Volume 8 l, number 5
OPTICS COMMUNICATIONS
I March 1991
Plastic optical isolator Shinzo Muto, Nobuo Seki, Shin-ichiro Ichikawa and Hiroshi Ito Department of Electrical Engineering and Computer Science, Faculty of Engineering, Yamanashi University. 4-3-11 Takeda, Kofu, 400 Japan Received 6 August 1990; revised manuscript received 19 October 1990
A plastic optical isolator was constructed for the blue light region and tested at the 488-nm line of an argon laser for the first time. As the Faraday rotator of the isolator, poly-a-methylstyrene rods were placed in the magnetic field of five permanent magnets. This plastic isolator had about 15 dB isolation and about 5 dB insertion loss.
1. Introduction A n optical isolator is an indispensable device in optics because it can prevent optical feedback which often seriously i m p a i r s optical systems [ 1 - 4 ] . Therefore, several optical isolator devices have been developed by using a variety o f magnetooptical materials: i.e. the garnet crystals ( t e r b i u m a l u m i n i u m garnet, yttrium iron garnet, etc.) [ 5 ], crystal o f ZnSe [6], glasses (heavy lead glass, t e r b i u m - p h o s p h a t e glass, etc. ) [ 1,7 ], a n d so on. However, almost all o f t h e m are expensive and operate as a F a r a d a y r o t a t o r at longer wavelength region than red. There is also need for a short wavelength isolator in m a n y experimental setups. Then, we a t t e m p t e d to construct such an isolator using an inexpensive plastic m a t e r i a l a n d p e r m a n e n t magnets. The e x p e r i m e n t for the 488-nm line o f argon laser presented a possibility o f a bulktype or fiber-type plastic optical isolator. This p a p e r reports on these properties.
2. Experimental Poly-a-methylstyrene ( P a M S ) was used as a plastic F a r a d a y rotator since it has a wide t r a n s p a r e n t region from 400 n m to near infrared a n d a d i a m a g netic property [ 8 ]. In this experiment, the r o d samples o f the P a M S with 5 m m d i a m e t e r were prepared. Its Verdet constant was m e a s u r e d at the three lines o f a H e N e laser (543.5, 594.1 a n d 632.8 n m )
a n d the 488-nm line o f an argon laser. In general, plastic materials have a large birefringence which diminishes the effective value o f the Verdet constant. However, the carefully p r e p a r e d P a M S rod in this e x p e r i m e n t h a d relatively lower birefringence. Therefore, the m e a s u r e d values o f the Verdet constant b e c a m e slightly large as c o m p a r e d with that o f ref. [ 7 ]. The d a t a are s u m m a r i z e d in table I. These values well fitted to the equation V= --0.0332-1-248002 -2 ,
( 1)
with 2 expressed in n m a n d V in rain O e - ~ c m - ~. Furthermore, the absorption coefficient o f the P a M S rod was measured to be lower than 0.2 c m in the spectral region b e y o n d 420 nm. F r o m these results, it is found that the P a M S rod can be used as the F a r a d a y rotator material for the blue light isolator. The practical structure o f the plastic optical isolator, which was constructed for the 488-nm line o f Table 1 Verdet constant of PaMS plastic rods as a function of the wavelength. Wavelength (nm)
Verdet constant (min Oe-i cm-i )
488.0 543.5 594.1 632.8
0.071 0.051 0.037 0.029
0030-4018/91/$03.50 © 1991 - Elsevier Science Publishers B.V. ( North-Holland )
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OPTICSCOMMUNICATIONS
optical isolator at 0:45 °
0
A
.
mognet 24
50
1 March 1991
i0
T
insertion loss
%-5
• pol.orizer
(PtxlvI5, ~1~19) 5
i
" ~
' or~lyzer
Fig. 1. Structure of plastic optical isolatorand experimental setup.
an argon laser, is shown in fig. 1. The permanent magnets employed here are circular cylinder types with a concentric hole of diameter 5 mm and a 24 mm length. To obtain a higher magnetic field intensity on axis, the above five permanent magnets were arranged in a series with a separation of 50 mm. By using this arrangement, an average magnetic field intensity of 4 kOe was obtained in each hole of the five permanent magnets. Under this condition, a total PctMS rod length of 19 mm X 5 was required to obtain a Faraday rotation of 0= 45 °. The end-surfaces of the five PctMS rods with 19 mm length were polished at small angles to reduce backreflection and were inserted into the above magnets without antireflective coating. To avoid the influence of radial nonuniformity of the birefringence axis [ 5 ] and the magnetic field provided by the magnets [2,3], the position of the laser beam with a spot size of 0.8 mm was set to the center of the PaMS rods. The transmitted laser power through this isolator device was measured as a function of the rotating angle of the analyzer. The result is shown in fig. 2. As can be seen from this figure, the isolator operation with about 15 dB isolation and about 5 dB insertion loss is obtained at 0 = 4 5 ° . The low isolation obtained here is mainly caused by light propagation through birefringent long PaMS rods. In addition, the relatively high insertion loss is attributed to absorption (3.0 riB) and Fresnel reflection from the end surfaces (2.2 dB) of the five PaMS rods. Therefore, if we can use the shorter P~tMS rod with antireflective coating and strong permanent magnets, an improvement of the performance of the plastic optical isolator can be obtained.
274
c-
-2G
-90
-45 0 45 90 Rotating angle of anotyzer O (deg.)
Fig. 2. Transmitted light intensity through the isolator device as a functionof the rotatingangle of the analyzer.
3. Conclusions The operation of a plastic optical isolator using PaMS rods and permanent magnets was demonstrated for the first time. This isolator presented about 15 dB isolation and about 5 dB insertion loss at 488 nm. A further improvement of the characteristics can be obtained by using stronger permanent magnets and by decreasing the birefringence for the shorter plastic rod. These results also showed the possibility of a plastic fiber isolator and a plastic fiber sensor for higher current or magnetic field.
Acknowledgement This work has been supported by a Scientific Grant of the Ministry of Education in Japan (General C, No. 02805039).
References [ I ] W.G. Driscoll, ed., Handbookof optics (McGraw-Hill,New York, 1972) pp. 17-21. [2] D.J. Gauthier, P. Narum and R.W. Boyd, Optics Lett. I 1 (1986) 623.
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OPTICS COMMUNICATIONS
[ 3 ] C.B. Carlisle and D.E. Cooper, Optics Comm. 74 ( 1989) 207. [ 41 K.W. Chang and W.V. Sorin, Optics Lett. 15 ( 1990) 449. [ 51 S. Matsumoto and S. Suzuki, Appl. Optics 25 ( 1986) 1940. [6] J.A. Wunderlich and L.G. DeShazer, Appl. Optics 16 ( 1977) 1584.
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[ 7 ] A.B. Villaverde and E.C.C. Vasconcellos, Appl. Optics 2 1 (1982) 1347.
[ 81 S. Muto, S. Ichikawa, T. Nagata, A. Matsuzaki and H. Ito, J. Appl. Phys. 66 ( 1990) 39 12.
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