Patent Document

FIELD OF THE INVENTION 
   The present invention relates to a modulator for an optical communication system; and, more particularly, to an optical modulator for modulating a signal by using a deflector integrated with a dynamic single mode laser diode (DSM-LD). 
   DESCRIPTION OF RELATED ARTS 
   Recently, various analog and digital optical communication systems have been introduced for processing mess traffic amount of information and providing various communication services to users. An early stage, a time division multiplex (TDM) method is used for transmitting data in optical communication systems. In order to transmit mess amount of data in a short period time, a wavelength division multiplex (WDM) method is introduced. The WDM method transmits mess amount of data by dividing a wavelength of an optical signal and using divided wavelengths of the optical signal. The WDM method was further developed to a dense wavelength division multiplex (DWDM) method that densely divides the wavelength of the optical signal and uses the densely divided wavelength for transmitting data. The DWDM method transmits data in a speed of tera bits per second (Tbps). 
   The Fabry Perot—laser diode (FP-LD), characterized by its wide optical gain spectrum and multimode operation, has limited used in the application of an optical communication system with the DWDM method. On the other hand, a single mode operation becomes feasible by introducing a period corrugation along the propagation path. This periodic corrugation, namely grating, backscatters all waves propagating along one direction and finally acts as an optical band-pass filter so that only wavelength components close to the Bragg wavelength will be coherently reinforced. Other wavelength terms are effectively cut off as a result of destructive interference. 
   The laser diodes with the grating are classified into distributed feedback—laser diode (DFB-LD), a distributed Bragg reflector—laser diode (DBR-LD) and a distributed reflector—laser diode (DR-LD). These types of laser diodes have been widely used as a semiconductor optical source in coherent optical communication systems as well as DWDM systems because they can be operated as a single mode even in a direct modulation (DM). Especially, the DFB-LD has been widely used owing to easy fabrication, high reliability, and high power. 
   The DFB-LD includes a resonance structure provided with a grating and an active layer formed near the grating in a semiconductor material. The DFB-LD is a resonator having a predetermined length “L”. The DFB-LD generates an optical gain at a medium of the active layer by an electric current injected from external, the optical gain is amplified in the resonance structure and finally, the DFB-LD generates a coherent light beam owing to the grating. The light beam is transferred to an optical fiber and transmitted through the optical fiber. 
   In a direct modulation (DM), the DFB-LD generates the light beam according to a bit stream such as “1010”. That is, the DFB-LD generates the light beam by receiving the electric current when corresponding bit of the bit stream is “1” and outputs the light beam to the optical fiber. In contrary, the electric current is not applied to the DFB-LD when corresponding bit of the bit stream is “0” so the DFB-LD does not generate the light beam. 
   In the DM, the DSM-LD generates a chirp which is a phenomenon that a wavelength of the light beam is fluctuated by the modulation of electric current. This is due to the variation in the refractive index of a medium and results in broadening a spectral line-width of the light beam. This chirped bit stream is broadened and distorted while it travels through the nonlinear dispersive media such as optical fiber. Therefore, it is difficult to apply to DWDM system where the wavelength of light beam is divided less than 0.8 nm (100 GHz). 
   For overcoming the above mentioned disadvantage of the DM cause by the chirp, an indirect modulation is introduced. The indirect modulation uses an external modulator to modulate a signal. 
   In the indirect modulation, the DFB-LD continuously generates a light beam and outputs the generated light beam to the external modulator. The external modulator passes or un-passes (ON-OFF) the light beam to an optical fiber according to an external electric signal. Since the DFB-LD is not operated for modulation, there is no variation of injection current for changing the refractive index of the medium and thus, the chirp can be fundamentally eliminated. 
   Recently, an optical modulator for the indirect modulation is implemented by using an electro-absorption (EA) type modulator or a Mach-Zehnder type modulator as the external modulator integrated with the DSM-LD in a monolithic type and a hybrid type. 
     FIG. 1  is a diagram showing a conventional optical modulator integrated with a Mach-Zehnder type modulator. 
   As shown, the conventional optical modulator  100  includes a DFB-LD  110  and a Mach-Zehnder type modulator  120 . 
   The DFB-LD  110  continuously generates a light beam and outputs the light beam to the Mach-Zehnder type modulator  120 . 
   The Mach-Zehnder type modulator  120  passes or un-passes (ON-OFF) the light beam to the optical fiber (not shown) by applying an electric current or a voltage to arms  121 A and  121 B. In a case of using one arm  121 A, either the electric current or the voltage can be applied and in a case of using two arms  121 A and  121 B, which is called a push-pull method, the voltage is applied for changing a phase of light beam propagated through the arms  121 A and  121 B. That is, the light beam is divided to a two light beams and divided light beams are propagated through two arms  121 A and  121 B. Each of the light beams is controlled to change it&#39;s phase according to the electric current or the voltage applied to the arms  121 A and  121 B. The electric current or the voltage is applied to the electrodes  123 A,  123 B to generate an optical field. The optical field changed a phase of the light beams propagated through the two arms  121 A and  121 B. The phase changed light beams are combined at an output end of the Mach-Zehnder modulator  120 . The light beams propagated through the two arms are passed to the optical fiber (not shown) through an anti-reflection coating  122  by a constructive interference of two phase changed light beams or un-passed to the optical fiber (not shown) by a deconstructive interference. 
   The conventional optical modulator  100  reduces an amount of chirp comparing to the DM. In the case of the push-pull method, negative chirp may be generated and thus, a transmission characteristic can be improved. Furthermore, an extinction ration (ER) can be improved. However, a length of both arms  121 A and  121 B must be longer than a predetermined length for sufficient phase modulation. Therefore, there is a limitation of maximum modulation speed because of parasitic capacitances, a manufacturing conventional optical modulator  100  becomes complicated and a size of the optical modulator becomes larger. Moreover, the phase of the light beam is distorted by a refractive index at an output end of the Mach-Zehnder modulator  120 . 
     FIG. 2  is a diagram illustrating a conventional optical modulator integrated with an electro-absorption (EA) type modulator. 
   As shown, the conventional optical modulator  200  includes a DFB-LD  210  and an electro-absorption (EA) type modulator  220 . 
   The DFB-LD  210  continuously generates a light beam by using a resonance effect in an active layer  211  and outputs the light beam to the electro-absorption (EA) type modulator  220 . 
   The electro-absorption (EA) type modulator  220  passes or un-passes (ON-OFF) the light beam to the optical fiber (not shown) by absorbing the light beam in a waveguide medium  221  having a quantum well structure based on a quantum confined stark effect (QCSE). A reverse voltage is applied to the waveguide medium  221  of the EA type modulator  220  and an electric field is excited at the waveguide medium  221 . The electric field occurs the QCSE to absorb the light beam inputted to the waveguide medium  221 . The QCSE is a phenomenon that an absorption spectrum in quantum wells is shifted to the longer wavelength by an electric filed. 
   A Stark effect, a Frantz Keldysh effect or an exaction quenching may be used for absorbing the light beam. 
   In the electro-absorption (EA) type modulator  220 , the absorption spectrum of the excition must be sharp and a wavelength of absorption peak must be very close to a wavelength of the light beam for obtaining high extinction ratio and high optical absorption within less voltage. However, when the wavelength of absorption spectrum is closed to the wavelength of the light beam, an optical output of the electro-absorption modulator is very weak caused by a no-bias absorption, which absorbs the light beam when an electric field is not applied. Accordingly, a design of the electro-absorption modulator is very complicated for obtaining high extinction ratio and high optical absorption within less voltage since a relation between a wavelength of the light beam generated from the DFB-LD and a wavelength of peak absorption spectrum is must be considered to reduce the basic absorption. 
   Furthermore, the extinction ratio more than 20 dB cannot be obtained in the electro-absorption modulator having the quantum well structure since a Hole Pile-up is occurred in proportion to a modulation speed. That is, heavy holes are accumulated in the quantum well in proportion to the modulation speed is getting fast. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a modulator including a dynamic single mode laser diode integrated with a deflector for modulating a signal by changing a direction of a light beam oscillated from the dynamic single mode laser diode by using the deflector to thereby control a coupling efficiency between the deflector and an optical fiber. 
   It is another object of the present invention to provide a modulator by using a dynamic single mode laser diode integrated with a deflector for eliminating a chirp generated by a direct modulation and having a high extinction ratio. 
   It is still another object of the present invention to provide a modulator by using a dynamic single mode laser diode integrated with a deflector for eliminating a chirp generated by a reflectivity of an output end of the deflector. 
   It is further still another object of the present invention to provide a small and simple structured modulator by using a dynamic single mode laser diode integrated with a deflector. 
   In accordance with an aspect of the present invention, there is provided an optical modulator for coupling a light beam to an optical fiber, the optical modulator including: a laser diode for generating the light beam; and a deflector for deflecting a direction of the light beam according to an electric signal externally applied and outputting the defected light beam to the optical fiber, wherein the laser diode and the deflector are integrated with a multi-layer semiconductor structure in such a way that the light beam is modulated by changing a deflection angle of the deflector 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become better understood with regard to the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagram showing a conventional Mach-Zehnder semiconductor optical modulator integrated with a Mach-Zehnder type modulator in accordance with a prior art; 
       FIG. 2  is a diagram illustrating a conventional electro-absorption semiconductor optical modulator  200  integrated with an electro-absorption (EA) type modulator; 
       FIGS. 3A and 3B  are a top view and a side elevation view of an optical modulator in accordance with a preferred embodiment of the present invention; 
       FIG. 4A  is a top view of an optical modulator in accordance with another preferred embodiment of the present invention; 
       FIG. 4B  is a side elevation view of the optical modulator, which is a cross sectional view taken along with a line II–II′ of the optical modulator in  FIG. 4A ; 
       FIG. 5  is a graph showing a beam propagation characteristic of the optical modulator in  FIG. 4A ; and 
       FIG. 6  is a graph showing a coupling efficiency between the optical modulator  400  in  FIG. 4A  and an optical fiber. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, an optical modulator for modulating a signal by using a deflector integrated with a dynamic single mode laser diode (DSM-LD) in accordance with a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 3A  is a top view of an optical modulator in accordance with a preferred embodiment of the present invention and  FIG. 3B  is a side elevation view of the optical modulator, which is a cross sectional view taken along with I–I′ of the optical modulator in  FIG. 3A . 
   As shown, the optical modulator  300  includes a DFB-LD  310  and a deflector  320  integrated in a multi-layer semiconductor material and the optical modulator  300  is coupled to an optical fiber  330 . 
   The DFB-LD  310  includes an n-InP layer  314  having a grating  314 A, an active layer  313  formed on the n-InP layer for generating a light beam, a P-clad layer  312  formed on the active layer and an electrode  311  formed on the P-clad layer  312  for applying an electric current to the active layer for generating the light beam, wherein the electrode  311  is a metal layer. The DFB-LD  310  continuously generates the light beam and outputs the light beam through the active layer  313  to the deflector  320 . 
   The deflector  320  includes a passive core  323  coupled to the active layer  313  of the DFB-LD  310  for receiving the light beam from the DFB-LD  310  and passing the light beam to the optical fiber  330 , a pattern layer  322  including a deflection pattern  322 A for changing a refractive index of the passive core  323  and an electrode  321  for applying the electric current to the deflection pattern  322 A. 
   The deflector  320  receives the light beam from the DFB-LD  310  and deflects a direction of the light beam propagated through the passive waveguide  323  according to the electric current applied to the deflection pattern  322 A. By changing the direction of the light beam, an optical coupling efficiency between the optical modulator  300  and the optical fiber  330  is controlled for modulation. 
   In a case that the electric current is not applied to the deflection pattern  322 A of the deflector  320 , the deflector  320  passes the light beam to the optical fiber  330  without deflecting the direction of the light beam and thus, the light beam is transmitted to the optical fiber  330 . The light beam is passed through a path A to a core  332  of the optical fiber  330 . 
   In a case that the electric current is applied to the deflection pattern  322 A of the deflector  320 , the deflector  320  deflects the direction of the light beam to propagate to the clad  331  of the optical fiber  330 . That is, a direction of the light beam is changed to a path B by the deflector  320  and the light beam is propagated toward to a clad  331  of the optical fiber  330 . Therefore, the light beam is not passed to the fiber core  332 . That is, the deflector  320  reduces the optical coupling efficiency of the light beam to the optical fiber. 
   The direction of the light beam is deflected by the deflection pattern  322 A. The electric current is applied to the deflection pattern  322 A through the electrode  321  and the deflection pattern  322 A changes the refractive index of a medium of the passive core  323 . 
   The electric current applied to the deflect pattern changes a refractive index and a medium loss of the passive waveguide because a complex dielectric constant of the medium is influenced by the electric current. As increasing amount of electric current applied to the medium, a band-gap shrinkage, a free-carrier absorption and a plasma effect are additionally occurred with the anomalous dispersion and they also change the refractive index of the medium of the passive waveguide. Therefore, the refractive index of the medium is varied according to the amount of the electric current applied to the medium. 
   The deflection pattern  322 A has a shape of a triangle, which is asymmetric shape based on a Z-axis in  FIG. 3A . By the shape of the deflection pattern  322 A, the amount of the electric current applied to the medium of the pass waveguide  322  is varied. That is, the refractive index of the medium of the pass waveguide  322  is changed according to the amount of the electric current varied by the shape of the deflection pattern  322 A. A vertex portion of the deflection pattern applies less amount of the electric current comparing to a base portion of the deflection pattern. The refractive index of the medium coupled to the vertex portion of the deflection pattern  322 A is less influenced than the medium coupled to the base portion of the deflection pattern  322 A. Therefore, the light beam is deflected to a direction from the base of the deflection pattern  322 A to the vertex of the deflection patterns  322 . 
   In the preferred embodiment of the present invention, the deflection pattern  322 A of the triangle shape having N-P semiconductor structure and the pattern layer  322  has P-N semiconductor structure. Therefore, the electric current applied to the deflection pattern  322  does not flow to the pattern layer  322 . 
   In the preferred embodiment of the present invention, a medium of InGaAsP is used as the medium of the passive waveguide  323 . The refractive index of the medium at a specific wavelength can be varied according to a loss spectrum of the medium. In a case of an InGaAsP medium having 1.3 μm of band-gap wavelength, a difference of the refractive index varied is 0.06 at a wavelength of 1.55 μm. 
   In the preferred embodiment of the present invention, the electric current is used for changing the refractive index of the medium but an electric voltage can be used for the same. 
   The DFB-LD  310  and the deflector  320  are integrated in a multi-layer semiconductor material within a monolithic type or a hybrid type. The deflection pattern  322 A of the deflector  320  must be an asymmetric shape base on a Z-axis, which is a direction of propagation of the light beam. 
     FIG. 4A  is a top view of an optical modulator in accordance with another preferred embodiment of the present invention and  FIG. 4B  is a side elevation view of the optical modulator, which is a cross sectional view taken along with a line II–II′ of the optical modulator in  FIG. 4A . 
   As shown in  FIG. 4A , the optical modulator  400  including a DFB-LD  410  and a deflector  420  integrated in a multi-layer semiconductor material. The deflector  420  includes a deflection pattern  421  having three triangle shapes for minutely controlling a deflection of a light beam. 
   In the preferred embodiment of the present invention in  FIG. 4A , a length of the optical modulator  400  is 100 μm. A width of active layer  412  of the DFB-LD  410  is 3 μm and a refractive index of the active layer  412  is “3.33”. A refractive index of a P-clad of the DFB-LD  410  is “3.30” and a refractive index of the deflector  420  is “3.24”. 
   Lengths of three triangle shapes in the deflection patterns  421  are 10 μm, 12.5 μm, and 15 μm and heights are 15 μm respectively. 
   According to the structure of the optical modulator  400  in  FIG. 4A , a variation of the refractive index of the deflector  420  is 0.06. 
   The DFB-LD  410  has a laterally weakly index guide structure and the deflector  410  has a slab waveguide structure in the preferred embodiment in  FIG. 4A . In the structure of the optical modulator  400  in  FIG. 4A , a width of the light beam outputted from the DFB-LD  410  may become wider. The DFB-LD  410  can be implemented to have a buried hetero-structure such as a strong index guide structure. In this case, the width of the light beam may become much wider because widths of a waveguide and a mode are very narrow. For preventing spreading the width of the light beam, an optical spot size converter may be inserted between the DFB-LD  410  and the deflector  420 . 
   As shown in  FIG. 4B , the DFB-LD  410  includes an n-InP layer  411  including a grating, an active layer  412  formed on the n-InP layer  411 , a p-InP layer  413  formed on the active layer  412 , a p-Etch stop layer  414  formed on the p-InP layer  413 , a p-InP layer  415  formed on the p-Etch stop layer  414 , an InGaAsP layer  416  formed on the p-InP layer  415  and a metal layer  417  formed on the InGaAsP layer  416  as an electrode. The deflector  420  includes the n-InP layer  411  which is extended from the DFB-LD  410 , a passive core  422  formed on the n-InP layer  411  and a pattern layer  423  having a p-InP layer  423 A, an undoped-Etch stop layer  423 B, a n-InP  423 C and a P-Etch step layer  423 D formed in order. The pattern layer  423  includes the deflection pattern  421  formed on a portion A of the pattern layer  423 . The deflection pattern  421  includes an n-InP layer  421 A, a p-Etch stop layer  421 B, a P-InP layer  421 C, an InGaAsP layer  421 D and a metal layer  421 E. As shown, the deflection pattern  421  has an N-P semiconductor structure and the pattern layer  423  has a P-N semiconductor structure. Therefore, electric current flowing in the deflection pattern  421  is not flow to the pattern layer  423 . 
     FIG. 5  is a graph showing a beam propagation characteristic of the optical modulator in  FIG. 4A . 
   As shown, at a point of 200 μm on a Z axis, the graph clearly shows that the light beam is maximally deflected to approximately 6.3 μm based on an X axis. That is, a maximum angle of deflection is 22 degrees. 
     FIG. 6  is a graph showing a coupling efficiency between the optical modulator  400  in  FIG. 4A  and an optical fiber. The optical fiber includes a 9 μm diameter core and a 125 μm diameter clad. A difference of refractive indexes between the core and the clad is 0.01. 
   A solid curve a represents the coupling efficiency when a refractive index of the deflector  420  is not changed and a dotted curve b represents the coupling efficiency when a refractive index of the deflector  420  is changed by applying an electric current to the deflector  420 . The curves a and b shows coupling efficiency based on a lateral direction of deflection of the light beam and thus, the coupling efficiency may be decreased if a vertical direction of deflection of the light beam is considered. 
   The graph shows that the coupling efficiency between the optical modulator  400  and the optical fiber is approximately −3 dB when the refractive index of the deflector  400  is not changed. 
   In a case that the refractive index is not changed, the light beam is spread when the light beam travels through the slab waveguide of the deflector  420 . However, the coupling efficiency is maintained at approximately −3 dB since a width of a waveguide mode of the optical fiber is much wider than a width of optical distribution in the slab waveguide. 
   In contrary, the coupling efficiency is incredibly decreased when the refractive index of the deflector  420  is changed by applying the electric current to the three triangle patterns of the deflector  420 . The coupling efficiency is gradually deflected when the light beam is passed through each of three triangle shaped deflection patterns of the deflector  420 . 
   At an output end of the deflector  420 , the coupling efficiency is approximately −28 dB and an extinction ratio is approximately 25 dB, which is calculated by 10 log(power_on/power_off). If a loss, which is increased according to an amount of the electric current applied to the deflector  420 , is considered, the expected extinction ratio would be increased. 
   As mentioned above, the optical modulator using a deflector in accordance with the present invention can control a coupling efficiency by deflecting the light beam. 
   Also, the optical modulator using the deflector can eliminate the chirp generated by direct modulation method by using the deflector for changing a direction of light beam oscillated from a laser diode. 
   Furthermore, the optical modulator using the deflector can be manufactured in small sized and simple structure comparing to a conventional Mach-Zehnder optical modulator. The optical modulator is less influenced by reflection caused by the external electric signal modulation because a variation of amplitude of light beam propagated through the deflector according to external electric signal is much smaller than the electro-absorption modulator. Accordingly, manufacturing of the deflector is much easier since the deflector of the present does not require extremely low reflectivity such as below than 0.01%. 
   Moreover, the optical modulator having the deflector has higher extinction ratio and a lower refractive rate comparing to a conventional electric absorption optical modulator. 
   The present invention can produce less amount of chirp since amplitude of a laser beam generated from a light source is not changed when a deflector is turned-on and off for changing a direction of the laser beam. That is, an amount of a laser beam reflected from the deflector to the light source is almost same. 
   The present application contains subject matter related to Korean patent application No. KR 2003-0071839, filed in the Korean patent office on Oct. 15, 2003, the entire contents of which being incorporated herein by reference. 
   While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirits and scope of the invention as defined in the following claims.

Technology Category: g