JP2005209988A - Method for driving semiconductor laser and optical disk device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for driving a semiconductor laser which can obtain good RIN (relative intensity noise) in such a state that part of exiting light flux is returned as return light when especially a blue-violet laser is used as the semiconductor laser, and an optical disk device using the method. <P>SOLUTION: When the semiconductor laser is driven by a drive current having a high frequency component multiplexed with a D.C. component, a threshold (350 mW for the blue-violet laser) is provided for a ratio P/D of a peak value P in the exiting power waveform of the semiconductor laser to a duty ratio D. The magnitude of the D.C. component of the drive current of the semiconductor laser and the frequency and the amplitude of a high frequency component thereof are determined so that the P/D value is not lower than the threshold. That is, the RIN of the semiconductor laser is lowered so that, in the single power waveform of the semiconductor laser, a single isolated sharp pulse appears once in one period of the high frequency current. Using such a driving method, the semiconductor laser is driven as the light source of the optical disk device. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a semiconductor laser driving method and an optical disk apparatus using the same, and more particularly, to a semiconductor laser driving method for suppressing an increase in laser noise caused by a part of a light beam emitted from a semiconductor laser returning to the semiconductor laser. The present invention relates to an optical disc apparatus using the above.

  DVD ± RW and DVD-RAM are expanding the market as alternatives to VTRs. However, as terrestrial digital broadcasting started in 2003 and digital HD (High Definition) TV broadcasting is scheduled to start in 2005, optical discs, which are recording media for video content, are required to have higher capacity. ing. An effective means for realizing large-capacity recording on an optical disk is to use a short wavelength light source and a high NA (numerical aperture) objective lens. A blue-violet semiconductor laser and a high NA objective lens of 0.65 or more were used. Development of optical discs exceeding 20 GB / single side is also being actively conducted.

  An LD (semiconductor laser) used for a light source of an optical disk device oscillates in a single mode in the absence of return light, and therefore the laser noise of the LD itself is low. However, in an optical head that records and reproduces data on an optical disk, a part of the light beam reflected from the optical disk returns to the LD that is the light source. For this reason, an external resonator is formed by the surface of the optical disk and the end face of the LD, a longitudinal mode different from the longitudinal mode inherent to the internal resonator of the LD is generated, and energy is exchanged between the two. For this reason, laser noise increases greatly. Conventionally, in order to reduce the laser noise, a method of superposing a high-frequency current on a driving DC current to the LD has been adopted. As a result, a large number of longitudinal modes of the LD are generated to suppress the oscillation of the longitudinal mode caused by the reflected light beam from the optical disc, thereby reducing the laser noise. However, despite the fact that sufficient high-frequency superposition is performed, a phenomenon in which laser noise increases at a specific light output has been found, and a method for driving an LD under conditions in which the laser noise does not appear has been proposed. (For example, refer to Patent Document 1).

  FIG. 8 shows the relationship between LD RIN (Relative Intensity Noise) and average output power, which are described in this document. The LD used is a refractive index guided AlGaInP LD having a wavelength of 650 nm. RIN is low (curve B) if the reflected light from the optical disk does not return to the LD even when high-frequency superimposition is off. However, when a part of the reflected light flux returns to the LD, the laser noise increases remarkably, and although not shown in FIG. 8, RIN> −110 dB / Hz may be obtained. Therefore, high frequency superimposition is turned on to suppress an increase in laser noise (curve A). However, since the LD oscillates in a plurality of longitudinal modes when the high frequency superimposition is on, steady energy exchange occurs between these longitudinal modes, and RIN becomes higher than that in the case of the curve B. However, even when there is return light, the change in RIN from when there is no return light is much smaller than when high-frequency superimposition is off, and RIN tends to decrease as the average output power of the LD increases. It can be said that the maximum value is obtained at specific average output powers P2 and P3. When the high-frequency superimposition is on, the light emission waveform of the LD is pulsed. However, when there is return light from the optical disc, a new oscillation mode is generated with specific average output powers P2 and P3, so one cycle of the high-frequency current is generated. This is because two or more pulses of light emission are generated and laser noise increases accordingly.

In the above document, in order to avoid the occurrence of such oscillation mode, as shown in FIG. 8, when the average output power of the LD was P ^*, the margin ± ΔP * ⁼ ± 0.5mW for P ^* The high-frequency superposition conditions are set so that P2 and P3 are not included in the included (P ^* ± 0.5) mW range (Condition 1). Further, when the average output power is smaller than 10 mW, the frequency fm of the high frequency superimposed current is made as high as possible, and the interval between P2 and P3 is widened to reduce the laser noise. Thus, fm is set so as to satisfy the condition of fr ≧ fm ≧ fr / 5 (condition 2). Further, in order not to generate a plurality of pulsed light emission per cycle of the high-frequency current, a rectangular wave is used for the waveform of the LD driving current, the top level pulse width of this rectangular wave is set to Wp, and the LD oscillation delay time ( The pulse width Wp is limited so as to satisfy Td + (1 / fr) ≦ Wp ≦ Td + (2 / fr), where Td is the time from the rise of the rectangular wave to the rise of the pulsed light emission waveform of the LD. (Condition 3). At this time, the LD is oscillated at the top level of the rectangular wave, and the LD is not oscillated at the bottom level. By satisfying the above conditions, RIN is suppressed in a refractive index guided AlGaInP-based LD having a wavelength of 650 nm.
JP 11-54826 A (page 3-6, FIG. 2-4)

  However, in an optical head capable of recording and reproducing, it is necessary to set the reproducing power in consideration of the following conditions. 1. The LD must be used within the maximum rated power, and the minimum light utilization rate of the optical head (ratio of the output power from the objective lens to the output power of the LD) from the maximum rated power and the recording power to the optical disc. Determined. The light utilization factor of the optical head is set to be larger than the minimum light utilization factor, and the emission power of the LD at the time of recording is determined from the light utilization factor and the recording power to the optical disc. 2. The reproduction power of the optical disc needs to be about 1/10 of the recording power in order to prevent erasure at the time of reproduction of the recording mark recorded at the time of recording. For this reason, in an optical head capable of recording and reproduction, it may be difficult to set the output power of the LD during reproduction so as to satisfy the above-mentioned condition 1.

  Furthermore, when a blue-violet LD having a wavelength in the range of 390 to 420 nm is assumed as the light source for the optical head, there is a blue-violet LD having a relaxation oscillation frequency fr of about 1.5 GHz. The high frequency superposition frequency fm satisfying 2 is 1.5 GHz to 300 MHz. However, in an optical head equipped with a blue-violet LD, when fm is within this frequency range, for example, 350 MHz, the RIN is worst and becomes -110 dB / Hz or higher. In the reproduction of an optical disk, it is desirable that RIN is −125 dB / Hz or less. Therefore, even if fm of blue-violet LD is a value that satisfies Condition 2, it is not always possible to obtain a desired RIN. In addition, the oscillation delay time Td of the blue-violet LD is about 1 ns, and considering the fr value of 1.5 GHz, Wp satisfying the condition 3 is 1.67 to 2.34 ns. When a rectangular wave having a pulse width in this range is used as a high-frequency superimposed waveform, RIN may be a high value exceeding -120 dB / Hz, and a rectangular wave having a pulse width that satisfies the condition 3 is not always desired. Can't get you. Therefore, the blue-violet LD has a problem that it is difficult to always obtain a desired RIN only under the conditions of the prior art.

  The present invention has been made in view of the above problems, and the object thereof is a semiconductor laser capable of realizing good RIN in a state in which a part of an emitted light beam returns as return light particularly when a blue-violet LD is used. And an optical disc apparatus using the same.

In order to achieve the above object, according to the present invention, there is provided a semiconductor laser driving method for driving a semiconductor laser with a driving current in which a high-frequency current is superimposed on a direct current, and a peak value of an emission power waveform of the semiconductor laser A method of driving a semiconductor laser is provided, wherein a ratio P / D between P and a duty ratio D of the waveform is equal to or greater than a predetermined threshold value.
Preferably, the emission power waveform of the semiconductor laser is a waveform in which an isolated single peak pulse appears once in one period of the high-frequency current, and the wavelength of the semiconductor laser is in a range of 390 to 420 nm. The P / D is 350 mW or more.

  In order to achieve the above object, according to the present invention, there is provided an optical disc apparatus using a semiconductor laser as a light source, wherein the semiconductor laser is driven using the semiconductor laser driving method. An apparatus is provided.

  In the semiconductor laser driving method according to the present invention, a threshold value is provided for the ratio P / D between the peak value P of the emission power waveform of the semiconductor laser and the duty ratio D, and the P / D value is equal to or greater than this threshold value. The magnitude of the direct current component of the drive current of the semiconductor laser and the frequency and amplitude of the high frequency component are determined, and the output power waveform of the semiconductor laser is one isolated single peak pulse in one cycle of the high frequency current. Since the waveform appears twice, the RIN of the semiconductor laser can be kept low.

  In addition, since the optical disk apparatus according to the present invention uses a semiconductor laser driven by such a driving method as a light source, noise can be suppressed low during recording / reproduction of the optical disk, particularly reproduction.

Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram for explaining a method of driving a semiconductor laser according to an embodiment of the present invention. A high-frequency current source 16 and a constant current source 15 are connected to a blue-violet LD 1 having a wavelength in the range of 390 to 420 nm via a capacitor C and a coil L, respectively. The blue-violet LD1 is driven by a drive current in which the high-frequency current supplied from the high-frequency current source 16 and the constant DC current supplied from the constant current source 15 are superimposed.

  Next, the light emission operation of the blue-violet LD1 when such a drive current is applied to the blue-violet LD1 will be described. For simplicity of explanation, consider a case where the waveform of the high-frequency current supplied from the high-frequency current source 16 is a rectangular pulse waveform. FIG. 2 is a waveform diagram of the drive current for driving the blue-violet LD1 and the emission power from the blue-violet LD1. The horizontal axis is time. In FIG. 2, the amplitudes of the direct current and the high frequency current are set so that the blue-violet LD1 emits light at the drive current level A and does not emit the blue-violet LD1 at level B. The time width (pulse width) at which the drive current is continuously at level A is set to be 50% of the time width of one cycle of the drive current. Further, the DC component of the drive current is set to a value lower than the threshold current of the blue-violet LD1 so that the average emission power of the blue-violet LD1 becomes the desired value Po, and the frequency and amplitude of the high-frequency current superimposed on the DC component are set. Is decided. As will be described later, the desired value Po is the average emission power of the blue-violet LD1 required when reproducing the signal recorded on the optical disk when the blue-violet LD1 is incorporated in the optical disk apparatus as a light source. Equivalent to.

  As shown in FIG. 2, since the blue-violet LD1 has an oscillation delay time, it starts to emit light with a slight delay from the rise of the rectangular wave of the drive current. However, due to the relaxation oscillation of the LD, the waveform of the emission power of the blue-violet LD1 decreases immediately after reaching the peak value. When the decreasing timing substantially matches the falling of the drive current, the emission power of the blue-violet LD1 quickly returns to zero. Both rising and falling of the output power show a curve that becomes steeper as the peak value of the waveform of the output power is higher due to the response characteristics of the blue-violet LD1. Accordingly, the output power waveform is a pulse waveform having a sharp peak with a peak peak, and a substantially inverse sawtooth waveform with a tail before and after the peak. For example, if the frequency of the driving current is 300 MHz, the time for which the driving current is continuously at level A is 1.67 ns, and if the LD response delay is 1.5 ns, the LD starts to emit light. The maximum time width until the end of light emission is 0.17 ns, which is a very short time. Light is emitted only during that short time, and single pulse light emission with a steep and small duty ratio that immediately returns to 0 is performed. . That is, the time width from the start of light emission to the end of light emission in the LD output power waveform (hereinafter referred to as “light emission duration width”) T2 is 0.17 ns. Thus, the output power waveform of the LD when a rectangular wave is used for the high-frequency current shows a steep rise and fall, but the frequency of the high-frequency current is selected to be lower than the relaxation oscillation frequency. The output power waveform of the LD has a tail. Therefore, when assuming a triangular wave with the light emission duration width T2 as the base and the peak of the LD output power waveform as the apex, the actual light emission time width in the portion where the LD output power is high is assumed in the power portion. The emission energy of one pulse is smaller than the energy given by (1/2 × peak power × T2) with respect to the triangular wave. Here, a value (T2 / T1) obtained by dividing the light emission duration time T2 by the time width T1 of one period (= one period of high-frequency current) of the output power waveform of the LD is defined as a duty ratio D, and the LD When the top level power measured from the output power 0 is defined as the peak power P, even if the duty ratio D is 0.1 and the peak power P is 60 mW, the average power is a square wave of light emission waveform. As a matter of course, it is smaller than 3 mW when assuming that it is a triangular wave. Actually, the average power is about 2.5 mW under the conditions of peak power P = 60 mW and duty ratio D = 0.15.

  FIG. 3 shows a change in RIN with respect to the frequency of the high-frequency current supplied from the high-frequency current source 16, and FIG. 4 shows the duty ratio D of the output power waveform with respect to the frequency of the high-frequency current supplied from the high-frequency current source 16. The change of the peak value P is shown. In any of the figures, the magnitude of the direct current component and the amplitude of the high frequency component of the drive current are adjusted at each frequency so that the average emission power of the blue-violet LD1 is 2.5 mW. The emission power waveform of the blue-violet LD1 is a part of the emitted light beam collected by a lens and incident on an optical fiber arranged behind the lens, and the output is provided with a high-speed photoelectric conversion element. A high-speed waveform display device (for example, a sampling type optical oscilloscope) was used for observation.

  As is apparent from FIG. 3, there is a frequency region of a high-frequency current in which RIN decreases rapidly. As described above, RIN is preferably −125 dB / Hz or less in order to achieve good reproduction characteristics in the optical disc apparatus. Region A in FIG. 3 is a region where RIN is −125 dB / Hz or less. As is clear from FIG. 4, the duty ratio D of the output power is small in the region B, and the peak value P of the output power is large in the region C.

  FIG. 5 shows the time of emission power (arbitrary unit) of the blue-violet LD in the region B [(b)] of FIG. It shows a change. The drive current waveform of the blue-violet LD is shown only on the lower frequency side [(a)] than the region B, but the blue-violet color on the higher frequency side [(c)] than the region B [(b)] The same applies to the drive current waveform of the LD. As apparent from FIG. 5B, in the region B of FIG. 4, the emission power waveform of the blue-violet LD has only a single peak isolated in one cycle, and the emission duration width T2b is very large. The waveform is very narrow. On the other hand, as is clear from FIG. 5A, on the lower frequency side than the region B in FIG. 4, the emission power waveform of the blue-violet LD becomes a waveform having two peaks in one cycle, and the total emission The ratio of the duration time T2a to one cycle is larger than the ratio of the light emission duration time T2b to one cycle in the region B shown in FIG. Although not shown in the figure, the number of peaks of the output power waveform increases to 3 and 4 as the frequency goes further, and the emission duration width becomes wider. When a plurality of peaks are observed in the output power waveform as described above, the maximum peak value is used as the peak value in FIG. Further, as is clear from FIG. 5 (c), the emission power waveform of the blue-violet LD shows only a single peak in one period on the higher frequency side than the region B in FIG. As in FIG. 5A, the ratio of the light emission duration width T2c to one period is 1 of the light emission duration time width T2b in the region B shown in FIG. 5B. It becomes larger than the ratio to the period. As described in Japanese Patent Laid-Open No. 11-54826, the waveform of the output power on the lower frequency side than the region B indicates that a plurality of relaxation oscillation pulses are generated per period. On the lower frequency side than B, it is considered that the increase in laser noise accompanying this occurs.

  FIG. 6 shows the peak value / duty ratio (P / D) of the output power obtained from the data of FIG. 4 as a function of the frequency of the high frequency current supplied from the high frequency current source. Region A in FIG. 6 is a frequency region where RIN in FIG. 3 is −125 dB / Hz or less.

  From FIG. 3 and FIG. 6, there is a clear correlation between RIN and P / D. That is, in region A where RIN is −125 dB / Hz or less, P / D is relatively larger than that in other frequency regions. From FIG. 6, it is clear that when the ratio P / D between the peak value P and the duty ratio D is greater than 350 mW, RIN of −125 dB / Hz or less can be satisfied. Since there is a variation in the characteristics of high frequency superposition for each LD, there is a difference in RIN with respect to frequency for each LD, but the relationship between P / D and RIN shows the above-mentioned tendency, and if P / D is made larger than 350 mW The desired RIN can be obtained.

  Increasing P / D, that is, decreasing the duty ratio D and increasing the peak value P as in the above condition, indicates that the emission waveform of the blue-violet LD is equal to one period of the emission waveform in light of FIG. A waveform in which an isolated single peak pulse appears once in (one period of high-frequency current), suppresses the generation of other light emission waveforms, and fluctuations in power due to competition between those waveforms, and therefore blue-violet It is considered that fluctuation of the average output power of the LD is suppressed, and accordingly, RIN is suppressed low.

  In FIG. 6, the condition that P / D is larger than 350 mW is 230 to 310 MHz from FIGS. 3 and 6 in terms of the frequency of the high-frequency current, and the pulse of the drive current (rectangular pulse) shown in FIG. When converted to a width (1/2 cycle), it is 2.17 to 1.61 ns. The pulse width in this range is included in the range of Condition 3 shown in the conventional example, but it is difficult to suppress the RIN of the blue-violet LD only with Condition 3 of the conventional example as described above.

  In consideration of the individual difference of blue-violet LD, the frequency of the high-frequency current that can actually be used is 200 to 500 MHz. When the blue-violet LD is driven at this frequency, as shown in FIG. 8, the interval between the average output powers that give a maximum value to RIN is widened, and the blue-violet LD is not affected by the laser noise associated therewith. In order to provide a sufficient margin for the average output power, it is preferable that the relaxation oscillation frequency of the blue-violet LD is 3 GHz or less and the frequency of the high-frequency current or more. Since the pulse width of the drive current at the frequency of these high-frequency currents is 1 to 2.5 ns, the time from the rise of the drive current to the rise of the emission power of the blue-violet LD (oscillation delay time) is 0.5 to About 2.5 ns is a preferable condition for causing single peak pulse emission.

  In the above embodiment, the driving current of the blue-violet LD has been described as a rectangular wave. However, even when the driving current of the blue-violet LD is a sine wave, the driving current of the driving current is set so that P / D is larger than 350 mW. By determining the magnitude of the direct current component, the frequency of the high frequency component, and the amplitude, RIN can be set to −125 dB / Hz or less. Further, the pulse width of the drive current need not be limited to 50% of the time width of one cycle of the drive current.

  FIG. 7 is a block diagram of an optical system of the optical disc apparatus according to the present invention. As the light source, a blue-violet LD 11 similar to the blue-violet LD 1 of FIG. 1 is used. Further, the blue-violet LD 11 is connected to an LD drive circuit 6 similar to the LD drive circuit including the constant current source 15 and the high-frequency current source 16 shown in FIG. 1 and the coil L and the capacitor C connected thereto. Has been. The linearly polarized light beam driven by the LD driving circuit 6 and emitted from the blue-violet LD 11 is converted into a parallel light beam by the collimator lens 22 and enters the polarization beam splitter 23. Here, the left-right direction of the paper surface is the X direction, the vertical direction of the paper surface is the Z direction, and the direction orthogonal to the X direction and the Z direction is the Y direction. The polarization beam splitter 23 transmits P-polarized light, that is, linearly polarized light having a polarization axis in the Z direction, and reflects S-polarized light, that is, linearly polarized light having a polarization axis in the Y direction. Since the light beam emitted from the blue-violet LD 11 is adjusted to be P-polarized light, it passes through the polarization beam splitter 23. The light beam that has passed through the polarization beam splitter 23 becomes circularly polarized light by the quarter-wave plate 25, its traveling direction is bent by 90 degrees by the mirror 26, and after being condensed on the recording surface of the optical disk 28 by the objective lens 27, Reflected by the optical disk. The light beam reflected by the optical disk 28 becomes a parallel light beam again by the objective lens 27, passes through the mirror 26, passes through the quarter-wave plate 25, and becomes linearly polarized light whose polarization direction is rotated by 90 degrees with respect to the forward light beam. Polarized light enters the polarizing beam splitter 23. Therefore, the light beam is reflected by the polarization beam splitter 23 and enters the re-condensing lens 29. Then, this light beam is condensed by the re-condensing lens 29 and enters the photodetector 31. The photodetector 31 includes a plurality of light receiving elements (not shown), and a focus and track error signal and a reproduction signal are generated using detection signals from the respective light receiving elements.

  In the LD drive circuit 6 that drives the blue-violet LD 11 so that the ratio P / D of the peak value P and the duty ratio D is greater than 350 mW in the emission power waveform of the blue-violet LD 11, the magnitude of the direct current component of the drive current Set the frequency and amplitude of the high-frequency component. As a result, RIN can be set to −125 dB / Hz or less, and the reproduction characteristics in the optical disc apparatus can be improved.

  In the above description, a blue-violet LD having a wavelength of 390 to 420 nm is used as the semiconductor laser. However, in the semiconductor laser driving method and the optical disk apparatus of the present invention, the semiconductor laser is not limited to the blue-violet LD. Any semiconductor laser may be used. In that case, the DC component of the drive current of the semiconductor laser is large so that the ratio P / D between the peak value P of the output power waveform of the semiconductor laser and the duty ratio D is equal to or greater than the threshold inherent to the semiconductor laser used. The frequency and amplitude of the high frequency component are set.

4 is a block diagram for explaining a method of driving a semiconductor laser according to the present invention. FIG. FIG. 2 is a waveform diagram of drive current and output power of the blue-violet LD of FIG. 1. The curve figure which shows the relationship between the frequency of the high frequency current of FIG. 1, and RIN of blue purple LD. FIG. 2 is a curve diagram showing the relationship between the frequency of the high-frequency current in FIG. FIG. 2 is a waveform diagram of the emission power of a blue-violet LD using the frequency of the high-frequency current in FIG. 1 as a parameter. The curve figure which shows the relationship between the frequency of the high frequency current calculated | required from FIG. 4, and the peak value / duty ratio of the emission power of blue violet LD. 1 is a block diagram of a part of an optical disc apparatus according to the present invention. The curve figure which shows the relationship between the average output power of a conventional semiconductor laser, and RIN.

Explanation of symbols

1,11 Blue-violet LD
6 LD drive circuit 15 Constant current source 16 High frequency current source 22 Collimator lens 23 Polarizing beam splitter 25 1/4 wavelength plate 26 Mirror 27 Objective lens 28 Optical disk 29 Re-condensing lens 31 Photo detector

Claims (7)

A semiconductor laser driving method for driving a semiconductor laser with a driving current in which a high-frequency current is superimposed on a direct current, wherein the ratio P between the peak value P of the emission power waveform of the semiconductor laser and the duty ratio D of the waveform A method for driving a semiconductor laser, wherein / D is equal to or greater than a predetermined threshold value. 2. The semiconductor laser driving method according to claim 1, wherein the emission power waveform of the semiconductor laser is a waveform in which an isolated single peak pulse appears once in one period of the high-frequency current. 3. The method of driving a semiconductor laser according to claim 1, wherein the wavelength of the semiconductor laser is in a range of 390 to 420 nm. 4. The semiconductor laser driving method according to claim 3, wherein the P / D is 350 mW or more. 5. The method of driving a semiconductor laser according to claim 3, wherein a relaxation oscillation frequency of the semiconductor laser is 3 GHz or less and a frequency of the high-frequency current or more. The high-frequency current is a rectangular wave pulse current, and the time from the rise of the pulse to the rise of the emission power of the semiconductor laser is in the range of 0.5 to 2.5 ns. 6. A method for driving a semiconductor laser according to any one of items 1 to 5. An optical disc apparatus using a semiconductor laser as a light source, wherein the semiconductor laser is driven using the semiconductor laser driving method according to claim 1.

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