Modulating a laserwave 150mW 532 dpss

[tocket]

Ok.. whether it's mHz or MHz, it still doesn't make any sense. 1 milliHertz is 1/1000 of 1Hz, which is an incomplete wave. Even 100 milliHertz is an incomplete wave. In order to have a complete wave, the frequency must equal 1Hz or more.. So obviously 1m(illi)Hz makes no sense.

I'm curious, why would periodical functions be limited to frequencies above 1 Hz? What is the reasoning behind such a statement?

Maybe he means a frequency of .1hertz (10 second pulse duration with a 50/50 duty cycle) and he's using the metric prefix of milli? That's the way I interpreted it anyway. (Though I agree that it's absolutely *not* a standard way of stating frequency.)

Adam

I'm not sure what the standard way of stating a frequency is, but I guess you'd always get away with using scientific notation. It does become rather inconvenient and hard to read though, especially as the forum does not support superscript. I think the notation bart used is the most suitable.

I can see how this very simple, efficient and logical notation can confuse you americans, since you're used the overly complicated and highly illogical English system. This is not an attack against you, just your system.


Now, back to the topic. When I visited the laser physics department at my university we did some measurements using a Si-photodiode. What they did was to take the reflection off a piece of glass and terminate that on the photodiode. I don't know how necessary this is, but if you look at the datasheet for the photodiode they don't show the linearity at this high radiant flux, so there might be something to it.

I'm still interested in seeing a "typical" 10 mHz response, even if it's not reproducible.

With "Minimal 1 hour stabilisation time on infinite heatsink." do you mean that you let it run for at least 1 hour at the frequency you were testing before starting data acquisition?

[ElektroFreak]

I'm curious, why would periodical functions be limited to frequencies above 1 Hz? What is the reasoning behind such a statement?

My reasoning is that never have I heard, seen or otherwise encountered the term milliHertz anywhere but here. Ever. To my knowledge (and others it would seem, each VERY well-qualified) such a term is not the standard form. It is not simple or logical to state 10/1000 of 1Hz as being the same as a 10s pulse, with 50% duty cycle. A simpler, clearer way to say the same thing would be by stating clearly: 10s pulse duration, with a 50% duty cycle. Also, I did just say that I didn't think about it in the same way that Buffo did, thereby acknowledging the fact that I might have misunderstood..

I'm not sure what the standard way of stating a frequency is, but I guess you'd always get away with using scientific notation. It does become rather inconvenient and hard to read though, especially as the forum does not support superscript. I think the notation bart used is the most suitable.

I can see how this very simple, efficient and logical notation can confuse you americans, since you're used the overly complicated and highly illogical English system. This is not an attack against you, just your system.

Let's keep your issues with America and our systems out of this shall we? I get a little sick of you foreigners assuming we're all stupid because you don't understand our system..

Now, back to the topic. *couldn't be soon enough for my taste* When I visited the laser physics department at my university we did some measurements using a Si-photodiode. What they did was to take the reflection off a piece of glass and terminate that on the photodiode. I don't know how necessary this is, but if you look at the datasheet for the photodiode they don't show the linearity at this high radiant flux, so there might be something to it.

I'm still interested in seeing a "typical" 10 mHz response, even if it's not reproducible.

With "Minimal 1 hour stabilisation time on infinite heatsink." do you mean that you let it run for at least 1 hour at the frequency you were testing before starting data acquisition?

15 chars..........

[tocket]

I suspected it was a sore point... I'll just clarify that I don't think and certainly never said that americans are stupid. I do think that the English system is though. There are also areas where I think the SI system is lacking...

It is not simple or logical to state 10/1000 of 1Hz as being the same as a 10s pulse, with 50% duty cycle. A simpler, clearer way to say the same thing would be by stating clearly: 10s pulse duration, with a 50% duty cycle.

Well, that might be because it's not the same. A square wave with a pulse duration of 10s has a frequency of 50 mHz.

I like bart's way of specifying the signal because it's unambiguous. The only information it doesn't provide is the amplitude. A square wave has a duty cycle of 50% by definition. If the duty cycle is something else it's technically a pulse wave (though I wouldn't blame anyone for using these terms incorrectly, I probably have too).

My curiosity grows though, what is the standard form for frequencies? Perhaps they differ between the disciplines of science. I'm a chemical engineer myself and the only frequencies we use are in the MHz.

[ElektroFreak]

Hz, KHz, MHz, etc are the standard terms for frequencies.. In order for something to have a frequency, it is implied that there are one or more repetitive occurrences. I've never worked with "frequencies" that are less than 1Hz because by definition such a thing wouldn't have a frequency. It wouldn't be a complete single occurrence (or waveform).. That's why I have no idea what Bart means by milliHertz, since the term would seem to describe an incomplete wave. I'm really not sure what he means, which is why originally posted here. When describing fractions of 1Hz, the only way I have ever seen it described is in fractions of wavelengths (1/4 wave, 1/2 wave, etc..). Also, the same can be expressed in degrees, but I can't see how either applies here.. Since Bart is dealing in repetitive complete waveforms here, Hz, kHz, MHz, etc should be the only applicable terms.

[buffo]

I agree with Electrofreak... It's very confusing to speak of something with a frequency of less than 1 hertz. In that case you don't speak about frequency, you speak about pulse duration and duty cycle.

While I am not as experienced as Electrofreak, I do have nearly 30 years invested between Ham radio, Sonar, Nuclear Physics, and most recently, Lasers, and I, too, have *never* come across the term "millihertz" in my entire life. It is possible that all of these disciplines normally do not have repeating processes that act over periods longer than a second (most are considerably shorter), but one would think I would have come across the term at least once if it was in the common scientific parlance.

The only reason I figured out what Bart was trying to say is that I considered the language barrier and then remembered that he's also used to using standard units rather than English units. Then I looked at his test results and saw that there was a power of 10 progression, and that the one following "100 mHz" was 1Hz, so I figured he must have meant .1Hz, regardless of how strange that sounded.

As for the English units being stupid, it's only because you were raised with something different. While I agree that some calculations are easier with standard units, that is not always the case. Also, there are cases when English units offer finer granularity of measurement (without resorting to a decimal). In short, if you had been raised with the system, it would not seem stupid at all.

Adam

[tocket]

I think I understand now. Yes, in order to attribute a frequency to something it should be periodical. Periodical functions have both a period and frequency and they are the inverse of each other. If the frequency is low it can be more convenient to use the period, rather than the frequency.

You will no doubt agree that a frequency of for example 100 Hz gives a period of 1/(100 s^-1) = 10 ms. There is no limit to how long the period can be however (if you think about it, why would there be?). Thus, if the period is 10 s, the frequency must be 1/10 s = 0.1 s^-1 = 100 mHz.

Simply put, everything that has a period also has a frequency. There is no reason that the wave has to be chopped after 1 s (unless your DSO can't handle more ).

[ElektroFreak]

I think there's still some confusion, because I don't agree with your math. This part is accurate: 1/(100 s^-1) = 10 ms, but if the frequency increases, the period must decrease. Also, the definition of 1Hz is 1 cycle per second, so a 10s period is far less than 1Hz. Perhaps even a "milliHertz" (actually 1/10 Hz), as bart would say, but I would just say a 10 sec square wave. If 100Hz gives us a period of 10ms, then 100MHz would be 0.00001 seconds, or 10 microseconds.

[tocket]

Are the prefixes the source of the confusion?

if the frequency increases, the period must decrease.

Yes, and that also works the other way around since they are the inverse of each other! If the period increases, the frequency must decrease.

A 10 s period is 0.1 Hz, or 100 mHz (the SI system states that if this number stands by itself it is actually more correct to write it as 0.1 Hz, as that has less 0s in it - but 0.01 Hz should be written as 10 mHz).

You can explore this easily, as Wolfram|Alpha understands simple queries like http://www.wolframalpha.com/input/?i=square+wave+50+mHz

[ElektroFreak]

I believe I see where you're coming from, I misread your prefix in your last post, but that winds up being such a misleading term that I question it's use in a case like this. While it might be technically accurate for .01Hz after much thought, it doesn't convey the right message at first glance. It's very confusing.. I'm sure that accounts for the fact that it isn't commonly used.

[-bart-]

Maybe the message is only meant to be conveyed to those who grasp the concept of mHz at first glance.

I think ElectroFreak is bringing this whole mHz misconception up, to somehow make his absurd assumption plausible that I modulated a laserwave at 100Mhz. Even though the corresponding jpg clearly shows 2,5 seconds per division.