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A set of rapid prototyping techniques are combined to construct a laterally-tilted Bragg grating refractometer in a novel planar geometry. The tilted Bragg grating is fabricated in a silica-on-silicon planar substrate using a dual beam direct UV writing (DUW) technique. Lateral cladding mode confinement is subsequently achieved by physically micromachining two trenches either side of the direct UV written waveguide. The resulting device is demonstrated as an effective refractometer, displaying a comparable sensitivity to tilted Bragg gratings in a fiber optical geometry, but with the added advantages of planar integration.
©2011 Optical Society of America
1. Introduction
Bragg grating refractometers have been subject to continuous development over recent years for applications in chemical and biochemical sensing [1,2]. Since being first reported in optical fiber [3] these components have subsequently been demonstrated in planar optical platforms [4,5]. The motivation behind this fiber to planar platform transition can be largely attributed to the added advantages associated with planar integration, including compact multi-parameter sensing [5] and the potential to be seamlessly interfaced with microfluidics [6].
Tilted Bragg gratings are a particular type of Bragg grating, used in fiber-based refractometry. Unlike conventional (un-tilted) fiber Bragg grating refractometers [3], they attain evanescent field exposure by coupling light from a guided core mode into a set of cladding modes [7]. As the measured analyte is well separated from the waveguide core, these structures have an associated low cross sensitivity between unwanted environmental perturbations, including temperature and strain [7–15]. These structures have also been demonstrated as a means to couple light into Plasmon resonance modes, opening-up a whole new branch of applications [8]. By coupling light into cladding modes rather than removing away the cladding to expose the evanescent mode, tilted fiber Bragg gratings have the advantage of not requiring the use of hazardous chemicals, such as hydrofluoric acid or complicated processes such as those associated with lapping and polishing of cladding layers, as is generally required for typical Bragg grating refractometers [3].
In contrast to the developments made in fiber [7–15], little has been reported on tilted Bragg gratings in a planar platform. A recent patent application for a refractometer device [16] has proposed the use of tilted planar Bragg gratings (TPBGs) but only in geometries in which the measurand is in contact with the core of the waveguide. The majority of other reported TPBG devices are focused towards signal processing and interrogation [17–19], and not refractometry. The reason little experimental development has been made on TPBG refractometers may be partially due to the difficulty in achieving a discrete set of resolvable cladding modes. The work reported achieves this through physically micromachining two trenches either side of a direct UV written tilted Bragg grating, illustrated in Fig. 1 . This novel technique for fabricating integrated components uses a flexible computer aided design and manufacture system. The ridge features formed can achieve deep (>100 μm) vertical side walls (<0.008 radians of wedge), meaning reduced polarization cross talk and a relatively low side-wall surface roughness (<15 nm Ra), resulting in a reduced level of scattering loss. Although alternative micro-manufacturing techniques including focused ion-beam, laser milling, and wet-etching may perform better than physical micromachining in one of these areas [20], comparatively they cannot deliver all three desired aspects without compromise. Physical micromachining performs well for all three outlined aspects of fabrication.
Fig. 1 (a) A schematic of the tilted planar Bragg grating refractometer fabricated (b) a micrograph illustrating the cross-sectional view of a 40 μm width ridge with buried waveguide – the fact that the bottom of each trench is not entirely horizontal is an artifact due to blade wear and does not affect the evanescent interaction of the mode which occurs in the silica layers.
Importantly these devices differ from recently proposed structures [16] in that the measurand fluid is well separated from the core waveguide, and is contained in the micrometer scale physically micromachined channels. This fabrication technique also has associated benefits of rapid prototyping (1 mms−1 cutting speeds), removes all dependency on hazardous chemical etchants and offers the ability to cut any desired ridge width (demonstrated down to 10 μm) through a fully automated computer design and manufacture system. As ridge width can be easily controlled during fabrication, the spectral density of cladding modes can be controlled. There are applications where fewer resonances would be beneficial, for example tracking the evolution of one particular resonance in a spectral window. An additional benefit of the physically micromachining is that the feature size of the trenches is micrometer order, meaning they can be seamlessly adapted to form microfluidic channels.
The precision control of direct UV writing can attain integration of multiple TPBG components, of different tilt angles on a single monolithic chip. As tilt angle can be engineered to optimally access certain refractive index windows [7,12], so chips can be designed to offer multiple sensitivity ranges that can be accessed in parallel. This allows greater functionality and integration upon a compact device footprint. Another benefit of the planar configuration is that the grating tilt plane is automatically aligned with the principle axis of the waveguide, which is feasible in optical fibers [15] but not as trivial.
This paper reports the novel fabrication of an integrated TPBG refractometer and compares its sensitivity to similar structures reported in optical fiber [7,10,15]. The following section addresses the theory of mode coupling into cladding and radiation modes. Through monitoring these modes, using a standard area scheme [11,21] device sensitivity is achieved.
2. Concept and theory
A typical Bragg grating consists of a refractive index modulation in the core of an optical waveguide, which at the phase matching condition acts to couple the fundamental forward-propagating core mode to a counter-propagating core mode. This is effectively a spectral notch filter as a narrow peak is measured in reflection, corresponding to a notch in transmission. A tilted Bragg grating consists of a refractive index modulation that is purposely blazed relative to the waveguide’s axis, illustrated in Fig. 1. The effect of this is to enhance coupling between the forward-propagating core mode and counter-propagating cladding modes. The counter-propagating cladding modes attenuate quickly and have little coupling into the launch waveguide and so are not usually observed in reflection. However, they can be observed in transmission as a series of narrow transmission dips (resonance features) seen at shorter wavelengths than the main Bragg reflection peak. The effective index of the ith cladding mode nclad,i, is calculated from the resonance position λi using the phase matching condition [9].
where ncore is the effective refractive index of the core mode, Λ is the nominal grating period and θ is the angle of tilt, illustrated in Fig. 2 . The solutions associated with Eq. (1) can give a finite set of distinguishable spectral resonances, dependent upon the width of the cladding. For fiber platforms this is determined by the diameter of the optical fiber. In this work, the grating is tilted in the plane of the device (lateral tilt). Thus, the spectral spacing of cladding modes are determined by the lateral dimension of the ridge.Fig. 2 The dual beam direct UV writing technique for fabricating tilted Bragg gratings. Each exposure of the focused spot defines only a few Bragg grating periods.
A change in refractive index of a fluid contained within the trenches alters the respective cladding mode solutions, which in turn results in a change in the phase matching conditions, of Eq. (1). If the refractive index of the material in the trenches is equal to that of the cladding mode then the resonance dip shall vanish as a continuum of radiation modes. It is through interpreting the guided and radiation modes that sensitive measurements are achieved.
The following section proceeds to detail the fabrication processes that have been combined to realize this unique planar device.
3. Device fabrication
The device is fabricated from a planar silica-on-silicon substrate. The substrate was fabricated by firstly growing a thick (~16.5 μm) thermal oxide upon a silicon wafer (~1 mm thick). Upon this thermal oxide, two silica layers were deposited and consolidated using flame hydrolysis deposition (FHD). The thick thermal oxide, upon which these layers are consolidated, forms an underclad layer. The first FHD layer forms a core layer, 6.2 μm thick and the second layer an overclad, 15.1 μm thick. The deposited core layer was doped with germanium and boron in order to provide photosensitivity to UV light. Photosensitivity was further enhanced through hydrogen loading the sample for 1 week at 12 MPa prior to UV writing.
To fabricate single mode waveguides and Bragg gratings a dual beam interferometer technique was implemented [22], illustrated in Fig. 2. This fabrication process splits and focuses two coherent UV laser beams to create a single spot with a diameter of ~6 μm. The arms of this interferometer are set such that the interference pattern at the focus has a period of ~530 nm. Through amplitude modulating the beam with respect to translation this technique is capable of defining both single mode waveguides and Bragg gratings, with a typical waveguide refractive index change of 5x10−3 and a mode profile matched to SMF-28. The sample was translated with respect to the focused spot using a set of computer control stages. Using the two linear degrees of freedom and a rotational degree of freedom any Bragg grating with desired tilt can be fabricated, alongside a range of 2-dimensional integrated optical components [23–25]. As illustrated in Fig. 2, a tilt angle was fabricated by rotating the sample by φ about the y-axis, with respect to the x-axis. As the phase planes of the interference pattern run perpendicular to the x-axis, angle φ is related to the tilt angle of the grating θ, such that θ ≡ φ.
Subsequent to the DUW a ridge was formed which contained the waveguide. The ridge was formed using two plunge cuts with a dicing saw (Loadpoint, Microace). These cuts were made with a diamond impregnated nickel bonded blade, 70 μm wide and with an outer diameter of 56.5 mm, the parallelism of the trenches were better than 0.01°. Although this technique has been observed to cut ridge widths as thin as 10 μm the lateral width of the ridge reported is comparable to the cladding diameter of previously reported tilted fiber Bragg gratings in SMF-28 [7,10,15]. Through engineering the tilt angle it is understood that sensitivity to low refractive indices can be attained [7,12]. A tilt angle of 7° has been reported as this angle of tilt offers a quantitative data set for the refractive index fluids available. The fabricated device consisted of a 5 mm long tilted Bragg grating that resided in a ridge of width 134 ± 1 μm.
Light was coupled into and out of the single mode waveguide using two polarization maintaining (PM) fiber pigtails, secured using UV curing epoxy. After curing, both pigtails were mechanically robust and had an associated low coupling loss (<0.3 dB). A plastic cover with two ports (input/output) was bonded on top of the two trenches, making a pair of encapsulated microfluidic channels. Refractive index fluids were delivered and flushed out of the microfluidic channels using dispensing needles.
4. Refractive index sensitivity
Using a spectrally broadband SLED source, Optical Spectrum Analyzer (OSA), fiber polarizer and polarization controller the device’s transmission spectra was analyzed. The polarizer and PM pigtails ensured single polarization modes were coupled into the device. Defining the transverse direction being parallel to the planar layer, the transverse electric (TE) and transverse magnetic (TM) polarizations were independently measured.
Figure 3 illustrates the TM transmission spectra of the device containing four different refractive index fluids (Cargille, Series A/AA/AAA), of refractive index n, contained within the trenches. The transmission resolution of the OSA was set to 0.1 nm.
Fig. 3 Measured TM transmission spectrum of a 5 mm long θ = 7° – tilted planar Bragg grating with various external refractive indices, n.
To quantify the refractive index sensitivity of the device a normalized area technique [10] was implemented. This technique quantifies the envelope of cladding modes by evaluating the modulation depth of the resonance features. The envelope of the cladding mode resonances was analyzed using a standard deviation method. This technique tracked the standard deviation of a 1.4 nm wide spectral window over the transmission spectra. The width of this window was such to ensure a single resonance was encompassed. Figure 4 depicts the spectral evolution of standard deviation for a variety of refractive index fluids.
Fig. 4 Measured envelope curves for the TM transmission spectra of a θ = 7° – tilted Bragg grating, surrounded by different refractive indices, n.
To obtain a representation of this spectral area numerical trapezoidal integration was performed. Figure 5 illustrates the integrated resonant mode area for TE and TM polarizations. This plot has been normalized with respect to the TM polarization with no index fluid in the trenches. It must be noted that just as with tilted fibre Bragg gratings [10,21], there is a polarization dependent variation in the transmission spectra. For TM polarizations the normalized area A of the cladding mode envelope is greater for indices of 1.36 and 1.37 than for an index of 1.0. In comparison, for TE polarizations, the normalized area for the measured refractive indices is always less than that for a refractive index of 1.0. Such polarization dependencies have also been reported in fibre, where the grating tilt plane breaks the symmetry of the waveguide [10,21].
Fig. 5 The evolution of normalized area A with refractive index n for both TE and TM polarizations (insert) a linear fit of the TM data between refractive indices of 1.37 to 1.42.
The refractive index sensitivity of the device is greatest over the range of 1.37 to 1.42 r.i.u. (refractive index units). The TM polarization demonstrates the largest sensitivity in this range having a dA/dn of −14.3 (dimensionless units). The insert in Fig. 5 illustrates the linear approximation, with respect to the recorded data in this range. The data about this fit line has a standard error of 0.003 normalized area units, corresponding to a refractive index resolution of 2.1x10−4 r.i.u. As the typical thermal response of the Cargille fluids in this range is −4x10−4 r.i.u °C−1 and the variations in laboratory temperature during experimentation was ± 0.5 °C, there is an uncertainty in the oils refractive index of ± 2x10−4 r.i.u. It is understood that the refractive index uncertainty in the oil due to temperature variations limit device resolution. To infer the repeatability and resolution, thermal variations in the laboratory ( ± 0.5 °C) were addressed.
Thermal sensitivity of the device was calibrated by placing it on a temperature controlled hot plate. For a refractive index fluid of 1.3826 the thermal sensitivity (dA/dT) between 20°C and 50°C was 7.3x10-2 oC−1. It must be noted that this was greater than the thermal sensitivity of air filled trenches, which had a dA/dT of 7.2x10-3 oC−1. This result illustrates that the fluid filled channels result in a larger thermal response due to the large and negative dn/dt ( = −4.12x10−4 r.i.u °C−1) compared to air.
To infer the repeatability and resolution, device temperature was simultaneously monitored with a thermocouple and compensated for through using the measured thermal response value. For a 1.3826 index oil contained within the trenches the measurement repeatability of data acquisitions was measured. A five scan standard error, over 200 data acquisitions, had a maximum value of 4.0x10−4 normalized area units. This result infers that through thermal compensation or stabilization, the maximum refractive index resolution and repeatability achievable is 2.8x10−5 r.i.u.; a value that is comparable to resolutions obtained for thermally stabilized tilted fiber Bragg gratings [7,10,15].
5. Conclusion
A tilted planar Bragg grating refractometer has been demonstrated in a silica-on-silicon platform. Uniquely fabricated by combining direct UV writing and physical micromachining, the device achieved a refractive index resolution better than 1x10−4, which is comparable to the resolution of reported tilted Bragg gratings in fiber.
Through using the unique set of computer controlled techniques the system is capable of fabricating a variety of tilted Bragg gratings with any desirable lateral width and tilt angle upon a single monolithic chip. This opens up a new avenue for tilted Bragg gratings, with a greater degree of fabrication flexibility and advantages associated with planar integration.
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