Optical Fiber Technology xxx (2014) xxx–xxx

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Optical Fiber Technology

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Invited Papers

Ultrafast fibre laser sources: Examples of recent developments

http://dx.doi.org/10.1016/j.yofte.2014.07.0051068-5200/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author.E-mail address: edmund.kelleher08@imperial.ac.uk (E.J.R. Kelleher).

Please cite this article in press as: M. Zhang et al., Ultrafast fibre laser sources: Examples of recent developments, Opt. Fiber Technol. (2014),dx.doi.org/10.1016/j.yofte.2014.07.005

M. Zhang, E.J.R. Kelleher ⇑, S.V. Popov, J.R. TaylorFemtosecond Optics Group, Department of Physics, Blackett Laboratory, Imperial College London, London SW7 2BW, UK

a r t i c l e i n f o a b s t r a c t

Article history:Available online xxxxShort Pulse Optical Fiber

Keyword:Mode-locked fiber lasers

We summarise a number of recent experimental developments in the field of ultrafast compact all-fibrelasers, including: ionically-doped coloured glass saturable absorbers; Tm:fibre lasers utilising graphenearound 2 lm; alternative layered materials including MoS2; passively synchronised, coupled-cavity ultra-fast dual-wavelength fibre lasers; and schemes for the generation of high repetition rate femtosecondpulses based on phase modulation, and spectral masking of CW radiation. The breadth of light sourcescovered in this review highlights the diversity of approaches in ongoing research in the field of ultrafastfibre optics.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction of recent results from the continuing research effort focussed on

Optical fibres provide a convenient platform for the developmentand monolithic integration of a laser system; consequently, fibretechnologies have significantly impacted upon the field of laser engi-neering, and hold a dominant position in the commercial laser mar-ket. Ultrafast fibre lasers – light sources that emit bursts of radiationon time-scales of the order of, or less than, several tens of picosec-onds – have been widely studied, with emphasis focussing on thenature of the gain media and the modulation mechanisms anddynamics that promote short-pulse generation. The near-infrared(�1.0–2.0 lm, near-IR) region is now well addressed by silica glassfibre amplifiers, including active media doped with ytterbium(Yb); bismuth (Bi); erbium (Er) and thulium (Tm), allowing almostcontinuous coverage of this wavelength region [1–5]. Raman gainin silica optical fibres has also been shown to support mode-lockedoperation [6,7], and can thus be adopted in regions not covered byrare-earth doped fibre media, although this approach often leadsto excess system noise [6,8]. Modulation schemes, both passiveand active, as well as dispersion and nonlinearity engineering ofthe laser cavity, permit modes of pulsed operation from the femto-second scale, with bandwidth-limited durations, to several nanosec-onds, with predominantly linear chirp and high energy, that can berecompressed after stages of amplification in master-oscillator,power fibre-amplifier (MOPFA) architectures.

Although short-pulse fibre laser development is entering itsfourth decade the field remains fertile, benefitting from disruptivetechnological advances in parallel scientific disciplines such as,materials science and engineering. Here, we summarise a selection

the development of pulsed fibre laser geometries. In particular,we consider: the use of ionically-doped coloured glass as saturableabsorbers to mode-lock fibre lasers; the current status of mode-locked lasers operating in the 2 lm spectral range, utilising Tm-doped fibre amplifier technology; early reports of MolybdenumDisulfide (MoS2) as a novel two-dimensional (2D) ultrafast opticalswitch for short pulse generation; passively coupled-cavity mode-locked oscillators, that provide a convenient monolithic source ofsynchronous dual-wavelength near-IR generation; and high-repe-tition rate ultrashort pulse sources based on fibre amplificationand spectral masking of phase-modulated signals from continu-ous-wave (CW) diode seed lasers.

2. Ionically-doped coloured glass for mode-locking fibre lasers

Saturable absorbers (SAs) are optical devices that exhibit anintensity dependent transmission: asborbing lower intensity lightmore strongly than higher intensity light, and thus promoting pulsedoperation. Such nonlinear absorbers are widely deployed in mode-locked lasers to initiate self-starting and stable short-pulse opera-tion [9,10]. Key parameters for a saturable absorber are:

i Absorption wavelength – defining its region of operation thathas to coincide with the gain bandwidth of the correspond-ing lasing medium.

ii Recovery time – setting a limit on the switching speed of thedevice that can affect the duration of the achievable pulses.

iii Saturation intensity – the light intensity necessary to satu-rate the absorption of the device; and should be equivalentto the intra-cavity intensities reached in the laser system.

iv Modulation depth – the maximum change of absorption.

http://

Fig. 1. (a) Schematic of the Z-scan setup. OC, output coupler; SCF, small-core fibre;R, reference; POW, optical power meter; SA, scanning arm; ATS, automatedtranslation-stage; T, transmitted signal. (b) Representative dataset from open-aperture Z-scan measurement (after Ref. [29]). (c) Linear transmission spectrum ofthe ionically-doped glass sample. The vertical black line denotes the wavelength ofthe pump source used in the open aperture Z-scan measurements.

2 M. Zhang et al. / Optical Fiber Technology xxx (2014) xxx–xxx

v Damage threshold – the maximum light intensity that adevice can tolerate before catastrophic damage occurs;and must be higher than the intra-cavity intensities reachedin the laser system.

Materials possessing the aforementioned key properties are ofsignificant interest, and thus intensively studied. To date, a numberof SAs, with parameters specific for use in fibre lasers, have beeninvestigated and employed to achieve reliable mode-locked perfor-mance including, semiconductor-based absorbers [11]; carbonnanotubes (CNTs) [12,13]; and more recently, graphene [12,14–16]. Although these technologies offer many advantages, such asbroadband and reliable operation, they are not without limitations,namely, relatively high production costs and low damage thresh-olds. In contrast, ionically-doped coloured glasses [17–19] usedas a saturable absorber in fibre lasers may solve these issues, pro-viding a more thermally robust solution allowing potential forhigher achievable output powers, supported by the performanceof mode-locked solid-state lasers based on this approach [18,20–28], as well as offering the potential for broadband wavelengthoperation.

Using a Z-scan technique we measured the nonlinear transmis-sion of a low-cost, commercially available ionically-doped col-oured glass filter (Schott RG1000). We demonstrated that thisdevice offered sufficient intensity contrast (or modulation depth)to promote mode-locking of a low power Yb-doped fibre laser. Thisproof-of-concept demonstration of mode-locking a fibre laser,importantly, where the requirements on the absorber differ signif-icantly to solid-state counterparts due to differences in the gaindynamics and the magnitude of the nonlinearity and dispersion,could open the possibility of achieving elevated output energiesfrom all-fibre, passively mode-locked pulsed lasers.

2.1. Z-scan characterisation of nonlinear absorption

The nonlinear absorption of the ionically-doped coloured glass(Schott RG 1000) was characterised using an open-aperture Z-scanmeasurement. The pump source, providing picosecond pulses cen-tred at 1063 nm, was a commercial Yb-doped mode-locked fibrelaser (Fianium Inc.), followed by an Yb-doped fibre amplifier (IPGPhotonics). The output was split using a 93%:7% fused fibre coupler[see Fig. 1(a)]. The smaller fraction of pump light was detectedusing an optical power meter, forming a reference arm. The 93%was coupled into a small-core, high numerical aperture fibre(Nufern UHNA3), the output of which was imaged onto the sampleby a pair of 12 mm focal length lenses.

A 2 � 2 mm square piece of the the Schott glass, with a thick-ness of 240 lm, was placed in the sample arm and moved throughthe focal point by an automated translation stage. Light transmit-ted through the sample was collected by a third lens and detectedby a second photodiode. In order to fully saturate the sample’sabsorption, the pump source was amplified to 0.5 W, producing8.9 ps pulses at 50 MHz repetition rate. The corresponding maxi-mum intensity at focus was approximately 17 GW cm�2.

The power on the sample and reference arms were recorded,moving the sample incrementally through the focal plane, inte-grated with the precision stepper motor. This approach allowedrapid, accurate and reproducible acquisition of multiple datasets.A dataset from a representative scan, at a fixed transverse positionon the ionically-doped glass sample, is shown in Fig. 1(b). The redcurve is a fit to the experimental data based on the slow saturableabsorption model [30,31]. The relaxation time of the ionically-doped glass was estimated to be on the order of tens of picosec-onds, based on literature values from Chen et al. [17] and Yuma-shev et al. [32]. This is much longer than the duration of thepump pulse and so the use of the slow saturable absorption model

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is valid. Accordingly, the saturation fluence is 3.0 mJ cm�2. The 1=evalue of 8.8 mJ cm�2 is highlighted with a solid black line. Themodulation depth, which is the contrast between the fully absorb-ing and fully saturated state of the device, is highlighted with asolid black arrow and is �3% for the 240 lm thick ionically-dopedglass sample used in this measurement.

The density of the active ions was not homogeneous across thetest sample. The sample inhomogeneity was mapped by rasterscanning the sample. The nonlinear saturation curve was measuredat 0.5 mm spatial increments in the XY plain, across the 2 � 2 mmsample, with a uniform thickness of 240 lm. Fig. 2 plots the vari-ation in the modulation depth and saturation fluence based onthe resulting data. The resolution (or minimal spatial increment)was limited by the achievable spot size through the Z-scan systemoptics. As shown in Fig. 2, the modulation depth and the corre-sponding saturation fluence of the investigated sample varies sig-nificantly from 0% to 3% and 0 to 8.8 mJ cm�2, respectively. It isclear that the density of the absorbing ions, present in the glasshost, is non-uniform. Improved doping control would lead to ahigher degree of sample homogeneity and enhanced deviceperformance.

To characterise the linear absorption, the spectrally dependenttransmission was measured using a commercial spectrophotome-ter, plotted in Fig. 1(c). The dashed line shows the region wherenonlinear saturation measurements were performed using theopen-aperture Z-scan technique. A 240 lm thick sample has a cor-responding linear transmission of 80%, at 1063 nm. Based on theabsorption curves shown in Refs. [28,33], it indicates that the ionicdoping of this coloured glass is likely to be CuInSSe, although theexact composition data is not available from the manufacturer.

2.2. All-fibre soliton-laser

Although ionically-doped coloured glass filters have beenwidely employed to mode-lock solid-state bulk lasers, onlyrecently have they been demonstrated in the context of fibre sys-tems [29], where the demands on the pulse-shaping action of the

es: Examples of recent developments, Opt. Fiber Technol. (2014), http://

Fig. 2. Mapping of the sample inhomogeneity. (a) Variation in modulation depth; (b) Variation in saturation fluence. After Ref. [29].

M. Zhang et al. / Optical Fiber Technology xxx (2014) xxx–xxx 3

absorber device differ due to large round-trip gain, and high cavitynonlinearity and dispersion. In order to demonstrate the mode-locking potential of ionically-doped glass an Yb-doped travellingring cavity fibre laser was constructed.

The configuration of the all-fibre laser is shown in Fig. 3(a). Acommercial fibre amplifier (consisting of 0.6 m double-clad Yb-doped fibre and a 4 W counter pumped multi-mode diode at980 nm) was used, providing amplification around 1065 nm. Apolarisation-independent inline fibre circulator ensured unidirec-tional propagation and incorporated with a chirped fibre Bragggrating (CFBG), allowed dispersion control of the cavity. The FBGalso provided spectral filtering for mode-locking stabilisation. Achirped FBG, with a passband of 3.8 nm, was used to achieve netanomalous dispersion, and operation in the soliton regime. Afibre-based polarisation controller was added to optimise long-term stable mode-locked operation although this element wasnot essential for the mode-locking action. The output was deliv-ered through a 20% fused-fibre coupler to both spectral and tempo-ral diagnostics (50 GHz Tektronix sampling scope and abackground free intensity autocorrelator). Mode-locking was initi-ated and maintained by the inclusion in the cavity of the commer-cial ionically-doped glass (Schott RG1000) characterised above.The device comprised three individual thin layers, due to the rela-tively low modulation depth of each single layer. All samples werepolished to a uniform thickness of 240 lm, and integrated into thecavity by sandwiching between two angled fibre connectors, withindex matching gel.

To compensate the normal dispersion of the single-mode fibrearound 1.06 lm for the generation of shorter pulses, a chirpedFBG, with a negative dispersion of 35.71 ps nm�1 was employed.The fundamental frequency of the laser operating in the averagesoliton regime was 7.3 MHz, with a corresponding single-pulseenergy of 0.28 pJ. Fig. 4(a) shows the output spectrum, with afull-width at half maximum (FWHM) of 0.37 nm, centred at1065.2 nm, corresponding to a transform limited pulse durationof 3.2 ps. Fig. 4(b) shows the background free intensity autocorre-lation trace. The deconvolved pulse duration is 4.1 ps (1.28 timesthe transform limit), based on a sech2 profile, which closely fitsto the experimental data.

The cavity contained no polarisation-dependent elements, andwithout the ionically-doped glass no mode-locking could beobtained. Pulse formation was stabilised by the filtering effect ofthe CFBG, with the steady-state pulse duration largely defined bythe combined effect of its narrow reflection band and strong anom-alous dispersion. It should be noted that, in this case, a large inser-

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tion loss was introduced due to the finite separation,approximately 720 lm, of the fibre connectors containing the mul-tiple glass absorber layers. This large loss contributes to the mod-est output powers and low pulse energies achieved, but could bereadily improved by increasing the doping concentration and usinga single ionic-glass layer.

3. Tm-doped ultrafast fibre lasers

Ultrafast lasers operating at wavelengths around �2 lm [34–42] are essential to address demands for mid-infrared (mid-IR)sources for a variety of applications, ranging from molecular spec-troscopy and biomedical diagnostics [43,44], to remote sensing[45,46] and medical treatment. The mid-IR spectral region is par-ticularly important because of the large number of absorption linesof several gas species (e.g. CO2, SO2, CH4 etc.) [47,48]. Since liquidwater (the main constituent of human tissue) absorbs morestrongly around 1.9–2 lm (�100 cm�1) than at �1.5 lm(�10 cm�1) and �1 lm (�1 cm�1), �2 lm laser sources are prom-ising for medical diagnostics and laser surgery. In addition, lightdetection and ranging (LIDAR) [49] measurements and opticalfree-space telecommunications can be performed within the 2–3 lm atmospheric transparency window [50].

Tm-doped fibre-based oscillators have attracted considerableattention because it is possible to pump the Tm ions at �790 nmthrough a cross-relaxation process, where efficient diodes are read-ily available [1,51]. Tm-doped fibre is well known to have a broadand smooth fluorescence profile [1], which is suitable for short-pulse generation. To date, a number of mode-locked Tm-dopedfibre laser have been reported. Nonlinear polarisation evolution(NPE) [52] was used by Nelson et al. [38] to demonstrate a 500-fs-pulse oscillator. Sharp et al. used a semiconductor saturableabsorber mirror (SESAM) [10] in Tm-doped fibre laser to achieve190 fs pulses [40]. To achieve shorter pulses, intra-cavity gratingpairs could be used for dispersion compensation. Recently, 45 fspulses were produced in a stretched pulse Tm-doped ZBLAN fibrecavity reported by Nomura et al. [39]. NPE was used to initiateand stabilise the mode-locked pulses in this configuration andoffered excellent performance but sacrificed the benefits of all-fibre integration due to the use of bulk optics elements.

In ultrafast fibre lasers, NPE and SESAMs are commonly usedbecause they provide fast amplitude modulation. These tech-niques, however, have disadvantages: NPE suffers from bulky con-struction and environmental sensitivity, SESAMs have complex

es: Examples of recent developments, Opt. Fiber Technol. (2014), http://

Fig. 3. Ultrafast mode-locked fibre laser cavity schematics. (a) Fibre-laser mode-locked with an ionically-doped glass-based saturable absorber; CFBG, chirped fibre Bragggrating employed for dispersion compensation and solitonic operation. (b) Ultrafast 2 lm Tm-doped fibre laser. (c) Synchronously coupled-cavity, dual-wavelength fibrelaser.

4 M. Zhang et al. / Optical Fiber Technology xxx (2014) xxx–xxx

fabrication and packaging, as well as limited bandwidth of opera-tion. Polymeric composites, e.g. nanotubes and graphene, arepromising saturable absorbers due to ultrafast recovery time[53–55], broadband operation [16,56], ease of fabrication and inte-gration into all-fibre cavity designs [12,36,57–59]. While broad-band operation can be achieved using a distribution of nanotubediameters [56], this is an intrinsic property of graphene due tothe gapless linear dispersion of Dirac electrons [15,16,60,61].Nanotubes have been used for mode-locking fibre [15,16,56,59,62,63], waveguide-based [64,65], solid-state [66,67] and semicon-ductor lasers [68], covering from �0.8 to �2 lm wavelength range.Ultrafast pulse generation at 0.8 lm [69], 1 lm [70], 1.3 lm [71],

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and 1.5 lm [12,16,60,72,73] was demonstrated by exploitinggraphene saturable absorbers. To date, only a few Tm-doped all-fibre laser mode-locked with graphene have been demonstratedin the literature [4,74,75]. Here, we discuss the demonstration ofTm-doped fibre laser mode-locked by a graphene saturableabsorber.

The configuration of the all-fibre integrated Tm-doped laser isshown in Fig. 3(b). A Tm-doped fibre amplifier (IPG Photonics),consisting of �15 m Tm-doped fibre, with integrated optical isola-tor (ISO) and providing 25 dB small signal gain at 1.94 lm in abroad gain bandwidth (FWHM = 60 nm), was followed by a fibrepigtailed air–gap used to incorporate a bandpass filter for pulse

es: Examples of recent developments, Opt. Fiber Technol. (2014), http://

Fig. 4. (a) Spectrum of the all-fibre soliton laser mode-locked using an ionically-doped glass saturable absorber. (b) Corresponding intensity autocorrelation. After [29].

M. Zhang et al. / Optical Fiber Technology xxx (2014) xxx–xxx 5

stabilisation, with 80% peak transmission and a FWHM transmis-sion bandwidth of 11 nm, centred at �1.94 lm. A 10% fused-fibreoutput coupler was employed. A polarisation controller allowedadjustment of the intra-cavity polarisation. The overall cavitygroup velocity dispersion was estimated to be �59.7 ps2 km�1

and was deduced from the solitonic spectral sidebands observedwhen the oscillator was operating without the bandpass filter[76]. The graphene-polymer composite saturable absorber (GSA)[15,60] was prepared by sandwiching 2 mm2 of the compositebetween two fibre connectors, adhered with index matching gel.The integrated device had �4 dB total insertion loss (includingthe linear absorption of the few-layer graphene-film).

Self-starting mode-locked operation was initiated and stabi-lised by the saturable action of the GSA. The optical spectrum[see Fig. 5(a)] was centred at 1.94 lm (defined by the transmissionband of the filter), and had a FWHM bandwidth of 2.1 nm. The cor-responding intensity autocorrelation trace is shown in Fig. 5(b),and is fitted with a sech2 function that well represents the tempo-ral pulse profile. Based on the fit, the FWHM duration, after decon-volution, was 3.6 ps, resulting in a time-bandwidth product of 0.59,indicating slight chirp. The output power was �2 mW. Althoughthe laser operated in the anomalous dispersion regime, and thuspulse-shaping was dominated by average solitonic effects, thespectral sideband signature of deviation from average soliton oper-ation could not be observed, due to the limited resolution of thespectrometer used to measure the optical spectrum.

Fig. 5(c)–(f) show plots of the fundamental (frep) and 60th(60f rep) harmonics of the radio frequency (RF) spectrum over twodifferent frequency spans: 1 MHz and 8 kHz. The wide-span spec-tra indicate that the pulse stability is high, with peak to noise-floorratio limited by the 300 Hz resolution of the analyser (the noisefloor of the analyser is plotted in green). No sidebands at aharmon-ic cavity frequencies were observed over the 1 MHz span, suggest-ing good pulse-train stability and absence of Q-switch instabilities[77]. The narrow-span spectra (at 30 Hz resolution) reveal a low-level pedestal component �70 dB down from the central beat-note[see Fig. 5(d)].

The stability of the Tm-doped laser was calculated using thefundamental frequency and 60th harmonic. The energy fluctuationwas DE

E � 1:1� 10�3, based on the method outlined in Ref [77].Compared to a practical low-noise Yb-doped fibre laser with�0:3� 10�3 energy fluctuation [78], it shows that despite a verysimple cavity consisting of non-polarisation maintaining (PM)fibre, the laser emits high-quality pulses.

4. MoS2-based ultrafast fibre lasers

Low-dimensional nanomaterials present great potential forphotonic applications due to their remarkable optical and electricalproperties. Over recent years, the appearance of carbon nanotubes

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and graphene, one-dimensional (1D) and 2D materials respec-tively, have boosted the tremendous research activity in saturableabsorbers for passive mode-locking of lasers. Significant efforts arecurrently being directed towards the study of other 2D nanomate-rials, for example metal dichalcogenides for photonic and optoelec-tronic applications [79,80]. MoS2 is one such layered material,which exhibits saturable absorption [81,82], high third-order non-linear susceptibility [83] and ultrafast carrier dynamics [84]. Wide-band operation from the visible to near-infrared of a few-layerMoS2-based devices has been suggested, arising from the directvisible band gap which can be altered by defects states in the mate-rial [85]. Wang et al. reported that both Mo and S atomic defectscan reduce the band gap, enabling longer wavelength photons tobe absorbed. While a large number of Mo defects results in metallicbehaviour and no saturable absorption, an increase in S defectstates can yield semiconductor behaviour with band gap energyas low as 0.8 eV (1.55 lm) [85]. Strong saturable absorption canthen be observed from excitation by many photons with energiesexceeding the band gap due to the filling of conduction band statesand Pauli Blocking [86].

The combination of ultrafast carrier dynamics and widebandoperation means MoS2 could be used for the generation of ultra-short pulses. To date, few-layer MoS2 saturable absorbers havebeen demonstrated to Q-switch a fibre laser at 1.06 lm [87],solid-state lasers at 1.06 lm, 1.42 lm and 2.1 lm, producingpulses with hundreds of nanosecond durations [85]. The use ofMoS2 has also very recently been demonstrated in mode-lockedlasers. The lasers operated at 1.054 lm, producing 800 ps pulses[86]; and at 1.55 lm, producing 880 fs pulses at a repetition rateof 15 MHz, with pulse energy of 66 pJ, limited by the solitonic con-dition of the cavity [88]. Such sources may find application in areassuch as metrology, spectroscopy and biomedical applications.MoS2 represents an alternative technology to graphene as amode-locker in ultrafast lasers, and is particularly useful becauseof its direct bandgap in the visible spectral region.

5. Passively coupled-cavity lasers

The combination of rare-earth doped fibre gain media and sat-urable absorber device technology provides a powerful and flexibleapproach to generate ultrashort pulses. We discussed recent exper-imental examples of mode-locked fibre lasers emitting in a singlewavelength region above. For a number of applications, such aspump probe spectroscopy, a dual-wavelength ultrafast source isdesirable [89]. An important aspect of such sources is the abilityto control the relative arrival time of the pulses; in other words,synchronism of the dual-wavelength pulse-trains is necessary.One approach to achieve such synchronisation is through theinclusion of a common saturable absorber in a coupled cavity con-figuration [3]. Single-wall carbon nanotubes, possessing resonant

es: Examples of recent developments, Opt. Fiber Technol. (2014), http://

Fig. 5. Ultrafast Tm-doped fibre laser properties: (a) optical spectrum; (b) intensity autocorrelation; (c)–(f) RF spectra. (c) frep, wide-span; (d) frep, narrow-span; (e) 60f rep,wide-span; (f) 60f rep, narrow-span. Adapted from [4].

6 M. Zhang et al. / Optical Fiber Technology xxx (2014) xxx–xxx

transitions in the electron density of states at energies correspond-ing to the operating wavelengths of a specific dual-wavelengthlaser source, can act as simultaneous mode-locker and passive syn-chroniser [3]. Similarly, graphene – widely heralded as a broad-band mode-locker [15,16,60,61] – can be used as a passiveelement to synchronise lasers at any wavelength in the infrared,due to the fact that graphene possesses a linear dispersion of elec-trons, with a point bandgap at the Dirac point [60,90].

The configuration of the dual-wavelength all-fibre laser is shownin Fig. 3(c), with the top and bottom parts representing the Er- andYb-laser respectively. In the Er-laser, a commercial fibre amplifiermodule (IPG Photonics), consisting of 1.5 m double-clad Yb/Er-cod-oped fibre counter pumped by a 4 W multi-mode diode at 980 nm,with 23 dB gain, was used to provide amplification around1.55 lm. The amplifier module was followed by an inline optical iso-lator to ensure unidirectional lasing operation. The output signalwas delivered through a 50:50 fused-fibre output coupler. A polari-sation controller was added to adjust the polarisation state withinthe cavity, but was not essential for the mode-locking action. Thecavity length of the Er-laser could be changed by a maximum of9 cm through a fibre-pigtailed optical delay line, corresponding toa temporal delay of 300 ps. The Er-laser cavity had a total length of

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15 m, comprising 13.8 m of single-mode fibre (with a group velocitydispersion (GVD) at 1.5 lm of �17 ps2 km�1 [91]), and 1.2 m of Er-doped fibre (GVDEr = +0.207 ps2, delivering a small-signal gain of23 dB and a GVD�@1534nm = 0.009 ps2 dB�1 [92]). The resulting net cav-ity GVD was �0.028 ps2.

The Yb-laser, forming the lower half of the coupled-cavity sys-tem, was constructed from a commercial fibre amplifier module(IPG Photonics) consisting of 0.6 m of double-clad Yb-doped fibreand a 4 W counter pumped multi-mode diode at 980 nm, providingamplification in a band around 1.06 lm; a 20% output coupler, anda polarisation controller. A polarisation-independent inline fibrecirculator was employed to incorporate a CFBG, providing anoma-lous dispersion (GVD = �21.6 ps2) for operation in the average sol-iton regime, while ensuring unidirectional propagation.

Passive mode-locking and synchronism of the coupled cavitieswas achieved using two distinct devices: a polymeric film embed-ded with single-wall carbon nanotubes [3]; and a polymeric filmembedded with few-layer graphene flakes. We proceed by discuss-ing the system performance using the single-wall nanotube-baseddevice.

A transparent carboxymethylcellulose film, homogeneouslyembedded with single-wall carbon nanotubes (SWNTs), was

es: Examples of recent developments, Opt. Fiber Technol. (2014), http://

Fig. 6. Linear transmission spectra of polymeric films embedded with low-dimensional nanomaterials: (a) single-wall carbon nanotubes; (b) few-layergraphene flakes.

M. Zhang et al. / Optical Fiber Technology xxx (2014) xxx–xxx 7

integrated into a common section of the coupled cavity systemusing fibre connectors – in a fashion typical of SWNT-based devicesused as saturable absorbers in single-cavity fibre laser sources.

Stable, self-starting mode-locking was achieved independentlyin both sub-cavities. In the non-synchronised state, the spectraland temporal intensity of the Er-laser pulses were maintainedthroughout the entire tuning range of the intra-cavity delay line.In this mode, the sub-cavities behaved like two independent lasers.

When the repetition rate of the Er-laser (13.08 MHz) was pre-cisely adjusted, using the intra-cavity opitcal delay line, to exactlytwice the frequency of the Yb-laser (6.54 MHz), synchronisation ofthe sub-cavity pulse-trains was achieved. As the pump power ofthe Yb-laser varied, two modes of synchronisation were observed:either the repetition rate was locked at the fundamental frequency(one pulse per round-trip period) or harmonic operation wasachieved (with two pulses per round-trip period); the latter wasobserved with the increase of the pump power typical for har-monic mode-locking in the soliton-like regime.

The spectrum of the Er-laser [see Fig. 7(a)] was centred at1535.5 nm, with a FWHM of 1.48 nm. The corresponding pulseduration, measured using intensity autocorrelation, was 2.1 ps(deconvolved), and was well fitted by a sech2 pulse-shape [seeFig. 7(b)]. The corresponding spectra and autocorrelation of theYb-laser are shown in Fig. 7(b) and Fig. 7(d), respectively, havinga centre wavelength of 1076.1 nm, a FWHM spectral bandwidthof 0.24 nm, and a deconvolved pulse duration of 6.1 ps, again, wellfitted using a sech2 pulse-shape.

Fig. 8(a) plots the change in repetition frequency of the Er-laserwith cavity length detuning, normalised to the repetition fre-quency of the Yb-laser. It can be seen that over a detuning range,corresponding to a cavity length change greater than �0.5 mm,the Er-laser repetition frequency is locked to that of the Yb-laser(i.e. frep;Er ¼ 2f rep;Yb). Outside of this range the two lasers are effec-tively independent oscillators. The maximum locking range corre-sponded to a cavity length mismatch of 1.4 mm.

It is important to note that the transition from the non-syn-chronised to the synchronised regimes described above did not

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modify the pulse parameters within the accuracy of the diagnos-tics. In synchronous lasers adopting a master–slave configuration,it is well known that cross-phase modulation (XPM) can induce awavelength shift in the slave laser that can compensate for cav-ity-length mismatch by a change in the pulse group-velocity[93–97]. However, this feature was not observed in our system,which may be due to the low intra-cavity peak powers (�1.48 Wfor Yb-laser and�14.28 W for Er-laser). We believe that synchroni-sation here is stabilised by the nonlinear coupling of the E11 and E22

states in semiconducting SWNTs [98].The timing jitter between sub-cavity pulses was measured by

optical cross-correlation [99]. The output pulses of the Er- andYb- lasers were amplified to 2.8 mW and 6.0 mW, by a single-modeEr-amplifier and a Yb-amplifier, respectively. A 1.06/1.55 lmWDM was used to combine the synchronised Er- and Yb-pulsesafter amplification. The signal at the output port of the WDMwas collimated through an aspheric lens and coupled into an auto-correlator modified for a cross-correlation measurement. TheFWHM of the cross-correlation pulse was 6.5 ps, shown in Fig. 9(a).

To determine the timing jitter between the two synchronisedsub-cavities, the half-maximum intensity of the cross-correlationtrace was recorded with 8000 points in one second, correspondingto a Nyquist frequency of 4 kHz. The data was processed using astandard fast Fourrier transform (FFT) method. The power spectraldensity (PSD) was used to represent the mean squared phase fluc-tuation at Fourier frequency f from the carrier in a measurementbandwidth of 1 Hz. The calibrated PSD and the integrated root-mean squared (RMS) timing jitter of 600 fs, in a Fourier frequencyrange from 1 Hz to 4 kHz, is shown in Fig. 9(b). Beyond 1 kHz thePSD decays while the integrated RMS timing jitter appears to sat-urate (�530 fs RMS jitter from 1 Hz to 1 kHz), indicating that lowfrequency noise was the dominant contribution to the jitter.

The limitation of using a SWNT-based polymeric-film device isthat it has a limited range over which it can operate. Due to theunique band-structure properties of graphene, it can be used as awide-band absorber and thus is a highly desirable material foruse as a mode-locker – as has been widely studied [15,16,60] –but also as a passive synchroniser, in a fashion similar to the SWNTdevice, except not being restricted to a narrow operation windowdictated by absorptive resonances [see Fig. 6(b)]. Dual-wavelengthultra-short pulse operation can be achieved, and the sub-cavitieshave similar pulse properties to the system based on SWNTs [seethe spectra and corresponding intensity autocorrelation traces inFig. 7(e)–(f)]. Similarly, the locking range – where both sub-cavitiesremain in a synchronised state – is greater than 1 mm, althoughnot quite as broad as the SWNT case. We believe this is due tothe strong nonlinear coupling in the saturable absorber device.Fig. 8(c) shows the RF spectra normalised to the synchronised fre-quency fsync for cavity detuning DL corresponding to (i) �0.6 mm;(ii) 0 mm and (iii) +1.0 mm. It can be seen that outside the lock-ing-range a second peak can be resolved at �fsync, correspondingto the repetition frequency of the asynchronous sub-cavity.

6. Single-pass short-pulse generation

As a result of typical operational fibre lengths in conventionalfibre lasers, pulse repetition rates in passively mode-locked sys-tems usually are in the range of tens of megahertz. Substantiallylower repetition rate pulse rates can be readily achieved usingextended cavities such as giant chirp configurations. However,for some applications, higher repetition rates in the gigahertzrange, are required. This is difficult to achieve with conventionalpassive configurations, but can usually be obtained with activelymode-locked systems. Once configured, however, there is littleflexibility on operational repetition rates and pulse durations tend

es: Examples of recent developments, Opt. Fiber Technol. (2014), http://

Fig. 7. Optical properties of the coupled-cavity ultrafast fibre lasers: (a)–(d), using a polymeric film embedded with SWNTs; (e)–(f), using a polymeric film embedded withfew-layer graphene flakes. (a,c,e,g), optical spectra; (b,d,f,h) intensity autocorrelation traces.

8 M. Zhang et al. / Optical Fiber Technology xxx (2014) xxx–xxx

to be fixed also by the cavity parameters and offer little selectivity.For flexibility and selectivity of repetition rate, pulse duration andwavelength, a simple single pass technique has been developedutilising spectral masking of a phase modulated CW signal in asso-ciation with single pass amplification and adiabatic soliton shapingto provide a versatile wavelength diverse source. It is well knownthat the spectral windowing of self phase modulated (SPM) pulsesin an optical fibre leads to the generation of temporally shorterpulses albeit at the expense of energy loss [102]. The spectral

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masking of a phase modulated signal originally developed byMamyshev [103], was simply based upon the introduction of atime varying optical frequency shift of a narrow linewidth CW sig-nal through the application of a periodic phase modulation; theprinciple of the technique is shown in Fig. 10.

A phase modulator is used to impose a sinusoidal instantaneousphase shift DU on a CW signal [see Fig. 10(a)], since Dx ¼ d/=dt,this will impose a cosinusoidal variation in the instantaneousoptical frequency x of the signal [Fig. 10(b)]. Subsequently using

es: Examples of recent developments, Opt. Fiber Technol. (2014), http://

Fig. 8. Locking ranges of the synchronous ultrafast dual-wavelength fibre lasers: (a)based on a SWNT device; and (b) based on a graphene device. (c) RF spectra centredat the synchronisation frequency (fsync:) for cavity detuning lengths of (i) �0.6 mm,(ii) 0 mm, (iii) +1.0 mm (graphene-based system).

Fig. 9. (a) Cross-correlation of the dual-wavelength ultrafast fibre laser source,simultaneously mode-locked and passively synchronised using a device based onSWNTs. (b) Corresponding power spectral density, and integrated timing jitter.

M. Zhang et al. / Optical Fiber Technology xxx (2014) xxx–xxx 9

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a long frequency band pass filter with a sharp edge as is shown inFig. 10(c), or alternatively a low frequency band pass filter can beutilised, where the absorption is represented by the hashed areain Fig. 10(b), a temporally shortened pulse can be extracted atthe repetition rate of the applied modulation [see Fig. 10(c)]. Asonly the extremities of the sinusoidally modulated signal areselected, there is minimal optical frequency variation and so thechirp on the generated pulses is relatively low. The use of a sigmoi-dal filter generates pulses that are closely fitting to a sech2 shape,although a Gaussian filter tends to reduce noise effects and giverise to smoother generated pulses. A typical experimental configu-ration is shown in Fig. 11.

A CW single frequency laser diode was modulated by a lithiumniobate phase modulator at up to 10 GHz. The signal was subse-quently amplified, single pass, in an Er-doped fibre amplifier(EDFA) before being spectrally filtered using a tuneable band-passfilter. The filter had a band-pass of 0.6 nm with sideband suppres-sion of over 40 dB and was tuneable over a range of 80 nm. Afterspectral selection, the signal had a duration of 30 ps at 10 GHz.In order to amplify and controllably compress the seed pulsesadvantage was taken of the adiabatic compression of solitons usinga distributed Raman gain configuration [see Fig. 11(b)]. It is knownthat the energy of a soliton Es is given by

Es ¼ 1:762jb2jcT

; ð1Þ

where b2 is the GVD, c the nonlinear parameter, and T the full widthhalf maximum duration of the soliton. It can be seen that if theenergy of the soliton increases in a fibre with all other parametersconstant, then the pulse duration will decrease. In order that energyis not exchanged to the continuum as dispersive radiation, thechange in the pulse energy must proceed slowly or adiabatically. Ageneral rule is that the pulse energy should change by about 10%

Fig. 10. Schematic of the principle of operation of the spectral masking of a phasemodulated pulse for single pass short pulse generation. (a) phase modulation of aCW signal, leads to an instantaneous frequency shift (b), which on spectral masking,shown as the shaded area, leads to (c) the generation of pulses. After [100,101].

es: Examples of recent developments, Opt. Fiber Technol. (2014), http://

Fig. 11. Experimental schematic of system for pulse generation using spectralmasking of phase modulated signals followed by amplification and pump powercontrolled adiabatic soliton compression. After [100].

10 M. Zhang et al. / Optical Fiber Technology xxx (2014) xxx–xxx

over a soliton period. As the soliton period is proportional to thesquare of the pulse duration, then as the pulse compresses the char-acteristic soliton period decreases and the energy change is experi-enced over a markedly shorter distance. A counter-pumped Ramangain configuration provides such a gain profile. In the exampleabove, Raman gain was provided in a 21 km length of dispersionshifted fibre, with a zero dispersion at 1464 nm. This was pumpedby a Raman fibre laser providing 5 W at 1464 nm, such that gainaround 1550 nm was in the anomalously dispersive regime. A coun-ter propagating pump geometry was used and as the pulses propa-gated along the fibre they experienced increasing gain per unitlength, but since the soliton pulse decreased in duration this config-uration ensured the Raman gain per soliton length remained rela-tively low and constant, inhibiting the generation of dispersiveradiation. A second EDFA was used to ensure that the launchedpulses were at fundamental soliton power. Intensity autocorrela-tions were taken as the pump power to the distributed Ramanamplifier was increased. Fig. 12 shows the measured decrease inthe output pulse duration with this increasing pump power. Clean,background free, soliton pulses as short as 290 fs were obtainedwith average powers up to 1.6 W [100,101].

Through tuning the input wavelength, similar operation wasachieved throughout the complete gain bandwidth of the Er ampli-fiers used above and the technique has been extended to operate invarious wavelength ranges from 1300 nm in conventional gainfibres up to 2000 nm. Below 1270 nm, however, anomalous disper-sion can only be provided by photonic crystal fibres (PCF), but typ-ical seed pulses from the phase modulator, although having beenoperated up to 20 GHz repetition rates, are still only in the regionof 10 ps. Consequently, associated soliton lengths are in the kilo-

Fig. 12. Variation of output soliton pulse duration and average power withincreasing pump power. After [100].

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metre range and so both loss and cost prohibit extension of thistechnique to shorter wavelength employing PCF. Additional com-pression of the pulses can be carried out before adiabatic compres-sion using conventional fibre grating pulse compressors forexample Ref. [101], where pulse compression to 190 fs in taperedPCF has been demonstrated at 20 GHz, centred at 1060 nm.

7. Conclusion

In conclusion, we have summarised a number of recent exam-ples from the ongoing research efforts focussing on the develop-ment of ultrafast fibre lasers. Specifically, it was demonstratedthat ionically-doped coloured glasses can be employed as efficientsaturable absorbers in fibre lasers. This approach extends the avail-able ultrafast laser technologies, offering the potential of scaling upaverage cavity powers beyond the damage thresholds of compet-ing technologies based on nanomaterial composite devices, suchas SWNTs and graphene embedded in polymeric films. Theseresults suggest that ionically-doped coloured glasses could presenta low-cost alternative to nanomaterials, that is robust againstphoto-degradation and thermal damage.

Tm-doped fibre lasers operating around 2 lm were reviewed,and first demonstrations of such systems mode-locked with graph-ene-based polymer composites were summarised. The numerousapplications that benefit from short-pulse sources operating in thiswavelength region will mean that the development of Tm-dopedfibre laser technologies will continue to be a vibrant area of laserresearch.

The importance of graphene, specifically in fibre laser technol-ogy, and other low-dimensional nanomaterials that have beenextensively studied in the context of optics – as well as otherscientific disciplines – was discussed, and we anticipate it to havean even more prominent role in the development of ultrafast pho-tonic systems in the future; the emergence of other layered mate-rials, such as MoS2 has been considered, and first reports of pulsegeneration in Q-switched and passively mode-locked fibre lasershave been reviewed.

The role of nanomaterials in the context of fibre lasers is notlimited to ultrafast switching; results of SWNT- and graphene-based devices used as a simultaneous mode-locker and passivesynchroniser of a two-colour short-pulse laser has been summa-rised. It is anticipated that interest in these highly nonlinear mate-rials will increase, in particular four wave-mixing and nonlinearconversion due to their exceedingly high third-order nonlinearresponse.

Finally, a scheme for the development of high repetition rateultrafast fibre lasers was outlined, based on the phase modulationand spectral masking of a CW integrated diode. It is imperative thatthis robust technique for the generation of gigahertz pulse-trainsof femtosecond-scale optical pulses will be extended to otherwavelength regions throughout the infrared, in particular withthe use of Tm-doped fibre amplifier technology.

Acknowledgments

The authors would like to thank B. H. Chapman, T. H. Runcorn,R. T. Murray, R. I. Wooward, L. Chen, A. C. Ferrari, T. Hasan and E.Obratzsova. M. Zhang acknowledges funding from EPSRC (EP/K03705) and E. J. R. Kelleher acknowledges funding from the RoyalAcademy of Engineering, through a Royal Academy of EngineeringFellowship.

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