Hi,
New on this forum.
Is it be possible to use an high laser diode in pulsed mode to overcome stability/vibration challenges and in particular for making portrait or pets holograms?
many thanks
pulsed holography
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BobH1357
Re: pulsed holography
Those making pulsed laser holograms typically use from 700mJ to 2J of energy per pulse. Their lasers also have etalons to give long coherence length (single longitudinal mode) and have a very nice spatial mode. If you can find a diode laser with those characteristics, please post about it here!
Re: pulsed holography
Yeah, I thought first about the energy levels a single pulse would need to provide. There are not many 10,000 Watt diode lasers out there.
However, why does it need to be all in one pulse? Isn't pulsed mode (with the diode being pulsed continually with, say, a 50% duty cycle) used in many laser diode testing and evaluation setups? I had thought that was because thermal effects and other stresses on the diode were reduced. So, the question I have is how much differently does a laser diode behave in CW versus pulse modes?
However, why does it need to be all in one pulse? Isn't pulsed mode (with the diode being pulsed continually with, say, a 50% duty cycle) used in many laser diode testing and evaluation setups? I had thought that was because thermal effects and other stresses on the diode were reduced. So, the question I have is how much differently does a laser diode behave in CW versus pulse modes?
World's worst holographer
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BobH1357
Re: pulsed holography
Pulsed mode for a diode laser allows it to get rid of heat and stay at a higher power level for a given current. Can't use it for holography because each pulse is too weak to expose a plate and multi=pulse exposure over time allows motion of subjects.
Re: pulsed holography
The pulse width is also a factor. All films require a certain amount of energy to maximise whatever physical change is caused by the actinic radiation (hardness, reduction of silver salts etc) - the exposure. The power of a laser is a figure determining the amount of energy per second. Thus a 10,000 W laser would give 10,000 Joules of energy in 1 second (and probably fry most films!).
For example, Slavich silver films have an exposure of about 200 microJoules (200 x 10^(-6) J). Using Bob's figure of 700 mJ the time necessary to transfer the right amount of energy to the film to cause the physical change (reduction of silver salts, in this case), ~200 uJ - is 200/700,000 = ~0.2 msec. If the pulse width was much larger, you'd be transferring too much energy to the film; if much lower you'd not tranfer enough energy to the film.
To determine the pulse width necessary, you need to determine the speed of the motion you're trying to capture. Making assumptions such as that the object must not move by a quarter wavelength over the period of the exposure (ideally, it must not move at all, but you're using a pulse laser because it moves), then, for an object moving at velocity v, the exposure time, t = lambda/(4v). This is a bit simplified, since the direction of the velocity is also important and I haven't taken that into consideration. It's the phase of the object wave, relative to the reference phase, that's important. In terms of the energy of the pulse, E, t = S/E (where S is the sensitivity of the film). Thus, E = 4Sv/lambda.
For example, Slavich silver films have an exposure of about 200 microJoules (200 x 10^(-6) J). Using Bob's figure of 700 mJ the time necessary to transfer the right amount of energy to the film to cause the physical change (reduction of silver salts, in this case), ~200 uJ - is 200/700,000 = ~0.2 msec. If the pulse width was much larger, you'd be transferring too much energy to the film; if much lower you'd not tranfer enough energy to the film.
To determine the pulse width necessary, you need to determine the speed of the motion you're trying to capture. Making assumptions such as that the object must not move by a quarter wavelength over the period of the exposure (ideally, it must not move at all, but you're using a pulse laser because it moves), then, for an object moving at velocity v, the exposure time, t = lambda/(4v). This is a bit simplified, since the direction of the velocity is also important and I haven't taken that into consideration. It's the phase of the object wave, relative to the reference phase, that's important. In terms of the energy of the pulse, E, t = S/E (where S is the sensitivity of the film). Thus, E = 4Sv/lambda.