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Optically-Pumped SESAM for Fast Switching between Continuous Wave and
Passively Mode Locked Regimes of a Femtosecond Pulse Cr4+:forsterite Laser
Christopher G. Leburn, Christian T. A. Brown, Wilson Sibbett
J. F. Allen Research Laboratories, SUPA, School of Physics and Astronomy,
University of St Andrews, St Andrews, KY16 9SS, UK
Introduction
• Femtosecond mode locked lasers are significant scientific tools with wide ranging applications that include
spectroscopy and biophotonics [1, 2]. Currently, such laser systems usually operate in a fixed pulsed regime.
• There are several applications, particularly in the area of biophotonics, that would profit from the ability to
switch between a pulsed or cw regime. These include experiments involving optical transfection of trapped
cells [3] and optical tweezing [4].
Abstract: We report on fast switching between continuous-wave mode-locked and continuous-wave operation
of a Cr4+:forsterite femtosecond laser operating at 1300 nm, by means of a GaInNAs SESAM that is optically
excited by an external diode laser.
Results
Temporal response of rapid state-switching Intensity autocorrelation and spectral output of modelocked laser
of the laser when the EDL is activated and
deactivated
• Actively mode locked lasers have demonstrated this type of regime change, but the electronic modulators
required for this complicate the system and add cost to the overall setup. Mechanical methods have been
implemented to switch between cw and femtosecond operation of passively mode locked systems however
switching times have been relatively long (ms to s) [5]. Also picosecond pulses are not suitable for most
biological applications [6, 7].
• Here we demonstrate a Cr4+:forsterite laser system operating at the biologically relevant wavelength region
of 1300 nm [8] which contains a SESAM [9] that is optically pumped from outside the cavity to switch the laser
from a state where the laser is producing sub-300 fs pulses to a state where the laser operates in the cw
regime.
Experimental setup
 Mode-locked operation of the laser was obtained through use of a quantum-well
SESAM.
 Mode-locked output powers >100 mW
 With prisms in place a spectral bandwidth of 6.3 nm could be supported – this
corresponded to a pulse duration of 290 fs
 When EDL is activated the femtosecond pulse train diminishes to a cw state in less than 25 µs
 EDL power required to switch state >600 mW
 With EDL deactivated, pulse build up time ~100 µs. This is due to the >1500 round trips required to
establish a stable pulse train [10]
 No Q-switching or relaxation-oscillation-induced spiking observed
Conclusions & future work
Here we have demonstrated a simple technique that can be employed to change rapidly the state of a
broadband femtosecond laser, from a cw mode locked state to a cw state. A switching time < 25 µs was
observed when switching from the cw mode locked state to the cw state. A longer interval of ~100 µs was
observed when moving from the cw state to the self-starting cw mode locked state. Local heating within the
SESAM and the subsequent shift in the quantum well absorption is the mechanism responsible for this change
of state [11]. Assessments with quantum dot SESAM devices are now underway to see if there are significant
differences in switching dynamics between quantum-well and quantum-dot SESAMs.
References
[1] J. Mandon, et al., "Fourier transform spectroscopy with a laser frequency comb”, Nat. Photonics, 3, 99, (2009).
[2] A. Vogel, et al., "Mechanisms of femtosecond laser nanosurgery of cells and tissues”, App. Phys. B, 81,1015, (2005).
[3] D. Stevenson, et al., "Femtosecond optical transfection of cells: viability and efficiency”, Opt. Express, 14, 7125, (2006).
[4] B. Agate, et al., "Femtosecond optical tweezers for in-situ control of two-photon fluorescence”, Opt. Express, 12, 3011, (2004).
[5] A. McWilliam, et al., "Quantumdot-based saturable absorber for femtosecond mode-locked operation of a solid-state laser”, Opt. Lett., 31, 1444, (2006).
The University of St Andrews is a charity registered in Scotland : No SC013532
[6] V. G. Savitski, et al., "Optically-pumped saturable absorber for fast switching between continuous-wave and passively mode-locked regimes of a Nd:YVO4”, Opt. Express, 17, 5373, (2009).
[7] D. J. Stevenson, et al., "Single cell optical transfection," J. Royal Soc. Interface, In press – currently online only, (2010).
[8] J. H. Lee, et al., "Noninvasive in vitro and in vivo assessment of epidermal hyperkeratosis and dermal fibrosis in atopic dermatitis”, J. Biomed. Opt., 14, 15, (2009).
[9] A. McWilliam, et al., "Low-loss GaInNAs saturable Bragg reflector for mode-locking of a femtosecond Cr4+: forsterite-laser”, IEEE Phot. Tech. Lett., 17, 2292, (2005).
[10] C. Hönninger, et al., "Q-switching stability limits of continuous-wave passive mode locking”, J. Opt. Soc. Am. B, 16, 46, (1999).
[11] V. G. Savitski, et al., "Optically-Pumped Saturable Bragg Reflectors: Non-linear Spectroscopy and Applications in Ultrafast Lasers”, J. Quant. Electron., in press.
Website
http://www.st-andrews.ac.uk/~wsquad
e-mail: - [email protected]
Poster ME31 - 19th July -Ultrafast Phenomena 2010 – Snowmass, Colorado, USA