Development and characterisation of phosphate glasses for athermalisation

Lightwave devices such as gratings have a drawback where the central wavelength of the devices is sensitive to changes in temperature, which compromises their performance. This problem is particularly acute in systems that employ Wavelength Division Multiplexing (WDM), where the stability of wavelength with regards to temperature is essential. An athermal waveguide, which is inherently insensitive to temperature, is an ideal choice to solve this problem instead of using a passive temperature control unit, which could limit the optimum performance of a WDM system. Generally, athermal glasses must exhibit negative refractive index change with temperature (dn/dT) to counter the effect of thermal expansion. Phosphate glasses are ideal candidates for athermalisation as they exhibit negative dn/dT. An interferometer set-up has been constructed to specifically measure dn/dT of the glasses. Three different phosphate glass families were prepared and characterised in terms of their optical and thermal properties. The properties of the potassium aluminophosphate glasses were strongly influenced by the strengthening effect of Al3+, the trend observed for the binary phosphate glasses depended on the availability of terminal oxygens to coordinate the cations while the change in the properties of borophosphate glasses depended on the effect and coordination number of B3+. Glasses with negative dn/dT have been achieved from the various compositions of glasses studied. The results showed that there was a minimum thermal expansion coefficient (α) for each glass system, beyond which α would overcome the effect of polarisability change with temperature so that the glasses exhibited negative dn/dT. Semi-empirical equations linking dn/dT and α have been defined, enabling the estimation of dn/dT of any glass within the same system. This research work represents the first comprehensive measurement of dn/dT of phosphate glasses. Both fibre and planar waveguides were fabricated. Extrusion was used to prepare the preform for fibre drawing. The extrusion process used a two-die assembly to extrude dissimilar glasses, and is the first demonstration of its kind. The spin-coating process was used to deposit layers of glass on glass substrates to produce planar structures. The success of the extrusion and spin-coating work showed that these processes could be used for combinations of glasses with dissimilar thermal properties. Femtosecond writing was investigated on the phosphate glasses and channel waveguides have been successfully defined

Compositional effects on the optical and thermal properties of potassium aluminophosphate glasses

The K2O–Al2O3–P2O5 glass system has been examined and various compositions have been melted. Their optical and thermal properties have been measured to assess their potential for athermalisation. The addition of alumina (Al2O3) increases the refractive index (n) and glass transition temperature (Tg) and decreases the thermal expansion coefficient (α), consequently leading to positive thermo-optic coefficient (dn/dT). In addition to thermal expansion, polarisability of the glass also affects dn/dT. Generally, glasses must exhibit negative dn/dT to counter thermal expansion, in order to have potential application in athermalisation

Two-die assembly for the extrusion of glasses with dissimilar thermal properties for fibre optic preforms

Extrusion of a preform based on core and cladding glasses with different thermal properties is demonstrated. The core glass (50BaO-8B) exhibits refractive index of 1.594 and softening temperature of 650 °C. Due to the softer nature of the core glass compared to the cladding glass, a novel two-die assembly method is employed for extrusion. The results show that the core glass flows through the cladding glass and be extruded first before stabilising to enable the extrusion of a cane of core and cladding glasses

Thermo-optic coefficients of potassium alumino-metaphosphate glasses

Metaphosphate compositions of the K2O–Al2O3–P2O5 system have been melted and their properties measured to assess their potential for athermalisation. The results show that the addition of alumina (Al2O3) increases the refractive index and glass transition temperature (Tg), decreases the thermal expansion coefficient (α) and leads to a more positive thermo-optic coefficient (dn/dT). Glasses must exhibit negative dn/dT as a requirement for athermalisation purposes

Compositional effects on the optical and thermal properties of sodium borophosphate glasses

A series of sodium borophosphate glasses of the composition (1-x)NaPO3-xB2O3 have been synthesised from Na2CO3, B2O3 and P2O5 and their optical and thermal properties investigated. The results show that refractive index (n) and glass transition temperature (Tg) show a maximum at about B/(B+P) = 0.6 while thermal expansion coefficient (α) and thermo-optic coefficient (dn/dT) change monotonically with the B/(B+P) ratio. These observations can be interpreted based on the incorporation of BO3 and BO4 units into the glass structural network

Optical and thermal properties of binary calcium phosphate and barium phosphate glasses

Binary calcium phosphate and barium phosphate glasses corresponding to xCaO–(100-x)P2O5 and xBaO–(100-x)P2O5, respectively, have been prepared in the range of 20≤x≤50. Assessment of the optical and thermal properties reveal that refractive index (n) and glass transition temperature (Tg) show a minima while thermal expansion coefficient (α) and thermo-optic coefficient (dn/dT) change monotonically as the amount of modifying oxides, CaO and BaO, increase. α > 9 × 10-6/°C is required for the calcium phosphate and barium phosphate glasses to exhibit negative dn/dT

Compositional effects on the thermo-optic coefficients of potassium aluminophosphate glasses

The central wavelength of lightwave devices such as gratings is sensitive to temperature, due to the temperature dependence of the devices' optical path lengths. One way to solve this problem is to use an athermal device. Phosphate glasses are ideal hosts for such devices as they exhibit negative thermo-optic coefficient (dn/dT), which counters the effect of expansion. For this study, fourteen different compositions of potassium aluminophosphate glasses, of the K2O-Al2O3-P2O5 system, were prepared with O/P ranging from 2.8 to 3.1. Refractive indices, glass transition temperature (Tg), coefficient of thermal expansion (CTE) and thermo-optic coefficient (dn/dT) measured for the glasses are what is expected with the addition of aluminium : an increase in Tg and a reduction in CTE. Tg changes in the range of 272°C to 621°C while CTE decreases from 21x10-6/°C to 7x10-6/°C as the aluminium content increases. We are able to achieve dn/dT value as negative as -16x10-6/°C. In addition, the glasses are thermally stable, judging by the large difference between Tg and the temperature of onset of crystallisation. Athermal glasses have been achieved at low aluminium content with high CTE, which in turn leads to negative dn/dT. These results are consistent with the polarisability effect in the glasses as low aluminium content lowers polarisability, which gives rise to negative dn/dT. The CTE of glasses must be more than 10x10-6/°C to achieve negative dn/dT. Any deficit or excess P2O5 does not seem to affect the CTE and dn/dT

Compositional effects on the thermo-optic coefficients of barium borophosphate glasses

Thermal stability of wavelength is crucial in Dense Wavelength Division Multiplexing (DWDM) systems. Therefore there is a need to address the problem with lightwave devices such as gratings, where the central wavelength changes with temperature. Using an athermal device is one method to eliminate this problem. We are currently investigating borophosphate glasses as hosts for athermal devices as both Bg, CTE and dn/dT as a function of glass compositions will be presented. The practicality of using such athermal glasses for lightwave devices will also be presented, including the various techniques to produce waveguides in these glasses

UV-written channel waveguides in Er³⁺-doped Bi₂O₃-based glass

The present paper describes the fabrication of optical channel waveguides in an Er3+-doped Bi2O3-based glass using direct laser writing with a UV beam at 244 nm. A buried positive refractive index change was achieved through this method and a description of the observed phenomena is given, based on optical and electron microscopy