Pinhole spatial filter

Figure 9.29 Optical diagram of a Raman microscope. A pinhole spatial filter consists of a pinhole confocal diaphragm (Dj and D2). (Reproduced with permission from G. Turrell and J. Corset, Raman Microscopy, Developments and Applications, Academic Press, Harcourt Brace Company, London. 1996 Elsevier B.V.)...

After the preamplifier, the beam is expanded to 2 mm, collimated and imaged onto a 1 mm aperture, producing a flat-top intensity profile. A 3-element telescope relays the aperture plane to the amplifier with a collimated 0.5-mm diameter. The telescope contains a spatial filter pinhole. The nominal power levels are 3 mW into the preamp, 500 mW out of the preamp and 200 mW out of the aperture. A 6° angle of incidence bounce beam geometry is utilized in the amplifier cell. The "bounce" foofprinf overlaps with the 4 pump beam fibers, arranged in 2 time sefs of 13 kHz. The pump fibers have f 50-60% fransmission. The amplifier brings the power up to < 20 W at 26 kHz. 

Figure 12.23. Schematic of Kaiser Mark II fiber-optic probe head. A holographic diffraction grating and pinhole act to remove silica scattering from the laser light and improve beam quality by spatial filtering. (Adapted from Reference 1 with permission.)...

Figure 3. Confocal optical detection channel demonstrating the concept of spatial filtering. A microscope objective lens collect the light emitted from a point light source or a single molecule. The image appears as a diffraction pattern (Airy pattern, see insert). The diameter of the pinhole placed in the image plane is such that only light from the bright central spot can pass onto the detector. Radiation from an out-of-focus light source in the sample is efficiently discriminated.

Lasers with short pulses are not used in Raman spectrometers, mainly because the detectors in Raman spectrometers are tuned to high sensitivity. Such detectors are very easy to saturate and this is a case where short and intense laser pulses are employed for excitation of Raman scattering. It must be noted, that gas lasers are not perfect sources of monochromatic radiation. Together with intense coherent radiation such lasers produce weak incoherent radiation, caused by a different transition between electronic energy levels of the gas. The intensity of this incoherent and noncollimated radiation can be suppressed by increasing the distance between the laser and the sample, by placing a spatial filter (consisting of two lenses and a pinhole) or a narrow-band filter (usually an interference filter) into the laser beam. 

At this point, the fluorescence from each organelle can be collected by a microscope objective. As with all LIF detectors, care must be taken to choose an objective with high numerical aperture (NA) that can maximize the collection efficiency. The collected fluorescence can then be spatially filtered with a pinhole placed at the image plane to remove out-of-focus scatter from the buffer-cuvette interfaces. The size of the pinhole should be matched with the magnification of the collection objective... 

Fig. 2 Typical Brillouin scattering setup using a tandem of triple-passed Fabry-Perot interferometers. M mirrors, SF spatial filters. FP Fabry Perot interferometers, c corner cubes. IF interferential filter, 1-12 half-wave plate. PM photomultipliers, P pinholes. A Gian analyzer. S fast shutter. G glass plate, and PC personal computer for data acquisition. (From Ref [13].) (View this art in color at WWW. dekker. com.)...

The high contrast of the spectrometer is achieved either by triple passing each FP or the whole tandem setup. It results theoretically in an elevation of the apparatus function of each interferometer to the cube, which strongly enhances the contrast defined as the ratio of the maximum transmission over its minimum value. In order to achieve this high contrast, the tandem sits in a highly collimated beam between spatial filters, which also allows the stray light at the entrance to be diminished and the bandwidth at the exit pinhole in front of a photomultiplier or an avalanche diode to be selected (Fig. 2). 

The spatial filter may also be used to expand the beam (in order to vary the input beam diameter into the microscope objective, see Section 3.6). In a system where the pinhole size (PI) is matched to the lens (LI) spot size, then the ratio of the focal lengths of the collimating and focusing lenses give the factor of expansion. In our case the pinhole is smaller than the spot size. So the collimated beam diameter can be calculated, to an approximation, from equation 3.1 and in this case the collimated spatially filtered beam diameter is of the order 3-4 mm in diameter, which is smaller than the back aperture diameter of the microscope objective (around 5-6 mm). 

Two-wave laser interference (Fig. 20(a)) may be used to record periodic elements with subwavelength features (A 100 nm). The UV laser (for example helium cadmium, argon-ion, or excimer laser) beam is focused down with an objective lens and passed through a matching pinhole to spatially filter the beam. The central part of the emerging spherical wave, which has nearly planar phase fronts, illuminates the sample directly. A part of the wave is reflected towards the sample as shown. These two waves interfere to produce a standing, periodic intensity pattern with period... 

Figure 13. The laser light is focused via the scanner (b) through the tube lens (c) and the objective (d). and illuminates a small spot in the specimen (e). Emitted light emanating from the focal plane and the planes above and below (dotted and dashed lines) is directed via the scanner to the dichroic beam splitter (a) where it is decoupled and directed onto a photomultiplier (i). A pinhole (h) in front of the photomultiplier is positioned at the crossover of the light beams emerging from the focal point. This plane corresponds to the intermediate image of the Kohler illumination described in Section 29.1.3. Light emanating from above and below the focal point has its crossover behind and before the pinhole plane so that the pinhole acts as a spatial Filter. Numerous papers elucidate the basic aspects of confocal image formation [59] - [64].

In LSCM, light emitted from the object is focused on a confocal pinhole which acts as a spatial filter light from the focal plane passes the pinhole, while light from other planes is effectively suppressed. The pinhole thus ensures that the information only arrives from a particular level of the specimen with a very high resolution (AF) along the... 

A half-wave plate (HWP) and a polarizer (GLP) are positioned after the oscillator and are used to variably attenuate the laser output power to the desired input power required by specific experiments. Using a beam sampler (M ), a small portion of the laser beam is directed into a beam diagnostic unit (AC). In it, the laser pulse is characterized both in the time and frequency domains by employing an autocorrelator and a spectrometer. The laser beam is then expanded to match or overfill the back aperture of the objective lens. This is accomphshed using two positive lenses with the appropriate focal lengths. At the focal point of the first lens, a pinhole (SF) is carefully positioned to spatially filter the laser beam. An electro-mechanical shutter (S), used to control laser exposure times in the sample, is placed before this assembly. If exposure times shorter that a few milliseconds are required, faster response shutters such as acousto- or electro-optic modulators can be used. 

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