Theory - External-Cavity Diode Lasers (Assembly)

In this module you will learn about lasing and in particular about the external-cavity diode laser. We will start by discussing types of atom-light interactions that will be used to create a laser. We will then discuss what a laser is and the key parts of a laser. From here, we will specifically look at the diode laser before moving on to the components of an ECDL.

Goals

By the end of this module, you should

• Have a lay understanding of why the laser in important.

• Know what a laser is at a conceptual and technical level.

• Know the key components of an ECDL and the reason we are using the ECDL in our experiment.

• Be able to draw schematics of lasing and the ECDL to use during the build phase of your project.

Deliverables

• Drawn schematic for lasing with annotations to understand the three main components.

• Drawn schematic for the ECDL with annotations to understand the components and how they connect to lasing.

Theory - External-Cavity Diode Lasers (Assembly)

The importance of the laser

You see this word used all the time, and you actually encounter lasers in everyday life! Nowadays, we have laser pointers, laser surgery, and laser scanners! But aside from everyday uses, the laser has allowed for huge scientific advances.

For us, the laser has allowed for incredible advances in atomic physics. Without the laser, we would have no way to cool atoms, trap atoms, and image atoms!

What is a laser?

LASER is actually an acronym that stands for Light Amplification by Stimulated Emission of Radiation.

So how does a laser work? One way for us to generate a bunch of light would be to take a bunch of atoms in the excited state and stimulate them to decay. This would give us a bunch of photons. However, we would eventually run out of excitations. We can fix this by providing an energy source that continues to pump the atoms into the excited state. We then should have some mechanism to make sure that the light gets amplified.

Lasing happens when stimulated emission builds up, cascading a chain reaction that continuously amplifies a light source with more coherent (same general, frequency, phase, and direction) photons. In order to achieve regular continuous stimulated emission, we need to drive a scenario known as population inversion, where the atoms in the laser gain medium are predominantly occupying the excited state over the ground state.

Laser components

A typical laser has three main components.

• A pump source: provides energy to pump the atoms into the excited state (when more atoms are in the excited state than ground state, we call this population inversion).

• A gain medium: the material that contains the atoms that will undergo population inversion.

• A resonator: a device, often a cavity, that acts as a feedback source to amplify the light.n

There are 5 main types of lasers that exist today

• Gas lasers (HeNe laser)

• Solid-state lasers (Ruby Laser)

• Fiber lasers

• Dye Lasers

• Semiconductor lasers (Laser Diodes)

We will be mainly using semiconductor lasers for our experiment. An illustration of the components for the laser diode is shown in the diagram below.

INSERT DIAGRAM

Component	Example
Pump source	Back-reflected seed light from the grating
Gain Medium	Charged semiconductor
Resonator	Facets of the laser diode

Laser Theory

A laser is an acronym that stands for Light Amplification by Stimulated Emission of Radiation. We detail a brief review of the semi-classical theory of laser operation below.

Imagine a two-level atom, with a ground state with energy $E_g$ and an excited state with energy $E_e$. Three types of atom-light interactions can occurs:

Lasing happens when stimulated emission builds up, cascading a chain reaction that continuously amplifies a light source with more coherent (same general, frequency, phase, and direction) photons (the particles that constitute light). In order to achieve regular continuous stimulated emission, we need to drive a scenario known as population inversion, where the atoms in the laser gain medium are predominantly occupying the excited state over the ground state.

Note: population inversion is not feasible in a simple, pure, two-level system.

Question: why?

<the photons that cause the excitation can equally well drive the system down again. Some external effects can make lasing possible with 2-level systems alone, such as systems that filter away excited states. I believe some masers used 2-level ammonia passing by quadrupole fields that separated excited states from non-excited states>

Thus, to lase light, we have to increase the complexity of the system. Typically, the choice is to engineer gain media with multi-level systems to achieve this .

What is an ECDL?

Before we go through all the components, let's talk about what we are building and why.

Remember what our end goal is - we need a laser to use for us to cool and trap atoms that have a certain transition frequency. To accomplish this, we need to have a laser that sits exactly at that atomic transition frequency. So, we need a laser that is not only practical to build, but has some nice tuning features.

ECDL is the acronym for external cavity diode laser. As you might expect, we will build this laser using:

• A laser diode (Pump source and gain medium)

• An external cavity (Resonator)

• Additional electronics and pieces for feedback.

We've already talked about the laser diode, so let's talk about the other components. We will build our ECDL using the Littrow configuration. The Littrow configuration consists of:

• A lens

• A diffraction grating

• A laser diode.

Try it yourself: Even before reading on, given your knowledge of a laser, can you think of a way to arrange these components to build a laser? As a hint:

• Remember that the laser diode is the gain medium and the pump source is the electrical current that is provided.

• The lens allows for light to pass through it in both directions. This is a collimating lens which means it will narrow the beam and cause the direction of motion to be much more aligned.

• The diffraction grating will allow the zeroth order light to be reflected like normal classical optics, but the first order (and other higher orders) can be reflected back at different angles.

Figure (NEED FIGURE) shows the schematic for the ECDL in the Littrow configuration. We have our diode that serves as the gain medium and pump source. The output light travel through the collimating lens. The space between the diode and diffraction grating is our resonator.

Concept Check: Draw your own schematic for an ECDL in the Littrow configuration. What are the benefits of the Littrow configuration? What are the drawbacks? Think about this both from a physical perspective and an engineering perspective. (What experiments would this be good for? What would it not be good for? How involved is the build? How much flexibility do we have? How expensive is the build?)

Concept Check: We didn't discuss it, but figure <?> shows another ECDL configuration known as the Littman-Metcalf configuration. What might be the advantages and disadvantages of this configuration?

Concept Check: We mentioned that ECDLs have some fine-tuning abilities for the output frequency. How does the ECDL do this? Why is this important for our MOT experiment?

Next Steps

With your schematic in hand, move to the next section on the ECDL assembly. Everything we just did is technically still theory. Now, you'll want to match all these theoretical components to actually parts that can in your kit!