JPH02257441A - System for controlling emissive light quantity of laser diode - Google Patents

Abstract

PURPOSE:To decrease the time of monitoring and to keep a laser output constant at the time of recording by measuring emissive light quantity and a current value at the time of a reproducing output, determining necessary current quantity based on the measured values and executing control. CONSTITUTION:At first, the value of a necessary constant is calculated from temperatures T1 and T2 at the two points of an output/current characteristic, which defines the temperature of a laser diode 26 to be used for the initial set of a control system as a parameter, and an arithmetic formula is stored in the internal part of a control part 12. When it is the time of reproducing and a sample/hold circuit 32 is made active by an external signal, the light quantity of the laser diode 26 is supplied as a feedback signal to the control part 12. According to this feedback signal, the driving current value at the time of reproducing and recording is updated and outputted to a reading level and a writing level based on the arithmetic formula in the control part 12. Thus, the light quantity can be controlled at the time of recording and the time of sampling can be reduced. Then, light can be emitted with the light quantity corresponding to a target value at the time of reproducing.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for controlling the amount of light emitted from a laser diode in an apparatus for recording and reproducing information using a laser diode as a light source.

(Prior Art) Generally, in an optical recording/reproducing device, information is written by focusing a laser beam on a recording medium to thermally and chemically form minute pits.

During reproduction, the written location is traced using light weakened to such an extent that the above-mentioned changes do not occur, and a reproduction signal is obtained from the intensity of the returning light. In such a system, the optimum amount of light for recording is almost determined by the medium. However, due to the characteristics of laser diodes, the output varies greatly depending on the temperature, and in actual use it is not possible to set the current value that drives the laser constant. By constantly monitoring the amount of laser light and providing feedback, the amount of current is controlled to obtain a constant amount of light necessary for recording (or playback).

(Problem to be Solved by the Invention) When using the above control method, when a write signal is received during recording, the laser output (light intensity) is increased from the reading level to the writing level. , it is necessary to capture the current level of the laser output in sufficient detail with respect to the writing signal width. In other words, as shown in Figure 5, which shows the relationship of sampling to the reproduction level and recording level in the conventional technology. The output of the laser diode is sampled many times in order to constantly monitor changes in the output due to changes in the laser diode's temperature, and by feeding back the amount of light obtained by sampling, the required constant The amount of light is obtained.

If you do not do this, it is equivalent to reducing the number of samplings for monitoring), and in the worst case, overshoot etc. will occur, especially at the rise of the laser output.
There is a possibility that the reproduced waveform will be distorted with respect to the waveform that was intended to be recorded, making it impossible to obtain correct information. In other words, when the laser output rises, high speed is required and the laser output changes rapidly, so if the number of sampling times is reduced, the feedback control will not be able to follow it and distortion of the reproduced waveform will occur due to overshoot as described above. .

Therefore, it is necessary to measure the amount of laser light at a speed several tens of times faster than the highest recording frequency and adjust the amount of current flowing. In such cases, the higher the recording frequency, the higher the speed of the control system is required, and the design becomes more difficult in terms of stability and cost.

Therefore, the purpose of the present invention is to measure the amount of light emitted and the current value only during playback output without monitoring the amount of laser light during recording, and based on that, calculate the amount of current required during recording. By determining the amount and controlling it, we significantly reduced the number of times of monitoring, narrowed the frequency bandwidth required for the circuit, and achieved the original purpose of ``maintaining a constant laser output during recording.'' It is something to do.

(Means for Solving the Problem) In order to achieve the above-mentioned object, the present invention, in principle,
In a control method for the amount of laser diode emitted light in a device that records and reproduces information using a laser diode as a light source, the laser diode drive required to emit the recording output value from the laser output during reproduction and the laser diode drive current value. A control method is adopted in which the amount of light for recording is kept constant by calculating a current value and flowing the current amount of the calculated laser diode drive current value through the laser diode during recording.

Further, as a specific preferable control method, the present invention provides a method for controlling the amount of light emitted from a laser diode in a device that records and reproduces information using a laser diode as a light source. a target light amount setting means for setting a target light amount of the laser diode, a sample/hold means for sampling and holding the light amount of the laser diode during reproduction, and a laser diode that emits the target light amount during reproduction set by the target light amount setting means. A reproduction drive current value updating means updates the reproduction drive current value based on the sample/hold value of the sample/hold means, and a laser diode is configured to emit a target light amount during recording set by the target light amount setting means a recording drive current value updating means that calculates and updates a recording drive current value based on the sample/hold value of the sample/hold means; and a reproduction drive current value updated by the reproduction drive current value updating means. a recording current supply means for supplying a current to the laser diode, a recording current supply means for supplying the laser diode with a current based on the recording drive current value updated in the recording drive current value updating means; This invention employs a method for controlling the amount of light emitted from a laser diode, which is characterized by having a current supply means and a switch for switching the recording current supply means.

(Example) Next, the present invention will be explained with reference to the drawings.

FIG. 1 shows reproduction levels for detailed explanation of the present invention;
FIG. 2 is a graph showing the relationship between sampling and recording level; FIG. 2 is a graph showing laser output (light amount) vs. current characteristics with temperature as a parameter; FIG. 4 is a graph showing the output-current characteristics of a commercially available laser diode, and FIG. 4 is a schematic block circuit diagram showing a preferred control device to which the principles of the present invention are applied.

First, the present invention will be described in detail with reference to FIGS. 1 and 2. FIG.

As shown in Figure 2, a laser diode's output (
To summarize, the principle of the present invention is to calculate a current value for obtaining a predetermined oscillation output value during recording from a current value during reproduction and an oscillation output value without specifying the current temperature, The purpose is to cause the laser diode to emit light based on the current value. This point will be explained in detail below.

The only information currently available is the oscillation output value at low output (reading) (playback) and the current value flowing through the laser at that time. Let us assume that the information we want to know is how much current needs to flow in order to oscillate the high output value (during writing) (during recording) set at this time. In this case, information is missing. The following describes how to find the current value without being aware of the temperature at that time.

Converting the relationship between “laser output current” into a mathematical formula, P=n
(T) (I op-I th (T) )-1l
lP: Output light amount n (T): Oscillation efficiency (function of temperature T) Iop
: Current value Ith (T): Function n of oscillation threshold current (T) (T
) = n (Ta) −K i (T−Ta) −
(llaI th ?T)=KtXexp (T/To
)---(llbKi, Kt: Constant Ta: Arbitrary temperature value To = Characteristic temperature n (Ta): Oscillation efficiency at Ta (A) Determination of Kt, To, Ki Based on the output-current characteristic table or actual measurement, Two types of temperature (
Ith(TI), Ith(T2) at
seek. (See Figure 2) The curve is approximated by a straight line, and the light output is 0.
Let the intersection with Ith be Ith.

Ith (Tl)=Kt exp (Tl/To)I
th (T2)=Kt, exp (T2/To)↓ To= (T2-Tl)/('logIth(T2
)−1oglth(TI))Kt=1th(TI
)/exp(TI/To) Also, using the curve in Figure 2,
(1) -a equation Ta, n(Ta) are T1, n(Tl
) and similarly, T2, n(T) for T, n(T).
Ki can also be found by substituting 2).

(B) Derivation of current value (I w ) corresponding to laser output (P w ) during writing When the laser output value during reading is Pr and the current value is Ir at a certain temperature “T”, the write output is calculated as follows: P
Find the current value Iw that should flow when w is set. However, it is assumed that the temperature "T" itself is unknown.

From formula (1), Pr=n (T) (I r−I th (T) )
−(′2JPw=n (T) (I w −I t
h (T) ) = 1311th (T) is deleted and Iw = I r+ (Pw-Pr)/n (T)-441
(C) Derivation of n (T) [1)-a, 11)-b In the formula, n (Ta), Ta,
Keeping in mind that Kt and To are already known, consider the following three equations.

I th (T) = I r-P r/n (T)
=・(51 ((from 2J formula) n (T) =n (Ta) −K i (T−Ta)
=・(61 (from formula 11-a) I th (T)=KtXexp (T/Ta)=・(
71 (from [11-b equation)] Eliminate T, I th (T), Kt*exp[(1/To)*((n(Ta)-n(T
))/Ki+Ta)]=Ir-Pr/n(T)-(EA Such an equation for n(T) is obtained.

To solve (2), take the logarithm of both sides and [(n (T
a)-n(T))/Ki+Ta]/To=lo
g[(Ir-Pr/n (T))/K i ]...(
9) In order to make it easier to calculate on an electric circuit such as a microcomputer, we will calculate the value using an iterative method as an example.

a, assign either the value of n(TI) or n(T2) to the right-hand side n(T), and solve for n(T) as a -order equation.

b. Again, substitute the new value obtained in "a," to n (T) on the right side of equation (9), and solve for n (T).

C0 When such an operation is repeated, n (T) converges to the true value, and it almost converges after about 4 to 5 repetitions.

Now that n(T) is known above, the writing current Iw can be determined from the equation.

In summary, for the laser diode to be used, obtain the oscillation threshold currents Ith (TI), Ith (T2) and oscillation efficiencies n (TI), n (T2) at any two points whose temperatures are known in advance. This means that the oscillation efficiency at that temperature can be calculated from the laser output and current when outputting for reading, and the current value that should be passed during writing can be calculated.

Let's check if the above results actually match.

The semiconductor laser "LN98" manufactured by Matsushita Electronics Co., Ltd. has the output-current characteristics shown in Figure 3 because of product standard data.
50", the current required for an appropriately set output value is calculated from the laser output and current value at a given temperature, and compared with the actual current value.

Figure 3 shows one “output-current characteristic” of “LN9850”
First, from the curves at 20°C and 50°C, 1th (20°C) = 39.0 (mA) 1th (
50℃)=47.5 (mA)n(20℃)=0.7
It can be read as 22 n (50°C) = 0.680.

From “2.(A)”, using these values, To=152
(K) Kt=5.674 Ki=0.0014
is obtained.

Now that the preparations are complete, we will check whether the calculated results match the actual measurements for the 40°C curve as an example.

From the graph, Pr=18mW, I as the output when reading
Take r=70mA and based on this, Pw=40mW
Find the current required when you want to. (*The information of 40℃ is not used in the calculation.) In equation (9), Ta=293 (K)...-20℃, n (
Set T)=X, and first find X.

[(110,0014)*(0,722-X)+293
1/152=log[(70-18/x)15.674
Assign 0.722 to the right side X, x=o, 691〃x to 0
．． 691 /7 X=0.697n 0.
697 77 X=0.696II x
0. 696 II X=0
．． 696n (T)=0.696 was obtained. Therefore, from equation (4), I w = 70.0 + (40-18) 10.696 =
101.6 (mA)Actually, the current required when outputting 40mW on a curve above 40℃ is approximately 102mA, as read from the graph.
A, and the calculated value and the measured value are in good agreement.

Next, a preferred specific control system to which the above-described principle of the present invention is applied will be described with reference to FIG.

The control device 10 has a control section 12, and this control section 1
2 is configured so that target values of laser output for write level and read level, which will be described later, are set externally. 0For example, digital data corresponding to a write level of 1 mW and a read level of 7 mW is set externally. . The control unit 12 also operates as shown in FIGS.
It has a control system model with the calculation formula explained in relation to the figure, and based on this model, the laser output during playback and recording is determined from the current value corresponding to the light amount obtained by the feedback signal described later. The drive current value is updated for each sampling (for each sampling during playback) so as to obtain the target value. Note that the drive current value during reproduction may be controlled by a normal feedback loop without being updated, and only the drive current value during recording may be controlled using an internal model.

The read level is determined by the D/A converter 1 which converts the drive current value into a voltage analog quantity when reproducing the updated digital quantity.
4 and a voltage-current conversion circuit 16 that converts the voltage analog amount into a current amount proportional to the voltage analog amount, and supplies a current corresponding to the updated playback drive current value to the laser diode 26.

The write level similarly includes a D/A converter 18 that converts the updated driving current value during recording of the digital quantity into a voltage analog quantity, and a voltage-current conversion circuit 20 that converts it into a current quantity proportional to the voltage analog quantity. , a current corresponding to the updated recording drive current value is supplied to the laser diode 26.
supply to.

It should be noted here that, as in the embodiment of the present invention, the drive current during playback is always passed through the laser diode 26, and during recording, the drive current during recording is passed through the laser diode 26 so that it is added to the drive current during recording. With this configuration, the recording drive current value at the write level is the value obtained by subtracting the reproduction drive current value from the target value given to the control section.

A read level voltage-to-current conversion circuit 16 is connected to the anode of the laser diode 26, and a write level voltage-to-current conversion circuit 20 is connected to the laser diode 26 via a switch 22. The switch 22 is configured to be switched between the laser diode 26 or the dummy resistor 24 at high speed by an external write power timing signal (binary signal corresponding to "0" and "1") during recording.

The amount of light from the laser diode 26 is determined by the photodiode 28
The signal is detected by the amplifier 30, sampled and held by the sample/hold circuit 32, converted into a digital quantity by the A/D converter 34, and returned to the control section as a feedback signal.

The sample/hold circuit 32 is controlled by an external signal so that it operates only during reading (playback). That is, the sample/hold circuit 32 samples and holds the light amount of the laser diode only once during reproduction. Therefore, the number of samplings is extremely small compared to the number of samplings in the conventional technology.

Next, the operation of the control system of the present invention will be summarized. First, as an initial setting of the control method, the necessary constant values (for example, Kt, To, Ki, Ta oscillation efficiency n (T) etc.) are determined, and the arithmetic expressions explained in connection with FIGS. 1 and 2 are stored in the control unit 12. When the sample/hold circuit 32 is activated by an external signal, the light intensity of the laser diode 26 is output to the control unit 12 as a feedback signal.
is supplied to Based on this feedback signal, the model at the time of reproduction and recording is updated based on the arithmetic expression in the control section 12. That is, the drive current values during reproduction and recording are updated and output to read level and write level. Note that, as described above, this reproduction drive current may be controlled by a feedback loop. The updated drive current during reproduction is supplied to the laser diode 26, and the laser diode 26 emits light with an amount of light corresponding to the target value during reproduction.

On the other hand, the updated recording drive current flows through the switch 22 to the dummy resistor 24 and is not supplied to the laser diode 26. At the time of recording, the switch 22 is switched to the laser diode 26 based on the write power timing signal. When the recording drive current is added to the playback drive current and flows into the laser diode 26, the laser diode 26 emits light with an amount of light corresponding to the target value during recording.When recording ends and it is time to play again, Similarly to the above, a feedback signal is given to the control section 12 by an external signal given to the sample/hold circuit, and the model inside the control section is updated again.

(Effects of the Invention) As explained above, the present invention has the advantage of measuring the "output-current characteristics" of the laser diode to be used for the first time at two temperature points, thereby achieving a recording time that meets the initial purpose. It is possible to control the amount of light, and since the number of samplings is extremely small compared to a single constant monitor system, the frequency range used for control can be several orders of magnitude lower, so the higher the writing frequency with the laser, the more , which is advantageous in terms of design.

[Brief explanation of drawings]

FIG. 1 shows reproduction levels for detailed explanation of the present invention;
FIG. 3 is a diagram showing the relationship between sampling and recording level. Figure 2 shows laser output (light intensity) with temperature as a parameter.
- It is a graph showing current characteristics. FIG. 3 is a graph showing the output-current characteristics of a commercially available laser diode used to confirm the principle of the present invention. FIG. 4 is a schematic block diagram illustrating a preferred control system incorporating the principles of the present invention. FIG. 5 is a schematic diagram showing a conventional control method. 10...Control device. 12... Control unit, 14.18... D/A converter, 16.20... Voltage-current conversion circuit, 22... Switch, 24... Dummy resistor, 26... Laser diode, 28... Photodiode, 32... Sample/hold circuit, 34... A/D converter.

Claims (5)

[Claims] (1) Record information using a laser diode as a light source,
In a method for controlling the amount of laser diode emitted light in a playback device, the laser diode drive current value necessary to emit the recording output value is calculated from the laser output during playback and the laser diode drive current value, and the calculated laser diode drive A control method characterized by keeping the amount of light for recording constant by passing a current amount of current value through a laser diode during recording. (2) The control method according to claim 1, wherein the laser diode drive current value during recording is calculated based on an output-current characteristic using the temperature of the laser diode as a parameter. (3) The control method according to claim 1, wherein calculation of the laser diode drive current value during recording is updated every time during reproduction. (4) Recording information using a laser diode as a light source,
A control method for the amount of light emitted by a laser diode in a playback device includes a laser diode used as a light source, a target light amount setting means for setting the target light amount of the laser diode during playback and recording, and sample and hold of the light amount of the laser diode during playback. and a reproduction drive that updates the reproduction drive current value based on the sample/hold value of the sample/hold means so that the laser diode emits the target light quantity during reproduction set by the target light quantity setting means. a current value updating means; and a recording drive current value for calculating and updating a recording drive current value based on the sample/hold value of the sample/hold means so that the laser diode emits the target light amount during recording set by the target light amount setting means. drive current value updating means; reproduction current supply means for supplying a current to the laser diode based on the reproduction drive current value updated in the reproduction drive current value updating means; a recording current supply means for supplying a current to the laser diode based on the recorded recording drive current value; and a switch for switching between the reproduction current supply means and the recording current supply means. Light intensity control method. (5) The control method according to claim 4, wherein the recording drive current value is calculated based on an output-current characteristic using the temperature of the laser diode as a parameter.