Patent Publication Number: US-2022224416-A1

Title: Optical transmitter and method of manufacturing optical transmitter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2021-001828 filed on Jan. 8, 2021, with the Japanese Patent Office, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The disclosures herein relate to an optical transmitter and a method of manufacturing an optical transmitter. 
     BACKGROUND 
     Optical transmitters used in optical transceivers and the like include optical modulators, and automatic bias control (ABC) is performed on the optical modulators. 
     However, there is a case in which existing optical transmitters cannot easily stabilize the ABC of the optical modulator. 
     RELATED-ART DOCUMENTS 
     Patent Document 
     
         
         [Patent Document 1] Japanese Laid-Open Patent Publication No. 2009-288509 
         [Patent Document 2] Japanese Laid-Open Patent Publication No. 2012-128165 
         [Patent Document 3] U.S. Pat. No. 8,041,228 
       
    
     SUMMARY 
     According to an aspect of the embodiment, an optical transmitter includes an optical modulator configured to modulate an input light and output an optical signal, a bias controller configured to control a bias applied to the optical modulator, an attenuator configured to attenuate the optical signal output from the optical modulator and output the attenuated optical signal, a positive monitor configured to detect a forward optical signal included in the attenuated optical signal output from the attenuator, a negative monitor configured to detect a complementary optical signal included in the attenuated optical signal output from the attenuator, and a controller configured to control the bias controller based on a sum of a first dither of a positive signal output from the positive monitor and a second dither of a negative signal output from the negative monitor. 
     The object and advantages of the embodiment will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram illustrating an optical transmitter according to a reference example; 
         FIG. 2  is a block diagram illustrating an optical transmitter according to an embodiment; and 
         FIG. 3  is a time chart illustrating an operation of the optical transmitter according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     According to an aspect of the embodiment, an optical transmitter includes an optical modulator configured to modulate an input light and output an optical signal, a bias controller configured to control a bias applied to the optical modulator in order to adjust differential static phase of Machzehnder interferometer to the optimum point, an attenuator configured to attenuate the optical signal output from the optical modulator and output the attenuated optical signal, a positive monitor configured to detect a branched optical power proportional to the output power of an output optical signal from the attenuator, a negative monitor configured to detect an optical power complimentary to (i.e., negatively proportional to) the output power of the output optical signal from the attenuator, and a controller configured to control the bias based on a sum of a first dither detected by the positive monitor and a second dither detected by the negative monitor. 
     In the following, an embodiment of the present disclosure will be specifically described in detail with reference to the accompanying drawings. In the present specification and the drawings, components having substantially the same functional configuration are referenced by the same reference numerals and overlapped description may be omitted. 
     Reference Example 
     First, a reference example will be described.  FIG. 1  is a block diagram illustrating an optical transmitter according to the reference example. 
     As illustrated in  FIG. 1 , an optical transmitter  900  according to the reference example includes an optical modulator  910 , a bias controller  920 , an attenuator  930 , a positive monitor  941 , a negative monitor  942 , and a controller  950 . 
     The optical modulator  910  modulates a continuous wave (CW) light output from a light source  960  as an input light and outputs the modulated light as an optical signal. The bias controller  920  adds a bias to an input data signal (an electric signal) based on control of the controller  950  and generates a drive signal to drive the optical modulator  910 . The attenuator  930  attenuates the optical signal output from the optical modulator  910  and outputs the attenuated optical signal. The positive monitor  941  monitors a forward optical signal output from the attenuator  930  to output an electric signal proportional to the forward optical signal, which is a monitor value. The negative monitor  942  monitors a complementary optical signal output from the optical modulator  10  to output an electric signal negatively proportional to the attenuator output, which is a monitor value. The positive monitor  941  and the negative monitor  942  are, for example, photodiodes (PD). 
     The optical transmitter  900  has two operation modes: Tx_Disable and Tx_Enable. In the operation mode Tx_Disable, the light source  960  is turned off, the attenuator  930  is in a closed state, and the controller  950  controls the bias controller  920  based on an output signal from the negative monitor  942 . In the operation mode Tx_Enable, the light source  960  is turned on, the attenuator  930  is in an open state, and the controller  950  controls the bias controller  920  based on an output signal from the positive monitor  941 . 
     As described, in the optical transmitter  900 , the bias of the optical modulator  910  is controlled based on different output signals between two operation modes of Tx_Disable and Tx_Enable. Thus, it may be difficult to stabilize the modulator bias by ABC. Additionally, in the optical transmitter  900 , a shift of the bias point from the optimum point, what is called “bias shift”, may be caused if the negative monitor  942  is used for adjusting the bias point. In such a case, when the operation mode transitions from Tx_Disable to Tx_Enable, the bias is not easily stabilized at the beginning of Tx_Enable, and the wait time until the bias is stabilized may be longer. 
     The inventor of the present application, in view of such characteristics of the optical transmitter  900 , has made an intense investigation to stabilize the bias of the optical modulator, and as a result, has arrived at the following embodiment. 
     Embodiment 
     Next, an optical transmitter according to the embodiment will be described.  FIG. 2  is a block diagram illustrating the optical transmitter according to the embodiment. 
     As illustrated in  FIG. 2 , an optical transmitter  100  according to the embodiment includes an optical modulator  10 , a bias controller  20 , an attenuator  30 , a positive monitor  41 , a negative monitor  42 , and a controller  50 . 
     The optical modulator  10  modulates the CW light output from the light source  60  as an input light and outputs the modulated light as an optical signal. The optical modulator  10  includes, for example, a Mach-Zehnder (MZ) modulator. The optical modulator  10  operates using an applied bias. The optical modulator  10  includes a phase shifter  11 A provided in an optical waveguide  16 A and a phase shifter  11 B provided in an optical waveguide  16 B. The optical waveguide  16 A is one waveguide of two waveguides branched from an input section and the optical waveguide  16 B is the other waveguide of the two waveguides. The optical modulator  10  further includes a phase shifter  12 A and a phase modulator  13 A provided on an optical waveguide  17 A and a phase shifter  12 B and a phase modulator  13 B provided on an optical waveguide  17 B. The optical waveguide  17 A is one waveguide of two waveguides branched from the optical waveguide  16 A and the optical waveguide  17 B is the other waveguide of the two waveguides. The optical modulator  10  further includes a phase shifter  12 C and a phase modulator  13 C provided on an optical waveguide  17 C and a phase shifter  12 D and a phase modulator  13 D provided on an optical waveguide  17 D. The optical waveguide  17 C is one waveguide of two waveguides branched from the optical waveguide  16 B and the optical waveguide  17 D is the other waveguide of the two waveguides. The optical waveguide  17 A and the optical waveguide  17 B are coupled to the optical waveguide  18 A, the optical waveguide  17 C and the optical waveguide  17 D are coupled to the optical waveguide  18 B, and the optical waveguide  18 A and the optical waveguide  18 B are coupled to one optical waveguide (an output section). 
     The bias controller  20  adds a bias to an input data signal (an electric signal) output from a digital signal processor (DSP) or the like based on control of the controller  50  and generates a drive signal to drive the optical modulator  10 . That is, the bias controller  20  controls the bias applied to the optical modulator  10 . 
     The attenuator  30  attenuates an optical signal output from the optical modulator  10  and outputs the attenuated optical signal. The attenuator  30  includes a first phase shifter  31  and a second phase shifter  32 . The first phase shifter  31  shifts a phase of the branched part of the optical signal output from the optical modulator  10  and outputs the phase-shifted optical signal. The second phase shifter  32  shifts a phase of the other branched part of the optical signal output from the optical modulator  10  to a direction opposite to the phase shift in the first phase shifter and outputs the inversely-phase-shifted optical signal. The optical signal output from the attenuator  30  includes coupled outputs from the first phase shifter and the second phase shifter. The coupled outputs include dithers. 
     The positive monitor  41  monitors the forward optical signal output from the attenuator  30  and outputs an electric signal proportional to the output power of the output of the attenuator  30  (i.e., an electric positive signal), which is a monitor value. The negative monitor  42  monitors the complementary optical signal output from the attenuator  30  and outputs an electric signal complementary to the output power of the output of the attenuator  30  (i.e., an electric negative signal), which is a monitor value. The positive monitor  41  and the negative monitor  42  are, for example, PDs. The electric positive signal and the electric negative signal also include dithers. 
     The controller  50  controls the bias controller  20  based on the sum of a first dither of the positive signal output from the positive monitor  41  and a second dither of the negative signal output from the negative monitor  42 . For example, the sum of the first dither and the second dither is within a predetermined range, and the sum of the first dither and the second dither is preferably substantially constant. The controller  50  controls the bias controller  20  to generate, for example, a bias on which the dither is superimposed. 
     Here, the operation of the optical transmitter  100  will be described. The operation is an example of a control method of the optical transmitter  100  that is performed mainly by the controller  50 .  FIG. 3  is a time chart illustrating the operation of the optical transmitter  100  according to the embodiment. The optical transmitter  100  has two operation modes of: Tx_Disable and Tx_Enable. Tx_Disable is an operation mode in which optical transmission is stopped. In contrast, Tx_Enable is an operation mode in which optical transmission is performed. The controller  50  controls the bias controller  20  so as to generate a bias on which the dither is superimposed in both Tx_Disable and Tx_Enable. Tx_Disable is an example of a first operation mode, and Tx_Enable is an example of a second operation mode. 
     When the optical transmitter  100  is in the operation mode Tx_Disable, the controller  50  is in a Tx_Disable state, and the optical modulator  10  is controlled with a first gain for feedback control loop of the bias according to the dither. Additionally, the attenuator  30  is in a closed state. Thus, the second dither of the negative signal output from the negative monitor  42  is larger than the first dither of the positive signal output from the positive monitor  41 . 
     When the optical transmitter  100  is in the operation mode Tx_Enable, the controller  50  is in a Tx_Enable state, and the optical modulator  10  is controlled with a second gain. Additionally, the attenuator  30  is in an open state. Thus, the first dither of the positive signal output from the positive monitor  41  is larger than the second dither of the negative signal output from the negative monitor  42 . 
     When the operation mode is switched from Tx_Disable to Tx_Enable, a certain transition period occurs. In the transition period, the controller  50  enters a transition state, and the optical modulator  10  stops simultaneously with the cancellation of Tx_Disable. The controller  50  controls the bias controller  20  based on the sum of the first dither and the second dither even during the transition period. Additionally, the attenuator  30  transitions from the closed state to the open state, and accordingly the second dither is gradually reduced and the first dither gradually increases. When the attenuator  30  is in the open state, the operation of the optical modulator  10  starts, and when the bias of the optical modulator  10  is stabilized at the second gain, the operation mode becomes Tx_Enable. 
     The controller  50  controls the bias controller  20  based on the sum of the first dither and the second dither in the operation mode Tx_Disable, in the operation mode Tx_Enable, and in the transition period between Tx_Disable and Tx_Enable. As described above, the first dither and the second dither change, but the sum of the first dither and the second dither is substantially constant and within a predetermined range. Therefore, according to the present embodiment, the first gain and the second gain can be easily matched, and the bias of the optical modulator  10  can be easily stabilized. 
     Additionally, because the bias controller  20  is continuously controlled based on the sum of the first dither and the second dither, a shift of the bias does not easily occur, and thus the bias of the optical modulator  10  is stabilized at the second gain within a short period of time after the operation of the optical modulator  10  has started. Therefore, the wait time until the gain is stabilized can be reduced. 
     The optical transmitter  100  can be used, for example, in an optical transceiver. 
     According to at least one embodiment of the present disclosure, the bias of the optical modulator can be easily stabilized. 
     Although the preferred embodiment and the like have been described in detail above, the present invention is not limited to the above-described embodiment and the like. Various modifications and substitutions can be made to the above-described embodiment without departing from the scope of the claims. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.