Patent Publication Number: US-2023134115-A1

Title: Wavelength stabilizer and optical module including same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Korean Patent Application No. 10-2021-0145879 filed on Oct. 28, 2021 in the Korean Intellectual Property Office, which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to an optical module, and more particularly, to a wavelength stabilizer for stabilizing the wavelength of laser light outputted from a laser diode (LD), and an optical module including the same. 
     Description of Related Technology 
     An optical module is a device that performs transmission and reception functions using laser light. If the channel spacing is sufficiently secured in the optical module, there may be no problem in optical communication even if a shift of the output wavelength of some laser light occurs due to a change in ambient temperature or deterioration of the laser. 
     SUMMARY 
     Accordingly, the present disclosure is intended to provide a wavelength stabilizer that performs wavelength stabilization using thermal characteristics of a laser diode without using additional components such as an etalon filter, and also provide an optical module including the same. 
     In addition, the present disclosure is intended to provide a wavelength stabilizer capable of solving a wavelength shift problem due to deterioration of a laser diode or a change in ambient temperature, and also provide an optical module including the same. 
     In addition, the present disclosure is intended to provide a wavelength stabilizer capable of simplifying the structure of an optical module, and also provide the optical module including the wavelength stabilizer. 
     According to the present disclosure, a wavelength stabilizer for an optical module that stabilizes a wavelength of laser light outputted from a laser diode may include a controller constantly maintaining a junction temperature of the laser diode. 
     The wavelength stabilizer may further include a thermoelectric cooler allowing the laser diode to be mounted thereon, and adjusting the temperature of the laser diode under control of the controller. 
     The controller may constantly maintain the junction temperature of the laser diode through the thermoelectric cooler. 
     The laser diode may include a substrate being in contact with the thermoelectric cooler and exchanging heat with the thermoelectric cooler, and a laser chip mounted on the substrate and outputting laser light. 
     The wavelength stabilizer may further include a current measurer measuring a current applied to the laser chip, a voltage measurer measuring a voltage applied to the laser chip, and a temperature measurer measuring a temperature of the substrate. 
     The controller may constantly maintain the junction temperature of the laser diode by adjusting the temperature of the substrate through the thermoelectric cooler. 
     Based on (3) of Equation 1 below, the controller may calculate the temperature (T s2 ) of the substrate such that a temperature change amount (T s1 −T s2 ) of the substrate written on a left side is equal to a value written on a right side. 
       [Equation 1] 
         T   j1   =T   s1   +R   th ( I   1   V   1   −P   1 )  (1)
 
         T   j2   =T   s2   +R   th ( I   2   V   2   −P   2 )  (2)
 
     Under the condition T j1   =T   j2 , 
         T   s1   −T   s2   =R   th ( I   2   V   2   −I   1   V   1 )+ R   th ( P   1   −P   2 )  (3)
 
     (T j1 : Junction temperature at t1 
     T j2 : Junction temperature at t2 
     T s1 : Substrate temperature at t1 
     T s2 : Substrate temperature at t2 
     R th : Thermal resistance [° C./W] 
     P 1 : Light output at t1 
     P 2 : Light output at t1) 
     The wavelength stabilizer may further include a light output measurer measuring a relative value (P 2 /P 1 ) of a light output (P 1 ) at t1 and a light output (P 2 ) at t2. 
     The controller may calculate the temperature (T s2 ) of the substrate by using Equation 2 below. 
       [Equation 2] 
         T   s1   −T   s2   =R   th ( I   2   V   2   −V   1 )+ P   1 R th (1 − P   2   /P   1 )  (4)
 
     When P 1 =P 2  and V 1 ≈V 2  in an automatic power control (APC) mode, and when the current measurer measures a current relative value (I 2 /I 1 ), the controller may calculate the temperature (T s2 ) of the substrate by using (7) of Equation 3 below. 
       [Equation 3] 
         T   s1   −T   s2   =R   th ( I   2   V   2   −I   1   V   1 )+ R   th ( P   1   −P   2 )  (3)
 
         T   s1   −T   s2   =R   th ( I   2   V   2   −I   1   V   1 ) (when  P   1   =P   2 )  (5)
 
         T   s1   −T   s2   =I   1   V   1   R   th ( I   2   V   2   /I   1   V   1 −1)  (6)
 
         T   s1   −T   s2   =I   1   V   1   R   th ( I   2   /I   1 −1) ( V   1   ≈V   2 )  (7)
 
     In addition, according to the present disclosure, an optical module may include a wavelength stabilizer stabilizing a wavelength of laser light outputted from a laser diode. The wavelength stabilizer may include a controller constantly maintaining a junction temperature of the laser diode. 
     According to the present disclosure, the wavelength stabilizer for the optical module performs wavelength stabilization using thermal characteristics of the laser diode without using additional components such as an etalon filter. That is, the wavelength stabilizer can constantly maintain the junction temperature of the laser diode, thereby stabilizing the wavelength of the outputted laser even if the laser diode deteriorates or the ambient temperature changes. As such, the junction temperature of the laser diode is constantly maintained by adjusting the temperature of the laser diode through the thermoelectric cooler on which the laser diode is mounted. 
     According to the present disclosure, because the wavelength stabilizer for the optical module can perform the wavelength stabilization by maintaining the junction temperature of the laser diode through the thermoelectric cooler, there is an advantage that the structure of the optical module is simplified and the manufacturing cost is lowered, compared to applying the etalon filter. 
     In addition, according to the present disclosure, the wavelength stabilizer can be used in the optical module for WDM or DWDM. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating an optical module including a wavelength stabilizer according to an embodiment of the present disclosure. 
         FIG.  2    is a LIV graph for explaining a wavelength stabilization method using the wavelength stabilizer of  FIG.  1    when there is no deterioration in a laser diode. 
         FIG.  3    is a LIV graph for explaining a wavelength stabilization method using the wavelength stabilizer of  FIG.  1    when there is deterioration in a laser diode. 
     
    
    
     DETAILED DESCRIPTION 
     However, the shift of the output wavelength of laser light in a channel spacing of 100 GHz or less as in an optical module for wavelength division multiplexing (WDM) causes a serious problem, so a wavelength stabilization technology is required. The wavelength stabilization is to prevent the shift of the output wavelength of laser light used for optical communication and thus maintain a constant output wavelength. 
     As a wavelength stabilization method, a method using the Fabry-Perot filter is used. This method is to monitor the current wavelength of laser light with the Fabry-Perot filter by using a part of the output of the laser and adjust the temperature of the laser through feedback from the circuit to the laser, thereby maintaining the laser wavelength desired by the user. As the Fabry-Perot filter, the etalon filter is used. The optical module to which the etalon filter is applied is also available for dense WDM (DWDM) with more than tens of channels. 
     The optical module to which the etalon filter is applied monitors the output light of the laser that has passed through the etalon filter, and adjusts the temperature of the laser with a thermoelectric cooler so that the laser can output the light of a desired wavelength through a wavelength stabilization algorithm based on the monitored output light. 
     As such, typically, the etalon filter and the module for performing a wavelength stabilization algorithm are essential for wavelength stabilization. 
     In addition, fine-tuning the angle of the etalon filter through the laser output light monitoring and the wavelength stabilization algorithm is a very difficult technique. 
     Also, in case of applying the etalon filter to the wavelength stabilization, the structure of the optical module becomes complicated and the manufacturing cost is also increased. 
     Now, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. 
     However, in the following description and the accompanying drawings, well known techniques may not be described or illustrated in detail to avoid obscuring the subject matter of the present disclosure. Through the drawings, the same or similar reference numerals denote corresponding features consistently. 
     The terms and words used in the following description, drawings and claims are not limited to the bibliographical meanings thereof and are merely used by the inventor to enable a clear and consistent understanding of the disclosure. Thus, it will be apparent to those skilled in the art that the following description about various embodiments of the present disclosure is provided for illustration purpose only and not for the purpose of limiting the disclosure as defined by the appended claims and their equivalents. 
       FIG.  1    is a diagram illustrating an optical module including a wavelength stabilizer according to an embodiment of the present disclosure. 
     Referring to  FIG.  1   , the optical module  100  according to an embodiment includes a wavelength stabilizer  90  that stabilizes the wavelength of laser light outputted from a laser diode  20 . The wavelength stabilizer  90  includes a controller  70  that constantly maintains the junction temperature of the laser diode  20 . 
     The wavelength stabilizer  90  according to an embodiment may further include a thermoelectric cooler  10 , a current measurer  30 , a voltage measurer  40 , a temperature measurer  50 , and a light output measurer  60 . 
     In particular, the wavelength stabilizer  90  according to an embodiment performs wavelength stabilization by using the thermal characteristics of the laser diode  20  without the use of additional components such as a typical etalon filter. That is, the wavelength stabilizer  90  can constantly maintain the junction temperature of the laser diode  20 , thereby stabilizing the wavelength of the outputted laser light even if the laser diode  20  deteriorates or the ambient temperature changes. 
     The optical module  100  according to an embodiment is a device that performs transmission and reception functions using laser light, and is referred to as an optical transceiver, an optical transponder, and the like. For example, the optical module  100  including the wavelength stabilizer  90  according to an embodiment may be used for WDM or DWDM. 
     In the optical module  100  according to an embodiment, the wavelength stabilizer  90  stabilizes the wavelength of the laser light outputted from the laser diode  20 . 
     Now, the wavelength stabilizer  90  according to an embodiment will be described in detail. 
     The laser diode  20  includes a substrate  21  and a laser chip  23  mounted on the substrate  21  and outputting laser light. The substrate  21  receives a control signal and power required for the operation of the laser chip  23  and operates the laser chip  23 . 
     Heat generated by the laser chip  23  is dissipated to the outside through the substrate  23  and the thermoelectric cooler  10 . 
     The thermoelectric cooler  10  allows the laser diode  20  to be mounted thereon, and adjusts the temperature of the laser diode  20  under the control of the controller  70 . For example, the thermoelectric cooler  10  dissipates heat generated during the operation of the laser diode  20  to the outside so that the laser diode  20  can maintain a constant temperature. Conversely, when the ambient temperature is lower than a suitable temperature for operating the laser diode  20 , the thermoelectric cooler  10  can apply heat to the laser diode  20 . In particular, for wavelength stabilization, the controller  70  constantly maintains the junction temperature of the laser diode  20  through the thermoelectric cooler  10 . 
     In order for the thermoelectric cooler  10  to effectively adjust the temperature of the laser diode  20 , the substrate  21  of the laser diode  20  is mounted on the thermoelectric cooler  10 . That is, the substrate  21  has an upper surface for mounting the laser chip  23  and a lower surface being in contact with the thermoelectric cooler  10 . The substrate  21  exchanges heat with the thermoelectric cooler  10  through surface contact, so that the temperature of the laser diode  20  is adjusted. 
     The controller  70  controls the temperature of the thermoelectric cooler  10  by adjusting the current applied to the thermoelectric cooler  10 . 
     The reason why the wavelength stabilizer  90  constantly maintains the junction temperature of the laser diode  20  in an embodiment is to stabilize the wavelength of the laser light outputted from the laser diode  20 . That is, the reason is that the wavelength of the laser light outputted from the laser diode  20  is influenced by the junction temperature of the laser diode  20 , namely, the junction temperature of the laser chip  23 . 
     In other words, if the junction temperature of the laser diode  20  is kept constant, the wavelength of the laser light can be stabilized. 
     In order to constantly maintain the junction temperature of the laser diode  20 , the thermoelectric cooler  10  adjusts the temperature of the substrate  21  in an embodiment. 
     The temperature of the substrate  21  to constantly maintain the junction temperature of the laser diode  20  can be calculated as follows. 
     The current measurer  30  measures a current applied to the laser chip  23 . 
     The voltage measurer  40  measures a voltage applied to the laser chip  23 . 
     The temperature measurer  50  measures the temperature of the substrate  21 . A point at which the temperature of the substrate  21  is measured may be a soldering point. As the temperature measurer  50 , a thermocouple may be used. 
     Then, based on the current measured by the current measurer  30 , the voltage measured by the voltage measurer  40 , and the temperature of the substrate  21  measured by the temperature measurer  50 , the controller  70  adjusts the temperature of the substrate  21  through the thermoelectric cooler  10  and thereby constantly maintains the junction temperature of the laser diode  20 . 
     As shown in (3) of Equation 1 below, the controller  70  may calculate the temperature (T s2 ) of the substrate  21  such that the temperature change amount (T s1 −T s2 ) of the substrate written on the left side is equal to a value written on the right side. The temperature (T s2 ) of the substrate  21  denotes the substrate temperature at t2, which is a certain time point after a time point t1. The time point t1 may be an initial time point. 
       [Equation 1] 
         T   j1   =T   s1   +R   th ( I   1   V   1   −P   1 )  (1)
 
         T   j2   =T   s2   +R   th ( I   2   V   2   −P   2 )  (2)
 
     Under the condition T j1   =T   j2 , 
         T   s1   −T   s2   =R   th ( I   2   V   2   −I   1   V   1 )+ R   th ( P   1   −P   2 )  (3)
 
     (T j1 : Junction temperature at t1 
     T j2 : Junction temperature at t2 
     T s1 : Substrate temperature at t1 
     T s2 : Substrate temperature at t2 
     R th : Thermal resistance [° C./W] 
     P 1 : Light output at t1 
     P 2 : Light output at t1) 
     In Equation 1, the thermal resistance (R th ) denotes a thermal resistance between points at which the temperature of the substrate  21  is measured at the junction of the laser chip  23 , and is provided as a set value. In this case, the point at which the temperature of the substrate  21  is measured denotes a point at which the temperature measurer  50  measures the temperature of the substrate  21 . 
     Hereinafter, a process of calculating the temperature (T s2 ) of the substrate  21  by Equation 1 will be described with reference to  FIG.  2   .  FIG.  2    is a LIV graph for explaining a wavelength stabilization method using the wavelength stabilizer  90  of  FIG.  1    when there is no deterioration in a laser diode  20 . 
     In  FIG.  2   , the solid line is an L-I curve showing a change in the light output of the laser diode  20  according to the applied power. The dashed-dotted line is an I-V curve showing a change in the voltage of the laser diode  20  according to the applied power. 
     First, from (1) and (2) in Equation 1, the junction temperature (T j1 ) at t1 and the junction temperature (T j2 ) at t2 can be calculated. Here, T j1  is given as a value set as the initial junction temperature. 
     If Equation 1 is expanded under the condition of T j1 =T j2  using (1) and (2) of Equation 1, (3) of Equation 1 can be obtained. 
     In (3) of Equation 1, the temperature (T s2 ) of the substrate  21  at t2 can be calculated. That is, because the remaining variables except T s2  are measured, calculated, or given as set values, T s2  can be calculated. 
     The controller  70  stabilizes the wavelength of the laser light by controlling the operation of the thermoelectric cooler  10  so that the substrate  21  has T s2  at the calculated t2. 
     The light output measurer  60  measures the light output of the laser diode  20 . If the light output measurer  60  can measure both the light output (P 1 ) at t1 and the light output (P 2 ) at t2, the controller  70  can calculate the temperature (T s2 ) of the substrate  21  by (3) of Equation 1. 
     On the other hand, the light output (P 1 ) at t1, which is the initial value of the light output, can be given as a set value, and the light output measurer  60  cannot measure the absolute value of the light output (P 2 ) at t2, but it is possible to measure the relative value (P 2 /P 1 ) of the light output (P 1 ) at t1 and the light output (P 2 ) at t2. 
     In this case, the controller  70  may calculate the temperature (T s2 ) of the substrate  21  by using (4) of Equation 2 obtained by modifying (3) of Equation 1. 
       [Equation 2] 
         T   s1   −T   s2   =R   th ( I   2   V   2   −I   1   V   1 )+ P   1   R   th (1 −P   2   /P   1 )  (4)
 
     Meanwhile, the reason why the wavelength stabilization is required in the optical module  100  is that the deterioration of the laser diode  20  occurs due to the use of the optical module  100 . The deterioration of the laser diode  20  causes a shift in the wavelength of the laser light. 
       FIG.  3    is a LIV graph for explaining a wavelength stabilization method using the wavelength stabilizer  90  of  FIG.  1    when there is deterioration in a laser diode  20 . 
     Referring to  FIG.  3   , when the laser diode  20  is deteriorated, the light output is reduced for the same current. 
     Therefore, in optical communication, an automatic power control (APC) mode for a constant light output is generally used. Even if the laser diode  20  is deteriorated and the light output is reduced, the APC mode allows the light output to be constantly maintained by increasing the intensity of a driving current inputted to the laser diode  20 . 
     In  FIG.  3   , A denotes an L-I curve showing a change in the light output of the laser diode  20  before deterioration, and B denotes an L-I curve showing a change in the light output of the deteriorated laser diode  20 . That is, the change in the light output of the laser diode  20  shows the curve B shifted to right due to deterioration from the curve A before deterioration. 
     In addition, A′ is an I-V curve showing a change in the voltage of the laser diode  20  before deterioration, and B′ is an I-V curve showing a change in the voltage of the deteriorated laser diode  20 . The curve A′ represents a voltage (V 1 ) for a current (I 1 ) at t1, and the curve B′ represents a voltage (V 2 ) for a current (I 2 ) at t2. 
     According to an embodiment, in the APC mode, the temperature (T s2 ) of the substrate  21  can be calculated by Equation 3 modified from (3) of Equation 1. 
       [Equation 3] 
         T   s1   −T   s2   =R   th ( I   2   V   2   −I   1   V   1 )+ R   th ( P   1   −P   2 )  (3)
 
         T   s1   −T   s2   =R   th ( I   2   V   2   −I   1   V   1 ) (when  P   1   =P   2 )  (5)
 
         T   s1   −T   s2   =I   1   V   1   R   th ( I   2   V   2   /I   1   V   1 −1)  (6)
 
         T   s1   −T   s2   =I   1   V   1   R   th ( I   2   /I   1 −1) ( V   1   ≈V   2 )  (7)
 
     That is, in (3) of Equation 3, because P 1 =P 2  in the APC mode, the temperature (T s2 ) of the substrate  21  can be calculated by modified (5) of Equation 3. 
     The voltage measurer  40  measures a voltage applied to the laser chip  23 . 
     At this time, the voltage (V 1 ) at t1 and the voltage (V 2 ) at t2 have almost the same value (V 1 ≈V 2 ). 
     The current measurer  30  measures the current applied to the laser chip  23 . If the current measurer  30  can measure both the current (I 1 ) at t1 and the current (I 2 ) at t2, the controller  70  can calculate the temperature (T s2 ) of the substrate  21  by (5) or (6) of Equation 3. 
     On the other hand, the current (I 1 ) at t1, which is the initial value of the current, can be given as a set value, and the current measurer  30  cannot measure the absolute value of the current (I 2 ) at t2, but it is possible to measure the relative value (I 2 /I 1 ) of the current (I 1 ) at t1 and the current (I 2 ) at t2. In this case, the controller  70  may calculate the temperature (T s2 ) of the substrate  21  by using (7) obtained by modifying (6) of Equation 3. 
     As described above, the wavelength stabilizer  90  according to embodiments of the present disclosure can constantly maintain the junction temperature of the laser diode  20 , thereby stabilizing the wavelength of the outputted laser even if the laser diode  20  deteriorates or the ambient temperature changes. That is, the junction temperature of the laser diode  20  is constantly maintained by adjusting the temperature of the laser diode  20  through the thermoelectric cooler  10  on which the laser diode  20  is mounted. 
     Because the wavelength stabilizer  90  according to the present disclosure can perform the wavelength stabilization by maintaining the junction temperature of the laser diode  20  through the thermoelectric cooler  10 , there is an advantage that the structure of the optical module  100  is simplified and the manufacturing cost is lowered, compared to applying the etalon filter. 
     While the present disclosure has been particularly shown and described with reference to an exemplary embodiment 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 scope of the present disclosure as defined by the appended claims.