Abstract
An acousto-optic (AO) holographic display unit based on a suspended waveguide membrane was developed. The AO unit consists of a wide bandwidth chirp interdigital transducer (IDT) on a 20 µm thick suspended crystalline 128° YX LiNbO3 membrane, a light blocker with a 20 µm hole near the entrance, and an active lens near the exit. The 20 µm thickness of the floating membrane significantly enhanced surface acoustic wave (SAW) confinement. The light blocker was installed in front of the AO unit to enhance the coupling efficiency of the incident light to the waveguide membrane and to remove perturbations to the photodetector during measurement at the exit region. The active lens was vertically attached to the waveguide sidewall to collect the diffracted beam without loss and to modulate the focal length in free space through the applied voltage. As SAWs were radiated from the IDT, a Bragg grating with periodic refractive indexes was formed along the waveguide membrane. The grating diffracted incident light. The deflection angle and phase, and the intensity of the light beam were controlled by the SAW frequency and input power, respectively. The maximum diffraction efficiency achieved was approximately 90% for a 400 MHz SAW. COMSOL simulation and coupling of mode modeling were performed to optimize design parameters and predict device performance.
Highlights
Various types of holographic display technologies are currently available, these displays are not suitable for use in portable devices because of their large volume and power consumption, high cost, and system complexity
4” LiNbO3 and silicon, were bonded together and treated by chemical mechanical polishing (CMP) to reach to the desired thickness of the piezo membrane, which was processed by Nanoln (Jinan Jing zheng Electronics Co.) on customer demand (Figure 5a)
After completing the interdigital transducer (IDT) patterns on the top surface, a 20 μm thick PR (AZ 4620) layer was patterned on the back side of the wafer as a masking layer for the subsequent deep reactive ion etch (DRIE) process in which the top surface was protected with PR as well (Figure 5c)
Summary
Various types of holographic display technologies are currently available, these displays are not suitable for use in portable devices because of their large volume and power consumption, high cost, and system complexity. Most of the current technology in SLMs employs optical addressing technology that either moves the light source directly to form a 3D image or uses a secondary light source with no mechanical movements [5,6]. The weaknesses of these systems include bulky dimension, high power consumption, small bandwidth, blurry images due to small deflection angles, small viewing displays, and poor scalability. Among the possible SLM technologies, acousto-optic (AO) based SLMs are considered to be the most promising candidates for overcoming these limitations as they do not require motors for beam adjustment, and have high light efficiency, scalability, and portability [7,8,9].
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