Coherance determined by Driver?

On: Tue, Jan 08, 02 07:12:40 PM

Jim McPherson wrote:

As I read the web site about the D&S didoe driver it sounds as if you use the D&S driver you will get better coherence in the beam. Is coherence determined by the quality of the driver or am I reading the page wrong. What sets the D&S driver apart from other drivers? -Jim
RESPONSES

Colin Kaminski - Fri, Jan 11, 02 10:50:49 AM

I am not sure if anyone is qualified to give you a definitive answer on this forum but this is what I have found out. I will start simple so this post can be followed be beginners. When we talk about coherence we are talking about two things: 1. Spatial coherence is the relative phase across the width of the beam. We are looking for TEM 00. This is defined by the laser cavity. Not all diodes are TEM 00. 2. Temporal Coherence is a measurement of how narrow is the frequency band the laser puts out. This is what we measure when we check the coherence length. Temporal coherence is limited by the laser cavity. If the diode driver does things to change the laser cavity then it can affect the temporal coherence. For example as the diode heats up the cavity changes size and therefore frequency. If this change happens during an exposure then fringe brightness can ban be affected or the image can be erased all together. When talking with experts I have found there is some disagreement as to which is a better driver design for holography, constant output power or constant current. As I see it constant output power is the better choice and I expect it is what the D & S uses. The D & S uses the photodiode so I assume it runs constant output power. Now since we are extracting an exact amount of power from the diode and the efficiency stays about the same, we have a fairly constant thermal load. And I believe a fairly constant current draw. Once the D&S warms up the fringes are very stable. Another problem is when the room changes temperature. It only takes a small change in diode temperature to change the frequency of the diode. In fact the diode changes frequency gradually then jumps. This is what is called a mode hop. It is important to note the frequency changes as a function of temperature even when the diode is in between mode hops. I am presently working with Jonathan H. on an experimental peltier temperature controller for the D & S laser. We will post results when we have them. 63.193.192.221

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