Patent Abstract:
The object of the present invention is to provide an optical module for a WDM communication system, in which the oscillation wavelength is on the grid of the WDM regulation. The optical output power and the oscillation wavelength can be controlled independently. The present module includes a semiconductor light-emitting device, a wedge shaped etalon device and two light-receiving devices. The etalon contains a second portion, on which anti-reflection films are coated, and a first portion. One of the receiving devices detects light transmitted through the second portion of the etalon, while the other device detects light through the first portion. A signal from the former device controls the output power of the light-emitting device, while a signal from the latter receiving device controls the oscillation wavelength of the laser.

Full Description:
CROSS REFERENCE RELATED APPLICATIONS 
     This application contains subject matter that is related to the subject matter of the following application, which is assigned to the same assignee as this application and filed on the same day as this application. The below listed application is hereby incorporated herein by reference in its entirely: 
     “Light-Emitting Module” by Yabe et. al. 
     “Optical Module” by Takagi et. al. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates to an optical module, especially relates to an optical signal source used in a WDM (Wavelength Division Multiplexing) communication. 
     2. Related Prior Art 
     In the WDM communication, the wavelength interval to the adjacent channel is defined to be 0.8 nm. This regulation means that the absolute accuracy superior than ±0.1 nm is required for the signal wavelength of respective channel. A semiconductor laser, such as DFB laser (Distributed Feedback Laser) and DBR (Distributed Bragg Reflector), is utilized for the signal source of the WDM system. 
     These feedback lasers have a sharp oscillation spectrum with a typical bandwidth less than 50 GHz. However, since the Bragg grating formed within a laser chip solely determines the oscillation wavelength, it would be quite difficult to get the desired wavelength due to the uncertainty of the manufacturing process parameter. 
     It is also known that the oscillation wavelength of individual lasers can be adjusted by the feedback control after the completion of the production. The method is: 1) dividing the output light from the optical module, 2) monitoring the divided light with a spectrum analyzer, and 3) adjusting the temperature of the laser and the injection current to the laser, thus controlling the oscillation wavelength. However, this technique uses the optical spectrum analyzer and is quite impossible to apply to the WDM system, which requires a plurality set of such large-scale equipment for respective optical signal source. 
     Another example is disclosed in U.S. Pat. No. 5,825,792, in which a parallel plates etalon is used for the controlling of the oscillation wavelength. In the &#39;792 patent, two optical detectors monitor a divergent light emitted from the back facet of the laser through the etalon placed with an angle for the light. By feed backing the differential signal of two detectors to a temperature of the laser, the oscillation wavelength is effectively adjusted. This method realizes the precisely controlled oscillation wavelength, but requires a precise adjustment of the rotational angle of the etalon to the divergent light beam of the laser. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a light-emitting module that enables to control both of the oscillation wavelength and the optical output power with high accuracy within a compact sized housing. 
     A light-emitting module according to the present invention comprises a second optical detector for monitoring a light from the semiconductor laser not through an etalon and a first optical detector for monitoring light from the laser through the etalon. The light through the etalon reflects optical properties both of the laser and the etalon, while the light not through the etalon merely shows the properties of the laser. The optical property of the etalon depends on a thickness and shows the transmittance with a periodicity. 
     Another aspect of the invention is that the etalon has a second portion on which an anti-reflection film is coated and a second portion. Light transmitted through the second portion does not show a periodic behavior based on the thickness of the etalon and merely reflects the characteristic of the laser. On the other hand, light through the first portion on which any anti-reflection film is provided has periodic characteristics reflecting the etalon and the laser. 
     The fluctuation of the oscillation wavelength of the laser appears as a phase shift of the periodic characteristic of light transmitted through the etalon. Therefore, by monitoring light through the etalon, the just present oscillation wavelength is detected and by monitoring light not through the etalon, the present power of the laser is obtained. 
     In the invention, it is preferable to split light from the laser before the etalon and to detect split light for monitor light not through the etalon. The light splitting device can locate either in the front side of the laser or the backside of the laser. 
     It is further preferable to place a lens between the laser and the etalon device for converting divergent light from the laser into a collimated light. Moreover, by using a wedge shape etalon, the oscillating wavelength of the laser can be selected merely sliding the etalon along a direction normal to the optical axis. 
     The present invention provides a thermoelectric cooler for adjusting temperature of the laser. The temperature is controlled by the signal from the detector that monitors light through the etalon, thus defines the oscillation wavelength. 
     The invention may also provide an adjusting circuit of the output optical power of the laser. 
     The signal from the detector that monitors light not through the etalon can maintains the output power of the laser. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a perspective view showing the module; 
     FIG. 2 is a cross sectional view showing the primary assembly of the module; 
     FIG.  3 ( a ) and FIG.  3 ( b ) show examples of the wedge etalon device; 
     FIG.  4 ( a ) is an exemplary diagram showing light signal not through the etalon device and FIG.  4 ( b ) is a schematic diagram of an optical signal source for the WDM transmission using present optical module; 
     FIG.  5 ( a ) is a perspective view showing an embodiment of the primary assembly, FIG.  5 ( b ) is a side view and FIG.  5 ( c ) is a plane view of the assembly showing in FIG.  5 ( a ); 
     FIG.  6 ( a ) is a view showing the second embodiment, FIG.  6 ( b ) is a plane view and FIG.  6 ( c ) is the side view of the embodiment of FIG. 6; 
     FIG.  7 ( a ) is a view showing the third embodiment of the invention, FIG.  7 ( b ) is a plan view and FIG.  7 ( c ) is a side view of the embodiment; 
     FIG.  8 ( a ) shows a perspective view of the fourth embodiment, FIG.  8 ( b ) is a plan view and FIG.  8 ( c ) is a side view of the embodiment; 
     FIG.  9 ( a ) is a perspective view of the fifth embodiment and FIG.  9 ( b ) is a place view of the fifth embodiment; and 
     FIG.  10 ( a ) shows a perspective view of the sixth embodiment, and FIG.  10 ( b ) is a plane view. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiments of the optical module will be described in referring to drawings. In the description, elements identical to each other will be referred to with numerals identical to each other without their overlapping explanations. 
     FIG. 1 is a perspective view showing the laser module of the present invention and FIG. 2 is a cross sectional viewing of the module. 
     The module comprises a primary assembly  10  and housing  12  containing the assembly  10  therein. The primary assembly  10  is placed on the base plate of the housing and sealed with an inert gas such as dry nitrogen in the housing. The housing  12  comprises a body  12   a , a cylinder  12   b  guiding an optical fiber into the primary assembly and a plurality of leads  12   c.    
     The primary assembly  10  contains a laser diode  16 , auxiliary members  24 ,  26 ,  28  and a lens holder  32 . Auxiliary members  24 ,  26 ,  28  mount the laser diode  16 , a photo diode  18 , and a lens  21 , respectively. The auxiliary member  24  is placed on a thermoelectric cooler  34 . The cooler  34  enables to control the temperature of the laser diode  16  by adjusting a supply current to the cooler. A Peltier element is a typical device for the cooler  34 . The auxiliary member is made of material having a good thermal conductivity, such as Aluminum Nitride (AlN). 
     An opening sealed by a hermetic glass for coupling the primary assembly to the cylinder  12   b  is provided on one wall of the housing  12 . Light emitted from the laser diode  16  is passing through the opening and entering one tip of an optical fiber  14 . Another lens holder  38  is held at the edge of the cylinder  12   b . An optical isolator  40  cutting the light propagating form the optical fiber  14  to the laser diode  16  is provided between the lens holder  38  and the cylinder  12   b.    
     The optical fiber  14  is inserted at the edge of the cylinder  12   b . A ferrule  42  covers the tips of the fiber  14 . The lens holder  38  holds a sleeve  44 . Inserting the ferrule  42  into the sleeve  44 , the optical position of the ferrule to the housing  12  is defined. Thus, the fiber  14 , the lens holder  38  and the primary assembly  10  are optically aligned with each other. 
     Referring to FIG. 2, the auxiliary member  24  comprises a device-mounting portion  24   a  and a lens-supporting portion  24   b . The lens-supporting portion  24   a  provides an opening to secure the lens holder  32 , in which a lens  32   a  to collimate light emitted from the laser diode  16  is inserted. 
     The laser diode  16  comprises a first facet  16   a , a second facet  16   b , and an active layer (a light-emitting layer) provided between the first and the second facet. The laser is placed on the auxiliary member  26 . A pair of facet  16   a  and  16   b  of the laser  16  forms an optical cavity. Since the reflectivity of the first facet  16   a  is lower than that of the second facet  16   b , it is enables to take out the light through the first facet  16   a . The first facet  16   a  couples to the optical fiber  14  through lenses  32   a  and  38   a . It is preferable to use the DFB laser (Distributed Feedback Laser) for the light-emitting device  16 . However, a Fabry-Perott type laser is also applicable. On the first facet of the laser provides an anti-reflection coating, while a high-reflection coating is preferred to be on the second facet. A SiN (Silicon Nitride) and amorphous Si are used as coating materials. 
     An etalon device  18  is placed on the auxiliary member  24 . One surface of the etalon optically couples to the facet  16   b  of the laser, while the other surface of the etalon couples to the photo diode  20 , which contains a first light detector  20   a  and a second light detector  20   b.    
     Next is an explanation of the etalon device as referring FIG.  3 . As shown in FIG. 3, etalons ( 18 ,  19 ) have a pair of surface making an angle α with each other. The angle α is preferable to be greater than 0.01° and smaller than 0.1°. Etalons shown in FIG. 3 are wedge type etalon. Only by sliding the wedge etalon along the direction X, to which the surface is inclined, the locking wavelength of the laser module is adjusted. The wedge etalon is usable compared to the parallel plate etalon in this point of view. 
     In FIG. 3, the etalon  18  comprises a light-entering surface  18   a  and a light-emitting surface  18   b . The angle α between two surfaces is set so as to make multiple interference between the incident light from the surface  18   a  and the reflected light from the surface  18   b . In another aspect, the etalon  18  comprises a first portion  18   x  and a second portion  18   y . The first portion has a reflective film  18   c  on the light-entering surface  18   a  and another reflective film  18   d  on the light-emitting surface. Also, the second portion  18   y  has anti-reflective films  18   e  and  18   f  on the light-entering surface  18   a  and the light-emitting surface  18   b , respectively. The films  18   e  and  18   f  on the second portion  18   y  suppress the reflection at both surfaces so that the periodicity on the transmission spectrum due to multiple reflection at the surface of the etalon disappears. Films ( 18   c  to  18   f ) on the surface are composed of multi layered materials. 
     FIG.  3 ( b ) shows another example of the etalon. This etalon has two films ( 19   c ,  19   d ) on respective surfaces ( 19   a ,  19   b ). Both films adjust the reflectivity at respective surfaces so that the periodic characteristic of the transmittance of the etalon may appear on the spectrum, which depends on the position X. 
     FIG.  4 ( a ) shows a typical diagram obtained by the photo detector  20   b . The horizontal axis denotes the wavelength of light emitted from the laser  16 , while the vertical axis corresponds to the signal monitored by the detector  20   b . FIG.  4 ( b ) is a schematic diagram of a light source using the optical module of the present invention. The light source comprises a laser module  1 , a first circuit block  48  for controlling the wavelength and a second circuit block  52  for controlling the optical power. The first block  48  couples to the detector  20   b  through the line  50   a  and also couples to the thermoelectric cooler  34  through the ling  50   b . The first block receives the signal from the detector  20   b  and output the driving signal for the cooler  34 . The temperature of the cooler is adjusted by the driving signal so as to compensate the wavelength shift of the emitting light, accordingly. 
     Namely, when the wavelength of the laser shifts to the shorter from λ LOCK , the monitor current of the detector  20   b  increase. Responding the monitor current, the circuit  48  drives the cooler so that the laser  16  emits light with longer wavelength. When the wavelength shifts to longer side from the λ LOCK , an reverse control may occur. 
     The second block  52  couples to the photo detector  20   a  through the ling  54   a  and the laser  16  through the line  54   b . Receiving the monitored signal from the detector  20   a , the block  52  drives the laser  16  so as to maintain the output power of the laser. 
     From FIG. 5 to FIG. 10 show various assemblies applicable to the present optical module. 
     (First Embodiment) 
     In FIGS.  5 ( a ) and  5 ( b ), the primary assembly  10   a  aligns the laser  16 , the lens  17 , the etalon  18 , and the photo diode  20  along the predetermined axis on the surface  24   c . This embodiment provides the etalon  18  of FIG.  3 ( a ), in which light transmitting through the second portion  18   y  does not show the periodic characteristics. The lens shapes a flat bottom surface  17   a , a flat top surface  17   c , and curved side surface  17   b . The top of the lens is cut to be flat so as not to enter light reflected by the etalon  18  into the laser  16 , which results on a small sized package. Further, the flat bottom surface of the lens enables to assemble it directly on the auxiliary member  24   a  without any lens holder. The lens  32  and the lens holder  32   a  are not shown on the lens-supporting portion in FIG.  5 ( a ) 
     Tow optical detectors  20   a  and  20   b  are arranged side by side on the photo diode  20 . The detector  20   a  receives light transmitted through the second portion  18   y  of the etalon, while the second detector  20   b  receives light from the first portion  18   x  of the etalon. The width of the first detector  20   a  along the inclined direction of the etalon is larger than that of the second detector  20   b . The width of the second detector  20   b  along a direction normal to the inclined direction is larger than the width along the inclined direction. By this configuration, the sensitivity for the wavelength variation and the magnitude of light are enhanced. 
     In FIG.  5 ( c ), a light beam A 1  enters the optical fiber  14  through two lenses  32   a  and  28   a . Another beam A 2 , emitted from another facet  16   b  of the laser  16 , enters the lens  17 . The lens  17  generates two beams A 3  and A 4  collimated with each other. The beam A 3  reaches the detector  20   a  through the second portion  18   y , in which the periodic characteristics does not appear. In this configuration, beams A 3  and A 4  reflect the spectrum of the laser  16 , and also the beam  6  reflects the optical properties of the etalon  18 . 
     (Second Embodiment) 
     FIG. 6 shows the second embodiment of the invention, in which the etalon of FIG.  3 ( b ) is applied. The primary assembly  10   b  has a photo diode  21  replaced from the photo diode  20  in the first embodiment. The assembly  10   b  aligns the laser  16 , the lens  17 , the etalon  19 , and the photo diode  21  on the surface  24   c  along the predetermined axis. In this configuration, the detector  21   a  on the photo diode opposes the lens  17 , while the detector  21   b  opposes the etalon  19 . 
     Detectors  21   a  and  21   b  have an up-and-down arrangement. The detector  21   a  receives light passing over the etalon, while the detector  21   b  receives light passing through the etalon. The shape of respective detectors is same as the first embodiment. The height of the etalon  19  and the position of the first detector  21   a  are decided so that the detector  21   a  directly receives light from the lens  17 . Further, the shape of lens  17  is also determined by the condition that the detector  21   a  directly receives light. 
     The light beam B 1  from the facet  16  of the laser  16  enters the fiber  15  through a pair of lens  32   b  and  38   a . Another beam B 2  emitted from the facet  16   b  enters the lens  17 . The lens  17  generates two collimated beams B 3  and B 4 . The beam B 4  directly enters the detector  21   a  without passing the etalon. The portion of the beam B 3  enters the etalon and makes the beam B 5  that reaches the detector  21   b . The B 4  involves the wavelength characteristic only of the laser  16 , while the beam B 5  reflects the characteristics both of the laser and the etalon. 
     (Third Embodiment) 
     FIG. 7 shows the third embodiment of the invention. In this embodiment, the photo diode has detectors  22   a  and  22   b  instead of detectors appeared in previous embodiment. The primary assembly  10   c  aligns the laser  16 , the lens  17 , the etalon  19 , and detectors ( 22   a ,  22   b ) on the surface  24   c  along the predetermined axis. In this configuration, the detector  22   a  faces the lens  17 , while the detector  22   b  opposes the etalon  19 . Two detectors  22   a  and  22   b  are independently to each other. The shape of the light sensitive region of respective detectors ( 22   a ,  22   b ) is same as the shape previously explained. 
     The etalon  19  has a flat top surface  19   g  to place the detector  22   a  thereon. This configuration, in which a distance from the laser to the detector  22   a  is shortened compared to the case in FIG. 6, enhances the magnitude of the received light. The size and its curvature of the lens  17  are determined by the condition that the detector  22   a  receives collimated light from the lens. 
     In this embodiment, a beam C 1  emitted from the facet  16   a  enters the optical fiber  14  through two lenses  32   a  and  28   a  Another beam C 2  emitted from the facet  16   b  enters the lens  17  and is converted to collimated beams C 3  and C 4 . The beam C 3  directly enters the detector  22   a ; therefore, the beam C 3  only reflects the characteristic of the laser  16 . On the other hand, since another beam C 4  enters the detector  22   b  through the etalon  19 , the output from the detector  22   a  involves the contribution of the laser  16  and the etalon  19 . 
     (Fourth Embodiment) 
     FIG. 8 shows the fourth embodiment of the invention. In this embodiment, the primary assembly contains a member  29  for mounting the etalon  19  thereon and for attaching the detector  22   a  thereto. To adjust the wavelength, to which the laser oscillation is fixed, is realized by sliding the etalon on the surface  29   a  of the member. Other compositions of the assembly are same with the case of the third embodiment. 
     (Fifth Embodiment) 
     FIG. 9 shows the fifth embodiment of the invention. This embodiment contains the laser  16 , the lens  17 , the beam splitter  23   a , the etalon, and two detectors ( 22   a ,  22   b ) on the auxiliary member  24 . The beam splitter  23   a  optically couples to the lens and the detector  22   a  attached to another member  31 . Light from the splitter  23   a  reaches the detector  22   b  through the etalon  19 . 
     In this arrangement, a beam D 1  emitted from the facet  16  enters the optical fiber  14  through two lenses  32   a  and  38   a . Another beam D 2  emitted from the facet  18   b  enters the lens  17  and is converted to collimated beam D 3  by the lens. The splitter  23   a  divides the collimated beam D 3  into two beams D 4  and D 5 . The beam reaches the detector  22   a  without passing through the etalon, so the beam reflects the spectrum only of the laser  16 . On the other hand, one of the divided beams D 5  reaches the detector  22   b  through the etalon, so the output from the detector  22   b  contains the spectrum both of the laser  16  and the etalon  19 . 
     (Sixth Embodiment) 
     Embodiments previously described utilize light emitted from the facet  16   b  of the laser to control the wavelength and the output power of the laser. Another example will be explained in which light from the front facet  16   b  of the laser is referred for the control. 
     In FIG. 10, the primary assembly of the module contains the detector  22   a  on the front side of the laser  16  and the detector  22   b  on the backside of the laser. The thermoelectric cooler  34  place an auxiliary member  24  and another member  25  thereon. The laser  15 , the lens  17 , the etalon, and the detector  22   b  are mounted on the auxiliary member  24 . The splitter  23   b  and the detector  22   a  are mounted on the member  25 . The splitter  23   b  optically couples to the laser  16  through the lens  32   a , the fiber  14 , and the detector  22   a . The back facet  16   b  of the laser optically couples to the detector  22   a  through the etalon  19 . 
     A light beam E 1  emitted from the facet  16   a  enters the splitter through the lens  32   a . The splitter  23   b  divides the beam E 1  into two beams E 4  and E 5 . The beam E 5  enters the detector  22   a , in which only the spectrum of the laser is contained. Another beam E 4  enters the fiber  14  through the lens  38   a . On the other hand, the beam E 2  emitted from the back facet  16   b  enters the lens  17  and is converted into the collimated beam E 7 . The beam E 7  reaches the detector  22   b  through the etalon; therefore, the output from the detector  22   b  contains the spectrum both of the laser and the etalon. 
     Since various embodiments previously mentioned use a wedge type etalon not a parallel-plate type etalon for the wavelength discriminate device, it enables to reduce a region to place the etalon. In the parallel-plate etalon, a free spectral rang, which means a period appeared in the transmittance spectrum, is determined by an angle of incident light. Since the free spectral range closely relates to the wavelength interval, it is inevitable to rotate the etalon for adjusting the free spectral range and to obtain a desired wavelength interval. Therefore, it requires for the etalon to rotate in the case of the parallel plate type. On the other hand, only sliding adjusts the wavelength in the wedge type etalon. 
     From the invention thus described, it will be obvious that the invention may be varied in many ways. Although various types of auxiliary member are described, other combination of members are considered to be within the scope of the present invention. The present invention is not restricted to the L-shaped member. Further, the light-receiving device may integrally contain two detectors or may be discrete device independently to each other. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.

Technology Classification (CPC): 7