Megawatt peak power tunable femtosecond source based on self-phase modulation enabled spectral selection

Abstract

Wavelength widely tunable femtosecond sources can be implemented by optically filtering the leftmost/rightmost spectral lobes of a broadened spectrum due to self-phase modulation (SPM) dominated fiber-optic nonlinearities. We numerically and experimentally investigate the feasibility of implementing such a tunable source inside optical fibers with negative group-velocity dispersion (GVD). We show that the spectral broadening prior to soliton fission is dominated by SPM and generates well-isolated spectral lobes; filtering the leftmost/rightmost spectral lobes results in energetic femtosecond pulses with the wavelength tuning range more than 400 nm. Employing an ultrafast Er-fiber laser and a dispersion-shifted fiber with negative GVD, we implement an energetic tunable source that produces ~100-fs pulses tunable between 1.3 µm and 1.7 µm with up to ~16-nJ pulse energy. Further energy scaling is achieved by increasing the input pulse energy to ~1-μJ and reducing the fiber length to 1.3 cm. The resulting source can produce >100-nJ femtosecond pulses at 1.3 µm and 1.7 µm with MW level peak power, representing an order of magnitude improvement of our previous results. Such a powerful source covers the 2nd and the 3rd biological transmission window and can facilitate multiphoton deep-tissue imaging.

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“…The other one employs self-phase modulation (SPM) in an optical fiber, which can substantially broaden a narrowband input spectrum to >400 nm bandwidth with well-isolated spectral lobes. For example, using an ultrafast EDFL centered at 1550 nm wavelength as the pump source, the SPM-dominated nonlinear effect can typically extend the spectrum from 1300 to 1700 nm in optical fibers within only a few cm long [26][27][28][29] . The pulse energies for 1300 nm and 775 nm are~10 nJ (corresponding to 300 mW average power) after wavelength conversion.…”

“…Using optical filters to select the leftmost spectral lobe, we achieve nearly transform-limited pulses at 1300 nm, enabling SHG and THG imaging. We dub this methodology SPM-enabled spectral selection [26][27][28][29][30][31] . A flip mirror is introduced, allowing to choose different excitation beams (775 nm/1300 nm) entering the scanning microscope (MPM-2PKIT, Thorlabs), which consists of a resonant scanner (8 kHz) and a mirror galvanometer (up to 30 Hz) to sweep the field of view.…”

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“…For example, soliton self-frequency shift in combination with frequency doubling can generate 6.5-nJ, 86-fs pulses at 1150 nm [62] and 32-nJ, 99-fs pulses at 1200 nm [63]. Recently we demonstrated a new approach to generate wavelength widely tunable (>400 nm) and nearly transform-limited femtosecond pulses for NLOM [53,[64][65][66][67]. The core concept is to employ self-phase modulation (SPM) [68] in optical fibers to significantly broaden a narrowband input optical spectrum followed by filtering the leftmost or the rightmost spectral lobes.…”

“…The core concept is to employ self-phase modulation (SPM) [68] in optical fibers to significantly broaden a narrowband input optical spectrum followed by filtering the leftmost or the rightmost spectral lobes. This method-dubbed as SPM-enabled spectral selection (SESS)-allows us to generate >10-nJ, ~100-fs pulses at 1215 nm from a large-mode-area fiber pumped by an Yb-fiber laser [65], or >15-nJ, ~100-fs pulses at 1300 nm or 1700 nm from a dispersion-shifted fiber (DSF) pumped by an Er-fiber laser [66]. With a lower-repetition-rate energetic pump source, SESS can produce >100-nJ, ~100-fs pulses at 1250 nm with ~MW peak power [66], which is highly desired by deep-tissue imaging.…”

“…In this paper, we employ SESS in 9-cm DSF to generate pulses at 1250 nm for SHG/THG microscopy. The DSF has a 10-µm mode-field diameter and −10 fs 2 /mm group-velocity dispersion at 1550 nm as used in [66,67]. Figure 3(a) shows the broadened spectrum that spans more than 500 nm for 85-nJ pulses coupled into the DSF.…”