Do You Know About Mid-infrared Solid-state Lasers?(Part 2)

Mid-infrared solid-state laser with a wavelength of 3~ 5μm has the advantages of high efficiency, small size, and lightweight, and has important application value in the industry, medical treatment, military, etc. The development of large-size, high-quality mid-infrared laser crystal and infrared high-power laser pump sources with longer output wavelengths has become one of the future development directions of mid-infrared solid-state laser.

We then talk about the generation technology of mid-infrared solid-state laser.

(2) Holmium doped solid laser

The 2μm band coherent light source has a relatively high transmittance in the air and is an ideal light source for wind speed measurement, coherent Lidar, remote sensing, and other applications.

The gain medium doped with holmium ion can directly generate a laser of about 2.1 µm. Holmium ions have absorption peaks near-visible light and 1.9µm. Earlier holmium lasers were pumped by the flash lamp, and co-doped ions such as Tm3+ were added to the gain medium as sensitizers, which was not conducive to obtaining high conversion efficiency at room temperature.

At present, the ideal way is to use a 1.9µm laser generated by a thulium-doped laser to directly pump holmium crystal, or use a semiconductor laser of about 1908 nm as a pumping source, which can achieve stable and efficient holmium laser output at room temperature.

(3) Erbium-doped solid-state laser

The 4I 11/2 → 4I 13/2 transition of Er3+ can produce laser in the band of 2.7 ~ 3 μm in different substrates, which can be directly obtained by xenon lamp and LD pumping high doping concentration of Erbium material. The relatively mature materials studied include Er: YAG, Er: YLF, Er: YSGG, Er: GSGG, Er: BYF, etc. In recent years, there have also been studies on oxide laser ceramics as matrix materials, such as Er: LuO3, Er: Y2O3, etc.

GSGG crystal thermal conductivity is low, there is a serious thermal lens effect, and it is difficult to achieve high repetition frequency, high power, and high beam quality of mid-infrared laser output; YSGG matrix material can be used for low repetition frequency medium and small power solid-state laser, and the phonon energy is low, and the influence of multi-phonon non-radiative transition is small.

YAG crystal matrix growth technology is mature, easy to dope, has high thermal conductivity, high laser damage threshold, and excellent physical and chemical properties. Compared with YAG crystal, YLF crystal structure stress, and thermal stress are larger, there is a certain thermal lens effect, and the crystal growth process is difficult. The pumping mode of Er: YAG laser is mainly divided into xenon lamp pumping, LD side pumping, and LD end pumping, which can output 2940 nm laser with peak power and large energy.

(4) Transition metal element chromium iron doped solid laser

Transition metal ions Cr2+, Ni2+, Co2+, and Fe2+ exhibit better mid-infrared laser properties in group Ⅱ-Ⅵ semiconductor materials, especially the semiconductor crystals doped with Cr2+ ions, such as Cr2+:ZnSe, Cr2+:ZnS, have good fluorescence properties at room temperature, wide tuning range, and high quantum efficiency. Cr2+:ZnSe has a wavelength tuning range of about 2200-2700 nm, and Cr2+:ZnS crystals have an output range of 2100-2700 nm.

(5) Mid-infrared laser based on nonlinear technology

①Difference-frequency mid-infrared solid-state laser

When two laser beams with frequency differences are incident on a nonlinear crystal, a new laser whose frequency is the difference of the frequency of the two laser beams is generated. Like any other nonlinear process, this process must reach certain threshold conditions. Based on the difference frequency technology, light sources in the visible to 30 µm range can be obtained, and in most cases are used to achieve far infrared waves.

②Medium infrared parametric oscillation laser

If the nonlinear medium is placed in the optical resonator, the pumped light is incident on the nonlinear crystal, producing two new low-frequency lights (signal light and idle light), the pumped light, signal light, and idle light many times through the nonlinear medium, when the gain of the signal light wave and idle light is greater than their loss in the resonator, the laser oscillation is formed in the resonator.

This is the optical parametric oscillator (OPO). Through the coating design of the resonator mirror, the desired laser frequency output can be selected.

As shown in the figure. ωp is the pumping optical frequency, ωs is the signal optical frequency, ωi is the idle optical frequency, and meets the relationship of ωp = ωs+ωi.

The resonator of an optical parametric oscillator can be resonant to both signal light and idle light or to one of the frequencies. The former is often called a double-resonance parametric oscillator (DRO), and the latter is often called a single-resonance optical parametric oscillator (SRO).

The three beams of light propagating in the crystal need to meet the phase matching condition, that is, related to the refractive index of the optical wavelength in the crystal, if the pumped light incident at a fixed wavelength, the change of the refractive index of the nonlinear crystal will change the wavelength of the signal light and the idle light, so as to obtain a new phase matching condition and achieve wavelength tuning.

The Angle tuning can be achieved by using the relationship between anisotropic crystal birefringence and Angle, or temperature tuning can be achieved by changing temperature. Periodic tuning can also be performed by changing the crystal period for periodic polarized crystals.

The Angle tuning can be achieved by using the relationship between anisotropic crystal birefringence and Angle, or temperature tuning can be achieved by changing temperature. Periodic tuning can also be performed by changing the crystal period for periodic polarized crystals.

Nonlinear crystals are the key components of mid-infrared parametric oscillation lasers. Common mid-infrared nonlinear crystals include KTP, KTA, ZnGeP2 (ZGP), AgGaS2, LiNbO3 (LN), LiTaO3 (LT), PPLN, PPLT, PPKTP, PPKTA. PPLN, PPLT, PPKTP, and PPKTA belong to period-polarized crystals and have high conversion efficiency. Adding MgO into PPLN and PPLT can improve the damage threshold of the crystals.

Contact information:

If you have any ideas, feel free to talk to us. No matter where our customers are and what our requirements are, we will follow our goal to provide our customers with high quality, low prices, and the best service.