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.
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 (%)|
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
|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: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).
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.