Optical recording and reading equipment

This application is a Continuation application of U.S. application Ser. No. 11/192,082 filed Jul. 29, 2005. Priority is claimed based on U.S. application Ser. No. 11/192,082 filed Jul. 29, 2005, which claims priority to Japanese Application No. 2004-341380 filed on Nov. 26, 2004, all of which is hereby incorporated by reference into this application.

The present invention relates to a data recording and reading equipment such as any of optical disk drives of CD-R, CD-RW, DVD-R, DVD-RW, DVD-RAM, Blu-Ray disk, and HD-DVD for driving/controlling laser beam waveforms when recording/reading data on/from data recording media.

Such optical recording disk drives have all been expanded in capacity and CD drives using conventional infrared laser beams (wavelength: 780 nm), DVD drives using red laser beams (wavelength: 650 nm), and blu-ray disk drives (BD) and high density (HD) DVD drives using blue laser beams are already under manufacturing. In optical disk drives such as optical disk reading/recording drives as described above, each high frequency signal (read RF signals) read from an optical disk includes so-called laser noise generated from the beam irradiated on the optical disk. The characteristic of this laser noise do not differ so much between LSs of CD (wavelength: 780 nm) and DVD (wavelength: 650 nm), since compound semiconductor materials used for those disks, as well as their LD wavelengths are almost the same. In the case of the blue laser used for the next generation DVD drives, however, the characteristic of the LD comes to be different from that of the conventional ones, since a special compound semiconductor material referred to as GaN is used for the blue laser.

For example, the laser noise characteristic is represented as RIN (Relative Intensity Noise). According to the BD standard, this RIN is ruled as −125 dB/Hz or under. FIG. 2 shows a relationship between system noise and laser noise. The system noise is composed of all types of noise such as laser noise, amplification noise, etc. generated from the subject system. At present, the BD employs a one-time speed. In case the speed is improved to a 2-time speed or 4-time speed in the future, however, such system noise is required to be reduced and the main system noise, that is, laser noise is required to be further reduced as the operation of the BD is speeded up more and more.

When writing data into an optical disk drive, the required output value is 30 mW or over at present in case a blue laser is used. On the contrary, the blue laser power on the surface of the disk when in reading is ruled as 0.35 mW+0.1 by the BD standard. And, in order to satisfy the requirement, the output blue laser power when in reading is required to be 2 mW or under. The conventional blue laser cannot satisfy both of the requirements, that is, large output power and less noise generation with any ordinary methods that use the BD respectively.

In order to realize highly accurate data reading, there have been proposed some methods for removing laser noise from read RF signals. For example, the patent document 1 (JP-A No. 183970/2002) discloses a first multiplier 16-1 shown in FIG. 3, in which a read RF signal rf (t) is multiplied by a DC component of an APC monitor output signal m (t) output from an LPF 15 and the signal that is a result of the multiplication is supplied to a calculator 17. In a second multiplier 16-2, a read RF signal rf(t) is multiplied by a laser noise component of the APC monitor output signal m output from an HPF 18 and the signal that is a result of the multiplication is supplied to the calculator 17. And, in the calculator 17, the signal output from the multiplier 16-2 is reduced from the signal output from the multiplier 16-1 to remove both of the laser noise additive noise component and the modulation noise component. Each optical disk drive provided with a reading mechanism is designed to realize desirable reading.

The patent document 2 (JP-A No. 229636/2003) discloses a method that forms an attenuator for adjusting an optical output of the laser beam in a semiconductor laser element that includes a laser beam source for irradiating a laser beam onto a semiconductor substrate and a modulator for modulating the laser beam in the same laser structure as that of the laser beam source.

However, in the case of a method for canceling laser noise with use of a conventional circuit, the circuit configuration must be modified significantly. And, this comes to cause the system design to be complicated and the manufacturing cost to increase.

This is why there has been desired to realize a method that can cancel such laser noise without modifying the circuit configuration. FIG. 4 shows the characteristic of the RIN (Relative Intensity Noise) of blue lasers. As shown in FIG. 4, in case the beam power is low, the RIN increases. And, in case the beam power is high, the RIN decreases. The output beam power at the root of the LD goes down to 2 mW or under when in reading from an optical disk due to the beam output from the surface of the disk and the limited efficiency of the optical pick-up for using the beam. The conventional laser thus causes the RIN to goes over −125 dB/Hz, thereby the read signals are degraded in quality. As shown in FIG. 4, the RIN depends on the laser power so that the more the laser power increases, the more the RIN decreases. This phenomenon is used to reduce the laser noise when in reading from optical disks. It is assumed that the utility efficiency of the beam from LD output to disk surface is 25%. The LD output is 1.4 mW at a reading power of film surface 0.35 mW and the RIN goes over −125 dB/Hz at that time.

In this case, a filter is disposed so as to follow the output of the laser so that the filter functions to lower the transmissivity of the laser beam (50%) when in reading. Then, the laser output is increased while the reading power on the surface of the disk is set at 0.35 mW, thereby reducing the noise up to −125 dB/Hz that is a target value of the noise reduction. On the other hand, when in reading/erasing, the transmissivity of the filter is raised not to lower the power efficiency of the laser beam. When in reading and erasing, the laser beam output becomes larger than that when in reading, so that the laser noise problem as described above is ignorable.

A visible light filter such as a liquid crystal attenuator that is slow in response should not be used to avoid another problem occurrence. FIG. 5 shows a concept of the response time in a process from reading to recording. This liquid crystal attenuator requires about 50 ms for the response time of transmissivity switching. And, this response time, when it is 50 ms, comes to be equivalent to a time within which the subject disk rotates once. In case the laser beam power is not stabilized while the disk rotates once in a switching process from recording to reading or from reading to recording, it causes a problem of the deviation of the focusing servo. This focusing servo deviation is a serious problem that disable detection of disk positions, laser spot defocusing, etc. Those problems are thus required to be solved to employ any of such visible light filters.

Under such circumstances, it is an object of the present invention to provide a data recording and reading equipment capable of reducing the laser noise as described above easily and solving the above conventional response time problems.

In order to solve the above conventional problems, the present invention uses a modulator that modulates high speed signals in optical communications. In each optical fiber communication system, chirping of comparatively large waveforms comes to limit both transmission distance and modulation speed in direct intensity modulation by a laser diode. In case a signal light having chirping passes through an optical fiber having chromatic dispersion (wavelength dispersion), the waveform is usually distorted.

In order to avoid this problem, an external light modulator that does not cause chirping so easily is employed for optical communications. For example, it is reported that such an external light modulator is an MZ modulator that uses LiNbO3 crystal or compound semiconductor crystal. There is also an electro absorption type optical modulator (EA modulator) proposed as another external light modulator that can be driven with a power lower than that of the MZ modulator and can be reduced more in size than the MZ modulator. The EA modulator absorbs a carrier beam according to an applied voltage (see FIG. 6) to generate an intensity-modulated signal beam. For example, an EA modulator that uses compound semiconductor crystal is reported as such a modulator.

This practical EA modulator is provided as a semiconductor chip manufactured with a semiconductor laminating technique. The EA modulator is easily united with a laser diode used as a carrier beam source. Consequently, high output and size reduction can be realized by reducing the coupling loss between the carrier beam source and the modulator. For example, there is a report about an EA-DFB laser semiconductor chip obtained by uniting a DFB (Distributed FeedBack)-LD (laser diode) with an EA modulator into one monolithically.

Hereunder, a general EA-DFB laser semiconductor chip will be described with reference to FIG. 7. FIG. 7 shows a configuration of the EA-DFB laser semiconductor chip in which a modulator and a laser are integrated. An output beam of this semiconductor laser element 10 is inputted to a modulator 11 disposed adjacently, then passed through a beam modulator 2 in the normal state. In case a high frequency modulation signal is applied to the laser element 10, the laser beam is transmitted/absorbed repetitively according to a signal voltage change to become high frequency beam signals.

In this case, modulation is made by the laser beam itself on the subject optical disk. A DC voltage is applied to the modulator to change the voltage between reading and writing. The response time at a power change from reading to writing in the modulator is also improved up to 40 Gb/s (response time: 15 ps, pico: 10⁻¹²) (see FIG. 8). And, because the modulator response time is lower enough than the rotation speed of the present optical disks that is limited at 10,000 rpm (revolutions per minutes (cycle: about 6 milliseconds, milli: 10⁻³), the modulator design can be modified without causing any problem to correspond to the laser power of those optical disks.