Fig. 1. c-axis view of alexandrite structure
[21] and the image of alexandrite crystals grown by the Czochralski method in Crystech Co., Ltd.
Fig. 2. Simplified energy level diagram for the alexandrite crystal
[22] Fig. 3. Alexandrite absorption spectrum for Cr
3+ dopant concentration of 0.063 at.% and alexandrite fluorescence rate spectra at 300 K
[23] Fig. 4. Schematic diagram of the single bounce alexandrite slab laser, the double-bounce alexandrite slab laser and the extended double bounce alexandrite slab laser
[12] Fig. 5. Schematic diagram of diode-pumped alexandrite vortex laser
[44] Fig. 6. Pumping configuration with two diode modules and the resonator configuration of the alexandrite laser
[14] Fig. 7. Experimental arrangements for fiber-delivered polarized diode single-end-pumped alexandrite laser and double-pass-end-pumped alexandrite laser
[45] Fig. 8. Effective emission cross section spectra of alexandrite for E∥
b polarization at different crystal temperatures and variation of small signal gain with temperature
[22] Fig. 9. Thermal lens dioptric power and laser power as a function of the absorbed pump power
[48] Fig. 10. Schematic of double-end-pumped L-shaped alexandrite laser
[13] Fig. 11. Schematic layout of the blue LD pumped alexandrite laser system at cryogenic temperatures and within the temperature range of 300~400 K
[49] Fig. 12. Schematic layout of alexandrite laser system for Q-switching
[9] Fig. 13. Schematic of red LD-pumped Q-switched alexandrite laser and cavity-dumping Q-switched alexandrite laser
[53] Fig. 14. Schematic of Q-switched diode-pumped alexandrite ring laser
[55] Fig. 15. Schematic of W-level Q-switched diode-pumped alexandrite ring laser
[56] Fig. 16. Setup of the experiment for LED-pumped alexandrite laser and the tunable multipass amplifier for a CW Ti:sapphire laser
[59] Fig. 17. Schematic diagram of 532 nm pumped KLM
[62] and QD-SESAM passively mode-locked
[63] alexandrite laser
Fig. 18. Schematic diagram of 532 nm laser pumped MPC KLM alexandrite laser
[64] and graphene passively mode-locked alexandrite laser
[65] Fig. 19. Experimental setup for narrow linewidth alexandrite regenerative amplifier
[66] Fig. 20. Experimental setup for chirped pulse amplification of 300 fs pulses in an alexandrite regenerative amplifier
[67] Fig. 21. System schematic of the alexandrite-pumped alexandrite regenerative amplifier
[68] Fig. 22. Experimental setup diagram of dual-end-pumped alexandrite laser
[81] Fig. 23. Laser spectrum at maximum output power and beam qualities at different output powers
[81] Fig. 24. Laser-tissue absorption spectrum
[90] Fig. 25. Demonstration of a potassium layer measurement with a LD-pumped alexandrite laser
[55] and a fashlamp-pumped alexandrite laser
[100] Fig. 26. Multiphoton microscopy images of a mouse popliteal lymph node
[112] | Gain medium | Cr3+:BeAl2O4 (Alexandrite) | Ti3+:Al2O3 (Ti:sapphire) | Cr3+:LiCaAlF6 (Cr:LiCAF) | Cr3+:LiSrAlF6 (Cr:LiSAF) | Cr3+:LiSrGaF6 (Cr:LiSGaF) | Yb3+:Y3Al5O12 (Yb:YAG) |
|---|
| Mass density ρ/(g·cm-3) | 3.69 | 3.98 | 2.99 | 3.45 | 3.89 | 4.56 | | Melting point/°C | 1 870 | 2 040 | 810 | 766 | 716 | 1 970 | Specific heat capacity Cp/(J·g-1·°C-1) | 1.05 | 0.761 | 0.935 | 0.842 | 0.76 | 0.59 | | Mohs hardness | 8.5 | 9 | ~4 | 3~4 | ~4 | 8.5 | Knoop hardness/ (kg·mm-2) | 1 600~2 300 | 1 800 (∥c) 2 200 (∥a) | - | 197 | - | 1 320 | Thermal conductivity κ/(W·m-1·K-1) | 23 (∥a-b-c) | 30.3 (∥a) 32.5 (∥c) | 4.58 (∥a) 5.14 (∥c) | 1, 1.8 (∥a) 1.68, 3 (∥c) | 1.3 (∥a) 2.6 (∥c) | 10 | Thermal expansion coefficient α/(×10-6K-1) | 6 (∥a) 6 (∥b) 7 (∥c) | 4.8 & 5.3 | 22, 21 (∥a) 3.6, 3.1 (∥c) | 22.2, 25, 26 (∥a) -9.8, -10, -8.1 (∥c) | 12, 23 (∥a) 0, -5.4 (∥c) | 6.7 | Thermal diffusivity D/(×10-3 cm2·s-1) | 60 | 92.5 | 16.4 (∥a) 18.4 (∥c) | 6 (∥a) 10 (∥c) | 4.4 (∥a) 8.8 (∥c) | 37 | Young modulus E/(×109 Pa) | 469 | 335 | 96 | 109 (avg) 85 (∥c) 120 (∥a) | - | 280, 310 | | Poisson's ratio ν | ~0.25 | 0.29 | 0.25 | 0.3 | - | 0.3 | Tensile (fracture) strength σf/(×106 Pa) | 457~948 (∥a) 520 (∥b) | 400 | - | 38.5±8 | - | 200 | Fracture toughness K1c/(×106 Pa·m1/2) | 2.6 | 2.2 | 0.31, 0.18~0.37 | 0.33, 0.4 | - | 1.4 | Thermal figure of merit RT'/(W·m-1/2) | 14 | 22 | 0.53 | 0.42 (∥a) 0.80 (∥c) | 0.55 | 5.1 | Damage threshold/ (J·cm-2) | 270 @ 12 ns | 7.8 @ 0.5 ps 80 @ 50 ps 210 @ 8 ns | 20~25 @ 50 ps | 1.5 @ 20 ps 8~24 @ 50 ps | 20~26 @ 50 ps | 110 @ 4.5 ns |
|
Table 1. The related thermo-mechanical parameters of the alexandrite, Ti:Sapphire, Cr:LiCAF, Cr:LiSAF, Cr:LiSGaF, and Yb:YAG crystal
[17] | Gain medium | Cr3+:BeAl2O4 (Alexandrite) | Ti3+:Al2O3 (Ti:sapphire) | Cr3+:LiCaAlF6 (Cr:LiCAF) | Cr3+:LiSrAlF6 (Cr:LiSAF) | Cr3+:LiSrGaF6 (Cr:LiSGaF) | Yb3+:Y3Al5O12 (Yb:YAG) |
|---|
| Birefringence | Biaxial | Negative uniaxial | Positive uniaxial | Positive uniaxial | Positive uniaxial | Isotropic | | Refractive index n | 1.736 7 (∥a) 1.742 1 (∥b) 1.734 6 (∥c) | 1.765 5 (∥a) 1.757 3 (∥c) | 1.380 (∥a) 1.380 8 (∥c) | 1.387 3 (∥a) 1.394 0 (∥c) | 1.389 3 (∥a) 1.391 (∥c) | 1.82 | | Nonlinear refractive index n2/(×10-16 cm2·W-1) | 2 3.54 | 3.2 | 0.4 0.36~0.66 | 0.8 0.52-2.15 | 1.2 | 6.9 | Temperature dependence of refractive index dn/dT/(×10-6 ·K-1) | 5.5, 9.4 (∥a) 7, 8.3 (∥b) 14.9 (∥c) | 13 | 4.2, -7.3 (∥a) 4.6, -4.9 (∥c) | -2.5, -4.5 (∥a) -4, -9.1 (∥c) | -7, -2.7 (∥a) -1.8 (∥c) | 9.9 | | Group velocity dispersion/(fs2·mm-1) | 60.7 | 56.6 | 24 | 22.7 | ~25 | 66.6 | | Pump wavelength/nm | 550 (∥a) 595 (∥b) 570 (∥c) | 480 | 630 | 650 | 630 | 940 | | Absorption bandwidth/nm | 90 (∥a) 80 (∥b) 70 (∥c) | 125 | 90 | 100 | 85 | 12.5 | | Peak absorption cross section σab/(×10-20 cm2) | 3.9 (∥a) 19 (∥b) 9 (∥c) | 6.4 (∥c) 2.6 (∥a) | 1.3 (∥c) 0.9 (∥a) | 4.5 (∥c) 2.5 (∥a) | 3 (∥c) 1.5 (∥a) | 0.83 | | Maximum gain wavelength/nm | 750 | 790 | 780 | 855 | 840 | 1 030 | | Tuning range/nm | 714~818 (300 K) | 660~1180 | 720~887 | 770~1 110 | 777~977 | 1 016~1 108 | | Peak emission cross section σem/(×10-20 cm2) | 0.7 @ 22 °C | 41 (∥c) 15 (∥a) | 1.3 (∥c) 0.9 (∥a) | 4.8 (∥c) 1.6 (∥a) | 3.3 (∥c) 1.4 (∥a) | 2.1 | | Room-temperature fluorescence lifetime τf /µs | 262 | 3.2 | 175 | 67 | 88 | 940 | | σemτf/(×10-26 cm2·s) | 183 @ 22 °C | 131 | 228 | 322 | 290 | 1 975 | | Crystal figure of merit | 3 000 | 150 | 2 150 | 3 300 | ~2 000 | - | Gain saturation fluence Jsat/(J·cm-2) | 38 @ 22 °C | 0.6 (∥c) | 19.1 (∥c) | 4.8 (∥c) | 7.5 (∥c) | 8.8 9.2 |
|
Table 2. The spectroscopic and laser parameters of the alexandrite, Ti:Sapphire, Cr:LiCAF, Cr:LiSAF, Cr:LiSGaF, and Yb:YAG crystal
[17] | Year | Wavelength/nm | Power/W | Chip structure | Institution |
|---|
| 2008 | 643 | 12 | 0.4 mm bar, 20 emitting points, 40 µm×1.5 mm | Mitsubishi Electric Co., Japan[25] | | 2012 | 635 | 1.2 | 5 µm×250 µm ridge-waveguide, 2 mm resonator | FBH, Germany[30] | | 2014 | 639 | 2.3 | Single emitting region, 150 µm×3 mm | nLight Co., US[29] | | 2017 | 644 | 20.1 | 1 cm bar, 25 emitting points, 60 µm×0.7 mm | Sony Co., Japan[31] | | 2018 | 638 | 6 | Three emitting region, 180 µm×1.5 mm | Mitsubishi Electric Co., Japan [27] | | 2019 | 638 | 4.5 | Double emitting regions, 150 µm×1.5 mm | Ushio Opto Semiconductors Inc., Japan[28] | | 2019 | 640 | 3.9 | Single emitting region, 100 µm×1.5 mm | Huaguang Optoelectronics Co., China [32] |
|
Table 3. The research progress of 640 nm red LDs
| Year | Pump source | Pump parameters | Laser output performance | Laser wavelength | Ref. |
|---|
| 1979 | Xe flash lamp | - | 500 mJ, 200 µs; 70 mJ, 120 ns | 701~794 nm | [1] | | 1980 | Flash lamp | 500 J, 1.5 kW | 500 mJ, 33 ns, 5 Hz | 701~818 nm | [2] | | 1980 | Flash lamp | 3.2 kW | CW 6.5 W | 765 nm 744~788 nm | [36] | | 1980 | Flash lamp | - | 500 mJ, 20 ns | 680.4 nm | [38] | | 1985 | Hg arc lamp | 6 kW | CW 60 W | - | [18] | | 1985 | Xe arc lamp | 8 kW | CW 20W | - | [18] |
|
Table 4. Results of experiments based on flash lamp pumped alexandrite laser
| Year | Pump source | Pump wavelength/nm | Pump power/W | Slope efficiency | Output power/W | Tuning range/nm | Remark |
|---|
| 1983[39] | Krypton ion laser | 647.1 | 1.9 | 51% | 0.6 | 726~802 | Krypton ion laser pumping | | 1993[40] | Dye laser | 645 | 0.36 | 64% | 0.15 | 753.4 | Dye laser pumping | | 1993[40] | Red LD | 640 | 2×0.25 | 28% | 0.025 | 753 | - | | 2016[41] | Green laser | 532 | 11 | 26% | 2.6 | 715~800 | Green laser pumping | | 2006[42] | Green laser | 532 | 5 | 31% | 1.4 | 730~780 | - | | 1990[43] | Red LD | 680.4 | 0.01 | 25% | - | 751 | First LD pumping | | 2005[11] | Red LD | 680.4 | 10 | 24% | 1.3 | 750 | - | | 2013[8] | Red LD | 680.4 | 0.865 | 34% | 0.2 | - | - | | 2014[9] | Red LD | 639 | 64.5 | 49% | 26 | 730~792 | Highest output power with LD pumping | | 2017[12] | Red LD | 638 | 56 | 37% | 12.2 | 755.3 | - | | 2020[14] | Red LD | 637 | 25 | - | 6.5 | 752 | - | | 2018[47] | Red LD | 636 | 3.07 | 54.4% | 1.22 | 714~818 | Longest tuning range | | 2020[13] | Red LD | 636 | 34 | 54.9% | 12.7 | 725~808 | Highest slope efficiency with LD pumping | | 2017[49] | InGaN blue LD | 444 | 3.5 | 20% | 0.326 | 750 | First blue LD pumping | | 2019[50] | InGaN blue LD | 445 | 3.5 | 39% | 0.57 | 749.5 | - | | 2020[16] | Yellow laser | 589 | 7.7 | 41% | 2.51 | 727.2~787.3 | First yellow laser pumping |
|
Table 5. Results of continuous-wave alexandrite lasers (non-flashlamp pumping)
| Year | Pump source | Q-switching | Pulse energy | Pulse width | Repetition frequency | Output power | Ref. |
|---|
| 2014 | Red LD | Pockels cell | 0.74 mJ | 92 ns | 1 kHz | 740 mW | [9] | | 2014 | Red LD | Pockels cell | 0.7 mJ | 58 ns | 100 Hz | 70 mW | [9] | | 2016 | Red LD | Pockels cell | 0.8 mJ | 350 ns | 35 Hz | 28 mW | [52] | | 2016 | Red LD | Pockels cell | 6.2 mJ | - | 100 Hz | 62 mW | [52] | | 2016 | Red LD | Pockels cell (cavity dumped) | 510 µJ | 3 ns | Multi-kHz | - | [53] | | 2018 | Red LD | SESAM | | 550 ns | 27 kHz | - | [54] | | 2018 | Red LD | Pockels cell | 1 mJ | 420 ns | 150 Hz | 150 mW | [55] | | 2018 | Red LD | Pockels cell | 1.7 mJ | 850 ns | 500 Hz | 850 mW | [56] | | 2019 | Red LD | Self-Q-switching | 9.8 μJ | 660 ns | 135 kHz | 1.32 W | [57] |
|
Table 6. Results of Q-switched experiments based on alexandrite crystal
| Year | Pump source | Mode-locking | Laser wavelength | Pulse width | Repetition frequency | Output power | Ref. |
|---|
| 1982 | Flash lamp | Organic dye | 725~745 nm | 8 ps | 12.5 Hz | - | [60] | | 2016 | 532 nm laser | KLM | 755 nm | 170 fs | 80 MHz | 780 mW | [62] | | 2018 | 532 nm laser | QD-SESAM | 775 nm | 380 fs | 79.9 MHz | 295 mW | [63] | | 2018 | 532 nm laser | KLM | 750 nm | 70 fs | 5.6 MHz | 4 mW | [64] | | 2018 | 532 nm laser | Graphene | 750 nm | 65 fs | 5.56 MHz | 8 mW | [65] |
|
Table 7. Results of mode-locking experiments based on alexandrite crystal
| Year | Crystal | Type | Crystal dimensions | Wavelength | Pulse energy | Pulse width | Repetition frequency | Highest conversion efficiency/％ | Ref. |
|---|
| 1983 | RDP | Type I SHG | 10 mm×10 mm×25 mm | 0.36~0.40 µm | 5 mJ | 0.1 µs | - | - | [69] | | 1988 | BBO | Type I SHG | 9 mm×5 mm×7 mm | 378 nm | 105 mJ | - | 4 Hz | 31% | [70] | | 1989 | BBO | Type I SHG | 4 mm×9 mm×7 mm | 378 nm | ~19 mJ | - | 10 Hz | 26% | [72] | | 1989 | BBO | Type I THG | 8 mm×4 mm×7.5 mm | 252 nm | ~7.5 mJ | - | 10 Hz | 10%* | [72] | | 1994 | BBO | Type I SHG | 8 mm | 373 nm | ~72 mJ | - | 60 Hz | 28.6% | [71] | | 1994 | KDP | Type I THG | - | 248 nm | 15 mJ | - | 100 Hz | - | [71] | | 1998 | BBO | Type I SHG | 5 mm×5 mm×5 mm | 375 nm | 90 mJ | 240 µs | 10 Hz | - | [73] | | 2001 | BBO | Type I SHG | 5 mm×5 mm×10 mm | 365 nm | 186 mJ | 220 µs | - | 4.2% | [74] | | 2007 | LBO | Type I SHG | 5 mm×4 mm×5 mm | 0.36~0.388 µm | 0.87 mJ | - | - | 3.5% | [75] | | 2016 | BBO | Type I SHG | 4 mm×4 mm×10 mm | 379 nm | 184 µJ | - | 1 kHz | 47% | [53] |
|
Table 8. The research progress of ultraviolet alexandrite laser
| Property | CNRS | NASA | MPI | IAP | Arecibo | PNL |
|---|
| Application | DIAL | DIAL | DIAL | Res fl | Res fl | Lab | | Seeder | none | none | Ti:Al2O3 | Diode | Diode | Diode | | Wavelength range /nm | 727~732 | 725~785 | 720~780 | 770 | 770+385 | 750 | | Linewidth /MHz | <560 | 560 | <150 | <20 | <50 | <20 | | Freq stability /MHz | <110 | <400 | 63 rms | - | - | 16 | | Spectral purity | >99.95% | >99.85% | >99.99% | >99% | - | - | | Pulse width /ns | <500 | 200 | <200 | 275 | 100~300 | 140 | | Pulse energy /mJ | 30 | 30 | >50 | 100 | 200 | 250 | | Pulse rate /pps | 20 | 10 | >15 | 25 | 20 | 20 | | Installation | Ground | Aircraft | Ground | Ship | Ground | Ground |
|
Table 9. Some working lidars based on alexandrite lasers pumped by flashlamp
[92] 
![c-axis view of alexandrite structure[21] and the image of alexandrite crystals grown by the Czochralski method in Crystech Co., Ltd.](/richHtml/gzxb/2020/49/11/77/img_1.png)
![Simplified energy level diagram for the alexandrite crystal[22]](/richHtml/gzxb/2020/49/11/77/img_2.png)
![Alexandrite absorption spectrum for Cr3+ dopant concentration of 0.063 at.% and alexandrite fluorescence rate spectra at 300 K[23]](/Images/icon/loading.gif){kind=icon}
