Tunable Bandpass Filters

Most implementations of filter-based nondispersive spectrometers occur in Raman microscopy and imaging, where their large clear aperture is particularly valuable. These applications are discussed further in Chapter 11, but the nature and performance of tunable filters as wavelength analyzers are outlined here. 

A narrow bandpass filter chosen to match a Raman band can serve as a very simple Raman spectrometer, provided only a limited range of Raman shifts is of interest. For example, a 900 nm bandpass interference filter transmits a band centered at 1627 cm relative to a 785 nm laser. The center of the passband may be angle tuned over a limited range, and it is possible to construct a spectrum from several measurements of intensity vs. angle (6). However, the 

Strong scatterers. In addition, the frequency may be switched rapidly among several Raman bands to provide near-simultaneous monitoring of several spectral features. 

A third type of tunable filter that has been applied to Raman spectroscopy is a conceptual extension of the interference filter. A conventional dielectric interference filter transmits light most efficiently when Eq. (9.2) is satisfied (11)  

A different approach to liquid crystal filters led to the development of a nondispersive Raman spectrometer used commercially in a Raman microscope 

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Acousto-optic Filters. The newest type of spectrometer to become commercially available is the acousto-optic tunable filter (AOTF). An AOTF is a sohd-state, electronically tunable bandpass filter based on the diffraction of optical waves by acoustic waves in an optically anisotropic crystal. 

An experimental investigation was made of the statistical fluctuations in neutron intensity which occur in a nuclear reactor. An ion chamber was exposed to reactor flux, and the fluctuations in its output current were analyzed in a tunable bandpass filter to get the frequency spectrum of these fluctuations, which has the shap>e of the square modulus of the transfer function. Results are presented of some measurements made on various low-power experimental reactors at Argonne National Laboratory. For reactors with prompt neutron lifetime between 15 and 70 /isec, the quantity 1//3 was determined within 5 per cent or better from a least squares fit to the transfer function thus measured. 

This Synchroscan [68] streak camera system has been used to study the time resolved fluorescence of trans-stilbene in the picosecond time regime. The experimental arrangement [69] is shown in Fig. 20. An acousto-optically mode-locked argon ion laser (Spectra Physics 164), modulated at 69.55 MHz was used to pump a dye laser. The fundamental of this dye laser, formed by mirrors M, M2, M3 and M4, was tunable from 565 to 630 nm using Rhodamine 6G and second harmonic output was available by doubling in an ADP crystal placed intracavity at the focal point of mirrors M5 and M6. The peak output power of this laser in the ultraviolet was 0.35W for a 2ps pulse which, when focused into the quartz sample cell of lens L, produced a typical power density of 10 KW cm-2. Fluorescence was collected at 90° to the incident beam and focused onto the streak camera photocathode with lens L3. The fluorescence was also passed through a polarizer and a bandpass filter whose maximum transmission corresponded to the peak of the trans-stilbene fluorescence. 

Figure 9.1. Schematic of Raman spectrometer based on an acousto-optic tunable filter (AOTF). BP, bandpass filter BR, band reject filters APD, avalanche photodiode. (Adapted from Reference 9, with permission.)...

Select an appropriate dichroic mirror to reflect the 514-nm excitation light to the specimen. Select and an appropriate bandpass filter in the emission filter wheel (550/40, Chroma). In addition to the emission filter, we place a 514-nm notch filter (Semrock) into the emission filter slot of the dichroic mirror cube to block unwanted excitation light from reaching the detector, and we use a 514-nm excitation filter to clean up unwanted wavelengths from the argon gas laser that are not blocked by our acousto-optical tunable filter. 

Tunable single element bandpass filters have also been used as Raman monochromators. The most popular are acousto-optic tunable filters and liquid crystal tunable filters. These filters are electrically tunable and are well suited to Raman imaging. 

Early receiver designs used a tunable local oscillator (LO) to select the desired station and a bandpass filter to reject stations at nearby frequencies, out-of-band noise, and interference. The filter was ganged with the local oscillator so that its center frequency tracked the oscillator frequency (Fig. 12.29). However,... 

Receiving equipment for aircraft astrophysical investigations in the far infrared has been developed and flown in 1974-1975 on board the AN-30 aircraft. Four types of infrared and submillimeter detectors mainly of photoresistor type are used in the wavelength region from 10 mkm to 1 mm. Thermal background at the detectors is limited by cooled bandpass filters and by optimal matching of the detectors with the telescope optics. The spectral filters used are combinations of quasiresonance metal mesh filters of different structure and Q-factor from 2 to 7 with the Yamada cut-off powder filters. A tunable Fabry-Perot interferometer, a polarimeter for linear polarization measurements and an aircraft 25 cm-telescope are briefly described. 

An alternative to the angle-tuned filter approach is to employ a stationary dielectric bandpass filter in combination with a tunable laser source [16]. The fixed filter eliminates... 

The total transmittance of a multiple-stage filter is the product of the individual filter stage transmittances and has a sine function profile, shown in Fig. 8D. An LCTF based on the Lyot filter geometry having a maximum peak transmittance of 16%, a continuously tunable bandpass of 7.6 cm and a free spectral range of greater than 4600 cm" (500-650 nm) has been demonstrated [23]. However, a significant fraction of peak transmission loss in the Lyot LCTF is due to absorption by the polarizers and imperfect wave-plate action [24]. 

The distance between the mirrors can be adjusted with a MEMS actuator to enable a tunable interferometer. When the distance between the mirrors is adjusted to 2/2, the light passes through the interferometer while light with other wavelengths is blocked. The Fabry-Perot interferometer acts as a narrow bandpass filter for light, with the bandpass is dependent on the finesse of the optical cavity. 

G dye laser pumped by a pulsed nitrogen laser. The bandwidth of the tunable laser output was 300 MHz, but this was reduced still further by passing the beam through an external confocal interferometer approximately 1 m long. This interferometer serves as a very narrow bandpass filter and an output linewidth of approximately 7 MHz was obtained. 

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