2.1 Ultrafast solid-state lasers - ETH - the Keller Group

36 2.1.2 Definition of Q-switching and mode-locking [Ref. p. 134
In passive Q-switching the shutter is replaced by an intracavity saturable absorber. The saturable
absorber starts to bleach as the intensity inside the laser continues to grow from noise due
to spontaneous emission. Thus the laser intensity continues to increase which in turn results in
stronger bleaching of the absorber, and so on. If the saturation intensity is comparatively small,
the inversion still left in the laser medium after the absorber is bleached is essentially the same as
the initial inversion. Therefore, after bleaching of the saturable absorber the laser will have a gain
well in excess of the losses and if the gain cannot saturate fast enough, the intensity will continue
to grow and stable Q-switching can occur. A large modulation depth of the saturable absorber
then results in a high Q-switched pulse energy.
Typically the pulse repetition rate in Q-switched solid-state lasers is in the hertz to few megahertz
regime, always much lower than the cavity round-trip frequency. Picosecond pulse durations
canbeobtainedwithQ-switched diode-pumped microchip lasers [89Zay, 97Zay] with pulses as short
as 115 ps for active Q-switching using electro-optic light modulators [95Zay] and 37 ps for passive
Q-switching using SESAMs [99Spu1, 01Spu3]. For LIDAR applications passively Q-switched
Er:Yb:glass microchip lasers around 1.5 μm are particularly interesting [98Flu, 01Hae1]. The performance
of Q-switched microchip lasers bridge the gap between Q-switching and mode-locking
both in terms of pulse duration (nanoseconds to a few tens of picoseconds) and pulse repetition
rates (kilohertz to a few tens of megahertz) (Table 2.1.3).
2.1.2.2 Mode-locking
Mode-locking is a technique to generate ultrashort pulses from lasers. In cw mode-locking the pulses
are typically much shorter than the cavity round trip and the pulse repetition rate (from few tens
of megahertz to a few hundreds of gigahertz) is determined by the cavity round-trip time. Typically
an intracavity loss modulator (i.e. a loss modulator inside a laser cavity) is used to collect the laser
light in short pulses around the minimum of the loss modulation with a period given by the cavity
round-trip time T R =2L/v g ,whereL is the laser cavity length and v g the group velocity (i.e.
the propagation velocity of the peak of the pulse intensity). Under certain conditions, the pulse
repetition rate can be some integer multiple of the fundamental repetition rate (i.e. harmonic
mode-locking) [72Bec]. We distinguish between active and passive mode-locking.
For active mode-locking (Fig. 2.1.3), an external signal is applied to an optical loss modulator
typically using the acousto-optic or electro-optic effect. Such an electronically driven loss modulation
produces a sinusoidal loss modulation with a period given by the cavity round-trip time
T R . The saturated gain at steady state then only supports net gain around the minimum of the
loss modulation and therefore only supports pulses that are significantly shorter than the cavity
round-trip time.
For passive mode-locking (Fig. 2.1.3), a saturable absorber is used to obtain a Self-Amplitude
Modulation (SAM) of the light inside the laser cavity. Such an absorber introduces some loss to the
intracavity laser radiation, which is relatively large for low intensities but significantly smaller for
a short pulse with high intensity. Thus, a short pulse then produces a loss modulation because the
high intensity at the peak of the pulse saturates the absorber more strongly than its low intensity
wings. This results in a loss modulation with a fast initial loss saturation (i.e. reduction of the loss)
determined by the pulse duration and typically a somewhat slower recovery which depends on the
detailed mechanism of the absorption process in the saturable absorber. In effect, the circulating
pulse saturates the laser gain to a level which is just sufficient to compensate the losses for the
pulse itself, while any other circulating low-intensity light experiences more loss than gain, and
thus dies out during the following cavity round trips. The obvious remaining question is – how
does passive mode-locking start? Ideally from normal noise fluctuations in the laser. One noise
spike is strong enough to significantly reduce its loss in the saturable absorber and thus will be
more strongly amplified during the following cavity round trips, so that the stronger noise spike
Landolt-Börnstein
New Series VIII/1B1