Abstract:
A high accurate SOI optical waveguide Michelson interferometer sensor for temperature monitoring combines a waveguide coupler, waveguide, or splitter with two silicon-on-insulator Bragg gratings.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a temperature sensor, and more particularly to a Michelson interferometer temperature sensor by combining a silicon-on-insulator waveguide coupler with two silicon-on-insulator waveguide Bragg gratings. 
     BACKGROUND OF THE INVENTION 
     According to the present developments of the optical sensor, the fiber Bragg grating is still one of the major components for providing physical measurements. But the manufacturing time of the fiber Bragg grating is longer than that of the semiconductor grating, and the yield thereof is less than that obtained using semiconductor techniques. In addition, the size of the fiber Bragg grating is larger than that of the semiconductor waveguide, and thus the cost cannot be reduced. 
     Since the optical fiber communication network develops very fast, each reliable subscriber needs many highly accurate optical sensors built in key components. Developing low cost optical waveguide sensors based on the semiconductor manufacturing process will therefore be a trend in related fields. 
     Silicon is very easy to acquire and very cheap, and has been the major material in the IC manufacturing process, so the present invention uses silicon-on-insulator as the substrate. The advantages of high bandwidth and low power loss of MOSFET based on silicon-on-insulator lead it to be the best choice for the future OEIC (Optoelectronic Integrated Circuit). 
     The present invention applies the technology of IC semicoductor manufacturing process to the field of optical sensor to reduce the size of the component significantly. 
     DESCRIPTION OF THE PRIOR ART 
     A. D. Kersey and T. A. Berkoff disclosed in IEEE Photonics Technology Letters, vol. 4, no. 10, 1992, page 1183˜1185 that the fiber Bragg grating was used as a temperature sensing component, having an accuracy of 0.05° C. That result proved the feasibility of using Bragg grating as a temperature sensor. But the manufacturing cost is too high, and the effect is not so good as the temperature sensor of the present invention designed by using silicon-on-insulator waveguide grating. 
     Wei-Chong Du, Xiao-Ming Tao and Hwa-Yaw Tam disclosed in IEEE Photonics Technology, vol. 11, no. 1, 1999, page 105˜107 that the reflective spectrum of the fiber Bragg grating was used to analyze the variation of the temperature. However, the present invention uses two reflective gratings and Michelson interferometer effect to reduce reflective spectrum linewidth and achieve a more accurate temperature monitoring. 
     A. D. Kersey and T. A. Berkoff disclosed in IEEE Photonics Technology Letters, vol. 8, no. 9, 1996, page 1223-1225 that the fiber Bragg grating was successfully used as a temperature and pressure sensor, but the cost can&#39;t be reduced. The present invention utilizes the semiconductor technology to lower the cost. 
     T. W. Ang, G. T. Reed, A. Vonsovici, A. G. R. Evans, P. R. Routley and M. R. Josey disclosed in IEEE Photonics Technology Letters, vol 12, no. 1, 2000, page 59˜61 that the effect of the silicon-on-insulator waveguide grating was analyzed, and proved the feasibility of the silicon-on-insulator waveguide grating sensor. 
     Eric Udd disclosed in U.S. Pat. No. 5,591,965 (1997) that a sensor system was designed by a plurality of fiber gratings, and therefore proved the feasibility of multiplex physical measurement by fiber grating. However, the present invention does not use the periodic fiber grating shown in U.S. Pat. No. 5,591,965, but rather utilizes a semiconductor manufacturing process for commercialization. 
     Mark F. Krol disclosed in U.S. Pat. No. 96,075,907 (2000) that a plurality of fiber gratings of long period were arranged in a fiber network for monitoring the physical quantities of many points, such as temperature . . . and so on. U.S. Pat. No. 96,075,907 proved that an optical temperature sensor is very useful in fiber network, but the technology thereof is different from the present invention. 
     Stephen James Crampton, Arnold Peter Rosc and Andrew George Rickman disclosed in U.S. Pat. No. 5,757,986 (1998) that an optical modulating component was designed using a silicon-on-insulator waveguide, proving that the silicon-on-insulator waveguide is very useful and marketable. 
     OBJECT OF THE INVENTION 
     The present invention provides a Michelson interferometer temperature sensor including a 2×2 waveguide coupler and two waveguide Bragg gratings based on silicon-on-insulator substrate. The temperature can be read out through the sensor. The temperature variation can induce the wavelength response variation which results in the power variation. Since the thermal-optical expansion coefficient of the silicon-on-insulator is higher than that of a fiber, it can enhance the reslution of temperture measurement significantly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows schematically a silicon-on-insulator optical waveguide Michelson interferometer temperature sensor according to the present invention. 
     FIG. 2 shows schematically the structure of a 2×2 silicon-on-insulator optical waveguide coupler according to the present invention. 
     FIG. 3 shows schematically a side view of the silicon-on-insulator optical waveguide Bragg grating. 
     FIG. 4 shows a comparison of the reflective optical power spectrum between the SOI optical waveguide Michelson interferometer temperature sensor and the conventional fiber Bragg grating. 
     FIG. 5 shows a comparison of the rift of the reflective power spectrum induced by temperature variation between the SOI optical waveguide Michelson interferometer temperature sensor and the conventional fiber Bragg grating. 
    
    
     BRIEF DESCRIPTION OF THE REFERENCE NUMBERS 
       1  2×2 silicon-on-insulator optical waveguide coupler 
       2  silicon-on-insulator optical waveguide Bragg grating 
       3  reflective output electric field (Er 1 ) of port  1   
       4  reflective output electric field (Er 2 ) of port  2   
       5  reflective output electric field (Er 3 ) of port  3   
       6  reflective output electric field (Er 4 ) of port  4   
       7  input port length (l 1 ) of port  1   
       8  input port length (l 2 ) of port  2   
       9  output port length (l 3 ) of port  3   
       10  output port length (l 4 ) of port  4   
       81  optical power detector 
       82  microprocessor 
       83  laser 
       84  fiber 
       11  width of silicon-on-insulator optical waveguide 
       12  single mode ridge-type waveguide 
       13  single mode S-type ridge waveguide 
       14  single mode parallel-coupling waveguide 
       15  silicon guiding layer 
       16  silicon dioxide insulating layer 
       17  silicon substrate 
       21  amorphous silicon layer 
       22  sinusoidal grating in silicon layer 
       23  period of the grating 
       24  length of the grating 
     DESCRIPTION OF THE INVENTION 
     The present invention provides a Michelson interferometer temperature sensor by combining a 2×2 (two input ports and two output ports) silicon-on-insulator waveguide coupler  1  with two silicon-on-insulator waveguide Bragg gratings  2 , as shown in FIG.  1 . 
     Silicon-on-insulator waveguide coupler  1  comprises a single mode ridge-type waveguide  12 , a single mode S-type ridge waveguide  13 , a single mode parallel-coupling waveguide  14 , a silicon guiding layer  15 , a silicon dioxide insulating layer  16  and a silicon substrate  17 , as shown in FIG. 2 
     Silicon-on-insulator waveguide Bragg grating  2  comprises an amorphous silicon layer  21 , a sinusoidal silicon grating layer  22 , a silicon dioxide insulating layer  16  and a silicon substrate  17 , as shown in FIG.  3 . 
     Silicon-on-insulator waveguide coupler  1  and silicon-on-insulator waveguide Bragg grating  2  are both formed on silicon dioxide insulating layer  16  and silicon substrate  17 , and are contacted with each other. 
     When a light of 1.55 μm wavelength is projected through the silicon guiding layer  15  of the single mode ridge-type waveguide  12 , since the refraction index (nsi=3.5) of the silicon guiding layer  15  is higher than those of the air (nair=1 and the silicon dioxide insulating layer  16  (nsio 2 =1.5), the light will be confined within the waveguide due to the effect of total reflection. Since the outer diameter of a conventional fiber is about 125 μm, the present invention designed a single mode S-type ridge waveguide  13  (as shown in FIG.  2 ), then let the distance between waveguides being larger than 125 μm, so as to connect the silicon-on-insulator waveguide coupler  1  and the fiber conveniently. When the light passes through the fiber, single mode ridge-type waveguide  12  and then enters the single mode parallel-coupling waveguide  14 , the light will be coupled to another parallel single mode ridge-type waveguide  12  due to the weak coupling effect. 
     By the couple-mode equation, the present invention designed an optimal silicon-on-insulator waveguide Bragg grating having waveguide width  11  of 6 μm, sinusoidal silicon grating layer  22  of 1.5 μm, grating period  23  of 0.2215 μm, grating length  24  of 100 μm, silicon dioxide insulation layer  16  of 0.4 μm, and amorphous silicon layer  21  of 1 μm. By calculating of the couple-mode equations as shown below, the reflective optical power distribution presented by 1.55 μm light passing through the waveguide Bragg grating can be written as:                P   R     =           (       πΔ                 nf     c     )     2            sinh   2          [             (       πΔ                 nf     c     )     2     -       (         2      π                 nf     c     -     π   Λ       )     2            L     ]                           (       πΔ                 nf     c     )     2     -       (         2      π                 nf     c     -     π   Λ       )     2            2                     cosh   2          [             (       πΔ                 nf     c     )     2     -       (         2      π                 nf     c     -     π   Λ       )     2            L     ]         +                   (         2      π                 nf     c     -     π   Λ       )     2            sinh   2          [           (       πΔ                 nf     c     )          2     -         (         2      π                 nf     c     -     π   Λ       )     2        L       ]                         (   1   )                                
     in which c represents the light speed, f is the operating frequency for the grating, n is the refraction index, Δn is the refraction index difference between the grating layer and the covering layer. 
     When the thermal expansion property is considered into the waveguide Bragg grating, the relation between external temperature variation ΔT and the drift of the reflective optical spectrum f R  is shown as below:                f   R     =     c       [     1   +       (     E   +     T   0       )        Δ                 T       ]        2      n                 Λ               (   2   )                                
     in which E is the thermal expansion coefficient (2.6×10 −6 /° C.), T 0  is the thermaI-optical coefficient (8.6×10 −4 /° C.). 
     FIG. 1 shows schematically a silicon-on-insulator optical waveguide Michelson interferometer temperature sensor according to the present invention, in which the relation between the input electric field E in  and output electric field E ri  can be obtained by matrix algebra method as shown below:                E   r1     =           r                          -   β                     l   1             (     1   -   K     )            [       K                          -   2        β                   l   3           +       (     1   -   K     )                 -   2        β                   l   4             ]            E     i                 n                 (   3   )                 E   r2     =       [         i        K           K       1   -   K         +       1   -   K                r        (              -   2        β                   l   3         +            -   2        β                   l   4           )                 -     β        (       l   1     +     l   2       )             ]          E     i                 n                 (   4   )                                
     in which K is the coupling constant of the 2×2 waveguide coupler, r is the reflectivity of the wave guide Bragg grating, β is the waveguide propagation constant, l i  represents the length of one of the four input/output ports. 
     When equations (1) and (2) are substituted into the above-mentioned equations (3), (4), (5), (6) for simulation, the drift condition of the light passing through the waveguide Michelson Interferometer due to the environmental temperature can be obtained. The following embodiment describes the simulation result of the present invention. 
     Embodiment 
     In order to verify the feasibility of the present invention, a numerical analysis is employed to prove that the silicon-on-insulator optical waveguide Michelson interferometer temperature sensor according to the present invention can measure the environmental temperature. An optical light of 1.55 μm is considered to simulate the function and feature of the silicon-on-insulator optical waveguide Michelson interferometer temperature sensor. 
     FIG. 4 shows a comparison of the reflective optical power spectrum between the SOI optical waveguide Michelson interferometer temperature sensor and the conventional fiber Bragg grating temperature sensor, using wavelength of 1.55 μm, grating period  23  of 0.2215 μm, grating length  24  of 100 μm. According to the comparison of the reflective optical power spectra shown in FIG. 4, it is found that the SOI optical waveguide Michelson interferometer temperature sensor of the present invention has a narrower reflective optical spectrum when adopting the same grating period, and therefore has a higher accuracy than that of the fiber grating temperature sensor. 
     FIG. 5 shows a comparison of the drift of the reflective power spectrum by temperature variation of 10° C. between the SOI optical waveguide Michelson interferometer temperature sensor and the conventional fiber Bragg grating. By analyzing FIG. 5, it is found that the drift of the reflective power spectrum per 1° C. of the SOI optical waveguide Michelson interferometer temperature sensor is 20 times more than that of the conventional fiber Bragg grating temperature sensor. This result proves that the temperature sensing feature of the SOI optical waveguide Michelson interferometer temperature sensor is much more sensitive than that of the conventional fiber Bragg grating temperature sensor. 
     Features and Effects 
     The feature of the present invention is to combine the integrated circuit and the integrated optical sensor based on SOI substrate, and to reduce the size of an optical sensor, enhance the accuracy of temperature sensing by an SOI optical waveguide Michelson interferometer temperature sensor, therefore improve the effects of the temperature sensor. The effects of the SOI optical waveguide Michelson interferometer temperature sensor according to the present invention are as below: 
     1. Temperature sensing: can be used as an industrial sensor, a temperature controller for silicon IC wafer and a biomedical sensor. 
     2. High accuracy. 
     3. Narrow FWHM (Full Wavelength Half Maximum) 
     4. When it is used in optical communication network monitoring, the reliability of the network data transmission is enhanced.