Fiber Bragg gratings inscribed using 800 nm femtosecond laser and a phase mask in single-and multi-core midIR glass fibers

For the first time, Fiber Bragg grating (FBG) structures have been inscribed in single-core passive germanate and three-core passive and active tellurite glass fibers using 800nm femtosecond (fs) laser and phase mask technique. With fs peak power intensity in the order of 10 11 W/cm 2 , the FBG spectra with 2 nd and 3 rd order resonances at 1540 and 1033nm in the germanate glass fiber and 2 nd order resonances at ~1694 and ~1677nm with strengths up to 14dB in all three cores in the tellurite fiber were observed. Thermal responsivities of the FBGs made in these mid-IR glass fibers were characterized, showing average temperature responsivity ~20pm/°C. Strain responsivities of the FBGs in germanate glass fiber were measured to be 1.219pm/με. 2009 Optical Society of America OCIS codes: (320.7140) Ultrafast process in fibers; (060.3735) Fiber Bragg gratings. References and links 1. S. Shen, A. Jha, X. Liu, M. Naftaly, K. Bindra, H. J. Bookey, and A. K. 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Introduction
Germanate and tellurite glass fibers have attracted considerable attention to be explored for fiber devices in near and mid-IR regions.Their high refractive index and optical nonlinearity, resistance to corrosion, low melting temperature and good transmission properties from the visible to mid-IR region (0.35-6µm) [1] make them promising fiber hosts for bio/chemical and gas sensing [2][3], nonlinear optical signal processing [4] and optical amplifier and laser [5][6] devices.In recent years, we have also seen that there are increasingly reports for multi-core fibers (MCFs).MCFs can offer new device designs such as arrayed fiber sensors for load and bend measurement with direction recognition and temperature compensation capability [7][8] and arrayed fiber lasers and amplifiers which can scale up output powers [9].Very recently, germanate and tellurite glass fibers with single-and three-core have been fabricated [10][11], and more significantly, the three-core tellurite glass fiber has been actively doped with Ho 3+ /Tm 3+ /Yb 3+ , Er 3+ /Ce 3+ , and Tm 3+ /Yb 3+ respectively in each core and the emissions from the visible to mid-IR have been observed [12].
Single-and multi-core passive and active germanate and tellurite glass fibers represent a new class fiber host for devices which will extend photonics applications to mid-IR range, which is in increasing demands.Fiber Bragg grating (FBG) structures have been proven as one of the most functional in-fiber devices and vastly produced in silicate fibers by UVinscription for countless laser and sensor applications.However, because germanate and tellurite fibers absorb UV greatly, FBG structures cannot be produced in such fibers by UVinscription.In recent years, femtosecond (fs) lasers have been developed for laser machining and microstructuring in a variety of glass fibers and planar substrates.A number of papers have been reported on fabrication of FBGs and long-period gratings in optical fibers and also on the photosensitivity mechanism using 800 nm fs lasers [13][14][15][16][17][18].
In this paper, we demonstrate for the first time the fabrication of FBGs in single-core passive germanate and three-core passive and active tellurite glass fibers by 800 nm fsinscription.Using a 1697.33 nm period phase mask, the 2 nd order FBG resonances with strength up to ~14 dB have been achieved in these mid-IR fibers and their thermal and strain responsivities have been characterized.

Fabrication and characteristics of germanate and tellurite glass fibers
The used germanate and tellurite fibers were drawn from glass compositions (mol%): 55GeO 2 -30PbO-11Na 2 O-4Ga 2 O 3 (GPNG) and 79TeO 2 -14ZnO-7Na 2 O (TZN), respectively for passive fibers, whereas for the active fiber the composition was 80TeO 2 -10ZnO-10Na 2 O.The methods for fiber fabrication are described elsewhere [10][11].The GPNG fiber has an undoped single-core with a diameter of ~8 µm surrounded by a cladding of ~126 µm diameter.The refractive indices of the core and cladding are 1.8622 and 1.8505 at 633 nm, respectively.The passive TZN fiber has three cores with diameters of ~12 µm and arranged equilaterally from the center with a separation of ~35 µm between each core, and surrounded by a cladding of ~130 µm diameter.The refractive indices of the TZN fiber core and cladding are measured as 2.0475 and 2.0224 at 633 nm, respectively.The active three-core TZN fiber has a similar form to the passive one, but with smaller core diameters, ~6 µm, and all three cores were doped with Er 3+ /Ce 3+ .However, it has a double-clad structure with the inner and outer cladding diameters of ~68 and ~120 µm with refractive indices of 2.0224 and ~1.9900 at 633 nm, respectively.The cross section images of the single-core GPNG and three-core passive and active TZN fibers are shown in Figs.1(a)-1(c).Both GPNG and TZN fibers have much lower softening point for glass fabrication at approximately 480 °C and 320 °C, respectively, which are substantially lower than that (~1200 °C) of silica glass.

FBG structures inscribed using femtosecond laser and phase mask
The FBG structures were produced in the GPNG and TZN fibers using fs-inscription through a custom-designed phase mask with a period of 1697.33 nm.Because of high refractive indices of the GPNG and TZN fibers, the phase mask was aiming to fs-inscribing 2 nd order FBGs around 1500-1800 nm.The 800 nm fs laser power was from an amplified Ti:sapphire system with a repetition rate of 1kHz and a maximum output energy of ~1 mJ.The pulse duration measured by an autocorrelator was <120 fs.The output beam with a radius of 5 mm was focused in y-axis by a cylindrical lens of 30 mm focal length and through the phase mask to the fiber core.Due to the focusing effect of the high refractive index of the cladding, the laser beam was highly focused.Hence the width of fs-inscribed grating structure along x-axis, as shown in Fig. 2(b), is merely ~2 µm.Such a thin-layer of FBG structure would not give a high reflectivity and also in the experiment it was easy to miss the core region.To ensure maximum coverage of fs-inscription in the core region, the incident beam was scanned along the x-axis from -30 to 30 µm with an incremental step of 2 µm for the single-core GPNG fiber and from -50 to 50 µm for the three-core TZN fibers.After each shift, the translation stage was paused for 3 min letting the fs-exposing the focused fibre position.In order to eliminate the phase error which might be induced by the multi-scan along x-axis, the fibre was placed very close to the phase mask in the y-axis direction.It was noted that the fs power level is critical for inscribing FBG structure in the core without burning the fiber.Via systematical tests, we identified critical fs energy levels of ~22 and ~14 µJ to be suitable for inscribing FBGs in the GPNG and TZN fibers.After fs-inscription, we first examined the FBG fringe structures under the microscope.For all inscribed GPNG and TZN fiber samples, we have seen clear fringe structures.Fig. 3(a) shows the top view (from the laser incident direction) image of an inscribed three-core TZN fiber and we can see all the three cores are fully covered by the continuous fringes.It is noted that, the different sizes of the imaged cores are due to different focus position of the microscope.Fig. 3(b) exhibits the side view image, the grating structure covering the almost the whole section depicts a typical three beams interference pattern.We measured the fringe space from the microscope image is 1.687 µm, which is close to the phase mask period, instead of a half of it.This is attributed to the Talbot effect [19] induced by the group-velocity walk-off of phase mask order pairs [14].

Transmission properties
A super continuum broadband light source from Koheras with the output wavelength up to 1800nm was employed to examine the grating spectra of the fabricated FBGs in single-core GPNG and three-core TZN fibers using butt-coupling method.In order to ensure the light can be coupled into the individual cores of the three-core TZN fibers, we first examined the nearfield patterns from the output end of the fiber for different coupling positions.Figures 4(a)-4(e) are the near-field images captured by an IR camera when the light was coupled into the outer cladding, inner cladding and the three individual cores, respectively.When in the nearfield the cores were seen clearly, as shown for example in Figs.4(c)-4(e), the output signal was then butt-coupled using a fiber lead to an optical spectrum analyzer to view FBG spectrum.Following this procedure, the spectra of all FBGs made in GPNG and TZN fibers were fully characterized.Due to higher refractive index and larger size of the core, the passive three-core TZN fiber is actually multimode fiber at ~1694 nm.This is evidenced by the multiple Bragg resonance feature exhibited by the FBGs in all three cores, as shown in Figs.6(a)-6(c).In the fabrication, the fs beam was more focused to the core A, thus we see the grating in core A is the strongest, giving 14.3 dB transmission loss for the lowest mode and the FBG resonances in core B and C achieved a similar strength in an order of ~4 dB.As a prototype device, this result is encouraging and more uniform FBG structures in all three cores should be possible by rotating the three-core fiber and focusing the fs beam to the targeted core during the inscription as shown in Fig. 2(b).FBG structures were also fs-inscribed in the active three-core TZN fiber and the 2 nd order FBG spectra of the three cores are shown in Figs.7(a)-7(c).Here, we only see single resonance in each core indicating the fiber is single mode structure, because its core diameters are smaller than that of passive TZN fiber.The 2 nd FBGs are more uniform across the three cores, as we see the strengths of the three resonances at 1678.05 nm, 1676.05nm and 1676.85 nm in the three cores are 10.6, 5.6 and 6.8 dB, respectively.The refractive index of the TZN fiber core at ~1677 nm can be estimated as ~1.9760.It is noted from Fig. 6 and 7 that some pronounced type of Fabry-Perot structures appear at the longer wavelength side of the Bragg resonances for the TZN fibers, which is different from the normal FBG spectrum.A similar phenomenon was observed for UV-inscribed B/Ge fiber [20] and the simulation suggested that this may be caused by a negative refractive index modulation induced during inscription.This explanation needs to be verified by further experiment in TZN fibers using fs-inscription.

Thermal responses of the FBGs in germanante and tellurite glass fibers
We then further examined the thermal responses of the fs-inscribed FBGs in GPNG and TZN fibers by subjecting the grating fibers to elevating temperatures.The wavelength shifts of the 2 nd and 3 rd order Bragg resonances of GPNG fiber at 1540 nm and 1033 nm versus the temperature changes from 10 °C to 70 °C with increments of 5 °C are shown in Fig. 8(a).The measured thermal responsivities are 24.71 and 16.80 pm/°C, respectively.Fig. 8(b) shows the thermal responsivities of the active three-core TZN fiber for elevating temperature from 10 °C to 60 °C.The thermal responsivities of cores A -C are measured as 20.21, 18.61 and 19.25 pm/°C, respectively.
The thermal responsivity can be expressed as: is the thermal expansion coefficient, and β = dT dn is the thermo-optic coefficient.For GPNG fiber, α and β given in [21] are 1.09×10 -5 /°C and 9.0×10 -6 /°C.Therefore, using the value of refractive index n eff =1.8156, the thermal responsivity for 1540 and 1033 nm can be calculated as 24.43 and 16.39 pm/°C.For TZN fiber, using α=1.86×10 - /°C, β=-1.64×10 - /°C from [22] and n eff =1.9760, we can derive the thermal responsivity for 1676 nm with the value of 17.26 pm/°C.From the above calculation, the experimental thermal responsivities are in good agreement with the theoretical ones.

Strain responses of the FBGs in germanante glass fibers
The strain response of the fs-inscribed FBGs in GPNG fibers was also examined by applying longitudinal strain to the grating fibers.The strain sensitivity and normalized strain sensitivity of the 2 nd order Bragg resonance of GPNG fiber at 1540 nm were measured to be 1.219 pm/µε and 0.7912×10 -6 /µε respectively, as shown in Fig. 9(a).These values are consistent with results from strain applied to the fiber configured as a Fabry-Perot interferometer [22].The optical phase change with strain and the normalized strain sensitivity of GPNG fiber using Fabry-Perot interferometer at 1540nm are 5902×10 3 rad/m and 0.7910×10 -6 /µε respectively, as shown in Fig. 9(b).The strain responses of germanate glass fiber from the FBG and Fabry-Perot interferometer measurement were consistent with each other with only 0.025% difference in the normalized strain sensitivity.Strain sensitivity of FBGs can be expressed as: Optical phase change per unit strain per unit fiber length of the Fabry-Perot interferometer can be express as: Then the normalized strain sensitivity of the fiber from FBGs and F-P interferometer can be expressed as: where β is the propagation constant of the mode in the fiber, ε is the longitudinal strain applied to the fiber, µ is Poisson's ratio, p 11 and p 12 are the strain-optic coefficients.Using p 11 =0.225, p 12 =0.235and µ=0.232 for germanate glass with composition (mol%) of 30PbO-10Bi 2 O 3 -60GeO 2 from [23], the strain sensitivity of FBGs and optical phase change with strain of F-P interferometer for 1540nm can be calculated as 1.270 pm/µε and 6318×10 rad/m, respectively.The normalized strain sensitivity of germanate glass fiber at 1540nm can be derived with the value of 0.8243×10 -6 /µε, which is 4% higher than the value from experimental measurement.From the above calculation, the experimental strain responses are in good agreement with values calculated using published data for fiber with similar compositions, and the small difference between these experimental and predicted values may be due to the slightly different glass compositions being compared.The strain properties of TZN fiber were also examined using the F-P interferometer method as introduced in [22].The optical phase change with strain and the normalized strain sensitivity of TZN fiber at 1540nm are 5621×10 3 rad/m and 0.6787×10 -6 /µε respectively, and the Young's modulus was measured to be 36.7 GPa.Table 1 gives the strain properties of GPNG fiber and TZN fiber from optical measurement and calculation.
From Table 1, we can conclude that the Fabry-Perot experiments show that the phase change per unit length per unit strain (dφ/dL) in tellurite fiber is 4.8% smaller than in the germanate fiber; this proportion is consistent with values calculated using published data for fibers of similar compositions, although the absolute values from experiments are ~6.6%lower than those derived from published properties.Even though tellurite fiber has a higher refractive index, its effect is offset by a larger strain-optic term, which reduces the phase change under strain.However, if we consider the phase change per unit length per unit stress ( ), the smaller Young's modulus of tellurite fiber compared to germanate leads to a larger response by a factor in the ratio of the measured moduli (51.8 / 36.7)i.e. ~1.4.Hence tellurite fiber would be advantageous as a load sensing element.

Conclusions
We have for the first time successfully inscribed FBGs in germanate and tellurite glass singleand multi-core fibers using 800 nm femtosecond laser with pulse width of 120 fs and a phase mask with the pitch of 1697.33 nm.With peak power intensities in the order of 10 11 W/cm 2 , FBGs in GPNG and TZN fibers were achieved.In the GPNG fiber, the 2 nd and 3 rd order Bragg resonances are at 1540 and 1033 nm with strengths of 6.5 and 1.2 dB.In the passive and active three-core TZN fibers, the 2 nd order resonances at situated at ~1694 and ~1677 nm with strongest peaks exhibiting reflectivities of 14.3 and 10.6 dB, respectively.
In addition, we have characterized thermal responsivities of the FBGs made in two mid-IR fibers.The thermal responsivities of the 2 nd and 3 rd order Bragg resonances in GPNG fiber were measured as 24.7 and 16.8 pm/°C and that of the 2 nd order resonances of the three individual cores of TZN fiber were measured as 20.2, 18.6 and 19.3 pm/°C, respectively.The experimental thermal responsivities are in good agreement with the theoretically calculated ones.The strain and normalized strain sensitivity of 2 nd order FBGs in GPNG fiber were also measured as 1.219 pm/µε and 0.7192×10 -6 /µε respectively, which is consistent with the value of 0.7190×10 -6 /µε from F-P interferometer measurement.The experimental strain responses are also in good agreement with values calculated using published data for fiber of similar compositions.The strain properties measurement of germanate and tellurite fiber also shows that tellurite fiber would be advantageous as a load sensing element because of the smaller Young's modulus compared to germanate fiber.

Fig. 1 .
Fig. 1.Microscopy images of the cross-sections of (a) GPNG single-core fiber, (b) TZN passive and (c) active three-core fibers.Note: the D-shape-like end images are attributed to cleaving defects, as the fibres are not standard and fragile to achieve nice-finish cleaving.

Fig. 2 .
Fig.2.Schematic diagram of FBG inscription system using fs laser and phase mask with a TZN MCF mounted.The x axis in (b) is parallel to the phase mask and perpendicular to the grooves; and the y axis is normal to the phase mask.

Fig. 3 .
Fig. 3. Microscopy images of grating structure from (a) the top view and (b) side view of the TZN passive fiber.In both (a) and (b), the periods of the structures are measured as 1.687 µm.

Figure 5 (
Figure5(a) shows the 2 nd order Bragg resonance of the GPNG fiber at 1540.42 nm with the strength of 6.5 dB.We also observed a very weak 3 rd order resonance at 1033.74 nm with the strength of only 1.2 dB, as shown in Fig.5(b).From the wavelengths of these two resonances, we calculated the refractive indices of the GPNG fiber as ~1.8156 and ~1.8271 at 1540 and 1034 nm, respectively.It is noted that the overall transmission losses in Figs.5-7are about 6-12dB, which are mainly attributed to the butt-coupling, as GPNG and TZN fibre ends cannot be cleaved perfectly like normal commercial fibers and unmatched numerical apertures of the high index (~1.82and ~1.98) mid-IR fibers and the low index (~1.45)normal single mode fiber.

Fig. 7 .
Fig. 7. 2 nd order FBG transmission spectra in (a) -(c) the core A-C of the active TZN MCF.

Fig. 8 .
Fig. 8.The wavelength shift of (a) the 2 nd and 3 rd order resonances of the GPNG fiber at ~1540 and 1033 nm and (b) the 2 nd order resonances of the core A-C of the active three-core TZN fiber at ~1677 nm against temperature change.

Fig. 9 .
Fig.9.The strain sensitivity of the 2 nd order resonances of (a) the GPNG fiber at ~1540 nm and (b) the optical phase change with strain per unit length of GPNG fiber at ~1540 nm from F-P interferometer.