JPH0883434A - Bias adjusting device of eo modulator - Google Patents

Abstract

PURPOSE: To obtain a stable modulated light signal by preparing a light different from an input light to be modulated by an EO modulator and by inputting the signal having the phase difference of a 1/4 wavelength with the input light to be originally modulated when it is used as a second input light. CONSTITUTION: A light 21 separated by a beam splitter 19 advances toward a 1/4 frequency plate 24 via mirrors 22, 23 and a second input light 25 transmitted through the 1/4 frequency plate is inputted to a modulator 3 in the direction opposite to that of the input light 20 and then is separated by a polarizing beam splitter 28 after being reflected by the beam splitter 19. When a header signal is made to be a modulation signal, the modulation characteristics curve of the EO modulator becomes a deviated curve with respect to impression voltages by inputting the light (the second input light 25) being different from the input light to be originally modulated to the modulator 3 and shifting the phase by the 1/4 wavelength. When a feedback control is performed at the bias point of 50% of the mean value of a modulation light signal strength with respect to the second input light, a control is made possible at the bias point of 0% of the mean value of a modulated light strength with respect to the input light 20 to be originally modulated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides an electro-optical effect suitable for modulating a pulse signal, particularly for manufacturing an optical disk master for recording header signals such as track numbers and sector numbers in an optical disk master manufacturing apparatus. The present invention relates to an optical modulator used.

[0002]

2. Description of the Related Art In particular, an optical disk master manufacturing apparatus is given as an example in which pulse signal modulation is required. An optical disk master is manufactured by irradiating a substrate coated with a photoresist with laser modulated light to form an optically detectable form, for example, a pit row, and recording by laser light is an optical disk master manufacturing. It is performed using the device. This device usually has an optical modulator that modulates laser light based on the input signal of the formatter. As such an optical modulator, one using an electro-optic effect (EO optical modulator) and one using ultrasonic light deflection (AO optical modulator)
There is. Among them, the EO type optical modulator is an electro-optical modulator (E-optical modulator) that uses an electro-optical effect to convert an electric signal into an optical signal.
O element). This EO optical modulator has a wide band and power resistance, but on the other hand, it has a very large drift and generally exhibits temperature dependence. Therefore, in order to prevent the modulation degree of the laser light from changing due to the temperature change, the auto bias control for automatically adjusting the bias input to the modulator is connected to the modulator. A conventional optical modulator will be described below with reference to the drawings.

FIG. 2 is a diagram showing the modulation characteristics of an EO modulator, in which the vertical axis represents the relative intensity of transmitted light and the horizontal axis represents the applied voltage at an arbitrary bias point. The signal waveform on the right side of the figure shows the waveform of the modulated light intensity signal io when modulated by the modulation signal Ei. The output light intensity with respect to the bias voltage Eb is B1, and the output light intensity with respect to the applied voltage (Eb + ei) is (B1 + bi). Therefore, when the sinusoidal modulation signal Ei is superimposed on the bias voltage Eb,
The output light intensity changes between A1 and C1, and the modulated light intensity signal io is obtained. The linearly polarized light incident on the modulator becomes elliptically polarized light due to the phase difference proportional to the voltage applied to the EO crystal element. The relative intensity of the light becomes the sine-square characteristic of the applied voltage by extracting the linearly polarized light component of this light with the polarization beam splitter. Since the modulation characteristic IM of the EO modulator has a sine-square characteristic, if the bias point Eb is offset, the modulated light intensity signal i becomes a distorted waveform signal and the average value level of the modulated light signal i is B1 = 50%. When is the best.

This characteristic is applied to a conventional bias adjusting device, and a conventional optical modulator equipped with the conventional bias adjusting device will be described with reference to FIG.

FIG. 3 is a block diagram of a conventional EO optical modulator. In the figure, 1 is a modulation amplifier for supplying the modulation signal from the input terminal 2 to the optical modulator 3, and 5 is an E of the modulator 3.
Output light elliptically polarized by applying an electric field to the O crystal element, 4 is P polarization of the output light 5 from the modulator 3, a polarization beam splitter for separating S polarization components, and 6 is an output separated from the polarization beam splitter 4. A photodetector that detects the S-polarized component 16 of the light 5 and converts it into an electrical signal, and 7 is a photodetector 6
An output amplifier for amplifying the output signal of the output amplifier 8, an average value circuit 8 for outputting a signal b of the average value level of the output a of the output amplifier 7,
Reference numeral 9 is a beam splitter for extracting a part of the light 18 of the output light 10 consisting of the linearly polarized light component transmitted through the polarization beam splitter 4, 11 is light for detecting the output light 10 from the beam splitter 9 and converting it into an electric signal. A detector, 12 is an output amplifier that amplifies the output signal of the photodetector 11 to the same value as the output a of the output amplifier 7, and 13 is an output amplifier 12
An average value circuit for outputting a signal d of the average value level of the output c, 14 is a differential amplifier having two inputs of the output b of the average value circuit 8 and the output d of the average value circuit 13, and 15 is a differential It is a bias amplifier that amplifies the output of the amplifier 14 and supplies it to the modulator 3 as a bias voltage. This circuit is all DC coupled.

Next, the operation of this circuit will be described.
The output light 5 that has been elliptically polarized by the modulator 3 is separated into P-polarized and S-polarized linear components by the polarization beam splitter 4, the reflected S-polarized component is directed to the photodetector 6, and the P-polarized component is the beam. Head towards splitter 9. At this time, the output light 10 separated by the polarization beam splitter 4
The ratio of the light amounts of the reflected light 16 and the reflected light 16 is 1: 1.
The output light 10 is split by the beam splitter 9, a part of which is directed to the photodetector 11, and most of the other output light 17 is output.
Is the output of the entire modulator.

The photodetector 6 detects the output light 16, converts it into an electric signal, and outputs it to the output amplifier 7. The output amplifier 7 amplifies the input signal and outputs the output a to the average value circuit 8. The output light 16 is a signal that has been modulated when passing through the modulator 3, and the average value circuit 8 outputs the signal b of the average value level of this signal to one input of the differential amplifier 14. on the other hand,
The photodetector 11 detects the output light 18, converts it into an electric signal, and outputs it to the output amplifier 12. The output amplifier 12 corrects the split light intensity ratio of the beam splitter 9, outputs a signal corresponding to the light intensity of the output light 10, and outputs its output c to the average value circuit 13. The output light 10 is a signal having a linear polarization component that has passed through the polarization beam splitter 4, and the average value circuit 13 outputs a signal d having an average value level of this signal.
This output signal d becomes the other input signal of the differential amplifier 14. The differential amplifier 14 amplifies a difference signal between the output signal b of the average value circuit 8 and the output signal d of the average value circuit 13 and outputs the amplified signal to the bias amplifier 15. The bias amplifier 15 amplifies the output signal of the differential amplifier 14 to a required level and supplies a bias voltage to the modulator 3. The output light 17 has a light intensity in FIG.

The modulated signal is input to the input terminal 2, amplified by the modulation amplifier 1, and supplied to the modulator 3. Further, the bias signal and the modulation signal are simultaneously added to the modulator 3.
The average value of the modulation signal from the input terminal 2 is 0 and is applied around the bias point Eb of the output of the bias amplifier 15. Separated light 16 and transmitted light 10 of the polarization beam splitter 4
The sum of the light quantities of is the same as that of the output light 5. That is, separated light 1
If the bias point Eb is feedback-controlled so that the output forces of the light amount signal b of 6 and the signal d of which the gain is corrected to correspond to the light amount of the output light 10 become equal, the average value level of the modulated optical signal i becomes B1 = It will be 50%.

Therefore, in the optical modulator 3, the output light 17 is obtained by performing the modulation according to the modulation signal from the input terminal 2 with the bias point according to the difference signal of the output of the differential amplifier 14 as the center.

Here, as shown in FIG. 2, the average value level B1 of the modulated light intensity signal when an appropriate bias is supplied.
Is half of the maximum output light intensity. This is A1 = 1
When 00% and C1 = 0%, it is apparent from the sine-square curve, and the bias control is performed based on this principle.

In this way, the difference signal between the average value signal b representing the proper bias level and the comparison signal d representing the modulation state is generated by the differential amplifier 14 and supplied to the modulator 3 via the bias amplifier 15. By doing so, it is possible to control the drift of the bias point due to, for example, temperature change. As a result, it is possible to always obtain a stable modulated optical signal.

[0012]

In the above conventional method,
Even if the input signal is a pulse waveform having binary values of 0 and 1, the average value is the center of its amplitude (0.
Output is 0% and 100% if it is 5V, 0V if ± 1V)
There was no problem as it became a binary level pulse of. However, when the pulse duty changes, the average value does not become the center of the amplitude. When the header signal is input as a modulation signal to the block diagram of the conventional optical modulator shown in FIG. 3, when the duty of the input signal is low, the signal d representing the intensity of the output light 10 becomes low, and conversely the intensity of the separated light 16 becomes high. The signal b that represents As a result, the bias point shifts to the right as shown in FIG. That is, although the input is "0", light is output and the level of "1" is also distorted. If the gain ratio of the signals b and d is adjusted according to the duty of the signal, it can be controlled if the average value level of the modulated optical signal i is in the range of B1 = 20 to 80%, but if the duty is lower, the bias point becomes Even if it fluctuates, there is a problem that the output of the differential amplifier 14 is small and stable control cannot be performed.

When an EO modulator is used to modulate an instantaneous pulse output such as a header signal such as a track number or a sector number of a recording area on an optical disk,
Since the waveform duty ratio is such that the average value of the output of the header signal is 20% or less, a stable modulated signal cannot be obtained. The signal level of the signal b which is the input signal of the differential amplifier 14 represents the half of the output light 5, and the signal level of the signal d which is the other input signal represents the average value of the output light 10. It is a signal, and the level of the signal b is clearly high, and the differential amplifier 14 amplifies the difference signal. In FIG. 4 showing the modulation characteristics when the header signal is used as the modulation signal, the modulated signal i ′ is output centered on the bias level Eb ′ according to the difference signal. At this time, the modulated signal i ′ is output according to the bias level. Since the average value level changes, when the bias level shifts to the higher side, the average value level of the modulated signal also increases and i ′ becomes a distorted waveform signal. Also, when the bias level shifts to the lower side, the average value level of the modulated signal becomes low and i'becomes a distorted waveform signal.

An object of the present invention is to provide an optical modulator which can modulate a digital signal such as the header signal of an optical disc by applying the modulation characteristic of the conventional EO type modulator. Especially.

[0015]

Means for Solving the Problems The above-mentioned object is that when the input light originally intended to be modulated by the EO modulator is the first input light, a light different from the first input light is inputted to the modulator, This is achieved by inserting a ¼ wavelength plate for the second input light before inputting this to the modulator as the second input light. The light sources of the first and second input lights may be the same or different light sources.

[0016]

In the present invention, when a light different from the input light modulated by the EO modulator is prepared and this light is used as the second input light, a phase difference of ¼ wavelength from the input light originally modulated is produced. By inputting with the input, it is 1 / from the origin on the modulation characteristics of the EO modulator, that is, the normal proper bias point for the input light.
The point that is shifted by four wavelengths is the central operating point. The optical disk header signal has an average output value of 20% or less or 80
Since the duty ratio of the waveform is not less than%, 0 to 20%, on the contrary, corresponds to 50 to 80% for the second input light having a phase difference, so that stable control can be performed.

[0017]

EXAMPLES The present invention will be described below with reference to examples.

FIG. 1 shows a block diagram of an optical modulator according to an embodiment of the present invention. In the figure, 1 is a modulation amplifier for supplying the modulation signal from the input terminal 2 to the optical modulator 3, and 19
Is a beam splitter for extracting a part of light 21 of the first input light 27 to the modulator 3, and 5 is elliptically polarized output light by applying an electric field to the EO crystal element in the modulator 3, 2
Reference numerals 2 and 23 are mirrors for reflecting the partial light 21 extracted from the beam splitter 19, 24 is a wave plate for shifting the phase of the light 21 reflected by the mirror 23 by a quarter wavelength, and 25 is an input light 20. Is the second input light which is out of phase by 1/4 wavelength and is input to the modulator 3 from the opposite direction,
Reference numeral 26 designates a second light transmitting element for transmitting the output light of the input light 20 to the modulator 3.
, A P-polarization of the light reflected by the beam splitter 19 and transmitted through the modulator 3, and a polarization beam splitter for separating the S-polarization component, and 6 is an S-polarization component separated from the polarization beam splitter 28. A photodetector for detecting an output light 29 having a light and converting it into an electric signal, 7
Is an output amplifier for amplifying the output signal of the photodetector 6, 8 is an average value circuit for outputting a signal b of the average value level of the output a of the output amplifier 7, and 11 is a linearly polarized light component transmitted through the polarization beam splitter 28. A photodetector that detects the output light 30 and converts it into an electric signal, 12 is an output amplifier that amplifies the output signal of the photodetector 11, and 13 is an average value that outputs a signal d of the average value level of the output c of the output amplifier 12. A circuit, 14 is a differential amplifier which receives the output b of the average value circuit 8 and the output d of the average value circuit 13 as two inputs, and 15 amplifies the output of the differential amplifier 14 and outputs it to the modulator 3 as a bias voltage. It is a bias amplifier to supply. The polarization beam splitter uses a Wollaston prism, a Glan-Thompson prism, which utilizes birefringence, and a device in which thin films are multilayer-deposited so as to satisfy the Brewster condition. The photodetector uses a photodiode.

Next, the operation of this optical modulator will be described with reference to FIG. 5 showing signal waveforms. The light 21 separated by the beam splitter 19 goes through mirrors 22 and 23 to 1 /
Heading to the four-wave plate 24. The second input light 25 transmitted through the quarter-wave plate enters the modulator 3 in the opposite direction to the input light 20, is reflected by the beam splitter 19, and is then separated by the polarization beam splitter 28. At this time, the ratio of the amounts of the transmitted light 30 and the reflected light 29 separated by the polarization beam splitter 28 is 1: 1. As shown in FIG. 5, EO modulation is performed when a header signal is used as a modulation signal by inputting light (second input light 25) different from the input light that is originally modulated into the modulator 3 and shifting it by ¼ wavelength. In terms of the modulation characteristic of the device, the characteristic curve deviates from the applied voltage.
If feedback control is performed on the second input light at the bias point of the average value of the modulated light intensity signal of 50%, for the input light 20 to be originally modulated, at the bias point of the average value of the modulated light intensity of 0%. You will have control. By taking the center operating point at the point where the 0 level of the header signal deviates from the conventional proper bias level Eb, that is, the point D, the input light 20 to the modulator 3 can hold the bias point, and the header signal Such digital signals can be modulated. The bias voltage range is ± 300V, and the modulation voltage range is 0 to 80V.

As another embodiment, the second input light 25
Is the same light source as the light 20 originally intended to be modulated,
Instead, another light source having a different wavelength is provided and the light is used as the second input light. As a result, the same effect as shifting the phase can be obtained, and a digital signal such as the header signal can be modulated.

FIG. 6 shows another embodiment of the present invention. First
In the figure, the second input light is the same light source as the light to be originally modulated, but here the light 32 from the light source 31 having a different wavelength is used as the second input light to
It has the effect of shifting the phase, similar to a four-wave plate.
33 is a color separation mirror that reflects the input light 32 and transmits the output light 5, 34 is a color separation mirror that reflects the input light 32 and transmits the input light 27, and 35 is light reflected by the mirror 34 and transmitted through the modulator 3. Of the P-polarized beam and the S-polarized component of
11 is a polarization beam splitter 35 that detects the output light having the S-polarized component separated from
A photodetector that detects the output light consisting of the linearly polarized light component that has passed through and converts it into an electrical signal. This allows the EO modulator to modulate a digital signal such as a header signal,
Moreover, the number of optical components can be reduced.

FIG. 7 shows another embodiment of the present invention. In the above embodiments, the second input light is transmitted through the modulator 3 from the opposite direction to the light originally intended to be modulated.
The light 32 from the light source 31 having a different wavelength is input as the second input light from the same direction as the input light 20 to obtain the same effect as shifting the phase. 33 is the input light 32
And a color separation mirror that reflects the input light 27 and transmits the input light 27. Reference numeral 34 reflects the output of the input light 32 that has passed through the modulator 3 and the output light 5
, 35 is a color separation mirror, 35 is P-polarized light of the light reflected by the mirror 34 and transmitted through the modulator 3, a polarization beam splitter for separating S-polarized light component, and 6 is an output having S-polarized light component separated from the polarization beam splitter 35. A photodetector that detects light and converts it into an electric signal, and 11 is a photodetector that detects output light composed of a linearly polarized light component that has passed through the polarization beam splitter 35 and converts it into an electric signal. In this way, if the light from another light source having a different wavelength is used, the digital signal such as the header signal can be modulated by the EO modulator even in the same input direction as the light to be originally modulated.

[0023]

As described above, according to the present invention, the modulated signal is an instantaneous pulse output like the header signal of the optical disk,
It has become possible to modulate a digital signal having a large waveform duty and an average output value of 20% or less or 80% or more by the EO modulator. It is particularly suitable for producing a high-precision optical disk original plate.

[Brief description of drawings]

FIG. 1 is a block diagram of an EO optical modulator according to an embodiment of the present invention.

FIG. 2 is a diagram showing the modulation characteristics of an optical modulator utilizing the electro-optical effect, and the waveforms of a modulation signal and a modulated light intensity signal when a proper bias is applied.

FIG. 3 is a block diagram of a conventional optical modulator.

FIG. 4 shows a configuration of a conventional optical modulator device,
It is a figure which shows the modulation | alteration characteristic when the header signal of an optical disk is made into a modulation signal, the waveform of a modulation signal at the time of a proper bias, and a modulated light intensity signal.

FIG. 5 is a modulation characteristic and a modulation signal of the optical modulator in the configuration of the EO optical modulator according to the embodiment of the present invention,
It is a figure which shows the waveform of a modulated light intensity signal.

FIG. 6 is a block diagram of an optical modulator according to another embodiment of the present invention.

FIG. 7 is a block diagram of an optical modulator according to another embodiment of the present invention.

[Explanation of symbols]

 2 Input terminal for input modulation signal 3 Optical modulator 4 Polarization beam splitter 6, 11 Photodetector 14 Differential amplifier 15 Bias amplifier 24 1/4 wavelength plate 31 Second light source 33, 34 Color separation mirror

Claims (2)

[Claims] 1. A bias adjusting device for an EO modulator, which comprises an EO optical modulator that records information by using a laser beam and uses an electro-optical effect, and a bias control that automatically adjusts a bias input to the modulator. In the above, when the input light originally intended to be modulated by the EO modulator is the first input light,
Light different from the input light is input to the EO modulator, and this is used as the second input light.
A bias adjusting device for an EO modulator, which outputs a modulated signal by inserting a four-wave plate. 2. The bias adjusting device for an EO modulator according to claim 1, wherein the light sources for the first and second input lights are different light sources.