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We report smooth and broad continuum generation using a compact femtosecond Ti:Sapphire laser as a pump source and a tapered photonic crystal fibre as a nonlinear element. Spectral output is optimised for use in optical coherence tomography, providing a maximum longitudinal resolution of 1.5 μm in free space at 809 nm centre wavelength without use of additional spectral filtering.
©2006 Optical Society of America
1. Introduction
Light sources based on supercontinuum (SC) generation provide a combination of desirable features: high fibre output power, a broad and controllable spectrum, low noise, and a high degree of spatial coherence that enables tight focusing. These features make SC sources ideal for several applications such as frequency metrology, femtosecond-pulse phase stabilization, ultrashort pulse compression, spectroscopy of materials and photonic structures, and fibre characterization. The extremely broad bandwidth of SC sources makes them particularly interesting for optical coherence tomography (OCT) imaging systems, since the longitudinal image resolution attainable in OCT is inversely proportional to the bandwidth and proportional to the square of the centre wavelength of the light source [1].
With the development of photonic crystal fibres (PCFs), supercontinuum generation has attracted much attention in the past few years [2, 3]. PCFs guide light in a central region of pure silica (core) surrounded by an ordered array of microscopic air holes (low-index cladding), that allow strong control of waveguide dispersion contribution and of light confinement (effective nonlinearities). Hartl et al. [4] reported the first OCT system based on SC generation with PCF. They used a femtosecond Ti:Sapphire laser with a PCF to achieve a longitudinal OCT image resolution of 2.5 μm in the spectral region 1.2 μm to 1.5 μm. Further high resolution OCT systems have also been reported, with a longitudinal resolution of 1.3 μm [5] and <1 μm [6] in the spectral region of 800 nm to 1400 nm and of 550 nm to 950 nm, respectively. To go beyond the laboratory prototype, a portable SC source achieving sub-2 micron longitudinal resolution OCT in the wavelength range around 1375 nm has been recently demonstrated [7]. However, the SC used in these studies [4–7] were generated by launching femtosecond laser pulses at wavelengths where the group-velocity dispersion (GVD) was anomalous. Under these conditions SC generation relies on the excitation of unstable solitons that produce severe spectral variations. Filters can be used to tailor the SC spectrum but these induce power-loss and increase the complexity of the system. Furthermore, SC spectra generated in this regime tend to be noisy and unstable, resulting in lower dynamic range of the OCT system and therefore in lower penetration depth and scanning speed [8–10].
In this paper, we report a compact, portable, powerful light source suitable for ultrahigh resolution OCT imaging systems, which can be deployed outside the laboratory environment. The SC is generated using a compact Ti:sapphire laser that emits relatively low energy (~2 nJ) femtosecond pulses at a wavelength of 809 nm. By tapering a PCF to control the dispersion curve in the taper waist, a spectrally flat continuum was generated by self-phase modulation (SPM). The yielded a bandwidth of up to 177 nm at 809 nm centre wavelength, at an output power of 108 mW (78% of input power). Finally, ultrahigh resolution OCT imaging was demonstrated on selected material samples using the novel source. The output power of the laser was slightly increased to broaden the spectrum to 194 nm, providing a longitudinal resolution down to 1.5 μm.
2. Experimental conditions
The pump source was a semiconductor-saturable-absorber mode-locked femtosecond Ti-sapphire laser (High Q Laser Production) that emits pulses of 45 fs duration at a wavelength of 809 nm with a pulse repetition rate of 70 MHz and an average output power of 140 mW. This is a compact (53 cm × 20 cm × 8cm) and robust turnkey system. An electron micrograph of the PCF used as a nonlinear element is shown in Fig. 1(a). The fibre has a pitch (ʌ) of 3.2 μm, a ratio of the hole diameter to pitch (d/ʌ) of 0.48 and a core diameter of 5 μm.
Fig. 1. (a) Scanning electron microscope “SEM” image of the photonic crystal fibre used. (b) Simulated dispersion curves of the periodic pattern of small holes from this fibre, with different periodicities (ʌ) and fibre diameters (D).
Since the ratio d/ʌ is close to 0.4, the PCF is almost an “endlessly single mode fibre” [11], which means that it is single-mode over an extended wavelength range. The GVD of the fundamental mode has been simulated for different values of ʌ for the same periodic pattern of small holes (d/ʌ = 0.48), using the method described in [12]. The result is that smaller values of the pitch shift the first and the second zero-crossings of the GVD to shorter wavelengths, but below a certain value (ʌ = 1.2 μm for this PCF) the GVD is normal for all wavelengths. Furthermore, for this particular periodic air-hole pattern (d/ʌ = 0.48) the GVD at the pump wavelength is always normal even if the periodicity is small. These features are particularly significant in the study of SC generation by self phase modulation. The dispersion curves with normal GVD are more favourable to generate a broad spectrum and for compensating spectral oscillations generated by SPM, unlike SC generated in ultrahigh-numerical-aperture step index fibre [13].
3. Supercontinuum from untapered fibre
The dispersion curve of PCF initially used is shown by the black curve in Fig. 1(b). The GVD of the fundamental mode is normal at wavelengths below the zero dispersion wavelength of 1040 nm, with a value of -75 ps/nm/km at the pump wavelength. Since the zero dispersion wavelength is much longer than the pump wavelength, SC generation process is expected to be dominated by SPM.
Laser pulses were launched into a 1-meter length of PCF using a 20x objective lens. The observed output spectrum of the PCF and the laser output spectrum (red curve) are shown in Fig. 2. Spectra were recorded using an optical spectrum analyser (Ando, Model AO-6315B). It is noticeable that a 1-meter length is not necessary to generate this spectrum since the dispersion length at the pump wavelength is around 9.6 cm [14]. The 1-meter fibre length simply allows easier handling.
Fig. 2. Measured laser output spectrum (red curve in linear scale) and output spectrum from 1-meter length of the PCF-used (bold and thin black curves respectively for linear and logarithmic scales).
The generated continuum has a bandwidth at half maximum of 79 nm, for an average fibre output power of 100 mW.
4. Spectral broadening by tapering a photonic crystal fibre
In order to increase the spectral broadening, SC generation has been investigated in the dispersion regimes plotted in Fig. 1(b) by using a tapering process to scale down the PCF. The tapering process involves heating and stretching a fibre to form a narrow waist connected to untapered fibre by taper transitions. A “flame brush” that travels to and fro along the fibre produces waists of uniform and predictable diameter. The travel distance changes as tapering proceeds, giving control of waist length independently of transition length [15]. The use of this process allows the study of SC generation with different PCF parameters, starting from only one fibre. The decrease in pitch reduces the effective area, increasing the non-linear response. Furthermore, since the fibre is tapered only locally, light can be launched into the untapered end of the fibre, allowing much better coupling efficiency and good mechanical stability. The fibre was tapered ‘fast and cold’ to minimise the degree of hole collapse due to surface tension while tapering [16]. Tapering enabled us to quickly and conveniently make suitable lengths of PCFs with a range of different parameters, without having to separately draw a number of different fibres.
The PCF was tapered to different waist diameters (80 μm, 70 μm, 60 μm, 50 μm, 40 μm) at a constant taper waist length of 100 mm. Transmission losses were measured during the fabrication process, and in all cases were lower than 0.1 dB. For each device, laser pulses were launched into the untapered end of the fibre close to the taper transition. A 1 m length of untapered fibre after the tapered region is fed into an optical spectrum analyser. The SC spectra generated with the PCF-tapers are shown in Fig. 3, together with the corresponding dispersion curves. These results illustrate the dependence on the GVD and the effective area.
Fig. 3. Measured SC spectra from 1 m length of the PCF (a); at the fibre input a taper is added with a waist diameter of 80 μm (b), 70 μm (c), 60 μm (d), 50 μm (e) and of 40 μm (f). In each plot, the simulated dispersion curve associated with the taper waist diameter is given in blue.
Tapers with a waist diameter of 30 μm were also fabricated, but as expected they were very lossy due to confinement and bend loss. The widest (FWHM) SC was obtained at a taper waist diameter of 40 μm. Different PCF tapers have been fabricated with this waist diameter, yielding similar behaviour. A smooth SC with a bandwidth of 177 nm at half maximum and an average fibre output power of 108 mW (corresponding to a coupling efficiency higher than 78%) was obtained with the best sample, as shown in Fig. 4(a).
Fig. 4. (a) Normalized output spectrum of the widest SC generated from a PCF taper with a waist diameter of 40 μm. Simulated dispersion curve of the taper waist cross-section from parameters measured by SEM imaging (blue curve). (b) Normalized output spectrum from the same PCF taper as (a), with average input power increased from 140 mW to 190 mW (respectively in linear and logarithmic scales for bold and thin curves).
However, even though the tapering process allows to scale down the PCF to sub-micrometer scales [16], SEM imaging of the waist cross section of this taper shows slight hole collapse that is due to the difficulty of tapering fibres with low air filling fraction. The final diameter of the holes was less than 5 % smaller than they would been in the absence of hole collapse due to surface tension while tapering. The corrected dispersion curve simulated with the measured values (ʌ = 0.97 μm, d/ʌ = 0.32) is also plotted in Fig. 4(a). The GVD is normal in the spectral range where the SC is generated, with an almost constant value of -200 ps/km/nm. This large normal dispersion leads to a dispersion length for the pump pulses of around 3.6 cm, so that the 10 cm waist length is not a limitation in the continuum generation; more compact tapers can thus be used. In contrast to SC generated by tapering a PCF with anomalous dispersion [17], the dispersion variation along the taper transition (26.8 mm) has a small impact on SC generation by SPM. Dispersion variations at the pump wavelength, are small compared to the final dispersion curve at the taper waist.
By increasing the average output power of the laser from 140 mW to 190 mW, the SC spectrum is broadened to a FWHM bandwidth of 194 nm, although the broadening is accompanied by weak oscillations as shown in Fig. 4(b). It is noticeable that, contrary to SC generated by SPM around the zero dispersion wavelength or in a dispersion-free medium, these spectral oscillations are weak and mainly due to the influence of higher order dispersion [14].
5. Application to optical coherence tomography imaging
The applicability of the SC source shown in Fig. 4(b) to OCT was evaluated at first by measuring the field autocorrelation in a Michelson interferometer setup with fully dispersion matched arms. The maximum obtainable free-space resolution of 1.5 μm, according to the FWHM interference envelope of the signal, is shown in Fig. 5. Sidelobes observed in the autocorrelation function, caused by residual modulations in the spectrum, were equal or less than 5% of the signal maximum.
Fig. 5. Envelope of interference signal of the widest SC (black curve: linear scale, blue curve: log. scale) with the sidelobe level marked (dashed line).
The usefulness of this light source for ultrahigh resolution imaging was then demonstrated by performing measurements on selected material samples in combination with our en-face scanning OCT system, as described in detail in [18] and based on a concept presented in [19].
Fig. 6. Ultrahigh resolution OCT images of selected material samples: cross-sectional images of (a, b) polyolefin foams with different pore size, (g) a 150 μm thick protective coating on wood. (c, d, h) Corresponding en-face OCT images of the foam specimens and the coating, (f) microscope image of a polished cross-section of the protective coating. (e) Demodulated interferogram (from a single depth- or A-scan) with a mirror as sample.
The actual depth resolution of the combined OCT - light source system was determined to be 2.1 μm in air, corresponding to ~1.4 μm in typical polymeric materials (Fig. 6(e)). The degradation when compared to the above measured value of 1.5 μm is due to incomplete compensation of higher order dispersion between the sample and reference arms in our heterodyne Mach-Zehnder interferometer: only one BK7 prism pair and a dense flint glass block are used to compensate two acousto-optic modulators needed for heterodyne detection and a beam expansion telescope (see Fig. 1 of ref. [18]). The lateral resolution as well as the system sensitivity have been found to be identical to the setup in [18] when applying the same scan speed and incident power (lateral resolution better than 4 μm, 100 dB system sensitivity).
For demonstration purposes and comparison, images taken from selected material samples, some identical to those presented in [18], are given in Fig. 6: Figures 6(a-d) show ultrahigh-resolution cross-sectional and en-face OCT images of different polyolefin foam specimens with average pore sizes of 500 μm and 80 μm, respectively. In Fig. 6(g, h) a 150 μm thick protective coating with embedded ceramic particles on wood has been imaged and compared to a light microscope image taken from a polished cross-section (Fig. 6(f)). In the images the fine particles in the coating, as well as small structures within the cell walls of the foam samples, are clearly resolved and demonstrate the utility of the SC as a light source for material investigation with ultrahigh-resolution OCT systems. Unlike in vivo OCT measurements that benefit from a broadband light source with a centre wavelength near 1 μm [20], for the study of inanimate objects the high power and high resolution of the current system will be of advantage.
6. Conclusions
We have demonstrated a smooth, broad and powerful SC light source centred around 809 nm. This SC is generated by a compact Ti:Sapphire laser that emits relatively low energy femtosecond pulses. The spectral smoothness results from a tailored dispersion curve obtained by tapering a PCF, allowing good control of the PCF parameters and a high coupling efficiency. Since the SC is mainly generated by SPM, additional spectral filtering was not required, and the pulse coherence was preserved. We have used this compact and portable light source to demonstrate ultrahigh resolution OCT imaging of selected material samples.
Acknowledgments
Part of this work was supported by the European Commission (FP6 CRAFT Project: COOP-CT-2003-507825) and the Austrian Science Fund FWF (Project: P16585-N08). In addition some of the authors (UAR) wish to thank C.K. Hitzenberger and M. Pircher from the Medical University of Vienna for support in the setup of the en-face OCT system and E. Schlotthauer (FH Wels) for the microscope image.
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