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Abstract
In this work, a double-end diffusion bonded Nd:YVO4 self-Raman laser was designed to drive an intracavity, noncritically-phase-matched KTiOAsO4 (KTA) optical parametric oscillator (OPO). Both conversion efficiency and output power at 1.7 µm (the wavelength of the OPO signal field) were improved by effectively reducing the thermal lens effect and increasing the effective length of self-Raman medium. At an incident pump power of 15.4 W, the output power for 1742 nm output laser reached 2.16 W with a conversion efficiency of 14%, and the output having a pulse width of 10.5 ns and a pulse repetition frequency of 90 kHz. The competition between the OPO and cascaded Raman laser was observed when the incident pump power was above 12.4 W. The results highlight that in order to improve output power at 1742 nm, it is critical that both the cascaded, second-Stokes field at 1313 nm and the signal field generated at 1534 nm from the 1064 nm field driving the KTA-OPO be minimized, if not completely suppressed. This laser system combining the processes of stimulated Raman scattering and optical parametric oscillation for the generation of laser emission at 1742 nm may find significant application across a broad range of fields including biological engineering, laser therapy, optical coherence tomography and for the generation of mid-infrared laser wavelengths.
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1. Introduction
Stimulated Raman Scattering (SRS) is an efficient third order nonlinear effect which is used for laser wavelength conversion and does not require actively achieving the phase match condition, something which is required by second order nonlinear effects. Combining the SRS process with second order nonlinear effects opens the door to a host of laser wavelengths ranging from the visible to the infrared. In particular, the use of SRS in conjunction with second harmonic generation (SHG) and sum-frequency generation (SFG) for visible band laser emission has been the subject of extensive research [1–5]. Furthermore, the combination of SRS and optical parametric oscillation (OPO) has been applied for the generation of laser wavelengths in the near- and mid- infrared waveband [6–8]. Wavelengths in the 1.7 µm range have become of particular interest due to its use in the detection of a range of chemical compounds, biomedical imaging, and for mid-infrared laser generation. The effective detection of chemical compounds using 1.7 µm laser emission arises from the strong absorption peak at 1.7 µm by a wide range of chemical bonds including O-C, N-O, O-H, and C-H [9–11]. Laser wavelengths in the 1.7 µm range are also applied for efficient wielding of a range of high density polymers [12,13]. 1.7 µm radiation also lies in the low absorption window of water molecules and hence this wavelength is also applied in low noise water-based spectroscopy [14,15]. In comparison with other commonly used wavelengths which occupy the biomedical imaging window (such as 1.3 µm), laser emission at 1.7 µm has lower Rayleigh scattering which yield enhanced tissue penetration depth, a characteristic which is beneficial to applications such as tissue characterization by optical coherence tomography [16]. High power lasers at 1.7 µm also play a key role as a pump source for mid-infrared laser generation [17,18].
Owing to the aforementioned applications, there has been rising interest in methods of generating laser emission in the 1.7 µm wavelength range. To date, methods which have been used for the generation of laser wavelengths in the 1.7 µm range including thulium doped fibers [19,20], bismuth doped fibers [21], InGaAsP-based semiconductor lasers [22], quantum dots lasers [23] and nonlinear frequency conversion of all-solid-state lasers. In particular, all-solid-state lasers utilizing nonlinear frequency conversion has been the subject of numerous research because of its robustness, and its use of conventional pump sources. In 2011, Yang et al. investigated the application of a critically phase-matched (CPM) OPO for the generation of 1.9 µm and 1.7 µm dual spectral line emissions using a KTA crystal pumped using the dual emission lines of an Nd:YAG laser at 1319 nm and 1064 nm respectively [24]. They demonstrated an average output power of 0.9 W at 1.7 µm with a conversion efficiency of less than 1%. Cascaded Raman conversion driven by the 1.3 µm fundamental line from a neodymium doped crystal laser has also been demonstrated for the generation of 1.7 µm emission. In 2014, Du et al. demonstrated a 0.99 W cascaded Raman laser emitting at 1764nm from a composite Nd:YVO4 crystal, with a diode-to-second-Stokes conversion of 2.9% [25]. In 2020, Zhao et al. reported an output power of 2.7 W and a corresponding conversion efficiency of ∼5.5% from a cascaded YVO4 Raman laser emitting at 1715.2 nm and driven by an Nd:YLF laser at 1314 nm [26].
Though the SRS process is highly efficient, the 1.3 µm emission line from neodymium-doped crystals has a small emission cross-section and this limits the overall diode-to-Stokes conversion efficiencies that can be achieved from these systems. An alternative approach to generating laser emission in the 1.7 µm range which also makes use of SRS conversion is the combined application of SRS and OPO nonlinear processes. In 2020, both an 8th-order cascaded KY(WO4)2 Raman laser using the 87 cm−1 Raman mode; and a YVO4 Raman laser using the 890 cm-1 Raman mode driven by the 1535 nm signal field from a noncritically phase matched (NCPM)-KTA OPO were used for the generation of 1.7 µm laser emission [8]. In the former case, 1.26 W multi-Stokes emission in the wavelength range spanning 1.5-1.7 µm was demonstrated; and in the latter case, 1.05 W of first Stokes emission at 1778nm was demonstrated. In 2018, our group firstly reported the application of a KTA-OPO driven by a 1.18 µm Nd:YVO4 Raman laser producing 1.7 µm laser emission [27]. In that case, Watt-level signal emission at 1742 nm with a diode-to-signal optical conversion efficiency of ∼10% was achieved. The output power scaling of the self-Raman laser system was significantly limited by thermal effects arising from the simultaneous lasing and Raman conversion processes. From our experience, a particularly effective method for managing the thermal effects (such as thermal lensing and end-face bulging) which can lead to laser resonator instabilities, is the use of composite self-Raman crystals [28]. In this paper, a double-end diffusion bonded Nd:YVO4 Raman laser was designed to drive an intracavity, non-critically phase-matched (NCPM) KTA-OPO. Both conversion efficiency and output power at a wavelength of 1742 nm were improved by effectively reducing the thermal lensing effect and increasing the effective length of self-Raman medium. Under an incident pump power of 15.4 W, a maximum average output power of 2.16 W was obtained, with a diode-to-signal conversion efficiency of up to 14.0%.
2. Experimental setup design
The self-Raman operating and performance characteristics of Nd:YVO4 has been researched extensively ever since its first demonstration by Chen et. al. [29]. Nd:YVO4 has a very high laser emission cross section and a high Raman gain (along the 890 cm−1 shift), which have made it one of the most extensively used crystals for the generation of laser wavelengths in the 1.18 µm range. Based on our experience using this crystal, overall laser efficiencies exceeding 30% can be achieved using Nd:YVO4 self-Raman laser architectures [30]. As such, we believe that it is an excellent candidate to pump an OPO. To minimize the strong thermal lensing which manifests in these self-Raman laser designs, we make use of a three-stage, diffusion bonded composite self-Raman crystal. In this work, we use a YVO4/Nd:YVO4/YVO4 composite crystal with a doping concentration of 0.3 at %. The crystal had the total length of 30 mm and an aperture of 3 × 3 mm2. The doped section of the crystal had a length of 10 mm, and the two undoped sections had lengths of 3 mm and 17 mm. The crystal was oriented within the laser cavity such that the 3 mm long undoped section was facing towards the resonator input mirror. Diffusion bonding of these two undoped sections to the Nd:YVO4 crystal effectively reduced the thermal lensing effect and increased the effective length of self-Raman medium, which contribute to improve the Raman conversion efficiency and output power with higher incident pump power. The laser cavity was pumped using an 808 nm fiber-coupled diode with a core diameter of 200 µm and a numeric aperture of 0.22 (LIMO GmbH). The pump beam was re-imaged and focused into a spot with a diameter of 320 µm, incident onto the front facet of the composite crystal. An acoustic-optic module (AO, Gooch & Housego Co., QS041-10G-GH12) with a variable repetition rate was inserted next to the laser crystal for Q-switched pulse operation.
A NCPM-KTA crystal with dimensions of 4 mm × 4 mm × 25 mm was used to convert the first-Stokes wave from the self-Raman crystal at 1176 nm to the signal wavelength of 1742 nm. Efficient operation of the OPO was maintained by virtue of the zero walk-off effects of the NCPM condition. NCPM also corresponds to the highest effective nonlinear coefficient (deff) with respect to the signal laser at 1742 nm and the idler laser at 3.62 µm. In this work, we noted that the collinearly generated idler field was strongly absorbed by the BK7-based cavity mirrors. Shown in Fig. 1 is a schematic of the experimental laser system. The laser resonator comprises two coupled cavities, one to support development and oscillation of the Stokes field generated in the Nd:YVO4 crystal, and the other which is resonant for the signal field of the KTA-OPO. The KTA crystal was wrapped in indium foil and placed in a water-cooled copper block and maintained at a temperature of 25°C.
Fig. 1. Schematic showing the experimental layout of the coupled-cavity SRS/OPO laser resonator designed for generation of laser output at 1742 nm.
The overall length of the laser resonator was 110 mm, with the OPO section comprising a length of 30 mm. The resonator for the fundamental laser field (1064 nm) and the first-Stokes field (1176 nm) is comprised of input mirror M3 and a concave mirror M1 (ROC = 320 mm). The input mirror M3 was high transmittance (HT, T > 95%) coated at the 808 nm pump wavelength and high-reflection coated (HR, R > 99.9%) at 1064 nm and 1176 nm. M1 was coated HR (R > 99.9%) at 1064 nm and 1176 nm, and partially transmitting (T∼6.5%) at 1742 nm. A plane mirror M2 which was anti-reflective (AR) coated at 1064 nm and 1176 nm, and HR (R > 99.5%) coated at 1.7 µm, was inserted between the AOM and KTA crystal to form the OPO cavity with M1. It should be noted that due to this cavity design, OPO conversion of the fundamental field at 1064 nm was also observed, generating a signal field at 1535 nm. The OPO conversion of the fundamental field is undesirable and was a loss mechanism on the fundamental field, thereby limiting the overall power available to be converted to the first-Stokes at 1176 nm. To aid in limiting the generation of the 1535 nm field, the output coupler M1 was also HT coated (T > 80%) at 1535 nm, and M2 was HT coated (T∼ 27%) at 1535 nm.
3. Experimental results and discussion
The output performance of the laser system was examined while operating across a range of pulse repetition frequencies (PRFs) and incident pump powers. The highest overall output power at 1742 nm was achieved with the system operating at a PRF of 90 kHz and for an incident pump power of 15.4 W. Here a maximum output power of 2.22 W and a diode-to-signal conversion efficiency of 14.4% was achieved. The spectral output of the laser was measured while the system was operating at maximum output power with the use of a grating monochromator (Omni-λ 500, resolution 0.05 nm, ZOLIX); this is plotted in Fig. 2. The signal wavelength was centered at 1742 nm. An unexpected emission line at 1313 nm was observed in addition to the spectrum of the signal field. This emission line arises from the cascaded second-Stokes shift of the fundamental field by the 890 cm−1 shift in YVO4. After carefully examining the transmittance curves of the laser resonator mirrors (plotted in Fig. 3), it was found that mirrors M1 and M3 both had relatively high transmittance values at 1313 nm of 47% and 64% respectively. However, the threshold for the second-Stokes wave in cascaded Raman process is low, so it can not completely inhibit the production of 1313 nm laser. As shown in Ref. [31], output power of 1.4W (pump power of 34 W) for second-Stokes wave was obtained despite the transmittance of both cavity mirrors in two-mirror self-Raman cavity reaching 70% and 86%, respectively.
A filtering mirror M0 with the same coating as M2 was used to separate the co-propagating 1313 nm field from the 1742 nm field. Further examination of the output characteristics of this system revealed that the 1313 nm emission firstly appeared at an incident pump power of 12.4 W, for which the output power at 1742 nm was 1.51 W. The power of the 1313 nm emission increased as the incident pump power increased. Reaching a maximum value of 60 mW at an incident pump power of 15.4 W and the system operating at a PRF of 90 kHz. The presence of the 1313 nm, second-Stokes emission line in the output of this laser system highlights the complex interplay that occurs between the different fields which resonate within the system.
The power-transfer curve (1742 nm emission as a function of incident pump power) of this laser system while operating at PRFs of 60 kHz and 90 kHz are plotted in Fig. 4. The threshold for emission at 1742 nm was found to be sensitive to the PRF, climbing from 4.69 W at 60 kHz, to 5.39 W at 90 kHz. The power transfer characteristics of the 1742 nm output was highly non-linear with the incident pump power. This characteristic was always observed in the self-Raman laser operation with the use of the three-stage, diffusion bonded composite crystal. In our previous work of selective frequency-mixing in the self-Raman laser using similar composite crystal [1], it can be seen that this nonlinear power-transfer characteristic only occurs for the Stokes wave, and is not seen for the fundamental wave. According to the experiment performance, we speculate that the power-transfer characteristic was associated with the use of the three-stage, diffusion bonded composite self-Raman crystal. The difference of thermal effect between Nd:YVO4 and pure YVO4 in the composite crystal affect the Raman threshold in both part crystal, which would result in sudden changes in the thermal effect and cavity modes. The Raman threshold difference and the change of cavity modes might lead to the fluctuation of the total Raman efficiency near the threshold. From Fig. 4, it could be seen that for PRF of 60 kHz, the average output power dropped after reaching 1.8 W for an incident pump power of 14.5 W. At the higher PRF of 90 kHz, the roll-over point moved to an incident pump power of 15.4 W, indicating the power-scaling limit of the laser resonator. The highest average output power at 1742 nm achieved from this system was 2.16 W for an incident pump power of 15.4 W and PRF of 90 kHz. This corresponded to a pulse energy of 24 µJ and a peak power of 2.2 kW. The diode-to-signal conversion efficiency reached a value of 14%. Therefore, both output power and conversion efficiency were remarkable improved with compared to the previous work (output power of 1.2 W and conversion efficiency of 10%) with common self-Raman crystal.
Fig. 4. Plot of the power-transfer characteristics of the coupled-cavity, self-Raman/OPO laser resonator generating output at 1742 nm. Data is plotted for two PRFs of 60 kHz and 90 kHz.
The temporal pulse profile of the 1742 nm output was measured using an InGaAs free-space photo detector (5 GHz, Thorlabs Model DETO8C/M) which was connected to a 500 MHz oscilloscope (Model: DPO2052B). The pulse characteristics of the signal field are plotted in Fig. 5, with the laser operating at a PRF of 90 kHz. The FWHM pulse width of the 1742 nm output was ∼10.5 ns and the pulse-to-pulse stability was about ±9%. The pulse properties of the 1313 nm field were also measured, and a representative pulse is plotted in Fig. 6. The FWHM pulse width of the 1313 nm field was ∼4 ns. In the case of lasers employing SRS, the generated Stokes fields typically have shorter pulse widths than the fundamental field. This is due to the fact that only the peak of the fundamental pulse is typically converted by the SRS process. Similarly, cascaded SRS pulses also exhibit this pulse shortening characteristic. This is the primary reason why the 1313 nm field exhibited a shorter pulse width than the 1742 nm field. In this experiment, we did not make the consideration for a special coating in the cavity mirror to suppress the generation of 1313 nm laser from the cascaded Raman process, which in some extent compromise the pulse stability and the overall efficiency for 1.7 µm laser. It also reminded us that in the future we should add anti-reflection coating for 1313 nm wavelength to eliminate the competition of the cascaded Raman for similar systems.
Fig. 5. Plots of the temporal characteristics of the 1742 nm output, showing a representative pulse (upper plot) and a pulse train (lower plot).
Fig. 6. Plot of a representative output pulse at 1313 nm, the second-Stokes line generated by the Nd:YVO4 crystal.
4. Conclusion
In conclusion, efficient 1742 nm laser output has been produced using a coupled-cavity, self-Raman/OPO laser. Key to the improvement of output power was the use of a composite YVO4/Nd:YVO4/YVO4 self-Raman laser crystal. The highest output power at 1742 nm achieved from this system was 2.16 W (corresponding to a peak power of 2.2 kW), with a diode-to-signal conversion efficiency of 14.0%. Due to the coupled-cavity design of this laser resonator, we observed the presence of the second-Stokes laser at 1313 nm. While the overall intensity of this laser was low, it did act as a loss mechanism which limited the overall power that could be generated at the desired output wavelength of 1742 nm. This highlights that further gains from this laser cavity design can be made through targeted suppression of this second-Stokes field. Also, suppression of the second-Stokes laser should also further reduce the level of thermal loading within the self-Raman crystal and facilitate further power scaling of the system. We believe that this work demonstrated a great potential for the combination of SRS and OPO non-linear processes for the generation of diverse laser wavelengths. In particular, we believe that the demonstrated efficient 1742 nm laser output may find applications in optical coherence tomography (OCT) and mid-infrared laser generation.
Funding
National Natural Science Foundation of China (62075167, 62275200, 62205251); Basic Scientific Research Project of Wenzhou City (G20220014).
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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