1-THz bandwidth C- and L-band optical sampling with a bit rate agile timebase

Optical waveform monitoring based on a free-running mode-locked femtosecond fibre laser and four-wave mixing in a highly nonlinear fibre

“…After photodetection, the enve− lope of the detected pulses, which is indicated by the dashed line in Fig. 2(a) (4), is analogous to the waveform of the origi− nal optical signal under test except for the time scale. Such elec− tronic pulses at a low pulse rate of f s must be fed into an A/D converter, so that a computer can be used to efficiently process the electronic data in order to obtain the synchronization infor− mation of the acquired samples, time step DT of optical wave− form sampling, pulse width, rise or fall time, and bit period of an optical signal, as well as other time−related signal parame− ters.…”

“…This, in turn, corresponds to a subpicosecond temporal resolution for the developed waveform measurement system. All−optical waveform sampling [1,2] is a very useful tech− nique which can be employed to directly measure the wave− forms of ultrashort optical pulses generated by ultrafast lasers [1,4,5] or propagating in optical fibres [6] (or waveguides) and the eye diagrams of ultrahigh−speed optical data signals in optical fibre communications [5−7], respectively. This is achieved by using optical signal processing to remove the bandwidth limitation of electronic circuits as encountered in traditional electronic oscilloscopes.…”

“…At present, all−optical waveform sampling is normally im− plemented by exploiting the nonlinear optical effects in opti− cal media such as optical fibres [1,2,7], waveguides [3,12,13] and crystals [4,5]. These include the second−order susceptibil− ity c 2 in nonlinear crystals (e.g., KTiOPO 4 (KTP) [14,15] and periodically poled LiNbO 3 (PPLN) crystals [4,5]) and the third−order susceptibility c 3 (e.g., cross−phase modulation and four−wave mixing) in nonlinear optical fibres [1,2,16], semiconductor optical amplifiers (SOAs) [3,17], and silicon nanowire [12] or chalcogenide planar waveguide [13], respec− tively.…”

“…These include the second−order susceptibil− ity c 2 in nonlinear crystals (e.g., KTiOPO 4 (KTP) [14,15] and periodically poled LiNbO 3 (PPLN) crystals [4,5]) and the third−order susceptibility c 3 (e.g., cross−phase modulation and four−wave mixing) in nonlinear optical fibres [1,2,16], semiconductor optical amplifiers (SOAs) [3,17], and silicon nanowire [12] or chalcogenide planar waveguide [13], respec− tively. Nevertheless, those schemes can have their individual advantages and respective drawbacks in terms of operational speed, wavelength range, manufacturing cost, complexity and physical size of the resulting optical waveform samplers (also called sampling gates).…”

“…For example, the use of sun−frequency generation (SFG) in nonlinear crystals allows to implement ultrafast optical samplers which are capable of measuring the optical data signals up to 640 Gbit/s [5], while the resulting samplers are normally bulky and their operational wavelength range is also restricted by the phase−matching condition of nonlinear crystals [7,9]. Although a PPLN crystal can be employed to sample optical signal waveforms with higher temporal resolution and larger SFG conversion efficiency than the KTP crystal, a drawback of the former is the requirement for an accurate control of operation temperature [4,13] at the expense of increasing the system complexity and cost. On the other hand, if the SOA−based waveform samplers are used, their operational speed can be ultimately limited by the rela− tively long gain recovery time in SOAs, and the amplified spontaneous emission noise of a SOA can also cause a degra− dation of signal−to−noise ratio for the optically sampled signal.…”