Near-infrared to mid-infrared tunable laser selection plan

This article aims to discuss some considerations and program suggestions when selecting near-infrared to mid-infrared light sources. This article mainly briefly introduces and compares the four major categories of optical parametric oscillators (OPO), optical parametric amplifiers (OPA), quantum cascade lasers, and supercontinuum light sources.

1.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).

2.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 can see 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.

3.Near-infrared to mid-infrared tunable laser product types and selection characteristics

Many technologies can produce near-infrared to mid-infrared lasers, such as various types of lead salt lasers based on early ternary lead compounds or quaternary compounds, as well as common doped insulator bulk lasers, various fiber lasers, and carbon dioxide gas lasers. Wait, here we focus on several laser principle technologies and products that can be tuned in a wide range of wavelengths from near-infrared to mid-infrared.

①Optical parametric oscillators, amplifiers and generators (OPO and OPA)

In a nonlinear frequency conversion system, a near-infrared laser, pump optical parametric oscillator (OPO), amplifier (OPA) or generator (OPG) can be used to generate idler light in the mid-infrared spectral region, such as:
In nanosecond OPO mid-infrared lasers, Q-switched lasers can be used as pump sources. Common crystalline materials used for such applications are zinc germanium phosphide (ZGP, ZnGeP2), silver gallium sulfide and selenide (AgGaS2, AgGaSe2), gallium selenide (GaSe) and cadmium selenide (CdSe).
Since many of these materials are opaque in the 1 μm region, it is often necessary to use OPOs in series: the first OPO converts the 1 μm laser radiation to a longer wavelength, which is then used to pump the actual mid-infrared OPO. The latter's signal and idle frequency can both be in the mid-infrared spectrum region.
The 1064 nm mode-locked picosecond Nd:YVO4 laser can also be used to synchronously pump OPO and LiNbO3 crystals, allowing idler light output to reach 4 μm or even 4.5 μm. Its wavelength limitation is mainly superior to increasing idler light absorption at long wavelengths. Therefore, OPOs based on this principle usually have a resonance signal. Such a device could easily generate pulses with energy in the tens of millijoules. The output wavelength is tunable over hundreds of nanometers.

②CWOPO

Compared with the pulse excitation of general OPO, recent CWOPO technology products provide mid-infrared lasers based on the following framework:

1) DFB fiber lasers and amplifiers;

2) DFB fiber laser control;

3) OPO optical part and control;
This type of product can provide continuously adjustable output wavelength in the mid-infrared range of 1435-4138 nm (6969-2416 cm-1). At the same time, compared to pulse OPO, this type of product can provide excellent line width. (<100 MHz). This makes it possible for such products to be optimized in applications such as infrared calibration and spectral analysis.

③Quantum cascade laser

Quantum cascade lasers are a relatively new development direction in the field of semiconductor lasers.

The difference between quantum cascade lasers and early mid-infrared semiconductor lasers based on inter-band transitions is that it works based on inter-sub-band transitions.

This enables quantum cascade lasers to engineer the details of the semiconductor layer structure so that the transition photon energy (and therefore wavelength) can be varied over a wide range. In addition, some important wavelength tuning ranges (sometimes exceeding 10% of the central wavelength) can also be covered through external cavity devices.

Although cryogenic cooling is currently required to achieve optimal performance, many quantum cascade lasers can still be operated at room temperature, even continuously. Quantum cascade lasers can also be used to generate pulsed lasers with pulse times even well below 1 ns, although the peak power is rather limited.

In terms of power, although its output power can reach 1 W through optimization, the output power of this type of laser is still lower than that of common infrared lasers. Because, in the field of quantum cascade lasers, which are mainly used in spectroscopy, quantum cascade lasers are limited to transitions with lower phonon energy.

Here are some common parameters and types:
CW-DFB laser tube 800 cm-1-2320 cm-1
Pulsed DFB laser tube 700 cm-1-2350 cm-1
Refrigerated DFB laser tube 645 cm-1-2370 cm-1

OPO (optical parametric oscillator) and quantum cascade are two commonly used technologies in mid-infrared laser generation, and they have some significant application differences.

OPO (Optical Parametric Oscillator, optical parametric oscillator)：

OPO is a nonlinear optical device that uses parametric processes in nonlinear optical crystals or optical fibers to generate new wavelengths, including the mid-infrared band. OPO excites parametric oscillations through a pump light source, where nonlinear materials in the oscillator split the pump light into signal light and auxiliary light. The signal light wavelength is tunable into the mid-infrared range, while the auxiliary light acts as feedback to the pump light source. OPO has high conversion efficiency and wide frequency tuning range, so it is widely used in mid-infrared laser research and applications.

Application difference: OPO is suitable for applications requiring frequency tunability. By adjusting the frequency of the pump light or the phase matching conditions of the nonlinear crystal, continuously tunable laser output can be achieved in the mid-infrared range. OPO can be used in spectral analysis, gas detection, biomedical imaging and other fields, and is especially suitable for applications that require high-sensitivity analysis or microscopic imaging in the mid-infrared band.

Quantum Cascade:

The quantum cascade laser is a laser based on a semiconductor superlattice structure that generates mid-infrared laser light through a quantum cascade process. In a quantum cascade laser, electrons release energy through a step-by-step transition process between multiple energy bands, producing continuously tunable mid-infrared radiation.

Application differences: Quantum cascade lasers have higher power and narrower spectral linewidth, and are suitable for high-resolution spectral measurement, lidar, infrared imaging and other fields. Quantum cascade lasers can also work in high-temperature environments, so they are suitable for applications that require mid-infrared lasers under harsh conditions, such as industrial inspection, environmental monitoring, etc.

To sum up, OPO is mainly used for applications with high frequency tunability, while quantum cascade lasers are more suitable for high power, narrow linewidth and high temperature.

The specific comparison of parameter value differences varies by product model and manufacturer. The following are examples of some common parameter comparisons:

——Frequency tunability:

OPO: Continuously tunable mid-infrared laser output can be achieved, with a frequency range usually from hundreds of megahertz to several gigahertz or wider.

Quantum cascade: The frequency tuning range is relatively narrow, usually tens to hundreds of megahertz or narrower.

——Output power and efficiency:

OPO: The output power is usually in the range of several hundred milliwatts to several watts, and the conversion efficiency can reach more than 10%.

Quantum cascade: The output power is usually in the range of tens to hundreds of milliwatts, and the conversion efficiency can reach more than 20%.

——Spectral linewidth:

OPO: The spectral linewidth is narrow, usually in the range of several gigahertz to tens of megahertz.

Quantum cascade: The spectral linewidth is relatively broad, usually in the range of tens of gigahertz to hundreds of megahertz.

——Operating temperature:

OPO: It usually needs to work at a more stable room temperature or close to room temperature.

Quantum cascade: Can work at higher operating temperatures, usually above room temperature, even up to tens of degrees Celsius.

It should be noted that these values are only for general reference and do not represent the specific parameters of all commercial products. Actual parameters depend on product model, technological advances, and manufacturer's design and performance requirements. When selecting a specific commercial product, it is best to refer to the product specification sheet and technical documentation provided by the manufacturer for accurate parameter information.

④Supercontinuum light source

There are some light sources based on supercontinuum generation that span a large portion of the mid-infrared band. Such a light source could operate based on certain mid-infrared optical fibers, through which intense light pulses are sent to create strong nonlinear interactions.

If tunable narrow linewidth light is required, tunable filters can be used to extract the desired spectral components from the broad spectrum light. In some cases, the entire spectrum is utilized. One example is optical coherence tomography (OCT). This process is often performed at shorter wavelength bands. However, the advantage of mid-infrared light in this application is that mid-infrared light is less scattered. Compared with shorter wavelength bands, it has The ability to penetrate deeper.

Currently, the most popular commercial mid-infrared (mid-IR) light sources are optical parametric oscillators (OPOs) [1] and amplifiers (OPAs) [2], and quantum cascade lasers (QCLs) [3]. They have achieved very good performance and proven useful in many important applications. However, it should be noted that OPO/OPA are complex, susceptible to vibration, require frequent maintenance, and are difficult to scale up. QCLs can cover a significant emission band of ~3.5–12 μm, but they emit low output power with limited tunability per laser output wavelength. This has led to the need to find new alternative solutions for these laser sources. In this context, high-power mid-infrared supercontinuum generators appear to be of great interest, mainly due to their unique properties, the most important of which are their broad spectrum spanning thousands of nanometers, high spectral power density (>1 mW/ nm ), it has wider bandwidth, higher spatial coherence, directionality and brightness than traditional lasers.

⑤Micro mid-infrared light source

There are currently many attempts to develop photonic integrated circuits for mid-infrared applications, such as those based on silicon photonics platforms. Unfortunately, it is not easy to implement a mid-infrared light source on a chip, which has led to research on many possible methods. One example is integrating light sources onto other semiconductors, and although this presents technical difficulties, there are also examples involving flip-chip bonding technology. Another possibility is to integrate blackbody emitters (→ thermal radiation) or luminescent materials, although this does not result in spatially coherent radiation.

There are other methods based on nonlinear frequency conversion, utilizing Kerr nonlinearity for four-wave mixing or stimulated Raman scattering. And using microresonators, frequency combs can also be generated.

besides

The following are some mid-infrared light sources that are less frequently used. Because they are not widely used, they will not be discussed in too much detail here, such as free electron lasers and frequency-doubled CO₂ lasers.

Based on the above, the following is a reference for comparison and selection of various laser types: