**5. Conclusion**

A brief review of the photonic crystal fibre and the modal interferometers based on PCF are presented in this chapter. Along with the review the operating principle and the fabrication of a reflection type PCF based modal interferometer are also explained in the chapter. The dependence of the interferometer fringe spacing on the length of PCF is also explained and demonstrated experimentally. The experimental investigation and demonstration of a humidity sensor based on a PCF interferometer is presented in the chapter with a brief explanation of the operating principle of the sensor. The water vapour adsorption/desorption phenomena on silica surface are briefly addressed to explain the operating principle of the sensor. The chapter includes the experimental investigation of the relative humidity response of the sensor and the dependence of its sensitivity on the length of PCF. It is shown that a device with a longer length of the PCF section is more sensitive to relative humidity changes. A dew sensor based on PCF interferometer is presented along with the explanation of its sensing principle. The chapter presents the temperature dependence of the PCF interferometer and the dependence of its sensitivity on the length of the PCF. The dew sensing performances of PCFIs with different lengths and at different ambient relative humidity values are also presented. Based on the explained dew sensor a novel dew point hygrometer using PCF interferometer is also proposed in the chapter.

#### **6. References**

172 Photonic Crystals – Introduction, Applications and Theory

temperature of the PCFI back to room temperature the interference peaks also shift back to their initial position. This shows the reversibility of the sensor. Because of the small size of the sensor head and the high sensitivity to adsorbed water vapour the demonstrated sensor response time is in seconds which is relatively fast compared to existing dew point hygrometers that take several minutes for a single measurement. The simple fabrication method, small size and the all-silica nature of the demonstrated sensor head suggest that with some simple additions such as attaching a TEC element with temperature feedback on

to the PCFI, the combination can be used as a dew point hygrometer.

Fig. 11. Interference peak shift of PCFI with respect to temperature at three ambient

A brief review of the photonic crystal fibre and the modal interferometers based on PCF are presented in this chapter. Along with the review the operating principle and the fabrication of a reflection type PCF based modal interferometer are also explained in the chapter. The dependence of the interferometer fringe spacing on the length of PCF is also explained and demonstrated experimentally. The experimental investigation and demonstration of a humidity sensor based on a PCF interferometer is presented in the chapter with a brief explanation of the operating principle of the sensor. The water vapour adsorption/desorption phenomena on silica surface are briefly addressed to explain the operating principle of the sensor. The chapter includes the experimental investigation of the relative humidity response of the sensor and the dependence of its sensitivity on the length of PCF. It is shown that a device with a longer length of the PCF section is more sensitive to relative humidity changes. A dew sensor based on PCF interferometer is presented along with the explanation of its sensing principle. The chapter presents the temperature dependence of the PCF interferometer and the dependence of its sensitivity on the length of the PCF. The dew sensing performances of PCFIs with different lengths and at different ambient relative humidity values are also presented. Based on the explained dew sensor a novel dew point hygrometer using PCF

humidity values of 40, 60 and 80 %RH.

interferometer is also proposed in the chapter.

**5. Conclusion** 


**9** 

*1Poland 2Germany* 

**Arc Fusion Splicing of Photonic Crystal Fibres** 

Arc fusion splicing is an established method for joining optical fibres in communication networks, ensuring splice loss down to 0.05 dB and excellent reliability. Telecom fibres are covered by IEC 60793 and ITU-T G.651.1-G.657 standards, with common material (fused silica) and cladding size (125 μm). Splicing equipment for these fibres is widely available. Fusion splicing of specialty fibres, like dispersion compensating fibres (DCF), polarizationmaintaining fibres (PMF), rare-earth doped active fibres and photonic crystal fibres (PCF) having varying, not standardized designs, dimensions and materials is considerably more difficult. Some, like PMF and many PCFs lack axial symmetry, requiring rotational alignment before fusion. However, this functionality is not available in common splicing machines. A length of specialty fiber enclosed inside a device like optical amplifier, sensor or dispersion compensator is usually spliced at both ends to telecom single mode fibres

Splicing procedure must be tailored to particular fibre, often in a time-consuming trial-anderror way. Special solutions, like fiber pre-forming or insertion of intermediate fiber are sometimes needed. Dedicated splicing machines for such fibres employing both arc fusion

Fusion splicing of photonic crystal or other "holey" fibres with numerous tiny, ca. 0.5-4 μm gas-filled holes is particularly hard because holes collapse quickly once glass is heated to

There is a need to splice "holey" or "microstructured" fibres for characterization and experiments using typical tools and equipment. PCF-SMF splices are most common, as PCFs need to be connected to test instruments, optical devices and circuits incorporating or designed for SMFs. Splicing PCF to a SMF requires special fiber handling and machine settings different from splicing SMF to SMF, like reduced arc power and fusion time shortened to 0.2-0.5 s. More difficult splicing of two lengths of PCF is much less common.

Fusion splicing has the advantage of gas-tight sealing a length of PCF, which is of importance in making gas-filled absorption cells or protecting the fibre against penetration

(SMF) for connections to other components and external interfaces.

of humidity, dust or vapours in hostile operating environment.

(OFS, 2008) and hot filament methods (Vytran, 2009) exist, but are expensive.

melt; this disturbs radiation guiding, introduces loss and causes fiber shrinkage.

**1. Introduction** 

Krzysztof Borzycki1 and Kay Schuster2 *1National Institute of Telecommunications (NIT)* 

*2Institute of Photonic Technology (IPHT)* 

