Abstract:
Disclosed is a wavelength locker and a method for locking wavelengths for an optical transmitter which includes a semiconductor light source for projecting light for optical communication from the front face thereof, the oscillating wavelength of the optical transmitter changing according to the working temperature, and a heater for maintaining the working temperature of the semiconductor light source at a predetermined value. The wavelength locker includes: a detector for detecting the light projected from the rear face of the semiconductor light source; a Fabry-Perot filter which is interposed between the semiconductor light source and the detector and comprises a spacer formed from a non-linear optical material, the transmittance of the Fabry-Perot filter shifting with respect to the wavelength depending on the change of the transmittance of the spacer; and, a control circuit for changing the refractive index of the Fabry-Perot filter so that the Fabry-Perot filter exhibits a predetermined transmittance in relation to the oscillating wavelength of the semiconductor light source.

Description:
CLAIM OF PRIORITY 
   This application claims priority to an application entitled “WAVELENGTH LOCKER FOR OPTICAL TRANSMITTER,” filed in the Korean Industrial Property Office on Mar. 11, 2002 and assigned Serial No. 2002-12836, the contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to an optical transmitter. In particular, the present invention relates to a wavelength locker is provided in the optical transmitter for locking the wavelength of an optical signal. 
   2. Description of the Related Art 
   The wavelength-division-multiplexing (WDM) technology has been a great area of research as it can allow for the transmission of increasing information without largely changing the backbone network of an optical communication system. Consequently, the possible employment of both the 100-GHz spacing wavelength-division-multiplexing technology which is currently used in the existing system, as well as the 50-GHz dense wavelength-division-multiplexing (DWDM) technology has been of great interest. In order to implement such a dense wavelength-division-multiplexing technology, it is necessary for an oscillating wavelength to be maintained within an error range not to exceed 20 pm. Because it is difficult to implement the dense wavelength-division-multiplexing technology merely by securing the stability of an oscillator itself, optical transmitters are widely used and are individually provided with a wavelength locker disposed outside the oscillator. Such wavelength lockers are classified as either an exterior type which is provided outside of optical transmitters, or an interior type which is integrated inside of optical transmitters. In view of easy system construction, price and other factors, it is expected that the demand for the interior type wavelength lockers will increase. 
     FIG. 1  schematically illustrates an optical transmitter provided with a wavelength locker of the prior art. The optical transmitter comprises a laser diode (LED)  110 , a Fabry-Perot filter  120 , a photodiode (PD)  130 , an analog/digital converter (ADC)  140 , a controller  150 , a digital/analog converter  160 , a bias circuit  170 , and a thermoelectric cooler (TEC)  180 . 
   The laser diode  110  is a semiconductor component, which projects light with a predetermined wavelength through its front and rear faces. The light projected through the rear face is used for monitoring the oscillating wavelength of the laser diode  110 . That is, the stabilized optical transmission can be executed by monitoring the oscillating wavelength of the laser diode  110  and correcting a wavelength error when the error occurs. The wavelength of the laser diode  110  changes as the operating temperature of the laser diode  110  changes. 
   The Fabry-Perot filter  120  may consist of a spacer formed from a linear optical material and two reflecting layers each being formed on one of the front and rear faces of the spacer, wherein the Fabry-Perot filter  120  has a predetermined transmittance spectrum depending on the thickness and refractive index of the spacer. That is, if the light projected through the rear face of the laser diode  110  is incident to the Fabry-Perot filter  120 , the power of the transmitted light will vary depending on the wavelength of the light. Therefore, it is possible to find the wavelength of the light by measuring the power of the light projected from the Fabry-Perot filter  120 . 
   The photodiode  130  converts the light that is incident to the Fabry-Perot filter  120  into an electrical signal and outputs the electrical signal. 
   The analog/digital converter  140  converts the electrical signal into a digital signal and outputs the digital signal to the controller  150 . 
   The controller  150  perceives the power of the light from the electrical signal and finds the wavelength of the light based upon the power of the light. If the wavelength determined from this operation does not match a predetermined wavelength value, then the controller  150  outputs a control signal for correcting the wavelength error. 
   The digital/analog converter  160  converts the control signal into an analog signal and outputs the analog signal to the bias circuit  170 . 
   The bias circuit  170  applies electric current to the thermoelectric cooler  180  in response to the control signal output by the controller  150 . That is, the level of the applied electric current is determined by the control signal. 
   The thermoelectric cooler  180  is operated by the electric current applied by the bias circuit  170  and functions to maintain the temperature of the laser diode  110  to be constant at a predetermined temperature. 
     FIG. 2  is a flowchart showing the setting procedure of the optical transmitter shown in FIG  1 .  FIG. 3  is a drawing illustrating the setting procedure shown in FIG  2 . The setting procedure consists of a channel-setting step  210 , a working temperature-setting step  220 , and a Fabry-Perot filter-aligning step  230 . 
   The channel-setting step  210  is a step for setting a specific channel from a group of usable International Telecommunication Union (ITU) channels. Referring to  FIGS. 3   a  and  3   b , one channel  310  is selected from a series of ITU channels and the oscillating wavelength of the laser diode  110  is biased to an edge of the set channel  310  prior to the working temperature-setting step  220 . 
   The working temperature-setting step  220  adjusts the working temperature of the laser diode  110  in order to match the oscillating wavelength to the center of the selected channel  310 . According to this step, it can be seen that the oscillating wavelength moves from the edge to the center of the set channel  310  ( 320 → 350 ). 
   The Fabry-Perot filter-aligning step  230  is a step for aligning the oscillating wavelength of the laser diode  110  and the transmittance of the Fabry-Perot filter  120 . Referring to  FIG. 3   c , the oscillating wavelength of the laser diode  110  is laid in the flat region of the transmittance spectrum prior to the Fabry-Perot filter-aligning step  230 . For this reason, a problem arises because it is impossible to find such a wavelength change, and because the power of the light projected from the Fabry-Perot filter  120  changes slightly even if the oscillating wavelength of the laser diode  110  fluctuates within the flat region. Therefore, the Fabry-Perot filter  120  is aligned ( 340 → 350 ) in such a manner that the oscillating wavelength can be fixed in a wavelength region where the change of the transmittance of the Fabry-Perot filter  120  is large—i.e. close to a wavelength that exhibits a mean transmittance of the maximum and minimum transmittances. At this time, the aligning step  230  is conducted by aligning the incident angle θ of the light that is incident in the Fabry-Perot filter  120  and the optical axis of the Fabry-Perot filter  120 . That is, the aligning step  230  is conducted by aligning the optical axis of the laser diode  110  and the optical axis of the Fabry-Perot filter  120 . It can be seen that the oscillating wavelength of the laser diode  110  is fixed close to the wavelength which exhibits the mean transmittance of the maximum and minimum transmittances of the Fabry-Perot filter  120  while passing through the aligning step  230 . 
     FIG. 4  shows a transmittance spectrum for the Fabry-Perot filter  120  shown in  FIG. 1  in relation to the incident angle θ. The transmittance spectrum is obtained when the thickness of the spacer is 20 μm and the coefficient of finess is 10. From the drawing, it can be seen that the transmittance spectrum rapidly moves to the shorter wavelength side ( 410 → 420 → 430 ) as the incident angle θ increases by increments of 1° from 0°. That is, it can be seen that the wavelength moves 0.3 nm every time the incident angle θ changes 1°, and thus the aligning error of the Fabry-Perot filter  120  should be limited to approximately 0.1° considering that a 50-GHz ITU channel grid is 0.4 nm. However, there are problems in that it is extremely difficult to conduct such an alignment manually, and the separate use of an expensive active alignment installation will excessively increase production costs. 
   As described above, wavelength lockers for optical transmitters of the prior art experience problems with the alignment of the Fabry-Perot filter. Either manual alignment is required which is an extremely difficult procedure considering the precision required, or an active alignment installation must be used which is a very expensive procedure increasing the production cost of the optical transmitter. 
   Accordingly, there is a need to provide a wavelength locker for an optical transmitter that does not require the fine geometrical alignment of the Fabry-Perot filter. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the invention, there is provided a wavelength locker for an optical transmitter which includes a semiconductor light source for projecting light for optical communication from the front face thereof, an oscillating wavelength of the optical transmitter changing with the working temperature of the semiconductor light source, and a heater for maintaining the working temperature of the semiconductor light source at a predetermined value, wherein the wavelength locker comprises: a detector for detecting the light projected from the rear face of the semiconductor light source; a Fabry-Perot filter which is interposed between the semiconductor light source and the detector including a spacer formed from a non-linear optical material, wherein the transmittance of the Fabry-Perot filter changes in relation to the wavelength depending on the change of the transmittance of the spacer; and, a control circuit for changing the refractive index of the Fabry-Perot filter so that the Fabry-Perot filter exhibits the preset transmittance in relation to the oscillating wavelength of the semiconductor light source. 
   According to another aspect of the invention, there is also provided a method of locking the wavelength of an optical transmitter, comprising the steps of: detecting a light projected from the rear face of a semiconductor light source; providing a Fabry-Perot filter with means for shifting transmittance of the filter with respect to its wavelength in a predetermined manner; and, providing a control circuit for changing the refractive index of the Fabry-Perot filter to exhibit the predetermined transmittance in relation to the oscillating wavelength of the semiconductor light source. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
       FIG. 1  schematically shows the construction of an optical transmitter provided with a wavelength locker of the prior art; 
       FIG. 2  is a flowchart showing the setting procedure of the optical transmitter shown in  FIG. 1 ; 
       FIG. 3  a drawing for illustrating the setting procedure shown in  FIG. 2 ; 
       FIG. 4  shows a transmittance spectrum for the Fabry-Perot filter shown in  FIG. 1  in relation to the incident angle; 
       FIG. 5  schematically shows the construction of a wavelength locker for an optical transmitter in accordance with the first embodiment of the present invention; 
       FIG. 6  is a cross-sectional view showing the construction of the Fabry-Perot filter shown in  FIG. 5 ; 
       FIG. 7  shows a transmittance spectrum for the Fabry-Perot filter in accordance with the present invention in relation to the incident angle; 
       FIG. 8  is a flowchart which shows the procedure in which the controller shown in  FIG. 5  works in the second control mode; 
       FIG. 9  schematically shows the construction of a wavelength locker for an optical transmitter in accordance with the second embodiment of the present invention; and, 
       FIG. 10  is a cross-sectional view showing the construction of the Fabry-Perot filter shown in FIG.  9 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. It shall be noted that in the drawings, identical components are indicated by identical referential numerals and symbols. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted as they would obscure the invention in unnecessary detail. 
     FIG. 5  schematically shows the construction of a wavelength locker for an optical transmitter in accordance with the first embodiment of the present invention.  FIG. 6  is a cross-sectional view showing the construction of he Fabry-Perot filter shown in FIG.  5 . The optical transmitter comprises a heater  510 , a substrate  520 , a submount  530 , a semiconductor light source  540 , a support  550 ,  555 , a Fabry-Perot filter  570 , a detector  580 , and a control circuit  600 . 
   The heater  510  is operated by electric current applied thereto and functions to maintain the temperature of the semiconductor light source  540  to be constant at a predetermined value. A thermoelectric cooler may be used for the heater  510 . 
   The substrate  520  is secured to the top surface of the heater  510  with a bonding material formed from InAg, having a melting point of approximately 130° C. The substrate may be made from Kovar so that laser welding can be easily performed. 
   The submount  530  is secured to the top surface of the substrate  520  with a bonding material made from AgSn, having a melting point of approximately 210° C. 
   The semiconductor light source  540  is secured to the top surface of the submount  530  with a bonding material formed from AuSn, having a melting point of approximately 280° C. The semiconductor light source  540  projects light having a predetermined wavelength through the front and rear faces thereof and its oscillating wavelength changes with its working temperature. The light projected through the front face is used for optical communication and the light projected through the rear face is used for monitoring the oscillating wavelength of the semiconductor light source  540 . It is possible to use a laser diode, a light emitting diode (LED) or the like for the semiconductor light source  540 . 
   The support comprises a ferrule  555  and a submodule  550  and is secured to the top surface of the substrate  520 . 
   The submodule  550  is formed from the identical material to that of the substrate  520  and aligned facing the front face of the semiconductor light source  540 . 
   The ferrule  555  has a diameter of approximately 2.7 mm and is fixed through the submodule  550  using laser welding. The ferrule  555  is provided with a cylindrical hole (not shown), through which an optical fiber  560  is inserted, wherein the end of the optical fiber  560  faces the front face of the semiconductor light source  540 . 
   The Fairy-Perot filter  570  is interposed between the semiconductor light source  540  and the detector  580  and secured to the top surface of the substrate  520  with the bonding material made of AuSn, having a melting point of 280° C. The Fabry-Perot filter  570  includes a spacer  572  formed from a non-linear optical material, two reflecting layers  573  and  574  each being deposited on one of the front and rear surfaces of the spacer  572 , and two electrodes  577  and  578  each being layered on the opposite sides of the spacer  572 . Due to the characteristics of the non-linear optical material, the refractive index of the Fabry-Perot filter  570  changes depending on the electric field applied thereto or the change of its internal temperature. Therefore, if an electric field of a predetermined magnitude is applied to the Fabry-Perot filter  570  through the two electrodes  577  and  578 , the refractive index of the spacer  572  changes predictably and thus the transmittance spectrum of the Fabry-Perot filter  570  will shift in relation to the wavelength. 
     FIG. 7  shows a transmittance spectrum for the Fabry-Perot filter  570  in accordance with the present invention with respect to the refractive index of the spacer  572 . The spectrum is obtained when the thickness of the spacer  572  is 50 μm and the coefficient of finess is  20 . From the chart, it is apparent that, as the refractive index of the spacer  572  increases by an increment of 0.0007 from 1.4486, the corresponding transmittance spectrum shifts to the longer wavelength side ( 710 → 720 → 730 → 740 → 750 ). 
   The detector  580  converts the light that is incident through the Fabry-Perot filter  570  into an electrical signal and outputs the electrical signal. For the detector  580 , a photodiode chip, a charge coupled device (CCD) or the like may be used. The detector  580  is secured to the front surface of the substrate  520  with the bonding material made of AuSn having a melting point of approximately 280° C. 
   The control circuit  600  has two control modes. The first control mode is associated with optical communication. The second control mode is associated with setting the working condition of the Fabry-Perot filter  570 . At the first control mode, the control circuit  600  corrects the wavelength error when the wavelength does not match with the predetermined wavelength value. At the second control mode, the control circuit  600  changes the refractive index of the Fabry-Perot filter  570 , so that a wavelength at which the transmittance of the Fabry-Perot filter  570  has a mean value of the maximum transmittance and the minimum transmittance is close to or preferably conforms to the oscillating wavelength of the semiconductor light source  540 . The control circuit  600  comprises an analog/digital converter  610 , a controller  620 , first and second digital/analog converters  630  and  650 , and first and second bias circuit  640  and  660 . 
   The analog/digital converter  610  converts the electrical signal output by the detector  580  into a digital signal and outputs the digital signal to the controller  620 . 
   The first digital/analog converter  630  converts the first control signal output by the controller  620  into an analog signal and outputs the analog signal to the first bias circuit  640 . 
   The first bias circuit  640  applies an electric current to the heater  510  in response to the first control signal. That is, the level of the applied electric current is determined by the first control signal. 
   The second digital/analog converter  650  converts the second control signal output by the controller  620  into an analog signal and outputs the analog signal to the second bias circuit  660 . 
   The second bias circuit  660  applies an electric voltage of a predetermined magnitude to the two electrodes  577  and  578  of the Fabry-Perot filter  570  in response to the second control signal. That is, the level of the applied electric voltage is determined by the second control signal. 
   The controller  620  perceives the power of the light from the electric signal when working at the first control mode and then determines the wavelength of the light from the power of the light. The controller  620  outputs the first control signal for correcting the wavelength error when the wavelength of the light does not match the predetermined wavelength value. 
     FIG. 8  is a flowchart showing the setting procedure of the controller  620  at the second control mode. The setting procedure consists of a channel-setting step  810 , a working temperature-setting step  820 , and a Fabry-Perot filter-aligning step  830 . 
   The channel-setting step  810  is a step setting a certain channel from the usable ITU channels, wherein one channel is selected from a series of ITU channels. 
   The working temperature-setting step  820  is a step for adjusting the temperature of the semiconductor light source  540  in order to match the oscillating wavelength of the semiconductor light source  540  with the center of the selected channel. According to this step, the oscillating wavelength of the semiconductor light source  540  shifts towards the center of the selected channel. 
   The Fabry-Perot filter-aligning step  830  is a step for aligning the wavelength at which the transmittance of the Fabry-Perot filter  570  has a mean value of the maximum transmittance and the minimum transmittance with the oscillating wavelength of the semiconductor light source  540 . In this step, the refractive index of the Fabry-Perot filter  580  is changed so that the wavelength at which the transmittance of the Fabry-Perot filter  580  has the mean value of the maximum transmittance and minimum rate is close to or preferably conforms to the oscillating wavelength of the semiconductor light source  540 . At this time, the aligning step is performed in such a manner that the magnitude of the electric field applied to the Fabry-Perot filter  570  is increased. That is, the controller  620  finds the aligned state from the power of the electrical signal inputted from the analog/digital converter  610  and raises or lowers the magnitude of the electric field applied to the Fabry-Perot filter  570  until the transmittance of the Fabry-Perot filter  570  is close to or preferably conforms to the mean value of the maximum transmittance and minimum transmittance. For this purpose, the controller  620  outputs the second control signal and the second digital/analog converter  650  converts the second control signal into a digital signal and outputs the digital signal. 
   Thereafter, the second bias circuit  660  applies an electrical voltage of a predetermined magnitude to the two electrodes  577  and  578  of the Fabry-Perot filter  570  in response to the second control signal and the applied electrical voltage produces a corresponding electric field within the Fabry-Perot filter  570 . 
     FIG. 9  schematically shows the construction of a wavelength locker for an optical transmitter in accordance with the second embodiment of the present invention.  FIG. 10  is a cross-sectional view showing the construction of the Fabry-Perot filter shown in FIG.  9 . 
   The optical transmitter comprises a first heater  910 , a substrate  920 , a submount  930 , a semiconductor light source  940 , a support  950 ,  955 , a Fabry-Perot filter  980 , a detector  990 , and a control circuit  1000 , which includes a second heater  970 . Below, only the Fabry-Perot filter  980  and the control circuit  1000  are described in order to avoid overlapping. 
   The second heater  970  is secured to the top surface of the substrate  920  with the bonding material made of AgSn and is operated by an electric current applied thereto. The second heater  970  functions to maintain the temperature of the Fabry-Perot filter  980  to be a constant predetermined value. An electric heat wire may be used for the second heater  970 . 
   The Fabry-Perot filter  980  is interposed between the semiconductor light source  940  and the detector  990  and secured to the top surface of the second heater with the bonding material made of AuSn having a melting point of 280° C. The Fabry-Perot filter  980  includes a spacer  982  formed from a non-linear optical material, and two reflecting layers  984  and  986  each being deposited on one of the front and rear surfaces of the spacer  982 . The Fabry-Perot filter  980  has a predetermined transmittance spectrum depending on the thickness and refractive index of the spacer  982 . That is, if the light projected through the rear face of the semiconductor light source  940  is incident through the Fabry-Perot filter  980 , the power of the transmitted light depends on the wavelength of the light. Therefore, if the power of the light transmitted through the Fabry-Perot filter  980  is measured, it is possible to determine the wavelength of the light. Due to the characteristics of the non-linear optical material, the refractive index of the Fabry-Perot filter  980  changes depending on the electrical field applied thereto, or depending on the change of its internal temperature. Therefore, if the internal temperature of the Fabry-Perot filter  980  is changed, the refractive index of the spacer  982  changes accordingly and thus the transmittance spectrum of the Fabry-Perot filter  980  will shift in relation to the wavelength similar to that shown in FIG.  7 . 
   The control circuit  1000  has two control modes. The first control mode is associated with the optical communication. The second control mode is associated with setting the working condition of the Fabry-Perot filter  980 . At the first control mode, the control circuit  1000  corrects the wavelength error when the wavelength does not match with the predetermined wavelength value. At the second control mode, the control circuit  1000  changes the refractive index of the Fabry-Perot filter  980 , so that a wavelength at which the transmittance of the Fabry-Perot filter  980  has a mean value of the maximum transmittance and the minimum transmittance is close to or preferably conforms to the oscillating wavelength of the semiconductor light source  940 . The control circuit  1000  comprises an analog/digital converter  1010 , a controller  1020 , first and second digital/analog converters  1030  and  1050 , first and second bias circuit  1040  and  1060 , and the second heater  970 . 
   The analog/digital converter  1010  converts the electrical signal output by the detector  990  into a digital signal and outputs the digital signal to the controller  1020 . 
   The first digital/analog converter  1030  converts the first control signal generated by the controller into an analog signal and outputs the analog signal to the first bias circuit  1040 . 
   The first bias circuit  1040  applies electric current to the first heater in response to the first control signal. That is, the level of the applied electric current is determined by the first control signal. 
   The second digital/analog converter  1050  converts the second control signal generated by the controller  1020  into an analog signal and outputs the analog signal to the second bias circuit  1060 . 
   The second bias circuit  1060  applies electric current to the second heater  970  in response to the second control signal. That is, the level of the applied electric current is determined by the second control signal. 
   The controller  1020  perceives the power of the light from the electrical signal when working at the first control mode and then determines the wavelength of the light from the power of the light. The controller  1020  outputs the first control signal for correcting the wavelength error when the wavelength of the light does not match the predetermined wavelength value. 
   The setting procedure of the controller  1020  at the second control mode consists of a channel-setting step, a working temperature-setting step, and a Fabry-Perot filter-aligning step. 
   The channel-setting step is a step setting a certain channel from the usable ITU channels, wherein one channel is selected from a series of ITU channels. 
   The working temperature-setting step is a step for adjusting the temperature of the semiconductor light source  940  in order to match the oscillating wavelength of the semiconductor light source  940  with the center of the selected channel. According to this step, the oscillating wavelength of the semiconductor light source  940  shifts towards the center of the selected channel. 
   The Fabry-Perot filter-aligning step is a step for aligning the wavelength at which the transmittance of the Fabry-Perot filter  980  has a mean value of the maximum transmittance and the minimum transmittance with the oscillating wavelength of the semiconductor light source  940 . In this step, the refractive index of the Fabry-Perot filter  980  is changed so that the wavelength at which the transmittance of the Fabry-Perot filter  980  has the mean value of the maximum transmittance and minimum transmittance is close to or, preferably conforms to the oscillating wavelength of the semiconductor light source  940 . At this time, the aligning step is performed in such a manner that the internal temperature of the Fabry-Perot filter  980  is increased. That is, the controller  1020  finds the aligned state from the power of the electrical signal output by the analog/digital converter  1010  and raises the internal temperature of the Fabry-Perot filter  980  until the transmittance of the Fabry-Perot filter  980  is close to or preferably conforms to the mean value of the maximum transmittance and minimum transmittance. For this purpose, the controller  1020  outputs the second control signal, and the second digital/analog converter  1050  converts the second control signal into a digital signal and outputs the digital signal. Thereafter, the second bias circuit  1060  applies electric current to the second heater  970  in response to the second control signal and the second heater  970  is operated by the applied electric current and thus changes the internal temperature of the Fabry-Perot filter  980  to the selected value. 
   As described in the above, the wavelength locker for an optical transmitter in accordance with the present invention has the advantage of not requiring the fine geometrical alignment of a Fabry-Perot filter such as angular alignment, because the wavelength locker aligns the oscillating wavelength of a semiconductor light source and a wavelength which has a mean value of the maximum transmittance and the minimum transmittance on the transmittance spectrum by adjusting the refractive index of a spacer provided in the Fabry-Perot filter. The spacer being formed from a non-linear optical material and the transmittance of the Fabry-Perot filter shifts in relation to the wavelength according to the change of the transmittance of the spacer. 
   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.