**2. Photonic crystal fiber ring laser**

Photonic crystal fibers (PCFs) have generated great interest over the past few years, growing from a research-oriented field to a commercially available technology. The PCFs were first developed by Philip Russell in 1998, and can be designed to possess enhanced properties over (normal) optical fibers. They can be divided into two fundamental classes, solid-core and hollow-core as shown in Figure 1.

Fig. 1. Photonic Crystal Fibers Types, (a) Solid core PCF, (b) Hollow core PCF.

The solid core PCF is used in this report that is two dimensions (it has a periodic geometry in two directions and is homogeneous in the third) and we already introduced physical properties of that in table 1.

Figure 2 shows an electron micrograph of the cross section of this solid core PCF. Despite the hexagonal structure of the cladding, the mode field is very similar to that of the fundamental mode of a conventional fiber. The optical properties of PCFs rely on the specification of the size, shape and arrangement of the holes that surround a solid core to

polarization controller (PC) is used in the cavity to change both the number of lasing lines

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

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

Photonic crystal fibers (PCFs) have generated great interest over the past few years, growing from a research-oriented field to a commercially available technology. The PCFs were first developed by Philip Russell in 1998, and can be designed to possess enhanced properties over (normal) optical fibers. They can be divided into two fundamental classes, solid-core

**(a) (b)** 

Fig. 1. Photonic Crystal Fibers Types, (a) Solid core PCF, (b) Hollow core PCF.

The solid core PCF is used in this report that is two dimensions (it has a periodic geometry in two directions and is homogeneous in the third) and we already introduced physical

Figure 2 shows an electron micrograph of the cross section of this solid core PCF. Despite the hexagonal structure of the cladding, the mode field is very similar to that of the fundamental mode of a conventional fiber. The optical properties of PCFs rely on the specification of the size, shape and arrangement of the holes that surround a solid core to

and spacing of the multi-wavelength laser [8, 9].

lower threshold pump power [16].

producing amplifiers and fiber lasers.

**2. Photonic crystal fiber ring laser** 

and hollow-core as shown in Figure 1.

properties of that in table 1.


form a cladding. These parameters can easily be tailored to increase fiber nonlinearity, which is difficult to achieve using conventional fibers.

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

Fig. 2. The Scanning Electron Micrograph (SEM) of the PCF cross section and an enlarged view of the central "holey" cladding.

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

Multi-Wavelength Photonic Crystal Fiber Laser 257

The BP is injected into the ring cavity and then PCF via the circulator to generate the backward propagating Stokes light at opposite direction. However, since the PCF length is not sufficient enough, the back-scattered light due to Rayleigh scattering is relatively higher than the Stokes light. Both back-scattered pump and the Stokes lights are amplified by the bi-directionally pumped Bi-EDF and it oscillates in the ring cavity to generate first Stokes in an anti-clockwise direction. This oscillation continues and when the intensity of the first Brillouin Stokes is higher than the threshold value for Brillouin gain, the second order SBS is generated in clockwise direction and this signal is blocked by the isolator in the cavity. However, the back-scattered light from second SBS will be amplified by the Bi-EDF. Hence, the nonlinear gain by both PCF and Bi-EDF only amplifies the Stokes light and thus the Stokes light is more dominant and laser is generated at the Stokes wavelength. The spacing between the BP and the BFL is obtained at approximately 10 GHz, which is equivalent to the

The operating wavelength of the BFL is determined by the bi-directionally pumped Bi-EDF gain spectrum which covers the L-band region from 1560 nm to 1620 nm as well as the cavity loss. For comparison and the effect of different cavity resonators, three kinds of output couplers selected. Figure 4 shows the free running spectrum of the BEFL, which is obtained by turning off the BP for three different output coupler ratios; 80/20, 90/10 and 95/5. The output laser is taken from the leg with a lower portion. The peak wave generated at approximately 1574 nm with bandwidth of approximately 3 nm due to the difference between Bi-EDF's gain and cavity loss is the largest in this region. The chosen BFL operating wavelength must be within or close to the bandwidth of free running BFL. Therefore, the BP is set within 1574 nm region which is within the lasing bandwidth of the free running BFL. At the coupling ratio of 80/20, the free-running BFL exhibits the highest peak power of approximately -6 dBm with 20 dB bandwidth of approximately 1 nm. The cavity loss is the

lowest with 80/20 coupler and therefore the peak power is the highest.

Fig. 4. Free-running spectrum of the BEFL using 80/20, 90/10 and 95/5 couplers.

Stokes shift in the single mode fiber (SMF).

giving rise to backward propagating frequency shifted light [13]. In BFL applications, the required gain medium length can be substantially reduced using a holey fiber to replace the conventional SMF-28 Fiber of Corning Inc.[21]. However, most of the earlier works on PCF based BFLs are mainly on a single wavelength operation [21].

In this research, the fibre ring structure based on PCF can be used to make a very stable wavelength and narrow line-width laser. A conceptual structure of such a laser is very similar to a fibre ring resonator. In the ring configurations, a very short length of PCF (20 m) is added in the ring cavity BEFL in the proposed configurations to achieve a stable single and multi-wavelength laser generation.
