JPH087319A - Circuit for automatically adjusting amount of light of laser diode - Google Patents

Abstract

PURPOSE:To obtain a circuit for automatically adjusting an amount of light from a laser diode, which can reduce the number of external parts and pins and obtain an RF signal of an optimum level. CONSTITUTION:A signal level of an RF signal obtained by adding output signals from photodiodes 12-1 to 12-4 at an adder 15 is detected by a level detecting circuit 16. The detected level V1 is compared with a predetermined reference level V2 at a comparator 17. A driving current for a laser diode 11 is controlled by a driver 20 on the basis of a difference of the levels (V1-V2).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode automatic light quantity adjusting circuit, and more particularly to a laser diode light quantity automatic adjusting circuit used as a light source of an optical pickup in an optical disk player such as a CD (compact disk) player. .

[0002]

2. Description of the Related Art In an optical disc player such as a CD player, a laser diode is used as a light source of an optical pickup. In this laser diode, the amount of light (light output) changes with temperature. If the light quantity of the laser diode changes, it adversely affects the gain characteristics of the servo and the like, so it is necessary to maintain the light quantity constant. For this reason, a light amount automatic adjustment circuit is used that automatically receives the light from the laser diode by the light receiving element and controls the drive current of the laser diode so that the output is constant, thereby automatically adjusting the light amount of the laser diode.

In the conventional automatic light amount adjusting circuit, as shown in FIG. 4, in order to receive the laser light emitted from the laser diode 41, for example, four photodiodes 42-1 to 42 for signal reading and servo. In addition to -4, a dedicated photodiode 43 was provided for adjusting the amount of light. The output of this photodiode 43 is taken into the inside of the IC through the terminal 44, and the amplifier 45 outputs I (current).
After being converted to -V (voltage) and amplified by the amplifier 46,
It is taken out of the IC through the terminal 47. Then, the bias voltage of the current source 48 is changed by this output voltage to control the direct current flowing through the laser diode 41.

On the other hand, four photodiodes 42-1 to 4-4
Reference numeral 2-4 constitutes each element of a so-called four-division photodetector for signal reading and focus servo, and the sum of output signals is used as a read signal (hereinafter referred to as RF signal). That is, the four photodiodes 42-1 to
Each output signal of 42-4 is input to I through terminals 49-1 to 49-4.
The signal is taken into C, subjected to IV conversion in the amplifiers 50-1 to 50-4, and then added to each other in the adder 51 to become an RF signal.

Here, even if the light quantity of the laser diode 41 is adjusted to be constant, the RF signal is changed between the systems due to the difference in the sensitivity of the photodiodes 42-1 to 42-4 and the difference in the subsequent blocks. There will be a difference in the amplitude of. Therefore, conventionally, the resistance R is placed between the terminals 52 and 53.
Externally and adjust the amplification amount of the adder 51 by this resistor R, or adder 5 as shown by the broken line in FIG.
An AGC (automatic gain control) circuit 54 is provided in the subsequent stage of 1, and the RF signal has a constant amplitude by adjusting the amplification amount of the adder 51 by the AGC circuit 54.

[0006]

As described above, in the conventional automatic light quantity adjusting circuit, the dedicated photodiode 43 is used for adjusting the light quantity of the laser diode 41. Therefore, the number of external parts and the IC There was a problem that the number of pins of the increased. Further, since the light quantity of the laser diode 41 is adjusted independently of the adjustment of the signal level of the RF signal or the like, when the external resistor R is used to adjust the level of the RF signal or the like, As the frequency becomes higher, it becomes difficult to cope with it, and the inter-pin capacitance Cpin is inserted in parallel with the external resistor R to deteriorate the frequency characteristic, and when the AGC circuit 54 is used, the DC offset becomes large. There was a problem. There is also a method of incorporating an external resistor R inside the IC and selecting the conversion resistor by switching, but in this case, it becomes impossible to select a linear voltage gain.

The present invention has been made in view of the above problems, and an object of the present invention is to reduce the number of external parts and the number of pins and to achieve an optimum level of R.
An object of the present invention is to provide a circuit for automatically adjusting the light amount of a laser diode that can obtain an F signal.

[0008]

An automatic light quantity adjusting circuit for a laser diode according to the present invention uses a laser diode as a light source of an optical pickup, and receives reflected light from a disk based on laser light emitted from the laser diode, In an optical disc player configured to generate an RF signal based on the received light output, a signal level detecting means for detecting a signal level of the RF signal and a comparing means for detecting a level difference between the detected level and a predetermined reference level. , And a control means for controlling the drive current of the laser diode according to the detection level difference.

[0009]

In the automatic light amount adjusting circuit having the above structure, the signal level of the generated RF signal is detected by the signal level detecting means. The signal level of this RF signal fluctuates due to variations in the light quantity of the laser diode, variations in the sensitivity of the light receiving element, and the like. The detection level of the RF signal is compared with a predetermined reference level by the comparison means, and the level difference is detected. Then, the drive current of the laser diode is controlled according to the level difference between the RF signal level and the reference level.

[0010]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention. In FIG. 1, a laser diode 11 is used as a light source of an optical pickup (not shown) in an optical disc player such as a CD player. The laser light emitted from the laser diode 11 is irradiated as a light spot on the signal surface of a disc (not shown), and the reflected light is incident on, for example, four photodiodes 12-1 to 12-4.

Four photodiodes 12-1 to 12-4
Constitute each element of the four-division photodetector for signal reading and focus servo. FIG. 3 shows an example of the configuration of the 4-division photodetector. Center O of this 4-split photodetector
The reflected light from the disk enters so that the centers of the laser light spots SP substantially coincide with each other. The output signals of the four photodiodes 12-1 to 12-4 are supplied to the terminal 13-1.
13-14 is taken into the IC through the amplifier 14-1.
After being IV converted at 14-4, they are added to each other by the adder 15 to become an RF signal. That is, the sum of the output signals of the photodiodes 12-1 to 12-4 becomes the RF signal.

The amplifiers 14-1 to 14-4 have a non-inverting (+) input of a predetermined positive voltage E1 and have a photodiode 12
-1 to 12-4, the operational amplifiers OP1-1 to OP1-4 and the operational amplifiers OP1-1 to OP1-4, each of which has an inverted (-) input.
The feedback resistors Rf1-1 to Rf1-4 are connected between the inverting input terminal 1-4 and the output terminal. Further, the adder 15 uses the input resistances Rin-1 to Rin-4 and a predetermined positive voltage E2 as non-inverting inputs and also has the input resistances Rin-1 to Ri.
An operational amplifier OP2 which receives each output signal of the amplifiers 14-1 to 14-4 input via n-4 as an inverting input, and a feedback resistor connected between the inverting input terminal and the output terminal of the operational amplifier OP2. And Rf2.

Although a specific circuit configuration is not shown,
In FIG. 3, the outputs of the photodiodes on the diagonal lines, that is, the outputs of the photodiode 12-1 and the photodiode 12-3, and the outputs of the photodiode 12-2 and the photodiode 12-4 are added, and the difference between the added outputs is calculated. As a result, a focus error signal by the so-called astigmatism method can be obtained. This focus error signal is used as a control signal of a focus servo circuit that controls the distance between the optical pickup and the signal surface of the disc to always be constant.

The RF signal is supplied to a signal processing system (not shown), subjected to predetermined signal processing and output as an audio signal, and is also supplied to the signal level detection circuit 16 to detect the signal level. Signal level detection circuit 1
For example, 6 has an integrating circuit configuration.
The detection level V1 by the signal level detection circuit 16 becomes one input of the comparator 17. Comparator 17
Uses the reference level V2 supplied from the DC power supply 19 via the terminal 18 as the other input, and obtains the level difference (V1-V2) of the detection level V1 with respect to the reference level V2 by comparing both inputs.

The reference level V2 is set corresponding to the target level so that the signal level of the RF signal is always the target level. The level difference (V1-V2) signal obtained by the comparator 17 is given to the driver 20. The driver 20 controls the drive current of the laser diode 11 by changing the bias voltage of the current source 22 via the terminal 21 according to the level difference (V1-V2) signal given from the comparator 17.

In the automatic light amount adjusting circuit having the above structure, the signal level of the RF signal obtained as the sum of the output signals of the photodiodes 12-1 to 12-4 is detected by the signal level detecting circuit 16, and this detection level V1 is detected. The comparator 17 compares the reference level V2 set corresponding to the target level of the RF signal with the comparator 17, and the level difference (V1-V2) signal as the comparison output is supplied to the driver 20.

When the detection level V1 of the RF signal is smaller than the reference level V2, the level difference (V2
A negative voltage corresponding to −V1) is applied to driver 20. As a result, the driver 20 becomes the laser diode 1
The bias voltage of the current source 22 is controlled so that the driving current of No. 1 is increased and the amount of light of the laser diode 11 is increased.
On the other hand, when the detection level V1 of the RF signal is higher than the reference level V2, a positive voltage corresponding to the level difference (V1-V2) is applied to the driver 20. This allows
The driver 20 controls the bias voltage of the current source 22 so as to reduce the drive current of the laser diode 11 and reduce the light amount of the laser diode 11.

As described above, by detecting the signal level of the RF signal, comparing the detected level V1 with the reference level V2, and controlling the light quantity of the laser diode 11 based on the level difference, the light quantity of the laser diode 11 is controlled. Even if a dedicated photodiode for adjustment is not used, it is possible to compensate for a change in the amount of light due to the temperature of the laser diode 11, and to adjust the RF signal due to variations in the sensitivity of the photodiodes 12-1 to 12-4. The change in signal level can also be compensated.

Therefore, unlike the conventional example (see FIG. 4), it is not necessary to externally attach the resistor R for adjusting the level of the RF signal or the like, or to use the AGC circuit 54, so that a dedicated photodiode is not necessary. In addition to being able to reduce the number of external parts and the number of IC pins,
Deterioration of frequency characteristics due to external resistor R, or AGC
The problem of DC offset due to the circuit 54 does not occur. Further, if the reference level V2 is changed linearly, the voltage gain of the adder 15 can also be changed linearly.

By the way, in an optical disc player such as a CD player, in a mode in which a laser light spot emitted from the laser diode 11 onto the signal surface of the disc jumps over a track (pit row) of the disc, a so-called track jump or the like is performed temporarily. As a result, the RF signal becomes unstable. Therefore, in such a mode, if the light amount is automatically adjusted based on the signal level of the RF signal as it is, the light amount of the laser diode 11 becomes rather unstable.

FIG. 2 is a circuit diagram showing another embodiment of the present invention made assuming such a case. In FIG.
The same reference numerals are attached to the same parts as. In the present embodiment, the hold circuit 23 that holds the level difference detected by the comparator 17, and the detection level difference detected by the comparator 17 in the normal play mode according to the mode signal are selected and given to the driver 20 to perform a track jump or the like. In the mode in which the operation is performed, the changeover switch 24 which selects the level difference held in the hold circuit 23 and gives it to the driver 20 immediately before that is added.

The mode for performing the track jump or the like can be discriminated by the player itself, and it is possible to easily give a signal indicating the state to the changeover switch 24 as a mode signal. As described above, the hold circuit 23 for holding the output of the comparator 17 is provided, and in the mode in which the RF signal becomes unstable, the output of the hold circuit 23 is changed to RF.
In the stable signal mode, a changeover switch 24 for selecting the output of the comparator 17 is provided.
By switching 4 according to the mode signal,
It is possible to stably adjust the light amount of the laser diode 11 regardless of whether the RF signal is stable or unstable.

In the above embodiment, the case where the present invention is applied to the adjustment of the light amount of the laser diode of the optical pickup in the CD player has been described, but the present invention is not limited to this, and it is an MD (mini disk) player or a CD-R.
It can be applied to the adjustment of the light quantity of a laser diode of an optical pickup in all optical disc players such as OM drivers.

[0024]

As described above, according to the present invention,
A dedicated light receiving element is provided by detecting the signal level of the RF signal, detecting the level difference between the detected level and a predetermined reference level, and controlling the drive current of the laser diode based on the level difference. Even if it is not necessary, it is possible to compensate for the change in the amount of light due to the temperature of the laser diode, so that it is possible to reduce the number of external components and the number of IC pins.

Further, even if an external resistor is not used or an AGC circuit is not used, the change in the signal level of the RF signal caused by the variation in the sensitivity of the light receiving element can be compensated for at the same time, so the number of external parts is increased. And the number of IC pins can be further reduced, and the frequency characteristics deteriorate due to external resistors.
Alternatively, the problem of DC offset due to the AGC circuit does not occur, and thus it is possible to obtain the RF signal of the optimum level.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the present invention.

FIG. 3 is a configuration diagram showing an example of a four-division photodetector.

FIG. 4 is a circuit diagram showing a conventional example.

[Explanation of symbols]

11 Laser diode 12-1 to 12-4
Photodiodes 14-1 to 14-4 Amplifier 15 Adder 16 Signal level detection circuit 17 Comparator 23 Hold circuit 24 Changeover switch

Claims (2)

[Claims] 1. An optical disc having a structure in which a laser diode is used as a light source of an optical pickup, the reflected light from the disc based on the laser light emitted from the laser diode is received, and a read signal is generated based on the received light output. In the player, a signal level detecting means for detecting a signal level of the read signal, a comparing means for detecting a level difference between a detection level of the signal level detecting means and a predetermined reference level, and a detecting level difference of the comparing means. And a control means for controlling the drive current of the laser diode. 2. An automatic light quantity adjusting circuit for a laser diode according to claim 1, further comprising holding means for holding a detection level difference by the comparing means, and a holding level of the holding means immediately before that in a predetermined operation mode. And a switching means for supplying the difference to the control means instead of the detection level difference by the comparing means.