Design of acatadioptric panoramic lens with a long focal length

Junhua Wang

Supervisor: Min Xu

In contemporary information times, the information acquiring technology is in increasing high requirement. In the realm of optical imaging, the lens designed are always hoped to be characterized with long focal length, large angle of view and large relative aperture[1].Panoramic lens can realize the 360° panoramic annular imaging of the surroundingobjects by using a special panoramic imaging device[2]. Comparing with other panoramic lens, the two catadioptric panoramic lens have the advantages of a larger field of view, a larger relative aperture and a longer focal length, great real time response, high reliability[3-7].

Through the analysis of the imaging relationship and the structure characteristics of the two catadioptric panoramic lens, the main factors affecting the panoramic lens imaging quality are: the lateral chromatic aberration generated by panoramic annular lens is difficult to be corrected; the stray light rays entering the panoramic annular lens are hard to be suppressed[8-9]. The solution to the above problems is forminga special conjugation between the aperture stop and the entrance pupil, thus the conjugation between the aperture stop and "annular entrance pupil" are realized.

Using a high order aspheric surface in the panoramic annular lens can increase the optical designing variables, and the special conjugation relation is formed, meanwhile the special imaging relations are achieved and this improves the panoramic lens focal length.

The initial parameters of the panoramic annular lens are worked out based on the structure of annular lens, the path of the chief rays, the aplanatic principle, the law of reflection, the aperture stop and the "the annular  entrance pupil" conjugation relationship. Then the initial parameters are entered into the Zemax software, and after the optimization, the focal length of 10.787mm is calculated and the angle of view of the lens is 360°×（45°~85°）. The designing results show thatthe panorama lens has a simple structure and good imaging quality.

In addition, the tolerance analysis, the mechanical designing and the assembly adjustment are made, and then the panoramic image is obtainedby the digital camera with macro objective distance mode. As a result, the resolution of the panoramic image is high and with littlelateral distortion.

References

[1]CUIQingfeng Design of hybrid diffractive-refractive imaging optical systems[J] Infrared and Laser Engineering.2006, 35(1):12-15.

[2]GREGUSS P．The tube peeper：a new concept in endoscopy[J]．Optics and Laser Technology，1985，17(1)：41-45．

[3]Powell I．Panoramic lens[J]．Appl.Opt，1994，33(31)：7356-7361．

[4]Powell I．Panoramic lens[P]．U.S. Patent：473,474．1995：

[5]Gachter S，Pajdla T，Micusik B．Minor Design for an Omnidirectional Camera with a Uniform Cylindrical Projection when Using the SVAVISCA Sensor[R]．Center for Machine Perception，Czech Technical University，2001．

[6]SuX，ZengJ．Catadioptricomnidirectionalsystemwithun-distorted Imaging for horizontal scene(A)．Proceedings of SPIE，2002，4927：23-28．

[7] Powell I．Design study of an infrared Panoramic optical system[J]．Appl. Opt，1996，35 (31)：6190-6194．

[8] S. Niu，J. Bai，X. Y. Hou， G. G. Yang．Design of a panoramic annular lens with a long focal length[J]．Appl. Opt，2007，46(32)：7850-7857．

[9] Z. Huang，J. Bai，and X. Y. Hou．Design of panoramic stereo imaging with single optical system[J]．Opt. Express，2012，20(6)：6085–6096．

Synthesis and Properties of Fe-N Magnetic Films

Bo Yu

Supervisor:Bin Ma

Recently, the information explosion stresses the lack of data storage year by year, which obliges the storage capability to be raised urgently. As we know, a hard storage layer such as FePt, whose perpendicular magnetocrystalline anisotropy constant K_u is as high as 7´10⁷erg/cc and smallest thermal stable grain size reaches 2.4nm, is an ideal storage material^[1]. However, the magnetic storage has met with some difficulties, called the ”Trilemma”. That is to say the writability, the thermal stability and the signal to noise ratio restrict each other, which has a great influence on the traditional hard disk storage^[2]. In order to make the high K_u media writable, exchange coupled composite (ECC) media consisting of a hard storage layer and a soft switching-assisted layer has been proposed.

 Fe-N thin films have been widely studied. They are excellent soft magnetic storage materials due to their variety of structural and high corrosion/wear resistance and good mechanical properties. The research of ferromagnetic iron nitride compounds are mainly focused on three aspects: magnetic film materials, magnetic powders and the theoretical calculation^[3-5]. However, Fe-N thin films have many different phases, such as Fe₂N, Fe₃N, γ’-Fe₄N and α’’-Fe₁₆N₂ and so on. Many experiments have been carried on to study the synthesis and properties of the single phase Fe-N magnetic films^[6-7]. So far the single phase Fe-N magnetic films have not been synthesised. Under the background all above, we take a lot of experimental work on fabrication and property study of Fe-N magnetic films.

In our work, we fabricated iron nitride ferromagnetic film by using the DC Magnetron Sputtering. We bubbled into N₂ with different flow ratios into the sputtering cavity. When the N₂ flow ratio is 10%, we have successfully fabricated the single phase Fe₄N magnetic films, whose diffraction peaks could be observed obviously. Then, we have researched the in-plane M-H loops and the angle dependence of single phase Fe₄N magnetic film in room temperature by using the vibrating sample magnetometry (VSM). In order to get higher saturation magnetization, we have fabricated [Fe4N/Fe]_N composite films in MgO substrate. In these composite films we choose 10% and 20% N₂ flow ratios to deposit the Fe-N films. We have compared the in plane M-H loops of different layers of [Fe₄N/Fe]_N in room temperature and got a lower Ms than pure Fe. Then the composite films were post-annealed at 400, 450, 500 degree celsius.

From our experiments, we have showed the structural and magnetic properties of nanocrystalline Fe-N thin films were significantly influenced by deposition conditions. Different N₂ flow ratios and sputtering temperatures can lead to the difference of saturation magnetization and the magnetic coercivity. We have successfully fabricated the single phase Fe₄N magnetic films. The angle character of the Fe₄N film with 30nm relates to the domain wall movement. The decrease of the saturation magnetization of [Fe₄N/Fe]_N multilayer films relates to the interface “phase”. The different deposition conditions will lead to the different magnetic properties of the interface phase. Our next work is to improve the magnetic properties of the interface phase. Our results would make Fe-N films more attractive to be the suitable materials instead of the rare earth materials.

References

[1]  Honghua Guo, Haigang Chu, Jialin Liao, Bin Ma, Zongzhi Zhang, Qingyuan Jin, IEEE Trans. Magn. 2013, 49, 7.

[2]  M. Mallary, A. Torabi, M. Benakli, IEEE Trans. Magn. 2002, 38, 4.

[3]  Jianping Wang, Nian Ji, Xiaoqi Xu, Yiming Wu, IEEE Trans. Magn. 2012, 183, 5.

[4]  Kohetsu Yamanaka, Yuki Onuma, Shohei Yamashita, Yuji Masubuchi, Takashi Takeda, Shinichi Kikkawa, Journal of Solid State Chemistry. 2010, 108, 2236-2241.

[5]  H. Sims, W. H. Butler, M. Richter, K. Koepernik, Physical review B. 2012, 86, 174422.

[6]  H. Takahashi, M. Igarashi, A. Kaneko, H. Miyajima, Y. Sugita, IEEE Trans. Magn. 1999, 35, 5.

[7]  V. Hari Babu, J. Rajeswari, S. Venkatesh, G. Markandeyulu, Journal of Magnetism and Magnetic Materials. 2013, 339, 1-5.

Design of non-polarizing cut-off filters based on dielectric-metal-dielectric stacks

Dongdong Zhao

Supervisor: Yuxiang Zheng

Cut-off filters are widely used in optical system, especially in multichannel optical system [1]. To separate different wavebands, for instance, visible and infrared light, dielectric-metaldielectric (DMD) stacks are often used in cut-off filters [2]. They are also used in band passfilters to get narrow pass band and wide cut-off region [3]. For the extinction coefficients ofsome metal materials, such as Ag, Au, Al, etc., are approximately proportional to wavelengthin visible/infrared region, very thin metal layer induces bigger change of equivalentoptical admittance of film at longer wavelength, which induces higher reflectance at longerwavelength. In many situations, optical films are operating at oblique incidence, whichinduces different transmission properties of s- and p-polarized light. In some advancedoptical systems, polarization state of the light contains key information, and very low linearpolarization sensitivity (LPS) is required. LPS is defined as [4]

Where, T is transmittance of different linear polarized light. It means all the optical elementsmust be designed and fabricated non-polarizing. A lot of works have been done on thesubject of polarization of optical films. On the subject of designing non-polarizing cutoff filters (edge filters, heat mirrors or dichroic beam-splitters), several approaches to thedesign of non-polarizing all-dielectric cut-off filters have been suggested in the references. Few works have been done on the design of non-polarizing cut-off filters based onDMD stacks.

Cut-off filters are usually operating at oblique incidence andexhibit polarization dependence properties. We propose a simple approachto design cut-off filters with low linear polarization sensitivity (LPS) basedon dielectric-metal-dielectric (DMD) stacks. The designing method isderived from the theory of optical film characteristic matrix. Theadmittance loci of the film are adjusted to achieve similar spectralproperties of s- and p-polarized light at oblique incidence. Different filmstructures are designed non-polarizing at different angles of incidence withthe method. The results show that the designing method is efficient fordesigning non-polarizing cut-off filters, which are widely used in nonpolarizing optical system.

References

1. P. N. Slater, Remote Sensing: Optics and Optical Systems (Addison-Wesley, 1980).

2. P. Ma, F. Lin, and J. A. Dobrowolski, Appl. Opt. 50(9), C201–C209 (2011).

3. P. H. Berning and A. F. Turner, J.Opt. Soc. Am. 47(3), 230–239 (1957).

4.W. L. Barnes, T. S. Pagano, and V. V. Salomonson, IEEE Trans. Geosci. Rem. Sens. 36(4), 1088–1100 (1998)

Time:  6:30 pm, Thursday, 2014.11.20

Location:Optical Building. Room 525