Femtosecond Laser Micromachining
Open Notch FFP Sensor Fabricated by Femtosecond Laser
The advancement in fs-laser technology has opened a new window of opportunity for precise fabrication of microdevices with true three-dimensional (3D) configurations. Fs lasers could emit ultraintense laser pulses, offering the ability to ablate most solid materials with high surface quality, and/or to change their refractive index without special pretreating and doping. Fs laser pulses with extremely high peak power produce almost no thermal damage as the pulse duration is shorter than the thermalization time . Due to the multiphoton nature of the interaction, the ablation process can be conducted on the material surface as well as within its bulk. Fs lasers have been successfully used for directly writing optical waveguides [7,8] and micromachining microchannels and microchambers in glasses .
When a fs laser is focused onto optical fiber, the energy absorption takes place through nonlinear phenomena such as multiphoton absorption (a number of photons could be absorbed simultaneously, when material is irradiated by high-power laser light beam), tunneling, and avalanche ionization. As long as the absorbed energy is high enough, catastrophic material damage occurs, which leads to the formation of special structures . Intensive index modulation of up to 0.001 could be induced in fiber by fs laser. If the absorbed energy increases further, a microstructure in optical fiber can be formed with designed shape and size.
Fiber-optic micro extrinsic FP interferometers (MEFPIs) could be fabricated by using fs laser to mill the fiber. The schematic diagram of fs-laser micromachining system is shown in Figure 3.1. A home- built chirped-pulse amplified Ti:sapphire laser system with high pulse energy of up to 100 pJ for 100 fs pulses with a repetition rate of 1-5 kHz was used. After being reflected from a dichroic mirror, the light was
Figure 3.1 Schematic diagram of the fs-laser system. (From Rao, Y. J. et al. 2007. Optics Express, 15(21), 14123-14128.) focused onto the silica fiber by an objective lens (50x magnification, 0.65 numerical aperture [NA]). A light-emitting diode (LED) was used to illuminate the sample so that the MFPI sensor after ablation can be monitored in real time by using a charge-coupled device (CCD) camera attached to a phase-contrast optical microscope. A computer- controlled three-axis translation stage (100 nm resolution at X direction, 125 nm at Y direction, and 7 nm at Z direction; PI, German) was employed to carry out the required movements of the silica fiber.
The laser wavelength was 800 nm with a pulse width of 120 fs and a repetition rate of 1 kHz. The focused spot size was 5 |!m and the pulse energy was 20 |lJ. The SMF (Corning: SMF-28) and the PCF (Crystal Fiber: ESM-12-01) were mounted, respectively, onto the translation stage and moved at a speed of ~300 ^m/s. A single-pass exposure over an area of 80 |!m X 30 ^m was carried out and such a process was repeated by several times until the ablated FP cavity formed to meet the requirements .
Figure 3.2 displays the optical micrograph of an MFPI with an 80-^.m cavity length based on SMF, and the related reflective spectrum is shown in Figure 3.3. It can be seen from Figure 3.3 that the interferometric fringes are good enough for sensing applications, but the fringe visibility is relative low. The poor fringe visibility was mainly caused by three reasons: (1) the light scattering loss at the laser-ablated surface; (2) the non-perpendicular surface orientation with respect to the fiber axis; (3) the coupling loss as a result of recoupling the light reflected from the second end face of the FP cavity back into the fiber core [ ].
Figure 3.2 Optical micrograph of an MFPI with an 80-цт cavity length on the SMF. (From Rao, Y. J. et al. 2007. Optics Express, 15(21), 14123-14128.)
Figure 3.3 Reflective spectrum of an MFPI with an 80-цт cavity length on the SMF. (From Rao, Y. J. et al. 2007. Optics Express, 15(21), 14123-14128.)
Figure 3.4 shows the optical micrograph of an MFPI sensor with a 75-p.m cavity length based on the PCF. It can be seen that the two sides of such a PCF-based MFPI are neat and parallel. Comparing with the SMF-based MFPI, the fringe visibility of the PCF-based MFPI was improved by a few dB as shown in Figure 3.5; this is mainly due to the following reasons: (i) the PCF is entirely made from pure fused silica, which reduces the sputtered remains as well as light scattering ; (ii) the cladding of the PCF is a 2D photonic crystal structure with air holes distributed along the length of the fiber, diffusing the laser-ablation-generated heat and pressure quickly, and hence can effectively reduce the thermal damage on the cross section of the PCF ablated.
Figure 3.4 Optical micrograph of an MFPI with a 75-^m cavity length based on the PCF. (From Rao, Y. J. et al. 2007. Optics Express, 15(21), 14123-14128.)
Figure 3.5 Reflective spectrum of the PCF MFPI. (From Rao, Y. J. et al. 2007. Optics Express, 15(21), 14123-14128.)
For achieving better fringe visibility, it is believed that the surface roughness can be reduced by reducing the laser scanning steps, of course, at the expense of a long device fabrication time. The nonperpendicular surface orientation can also be minimized by careful adjustment of the stages. The coupling loss increases with the length of the FP cavity. As a result, it may eventually limit the practical length of the cavity. As shown in Figure 3.6, when the cavity length is much shorter (~30 p.m), fringe visibility is obviously improved to ~14 dB .
The strain responses of the MFPIs based on the SMF in Figure 3.2 and PCF in Figure 3.4 are studied, respectively. The MFPI was fixed on a translation stage with a resolution of 1 |Im. The output of the
Figure 3.6 (a) SEM images of fiber in-line MFPI device with a ~30-|am cavity length fabricated by fs-laser ablation. (b) Interference spectrum of the EFPI device. (From Wei, T. et al. 2008. Optics Letters, 33(6), 536-538.) tunable laser in the optical spectrum analyzer (OS A) was coupled into the MFPI through a 2 X 2 coupler, and the reflected light was returned via the same coupler to the receiver of the OSA. The tunable laser light is scanned and coupled into the optical device under test. At the same time, a small portion of the laser light is divided by a coupler, passed by a wavelength reference, and finally detected by a photon detector for calibration of the certain wavelength of the tunable laser. When comparing the detected signal from sensor with the reference light, the sensor spectrum could be obtained. The wavelength shift of the SMF-based MFPI sensor and that of the PCF-based MFPI sensor were measured at 1550-nm wavelength band, and their relationships between the wavelength shift and strain variation are shown in Figure 3.7a and b, respectively. Experimental results show that the wavelength-strain sensitivity of the SMF-MFPI sensor and the PCF-MFPI sensor are 0.006 and 0.0045 nm/|lm, respectively.
Furthermore, such an open EMFPI could be used to measure refractive index. To evaluate its capability for refractive index measurement, the fiber FPI device was tested using various liquids including methanol, acetone, and isopropanol at room temperature. The interference spectra of the device immersed in various liquids are also shown in Figure 3.8 for comparison. The signal intensity dropped when the device was immersed in liquids as a result of the reduced refractive index contrast and thus lower Fresnel reflections from the cavity end faces. However, the interference fringes maintained a similar visibility. The spectral distance between the two adjacent valleys also decreased, indicating the increase in refractive index of the medium inside the cavity .
Figure 3.7 (a) Wavelength-strain relationship of the SMF-MFPI sensor. (b) Wavelength- strain relationship of the PCF-MFPI sensor. (From Rao, Y. J. et al. 2007. Optics Express, 15(21), 14123-14128.)
Figure 3.8 Interference spectra of the FPI device in air, methanol, acetone, and isopropanol. (From Wei, T. et al. 2008. Optics Express, 16(8), 5764-5769.)