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RESEARCH PRODUCT

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

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International audience; High-repetition-rate optical frequency-comb sources emitting picosecond pulses play important roles in variousscientific researches and industrial applications. Such ultrafast pulse sources are mostly generated in opticalcavities or microresonators. By means of the wavelength-conversion techniques, it is possible to transfer thecavity-based near-IR robust and compact sources to the mid-IR or to the visible wavelength regions [1-2], forwhich there is an increasing demand, for biophotonics and other applications. Here we demonstrate the generationof high-repetition-rate picosecond pulses in the visible wavelength range by using a fully optical cavity-lessconfiguration. First, we developed a tunable C-band picosecond pulse generator whose high-repetition-rate is alsotunable from 20 to 40 GHz. This near-IR optical cavity-less system makes use of high reliability componentsdeveloped for telecommunications [3]. Next, it was used to directly pump a nonlinear periodically poled ridgeLiNbO3 (PPLN) waveguide fabricated on silicon substrate [4]. This highly efficient χ(2)-type nonlinear material,which performs ultrafast-response of second harmonic generation (SHG), will offer us the opportunities to developfiber-coupled frequency doubling modules for fiber-integrated picosecond pulse sources in the visible. Figure 1(a)presents the experimental setup, where the generation of stable 40-GHz pulse trains is obtained through thenonlinear compression of an initial beat-signal in a cavity-less optical-fiber-based device. The initial sinusoidalbeating is generated by using a commercial LiNbO3 intensity modulator driven by a half repetition-rate externalRF clock and then amplified by means of Er-doped fiber amplifier. Moreover, we imposed a RF phase modulationto suppress Brillouin backscattering into the 2.2-km-long compression fiber. High-quality 6-ps Gaussian pulses ata repetition rate of 40 GHz are then obtained with an average power of 400 mW at the fiber output as shown inFig. 1(c1-1). The corresponding spectrum is reported in Fig. 1(b1) with a FWHM bandwidth about 100 GHz. Thispulse source is then injected into the fundamental mode of our PPLN waveguide by means of a lensed fiber. Thecenter wavelength and state of polarization of the near-IR pulse train is set to match the optimum SHG conversionof the PPLN waveguide whose temperature is stabilized near room temperature. The 20mm long SHG waveguidegives a normalized conversion coefficient of 40%/W. After beam collimation at the waveguide output, an opticalprism and an optical diaphragm are used to efficiently filter out the SHG signal and reject the residual pump andspectra generated by other nonlinear processes. The SHG signal is then collected into a multimode fiber andanalyzed by means of an optical spectrum analyzer (~15 GHz resolution) and our 45-GHz photodiode –oscilloscope system. As shown in Fig. 1(b2), the SHG spectrum exhibits a 40-GHz frequency-comb profilecentered at ~771 nm whose bandwidth is ~100 GHz. The SHG average power is measured to be about 30 mW.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 limitations of our detection system. The same behaviour and pulse durationis obtained when applying the same detection device to the measurement of our initial near-IR picosecond pulsetrain (see Fig. 1(c1-2)).