September 1, 1992 / Vol. 17, No. 17 / OPTICS LETTERS
1219
Simple method to start and maintain self-mode-locking of a
Ti:sapphire laser
Y. M. Liu, K. W Sun, P. R. Prucnal, and S. A. Lyon
Department of Electrical Engineering,Princeton University,Princeton,New Jersey08544
Received May 8, 1992
By periodically moving one of the end mirrors of a Ti:sapphire laser using a shaker, we have been able to start
and maintain self-mode-locking of the laser. The resulting laser is stable over a long period with low-amplitude
noise (3.3%)and low random timing jitter (4.8 ps). In addition, there is a deterministic timing variation on the
order of subnanoseconds, caused by the moving mirror, which can be reduced by employing a feedback system.
Pulses as short as 43 fs have been obtained.
Mode-locked Ti:sapphire lasers have proven to be reliable femtosecond light sources in the near-infrared
regime. The mode locking of these lasers has
been actively investigated by a number of research
groups. Various techniques, such as coupled-cavity
mode locking,' active mode locking,2' 3 coupled-cavity
resonant passive mode locking,4 synchronous pumping,' and self-mode-locking,
6
have been used to gen-
cess to self-start from the most intense noise pulse
of the laser. Therefore an induced strong fluctuation is required. This fluctuation can be initiated
eleeither by using the traditional mode-locking
23
ments (e.g., a dye jet7 or an active mode locker ' ) or
by perturbing the cavity by using, e.g., a moving
external-cavity mirror 0"'" or cavity-length variation
by a pair of rotating Brewster plates inside the cav-
erate ultrashort pulses. Since self-mode-locking was
demonstrated by Spence et al.6 in a multitransversemode cavity, it has also been observed in three other
mode-locking configurations mentioned above.2 5
However, self-mode-locking is not self-starting, 6
thus one must perturb the cavity (e.g., tap a mirror) to
initiate mode locking. Although self-mode-locking
can be initiated by using a dye jet in the cavity,7 an
active mode locker,2' 3 or a synchronously modelocked pump laser,5 these techniques complicate the
Ti:sapphire laser cavity.
In this study we move one of the end mirrors of
the Ti:sapphire cavity8 to start self-mode-locking of
the laser by mounting one of the cavity mirrors (the
total reflector in this case) on a shaker of the variety typically used in autocorrelators. The resulting
self-mode-locked laser operates stably from day to
day with low timing jitter and low noise. In our
laser, the moving mirror is used only as a starting
mechanism for self-mode locking. More than two
ity' 3 and a moving main-cavity mirror.8 If the
Ne gas laser when one of the end mirrors was moved
above a critical velocity. Recently, this moving mirror was used in the external cavity of a Ti:sapphire
laser'l to generate 5-ps pulses. However, self-modelocking was not observed, possibly because the degree of external-cavity feedback was not appropriate
In the setup we use a longitudinally pumped,
astigmatism-compensated four-mirror Ti:sapphire
laser cavity6 with the total reflecting end mirror
decades ago Smith 9 reported mode locking of a He-
and the intracavity
peak power was too low. More
recently, such a laser has been self-mode-locked to
generate 47-fs pulses by using a suitable externalcavity alignment."
It is believed that the mechanism for self-modelocking of a Ti:sapphire laser is based on the optical
Kerr nonlinearity (Kerr-lens self-focusing) inside
the Ti:sapphire rod.'2 However, the nonlinearity is
usually not strong enough for the mode-locking pro0146-9592/92/171219-03$5.00/0
main-cavity mirror is moved at a sufficient velocity
to sweep the frequencies of the competing longitudinal modes in an interval less than their interaction time, then the energy exchange between the
competing modes under the gain curve can be reduced. As a result, each mode will retain a definite
phase relationship with the other modes, and strong
mode beating at harmonics of the cavity repetition
rate will result. In our laser, we observe a strong
amplitude modulation occurring every half-period of
the shaking, caused by mode beating at twice the
cavity repetition rate. The modulation is similar to
the initial modulation caused by a saturable absorber for self-mode-locking.'4 If the mirror moves
fast enough, the most intense mode-beating pulse
will be strong enough to initiate mode locking.
This observation is different from either the
Q switching observed by Rizvi et al." or the picosecond fluctuations caused by mode dragging as suggested by Spinelli et al."3
mounted on a shaker. Two SF-14 prisms are placed
40 cm from tip to tip in the cavity to control the
group-velocity dispersion and the pulse width. We
also employ an adjustable slit positioned between
the two prisms to limit the transverse beam size and
to improve the mode-locking stability once the laser
is self-mode-locked. 4 The Ti:sapphire crystal is
pumped with all lines of an argon-ion laser. The
threshold pump power for the self-mode-locked operation is less than 4 W When the laser is pumped
with a power of 10 W, we obtained self-mode-locking
for center wavelengths between 750 and 920 nm
© 1992 Optical Society of America
1220
OPTICS LETTERS
/ Vol. 17, No. 17 / September 1, 1992
eration. However, if the shaker remains on, mode
locking is never observed to cease. In addition, we
have observed that there is no discernable changes
in the pulse width or the corresponding optical spec-
trum if the shaker is turned on.
With the shaker on, we did not observe any amplitude modulation or dropouts in the self-mode-locked
pulse train. To examine the shaker-induced noise
more carefully, Fig. 3 shows the low-frequency
(a)
:.D_
797-
C
797.9
847.9
897.9
Wavelength
(nm)
(b)
amplitude-noise sidebands of the Ti:sapphire laser
with the shaker both on and off, where the shaker is
operating at 25 Hz with an amplitude of 8 Am.
Note that as shown in Fig. 3 with the shaker on, the
noise at the shaker frequency (25 Hz) is approximately 1 dB larger than that with the shaker off,
which is caused by the imperfect axial movement of
the shaking mirror. This noise can be neglected,
however, because it is at least 6 dB less than the
noise at the power line frequency (60 Hz), as shown
in Fig. 3. In addition, the noise at lower frequencies
is reduced when the shaker is on. In another measurement, the self-mode-locked Ti:sapphire laser is
characterized with a microwave spectrum analyzer
(Hewlett-Packard HP 47000A). A fast GaAs photodetector is used at the input to the spectrum analyzer. The measured first-order power spectra of
the mode-locked pulse train with the shaker both off
Fig. 1. (a) Sweep intensity autocorrelation trace of the
43-fs pulses and (b) the corresponding spectrum with a
FWHM bandwidth of 20.8 nm.
eu-
with a midrange (800-900 nm) optics set. Pulses
as short as 43 fs have been generated. Figure 1(a)
is the sweep intensity autocorrelation trace of the
43-fs pulse. Figure 1(b) is the corresponding spectrum with a FWHM bandwidth of 20.8 nm. If we
assume a sech2 pulse, the pulse width-bandwidth
E
-0
10 -
Self-mode locking
E
product is 0.36, which is close to the Fouriertransform-limited value of 0.32. As mentioned
above, to start and maintain more stable self-modelocking, the peak amplitude of the shaking mirror
must exceed a certain threshold for a given shaking
frequency. Once this threshold is reached, the measurements indicate that the self-mode-locking process begins within approximately 0.5-1.6 ms, which
does not depend on the slit size. In addition, to stabilize the mode-locked pulse train and the pulse
width we decrease the slit size, and the output power
is reduced by 30% from 450 mW (cw) to 315 mW
n.
0
N
to start self-mode-locking versus the frequency
C
a)
0
similar to that of French et al.,'0 who found a minimum shaker velocity of 5 mm/s. We have also ob-
served that the threshold shaking amplitude for
starting self-mode-locking is lowered if the pump
power is increased. If the shaker is turned off, the
self-mode-locked laser eventually reverts to cw op-
30
40
50
Fig. 2. Threshold condition for a shaker to start and
maintain self-mode-locking of a Ti:sapphire laser. At
threshold, the product of the shaker frequency and amplitude is approximately constant, which corresponds to a
shaker peak velocity of approximately 0.26 mm/s.
m
approximately 0.26 mm/s. This observation is
20
Shaker Frequency (Hz)
(mode locked) for 6-Wpump power.
Figure 2 is a plot of the threshold peak amplitude
when the laser pump power is 10 W, the mode-locked
output power is 550 mW,and the center wavelength
is 870 nm. At threshold, the product of the shaker
frequency and the amplitude is approximately constant, which corresponds to a shaker peak velocity of
10
-40
a-
W
-80
0.
U,
B -100
0
.0
Z -120
10
100
1000
Frequency
(Hz)
Fig. 3. Conmparisonof the low-frequency amplitude fluctuations of the self-mode-locked Ti:sapphire laser with the
shaker both on and off.
September 1, 1992 / Vol. 17, No. 17 / OPTICS LETTERS
0
Offsetfrequency(kHz)
-1.5
by employing a combination of a photodetector
and an electronic feedback circuit. The shaker is
turned off once self-mode-locking is initiated. If
the self-mode-locking is interrupted, the shaker is
turned on again.
In conclusion, by simply mounting one of the end
mirrors on a shaker, we have demonstrated a selfmode-locked Ti:sapphire laser with long-term stability, pulse energy fluctuations of less than 3.3%, and
a random timing jitter less than 4.8 ps. Fouriertransform-limited pulses as short as 43 fs have been
generated by this laser. Since this method of starting self-mode-locking of a Ti:sapphire laser is wavelength independent, it is applicable over the full
tuning range of the Ti:sapphire crystal as well as
other self-mode-locked lasers.
1.5
Fig. 4. First-order power spectra of the self-mode-locked
Ti:sapphire laser with the shaker both on and off. The
resolution bandwidth
is 30 Hz.
3.3% and 4.8 ps, respectively, for offset frequencies
above 50 Hz. This random noise is mainly caused
by mechanical vibrations of the optics, which are
mounted 6.5 in. (16.5 cm) above the optics table, and
the pump-induced fluctuations.
In addition to the random timing jitter of the
laser, the moving mirror also imposes a periodic
timing variation, which can be identified from the
broadened spectrum with the shaker on in Fig. 4.
The maximum timing variation AT of the pulse
train is calculated as the timing variation accumulated over one period (1/fm ) of the shaker oscillation,
6
AT
AL
2lTLfmn
References
1. J. Goodberlet, J. Wang, J. G. Fujimoto, and P. A.
and on are shown in Fig. 4. Both spectra exhibit
similar random amplitude- and phase-noise sidebands. The random phase-noise contribution can
be identified by the quadratic increase in its spectral
power with the harmonic order, while the amplitude
noise spectral power is independent of the harmonic
order.'6 From the measurements of the first- and
fifth-order power spectra the random pulse-energy
fluctuation and timing jitter are calculated to be
where'
1221
(1)
Schulz, Opt. Lett. 14, 1125 (1989).
2. J. D. Kafka, M. L. Watts, and T. Baer, in Digest
of Conferenceon Lasers and Electro-Optics (Optical Society of America, Washington, D.C., 1991),
paper JMB3.
3. P. F Curley and A. I. Ferguson, Opt. Lett. 16, 1016
(1991).
4. U. Keller, G. W 'tHooft, W. H. Knox, and J. E.
Cunningham, Opt. Lett. 16, 1022 (1991).
5. C. Spielmann, E Krausz, T. Brabec, E. Wintner, and
A. J. Schmidt, Opt. Lett. 16, 1180 (1991).
6. D. E. Spence, P. N. Kean, and W Sibbett, Opt. Lett.
16, 42 (1991).
7. N. Sarukura, Y.Ishida, and H. Nakano, Opt. Lett. 16,
153 (1991).
8. Y. M. Liu, K. W Sun, P. R. Prucnal, and S. A. Lyon, in
Digest of OpticalSocietyof America Annual Meeting
(Optical Society of America, Washington, D.C., 1991),
p. 61.
9. P. W Smith, Appl. Phys. Lett. 10, 51 (1967).
10. P. M. W French, D. U. Noske, N. H. Rizvi, J. A. R.
Williams, and J. R. Taylor, Opt. Commun. 83, 185
(1991).
11. N. H. Rizvi, P. M. W French, and J. R. Taylor, Opt.
Lett. 17, 279 (1992).
12. M. Pichg, N. McCarthy, and E Salin, in Digest of Optical Society of America Annual Meeting (Optical Society of America, Washington, D.C., 1990), paper MB8.
For example, with a shaker amplitude AL = 1 /-km
and frequency f m = 100 Hz, Eq. (1) yields a maximum timing variation AT = 0.9 ns over a shaking
period of 10 ms. Although this amount of timing
variation is not a problem for some applications,
13. L. Spinelli,
such as pump-probe experiments, it can present
16. G. T. Harvey,
some difficulties in synchronous systems. The timing variation caused by the shaker can be reduced
B. Couillaud,
N. Goldblatt,
and D. K.
Negus,in Digestof Conferenceon Lasers and ElectroOptics (Optical Society of America, Washington, D.C.,
1991), paper CPDP7.
14. N. Sarukura and Y. Ishida, Opt. Lett. 17, 61 (1992).
15. D. von der Linde, Appl. Phys. B 39, 201 (1986).
M. S. Heutmaker,
P. R. Smith,
M. C.
Nuss, U. Keller, and J. A. Valdmanis, IEEE J. Quantum Electron. 27, 295 (1991).