CONSCRIPTION AND CONFISCATION

LASER stands for light amplification by stimulated emission of radiation and is a device that allows the amplification of light in a coherent, monochromatic and unidirectional manner in the optical spectrum region. The common characteristic of all types of laser devices is the population inversion in the active gain medium in the laser cavity. In order to generate laser, three basic conditions must be satisfied. First, the laser should have an active gain medium. Secondly, the gain medium should be placed in between the reflective optical cavity to allow circulation of photons. Finally, the external pump source is needed to generate population inversion. All optical fiber lasers are built with these three basic elements. Several configurations of laser cavity will be discussed afterwards.

2.3.1 Laser cavity

The most common laser cavity used for optical fiber laser is a Fabry-Perot resonator as shown in Figure 2.8. It is typically constructed by placing an active gain medium in between two planar dielectric mirrors. The mirrors serve as an input and output coupler. Both ends of the fiber are either perpendicularly cleaved or polished flat. The pump power provided by the pump source is directly coupled into the fiber by splicing or by high transmittance mirror through the input coupler (mirror), which is transparent to the pump light and highly reflective to the generated light emission. The mirrors introduce optical feedback to the laser beam in the cavity, therefore causes a population inversion. The laser beam that propagates back and forth in the cavity enables amplification due to stimulated emission. The generated light leaves the laser cavity through the output coupler (mirror). To ensure that the laser can be realized, the population inversion which produces the gain within the cavity must be sufficient to compensate for the fraction of energy loss due to all causes. Several reflecting elements

such as fiber Bragg grating (FBG), selective wavelength filter and injection locking can also be used to replace the dielectric mirror.

Figure 2.8: Schematic diagram of a Fabry-Perot fiber resonator.

The laser setup can also be configured in a ring cavity as shown in Figure 2.9. The cavity allows light to oscillate in both directions thus generates bidirectional output which limits the efficiency of the laser. The constructive interference between the counterpropagating signals produce a standing wave which induces the spatial hole burning. This effect allows the oscillation of several longitudinal modes in the laser cavity (Digonnet, 2002). The bidirectional signal light travelling in the cavity can be eliminated using an optical isolator by forcing the light travel in a unidirectional operation. However, the small amount of loss introduced by the isolator will increase the threshold of the laser. Another method to reduce the effect of spatial hole burning is to introduce the polarization controller (PC) that modifies the polarization state of light and allows continuous adjustment of the birefringence within the cavity to balance the gain and loss. The ring laser cavity offers the advantages of simplicity in design, low cost and low threshold operation. Ring cavities are commonly designed for pulse laser applications (Nelson et al., 1997; Panasenko et al., 2006).

Pump Light

Active gain medium

High reflecting mirror, R1 Partially transmitting mirror, R2 Laser output Oscillating cavity, L

Figure 2.9: Schematic diagram of all-fiber ring laser resonator.

2.3.2 Important Laser parameters

Laser threshold, output power and efficiency are the important laser parameters. Laser threshold refers to the operation circumstance of a laser which laser emission just starts to occur. It is defined as the minimum amount of pump/input power or energy required to start the lasing action during which the gain coefficient becomes larger than the losses in the cavity. The output is considered to be a laser when the pump power is sufficiently high such that population inversion is achieved and therefore, the energy of the system has reached the lasing threshold. According to Figure 2.8, light beam will oscillate in the cavity length, L between two reflecting mirrors which are high reflecting, R1 and partially transmitting, R2 hence the reflection coefficient is less than

one. The fraction of light that remains after a full round trip passage through the laser is denoted by (Reisfeld et al., 1977):

<I4Y _{+ /}

/4 (2-7)

where  is the measured loss in a single passage and is positive, therefore:

 + −₄ln //4 (2-8) Laser output Coupler Active gain medium Pump Light Output coupler

The intensity of the radiation increased as the laser oscillates in the cavity due to the continuous population inversion by a factor of <[\ where  is amplification coefficient.  is given by  + ]∆, where ] is the absorption coefficient of a pump wavelength and ∆ is the population inversion. By comparing the increased intensity of the radiation in a passage and the fraction of light remains in the cavity, the threshold of the laser oscillation is attained which satisfies the condition below (Reisfeld et al., 1977)

 >  (2-9)

The threshold power usually depends on the gain per unit pump power, the round trip cavity losses, and how strong the pump, signal and dopants are confined (Armitage, 1988).

The slope efficiency is one of the important parameters in characterizing a laser. The efficiency of the laser is given by the ratio of output power of the laser, _, over the absorbed pump power, _. According to (Digonnet, 2002), the output power, _ is given by:

 +_`_M_Ja%a%_cb(d!− a* (2-10)

where ℎ_! and ℎ_$ is the signal photon energy and pump photon energy respectively. e_ is the power transmission of the output coupler, f_ is the round-trip loss, _d! is the total pump power absorbed by the dopant and _a is the threshold power. The Eq. (2-10) states that the output power grows linearly with the absorbed pump power. Thus, the slope efficiency, defined as the output power divided by the absorbed pump power,  + d!− a is (Digonnet, 2002):

The efficiency depends on the slope of the graph. This is due to the active pump photon compared to the signal photon in order to excite ions to the higher level, and the energy difference between them is wasted usually in terms of phonons.