Re: Low-coherence and high-coherence

On 1/6/06 3:47 AM, in article dpllcp$ldn$1@xxxxxxxxxxxxxxxxxxxxxxx,
"Alexander Dräbenstedt" <alexspamander@xxxxxxxxxxxxxxx> wrote:

> With low coherence interferometry you use a short coherence length (~?m) to
> map a topography that can be relativily high and can have steps. The
> measurement device steps through the full height of the object and observes
> the passing of the contrast maximum of the interference fringes in each
> point. The distance at which this maximum appears corresponds to the height
> of that point.
>
> In high coherence length interferometry the coherence length is as long as
> the full height of the objects topography. The measurement device steps only
> through a distance change of half lambda to record a full cycle of
> interference fringes in every point. With that in each point the phase of
> the fringes can be computed that corresponds to: (height of topography)
> modulo (lambda/2). Using phase unwrapping the absolute topography can be
> restored if the object has smooth slopes connecting all points.
>
> Alexander
>
>
> "wytnij_to" <"(wytnij_to)pawel_gasior"@o2.pl> schrieb im Newsbeitrag
> news:dpge4u$5gg$1@xxxxxxxxxxxxxxxxxxxxxxxx
>> Hi!
>> I've got one question: what is a difference between low-coherency and
>> high-coherency intereferometry? Is it the point about coherence length??
>>
>> Best regards
>> Gasik
>
>
If I interpret your post correctly, the low coherence interferometry you are
talking about uses a source such as a multi-longitudinal mode laser source.
The autocorrelation function for such a source is periodic over long time
with a period of the round trip resonator transit time. Between such times,
the autocorrelation is close to zero and the light appears to be incoherent.

Is that what you are talking about?

Bill

-- Ferme le Bush


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