RU2454765C1 - Apparatus for offsetting current of semiconductor laser and monitoring operating capacity of analogue fibre-optic communication lines - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: apparatus includes circuits for controlling laser current, monitor photodiode current and ambient temperature. Power values set by a microcontroller are maintained by a comparator. The apparatus also has laser current offset circuits for modulation with a broadband analogue signal and controlling modulation power and a circuit for setting the required laser radiation power depending on temperature of the medium with a ripple filter. Current offset and laser radiation power are controlled such that the free generation stability type set using a priori obtained table data is maintained.

EFFECT: providing maximum dynamic range and maintaining non-uniformity of the amplitude-frequency characteristic of the fibre-optic line for transmitting analogue high-frequency signals in a wide range of operating temperatures.

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Description

The invention relates to radio engineering and optoelectronics, namely to fiber-optic systems for transmitting analog signals, and can be used in devices for transmitting analog signals via fiber-optic cable for radio communication systems and cable television (CATV).

A number of temperature compensation schemes for the characteristics of a semiconductor laser are known, which are based on maintaining a constant average current through a monitor photodiode (ie, a constant average laser power) ([1] - US Patent 6859473), ([2] - US Patent 5970078). The circuit in ([3] - US Patent 6226114) controls the offset of the laser according to a predefined table without a monitor photodiode. In the work ([4] - V.A. Malyshev, N. A. Mikhailov. Taking into account the features of generation-recombination processes in the analysis of the frequency characteristics of the modulation of injection pumped semiconductor lasers [Text] // Radio Engineering, Moscow, 2010 No. 3, p.67-70) it is shown that when spontaneous radiation enters the operating mode of the cavity of a single-mode laser, the stability mode of free generation changes, which can be maintained by changing the laser bias current. There is also a known scheme for monitoring and stabilizing the current and output laser power ([5] - US Patent 7166826), which uses a monitor photodiode for temperature compensation and stabilization of the radiation power of a semiconductor laser, as well as a digital device for storing the table of the required calibration of the bias parameters.

This stabilization scheme of the modulation characteristics of the laser diode, adopted as a prototype, has the following disadvantages:

- the circuit is not intended for modulation by analog microwave signals (microwave) signals, and accordingly, there is no way to fill in the table values in the program control module;

- there is no circuit switching circuit for direct modulation of the laser by an analog high-frequency signal.

The aim of the invention is a device for biasing the laser current, providing the maximum dynamic range and maintaining uneven amplitude-frequency characteristics (AFC) of the fiber-optic transmission line of analog high-frequency signals in a wide range of operating temperatures.

To achieve the goal, a power stabilization circuit is proposed, which includes control circuits for laser current, monitor photodiode current, ambient temperature, and a comparator that supports power level values set by the microcontroller.

According to the invention, the circuit comprises laser current bias circuits for modulating a broadband analog signal and controlling the modulation power, a circuit for setting the required laser radiation power depending on the temperature of the medium with a smoothing filter, wherein the current bias and laser radiation power are controlled so that the type of stability of free generation is maintained established by pre-obtained tabular data obtained from experimental verification of the type of laser used in the circuit.

A table of calibration data is obtained when measuring the amplitude-frequency response of a modulation circuit containing a laser of the type used in the device and a minimum of distorting elements in the signal path, depending on the ambient temperature and laser current. The values of the laser radiation power depending on the temperature for the calibration table are fixed when there is a transition of a nonmonotonic amplitude-frequency characteristic of the modulation circuit with a maximum on the characteristic whose frequency is nonzero to a monotonically decreasing characteristic.

The combination of distinctive features and properties of the proposed laser current bias device from the scientific and technical literature are unknown, therefore, it meets the criteria of novelty and inventive step.

Figure 1 shows an electrical diagram of a current bias device for a semiconductor laser and health monitoring for analog fiber optic communication lines.

Figure 2 shows the electrical circuit of the device receiving part for obtaining calibration coefficients.

The proposed circuit of the laser current bias device shown in figure 1, contains:

- laser diode 1.11, monitor photodiode 1.8;

- the bias circuit with a microwave signal in the form of a terminating resistor 1.5, an isolation capacitor 1.7 and a low-pass filter 1.15;

- a laser bias installation circuit, consisting of a limiting resistor 1.12, a control transistor 1.13, a comparator 1.10, a digitally controlled resistor 1.9 that sets the level of radiated power of the DAC 1.2, connected through a filter 1.4 smoothing the voltage jumps;

- microwave signal control circuit in the form of a microwave power coupler 1.1, detector 1.6 and an analog-to-digital converter (ADC) 1.19;

voltage control circuit on the monitor photodiode in the form of ADC 1.20; laser current control circuit in the form of resistors 1.14, 1.16 and 1.18, operational amplifier 1.17 and ADC 1.21;

- microcontroller 1.3, temperature sensor 1.22.

The diagram of the device receiving part for obtaining calibration coefficients, shown in figure 2, contains a photodiode 2.1, a resistor 2.2, a separation capacitor 2.3, a resistor 2.4 and a microwave amplifier 2.5.

The principle of stabilization of the modulation characteristics of the laser is based on compensation for changes in the stability mode of the generation when spontaneous radiation enters the working mode. The invention is effective only when using single-mode lasers. Power stabilization is based on the well-known principle of maintaining the laser current 1.11 in such a way that the current is maintained in the circuit of the monitor photodiode 1.8, which is proportional to the power radiated by the laser. The voltage incident on the resistor 1.9 in the circuit of the monitor photodiode 1.8 is compared by the comparator 1.10 with the voltage of the DAC 1.2 installed through the smoothing filter 1.4 and the laser bias current is regulated through the transistor 1.13. The required radiation power is set according to the algorithm laid down in the microcontroller 1.3, which depends on the ambient temperature measured by the temperature sensor 1.22. The microwave power supplied to the laser is controlled by its branching and detection by elements 1.1, 1.6, and 1.19. The laser current is controlled by measuring the voltage on the monitor resistor 1.14 elements 1.16, 1.17, 1.18 and 1.21. The current in the circuit of the monitor photodiode 1.8 is controlled by measuring the voltage across the resistor 1.9, ADC 1.20.

To obtain a calibration data table for the microcontroller, the circuits shown in Figs. 1 and 2 are collected and a fiber optic line is connected between the laser diode 1.11 and the photodiode 2.1. Observing the amplitude-frequency characteristic of the modulation circuit at room temperature, the laser bias current is gradually increased from zero to the moment when there is a transition of a non-monotonic amplitude-frequency characteristic with a maximum at a modulation frequency other than zero to a monotonically decreasing characteristic. The temperature, the control voltage on the comparator 1.10 (current of the monitor photodiode), and the laser current are recorded. Then put the transmitting part of the installation, shown in figure 1, in the chamber of heat and cold and repeat the measurements for high and low operating temperatures of the medium. The obtained values of the control voltage on the comparator 1.10 at two temperature points are linearly interpolated by the temperature coordinate in the microcontroller 1.3 to ensure the optimal operating point for the laser bias current.

The invention allows to provide the maximum dynamic range of devices for transmitting analog signals when the temperature of the environment is maintained by maintaining the stability of free generation by adjusting the bias current and laser radiation power compared to the prototype, in which the average laser radiation power is stabilized.

At the filing date, a device was developed and manufactured for displacing the laser current and monitoring its operability for analog fiber-optic communication lines with a working frequency range up to 2.5 GHz and a working temperature range from minus 40 to 70 ° C. The developed device can be used to provide the optimal laser modulation mode in the transmission paths of analog signals.

Claims (1)

A semiconductor laser current bias device and operability control for analog fiber-optic communication lines, including laser current, monitor photodiode current, ambient temperature control circuits, a comparator supporting power level values set by the microcontroller, characterized in that it contains laser current bias circuits for modulation broadband analog signal and control the modulation power, the circuit setting the required laser radiation power depending on the temperature of the medium with lazhivayuschim filter, wherein the DC offset and power of the laser radiation are controlled so as to maintain the free-running resistance type, installed on a previously made table data.

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