Ridge-waveguide-based polarization insensitive

INSTITUTE OF PHYSICS PUBLISHING

MEASUREMENT SCIENCE AND TECHNOLOGY

doi:10.1088/0957-0233/17/7/013

Meas. Sci. Technol. 17 (2006) 1752–1756

Ridge-waveguide-based polarization insensitive Bragg grating refractometer X Dai, S J Mihailov, C L Callender, C Blancheti`ere and R B Walker Communications Research Centre, 3701 Carling Avenue, PO Box 11490 Station H, Ottawa, Ontario K2H 8S2, Canada

Received 6 February 2006, in final form 27 April 2006 Published 7 June 2006 Online at stacks.iop.org/MST/17/1752 Abstract A highly sensitive waveguide Bragg grating (WBG) sensor for measuring small changes of the refractive index of the surrounding liquid is presented. By using an open top ridge waveguide with a small core, the evanescent field interaction of the guided mode with the liquid analyte on the top of the waveguide is enhanced. The sensitivity measured via a shift in the resonance wavelength of the Bragg grating as high as 1 pm of wavelength shift for a change of 4 × 10−5 in the refractive index around 1.402 is realized. With a polarization insensitive Bragg grating, the polarization dependence of the sensor is improved. A theoretical analysis for the sensitivity of ridge waveguide sensors is given. The experimental results are in good agreement with the theoretical analysis. Keywords: high sensitivity, evanescent field, Bragg grating,

open top ridge waveguide, polarization insensitive, mode conversion, refractometer

1. Introduction The evanescent field surrounding a Bragg grating inscribed in an optical waveguide has been used as a refractometer for the measurement of small refractive index changes of liquid samples [1–4]. To increase the interaction of the surrounding medium with the evanescent field about the waveguide core, the fibre cladding is removed at the Bragg grating location. But, as a device, the strength and durability of the fibre Bragg grating without the cladding are poor. The one-side-etched waveguide Bragg grating sensor [1, 2] is developed to make a mechanically stronger device. However, since only one surface of the waveguide Bragg grating sensor is etched, the sensor has lower sensitivity and strong polarization dependence. In this paper, by using an open top ridge waveguide and inducing a polarization insensitive Bragg grating (PIBG) [6, 8], the sensitivity and polarization dependence of the waveguide Bragg grating sensor are improved. The structure of the open top ridge waveguide is shown in figure 1. When the light source is coupled into the core of the waveguide, the evanescent field of the guided core mode, which emanates from both the top and sides of the ridge waveguide, can be accessed by the surrounding 0957-0233/06/071752+05$30.00

liquid. The effective refractive index n of the waveguide mode is a function of the size of the waveguide cores a and b, the refractive index of the waveguide core ng, the refractive index of the surrounding liquid nt and the refractive index of the waveguide bottom cladding ns. n is sensitive to changes in nt. When the Bragg grating is induced in the waveguide, any small change of n will result in a shift of the Bragg wavelength λb when the temperature is fixed. Therefore, by monitoring the shift of λb, a change of the refractive index of the surrounding liquid nt can be measured. In waveguide structures, the resonance wavelength λb of a Bragg grating is written as (1) λb = 2n, where  is the grating period and n is the average effective refractive index of the waveguide. The shift of the Bragg wavelength λb is given by λb = 2 n. (2) The evanescent field interaction of the guided mode with the surrounding liquid is described by the sensitivity S of the waveguide evanescent field. S is defined by the rate of change of the modal effective index under an index change of the top cladding as S = dn/dnt. Equation (2) can be rewritten as λb = 2S nt . (3)

© 2006 IOP Publishing Ltd Printed in the UK

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Ridge-waveguide-based polarization insensitive Bragg grating refractometer

Ge doped SiO2

a nt b

core size 5.7 µm × 5.6 µm, a measurement sensitivity of 4 × 10−5 pm−1 for nt around 1.402 is realized.

2. Theoretical analysis

ng nS

Thermal Oxide SiO2 Si

Figure 1. The structure of the open-top ridge waveguide without a top cladding.

By increasing the values of  or S, λb can be made sensitive to the change of the analyte nt. In a previous work [1], the method of decreasing the thickness of residual cladding or applying a thin-film layer with a high refractive index between the etched fibre and the analyte layer has been used to increase S by shifting the field distribution of the guide mode towards the analyte layer. However, in practice, this technique increases the complexity of the device fabrication. In this paper, the sensitivity is increased by reducing the waveguide core diameter with a constant working wavelength and by using an open top ridge waveguide to enhance the intensity of the evanescent field. In the theoretical analysis, the dependence of the sensitivity S on the structure parameters of the ridge waveguide is considered. From equations (6) and (7), the sensitivity S can be enhanced by decreasing the core diameter, increasing the working wavelength or both. In the experiments, Bragg gratings were induced in the open top ridge waveguides using a phase mask and excimer laser radiation at 193 nm. The Gedoped silica on silicon ridge waveguides were made by flame hydrolysis deposition (FHD), standard photolithography and reactive ion etching (RIE). The ridge waveguide core sizes were varied from 5.7 µm × 5.6 µm to 7.5 µm × 5.6 µm. A series of commercially available refractive index matching liquids were used to test the devices. Experimental results indicate that the sensor with the open top ridge waveguide has a higher sensitivity than the previously reported sensors with side-etched waveguides. However, since the small core waveguide has a larger birefringence when the refractive index of the analyte nt is much lower than that of the bottom cladding ns, it exhibits strong polarization dependence. Using the technique of waveguide birefringence control with UV irradiation [5, 6], a polarization insensitive Bragg grating is fabricated in the small core size ridge waveguide. The wavelength instability in measurement due to the variation of light polarization is improved from 0.4 nm to 0.05. The coupling of the single mode probe fibres and the ridge waveguide is easily changed due to the liquid flowing into the coupling region between the probe fibre and the waveguide structure perhaps exciting modes in the sub-cladding layer of the waveguide. To avoid the fibre waveguide coupling distortion, UV optical adhesive epoxy OG175 is used to hold the liquid. Cargille liquids are also used as temperature control in the stability measurement of the sensor because the thermooptic and thermo-mechanical effects of the silica waveguide can be balanced by the thermal dependence of the refractive index of the Cargille liquid. With the waveguide device of

There are several approaches such as the effective-index method, Marcatili’s method, the mode-matching method and the finite-element method that are typically used to analyse ridge waveguide structures [7–10]. A simple numerical method [11] was developed to analyse the dispersion characteristics of the guided modes of a strip waveguide. The effective indices nTE and nTM of the TE and TM modes of the ridge waveguide were given by
2 2
 1/2  2 2 n2TM = n
2 4a k 1 − 2 ak n2g − n2t (4) TM − m π and


2 2
 1/2 2 2 4a k 1 − 2 ak n2g − n2t n2TE = n
2 TE − m π  1/2
 + 2 n2g − n2t ak n2g ,

(5)

n
TE

and n
TM where k = 2π/λ is the free-space wave number. are the effective indices of TE and TM modes of a three-layer slab, respectively, a is the width of the ridge waveguide and ng, ns and nt are the refractive indices of the core, the substrate and the surrounding regions, respectively. Here, using equations (4) and (5), the sensitivities of TM and TE modes of the ridge waveguide are derived as
STM = ∂nTM /∂nt = STM (n
TM /nTM ) 
 3/2  + (nt /nTM )(m2 π 2 /4a 3 k 3 ) 1 n2g − n2t

(6)

and
(n
TE /nTE ) + (nt /nTE )(m2 π 2 /4a 3 k 3 ) STE = ∂nTE /∂nt = STE 
 2
  1/2  3/2 × 1 ng − n2t + 1 n2g n2g − n2t , (7)


= ∂n
TE /∂nt and STM = ∂n
TM /∂nt. STE and where STE
STM are the sensitivities of TE and TM modes of the threelayer slab, respectively. STE and STM are the sensitivities of TE and TM modes of the ridge waveguides, respectively. Considering 4a3k3 in equation (6) or (7), the sensitivity S can be enhanced by decreasing the core diameter a, increasing the working wavelength λ (k = 2π/λ) or both. As n
TE ≈ nTE and n
TM ≈ nTM, the first terms in equations (6) and (7) are

or STM . By the normalized analysis of a slab dominated by STE
waveguide evanescent wave sensor, the expressions of STE and
STM were given in [12], and optimization results for all slab waveguide sensors were achieved. However, these results cannot be applied directly to ridge waveguides due to the structural difference between the three-layer slab waveguides and the ridge waveguides. The structural characteristic of ridge waveguides is reflected in the second term of equation (6) or (7) by the parameters a, ng and nt. We are interested in the contribution of the second term to STE or STM. The contribution of ridge waveguides to the sensitivity is given
. The results are plotted in figure 2 by S(TE/TM) − S(TE/TM) for refractive indices between 1 and 1.454. It is clear that the contribution increases slowly in the low refractive index region, but increases rapidly as the index of the surrounding medium approaches that of the waveguide. The contribution also increases as the core width decreases. As the analyte index nt approaches the core index ng = 1.4545, the contribution is maximized and the birefringence becomes zero.

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S(TE/TM) – `S(TE/TM)

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7µm×6µm 6µm×6µm 5µm×6µm

Refractive index nt of liquid analyte

Figure 2. The variation of sensitivity (solid line for TM mode, dashed line for TE mode) with core size and the refractive index nt in the range 1–1.4545.

Input fiber

Epoxy

Output fiber

Er Broadband source

Optical Spectrum Analyzer Bragg grating

Polarizer and Polarization Controller

Figure 3. The configuration of the measurement setup.

3. Experiments and results 3.1. The configuration of the ridge waveguide Bragg grating (WBG) sensor The configuration of the sensor is shown in figure 3. The Bragg grating was induced on a Ge-doped SiO2 ridge waveguide. A section of Bragg grating is used to sense the refractive index changes of the liquid on the top of the waveguide. To monitor the spectral response of the gratings, broadband light from a 980 nm pumped erbium-doped fibre was coupled via a single mode fibre into the core of the open top ridge waveguide. The transmitted light was out-coupled to a single mode fibre and monitored by using an optical spectrum analyser. The coupling of the single mode probe fibres and the ridge waveguide is easily changed due to the liquid flowing into the coupling region between the probe fibre and the waveguide structure perhaps exciting modes in the sub-cladding layer of the waveguide. To avoid the coupling distortion of the fibre and the waveguide, UV optical adhesive epoxy OG175 with a refractive index of 1.452 is used to hold the liquid. This effectively ensures that the test liquid remains in the central part of the sensor and blocks the liquid flow over the waveguide ends. 3.2. Bragg grating fabrication in the ridge waveguides Ridge waveguides were fabricated in a 6 ± 0.5 µm thick Gedoped SiO2 layer grown by FHD on a 7 µm layer of thermal silicon dioxide on silicon. Waveguides can also be fabricated in similar layers grown by plasma-enhanced chemical vapour deposition (PECVD) in the lab. The ridges were produced 1754

using standard photolithography and RIE using a CHF3/O2 gas mixture. The core layer index ng was measured before etching by prism coupling at 1537 nm to be 1.4545 ± 0.0004. The bottom cladding index ns was 1.4436, which was 0.75 ± 0.07% less than ng. The refractive index of the UV epoxy was 1.452 measured at 1537 nm, which is smaller than ng. The dimensions of the waveguide cores were designed to be 9 µm × 6 µm, 8 µm × 6 µm, 7 µm × 6 µm and 6 µm × 6 µm, respectively. The distance between two adjacent ridges was 75 µm. On a wafer patterned with different core size ridge waveguides, Bragg gratings were written using a single zeroorder nulled phase mask and ArF excimer laser with an emission wavelength of 193 nm. For the fabrication of a polarization insensitive Bragg grating, the waveguide was hydrogen-loaded at 110 atm H2, 352 K, 48 h [6]. In order to induce a positive UV-induced birefringence, the beam width must be greater than twice the thickness of the deposited layers (core layer and buffer layer) in order to achieve enough UVinduced stress relief. With a UV cylindrical lens, the laser beam was focused to a spot size of 5 mm × 300 µm onto the wafer surface. A strong Bragg grating with n ∼ 9 × 10−4 was induced in the hydrogen-loaded 6 µm × 6 µm waveguide with 40 Hz, 100 mJ cm−2/pulse of polarized UV irradiation (oriented normal to the waveguide axis). The birefringence (wavelength separation) of the TM and TE modes of the Bragg grating decreased from 0.4 nm to 0.2 nm. The phase mask was then removed and the waveguide was trimmed with a 50 mJ cm−2/pulse irradiation of UV until both the TM and TE modes overlapped and a PIBG formed. Finally, the PIBG device was annealed at 150 ◦ C for 2 days to remove the hydrogen residue in the waveguide [5]. The total UV exposure for the fabrication of the Bragg grating with −21 dB transmission and 0.7 nm bandwidth is 1 kJ cm−2. 3.3. The sensitivity of ridge waveguide Bragg grating sensor To monitor the spectral responses of the grating for TE and TM modes, broadband light from a 980 nm pumped erbiumdoped fibre was passed through a polarizer and a polarization controller and coupled via a single mode fibre to the ridge waveguide. The transmitted light was butt-coupled to a single mode fibre and monitored using an optical spectrum analyser. A series of refractive index matching liquids was used to test the devices with the waveguide core sizes of 7.7 µm × 5.6 µm, 6.6 µm × 5.6 µm and 5.7 µm × 5.6 µm, respectively. The spectra of the device with the waveguide core size of 6.6 µm × 5.6 µm are shown in figure 4. The Bragg wavelength shift was sensitive to the change of the analyte index nt. With each measurement of a new analyte, the probe fibres were removed and the waveguide surface was cleaned with methanol to remove the previous analyte. The probe fibres were then recoupled to the waveguide. In figure 4 the transmission spectrum of the grating for each analyte is normalized to 1. The results in figure 5 indicate that the devices were polarization dependent. The sensitivity increased with decreasing waveguide core width. The sensitivity increased slowly in the low analyte index, but increased more rapidly as the analyte index nt approached that of the waveguide core ng. As the analyte index approached that of the core

Ridge-waveguide-based polarization insensitive Bragg grating refractometer Sample#1, 7x6, No3

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1.00 0.95 0.90 0.85 0.80 0.75 0.70

TE mode of waveguide 5.7µm x 5.6µm TM mode of waveguide 5.7µm x 5.6µm

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Wavelength (nm)

Normalized Transmissions

1.05

1558 1557 1556 1555 1554

1

1.452

0.65

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1.464

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1.33

0.60

1.44

1.39

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0.55 1552

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1558

1560

1.0

1562

Wavelength (nm)

Figure 4. The spectra of Bragg wavelength shifting with the change of the refractive index of the analyte liquid.

Wavelength (nm)

1559 1558 1557

TE mode of waveguide 5.7µm x 5.6µm TE mode of waveguide 6.6µm x 5.6µm TE mode of waveguide 7.7µm x 5.6µm TM mode of waveguide 5.7µm x 5.6µm TM mode of waveguide 6.6µm x 5.6µm TM mode of waveguide 7.7µm x 5.6µm

1.5

Figure 6. The Bragg wavelength as a function of the test liquid refractive index for the device with the polarization insensitive Bragg grating. 1556.0

±5 pm for nt=1.424

1555.5 1555.0

Wavelength (nm)

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1.1 1.2 1.3 1.4 Refractive index nt of liquid analyte

1556 1555 1554 1553

±1 pm for nt=1.402

1554.5 1554.0

±3 pm for nt=1.33

1553.5 1553.0 1552.5 1552.0

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100

200

300

400

500

600

Time (min.)

1550 1.0

1.1

1.2

1.3

1.4

1.5

Refractive index nt of liquid analyte

Figure 5. The Bragg wavelength as a function of the test liquid refractive index for the devices whose waveguide cores were 7.7 µm × 5.6 µm, 6.6 µm × 5.6 µm and 5.7 µm × 5.6 µm.

index, the sensitivity was increased to the maximum and the birefringence became zero. The small core size 5.7 µm × 5.6 µm device had the highest sensitivity, but had a strong polarization sensitivity, especially at low analyte index nt. By fabricating the polarization insensitive Bragg grating on the 5.7 µm × 5.6 µm core size waveguide, the polarization instability of the device was improved from 0.4 nm to 0.05 nm for a liquid refractive index of around 1.3 as shown in figure 6. The experimental results were in good agreement with the theoretical results given in section 2. 3.4. The measurement stability of the sensor The stability of the sensor was tested by Agilent 8164 A Lightwave Measurement System which has a 1 pm resolution in wavelength. The environmental temperature monitored by a thermometer is 22 ◦ C ± 0.5 ◦ C. The optical adhesive was used to protect the coupling of the fibre and waveguide to avoid the analyte liquid flowing towards the ends of the ridge waveguide, leading to a shift in the measured Bragg wavelength. One drop of Cargille liquid with refractive indices of 1.33, 1.402 and 1.424, respectively, was deposited on the top surface of the ridge waveguide Bragg grating sensor, and the stabilities were tested by observing the shift of the Bragg wavelengths

Figure 7. The test result for the measurement stability of the sensor. Corresponding to the liquids of nt = 1.33, 1.402 and 1.424, the Bragg wavelength fluctuations are ±3 pm, ±1 pm and ±5 pm, respectively.

with time. As shown in figure 7, the Bragg wavelength fluctuations are ±3 pm, ±1 pm and ±5 pm, respectively.

4. Discussion The wavelength shift without the epoxy layer is due to the liquid flowing over the end of waveguide which changed the coupling condition of the single mode fibres with the ridge waveguide and perhaps resulted in mode excitation through the fluid or through the sub-cladding layer waveguide. The Bragg wavelength fluctuation in the measurement stability of the sensor is affected by the counter-balancing of the thermal dependence of the refractive index of the matching oil and the thermo-optic and thermo-mechanical effects of the silica waveguide. For the liquids whose refractive indices are 1.33, 1.402 and 1.424, their index variations with temperature are −3.6 × 10−4 ◦ C−1, −4 × 10−4 ◦ C−1 and −4 × 10−4 ◦ C−1, respectively, which would result in a negative change to the effective index of the waveguide grating, hence, a negative wavelength shift with increasing temperature. The variation in the effective index of the waveguide grating resulting from thermo-optic and thermo-mechanical effects results in a positive wavelength shift with temperature. By the sensitivity curve of the Bragg wavelength versus the analyte nt in figure 6, we have dλ/dnt = 12 nm, 25 nm and 52 nm for liquid 1755

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indices 1.33, 1.402 and 1.424, respectively. Therefore, the Bragg wavelength shifts due to the thermal dependence of the refractive index of the liquid would be −4.5 pm ◦ C−1, −10.2 pm ◦ C−1 and −20.5 pm ◦ C−1, respectively, which are balanced by the thermo-optic effect of the silica waveguide which results in a Bragg wavelength shift dλ/dt of ±11 pm ◦ C−1 in the absence of the oils. In figure 7, it took a few minutes for the signal to stabilize which is determined by the time that the test fluid takes to efficiently cover the ridge waveguide. The bandwidth was narrower as the fluid became a new top cladding replacing the air.

5. Conclusion Since the open top ridge waveguide can provide a 3D space for the evanescent field to sense the surrounding liquid, a Bragg grating ridge waveguide device exhibits higher sensitivity than side-etched waveguide Bragg grating devices. The temperature sensitivity of the waveguide grating could be reduced by using the thermal dependence of the refractive index of the Cargille liquid since its index variation with temperature is opposite to that of the silica waveguide. With the waveguide device of dimension 5.7 µm × 5.6 µm, a measurement sensitivity of 4 × 10−5 pm−1 for nt around 1.402 is realized. A smaller size waveguide will have higher sensitivity, but will also have larger birefringence and coupling loss with standard single mode fibre. This device can be applied to measure small refractive index changes in sensing particular chemical reactions and in biological analysis for aqueous solutions as well as for solvent-based liquids. This work will be studied in the near future. This technology offers many advantages over previously proposed waveguide sensors, including enhanced sensitivity, better polarization stability and simpler fabrication processes.

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