Development of digital hologram research, present situation and problems
The digital hologram dry plate with a photoelectric sensor member ( such as a CCD or CMOS) instead of recording a hologram , the hologram is then stored in the computer , the computer simulation process to achieve the objects to be recorded of the holographic reproducing and processing of the optical diffraction . The digital holography with traditional optical holographic compared with the advantages of low production cost, the imaging speed, the recording and reproducing flexible . In recent years, with the computer , especially the development of the high-resolution CCD digital holographic technology and its applications are more and more concerned about the scope of its application has been involved in the morphology measurements , deformation measurements , particle field testing , digital holographic microscopy , forgery , three-dimensional image recognition, medical diagnosis and many other areas . This paper describes the development and research focus of the digital holography and the problems .
The hologram technology 's development
The holography imaging process is divided into two steps : record and wavefront wavefront reproduction . Its main feature is that it not only records the amplitude information of the object , and record the phase information of the object , and more truly reflect the original objects . The hologram recording means and the different ways of reproduction generally holographic technology can be divided into three categories .
Optical hologram : The hologram recording process is optical process , the reproduction process is the use of optical illumination to achieve, this holographic process is a conventional optical hologram ;
Calculation hologram : the use of computer simulation of propagation of light , through the computer to form a hologram formed print hologram Microform Motherboard.
Also available laser direct write system forming a computer hologram CGH or using a liquid crystal light valve or spatial light modulator display holograms, using the illumination optical reproducing such a holographic method is referred to as a computer generated hologram ; one of the important characteristics of a hologram is that it scale small recording interference fringes reached nanoscale , which resulted in too much information for the computer generated hologram , a hologram contains coded reproduced is a calculation amount is too large , so that the Fourier spectrum , and therefore early CGH able to handle the images are simple symbols and stripes .
Digital hologram : digital holography is a new holographic imaging method proposed by the of Gu Dexin door in 1967 , CCD optocoupler devices instead of traditional dry version recorded holograms by computer in digital form on the holographic Figure reproduced , constrained by a variety of conditions but then , there has been no significant progress , with the advent of the development of computer technology and high resolution CCD , charge-coupled device , digital holographic technology before they get rapid development .
Development of digital hologram
To one aspect of the development of the digital hologram is mainly due to the shortcomings of optical holography in the recording medium itself is difficult to achieve high sensitivity requirements , forcing the scientists search for the high sensitivity of the holographic technique . The other hand , with the rapid development of microelectronics and computer technology , CCD instead of film as a Fourier transform coding stripes show carrier also greatly improved the calculation speed of modern computers , digital holography using CCD charge-coupled device to replace the traditional optical holography in the recording medium to record the hologram , the reconstruction process is completed , the development of holographic technology and recording media has laid a solid foundation for the emergence of digital holographic computer .
Charge Coupled Device (CCD) is a semiconductor solid 1970 Bell Labs invented device has the advantages of small size , light weight , high sensitivity , long life , low power consumption and dynamic range , it is a simple structure of silicon photoelectric conversion function , storage capabilities, and scan functions are combined , so it does not need to warm-up , easy to use , the ability of anti- vibration and shock as well as damage caused by anti- ability , and are not easily by the lag and the impact of the smear . CCD detector in real time the two-dimensional optical reproducing an image signal is converted to a timing electrical signal , which has a simple, fast , compatible with the electronic signal processing system , easy to interface with computer advantages , the drawback is the spatial resolution of the optoelectronic receiver and lower gray levels .
Research of current digital hologram situation
Study of the spatial light modulator
The core component of the digital holography system is a spatial light modulator . By two-dimensional picture it may be directly input to the photographic optical path in the hologram , the completion of the related records. Under the control of the source signal , it can be modulated in a parameter of the lightwave modulation amplitude , for example , by absorption through a refractive index modulation phase , through the rotation of the polarization plane modulation states of polarization , and so on , so that the source signal by the load The information is written into the light waves into .
Currently there are two: addressing the thin-film transistor liquid crystal display TFT-LCD and digital micro-mirror DMD . Electrical signal addressed liquid crystal display device LCD can directly receive an analog or digital electronic signals from the CCD camera or a computer , which has a programmable control and flexibility , the liquid crystal display by the number of two - dimensional matrix structure pixels , each pixel equivalent to a small liquid crystal cell , the liquid crystal cell is generally equipped with a twisted by 90 degrees , the nematic liquid crystal , it is possible to independently control them by electrical signals , respectively . However, due to the lower resolution and contrast of the LCD screen affect the quality of the hologram , whereas the DMD system developed in recent years has a great potential for development , and its micro- structure is fixed on two support column Lm order reflection mirror prime unit , and only at a specific angle ( the angle of the optical axis is 10 or 12 degrees ) is incident to these micro mirror light incident on the projection lens can be imaging . The system has a higher light energy utilization , of more grayscale and greater pixel resolution .
Digital hologram image reproduction zero-order image