Transcript
Vol . B10 , No. 3
CHINESE JOURNAL OF LASERS
J une , 2001
The Study of Full Light Speech Signal Collection System J IA Bo J IN Yaqiu ZHANG Wei ( Center for Wave Scatter and Remote Sensing , School of Information Science and Engineering of Fudan University , Shanghai 200433 , China)
HU Li YE Kunzhen ( National Key Laboratory of Optical Fiber Communication , University of Electronic Science and Technology of China , Chengdu 610054 , China) Received Sep . 1 , 2000 ; Revised manuscript received Nov. 16 , 2000 Abstract The demodulation character of 3 ×3 optic fiber couplers is analyzed , and the application in the coherent communication system and speech signal collection is pointed out in the paper. By the experiment , the feasibility of speech signal collection system by the way of the all light is verified. Key words optic fiber couplers , speech signal collection , full light
1 Introduction Optic fiber couplers have been widely used in the optical communication and optical sensors. Besides their power distribution functions , the couplers′coherent demodulation character which has been used in the field of [1 ,2 ] optic interferometer is also important , but that was little presented to particularly discuss. In the paper , on the ground of analysis of the couplers′demodulation character , a new speech signal collection system which depends on the optical interference is constructed. This is the first system used to pick2up speech signal up to present. The work for completing speech signal collection by the way of optical interference has [3 ] been doing since early seventy years , but the practical design scheme has not been appeared until the all fiber velocity interference emerges , the main speech signal collection depends on the way of speech signal being changed to electric signal. With regard to the main reasons , the following two shortcomings should be pointed out. ( 1) The light polarization state is unstable in the former speech signal collection system by the optical interference way. The systemic stability is not guaranteed. ( 2 ) The systemic sensitivity is varied with different phase of coherent light signal , the high accuracy of
speech signal collection can not be gained. With the development of the all2fiber [4 ] interferometer in the sensor field , the more reasonable system successfully used in the velocity measurement has been reported , all these make the speech signal collection by all light become possible. In the developing course of the full light communication , the full light speech signal collection possesses utility [5 ] merits . For example , in the light telephone transmission system , speech signal usually modulates the light with the acoustic2light modulator , after transmission in optical cable , it is restored by the sound2light demodulator in the receiver port. If the optical speech signal collection system is introduced , the sound2light modulator needn′ t be used , the light telephone transmission system will become more simple and compact. This change will directly boost the full light communication development. In the Ref . [ 6 ] , the optic microphone (which also can be regarded as optical speech signal collection system) was first presented. According to Ref . [ 2 ] , the optic microphone system is not ripe and can not be used in the speech signal collection practically. The speech signal collection system in the paper has the hybrid structure of Sagnac and Michelson interference , which eliminates the influence of light
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186 Vol . B10 , No. 3 CHINESE JOURNAL OF LASERS J une , 2001
polarization state. There are two output signals ( there is phase difference between the signals) in the system , that improves the system′s accuracy. By the experiments , the full light speech signal collection system can not only complete the speech signal collection perfectly but also have the high repeatability and stability. All these make the system have high2 priced in the optical speech signal collection field. At the beginning of the paper , the potential application of the optic fiber coupler in the coherent light communication and speech signal collection is indicated by analysing the demodulation character of a 3 ×3 optical fiber coupler. Secondly , the constitute of optical speech signal collection system is presented. Finally , feasibility of the optical speech signal collection is verified through the experiment.
2 Principle analysis Taking advantage of the coherent character of the two light with different phase in the 3 ×3 coupler , the optical speech signal collection system is constructed. The 3 ×3 coupler plays a key role in the system , so its demodulation character is analyzed first. As Fig. 1 shows , on the assumption that the light signals at the input port 2 and 3 are t ( ) A in2 = B exp i∫ 0 F1 t d t t
( ) A in3 = B exp i∫ 0 F2 t d t
( 1)
Fig. 1 The input and output signal of 3 ×3 optic coupler
In which , B is amplitude of the input light , F1 ( t ) and F2 ( t ) are both light frequency. Based on the coupling theory , the 3 ×3 couplers′ demodulation character is [7 ,8 ] derivated . Let aj ( j = 1 ,2 ,3) is complex and indicates the light amplitude in the different waveguide of 3 ×3 coupler. aj should satisfy the
below set of linear differential equations d ajΠd z + ikj , j +1 aj +1 + ikj , j +2 aj +2 = 0 ( 2) In which z is coupling length of coupler , j = 1 ,2 ,3 , j → j + 3 , kjk = k kj is the coupling coefficient between the jth waveguide and kth waveguide. In order to simplify the calculation , supposing k 12 = k23 = k 31 = k , the solution of Eq. ( 2) is aj ( z ) = cj e
ikz
+ de
- 2 ikz
3
, ∑cj = 0 ( 3) j =1
where the cj and d are coefficients in the solution of the Eq. ( 2) . If the 3 ×3 coupler′ s coupling length is L , the input signal is shown as Eq. ( 1) , the output signals are 2 3 PⅠ = a1 (φ ,L ) = a1 (φ ,L ) ×a1 (φ ,L ) = (4Π9) B 2 (1 - cos3 kL ) (1 + cosφ) (4) 2 2 PⅡ = a2 (φ ,L ) = (1Π9) B [7 + 2cos3 kL 2cosφ(1 - cos3 kL ) - 6sinφsin3 kL ] (5) 2 2 P Ⅲ = a3 (φ ,L ) = (1Π9) B [7 + 2cos3 kL 2cosφ(1 - cos3 kL ) + 6sinφsin3 kL ] (6) where t φ =∫ ( ) ( ) ( 7) 0 [ F1 t′ - F2 t′]d t′
If the coupler′s splitting ratio is 3 dB , based on the Eqs. ( 2 ) and ( 3 ) , result kL is equal to 40 degrees. With substituting the result into Eq. ( 6) , the Eqs. ( 4) ～ ( 6) become 2 t 2B ( ) (8) PⅠ = 1 + cos 2π∫ 0 F1 - F2 d t 3 2 t 2B ( ) PⅡ = 1 + cos 2π∫ 0 F1 - F2 d t + 120° 3 (9) 2 t 2B ( ) PⅢ = 1 + cos 2π∫ 0 F1 - F2 d t - 120° 3 (10) In the Eqs. ( 8 ) ～ ( 10 ) , the frequency difference is appearance Δ F = F1 - F2 ( 11) In the above equation ,supposing F2 ( t ) = f 0 is frequency of local light , F1 ( t ) = f 0 + f ( t ) is that of modulated light , Δ F is equal to f ( t ) . This indicates that signal coherent detection can be realized with the 3 ×3 optic coupler , which is of significance in coherent communication. Using the 3 ×3 optic coupler , the optical speech signal collection system is constituted as
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The Study of Full Light Speech Signal Collection System
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shown in Fig. 2.
Fig. 2 Optical sound signal collection system
Because the optic delay line exists , when the sound source oscillating , the two coherent optical paths abocd and dcoba is not same. The light through the path ab earlier arrives point o than that through path ab to point o . The reflected light will produce Doppler frequency [10 ] shift information . Through the 2 ×2 coupler , the two coherent lights correspond to frequency shift of different time′s Doppler , in terms of Eq. ( 11 ) , the light frequency difference is given as Δ F ( t ) = 2 [ V ( t ) - V ( t - τ) ] = λ 2τ ( τ ) ( 12) λV t - Π2
In which , λ is the laser wavelength , V ( t ) is the velocity of the reflecting surface at time t . Submiting the Eq. ( 11 ) to the Eqs. ( 9 ) ( and 10 ) , the output signals of the optical speech signal collection system are obtained. From Eq. ( 12 ) , if delay τ increases , the coherent light signals′frequency increases also.
3 Experiments Using the optical speech signal collection ( OSSC ) system ( shows as Fig. 2 ) , the experiment system is shown as Fig. 3. In the experiment , the speaker is employed as sound source.
Fig. 3 Experimental system
The wave of output signals is shown in Fig. 4 , the speaker driving voltage is 2 V , the frequency is 183 Hz , the optic delay line length is 750 m. From the figure , we can see that there is phase difference between two output signals , this makes it impossible that the two signals′ sensitivity are zero. If there is only one output signal , the systemic sensitivity will decrease to zero at many special points. All these indicate that the two output signals with difference phase are important and ensure for the systemic sensitivity. The signal wave fits quite well to the Eqs. ( 9) and ( 10) , this verifies that the former theory analysis is proper. Using the calculation
Fig. 4 The interference fringe of optical speech signal collection system
software , the profile of speaker amplitude can be gained , thus the speech signal character is fully recorded by the optical speech signal
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188 Vol . B10 , No. 3 CHINESE JOURNAL OF LASERS J une , 2001
collection system. In our experiment system , the software plays the role of the optical2acoustic demodulator in optical transmission system.
4 Results Based on the analysis of the 3 × 3 demodulation character of optic fiber coupler , the couplers′application in the light coherent communication and optical fiber sensor fields is pointed out. Using the 3 × 3 optic fiber couplers , an optical speech signal collection is constructed and through the experiments , a perfect optical coherent signal graph is got. The experiment results quite fit the theory analysis. Our experiment on the speech signal collection with full light is the novel approach , expecting that will bring new influence in the all light communication and voice E2mail research. References 1 S. K. Sheem , R. P. Moeller. Wavelength monitoring of signal2mode diode laser using guided2wave interferometer. Opt . Lett . , 1980 , 5 ( 5) :179～181 2 J . Breguet , N. Gisin. Interferometer using a 3 ×
3 coupler and Faraday mirrors. Opt . Lett . , 1995 , 20 ( 12) :1447～1449 3 J . H. Cole , R. L. Jahnson. Fiber optic2 detection of sound. J . Acoust . Soc . Am . , 1977 , 62 ( 5) :1136～1138 4 Bo Jia , Li Hu , Hua Tan et al . . Fiber2optic interferometer for measuring low velocity of diffusely reflecting surface. Microwave and Opt . Technol . Lett . , 1999 , 22 ( 4) :231～234 5 ZHOU Ming2Tian , WANG Wen2Yong. TCPΠIP Net work theory and technology. Published by Qing2Hua University , 1999. 243 ～ 244 ( in Chinese) 6 J . Breguet , J . P. Pellaux , N. Gisin. Sens. Actuators A ( to be published) 7 Sang K. Sheem. Optical fiber interferometers with [ 3 ×3 ] directional couplers : Analysis. J . Appl . Phys . , 1981 , 52 ( 6) :3865～3872 8 Sang K. Sheem , Fiber2optic gyroscope with [ 3 × 3 ] directional coupler. Appl . Phys . Lett . , 1980 , 37 ( 10) :869～871 9 J I Yue2Feng et al . . Optical fiber digicom foundation. Published by Science and Technology Press , 1994. 200～203 ( in Chinese) 10 L. M. Barker , R. E. Hollenbach. Laser interferometer for measuring high velocity of any reflecting surface. J . Appl . Phys . , 1972 , 43 : 4669～4675
Biography JIA Bo was born in 1971 , Sichuan province. He received his M. S. degree and Ph. D. degree in the National Key Laboratory of Optical Fiber Communication , University of electronic Science and Technology of China in 1997 and 2000 , respectively. Now he is postdoctoral of School of Information and Engineering of Fudan University. His main work is about optic2fiber communication and optic2fiber sensors.
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