**6. Summary**

248 Photonic Crystals – Introduction, Applications and Theory

increases 10% and decreases 10%. It is clear that the measurement will deviate from true

47 48 49 50 51 52 53 54 55 **Temperature(**℃**)**

> Power constanting Power increasing Power decreasing

47 48 49 50 51 52 53 54 55

**Temperature(**℃**)**

Fig.33 shows the relationship of the differential ΔW with temperature. The fitting function can be written as Y= -0.0075X3 + 1.138X2 - 57.382X + 959.39 with the fitting degree of R2 = 0.9976 when temperature rises from 47°C to 55°C. When the power of the light source increases 10%, keeps constant and decreases 10%, the relationship of the differential ΔW with temperature remains stable and the differential algorithm based on a PCF-LPG eliminates the noises effectively. When the power of the light source changes ±10%, the measured temperature is 49.83 °C and the relative error is 0.34%. Compared with the result without the differential ΔW, the accuracy of the HiBi-FLM sensor rises from

Fig. 32. Relationship of W2 and temperature when the power of light source changes.

Fig. 31. Relationship of W1 and temperature when the power of light source changes

value and the accuracy of temperature sensor is low.

Power constanting Power increasing Power decreasing

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008

**Intensity(mw)**

~90.5% to ~99.7%.

**Intensity(mw**)

In this chapter, we have presented the basic operation principle of LPGs, and demonstrated the special properties of PCF-LPGs. By use of a dispersion factor , a deeper understanding of the behavior of LPG in the ESM-PCF has been achieved. Both the theoretical and experimental results clearly reveal the significant effect of the waveguide dispersive characteristics of the cladding modes on the strain and temperature characteristics of the LPG in the ESM-PCF. By selecting proper grating period, it is possible to design a LPG with specific strain and temperature properties.

We have shortly reviewed PCF-LPG fabrication methods and mainly described the fabrication method by using a CO2 laser. The PCF-LPG fabricated under the theoretical design shows a good agreement with the theoretical predictions. Finally, applications of the PCF-LPG in optical fiber sensors have been demonstrated and discussed fully.

### **7. References**


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**12** 

*1Iran 2Malaysia* 

**Multi-Wavelength Photonic Crystal Fiber Laser** 

The fiber lasers have some advantages compared to bulk-optics systems like compact size, high efficiency and high beam quality. The lasers in time-domain can be categorized into two groups "continuous wave fiber lasers" or "pulsed fiber lasers", and in wavelength domain as single wavelength or multi-wavelength. Such lasers were made as early as 1976 and have remained an active topic of study since then [2, 3]. Fiber lasers can be used to generate CW radiation as well as ultra-short optical pulses. The wavelength division multiplexing (WDM) techniques have shown to unlock the available fiber capacity and to increase the performances of broadband optical access networks. One of the essential components is the creation of new low-cost laser sources. Candidates for such applications are multi-wavelength fiber ring lasers as they have simple structure, are low cost, and have

Recently, multi-wavelength lasers have caused considerable interests due to their potential applications such as WDM systems, fiber sensors and fiber-optics instrumentations. Requirements for multi-wavelength sources include; stable multi-wavelength operation, high signal to noise ratio and channel power flattening. Compared to a system that uses a number of discrete semiconductor diode laser [4], it is physically simpler to produce a multiple wavelength source using a single gain medium including a wavelength selective element. In order to define lasing wavelengths, wavelength selective comb filters have been included in the laser cavity. A multi-wavelength laser is highly desirable for the cost and size reduction, improvement of system integration and compatible with optical communication networks. For the past one decade or so, EDFs have been extensively

In Erbium doped fiber laser (EDFL), the Erbium ions possess split Stark sublevels with multiple allowed transitions possibility of having oscillations at more than one wavelength. Therefore, the multi-transitions can be achieved in this fiber laser due to the depletion of Stark sub-levels which is selective and depends on the polarization of the wave. However, the outputs of the EDFLs are not stable at room temperature due to homogeneous broadening of lasing modes [5]. To increase the in-homogeneity one can cool Er+3 doped fiber at liquid nitrogen temperature [6, 7]. Generally, in order to produce the multiwavelength, we have to employ intra-cavity filter in the EDFL cavity. In some works, a

studied and developed as a gain medium for the multi-wavelength laser.

**1. Introduction** 

a multi-wavelength operation.

*1Department of Electrical Engineering, Isfahan University of Technology, Isfahan 2Department of Electrical Engineering, University of Malaya, Kuala Lumpur* 

S. Shahi1, M. R. A. Moghaddam2 and S. W. Harun2

