KR100277716B1 - Bias Stabilizer of Optical Modulator Using Lowest Point of Output Optical Signal - Google Patents

Abstract

1. TECHNICAL FIELD OF THE INVENTION

The present invention relates to a bias stabilization device of an optical modulator.

2. The problem to be solved by the invention

The present invention provides a bias stabilization device for an optical modulator capable of adaptively and accurately controlling the bias by selectively extracting and measuring an off state of an output optical signal and minimizing the measured value. There is a purpose.

3. Summary of Solution to Invention

The present invention includes sampling and holding means for capturing, sampling and holding the lowest point of an output optical signal input from the outside according to a sampling trigger signal generated therein; Analog signal processing means for amplifying a signal output from the sampling and holding means and converting the signal into a digital signal; Count processing means for receiving an output signal of the analog signal processing means for up counting and down counting; Digital signal processing means for converting an output signal of the count processing means into an analog signal; And signal converting and amplifying means for converting an output signal of the digital signal processing means into a voltage signal and then amplifying and outputting the converted signal.

4. Important uses of the invention

The present invention is used to stabilize the bias of an optical modulator.

Description

Bias Stabilizer of Optical Modulator Using Lowest Point of Output Optical Signal

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bias stabilization device for an optical modulator, and more particularly, to a bias stabilization device for stabilizing output characteristics of an optical modulator by using a lowest point of an output optical signal having a burst mode traffic characteristic.

In general, external light modulators exhibit periodic output characteristics depending on the phase difference of both arms.

1A and 1B are characteristic diagrams illustrating output signal types of an optical modulator according to an initial bias operating point with respect to the same electrical input signal.

1A illustrates a case where an electrical signal is well converted to an optical signal.

However, as shown in FIG. 1B, when the operating point moves, the optical signal is distorted, making it difficult to distinguish the logical signal level (that is, the high bit and the low bit). The output signal level of the signal can be changed.

Therefore, it is necessary to adjust the operating point, and for this purpose, a separate bias electrode is generally installed in the optical modulator to adjust the operating point.

However, the operating point of the initial optical modulator determined by the DC bias voltage applied to the optical waveguide layer is changed in time due to the photorefraction effect of the medium and the DC drift due to the accumulation of space charge and the change of ambient temperature. Will change accordingly.

Therefore, in order to continuously maintain the same operating point as the initial stage, an apparatus for stabilizing an output optical signal that adaptively changes a bias voltage according to an output state of an optical modulator is required.

In the case of modulating an analog signal or digital transmission, signal distortion becomes a problem due to the periodic output characteristics of the optical modulator. Therefore, the operating point is fixed to the point where the linearity of the output characteristic curve of the optical modulator is the best. In such a case, when a small shake signal is usually placed on the bias electrode, or a signal to be modulated is modulated into a small shake signal of low frequency, the second harmonic of the optical signal modulated by the shake signal has the smallest linearity. Since this is the best point, that is, the relative phase difference becomes π / 2, the method using this is generally used.

However, when the electro-optic modulation system is to be used as the photogate, it is important that the ON and OFF states of the output light maintain the maximum extinction ratio. That is, the off state should be the minimum light modulator transmission state. Therefore, for the photogate application, when the electrical input signal is off, the bias voltage of the optical modulator should be adjusted to the lowest point of the optical modulator output. To this end, when the shake signal method as shown in FIG. 2 is used, since the fundamental frequency component is the smallest at the lowest point of the transmission curve, the bias voltage may be controlled by a method of minimizing the basic shake frequency component of the output light. However, in this case, the traffic characteristics of the input signal are uniformly defined (scrambled data) or uniformly defined. If the input signals exhibit burst characteristics such as being continuously on or off, the output is stabilized. Because the size of the obstacle signal for the system depends on the traffic characteristics, it is impossible to control the optical modulator. As described above, the reason why the conventional shaking method is difficult to adaptively control according to the traffic characteristics is that the fault signal is extracted in both the on and off states of the output light.

FIG. 2 is a block diagram illustrating a stabilization apparatus for an optical modulator using a conventional dithering method, wherein a shake signal generator 211, a band pass filter 212, a frequency multiplier 213, and a phase controller ( 214, lock-in amplifier 215, low pass filter 216, integrator 217, direct current (DC) bias supply unit 218, adder 219 And a low noise Mach-Zender (LN-MZ) optical modulator 220, an optocoupler 221, and a low noise amplifier 222.

Referring to the stabilization device of the optical modulator by the conventional shake method having the structure as described above is as follows.

A small electric dithering signal of 1 to 100 kHz is applied to the input optical signal to extract the fundamental or second harmonic signal of the modulated shake signal and use it as a disturbance signal. DC bias voltage is controlled through control.

If the second harmonic signal of the shake signal is extracted and used as a disturbance signal, the disturbance signal value becomes '0' at the point where the linearity is best, and the addition of the disturbance signal changes around this point.

Therefore, if only the reference voltage of the comparison circuit is adjusted according to the sign of the output signal of the lock-in amplifier 215, the position of the operating point can be stabilized at the point where the linearity is maximum.

However, in the case of the stabilization device of the conventional optical modulation by the conventional shake method as described above, when the input signal exhibits a burst characteristic, such as when the input signal is continuously on or off, the failure signal for output stabilization Since the size varies depending on the traffic characteristics, there is a problem in that the adaptive control of the optical modulator is impossible.

FIG. 3 is a block diagram illustrating a bias stabilization device for an optical modulator using a conventional input / output comparison method, and includes a resistor diode 311, an optical modulator 312, an optical coupler 313, a photodiode 314, and the like. Low pass filters 315 and 317, modulated signal generator 316, reference voltage generator 318, differential amplifier 319, preamplifiers 320 and 321, and proportional integrator 322 and an optical modulator driver 323.

Referring to the bias stabilizer of the optical modulator using the conventional input-output comparison method having the structure as described above is as follows.

Filtering a portion of the optical modulator 312 input electrical signal using the low pass filter 317 can obtain a reference voltage proportional to the amount of traffic in the signal. In addition, if the optical signal output of the optical modulator 312 is detected by the photodiode 314 and then filtered using the low pass filter 315, a direct current voltage proportional to the traffic amount of the output optical signal can be obtained. . When the output signal of the optical modulator 312 is modulated in the same manner as the input electrical signal, the value of the DC voltage and the electrical reference voltage of the output of the optical modulator 312 should be equal.

Therefore, when the difference between the reference voltage of the electrical signal and the voltage of the output optical signal is used as a failure signal, an output optical signal modulated in the same manner as the input electrical signal can be obtained.

However, since the bias stabilization device of the optical modulator using the conventional input / output comparison method as described above uses a fault signal proportional to the absolute value of the DC voltage, stability against the bias of the optical modulator can be maintained due to the ambient temperature. There was no problem.

Accordingly, the present invention has been made to solve the above problems, by selectively extracting and measuring the off state of the output optical signal having the burst mode traffic (characteristics) and minimize the measured value to control the bias Accordingly, an object of the present invention is to provide a bias stabilization device for an optical modulator capable of accurately and stably controlling the adaptive bias regardless of the traffic characteristics of the output optical signal.

1A and 1B are characteristic diagrams showing the output signal shape of an optical modulator according to an initial bias operating point for the same electrical input signal.

2 is a block diagram illustrating a stabilization device of an optical modulator by a conventional dithering method.

3 is a block diagram illustrating a bias stabilization device of an optical modulator using a conventional input / output comparison method.

4 is a timing diagram for generating a sample triggering signal source required to selectively extract only an off state of an output optical signal by a sample and hold method applied to the present invention. .

5 is a characteristic diagram for explaining the lowest bias holding of an optical signal input to an optical modulator used in the present invention.

Figure 6 is a block diagram of an embodiment of a bias stabilization device of an optical modulator using the lowest point of the output optical signal according to the present invention.

Explanation of symbols on the main parts of the drawings

610: sampling and holding unit 611: sampling trigger signal generator

612: preamplifier 612: sampling and holding machine

620: analog signal processing unit 621: filtering unit

622: gain control unit 623: A / D conversion unit

630: count processor 631: signal driver

632: up-down count control unit 633: system clock generation unit

634: processor 640: digital signal processing unit

641: latch portion 642: D / A conversion portion

650: signal conversion and amplification unit 651: current / voltage conversion unit

652: gain control unit

The present invention for achieving the above object is, in the bias stabilization device for stabilizing the bias of the optical modulator, according to the sampling trigger signal generated inside, capture the lowest point of the output optical signal input from the outside Sampling and holding means for sampling and holding; Reference voltage generating means for generating a reference voltage; Analog signal processing means for amplifying the analog signal output from the sampling and holding means by filtering to remove the obstacle, and converting the analog signal into a digital signal according to the reference voltage; Count processing means for receiving an output signal of the analog signal processing means and outputting a counted signal by counting up and down counting; Digital signal processing means for latching the digital signal counted by the count processing means and converting the digital signal into an analog signal according to the reference voltage; And signal conversion and amplifying means for converting the output signal of the digital signal processing means into a voltage signal and then amplifying and outputting the converted signal.

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to FIGS. 4 and 6.

4 is a timing diagram for generating a sample triggering signal source required to selectively extract only an off state of an output optical signal by a sample and hold method applied to the present invention. As an example, first, the inverse data (/ DATA) is taken from the electrical input data (DATA Electrical), and then, through an AND operation with the system clock signal, a sample trigger signal (/ DATA) for extracting an off state value of the output light is obtained. & CLK) is generated. This sample trigger signal is used to capture the on and off points of the output optical signal.

5 is a characteristic diagram for explaining the lowest bias holding of an optical signal input to an optical modulator used in the present invention.

Referring to Fig. 5, in order to track the minimum point of the off-state light intensity of the output optical signal, the output light input signal level A previously entered at any operating point of the output light signal and the currently incoming output light input signal level Comparing (B), in the rising section of the output light signal, counting in the downward direction when A <B (DOWN count mode), counting in the upward direction when A> B (UP count mode), and A = In the B or rising period of the output light signal, the external bias value is adaptively adjusted and stabilized so as to maintain the operation minimum near the zero point of the output light signal at all times.

FIG. 6 is a block diagram of an embodiment of a bias stabilization device of an optical modulator using a sample and hold method according to the present invention.

As shown in FIG. 6, the bias stabilization apparatus of the optical modulator according to the present invention captures the lowest point of an output optical signal input from the outside according to a sampling trigger signal generated therein, and performs sampling and holding. Amplifying the sampling and holding unit 610, the reference voltage generator 617 for generating the reference voltage, and the analog signal output from the sampling and holding unit 610 by filtering to remove the obstacle. The analog signal processor 620 converts the digital signal according to the reference voltage provided from the reference voltage generator 617 and the output signal of the analog signal processor 620, and counts up or down. A count processor 630 for counting and outputting a counted signal, and a digital signal counted by the count processor 630, and latching the latched digital signal according to the reference voltage provided from the reference voltage generator 617. And a digital signal processing unit 640 and, after converting the output signal of the digital signal processor 640 into a voltage signal, amplifies the signal to output converting and amplifying unit 650 to convert the analog signal.

The sampling and holding unit 610 includes a sampling trigger signal generator 611 and a polarization detector 601 for providing a sampling trigger signal to be used for capturing the lowest point of the output optical signal transmitted from the external polarization detector 601. By the preamplifier 612 for amplifying the gain of the output optical signal transmitted from the &lt; RTI ID = 0.0 &gt;) &lt; / RTI &gt; and at the rising edge of the sampling trigger signal provided by the sampling trigger signal generator 611. A sampling and holding unit 613 is provided for capturing, sampling, and holding the lowest point of the amplified output optical signal.

The analog signal processor 620 may filter an output signal of the sampling and holding unit 613 of the sampling and holding unit 610 to remove an obstacle, and amplify the output signal of the filtering unit 621. A gain control unit 622 for adjusting the gain, and an A / D converter 623 for converting the analog signal output from the gain control unit 622 into a digital signal.

The count processor 630 inputs a signal driver 631 for converting and driving the output signal of the analog signal processor 620 into a signal suitable for counting, and a digital signal transmitted through the signal driver 631. An up-down count control unit 632 for controlling the up count and down count, a system clock generator 633 for generating a system clock, and a digital signal input from the signal driver 631. And a processor 634 for outputting a counted up or down counted signal under the control of the system clock and up-down count control unit 632 transferred from the system clock generator 633.

The digital signal processing unit 640 includes a latch unit 641 for latching the digital signal counted by the count processing unit 630 and a latch unit 641 in accordance with the reference voltage transmitted from the reference voltage generator 617. And a D / A conversion section 642 for converting the digital signal latched by the &lt; RTI ID = 0.0 &gt;

The signal converter and amplifier 650 is configured to convert the current signal output from the digital signal processor 640 into a voltage signal, and outputs the output signal of the current / voltage converter 651 and the current / voltage converter 651. A gain adjusting unit 652 for amplifying and adjusting gain is provided.

Referring to the operation of the bias stabilization device of the optical modulator of the present invention having the structure as described above in detail as follows.

The sampling trigger signal generator 611 obtains the inverse data signal / DATA from the electrical input data signal DATA as shown in FIG. 4, and the system clock signal and logic for synchronizing the packet data signal. Through the multiplication operation, an ECL (Emitter coupled logic) trigger signal for capturing the off state of the output optical signal of the polarization detector (PD) 601 is provided to the sampling and holding unit 630.

The preamplifier 612 amplifies the gain of the micro current input from the 20 Mb / s polarization detector 601 that is impedance matched to 50 Ω at the input terminal to a high signal-to-noise ratio (S / N) of about 20 dB. Transfer to sampling and holding machine 613.

The sampling and holding unit 613 functions as an analog memory for storing the lowest point of the final input signal level within an arbitrary time interval, and has a polarization of 20 MHz to 200 MHz amplified to 20 dB by the preamplifier 612. The detection signal is triggered at the rising edge of the sample triggering signal provided from the sampling trigger signal generator 611 to capture the lowest point of the output light data, and the signal voltage level is set to an internal capacitor. Store continuously (not shown).

For example, the sampling and holding unit 613 has an acquisition time of 16 ns for sample and hold for a tracking bandwidth of up to 250 MHz, and updates the memory contents every 6 ms. ), Which uses AD9100 (commercially available).

Filtering unit 621, the output signal of the high-speed sampling and holding machine 613 in the form of a pulse signal almost close to direct current, primarily through an internal low pass filter (not shown) consisting of a capacitor and a resistor The ripple is removed by filtering, and the ripple is removed. Secondly, it is filtered through an inverted multiple feedback low pass filter provided therein to remove the common mode error more precisely. 622).

In this way, the signal filtered by the filtering unit 621 is amplified and adjusted to an appropriate level by the gain adjusting unit 622 is transferred to the A / D converter 623. In this case, the gain adjusting unit 622 amplifies the gain of the signal output from the filtering unit 621 through an amplifier provided inside the non-inverting mode.

The A / D converter 623 treats the sequence of the sequence generated internally at the rising edge of the clock as a digital output, and the order determined at one clock time is a 1-bit value. To be printed. That is, the A / D converter 623 converts the analog signal output from the gain adjuster 623 into a digital signal according to the reference voltage generated from the reference voltage generator 617 and outputs the digital signal to the signal driver 618. do.

The A / D conversion unit performing the conversion operation will be described in detail.

The A / D converter 623 outputs data through a delay time controlled by the control at the rising edge of the next clock through the encoding and latching functions. At this time, the A / D converter 623 is 20 Mb. In order to operate at / s, one clock is 50 ns, so there must be a margin (i.e. duty cycle of 25 ns or less) within the marginal width (i.e. the duty ratio is 50%). The output is normal. Here, the A / D converter 623 counts 256 times by counting once every 40 ns when using a 25 MHz clock, and it takes 10 μs to count the full scale.

In addition, since the A / D converter 623 has a maximum conversion time of 40 μs in the case of DC, at least 1/40 μs to 25,000 times / s can be converted continuously, and when operating at ± 6 V, 1 bit The resolution is 50 mV (ie 12 x (1/256) = 0.05 V). If the clock is stopped during A / D conversion, the A / D converter 623 maintains the input value at the time of stopping, which is stored as final data unless power supply is interrupted.

The reference voltage generator 617 provides a reference voltage to the A / D converter 623 and the D / A converter 642. The reference voltage generator 617 inverts the converted output voltage value therein. An amplifier (not shown) includes a function of adjusting a reference voltage value supplied to an external optical modulator by adjusting to an appropriate amplification factor of several times. In addition, the reference voltage generator 617 may improve accuracy and reliability by using an ultra low noise precision high speed operational amplifier for gain control and zero adjustment.

The signal driver 631 is for driving an address and a control signal of a data bus. When a digital signal converted by the A / D converter 623 is input, the signal driver 631 receives an address through the bus and receives a processor 634. The processor 634 to be in a read enable state, and when the processor 634 transfers data to the D / A converter 642, the processor 634 becomes write enable to the latch unit 641. Function. As an example, 74AC244 (commercially available) or the like may be used for the signal driver 631.

When the output optical signal value currently input through the processor 634 is greater than the input reference signal value previously input through the processor 634, the up-down count control unit 632 turns the output value of the comparator included therein to be low, thereby processing the processor 634. Is operated in the DOWN count mode, and if the output optical signal value currently input through the processor 634 is smaller than the input reference signal value previously input, the output value of the comparator provided therein is high. Until the input value is reached, the processor 634 controls to operate in the up count mode.

When the input data transmitted through the processor 634 becomes equal to the reference value, the output of the comparator included in the up-down count control unit 632 is output by using an internal switch to operate to maintain the state. By positioning the operating point of the light to the desired lowest point provides a function to set the initial operating point, after which it does not deviate significantly from the initial operating point. In addition, the up-down count control unit 632 up-counts or down-counts the address count by 8 bits, and when the count bit increases, the reference voltage is larger than the input data voltage, and the internal comparator is switched low to move in the downward direction. Processor 634 is controlled to count.

The processor 634 is controlled by the up-down count control unit 632 while transferring a signal transmitted from the signal driver 631 to the up-down count control unit 632 according to the system clock provided from the system clock generation unit 633. Up count or down count is output to the latch unit 641.

The processor 634 which performs such an operation will be described in detail.

The processor 634 first reads the 8-bit digital data transmitted through the signal driver 631 from the program counter and the stack pointer by internal software and resets the data to the internal memory at a time. After that, the incoming signal level is read and stored in the internal register. Then, the current input signal B and the previously entered data A are compared and counted in the positive direction. Whether to count in the direction is determined and controlled respectively in the falling curve and the rising curve of the signal as shown in FIG.

That is, the processor 634 controls the down count mode to decrease the output when A <B in the rising curve, and controls the up count mode to increase the output when A> B, or A = In the case of B, the current signal level is maintained in such a manner that the lowest point is continuously maintained near the zero point of the output light signal. On the other hand, the opposite logic holds in the downward curve. That is, this logic is easy to understand when it is noted that the data A has a '+' reference value in the rising curve of the output optical signal and the '-' reference value in the falling curve.

On the other hand, when data does not come in, the level finally entered is maintained and used as a comparison reference for the next signal level (about 100 ns pause time).

For example, the processor 634 may use ATEL's low-cost AT89C2051 (used product), which incorporates a 2 Kbyte flash memory for processing an algorithm for tracking the lowest point of the output optical signal.

The latch unit 641 latches data counted up or down by the processor 634 and is controlled by the processor 634 to output digital data to the D / A converter 642 at the rising edge of the next clock. do.

The D / A converter 642 converts the digital signal input from the latch unit 641 into an analog signal according to the reference voltage provided from the reference voltage generator 617, and transmits the digital signal to the current / voltage converter 651. . Here, the analog signal converted by the D / A converter 623 is a bias current value corresponding to the count value of the processor 634.

On the other hand, the D / A converter 642 uses the same clock timing element as the A / D converter 623.

The current / voltage converter 651 converts the current value transferred from the D / A converter 642 into a voltage value in accordance with the load condition through an inverting amplifier provided therein and outputs it to the gain controller 652.

The gain adjuster 652 amplifies the voltage transmitted from the current / voltage converter 651 to adjust the gain and outputs the gain. The gain is adjusted by the gain adjuster 652 and the output signal is biased by the optical modulator. Is input as.

Although the technical idea of the present invention has been described in detail according to the above preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

As described above, the present invention stabilizes the output characteristics of the optical modulator by using the lowest point of the output optical signal having the burst mode traffic characteristic, thereby adaptively independent of the traffic characteristics of the output optical signal. It is possible to control the bias accurately and stably, and to cope with the modulated signal in the burst mode, and to obtain the bias stabilization effect of the accuracy and the high speed, in particular, and to be widely applied in the optical exchange system as well as the transmission device. It has an effect.

Claims (7)

In the bias stabilizer for stabilizing the bias of the optical modulator, Sampling and holding means for capturing, sampling and holding the lowest point of an output optical signal input from the outside according to a sampling trigger signal generated therein; Reference voltage generating means for generating a reference voltage; Analog signal processing means for amplifying the analog signal output from the sampling and holding means by filtering to remove the obstacle, and converting the analog signal into a digital signal according to the reference voltage; Count processing means for receiving an output signal of the analog signal processing means and outputting a counted signal by counting up and down counting; Digital signal processing means for latching the digital signal counted by the count processing means and converting the digital signal into an analog signal according to the reference voltage; And Signal conversion and amplification means for converting the output signal of the digital signal processing means into a voltage signal and then amplified and output Bias stabilization device of the optical modulator using the lowest point of the output optical signal comprising a. The method of claim 1, The sampling and holding means, Sampling trigger signal generating means for providing the sampling trigger signal to be used to capture the lowest point of the output optical signal; Amplifying means for amplifying the gain of the output optical signal; And Sampling and holding unit for capturing, sampling and holding the lowest point of the output optical signal amplified by the amplifying means at the rising edge of the sampling trigger signal Bias stabilization device of the optical modulator using the lowest point of the output optical signal comprising a. The method of claim 1, The analog signal processing means, Filtering means for filtering an output signal of the sampling and holding means to remove an obstacle; First gain adjusting means for amplifying the output signal of the filtering means to adjust the gain; And Analog / digital converting means for converting the analog signal output from the first gain adjusting means into a digital signal Bias stabilization device of the optical modulator using the lowest point of the output optical signal comprising a. The method of claim 1, The count processing means, Signal driving means for converting and driving the output signal of the analog signal processing means into a signal suitable for counting; Up-down count control means for controlling the up count and down count by receiving a digital signal transmitted through the signal driving means; System clock generating means for generating a system clock; And A count processor which receives a digital signal from the signal driving means and outputs a counted up and down counted signal according to control of the system clock and the up-down count control means Bias stabilization device of the optical modulator using the lowest point of the output optical signal comprising a. The method of claim 1, The digital signal processing means, Latch means for latching a digital signal counted by said count processing means; And Digital / analog converting means for converting the digital signal latched by the latching means into an analog signal according to the reference voltage Bias stabilization device of the optical modulator using the lowest point of the output optical signal comprising a. The method according to any one of claims 1 to 5, And a signal output from the digital signal processing means is a current signal. The method of claim 6, The signal conversion and amplification means, Current / voltage conversion means for converting the current signal output from the digital signal processing means into a voltage signal; And Second gain adjusting means for amplifying the output signal of the current / voltage converting means to adjust the gain Bias stabilization device of the optical modulator using the lowest point of the output optical signal comprising a.