Static and dynamic characterization of Fiber Bragg Gratings using a LabVIEW platform

Author(s):
D. Tosi - PHOTONLAB - ISTITUTO SUPERIORE MARIO BOELLA
M. Oliviero - PHOTONLAB - ISTITUTO SUPERIORE MARIO BOELLA
G. Perrone - PHOTONLAB - ISTITUTO SUPERIORE MARIO BOELLA

Industry:
Research

Products:
Data Acquisition, LabVIEW, GPIB

The Challenge:
Realization of an advanced LabVIEW software interface for an optical spectrum analyzer, which permits a full characterization of a Fiber Bragg Grating, evaluating both the static spectral response and its dependence on temperature and strain.

The Solution:
LabVIEW provides a software interface for a spectrum analyzer, interrogated via GPIB, implementing both single sweep and continuous sweep functions. The mechanical and thermal spectral characterizations are performed by dynamically tracking the whole spectrum and the applied temperature/strain, acquired by a NI DAQ card.

Short Summary
Fiber Bragg Gratings, commonly used in telecommunications and sensing, are optical devices that present a frequency-selective temperature/strain-dependent spectral response. In the present work, the process of characterization of gratings is described, exploiting the possibilities offered by acquisition devices. An optical spectrum analyzer is connected to a PC with a GPIB, and a LabVIEW platform provides a suitable interface that permits to configure the instrument to obtain a measure of the spectral response in a suitable format. Using a climatic chamber, and tracking the temporal evolution of temperature and spectrum, it is possible to evaluate the temperature dependence of the FBG.

Article

Introduction
A Fiber Bragg Gratings (FBG) is an optical fiber component that presents a frequency-dependent spectral response; in particular, it has a high reflectivity for a specific wavelength, the so-called Bragg wavelength, and a low reflectivity for the other wavelengths. For this reason, it is possible to use a FBG in two different ways: in transmission, using it as a mirror, or in reflection, operating on the reflected signal. Because of their spectral response properties, FBGs are usually used as filters for optical communications and in sensing applications.

FBG static characterization
The static characterization of a FBG consists in the evaluation of its spectral response, both in transmission and reflection mode. This measurement requires a broadband source, which emits a constant power within the FBG bandwidth, and an optical spectrum analyzer (OSA), that evaluates the spectral response of the input signal; in reflection mode, it is also required a directional coupler or an optical circulator to operate with the signal reflected by the grating.

In order to realize an automatic platform for FBG spectral characterization, the optical spectrum analyzer has been interconnected to a PC, with a NI GPIB-USB-HS cable, allowing a bidirectional communication between the instrument and the processing unit. Then, it is possible to control the OSA with LabVIEW, permitting to appropriately configure the instruments via PC and acquiring data in a suitable way, as shown in Fig. 1.

Hence, a LabVIEW program, running on standard Windows XP operative system, has been realized; the main features of the program are a user-friendly interface (similar to Fig. 2), that reproduces the standard commands of a spectrum analyzer, and the possibility to interrogate several types of OSA, adjusting the interrogation instructions for the specific OSA model. The functioning of the program is divided into two phases: firstly, the user configures the instrument, setting all the fundamental parameters (OSA type, start/stop wavelength, resolution bandwidth reference level, and dB/div scale), the measurement type (transmission/reflection) and the output layout options; then, by starting the acquisition phase, the spectrum, together with all the related parameters (peak position, peak amplitude, -3dB bandwidth, sweep time and video bandwidth, verifying the correctness of the measurement), is acquired and stored in the selected form. Notice that the spectral parameters are evaluated by positioning the appropriate markers provided by the OSA, pointing the minimum or maximum peak if the measurement is performed respectively in transmission or in reflection.

Two different versions of this program have been made, implementing both a single scan and a continuous sweep. In the first case, the program performs only one instrument interrogation, and stores the spectral response as a function of wavelength in the output file. In the second version, the acquisition is periodically repeated (the sampling rate is set by the user), acquiring each time the spectrum and the spectral parameters; the output consists in one file containing the peak amplitude, the peak position and the bandwidth as a function of time, and a series of progressively named files containing the whole spectral response for each sampled acquisition. The first version of the program executes a fast acquisition of the FBG spectrum, while the second version permits also to track dynamic variations of the spectral characteristics, and is particularly useful for monitoring the spectral temporal evolution.

FBG dynamic characterization
The dynamic characterization of a FBG is the evaluation if its spectral response as a function of strain and/or temperature variations. From the theoretical point of view, the Bragg wavelength has a linear dependence on axial strains (~1.2pm/με) and on temperature (~10pm/°C); with an appropriate experimental setup it is possible to verify this behavior.

The measurement of the strain-response of a FBG is performed by placing the grating into a manual extensimeter: a ~60cm fiber span containing the FBG is blocked at its edges, and two micropositioners permits to apply an accurate mechanical extension to the fiber, with a resolution of 0.5μm; the correspondent axial strain can be calculated as ΔL/L. Using the previously described equipment for static characterization, it is possible to obtain, for each strain value, the related spectral response: the results show that the whole spectrum linearly shifts towards the higher wavelengths as the strain increases, with a 1.10pm/με coefficient.

The evaluation of the FBG temperature response, instead, can be automatically carried out by using the appropriate equipment. The principle of operation is shown in Fig. 3a: the measurement setup is arranged as for standard spectral response characterization, except than the FBG is inserted into a climatic chamber, that permits to reproduce a time-variant personalized environment (temperature + humidity). Hence, applying a temperature temporal ramp, and measuring the correspondent spectrum, it is possible to obtain the FBG temperature response.

Exploiting the potentialities of NI DAQ devices, it is possible to automatically determine this characteristic, acquiring temperature and spectrum at the same time and processing the data via software. The climatic chamber temperature is measured with a LM35, an electronic temperature sensor that returns an output voltage proportional to the measured temperature (10.0mV/°C); the output of the LM35 circuit is connected to a NI CB-68LP board, and acquired with a NI DAQ Card-6036E (16-bit, 200KS/s). The OSA, instead, is connected with a NI GPIB-USB-HS cable, as previously specified. Starting from the continuous sweep LabVIEW program, and adding the temperature acquisition from the DAQ card, implemented through a DAQ Assistant, a new LabVIEW 8.0 program for thermal characterization has been realized (Fig. 2): for each sampling instant, both the FBG spectral response and temperature are acquired (temperature data are also block-averaged to reduce uncertainty), and stored into a progressively named output file. The final result is a 3D-graph that plots the FBG spectral response as a function of temperature, as shown in Fig. 3b: as expected, the whole spectrum linearly shifts with temperature, without modifying its shape; the estimated temperature coefficient is 13.22pm/°C.

Conclusion
In this work, a description of the characterization process of a FBG has been presented; exploiting the possibilities of data acquisition and instruments communications devices, an automatic platform for full characterization of fiber Bragg gratings has been realized.

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