Introduction Fiber Fabry-Perot (FP) is divided into intrinsic cavity and extrinsic FP cavity. The intrinsic fiber FP cavity is difficult to manufacture, while non-intrinsic fiber FP cavity sensor is relatively simple to fabricate. Intrinsic optical fiber FP sensor has the advantages of high sensitivity, anti-electromagnetic interference, corrosion resistance, good electrical insulation, and easy to form a telemetry network with optical fiber transmission system. It is widely used to measure strain, pressure, vibration, acceleration, and refractive index. Such physical quantities, however, traditional extrinsic FP cavities are mainly formed by processing the end surfaces of two single-mode fibers into mirror-reflecting surfaces and filling them into a sealed glass tube. There are still many problems in the production process: 1) Full manual The manufacturing process makes the FP sensor less repeatable. When the FP cavity is formed by butt-joining two fiber ends, the cavity length is difficult to control. Therefore, it would be very difficult to make a batch of FP cavities with the same cavity length; 2) FP cavity In the production of the two optical fiber end surfaces are easily contaminated by gas dust in the air, and the capillary used to connect the two optical fiber end surfaces during docking is also more likely to damage the optical fiber end surface.
In this paper, a miniature FP sensor fabricated directly on a Photonic Crystal Fiber (PCF) is reported. It is fabricated using a near-infrared femtosecond laser directly written on a PCF. The method is simple and can realize optical fiber. The large-scale production of FP cavities has good application prospects. The temperature and strain characteristics of FP sensors fabricated by this method were also tested.
1 Femtosecond Laser Fabrication of Micro FP Cavity Structure Near-infrared femtosecond lasers are used in the fabrication of this PCF FP cavity. The processing system is shown in Figure 1. PCF is a refractive index light guide type PCF (ESM-12-01) whose main component is fused silica SiO2. The laser source used is a Ti-Gem laser regenerative amplifier (Spitfire-F, Spectra-Physics) with a wavelength of 800 nm, an energy of 100 μJ, a pulse width of 100 fs, and a repetition rate of 1 to 5 kHz. The laser light emitted from the femtosecond laser first passes through a 10 μm-diameter spatial light filter to improve the quality of the laser beam and expand the beam. A vacuum tube or a micro lens can be added to the aperture of the spatial filter. Prevents breakdown of air leading to ionization (adding vacuum tubes is cheaper and less fitting; microlenses are simple and expensive). It is then reflected by the dichroic mirror and focused on the PCF to be processed through the objective lens (magnification 20, numerical aperture NA=0.45). The energy emitted by the laser is controlled by an attenuator consisting of a half-wave plate and a polarizer. A computer-controlled three-dimensional mobile platform (PI, German) is used to precisely control the position of the PCF in the x, y, and z directions. The accuracy in the z direction is 100 nm, the accuracy in the y direction is 125 nm, and the z direction is Accuracy is 7nm. The processed PCF is illuminated with a light emitting diode and the formation of the fiber FP cavity is monitored on the display in real time by the CCD camera.
Fig. 1 Femtosecond laser micromachining system
In the processing process, the femtosecond laser interacts with the optical fiber (silicon) in a very short time and in a very small space, and the energy injected into the towel in the action area is effectively accumulated, and the temperature in the action area is within the moment It rises sharply and far exceeds the melting and vaporization temperatures of silicon, making silicon highly ionized into high-temperature, high-pressure, high-density plasmas. The silicon in the final area of ​​action is removed in the form of plasma eruptions. The eruption of the plasma takes away almost all of the heat, and the temperature in the area of ​​action basically returns to the state before processing, thereby avoiding the existence of hot melting in this process, realizing a relative sense of cold processing, and greatly weakening the traditional processing. Due to the many negative effects of the medium heat effect, the processed two ends are relatively flat. During processing, most of the dust is removed in the form of plasma eruptions. The small amount of dust remaining in the chamber can be washed away by ultrasonic or hydrofluoric acid solution. A rectangular groove with a length, width, and depth of 75 μm, 30 μm, and 80 μm was fabricated in the experiment. The two end faces of the hollowed out part of the core formed a miniature extrinsic FP cavity with a cavity length of approximately 75 μm. As shown in Figure 2, the interference spectrum is shown in Figure 3.
Figure 2 Photographs and structures of photonic crystal fiber (EMS-12-01) face and FP cavity
Fig. 3 Photorefractive spectrum of photonic crystal fiber FP cavity
2 Experimental results and discussion The micro FP cavity structure of Figure 2 was placed in a thermostatic chamber to test the variation of its cavity length with temperature. The spectrometer was used to monitor the real-time and collect data. The temperature variation in the experiment is -20°C to +100°C, and data is collected once the temperature is stable at 10°C. The relationship between the wavelength and temperature change of the micro-FP cavity interference line and the experimental results of cavity length variation with temperature are shown in Fig. 4.
Fig. 4 Relationship between wavelength and cavity length and temperature change
From the experimental results, it can be seen that the maximum variation of the FP cavity length is 0.115 μm and the temperature coefficient is 0.958 nm/°C in the process of changing the temperature from -20°C to 100°C. The experimentally measured streaks drift around 1540 nm is 0.206 nm.
The two ends of the PCF are respectively fixed on the left and right micro-movement tables, and the pre-stress is applied to straighten the fibers. The micro-FP cavity is located in the middle of the fixed PCF, and the micro-movement table at one end is fixed, and the PCF is stretched at the other end. The moving range of the micro-moving stage is 0-130 μm, and the experimental results of the micro FP cavity strain characteristics are shown in FIG. 5 . In the range of 0 to 1500 με, the wavelength drifts 5.43 nm and the sensitivity is 0.0036 nm/με.
Fig. 5 Relationship between peak position and strain in the 1550 nm region
3 Conclusions This paper reported a method for fabricating miniature FP sensors using near-infrared femtosecond lasers on total-reflection-type photonics fibers. This method can fundamentally overcome the manual operation, inefficiency, and repetition of traditional methods for fabricating FP cavities. The disadvantages of the piece and the PCF FP cavity produced by this method as a fiber strain sensor have been evaluated for temperature and strain. As a result, PCF FP cavity produced by Yueming has a high sensitivity and excellent linearity as an optical fiber strain sensor. The PCF FP cavity is cut once on the optical fiber without mechanical splicing. As an all-fiber online device, the PCF FP cavity can be widely used for strain measurement in a general environment. In addition, it is also imprisoned directly on the optical fiber, cavity length is short, stable and other characteristics, easy to embed in materials or structures to form a distributed multi-point detection system, and thus has a wide range of applications in the field of optical fiber potassium energy materials and structures. prospect.
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