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Few-Cycle, Carrier–Envelope-Phase-Stable Laser Pulses from a Compact Supercontinuum Source

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Few-Cycle, Carrier–Envelope-Phase-Stable Laser Pulses from a Compact Supercontinuum Source Erschienen in: Journal of the Optical Society of America B ; 36 (2019), 2. - S. A93-A97 https://dx.doi.org/10.1364/JOSAB.36.000A93 Few-cycle, carrier–envelope-phase-stable laser pulses from a compact supercontinuum source 1,2,3, 1 1 4 WILLIAM P. P UTNAM, *PHILLIP D. KEATHLEY, JONATHAN A. COX, ANDREAS LIEHL, 4 1,2,5 ALFRED LEITENSTORFER, AND FRANZ X. KÄRTNER 1Research Laboratory of Electronics, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, Massachusetts 02139, USA 2Department of Physics and The Hamburg Center for Ultrafast Imaging, University of Hamburg, Luruper Chaußee 149, 22761 Hamburg, Germany 3NG Next, Northrop Grumman Corporation, 1 Space Park Blvd., Redondo Beach, California 90278, USA 4Department of Physics and Center for Applied Photonics, University of Konstanz, 78457 Konstanz, Germany 5Center for Free-Electron Laser Science, Deutsches Elektronen-Synchrotron (DESY), Notkestraße 85, 22607 Hamburg, Germany *Corresponding author: [email protected] We report on a few-cycle, carrier–envelope-phase-stable laser source based on supercontinuum generation driven by an amplified Er:fiber-based system. Laser pulses from an Er:fiber oscillator are amplified, and these amplified pulses generate a stable supercontinuum in a highly nonlinear optical fiber. The short- and long-wavelength tails of this continuum are used in an f -to-2f interferometer to stabilize the carrier–envelope phase (CEP) of the Er:fiber oscillator via an acousto-optic modulator. Compressing the central part of the continuum, we generate a train of CEP-stabilized laser pulses with a central wavelength of 1170 nm, duration of 9.1 fs (∼2.3 optical cycles), and repetition rate of 78.4 MHz. Characterizing the CEP stability of our output pulse train with an out-of-loop f -to-2f interferometer, we find a phase jitter of only 157.4 mrad when integrating the radiofrequency spectrum from 5 mHz to 5 MHz. 1. INTRODUCTION have demonstrated passively CEP-stabilized, fiber-based laser – Laser sources with stabilized carrier–envelope phases (CEPs) sources with center wavelengths in the near infrared [12 16]. are critical for strong-field optical physics experiments and The CEP noise of these systems has been investigated [13,14], frequency comb applications. In the time domain, the CEP dic- and stabilizing their repetition rate, their frequency comb per- tates the exact shape of the optical electric field of a laser pulse. formance has been analyzed [15,17]. Driven by the optical field, intense light–matter interactions In this brief paper, we report on an Er:fiber-based system f around gas atoms [1,2] and nanostructures [3–6] have been with an actively stabilized and tunable CEO as well as with shown to sensitively respond to the CEP. Moreover, in the fre- few-cycle output pulses and a broadband output spectrum. quency domain, the carrier–envelope offset frequency, f CEO, to- Similar to the DFG-based sources mentioned above, our source gether with the repetition rate, f R, determines the exact position uses laser pulses from an amplified Er:fiber oscillator to generate of each spectral line in a frequency comb. The carrier–envelope stable supercontinuum spectra. In contrast to the DFG-based offset frequency is the frequency at which the CEP changes in a sources, however, we use the spectral wings of the supercontin- f comb’s optical pulse train, and control of the f CEO has enabled uum to actively stabilize the CEO from 0 MHz to 20 MHz, and the wide-ranging applications of frequency combs, from novel our compact source uses only a single amplifier stage to generate spectroscopy to precision measurement [7–9]. pulses of ∼2.3 optical cycles in duration and with a spectral Over the past two decades, CEP-stabilized sources have bandwidth of 400 nm (centered around 1170 nm). been dominated by solid-state lasers, in particular Ti:sapphire In the following, we first describe the layout of our laser systems. Compared to solid-state lasers, fiber lasers offer attrac- source and present measurements of the few-cycle duration tive stability and a compact footprint. Using laser pulses from output pulses. Next, we describe the CEP stabilization amplified Er:fiber oscillators, researchers have generated stable scheme, and we show measurements of our source’s CEP noise. supercontinuum spectra [10–12]. Recently, these stable, broad- Lastly, we briefly discuss our results and compare our source’s band spectra have been used to produce difference frequency measured CEP noise to that from a state-of-the-art Ti:sapphire generation (DFG), and amplifying this DFG light, researchers laser. Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-2-zchy4i30c57n1 2. SYSTEM LAYOUT AND PULSE MEASUREMENTS The overall setup of our source is shown in Fig. 1. The oscil- lator is a stretched-pulse Er:fiber laser passively modelocked via nonlinear polarization rotation. It includes an intra-cavity glass (BK7) wedge for coarse adjustment of the oscillator’s carrier–envelope offset frequency, f CEO osc:, and an intra- cavity piezo-actuated mirror for locking the repetition rate, f R (not implemented in this work). The oscillator’s output, labeled A in Fig. 1, has a central wavelength of ∼1577 nm and a full-width-at-half-maximum (FWHM) spectral band- width of ∼68 nm. The repetition rate is f R 78.4 MHz, and the average power is ∼30 mW (pulse energy ∼380 pJ). (Further oscillator details are included in Ref. [11].) The output from the oscillator is sent to an acousto-optic modulator (AOM), which is used as a frequency shifter to control the f CEO [11,17,18]. The diffracted light from the AOM is Doppler shifted by the acoustic frequency, f 175 5 MHz f s , and the CEO at the AOM output is then Fig. 2. Laser spectrum and pulse measurement. (a) Output spec- f f f f 5 MHz CEO CEO osc: s. Tuning s over its range, trum and group delay. The output optical spectrum is shown in blue we can therefore control the f CEO. Additionally, with a several- (normalized spectral intensity axis on the left). The group delay is hundred-micrometer beam diameter in the AOM, we are able shown in red (group delay axis on the right). (b) Pulse measurement. to adjust the f CEO with a bandwidth of several MHz. A similar The measured intensity envelope of the output pulse is shown in blue, f CEO control scheme was implemented in Ref. [11], and and the transform-limited pulse is shown with the black dashed line. further details can be found therein. (c) Interferometric autocorrelation (IAC) measurement. An IAC mea- The output of the AOM is amplified in an Er-doped fiber surement of the output pulse is shown (blue dots), and the expected IAC calculated from the measured pulse shown in part (b) is plotted amplifier (EDFA), compressed in a silicon prism compressor (orange line). The dashed black lines show the relevant normalized and coupled into a germanium-doped, highly nonlinear fiber intensity values of 0, 1, and 8. (HNF). After amplification, at the output of the silicon prism compressor (labeled B in Fig. 1), the pulse train has ∼375 mW ∼4 5nJ average power ( . pulse energy) and pulse duration of components are sent to an f -to-2f interferometer for f CEO ∼60 fs. In the HNF, these amplified, compressed pulses gen- detection. Following these two pick-offs, a spatial filter is erate an octave-spanning supercontinuum. We note that nearly inserted to select the dispersive wave spectrum for the output identical supercontinuum generation has been previously dem- and to tune the exact output spectrum. onstrated, and further HNF and supercontinuum generation The output of our source is labeled by the circled C in Fig. 1. details may be found in Refs. [10–12]. The supercontinuum The output has an average power of ∼17 mW, a center wave- spectrum consists of a short wavelength, dispersive wave part length of 1170 nm, and a pulse duration of 9.1 fs (∼2.3 optical and a longer wavelength, soliton component. Following the cycles). The measured output spectrum and group delay are HNF, the supercontinuum light enters an SF10 prism com- shown in Fig. 2(a). The spectrum has a FWHM bandwidth pressor. In the SF10 prism compressor, two pick-off mirrors of 400 nm; the group delay is measured with a two-dimensional are positioned. One mirror redirects the shortest wavelengths spectral shearing interferometer (2DSI) [19] and shows residual of the dispersive wave, around 900 nm, and the other mirror third-order dispersion (TOD). The intensity envelope of the la- redirects the soliton component around 1800 nm; these two ser pulse, reconstructed from the group delay measurement, is displayed in Fig. 2(b). The pulses show satellite features around the central peak due to the TOD; however, 77.4% of the pulse energy remains in the central pulse lobe. The transform-limited pulse is also shown in Fig. 2(b) and has a FWHM of 8.3 fs. Lastly, in Fig. 2(c) we present both measured and calculated in- terferometric autocorrelation (IAC) traces of our few-cycle pulse, which confirm the accuracy of our 2DSI pulse reconstruction. The IAC measurement shows the expected 1-to-8 back- ground-to-peak ratio and very closely agrees with the IAC calcu- lated from our reconstructed pulse (even for large IAC delays). Fig. 1. Source setup. The source consists of an Er:fiber oscillator 3. CEP STABILIZATION (tan box on the left) and a supercontinuum generation setup (gray box on the right). In the illustration: iso., fiber isolator; WPs, wave- The supercontinuum’s spectral components near 900 nm and plates; PC, prism compressors; PZT, piezo-actuated mirror.
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