Active mode-locking

If the modulator is placed inside the laser resonator with mirror separation d and mode frequencies = Uq m c/2d (m = 0,1,2,. ..) the sidebands coincide with resonator mode frequencies if the modulation frequency f equals the mode separation Ai/ = c/2d (Fig. 11.8c). The sidebands can then reach the oscillation threshold and participate in the laser oscillation. Since they pass the intracavity modulator they are also modulated and new sidebands = i/q 2f are generated. This continues until all modes inside the gain profile participate in the laser oscillation. There is, however, an important difference to normal multimode operation The modes do not oscillate independently but are phase-coupled by the modulator. At a certain time tg the amplitudes of all modes have their maximum at the location of the modulator and this situation is repeated after each cavity round-trip time T = 2d/c. We will discuss this in more detail  

For equal mode amplitudes Aj = Aq (11.8) gives the total time-dependent intensity 

If the amplitude Aq is time independent (CW laser), this represents a sequence of equidistant pulses with the separation 

Note For equal amplitudes = Aq the time-dependent intensity I(t) in (11.9) corresponds exactly to the spatial intensity distribution I(x) of light diffracted by a grating with N grooves, which are illuminated by a plane wave. One has to replace fit by the phase difference (j between neighbouring interferring partial waves, see Eq.(4.4) in Sect.4.2 and compare Figs. 4.21 and 11.9. 

In real mode-locked lasers the amplitudes Aj are generally not equal. Their amplitude distribution Aj depends on the form of the spectral gain profile. This modifies (11.9) and gives slightly different time profiles of the mode-locked pulses but does not change the principle considerations. 

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Fig. 5. High repetition rate pulsed excitation systems for picosecond absorption and emission studies, (a) Pulsed e -beam with Cerenkov or laser probe pulses (b) actively mode-locked, synchronously pumped argon ion jet stream dye laser. See text for further details.

Ling WJ, Zhang SG, Zhang MX, Dong Z, Li K, Zuo YY et al (2010) High power continuous-wave actively mode-locked diode-pumped Nd YAG laser. Chin Phys Lett... 

Shcherbakov AS, Kosarsky AY, Moreno Zarate P, Campos Acosta J, Il in YV, Tarasov IS (2011) Characterization of the train-average time-frequency parameters inherent in the low-powta- picosecond optical pulses generated by the actively mode-locked semiconductor laser with an exbsnal single-mode fibta- cavity. Optik 122 136-141... 

Wojcik AK, Malara P, Blanchard R, Mansuripur TS, Capasso F, Belyanin A (2013) Gentsation of picosecond pulses and frequency combs in actively mode locked external ring cavity quantum cascade lasers. Appl Phys Lett 103... 

Yin K, Zhang BB, Yang WQ, Chen H, (Then SP, Hou J (2014) Flexible picosecond thulium-doped fibCT laser using the active mode-locking technique. Opt Lett 39 4259-4262... 

Fig. 6.8 Active mode locking (a) sideband generation (b) experimental arrangement with a standing ultrasonic wave inside the laser resonator (c) idealized output pulses...

Time-Resolved Laser Spectroscopy active mode locking of cw lasers... 

PASSIVE MODE-LOCKING--10 ACTIVE MODE-LOCKING... 

Active mode locking utilizes a time-varying element within the laser cavity, such as an integrated electro-optic amplitude or phase modulator driven by an external frequency reference, to promote phase locking of the cavity modes. The advantage of active mode locking is that pulses are generated at a stable repetition rate. The schematic of a typical actively mode locked fiber laser is shown in Fig. 9. 

FIGURE 9 Schematic of an actively mode locked fiber laser. The modulator drive frequency must be a harmonic of the cavity fundamental frequency and determines the pulse repetition rate. 

Hybrid mode locking techniques combine the best features of other mode locking methods. For example, an active mode locking element may be inserted into a passively mode locked fiber laser cavity to produce an ultrashort pulse laser with well-defined repetition rate. Similarly, semiconductor saturable absorbers may be inserted into passively mode locked fiber lasers to initiate soliton generation, resulting in an ultrashort pulse laser with low selfstarting threshold. 

For excitation and detection of fine structure beats with a frequency of 517 GHz subpicosecond light pulses are necessary. For this purpose we used a synchronously pumped mode-locked dye laser with saturable absorber in the dye solution. The dye laser generates light pulses of about ItOO fs duration at a pulse rate of 8U MHz. It is pumped by a frequency doubled actively mode-locked Nd YAG laser and tuned to a wavelength of 589.3 nm for resonant excitation of the Na atoms into the 3p fine structure states. 

Ortigosa-Blanch, A., Mora, J., Capmany, J., Ortega, B. Pastor, D. (2006). Tunable radiofrequency photonic filter based on an actively mode-locked fiber laser. Optics Letters, Vol. 31, No. 6, pp. 709-711, Issn 0146-9592. 

Passive mode locking is a technique that demands less experimental effort than active mode-locking it can be applied to pulsed as well as to cw lasers. Pulse widths below 1 ps have been realized. Its basic principles can be understood as follows ... 

Table 11.1. Summary of different mode-locking techniques. With active mode locking an average power of 1 W can be achieved...

Soliton ring fiber lasers can be also realized with active mode locking by polarization modulation [11.77], or by additive-pulse mode locking (APM). In the latter technique the pulse is split into the two arms of an interferometer and the coherent superposition of the self-phase modulated pulses results in pulse shortening [11.55]. 

The quality of the actively mode-locked pulse trains and the smoothness of one single pulse depends on the acousto-optical modulator. In Fig. 5 a mode-locked pulse train and a single pulse of 3 ns halfwidth are shown. A further result of this measurement is that only one pulse is selected by the pulse cutting system. 

For the one-color picosecond pump probe experiments (see Sect. 3.2.2), an astigmatically compensated dye laser (Spectra Physics model 375) with rhodamine 6G/rhodamine B is used [190]. The dye laser is synchronously pumped by an actively mode-locked argon ion laser (Spectra Physics model 171). In the synchronous pumping process, the output pulses of the mode-locked ion laser are used to excite the dye laser, whose cavity length has been extended to be equal to the ion laser s cavity 1.8 m) [187]. The transmission of the dye laser s output coupler is 22%. With this setup the emitted dye laser pulses have a pulse duration of 1.38 ps and a bandwidth of 40 cm (see Fig. 2.3). The pulse repetition rate is 82.5 MHz and an average power of 120 mW is obtained over the tuning from 600 nm to 630 nm. 

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