<title>Rational harmonic mode-locking fiber laser</title>

Abstract

Optical pulse sources with repetition rate approaching Terahertz is very import for many photonics applications including ultra-high speed optical communication and generation of sub-mm waves. Both active and passive mode locked fiber lasers are the appropriate choice for this purpose because of the availability ofErbium doped fiber amplifier. In general, the mode locking occurs with a repetition rate of nf, where n is an integer and f, is the longitudinal mode frequency spacing. This is called harmonic mode locking. In the case ofrational harmonic mode locking, the repetition rate is (np + 1) f,, where p is also another integer. For the case ofactive mode locking, this is obtained when the modulation frequency to the amplitude or phase modulator used for mode locking is given by (n + l/p) f. For the case of passive mode-locking, the rational harmonic mode-locking occurs when the saturable absorber in a ring laser is offset by a fraction p/L from the center where L is the length of the cavity. We have developed a theory of the rational mode locked fiber laser. The results of the theory are compared with experimental results obtained from a l.5p fiber laser actively mode-locked with a LiNbO3 electro-optic phase modulator.

Subharmonic Generation in a Bandwidth Clamped Actively Mode-Locked Fiber Laser

“…where n and P are integer, and p m is a proper fraction. Many previous works indicate that the repetition frequency of the mode-locked pulse train is the lowest common multiple of the cavity resonance frequency and the modulation frequency [4], [5], [14], [15]. Therefore, for any value of m, the subharmonic frequency ω m can not be locked under the condition of g < ω m .…”

“…The subharmonic generation mechanism can be explained by the mode-locking cavity theory [14], [15]. The schematic AML-FRL and its spectral description are shown in Fig.…”

“…Let a(t) be the envelope of the optical pulse, a q (ω) denotes the qth Fourier component of a(t), and a (k) q (ω) is the kth circulation of a q (ω) inside the cavity. Then the recursion equation for a (k) q (ω) can be written as [14], [15]:…”