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Characteristics of 2µm Lasers

Characteristics of 2µm Lasers

Lasers with a wavelength of 2µm have advanced significantly in recent years and offer high efficiency, stability, and ease of use. 2µm lasers are ideal for highly precise applications including medical surgery and plastic processing because of their unique absorption characteristics, which allow them to create very small and precise cuts in both biological tissue and plastics with minimal localized heating. The absorption characteristics of 2µm lasers give them an advantage over 1µm lasers for certain applications.

Laser Design

The first 2µm lasers were very large, expensive, liquid-nitrogen cooled devices. Today there are 2µm diode lasers 30mm long and fiber lasers that are even smaller. Due to their inherent user-friendliness and broad range of energy delivery, including both pulsed and continuous wave (CW), 2µm lasers will improve efficiency and enable new procedures in various applications. Advances in technology are driving down costs and decreasing the size, while simultaneously improving performance. In some cases, researchers are developing components out of optical fibers, which can dramatically decrease the cost.

Laser Operation, Elements, and Power

For 2µm lasers, two rare earth elements have proven to be laser gain dopants that provide high power for both CW and pulsed laser operation: Thulium (Tm3+) and Holmium (Ho3+). The ions from these elements achieve laser emission in many different host crystals and glass fibers (see Table 1). For CW operation, thulium lasers proved a better option, while holmium proved better for pulsed and q-switched lasers due to the higher gain of the holmium-doped crystals. Thulium lasers are also advantageous because the ions can be excited with commercially available laser diodes around 800nm, while holmium can only be excited by a 1.9µm pump source.

2µm lasers with average powers around 100W are ideal for a number of industrial processes. Table 1 shows powers achieved by thulium-doped lasers and table 2 show powers achieved by holmium-doped lasers.

Laser Host Material Pump  Wavelength (nm) Emission Wavelength (nm) CW Output Power (W) Slope Efficiency (%)
YAG 805 2013 115 52
YAG 800 2013 120 Not Reported
YLF 792 1910 55 49
YLF 790 1912 148 32.6
LuO3 796 2070 1.5 61
Germanate 800 1900 64 68
Silica Fiber 793 2050 110 55
Silica Fiber 1567 1940 415 60
Silica Fiber 790 2040 885 49.2
Table 1: Published output powers, pump wavelengths, and emission wavelengths for CW thulium-doped lasers (Scholle et al., 2010).
Laser Host Material Pump Source Pump
Wavelength (µm)
Emission Wavelength (nm) CW Output Power (W) Pulse Energy (mJ) Slope Efficiency (%)
Ho:YAG Tm: YL 1.95 2090 1.6 Not Reported 21
Ho:YAG Tm:YLF 1.9 2090 Not Reported 50 Not Reported
Ho:YAG Tm Fiber 1.905 2097 6.4 Not Reported 80
Ho:YLF Tm Fiber 1.94 2050 43 40 42
Ho:YAG Tm Fiber 1.908 2100 10 15 52
Ho:YAG Tm:YLF 1.908 2090 9.4 Not Reported 40
Ho:YAG Tm:YLF 1.91 2100 14 Not Reported 16
Ho:YAG Diode 1.91 2120 40 3.5 57
Ho:YAF Tm Fiber 1.94 2065 12.4 10.9 47
Ho:YAG Tm Fiber 1.908 2090 18.7 Not Reported 80
Table 2: Published output powers, pump wavelengths, and emission wavelengths for holmium-doped lasers (Scholle et al., 2010). 

Eye Safety

2µm wavelengths are included in the wavelength range deemed safe for eyes, which starts around 1.4µm. This range is deemed safe to the eye because laser radiation around 1.4µm-2.4µm is strongly absorbed in the vitreous body of the eye and does not reach the retina, which is responsible for sending nerve impulses to the brain. In addition, the intensity threshold for irreversible eye damage is much higher around 2µm than for shorter wavelengths, such as 1µm. While the user-friendly design and protection of the retina are beneficial, 2µm beams can also damage other parts of the eye besides the retina, requiring all laser safety precautions to still be taken.

Compatible Optical Materials

Optical materials with good transmission in the 2µm spectral range each have their own unique advantages that lend to various applications. Zinc Selenide (ZnSe) is arguably the most preferred material for lenses, windows, output couplers, and laser beam expanders operating at 2µm for its low absorptivity at infrared wavelengths and visible transmission. Transmission in the visible spectrum allows for a visible guide beam to be used along with a 2µm beam. Calcium Fluoride (CaF2) is another substrate option for 2µm lasers because its transmission is above 90% between 0.25-7µm, it is available in large sizes, and it is cheaper than similar substrates such as barium fluoride (BaF2). Other substrates that transmit at 2µm include IR-grade fused silica, germanium, magnesium fluoride (MgF2), N-BK7, potassium bromide (KBr), sapphire, silicon, sodium chloride (NaCl), and clear-grade zinc sulfide (also known as Cleartran™).

IR Material Comparison
Name Properties / Typical Applications
Calcium Fluoride (CaF2) Low Absorption, High Refractive Index Homogeneity
Used in Spectroscopy, Semiconductor Processing, Cooled Thermal Imaging
Fused Silica (FS) Low CTE and Excellent Transmission in IR
Used in Interferometry, Laser Instrumentation, Spectroscopy
Germanium (Ge) High nd, High Knoop Hardness, Excellent MWIR to FIR Transmission
Used in Thermal Imaging, Rugged IR Imaging
Magnesium Fluoride (MgF2) High CTE, Low Index of Refraction, Good Transmission from Visible to MWIR
Used in Windows, Lenses, and Polarizers that Do Not Require Anti-Reflection Coatings
N-BK7 Low-Cost Material, Works Well in Visible and NIR Applications
Used in Machine Vision, Microscopy, Industrial Applications
Potassium Bromide (KBr) Good Resistance to Mechanical Shock, Water Soluble, Broad Transmission Range
Used in FTIR spectroscopy
Sapphire Very Durable and Good Transmission in IR
Used in IR Laser Systems, Spectroscopy, and Rugged Environmental Equipment
Silicon (Si) Low Cost and Lightweight
Used in Spectroscopy, MWIR Laser Systems, THz Imaging
Sodium Chloride (NaCl) Water Soluble, Low Cost, Excellent Transmission from 250nm to 16μm, Sensitive to Thermal Shock
Used in FTIR spectroscopy
Zinc Selenide (ZnSe) Low Absorption, High Resistance to Thermal Shock
CO2 Laser Systems and Thermal Imaging
Zinc Sulfide (ZnS) Excellent Transmission in Both Visible and IR, Harder and More Chemically Resistant than ZnSe
Used in Thermal Imaging
Table 3: Comparison of the properties of common IR substrates.

To learn more about the promising applications of 2µm laser optics, download the whitepapers below. 

2µm Medical Laser Applications
2µm Medical Laser Applications
2µm Materials Processing Applications
2µm Materials Processing Applications

Scholle, Karsten, Samir Lamrini, Philipp Koopmann, and Peter Fuhrberg. "2 µm Laser Sources and Their Possible Applications." InTechOpen. InTech, 01 Feb. 2010. Web.

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