JP3619694B2 - Oscillator circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an oscillation circuit for driving a laser element by a high frequency superposition method.
[0002]
[Prior art]
In recent years, optical disk technology has undergone significant development. The optical disk can be reproduced in a non-contact manner with the optical disk by laser light. Although there was a CD as an optical disk, it is now widely accepted by the market due to its small size and high-speed accessibility. Under such circumstances, DVD (Digital Video Disc) has appeared. In the DVD technology, similarly to the CD technology, data is obtained by irradiating an optical disk with laser light and the brightness of the reflected light, and has the same diameter as the CD. Therefore, the DVD technology can apply the development and production technologies of CDs so far. On the other hand, DVD has advantages not found in CD. That is, it has a large storage capacity (4.7 GB, CD audio is 740 MB). For example, it is a storage capacity that can store all or more image data corresponding to one movie. 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, data writing and reading have been performed. It is used as a DVD-RAM that can be freely performed.
[0003]
Although the difference in storage capacity is described as a difference between DVD and CD, one of the factors is that the track pitch of DVD is narrower than that of CD as shown in FIG. As the track pitch becomes narrower, the size of each pit itself becomes smaller. Since DVD is also a technique for obtaining data by irradiating a laser beam, when irradiating a DVD with a laser, it is necessary to sufficiently squeeze the laser beam spot so that the target pit is irradiated and the adjacent pit is not irradiated. There is. Therefore, as shown in FIG. 3, the spot of the DVD must be sufficiently smaller than the CD. Therefore, in a DVD player, the laser beam wavelength can be made shorter than that of a CD so that the laser beam has a small focal point on the disk surface. The wavelength of the laser beam for DVD is 635/650 nm, and that for CD is 780 nm.
[0004]
FIG. 4 shows an APC (auto power control) circuit for driving a conventional laser element. Laser light output from a laser diode LD is not only irradiated on a DVD but also received by a photodiode PD. Light is converted into an electrical signal by the photodiode PD, and at this time, an electrical signal corresponding to the amount of laser light is obtained. In accordance with this electrical signal, the APC circuit is controlled so that the amount of emitted laser light is constant.
[0005]
In a DVD, a laser that emits light in a multimode that emits laser light having a plurality of wavelengths as shown in FIG. 5 may be used. Single-mode lasers have the disadvantage that the oscillation frequency is likely to fluctuate due to the temperature characteristics of the laser itself and the return light noise, and mode jumps are likely to occur, resulting in fluctuations in the optical output and noise. is there. However, a laser diode that emits light at a wavelength of 635/650 nm by a multimode has been developed, but has disadvantages such as low reliability and high cost.
[0006]
Therefore, in general, a method of obtaining a multi-mode laser beam by driving a single-mode laser diode using a high-frequency superposition method has been adopted. The high-frequency superimposing 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 razor diode is of the self-oscillation type, it tends to be multimode at the time of startup. As a circuit for achieving the harmonic superposition method, an oscillator OSC for applying a high-frequency signal to the laser diode LD is provided as shown by a dotted line in FIG. The rising state is continued by applying a high-frequency signal to the laser diode from the outside, and laser light including a plurality of wavelengths is emitted. Therefore, by using the harmonic superposition method, it can be seen as if a multimode laser diode is used using a single mode laser diode. Since laser diodes that emit laser light of 650 nm in a single mode are widely available on the market as general-purpose products, the harmonic superposition method is currently an effective means.
[0007]
[Problems to be solved by the invention]
However, conventionally, the output signal of the oscillator is directly applied to the laser diode. In addition, when an oscillator is integrated, a resonance circuit is connected as an external element, but the output level of the oscillator may vary due to variations in characteristics of the external element. Even if an external element is carefully selected, there is a variation, so that the output level of the oscillator cannot be stably obtained.
[0008]
[Means for Solving the Problems]
The present invention relates to 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, and the operation of the output amplifier is performed in a linear region and a saturation region. It is characterized by being carried out at the boundary.
[0009]
Further, it is characterized by being integrated on the same semiconductor substrate.
[0010]
Particularly, the input bias of the output amplifier is set so that the amplification operation is performed at the boundary between the linear region and the saturation region.
[0011]
According to the present invention, since the output signal from the oscillator is amplified at the boundary between the linear region and the saturation region in the output amplifier, output fluctuation can be suppressed.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a diagram showing an embodiment of the present invention. 1 is an oscillator that oscillates at an oscillation frequency corresponding to the resonance frequency of the resonance element LC, outputs an oscillation signal, and 2 amplifies an output signal from the oscillator 1. It is an output amplifier. The oscillator 1 and the output amplifier 2 are configured to be integrated on the same semiconductor substrate. The same circuits as those in the conventional example of FIG. 4 are denoted by the same reference numerals as those in FIG.
[0013]
In FIG. 1, the oscillator 1 oscillates at several hundred MHz according to the resonance frequency of the resonance element LC. The 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 DC removed by the coupling capacitor C. The output signal after DC removal is superimposed on the bias from the APC circuit and then 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 self-excited oscillation type single-mode laser diode LD. The laser diode LD oscillates by the oscillation signal with the bias superimposed, but the laser diode LD is turned on when the amplitude is positive, and the laser diode LD is turned off when the amplitude is negative. Accordingly, laser light is emitted from the laser diode LD every unit time.
[0014]
Thus, the laser diode LD rises every unit time, so that laser light including a plurality of wavelengths is emitted from the single mode type laser diode LD in multimode. Since the laser diode LD is turned on at a high frequency of several hundred MHz, the laser diode LD appears to emit light continuously. Therefore, it appears that the laser diode LD is oscillating in multimode.
[0015]
FIG. 2 is a diagram showing a specific circuit example of the oscillator 1 and the output amplifier circuit 2 of FIG. The oscillator 1 is a Colpitts type oscillator, and includes a transistor Tr1 having a resonance circuit LC connected to the base, and two capacitors C1 and C2 connected in series between the base of the transistor Tr1 and the ground GND, and the capacitor C1. And the connection point of C2 are connected to the emitter of the transistor Tr1 and the base of the transistor Tr1 through the resistor R1. Since the Colpitts oscillator is a well-known circuit, its description is omitted.
[0016]
The output amplifier 2 includes a transistor Tr2 to which an oscillation signal is applied to the base, and resistors R2 and R3 that set the base bias of the transistor Tr2. 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 amplified by the current amplification factor β2. The amplified signal is generated at the collector of the transistor Tr2 and supplied to the laser diode LD (not shown) via the output terminal.
[0017]
Thus, the oscillator and output amplifier of FIG. 1 can be configured with a simple circuit as shown in FIG. Therefore, even if integrated on the same semiconductor substrate, it can greatly contribute to miniaturization without increasing the chip area.
[0018]
Therefore, when 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.
[0019]
In general, as shown in FIG. 7, the input / output characteristics of a transistor are linear in a region where the input current is small and saturated in a region where the input current is large. In the output amplifier 2, the input bias of the transistor T r 2 is set by resistors R2 and R3. In the present invention, the values of the resistors R2 and R3 are adjusted so that the base bias of the transistor Tr2 becomes a predetermined value. That is, the input base bias to a region where the transistor Tr2 has Kakaru saturated from the linear region is adjusted to the input signal. Therefore, the transistor Tr2 operates in the region A in FIG. 7 with respect to the oscillation signal that is input. Thus, some of the oscillating input signal is amplified in the saturation region of the transistor Tr2, Ru part is amplified in the linear region.
[0020]
As shown in FIG. 7, in the vicinity of the boundary between the linear region and the saturation region, the slope of the output with respect to the input is gentler than other regions, that is, the amplification factor is set low. When the transistor Tr2 is operated at the boundary between the linear region and the saturation region, it is possible to prevent the output of the transistor Tr2 from fluctuating due to variation and increasing output. Therefore, an output signal with a stable level can be generated from the output amplifier 2 .
[0021]
As another effect, since the amplification factor of the output amplifier is set low in the area A of FIG. 7, it is possible to prevent a large output from being output, and as a result, the laser diode LD of the laser diode LD due to an excessive current from the output amplifier is prevented. Destruction can be prevented.
[0022]
【The invention's effect】
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. Further, since the output amplifier is operated at the boundary between the linear region and the saturation region, variations in the output signal can be suppressed.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of the present invention.
FIG. 2 is a circuit diagram illustrating a specific circuit example of FIG. 1;
FIG. 3 is a diagram showing pits and light spots on CDs and DVDs.
FIG. 4 is a block diagram showing a conventional circuit by a high frequency superposition method.
FIG. 5 is a characteristic diagram showing a spectrum when a laser is emitted in a single mode or a multimode.
FIG. 6 is a characteristic diagram showing characteristics of a laser diode LD.
FIG. 7 is a characteristic diagram showing an input / output relationship of the output amplifier 2;
[Explanation of symbols]
1 Oscillator 2 Output amplifier

Claims (3)

In an oscillation circuit for driving a laser element by a high frequency superposition method,
An oscillator, and an output amplifier that outputs an output oscillation signal of the oscillator to a laser element,
An oscillation circuit characterized in that an operation of an output amplifier is performed at a boundary between a linear region and a saturation region. 2. The oscillation circuit according to claim 1, wherein the oscillation circuit is integrated on the same semiconductor substrate. 2. The oscillation circuit according to claim 1, wherein an input bias of the output amplifier is set so that an amplification operation is performed at a boundary between a linear region and a saturation region.

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