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The fabrication and characterization of high quality ultra-long (up to 1m) fiber Bragg gratings (FBGs) is reported. A moving phase mask and an electro-optic phase-modulation (EOPM) based interferometer are used with a high precision 1-meter long translation stage and compared. A novel interferometer position feedback scheme to simplify the fabrication process is proposed and analyzed. The ultra-long uniform FBGs show near perfect characteristics of a few picometers bandwidth, symmetrical, near theory-matching group-delay and transmission spectra. Grating characterization using optical backscattering reflectometry and chirped FBGs are also demonstrated. Limitations of the schemes are discussed.
© 2014 Optical Society of America
1. Introduction
Fiber Bragg grating fabrication techniques have been much refined since their discovery in 1978 by Hill et al. [1], but high quality ultra-long FBG fabrication (>10 cm) remains an experimental challenge. The traditional holographic fabrication technique relying on a phase mask [2] limits the maximum grating length to that of the phase mask. Longer FBGs were first obtained by stitching together a large number of FBGs written sequentially using the phase mask technique [3]. Following this scheme for ultra-long FBG fabrication, techniques were developed to lift the dependence on the phase mask and to allow the fabrication of ultra-long gratings of arbitrary profiles [4–8]. However, the fabrication of very high quality FBGs remains a challenge. Recently (2011), Chung et al. [8] published a modified scheme of the piezo-mounted phase mask technique, but noted the practical implementation as being extremely challenging and experimental results were far from ideal.
Ultra-long FBGs fabrication is motivated by applications such as dispersion compensation [9], narrow-band optical add-drop multiplexers [10], fiber lasers [11], speciality gratings for pulse shaping [12] and lately in signal processing for RF, microwave and THz regions [13]. Many of these applications become possible only if one can control the amplitude and phase of the grating with great precision. This has not been possible for long FBGs.
In this paper, we present recent results on fabrication of unmatched quality ultra-long FBGs. The fabrication is based on a piezo-mounted phase mask technique analog to Petermann et al. [7], as well as an electro-optic phase-modulator (EOPM) based technique developed and presented in a previous article by the authors [14]. In the first case, the phase mask is mounted on a piezoelectric stage driven under a ramp signal. With a certain repetition rate, a continuously moving fringe pattern is created at a fixed position, which is synchronized to a moving fiber. In the latter case, the same moving fringe pattern is obtained by introducing EOPM (Pockels cells) in each arm of a Talbot interferometer and driving them under the same, but opposing ramp signals. This technique has the advantage of having no mechanical inertia suffered by the moving phase-mask technique, and allows a significantly faster writing speed by several orders of magnitude – a great advantage for long FBGs. This improvement can be important in the case of hydrogen-loaded fiber for which the photosensitivity decays in time through hydrogen out-diffusion. Both techniques can be used to obtain arbitrary chirp, apodization and phase shift by varying the applied voltage and ramp signal frequency as shown in [15].
2. Fabrication setup
The EOPM technique was previously used in a configuration in which the fiber mount was fixed and the fiber was pulled with the fiber attached to a rotation stage [14]. This makes the optical alignment process trivial, but phase errors arising from frictional forces between the fiber and the mount, renders the writing speed unstable during the writing process, causing uncontrolled random phase-shifts to be incorporated in the grating [16]. Using a linear translation stage to move the fiber eliminates these issues.
Figure 1(b) shows a schematic of the piezo-mounted phase mask interferometer (mounted on another module that can be swapped easily), in which the phase mask is mounted on a piezoelectric stage, decoupled from the interferometer module to limit the impact of mechanical vibrations on the grating quality. Figure 1(a) shows the EOPM interferometer. In this case, the phase mask can be placed on the same mount since this technique is vibration free.
Fig. 1 Schematics of the grating writing Talbot interferometers. (a) EOPM interferometer (b) Piezo mounted phase mask interferometer.
The main challenges for high quality ultra-long fiber Bragg gratings are in obtaining a smooth and precise fiber movement, maintaining fiber alignment with respect to the beams over the translation length, and controlling the exposition parameters so that a single exposition is sufficient to obtain a strong and uniform FBG, provided that the fibre parameters are unchanging over the grating length. Multiple exposures are possible, but require a way to re-synchronize the fringe phases with the previously written grating, which can be challenging to implement experimentally. In our recent implementation, a 1-meter long Aerotech air bearing translation stage assisted by a Renishaw laser interferometer encoder is used to move the fiber. The use of an interferometer allows this translation stage to have a high positional precision during inscription, effectively minimizing possible grating errors due to dynamic misalignment. By using a continuous writing scheme instead of a multiple exposition scheme, off positioning of the translation stage can be averaged out easily. The writing setup itself is mounted on a granite stage to further minimize vibrations.
The fiber is held by two vacuum clamps at the ends of a 1 meter long invar bar. Invar was chosen for its stability with temperature changes that could occur during the fiber exposition. Six additional fiber supports are inserted between the clamps to reduce vibrations and mechanical bending of the fiber under its own weight. Even if they were perfectly aligned in a straight line, maintaining fiber alignment with respect to the UV-beams is not possible as the fiber hangs between supports. This effect is compensated for by mounting the interferometer on a two axis translation stage to follow the measured deviation in the position of the fiber along the 1 meter stage. This feature is critical for femtosecond writing of gratings, but is shown here that it can also be useful for UV writing. The fiber position mapping for a given fiber tension is done with two perpendicular cameras that track and record the fiber’s position before writing.
3. Writing process analysis
The interferometer position feedback scheme allows the beam position to stay within a 1 um deviation from the core center while minimizing additional inscription errors in the grating index profile. This also simplifies the otherwise critical alignment of the translation stage with respect to the interferometer. Figure 2 shows an example of fiber position mapping. It can be argued that this scheme can worsen the grating quality by introducing its own sources of errors. As Fig. 3 shows, these errors come from the slight slipping of the UV beam along the periods of the phase mask that occurs when there is a slight misalignment of the incoming UV beams or when the phase mask is not perpendicular to the fiber. The effect is equivalent to the grating writing technique of Cole et al. [15] where the phase mask or the fiber is moved at a small speed compared to the scanning speed to induce chirp and apodization along the grating.
Fig. 2 Example of fiber position mapping showing deviation versus fiber position along its length for both axes.
Fig. 3 Illustrations of (a) misaligned phase mask and (b) non perpendicular UV beam. As the interferometer is moved, the beam scans across the phase mask (in red).
If the phase mask is oriented incorrectly at an angle θ to the fiber axis, the change in the Bragg wavelength from moving the beam along the phase mask by Δy over a fiber displacement of Δx, is given by:
where is initial Bragg wavelength.Fig. 4 shows that a misaligned phase mask can have a clear impact on the spectrum of grating with a few picometers bandwidth by inducing an undesired chirp. Experimentally, this angle can be minimized easily by positioning the zeroth order beam and the high order diffracted beams to pass through the core of the fiber. For a 1-meter long uniform FBG with a ~2.5 pm simulated bandwidth, this alignment is critical to obtain theory matching spectra.
Fig. 4 Bragg wavelength shift as a function of the fiber position for different phase mask angle for the data shown in Fig. 2.
The impact of an incident beam that is not perpendicular to the phase mask will be similar and is shown in Fig. 5. Moving the interferometer vertically can only be done with the EOPM technique since the phase mask for the piezo technique is not directly mounted on the interferometer. As the interferometer is moved in the vertical direction, the Bragg wavelength changes with the following relationship:
which essentially gives the same dependence as previously for a misaligned phase mask. This angle can be minimized by simply reflecting the beam back along its own trajectory, and shows that the potential errors induced by slightly moving the interferometer can be effectively minimized with proper care. Proper alignment of the fiber is still important as any small deviation angle causes a small period change.Fig. 5 Bragg wavelength shift as a function of the fiber position for different incident beam angle for the data shown in Fig. 2.
4. Results
4.1 The EOPM interferometer
Figure 6 shows the transmission spectrum of a 30 cm long weak FBG fabricated with the EOPM technique and measured with a Luna optical vector analyzer (OVA) with a resolution of 1.58 pm. The hydrogen loaded fiber was exposed to 100 mW 266 nm Q-switched frequency quadrupled light from a Spectra Physics Nd:YVO4 laser. The beam was focused using two cylindrical lenses to a spot size of 50 × 200 µm. The total exposure time was 30 minutes, corresponding to a writing speed of 166 µm/s. A grating of such length and -7 dB transmission dip has a theoretical bandwidth between the first zeros of ~6 pm, which only gives ~5 measurable points within the bandwidth. This is very close to the 6.4 pm bandwidth observed in the transmission spectrum shown in Fig. 5. Longer gratings of narrower linewidth cannot be measured with precision using the OVA.
Fig. 6 Experimental (written with the EOPM technique) and computed transmission spectrum of a 30 cm FBG.
Figure 7 shows the group delay measurement of the same 30 cm long FBG. The group delay is symmetrical with respect to the Bragg wavelength and has a maximum delay of approximately -600 ps. From Fig. 6, we can also approximate the bandwidth to contain 5 points, in agreement with the theoretical bandwidth.
Figure 8 shows the dispersion spectrum calculated from the previous results. This high dispersion value may have promising applications for signal processing or for nonlinear optical applications. As will be apparent from the subsequent measurements of transmission and reflection using a different technique for longer gratings of high quality, we can surmise that the agreement between the simulated and measured dispersion is probably much better than the few points indicate in Figs. 7 and 8.
Fig. 8 Experimental and computed dispersion spectra of a 30 cm long FBG written by the EO modulater technique.
High quality 1-meter long FBGs with strong reflectivity were also obtained with the EOPM technique. Figure 9 shows the transmission and reflection spectrum of a −28 dB reflectivity, 1 meter long FBG characterized by tuning the wavelength of a single frequency Tunics laser with a linewidth of ~100 MHz (~0.77 pm) across the bandwidth of the FBG while measuring the transmitted or reflected power synchronized in time using an Ando optical spectrum analyzer set at a fixed wavelength with a resolution of 1nm. This technique allows the ASE from the laser to be suppressed significantly to better resolve the details of the spectra [17]. The measured FW to first zeroes (FWFZ) bandwidth corresponds to the simulated bandwidth of ~2.5 pm (~325MHz). The FBG was obtained at a fast writing speed of 750µm/s. The hydrogen loaded fiber was exposed to 100 mW 266 nm laser. The total writing time being only 22 minutes due to the high speed phase modulation allowed by this technique, errors from such sources such as room temperature variation, laser power variation and hydrogen diffusion are minimized. With a maximum writing speed being set to 20mm/s, a 1 meter long FBG can be written in less than a minute if desired. The reflection spectrum is reasonably symmetrical, showing stronger side bands only on the long wavelength side, hinting at a slight parabolic concave chirp, possibly due to the lay of the fiber. However, it is clear from the measured spectra that the agreement with the simulation can be considered to be good, indicating the excellent control over the long length of the grating.
Fig. 9 Experimental (colored) and computed (in dash) of (a) transmission spectrum and (b) reflection spectrum of a 1 meter long FBG written by the EOPM technique.
To further attest the capability of the technique, a shorter, strong 100 mm long FBG was written at a writing speed of 100 um/s. These characteristics allow the spectrum to be resolved conventionally with the 3 pm resolution JDSU Swept Wavelength System passive component spectrum analyser (SWS-OMNI). Figure 10 shows the experimental and computed spectra of the 100 mm long grating. The spectrum shows a −16.9 dB transmission dip and a theory matching bandwidth of ~24 pm. The characteristic of interest in the spectrum lies in how well the side lobes follow the theoretical spectrum, showing the high quality of the technique amplitude and phase control.
Fig. 10 Experimental (colored) and computed (in dash) of (a) transmission spectrum and (b) reflection spectrum of a 100 mm long uniform FBGs written by the EOPM technique. Sampling at 3pm, masks the resolution in the measured spectra.
4.2 Piezo-mounted phase mask interferometer
Figure 11 shows the computed and simulated transmission and reflection spectrum of a −1.6 dB reflection and 1 meter long grating now written with the piezo mounted phase mask interferometer and characterized by the wavelength tuning technique [17]. The hydrogen loaded fiber was exposed to only 5 mW of 213 nm, Q-switched, fifth harmonic radiation using a Xiton laser and moved at 180 um/s. As previously shown by the authors [18], the fiber’s photosensitivity at this wavelength is very high and in this case, only a small amount of power is required to obtain the required index change. This laser could only be used safely with the piezo mounted phase mask interferometer because of its shorter wavelength.
Fig. 11 Experimental (colored) and computed (in dash) of (a) transmission spectrum and (b) reflection spectrum of a 1 meter long FBGs written by the piezo-mounted phase mask technique.
By having the interferometer feedback applied to only one axis, this technique gains from an optimised fiber alignment along the z-axis. This deviation from a straight line is always affected by gravity, whatever the precision of the mounts’ alignment. For the fiber position mapping shown in Fig. 2 the grating’s quality was generally inferior to that obtained with the EOPM interferometer, as can be observed in Fig. 11 where the effects of a noticeable parabolic concave chirp appears on the long wavelength side. Contrary to the EOPM technique, such a spectral quality as shown in Fig. 9 was experimentally difficult to reproduce and obtain on a consistent basis. Furthermore, obtaining high reflectivity uniform FBGs of comparable quality to the EOPM technique with this piezo-mounted phase mask interferometer technique was not possible; since the phase mask cannot be moved rapidly without mechanical damping, a lower contrast arises from a higher ramp frequency. This can be minimized by having a ramp amplitude which corresponds to multiple grating periods per cycle. In the current configuration, it was possible to reach about 20 periods per cycle at low speed without suffering significant mechanical damping. By raising the number of periods per cycle, the natural apodization arising from Bragg wavelength detuning is exacerbated, limiting the maximum tunability of the Bragg wavelength.
Other issues can have an impact on the quality of gratings of such a long length and low index modulation, such as fiber cleanliness, laser power stability, temperature variations and uniformity in the fiber’s core diameter (and hence neff) and photosensitivity. Proper care needs to be taken to reduce these sources of errors as much as possible. Temperature was passively kept in a +/- ~0.1°C range during all the writing processes.
4.3 Optical backscatter reflectometer analysis
A useful tool to characterize long fiber Bragg grating is an optical backscatter reflectometer (OBR) as it can analyse the reflectivity amplitude along the length of the grating as well as the spectrum from a portion of it. A Luna OBR was used to evaluate the impact of the interferometer position-feedback on the FBG characteristics. Figure 12 shows the reflected amplitude with respect to position along a 1 meter long grating.
Fig. 12 Backscattering amplitude of two 50 cm long sections of a single 1 m long FBG. The left side of the FBG was written with the position feedback on and the right half of the FBG with interferometer fixed without feeback.
The first half of this grating was fabricated with the position feedback technique in the “on” position for both axes (using the EOPM interferometer) and the second half of the grating was fabricated with the interferometer at a fixed position under the same conditions. As the temperature was kept stable within a +/− 0.1°C window, it cannot account for the observed change in structure in the reflection amplitude. This shows the potential benefit of the interferometer position-feedback scheme.
The spectra from each half of the grating was also obtained from the OBR with a wavelength resolution of 1.2 pm and is shown in Fig. 13. The FBG obtained with the position feedback shows a narrow peak of ~3 pm. The FBG obtained with a classical fixed interferometer shows a weaker and slightly broadened peak of ~10 pm. The additional structure on right side of the reflection peak can be linked to a slight parabolic concave chirp, which is expected from the fiber’s position along the moving axis as per Fig. 2. This further shows the novel scheme’s potential for obtaining high quality long FBGs.
Fig. 13 Reflectivity spectra of two 50 cm long FBG written consecutively with and without the position feedback scheme.
4.4 Fabrication of chirped FBGs
The techniques presented here can be used to obtain arbitrary chirped and apodized gratings as was previously shown by the authors in [14]. Chirp, phase and amplitude can be controlled by applying the proper signal to the EOPM/piezo stage to obtain a specific filter characteristic.This section shows preliminary results on chirped FBGs. A chirped grating can be obtained by either sweeping the frequency of the EOPM/piezo or by varying the speed of the moving fiber. The frequency sweeping scheme can be used to obtain continuously chirped gratings and the varying speed scheme can be used to obtain step-chirped gratings as the translation stage is not designed to operate at a fixed and very low acceleration which is necessary to obtain a continuously chirped grating. Frequency sweeping was limited to a 500s window by the Agilent 33120A function generator and was then only used for short chirped gratings. With longer sweep times, longer gratings may be written. However, the principle remains the same.
Figure 14 shows the transmission spectrum of a 5 mm long continuously chirped FBG obtained by sweeping the piezo frequency. The piezo mounted phase mask interferometer was chosen for this grating as the B/Ge doped fiber [17] shows particularly high photosensitivity to the 213 nm wavelength laser, which is necessary to obtain strong chirped FBGs. The additional structures on both side of the transmission dip can be attributed to truncated apodization for this short grating.
Fig. 14 Experimental and computed transmission spectrum of a 5 mm long continuously chirped FBG fabricated by sweeping the piezo frequency.
Figure 15 shows the transmission spectrum of a 1 meter long step-chirped FBG in the B/Ge doped fiber (not hydrogen-loaded) fabricated by varying the writing speed between 14.992 and 15.028 µm/s in 35 increments with the piezo mounted phase mask technique with a writing power of 5 mW at a wavelength of 213 nm. This low speed results in a total writing time of 18.5 hours! At this speed, the grating is written at a rate of 2 periods per piezo ramp cycle, which limits the induced lowering of the contrast at the edges of the chirped grating. The maximum chirp for a certain grating length and strength using this varying writing speed technique is limited by the programmed speed resolution of 0.001 µm/s. The impact of such a limitation can be seen in Fig. 15 where ripples at the edges of the grating spectrum correspond to individual speed increments. Writing conditions such as temperature, hydrogen content and beam quality over such an extended period of time can vary so that it effectively affects grating’s quality. Under such circumstances, the use of the faster EOPM interferometer can be more appropriate. However, our scheme clearly demonstrates the possibility of extremely long writing times.
Fig. 15 Experimental and computed transmission spectrum of a 1 meter long step-chirped FBG fabricated by varying the writing speed.
It is also possible to obtain chirped gratings by slowly adding a bias to the voltage signal, but since it is already operating at a high voltage, the potential maximum chirp is limited to smaller values of typically less than a nanometer. As demonstrated by the authors in [14], a large detuning of the Bragg wavelength will suffer from lower contrast as it gets further away from the “natural” Bragg wavelength. This may be corrected by varying the modulation amplitude during writing process. Further chirped FBGs results and other types of high quality gratings will be demonstrated in a future publication.
5. Conclusion
In conclusion, we have demonstrated the use of two types of Talbot interferometers, one based on the moving phase mask and the other on the EOPM for the writing of high quality, ultra-long fiber Bragg gratings. Our novel approach for fabricating high quality, long fiber Bragg gratings relies on interferometer position feedback control. The alignment of the phase mask relative to the fiber and the incident writing beam has a significant impact on the spectra of ultra-long gratings, inducing chirp, adversely affecting its design characteristics. Near perfect spectrum of a 300 mm FBG with a −7 dB transmission dip, exhibited a measured bandwidth of only 6.4 pm close to the simulated bandwidth of 6 pm. Uniform FBGs up to 1 meter long with ~two million periods nearly all in phase were also demonstrated using the EOPM technique. This technique has demonstrated outstanding FBGs with near perfect, bandwidth limited characteristics. The narrowest bandwidth achieved so far for a strong −28 dB reflectivity, 1 m long FBG matched the simulated bandwidth of ~2.5 pm (~325MHz). These are, to our knowledge, the smallest measured bandwidths reported for any such uniform grating of these lengths.
Both optical backscattering reflectometry and tunable laser scanning techniques were unable to fully resolve their spectra, indicating shortcomings of these techniques. A modified laser tuning scheme had to be used to fully resolve the 1 m long gratings. However, the OBR was only able to confirm the enhancement of the grating’s quality when using a feedback scheme. Up to 1-meter long chirped gratings were also demonstrated with good quality despite a writing time of over 18 hours with the moving phase-mask technique, showing the outstanding capability of the writing system. Also demonstrated was the capability of the EOPM interferometer to significantly increase the writing speed of ultra-long FBG by orders of magnitude compared to the moving phase-mask scheme. Strong, near perfect 100 mm long uniform gratings were also demonstrated.
Our measurements also indicate that the grating spectrum is probably not limited by the fiber parameters such as core diameter and core-cladding refractive index difference (variations in the neff of the fiber mode).
These results open the possibility of fabricating high quality complex filters for signal processing as well as ultra high quality factor cavities for fiber lasers, especially with the EOPM technique.
Acknowledgments
This research was supported by the Canadian Foundation for Innovation, the Govt. of Canada’s Canada Research Chairs program and the Natural Sciences and Engineering Council of Canada’s Discovery grants program.
References and links
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