1. Light Source (Laser)
The basic components of a laser can be divided into three parts: a pump source (which provides energy to achieve population inversion in the working medium); a working medium (which has a suitable energy level structure that enables population inversion under the action of the pump, allowing electrons to transition from high energy levels to the lower level and release energy in the form of photons); and a resonant cavity.
The properties of the working medium determine the wavelength of the laser light emitted.
The mainstream laser with an 808nm wavelength is a semiconductor laser. The band gap energy of the semiconductor determines the wavelength of the laser light emitted, making 808nm a relatively common operating wavelength. The 808nm type of semiconductor laser is also one of the earliest and most intensively researched. Its active region consists of either aluminum-containing materials (such as InAlGaAs) or aluminum-free materials (such as GaAsP). This type of laser offers advantages such as low cost, high efficiency, and long life.
1064nm is also a classic wavelength for solid-state lasers. The working material is a neodymium (Nd)-doped YAG (yttrium aluminum garnet Y3AI5012) crystal. The aluminum ions in the YAG crystal interact synergistically with the Nd-doped cations, creating a suitable spatial structure and energy band structure. Under the action of excitation energy, the Nd cations are excited into an excited state, undergoing radioactive transitions and generating lasing. Furthermore, Nd:YAG crystals offer excellent stability and a relatively long operating life.
1550nm lasers can also be generated using semiconductor lasers. Commonly used semiconductor materials include InGaAsP, InGaAsN, and InGaAlAs.
2. Uses & Applications
The infrared band has numerous applications, such as optical communications, healthcare, biomedical imaging, laser processing, and more.
Take optical communications as an example. Current fiber-optic communications utilize quartz fiber. To ensure that light can carry information over long distances without loss, we must consider which wavelengths of light are best transmitted through the fiber.
In the near-infrared band, the loss of ordinary quartz fiber decreases with increasing wavelength, excluding impurity absorption peaks. Three wavelength "windows" with very low loss exist at 0.85 μm, 1.31 μm, and 1.55 μm. The emission wavelength of the light source laser and the wavelength response of the photodetector photodiode must align with these three wavelength windows. Specifically, under laboratory conditions, the loss at 1.55 μm has reached 0.1419 dB/km, approaching the theoretical loss limit for quartz fiber.
Light in this wavelength range can penetrate biological tissue relatively well, and has applications in areas such as photothermal therapy. For example, Yue et al. constructed heparin-folate targeted nanoparticles using the cyanine near-infrared dye IR780, which has a maximum absorption wavelength of approximately 780 nm and an emission wavelength of 807 nm. At a concentration of 10 mg/mL, laser irradiation (808 nm laser, 0.6 W/cm² power density) for 2 minutes increased the temperature from 23°C to 42°C. A 1.4 mg/kg dose was administered to mice bearing folate receptor-positive MCF-7 tumors, and the tumors were irradiated with 808 nm laser light (0.8 W/cm²) for 5 minutes. Significant tumor shrinkage was observed over the following days.
Other applications include infrared lidar. The current 905 nm wavelength band has weak weather interference capabilities and insufficient penetration into rain and fog. Laser radiation at 1.5 μm falls within the atmospheric window of 1.5–1.8 μm, resulting in low attenuation in air. Furthermore, 905 nm falls within the eye-hazardous band, requiring power limitation to minimize damage. However, 1550 nm is eye-safe, so it also finds applications in lidar.
In summary, lasers at these wavelengths are both mature and cost-effective, and they exhibit excellent performance in various applications. These factors combined have led to the widespread use of lasers in these wavelengths.