Grating formation with multiple phase mask orders

The ideal phase mask is designed to produce uniform FBGs patterns suppressing zeroth order diffraction and maximizing first order diffraction. As already noted the zeroth order is not fully suppressed; a few percent (3%) transmits while each of the first orders has nearly 35% of total transmitted power. Therefore, the resultant FBG pattern is not purely uniform as it results from the superposition of multiple interference patterns. Various studies have been conducted experimentally and numerically to investigate the contribution of other orders in addition to the first orders. Dragomir et al. (2003) used Differential Interference Contrast (DIC) microscopy to obtain non-destructive images of Type I FBG fabricated using the phase mask method. These images confirmed the formation of a complex structure that is due to the interference of various diffraction orders. These results are depicted in Figure 2.12 and Figure 2.13 in which Dragomir (2004) and Rollinson et al. (2005) reported DIC images of FBGs.

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Figure 2.12: DIC images of the fibre core containing Bragg grating, recorded for two different fibre orientation (a) the images was taken from a direction parallel to the writing UV laser beam and (b) after rotating 90 degrees from (a) (Dragomir, 2004)

Figure 2.13: DIC microscopy images of FBG in core region of Type 1 fibre at different orientation to the writing beam (a) Images of fibre orientation is perpendicular to the writing beam (b) Images of fibre orientation is parallel to the writing beam (c) schematic diagram of writing technique and image planes orientation to the writing beam (d) Schematic diagram of (b) showing the interleaving planes belongs to index modulation of Ʌpm and Ʌpm/2; ZT is Talbot length (Rollinson, 2012)

Although it is said that the zeroth and higher order are suppressed to maximize the first orders it is impractical to eliminate them entirely in the manufacturing process. Generally the phase masks for FBG fabrication manufactured with zeroth order transmit less than 3% of the diffracted waves, while first orders

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transmit more than 35% each and the rest of the transmission is via the other possible higher orders (Othonos & Kyriacos, 1999). The possibilities of fringe formation by other orders except ±1 orders have resulted in concern about how fabrication processes could impact on spectral properties. Investigation of the real structure of FBG patterns have been reported via various approaches such as: numerical analysis (Dyer, Farley & Giedl, 1995; Kouskousis et al., 2013; Mills et al., 2000; Tarnowski & Urbanczyk, 2013), experimental analysis (Rollinson et al., 2005) and microscopic analysis (Dragomir et al., 2003; Goh et al., 2014; Kouskousis, 2009; Yam et al., 2009). These analyses show how the existence of a complex FBG structure is as a result of the contribution of multiple orders and their interference. Figure 2.14 shows the modelled refractive index variation in the fibre core obtained by Kouskousis et al. (2013) considering interference of 0 - ± 4th _{orders. According to these analyses, it was observed and confirmed that}

the complex structure includes multiple periodic structures at certain distances from the phase mask which is due to the Talbot effect (discussed in the subsequent paragraph). Although the existence of complex FBG structures and how these relate to the resultant spectrum has been considered, there is need for more extensive modelling to improve our understanding.

Figure 2.14: Modelled intensity variation in the core area of the fibre due to contribution of multiple diffraction orders from phase mask fabrication (Kouskousis et al., 2013) As shown in Figure 2.13 (b), the complex structure contains replicates of structures at multiples of defined distances from the phase mask due to the Talbot effect, which was discovered by Talbot in 1836. This effect has also been

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observed in atom optics. As a result of the fundamental Fresnel diffraction effect, it has gained a great attention over wide range of applications such as real time data acquisition, generation of ultra-high speed tuneable pulse source, determination of dispersion parameters of optical fibre links and a number of spectroscopic techniques (Chavel & Strand, 1984; Chen & Azana, 2005; Guigay et al., 2004; Liu, 1988a, 1988b; Mehta et al., 2006). As a result of near field diffraction and when a plane wave transmits through periodic structure, the resultant wave-front replicates the periodic structure at a certain distance. That distance is called the Talbot length (Talbot, 1836). The phenomenon of a self- imaging distance was first defined in theoretically by Rayleigh (1881) as given by:

𝑍_𝑇 = 𝜆 1 − √1 − 𝜆2 _𝛬 𝑝𝑚 2 ⁄ Equation 2.13

Later he simplified Equation 2.13 considering the relation between 𝛬_𝑝𝑚 and 𝝺

(when wavelength is considerably small); the result was Equation 2.14 as given below:

𝑍_𝑇 = 2𝛬𝑝𝑚 2 𝜆

Equation 2.14

Measurement of the Talbot length using the equation above became popular and position of Talbot planes were calculated using Fresnel-Kirchhoff diffraction and Fourier optics. However, the difficulty of measuring all Talbot planes became a disadvantage of this methodology. That was overcome by Latimer (Latimer, 1993a, 1993b, 1993c) by introducing these patterns as a multiple-slit diffraction pattern produced by conventional mechanism instead of Fourier images of the gratings.

Investigation of the Talbot effect has been further established with finite element analysis and scalar theory analysis of Fresnel-diffraction. Those analyses provide the information of the amplitude and phase. The experiments and simulation were performed to investigate the effect of exposure condition of laser confirming the deviation of the expected grating structure due to the contribution of other orders (Dyer, Farley & Giedl, 1995). Direct imaging of Talbot patterns using tapered optical fibre to the tip by Mills et al, who first used an imaging technique, has

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further confirmed the theoretical models based on diffraction theory (Mills et al., 2000). They also explained the reason to have repeated lengths is a result of the interaction of individual diffraction orders. Mills’ further work on scanning the intensity pattern along the optical fibre produced results that were compatible with X-ray diffraction theory, which is used to describe the electric pattern field behind the phase mask. According to his explanation, the Talbot length has been simplified, as in Equation 2.15, when the numbers of diffraction orders are small:

𝑍𝑇 =

2𝜋

[(𝑘2_{− 𝑚}2_𝐺2₎1/2_{− (𝑘}2_{− 𝑛}2_𝐺2_)]

Equation 2.15

where 𝑚 and 𝑛 are integers representing the diffraction orders 𝑚 < 𝑛, and G is the unit reciprocal lattice vector which can be calculated using 2π Λ⁄ pm of the

phase mask with periodicity Λpm.