optical pumping (original) (raw)

Author: the photonics expert (RP)

Definition: electronically exciting a medium with light, or specifically populating certain electronic levels

Categories: article belongs to category laser devices and laser physics laser devices and laser physics, article belongs to category quantum photonics quantum photonics, article belongs to category methods methods, article belongs to category physical foundations physical foundations

Related: Tutorial on Fiber Amplifiers Part 5: Forward and Backward Pumpingin-band pumpingend pumpingside pumpingpump absorptionRabi oscillationsdiode-pumped laserslamp-pumped lasers

Page views in 12 months: 2749

DOI: 10.61835/wkk Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

Content quality and neutrality are maintained according to our editorial policy.

Contents

What is Optical Pumping?

Optically Pumped Lasers and Amplifiers

Pump Sources for Optical Pumping

Various Aspects

Efficiency of Pump Absorption

Using Multiple Electronic Levels

In-band Pumping

Tandem Pumping

Side Pumping Versus End Pumping

Requirements on the Pump Light

Optical Pumping in Fundamental Physics

Frequently Asked Questions

Summary: This article provides a comprehensive overview of optical pumping, the process of using light to excite a medium. It primarily focuses on the application in lasers and amplifiers, where the goal is to achieve a population inversion in the gain medium. The text covers various optically pumped laser types, including solid-state, fiber, and semiconductor lasers.

Different optical pump sources like laser diodes and lamps are explained, along with crucial pumping schemes such as three-level, four-level, in-band, and tandem pumping. Pumping geometries like side pumping and end pumping are compared. The article also touches on the use of optical pumping in fundamental physics, for example in laser cooling and isotope separation.

What is Optical Pumping?

Optically pumping some medium essentially means to inject light to electronically excite the medium or some of its constituents (e.g. some ions) into other (usually higher-lying) energy levels.

Specifically in the context of lasers or laser amplifiers, the goal of pumping is to achieve a population inversion in the laser gain medium and thus to obtain optical amplification via stimulated emission for some range of optical frequencies. (The width of that range is called the gain bandwidth.)

In other cases, such as in spectroscopic measurements, the goal can be to selectively populate a specific electronic level (e.g. some hyperfine sublevel), which does not necessarily have an energy well above the ground state.

The dynamics of optical pumping processes can often be described with rate equation modeling. However, this disregards some aspects of the quantum nature of the atom–photon interaction. More comprehensive physical models exist which can also describe coherent phenomena such as Rabi oscillations, but are relevant only in special situations.

A frequently used alternative to optical pumping is electrical pumping, applied particularly to laser diodes and gas lasers. Compared with electrical pumping, optical pumping can more specifically reach individual target levels if light with a small optical bandwidth is used.

Optically Pumped Lasers and Amplifiers

Various types of lasers can be optically pumped:

pumping of neodymium laser ions

Figure 1: Neodymium ions (e.g. in a YAG laser) can be optically pumped to some high-lying energy levels, from where they quickly decay (green arrow) into the upper laser level, from where stimulated emission at the laser wavelength of 1064 nm (for example) can occur.

Tutorials

Tutorial Fiber Amplifiers, Part 5: Forward and Backward Pumping

Fiber Amplifiers Part 5: Forward and Backward Pumping

We discuss the merits of forward and backward pumping, or bidirectional pumping, of fiber amplifiers. Depending on whether the power efficiency, ASE effects or noise issues are of highest interest, different configurations can provide the best performance.

Pump Sources for Optical Pumping

Common types of optical pump sources are:

In rare cases, light-emitting diodes (LEDs) are used for pumping lasers. This can be a cheap solution with long lifetime, but challenges arise from the diffuse light emission, making it harder to direct the pump light exactly where it is needed.

Various Aspects

Efficiency of Pump Absorption

A requirement for achieving a high power conversion efficiency in an optically pumped laser is that the pump light is efficiently absorbed in the gain medium. This can be the case if the gain medium is sufficiently long, has a high doping concentration, and a sufficiently wide optical frequency range for the pump light.

In some cases, the pump absorption efficiency can be increased by arranging for multiple passes of the pump light through the gain medium. This technique is often used e.g. in thin-disk lasers.

Using Multiple Electronic Levels

In the simplest case, the optically pumped medium absorbs light, and each absorbed photon excites one atom or ion into a higher-lying electronic level. The energy difference of the involved electronic levels must match the photon energy of the pump light. Once some degree of excitation of the medium has been achieved, the same kind of light can also cause stimulated emission, bringing excited atoms or ions back to the lower level. For that reason, a medium with a simple two-level scheme cannot reach a population inversion.

Optical pumping in lasers involves at least three different energy levels. In the simplest case, laser-active ions are pumped from their ground state to a higher-lying level, from where they undergo a quick radiative or non-radiative decay into an excited level with somewhat lower energy. As pump light cannot cause stimulated emission from this level down to the ground state (due to the too high photon energy), a population inversion can be achieved for a sufficiently high pump intensity. By introducing a fourth energy level, one can obtain a four-level laser system, where the laser transition involves two intermediate levels. If the lower level population is kept small due to a quick decay to the ground state, population inversion can be achieved even when only a tiny fraction of the ions is excited. The article on four-level and three-level laser gain media explains the details.

In solid-state lasers, one is usually dealing with Stark level manifolds rather than with individual sub-levels. Two such manifolds are then sometimes sufficient for reaching population inversion; see the following section on in-band pumping.

In-band Pumping

Solid-state laser gain media often exhibit slightly non-degenerate Stark level manifolds. Due to the energy variations within each manifold, optically pumped laser operation is possible even with only two involved manifolds: ions are pumped from the lower manifold (usually the ground-state manifold) to some higher manifold, and the laser transition directly leads from there back to the lower manifold, with no intermediate manifolds.

This pump scheme, called in-band pumping, can be used with various laser-active ions:

In-band pumping often leads to a small quantum defect, but also often to significant effects of reabsorption from the lower laser level (→ quasi-three-level behavior). Also, the achievable degree of excitation (and thus the laser gain) can be limited by stimulated emission caused by the pump light.

For more details, see the article on in-band pumping.

Tandem Pumping

Some fiber amplifiers, for example based on ytterbium-doped fiber, can be more efficient and powerful when pumped at a relatively long wavelength, e.g. around 1020 nm. As such a wavelength is hard to obtain from laser diodes, one may use the concept of tandem pumping: have a first Yb-doped fiber laser (or several such lasers) emitting around 1020 nm and use that radiation to pump another Yb-doped fiber laser emitting e.g. around 1100 nm. That way, the quantum defect is rather small in each laser, which minimizes the heat generation and thus allows for higher output powers.

Side Pumping Versus End Pumping

In an optically pumped laser, pump light can be injected into the gain medium from different directions. Side pumping means that the light is injected in directions which are roughly perpendicular to that of the laser beam. In the case of end pumping, the pump light is approximately collinear with the laser beam. The pump geometry has implications for the required beam quality (see below), but also the achieved power conversion efficiency, the laser gain, and the laser beam quality.

Requirements on the Pump Light

Pump light for optical pumping has to fulfill a number of requirements:

Optical Pumping in Fundamental Physics

Techniques of optical pumping are also used outside the area of laser physics, often in fundamental research. For example, various methods of laser cooling essentially depend on optical pumping of atoms or ions, for example suspended in an optical trap. In many cases, certain selection rules play a vital role, which reflect principles like the conservation of angular momentum: The absorption of photons of circularly polarized light must accordingly change the angular momentum of the atoms, i.e., it must lead to final states having correspondingly modified angular momentum. Therefore, it is possible, for example, to selectively pump certain states within a manifold of states having essentially the same excitation energy. In many cases, such processes involve multiple steps of optical pumping and spontaneous emission until the final target states are largely populated.

Interestingly, optical pumping can reduce the entropy of atoms or ions; such processes are associated with an increase of entropy of the light field, such that the total entropy is never decreased and in fact often increased.

Optical pumping can also be utilized for isotope separation because different isotopes of a substance exhibit slightly different transition energies.

Frequently Asked Questions

What is optical pumping?

Optical pumping is the process of using light to electronically excite a medium. In the context of lasers, its purpose is to create a population inversion in the laser gain medium, which is necessary for optical amplification through stimulated emission.

What are common pump sources for optical pumping?

What is the difference between optical and electrical pumping?

Optical pumping uses light to supply energy, whereas electrical pumping uses an electric current. Optical pumping is essential for electrically insulating media like doped-insulator solid-state lasers, while electrical pumping is common for laser diodes and gas lasers.

Why can a simple two-level system not be used for a laser?

In a two-level system, the same light used for excitation also causes stimulated emission from the upper level back to the lower level. This prevents the population of the upper level from exceeding the lower one, making a population inversion impossible.

What is in-band pumping?

In-band pumping is a scheme where ions are optically excited from a lower energy manifold to a higher one, and the laser transition occurs directly back to the same lower manifold. This method, common in ytterbium-doped media, often leads to a small quantum defect.

What is the difference between end pumping and side pumping?

In end pumping, the pump light travels roughly collinearly with the laser beam, typically requiring good pump beam quality. In side pumping, the pump light is injected from a direction perpendicular to the laser beam.

What is tandem pumping?

Tandem pumping is a technique where one laser is used to pump another. For example, a ytterbium-doped fiber laser at 1020 nm can pump another Yb-doped fiber laser to emit at 1100 nm, minimizing heat generation in each stage due to a small quantum defect.

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.