# He-Ne Laser Wavelengths & Quantum Effects

• WolfOfTheSteps
In summary: The underlying physics of semiconductor lasers is explained in detail.In summary, there is no clear answer to the question about at what He-Ne laser power quantum effects become important. The first part of the question, regarding the number of photons per second emitted by a low power He-Ne laser, has an answer of 1.7 x 10^15 photons/second. However, the second part of the question, which suggests that changing the power of the laser may affect the relevance of quantum effects, may be a typo as the quoted wavelength of 336nm is incorrect for a He-Ne laser. Other recommended books on lasers include "Understanding Lasers: An Entry-Level Guide" by Jeff Hecht, "Principles
WolfOfTheSteps

## Homework Statement

How many photons per second does a low power (1 mW) He-Ne laser ($\lambda=336$nm) emit?

At what He-Ne laser power do you expect quantum effects to become important?

## The Attempt at a Solution

I got the answer to the first part, $1.7\times 10^{15}$ photons/second.

But the second part makes no sense to me. The book doesn't talk about lasers at all in this chapter or the ones before it. I looked HeNe lasers up on google, and I saw statements like these:

He-Ne lasers as used for holography operate at a wavelength of 632.8 nm, with a power ranging from 0.5 mW to 100 mW

So apparently, the power does not effect the wavelength! And so for the HeNe laser in the question, the wavelength will always be $\lambda=336$nm. So how could changing the power possibly effect the wavelengths and hence the relavence of quantum effects? Is this a trick question?Thanks!

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It makes no sense to me either. Obviously it made no sense to anyone at PF or someone would have answered.

Yeah, I'm not surprised. This books sucks horribly. It's littered with typos and the author doesn't even know how to use LaTeX! (For example, she uses a '.' (period) to represent dot products! And she randomly switches the usage of the Greek letters nu and upsilon, as if they were the same letter. I have no confidence in this text!)

I think... in general (?)... you need to look at when the photon energy becomes comparable to the bandwidth of the gain.

I think the first major typo is the wavelength. A HeNe laser operates at
633 nm and not 366 nm as indicated in your quote-box.

Edgardo said:
I think the first major typo is the wavelength. A HeNe laser operates at
633 nm and not 366 nm as indicated in your quote-box.

Yes, definitely seems like another typo.

If any of you are teachers, make sure you don't use https://www.amazon.com/dp/0387281525/?tag=pfamazon01-20. I'm embarrassed that the author is a prof at my university.

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If you are searching for good books, I can recommend the following:

1) Understanding Lasers: An Entry-Level Guide (IEEE Press Understanding Science & Technology Series) by Jeff Hecht
This book is easy to read and serves good as an introduction.

2) Principles of Lasers by Orazio Svelto
This book has more details.

3) Lasers (Hardcover) by A. E. Siegman
This is the bible on lasers.

4) Fundamentals of Photonics by Saleh, Teich
Contains many topics besides lasers.

I'd recommend:

Semiconductor-Laser Physics (Hardcover)
by Weng W. Chow (Author), Stephan W. Koch (Author), Murray, III Sargent (Author)

## 1. What is the wavelength of a He-Ne laser?

The wavelength of a He-Ne laser is typically 632.8 nanometers (nm). This is in the red portion of the visible light spectrum.

## 2. How does a He-Ne laser produce light of a specific wavelength?

A He-Ne laser works by using a gas mixture of helium and neon, which is excited by an electric current. This excitation causes the electrons to jump to higher energy levels, and when they return to their original states, they emit photons of light with a specific wavelength.

## 3. What are quantum effects in relation to He-Ne laser wavelengths?

Quantum effects refer to the behavior of subatomic particles, such as electrons, which are responsible for the specific wavelength of light emitted by a He-Ne laser. These effects are based on the principles of quantum mechanics, which describe the behavior of particles at a very small scale.

## 4. Can the wavelength of a He-Ne laser be changed?

Yes, the wavelength of a He-Ne laser can be changed by adjusting the gas mixture and the electric current. However, the range of possible wavelengths is limited and still falls within the red portion of the visible light spectrum.

## 5. What are some practical applications of He-Ne lasers and their specific wavelengths?

He-Ne lasers have many applications, including in barcode scanners, alignment of optical instruments, and as a light source in interferometers. The specific wavelength of 632.8 nm is also used in spectroscopy and as a reference for other types of lasers.

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