Lasers and how to measure their output (2023)

Laser is an acronym for light amplification by stimulated emission of radiation. Lasers, of course, started out as comprised of discrete components. The most basic laser consists of a cylinder containing the lasing medium, a material that emits light when exposed to an outside source of energy. The gain medium can be gas, liquid, solid or plasma. The first working laser had for a gain medium a doped synthetic ruby rod. Today many lasers have gas lasing media.

Lasers and how to measure their output (1)The cylinder is known as the optical cavity and is sized so as to resonate at the desired light frequency. At either end of the cylinder is a mirror. The reflective surfaces of the mirrors face inward and are precisely aligned toward one another. The reflected surfaces allow photons to bounce back and forth through the gain medium, their energy rising from the outside energy source which is known as the pump. One mirror is totally reflective. The other is typically 95% reflective, allowing a portion of the photons to be emitted to form the coherent laser beam.

The lasing process begins with the pumping of energy into the gain medium. The excitation mechanism is often light. Pumping puts most of the gain medium molecules into an excited state. Some of the pumped-in energy is stored in electrons within the excited molecules. This energy gets released as electrons spontaneously drop to lower or ground state. Some of the photons released by this spontaneous emission leave the gain medium through the sides, while other photons act as triggers to produce stimulated emission, arising when released photons pass near molecules still in the excited state.

Photons released by stimulated emissions are coherent and traveling in the same direction. Thus there results an avalanche of photons as light travels through the active medium. The photons strike the partially reflective mirror which allows a percentage of light to exit as the laser beam. The rest of the light reflects back into the gain medium. The lasing process continues so long as energy is pumped into the gain medium and molecules remain in an excited state.

There are, however, variations. In some designs, the mirrors are omitted because the photons gain sufficient energy in a single pass.

Lasers can be characterized as continuous mode or pulsed mode, depending upon whether the output is on all the time or whether it operates on a fixed duty cycle. A continuous-mode laser can operate in pulse mode, but the pulse-mode laser cannot usually be operated at rated power in continuous mode because it would quickly overheat. One rationale for the pulse-mode laser is that it is capable of putting out more powerful short pulses compared to a continuous mode laser.

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Lasers and how to measure their output (2)

There are many different types of lasers, classified primarily according to the lasing medium. Gas lasers are common, particularly the helium-neon (HeNe) laser, which can operate over a variety of frequencies. Most are designed to operate at 633 nm. They are relatively inexpensive and ubiquitous in optical research facilities.

Carbon dioxide (CO2) lasers can put out hundreds of watts, which can be concentrated at a small target, making for enormous temperature rise. The emission is infrared, at 10.6 μm, and the efficiency is relatively high, typically greater than 30%. A major application is metal cutting and welding.

Metal-ion gas lasers, such as helium-silver (HeAg) and neon-copper (NeCu), operate in the ultraviolet regime. The gain media have narrow (0.5 pm) spectral emission, so they are used in specialized spectroscopy applications.

Chemical lasers are powered by chemical reactions, essentially explosions, so many of these are single-use devices.

Solid-state lasers, with crystalline or doped-glass rods, are successors of the original chromium-doped ruby lasers. Population inversion takes place in the dopant. They are used for cutting and welding as well as spectroscopy and pumping dye lasers. These are not to be confused with semiconductor (diode) lasers, which produce infrared beams.

Dye lasers use an organic dye in a liquid solution as the lasing medium. The advantage over gas and solid-state lasing media is that a wider range of wavelengths is possible often spanning in excess of 100 nm. This characteristic is useful in tunable and pulsed lasers. The pulses can be as short as 16 fsec, permitting enormous peak power.

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Besides liquid lasing media, dye lasers sometimes take the form of solid-state lasers, with organic dye as the dopant. A high-energy source is needed to pump the lasing medium above its lasing threshold. This source can be a fast discharge flashtube or another laser. The resonant cavity is usually water-cooled. Because the presence of light can decompose organic dyes, the lasing solution is usually recirculated through a large external tank.

Then there are diode lasers. Here a p-n junction is formed by two layers of doped gallium arsenide. The length of the p-n junction bears a precise relationship with the wavelength of the light to be emitted. A highly reflective surface sits at one end of the p-n junction and a partially reflective surface at the other, forming a resonant cavity for the photons.

In operation, the laser diode is forward-biased. As electrons move through the junction, there is charge carrier recombination just as in an ordinary diode. As electrons fall into holes to recombine, they release photons. A released photon can strike an atom, causing another photon release. As the forward current rises, more electrons enter the depletion region and cause more photon emissions. Eventually some of the photons randomly drifting within the depletion region strike the reflected surfaces perpendicularly. These reflected photons move along the depletion region, striking atoms and releasing additional photons via the avalanche effect.

The back-and-forth movement of photons rises as more photons get generated until laser light forms from the photons that pass through the partially reflective end of the p-n junction. Photons produced this way have the same energy level, phase relationship, and frequency. Laser diodes have a threshold level of current above which the laser action happens and below which the diode behaves essentially as an ordinary LED.

There are several types of diode lasers. The double heterostructure laser diode has an additional confinement layer of a different material sandwiched between the two p and n-type materials. Each junction between different materials is called a heterostructure. The advantage of this structure is that it confines the active region to a thin layer which gives better optical amplification.

The quantum well laser diode has a thin middle layer which acts as a quantum well. Electrons use the quantum energy levels when dropping from higher to lower energy levels. This action improves efficiency.

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The motivation for the separate confinement heterostructure laser diode is that the thin middle layer in the quantum-well laser diode is so small that it harms the ability to confine emitted light effectively. To compensate, the separate confinement heterostructure laser diode adds two layers over the three initial layers. These layers have a lower refractive index and help confine the emitted light.

All the previously discussed laser diodes have an optical cavity perpendicular to the current flow. In vertical cavity surface emitting laser diodes (VCSEL), the optical cavity is along the axis of current flow. The partially reflecting mirrors sit near the ends of optical cavity.

The primary measuring device for lasers is the optical power meter (or laser power meter). It measures the optical power (delivered energy per unit time) in a light beam. Typically, it displays only the average power when receiving a pulse train with a high pulse repetition rate. Other instruments, called optical energy meters, measure pulse energies.

Lasers and how to measure their output (4)

Most power meters are based on the principle of a thermal detector: They convert optical power to heating power via an absorber structure with a black coating. They use a thermopile to measure the resulting temperature rise (or actually the temperature difference between the absorber and the mount). Thermal power meters are useful for average powers between about 10 mW and several kilowatts. Thermal power meters are moderately precise, have a sensitivity that is fairly independent of wavelength, and are relatively slow.

Optical power meters can also be made with photodiodes, usually based on silicon, germanium for near infrared, and indium gallium arsenide, though the latter type can be expensive. When a laser is directed at a photodiode detector, the light creates a current proportional to the light intensity and dependent on the wavelength. Photodiode optical power meters can measure power in the microwatt region or possibly even lower and can measure higher optical powers with a suitable attenuator. But they are more fragile than thermal power meters because they are more easily damaged by high optical intensities.

The most common specifications for laser power meters include average power, power density, energy, energy density, repetition rate, and so forth. Meter manufacturers often advise users to let the laser of interest reach a stable point before measuring its power, typically between 20 and 30 minutes, because the properties of the optical components tend to change as the laser reaches equilibrium. Other advice includes ensuring the beam is 40 to 60% of the meter’s optical aperture because a beam too small increases the possibility the detector will be damaged during the measurement. This is particularly true for photodiode-based devices; even before damage occurs, there may be local saturation effects that cause bad readings.

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It is also good to keep in mind that the thermal-based detectors generally have a NIST-traceable sensitivity value valid for only one specific laser wavelength. Lasers somewhere else in the detector’s spectral range need a wavelength correction factor. The correction is often automatic but can also be a manual process.

Photodiode-based meters may also handle a wide wavelength range. Those with Si-based diodes generally work from near UV to somewhat beyond IR-A, but with varying responsivity.

Electrical noise sets the minimum measurement range, and the noise typically comes from the sensor head. Manufacturers generally set the lowest range to be such that measurement uncertainty stays at least 10 to 20 dB below the maximum power. Photodiode-based power sensors are more sensitive than thermal versions–their lowest measurement ranges are often in the microwatt region or less. Sensitive measurements like this generally mean working in a dark laboratory.

Typical measurement accuracies are on the order of ±3% or ±5% for powers near the maximum in a specific measurement range. Measurements well below the maximum may be less accurate.

Finally, because they use heating as the measurement of optical power, thermal power sensors are relatively slow, on the order of 0.2 sec to a few seconds. Ordinary photodiode-based power meters are faster but not appreciably so.

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