JP2503530Y2 - Optical output control circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an optical output control circuit used in a write-once optical disc.

[Prior Art] FIG. 5 shows a conventional example of a light output control circuit of this type. In the figure, LD is a laser diode. In this laser diode, when the output pulse Vp of the pulse voltage source Ps is lower than the output of the DC power source E1 and the transistor Tr1 is off,
When the constant current Id0 is supplied by the constant current source Is1 and Vp becomes higher than E1 and the transistor Tr1 is turned on, the constant current ΔId by the constant current source Is2 flows so as to be added thereto. That is, Vp
Is higher than E1, a forward current of Id = Ido + ΔId (1) flows through the laser diode LD. Figure 6 (a) shows a laser diode LD
The optical output (Po) -current (Id) characteristic diagram and (b) diagram of FIG. 4 show the waveform of the forward current Id shown in the equation (1). When this forward current is supplied to the laser diode LD, (c) As shown, an on / off optical output Po is obtained, and information is recorded on an optical disc (not shown) by the optical output Po. In FIG. 6C, Pw indicates the optical power at the time of writing, and Pr indicates the optical power at the time of non-writing. In this case, if the optical power Pr at the time of non-writing is larger than the predetermined value, the information will be written on the optical disk, and if it is extremely small, the
The focus / servo of the head does not work. Therefore, Pr and Pw must be kept constant. by the way,
The threshold current Ith of the laser diode LD changes with time due to temperature change or deterioration phenomenon, which affects the values of Pw and Pr. In order to compensate for this change, generally, the light emitted from the rear of the laser diode LD is received by a photodiode Pd (Pin diode) as shown in FIG.
The output current Ipd of the above is passed through the resistance element Rf, and the voltage drop thereof is compared with the set voltage Es in the comparator U1. Then, the comparison result is negatively fed back to the constant current source Is1 to control Ido, whereby the power Pr at the time of non-writing is kept at a constant value. However, even with such a scheme, there is almost no photodiode PD that ideally responds to a square wave, except for the avalanche photo diode, so it is extremely difficult to accurately control Pr at high repetition frequencies. Is.

Therefore, conventionally, a capacitor Cf is connected in parallel with the feedback resistor Rf, and its time constant Cf · Rf is set sufficiently longer than one cycle of the repetition frequency of the output pulse Vp in the pulse voltage source Ps as shown in FIG. The means for detecting the average value of the optical output Po and controlling the constant current source Is1 by this average value output, thereby controlling Pr to a constant value has been taken. But,
In this case, the set voltage Es compared with the voltage drop occurring in the feedback resistance Rf in the comparator U1 is compared with the time constant Cf.
You have to change it to the average value Esa with Rf.
In this case, assuming that Eso is the original set voltage during non-writing, (Esa-Eso) is a value calculated by the value of (Pw-Pr) and the duty ratio of the pulse Vp. It is premised that the Id-Po characteristic of the laser diode LD shown in FIG. 6 (a) is constant. If the duty ratio is 80%, Pw = 20mW, Pr = 2mW, the Id-Po characteristic slope becomes 3/4 due to deterioration of the laser diode LD, etc. As shown in the figure, the value of Pr is calculated to be as large as 4.1mW, and there is a risk of writing even when not writing.

[Problems to be solved by the invention]

The present invention has been made in order to solve such a problem, and an object thereof is to provide an optical output control circuit in which the fluctuation of the optical power during non-writing is small.

[Means for solving problems]

In order to achieve the above object, the present invention supplies a laser diode with a current obtained by superimposing a constant current and a pulse current as a forward current and receives light from the laser diode.
The voltage drop according to the output current of the pin diode is compared with the set voltage, and the optical output power is controlled by controlling the magnitude of the constant current according to the comparison result. In the output control circuit, the voltage drop is applied to its non-inverting input terminal, the inverting input terminal is connected to the reference potential point via the resistance element R1, and is connected to the output terminal via the resistance element R2,
And an amplifier comprising a series circuit of a resistance element R3 connected in parallel to the resistance element R1 and a capacitor C, a capacitor Cd which is charged by supplying a constant current, and between the capacitor Cd and the output terminal of the amplifier. A minimum value detection circuit for detecting the minimum value of the output of the amplifier consisting of a diode connected to the resistance element R3 and R1, when the time constant of the relaxation response due to the diffusion current of the Pin diode is τ ₀ . R2
And the value of the capacitor C are selected according to the following relationship, and the output of the minimum value detection circuit is compared with the set voltage.

Note R3 = {(R1 ・ R2) / (R1 + R2)} × 1 / K C = τ ₀ / R3 (1 + K) where K is a constant [circuit for explaining the present invention] FIG. 3 illustrates the present invention It is a circuit for doing. In FIG. 3, U2 is a non-inverting type amplifier configured by using an operational amplifier, the non-inverting input terminal of which is connected to one end of the feedback resistor Rf, and the inverting input terminal is connected to the reference potential point via the resistance element R1. In addition to being connected, it is also connected to the output terminal via the resistance element R2. D is a diode, Is3 is a constant current source, and Cd is a capacitor. The cathode electrode of diode D is an amplifier
Connected to the output terminal of U2, the anode electrode is a constant current source Is3
Of the operational amplifier U1, and one input terminal of the operational amplifier U1. The set voltage source Es is connected to the other input terminal of the operational amplifier U1. This set voltage source Es has a constant value. The output Ido of the constant current source Is1 is controlled by the output of the operational amplifier U1. Note that, in FIG. 3, the other parts are the same as those in the circuit in FIG. 5, so the same reference numerals as those in FIG. 5 are given and the description of those parts is omitted.

In such a configuration, if the voltage drop that occurs in the feedback resistor Rf due to the output current Ipd of the photodiode Pd is Vin, this Vin is amplified by the amplifier U2 and its amplified output V
2 is added to the cathode electrode of diode D. on the other hand,
The output current Ib of the constant current source Is3 charges the capacitor Cd to a positive voltage. When this charging voltage is about to exceed the minimum value of the output V2 of the amplifier U2, a current flows through the diode D, and as a result, the minimum value of V2 is held in the capacitor Cd. When the response characteristic of the photodiode Pd is ideal, the voltage V2 corresponds to the optical output Po of the laser diode LD, and its minimum value corresponds to the optical power Pr at the time of non-writing. That is, the optical power Pr at the time of non-writing is directly detected by the terminal voltage of the capacitor Cd. This detected voltage is compared with the reference voltage Es in the comparator U1, and the output Ido of the constant current source Is1 is controlled by the voltage of the difference. As a result, the optical power Pr at the time of non-writing is controlled to a constant value.

However, it operates at high speed as a photodiode PD.
Even if a PIN diode is used, this PIN diode has "relaxation response characteristics due to diffusion current" as shown in Fig. 4.
It has been found that an error occurs in Pr even in the circuit of FIG. 3 due to that phenomenon. That is,
At high frequencies on the order of 10MHz, the PIN diode outputs a square-wave Ipd for square-wave light, but for long pulses, the Ipd gradually increases as shown in Fig. 4, and the final The reached value is proportional to the optical output Po of the laser diode LD. This phenomenon in which the current gradually changes is an excessive phenomenon until the minority carriers accumulate in the steady state due to diffusion.
Except for the photodiode, which is so small that this phenomenon can be ignored by a strong electric field, such as a photodiode, it is large or small. This phenomenon is due to the "relaxation response characteristic due to diffusion current" as described above, and it is assumed that the slow increase in the current due to this characteristic is k times as high as the fast pulse response as shown in FIG. This characteristic corresponds to the fact that the sensitivity of the Pin diode is reduced to 1 / (1 + k) at a high frequency of 10 MHz or more compared to the case where the frequency is extremely low. For example, k is 0 at a high frequency.
When Pw = 20 mW, Pr = 0.5 mW, and duty ratio 75% in 5, the calculation must set Es corresponding to Pr = 5.0 mW as the set voltage Es, resulting in a large error.

Here, the present invention utilizes FIG. 3 and improves so that the “relaxation response characteristic due to diffusion current” of the PIN diode does not affect the optical output power of the laser diode at the time of non-writing.

FIG. 1 is a connection diagram of an embodiment of the circuit according to the present invention. In FIG. 1, R3 is a resistance element and C is a capacitor. The resistance element R3 and the capacitor C are connected in series, and this series circuit is connected in parallel to the resistance element R1. In FIG. 1, the other parts are the same as those in FIG. 3 and are therefore given the same reference numerals as those in FIG. 3, and since the basic operation is also the same as in FIG. 1, re-explanation thereof will be omitted. .

The feedback resistance P due to the current Ipd flowing in the photodiode PD
The voltage drop occurring at f is Vin, and the result of amplifying this Vin by the amplifier U2 is V2. By using the PIN diode as the photodiode PD, Ipd gradually increases with time because of the "relaxation response characteristic due to the diffusion current" as described in FIG. Therefore, likewise, Vin gradually increases as shown in FIG. The output V2 obtained by amplifying this Vin by the amplifier U2 naturally cannot obtain an ideal step response, and as a result, an error occurs in the optical power Pr at the time of non-writing at a high frequency as explained in FIG. However, even if Vin is a waveform as shown in Fig. 2 (b), V2
When Pr becomes an ideal step response as shown in Fig. 2 (a), Pr is controlled to a constant value. In the present invention, the resistance element R3 and the condesan C are inserted and connected so that V2 has an ideal step response. This will be described below.

Now, when the time constant of the change of Vin is τ ₀ , Vin shown in FIG. 2B can be expressed as Vin = Va {1 + K (1−ε ^{−t / τ0} )} (2). Note that k represents a constant. Amplifier U2
The current flowing through the resistance element R2 due to the output V2 of
And split into R3. When the current flowing through the resistance element R3 is I3, the capacitor C is charged by this current I3. Assuming that the charging voltage of the capacitor C is Vc, Vc is shown in Fig. 2 (c).
It is represented as.

Now, assume that this voltage Vc is Vc = Va (1 + k) (1-ε- ^{t / τ0} )} (3). Assuming that the voltage applied to the series circuit of the resistance element R3 and the capacitor C is V1, if the amplifier U2 is not saturated, V1 = Vin (4), so I3 = (Vin−Vc) / R3 = (Va.ε ^{−t / τ0} ) / R3 (5) V2 is represented by V2 = Vin + R2 [Vin / R1) + I3]. Therefore, according to this equation and the equations (2) and (5), V2 = Va [1 + k (1-ε- ^{t /} τ ₀ )] + R2 {[Va [1+
^{k (1-ε -t / τ} 0)] / R1 ] _{^{+ Vaε -t / τ 0) /}} R3} = Va (1 + k) -Vakε -t / τ 0 + (R2 / R1) Va (1+
k) - (R2 / R1) Vakε -t / τ 0 + (R2 / R3) Vaε -t / τ 0 = Va (1 + k) (1 + R2 / R1) + Vaε -t / τ 0 [-k-
(R2 / R1) k + ( R2 / R3)] = Va (1 + k) (1 + R2 / R1) + Vaε -t / τ 0 [(R2 / R
3) -k [1+ (R2 / R1)]] (6). As shown in FIG. 2 (a), in order for V2 to be constant after t = 0, the value of the resistance element R3 is first set to (R2 / R3) -k [1+ (R2 / R1)] = 0 ( It is necessary to choose to satisfy 7).

Then, the current Ic flowing through the capacitor C is multiplied by C by differentiating the equation (3) at t = 0 after, Ic = C · dVc / dt = C · Va (1 + k) (1 / τ 0) ε - ^{t /} τ ₀ (8) Now, when the value of C is selected so that the value of the capacitor C becomes C (1 + k) (1 / τ ₀ ) = 1 / R3 (9), (5), (8 ), (9)
Since Ic = I3 (10) from the equation, the assumption of equation (3) is satisfied. That is, the output current waveform of the PIN diode such as the second (b) Vin can be obtained by using the circuit of FIG. 1 according to the present invention to obtain an ideal step response and eliminate the setting error of Pr. .

[Advantages of the Invention] As described above, according to the present invention, it is possible to obtain the optical output control circuit having a small setting error of the optical power at the time of non-writing.

[Brief description of drawings]

FIG. 1 is a connection diagram of an embodiment of an optical output control circuit according to the present invention, FIG. 2 is a waveform diagram for explaining the operation of the circuit of FIG. 1, and FIG. 3 is an optical diagram for explaining the present invention. Connection diagram of control circuit, FIG. 4 is a characteristic diagram of a Pin diode, FIG. 5 is a connection diagram of an example of a conventional circuit, FIG. 6 is a characteristic diagram of a laser diode, and FIGS. 7 and 8 are FIG. It is a figure for demonstrating operation | movement of a circuit. LD ... Laser diode, PD ... Pin diode, U2
…… Amplifier, R1 to R3 …… Resistance element, C, Cd …… Capacitor.

Claims (1)

(57) [Scope of utility model registration request] 1. A current in which a constant current and a pulse current are superimposed is supplied to a laser diode as a forward current, and a voltage drop corresponding to an output current of a pin diode that receives the light of the laser diode is compared with a set voltage. In a light output control circuit for writing information on an optical disk by controlling the light output power by controlling the magnitude of the constant current according to the comparison result, the voltage drop is applied to its non-inverting input terminal. The inverting input terminal is connected to the reference potential point via the resistance element R1, is connected to the output terminal via the resistance element R2, and is connected in parallel to the resistance element R1. The series circuit of the resistance element R3 and the capacitor C is connected. Amplifier, a capacitor Cd that is charged by supplying a constant current, and a diode connected between the capacitor Cd and the output terminal of the amplifier. A minimum value detection circuit for detecting the minimum value of the output of the amplifier, and the resistance elements R3, R1, R2 and the capacitor C when the time constant of the relaxation response due to the diffusion current of the Pin diode is τ _0. Is selected according to the following relationship, and the output of the minimum value detection circuit is compared with the set voltage. Note R3 = {(R1 ・ R2) / (R1 + R2)} × 1 / K C = τ ₀ / R3 (1 + K) where K is a constant