Acousto-optic-modulator-stabilized low-threshold mode-locked Nd:YVO 4 laser

1. Acousto-optic-modulator-stabilized low-threshold mode-locked Nd:YVO4 laser Speaker : Wei-Cheng Lin Advisor : Jia-Hon Lin

2. Outline • Introduction • Experiment setup • Results & Discussions • Summary

3. Introduction • Picosecond pulses with microjoule energy immensely used in: • nonlinear optics • precise fabrication of microstructures • medical science • Simultaneous Q switching and mode locking (QML) in solid-state lasers is a relatively efficient and simple way to achieve picosecond pulses of approximately microjoule energy.

4. Introduction • Compared to continuous wave (cw) mode-locking regime , the QML enhances the pulse energy by several orders of magnitude.

5. Introduction • QML pulse generation has been reported using the techniques: • simultaneous active mode locking and active Q switching • simultaneous passive mode locking and passive Q switching • simultaneous passive mode locking and active Q switching

6. Introduction • the technique of nonlinear mirror (NLM) based saturable absorber for mode locking was used. • The setup consists of a frequency doubling crystal placed near a dichroic output coupler (DOC) with high reflectivity at the second harmonic (SH) wavelength. • If the SH experiences appropriate phase shift with the fundamental wave FW , then the SH power is totally reconverted into the FW during the second pass through the nonlinear crystal. • second harmonic generation (SHG) is a second order nonlinear optical process NLC along with the DOC behaves like a NLM

7. Experiment setup • RM: (1) two lenses with focal lengths of 15 and 12 mm (2)rear face: antireflection coated at 808 nm (3) other side: high reflectivity (99.5%) at 1064 nm • Nd:YVO4 crystal : (1)concentration: 0.5% Nd3+ (2)antireflection coated of 1064 and 808 nm on both faces • M1 and M2 : radii of curvature of 500 and 250 mm • Z-shaped cavity: length of 81.7 cm • KTP crystal: antireflection coated of 1064nm and 532 nm on both faces • DOC: reflectivities of 100% at 532 nm and 78% at 1064 nm

8. Experimental method • RM: The pump beam coming from the fiber is imaged on the crystal through the rear mirror RM • Nd:YVO4 crystal: (1)end pumped at 808 nm by a fiber-coupled LDA (2)tilt of 2° to reduce the effect of undesired reflections inside the cavity • AQ-QS: (1) with faces cut for Brewster’s angle at 1064 nm inserted in the cavity (2) driven by a radio frequency of 27.2 MHz with a modulation in the frequency range of 1–50 kHz • M1 and M2: focus the beam onto the NLC • Z-shaped cavity(81.7cm): optimized to be 250, 405, and 162 mm, respectively, for stable operation • Finally, The train of mode-locked pulses is detected by a silicon photodiode with a rise/fall time of 0.5 ns and traced by a 500 MHzoscilloscope.

9. Experimental Results • When AOQS is made on, the laser is found to operate in the QML mode at a threshold pump power of 5.5 W • input pump power ≤ 6.0W => QML pulses unstable • 5.5W≤ pump power ≤ 6.0W =>amplitude fluctuation but with nearly full depth of modulation • 6.0W≤ pump power => QML pulses stable and having full depth of modulation • scaling down the current => a stable mode locking up to the threshold pump power of 5.4 W • The QML pulse train for a pump power of 9.0 W

10. The laser diode current scaling for the output power and the pulse width • slope efficiency : 73% • (1)Keeping the current constant (2)increasing the AOQS modulation frequency • The stable QML operation: occurring for modulation frequencies in the range of 27–50 kHz, with an optimum around 40 kHz pulse width

11. Noncollinear SHG intensity autocorrelation trace • The pulse width is measured by the technique of second order nonlinear optical intensity autocorrelation with a 3 mm long BBO crystal in a noncollinear geometry • pulse width = 11.5 ps • The peak power calculated from the measured average output power for the stable QML pulse trace corresponding to the modulation frequency of 40 kHz is about 719 kW

12. QML pulse stability max ( kw) max/2 • If the energy of most of the mode-locked pulses inside the FWHM of the QML envelope exceeds The QML pulses stable the critical energy Ec ( ps)

13. QML pulse stability • Ec = • T: cavity round-trip time • EL: the energy of a mode-locked pulse in the cavity • PA: saturation powers of the NLM absorber • : maximum nonlinear loss modulation =1−R • R: reflectivity of the output coupler • During the Q-switching operation, the total laser output energy is distributed in the nanosecond envelopes. cw operation cannot exceed Ec at avery low pump the QML pulses have sufficient energy to sustain mode locking operation

14. saturation power of gain medium and NLM absorber • : saturation power of the gain medium • Rg: the cavity mode radius (240 μm) • : saturation intensity • : upper state lifetime • : gain cross section • : lasing frequency • PL = 1.09 W • : saturation power of the NLM absorber • : linear loss in the cavity • : maximum nonlinear loss • : intracavity power of the FW • : conversion efficiency from the FW to SH

15. saturation power of gain medium and NLM absorber • Inthe low conversion efficiency limit, it is given as • l : length of the KTP crystal • : plane wave impedance • deff= 3.18 pm/V = 4.8 kW • Assuming k = 0 Ec =190.5 nJ • Since the pulse energy of all the QML pulses inside the FWHMof theQML envelope is more than Ecat a pump power of 6.0 W, we obtain stable QML pulses.

16. QML pulse stability • For low conversion efficiency, the limiting pulsewidth is given approximately by 3 • : crystal parameter • u1 , u2 : group velocities of the fundamental and second harmonic • Lcr : crystal length • For a 10 mm long KTP, is calculated to be 4.2 ps • SHG intensity (fundamental intensity) • The stability of the generated pulse train can be assessed from the autocorrelation trace 2 ∝

17. Conclusions • A stableandefficient Nd:YVO4 laser , actively Q switched by an acousto-optic modulator and passively mode-locked by a KTP crystal based NLM can be developed. • A improvement in threshold pump power and output peak power for stableQML operation has been achieved as compared to the LBO based system. • The active Q switching is proven to be efficient for generating ultrashort pulses of 11.5 ps with a high peakpower of more than 719 kW for a pump power of 10 W as well as in stabilizing the QML operation.