Optical Cavity-Less 40-GHz Picosecond Pulse Generator in the Visible Wavelength Range

Abstract

High-repetition-rate optical frequency-comb sources emitting picosecond pulses play an important role in various scientific areas and industrial applications. Such ultrafast pulse sources are mostly generated in optical cavities or microresonators. By means of the wavelength-conversion techniques, it is then possible to transfer such robust and compact near-IR sources from the visible to the mid-IR wavelength regions [1–3], for which there exists an increasing demand, for biophotonics and other applications. Here we demonstrate the generation of high-repetition-rate picosecond pulses in the visible wavelength range by using an optical cavity-less configuration. To this aim, we first developed a tunable C-band picosecond pulse generator at a repetition-rate of 40 GHz. This near-IR optical cavity-less system makes use of highly reliable components developed for optical communications [4]. Next, it was used to directly pump a nonlinear periodically poled ridge LiNbO 3 (PPLN) waveguide fabricated on silicon substrate [5]. This highly efficient χ(2)-type nonlinear material, which provides ultrafast-response of second harmonic generation (SHG), enables us to develop a fiber-coupled frequency doubling module for fiber-integrated picosecond pulse sources in the visible. Figure 1(a) depicts the experimental setup. The generation of stable 40-GHz pulse trains is based on the nonlinear compression of an initial beat-signal in a cavity-less all-fiber system. The initial 40-GHz sinusoidal beating is generated thanks to a LiNbO 3 intensity modulator driven at its null transmission point by a half repetition-rate 20-GHz RF clock and then amplified by means of an Er-doped fiber amplifier. Moreover, we imposed a 70-MHz RF phase modulation to suppress Brillouin backscattering in the 2.2-km long SMF compression fiber. As shown experimentally in Fig. 1(c1–1), high-quality 6-ps Gaussian pulses at a repetition rate of 40 GHz are then obtained at the fiber output with an average power of 400 mW. The corresponding spectrum is reported in Fig. 1(b1) with a FWHM bandwidth close to 100 GHz. This pulse source is then injected into the fundamental mode of our PPLN waveguide by means of a lensed fiber. The center wavelength and state of polarization of the near-IR pulse train is set to match the optimum SHG conversion in the PPLN waveguide whose temperature is stabilized near room temperature. The 20-mm-long SHG waveguide gives a normalized conversion coefficient of 40%/W. After beam collimation at the waveguide output, an optical prism and an optical diaphragm are used to filter out the SHG signal and reject the residual pump and spectra generated by other nonlinear processes. The SHG signal is then collected into a multimode fiber and monitored by means of an optical spectrum analyzer (∼5 GHz resolution) and a 45-GHz photodetector. As shown in Fig. 1(b2), the SHG spectrum exhibits a 40-GHz frequency-comb profile centered at ∼771 nm whose bandwidth is ∼100 GHz. The temporal profile of our 40-GHz pulse train converted into the visible is shown in Fig. 1(c2) and exhibits a 40-GHz sinusoidal waveform due to the limited bandwidth of our detector. Similar waveform and duration are obtained for our initial near-IR pulse train when applying the same detection limitation (Fig. 1(c1–2)).