1. Introduction

Tunable optical fiber devices are attractive for many important operations in optical communication systems such as dynamic chromatic dispersion compensation, programmable adding and dropping of wavelength channels, dynamic gain equalization as well as for sensing applications [1–6]. These devices naturally incorporate the beneficial attributes of optical fiber technology such as immunity to electromagnetic interference, small size, high sensitivity and large bandwidth [7]. They also avoid many of the disadvantages such as difficult alignment, challenging reliability requirements and optical coupling typical of bulk optics, planar waveguides or conventional microelectromechanical systems operation [8].

On this line of argument, several designs have been explored in the last years such as the possibility to combine the potentialities of pumped microfluidrics and long period gratings to realize tunable broadband attenuators [9]. Also, a tuneable optical fiber that incorporates multiple microfluidic plugs into the interior of fiber microchannels has been investigated [10]. In this case, the propagation characteristics of certain optical modes of these fiber waveguides can be usefully manipulated by controlling the positions and optical properties of these plugs, using actuators and pumps formed on the fiber surface. This type of tunable microfluidic fiber can form the basis of wavelength and depth adjustable narrow-band filters. Recently, Domachuk et al [11] demonstrated a class of highly compact refractometers integrated onto a planar microfluidic geometry that exhibits high resolution refractive index measurements in 50 μm fluid channels utilizing a Fabry–Perot cavity formed between resonant fiber Bragg grating (FBG) reflectors. This cavity forms a resonant peak in the transmission spectrum which is dependent upon the refractive index of the fluid in the microfluidic channel.

On this line of argument, the authors focused their attention on photonics devices based on microstructured Fiber Bragg Gratings (FBGs) [12–13]. The final objective is to develop new in-fiber devices capable to provide the bases for multi-function integrated optoelectronic systems for sensing and communication applications, well suited for microfluidrics integration. The basic idea proposed in the recent years relies on the partial or total cladding layer stripping along a standard FBG to force a strong interaction between the core guided mode and external medium, not possible in standard devices. A first device was proposed involving uniformly thinned FBGs where surrounding refractive index (SRI) changes lead to spectral shifts of the grating spectral response [14]. Successively, more complex configurations involving microstructured FBGs (MSFBG) have been also proposed [12-13] as high performance technological platform to develop advanced active and passive photonic devices. MSFBGs in fact, rely on the generation of geometrical defects along the periodic structure of the grating consisting in selective stripping of the cladding layer in well-defined regions within the grating length. The defect, by breaking the structure periodicity, provides the formation of a defect state (allowed wavelength states) within the original grating stop-band. As matter of fact, the defect state can be accurately controlled by acting on the geometrical and physical features of the perturbation as well as the refractive index surrounding the stripped region [15].As expected, the proposed structure enables the tuning of the defect state wavelengths by easily acting on the SRI opening the possibility to develop new devices for sensing and communications applications. A wide theoretical investigation of the characteristics and potentialities of MSFBG and complex structures based on multi-defect and high refractive index coating MSFBG has been provided in previous works [15–16].

In all investigated case, fabrication steps involving wet chemical etching in hydrofluoric acid (HF) solution have been used enabling the rapid prototyping of high sensitivity in-fiber refractometers [13].

Even if this approach is very attractive in terms of cost and easy implementation allowing a fast device prototyping, it exhibits a low control capability of the geometrical features of the defect. This aspect can be considered the main limitation to realize advanced MSFBGs [16].

In order to overcome this limitation and allow an easy and reliable fabrication of MSFBGs based devices, here, we present the assessment and the optimization of a novel process allowing the accurate definition of the stripped region profile and dimensions. The novel method involves the use of suitable polymeric coatings uniformly deposited along the grating structure as protective layers against the chemical etching whereas UV laser micromachining tool has been employed for the selective removing of the polymeric masking enabling the selective cladding stripping.

2. Novel MSFBG fabrication approach

The novel method, here proposed, relies on the use of polymeric overlays uniformly deposited along the grating structure in combination with UV laser micromachining for coating patterning and successive selective wet chemical etching. On this line of argument, the fabrication stage can be organised as the sequence of three main steps:

The following subsections accurately present all the steps of the proposed procedure as well as details of the adopted tools.

2.1 Uniform overlay deposition

The first step of the proposed approach is based on the deposition of an uniform polymeric coating along the fiber grating in order to prevent the HF etching. To this aim, a commercial Fiber Recoater machine was adopted, allowing fast recoating of bare-glass fibers by UV curable polymeric materials. It uses a two-part mating quartz glass mold (see Fig. 1(a)), which, once closed, form a circular cross-section mold cavity around the exposed section of fiber. Two V-groove channels on both sides of the mold plates provide that the fibre will be automatically centered when it is inserted into the mold, whereas two external clamps keep the fiber in fixed strain state. A volumetric pump automatically injects a pre-programmed amount of recoat material into the mold cavity, which is then cured by UV exposure. The amount of recoat material is automatically computed taking into account the selected fiber length need to be coated and the coating thickness, where the maximum recoat length is limited to 50 mm. With regards to the coating thickness, it depends on the circular mold cavity size. Finally, efficient curing is assured since the adopted recoater has a UV sensor monitor which automatically compensates for lamp intensity by adjustment of the UV curing time. Differently, the curing time can be manually selected between 15 s and 60 s. Figure 1(b) plots a schematic diagram of the FBG coated by an uniform polymeric layer.

 

Fig. 1. (a) Quarz molds of the recoater machine; (b) Schematic diagram of the uniformly coated FBG.

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It is worth noting that to meet the needs of the proposed technique for MSFBG fabrication, the coating material needs to satisfy two important requirements:

To this aim several coating types in terms of material features and thickness have been investigated to fulfil both requirements. In particular Acrylate, DSM Desotech 950-200 and Polyamide coating with thickness of 20 μm (coated fiber diameter 165µm) and 62.5 µm (coated fiber diameter 250µm) have been investigated. Several experimental tests proved that all the adopted materials exhibit good micro-machining capability, while Acrylate and DSM Desotech 950–200 were not able to act as efficient HF masks. On the contrary, both requirements have been achieved by using polyamide coatings as protective layers.

2.2 UV laser micromachining

The UV laser micromachining system adopted in our experiments consists of an excimer laser (ArF, λ=193 nm) with pulse width of 5–6 ns. The system contains several fixed masks having different shapes (circles, squares) and sizes (250÷2500 μm).Besides, it includes a motorized rectangular aperture (MRA) mask, characterized by horizontal and vertical dimensions separately selectable in the range 0÷4000 μm, with a resolution of about 10 μm. Note that the MRA dimensioning is completely independent on the other masks, but its shape results projected upon that of the selected fixed mask. If necessary, this is useful to realize particular beam shapes. The spot size of the focused beam on the target is defined by the size and shape of the utilized masking aperture and by the demagnification of the focusing objective. The adopted laser system exhibits a demagnification coefficient of 10:1, as provided by the manufacturer. Nevertheless, a CCD(charged-coupled device) camera provides a real-time monitoring of the laser operation on the sample by a dedicated monitor. At the same time it provides a real-time control of the sample positioning at the focal distance from the objective which can be adjusted by proper micro-positioning system acting on the vertical position of the focal lens.

With regard to the sample positioning, the micromachining system is equipped with an additional X-Y micro-positioning system with resolution of 1 μm and maximum excursion of 10cm for each axis. In addition, the laser system has been properly equipped by a suitable rotating stage (see Fig. 2(a)) allowing the housing of the optical fiber and its rotation during the firing process. This feature is extremely important enabling the possibility to produce overlay patterning with azimuthal symmetry. To this aim, two motorized mandrels capable to host and rotate the fiber in both directions with selectable rotation angle and speed have been employed. However, the optical fiber arrangement inside the rotative stage represents a critical step. In fact, the proper positioning of the fiber avoids any fiber moving during the rotation enabling a precise ablation of the polymeric coating. To this aim, two identical weights (usually weights of 50 g.) were fixed at both fiber ends ensuring a constant strain state along the sample and thus to repeatable micromachining operations.

All the described units such as motorized steppers, rotating stage and vision system as well as the laser features are completely computer assisted by a dedicated personal computer.

 

Fig. 2. (a) Rotative stage of the UV laser micromachining system; (b) Schematic diagram a micro-structured polymeric coating.

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It is worth noting that in order to achieve a complete coated microstructure, the laser fluence needs to be properly selected. The used system allows controlling the laser fluence on the target by fixing the output laser energy between 0–18mJ and by acting on an external beam splitter leading to the possibility to provide the desired ablation features. Here,the laser fluence was selected taking into account several aspects. High laser fluence are desired to remove the polymeric coating very easily, enabling a complete microstructuring of the polyamide layer without time expensive operations. On the other side, the laser fluence should be significantly lower than the ablation threshold of silica fibers at the laser wavelength. However, the ablation thresholds of glass substrates are typically much higher than those of polymeric materials [17–18], leading to the possibility to meet both requirements. K. Awazu, in fact, demonstrated that thermally grown SiO₂ films on silicon exhibits ablation threshold of 1 J/cm² whereas it passes to 2.5 J/cm² for bulk SiO₂ adopting an ArF excimer laser operating at λ = 193nm with a pulse duration of 14 ns [19]. On the other side, Yip et al. demonstrated that significantly morphological effects on polyamide fiber were achieved by similar eximer laser (at 193 nm) with fluence of 50 J/cm² [20].

Figure 2(b) shows a schematic diagram of a complete micro-structured polymeric coating in the center of the grating region where L_POL indicates the length of the uncoated region.

2.3 Wet chemical etching

This section deals with the last step of the proposed fabrication procedure for MSFBG device. The objective of this step is to remove the cladding layer where the polymeric film has been removed by UV radiation. To this aim, simple and low cost process based on wet chemical etching in HF solution [12–14] was selected.

The polymeric coating has a double role of protection against external stress and as an efficient mask for the HF etching. To this aim, a V-groove channel (as shown in Fig. 3(a)) able to house the fiber grating leaving it free from any mechanical stress was used. Additional pipes allow inserting of the acid solution, water and test liquids inside the V channel. A schematic diagram of the structure is plotted in Fig. 3(b), where sharp profile has been considered without taking into account the isotropic nature of the wet chemical etching [15]. A critical issue that needs to be addressed during the holder design stage regards the capability to achieve azimuthally symmetric structure after the wet etching process. In the configuration proposed in Fig. 3(a), the coating of the lateral regions will ensure a spatial separation of the micro-machined region from the holder walls of the order of 20 μm (minimum coating thickness used in our experiments). This would ensure a correct azimuthally symmetric etching and thus avoiding birefringence effects and polarization induced dependence of the defect state.

 

Fig. 3. (a) Holder for etching of the grating with micro-structured polyamide coating; (b) Schematic diagram of the cladding layer stripping along the micro-machined region.

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3. Experimentals

To prove the potentiality of the proposed approach, here, experimental results on the first MSFBG prototype are presented. To this aim, we used a 6.0mm long FBG with resonant wavelength of 1559.10 nm, a FWHM of 0.8 nm and a maximum reflectivity of 96%, whereas its localization along the fiber was provided with a resolution of 1–2 mm.

The UOD step is aimed to cast a proper overlay coating along the grating structure. To this aim, a polyimide overlay of approximately 20 μm was preferred in order to facilitate the micromachining operation. First, the fiber was carefully washed by pure alcohol and arranged in the recoater machine. The polyamide coating was deposited along the 4.0 cm of the bare fiber with an insertion rate of 3.0 mm/s and UV cured for about 30 seconds. Figures 4(a) and 4(b) compare stereo-microscopy images of the bare and coated FBG, respectively.

 

Fig. 4. Stereo-microscopy images of (a) bare and (b) coated FBG.

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A MSFBG with stripped cladding length of L_POL=180 μm was selected as target with regards to the LM step. To this aim, the MRA was used to shape the laser beam to 1800×2500 μm², leading to a spot size of 180×250 μm² at the focal point.

As above mentioned the laser fluence on the target needs to be properly selected to completely remove the polymeric coating and at same time to avoid any silica fiber modifications. Several experiments proved that both requirements are satisfied when the laser energy was selected to be 5 mJ and the external splitter fixed to 40%.

Also the repetition rate needs to be properly selected, in our experiment it was set to 10 Hz combined with an angular speed of the rotative stage of 10°/second to enable a uniform thinning of the polymeric coating in the selected area.Figure 5 illustrates a stereo-microscopy image of the coated and micromachined FBG with L_POL = 180 μm +/- 5 μm.

 

Fig. 5. Grating with micro-machined polyamide coating with L_POL=180 μm

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Once the polymeric coating is patterned, the process is concluded with HF operation aimed to remove the cladding layer in the micro-machined region. Here a 24% HF solution was selected [12–14] combined with the V-groove holder presented in Fig.3(a). To monitor the effects of the fiber etching on the spectral response of the grating, a simple optoelectronic setup was used. It consists in a broadband superluminescent diode (2 mW) operating at 1550 nm with 40 nm FWHM (Full Width Half Maximum), a directional 3 dB coupler to collect the reflected spectrum from the grating and an optical spectrum analyzer (AQ6317C) with resolution of 10 pm. The optoelectronic setup was computer assisted by GPIB controller and a LabView plug-in enabling automated spectral measurements every 45 seconds.

Figure 6(a) plots the reflected spectra of the MSFBG during the etching process and compares them with the unperturbed response. For the first 167 minutes of the etching process, no effect on the spectral response was observed revealing a residual thickness in the etched area higher than 20 μm. As the etching process proceeds, the etched region begin to act as a distributed phase shift in the middle of the grating structure leading to the formation of a defect state inside the grating bandwidth, close to the right edge of the original FBG spectrum. As the phase delay related to the thinned region grows up, a shift of the defect state wavelength towards lower wavelengths occurs. It is worth noting that the defect state exhibits a non-zero reflectivity. This effect could be due to a not perfect centered etched region along the grating structure [15] leading to an unbalanced Fabry-Perot cavity. As evident in Fig. 6(a), the second effect due to the grating microstructuring relies on the increase of the reflected bandwidth to approx. 1 nm as expected from numerical analysis [15].

A clear analysis of the defect state wavelength shift during the etching process is provided in Fig. 6(b). Once the defect state is formed, it exhibits a blue shift accompanied with a slope increase as the etching process proceeds. In our experiment the defect state wavelength shift versus the etching time reaches the maximum value of 300 pm/minutes after 172 minutes. The acid solution was then removed and the holder was washed with pure water and successively filled with calcium oxide (CaO) to neutralize the acid solution. The basic solution quickly slows down the etching process, and the chemical process completely stops after 34 minutes. In addition, after 175 minutes the defect state moves out of the grating bandwidth (though the left edge of band) and a new defect state appears at the right edge. Observed results are also in good agreement with the theoretical analysis reported in [15] where the MSFBG spectral responses as function of the fiber diameter have been theoretically analysed. In fact, due to the relationship between the spectral position of the defect state and the phase delay related to the thinned region, a blue shift of the defect wavelength is expected as the fiber diameter decreases during the etching process.

Finally, Fig. 7 compares the reflected spectra of the unperturbed grating and the MSFBG realized with the proposed approach and air as surrounding medium. As evident, the micro-structured device exhibits a defect wavelength located at 1558.95 nm with a bandwidth of 160 pm.

 

Fig. 6. Etching process monitoring: (a) Spectral response of the grating during the etching process; (b) Defect state wavelength versus the etching time.

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Fig. 7. Comparison between unperturbed FBG and MSFBG spectrum in air.

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Once the MSFBG is realized, we investigated the obtained structure in terms of the geometrical profile in correspondence to the etching region to prove the validity of the proposed approach. Figure 8(a) reports a stereo-microscopy image of the etched region forming the MSFBG. First, the isotropic nature of the wet etching leads to undercutting effects and thus to tapered regions located at the edges of the uniformly thinned part. The tapered regions extend under the mask layer for a few hundreds of microns, however, they play a negligible role in the MSFBG operation. In fact, tapered regions with diameter less than 20 μm are able to contribute to the distributed phase shift [14]. Fortunately, in our case, they extend for 70-80 μm on both sides and thus are not able to perturb the spectral response due to their very limited length if compared to the grating length. To better clarify this aspect, both ideal and real profiles of the MSFBG are reported in Fig. 8(b) where the tapered regions have been estimated to be linear. Here, the slopes of the tapered regions are found to mach the real profile in the region with diameter less then 20 μm.

The uniform thinned region (diameter changes less than 1 μm) was found to be approximately L_Th=136 μm +/- 5 μm, and thus it is significantly shorter than the grating region without the polymeric coating (L_POL). This effect could be attributed to different etching regime at the mask edges leading to a strong dependence of the final profile on the ratio between the mask length and the etching depth [21].

Finally, by microscopic photography analysis the diameter of the uniform region was estimated to be about 6 μm, slightly lower than the core diameter.

It is worth noting that the mismatch between real and ideal behaviour can be easily taken into account at the design level and thus it is possible to realize the desired structure in terms of spectral characteristics for specific applications [15].

 

Fig. 8. (a) Optical photogram of the etched region forming the MSFBG: (b) Schema of real and ideal profiles.

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In conclusions, the results reported in this section demonstrate the effectiveness of the proposed technique as a simple and cost effective methodology for MSFBGs based devices. The resolution offered by the laser micro-machining technique allows the realization of the desired MSFBG and thus extend the functionalities of the proposed structure [16]. In fact, previous fabrication approach based on a masking procedure involving Teflon support and epoxy resin [12–13] was able to provide thinned region length higher than 500μm with a resolution of about 100μm. Also, the final structure was characterized by a thinned fiber region with lateral regions coated with thick resin resulting in a difficult handling and less robust devices.

Unfortunately, in the above results, a critical point to be improved is the correct identification of the grating location. Nevertheless, this problem is not intrinsically related to the fabrication process and it can be easily eliminated by careful identification of the grating during the write operation.

3.1 SRI based tuning

The main advantage of MSFBGs relies on the possibility to tune the defect state wavelength within the reflected bandwidth of the grating by acting on the SRI [12–13, 15]. This aspect, widely theoretically investigated and experimentally demonstrated in previous works, gives to MSFBG an unique characteristics if compared to the standard phase shift gratings (PSGs) [22–23]. It allows adopting MSFBG devices to realize high performance transducer for chemical sensing or narrowband selective and tunable filter for telecommunications applications.

In this section the defect state sensitivity to SRI is tested with regards to the device fabricated with the proposed approach. To this aim, aqueous glycerin solutions at different concentrations were efficiently used as test liquids to simulate SRI varying in the range 1.33-1.45 whereas reflected spectra measurements have been carried out by the same optoelectronic setup above presented.

 

Fig. 9. Dependence of MSFBG on the SRI: (a) spectral response for several SRI; (b) Defect state wavelength shift versus SRI.

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Figure 9(a) compares the realized MSFBG spectral response to several SRI where the dependence of the defect wavelength on the SRI is clearly evident. As observable, the defect state shifts towards higher wavelengths of 316 pm as the SRI changes from 1.33 to 1.404 and of 179 pm from 1.40 to 1.423. According to the effective refractive behavior [14] and previous works [12–13], the defect state shift exhibits an increase in the sensitivity as SRI approaches the silica refractive index. The defect state behavior is clearly reported in Fig. 9(b). As observed, a shift of 680 pm (from 1558.97 nm to 1559.65 nm) as the SRI changes from 1.33 to 1.44 was measured.

Finally, Fig. 10 shows the behavior of the tuning sensitivity, defined as the derivative of the defect state wavelength shift with respect the SRI and expressed in terms of nanometers per refractive index unit (RIU). Based on the obtained results, the tuning sensitivity passes from 2.5 nm/RIU around the water refractive index (SRI=1.33) to 14.6 nm/RIU for SRI=1.44.

 

Fig. 10. Sensitivity of the defect state central wavelength versus SRI.

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4. Conclusion

In conclusions, this paper presents a novel approach for a cost effective and accurate MSFBGs fabrication. The proposed method involves UV laser micromachining of polymeric overlay and wet chemical etching for cladding removal. The experimental results demonstrated the effectiveness of the UV micromachining tool as a valuable technology for polymeric coating patterning with azimuthal symmetry and fine control of the structure geometry. Also, a polyimide coating is demonstrated to act as an excellent mask during HF etching operation. This technological assessment would enable the possibility to extend the MSFBG functionalities especially if integrated with microfluidic components and to prototype new advantageous and multifunctional photonic devices.