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Abstract: 1st-Stokes and 2nd-Stokes dual-wavelength operation within a diode-side-pumped Q-switched Nd:YAG/BaWO4 intracavity Raman laser was realized. Using an output coupler of transmission of 3.9% at 1180 nm and transmission of 60.08% at 1325 nm, the maximum output power of 8.30 W and 2.84 W at a pulse repetition rate of 15 kHz for the 1st Stokes and the 2nd Stokes laser were obtained, respectively. The corresponding optical conversion efficiency from diode laser to the 1st Stokes and 2nd Stokes laser are 5.0% and 1.4%, respectively. With the pump power of 209 W and a pulse repetition rate of 15 kHz, the 1st Stokes and the 2nd Stokes pulse widths were 20.5 ns and 5.8 ns, respectively. The stable simultaneous Q-switching and mode locking of the 2nd Stokes laser without mode locking component was obtained at the pump power of about 29~82 W. The estimated mode-locked pulse width was approximately 31 ps at the pump power of 50 W and a pulse repetition rate of 15 kHz.
©2012 Optical Society of America
1. Introduction
Stimulated Raman scattering (SRS) is a very efficient method and one of the most important nonlinear frequency conversion process to produce new laser lines [1–7]. In recent years, with emergence of excellent Raman crystals, all-solid-state Raman lasers have attracted much attention for the advantages of high conversion efficiency, compactness, good mechanical and thermal properties. The most commonly known Raman active media include: YVO4, GdVO4, SrWO4, KGd(WO4), BaWO4, PbWO4 and Ba(NO3)2. The BaWO4 crystal, with a Raman shift of 925 cm−1 and Raman gain of 8 cm/GW at 1064 nm, is a promising Raman material for picosecond and nanosecond pulse. There have been many investigations on BaWO4 Raman lasers [8–16]. P. Cerny et al. investigated the SRS characteristics of picosecond laser pulses in BaWO4 crystal using single-pass, double-pass, and external cavity configurations [1, 8, 9]. C. Zhang et al. investigated the SRS characteristics of nanosecond laser pulses in BaWO4 crystal [14]. For intracavity Raman laser, Y. F. Chen et al. reported an actively Q-switched Nd:YAG/BaWO4 Raman laser in 2005. With the incident pump power of 9.2 W, 1.56 W of 1181 nm 1st-Stokes output power was generated at a pulse repetition rate (PRR) of 20 kHz [10]. In 2007, S. T. Li et al. reported an diode-side-pumped intracavity frequency-doubled Nd:YAG/BaWO4 Raman laser, generating a 3.14 W at 590 nm [11]. In 2009, a 3.36 W continuous wave (CW) 1180 nm 1st-Stokes laser was obtained in a Nd:YVO4/BaWO4 Raman laser [12]. In 2010, A. J. Lee et al. reported the generation of 2.9 W CW yellow laser emission from an intracavity Nd:GdVO4/BaWO4/LBO Raman laser [13]. And in the same year, Z. H. Cong et al. reported an 8.3 W diode-side-pumped Q-switched intracavity frequency-doubled Nd:YAG/BaWO4 Raman laser [15].
In fact, SRS is a cascading nonlinear frequency conversion process. When the first Stokes optical field reaches adequate power intensity, it can act as a pump source for SRS to generate a second Stokes wave [17, 18]. Cascading to higher Stokes orders has been reported in external resonator Raman lasers [19, 20] and intracavity Raman lasers [21–24]. For BaWO4 Raman laser, the 2nd Stokes radiation at 1325 nm generated by Raman shift of 925 cm−1 of BaWO4 with 1064 nm laser as the fundamental wave has potential use in the terahertz (THz) generation with the nonlinear optical method, such as by difference frequency with fundamental frequency of 1319 nm laser generated by Nd:YAG, 1342 nm laser generated by Nd:YVO4, or the 2nd Stokes laser of 1313 nm generated by Nd:YVO4 [18].
In 2009, A. S. Grabchikov et al. demonstrated the first CW Raman laser operating simultaneously at frequencies of two Stokes components [25]. In 2010, two-Stokes laser of milliwatt level was generated in passively Q-switched end-diode-pumped microchip Nd:LSB/BaWO4 laser [26]. Compared with end-pumped Raman laser, the side-pumped Raman laser has lower conversion efficiency, but it can generate higher output power. V. G. Savitski et al. reported a CW intracavity Raman lasers, in which a 150 W side-pumped Nd:YLF module was used [27]. In our paper, a similar side-pumped Nd:YAG module was employed. As far as we know, there are not relevant reports on 1st-Stokes at 1180nm and 2nd-Stokes at 1325 nm dual-wavelength operation within diode-side-pumped Nd:YAG/BaWO4 Raman laser. In this paper, 1st-Stokes and 2nd-Stokes dual-wavelength stable operation in diode-side-pumped Nd:YAG/BaWO4 Raman laser was presented for the first time. The maximum output power of 8.30 W and 2.84 W at a PRR of 15 kHz for 1st Stokes and 2nd Stokes wave were obtained, respectively. The corresponding optical conversion efficiency from diode laser to 1st Stokes and 2nd Stokes laser is 5.0% and 1.4%, respectively.
Generally, the pulse widths of the Q-switched lasers range from several ns to hundreds of ns. And the continuous wave mode-locked (CWML) lasers can generate the ultrashort optical pulses with the pulse width of picosecond scale. For the simultaneously Q-switched and mode-locked (QML) lasers, the mode-locked pulses underneath the Q-switched envelope have the duration of the range from dozens of picosecond to sub-nanosecond. There are many mode-locking pulses in the singly Q-switched envelope and each one has different pulse energy. And QML pulses can have higher peak powers than that of CWML pulses. In the experiment, the phenomenon of QML of Raman laser was observed although there is no any other mode locking component in the cavity. And the picosecond pulses of 2nd-Stokes were obtained in intracavity Raman laser for the first time.
2. Experimental setup
The experimental setup is shown schematically in Fig. 1 . The resonator is a plane-plane configuration. Compared with a convex-plane resonator [28], the plane-plane cavity can be adjusted easily. Through rapid adjustment, it is easy to make Raman wave generate and operate in an efficient, stable, optimal situation. Compared with a concave-plane resonator [15], the plane-plane resonator can bear some thermal-lensing effect and makes the laser operating more efficiently under high pump power. The rear mirror (RM) is high reflection (HR) coated (R>99.9%) at 1064, 1180 and 1325 nm. In order to get suitable output couplers (OC), three OC mirrors were used in the experiment, respectively, and they have different transmissions at the 1st and 2nd Stokes wavelengths (detailed in Table 1 ). All of the three OC mirrors are coated for HR at 1064 nm (R>99.98%). A dichroic mirror coated for HT at 1180 nm (T>98.6%) and HR at 1325 nm (R>99.5%) was used to separate the second Stokes from the first Stokes laser. The Nd:YAG module (Northrop Grumman, USA) is consisted of a Nd:YAG rod (1.0 at. %, ∅3 mm × 63 mm), a cooling sleeve, a diffusive optical pump cavity and three diode array modules operating at 808 nm. It was used as the fundamental laser source. The total pump power for Nd:YAG module is 220 W. The 46-mm-long AO Q-switch (Gooch and Housego) has anti-reflection (AR) coatings on both faces at 1064, 1180, 1325 nm (T >99.8%) and is driven at 27.12 MHz center frequency with the rf power of 50 W. The Raman active medium is an a-cut BaWO4 crystal with the size of 5 × 5 × 46.6 mm3. Both ends of the Nd:YAG and BaWO4 crystals are AR coated at 1064 and 1180 nm (R<0.2%). The Nd:YAG laser module and the Q-switch were water cooled to be 24 °C and 19 °C, respectively. The BaWO4 crystal was wrapped with indium foil and mounted in water-cooled copper blocks. And the water temperature was maintained at 19 °C. The overall cavity length was about 25 cm.
Table 1. The Transmission of the Output Couplers
The average output power was measured by a power meter (Molectron PM30) connected to Molectron EPM2000 (Coherent Inc.). The emission spectra of the laser were monitored and measured by an optical spectrum analyzer (Yokogawa AQ6315A) of a spectra range from 350 to 1750 nm.
3. Experimental results and discussions
First we studied the Q-switched Nd:YAG laser performance at 1064 nm. For this investigation, an output coupler with partial reflection at 1064 nm (T = 17.9%) was used instead of the Raman cavity output coupler. Figure 2 depicts the average output power at 1064 nm with respect to the incident pump power at PRR of 5, 10, 15 kHz and CW mode. The threshold for 1064 nm oscillation was approximately 12.3 W. According to Fig. 2, with lower PRR, the lower of the average output power was obtained.
Then, by using the three output couplers (number 1, 2 and 3) in Table 1, the operation of 1st Stokes at 1180 nm was investigated. Figure 3 shows the dependence of the output power at 1180 nm on the incident pump power at a PRR of 15 kHz. It can be seen that the output power strongly depended on the transmission of OC. The slope efficiency is higher when the transmission of OC is higher. The maximum output power at 1180 nm for OC1, OC2 and OC3 is 20.5, 14.2 and 8.3 W, respectively. The corresponding optical conversion efficiency from diode laser to the first Stokes is 10.8%, 8.0% and 5.0%, respectively. For all pump powers, no 2nd-Stokes was observed for OC1, OC2 due to high loss caused by the high transmission at 1325 nm. And for OC3, the transmission at 1180 nm is low, and the 1st Stokes optical field reaches adequate power intensity in cavity, it contributes to generate the 2nd Stokes wave. In this experiment, the 2nd Stokes was observed when the pump power exceeded 60.2W at a PRR of 15 kHz.
Fig. 3 Dependence of the 1st Stokes output power on the incident pump power for different output couplers.
By using OC3 output coupler, we demonstrated the performance of 1st-Stokes and 2nd-Stokes dual-wavelength operation. We obtained output power of 1st-Stokes and 2nd-Stokes laser as a function of LD pump powers and PRRs. Figure 4 shows the dependence of the output power on the incident pump power at PRRs of 5, 10, and 15 kHz. It can be seen that the output power strongly depended on the PRR. Under low pump powers, the thermal effect in the medium is not obvious at low PRR or high PRR, but the single pulse energy is higher at lower PRR (5 kHz) than that at higher PRR (10 kHz and 15 kHz), and higher pulse energy usually leads higher conversion efficiency from the fundamental laser to the Stokes laser. So under low incident pump power, the Stokes average output power at 5 kHz PRR is higher than the powers at 10 and 15 kHz PRRs. However, it has been experimentally evidenced that the fractional thermal loading of Q-switched Nd-doped laser significantly increases with decreasing PRR in the range of 1-10 kHz [29]. According to [30], the higher fractional loading in Raman crystal influences the performance of Raman lasers due to thermally induced Raman gain suppression originated after temperature broadening of the Raman line. Moreover, the Nd:YAG module suffers from significant depolarisation effects, which could influence performance of the Raman laser at high pump powers. So under low incident pump power, the Stokes laser power at 5 kHz PRR is higher than the powers at 10 and 15 kHz PRRs. And under high incident pump power, the Stokes laser power at 5 kHz PRR is lower than the powers at 10 and 15 kHz PRRs. Because the transmission of OC3 at 1325 nm (T = 60.08%) is high, the loss is high. So the output powers of 2nd-Stokes are much lower than the output powers of 1st-Stokes in this experiment. As can be seen, the maximum output power reaches 8.30 W and 2.84 W at a PRR of 15 kHz for 1st Stokes and 2nd Stokes wave, respectively. The corresponding optical conversion efficiency from diode laser to 1st Stokes and 2nd Stokes laser is 5.0% and 1.4%, respectively. For the performance of the Raman laser at PRR of 5 kHz, the 2nd Strokes laser output has its peak when the pump power is around 140W. Higher pump power would result in lower output, so the chart ignored the part beyond the saturation. In Fig. 4, the output power of the 1st Stokes decreases under the high pump power, however, the output power of the 2nd Stokes increases. The reason of the phenomenon may be as follows. When the power of the 1st Stokes is high enough, the 2nd Stokes laser will be generated, leading to the increase of the conversion efficiency from 1st-Stokes to 2nd-Stokes and the decrease of the 1st Stokes output power. In addition, the low transmission at 1st-Stokes wavelength of the output coupler OC3 and severe thermal effect under incident pump power limit the output power of the 1st Stokes.
Fig. 4 Output power of the 1st Stokes and 2nd Stokes versus the incident pump power at different PRRs.
The output spectrum of 1st-Stokes and 2nd-Stokes lines were measured by an optical spectrum analyzer, as depicted in Fig. 5 . It can be seen that the frequency shift between the 1st Stokes and the fundamental laser and the shift between 1st-Stokes and 2nd-Stokes agree very well with the optical vibration modes of tetrahedral WO4−2 ionic groups (925 cm−1). Because of the higher conversion efficiency from the fundamental laser to the Stokes laser and the low transmission at 1064 nm of the output coupler OC3, the spectral line of 1064 nm can hardly be observed. In this experiment, the spectral line of 1228 nm was observed when the pump power exceeded 122 W at a PRR of 5 kHz, as depicted in Fig. 6 . In fact, BaWO4 crystal has another important Raman shift of 332 cm−1 [31]. By this Raman shift, the first Stokes radiation of 1228 nm could be generated with 1180 nm laser as the fundamental wave. Recently, the first Stokes radiation of 1103 nm generated by Raman shift of 332 cm−1 with 1064 nm laser as the fundamental wave was reported [32]. As far as we know, for Nd:YAG/BaWO4 intracavity Raman laser, the 1st Stokes radiation of 1228 nm generated by Raman shift of 332 cm−1 with the 1st Stokes radiation of 1180 nm generated by Raman shift of 925 cm−1 with 1064 nm laser was reported for the first time in this paper. The generation of 1228 nm is caused by the high pump power (higher than 122 W in this experiment) and the low PRR (lower than 10 kHz in this experiment). In this experiment, for all pump powers, no other higher order Stokes than the 2nd-Stokes was observed.
Fig. 5 The spectra of the 1st Stokes and 2nd Stokes dual-wavelength operation in intracavity Raman laser.
The pulse temporal behavior was recorded with a digital phosphor oscilloscope (TDS 5052B, 500 MHZ bandwidth, 5 G Samples/s, Tektronix Inc.) and an InGaAs photodiode (1.5 GHZ bandwidth, 850~1650 nm response spectrum, and 0.3 ns response time). Typical pulse shapes of the 1st Stokes and the 2nd Stokes lasers are shown in Fig. 7 . The 1st and 2nd Stokes were measured spontaneously with two InGaAs probes, due to the limitation of experimental instrument, the signal of the 2nd Stokes was transmitted into oscilloscope directly, while the signal of the 1st Stokes was coupled with a long optical fiber into the other InGaAs probe before being transmitted into the oscilloscope. So the delay of the 1st Stokes signal gives rise to the result of the 1st and 2nd Stokes appearing to rise at the same time. With the pump power of 209 W and a PRR of 15 kHz, the 1st Stokes and 2nd Stokes pulse widths were 20.5 ns and 5.8 ns, respectively. The corresponding pulse energy and peak power of the 2nd Stokes laser were 0.19 mJ and 33 kW, respectively. It can be seen that SRS frequency conversion leads to pulse-shortening of the Stokes components. In Fig. 6, it also can be seen that the Q-switched pulse has many narrow pulse modulation. In some situations, simultaneous Q-switching and mode locking (QML) was observed in Raman lasers [33–36]. In our experiment, the stable QML of Raman laser without mode locking component was obtained at the pump power of about 29~82 W. In order to observe the picosecond-pulse of the 2nd Stokes underneath the Q-switched envelope, the pulse train of the 2nd Stokes was recorded by a fast InGaAs p-i-n photodiode detector (12 GHz bandwidth, 500~1650 nm response spectrum, and 34 ps rise time, Newport Inc.) and a TDO 90804A digital oscilloscope (8 GHZ bandwidth, 40 G Samples/s, Agilent Inc.). Figure 8 gives the pulse train of the 2nd Stokes at the pump power of 50 W and a PRR of 15 kHz. It can be seen that the pulse width of the Q-switched envelope is about 9.2 ns. In the Q-switched envelope, there are eight mode-locked pulses, and the time interval between two neighbor mode-locked laser pulses was about 2.08 ns which was equal to the resonator roundtrip time (, where is the optical length of the laser resonator and is the speed of the light.). This confirms the existence of the mode-locking regime in the Raman laser. The mode-locked pulse width on the oscilloscope was about 76 ps. Taking the response times of the detector and oscilloscope into account, the real mode-locked pulse width was less than 76 ps. And the mode-locked pulse width can be estimated by the formula [37], which describes the relationships among the measured rise time , the real rise time of the pulse , the rise time of the probe and the rise time of oscilloscope , the estimated width of mode-locked pulse is about 31 ps. With increased pumping power, the pulsewidth of the mode-locked pulses were getting larger. And the mode-locking modulation was not obvious under high pump power. The mechanism causing mode-locking in the Raman laser is synchronous pumping [38, 39]. Synchronous pumping is also possible even when the pump radiation is not mode-locked but is multimode with a beat frequency between adjacent longitudinal modes equal to the frequency of intermode beats in the Raman laser. This occurs, for example, in intracavity Raman conversion, when the Raman medium is placed in the cavity of the pump laser [38]. The multimode pump radiation displays modulation with a beat frequency equal to the cavity round trip time. The modulation amplitude of the beats was not higher than 10%. Nevertheless it sufficed to cause mode-locking of the first Stokes. At cascade Raman generation the second Stokes is excited by the previously generated first Stokes. These beatings of the first Stokes intensity become a synchronous pump for the second Stokes. And, as it known [39], synchronous pumping can results in mode locking of the second Stokes.
Fig. 7 Typical Q-switched pulse at the pump power of 209 W and a PRR of 15 kHz. (The relative timings of the first and second Stokes were not set up.)
Fig. 8 The picosecond-pulse of QML for the 2nd Stokes at the pump power of 50 W and a PRR of 15 kHz.
4. Conclusion
In summary, a diode-side-pumped actively Q-switched Nd:YAG/BaWO4 two-Stokes dual-wavelength operating laser has been demonstrated. With an output coupler of transmission 3.9% at 1180 nm and 60.08% at 1325 nm, as much as 8.30 W of average power at the 1st Stokes wavelength and 2.84 W at the 2nd Stokes wavelength were generated at a pulse repetition rate of 15 kHz, corresponding to an optical-to-optical conversion efficiency of 5.0% and 1.4%, respectively. With an incident pump power of 209 W, 20.5 ns of 1180 nm pulse width and 5.8 ns of 1325 nm pulse width at a repetition rate of 15 kHz were obtained. The QML of the 2nd Stokes laser was realized in this experiment due to synchronous pumping. With an incident pump power of 50 W and a repetition rate of 15 kHz, about 31 ps of 1325 nm mode locking pulse width was obtained.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No. 60908010, 11004122), Open project of State Key laboratory of Crystal Material, (No. KF1010), and Special Grade of China Postdoctoral Science Foundation (No. 201003632).
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