JP2005346823A - Optical pickup and its drive method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical pickup in which information can be read by reducing influence of returning light at the time of low output operation for semiconductor laser without externally performing high frequency overlap and the information can also be written with high output operation of the semiconductor laser. <P>SOLUTION: Laser 21, as a light source of an optical pickup 20, is equipped with a first electrode 31 to which voltage is individually impressed and a second electrode 32 arranged closer to an optical recording medium 27 than the first electrode. When the information is read, the voltage is impressed to the first electrode with a first voltage value so that it becomes a gain area, and the voltage is impressed to a second electrode with a second voltage value lower than the first voltage value so that it becomes a saturable absorption area, and mode synchronous oscillation is performed. Resonator optical path length Li inside the laser and external optical path length Le are configured so that the following formula [(A+1/4)<Le/Li<(A+3/4); A is integer] is satisfied. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical pickup suitably used for, for example, a computer, a video recording / playback apparatus, an in-vehicle optical disc apparatus, and the like, and a driving method thereof.

  An optical recording medium such as a compact disk (abbreviated CD), a digital versatile disk (abbreviated DVD), a mini disk (abbreviated MD), a magneto-optical disk (abbreviated MO), a Blu-ray disk using a blue-violet laser, or the like ( Here, in an optical disk recording / reproducing apparatus that records / reproduces information on / from an optical recording medium (representatively referred to as an optical recording medium), an optical pickup used for reading / writing data is a light source such as a semiconductor laser. There is a problem that read or write errors occur due to noise.

  A semiconductor laser, which is widely used as a light source for an optical pickup, normally oscillates in a single longitudinal mode. However, when return light reflected by a recording medium enters the semiconductor laser, it shifts to another single longitudinal mode. Sometimes. When such a single longitudinal mode transition occurs, a change in light output level, that is, noise occurs.

  One of the conventional techniques for avoiding the noise problem caused by the return light is to superimpose a single longitudinal mode oscillation into a multiple longitudinal mode oscillation by superimposing a high frequency current of about several hundred megahertz together with a direct current when driving a semiconductor laser. There is one that reduces the influence of the return light noise by changing it (see Patent Document 1).

However, since the technique disclosed in Patent Document 1 requires a high-frequency oscillator, an electromagnetic interference (EMI: EMI) for shielding an electromagnetic wave generated from the high-frequency oscillator is associated therewith.
Further countermeasures against Electro Magnetic Interference are required. Since the electromagnetic wave shield is a large-scale covering the high-frequency oscillator and the semiconductor laser, it hinders downsizing required for the optical pickup. Further, in the case of a blue-violet semiconductor laser having a high driving voltage and high impedance, there is a problem that it is difficult to apply high-frequency superposition.

  As a method for reducing noise without using a high-frequency oscillator, an optical pickup using a self-excited oscillation type semiconductor has been proposed. By appropriately setting the active layer thickness and waveguide of the semiconductor laser, the semiconductor laser self-oscillates without using an external oscillator, and the self-excited oscillation multiplexes the longitudinal mode. Is reduced. Although this self-excited oscillation occurs during a low output operation for reading information from a recording medium, there is a problem that it stops during a high output operation necessary for writing information on the recording medium. As a conventional technique for solving such a problem, by mounting two self-excited oscillation type low-power semiconductor lasers and two high-power semiconductor lasers, a high-frequency superposition circuit is not required and information writing is required. An optical pickup that enables high output operation has also been proposed (see Patent Document 2).

  However, in the optical pickup disclosed in Patent Document 2 in which two self-excited oscillation type low-power semiconductor lasers and two high-power semiconductor lasers are mounted, it is cumbersome to align the optical axes of the two semiconductor lasers during assembly adjustment of the apparatus. Adjustments are necessary, causing problems such as reduced production efficiency.

  In addition, since a blue-violet semiconductor laser using a GaInN-based material for the active layer has a high operating voltage and high impedance, it is difficult to perform high-frequency superposition compared to other semiconductor lasers. Therefore, when a blue-violet semiconductor laser is used as a light source, the polarization of the return light is orthogonalized using a quarter-wave plate, and combined with a polarization beam splitter, the return light is returned to the blue-violet semiconductor laser. Has been proposed (see Patent Document 3).

  However, because the quarter-wave plate is wavelength-dependent, setting a quarter-wave plate effective for a blue-violet semiconductor laser emits laser light having a wavelength different from that of the blue-violet semiconductor laser. There is a problem with lasers that the amount of feedback of the return light is insufficiently suppressed.

  In another conventional technique, a semiconductor laser capable of performing self-excited oscillation or mode-locked oscillation has been proposed as a laser light source having adjustable power and coherence (see Patent Document 4).

  FIG. 8 is a diagram showing a simplified configuration of the semiconductor laser 1 in the prior art. In the conventional semiconductor laser 1, current injection into the waveguide is performed in three regions. A voltage V1 is applied to the first contact 2 and the third contact 4 from the first voltage source 5 through the first and third resistors 7 and 9, respectively. The voltage V 2 is applied from the voltage source 6 through the second resistor 8.

  When the voltage V1 and the voltage V2 are both positive (+), the operation is the same as when the contacts are not divided, and the semiconductor laser 1 oscillates with high output and high coherence. When the voltage V1 is + and the voltage V2 is negative (−), the semiconductor laser 1 oscillates by itself and operates with high output and low coherence. In this way, a low output operation and a high output operation can be performed using one semiconductor laser 1. In the semiconductor laser 1 of the prior art, when the voltage V2 is −, it oscillates in a self-excited oscillation mode with a repetition frequency of about 1 GHz, which is typically related to the relaxation effect, but a mode-locking operation with a repetition frequency of about 10 GHz. It is also possible.

  The difference between self-excited oscillation and similar mode-locked oscillation in the semiconductor laser will be described below.

  (A) In a mode-locked laser, pulsed light having a width shorter than the cavity length reciprocates in the laser cavity. On the other hand, in the self-excited oscillation laser, oscillation and stop are repeated throughout the laser. Such spatially short pulsed light is called mode-locked oscillation because it occurs when the phases of a plurality of longitudinal modes (oscillation wavelengths allowed to exist with respect to the laser resonator optical path length) are aligned. Although a plurality of longitudinal modes occur in normal self-oscillation operation, the phase relationship is not constant unlike mode-locked oscillation.

  (B) In a mode-locked laser, assuming a resonator optical path length of about 3 mm (resonator length × refractive index), a very high-speed pulse train of about 50 GHz is generated (light velocity is c, and laser optical path length is L is obtained at a mode-locked oscillation frequency f = c / 2L). On the other hand, the self-oscillation laser generates a relatively slow pulse train of about several hundred to 1 GHz. Since the oscillation frequency of the mode-locked laser is substantially determined by the laser cavity length, the fluctuation due to the operating state is small. On the other hand, the oscillation frequency of the self-excited oscillation laser changes greatly by changing the optical output or the like.

  (C) Since the mode-locked laser performs pulse oscillation only by providing a saturable absorber in a part of the waveguide, for example, an internal structure for a high-power laser can be used. On the other hand, the normal self-excited oscillation laser has its internal structure such as the active layer thickness of the laser changed for self-excited oscillation, so there is a limit that the kink level becomes low and high output operation cannot be performed. .

  As described above, since the mode-locked laser can use the internal structure for a high-power laser, it is possible to write information to a recording medium by operating at a high output, and it can be used in a plurality of longitudinal modes with the same phase. Since a high-speed pulse train can be generated, it is possible to read out information from the recording medium (low output) by reducing the influence of return light without performing high-frequency superposition.

  However, the characteristics of the mode-locked semiconductor laser are greatly different depending on the position of the saturable absorber. In the case of the semiconductor laser 1 of the prior art, although the noise reduction effect can be exhibited by multiplexing the longitudinal modes, A part of the laser beam that has been reflected follows the optical path as return light reflected by the recording medium and enters the third contact region 4. Since this third contact region 4 is a gain region, the return light is amplified. There is a problem that it becomes a strong noise.

Japanese Patent Publication No.59-9086 JP 2003-243759 A JP 2003-323735 A JP-A-8-330680

  The object of the present invention is to enable reading of information by reducing the influence of return light without performing high-frequency superimposition from the outside during low output operation of the semiconductor laser, and also to write information by high output operation of the semiconductor laser. An optical pickup and a driving method thereof are provided.

  Another object of the present invention is to provide an optical pickup capable of reducing noise without performing high frequency superposition on any wavelength oscillated at a plurality of different center wavelengths, and a driving method thereof.

The present invention provides an optical pickup that uses a laser as a light source, reads information recorded on an optical recording medium using laser light emitted from the laser, and writes information on the optical recording medium.
The laser
A first region to which a voltage is individually applied and a second region disposed closer to the optical recording medium than the first region;
When reading the information recorded on the optical recording medium, a voltage is applied at the first voltage value so as to be a gain region in the first region, and the first region so as to be a saturable absorption region in the second region. The optical pickup is characterized in that a voltage is applied at a second voltage value lower than the voltage value and mode-locked oscillation is performed.

The present invention is characterized in that the following equation is satisfied, where Li is the resonator optical path length in the laser, and Le is the external optical path length from the laser light emitting surface of the laser to the optical recording medium.
(A + 1/4) <Le / Li <(A + 3/4); A is an integer

Further, according to the present invention, the laser includes a light emitting layer, and the light emitting layer is a semiconductor laser made of Al _y Ga _x In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1). It is characterized by.

  The present invention is also characterized in that the laser has a plurality of waveguides that oscillate at different center wavelengths.

In the present invention, the laser
Having M waveguides oscillating at different center wavelengths;
When the resonator optical path length in each waveguide in the laser for each central wavelength is Li _j (j is an integer from 1 to M), and the external optical path length from the laser light emitting surface of the laser to the optical recording medium is Le The following formula is satisfied for each central wavelength simultaneously.
(A _j +1/4) <Le / Li _j <(A _j +3/4); A _j is an integer

According to the present invention, the laser has two first and second waveguides,
The resonator optical path length in the first waveguide is Li ₁ , the resonator optical path length in the second waveguide is Li ₂ (where Li ₂ > Li ₁ ), and the external optical path from the laser light emitting surface of the laser to the optical recording medium When the length is Le and Le ₀ is defined by the following formula [1 / Li ₁ -1 / Li ₂ = N / Le ₀ ; N is an integer], the following formulas are satisfied.
_{3Le 0/4 <Le <5Le} 0/4
(A ₁ +1/4) <Le / Li ₁ <(A ₁ +3/4); A ₁ is an integer (A ₂ +1/4) <Le / Li ₂ <(A ₂ +3/4); A ₂ is an integer

  In the invention, it is preferable that the laser is provided with a window region that is disposed closer to the optical recording medium than the second region to suppress light absorption and emission.

The present invention uses a laser including a first region to which a voltage is individually applied and a second region disposed closer to the optical recording medium than the first region as a light source, and uses laser light emitted from the laser as a light source. In an optical pickup driving method for reading information recorded on an optical recording medium and writing information on the optical recording medium,
When reading information recorded on an optical recording medium,
A voltage is applied to the first region at a first voltage value so as to be a gain region, and a voltage is applied to the second region at a second voltage value lower than the first voltage value so as to be a saturable absorption region. Applied to make the laser mode-oscillate,
When writing information to an optical recording medium,
A driving method of an optical pickup, characterized in that a laser is oscillated by applying a voltage at a voltage value such that both the first region and the second region are gain regions.

In the present invention, when the resonator optical path length in the laser is Li and the external optical path length from the laser light emitting surface of the laser to the optical recording medium is Le, the external optical path length Le is set so as to satisfy the following formula: Then, the laser is oscillated.
(A + 1/4) <Le / Li <(A + 3/4); A is an integer

In the present invention, the laser has M waveguides that oscillate at different center wavelengths.
When the resonator optical path length in each waveguide in the laser for each central wavelength is Li _j (j is an integer from 1 to M), and the external optical path length from the laser light emitting surface of the laser to the optical recording medium is Le ,
The laser is oscillated by setting the external optical path length Le so that the following equation is satisfied at the j-th center wavelength in the j-th waveguide.
(A _j +1/4) <Le / Li _j <(A _j +3/4); A _j is an integer

  According to the present invention, the laser includes a first region to which voltages are individually applied and a second region disposed closer to the optical recording medium than the first region, and is recorded on the optical recording medium. When reading information, a voltage is applied to the first region at a first voltage value so as to be a gain region, and a second voltage lower than the first voltage value is set to be a saturable absorption region in the second region. A voltage is applied at a voltage value, and mode-locked oscillation is performed. The return light emitted from the laser and reflected by the optical recording medium first enters the second region, which is the saturable absorption region, before entering the first region, which is the gain region. Thus, since the return light is absorbed in the saturable absorption region and then enters the gain region, the return light is not amplified and noise due to the return light is suppressed. As a result, the optical disc recording / reproducing apparatus can be reduced in size and cost, and the optical pickup can be reduced in weight, so that high-speed access to information is possible. Here, the saturable absorption region is a region that operates so as to absorb light with respect to weak light less than the saturated absorption amount and operates so as to transmit light with respect to strong light that exceeds the saturated absorption amount. I mean.

  Further, according to the present invention, the following formula [(A + 1/4) <Le / Li <(A + 3/4); A is an integer] is provided between the resonator optical path length Li in the laser and the external optical path length Le. Satisfied. As a result, when the return light reflected by the optical recording medium returns to the vicinity of the light emitting surface of the laser, the pulse inside the laser is on the rear surface side opposite to the light emitting surface of the laser. In the second region of the saturable absorption region near the surface, the reached light is weak and does not transmit light. Therefore, since the return light cannot enter the first region which is the gain region in the laser, noise due to the return light is suppressed.

Further, the present invention is a semiconductor laser in which a light emitting layer provided in the laser is made of Al _y Ga _x In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1). AlGaInN-based semiconductor lasers are difficult to apply high-frequency superimposition and cannot reduce noise due to high-frequency superimposition. However, by using an optical pickup that uses a mode-locked laser, low noise can be achieved without high-frequency superimposition. Therefore, an optical pickup that is small in size and low in cost can be provided.

According to the present invention, the laser has a plurality of, for example, M waveguides that oscillate at different center wavelengths, and the resonator optical path length Li _j (j is 1 to M) in each waveguide in the laser for each center wavelength. When the external optical path length is Le, the following equation [(A _j +1/4) <Le / Li _j <(A _j +3/4); A _j is an integer. ] Is satisfied. This makes it possible to simultaneously reduce noise due to return light to the mode-locked laser with respect to a plurality of center wavelengths with a simple configuration. Here, the center wavelength means a weighted average wavelength of a spectrum oscillating at a plurality of slightly different wavelengths (longitudinal modes) at the time of mode-locked oscillation.

According to the invention, the laser has two first and second waveguides, the resonator optical path length in the first waveguide is Li ₁ , and the resonator optical path length in the second waveguide is Li ₂ . to _(However, Li 2> Li ₁₎ when the formula _{[1 / Li 1 -1 / Li} 2 = N / Le 0; N is an integer] by starting from the Le ₀ defined by the external light path length Le, 3Le _0/4 <by setting probed with a range of Le _<5Le 0/4, the following respective formulas _{[(a 1 +1/4) <Le} / Li 1 <(a 1 +3/4); a 1 is The external optical path length Le satisfying [integer], [(A ₂ +1/4) <Le / Li ₂ <(A ₂ +3/4); A ₂ is an integer] is easily obtained. As a result, in the case of a laser that oscillates at two different center wavelengths, the return light noise can be reduced for any center wavelength.

  According to the invention, the laser further includes a window region that is disposed closer to the optical recording medium than the second region to suppress light absorption and emission. Since this window region does not absorb light, heat generation due to light absorption in the vicinity of the light exit surface where the window region is formed is suppressed. This suppresses the deterioration of the light exit surface during the high power operation of the laser. The window region does not affect the mode-locked oscillation operation and noise.

  According to the present invention, when the optical pickup reads information recorded on the optical recording medium, the optical pickup applies a voltage at the first voltage value so as to be a gain region in the first region, and When a voltage is applied at a second voltage value lower than the first voltage value so as to be a saturable absorption region, the laser is mode-locked and information is written to the optical recording medium, the first region and the second region The laser is driven to oscillate by applying a voltage with a voltage value such that both of the regions become a gain region. This driving method allows mode-synchronized oscillation operation at low output for reading information and normal oscillation operation at high output for writing information, thus enabling recording / reproduction with reduced noise caused by return light. .

  According to the present invention, the resonator optical path length Li in the laser and the external optical path length Le satisfy the following formula [(A + 1/4) <Le / Li <(A + 3/4); A is an integer]. As described above, the external optical path length Le is set to oscillate the laser. As a result, it is possible to maintain a condition in which the noise due to the return light is small even with respect to various fluctuations such as fluctuations in the resonator optical path length Li accompanying temperature changes of the laser.

According to the present invention, the laser has M waveguides that oscillate at different center wavelengths, and the resonator optical path length in each waveguide for each center wavelength is Li _j (j is an integer from 1 to M). Then, in the j-th waveguide, the following equation [(A _j +1/4) <Le / Li _j <(A _j +3/4); A _j is an integer] is satisfied at the j-th central wavelength. Thus, the laser is oscillated with the external optical path length Le set. This driving method can reduce the return light noise even when the center wavelength to be oscillated differs. Although the conditions for reducing the return optical noise simultaneously for a plurality of center wavelengths are severely restricted, such as the actual external optical path length Le may be longer than the desired external optical path length, the external optical path length Le is By making it variably settable, the actual external optical path length Le can be set to a value close to a desired external optical path length.

  FIG. 1 is a diagram showing a simplified configuration of the optical pickup 20 according to the first embodiment of the present invention, and FIG. 2 is a cross-sectional view showing the configuration of the semiconductor laser 21 as viewed along the section line II-II in FIG. FIG.

  The optical pickup 20 includes a semiconductor laser 21 that is a light source, a collimator lens 22, a beam splitter 23, an objective lens 24, a condenser lens 25, and a multi-segment photodetector 26.

  The collimator lens 22 is disposed closest to the semiconductor laser 21 and makes light emitted from the semiconductor laser 21 substantially parallel. The beam splitter 23 transmits the light substantially made parallel by the collimator lens 22 and reflects the return light reflected by the optical recording medium 27. The objective lens 24 condenses the light transmitted through the beam splitter 23 on the information recording surface of the optical recording medium 27. The light condensed by the objective lens 24 is reflected by the optical recording medium 27 to become return light, passes through the objective lens 24 again, is reflected by the beam splitter 23, and is guided to the condenser lens 25. The light incident on the condensing lens 25 is condensed and irradiated to the multi-divided light detector 26 by the condensing lens 25, and a pit signal, a focus error signal, and a tracking error signal are detected by the multi-divided light detector 26.

  The semiconductor laser 21 that is a light source includes a first electrode 31 that is a first region to which a voltage is individually applied and a second electrode 32 that is a second region. The second electrode 32 is disposed closer to the optical recording medium 27 than the first electrode 31, and the first electrode 31 and the second electrode 32 are formed continuously through the separation part 33.

  Since laser light is emitted from the surface of the second electrode 32 facing the collimator lens 22, this surface is referred to as an emission surface 34. On the emission surface 34, for example, a front reflection film 35 having a reflectance of 30% made of a multilayer film of titanium oxide and silicon oxide is formed. On the end surface 36 (referred to as the rear surface 36 for convenience) of the first electrode 31 located on the opposite side of the emission surface 34, a rear surface reflection film 37 having a reflectance of 90% made of, for example, a multilayer film of titanium oxide and silicon oxide is formed. The

  A ridge serving as a waveguide 38 is formed in the semiconductor laser 21, and the oscillated pulse light travels through the waveguide 38 while being repeatedly reflected between the emission surface 34 and the rear surface 36. The length of the waveguide 38 is the resonator length, and the resonator length multiplied by the refractive index (refractive index × resonator length) is the resonator optical path length Li.

  An example of the resonator optical path length Li for the semiconductor laser 21 of the present embodiment is as follows. The length of the first electrode 31 part: 530 μm, the length of the second electrode 32 part: 55 μm, the length of the separation part 33: 15 μm, the sum of these resonator lengths: 600 μm (= 0.6 mm), The resonator optical path length Li obtained by multiplying the resonator length by the refractive index 2.5 (0.6 mm × 2.5) is 1.5 mm.

  On the other hand, the external optical path length Le from the emission surface 34 of the semiconductor laser 21 to the optical recording medium 27 is given by the integral of refractive index × length, and in this embodiment, the external optical path length Le is set to 38.25 mm. The

  The time ti required for the pulsed light oscillated in the semiconductor laser 21 and reflected by the front reflective film 35 to travel through the waveguide 38 and be reflected by the rear reflective film 37 to reach the front reflective film 35 again is expressed by the formula: (2Li / c; where c is the speed of light). On the other hand, the time te required for the light emitted from the emission surface 34 of the semiconductor laser 21 to reach the emission surface 34 again as return light reflected by the optical recording medium 27 is given by the formula: (2Le / c). . Therefore, the ratio (te / ti) of the required time is given as a ratio (Le / Li) between the external optical path length Le and the resonator optical path length Li. In the optical pickup 20 of the present embodiment, the ratio (Le / Li) between the external optical path length Le and the resonator optical path length Li is 25.5 (= Le / Li = 38.25 mm / 1.5 mm).

  The relationship between the required time ratio (te / ti) and the amount of light on the emission surface 34 of the semiconductor laser 21 will be described below. FIG. 3 is a diagram showing the relationship between time and optical path length. In FIG. 3, the horizontal axis is time t, the vertical axis is the optical path length L, and the zero point of the optical path length axis is the emission surface 34 of the semiconductor laser 21, and the external optical path length is on the + side of the optical path length axis. Le indicates the resonator optical path length Li on the negative side of the optical path long axis.

  FIG. 3A shows a state in which the timing at which the return light reaches the emission surface 34 coincides with the timing at which the pulsed light reflected by the rear reflection film 37 reaches the emission surface 34. At this time, the ratio of required time (te / ti), that is, the ratio (Le / Li) between the external optical path length Le and the resonator optical path length Li is represented by an integer, which means that a large amount of light reaches the emission surface 34 at a time. To do.

  On the other hand, in FIG. 3B, the timing at which the return light reaches the emission surface 34 is different from the timing at which the pulsed light reflected by the rear reflective film 37 reaches the emission surface 34, and the return light reaches the emission surface 34. In this case, a state in which the pulsed light reaches the rear reflective film 37 is shown. At this time, the ratio of required time (te / ti), that is, the ratio (Le / Li) between the external optical path length Le and the resonator optical path length Li is represented by (integer +1/2), and the amount of light that reaches the emission surface 34 at a time. Means less.

  When a low output operation for reading information recorded on the optical recording medium 27 is performed, a voltage is applied to the first electrode 31 at a first voltage value, for example, about +4.6 V so as to be a gain region, and the second electrode 32 is applied. Is placed closer to the optical recording medium 27 than the first electrode 31 by applying a voltage at a second voltage value lower than the first voltage value, for example, 0 V, so as to be a saturable absorption region and mode-oscillating. The second electrode 32 operates as a saturable absorber.

  Further, the ratio (Le / Li) between the external optical path length Le and the resonator optical path length Li is set to (integer +1/2), and the mode-locked oscillation is set by setting so that the amount of light reaching the emission surface 34 at a time is reduced. At this time, the second electrode 32 operating as a saturable absorber acts as a closed shutter that absorbs a small amount of light (weak light less than the saturated absorption amount), so that the return light is the gain region. It is possible to reduce the influence of noise due to return light without reaching one electrode 31.

As described above, on the emission surface 34 of the semiconductor laser 21, the return light and the pulse light repeatedly reflected in the waveguide 38 are not superimposed, and the conditions for operating the second electrode 32 as a closed shutter, that is, the return light. Is returned to the emission surface 34, the pulse light in the semiconductor laser 21 is near the rear surface, and the condition that the second electrode 32, which is the saturable absorption region, operates as a closed shutter, is expressed by equation (1). Given.
(A + 1/4) <Le / Li <(A + 3/4); A is an integer (1)

  In the optical pickup of the present embodiment, the ratio (Le / Li) between the external optical path length Le and the resonator optical path length Li is 25.5, that is, (25 + 1/2), which satisfies the condition of Expression (1). Is set as follows.

Note that the condition of equation (1) is relaxed as the pulse width becomes narrower. For example, if the pulse width is 1/10 of the pulse period, the condition is relaxed to the condition expressed by the equation (2).
(A + 1/10) <Le / Li <(A + 9/10); A is an integer (2)

  It should be noted here that the internal optical path length Li is a value derived from the mode synchronization frequency f = 2c / Li. Although there is a method for deriving the effective internal optical path length from the wavelength difference between the two wavelengths that oscillate in the longitudinal mode, the value obtained by this method is different from the value derived from the mode-locked frequency. Affected by chromatic dispersion.

  When information is written to the optical recording medium 27, a voltage of about 6 V is applied to both the first electrode 31 and the second electrode 32 in accordance with the write signal. As a result, both the first electrode 31 and the second electrode 32 operate as a gain region, and for example, a high output operation with a maximum pulse output of 60 mW can be performed. When erasing the information written in the optical recording medium 27, the same voltage is applied to both the first electrode 31 and the second electrode 32, and for example, oscillation is performed with a continuous (CW) output of 30 mW.

Returning to FIG. 2, a configuration example of the semiconductor laser 21 will be described. The semiconductor laser 21 includes an n-type GaN lower contact layer 212, an n-type Al _0.1 Ga _0.9 N lower cladding layer 213, an n-type GaN lower guide layer 214, In _v Ga ₁₋ on an n-type GaN substrate 211. _From an alternately laminated structure (barrier layer / well layer /... well layer / barrier layer) of _v N (0 ≦ v ≦ 1) barrier layers and In _w Ga _1-w N (0 ≦ w ≦ 1) well layers A multiple quantum well active layer 215, a p-type Al _0.2 Ga _0.8 N evaporation preventing layer 216, a p-type GaN upper guide layer 217, and a p-type Al _0.1 Ga _0.9 N upper cladding layer 218, a p-type GaN upper contact layer 219 is formed. In the semiconductor laser 21, a ridge stripe portion 220 to be the waveguide 38 is formed, and a SiO ₂ dielectric film 221 is formed so as to cover the side surface of the ridge stripe portion 220. Further, a p-side electrode 222 is formed on the p-type GaN contact layer 219, and an n-side electrode 223 is formed below the n-type GaN substrate 211.

  The p-side electrode 222 is formed in a laminated structure composed of Pd / Mo / Au in order from the side closer to the p-type GaN upper contact layer 219, and the p-side electrode 222 is divided into the first electrode 31 and the second electrode 32. The separation portion 33 between the first electrode 31 and the second electrode 32 is removed. The n-side electrode 223 is formed by depositing Ti / Al sequentially from the side closer to the n-type GaN substrate 211.

Since the GaInN laser used in this embodiment has a short wavelength, a voltage of 1.24 / wavelength (μm) = 3 V or more is essential. In addition, AlGaN such as the p-type Al _0.1 Ga _0.9 N upper cladding layer used for the cladding is difficult to increase the active carrier concentration and the mobility of the material is small, so that the resistance value is lower than that of AlGaAs and AlGaInP. Is expensive. As a result, in the AlGaAs laser and the AlGaInP laser, the operating voltage at the time of low output operation of about 3 mW is as low as about 2.5 V, but a much larger voltage is required, which is essentially high impedance. Therefore, it has a problem that it is difficult to apply high-frequency superposition. However, by using the optical pickup using the mode-synchronized laser of the present invention, it is possible to achieve low noise without applying high-frequency superposition.

  Note that the lengths of the first electrode and the second electrode in the optical pickup of the present invention are not limited to the values exemplified in this embodiment, and are given to the second electrode when reading information from the optical recording medium. Mode-locked oscillation sufficiently occurs depending on the applied voltage (which is lower than the voltage applied to the first electrode, which is the gain region, and is not limited to 0 V as illustrated) and the thickness of the multiple quantum well active layer It may be adjusted to such a value.

The multiple quantum well active layer 215 in the semiconductor laser 21 in the optical pickup 20 of the present invention includes an In _v Ga _1-v N (0 ≦ v ≦ 1) barrier layer and an In _w Ga _1-w N (0 ≦ w ≦ 1). Although the light-emitting layer has an alternately laminated structure with the well layer (barrier layer / well layer /... Well layer / barrier layer), the present invention is not limited to this, and for the purpose of wavelength adjustment, Al _y Ga _{x In 1-x-y N} (0 ≦ x ≦ 1,0 ≦ y ≦ 1, x + y ≦ 1) barrier _{layer, Al q Ga p In 1-} p-q N (0 ≦ p ≦ 1,0 ≦ q ≦ 1, p + q ≦ 1) A well layer may be used.

  FIG. 4 is a simplified diagram showing the configuration of an optical pickup 40 according to the second embodiment of the present invention, and FIG. 5 is a cross-sectional view showing the configuration of the semiconductor laser 41 as viewed from the section line VV in FIG. FIG. The optical pickup 40 of the present embodiment is similar to the optical pickup 20 of the first embodiment, and corresponding portions are denoted by the same reference numerals and description thereof is omitted.

  The optical pickup 40 is characterized in that a semiconductor laser 41 included in the optical pickup 40 has two first and second waveguides 42 and 43 that oscillate at different center wavelengths. Such a semiconductor laser 41 is a two-wavelength semiconductor laser in which, for example, a semiconductor laser oscillating at a wavelength of 784 nm for CD and a semiconductor laser oscillating at a wavelength of 656 nm for DVD are integrated on one chip.

  The first and second waveguides 42 and 43 of the semiconductor laser 41 are configured similarly to the waveguide 38 of the semiconductor laser 21 in the first embodiment, and the first electrodes 44a and 44b, which are gain regions, and the saturable absorption region. The second electrodes 45a and 45b, and the separators 46a and 46b that divide the electrodes into first electrodes 44a and 44b and second electrodes 45a and 45b, respectively. The length of the first electrodes 44a and 44b in the resonator direction is 530 μm, the length of the second electrodes 45a and 45b in the resonator direction is 50 μm, and the lengths of the separation portions 46a and 46b are 20 μm. The instrument length is 600 μm (= 0.6 mm).

Since the equivalent refractive index for the guided light of the semiconductor laser oscillating at a wavelength of 784 nm is 3.36, and the equivalent refractive index for the guided light of a semiconductor laser oscillating at a wavelength of 656 nm is 3.44, the first waveguide in the semiconductor laser 41 The resonator optical path length Li ₁ of 42 is resonator length × refractive index = 0.6 mm × 3.36 = 2.016 mm, and the resonator optical path length Li ₂ of the second waveguide 43 is resonator length × refractive. Ratio = 0.6 mm × 3.44 = 2.004 mm.

  On the emission surface 34 of the semiconductor laser 41, a front reflective film 47 made of alumina (reflectance 8% for light with a wavelength of 656 nm and reflectivity 20% for light with a wavelength of 784 nm) is formed. A rear reflective film 48 (a reflectance of 60% or more with respect to light of both wavelengths) formed of a multilayer film of alumina and amorphous Si is formed.

  When performing a low output operation for reading information recorded on the optical recording medium 27, the second electrodes 45a and 45b are respectively applied with a voltage so as to have the same potential (0 V) as an n-side electrode 432 described later, and are mode-locked. When performing high output operation of oscillating and writing information on the optical recording medium 27, the first electrodes 44a and 44b and the second electrodes 45a and 45b are normally operated by applying voltages so that they have the same potential. .

In the optical pickup 40 of the present embodiment, in both the first waveguide 42 and the second waveguide 43, in order to reduce noise due to return light during mode-locked oscillation operation, the expressions (3) and (4) ) Is set such that the external optical path length Le is satisfied at the same time.
(A ₁ +1/4) <Le / Li ₁ <(A ₁ +3/4) (3)
(A ₂ +1/4) <Le / Li ₂ <(A ₂ +3/4) (4)

Hereinafter, one method for setting the external optical path length Le will be described. First, Le ₀ defined by equation (5) is obtained. However, in the present embodiment, Li ₂ > Li ₁ as described above.
1 / Li ₁ -1 / Li ₂ = N / Le ₀ ; N is an integer (5)

Here, in the case of N = 1, Le ₀ is given by [Li ₁ · Li ₂ / (Li ₂ −Li ₁ )], and substituting the above values, it is 86.688 mm. Further, from the viewpoint of optical arrangement in the optical pickup, it is preferable that the expression (6) is satisfied between Le ₀ and the external optical path length Le, and is limited to the range where the expression (6) is satisfied. By selecting the optical path length Le, a desired external optical path length Le can be easily selected.
_{3Le 0/4 <Le <5Le} 0/4 ... (6)

In the present embodiment, the external optical path length Le is set to 89.7 mm. As a result, Le / Li ₁ = 44.5 and Le / Li ₂ = 43.5, which satisfy the expressions (3) and (4). Therefore, any of the first and second waveguides 42 and 43 is satisfied. In this case, when the return light reaches the emission surface 34, the second electrodes 45a and 45b operate as a closed shutter, and return light noise is reduced. The appropriate external optical path length Le that satisfies the expressions (3), (4), and (6) is also adjusted by changing the layer configuration of the semiconductor laser to change Li ₁ and / or Li _2. be able to.

  Hereinafter, a configuration example of the semiconductor laser 41 will be described with reference to FIG. As described above, the semiconductor laser 41 is a two-wavelength semiconductor laser for CD and DVD. The laser formed on the n-type GaAs substrate 411 is a CD semiconductor laser 420, which includes an n-type GaAs buffer layer 412, an n-type AlGaAs cladding layer 413, an AlGaAs guide layer 414, a multiple quantum well active layer 415, and an AlGaAs guide layer. 416, a p-type AlGaAs cladding layer 417, a p-type GaAs contact layer 418, and a p-side electrode 419 are sequentially formed, and light is guided along the first ridge 421 that is the first waveguide 42.

  Another laser formed on the n-type GaAs substrate 411 is a DVD semiconductor laser 430, which includes an n-type GaAs buffer layer 422, an n-type AlGaInP cladding layer 423, an AlGaInP guide layer 424, a multiple quantum well active layer 425, An AlGaInP guide layer 426, a p-type AlGaInP cladding layer 427, a p-type GaAs contact layer 428, and a p-side electrode 429 are sequentially formed, and light is guided along the second ridge 431 that is the second waveguide 43. An n-side electrode 432 is formed on the lower side of the n-type GaAs substrate 411.

Although the optical path length of the optical pickup in the present embodiment is fixed, the distance between the collimator lens 22 and the objective lens 24 is changed by an actuator or the like in response to fluctuations in the oscillation wavelength due to temperature or the like. Therefore, a method of always maintaining an optimum state in which return light noise is most suppressed may be used. When this method is used, the optical pickup length can be reduced by reducing the external optical path length Le without being restricted by the restriction that the external optical path length Le is a value in the vicinity of Le ₀ .

  FIG. 6 is a simplified diagram showing the configuration of an optical pickup 50 according to the third embodiment of the present invention, and FIG. 7 is a cross-sectional view showing the configuration of the semiconductor laser 51 as seen from the section line VII-VII in FIG. FIG. The optical pickup 50 of the present embodiment is similar to the optical pickup 20 of the first embodiment, and corresponding portions are denoted by the same reference numerals and description thereof is omitted.

  The semiconductor laser 51 is a DVD semiconductor laser that oscillates at a wavelength of 656 nm, for example. The semiconductor laser 51 is a first window region 55 which is disposed closer to the optical recording medium 27 than the second electrode 53 which is the second region and suppresses light absorption and emission, and a first region. A second window region 56 having characteristics similar to those of the first window region 55 is provided closer to the rear surface 36 than the first electrode 52. The first electrode 52 and the second electrode 53 are divided by the separation unit 54 as in the optical pickup 20 of the previous embodiment.

  A front reflective film 57 made of alumina, for example, having a reflectivity of 6% is formed on the front surface 34 that is the surface of the first window region 55 near the optical recording medium 27, and is opposite to the optical recording medium 27 in the second window region 56. On the rear surface 36 which is the side surface, a rear surface reflection film 58 made of, for example, a multilayer film of alumina and titanium oxide and having a reflectance of 95% or more is formed.

  The first and second window regions 55 and 56 hardly absorb light as described above. Therefore, heat generation at the front surface 34 portion which is a light emitting surface and the rear surface 36 portion which is a light reflecting surface is suppressed. In particular, optical damage (COD: Catastrophic Optical) in which light that is repeatedly reflected is emitted and the front surface 34, which is a light emission surface irradiated with return light, absorbs light and rises in temperature and melts its end surface. Although there is a risk that damage will occur, the generation of COD is prevented by providing the first window region 55 that hardly absorbs light as in the present embodiment.

  Since this window region does not amplify the return light noise, even if it is provided on the side closer to the front surface 34 that is the light emitting surface than the second electrode 53 that becomes the saturable absorption region at the time of mode-locked oscillation, there is an adverse effect. In addition, since it does not inhibit the mode-locked oscillation, it can be disposed in a portion closer to the rear surface 36 than the first electrode 52 serving as a gain region.

  Hereinafter, a configuration example of the semiconductor laser 51 will be described with reference to FIG. The semiconductor laser 51 includes an n-type GaAs buffer layer 512, an n-type AlGaInP cladding layer 513, an AlGaInP guide layer 514, a multiple quantum well active layer 515, an AlGaInP guide layer 516, and a p-type AlGaInP cladding layer 517 on an n-type GaAs substrate 511. The p-type GaAs contact layer 518 and the p-side electrode 519 are formed, and light is guided along the ridge 521 which is a waveguide. An n-side electrode 520 is formed on the lower side of the n-type GaAs substrate 511.

  In the semiconductor laser 51 of the present embodiment, as shown in FIG. 7B, the multiple quantum well active layer 515 includes first to third GaInP well layers 515A, 515C, and 515E each having a thickness of 6 nm. 6 nm first and second AlGaInP barrier layers 515B and 515D are stacked so that the well layers and the barrier layers alternate.

  As a method for forming the first and second window regions 55 and 56 in the semiconductor laser 51, for example, a portion from which an n-type GaAs buffer layer 512 to a p-type GaAs contact layer 518, which are semiconductor layers, are formed and then a window region is to be formed. 7 includes forming a ZnO film not shown in FIG. 7 on the p-type GaAs contact layer 518 and performing a heat treatment at a temperature of about 550 ° C. to 650 ° C. By performing such a heat treatment after the formation of the ZnO film, the laminated structure of the well layer and the barrier layer in the multiple quantum well active layer 515 is averaged, and light absorption here is suppressed. Therefore, it is desirable to provide a window region in the vicinity of the light exit surface (front surface 34) where COD due to light absorption occurs during high output operation.

As described above, in the present embodiment, the optical recording medium is related to a disk such as a CD and a DVD. However, the present invention is not limited to this, and the optical recording medium does not need to have a disk shape, and a laser. The present invention can be applied to all optical recording media that need to suppress the return light noise when reading optical information using. Although an internal resonator type semiconductor laser is used as the laser, the present invention is not limited to this. For example, an external resonator type semiconductor laser that emits light from the rear surface of the laser and then returns the light to the laser by an external reflecting mirror. May be used. Further, the laser is not limited to a semiconductor laser, for example, SHG (Second
A combination of a Harmonic Generation) laser and a saturable absorber may be used.

  It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a figure which simplifies and shows the structure of the optical pick-up 20 which is 1st Embodiment of this invention. It is sectional drawing which shows the structure of the semiconductor laser 21 seen from the cut surface line II-II of FIG. It is a figure which shows the relationship between time and optical path length. It is a figure which simplifies and shows the structure of the optical pick-up 40 which is the 2nd Embodiment of this invention. FIG. 5 is a cross-sectional view showing a configuration of a semiconductor laser 41 as seen from a section line VV in FIG. 4. It is a figure which simplifies and shows the structure of the optical pick-up 50 which is the 3rd Embodiment of this invention. It is sectional drawing which shows the structure of the semiconductor laser 51 seen from the cut surface line VII-VII of FIG. It is a figure which simplifies and shows the structure of the semiconductor laser 1 in a prior art.

Explanation of symbols

20, 40, 50 Optical pickup 21, 41, 51 Semiconductor laser 22 Collimator lens 23 Beam splitter 24 Objective lens 25 Condensing lens 26 Multi-segment photodetector 27 Optical recording medium 31, 44, 52 First electrode 32, 45, 53 Second electrode 33, 46, 54 Separating part 34 Emission surface 35, 47, 57 Front reflective film 36 Rear surface 37, 48, 58 Rear reflective film 38, 42, 43 Waveguide 55 First window region 56 Second window region

Claims (10)

In an optical pickup that uses a laser as a light source, reads information recorded on an optical recording medium using laser light emitted from the laser, and writes information on the optical recording medium.
The laser
A first region to which a voltage is individually applied and a second region disposed closer to the optical recording medium than the first region;
When reading the information recorded on the optical recording medium, a voltage is applied at the first voltage value so as to be a gain region in the first region, and the first region so as to be a saturable absorption region in the second region. An optical pickup, wherein a voltage is applied at a second voltage value lower than the voltage value and mode-locked oscillation is performed. 2. The optical pickup according to claim 1, wherein when the resonator optical path length in the laser is Li and the external optical path length from the laser light emitting surface of the laser to the optical recording medium is Le, the following equation is satisfied.
(A + 1/4) <Le / Li <(A + 3/4); A is an integer The laser has a light emitting layer,
3. The light according to claim _{1, wherein the} light emitting layer is a semiconductor laser made of Al _y Ga _x In _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1). pick up. The laser
3. The optical pickup according to claim 1, further comprising a plurality of waveguides that oscillate at different center wavelengths. The laser
Having M waveguides oscillating at different center wavelengths;
When the resonator optical path length in each waveguide in the laser for each central wavelength is Li _j (j is an integer from 1 to M), and the external optical path length from the laser light emitting surface of the laser to the optical recording medium is Le 5. The optical pickup according to claim 4, wherein the following expression is satisfied simultaneously for each central wavelength.
(A _j +1/4) <Le / Li _j <(A _j +3/4); A _j is an integer The laser has two first and second waveguides;
The resonator optical path length in the first waveguide is Li ₁ , the resonator optical path length in the second waveguide is Li ₂ (where Li ₂ > Li ₁ ), and the external optical path from the laser light emitting surface of the laser to the optical recording medium The length is Le, and when defining Le ₀ by the following formula [1 / Li ₁ −1 / Li ₂ = N / Le ₀ ; N is an integer], the following formulas are satisfied: The optical pickup described.
_{3Le 0/4 <Le <5Le} 0/4
(A ₁ +1/4) <Le / Li ₁ <(A ₁ +3/4); A ₁ is an integer (A ₂ +1/4) <Le / Li ₂ <(A ₂ +3/4); A ₂ is an integer The laser
The optical pickup according to claim 1, further comprising a window region that is disposed closer to the optical recording medium than the second region to suppress light absorption and light emission. Optical recording using a laser having a first region to which a voltage is individually applied and a second region disposed closer to the optical recording medium than the first region as a light source, and using laser light emitted from the laser In an optical pickup driving method for reading information recorded on a medium and writing information on an optical recording medium,
When reading information recorded on an optical recording medium,
A voltage is applied to the first region at a first voltage value so as to be a gain region, and a voltage is applied to the second region at a second voltage value lower than the first voltage value so as to be a saturable absorption region. Applied to make the laser mode-oscillate,
When writing information to an optical recording medium,
A driving method of an optical pickup, characterized in that a laser is oscillated by applying a voltage with a voltage value such that both the first region and the second region are gain regions. When the resonator optical path length in the laser is Li and the external optical path length from the laser light emitting surface of the laser to the optical recording medium is Le, the laser is oscillated by setting the external optical path length Le to satisfy the following formula: 9. The method of driving an optical pickup according to claim 8, wherein:
(A + 1/4) <Le / Li <(A + 3/4); A is an integer The laser has M waveguides that oscillate at different center wavelengths;
When the resonator optical path length in each waveguide in the laser for each central wavelength is Li _j (j is an integer from 1 to M), and the external optical path length from the laser light emitting surface of the laser to the optical recording medium is Le ,
10. The method of driving an optical pickup according to claim 9, wherein the laser is oscillated by setting the external optical path length Le so that the following expression is satisfied at the jth center wavelength in the jth waveguide.
(A _j +1/4) <Le / Li _j <(A _j +3/4); A _j is an integer

Images