Laser is the abbreviation of "light amplification through stimulated emission of radiation". The unique characteristics of laser beam output are: the electromagnetic energy and output wave have good monochromaticity, high coherence, and consistency in phase, time, and propagation direction. This is true whether the laser output is in the visible or invisible part of the spectrum. Most lasers have a fixed output wavelength (λ), but some can also set the wavelength to one of several discrete values.
In May 1960, physicist Theodore H. Maiman of the Hughes Research Laboratory in Malibu, California, USA demonstrated the world's first laser. He used ruby ??(CrAlO3) and a photographic flash lamp as a laser pump source, producing a red beam with a wavelength of 694 nm. A 30-year patent dispute has raged between three physicists over who should claim scientific discovery and patent rights for a laser concept.
A laser has three basic components:
The essential condition for generating laser light is to excite most of the electrons in the resonant cavity to a higher energy level, which is called particle number inversion. change. For electrons, this is an unstable state. Therefore, after briefly staying in this state, they decay back to their original energy state in two ways:
This stimulated transition releases energy in the form of photons, and the stimulated emission of photons has the same energy as the incident photon. phase, wavelength and direction of propagation. The emitted photon propagates back and forth in the optical resonant cavity and passes through the laser material between the total reflection mirror and the partial reflection mirror. This causes the light energy to continue to increase until sufficient energy is accumulated, and a laser beam passes through the partial reflection mirror. Launched.
As with all component selections, there is no single "best" laser because different applications require different wavelengths, power levels and other specifications, often depending on the physical factors of the specific situation. Helium-neon lasers are often well suited for numerous industrial and testing projects, such as Raman spectroscopy—a non-destructive optical inspection technique that does not require direct physical contact with the sample.
This spectroscopic analysis provides fast and accurate chemical analysis of solids, powders, liquids and gases and is suitable for materials analysis, microscopy, pharmaceuticals, forensic identification, food fraud identification, chemical process monitoring and various applications. a homeland security function. For these applications, helium-neon lasers have many attractive properties: stable output wavelength and power, super monochromatic red output at λ = 632.8 nm (often simplified to 633 nm), narrow beams, low divergence, and no random Good output coherence and stability over distance and time.
A helium-neon laser consists of a hollow glass tube with an inward-facing mirror. The tube is filled with 85-90% helium and 10-15% neon (the actual laser medium) at a pressure of approximately 1 Torr (0.02 lb/in2). There are also two inward reflectors in the glass tube, which are placed at both ends of the discharge tube. One of them is a highly reflective plane mirror, and the other is a concave output coupling mirror with a transmittance of about 1% (Figure 1).
During the pumping process, a high-voltage pulse (approximately 1000 V to 1500 V DC, 10 to 20 mA) is applied to the mixed gas to discharge. The actual lasing results from carrier deexcitation between electron shell levels of Ne atoms (e.g. transition from 3s to 2p). The transition from 3s to 2p results in a primary output at 632.8 nm. Additionally, other energy level transitions occur, resulting in outputs at 543 nm, 594 nm, 612 nm, and 1523 nm, but the 632.8 nm output is the most useful.
In the early days of laser development, laser units and power supplies were often handmade. Today, lasers are readily available off-the-shelf components, especially widely used products such as helium-neon gas lasers. Moreover, such devices are available in a wide range of power ratings, as exemplified by the two lasers in Excelitas Technologies' REO series.
The first example is the model 31007, which is at the lower end of the power range of the series and is capable of delivering 0.8 mW (minimum) with a beam diameter of 0.57 mm and a beam divergence of 1.41 mrad (Figure 2 ). This laser tube requires 1500 V/5.25 mA during operation, is approximately 178 mm long, 44.5 mm in diameter, and has a Center for Devices and Radiological Health (CDRH)/CE safety level IIIa/3R.
At the higher end of the REO Series power range, the Model 30995 is a 17 mW (typ), 25 mW (max) laser requiring 3500 V/7 mA applied. The laser tube is approximately 660 mm long, with a beam width of 0.92 mm and a divergence of 0.82 mrad. This laser has the stricter IIIb/3B CDRH/CE safety rating.
There are many reasons to choose the lowest power laser that will do the job. Lower power means fewer safety hazards, lower regulatory requirements, smaller laser tubes, lower costs, and smaller power supplies.
Power supply is critical to the performance of laser devices. For HeNe lasers, the laser tube first requires an application of approximately 10 kV DC (breakdown voltage) to initiate the excitation process. In addition, a steady-state maintenance voltage of 1 to 3 kV DC is required, as well as a current of less than 10 mA. Although the power levels are not that high (only 20 to 30 W), few engineers have the resources, training, or time to design a suitable power supply for this voltage, especially given safety and regulatory requirements and concerns about creepage distances and Certification of factors such as electrical clearance, in addition to basic electrical and electromagnetic (EMI) performance.
Why is the starting voltage higher than the sustaining voltage? The helium-neon laser is a "negative resistance" component. As the current increases, the voltage across the laser tube will decrease. The same problem occurs with simple neon light bulbs, such as the famed but now obsolete NE-2 "nixie tube" bulb. Its breakdown or "arcing" voltage is approximately 90 V (AC or DC), after which the operating voltage drops to approximately 60 V. In the past, one way designers used to provide a higher startup voltage and then a lower operating voltage was to use a series ballast resistor of approximately 220 kΩ (Figure 3).
However, this simple solution does not work for helium-neon laser tubes in commercial applications. The first is safety and regulatory requirements. Second, the power supply must be properly matched to the laser tube for optimal performance, and the starting voltage must remain within tolerances. Thirdly, the stability of the power supply output voltage and current is crucial to maintaining the stability of the laser.
For these reasons, Excelitas Technologies offers plug-and-play power supplies for lower power helium-neon lasers that meet technical and regulatory requirements. For example, the 39783 power supply operates from 100 to 130 V AC and 200 to 260 V AC (50 to 400 Hz), provides 1500 to 2400 V, starts at a voltage above 10 kV DC, and operates at 5.25 mA (Figure 4). Tight current regulation is important for stable helium-neon laser tube performance, so the 39783 maintains it at 0.05 mA. The power supply has a modest footprint of 241 x 133 mm and a height of 54 mm. It also comes with a physical key lock for security.
For larger helium-neon laser tubes, Excelitas has introduced the 39786 power supply in the same package size. The power supply offers a higher output of 3200 to 3800 V, with a starting voltage of over 12.5 kV and a DC current of up to 7.0 mA.