JPS63239626A - Signal detecting method - Google Patents

Abstract

PURPOSE:To reproduce optically recorded information with a high SN ratio by setting up a semiconductor laser at a low reflection factor part of an optical recording medium to a natural radiation state, setting up the semiconductor laser at a high reflection factor part to a laser oscillation state and detecting the quantity of light in both the states by a photodetector. CONSTITUTION:Naturally radiated light 40 from the semiconductor laser 1 is radiated to the optical recording medium 4 having the high reflection factor part and the low reflection factor part, reflected light 41 from the medium 40 is made incident upon a light emitting layer of the laser 1, and when the medium 4 has a high reflection factor, the laser 1 is turned to the oscillation state by the reflected light to detect the existence of the laser oscillation state. Thus, an avalanche resonance state oscillating the semiconductor laser like avalanche on the basis of natural radiation beams radiated from the laser 1 and reflected by the part having the high reflection factor and the no-laser oscillation state of the semiconductor laser 1 based upon the reflected light from the part having the low reflection factor are detected. Since an area having large noise at a part near the oscillation threshold of the laser 1 is not used by using said method, the SN ratio and signal modulation can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a signal detection method that utilizes avalanche resonance of light between a semiconductor laser and an optical recording medium, and improves the quality of an information reproduction signal.

[Conventional technology]

Conventional optical heads of this type include, for example, Miyazawa et al.: "Semiconductor laser pickup for PCM deck.

° Electronic Materials, p. 67. As stated in February 1979 issue, it had the structure shown in Figure 10. That is, the light emitted from the semiconductor laser 1 is transmitted through the coupling lens 2 and the condensing lens 3.
The light is then focused onto the optical recording medium 4. The reflected light from the optical recording medium 4 is returned to the semiconductor laser 1 through an optical path opposite to that described above.

The optical output in this case is detected by a photodetector 5 installed at the rear end of the semiconductor laser 1. 6 and 7 are wobbling elements for obtaining a focus error signal and a tracking error signal, and use, for example, a PZT element. The oscillators 8 and 9 drive the PZT element 6° and the PZT element 7 to slightly vibrate in a direction perpendicular to the optical recording medium 4. The focus error signal and the tracking error signal are obtained by phase detecting the feedback light at this time using phase detectors 10 and 11. In the figure.

12 is a support spring, 13 is a focus control actuator, 1
4 is a track control actuator.

[Problem that the invention seeks to solve]

In the conventional optical head as described above, the semiconductor laser driving current during information reproduction is greater than the threshold current, and the laser is always in an oscillating state. When used in this manner, noise in the vicinity of the one-oscillation threshold is generated when a low-output oscillation state is detected to detect a low-reflectance portion of an optical recording medium.

Therefore, there was a drawback that the SN ratio of the reproduced signal was low. Furthermore, if an attempt was made to improve the S/N ratio by increasing the drive current, there was a problem in that the optical recording medium would be destroyed by the laser beam reflected back from the high reflectance portion.

An object of the present invention is to set a semiconductor laser in a low reflectance part of an optical recording medium to a spontaneous emission state and a semiconductor laser in a high reflectance part to a laser oscillation state.

It is an object of the present invention to provide a signal detection method for reproducing optically recorded information with a high SN ratio by detecting the amount of light in both states with a photodetector.

Measures to solve problem C] In order to achieve the above objects, the present invention has two aspects.

By irradiating spontaneous emission light from a semiconductor laser onto an optical recording medium that has a high reflectance area and a low reflectance area.

Reflected light from an optical recording medium is made incident on a light emitting layer of the semiconductor laser, and when the optical recording medium has a high reflectance, the reflected light causes the semiconductor laser to be in an oscillation state, and the presence or absence of a laser oscillation state is detected. By doing so, it is possible to detect information on an optical recording medium.

Further, the driving current when causing the semiconductor laser to emit spontaneous emission light is below a threshold value that causes the semiconductor laser to be in an oscillation state by the reflected light when the optical recording medium is a low reflectance portion, and the optical recording medium 2. The threshold current for causing the semiconductor laser to oscillate due to the reflected light when is a high reflectance portion; 2. It is desirable to cause a semiconductor laser to emit spontaneous emission light and to make the spontaneous emission light enter a light emitting layer of the semiconductor laser.

[Effect]

In this way, the avalanche resonance state in which the semiconductor laser oscillates in an avalanche manner due to the spontaneous emission light of the semiconductor laser reflected by the part with high reflectance, and the spontaneous emission light of the semiconductor laser reflected by the part with low reflectance. By detecting the state in which the semiconductor laser does not oscillate, it is possible to read out information stored in the optical recording medium in the state of high reflectance and low reflectance.

[Principle of the present invention]

FIG. 1 shows the basic configuration of a signal detection method using avalanche resonance consisting of a semiconductor laser and a medium placed opposite to one end of the semiconductor laser. The reflective surface for resonance is composed of three surfaces: two end surfaces of the semiconductor laser and a medium surface. The laser oscillation state due to avalanche resonance of the semiconductor laser is obtained by detecting the output light from the side without the medium with a PIN diode.

In this signal detection method using avalanche resonance, the internal mode in the semiconductor laser and the external mode between the end facet and the medium are matched.

The higher the medium reflectance, the lower the laser oscillation threshold for generating avalanche resonance. therefore.

Depending on the medium reflectance, a relationship between current and optical output <EL characteristic> as shown in FIG. 2 can be obtained. Therefore, if the drive bias current is set to the spontaneous emission state of the semiconductor laser below the threshold value of the low reflectance part, the semiconductor laser will emit light in an avalanche due to the spontaneous emission light of the semiconductor laser reflected from the part with high reflectance. An avalanche resonance state in which the semiconductor laser oscillates and a state in which the semiconductor laser does not oscillate due to natural light of the semiconductor laser reflected from a part with low reflectance are defined as P.
Information on an optical recording medium is detected by an IN diode. Conventionally, the optical output characteristics of a semiconductor laser have been analyzed using a rate equation that expresses the relationship between the electron density N and the photon density 5 over time.

This method was applied to the invention shown in FIG. That is,
If the proportion of feedback light that couples with light in the semiconductor laser active layer (coupling coefficient) is ε, the steady state rate equation for electron density and photon density is expressed as J − using current density and spontaneous emission light coefficient. N - NS = O - one axis (1) S (N - 100) + CN = O ---- (2). Here, it is assumed that the current density, electron density, and photon density are uniform within the active layer, and that the laser oscillates in a single mode.

The I-L characteristics calculated using the above rate equation are shown in FIG. 3 by solid lines. In addition, the actual measured values of the cSP laser used in the experiment (no optical feedback, ε=0) are indicated by circles in the same figure. Figure 3 (
a) is the result of fitting the spontaneous emission coefficient C, and C
=5xlO-4. FIG. 3(b) shows the results of calculating the 1-L characteristics for various ε in the case of optical feedback using this C.

By evaluating ε for various operating conditions (spacing, medium reflectance, laser end face reflectance, etc.), the data signal detection mechanism can be theoretically elucidated.

In the present invention shown in FIG. 1, the effective reflectance RLEFF when looking to the right from the semiconductor laser is R,''=RL(1+(1-R,)2RFq/RL) if the phase term due to subesing is ignored.
---= (3). Here, R5 is the end face reflectance, R2 is the medium reflectance, and ■ is the light transmittance. This amount depends on the beam spread of the feedback light, and is determined by the geometrical conditions of the resonator (active layer thickness, stripe width, spacing, etc.) used in the present invention.

The coupling coefficient ε is expressed using RLEFF as follows.

ε=1-1□H110TH=In(R,”'/R1)/(2aL-In(RoR
,)) ---(4) Here, 'TH is the oscillation threshold when there is optical feedback, 10T, and A is the threshold when there is no optical feedback. Further, α is the absorption coefficient, L is the resonator length of the semiconductor laser, and Ro is the reflectance of the laser end face on the side opposite to the medium.

〔Example〕

FIGS. 4(a) and 4(b) show a first embodiment of the present invention. Figure 4 <a> is a usage state diagram of the optical head, Figure 4 (b)
)' is a configuration diagram of an optical head. The optical head 21 is divided into the semiconductor laser 1 and the photodetector 5 by a separation groove 32 having a width and depth of several μm. 33 is a semiconductor laser substrate, 3
4 is an active layer, 35 is an insulating layer, 36 is a semiconductor laser electrode,
37 is a photodetector electrode, 38 is a light receiving section, and 39 is a common electrode. As shown in FIG. 4(a), the optical head 21 is floated close to the optical recording medium 4. As shown in FIG. That is, the optical head 2
1 is an arm 2 that can move at high speed in the radial direction of the optical recording medium 4;
It is used by being attached to a slider 24 attached to a load spring 23 on 2. Thereby, the focus control of the optical head 21 is maintained at a constant spacing value determined by the load of the load spring 23, the shape 9 weight of the slider 24, and the traveling speed of the optical recording medium 4. Natural light 40 from the semiconductor laser 1 is reflected by the optical recording medium 4 and one reflected light 41 returns to the semiconductor laser 1, and the light output (avalanche resonance signal output) 42 at that time is detected by the light receiving section 38.

As a result, as shown in FIG. 2, the optical output 42 corresponds to the change in the reflectance of the optical recording medium 4 (the presence or absence of information bits), and the spontaneous emission light LM (L) of the semiconductor laser 1 changes depending on the amount of feedback of natural light. State of avalanche-like laser oscillation due to increase a (H)
and changed.

In order to quantitatively clarify the relationship between noise and operating conditions, the effective value of the noise N (mV) is expressed as the frequency characteristic N (f).
Using, N, rns=-Jrjz,, Nz<+>dft<r2-
fl> --- Defined in (5). Here, f1=
30kHz, f2 = 3MHz, resolution 10kHz
It is z. Below, the relationship between Nrffls, injection current, medium reflectance, and spacing will be discussed.

FIG. 5 shows the current dependence of N and ms, and also shows the IL characteristics. At current levels up to the threshold, N
increases with optical output, but tends to saturate and decrease at higher current levels. The peak value of N is about twice as high with optical feedback as with no optical feedback.

FIG. 6 shows the current dependence of N/P depending on the medium reflectance. It can be seen that as the medium reflectance increases, the peak value of the I-N/P curve becomes higher and the width becomes narrower. FIG. 7 shows the current dependence of N/P due to spacing.

It can be seen that the I-N/P curve shifts toward lower currents as the spacing decreases.

However, the current value at which N 2 /P is maximum is approximately 1.02 times the respective threshold value, regardless of the presence or absence of optical feedback, medium reflectance, and spacing.

Therefore, as is clear from Fig. 5, the signal detection method for obtaining a high-quality data signal is to use a spontaneous emission region (+<lL) with less laser noise for a low reflectance region and a high reflectance region. The driving current 1 of the semiconductor laser should be set in the laser oscillation range (IllH) where the laser noise is small.

In order to optimize the operating conditions, a data signal 5N ratio defined by the following equation is introduced.

SNR = 201og. S, , /N,ITl, −=
(6).

Here 5. − is the data signal amplitude, defined by the following equation.

S,, = pH-pL --(7) S shown in Figure 8
IH where noise peaks due to drive current dependence of N ratio
The SN ratio becomes too small, and as the drive current 1 increases, the 5N ratio increases. However, the upper limit of the drive current 1 is limited to a value at which the optical recording medium is destroyed by the optical output, and no influence from 1-2 has been observed.

Next, in order to evaluate the data signal waveform due to the injection current,
The composite resonance output of the high reflectance part and low reflectance part is adjusted to pH
, pL, the modulation degree M is defined by the following equation.

M=(P', -PL)/(P'+PL) -(8) 9th
The figure shows the current dependence of the modulation degree observed using reflectance RF: 0.30 and 0.15. The horizontal axis is the laser oscillation threshold value "TH"'C minus the normalized current value when there is no optical feedback. ○ indicates the actual measured value, and the solid line indicates the theoretical value based on the rate equation taking into account the coupling efficiency of the feedback light. From the figure, the current value I at which the modulation degree is maximum is +H<1. It can be seen that < 1°. Also, this current value 1. is almost the same as the drive current value when the medium permissible reproduction output is 1 mW in FIG.

FIG. 4(c) shows an example corresponding to a modification of the experimental example shown in FIG. 4(b). Namely.

FIG. 4C shows a structure in which a waveguide lens section 60 is disposed at the tip, which can reduce the light beam spot 61 on the optical recording medium 4 and improve the recording density. Reference numeral 62 is an etched mirror surface formed by microfabrication technology and is in contact with the buffer layer 63 (for example, 5i02) forming the waveguide lens portion 60 and the waveguide N64 (for example, glass 7059). A Luneburg lens 65 is made of a dielectric material (for example, 5i-N) with a higher refractive index than the waveguide H64, and has a circular circumference and a semicircular surface. The operation is the same as in FIG. 4(b).

Note that, as is clear from equation (4), by applying an antireflection film to the laser end face on the optical recording medium side, +RL can be reduced and the coupling coefficient ε can be increased. As a result,
The feedback rate of spontaneously emitted light increases, making it easier for the semiconductor laser to oscillate in an avalanche manner.

〔Effect of the invention〕

As explained above, if the signal detection method according to the present invention is used, the noisy region near the two-oscillation threshold is not used, so the S/N ratio and signal modulation can be improved. Also, since it is used in a natural light emitting area, a low drive current is sufficient.

In addition to suppressing the heat generation of the semiconductor laser.

The lifespan of semiconductor lasers can be extended.

[Brief explanation of the drawing]

Figure 1 is a basic configuration diagram of a signal detection method that best represents the features of the present invention, Figure 2 is a diagram of the principle of signal detection according to the present invention, and Figure 3 is a diagram of the relationship between current and optical output of the optical head according to the present invention. ,
Fig. 4 is an example of an optical head using the present invention, Fig. 5 is a diagram showing the current dependence of noise of the present invention, and Fig. 6 is a diagram showing the current dependence of noise with medium reflectance as a parameter. , FIG. 7 is a diagram showing the current dependence of noise with spacing as a parameter, and FIG. 8 is a diagram showing the current dependence of the SN ratio. Figure 9 is a diagram showing the current dependence of the modulation degree, and Figure 10 is for conventional light that uses complex resonance. FIG. In the figure, 1 is a semiconductor laser and 4 is an optical recording medium. 5 is a photodetector, 31 is an optical head, and 32 is an insulating groove. 38 is a light receiving section, and 42 is a light output.

Claims (2)

[Claims] (1) Spontaneous emission light from a semiconductor laser is irradiated onto an optical recording medium having a high reflectance part and a low reflectance part, and the reflected light from the optical recording medium is made incident on the light emitting layer of the semiconductor laser, and the optical recording medium is The semiconductor laser is brought into an oscillation state by the reflected light in a high reflectance portion of the medium, and is brought into a spontaneous emission state in a low reflectance portion of the optical recording medium, and the amount of light in the laser oscillation state and the spontaneous emission state is detected. A signal detection method characterized by reproducing information from a recording medium. (2) When the semiconductor laser emits spontaneous emission light, the driving current is set to be below a threshold value that causes the semiconductor laser to be in an oscillation state by the reflected light when the optical recording medium is a low reflectance portion, and the 2. The signal detection method according to claim 1, wherein when the recording medium has a high reflectance portion, the reflected light causes the semiconductor laser to oscillate at a threshold value or higher.