Heat-treatment of fiber Bragg grating by arc discharge

Optical Fiber Technology 48 (2019) 70–75

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Heat-treatment of fiber Bragg grating by arc discharge a

b

b

a,b,⁎

Jingtao Xin , Xiaoping Lou , Mingli Dong , Lianqing Zhu a b

T

Beijing Engineering Research Center of Optoelectronic Information and Instruments, Beijing Information Science and Technology University, Beijing 100192, China Beijing Key Laboratory for Optoelectronic Measurement Technology, Beijing Information Science and Technology University, Beijing 100192, China

ABSTRACT

To develop a quick and effective technology to anneal the UV inscribed fiber Bragg grating, a heat-treatment method carried out by arc discharge scanning was studied. The spectral degeneration of UV-inscribed FBG under arc discharge was investigated. It is found that the transmission spectrum intensity was reduced, the 3 dB bandwidth is narrowed and the central wavelength was blue-shifted. Increasing the number of scanning, the transmission spectrum intensity, 3 dB bandwidth and central wavelength of the FBG change slowly until finally they remain unchanged. The similar annealing process was also carried out for femtosecond laser inscribed FBG. It was found that the spectrum remains unchanging after heat-treatment. For the stable thermal property, femtosecond laser inscribed FBG was used to investigate the spectrum response of arc discharge. The discharge leads to the non-uniform temperature field on the FBG, resulting in significant chirp broadening which can be used to approximate calculate the temperature gradient on the FBG. The heat-treated method have potential applications in annealing FBG and transient high temperature measurement.

1. Introduction For inscription of fiber Bragg gratings (FBGs) and long-period fiber gratings (LPFGs), ultraviolet (UV) laser such as KrF excimer laser radiation at 248 nm and the second harmonic of a cw argon gas laser at 244 nm are widely used [1–5]. The refractive index change in fibers at these wavelengths is connected with the absorption band of defects in germanosilicate glass and is mainly of a single-photon nature. Another very important approach to inscription is to expose fibers to femtosecond Ti:sapphire laser radiation at 800 nm and thus to produce the index changes through multiple photon absorption [6–11]. Because of its simplicity and flexibility, ultraviolet exposure with phase mask method is the most important technique to produce commercial FBG. This method has a strong dependence on the photo-sensitivity of the optical fiber. Hydrogen-loading was used before the inscription. After the completion of the inscription, unreacted hydrogen molecules and unstable Ge-OH, Si-H and Si-OH bonds remain in the fiber, leading to the instability of the FBG [12–14]. Therefore, it is necessary to anneal the FBG before it was used as a sensor. After annealing, the transmission spectrum intensity and 3 dB bandwidth of the FBG decrease and the central wavelength is blue-shifted. At present, the conventional method to anneal FBGs is carried out by putting FBG in a 100 °C furnace for 24 h. Therefore, it is necessary to find an efficient annealing method. Arc discharge can generate an instantaneous high temperature up to 3000 C [15], which has been widely used in fiber fusion splicing

[16,17] and fabrication of basic devices such as fiber tapers [18], fiber couplers [19], pump power combiners[20] and long-period fiber gratings (LPFGs) [21,22]. In this paper, transient high temperature field created by arc discharge is designed to anneal the UV laser inscribed FBG. It was found that the transmission spectrum intensity is reduced, the 3 dB bandwidth is narrowed and the central wavelength is blueshifted after arc discharging. Increasing the number of scanning, the transmission spectrum intensity, 3 dB bandwidth and central wavelength of the FBG change slowly until finally they remain unchanged. To confirm the annealing effect, the arc plasma treated FBG was annealed again in the high temperature furnace at 24 h. It was found that the spectrum remains unchanged which verified the feasibility of the annealing method. The similar annealing process for femtosecond laser inscribed FBG was carried out. The transmission spectrum is almost unchanged and the spectrum is red shifted of 0.022 nm. The experimental results also verified excellent high temperature resistance of femtosecond laser inscribed FBG. The relationship between the electrode position and the spectrum response was investigated. It was found that the transmission spectrum is symmetric with the position of electrodes. Especially, when the electrodes are locked in the center of FBG, the transmission spectrum intensity is 8 dB less than initial spectrum. This characteristic has potential application in optic switching devices. The discharge electrode generates a non-uniform temperature field on the FBG, resulting in significant increase of chirp bandwidth. The width of the chirp broaden can be used to estimate the temperature gradient on the FBG.

⁎ Corresponding author at: Beijing Engineering Research Center of Optoelectronic Information and Instruments, Beijing Information Science and Technology University, Beijing 100192, China. E-mail address: [email protected] (L. Zhu).

https://doi.org/10.1016/j.yofte.2018.12.027 Received 12 September 2018; Received in revised form 7 December 2018; Accepted 21 December 2018 1068-5200/ © 2018 Published by Elsevier Inc.

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(a)

be moved along the whole region of the FBG. The 3SAE Technologies Polyimide Fiber Preparation Unit II [18] was used as the electrode discharge device. This device has three electrodes to produce isothermal plasma from 100 °C to 3000 °C. To be convenient, a coordinate is defined to depict the position of the electrodes along the fiber grating which is shown in Fig. 1(b). A scanning discharge method is used to anneal the FBG. The discharge power is set to a fixed value of 40mw, the electrodes moving at a speed of 0.2 mm/s. Fig. 2 shows the evolution of the transmission spectrum of UV-inscribed FBG when is scanned different number of times. Fig. 3 plots the curves of transmission intensity, center wavelength and 3 dB bandwidth for different numbers of scanning. It can be seen that the transmission intensity, central wavelength and 3 dB bandwidth of FBG were changed quickly at the first 10 times of scanning. Increasing the numbers of scanning, the transmission intensity of FBG tends to be stable and the shift of spectrum was slowed down. After scanning 50 times, the transmission spectrum intensity remains at 5.1 dB, the central wavelength is 1551.27 nm (blue shift of 0.38 nm) and the 3 dB bandwidth is 0.137 nm. This experimental results are the same as the high temperature annealing. To confirm the annealing effect, the arc discharge treated FBG was annealed again in the high temperature furnace for 24 h at 100 °C. The transmission spectrum and reflection spectra before and after annealing are shown in Fig. 4. It was found that the spectral remains unchanged. This phenomenon indicates that the annealing of FBG can be realized by arc discharge. This annealing method has obvious advantages regarding high efficiency and reduce time consumption, which allows to accomplish the annealing within a few minutes. The grating stability experiments of arc discharge annealed FBG and the conventional annealed FBG through time were carried out. Through a month's observation, we couldn’t found slight degeneration of the reflectivity and the center wavelength of FBGs only affected by temperature. Those two kinds of FBG behave in the same way. The response to external perturbations like temperature and strain were investigated. It were found that those two kinds of FBG behave in the same way. Arc discharge annealed FBGs and conventional annealed FBG have the same temperature and stain sensitivity coefficients which can be seen in the Fig. 5.

Electrodes

ASE

3-dB coupler FBG

OSA

Optical Switch

(b)

Electrodes

-3

-2

-1

0

1

2

3

Position/mm

Transmission (dBm)

Fig. 1. Schematic diagram of experimental setup. (a) set up; (b) Schematic diagram of scanning.

-30 N=0 N=1 N=10 N=20 N=50

-35

-40

-45 1551.2

1551.6

1552.0

1552.4

Wavelength (nm)

1552.8

Fig. 2. The transmission spectrum of UV-inscribed FBGs at different number of scanning.

2. Spectral properties of UV-inscribed FBG under arc discharge

3. Spectral properties of femtosecond laser inscribed FBG under arc discharge

A schematic diagram of the experimental setup to investigate the spectral characteristics of arc discharge heated FBG is shown in Fig. 1(a). The light of a super luminescent diode (SLD) was sent to a FBG through a 3-dB coupler. Both transmission and reflection light are monitored which is switched by a 1 × 2 optical switch. The FBG is fixed by the holders of the electrode discharge device and the electrodes can

The same treatment was carried out for femtosecond laser inscribed FBG. The transmission spectrum under different scanning times is shown in Fig. 6. It was found that the transmission intensity was nearly unchanged and the central wavelength only shifted 0.022 nm to the red.

Fig. 3. The change of spectrum parameters at different number of scanning (a) transmission; (b) central wavelength; (c) 3 dB-bandwidth.

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Fig. 4. Comparison of the spectra before and after annealing (a) Transmission; (b) Reflection.

Wavelength/nm

1577.5

1578

(a) Arc discharge Traditional Linear Fit-1 Linear Fit-2

1577

1576.5 1576

y=0.012439x+1576.6

1575.5 1575

Wavelength/nm

1578

(b)

y=0.00112x+1576.6

1576 1575

y=0.00114x+1574.8

y =0.012505x + 1574.8

20

40

60 80 Temperature/

100

Arc discharge Traditional Linear Fit-1 Linear Fit-2

1577

1574

0

200

400

600

800

1000

Strain/

Fig.5. Variation of center wavelength with temperature and strain changes. (a) temperature; (b) strain.

Transmission(dBm)

z = 0 mm, the transmission intensity of FBG gradually decreased. When the scanning is near z = 0 mm, the transmission spectrum became flatten. From z = 0 mm to z = 3 mm, the transmission intensity of FBG gradually increased. Fig. 8 shows the transmission spectrums at z = 0 mm, z = 1 mm, z = 2 mm and z = 3 mm. It can be seen that the transmission spectrum is gradually broadened and red-shifted. When z = 0 mm, the transmission spectrum became flatten which is 8 dB higher than that of z = 3 mm. Fig. 9 shows the reflection spectrums at z = 0 mm, z = 1 mm, z = 2 mm, z = 3 mm. it can be seen that, as the electrodes move toward the center of the FBG, the reflection spectrum became widen and redshift, and the peak value of the reflection spectrum decreases gradually. The broadening of the reflection spectrum was caused by the increasing of the temperature gradient on the FBG. The red-shift of the reflection spectrum was caused by the uniform and entire temperature increasing on the FBG. When z = 0 mm, the reflection spectrum is the same as a chirped FBG. The arc excitation created a non-uniform temperature field on FBG which caused the chirped bandwidth of the spectrum. The width of the chirp broaden can be used to estimate the temperature gradient on the FBG. When z = 0 mm, the chirp broaden of the spectrum reaches 8 nm which indicate a large temperature gradient were formed on the FBG. Because of the non-uniform temperature distribution of the FBG, the grating reflects different wavelengths. The superposition of these different wavelength reflections forms a reflected bandwidth similar to the aperiodic gratings. The bandwidth of chirp spectrum BW can be expressed as the difference between the maximum resonant wavelength

Wavelength(nm) Fig. 6. Transmission spectrum of femtosecond laser inscribed FBG under different times of scanning.

This phenomenon also confirmed the good resistance to high temperature of femtosecond laser inscribed FBG. Due to the high temperature stability, femtosecond laser inscribed FBG is used to study the spectrum characteristics during the process of heat treatment. The relationship between the electrode discharge position and the spectrum was studied. In this experiment, the discharge power is set to 40mw, electrodes moving at the speed of 0.01 mm/s. In the process of scanning discharge along the FBG from right to left, it can be seen that the transmission spectrum is symmetrical which is shown in Fig. 7. From z = -3mm to

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Intensity/dBm

Central wavelegth(nm)

1595

Positio

n(mm)

th eng

)

(nm

vel

Wa

1590

1585

1580

Fig.7. Transmission spectrum under different discharging positions.

data linear fit

y=0.014372x+1579.8 200

400

600

Temperature(

800

1000

)

Fig. 10. The calibration results of femtosecond laser inscribed FBG at high temperature.

Where neff is the effective refractive index of the fiber core, is the grating period, is the expansion coefficient and is the thermo-optic coefficient. To be convenient, a global thermal coefficient is defined as −1 ] which represent the thermal sensitivity of the FBG. The T [nm°C bandwidth broadening is approximately as linearly proportional to the maximum temperature gradient on the FBG. The temperature sensitivity coefficient of femtosecond laser inscribed FBG at high temperature was calibrated in a muffle furnace. Fig. 10 shows the calibration results. In the range from 100 °C to 1000 °C, the center wavelength of the fiber grating has a good linear relationship with the temperature. The linearity is 0.99982 and thermal sensitivity coefficient is 14.04 pm/°C. Fig. 11 shows the reflection spectrum at discharging power of 30mW, 60mw, 90mw and 120mw. As discharging power increases, the chirp broaden get wider. When the discharging power reaches 120mw, the reflected spectrum width is about 12.4 nm. From the reflected bandwidth of the FBG, the temperature of the discharging can be roughly measured. The gradient temperature study for the UV-inscribed FBGs were carried out. When the power is lower than 50mW, the experimental phenomenon is same as femtosecond-inscribed FBGs which can be seen in Fig. 12. But when the power is large than 50mW, the spectrum of the FBG were obviously deformed which can be seen in Fig. 13(b). Because the structure of the UV-inscribed FBGs were destroyed by the arc discharge. For UV-inscribed FBGs, this methods cannot measure the temperature more than 400 °C.

Fig.8. Transmission spectrum under discharging at z = 0 mm, z = 1 mm, z = 2 mm, z = 3 mm.

4. Conclusion The influence of electrode discharge on the spectral characteristics of two kinds of FBG is studied. It is found that the transmission spectrum intensity is reduced, the 3 dB bandwidth is narrowed and the central wavelength is blue-shifted after heat of electrical arc discharge for UV inscribed FBG. This heat-treatment methods can be used as an efficient way to anneal UV inscribed FBG. For femtosecond laser inscribed FBG, the spectrum is nearly unchanged after the heat-treatment. The relationship between the electrode discharge position and the spectrum was studied. It was found that the transmission spectrum is symmetric with the position of electrodes. The discharge generate a non-uniform temperature field on the FBG, resulting in significant chirp

Fig.9. Reflection spectrum under discharging at z = 0 mm, z = 1 mm, z = 2 mm, z = 3 mm. max and BW

the minimum resonant wavelength

= max min = 2neff ( + ) Tgrad = T Tgrad

min

[23–24].

(1)

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Reflected spectrum width/nm

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12

Experiments Linear 8 6

y = 0.079545x +2.6864

4 20

40

60

80

100

Discharge power/mW

120

Reflection/dBm

Fig.11. Reflected spectrum of femtosecond laser inscribed FBG under different discharging power, (a) reflection spectrum; (b) curve of reflected spectrum width under different discharging power.

2.55nm (182 )

3.05nm (218 )

3.52nm (251 )

4.08nm (291 )

4.50nm (321 )

Wavele gt

h/nm

4.97nm (355 )

Power/mW

Fig.12. Reflected spectrum of UV-inscribed FBG under different discharging power.

-35

-35

(b)

-40

Reflection/dBm

Reflection/dBm

(a)

-45

-50

-55

-40

-45

-50

-55 1572

1574

1576

Wavelegth/nm

1578

1580

1572

1574

1576

Wavelegth/nm

1578

1580

Fig.13. The reflective spectrum of the UV-inscribed FBGs after discharging (a) discharged power less than 35 mW (b) discharged power more than 50 mW.

broadening. The chirp broaden property have potential application in FBG spectral regulation.

and Innovative Research Team in University, PCSIRT (IRT-16R07), the Project Plan of Beijing Municipal Commission of Science and Technology (Z151100003615010), the Project Plan of Beijing Municipal Commission of Education for Enhancing Innovation Capability in 2015 under Grant (TJSHG201510772016).

Funding sources This work was supported by the Program for Changjiang Scholars 74

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Appendix A. Supplementary data

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