Femtosecond laser micromachining of Fabry-Perot cavity in fibre Bragg grating Rodrigo Fiorin*a,b, Fernando N. Cidadea, Jociel L.S. Adachia, Lucieli Rossia, Valmir de Oliveiraa, Ilda Abea, Hypolito J. Kalinowskia a Federal University of Technology - Paraná (UTFPR), 80230-901, Curitiba, Brazil; b Faculdades Integradas do Brasil - UniBrasil, 82821-020, Curitiba, Brazil. ABSTRACT A 10 m (length) 75 m (depth) open channel is fabricated in fibre Bragg gratings (FBG) by femtosecond laser micromachining. The FBG Fabry-Perot (FP) cavity formed by this complex structure has a length of 4 mm; value estimated from interference spectrum for the air open channel. Reflection spectra of FBG FP cavity as a function of the temperature shows the cavity phase change. The sensor is thermally characterized by filling polymer in the channel and subsequent UV curing; the results show a period shift of approximately 12 x10-3, value obtained of interferometer pattern for 30C temperature range. Keywords: femtosecond laser, micromaching, Fabry-Perot cavity, fibre Bragg gratings
1. INTRODUCTION The femtosecond pulsed laser technology has been largely used to the fabrication of photonic microstructures1-4. Recent works show that the femtosecond laser micromachining can be used to fabricate cavity in optical fibre2-5. Compared with other fabrication methods, the femtosecond laser technology has the advantage of high resolution in the construction of structures with reduced dimensions and flexibility4,5. Cavities based on Bragg grating has been widely reported in the literature due to high sensitivity to temperature and strain5,6. In this paper are presented the results of production of an open channel in fibre Bragg gratings by using femtosecond laser micromachining. The sensor FBG FP cavity formed by this complex structure is demonstrated; the temperature characterization and the channel filled with different refractive index materials are presented.
2. METHDOLOGY FBGs are written into standard single-mode optical fibre (G-652) non-hydrogenated by using a phase mask interferometer illuminated by UV light. The grating recording employs an Excimer laser at 248 nm (KrF, Coherent Xantos XS 500) with pulse energy of 6 mJ per pulse and repetition rate of 250 Hz and phase masks (Ibsen) with pitch of 1064.6 nm and 1067.4 nm. For the experiments, two FBGs are written in a standard single-mode optical fibre, namely, the sensor grating and the reference grating. The length of the sensor grating is approximately 10 mm and it is obtained by sweeping technique. The principle consists of focusing the UV beam (3 mm of diameter) via a cylindrical lens on the core of the fibre positioned behind a phase mask. UV laser beam is swept along the phase mask by a mirror positioned on a motorized translation stage. The velocity of the UV laser beam sweep is 0.03 mm/s. The velocity of the UV laser beam sweep and the laser pulse energy can be modified to obtain specific gratings. The estimated length of the reference grating is approximately 3 mm derived from the laser beam diameter. Total exposition time is in the range from 1 to 5 minutes, depending on the grating length. FP cavities are fabricated on a single-mode fibre by femtosecond laser pulses generated by an amplified Ti:Sapphire system with central wavelength of 800 nm, and repetition rate of 1 kHz. The laser pulse width is of 100 fs and the used energy is 100 nJ per pulse. The laser beam is focused in the sample by a 20X microscope objective with NA of 0.25. The sensor grating is fixed in a glass slide and it is moved using a 3-axis stage flexure platform with submicron resolution. *
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The used nano positioner is a Thorlabs Nanomax 600 series, with six programmable actuators; 3 for linear displacement positioners along each orthogonal axis and 3 other for yaw, pitch and roll angular movements. It presents a maximum resolution of 20nm / 0,1 arcsec. The step motor actuators are models DVR001, which have maximum linear speed of 0,5 mm/s and 4 mm range. FP cavity is engraved on the center of sensor grating. The engraving technique is based in a destructive fabrication, the femtosecond laser ablation effect is directly used to open channel (trench) written in a nonstop movement. In the focal plane the transversal section has an elliptical shape 7, with its minor axis in the horizontal plane perpendicular to the movement direction. The structure of a FBG FP cavity consists of two identical FBGs (sensor grating) separated by a short cavity (FP cavity). Figure 1(a) shows the structural schematic of the FBG FP cavity and the reference sensor. The scanning electron microscope (SEM) image of FP cavity is showed in figure 1(b). The image obtained from the side view of the fibre shows the elliptical shape of the laser focus transversal section. The cavity length is about 10 m as estimated from the SEM and the depth of FP cavity is around 75 m, indeed passing the fibre core. Amplified spontaneous emission as broadband light source, optical circulator and optical spectrum analyzer with resolution of 50 pm are used to monitor the reflection spectrum of the grating during the fabrication of cavity and the characterization process. open channel
(a)
(b)
reference grating
sensor grating
Figure 1. FBG FB cavity: (a) the structural schematic of sensor and (b) SEM image.
3. EXPERIMENTAL RESULTS The optical reflection spectrum from the reference grating is monitorized in the process of cavity fabrication. When the laser focus reaches the fibre core, the reflection spectrum of the grating disappears. Figure 2 shows the initial two peaks reflection spectrum corresponding to the sensor grating and the reference grating. After exposition of the fibre to femtosecond laser for forming the cavity, the reflection spectrum shows only one peak, corresponding to the partial reflection of the sensor grating. The temperature dependence of the reflection band of the FBG FP cavity is characterized using a cooling/heating system. This unit is assembled with thermoelectric cooler, controlled by a dedicated electronic circuit. The reference grating is positioned outside of the temperature variation system. Figure 3(a) shows the optical reflection spectra from sensor grating as a function of temperature variation. It can be seen that the reference grating reappears when the FBG FP cavity is subject to temperature variation. The changes in the spectrum of reference grating can be related to the modification of the cavity length; when the cavity is heating there is a growing of the reflected peak power from the reference grating. The interference function of the cavity is determined by the phase difference between the light reflected by the two gratings and the air cavity. Thus, the reflection spectrum of FBG FP cavity consists of the reflection spectrum of sensor grating modulated by the cavity phase change. When a temperature is applied to FBG FP cavity structure, it leads to a variation in both the sensor grating and the interference function, whereas each one will present different sensibilities; it can be seen in Figure 3(b). The peak power of the sensor grating varies linearly with temperature. The calibration line for the temperature variation is determined and the corresponding slope (sensitivity of the Bragg wavelength to temperature changes) is almost the same determined for ordinary gratings. The value obtained with best-fit to experimental points is 8.32 pm/C. The interference function of cavity can be determinate by the phase difference between the light reflected formed by the two gratings and the open channel. Two adjacent interference minimums (1 and 2) have a phase difference of 2. The optical length of the cavity (L) can be calculated by:
L
1 12 2n 2 1
(1)
where n is the effective refractive index of the cavity. The period of interference function of cavity, for the air open channel length of 10 m (estimated from SEM image), calculated by the Equation 1, is around 100 nm (n=1.0003), almost light source bandwidth. This interference pattern cannot be observed. However, another interference function can be seen throughout the spectrum. This pattern has a short period that corresponding to optical length of approximately 3 mm (n=1.4560, 1 = 1548.49 nm and 2 = 1548.81 nm). This cavity can be related to complex structure that consists of two reflectivity gratings formed by the air open channel. Due to the fact that the micrometer channel is fabricated approximately in a middle of original grating (10 mm length), the cavity structure length can be associated to the center of two gratings formed. Other studies are being performed by the authors in order to verify this result.
REFLECTED POWER(dBm)
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-52 1540
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1546
WAVELENGTH (nm)
-44 -46
-44 -46 -48 0 1 2 3
2
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E
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PE RA TU R
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14 20 1544
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1550
23
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TE M
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REFLECTED POWER (dBm)
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ERA TUR E
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REFLECTED POWER (dBm)
Figure 2. The reflection spectra from the sensor grating structure before (circle points) and after (square points) forming the cavity.
8 1543.5
1544.0
1544.5
1545.0
WAVELENGTH (nm)
Figure 3. Evolution of the reflection spectrum of FBG FP cavity as a function of the temperature. The reference grating is not subject to temperature variations. (b) Detail of the spectrum of sensor grating modulated by cavity phase change.
Figure 4(a) shows the interference spectrum of the FBG FP cavity obtained with the open channel filled with air and isopropyl alcohol (n=1.3747 to 25°C). No variation in the period of cavity interference function has been noted; however it is possible to see the reduction in the interference pattern visibility. The interference pattern visibility is obtained when the open channel is filled with air is 2.8 dBm and with isopropyl alcohol is 1.5 dBm. The intensity dropped when the channel is filled with liquids with major reflective index that can be related to the reduction of cavity length. When the liquid reflective index inside of channel has approximately the reflective index of fibre core, the interference pattern will disappear. The 10 m filled channel will not be noticed because the grating has the period of about 1 m and the grating length will be 10 mm again (back to the original grating). Figure 4(b) shows the interference spectra of the FBG FP cavity and air open channel (air cavity) for different temperatures. It is possible to notice the variation in the period of cavity interference function and the phase shift of the pattern. The first column of Table 1 shows the values of cavity interference period for different temperatures for air. The values are obtained with fitting a sine non linear curve to the
experimental points. For a temperature range of 30C, the period shift is of approximately 3x10-3. In order to verify the shift in the interference period, the temperature experience has been realized for open channel filled with a clear polymer (NOA68). The channel is filled with the liquid photopolymer and then it is cured when exposed to ultraviolet light. The refractive index of this polymer provided by the manufacturer is 1.54. Table 1 shows in the second column the values obtained for cavity interference period for polymer channel (polymer cavity). The same temperature range shows a period shift of approximately 12 x10-3. (a)
-53
(b)
-53 Adj. R-Square
REFLECTED POWER (dBm)
REFLECTED POWER (dBm)
Equation
-54
AR AR AR AR
-55
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-58
Air Isopropyl alcohol 1548.0
1548.5
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-56 T( C) 50 40 30 20
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WAVELEGTH (nm)
1548.5
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Figure 4. Interference spectrum of FBG FP cavity: (a) obtained for the air open channel and for channel filled with isopropyl alcohol; (b) obtained when the temperature is change (air open channel). Table 1. Temperature dependence of interference period obtained for air cavity and polymer cavity. Air cavity Temperature (C) 20 30 40 50
Period 0.3166 0.3156 0.3146 0.3136
Polymer cavity Temperature (C) 50 60 70 80
Period 0.2716 0.2690 0.2664 0.2594
4. CONCLUSION We demonstrated a Fabry-Perot interferometer cavity fibre sensor based in a structure formed by fibre Bragg gratings and open channel. The open channel is fabricated by femtosecond laser micromachining. The operation of sensor and preliminary experimental results are reported.
ACKNOWLEDGMENTS The authors acknowledge financial support and scholarships received from the Brazilian agencies CNPq, CAPES, FINEP and Fundação Araucária.
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