1. Introduction

As it is known, fiber Bragg gratings (FBGs) have been widely used in the areas of long-haul optical fiber communication, highly sensitive sensing, fiber laser and nonlinear optics since the end of last century [1]. Recently, due to the wavelength-scale feature, tight optical confinement and excellent compatibility with present optical fiber systems, optical microfibers have been attracting extensive interests in various areas [2], such as optical sensing, nonlinear optics and nanophotonics. Hence, it is of great interest to inscribe Bragg gratings in microfibers, i.e., microfiber Bragg gratings (MFBGs), which is promising in micro/nanophotonics. So far, there have been several approaches to fabricating MFBGs, including focused ion beam (FIB) milling [3–5], ultraviolet (UV) laser exposure [6, 7], modified visible/infrared light interference methods [8, 9] and femtosecond laser (Fs-laser) micromachining [10]. FIB technique is an effective method of fabricating geometrical MFBGs. Nevertheless, it cannot support real-time measuring of grating spectra. On the other hand, conventional 244/248nm UV-laser writing is not suitable for the fabrication of MFBGs since regular silica microfibers are low-photosensitive. Additional processes to improve the photonic sensitivity of microfibers may cause large losses or even breakage of microfibers. Inscription of Bragg gratings in microfibers drawn from single-mode fibers, without any additional process, has been demonstrated by Y. Ran et al. using far UV laser exposure [7]. The index change is due to the two-photon excitation mechanism. In this method, tens or even hundreds of seconds are needed to obtain MFBGs with high transmission-extinction ratios [11]. The Fs-laser writing technique based on multi-photon excitation mechanism can efficiently bring down the inscription time, and the index change induced by ultrashort pulses is more localized in fibers [12]. Besides, owing to the ultra-high peak power of Fs-laser, micromachining of various materials can be achieved without any additional process [12, 13]. X. Fang et al. reported fabrication of MFBGs using Fs-laser irradiation [10]. Nevertheless, geometrical damage in the whole microfiber cross-section was observed, resulting in high loss. By drilling shallow nanoholes in microfibers, the loss of gratings can be reduced and a strong RI modulation of guided modes can be achieved. Generally, the diameter of microfiber is several micrometers. Due to diffraction effect, it is quite a challenge to obtain nanoholes in microfibers using Fs-laser micromachining. Moreover, the aging properties of resulted gratings still need to be investigated.

In this paper, we report on the inscription of high-index-contrast MFBGs by drilling periodic nanoholes in microfibers using Fs-laser ablation technique. With diameter ranging from about 400 nm to about 550 nm, these nanoholes have depths up to about 800 nm and are benefited from the resolution of Fs-laser micromachining beyond diffraction limit. By using a uniform phase mask, MFBGs with transmission dips down to −23 dB and 3dB-bandwidths as much as 1.14 nm are obtained. The corresponding effective negative RI change is on the level of −10⁻³. Besides, the gratings with strong sidelobes in the long-wavelength range are well Gaussian apodized. Positive RI change of silica also occurs in the inscription process, which is not stable and decreases with time. After two weeks’ placement at room temperature, the reflection peak is blue-shifted and peak reflectivity is improved about 3 dB. This enhanced resonance can be attributed to the fact that the net negative RI change is strengthened. These geometrical gratings have high reliability in the following time and pave ways for high-sensitive optical sensors and micro/nanophotonics.

2. Fabrication of MFBGs

Utilizing fiber-tapering technique, microfibers with diameters down to approximate 3 μm and lengths of 75 mm were drawn from single-mode fibers (SMFs, YOFC-MKD-101). The insertion losses of microfibers are less than 1 dB at 1550nm wavelength.

To perform the inscription of Bragg gratings, an 800nm-wavelength Fs-laser system (Spectra-Physics) with the pulse duration of 120 fs and the repetition frequency of 1 kHz was utilized. The maximum single pulse energy is 3 mJ, which can be adjusted continuously by rotating a variable optical attenuator. The circular laser beam, which has a Gaussian profile with 1/e diameter, 2w₀, of 6 mm, was focused using a cylindrical lens through a uniform silica phase mask with a period Λ = 2202 nm onto the microfiber. Here, the cylindrical lens with a short focal length f = 40 mm was utilized and it is crucial to compress the beam width down to wavelength-scale in x-axis. The ± 1 orders contained 66% of the beam energy incident on the mask. The microfiber was fixed on two XYZ translation stages in the fabrication process and was placed nearly at the focal point. Hence, by slightly stretching the microfiber to resist air turbulence, high-efficient fabrication of MFBGs can be ensured.

Ultrafast IR laser with pulse energy of 400 μJ was used to write MFBGs. White light was observed during the fabrication process, indicating that Type II-IR Bragg grating had been formed [14]. The spot width along x-axis (2w₀’) at the focal point is as small as approximate 1.7 μm, calculated by w₀’ = λf/πw₀, where λ is the incident wavelength. Hence, the laser intensity as high as approximate 3.7 x 10¹⁴ W/cm² at the focal point can be generated, which is greatly above the threshold intensity for the ablation of silica fibers [14] and facilities the generation of MFBGs with nanostructures.

3. Experimental results and analysis

Figure 1(a) shows an optical microscope (AxioLab A1, ZEISS) image of the central part of a MFBG with the fiber diameter of 3.3 μm, demonstrating that periodic nanoholes are successfully carved in the microfiber by the ultra-fast pulses. As can be seen from the scanning electron microscope (SEM) image in Fig. 1(b), the width of nanoholes is approximate 550 nm, i.e., a quarter of the pitch of the phase mask. This is a feature of fiber Bragg gratings fabricated with the phase-mask approach. The nanoholes are not circular and have a height in the range of 520-600 nm. The formation of nanoholes can be attributed to the resolution of Fs-laser micromachining beyond diffraction limit [13]. The Fs-laser beam is highly spatially compressed along x-direction, of which the width is smaller than the microfiber diameter. Then the line light source is further divided into micro-beams due to the interference of Fs-laser after the phase mask. Owing to the threshold property of Fs-laser micromachining, periodic nanoholes can be obtained with these micro-beams [13]. This is schematically presented by Fig. 2 . Assume that the intensity of each micro beam has a Gaussian profile, shown in Fig. 2(a). I_th1 is the threshold laser intensity to remove material directly from the host. When laser intensity is below I_th1 but higher than I_th2, only the RI of material exposed will be changed. For silica, the RI change is positive and weakens with time. Since the intensity of the central part of laser beam is ultra-high, it results in plasma in local areas and roughly modifies the surface of microfiber. This leads to the variation in the size of nanoholes. As can be seen in Fig. 1(c), circular nanoholes with smooth surface and diameters down to 400 nm are obtained at the grating edges where the laser intensity drops. As a result, the hole quality can be conveniently controlled by simply adjusting the Fs-laser power. Moreover, the maximum depth of nanoholes is estimated from Fig. 1(d) to be about a quarter of the microfiber diameter, i.e., 800 nm, which implies that a deep RI modulation may be resulted in for the grating.

 

Fig. 1 Local optical (a, d) and scanning electron (b, c) microscope images of a MFBG. (a) and (b) The central part of the grating; (c) The grating edge. (d) Side view of the MFBG. The red arrow denotes the incident direction of Fs-laser.

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Fig. 2 Illustration of Fs-laser ablation. (a) The profile of Gaussian beam intensity in x-axis. (b) The exposure area of Fs-laser.

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Reflection spectrum of the MFBG obtained are depicted in Fig. 3(a) , recorded by an optical spectrum analyzer (AQ6370B, Yokogawa, Japan) with a resolution of 0.02 nm. The light source is an erbium-doped fiber amplifier (EDFA) with a flat amplified spontaneous emission (ASE) spectrum and it covers the wavelength range from 1530 nm to 1565 nm. As can be seen, a reflection peak over 15 dB centered at 1548nm wavelength is achieved. Moreover, this grating is well Gaussian-apodized and has strong sidelobes owing to nonuniform “dc” index change of the grating [15]. The apodization can be attributed to the Gaussian intensity profile of Fs-laser beam. The presence of strong sidelobes only in the long-wavelength range indicates that deep negative RI modulation of grating is obtained. Furthermore, Fig. 3(b) shows the transmission spectrum (the red curve) of MFBG with a dip of −23 dB. By Using the coupled-mode theory [16] and assuming that the grating with a Gaussian profile is sinusoidal, a theoretical transmission spectrum is obtained as the blue-dotted curve presented in Fig. 3(b). The DC and the AC components of the effective RI modulation of the grating are estimated to be −2.3 x 10⁻³ and −1.1 x 10⁻³, respectively. There is a mismatch in the bandgap between the experimental and calculated curves. This may stem from the polarization-dependent coupling of the forward- and backward-propagating optical waves, which is neglected in the simulations.

 

Fig. 3 Reflection (a) and transmission (b) spectra of the MFBG. The blue-dotted line show the calculated spectrum assuming effective sinusoidal modulation of effective index under Gaussian apodization.

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Then the aging property of MFBG was investigated. Figure 4(a) shows the reflection spectra of MFBG after the inscription (black curve) and two weeks’ placement (red curve) at room temperature. A prominent blue-shift up to 1.35 nm of Bragg wavelength is clearly illustrated. This indicates that the resonance can be simply enhanced for such kind of gratings. Figure 5(a) schematically shows the RI change along microfibers after exposure to the optical filed behind the phase mask. As a result of the ultra-high laser intensity, some material is removed, leaving periodic nanoholes in the microfiber. Apart from the center of Fs-laser beam, the intensity gradually decreases. Thus, the depth of nanoholes is reduced. What’s more, when the incident light passes through the microfiber, the intensity drops due to scattering, absorption of silica and even diffraction, which may still higher than I_th2 and causes RI change of silica. The Fs-laser induced RI change is positive [14]. The black area in Fig. 5(a) in the microfiber represents where the RI change occurs and the color depth denotes its strength. The variation of effective RI change along the microfiber axis is qualitatively shown in Fig. 5(b). The black and the red curves denote the variation after the fabrication of MFBG and two weeks’ placement at room temperature, respectively. The laser-induced RI change of silica is not stable and weakens when the grating is placed for a long time. Hence, the net negative RI modulation of the whole grating is reversely enhanced. The overall average index decreases, which makes the resonant peak shifted to short wavelengths. This is in accordance with the aging property of conventional Type II-IR FBG [14]. Additionally, the reflection extinction ratio is improved up to nearly 20 dB and the reflection spectrum is maintained in the following times. This indicates that the gratings own high stability for lab usage.

 

Fig. 4 (a) Reflection spectra of MFBG after inscription (black curve) and two weeks’ placement (red curve) at room temperature, respectively. (b) Reflection spectra under the injection of transvers-electric (TE, blue curve) and transverse-magnetic (TM, red curve) waves.

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Fig. 5 (a) Schematic diagram of the formation of MFBG. The red curve denotes the intensity of interference field after the phase mask. (b) Variation of effective RI change along the grating after the fabrication (black curve) and two weeks (red curve).

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Moreover, the polarization property of MFBG was studied. Figure 4(b) shows the reflection spectra of MFBG under the injection of two orthogonally-polarized optical fields: transverse-electric (TE, blue curve) wave with polarization along x-axis and transverse-magnetic (TM, red curve) wave in y-direction, respectively. Since the microfiber was ablated on only one side, the axis symmetry was broken. Hence, it can be inspected that the fundamental modes on orthogonal polarization states have different effective RI. This difference in effective RI results in the slight separation of Bragg wavelengths for two orthogonal polarizations. As can be seen, the peak reflectivity of the TE mode is about 2dB higher than that of the TM mode. Figure 6 shows the simulated fundamental modes of a 3.3-μm-diameter silica microfiber with a 0.8-μm-deep air slit using COMSOL Multiphysics 4.2a at 1550nm wavelength. The effective RI of both TE (Fig. 6(a)) and TM (Fig. 6(b)) modes is 1.3959 and 1.3965, respectively. Since the RI difference is quite small, the central resonant wavelength for both polarizations varies little. Although the mode distributions on both polarizations are nearly the same, the TE mode has more fields in the air-slit and is more strongly modulated by the nanoholes. The coupling coefficient of the forward- and backward- propagating fields can be described as [16]

 

Fig. 6 Fundamental modes of a 3.3-μm-diameter silica microfiber with an air-slit (wavelength @1550 nm). (a) TE mode; (b) TM mode.

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where Δε is the permittivity variation in the cross-section of microfiber, e_f and e_b are the amplitudes of normalized transverse forward- and backward- propagating fields with an angular frequency ω. According to Eq. (1), κ_TM is smaller than κ_TE, which accounts for the difference in the maximum reflectivity. Thus, these gratings may be favored in polarization-sensitive applications.

4. Discussion and conclusion

Nanoholes induced MFBGs were successfully fabricated with Fs-laser ablation method. To further optimize the fabrication of such type of MFBGs, the threshold for ablating microfibers is of importance. However, the width of Fs-laser beam at the focal point is smaller than the diameter of microfiber, i.e., about two micrometers. The 800nm lightwave may not be totally received by microfiber because the diffraction effect needs to be considered. Besides, the microfiber has a curved surface which causes scattering of the incident wave. As a result, it is difficult to precisely calculate the laser power passing through the microfiber. In addition, the alignment of microfiber is critical since it is necessary to place the microfiber at the focal point to generate nanoholes. Hence, it is still an issue to accurately measure the threshold properties of MFBGs in the fabrication process, which will be studied in our future work.

In summary, we demonstrate the fabrication of nanoholes induced MFBGs using femtosecond laser ablation technique. By tightly focusing the Fs-laser beam through the phase mask, periodic nanoholes are drilled along the microfiber axis only after seconds’ exposure to ultra-fast optical pulses. The effective negative RI change is over −10⁻³, resulting in high-contras-index MFBGs with transmission dips up to −23 dB and 3-dB bandwidths as much as 1.14 nm. Excellent Gaussian apodization for MFBGs is obtained and strong sidelobes arise in the long-wavelength range on the spectra. In addition to the maximum reflectivity improved about 3 dB, the resonant wavelengths endure an obvious blue-shift of 1.35 nm when the gratings are placed for two weeks at room temperature. The geometrical MFBGs show good stability in the following times and can find applications in optical sensing, nonlinear optical devices and atom photonics.