Acousto-optic modulator: Application in cold atom cabinets
As the core component of the all-fiber laser link in the cold atom cabinet, the optical fiber acousto-optic modulator will provide high-power frequency-stabilized laser for the cold atom cabinet. Atoms will absorb photons with a resonant frequency of v1. Since the momentum of photons and atoms is opposite, the speed of atoms will decrease after absorbing photons, thereby achieving the purpose of cooling atoms. Laser-cooled atoms, with their advantages such as long probing time, elimination of Doppler frequency shift and frequency shift caused by collision, and weak coupling of the detection light field, significantly improve the precise measurement capability of atomic spectra and can be widely applied in cold atomic clocks, cold atomic interferometers, and cold atomic navigation, among other fields.
The interior of an optical fiber AOM acousto-optic modulator mainly consists of an acousto-optic crystal and an optical fiber collimator, etc. The modulated signal acts on the piezoelectric transducer in the form of an electrical signal (amplitude modulation, phase modulation or frequency modulation). By changing the input characteristics such as the frequency and amplitude of the input modulated signal, the frequency and amplitude modulation of the input laser are achieved. The piezoelectric transducer converts electrical signals into ultrasonic signals that vary in the same pattern due to the piezoelectric effect and propagates them in the acousto-optic medium. After the refractive index of the acousto-optic medium changes periodically, a refractive index grating is formed. When the laser passes through the fiber collimator and enters the acousto-optic medium, diffraction occurs. The frequency of the diffracted light superimposes an ultrasonic frequency on the original input laser frequency. Adjust the position of the optical fiber collimator to make the optical fiber acoust-optic modulator work in the best state. At this time, the incident Angle of the incident light beam should satisfy the Bragg diffraction condition, and the diffraction mode should be Bragg diffraction. At this time, almost all the energy of the incident light is transferred to the first-order diffraction light.
The first AOM acouto-optic modulator is used at the front end of the system’s optical amplifier, modulating the continuous input light from the front end with optical pulses. The modulated optical pulses then enter the system’s optical amplification module for energy amplification. The second AOM acouto-optic modulator is used at the back end of the optical amplifier, and its function is to isolate the base noise of the optical pulse signal amplified by the system. The front and back edges of the light pulses output by the first AOM acouto-optic modulator are symmetrically distributed. After entering the optical amplifier, due to the gain of the amplifier for the pulse leading edge being higher than that for the pulse trailing edge, the amplified light pulses will show a waveform distortion phenomenon where energy is concentrated at the leading edge, as shown in Figure 3. To enable the system to obtain optical pulses with symmetrical distribution at the front and back edges, the first AOM acouto-optic modulator needs to adopt analog modulation. The system control unit adjusts the rising edge of the first AOM acouto-optic modulator to increase the rising edge of the optical pulse of the acoust-optic module and compensate for the gain non-uniformity of the optical amplifier at the front and back edges of the pulse.
The optical amplifier of the system not only amplifies the useful optical pulse signals, but also amplifies the base noise of the pulse sequence. To achieve a high system signal-to-noise ratio, the high extinction ratio feature of the optical fiber AOM modulator is utilized to suppress the base noise at the rear end of the amplifier, ensuring that the system signal pulses can pass through effectively to the greatest extent while preventing the base noise from entering the time-domain acousto-optic shutter (time-domain pulse gate). The digital modulation method is adopted, and the TTL level signal is used to control the on and off of the acoust-optic module to ensure that the rising edge of the time-domain pulse of the acoust-optic module is the designed rising time of the product (i.e., the minimum rising time that the product can obtain), and the pulse width depends on the pulse width of the system TTL level signal.
Post time: Jul-01-2025