What is Laser Pumping?
Laser pumping refers to introducing energy into a laser system to produce a population inversion, where there are more atoms or molecules in an excited state than in the ground state. This increases the probability of stimulated emission of light and enables lasing to occur.
Pump It Up
Depending on the laser type, pumping can be achieved through various methods, including optical pumping, electrical pumping, and chemical pumping. Regardless of the pumping method used, the key to achieving laser action is to produce a population inversion in the gain medium. That’s because laser action is based on a process first described by Einstein called stimulated emission. This occurs when an excited atom or molecule in the gain medium is stimulated by an incoming photon to emit a second photon of the same wavelength and phase.
This amplification process results in coherent light that is in phase and has a single wavelength and direction. However, the probability of the excited particle undergoing stimulated emission depends very strongly on how many excited particles there are. So you need more of these in the excited state than the ground state. Otherwise, other mechanisms will dominate and the pumped energy is lost instead as heat or random light (spontaneous emission). The crossover point is sometimes called the pumping threshold.
Let’s take a look at how this works in the most common types of lasers.
Laser types are usually categorized by their choice of gain medium. This is the material that actually converts the pump energy into laser light. The gain medium may be a solid-state crystal or glass, a semiconductor chip, a gas plasma, or a liquid.
Optical Pumping Solid State Lasers
In optical pumping, the excitation light must have a wavelength that matches the absorption spectrum of the gain medium. When the gain medium absorbs the excitation light, its electrons are promoted to higher energy levels, resulting in a population inversion.
Optical pumping is the most common method of pumping solid-state lasers where the gain medium is a piece of glass or crystal. For many years the excitation light was provided by intense flashlamps, which are high-intensity light sources that emit short bursts of light. Flashlamps typically emit intense white light, which is then focused onto the gain medium. The first ever laser was a solid-state laser of this type: a ruby laser pumped by a flashlamp.
Diode-Pumped Solid-State (DPSS) Lasers
Unfortunately, flashlamps produce light across a broad spectrum of wavelengths, but a solid-state gain medium usually only absorbs at one or more very specific wavelengths. So most of the energy from the flashlamps ends up as heat. This requires active water cooling. It also limits the ability to adjust the laser power without spoiling the quality of the output beam because of a problem called thermal lensing.
A solution that reduced this heating problem was found by replacing the flashlamps with diode lasers, semiconductor chips that are electrically pumped – see below. The diode lasers are designed to produce light only at the wavelength where the solid-state gain medium is known to absorb light. This type of laser is not surprisingly called a diode-pumped solid state (DPSS) laser.
Optical Pumping of Other Lasers
In a dye laser, the gain medium is in a liquid form: a solvent containing a fluorescent dye. These lasers are optically pumped, sometimes by another laser, sometimes by flashlamps. Always a minor technology, dye lasers were previously used in scientific research because of their wavelength tunability. But today most applications needing wavelength tuning have moved to solid-state alternatives based on titanium: sapphire (Ti:S) or ytterbium gain media. However, pulsed dye lasers pumped by flashlamps are occasionally used in a handful of niche applications such as lithotripsy.
Titanium: sapphire lasers are solid-state lasers where the gain medium is a sapphire crystal doped with titanium ions. These lasers are optically pumped by some type of green laser. They are widely used in science because their broad wavelength range lasers supports applications needing simple tunability, such as fluorescent microscopy and flow cytometry. It also enables pulsed operation with pulses as short as a few femtoseconds, using a method called mode-locking.
Other lasers use optical pumping by diode lasers, including ytterbium-doped glass and ytterbium-doped fiber, as well as lasers based on fibers doped with other rare earth metals.
Electrical Pumping of Gas Lasers
Electrical pumping is another method of laser pumping; it involves passing an electric current through the gain medium to excite the atoms or molecules. This is the pumping mechanism used in virtually all gas lasers, where the flow of electricity through a low-pressure gas creates a plasma.
Electrical pumping is used to power excimer lasers. These are powerful gas lasers that emit pulses of deep UV laser light with very high pulse energies. The unique performance regime of excimer lasers is key to several critical processes in the manufacturing of high-performance displays, including those based on OLED and the latest micro-OLED technologies. Excimer lasers are also used in refractive ophthalmic procedures (e.g., LASIK) used to correct vision issues. In addition, they are becoming established as workhorse lasers sources in many emerging pulsed laser deposition (PLD) applications.
Continuous-wave (CW) gas lasers such as argon ion lasers and helium-neon lasers were standout examples that relied on electrical pumping and at one time dominated laser applications needing visible wavelengths. While they produced a high-quality beam, their limited wavelength choices, and extremely low electrical efficiency meant that they are only niche products today. Their former applications are often now serviced by semiconductor lasers, DPSS lasers, or optically-pumped semiconductor lasers (OPSLs) – see below.
Electrical Pumping of Semiconductor Lasers
Electrical pumping is commonly used in diode lasers, which are often also called semiconductor lasers, where a p-n junction is used to create a population inversion. A p-n junction is a boundary between two types of semiconductors, where the p-type semiconductor has an excess of positively charged holes, and the n-type semiconductor has an excess of negatively charged electrons. When a voltage is applied across the p-n junction, electrons and holes are injected into the semiconductor, creating a population inversion and resulting in laser light.
The small size and relatively low cost of diode lasers mean that today they are by far the most common laser type using electrical pumping. And the diode lasers themselves are then widely used to pump other laser types. High-power diode lasers are also directly used in applications such as plastic welding and metal cladding/hardening.
Optically Pumped Semiconductor Lasers
This brings us to an important and unique type of laser called the optically pumped semiconductor laser, or OPSL. This laser includes a special type of semiconductor chip that is pumped not by electricity, but by the light from one or more diode lasers. The OPSL has several unique advantages. Its semiconductor details can be designed for any specific wavelength across a wide part of the near-infrared spectrum. The near-IR wavelength can then be frequency-doubled to a visible or even frequency-tripled to deliver UV output, giving models at an unmatched choice of wavelengths. Just as important, the output power can be scaled from a few milliwatts to as much as 20 watts.
Examples of OPSLs include the Verdi, Sapphire, Genesis, and OBIS families of lasers from Coherent. These lasers are widely used in life sciences, particularly for flow cytometry and confocal microscopy. OPSLs are also used in spectacular multi-color laser light shows because they enable a broader palette of colors than any other laser type.
Chemical pumping is a rarely used method of laser pumping, and it involves using a chemical reaction to produce a population inversion in the gain medium. Chemical pumping is used in very specialized gas lasers, where a chemical reaction is used to excite the atoms or molecules in the gas. The most common chemical pumping method is the combustion of hydrogen and fluorine gas in a chemical laser, which results in a population inversion and laser light.
In conclusion, laser pumping is a critical process for the production of coherent, high-intensity light in laser systems. Whether achieved through optical, electrical, or chemical means, the key to laser pumping is the production of a population inversion in the gain medium, which allows for stimulated emission and the production of laser light.