Controlling Laser Light by Acousto-optic Modulator, also Called Bragg Cell
A significant part of APE’s products work with specially configured AOMs and high frequency drivers adapted to the respective application. Dependig on the application (e.g. OPO’s, CARS/SRS, customized ultrafast laser setups) we design optimized AOM packages.
Our modulators are optimized for ultra-short laser pulses and fast switching. Multiple channels and synchronized switching is possible.
AOM At a Glance
The acousto-optic modulator – AOM – modifies the intensity of the laser beam. For example, it is used as an acousto-optical chopper for Lock-In detection systems. Its advantage over mechanical choppers is a higher modulation frequency. Besides, it doesn’t generate mechanical vibration which could have negative effects on the signal detection as it doesn’t contain flexible parts. In analog operation the laser intensities can be adjusted at will and time modulated.
AOM-based Dual Frequency Shifter
AOM Dual Frequency Shifter DFD-40/80
APEs acousto-optical dual frequency shifter DFD-40/80 allows for a defined shift of the laser frequency of narrow-band laser systems as used in e.g. experiments for Bose-Einstein condensates as part of the atom trap.
The DFD-40/80 is based on the acousto-optical effect. The DFD-40/80 has two channels driven by the same master clock oscillator; hence they are phase-locked but the frequency and amplitude can be individually adjusted. This makes the DFD-40/80 the ideal source for heterodyne experiments. It consists of the RF-driver electronics and two acousto-optical modulator elements.
- Complete instrument with driver electronics and modulators
- Two phase-locked, independently adjustable output channels
- Quartz stable oscillator
- RF-safety circuit to avoid damage by overload
- Microprocessor control
Technical Design Considerations on AOMs
The following considerations need to be taken into account for the design of a proper AOM package:
- Material of the optical modulator
- Dispersion (especially low disperation for short-pulsed lasers)
- Damage threshold (especially high theshold for high pulse energies or small (focussed) beams
- Optical Bandwidth / Wavelength Transparency Window (especially important for shortpulse (broadband) applications)
- Carrier Frequency of the AOM and AOM driver
- Beam Diameter or size of AOM aperture (Rise time of the AOM, Damage Threshold)
- Type of driver (analouge or digital), Synchronisation requirements (digital driver)
- Required contrast ratio
- Diffraction efficiency
The main focus in choosing suitable AOMs is on the required speed. There are essentially two categories: Fast modulators (i.e. 40 MHz, 80 MHz and above) and low speed modulators (i.e. typically kHz).
Fast modulators can have very small rise times, e.g. in the range of 10 ns. The rise time is proportional to the time required for the acoustic wave of the AOM to transit the optical beam. Because of this, the rise time is influenced by the beam diameter, respectively the aperture size, of the modulator. Due to that it is hard to find AOMs for large beam diameters in combination with short rise times. Therefore the input beam often must be focused into the modulator to achieve a short rise time. On the other hand, the deflection efficiency decreases with decreasing beam diameter, because only a smaller part of the acoustic field interacts with the beam. In addition, a focussed beam require AOMs that are optimized for high laser damage thresholds.
Low speed modulators do not have this speed constraint and can accept larger input beams.
A significant part of APE’s products work with specially configured AOMs and high frequency drivers adapted to the respective application. Dependig on the application (e.g. OPO’s, CARS/SRS, etc, customized ultrafast setups) we can offer optimized AOM packages. Please contact us for detailed technical advice.
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