fibers

Article Properties of Scalable Chirped-Pulse Optical Comb in Erbium-Doped Ultrafast All-Fiber Ring

Ilya O. Orekhov 1, Dmitriy A. Dvoretskiy 1,* , Stanislav G. Sazonkin 1, Yan Z. Ososkov 2, Anton O. Chernutsky 1, Aleksander Y. Fedorenko 1, Lev K. Denisov 1 and Valeriy E. Karasik 1

1 Department of Laser and Optical-Electronic Systems, Bauman Moscow State Technical University, 105005 Moscow, Russia; [email protected] (I.O.O.); [email protected] (S.G.S.); [email protected] (A.O.C.); [email protected] (A.Y.F.); [email protected] (L.K.D.); [email protected] (V.E.K.) 2 Dianov Fiber Optics Research Center, Prokhorov General Physics Institute of the Russian Academy of Sciences, 117942 Moscow, Russia; [email protected] * Correspondence: [email protected]

Abstract: We report on a scalable chirped-pulse Er-doped all-fiber laser, passively mode-locked by single-wall carbon nitride nanotubes. The average output power is ~15 mW, which corresponds to a peak power of ~77 W, and pulse energy of ~1.9 nJ and was achieved using a single amplification stage. We observed chirped-pulse generation with a duration of ~24.6 ps at a relatively low repetition rate of ~7.9 MHz, with a signal-to-noise ratio of ~69 dB. To characterize the short-term stability of the obtained regime, we have measured the relative intensity noise of the laser, which is <−107 dBc/Hz in the range of 3 Hz–1000 kHz. It should be noted that the standard deviation of root mean square of  average power does not exceed a magnitude of 0.9% for 3 h of measurement. 

Citation: Orekhov, I.O.; Dvoretskiy, Keywords: erbium-doped fiber ; generation; femtosecond; highly nonlinear D.A.; Sazonkin, S.G.; Ososkov, Y.Z.; fiber; passive mode-locking; nonlinear polarization evolution effect Chernutsky, A.O.; Fedorenko, A.Y.; Denisov, L.K.; Karasik, V.E. Properties of Scalable Chirped-Pulse Optical Comb in Erbium-Doped Ultrafast 1. Introduction All-Fiber Ring Laser. Fibers 2021, 9, 36. Over the last decade, there has been an increase in the demand for high-energy and https://doi.org/10.3390/fib9060036 high-power laser systems of ultrashort pulses (USP) in areas such as various scientific applications [1], medical surgery [2,3], and industrial micromechanical processing [4,5]. Academic Editor: Pavel Peterka However, the creation of such high-energy systems is a nontrivial task, which is caused by the complex nonlinear dynamics of ultrashort pulses. Recent studies show [6] that the Received: 14 April 2021 chirped-pulse regime, also called dissipative solitons, can be a potential solution to this Accepted: 1 June 2021 Published: 2 June 2021 problem. Dissipative solitons are localized formations of the electromagnetic field, which are balanced by the exchange of energy with the environment in the presence of nonlinearity,

Publisher’s Note: MDPI stays neutral , and/or diffraction [7]; the speed limits imposed by an electron sampler [8]. with regard to jurisdictional claims in The method of amplifying chirped pulses is currently widely used for high-power optical published maps and institutional affil- amplification [9]. One of the advantages of dissipative solitons is an operation in the iations. region of normal dispersion of group velocities, which will increase the pulse energy and reduce the influence of nonlinear effects [10]. Today, many research groups are conducting research related to the generation and amplification of chirped pulses. However, most of the research is based on the use of fibers with an increased mode area. Thus, using large mode area (LMA) fibers to overcome nonlinearity, Sobon et al. demonstrated a 50 MHz Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Er-Yb co-doped chirped pulse amplification system, generating 835 fs compressed pulses This article is an open access article with an average power of 8.65 W [11]. The research team, led by M. Tang, generated distributed under the terms and dissipative solitons with a pulse energy of about 4 nJ and a pulse duration of about 30 ps, conditions of the Creative Commons using an Er-doped double concentric fiber with a core combining high normal dispersion Attribution (CC BY) license (https:// and large mode area [12]. Thus, since the appearance of the first works on the use of creativecommons.org/licenses/by/ LMA fiber to create high-power fiber lasers, this approach has become widespread in this 4.0/). area [6,10,13].

Fibers 2021, 9, 36. https://doi.org/10.3390/fib9060036 https://www.mdpi.com/journal/fibers Fibers 2021, 9, 36 2 of 7

However, it should be noted that the most common approach to LMA fiber fabrication is to complicate the fiber structure to obtain the lowest possible mode coupling during radiation propagation. This approach greatly complicates the fiber manufacturing process and therefore increases its cost, which in turn increases the cost of developing a laser. In this article, we used dispersion/nonlinearity and /loss control to create a chirped-pulse all-fiber USP laser, similar to the approach recently demonstrated in Bismuth-doped fiber lasers with a highly nonlinear cavity design by Khegai A. et al. [14]. For the first time, as far as we know, we propose to use a combined approach, which includes the use of carbon nanotubes and the regime of generation of dissipative solitons, which are known to have the best energy properties for further scaling [7]. We have developed an all-fiber erbium laser, operating in the second-order normal dispersion region, and single-wall carbon nitride nanotubes (C: BNNTs), used in lasers as a saturable absorber (SA) to reduce laser amplitude noise and provide reliable self-starting mode-locking (ML) [15,16]. As a result of the study, the stable generation of chirped pulses with a duration of 24.6 ps and a repetition rate of ~7.9 MHz was obtained. The average output power is ~15 mW, which corresponds to a peak power of ~77 W and pulse energy of ~1.9 nJ after one amplification stage. It should be noted that we measured the relative intensity of laser noise (RIN), which is <−107 dBc/Hz in the range 3 Hz–1000 kHz and the standard deviation(STD) of the average power root mean square(RMS) over a time of 3 h does not exceeded a value of 0.9%. These studies confirm a high level of generation stability compared to other devices of the same class [6,7].We hope that the presented concept opens up new opportunities for the development of low-cost high-energy mode-locked Er-doped fiber lasers.

2. Materials and Methods The experimental setup of the ML fiber laser is shown in Figure1. The SWCNTs: BNNTs film was used as a slow passive SA inserted into a module formed by two FC/APC connectors. The SA used in the work has a response time of up to 0.5 ps and a modulation depth of about 12% (at 1550 nm), which is sufficient for the reliable start-up of USP generation [15]. Unsaturated losses of the SA used in the work were about 29.2 ± 0.5% [16]. To provide unidirectional generation, a commercially available polarization insensitive (ISO) was used. To adjust the mode locking, a polarization controller (PC) was included into the cavity on one side of the ISO. The all-fiber ring cavity was formed by a 7.07 m active erbium-doped fiber (Lucent HE980) with a low signal core absorption of ~3 dB/m at the pump wavelength, with dispersion of D ~−54.6 ps/(nm·km) at 1550 nm and 18.5 m SMF-28 fiber (Corning Corp.) with dispersion of D ~17.4 ps/(nm·km) at 1550 nm. The lengths of the fibers were calculated to shift the total net-cavity GVD at 1550 nm into the slightly positive region and to ensure the chirped-pulse generation regime. 2 The total net-cavity β2 is estimated to be +0.12 ps at 1550 nm. A single-mode operating at the central wavelength of 980 nm with a maximum output power of 330 mW was used as the pump source for the erbium-doped fiber (EDF). Two wavelength demultiplexing filters (WDMs 980/1550) were used for inputting and outputting the pump radiation. The laser radiation was coupled out of the cavity through a fiber-optic coupler with a division ratio of 70/30. All-normal dispersion erbium-doped fiber amplifier (EDFA) with counter pumping was used to increase the linear chirp of the amplified pulses. EDFA also includes two wavelength demultiplexing filters for pump supply of the 10 m erbium-doped fiber with the same parameters as used inside the cavity. Fibers 2021, 9, x FOR PEER REVIEW 3 of 7 Fibers 2021, 9, 36 3 of 7

FigureFigure 1. 1. TheThe experimental experimental setup setup of of the the of of the the all all-fiber-fiber EDF EDF USP USP ring ring laser laser with with C:BNNTs C:BNNTs and and linear linear erbium erbium-doped-doped fiber fiber amplifieramplifier (EDFA) (EDFA)..

3. ExperimentalA single-mode Results laser diode and Discussion operating at the central wavelength of 980 nm with a max- imumWe output have power implemented of 330 mW a stable was single-pulse used as the self-startingpump source generation for the erbium of ultrashort-doped pulses fiber (EDF).by adjusting Two wavelength the PC. The demultiplexing inset of Figure2 filters shows (WDMs the typical 980/1550) output were pulse used trace for for inputting obtained andUSP outputting generation the (using pump the radiation.oscilloscope The Infinium laser r MSO9254A;adiation was Keysight coupled Technologies, out of the cavity Santa throughRosa, CA, a fiber USA)-optic at a coupler repetition with rate a division of 7.925 MHzratio of corresponding 70/30. All-normal to a total dispersion cavity lengtherbium of- doped~25.57 fiber m. The amplifier ML threshold (EDFA) was with observed counter atpumping an average was pump used to power increase of ~33 the mW, linear which chirp is oflower the amplified than usually pulses. reported EDFA in other also refsincludes connected two wavelength with a chirped-pulse demultiplexinggeneration filters [17 –for19] . pumpThe average supply outputof the 10 power m erbium of the-doped pulses fiber is ~1 with mW. the It shouldsame parameters be noted thatas used such inside a low thethreshold cavity. ML compared to only ML based on NPE [20] is, on the one hand, explained by the use of C: BNNTs SA, which reliably triggers ML at low pump power, as well as complex 3.dissipative Experimental dynamics, Results where and D theiscussion stable regime is only possible when there is a balance not only between dispersion and nonlinearity, but also in the intracavity gain and losses [7]. We have implemented a stable single-pulse self-starting generation of ultrashort Figure3 shows an experimentally observed optical spectrum before amplification at a pulses by adjusting the PC. The inset of Figure 2 shows the typical output pulse trace for central wavelength of 1558 nm, with a full width at half maximum (FWHM) of 15 nm obtained USP generation (using the oscilloscope Infinium MSO9254A; Keysight Technol- (obtained using the Yokogawa AQ6370D spectrum analyzer). Remarkably, the spectrum ogies,is flat-top Santa shaped Rosa, CA, with USA) steep at edges, a repetition which arerate inherent of 7.925 toMHz chirped-pulse corresponding generation to a total [7]. cavityUnlike length most of of ~25.57 the works m. The on dissipativeML threshold solitons was observed [21,22], no at spectralan average filter pump is used power for theof ~33mode-locking mW, which stabilization. is lower than The usually effect reported of spectral in filteringother refs in connected our laser iswith initiated a chirped by the- pulseeffect generation of temporal [1 filtering7–19]. The of C:average BNNT output itself since power the of generated the pulses optical is ~1 spectrummW. It should is limited be noted that such a low threshold ML compared to only ML based on NPE [20] is, on the by the transmission spectrum of the used nanotubes (∆λtr ~30 nm) [16]. one hand,However, explained such by a low the MLuse thresholdof C: BNNTs caused SA, difficultieswhich reliably in recording triggers theML pulse at low duration. pump power,Since theas well peak as outputcomplex power dissipative of the dynamics, master oscillator where the was stable too lowregime to correctlyis only possible record whenlong there pulse is widths, a balance the not EDFA only was between used todispersion amplify and the pulses.nonlinearity No pulse, but also breaking in the wasin- tracavityobserved gain after and amplification losses [7]. Figure of up to3 shows ~15 mW. an Figureexperimentally2 shows the obser intensityved optical autocorrelation spectrum before(AC) traceamplification after 10 at times a central averaging wavelength (obtained of 1558 by usingnm, with the a autocorrelator full width at half FR-103WS; maxi- mumFEMTOCHROME (FWHM) of 15 RESEARCH nm (obtained INC., using Berkeley, the Yokogawa CA, USA) andAQ6370D its Gaussian spectrum fitting. analyzer). The red Remarkably,dashed line in the Figure spectrum2 shows is the flat Gaussian-top shaped fitting with to the steep envelope edges, ofwhich the experimental are inherent data to chirped(black dashed-pulse generation line). From [7]. the Unlike fitting, most the autocorrelation of the works on width dissipative was estimated solitons to [2 be1,2 31.82], no ps spectralwhich corresponds filter is used to for 24.6 the ps mode chirped-pulse-locking stabilization. width at FWHM. The effect It results of spectral in a pulse filtering width in of ourassuming laser is Gaussianinitiated by pulse the approximation.effect of temporal It shouldfiltering be of noted C: BNNT that theoretically,itself since the the generated obtained opticalpulses spectrum can be compressed is limited by in the the transmission case of full compensation spectrum of the of theused chirp nanotubes to a duration (∆λtr ~30 of nm)about [16 170]. fs, in accordance with the formula for pulses with the shape sech2 [23]:

1 τpulse = 0.315 (1) ∆ν(FWHM) Fibers 2021, 9, 36 4 of 7

where ∆ν(FWHM) is the width (FWHM) of the optical spectrum (about 1.86 THz in our case). The time-bandwidth product (TBP) of the pulses is equal to 35.9, indicating the presence of Fibers 2021, 9, x FOR PEER REVIEWuncompensated chirp in the pulses. Relatively long pulses increase the intensity threshold4 of 7 Fibers 2021, 9, x FOR PEER REVIEWof the nonlinear effects, such as self-phase modulation (SPM) and stimulated Raman4 of 7 scattering (SRS) in fiber, which could affect the laser performance in future applications. The inset of Figure2 shows the typical output pulse train for USP generation (obtained by using the oscilloscope Infinium MSO9254A; Keysight Technologies, Santa Rosa, CA, USA).

(a) (b) (a) (b) Figure 2. (a) An USP intensity autocorrelation and Gaussian fitting with a typical pulse train (inset); (b) Chirped-pulse FigurespectrumFigure 2. 2.(a before)(a An) An USP amplification USP intensity intensity autocorrelation of autocorrelation the all-fiber EDF and and USP Gaussian Gaussian ring laser fitting fitting with with with C:BNNTs. a typical a typical pulse pulse train train (inset); (inset); (b) (b Chirped-pulse) Chirped-pulse spectrumspectrum before before amplification amplification of of the the all-fiber all-fiber EDF EDF USP USP ring ring laser laser with with C:BNNTs. C:BNNTs.

FigureFigure 3. 3.RF RF spectrum spectrum of of the the output output pulse pulse train train at at the the fundamental fundamental repetition repetition frequency frequency (the (the central cen- peaktralFigure peak corresponds 3. corresponds RF spectrum to the to ofrepetition the the repetition output rate pulse rate of ~7.925 trainof ~7. at925 MHz, the MHz, fundamental RBW RBW = 300 = 300 Hz).repetition Hz Inset). Inset frequency shows shows an an (the average average cen- outputoutputtral peak optical optical corresponds powerpower duringduring to the therepetitionthe measurement rate of ~7.925 and and a a full MHz, full scale scale RBW (up (up = to300 to 200 200 Hz). MHz) MHz) Inset RF shows RF spectrum spectrum an average of of modeoutput-locked optical pulses power (RBW during = 3 the kHz). measurement and a full scale (up to 200 MHz) RF spectrum of mode-locked pulses (RBW = 3 kHz). mode-locked pulses (RBW = 3 kHz). ToHowever, characterize such the a low short-term ML threshold stability caused of the obtaineddifficulties USP in generation,recording the we pulse measured dura- thetio radion. SinceHowever, frequency the peak such (RF) output a low spectrum powerML threshold at of the the fundamental master caused oscillator difficulties oscillation was in too frequency recording low to ofcorrectly the ~7.925 pulse MHz,record dura- shownlongtion. pulse Since in Figure widths, the peak3 (with the output EDFA a FEMTO power was used 200of the MHz to master amplify HCA-S-200M oscillator the pulses. was InGaAs No too pulse low photodetector tobreaking correctly was andrecord ob- usingservedlong ESApulse after FSL widths, amplification 3 model.03; the EDFA Rohdeof up was to & used~15 Schwarz mW. to amplify Figure GmbH the2 & shows Co.pulses. KG, the No Munich, intensity pulse breaking Germany) autocorrela was withtion ob- a(AC) signal-to-noiseserved trace after after amplificationratio 10 times (SNR) averagingof ~69 up dBto ~15 (with (obtained mW. ESA Figure resolution by using2 shows 300 the the Hz). autocorrelator intensity The inset autocorrelation in FR Figure-103WS;3 showsFEMTOCHROME(AC) the trace RF after spectrum 10 RESEARCH times in the averaging frequency INC., Berkeley, (obtained range 30CA, by kHz–200 USA) using and MHz the its autocorrelator Gaussian (resolution fitting. bandwidth FR-103WS; The red ~3dashedFEMTOCHROME kHz). Itline should in Figure be RESEARCH noted 2 shows that in theINC., addition Gaussian Berkeley, to the fitting CA, main USA)to peak the and atenvelope a its frequency Gaussian of the of fitting. ~7.925experimental The MHz, red datadashed (black line dashed in Figure line). 2 showsFrom the the fitting, Gaussian the fittingautocorrelation to the envelope width was of the estimated experimental to be 31.8data ps (black which dashed corresponds line). From to 24.6 the ps fitting, chirped the-pulse autocorrelation width at FWHM. width wasIt results estimated in a pulse to be width31.8 ps of which assuming corresponds Gaussian to pulse 24.6 psapproximation. chirped-pulse It width should at FWHM.be noted It that results theore in tically,a pulse thewidth obtained of assuming pulses Gaussiancan be compressed pulse approximation. in the case of It fullshould compensation be noted that of the theoretically, chirp to a durationthe obtained of about pulses 170 can fs, bein compressedaccordance with in the the case formula of full for compensation pulses with ofthe the shape chirp sech to 2a [2duration3]: of about 170 fs, in accordance with the formula for pulses with the shape sech2 [23]: Fibers 2021, 9, x FOR PEER REVIEW 5 of 7

1 휏푝푢푙푠푒 = 0.315 (1) ∆휈(퐹푊퐻푀)

where ∆휈(퐹푊퐻푀) is the width (FWHM) of the optical spectrum (about 1.86 THz in our case). The time-bandwidth product (TBP) of the pulses is equal to 35.9, indicating the pres- ence of uncompensated chirp in the pulses. Relatively long pulses increase the intensity threshold of the nonlinear effects, such as self-phase modulation (SPM) and stimulated Raman scattering (SRS) in fiber, which could affect the laser performance in future appli- cations. The inset of Figure 2 shows the typical output pulse train for USP generation (ob- tained by using the oscilloscope Infinium MSO9254A; Keysight Technologies, Santa Rosa, CA, USA). To characterize the short-term stability of the obtained USP generation, we measured the radio frequency (RF) spectrum at the fundamental oscillation frequency of ~7.925 MHz, shown in Figure 3 (with a FEMTO 200 MHz HCA-S-200M InGaAs photodetector and using ESA FSL 3 model.03; Rohde & Schwarz GmbH & Co. KG, Munich, Germany) Fibers 2021, 9, 36 with a signal-to-noise ratio (SNR) ~69 dB (with ESA resolution 300 Hz). The inset in Figure5 of 7 3 shows the RF spectrum in the frequency range 30 kHz–200 MHz (resolution bandwidth ~3 kHz). It should be noted that in addition to the main peak at a frequency of ~7.925 MHz, we observed two more side peaks (see Figure 3), shifted by ~30 kHz, which indicate the presencewe observed of modulation two more of sidethe output peaks (seesignal Figure intensity.3), shifted However, by ~30 the kHz, intensity which modulation indicate the factorpresence ∆I of ofthe modulation pulse train ofis theestimated output to signal be ~− intensity.39 dB, and However, these intensity the intensity fluctuations modulation do − notfactor significantly∆I of the affect pulse the train generati is estimatedon characteristics. to be ~ 39 Most dB, and likely, these this intensity feature fluctuationsis associated do withnot the significantly process of affect pulse the formation generation and characteristics. can be eliminated Most by likely, optimizing this feature the islengths associated of with the process of pulse formation and can be eliminated by optimizing the lengths of active and passive fibers in the cavity. There is also a slight intensity modulation in the RF active and passive fibers in the cavity. There is also a slight intensity modulation in the RF spectrum (see inset to Figure 3), which is caused by current modulation on the pump di- spectrum (see inset to Figure3), which is caused by current modulation on the pump diode ode driver. driver. To characterize the long-term stability of the obtained generation, we measured the To characterize the long-term stability of the obtained generation, we measured the output average power stability. The inset of Figure 3 shows the stability of the average output average power stability. The inset of Figure3 shows the stability of the average optical output power with a standard deviation of ~0.9% RMS over a time of 3 h, deter- optical output power with a standard deviation of ~0.9% RMS over a time of 3 h, deter- mined mainly by temperature drift (measured with a PM200 power meter with an InGaAs mined mainly by temperature drift (measured with a PM200 power meter with an InGaAs S145C detector, Thorlabs Inc., Newton, NJ, USA). Additionally, we have measured the S145C detector, Thorlabs Inc., Newton, NJ, USA). Additionally, we have measured the relative intensity noise (RIN) of a fiber laser with a maximum value of <−107 dBc/Hz, de- relative intensity noise (RIN) of a fiber laser with a maximum value of <−107 dBc/Hz, picteddepicted in Figure in Figure 4, in4 the, in therange range 30 Hz 30– Hz–100100 kHz kHz (at (at250 250 Hz Hzresolution resolution using using ESA ESA SR770FFT, SR770FFT, StanfordStanford Research Research Systems. Systems. CA CA,, USA). USA). The The peak peakss on on ~12 ~12 kHz kHz and and ~91 ~91 kHz kHz are are due due to to radioradio-frequency-frequency interference interference

Figure 4. RIN of the chirped-pulse USP fiber laser and the receiving system noise floor. Inset: RIN in Figure 4. RIN of the chirped-pulse USP fiber laser and the receiving system noise floor. Inset: RIN the high-frequency range. in the high-frequency range. The inset of Figure4 shows the RIN in the high frequency range from 30 kHz to 1 MHz (300 Hz resolution using Rohde & Schwarz ESA, Munich, Germany) and corresponds to the level of PD + ESA noise floor by the black curve. It is obvious that in high-frequency range, the intensity noise level is lower than <−130 dBc/Hz and does not affect the output laser radiation. The main contribution in the total RIN level is described by 30 kHz peak that could be further suppressed as described previously.

4. Conclusions In summary, the generation of stable chirped pulses in a ring resonator with a slightly positive second-order dispersion of +0.12 ps2 was obtained. We observed chirped pulses with a duration of about 24.6 ps at a central wavelength of 1558 nm, with an average output power of 15 mW, corresponding to a maximum peak power of 77 W and maximum pulse energy of <1.9 nJ, after one amplification stage. The RIN value was <−107 dBc/Hz (3 Hz–1000 kHz) at a repetition rate of 7.925 MHz, with a signal-to-noise ratio of ~69 dB (with a resolution of ~300 Hz) in an erbium-doped ring laser with mode-locking, based on single-walled C: BNNTs SA. It should be noted that the standard deviation of the average output power did not exceed ~0.9% RMS in 3 h of measurements. Thus, the presented system is a splendid example of a high-energy laser, which can be used as a master oscillator Fibers 2021, 9, 36 6 of 7

in medical surgery and in industrial micromechanical processing, as well as a seeding laser in distributed temperature and strain fiber sensor systems.

Author Contributions: Conceptualization, S.G.S., L.K.D. and V.E.K.; Data curation, Y.Z.O., A.O.C. and A.Y.F.; Formal analysis, A.O.C. and L.K.D.; Funding acquisition, D.A.D.; Investigation, I.O.O.; Project administration, S.G.S.; Software, I.O.O.; Supervision, D.A.D. and V.E.K.; Validation, L.K.D.; Writing—original draft, I.O.O.; Writing—review & editing, D.A.D. All authors have read and agreed to the published version of the manuscript. Funding: The reported study was funded by RSF, according to the research project No. 19-72-00090. Anton Chernutsky was supported by RFBR according to project No. 19-32-90185. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are provided upon request. Contact [email protected]. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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