JP2000216483A - Oscillation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably obtain a driving signal for driving a laser diode. SOLUTION: An oscillation output signal from an oscillator 1 is applied to a laser diode LD after being amplified by an output amplifier 2. The laser diode LD is turned 'on' at every high frequency. Since a laser beam is emitted by multi-mode at rise of a laser diode, the laser beam is continuously emitted by mult-mode. Furthermore, since the oscillator 1 is set at a large output level and a wide dynamic range, a stable oscillation output can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oscillation circuit for driving a laser element by a high frequency superposition method.

[0002]

2. Description of the Related Art In recent years, optical disk technology has been remarkably developed. The optical disk can be reproduced without contact with the optical disk by laser light. Although there was a CD as an optical disc, it is now widely accepted in the market due to its small size and high-speed accessibility. Under such circumstances, DVD (digital video disc) has appeared. Like the CD technology, the DVD technology irradiates a laser beam onto an optical disc to obtain data by the brightness of the reflected light, and has the same diameter as the CD. Therefore, the DVD technology can apply the conventional CD development and production technology. On the other hand, DVDs have merits not found in CDs. It has a large storage capacity (4.7 GB, CD audio is 740 MB),
For example, the storage capacity is such that all or more image data corresponding to one movie can be stored. Therefore, when the DVD player first appeared on the market, it was intended to play back software such as movies, but since then it has been used as a DVD-ROM in personal computers, and recently writing and reading of data has been difficult. It is used as a DVD-RAM that can be freely operated.

The difference between the DVD and the CD is the difference in the storage capacity. One of the factors is that the track pitch of the DVD is narrower than that of the CD as shown in FIG. Then, at the same time as the track pitch becomes narrow, the size of each pit itself also becomes small. DV
Since D is also a technique for obtaining data by irradiating laser light, when irradiating a DVD with laser, it is necessary to irradiate the target pit and narrow the laser light spot sufficiently so as not to irradiate the pit next to it. There is. Therefore, as shown in FIG. 3, the spot of the DVD must be sufficiently smaller than the CD. Therefore, in the DVD player, in order to make the laser beam easily focus on the disk surface with a small focus,
The wavelength of the laser beam can be made shorter than that of a CD.
The wavelength of the DVD laser light is 635/650 nm.
And 780 nm for CD.

FIG. 4 shows a conventional AP for driving a laser element.
This is a C (auto power control) circuit, and the laser light output from the laser diode LD is not only irradiated onto the DVD but also received by the photodiode PD. Light is converted into an electric signal by the photodiode PD, and at this time, an electric signal corresponding to the amount of laser light is obtained. The APC circuit is controlled in accordance with the electric signal so that the light amount of the emitted laser light becomes constant.

[0005] In DVD, a laser that emits laser light of a plurality of wavelengths in a multi-mode may be used as shown in FIG. Single mode lasers have the drawback that the oscillation frequency is likely to fluctuate due to the temperature characteristics of the laser itself and the return light noise, and that mode jumps are likely to occur, so that the optical output fluctuates and noise tends to occur. is there. However, although a laser diode that emits light at a wavelength of 635/650 nm by multi-mode has been developed, it has disadvantages such as low reliability and high cost.

Therefore, generally, using a high frequency superposition method,
A method of driving a single-mode laser diode to obtain a multi-mode laser beam has been used. The high-frequency superposition method utilizes the fact that a laser diode capable of emitting short-wavelength laser light (single mode) as shown in FIG.
In particular, if the laser diode is a self-excited oscillation type, it tends to be in a multi-mode at the time of rising. As a circuit for achieving the harmonic superposition method, a laser diode L as shown by a dotted line in FIG.
An oscillator OSC for applying a high-frequency signal to D is provided. By applying a high-frequency signal to the laser diode from the outside, the rising state is continued, and laser light including a plurality of wavelengths is emitted. Thus, by using the harmonic superposition method, it is possible to use a single-mode laser diode and look as if using a multi-mode laser diode. 650n in single mode
Laser diodes emitting m laser light are widely available on the market as general-purpose products, and the harmonic superposition method is one of the effective means at present.

[0007]

However, conventionally,
When an oscillator is integrated, a resonance circuit is connected as an external element, but the oscillation frequency and output level of the oscillator sometimes vary due to variations in characteristics of the external element. Even if an external element is carefully selected, the oscillation frequency and output level of the oscillator cannot be stably obtained because of the variation.

[0008]

According to the present invention, there is provided an oscillation circuit for driving a laser element by a high frequency superposition method, comprising: an oscillator; and an output amplifier for outputting an output oscillation signal of the oscillator to the laser element. The oscillator is set to obtain a desired output level and output dynamic range.

In particular, the oscillator comprises a transistor having a base connected to a resonant element, and first and second transistors connected in series between an emitter and a collector of the transistor and having a connection point connected to the base of the transistor. A resistor, first and second capacitors connected in series between the base of the transistor and ground and having a connection point connected to the emitter of the transistor, and a third capacitor connected between the emitter and the collector of the transistor. Characterized by comprising:

Further, the oscillator and the output amplifier are integrated on the same semiconductor substrate.

According to the present invention, a large output level and a wide output dynamic range can be obtained according to the capacity of the third capacitor, and thus a stable oscillation output can be obtained.

[0012]

FIG. 1 is a diagram showing an embodiment of the present invention. Reference numeral 1 denotes an oscillator which oscillates at an oscillation frequency corresponding to the resonance frequency of a resonance element LC and outputs an oscillation signal, and 2 denotes an oscillator 1 Is an output amplifier that amplifies the output signal of the first embodiment. The oscillator 1 and the output amplifier 2 are configured to be integrated on the same semiconductor substrate. Note that the same circuits as those in the conventional example in FIG. 4 are denoted by the same reference numerals as in FIG. 4, and description thereof will be omitted.

In FIG. 1, an oscillator 1 includes a resonance element LC
Oscillates at several hundred MHz according to the resonance frequency of An oscillation signal from the oscillator 1 is amplified by the output amplifier circuit 2. The output amplified signal of the output amplifier circuit 2 is subjected to DC removal by the coupling capacitor C. The output signal after DC removal is
After being superimposed on the bias from the APC circuit, it is applied to the laser diode LD. The relationship between the laser diode LD and the oscillation amplification signal is as shown in FIG. 6, and the bias matches the oscillation start current of the laser diode LD which is a self-excited oscillation type and single mode. The laser diode LD oscillates by the oscillation signal on which the bias is superimposed. When the amplitude is positive, the laser diode LD is turned on, and when the amplitude is negative, the laser diode LD is turned off. Therefore, laser light is emitted from the laser diode LD every unit time.

As described above, the laser diode LD rises every unit time, so that the single mode type laser diode LD emits laser light in a multi-mode, that is, including a plurality of wavelengths. Since the laser diode LD is turned on at a high frequency of several hundred MHz, it looks as if the laser diode LD emits light continuously. Therefore, it appears that the laser diode LD oscillates in multi-mode.

FIG. 2 shows the oscillator 1 and the output amplifier 2 of FIG.
3 is a diagram showing a specific circuit example of FIG. The oscillator 1 is a Colpitts-type oscillator and includes a transistor Tr1 having a base connected to the resonance circuit LC, and two capacitors C1 and C2 connected in series between the base of the transistor Tr1 and the ground GND. And C2
Is connected to the emitter of the transistor Tr1 and the base of the transistor Tr1 via the resistor R1. Since the Colpitts oscillator is a well-known circuit, the description of its operation is omitted.

The output amplifier 2 includes a transistor Tr2 to which an oscillation signal is applied to a base, and a transistor Tr2.
The resistors R2 and R3 set the base bias of R2. Note that the resistors R2 and R3 also serve as feedback resistors. The oscillation signal of the oscillator 1 is applied to the base of the transistor Tr2 and is amplified by the current amplification factor β2. The amplified signal is generated at the collector of the transistor Tr2 and supplied to a laser diode LD (not shown) via an output terminal.

As described above, the oscillator and the output amplifier of FIG. 1 can be constituted by a simple circuit as shown in FIG.
Therefore, even if integrated on the same semiconductor substrate, it is possible to greatly contribute to miniaturization without increasing the chip area.

Therefore, when the present invention is applied to a pickup used in an optical disc reproducing apparatus such as a DVD player or an MD player, the size of the pickup itself can be reduced.

In the oscillator 1, a capacitor C is connected between the collector and the emitter of the transistor Tr1.
3 are connected. By changing the capacitance of the capacitor C3, the output characteristics of the oscillator 1 change. When the capacitor C3 is increased, a wide dynamic range of the oscillator 1 can be secured, but the magnitude of the output signal from the oscillator 1 decreases. On the other hand, when the capacitance of the capacitor C3 is reduced, the output of the oscillator 1 can be increased, but a wide dynamic range cannot be secured. In the present invention, the capacity of the capacitor C3 is determined by making a compromise between the magnitude of the output of the oscillator 1 and the wide dynamic range of the oscillator 1. Therefore, as a result of measuring the output magnitude and the dynamic range of the oscillator 1 while actually changing the capacitor C3, it is desirable to set the capacitance of the capacitor C3 to 2 pF.

[0020]

According to the present invention, a high output level and a wide dynamic range can be ensured according to the capacity of the capacitor, so that a stable oscillation output can be obtained. Further, since the output signal of the oscillator is amplified by the output amplifier, a stable output signal can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific circuit example of FIG. 1;

FIG. 3 is a diagram showing pits and light spots on a CD and a DVD.

FIG. 4 is a block diagram showing a conventional circuit based on a high-frequency superposition method.

FIG. 5 is a characteristic diagram showing a spectrum when a laser emits in a single mode or a multi mode.

FIG. 6 is a characteristic diagram showing characteristics of the laser diode LD.

[Explanation of symbols]

 1 oscillator 2 output amplifier

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masaaki Nakatani 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (72) Inventor Yoshiaki Matsumiya 2-chome, Keihanhondori, Moriguchi-shi, Osaka No. 5 Sanyo Electric Co., Ltd. F term (reference) 5D119 AA12 BA01 DA01 DA05 FA05 HA41 HA68 5F073 BA05 EA15 GA03 GA12 GA38

Claims (3)

[Claims] 1. An oscillation circuit for driving a laser element by a high-frequency superposition method, comprising: an oscillator; and an output amplifier for outputting an output oscillation signal of the oscillator to the laser element. An oscillation circuit which is set so as to obtain an output dynamic range. 2. The oscillator according to claim 1, wherein the oscillator includes a transistor having a base connected to a resonance element, and first and second transistors connected in series between an emitter and a collector of the transistor and having a connection point connected to a base of the transistor. Resistance and
First and second capacitors connected in series between the base of the transistor and ground and connected at the connection point to the emitter of the transistor, and a third capacitor connected between the emitter and the collector of the transistor. The oscillation circuit according to claim 1, wherein: 3. The oscillation circuit according to claim 1, wherein the oscillator and the output amplifier are integrated on a same semiconductor substrate.