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Intracavity coupled Raman conversions in KTP and KTA driven by a laser diode (LD) pumped Nd:YAG/Cr4+:YAG 1064 nm laser is demonstrated in this paper. Simultaneous pulse generation of orthogonally polarized dual-wavelength at 1091 and 1095 nm are achieved by balancing the Raman gain of KTP and KTA. Under the LD pump power of 8.1 W, the maximum average output powers at 1091 and 1095 nm are 170 and 150 mW, respectively. The corresponding pulse width and repetition rate are measured to be 3.3 ns and 11.2 kHz, with the pulse peak powers calculated to be 4.6 and 4.1 kW, respectively. The laser source with such small wavelength separation and orthogonal polarization provides the interest for terahertz generation in the 1 THz range. Our study provides a simple and flexible method to achieve orthogonally polarized dual-wavelength laser source by Raman-based intracavity coupled nonlinear frequency conversions.
©2012 Optical Society of America
1. Introduction
Simultaneous dual-wavelength emission is of fundamental scientific interest and has applications in various fields, such as medical instrumentation, holography, lidar, and laser spectroscopy [1–3]. Especially, the dual-wavelength lasers with orthogonal polarizations have shown great potential for laser interferometry and precision metrology [4,5]. Nd3+ doped laser crystals have been proved to be promising materials to realize the dual-wavelength oscillation due to their many sharp fluorescence lines induced by the transitions from 4F3/2→4I11/2, 4F3/2→4I13/2, and 4F3/2→4I9/2 [6–9]. A common characteristic of such dual-wavelength lasers is that the two lasing wavelengths have the same polarization. By adopting additional polarization component or other birefringent gain medium, researchers have realized the orthogonally polarized dual-wavelength lasing [10,11]. The laser crystal with comparable polarization-dependent fluorescence lines can naturally reduce the complexity and cost of the device. Simultaneously passively mode-locked at 1063 and 1065 nm with orthogonal polarization in Nd:GdVO4 have been demonstrated by W. D. Tan et al [12]. Recently, Y. P. Huang et al. have demonstrated the orthogonally polarized dual-wavelength laser at 1085.7 and 1088.5 nm (4F3/2→4I11/2) with a single Nd:LuVO4 crystal [13]. However, this type of dual-wavelength laser based on the energy levels of laser-active ions is always accompanied with gain competition effect resulting from the fact that two laser transitions share the same upper laser level. When the output power at one wavelength grows rapidly, the power at the other wavelength always grow at a lower rate or it can even be saturated. This will degenerate the performance of dual-wavelength laser.
Nonlinear frequency conversions (NFCs) have been widely used to realize dual-wavelength emission [14–17]. Doubly resonant OPO can realize the simultaneously dual-frequency output at the signal and idler wavelength. However, the advantage of this device is offset by a reduction in operating stability and tunability. Stable pump radiation and actively controlled OPO cavity length is generally required to maintain the doubly resonant condition. Cascaded Raman conversion in one Raman medium can also result in multi-wavelength emission. However, there exist some problems for this method. Firstly, the wavelength separation between the cascaded Stokes is relatively large. This may reduce the potential of such device in the application of THz generation. Secondly, the cascaded Stokes radiations always have the same polarization. Thirdly, the cascaded Raman conversion is always accompanied with competition effect resulting from the interactive characteristics of cascaded conversion process, as demonstrated in [18].
The intracavity coupled nonlinear frequency conversions (ICNFCs) have been validated as efficient approaches for achieving the orthogonally polarized dual-wavelength output [19–21]. So-called “Coupled NFCs” is introduced relative to the conventional cascaded NFCs. ICNFCs can be understood as follows. The orthogonally polarized fundamental wave from one Nd:YAG-like crystal can be used to simultaneously intracavity pumped one or two nonlinear crystals, outputting the target dual-wavelength wave. Though the two intracavity NFCs can proceed in parallel, they are coupled by the fundamental wave from the same laser transition in one gain medium though with different polarizations. Therefore, ICNFCs can intrinsically avoid the gain competition from the multi-line laser transitions as well as the negative influence on conversion efficiency. Furthermore, it is also found that synchronization of the orthogonally polarized dual-wavelength pulses can be realized by optimizing the nonlinear optical crystal parameters, a very simple passive method [21]. In a word, the ICNFCs can be characterized by decreased competition effect, and good potential for synchronizing the orthogonally polarized dual-wavelength emissions.
As members of the “KTP family,” KTP and KTA exhibit large optical nonlinearity, high optical damage threshold and excellent thermal stability, which make them useful for the second-order NFCs. Besides these applications that exploit the χ(2) nonlinearities, KTP and KTA have been presenting large χ(3) nonlinearities to realize efficient stimulated Raman scattering (SRS) frequency conversions [22–26]. The most prominent Raman shifts of KTP and KTA are respectively 267 and 233 cm−1 (both corresponding to X(ZZ)X Raman configuration), which are associated with the TiO6 octahedron torsional modes. Owing to this slight difference in Raman shift between the two crystals, orthogonally polarized dual-wavelength at 1.09 μm can be simultaneous generated by intracavity coupled Raman conversions in KTP and KTA that are placed within a 1064 nm fundamental cavity.
In this paper, intracavity coupled SRS in KTP and KTA driven by a LD pumped Nd:YAG/Cr4+:YAG 1064 nm laser is demonstrated. Simultaneous pulse generation of orthogonally polarized dual-wavelength at 1091 and 1095 nm are achieved by balancing the Raman gain of KTP and KTA. Under the LD pump power of 8.1 W, the maximum average output powers at 1091 and 1095 nm are 170 and 150 mW, respectively. The corresponding pulse width and repetition rate are measured to be 3.3 ns and 11.2 kHz, with the pulse peak powers calculated to be 4.6 and 4.1 kW, respectively. The laser source with such small wavelength separation and orthogonal polarization provides the interest for 1 THz generation through difference frequency generation. Our study provides a simple and flexible method to achieve simultaneous dual-wavelength laser source by SRS-based ICNFCs. The combination of Raman mediums with different Raman shifts will open a door for the application of ICNFCs to obtain expected dual-wavelength emissions.
2. Analysis on the intracavity coupled SRS conversion
According to the rate equation model describing intracavity Raman laser, the Raman gain is an important parameter that determines the Stokes pulse dynamics [27–29]. Synchronization of the Stokes pulses can be achieved by balancing the Raman gain of KTP and KTA. The Raman gain can be expressed by the product of the Raman gain coefficient and length of the Raman crystal. According to [30], the Raman gain coefficient can be expressed by
where λs and λp are the Stokes and pump wavelength, Ip is the pump intensity, N is the number density of molecules, ns is the refractive index of Stokes, h is Planck’s constant, c is velocity of light in vacuum, dσ/dΩ is spontaneous Raman scattering cross section, and Δν is the full-width half-maximum Raman line width. Obviously, the parameters λp, Ip, h, and c remain the same for KTP and KTA under the same excitation source. Moreover, due to the fact that both the Stokes are associated with TiO6 octahedron torsional modes from the isostructural crystals, KTP and KTA can be considered to have the same value of N. In fact, the relative Raman coefficient is enough to demonstrate the Raman activity of the two crystals. Figure 1 shows the Raman spectrum of KTP and KTA obtained with the same Raman spectrum analyzer. The relative Raman scattering cross sections can be expressed by MIR, where IR is relative Raman signal intensity determined from Fig. 1, and M is correction factor indicating the effects caused by the refractive index difference between the two crystals [31]. Correction factor M is given bywhere n and R are the refractive index and the reflectivity at the light-passing surfaces of Raman crystal, respectively. Superscripts P and S denote the pump and Stokes waves, respectively. Eventually, the Raman coefficient of KTP relative to KTA can be obtained with Eqs. (1) and (2), with the results presented in Table 1 . The refractive indices of 1095 nm in KTP and 1091 nm in KTA are 1.829 and 1.867, respectively. Since both the crystals have anti-refraction coatings on the two light-passing surfaces, the reflectivity R is measured to be 0.01 for the pump and Stokes waves. As can be seen from Table 1, the Raman gain coefficient of KTP shows little difference with that of KTA, indicating that slight modification of length of crystal can balance the Raman gain of KTP and KTA.
Table 1. Corresponding parameters for calculating relative Raman gain coefficients
3. Experimental setup
The experimental arrangement of the compact dual-wavelength laser was shown schematically in Fig. 2 . The fiber-coupled LD pump source was with a core diameter of 400 μm and numerical aperture of 0.22. Its radiation was coupled into the laser crystal by a focusing optical system (consisting with two lenses) with a 25 mm focal length and 95% coupling efficiency. The radius of the pump beam within the laser crystal was around 400 μm. The laser resonator was made up of an input mirror M1, Nd:YAG/Cr4+:YAG composite crystal, KTP, KTA, and an output coupler M2. M1 was a plano-concave mirror with 200 mm radius of curvature. It was antireflection coated at 808 nm on the plane surface, high-reflection coated at 1064, 1091, and 1095 nm and high-transmission coated at 808 nm on the concave surface. The plano-parallel output coupler M2 was highly reflective at the pump wavelength and with partial transmission coatings at the Stokes wavelength (T = 5%@1091 and 1095 nm).
Fig. 2 Schematic diagram of the coupled SRS laser intracavity pumped by a diode-pumped passively Q-switched Nd:YAG/Cr4+:YAG laser.
The composite crystal was comprised of a 1.0 at. % Nd:YAG with the dimensions of 4.5 × 4.5 × 6 mm3 and a 4.5 × 4.5 × 1.5 mm3 Cr4+:YAG saturable absorber with the initial transmission of 85% at 1064 nm. The Cr4+:YAG was diffusion-bonded to the un-pumped facet of the Nd:YAG, as shown in Fig. 2. The composite crystal had the antireflection coatings at 1064 nm on both surfaces. Both the X–cut 4 × 4 × 21 mm3 KTP and 4 × 4 × 20 mm3 KTA crystals were antireflection coated at the Stokes and pump wavelength on the light-passing faces. As for the SRS conversion, the fundamental wave with the polarization being parallel to the Z-axis of KTP and KTA crystal could be consumed. In order to achieve coupled SRS conversion, KTP and KTA were placed in the resonator with their Y axes perpendicular to each other. Consequently, the generated Stokes wave from KTP and KTA are naturally orthogonal polarized due to the X(ZZ)X Raman configuration. Furthermore, the slightly lengthened KTP crystal is used in the experiment to compensate for the weakness from its lower Raman gain coefficient. Both the Nd:YAG/Cr4+:YAG composite crystal and Raman crystals were wrapped with indium foil and mounted in copper block cooled by water at a temperature of 20 °C. The overall cavity length was as short as 50 mm.
4. Experimental results and discussions
Figure 3 shows the average output powers at 1091 and 1095 nm with respect to the LD pump power in the optimal situation. To begin with, an optical spectrum analyzer (AvaSpec-3648-NIR256-2.2) with the resolution of 0.5 nm is used to measure the spectrum of the laser output. It is found that 1091 and 1095nm start to oscillate almost at the same time with the LD pump power increased to 4.4 W. This validates the dual-wavelength scheme by χ(3)-based ICNFCs is practicable. With further increasing the LD pump power, both the average output powers of Stokes are increased linearly, indicating the weakened competition effect in ICNFCs. Under the LD pump power of 8.1 W, the maximum average output powers for Stokes 1091 and 1095 nm are measured to be 170 and 150 mW, respectively. Figure 4 presents the laser emission spectrum from the pulsed laser with maximum total output power. As can be seen, the laser operates at dual-wavelength locating at 1091 and 1095 nm, with the linewidth of 0.8 and 0.84 nm, respectively. Weak fundamental wave at 1064 nm is also observed in the spectrum. The laser source with such small wavelength separation and orthogonal polarization provides the interest for 1 THz generation through difference frequency generation in a nonlinear crystal placed right after the output coupler M2, as demonstrated in [32]. Decreased competition between the dual-wavelength radiations is necessary for scaling up the THz power.
We estimate the performance of the passively Q-switched Nd:YAG/Cr4+:YAG laser with an T = 10%@1064 nm output coupler. When the cavity length is set as 30 mm, the maximum average output power at 1064 nm of 1.8 W is obtained at the LD pump power of 8.1 W. However, when the cavity is lengthened to 50 mm, the maximum average output power at 1064 nm is only 900 mW under the same experimental conditions. The severe thermal effect of the composite Nd:YAG/Cr4+:YAG crystal induced by the bad crystal quality can account for such distinguished difference between the two situations. If the effective conversion efficiency can be defined by the ratio of average output powers of Stokes to maximum output power of 1064 nm fundamental laser, the corresponding value of this dual-wavelength device is up to 35.6%, indicating that the method of intracavity coupled Raman conversions is effective. Actually, high output power can be expected by adopting high quality laser crystal as well as optimizing the transmission of the output coupler. However, since the focus of this study is on the demonstration of effectiveness of SRS-based ICNFCs, the power scaling of this device is not considered in this paper.
The dual-wavelength pulse characteristic is recorded by a Tektronix DPO7104 digital oscilloscope (1 GHz bandwidth, 5 Gs/s sampling rate) and a high-speed photodetector (New Focus Model 1623, InGaAs detector with rise time of 1 ns). When the dual-wavelength output signal is received by the photodetector, stable single pulse is observed on the screen of oscilloscope, indicating there should be no delay between the two Stokes signals under our measuring conditions. The dependence of pulse width and repetition rate on the LD pump power is also measured, with the results plotted in Fig. 5 . As can be seen, the pulse repetition rate presents variation of 5.1-11.2 kHz with the LD pump power increased from 5.3 to 8.1 W. The corresponding pulse width is decreased from 7.8 to 3.3 ns. The phenomenon that the pulse widths generated by a passively Q-switched laser were decreased with the pump power can be understood as follows [33]. During the interval when the passive Q-switch is open (corresponding to the bleaching of the saturable absorber), the inversion population will continue to accumulate due to the pumping process. Furthermore, the thermal effects that occur in the laser crystal as well as in the absorber will also depend on the pump power. These effects will make the initial inversion density change with pump power, resulting in the variation of pulse width accordingly. Figure 6 shows the 3.3 ns single pulse shape, with the inset displaying the corresponding pulse train at 11.2 kHz repetition rate.
Based on the results mentioned above, it can be found that the method of intracavity coupled Raman conversions has wider applicability than the early reported results [19–21]. Obviously, the number of nonlinear crystals possessing the potential for simultaneous realization of OPO and SRS in one crystal is quite limited. However, Raman mediums with different Raman shifts are easily obtained, impregnating new activities for the development of SRS-based ICNFCs.
5. Conclusions
In this paper, intracavity coupled SRS in KTP and KTA driven by a LD pumped Nd:YAG/Cr4+:YAG 1064 nm laser is demonstrated. Simultaneous pulse generation of orthogonally polarized dual-wavelength at 1091 and 1095 nm are achieved by balanced Raman gain of KTP and KTA. Under the LD pump power of 8.1 W, the maximum average output powers at 1091 nm and 1095 nm are 170 and 150 mW, respectively. The corresponding pulse width and repetition rate are measured to be 3.3 ns and 11.2 kHz, with the pulse peak powers calculated to be 4.6 and 4.1 kW, respectively. Our study provides a simple and flexible method to achieve simultaneous dual-wavelength laser source by SRS-based ICNFCs. The combination of Raman mediums with different Raman shifts will open a door for the application of ICNFCs to obtain expected dual-wavelength emissions.
Acknowledgments
This work was supported by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). This work was also supported by the National Natural Science Foundation of China (Grants: 91022003 and 51021062).
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