Lossless equalization of frequency combs

Abstract

Frequency combs obtained by sinusoidal phase modulation of narrow-band continuous-wave lasers are widely used in the field of optical communications. However, the resulting spectral envelope of the comb is not flat. In this Letter, we propose a general and efficient approach to achieve flat frequency combs with tunable bandwidth. The idea is based on a two-step process. First, efficient generation of a train with temporal flat-top-pulse profile is required. Second, we use large parabolic phase modulation in every train period in order to map the temporal intensity shape into the spectral domain. In this way, the resulting spectral envelope is flat and the size is tunable with the chirping rate. Two different schemes are proposed and verified through numerical simulations.

VCSEL-Based Optical Frequency Combs: Toward Efficient Single-Device Comb Generation

“…Because of this, the measured values of extinction ratio and flatness are not conclusive, and the actual values could be better and worse, respectively. The key advantages of this OFCG is that no external EOMs [11] or further components are needed to obtain such a flat spectrum [11][12][13][14]; and that the amount of power needed is more than one order of magnitude lower than typical OFCG implementations [12], [22]. Furthermore, and unlike MLLDs, the frequency spacing between lines can be easily changed using the external CW RF synthesizer in a continuous way and with high frequency resolution, as it is also the case when EOMs are used.…”

“…Nevertheless, if standard edge-emitting lasers are used, the amount of direct modulation power needed for an OFCG featuring 8-10 lines is about 0.5-1 W, which makes necessary the use of Radiofrequency (RF) power amplifiers [4], [10]. Moreover, the generated optical spectra are not flat, and additional stages based on nonlinear techniques and comprising several external components are usually needed to obtain flat-topped pulses, such as cascaded Intensity or Phase Electro-Optical Modulators (EOMs) [11][12][13] and Four Wave Mixing (FWM) [14].For this reason, different types of semiconductor lasers have been used under GS regime in the search of compact, commercial devices based pulsed sources, and, Vertical Cavity Surface Emitting Lasers (VCSELs) are promising candidates [15][16][17][18][19]. VCSELs need very few current to operate (under 10 mA), and their integration capabilities as well as the capacity for on-chip testing allow for low cost optical subsystems with an excellent energy efficiency.…”

“…Nevertheless, if standard edge-emitting lasers are used, the amount of direct modulation power needed for an OFCG featuring 8-10 lines is about 0.5-1 W, which makes necessary the use of Radiofrequency (RF) power amplifiers [4], [10]. Moreover, the generated optical spectra are not flat, and additional stages based on nonlinear techniques and comprising several external components are usually needed to obtain flat-topped pulses, such as cascaded Intensity or Phase Electro-Optical Modulators (EOMs) [11][12][13] and Four Wave Mixing (FWM) [14].…”

A Survey of Optical Carrier Generation Techniques for Terabit Capacity Elastic Optical Networks

“…Torres-Company et al provided theoretical analysis and a novel generalized approach to understanding the shape of optical frequency combs ,using the notion of time-to-frequency mapping [86]. A user-defined envelope of optical frequency combs can be achieved following a two-step process as shown in Fig.…”

“…If the amount of chirp is sufficiently large, the optical spectral envelope will be a scaled replica of the tailored intensity pulse" [10]. It implies that for a flat frequency comb we need a flat-top pulse train and a pure quadratic phase modulation on each pulse to translate temporal intensity shape into spectral domain i.e., time-tofrequency mapping [86]. The pulse train can be generated by intensity modulating a CW laser and then introducing a quadratic phase modulation using a PM.…”