JP2008085529A - Optical transmission system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain optimal amplification characteristics regardless of optical transmission conditions by setting a bias most suitable for optical transmission at a photoelectric conversion part in an optical transmission system. <P>SOLUTION: The optical transmission system is provided with a photodiode 13 which photoelectrically converts an optical signal inputted via an optical cable 3, a power amplification part 14 for amplifying an RF signal which is outputted from the photodiode on the basis of a bias voltage, and a bias setting part 25 that generates the bias voltage on the basis of a voltage outputted from the photodiode. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical transmission system in which transmission loss in an optical cable is taken into consideration and optimum distortion and noise characteristics can be obtained regardless of the magnitude of the transmission loss.

  In data transmission systems, optical transmission systems using optical cables that enable transmission and reception of high-capacity data with high speed and low loss are widespread, and optical transmission systems convert electrical signals into optical signals. A photoelectric conversion unit that converts an optical signal into an electrical signal.

  FIG. 6 shows an outline of a conventional optical transmission system.

  In the figure, 1 is a master station in the optical transmission system, 2 is a slave station, and the master station 1 and the slave station 2 are connected by optical cables 3 and 4 which are transmission paths. The optical cable 3 indicates a downstream transmission path, and the optical cable 4 indicates an upstream transmission path.

  The master station 1 includes a master station electro-optic converter 5 and a master station photoelectric converter 6, and the slave station 2 includes a slave station photoelectric converter 7 and a slave station electro-optic converter 8. The station electrical / optical converter 5 and the slave station photoelectric converter 7 are connected by the optical cable 3, and the master station photoelectric converter 6 and the slave station electrical / optical converter 8 are connected by the optical cable 4. Yes.

  The master station electro-optic converter 5 amplifies the received radio frequency signal (hereinafter referred to as RF signal) by the power amplifying unit 11 and then inputs the RF signal to a laser diode (hereinafter referred to as LD) 12. The signal is converted into an optical signal and sent to the optical cable 3.

  The optical signal transmitted by the optical cable 3 is received by the slave station photoelectric conversion unit 7, and the received optical signal is converted into an RF signal by a photodiode (hereinafter referred to as PD) 13, and then in the power amplification unit 14. After performing necessary processing such as amplification, the signal is adjusted and output through the variable attenuator 15 for optical loss correction so that the gain of the slave-station photoelectric conversion unit 7 becomes constant.

  The slave station electro-optic converter 8 of the slave station 2 has the same configuration as the master station electro-optic converter 5, and the master station photoelectric converter 6 of the master station 1 is the slave station photoelectric converter. The RF signal received by the slave station 2 is converted into an optical signal by the slave station electro-optical converter 8, and the optical signal is transmitted through the optical cable 4 and the master station 1. And is converted into an RF signal in the master-station photoelectric conversion unit 6.

  Optical cables have low transmission loss and are used for long-distance transmission. The transmission loss of light is not normally a fixed value, but has a range of 0 dB to NdB. Further, there is a 1: 2 relationship between the optical power dBm and the electrical power dBm, and the transmission loss of 0 dB to NdB of the optical power has a wide dynamic range of 0 dB to 2 NdB of the electrical power. The gain difference between the optical power dBm and the electrical power dBm is absorbed by the variable attenuator 15.

  On the other hand, the RF signal immediately after being converted by the PD 13 is input to the power amplifier 14. Therefore, the power amplifier 14 performs signal processing such as amplification on an RF signal having a wide dynamic range in which the gain between the optical power dBm and the electrical power dBm is not adjusted.

  Further, the amplification characteristics of the power amplifying unit 14, particularly the first-stage amplifier in the power amplifying unit 14, greatly affect the characteristics of the entire apparatus.

  In the conventional optical transmission system described above, the bias value is narrowed down to one point where noise characteristics can be obtained well with respect to the above-described wide dynamic range in the signal processing in the slave-station photoelectric conversion unit 7, particularly in the first stage amplification. Is determined.

  For this reason, the bias value is not adapted to the optical transmission conditions in the entire range, and the first-stage amplifier cannot always be said to have optimum amplification characteristics.

  FIG. 7 shows the ACLR characteristics (adjacent channel leakage power) in the conventional first-stage amplifier. When the optical transmission loss is small (the portion surrounded by the broken line in FIG. 7), the input power increases and distortion occurs. Is occurring.

  In addition, there exists a thing shown by patent document 1 as an optical transmission system and an optical receiver.

JP 2006-20368 A

  In view of such circumstances, the present invention is such that an optimum amplification characteristic can be obtained regardless of the optical transmission conditions by setting an optimum bias for optical transmission conditions such as distance in the photoelectric conversion unit. It is.

  The present invention provides a photodiode that photoelectrically converts an optical signal input via an optical cable, a power amplifying unit that amplifies an RF signal output from the photodiode based on a bias voltage, and is output from the photodiode. The present invention relates to an optical transmission system including a bias setting unit that generates the bias voltage based on a voltage.

  According to the present invention, a photodiode that photoelectrically converts an optical signal input via an optical cable, a power amplifier that amplifies an RF signal output from the photodiode based on a bias voltage, and an output from the photodiode A bias setting unit that generates the bias voltage based on the voltage to be applied, the bias voltage applied to the power amplification unit is a voltage value suitable for a signal input to the power amplification unit, and the power The amplifying unit is provided with the optimum bias voltage for the optical transmission conditions, and exhibits excellent effects of optimizing distortion and noise characteristics.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  The outline of the optical transmission system according to the present invention will be described with reference to FIG. In FIG. 1, the same components as those shown in FIG. 6 are denoted by the same reference numerals.

  The master station 1 includes a master station electro-optic conversion unit 21 and a master station photoelectric conversion unit 22, and the slave station 2 includes a slave station photoelectric conversion unit 23 and a slave station electro-optic conversion unit 24. An optical conversion unit 21 and the slave station photoelectric conversion unit 23 are connected by an optical cable 3, and the master station photoelectric conversion unit 22 and the slave station electrical / optical conversion unit 24 are connected by an optical cable 4.

  The master station electro-optic converter 21, the master station photoelectric converter 22, and the slave station electro-optic converter 24 are the same as the master station electro-optic converter 5, the master station photoelectric converter 6, and the slave shown in FIG. Since it is equivalent to the local electro-optical converter 8, the description thereof is omitted below.

  The slave station photoelectric conversion unit 23 will be described.

  The slave station photoelectric conversion unit 23 includes a PD 13, a power amplifying unit 14, a variable attenuator 15, and a bias setting unit 25, and an optical signal transmitted by the optical cable 3 is input to the PD 13. The signal is photoelectrically converted to an RF signal. The RF signal is subjected to necessary signal processing such as amplification by the power amplifier 14. In the figure, the first-stage amplifier constituting the power amplifying unit 14 is shown.

  The RF signal amplified by the power amplifying unit 14 corrects a gain difference based on the difference in dynamic range between the optical signal and the RF signal by the variable attenuator 15, and the gain of the slave station photoelectric conversion unit 23 is corrected. Adjust to be constant.

  The bias voltage applied to the power amplifying unit 14 is set by the bias setting unit 25.

  The bias setting unit 25 includes an AD converter 26, a calculation unit (CPU) 27, a storage unit 28, a DA converter 29, and an amplifier 30.

  FIG. 2 shows the relationship between the optical transmission loss and the received light voltage. In addition, an appropriate bias voltage applied to the power amplifying unit 14 with respect to the received light voltage is shown. An appropriate bias voltage is obtained in advance by experiment, calculation, or the like. Table data (optical transmission loss correction bias table) regarding the relationship between the received light voltage and the appropriate bias voltage is stored in the storage unit 28.

  A part of the output signal (RF signal) from the PD 13 is branched and input to the AD converter 26. The AD converter 26 performs A / D conversion and sends it to the arithmetic unit 27 as a digital detection signal. The calculation unit 27 compares a detection signal (light reception voltage) with the table data, calculates a bias voltage corresponding to the detected light reception voltage, and outputs it to the DA converter 29. The DA converter 29 sends an analog bias signal obtained by D / A conversion to the amplifier 30.

  Thus, the bias setting unit 25 generates a bias voltage based on the voltage output from the PD 13 and applies it to the power amplification unit 14.

  The amplifier 30 amplifies an analog bias signal and applies an appropriate bias voltage corresponding to the received light voltage output from the PD 13 to the power amplifier 14. The RF signal output from the PD 13 is amplified by the power amplifier 14 at an amplification factor appropriate for the voltage. The RF signal output from the power amplifying unit 14 is adjusted by the variable attenuator 15 so that the gain of the sub-station photoelectric conversion unit 23 is constant.

  In the above-described embodiment, the bias setting unit 25 may be provided for the master-station photoelectric conversion unit 22 as well.

  FIG. 3 shows the ACLR characteristics when the present invention is implemented, and the distortion in the state where the optical transmission loss is small (the portion surrounded by the broken line in FIG. 3) is eliminated.

  Next, a specific example of bias voltage setting by the bias setting unit 25 will be described.

  The optical transmission loss correction bias table shown in FIG. 2 can be simplified and stored in the storage unit 28. FIG. 4 shows a simplified optical transmission loss correction bias table.

  In the optical transmission loss correction bias table shown in FIG. 4, the light reception voltage set at a required step, for example, the light reception voltage is set at a step of 0.5 V, and the bias value corresponding to each step is set. The bias value is obtained based on circuit characteristics obtained in advance or actually measured values.

  FIG. 5 shows a flow in which the bias setting unit 25 sets a bias voltage value based on the optical transmission loss correction bias table shown in FIG.

  When the PD 13 receives an optical signal and outputs a light reception voltage, the calculation unit 27 reads an optical transmission loss correction bias table from the storage unit 28, and a bias corresponding to the light reception voltage from the optical transmission loss correction bias table. Find the value. The obtained bias value is output to the power amplifier 14 through the amplifier 30.

  When the output light reception voltage does not match the light reception voltage set in the optical transmission loss correction bias table, for example, when the light reception voltage is 0.6V, the calculation unit 27 is set to 0.5V. The bias value with respect to 0.6V is calculated by an interpolation process such as reading the bias value and the bias value with respect to 1.0V, and appropriately dividing between 0.5V and 1.0V.

  Thus, even when the received light voltage is 0.6V, the bias setting unit 25 outputs an appropriate bias value.

1 is a schematic configuration diagram of an optical transmission system according to an embodiment of the present invention. It is a figure which shows the optical transmission loss correction | amendment bias table used for this embodiment. It is a diagram which shows the ACLR characteristic in this embodiment. It is a figure which shows the simplified optical transmission loss correction bias table. It is a flowchart which shows the flow which calculates | requires a bias value with the simplified optical transmission loss correction | amendment bias table. It is a schematic block diagram of the conventional optical transmission system. It is a diagram which shows the ACLR characteristic in the conventional optical transmission system.

Explanation of symbols

1 Master station 2 Slave station 3 Optical cable 4 Optical cable 13 PD
DESCRIPTION OF SYMBOLS 14 Power amplification part 15 Variable attenuator 21 Parent station electro-optic conversion part 22 Parent station photoelectric conversion part 23 Slave station photoelectric conversion part 24 Slave station electric light conversion part 25 Bias setting part 27 Calculation part 28 Storage part

Claims (1)

  A photodiode that photoelectrically converts an optical signal input via an optical cable, a power amplifier that amplifies an RF signal output from the photodiode based on a bias voltage, and the voltage output from the photodiode An optical transmission system comprising a bias setting unit for generating a bias voltage.

Images