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The tunable Stokes laser characteristics based on the stimulated polariton scattering in KTiOPO4 (KTP) crystal and the intracavity frequency doubling properties for the Stokes laser are investigated for the first time. When the pumping laser wavelength is 1064.2 nm, and the angle between the pumping and Stokes beams outside the KTP crystal changes from 1.875° to 6.750°, the obtained tunable Stokes laser wavelength varies discontinuously from 1076.5 nm to 1091.4 nm with four gaps. When the pumping pulse energy is 120.0 mJ, the maximum Stokes pulse energy is 46.5 mJ obtained at the wavelength of 1086.6 nm. By inserting a LiB3O5 (LBO) crystal into the cavity, the obtained frequency-doubled laser wavelength is inconsecutive tunable from 538.5 nm to 543.8 nm. The maximum frequency-doubled laser pulse energy is 15.9 mJ at the wavelength of 543.5 nm.
© 2015 Optical Society of America
1. Introduction
Potassium titanyl orthophosphate (KTiOPO4, KTP) is an excellent nonlinear crystal. It has many desirable optical properties: a wide transparency range (0.35-4.5 μm) [1], a high optical damage threshold of 500 MW/cm2 [2], a high nonlinear coefficient (d 33) of about 15.4 pm/V at frequency doubling of 1064 nm laser [3], high thermal and chemical stability and good resistance to humidity. Therefore, it has been investigated and applied extensively since its first report in 1976 [1].
Based on its second-order nonlinearity, KTP crystal has been widely used in optical parametric oscillators (OPOs) [2,4–12 ], second-harmonic generation (SHG) [13,14 ], sum-frequency generation (SFG) [15] and difference frequency generation (DFG) [16,17 ].
Based on the stimulated Raman scattering (SRS) in the KTP crystal (a third-order nonlinear effect), many Raman lasers have been constructed and investigated [18–24 ]. Most researches focused on the first-Stokes laser operation [21–23 ]. Some studied the cascade SRS emission to get higher-Stokes laser operations [24]. The Raman laser wavelength is determined by the pumping laser wavelength and the Raman shift of the material. Generally speaking, a Raman material may has one Raman shift or many shifts. However, these shifts are fixed. Therefore, it is difficult for a Raman laser to be tunable [25].
The stimulated polariton scattering (SPS) can occur in some crystals. KTP crystal is one of them. Polaritons are the coupled phonon-photon waves. Stimulated polariton scattering is a process in which the second-order nonlinear effect and the third-order nonlinear effect work together [26]. In the process of the SPS, three waves including the pumping wave, the generated Stokes wave and the polariton wave interact in the overlapped beam area. The momentum conservation and energy conservation must be satisfied simultaneously:
where k s, k pu, and k po are the wave vectors of the Stokes, pumping and polariton waves, νs, νpu, and νpo are the frequencies of the three waves, h is the Plank constant. Because of the large refractive index and large dispersion coefficient of the crystal for polariton wave, only noncollinear phase matching can be realized, shown in Fig. 1 . For a given pumping laser wavelength, the Stokes laser and polariton wavelengths can be tunable by changing the angle θ between the pumping and the Stokes waves. Because the polariton wave frequency in KTP crystal locates in the terahertz spectral range, the SPS in KTP crystal was used to generate tunable THz wave in 2014 [27]. This paper focuses on the tunable Stokes laser generation based on SPS in KTP crystal and the frequency-doubled Stokes laser operation. We realized a discontinuously tunable Stokes laser whose wavelength covered a range from 1076.5 nm to 1091.4 nm by changing the angle between the pumping (1064.2 nm) and Stokes beams outside the KTP crystal θext from 1.875° to 6.750°. The obtained maximum Stokes pulse energy was 46.5 mJ at 1086.6 nm when the pumping pulse energy was 120.0 mJ. With an LBO crystal in the cavity, the frequency-doubled laser operation with inconsecutive tunable wavelength from 538.5 nm to 543.8 nm was achieved and the maximum pulse energy of 15.9 mJ was obtained at 543.5 nm.2. Experimental setup
The experimental setup is schematically depicted in Fig. 2 . The x-cut KTP crystal had a size of 6 × 6 × 32 mm3. The pump source was a Q-switched Nd:YAG laser operating at 1064.2 nm with the pulse duration, beam diameter and repetition rate of 8.0 ns, 3.0 mm, and 1 Hz, respectively. The laser beam had a top-hat intensity profile. The polarization of the pump wave after the half wave plate was parallel to the z axis of the KTP crystal. The Stokes oscillator was constituted by the KTP crystal and two parallel plane mirrors M1 and M2. It was mounted on a rotating stage. The external angle θext between the pumping and oscillating Stokes beams could be adjusted to tune the Stokes laser wavelength by rotating the stage. M1 was coated with high reflectivity (R > 99.8%). Several mirrors were used as the output coupler (M2). The transmissions were 27%, 38%, 50%, 60%, 78%, respectively. The Stokes laser characteristics were investigated when there was no the LBO crystal and the cavity length was 18 cm. When the LBO crystal was inserted into the cavity, the cavity length was 29 cm and the frequency-doubled Stokes laser characteristics were studied. The energies of the pumping and Stokes pulses were detected by an energy sensor connected to an energy meter (Labmax Top, Coherent Inc.). The wavelengths were measured by an optical spectrum analyzer (YOKOGAWA AQ6315, 350–1750 nm). The pulses were monitored by a Si biased detector (THORLABS, DET10A/M, 200-1100 nm) connected to a digital phosphor oscilloscope (Tektronic TDS5052B, 500MHz, 5GS/s).
3. Experimental results of the Stokes laser
Unless otherwise noted, the experimental results were obtained when the pumping pulse energy was 120.0 mJ and the transmission of the Stokes output mirror was 60%. Considering the experimental operability, the angle (θext) between the pumping and Stokes beams outside the KTP crystal was varied from 1.875° to 6.750°, corresponding to the range from 1.073° to 3.856° inside the crystal. In order to separate the pumping spot and the Stokes spot completely, the minimum external angle was selected at 1.875°. As shown in Fig. 3 , with the increase of θext from 1.875° to 2.125°, from 2.250° to 3.000°, from 3.125° to 3.625°, from 3.750° to 5.625° and from 5.750° to 6.750°, the Stokes wavelengths showed a rising trend from 1076.5 nm to 1077.0 nm, from 1080.7 nm to 1081.3 nm, from 1082.6 nm to 1084.0 nm, from 1085.3 nm to 1087.2 nm and from 1090.6 nm to 1091.4 nm, respectively. The existence of these four wavelength gaps was confirmed by a further detailed measurement. The transverse optical (TO) A1 mode associated with the stimulated polariton scattering in KTP crystal is 268.5 cm−1 [27]. There are several much smaller A1 modes below 268.5 cm−1 that are strongly infrared absorbing. We believe that the four wavelength gaps are caused by these smaller A1 modes [28].
Figure 4 indicates the measured Stokes laser output energy as a function of the wavelength for a given pumping pulse energy of 120.0 mJ. From Fig. 4, we can see that the maximum measured Stokes energy was 46.5 mJ, obtained at the wavelength of 1086.6 nm in range IV. The pulse energy of another peak was 36.1 mJ at the wavelength of 1081.1 nm in range II. Additionally, in range III and V, weak peaks of 23.8 mJ and 12.2 mJ were found at 1082.8 nm and 1090.8 nm, respectively. The measured Stokes pulse energies at large angles (5.750° - 6.750°) in range V were smaller compared with those in the other ranges. The Stokes pulse energy depends on several factors, such as the second- and third-order nonlinearities, the absorption of the smaller A1 modes for the polariton, and the interaction area of the three waves. The nonlinear coefficients are the function of the frequency. The absorption of the smaller A1 modes for the polariton can weaken the interaction of the three waves. This results in the existence of the gaps and the smaller Stokes pulse energies near these gaps. When the angle between the pumping and Stokes beams is relative large, the interaction area becomes relative small. The smaller the interaction area is, the smaller the Stokes pulse energy is.
Figure 5 presents the Stokes laser input-output characteristics at 1076.7 nm, 1081.3 nm and 1086.6 nm, respectively, with the maximum output energies of 8.5 mJ, 32.2 mJ and 46.5 mJ. The results indicated that the output energy increased approximately linearly with the increase of the pumping pulse energy when it was above the threshold of Stokes generation. As the pulse energies show no trend of saturation, as long as below the damage threshold of the KTP crystal, these values could be improved with higher pumping energies.
The waveforms were recorded when the incident pumping pulse energy was 120.0 mJ, the residual pumping pulse energy was 73.4 mJ and the Stokes pulse energy was 37.5 mJ at the wavelength of 1085.8 nm, and the results were demonstrated in Fig. 6 . Although these waveforms were recorded by two identical detectors, we obtained their relative heights approximately by using their pulse energies and shapes. The temporal relations of the three waveforms were obtained by using the temporal relations between the pumping pulse and the residual pulse and that between the residual pulse and the Stokes pulse. It can be seen that the Stokes pulse was generated along with the consumption of the pumping pulse.
Different plane mirrors with transmissions of 78%, 60%, 50% and 38%, and 27% were used as the Stokes output mirror, respectively, and the results are presented in Fig. 7 . As we can see, when the external angle θext changed from 1.875° to 5.625°, for the wavelengths below 1090 nm, the optimum transmission for getting the maximum Stokes pulse energy was not far from 60%. For the large angles from 5.750° to 6.750°, the optimum transmission was about 50%.
Fig. 7 Stokes pulse energies as a function of the external angle with different Stokes output mirrors.
4. Intracavity second harmonic generation
To obtain the frequency-doubled Stokes laser pulses, we inserted an LBO crystal in the Stokes cavity as shown in Fig. 2. The LBO crystal with the dimension of 7 × 7 × 15 mm3 was cut in θ = 90° and φ = 10.4°. Its two end faces were antireflection coated (R < 0.5%) at the wavelength ranges from 1064 nm to 1096 nm and from 532 nm to 548 nm. In the second harmonic generation experiment, the two cavity mirrors were different with those used in the Stokes laser generation, so we marked them as M3 and M4. M3 was coated with high reflectivity (R > 98%) at 1064-1100 nm and 532-546 nm. M4 was coated with high reflectivity (R > 98%) at 1064-1100 nm and high transmission (R < 2%) at 532-546 nm.
Figure 8 gives the frequency-doubled Stokes laser wavelength as a function of external angle θext. It can be seen that, with the external angle θext varied from 1.875° to 2.000°, from 2.250° to 3.000°, from 3.125° to 3.375°, from 3.625° to 5.625°, the frequency-doubled laser wavelengths rose from 538.5 nm to 538.6 nm, from 540.4 nm to 540.9 nm, from 541.5 nm to 542.1 nm and from 543.1 nm to 543.8 nm, respectively. The second harmonic generation of the pulses corresponding to the range V in Fig. 4 was not observed because the intensities of the Stokes pulses were not strong enough. Figure 9 gives the frequency-doubled laser pulse energy as a function of the wavelength. Figure 10 shows the frequency-doubled laser input-output characteristics at 538.5 nm, 540.7 nm, and 543.4 nm, and the obtained maximum pulse energies were 6.1 mJ, 11.2 mJ and 15.8 mJ, respectively.
5. Discussion
Both OPO and SPS in KTP crystal can be used to generate tunable laser beams. We would like to compare the characteristics of the OPO and SPS. OPO is based on the second order nonlinear effect, while both the second- and third-order nonlinear effects are involved in the process of SPS. Both OPO and SPS require phase matching. However, the collinear phase matching in OPO can be realized while in SPS only noncollinear phase matching can be obtained. The tunable range in OPO is relative large and the tunable wavelength is far from the pumping wavelength [5–7 ] while the tunable range in SPS is relative small and the tunable wavelength is near the pumping wavelength. The conversion efficiency from the pumping pulse energy to the Stokes pulse energy of the SPS is lower than the conversion efficiency from the pumping pulse energy to the signal pulse energy of OPO [8,9 ].
There are some similarities and differences between SRS and SPS. SRS is based on the third order nonlinear effect, while SPS involves both second- and third-order nonlinear processes. The Stokes laser wavelength based on SRS is difficult to be tunable while the Stokes laser wavelength based on SPS is easy to be tunable. For Stokes laser generation by SRS, there is no need for phase matching, the Stokes laser beam and the pumping laser beam are collinear [18–24 ]. For anti-Stokes laser generation by SRS, phase matching is needed, the Stokes laser beam, the pumping laser beam and the anti-Stokes laser beam are noncollinear [29]. SPS needs phase matching, the Stokes laser beam, the pumping laser beam and the polariton wave must be noncollinear to realize the phase matching.
6. Conclusion
In conclusion, the tunable Stokes laser characteristics based on SPS in KTP crystal have been investigated for the first time. For a given pumping laser wavelength of 1064.2 nm, when the external angle between the pumping and Stokes beams changed from 1.875° to 6.750°, the obtained tunable Stokes laser wavelength varies discontinuously from 1076.5 nm to 1091.4 nm with four gaps, which were from 1077.0 nm to 1080.7 nm, from 1081.3 nm to 1082.6 nm, from 1084.0 nm to 1085.3 nm and from 1087.2 nm to 1090.6 nm, respectively. For a given pumping pulse energy of 120.0 mJ, the obtained maximum Stokes pulse energy at 1086.6 nm was 46.5 mJ. By inserting an LBO crystal into the Stokes laser cavity, the intracavity frequency doubled Stokes laser properties were studied. The obtained tunable laser wavelength varies discontinuously from 538.5 nm to 543.8 nm with three gaps, which were from 538.6 nm to 540.4 nm, from 540.9 nm to 541.5 nm and from 542.1 nm to 543.1 nm, respectively. The maximum frequency-doubled laser pulse energy at 543.5 nm was 15.9 mJ.
Acknowledgments
This research is supported by the National Natural Science Foundation of China (11174185, 61475087, 11204160), the Natural Science Foundation of Shandong Province (ZR2014FM024), and the Fundamental Research Funds of Shandong University (2014QY005).
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