Mid-infrared laser can be used for gas remote detection, photoacoustic spectroscopic measurement, isotope precision analysis, and in the field of optoelectronic countermeasures, it is also a powerful tool to check and balance the infrared guidance of unmanned aircraft, space-based and air-based missile defense systems. The difficulty of obtaining mid-infrared laser is firstly because the bandgap of mid-infrared laser crystals is generally large, and the emission in the mid-infrared band (3 μm~5 μm) is difficult; semiconductor quantum cascade lasers (QCLs) utilize the energy level design method to make the semiconductor have multiple narrower cascade bandgaps, and emit longer wavelengths, and the wavelengths are generally in the mid-infrared band of 4 μm~12 μm, but due to the divergence of QCLs However, due to the large dispersion angle of QCL, the wavelength linewidth is very wide, the peak power is low, the application fields are greatly restricted, and the QCL with higher power is embargoed by foreign countries, which is difficult to obtain domestically. Another reason is the low quality and high price of optical components in the mid-infrared wavelength band, especially high-quality nonlinear optical crystals, such as lithium niobate crystals with periodical polarization, and many substrates and processes require imported equipments, which limits the way of generating mid-infrared lasers by nonlinear methods.
In order to solve this problem, Nanjing University Zhu Shining group, using parametric cascade technology to overcome the lithium niobate absorption loss in the 5μm band, to solve the problem of continuous wave mid-infrared laser generation as far as 5.19μm, and the wavelength can be tuned from the near infrared to mid-infrared wavelength band continuously, with a simple structure, a wide range of wavelength tuning, narrow linewidth, the advantages of good beam quality, and is expected to be applied to gas detection, It is expected to be used in gas detection, photoelectricity countermeasures and other fields. The technology is composed of magnesium-doped periodically polarized lithium niobate (MgO:PPLN), and the cascading optical parametric oscillation process (TOPO) is realized by using a single cycle. The cascade process consists of optical parametric oscillation (OPO) signal light cascade pumping light, and simultaneously outputs three mid-infrared wavelengths that can be wavelength-tuned, and through the reduction of intracavity loss, the continuous-wave resonance output is realized, and the maximum output power is more than 2.6 W at 3896 nm. The innovation of this technology lies in the cascade of optical parametric oscillations through a single cycle, and the reduction of intracavity loss through optical processing, so that the continuous-wave resonance OPO can be used as a continuous wave resonator. The continuous-wave resonant OPO is capable of long-wave cascade processes.
Wavelength tuning of OPO and TOPO
Based on years of accumulation in the field of lithium niobate superlattice materials and laser technology, the tunable OPO developed by the group has been successfully transformed into various types of mid-infrared lasers covering continuous wave, nanosecond pulse, picosecond ultra-fast, etc., which have been highly praised by users from various universities, research institutes and military units. At present, the longest output wavelength of tunable laser products can reach 4.65μm, and the highest output power is more than 10W.
