Abstract:
An optical device for use in generating a high frequency optical signal includes a light source for generating a pumping light beam, a first oscillator for generating a first light beam with a plurality of modes by using the pumping light beam, an optical element for selecting a first mode from the modes, a second oscillator for generating a second light beam of a second mode by using the selected first mode; and an optical coupler for coupling the selected first mode to the second mode to induce a beat phenomenon therebetween. In the optical device, the first oscillator generates the high frequency optical signal by the beat phenomenon.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optical device for generating a high frequency optical signal; and, more particularly, to an optical device incorporating therein a ring oscillator and a linear oscillator for inducing a beat phenomenon which is used for generating a high frequency optical signal. 
     DESCRIPTION OF THE PRIOR ART 
     As is well known, a semiconductor laser has achieved substantial success as a light source and an oscillator in fiber optic communication systems because of their capability to provide a high speed, a direct current modulation and their relative low cost per component. 
     However, there is still a demand for developing a high frequency light source to implement a wireless multimedia technology in coupled with the high speed fiber communication network. In order to meet this demand, several studies for applying the semiconductor laser diode to the high frequency light source have been developed recently. 
     But, the semiconductor laser diode has an inherent shortcoming that it has a low modulation frequency range. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an optical device incorporating therein a ring oscillator and a linear oscillator coupled to the ring oscillator for generating a high frequency optical signal by using a beat phenomenon between the ring and the linear oscillators. 
     In accordance with one aspect of the present invention, there is provided an optical device for modulating a high frequency optical signal, comprising: a light source for generating a pumping light beam; a first oscillator for generating a first light beam with a plurality of modes by using the pumping light beam; an optical element for selecting a first mode from the modes; a second oscillator for generating a second light beam of a second mode by using the selected first mode; and an optical coupler for coupling the selected first mode to the second mode to induce a beat phenomenon therebetween, whereby the first oscillator outputs the high frequency optical signal generated by the beat phenomenon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
     FIG. 1 shows a schematic view representing an optical device incorporating therein a ring oscillator and a linear oscillator coupled to the ring oscillator for generating a high frequency optical signal in accordance with a first preferred embodiment of the present invention; 
     FIG. 2 is a three-dimensional graph illustrating a gain controlled by changing an orientation angle of a mode controller incorporating the ring oscillator; 
     FIG. 3 represents a graph of the index of birefringence versus wavelengths in accordance with the first preferred embodiment of the present invention; 
     FIG. 4 illustrates a graph of a total gain versus wavelengths in accordance with the first preferred embodiment of the present invention; 
     FIG. 5 depicts a graph of beat frequency versus orientation angles of the mode controller in the ring oscillator; 
     FIG. 6 presents a schematic view representing an optical device incorporating therein a pair of ring oscillators for generating a high frequency optical signal in accordance with a second preferred embodiment of the present invention; 
     FIG. 7 represents a graph of the index of birefringence versus wavelengths in accordance with the second preferred embodiment of the present invention; and 
     FIG. 8 illustrates a graph of a gain versus wavelengths in accordance with the second preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There are provided in FIGS. 1 to  8  schematic views of optical devices  100 ,  200  for generating high frequency optical signals and graphs showing experimental data thereof in accordance with preferred embodiments of the present invention. 
     In FIG. 1, there is provided a schematic view of the inventive optical device  100  comprising a light source, e.g., a laser diode, for generating a pumping light beam, a ring oscillator  50 , a linear oscillator  60  and an optical coupler  30  for coupling the ring oscillator  50  to the linear oscillator  60 . The ring oscillator  50  includes a wavelength division multiplexer (WDM)  10 , a light amplifying fiber (LAF)  12 , a first dispersion shifted fiber (DSF)  14 , an isolator  16 , a first mode controller  18  and an output port  20 . The linear oscillator  60  includes a first and a second mirrors  40 A,  40 B, a second mode controller  44  and a second DSF  42 . 
     In the optical device  100 , the pumping light beam is supplied into the ring oscillator  50  by using the WDM  10 , wherein a wavelength of the pumping light beam has a range from approximately 960 nm to approximately 980 nm. The supplied pumping light beam is amplified by the LAF  12  incorporated into the ring oscillator  50 . It is possible that an erbium doped fiber can be utilized as the LAF  12 . And then, the amplified light beam is inputted to the DSF  14  so as to compensate dispersions caused by the ring oscillator  50 . 
     Thereafter, the dispersion compensated light beam is transmitted to the mode controller  18  through the isolator  16  that makes a light beam transmit to ensure the desired unidirectional operation of the ring oscillator  50  after passing therethrough. After the light beam passes through the mode controller  18 , a portion of the light beam travels to the output port  20  and the remaining portion of the light beam travels to the linear oscillator  60  through the optical coupler  30 . If the mode controller  18  does not operate, the light beam oscillated in the ring oscillator  50  has a first natural mode, which is determined by a birefringence of the ring oscillator  50 . 
     On the other hands, the remaining portion of the light beam is inputted to the linear oscillator  60  by using the optical coupler  30 . In the first preferred embodiment, the optical coupler  30  includes four ports so as to couple two of them to the ring oscillator  50  and to couple the other to the linear oscillator  60 . It is preferable that a portion, e.g., 50%, of the light beam is fed into the output port  20  and the remaining portion, e.g., 50%, of the light beam is fed into the linear oscillator  60 . And also, the remaining portion of the light beam is changed to a second natural mode by a birefringence of the linear oscillator  60 . If the second mode controller  44  does not operate, this second natural mode of the light beam is oscillated from the first mirror  40 A to the second mirror  40 B with passing through the second mode controller  44  and the second DSF  42  in the linear oscillator  60 . The first and the second mirrors  40 A,  40 B are attached to ends of the linear oscillator  60 , respectively. It is preferable that the LAF  12 , the first and the second DSFs  14 ,  42  are approximately 20 m, 60 m and 20 m, respectively. 
     In the first preferred embodiment, the mode controller  18  can change the first natural mode into a new mode. After the remaining portion of the light beam is inputted to the linear oscillator  60 , the mode controller  18  is operated to change the first natural mode into the new mode. There is occurred a beat phenomenon between the second natural mode and the new mode. It is preferable that the mode controller  18  is placed between the isolator  16  and the optical coupler  30 . The mode controller  18  is capable of changing the first natural mode by controlling an orientation angle thereof. After the mode is changed at the mode controller  18 , a light beam having the selected mode is oscillated in the ring oscillator  50 . 
     In the output port  20 , an amplified optical signal having a beat frequency is outputted, wherein the beat frequency can be obtained by the beat phenomenon. The beat frequency can be modulated by changing an orientation angle of the mode controller  18 . It is possible that the beat frequency is modulated by changing an orientation angle of the mode controller  44  in the linear oscillator  60 . 
     FIG. 2 is a three-dimensional graph illustrating a gain controlled by changing an orientation angle of the mode controller  18 . As shown in FIG. 2, the gain of the ring oscillator  50  is sensitively changed in response to the birefringence thereof. 
     FIG. 3 shows a graph representing a relationship between the index of birefringence and wavelengths with respect to the manufacturing conditions of the optical fiber in accordance with the first preferred embodiment of the present invention. 
     FIG. 4 illustrates a graph of a total gain versus wavelengths in accordance with the first preferred embodiment of the present invention. This graph is obtained by summing the gain of the birefringence to the gain of the LAF  12 . 
     FIG. 5 depicts a graph of beat frequency versus orientation angles of the mode controller in the ring oscillator in accordance with the first preferred embodiment of the present invention. This implies that the beat frequency can be modulated in this range by controlling the mode controller  18 . 
     In comparison with the prior art, the first preferred embodiment of the present invention can generate a high frequency optical signal by utilizing a beat phenomenon between a first optical signal and a second optical signal. This is achieved by coupling a ring oscillator to a linear oscillator. 
     In FIG. 6, there is provided a schematic view of the inventive optical device  200  comprising a light source, e.g., a laser diode, for generating a pumping light beam, a first ring oscillator  250 , a second ring oscillator  260  and an optical coupler  230  for coupling the first ring oscillator  250  to the second ring oscillator  260 . The first ring oscillator  250  includes a wavelength division multiplexer (WDM)  210 , a light amplifying fiber (LAF)  212 , a first dispersion shifted fiber (DSF)  214 , an isolator  216 , a first mode controller  218  and an output port  220 . The second ring oscillator  260  includes a second mode controller  244  and a second DSF  242 . 
     In the optical device  200 , the pumping light beam is supplied into the first ring oscillator  250  by using the WDM  210 , wherein a wavelength of the pumping light beam has a range from approximately 960 nm to approximately 980 nm. The supplied pumping light beam is amplified by the LAF  212  incorporated into the first ring oscillator  250 . It is possible that an erbium doped fiber can be utilized as the LAF  212 . And then, the amplified light beam is inputted to the DSF  214  so as to compensate dispersions caused by the first ring oscillator  250 . 
     Thereafter, the dispersion compensated light beam is transmitted to the mode controller  218  through the isolator  216  that makes a light beam transmit to ensure the desired unidirectional operation of the first ring oscillator  250  after passing therethrough. After the light beam passes through the mode controller  218 , a portion of the light beam travels to the output port  220  and the remaining portion of the light beam travels to the linear oscillator  260  through the optical coupler  230 . If the mode controller  218  does not operate, the light beam oscillated in the first ring oscillator  250  has a first natural mode, which is determined by a birefringence of the first ring oscillator  250 . 
     On the other hands, the remaining portion of the light beam is inputted to the second ring oscillator  260  by using the optical coupler  230 . In the second preferred embodiment, the optical coupler  230  includes four ports so as to couple two of them to the first ring oscillator  250  and to couple the other to the second ring oscillator  260 . It is preferable that a portion, e.g., 50%, of the light beam is fed into the output port  20  and the remaining portion, e.g., 50%, of the light beam is fed into the second ring oscillator  260 . And also, the remaining portion of the light beam is changed to a second natural mode by a birefringence of the second ring oscillator  260 . If the second mode controller  244  does not operate, this second natural mode of the light beam is oscillated into the second ring oscillator  260  with passing through the second mode controller  244  and the second DSF  242 . 
     In the preferred embodiment, the mode controller  218  can change the first natural mode into a new mode. After the remaining portion of the light beam is inputted to the second ring oscillator  260 , the mode controller  218  is operated to change the first natural mode into the new mode. There is occurred a beat phenomenon between the second natural mode and the new mode. 
     In the output port  220 , an amplified optical signal having a beat frequency is outputted, wherein the beat frequency can be obtained by the beat phenomenon. The beat frequency can be modulated by changing an orientation angle of the mode controller  218 . It is possible that the beat frequency is modulated by changing an orientation angle of the mode controller  244  in the second ring oscillator  260 . 
     FIG. 7 illustrates a graph of a gain versus wavelengths in accordance with the second preferred embodiment of the present invention. This graph is obtained by summing the gain of the birefringence to the gain of the LAF  212 , wherein a dotted line and a solid line represent a first and a second preferred embodiments, respectively. 
     FIG. 8 depicts a graph of beat frequency versus orientation angles of the mode controller in the ring oscillator in accordance with the second preferred embodiment of the present invention. This implies that the beat frequency can be modulated in this range by controlling the mode controller  218 . 
     In comparison with the first preferred embodiment, the second preferred embodiment can easily generate a high frequency optical signal by coupling a first ring oscillator to a second ring oscillator. 
     While the present invention has been described with respect to the particular embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.