Saturable Absorber Mirrors For Passive Mode-locking R. Hohmuth1 3, G. Paunescu2, J. Hein2, C. H. Lange3, W. Richter1 3, 1 Institut fuer Festkoerperphysik, Friedrich-Schiller-Universitaet Jena, Max-Wien-Platz 1, 07743 Jena, Germany, tel: +493641947444, fax: +493641947442, [email protected] 2 Institut fuer Optik und Quantenelektronik, Friedrich-Schiller-Universitaet Jena, Max-Wien-Platz 1, 07743 Jena, Germany 3 BATOP GmbH, Th.-Koerner-Str. 4, 99425 Weimar, Germany Introduction Saturable Absorber Mirror (SAM) Saturable absorber mirrors (SAMs) are inexpensive and schematic laser set-up compact devices for passive mode-locking of diode pumped solid state lasers. Such laser systems can provide ultrashort cavity, length L -cavity with gain medium pulse trains with high repetition rates. Typical values for pulse pulse -high reflective mirror and TRT=2L/c ) t output mirror with partially ( duration ranging from 100 fs up to 10 ps. For instance a I y t transmission i s Nd:YAG laser can be mode-locked with pulse duration of 8 ps n e -saturable absorber as t n and mean output power of 6 W. I modulator On this poster we present results for a Yb: KYW laser passive =>pulse trains spaced by mode-locked by SAMs with three different modulation depths round-trip time TRT=2L/c time t between 0.6% and 2.0%. SAMs were prepared by solid- gain medium high reflective mirror (pumped e.g. output mirror source molecular beam epitaxy with a low-temperature (LT) with saturable absorber by laser diode) (SAM) grown InGaAs absorbing quantum well. LT InGaAs quantum well SAM design SAM reflection spectrum Ta O or SiO / dielectric cover Requirements for passive mode- 25 2 conduction 71.2 nm GaAs / barrier E 7 nm c1 71.2 nm GaAs / barrier locking band Ec LT InGaAs 74.7 nm GaAs The mode-locking regime is stable agaist the onset of quantum well InGaAs 88.4 nm AlAs multiple pulsing as long as the pulse duration is smaller energy : 25x Bragg mirror than t . (*) n gap Eg min · 74.7 nm GaAs h 88.4 nm AlAs Dg, f æ ö 350 µm GaAs / substrate 1 EP Ev1 valence t » f ç ÷ band E min ç ÷ GaAs v 2 DR è EA ø -tunable absorption wavelength via -layers are grown by solid-source molecular beam epitaxy Dg,f gain and filter losses DR modulation depth thickness and In content -Bragg-mirror with wide high reflection band (R>99.7%) EP pulse energy -fast recovery time of the absorbing -embedded absorbing InGaAs quantum well EA saturation energy low-temperature (LT) grown InGaAs -dielectric protection layer f(EPA/E ) a function which has a minimum at EPA/E between 2 film and 3 Experimental set-up for the mode-locked Yb:KYW laser Autocorrelation trace and optical spectrum of 100-fs pulses OC1 1,0 1,0 data 2 SF 10 sech fit prism 2 0,8 0,8 OC2 ] . u . ] . a [ u . 0,6 0,6 y a t SF 10 [ i s y Laser prism 1 t n i s e Diode t n n I e M3 t 0,4 0,4 l n a Plane I r t H Mirror R = 20 cm c e S M4 0,2 p S 0,2 SAM 0,0 0,0 M1 M2 -400 -300 -200 -100 0 100 200 300 400 1000 1010 1020 1030 1040 1050 1060 1070 1080 R = 10 cm R = 10 cm delay [fs] Wavelength [nm] Focusing Yb:KYW lenses d = 1 mm The autocorrelation trace (left) and the corresponding optical spectrum (right) of the shortest pulses delievered from the Yb:KYW laser. They were obtained using a 2% modulation depth saturable absorber and an 1% M1 -M3, curved mirrors; R, radius of curvature; M4, high-reflective plane mirror; transmision output coupler. The red line shows the theoretical curve assuming a pulsewidth of 100-fs and a OC1-OC2, output coupler with different transmissions; SAM, saturable absorber mirror sech2 - pulse shape. The spectrum is centered at 1041 nm and has a bandwidth of 14.2 nm FWHM. The output Active media: 1-mm thick Yb:KYW crystal, 5 at% of Yb3+ ions, pumping along b axis power was 52 mW at a pump power of 4.4 W. Pumping system: fiber coupled high-brightness laser diode, output power up to 5 W Measured pump spot diameter in the crystal: 100 µm Laser beam radius onto the SAM: ~ 83 µm Dispersion compensation: SF10 prisms separated by 34 cm Pulse train and frequency spectrum 0,045 0,040 0,035 ] Laser parameters achieved for different modulation . u . 0,030 a [ l depths DR a 0,025 n g i 0,020 P (mW) T (%) P (W) s DR (%) tFWHM (fs) out OC pump e d 0,015 o i d 0,010 o 0.6 160 53 1 3.0 t o 0,005 h 198 292 4 4.5 P 0,000 -0,005 1.2 140 63 1 3.2 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 156 228 4 4.5 t [ns] 2.0 100 52 1 4.4 The pulse train was recorded with a silicon photodiode with a rise time of 2 ns and a 500 MHz - oscilloscope. 134 136 4 4.5 The frequency spectrum does not show bandsides. tFWHM pulsewidth, Pout laser output power, TOC output coupler transmission, Ppump pump power. The minimum pulsewidth and the maximum output power are indicated with bold letters. For a given value of the modulation depth, tmin is dependent on the total cavity loss (including Summary the output coupler transmission TOC) and on the intracavity pulse energy. In order to get the Saturable absorber mirrors (SAMs) were grown by low-temperature solid-source molecular beam epitaxy. minimum pulsewidth, we tested output couplers with different transmissions. The pulse energy SAMs with modulation depths of 0.6%, 1.1% and 2.0% were used to mode lock a diode-pumped Yb:KYW laser was modified by varying the pump power. at around 1040 nm. Similar results were obtained with a Yb:KGW laser. (**) The mode-locking regime was selfstarting and stable. For some hours of continuously operation, we did not notice any damage. (*) F. X. Kärtner, I. Aus der Au and U. Keller, "Mode-Locking with Slow and Fast Saturable Absorbers What's the Difference?", Special Issue on Ultrafast Electronics, Photonics and Optoelectronics, IEEE The minimum achievable pulse duration and the output power decrease by increasing the modulation depth. J. Select. Topics Quantum Electron. 4, 159-168 (1998). (**) G. Paunescu, J. Hein, R. Sauerbrey, "100-fs diode-pumped Yb:KGW mode-locked laser", Appl. Pulses as short as 100 fs were obtained with a 2% modulation depth saturable absorber. Phys. B (2004)..