Fiber Optic Temperature Sensor Based on Harmonic Vernier Effect of Sagnac Interferometer

Fiber optic interferometer has the advantages of small size, light weight, anti-corrosion, anti-electromagnetic interference, high sensitivity, etc., and is widely used in the measurement of temperature, humidity, magnetic field and other parameters. In recent years, researchers have dramatically improved the measurement sensitivity of interferometric fiber-optic sensors by cascading or paralleling two fiber-optic interferometers to produce an optical Vernier effect. When the free spectral ranges of the two fiber optic interferometers are close but not equal, the resulting Vernier effect is called the normal Vernier effect, when the free spectral range of one fiber optic interferometer is about an integer multiple of the other fiber optic interferometer, the resulting Vernier effect is called the harmonic Vernier effect. In this paper, a parallel optical fiber temperature sensor based on two Sagnac interferometers is proposed, where the interferometers SI1 and SI2 are connected to the two outputs of the fiber-optic coupler C3, where SI1 is a reference interferometer and SI2 is a sensing interferometer. When the length of the Panda fiber in the SI2 interferometer is approximately i+1 of the length of the Panda fiber in the SI1 interferometer (i is 1, 2, 3 center dot center dot center dot) times, the two interferometers will produce an i-order harmonic Vernier effect. When i is 0, it produces an normal Vernier effect, at which time there will be a single envelope in the interference spectrum. When i is 1, it produces a first-order harmonic Vernier effect, at which time there will be a double envelope in the interference spectrum. When i is 2, it produces a second-order harmonic Vernier effect, at which time there will be a triple envelope in the interference spectrum. In other cases, the order is analogous. We have numerically simulated the theoretical analysis and the free spectral range of the interferometric spectrum of the length interferometer SI1 with a Panda fiber of 520 mm at constant temperature is 9.13 nm. The SI2 interferometers with lengths of 572 mm, 953 mm, and 1 430 mm of the Panda fiber have free spectral ranges of 8.30 nm, 4.98 nm, and 3.32 nm, respectively. When the temperature is increased from T-0 degrees C to T-0+1 degrees C, the interference spectra of SI2 interferometers with different Panda fiber lengths are all shifted to the short-wave direction, and the shifts are all about 1.89 nm, which is consistent with the theoretical analysis. The parallel interference spectra of interferometer SI1 and SI2 with Panda fiber lengths of 572 mm, 953 mm and 1 430 mm show single, double and triple envelopes respectively, indicating that the two interferometers produce the normal Vernier effect, first-order and second-order harmonic Vernier effects, respectively, and from the theoretical calculations. It can be seen that the amplification of the normal Vernier effect, first-order and second-order harmonic Vernier effects are all 11 times. When the temperature increases from T-0 degrees C to T-0+1 degrees C, the single envelope moves in the short-wave direction, while the double and triple envelopes both move in the long-wave direction, which is opposite to that of the single SI2. In addition, the shifts of the single, double and triple envelopes are all about 20.7 nm due to the fact that the Vernier magnification is the same for the normal Vernier effect, first-order and second-order harmonic Vernier effects. It is experimentally concluded that the interference spectra of SI2 are blueshifted in the temperature range from 40 degrees C to 50 degrees C, and the shifts are all about 1.89 nm, which is consistent with the theoretical analysis and simulation results. The temperature sensitivity of the sensor corresponding to the normal Vernier effect is -20.67 nm/degrees C, the temperature sensitivity of the sensor corresponding to the first-order harmonic Vernier effect is 21.34 nm/degrees C, the temperature sensitivity of the sensor corresponding to the second-order harmonic Vernier effect is 21.18 nm/degrees C, and the fiber optic sensors corresponding to the harmonic Vernier effect and the normal Vernier effect have almost the same temperature sensitivities, which are both about 21 nm/degrees C. This results are consistent with the theoretical analysis and simulation results. The above experimental results show that the temperature sensitivity of the SI2 interferometer is independent of the length of the Panda fiber, although the magnification is the same, the harmonic Vernier effect and the normal Vernier effect correspond to the detuning of the length of the Panda fiber are obviously different, the detuning corresponding to the normal Vernier is 52 mm, and the detuning corresponding to the first-order and second-order harmonics is -87 mm and -130 mm, respectively. This shows that the higher the order, the larger the detuning amount, which is approximately a multiple increase. The above experimental results are consistent with the theoretical analysis. Since the larger the detuning amount, the easier the Vernier magnification can be controlled and realised, the harmonic Vernier effect is obviously superior to the normal Vernier effect from the preparation point of view. This study can provide an important reference for the subsequent study of optical Vernier effect.