Micro Fabry-Perot (F-P) interferometers (MFPIs) are machined in a single-mode fiber (SMF) and a photonic crystal fiber (PCF) by using a near-infrared femtosecond laser, respectively. The strain and temperature characteristics of the two MFPIs with an identical cavity length are investigated and the experimental results show that the strain sensitivity of the PCF-based MFPI is smaller than that of the SMF-based MFPI due to their different waveguide structures, while the two MFPIs have close temperature sensitivities which are much smaller than that of an in-line SMF etalon sensor reported previously. These MFPIs in silica fibers are compact, stable, inexpensive, capable for mass-production and easy fabrication, offering great potentials for wide sensing applications.
©2007 Optical Society of America
Fiber-optic Fabry-Perot interferometers (FPIs) have been successfully used as optical sensors in health monitoring of composite materials, civil engineering structures, space aircrafts, and medicine, etc, due to their distinct advantages over conventional fiber-optic sensors, such as small size, relatively low temperature cross-sensitivity, immunity to electromagnetic interference, high resolution, low cost, etc, becoming one of the best candidates towards realizing so-called smart materials and structures [1–3]. Most of the sensors based on the FPI are constructed by inserting or splicing two silica fibers into a glass capillary [4–5] or to a hollow-core fiber . The major problems remained at present are: (i) Good reproducibility is difficult to achieve due to the manual operation of the whole manufacturing process and the calibration of each individual sensor is required after fabrication; (ii) The cleaved fiber ends are easily contaminated by air-borne dust and may be damaged by possible touch to the capillary or the hollow-core fiber; (iii) The repeatability of the F-P cavity length pre-set is strongly affected by the process of mounting the two fibers into the capillary or the cleaved length of the hollow-core fiber.
In this paper, we report the first demonstration to directly machine a micro FPI (MFPI) on a single-mode optical fiber (SMF) and a photonic crystal fiber (PCF) by using a near-infrared femtosecond (fs) laser. This MFPI has a micro-rectangular notch structure located in the optical fiber with a typical size of tens of micrometers. It is obvious that such a direct micromachining process with fs laser can overcome the drawbacks of the manual operation above-mentioned, due to the elimination of any manual assembly.
2. Fabrication of the MFPIs
The use of fs lasers for direct ablation of micro-structures on silica materials has been reported previously [7–12]. When fs laser pulses are focused into the silica material, a micro explosion or Coulomb explosion occurs on the focusing point by a nonlinear ionization process and makes a micro-structure shaped. Compared with long pulse lasers, when using fs laser pulses for material processing, the energy is deposited in an efficient, fast and localized way. Therefore, the thermal and mechanical damage of the material surrounding is minimal, and the deformation and ablation thresholds can be well-defined. The schematic diagram of the fs laser system for the fabrication of MFPIs is shown in Fig. 1. A home-built chirped pulse amplified Ti:sapphire laser system with high pulse energy of up to 100μJ for 100fs pulses with a repetition rate of 1–5 kHz was used. After being reflected from a dichroic mirror, the light was focused onto the silica fiber by an objective lens (50× magnification, NA=0.65). 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 (100nm resolution at X direction, 125nm at Y direction, and 7nm at Z direction, PI, German) was employed to carry out the required movements of the silica fiber.
In the experiment, the laser wavelength was 800nm with a pulse width of 120fs and a repetition rate of 1kHz. The focused spot size was ~5μm and the pulse energy was ~20μJ. The SMF (Corning: SMF-28) and the PCF (Crystal Fiber: ESM-12-01) were mounted on the translation stage and moved with a speed of ~300μm/sec, respectively. A single-pass exposure over an area of 80μm×30μm was carried out and such a process was repeated by several times until the ablated F-P cavity formed to meet the design requirements.
Figure 2 displays the optical micrograph of a MFPI with a 80μm cavity length based on the SMF, while the reflective spectrum of the MFPI is shown in Fig. 3, which was measured by using a high accuracy optical spectrum analyzer (OSA) (Si720, Micron Optics, USA) with a wavelength resolution of 0.25pm and a wavelength precision of 1pm over a spectral range of 1520–1570nm. It can be seen from Fig. 3 that the interferometric fringes are good enough for sensing applications, but the fringe visibility is relative low due to the rugged surfaces of the F-P cavity caused by some sputtered remains adhering to the surfaces machined, resulting in relatively strong light scattering inside the F-P cavity. We are currently working on the improvement in the surface quality of the cavity by optimizing the laser ablation parameters and using the etching technique.
Figure 4 shows the optical micrograph of a MFPI sensor with 75μm cavity length based on the PCF. It can be seen that the two sides of such a PCF-based MFPI are neat and parallel, resulting in relatively high-quality fringes as shown in Fig. 5. Comparing with the SMF-based MFPI, the fringe visibility of the PCF-based MFPI was improved by a few dB, this is mainly due to the following reasons: (i) The PCF is entirely made from undoped fused silica, this may reduce the sputtered remains and hence decrease the light scattering ; (ii) The cladding of the PCF is a 2-D photonic crystal structure with air holes distributed along the length of the fiber, making the heat and pressure, generated by laser ablation, to diffuse quickly and hence can effectively reduce the thermal damage on the cross-section of the PCF ablated.
3. Characteristics of the MFPIs
3.1 Strain responses of the MFPIs
The strain responses of the MFPIs based on the SMF and PCF are studied, respectively. The MFPI was fixed on a translation stage with a resolution of 1μm. The output of the tunable laser in the OSA was coupled into the MFPI through a 2×2 coupler and the reflected light was returned back via the same coupler to the receiver of the OSA. As the cavity length of the MFPI varies with the applied strain and the interferometric fringes shift accordingly, the shift of a certain wavelength can be used to measure the strain change . According to the interference theory, the value of the round-trip propagation phase shift in the F-P cavity can be expressed as:
where n is refractive index of the cavity, L is the length of the cavity and λ is the certain free space optical wavelength.
When the strain is applied to the MFPI, Φ is modified as:
where ΔL is the cavity length change, Δλ is the optical wavelength shift. As the number of the fringes remains unchanged in the experiment due to ΔΦ<<π, i.e. Φ = Φ', the strain ε can be given by:
Eq. (3) indicates that the strain varies linearly with the wavelength shift.
The schematic diagram of the MFPI sensor is shown in Fig. 6. When a longitudinal stress is applied to the F-P cavity, the distribution of the longitudinal stress becomes not uniform on the cross section of the fiber. The largest longitudinal stress occurs at the root of the cavity, namely the position of a. The longitudinal stress decreases with the increment of the distance from a, making the cavity lean backwards with an angle of θ. In fact, the cavity can be separated into several regions with different longitudinal stress. According to the definition of Poisson’s coefficient, we know that the transverse strain of each region is also different. Therefore, such a micro F-P cavity would experience a 2-D strain distribution under a longitudinal strain.
As the holes of PCF are arranged in a hexagonal honeycomb pattern across the cross section of the fiber, such a special structure provides the MFPI sensor with a great potential to respond the external force from different directions in a decentralized and homogeneous way [13–15]. Thus, when comparing with the SMF-MFPI sensor, the mechanical strength of the PCF-MFPI sensor can be improved considerably, hence, the PCF-MFPI sensor should have smaller θ, i.e. smaller strain sensitivity, than the SMF-based MFPI sensor under the same longitudinal strain condition.
The wavelength shift of the SMF-based MFPI sensor and the PCF-based MFPI sensor were measured at the 1550nm region and their relationships between the wavelength shift and the strain change are shown in Fig. 7 and Fig. 8, respectively. Experimental results show that the wavelength-strain sensitivity of the SMF-MFPI sensor and the PCF-MFPI sensor are 0.006nm/με and 0.0045nm/με, respectively. Hence, the SMF-MFPI sensor is more sensitive to strain than the PCF-MFPI sensor, which is in agreement with the theoretical analysis mentioned above. In addition, it can be obtained that the phase-strain sensitivity of the SMF-MFPI sensor is ~2.51×10-3rad/με, which is about five times larger than that of an in-line SMF etalon sensor of 0.49×10-3rad/με reported previously , this is mainly due to the smaller cross-section area of the MFPI sensor stressed, when compared with that of the in-line SMF etalon sensor, leading to a relative larger cavity length change under the same strain.
3.2 Temperature responses of the MFPI sensors
The temperature characteristics of the MFPI sensors are also studied. In our experiment, the sensors were placed in a furnace and the temperature was increased with a step of 10°C from the room temperature to 100°C. The temperature responses of the SMF-MFPI sensor and the PCF-MFPI sensor are shown in Fig. 9 and Fig. 10, respectively. It can be seen that for a temperature range from 20°C to 100°C, the wavelengths of both the sensors shift towards the short-wavelength direction linearly. Experimental results show that the wavelength-temperature sensitivity of the SMF-MFPI sensor is -0.0021nm/°C which is close to that of the PCF-MFPI sensor, i.e. -0.002nm/°C, this is due to very similar thermal expansion coefficients between the fused silica of the SMF and the pure silica of the PCF. Correspondingly, the phase-temperature sensitivity of the SMF-MFPI sensor is -0.87×10-3rad/°C. Hence, the temperature sensitivity of the SMF-MFPI sensor is about 11 times smaller than that of the inline SMF etalon sensor of ~0.01rad/°C , and it is negative, this is mainly because the two endfaces of the MEFPI cavity would expand towards the cavity centre with the increment of temperature, leading to such a decrease in cavity length.
In this paper, novel MFPI cavities with sizes of 80μm×30μm and 75μm×30μm are directly machined on a conventional single-mode fiber and an endless single-mode photonic crystal fiber, respectively, based on the use of the micro-explosion mechanism of near-infrared fs laser pulses at 1 kHz. This new type of micro fiber-optic sensors offers a number of advantages over conventional EFPI sensors assembled manually, in particular, better thermal stability, smaller size, greater potential for mass-production with good reproducibility and lower cost. It is found that the MFPI sensor based on the PCF can achieve better fabrication quality for the F-P cavity than the MFPI sensor based on the SMF. Experimental results show that the PCF-MFPI sensor has smaller strain sensitivity of 0.0045nm/με than that of the SMF-MFPI sensor, i.e. 0.006nm/με, which is in agreement with the theoretical analysis. The phase-strain sensitivity of the MFPI sensor is about five times larger than that of an in-line SMF etalon sensor. In addition, the temperature sensitivities of the two kinds of MFPI sensors are close due to their very similar host materials and about 11 times smaller than that of an in-line SMF etalon sensor. Accordingly, it is anticipated that these MFPI sensors could find wide applications in practice due to their outstanding advantage of being high strain sensitivity and low temperature sensitivity.
This work is supported by the Key Project of Natural Science Foundation of China (Grant 60537040).
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