Few-cycle all-fibre temporally coherent supercontinuum.
Ultrafast fibre lasers appear as an alternative to Ti:Sa oscillators and OPCPA systems, since they are compact, air-cooled, turn-key, cost-effective and maintenance and alignment-free17,18. However, the natural gain spectra of rare-earth elements (the active media in fibre lasers) limit their emission bandwidths and pulse durations to a few tens of nm and a few hundreds of femtoseconds, respectively. Nevertheless, in the last years, research on supercontinuum (SC) generation has demonstrated successful approaches to obtain very high-quality few-cycle pulses using ultrafast fibre lasers as exciting sources of particular nonlinear effects in photonic crystal fibres (PCFs): pumping at a wavelength within the flattened top of the convex dispersion curves of all-normal dispersion (ANDi) PCFs (whose dispersion lies completely in the normal dispersion region), spectral broadening appears due to the action of self-phase modulation (SPM) and optical wave breaking (OWB)19. In this way, highly coherent SC emission can be generated, preserving compressible pulses in the temporal domain. Different approaches to generate temporally coherent SC using ANDi PCFs have been proposed. In some configurations the seed stage (or pump stage) of the coherent SC source is built with free-space optics technology: e.g., Ti:Sa lasers, optical parametric oscillators (OPOs) or master oscillator power amplifiers (MOPAs)13,14,20−24. In other configurations the seed stage is a fibre laser but light coupling to the ANDi PCF is performed with free-space optics15,25−27. Chow et. al.28 proposed an all-fibre configuration based on ANDi PCFs for the 1.55 µm band, but this work lacked a demonstration of transform limited or near-to-transform limited compression of the generated SC, probably due to the long length of the ANDi fibre used in the experiment (64 m). To the best of our knowledge, none of the current state-of-the-art configurations that demonstrate generation of transform-limited or near-to-transform-limited few-cycle temporally coherent supercontinua using ANDi PCFs are an all-fibre configuration (said all-fibre configuration understood as a monolithic fibre-optic configuration where all its stages are fibre based and coupled to each other by a fibre splice or a fibre-based transition). In this section we describe an all-fibre configuration for generating temporally coherent supercontinuum that provides transform-limited few-cycle pulses with durations as short as 13.0 fs. Recently we have reported first evidence of the ability of this all-fibre configuration to deliver few-cycle transform-limited pulses29. In this section we describe in detail the monolithic architecture of the system, the process of design, manufacture and optimization of ANDi PCFs according to the diagnosis of different emission regimes depending on the fibre geometry and we present results of few-cycle emission of optimized time-domain quality, which are confirmed by independent pulse duration measurement techniques: d-scan, and interferometric autocorrelation.
Temporally coherent spectral broadening in ANDi PCFs is achieved by SPM and OWB using short lengths of ANDi PCFs (few to tens of cm) pumped by input pulses of high peak power (few to tens of kW) and very short duration (few to hundreds of fs). As an example, Heidt et al.19 showed that a spectral broadening of > 100 nm (with central wavelength at 1060 nm) can be obtained while maintaining perfect coherence by pumping 1 m of ANDi PCF with input pulses of 5 kW peak power and 200 fs duration. In our monolithic configuration, the ANDi PCF is a few tens of cm long and is excited by input pulses of > 15 kW peak power and < 250 fs duration. Ti:Sa lasers, OPOs or MOPAs, with laser generation architectures based on free-space configurations, deliver naturally pulses of this type. However, for an all-fibre laser architecture it is a challenge to offer pulses with these properties to be delivered to an ANDi PCF because the light is completely confined in the cores of the optical fibres, whose core diameters are typically below 10 mm. A peak power of 15 kW inside guiding fibres of 10 mm core diameter yields intensities of > 15 GW/cm2 which, for propagation lengths of a few cm, are above the threshold for many undesired nonlinear effects that distort the laser pulses propagating inside the fibres and, particularly, destroy the temporal coherence of the laser pulses. To overcome this problem, we use a chirped pulse amplification (CPA)30 system with an all-fibre configuration (see Fig. 1), where the temporal compression stage is built with a hollow core photonic bandgap fibre (HC PBGF). Maintaining the temporal coherence throughout its stages, such configuration delivers pulses of > 15 kW peak power and < 250 fs duration, to be used as exciting pulses of the ANDi PCF. The end of the HC PBGF is fusion spliced to the ANDi PCF. To maintain the integrity of the structure of both fibres, the arc discharge parameters of the fusion procedure are set for a weak splice, providing a coupling efficiency of 0.4 or greater.
Figure 1. a), All-fibre configuration of a temporally coherent supercontinuum source of few-cycle pulses. GDD: group delay dispersion; GVD: group velocity dispersion; b), Qualitative representation of the spectral and temporal properties of the pulsed optical signal as it evolves throughout the stages of the all-fibre source (stages 1 to 5) and at the output of the temporal compressor (stage 6) used to compress the pulse down to its Fourier limited duration. Dli and Dti: spectral bandwidth and temporal duration of the pulses, respectively, at the output of i-th stage; c), Properties of the pulsed signal at the output of each stage, for an example of implementation of the all-fibre configuration where the active fibre is Yb-doped, thus with laser emission in the 1 mm band. MFD: mode field diameter, lc: central wavelength, PRR: pulse repetition rate, DlFWHM: spectral bandwidth at full width at half maximum, DtFWHM: pulse duration at full width at half maximum, Pavg: average power, Pp: pulse peak power, Ip: pulse peak intensity. Values are given for a pump laser diode wavelength and average power of 976 nm and 3.75 W, respectively.
The first stage (stage 1) corresponds to the seed of the system, a passively mode-locked all-fibre oscillator that delivers transform limited pulses of hundreds of femtoseconds duration and MHz range pulse repetition rate (PRR) with a central emission wavelength lc. PRR and lc remain unchanged throughout all stages of the system. Stage 2 is composed of a polarization maintaining (PM) single mode optical fibre and a PM fused fibre combiner. The fibres of this stage have a normal group velocity dispersion (GVD > 0). The function of stage 2 is to stretch the pulses temporally so they can be amplified in the next stage without generating nonlinear effects (the peak power remains below the threshold of generation of nonlinear effects in the optical fibre core). Also, it pre-compensates the anomalous dispersion that the pulses will experience in stage 4. The fused fibre combiner launches light from a laser diode into the active fibre of the next stage. Stage 3 is composed of a double clad PM rare-earth doped active fibre, with normal group velocity dispersion (GVD > 0). The function of stage 3 is to amplify the pulses to the maximum possible peak power without biasing nonlinear effects that would distort the temporal and spectral shape of the pulses. The amplification is produced progressively in the fibre by stimulated radiation in the active rare earth ions of the fibre core, which are pumped to excited states by the light coming from the laser diode of the previous stage through the first clad of the fibre (this clad is passive, i.e., undoped). Stage 4 is composed of a hollow core photonic bandgap (HC PBG) microstructured fibre with anomalous group velocity dispersion (GVD < 0). The function of stage 4 is to compress the pulses to achieve the peak power required at the input of the ANDi PCF to efficiently generate temporally coherent spectral broadening by SPM and OWB. Also, its length is chosen so that the net group delay dispersion (GDD) suffered by the pulses from the oscillator output is slightly anomalous (− 0.015 ps2 in the example of implementation of Fig. 1). This way, the pulse still undergoes compression in the first segment of the ANDi PCF and SPM efficiency is optimized by having the maximum peak power achievable inside the ANDi PCF. Since the material of the HC PBG fibre core is air, nonlinear effects due to high peak power are avoided, hence the pulse does not experience additional spectral broadening. Stage 5 is composed of a highly nonlinear ANDi PCF a few tens of cm long. GVD in this fibre is normal within a very broad spectral bandwidth (broader than 300 nm, centred at 1060 nm, in the implementation example of Fig. 1), with a quasi-flat and symmetric shape (see Fig. 2-d2). The function of this stage is to spectrally broaden the spectrum of the pulsed signal by SPM under near-zero-dispersion conditions, which preserves the temporal coherence of the pulses so that they remain compressible down to pulse durations corresponding to the Fourier limit of their spectrum. At the end of stage 5, the pulsed signal is delivered to free-space and collimated. Stage 6 is composed of a free-space temporal compressor of fixed anomalous dispersion (GDD < 0) and variable normal dispersion (GDD > 0). It compresses the temporally coherent pulses from the output of stage 5 down to close to their Fourier limited durations (12.2 fs in the example of implementation of Fig. 1). The variable compression is introduced by a pair of glass wedges placed in the path of the optical signal. Dispersion is varied by changing their relative position (insertion length), therefore changing the amount of glass material effectively traversed by the optical signal.
The ANDi fibre of Sect. 5 was manufactured with F300 silica and exhibits a standard PCF solid core design: the solid-core results from a missing hole that is surrounded by N rings of holes running along the fibre longitudinal axis. Holes are displayed in a periodic manner with a triangular lattice. The air holes have a diameter d. The distance between hole centres, called pitch, is Λ. The microstructure of air holes is surrounded by a jacket of uniform silica that confers the fibre a typical diameter of 125 µm. Efficient confinement of the light inside the core is obtained for a value of N equal to 7. The required properties of the GVD curve are calculated using the semi-empirical model proposed by Saitoh et al.31. Considering a central wavelength of 1060 nm, a set of potentially valid dispersion curves is obtained combining the values of Λ and d within the following ranges: Λ = [0.50 ; 0.64] µm ; d = [1.50 ; 1.70] µm. ANDi PCFs with optimum values of Λ and d have been manufactured through an optimisation iterative process (see section Methods). Figure 2 summarizes the results of the optimisation process of the ANDi PCF manufacture, from design to performance on temporally coherent spectral broadening. Figure 2a shows a scanning electron microscope (SEM) image of the cross section of the manufactured ANDi PCF that presents the largest temporally coherent spectral broadening (fibre 2). Figure 2b shows a map of calculated values of Λ and d that provide adequate dispersion curves for temporally coherent spectral broadening. The red region denotes the condition of the dispersion maximum belonging to the exciting laser bandwidth (1060 ± 15 nm). The blue region extends the condition to a broader bandwidth (1060 ± 30 nm). Section Methods offers an extended explanation of the elaboration of this map. Figures 2c1-3 are detailed SEM images of the core region of representative ANDi PCFs manufactured during the optimisation process, with different values Λ and d (measured from the corresponding SEM images) and numbered from 1 to 3. Figures 2d1-3 show the calculated dispersion curves for fibres 1 to 3, respectively. Finally, Figs. 2e1-3 show the spectra at the output of the ANDi PCF (stage 5 in Fig. 1) obtained with fibres 1 to 3, respectively, for the same exciting conditions (output signal of stage 4 in Fig. 1) and same length (20 cm), at 3 different driving currents of the laser diode that pumps the amplifying stage (stage 3 in Fig. 1). Fibre 1 presents a dispersion curve out of the target of ANDi condition, being the dispersion parameter D ∈ [0; + 5 ps/nm km] within the exciting laser bandwidth. Consequently, an asymmetric spectral broadening is observed, governed by temporally incoherent dynamics of first stages of SC construction in an anomalous GVD regime: modulation instability (MI), soliton fission and Raman self-frequency shift32,33. Contrarily, fibre 2 presents an ANDi curve, being the dispersion parameter D ∈ [-5; -25 ps/nm km] within a full bandwidth of 300 nm, centred at a maximum dispersion wavelength of 1060 nm. SPM governs a symmetric temporally coherent broadening. The near-zero dispersion of the ANDi fibre limits pulse temporal stretching, thus driving high SPM efficiency. The evolution of spectral broadening in fibre 2 is shown in Fig. 2e2. The average pump power of 3.75 W is slightly above the average pump power limit below which spectral broadening is due to SPM only. The shoulder peak at 930 nm in the short-wavelength edge of the spectrum indicates the beginning of the appearance of OWB effects, which are to be avoided to maintain “pure” temporally coherent spectral broadening by SPM19. Fibre 3 presents an ANDi curve as well, being D ∈ [-25; -40 ps/nm km] within a full bandwidth of 300 nm, centred at a maximum dispersion wavelength of 1045 nm. These values are more distant from zero dispersion than those of fibre 2. Consequently, SPM still broadens the spectrum while preserving temporal coherence, but with less efficiency (compared to fibre 2), because the pulse suffers larger temporal stretching.
To confirm that our pulses at the output of the ANDi PCF are temporally coherent (thus compressible to close to their Fourier transform limit), we use the free-space temporal compressor of stage 6 (Fig. 1) and compare results of two different methods of pulse characterization in the time domain: d-scan technique and interferometric autocorrelation. The d-scan technique is based on performing a dispersion scan to the pulses with a pulse compressor while measuring the spectrum of the resulting second-harmonic (SH) signal. From this measurement, the phase and the amplitude properties of the electric field of the pulses are retrieved34. Figures 3a1-4 show the properties of the pulse measured with the d-scan technique, for the case of ANDi fibre 2 and a pump diode average output power of 3.75 W. The compressor is designed to provide a range of net GDD to the pulse from − 1000 fs2 (0 mm insertion length, Figs. 3a1-2) to + 1500 fs2 (17 mm insertion length, Figs. 3a1-2). Details on the design of this compressor and corresponding application of the d-scan technique have been reported previously by us35. The compressor is adjusted to achieve the best quality pulse (shortest FWHM pulse width and highest peak power ratio between main peak and side-lobes). In these conditions, the pulses present very low GDD, third order dispersion (TOD) and fourth order dispersion (FOD) values (+ 276 fs2, -5162 fs3 and − 1621 fs4, respectively) with 56 ± 4 % of the energy in the main peak and with peak power ratio between the main peak and the side-lobes greater than 8. The measured pulse width at FWHM of the compressed pulse is 13.0 fs (3.7 optical cycles). This result demonstrates the high degree of temporal coherence of the pulsed signal, since it is very close to the Fourier limited duration supported by its optical spectrum (12.2 fs). The fact that the pulses are properly compressed by a compressor of negligible TOD proves that the pulses suffer very low TOD (comparable to that of the compressor) while being spectrally broadened at the ANDi fibre. This effect results from the flatness of the dispersion curve of the ANDi fibre (Fig. 2d2). Average and peak power of the beam at the output of the compressor are Pavg = 160 mW and Pp =169.2 kW, respectively. Focused on a sample area of e.g., 10 mm2, the photon irradiance of the beam during pulse propagation on the sample would be 8.5 x 10 30 photons s − 1 cm − 2 (neglecting losses in the optics), a figure well above typical multiphoton excitation thresholds. Figure 3b shows an autocorrelation trace of the pulses obtained at the output of stage 6 with an interferometric autocorrelator. This trace corresponds to the pulsed signal compressed to its minimum duration by the same variable dispersion compressor employed for the d-scan technique. From this trace, an estimation of the FWHM of the pulse intensity profile of 12.6 fs is obtained. Despite not providing unambiguously the real form of the pulse intensity profile36,37, the autocorrelation method is widely known and trusted by microscopists. Hence, the fact that it produces, independently, an estimated result of the pulse width comparable to the accurate result of the d-scan method, is relevant for a straightforward use of the source in current NLO microscopy setups. To the best of our knowledge, with the measurement of a temporal pulse width of 13.0 fs (3.7 optical cycles) we report the shortest pulses obtained to date from an all-fibre source in the 1 mm spectral region, being > 3 times shorter than the shortest pulses delivered by previously reported all-fiber sources in this region, which, limited mainly by the gain spectrum of ytterbium and by fibre dispersion and nonlinearity management constraints, do not support durations below 42 fs38 − 41.
Broadband multispectral and multimodal nonlinear imaging
The output from stage 6 was coupled to an adapted inverted confocal microscope (Eclipse TE2000-U, Nikon) (Fig. 4a) modified for nonlinear imaging experiments. The variable dispersion compressor described above was used to pre-compensate the dispersion of the optical elements in the path of the microscope towards the sample, where the pulse duration was maintained below an estimated duration of 16 fs (Fig. 4c). The coupling system of the external illumination source included two galvanometric mirrors (Cambridge Technology, UK) and a telescope arrangement. We used a dichroic mirror (FF825-SDi01, Semrock) for sending the pulses to the illumination objective. The generated fluorescent light captured using this objective was collimated and sent through the same dichroic mirror, in a non-descanned configuration, for detection using a photomultiplier tube (PMT) detector (H9305-04, Hamamatsu). Three different microscope objectives, with different refractive index immersion media, numerical apertures (NA) and magnifications were used (see table with properties in Fig. 4). Dispersion pre-compensation was performed for every objective. The few-cycle all-fibre source described above was used as the illumination source for all image acquisitions, except for images in Figs. 5F,H that were generated with a pulsed Ti:Sa laser (MIRA 900-F, Coherent, 200 fs nominal pulse width) operating at a central wavelength of 810 nm.
Possible leakage of fundamental laser light was filtered with a BG40 filter (FGB37-A, Thorlabs). The two-photon excited fluorescence (TPEF) signal was filtered with fluorescence filter cubes (DAPI, FITC and TRITC: Standard series, Nikon) and collected in the backward direction. We used a 25x NA1.10 water immersion objective (Apo LWD, Nikon) for the second harmonic generation (SHG) signal collection in the forward direction. The SHG signal was filtered with a bandpass filter (FF01-542/27 − 25, Semrock).
The dispersion pre-compensation performance of the variable compressor was evaluated by varying the glass wedges insertion to adjust the duration of the pulse at the sample plane. For each image, we measured the mean intensity of the generated TPEF signal as the function of glass insertion. The results can be seen in Figs. 4b1-3. The induced dispersion per glass insertion length was + 147 fs2 / mm. We see that the pre-compensation system can efficiently be used to maximise the fluorescent signal from the sample. A maximum TPEF intensity depending on the insertion of the glass was observed. This indicates that the system is capable of pre-compensating the dispersion introduced by the different microscope objectives. The discrepancies found in the optimum insertion length when imaging different samples can be attributed to changes in the refractive index of the samples.
Using the 25x objective under the optimised GVD settings of the compressor, we have successfully imaged several samples. Importantly, good fluorescence signal and depth were achieved using an average power of the pulsed beam of ~ 4 mW, measured at the sample plane (the average power of the beam at the output of the variable compressor was 160 mW; it was attenuated by a variable neutral density filter and by intrinsic losses of the optical components through the microscope optical path). The maximum penetration achieved corresponds to 220 µm (Figs. 5A-D) in depth of the tail of a transgenic line zebrafish embryo (Caax-GFP) expressing GFP in all cell membranes. Zebrafish embryos are transparent, so they allow imaging at these large penetration depths. To test the penetration capabilities of the laser within a scattering tissue two excised retinas were stained with the same cytoskeleton marker (phalloidin), each conjugated with a different fluorescent dye: AlexaFluor 405 and 647, to be excited with a Ti:Sa Coherent MIRA 900-F laser (ΔλFWHM = 10 nm, λc = 810 nm) and with our few-cycle all-fiber source (ΔλFWHM = 150 nm, λc = 1060 nm), respectively. We proceeded to record the full retina (~ 170 µm) with cellular resolution, acquiring z-stack images (Figs. 5G-H). For both lasers, we used the same laser power at the sample plane and similar step spacing for constructing the z-stacks. Then lateral re-slices of the z-stack images were performed. Figures 5E-F show the comparison of the lateral re-slice TPEF images acquired with our few-cycle all-fibre source and with a Coherent MIRA 900-F laser. In the image acquired with our few-cycle all-fibre source, all synaptic (bright regions) and nuclear (gap regions) layers that characterize the tissue can be distinguished (Fig. 5E). However, in the images acquired with the Coherent MIRA 900-F laser it was only possible to distinguish four layers (Fig. 5F). Larger imaging depth was achieved using the few-cycle all-fibre source, with which properly resolved images of the retina deeper layers ONL and OS (Figs. 5E,G) were obtained. It is interesting to mention that the rat retina is highly autofluorescent to light in the blue-green region of the spectrum. In addition, the external segment of the photoreceptor cells where opsins (photopigments) are packaged, is highly absorbing to visible light. Therefore, illumination sources in the IR spectrum combined with red fluorescent dyes are ideal for depth imaging to prevent the autofluorescence generation/distortions in this tissue.
To test the capability of our few-cycle all-fibre source to nonlinearly excite multiple markers we proceeded to acquire TPEF images of multiple fluorophores: GFP, SYTOX Green, Alexa Fluor 568, tagRFP and Alexa Fluor 647. We also acquired SHG images of unlabelled tissues. Care was taken to use the corresponding filters for acquiring the TPEF signals. Figure 6A shows an image of a mouse intestine section stained with SYTOX Green (FITC filter) labelling the nuclei shown in yellow, and Alexa Fluor 568 phalloidin (TRITC filter) labelling the actin filaments shown in blue. Figure 6B shows the rhizome of Convallaria majalis stained with Fast Green and Safranin. Chloroplasts are shown in green (FITC filter) and cell walls are shown in red (TRITC filter). In Fig. 6C, it is also possible to see the autofluorescence of a pollen grain. The large emitted autofluorescence spectrum was detected using two different fluorescence filters, FITC filter shown in green and TRITC filter shown in red. We have also been able to image in vivo samples with cellular resolution. In particular, paralysed C.elegans specimens. In Fig. 6D, we can see a C.elegans OH15500 strain expressing tagRFP in all neurons. The SHG signal revealed all the different structures of the pharynx of the animal: corpus, isthmus and posterior bulb. Moreover, we can see the muscle fibres and their contraction during swallowing over time. These structures are a high-priced reference while imaging the neurons with TPEF. By using different emission filters, we have been able to split the fluorescence from the different fluorophores to visualize multiple structures with a single illumination source.