JP2003006906A - Laser driving device and its method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To equivalently reduce superimposed frequency for a laser light source while its peak power is suppressed, in the case of employing a method for superimposing a high frequency current on the driving current of the laser light source. SOLUTION: In superimposing a high frequency current on the driving current of the laser light source, a frequency signal having a sampling frequency is modulated by the PNM method to generate a superimposing pulse signal; on the basis of this superimposing pulse signal, a high frequency switch current is superimposed on the laser driving current. As a result, while the peak power of the laser light source is suppressed, a full ON-time of the laser light source is secured. Accordingly, when a laser is emitted to an optical recording medium to reproduce information, danger is eliminated for the loss information recorded on the optical recording medium. Additionally, effectively using relaxation oscillation by the high frequency superimposing method, noise can be reduced to improve the S/N of a PD amplifier on the light receiving side.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser driving device and a driving method for controlling a laser light source provided in, for example, a reproducing device for various optical disks by high frequency superposition.

[0002]

2. Description of the Related Art Conventionally, when driving a laser diode used as a light source for reading a signal of an optical disk in a reproducing apparatus for various optical disks, by superposing a high frequency current on a driving current of the laser diode, the optical disk is A laser drive device has been proposed in which noise generated in reflected light from a laser is reduced and RIN (relative noise intensity) is improved (for example, Japanese Patent Laid-Open No. 7-93).
794 and Japanese Patent Laid-Open No. 2000-149302). Further, in a reproducing device that converts the amount of reflected light of an optical disk into a reproducing signal level in which such a laser driving device is used, the reproducing signal level increases in proportion to the laser irradiation light amount, so that the reproducing signal from the photodetector on the light receiving side is The S / N ratio of the PD amplifier for taking out the signal did not cause much problem.

By the way, for example, in a ROM disk (reflectance of about 80%) such as a laser disk for reproduction and a compact disk (CD), the irradiation power (read power) during reproduction is 0.2 to 1.0 mW. . In addition, the read power of the rewritable disc is
O) is about 0.5 to 1.0 mW, and a digital video disc (DVD) is about 0.4 mW.

However, in the ROM disc, the data was not erased by the read power, but in the case of the rewritable disc, as the read power is increased, the power at the time of recording becomes closer to that of the disc. There is a problem that the recorded information is gradually erased. Also,
Recently, the density of optical discs has been increasing, for example 400 nm.
It is expected that a track pitch of 0.6 μm and a shortest recording wavelength of 0.2 μm will be put into practical use by using a laser having a wavelength.
In such a case, if the spot of the laser beam is narrowed down to make it smaller, the power per unit area rises, and the recorded information on the disc becomes easier to erase.

On the other hand, the S / N characteristic of the PD amplifier has been improved year by year, and the input conversion noise current is 5 to 10 p compared with the conventional one.
It is about A / (Hz ^1/2 ) to 1.0 pA / (Hz ^1/2 ), but due to the reduction of read power, shortening of the wavelength of the reproduced signal, and widening of the band due to the request for double speed reproduction as described above. S
/ N is becoming scarce. For example, if the band is doubled, it is necessary to improve the S / N by about 3.0 dB.

[0006]

However, in the laser driving device using the high frequency superposition method for improving the RIN (relative noise intensity) of the laser as described above, the on-time of the laser is positively used in order to positively utilize the relaxation oscillation. Becomes smaller, so that a high frequency of 300 to 600 MHz is obtained in order to obtain the necessary read power. Then, the superposed frequency is 100 to 200 MH.
When the power is reduced to z, the power of the peak portion is tripled, and the problem of erasing the recorded information described above is raised. That is, in the conventional laser drive device using the high frequency superposition method, when the superposition frequency for the laser diode is lowered, the peak power becomes large and the problem that the recording signal disappears occurs. It becomes necessary to increase the frequency, and it becomes impossible to secure a sufficient ON time of the laser diode, which causes a problem such as a decrease in S / N of the PD amplifier.

Therefore, an object of the present invention is to suppress the peak power of the laser light source and equivalently reduce the superimposing frequency for the laser light source, and to secure a sufficient on-time of the laser light source. It is an object of the present invention to provide a laser driving device and a driving method capable of effectively utilizing vibration and improving the S / N of a PD amplifier.

[0008]

In order to achieve the above object, the present invention comprises a drive current supply section for supplying a laser drive current to a laser light source and a high frequency current superposition section for superposing a high frequency current on the laser drive current. In the laser driving device having the above, the high frequency current superimposing section generates a superimposing pulse signal by modulating a frequency signal having a sampling frequency by a PNM method, and a superimposing pulse signal generated by the PNM modulating means. Current control means for superposing a high frequency switch current on the laser drive current based on the above. Further, according to the present invention, in a laser driving method of supplying a laser driving current to a laser light source and superimposing a high frequency current on the laser driving current, a superposition pulse signal is generated by modulating a frequency signal having a sampling frequency by a PNM method. Then, a high frequency switch current is superimposed on the laser drive current based on the superimposing pulse signal.

In the laser drive device of the present invention, when a high frequency current is superimposed on the drive current of the laser light source, a frequency signal having a sampling frequency is modulated by the PNM system to generate a superposition pulse signal, and this superposition pulse signal is superposed. The high frequency switch current is superimposed on the laser drive current based on the pulse signal. Therefore, by using the PNM method, the peak power of the laser light source can be suppressed, and the superposition frequency with respect to the laser light source can be reduced equivalently. By securing a sufficient on time of the laser light source, relaxation oscillation by the high frequency superposition method is achieved. Can be effectively used, and the S / N of the PD amplifier can be improved.

Further, similarly in the laser driving method of the present invention,
When superimposing a high frequency current on the drive current of the laser light source,
A frequency signal having a sampling frequency is modulated by the PNM method to generate a superposition pulse signal, and the high frequency switch current is superposed on the laser drive current based on the superposition pulse signal. Therefore, by using the PNM method, while suppressing the peak power of the laser light source,
The superposition frequency for the laser light source can be reduced equivalently, and by ensuring a sufficient on-time of the laser light source, relaxation oscillation by the high frequency superposition method can be effectively used and the S / N of the PD amplifier can be improved.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a laser driving device and a driving method according to the present invention will be described below. The embodiments described below are preferred specific examples of the present invention, and various technically preferable limitations are given. However, the scope of the present invention is not limited to the present invention in the following description. Unless otherwise stated, the present invention is not limited to these embodiments. In the present embodiment, in a laser driving device using high frequency superposition, the superposition high frequency current is P
By controlling using the NM, the superposed frequency is reduced equivalently and the S / of the output signal in the PD amplifier on the light receiving unit side is reduced.
To be able to improve N. That is, the frequency superimposition on the laser drive signal is an on / off operation of the laser power, and the peak of the laser power reaches several times to several tens times the average read power.

Therefore, if this light irradiation is regarded as sampling, the reproduction signal level obtained at that time is proportional to the amount of light, so that the reproduction signal level becomes several to several tens of times.
Here, if the superposed frequency on the laser drive signal can be reduced to the band of the PD amplifier, the peak of the output signal of the PD amplifier can be sampled and a reproduction signal with a good S / N can be obtained. Therefore, in the present embodiment, as a method of changing the frequency equivalently without changing the superimposed waveform, the PNM is used.
(Pulse number modulation)
Is used, a high-quality reproduction signal can be obtained while suppressing the peak power of the laser light source.

FIG. 1 is a block diagram showing a configuration example of a laser driving device according to an embodiment of the present invention, and FIG. 2 is an explanatory diagram showing a concrete example of operation waveforms of the laser driving device shown in FIG. . This laser drive device has a drive transistor (Q3) 10, a current-voltage conversion amplifier 20, a loop filter 30, an error amplifier 40, and a photodiode (PD) as means for driving the laser diode (LD) 1. 50 and a variable resistor 60 for power adjustment. Further, as means for superposing the high frequency current, an oscillator (OSC) 110 and a 1/8 counter (gate means)
120, a gate pulse generator 130, a high frequency gate section 140, and a current control transistor pair (Q1-
Q2) 150, variable current source 160, DC power source 170
Have and.

The drive transistor (Q3) 10 supplies a direct current drive current to the laser diode 1. The light emitting power of the laser diode 1 is detected by the photodiode 50, and the output detection current is a current-voltage conversion amplifier. Feedback to 20. The current-voltage conversion amplifier 20 supplies the detection voltage of the level corresponding to the detection current from the photodiode 50 to the error amplifier 40,
The error amplifier 40 uses the adjustment reference voltage value set by the power adjustment variable resistor 60 and the current-voltage conversion amplifier 2
The detected voltage value from 0 is compared, and a drive voltage having a level corresponding to the difference is supplied to the base of the drive transistor 10,
The drive current of the drive transistor 10 is controlled to be maintained at a constant value corresponding to the adjusted reference voltage value. Further, the loop filter 30 includes the drive transistor 1 as described above.
It is intended to stabilize the operation of the feedback control system of 0.

On the other hand, the oscillator (OSC) 110 oscillates a high frequency signal based on the reference high frequency signal (HF), and in this example, the reference high frequency signal (HF) of 200 MHz.
FREQ) (FIG. 2A) outputs a high-frequency signal of 800 MHz as the original oscillation frequency (FIG. 2).
(B)). In addition, the 1/8 counter 120 includes the oscillator 11
A high frequency signal output from 0 is counted and converted into a sampling frequency signal having a frequency of 1/8.
That is, in this example, the original oscillation frequency of 800 MHz is set to 1
/ 8 to obtain a sampling frequency of 100 MHz (FIG. 2 (c)). The gate pulse generator 130 has a preset number parameter (N) corresponding to the sampling frequency signal from the 1/8 counter 120.
The pulse signal (pulse signal for superimposition) by the PNM is output. In this example, the case where N = 2 is described (FIG. 2D).

The high frequency gate section 140 supplies a drive switch voltage to the gate of the input side transistor Q1 of the current control transistor pair (Q1-Q2) 150 based on the gate pulse output from the gate pulse generator 130. is there. Current control transistor pair (Q1
-Q2) 150 drives the output side transistor Q2 according to the operation of the input side transistor Q1 and superimposes the collector current of the output side transistor Q2 on the drive current of the laser diode 1 as a high frequency superimposed current (high frequency switch current). Is. The variable current source 160 variably controls the current level of the high frequency superimposed current by the switching operation of the current control transistor pair (Q1-Q2) 150 based on the current level control signal (HF LEVEL).
The DC power supply 170 supplies a drive voltage to the gate of the output side transistor Q2.

In the conventional laser driving device, the double-lined block shown in FIG. 1, that is, the 1/8 counter 120, the gate pulse generator 130, and the high frequency gate section 140.
Is not provided, and the high frequency signal from the OSC is directly input to the gate of the transistor Q1. The pulse width of OSC is fixed, and only the frequency is variable. On the other hand, in the laser drive device shown in FIG. 1, as shown in the operation waveform in FIG.
A high frequency signal of Hz is converted into a sampling frequency of 100 MHz by a ⅛ counter 120, and this is converted into a gate pulse modulated by a PNM of N = 2 by a gate pulse generator 130, and the high frequency current switched by this gate pulse is a laser. It is superimposed on the drive current of the diode 1.

That is, in this example, two waves are extracted from the high frequency signal of 800 MHz and supplied to the transistor Q1. The superposed signal supplied to the transistor Q1 has two waves but a repetition frequency of 100 MHz, and can equivalently perform a laser switching operation of 100 MHz. If N is changed with the laser power kept constant,
As shown in FIG. 3, the peak power is twice as high as N = 1 (FIG. 3 (b)) with respect to the reference high frequency signal (FIG. 3 (a)).
1 times when N = 2 (the same as the example in FIG. 3C), N = 4
It is 1/2 times (Fig. 3 (d)), but the repetition frequency is 1
00 MHz does not change. With such a method, the superposition frequency is equivalently reduced, and control is performed while suppressing the superposition level.

Next, the detailed principle of the laser driving method by the laser driving device according to the present embodiment as described above will be described in comparison with the conventional laser driving method. Figure 4
An example of the laser emission waveform at the time of reading by the high frequency superposition method is shown, the vertical axis shows the laser power level, and the horizontal axis shows the time. 5 is an explanatory diagram showing an example of a disc reproduction signal in the optical disc reproducing device using the laser light shown in FIG.

As shown in FIG. 4, the RIN (relative noise intensity) is improved by intermittently turning on the laser. D at this time
The duty ratio becomes approximately 1: 7.5 at the point of 50% from the peak of the laser power level, becomes approximately 1:15 at the point of 90%, and the frequency is approximately 400 MHz. Since the read power is defined by the DC power, the integrated value of this waveform is 0.3 mW, but the peak power is 2.7 mW, which is the area integrated power (see FIG. 5B). Here, when the emission time is fixed, the peak power increases when the superimposing frequency is lowered,
If the superposition frequency is increased, the peak power will decrease. Since the laser has a natural vibration frequency and causes relaxation vibration at the time of pulse lighting, in order to positively utilize this, processing for improving RIN by adjusting the superposed wavelength to the natural vibration frequency is also performed.

The reproduction of the phase change type disc is performed by irradiating the disc surface with a fixed laser power and reading the change in the reflectance of the disc, which is the power obtained by multiplying the irradiation power by the reflectance. If the capacity of the photodetector (PD) is large and the band of the PD amplifier is sufficiently low with respect to the superposition frequency, the reflected light amount when the average value of the read power is applied to the disc can be obtained. FIG. 5A shows an example of the reflectance of the recorded disc. If the PD capacity is made sufficiently small and the band of the amplifier is made sufficiently high (for example, 5 times or more) with respect to the superposed frequency, the PAM (pu
LSE AMPLITUDEMODULATION) signal is obtained.

FIG. 5C shows a reproduction signal obtained by irradiating a disk having the reflectance as shown in FIG. 5A with a laser as shown in FIG. 5B. The band amplifier only gives the integrated signal and the wide band amplifier gives the PAM signal. The peak envelope signal of the PAM signal is several times the signal wave. Fig. 5 (d) shows a playback PA
This is a magnified version of the M signal, and the crest value of the pulse is composed of the signal level, LD noise, and noise of the PD amplifier, and the portion having no pulse becomes amplifier noise. Note that FIG.
Shows a case where the LD emits a perfect rectangular wave and is completely turned off, and shows a case where no noise is generated when the LD itself is turned off.

FIG. 6 shows an example of the structure of a signal according to the PAM signal processing method. First, the case where the PAM signal is used before the PNM signal used in this example will be described. Figure 6
(A) shows a reproduced signal by the current narrow band amplifier, and there is amplifier noise other than the signal reproduction time, and the contribution rate of the amplifier noise is large. Here, in order to set the contribution rate to 1, the pulse occupancy rate may be set to 1. That is, if the LD is turned on by DC, the contribution rate of the amplifier noise is reduced. However, the LD noise cannot be improved. Figure 6
As shown in (b), when the reproduced PAM signal is sampled, the structure of the PAM signal is preserved, so that the ratio of the reproduced signal level, the LD noise, and the amplifier noise does not change.
Since the signal has a 1 / pulse occupation ratio as compared with FIG. 6A, the apparent amplifier S / N can be improved.

By the way, if the LD noise is sufficiently small,
C / N (carrier to noise ratio) is improved as it is, but L
When the D noise is large, the C / N is not improved merely by increasing the level. On the other hand, in this type of optical disc reproducing apparatus, as disclosed in, for example, Japanese Patent Laid-Open No. 10-124919, a technique for canceling laser noise by using two received light signals (LNC; laser noise canceller in this description). It has been known. When such an LNC is operated, LD noise is −
Although it becomes 30 dB, C / N is determined by the amplifier noise unless the amplifier noise is reduced. Therefore, by using the above PD amplifier output as a PAM signal in combination with such LNC, the C / N can be improved.

Incidentally, FIG. 6C shows an example in which the laser-on portion of the laser reproduction signal subjected to high frequency superposition on the PD amplifier side is subjected to peak detection and the reproduction signal is sampled by the peak detection signal. (D) is also P
This is an example of the case where the laser off portion of the laser reproduced signal superposed on the high frequency is clipped on the D amplifier side and the reproduced signal is sampled from the signal of only the laser on portion.
This is a method of reducing the laser noise of the amplifier, but since it is not directly related to the present invention, its explanation is omitted.

FIG. 7 shows the relationship between sampling and spectrum. If sampling is performed at a frequency twice or more the transmission band, faithful transmission becomes possible. In particular, in order to easily manufacture the interpolation filter, the larger the multiple, the better. FIG. 8 is an explanatory diagram showing various pulse modulation methods, including pulse amplitude modulation (PAM), pulse frequency modulation (PFM), pulse phase modulation (PPM), pulse width modulation (PWM), and pulse number modulation (PNM). , Pulse code modulation (PCM). In this example, the PNM is used to generate the superimposed frequency current. In other words, by expressing the read power corresponding to PAM by the number of pulses, the peak power of the high frequency modulation signal (HFMOD) is suppressed, and the high frequency modulation signal (HFM) is controlled by the period (sampling frequency) of the PNM.
OD) frequency is determined. The optical cutoff frequency of DVR is 24MHz, and the minimum sampling frequency is 48M.
It becomes Hz. In this case, it is assumed that the sampled spectrums are close to each other, so that the frequency is 100 times as large as 100 MHz. Conventional HFMOD
If the frequency is 200 MHz, the sampling frequency is 100
If the frequencies of MHz and PNM are 800 MHz and N = 2, an equivalent read power can be obtained and an equivalent HFMOD can be obtained.
The frequency is 100 MHz.

FIG. 9 shows a PNM frequency of 200 MHz,
An example of the pseudo reproduction signal confirmed by actual measurement when N = 2 and the sampling frequency is 50 MHz is shown. The signal frequency is 2.0 MHz and the PD amplifier band 150
The case of MHz / −3.0 dB is shown. Here, the pseudo reproduction signal is a signal obtained by modulating an LD at 2.0 MHz and amplifying it by an optically coupled PD amplifier, and in actual reproduction, the reflectance of the recording disk is multiplied.
In the figure, the signal waveform Sc is a superposed waveform and the sampling frequency is 50 MHz, so solitary waves are observed, but they overlap when the PD amplifier band is narrow (10 n / div). Further, the signal waveform Sa is a pseudo reproduction waveform, which is a PAM signal having a DC component and an envelope level of 200 mV.
When sampled at pp, 200 mVp-p is obtained (200 mV / div, 100 nS / div). Also,
The signal waveform Sb is a reproduced signal when the signal waveform Sa is band-limited to 20 MHz, and the signal level is ⅕ of 50.
mVp-p, and the ripple is the sampling frequency of 50 MH
z (50 mV / div, 100 nS / div).

With the laser driving device and driving method according to the present embodiment as described above, the following effects can be obtained. (1) An equivalent superposition frequency can be reduced while keeping the peak power as it is. (2) Since the current on-time of the laser diode can be maintained, RIN is not lowered. (3) Peak detection and bottom clipping are possible without extending the band of the PD amplifier to the superposed frequency.
Can be improved by several dB to several dozen dB. (4) If LNC is used together, conventional read power, PD
With the amplifier as it is, the S / N can be improved, and higher density recording / reproducing can be performed. (5) If the LNC is used together, the read power can be reduced by the S / N improvement, and the power reduction of several dB to several tens dB can be realized, and the record information is erased illegally due to the increase of the read power. Can solve the problem.

[0029]

As described above, according to the laser driving apparatus of the present invention, when the high frequency current is superimposed on the driving current of the laser light source, the frequency signal having the sampling frequency is set to P
Modulate by the NM method to generate a superposition pulse signal,
Since the high frequency switch current is superposed on the laser drive current based on the superposition pulse signal, the PNM
By suppressing the peak power of the laser light source by the modulation signal by, the superposition frequency for the laser light source can be reduced equivalently, and by ensuring sufficient on-time of the laser light source, relaxation oscillation by the high frequency superposition method is effectively used. it can. Further, when this laser driving device is used in a reproducing device for an optical recording medium, there is an effect that the S / N in the PD amplifier on the light receiving side can be improved.

Further, according to the laser driving method of the present invention, when a high frequency current is superposed on the driving current of the laser light source, a frequency signal having a sampling frequency is modulated by the PNM system to generate a superposition pulse signal, The high frequency switch current is superimposed on the laser drive current based on the superimposing pulse signal. Therefore, by using the PNM method, the peak power of the laser light source can be suppressed, and the superposition frequency with respect to the laser light source can be reduced equivalently. By securing a sufficient on time of the laser light source, relaxation oscillation by the high frequency superposition method is achieved. Can be used effectively. Also, when this laser driving method is used in a reproducing apparatus for an optical recording medium,
There is an effect that the S / N in the PD amplifier on the light receiving side can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a laser driving device according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a specific example of operation waveforms of the laser driving device shown in FIG.

FIG. 3 is an explanatory diagram showing an example of a PNM waveform in the laser driving device shown in FIG.

FIG. 4 is an explanatory diagram showing an example of a laser emission waveform at the time of reading by a high frequency superposition method.

5 is an explanatory diagram showing an example of a disc reproduction signal in the optical disc reproducing device using the laser light shown in FIG.

FIG. 6 is an explanatory diagram showing a configuration example of a signal according to a PAM signal processing method.

FIG. 7 is an explanatory diagram showing the relationship between sampling and spectrum.

FIG. 8 is an explanatory diagram showing various pulse modulation methods.

9 is an explanatory diagram showing an example of actual measurement of a pseudo reproduction signal when the method using the laser drive device shown in FIG. 1 is used.

[Explanation of symbols]

1 ... Laser diode, 10 ... Driving transistor,
20 ... Current / voltage conversion amplifier, 30 ... Loop filter, 40 ... Error amplifier, 50 ... Photodiode, 60 ... Power adjustment variable resistor, 110 ... Oscillator, 120 ... 1/8 counter, 130 ...... Gate pulse generator, 140 …… High frequency gate section, 150…
… Current control transistor pair, 160… Variable current source,
170 ... DC power supply.

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Claims (7)

[Claims] 1. A laser drive device comprising: a drive current supply section for supplying a laser drive current to a laser light source; and a high-frequency current superposition section for superposing a high-frequency current on the laser drive current, wherein the high-frequency current superposition section is a sample. PNM modulating means for generating a superimposing pulse signal by modulating a frequency signal having a conversion frequency by the PNM method, and superimposing a high frequency switch current on the laser drive current based on the superimposing pulse signal generated by the PNM modulating means. A laser drive device comprising: a current control unit. 2. The high frequency current superimposing section generates an high frequency signal having an original oscillation frequency based on a reference high frequency signal, and divides the high frequency signal generated by the oscillation means to divide the sampling frequency. 2. The laser driving device according to claim 1, further comprising a frequency dividing unit that outputs the frequency signal of. 3. The laser drive device according to claim 1, wherein the current control unit includes a current control transistor pair and a current level control unit that variably controls the output current level of the transistor pair. 4. The laser driving device according to claim 1, wherein the PNM modulating means modulates a frequency signal having the sampling frequency according to a variable setting of a PNM parameter. 5. A reproducing device for reproducing information on the optical recording medium by irradiating the optical recording medium on which information is recorded with laser light and detecting reflected light with a photodetector. The laser drive device according to claim 1. 6. The laser driving device according to claim 5, wherein the optical recording medium is an optical disc. 7. A laser driving method for supplying a laser driving current to a laser light source and superimposing a high frequency current on the laser driving current, wherein a superposition pulse signal is generated by modulating a frequency signal having a sampling frequency by a PNM method. Then, a high frequency switch current is superimposed on the laser drive current based on the superposition pulse signal.