Chirp-adjustable square-wave pulse in a passively ... - OSA Publishing

Vol. 26, No. 18 | 3 Sep 2018 | OPTICS EXPRESS 23926

Chirp-adjustable square-wave pulse in a passively mode-locked fiber laser SHUJIE LI, ZHIPENG DONG, GEN LI, RUISHAN CHEN, CHUN GU, LIXIN XU,* AND PEIJUN YAO Department of Optics and Optical Engineering, University of Science and Technology of China, Hefei 230026, China *[email protected]

Abstract: A passively mode-locked fiber laser to generate chirp-adjustable square-wave pulses is reported. A simple chirp measurement system is designed to study the output chirp of the fiber laser. The results indicate that the chirp of the square-wave pulses in our fiber laser can be adjusted by the polarization controllers inside the cavity. Three typical chirp states, including random chirp, V-shaped chirp and linear chirp, are achieved. This kind of fiber laser cannot only help to further understand the characteristics of square-wave pulse but also serve as multifunction light source for potential applications. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction In recent years, much attention has been paid to square-wave pulse passively mode-locked fiber lasers for their potential applications in such as all-optical square-wave clocks, optical sensing and laser micromachining [1–4], etc. In 2008, W. Chang et al. theoretically found the phenomenon of dissipative soliton resonance (DSR) and showed that, under certain condition, the energy of soliton pulse can increase indefinitely while keeping its amplitude [1]. Subsequently, a great deal of studies on DSR fiber laser appeared [5–13]. The X. Liu team and the L. Zhao team respectively studied the DSR pulses in fiber lasers and elaborated their pulse characteristics [5–9]. They showed that the DSR pulse has a low linear chirp across its central region and large nonlinear chirps at its two edges. In addition to DSR pulse, other types of square-wave pulses also arouse researchers' interest, such as randomly chirped square-wave pulse [2], noise-like square-wave pulse [14–18], square-wave pulse with large tuning range [19,20], and so on. K. Özgören et al. reported a 1 ns-long randomly chirped square-wave pulse and pointed out that the pulse was incompressible [2]. The A. Luo team carried out a series of studies on the noise-like square-wave pulse [15–18], they showed that the noise-like square-wave pulses can be generated in several long-cavity fiber lasers with certain cavity parameters. X. Zhang et al. reported the generation of square-wave pulse with ultra-wide tuning range in a mode-locked fiber laser, and showed that high nonlinearity can increase the tuning range of pulse width [3]. L. Mei et al. obtained the width and amplitude tunable square-wave pulse using a dual-pump figure-8 fiber laser [4]. In addition, the energy of square-wave pulse generated from fiber laser, or fiber amplifier, is constantly improved [21–28]. Generally, the square-wave pulse has a large chirp, and its time-bandwidth product 4

can reach to 10 [26,28]. In past reports on square-wave pulse chirp, the DSR chirp has been studied fully [5–9], while other chirp characteristics of square-wave pulses received little attention. However, the pulse chirp is closely related to its temporal profile and spectrum, and square-wave pulses of different chirps may have different applications [2,8]. Therefore, the chirp characteristics of square-wave pulses are worthy of further study. In this paper, we report on a passively mode-locked fiber laser to generate chirpadjustable square-wave pulses. A chirp measurement system is designed to study the output pulse chirp. We show that the chirp of the square-wave pulse can be adjusted just by the polarization controllers inside the cavity, and three typical output chirp states, including random chirp, V-shaped chirp and linear chirp, were obtained. To the best of our knowledge, #340919 Journal © 2018

https://doi.org/10.1364/OE.26.023926 Received 31 Jul 2018; revised 26 Aug 2018; accepted 27 Aug 2018; published 30 Aug 2018


it is the first time to report on the generaation of chirp-aadjustable square-wave pulsee in fiber nd of fiber laser cannot only y help to furtheer study the chharacteristics oof squarelaser. This kin wave pulse bu ut also serve ass multifunction n light source fo for potential appplications. 2. Experime ental setups 2.1 Structure e of the fiber laser l The experimeental setup of th he passively mode-locked m fibber laser is shoown in Fig. 1. IIt consists of two loops. The right loop p and the left are connected with a 3 dB ccoupler. The riight loop, onlinear ampliifying loop mirror m (NALM)), acts the rolee of artificial saturable also called no absorber. Thee NALM contaains a 5-centim meter-length heeavily doped Y YDF (LIEKKI Yb12004/125), two 915/980 9 waveleength division multiplexers ((WDM), a 9155 nm single m mode laser diode with 30 00 mW maxim mum output, a polarization p coontroller (PC), and a section of single mode fiber (C Corning HI106 60, 45 m lengtth). The left looop is composeed of a band-ppass filter with transmisssion band rang ging from 960 nm to 990 nm m, a 10% outpput coupler, a ssection of single mode fiber (Cornin ng HI1060, 120 1 m lengthh), a polarizattion controller, and a polarization in ndependent iso olator (PI-ISO)). The PI-ISO eensures unidireectional operatiion in the left loop. The filter is used to t eliminate thee ASE around 1030 nm to ennsure the laser operating he total cavity length l is 175 m. m at 980 nm. Th

Fig. 1. Experimental setup schematic of the fiber laaser. WDM, wavvelength divisionn multip plexer; YDF, ytteerbium-doped fibeer; PC, polarizatioon controller; SM MF, corning singlee mode fiber HI1060; PI-ISO, polarization independent i isolattor.

For the monitoring m an nd measuring, the followinng equipment was used: a 4 GHz oscilloscope (Teledyne LeeCroy WAVER RUNNER 6400ZI), 6 GHz photodetectorrs, a RF ANDO AQ63117B) and a pow wer meter spectrum anallyzer, an opticaal spectrum analyzer (OSA, A (Thorlab PM1 100D/S145C). 2.2 Chirp me easurement system s To study the output chirp characteristic c of o the fiber laaser, we designned a simple ssystem to wn in Fig. 2), which w containss two fiber Braagg gratings (F FBG1 and measure pulsee chirp (as show FBG2), a secttion of single mode m fiber of 5 m length, thhree 3 dB coupplers, and a polarization independent isolator. i The ou utput pulse of fiber laser wass spectrally sam mpled by fiberr gratings (FBG1 and FBG2), F and th he chirp charaacteristic is deetermined accoording to the temporal waveform varriation of different spectral co omponents of the pulse. Connsidering that thhe output spectrum is ranged r from 974 9 nm to 981 nm, the speecific parametters of the FB BGs were designed. Thee central waveelength of the FBG1 F can be ttuned from 9777.5 to 980.5 nnm with a constant refleectivity of 99.9 9% and a refleection bandwiddth of 0.2 nm m. Similarly, thhe central wavelength of FBG2 can bee tuned from 974.5 9 to 977.5 nm with a connstant reflectivvity of 99.


9% and a reflection bandwid dth of 0.2 nm. The total specctral sampling range of FBGss is 974.5 ∼980.5nm, wh hich nearly cov vers the outputt spectrum of thhe fiber laser.

Fig. 2. 2 Experimental setup schematic for pulse chirp m measurement. PI--ISO, polarizationn indepeendent isolator; FBG, F fiber Bragg grating; SMF, ccorning single moode fiber HI1060;; OSA, optical spectrum analyzer. a

For the pu ulse chirp meassurement, the incident i pulse w was divided innto two beams by a 3dB coupler, one beam b from thee output port B was sent to thhe oscilloscoppe (displayed inn channel 2) and OSA for f the waveforrm and spectru um measuremeent. Another beeam passed thrrough the spectral samp pling system co onsisting of FB BG1, 5 m singlee mode fiber annd FBG2. The reflected pulses of FBG G1 and FBG2 were output from fr port A, annd then sent too a fast photoddiode and monitored by y the oscillosccope (displayeed in channell 1). The corrresponding waavelength components of o reflected pullses were meassured by the O SA. The sectioon of SMF betw ween two FBGs was em mployed to co ompletely sepaarate the two rreflected pulsees from the FB BG1 and FBG2 in time domain. Wh hen channel 1 was triggeredd by channel 22, the waveforrms from pling system were w displayed in channel 1. The waveform m of the refleccted pulse spectral samp versus the refflection wavellength in the range r of 974.55 ∼977.5 nm w was obtained bby tuning FBG2’s centrral wavelength h when the cen ntral wavelengtth of FBG1 w was fixed. Simiilarly, the chirp characteeristics in the range r of 977.5 ∼980.5 nm weere obtained byy tuning FBG1’s central wavelength and a fixing FBG2. Finally, the t total chirpp characteristiccs of the outpput pulse ranging from 974.5 nm to 98 80.5 nm were obtained. o 3. Experime ental results and analysis s In our fiber laser, l when thee pump powerr exceeded thee threshold puump power of about 90 mW, stable mode-locking m square-wave pu ulses were emiitted by simplyy adjusting thee two PCs inside the cav vity. After anallyzing the outp put with the chiirp measuremeent setup, we ffound that the output pu ulse has compllex chirp charaacteristics. Annd three typicaal chirp states of output pulse were ob bserved just by y adjusting thee PCs, includinng random chirrp, V-shaped cchirp, and linear chirp (aas shown in Fig g. 3, Fig. 5, and d Fig. 7, respecctively). For the sttate of random m chirp, the outtput waveform m of the fiber llaser at pump power of 180 mW is shown in Fig. 3(a). It is cleaar that the pulsse exhibits a rrectangle profiile with a 5 ns. To confirrm the stability y of the pulse train, the RF sspectrum was m measured duration of 15 by a RF specttrum analyzer. As shown in the t inset of Figg. 3(a), the pullse repetition ffrequency is 1.14 MHz,, corresponding g to the total cavity length of 175 m. Thhe resolution bbandwidth (RBW) of thee RF spectrum m analyzer is 10 00 Hz. The siggnal-to-noise rratio (SNR) exxceeds 50 dB. The correesponding outp put spectrum is presented in F Fig. 3(b). The rresolution of thhe optical spectrum anallyzer is 0.01 nm. n The spectru um is smooth, with a centrall wavelength oof 978 nm and 3 dB speectral width off 4.01 nm. Thee waveform off the pulse refflected by FBG Gs versus reflection waavelength is presented in Fig. 3(c). Appaarently, the teemporal waveeforms of different specctral componen nts vary irregu ularly with theeir wavelengths. The duratioons of the reflected pulses centered at 977.26, 977.97 7, and 978.88 nm are approxximately 15 ns, equal to


that of the outtput square-waave pulse. The reflected pulsees centered at 9974.54, 975.111, 975.69, and 976.48 nm n have duratiions of approx ximately 6, 11,, 12.5, and 13.5 ns, respectivvely, and their temporall positions are closer to the trrailing edge off the square-waave pulse. The reflected pulses centereed at 979.32 and a 979.98 nm m have duratioons of approxiimately 13.5 aand 8 ns, respectively, and their temp poral positionss are closer to the front edgee of square-waave pulse. onfirm the chirrp characteristtics, the corressponding specctrogram was m measured To further co with a spectraal sampling intterval of 0.2 nm m and is given in Fig. 3(d). A As can be seenn from the spectrogram, it is a compleex nonlinear chirp c profile. S Since the specctral componennts of the h well-defin ned temporal positions p like a linearly chirped pulse, we consider it pulse do not have to be a random mly chirped pu ulse.

Fig. 3. 3 Randomly chirp ped pulse output: (a) waveform of single pulse, insett: RF spectrum off pulse train; (b) output sp pectrum; (c) waveeform of the pulse reflected by FBG Gs versus reflectionn wavellength; (d) measurred spectrogram.

Fig. 4. 4 Pulse waveform m (a) and output spectrum (b) veersus pump powerr, when the laserr operatting in the random m-chirp state. Pum mp power = 100, 140, 180, 220, 2260 and 300 mW, respecctively.


The pulse waveform as a function of pump power iin the random--chirp state is shown in Fig. 4(a). Wh hen the pump powers were tuned to 100,, 140, 180, 2220, 260, and 3300 mW, respectively, the correspond ding pulse durrations of 6.5,, 10.8, 14.9 23.6, and 25.8 ns, were observed. The pulse width increases lineearly with thee pump powerr. The output spectrum ump power is presented in Fig. 4(b). Thee spectra are smooth, and tthe 3 dB versus the pu spectral width hs at the pump power of 100 and 300 mW are 3.80 and 44.12 nm, respecctively. It was confirmeed by chirp measurement m th hat the pulse cchirps at diffeerent pump poowers are similar to the one in Fig. 3(d d). ulse waveform m at pump poweer of 180 mW is shown For the staate of V-shapeed chirp, the pu in Fig. 5(a). The T pulse preseents a rectanglee profile with a duration of 17.8 ns. As shown in the inset of Fig. 5(a), the pulsee repetition ratte is 1.14 MH Hz, and the SN NR exceeds 500 dB. The g output spectrrum is shown in i Fig. 5(b), w with a 3 dB speectral width of 3.98 nm. corresponding The waveform m of reflected pulse versus reflection r wavvelength is deppicted in Fig. 55(c). It is clear that thee waveform off reflected pulsse vary irregullarly with its wavelength. C Compared with Fig. 3(c)), the chirp chaaracteristic heree is more compplicated. The sppectrogram off this state is shown in Fiig. 5(d). It indiicates that the chirp c is non-moonotonous acrooss the pulse, aappearing “V” shape profile. It is a po ositive chirp profile p during 00∼6 ns while a negative chirrp profile during 6∼17.8 8 ns. From an nother point off view, the shhorter wavelenngth componennts of the pulse are locaated at the centtral part of the pulse, while thhe longer waveelength compoonents are more close to the two edges.

Fig. 5. 5 Pulse output off V-shaped chirp: (a) waveform of single pulse, insett: RF spectrum off pulse train; (b) output sp pectrum; (c) waveeform of the pulse reflected by FBG Gs versus reflectionn wavellength; (d) measurred spectrogram.

The outpu ut waveform veersus pump po ower in the statte of V-shapedd chirp is show wn in Fig. 6(a). When pump p powers were w tuned fro om 100 mW tto 300 mW, thhe correspondiing pulse durations incrreased linearly from 9.6 ns to o 30.1 ns. Figuure 6(b) showss the output speectrum of the fiber laserr versus pump p power. The spectra s are smoooth, and the 3 dB spectral widths at pump power of 100 and 30 00 mW are 3.77 nm and 4.100 nm, respectivvely. It’s notedd that the p under diifferent pump powers p all havve V-shaped chhirps. square-wave pulses


Fig. 6. 6 Pulse waveform (a) and output speectrum (b) versus ppump power, wheen the fiber laser inn the staate of V-shaped ch hirp. Pump power = 100, 140, 180, 2220, 260 and 300 m mW, respectively.

For the lin near-chirp statte, the output waveform w at ppump power of 180 mW is shown in Fig. 7(a). Thee pulse has a du uration of 18.2 ns. Different ffrom the squaree-wave pulses shown in Fig. 3(a) and d Fig. 5(a), thee front edge of o the pulse is much higher than the trailiing edge, exhibiting a trriangular waveeform. The corrresponding RF F spectrum is ggiven in the insset of Fig. 7(a), and the SNR exceeds 50 dB, indicaating a stable mode-locked ppulse train. Thhe output ound 978.5 nm m and 979.8 nnm, as shown in Fig. 7(b). T The chirp spectrum has two peaks aro i given in Fig. 7(c) - 7(d). T The temporal poositions of the reflected characteristicss of the pulse is pulses are graadually approaaching the fron nt edge of pullse as reflectioon wavelengthss become longer, as sho own in Fig. 7(cc), which mean ns a monotonoous chirp acrosss the pulse. Itt’s further confirmed fro om Fig. 7(d) that t the pulse chirp is nearlyy linear acrosss its central reegion and nonlinear at both b edges. Besides, B the ch hirp have a noon-centrosymm metric profile, with the intensity of th he pulse front higher h than thatt of the trailingg edge.

Fig. 7. Linearly chirped d pulse output: (a) waveform w of singlle pulse, inset: RF spectrum of pulsee train; (b) output spectrrum; (c) waveform m of the pulse reeflected by FBGss versus reflectionn wavellength, (d) measured spectrogram.


The tempo oral waveform m of the linearly chirped pulsse versus pumpp power is preesented in Fig. 8(a). Obv viously, the pu ulse width is well w tuned witth pump poweer. The output spectrum versus the pum mp power is sh hown in Fig. 8((b), and each oof the spectra hhas two peaks. Compare Fig. 8 with Fig. 7(d), and we w can find thaat the pulse froonts in Fig. 8(a) are correspoonding to ks in Fig. 8(b). Furthermore,, it was confirm med that the ppulse chirps at different the right peak pump powers are similar to the one in Fig.. 7(d).

Fig. 8. 8 Pulse waveform m (a) and output sp pectrum (b) versuss pump power, whhen the laser in thee linear-chirp state. Pump p power = 100, 140 0, 180, 220, 260 annd 300 mW, respeectively.

The squarre-wave pulse is generated in i the fiber laaser due to thee peak power clamping effect. Accord ding to Mei [4 4], there is non nlinear switchiing in the fibeer laser, and puulse peak power will bee clamped wheen reaching thee switching pow wer. In our fiber laser setup, the 45 m SMF is used d to increase nonlinearity n in n the NALM, and the highh nonlinearity helps to decrease the switching pow wer [3]. Thus the square-waave state of thhe fiber laser is easily achieved. Thee 120 m SMF in the unidirecctional ring is employed to inncrease the puulse width tunability. It can c be physicaally understood d that the long SMF increasess the cavity lenngth, thus allowing morre energy to be extracted during the tim me of one caavity round-triip. If the switching pow wer remains un nchanged, the pulse p in long caavity will havee large pulse w width [29]. In the experim ment, differentt pulse temporaal heights and widths are observed by adjuusting the PCs when fix xing pump pow wer, as shown in Fig. 3(a) annd Fig. 5(a), w which is consisstent with the multistabiility of square-wave pulse rep ported in [23]. This phenomeenon can be unnderstood from the view wpoint that th he transmittancce of NALM is related to input polarizaation and birefringence within the NALM N in addiition to the reelative nonlineear phase shifft. So the nsmittance of NALM N changees when adjustting PCs, leadding to switchinng power nonlinear tran varying. uring process, the t dispersion introduced byy FBGs, 5 m S SMF, and During the chirp measu w lead to durration variationns of incident square-wave ppulse and fiber pigtails of couplers will t pulse durattion variations are relatively smaller, compared with reflected pulses. However, the wave pulse (>1 10 ns) and the reflected pulsees (a few nanoseconds). the durations of the square-w n introduced bby chirp measuurement system m can be Therefore, thee influence off the dispersion neglected when studying th he chirp acrosss the whole squ quare-wave pullse. It is indicaated from t the outpu ut square-wavve pulse indeeed has compllex chirp the experimeental results that characteristicss, and the chirp p state can be adjusted by addjusting the PC Cs inside the ccavity. To the best of ou ur knowledge, it is the first time to reportt on the chirp-adjustable squuare-wave pulse in fiber laser. The three typical chirped pulses obtaained in the eexperiment aree unique. Althhough the randomly chirped square-w wave pulse repo orted in [2] is similar to thee one in Fig. 33(d), their omewhat differrent. For the ppulse in [2], tthe temporal w widths of chirp characteristics are so nts are equal to t that of the unfiltered pullse, while the temporal different specctral componen widths of thee spectral com mponents in Fig g. 3(d) vary irrregularly withh their waveleengths. In addition, the DSR D pulse rep ported in [8] is similar to thee one in Fig. 7((d), and their cchirps are


linear in the central part of the pulse and nonlinear at the edges of pulse. However, the DSR pulse in [8] has a centrosymmetric chirp profile and a rectangular temporal waveform, while the linearly chirped pulse here possesses a non-centrosymmetric chirp profile and a triangular temporal waveform. Here, we think the giant-chirp pulse can be compressed because of the clear linear-chirp profile. According to Smirnov [30], the maximum compression ratio theoretically depends on the spectrum width Δν and frequency fluctuation ( | 2δν | ). If the frequency fluctuation of the giant-chirp pulse is similar to that of the intermediate regime in [30], we think that the pulse compression can be achieved by conventional grating pairs, but the resulting pulse width is far from the transform limit. 4. Conclusion In conclusion, we have demonstrated a passively mode-locked fiber laser to generate chirpadjustable square-wave pulses. The fiber laser is based on the NALM technology and works at the 980 nm band. A simple chirp measurement system is employed to demonstrate that the output square-wave pulses have chirps adjustable by the PCs inside the cavity. By adjusting the PCs, three typical chirp states, including random chirp, V-shaped chirp and linear chirp, are achieved. This type of chirp-adjustable square-wave pulse fiber laser cannot only help to further understand the characteristics of square-wave pulse but also serve as multifunction light source for potential applications. Funding National Natural Science Foundation of China (Grant No. 61675188); Open Fund of Key Laboratory Pulse Power Laser Technology of China (Grant No. SKL2016KF03). References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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