What do you think of when it comes to adamas? You may think of the eternal meaning carried by it as rough diamonds. However, in the eyes of physics researchers, single-crystal diamond is a Raman gain medium with excellent performance, and diamond Raman laser is an important technical means to obtain high-power continuous-wave single-frequency lasers of special bands.
A team from Hangzhou Institute for Advanced Study (HIAS), UCAS led by Feng Yan (research fellow) and Yang Xuezong (associate research fellow), as well as other cooperators, introduced a second-order nonlinear crystal (LiB3O5, LBO) into a stationary-wave diamond Raman resonator and regulated its phase matching, greatly improving the output power of single-frequency diamond Raman lasers. In their experiment results, the continuous wave power of 1178 nm Raman laser reaches 20 W.Combining the respective advantages of diamond Raman laser technology and fiber laser technology, this study provides a new feasible scheme for acquiring high-power single-frequency lasers of special wave bands.
The research results have been published in Applied Physics Letters through an article titled "High-power continuous-wave single-frequency diamond Raman laser at 1178 nm", and the article was selected as an Editor's Pick for that very edition.
Single-Crystal Diamond Assists Single-Frequency Laser
The 1.2 μm single-frequency laser is of important research and application value in many fields, including biomedical, remote sensing and communication.Such applications require that the wavelength and linewidth of a single-frequency laser be coupled with the energy level transition of a specific atom or molecule, or that a single-frequency laser meet the coherence requirement of an interferometric sensor.
Considering an efficient gain medium for 1.2 μm is unavailable, it is extremely challenging to realize high-power laser output in this band.Stimulated Raman scattering (SRS) is an efficient means to extend laser wavelength. SRS is superior to other nonlinear frequency conversion technologies (e.g. optical parametric oscillator (OPO), four-wave mixing (FWM)) in terms of flexible output wavelength and self-phase matching. Fiber optic raman laser using silica fiber as the gain medium dominates for present. It has realized hundred-watt-level single-frequency continuous laser output, but it is difficult to improve the power due to the stimulated Brillouin scattering effect in the fiber. Although ionically polymerized Raman crystals (e.g. calcium tungstate, yttrium vanadate, barium nitrate) have larger frequency shifts and higher Raman gain coefficients than silica fibers, severe crystal thermal effect hinders their development towards high-power continuous lasers.
Single-crystal diamond, when used as a Raman gain medium, has the advantages of large Raman gain coefficient (10 cm/GW), wide Raman shift (1332.3 cm-1), high thermal conductivity (2200 W/m・K) and wide light transmission range (>230 nm). Relevant studies have made important breakthroughs in acquiring high-power single-frequency continuous wave Raman laser. For example, the diamond Raman laser in combination with intracavity frequency doubling provides 38W 620 nm red light, 22 W 589 nm sodium guiding star laser, and 8 W 590-625 nm tunable single-frequency lasers in the yellow-red band. Therefore, diamond Raman laser is an important technical means to obtain high-power continuous-wave single-frequency lasers in special bands.
Introduction to the Research Project
The research team employed single-frequency diamond Raman laser technology and nonlinear longitudinal mode suppression technology, and adopted a 1018 nm ytterbium-doped fiber laser as the pump source. They realized the output of 1178 nm near-infrared laser from a simple stationary wave resonator, with the maximum continuous wave output power up to 20 W. Figure 1 shows the structure of the single-frequency diamond Raman laser device.
Fig. 1 Structure of 1178 nm Diamond Raman Laser Device
The Raman gain spectrum line has a similarity homogeneous broadening characteristic. The longitudinal mode at the gain peak oscillates preferentially after the Raman light reaches the threshold when the pump light’s spectral width is less than the width of the diamond Raman gain line (45 GHz).In a stationary wave resonator, Raman gain has a spatial resonance enhancement effect. The preferentially oscillating longitudinal mode has a higher Raman gain. Meanwhile, Raman gain has no spatial hole-burning effect. So single longitudinal mode Raman laser output can be realized in a stationary wave resonator. However, with the increase of power, the stability of the single longitudinal mode of Raman laser decreases and multiple longitudinal modes may be easily excited due to thermo-induced cavity length fluctuations, weak pump resonance and other factors. When a second-order nonlinear crystal is inserted into a Raman resonator and certain phase matching is satisfied, the crystal can introduce a nonlinear gain competition between longitudinal modes, thus greatly improving the stability of single longitudinal mode Raman light.
By actively regulating the phase matching of the second harmonic crystal, the loss of fundamental frequency light caused by the second harmonic conversion can be reduced on the condition that the multiple longitudinal modes are effectively suppressed, so that the output of high-power single longitudinal mode fundamental frequency Raman light is realized.A 1178 nm single longitudinal mode Raman laser was output with a power of 20 W when the maximum pump light power was 82 W, with a conversion efficiency of 24% and a slope efficiency of 38%. FIG. 2 shows how the powers of Raman light, frequency doubling light and residual pump light vary with the temperature of the second harmonic crystal and the longitudinal mode characteristics of Raman light.
FIG. 2 (a) Power Changes of Residual Pump Light, Raman Light and Frequency Doubling Light during Temperature Regulation of Frequency Doubling Crystal; (b) Stokes Diagrams under Single Longitudinal Mode and Multiple Longitudinal Modes Respectively Measured by Fabry-Perot (F-P) Scanning Interferometer When the Temperatures of Frequency Doubling Crystals were 38 °C and 36 °C (the blue dotted line refers to “built-in scanning voltage signal”, and the red dotted line “output signal”)
At the max. output power of 82 W, the linewidth of 1018 nm pump light was 11.6 GHz, the central wavelength of the 20 W Raman light was 1178.1 nm, and the linewidth of the single longitudinal mode Raman light was 67 MHz (the resolution limit of F-P scanning interferometer).Therefore, the power spectral concentration enhancement factor from multi-longitudinal mode pump light to single-longitudinal mode Raman light exceeded 173. FIG. 3 shows the spectral test results of pump light and Raman light.
Fig. 3 (a) Pump Light and Raman Light Spectra Recorded by a Spectrometer with a Resolution of 0.03 nm; (b) Raman Signal Measured by a F-P Scanning Interferometer. The inset is an amplification of a single signal peak. The half-peak width is 67 MHz (the resolution limit of the meter).
Conclusions and Outlook
Based on a stationary wave cavity diamond Raman scheme, the study enhanced the output power of 1178 nm laser to 20 W by introducing nonlinear longitudinal mode gain competition and using technically mature ytterbium-doped fiber laser as the pump source.The all-fiber pump laser combined with a simple stationary wave Raman resonator structure makes a compact and stable laser system.
Compared with silica fiber Raman, diamond crystal Raman has a larger Raman shift, which makes it easier to realize laser output in a longer wave band.Furthermore, the SBS in a single-frequency diamond Raman laser can be effectively suppressed by optimizing the resonator parameters. Diamond crystals have an ultra-high thermal conductivity and a low coefficient of thermal expansion. The thermal lens effect is weak during high-power operation. In conclusion, diamond Raman laser technology provides a promising technical solution to single-frequency lasers with a hundred-watt or kilowatt power.
Brief Introduction to the Main Authors of the Published Article
First author:Sun Yuxiang, a doctoral candidate jointly trained by Hangzhou Institute for Advanced Study, UCAS and Nanjing University of Science and Technology, and a visiting scholar to Macquarie University Photonics Research Centre, Australia (2019-2020), mainly engaged in research on solid-state lasers, diamond Raman lasers, and crystal thermal analysis.
Corresponding author: Yang Xuezong, "Cultivated Talent" and specially-appointed associate research fellow of the School of Physics and Optoelectronic Engineering, Hangzhou Institute for Advanced Study, UCAS.He holds doctorates in Optical Engineering, and Physics and Astronomy respectively of Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS), and Macquarie University, Australia. He is mainly engaged in basic and applied research on high-power fiber laser, fiber Raman laser, diamond Raman and Brillouin laser, and nonlinear frequency conversion technology.
Access to the research article:
https://aip.scitation.org/doi/10.1063/5.0107200
Source | School of Physics and Optoelectronic Engineering
Typesetter | Shen Yuanyuan
Executive Editor | Jiang Xuchen
