All-fiber interferometer-based repetition-rate

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All-fiber interferometer-based repetition-rate stabilization of mode-locked lasers to 10-14-level frequency instability and 1-fs-level jitter over 1-s DOHYEON KWON AND JUNGWON KIM* School of Mechanical and Aerospace Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea *Corresponding author: [email protected] Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX

We report on all-fiber Michelson interferometer-based repetition-rate stabilization of femtosecond mode-locked lasers down to 1.3×10-14 frequency instability and 1.4-fs integrated jitter in 1-s time scale. The use of a compactly packaged 10-km-long SMF-28 fiber link as a timing reference allows the scaling of phase noise at 10-GHz carrier down to -80 dBc/Hz at 1-Hz Fourier frequency. We also tested a 500-m-long low-thermal-sensitivity fiber as a reference and found that, compared to standard SMF-28 fiber, it can mitigate the phase noise divergence by ~10 dB/dec in the 0.1 – 1 Hz Fourier frequency range. These results suggest that the use of a longer low-thermalsensitivity fiber may achieve sub-fs integrated timing jitter with sub-10-14–level frequency instability in repetition-rate by a simple and robust all-fiber-photonic method. OCIS codes: (140.3425) Laser stabilization; (140.4050) Mode-locked lasers; (140.7090) Ultrafast lasers; (060.2840) Heterodyne; (320.7160) Ultrafast technology http://dx.doi.org/10.1364/OL.99.099999

Low-jitter femtosecond mode-locked lasers (MLLs) and optical frequency comb sources have made a great contribution to various high-timing-precision applications. As an ultralow-noise master source, low-jitter femtosecond mode-locked lasers have been used for synchronization of X-ray free-electron lasers [1-4], generation of ultralow-phase-noise microwave signals [5-7], time-of-flightbased ranging [8,9] and strain-sensing [10], dual-comb spectroscopy [11,12], photonic analog-to-digital converts [13], and photonics-based radars [14], to name a few. Naturally, suppression of timing jitter (i.e., phase noise in repetition-rate) in femtosecond MLLs and frequency combs is highly desirable for further advances in existing and emerging timing-critical applications. Recent advances in noise performances of free-running femtosecond MLLs have enabled sub-femtosecond timing jitter in the fast time scale (e.g., 1 km) fibeer delay arm are a used for ML LL repetition-raate n Fig. 1) and out-of-loop absolu ute staabilization (“in-lloop system” in ph hase noise meaasurement (“out-of-loop system m” in Fig. 1). In esssence, the fiberr interferometerr system is desiggned to detect th the freequency noise of the differen nce frequency between b the tw wo sp pectral modes off MLL output [i.ee., (m-n)frep betweeen (nfrep + fceo) an nd (m mfrep + fceo), wheree frep and fceo are repetition-rate r an nd carrier-envelop peoff ffset frequency, reespectively] with h high sensitivity y. In this work, w we ussed 1540 nm and a 1560 nm wavelength com mponents, which reesults in 2.5 THz frequency differrence. An acoustto-optic frequen ncy sh hifter (AOFS) driiven by fao (50 M MHz in this experriment) is insertted in the fiber delay d for syncchronous detecction. Thus, the th ph hotodetected po ower of the in nterferometer ou utput carries the th freequency noise at a (nfrep + fceo+ 2faoo) and (mfrep + fceoo + 2fao). Finally, the t freequency mixer rejects the com mmon-mode (ffceo + 2f ), and the t ao c reesulting baseban nd mixer output t is the frequenccy noise of (m-n)ffrep m magnified by the fiber delay timee (τ). This error ssignal can be used forr stabilizing the MLL repetition-rate to the stab bility of fiber dellay lin ne (i.e., achievin ng δfrep/frep=δτ/τ) by b intra-cavity actuators such as piezo-electric tran nsducer and eleectro-optic mod dulator (as show wn “in n-loop system” iin Fig. 1). The errror signal from m the mixer can b be alsso used for measuring the out-o of-loop repetition n-rate phase noiise of f the MLL under test (as shown ““out-of-loop systtem” in Fig. 1).

MLL soource, the timingg jitter of which iis improved by ~ ~10 dB in the high Fo Fourier frequenccy range. The use of a loweer-jitter MLL enabless lower overall rrepetition-rate p phase noise overr broad range of Fourrier frequency. SSecond, longer fib ber delay is used to scale the residuaal phase noise sp pectrum. Since th the frequency no oise detection sensitivvity scales with tthe delay time ((τ) [27], lower reesidual phase noise ccan be achieved with longer fiber delay. Note th hat there is a her sensitivity (w which scales wiith τ) and the trade-ooff between high achievaable bandwidth h (which scales with 1/τ). In th his work, we found tthat the optimall fiber delay len ngth is ~10-km for achieving the moost suppressed p phase noise speectrum and inteegrated jitter over brroad (e.g., >100 kHz) bandwidtth. Note that thee 10-km-long fiber lin nk is packaged w within a compaact size (16-cm d diameter and 4-cm h height) using an advanced fiberr winding techniique that has been eemployed for m manufacturing co ommercial-levell FOGs. Such compacct and robustt fiber-link paackaging also enables the reductiion of various technical noisse (induced byy mechanical vibratioons and tempeerature change)) in the low (ee.g.,