FR2550645A1 - ELECTROLUMINESCENT DEVICE AND OPTICAL SIGNAL PROCESSING SYSTEM USING SUCH A DEVICE - Google Patents

Abstract

THIS ELECTROLUMINESCENT DEVICE INCLUDES AN ELECTROLUMINESCENT ELEMENT 1 EMITTING A LASER BEAM, A DEVICE 2 SERVING TO PRODUCE A DIRECT CURRENT, A DEVICE 3 SERVES TO PRODUCE A CURRENT CONTAINING AN ALTERNATIVE CURRENT COMPONENT WITH A FREQUENCY BETWEEN 12 AND 2 TIMES RESONANCE OF THE ELECTROLUMINESCENT ELEMENT, AND OF THE MEANS L, C TO SUPERIMPOSE THE SIGNAL INCLUDING THE ALTERNATIVE CURRENT COMPONENT TO THE DIRECT CURRENT AND TO PRODUCE A SIGNAL COMMANDING THE ELECTROLUMINESCENT ELEMENT AND WHOSE MAXIMUM CURRENT IS GREATER THAN A THRESHOLD OF L 'LASER EFFECT, WHILE THE MINIMUM CURRENT IS LOWER THAN THIS THRESHOLD VALUE. APPLICATION IN PARTICULAR TO READING DEVICES AND RECORDING OF INFORMATION ON AN OPTICAL VIDEO DISC.

Description

The present invention relates to an electroluminescent device which emits

  a laser beam, and an optical signal processing system, which uses this

electroluminescent device.

  In recent years, the use of an optical disk device, in which the information is recorded or reproduced through the use of a laser beam that delivers light having good homogeneity

  and a high intensity, has been a great success.

  By way of example, the operation of reproducing the information of an optical disk can be summarized as follows. A laser beam emitted for example by a semiconductor laser diode is focused by an objective on the surface of a disk, on wherein the information is recorded, and a change in the amount of light reflected by the disc is detected to reproduce a signal. The optical disc has a stored information density which is 10 to 100 times greater than that of the disc. Furthermore, it retains the stored memory. This is why it is expected that the optical disk will be used in different fields, and

  Optical video discs have already appeared on the market.

  Prior to the present invention, the inventors have carried out various studies on an optical video disc using a semiconductor laser.

  proved that the parasitic noise of the semiconductor laser (fluctuation of the optical output of the semiconductor laser) has a detrimental influence directly on the quality of the reproduction image of the optical video disc.

  The same causes of the semiconductor laser's noise can be roughly divided into two classes. In one of these classes, when the longitudinal mode of a laser beam shifts from a certain mode to a

  next in the case of a variation of the ambient temperature, the temperature of the casing or the like of the semiconductor laser.

  conductors, unwanted noise develops The parasitic noise thus developed is referred to as "parasitic noise of jumping or jittering mode" As another cause of stray noise, some of the light reflected from the surface of the optical disk returns to the semiconductor laser, so that the state of the semiconductor laser when the laser effect is obtained becomes unstable, resulting in the development of the parasitic noise This parasitic noise ain10 if produced is referred to as the "SCOOP noise"

  ("Self-C Oupled Optical Pick-up" stands for self-coupling optical sensing).

  The elements indicated above relate to the case where the noise appears in the laser beam itself emitted by the semiconductor laser In an optical signal processing system used for the optical disk or the like, another factor causes the appearance spurious noise, apart from the unwanted noise indicated above, in the output signal of the semi-conductor laser That is to say that a spurious noise develops during the operation of reproducing or reading the information in such a way that lights reflected by an optical component, for example a cylindrical lens, and by the disk which is a recording medium, or the reflected light and the laser beam emitted by the laser.

  Semiconductors interfere with each other, which results in the formation of interference fringes on a photodetector.

  This unwanted noise is referred to as "spurious noise in loose form". It reduces the signal-to-noise ratio in optical communication or on the optical disk. For example in the optical video disc, a reproduced image is adversely affected. so that white spots appear irregularly on the image or

  although a phenomenon occurs in which the gradation of the entire image varies over time.

  The specification of the patent application published in Japan under No. 56-37,834 describes a technique in which a current, consisting of a high frequency current superimposed on a DC bias current, is applied to a single mode semiconductor laser. to "plug" and "unplug" the semiconductor laser, so as to switch from the longitudinal mode to a multiple mode to reduce parasitic noise of mode jumping and noise S Coop However studies by the authors of the 10 present The invention has revealed that this method can not be considered to be of satisfactory efficacy to prevent

  the development of SCOOP spurious noise, although one could expect the same effect with regard to the parasitic noise of mode jump.

  The researches carried out by the authors of the present invention have also revealed that the parasitic noise SCOOP is not sufficiently reduced when, in addition to the means indicated above, no attempt is made to select the combination of the value 20 f of the high frequency to be superimposed in the laser and the length L of the optical path between a semiconductor laser chip and the reflecting part so as to have the relation f = c / 2 L- (c designating the speed of

the light).

  It has also been found that the above-mentioned method can not be a provision or solution obviating the mode jump noise and that the reduction of this parasitic noise can not be expected unless one takes in a sufficient manner of the arrangements against the diffuse light, such as for example the coating of the surface of each optical component with an anti-reflection film. It is an object of the present invention to provide a light-emitting device which makes it possible to reduce the parasitic noise SCOOP without taking into account the combination of a length of an optical path and a modulation frequency, and which can also reduce the appearance of the parasitic noise of mode jump as well as spurious noise in the form of tasks Another object of the present invention is to provide an optical signal processing system, in which the parasitic noise can hardly develop

  during the recording or reproduction of the signals.

  The outline of the present invention will be summarized below.

  The invention provides for the use of a laser powered by a current whose high frequency is close to the relaxation oscillation frequency (resonance frequency) of the laser and which has a degree of modulation sufficient to modulate the high speed laser by placing in the "plugged-in" state and the "disconnected" state, which has the effect of passing a laser beam of the longitudinal mode in a multiple mode, and further the spectral bands of the individual modes Multi-mode operation is apparently enlarged, which reduces the coherence of the laser beam, which makes it possible to achieve the above-mentioned objective of reducing noise. More precisely, this problem is solved by means of an electroluminescent device, characterized in that it comprises an electroluminescent element emitting a laser beam, a device for producing a direct current, a device for generating a signal having an ac component having a frequency of 1/2 to 2 times the resonance frequency of said electroluminescent element cent, and a device for superimposing the signal comprising the ac component to the DC current and for producing a signal electroluminescent element, whose maximum intensity of the current becomes greater than a threshold current intensity of the laser effect of the electroluminescent element and whose minimum intensity of the current becomes lower than the intensity of the effect threshold current

laser of said element.

  The digital signal processing system according to the invention is characterized in that it comprises an electroluminescent device which comprises an electroluminescent element for emitting a laser beam, a device for producing a direct current, a device for producing a a signal having an ac component having a frequency of between 1/2 and 2 times the resonant frequency of said light emitting element, and a device for superimposing the signal comprising the ac component to the direct current andcan produce a control signal of the electroluminescent element, whose maximum current intensity is greater than a threshold current intensity at the laser effect of the electroluminescent element and whose minimum intensity of the current becomes lower than the intensity of the current of the electroluminescent element. laser effect threshold of said element, a medium on which a signal is recorded stré, and a device for optically detecting the signal recorded on said medium,

  thanks to the use of the laser beam.

  Other features and advantages of the present invention will emerge from the description given hereinafter with reference to the accompanying drawings, of which:

  Figs. 1 (a) 1 (c) and 2 (a) 2 (c) are diagrams showing the transient characteristics of the spectral waveforms of a semiconductor laser for explaining the principle of the present invention;

  Fig. 3 is a diagram showing the basic arrangement of the circuit of the present invention; FIGS. 4 (a) 4 (c) are waveform diagrams showing the variation over time of a laser output signal in the case where the laser is

  mandated by the superposition of a high frequency current; Fig. 5 is a perspective view of an electroluminescent device according to one embodiment of the present invention; Figure 6 is a schematic sectional view of the electroluminescent device of Figure 5, taken along the line X-X '; and

  Fig. 7 is a view schematically showing the overall arrangement of an optical signal processing system, which uses the light emitting device shown in Fig. 5.

  The preferred embodiments of the invention will be described below.

  The important feature of the present invention resides in the apparent thickening of the spectral width of a laser beam, i.e. in other words in increasing the variation of the wavelength X of the laser beam as a function of time, with

  As a result, the coherence of the laser is lowered, thereby reducing the occurrence of unwanted noise.

  As a means of obtaining the above state, the present invention adopts a technique, according to which a high frequency current is superimposed on a direct current in order to modulate the laser by placing it in the "connected" and "disconnected" state. "which has the effect of switching from a longitudinal mode of producing the laser effect to a multiple mode and furthermore that the frequency of the high frequency current to be superimposed is brought to the vicinity of the frequency of resonance (relaxation oscillation frequency) of the laser device so as to modulate the laser at high speed.

  resonant frequency in this description is meant to

  designate a modulation frequency fr in the case where the degree of modulation of an optical output signal becomes very high in a state of resonance established when the frequency (modulation frequency) of an alternating current applied to the semiconductor laser is In general, in the semiconductor laser, the resonant frequency is between 1 GHz and 4 GHz as a result of the interaction of the laser beam and the carriers. The principle of the present invention indicated above will now be explained. Referring to FIGS. 1 (a) 1 (c) and 2 (a) 2 (c), the transient characteristics of a semiconductor laser and spectral waveforms of laser beams are shown in FIGS. The pulsed current as shown in FIG. 1 (a) is applied to the laser with a bias current I maintained in the vicinity of the threshold current (Ith) of the laser, the "fluctuations in light" being The resonant frequency f of the laser intervenes in the rising portion of the laser output light. Light fluctuations are amorphous oscillations peculiar to the semiconductor laser, referred to as "relaxation oscillations". After the lapse of a certain time interval, the optical output signal P is maintained at a predetermined value Po O (Figure l (b)). It is believed that the relaxation oscillations result from the modulation of the laser light by the components of the resonant frequency fr 'which occur when the step-change of the current

  applied is developed in Fourier series.

  In theory, the frequency of resonance

  and the relaxation oscillation frequency of the laser device coincide. This is why, in the present description,

  we will consider that the resonant frequency and the frequency

  relaxation oscillation are synonyms.

  In the relaxation oscillation portion, the optical output signal P (i.e. the carrier density in an active layer) varies greatly with time. This means that the variation of the refractive index in the As the active layer becomes larger and the variation b X as a function of time, the wavelength of the laser light existing inside the active layer becomes high. When the period T 1 of the applied pulse is sufficiently greater than a relaxation oscillation duration T 2, the fluctuations of the light imparted to the relaxation oscillations can be neglected, and the modulated output signal of the laser light can be considered as the predetermined output signal Po The laser device is controlled at the "connected" state and in the "disconnected" state by a pulse wave (a current wave

  alternating high frequency can be perfectly used), wherein the applied pulse period T 1 is sufficiently greater than the relaxation oscillation duration T 2.

  Therefore, when the laser device has fallen into the state in which a plurality of laser lights having substantially different wavelengths exist within a resonator, i.e. when the multiple mode has been established, the spectral waveform of the laser output light becomes that represented on the

  Figure 1 (c), and the spectral width (half-width of Ps energy of each spectrum) A is small.

  Here, when the period T 1 of the applied pulse current is synchronized with the relaxation oscillation duration T 2 as shown in FIG. 2 (a), the optical output signal of the only part of the relaxation oscillation is produced in the form shown in FIG. 2 (b) When the laser beam has been brought to its multiple operating mode, under the effect of the control of the laser in the "connected" and "disconnected" state with such a current impulse (high frequency alternating current) preceding the very short period T 1, the spectral waveform of the laser output light becomes that represented in FIG. 2 (c) That is to say that the variation> (spectral width), as a function of time, of the laser beam wavelength X in the multiple mode, is important The reason for this is that, since the wavelength of each of the laser lights in the mode multiple, is further modified essentially nt as a function of time under the effect of the modification of the relaxation oscillation,

  the energy contained in each mode is dispersed so as to provide the spectrum which is dense and extended unlike the peak spectrum as shown in Figure 1 (c).

  In such a state, the coherence of the laser output light is reduced to a certain value.

  As a result, even when the light reflected by an optical system returns to a laser chip, spurious noises can hardly develop. Furthermore, interference between the light reflected by the optical system and the unwanted noise before During this process, the present invention is characterized by extracting only the fluctuating portions of the laser light under

  the shape of a continuous light pulse.

  FIG. 3 is a diagram showing the fundamental arrangement of the circuit for carrying out the present invention. A semiconductor laser diode 1 is controlled by the superimposed currents provided by a DC power source 2 and a current source. The symbols L and C in the figure respectively denote a coil and a capacitor, which are

  mounted in such a way that the two current sources can independently control the semiconductor laser.

  FIG. 4 (a) is a diagram showing the current characteristic (Io) optical output (P) of the semiconductor laser The semiconductor laser is driven by the current: I = I + t AI cos (2 rf t) (1) ) in which the high frequency current AI cos (211 ft) 35 (f: frequency, t: time) is superimposed on the direct current Io. FIG. 4 (b) represents the variation over time of the control current of the laser. moment, the variation of the output of the laser as a function of time (t) becomes that represented in FIG. 4 (c) That is to say that we have: L = L + L cos (2-Wf t ) for I (t)> Ith (2) L = O for I (t) Ith (2) Here, Ith denotes a laser effect threshold current and L and L respectively denote a DC light output and an AC light amplitude corresponding to Io and Ỉ The laser effect appears only when the control current I of the laser exceeds Ith, so that the output L of the laser light becomes The modulation of the semiconductor laser in the "plugged-in" state or in the "disconnected" state, achieved in this manner, is a prerequisite for bringing the laser beam into the mode. In addition, according to the present invention, and as previously indicated, the frequency f of the high-frequency alternating current to be superimposed on the laser direct control current has a value close to the frequency of the laser. relaxation oscillation (resonant frequency) fr of the laser device to be controlled By way of example, the inventors have verified, from experience, that in the case of the use of a semiconductor laser having the structure MCSP (modified planar structure with channel-equipped substrate), having a wavelength of 780 nm and an Ith value of 50 m A, with a resonant frequency equal to about 1.8 GHz, the spurious noise reduction effect is intense when the frequency of the high frequency current to be superimposed on the direct current is set to 1/2 2 times the resonance frequency fr 'namely between 900 M Hz and 3.6 G Hz It has also been verified that, in the case of a semiconductor laser having the BH (hetero gene-enselevie) structure, which has a resonant frequency of about 2 GHz, the spurious noise reduction effect is intense when the high frequency value to be superimposed is maintained in a manner similar to 1/2 2 times the resonant frequency. The frequency of the high frequency current to be superimposed is determined in FIG. present case depending on the resonant frequency of the laser device to be used and the rate

  required noise reduction.

  Hereinafter, an example of an electroluminescent device (a semiconductor laser module device) embodying the fundamental arrangement of the circuit of the present invention shown in FIG. 3 will be discussed with reference to FIGS. 5 is a perspective view of the electroluminescent device, while FIG. 6 is a sectional view of the electroluminescent device of FIG. 5, taken along the line XX '. The electroluminescent device 4 is constituted by a diode device. laser 5, a housing 10, external connection terminals 6, 7, 8, 9, etc. A ceramic printed circuit board 17, on which a coil 14,

  transistors 15, 16, etc. are mounted, is fixed inside the housing 10 by means of a connecting element 18.

  The terminals 13 of the laser diode device penetrate inside the casing 10 and the printed ceramic circuit board 17 via through holes (not shown) and are attached by a solder 19 although this is not shown. an aluminum wiring (A 1) is arranged or structured on the printed circuit board 17 to individually connect the coil 14, the transistors 15, 16, the laser diode device 5, etc. Thus a laser oscillator circuit is built on the ceramic printed circuit board 17 In the part

  of a junction pad (not shown), the aluminum wiring is connected for example to the outer connection terminal 8 via a tinned copper wire 12.

  The external connection terminals 6, 7, 8 and 9 are a power supply source terminal for a high frequency generator circuit, a DC power source terminal for the laser, a ground terminal, and a power source terminal. When the desired supply voltages are applied to the respective terminals, the laser beam 1 is emitted by the laser diode device 5. This laser beam is sent to a recording medium or medium. With the aid of optical means such as lenses, so as to perform the reading of a recorded signal. This case is illustrated in FIG. 7. This figure represents a schematic view making it possible to explain the general arrangement of a system. First of all, the mounting of the laser diode device 5 will be explained briefly. A copper rod 21 is mounted vertically on the central part of the surface. The upper flange 35 is made of a metal having good thermal conductivity, such as copper. A semiconductor laser element (chip) 23 is attached to a side face of the rod 21 by means of an auxiliary support. The chip 23 has two faces, namely upper and lower faces for the emission of laser beams 11. A photodetector (photodiode) 25 for controlling the output signal, which receives the laser beam 11, is arranged Below the lower emissive face The chip 23 and the photodetector 25 are respectively connected to the terminals 13 via gold wires (Au). The laser beam exits through a transparent window 34, which is formed in a part of the

housing 20 of the laser.

  Hereinafter will be described as a whole the optical signal processing system The beam

  laser 11 sent by the laser chip 23 is trans35 formed by a coupling lens 26 into a colliding beam.

  The light of the circular polarization is quenched directly into a polarization prism 27, then passes through a quarter-wave plate 28 so as to receive a circular polarization. The light of the circular polarization is refined to a few microns by an objective 29 and penetrates for example into the Cell or housing 31 for disc information 30 which constitutes a medium or medium for recording signals The light reflected by the disc contains information on the presence or absence of the information slot. The reflected light passes through the plate. quarter-wave 28 and is retransformed with a linear polarization which is reflected back into the polarization prism, and is then condensed by a cylindrical lens 32 so as to fall on a photodiode (detector) 33 There the optical signal is transformed into a signal electrical and a reproduced signal is obtained When the light emitting device according to the present invention is used as a sou In the case of light, the coherence of the laser output signal is reduced to a certain extent. Even if the reflected light in the optical system returns to the laser chip, it is difficult for interference to occur within the resonator. of the laser, with the result that the noise can hardly develop further, since the interference between the lights reflected by the optical components can be reduced, the interference fringes, which are the cause of the development of noise , do not appear on the photocathode of the photodiode 33, and it is possible to accurately reproduce only the signal recorded on the disc. The present invention has the following effects: (1) A laser is modulated by being "plugged" and "unplugged" by superimposing a high frequency alternating current to a laser control current

  (DC current), which has the effect that a long-term

  dinal production of the laser effect is transformed into a multiple mode That is why it is possible to reduce the parasitic noise of mode jump that is related to changes in ambient temperature, control current, etc. of the laser. (2) Furthermore, the frequency of the high frequency alternating current to be superimposed on the direct current is brought close to the resonant frequency (relaxation oscillation frequency) of the laser, which has the effect that the spectral width ( the variation of the wavelength> as a function of time) A> of each of the laser lights in the multiple mode is increased, which reduces the coherence of this fact, likewise when the relation between the frequency of the laser effect and the optical path length of the optical system and the laser are not taken into account at all, the SCOOP spurious noise attributed to the back-progression of

  light from the optical system can be reduced.

  (3) It is possible to reduce stray noise in the form of tact, which occurs as a result of the interference of the light reflected by the optical components. This makes it unnecessary to take steps against

  diffuse light for individual optical components.

  (4) Taking into account the items (1) (3) above, it is possible to improve the operational efficiency of an optical signal processing system, consisting of a

optical disk.

  Although, in the foregoing, the invention has been concretely described in connection with embodiments, it goes without saying that said invention is in no way limited to the embodiments mentioned above, but that various modifications may For example, the control or control current used to control the

  In the "connected" and "disconnected" state, an AC component having a frequency of

  essentially equal to or close to the frequency of

  resonance of the laser, and is not limited to a sine wave, a square pulse wave or the like.

  In the foregoing, the invention has been described primarily with reference to the case of its application to the light emitting device and the optical signal processing system, which are basic areas of use, but is not limited thereto. For example, in the case of the application of the present invention to a light propagation device which has an electroluminescent element and optical fibers, it is also possible to prevent the interference noise produced by the interferences at the level of the connecting end between the optical fiber and the semiconductor laser and at the connection end between the fibers or between the fiber and any other optical component.

  The present invention is applicable to all devices comprising at least one semiconductor laser or a luminescent element.

Claims (6)

  Electroluminescent device, characterized in that it comprises: (1) an electroluminescent element (1; 5,23) which emits a laser beam (11); (2) a device (2) for producing a direct current; (3) a device (3) for producing a signal having an alternating current component having a frequency of 1/2 to 2 times a resonant frequency of said light emitting element, and (4) a device for superimposing the signal containing the ac component to the direct current and producing a control signal of the electroluminescent element (1; 5,23) whose maximum intensity of the current becomes greater than an intensity of the effect threshold current said electroluminescent element and whose minimum current intensity becomes less than   the laser effect threshold intensity of said element.   2 Electroluminescent device according to   Claim 1, characterized in that said electroluminescent element (1; 5,23) is a semiconductor laser.   Electroluminescent device according to claim 1, characterized in that said electroluminescent element (1; 5,23), the device for producing the direct current, the device for producing the signal including the AC component and said device for to produce the control signal of the element   electroluminescent are contained in a single housing (10).   Optical signal processing system, characterized in that it comprises: (1) an electroluminescent device (5, 23) which comprises (2) a light-emitting element (23) for emitting a laser beam (11), (3) ) a device for producing a direct current, (4) a device for producing a signal having an alternating current component having a frequency of 1/2 to 2 times a resonant frequency of said light emitting element, and (5) a device for superimposing the signal containing the ac component to the dc current and producing a control signal of the light emitting element (5,23) whose maximum current intensity becomes greater than an intensity of the effect threshold current laser of said electroluminescent element and whose minimum current intensity becomes lower than the intensity of the laser effect threshold current of said element, (6) a support (30) on lacquer l a signal is recorded, and   (7) a device (26-29 and 32) for optically detecting the signal recorded on said medium by use of the laser beam.   An optical signal processing system according to claim 4, characterized in that said medium (30) on which the signal is recorded is   a disc on which a digital signal is recorded.   Optical signal processing system according to claim 4, characterized in that said device (26-29 and 32) for optically detecting the signal recorded on said medium (30) comprises a coupling lens (26), a prism of polarization (27), a quarter-wave plate (28), a lens (29) and a lens cylindrical (32).   Optical signal processing system according to claim 4, characterized in that said   electroluminescent element (23) is a semiconductor laser. 35 8 Optimal signal processing system   4. according to claim 4, characterized in that it further comprises means for recording the signal on said support (30) through the use of the laser beam (11).