Source: http://proxy.osapublishing.org/josab/abstract.cfm?uri=josab-34-2-329
Timestamp: 2019-04-22 16:56:22+00:00

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The passive mode locking of vertical external cavity surface emitting lasers (VECSELs) enables the generation of high brightness ultrashort pulses at high repetition rates with unmatched performance. The peak power achievable with sub-200-fs pulse duration is mostly limited by the stability of the fundamental mode-locking regime as side pulses or harmonic mode locking emerges at high pump power. Here, we study a colliding pulse mode-locked VECSEL generating a pulse duration as short as 128 fs, with an average power of 90 mW per beam and a repetition rate of 3.27 GHz. The relevant laser parameters under different pumping regimes before and after the emergence of a side pulse are then used as input parameters for the simulation of the pulse interactions in the saturable absorber. We present a new comprehensive model for the calculation of saturable losses in the saturable absorber mirror and we study the energy transfer between the two counter-propagating pulses. This study reveals how a colliding pulse scheme reduces the saturation fluence of the absorber by a factor 2.9 and suppresses the mode competition between the two counterpropagating pulses of the ring cavity.
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Maas, D. J. H. C.
Fig. 1. Schematic layout of the VECSEL setup (right) and design of the full VECSEL structure (left).
Fig. 2. (a) Reflectivity spectra of the VECSEL structure with and without the gold reflector measured at room temperature (black and gray lines, respectively), together with the simulated spectra (dotted lines). (b) Measured group delay dispersion spectrum of the SESAM and VECSEL gain structure. The dotted line represents the sum of the VECSEL and SESAM GDD, and the vertical bars indicate the standard deviation over 20 measures.
Fig. 3. Measured non-collinear SHG autocorrelation of the single pulse operation output, and simulated autocorrelation of a sech 2 pulse with a FWHM of 128 fs.
Fig. 4. Measured optical spectrum of the output beam consisting of a single 128 fs pulse per round trip, and simulated spectrum of an unchirped 128 fs sech 2 pulse.
Fig. 5. Microwave spectrum of the laser output, with a RBW of 100 kHz and a 40 GHz span (left), and with a RBW of 1 kHz and a span of 2 MHz (right).
Fig. 6. Measured non-collinear autocorrelation of the side pulse operation output, and simulation of the autocorrelation of a sech 2 pulse with a FWHM of 130 fs followed by a 130 fs side pulse delayed by 430 fs with a relative intensity of 56%.
Fig. 7. Measured optical spectrum of the side pulse operation output, and simulation of the transform limited spectrum of two successive sech 2 130 fs pulse delayed by 430 fs.
Fig. 8. Field intensity distribution of two Gaussian beams colliding with an angle of 7° at z = 0 .
Fig. 9. (a) Microscopically calculated QW absorption spectra for various carrier density N . (b) Simulated absorption losses as a function of the carrier density N of the SESAM.
Fig. 10. Experimentally measured and simulated reflectivity change in the SESAM after excitation with a 80 fs pulse at 990 nm with a fluence of 25 μJ / cm 2 .
Fig. 11. Carrier density in the SESAM QW generated by the colliding of two pulses of (a) 50 nJ / cm 2 , and (b) 50 μJ / cm 2 . The incident beams are centered at ( x = 0 , y = 0 ) and are axially symmetric.
Fig. 12. Simulated carrier density in the SESAM at the center of the beam ( x = y = 0 ) , from a single 128 fs pulse of 51.8 μJ / cm 2 (90 mW average output power), and a double 130 fs pulse of 87.4 μJ / cm 2 (152 mW output power) separated by 430 fs.
Fig. 13. Simulated absorption losses from a single pulse of 90 mW and a dual pulse of 152 mW separated by 430 fs.
Fig. 14. Simulated pulse intensity before and after reflection on the SESAM for (a) single pulse operation and (b) side pulse operation. The SESAM losses are artificially increased by a factor 30 for illustration purposes.
Fig. 15. Absorption losses from the SESAM as a function of intracavity fluence for different delays of the two counterpropagating pulses.
Fig. 16. Time traces of the two counterpropagating output beams in a CW regime (left) and in a mode-locked regime (right).
(3) I ( x , y , z , t ) = [ E l + E r ] × [ E l ¯ + E r ¯ ] .
(4) I ( x , y , t ) = e − 2 x 2 + y 2 w 0 2 [ E r 0 2 sech 2 ( t τ ) + E l 0 2 sech 2 ( t − t d τ ) + … 2 E r 0 E l 0 sech ( t − t d τ ) sech ( t τ ) cos ( π θ x λ ) ] .
(11) P lost ( t ) = h ν η × ∂ ∂ t ( ∬ x y N gen ( x , y , t ) d x d y ) .
(12) E lost = ∫ P lost ( t ) d t .

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