GB2144912A - Light emitting device and optical signal processing system employing the same - Google Patents

Abstract

A laser diode is energised by a direct current on which is superimposed an alternating current of a frequency between 1 DIVIDED 2 and 2 times the resonance (relaxation) frequency of the laser diode (5). The energising signal has a highest current value greater than the lasing threshold of the laser and a lowest current value smaller than the lasing threshold. The laser generates multiple modes with a broadened spectrum and reduced coherence. This reduces mode hopping noise. The laser (5) and its drive components 14 to 16 are mounted on a ceramic circuit board (17) and contained within a housing (10). The laser can form part of an optical signal processing system, eg for an optical video disc, which due to the reduced mode hopping noise, scoop noise and speckle noise provides a system which is substantially free from noise during signal recording and reproduction. <IMAGE>

Description

SPECIFICATION Light emitting device and optical signal processing system employing the same The present invention relates to a light emitting device which emits a laser beam, and an optical signal processing system which employs the light emitting device.

In recent years, an optical disk device has come into the limelight in which information is recorded or played back using a laser beam that affords light of good homogeneousness and high intensity.

By way of example, the information playing-back operation of an optical disk is as summarized below. A laser beam emitted from, for example, a semiconductor laser diode is focused by an objective on a disk surface on which information is stored, and variation in the quantity of reflected light from the disk is detected, thereby to reproduce a signal. The optical disk is 10-100 times higher in the density of stored information than a magnetic disk. In addition, it preserves the stored information well. Therefore, the utilization of the optical disk in various fields is expected, and optical video disks etc. have already been put on the market.

We have carried out experiments on an optical video disk employing a semiconductor laser.

As a result, it has been revealed that the noise of the semiconductor laser (fluctuation in the optical output of the semiconductor laser) exerts bad influence directly on the playback picture quality of the optical video disk.

The causes of the noise of the semiconductor laser itself are broadly classified into two. As one of them, when the longitudinal mode of a laser beam shifts from a certain mode to the next mode in the case of change in the ambient temperature, or the package temperature of the semiconductor laser, noise is developed. The noise thus developed is called "mode hopping noise".

As the other cause of the noise, part of the light reflected from the optical disk surface returns to the semiconductor laser itself, so that the lasing state of the semiconductor laser becomes unstable to develop noise. The noise thus developed is called "SCOOP (Self-COupled Optical Pick-up) noise".

The above is the case where the noises develop in the laser beam itself emitted from the semiconductor laser. In an optical signal processing system for an optical disk, another factor causes noise besides the foregoing noises in the output of the semiconductor laser. That is, noise develops in the information play-back operation in such a manner that light reflected from an optical component, e.g. a cyindrical lens and from the disk being a recording medium, or the reflected light and the laser beam emitted from the semiconductor laser interfere with each other, to give rise to interference fringes on a photodetector. This noise is called "speckle noise". It reduces the signal-to-noise ratio in the optical communication or the optical disk.In, for example, the optical video disk, a reproduced picture is advsersely affected in such a manner that white speckles appear irregularly in the picture or that a phenomemon arises in which the clarity of the whole picture fluctuates with time.

Japanese Patent Publication No. 37834/81 discloses a technique in which current with a high frequency current superimposed on a D.C. bias current is applied to a single-mode semiconductor laser to drive the semiconductor laser "on" and "off", thereby to turn the longitudinal mode into a multiple mode so as to reduce the mode hopping noise and the SCOOP noise. We have found that this method cannot be said satisfactorily effective for preventing the development of the SCOOP noise although some effect can be expected for the mode of hopping noise.

We have also found that the SCOOP noise is not sufficiently reduced when, in addition to the above expedient, any contrivance is not made for selecting the combination of the value f of the high frequency to be superimposed on the laser with the optical path length L between a semiconductor laser chip and the reflective part so as to hold f/2L (where c denotes the velocity of light).

It has also been revealed that the foregoing method cannot become a measure against the speckle noise and that the reduction of the noise cannot be expected unless any measure against stray light, such as coating the surface of each optical component with an antireflection film, is sufficiently taken.

It is an object of this invention to provide a light emitting device which can reduce the SCOOP noise without considering the combination of an optical path length and a modulation frequency and which can also reduce the occurrence of the mode hopping noise as well as the speckle noise.

Another object of this invention is to provide an optical signal processing system in which little or no noise is generated during signal recording or reproduction.

According to the present invention there is provided a light emitting device including: (a) a light emitting element which emits a laser beam; (b) means to generate a direct current; (c) means to generate a signal which includes an alternating current component of a frequency between 3 and 2 times of a resonance frequency of said light emitting element; and (d) means to superimpose the signal including the alternating current component on the direct current, and to generate a light emitting element-driving signal whose highest current value becomes greater than a lasing threshold current value of said light emitting element and whose lowest current value becomes smaller than the lasing threshold current value of said element.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein: Figures l(a)-l(c) and Figures 2(a)-2(c) are diagrams showing the transient characteristics and spectral waveforms of a semiconductor laser for explaining the principle of the present invention; Figure 3 is a circuit diagram showing the fundamental circuit arrangement of the present invention; Figures 4(a)-4(c) are waveform diagrams showing the time variation of a laser output in the case where a laser is driven by superimposing a high frequency current; Figure 5 is a perspective view of a light emitting device which is an embodiment of the present invention; Figure 6 is a schematic sectional view of the light emitting device of Fig. 5 taken along X-X;; and Figure 7 is a view schematically showing the outline of an optical signal processing system which employs the light emitting device shown in Fig. 5.

The important feature of the present invention consists in apparently thickening the spectral width of a laser beam, in other words, increasing the variation of the wavelength N of the laser beam versus time, with the result that the coherency of a laser is lowered to reduce the occurrence of noise. As an expedient for realizing the above state, the present invention adopts a technique in which a high frequency current is superimposed on a direct current to modulate the laser "on" and "off", thereby to turn a lasing longitudinal mode into a multiple mode, and further, the frequency of the high frequency current to be superimposed is rendered close to the resonance frequency (relaxation oscillation frequency) of the laser device so as to modulate the laser at high speed.The resonance frequency in this specification is intended to mean a modulation frequency ff in the case where the modulation degree of an optical output becomes very great in a resonance state established when the frequency (modulation frequency) of an alternating current applied to the semiconductor laser is varied. In general, in the semiconductor laser, the resonance frequency lies between 1 GHz and 4 GHz owing to the interaction of the laser beam and carriers. Now, the principle of the present invention stated above will be elucidated with reference to Figs. 1(a)-i (c) and Figs. 2(a)-2(c). In these figures, the transient characteristics of a semiconductor laser and the spectral waveforms of laser beams are shown.

When a pulse current as shown in Fig.l(a) is impressed on the laser with a bias current lo held in the vicinity of the threshold current (Ith) of the laser, the ''fluctuations of light" repeated at the resonance frequency f, of the laser arise in the rise part of laser output light. The fluctuations of light are damped oscillations peculiar to the semiconductur laser, called "relaxation oscilla tions". After lapse of a certain period of time, the optical output P is held at a predetermined output PO (Fig. 1(b)).

The relaxation oscillations are considered to result from the modulation of the laser light by the components of the resonance frequency fr which appear when the step-like change of the impressed current is Fourier-expanded.

Theoretically, the resonance frequency f, and relaxation oscillation frequency of the laser device agree. In this specification, therefore, the resonance frequency and the relaxation oscillation frequency shall be treated as synonyms.

In the relaxation oscillation part, the optical output P (that is, the density of carriers in an active layer) changes greatly with time. This signifies that the change of the refractive index of the active layer becomes great and that the variation AN versus time, of the wavelength A of the laser light existing within the active layer becomes great.When the period T, of the impressed pulse is sufficiently greater than a relaxation oscilaation time T2, the fluctuations of light attributed to the relaxation oscillations can be neglected, and the modulated output of the laser light can be regarded as the predetermined output PO. The laser device is driven "on" and "off" by such a pulse wave (a high frequency A.C. 'Nave may well be used) in which the period T1 of the impressed pulse is sufficiently greater than the relaxation oscillation time T. Thus, when the laser device has fallen into the state in which a plurality of laser lights of subtly different wavelengths exist within a resonator, that is, the multiple mode has been established, the spectral waveform of the laser output light becomes as shown in Fig. 1(c), and the spectral width (the half-width of the energy Ps of each spectrum) AN is small.

Here, when the period T1 of the impressed pulse current is synchronized with the relaxation oscillation time T2 as shown in Fig. 2(a), the optical output of only the relaxation oscillation part is produced as shown in Fig. 2(b). When the laser beam has been brought into the multiple mode by driving the laser device "on" and "off" with such pulse current (high frequency alternating current) of the very short period T1, the spectral waveform of the laser output light becomes as shown in Fig. 2(c). That is, the variation AN (spectral width) versus time, of the wavelength A of the laser beam in the multiple mode is great.The reason is that, since the wavelength of each of the laser lights in the multiple mode is further changed subtly with time by the change of the relaxation oscillation, energy possessed by each mode disperses to become the spectrum which is observed and wide in contrast to the sharp spectrum as shown in Fig.

1(c).

Under such state, the coherency of the laser output light lowers to some extent. As a result, even when light reflected by an optical system returns to a laser chip, little or no noise is generated. In addition, the interference between lights reflected by the optical system is substantially eliminated, and noise developed during information playback can also be reduced.

In this manner, the present invention is characterized by taking out only the fluctuating parts of the laser light as a continuous light pulse.

Fig. 3 is a diagram which shows the fundamental circuit arrangement for performing the present invention. A semiconductor laser diode 1 is driven by the superimposed currents of a direct current source 2 and a high frequency current source 3. Symbols L and C in the figure denote a coil and a capacitor respectively, which are inserted so that the current sources can drive the semiconductor laser independently.

Fig. 4(a) is a diagram showing the current (IO)~optical output (P) characteristic of the semiconductor laser. The semiconductor laser is driven by the current: I=l0+Al'cos(2"ft) (1) in which the high frequency current Al cos (27rut) (f: frequency, t: time) is superimposed on the direct current lo Fig. 4(b) shows the time variation of the laser driving current. At this time, the variation of the laser output versus time (t) becomes as shown in Fig. 4(c).That is: L= LO+ALcos(277ft) for l(t) > lthl L=O for l(t) < lthJ for Here, Ith denotes a lasing threshold current, and L0 and AL denote a D.C. light output and an A.C. light amplitude corresponding to lo and Al, respectively.Lasing occurs only when the laser driving current I exceeds Ith, so that the laser light output L becomes continuous light pulse oscillation. (Modulating the semiconductor laser "on" and "off" in this manner is an indispensable condition for bringing the laser beam into the multiple mode as stated before.) Further, in the present invention, as stated before, the frequency f of the high frequency alternating current to be superimposed on the laser driving direct current has a value near the relaxation oscillation frequency (resonance frequency) f, of the laser device to be driven.By way of example, we have found by experiment that, in the case of employing a semiconductor laser of the MCSP (Modified Channeled Substrate Planar) structure having a wavelength of 780 nm and an 1th of 50 mA, the resonance frequency f, of which is about 1.8 GHz, the effect of reducing noise is high when the frequency of the high frequency current to be superimposed on the direct current is set at 3-2 times of the resonance frequency f,, namely, between 900 MHz and 3.6 GHz. It has also been verified that, in the case of a semiconductor laser of the BH (Buried-Hetero) structure, which has a resonance frequency f, of about 2 GHz, the noise reducing effect is high when the frequency value of the high frequency to be superimposed is similarly held at ±2 times of the resonance frequency f,.The frequency of the high frequency current to be superimposed is determined upon occasion in accordance with the resonance frequency of the laser device to be used and a noise reduction rate required.

Next, an example of a light emitting device (a semiconductor laser module device) embodying the fundamental circuit arrangement of the present invention shown in Fig. 3 is illustrated in Figs. 5 and 6. Fig. 5 is a perspective view of the light emitting device, while Fig. 6 is a sectional view of the light emitting device of Fig. 5 taken along X-X'. The light emitting device 4 is composed of a laser diode device 5, a housing 10, external connection terminals 6, 7, 8 and 9. A ceramic circuit board 17 on which a coil 14, transistors 15 and 16 are installed is fixed inside the housing 10 by a binder 18. The terminals 13 of the laser diode device penetrate the housing 10 and ceramic circuit board via 17 through-holes (not shown), and are fixed by solder joints 19.Although not shown, aluminium (Al) wiring is patterned on the ceramic circuit board 17 so as to individually connect the coil 14, the transistors 15, 16 and the laser diode device 5. Thus, a desired laser oscillator circuit is constructed on the ceramic circuit board 17.

In the part of a bonding pad (not shown), the aluminium wiring is connected to, e.g. the external connection terminal 8 through a tin-plated copper wire 12. The external connection terminals 6, 7, 8 and 9 are a power source terminal for a high frequency generator circuit, a laser direct current source terminal, an earth terminal and a laser beam monitoring output terminal, respectively. When desired supply voltages are applied to the respective terminals, the laser beam 11 is emitted from the laser diode device 5. This laser beam is fed to a recording medium by optical means such as lenses, to read out a recorded signal. This situation is illustrated in Fig. 7. The figure is a schematic view for explaining the outline of an optical signal processing system. First, the setup of the laser diode device 5 will be briefly explained.A stem 21 made of copper is erected upright on the central part of the upper surface of a flange 35 which is made of a metal of good thermal conductivity such as copper. A semiconductor laser element (chip) 23 is fixed on one side face of the stem 21 through a silicon submount 22. The chip 23 has two faces, upper and lower faces for emitting laser beams 11. An output monitoring photosensor (photodiode) 25 which receives the laser beam 11 is located under the lower emissive face. The chip 23 and the photosensor 25 are respectively connected to the terminals 13 through gold (Au) wires 24. The laser beam, is emitted by passing through a transparent window 34 which is provided in a part of a laser package 20.

Next, the optical signal processing system will be outlined. The laser beam 11 emitted from the laser chip 23 is turned by a coupling lens 26 into a collimated beam, which directly enters a polarizing prism 27 and then passes through a quarter waveplate 28 to be made circularly polarized. The beam of the circularly polarized light is focused to a point having a width of several microns by an objective 29, and enters, for example, the information pit 31 of a disk 30 which is in the form of a single recording medium. Light reflected from the disk has information on the presence or absence of the pit. The reflected light passes through the quarter waveplate 28 and is reconverted into linearly polarized light, which is reflected within the polarizing prism and then condensed by a cylindrical lens 32 to fail on a photodiode (detector) 33.Here, the optical signal is converted into an electric signal, and a reproduced signal is obtained. When the light emitting device of the present invention is employed as a light source, the coherency of the laser output is reduced to some extent. Even when the reflected light in the optical system returns to the laser chip, little or no interference takes place within the laser resonator with the result that little or no noise is generated. Moreover, since the interference between the lights reflected by the optical components can be reduced, interference fringes which form the cause of noise development do not appear on the photocathode of the photodiode 33, and it is permitted to precisely reproduce only the signal recorded on the disk.

The present invention has the following advantages:~ (a) A laser is modulated "on" and "off" by superimposing a high frequency alternating current on a laser driving current (direct current), whereby a lasing longitudinal mode is turned into a multiple mode. It is therefore possible to reduce the mode hopping noise attendant upon the changes of the ambient temperature, or driving current of the laser.

(b) Further, the frequency of the high frequency alternating current to be superimposed on the direct current is rendered near the resonance frequency (relaxation oscillation frequency) of the laser, whereby the spectral width (the variation of the wavelength N versus time) AN of each of laser lights in the multiple mode is expanded to lower the coherency. Therefore, even when the relationship between the lasing frequency and the optical path length of the optical system and the laser is not considered at all, the SCOOP noise attributed to the light retrogressing from the optical system can be reduced.

(c) It is possible to reduce the speckle noise which occurs due to the interference between lights reflected by the optical components. This makes it unnecessary to take any measure against stray light for the individual optical components.

(d) Owing to the above items (a)-(c), the performance of an optical signal processing system represented by an optical disk can be enhanced.

In a modified form, the drive current for driving the laser "on" and "off" may include an A.C. component of a frequency which is substantially equal or close to the resonance frequency of the laser, and it is not restricted to a sine wave, a square pulse wave, or the like.

Moreover, whilst the invention has been principally described with reference to the case of applying it to the light emitting device and the optical signal processing system which are the background fields of utilization, it may equally be applied to a light propagation device which has a light emitting element and optical fibers, in order to prevent interference noise at the connection end between the optical fiber and the semiconductor laser and at the connection end between the fiber and any other optical component. The present invention is applicable to any device having, at least, a semiconductor laser or light emitting element.

Claims (10)

1. A light emitting device, including: (a) a light emitting element which emits a laser beam; (b) means to generate a direct current; (c) means to generate a signal which includes an alternating current component of a frequency between 3 and 2 times of a resonance frequency of said light emitting element; and (d) means to superimpose the signal including the alternating current component on the direct current, and to generate a light emitting element-driving signal whose highest current value becomes greater than a lasing threshold current value of said light emitting element and whose lowest current value becomes smaller than the lasing threshold current value of said element.

2. A light emitting device according to claim 1, wherein said light emitting element is a semiconductor laser.

3. A light emitting device according to claim 1 or 2, wherein said light emitting element, the direct current generating means, said means to generate the signal including the alternating current component, and said means to generate the light emitting element-driving signal are housed within a single package.

4. An optical signal processing system, including: (a) light emitting device according to any one of the preceding claims; (b) a medium on which a signal is recorded; and (c) means to optically detect the signal recorded on said medium, by the use of the laser beam.

5. An optical signal processing system according to claim 4, wherein said medium on which the signal is recorded is a disk on which a digital signal is recorded.

6. An optical signal processing system according to claim 4 or 5, wherein said means to optically detect the signal recorded on said medium comprises a coupling lens, a polarizing prism, a quarter waveplate, an objective, and a cylindrical lens.

7. An optical signal processing system according to any one of the preceding claims 1 to 4, wherein said light emitting element is a semiconductor laser.

8. An optical signal processing system according to claim 4, further including: (d) means to record the signal on said medium by the use of the laser beam.

9. A light emitting device constructed substantially as herein described with reference to and as illustrated in Figs. 5 and 6 of the accompanying drawings.

10. An optical processing system constructed substantially as herein described with reference to and as illustrated in Fig. 7 of the accompanying drawings.