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We compare and contrast novel techniques for the fabrication of chirped broadband fiber Bragg gratings by ultrafast laser inscription. These methods enable the inscription of gratings with flexible period profiles and thus tailored reflection and dispersion characteristics in non-photosensitive optical fibers. Up to 19.5 cm long chirped gratings with a spectral bandwidth of up to 30 nm were fabricated and the grating dispersion was characterized. A maximum group delay of almost 2 ns was obtained for linearly chirped gratings with either normal or anomalous group velocity dispersion, demonstrating the potential for using these gratings for dispersion compensation. Coupling to cladding modes was reduced by careful design of the inscribed modification features.
© 2016 Optical Society of America
1. Introduction
Since their first demonstration by Hill et al. almost four decades ago [1], fiber Bragg gratings (FBGs) have become indispensable components of virtually all fiber-based optical networks and sensor systems. In particular chirped FBGs, i.e. broadband fiber gratings with variable period profile (see Fig. 1), have found important applications in chirped pulse amplification, amplified spontaneous emission suppression, dispersion compensation and sensing [1–4].
Fig. 1 Schematic representation of a chirped fiber Bragg grating. As the Bragg period is varied along the length of the fiber, different spectral components of the injected signal are reflected off different sections within the core of the fiber which can result in a high group delay dispersion and a broadband reflectivity spectrum.
The standard fabrication method for FBGs relies on the fibers’ photosensitivity, which means that the refractive index of the core changes with exposure to UV light. However, as this places significant limitations on the types of fibers that can be used, alternative manufacturing methods are being pursued. Over the past two decades, the femtosecond laser direct-write technique has matured into an efficient and flexible method for the fabrication of waveguides and gratings in bulk glass and optical fibers [5]. Femtosecond laser inscribed uniform and chirped fiber Bragg gratings have been demonstrated in silica, fluoride, phosphate and photonic bandgap fibers [6–14]. Furthermore, Large Mode Area (LMA) fibers were successfully modified via a phase-mask-based femtosecond inscription method [15]. In contrast to conventional UV laser fabrication, the use of femtosecond lasers allows direct inscription of FBGs into virtually all (i.e. also non-photosensitive) fibers through nonlinear ionization processes. Furthermore, modifications can be positioned with complete control anywhere within the core or cladding of the fiber, enabling more sophisticated grating designs [16,17].
Several methods for realizing chirped period profiles via the femtosecond laser phase mask scanning technique were demonstrated by Thomas and Voigtlander et al. [18–20]. Variations in the grating period were achieved by applying fiber bending, deformed wavefront or a spatial light modulator allowing the Bragg wavelength to deviate a few nanometers from the design wavelength of the phase mask. Using a highly chirped phase mask, Bernier et al. were successful in fabricating ultrabroadband FBGs via filamentation of high-energy femtosecond laser pulses [21]. However, the need for a specifically designed pre-fabricated phase mask limits the flexibility of this approach. In order to allow for arbitrary grating designs, a different fabrication technique is required. In the point-by-point (PbP) method, a femtosecond laser beam is tightly focused into the core of an optical fiber, modifying a volume that is limited to the focal volume of the laser beam. By translating the fiber relative to the focus, a periodic pattern can be inscribed [9]. Depending on the pulse energy of the writing laser, two different types of modification can be distinguished: Type I index changes or Type II damage modifications [22–25]. Whereas the latter typically results in stronger gratings, an advantage of gratings based on Type I modifications over those based on Type II modifications is the lower level of broadband losses. Aslund et al have shown that in Type II PbP gratings, Mie scattering losses dominate over other loss mechanisms [26]. Average loss values for Type II PbP gratings were estimated to be 1-2 dB/cm for the highest net reflectivity [27]. As a result of light scattering, the reflectivity spectrum of Type II IR inscribed chirped FBGs is not constant over the grating bandwidth and lower levels of reflectivity are obtained for wavelength components that correspond to a Bragg period spaced further away from the injection side of the grating, see Fig. 1. Thus, the reflectivity spectrum depends on which side of the FBG is used as the signal input. In contrast, a less than 0.1 dB/cm loss level was demonstrated for uniform gratings based on Type I modifications [28]. In addition, it has been shown that gratings based on Type I and Type II modifications, respectively, exhibit different annealing properties [29].
In this paper we compare and contrast three different methods for the fabrication of chirped fiber Bragg gratings via direct inscription with femtosecond laser pulses. We demonstrate that a nonlinear variation in Bragg period along the length of the grating can be used to offset wavelength dependent losses in PbP gratings based on Type II modifications and show that a fairly uniform reflectivity can be obtained across a 12 nm bandwidth. Further, for the first time to our knowledge, we utilize a continuous core-scanning technique to inscribe linearly chirped gratings with a bandwidth of up to 30 nm and a group delay of up to 2 ns. Finally, we introduce a modified core-scanned technique that offers an improved flexibility in grating design as well as a reduced coupling to cladding modes.
2. Methodology
A Ti:Sapphire femtosecond laser (Spectra Physics Hurricane), emitting pulses with a duration of 112 fs at a wavelength of 800 nm was used for the inscription. The laser pulses were focused inside the core of an optical fiber (Corning SMF-28e) using a 20 × oil-immersion objective with an NA of 0.8. Depending on the inscription method, the pulse energy, repetition frequency and pulse overlap were varied as summarized in Table 1.
Table 1. Inscription parameters
The exact setups for the PbP and the core-scanned inscription method have been described elsewhere and can be found in [16] and [28], respectively. In order to inscribe chirped gratings, the fiber was not translated with constant velocity (as in [16] and [28]), but an acceleration profile was programmed into the stage controlling the longitudinal movement of the fiber. Resulting traces for different inscription methods are presented in Fig. 2.
Fig. 2 Schematic (left) and DIC image (right) of the femtosecond laser inscribed patterns within the fiber for the three different inscription methods: (a) point-by-point, (b) continuous core-scanned and (c) modified core-scanned.
The modified core-scanned inscription method is schematically outlined in Fig. 3 and the resulting square-shaped trace that is formed in the core of the fiber is shown in Fig. 2(c). The fiber was placed into a groove, 130 µm in width, that was cut into a glass substrate using a picosecond laser and subsequently covered by a microscope coverslip. Index-matching oil was applied between the two glass plates and between the coverslip and the objective to eliminate aberrations. The glass plates with the fiber resting in-between were moved as a single unit by a programmable, high-resolution air bearing translation stage (Aerotech ABL2000 for the fiber axis and FibreAlign 130 US for the transverse axes).
Fig. 3 Schematic of the modified core-scanned inscription setup. 1: focusing objective; 2: glass substrate with V-grove to hold the fiber and a 100µm thick glass coverslip on top; 3: Aerotech FA130 US translation stage, controlling the movement of the x- and z-axis; 4: Aerotech ABL2000 air-bearing translation stage, controlling the movement of the y-axis; 5: silica glass fiber with protective coating removed.
All gratings were inscribed with a Bragg wavelength close to 1.5 µm in order to simplify their characterization using standard telecommunications equipment. Reflectivity spectra and insertion losses were measured using a swept wavelength system (JDSU SWS 15100), tunable across the C-band (1520 nm to 1570 nm). The insertion losses per grating length were estimated to match the results, reported in [27] and [28] for the Type II-IR and Type I-IR modification, respectively. Transmission spectra were referenced to an unmodified fiber of the same length whereas reflection spectra were referenced to a 100% reflective mirror. Group delay measurements were performed utilizing the built-in capability of the SWS system with a 0.1 nm measurement step.
3. Results and Discussion
3.1 Point by point chirped gratings
A high level of scattering losses is typical for Type II PbP gratings, of which each period consists of a chain of sub-micron sized voids [30]. In the case of narrow-bandwidth (uniform) gratings that are used in high power fiber laser applications those losses can be tolerated as their impact on the overall system performance has been shown to be minimal [31]. However, the situation changes in the case of the linearly chirped gratings with broad reflectivity spectra. Wavelength components that are reflected off grating periods located further away from the injection side experience higher scattering losses and thus produce a weaker reflection response than wavelength components that are reflected off periods close to the injection side of the FBG. Thus, chirped gratings with a bandwidth as small as only 370 pm have been demonstrated using the PbP method to date [16]. In this paper, for the first time to our knowledge, we demonstrate that the fabrication of gratings with bandwidths up to 12 nm is possible if the scattering losses are taken into account by specifically designing PbP gratings with a Bragg period that varies nonlinearly along the length of the grating. With respect to the injection side, the increment change in period has progressively been slowed down towards the back end of the grating. This means that the effective grating length per unit bandwidth has been increased for wavelength components reflected off FBG segments further away from the injection side. The resulting increase in reflectivity for those wavelength components now offsets the higher level of scattering losses. Figure 4 shows the measured reflection spectrum of a 1.8 cm long 1st-order chirped PbP grating with nonlinear chirp. It can be seen that the reflectivity response is relatively flat for the short-period injection side (i.e. the design-input side), while probing the reflectivity spectrum from the long-period side (i.e. the back end of the grating) results in a strongly reduced reflectivity response towards shorter wavelengths. The grating bandwidth reached 12 nm and the measured group delay between the shortest and the longest reflected wavelength components was measured to be 150 ps as expected from a 1.8-cm long grating. The average reflectivity reached ~50% while relatively high levels of ripples are visible in the reflection spectrum (up to ~5dB) and in the group delay response (~11%). In addition, it can also be seen in Fig. 4 that due to the nonlinear chirp that is required to offset the scattering losses, the introduction of higher order dispersion is unavoidable which limits the freedom the grating offers in terms of dispersion engineering.
Fig. 4 Reflection spectrum of a 1.8 cm long nonlinearly chirped point-by-point grating with a 12 nm bandwidth. The black trace shows the reflection response measured from the short-period side, while the red trace shows the response measured from the long-period side. Also shown is the dispersion characteristic of the grating with clearly visible higher-order dispersion.
3.2 Core-scanned chirped gratings
We have recently introduced a novel continuous core-scanned method for the fabrication of strong narrowband first-order gratings with low scattering loss [28]. In this work we have now applied this method to fabricate a range of linearly chirped 1st-order gratings based on Type I modifications written across the core of the fiber (see Fig. 2(b)). A typical reflection spectrum for a 10 nm bandwidth grating is shown in the Fig. 5.
Fig. 5 Reflection spectrum of a 19.5 cm long linearly chirped continuous core-scanned grating with a 10 nm bandwidth, which exhibits properties that are independent of the injection side of the grating. Also shown are the measured total group delays for the short and the long-period injection sides showing that the FBG can selectively introduce normal or anomalous dispersion.
The reflectivity reached an average of 40% over the grating bandwidth and the reflectivity spectrum is virtually independent of the injection side of the grating in stark contrast to the nonlinearly chirped point-by-point grating shown in Fig. 4. The ripple in the reflection- and group delay spectra was also lower, measured to be 3 dB and 8%, respectively. Due to the linear chirp, the magnitude of the group delay dispersion (GDD) was constant over the grating bandwidth and was measured to be about 10 ps/(nm·cm). Since the losses are reduced, the grating can be used from either injection side, therefore providing either normal or anomalous dispersion. Table 2 summarizes our results using the continuous core-scanned method to fabricate linearly chirped FBGs with a bandwidth from 5 nm to 30 nm and a physical length of 19.5 cm. The average in-band reflectivity of the gratings ranged from 40% for the 5 nm wide grating to 13% for the 30 nm wide grating while the magnitude of the corresponding dispersion parameter Dλ ranged from 1.98 - 11.69 ps/(nm·cm).
Table 2. Results for chirped core-scanned gratings
All gratings were inscribed over a total length of 19.5 cm (the maximum travel range of our translation stages) to demonstrate the potential of this approach for writing long FBGs with very high levels of group delay. The total group delay that is introduced over the grating bandwidth can be estimated by the difference in effective path length for the shortest and the longest wavelength components in the reflection spectrum, respectively. For Corning SMF-28e fiber with a refractive index n = 1.468 at 1550 nm and a fiber length of L = 19.5cm, the total group delay thus becomes GD = 2·L·n/c = 1908 ps in agreement with the measured value of 1930 ps. The dispersion of SMF-28e fiber at a signal wavelength of 1550 nm can be estimated as Dλ = S0/4·(λ-λ04/λ3) = 16.17 ps/(nm·km), where S0 = 0.086 ps/(nm2·km) is the zero dispersion slope and λ0 = 1313 nm is the zero dispersion wavelength. Our results suggest that a core-scanned grating inscribed over a length of 19.5 cm could be used to compensate the dispersion of approximately 120 km of standard single-mode fiber.
The possibility to write FBGs over such long distances can also be beneficial in sensing applications. Gilbertson et al. showed that chirped fiber Bragg gratings can be used to measure the detonation wavefront position and velocity changes across material interfaces for high explosives. It was found that longer gratings result in an improved mapping accuracy in measurements used for the explosion control [32]. Further, chirped FBGs could also be used as distributed temperature and pressure sensors, even in extreme environments such as nuclear power plants [33] where a very long single chirped grating could potentially replace multiple narrowband FBGs in multi-wavelength sensing systems.
In order to maximize the refractive index contrast, and thus the strength of the gratings, the energy of the writing pulses was kept just below the damage threshold of the fiber core. A previous study of a femtosecond inscription in a silica fiber showed that the refractive index contrast could also be improved with an increased inscription pulse density, i.e. a larger overlap between successive writing pulses. However, we found that an increase in pulse overlap also results in reduced grating strength, which we attribute to the increase in the thickness of the modification planes. For that reason, the pulse density was kept below 2 −2.5 pulses/µm. All the core-scanned gratings were inscribed at first-order in order to avoid excessive tilt of the modification planes (see Fig. 2(b)) and associated increase in grating loss. To overcome these issues, the modified core-scanned technique was developed (see Fig. 2 (c)) to enable the fabrication of gratings with highly increased inscription pulse densities.
3.3 Modified core-scanned chirped gratings
The modified core-scanning method was used to fabricate linearly chirped 4th-order gratings with high pulse overlap. Figure 6 shows a typical reflection spectrum of a 10 mm long grating that was inscribed with the pulse overlap of 100 pulses/µm. The bandwidth of the FBG was 5 nm and the reflection reached ~25% with a relatively low ripple of ~1dB compared to the ~3dB for the core-scanned gratings. The ripples in the reflectivity spectra were found to be almost independent of the grating strength. Again, probing the grating from either end did not significantly change the reflection spectrum. The reduction in reflectivity ripple is believed to be a direct result of the increased Bragg period for the 4th-order grating as the inscription process is limited by the resolution of the translation stages which restricts the minimum incremental increase in Bragg period over the length of the grating. For a 4th-order grating, the increment length change is quadrupled compared to a first-order grating, resulting in a smoother reflectivity response.
Fig. 6 Reflection spectra for a 10 mm long 4th-order chirped FBG inscribed with the modified core-scanned technique with light injected at either ends of the grating.
As discussed earlier, an additional source of loss for continuous core-scanned gratings is attributed to the non-vertical positioning of the grating planes. An analysis of the transmission spectra for gratings with vertically oriented planes (modified core-scanned, Fig. 2(c)) and tilted planes (continuous core-scanned, Fig. 2(b)), reveals a remarkable difference in cladding mode resonances. Albert et al. have shown that the coupling strength to backwards propagating cladding modes is proportional to the tilt angle [34]. For the core-scanned gratings, inscribed at first order at 1550 nm, the tilt angle of the individual planes was calculated to be around 1.5° in our case and the tilt angle is proportional to the Bragg wavelength. As a result, losses due to coupling to cladding modes will increase with the Bragg wavelength or grating order. A comparison of the cladding mode resonances for modified core-scanned (vertical planes) and continuous core-scanned gratings (tilted planes) is shown in Fig. 7. Both gratings were inscribed in first order over a length of 10 mm in SMF-28e at a Bragg wavelength of 1545 nm. It can be seen that the grating written with the modified core-scanned technique has a reduced level of cladding mode coupling. The ratio of the coupling coefficient for the cladding modes to the coupling coefficient for the Bragg wavelength is κcladding/κBragg = 0.15 for the modified core-scanned grating, compared to κcladding/κBragg = 0.23 for the core-scanned grating Fig. 7.
Fig. 7 Cladding mode resonances in the transmission spectrum of a modified core-scanned grating with vertical planes (red trace) and a continuous core-scanned grating with tilted planes (black trace). Both gratings are 10 mm long uniform gratings and have been inscribed with identical pulse energy and overlap.
Another mechanism responsible for an increase in cladding mode resonances is a displacement of the grating position from the center of the core. Thomas et al showed, that the cladding mode resonances can span over a full octave in the spectrum and become very pronounced (in excess of 20 dB) for inhomogeneous and asymmetric FBGs [35]. Thus, in our experiments a correct alignment (symmetrical to the fiber axis) of the modification planes to the core of the fiber plays a pivotal role and was carefully maintained during the inscription process.
Cladding mode resonances, although an unwanted side effect in most application, can also be exploited in fiber sensing systems as cladding modes are highly susceptible to changes in the refractive index of the surrounding atmosphere [34]. The modified core-scanned technique offers almost significant flexibility over the grating design, hence highly tilted FBGs could also be inscribed in a variety of optical fibers. Moreover, the grating planes could potentially be moved from the core of the fiber into the cladding for mode-filtering applications in multimode fibers [36].
4. Conclusions
Direct femtosecond laser inscription of chirped fiber Bragg gratings has been demonstrated and three different methods have been compared and contrasted. By nonlinearly chirping point-by-point gratings based on Type II modifications, a flat reflectivity response could be obtained over a 12 nm bandwidth. Utilizing a continuous core-scanning approach based on Type I modifications, linear chirped gratings with a bandwidth ranging from 5 nm to 30 nm have been fabricated with greatly reduced losses compared to PbP-written gratings. Those gratings also offer very high values of group delay dispersion and can be used to introduce normal as well as anomalous group velocity dispersion simply by changing the light injection side. Finally, a modified core-scanned method has been introduced that offers complete flexibility over the design of the grating planes which can be utilized to reduce unwanted coupling to cladding modes.
Acknowledgments
This research was conducted with the Australian Research Council Centre of Excellence for Ultrahigh Bandwidth Devices for Optical Systems (project number CE110001018) and the assistance of the LIEF programs. This work was performed in-part at the OptoFab node of the Australian National Fabrication Facility, utilizing NCRIS and NSW state government funding. S. Antipov acknowledges the support of an international Macquarie University Research Scholarship. R. Williams acknowledges the support of a Macquarie University Research Fellowship.
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