Ultrashort Laser Pulse Phenomena, 2nd Edition

346 Ultrashort Sources II: Examples

• when attempting to achieve the shortest pulse durations, the pulse frequen-

cies are randomly distributed [4].

One solution to these problems is to combine the techniques of passive and

active mode-locking in a hybrid system [14,15]. Depending on the optical thick-

ness of the absorber, the hybrid mode-locked laser is either a synchronously

mode-locked laser perturbed by the addition of saturable absorption or a pas-

sively mode-locked laser pumped synchronously. The distinction is obvious to

the user. The laser with little saturable absorption modulation will have the noise

characteristics and cavity length sensitivity typical of synchronously pumped

lasers, but a shorter pulse duration. The laser with a deep passive modulation

(concentrated saturable absorber for a dye, or a large number of MQW for a

semiconductor saturable absorber) shows intensity autocorrelation traces identi-

cal to those of the passively mode-locked laser [16]. The sensitivity of the laser

to cavity detuning decreases. The reduction in noise can be explained as being

related to the additional timing mismatch introduced by the absorber, which par-

tially compensates the pulse advancing influence of the gain and spontaneous

emission [17].

6.3. ADDITIVE PULSE MODE-LOCKING

6.3.1. Generalities

There was in the late 1980s a resurrection of interest in developing additive

pulse mode-locking (APML), a technique involving coupled cavities. One of the

basic ideas—to establish the mode coupling outside the main laser resonator—

was suggested in 1965 by Foster et al. [18] and applied to mode-locking a He-Ne

laser [19]. In that earlier implementation, an acousto-optic modulator is used to

modulate the laser output at half the intermode spacing of the laser. The frequency

shifted beam is reflected back through the modulator, resulting in a first-order

diffracted beam, which is shifted in frequency by the total mode spacing, and

reinjected into the laser cavity through the output mirror. The output mirror of

the laser forms, with the mirror used to reinject the modulated radiation, a cavity

with the same mode spacing as the main laser cavity. If the laser is close to

threshold, a small extracavity modulation fed back into the main cavity can be

sufficient to lock the longitudinal modes.

Unlike this technique more recent APML implementations are based on pas-

sive methods. In the purely dispersive version, pulses from the coupled cavity

are given some phase modulation, such that the first half of the pulse fed back

into the laser adds in phase with the intracavity pulse, while the second half

Additive Pulse Mode-Locking 347

Gain

Mo

Main laser

(a)

(b)

Laser pulse

Shortened pulse

Addition

Cancellation

Modulated

feedback pulse

Feedback

Output

Fiber



Figure 6.3 A typical additive pulse mode-locked laser (a). At the output mirror M

0

, the pulse of

the main cavity [(b), top left] adds coherently to the pulse of the auxiliary cavity [(b), bottom left],

to result in a shortened pulse [(b), right]. (Courtesy E. Wintner.)

has opposite phase [20]. At each round-trip, the externally injected pulse thus

contributes to compress the intracavity pulse, by adding a contribution to the

leading edge and subtracting a certain amount from the trailing edge, as sketched

in Figure 6.3. This technique has first been applied to shortening pulses gen-

erated through other mode-locking mechanisms. A reduction in pulse duration

by as much as two orders of magnitudes was demonstrated with color-center

lasers [21–24] and with Ti:sapphire lasers [25].

It was subsequently realized that the mechanism of pulse addition through

a nonlinear coupled cavity is sufficient to passively mode-lock a laser. This

technique has been successfully demonstrated in a Ti:sapphire laser, [26]

Nd:YAG [27,28] Nd:YLF [29,30], Nd:glass [31], and KCl color-center

lasers [32]. A detailed description of the coherent addition of pulses from the

main laser and the extended cavity which takes place in the additive pulse mode-

locking has been summarized by Ippen et al. [33].

Coherent field addition is only one aspect of the coupled cavity mode-locked

laser. The nonlinearity from the coupled cavity can be, for example, an amplitude

modulation, as in the “soliton” laser [34], or a resonant nonlinear reflectivity via

a quantum well material [35].