Solving Homework Questions on He-Ne Lasers

[WolfOfTheSteps]

Is this forum ok for posting EE student questions? I guess I'll find out.

Homework Statement

How many photons per second does a low power (1 mW) He-Ne laser (\lambda=336nm) 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=336nm. So how could changing the power possibly effect the wavelengths and hence the relavence of quantum effects? Is this a trick question?


Thanks!

 

[bdrosd]

I don't think there is a quantitative answer to the second part of the question. Power is the rate at which energy is transferred. The quantum theory of light implies that light comes in particles (photons). If you are measuring some particular effect that occurs in a similar (or shorter) time than the time between photons arriving, then the effect will be erratic (i.e. statistical). For example, if you are measuring the heating of a block of material by the laser beam striking it, the heating requires a much greater time than that between individual photons arriving. Hence, the heating rate is "smooth" and there is no need to consider it a quantum effect. If, on the other hand, you are considering the promotion of an electron from the valence to the conduction band in a solar cell, the subsequent current will not be continuous if measured on a short enough time scale compared to the production rate of mobile electrons. Based on the question as stated, I suspect this is a little more detail than was intended but I believe this is the key point.

 

[WolfOfTheSteps]

Yeah, this seems strange. While I understand what you are saying, I have a feeling that is not what the book wants. This book has a crapload of typos, too. I wonder if the word "power" should have been "wavelength." In my google search I did see that they make HeNe lasers that produce light at different wavelengths.

It's also the first question in the chapter, so it's most likely considered "easy."

Also, since the TA doesn't speak english, I know he is just going to compare the answers to an answer sheet... so even if I give a good response based on your info, I probably won't get any points.

The original question actually had some more in it, that I omitted. The actual paragraph after the quantitative (photon/second) part read like this:

At a given power of an electromagnetic wave, do you expect a classic wave description to work better for radio frequencies, or x-rays? Why? At what He-Ne laser power do you expect quantum effects to become important?

Since those two questions are in the same paragraph, it seems like they must both be talking about wavelength effects, not time effect.

Arghgh.

 

[bdrosd]

The radio frequency photon will have much less energy than an X-ray. Hence for a given amount of energy transmitted, there will be a much greater number of RF photons and so, less influence of quantum effects. But as far as a number as to where quantum effects are going to become important at, its not clear to me? Anybody else have an opinion?

 

[dlgoff]

You got me curious, so I looked in my old "Introduction to Modern Optics" text by Grand Fowles. He gives a table about the types/reagions of radiation. There are three types; "Wave" region (radio & microwaves), "Optical" region (infrared, visible, & ultraviolet), and "Ray" region (x-ray & gamma rays). They each have an associated quantum energy in electronvolt units.

 

[Integral]

IMHO, A laser is fundamently a Quantum device. There is no point at which it "becomes" quantum, it is ALWAYS a quantum device. This is true regardless of power or wavelength.

 

[WolfOfTheSteps]

I think the problem is with the book. https://www.amazon.com/dp/0387281525/?tag=pfamazon01-20. Unfortunately I have to use it.