High-power dual-wavelength intracavity diamond Raman laser

Abstract

Diamond crystals have garnered significant attention for applications in photonics, owing to their characteristics of ultra-high thermal conductivity, extremely broad spectral transmission range, and high Raman gain coefficient. These characteristics are particularly advantageous for the development of high-power, multi-wavelength lasers. In this work, we utilize single crystal diamond as a Raman gain medium, for intracavity wavelength-conversion of emission from a side-pumped, electro-optically Q-switched Nd:YAG module. Cascaded Raman conversion is demonstrated and dual-wavelength, pulsed laser emission at wavelengths of 1.2 μm (first-Stokes wavelength) and 1.5 μm (second-Stokes wavelength) is achieved (with a maximum combined pulse energy of 6.19 mJ). The relative ratio of output energy at the two Stokes orders could be varied by tuning the characteristics of the output coupling mirror and the length of the laser resonator. When operating at maximum pump power, the pulse width of the second-Stokes field was approximately half that of the first-Stokes field (second-Stokes ∼10 ns, first-Stokes ∼18 ns), resulting in maximum peak powers of 174 kW (at 1.2 μm) and 220 kW (at 1.5 μm). To the best of our knowledge, this is the highest reported peak power generated from a dual-wavelength, intracavity diamond Raman laser. This work highlights the diversity and utility of diamond as an effective wavelength-conversion component in high-power lasers, and stands as the current state-of-the-art in high pulse-energy, high peak-power, multi-wavelength intracavity Raman lasers.

Keywords:

• Diamond
• Raman laser
• intracavity
• high-power
• dual-wavelength

1. Introduction

Multi-wavelength lasers are enabling technologies used across a diversity of applications including precision laser spectroscopy, resonant laser interferometry, and LIDAR [1, 2]. The predominant methods for obtaining multi-wavelength laser output include: lasing on multiple spectral emission lines of a laser gain medium, beam combining of emission from multiple lasers using devices such as gratings, and converting a single laser wavelength to multiple wavelengths via nonlinear optical frequency conversion. Lasing on multiple spectral emission lines of a gain medium has inherent challenges, particularly the control of energy/power at each emission line; also, depending on the gain medium, the span/diversity of wavelengths which may be generated can be limited. Beam combining emission from multiple lasers is inherently costly owing to the need for multiple laser resonators and/or pump systems; also, optics used for combination can be bulky and, in some cases, very lossy. In contrast to these methods, nonlinear optical frequency conversion as a whole, offers significantly greater flexibility for the generation of laser emission across a broad wavelength span. Nonlinear optical conversion processes include second-order nonlinear effects [3] such as second harmonic generation (SHG), sum frequency generation (SFG), and optical parametric generation and oscillation (OPG/OPO), as well as third-­order nonlinear effects such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS) [4–6]. In contrast to second-order nonlinear processes, SRS and SBS are inherently phase matched and can also cascade in the sense that the SRS and SBS processes can be applied to successively generated Stokes fields.

The SRS process is more widely utilized (in comparison to SBS) for wavelength conversion of laser emission due to the broad range of Stokes-shifts available from a wide range of Raman-active media (in gaseous, liquid and crystalline forms). Common crystalline Raman media include BaWO₄, KGW and Ba (NO₃)₂. Among these crystals, Shen H et al. and Zhang H et al. has demonstrated dual wavelength Raman laser outputs of 1180 nm/1325 nm [7] and 1240 nm/1376 nm [8] using BaWO₄ in an intracavity, cascaded Raman laser, respectively. Sun Y et al. [9] and Tu Z et al. [10] achieved dual wavelength laser output at 1133/1156 nm, 1137.8/1151.9 nm, and 1470/1490 nm, 1461/1499 nm using two orthogonal Raman frequency shifts of KGW crystals, respectively. While multi-wavelength laser output has been achieved through cascaded Raman conversion and using different Raman frequency shifts within these crystals, achieving high power scaling of multi-wavelength lasers is still a challenge due to the relatively low thermal conductivity and thermal diffusion characteristics of many crystalline Raman media.

With the further development of diamond crystal growth technology [11], optical grade diamond crystals with excellent optical and thermal properties are playing an increasingly important role in the field of lasers. In contrast to other, diamond crystals have ultra-high thermal conductivity (>2000 Wm⁻¹K⁻¹) and high Raman gain coefficient (∼10 cm/GW @ 1 μm) [12–15]. Thanks to these characteristics, Raman lasers which utilize diamond as the SRS medium have now achieved Raman laser output with continuous wave powers of up to 154 W and a pulse energy of up to 9.7 mJ (with a pulse width of 8 ns and corresponding peak power of 1.2 MW) [16]. Diamond has an extremely wide spectral transmission range and the longest Raman frequency shift (1332 cm⁻¹) among known Raman crystals, two properties which are advantageous for the generation of multi-wavelength laser output with wide frequency intervals [12, 16]. For example, by utilizing a fundamental laser field at 1 μm (such as that from a Nd:YAG or Nd:YVO₄ laser) SRS in a diamond crystal can be used to obtain emission at wavelengths of 1.2 μm (first-Stokes) and 1.5 μm (second-Stokes); wavelengths which are located in the important atmospheric transmission window [17, 18]. The diversity of wavelengths which have been demonstrated via SRS in diamond spans the deep ultraviolet, visible, near-infrared and mid infrared wavelength ranges [19–24]. Extracavity SRS diamond lasers emitting simultaneously at 1.2 μm and 1.5 μm have been demonstrated in the continuous wave, ­nanosecond-pulsed and picosecond-pulsed regimes [25–27]. S. Reilly et al. [28] and D. T. Echarri et al. [29] demonstrated emission at three wavelengths (573/620/676 nm), and broadly-tunable emission (420-600 nm) in the visible wavelength range, respectively, through cascaded SRS in diamond. Li M et al. utilized a ring diamond Raman oscillator design to achieve dual wavelength Raman laser output at 964 nm and 1101 nm with single longitudinal mode operation [30]. From these prior works, it is apparent that SRS shifting in diamond is a highly effective method for generating multi-wavelength laser emission.

In this letter, we demonstrate an intracavity, cascaded diamond Raman laser generating simultaneous, high pulse energy emission at 1.2 μm (first-Stokes) and 1.5 μm (second-Stokes). An electro-optic Q-switched, side pumped Nd:YAG laser was used to generate the fundamental, 1064 nm field. Considering the severe thermal effects which can manifest in the side pumped Nd:YAG crystal, the laser was set to operate at a repetition rate of 50 Hz. By changing the output transmittance of the laser resonator, the output pulse energy of the first- and second-­Stokes fields could be varied (in this work, we demonstrate first-Stokes to second-Stokes pulse energy ratios of 1.43 and 0.17). During the cascaded Raman conversion process, the pulse width of the second-Stokes field is approximately half that of the first-Stokes field, resulting in high peak-power outputs of 174 kW at 1.2 μm and 220 kW at 1.5 μm, these values representing the highest currently known peak power outputs from a dual-wavelength intracavity diamond Raman laser.

2. Experiments

The setup of the intracavity cascaded diamond Raman laser is shown in Figure 1. The fundamental frequency gain medium was a laser diode (LD) side pumped Nd:YAG crystal, with dimensions Φ3 × 75 mm and Nd³⁺ ion doping concentration of 0.6 at.%. Its end faces were anti-reflection coated at 1064 nm to minimize any off-axis intracavity losses. The electro-optic Q-switch consisted of a BBO electro-optic Q-switching module (EOM) with a length of 25 mm, a polarizer (P), and a quarter wave plate (λ/4). Based on the characteristics of the thermal lens generated within the Nd:YAG crystal (determined prior to the experiment), to ensure that the oscillator could operate across a wide range of stability zones, the repetition frequency of the Q-switch was set to 50 Hz. A convex lens (F) with a focal length of 300 mm was placed intracavity and positioned 55 mm from the Nd:YAG crystal. It was used to increase the thermal stability range of the cavity, and to control the mode size of the fundamental field within the cavity so as to maximize the size of the TEM₀₀ mode in the Nd:YAG gain medium and avoid excitation of higher-order transverse modes. The size of the fundamental field mode in the diamond crystal was controlled so as to be little influenced by thermal effects within the Nd:YAG crystal, ensuring stable SRS conversion. The diamond (single crystal) had dimensions 2 × 4 × 7 mm³ was uncoated, and was cut for propagation along the <110> direction; it was placed on a copper heat sink.

Figure 1. Schematic diagram of the intracavity cascaded diamond Raman laser.

The fundamental field cavity was formed by mirrors M1 and M3. M1 was a flat mirror coated high-reflecting at 1 μm, mirror M3 had a radius of curvature of 150 mm and was coated high-reflecting at 1 μm and partially transmitting at 1240 nm (T = 54.9%) and 1485 nm (T = 95.4%). The length of the cavity was 712 mm. The Raman cavity was a dual concave cavity formed by mirrors M2 and M3. M2 was a concave mirror with a radius of curvature of 100 mm that was anti-reflection coated at 1064 nm and high-reflection coated at 1240 nm and 1485 nm. The distance between M2 and F was 220 mm. The total length of the Raman cavity was 187 mm, and the distances between the diamond crystal and M2 and M3 were 70 mm and 110 mm, respectively. The size distribution of fundamental field mode and the first- and second-Stokes field modes in the cavity is shown in Figure 2. According to ABCD transfer matrix theory, within the thermal stability range of the cavity, the radius sizes of TEM₀₀ fundamental frequency mode at the Nd:YAG and diamond crystals were 913-993 μm and 151-139 μm, respectively. The radius of the first-Stokes (1.2 μm) and second-Stokes (1.5 μm) fields within the diamond were 150 μm and 165 μm, respectively.

Figure 2. Plot showing the distribution of different oscillating modes within the laser resonator.

3. Results and discussion

Figure 3a shows a plot of the energy-transfer characteristic of the first- and second-Stokes (and combined) fields as a function of the laser diode pump current. It can be seen that both the first- and second-Stokes fields were generated almost simultaneously, with a generation threshold current of ∼30 A. The output energy of both Stokes fields increased somewhat linearly with the increase in pump current; this is a characteristic which is different to that observed from cascaded emission from an extracavity diamond Raman laser [31]. At a peak pump current of 90 A, which is below the damage threshold of the crystal end face, the output energies for the first and second Stokes fields were measured to be 3.14 mJ and 2.20 mJ, respectively, with an energy ratio of 1.43 and a total energy of 5.34 mJ. Based on the electro-optical conversion efficiency, this corresponds to a total conversion efficiency from the LD to the Raman of approximately 3.4%. Due to the presence multiple, simultaneous laser energy conversion processes within the oscillator and the thermal effect of the fundamental gain medium, the total output energy RMS is about 5%. It can be seen that the energy generation efficiency of the second-Stokes increased with pump current. When the pump current was 50 A, the output energy at 1.5 μm accounted for 31.7% of the total energy. When the pump current increased to 90 A, the output energy at 1.5 μm increases to 41.3% of the total energy. It is predicted that as the pump current further increases, laser output with the same pulse energy at both wavelengths can be obtained.

Figure 3. (a) Plot of the output pulse energy of the cascaded first- and second-Stokes emission lines (output coupler with T = 54.9% @ 1240 nm/95.4% @ 1485 nm); and (b) plot of the emission spectrum of the laser (inset shows the spatial beam profile of the first and second-Stokes emission).

Figure 3b illustrates the output spectrum of the cascaded diamond Raman laser. The emission is clearly visible at 1240 nm and 1485 nm, corresponding to the first- and second-Stokes shifts of the fundamental emission line at 1064 nm, with linewidths of approximately 30 GHz and 19 GHz, respectively. The insert in Figure 3b shows the spatial beam profiles of the first- and second Stokes emissions, respectively.

The temporal characteristics of the laser emission were also recorded as a function of the diode pump current. Shown in Figure 4a and b are the temporal pulse characteristics of the fundamental field when operating below threshold for the SRS process and when above threshold for SRS. When above threshold for SRS, it can be seen that a significant portion of the fundamental field pulse is “consumed” in the generation of the Stokes fields. Figure 4c shows the evolution of the temporal characteristics of the first- and second-Stokes pulses as the diode pump power is increased. The inherent gain threshold characteristics of the SRS process make the pulse width of the generated Stokes naturally shorter than the pulse width of the pump [32]. It can be seen that the first-Stokes field pulse width is narrower than that of the fundamental field, and the second-Stokes field pulse width is narrower than that of the first-Stokes field. In this work, the effective ratio of first-Stokes pulse width to second-Stokes pulse width evolved with diode pump current. At a pump current of 50 A, the second-Stokes pulse width (∼5 ns) was ∼4.8 times narrower than that of the first-Stokes pulse width (∼24 ns). At the maximum pump current of 90 A, the ratio of pulse width decreased to ∼1.8 (second-Stokes pulse width ∼10 ns, first-Stokes pulse width ∼18 ns). This evolution in pulse widths was a consequence of the efficiency by which the second­Stokes field was generated. As the pump current increased, as did the energy in the first-Stokes field, this led to an increase in the generation efficiency of the second-­Stokes field and greater ‘depletion’ of the first-Stokes field. As a consequence, the measured pulse width of the first-Stokes field decreased and the measured pulse width of the second-Stokes field increased. The corresponding maximum peak powers were 174 kW (at 1240 nm) and 220 kW (at 1485 nm), respectively.

Figure 4. Plots showing the temporal characteristics of the fundamental field (a) below threshold for SRS; and (b) above threshold for SRS. Shown in (c) are the output pulse characteristics of the first- and second-Stokes fields for diode pump currents of 50 A, 70 a and 90 A (output coupler with T = 54.9% @ 1240 nm/95.4% @ 1485 nm).

Dual wavelength Raman laser output with different energy ratios was achieved by changing the output transmittance parameters of M3. The characteristics of the system when operating with M3 changed to high-­reflecting at 1064 nm, T = 4.17% at 1240 nm, T = 99.3% at 1485 nm, and with a radius of curvature of 100 mm were examined. Here, the length of the Raman cavity was also changed with the distance between F and M2, and M2 and M3 changed to 235 mm and 117 mm, respectively. The corresponding radii of the fundamental, first-Stokes, and second-Stokes fields in the diamond crystal were 129-134 μm, 140 μm and 154 μm, respectively, when considering the thermal stability range of the fundamental field cavity.

The energy-transfer characteristics of the laser operating with this altered configuration are shown in Figure 5. The first- and second-Stokes fields again had almost identical generation thresholds, and the output energy increased uniformly with diode pump current; the rate of increase in second-Stokes output energy was significantly higher than that of the first-Stokes. This can be attributed to the reduced out-coupling of the first-Stokes field and greater out-coupling of the second-Stokes field. At a maximum pump current of 110 A, the first- and second-Stokes output energies were 0.91 mJ and 5.28 mJ, respectively, with a corresponding total energy of 6.19 mJ, and a first- to second-Stokes energy ratio of 0.17. From these results, it can be concluded that multi-­wavelength diamond Raman laser output with different energy ratios can be achieved by changing the transmittance of the resonator output coupler. It is worth noting that within the available pump current range, we observed no signs of roll-over or clamping effect in the pulse energy output at the first- and second-Stokes wavelengths within the strength endurance limit of the diamond crystal. Progression of this work will focus on optimization of the polarization of the fundamental field and better management of thermal effects within the laser gain medium [33]. This will lead to more efficient and better energy-scaling of multi-wavelength Raman laser emission with different repetition rates.

Figure 5. Plot showing the pulse energy scaling characteristics of the first- and second-Stokes outputs from the modified resonator (output coupler T = 4.17% @ 1240 nm/99.3% @ 1485 nm).

Compared to extracavity Raman laser designs, intracavity Raman lasers in general, have a more compact structure, benefit from high intensity laser fields, have low SRS thresholds, and generate high output power. Table 1 summarizes the key output parameters of reported multi-wavelength pulsed diamond Raman lasers with intracavity and extracavity designs. In all prior reports, regardless of whether the laser used an intracavity or extracavity design, the outputs from multi-wavelength Raman lasers are uneven in that most of the output energy is biased to a single wavelength. While extracavity designs may have greater flexibility in terms of wavelength conversion, the work detailed herein demonstrates the highest peak power, cascaded diamond Raman laser output from an intracavity design.

Table 1. Comparison of output parameters of multi-wavelength pulsed diamond Raman lasers with intracavity and extracavity designs.

Pump method	Energy/Power	Peak power	Wavelength (nm)	Reference
Intracavity	375 mW@1240 nm	1.3 kW@1240 nm	1240/1485	[34]
	0.6 W@1240nm	2.5 kW@1240 nm	1240/1485	[35]
	3.14 mJ@1240 nm/ 2.20 mJ@1485 nm	174 kW@1240 nm/ 220 kW@1485 nm	1240/1485	This work
Extracavity	1.95 W@620 nm	–	573/620	[36]
	0.67 mJ	80 kW	573/620	[37]
	134 mW	–	573/620/676	[27]
	1.1 W@1194 nm/ 0.52 W@1419 nm	–	1194/1419	[38]
	14.5 W	–	1240/1485	[39]
	1.5 W@1240 nm/ 13 W@1485 nm	–	1240/1485	[25]
	2.75 W @1240 nm/ 1 W@1485 nm	–	1240/1485	[26]
	0.26 mJ@1240 nm/ 0.18 mJ@1485 nm	25.5 kW@1240 nm/ 23 kW@1485 nm	1240/1485	[40]
	1.2 mJ@1240 nm/ 0.7 mJ@1485 nm	–	1240/1485	[41]

4. Conclusion

We have reported the design and operation of an intracavity Raman laser utilizing a side-pumped Nd:YAG laser crystal and diamond as the SRS medium, to generate simultaneous, dual-wavelength emission at 1.2 μm (first-Stokes) and 1.5 μm (second-Stokes). The laser operated at a repetition rate of 50 Hz, and the maximum combined pulse energy (for both output wavelengths) was 5.34 mJ. The corresponding peak powers were of 174 kW at 1.2 μm and 220 kW at 1.5 μm, which are to the best of our knowledge, the highest peak powers produced from a dual-wavelength diamond Raman laser. By modifying the laser resonator by way of output transmittance and cavity length, the relative ratio of output pulse energy in each Stokes emission line could be changed. While there are currently some disparities in the output power of diamond Raman lasers compared to the Raman lasers that utilize large-sized crystals [42, 43], we believe that as the size of optical-grade diamond crystals increases, diamond Raman lasers will demonstrate tremendous potential for further enhancing the output power and expending wavelength of lasers. In this work, we demonstrate first- to second-­Stokes output pulse energy ratios of both 1.43 and 0.17. We anticipate that interest in diamond Raman lasers will continue to expand (driven by the excellent optical, thermal and Raman-active properties of diamond crystals) and here, Raman lasers with a diversity of output characteristics (such as single longitudinal mode, cascaded frequency, and high beam quality) [44–48] will be developed in order to service a broad range of applications.

Author contributions

Hui Chen: Conceptualization, Investigation, Data collection, Writing-original draft, Visualization. Yufan Cui: Data collection, Investigation, Theoretical analysis. Xiaowei Li: Data collection, Investigation, Visualization. Boyuan Zhang: Data collec­tion, Investigation, Formal analysis. Yunpeng Cai: Data collection, Investigation, Formal analysis. Jie Ding: Writing-review & editing, Formal analysis. Yaoyao Qi: Writing-review & editing, Formal analysis. Bingzheng Yan: Writing-review & ­editing, Formal analysis. Yulei Wang: Supervision, Con­ceptualization, Methodology, Formal analysis. Zhiwei Lu: Supervision, Con­ceptualization, Methodology, Formal analysis, Funding acquisition. Zhenxu Bai: Writing-review & editing, Supervision, Conceptualization, Methodology, Formal analysis, Project administration, Funding acquisition.

Acknowledgments

The authors thank Prof. Richard P. Mildren (Macquarie University) for useful discussion, Dien Tech Co., Ltd. and Element Six for providing the coated CVD diamond.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The authors are grateful for the financial support by the National Natural Science Foundation of China (61927815), the Natural Science Foundation of Tianjin City (22JCYBJC01100), the Natural Science Foundation of Hebei Province (F2023202063), the Program of State Key Laboratory of Quantum Optics and Quantum Optics Devices (KF202201), the Funds for Basic Scientific Research of Hebei University of Technology (JBKYTD2201) and the Postgraduate Innovation Ability Training Program of Hebei Province (CXZZSS2021039).