Do You Know About Mid-infrared Solid-state Lasers?(Part 1)

Mid-infrared band refers to a certain band in the infrared wavelength range, due to different application requirements, the range of mid-infrared wavelength has different definitions in different application fields. The International Lighting Association defines mid-infrared as 3-1000μm; In the military generally limited to 3-5 μm; In the field of laser technology, the mid-infrared laser wavelength range generally refers to a 2-5 μm band.

(1) Space communication

The mid-infrared band is located in the absorption window of the atmosphere. As can be seen from Figure 1, in the mid-infrared band, the transmittance of most wavelengths is above 60%, some of them are as high as 90%, and a small number of wavelengths are very low due to the absorption and transmittance of CO2, H2O, and O3 molecules. Therefore, the mid-infrared laser can realize long-distance transmission in the atmosphere and has a wide range of applications in remote sensing, detection, and other fields.

The mid-infrared band of 3-5 μm is the window of low loss, weak turbulence, and weak background noise of the atmosphere, which can overcome the influence of atmospheric channels well, and is the ideal band for long-distance laser communication in space.

The high-speed data to be transmitted is encoded and loaded onto the optical carrier output by the mid-infrared laser source to form the mid-infrared laser signal, and then amplified by the optical power and the transmitting antenna to expand the beam. The purpose of expanding the beam is to compress the divergence Angle of the beam and reduce the divergence loss of the laser beam in the atmosphere, and then transmit through the atmospheric channel to the receiving end. It is transmitted by the receiving antenna and converted by the medium infrared photodetector, and finally processed by the data processing unit such as the line decoder, the original high-speed data is obtained.

(2) Medical applications

Water molecules are an important part of biological tissue (water absorption spectrum is shown in Figure 3). The thermal effect of water molecules on the intense absorption of 1.9-2μm laser can achieve rapid hemostasis and reduce the damage to human tissue during surgery. Therefore, lasers in this band are widely used in clinical surgery.

The cases that have been used in clinical surgery include the resection of benign and malignant tumors such as angioceratoma and brain tumor, nasal surgery such as nasal polyps, follicular hyperplasia of the posterior pharyngeal wall, hypertrophy of the inferior turbinate, endometrial transposition, glandular cystitis, prostatic hypertrophy, lithotripsy, laser myocardial perforation, joint synoviectomy, joint cyst and other soft tissue resection and the treatment of osteoarthritis, etc.

This medical method has the advantages of less or no bleeding, no need for tamponade, small injury, quick healing of the injured surface, and a simple surgical method.

(3) Military applications

Directional infrared jamming technology is a kind of infrared active jamming technology, after the laser beam reaches a certain beam expansion ratio, when there is a missile approaching, the tracking device is used to direct the jamming energy to the direction of the incoming missile, causing the missile seeker to fail and deviate from the target.

The United States Naval Laboratory has successfully developed the multi-band Anti-ship Tactical Electronic Warfare System (MATES) for the Integrated Electronic Warfare System (AIEWS), which uses a light source mainly in the spectral range of mid-wave infrared and far-infrared laser devices.

(4) Industrial processing

Transparent plastics have a small absorption of 1μm band, while most organic materials have enough absorption of 2μm, so they can be directly used in the cutting, welding, engraving, and other processing fields of transparent materials. With the increasing popularity of laser 3D printing technology, the 3D printing manufacturing of transparent organic materials will develop more rapidly.

(5) Gas monitoring

The mid-infrared band concentrates the absorption lines of a large number of gas molecules, and its absorption intensity is 2-3 times stronger than that of the near-infrared band. Therefore, the mid-infrared laser has a wide range of civilian value in the field of trace gas detection. Since the absorption peaks of CO2, CH4, and C2H6 are in the bands of 2.8 μm, 3.2 μm, and 3.3 μm respectively, the continuous mid-infrared laser can be applied to molecular spectroscopy to improve the sensitivity of trace gas monitoring.

Mid-infrared solid-state laser generation technology

For solid-state laser technology, the generation methods of the mid-infrared band can be divided into doped ion direct emission and nonlinear conversion technology.

The direct emission of doped ions is the emission of mid-infrared photons through the energy level transition of ions. Common solid-state activators include rare earth ions (Tm3+, Ho3+, Er3+, etc.) and transition metal ions (Fe2+, Cr2+, etc.).

Nonlinear frequency conversion techniques include difference frequency, optical parametric oscillation, and stimulated Raman scattering, which are mainly determined by the properties of nonlinear crystals.

(1) Thulium-doped solid-state laser

The emission band of thulium laser is at the absorption peak of water molecules (1.92-1.94μm), so thulium laser is a promising medical laser with high efficiency and low thermal damage when applied in surgery. In addition, thulium-doped lasers can be used as pumping sources for holmium-doped laser systems and mid-infrared parametric lasers.

The absorption peak of thulium-doped materials is around 790 nm, which is suitable for semiconductor pumping. Common thulium-doped matrix materials include YAG, YLF, LuAG, YAP, etc. In recent years, new gain media based on sesquioxide ceramics, such as Tm: Lu2O3, and Tm:(Lu, Sc)2O3, have also been widely studied.

The energy level of thulium ion is broadened under the action of the crystal field of the matrix material, and the energy level width and band interval are different, but the basic characteristics are similar, and the emission spectrum lines are mainly concentrated in the range of 1.9-2.1 μm. Tunable output with narrow linewidth can be achieved by tuning elements such as volume Bragg grating with its wide fluorescence spectrum.

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