Low noise all-fiber chirped pulse amplification of a coherent supercontinuum at 2 m
We report the amplification of an all-normal dispersion supercontinuum pulse in a Thulium / Holmium co-doped all-fiber chirped pulse amplification system. With a -20 dB bandwidth of more than 300 nm in the range 1800-2100 nm the system delivers high quality 66 fs pulses with more than 70 kW peak power directly from the output fiber. The coherent seeding of the entire emission bandwidth of the doped fiber and the stability of the supercontinuum generation dynamics in the silicate glass all-normal dispersion photonic crystal fiber result in excellent noise characteristics of the amplified ultrashort pulses.
Dispersion engineering in solid-core photonic crystal fibers (PCF) has enabled unprecedented control over nonlinear phenomena and consequently also over spectral, temporal, and coherence properties of supercontinuum (SC) pulses Mogilevtsev1998 (); Dudley2006 (); Dudley2010 (); Alfano2016 (). While this was exploited in many applications Dudley2010 (); Alfano2016 (); Udem2002 (), an important drawback is given by the fact that small core diameters and large refractive index steps are required for effective dispersion engineering Knight2000 (); Poletti2011 (). As a result, nonlinear fiber optic methods for shaping ultrashort pulses in the spectral or temporal domain usually remain restricted to relatively low pulse energies of a few nano Joule, limited by the damage threshold of small-core PCF Seidel2018 ().
One approach for boosting the pulse energy is using the entire or partial spectral bandwidth of low energy SC pulses for coherent seeding of broadband amplifiers. To date and in the context of optical fiber amplifiers, this has mainly been explored using the solitonic part of conventional SC pumped in the anomalous dispersion region of PCFs or highly nonlinear fibers Kumkar2012 (); Coluccelli2013 (); Tan2016 (); Sobon2018 (). In contrast to these previous works we propose the use of SC pulses generated in all-normal dispersion (ANDi) fibers as seed sources, because they offer several key advantages that become especially relevant in the context of ultrashort pulse amplification Heidt2016 (): (i) noise amplifying nonlinear dynamics are suppressed in ANDi fibers such that the generated SC can have lower relative intensity noise (RIN) than the pump laser Heidt2017 (); Rao2018 (); Genier2019 (). In contrast, conventional SC and Raman-shifted solitons amplify pump laser RIN Coluccelli2013 (); Newbury2003 (). (ii) While conventional SC can exhibit significant temporal jitter between wavelength components, the jitter of wavelength components far from the pump is reduced by 2 orders of magnitude in normal dispersion SC, reaching attosecond precision Rothhardt2012a (). This is essential for applications requiring precise synchronisation of the amplified pulses to the pump or other lasers. (iii) In contrast to their conventional counterparts, ANDi SC preserve the temporal integrity of the pump pulse and exhibit flat, smooth and stable spectra that can be compressed to high quality single-cycle pulses Heidt2011b (); Demmler2011 (). This enables, for example, the synchronous ultrafast seeding of multiple amplifiers operating at different wavelengths.
These highly beneficial properties have been exploited in strong-field physics applications, for example, where carrier-envelope stablilized ANDi SC pulses, amplified to more than 20 W average power in an optical parametric chirped pulse amplifier, were instrumental in enabling the first demonstrations of high harmonics generation and isolated attosecond pulses at high repetition rate and average power Rothhardt2012b (); Krebs2013 (); Rothhardt2017 (). While these results illustrate the benefits of using ANDi SC pulses for coherent ultrafast seeding of broadband amplifiers, this approach has so far not been applied to fiber optic systems.
In this Letter we investigate the potential of using ANDi SC pulses for the broadband coherent seeding of fiber amplifiers. We chose an ANDi PCF optimised for pumping at 1560 nm to create SC pulses suitable for seeding the large gain bandwidth of a Thulium/Holmium (Tm/Ho) co-doped fiber chirped pulse amplifier (FCPA) in the range 1750-2200 nm. The FCPA is realized in an all-fiber design, i.e. both dispersive pulse stretching and compressing is accomplished in optical fibers. We show that the stability of the SC generation dynamics in ANDi fibers as well as the suppression of amplified spontaneous emission (ASE) noise by broadband coherent seeding of the amplifying medium results in excellent noise characteristics of the amplified ultrashort pulses. The system supports a -20 dB bandwidth of more than 300 nm and delivers high quality 66 fs pulses with more than 70 kW peak power directly from the output fiber.
The experimental setup is illustrated in Fig. 1. We employ an FCPA scheme, consisting of a SC generation and dispersive pulse stretching stage, a Tm/Ho co-doped fiber amplifier stage, and a fiberized pulse compression stage. While the pump pulses for the SC generation are free-space coupled to the PCF, the subsequent FCPA system is realised in an all-fiber configuration such that the amplified and compressed pulses are directly available at the fiber exit of the system.
Coherent SC generation in an in-house drawn 20 cm long ANDi PCF is used to spectrally broaden the 1560 nm pump pulses sufficiently for seeding the entire gain bandwidth of the Tm/Ho amplifier in the 1750 - 2200 nm range. The PCF has a core with diameter of 2.3 m surrounded by a photonic crystal lattice realized in an all-solid design using two thermally compatible silicate glasses Klimczak2016 (). Schott SF6 is used as core and lattice glass, Schott F2 for lattice inclusions and tube glass. The fiber design provides high nonlinearity ( 214 (W km) at 1560 nm) and normal group velocity dispersion (GVD) over the entire wavelength region of interest, as shown in the measured GVD curve in Fig. 2 (a). A commercial ultrafast Erbium-doped fiber amplifier system (Toptica), delivering 100 fs pulses with 80 MHz repetition rate at a central wavelength of 1560 nm, is used for pumping the PCF near its minimum dispersion wavelength with an estimated coupled peak power of 20 kW. The resulting full SC spectrum, shown in logarithmic scale in the inset of Fig. 3 (a), covers the wavelength range 1120 - 2150 nm at a -20 dB level.
Figure 2 (b) shows a typical measured projected axes spectrogram of the SC pulse in linear scale at the exit of the ANDi PCF under similar pumping conditions, using the spectrally filtered pump laser with about 10 nm full-width at half-maximum (FWHM) spectral bandwidth for gating. Due to the normal GVD design of the fiber and the short pump pulse duration, the SC generation process is dominated by coherent, noise-insensitive nonlinear processes, i.e. self-phase modulation (SPM) and optical wave braking, which preserve the temporal integrity of the pump pulse Heidt2017 (). Consequently, a highly coherent SC pulse is generated with a chirp that is predominantly linear at the long wavelength wing between 1750 - 2200 nm, i.e. very well suited as seed for the Tm/Ho amplifier.
The SC pulses are further dispersively stretched in 6.8 m of UHNA 7 fiber (Nufern; core diameter 2.4 m, 0.41 NA), which has normal dispersion in the Tm/Ho amplification window Ciacka2018 (). The measured GVD curve of this fiber is shown in Fig. 2 (a) and compared to the GVD of a standard single-mode fiber (SM 2000, Thorlabs). The choice of stretching fiber is a critical issue for the performance of the system as it should ideally compensate not only second order, but also higher order dispersion terms of the active and passive fibers used in the system. This applies especially to all-fiber CPA systems as the compression in optical fiber necessarily implies an increased propagation distance through dispersive media compared to free-space compression, and therefore also an increased influence of higher-order dispersion on the pulse shape and quality. The measured second and third order dispersion coefficients of UHNA 7 at 1900 nm are fs/mm and fs/mm, respectively, thus compensating both GVD and third order dispersion of the SMF ( fs/mm and fs/mm). In addition, UHNA 7 can also act as an efficient mode field adapter between PCF and SMF, because its small core with high NA efficiently collects the light exiting the PCF while the expansion of the core under fusion splicing can be used to minimize splice loss to SMF to ¡ 0.2 dB. This allows seeding the subsequent amplifier with approx. 11 mW of average power contained in the relevant spectral range of 1750-2200 nm of the SC pulse.
The amplifier consists of 140 cm of Tm/Ho co-doped single-mode fiber (CorActive Th512; 9 m core diameter, 0.16 NA, 150 dB/m core absorption at 790 nm), backward core-pumped via a wavelength-division multiplexer by an in-house built Erbium/Ytterbium co-doped single-mode fiber laser operating at 1560 nm. The length of the Tm/Ho doped fiber was optimized for maximum FWHM spectral bandwidth of the amplified signal. Since the pulses are positively chirped at the exit of the amplifier, temporal recompression can simply be achieved by adding an appropriate length (about 1.2 m) of standard single-mode fiber (SM2000, Thorlabs) exhibiting anomalous dispersion in the 2 m window. Spurious reflections inside the amplifier are avoided by an in-line isolator at the amplifier input and an angle-cleaved fiber end-facet at the exit of the system. The polarization state is controlled by a half-wave plate in front of the ANDi PCF and a polarization controller near the output. The seeding level is sufficient to operate the amplifier system near saturation, delivering a maximum of 0.5 W average output power at a pump power of 2.5 W. Further power scaling is limited by nonlinear effects and related pulse distortions in the compression fiber.
The spectrum of the amplified signal in relation to the SC seed is shown in Fig. 3. The SC spectrum is sufficiently broad to coherently seed the entire gain bandwidth of the amplifier. As a consequence, incoherent amplified spontaneous emission (ASE) noise is not observed in the spectral domain, and is also not expected to build up temporally between pulses due to the high repetition rate in comparison to the long ( ms) excited state life time of Tm- and Ho-ions. The amplified signal has a central wavelength of 1900 nm, a -20 dB spectral bandwidth of 310 nm (1798-2108 nm) and a FWHM of 100 nm. The Fourier-limited pulse duration is 45 fs, obtained from direct Fourier-transformation of the measured spectrum assuming flat spectral phase, resulting in a time-bandwidth product of TBP = 0.375. This indicates a pulse shape half way in between Gaussian and . Note that the spectrum does not significantly change or broaden in the compression fiber, i.e. we do not employ additional nonlinear compression to shorten the pulse.
Figure 4 (a) shows the measured autocorrelation (AC) trace of the system output with a FWHM of 98 fs. Applying the same deconvolution factor as obtained for the Fourier-limited pulse results in a pulse duration of 66 fs. The AC trace is exceptionally clean for an all-fiber CPA system; it does not exhibit any major side peaks, only the broadened base of the main peak indicates the presence of a small pedestal. The absence of further side pulses was checked using an AC scan window of 150 ps as well as a fast photodiode and oscilloscope combination with 30 ps rise time.
For further evaluation of the pulse quality we attempt to computationally reproduce the measured AC trace by adding third and fourth order spectral phase terms to the measured spectrum, taking the Fourier transform and calculating the AC trace of the corresponding pulse. The uncompensated third order phase is assumed to stem predominantly from linear propagation through the FCPA system. Therefore, it is calculated from the measured GVD curves in Fig. 2 (a) and the lengths of the different fiber types used in the system to approximately ps. The fourth order spectral phase is then determined in an iterative procedure finding the best fit to the experimentally measured trace. This allows for nonlinear phase contributions resulting from the SC generation process in the ANDi fiber. A good fit is found for ps, as shown in Fig. 4 (a). While the calculation results in a slightly longer pulse duration than measured experimentally, the shape of the AC trace is well reproduced. The corresponding deconvoluted pulse shape shown in Fig. 4 (b) reveals that 75% of the energy is contained in the central peak, with the remaining part mostly located in a low-level pedestal at the leading pulse edge. Using the measured pulse energy and duration and correcting for the energy content of the pedestal, we estimate the peak power of the output pulse to approximately 71 kW.
Recently there have been reports of instabilities and coherence degradation of ANDi SC pulses related to polarization modulation instability (PMI), in particular in non-polarization-maintaining fibers as we use in this system Liu2015 (); Gonzalo2018 (). PMI amplifies spectral components seeded by shot noise and leads to significant pulse-to-pulse fluctuations in spectral phase and polarization state of the pulse, which could severely degrade the performance of the FCPA system. Therefore, we examined the presence of PMI by propagating the system output through a polarizer, which converts the polarization state fluctuations to amplitude fluctuations, and characterizing the pulse train with a fast photodiode and radio-frequency (RF) spectrum analyzer. PMI would then be detectable by a broadband noise component around the repetition frequency of the system Liu2015 (). Figure 5 compares the RF spectra of the 1560 nm femtosecond laser system used to pump the ANDi PCF and the FCPA output centered around 1900 nm. Both spectra were recorded with the same incident power level on the photodiode and a resolution bandwidth (RBW) of 1 kHz. No significant changes in the FCPA spectrum were detected when rotating the polarizer. The two spectra are virtually indistinguishable, i.e. the noise characteristics of the FCPA system are determined by the seed laser. Hence, neither the SC generation process nor the subsequent amplification contribute any significant amplitude noise, and broadband noise associated with PMI is not observed. In fact, when zooming in on the base level of the fundamental frequency peak, the noise band of the FCPA system is located slightly below the noise of the seed laser. Following the procedure outlined by von der Linde Linde1986 (), we calculate r.m.s. pulse energy fluctuations of for the 1560 nm seed laser and for the FCPA. The minute difference lies within the measurement error. This should be contrasted to the results obtained when using the solitonic part of anomalously pumped SC for seeding Tm/Ho fiber amplifiers, where the noise of the amplified output can be up to four times higher than the intensity fluctuations of the laser pumping the SC Coluccelli2013 (). We attribute these excellent noise characteristics of the amplifier to three factors: (i) the high coherence and stability of the SC generation process in the ANDi fiber, which can reduce the RIN of the pump laser Genier2019 (); Rao2018 (); (ii) the broadband coherent seeding of the entire emission bandwidth of the amplifying medium, which minimizes ASE noise; and (iii) the operation of the amplifier near saturation, which dampens output power fluctuations with respect to seed level variations.
In conclusion, we demonstrated a Tm/Ho co-doped all-fiber chirped pulse amplification system operating in the 1750-2200 nm spectral region, coherently seeded by a SC pulse generated in an in-house drawn silicate glass PCF with all-normal dispersion profile. The system supports a -20 dB spectral bandwidth of more than 300 nm and delivers 66 fs pulses with 70 kW peak power directly at the output fiber. The design exploits the excellent stability of the SC generation dynamics in ANDi fibers as well as the suppression of ASE noise by broadband coherent seeding of the doped fiber to provide excellent low-noise amplification performance. These proof-of-principle experiments highlight the benefits of using ANDi SC pulses for coherent ultrafast seeding of broadband fiber optic amplifiers. In future, the environmental stability and handling will be improved by implementing the entire system in a polarization-maintaining (PM) architecture, as PM-ANDi PCF and normally dispersive PM fibers for pulse stretching are now available. Free-space compression and additional power amplifier stages will allow for further energy and peak-power scaling.
Funding. Swiss National Research Foundation (SNSF) (NCCR MUST). Fundacja na rzecz Nauki Polskiej (FNP) (First TEAM/2016-1/1). Schlumberger Foundation, Faculty for the Future.
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