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A diode-end-pumped simultaneously Q-switched and mode-locked self-Raman YVO4/Nd:YVO4/YVO4 laser at first-Stokes wavelength of 1175.9 nm was demonstrated. The shortest mode-locked pulse width of the laser was obtained to be ~23.57 ps, with the corresponding time-bandwidth product of ~0.51. The maximum average output power, the highest pulse energy and the highest peak power were obtained to be 1.83 W, 6.1 μJ and 220 kW, respectively. The nonlinear Raman process improved the Q-switched mode-locking performance of the Stokes pulses.
©2013 Optical Society of America
1. Introduction
Neodymium doped vanadate crystals such as Nd3+:YVO4 [1,2], and Nd3+:GdVO4 [3–5] crystals, have been widely applied in diode-pumped solid-state lasers due to their broad absorption bands, larger absorption and emission cross-sections. Additionally, vanadate crystals possess efficient third-order nonlinear optical responses that are related to the stimulated Raman scattering (SRS) and the Kerr-lensing effect [6,7]. Kerr-lens mode-locked (KLM) [7–9] and self-SRS [10–13] lasers with neodymium doped vanadate crystals have been efficiently demonstrated, respectively. SRS has been recognized as an effective method for laser frequency conversion. Recently, SRS has been found as a simple mode-locking method of generating picosecond pulse trains, due to its nonlinear conversion process [14–16]. So in self-SRS lasers with neodymium doped vanadate crystals, the fundamental fields can be mode-locked through KLM mechanism, while the Stokes fields can be mode-locked through both KLM and SRS mechanisms. The mode-locking performance of Stokes fields should be better than that of fundamental fields.
Over the last decade, the passively and actively Q-switched self-Raman lasers have been commonly demonstrated [10–13]. However, it was rarely reported on the self-Raman mode-locked lasers, which had the common advantages of Raman lasers and mode-locked lasers. In 2008, a picosecond Nd:YVO4 self-Raman laser passively mode-locked by a saturable absorber was reported, which consisted of two coupled resonators [17]. In 2012, passively Q-switched mode-locking in a compact Nd:GdVO4/Cr:YAG self-Raman laser was demonstrated [18]. And in the same year, a diode-side-pumped simultaneously Q-switched and mode-locked (QML) Nd:YAG/BaWO4 two-Stokes dual-wavelength operating laser was reported [19].
Compared with the continuous-wave mode-locked (CWML) lasers, QML lasers can generate ultrashort optical pulses with higher peak power [19–22]. In this paper, we report a diode-end-pumped QML YVO4/Nd:YVO4/YVO4 self-Raman laser at first-Stokes wavelength of 1175.9 nm. The maximum average output power, the shortest mode-locked pulse width and the highest peak power were obtained to be 1.83 W, ~23.57 ps and 220 kW, respectively. The experimental results indicated that self-SRS could contribute significantly to the improvement of the Q-switched mode-locking performance of the first-Stokes field.
2. Experimental setup
The experimental setup of the QML first-Stokes self-Raman laser is shown in Fig. 1. The cavity configuration is a simple flat-flat resonator. A linear flat–flat cavity is an attractive design because it reduces complexity and makes the system compact and rugged. The pump source was a commercially available high-power fiber-coupled diode-laser-array. The core diameter and numerical aperture (N.A.) of the fiber were 0.4 mm and 0.22, respectively. The pump beam from the fiber end at 808 nm was focused into the laser crystal with the spot size of 0.4 mm in diameter by an optical imaging system. The pump mirror M1 was a flat mirror, high-transmittance (HT) coated at 808 nm (T>98.8%) and high-reflection (HR) coated at 1064 nm (R>99.93%) and 1176 nm (R>99.95%). The output coupler M2 was a flat mirror with reflectivity of ~82.4% at 1176 nm, HR coated at 1064 nm (R>99.93%).
The 3 × 3 × 30 mm3, a-cut YVO4/Nd:YVO4/YVO4 composite crystal was employed as self-Raman material. The 10-mm-long 0.3 at.% Nd3+ doped Nd:YVO4 crystal was bounded with a 2-mm-long pure YVO4 at the pumped end and a 18-mm-long pure YVO4 at another end. Both of its two ends were anti-reflection (AR) coated at 808 nm, 1064 nm and 1176 nm. The composite laser crystal was wrapped with indium foil and mounted in a water-cooled copper block for removing the heat generated in the crystal. The temperature of the water was kept to be about 17 °C during the experiments. A 35-mm-long acousto-optical Q-switch (AOS) AR coated at 1064 nm on both of its facets was placed close to the composite laser crystal. Its repetition rate could be tuned from 1 kHz to 100 kHz continuously. The distance between M1 and M2 was kept to be about 87 mm. A monochromator as an optical splitter was placed behind M2.
The mode-locked pulses were detected by a high speed InGaAs photodetector (New Focus Model 1014, 45 GHz IR Photodetector, with rise time of 9 ps) whose output signal was connected to a digital oscilloscope (Tektronix DPO70404B) with 4 GHz electrical bandwidth and rise time of 98 ps. The spectral information was monitored by two optical spectrum analyzers (YOKOGAWA AQ6370B with the resolution of 0.02 nm and StellarNet EPP2000). The mode-locked pulse width was measured by a homemade autocorrelator.
3. Results and discussions
The laser cavity was first aligned to obtain the maximum average output power. With fine adjusting of the cavity, the amplitude instability was minimized to obtain a nearly perfect stable mode-locking operation with the modulation depth of 100%. Without the optical splitter, the output spectral information of QML self-Raman laser was measured at the incident pump power of 30 W and PRF of 30 kHz, as shown in Fig. 2. The wavelengths of the fundamental, the first-Stokes and the second-Stokes components were measured to be 1064.4 nm, 1175.9 nm and 1313.5 nm, respectively. The frequency shift was about 890 cm−1 of each adjacent wavelengths interval, corresponding to the optical vibration modes of the tetrahedral VO43- ionic groups. The optical spectrum indicates that the fundamental and second-Stokes Raman emission could hardly be observed in the output radiation.
Fig. 2 Optical spectrum of QML self-Raman laser at the incident pump power of 30 W and PRF of 30 kHz.
With placing the optical splitter behind M2, the average output power at 1064.4 nm and 1175.9 nm were measured separately versus incident pump power at different pulse repetition frequencies (PRFs) in mode-locking operation, as shown in Fig. 3(a). It can be seen that the output average power strongly depended on the PRFs of AOS. The maximum output power at 1175.9 nm for 20, 30 and 40 kHz is 1510, 1830 and 1520 mW, respectively. The corresponding optical conversion efficiency is 5.8%, 6.1% and 5.1%, respectively. The maximum output power of 1830 mW was not saturated at the highest incident pump power of 30W. In order to protect the end face of crystal from damage, we didn’t increase the pump power over 30 W. Figure 3(a) also shows that the output power of fundamental wave was extremely low compared with that of first-Stokes emission. The maximum output power at 1064.4 nm for 20, 30 and 40 kHz is 25, 30 and 38 mW, respectively. At a PRF of 20 kHz, the threshold of the fundamental emission and the first-Stokes emission were about 2 and 7 W, respectively. The thresholds of first-Stokes emission were measured to be 7, 9 and 10 W at different PRFs of 20 kHz, 30 kHz and 40 kHz, respectively. The average output power of second-stokes at 1313.5 nm was too low to be measured by power meter.
Fig. 3 (a) Average output power and (b) Q-switched envelope duration versus incident pump power at different PRFs in mode-locking operation.
Q-switched envelope duration (Full Width Half Maximum, FWHM) of first-Stokes emission versus incident pump power at different PRFs in mode-locking operation is shown in Fig. 3(b). It shows that Q-switched envelope duration depended much on the incident pump power. The shortest Q-switched envelope duration of ~7.2 ns was obtained at the incident pump power of 30W and PRF of 30 kHz, as shown in Fig. 4. The pulse trains indicate full modulation and complete mode-locking. In the Q-switched envelope, there are 17 mode-locked pulses, and the time interval between two neighbor mode-locked laser pulses was ~0.94 ns (corresponding repetition rate of mode-locked laser pulse is ~1.06 GHz), which was approximately equal to the resonator roundtrip time. Taking into account the influence of the refractive index of laser crystal and AOS, the optical path length of the resonator was calculated to be about 140 mm, and the roundtrip time was calculated to be 0.937 ns. This confirmed the existence of the mode locking regime in the QML self-Raman laser. The radio frequency of the AOS was 70 MHz, which was very different from the repetition rate of mode-locked laser pulse. It indicated that the mode-locking phenomenon in our experiment didn’t result from the AOS modulation. The inset illustration shows the Q-switched pulse train with the time interval of 33.4 μs. The mode-locked pulse width and linewidth at the maximum average output power of 1.83 W were measured to be ~27.44 ps and ~0.094 nm, respectively. By calculating the pulse area in Fig. 4, we can estimate that the pulse energy of the highest pulse in the pulse train is about 10% of the total energy of the whole Q-switched pulse envelope. And then the highest pulse energy and peak power of mode-locked pulse were calculated to be about 6.1 μJ and 220 kW, respectively. The output beam profile was measured by a CCD system at the incident pump power of 30 W and PRF of 30 kHz, as shown in Fig. 5. It indicated that the QML self-Raman laser operated in the TEM00 mode. The M2 factors at the horizontal and vertical axes were measured to be 1.33 and 1.72, respectively.
Fig. 4 The temporal pulse profile of the QML laser at the incident pump power of 30 W and PRF of 30 kHz.
Fig. 5 Output beam profile of the laser at the incident pump power of 30 W and PRF of 30 kHz: (a) 2D display of transverse section; (b) 3D display of laser intensity of transverse section.
The shortest mode-locked pulse width was obtained at the pump power of 30 W and PRF of 40 kHz. The FWHM of the autocorrelation trace was measured to be ~33.33 ps, as shown in Fig. 6(a). Assuming a Gauss-shaped temporal profile, the pulse width was thus estimated to be ~23.57 ps. And the corresponding linewidth was measured to be ~0.100 nm around the central wavelength of 1175.9 nm, as shown in Fig. 6(b). The time-bandwidth product was calculated to be ~0.51, which was close to the value of 0.44 for the transform-limited Gaussian pulses. The mode-locked pulse widths were measured to be 28.14, 27.44 and 23.57 ps for the PRFs of 20, 30 and 40 kHz at the pump power of 30 W, respectively. It indicated that the mode-locked pulse width varied non-significantly with the PRF.
Fig. 6 (a) Autocorrelation trace of the output pulses at the incident pump power of 30 W and a PRF of 40 kHz; (b) the corresponding output spectrum.
Separated by a monochromator, the temporal pulse profile of the fundamental emission at 1064.4 nm and the first-Stokes emission at 1175.9 nm were measured at the incident pump power of 20 W and PRF of 15 kHz respectively, as shown in Fig. 7. In the fundamental Q-switched envelope, the measured envelope duration was 21 ns, and the corresponding modulation depth was ~40%, as shown in Fig. 7(a). The mode-locking of fundamental wave should be owed to the Kerr-lens mode locking (KLM) [7, 8]. Figure 7(b) shows that the full modulation and complete mode-locking of the first-Stokes emission Q-switched envelope is achieved. And the corresponding measured envelope duration was ~9 ns. From the comparison of Fig. 7(a) and Fig. 7(b), it indicates that SRS process plays a significant role in increasing modulation depth and envelope duration compression for the first-Stokes mode-locked pulses, due to its nonlinear conversion. However, SRS process is not beneficial to the mode-locking of the fundamental emission. The Stokes field depends greatly on the fundamental filed intensity. In the QML fundamental pulse envelope, the Raman gain on the top of pulse is higher than that at the bottom of pulse. It’s a nonlinear Raman gain for the Stokes filed, while it’s also a nonlinear loss for the fundamental filed. The higher the intensity of the fundamental filed, the higher the loss of the fundamental filed. This situation is opposite to that of the passive mode-locking with a saturable absorber. Therefore, the Q-switched mode-locking performance of the first-Stokes pulses was much better than that of the fundamental pulses.
Fig. 7 The temporal pulse profiles of (a) the fundamental emission at 1064.4 nm and (b) the first-Stokes emission at 1175.9 nm at the incident pump power of 20 W and PRF of 15 kHz.
Without the optical splitter, the average output power of the QML self-Raman laser at first-Stokes wavelength of 1176 nm was measured to be 1.17 W at the incident pump power of 20 W and the PRF of 15 kHz. The output power consists of a majority of first-Stokes emission and a small quantity of fundamental emission. Because SRS efficiency is dependent on the intensity of fundamental wave, the first-Stokes intensity is more sensitive to the fundamental intensity in the resonator. So, the development of the first-Stokes field was measured with fine adjusting the angle of output coupler. The temporal profiles with the corresponding spectra at different output powers of 13, 15, 17, 21 and 80 mW, were shown in Fig. 8, respectively. At the output power of 13 mW, only the fundamental envelope was measured, with the envelope duration of ~19 ns. When the output power was increased to 15 mW, a very small quantity of first-Stokes emission was generated, as shown in Fig. 8(2). By comparing Fig. 8(2a) with Fig. 8(1a), we can deduce that the right raising envelope in Fig. 8(2a) must be the first-Stokes envelope. The interval of the two envelope peak was ~18 ns, which corresponded to about 18 round-trip times. With the increasing of the output power, the intensity of the first-Stokes envelope grew rapidly, while the intensity of the fundamental envelope had little change. When the output power was increased to 80 mW, the first-Stokes envelope duration of ~11 ns with 100% modulation depth was obtained, as shown in Fig. 8(5). The modulation depths of all of the fundamental envelopes were about 40%, while the first-Stokes envelopes displayed full modulation. It also indicated that SRS could improve the Q-switched mode-locking performance of the Stokes pulses.
Fig. 8 (1)(2)(3)(4)(5): (a) Temporal profiles of the output pulses, and (b) the corresponding output spectra of the laser at different output powers.
4. Conclusion
In conclusion, simultaneous Q-switching and mode-locking in a diode-end-pumped self-Raman YVO4/Nd:YVO4/YVO4 laser was achieved. The shortest mode-locked pulse width of the first-Stokes emission at 1175.9 nm was obtained to be ~23.57 ps, with the corresponding time-bandwidth product of ~0.51. The maximum average output power, the highest pulse energy and the highest peak power were obtained to be 1.83 W, 6.1 μJ and 220 kW, respectively. The experimental results confirm that SRS process is much beneficial to the mode-locking of the Stokes pulses.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No.10804074), the Science and Technology Project of Shenzhen (No. JCYJ 20120613105141482, No. JC201005280473A), and the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20124408120004).
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