**1. Introduction**

252 Photonic Crystals – Introduction, Applications and Theory

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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 a multi-wavelength operation.

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

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

Multi-Wavelength Photonic Crystal Fiber Laser 255

form a cladding. These parameters can easily be tailored to increase fiber nonlinearity,

~1.5@1.55µm

0.82@1.55 µm 1.18@1.48 µm

**Fiber Type PCF Bi-EDF Length(m)** 20 2.15 **Numerical Aperture (NA)** 0.2 0.2 **Core( μm )** 4.8 5.4 **Cladding( μm )** 125 125.7 **Mode field diameter( μm )** 4.2 6.12 **Zero dispersion wavelength(nm)** 1040 1513 **Cut off wavelength (nm)** 1000 1180 **Effective area(μm)2** 27.5 29.4 **V-number** 1.94 2.18 **Material** Pure silica Bi2o3-Er doped

**Brillouin gain, gB(m/W)** 5×10-7 3.8×10-7

**(ps/nm.Km)** ~70 -120

**(w.km)-1@1550nm** ~33.8 ~60

**1.55 μm** 1.46/1.45 2.03/2.02

Fig. 2. The Scanning Electron Micrograph (SEM) of the PCF cross section and an enlarged

The highly nonlinear PCFs have many applications such as wavelength conversion [18] and Brillouin fiber lasers (BFLs) [13]. So far, few reports have been published on the Brillouin effects in PCFs [18, 19, 20]. The stimulated Brillouin scattering (SBS) is a nonlinear effect that results from the interaction between intense pump light and acoustic waves in a fiber, thus

which is difficult to achieve using conventional fibers.

**Insertion loss (dB)** ~2@1.06µm

**Chromatic dispersion @1550nm** 

**Nonlinear coefficient,** 

**Refractive index of core/cladding at** 

view of the central "holey" cladding.

Table 1. The physical parameters of PCF and Bi-EDF

polarization controller (PC) is used in the cavity to change both the number of lasing lines and spacing of the multi-wavelength laser [8, 9].

There are also other methods to get simultaneous multi-wavelength outputs such as multiwavelength Raman lasers [10, 11], multi-wavelength generation using semiconductor optical amplifiers (SOA) [12] and multi-wavelength Brillouin fiber lasers (BFLs) [13,14]. Special fibers such as dispersion compensating fibers (DCFs) have been used to increase the Raman gain in multi-wavelength Raman fiber lasers where the output power are limited only by the available pump sources [15]. Furthermore, the BFL is easier to be generated due to the lower threshold pump power [16].

Of the various approaches, the interest on the multi-wavelength fiber laser is increasing due to the improvements in number of lasing lines and power flatness. Furthermore, the Brillouin Erbium fiber laser (BEFL) is easier to be generated due to the lower threshold pump power for achieving the stimulated fiber laser [17]. Recently, the hybrid of EDFAs and new compact optical fibers like PCFs as a gain medium have many applications for producing amplifiers and fiber lasers.
