Abstract:
The present invention provides a wideband modulated signal generating device capable of realizing an always stable operation and obtaining an intended wideband modulated signal in spite of a shift in the optimal bias voltage due to DC drift occurring in an optical intensity modulation section. In the wideband modulated signal generating device, a DC power supply control section  50  controls a first DC power supply  51  and a second DC power supply  52  for applying first and second bias voltages to an optical intensity modulation section  30  based on a signal level detected by a level detecting section  70 , and controls a third DC power supply  53  for applying a third bias voltage to an optical intensity modulation section  30  based on a distortion level detected by a distortion level detecting section  81 , thus compensating for a shift in the optimal bias voltage occurring due to DC drift.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a wideband modulated signal generating device for generating a wideband modulated signal (a phase-modulated signal or a frequency-modulated signal), and more particularly to a wideband modulated signal generating device using an external optical modulator capable of a bias voltage control of making the bias voltage applied to the external optical modulator follow the fluctuations of the optimal bias voltage due to DC drift.  
         [0003]     2. Description of the Background Art  
         [0004]     Examples of conventional wideband modulated signal generating methods using the wideband property of light include a method for generating a wideband modulated signal through a heterodyne detection using the chirp characteristics of semiconductor lasers (e.g., Non-Patent Document 1).  
         [0005]     Non-Patent Document 1: K. Kikushima, et al., “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems”, IOOC &#39;95 Technical Digest, Vol. 5 PD2-7, pp. 33-34  
         [0006]      FIG. 10  is a block diagram showing a configuration of a conventional wideband modulated signal generating device. The operation, etc., of the wideband modulated signal generating device are discussed in detail in Non-Patent Document 1. Referring to  FIG. 10 , the wideband modulated signal generating device includes an optical frequency control section  900 , a signal source  901 , a local light source  902 , an optical modulation section  903 , a light combining section  904 , and a light detecting section  905 .  
         [0007]     With the wideband modulated signal generating device having such a configuration, the signal source  901  outputs an electric signal being the original signal to be subjected to an angular modulation. The optical modulation section  903  may be a semiconductor laser. Typically, where the injected current is constant, a semiconductor laser oscillates to output light having a constant optical frequency f 1 . When the current injected into the semiconductor laser is amplitude-modulated, the frequency of the output light is also modulated, thus outputting an optical frequency-modulated signal centered about an optical frequency f 1 . With such a nature, the optical modulation section  903  converts the electric signal outputted from the signal source  901  to an optical frequency-modulated signal. The local light source  902  outputs an unmodulated optical signal having a constant optical frequency f 2 .  
         [0008]     The optical frequency-modulated signal outputted from the optical modulation section  903  and the optical signal outputted from the local light source  902  are combined together by the light combining section  904  and inputted to the light detecting section  905 . The light detecting section  905  may be a photodiode having squared detection characteristics, or the like. The light detecting section  905  outputs a beat signal between two input optical signals at a frequency f C  (=|f 1 −f 2 |) corresponding to the difference between the optical frequencies of the two optical signals. This is called an optical heterodyne detection.  
         [0009]     The beat signal thus obtained is an angle-modulated signal (frequency-modulated signal) having a carrier frequency f C  with the original signal being the electric signal outputted from the signal source  901 . The optical frequency control section  900  controls one or both of the center optical frequency f 1  of the optical signal outputted from the optical modulation section  903  and the optical frequency f 2  of the optical signal outputted from the local light source  902  so as to stabilize the center frequency f C  of the angle-modulated signal outputted from the light detecting section  905 .  
         [0010]     As described above, the wideband modulated signal generating device uses a high modulation efficiency of optical signal processing (a high efficiency that is 10 times or more higher than that obtained with a general electric circuit), whereby it is possible to easily produce an angle-modulated signal having a very high frequency and being wideband (with a large frequency or phase deviation), which is difficult to produce with a general electric circuit. However, a light source such as a semiconductor laser typically has greater phase noise (a greater oscillation spectral line width) as compared with an electric oscillator.  
         [0011]     Referring to  FIGS. 11A  to  11 C, phase noise contained in signals outputted from the various components of the conventional wideband modulated signal generating device will be discussed.  FIG. 11A  is a schematic diagram showing a frequency spectrum of an optical signal outputted from the local light source  902 .  FIG. 11B  is a schematic diagram showing a frequency spectrum of an optical signal outputted from the optical modulation section  903 .  FIG. 11C  is a schematic diagram showing a frequency spectrum of a signal outputted from the light detecting section  905 .  
         [0012]     In  FIG. 11A , the phase noise (the oscillation spectral line width) of the local light source  902  is denoted as Δν 1 . In  FIG. 11B , the phase noise (the oscillation spectral line width) contained in the optical signal outputted from the optical modulation section  903  is denoted as Δν 2 . The angle-modulated signal obtained as the beat signal between these optical signals has phase noise (Δν 1 +Δν 2 ) corresponding to the sum of two phase noise, as shown in  FIG. 11C . This is because there is no phase level correlation between light waves outputted from the light sources, and thus the phase noise are simply added together. When an angle-modulated signal is demodulated, the phase noise is also demodulated to become substantial white (intensity) noise. Thus, the conventional wideband modulated signal generating device has the characteristic problem that the quality of the demodulated signal deteriorates significantly due to the noise.  
         [0013]     The conventional wideband modulated signal generating device shown in  FIG. 10  needs to successively monitor the optical frequencies of the two light sources (or the difference therebetween) in order to stabilize the frequency of the angle-modulated signal. Thus, the conventional device has the characteristic problem that it requires a complicated component such as a control circuit for monitoring/adjustment (e.g., the optical frequency control section  900 ).  
         [0014]     In order to solve the problem, a conventional wideband modulated signal generating device disclosed in Japanese Laid-Open Patent Publication No. 2001-133824 (hereinafter referred to as “Patent Document 1”) employs a configuration as shown in  FIG. 12 . Referring to  FIG. 12 , the conventional wideband modulated signal generating device splits the light outputted from a light source  2000  into two, one of which is subjected to a predetermined optical intensity modulation through an optical intensity modulation section  2003  with the original signal being a first electric signal having a predetermined frequency f C  outputted from a first signal source  2007  to thereby produce an optical intensity-modulated signal. The other light is subjected to an optical angular modulation through an optical angle modulation section  2004  with the original signal being a second electric signal outputted from a second signal source  2008  to thereby produce an optical angle-modulated signal. A light combining section  2005  combines together the optical intensity-modulated signal produced by the optical intensity modulation section  2003  and the optical angle-modulated signal produced by the optical angle modulation section  2004 .  
         [0015]     A light detecting section  2006  homodyne-detects the optical intensity-modulated signal and the optical angle-modulated signal, which have been combined together by the light combining section  2005 , and produces, as the difference beat signal therebetween, an angle-modulated signal centered about the frequency f C  with the original signal being the output signal from the second signal source  2008 . The optical angle-modulated signal and the optical intensity-modulated signal each have the same phase noise Δν as that of the light source  2000 , and these phase noise are canceled out by each other in the angle-modulated signal being the difference beat component. Specifically, even if the optical frequency of the optical angle-modulated signal fluctuates up and down due to the influence of the phase noise, the optical frequency of the optical intensity-modulated signal undergoes the same fluctuations, whereby the frequency difference between these signals is always constant irrespective of the frequency fluctuations. Therefore, with the conventional wideband modulated signal generating device shown in  FIG. 12 , it is possible to obtain an angle-modulated signal with desirable noise characteristics.  
         [0016]     However, for the optical intensity modulation section  2003  of the conventional wideband modulated signal generating device shown in  FIG. 12 , it is necessary to provide an optical SSB modulator capable of performing a double-sideband suppressed-optical carrier modulation or a single-sideband suppressed-optical carrier modulation. For example, for an optical SSB modulator capable of performing a single-sideband suppressed-optical carrier modulation, there are very limited bias conditions under which carrier light or unnecessary sideband light components are canceled out by each other, whereby unnecessary light components are produced even by slight changes in the bias voltage. With an optical SSB modulator using lithium niobate, there is a phenomenon called “DC drift” where the point of operation shifts over time. Thus, even when the bias voltage does not change, the optimal point of operation may shift over time, thus producing unnecessary light components. Therefore, the optical signal outputted from the optical intensity modulation section  2003  may include unnecessary light components as shown in  FIG. 13A .  
         [0017]      FIG. 13C  shows the spectrum of an electric signal obtained when the optical signal outputted from the optical intensity modulation section  2003  (see  FIG. 13A ) and the optical signal outputted from the optical angle modulation section  2004  (see  FIG. 13B ) are combined together and homodyne-detected by the light detecting section  2006 , where there are unnecessary light components. As shown in  FIG. 13C , the electric signal homodyne-detected by the light detecting section  2006  includes an unnecessary component having the same center frequency as that of the intended angle-modulated signal, an unnecessary component whose center frequency is DC. When an angle-modulated signal including such unnecessary wave components is demodulated, the distortion characteristics may be deteriorated (see, for example, Non-Patent Document 2).  
         [0018]     Non-Patent Document 2: Ohira, et al., “Study Of Wideband Fm Modulation Scheme Using Optical Homodyne Detection—System Proposal And Basic Characteristics Of Wideband Modulator—”, IEICE Technical Report  
         [0019]     The distortion characteristics are shown in  FIGS. 14A and 14B .  FIG. 14A  shows the distortion characteristics with respect to the level difference between the J +1  component being the upper sideband and the optical carrier J 0  component.  FIG. 14B  shows the distortion characteristics with respect to the level difference between the J +1  component being the upper sideband and the J −1  component being the lower sideband. As can be seen from  FIGS. 14A and 14B , both of the unnecessary wave components substantially influence the distortion characteristics. Thus, it can be seen that it is necessary to suppress unnecessary light components outputted from the optical intensity modulation section  2003  in order to obtain an angle-modulated signal that has desirable distortion characteristics when demodulated.  
         [0020]     In order to address the problem, a conventional optical SSB modulation device disclosed in Japanese Laid-Open Patent Publication No. 2004-302238 (hereinafter referred to as “Patent Document 2”) performs a control as follows for the fluctuations over time of the optimal point of operation due to DC drift, or the like.  FIG. 15  is a block diagram showing a configuration of a conventional optical SSB modulation device. Referring to  FIG. 15 , the optical SSB modulation device includes an optical input terminal  3000 , an optical output terminal  3005 , an optical SSB modulator  3003  including a pair of modulation electric signal input terminals  3004   a  and  3004   b , a power supply  3001 , and a voltage control circuit  3002 .  
         [0021]      FIG. 16  shows a block diagram showing an internal configuration of the optical SSB modulator  3003 . Referring to  FIG. 16 , the optical SSB modulator  3003  includes an optical input terminal  3010 , an optical output terminal  3011 , sub-interferometers  3013   a  and  3013   b , a main interferometer  3012   c , RF electrodes  3014   a ,  3014   b  and  3014   c , DC electrodes  3015   a ,  3015   b  and  3015   c , and Y branches  3016 ,  3017   a ,  3017   b ,  3018   a ,  3018   b  and  3019 .  
         [0022]     Referring to  FIGS. 15 and 16 , the optical input terminal  3000  of the optical SSB modulation device receives carrier light generated from a light source such as an LD (laser diode), and electric signals, with which the carrier light is to be modulated, are applied to the pair of modulation electric signal input terminals  3004   a  and  3004   b . The modulated output light from the optical SSB modulator  3003  is outputted from the optical output terminal  3005 .  
         [0023]     The voltage control circuit  3002  monitors a portion of the modulated output light, and controls the bias voltage generated by the power supply  3001 . The bias voltage thus controlled is applied to the optical SSB modulator  3003  via the power supply  3001 . Thus, the voltage control circuit  3002  monitors a portion of the output light from the optical SSB modulator  3003  to detect the power of the angular frequency component and to output a control signal for controlling the bias voltages to be applied to the sub-interferometers  3013   a  and  3013   b  and the main interferometer  3012   c  of the optical SSB modulator  3003 . Specifically, the voltage control circuit  3002  controls the bias voltages to be applied to the sub-interferometers  3013   a  and  3013   b  so that the power of the carrier light, among other output light components, is minimized and controls the bias voltage to be applied to the main interferometer  3012   c  so that the power of the unnecessary sideband light, among other output light components, is minimized.  
         [0024]     Patent Document 2 also discloses configurations as shown in  FIGS. 17 and 18 , as conventional optical SSB modulation devices. As compared with the configuration shown in  FIG. 15 , the conventional optical SSB modulation device shown in  FIG. 17  further includes an optical filter  3006  for separating the modulated output light outputted from the optical output terminal  3005  into angular frequency components. Referring to  FIG. 17 , a portion of the modulated output light from the optical SSB modulator  3003  is separated by the optical filter  3006  into carrier light and unnecessary sideband light components. The voltage control circuit  3002  monitors the power of the carrier light, and controls the power supply  3001  so that the DC electrodes  3015   a  and  3015   b  of the sub-interferometers  3013   a  and  3013   b  receives bias voltages such that the power of the carrier light is minimized. The voltage control circuit  3002  monitors the power of the unnecessary sideband light, and controls the power supply  3001  so that the DC electrode  3015   c  of the main interferometer  3012   c  receives a bias voltage such that the power of the unnecessary sideband light is minimized.  
         [0025]     As compared with the configuration shown in  FIG. 15 , the conventional optical SSB modulation device shown in  FIG. 18  further includes a PD (photodetector)  3007  for converting a portion of the modulated output light from the optical output terminal  3005  to an electric signal. Referring to  FIG. 18 , a portion of the modulated output light from the optical SSB modulator  3003  is converted by the PD  3007  to an electric signal. The voltage control circuit  3002  monitors the carrier light component based on the electric signal, and controls the power supply  3001  so that the DC electrodes  3015   a  and  3015   b  of the sub-interferometers  3013   a  and  3013   b  receive bias voltages such that the power of the carrier light is minimized. The voltage control circuit  3002  monitors the unnecessary sideband light component based on the electric signal, and controls the power supply  3001  so that the DC electrode  3015   c  of the main interferometer  3012   c  receives a bias voltage such that the power of the unnecessary sideband light is minimized. Thus, as does the two conventional devices described above, the optical SSB modulation device shown in  FIG. 18  is capable of always producing a practical, high-quality optical SSB-modulated signal.  
         [0026]     The conventional optical SSB modulation device shown in  FIG. 18  controls the bias voltages based on respective signal components, i.e., the carrier light converted by the PD to an electric signal and the unnecessary sideband light, whereby it is possible to always output a high-quality SSB-modulated optical signal without being influenced by DC drift. Thus, there is an advantageous effect that the reliability of optical communications is practically improved. Moreover, the device can be realized with a simple configuration because the power of the carrier light and that of the unnecessary sideband light can be monitored with a simple configuration such as a PD.  
         [0027]     With the conventional optical SSB modulation devices shown in  FIGS. 15 and 17  to  18 , the power of the carrier light and that of the unnecessary sideband light from the optical SSB modulator  3003  need to be monitored in order to control the bias voltages to be applied to the sub-interferometers  3013   a  and  3013   b  and the main interferometer  3012   c . However, the monitoring cannot always be done successfully. Specifically, with the conventional optical SSB modulation device shown in  FIG. 15 , the power of the carrier light and that of the unnecessary sideband light from the optical SSB modulator  3003  are not separated from each other, whereby it is impossible to control the bias voltages by monitoring the power of each light component.  
         [0028]     With the conventional optical SSB modulation device shown in  FIG. 17 , the optical filter  3006  is provided in order to separate the power of the carrier light and that of the unnecessary sideband light from the optical SSB modulator  3003  from each other. A conventional optical filter can be used where the frequency of the electric signal inputted to the optical SSB modulator  3003  is sufficiently high (e.g., 40 GHz or more). However, where the frequency of the input signal is on the order of 1 GHz, there is no existing optical filter capable of separating light components arranged with small intervals therebetween. Therefore, where the optical SSB modulator  3003  is driven with an input signal whose frequency is on the order of 1 GHz, it is impossible to control the bias voltages by monitoring the power of the carrier light and that of the unnecessary sideband light. With the conventional optical SSB modulation device shown in  FIG. 18 , the carrier light and unnecessary sideband light components need to be converted back to electric signals by the PD  3007  so that the power can be monitored in terms of the power of each electric signal. This cannot be done if the light components cannot be separated from each other.  
       SUMMARY OF THE INVENTION  
       [0029]     Therefore, an object of the present invention is to provide specific means for optimally controlling bias voltages even if DC drift occurs where the frequency of the electric signal inputted to the optical SSB modulator is relatively low (on the order of 1 GHz), and to realize a wideband modulated signal generating device having desirable modulation characteristics.  
         [0030]     The present invention is directed to a wide band modulated signal generating device. In order to attain the object set forth above, a first aspect of the present invention is directed to a wideband modulated signal generating device, including: a light source for outputting light; a light branching section for splitting the light outputted from the light source into first light and second light; an optical intensity modulation section for subjecting the first light to an optical intensity modulation or an optical amplitude modulation with an original signal being a first electric signal having a predetermined frequency f C  to output a resultant signal as a first optical signal; an optical angle modulation section for subjecting the second light to an optical angular modulation with an original signal being a second electric signal to output a resultant signal as a second optical signal; a light combining section for combining together the first optical signal and the second optical signal; a light detecting section having squared detection characteristics for converting an optical signal outputted from the light combining section to an electric signal to thereby output a wideband modulated signal having a carrier frequency f C  with an original signal being the second electric signal; first, second and third DC power supplies for applying first, second and third bias voltages, respectively, to the optical intensity modulation section; and a bias voltage control section for controlling the first bias voltage and the second bias voltage applied by the first and second DC power supplies to the optical intensity modulation section based on a level of an electric signal having an arbitrary frequency included in the wideband modulated signal outputted from the light detecting section, and controlling the third bias voltage applied by the third DC power supply to the optical intensity modulation section based on a level of a distortion component at an arbitrary frequency included in a demodulated electric signal outputted from the light combining section.  
         [0031]     According to the first aspect of the present invention, the first bias voltage and the second bias voltage are controlled based on the level of the electric signal having an arbitrary frequency included in the wideband modulated signal outputted from the light detecting section, and the third bias voltage is controlled based on the level of the distortion component at an arbitrary frequency included in the electric signal outputted from the light combining section and demodulated, whereby it is possible to follow the fluctuations of the optimal point of each bias voltage due to DC drift, or the like, and it is possible to produce a wideband modulated signal with desirable modulation characteristics.  
         [0032]     In a second aspect of the present invention, the bias voltage control section controls the first DC power supply and the second DC power supply to set the first bias voltage and the second bias voltage each to a predetermined bias voltage value, and then controls the third DC power supply to set the third bias voltage to a predetermined bias voltage.  
         [0033]     According to the second aspect of the present invention, with regard to the flow of controlling a plurality of bias voltages, the third bias voltage is controlled after the first and second bias voltages are controlled in view of the characteristics of the optical intensity modulation section, thus realizing an efficient bias voltage control.  
         [0034]     In a third aspect of the present invention, the bias voltage control section includes: a branching section for branching a portion of the electric signal from the light detecting section into two paths; a signal level detecting section for extracting a component of one of the electric signals from the branching section that is within a particular band and measuring a level of the component to detect the level of an electric signal having an arbitrary frequency included in a wideband modulated signal outputted from the light detecting section; a demodulation section for demodulating a wideband modulated signal included in the other one of the electric signals from the branching section; a distortion level detecting section for detecting a level of a distortion component at an arbitrary frequency included in a wideband modulated signal outputted from the demodulation section; and a bias voltage control section for controlling the first bias voltage and the second bias voltage applied by the first DC power supply and the second DC power supply, respectively, to the optical intensity modulation section so that the level of the electric signal having an arbitrary frequency detected by the signal level detecting section is less than or equal to a reference level, and controlling the third bias voltage applied by the third DC power supply to the optical intensity modulation section so that the level of the distortion component at an arbitrary frequency detected by the distortion level detecting section is less than or equal to a reference level.  
         [0035]     According to the third aspect of the present invention, the first and second bias voltages are controlled so that the level of the electric signal having an arbitrary frequency detected by the signal level detecting section is less than or equal to a reference level, and the third bias voltage is controlled so that the level of the distortion component at an arbitrary frequency detected by the distortion level detecting section is less than or equal to a reference level, whereby it is possible to produce an always stable wideband modulated signal.  
         [0036]     In a fourth aspect of the present invention, the signal level detecting section detects a level of a component of the second electric signal that has a lowest frequency.  
         [0037]     According to the fourth aspect of the present invention, signal level detecting section detects a component of the second electric signal that has the lowest frequency, whereby it is possible to easily detect the level of an electric signal having an arbitrary frequency included in the wideband modulated signal.  
         [0038]     In a fifth aspect of the present invention, the distortion level detecting section detects a distortion component occurring within a signal band of a highest frequency among other components of the second electric signal inputted to the optical angle modulation section.  
         [0039]     According to the fifth aspect of the present invention, the distortion level detecting section detects a distortion for a high frequency band, where the deterioration of the distortion characteristics is most pronounced, whereby it is possible to realize a bias voltage control with a high precision.  
         [0040]     In a sixth aspect of the present invention, where the second electric signal includes modulated signals of different modulation schemes, the distortion level detecting section detects a distortion component occurring within a signal band of a highest frequency among other components of the second electric signal that has been modulated by a modulation scheme for which the highest performance is required.  
         [0041]     According to the sixth aspect of the present invention, the distortion level detecting section detects the distortion within a signal band of the highest frequency among other components of a signal for which a predetermined performance is required strictly, whereby it is possible to realize a bias voltage control with a higher precision.  
         [0042]     In a seventh aspect of the present invention, a third electric signal is additionally superposed over the second electric signal inputted to the optical angle modulation section, and the signal level detecting section detects a level of the third electric signal.  
         [0043]     According to the seventh aspect of the present invention, even if the second electric signal is composed only of modulated components, the signal level detecting section detects an unmodulated third electric signal as a monitor signal, whereby it is possible to realize a bias voltage control with a higher precision. Since the level and the frequency of the third monitor electric signal can be determined arbitrarily, it is possible to realize a signal level detecting section more inexpensively with a simple configuration.  
         [0044]     In an eighth aspect of the present invention, the third electric signal has a frequency lower than that of the second electric signal.  
         [0045]     According to the eighth aspect of the present invention, since the third electric signal has a frequency lower than that of the second electric signal, it is possible to more easily detect the level of the electric signal having an arbitrary frequency included in the wideband modulated signal if the signal level detecting section uses the third electric signal as a monitor signal.  
         [0046]     In a ninth aspect of the present invention, the distortion level detecting section detects a distortion component produced by a fourth electric signal and a fifth electric signal when the fourth and fifth electric signals are superposed over the second electric signal inputted to the optical angle modulation section.  
         [0047]     According to the ninth aspect of the present invention, even if the second electric signal is composed only of modulated components, the distortion level detecting section detects a distortion component produced by the unmodulated fourth and fifth electric signals, whereby it is possible to realize a bias voltage control with a higher precision. Since the level and the frequency of the fourth and fifth electric signals can be determined arbitrarily, it is possible to realize a distortion level detecting section more inexpensively with a simple configuration.  
         [0048]     In a tenth aspect of the present invention, the fourth and fifth electric signals have frequencies such that a distortion component produced by the fourth electric signal and the fifth electric signal is not within a signal band of the second electric signal.  
         [0049]     According to the tenth aspect of the present invention, the frequencies of the fourth and fifth electric signals are selected so that a distortion does not occur within a signal band of the second electric signal, whereby it is possible to produce a wideband modulated signal of a higher quality.  
         [0050]     In an eleventh aspect of the present invention, the bias voltage control section includes: a first branching section for branching a portion of the electric signal from the light detecting section; a demodulation section for demodulating a wideband modulated signal included in an electric signal outputted from the first branching section; a second branching section for branching an electric signal outputted from the demodulation section into two paths; a signal level detecting section for detecting a level of an electric signal having an arbitrary frequency included in one of the wideband modulated signals outputted from the second branching section; a distortion level detecting section for detecting a level of a distortion component at an arbitrary frequency included in the other one of the wideband modulated signals outputted from the second branching section; and a bias voltage control section for controlling the first bias voltage and the second bias voltage applied by the first DC power supply and the second DC power supply, respectively, to the optical intensity modulation section so that the level of the electric signal having an arbitrary frequency detected by the signal level detecting section is less than or equal to a reference level, and controlling the third bias voltage applied by the third DC power supply to the optical intensity modulation section so that the level of the distortion component at an arbitrary frequency detected by the distortion level detecting section is less than or equal to a reference level.  
         [0051]     According to the eleventh aspect of the present invention, the first and second bias voltages are controlled so that the level of the electric signal having an arbitrary frequency detected by the signal level detecting section is less than or equal to a reference level, and the third bias voltage is controlled so that the level of the distortion component at an arbitrary frequency detected by the distortion level detecting section is less than or equal to a reference level, whereby it is possible to produce an always stable wideband modulated signal.  
         [0052]     In a twelfth aspect of the present invention, the signal level detecting section detects a level of a component of the second electric signal that has a frequency twice as high as a lowest frequency thereof.  
         [0053]     According to the twelfth aspect of the present invention, the signal level detecting section detects a frequency component that is twice as high as that of a component of the second electric signal having the lowest frequency, whereby it is possible to easily detect the level of the electric signal having an arbitrary frequency included in the wideband modulated signal.  
         [0054]     In a thirteenth aspect of the present invention, the distortion level detecting section detects a distortion component occurring within a signal band of a highest frequency among other components of the second electric signal inputted to the optical angle modulation section.  
         [0055]     According to the thirteenth aspect of the present invention, the distortion level detecting section detects a distortion for a high frequency band, where the deterioration of the distortion characteristics is most pronounced, whereby it is possible to realize a bias voltage control with a high precision.  
         [0056]     In a fourteenth aspect of the present invention, where the second electric signal includes modulated signals of different modulation schemes, the distortion level detecting section detects a distortion component occurring within a signal band of a highest frequency among other components of the second electric signal that has been modulated by a modulation scheme for which the highest performance is required.  
         [0057]     According to the fourteenth aspect of the present invention, the distortion level detecting section detects the distortion within a signal band of the highest frequency among other components of a signal for which a predetermined performance is required strictly, whereby it is possible to realize a bias voltage control with a higher precision.  
         [0058]     In a fifteenth aspect of the present invention, a sixth electric signal is additionally superposed over the second electric signal inputted to the optical angle modulation section, and the signal level detecting section detects a level of an electric signal having a frequency component twice as high as that of the sixth electric signal.  
         [0059]     According to the fifteenth aspect of the present invention, even if the second electric signal is composed only of modulated components, the signal level detecting section detects, as a monitor signal, a frequency component twice as high as that of the unmodulated sixth electric signal, whereby it is possible to realize a bias voltage control with a higher precision. Since the level and the frequency of the sixth monitor electric signal can be determined arbitrarily, it is possible to realize a signal level detecting section more inexpensively with a simple configuration.  
         [0060]     In a sixteenth aspect of the present invention, the sixth electric signal has a frequency lower than that of the second electric signal.  
         [0061]     According to the sixteenth aspect of the present invention, since the sixth electric signal has a frequency lower than that of the second electric signal, it is possible to more easily detect the level of the electric signal having an arbitrary frequency included in the wideband modulated signal if the signal level detecting section uses the sixth electric signal as a monitor signal.  
         [0062]     In a seventeenth aspect of the present invention, the distortion level detecting section detects a distortion component produced by a seventh electric signal and an eighth electric signal when the seventh and eighth electric signals are superposed over the second electric signal inputted to the optical angle modulation section.  
         [0063]     According to the seventeenth aspect of the present invention, even if the second electric signal is composed only of modulated components, the distortion level detecting section detects a distortion component produced by the unmodulated seventh and eighth electric signals, whereby it is possible to realize a bias voltage control with a higher precision. Since the level and the frequency of the seventh and eighth electric signals can be determined arbitrarily, it is possible to realize a distortion level detecting section more inexpensively with a simple configuration.  
         [0064]     In an eighteenth aspect of the present invention, the seventh and eighth electric signals have frequencies such that a distortion component produced by the seventh electric signal and the eighth electric signal is not within a signal band of the second electric signal.  
         [0065]     According to the eighteenth aspect of the present invention, the frequencies of the seventh and eighth electric signals are selected so that a distortion does not occur within a signal band of the second electric signal, whereby it is possible to produce a wideband modulated signal of a higher quality.  
         [0066]     As described above, with the wideband modulated signal generating device of the present invention, bias voltages to be applied to the optical intensity modulation section are controlled based on the signal level of a particular frequency included in the wideband modulated signal and the level of the distortion component thereof, whereby it is possible to always stabilize the operation of the optical intensity modulation section without being influenced by DC drift, and thus to obtain a wideband modulated signal of a high quality. Moreover, since an optical filter, which is required in conventional configurations, is not needed in the present invention, it is possible to realize a stable operation with a simple configuration even if the input signal to the optical intensity modulation section is an electric signal whose frequency is on the order of 1 GHz, whereby it is possible to obtain a high-quality wideband modulated signal, irrespective of the frequency of the input signal to the optical intensity modulation section.  
         [0067]     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0068]      FIG. 1  is a block diagram showing a configuration of a wideband modulated signal generating device according to a first embodiment of the present invention;  
         [0069]      FIG. 2  is a diagram showing a schematic internal configuration of an optical intensity modulation section  30  shown in  FIG. 1 ;  
         [0070]      FIG. 3  is a flow chart showing the process of controlling the bias voltage performed by a DC power supply control section  50 ;  
         [0071]      FIG. 4  is a flow chart showing the details of the process of controlling the first bias voltage (step S 100 );  
         [0072]      FIG. 5  is a flow chart showing the details of the process of controlling the second bias voltage (step S 200 );  
         [0073]      FIG. 6  is a flow chart showing the details of the process of controlling the third bias voltage (step S 300 );  
         [0074]      FIG. 7A  is a block diagram showing a modified configuration of the wideband modulated signal generating device according to the first embodiment of the present invention;  
         [0075]      FIG. 7B  is a block diagram showing a modified configuration of the wideband modulated signal generating device according to the first embodiment of the present invention;  
         [0076]      FIG. 8  is a block diagram showing a configuration of a wideband modulated signal generating device according to a second embodiment of the present invention;  
         [0077]      FIG. 9A  is a block diagram showing a modified configuration of the wideband modulated signal generating device according to the second embodiment of the present invention;  
         [0078]      FIG. 9B  is a block diagram showing a modified configuration of the wideband modulated signal generating device according to the second embodiment of the present invention;  
         [0079]      FIG. 10  is a block diagram showing a configuration of a conventional wideband modulated signal generating device;  
         [0080]      FIG. 11A  is a schematic diagram showing the frequency spectrum of an optical signal outputted from a local light source  902 ;  
         [0081]      FIG. 11B  is a schematic diagram showing the frequency spectrum of an optical signal outputted from an optical modulation section  903 ;  
         [0082]      FIG. 11C  is a schematic diagram showing the frequency spectrum of a signal outputted from a light detecting section  905 ;  
         [0083]      FIG. 12  is a block diagram showing a configuration of a conventional wideband modulated signal generating device;  
         [0084]      FIG. 13A  is a diagram showing the frequency spectrum of an optical signal outputted from an optical intensity modulation section  2003 ;  
         [0085]      FIG. 13B  is a diagram showing the frequency spectrum of an optical signal outputted from an optical angle modulation section  2004 ;  
         [0086]      FIG. 13C  is a diagram showing the frequency spectrum of an electric signal outputted from a light detecting section  2006 ;  
         [0087]      FIG. 14A  is a diagram showing the relationship between the degree of carrier suppression and the distortion characteristics;  
         [0088]      FIG. 14B  is a diagram showing the relationship between the degree of sideband suppression and the distortion characteristics;  
         [0089]      FIG. 15  is a block diagram showing a configuration of a conventional optical SSB modulation device;  
         [0090]      FIG. 16  is a block diagram showing an internal configuration of an optical SSB modulator  3003 ;  
         [0091]      FIG. 17  is a block diagram showing a configuration of a conventional optical SSB modulation device; and  
         [0092]      FIG. 18  is a block diagram showing a configuration of a conventional optical SSB modulation device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0093]     Wideband modulated signal generating devices according to preferred embodiments of the present invention will now be described with reference to the drawings. It is understood that the scope of the present invention is not limited to these preferred embodiments.  
       First Embodiment  
       [0094]      FIG. 1  is a block diagram showing a configuration of a wideband modulated signal generating device according to a first embodiment of the present invention. Referring to  FIG. 1 , the wideband modulated signal generating device includes a light source  10 , a light branching section  11 , a light combining section  12 , an optical angle modulation section (an optical phase modulation section)  20 , an optical intensity modulation section  30 , a light detecting section  40 , a DC power supply control section  50 , a first DC power supply  51 , a second DC power supply  52 , a third DC power supply  53 , a first branching section  61 , a second branching section  62 , a level detecting section  70 , a demodulation section  80 , and a distortion level detecting section  81 . The light source  10 , the light branching section  11 , the light combining section  12 , the optical angle modulation section  20 , the optical intensity modulation section  30  and the light detecting section  40  may be referred to collectively as the “wideband modulated signal generating section”. Similarly, the DC power supply control section  50 , the second branching section  62 , the level detecting section  70 , the demodulation section  80  and the distortion level detecting section  81  may be referred to collectively as the “bias voltage control section”.  
         [0095]      FIG. 2  is a diagram showing a schematic internal configuration of the optical intensity modulation section  30 . For example, the optical intensity modulation section  30  includes an optical input terminal section  31 , an optical output terminal  32 , a first MZ interferometer  33   a , a second MZ interferometer  33   b , a third MZ interferometer  33   c , a first electrode section  34   a  of the first MZ interferometer  33   a , a second electrode section  34   b  of the second MZ interferometer  33   b , and a third electrode section  34   c  of the third MZ interferometer  33   c , as shown in  FIG. 2 .  
         [0096]     The flow of signals in the wideband modulated signal generating device will now be described. The light branching section  11  splits the unmodulated light from the light source  10  into first light and second light. The first light is inputted to the optical intensity modulation section  30 . In the optical intensity modulation section  30  shown in  FIG. 2 , the first light outputted from the light branching section  11  is inputted to the two MZ interferometers  33   a  and  33   b  via the optical input terminal section  31 . The two MZ interferometers  33   a  and  33   b  modulate the input first light with two electric signals, which are obtained by superposing an electric signal having a predetermined frequency f C  from the first branching section  61  over the first bias voltage from the first DC power supply  51  and the second bias voltage from the second DC power supply  52 .  
         [0097]     Then, the optically modulated signal from the first MZ interferometer  33   a  and that from the second MZ interferometer  33   b  are given predetermined phases in the third MZ interferometer  33   c  by the third bias voltage from the third DC power supply  53  and are made to interfere with each other. Thus, the optical intensity modulation section  30  subjects the input first light to an optical intensity modulation (or an optical amplitude modulation) based on the amplitude of the first electric signal having a predetermined frequency f C  to output the resultant signals as the first optically modulated signal.  
         [0098]     The second light is inputted to the optical angle modulation section  20 . The optical angle modulation section  20  subjects the input second light to an optical angular modulation (an optical phase modulation or an optical frequency modulation) based on the amplitude of the second electric signal to output the resultant signal as the second optically modulated signal. The light combining section  12  combines together the first optically modulated signal from the optical intensity modulation section  30  and the second optically modulated signal from the optical angle modulation section  20 . The light detecting section  40  may be a photodiode having squared detection characteristics, or the like. Using the squared detection characteristics, the light detecting section  40  homodyne-detects the first optically modulated signal and the second optically modulated signal from the light combining section  12  to thereby produce a difference beat signal therebetween. The difference beat signal is a wideband modulated signal obtained by down-converting the second optically modulated signal from the optical angle modulation section  20  and subjecting the signal to an angular modulation, and has a center frequency of f C .  
         [0099]     The second branching section  62  branches a portion of the electric signal from the light detecting section  40  into two paths. The level detecting section  70  extracts a component within a particular band from one of the electric signals from the second branching section  62  and measures the level of the extracted component to thereby detect the level of an electric signal having an arbitrary frequency included in the wideband modulated signal outputted from the light detecting section  40 . Particularly, the level detecting section  70  simply detects the level of the electric signal having an arbitrary frequency included in the wideband modulated signal by detecting the signal component of the lowest frequency among other components of the second electric signal.  
         [0100]     The demodulation section  80  demodulates the wideband modulated signal included in the other one of the electric signals from the second branching section  62 . The distortion level detecting section  81  detects the level of the distortion component at an arbitrary frequency included in the wideband modulated signal from the demodulation section  80 . Particularly, the distortion level detecting section  81  detects the distortion component occurring within a signal band of the highest frequency (where the deterioration of the distortion characteristics is most pronounced) among other components of the second electric signal, whereby it is possible to realize a bias control with a higher precision. Where the second electric signal includes modulated signals of different modulation schemes, the distortion level detecting section  81  detects the distortion component occurring within a signal band of the highest frequency among other components of a modulated signal (among other signals of the second electric signal) that has been modulated by a modulation scheme for which the highest performance is required, whereby it is possible to realize a bias control with a higher precision. The DC power supply control section  50  controls the first bias voltage to be applied to the first DC power supply  51 , the second bias voltage to be applied to the second DC power supply  52  and the third bias voltage to be applied to the third DC power supply  53  based on the signal level detected by the level detecting section  70  and the distortion level detected by the distortion level detecting section  81 .  
         [0101]     The method for controlling the bias voltages in the wideband modulated signal generating device of the present invention will now be described. Specifically, the method by which the first bias voltage, the second bias voltage and the third bias voltage are controlled by the DC power supply control section  50  will now be described in detail with reference to the flow charts shown in FIGS.  3  to  6 .  
         [0102]      FIG. 3  is a flow chart showing the process of controlling the bias voltage performed by the DC power supply control section  50 . Referring to  FIG. 3 , the DC power supply control section  50  compares the signal level detected by the level detecting section  70  with a predetermined signal level prestored in a memory, or the like (step S 50 ). If it is determined that the signal level detected by the level detecting section  70  is less than or equal to the predetermined signal level, the process proceeds to step S 300  (the process of controlling the third bias voltage), skipping steps S 100  and S 200  (the process of controlling the first bias voltage and the process of controlling the second bias voltage). If it is determined that the signal level detected by the level detecting section  70  is greater than the predetermined signal level, the process proceeds to step S 100  (the process of controlling the first bias voltage).  
         [0103]      FIG. 4  is a flow chart showing the details of the process of controlling the first bias voltage (step S 100 ). Referring to  FIG. 4 , the DC power supply control section  50  increases the first bias voltage by a predetermined voltage value (step S 11 ). After the first bias voltage is increased, the DC power supply control section  50  compares the signal level re-detected by the level detecting section  70  with the previously detected signal level (step S 12 ). If it is determined that the signal level re-detected by the level detecting section  70  has increased from the previous signal level, the process proceeds to step S 13 . If it is determined that the signal level re-detected by the level detecting section  70  has decreased from the previous signal level, the process returns to step S 11  to repeat the same procedure. This procedure is repeated until the signal level re-detected by the level detecting section  70  is higher than the immediately previous signal level.  
         [0104]     Then, the DC power supply control section  50  decreases the first bias voltage value by a predetermined voltage value (step S 13 ). After the bias voltage is decreased, the DC power supply control section  50  compares the signal level re-detected by the level detecting section  70  with the previously detected signal level (step S 14 ). If it is determined that the signal level re-detected by the level detecting section  70  has increased from the previous signal level, the process proceeds to step S 15 . If it is determined that the signal level re-detected by the level detecting section  70  has decreased from the previous signal level, the process returns to step S 13  to repeat the same procedure. This procedure is repeated until the signal level re-detected by the level detecting section  70  is higher than the immediately previous signal level. Then, the DC power supply control section  50  brings the first bias voltage value back to the immediately previous value and stores the first bias voltage value, and exits the control of step S 100  (step S 15 ).  
         [0105]     After step S 100 , the process proceeds to step S 200  (the second bias voltage control) as shown in  FIG. 3 .  FIG. 5  is a flow chart showing the details of the process of controlling the second bias voltage (step S 200 ). Steps S 100  and S 200  are the same process except that bias voltages to be controlled are different from each other, and therefore step S 200  will not be further described below. Note that there is no specific order in which step S 100  (the process of controlling the first bias voltage) and step S 200  (the process of controlling the second bias voltage) should be performed, and substantially the same results are obtained when step S 100  (the process of controlling the first bias voltage) and step S 200  (the process of controlling the second bias voltage) are performed in the reverse order.  
         [0106]     After step S 200 , the process proceeds to step S 300  (the process of controlling the third bias voltage).  FIG. 6  is a flow chart showing the details of the process of controlling the third bias voltage (step S 300 ). Referring to  FIG. 6 , the DC power supply control section  50  compares the distortion level detected by the distortion level detecting section  81  with a predetermined distortion level prestored in a memory, or the like (step S 30 ). If it is determined that the distortion level detected by the distortion level detecting section  81  is less than or equal to the predetermined distortion level, the control of step S 300  is terminated while holding the value of the third bias voltage.  
         [0107]     If it is determined that the distortion level detected by the distortion level detecting section  81  is greater than the predetermined distortion level, the process proceeds to step S 31 , where the value of the third bias voltage is increased by a predetermined voltage value (step S 31 ). After the third bias voltage is increased, the DC power supply control section  50  compares the distortion level re-detected by the distortion level detecting section  81  with the immediately previous distortion level (step S 32 ). If it is determined that the distortion level re-detected by the distortion level detecting section  81  has increased from the previous distortion level, the process proceeds to step S 33 . If it is determined that the distortion level re-detected by the distortion level detecting section  81  has decreased from the previous distortion level, the process returns to step S 31  to repeat the same procedure. This procedure is repeated until the distortion level re-detected by the distortion level detecting section  81  is higher than the immediately previous distortion level.  
         [0108]     Then, the DC power supply control section  50  decreases the third bias voltage value by a predetermined voltage value (step S 33 ). After the bias voltage is decreased, the distortion level re-detected by the distortion level detecting section  81  is compared with the immediately previous distortion level (step S 34 ). If it is determined that the distortion level re-detected by the distortion level detecting section  81  has increased from the previous distortion level, the process proceeds to step S 35 . If it is determined that the distortion level re-detected by the distortion level detecting section  81  has decreased from the previous distortion level, the process returns to step S 33  to repeat the same procedure. This procedure is repeated until the distortion level re-detected by the distortion level detecting section  81  is higher than the immediately previous distortion level. Then, the DC power supply control section  50  brings the third bias voltage value back to the immediately previous value and stores the third bias voltage value, and exits the control of step S 300  (step S 35 ).  
         [0109]     The order of the voltage control operations (steps S 100 , S 200  and S 300 ) will now be discussed below. The first to third bias voltages and the first electric signal are applied to the three MZ interferometers  33   a ,  33   b  and  33   c  of the optical intensity modulation section  30 . The first MZ interferometer  33   a  and the second MZ interferometer  33   b  of the optical intensity modulation section  30  serve to suppress light from the light source  10  (i.e., the optical carrier component). The third MZ interferometer  33   c  serves to cancel out the single sidebands of the optically modulated signals modulated by the first MZ interferometer  33   a  and the second MZ interferometer  33   b.    
         [0110]     Where the first optically modulated signal component outputted from the optical intensity modulation section  30  includes, as unnecessary light components, both of the optical carrier component and the single sideband component, these components both have adverse influence on the distortion characteristics of the wideband modulated signal as described above with reference to  FIGS. 14A and 14B . Nevertheless, it is inefficient to monitor the distortion component included in the wideband modulated signal to control the three bias voltages. In view of this, the DC power supply control section  50  controls the first and second bias voltages by using as a monitor signal a signal component ((ii) PM·J 0  component in  FIG. 13C ), among other signal components generated by the wideband modulated signal generating device, that appears in the vicinity of DC only when there remains an optical carrier component, and then controls the third bias voltage by monitoring the distortion level occurring when the wideband modulated signal is demodulated, thus realizing an efficient control flow.  
         [0111]     The amount of time over which each bias voltage is held is set to be sufficiently short so that the bias voltage is not influenced by aging or temperature variations, and the amount by which each bias voltage is controlled is within such a range that the fluctuations of the signal level and the distortion are sufficiently small.  
         [0112]     As described above, according to the first embodiment of the present invention, the bias control of the optical intensity modulation section  30  in the wideband modulated signal generating section is realized by detecting the signal level and the distortion level at a particular frequency outputted from the wideband modulated signal generating device, thus eliminating the need for an optical filter, which is required in conventional devices. This solves the problem that it is impossible to separate the optical carrier component and the optical sideband when demodulating an electric signal whose frequency is on the order of 1 GHz, and realizes an always stable operation with a simple configuration, whereby it is possible to provide a wideband modulated signal generating device whose modulation quality is always high.  
         [0113]      FIG. 7A  shows an alternative configuration of the first embodiment. As compared with the configuration shown in  FIG. 1 , the wideband modulated signal generating device shown in  FIG. 7A  further includes an electric combining section  22  and a third electric signal source  23 . The third electric signal source  23  outputs the third electric signal as a monitor electric signal to be inputted to the optical angle modulation section  20 . The electric combining section  22  combines together the second electric signal and the third electric signal.  
         [0114]     The wideband modulated signal generating device shown in  FIG. 7A  is the same as the configuration shown in  FIG. 1  in terms of the signal flow and the process of controlling the bias voltage, and differs from the configuration shown in  FIG. 1  in that the third electric signal is detected by the level detecting section  70  and used for controlling the first bias voltage and the second bias voltage. The second electric signal may in some cases be a signal composed only of modulated components, e.g., a video signal. It is possible to realize a control with a higher precision by using an unmodulated electric signal such as the third electric signal as a monitor signal, rather than by using a modulated electric signal such as the second electric signal. Then, the level and the frequency of the third monitor electric signal can be determined arbitrarily, whereby it is possible to realize the level detecting section  70  more inexpensively with a simple configuration. Moreover, by setting the frequency of the third electric signal to be lower than that of the second electric signal, it is possible to easily detect the level of the third electric signal.  
         [0115]      FIG. 7B  shows another alternative configuration of the first embodiment. As compared with the configuration shown in  FIG. 7A , the wideband modulated signal generating device shown in  FIG. 7B  further includes a fourth electric signal source  24 . The fourth electric signal source  24  outputs the fourth electric signal as a monitor electric signal to be inputted to the optical angle modulation section  20 . A characteristic of this configuration is that a distortion component (f m1 +f m2  or f m2 −f m1 ) produced by the third electric signal (e.g., frequency f m1 ) and the fourth electric signal (e.g., frequency f m2 ) is monitored as the distortion component detected by the distortion level detecting section  81  to thereby control the third bias voltage.  
         [0116]     The frequency of the monitor signal is preferably such that the produced distortion component does not appear within the signal band of the second electric signal inputted to the optical angle modulation section  20 . As described above, the second electric signal may in some cases be a signal composed only of modulated components, e.g., a video signal. It is possible to realize a control with a higher precision by superposing together unmodulated electric signals such as the third and fourth electric signals so as to detect the distortion component produced by the two electric signals, rather than by using a modulated electric signal such as the second electric signal. Since the level and the frequency of the third and fourth monitor electric signals can be determined arbitrarily, it is possible to realize the distortion level detecting section  81  more inexpensively with a simple configuration.  
       Second Embodiment  
       [0117]      FIG. 8  is a block diagram showing a configuration of a wideband modulated signal generating device according to a second embodiment of the present invention. Referring to  FIG. 8 , the wideband modulated signal generating device includes the light source  10 , the light branching section  11 , the light combining section  12 , the optical angle modulation section  20 , the optical intensity modulation section  30 , the light detecting section  40 , the DC power supply control section  50 , the first DC power supply  51 , the second DC power supply  52 , the third DC power supply  53 , the first branching section  61 , a third branching section  63 , a fourth branching section  64 , the level detecting section  70 , the demodulation section  80 , and the distortion level detecting section  81 . The light source  10 , the light branching section  11 , the light combining section  12 , the optical angle modulation section  20 , the optical intensity modulation section  30  and the light detecting section  40  may be referred to collectively as the “wideband modulated signal generating section”. Similarly, the DC power supply control section  50 , the third branching section  63 , the demodulation section  80 , the fourth branching section  64 , the level detecting section  70  and the distortion level detecting section  81  may be referred to collectively as the “bias voltage control section”.  
         [0118]     The signal flow is the same as that of the first embodiment, and will not be further described below. A characteristic of the method for controlling the bias voltage of the second embodiment is that the level detecting section  70  detects the level of the electric signal having an arbitrary frequency included in the wideband modulated signal after the wideband modulated signal is demodulated by the demodulation section  80 . When a wideband modulated signal is demodulated, a component (the PM·J 0  component) that appears in the vicinity of DC as the beat component between the optical signal component (the J +1  component) angle-modulated with the second electric signal and the optical carrier overlaps with the second electric signal, which occurs when an intended wideband modulated signal is demodulated, whereby it may not be possible to detect the necessary signal level. In view of this, the level detecting section  70  realizes a control of the first and second bias voltages based on a signal that does not overlap with the frequency band of the second electric signal among other beat components between the J +2  component being the double-frequency component of the second optically modulated signal angle-modulated with the second electric signal and the optical carrier J 0  component.  
         [0119]     The first bias voltage control and the second bias voltage control performed by the DC power supply control section  50  are as shown in the flow charts of  FIGS. 3, 4  and  5 , except that different signals are detected. The third bias voltage control is as shown in the flow chart of  FIG. 6 .  
         [0120]     As described above, the wideband modulated signal generating device according to the second embodiment of the present invention controls the first and second bias voltages by using as a monitor signal the beat component between the J +2  component of the second optically modulated signal angle-modulated with the second electric signal, which occurs only when there remains an optical carrier component after demodulation, and the optical carrier component, and then monitors the distortion level to control the third bias voltage, thus realizing an efficient control flow.  
         [0121]      FIG. 9A  shows an alternative configuration of the second embodiment. As compared with the configuration shown in  FIG. 8 , the wideband modulated signal generating device shown in  FIG. 9A  further includes the electric combining section  22  and a fifth electric signal source  25 . The fifth electric signal source  25  outputs the fifth electric signal as a monitor electric signal to be inputted to the optical angle modulation section  20 . The electric combining section  22  combines together the second electric signal and the fifth electric signal.  
         [0122]     The wideband modulated signal generating device shown in  FIG. 9A  is the same as the configuration shown in  FIG. 8  in terms of the signal flow and the process of controlling the bias voltage, and differs from the configuration shown in  FIG. 8  in that a signal component that occurs at a frequency twice as high as that of the fifth electric signal is detected by the level detecting section  70  and used for controlling the first bias voltage and the second bias voltage. The second electric signal may in some cases be a signal composed only of modulated components, e.g., a video signal. It is possible to realize a control with a higher precision by using, as the original signal of the monitor signal, an unmodulated electric signal such as the fifth electric signal, rather than by using a modulated electric signal such as the second electric signal. Then, the level and the frequency of the fifth monitor electric signal can be determined arbitrarily, whereby it is possible to realize the level detecting section  70  more inexpensively with a simple configuration.  
         [0123]      FIG. 9B  shows another alternative configuration of the second embodiment. As compared with the configuration shown in  FIG. 9A , the wideband modulated signal generating device shown in  FIG. 9B  further includes a sixth electric signal source  26 . The sixth electric signal source  26  outputs the sixth electric signal as a monitor electric signal to be inputted to the optical angle modulation section  20 . A characteristic of this configuration is that a distortion component (f m5 +f m6  or f m6 −f m5 ) produced by the fifth electric signal (e.g., frequency f m5 ) and the sixth electric signal (e.g., frequency f m6 ) is monitored as the distortion component detected by the distortion level detecting section  81  to thereby control the fifth bias voltage.  
         [0124]     The frequency of the monitor signal is preferably such that the produced distortion component does not appear within the signal band of the second electric signal inputted to the optical angle modulation section  20 . As described above, the second electric signal may in some cases be a signal composed only of modulated components, e.g., a video signal. It is possible to realize a control with a higher precision by superposing together unmodulated electric signals such as the fifth and sixth electric signals so as to detect the distortion component produced by the two electric signals, rather than by using a modulated electric signal such as the second electric signal. Since the level and the frequency of the fifth and sixth monitor electric signals can be determined arbitrarily, it is possible to realize the distortion level detecting section  81  more inexpensively with a simple configuration.  
         [0125]     The wideband modulated signal generating device of the present invention is useful in, for example, generating a wideband modulated signal (a phase-modulated signal or a frequency-modulated signal).  
         [0126]     While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is understood that numerous other modifications and variations can be devised without departing from the scope of the invention.