For 10G optical transceivers used in military equipment, meeting the wide operating temperature range of military grades is a very important consideration. Therefore, under the wide temperature condition of -40℃ to 80℃, effective temperature compensation for the optical module is an important task for the control part of the 10G optical module in this design. The control part of this design adopts a small arm chip as the MCU chip design based on 10G SFP+ SR. The control part can provide complete monitoring functions and powerful alarm and control functions for the optical transceiver module. In addition, the package is small and the peripherals are simple. meet the requirements of this design.
In this design, the control circuit mainly has two functions, namely: digital diagnosis function and control function. Digital diagnosis function, that is, real-time detection of some important performance parameters and working status of optical modules through digital diagnosis circuits, including optical module ambient temperature, bias current, transmitted optical power, received optical power, various warning, and alarm signals, etc.; The control function is to control the function and working status of the optical module according to the detection result of the digital diagnosis function, including receiving part of the working rate control, communication with the host, laser bias current temperature compensation, laser modulation current temperature compensation, alarm output and shutdown protection, especially the control of the temperature compensation circuit is the key to the successful realization of the temperature compensation function.
Under ideal conditions, the threshold current of the laser is stable, so the constant bias current and modulation current can keep the luminous power and extinction ratio of the laser basically constant. However, in actual use, the change of temperature will change the threshold current of the laser, which requires the drive circuit to compensate the bias current and modulation current. This compensation is completed by the compensation circuit, which is also the difficulty in realizing the wide temperature operation of the optical module.
Temperature Compensation of Bias Current
The bias current of the laser directly affects the average luminous power of the laser. The 10G optical module light source of this design is a DFB laser, which belongs to a semiconductor device, and its characteristics are similar to that of a diode, that is, as the ambient temperature increases, its threshold current Ith will gradually increase. The increase of the threshold current will cause the reduction of the photoelectric conversion efficiency of the laser, so in order to keep the average output optical power unchanged, the bias current IBIAS must be compensated when the temperature changes.
There are three commonly used methods for temperature compensation of bias current: backlight current feedback method, thermistor method, and digital look-up table method. This design adopts the digital look-up table method to carry out the compensation control of the bias current. The digital look-up table method is an open-loop method. It uses the memory to build a look-up table corresponding to the temperature value and the bias current value, uses the temperature sensor to collect the temperature value, and then uses the MCU to look up the table according to the temperature to control the drive. The circuit generates the bias current value under this temperature condition to achieve the purpose of compensation. This method can achieve good compensation for the temperature-induced changes in the threshold current. Combined with the memory and digital detection functions of the MCU chip, the digital look-up table method temperature compensation control of this design can be easily realized. The temperature compensation circuit realized by the digital look-up table method in this design can control the transmitted optical power more precisely and keep it constant in a wide temperature range, and the accuracy is set to 4°C.
Temperature Compensation for Modulation Current
It is assumed that the average transmitted optical power remains unchanged after the bias current temperature compensation, but due to the change of the laser photoelectric conversion efficiency with the temperature, the amplitude of the laser output optical signal will also change, which will cause the optical signal extinction ratio to change. The change in the extinction ratio will cause a change of the bit error rate and the signal jitter, which is not conducive to the stable operation of the optical module. Therefore, in the whole temperature variation range, on the basis of bias current compensation, the modulation current should also be compensated, so that the extinction ratio can be maintained at an optimal working level.
There are generally two methods for the compensation of the modulation current, one is the K factor compensation method, and the other is the digital look-up table method. The same as the bias current temperature compensation method, the temperature compensation of the modulation current in this design also adopts the digital look-up table method to construct the corresponding relationship table between the temperature value and the modulation current, and then uses the temperature sensor to collect the temperature information, and then adjust the drive circuit according to the temperature value. The desired adjustment current value is generated, and the adjustment accuracy is set to 4°C.
