JPS6035344A - Light emitting device and optical signal processor using light emitting device - Google Patents

Abstract

PURPOSE:To reduce noise by applying a high-frequency current to a laser for on-off modulation and obtaining plural modes, and increasing the spectrum width of the modes apparently and reducing coherence. CONSTITUTION:The high-frequency current is superposed upon a DC current to impose modulation upon the laser, plural oscillation longitudinal modes are obtained, and the period of high frequency is approximated to a slow oscillation period to impose high-speed modulation upon the laser. When a pulse current shown in a figure (a) is applied while held almost at the threshold value current (Ith) of the laser of a bias current I0, ''light flicker'' (attenuation oscillation characteristic to a laser diode) of resonance frequency of the laser is generated as shown in a figure (b). At this time, when the period T1 of the applied pulse is much longer than the slow oscillation period T2, the width DELTAlambda of a multimode light spectrum is small (figure c). At this time, when the periods T1 and T2 are synchronized with each other, a light output of only the slow oscillation part shown in a figure b' is obtained, the spectrum waveform of the output light is as shown in a figure c', and variation DELTAlambda with wavelength lambda is increased, thereby reducing the coherence.

Description

Detailed Description of the Invention [Technical Field] The present invention relates to a light emitting device F7 equipped with a 9°0 device, a 4”t semiconductor laser element (4ii), and an optical P2/F7 using the same.
3. [Seudan technology] Semiconductor laser diodes (hereinafter simply referred to as lasers), which have good monochromaticity and emit strong light, are used as light sources for signal pickup of optical video disks, optical disk file memories, etc. ) is used. However, the problem of disturbance of image 1 etc. due to laser noise has not yet been solved. Normal laser noise is classified as 31'iD'3'f. These will be briefly explained below. Mode hopping noise
1se) is the laser diode chip temperature (case temperature) or drive! III This occurs when the oscillation mode changes when the current changes.

Scoop noise is noise generated when light emitted from a laser is reflected by an intermediate optical system or a disk serving as a recording medium and is reflected back to the laser. Noise, usually called speckle noise, is noise that occurs during information reproduction (original pickup) due to interference between light beams reflected by a disk or optical system, or between reflected light and laser emitted light. It is. As a countermeasure against these problems, for example, as shown in Japanese Patent Application Laid-Open No. 56-37834, by superimposing a high frequency current on a single mode laser drive current (DC), longitudinal modes are multiplied and mode hopping noise is reduced. , techniques for reducing scoop noise have been proposed. However, although this method can be expected to have some effect on mode hopping noise, it is not sufficiently effective in preventing scoop noise. The combination with the frequency f of the high frequency superimposed on the laser is f
It has become clear through research by the inventors of the present invention that noise cannot be sufficiently reduced unless it is selected so that NC/2° (C is the speed of light). Further, in the method described above, noise reduction could not be expected unless sufficient countermeasures were taken against speckle noise and stray light from each optical component.

[Purpose of the invention]

An object of the present invention is to provide a technique that can reduce scoop noise without considering the combination of optical path length and modulation frequency, and can also reduce mode hopping noise and speckle noise. The objects and novel features of this invention will become apparent from the description of this specification and the accompanying drawings.

[Outline of the invention]

A brief overview of some of the inventions disclosed in this application is as follows.

That is, by applying a high frequency current close to the laser's relaxation oscillation period (resonance frequency) and with a sufficient amplitude to the laser, and modulating the laser on and off at high speed, the laser beam is made into a multi-mode. Furthermore, the above-mentioned objective of reducing noise is achieved by apparently widening the spectral width of each multimode mode and reducing the coherence of laser light.

〔Example〕

A major feature of the present invention is to apparently widen the spectral width of the laser beam, or in other words, to increase the amount of change in the wavelength λ of the laser beam over time, thereby reducing the coherence of the laser beam. The objective is to reduce noise generation. The upper period is brought closer to the laser's unique vibration period called the laser's relaxation vibration period (resonance period),
The method is to modulate the laser at high speed. The M principle of the above-mentioned present invention will be explained below using Figures 1 and 2. 1 and 2 show the transient characteristics of laser modulation and the spectral waveform of laser light. Bias current■. When the pulse current shown in Fig. 1 (3) is maintained near the threshold current (Itb) of the laser and a pulse current as shown in Fig. 1 (3) is applied, light that is repeated nine times at the laser's resonant frequency fr is generated in the rising portion of the laser output light. This light fluctuation is called relaxation vibration, which is a damped vibration unique to the laser diode, and after a certain period of time, the optical output P is maintained at a constant output P. (Figure 1 0
])) In the relaxation oscillation part, the optical output P (that is, the carrier density in the active layer) changes greatly over time, and this is due to changes in the refractive index of the active layer and the multimode laser. This means that the amount of change Δλ in the wavelength λ of light with respect to time is increasing. Period T of applied pulse.

If is sufficiently larger than the relaxation oscillation period T2, the light fluctuation due to relaxation oscillation can be ignored, and the modulated output of the laser beam can be regarded as a constant output Po. The spectral waveform of the multimode laser output light when driven by a pulse wave (a high frequency alternating current wave may also be used) that is sufficiently large compared to 11 is as shown in Fig. 1 (C),
Spectrum ^ (half width of each spectrum energy Ps)
Δλ is small.

Here, if the circumferential tree IT1 of the applied pulse current is synchronized with the relaxation oscillation period T2 as shown in the 21st Buddha, the optical output of only the relaxation oscillation part can be obtained as shown in Figure □□□. . The spectral waveform of the laser output light in this case is as shown in Figure 2. In other words, the change in wavelength λ of multimode laser light over time △λ
(spectral width) increases, and the coherence of the laser output light decreases. If the coherence of laser light decreases,
Noise is less likely to occur even when the light reflected by the optical system returns to the laser chip. In addition, interference between light beams reflected by the optical system is less likely to occur, making it easier to avoid interference between light beams reflected by the optical system. It is also possible to reduce noise when 1'r5 is generated. As described above, the present invention is characterized in that only the fluctuation portion of the laser light is emitted as a continuous light pulse.

FIG. 3 is a diagram showing the basic circuit configuration for implementing the present invention. The semiconductor laser diode 1 is connected to the DC power supply 2 and the high frequency power supply 3 by a tatami current.
Two current sources are inserted to enable independent driving of the semiconductor laser.

FIG. 4(a) is a diagram showing the current (Io)-optical output Φ)0 characteristic of a semiconductor laser. Direct current laser.

, the high-frequency current △Icos (2πft) (f:
frequency.

t: current with superimposed time), I=I. 10△I-cos (2yrft) =-=
- Drive with (1). Figure 4) shows the time variation of the current generated by the laser. At this time, the change in time (1) of the laser emission direction is as shown in FIG. 4(C). That is,
L=Lo △Lcos(2yrft), I(t)>
Ith□−6□. )<□,,H2). Here Ith is the oscillation threshold current, IJOr
ΔL is the DC optical output and AC optical amplitude corresponding to IO + ΔL, respectively. Since the laser oscillates only when the laser drive current (2) exceeds Ith, the laser light output becomes continuous light pulse oscillation. (The ability to modulate the semiconductor laser on and off in this way is an essential chromatic property for making laser light into multiple modes.) Furthermore, in the present invention, as described above, a high-frequency alternating current is superimposed on the laser and drive direct current. The period f has a value close to the relaxation oscillation frequency (resonance frequency) fr of the driven laser. For example, according to the inventor's experiments, a semiconductor laser with a wavelength of 7
80nm, Ith 50mA MCSP (M
odifiedCbanneled 5ubstrat
e Planar) When a 47'J semiconductor laser is used, this relaxation oscillation frequency ifr is approximately 1.8 GH
2, and the frequency (1) of the high frequency current mentioned above is 7 to 2 times fr, that is, 900ΔfH2 or more, 3.
It was confirmed that a noise reduction effect of 6 GH□ or less is highly effective. Also, B H (Buried-Hetcro
) In the case of a dry semiconductor laser, the relaxation oscillation frequency f
r is approximately 2 GH□, and it was also confirmed that the noise reduction effect is high when the period of the superimposed high frequency wave is kept at 7 to 2 times fr.8The frequency of the superimposed high frequency current depends on the laser used. slow fl + vibration period and required i
It is determined at any time according to the noise reduction rate to be used.

Next, an example of a light emitting device (semiconductor laser module device) embodying the basic circuit chamber bottom of the present invention shown in FIG. 3 is shown in FIGS. 5 and 6. FIG. 5 is a perspective view of the light emitting device, and FIG. 6 is a sectional view taken along line xx' of the light emitting device shown in FIG. The light emitting device 9i4 is a laser diode mounting section, 5, package 10° external connection terminals 6, 7, 8.9
etc. is out of the range.

Inside the package 10 is a coil 14. A ceramic substrate 17 on which transistors 15, 16, etc. are mounted is fixed with an adhesive 18. The terminal 13 of the laser diode device is connected to the package 10 through a through hole (not shown). It penetrates the ceramic substrate 17 and is fixed by a solder 19. Aluminum (At) wiring patterns are formed on the ceramic substrate 17 (not shown), and the coil 14, the transistors 15 and 16, and the laser diode device 5 are individually connected thereto. In this way, a desired laser oscillation circuit is grown on the ceramic: ji5 board 1 board 7, and the aluminum wiring is connected to, for example, the external connection terminal 8 via the tin-plated copper heel 12 at the bonding pad (not shown). Connected. The external connection terminals 6, 7゜8.9 are the battery terminal of the high frequency generation circuit, the laser DC power supply terminal, the ground (ground) terminal, and the laser light monitor output terminal, respectively.
When a desired power source is applied to each node 1, a laser beam 11 is emitted from the laser diode device 5.

This laser light is guided to the recording medium by optical means such as a lens, and the recorded signal is read out. This situation is shown in FIG. FIG. 7 is a horizontal diagram for explaining the outline of the pickup device. First, the structure of the laser diode device 5 will be simply described. At the center of the upper surface of the flange 35 made of gold plated with good thermal conductivity such as copper, a stem 21 made of a chisel is vertically provided.

A semiconductor laser chip 23 is fixed to the side surface of the stem 21 (7) via a silicon submount 22. There are two emission surfaces of the laser beam 11 of the chip 23, an upper surface and a lower surface.
An output monitoring light-receiving element (four diode) 25 for receiving light of 1 is provided. The chips 23, 25i, and the receiver 25i are connected to the terminals 13 via gold (Au) wires 24, respectively. The laser light passes through a transparent window 34 provided in a part of the laser package 20 and is emitted.

Next, an overview of the optical pickup device (optical signal processing device) will be explained. The laser beam 11 emitted from the laser chip 23 is turned into parallel light by the collimator lens 26, enters the polarizing prism 27 as it is, passes through a 7-wavelength plate, and becomes circularly polarized light. This circularly polarized light is condensed to several microns by an objective lens 29, and is incident on an information bit 31 of a disk 30, which is a recording medium for example. The light reflected from the disk carries information about the presence or absence of bits.

This reflected light passes through a seven-wavelength plate, is converted into linearly polarized light again, is reflected within a polarizing prism, is concentrated by a cylindrical lens 32, and is incident on a photodiode (detector) 33. Here, the optical signal is converted into an electric signal VC to obtain a reproduced signal. If the light emitting device of the present invention is used as a light source, the coherence of the laser output light will be reduced to some extent, and even if the reflected light from the optical system returns to the laser chip, interference will occur within the laser resonator. difficult,
As a result, noise is generated and the noise is high. In addition, since interference between light beams reflected by optical components can be reduced, there will be no interference stripes on the light receiving surface of the photodiode 33 that can cause noise, and only the signals recorded on the disk can be accurately reproduced. becomes possible.

〔effect〕

(1) By superimposing a high-frequency alternating current on the laser drive current (direct current) and modulating the laser on and off, the oscillation mode is multiplied, so the ambient temperature of the laser,
Mode hopping noise caused by changes in the drive III current etc. can be reduced.

(2) Furthermore, by bringing the frequency θ of the high-frequency alternating current superimposed on the direct current closer to the laser resonance frequency (combination vibration frequency a), the spectral width (wavelength λ Change in time) △λ
increases and reduces coherence. Therefore, the scoop noise caused by the return υ light from the optical system can be reduced without any consideration of the relationship between the optical path length of the optical system and the laser and the laser oscillation frequency.

(3) Speckle noise caused by interference between reflected lights from optical components can be reduced. Therefore, there is no need to take special measures against stray light for each optical component.

(4) According to (1,) to (3) above, the performance of an optical pickup device (optical signal processing device) typified by an optical video disc can be improved.

Although the invention made by the present inventor has been specifically explained above based on Examples, this invention is not limited to the above Examples, and may be modified in various ways without departing from the gist thereof. Not even. For example, the high frequency alternating current superimposed on the laser may be a square pulse wave current.

[Original field]

In the above description, the invention made by the present inventor was mainly applied to a light emitting device and an optical pickup device, which are the background fields of application, but the invention is not limited thereto. Applying the present invention to a light propagation device having an element and an optical fiber,
It is also possible to prevent interference noise at the connection end between an optical fiber and a semiconductor laser, between fibers, or between another optical component and a fiber. The present invention is applicable to at least all devices having a semiconductor laser light emitting element.

FIG. 3 is a diagram showing the transient characteristics of laser modulation and the spectral waveform of laser light.

FIG. 5 is a perspective view of a light emitting device according to an embodiment of the present invention. FIG. 6 is a perspective view of the light emitting device shown in FIG. 5. 7 is a diagram schematically showing an outline of an optical pickup device using the light emitting device shown in FIG. 5. FIG.

1...Laser chip, 2...m current power supply, 3...
AC power supply, 4... light emitting device (semiconductor laser module 1)
- laser device), 5... laser diode device, 6, 7
,8゜9...Terminal, 10...Light emitting device package, 11...L/-Ser--)'e, 12...Tin plated copper wire, 13...Laser package terminal, 14
...Coil, 15, 16...Transistor, 17
...Ceramic substrate, 18...Wffi material, 19.
...Solder, 20...Laser cage, 21.
...Stem, 22...Silicon submount, 23.
...Laser chip, 24...Gold wire, 25...
Photodiode, 26...Collimator lens, 2
7...Polarizing prism, 28...Human wave plate, 29...
- Objective lens, 30... Disc, 31... Information pit, 32... Cylindrical lens, 33... Photodiode, 34... Transparent window, 35... Flange.

Figure 1 Figure 2 Figure 3 " Figure 4 Figure 5 Figure 6

Claims (1)

[Claims] 1. A light-emitting device comprising a laser element driven by superimposing a high-frequency variable current or high-frequency pulsed current on a direct current, wherein the frequency of the high-frequency alternating current or pulsed current is equal to the frequency of the high-frequency alternating current or pulsed current. A light emitting device characterized by having a value close to the relaxation oscillation frequency of a laser element. 2. A direct current generating means, a high frequency alternating current generating means or a pulse current generating means, and a high frequency alternating current generated from the high frequency variable current generating means or pulse current generating means, or a means for superimposing a pulse current, and a laser element driven by a current in which the high frequency 'I:5If, current or pulse current is superimposed on the niI direct current, and the high frequency alternating current generating means or the pulse current The generating means is configured to generate a high frequency alternating current or pulse current having a frequency close to the slow vibration frequency of the laser element. 3. Relaxation vibration. A laser element driven by superimposing a high-frequency alternating current or pulsed current with a frequency close to that of the DC chamber, a signal recording body recording a signal, and a laser beam emitted from the laser element. 11. An optical signal processing device comprising: a means for optically reading out a signal recorded on the signal recording medium using the signal recording medium.