Optical Spectrometers Prism Spectrometers Grating Spectrometers Interferential Spectrometers Hyperspectral Spectrometers Credit: www.national.com Experimental Methods in Physics [2011-2012] EPFL - SB - ICMP - IPEQ CH - 1015 Lausanne IPEQ - ICMP - SB - EPFL Station 3 CH - 1015 LAUSANNE
Optical spectroscopy light source sample light analyzer light detector 2
Content Optical spectroscopy Resolvance (resolving power) Luminosity Prism spectrometer Grating spectrometer Interferential spectrometer Fourier spectrometer Hyperspectral spectrometer 3
Optical spectroscopy Definition The emission spectrum of a source, primary or secondary, is characterized by its luminance L ( ). The absorption spectrum is characterized by the absorption factor A ( ). The role of dispersive devices (prism, grating, interferential) is to determine the functions A ( ) and L ( ) with the greatest precision. 4
Optical spectroscopy Keep in mind that the important physical parameter is... the frequency,,... even if we measure the wavelength! in the vacuum: λ v = c υ in a medium of index n: λ n = c n υ 5
Optical spectroscopy λ v λ n = (n 1) λ v Often in the air, it makes no difference between n and v In fact, in the visible range, the error is not negligible (1-2 Å) 6
Dispersive apparatus General properties F: entrance slit C: collimator (objective, mirror, ) D: dispersive element (prism, grating, ) O: objective (mirror) P: image plane (photodetector, CCD, ) 7
Dispersive apparatus Figures of merit Resolving power or resolvance R = λ Δλ Luminosity L = E M L s 8
Resolvance Ideal case: punctual source (eventually slit) The size of the image, in the plane P, is limited by the diffraction... this is the projection of the contour of the dispersive element in a plane which is perpendicular to a direction that follows the dispersion that is important for calculating the size of the image. 9
Resolvance Ideal case: incoherent punctual source D(x) = sin(π x /d ) o ( π x /d ) o 2 d o = f λ w g '' 10
Resolvance Rayleigh criterium: Ideal case: 2 punctual sources emitting at two different wavelengths ( and [ + ]) We consider that both wavelengths, and [ + ] are resolved if the centers of their diffraction pattern are separated by at least d 0 11
Resolvance 12
Resolvance R o = λ Δλ = w '' g dβ dλ The intrinsic resolvance of the system is limited by the size of the dispersive element (or rather by it s projection.), i.e. by the diffraction 13
Resolvance Influence of the width of the entrance slit 14
Resolvance Influence of the width of the entrance slit E(x) = F(ζ ) D(x ζ)dζ E = F D where F is the function "slit", and D(x) the diffracted intensity in the image plane, of an infinitely thin slit. 15
Resolvance Influence of the entrance slit width 16
Luminosity versus resolvance Luminosity versus resolvance - a compromise 17
Monochromator or spectrometer? Dispersive instruments. prism. grating. interferential Monochromator Monochromator + detector Bandpass filter - monochromator Spectroscope Spectrometer Spectrograph Monochromatic light source Spectral signature 18
The prism spectrometer a I( ) S C e O I( + ) S: ponctual source C: collimator O: objectif 19
The prism spectrometer a Snell s law d i i1 i t1 i i2 i t2 sin(i n = i1 ) sin(i = t2 ) sin(it1 ) sin(ii2) If the angle of deviation,, is a + d sin( n = 2 ) a sin(2 ) minimum 20
The prism spectrometer a I( ) S C e O I( + ) the resolving power is given by: m db dn 0 = = a = e Dm dm dm 21
The prism spectrometer Example: To estimate the resolving power, we consider the case of a prism spectrometer working around 0.5 µm (green light). dn n 0 = e. r - n b dm mr - m b e = o abbe (n y - 1) 170 5 nm? e = 25 mm 0. 1100 Crown 3400 Fl int 0.17 nmk Dm TK0.5 nm 22
The prism spectrometer Advantages - low sensitivity to polarization - no overlap between different orders - uniform efficiency over the whole spectrum - heavy duty, high damage threshold - scanning and imaging modes Disadvantages - n = n( ) non-linear dependance credit: www.antique-microscopes.com/chemistry/ - material has to be transparent over the whole spectrum - relatively low resolving power, compared to grating instruments (for similar luminosity...) - need for costly achromatic optics - need to control several angles... 23
The prism spectrometer source Pellin-Broca s prism At minimum deviation angle, the angle between incident and refracted beam is precisely 90 24
The prism spectrometer in the NIR D D ES ED ES ES C ES C R E R S S ED E S: white light source R: reference E: sample C: chopper ES: entrance slit of the spectrometer ED: dispersive element ES: exit slit of the spectrometer D: detector 25
The prism spectrometer in the NIR 26
The prism spectrometer in the NIR I(z) Beer s law I(z) = I 0 exp(- az) 0 z Transmittance I %T = I0 $ 100 Absorbance A =-log T = log b I 0 I l 27
The grating spectrometer 28
The grating spectrometer The dispersive element is a grating instead of a prism collimator and objective are replaced by mirror optics (achromatic over a wide spectral range) imaging mode possible using an image detector (CCD or CMOS array) placed in the exit plane 29
Grating... intuitive feeling Huygens principle = 0 0 th order = 2π 1 st order = 4π 2 nd order 30
Transmission or reflection gratings? transmission grating reflection grating refractive index modulations within a thin layer of material sandwiched between two glass substrates ruled gratings holographic gratings 31
Grating spectrometer - basic equations Notations = incident angle = diffraction angle k = diffraction order N = total number of grooves n = grooves density [grooves/mm] = wavelength [nm] b = grating step D V = + = total deviation angle 32
Grating spectrometer-basic equations D V = b - a D V is fixed by the geometry k = diffraction order n = grooves density sin(a) + sin(b) = 2 $ sin b a + b 2 l $ cos b b - a 2 l = 10-6 $ k $ n $ m 33
Grating spectrometer-basic equations example of configuration 34
Grating spectrometer imaging mode The image detector is placed in a plane which is not perpendicular to the axis defined by the central wavelength (to minimize the infuence of the aberrations). 35
Grating spectrometer Superposition of the different orders of diffraction k $ m =cste cannot be avoided the use of blocking filter can help 36
Grating spectrometer angular dispersion and intrinsic resolvance m 0 0 = = '' Dm wg db dm m 0 0 = = '' wg Dm k $ n $ 10-6 = w cos b g $ k $ n = k $ N m 0 0 = = wg $ Dm sin(a) + sin(b) 10-6 $ m 37
Grating spectrometer blazed gratings 1st order it is possible to concentrate most of the diffracted energy in the first order for a given wavelength if = The grating equation shows that the angles of the diffracted orders only depend on the grooves' period, and not on their shape. By controlling the crosssectional profile of the grooves, it is possible to concentrate most of the diffracted energy in a particular order for a given wavelength. A triangular profile is commonly used. This technique is called blazing 38
Grating spectrometer blazed gratings Usually the blazed angle is defined for a Littrow configuration to be independent of the angle of total deflection (D V is imposed by the geometry of the monochromator) 39
Grating spectrometer blazed gratings 40
Grating spectrometer Ebert- Fastie design one concave mirror one planar grating slits are placed in the focal plane of the mirror advantages simple inexpensive disadvantages off-axis configuration, performances strongly limited by aberrations 41
Grating spectrometer Czerny - Turner 42
Grating spectrometer Aberration in PGS systems 43
Aberration in PGS spectrometer Aberration in PGS systems 44
Grating spectrometer Concave gratings (ACGH) 45
Grating spectrometer Anamorphism 46
Grating spectrometer Bandpass and resolution 47
Grating spectrometer Quasi-littrow configuration 48
Grating spectrometer F - value N.A. = sin X 1 f/value = 2 NA 49
Grating spectrometer Radiometry and spectrometry... geometry extent 50
Grating spectrometer One example: Jobin-Yvon HR 250 51
Grating spectrometer Examples: Triax Series 52
PF spectrometer [Fabry-Perot] 53
Interferential spectrometer 54
Iterferential spectrometer 55
Interferential spectrometer S = 2nd cos i E trans = E 5 inc tt + trrte -jk 0 S + trrrrte -jk 02S +...? 56
Interferential spectrometer avec R = r 2 et T = t 2 4R F = (1 - R) 2 c c o 0 = = S 2nd cos i I trans = I inc b ro 1 + F sin 2 o 0 l 1 57
Interferential spectrometer 58
Interferential spectrometer FWHM = r 2 sin -1 1 2 c m. F r F = p 1 finesse o 0 = Do o 1 = $ o0 FWHM o = $ p =m $ p o0 0 F=380, = 30.6, 0 = 15 GHz, m = 40000 = 1.2 10 6 59
Interferential spectrometer 60
Hyperspectral spectrometer LCPF 61
Interferential spectrometer LCPF 62
Interferential spectrometer LCPF 63
Spectral imaging filter 64
Spectral imaging filter 65
Acousto-optic spectrometer.. AOTF Texte 66
Acousto-optic spectrometer.. AOTF Texte 67
Acousto-optic spectrometer.. AOTF Texte 68
Acousto-optic spectrometer.. AOTF Texte 69
FTIR spectrometer 70
FTIR spectrometer 71
FTIR spectrometer I o1 (x) = B(o 1 ) $ cos 2ro 1 x I o2 (x) = B(o 2 ) $ cos 2ro 2 x I (x) = I 1 (x)+ I 2 (x) 72
FTIR spectrometer The mesured intensity (interferogram) in fonction of the displacement, x, is given by: I(x) = 2r 1 3 # -3 B(o) $ cos(2rox)do we observe immediately that the spectral information B(V) is nothing else than the Fourier transform of the measured intensity: B(o) = 3 # -3 I(x) $ cos(2rox)dx 73
FTIR spectrometer 74
FTIR spectrometer 75
Hyperspectral Visual perception of colors 76
Hyperspectral 77
Hyperspectral 78
Hyperspectral 79
Hyperspectral 80
Hyperspectral 81
Hyperspectral 82
Hyperspectral 83
Hyperspectral 84
Hyperspectral 85
Minimum Deviation by a Prism i i1 a i t2 d sin(i n = i1 ) sin(i = t2 ) sin(it1 ) sin(ii2) i t1 i i2 a = i + i t1 i2 d = i + i -a i1 t2 arcsin n sin arcsin sin i d = i + ; $ ca- ' i1 1mE -a i1 n 86
Minimum deviation angle d i i1 a i t2 d i t1 i i2 n=1.5 i i1 87