Abstract:
A semiconductor monolithic integrated optical transmitter including a plurality of active layers formed on a semiconductor substrate is disclosed, which comprises: a distributed feedback laser diode including a grating for reflecting light with a predetermined wavelength and a first active layer for oscillating received light from the grating; an electro-absorption modulator including a second active layer for receiving light from the first active layer, wherein the received light intensity is modulated through a change of absorbency in accordance with an applied voltage; an optical amplifier including a third active layer for amplifying received light from the second active layer; a first optical attenuator between the first active layer and the second active layer; and a second optical attenuator between the second active layer and the third active layer.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to an application entitled “Semiconductor monolithic integrated optical transmitter,” filed in the Korean Intellectual Property Office on Jul. 8, 2003 and assigned Serial No. 2003-46204, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor optical device, and more particularly to a semiconductor optical transmitter.  
         [0004]     2. Description of the Related Art  
         [0005]     As Internet communication rapidly increases, the need for improved optical communication speed and facilities also increases. Further, rapid depreciation in optical part prices has promoted the construction of a new ultra-high speed communication environment. In such an environment, technologies are capable of realizing a modulation speed of more than 10 Gbps. In addition, long-distance transmission of more than 80 km without an erbium doped fiber amplifier (EDFA) have attracted considerable attention. Advantageously, the removal of high-priced optical amplifier parts in an optical fiber transmission line provides not only a reduction in price but also in the maintenance of facilities. Further, for an ultra-high speed long-distance transmission, a high-power optical transmitter and a high-sensitivity optical detector, which can transmit optical signals, are necessary even if a loss of optical fiber exists.  
         [0006]      FIG. 1  is an optical transmitter according to one example of the prior art. The optical transmitter  100  includes a distributed feedback laser diode (hereinafter, referred to as DFB LD)  110 , an isolator (hereinafter, referred to as ISO)  120  and a Mach-Zehnder modulator (hereinafter, referred to as M-Z MOD)  130 . The DFB LD  110  continuously outputs high-power lights and the M-Z MOD  130  modulates the inputted light into communication signals at high-speed. Since the DFB LD  110  easily distorts output light due to fed-back light such as reflected light, the ISO  120  is inserted between the DFB LD  110  and the M-Z MOD  130  in order to prevent the distortion. The ISO  120  passes light inputted in one direction and isolates light inputted in other direction.  
         [0007]      FIG. 2  is an optical transmitter according to another example of the prior art. The optical transmitter  200  includes a DFB LD  210 , a first and a second ISO  220  and  240 , an electro-absorption modulator (hereinafter, referred to as EA MOD)  230  and a semiconductor optical amplifier (hereinafter, referred to as SOA)  250 . The DFB LD  210  continuously outputs high-power light and the EA MOD  230  modulates the inputted light into communication signals at high-speed. The SOA  250  compensates for optical loss in the EA MOD  230  by amplifying and outputting the inputted light. Further, the SOA  250  partially compensates for frequency chirp that is generated in the EA MOD  230 . The first ISO  220  is disposed between the DFB LD  210  and the EA MOD  230 . The second ISO  240  is disposed between the EA MOD  230  and the SOA  250 . Each of the first and the second ISOs  220  and  240  passes light inputted in one direction and isolates light inputted in other direction.  
         [0008]     However, in such a conventional optical transmitter  100 , since each part is expensive, the entire cost of the optical transmitter  100  becomes very expensive. Further, in order to prevent transmission light from being distorted due to reverse-direction light such as reflected lights in connecting optical elements, it is essential to employ the ISO  120 . Moreover, each part has a size of several cm by several cm and optical fibers are used to connect parts with each other. Thus, the optical transmitter  100  has an increased overall size of several tens of cm by several tens of cm. To maintaining stable operation of each part, temperature must be kept constant. Power consumption of the optical transmitter  100  becomes very great since each part consumes a large amount of power.  
         [0009]     Further, in such a conventional optical transmitter  200 , as shown in  FIG. 2 , high-priced parts with wide amplification band and low polarization dependency must be employed. These parts are needed since the SOA  250  is affected by conditions of the wavelength of the DFB LD  210  and a polarization of light outputted in the EA MOD  230 .  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, the present invention has been made to reduce or overcome the above-mentioned limitations occurring in the prior art. One object of the present invention is to provide a semiconductor monolithic integrated optical transmitter capable of performing high-speed modulation, obtaining a high power and minimizing a distortion of light.  
         [0011]     An other object of the present invention is to provide a subminiature semiconductor monolithic integrated optical transmitter which has low power consumption, is ultra-miniaturized and low-priced. In addition, it can eliminate the necessity for consideration of a polarization dependency of a semiconductor optical amplifier.  
         [0012]     In accordance with the principles of the present invention, a semiconductor monolithic integrated optical transmitter is provided, that includes a plurality of active layers formed on a semiconductor substrate comprising: a distributed feedback laser diode including a grating for reflecting light with a predetermined wavelength and a first active layer for oscillating received light from the grating; an electro-absorption modulator including a second active layer for receiving light from the first active layer, wherein the received light intensity is modulated through a change of absorbency in accordance with an applied voltage; an optical amplifier including a third active layer for amplifying received light from the second active layer; a first optical attenuator between the first active layer and the second active layer; and a second optical attenuator between the second active layer and the third active layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0014]      FIG. 1  is an optical transmitter according to one example of the prior art;  
         [0015]      FIG. 2  is an optical transmitter according to another example of the prior art;  
         [0016]      FIG. 3  is a perspective view of a monolithic integrated optical transmitter according to a preferred embodiment of the present invention;  
         [0017]      FIG. 4  is a cross-sectional view of the monolithic integrated optical transmitter shown in  FIG. 3 ;  
         [0018]      FIGS. 5   a  and  5   b  are views illustrating variation of an absorption curve of the electro-absorption modulator shown in  FIG. 3 , according to the on and off state of light;  
         [0019]      FIG. 6  is a graph illustrating a modulation characteristic of the electro-absorption modulator shown in  FIG. 3 ;  
         [0020]      FIG. 7  is a graph illustrating a linear and non-linear amplification characteristic of a semiconductor optical amplifier shown in  FIG. 3 ; and  
         [0021]      FIG. 8  is a graph illustrating a transmission characteristic of the optical transmitter shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     Hereinafter, a preferred embodiment according to the present invention will be described with reference to the accompanying drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.  
         [0023]      FIG. 3  is a perspective view of a monolithic integrated optical transmitter according to a preferred embodiment of the present invention.  FIG. 4  is a cross-sectional view of the monolithic integrated optical transmitter shown in  FIG. 3 . The optical transmitter  300  has a monolithic integrated structure with a plurality of layers laminated on one semiconductor substrate  310 . The plurality of layers includes a distributed feedback laser diode (hereinafter, referred to as DFB LD)  450  for oscillating light, an electro-absorption modulator (hereinafter, referred to as EA MOD)  460  for an optical intensity modulation, and a semiconductor optical amplifier (hereinafter, referred to as SOA)  470  for an optical amplification. The optical transmitter  300  further includes a first and a second optical attenuator  350  and  360  for an optical attenuation, an antireflection layer  440  and a first to a third electrode  390 ,  400  and  410 .  
         [0024]     Hereinafter, manufacturing process of the optical transmitter  300  will be described with reference to  FIGS. 3 and 4 . The manufacturing process includes:  
         [0025]     1) The semiconductor substrate  310 , made from an n-InP, is made ready. A common electrode (not shown) is formed on a lower surface of the substrate  310 . Selectively, a lower clad layer, made from the n-InP, is formed on the semiconductor substrate  310 .  
         [0026]     2) A diffraction grating is formed on a predetermined area on the semiconductor substrate  310  in which the DFB LD will be formed. The diffraction grating may be formed on the semiconductor substrate  310  by etching the semiconductor substrate  310 . A photolithography process is used to form a photoresist layer with a grating pattern.  
         [0027]     3) A first to a third active layer  320 ,  330  and  340  in the DFB LD  450 , the EA MOD  460  and the SOA  470  are simultaneously grown on the semiconductor substrate using a selective growth process. Herein, the first to the third active layers  320 ,  330  and  340  are made from InGaAsP. They also have a multiple quantum well structure. Further, the first to the third active layers  320 ,  330  and  340  grow to have different energy bandgaps from each other.  
         [0028]     4) In forming the first to the third active layers  320 ,  330  and  340  on a waveguide structure by means of a photolithography process, boundary areas are etched at the same time. The boundary areas are between active layers and an end of the third active layer  340 , which is spaced away from the boundary areas.  
         [0029]     5) The first and the second optical attenuators  350  and  360  made from InP and a window  370  with the same material are also grown in the etched areas.  
         [0030]     6) An upper clad layer  380  made from p-InP is formed on the first to the third active layer  320 ,  330  and  340 , the first and the second optical attenuator  350  and  360 , and the window  370 .  
         [0031]     7) The first to the third electrodes  390 ,  400  and  410 , which correspond to the first to the third active layer  320 ,  330  and  340  in a one-to-one fashion, are formed on the upper clad layer  380 . Each of the first to third active layers  320 ,  330  and  340  and the corresponding first to third electrodes  390 ,  400  and  410  are vertically aligned.  
         [0032]     8) Parts of the upper clad layer  380  exposed between the first electrode  390  and the second electrode  400  and exposed between the second electrode  400  and third electrode  410  are etched to a predetermined depth. Therefore, a first and a second trench  420  and  430  are formed. Through these steps, the DFB LD  450 , the EA MOD  460  and the SOA  470  are formed on the semiconductor substrate  310 .  
         [0033]     9) An antireflection layer  440  made from a dielectric is coated on one end of the SOA  470  side, from both sides of the optical transmitter  300 .  
         [0034]      FIGS. 5   a  and  5   b  are graphs illustrating variation of an absorption curve of the EA MOD  460  according to the on and off state of light.  FIG. 6  is a graph illustrating a modulation characteristic of the EA MOD  460 .  FIG. 7  is a graph illustrating a linear and non-linear amplification characteristic of the SOA  470 , and  FIG. 8  is a graph illustrating a transmission characteristic of the optical transmitter  300 . Hereinafter, the operation of the optical transmitter  300  will be described with reference to FIGS.  3  to  8 .  
         [0035]     The DFB LD  450  shares the semiconductor substrate  310  and a common electrode. It also includes the first active layer  320 , the diffraction grating  315  and the first upper electrode  390 . Electrons from the semiconductor substrate  310  move to the first active layer  320  and holes from the upper clad layer  380  move to the first active layer  320 , when a current is applied to the first upper electrode  390  and an electric field is formed in the DFB LD  450 . Such reunion of the electrons and holes generates light in the first active layer  320 . The diffraction grating  315  adjusts an oscillation wavelength of the DFB LD  450 . In particular, the light generated in the first active layer  320  is filtered by the diffraction grating  315  on the basis of a predetermined wavelength. That is, from among the light inputted to the diffraction grating  315 , some light having a predetermined wavelength are diffracted by the diffraction grating  315 . The light is then guided along the first active layer  320  while oscillating a laser through a stimulated emission.  
         [0036]     The EA MOD  460  shares the semiconductor substrate  310  and a common electrode. It also includes the second active layer  330  and the second upper electrode  400 . The second active layer  330  has a characteristic in which absorbency changes according to a voltage applied to the second upper electrode  400 . In order to modulate intensity of light oscillated in the DFB LD  450 , the EA MOD  460  has an absorbency that is low in a low voltage and is high in a high voltage. An absorption edge wavelength of the EA MOD  460  is larger than that of the light.  
         [0037]     The SOA  470  shares the semiconductor substrate  310  and a common electrode. It also includes the third active layer  340  and the third upper electrode  410 . The third active layer  340  has a a gain that changes according to a current applied to the third upper electrode  410 . Further, the SOA  470  amplifies and outputs inputted lights. The length and wavelength in a gain peak of the SOA  470  are set to meet required gain and saturation output.  
         [0038]     The first optical attenuator  350  is located between the first and second active layers  320  and  330 . The second optical attenuator  360  is disposed between the second and third active layers  330  and  340 . Each of the first and the second optical attenuators  350  and  360  increases optical coupling loss between two adjacent active layers and attenuates inputted light.  
         [0039]     The window  370  is located at one end of the SOA  470  and diverges light inputted from the third active layer  340 . In this manner, light amplified in the course of progressing in the reverse direction after having been reflected in one end of the optical transmitter  300  is prevented from exerting a bad influence on the DFB LD  450 .  
         [0040]     The antireflection layer  440  is coated on one end of the SOA  470  side in the optical transmitter  300 . This minimizes light reflected by one end of the optical transmitter  300  together with the window  370 .  
         [0041]     The first to the third electrodes  390 ,  400  and  410  are formed on each upper portion of the DFB LD  450 , the EA MOD  460  and the SOA  470 , respectively. In order to insulate the first to the third electrode  390 ,  400  and  410  from each other, trench  420  is formed between the first electrode  390  and the second electrode  400  and trench  430  is formed between the second electrode  400  and the third electrode  410 .  
         [0042]      FIG. 5   a  is a view showing a characteristic curve of each component when the optical transmitter  300  outputs light  515  in an on state.  FIG. 5   b  is a view showing characteristic curve of each component when the optical transmitter  300  outputs light  515  in an off state.  FIGS. 5   a  and  5   b  show a characteristic curve  530  of the SOA  470 , a characteristic curve  510  of the DFB LD  450  and a characteristic curve  520  of the EA MOD  460 .  
         [0043]     The EA MOD  460  has the largest energy bandgap. The DFB LD  450  has the smallest energy bandgap. The absorption edge wavelength of the EA MOD  460  moves with respect to the oscillation wavelength of the DFB LD  450  according to the on and off state of the light  515 . The power of the light  515  decreases or increases according to an increase or decrease of absorbency. Amplified spontaneous emission (hereinafter, referred to as ASE) light  535  generated in the SOA  470  is outputted to both sides of the SOA  470 . Distortion of light  515  is caused when some of ASE lights  535  going toward the EA MOD  460  side are not absorbed but coupled to the first active layer  320 . The coupled light exert an unwanted influence on an output characteristic of the optical transmitter  300 . In order to prevent such influence, (1) the first optical attenuator  350  is located between the first active layer  320  and the second active layer  330 , and (2) the second optical attenuator  360  is disposed between the second active layer  330  and the third active layer  340 . Each of the first and the second optical attenuators  350  and  360  attenuates light inputted from an active layer located at one side and outputs the attenuated light to an active layer located at the other side. More particularly, the second optical attenuator  360  attenuates the ASE light  535  inputted from the third active layer  340  and enables the attenuated light to be coupled to the second active layer  330 . The first optical attenuator  350  attenuates the ASE lights  535  inputted from the second active layer  330  and enables the attenuated lights to be coupled to the first active layer  320 . For instance, as light outputted from an optical fiber in the air diverges, light inputted to the ends of optical attenuators  350  and  360  also diverge. Thus, only portion of the diverging lights are coupled to an active layer bordering to the other end of the first and the second optical attenuator  350  and  360 . Herein, the other light not coupled to the active layer are incident into the upper clad layer  380  and the semiconductor substrate  310  and then disappear. Further, the first optical attenuator  350  attenuates the power of the light inputted from the DFB LD  450  to the EA MOD  460 . This mitigates a so-called hole pile-up phenomenon in the EA MOD  460 . In the hole pile-up phenomenon, the absorbency of the second active layer  330  decreases when the intensity of the light inputted to the second active layer  330  exceeds a critical value.  
         [0044]      FIG. 6  shows a first to a fourth extinction ratio curve representing variation of the extinction ratio with respect to a voltage applied to the EA MOD  460 . The curves are distinguished according to a current I LD  applied to the DFB LD  450  and a current I SOA  applied to the SOA  470  is fixed. The extinction ratio of the EA MOD  460  is more than 15 dB and the extinction ratio characteristic is saturated due to a gain saturation phenomenon of the SOA  470  in a low voltage range with low absorbency.  
         [0045]     Referring to gain curve  600  shown in  FIG. 7 , an amplification rate (or gain) of the SOA  470  changes according to the intensity of the inputted light. When the inputted light has a small intensity, the SOA  470  has a constant amplification rate, even if the light intensity increases to a certain degree. In contrast, when the inputted light has a large intensity, a gain saturation phenomenon occurs, in which the amplification rate decreases as the light intensity increases. In  FIG. 7 , reference numerals A and A′ represent light in an off and on state when the SOA  470  operates in a linear gain range. Reference numerals B and B′ represent light in an off and on state when the SOA  470  operates in a non-linear gain range. When the SOA  470  operates in a non-linear gain range, an output extinction ratio of the SOA  470  is smaller than an input extinction ratio. Advantageously, due to such operation in the non-linear gain range, (1) frequency chirp with respect to light with a large intensity in an on state can be compensated and (2) a frequency chirp component of light outputted from the optical transmitter  300  can be reduced. Therefore, the optical transmitter  300  has an improved output and transmission distance in contrast with the prior art.  
         [0046]      FIG. 8  shows eye diagrams  710  and  720  before/after an optical transmission of an optical transmitter module including the optical transmitter  300 . For such a measurement, the optical transmitter module includes a thermoelectric element, an optical detector, an isolator and a wavelength filter together with the optical transmitter  300  in a package. The optical transmitter  300  is attached to the thermoelectric element for a temperature control. A resistor of 50 Ω is connected to the EA MOD  460  to enable the optical transmitter  300  to operate at a speed of 10 Gbps. Some of light outputted from the optical transmitter  300  is detected by the optical detector. Other light is coupled to an optical fiber after passing through the isolator and the wavelength filter. The wavelength filter removes ASE light from among the light outputted from the optical transmitter  300 . As shown in  FIG. 8 , both the eye diagram  720  after a transmission of 98 km (1600 ps/nm) and the eye diagram  710  before a transmission of 98 km (1600 ps/nm) show a clean state without a noise.  
         [0047]     Advantageously, a single packaging of the optical transmitter is possible, since the semiconductor monolithic integrated optical transmitter according to the present invention is similar in size to that of a conventional optical device, such as a conventional EA MOD, integrated DFB LD and SOA. Further, the optical transmitter&#39;s power consumption is similar to an individual optical element. Thus, the present invention realizes a semiconductor monolithic integrated optical transmitter that has a ultra-miniaturized size and a low price. In addition, optical transmitter has a similar performance as existing conventional high power ultra-high speed optical transmitters.  
         [0048]     Further, the semiconductor monolithic integrated optical transmitter of the present invention mitigates hole pile-up phenomenon of an EA MOD by adjusting an intensity of light inputted to an EA MOD. Further, the present invention performs long-distance transmission greater than a limited transmission distance when a SOA does not exist. This is accomplished by compensating for the frequency chirp of the EA MOD using the non-linear amplification characteristic of the SOA.  
         [0049]     Furthermore, the optical transmitter of the present invention is idle for use in communication systems, since the optical transmitter is an ultra-high speed long-distance transmission part with a low power and a small scale. Accordingly, facilities and maintenance costs are reduced and substituting a high power ultra-high speed optical transmitter including the existing individual optical parts. Furthermore, since the system construction cost is inexpensive, a system including the semiconductor monolithic integrated optical transmitter can be applied to construct various communication environments.  
         [0050]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.