physics

Photodarkening

Photodarkening is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Photodarkening rather than just read about it. In short: Photodarkening is an optical effect observed in the interaction of laser radiation with amorphous media (glasses) in optical fibers. Until the 2000s, such creation of color centers was reported only in glass fibers.

Key takeaways

Photodarkening belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Photodarkening to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Photodarkening from memory before moving on to harder problems.

Reference excerpt

Photodarkening is an optical effect observed in the interaction of laser radiation with amorphous media (glasses) in optical fibers. Until the 2000s, such creation of color centers was reported only in glass fibers. Photodarkening limits the density of excitations in fiber lasers and amplifiers. The experimental results suggest that operating at a saturated regime helps to reduce photodarkening.

Definition One could expect the term photodarkening to refer to any process when any object becomes non-transparent (dark) due to illumination with light. Formally, the darkening of the photo-emulsion also could be considered as photodarkening. However, recent papers use this term meaning reversible creation of absorbing color centers in optical fibers. One may expect that the effect is not specific for fibers; therefore, the definition should cover wide class of phenomena, excluding, perhaps, non-reversible darkening of photographic emulsions. According to the Encyclopedia of Laser Physics and Technology, photodarkening is the effect that the optical losses in a medium can grow when the medium is irradiated with light at certain wavelengths. Photodarkening can also be defined as reversible creation of absorption centers in optical media at the illumination with light.

Rate The inverse of the timescale at which photodarkening occurs can be interpreted as photodarkening rate.

Color centers Usually, photodarkening is attributed to creation of color centers due to resonant interaction of electromagnetic field with an active medium.

Possible mechanisms The phenomenon, similar to photodarkening in fibers, was recently observed in chunks of Yb-doped ceramics and crystals. At the high concentration of excitations, the absorption jumps up, causing the avalanche of the broadband luminescence. Increase of absorption can be caused by formation of color centers by electrons in the conduction band, created by several neighboring excited ions. (The energy of one or two excitations is not sufficient to pop an electron into the conduction band). This explains, why the rate of darkening is strong function of the intensity of the exciting beam (as in the case with optical fibers discussed above). In experiments, thermal effects are important; therefore only the initial stage of the avalanche can be interpreted as photodarkening, and such interpretation is not yet confirmed. A study from 2010 pointed out the role of thulium contamination. Through laser pump and signal absorption, and energy transfer from ytterbium; thulium is able to emit UV light, known to create color centers in silica glass. Although the actual mechanism of photodarkening is still unknown, a reliable setup to test the photodarkening properties of different types of fibers was reported in 2012.

References

Worked examples

Example 1 — a first encounter with Photodarkening

Start with the simplest possible case. Write down what Photodarkening claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Photodarkening before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Photodarkening ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Photodarkening

In research: Photodarkening appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Photodarkening in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Photodarkening is common in secondary-school and first-year university syllabi. It links to neighbouring topics Laser gain media, Laser science, Optical materials, so understanding it makes those chapters shorter.
In everyday life: Look for Photodarkening outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Photodarkening in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Photodarkening means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Photodarkening out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Photodarkening in simple terms?

Why does Photodarkening matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Photodarkening?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Photodarkening.