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Abstract
This article presents a dual-wavelength signal wave output system capable of generating a broad range of adjustable wavelength intervals. The setup involved the creation of a dual-wavelength cascaded Raman laser featuring composite cavities operating at 1176 nm and 1313 nm. Experimental investigations were carried out on an external cavity MgO:PPLN-OPO driven by the cascaded Raman laser. By setting the crystal polarization period to 27.6-34.4 µm and the temperature to 50-130°C, adjustable tunable output of dual-wavelength signal wave at 1176 nm-MgO:PPLN-OPO (1550-2294 nm) and 1313 nm-MgO:PPLN-OPO (1768-2189 nm) was achieved with a wavelength interval of 0-218 nm. Under the conditions of a period of 34.4 µm, temperature of 90°C, and an incident Raman power of 2.6 W, the highest conversion efficiency of Raman to dual-wavelength signal wave (2212, 2182 nm) was 34.2%. Furthermore, the maximum output power of dual-wavelength signal wave was recorded at 1.02 W with an incident Raman power of 3.33 W.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Dual-wavelength laser sources with different wavelength interval have found wide-ranging applications [1–3].For instance, dual-wavelength lasers with small wavelength interval are suitable for generation of terahertz (THz) waves [4,5] and differential absorption lidar (DIAL) [6–9]. Dual-wavelength lasers with large wavelength interval are crucial for precision measurements, as well as in chemical sensing, biomedical applications, and dual-wavelength interferometry [10–14]. The significance of dual-wavelength pulsed lasers with tunable wavelength interval is increasing, as they offer cost reduction and improved efficiency by allowing a wide range of wavelength intervals to be achieved with a single device.
The utilization of OPO is extensively employed in the implementation of dual-wavelength coherent light source. By utilizing a single-wavelength source as the pump source, it can generate dual-wavelength output in the form of the signal wave and the idler wave. When an OPO is operated near the degenerate point, a similar dual-wavelength output can be achieved [15,16]. However, in this case, the linewidth of the OPO broadens to hundred nanometers. To address this, the utilization of single-wavelength pumped two-period longitudinally bonded crystals has been explored, enabling the attainment of dual-wavelength output [4,17], but for each interaction process, only a portion of the crystal length can be utilized, resulting in limited conversion efficiency. Alternatively, the incorporation of two nonlinear crystals within a single OPO cavity presents an alternative approach to generate dual wavelengths. By placing two nonlinear crystals in the same OPO cavity, a dual-wavelength signal pulse can be obtained [18–21]. It is worth noting that the use of two separate nonlinear crystals in a single cavity can complicate the OPO cavity and impact its stability [22].
The dual-wavelength pump source pumped OPO is another solution for achieving dual-wavelength signal wave output. In contrast to the earlier method employing a single-wavelength pump source for the OPO, the stability and conversion efficiency can be effectively controlled. The dual-wavelength pump source can be achieved by using two gain media in the resonant cavity [23]. However, adopting the technique of coaxially pumping two laser crystals for the generation of dual-wavelength pump light introduces a more intricate structural design. Additionally, dual-wavelength pump sources can be realized by using a single gain medium. In 2010, Yang et al. accomplished the simultaneous oscillation of a Nd:YAG laser at 1.06 µm and 1.3 µm wavelengths. This laser was subsequently utilized to power a KTA OPO of type II critical phase-matching, leading to the production of dual-wavelength outputs at 1.7 µm and 1.9 µm [24]. It should be noted that the KTA OPO mentioned in the report only allows for dual-wavelength outputs with a relatively large wavelength interval. In 2015, Wang et al. employed Nd:GYSGG crystal to achieve simultaneous dual-wavelength oscillation of the fundamental wave. This crystal displayed comparable stimulated emission cross-sections at 1058.4 nm and 1061.5 nm. By utilizing the laser to pump the non-critical phase-matched KTP IOPO, they successfully generated dual-wavelength signal wave at 1562.1 nm/1567.4 nm, with a maximum output power of 750 mW [25]. Nevertheless, this configuration limited the output to dual-wavelength signal wave with a relatively narrow wavelength separation. The gain competition arising from the laser excursion process in dual-wavelength pump sources based on a single-gain medium leads to significant output fluctuations and pulse jitter [26].
All the aforementioned methods employ OPO to generate dual-wavelength output, albeit with limited wavelength tuning range or without any wavelength tuning, underutilized OPO crystal tuning capability, resulting in a fixed wavelength interval. The dual-wavelength cascaded Raman laser has a suitable wavelength interval, with the potential to be a superior dual-wavelength pumping source for OPO. By utilizing a dual-wavelength cascade Raman laser as the pump source for MgO:PPLN-OPO, it becomes possible to achieve a dual-wavelength signal wave output with a broad range of adjustable wavelength intervals, accomplished through the combination of period tuning and temperature tuning, effectively harnessing the crystal's tuning capability. Moreover, Raman lasers possess desirable characteristics including cascade effects, pulse width reduction, and spectral purification. Our previous work has demonstrated that the utilization of a single-wavelength Raman laser as a pump source can enhance the performance of single-signal wave OPO [27,28]. Consequently, all-solid-state cascade Raman lasers hold promise as efficient pumping sources for generating dual-wavelength signal wave in MgO:PPLN-OPO. Utilizing cascade Raman lasers as a dual-wavelength pump source for PPLN crystal allows for the generation of dual-wavelength signal wave output with a broad range of adjustable wavelength intervals. There is currently a lack of detailed report on this topic.
In this scholarly paper, we have successfully achieved a dual-wavelength signal wave tunable output using the dual-wavelength cascade Raman laser pumped MgO:PPLN-OPO. The utilization of the degenerate points of the tuning curves for the 1176 nm MgO:PPLN-OPO and 1313 nm MgO:PPLN-OPO enables the attainment of a dual-wavelength signal wave tunable output through the incorporation of both period tuning and temperature tuning. The output consisted of an OPO signal wave pumped at 1176 nm (ranging from 1550 nm to 2294 nm) and an OPO signal wave pumped at 1313 nm (ranging from 1768nm to 2189 nm). The wavelength interval of this output could be adjusted from 0 to 218 nm. To achieve this, we used a folded composite cavity 1176 nm, 1313 nm actively Q-switched Nd:YVO4/YVO4 cascaded Raman laser as the pump source. This provided a Raman wave with high beam quality and narrow pulse width. The mode matching between the Raman wave and signal wave was achieved through external cavity pumping. By setting the period Λ to 27.6 µm, 29.1 µm, 31.4 µm, and 32.1 µm, we were able to achieve a dual-wavelength stable output with the maximum wavelength interval near 218 nm. Additionally, by selecting the appropriate period and temperature, we could realize a tunable dual-wavelength signal wave output with small wavelength interval. At Λ=34.4 µm, varying the crystal temperature from 50 °C to 100 °C allowed us to achieve a tunable output with a wavelength internal from 0 to 44 nm. By further changing the crystal temperature, the wavelength interval can be further increased. At a crystal temperature of 90 °C and an incident Raman power of 2.6 W, we obtained a dual-wavelength signal wave (2212, 2182 nm) output power of 0.89 W, with a maximum conversion efficiency of 34.2%. Furthermore, the maximum output power of dual-wavelength signal wave was recorded at 1.02 W with an incident Raman power of 3.33 W.
2. Experimental setups
Figure 1 shows a schematic diagram of the experimental setup. The laser diode (LD) used in the experiment was at a wavelength of 880 nm. The pump light transmitted through a fiber, with a diameter of 400 µm and a numerical aperture of 0.22, entered the resonant cavity through a 1:2 coupled mirror set. The laser crystal employed was a YVO4-0.3 at.% Nd:YVO4 -YVO4 bonded crystal, cut in an a-axis orientation, with dimensions of 3 × 3 × (2 + 16 + 2) mm3. It had high transmission (T > 99%) to 880 nm on the incident side and increased transmission to 1064 nm on the front and back sides. The Raman crystal used was a YVO4 crystal, cut in an a-axis orientation, with dimensions of 3 × 3 × 30 mm3. It had transmittance enhancement to 1064 nm, 1176 nm, and 1313 nm at both ends. The Q-switched device employed was a Gooch & Housego acousto-optic element, with a length of 20 mm. Due to limitations of the experimental setup, a circulating water-cooled tandem connection was used to maintain a controlled temperature of 14 °C.
Fig. 1. Schematic diagram of the experimental setup for dual-wavelength cascaded Raman laser pumped MgO:PPLN-OPO
M1, a concave mirror with a curvature of 200 mm, is equipped with high transmittance films of 880 nm (T > 99%) and high reflectance films of 1064 nm (R > 99%). M2, a flat mirror, is coated with high reflectance films of 1064 nm (R > 99%), 1176 nm (T = 25%), and 1313 nm (T = 80%). M1 and M2 collectively create an optical resonance cavity for the fundamental frequency, with a total length of 140 mm. To segregate the Raman resonance cavity from the fundamental frequency resonance cavity, M3, a 45° folded mirror, is coated with 1064 nm high transmittance (HT) and 1176 nm, 1313 nm high reflectance (HR). M4, a plano-concave mirror with a curvature of 100 mm, is coated with high reflectance films of 1176 nm (R > 99%) and 1313 nm (R > 99%). Combining M4-M3-M2, the first- and second-order Raman optical resonance cavity is formed, with a cavity length of 80 mm. M2 acts as a dual-wavelength cascade Raman optical output mirror at 1176 nm and 1313 nm.
The pump source for the OPO is provided by the dual-wavelength cascade Raman wave output from M2. Spatial coupling was accomplished by using a convex lens L1 with a focal length of f1 = 100 mm. M5, a plano-concave mirror with a curvature of 100 mm, was coated with films of high transmittance at 1176 nm, 1313 nm (T > 99%), and high reflectance at 1500-2300 nm (R > 99%). M6, an additional plano-concave mirror featuring a curvature of 100 mm, was coated with films of high transmittance at 1176 nm, 1313 nm (T > 99%), partial transmittance (T = 15%/20%) within the range of 1500-2300 nm. M7 is a 45-degree mirror coated with 1176 nm, 1313 nm high reflectance (H > 98%) and high transmittance film (T > 99%) at 1500-2300 nm. The OPO resonant cavity comprises of M5 and M6, with the flat mirror M7 employed to filter out the effects of the remaining 1176 nm, 1313 nm pump light, and absorb the idler wave. The optical parametric oscillator crystal used was MgO:PPLN, measuring 1 × 12 × 35 mm3 with polarization periods of 27.6 µm, 29.1 µm, 31.4 µm, 32.1 µm, and 34.4 µm. Its temperature was regulated using a temperature-controlled furnace with an accuracy of 0.1 °C. Limited by the length of the temperature-controlled furnace, the minimum waist sizes of the signal wave beam waist inside the OPO cavity are 138 µm-164 µm and 147 µm-163 µm, respectively. In order to achieve good mode matching, the best waist spot radius ratios of Raman wave to signal wave were considered, with pump light spots at 1176 nm and 1313 nm being focused to 105 µm and 110 µm, respectively.
The polarization period and tuning temperature of MgO:PPLN crystals are calculated based on the quasi-phase matching condition of MgO:PPLN and its Sellmeier equation. The period tuning curve can be found in Fig. 2(a), while the temperature tuning curve is shown in Fig. 2(b). By combining period tuning with temperature tuning, it is possible to achieve finely adjustable dual-wavelength signal wave with a wide range of wavelength interval.
Fig. 2. (a) The MgO:PPLN-OPO period tuning curve for first-order Raman wave and second-order Raman wave pumping at 80°C. (b) The MgO:PPLN-OPO temperature tuning curve (34.4 µm) for first-order Raman wave and second-order Raman wave.
3. Results and discussion
3.1 Dual-wavelength cascaded Raman laser pumping source
In this scholarly investigation, the performance of the dual-wavelength cascaded Raman laser was examined. Figure 3 portrays the correlation between the output power at 1176 nm and 1313 nm, the pump power at 880 nm, at a repetition rate of 40 kHz. We separate the first-order and second-order Raman waves through a 45° beamsplitter coated with 1176 nm HT and 1313 nm HR. The threshold of 1176 nm is determined to be 9.8 W, whereas the threshold of 1313 nm is determined to be 13.2 W. It is worth noting that the crystal coating poses limitations on the maximum permissible pump power at a repetition rate of 40 kHz, which is determined to be 28.8 W. At this maximum pump power, the cascaded Raman wave showcases output powers of 2.51 W and 1.52 W at 1176 nm and 1313 nm, respectively, resulting in a combined power of 4.03 W and a conversion efficiency of 14%.
Fig. 3. (a) Average output power (b) conversion efficiency of 1176 nm, 1313 nm cascade dual-wavelength Raman laser.
In consideration of the stability of the pump source, the maximum pump power achieved is 25.8 W. Under these conditions, the power of the first-order Raman wave at 1176 nm is measured to be 2.29 W, while the power of the second-order Raman wave at 1313 nm is measured to be 1.32 W. Spectral information is collected using a Yokogawa AQ6370D spectrometer, as depicted in Fig. 4. The observed spectra reveal the presence of dual-wavelength Raman wave, along with a small amount of fundamental frequency light at 1064 nm. Detailed examination of the fine spectra of the first-order Raman wave at 1176 nm indicates a linewidth of approximately 0.1 nm, while the fine spectra of the second-order Raman wave at 1313 nm exhibit a linewidth of approximately 0.09 nm. The beam quality of the Raman wave at 1176 nm and 1313 nm was measured using the knife-edge method. The measured results are shown in the fitting curves in the insets (a) and (b) of Fig. 4. The M2 values in the X and Y directions were 1.98 and 1.81 for 1176 nm, 1.75 and 1.61 for 1313 nm. The power stability of the first-order Raman wave and second-order Raman wave was measured in the 60 minutes time range. At the highest power of 2.5 W for the first-order Raman wave, the RMS was 0.54%, and at the highest power of 1.5 W for the second-order Raman wave, the RMS was 0.75%.
Fig. 4. (1) 1176 nm, 1313 nm cascade dual-wavelength Raman optical spectra, inset: (a) 1176 nm beam quality, (b) 1313 nm beam quality, (c) 1176 nm fine spectrum, (d) 1313 nm fine spectrum. (2) Power stability of First and Second order Raman wave.
We employed an Agilent DSO9254A oscilloscope to examine the temporal properties of the dual-wavelength cascade Raman wave, as depicted in Fig. 5. When the pump power was set at 25.8 W, the pulse width of the fundamental wave was measured to be 8.89 ns, the pulse width of the first-order Raman wave was 3.43 ns, while that of the second-order Raman wave was 1.62 ns. Notably, a narrowing of the pulse width was observed, resulting in respective peak powers of 16.7 kW and 20.4 kW. It is noteworthy that the peak power of the second-order Raman wave exceeded that of the first-order Raman wave. Furthermore, it can be observed in Fig. 5(b) that the initiation time of the first-order Raman wave and second-order Raman wave is delayed, signifying that the production of higher-order Raman wave follows a cascade mechanism. We acquired a time interval of 3 ns for the first-order and second-order Raman wave by concurrently linking two photodetectors to an oscilloscope.
Fig. 5. Typical waveforms of (a) fundamental wave (b) first and second-order Raman wave (c) first-order Raman wave (d) second-order Raman wave at a pump power of 25.8 W (PRF = 40 kHz).
3.2 Dual wavelength signal wave MgO:PPLN-OPO with large wavelength interval
The utilization of the dual-wavelength cascade Raman laser for the purpose of pumping the MgO:PPLN OPO is the subject of investigation in this study. The analysis of the signal wave spectra was conducted by employing a spectrometer (Yokogawa AQ6375) while manipulating the operating temperatures and crystal polarization cycles. The signal wave spectra of the crystals with different polarization cycles were measured and plotted, with a gradient plotted every 10°C in the temperature range of 50 to 130°C. The corresponding signal wave tuning curves were plotted and depicted in Fig. 6. The tuning ranges of the signal wave, corresponding to the 1176 nm and 1313 nm, were determined for crystal polarization periods of 27.6 µm, 29.1 µm, 31.4 µm, and 32.1 µm. These ranges were found to be (1550.4 nm ∼1556.9 nm, 1767.5 nm ∼1773.3 nm), (1597.9 ∼ 1604.7 nm, 1816.9 ∼1823.0 nm), (1704.4 nm ∼1722.8 nm, 1917.2 nm ∼1928.6 nm), and (1745.1 nm ∼1763.9 nm, 1952.3 nm ∼1963.7 nm), respectively. For the pump source at 1176 nm first-order Raman wave, the wavelength change due to the variation in polarization period was 43.3 nm/µm, while the wavelength change temperature change was 1.58 nm/10°C. On the other hand, when the pump wave was 1313 nm second-order Raman wave, the wavelength change due to the alteration in polarization period was 41.1 nm/µm, and the wavelength change due to temperature change was 1.08 nm/10°C. These results indicate that the wavelength tuning rate is higher when pumping PPLN-OPO at 1176 nm compared to pumping it at 1313 nm.
Fig. 6. Signal wave wavelength-temperature tuning curves at 1176 nm, 1313 nm Raman pumped MgO:PPLN-OPO with different crystal polarization periods (27.6 µm, 29.1 µm, 31.4 µm, 32.1 µm, temperatures from 50 to 130°C).
At a temperature of 90°C, the signal wave spectrum was observed, as visually depicted in Fig. 7. The wavelengths corresponding to the centers of the signal waves, namely the first-order Raman pumped signal wave and the second-order Raman pumped signal wave, were measured to be 1554.1 nm and 1770.8 nm, 1601.8 nm and 1820.4 nm, 1712.8 nm and 1922.9 nm, and 1754.5 nm and 1958.0 nm, respectively. These measurements were obtained at polarization periods of 27.6 µm, 29.1 µm, 31.4 µm, and 32.1 µm.
The output characteristics of the MgO:PPLN-OPO were evaluated at a crystal temperature of 90 °C, and the obtained results are presented in Fig. 8. The maximum power of the cascade Raman wave entering the nonlinear crystal was limited to 3.33 W due to lens loss. In Fig. 8(a), the power output curves and efficiency curves are displayed for various crystal periods, namely 27.6 µm, 29.1 µm, 31.4 µm, and 32.1 µm. It is observed that the transmittance varies across different wavelengths due to the wide range of the M6 coating on the output mirror.
Fig. 8. Average output power (a) of dual-wavelength signal wave (b) dual-wavelength Raman wave-to-signal conversion efficiency as a function of the incident power at 1176 nm and 1313 nm for different periods (27.6 µm, 29.1 µm, 31.4 µm, 32.1 µm) (transmittance of 20%, temperature of 90°C).
When the transmittance of the output mirror with respect to the signal wave is relatively constant, the gains of the different signal waves differ. Notably, the gain is higher for larger periods at the same pump power density [29]. For instance, when the period (Λ) is equal to 32.1 µm, the MgO:PPLN-OPO yields a dual-wavelength signal wave at 1754.5 nm and 1958.0 nm, with an output power of 0.80 W and a maximum conversion efficiency of 25.4%. This conversion efficiency is higher than that achieved for the signal light at a period of Λ = 31.4 µm.
3.3 Dual wavelength signal wave MgO:PPLN-OPO with small wavelength interval
By increasing the grating period of the PPLN crystal, the wavelength interval becomes narrower. Temperature tuning allows for the achievement of dual-wavelength signal wave with similar wavelengths when the poling period is 34.4 µm. The experiments were conducted using different curvature cavities and output mirrors with varying transmittances, while setting the temperature to 90°C.
The threshold of the signal wave differs depending on the transmittance of the output mirror, with higher transmittance corresponding to a higher threshold. For a cavity curvature of 100 mm and output mirror transmittances of 15% and 20%, the thresholds are 0.896 W and 1.752 W, respectively. Furthermore, the output power of the signal wave varies with different transmittances. When the transmittance of the output mirror is between 15% and 20%, a higher transmittance leads to a higher output power of the signal wave. At a cascaded Raman wave power of 3.33 W at 1176 nm and 1313 nm, the highest output power of the signal wave corresponding to output mirrors with transmittances of 15% and 20% is 0.956 W and 1.02 W, respectively, with conversion efficiencies of 28.7% and 30.7%. Figure 9(b) illustrates the conversion efficiency of dual-wavelength Raman wave to signal wave as a function of the incident power of the dual-wavelength Raman wave. When the incident power of the dual-wavelength Raman light is 3.04 W, the output mirror with a transmittance of 15% exhibits the highest conversion efficiency, which is 29.8%. At this point, the output power of the signal light measures 0.906 W. The incident power of the dual-wavelength Raman wave corresponding to the highest conversion efficiency of Raman wave to signal wave with the output mirror of 20% transmittance is 2.6 W, and the output power of the signal wave is 0.89 W, with a corresponding efficiency of 34.2%.
Fig. 9. (a) Total output power, inset: dual-wavelength signal wave pulse waveform. (b) Conversion efficiency of 2.2 µm similar dual-wavelength signal wave, inset: dual-wavelength signal wave beam quality.
When using a cavity with a curvature of 50 mm and an output mirror with a transmittance of 20%, the threshold of the signal wave is lower at 0.513 W due to the smaller oscillation beam size and higher power density in the cavity. However, due to the length limitation of the temperature-controlled furnace, the mode matching inside the cavity is not as good as that at 100 mm, resulting in a lower conversion efficiency, with a maximum conversion efficiency of 29.6%. Considering factors such as the damage threshold of the PPLN crystal, the output mirror with a curvature of 100 mm and a transmittance of 20% is ultimately selected.
The temporal characteristics of the dual wavelength signal wave were examined, as depicted in the inset of Fig. 9(a). When the total power of the dual-wavelength cascaded Raman wave at 1176 nm and 1313 nm is 3.33 W, the signal wave pulse width is 1.66 ns. Notably, the pulse widths were further reduced when compared to the pulse widths at the 27.6 µm, 29.1 µm, 31.4 µm, and 32.1 µm periods. The generation of Raman wave is a cascaded process, so the dual-wavelength signal pulses are not strictly synchronized. Due to limitations imposed by the spectroscope coatings, we were unable to separate the dual-wavelength signal waves for observation. The inset of Fig. 9(b) demonstrates that the beam quality M2 was measured to be 3.61 and 3.58 in the XY direction using the knife-edge method.
The wavelength tuning of the MgO:PPLN-OPO was achieved by manipulating the heat sink temperature using a temperature-controlled furnace. The spectral data of the dual-wavelength signal wave were collected at regular intervals of 10 °C, and these spectra can be seen in Fig. 10(a)-(e). The central wavelength tuning curve is shown in Fig. 10(f). It was observed that the signal wavelength from the 1st-Stokes pumped MgO:PPLN-OPO showed a greater variation compared to that of the 2nd-Stokes pumped MgO:PPLN-OPO. This difference can be attributed to the fact that the MgO:PPLN-OPO, which is pumped at 1176 nm, is located closer to the degeneracy point. Consequently, as the system approaches the degeneracy point, both the wavelength and linewidth experience a rapid increase. Furthermore, as shown in Fig. 10(e), when the crystal temperature exceeds 100 °C, the 1176 nm-pumped MgO:PPLN-OPO signal wave becomes closer to the degeneracy point, resulting in a significant broadening of the linewidth.
Fig. 10. Spectrograms at different temperatures (a) 60, (b) 70, (c) 80, (d) 90, (e) 100°C. Spectrograms (f) temperature tuning curves of dual-wavelength signal wave.
When the temperature of the crystal approaches 80°C, the center wavelengths of the MgO:PPLN-OPO signal wave, which is pumped by the 1176 nm 1st-Stokes wave and the 1313 nm 2nd-Stokes wave, are approximately the same, specifically at 2176 nm. During this period, adjusting the crystal temperature can effectively modify the wavelength interval between the 1176 nm 1st-Stokes pumped MgO:PPLN-OPO signal wave and the 1313 nm 2nd-Stokes pumped MgO:PPLN-OPO signal wave. As Fig. 10(f) illustrates, at 50°C, the output signal wave of the MgO:PPLN-OPO, pumped by the 1176 nm 1st-Stokes wave, measures 2156 nm, while the output signal wave of the MgO:PPLN-OPO, pumped by the 1313 nm 2nd-Stokes wave, measures 2112 nm. This indicates a continuous adjustment of the wavelength interval ranging from 0 to 44 nm. By manipulating the crystal temperature further, the wavelength interval can be further extended. It is worth noting that the experimental data depicted in Fig. 10(f) aligns with the temperature tuning curve predicted by theory, although there are slight deviations possibly attributed to crystal period growth errors and temperature control precision.
We also observed the following phenomenon in the experiment, when the crystal temperature was 90°C, the dual-wavelength signal wave spectrum was collected as shown in Fig. 11. (Due to the proximity of the wavelengths of the dual-wavelength signal wave, limited by the spectrometer's coating process, we were only able to observe the relative spectral intensity of the dual-wavelength signal wave at a fixed position.) Figure 11(a) demonstrates that when the pump power was 2.02 W(1176 nm-1.641 W, 1313 nm-0.379 W), the signal wave relative spectral intensity of the 1st-Stokes pumped MgO:PPLN-OPO was higher than that from the 2nd-Stokes pumped MgO:PPLN-OPO. With the pump power increasing to 3.13 W (1176 nm-1.976 W, 1313 nm-1.154 W), although the average power of the 1st-Stokes in Fig. 11(b),(c) has always been higher than the 2nd-Stokes, the relative spectral intensity of the MgO:PPLN-OPO signal wave pumped by the 2nd-Stokes wave gradually increases and even exceeds that of the MgO:PPLN-OPO signal wave pumped by the 1st-Stokes wave. During this process, the 1st-Stokes Raman wave power curve tends to flatten, while the 2nd-Stokes wave power shows a larger increase, resulting in faster growth of the relative spectral intensity of the MgO:PPLN-OPO signal wave pumped by the 2nd-Stokes wave. Although the relative spectral intensity displayed in the spectrometer cannot entirely reflect the power of the two signal waves, this demonstrates the potential advantages of the 1313 nm 2nd-Stokes wave as the OPO pump source, with narrower pulse width, higher peak power, superior beam quality, and higher quantum efficiency.
Fig. 11. Variation of optical spectral intensity of 2.2 µm similar dual-wavelength signal wave at different pump powers at 90°C (a) 2.02 W, (b) 2.73 W, (c) 3.04 W, (d) 3.13 W.
The simulated tuning curve and experimental results of the dual-wavelength signal wave OPO pumped by cascaded Raman laser are presented in Fig. 12. By combining periodic tuning with temperature tuning, we were able to generate dual-wavelength signal wave output with a finely adjustable wavelength interval. The range of wavelength interval adjustment achieved was 0-218 nm, as illustrated in both Fig. 12 and Fig. 10(f).
4. Conclusion
The achievement of a tunable dual-wavelength interval of MgO:PPLN-OPO has been successfully realized by utilizing the 1176 nm first-order and the 1313 nm second-order cascaded Raman wave. The utilization of cascaded Raman wave leads to enhanced output performance due to its superior beam quality and narrower pulse width. The wavelength intervals can be finely adjusted by tuning the period, which can be further combined with temperature tuning for a more precise adjustment. By employing different periods (Λ) of 27.6 µm, 29.1 µm, 31.4 µm, 32.1 µm, and 34.4 µm, a tunable dual-wavelength output with wavelength intervals ranging from 0 to 218 nm has been achieved. For instance, the selection of appropriate periods and temperatures allows for the generation of tunable dual-signal wave output with precise wavelength interval. When Λ is set as 34.4 µm and the crystal temperature is adjusted from 50°C to 100°C, we can achieve continuous tunable output with wavelength interval in the range of 0-44 nm. Moreover, further changes in crystal temperature allowed for the increment of the wavelength interval. When an incident pump power of 2.6 W and a crystal temperature of 90°C are utilized, the resulting dual-wavelength signal wave (2212, 2182 nm) output power is 0.89 W, with a maximum Raman-to-signal efficiency of 34.2%, and LD-to-signal efficiency of 4.66%. With an incident power of 3.33 W for the cascaded Raman wave, we obtained a maximum output power of 1.02 W. With a Raman-to-signal efficiency of 30.7%, and LD-to-signal efficiency of 3.95%.
Funding
National Natural Science Foundation of China (62175181).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request
Supplemental document
See Supplement 1 for supporting content.
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