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Abstract
Applications of the type-I fiber Bragg gratings (FBGs) written through the coating (TTC) in strain sensing and tunable distributed Bragg reflector (DBR) fiber lasers were demonstrated. We reported the principle of selecting the distance between the fiber and the phase mask when writing type-I TTC FBGs. Type-I TTC FBGs written in commercially available acrylate-coated fibers with various geometries and their strain responses were demonstrated. Results showed that the strain sensitivity of FBGs increases as the core-diameter decreases, probably due to the waveguide effect. In addition, a continuously tunable DBR fiber laser based on TTC FBGs was achieved with a wavelength tuning range of 19.934 nm around 1080 nm, by applying a strain of 0-21265.8 µɛ to the laser resonant cavity. The wavelength tuning range was limited by the splice point between the gain fiber and the passive fiber for transmitting pump and signal lasers. When the pump power was 100 mW, the relative intensity noises were −97.334 dB/Hz at the relaxation oscillation peak of 880 kHz and −128 dB/Hz at frequencies greater than 3 MHz. The results open a potential scheme to design and implement continuously tunable fiber lasers and fiber laser sensors for strain sensing with a higher resolution.
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1. Introduction
Fiber Bragg gratings (FBGs) have become key components in the fields of fiber sensing and fiber lasers. Strain has received extensive attention in structural health monitoring in many fields, such as water conservancy and hydropower, petrochemical, civil engineering, and aerospace [1–4]. Among numerous strain sensors, FBG sensors have attracted widespread attention due to its advantages compared to the traditional electrical sensors, such as a high sensitivity, small size, good corrosion resistance, and robustness against an external electromagnetic radiation [5–10]. In addition, FBGs are also the cavity mirrors of fiber lasers, playing the role of feedback and coupling output. FBGs based fiber lasers could be achieved wavelength tuning by applying temperature or strain to the laser resonant cavity due to the spectral responses of FBGs to them, which are greatly meaningful in optical communication and fiber sensing systems [11]. Many researches have been conducted on temperature tuning. Huang et al. proposed a linear-cavity fiber laser using a Sagnac loop and a high temperature resisted FBG fabricated by femtosecond laser, and realized a tuning range of 11.13 nm around 1550 nm when the temperature of the laser resonant cavity varied from 300 to 1000 °C [12]. Compared to temperature, strain is a faster and simpler wavelength tuning method [13,14].
Strain sensitivity and range of FBGs are important parameters for fiber strain sensing and tunable fiber lasers, which are not only affected by the packaging materials of them, but also by the fiber geometry [9]. Currently, most of the researches have focused on the influence of the packaging materials on the sensitivity of the FBGs [10], and there is a little relevant research on the influence of the fiber geometry on the sensitivity of the FBGs. In addition, the maximum strain that FBGs could withstand is much less than the pristine fibers due to the FBGs inscription methods, which limited their performance in fiber strain sensing and tunable fiber lasers.
Traditional FBGs are inscribed in a photosensitive fiber by an ultraviolet (UV) laser [15]. In general, the coating on the optical fiber (i.e., acrylate or polyimide) is not transparent to UV lasers [16]. Hence, the inscription of FBGs using UV lasers typically involves the following steps, namely, i) photosensitization of the fiber core by loading hydrogen to the fiber [17] and ii) removal of the fiber coating and recoating after grating inscription [18]. Apparently, the latter steps will introduce mechanical cracks to the fiber surface, which greatly weakens the mechanical strength of the fiber and thus affects the ultimate tensile strength of the FBGs.
The emergence of femtosecond (fs) lasers has made it possible to write FBGs directly in non-photosensitive fibers through the coating (TTC), with both the phase mask (PM) [19] and the point-by-point (PbP) techniques [20–22]. It has been demonstrated that type-I TTC FBGs inscription with the PM technique could maintain the same mechanical strength as the pristine fibers [8,23–25]. While TTC FBGs inscribed with the PbP technique presents poor mechanical strength, with a mean breaking stress of only 15-20% compared to that of a pristine fiber [26]. As a result, the PM technique opens significant opportunities in the field of the FBG sensors. In order to write TTC FBGs with the PM technique, tight focusing geometries are needed that the intensity at the fiber core is sufficient for refractive index modification, while the intensity at the fiber coating is low enough to avoid damage. This places high demands on cylindrical objectives and the PMs.
The results of writing TTC FBGs in various fibers with the PM technique have been reported. In general, the PMs with a pitch of less than 1473 nm were used when writing TTC FBGs in large-mode-area fibers [27], because these fibers are easily to form a larger laser intensity ratio at the core and the coating interface, which is conducive to writing TTC FBGs. Additionally, when writing TTC FBGs in small-mode-area fibers, PMs with a uniform pitch of 1070 nm (defined as 1st order) were used to increase the laser intensity ratio at the core and the coating interface [8,16,23–25], which produce FBGs in silicon-based fibers with a first-order Bragg resonance around 1550 nm. But the 1st order masks usually have a bad zero-order cancellation [23]. What’s more, the distance between the fiber and the 1st order masks was only a few tens or hundreds of microns when writing TTC FBGs [23–25], which may easily damage the masks. Halstuch et al. reported a technique for writing TTC FBGs using a PM with a uniform pitch of 2140 nm [26]. But in this work, the fiber coating is greatly damaged during the FBGs inscription process, and the breaking stress is only ∼14% compared to the previous study [8,23–25], similar to the FBGs without coating. This is because a higher energy was required for the larger pitch masks as the grating growth was very slow [28], which is unfavorable for writing TTC FBGs. Hence, it’s extremely challenging to write TTC FBGs in small-mode-area fibers using PMs with a uniform pitch of larger than 1070 nm. In addition, the Bragg wavelength of TTC FBGs written in small-mode-area fibers is mainly concentrated at 1550 nm [8,16,23–25], and there is little relevant research on other wavelengths. This particular work will focus on using a PM with a uniform pitch of 1485 nm to write type-I TTC FBGs, and demonstrating their applications in fiber strain sensing and tunable fiber lasers. It should be noted that the devices reported in this paper have all been written through the coating, unless otherwise noted.
In this paper, we demonstrate the applications of the type-I TTC FBGs fabricated by femtosecond laser in strain sensing and tunable distributed Bragg reflector (DBR) fiber lasers. The principle of selecting the distance between the fiber and the PM when writing type-I TTC FBGs was discussed and type-I TTC FBGs inscribed in fibers with various geometries are presented. The strain response study shows that the strain sensitivity of FBGs increases as the core-diameter decreases due to the waveguide effect and the stress sensitivity of FBGs is inversely proportional to the square of the fiber radius. Besides, a wavelength tunable DBR fiber laser was constructed by directly inscribing a pair of type-I TTC FBGs in the ytterbium-doped fiber (YDF) and tuned with strain. A 19.934 nm tuning range at around 1080 nm was achieved by applying strain to the laser resonant cavity, which limited by the splice point between the gain fiber and the passive fiber for transmitting pump and signal lasers. The relative intensity noise (RIN) of the laser output is −128 dB/Hz at frequencies greater than 3 MHz with a pump power of 100 mW. It is of great significance for the fields which requires fast wavelength tuning and strain sensors with a higher resolution.
2. Experimental setup and methodology
2.1 Experimental setup
The inscribing light source is an amplified Ti-sapphire laser system (Coherent Inc., Astrella) that can produce up to 7 mJ pulses with a repetition of 1 kHz and a central wavelength of 800 nm. The pulse width was ∼35 fs. The Gaussian beam diameter is ∼12 mm and it is focused on the fiber core, through PM by an f = 15 mm plano-convex acylindrical lens (AL). The PM has a uniform pitch of 1485 nm, used for producing FBGs in silicon-based fibers with a second-order Bragg resonance around 1080 nm. It should be noted that we did not use the first-order Bragg resonance here, because it requires a uniform PM with a pitch of 742.5 nm, and the pitch of the PM is smaller than the wavelength of the inscribing light source, which is difficult to obtain. The laser beam was swept periodically across the fiber core at ∼3 µm/s by dithering the fiber using a piezo-actuated translation stage. During the grating inscription, the fiber is connected to an ytterbium broadband source via a circulator on one end and to an optical spectrum analyzer (Yokogawa Co., AQ6370D, OSA) on the other end for monitoring the transmission or reflection spectra. What’s more, precise alignment of the laser focus relative to the fiber core was achieved with the help of the nonlinear photoluminescence imaging technique as shown in Ref. [24].
2.2 Principle of selecting the distance between the fiber and the PM when writing type-I TTC FBGs
The distance between the fiber and the PM is crucial when writing type-I TTC FBGs. PM is a specially designed optical diffraction element, where fs-pulses incident perpendicular to PM are diffracted into multi-level diffraction pulses, as shown in Fig. 1. It’s necessary to place the fiber in the area where the incident pulses diffracted into the ±1-orders produce a pure two-beam interface pattern when writing type-I FBGs, so the distance L between the fiber and the PM should satisfy [29]:
Fig. 1. Schemes of avoiding fiber coating damage: (a) keep the 0th order focus inside the fiber cladding; (b) move the focus of the 0th order pulse away from the fiber surface.
Finally, the spatial dispersion caused by the broadband spectrum of the ultrashort pulses also has a significant impact on writing TTC FBGs. As can be seen form Fig. 2(a), the chromatic dispersion caused by the PM makes the red components (i.e., long-wavelength spectral components) of the incident pulses are closer to the PM compared to the blue components (i.e., short-wavelength spectral components). The differences in the positions of the blue components and red components foci along the z-axis could be calculated by [30]:
Fig. 2. Focal elongation caused by (a) chromatic dispersion of the PM and (b) chromatic aberration of the AL.
It is obvious that focal elongation caused by PM and AL are spatially complementary. When selecting a PM-AL combination, for a given m and ${\lambda _0}$, there would exist a $L = {L_{optim.}}$ where $\Delta z_{mask}^{chrom.} = \Delta z_{lens}^{chrom.}$. At this position, the focal elongation is the smallest, and the laser is most tightly focused. In other words, when writing TTC FBGs, placing the fiber at $L = {L_{optim.}}$ could maximize the laser intensity at the fiber core and minimize the laser intensity at the fiber coating simultaneously, which could minimize the damage to the fiber coating caused by the focused fs-pulses. However, as can be seen from Eq. (3), the chromatic dispersion caused by the PM will decrease when using a PM with a larger pitch. In this case, it’s necessary to increase L to counteract the chromatic aberration caused by AL, which means that the fiber is moved away from the PM. Obviously, it’s much more difficult to write TTC FBGs using a PM with a larger pitch compared to the PM with a smaller pitch.
3. Results and discussion
3.1 Type-I TTC FBGs inscribed in fibers with various geometries and their strain responses
The fibers with different geometries used in this work were commercially available from Fibercore, which were coated by acrylate polymer. The fiber parameters including fiber geometry, mode field diameter, numerical aperture, and coating thickness are listed in Table 1. The PM used here has a poor 0th diffraction order suppression (the 0th diffraction efficiency is approximately 5%), therefore we move the focus of the 0th order pulse away from the fiber surface. Based on the principle described above, we calculated the distance L between the fiber and the PM, as shown in Table 2. It can be seen that when the distance L is larger than 0.66 and 0.45 mm for fibers with diameters of 245 and 170 µm, respectively, the focus of the 0th order pulse has been away from the fiber surface. Furthermore, the focal elongation caused by PM and AL are cancelled when L = 1.18 mm. In order to find the position where the laser is most tightly focused, we first place the fiber approximately 1.2 mm behind the PM, and then fine-tune the position of the PM to determine whether the laser is tightly focused in the fiber core based on the intensity and length of the nonlinear fluorescence generated in the optical fiber.
Table 1. Fibers evaluated
Table 2. The distance between the fiber and the PM when writing type-I TTC FBGs
SM980 (5.8/125) was first used for writing TTC FBGs. The average power of the writing beam was set as 170 mW (i.e., the laser pulse energy was 170 µJ), and a FBG with a transmissivity greater than −40 dB at the Bragg resonance was written after a few-second exposure. The transmission spectra are shown in Fig. 3(a). In addition, it is noted that there exists noise in the FBG transmission spectra shown in Fig. 3(a), which is determined by both the intensity of the ytterbium broadband source and the power sensitivity of the OSA. The power sensitivity of the OSA we used is −85 dBm around 1080 nm, while the intensity of the ytterbium broadband source we used is approximately −45 dBm around 1080 nm, so the spectra shows noise below −40 dB.
Fig. 3. (a) The transmission spectra of a FBG inscribed through the acrylate coating. (b) Inspection of the fiber coating after the FBG inscription procedure.
The coating of the fiber was inspected under a fluorescence microscope and no damage was observed, as shown in Fig. 3(b). It should be noted that since we use a fluorescence microscope for observation, the period of the FBG (0.7425 µm) is close to the limit resolution of the microscope, and due to the small modulation of the grating, the grating structure cannot be observed under the microscope we used. Therefore, we do not provide a microscopic image of the grating structure here.
Next, TTC FBGs were written in all SM980 fibers with various geometries, as shown in Table 1. Among them, the cladding-diameter of the SM980 (3.7/125), SM980 (4.5/125), and SM980 (5.8/125) is the same, but with different core-diameters. SM980 (4.5/125) and SM980 (4.5/80) have the same core-diameter, but with different cladding-diameters. Figure 4 shows the transmission and reflection spectra of the FBGs written in those four fibers. The transmissivities of the four gratings are all approximately –4 dB and the Bragg wavelengths of the four gratings are 1082.182 nm, 1080.798 nm, 1080.14 nm, and 1080.512 nm, respectively. It should be noted that we did not inscribe FBGs with a high transmissivity here, but chose FBGs with a lower transmissivity, which is to facilitate finding the Bragg wavelength of FBGs during the strain testing. We also noted that the Bragg wavelength of the FBG written in fibers with a small-core-diameter was longer than the Bragg wavelength of the FBG written in fibers with a large-core-diameter when the fibers have the same cladding-diameter. Because the reduction of the core-diameter is achieved by increasing the Ge doping levels of the fiber core, which increases the effective index of the fiber core and hence creates a Bragg resonance at longer wavelength for a given PM. The higher Ge doping level also increases the photosensitivity of the fiber to ultrafast infrared radiation [16]. In addition, FBGs with a transmissivity greater than −8 dB cannot be written in fibers with a cladding-diameter of 80 µm, since the acrylate coating has been severely damaged at this time. This phenomenon could be further improved by: a) using an AL with a shorter focal length to increase the ratio between the laser intensity at the core and at the coating interface, or b) hydrogen or deuterium-loaded to the fibers prior to the inscription process to improve the photosensitivity of the fibers to ultrafast infrared radiation.
Fig. 4. Transmission/reflection spectra of FBGs written in fibers with: (a) SM980 (3.7/125); (b) SM980 (4.5/125); (c) SM980 (5.8/125); (d) SM980 (4.5/80).
The strain response of TTC FBGs inscribed in SM980 fibers with different geometries were studied. The schematic of the strain testing system is shown in Fig. 5. Both ends of the FBGs are fixed on the manual stages (MS) by an epoxy resin adhesive (ERA, product number: 801), with a displacement accuracy of 10 µm and the stroke is ±12.5 mm. It should be noted that since the ERA could corrode the acrylate coating, which may cause loosening at the glue point, so we removed the coating of the attached section of the fiber on the MS. The shift of the Bragg wavelength was monitored according to the increasing strain, using an OSA, and the testing results are shown in Fig. 6. Among them, Fig. 6(a) and (b) show the strain sensing characteristics of the TTC FBGs and the FBGs without acrylate coating, respectively. It can be seen that for fibers with a cladding-diameter of 125 µm, the maximum strain of TTC FBGs could achieve ∼31000 µɛ, while only ∼7000 µɛ for the FBGs without acrylate coating. The measured maximum strain of TTC FBGs is only ∼56% of that reported previously [8,23], because the fiber coating is corroded by the ERA in our experiments, causing the fiber to eventually break near the glue point (red cross in Fig. 5), so we did not obtain the ultimate tensile performance of our TTC FBGs. In addition, the maximum strain of TTC FBGs in the fiber with an 80 µm cladding-diameter was measured to be only ∼11000 µɛ. It implies that the acrylate coating may be greatly damaged during the inscription process, resulting in the decreased mechanical strength. At the same time, the coating of the fiber was also inspected under an optical microscope, the black coloring is observed and there is no shrinkage of the coating, as shown in Fig. 7. This indicates that a little damage to the coating is catastrophic for the mechanical reliability of the device.
Fig. 6. Measured wavelength sensitivity to strain for all four FBGs inscribed. (a) TTC FBGs; (b) FBGs without acrylate coating.
Furthermore, polymer optical fibers (POFs) may more suitable for measuring strain owing to their unique features, such as lower Young’s modulus, larger elastic strain limit, and higher fracture toughness [31]. Therefore, POFs may be used to further extend the range of the strain limit. However, some of the drawbacks of POFs are their high losses and their viscoelastic nature, which leads to they have a nonlinear response when strained and relaxed [32]. Besides, viscoelasticity could introduce hysteresis and creep effects when cyclic loading is applied to the fiber, which can influence the accuracy of the sensor reading. In addition, it well known that under high tension loading or cyclic tension loading, the fiber strength is reduced due to the stress-corrosion effects [33]. But fortunately, optical fibers also have an apparent fatigue endurance limit below which fatigue-induced damage and failure will not occur. Therefore, in real application, it is recommended that optical fibers operate under fatigue conditions below the endurance limit to avoid sensor failure [34].
Table 3 shows the strain sensitivity of FBGs inscribed in fibers with various geometries. For SM980 (5.8/125), the measured strain sensitivities of the TTC FBGs and FBGs without acrylate coating are 0.8538 and 0.9265 pm/µɛ, respectively. It could see that the FBGs without acrylate coating have higher strain sensitivity compared to the TTC FBGs, which may because the coating of the FBGs affects its strain transfer. Besides, for TTC FBGs, the measured strain sensitivities for FBGs written in SM980 (3.7/125), SM980 (4.5/125), and SM980 (5.8/125) are 0.9002, 0.8819, and 0.8538 pm/µɛ, respectively. Obviously, FBGs written in fibers with small-core-diameter are more sensitive to strain when the fibers have the same cladding-diameter, which may be caused by the waveguide effect [9]. FBGs without acrylate coating also follows the same rule. What’s more, it is noted that the measured strain sensitivities for TTC FBGs written in SM980 (4.5/125) and SM980 (4.5/80) are 0.8819 and 0.8688 pm/µɛ, respectively, which indicates that FBGs written in fibers with small-cladding-diameter present a decreased strain sensitivity, which is consistent with the results shown in Ref. [8]. While FBGs written in fibers with different-cladding-diameters of identical material characteristics have the same responses to temperature change, which enables us to realize simultaneous measurement of strain and temperature [35].
Table 3. Sensitivity of FBGs to strain
As mentioned above, changes in temperature or strain could tuning the Bragg wavelength of FBGs, so there may be crosstalk between temperature and strain in the above testing. Although we did not specifically consider temperature compensation during the testing, but the entire testing was completed in a constant temperature ultra-clean laboratory, and since the indoor temperature change did not exceed ±1 °C. In addition, the duration of each testing is very short. Hence, the effect of the temperature crosstalk on the testing could be ignored.
In addition, FBGs written in fibers with small-cladding-diameter are more sensitive to stress, because the sensitivity of FBGs to stress can be expressed as:
where ${S_\varepsilon }$ is the sensitivity of the FBGs to strain; r is the radius of the fiber; E is Young’s modulus. Obviously, the sensitivity of FBGs to stress scales inversely with the square of the fiber radius. In addition to being more sensitive to stress, fibers with small-cladding-diameter are more robust against bending. This allows smaller packaging sizes to be achieved, particularly useful for coiled sensors and the next generation small form factor telecoms equipment, which makes it more adapted for fiber smart sensing. In addition, when the optical-fibers-based sensors were embedded inside the composite materials, the mechanical performance of the materials could be unfavorably influenced [36], so fibers with small-cladding-diameter are very adequate for structure health monitoring of composite materials in their total life cycle [37], which can decrease the mismatch in size between embedded optical fibers and composite materials.3.2 Type-I TTC FBGs based continuously strain tunable DBR fiber laser
Further, we studied the application of the type-I TTC FBGs in DBR fiber lasers. The configuration of the DBR fiber laser is shown schematically in Fig. 8. The lowly-reflective FBG (LR-FBG) is first inscribed into the YDF through the coating by femtosecond laser and then the highly-reflective FBG (HR-FBG). The distance between the two FBGs was 1.8 cm. It should be noted that since we cannot directly observe the grating structure using our microscope, so we used a PM with a larger period to inscribe FBGs under the same conditions to estimate the length of the FBGs we inscribed. The lengths of the LR-FBG and HR-FBG were approximately 4 mm, and the effective cavity length was calculated to be 1.92 cm. The parameters of the YDF used in the experiment are shown in Table 4. In addition, the center wavelength of the laser diode (LD) we used here is 976 nm. Figure 9(a) shows the spectra of the LR-FBG, Fabry-Perot (F-P) cavity and the laser, the transmissivity of the LR-FBG is approximately −15.7 dB. The laser wavelength and power are 1079.988 nm and 31.43 mW when the pump power is set as 100 mW. The output power of the laser under different pump powers is shown in Fig. 9(b). It can be seen that the laser power increases linearly as the pump power increases and its slope efficiency is approximately 27.17%. It should be noted that there is currently no evidence to prove that using this technology could improve the slope efficiency of the DBR fiber lasers, because the laser slope efficiency is only determined by the gain coefficient, the length, and the loss of the resonator cavity. In Ref. [38], the author inscribed a ∼0.75% FBG through the coating, and used it as the output couple of an erbium-ytterbium co-doped fiber laser. Their results show that erbium-ytterbium co-doped fiber lasers with stripped and coated output couples have similar slope efficiencies.
Fig. 8. Experimental setup of the DBR fiber laser. Both FBGs are written in the gain fiber while maintaining the acrylate coating intact. ISO: isolator, WDM: wavelength division multiplexer.
Fig. 9. (a) Spectra of the LR-FBG, F-P cavity, and the laser. (b) Output power measurements under different pump powers.
Table 4. Gain fiber evaluated
Figure 10(a) shows the beat frequency spectrum of the laser, which measured by an electrical spectrum analyzer (Agilent Technologies, E1104B) with a resolution of 1 kHz. It can be seen that there exists a beat frequency signal at 5.344 GHz, which is equal to the longitudinal mode interval of the laser, indicating that the laser does not operate in a single-frequency state. Figure 10(b) shows the RIN at a pump power of 100 mW in the range of 0-20 MHz. The relaxation oscillation frequency is 880 kHz, with a peak value of −97.334 dB/Hz. The RIN is approximately −128 dB/Hz at frequencies of over 3 MHz.
Fig. 10. (a) The beat frequency spectrum of the laser. (b) RIN in the range of 0-20 MHz. (c) Laser output wavelength and intensity versus strain. (d) Laser output spectra at different strains.
Then, in order to study its wavelength strain tuning characteristics, both ends of the DBR laser were fixed to the two manual stages as shown in Fig. 5, respectively. The splice point between the LR-FBG and WDM was placed on the manual stage and covered by the ERA for protection. Both LR-FBG and HR-FBG were kept in the strain action range. The pump power is set to 100 mW during the testing process. The variation of the central wavelength and the spectra of the output laser for the resonant cavity with different strains were measured and shown in Fig. 10(c) and (d), respectively. It can be seen that the laser wavelength is red-shifted from 1080.064 nm to 1099.998 nm in the range of 0-21265.8 µɛ. The tunable range is up to 19.934 nm, which limited by the splice point between the gain fiber and the tail fiber of the WDM. The laser output wavelength and strain show a good linear relationship (regression coefficients of over 99.9%). The strain sensitivity of the DBR laser wavelength was 0.9345 pm/µɛ, which was higher than that of the TTC FBGs in SM980. The difference in strain sensitivity may be attributed to the different doping of the fiber. During the tuning, the signal-noise ratios of the laser output were always better than 60 dB, while the output power of the laser fluctuates within the range of −6.72 to −5.06 dBm. In addition, it is noted that the output power of the laser approximately varies periodically, which may be caused by the laser operates with multi-longitudinal modes and mode-hopping occur during the tuning process. This TTC FBG based DBR fiber laser could be used as not only a continuously tunable laser but also a strain sensor with a higher resolution.
It should be noted that limited by the splice point between the gain fiber and the tail fiber of the WDM, 19.934 nm is not the ultimate limit that this method could be tuned. Theoretically, this method could realize a wavelength tuning of 51.4 nm around 1080 nm, corresponding to ∼55000 µɛ [8,23]. In addition, it should be noted that due to the large bandwidth of the LR-FBG we inscribed, the laser does not operate in a single-frequency state even if use a shorter laser resonant cavity. Future research will focus on reducing the bandwidth of the LR-FBG to achieve single longitudinal mode output of the laser.
4. Conclusion
We demonstrate the applications of the type-I TTC FBGs in strain sensing and tunable fiber lasers. The principle of selecting the distance between the fiber and the PM when writing type-I TTC FBGs was discussed. Type-I TTC FBGs were successfully written in fibers with different geometries through the coating and their strain sensing were studied. It was found that the strain sensitivity of the TTC FBGs increases as the core-diameter decreases attributed to the waveguide effect. A TTC FBGs based continuously tunable DBR fiber laser was also constructed. The laser wavelength is red-shifted from 1080.064 nm to 1099.998 nm when the strain of the laser resonant cavity is varied from 0 to 21265.8 µɛ. The tunable range is up to 19.934 nm and the strain sensitivity is 0.9345 pm/µɛ. The RIN is close to −128 dB/Hz at frequencies of over 3 MHz when the pump power is 100 mW. It has broad application prospects in the fields of requiring fast wavelength tuning fiber lasers and strain sensors with a higher resolution.
Funding
National Key Research and Development Program of China (2019YFA0706402HZ); National Natural Science Foundation of China (62027822, 61905192).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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