Optical chopper

Evidently, the optical choppers are quite expensive, and therefore, the instrument manufacturers very often utilize the separation-in-space method for the routine measurement spectrophotometers. 

The prism at the outlet of the laser serves to separate the laser emission of the gas fluorescence and allows for a clean excitation of the sample. For excitation using solid-state lasers, this element is dispensable. The lens (element 5) collects the fluorescent signal and focuses on the aperture of the monochromator. The filter is used to eliminate excitation that is spread over the surface of the sample. The optical chopper serves to modulate the light at a defined frequency, which serves as reference for the lock-in amplifier. A data acquisition system controls the pace of the monochromator and reads the signal of the lock-in, generating the sample s emission spectrum. 

Figure 9.9. A differential radiometer with an optical chopper so the bolometer looks alternately at two sky different sky positions. The bolometer feeds an audio amplifier followed by a lockin amplifier.

We have developed a novel ultrasensitive detection method, thermal lens microscopy (TLM), for nonfluorescent species [13]. TLM is photothermal spectroscopy under an optical microscope. Our thermal lens microscope (TLM) has a dual-beam configuration excitation and probe beams [13]. The wavelength of the excitation beam is selected to coincide with an absorption band of the target molecule and that of the probe beam is chosen to be where the sample solution (both solvent and solute) has no absorption. For example, in determination of methyl red dye in water, cyclohexane, and n-octanol, a 514-nm emission line of an argon-ion laser and a 633-nm emission line of a helium-neon laser were used as excitation and probe beams, respectively [21], Figure 4 shows the configuration and principle of TLM [13]. The excitation beam was modulated at 1 kHz by an optical chopper. After the beam diameters were expanded, the excitation and probe beams were made coaxial by a dichroic mirror just before they were introduced into an objective lens whose magnification and numerical aper-... 

The first laser beam can be amplitude modulated ( lkHz) with an optical chopper (Fig. 2), which modulates the concentration of the excited metal atoms, M. Because the excited metal atoms have a much higher reactivity, the concentration of product molecules is also modulated. The modulated fluorescence excited by the second laser is then detected by a PMT and a lock-in amplifier. A monochromator or an optical filter is used to analyze the emission and to control the optical bandwidth detected by the PMT. This photochemical modulation and synchronous detection of the fluorescent signal is a very powerful technique for increasing the signal-to-noise (S/N) ratio. 

TRIR measurements are obtained from three different recordings of the emission spectrum of the IR source. These include a spectrum of the IR source without the sample in the beam path (/q), a spectrum of the IR source with the sample in the beam path (/), and the infrared intensity changes induced by photoexcitation (A/). The absorbance spectrum of the unexcited sample is derived from /q and /, A=log (/q//). Because the IR detector is AC coupled, measurements of /q and / are performed with an optical chopper (Stanford SR540) to modulate the IR light. 

The synchronous rectifier S is mechanically or electrically coupled to the sector mirror. It converts the amplified low-frequency output of the detector to direct current. The rectifier is phased with the optical chopper mirror so that the polarity of the rectified output indicates the condition of unbalance of the optical null system. That is, one polarity indicates more energy in the reference beam than in the sample beam. 

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