Flying over Scanning Holography
A laser beam is split into lights of different paths. Then the beam from a single path is modulated into a specific frequency by an AOM.
Two beams modulated into different frequencies are expanded through a beam expander, formulating a plane wave and a spherical wave, respectively.
The plane wave and spherical wave are then combined to generate a time dependent FZP,
and a scanning mirror unit is used to scan the object.
The light reflected from the object is collected to a single point through a lens
and is converted into an electrical signal using a photodetector with the features of a single-pixel detector.
01 Scan beam generation unit
The laser beam output from the light source is split into upper and lower path beams by the splitter. Through an acousto-optic modulator (AOM), the beams are transposed to different frequencies. The temporally modulated upper and lower path beams are then expanded by a beam expander which generates a plane wave (lower path beam) and a spherical wave (upper path beam).
The plane wave and the spherical wave transferred to the upper frequency are interfered with by the beam splitter and becomes a Fresnel zone plate (FZP) pattern in terms of space. In terms of time, the beams are beating with a frequency equal to the difference of each AOM modulation frequency. Such beams are called time-dependent FZP and are transmitted to the x-y scanning mirror.
02 Scanning mirror unit
A scanning mirror unit scans objects with a scanning mirror.
03 Photo-detection unit & Lock-in processing unit
The light reflected from the object is collected by a condensing lens and transmitted to a single-pixel photodetector (PD). A current is generated by the intensity of the light transmitted to the photodetector, which is then transmitted to the lock-in processing unit.
A cosine signal is obtained through the difference in the modulated frequency of each AOM. A sine signal, which is a q-phase signal, is also generated. Such signals are multiplied by the current signal generated by the PD. The signals that pass the low-pass filter become a cosine & sine hologram which is a convolution signal with the in-phase part of the time-dependent FZP and the reflection rate distribution of the object. This signal passes through the DAC (Digital to analog Converter) and is transmitted to the Digital Computer.