Experimental characterization of a Raman based distributed temperature sensor using a 1064 nm pump

. In this work, we present proof-of-concept of a Raman based distributed temperature sensor using standard telecommunication fiber as the sensing element. To allow coexistence with data transmission, a pump source with a wavelength of 1064 nm is used. The proposed DTS is characterized in the range of 20 °C to 100 °C, using the ratio between the Raman band’s powers linearized. We established a characteristic curve of the sensor with a sensitivity of 0.0018±0.0001 /°C, showing that the proposed DTS can detect temperature variations. This work is the first step forward in the development of distributed sensors that coexist with data transmission over the same optical fiber.


Introduction
Distributed optical fiber sensors (DOFSs) are intrinsic fiber sensors that provide continuous measurements over several kilometers using a unique interrogator unit.Such systems have been developed and optimized over the years, leading to several applications: structure health monitoring in tunnels, oil and gas pipelines, power cables, fire detection, subsea cable monitoring, and fluid leakages, among others.Of the DOFSs available for temperature sensing (distributed temperature sensors (DTSs)), the most mature and wellknown are based on the detection of spontaneous Raman backscattering using the optical-time domain reflectometry (OTDR) technique [1].
Recently, [2] reported the integration of DOFSs in telecommunications fiber networks together with data transmission for the detection of acoustic effects, road traffic, and environmental temperature.However, telecommunications channels were allocated for the sensor's pump, reducing the overall network's transmission capacity.
In this work, we propose a DTS that uses a pump source with a wavelength away from the standard telecommunications transmission windows, that can allow the coexistence of data transmission and sensing while maintaining the network's transmission capacity, within the same fiber.Here, we demonstrate proof-of-concept of a Raman OTDR based DTS, using standard single-mode fibers (SMFs) and a pump source with a wavelength of 1064 nm.Such systems could be applied to forest fire detection, taking advantage of already installed fiber networks through forests and along roads.

Raman OTDR based DTS design
The working principle of a typical Raman OTDR based DTS is schematized in Fig. 1.In the interrogator, the pump source launches light pulses with high peak power into the optical fiber.The generated backscattered light travels back to the interrogator, and the Raman Stokes (S) and anti-Stokes (AS) bands are detected and processed.Theoretically, the ratio between the powers of the S and AS bands is proportional to the fiber's temperature through an exponential relation [1].By linearizing this relation, we obtained the following: where  represents the ratio between the AS and the S band powers, λ and λ are the central wavelength of the S and AS bands, respectively, and T(z) is the fiber's temperature along the length of the fiber .
Error! Unknown switch argument.presents the Raman DTS interrogator proposed here.The system consists of a pump source (VFLS-1064-M-PL-25, CONNET) with wavelength of 1064 nm, 6.7 W peak power pulses and a 50 ns pulse width, which corresponds to a spatial resolution of 5 m [3], launched into ~400 m of SMF.The backscattered light is collected using a circulator and then filtered to separate the AS and S bands.To monitor the bands' powers, we used an avalanche photodiode (APD) (APD430C/M, THORLABS), an oscilloscope (DSOS804A, KEYSIGHT), and an optical switch to jump between the S and AS signals.

Experimental characterization
To characterize the Raman DTS, we placed the SMF spool inside a thermal chamber, and the temperature ranged between 20 ºC and 100 ºC, in 20 ºC steps.The S and AS powers are obtained from an average of 10 measurements for each temperature step, to decrease the impact of pump source power fluctuations.The ratio between the Raman bands is calculated considering ~ 250 m of SMF in the middle of the spool.
Fig. 3 displays the relation between the linearized ratio ( ) of AS and S bands' powers and temperature at which the fiber is submitted, given by (1).The plot shows a linear increase of the ratio with temperature, as expected.By adjusting a linear fit to the data, we can achieve a sensitivity of 0.0018±0.0001/°C.These results show that the proposed DTS can detect temperature variations in the range of 20 ºC to 100 ºC.

Conclusions
In summary, the work presents proof-of-concept of DTSs that use a wavelength pump of 1064 nm, outside the usual data transmission windows.Such a system can coexist with data transmission in already installed fiber networks without compromising the network's transmission capacity.
The proposed Raman OTDR based DTS was able to detect temperature variations in the range of 20º C to 100 ºC, and the system was characterized.The expected linear behavior was verified, and a characteristic curve of the sensor was established.
Even though this proof-of-concept is the first step for the development of DTSs that can coexist with data transmission in the same fiber, further improvements are required.

Fig. 1 .
Fig. 1. -Schematic of the working principle of the Raman based DTS.

Fig. 2 .
Fig. 2. -Schematic of the experimental setup of the Raman DTS proposed.

Fig. 3 -
Fig.3-Linearized ratio between the AS and S band's powers as function of the temperature.The linear regression represents the DTS's characteristic curve, given by  .