Near-infrared to mid-infrared tunable lasers

Different spectral range definitions.

Generally speaking, when people talk about infrared light sources, they are referring to light with vacuum wavelengths greater than ~700–800 nm (the upper limit of the visible wavelength range).

The specific wavelength lower limit is not clearly defined in this description because the human eye's perception of infrared slowly decreases rather than cuts off at a cliff.

For example, the response of light at 700 nm to the human eye is already very low, but if the light is strong enough, the human eye can even see the light emitted by some laser diodes with wavelengths exceeding 750 nm, which also makes infrared lasers a safety risk. --Even if it is not very bright to the human eye, its actual power may be very high.

Similarly, like the lower limit range of the infrared light source (700~800 nm), the upper limit definition range of the infrared light source is also uncertain. Generally speaking, it is about 1 mm.

Here are some common definitions of the infrared band:

Near-infrared spectral region (also called IR-A), range ~750-1400 nm.

Lasers emitted in this wavelength region are prone to noise and human eye safety issues, because the human eye focusing function is compatible with the near-infrared and visible light ranges, so that the near-infrared band light source can be transmitted and focused to the sensitive retina in the same way, but the near-infrared band light Does not trigger the protective blink reflex. As a result, the human eye's retina is damaged by excessive energy due to insensitivity. Therefore, when using light sources in this band, full attention must be paid to eye protection.

Short wavelength infrared (SWIR, IR-B) range from 1.4-3 μm.

This area is relatively safe for the eyes because this light is absorbed by the eye before it reaches the retina. For example, erbium-doped fiber amplifiers used in fiber optic communications operate in this region.

Mid-wave infrared (MWIR) range is 3-8 μm.

The atmosphere shows strong absorption in parts of the region; many atmospheric gases will have absorption lines in this band, such as carbon dioxide (CO2) and water vapor (H2O). Also because many gases exhibit strong absorption in this band Strong absorption characteristics make this spectral region widely used for gas detection in the atmosphere.

Long wave infrared (LWIR) range is 8-15 μm.

Next is far infrared (FIR), which ranges from 15 μm-1 mm (but there are also definitions starting from 50 μm, see ISO 20473). This spectral region is primarily used for thermal imaging.

This article aims to discuss the selection of broadband tunable wavelength lasers with near-infrared to mid-infrared light sources, which may include the above short-wavelength infrared (SWIR, IR-B, ranging from 1.4-3 μm) and part of the mid-wave infrared (MWIR, ranging is 3-8 μm).

Typical application

A typical application of light sources in this band is the identification of laser absorption spectra in trace gases (e.g. remote sensing in medical diagnosis and environmental monitoring). Here, the analysis takes advantage of the strong and characteristic absorption bands of many molecules in the mid-infrared spectral region, which serve as "molecular fingerprints". Although one can also study some of these molecules through pan-absorption lines in the near-infrared region, since near-infrared laser sources are easier to prepare, there are advantages to using strong fundamental absorption lines in the mid-infrared region with higher sensitivity.

In mid-infrared imaging, light sources in this band are also used. People usually take advantage of the fact that mid-infrared light can penetrate deeper into materials and has less scattering. For example, in corresponding hyperspectral imaging applications, near-infrared to mid-infrared can provide spectral information for each pixel (or voxel).

Due to the continued development of mid-infrared laser sources, such as fiber lasers, non-metallic laser materials processing applications are becoming more and more practical. Typically, people take advantage of the strong absorption of infrared light by certain materials, such as polymer films, to selectively remove materials.

A typical case is that indium tin oxide (ITO) transparent conductive films used for electrodes in electronic and optoelectronic devices need to be structured by selective laser ablation. Another example is the precise stripping of coatings on optical fibers. The power levels required in this band for such applications are typically much lower than those required for applications such as laser cutting.

Near-infrared to mid-infrared light sources are also used by the military for directional infrared countermeasures against heat-seeking missiles. In addition to higher output power suitable for blinding infrared cameras, broad spectral coverage within the atmospheric transmission band (around 3-4 μm and 8-13 μm) is also required to prevent simple notched filters from protecting infrared detectors.

The atmospheric transmission window described above can also be used for free-space optical communications via directional beams, and quantum cascade lasers are used in many applications for this purpose.

In some cases, mid-infrared ultrashort pulses are required. For example, one could use mid-infrared frequency combs in laser spectroscopy, or exploit the high peak intensities of ultrashort pulses for lasing. This can be generated with a mode-locked laser.

In particular, for near-infrared to mid-infrared light sources, some applications have special requirements for scanning wavelengths or wavelength tunability, and near-infrared to mid-infrared wavelength tunable lasers also play an extremely important role in these applications.

For example, in spectroscopy, mid-infrared tunable lasers are essential tools, whether in gas sensing, environmental monitoring, or chemical analysis. Scientists adjust the wavelength of the laser to precisely position it in the mid-infrared range to detect specific molecular absorption lines. In this way, they can obtain detailed information about the composition and properties of matter, like cracking a code book full of secrets.

In the field of medical imaging, mid-infrared tunable lasers also play an important role. They are widely used in non-invasive diagnostic and imaging technologies. By precisely tuning the wavelength of the laser, mid-infrared light can penetrate biological tissue, resulting in high-resolution images. This is important for detecting and diagnosing diseases and abnormalities, like a magical light peering into the inner secrets of the human body.

The field of defense and security is also inseparable from the application of mid-infrared tunable lasers. These lasers play a key role in infrared countermeasures, especially against heat-seeking missiles. For example, the Directional Infrared Countermeasures System (DIRCM) can protect aircraft from being tracked and attacked by missiles. By quickly adjusting the wavelength of the laser, these systems can interfere with the guidance system of incoming missiles and instantly turn the tide of the battle, like a magic sword guarding the sky.

Remote sensing technology is an important means of observing and monitoring the earth, in which infrared tunable lasers play a key role. Fields such as environmental monitoring, atmospheric research, and Earth observation all rely on the use of these lasers. Mid-infrared tunable lasers enable scientists to measure specific absorption lines of gases in the atmosphere, providing valuable data to help climate research, pollution monitoring and weather forecasting, like a magic mirror that provides insights into the mysteries of nature.

In industrial settings, mid-infrared tunable lasers are widely used for precision material processing. By tuning lasers to wavelengths that are strongly absorbed by certain materials, they enable selective ablation, cutting or welding. This enables precision manufacturing in areas such as electronics, semiconductors and micromachining. The mid-infrared tunable laser is like a finely polished carving knife, allowing the industry to carve out finely carved products and show the brilliance of technology.