Characterization of dual-electrode Mach-Zehnder modulator based optical frequency comb generator in two regimes

Abstract

Amplitude and phase characterization of the output from a dual-electrode LiNbO 3 Mach-Zehnder modulator based ultra-flat optical frequency comb generator is investigated for well-known "in-phase" operation and a new "out-of-phase" regime. The out-of-phase regime may be realized with a single-electrode Mach-Zehnder modulator.Optical frequency comb generators (OFCG) are useful for multi-wavelength sources for wavelength division multiplexing (WDM), optical arbitrary waveform generation, and spectroscopy due to their ability to generate a large number of precisely spaced spectral lines [1][2][3]. An ideal OFCG can create a broad and amplitude flattened comb with independently controlled center frequency and comb spacing. Modelocked lasers can be used as ultra-broadband OFCGs, but adjustment of the comb spacing and center frequency is often complex and difficult to control. However, OFCGs based on strong electro-optic (EO) modulation by a RF carrier on a continuous wave (CW) laser can produce a flat optical frequency comb (OFC) with independent adjustment of center wavelength and comb spacing [4], much like an ideal OFCG. These combs are simple to operate and easy to adjust. Typically, it is assumed that both phase modulation (PM) and amplitude modulation (AM) are required to generate a broadband and flat OFC [5]. Fig. 1(a) shows a schematic of a dual-electrode Mach-Zehnder modulator (DE-MZM) based OFCG capable of generating an ultra-flat OFC. Using only PM, under the "in-phase" operation mode the DE-MZM OFCG can generate a broad and flat OFC that appears as if it were created using both AM and PM [e.g., Fig. 1(g)]. There is a recently discovered regime of operation, "out-of-phase" which produces an identical spectral intensity (i.e., a broad and flat OFC)[6]. This mode can potentially be implemented using an x-cut, single-electrode, push-pull LiNbO 3 MZM with a single RF drive and unique electrode geometry [ Fig. 1(b)]. Description and characterization of the OFCG under the two primary modes of operation is the focus of this summary. Intensity (au)Intensity (au) Phase ( rad/div) Intensity (au)Phase (2 rad/div) Phase (2 rad/div) Intensity (au) Intensity (au)Phase (2 rad/div) Phase (2 rad/div) Intensity (au) Phase ( rad/div) "in-phase" "out-of-phase" Fig. 1. (a) Dual-electrode LN MZM OFCG operating at 10 GHz. (b) Single-electrode, push-pull LN MZM with asymmetrical electrode structure for "out-of-phase" operation. (c−f) Operation modes of DE-MZM OFCG. Low-drive biasing simulated output of DE-MZM OFCG (g,j) time-domain intensity (solid) and phase (dashed) and (h,k) spectral intensity (stems) and phase (dots) for peak-aligned (g,h) "in-phase" and (h,k) "out-of-phase" operation. Large drive biasing simulated output (i,l) spectral intensity (stems) and phase (dots) for peak-aligned (i) "in-phase" and (l) "out-of-phase" operation (βb = βa−π/2 for all cases). Note the spectral intensities are depicted on a linear scale.Referring back to Fig. 1(a), RF is applied to the traveling wave EO phase modulators in each arm of the MZ...