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Abstract
We demonstrate an efficient active Q-switched Ho:GdVO4 self-Raman laser at 2500 nm for the first time, to our knowledge. Using Ho:GdVO4 crystal as the gain medium for both the 2048nm fundamental laser and the 2500 nm Raman laser, the output performances of a new mid-infrared self-Raman laser were investigated. The maximum average output power of 1.45 W was achieved at an incident pump power of 22.5 W, with a slope efficiency of 25.8%, for an output transmittance of 30% and a pulse repetition frequency of 15 kHz. The maximum single pulse energy of 96.7 µJ with a pulse width of 11.35 ns was obtained, corresponding to the peak power of 8.5 kW. The beam quality was near diffraction limited with the M2 factors of 1.15 and 1.06 along the x and y directions. Moreover, adopting the two-end output way of the fundamental laser and the Raman laser, the Raman gain coefficient of Ho:GdVO4 crystal was estimated to be 1.14 cm/GW at 2048nm. This work shows that Ho:GdVO4 is an excellent self-Raman laser crystal for the generation of high power Raman laser at 2.5 µm.
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1. Introduction
Stimulated Raman scattering (SRS), a well-known third-order nonlinear process, is an efficient method for converting lasers to single or multi-Stokes wavelengths [1–3]. In the visible and near-infrared wavebands, many efficient Raman lasers whose conversion efficiency is close to the quantum limit have been reported [4–7]. The 2-3 µm mid-infrared Raman lasers have many important applications, such as medicine, gas spectroscopy, material processing, and environmental monitoring [8]. However, the Raman gain coefficient of Raman crystal evidently decreases as the fundamental laser wavelength increases, which limits the development of the 2-3 µm mid-infrared laser based on SRS effect [3].
In the past few years, the output performances of mid-infrared Raman lasers were actively explored in both intracavity and external-cavity configurations [9–18]. X. L. Zhang et al. investigated the output performances of the laser diode end-pumped actively Q-switched Tm,Ho:GdVO4/BaWO4 intracavity Raman laser at 2533 nm, and a maximum average output power of 800 mW was obtained [9,10]. However, the Tm,Ho:GdVO4 crystal needed cooling with liquid nitrogen to achieve efficient laser output. At room temperature, the mid-infrared Raman lasers were investigated by using the BaWO4, YVO4, diamond, and KGW crystals as the Raman gain media [11–18]. The highest average output power of 1.35 W was obtained from the Ho:YAG/BaWO4 external-cavity Raman laser at 2602 nm [12].
The self-Raman laser, as a type of compact Raman laser, has a simpler structure and lower cost, compared with others. In the near-infrared waveband, the efficient and high power self-Raman lasers have been widely reported based on the Nd:GdVO4 and Nd:YVO4 crystals [19–22]. However, in the mid-infrared waveband, only a weak self-Raman phenomenon was first observed in a passively Q-switched Tm:KY(WO4) laser [23]. At present, Ho:GdVO4 crystal has also been identified as an efficient laser gain medium in the 2 µm waveband at room temperature [24–29]. In our recent study, a dual-wavelength passively Q-switched Ho:GdVO4 self-Raman laser at 2473 nm and 2520 nm was demonstrated [30]. However, the efficient Raman laser output was not realized due to the coating quality of Cr:ZnS saturable absorber. Compared with the passively Q-switched devices, actively Q-switched devices have many advantages, such as low losses, high damage threshold, and controllable pulse repetition frequency (PRF) [27,31]. However, to our knowledge, there have been no reports on actively Q-switched self-Raman lasers in the 2-3 µm waveband, to date.
In this paper, we first report an efficient actively Q-switched Ho:GdVO4 self-Raman laser with watt-level output power and near diffraction limited beam quality. The maximum average output power was as high as 1.45 W at the PRF of 15 kHz. To our best knowledge, this is the highest output power for the crystalline Raman laser in the 2-3 µm waveband. The maximum single pulse energy was 96.7 µJ with a pulse width of 11.35 ns, corresponding to the peak power of 8.5 kW. The Raman gain coefficient of Ho:GdVO4 crystal was measured to be approximately 1.14 cm/GW at 2048 nm.
2. Experimental setup
The experimental setup of the actively Q-switched Ho:GdVO4 self-Raman laser is shown in Fig. 1. The pump source was a 1940 nm Tm-doped fiber laser (TDFL) with a core diameter of 25 µm, a numerical aperture of 0.11, and a maximum output power of 30 W. The TDFL was a single transverse (TEM00) mode fiber laser with a beam quality factor M2 of less than 1.1. Two focus lenses of 10 mm and 200 mm were used to reimage the pump laser. By using a laser beam profiler to measure the beam radii, the pump laser waist radius was nearly 200 µm in the center of gain crystal. A 0.5 at.% doped Ho:GdVO4 crystal was used as the shared gain medium of the fundamental and Raman lasers. The crystal, with dimensions of 3 × 3 × 20 mm3, was cut along the optical principle direction of the a-axis. Both end surfaces were antireflection coated at 1940-2600 nm. The Ho:GdVO4 crystal was wrapped with indium foil and mounted in a copper heat sink, which was kept at 15 °C with a thermoelectric cooler. A fused silica acousto-optics (AO) Q-switcher (QS041-10M-HI7, Gooch and Housego) with a 46 mm length and a 50 W maximum radio frequency power was used for the actively Q-switched self-Raman experiments. The AO Q-switcher was circulated with room temperature water to dissipate the heat. The single pass loss of the AO Q-switcher was approximately 15% at 2500 nm. To make only the 2048 nm fundamental laser pass through the Q-switcher and to reduce the insertion loss of 2500 nm Raman laser, an L-shaped coupling cavity was designed for the actively Q-switched self-Raman laser. The fundamental cavity was composed of mirrors M1, M2, and M4, with a cavity length of 130 mm. The Raman cavity was composed of mirrors M3 and M4, with a length of 35 mm. The input mirror M1 was a 45° dichroic flat mirror coated with high transmittance (T > 97.5%) at 1940nm and high reflection (R > 99.5%) at 2040-2600 nm. M2 was a plano-concave mirror with a curvature radius of 100 mm, which had a high reflection coating at 2040-2600 nm (R > 99.5%). The intracavity flat mirror M3 had high transmittance on both surfaces at 1940 nm and 2048 nm, moreover, one surface of it was also highly reflective (R > 99.5%) at 2400-2600 nm. The Raman output mirror M4 with a curvature radius of 100 mm was highly reflective at 1940-2060 nm and partially transmissive at 2400-2600 nm. The 45° dichroic flat mirror M5 with 1700-2100 nm high transmittance (T > 99.5%) and 2400-2600 nm high reflection (R > 99.5%) was used to separate the output Raman laser and eliminate the influence of the pump and fundamental lasers.
For an end-pumped solid-state laser with high pump power density, the influence of thermal focal length on the laser cavity mode should be considered. The effective thermal focal length can be written as [32]:
Fig. 2. (a) Thermal focal length of the Ho:GdVO4 crystal as a function of incident pump power. (b) Beam radii of the TEM00 mode fundamental and Raman lasers in the center of Ho:GdVO4 crystal as functions of thermal focal length. (c) Beam radii of the TEM00 mode fundamental laser (from mirror M2 to mirror M4) and Raman laser (from mirror M3 to mirror M4) as functions of position in the cavity, at the incident pump power of 13.6 W. (ϖF and ϖR are the average beam radii of fundament and Raman lasers in the Ho:GdVO4 crystal, respectively)
3. Results and discussion
3.1 Output performances of Raman laser
Firstly, the output performances of the continuous wave (CW) and actively Q-switched Ho:GdVO4 lasers were investigated by using a plano-concave mirror with a curvature radius of 100 mm and a transmittance of 40% at 2048nm to replace the mirror M4, as shown in Fig. 3. The threshold pump powers were nearly 6 W for the same central wavelength of 2048nm in all cases. At the incident pump power of 22.5 W, the average output powers were 4.32 W, 4.58 W, 4.75 W, and 5.16 W for 10 kHz, 15 kHz, 20 kHz, and CW, respectively. The corresponding pulse widths were 19.70 ns, 20.00 ns, and 23.57 ns for the PRFs of 10 kHz, 15 kHz, and 20 kHz, respectively.
Fig. 3. (a) Output power as the function of incident pump power and (b) output spectrum of CW and actively Q-switched Ho:GdVO4 lasers.
Subsequently, using two M4 mirrors with output transmittances of 10% and 30%, the output performances of the actively Q-switched Ho:GdVO4 self-Raman laser were investigated at three PRFs of 10 kHz, 15 kHz, and 20 kHz. The average output power as a function of incident pump power is shown in Fig. 4(a). The threshold pump power of the self-Raman laser increased with the PRF and the output coupler transmittance. Reducing the PRF significantly decreased the Raman threshold and made the self-Raman laser easier to generate. However, the self-focusing-induced damage of the Ho:GdVO4 crystal also occurred at a lower PRF of 10 kHz [4]. Therefore, the average output power showed obvious saturation at the PRF of 10 kHz for the two output transmittances due to the influence of the self-focusing effect, as shown in Fig. 4(a). The maximum average output power of 1.45 W was achieved at the incident pump power of 22.5 W, when the output transmittance and PRF were 30% and 15 kHz, respectively. To our knowledge, this is the highest average output power for a crystalline Raman laser in the 2-3 µm spectral region, to date. The slope efficiency was 25.8% and the optical-to-optical conversion efficiency was 6.4%. Under the non-lasering condition, the single pass absorption efficiency was 52% at the incident pump power of 22.5 W. Considering the single pass absorption efficiency, the optical-to-optical conversion efficiency, from the absorbed pump power to the self-Raman output power, should be significantly higher than 6.4%.
Fig. 4. (a) Average output power, (b) pulse width, (c) pulse energy, and (d) peak power of actively Q-switched Ho:GdVO4 self-Raman laser as the functions of incident pump power for the different PRFs and output transmittances.
The pulse width as a function of incident pump power is shown in Fig. 4(b). The pulse width decreased with the increase of incident pump power. The pulse width of the self-Raman laser was narrower at the output transmittance of 30% than at the output transmittance of 10%. For the output transmittance of 30% and PRF of 15 kHz, the pulse width of 11.35 ns was obtained at the maximum average output power of 1.45 W. The single pulse energy and peak power of the self-Raman laser as functions of incident power are shown in Fig. 4(c) and (d). Both the single pulse energy and peak power increased with the incident pump power, except when operating at the lower PRF of 10 kHz. The maximum single pulse energy of 96.7 µJ and peak power of 8.5 kW were achieved at the incident pump power of 22.5 W, PRF of 15 kHz, and output transmittance of 30%.
The output optical spectra of the Ho:GdVO4 self-Raman laser at the average output power of 1.45 W are shown in Fig. 5(a). The wavelengths of the fundamental and first Stokes lasers were 2048nm and 2500 nm, respectively, corresponding to the Raman frequency shift of 883 cm-1, which agreed very well with one optical vibration mode of the GdVO4 crystal [34]. In addition to the first Stokes laser, we also observed a very weak first anti-Stokes laser at 1734nm. It is worth noting that the higher-order Stokes and anti-Stokes lasers did not appear. At the maximum average output power of 1.45 W, the power stability of the 2500 nm self-Raman laser was measured, as shown in Fig. 5(b). The root-mean-square (RMS) deviation during 30 min was 1.02%, meaning that the output power of the self-Raman laser was very stable. The single pulse profile and pulse train of the Ho:GdVO4 self-Raman laser were monitored at the average output power of 1.45 W, as shown in Fig. 6(a) and (b). As can be seen, the output Raman pulses were relatively stable, with a pulse-to-pulse amplitude fluctuation within ±5%.
Fig. 5. (a) Optical spectra (Inset: Optical spectrum of first anti-Stokes laser) and (b) power stability at the average output power of 1.45 W.
At the incident pump power of 22.5 W, the beam qualities of the self-Raman laser were measured at maximum average output power for different output coupler transmittances, as shown in Fig. 7. The insets of Fig. 7 show the two-dimensional far-field beam distribution. It can be seen that the output beam was close to the TEM00 mode. The M2 factors were calculated to be 1.23 and 1.06 for the output transmittance of 10%, and 1.15 and 1.06 for the output transmittance of 30%, along the x and y directions. So, the beam quality of the Ho:GdVO4 self-Raman laser was near diffraction limited.
Fig. 7. M2 measurement of the Raman laser at the two transmittances of (a) 10% and (b) 30%. Inset: Two-dimensional far-field beam profile.
3.2 Evaluation of Ho:GdVO4 Raman gain coefficient
The gain coefficient of Raman crystal is a very important parameter. Evaluating the gain coefficients of typical Raman crystals used in the mid-infrared spectral region is of great interest for the development of mid-infrared Raman lasers. To our knowledge, the gain coefficients of BaWO4, KGW, and diamond have been measured in the mid-infrared waveband [11,14,35], however, the gain coefficient of GdVO4 crystal has not been measured in the mid-infrared waveband to date. We designed a two-end output cavity of the fundamental laser and the Raman laser to evaluate the Raman gain coefficient of Ho:GdVO4 crystal, by changing the mirror M2 from the high reflection to an output transmittance of 1.3% at 2048nm. When the Raman output mirror transmittance and the PRF were 10% and 15 kHz, respectively, the output performances of the 2048nm fundamental and 2500 nm Raman lasers were simultaneously explored. The average output powers of the fundamental laser and Raman laser as the functions of incident pump power are shown in Fig. 8. The threshold powers and the maximum average output powers of fundamental/Raman laser were 6.6/13.6 W and 0.45/1.09 W, respectively, where the maximums were achieved at an incident pump power of 22.5 W.
Fig. 8. Average output powers of fundamental laser and Raman laser as the functions of incident pump power.
As increasing the incident pump power, pulse profiles and pulse trains of fundamental laser and Raman laser were recorded synchronously, as shown in Fig. 9. The single pulse profiles of fundamental and Raman lasers at the Raman threshold of 13.6 W were shown in Fig. 9(a). At this time, the fundamental pulse width was 38.4 ns, and only a very weak peak of Raman laser was observed. Figure 9(b) shows the single pulse profiles of fundamental laser and Raman laser at the incident pump power of 14 W. It can be seen that the pulse width of fundamental laser was also 38.4 ns. Now, the Raman signal could be clearly observed, and the pulse of Raman laser had a significant delay compared to that of fundamental laser. Further increasing the incident pump power, the pulse profile of fundamental laser changed due to the Raman conversion. Figure 9(c) shows the pulse profiles of fundamental laser and Raman laser at the incident pump power of 22.5 W. It can be observed that the pulse of fundamental laser split into two pulses, consisting of a main pulse with a pulse width of 13.3 ns and a sub-pulse with a pulse width of 53.2 ns. This phenomenon was caused by the fact that there was still the residual energy of fundamental laser after the generation of Raman pulse, which resulted in the formation of a second pulse of fundamental laser. This similar phenomenon was also observed in the 1 µm Raman laser [36]. The pulse profile of Raman laser always maintained its normal shape without distortion. This indicated that no second or higher-order Raman conversion had occurred. Figure 9(d) shows the pulse trains of fundamental laser and Raman laser at the incident pump power of 22.5 W. As can be seen, the output pulses of fundamental laser and Raman laser were all relatively stable.
Fig. 9. Single pulse profiles of fundamental laser and Raman laser at different incident pump power: (a) Pin = 13.6 W, (b) Pin = 14 W, and (c) Pin = 22.5 W. (d) Pulses trains of fundamental laser and Raman laser at the incident pump power of 22.5 W.
The insets of Fig. 10(a) and (b) show the two-dimensional far-field beam distributions of fundamental laser and Raman laser at the incident pump power of 22.5 W, respectively. The output beams were all close to the TEM00 mode. The M2 factors in the x and y directions were measured to be 1.66 and 1.48 for the fundamental laser, and 1.17 and 1.07 for the Raman laser. A significant beam cleaning-up effect, based on the SRS effect, was observed from the fundamental laser to the Raman laser.
Fig. 10. M2 measurement of (a) fundamental laser and (b) Raman laser at the incident pump power of 22.5 W. Inset: Two-dimensional far-field beam profile.
According to Basiev [37], the SRS threshold condition can be approximated by:
where g is the effective Raman gain coefficient of Raman crystal, Ith is the measured threshold intensity of the fundamental laser, and Leff is the effective length of interaction between fundamental laser and Raman crystal. When the Raman medium is placed inside a cavity, the effective interaction length is defined by: where L is the length of Raman crystal and Neff is the effective number of passes of SRS radiation through the cavity. Neff is given by:At the Raman threshold, the average output power and pulse width of fundamental laser were 292 mW and 34.8 ns, respectively, corresponding to a peak power of 559 W. Therefore, based on the experimental parameters of Lr = 5.82 cm and R = 0.9, Neff was calculated to be approximately 130, giving rise to a Leff of 260 cm. For the pulse-pumped Raman lasers, the peak power should be used to calculate the threshold intensity. After obtaining the output peak power external to the cavity, the peak power of fundamental laser in the cavity can be calculated through the following formula [38]:
In addition to this work, the Raman gain coefficients of BaWO4, KGW, and diamond have been estimated in the 2 µm waveband, to date. Table 1 shows the measured Raman gain coefficients of several typical solid state Raman materials in the 2 µm waveband. It can be seen that diamond has a very high Raman gain factor near the 2 µm waveband. However, diamond has a relatively low transmittance in the 2.5-6.5 µm waveband owing to multi-phonon absorption, which leads to a significant decrease in the output efficiency of diamond Raman laser operating above 2.5 µm [18]. Except diamond, the gain coefficients of BaWO4, KGW, and Ho:GdVO4 crystals are close in the 2 µm spectral region. As known, the Raman gain coefficient decreases with the increase of fundamental laser wavelength. In this way, the Ho:GdVO4 crystal exhibits a greater advantage in Raman lasing at a longer fundamental laser wavelength, compared with the BaWO4 and KGW Raman crystals. The relatively high Raman gain, along with its ability to simultaneously generate the 2 µm fundamental laser and 2.5 µm Raman laser, makes the Ho:GdVO4 a more promising Raman crystal for generating high power self-Raman laser in the 2.5 µm mid-infrared waveband.
Table 1. Raman gain coefficient of Raman crystal in the 2 µm waveband.
4. Conclusion
In conclusion, the in-band pumped high power actively Q-switched Ho:GdVO4 self-Raman laser at 2500 nm has been first reported. The output performances of Ho:GdVO4 self-Raman laser were explored under the different PRFs and output coupler transmittances. The optimal results were achieved at the PRF of 15 kHz and the output transmittance of 30%. The maximum average output power was as high as 1.45 W at the incident pump power of 22.5 W and the slope efficiency was 25.8%. The maximum single pulse energy and peak power were 96.7 µJ and 8.5 kW, respectively. The M2 factors were 1.15 and 1.06 along the x and y directions, at the maximum average output power of 1.45 W. Moreover, we evaluated the gain coefficient of Ho:GdVO4 self-Raman crystal by adopting a two-end output way of the fundamental laser and the Raman laser. The Raman gain coefficient of Ho:GdVO4 self-Raman crystal was first measured to be 1.14 cm/GW at 2048 nm. The experimental results show that the Ho:GdVO4 as a promising self-Raman crystal can be used to generate high power Raman laser at 2.5 µm.
Funding
National Natural Science Foundation of China (62275194, 61775166).
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.
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