Open Access
Add to BibSonomy
Abstract
A flexible method of generating stable dual-wavelength laser pulses with tunable power ratio and pulse interval is proposed, through integrating a coaxially end-pumped laser with compound gain media and an intracavity pumped optical parametric oscillator (IOPO). A theoretical model was built by a set of time-domain coupling wave equations containing both the generation of two fundamental waves from a shared pump source and the conversion to signal waves through the parametric process. Simulations showed that by simply varying the pump focal position or pump wavelength, the gains in two laser crystals could be changed, leading to simultaneous change in average power ratio and time interval between two wavelengths. Experimental verifications were performed with combined laser crystals (Nd:YAG and a-cut Nd:YLF) and a nonlinear crystal (KTA), which enabled dual-wavelength signal output in the 1.5–1.6 μm eye-safe region and demonstrated coincident conclusions with theoretical results. As there was no gain competition between two fundamental waves, stable signal output was obtained. Moreover, various wavelength pairs in any wavelength ranges are possible by using different laser crystals and nonlinear crystals. It is believed that this is a promising method for generating simultaneous dual-wavelength laser pulses for applications in lidar, remote sensing, nonlinear frequency conversion, etc.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Dual-wavelength pulsed lasers are of great interest for the applications of precision measurement, spectroscopy, remote sensing, and frequency conversion to terahertz wave, and so on [1–4]. The most straightforward method is by two individual lasers with a pulse synchronization system, but the complication in both optics and electronics limits their applications in many areas. Simultaneous oscillation of two or even more wavelengths in one laser crystal is also feasible [5–8], however, the inherent gain competition prevents its stable operation. In our previous study, a coaxial end-pumping configuration with combined two laser crystals for compact and stable dual-wavelength pulse generation as well as its application for terahertz generation [9,10], were demonstrated. Although the output wavelength pairs were flexible by using different crystal combinations, the wavelength range must be within the emission band of the active ions, thus wavelength extension was severely restricted and tuning was impossible. The insistent demands of dual-wavelength laser sources in the eye-safe and mid-infrared ranges have to resort to the nonlinear optical method.
Optical parametric oscillators (OPOs) is the most popular solution for generating dual-wavelength laser pulses. Doubly-resonant OPOs operating near degeneracy are effective for generating two synchronized laser pulses with wavelengths close to each other [11,12]. For a singly-resonant OPO, dual-wavelength signal pulses could be obtained by adopting two different nonlinear crystals placed in the same OPO cavity [13]. There were also reports on dual-wavelength OPO sharing the same nonlinear crystal and pumped by different laser lines coming from a single laser gain medium, however, gain competition originating from the laser transition process definitely caused considerable output fluctuation and pulse timing jitter [14]. Additionally, a common defect for the above-mentioned OPOs is that the average power ratio as well as the pulse interval between two wavelengths were not tunable, which is a problem in the fields like differential absorption Lidar (DIAL), pump probe and temporal process monitoring.
In this paper, we demonstrated the integration of a singly-resonant IOPO and a stable dual-wavelength laser with coaxially diode-end-pumping scheme. A theoretical model was built to analyze the dynamics of both the evolution of two fundamental fields and the parametric interaction processes of the IOPO. In the experiment, combined Nd:YAG/Nd:YLF (a-cut) laser gain media and a KTA nonlinear crystal were employed to give dual signal wavelengths in the eye-safe range. The variation of both the average power ratio and the pulse interval at two wavelengths was realized simultaneously by changing the laser diode (LD) pump focal position inside the gain media, verifying the theoretical conclusions. This method is extremely flexible for dual-wavelength pulse generation and wavelength extension by using different laser and nonlinear crystals, while maintaining a compact, stable and costless configuration.
2. Experimental setup
The experimental diagram for the KTA IOPO driven by a coaxially pumped dual-wavelength laser is shown in Fig. 1. A 30-W fiber coupled LD (BWT Beijing Ltd.) with the central wavelength at 805 nm was used as pump source. The core diameter of the fiber was 400 μm and the numerical aperture (NA) was 0.22. The pump beam was focused into the laser crystal by a 1:2 coupling system. Practical measurements at the output power of 10 W showed that the beam waist diameter defined by the displacement of a knife-edge allowing 84% and 16% of the total power to pass through was 500 μm and the Rayleigh range was 60 mm, corresponding to the beam quality factor (M2) of around 47. Compared with the conventional IOPO setup, the principle character here was that the laser gain media was composed by two of different kinds (LC1 and LC2), which could enable the oscillation of two fundamental wavelengths. The inset of Fig. 1 is the detailed description of the compound gain media and the configuration of the LD pump beam. A parameter z was defined to indicate the pump focal position, where negative and positive values denoted the pump beams were focused in LC1 and LC2, respectively. LC1 was a 7-mm-long 0.6-at% doped Nd:YAG crystal for 1064-nm generation while LC2 was a 10-mm-long, 1-at% doped a-cut Nd:YLF crystal for 1047-nm generation. Two laser crystals were put close to each other with an air gap of 0.2–0.3 mm in between and they were anti-reflection (AR) coated at both the pump (around 800 nm) and fundamental wavelengths (around 1.06 μm) at all end faces. The fundamental laser cavity consisted of two cavity mirrors M1 and M3, and the OPO cavity was confined by mirrors M2 and M3 inside the laser cavity sharing M3 as the OPO output coupler. M1 (plano-concave with 500-mm curvature radius) had AR coatings at the LD pump wavelength and high-reflection (HR) coatings at the resonant fundamental wavelengths around 1.06 μm. M2 was coated for AR at the fundamental wavelengths and HR at the OPO signal wavelengths in the 1.5–1.6 μm range. M3 was HR coated around 1.06 μm and also had partial transmission (T = 20%) at 1.5–1.6 μm, ensuring high-intensity intracavity fundamental-wave power and efficient out-coupling for the OPO signal waves. The nonlinear crystal KTA was a-cut (θ = 90°, φ = 0°) for non-critical phase matching (NCPM), with the size of 4 × 4 × 20 mm3 and AR coatings at both the fundamental and signal wavelengths. The laser gain media, KTA crystal and AO Q-switch were cooled by circulating water at 20 °C. The overall laser cavity length was 120 mm and the OPO cavity length was 35 mm. A dichroic mirror (DM) which had HR coatings around 1.06 μm and AR coatings around 1.5 μm was used to characterize the OPO signal output.
Fig. 1 Experimental layout of the KTA IOPO in the eye-safe range driven by a coaxially pumped dual-wavelength laser. The inset is a detailed description of the focused LD pump beam and the compound gain media inside the laser cavity.
The selection of the compound gain media (Nd:YAG and a-cut Nd:YLF) was based on the consideration of their physical and optical properties. Two materials have similar but different absorption peaks (806 nm and 803 nm), making it feasible both for LD coaxial pumping and gain controlling by slightly tuning the LD wavelength. The good physical properties of Nd:YAG allow high pump power without damage when exposing to the pump beam before the other crystal. Additionally, only the polarization of the Nd:YLF laser should be taken into account for PM in the nonlinear crystal because Nd:YAG generates non-polarized beam. As a side effect, part of the fundamental power is wasted thus the efficiency is declined.
3. Theoretical model
Three stages should be considered during the generation of a signal pulse for IOPO in a Q-switching period: the accumulation of inverted population (before the Q-switch is opened), the foundation of the fundamental field and the parametric interaction process (after the Q-switch is opened). As the compound gain media gives two fundamental wavelengths which consume individual population inversion coming from the same pump beam, the time evolution of each signal generation is related to the other one and should be discussed simultaneously. At the first stage, the energy is transferred from the LD pump beam to population inversion in two laser crystals and the rate equation can be written as
where the subscript m (1 or 2) denotes two laser crystals and the corresponding wavelengths. The total population of active ions N and the inverted population n are normalized by the population inversion at the threshold of the fundamental laser. A is the spontaneous transition coefficient and W is the pump coefficient. W depends on the occupied volume of the pump beam in each crystal and the corresponding absorption coefficient [9], given byin which each of the crystal is equally divided into k parts in length, subscript j is the serial part number, ΔV is the pump volume and Δpabs,j is the absorbed pump power for every part. νp is the LD pump frequency, η is the quantum efficiency, and h is the Planck constant. Before the Q-switch is opened, the photon density inside the cavity can be ignored and the population inversion accumulates to maximum n(tc) until the Q-switch opens at the time tc. Obviously, W can be varied by the summation term which is related to both the pump absorption coefficients in two laser crystals and the pump focal position z, thus the energy storage (population inversion) in each laser crystal is tunable.The second and third stages are interdependent after the Q-switch is opened. According to the IOPO theories [15], the coupled equations of the population inversion, fundamental and signal intensities are expressed by
whereThe variation of signal photon density can be calculated using the Runge-Kutta algorithm by Eq. (3) through Eq. (6), where If and Is are normalized by the fundamental wave intensity at the threshold of the OPO, τa is the lifetime of the upper level of the laser transition, Llaser and LOPO are the cavity lengths of the fundamental laser and OPO, nf and ns are the average refractive indices for fundamental and signal waves in respective cavities, c is the light velocity in vacuum, rf and rs are the reflections of the output mirror at the fundamental and signal wavelengths, and x is the ratio between the OPO threshold fluence and the laser-medium saturation fluence. n(tc) is the initial population inversion at the second stage, calculated by Eqs. (1) and (2). The parameter F expresses the ratio between the laser cavity finesse and the OPO cavity finesse. As the population inversion n(tc) in each laser crystal greatly affects the solutions for the time-domain coupling wave equations, the time evolution of each signal wavelength can be tuned by changing the pump wavelength and the pump focal position z. The other parameters that may affect this process include laser upper level lifetime τa, spontaneous transition coefficient A, etc. However, τa and A are constants and should be similar for two laser crystals, or they will restrict the ability for signal pulse controlling. Since two OPO signal wavelengths are close to each other and in the same polarization, their OPO parameters (cavity length, nonlinear gain, cavity loss, etc) can be regarded as identical.The signal pulse energy is obtained by the time integration of Is, from which the output power at each signal wavelength can be calculated [16].
where fp is the pulse repetition rate, ws is the beam radii of the signal wave, and νs is the signal wave frequency. C is a constant coefficient determined by the normalization coefficient of Is,m and the non-ideal conditions during the parametric process like mode matching. Therefore, the relative output power at two signal wavelengths can be calculated from Eq. (7).4. Experimental results and discussion
Firstly, the performance of the coaxially end-pumped Q-switched laser with compound gain media working at 1047 nm and 1064 nm was evaluated by removing M2, the KTA crystal and replacing M3 with an output coupler around 1.06 μm (T = 10%). In order to enhance the pump absorption in the Nd:YLF crystal, the LD wavelength was tuned to 803 nm via temperature controlling. The output characteristics is shown in Fig. 2. When the pump beam was focused at z = −1 mm, the total average output power increased almost linearly with the LD pump power at the repetition rate of 6 kHz [Fig. 2(a)]. The maximum output power was 2.83 W with 10-W input LD pump power, corresponding to the conversion efficiency of 28.3%. Using a band-pass filter at 1064 nm (Thorlabs FL051064-3), two beams were separated and their powers were measured. The 1047-nm laser had a lower threshold but the 1064-nm laser grew faster at higher input pump power. Nearly equivalent output powers (1.4 W at 1064 nm and 1.43 W at 1047 nm) were realized when the LD pump power was 10 W and the single pulse energy reached 0.24 mJ at each wavelength. The output spectrum is also given in Fig. 2(a) with similar intensity at two wavelengths. Figure 2(b) shows the variation of power ratio with z between two wavelengths pumped at a constant level of 10 W. Compared with the results in [9], the efficiency achieved here was much higher for Q-switched operation (almost the same as that of continuous-wave operation in [9]) by optimizing the cavity, and the equivalent-power point was shifted to the same position where maximum output power was obtained by slightly varying the LD wavelength. It should be noted that not only the power ratio but also the pulse interval at two fundamental wavelengths can be changed by simply changing the pump focal position within several millimeters, as stated in [9], leading to alternate OPO signal pulses.
Fig. 2 Output characteristics of the fundamental laser at 1047 nm and 1064 nm with a pulse repetition rate of 6 kHz. (a) Output power versus LD pump power at z = −1 mm with the spectrum recorded by a Yokogawa AQ6370D optical spectrum analyzer (OSA) as the inset. (b) Power ratio variation versus z with constant LD pump power of 10 W.
With respect to the output characteristics of the KTA IOPO pumped by the coaxially pumped dual-wavelength laser mentioned above, similar measurements and analyzation were performed, shown in Fig. 3. The signal wavelengths were 1506 nm and 1535 nm, respectively, observed by the OSA. A band-pass filter at 1540 nm (Thorlabs PB1540-12) was used to separate the 1535-nm signal wave from the other. When the pump beam was focused at z = −1 mm, the OPO threshold was 2 W where only the 1506 nm was found resonant. The other signal wave at 1535 nm which was excited by the 1064-nm Nd:YAG laser, however, didn’t emerge until the LD pump power was increased to 5 W because the Nd:YAG laser had a higher threshold. Tuning the LD to longer wavelength to enhance the absorption of Nd:YAG could significantly change the status but the Nd:YLF laser would be greatly suppressed due to its limited absorption band. The maximum OPO output power was 724 mW (388 mW at 1506 nm and 336 mW at 1535 nm) when the input pump power was 10 W, corresponding to the optical-optical conversion efficiency of 7.24%. The variation of the power ratio at two signal wavelengths by changing z is shown in Fig. 3(b) where the comparison was also made, demonstrating good consistency. The parameters used in the simulation are Llaser = 120 mm, LOPO = 35 mm, tc = 1.67 × 10−4 s, τa,1 = 2.3 × 10−4 s, τa,2 = 4.8 × 10−4 s, rf,1 = rf,2 = 0.99, rs,1 = rs,2 = 0.8, C = 1 × 109.
Fig. 3 Output characteristics of the dual-wavelength KTA IOPO at 1506 nm and 1535 nm with the pulse repetition rate of 6 kHz. (a) OPO Output power versus LD pump power at z = −1 mm with the spectrum recorded by the OSA as the inset. (b) Power ratio variation versus z with constant LD pump power of 10 W.
As the coaxial pumping scheme is free from gain competition between two fundamental wavelengths, stable signal output could be obtained. For verification the power fluctuation of the signal output power during 1000 seconds was recorded with a Newport 818P-100-25 power meter and shown in Fig. 4(a). The LD pump power was 10 W, the pump beam was focused at the position of z = −1 mm and the Q-switching repetition rate was 6 kHz. The root mean-square (RMS) instabilities of signal powers for 1535-nm,1506-nm and both were 2.54%, 2.17% and 2.07%, respectively, as good as single-signal-resonant IOPOs with only one laser gain media. The beam quality factor (M2) of the dual-wavelength OPO was around 2 at maximum output power, measured by the knife-edge method and shown in Fig. 4(b). If the Q-switching repetition rate was set to 8 kHz and 10 kHz, the power stability and beam quality remained almost the same but the output power decreased to 662 mW and 570 mW, respectively, because the decay of accumulated fundamental pulse energy during a Q-switching period would induce the decline of nonlinear conversion efficiency.
Fig. 4 (a) Power fluctuation of the total signal power and power for each signal wave. (b) Beam quality factor (M2) of the signal beam. The LD pump power was 10 W, the Q-switching repetition rate was 6 kHz and the LD pump beam was focused at z = −1 mm).
The temporal behavior of the output fundamental and signal pulses was detected by a fast-response InGaAs detector (Thorlabs DET08C) when the LD pump power was 10 W and the pulse repetition rate was 6 kHz. As the relative gain in two laser gain media could be tuned by varying the pump focal position z, it was possible to manipulate the fundamental pulse building process which consequently affected the signal pulse evolution at each wavelength, shown in Fig. 5. Although comparable signal powers were obtained at z = −1 mm, the 1535-nm pulse had shorter build-up time and thus generated earlier than the 1506-nm pulse, shown in Fig. 5(a). The pulse widths at two wavelengths were both around 4.5 ns. The gain in the Nd:YAG crystal should be slightly decreased by changing the pump focal position closer to the Nd:YLF crystal, if pulse synchronization was necessary. At z = 0, two signal pulses were generated almost simultaneously, shown in Fig. 5(b) where the pulse width was 6 ns. If z was furtherly increased to z = 1 mm, the 1506-nm signal wave pulse would come out before the 1535-nm signal pulse, shown in Fig. 5(c). The experimentally measured pulse behavior fitted very well with theoretical results, shown in Fig. 5(d) through Fig. 5(f). The interval between two signal pulses could be roughly tuned from −60 ns to + 60 ns, beyond which there would be huge difference in peak power and width between two pulses. As to the fundamental pulses reflected by the dichroic mirror (DM), it was impossible to get a clear temporal waveform like the signal pulses, because two fundamental pulses overlapped with pulse width over 200 ns at each wavelength but a small interval of tens of nanoseconds.
Fig. 5 Experimental (a–c) and theoretical (d–f) temporal pulse shapes of the signal waves at different pump focal positions. (a) and (d): z = −1 mm; (b) and (e): z = 0; (c) and (f): z = 1 mm. The LD pump power was 10 W and the pulse repetition rate was 6 kHz.
Coaxial pumping provides a flexible scheme to generate stable dual-wavelength laser and extend to any wavelength range by OPO with simultaneously tunable average power ratio and pulse interval. Theoretically the same job can be done by changing the pump absorption coefficient via wavelength control of the pump LD, which brings in another degree of freedom. It should also be noted that equivalent power (z = −1 mm) and synchronized pulse (z = 0 mm) were not realized simultaneously in our experiment, which originated from the significant difference of two laser crystals. A smaller stimulated-emission cross section of Nd:YLF would lead to obvious delay in the foundation of fundamental pulse and consequently postponing the signal pulse generation, compared with Nd:YAG in the same condition. For the applications of nonlinear sum frequency generation (SFG) and difference frequency generation (DFG) where synchronized pulse and comparable output power are required, compound laser gain media with similar laser emission characteristics are indispensable. Potential candidates include Nd:YAG/Nd:YAP and a-cut/c-cut Nd:YLF, the former of which is more favorable because their different absorption peaks enable pump wavelength tuning to balance the gains in two laser crystals.
5. Conclusion
A realistic method to realize compact and stable dual-wavelength laser operation with simultaneously tunable power ratio and pulse interval was proposed, based on a singly resonant IOPO configuration driven by a coaxially end-pumped dual-wavelength laser with compound gain media. A theoretical model was built, indicating the feasibility of tuning the average output power and the time evolution process of the signal pulses. Using a Nd:YAG crystal, an a-cut Nd:YLF crystal and a KTA nonlinear crystal, experiments were performed and dual-wavelength laser operation in the eye-safe range was achieved. The average power ratio and time interval between the 1506-nm and 1535-nm pulses could be simultaneously and flexibly tuned by changing the pump focal position in the composite gain media, coincident with simulation results. By replacing the gain media and nonlinear material, various wavelength pairs at any designed wavelength range are possible, which have great potential in lidar, remote sensing, nonlinear frequency conversion, etc.
Funding
National Basic Research Program of China (“973” Project) (Grant No. 2014CB339802); National Natural Science Foundation of China (NSFC) (Grant No. 61675146); Natural Science Foundation of Tianjin City (18JCYBJC16700).
References and links
1. C. L. Wang, Y. H. Chuang, and C. L. Pan, “Two-wavelength interferometer based on a two-color laser-diode array and the second-order correlation technique,” Opt. Lett. 20(9), 1071–1073 (1995). [CrossRef] [PubMed]
2. B. Lashkari, S. S. Sean Choi, M. E. Khosroshahi, E. Dovlo, and A. Mandelis, “Simultaneous dual-wavelength photoacoustic radar imaging using waveform engineering with mismatched frequency modulated excitation,” Opt. Lett. 40(7), 1145–1148 (2015). [CrossRef] [PubMed]
3. Y. F. Chen, S. W. Tsai, S. C. Wang, Y. C. Huang, T. C. Lin, and B. C. Wong, “Efficient generation of continuous-wave yellow light by single-pass sum-frequency mixing of a diode-pumped Nd:YVO4 dual-wavelength laser with periodically poled lithium niobate,” Opt. Lett. 27(20), 1809–1811 (2002). [CrossRef] [PubMed]
4. K. Zhong, W. Shi, D. Xu, P. Liu, Y. Wang, J. Mei, and J. Yao, “Optically pumped terahertz sources,” Sci. China Technol. Sci. 60(12), 1801–1818 (2017). [CrossRef]
5. H. Yu, H. Zhang, Z. Wang, J. Wang, Y. Yu, Z. Shi, X. Zhang, and M. Jiang, “High-power dual-wavelength laser with disordered Nd:CNGG crystals,” Opt. Lett. 34(2), 151–153 (2009). [CrossRef] [PubMed]
6. K. Zhong, J. Yao, C. Sun, C. Zhang, Y. Miao, R. Wang, D. Xu, F. Zhang, Q. Zhang, D. Sun, and S. Yin, “Efficient diode-end-pumped dual-wavelength Nd, Gd:YSGG laser,” Opt. Lett. 36(19), 3813–3815 (2011). [CrossRef] [PubMed]
7. Y. P. Huang, C. Y. Cho, Y. J. Huang, and Y. F. Chen, “Orthogonally polarized dual-wavelength Nd:LuVO4 laser at 1086 nm and 1089 nm,” Opt. Express 20(5), 5644–5651 (2012). [CrossRef] [PubMed]
8. S. D. Liu, L. H. Zheng, J. L. He, J. Xu, X. D. Xu, L. B. Su, K. J. Yang, B. T. Zhang, R. H. Wang, and X. M. Liu, “Passively Q-switched Nd:Sc0.2Y0.8SiO5 dual-wavelength laser with the orthogonally polarized output,” Opt. Express 20(20), 22448–22453 (2012). [CrossRef] [PubMed]
9. Y. Liu, K. Zhong, J. Mei, M. Wang, S. Guo, C. Liu, D. Xu, W. Shi, and J. Yao, “Compact and flexible dual-wavelength laser generation in coaxial diode-end-pumped configuration,” IEEE Photonics J. 9(1), 1500210 (2017). [CrossRef]
10. Y. Liu, K. Zhong, J. Mei, C. Liu, J. Shi, X. Ding, D. Xu, W. Shi, and J. Yao, “Compact and stable high-repetition-rate terahertz generation based on an efficient coaxially pumped dual-wavelength laser,” Opt. Express 25(25), 31988–31996 (2017). [CrossRef] [PubMed]
11. S. C. Kumar and M. Ebrahim-Zadeh, “Yb-fiber-based, high-average-power, high-repetition-rate picosecond source at 2.1 μm,” Laser Photonics Rev. 10(6), 970–977 (2016). [CrossRef]
12. J. Mei, K. Zhong, Y. Liu, J. Shi, H. Qiao, D. Xu, W. Shi, and J. Yao, “Compact, efficient and widely tunable 2-μm high-repetition-rate optical parametric oscillators,” Opt. Commun. 426, 119–125 (2018). [CrossRef]
13. H. T. Huang, J. L. He, S. D. Liu, X. Q. Yang, H. W. Yang, Y. Yang, and H. Yang, “Synchronized generation of 1534 and 1572 nm by the mixed optical parameter oscillation,” Laser Phys. Lett. 8(5), 358–362 (2011). [CrossRef]
14. M. Wang, K. Zhong, J. Mei, S. Guo, D. Xu, and J. Yao, “Simultaneous dual-wavelength eye-safe KTP OPO intracavity pumped by a Nd:GYSGG laser,” J. Phys. D Appl. Phys. 49(6), 065101 (2016). [CrossRef]
15. T. Debuisschert, J. Raffy, J.-P. Pocholle, and M. Papuchon, “Intracavity optical parametric oscillator: study of the dynamics in pulsed regime,” J. Opt. Soc. Am. B 13(7), 1569–1587 (1996). [CrossRef]
16. F. Bai, Q. Wang, Z. Liu, X. Zhang, X. Wan, W. Lan, G. Jin, X. Tao, and Y. Sun, “Theoretical and experimental studies on output characteristics of an intracavity KTA OPO,” Opt. Express 20(2), 807–815 (2012). [CrossRef] [PubMed]
