KR20170109879A - Bias control apparatus for optical modulator in optical transmitter - Google Patents

Abstract

The present invention relates to a bias control apparatus for applying a bias signal to an optical modulator for an optical transmitter, the bias control apparatus comprising: a photodetector for detecting an output optical signal output from an optical modulator and providing a detection signal; And a bias signal output unit for generating a bias signal based on the detection signal provided from the optical detection unit and coupling an error detection signal to the generated bias signal and applying an error detection signal combined bias signal to the optical modulator .

Description

TECHNICAL FIELD [0001] The present invention relates to a bias control apparatus for an optical modulator for an optical transmitter,

The present invention relates to bias control of an optical modulator for an optical transmitter, and more particularly, to an apparatus for bias control of an IQ optical modulator used in an optical transmitter.

As the data transmission speed increases to over 100Gbps in long distance optical transmission technology that can transmit several hundred km or more, research and application of multi level modulation methods such as QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) have.

In addition, due to the development of long distance optical transmission technology, various modulation methods of multi-carrier type such as OFDM (Orthogonal Frequency Division Multiplexing) have become possible.

In this multi-level modulation scheme, not only the amplitude but also the phase characteristics of the light are modulated, so an IQ-optical modulator capable of quadrature modulation is used. Where I is the in-phase component of the signal and Q is the quadrature component.

1 shows a schematic structure of a general optical transmitter. Referring to the structure of the optical transmitter 10, a data signal to be transmitted is a signal for driving an optical modulator 13 Is amplified in a driver amplifier (11) with an appropriate voltage. Then, the data signal (Data Signal) amplified by the driver amplifier 11 is inputted to the optical modulator 13.

An optical signal generated in a laser diode (LD) 12 is in the form of a CW (continuous wave), and the optical signal is modulated by a data signal (Data Signal) And output.

The optical modulator 13 is configured to modulate the optical signal according to the data signal and perform electro-optic conversion of the signal. In general, the optical modulator 13 is implemented by using a lithium niobate (LiNbO ₃ ) waveguide and applying an electric signal to the electrode.

The driver amplifier 11, the laser diode 12 and the optical modulator 13 are controlled by the microprocessor 14 and the microprocessor 14 includes a driver amplifier 11, a laser diode 12, (13).

2 shows a structure of a generally used IQ optical modulator. Optical signals incident on the optical modulator 20 are divided into two optical signals, and then the optical signals are divided into a Mach-Zehnder interferometer (Mach-Zehnder Modulator, MZM) 21 and 22 of the second embodiment. Each of the MZMs 21 and 22 is driven by signals of In-phase Data (Data_I) and Quadrature Data (Data_Q), respectively.

The outputs of the MZMs 21 and 22 are combined again by the phase shifter 23. At this time, the phase difference of 90 degrees between the outputs of the MZMs 21 and 22 should be generated by using the DC voltage of Bias_90. The combined signal at the phase shifter 23 is output to the output of the optical modulator 20.

2, three DC bias voltages (Bias_I, Bias_Q, Bias_90) for optimizing the operation of the optical modulator 20 are required in addition to data signals Data_I and Data_Q applied from the outside.

Since the characteristics of the optical modulator 20 are changed according to the bias value and there is a bias drift, it is necessary to control these biases in order to optimize the performance.

As the prior art related to the bias control of the optical modulator, 'Auto bias control technique for optical 16-QAM transmitter with asymmetric bias dithering, Optics Express, vol.19, no. 26, B308 (2011) modulator bias control on external modulation link performance, International Topical Meeting on Microwave Photonics, pp. 121-124 (2000).

The above-mentioned prior art techniques apply bias control to bias signal values obtained by applying a tone of a certain frequency to a bias, and using a lock-in amplifier or the like to detect a value of a frequency of 1 or 2 times of the bias. .

However, the approach used in the prior art requires a relatively complex circuit such as a lock-in amplifier to detect a specific frequency. Also, since the bias change of the IQ-optical modulator is a slow change of several seconds, it does not require a high-speed circuit to track it.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an optical transmitter and a method for controlling the same, And a device capable of controlling the bias of the light modulator.

According to an aspect of the present invention, there is provided a bias controller for applying a bias signal to an optical modulator for an optical transmitter, the bias controller comprising: A photodetector for detecting an optical signal and providing a detection signal; And a bias signal output unit for generating a bias signal based on the detection signal provided from the optical detection unit and coupling an error detection signal to the generated bias signal and applying the bias signal combined with the error detection signal to the optical modulator do.

According to the present invention, the value of an error signal required for bias control of an optical modulator can be detected using a single photo-detector. Therefore, a circuit for controlling the bias of the optical modulator can be implemented in a simple form.

1 is a diagram showing a schematic structure of a commonly used optical transmitter.
2 is a diagram showing the structure of a commonly used IQ-optical modulator.
3 is a diagram illustrating an example of applying a bias control apparatus according to an embodiment of the present invention to an optical modulator for an optical transmitter.
4 is a diagram showing an output characteristic curve of an MZM (Mach-Zehnder modulator).
5 is a diagram illustrating a configuration of a bias signal output unit according to an embodiment of the present invention.
6 is a diagram illustrating an example of a value of an error detection signal used in generating a bias signal in the bias signal output unit according to the embodiment of the present invention.
7 is a diagram showing a change in RF output according to a phase change due to Bias_90.
8 is a diagram illustrating an example in which a bias control apparatus for an optical modulator for an optical transmitter according to an embodiment of the present invention is applied to a DP (Dual Polarization) -IQ optical modulator.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention and the manner of achieving them will become apparent with reference to the embodiments described in detail below with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Is provided to fully convey the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like numbers refer to like elements throughout.

In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. The following terms are defined in consideration of the functions in the embodiments of the present invention, which may vary depending on the intention of the user, the intention or the custom of the operator. Therefore, the definition should be based on the contents throughout this specification.

Hereinafter, a bias control apparatus for an optical modulator for an optical transmitter according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 3 is a diagram illustrating an example of application of a bias control apparatus according to an embodiment of the present invention to an optical modulator for an optical transmitter. The bias control apparatus 30 (hereinafter referred to as a bias control apparatus) Applies a bias signal to the optical modulator 20 'for stable optical signal output of the optical modulator 20' for the optical transmitter.

In particular, the bias controller 30 applies an error detection signal together with a bias signal to the optical modulator 20 ', checks an error detection signal from the output optical signal output from the optical modulator 20' And controls the bias signal so that the value of the signal is minimized.

At this time, the bias controller 30 may be configured to combine an error detection signal with a bias signal and to apply a bias signal coupled with an error detection signal to the optical modulator 20 '.

The error detection signal is an AC signal having a predetermined frequency. The bias control device 30 checks the voltage with respect to the error detection signal frequency and controls the bias signal so that the voltage becomes minimum.

Referring to an optical modulator 20 'receiving a bias signal from the bias controller 30 according to the present invention, the optical modulator 20' can be applied to the optical transmitter 10 shown in FIG. 1 (I) data D_I and Quadrature (Q) data D_Q, and modulates and outputs an optical signal according to I data D_I and Q data D_Q.

Hereinafter, an optical signal input to the optical modulator 20 'will be referred to as an' input optical signal ', and an optical signal output from the optical modulator 20' will be referred to as an 'output optical signal'.

The optical modulator 20 'receives an input optical signal, In-phase (I) data D_I and quadrature (Q) data D_Q and outputs the input optical signal D_I according to I data D_I and Q data D_Q. Modulates the signal and outputs an output optical signal.

More specifically, the optical modulator 20 'includes first and second modulators 21 and 22 arranged in parallel, and first and second optical signals output from the two modulators 21 and 22, respectively, And a phase shift portion 23 that has a phase difference of 90 degrees.

The first and second modulators 21 and 22 output the first and second optical signals, respectively. The first and second modulators 21 and 22 generate Mach And may be a Mach-Zehnder modulator (MZM) in the form of a Mach-Zehnder interferometer.

The first modulator 21 receives the input optical signal, I data D_I, and the first bias signal S_B1 input from the outside and outputs the first optical signal. The second modulator 22 modulates Q data D_Q and a second bias signal S_B2, and outputs the second optical signal.

The phase shifter 23 receives the first optical signal, the second optical signal and the third bias signal S_B3 and generates a phase difference between the first and second optical signals based on the third bias signal S_B3 And outputs the output optical signal at 90 degrees.

The bias control device 30 according to the present invention is for applying a bias signal to the optical modulator 20 'for stable optical signal output of the optical modulator 20' for the optical transmitter. The bias control device 30 includes a photodetector 31, And a bias signal output section 32.

The optical detector 31 is located at an output terminal of the optical modulator 20 'and detects an output optical signal output from the optical modulator 20'. For example, the built-in PD an embedded PD (photo detector).

The optical detecting unit 31 provides the detected output optical signal to the bias signal output unit 32. At this time, the detection unit 31 converts the detected output optical signal into an electrical signal and provides it to the bias signal output unit 32.

Hereinafter, an electrical signal supplied from the optical detecting unit 31 to the bias signal output unit 21 is referred to as a detection signal S_PD.

The bias signal output unit 32 generates a bias signal based on the detection signal S_PD provided from the optical detection unit 31 and applies the generated bias signal to the optical modulator 20 ' 20 '.

In particular, the bias signal output unit 32 confirms an error detection signal from the detection signal S_PD provided from the optical detection unit 31, and generates a bias signal based on the checked error detection signal.

Meanwhile, the bias signal output unit 31 generates the first to third bias signals S_B1, S_B2, and S_B3.

The first bias signal S_B1 is applied to the first modulator 21 of the optical modulator 20 'and the second bias signal S_B2 is applied to the second modulator 22 of the optical modulator 20' And the third bias signal S_B3 is applied to the phase shifter 23 of the optical modulator 20 '.

In addition, the bias signal output unit 31 combines the error detection signals with the first through third bias signals S_B1, S_B2, and S_B3 to generate first through third bias signals S_B1, S_B2, and S_B3 to the optical modulator 20 '.

Hereinafter, the configuration of the bias signal output unit and error signal generation according to the present invention will be described in detail with reference to the accompanying drawings. For convenience of description, characteristics of an MZM (Mach-Zehnder modulator) will be described first .

FIG. 4 shows an output characteristic curve of an MZM (Mach-Zehnder modulator). The output value of the MZM changes in the form of a sine function according to the value of the bias voltage applied to the MZM.

As shown in FIG. 4, the minimum output null of the MZM is the optimum position of Bias_I (S_B1 of the present invention) and Bias_Q (S_B2 of the present invention), and the minimum output (null) ) Is the optimum position of Bias_90 (S_B3 of the present invention).

5 is a diagram illustrating a configuration of a bias signal output unit according to an embodiment of the present invention. The bias signal output unit 50 shown in FIG. 5 may be applied to the bias signal output unit 31 shown in FIG. 6 is a diagram showing an example of the value of an error detection signal used in generating a bias signal in the bias signal output unit according to the embodiment of the present invention, and FIG. 7 is a graph showing an example of the value of the RF output according to the phase change by Bias_90 Fig.

5, the bias signal output unit 50 includes an amplification unit 51, a signal extraction unit 52, a first signal processing unit 53, a second signal processing unit 54, and a bias signal generation unit 55 ).

The amplification unit 51 receives an electrical signal, i.e., a detection signal S_PD, provided from the optical detection unit 32 of FIG. 3, amplifies the amplified signal to a predetermined size, and outputs the amplified signal to a transimpedance amplifier Lt; / RTI > For example, the bandwidth of the amplifier 51 may be about 2 GHz.

The signal extracting unit 52 is configured to extract a signal of a predetermined band from a detection signal amplified to a predetermined size by the amplifying unit 51 and extracts a signal having a low bandwidth of 100 kHz or less And may be a configured low pass filter (LPF).

The first signal processor 53 generates a voltage signal from the signal extracted by the signal extractor 52 and may be an RMS-to-DC converter and generates a DC voltage signal .

The second signal processor 54 may detect an RF power from a detection signal amplified to a predetermined magnitude by the amplifier 51 to generate a DC voltage signal and may be an RF DC / DC converter.

The bias signal generator 55 generates a bias signal based on the DC voltage signal applied from the first and second signal processors 53 and 54 and applies the generated bias signal to the optical modulator 20 'of FIG.

That is, the bias signal generator 55 generates the first to third bias signals S_B1, S_B2, and S_B3 and applies the generated bias signals S_B1, S_B2, and S_B3 to the optical modulator 20 '.

The bias signal generator 55 applies an error detection signal to the optical modulator 20 'of FIG. 3 together with the bias signal. The bias signal generator 55 generates a bias signal, For example, an AC signal having a frequency on the order of 8 kHz.

In generating a bias signal, the bias signal generator 55 uses a feedback error detection signal to generate a bias signal, and generates a bias signal such that the value of the feedback error detection signal is minimized.

The bias signal generator 55 generates first and second bias signals based on the DC voltage signal applied from the first signal processor 53 and generates a bias signal based on the DC voltage applied from the second signal processor 54 And generates a third bias signal based on the signal.

The bias signal generator 55 sequentially generates the first and second bias signals. When generating the first bias signal, the bias signal generator 55 combines an error detection signal with only the first bias signal, The bias signal is fixed to a predetermined DC voltage value.

Conversely, when generating the second bias signal, the bias signal generator 55 combines an error detection signal with only the second bias signal, and fixes the first bias signal with a predetermined DC voltage value.

That is, the bias signal generator 55 combines the error detection signals with the first and second bias signals at different times.

6, the bias signal generator 55 compares the values of the error detection signals coupled to the first and second biases to each other, and outputs a bias signal .

Further, as shown in FIG. 7, it can be seen that the phase of Bias_90 is 90 when the RF output is minimum. To this end, the bias signal generator 55 generates the third bias signal so that the value of the DC voltage signal applied from the second signal processor 54 is minimized.

Meanwhile, recently, an IQ-optical modulator which is widely used in an optical transmitter is a DP (Dual Polarization) -IQ optical modulator for utilizing both polarizations of an optical signal.

8 is a diagram illustrating an example of applying a bias control apparatus for an optical modulator for an optical transmitter according to an embodiment of the present invention to a DP (Dual Polarization) -IQ optical modulator. The DP- And two optical modulators 20 'are arranged in parallel, thus including four modulators.

At this time, one optical modulator 20'-1 receives I data (D_Ix) and Q data (D_Qx) for X-polarized light and the other optical modulator 20'- I data (D_Iy) and Q data (D_Qy).

The optical detectors 32'-1 and 32'-2 are respectively provided in the two optical modulators 20'-1 and 20'-2. The optical detectors 32'-1 and 32'- 2 provides the detection signal S_PDx for the output optical signal output from the optical modulator 20'-2 to the bias signal output unit 31 ', and the optical detection unit 32'- And provides the detection signal S_PDy for the optical signal to the bias signal output unit 31 '.

The bias signal output unit 31'generates the bias signals S_B1x, S_B2x and S_B3x based on the detection signal S_PDx and generates the bias signals S_B1y, S_B2y, S_B2x, S_B2y based on the detection signal S_PDy, S_B3y), and applies the generated bias signals to the optical modulators 20'-1 and 20'-2.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, And various alternatives, modifications, and alterations can be made within a range.

Therefore, the embodiments described in the present invention and the accompanying drawings are intended to illustrate rather than limit the technical spirit of the present invention, and the scope of the technical idea of the present invention is not limited by these embodiments and accompanying drawings . The scope of protection of the present invention should be construed according to the claims, and all technical ideas within the scope of equivalents should be interpreted as being included in the scope of the present invention.

20, 20 ', 20'-1, 20'-2: optical modulator
21, 22: Modulation section
23:
30: bias control device
31, 31 ', 50: bias signal output section
32, 32'-1, and 32'-2:
51:
52:
53: first signal processor
54: second signal processor
55: bias signal generator

Claims (1)

A bias control apparatus for applying a bias signal to an optical modulator for an optical transmitter,
A photodetector for detecting an output optical signal output from the optical modulator and providing a detection signal; And
And a bias signal output unit for generating a bias signal based on the detection signal provided from the optical detection unit and coupling the error detection signal to the generated bias signal and applying the bias signal combined with the error detection signal to the optical modulator
A bias control apparatus for an optical modulator for an optical transmitter.

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