JP2000216477A - 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 is applied to a laser diode LD after amplified by an output amplifier. The laser diode LD is turned 'on' at every high frequency. Since laser beam is emitted by multi- mode at rise of the laser diode LD, laser beam is continuously emitted by multi- mode. Since input impedance of an output amplifier 2 is made high and output impedance thereof is made low, the oscillator 1 and a laser element are matched.

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, the output signal of the oscillator has been directly applied to the laser diode. 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 the 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, an oscillation circuit for driving a laser element by a high frequency superposition method comprises an oscillator and an output amplifier for outputting an output oscillation signal of the oscillator to the laser element. The input impedance of the output amplifier is made high and the output impedance is made low.

Further, the oscillator and the output amplifier are integrated on the same semiconductor substrate. Furthermore, the output amplifier comprises an amplifier having a negative feedback loop.

According to the present invention, since the output signal of the oscillator is amplified and then applied to the laser element, an oscillation signal having a stable level can be applied to the laser element. In addition, since the input impedance of the output amplifier is raised and the output impedance is lowered, matching between the laser element and the oscillator can be achieved. As a result, even when the output load is small, a large output can be obtained stably. it can.

[0011]

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, when the laser diode LD rises every unit time, a laser beam including a plurality of wavelengths is emitted from the single mode type laser diode LD in a multi-mode mode. 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 circuit 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.

The collector current of the transistor Tr2 is fed back via the resistor R2. Transistor Tr
In FIG. 2, since the base current and the collector current are in anti-phase relationship with each other, a negative feedback loop is performed. The input impedance of the output amplifier 2 increases due to the negative feedback loop.
Generally, the input impedance of the BIP transistor is low, but the input impedance of the output amplifier 2 is increased by applying negative feedback.

The output impedance of the oscillator 1 is as high as several hundred Ω, and the load of the laser diode LD is as low as several Ω. In general, the input / output impedance of the output amplifier 2 used in a high frequency band of several hundred MHz is set low. However, by setting the input / output impedance of the output amplifier 2 of the present invention to several hundreds Ω and several tens Ω, respectively. The impedance of the oscillator 1 and the laser diode LD can be matched.

[0020]

According to the present invention, since the output signal of the oscillator is amplified by the output amplifier, a stable output signal can be obtained. In addition, by increasing the input impedance of the output amplifier and lowering the output impedance, matching between the oscillator and the laser element can be achieved, so that a high-output signal can be given to the laser element with a small load.

[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 AA01 AA06 BA01 FA05 HA41 HA68 5F073 BA06 EA27 GA12 GA38 5J081 AA03 BB02 CC10 CC11 DD03 EE02 EE03 FF17 FF23 GG05 LL05 LL09 MM01

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 characterized by low impedance. 2. The oscillation circuit according to claim 1, wherein the oscillator and the output amplifier are integrated on the same semiconductor substrate. 3. The oscillation circuit according to claim 1, wherein the output amplifier comprises an amplifier having a negative feedback loop.