Open Access
Add to BibSonomyAbstract
We report on a Nd:YVO4 laser mode-locked with a hybrid active and passive modulator consisting of a single partially poled KTP crystal. The periodically poled part provides negative cascaded Kerr-lensing, which together with intracavity soft and hard apertures gives passive modulation. Active phase modulation comes from the electro-optic effect by applying a voltage over the unpoled part of the crystal. The active modulation provides pulse lengths of about 95 ps, which initiate pulse shortening and self-sustained passive mode-locking by the cascaded Kerr effect. The repetition rate of the laser was 94 MHz and the output power was 350 mW, with a bandwidth of 0.235 nm and pulse lengths down to 6.9 ps.
©2006 Optical Society of America
1. Introduction
Generation of picosecond pulses by passive mode-locking of Nd:YVO4 lasers has been demonstrated by employing semiconductor saturable absorber mirrors [1] as well as by using cascaded second order nonlinearity and nonlinear mirror mode-locking [2–4], a technique first pioneered by Stankov et al. [5]. All these techniques generated mode-locked pulses with lengths of around 10 ps. The shortest mode-locked pulses in Nd:YVO4 were 2.3 ps and were generated by the additive-pulse mode-locking technique [6].
There are three distinct types of mode locking utilising cascaded second order effects, typically second harmonic generation followed by difference frequency generation. With the quadratic polarisation switching technique pulses as short as 2.8 ps have been demonstrated in Nd:YVO4 [7]. The nonlinear mirror mode locking technique has produced pulse lengths down to approximately 10 ps for Nd-doped crystalline hosts, limited by the group velocity mismatch between the fundamental and second harmonic wave. The modulation depth in this technique is mainly determined by the conversion efficiency.
The third method, cascaded Kerr lens mode locking, was first demonstrated by G. Cerullo et al. [8–9] and is employed in this work. If the cascaded nonlinear processes are not perfectly phasematched a nonlinear phase will be imprinted on to the circulating field [10]. The spatial intensity distribution of the beam translates as a curved phase front and this cascaded Kerr lens can also be used for cascaded mode-locking, together with an intracavity hard or soft aperture. The modulation depth of this process will depend on the particular cavity design, but as opposed to the case of nonlinear mirror modelocking, it can be large even for small conversion efficiencies. This reduces the need for high intracavity intensities and improves the reliability of the nonlinear crystal. We recently demonstrated this type of mode locking utilizing negative cascaded Kerr lens in temperature-detuned periodically poled KTP (PPKTP), which resulted in generation of a pulse length of 2.8 ps in Nd:GdVO4 [11]. It should be noted that the pulse length in this technique is not limited by the group velocity mismatch in the nonlinear crystal [12]. This is because the coherence length at large phase mismatch is shorter than the group velocity mismatch length.
In this work we employ the electro-optic effect together with second-order cascading in the same PPKTP crystal in order to realize a hybrid active-passive mode-locking. In particular, we demonstrate a diode-pumped Nd:YVO4 laser passively mode-locked by employing a defocusing cascaded Kerr lens realized with PPKTP. The mode-locking is initiated, enhanced and stabilized by electro-optic phase modulation thus realizing a regenerative mode-locking scheme with a single intracavity element. The hybrid scheme is not limited to cascaded Kerr lens mode-locking but should be beneficial to other passive mode-locking techniques using electro-optically active crystals. In our experiment the technique allowed for the generation of a stable continuous pulse train with pulses as short as 6.9 ps and a mode-locked bandwidth of 0.23 nm.
2. Experiment description and experimental results
The cavity design is schematically shown in Fig. 1. The end-pumped laser has a folded cavity with two lenses creating a tight intracavity focus in which the nonlinear crystal is placed. Describing the cavity in more detail, a Nd3+:YVO4 laser crystal (LC) with 0.5 atm% doping and dimensions 2×2-3 mm is pumped in a 400 μm diameter spot with a fiber-coupled CW diode laser operating at 808 nm. The laser crystal was placed 3 mm from the input coupler (IPC) and was slightly tilted in order to avoid back reflections. The folding mirror (M) had a radius of curvature of 750 mm and was placed 250 mm from the input coupler. In order to minimize astigmatism the full folding angle was kept below 6 degrees. The two lenses L1 and L2 had focal lengths of 50 mm and 100 mm, respectively, and were placed 268 mm apart. In order to mimimize the alignment sensitivity of the laser the focus inside the nonlinear crystal (NLC) was imaged with unit magnification by L2 upon the flat output coupler (OPC), which had a reflectivity of 95% at the fundamental wavelength. The full cavity length was 1575 mm.
A 7 mm long z-cut KTP crystal was placed in between the lenses. It was anti-reflection coated at 532 nm and 1064 nm. It had a periodically poled part of length 2 mm with a period of 9.01 μm, suited for second harmonic generation of 1064 nm. There was also a 4 mm long bulk region with silver paint electrodes on the top and bottom surfaces. A sinusoidal electric field of 45 V peak to peak voltage at the cavity repetition frequency of 94 MHz was applied over the electrodes rendering a phase modulation of 0.04π due to the second order electro-optic effect with the electro-optic coefficient r33. This is the active mode-locking part.
Fig. 2. Electro-optic modulation (above) and mode-locked signal (below). At the times 2 s and 8 s the beam was momentarily blocked inside the cavity and mode-locking is lost. When the electro-optic modulation is turned on again the mode-locking restarts.
The temperature of the crystal was kept at 95°C, well off the phase-matching peak of the periodically poled part at 22°C. The phase mismatch product ΔkL was close to 2π and the coherence length is thus about 1 mm, which is much shorter than the group velocity mismatch length of 11.3 mm for 7 ps pulses at 1064 nm in KTP. At this temperature detuning the cascaded second order interaction emulates the Kerr effect with a negative nonlinear index coefficient. All interacting waves were polarized vertically and the KTP crystal is placed with the z-axis vertically. This means that no polarization switching is taking place in the modulation. The cascaded Kerr lens together with the gain aperture of the laser crystal and the hard aperture (HA) placed 25 mm from L2 modulates the beam and constitute the passive mode locking. The exact off-focus position of the PPKTP crystal was of critical importance for stable pulse train generation in our previous passive-only cascaded Kerr lens mode-locking scheme [10], but was found to be not so critical in the hybrid passive-active technique. The sensitivity to mechanical vibrations was also significantly reduced. The hybrid scheme greatly simplifies finding the mode-locking region and maintaining stable operation. A benefit of the hybrid scheme when synchronising the cavity with other laser sources is that by modulating at exact the synchronisation frequency, the system is more stable and robust compared to passively-only mode locked systems.
In Fig. 2 we show the oscilloscope traces of the applied radio frequency (RF) modulation (upper curve) and the photodiode signal (lower curve) monitoring the laser output. The laser keeps mode locking when the active modulation is turned off as can be seen at the positions 0 s and 6 s. At approximately the times 2 s and 8 s the beam in the cavity was blocked momentarily and the laser drops out of mode-locking. When the electro-optic modulation is turned on again the mode-locking restarts, thus demonstrating that the partially poled KTP crystal indeed operates as a regenerative mode-locking element.
Fig. 3. Power spectral density of the optical signal, centered at the cavity repetition frequency of 94 MHz. The centre frequencies differs slightly for increased clarity.
At the applied phase modulation the active modulation frequency could be detuned up to ± 200 kHz from the cavity repetition frequency without loosing the regenerative action. Within that range there was no change in the pulse stability or pulse length. The active modulation did not contribute to the noise of the laser, as seen in Fig. 3, where the power spectral density is shown around the main frequency of 94 MHz. The main peak was found to have a FWHM width of less than 25 Hz, though the frequency drift due to cavity length fluctuations on the 1 s time scale was considerably larger.
Fig. 4. Intensity auto-correlation traces. The left-hand trace was recorded with electro-optic modulation present and the right-hand trace is from cascaded Kerr lensing only.
When pumped with 5.3 W of absorbed power the laser had a CW mode locked output power of 350 mW. The non-collinear auto-correlation pulse length (FWHM) assuming a sech2 pulse shape was found to be 6.9 ps with the electro-optic modulator running and with only the cascaded Kerr lens the pulses broadened to 7.5 ps (see Fig. 4). The FWHM bandwidth was 0.235 nm, which renders a time-bandwidth product of 0.43 and 0.47.
In order to look into the pulse generation from the active modulator, we modified the cavity slightly. It was changed into an active only mode-locked equivalent. Care was taken to avoid the cascaded Kerr lens effect, both in terms of reducing the cavity sensitivity to aperture effects and in terms of reducing the cascaded Kerr lensing produced by the nonlinear crystal. The nonlinear crystal was kept at room temperature and was moved away from the intracavity foci so that the cascaded Kerr lens was less efficient, with care taken to avoid diffractive losses at the crystal aperture. Finally we changed the pump power so that the output power remained at 350 mW. The cavity length and the phase modulation depth was unchanged. After these changes we had an actively phase modulated mode locked laser with no sensitivity to aperture effects. The pulse length was found to be about 95 ps, applying the autocorrelation fit used for Gaussian pulses. The pulse length is about 14 times longer that in the hybrid scheme, clearly demonstrating the drastic pulse shortening in the hybrid scheme. In Fig. 5 the intensity autocorrelation trace is shown, with the hybrid intensity autocorrelation trace as an inset for comparison. The modulation in the long-pulse trace we attribute to the pulse envelope modulation developing at relatively high phase modulation depths.
Fig. 5. Intensity auto-correlation traces. The black trace was recorded with electro-optic modulation only and smaller blue one is from the hybrid scheme.
3. Conclusions
In summary, we have demonstrated a combined actively and passively mode-locked laser. The active modulation is done with an electro-optic KTP phase modulator and the passive modulation is performed by cascaded Kerr lensing in a periodically poled part of the same KTP crystal. The mode-locking was initiated, enhanced and stabilized by the phase-modulator and when mode-locking the phase modulation could be turned off. A practical benefit of the hybrid scheme is enhanced stability when synchronizing the laser with external sources.
The repetition rate of the laser was 94 MHz and the output power was 350 mW, with a bandwidth of 0.235 nm. The active modulation provides pulse lengths of about 95 ps, which are shortened by the passive modulation. The pulse length was measured to be 6.9 ps with the active modulation employed and 7.5 ps with only passive mode-locking.
Acknowledgments
We gratefully acknowledge the GÖran Gustafssons foundation and the Carl Tryggers Foundation for financial support.
References and Links
1. L. Krainer, R. Paschotta, S. Lecomte, M. Moser, K. J. Weingarten, and U. Keller, “Compact Nd:YVO4 lasers with pulse repetition rates up to 160 GHz,” IEEE J. Quantum Electron. 38, 1331–1338 (2002). [CrossRef]
2. P. K. Yang and J. Y. Huang, “An inexpensive diode-pumped mode-locked Nd:YVO4 laser for nonlinear optical microscopy,” Opt. Commun. 173, 315–321 (2000). [CrossRef]
3. A. Agnesi, A. Lucca, G. Reali, and A. Tomaselli, “All-solid-state high-repetition-rate optical source tunable in wavelength and in pulse duration,” J. Opt. Soc. Am. B 18, 286–290 (2001). [CrossRef]
4. Y. F. Chen, S. W. Tsai, and S. C. Wang, “High-power diode-pumped nonlinear mirror mode-locked Nd:YVO4 laser with periodically poled KTP,” Appl. Phys. B 72, 395–397 (2001). [CrossRef]
5. K. A. Stankov and J. Jetwa, “A new mode-locking technique using a nonlinear mirror,” Opt. Commun. 66, 41–46 (1988). [CrossRef]
6. G. McConnell, A. I. Ferguson, and N. Langford, “Additive-pulse mode locking of a diode-pumped Nd:YVO4 laser,” Appl. Phys. B 74, 7–9 (2001). [CrossRef]
7. V. Couderc, F. Louradour, and A. Barthélémy, “2.8 ps pulses from a mode-locked diode pumped Nd:YVO4 laser using quadratic polarization switching,” Opt. Commun. 166, 103–111 (1999). [CrossRef]
8. G. Cerullo, S. De Silvestri, A. Monguzzi, D. Segala, and V. Magni, “Self-starting mode-locking of a CW Nd:YAG laser using cascaded second-order nonlinearities,” Opt. Lett. 20, 746–749 (1995). [CrossRef] [PubMed]
9. G. Cerullo, V. Magni, and A. Monguzzi, “Group-velocity mismatch compensation in continuous-wave lasers mode locked by second-order nonlinearities,” Opt. Lett. 20, 1785–1787 (1995). [CrossRef] [PubMed]
10. G. Toci, M. Vannini, and R. Salimbeni, “Pertubative model for nonstationary second-order cascaded effects,” J. Opt. Soc. Am. B 15, 103–117 (1998). [CrossRef]
11. S. J. Holmgren, V. Pasiskevicius, and F. Laurell, “Generation of 2.8 ps pulses by mode-locking a Nd:GdVO4 laser with defocusing cascaded Kerr lensing in periodically poled KTP,” Opt. Express 13, 5270–5278 (2005). [CrossRef] [PubMed]
12. X. Liu, L. Qian, and F. Wise, “High-energy pulse compression by use of negative phase shifts produced by the cascade χ(2):χ(2) nonlinearity,” Opt. Lett. 24, 1777–1779 (1999). [CrossRef]
