M.I Gijón-Sur

In the introduction to this chapter it was mentioned that the DGW technique allows insights to be gained on the behaviour of UV written structures over time and tem- perature. Limitations of the technique are apparent when for example, comparing thermally locked samples with freshly hydrogen loaded samples or when contrast- ing FHD samples with PECVD samples with a different intrinsic refractive index structure. Despite this, the data presented in the preceding sections and the previ- ous chapter may provide relevant detail on the PECVD samples developed as part

Chapter 8 Thermal Behaviour of UV Written Planar Waveguides

of this work.

Comparison of the thermally locked and freshly loaded samples from the two fabri- cation routes indicate the possibility that the hydrogen loaded PECVD samples are in fact behaving as though they are both thermally locked and hydrogen loaded. Factors which point towards this conclusion are;

• The near instant fluorescence on exposure to UV light is shared by both the ther- mally locked FHD samples and the hydrogen loaded PECVD layers.

•The relatively low dependence of effective index on fluence is shared by thermally locked FHD and the PECVD samples.

• The PECVD demonstrates slight photosensitivity without hydrogen loading al- though Bragg responses cannot be detected with the equipment in use.

•Hydrogen loading enhances the photosensitive response of the PECVD samples. • Observations on thick germanium doped PECVD layers and measurements of propagation loss indicate high levels of hydrogen and/or OH groups in the PECVD silica.

It is proposed therefore, that the thermal annealing stages of the PECVD deposition process are acting in a similar manner to the thermal locking procedure. It is possible that the hydrogen contained in the as-deposited PECVD layers undergoes the same reaction as the indiffused hydrogen present from the hydrogen loading process. Confirmation or contradiction of this assertion will require further study with more investigation into the effects of germanium concentration, hydrogen concentration and annealing regimes in particular.

8.8

Summary

Freshly hydrogen loaded FHD and PECVD samples along with thermally locked FHD samples have been studied using the DGW technique and thermal annealing studies.

In the FHD samples it is found that the effective index of waveguides in the hydro- gen loaded substrates is significantly more fluence dependent on UV fluence than in the thermally locked samples. This, coupled with observations of the UV induced fluorescence in the samples indicates that thermal locking may be a process compa- rable to that which occurs in the first moments of UV writing into freshly loaded samples.

Despite the wide variation of effective index with fluence displayed by the hydro- gen loaded FHD samples, thermal anneal causes the effective indices to converge towards a common value. It has been suggested that this is due to thermally re- versible, defect related mechanisms being predominantly responsible for the grating modulation and a more stable compaction or stress related mechanism being largely responsible for a more stable background refractive index level.

The effective index of PECVD samples reduces in an approximately linear fashion with increasing anneal but the Bragg grating refractive index modulation appears to show a sharp drop after annealing above 400‰. As with the FHD samples this

appears to be due to more than one mechanism being responsible for the refractive index change, specifically that the compaction or stress related background refractive index may degrade at a different rate to point defect related changes.

Observations also suggest that the PECVD samples may be partially thermally locked as a result of high levels of hydrogen in the structure and the thermal annealing pro- cess used in their production.

It is clear that there is considerable scope for further work into the complex field of photosensitivity in these layers. Nonetheless, the presented result clearly demon- strate the power of the DGW technique as an analysis tool for UV induced refractive index changes.

Chapter 8 REFERENCES

8.9

References

[1] T. Erdogan, V Mizrahi, P.J. Lemaire, and D. Monroe. “Decay of ultravioletin- duced fiber Bragg gratings”. J. Appl. Phys., 76(1):73–80, 1994.

[2] S.Kannan, J.Z.Y.Guo, and P.J.Lemaire. “Thermal Stability Analysis of UV- Induced Fiber Bragg Gratings”.IEEE Journal of Lightwave Technology, 15(8):1478– 1483, 1997.

[3] J. Canning. “Photosensitization and Photostabilization of Laser-Induced Index Changes in Optical Fibres”. Opt. Fib. Tech., 6:275–289, 2000.

[4] G.D. Emmerson, S.P. Watts, C.B.E. Gawith, V. Albanis, M. Ibsen, R.B.Williams, and P.G.R. Smith. “Fabrication of directly UV written channel waveguides with simultaneously defined integral gratings”.Electron. Lett., 38(24):1531–1532, 2002.

[5] L. Leick, A. Harpøth, and M. Svalgaard. “Empirical model for the waveguiding properties of directly UV-written waveguides”. App. Opt., 41(21):4325–4330, 2002.

[6] C.V. Poulsen, J. H ¨ubner, T. Rasmussen, L.U.A. Andersen, and M. Kristensen. “Characterisation of dispersion properties in planar waveguides using UV in- duced Bragg gratings”. Elec. Lett, 31(17):1437–1438, 1995.

[7] G.D. Emmerson. “Novel Direct UV Written Devices”. PhD Thesis, 2003. Univer- sity of Southampton.

[8] M. Fokine and W. Margulis. “Large increase in photosensitivity through mas- sive hydroxyl formation”. Opt. Lett., 25(5):302–304, 2000.

[9] G.E.Kohnke, D.W.Nightingale, P.G.Wigley, and C.R.Pollock. “Photosensitiza- tion of optical fiber by UV exposure of hydrogen loaded fiber”.OFC., San Diego, USA, page PD20/1PS20/3, 1999.

[10] A.Othonos and K.Kalli. Fiber Bragg Gratings. Fundamentals and Applications in Telecommunications and Sensing. Artech House, 1999.

[11] M. Douay, W.X. Xie, T. Taunay, P. Bernage, P. Niay, P. Cordier, B. Poumellec, L. Dong, J.F. Bayon, H. Poignant, and E. Delevaque. “Densification Involved in the UV-Based Photosensitivity of Silica Glasses and Optical Fibers”. IEEE Journal of Lightwave Technology, 15(8):1329–1342, 1997.

[12] R.M.Atkins, V.Mizrahi, and T.Erdogan. “248nm Induced Vacuum UV Spectral Changes in Optical Fibre Preform Cores: Support for a Colour Centre Model of Photosensitivity”. Electronics Letters, 29(4):385–387, 1993.

Chapter 9

Detection of Phase Transitions

9.1

Introduction

Sensing devices permeate all aspects of modern day society. Ranging from industrial to domestic applications, air travel to space travel, the role of sensors is fundamental to the smooth and safe running of a vast range of technologies. Unsurprisingly, such a technical challenge has triggered considerable academic and commercial interest. Optical techniques for sensing cover a broad range of techniques and applications [1, 2]. They can be both clean and compact, operate remotely and non-invasively, and provide a high level of sensitivity with low power consumption. The technolo- gies used for optical detection include particle scatter detection [3], surface plasmon resonance [4], Mach-Zehnder interferometers [5], grating couplers [1, 6], relief grat- ings [7], long period gratings [8] and fibre Bragg grating sensors [9].

The use of Bragg gratings as sensor elements is not new and fibre based gratings offer many advantages. Design and production of the Bragg sensing structure in planar waveguiding devices is a less well established procedure than the equivalent fibre process but the resultant sensing device offers a number of additional advantages. Whereas optical fibre without ruggedised cladding is a flexible but fragile material, planar silica on silicon structures are intrinsically more robust although somewhat larger in size. The principal strength of planar optical sensors lies in the scope for

integration of multiple functions, sensing or otherwise, onto a single detector-chip. This provides a route to low cost, flexible and compact devices that would be con- siderably more complex if realised in fibre technology.

The majority of fibre grating sensor designs are based around the shift in Bragg wa- velength due to strain or temperature. Strain on a grating alters the period due to the changes in the physical size and also due to stress effects on the refractive in- dex whereas temperature changes are detected due to thermal expansion and the temperature dependence of refractive index . However, it is also possible, by ex- posure of the waveguiding core layer, for a Bragg grating to act as an evanescent field sensor [10]. External changes in refractive index alter the effective index seen by propagating modes due to the interaction of the evanescent field. This technique has been demonstrated in UV written planar waveguides by removal of the upper cladding layer to expose the waveguide core [11, 12].

As discussed in chapter 2 the effective index, and therefore Bragg wavelength, of a waveguide or fibre Bragg grating is determined by the refractive index of both the core and the cladding. Thus a change in the cladding refractive index of a waveguide will result in a shift in the Bragg wavelength. This is the approach used here in the demonstration of planar Bragg sensors.

This chapter demonstrates the use of planar Bragg gratings as sensing components, specifically as refractometers used in the detection of phase transitions from one physical state to another. Initially focussing on the change of a liquid crystal from an ordered to isotropic state, the chapter then moves on to show how the technology is able to detect changes in, and identify the physical state of water.

A feature of this work is the use of wavelengths close to 1550nm. Although these wavelengths are common for telecommunications applications, refractive index mea- surements for substances such as liquid crystals and refractive index standards are generally quoted at visible wavelengths such as 589nm or 633nm (the sodium D- line and He-Ne lasers respectively) due to readily available, well characterised light sources. In itself this may prove to be a useful application as it allows substances that may be of use in the optical path of telecommunications components to be char-

Chapter 9 Detection of Phase Transitions

acterised.