there is a very simple although less scientific method: Just take a business card and shake it along the beam in a dark room. If the laser is pulsed, you will see a line of dots, just because your pulse "ketches" the card in different locations. That works with rep-rates up to several kHz. For rep-rates faster, you will need a fast detector.
Definetely, it is not possible to unambiguously determine the laser beam behavior by the eye. It could work only for quasi-continious mode of work for pulse repetition rate of tens Hertz and it is definitely not scientific method because of its essential subjectivity.
The most simple method will be using a fast photodetector and an oscilloscope with bandwidth enough to register fast processes in output laser radiation, which speed is determined by a type of laser active medium, cavity length, photon lifetime, etc.
Hi Sumit,
You can observe it with a detector and an oscilloscope which would be the best method i feel possible.
Sumit, I recommend you interferometric test in order to be visible by naked eye such as Micheson interferometer with optical delay to measure temporal correlation between two beams separated from one laser beam. Shigeo
there is a very simple although less scientific method: Just take a business card and shake it along the beam in a dark room. If the laser is pulsed, you will see a line of dots, just because your pulse "ketches" the card in different locations. That works with rep-rates up to several kHz. For rep-rates faster, you will need a fast detector.
Use a fast Si, Ge or GeAs photodiode or a PMT, a 50 ohm termination and an oscilloscope of more than 200 MHz bandwidth to ensure it is pulsed and measure its repetition rate, pulse duration (if its in nsec range). Please ensure that you shine a very small fraction of the laser beam onto the detector, either by using a OD 3 neutral density filter or reflecting the beam successively by simple glass plates or microscope slides.
If the repetition frequency is very low, you can see with naked eye. But if repetition frequency is high, it's impossible to see with naked eye. Oscilloscope is best method to detect.
Well, obviously photodetector with oscilloscope is simplest solution, but just for the fun of it there are other options:
Vincent, let me argue that you do not really need a fast "eye".
If I will take a fast chopper and and spin it with the frequency of the repetition rate of the laser minus some small say 10-20Hz offset, I will actually see light blinking at this difference frequency with my naked eye.
In fact, since chopperes do generally have 10-100 blades, you will have to spin it at frequencies of laser rep-rate devided by the number of blades - so not such a terrible idea.
Now regarding the interferometry option:
Interferometer will only measure longitudinal coherence of the laser (pulse) - which is simply the reciprocal of the bandwidth, so you will not really learn much about actual time behavior of the laser in this way. If the laser is mode locked, you might, indeed deduce some approximate data from this measurement.
Theoretically speacking, you can get similar data by "naked eye", just doing some rather simple statistics on speckles pattern at again my favorite business card :).
Go for fast detector :)
And Happy new year to all.
Vincent, that's a great one. If you modulate the intensity of the pulses fast enough, you could make it sound like "jingle bells"
Happy new year
The Laser medium itself gives you some hints on what is possible. Nitrogen, excimer, Ruby, and single hetero-junction diode lasers do not or even can not produce continuous output. On the other hand, an Argon ion laser (not necessarily, but most likely) gives continuous output.
Another method to consider is to convert the laser beam to an electrical signal with a photodiode sensor. Then analyze this signal with a spectrum analyzer . If the laser beam is pulsed, the spectrum analyzer, will show that it's electrical signal contains a spectrum of HARMONICS but the continuous beam will not!
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