Encrypted optical fiber tag based on encoded fiber Bragg grating array

Optical fibers are typically used in telecommunications services for data transmission, where the use of fiber tags is essential to distinguish between the different transmission fibers or channels and thus ensure the working functionality of the communication system. Traditional physical entity marking methods for fiber labeling are bulky, easily confused, and, most importantly, the label information can be accessed easily by all potential users. This work proposes an encrypted optical fiber tag based on an encoded fiber Bragg grating (FBG) array that is fabricated using a point-by-point femtosecond laser pulse chain inscription method. Gratings with different resonant wavelengths and reflectivities are realized by adjusting the grating period and the refractive index modulations. It is demonstrated that a binary data sequence carried by a fiber tag can be inscribed into the fiber core in the form of an FBG array, and the tag data can be encrypted through appropriate design of the spatial distributions of the FBGs with various reflection wavelengths and reflectivities. The proposed fiber tag technology can be used for applications in port identification, encrypted data storage, and transmission in fiber networks.


Introduction
An access network is a network that connects a user terminal to the operator's communication equipment. This type of network is usually only a few hundred meters to several kilometers in length and thus it is described vividly as the last kilometer. At present, networking based on passive optical networks (PONs) has greatly reduced the use of active devices, but PONs have also caused difficulties in detection of links, despite the advantage that they offer by reducing the number of communication rooms required. Generally, a PON network structure consists of an optical line terminal, an optical network unit, and a splitter [1]. Optical fibers are key components of access networks because of advantages that include high data transmission speeds, broad bandwidths, and network construction flexibility [2]. In the big data era, the exponential growth in data usage requires numerous optical fibers or fiber channels for data transmission, and it is essential to be able to identify and distinguish between the different optical fibers and fiber channels to ensure the operating functionality of the communication system. In addition, data encryption is an important factor in ensuring the safety and stability of the data transmission. Currently, physical labels that are attached to the fiber cables are often used for cable identification, but these labels have very limited lifetimes, and any potential user can access the labeled fiber cable information easily because it has no encryption function. Use of large numbers of physical labels may also lead to mess and confusion for the network users. To address this problem, Huawei Technologies Co., Ltd has proposed the 'optical iris' scheme for use in optical distribution networks, in which a unique identifier is provided for each optical fiber by writing a fiber tag [3].
A fiber Bragg grating (FBG) can be inscribed in an optical fiber as permanent markings and its reflection signal features (i.e. wavelength and intensity) are strongly dependent on the grating period and the refractive index modulation that occurs on the fiber material [4,5]. If FBGs with different reflection wavelengths and reflectivities are inscribed in the fiber channels, it then becomes possible to distinguish between the different fiber channels using their reflection signals [6][7][8][9]. Recently, the FBGs Co. has proposed and implemented optical channel identification technology based on these gratings [10]. However, a single FBG can only carry a limited amount of information. If a number of FBGs (forming an FBG array) are integrated within an optical fiber, then an encrypted fiber tag with a large information capacity can be realized, and the tag information can be encrypted further by encoding the FBGs (in terms of their spatial distributions, reflection wavelengths, and reflectivities) with binary data. The encrypted tag information can be decoded from the backscattering signal from the array, which is measured using an optical time-domain reflectometer (OTDR), but only if the code rule is known [11].
The femtosecond laser is a powerful tool for use in the fabrication of optical waveguides and their associated functional devices because of its ultrashort pulse duration, ultrahigh peak intensity, and high flexibility when tuning its fabrication parameters [12][13][14][15][16][17][18][19]. A femtosecond laser can introduce permanent refractive index modulations in materials and these lasers are thus widely used in microfabrication of optical devices in bulk glass [20][21][22], single crystals [23][24][25][26], and optical fibers [27][28][29][30]. In itself, the fabrication of FBGs using a femtosecond laser has attracted considerable attention since the procedure was initially proposed [31]. When compared with the traditional ultraviolet (UV) phasemask-based technique [32][33][34], femtosecond laser direct writing of FBGs is more flexible in terms of its ability to tune the reflection wavelength, and the fiber itself is not required to be photosensitive. There is also no need to peel off the fiber coating during the femtosecond laser writing process, which provides the fiber device with better mechanical strength. To date, four typical techniques have been proposed for FBG fabrication using a femtosecond laser, including the point-bypoint (PBP) method [35][36][37], the line-by-line method [38,39], the plane-by-plane method [40], and continuous core scanning technology [41]. Among these methods, the PBP inscription method is used most widely in FBG array preparation because of the simple design of its gratings and its efficient fabrication process [42].
In this paper, an encrypted optical fiber tag based on an encoded FBG array is proposed and demonstrated experimentally. A binary data sequence carried by an optical fiber tag is written into the fiber core in the form of FBGs with different spatial locations, resonant wavelengths, and reflectivities through accurate control of the femtosecond laser fabrication parameters. The encrypted data are recovered via analysis of the backscattered light signal from the fiber tag. The featured grating information (i.e. the wavelength and the reflection intensity) and the spatial distribution of the FBGs and the information carried in their specific channels can be recovered based on a code rule. After the proposed fiber tag based on the Bragg grating array is connected to the network, it can then be used as a channel marking and detection tool.

Working principle
The operating principle of the encrypted optical fiber tag is shown in figure 1. In this work, a femtosecond laser pulse chain (513 nm/290 fs/200 kHz; Light Conversion) was launched and was focused into the fiber core through an oil-immersed 100× objective lens with a numerical aperture of 1.32. This differs from the traditional PBP inscription method, in which a single scan of the laser with fixed power is used [41]. The proposed femtosecond laser pulse chain method for FBG array fabrication can achieve lower insertion losses than the conventional approach and it is more flexible in terms of tuning of the FBG's reflectivity. A motion controller (A3200; Aerotech) is used to control the number of pulses in the laser pulse chain. To ensure high precision fabrication, the entire femtosecond laser fabrication setup was positioned on an air-bearing stage (X-axis: ABL10100-LN; Y-axis: ABL10100-LN; Z-axis: ANT130V-5-CN1-PL2; Aerotech). A grating array was inscribed into the fiber core of a single-mode fiber (SMF) along the fiber core axis by the PBP technique using the laser pulse chain inscription method ( figure 1(a)). A charge-coupled device camera was used to monitor the inscription of the FBGs in real time. As shown in figure 1(b), in our experiments, the grating period was defined as Λ, the length L of each grating fragment was set to be 100 µm, and the space D between two adjacent grating fragments was fixed at 200 µm. A binary coding rule was applied to the fabricated FBG array, where a grating fragment was defined as code 1, while an unmodulated fiber fragment was defined as code 0. The morphology of the FBG formed by femtosecond laser modulation is shown in figure 1(c). Additional codes can be defined if FBGs with different reflection wavelengths and reflectivities are used. The fiber tag information can be expressed as a binary data sequence and stored within the fiber core in the form of the FBGs. The data carried by an optical fiber tag can be detected and restored using an optical backscatter reflectometer (OBR), as shown in figure 1(b). The OBR measures the complete scalar response of the device, including its phase and amplitude information, and has a sensitivity of −125 dB and spatial resolution of 20 µm. The relationship between the backscattered light intensity and the transmission distance can be reconstructed from the measured time-domain data, which are equivalent to the data obtained from a traditional OTDR. Analysis of the backscattered light signal acquired from the optical tag using a computer enables accurate depiction of the axial spatial distributions, reflectivities, and reflection wavelengths of the encoded FBG fragments, and the corresponding binary data sequence carried by the fiber tag can then be restored.

Results and discussion
To verify the feasibility of the proposed encrypted optical fiber tag, the effects of the spatial distributions, reflection wavelengths, and reflectivities of the FBGs on the tag performance are investigated systematically.
First, the spatial distributions of FBGs with the same featured wavelength and featured reflectivity are investigated for coding. We assume that all FBGs used in this case are ideally the same in principle; in practice, the reflectivities of individual FBGs may show small variations due to energy fluctuations in each laser pulse (see section 1, supporting information) and focal shifts in the laser beam at different modulation points in the fiber core (see section 2, supporting information). However, these variations will have very limited influence on our experiments and results if an appropriate threshold is selected.
A binary data sequence (i.e. the fiber tag information) was predefined as shown in figure 2, where each black bar represents a code 1, while each white bar represents 0. The binary data sequence was then replaced with a series of FBG fragments that were written into the fiber core in accordance with the rules described above. According to coupled-mode theory [23], the Bragg resonant wavelength λ of an mth-order FBG can be calculated using the following equation: where n eff is the effective refractive index of the fiber core and Λ is the grating pitch.
In the experiments, a second-order grating (Λ = 1.07 µm, Bragg wavelength ∼1550 nm) was selected to avoid overlap between the two modulation points while also maintaining relatively high reflectivity when compared with other higherorder gratings. The backscattered light signal from the fiber tag was measured using an OBR within the wavelength range from 1525 nm to 1610 nm, as shown in figure 2. The signal was obtained by normalizing the measured backscattered light amplitude in the time domain with respect to the number of data points within 1 mm, and then converting the resulting signal to a logarithmic scale. Therefore, the time-domain amplitudes can be displayed in units of dB mm −1 , which means that the datasets collected at the different wavelengths are comparable. As shown in figure 2, each grating reflection peak in the backscattered light signal corresponds to a code 1, and the spacing between two adjacent peaks is determined by the length of the unmodulated fiber fragment, i.e. the number of intervals D and the number of instances of code 0. If an appropriate filtering and smoothing method is used, the spatial locations of each reflection peak and the distances between the adjacent peaks can be identified easily; then, the spatial distribution of the FBG array can be confirmed, and the complete pre-defined binary data sequence is ultimately recovered based on the code rule.
The reflection wavelength and reflectivity are typically used to characterize an FBG, and these properties can also be used for FBG encoding. The performance of the proposed optical fiber tag based on grating segments encoded with spatial distributions of FBGs with different reflectivities was thus investigated next. In this case, FBGs with ideally the same wavelengths but apparently different reflectivities were designed for the FBG array.
Based on the code rule described above, a new gray bar was also defined as code 2, which can be written into the fiber core as a grating segment with a different reflectivity (a higher reflectivity was used in our experiment) to that of code 1, as shown in figure 3. FBGs corresponding to these different reflectivities can be distinguished clearly and extracted from the amplitudes of the backscattered light signals if appropriate thresholds are selected. The spatial distributions of these FBGs and their encoded information can thus be recovered. Through this method, FBG segments with different reflectivities (corresponding to different codes) can be used as basic units for data storage. The different reflectivities in the grating segments can be realized via fine control of the laser pulse energy. In this work, the different reflectivities of the FBGs were obtained by tuning the numbers of pulse exposures, which is a more efficient and precise technique for high-speed laser processing than adjustment of the laser power using a half-wave plate.
Next, the spatial distributions and wavelengths of the FBG segments were studied for use in data encryption in a similar fashion to that of the reflectivities used for encoding of the FBGs. A binary data sequence with five different predesigned grating periods, corresponding to five independent resonant wavelengths at 1534, 1546, 1558, 1570, and 1582 nm, was  inscribed into an SMF using a femtosecond laser, as shown in figure 4(a). The backscattered light signals from the fiber tag were then measured at the Bragg resonance wavelengths within a span of ∼5 nm using an OBR, and the results are shown in table 1 and figures 4(b)-(f). When the detected wavelength range matches the reflected wavelength of the encoded FBG, a backscatter signal that is stronger than that at the other wavelengths can be detected; this signal is sufficiently different (>5 dB) from the other encoded gratings to allow it to be identified easily and extracted from the measured data. Note that the information carried by an optical fiber tag encoded with wavelength can only be retrieved when the reflection wavelengths and the measurement range for each FBG fragment are identified.
Therefore, through reasonable design, multiple wavelengths can be used to realize complex information storage and encryption in these tags. The number of wavelength channels that can be used for data storage is dependent on the scanning range of the tunable laser in the OBR and on the spacing between adjacent grating segments. The former determines the minimum interval of the different wavelength channels, while the latter mainly affects the readability and the accuracy of the recovered tag information. To maximize the information storage capacity of the proposed optical fiber tags,  a smaller scanning range for the OBR and shorter distances between the adjacent encoded grating segments are preferred; however, in practice, the fabrication resolution and accuracy of the femtosecond laser should also be taken into account to optimize the grating periods, grating lengths, and spacings between gratings. Finally, an FBG array encoded with spatial distributions and different reflection wavelengths and reflectivities was also demonstrated, as shown in figure 5(a). A binary data sequence was predesigned, sliced, and encoded within FBG fragments with different reflectivities and wavelengths. The backscattered light signal from the optical fiber tag was first analyzed over a wide wavelength range from 1525 to 1610 nm, as illustrated in figure 5(b); the intensity information of all the gratings and their spatial distributions was restored, and the information stored in the fiber tag was then obtained. However, the decoded information based on the code rule for the spatial distributions and reflectivities of the FBGs was incorrect because the tag information is additionally encoded in wavelengths. The backscattered light signals were then measured and analyzed within wavelength ranges 2 and 4 (see table 1). The corresponding results are shown in figures 5(c) and (d), respectively. These results show that the grating fragments were detected with higher backscattered signal intensities near their reflection wavelength ranges without being affected by the other grating fragments. If an appropriate detection threshold is selected, then grating fragments with different reflectivities can be separated from the measured data, and the tag information can thus be recovered. It is thus demonstrated that data storage within different wavelength channels using grating fragments with different reflection intensities is feasible. Similarly, the wavelength characteristics can be used to distinguish the information contained in the different intensity channels.
However, because of the thermo-optic, thermal expansion, and elastic-optic effects, FBGs are used widely as temperature and strain sensors with high sensitivity, which will hinder the proposed application of the FBG array as a tag [43]. For use in communication systems, the temperature fluctuation range of the equipment must not exceed 100 • C. Therefore, the center wavelength shift of the Bragg grating must be less than 1 nm because the temperature response of a typical Bragg grating is 10 pm • C −1 [29]. This means that, when affected by temperature variations, the actual reflection wavelengths of the Bragg grating with respect to the different periods in the fiber tag will not deviate from the designed values by more than 1 nm. Therefore, when there is a suitable margin in the pre-design of the number of wavelength channels for the optical fiber tag, the temperature drift-induced error can be reduced considerably. At the same time, reasonable device packaging can eliminate the effects of external strain effectively [44].

Conclusions
We propose a new method for optical channel recognition and data encryption that uses FBGs with specially designed spatial distributions, reflection wavelengths, and reflectivities. In particular, an encrypted fiber tag is proposed and demonstrated experimentally based on an encoded fiber grating array that is fabricated by a PBP femtosecond laser pulse chain inscription method. The encrypted information contained in the fiber tag is restored by measuring the backscattering signal using an OBR. Systematic experiments have fully verified the feasibility of encryption of the proposed fiber tag using spatial distributions, different reflection wavelengths, and different reflectivities for the FBGs based on a predefined binary data sequence by varying the periods of the gratings and the number of laser pulses. It has also been demonstrated that the information carried by the optical fiber tag can only be decoded when the predefined code rule is known. The optical fiber tag technology proposed in this work is not only reliable for data encryption applications, but also is suitable for data storage within optical fiber access networks.