Abstract:
An apparatus and a method for modulating the optical intensity, with the noises suppressed by using a linear optical modulator, are disclosed, so that the characteristics of signals transmitted from an optical communication system can be improved, and that the resolution of the measured physical quantities can be improved in an optical measuring instrument. The method is carried out in the following manner. That is, the magnitude of frequency is measured by utilizing pilot signals and electrical signals to transfer negatively fed-back signals to an optical intensity modulator. The phase is delayed by generating pilot signals so as to suppress an amplitude noise from the pilot signal caused by nonlinear modulation at the optical intensity modulator. A linear modulation is carried out on the phase-delayed signals and on optical signals from an external source to provide a linear component. The bias voltage is adjusted in accordance with the negatively fed-back signals, and an optical intensity modulation is carried out on the linearly modulated signals and on the added signals, the added signals having been formed by combining the data signals and the pilot signals.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and a method for modulating the optical intensity with a noise suppressed by using a linear optical modulator; and more Particularly, to an apparatus and a method for modulating the optical intensity a noise suppressed by using a linear optical modulator, in which, in a method for adjusting the bias voltage of an optical modulator by using the pilot signals, the noises due to the pilot signals are suppressed, so that the characteristics of signals transmitted from an optical communication system can be improved, and that the resolution of the measured physical quantities can be improved in an optical measuring instrument. 
     DESCRIPTION OF THE PRIOR ART 
     An optical intensity modulator is usefully applied not only to the optical communication field but also to the other optical technology including the optical measurements. Further, the optical intensity modulator necessarily uses the bias voltage adjusting method to improve the performance in the measurement and in the optical communication. The method in this connection is divided into: a method of measuring the optical intensity to adjust the bias voltage; and a method of using the pilot signals to adjust the bias voltage. 
     The method of measuring the optical intensity to adjust the bias voltage has a simple signal processing structure, and is very sensitive to external influences. Therefore, this method is widely adopted for measuring the external physical quantities. 
     In contrast to this, the method of utilizing the pilot signals to adjust the bias voltage is not sensitive to the external influences, and therefore, the method is widely adopted in the optical communication system which requires a stable operation. This will be described referring to FIG.  1 . FIG. 1 illustrates the conventional optical intensity modulator using the pilot signals. 
     As shown in FIG. 1, in this conventional method for adjusting the bias voltage of the optical intensity modulator, first, the signals which are constant in the optical intensity are inputted into an optical intensity modulator  11 . Then the main signals which are to be transmitted are mixed together with electrical signals which have passed through a pilot signal generator  12  for adjusting the bias voltage. Then these mixed signals are supplied to the optical intensity modulator  11 . 
     Under this condition, the optical output signals of the optical intensity modulator consist of the main signals and the pilot signals to form the signals in which the optical intensity has been modulated. That is, they include data signals and pilot signals of, i.e., dc (direct current), fo, 2fo, 3fo . . . 
     That is, the optical signals which are outputted from the optical intensity modulator  11  include: dc, fo, 2fo, . . . , and data signals. However, the necessary components are the dc and data components, and therefore, the fo, 2fo, . . . components are noises. 
     Meanwhile, a part of the output optical signals of the optical intensity modulator  11  becomes monitoring signals to be transferred to a photo detector  14  which outputs electrical signals. Under this condition, signals which are same as the output signals of the optical intensity modulator  11  are also transferred to the photo detector  14 , but they are different only in the optical intensity. 
     Thereafter, the magnitude of the 2fo component is measured by a negative feedback part  15  by utilizing the 2fo component of the pilot signals and the dc, fo, 2fo, . . . data components of the photo detector  14 . The negative feedback circuit  15  includes a phase detector and a multiplier. If the value of the 2fo component which has been measured by the negative feedback circuit  15  is not zero, the level of the bias voltage is varied until the 2fo component becomes 0. 
     Now the conventional optical intensity modulator constituted as above will be described as to its bias voltage adjusting method. 
     The conversion of the electrical information signals to optical signals is carried out by the optical intensity modulator  11 . Under this condition, if the phase difference of the optical intensity modulator  11  is not exactly 90 degrees, then the extinction ratio of the ON/OFF signals is aggravated, resulting in that noises are generated. Accordingly, in order to realize a phase difference of 90 degrees, the pilot signals of the frequency fo are supplied to the optical intensity modulator  11 , thereby adjusting the bias voltage. 
     Thereafter, the beams which have been modulated by the pilot signals pass through an optical splitter to be detected by the photo detector  12 . Thus the frequencies which correspond to integer-multiples of fo are detected. Under this condition, if the 2fo components are made to be 0 by using the negative feedback circuit  13 , then the phase difference becomes 90 degrees. 
     However, the pilot signals other than the main signals (electrical information signals) modulate the optical signals, and therefore, the pilot signals themselves act as the noises, thereby generating noises. Accordingly, in this method using the pilot signals, the magnitude of the pilot signals has to be maintained at the minimum, but according as the magnitude of the pilot signals is reduced, the magnitude of the detected signals of the photo detector  12  becomes smaller, with the result that it becomes difficult to adjust the phase difference of the optical intensity modulator  11 . Further, the 2fo component which is detected by the photo detector  12  is very much smaller than the fo component, and therefore, the magnitude of the pilot signals which are supplied to the photo detector  12  can be a serious problem. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to overcome the above described disadvantages of the conventional techniques. 
     Therefore it is an object of the present invention to provide an apparatus and a method for modulating the optical intensity, with the noises suppressed by using a linear optical modulator, in which in a method for adjusting the bias voltage of an optical modulator by using the pilot signals, the input optical intensity is modulated by using a linear optical modulator, so that the error-inducing signal components are offset while maintaining the signal component for adjusting the bias voltage, thereby inhibiting the amplitude noises caused by the pilot signals. 
     In achieving the above object, the apparatus for modulating an optical intensity with amplitude noises suppressed by using a linear optical modulator according to the present invention includes: a pilot signal generating means for generating pilot signals so as to adjust a bias voltage; a phase delaying means for delaying the phase of the pilot signals after their generation by the pilot signal generating means; a linear optical modulating means for modulating optical signals from an external source by the phase-delayed signal; a signal adding means for adding together data information signals and the pilot signals from the pilot signal generating means; an optical intensity modulating means for modulating the linearly modulated signals from the linear optical modulating means to modulate the optical intensity by the added electrical signals from the signal adder and adjusting the bias voltage by negative feedback signals; a photo detecting means for detecting output signals of the optical intensity modulating means to convert them to electrical signals; and an amplitude measuring of a frequency component means for receiving the electrical signals from the optical detecting means and the pilot signals from the pilot signal generating means to measure the amplitude of the frequency component so as to generate negatively fed-back signals and so as to transfer them to the optical intensity modulating means. 
     In another aspect of the present invention, the method for modulating an optical intensity with amplitude noises suppressed by using a linear optical modulator according to the present invention includes the steps of: generating the pilot signal in order to adjust the bias voltage and applying the added signal to the optical intensity modulator, the added signals having been formed by combining the data signals and the pilot signals (first step); measuring the amplitude of the second harmonic frequency component from partly detected signals from among output signal and measured electrical signals to transfer negatively feed-back signals to an optical intensity modulator, the electrical signals having been converted from partly detected signals from among output signals, and then adjusting the bias voltage(second step); delaying the phase by generating pilot signals and modulating the optical signals from an external source to provide a linear component by delayed pilot signal, and suppressing the amplitude noise from the pilot signal (third step). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and other advantages of the present invention will become more apparent by describing in detail the preferred embodiment of the present invention with reference to the attached drawings in which: 
     FIG. 1 illustrates the conventional optical intensity modulator using the pilot signals; 
     FIG. 2 illustrates a preferred embodiment of the apparatus and method for modulating the optical intensity with the noise suppressed by using the linear optical modulator according to the present invention; and 
     FIG. 3 illustrates another preferred embodiment of the apparatus and method for modulating the optical intensity with the noise suppressed by using a laser diode of the linear optical modulator according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be described referring to the attached drawings. 
     FIG. 2 illustrates a preferred embodiment of the apparatus and method for modulating the optical intensity with the noise suppressed by using the linear optical modulator according to the present invention. 
     As shown in FIG. 2, the apparatus for modulating an optical intensity with amplitude noises suppressed by using a linear optical modulator according to the present invention includes: a pilot signal generator  21  for generating pilot signals as electrical signals of an arbitrary low frequency so as to maintain a bias voltage; a phase delayer  22  for delaying the phase of the pilot signals by shifting their phase by 180 degrees after their generation by the pilot signal generator  21 ; a linear optical modulator  23  for receiving the phase-delayed signals from the phase delayer and for receiving optical signals from an external optical source to linearly modulate them so as to provide only a linear component; a signal adder  24  for adding together data signals as the main signals and the pilot signals from the pilot signal generator  21 ; an optical intensity modulator  25  for receiving negatively fed-back signals to adjust the bias voltage, and for receiving the added signals from the signal adder and the linearly modulated signals from the linear optical modulator  23  to modulate the optical intensity; a photo detector  26  for detecting output signals of the optical intensity modulator  25  to convert them to electrical signals; and a negative feedback circuit  27  for receiving the electrical signals from the photo detector  26  and the pilot signals from the pilot signal generator  21  to measure the amplitude of second harmonic frequency component by using a phase detector or a multiplier so as to generate negatively fed-back signals and so as to transfer them to the optical intensity modulator  25 . 
     The optical intensity modulating apparatus using the linear optical modulator according to the present invention will be described in more detail. 
     When pilot signals of mo sin (2Pifot) are supplied to the optical intensity modulator, the optical output intensity I which is detected by the photo detector  26  is as follows: 
     
       
           I =( Io )/2 [1+cos{ Phio+mo  sin(2 Pifot )}]  &lt;Formula 1&gt; 
       
     
     where Io is the maximum optical intensity of the optical intensity modulator, Phio is the phase difference, mo (&lt;1) is the amplitude of the pilot signals, and fo is the frequency of the pilot signals. The intensities of the detected optical outputs for the different frequencies can be expressed by Formulas 2 to 5 as follows: 
     
       
         dc component: ( Io )/2 [1 +Jo ( mo )cos( Phio )]  &lt;Formula 2&gt; 
       
     
     
       
         fo component: ( Io )/2 2 sin( Phio )  J 1( mo )sin(2 Pifot )  &lt;Formula 3&gt; 
       
     
      2fo component: ( Io )/2 2 cos( Phio )  J 2( mo )cos(4 Pifot )  &lt;Formula 4&gt; 
     
       
         3fo component: ( Io )/2 2 sin( Phio )  J 3 sin(6 Pifot )  &lt;Formula 5&gt; 
       
     
     
       
         4fo component, 5fo component, . . . 
       
     
     In the magnitude of the dc component, when the value of phio becomes 90 degrees, the average output value becomes Io/2, and therefore, the extinction ratio of the ON/OFF signals can be made maximal. Meanwhile, when the phase difference is 90 degrees, the magnitude of the fo component becomes maximal, while the magnitude of the 2fo component becomes 0. 
     Accordingly, in adjusting the bias voltage, the method of maximizing the value of the fo component makes the signal processing very complicated, and therefore, it is not used, but the method of adjusting the value of the 2fo component to 0 is generally used. In the pilot signals, the value of mo is very much smaller than 1, and therefore, they have a large value compared with the other components. Accordingly, in the bias voltage adjusting method using the pilot signals, the value of the fo component is largest, and it produces the greatest amount of errors in transmitting the signals. That is, if the fo component is removed from the beams which are modulated by the optical intensity modulator  25 , then the greater part of errors can be suppressed. Thus in the present invention, the optical signals which are inputted into the optical intensity modulator  25  are modulated to the frequency fo in a direction opposite to the modulation direction of the optical intensity modulator  25 , and thus, the errors due to the pilot signals can be inhibited. The beams which are inputted into the optical intensity modulator  25  are deprived of the of component. Therefore, a linear optical modulator  23  is installed at the input terminal of the optical intensity modulator  25 , and then, the pilot signals are supplied. Under this condition, if signals which have been delayed by 180 degrees relative to the pilot signals for the optical intensity modulator  25  are supplied to the linear optical modulator  23 , then the fo component which causes the errors to the greatest degree is removed while maintaining the 2fo component intact. In this context, the phase delay is carried out by the phase delayer  22 . 
     Now the present invention will be described as to its operating method. 
     First, the pilot signals which are transferred from the pilot signal generator  21  are phase-shifted by 180 degrees by the phase delayer  22 . Under this condition, the output signals of the phase delayer  22  have the minimum value when the pilot signals have the maximum value, and vice versa (that is, they act to offset the pilot signals which are supplied to the optical intensity modulator). 
     Then the optical signals which are inputted from an external source are inputted into the input terminal of the linear optical modulator  23 , and the electrical signals which are outputted from the phase delayer  22  are supplied to the linear optical modulator  23  also. Under this condition, the signals which are outputted from the linear optical modulator  23  include only the dc and fo components. That is, they are linearly modulated signals which are linearly proportional to the electrical signals. 
     Thereafter, the linearly modulated optical signals which are outputted from the linear optical modulator  23  are inputted into the optical intensity modulator  25 , and then, the added signals which are formed by adding together the main signals (to be transmitted) and the electrical signals (which are transferred through the pilot signal generator  21  to adjust the bias voltage) are supplied to the optical intensity modulator  25 . 
     Under this condition, the optical signals which are outputted from the optical intensity modulator  25  are those which have been intensity-modulated by the pilot signals and the main signals. 
     These optical signals have been deprived of the fo component by the linear optical modulator  23 , and therefore, include only the dc 2fo, 3fo, . . . components owing to the main signals and the pilot signals component fo. 
     Meanwhile, a part of the optical signals which are outputted from the optical intensity modulator  25  are partly split into the monitoring signals to be transferred to the photo detector  26  by which the monitoring signals are converted into electrical signals. Under this condition, signals which are same as those outputted from the optical intensity modulator  25  are transferred to the photo detector  26 , but their intensity is different. 
     Thereafter, the negative feedback circuit  27  measures the magnitude of the 2fo component by utilizing the 2fo component of the pilot signals and the electrical signals (dc, 2fo, . . . , and data signals) which are outputted from the photo detector  26 . 
     The negative feedback circuit  27  includes a phase detector or a multiplier. If the 2fo component which has been measured by the negative feedback circuit  27  is not 0, then the bias voltage is varied until the value of the 2fo component becomes 0. 
     Accordingly, the optical signals which are outputted from the optical intensity modulator  25  includes the dc, 2fo and data signal components, and does not include the fo component as the noise except the 2fo, 3fo . . . components. That is, in FIG. 1, the fo component is very much larger than the 2fo, 3fo, . . . components, and therefore, the noise in the optical signals of the optical intensity modulator  25  is reduced to the utmost degree. 
     FIG. 3 illustrates another preferred embodiment of the apparatus and method for modulating the optical intensity with the noise suppressed by using a laser diode of the linear optical modulator according to the present invention. That is, the noises are inhibited through a direct modulation by a laser diode  33  which is the optical source. 
     The optical output strength P of the laser diode  33  is linearly proportional to the supplied current so as to output only the fo component as shown in Formula 6: 
     
       
           P =( I−Ith )* A =( Io+I   1  sin(2 Pifot )− Ith*A   &lt;Formula 6&gt; 
       
     
     where Ith is the threshold current value, and A is the proportionality coefficient. If I 1  is 0, then non-modulated beams are outputted, and therefore, they are used as the input optical signals in FIGS. 1 and 2. 
     Meanwhile, if I 1  sin(2Pifot) signals are supplied, then linearly modulated optical signals are outputted, and therefore, they have two functions as input signals and as a linear optical modulating device. Accordingly, if the pilot signals are supplied to the laser diode  33 , then the functions as shown in FIG. 2 can be obtained. That is, the input optical signals which have been linearly modulated by the laser diode are same as those of FIG.  2 . 
     Now the optical intensity modulating method of the present invention in which the noises are suppressed by using the laser diode will be described in detail. 
     As shown in FIG. 3, the laser diode  33  shows a linear variation in the current or voltage. Therefore, if a modulation is carried out on the frequency of fo, then 2fo or higher frequency components are not produced. Therefore, only the fo component can be eliminated by phase-delaying the pilot signals by 180 degrees, thereby making it possible to improve the data transmission quality. 
     In this method, in adjusting the bias voltage of the optical intensity modulator  35  by using the pilot signals, the frequency component which acts as the greatest noise can be suppressed without varying the magnitude of the useful frequency, thereby making it possible to reduce the transmission errors. 
     Further, far larger pilot signals can be used, and therefore, the bias voltage can be more precisely adjusted. 
     In the above, the present invention was described based on the specific embodiments and the attached drawings, but it should be apparent to those ordinarily skilled in the art that various changes and modifications can be added without departing from the spirit and scope of the present invention which will be defined in the appended claims. 
     According to the present invention as described above, the bias voltage of the optical intensity modulator is adjusted by using the pilot signals, and the inputted optical intensity is adjusted by using a linear optical modulator, so that the error-inducing signal components are offset while maintaining the signal component for adjusting the bias voltage intact, thereby suppressing the amplitude noises caused by the pilot signals. 
     Further, in the present invention, the bias voltage of the optical intensity modulator can be precisely adjusted, and therefore, the influence of the noise due to the pilot signals is diminished. Accordingly, the present invention can be usefully applied not only to the optical communication system but also to the optical measuring field. 
     Although the preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.