2.9 Erbium Doped Fiber Laser Analysis
2.9 Erbium Doped Fiber Laser
The EDFLs can operate in several wave-length regions, ranging from visible to far infrared. The 1.55 region has attracted the most attention because it coincides with the low-loss region of silica fibers used for optical-communications [14]. The erbium absorption line that best matches with available high power pump diodes is in the 980 regime. At this wave-length, the absorption cross section of erbium is so low that an excessive amount of fiber would be required for efficient pump absorption, and due to reabsorption inherent in a three level laser system, the overall laser efficiency would suffer due to the long fiber. Higher power diodes at 1480 are being developed for directly pumping erbium lasers in double …show more content…
Doped in a solid host, ion has allowable intra- shell transition from its first excited state to the ground state and the transition corresponds to a wave-length of minimum optical loss in silica based optical fibers (1.55 ). Thus, -doped materials are ideal candidates to make amplifiers for optical-communications [83].
2.10 Mode-Locking Techniques
There are two ways to understand how a mode-locked laser works. One can examine what happens in the time domain by thinking about what happens as laser light moves back and forth between the mirrors, or one can examine what happens in the frequency domain by thinking about how the longitudinal modes of the laser interfere with each other [20]. Fiber lasers operate simultaneously in a large number of longitudinal modes falling within the gain bandwidth. Multimode operation is due to a wide gain band-width compared with a relatively small mode spacing of fiber lasers. The total optical field can be written as [20] where , , and are the amplitude, phase, and frequency of a specific mode among …show more content…
Under mode locking, the laser output is in the form of a pulse train with a repetition rate equals to . The pulse width is estimated from Eq.(2.21) to be . Since represents the total band-width of all phase-locked modes, the pulse width is inversely related to the spectral band-width over which phases of various longitudinal modes can be synchronized [15].
The methods of mode-locking can be divided into two categories: active-mode-locking and passive mode-locking [84]. Passive mode-locking typically generates shorter pulses than active mode locking, but active mode-locking often produces pulses with less fluctuation and jitter [18].
Active mode-locking is usually achieved by placing, inside the laser cavity, either an amplitude modulator, which produces a periodic modulation in time of the cavity loss, or a phase modulator, which periodically varies the optical length of the resonator. In lasers with upper-state lifetimes shorter than the cavity round-trip time, active mode-locking can also be achieved by periodic modulation of the laser gain at a repetition rate equal to the longitudinal mode separation
The EDFLs can operate in several wave-length regions, ranging from visible to far infrared. The 1.55 region has attracted the most attention because it coincides with the low-loss region of silica fibers used for optical-communications [14]. The erbium absorption line that best matches with available high power pump diodes is in the 980 regime. At this wave-length, the absorption cross section of erbium is so low that an excessive amount of fiber would be required for efficient pump absorption, and due to reabsorption inherent in a three level laser system, the overall laser efficiency would suffer due to the long fiber. Higher power diodes at 1480 are being developed for directly pumping erbium lasers in double …show more content…
Doped in a solid host, ion has allowable intra- shell transition from its first excited state to the ground state and the transition corresponds to a wave-length of minimum optical loss in silica based optical fibers (1.55 ). Thus, -doped materials are ideal candidates to make amplifiers for optical-communications [83].
2.10 Mode-Locking Techniques
There are two ways to understand how a mode-locked laser works. One can examine what happens in the time domain by thinking about what happens as laser light moves back and forth between the mirrors, or one can examine what happens in the frequency domain by thinking about how the longitudinal modes of the laser interfere with each other [20]. Fiber lasers operate simultaneously in a large number of longitudinal modes falling within the gain bandwidth. Multimode operation is due to a wide gain band-width compared with a relatively small mode spacing of fiber lasers. The total optical field can be written as [20] where , , and are the amplitude, phase, and frequency of a specific mode among …show more content…
Under mode locking, the laser output is in the form of a pulse train with a repetition rate equals to . The pulse width is estimated from Eq.(2.21) to be . Since represents the total band-width of all phase-locked modes, the pulse width is inversely related to the spectral band-width over which phases of various longitudinal modes can be synchronized [15].
The methods of mode-locking can be divided into two categories: active-mode-locking and passive mode-locking [84]. Passive mode-locking typically generates shorter pulses than active mode locking, but active mode-locking often produces pulses with less fluctuation and jitter [18].
Active mode-locking is usually achieved by placing, inside the laser cavity, either an amplitude modulator, which produces a periodic modulation in time of the cavity loss, or a phase modulator, which periodically varies the optical length of the resonator. In lasers with upper-state lifetimes shorter than the cavity round-trip time, active mode-locking can also be achieved by periodic modulation of the laser gain at a repetition rate equal to the longitudinal mode separation