Bragg cell

Figure B2.1.1 Femtosecond light source based on an amplified titanium-sapphire laser and an optical parametric amplifier. Symbols used P, Brewster dispersing prism X, titanium-sapphire crystal OC, output coupler B, acousto-optic pulse selector (Bragg cell) FR, Faraday rotator and polarizer assembly DG, diffraction grating BBO, p-barium borate nonlinear crystal.

Large field of view angle (FOV). AOTFs, unlike conventional Bragg Cells, are capable of a large FOV (as large as a several degrees). 

The Ti-sapphire oscillator is extremely useful as a stand-alone source of femtosecond pulses in the near-IR region of the spectrum. Some ultrafast experiments, especially of the pump-probe variety (see below), can be conducted with pulses obtained directly from the oscillator or after pulse selection at a lower repetition rate. Far-IR (terahertz) radiation is usually generated using a semiconductor (usually GaAs) substrate and focused Ti-sapphire oscillator pulses [7]. If somewhat higher-energy pulses are required for an experiment, the Ti-sapphire oscillator can be cavity dumped by an intracavity acousto-optical device known as a Bragg cell. 

Simultane measurements in the radial and axial direction were possible, which were analysed by computer. Bragg cells were used for the preshift. 

Structural information on electrodes can also be obtained as a function of potential by X-ray using monochromatic synchrotron radiation and similar transmission cells to those used for XAS (Figure 7B). Transmission cells (Laue mode) are best suited to adsorbates, while reflection (Bragg) cells are used for thicker films. Laboratory rotating anode sources can also be used, and here the instrument usually demands a reflection cell geometry. Stable systems such as solid-state cells and batteries give better... 

Figure ( shows the 1S-2S two-photon spectrum with its two hyperfine components, recorded in this way. Shown below is a Doppler-free saturation spectrum of Tej, observed simultaneously with the cw dye laser at the fundamental wavelength. The tellurium spectrum appears shifted by 60 MHz towards lower frequencies, since a 120 MHz acousto-optic Bragg cell is employed as a chopper. The component i is thus found in nearly perfect coincidence with the hydrogen F 1- 1 resonance. The absolute frequency of the nearby Te2 component bj has recently been measured interferometrically to within 1) parts in 10 . Using this line as a reference and taking advantage of the auxiliary Te line ii, the frequency of the centroid of the two hyperfine components was measured to be f(1S-2S)... 

Bragg cell An acousto-optics modulator operates in such a way that only two diffracted light beams predominately exist as output laser beams. 

FIGURE 4 Typical Bragg cell or Bragg modulator operating at 40 MHz. 

FIGURE 11 Cascaded Bragg cells for realizing higher order spatial derivative inc is at the Bragg angle the relation between Einc(xcy) and E (xliy) Is given by Eq. (20). 

The intensity of the diffracted beam of a Bragg cell is propotional to the amplitude of the acoustic wave. Modulating the amphtude of the acoustic wave causes a corresponding modulation on the diffracted optical beam. This device can be used as a linear light modulator (Fig. 3.18). 

Note that the interaction bandwidth is inversely proportional to the acoustic frequency / and proportional to the acoustic wavelength A for a given aspect ratio L/A. The smaller value of 2 A/ao If and 2A/r// determines the overall bandwidth of the Bragg cell. At low frequencies the SAW excitation bandwidth limits the overall bandwidth. At high frequencies the SAW is limited by the interaction bandwidth. 

The SAW transit time over the interaction region determines the Bragg cells response speed r. Where D represents the guide wave width (or the interaction region) the response time is written as... 

This relationship places a lower limit on D, where one might seek to reduce it in order to increase the response time of the Bragg cell. 

The time bandwidth product is a nondimensional parameter defined as the product of the bandwidth 2 A/ and the SAW transit time r. For a Bragg cell not only is size of the absolute deflection important but so is the number of resolvable spots. For a guided wave with a width D the beam divergence angle is... 

The Bragg cell adds a fixed frequency shift fo to the diffracted beam, which then results in a measured frequency off a moving particle of... 

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