Fifty MZI fiber sensors are fabricated with the same trench depth of 60 μm but different lengths of 50, 65, 80, 100 and 115 μm. As shown in , the fringe visibilities of the processed fibers are all greater than 25 dB. The background loss increases as the trench length increases and it is greater than 11 dB in all the cases, which is similar to the previously reported experiment [1
]. The relatively high loss may be mainly due to the light scattering at the laser-ablated surface [11
]. At the trench lengths of 50, 65, 80, 100 and 115 μm, the FSRs are about 101, 74, 64, 53.5 and 38 nm, respectively. This indicates that FSR decreases as the trench length increases, which implies more interference orders at longer trench lengths.
Transmission spectra of the structures at different lengths.
The fabricated structure forms a MZI whose two main light transmission paths are (1) the remaining D-type fiber core; and (2) the cavity in the trench. The interference intensity is expressed by [17
are the intensities along the two light paths and ϕ
) is the phase difference; Δneff
(≈ 0.4682) is the difference between effective refractive index of the D-type fiber core and that of the trench cavity; λ
is the wavelength; L
is the trench length; and 0
is the initial interference phase. The fringe visibility depends on I1
, and is optimized when I1
. The interference changes in each ablation scanning cycle are shown in .
According to Equation (1)
, the phase difference of two adjacent minimum interference signals is 2π. Therefore:
are the wavelengths corresponding to the two adjacent minimum interference signals.
Thus, the trench length is:
which shows that the FSR decreases as the trench length increases. Based on the interference spectra in , the calculated trench lengths are 46.5, 62.8, 75.5, 92.8 and 126.1 μm, which are reasonably close to the experimental results: 50, 65, 80, 100 and 115 μm, respectively. The errors may mainly be caused by the simplification that the cladding effects and variation of Δneff
are not considered.
Gas sensing tests in air and acetone vapor were conducted. The sensor with a trench length of about 80 μm was put into a sealed stainless steel tube. The inner diameter and the length of the stainless steel are about 1 cm and 20 cm, respectively. The sensor transmission spectrum in air at room temperature is shown in .
Sensing test results in air and acetone vapor at room temperatures.
Then, 1.5 mL acetone was injected into the stainless tube. The transmission spectrum of the sensor in the acetone vapor was measured at room temperature. The spectrum scanning procedure is repeated several times until there is no obvious change compared with the preceding ones and the final sensor spectrum in acetone vapor is also shown in . The refractive index of acetone vapor is greater than that of air, between which the difference is on the order of magnitude of 10−4 RIU. Compared with the results in air, the interference dip wavelength shift in acetone vapor is about 6.5 nm. The sensitivity is about 104 nm/RIU for acetone vapor.
Temperature measurements were also conducted by using the proposed fiber sensor. The sensor with a trench length of about 85 μm was selected. The temperature changes from 200 °C to 875 °C at a step of 25 °C. The interference dip wavelength shows a red shift with the increase of the temperature, as shown in . It is mainly due to the change of Δneff or the effective refractive index change of the D-type fiber caused by the temperature variation. The temperature sensitivity estimated by least square linear fitting is 51.5 pm/°C.
The temperature sensing property of the sensor.