levels (m-ASK) with n-number of phase-shift keyed levels (n-PSK). The Optics Internetworking Forum (OIF) is developing stan- dards for optical transceivers which operate at a line rate of 100 Gbit/s for use in 100 GHz dense wavelength-division multiplexed (DWDM) networks. The OIF favours the use of a dual-polarization quadrature phase-shifted keyed (DP-QPSK) modulation format to achieve a line rate of 100 Gbit/s. Indeed, researchers have demonstrated optical fiber transmission experiments at a line rates of 112 Gbit/s using DP-QPSK [1–3], and now even 448 Gbit/s using DP-16QAM in conjunction with time-division multiplexing schemes [4,5]. Symbol rates for these ultra-high big rate channels are now at 56GBaud, demonstrating that there is a need for test instrumentation with measurement functionality at symbol rates up to 100GBaud. Scientists and engineers involved in the research and development of photonic technologies for 100 Gbit/s coherent optical communica- tion systems require test and measurement instruments which can in- terrogate both the amplitude and the phase of a data-encoded optical waveform. It is desirable that these coherent measurement techniques are simple to implement (both experimentally and mathematically) and that they are flexible enough to allow for the interrogation of a wide range of advanced coherent data formats. Here we describe the implementation of a simple system of time-resolved amplitude and chirp measurement, in which is the optical carrier frequency and band- width of the signal under test is flexible, can simultaneously process multiple optical channels and is able to characterise the constellation diagrams of advanced coherent modulation formats. 2. Background A method commonly used for high-speed time-resolved chirp measurement is frequency discrimination [6]. A frequency discrimina- tor is formed when the complex spectral field of a pulse train, E(ω), ex- periences a linear filter function of the form H(ω) =mω +c, where m is the linear slope of the filter and c is the intercept. Using the Fourier trans- form relationship ωE(ω) ⬄idE(t)/dt (where E(t) and E(ω) are Fourier transform pairs), time-resolved intensity measurements after the se- quential application of two linear filters (one with a slope of +m and one with a slope of –m) around the centre frequency of the input com- plex field can yield the frequency chirp of the temporal waveform through simple calculations [7]. While it is possible to perform ultra-high bandwidth chirp measure- ment using frequency-resolved measurements [8,9], spectral domain techniques generally requires the averaging of a repetitive pulse train (or short pseudo-random pattern). To achieve real-time refresh rates it is preferable to analyse temporal waveforms (eyes, noise, jitter) using time-resolved techniques [10,11]. There are two factors that limit high-bandwidth chirp measurement (>40 Gb/s) using conven- tional frequency discriminator-based techniques. The first limiting Optics Communications 285 (2012) 2039–2043 ⁎ Corresponding author. E-mail address: regan.watts@dcu.ie (R.T. Watts). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.12.064 Contents lists available at SciVerse ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom