1、The Study of Full Light Speech Signal Collection SystemAbstract 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 spee

2、ch signal collect ion system by the way of the all light is verified.Key words optic fiber couplers, speech signal collection, full light1 IntroductionOptic fiber couplers have been widely used in the optical communication and optical sensors. Besides their power distribution functions, the couplers

3、 coherent demodulation character which has been used in the field of optic interferometer 1, 2 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

4、 on the optical interference is constructed. This is the first system used to pick-up speech signal up to present. The work for completing speech signal collection by the way of optical interference has been doing since early seventy years 3 , but the practical design scheme has not been appeared un

5、til 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 polar ization state is unstable in the former

6、speech signal collection system bythe 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 ofspeech signal collection can not be gained. With the development of the all-fiber interf

7、erometer in the sensor field 4 , 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 pos

8、sesses utility merits 5 For example, in the light telephone transmission system, speech signal usually modulates the light with the acoustic-light modulator , after transmission in optical cable, it is restored by the sound-light demodulator in the receiver port. If the optical speech signal collect

9、ion system is introduced, the sound-light 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 theRef. 6 , the optic microphone (which also can be regarded as optical speech

10、 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 eliminat

11、es the influence of lightpolarization 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 perfectl

12、y but also have the high repeatability and stability. All these make the system have highpriced 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 indica

13、ted by analysing the demodulation character of a 3 x3 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 analysisTaking advantage of the c

14、oherent character of the two light with different phase in the 3x3 coupler, the optical speech signal collection system is constructed. The 3x3 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 in

15、put port 2 and 3 are （1）Fig. 1 The input and output signal of3x3 optic couplerIn which, B is amplitude of the input light, F1( t and F2( t are both light frequency. Based on the coupling theory, the 3x3 couplers demodulation character is derivated. 7, 8Let aj ( j = 1, 2, 3 is complex and indicates t

16、he light amplitude in the different waveguide of 3􀀂 3 coupler. aj should satisfy the below set of linear differential equations （2）In which z is coupling length of coupler, j= 1, 2, 3, j j + 3, kjk = kkj is the coupling coefficient between the j th waveguide and kth waveguide. In order to s

17、implify the calculation, supposing k 12 = k 23 = k 31 = k, the solution of Eq. ( 2 is （3）where the cj and d are coefficients in the solution of the Eq. ( 2 .If the 3x3 coupler s coupling length is L ,the input signal is shown as Eq. (1 , the output signals are （4） （5） （6）where （7）If the coupler s sp

18、litting ratio is 3 dB, based on the Eqs. ( 2 and ( 3 , result kL is equal to 40 degrees. With substituting the resultinto Eq. ( 6 , the Eqs. ( 4 ( 6 become （8） （9） (10Using the 3x3 optic coupler, the optical speech signal collection system is constituted as shown in Fig. 2.Fig. 2 Optical sound signa

19、l collect ion systemBecause 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 ear lier arr ives point o than that through path ab to point o. The reflected light will produce Doppler frequency shi

20、ft information 10 . Through the 2 x2 coupler , the two coherent lights correspond to frequencyshift of different time s Doppler, in terms of Eq. ( 11 , the light frequency difference is given as (12In which, ! is the laser wavelength, V( t is the velocity of the reflecting surface at time t .Submiti

21、ng 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 increases, the coherent light signals frequency increases also.3 ExperimentsUsing the optical speech signal collection( OSSC system ( shows as Fig. 2 , t

22、he experiment system is shown as Fig. 3. In the experiment, the speaker is employed as sound source.Fig. 3 Experimental systemThe wave of output signals is shown in Fig. 4, the speaker dr iving voltage is 2 V, the frequency is 183 Hz, the optic delay line length is 750 m.Fig. 4 The interference frin

23、ge of optical speechsignal collect ion system4 ResultsBased on the analysis of the 3x3 demodulation character of optic fiber coupler , the couplers application in the light coherent communication and optical fiber sensor fields is pointed out. Using the 3x3 optic fiber couplers, an optical speech si

24、gnal 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 br ing new influence in the all light communication and voice E-mail research.References2. J. Breguet，N. Gisin. Interferometer using a 3x3 coupler and Faraday mirrors. Opt. Lett.，1995,20(12：144714493. J. H. Cole，R. L. Jahnson. Fiber optic-detection of sound. J. Acoust. Soc. Am.，1977，62(5：113611384. Bo Jia，Li Hu，Hua T