1. Introduction

Biocompatible fiber optics has played a significant role in tackling various biological problems, such as laser scanning [1,2], molecules diagnosing [3,4], fluorescence collecting [5,6], etc. So far, typical biological fiber-optic applications employ continuous-wave (CW) laser sources for easier implementation and better cost-efficiency [7]. Recently, advanced optical technologies utilizing ultrashort laser pulses have created new opportunities for high-sensitivity, high-temporal and -spatial resolution applications [8,9], which however desire further efforts on investigating new biocompatible fibers with optimal performance for ultrashort laser pulses delivery and fluorescence collection. Biocompatible optical fibers capable of transform-limited pulse delivery and focusing in the near-infrared window are especially important for biophotonics deep inside the biological tissue [10–16], wherein applying multiphoton excitation and nonlinear optics can further broaden its horizon [17,18]. Compared with visible light biophotonics, the near-infrared counterparts exhibit following advantages: (1) multiphoton biophotonics enabled by the near-infrared laser allows deeper penetration into the highly scattering tissues and reduce phototoxic effect [19]; (2) multiphoton absorption is spatially confined to the tiny area around the perifocal region. Hence, it not only can further reduce the optical damage but also provide higher temporal and spatial resolutions [20].

In this regard, one of the most challenging issues in multiphoton (nonlinear) biophotonic applications is to ensure the high-quality transmission of ultrashort laser pulses in biocompatible fibers [15,21–23]. To address this pressing issue, photonic-crystal fibers (PCFs) with virtually zero chromatic dispersion and the capability of transmitting intensive laser are usually employed to reduce the temporal broadening effect on ultrashort laser pulses [24–26]. A promising alternative method is to adopt the concept of dispersion-varying fiber optics [27–29], which has not been investigated for biocompatible fibers. To this end, here we present a novel diameter-oscillating fiber (DOF) with high biocompatibility and good abilities of ultrashort pulse propagation and focusing. The DOF is designed with an oscillation landscape of dispersion and microlens at the endface, making it highly suitable for transmitting near-infrared laser pulses and focusing to a small spatial spot. To the best of our knowledge, such a DOF is demonstrated for the first time and anticipated to create new opportunities for multiphoton biophotonics in deep tissues.

2. Results and discussion

2.1 Design and fabrication of the diameter-oscillating fiber

The DOF is fabricated by using the technical steps as illustrated in Fig. 1(a), while the structure of DOF is sketched in Fig. 1(b). The DOF is designed with a step-index core/cladding structure by using PDMS elastomers. The PDMS elastomer is an organic silicon material with ultra-high flexibility, biocompatibility, and light transmittance, and has been widely used as a matrix material for stretchable fiber-optic sensors [30,31]. The total internal reflection of lightwave at the interface between the core and cladding is the most important factor for lightwave guiding. To achieve total internal reflection in the DOF, the refractive index (RI) of the core material needs to be larger than that of the cladding material. The curing process of PDMS is a crosslinking reaction between the base and curing agent under the appropriate heating condition. Changing the mixing ratio of the base and curing agent can adjust the crosslinking density of the polymer network of PDMS, and eventually adjust the RI of PDMS [32], as shown in Fig. 2(a). In this work, the base to curing agent ratios for the core and cladding are set to 5:1 (n ≈ 1.405) and 20:1 (n ≈ 1.400), respectively, as the PDMS sheets cured by these two ratios have a large RI difference and good transmittance, as shown in Fig. 2(b). In fabrication step 1, the fiber core is fabricated by sucking the high-RI PDMS precursor (base/curing agent, 5:1) into a Teflon tube mold and cured at 80 °C for 40 mins, and subsequently taken out from the Teflon tube mold by water injection. The diameter of the fiber core is determined by the inside diameter of the Teflon tube mold, such that we can simply fabricate DOFs with different core diameters by using different inner diameters of the Teflon tube mold. In fabrication step 2, the fiber core is dipped into the same PDMS precursor used in fabrication step 1 (base/curing agent, 5:1), and then vertically spun for a good homogeneity at a low spinning rate for a short time, in which way the fiber core is evenly coated with a thick layer of PDMS precursor. Due to surface tension and gravity, the thick layer of PDMS precursor can form a spherical shape, and the diameter-varying core is obtained by curing. In fabrication step 3, the same process as that of fabrication step 2 is repeated except that a low-RI PDMS precursor (base/curing agent, 20:1) is used to obtain core-cladding PDMS optical fibers. The thickness of the cladding layer can be controlled by adjusting the spinning rate and dipping time.

 

Fig. 1. (a) Schematic diagram of the DOF fabrication. (b) Structure of the DOF.

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Fig. 2. (a) Dispersion curves and (b) Transmission spectra of PDMS sheets fabricated with different base to curing agent ratios. Inset: the photograph of a PDMS sheet.

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2.2 Characterization of diameter-oscillating fiber

Figure 3(a) shows the cross-sectional and longitudinal images of DOFs with core/cladding diameters of 50/70, 130/160, and 300/350 µm. As can be observed, they exhibit uniform morphology and oscillating diameter along the fiber axis. The fabricated optical fibers have ultra-high flexibility and stretchability that they can be easily knotted and restored to their original condition after stretching to a strain of more than 200%, as shown in Fig. 3(b) and (d). To better couple lightwave into the DOF for characterization, we insert a commercial silica single-mode fiber (SMF) into the Teflon tube mold right after sucking the precursor solution and thermally cure together. Thereby, the SMF can be concentrically immersed in the PDMS fiber. It is worth noting that, when fabricating PDMS fibers with a core diameter of less than 125 µm, i.e., the same size as that of SMF, the silica SMF needs to be tapered for a smaller size to fit their size. The PDMS fibers show a good capability of light guiding, as shown in Fig. 3(c), where the 473-nm blue light is coupled. With a specific core/cladding RI difference, the core size of the PDMS fiber dominates the dispersion curve of the PDMS fiber. According to our experiments, the variation range of the DOF’s core diameter had a significant dependence on the spinning rate, spinning time and curing temperature, while the variation range of the diameter had a significant dependence on the PDMS precursor viscosity. As a result, we precisely adjust the variation range and period of the DOF’s core diameter by controlling the spinning rate, spinning time, curing temperature, and PDMS precursor viscosity in fabrication step 2. When keeping the spinning rate, spinning time and curing temperature constant, and increasing the viscosity of the PDMS precursor, the DOF’s core diameter keeps a variation range of 100–160 µm, and the variation period can be reduced from 1050 µm to 780 and 570 µm, as shown in Fig. 4(a). On the other hand, when keeping the viscosity of the PDMS precursor constant and increasing spinning rate, spinning time and curing temperature, the DOF’s core diameter keeps a variation period of 380 µm, and the variation range of the DOF can be flexibly changed, e.g., from 65 µm to 108, 90, and 72 µm, as shown in Fig. 4(b). When fabricating the cladding of the DOF in fabrication step 3, a microlens is also fabricated at the endface of the PDMS fiber, as shown in Fig. 5(a). The curvature of the microlens can also be precisely adjusted by controlling the spinning rate, spinning time and curing temperature in fabrication step 3. When increasing the spinning rate, spinning time and curing temperature, the curvature of microlenses increases. Figure 5(b) shows that the radius of microlens curvature can be adjusted to 35, 45, and 60 µm, when keeping the same fiber diameter, i.e., 60 µm in this case. To give more details about the fabrication of DOFs, we have summarized all the experimental parameters of DOFs’ fabrication, as shown in Table 1. Please note that, the PDMS precursor pre-curing time (i.e., 60 °C) in the table is used to control the viscosity of the PDMS precursor.

 

Fig. 3. (a) Microscopic images of the fabricated DOFs with different sizes. (b) Mechanical flexibility test of the DOFs. (c) Light guiding measurement of the DOFs. (d) Stretchability measurement of the DOFs.

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Fig. 4. (a) DOFs with different diameter variation periods. Inset: microscopic images of the DOFs with different diameter variation periods. (b) DOFs with different diameter variation ranges. Inset: microscopic images of the DOFs with different diameter variation ranges.

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Fig. 5. (a) Microscopic images of the fabricated DOFs with microlens. (b) Microlenses with different curvature

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Table 1. Key parameters of DOFs’ fabrication

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To validate the light-guiding capability of the fabricated DOFs, the light propagation measurement is performed. The transmission loss at the visible wavelength range (473 and 671 nm) and near-infrared wavelength range (1500 nm) are measured based on the cutback method. Please note that, in most multiphoton biophotonic applications, the excitation wavelength is usually in the near-infrared wavelength regime while the visible wavelength regime for the fluorescence signal. As a result, both visible and near-infrared wavelength windows are investigated in this work. The transmission loss of the normal PDMS optical fiber with a constant core diameter (i.e., 65 µm) is 0.1–0.2 dB/cm at these wavelengths, see Fig. 6(a), while the transmission loss of the DOF that varies from 65 to 110 µm is 2–2.5 dB/cm at these wavelengths, as shown in Fig. 6(b). The measured results manifest that the light transmission loss of the DOFs is more than ten times higher than that of normal PDMS fibers, which can be attributed to the fact that a large amount of high-order mode light excited in the DOFs leaks and causes a higher optical loss. In the meantime, the loss spectra at the range of 1020–1170 nm are also measured by using a broadband light source (i.e., a supercontinuum laser in this case) and a spectrometer, as shown in Fig. 6(c). The results further confirm that the transmission loss of DOFs is much higher than that of the normal PDMS fibers. It is also worth noting that the transmission loss of the DOF increases with the diameter variation range, e.g., from 0.45 dB/cm to 2.02 dB/cm at 1500 nm for the DOFs with diameter variation ranges of 65–72 µm and 65–110 µm, respectively, as shown in Fig. 6(d).

 

Fig. 6. (a) Light attenuation of normal PDMS fibers at wavelengths of 473, 671, and 1500 nm as a function of fiber length. (b) Light attenuation of DOFs at 473, 671, and 1500 nm as a function of fiber length. (c) Loss spectra of different PDMS fibers with 5-cm length. (d) Light attenuation of DOFs with different variation ranges at 1500 nm as a function of fiber length.

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To assess the laser focusing performance of the DOF for laser scanning applications, we perform measurements of laser beam focusing and ultrashort laser beam propagation. The microlens endface of the DOF is significantly important for obtaining laser focusing ability, which is highly valuable for improving the spatial resolution when applying to microscopy. To this end, a blue laser at 473 nm is coupled into the DOF, which is immersed in rhodamine B solution for better visualization through fluorescence excitation, as shown in Fig. 7(a). As can be observed, the laser beam exiting from the DOF with microlens endface exhibits obvious laser focusing, while it is diverged for the case of the normal DOF without microlens endface.

 

Fig. 7. (a) Laser focusing measurement of the DOF. (b) Calculated dispersion curves for the maximum (blue line) and minimum (red line) diameters of the DOF. (c) The fiber core diameter evolution of the DOF versus the fiber length. Inset: microscopic image of the DOF. (d) Calculated dispersion evolution of the DOF versus the fiber length.

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So far, we have verified that the DOF can enable laser focusing, which is of great interest for high-sensitivity and high-resolution biophotonic applications. However, in the cases of using ultrashort laser pulses, e.g., particularly for multiphoton excitation, the dispersion property of the optical fiber is largely important for preserving the pulse duration and thus the peak power of the laser pulse. This is because the non-dispersion-managed optical fiber can broaden the transform-limited laser pulse and thus reduce its peak power, yielding a worse efficiency of nonlinear excitation. In general, researchers usually utilize optical material with weaker chromatic dispersion to reduce the temporal broadening of the laser pulse. In this work, we propose to reduce the temporal broadening by specially designing the structure of the PDMS fiber, i.e., the diameter varying along the longitudinal axis. It is known that the dispersion property of the optical fiber largely depends on its core size, and we have shown before that we can flexibly fabricate PDMS fibers with core diameters varying with their lengths. As the variation range and period of the PDMS fiber can be precisely controlled, the diameter-varying DOF can provide a dispersion curve that periodically changes above and below the zero-dispersion baseline, in which way the net dispersion is minimized and thus suppresses the temporal broadening effect. To show this ability, we calculate the dispersion curve corresponding to the maximum and minimum fiber core diameter of an DOF with the diameter variation range of 40–120 µm, as shown in Fig. 7(b). At the typical multiphoton excitation wavelength range, e.g., 1400–1650 nm in this case, the DOF exhibits both positive and negative dispersion as its diameter varies from 40 µm to 120 µm. Particularly, at the wavelength range of 1520–1580 nm, both the minimum and maximum diameters successively pass through the zero-dispersion baseline. This means that the laser pulse with an optical spectrum covering 1520–1580 nm can experience a minimized temporal broadening. Based on the calculation results, we then fabricate an DOF with a diameter variation range of 40–120 µm, as shown in Fig. 7(c). Accordingly, the dispersion of the DOF versus the fiber length is calculated and shown in Fig. 7(d), which shows that the dispersion of the DOF periodically changes above and below the zero-dispersion baseline, such that the positive and negative dispersion can compensate each other and thus reduce the temporal broadening effect.

3. Conclusion

In summary, we have demonstrated a flexible diameter-oscillating fiber by using PDMS elastomers that shows good mechanical properties, such as good flexibility (easily knotting) and stretchability (more than 200% straining). This PDMS DOF exhibits high biocompatibility, which is suitable for biological implantation. As fabricated by using PDMS that has good transmittance at the near-infrared wavelength range, the DOF has a good light-guiding property with an acceptable propagation loss, i.e., about 2.02 dB/cm. The DOF has an oscillating diameter along the longitudinal axis and a microlens at the endface. The variation range and period of the DOF’s diameter can be flexibly adjusted by controlling the spinning rate, spinning time, curing temperature, and PDMS precursor viscosity. The experimental tests show that the DOF with a microlens endface exhibits a very good ability of laser focusing. The diameter varying property of the DOF, in addition, can provide a dispersion curve that oscillates above and below the zero-dispersion baseline, such that the positive and negative dispersion can compensate each other and thus reduce the temporal broadening of near-infrared laser pulses. It is anticipated that this new biocompatible DOF with good laser focusing and dispersion management is promising for nonlinear biophotonic applications and creates new opportunities for multiphoton biological applications in deep tissues.