光學元件、光電儀器的特性與應用

1. 光學元件、光電儀器的特性與應用 研究員/教授 李超煌 中央研究院應用科學研究中心 國立陽明大學生醫光電研究所

2. Optical Elements spherical lens mirror prism polarizer or non-plarizing beam splitter cylindrical lens filter window objective wave plate

3. Optical Spectrum

4. UV Category • Terrestrial Solar UV: 290 – 380 nm. • UV-A: 320 – 380 nm. Ozone is transparent. Cellular damage by photochemical reactions. • UV-B: 290 – 320 nm. Ozone is absorptive. DNA absorbs and induces many bioeffects. • UV-C: 190 – 290 nm. Air is transparent but ozone absorbs so heavily that we do not see this range at earth surface. • Vacuum UV: < 190 nm. Ionizing N2 and O2. • Extreme UV: < 50 nm. • Soft x-ray: < 30 nm. • X-ray: < 1 nm. Ref: Lasers in Medicine, edited by R. W. Waynant (CRC Press, London, 2002), Chap. 4.

5. Substrate Materials Mirror Pyrex: excellent mirror substrate; low coefficient of thermal expansion Zerodur: “zero” thermal expansion Lens or window UV fused silica: excellent transmissive properties from IR to UV Calcium Fluoride (CaF2): wider transmission bands than fused silica Glasses: BK7, SF14, etc: different transmission, dispersion....

6. Dispersion Dispersion: n is a function of l. Sellmeier equation: In catalogs of optical materials, the coefficients a, b, c ...can be found for various transparent materials.

7. Optical Surfaces Surface flatness: How flat the surface is. RMS amplitude of surface ripples When preservation of wavefront is critical, a l/10 to l/20 surface should be selected. Surface quality: How much the surface scatters. In the scratch-dig specification, the first number is the width of the largest scratch (in 0.1 mm), and the second is the diameter of the largest bubble or pit (in 10 mm). For demanding laser systems 20-10 to 10-5 scratch-dig is appropriate. If some scatter is tolerable, 40-20 can be used.

8. Coating • Reflective coatings • Metallic: broadband, insensitive to wavelength, angle of incidence, and polarization. But lower damage threshold. • Dielectric: reflectivity can be specified from low (10%) to near total reflection. Available either broadband or narrowband. Best for 0-45° angle of incidence. • High energy: resist optical damage of high power CW lasers and high energy pulsed lasers. Wavelength must be specified. • Ultrafast mirror coating: to minimize dispersion effects on ultrashort laser pulses. • Dichromatic coating: high transmission for wavelengths longer than a specific value and high reflection for wavelengths shorter than that.

9. Coating Anti-reflection coatings V-coating BBAR-coating

10. Damage Threshold • Fluence threshold: thermal effects. Energy fluence = pulse energy/beam area. • (Unit: J/cm2) • This is often noted on coatings for pulsed lasers. As a rule of thumb, the fluence threshold increases as a function of the square root of the time domain. For instance, if the damage threshold is 2 J/cm2 for 10 ns pulses, at the 1 ms time domain the coating can withstand 20 J/cm2. • Intensity threshold: electric field breakdown. Intensity = (peak) power/beam area. • (Unit: MW/cm2) • This is important for both cw and pulsed lasers. The intensity threshold • scales with wavelength, so the intensity threshold at 532 nm will be half of that at 1064 nm. Beyond either threshold, laser light can damage the optics.

11. Unit of Light Intensity vacuum impedance For E0 in V/m, the unit of intensity is then W/m2. In optics, however, W/cm2is used frequently. Light intensity can also be used to calculate the photon density r(m-3): I = chnr c: speed of light (m/s) hn: photon energy (J)

12. Cleaning of Optics Drop and Drag Brush hemostats methanol or acetone lens tissue For installed optics. Before first-time use or storage. Ref: Newport Resource 2006/2007, p. 688.

13. Selecting the Right Lens f/# = focal length/beam diameter On a lens, it means the lowest f/# this lens can achieve. spherical aberration focal spot diameter chromatic aberration

14. Selecting the Right Lens Plano-convex lenses: Focusing parallel rays of light to a point. Minimize spherical aberration in situations where the object and image are at unequal distances from the lens. For optimum performance, the curved surface should face the infinite conjugate. Bi-convex lenses: Minimize spherical aberration in situations where the object and image are at equal or near equal distances from the lens.

15. Aberrations Images are from http://micro.magnet.fsu.edu/ 15

16. Astigma Images are from http://micro.magnet.fsu.edu/ 16

17. q2/2 -f1 q1 d1 d2 d2 Beam Collimation and Expansion f1 f2 d2 d1 f d2/d1= f2/f1 f2 d1q1 = d2q2 d2 = q1f d1 d2/d1= f2/-f1 When used with high pulse energy lasers, use this configuration to avoid the unnecessary focus. Note: For minimum spherical aberration, the curved surfaces should face the parallel rays.

18. Beam Shaping with Cylindrical Lenses shorter focal length for wider divergence angle laser diode To shape an elliptic beam into a circular beam or vice versa.

19. Birefringent Effect O-wave polarization E-wave polarization • When the optical axis is unparallel to the crystal surface, the incident extraordinary wave does not obey Snell’s law. • In this condition, birefringence results in a double image. The polarization of the two images are orthogonal. Ref: R. Guenther, Modern Optics (John Wiley & Sons, New York, 1990), Chap. 13.

20. Polarization Optics: Waveplate Waveplates are birefringent crystals, which have different refractive indices for different polarizations. Retardation

21. Polarization Optics: Waveplate G = (2m+1)p: m-order half wave plate The half-waveplate can be used to rotate the polarization of linearly polarized light. Rotate the half-waveplate exactly q around the beam axis (in either direction) and we will have rotated the polarization of the beam by 2q. G = (2m+1/2)p: m-order quarter wave plate Quarter-waveplates are used to turn linearly-polarized light into circularly-polarized light, and vice versa. To do this, we must orient the waveplate so that equal amounts of fast and slow waves are excited.

22. Polarization Optics: Polarizer Broadband polarizer Glan-laser polarizer At this port, Tp/Ts = 100 - 1000 At this port, Tp/Ts > 105 multilayer dielectric coating air gap Tp/Ts is the extinction ratio, which is the most important specification of a polarizer.

23. Spectral Filters band pass filter interference filter long wavelength pass filter Color-glass (long pass) filters

24. Attenuation (Neutral Density) Filters Iin Iout Iout = Iinx 10-O.D. O.D.is optical density. Iin Iout Iout = Iinx 10-S(O.D.)

25. Grating angular dispersion Do not touch the face of gratings! Even lens-tissue is prohibited!

26. Acousto-optic (AO) Modulator from Wikipedia

27. Acousto-optic (AO) Deflector The acousto-optic deflector makes use of the acoustic frequency dependent diffraction angle, where a change in the angle Dqd as a function of the change in acoustic frequency Df given as qd l: optical wavelength va: velocity of acoustic wave from Wikipedia , at a given qdone can tune the modulation In addition, because period d to obtain different l. In this mode the device is an AO filter.

28. Electro-optic (EO) Modulator An electro-optic modulator (EOM) is a device used for controlling the power, phase or polarization of a laser beam with an electrical control signal. The most common devices are Pockels Cells. longitudinal mode transverse mode

29. Types of EO Modulation Phase Modulators The simplest type of electro-optic modulator is a phase modulator containing only a Pockels cell, where an electric field (applied to the crystal via electrodes) changes the phase delay of a laser beam sent through the crystal. The polarization of the input beam often has to be aligned with one of the optical axes of the crystal, so that the polarization state is not changed. Polarization Modulators Depending on the type and orientation of the nonlinear crystal, and on the direction of the applied electric field, the phase delay can depend on the polarization direction. A Pockels cell can thus be seen as a voltage-controlled wave plate, and it can be used for modulating the polarization state. Amplitude Modulators Combined with polarizers, Pockels cells can be used for amplitude modulation. The following figure is based on a Pockels cell for modifying the polarization state and a polarizer for subsequently converting this into a change of transmitted optical amplitude and power.

30. Objective Lens numerical aperture (NA) tube length thickness of cover glass magnification (M) M = b/a achromatic, planar focal plane

31. Objective Lens: Resolution and Focusing Rayleigh criterion: resolution ~ 0.61l/NA NA ~ 1/(2f/#) clearly resolved resolution limit To focus a laser beam, the smallest spot radius ~ 0.82l/NA. A better estimation is effective focal length

32. Light Source: Mercury-Arc Lamp i-line (365 nm) g-line (436 nm)

33. Light Source: Lasers

34. Diode-Pumped Solid-State (DPSS) Lasers

35. Excimer Lasers • XeCl: 308 nm • KrF: 248 nm • ArF: 193 nm • Typical Output: • Pulse duration: 10 – 50 nsec • Pulse energy: 0.2 – 1.0 J/pulse • Repetition rate: several hundred Hz

36. Excimer Laser and Photolithography

37. Light Source: Light-Emitting Diode (LED) Epoxy dome lens LED chip reflector cup anode cathode

38. Wavelengths of LEDs

39. Detection of Light Semiconductor detectors: photodiode Usually the photodiode is reverselybiased.

40. Detection of Light Photo-emission detectors: photocathode

41. Detection of Light Photomultiplier Tube (PMT):

43. Detection of Light CCD camera

44. Specifications of CCD or CMOS cameras • Pixel size (8 mm; 23 mm) • Pixel resolution (640 x 480; 1024 x 1024) • Spectral response (300 nm to 1000 nm) • Well depth (> 300,000 e-) • Dark current (20 e-/pixel/s @ 20 oC) • Dynamic range (> 85 dB) • Digital or analog • Bit depth or A/D levels (10 bit; 12 bit; 14 bit...) 44

45. Bandwidth of Digital Data Interfaces 45

46. CCD vs. CMOS CMOS: higher response speed, lower power consumption, compactness. CCD: larger dynamic range, smaller dark current. Ref: D. Litwiller, “CCD vs. CMOS: Facts and Fiction,” Photonics Spectra, Jan. 2001. 46

47. Receiver Noise Also called Physical source Equation Solid state Johnson Nyquist, white, thermal Thermal motion of charges in circuit components Vnoise = (4kTRB)1/2 k: Boltzmann’s constant T: absolute temperature R: resistance B: bandwidth in Hz 4k = 5.53*10-23 V2/Hz/K-W Solid state Shot Dark current, leakage current Statistical fluctuation in carriers at p-n junction Inoise = (2qIB)1/2 q: electron charge I: average dc current B: bandwidth in Hz Photo-emissive Quantum Photon Statistical fluctuation in arrival of signal photons Linear Noise in Optical Detection 47

48. Interference Linear superposition of two fields If light is from the same source, this term is zero. or where This term is optical path difference (OPD).

49. Interference Fringe (2m + 1)

50. Young’s Interferometer , m = 0, 1, 2, 3.... for bright fringe