Patent Document

CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority of Japanese Patent Application No. 2003-28555 filed on Feb. 5, 2003, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     A) Field of the Invention 
     The present invention relates to a semiconductor laser, and more particularly to a tunable twin-guide distributed feedback (TTG-DFB) type laser. 
     B) Description of the Related Art 
     Present backbone transmission systems for large capacity optical communication networks utilize wavelength division multiplexing (WDM) which can increase the capacity of transmission by multiplexing optical signals on a wavelength axis. As the degree of multiplexing is made high, the number of semiconductor lasers to be used as light sources increases. Backup light sources are also required as many as or larger than that of semiconductor lasers, resulting in annoying stock management. Simplifying the management of tunable optical light sources has been desired. Wavelength routing techniques have been paid attention, which dynamically change a routing destination depending upon the wavelength. 
     A distributed Bragg reflector (DBR) laser is known as one example of tunable lasers.  FIG. 4A  is a cross sectional view of a conventional tunable DBR laser. On a lower clad layer  100 , an active layer  101  and an upper clad layer  103  are formed. Along the longitudinal direction of an optical resonator, a gain region G, a phase adjusting region P and a DBR region R are defined in this order. In the DBR region R, a diffraction grating  102  is disposed near the active layer  101 . 
     Current is supplied to the gain region G by an electrode  104 , to the phase adjusting region P by an electrode  105 , and to the DBR region R by an electrode  106 . As the current is supplied to the gain region G, laser oscillation occurs. As the current is injected into the DBR region R, the oscillation wavelength can be changed. The oscillation wavelength is determined by the longitudinal mode nearest to the wavelength at which the loss of an optical resonator is minimum, so that the wavelength change is discrete. By adjusting the injection amount of current into the phase adjusting region P, it is possible to realize a quasi-continuous wavelength change. 
     With this method, however, it is necessary to control both the injection currents into the DBR region R and phase adjusting region P, resulting in a complicated control system. When the relation between wavelength and current shifts after a long time operation, it is difficult to follow a change in wavelength, leaving a reliability problem. 
     A TTG- DFB laser is known as a semiconductor laser which can eliminate the difficulty in controlling a tunable DBR laser. Refer to “Tunable twin-guide laser: A novel laser diode with improved tuning performance” by M. C. Amann, S. Illek, C. Schanen, and W. Thulk, Applied Physics Letters, Vol. 54, (1989), pp. 2532–2533. 
       FIG. 4B  is a cross sectional view of a TTG-DFB laser. A tuning layer  111 , an intermediate layer  112 , an active layer  113 , a diffraction grating  114  and an upper clad layer  115  are stacked in this order on a substrate  110  also serving as a lower clad layer. From the viewpoint of optical transmission mode, the layers of the tuning layer  111  to the diffraction grating  114  are disposed near each other so that they exist in the same mode. An electrode  117  is formed on the bottom surface of the substrate  110 , and an electrode  116  is formed on the upper clad layer  115 . 
     The intermediate layer  112  has a conductivity type opposite to that of the substrate  110  and upper clad layer  115 . The tuning layer  111  and active layer  113  are electrically independent from each other. Namely, current injected from the electrode  117  into the tuning layer  111  is controlled independently from that injected from the electrode  116  into the active layer  113 . 
     As current is flowed in the active region  113 , laser oscillation can be excited. As current is flowed in the tuning layer  111 , a refractive index of the tuning layer  111  changes due to the plasma effect. Because the effective refractive index in the laser transmission mode also changes, the Bragg wavelength determined by the diffraction grating  114  changes so that the oscillation frequency changes. In this manner, the oscillation wavelength can be changed in a continuous manner by controlling only the current to be injected into the tuning layer  111 . 
       FIG. 4C  is a cross sectional view showing another structure of a TTG-DFB laser. In the laser shown in  FIG. 4B , the diffraction grating  114  is disposed on the active layer  113  on the side opposite to the intermediate layer  112 . In the laser shown in  FIG. 4C , a diffraction grating  114  is disposed on a tuning layer  111  on the side opposite to an intermediate layer  112 . 
     SUMMARY OF THE INVENTION 
     In order to achieve the sufficient quality as a laser source, it is necessary to set a proper coupling coefficient κ (cm −1 ) between light propagating in an optical resonator and a diffraction grating. Generally, the tuning layer  111  is made of semiconductor material having a transition wavelength shorter than that of the active layer  113  so as not to absorb laser light. If the transition wavelength of the tuning layer  111  is set too short, the change in refractive index relative to the change in current cannot be made large. From this reason, the transition wavelength of the tuning layer  111  is set longer than those of the clad layers and the intermediate layer  112 . 
     Because the refractive index of the tuning layer  111  is large, an optical electric field distribution is attracted toward the tuning layer  111 . In the TTG-DFB laser having the structure shown in  FIG. 4B , if the optical electric field is attracted toward the tuning layer  111 , the optical electric field applied to the diffraction grating  114  decreases so that a proper coupling coefficient .kappa. cannot be obtained. 
     In the laser having the structure shown in  FIG. 4C , a proper coupling coefficient .kappa. can be obtained in the small tuning current range. However, as the tuning current is made large, the refractive index of the tuning layer lowers because of the plasma effect so that the optical electric field distribution is attracted toward the active layer  113 . Therefore, the optical electric field applied to the diffraction grating  114  decreases so that the coupling factor .kappa. lowers and the oscillation threshold value becomes high. Furthermore, because a loss by free carrier absorption increases, the oscillation threshold value rises synergistically. 
     An object of this invention is to provide a TTG-DFB laser capable of maintaining a proper coupling coefficient and having good characteristics as a communication light source. 
     According to one aspect of the present invention, there is provided a semiconductor laser comprising: an active layer for radiating light through stimulated emission by carrier injection; a tuning layer disposed spaced by some distance apart from the active layer in a thickness direction, the tuning layer having a transition wavelength shorter than a wavelength of the light radiated from the active layer; a diffraction grating layer disposed between the active layer and the tuning layer, a refractive index of the diffraction grating layer being periodically changed along an optical resonator direction; a first electrode for supplying the active layer with current; and a second electrode for supplying the tuning layer with current independently from the current to be supplied to the active layer. 
     Light generated in the active layer is confined mainly in the lamination structure from the active layer to the tuning layer. Because the diffraction grating layer is disposed between the active layer and tuning layer, a coupling coefficient between light and diffraction grating can be raised. A large coupling coefficient can be maintained even if the optical electric field is deviated toward the active layer or tuning layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view of a TTG-DFB laser according to an embodiment. 
         FIGS. 2A to 2F  are cross sectional views illustrating a manufacture method for a TTG-DFB laser according to an embodiment. 
         FIG. 3  is a graph showing optical electric field intensity distributions of a TTG-DFB laser of the embodiment along a stacking direction. 
         FIGS. 4A to 4C  are cross sectional views of conventional DBR and TTG-DFB lasers. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a cross sectional view of a TTG-DFB laser along a direction (light propagation direction) parallel to the longitudinal direction of an optical resonator, according to an embodiment of the invention. On a substrate  1  made of p-type InP, a buffer layer of p-type InP having a thickness of 1 μm is formed. A tuning layer  3 , an intermediate layer  4 , a diffraction grating layer  5  and a multiple quantum well active layer  6  are sequentially stacked in this order on the buffer layer  2 . The substrate  1  and the buffer layer  2  serve as a clad layer. 
     The tuning layer  3  is made of undoped Ga 0.283 In 0.717 As 0.611 P 0.389  and has a thickness of 0.28 μm. The intermediate layer  4  is made of n-type InP and has a thickness of 0.11 μm. The diffraction grating layer  5  has a structure such that a diffraction grating made of n-type Ga 0.217 In 0.783 As 0.472 P 0.528  is buried in the n-type JnP layer. The detailed structures of this layer will be later described. 
     The multiple quantum well active layer  6  is formed by alternately laminating well layers of undoped Ga 0.305 In 0.695 As 0.904 P 0.096 , each having a thickness of 5.1 nm and barrier layers of undoped Ga 0.283 In 0.717 As 0.611 P 0.389 , each having a thickness of 10 nm. The uppermost and lowermost layers of the multiple quantum well active layer are the barrier layers, and there are seven well layers. 
     On the multiple quantum well active layer  6 , a separated confinement hetero (SCH) layer  7  is formed which is made of undoped Ga 0.182 In 0.818 As 0.397 P 0.603  and has a thickness of 20 nm. On the SCH layer  7 , an upper clad layer  8  made of p-type InP and a contact layer  9  made of p + -type Ga 0.47 In 0.53 As are formed. 
     An upper electrode  10  is in ohmic contact with the contact layer  9 . A lower electrode  11  is in ohmic contact with the bottom surface of the substrate  1 . 
     The output end face of the optical resonator is non-reflection coated and the opposite end face is high-reflection coated. 
     Next, with reference to  FIGS. 2A to 2F , description will be given for a manufacturing method for the TTG-DFB laser of the embodiment shown in  FIG. 1 . 
     As shown in  FIG. 2A , on the substrate  1  of n-type InP, a buffer layer  2  of p-type InP having a thickness of 1 μm is grown by metal organic chemical vapor deposition (MOCVD). Each layer to be described hereinunder is also grown by MOCVD. Trimethylindium, triethylgallium, arsine and phosphine are used as the source materials for In, Ga, As and P, respectively, and hydrogen is used as carrier gas. Si is used as n-type impurities. 
     On the buffer layer  2 , a tuning layer  3  of undoped Ga 0.283 In 0.717 As 0.611 P 0.389  having a thickness of 0.28 μm and an intermediate layer  4  of n-type InP having a thickness of 0.11 μm are sequentially grown. On the intermediate layer  4 ; a first layer  5 A of n-type Ga 0.217 In 0.783 As 0.472 P 0.528  having a thickness of 30 nm and a second layer  5 B of n-type InP having a thickness of 10 nm are grown. 
     As shown in  FIG. 2B , grooves are formed, which are periodically disposed along the longitudinal direction of the optical resonator through the first and second layers  5 A and  5 B by electron beam exposure techniques. The period of grooves is set to about 240 nm. Each groove is formed through the first layer  5 A and exposes the intermediate layer  4  on the bottom surface thereof. The first layer  5 A of GaInAsP can be wet etched using mixed solution of H 2 SO 4 , H 2 O 2  and H 2 O as an etchant. By optimizing a mixture ratio, GaInAsP can be selectively etched relative to InP. A diffraction grating having GaInAsP regions, which are periodically disposed, can therefore be formed. 
     As shown in  FIG. 2C , a third layer  5 C made of n-type InP, the refractive index of which is different from the refractive index of the diffraction grating, is formed to bury the diffraction grating. The third layer  5 C has a thickness of 10 nm as measured on the second layer  5 B. A diffraction grating layer  5  is constituted of the first, second and third layers  5 A,  5 B and  5 C. 
     On the diffraction grating layer  5 , a barrier layers  6 B of undoped Ga 0.283 In 0.717 As 0.611 P 0.389  each having a thickness of 10 nm and well layers  6 W of undoped Ga 0.305 In 0.695 As 0.904 P 0.096  each having a thickness of 5.1 nm are alternatively stacked by seven cycles. A barrier layer  6 B is formed on the uppermost layer. A multiple quantum well active layer  6  is constituted of seven well layers  6 W and eight barrier layers  6 B. 
     On the multiple quantum well active layer  6 , an SCH layer  7  of undoped Ga 0.182 In 0.818 As 0.397 P 0.603  having a thickness of 20 nm is grown. On this SCH layer  7 , a first clad layer  8 A is grown which is made of p-type InP and has a thickness of 0.2 μm. 
       FIGS. 2D to 2F  are cross sectional views of the TTG-DFB laser along the direction perpendicular to the longitudinal direction of the optical resonator. As shown in  FIG. 2D , in order to leave a striped mesa  18  having the lamination structure from the tuning layer  3  to the first clad layer  8 A, the regions on both sides of the mesa  18  are etched down to the surface layer of the buffer layer  2 . In this case, a SiO 2  film  20  is formed on the first clad layer  8 A and used as an etching mask. The width of the mesa  18  is set to about 1 μm. 
     As shown in  FIG. 2E , by leaving the SiO 2  film ( FIG. 2D ) used as the etching mask, n-type InP is selectively grown on both sides of the mesa  18  to form a burying layer  15 . After the burying layer  15  is formed, the SiO 2  film  20  is removed to expose the first clad layer  8 A. On the first clad layer  8 A and burying layer  15 , a second clad layer  8 B of p-type InP is grown. The surface of the second clad layer  8 B is generally flat. The first and second clad layers  8 A and  8 B in the region where the mesa  18  is disposed correspond to the clad layer  8  shown in  FIG. 1 . 
     On the second clad layer  8 B, a contact layer  9  of p + -type Ga 0.47 In 0.53 As is grown. 
     As shown in  FIG. 2F , a partial region of the contact layer  9  and second clad layer  8 B is removed to expose a part of the surface of the burying layer  15 . An upper electrode  10  is formed on the contact layer  9 . A lower electrode  11  is formed on the bottom surface of the substrate  1 . A common electrode  12  is formed on the surface of the exposed burying layer  15 . Each of the upper and lower electrodes  10  and  11  has the lamination structure of an AuZn layer of 200 nm in thickness disposed on the substrate side and an Au layer of 3 μm in thickness disposed on the AuZn layer. The common electrode  12  has the lamination structure of an AuGe layer of 200 nm in thickness disposed on the substrate side and an Au layer of 3 μm in thickness disposed on the AuGe layer. 
     Current injected from the upper electrode  10  flows to the common electrode  12  via the multiple quantum well active layer  6 , diffraction grating layer  5 , intermediate layer  4  and burying layer  15 . Current injected from the lower electrode  11  flows to the common electrode  12  via the buffer layer  2 , tuning layer  3 , intermediate layer  4  and burying layer  15 . By controlling the voltage applied across the upper and lower electrodes  10  and  11 , excitation current flowing through the multiple quantum well active layer  6  and tuning current flowing through the tuning layer  3  can be controlled independently. 
     The substrate  1  is cleaved to separate it into laser chips. The output end face is non-reflection coated and the opposite end face is high-reflection coated. 
       FIG. 3  is a graph showing optical electric field intensity distributions of the TTG-DFB laser of the embodiment shown in  FIG. 1  as measured along the lamination direction. The abscissa represents a position along the lamination direction in the unit of “μm” and the ordinate represents an optical electric field intensity in the arbitrary unit. 
     Broken and solid lines in  FIG. 3  indicate the optical electric field intensity distributions at tuning currents I tune  of 0 and a positive value, respectively. In both cases, the optical electric field distributes mainly over the range from the tuning layer  3  to the multiple quantum well active layer  6 . The refractive index of the tuning layer  3  is relatively high at the tuning current I tune  of 0 so that the optical electric field exists slightly deviated in the tuning layer  3 . As the tuning current I tune  is flowed, the refractive index of the tuning layer  3  lowers relatively so that the optical electric field in the tuning layer  3  weakens and that in the multiple quantum well active layer  6  becomes high. 
     Because the diffraction grating layer  5  is disposed between the tuning layer  3  and multiple quantum well active layer  6 , the optical electric field intensity at the position of the diffraction grating layer  5  is high irrespective of whether the tuning current is flowed or not flowed. A relatively large coupling coefficient K can therefore be obtained. 
     If the tuning current is increased, the optical electric field distributes deviated in the multiple quantum well active layer  6  so that the optical electric field intensity in the diffraction grating layer  5  becomes large. An increase in the oscillation threshold value to be caused by free carrier absorption is therefore suppressed. Generally, as the tuning current is increased, Schottky noises to be caused by spontaneous emission increase so that a light emission spectrum is broadened. In the embodiment, as the tuning current is increased, the coupling coefficient κ becomes large so that broadening the light emission spectrum can be suppressed. 
     If the coupling coefficient κ at the tuning current of 0 is set to a proper initial value, the coupling coefficient κ at any tuning current will not lower than the initial value. Accordingly, the TTG-DFB laser of the embodiment has high resistance against reflected return light and suitable for application to integration purpose. 
     Also shown in  FIG. 3  for the purposes of comparison is the position of a diffraction grating layer  5 P of the conventional TTG-DFB laser shown in  FIG. 4C . Because the diffraction grating layer  5 P is disposed outside of the lamination structure sandwiched by the multiple quantum well active layer  6  and tuning layer  3 , the optical electric field intensity at the position of the diffraction grating layer  5 P is weak. As the tuning current is flowed, the optical electric field is deviated toward the multiple quantum well active layer  6  so that the optical electric field at the position of the diffraction grating layer  5 P becomes weak and it becomes difficult to maintain a proper coupling coefficient κ. 
     As in the embodiment, because the diffraction grating layer  5  is disposed between the multiple quantum well active layer  6  and the tuning layer  3 , a relatively large coupling coefficient .kappa. can be obtained irrespective of whether or not a tuning current flows. 
     In the above-described embodiment, a TTG-DFB laser for the 1.55 μm band has been described. TTG-DFB lasers for other bands may also be manufactured by changing the compositions of the multiple quantum well active layer, intermediate layer and tuning layer. 
     Also in the embodiment, although the active layer  6  radiating light through stimulated emission is made of multiple quantum wells, it may be made of a single semiconductor layer. Also in the embodiment, although the tuning layer  3  is made of a single semiconductor layer, it may have a multiple quantum well structure. In this case, the transition wavelength of the tuning layer  3  is set shorter than the wavelength of light radiated in the active layer. Light absorption in the tuning layer  3  can thus be prevented. 
     The present invention has been described in connection with the preferred embodiments. The invention is not limited only to the above embodiments. It will be apparent to those skilled in the art that other various modifications, improvements, combinations, and the like can be made.

Technology Category: h