JPH0157329B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical power control device using an optical modulator that changes light transmittance by changing a bias voltage.

2. Description of the Related Art Recording devices that record image signals and the like on a recording medium at high density using a light source such as a laser have been considered. To perform this recording, it is necessary to modulate the optical power of a light source such as a laser at high speed according to the recording signal, and for this purpose, an optical modulator is generally used.

Optical modulators include those that use acousto-optic effects,
There are various methods, including those that use changes in the refractive index of ADP crystals due to electro-optic effects. In terms of the uniformity of the transmitted light beam and the height of the maximum modulation frequency, changing the bias voltage changes the refractive index of the ADP crystal, which changes the polarization direction of the linearly polarized light beam and increases the transmittance. An optical modulator (hereinafter referred to as an E-O modulator) that uses a variable-type optical modulator is extremely superior.

Figure 1 shows the static characteristics of the bias voltage versus transmitted light power of the E-O modulator when the temperature is T _A °C.
The horizontal axis shows the bias voltage, and the vertical axis shows the transmitted light power. FIG. 2 shows the static characteristics of the bias voltage versus transmitted light power of the E-O modulator when the temperature is T _B °C. Further, +V _C and -V _C are the upper and lower limits of the bias voltage variable range, respectively.

When the temperature changes from T _A °C to T _B °C, the static characteristics change from Fig. 1 to Fig. 2, so the transmitted light power for a constant bias voltage differs.

For the above reasons, in general, when constructing a recording/reproducing device using an E-O modulator, the transmitted light power is monitored, a feedback loop is formed, and the bias voltage is controlled and adjusted. It is necessary to control the power of the light transmitted through the O modulator.

However, there are two problems when applying optical power control to the E-O modulator. One is Figure 1,
As shown in Figure 2, the static characteristics of bias voltage versus transmitted light power are sinusoidal, so there are two or more bias voltages that correspond to a constant value of transmitted light power. The problem is that the response speed of transmitted light power to a change in bias voltage is slow (rise time is 500 μ to 1 msec).

The present invention has the above two problems when applying optical power control to an E-O modulator, and when starting feedback control, the bias voltage to be controlled is controlled by a feedback loop. The purpose is to set a bias point that can be stably controlled.

First, a method of controlling the optical power of an E-O modulator will be explained using a conventional example shown in FIG.

1 is a linearly polarized light source such as a laser, and 2 is an E-
The O modulator has a bias terminal A and a recording signal input terminal B. During reproduction, the reproduction light power is set by the input voltage of the bias terminal A, and during recording, the transmitted light power is intensity-modulated according to the input signal of the recording signal terminal B to perform recording. 3 is a beam splitter, which irradiates a part of the output light of the E-O modulator 2 to a photodetector 4 such as a PIN diode.
The transmitted light power P is monitored. Reference numeral 5 denotes an amplifier that amplifies the output voltage of the photodetector 4. 6 is a differential amplifier, which calculates the difference between the output V ₁ of amplifier 5 and the reference voltage V ₂ (V ₂
−V ₁ ). 7 is an amplifier that amplifies the output voltage of the differential amplifier 6. A switch 8 switches between the output voltage V ₃ of the amplifier 7 and an arbitrary variable voltage V ₄ from +Vd to -Vd. 9 is switch 8
Amplify the output voltage switched by E
-O This is an amplifier added to the bias terminal of the modulator 2. If the amplification factor of amplifier 9 is Mo, then MoVd=
Make it V _C.

When switch 8 is switched to V ₃ ,
A feedback loop is formed by 3, 4, 5, 6, 7, and 9, and optical power control is performed. When switch 8 is set to _V4 ,
By changing the voltage of _V4 , the transmitted light power can be varied continuously. 10 is a signal generator for image signals such as a television camera, 11 is an FM modulator, and 12 is an FM modulator that amplifies the output of the FM modulator 11 and outputs an E.
-O This is an amplifier that is added to the recording signal terminal 2 of the modulator 2, and it modulates the intensity of the transmitted light power according to the recording FM signal and irradiates the recording medium to perform recording.

The operation of optical power control will be explained below. first,
As mentioned above, the static characteristics of the bias voltage versus transmitted light power of the E-O modulator when the temperature is T _A °C are as shown in Figure 1, but the l shown in the static characteristic curve is If we limit it to part _A , it becomes a straight line. When the temperature changes from T _A ℃ to T _B ℃,
The straight line _lA shown in FIG. 1 moves to the straight line _lB shown in FIG.

The linear section is the operating area for feedback control.
Assuming that l _A is used, as the linear part l _A moves to the linear part l _B as the temperature changes from T _A °C to T _B °C, the bias voltage is changed to feedback control as described below. Therefore, it is necessary to keep the transmitted light power constant by changing it.

Here, the linear operating range l _A when the temperature is T _A °C
is the transmitted light power P, the bias voltage V _IN , and the first
If the slope of l _A in the figure is m, then in Figure 1 the point (V _A ,
O), it can be expressed as P=m(V _IN −V _A ). Furthermore, in FIG. 3, since a proportional relationship is established between the transmitted light power P and the negative phase input V ₁ of the differential amplifier 6, it can be expressed as V ₁ =M ₁ P, where M ₁ is a proportionality constant. Further, if the product of the amplification factors of amplifier 7 and amplifier 9 is _M2 , it can be expressed as V _IN = _M2 ( _V2 - _V1 ).

P = m (V _IN - V _A ) V ₁ = M ₁ P V _IN = M ₂ (V ₂ - V ₁ ), and the simultaneous equations hold, but the unknowns are P,
Since there are three, V _IN and V ₁ , the solution is uniquely determined, and the transmitted light power is P=m(M ₂ V ₂ −V _A )/1+mM ₁ M ₂ .

The change in transmitted light power P when the temperature changes from T _A °C to T _B °C is Po (l _A ) in Figure 1 when the temperature is T _A °C, and when the temperature is T _B °C, Po in Figure 2
( _lB ), then Po( _lA )-Po( _lB )=m( _VB - _VA )/ ₁ + _mM1M2 .

m cannot be changed because it is determined by the E-O modulator, but M ₁ and M ₂ can be made very large by increasing the gain of the feedback loop, so Po( l _A ) and Po (l _B ) can be approximately equal. In this way, by increasing the loop gain of the feedback loop, it is possible to increase the accuracy of transmitted light power control with respect to temperature changes.

As mentioned above, in order to sufficiently increase the precision of optical power control against temperature changes, it is necessary to increase the gain of the amplifier in the feedback loop. When the loop is closed by switching switch 8 in Figure 3, the higher the loop gain, the less optical power control is applied and the bias voltage reaches the power supply voltage, causing the bias voltage to be controlled. It often does not return. That is, when the static characteristics of the E-O modulator are as shown in Figure 1, V ₄ in Figure 3
is, for example, OV, which is the voltage immediately after switching switch 8 from V ₄ to V ₃ and closing the feedback loop.
V ₁ and reference voltage V ₂ generally have an error. Since the loop gain _M3 of feedback control is generally set large to improve accuracy, the bias voltage becomes large even if the error is small. For this reason, the bias voltage immediately after closing the feedback loop jumps over the region La in FIG. 1, resulting in positive feedback control, and the bias voltage reaches the power supply voltage.

Therefore, when switching the switch 8 from _V4 to _V3 to close the feedback loop, the bias voltage often saturates at the power supply. Therefore, before closing the loop, we changed the value of V ₄ in advance so that the outputs V ₃ and V ₄ of amplifier 7 were equal, so that the change in bias voltage V _IN when switching switch 8 was reduced. After that, it is necessary to switch the switch 8, but the larger the loop gain, that is, the higher the accuracy of optical power control, the more difficult it is to make V ₃ and V ₄ equal.

It is an object of the present invention to provide a method for closing the feedback loop as described above and setting the bias voltage for optical power control to an appropriate value.

An embodiment of the invention is shown in FIG. In addition, the fourth
Components in the figure that are the same as those in FIG. 3 are designated by the same numbers. 13, 14, 15, and 16 are relays. 1
7 is a reset switch for resetting the bias voltage for optical power control. 18, when the reset switch 17 is turned on and off, the relay 1
This is a circuit that generates timing signals A, A, and C to control the signals 3, 14, 15, and 16. relay 13
is, when signal U is low level, γ and q are conductive,
When at high level, r and p are electrically connected. relay 14
is non-conductive when signal A is low level, and conductive when signal A is high level. Relays 15 and 16 are both non-conductive when signal A is low level, and conductive when signal A is high level.

FIG. 5 shows the relationship between the on/off operation of the reset switch 17 and the timing signals A, I, and C. FIG. 5a shows the on/off operation of the reset switch 17. FIG. 5b shows the change in signal A during the reset operation. FIG. 5c shows the change in signal A during the reset operation. 5d shows the change in signal U during the reset operation.

First, when the reset switch 17 is turned on as shown in FIG. 5a, the signal A changes from low level to high level as shown in FIG. 5b, and the relays 15 and 16 change from the off state to the on state. . Next, the signal A changes from a high level to a low level as shown in FIG. 5c, and the relay 14 changes from an on state to an off state. That is, with these two operations, the path of the control signal is switched from _l1 to _l2 . Next, the signal C changes from a low level to a high level as shown in d of FIG. 5, and the relay 13 switches from the conductive state between r and q to the conductive state between r and p.
V ₃ ′ becomes OV, and V ₃ ″ also becomes OV. Next, when the reset switch 17 is turned off as shown in FIG. 5, a, the signal U changes as shown in FIG. , goes from high level to low level, relay 1
3 switches from the conduction state between p and r to the conduction state between q and r. Since V ₃ ″ changes slowly from OV due to the presence of resistor R ₁ and capacitor C ₁ , the transmitted light power changes slowly, and the bias voltage can be set at the bias point where optical power is controlled. Then, within the feedback loop, the resistance R ₁ ,
Capacitor _C1 remains, but signal A changes from low level to high level as shown in Figure 5c, relay 14 is turned on, and signal A changes from high level to low level, relay 15, 16 is turned off, the control signal path is switched from _l2 to _l1 , and the initialization low-pass filter is removed from the feedback loop in subsequent optical power control.

FIG. 6 shows a specific configuration for generating the signals A, I and C of the time chart shown in FIG.
Further, a time chart of the circuit shown in FIG. 6 is shown in FIG.

The operation of the sequence circuit shown in FIG. 6 will be explained below. Reset switch 17 is normally turned off.

INV1 and INV2 are inverters. AND
1, AND2, AND3, AND4 are AND circuits,
JK-FF1, JK-FF2, and JK-FF3 are JK flip-flop circuits. CLOCK1, CLOCK
2. A clock pulse as shown at a' in the time chart of FIG. 7 is input to CLOCK3.

The reset operation is performed using reset switch 1 in Figure 6.
7 is turned on and then turned off. The reset switch 17 is turned off except during a reset operation. Therefore, when off, INV1 is low level and INV2 is high level, so the output of AND ₁ is low level,
J ₁ goes high level and K ₁ goes low level, so when a clock pulse is input, Q ₁ goes high level,
_Q1 becomes low level. Since the AND3 output becomes high level, _J2 becomes high level and _K2 becomes low level, and at the next clock pulse, _Q2 becomes high level and _Q2 becomes low level. Similarly, on the next clock pulse, _Q3 goes high and _Q3 goes low.

In this way, when the reset switch is turned off, Q ₁ ,
When the reset switch is turned on at time _T1 when Q ₂ and Q ₃ are at high level and ₁ , ₂ , and ₃ are at low level, INV1 is at high level and INV2 is at low level.
becomes low level, AND4 output becomes low level, _J3 becomes low level and K3 becomes high level, so at the first clock pulse, _Q3 becomes low level _{and Q3 becomes} high level. For this reason, the seventh
As shown in FIG. b, signal A( ₃ ) changes from low level to high level. Since ₃ becomes high level, the output of AND ₂ becomes high level. Since J ₂ becomes a low level and K ₂ becomes a high level, at the second clock pulse, Q ₂ becomes a low level and _{Q 2} becomes a high level. Therefore, as shown in FIG. 7c, the signal A (Q ₂ ) changes from high level to low level. ₂ becomes high level, so AND1
output becomes high level. J ₁ is low level,
Since _K1 goes high, at the third clock pulse, _Q1 goes low and _Q1 goes high. Therefore, as shown in FIG. 7d, the signal U( ₁ ) changes from low level to high level.

Next, when the reset switch is turned off at time _T2 , INV1 becomes low level and INV2 becomes high level, so the output of AND1 becomes low level,
J ₁ goes high level, K ₁ goes low level, and Q ₁ goes to the first clock pulse after reset switch off.
changes to high level and ₁ changes to low level. Therefore, as shown in FIG. 7d, the signal U( ₁ ) changes from high level to low level. Since _Q1 becomes high level, the output of AND3 becomes high level, and the output of AND2 becomes low level, so
J ₂ will be high level and K ₂ will be low level. On the second clock pulse after reset switch off,
Q ₂ changes to high level, ₂ changes to low level. Therefore, as shown in Fig. 7c, the signal I (Q ₂ )
changes from low level to high level. Since Q ₂ becomes high level, the output of AND4 becomes high level, J ₃ becomes high level, and K ₃ becomes low level. At the third clock pulse after reset switch off, _Q3 changes to high level and _Q3 changes to low level. Therefore, as shown in FIG. 7b, the signal A( ₃ ) changes from high level to low level.

As can be seen from the above explanation, the period during which the reset switch 17 is turned on is required to be at least three times the clock pulse period. As described above, according to the present invention, a low-pass filter is inserted in the feedback loop at the time of initial setting of the bias point for transmitted light power control, and the speed of change of the bias voltage is slowed down so that the bias voltage can be used as a positive feedback. This has the great effect of preventing the bias voltage from reaching the power supply voltage by jumping out into the region of You can get

[Brief explanation of drawings]

Figures 1 and 2 are diagrams showing the relationship between the bias voltage of the E-O modulator and the power of transmitted light, respectively, Figure 3 is a configuration diagram of a conventional optical power control device, and Figure 4 is an embodiment of the present invention. FIG. 5 is a waveform diagram illustrating the same operation, FIG. 6 is a block diagram showing the main part of an embodiment of the present invention, and FIG. 7 is a waveform diagram illustrating the same operation. 1... Laser, 2... E-O modulator, 3... Beam splitter, 4... Photodetector, 5, 7, 9,
12... Amplifier, 6... Differential amplifier, 8... Changeover switch, 10... Signal generator, 11... FM modulator.

Claims (1)

[Claims] 1 Control the transmitted light power to a constant level by applying a voltage obtained by photoelectrically converting the transmitted light power of an optical modulator using the electro-optic effect of a crystal to the bias voltage input of the optical modulator via a feedback loop. An optical power control device comprising: a photoelectric converter that converts transmitted optical power into a voltage value; a differential amplifier that amplifies a voltage difference between the output voltage of the photoelectric converter and a first predetermined voltage; and the differential amplifier a first switching means for supplying either the output voltage of the low-pass filter or the second predetermined voltage to the input of the low-pass filter; a second switching means for supplying a bias voltage input to the optical modulator, the first switching means supplying the second predetermined voltage to the input of the low-pass filter, and the second switching means The output voltage of the low-pass filter is supplied to the bias voltage input of the optical modulator, and then the first switching means changes the voltage supplied to the input of the low-pass filter from the second predetermined voltage to the voltage of the differential amplifier. After that, the second switching means changes the voltage supplied to the bias voltage input of the optical modulator from the output voltage of the low-pass filter to the output of the first switching means. An optical power control device characterized by controlling optical power by switching to voltage.