Femtosecond Alexandrite laser passively mode ...

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Femtosecond Alexandrite laser passively mode-locked by InP/InGaP quantum-dot saturab Shirin Ghanbari,Ksenia Fedorova,Andrey Krysa,Edik Rafailov,Arkady Major 07 December 17 07 December 17 313056

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Femtosecond Alexandrite laser passively mode-locked by InP/InGaP quantum-dot saturable absorber SHIRIN GHANBARI,1 KSENIA A. FEDOROVA,2 ANDREY B. KRYSA,3 EDIK U. RAFAILOV,2ARKADY MAJOR 1,* 1

Department of Electrical and Computer Engineering, University of Manitoba, Winnipeg, R3T 5V6, Canada Optoelectronics and Biomedical Photonics Group, School of Engineering & Applied Science, Aston University, Birmingham, B4 7ET, UK 3 EPSRC National Centre for III-V Technologies, University of Sheffield, Sheffield S1 3JD, UK *Corresponding author: [email protected] 2

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Received XX Month XXXX; revised XX Month, XXXX; accepted XX Month XXXX; posted XX Month XXXX (Doc. ID XXXXX); published XX Month XXXX

An Alexandrite laser passively mode-locked using an InP/InGaP quantum-dot semiconductor saturable absorber mirror (QD-SESAM) was demonstrated. The laser was pumped at 532 nm and generated pulses as short as 380 fs at 775 nm with an average output power of 295 mW. To the best of our knowledge, this is the first report on a passively mode-locked femtosecond Alexandrite laser using a SESAM in general and a quantum-dot SESAM in particular. OCIS codes: (140.3070) Infrared and far-infrared laser, (140.3538) Lasers, pulsed; (140.3600) Lasers, tunable; (140.4050) Mode-locked lasers; (140.7090) Ultrafast lasers, (140.7300) Visible laser. http://dx.doi.org/10.1364/OL.99.099999

Ultrafast laser sources in the near-infrared spectral region are highly desirable for a wide range of applications, including ultrafast spectroscopy [1, 2], nonlinear microscopy [3, 4], optical coherence tomography [5], and nonlinear frequency conversion [6, 7]. Ultrashort pulses are typically produced by laser gain media which have broad emission bandwidths. Currently, Ti:sapphire is the most commonly used broadband vibronic laser crystal that can directly generated a few cycle optical pulses [8, 9]. On the other hand, Alexandrite (Cr-doped chrysoberyl BeAl2O4) is another example of vibronic laser crystal [10] that has a wide (~100 nm) wavelength tuning range around 750 nm and high thermal conductivity (similar to Ti:sapphire) [11, 12]. Its other advantages are highly polarized output radiation and broad absorption bands that can be used for direct pumping with visible laser diodes [13, 14]. Unlike Ti:sapphire, Alexandrite can be directly pumped with red laser diodes because its absorption band covers most of the visible spectral range [10,13]. In addition, its larger στ product [15] presents a prospect of lower laser threshold. Furthermore, a diode-pumped Alexandrite laser was shown to produce >26 W of output power in the continuous wave regime [13] which significantly exceeds output from widely used Ti:sapphire lasers.

Despite these advantages, femtosecond laser mode locking was demonstrated only recently and resulted in the generation of 170 fs long pulses from a Kerr-lens mode-locked (KLM) Alexandrite laser [16]. At the same time, passive mode locking with a semiconductor saturable absorber mirror (SESAM) is an effective method that is widely used with Nd-doped and Yb-doped gain media to produce ultrashort pulses [20-27] because it does not require critical cavity alignment that is needed in case of KLM lasers [28, 29]. Therefore, the extension of SESAM mode locking to Alexandrite is a promising alternative. In this respect, quantum-dot SESAMs (QD-SESAMs), which can provide broadband absorption and emission spectra attributed to the inhomogeneous broadening associated with the size dispersion of QDs, are particularly attractive for generation of ultrashort pulses. It is worth mentioning that previous attempts of passive mode locking of Alexandrite using saturable dye absorbers could only produce picosecond (8-10 ps) pulses [17, 18]. On the other hand, recently SESAMs have been used for Q-switching of Alexandrite and produced 550 ns long pulses [19]. In this work we report on the first passively mode-locked Alexandrite laser using an InP/InGaP QD-SESAM that produced femtosecond pulses as short as 380 fs at 775 nm with an output power of 295 mW and pulse repetition frequency of 79.9 MHz. The results of our work open the way for the development of efficient diode–pumped ultrashort pulse Alexandrite lasers that can rival the currently used Ti:sapphire lasers in many industrial, scientific and medical applications. The experimental setup of the QD-SESAM mode-locked Alexandrite laser is schematically shown in Fig. 1. The designed 5-mirror cavity was similar to the one previously used in the KLM experiments [16]. The Brewster-cut Alexandrite crystal was 7 mm in length and doped with 0.155% of Cr. The crystal was mounted on a translation stage which helped to optimize its position with respect to the pump and cavity mode foci. The crystal was pumped at 532 nm by a frequencydoubled Nd:YVO4 laser with a maximum power of 7.3 W (Finesse, Laser Quantum). The pump was focused into a ~45 μm spot size diameter inside the crystal by a 150 mm focal length lens. About 85% of the pump power was absorbed in the crystal. Mirrors M1-M3 were

higghly reflecting (HR) ( in the 650 0-900 nm rangee. The Alexandrrite cryystal was placed between the two o folding mirrorss (M1 and M2) wiith 10 00 mm radii of curvature. c The ca avity contained the t InP/InGaP QDQ SE ESAM which was used as one of th he plane end mirrrors. Previously, the t QD D-SESAM used in n this work wass tested with Ti:sapphire lasers to prroduce picosecon nd pulses at 800 nm n and had a no on-saturable loss of ~1 1% [25]. In gen neral, the QD-SE ESAMs were sh hown to offer lo ow saaturation fluence e [25], controlla able modulation n depth [30] an nd brroadband operatting wavelength range [25, 31-35 5]. An appropriaate mode spot size on the QD-SESAM was w obtained by y using the focusing mirror M3 with th he radius of curv vature of 200 mm m. The delta cav vity geeometry (see Fig. 1) resulted in a spot size diametter of about 53 μm μ wiithin the Alexand drite crystal and approximately 171 1 μm on the QDQ SE ESAM. The effect of thermal lensiing [36] was also o considered in the t caavity design. Two o SF10 prisms (P P1 and P2) sepaarated by 345 mm m weere used to compensate for the positive p intracav vity dispersion. The T ou utput coupler (M4 4) had 3% transm mission around 750 nm.

he shorter 380 fs laser pulses ~4.1 nJJ and peak poweer of >9.5 kW. Th had en nergy of ~3.7 nJJ and similar peeak power. Optiical-to-optical efficien ncy was 5.4% forr the longer pulses and 4.8% fo or the shorter pulses w with respect to the absorbed p pump power. Th he transverse intensitty profile of the beam in the mo ode-locked regim mes is shown in Fig. 4(b). The laserr operated in tthe fundamentaal transverse mode. IIn both cases thee laser threshold d was at about 2 2.5 W and the mode llocking thresholld was approxim mately at 4.55 W of incident pump p power. Multi-pu ulse oscillation w was observed att higher than 7.12 W or 7.3 W of pu ump powers for longer and sh horter pulses, respecttively. The geneerated pulses in n this work aree longer than previou usly obtained o ones from the K KLM Alexandritte laser [16] probab bly due to the imperfect comp pensation of th he dispersion (disperrsive propertiess of our QD-SESSAM were not known) and generallly faster respo onse of the Kerrr-lensing effect employed in KLM.

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pulses with fit Fig. 2. Measured (a) aautocorrelation trrace of 420 fs p ng a sech2 intensiity profile and (b)) their spectrum. assumin Fig. 1. Schematicc of the QD-SESAM M mode-locked Alexandrite A laser..

Th he mode-locked d laser operation n was initiated and sustained by b th he InP/InGaP QD D-SESAM used ass one of the end mirrors. The used QD D-SESAM was originally desig gned for Ti:sap pphire [25], so it sh hifted the Alexan ndrite emission wavelength from the typical 75 55 nm m [12] to longeer wavelengths.. Reliable modee locking with an avverage output po ower of 325 mW W at the inciden nt pump power of 7.1 12W was achieeved. Pulses as short s as 420 fs in duration weere prroduced with 1.7 7 nm wide (FW WHM, full width at a half maximum m) sp pectrum centereed at 774 nm that t indicated a time-bandwid dth prroduct of 0.359. The fluence on the t saturable ab bsorber was abo out 59 90 μJ/cm2 and no damage waas observed. Th he autocorrelatio on traace of 420 fs pulses and th he correspondin ng spectrum are a displayed in Fig. 2(a) and Fig. 2(b), 2 respectivelly. In addition, by b op ptimizing the lasser cavity alignm ment, pulses as short as 380 fs in du uration with an a average outtput power of 295 mW weere prroduced at 7.3 W of an incidentt pump power. The T spectrum was w ceentered at 775 nm n and had a FWHM F bandwid dth of 1.8 nm th hat reesulted in a timee-bandwidth pro oduct of 0.341. The T fluence on the th saaturable absorber was about 535 5 μJ/cm2. Th he autocorrelatio on traace of the 380 fs pulses and the t correspond ding spectrum are a displayed in Fig g. 3(a) and Fig g. 3(b), respecttively. The rad dio freequency (RF) sp pectrum of the pulse train is shown in Fig. 4((a) an nd demonstratees high signal-to o-noise ratio in ndicative of stab ble m mode locking without w Q-swittched mode-loccked instabilitiees. Fu urthermore, stab ble single-pulse mode locking and a the absence of Q--switching instaabilities were co onfirmed using a long scan ran nge (2 200 ps) backgrou und-free intensiity autocorrelato or (Femtochrom me, FR R-103XL) and a fast oscilloscopee/photodetectorr with photodiode th hat had a combin ned resolution of o ~100 ps [37]]. Considering the th reepetition rate off ~79.9 MHz, 420 fs laser pulsses had energy of

Fig. 3. Measured (a) aautocorrelation trrace of 380 fs p pulses with fit assumin ng a sech2 intensiity profile and (b)) their spectrum.

Fig. 4. (a (a) RF spectrum of the 380 fs pullse train taken w with resolution bandwiidth of 1 kHz an nd (b) transversee beam intensity profile of the mode-loocked laser. d Alexandrite laaser using an In concclusion, a passivvely mode-locked InP/InG GaP QD-SESAM was demonstraated. Pulses with h duration as short ass 380 fs and 420 0 fs with an averaage output poweer of 295 mW and 325 mW, respectivvely, were generrated. The laser was pumped

with up to 7.3 W at 532 nm and operated around 775 nm. To the best of our knowledge, this is the first report of a passively mode-locked Alexandrite laser using a SESAM in general and a quantum-dot SESAM in particular. Due to the broad tuning range (~85 nm) of Alexandrite [12], a careful dispersion management [38] should lead to the generation of even shorter pulses. In addition, more efficient mode locking should be possible by designing QD-SESAMs specifically for Alexandrite laser crystal that has a gain peak around 755 nm wavelength [10, 11]. Also, low quantum defect of visible diode pumping [13, 14] should lead to efficient and powerful ultrafast Alexandrite oscillators that will be very attractive for various applications including nonlinear frequency conversion [39, 40] and ultrafast spectroscopy [41].

Acknowledgment. This research was supported by the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Innovation and the University of Manitoba.

References

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