Visible Triple-Wavelength Switchable Emission Generated in Passively Q-Switched Nd:YVO₄ Self-Raman Laser

Abstract

We report a passively Q-switched self-Raman laser using a dual-end composite c-cut Nd:YVO₄ crystal, which generates switchable visible emissions at 533 nm, 560 nm, and 589 nm. A Cr⁴⁺:YAG/YAG composite crystal served the role of a saturable absorber to achieve passive Q-switching. An angle-tuned BBO crystal was used to achieve the frequency mixing between the first-tokes wave and the fundamental wave. At an incident pump power of 9.5 W, the maximum average output powers were 425 mW for the 589 nm yellow laser, 193 mW for the 560 nm lime laser, and 605 mW for the 533 nm green laser, with corresponding pulse widths of approximately 3.8, 3.6, and 35.1 ns, respectively. This result shows that a passive Q-switching operation with self-Raman crystals presents a promising approach for compact multi-wavelength pulse laser sources.

1. Introduction

All solid-state multi-wavelength lasers have received growing attention for their versatility in applications such as laser medical treatment [1], photodynamic therapy [2], spectral imaging [3], and other fields requiring multiple wavelength sources. For instance, retinal photocoagulation therapy requires high-power wavelength-switchable visible lasers to target different pigments in the eye [4,5]. Similarly, dermatology benefits from selective interactions with melanin, hemoglobin, or collagen, necessitating lasers with tunable wavelengths [6]. Beyond biomedical applications, these lasers are critical in defense and remote sensing, where specific wavelengths are needed for environmental monitoring, biological agent detection, and underwater communication [7,8,9]. In recent years, stimulated Raman scattering (SRS) has attracted widespread attention owing to its ability to facilitate flexible frequency conversion, enabling the generation of new wavelengths which are typically hard to attain with traditional solid-state lasers [10]. By combining SRS with second-harmonic generation (SHG) or sum-frequency generation (SFG), researchers have unlocked the potential for wavelength-switchable visible light emission. This approach leverages the cascaded nature of SRS, where multiple Stokes orders can be generated and subsequently frequency-doubled or mixed to produce a broad spectrum of visible and ultraviolet (UV) outputs [11].

Recent advancements in multi-wavelength visible lasers have explored both continuous-wave (CW) and pulsed regimes. For CW lasers, Chen et al. [12] and Lee et al. [13] demonstrated multi-wavelength emission by using a self-Raman laser. Among the many Raman-active crystals, a special type of neodymium (Nd)-doped crystals has attracted much attention due to their self-Raman effect. Such crystals, such as Nd:YVO₄ [14,15,16,17], Nd:GdVO₄ [18,19] and Nd:KGW [20,21], can simultaneously serve as laser gain media and Raman scattering media, realizing the conversion from a fundamental frequency laser to Stokes light in a single crystal, thereby greatly simplifying the laser structure. Among them, the comprehensive performance of Nd:YVO₄ is particularly outstanding. It has a high absorption cross-section and a wide absorption bandwidth at the 808 nm pump wavelength, ensuring efficient and stable pump absorption; at the same time, its high stimulated emission cross-section makes it easy to achieve low-threshold and high-gain laser operations. More importantly, the strong Raman activity of its YVO₄ crystal makes it an ideal material for building compact and efficient self-Raman lasers [22]. These advantages jointly establish the core position of Nd:YVO₄ in medium- and low-power diode pumping and integrated Raman laser technology. CW lasers have a compact structure, but their disadvantages are more obvious in applications that require high peak power [23,24,25]. In contrast, pulse lasers with high peak power have good applications in laser medical treatment, precision machining, and so on. Recent advances in multi-wavelength pulsed lasers primarily employ active Q-switching [26,27]. For instance, Duan et al. reported an acousto-optically Q-switched Nd:YVO₄ laser producing multiple wavelengths via self-Raman conversion, achieving visible light output with switchable wavelengths of 532, 559, 588, 620, and 657 nm at the watt power level and above [28]. However, active Q-switching systems often involve complex cavity designs and external modulators, which can limit their practicality. To address these limitations, passive Q-switching with self-Raman crystals has emerged as a promising alternative. Passive Q-switching simplifies the system architecture, enhances compactness, and reduces costs compared to active methods [29,30,31,32]. In 2022, our group achieved a 780 mW average power at 589 nm from a passively Q-switched c-cut Nd:YVO₄ self-Raman laser [33]. Despite these successes, the selective frequency mixing between fundamental and Stokes waves in passively Q-switched self-Raman lasers remains underexplored, particularly for multi-wavelength operation.

This work presents, for the first time, a triple-wavelength switchable passively Q-switched Nd:YVO₄ self-Raman laser using an angle-tuned BBO crystal. By fine-tuning the BBO crystal’s orientation, we achieved SHG and SFG between the fundamental laser (1066 nm) and the first-Stokes laser (1178 nm), producing switchable outputs at 533 nm (green), 560 nm (lime), and 589 nm (yellow). Under an incident pump power of 9.5 W, the laser delivered maximum average output powers of 605, 193, and 425 mW for green, lime, and yellow emissions, respectively. At the maximum pump power, the pulse widths for yellow, lime, and green emissions were approximately 3.8, 3.6, and 35.1 ns with corresponding pulse repetition frequency of approximately 31.6, 28.3, and 34.7 kHz, respectively. These results highlight the potential of passively Q-switched self-Raman lasers as compact, efficient, and versatile sources for applications in laser medicine, display technologies, and biophotonics.

2. Crystal Analysis

As an excellent saturable absorber for passive Q-switch operations, Cr⁴⁺:YAG crystal offers several advantages, including a high damage threshold, superior thermomechanical properties, and robust photochemical stability. It exhibits a low saturation energy density and high absorption cross-section around the 1 μm wavelength range, making it particularly suitable for Nd-doped solid-state lasers [34]. A Cr⁴⁺:YAG/YAG composite crystal with dimensions of 3 × 3 × 5 mm³, was employed for the passive Q-switching of the fundamental laser radiation. The principal segment is a 2 mm length Cr⁴⁺:YAG crystal with an initial transmission of 85%.

The Raman gain spectrum of the YVO₄ crystal exhibits a pronounced peak at a frequency shift of 890 cm⁻¹, corresponding to a Raman gain coefficient of 4.5 cm/GW [35]. After undergoing the 890 cm⁻¹ shift, the fundamental wavelength of 1066 nm in c-cut Nd:YVO₄ crystal corresponds to the first-Stokes wavelength of 1178 nm, while the fundamental wavelength of 1064 nm in an a-cut Nd:YVO₄ crystal corresponds to the first-Stokes wavelength of 1176 nm. This interaction within the Nd:YVO₄ Raman laser resonator, combined with SHG or SFG, enables tunable output across a spectral range, including green, lime, and yellow light emissions. In the experiment, a dual-end composite Nd:YVO₄ crystal, with an overall dimension of 3 × 3 × 30 mm³, has been engineered to facilitate efficient self-Raman scattering. Both c-cut Nd:YVO₄ composite crystal and a-cut Nd:YVO₄ composite crystal were used for experimental comparison. The laser-active segment of the c-cut Nd:YVO₄ composite crystal consists of a 0.5 at.% Nd:YVO₄ crystal with dimension of 3 × 3 × 10 mm³. The Nd:YVO₄ crystal was bonded with a 3 mm length c-cut YVO₄ cap on the pump input side to improve thermal management, while a 17 mm length c-cut YVO₄ extension was added at the output side to enhance Raman conversion efficiency. The a-cut Nd:YVO₄ composite crystal maintains the same composite structure as the c-cut version, doped with Nd³⁺ at a concentration of 0.3 at.% in the laser-active segment.

To generate three visible wavelengths via SHG and SFG involving the fundamental and first-Stokes waves, both LBO and BBO have been used as nonlinear optical crystals. The phase matching conditions required for the operation of BBO and LBO crystals are shown in

Table 1. Phase matching parameters for the frequency mixing using LBO and BBO crystals in Nd:YVO₄ self-Raman laser.

Wavelength Conversion	1066 nm SHG	1066 nm and 1178 nm SFG	1178 nm SHG
Output wavelength/nm	533	560	589
LBO NCPM temperature/°C	146	87	40
LBO CPM angle	$\theta =90°,\phi =11.2°$	$\theta =90°,\phi =7.7°$	$\theta =90°,\phi =3.5°$
BBO CPM angle	$\theta =22.8°,\phi =0°$	$\theta =22.1°,\phi =0°$	$\theta =21.4°,\phi =0°$

. The non-critical phase matching (NCPM) LBO crystal demands temperature control over a range reaching 100 °C, which is cumbersome for practical applications. The critical phase matching (CPM) LBO crystal requires an angle adjustment of 7.7° to achieve switching for three visible wavelengths generation. And the critical phase matching (CPM) BBO crystal only requires 1.4° of angular adjustment to achieve rapid switching among the three wavelengths. Consequently, we employed an angle-tuned BBO crystal with a cut angle of (θ = 21.5°, φ = 0°) and dimensions of 4 × 4 × 7 mm³, enabling convenient and rapid switching of visible wavelengths. The intracavity crystals were carefully wrapped in indium foil to ensure efficient thermal conductivity and then mounted on a copper cooling block, maintaining a stable temperature of approximately 20 °C.

3. Experimental Setup

Figure 1 illustrates the experimental arrangement employed in the passively Q-switched self-Raman laser, designed to produce switchable multi-wavelength output. The laser cavity consisted of a pump input mirror (IM) with a 300 mm radius of curvature, and a plane output coupler (OC) mirror. The crystals are sequentially placed inside the cavity in the order of Nd:YVO₄ composite crystal (Castech Inc., Fuzhou, China), Cr⁴⁺:YAG/YAG composite crystal (Cryslaser Inc., Chengdu, China), and the BBO crystal (Castech Inc.). Those crystals had to be arranged as compactly as possible, taking into account the dimensional constraints of their respective mounts, while maintaining the total resonator length at approximately 87 mm. The pump source was a fiber-coupled (core diameter of 200 μm and numerical aperture of 0.22) laser diode operating at 808 nm. The pump beam through the coupling system consists of two lenses with focal lengths of 50 and 80 mm, and then irradiates on the Nd:YVO₄ part of composite crystal. The laser cavity supported both fundamental and Raman laser oscillation. The IM was coated with a high-reflection (HR, R > 99.9%) film for both the fundamental wavelength of 1.06 μm and the first-Stokes wavelength of 1.18 μm, while exhibiting high transmission (HT, T > 90%) at the pump wavelength of 808 nm. The OC mirror was designed with an HR coating for 1.06 μm and 1.18 μm, and an HT coating for the visible wavelength range from 530 to 600 nm. To achieve efficient visible emissions, the pump input surface of the dual-end composite Nd:YVO₄ crystal was anti-reflection (AR) coated at 808 nm, while the end surface next to the OC was HR coated in the 530–600 nm range to reverberate back visible emissions propagating backward. Additionally, both end faces of the dual-end composite Nd:YVO₄ crystal were AR coated at 1.06 μm and 1.18 μm. Similarly, AR coating was applied to both end faces of the Cr⁴⁺:YAG/YAG composite crystal and BBO crystal across the wavelength ranges of 530–600 nm and 1.06–1.18 μm. The BBO crystal was fixed on a rotatable platform for angle tuning.

4. Experimental Results and Discussion

During the experiment, both the c-cut Nd:YVO₄ composite crystal and the a-cut Nd:YVO₄ composite crystal were utilized for experimental investigation. When a c-cut Nd:YVO₄ composite crystal was used in the laser cavity, we successfully achieved three visible wavelength switchable emissions through precise angular rotation of the BBO crystal. Using a fiber optical spectrometer (Model 3648-2-USB2, Avantes Inc., Beijing, China) with a resolution of 0.07 nm, the measured center wavelengths of the yellow–lime–green three-wavelength switchable were 589.3, 559.9, and 533.3 nm, respectively, as shown in Figure 2.

In the experiment, we optimized and investigated the output performance of the yellow, lime, and green emissions with BBO angle tuning, respectively. Figure 3 shows the visible output power against incident pump power, measured using a thermal sensor power meter (Model PM310D, Thorlabs Inc., Newton, NJ, USA). The three visible light lasers have thresholds of around 2 W. For passively Q-switched lasers, as the pump power increases, the average output power and pulse repetition rate (PRF) of the fundamental wave increase simultaneously, while the pulse energy and pulse width remain relatively constant. Consequently, the threshold pump power for the first-Stokes laser in self-Raman conversion is essentially the same as that of the fundamental wave, leading to nearly identical threshold powers for the three visible-wavelength lasers generated via their SHG and SFG. This differs significantly from the case of acousto-optic Q-switching, where the threshold of the first-Stokes laser is markedly higher than that of the fundamental wave, resulting in substantial differences among the thresholds of the three wavelengths [36]. Under identical incident pump power conditions, an approximately linear increase was observed in the average output power of the visible lasers. The slope efficiencies derived from linear fitting were 5.8%, 2.7%, and 7.6%, respectively. When the incident pump power reached 9.5 W, the average output powers were 425 mW, 193 mW, and 605 mW for yellow, lime, and green emissions, with respective diode-to-visible conversion efficiencies of 4.5%, 2%, and 6.3%, respectively. Among these, the 533 nm wavelength laser demonstrated the highest average output power performance.

In addition, the average output power and efficiency of SFG are significantly lower than those of SHG. The main reason is that Q-switched Raman lasers exhibit pulse compression characteristics. The primary reason for this discrepancy lies in the temporal non-overlap between the fundamental laser and in the fact that the first-Stokes laser pulses significantly reduce the SFG conversion efficiency. The pulse waveforms of the three output wavelengths were measured using InGaAs free-space photodetectors (Model DETO8C/M, Thorlabs Inc., Newton, NJ, USA) and shown on a 500 MHz digital oscilloscope (Model DPO3052B, Tektronix Inc., Beaverton, OR, USA), as illustrated in Figure 4. For the green laser at 533 nm, lime laser at 560 nm, and yellow laser at 589 nm, the measured pulse widths were approximately 35.1, 3.6, and 3.8 ns, respectively. Notably, the SHG pulse width for the fundamental laser was substantially broader compared to both the SHG pulse width of the first-Stokes laser and the SFG pulse width produced by the fundamental and first-Stokes light. The pulse width characteristics of the SFG output further reflect the temporal relationship between the fundamental and first-Stokes laser pulses. It can be concluded that the broader pulse width of the fundamental light compared to the first-Stokes light results in a lower temporal overlap, thereby reducing the SFG conversion efficiency in Q-switched Raman lasers.

The train of pulses for each of the three visible-wavelength lasers are shown in Figure 5. The pulse repetition rate of three visible-wavelength lasers are related to the power density of the intracavity fundamental laser. It is precisely the varying losses imposed on the fundamental laser during the frequency conversion process that result in differences in their corresponding pulse repetition rate. The measured PRF of 533, 560, and 589 nm lasers were about 34.7, 28.3, and 31.6 kHz, respectively, for the pump power of 9.5 W. Because the fundamental and the first-Stokes wave do not overlap, conversion efficiency is reduced and pulse peak-to-peak stability worsens. Consequently, the 560 nm emission has the worst peak-to-peak stability.

Further, by replacing c-cut with a-cut Nd:YVO₄ composite crystals, we plan to study the multi-wavelength laser output characteristics using a-cut Nd:YVO₄ composite crystal and its differences from c-cut Nd:YVO₄ composite crystals. When using an a-cut Nd: YVO₄ composite crystal with the similar laser system, only obvious green light was observed during the experiment, reaching an average output power of 420 mW when the pump power was 9.5 W. However, by rotating the BBO angle, no other colors of light appeared, indicating that Raman conversion under passive Q-switching was not achieved. This issue may be due to the fact that the a-cut Nd: YVO₄ crystal has a larger stimulated emission cross-section (σ_a-cut = 25 × 10⁻¹⁹ cm²), which results in insufficient pulse peak power of the fundamental wave to initiate Raman conversion. And c-cut Nd: YVO₄ exhibits a lower stimulated emission cross-section (σ_c-cut = 6.5 × 10⁻¹⁹ cm²), which promotes shorter pulse duration and elevated peak power in Q-switching [37,38]. In 2004, Chen also discovered that a-cut Nd:YVO₄ could not achieve Raman conversion during his research on Nd: YVO₄ self-Raman lasers with passive Q-switching [39].

The significance of this study lies in demonstrating a simplified and compact multi-wavelength laser system that integrates passive Q-switching with self-Raman conversion. By eliminating active modulation requirements and leveraging the intrinsic properties of c-cut Nd:YVO₄ crystals, this approach provides a practical tunable visible laser source with high peak power. Although selective frequency mixing has not yet been achieved in the self-Raman configuration using a-cut Nd:YVO₄ crystal, future work should focus on increasing the fundamental wave power density and peak power, which could be achieved by optimizing the cavity structure (e.g., adopting a folded cavity) and the initial transmission of the saturable absorber. These enhancements would increase fundamental wave power density and peak power, potentially enabling improvement of Raman conversion efficiency and achievement of selective frequency mixing in the self-Raman configuration using an a-cut Nd:YVO₄ crystal. Further research directions include optimizing pulse stability and efficiency, as well as extending the operational wavelength range for specialized applications.

5. Conclusions

In summary, we first proposed a triple-wavelength switchable passively Q-switched self-Raman laser system with c-cut Nd:YVO₄ crystals. By fine-tuning the BBO crystal’s angle, we successfully achieved SHG and SFG between the fundamental laser and the first-Stokes laser, enabling wavelength-switchable outputs at 533 nm (green), 560 nm (lime), and 589 nm (yellow). With the incident pump power of 9.5 W, the maximum average output powers measured were 425 mW at 589 nm, 193 mW at 560 nm, and 605 mW at 533 nm., with respective pulse widths of approximately 3.8, 3.6, and 35.1 ns, respectively. Under maximum pump power conditions, the pulse repetition frequencies were approximately 31.6, 28.3, and 34.7 kHz, while the single-pulse energies were 13.4, 6.8, and 17.4 μJ for the yellow, lime, and green emissions, respectively. We also found that a-cut Nd:YVO₄ is difficult to achieve passive Q-switching self-Raman conversion with similar laser setup. These results indicate that the passively Q-switched self-Raman multi-wavelength switchable laser using a c-cut Nd:YVO₄ composite crystal is compact, easy to implement, and has broad prospects. This compact visible wavelength switchable laser has possible applications in fields like laser medicine, display, and biophotonics.