Detector crystal diode

Typically the source is tuned with the sample in place and then locked to match the cavity resonance frequency so as to achieve maximum energy storage and minimum reflected power. This reflected power is directed through a one-way coupler called a circulator to a crystal diode detector to convey information about sample absorption in the cavity. An iris opening to the cavity is adjusted to match the impedance of the cavity to that of the source so as to produce minimum reflection of radiation from the cavity. This condition gives maximum sensitivity for the impedance mismatch produced when sample absorption occurs in the cavity. 

Heterodyne detectors in the microwave and millimeter regions (hv< kT) include square-law mixers such as the crystal diode detector [7.93], the InSb photoconductive detector [7.94-96], the Golay cell [7.95], the pyroelectric detector [7.95], the metal-oxide-metal diode, and the bolometer [7.87]. The latter three types of detectors have also been used successfully in the middle infrared (at 10.6 pm) [7.97-100]. For this type of detector Johnson noise generally predominates, and the input SNR is given by [7.100]... 

Due to the pulsed radiation output of the LINAC the detectors and the detector electronics have to handle very high counting rates in very short periods. Therefore the detectors have to work in a mode, where the detector output is integrated for one or several beam pulses. For that purpose the crystals are coupled to photo- diodes. Their currents are read out and analysed by the electronic board, which has been developed for this special application. 

The energy-dispersive (EDX) solid state detector (SSD, Figs 4.6, 4.7) is made of lithium-drifted Si crystal (Si(Li)). Between a thin p-type and an n-type layer lies a high-resistivity Si crystal of centimeter dimensions. The front and end planes of the crystal are coated with Au and serve as electrodes. The crystal, cooled to 77 K by liquid nitrogen, represents a p-i-n diode (Fig. 4.7). An incident X-ray photon with... 

Stability may not be as much of a problem as with a diode source. However, there are problems with this method as well. The range of tunability is limited by the absorption properties of the nonlinear crystal which generates the difference frequency. At present, tunability is limited to wavenumbers >2500 cm-1 and conversion efficiencies are low. Typical laser powers in the CH2 experiments (82) were 20 n W (compared to the power of the CO lasers, 10 mW-1 W). This produces a situation where IR detectors, particularly fast ones, may be close to or background noise limited. However, it is clear that more applications of this technique will appear in the future. 

Lithium-Drifted Germanium Detectors. Two kinds of planar Ge(Li) diodes were fabricated in our Laboratory for the spectrometer (I). The chief consideration was maximum cross section with a resolution of less than 3 k.e.v. at 60Co FWHM (full width half maximum). The available single-crystal germanium material dictated the shape of the detector. The first detector was fabricated from a 15-cm. long Sylvania ingot with a trapezoidal cross section its finished dimensions were 6 cm. X 3 cm. X 1 cm. Our second detector was fabricated from circular Hoboken stock (NPC Metal and Chemical Co., Los Angeles) its finished dimensions were 2.8 cm. diameter X 1.2 cm. thick. 

Figure 6 Block diagram of the two-color optical parametric amplifier (OPA) and IR-Raman apparatus. CPA = Chirped pulse amplification system Fs OSC = femtosecond Ti sapphire oscillator Stretch = pulse stretcher Regen = regenerative pulse amplifier SHGYAG = intracavity frequency-doubled Q-switched Nd YAG laser YAG = diode-pumped, single longitudinal mode, Q-switched Nd YAG laser KTA = potassium titanyl arsenate crystals BBO = /J-barium borate crystal PMT = photomultiplier tube HNF = holographic notch filter IF = narrow-band interference filter CCD = charge-coupled device optical array detector. (From Ref. 96.)...

The field is modulated at some audio- or radio-frequency (usually -100 kHz), and this produces signals at the crystal detector whose output can be amplified at the modulation frequency. In general, sensitivity is higher at high-modulation frequencies because of the linear variation of excess crystal noise with the inverse of frequency. However, the recently developed Schottky diode makes it possible to obtain good sensitivity at relatively low frequency. 

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