We report soliton rains in a tunable Tm-doped fiber laser mode locked by carbon nanotubes. We also detect their second- and third-harmonics. We achieve a tunability of over 56 nm, from 1877 to 1933 nm, by introducing a polarization-maintaining isolator and two in-line polarization controllers. This makes our system promising as a tunable filter for ultrafast spectroscopy.We acknowledge funding from ERC Grant Hetero2D, EPSRC Grants Nos. EP/L016087/1, EP/K017144/1, EP/K01711X/1 and the China Scholarship Council

Ferrari, Andrea C.

Flahaut, Emmanuel

Fu, B.

Hasan, Tawfique

Hussain, Syed Asad

Popa, Daniel

Soavi, Giancarlo

Zhao, Z.

English

arXiv

International audienceWe report soliton rains in a tunable Tm-doped fiber laser mode locked by carbon nanotubes. We also detect their second- and third-harmonics. We achieve a tunability of over 56 nm, from 1877 to 1933 nm, by introducing a polarization-maintaining isolator and two in-line polarization controllers. This makes our system promising as a tunable filter for ultrafast spectroscopy

Hal-Diderot

Wavelength tunable soliton rains in a nanotube-mode locked Tm-doped fiber laser

We report soliton rains in a tunable Tm-doped fiber laser mode locked by carbon nanotubes. We also detect their second- and third-harmonics. We achieve a tunability of over 56 nm, from 1877 to 1933 nm, by introducing a polarization-maintaining isolator and two in-line polarization controllers. This makes our system promising as a tunable filter for ultrafast spectroscopy

Open Archive Toulouse Archive Ouverte

Fu, B

Popa, D

Zhao, Z

Hussain, SA

Flahaut, E

Hasan, T

Ferrari, AC

Apollo (Cambridge)

arXiv:1807.02722v1  [physics.optics]  7 Jul 2018Wavelength tunable soliton rains in a nanotube-mode locked Tm-doped fiber laserB. Fu1,2,3, D. Popa1, Z. Zhao1, S. A. Hussain1, E. Flahaut4, T. Hasan1, A. C. Ferrari11Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, UK2Beijing Advanced Innovation Center for Big Data-Based Precision Medicine, Beihang University, Beijing 100191, China3School of Instrument Science and Opto-Electronic Engineering, Beihang University, Beijing, 100191, China and4University Paul Sabatier, CIRIMAT/LCMIE, CNRS UMR 5085, Toulouse, FranceWe report soliton rains in a tunable Tm-doped fiber laser mode locked by carbon nanotubes. Wealso detect their second- and third-harmonic. We achieve a tunability of over 56nm, from 1877 to1933nm, by introducing a polarization-maintaining isolator and two in-line polarization controllers.This makes our system promising as a tunable filter for ultrafast spectroscopy.Soliton rains (SR) in mode-locked fiber lasers aredrifting pulses generated from the continuous wave(CW) background noise[1, 2] or the condensed soli-ton phase (CP) (i.e., a static pulse instead of drift-ing ones)[3–6]. These drift towards the next adjacentCP spontaneously[1–8]. They were first reported inRefs.[1, 2], and since then in many other mode-lockedfiber lasers[3–8]. SR are characterized by 3 proper-ties: drift, CP and CW background noise[1, 2]. SRwere first reported in Er-doped fiber lasers[1, 2], gen-erated from the CW background noise and drifting to-wards the CP, triggered by the injection of an exter-nal CW laser[1, 2]. SR were also generated from theCP in a normal dispersion Yb-doped fiber laser witha dual-filter effect[3], providing insights on the dissi-pative soliton SR dynamics[3]. SR have been stud-ied at 1µm[3–5] and 1.5µm[1, 2, 6, 7], with one re-port at 2µm based on a nonlinear amplified loop mirrorcavity[8]. However, Ref.[8] exploited a two-ring cavitydesign, with a rather long cavity (116m) compared tostandard (∼10m) soliton fiber lasers[1–7], and no wave-length tunability[8]. Therefore, to better understand thesoliton dynamics in different wavelength regions (e.g., the2µm standard soliton fiber lasers[9]), it is necessary tofind simple, wavelength tunable methods able to sup-port SR. Tm-doped fiber lasers operating in the 2µmregion an eye-safe band because liquid water (main con-stituent of human tissues) absorbs strongly[10], have po-tential applications in molecular spectroscopy[11], opti-cal communications[12], medicine[13] and light detectionand ranging (LIDAR)[14].Here, we report soliton rains in a broadband wave-length tunable Tm-doped fiber laser mode-locked by acarbon nanotube (CNTs) saturable absorber (SA). Thesecond- and third-harmonic SRs are also observed. Theoutput wavelength can be tuned from 1877 to 1933nm bytuning two in-line polarization controllers, making thispromising as tunable filter for ultrafast spectroscopy.CNTs can be used as SAs for mode-locking[15–23], asthey provide high modulation depths (∼17%[21]), pre-ferred for mode-locked fiber lasers[15–23] typically oper-ating with higher gain and cavity losses than their solid-state counterparts[24], which is much larger than whatEDFAWDMTDFPM-ISOPC2CNT-SA CouplerPC1LD FIG. 1: Fiber laser set up. LD: laser diode, EDFA:Erbium-doped fiber amplifier, WDM: wavelength-divisionmultiplexer, TDF: Tm-doped fiber, PC: polarization con-troller, PM-ISO: polarization-maintaining isolator, CNT-SA:carbon nanotube saturable absorber.is usually reported for single layer graphene devices[23,25, 26] (e.g., ∼0.5% in Ref.[26]). Here we use solu-tion processed CNTs to fabricate the SAs[22]. 0.03wt%purified laser ablation single-wall CNTs[27, 28] and0.02wt% of purified catalytic CVD grown double-wallCNTs (DWNTs)[29] are ultrasonicated separately for anhour with 0.7wt% sodium-carboxymethylcellulose (Na-CMC) polymer using a tip sonicator at 100W outputpower and 10◦C for 4 hours. The dispersions are thenultra-centrifuged. The topmost 60% is decanted, thenmixed and ultrasonicated again for 30mins, then drop-cast and the solvent evaporated in a desiccator, re-sulting in a∼30µm thick CNT-CMC composite. Themean inner and outer diameters of the DWNTs are∼1.1and 1.8nm[18], resulting in absorption peaks at∼1.1 and1.9µm[18, 30].A schematic of our Tm-doped fiber laser setup isshown in Fig.1. It consists of a 2.9m Tm-doped fiber(TDF, SCF-TM-8/125, with group velocity dispersion(GVD)∼69ps2/km at 1900nm), a wavelength-divisionmultiplexer (WDM) to couple the pump source (1560nmlaser diode (LD) amplified by an Er-doped fiber ampli-fier, EDFA) into the TDF, a polarization-maintaining21860 1880 1900 1920 1940-60-40-20-40 -20 0 20 40-120-100-80-60-40-200 -100 0 100 2000.000.010.020.030.040.05(c)Normalized intensity(a)Intensity (a.u.) Intensity (dBm)Wavelength (nm)~ 8 nm(b) Intensity (dBm)Frequency (f-f1) (kHz)f1 = 17.75 MHz(d)~75 dB Time (ns)-2 -1 0 1 20.00.20.40.60.81.0  Experiment Gauss fitTime (ps)1.41 x 506 fsFIG. 2: (a) Optical spectrum. (b) Temporal waveform. (c)Autocorrelation trace. (d) RF spectrum.isolator (PM-ISO, 0.81m PM 1550 fiber pigtail) to en-sure unidirectional operation, two in-line polarizationcontrollers (PCs) to adjust cavity polarization, and anoptical coupler with a 20% output. The CNT-SA issandwiched between the two fiber ferrules. The rest ofthe cavity is a standard single-mode fiber (SMF-28, withGVD∼67ps2/km at 1900nm), with length∼11.7m. Thetotal dispersion is∼-0.79 ps2. The output is monitored byan optical spectrum analyzer, an oscilloscope via an ex-tended InGaAs photodetector, an autocorrelator, a spec-trum analyzer, and a power meter.Mode-locking self-starts when the pump powerreaches∼500mW. The output power is 16.3mW and thepulse energy is 0.92nJ. As shown in Fig.2(a), the cen-tral wavelength is 1893nm with full width at half maxi-mum (FWHM)∼14.5nm. The arrows in Fig.2(a) indicatethe 8nm wavelength spacing caused by the Lyot birefrin-gence filter effect[31], attributed to the two PCs and thePM-ISO. This can be calculated as ∆λ = λ2/(L∆n)[31],where L is the length of the PM fiber and ∆n is themodal birefringence. The sharp dips in the spectrum areattributed to the intrinsic absorption peaks and molec-ular resonances of the Tm-doped fiber[32, 33]. Fig.2(b)shows the temporal waveform at the fundamental repeti-tion rate (FRR)∼17.75MHz, corresponding to the cavitylength∼11.7m. Fig.2(c) plots the autocorrelation trace ofthe pulses. A good fit is obtained by using a Gauss pro-file, with FWHM∼506fs. The time-bandwidth product(TBP) is 0.614, larger than the theoretical value (0.441)for transform-limited of Gauss-shaped pulse[34], indicat-ing minor chirping[34]. The signal-to-noise ratio (SNR)of the radio-frequency (RF) spectrum with 10Hz resolu-tion [Fig.2(d)] is 75dB at the FRR, which indicates stable1860 1880 1900 1920-60-40-20-4 -2 0 2 4-120-100-80-60-400 100 200 300 400 500-80-60-40Intensity (a.u.) Normalized intensity Intensity (dBm)Wavelength (nm)~ 8 nm(b)Time (ns)Time (ns)-2 -1 0 1 20.00.20.40.60.81.0  Experiment Gauss FitTime (ps)1.41 x 534 fs(a)(c)Intensity (dBm)  Intensity (dBm)Frequency (f-f1) (kHz)f1 = 17.75 MHz~60 dB(d)   Frequency (MHz)(e)Condensed phasesRoundtripDrifting pulses50 nsDirectionFIG. 3: (a) Oscilloscope trace of SR. (b) Optical spectrum.(c) Autocorrelation trace. (d) RF spectrum. (e) RF spectrumwith 500MHz span.mode-locking[35].By increasing the pump power and adjusting the PCs,we achieve SR. Fig.3(a) plots the SR oscilloscope tracewhen we increase the pump power to∼650mW. The out-put power is∼43.3mW. The red arrows indicate CPs thatradiate the drifting pulses (blue arrows) from left to right.During the SR process, the drifting pulses spontaneouslystart from one CP and vanish close to the next CP[3–8].The spacing of two adjacent CPs is the FRR[1, 2]. Thecentral wavelength of the spectrum [Fig.3(b)] is 1884nmwith FWHM∼10.4nm. Fig.3(c) shows the autocorrela-tion trace of the soliton pulses. A good fit is obtainedby using a Gauss profile, with FWHM∼534fs. The TBPis 0.469, close to the theoretical value[34]. The SNR ofthe RF spectrum with 10Hz resolution [Fig.3(d)] is 60dBat the FRR, smaller than the normal mode-locked SNR(∼75dB) caused by the soliton flow (i.e., SR), which de-creases the SNR[35]. We also measure the RF spectrumwith 500MHz span, Fig.3(e), and attribute its fluctuationto the soliton flow[35].SR in our laser generate from CP and drift towardsthe next CP. During the process, the amplitudes of thedrifting soliton pulses enlarge before they reach CP. Thisis different from the SR observed in Refs.[1–8], where31860 1880 1900 1920 1940-60-40-201860 1880 1900 1920 1940-60-40-20Normalized intensityNormalized intensityIntensity (a.u.)(b)  Intensity (dBm)Wavelength (nm)(c)(a)Time (ns)-2 -1 0 1 20.00.20.40.60.81.0  Experiment Gauss Fit Time (ps)1.41 x 545 fs(e) Intensity (dBm)Wavelength (nm)(d)-2 -1 0 1 20.00.20.40.60.81.0  Experiment Gauss FitTime (ps)1.41 x 542 fs(f)Condensed phasesRoundtripDrifting pulses50 nsDirectionCondensed phasesRoundtripDrifting pulses50 nsDirectionTime (ns)FIG. 4: Oscilloscope traces of (a) second and (b) third-harmonic soliton rains. Spectra of (c) second- and (d) third-harmonic SR. Autocorrelation traces of (e) second- and (f)third-harmonic SR.tens of drifting pulses within a roundtrip time equallydistribute their energy and aggregate during the flow ofthe soliton pulses[1–8]. Our laser has only two driftingpulses in a roundtrip [Fig.3(a)]. Thus, they can aggregateenergy before reaching the next CP, and subsequentlyrelease energy, i.e., radiate SR, in dynamic equilibrium.SR lasts tens of minutes if one does not operate the PC.By further increasing the pump power and adjustingthe PCs, we observe the second- and third-harmonic SR,Figs.4(a,b). Compared to the FRR SR, the CP and thedrifting direction of the pulses are unchanged. The drift-ing pulses still arise from CP and drift from left to rightfor a roundtrip route, with the amplitude enhanced whenthey pass the second- and third-harmonic, due to thesuperposition of the harmonics and the drifting pulses.The drift speed can be changed by adjusting the PCs,since SR are sensitive to the cavity polarization[4, 7, 8].1840 1860 1880 1900 1920 1940 1960 1980-60-40-20-60-40-20-60-40-20-60-40-20-60-40-20-60-40-20 Wavelength (nm) 1877 nm  1889 nm Intensity (dBm) 1900 nm  1915 nm  1925 nm  1933 nmFIG. 5: Wavelength tunable spectra.Figs.4(c,d) plot the output wavelengths corresponding tothe second- and third-harmonic SR. The central wave-lengths are 1889 and 1892nm, with FWHM∼14.2 and14.7nm, respectively. Figs.4(e,f) show the autocorrela-tion traces of the second- and third-harmonic SR. TheFWHM is 545 and 542fs, by using Gauss pulse pro-file, with TBP∼0.651 and 0.668, respectively, larger thanthe theoretical value for the transform-limited of Gauss-shaped pulse[34], indicating chirping.For SR generation, the drifting pulses move fasterthan the CP. This means that the SR have differentgroup velocity than the CP, corresponding to differentcentral wavelengths[1–3]. Thus, the SR spectra alwaysshow modulations (i.e., extra optical sidebands in addi-tion to Kelly sidebands[36]) because of the interactionsbetween different SRs and CP central wavelengths[1–8]. In our laser, the PM-ISO along with the two PCsact as a Lyot birefringence filter[31], enhancing spectralmodulation[37], which is beneficial for SR generation[3].By adjusting the PCs, we obtain SR tunability from1877 to 1933nm as shown in Fig.5. As indicated in Ta-ble I, the central wavelengths are 1877, 1889, 1900, 1915,1925 and 1933nm with FWHM∼14.6, 15.2, 3.1, 13.9, 3.5and 14.1nm, respectively. We assume Gaussian pulses for1877, 1889, 1915 and 1933nm (as confirmed by spectralmodulation[31, 37]), and sech2 for 1900 and 1925nm (asconfirmed by the Kelly sidebands[36]). The pulse dura-tion is 527, 523, 1284, 543, 1233 and 534fs, correspond-4TABLE I: Output properties of wavelength tunable spectraWavelength FWHM Pulse Duration TBP(nm) (nm) (fs)1877 14.6 527 0.6551889 15.2 523 0.6681900 3.1 1284 0.3311915 13.9 543 0.6171925 3.5 1233 0.3491933 14.1 534 0.605ing to TBP∼0.655, 0.668, 0.331, 0.617, 0.349 and 0.605,respectively, indicating chirping for the Gaussian pulsesand almost transform-limited behavior for the sech2 ones(0.315 holding for a transform-limited pulse[34]). Thedifferent wavelength and pulse profiles are attributed tothe birefringence filter effect[31, 37].In summary, we reported the fundamental repeti-tion rate, second- and third-harmonic soliton rains in awavelength tunable (1877-1933nm) Tm-doped fiber lasermode-locked by carbon nanotubes. 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Wavelength tunable soliton rains in a nanotube-mode locked Tm-doped fiber laser

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