High temperature stable fiber grating sensor and method for producing same

Field of the Invention

0001The present invention relates to a sensing apparatus and method for measurement of pressure, strain, temperature, or displacement or index of refraction of a high temperature environment and more particularly to a fiber Bragg grating sensor inscribed therein capable of measuring temperature, strains, and environment at temperatures up to 1000 °C.

Background of the Invention

0002Fiber Bragg grating sensors (FBG sensors) have demonstrated themselves to be attractive devices for sensing temperature and strain along an optical fiber. Variations in the spectral response of the grating result from period changes in the Bragg grating due to strains or temperature variations that are experienced by the in-situ optical fiber. These FBG sensors offer important advantages over other sensor technologies because of their electrically passive operation, electromagnetic interference (EMI) immunity, high sensitivity and multiplexing capabilities. Fiber Bragg gratings are simple, intrinsic sensing elements which traditionally have been UV photo-inscribed into photosensitive Ge-doped silica fiber. Each FBG sensor has a characteristic retro-reflective Bragg resonance or Bragg wavelength, which is dependent upon the periodicity of the grating photo-inscribed within the fiber and the effective refractive index difference in the grating regions of the optical fiber. The FBG sensors can then easily be multiplexed in a serial fashion along a length of single fiber. When embedded into composite materials, By continuing the exposure and grating inscription of the fiber in the type I regime such that the index modulation becomes about >3×10^-3, the threshold for type II grating formation is reduced in a continuous fashion until it traverses the grating inscription intensity, which in this instance was ∼1.5×10¹³ W/cm². The index modulation value is likely directly inversely proportional to the type II threshold intensity value.

0003Optical fibers with an array of FBG sensors allow for distributed measurements of load, strain, temperature and vibration of the material creating what has is commonly referred to as "smart structures" where the health and integrity of the structure is monitored on a real-time basis.

0004Typically fiber Bragg gratings are generated by exposing the UV-photosensitive core of a germanium doped silica core optical fiber to a spatially modulated UV laser beam in order to create permanent refractive index changes in the fiber core. Such a spatially modulated UV beam can be created by using a two-beam interference technique as disclosed in <patcit id="pcit0001" dnum="US4807950A"><text>US patent number 4,807,950 by Glenn et al</text></patcit>. or by using a phase mask as disclosed in <patcit id="pcit0002" dnum="US5367588A"><text>US patent number 5,367,588 by Hill et al</text></patcit>. The techniques taught by Glenn and Hill result in gratings that are typically referred to as Type I gratings.

0005A limitation of the prior-art UV-induced Type I fiber Bragg gratings, especially for high temperature sensor applications is that operation of the sensor at elevated temperatures results in the erasure or annealing of the UV-induced color centers and densification which are responsible for the induced index change of the grating. In fact, at temperatures approaching the glass transition temperature of the fiber, which for silica is approximately 1000 °C, total erasure of the induced index modulation results. The fiber also is modified at such high temperatures making it brittle with diffusion of the core material into the cladding. The fiber can easily be deformed by its own weight.

0006Another method for creating permanent photoretractive index changes in glasses employs the use of intense UV beams with fluences or energy/unit-area per laser pulse densities that approach those required to produce macroscopic damage of the glass. <patcit id="pcit0003" dnum="US5400422A"><text>Askins et al. in US patent number 5,400,422</text></patcit> teach a method for producing permanent photoretractive index changes in the photosensitive cores of Ge-doped optical fibers with single high intensity UV laser pulses. Such Bragg gratings resulting from macroscopic damage to the glass optical fiber are typically referred to as Type II gratings. The high intensity portions of the interference fringes created by two crossed UV beams split from a single UV beam create localized damage at the core-cladding interface within the fiber. Because the process for inducing index change is one of structural change due to localized physical damage to the glass, rather than due to UV photoinduced color center formation, the induced index change is more robust and does not decrease with elevated temperature. In fact Askins et al. disclose that gratings produced in this way cannot be removed by annealing until the fiber or waveguide approaches the material's glass transition temperature. The drawback of this approach for induction of index change is that the Bragg gratings produced in this fashion have relatively low refractive index modulations (Δn =10^-4) and are mechanically weak since the effective refractive index change results from periodic localized damage at the core-cladding interface. When the pulse duration is long (> a few tens of picoseconds) laser-excited electrons can transfer energy to the surrounding lattice faster than the thermal diffusion of the material can remove the energy resulting in damage. If the laser pulse continues to feed energy into the damage site, the damage can propagate beyond the irradiated zone. For damage grating structures written with long laser pulse durations greater than a few tens of picoseconds, the spectral quality of the resulting Bragg grating is often poor.

0007Another method for creating permanent photoretractive index changes in optical fiber employs the use of the process of "hydrogen-loading," as taught by<patcit id="pcit0004" dnum="US5287427A"><text> Atkins et al. in US. Pat. No. 5,287,427</text></patcit>, combined with UV-laser exposure of optical fiber that is manufactured with a core that is co-doped with fluorine. Subsequent to the UV exposure the fiber then undergoes a thermal post treatment at 1000 °C in order to induce a chemical composition grating as taught by <patcit id="pcit0005" dnum="US6334018B"><text>Fokine in US. Pat. No. 6,334,018</text></patcit>. As with the technique taught by Askins et al., the technique taught by Fokine also has the drawback that the induced index change of the Bragg gratings produced in this fashion have relatively low refractive index modulations (Δn =10^-4).

0008The fabrication of high temperature stable Bragg gratings using infrared ultrafast radiation and a phase mask, as taught by<patcit id="pcit0006" dnum="US6993221B"><text> Mihailov et al in US. Pat. No. 6,993,221</text></patcit> results in high temperature stable Bragg gratings with very high index modulations (Δn Δn >10^-3). As shown by <nplcit id="ncit0001" npl-type="s"><text>Smelser et al. Opt. Express., vol. 13, pp. 5377-5386, 2005</text></nplcit>, laser beam intensities greater than 4×10¹³<i>W</i>/<i>cm</i>² at the surface of the optical fiber result in the formation of thermally stable Bragg gratings similar to Type II UV-induced gratings, but with much higher index modulations. Another document by Smelser et. al. describing the formation of Bragg gratings using femtosecond pulses can be found in <nplcit id="ncit0002" npl-type="s"><text>Proceedings of SPIE Vol. 6796, pp. 67961E-1 - 67961E-13, 2007</text></nplcit>.

0009In the case of gratings fabricated using the techniques taught in <patcit id="pcit0007" dnum="US6993221B"><text>US. Pat. No. 6,993,221</text></patcit>, the index modulation results from a threshold type process of multiphoton absorption/ionization that results in plasma formation and the possible creation of microvoids. Although strong gratings can be formed using the approach taught in <patcit id="pcit0008" dnum="US6993221B"><text>US. Pat. No. 6,993,221</text></patcit>, the resulting gratings suffer from high scattering loss making it difficult to concatenate a large number of Bragg grating sensors into a sensor array on a single length of optical fiber. The threshold nature of the process also makes it more difficult to tailor the induced index profile of the grating in terms of its apodization, reflectivity and reflection bandwidth. Using the technique taught by <patcit id="pcit0009" dnum="US7031571B"><text>Mihailov et al in US Pat. No. 7,031,571</text></patcit>, Smelser et al. showed that very high index modulations (Δn >10^-3) could be created with laser beam intensities lower than 4 × 10¹³ W/cm² that did not possess high scattering loss however these gratings were not high temperature stable and the index modulation likely arising from color center formation and material densification, erased at temperatures > 800 °C.

0010The inventors have recently discovered that induction of extremely large index modulations (Δn > 3x10^-3) in Germanium doped telecommunication optical fiber using the technique disclosed by <nplcit id="ncit0003" npl-type="s"><text>Smelser et al. in Opt. Letters., vol. 29, pp. 2127-2129, 2004</text></nplcit> has the beneficial effect of being stable up to 1000 °C.

0011The thermal stability of the refractive index change that is generated using prolonged laser exposures consistent with a type I grating formation process may be caused by the extremely large index modulations that are generated. In one aspect of the present invention the inventors arrive at these high levels of index modulation for example 3x10^-3, through the process of hydrogen loading of Ge-doped silica fibers coupled with femtosecond infrared laser exposure through a phase mask.

0012In accordance with another aspect of the invention, an extremely high index modulation such as 3x10^-3 can be generated by femtosecond infrared laser exposure consistent with type I grating formation in optical fibers or waveguides which have high concentrations of core co-doping with for example Germanium. In <nplcit id="ncit0004" npl-type="s"><text>Grobnic et al Photon. Technol. Lett. vol. 20, no. 12, pp. 973-975, 2008</text></nplcit>, high NA, high Ge-doped core fibers are more photosensitive to femtosecond IR radiation than standard telecom (low Ge-core concentration) fibers. It is possible that large index changes induced in the high Ge-doped fibers have improved thermal stability. We believe that such high refractive index changes for example 3x10^-3 lower the intensity threshold sufficiently to yield a stable grating with minimal scatter.

0013It is an object of this invention to overcome the aforementioned limitations within the prior art systems for fabrication of high temperature FBG sensors by inducing large refractive index modulations in silica-based optical fibers that are relatively stable up to 1000 °C and which do not suffer from high scattering or insertion loss and.

Summary of the Invention

0014In accordance with this invention a method is provided as defined in claim 1. Further details of the present invention are defined by dependent claims 2-12.

0015In accordance with an aspect of this invention a method of writing a grating in a doped glass is provided wherein the non-linear absorption to IR irradiation is increased by hydrogen or deuterium loading of the doped glass prior to irradiating with infrared light having an intensity of at least 10⁹ W/cm² and less than 10¹³ W/cm², and wherein the hydrogen or deuterium loading of the doped glass lowers the intensity threshold of the glass. Preferably the glass is Ge doped. Aside from enhanced photosensitivity of Ge-doped silica caused by hydrogen loading, other dopant materials used for waveguides in silica that are photosensitive to UV exposure also benefit from improved photosensitivity by hydrogen loading. These include co-doping of Ge with Boron (B) or Tin (Sn). Other dopants that exhibit enhanced photosensitivity, exclusive of Ge, when hydrogen loaded are phosphorous (P₂O₅), silicon oxynitride (SiON or nitrogen doped), aluminosilicate (Al₂O₃) and aluminosilicate codoped with terbium (Tb³⁺:Al₂O₃), or cerium (Ce⁺³:Al₂O₃), or erbium (Er⁺³:Al₂O₃) or ytterbium (Yb⁺³:Al₂O₃) or Europium (Eu²⁺) or Tantalum (TaO₅) or Thulium. It is believed that the photosensitivity to high intensity ultrafast IR radiation of these dopants, or combination thereof, would be enhanced through hydrogen or deuterium loading.

0016In accordance with an another aspect of this invention the photosensitivity of said optical waveguide is arrived at through high concentrations of core dopants such as Germanium, but in the absence of hydrogen or deuterium loading.

Brief Description of Drawings

0017Exemplary embodiments of the invention will now be shown in conjunction with the drawings in which: <ul id="ul0001" list-style="none"><li><figref idref="f0001">Fig. 1 (a)</figref> shows the output of an amplified ultrafast laser being focused through a silica phase mask onto the core of an optical fiber.</li><li><figref idref="f0001">Fig. 1 (b)</figref> shows the diffracted phase mask orders combining to produce an interference pattern that is then inscribed into the photosensitive material.</li><li><figref idref="f0002">Fig. 2</figref> shows the evolution of the transmission loss at the Bragg wavelength (1550 nm) and the wavelength shift for a grating fabricated in hydrogen loaded SMF-28 fiber with a 4.28 mm pitch phase mask. The total wavelength shift is ∼1.8 nm, corresponding to a peak induced index change of ∼4×10^-3.</li><li><figref idref="f0003">Fig. 3</figref> shows the grating spectra that result from the fabrication of a low Δn (<1×10^-3) and high Δn (>3×10^-3) ultrafast infrared induced fiber Bragg grating in hydrogen loaded SMF-28 fiber with a 4.28 mm pitch phase mask.</li><li><figref idref="f0004">Fig. 4</figref> shows the isochronal annealing curves of 2 sets of 3 gratings. Each grating was annealed in steps of ∼150 °C per hour starting at -25 °C or room temperature. The set of 3 high Δn gratings retain in excess of 60 % of their initial peak index change at 1000 °C while the low Δn are completely annealed out below 900 °C.</li><li><figref idref="f0005">Fig. 5</figref> shows the isochronal annealing curve and wavelength shift for a grating that has been preannealed at a temperature of 1000 °C. Clearly there is no additional degradation in the induced index change and the wavelength shift is nearly linear with a slope of ∼15 pm/°C. There was no hysteresis in the initial wavelength after the grating was cooled back down to room temperature.</li><li><figref idref="f0006">Fig. 6</figref> is a plot of number of pulses versus grating strength indicating the evolution of the grating strength comparing a high energy type II grating versus writing a type I followed by what we believe is a type II grating but having less damage than a standard type II grating.</li><li><figref idref="f0006">Fig. 7</figref> is a graph indicating the spectral characteristics of a type II grating.</li><li><figref idref="f0007">Fig. 8</figref> is a graphs of temperature versus refractive index and shows the evolution of the grating refractive index after one hour at room temperature and 1000 °C.</li></ul>

Detailed Description of the Invention

0018Germanium doped silica based optical devices, specifically those formed in optical fiber, are an attractive medium for high temperature sensing applications (&lt;1100 °C). Fiber Bragg gratings are an attractive sensing device as the wavelength of the Bragg resonance is temperature and strain dependent.

0019The fabrication of phase mask assisted ultrafast infrared induced fiber Bragg gratings is described in <patcit id="pcit0010" dnum="US7031571B"><text>US Patent number 7,031,571 by Mihailov et al.</text></patcit>, incorporated herein by reference. Ge-doped <i>Corning</i> SMF-28 fiber was loaded with molecular hydrogen at a pressure of 2600 psi and a temperature of 23 °C for 14 days and kept at -40 °C until it was exposed. Fiber Bragg gratings were then inscribed by focusing the output of an amplified Ti:Sapphire femtosecond 800 nm laser through a silica phase mask onto a fiber sample, as shown in <figref idref="f0001">Fig. 1a</figref> where the laser beam 100 is focused through lens 102 onto a phase mask 104. The output light from the phase mask 104 is then focused on the core 108 of the optical fiber 110.

0020In the writing of a Bragg grating the repetition rate of the laser was set at 100 Hz with a temporal pulse length of 125 fs. The focal length of the lens was 30 mm and the pitch of the phase mask was 4.28 µm. The fiber was positioned 3 mm beyond the phase mask in order to ensure that, due to phase mask order walk-off, only a pure two-beam interference pattern was inscribed in the fiber, as shown in <figref idref="f0001">Fig. 1 (b)</figref>. The incident laser pulse energy for all the gratings was set at 450 µJ, resulting in a peak interference field intensity of ∼1.5x10¹³ W/cm², which is below the threshold intensity required to produce a grating in unloaded SMF-28 fiber as shown in <nplcit id="ncit0005" npl-type="s"><text>Smelser et al. Opt. Exress., vol. 13, pp. 5377-5386, 2005</text></nplcit>. The incident laser beam had a lie intensity spot radius of∼3.2 mm. For a 30 mm focal length lens the resulting free space focal spot would be -5 µm x 6.4 mm. As the 5 µm focus is less than the -8 µm diameter of the fiber core, the beam is scanned vertically with a piezo-actuated stage ±15 µm with a total sweep period of 20 s. The evolution of the transmission loss of the gratings at 1550 nm was recorded continuously with a broadband erbium source and a spectrum analyzer.

0021For non-H₂-loaded low Ge-doped standard telecom fiber, the characteristic intensity threshold for Type II grating formation was shown by <nplcit id="ncit0006" npl-type="s"><text>Smelser et al. Opt. Express., vol. 13, pp. 5377-5386, 2005</text></nplcit> to be 4×10¹³W/cm². This characteristic intensity threshold is dependent upon the characteristics of the waveguide and every waveguide has an inherent characteristic intensity threshold. It has been demonstrated in <nplcit id="ncit0007" npl-type="s"><text>Smelser et al. Opt. Letters., vol. 29, pp. 2127-2129, 2004</text></nplcit> that fiber Bragg gratings formed using ultrafast lasers below the aforementioned Type II grating threshold intensity in both hydrogen loaded and unloaded Ge-doped telecommunications fiber are not stable at temperatures exceeding 800 °C when the initial induced peak index modulation is less than 1×10^-3. This type of fiber grating formation has been referred to as the Type I ultrafast IR induced regime. The low temperature stability of this type of index change is clearly demonstrated in <nplcit id="ncit0008" npl-type="s"><text>Smelser et al. Opt. Letters., vol. 29, pp. 2127-2129, 2004</text></nplcit> and by <nplcit id="ncit0009" npl-type="s"><text>Smelser et al. Opt. Express., vol. 13, pp. 5377-5386, 2005</text></nplcit>.

0022Phase masks with pitches that are integer multiples of the smallest pitch required to produce a resonance at 1550 nm in Ge-doped silica (1.07 mm) can easily be used to fabricate gratings with ultrafast laser sources. The modulation profile of the induced index change that results from ultrafast grating fabrication is not a pure sinusoid, where UV induced index change is initially sinusoidal, and will produce higher order resonances without the need for saturation of the induced index change. For larger phase mask pitches the magnitude of the resonance at 1550 nm is smaller than the it would be for a 1.07 mm pitch mask due to a smaller Fourier component corresponding to that Bragg resonance (<nplcit id="ncit0010" npl-type="s"><text>Smelser et al. Opt. Letters., vol. 32, 1453-1455, 2007</text></nplcit>). For high order phase masks, such as those with pitches of 3.21, 4.28, 5.35 mm's and higher, a very large index change will be induced in the medium before a significant resonance is observed at ∼ 1550 nm. The peak induced index change for a fiber grating is most reliably determined by the magnitude of the shift of the wavelength of the Bragg resonance as the grating is formed. The wavelength shift during the formation of the grating is an indicator of the contribution of the induced index change to the effective index of the core mode of the fiber. The evolution of the Fourier components of ultrafast induced gratings, as shown in <nplcit id="ncit0011" npl-type="s"><text>Smelser et al. J. Opt. Soc. Am. B., vol. 25, pp. 877-883 (2008</text></nplcit>), suggests that a total wavelength shift of ∼2.5 nm corresponds to a total peak induced index change of ∼ 5×10^-3 .

0023The spectral evolution of a grating written with a 4.28 mm pitch phase mask and the corresponding wavelength shift in Hydrogen loaded SMF-28 fiber is shown in <figref idref="f0002">Fig. 2</figref>. The loss at ∼1550 nm increases until it reaches a turning point, after which it appears to be arrested and begins to disappear. As the irradiation continues the loss reappears and eventually grows until it approaches 30 dB or higher. The peak loss shifts to a longer wavelength for the entire duration of the grating growth. The grating spectra for gratings fabricated with a transmission loss of 5 dB and 30 dB are shown in <figref idref="f0003">Fig. 3</figref>. The wavelength shift for the 30 dB grating is ∼1.8 nm. This would correspond to a peak index change of ∼4×10^-3.

0024The average isochronal annealing curves of a set of 3 small (<1×10^-3) and a set of 3 large (>3×10^-3) peak index change gratings are compared in <figref idref="f0004">Fig. 4</figref>. The error bars represent the standard deviation from the mean. The gratings have been annealed in steps of 150 °C from room temperature up to 1000 °C with the grating resting at each temperature for one hour. The gratings that have been formed with large index change retain a large amount (in excess of 60 %) of their initial induced index change for temperatures at temperatures of 1000 °C. Further exposure of the high index change grating to temperatures of 1000 °C for 100 hours resulted in no further reduction of index modulation. The small index change gratings are clearly completely annealed out at a temperature of ∼850 °C. By continuing the exposure and grating inscription of the fiber in the type I regime such that the index modulation becomes greater or equal to about 3×10^-3, the threshold for type II grating formation is reduced in a continuous fashion until it traverses the grating inscription intensity, which in this instance was ~1.5×10¹³ W/cm². We believe that the index modulation value is directly inversely proportional to the type II threshold intensity value.

0025The temperature dependent wavelength shift and normalized refractive index change for a large Δn grating formed with a 4.28 µm phase mask in hydrogen loaded SMF-28 fiber after it has been pre-annealed at a temperature of 1000 °C for 100 hours is shown in <figref idref="f0005">Fig. 5</figref>. There is no further diminishment of the peak index change and the wavelength shift is nearly linear with a slope of ∼15 pm/°C. No hysteresis in the initial wavelength at room temperature is observed after cycling.

0026The use of a higher order grating is advantageous as large peak index changes, corresponding with high temperature stability, can be induced in the medium before a large resonance is observed at 1550 nm. This is, however, not a necessary requirement, as large index changes induced with lower order masks should also exhibit heightened temperature stability.

0027In accordance with this invention, the high temperature stable grating sensor made with prolonged exposures of hydrogen loaded fiber is different from our previous disclosures in that previous high temperature stable gratings for sensors required exposures with intensities that resulted in standard type II structures (multiphoton ionization, void formation, etc.

0028With the technique disclosed here of using 'low intensities' as defined in our hydrogen loading patent <patcit id="pcit0011" dnum="US7515792B"><text>US 7, 515,792</text></patcit>, a thermally stable type I structure is created. However previously, lower values of index change that produced the gratings in the type I regime were not thermally stable and erased at higher temperatures. However, in accordance with this invention, by prolonging the low intensity exposure such that a very large index change is induced in the fibre (> 0.005), the majority of the index change is stable at 1000 °C.

0029A disadvantage of a standard type II process absent hydrogen or deuterium loading for producing high temperature stable grating sensors is that because the induced index change is generated through an all or nothing type of threshold process, it is more difficult to tailor the induced index profile of the grating (apodization, amount of reflectivity etc). With a couple of dozen pulses from the laser, very high index modulations are created that are thermally stable but sometimes difficult to finely control grating reflectivity and bandwidth. Disadvantageously the type II structures result in scattering or insertion loss (∼20 % per device), which makes concatenation of several of these devices in a sensor array not practicable. With a type I process, where a similar thermal stability is achieved but with 15,000 pulses, better control of the filter spectrum is possible. It is our understanding that the insertion loss of the thermally stable type I grating is similar to that of a standard type I grating so potentially hundreds of thermally stable type I gratings could be concatenated into a sensor array.

0030Advantageously, hydrogen or deuterium loading of the waveguide allows one to write what we believe to be a type II grating in a type I regime where more control is achieved by lowering the characteristic intensity threshold of the waveguide or fiber. Furthermore, prolonged writing provides a large refractive index difference of about 3x10^-3 which unexpectedly provides a grating that remains stable at 1000 °C so that at least 60% of the grating remains.

0031We have also discovered, in an alternative embodiment of this invention, that it possible to reduce the threshold intensity required to write a grating that will have desired characteristics of lower scattering than a standard type II grating would have, and permanence so that at least 60% of the grating is not erased when exposed to temperatures up to 1000 °C for long periods of time. As with the previously described examples, this embodiment although not requiring exposure to hydrogen or deuterium also requires a step of lowering the characteristic intensity threshold of the waveguide by at least 25% before irradiating the waveguide with femtosecond pulses of light.

0032While there are presently a number of techniques to fabricate high temperature FBGs, such as type IIA, FBGs in nitrogen doped silica fiber, and low reflectivity chemical composition gratings, we have noticed that only the Type II and chemical composition FBGs display stability at 1000 °C. However, standard type II Bragg gratings are basically damage gratings resulting from the interaction of high-energy radiation with the silica host. Initially type II FBGs were made with high intensity single UV pulses resulting in gratings with low spectral quality.

0033Recently high quality multiple-pulse type II gratings were inscribed in the core and cladding of different optical fibers with femtosecond infrared (IR) pulsed radiation using the phase mask method. In the case of type II gratings made with ultrafast radiation and a phase mask, the gratings are not pre-treated or pre-conditioned during fabrication to lower the threshold intensity and are fabricated by placing the fiber close to the phase mask so that the multiple beam interference will generate the high intensity pattern required. The resulting refractive index growth, as denoted by the increase of the strength of the Bragg resonance with the number of laser pulses, is complex and very rapid. This is shown in <figref idref="f0006">Fig. 6</figref>. After an initial exposure period with little or no growth in grating reflectivity, the spectral response grows very rapidly as compared to the growth of a type I grating. This growth pattern makes the control of the grating strength relatively difficult.

0034In accordance with this invention a new writing process for type II FBG structures is provided, wherein initially a defect/compaction related type I grating structure is written in the fiber core, followed by a writing process characteristic of type II gratings. Advantageously the resulting grating has the thermal characteristics of a type II grating but is written at lower energy densities than the normal type II femtosecond gratings and is of better spectral quality. In an alternative embodiment this method is also useful for writing gratings into any region of an optical waveguide such as the cladding and is not only limited to writing within the core region.

Experiment and Results

0035FBGs were written using a regeneratively amplified 800-nm ultrafast Ti:sapphire (<i>Spectra Physics-Spitfire</i>) laser operating in a femtosecond regime. The laser beam was focused through a 30 mm focal length cylindrical lens and a zero-order-nulled silica phase mask with 2.14 µm pitch into the core of SMF28 fiber. According to the Gaussian beam approximation the focal spot width was computed to be ∼10 µm with a length of 6.4 mm. The beam was scanned across the waveguide by dithering the focusing lens with 20 µm amplitude at 0.05-Hz frequency. The FBGs were monitored during the inscription time using an ANDO spectrum analyzer and a broadband source. The Type I grating is written first by positioning the fiber 2 mm behind the phase mask to ensure that the interference pattern generated by the phase mask was due to only the ±1 diffracted orders and the exposure was made with 1 mJ energy pulses at a repetition rate of 100 Hz. After the type I grating is written, the beam is blocked and the fiber moved closer to the phase mask at a position of approximately 500 µm behind the phase mask so that the multiple beam interference pattern will supply the high radiation intensity required for writing type II grating structures. The fiber and the lens are then re-aligned. The energy of the ultrafast pulses is set to 750 µJ and the laser repetition rate to 5 Hz. During the type II grating inscription, only a small erasure of the type I grating is observed. The type II structures continue to grow as can be seen in <figref idref="f0006">Fig. 6</figref>. The period of the type II grating is the same as the mask while the period of the type I grating is half the period of the mask

0036Prewriting the fiber with a Type I grating in the manner described preconditions the fiber so that the threshold intensity of the waveguide is lowered preferably by at least 25%. The steps of prewriting the fiber with a Type I grating are disclosed in <patcit id="pcit0012" dnum="US7031571B"><text>U.S. patent 7,031,571</text></patcit> incorporated herein by reference.

0037The spectral characteristics of this type II grating are shown in <figref idref="f0006">Fig. 7</figref> and are similar to those of type II gratings inscribed directly into the fiber with high energy, for example the high cladding mode. The broadband loss for high-energy type II gratings can range from 1 to 5 dB. In the case of the low energy type II grating, the broadband loss is below 0.5 dB for a large index modulation and the strength of the grating is easier to control since the grating growth is less rapid.

0038For high-energy type II gratings without the pre-existence of a type I structure, absorption of the radiation in the fiber is initially very low and increases along with the growth of the type II grating. It is likely then that the formation of the type II grating is helping to couple radiation into the fiber. This can also explain the initial requirement for high intensity radiation. When the type I grating pre-exists in the fiber, coupling of subsequent radiation facilitates type II grating growth at lower intensities and at a slower pace. The experiment was repeated using a high Germania doped fiber. In this instance it was possible to write type II gratings with less than 500 µJ/pulses.

0039In order to evaluate the stability of the refractive index generated in the writing process described above, the grating was annealed in a micro-oven at temperatures up to 1000 °C for 1 hour at each temperature. The results are presented in <figref idref="f0007">Fig 8</figref>. Up to approximately 400 °C, there is no degradation of the grating. Between 400-700 °C, the grating looses a small amount of strength probably due to erasure of the initial type I grating and then shows high thermal stability up to 1000 °C. The grating was further annealed at 1000 °C for 10 hours and showed no significant degradation and at least 60% of the grating remained.

0040In summary, there are at least two ways in which the characteristic intensity of an optical fiber can be lowered: by hydrogen or deuterium loading prior at writing a grating with femtosecond pulses or by prewriting a grating in the optical fiber as is described above. By performing this lowering of the intensity threshold a substantially permanent grating with little scatter results.