MMS â Monolithic M i n i a t u re - S p e c t ro m e t e r - Photon Lines

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It should be noted he<strong>re</strong> that a wide dynamicrange is only obtainable if the PDA (photodiodea rray) is near the saturation limit. The aim isalways to <strong>re</strong>ach high light intensity – he<strong>re</strong>, thehigh sensitivity of the <strong>MMS</strong> modules is a benefit.Dynamic range = ADC/∆σ r a n g eFor instance, when using a 14bit converter –this cor<strong>re</strong>sponds to 16,384 steps or inc<strong>re</strong> m e n t sand a noise of ∆ σ = 1 count, a signal (full-scaledisplay) can <strong>re</strong>ally be divided into 16,384 inc<strong>re</strong>ments.Hence, the lowest measurable change is1/16,384 of the saturation signal. At a noise of4 counts an uncertainty of 4 counts also exists,i.e. a change of 4/16,384 of the saturation signalcan only be definitively measu<strong>re</strong>d or the signaldivided into 4,096 inc<strong>re</strong> m e n t s .LinearityThe p<strong>re</strong>vious <strong>re</strong>marks will be completely accurateonly if the detector and the post-detecto<strong>re</strong> l e c t ronics provide ideal linearity. For quantification,the admissible deviation must be specified.F o rt u n a t e l y, the behaviour of modern semiconductordetectors is almost perfectly linear within awide range. Befo<strong>re</strong> saturation (the ext<strong>re</strong>me caseof non-linearity) is <strong>re</strong>ached, however, the inc<strong>re</strong> a s eof the cur<strong>re</strong>nt (carrier of the intensity inform a t i o n )supplied is no longer linear to the number of photonsstriking the photosensitive material. For this<strong>re</strong>ason, the range of linearity is smaller than thedynamic range.External influencesAs the graphic shows, a change of the tempe r a t u <strong>re</strong> T does not cause any change in sensitivityof silicon, the sensitivity in fact inc<strong>re</strong> a s e sslightly in the range of up to 1100 nm when thet e m p e r a t u <strong>re</strong> is raised. In the case of InGaAs photodiode arrays, the sensitivity also changes by lessthan 1% in the range of 1 to 1.55 µm with tempe r a t u <strong>re</strong>s ranging from -50 and +50 °C. Me<strong>re</strong> l youtside the specified range, does the diff e <strong>re</strong> n tcoating lead to an inc<strong>re</strong>ased i n fluence of the tempe r a t u <strong>re</strong>. (Falling temperatu<strong>re</strong> s lead to <strong>re</strong> d u c e dsensitivity on the band edge).12
In addition, the signal-to-noise ratio of thephotodiode array used does not degrade withi n c <strong>re</strong>asing T. lt is only the dark cur<strong>re</strong> n t Id a r k w h i c hi n c <strong>re</strong>ases with rising temperatu<strong>re</strong>, <strong>re</strong>sulting in a<strong>re</strong>duction of the dynamic range. The<strong>re</strong> f o <strong>re</strong>detectors, in particular InGaAs diode arrays, a<strong>re</strong>often cooled.I d a r k(T+7K) = 2I d a r k(T) (Silicon)I d a r k (T+10K) = 2I d a r k (T) (InGaAs)In this context it should be mentioned thatthe amounts of light to be measu<strong>re</strong>d a<strong>re</strong> alsosubject to fluctuations. The instability of the lights o u rce is often the limiting factor.SensitivityThe „smallest detectable change“ is a <strong>re</strong> l a t i-ve specification. Much mo<strong>re</strong> difficult to specify isthe lowest detectable amount of light or: howmany photons a<strong>re</strong> needed for the detection electronics to <strong>re</strong> c o rd a change. The difficulties <strong>re</strong> s u l tf rom determining the light intensity of a lights o u rce and the coupling eff i c i e n c y. Furt h e rm o <strong>re</strong> ,these parameters a<strong>re</strong> wavelength-dependent.T h e <strong>re</strong> is, on the one hand, a di<strong>re</strong>ct dependence,as all components featu<strong>re</strong> wavelengthdependent efficiencies – including the coupling –in device; on the other, the<strong>re</strong> is a dependence, asthe bandwidth is of decisive importance for sensitivitymeasu<strong>re</strong>ments. The simplest case is a lights o u rce with a very narrow band, as displayed bymost of the lasers. lf the bandwidth of the lights o u rce used is markedly smaller than the bandwidthof the spectrometer used, the situation isc l e a r. The <strong>MMS</strong> value of mo<strong>re</strong> than 10 13 c o u n t s / W shas been measu<strong>re</strong>d with a <strong>re</strong>d HeNe laser. At1550 nm, the <strong>MMS</strong> NIR still exhibits a sensitivityof mo<strong>re</strong> than 10 14counts/ Ws (in the case of a14 bit conversion).S c a t t e <strong>re</strong>d lightThe specification of scatte<strong>re</strong>d light data isonly useful in connection with the measuring13
Page 4 and 5: The quasi-monolithic principleC o n
Page 8 and 9: Back to the basics: The specificati
Page 10 and 11: lf the maximum of a line is imaged
Page 14 and 15: i n s t ructions. Scattere<
Page 16 and 17: ControlDiode arrayDiode arr a yThe
Page 18 and 19: ApplicationsThe flexible design of
Page 20 and 21: I n t e rf e rence
Page 22: SpecificationsPast ModulesM o d u l

MMS â Monolithic M i n i a t u re - S p e c t ro m e t e r - Photon Lines

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MMS â Monolithic M i n i a t u re - S p e c t ro m e t e r - Photon Lines