SEMICONDUCTOR LASER LIGHT MODULATION AND DEMODULATION SYSTEM

Abstract: A semiconductor laser light modulation and demodula­ tion system to be used in light communication system is realized. In the modulation system, strength modulated pressure wave or electromagnetic wave is directly applied to an active region of a semiconductor laser element (1) and the frequency modulation is effected by varying the effective length of a laser resonator formed by the laser element (1). In the demodulation system, a modulation signal is derived by an interference of the received laser light beam which is split into two secondary beams which are then joined together after having passed through an interferometer having arms of different path lengths.

Description: The present invention relates to a semiconductor laser light modulation and demodulation system in which the semiconductor laser light is modulated and demodulated for realizing a light communication system. More particularly, the present invention aims at stably modulating and demodulating the frequency of semiconductor laser light which does not have a narrow spectrum band width and hence exhibits an oscillating frequency which is usually unstable. Semiconductor laser light devices have hitherto been used conveniently in view of their compact size and easy handling. This kind of semiconductor laser device is applied for instance in a light communication system. Such optical communication systems may be sub-classified into two systems. One of which is an incoherent system, in which the strength or intensity of the laser light is modulated. The other one is a coherent system, in which the phase or frequency of the laser light is modulated. So far, only the incoherent system of modulating the intensity of the laser light has been used. As a principle of the laser light intensity modulation, a modulation signal is superposed on the driving current supplied to the semiconductor laser element and the variation of the laser light intensity at the oscillator output varying according to the modulation signal is utilized. However, in such an incoherent intensity modulation of the laser light, the variation of the strength of the laser light oscillation output is inevitably accompanied by other variations, like an irregular hopping of the oscillation mode of the laser light due to variation of the driving current. Accordingly, a good signal to noise ratio in the laser light oscillation output has not been obtained. Further considering the drift speed of the carrier of the laser light, there is a disadvantage in that the upper limit of the signal frequency being able to be modulated is limited up to some GHz order. On the other hand, the coherent system, in which the laser light is frequency or phase modulated, is still just in a beginning stage or research stage to check the possibility of realization based on so-called "analogy" with the frequency modulation of electro-magnetic waves. Moreover, the semiconductor laser light has a wider spectrum band width and the oscillating frequency itself is unstable. Thus the present problem is how to obtain a narrower frequency spectrum of such a semiconductor laser light and also how to stabilize the oscillation. Under such situation of the development of the technique of this field, a system to actually realize the frequency modulation or phase modulation by intentionally varying the frequency or phase of the laser light has never been achieved so far. Concerning the demodulation aspect, there has been proposed a demodulation method intended for phase modulated signals, using a Michelson type inter ferometer with an intensity detector at the receiving end (JP - A - 57 24 921, NIPPON DENSHIN DENWA KOSHA, 09-02-82). The object of the present invention is to mitigate the above-mentioned disadvatanges of conventional systems and to realize a novel semiconductor laser light modulation and demodulation system to be used in coherent laser light communication systems by applying a frequency modulation to the oscillating laser light output of the semiconductor laser element which inherently has an unstable oscillation frequency. In the semiconductor laser light modulation and demodulation system according to the present invention, the frequency of the laser light oscillation output of a semiconductor laser element is modulated with a modulation signal by causing an oscillation of the semiconductor laser element by supplying a current exceeding a certain threshold value and by applying modulated pressure wave or electromagnetic wave to an active region of said semiconductor laser element to cause a variation in the effective length of the laser resonator formed by said semiconductor laser element. The system is further characterized in that said laser light oscillation output is divided into two secondary beams which travel along two light paths having different path lengths and are thereafter joined together to cause an interference therebetween whereby the frequency component introduced by the pressure wave or the electromagnetic wave and included in the intensity variation of said joined laser lights is demodulated and the modulation signal is derived therefrom. For a better understanding of the invention, reference is made to the accompanying drawing, in which : The single drawing figure accompanied herein is a block diagram showing one example of the semiconductor laser light modulation and demodulation system according to the present invention. The invention will now be described by refering to the accompanying drawing. The drawing shows one embodiment of a semiconductor laser light modulation and demodulation system according to the present invention. As can be seen from the drawing, the system comprises a semiconductor laser element 1, an automatic power and temperature controlling device 2 for controlling supply of the power to the semiconductor laser element 1 and for controlling the operating temperature thereof automatically, an oscillator 3 for generating a carrier wave to be modulated as the signal of the semiconductor laser element 1, a signal source 4 for generating a modulation signal, a modulator 5 for intensity modulating the carrier wave supplied from the oscillator 3 by the modulation signal fed from the signal source 4, a power amplifier 6 for amplifying the modulated carrier wave supplied from the modulator 5, an ultrasonic vibrator 7 for generating an elastic wave having the intensity modulated by the modulation signal according to the applied power amplified and modulated carrier wave fed from the power amplifier 6, and a horn 8 for converging said modulated elastic wave and for applying it concentratedly to an active region of the semiconductor laser element 1. By the above-mentioned constituting elements 1 to 8, a modulation system is formed. In the above-mentioned modulation system, the semiconductor laser element 1 emits a modulated laser light in a manner more fully described hereinbelow. The emitted modulated laser light is transmitted via an optical fiber 9 to a demodulation system including various elements 10 to 16, which will be referred to hereinbelow. Namely, the demodulation system comprises a light beam splitter 10 consisting for instance of a half-silvered plate, reflecting mirrors 11 and 12 for reflecting the respective light beams splitted from the beam splitter 10, a slit 13 for limiting the width of the light beam emitted from the beam splitter 10, a photo-electrical converting element 14 for converting the light beam passing through the slit 13 into an electric signal, an amplifier 15 for amplifying the electric signal derived from the photo-electric converting element 14, and an output device 16 for utilizing the output from the amplified electric signal fed from the amplifier 15 for instance by recording or by displaying it. The operation of the laser light modulation and demodulation system will be explained hereinafter. The semiconductor laser element 1 transmits output laser light of a constant strength and a constant mode oscillation under control of the automatic power and temperature controlling device 2. On the other hand, the carrier wave supplied from the oscillator 3 to the modulator 5 has an intensity which is modulated by the modulation signal fed from the signal source 4. Then the modulated signal is applied to the ultrasonic vibrator 7 via the power amplifier 6 and irradiates an elastic wave the strength of which is modulated by the modulation signal. The intensity modulated elastic wave is made convergent in the horn 8 and is concentratedly applied to a laser resonator (not shown) formed mainly by the semiconductor laser element 1 from normal direction thereto. By this the density of the active region of the semiconductor laser element 1 is varied according to the intensity variation so that the index of refraction and hence the active length of the laser resonator is varied and thus the effective length of the laser resonator, which is a product of the length of the resonator and the index of refraction is varied accordingly. As a result, the laser light oscillation output of the semiconductor laser element 1 is frequency modulated by the modulation signal. The modulated signal is in proportion to the intensity variation of the applied elastic signal. The frequency modulated laser light beam modulated in the modulation system in a manner mentioned in the foregoing is transmitted to the demodulation system via the optical fiber 9 and is divided into two parts by the beam splitter 10. More specifically, the received laser light beam is splitted into two secondary beams. The first secondary beam is a beam reflected by the half-silvered plate 10 arranged in a direction at 45° with respect to the axis of the incident light beam, so that this first secondary beam has a direction normal to the incident direction. The second secondary beam is a laser beam which crosses the half-silvered plate 10 without deviation and therefore has the same direction as the incident laser beam direction. The two secondary beams are then reflected by the reflecting mirrors 11 and 12, and return to the split point where they are joined into one beam after having passed through the splitter and had their directions changed again by 90° by the beam splitter as can be seen from the drawing. At the splitting and joining of the laser beams, the distances between the beam splitter 10 and the two reflecting mirrors 11 and 12, respectively, are shifted by a certain suitable amount from each other so that a suitable difference in the path lengths of the two laser beams is produced. Thus an interferometer having unequal arm lengths is formed by the beam splitter 10 and the two reflecting mirrors 11 and 12. Accordingly, there will be an interference between the two laser beams once splitted and then joined together, and by this reason, an intensity variation occurs in accordance with the originally applied frequency modulation. By applying to the photo-electric converter element 14 via a slit 13 the beams joined by the beam splitter 10, an intensity variation according to the frequency modulation is detected and this intensity variation can be detected by the photo-electric converter element 14 and it is converted into a voltage signal. This voltage signal is fed to an amplifier 15 and is amplified by tuning with a carrier frequency and the modulated signal can be obtained again. The demodulated signal is used in the output device 16. The illustrated semiconductor laser light modulation and demodulation device may be modified for its construction partly or greatly. For instance, the laser light oscillation output of a semiconductor laser element having conventional construction may be applied to a known photo-electric effect element and the frequency or the phase thereof may be varied according to a modulating signal. The interferometer with unequal arms in the demodulation system may be replaced by an interferometer having a similar construction, with an FM demodulation circuit for an FM electromagnetic wave, and a same modulation signal may be obtained by demodulation after having obtained a stabilization of the oscillation frequency of the semiconductor laser element. The combination of the ultrasonic vibrator 7 and the converging horn 8 in the modulating system in the illustrated construction can be modified in a manner to form a combination of an antenna of a millimeter wave or microwave and a coupler, or a combination of a modulatable light emitting element and an optical converging element. The same function and effect can also be obtained by such a construction. In the semiconductor laser light modulation and demodulation system having the illustrated construction, the input system 3 to 8 to the semiconductor laser element 1 may be omitted and a pressure wave such as vibration or an electromagnetic wave like an optical light may be directly applied to the semiconductor laser element 1 from outside and by arranging to detect frequency variation of the laser light according to the applied power, sensors of various types being respondable with various pressure wave or electromagnetic wave may be obtained. As can clearly be understood from the foregoing explanation, the output laser light of a semiconductor laser oscillation can be frequency modulated by applying the modulation signal in superposition with a pressure wave or an electromagnetic wave directly to an active region of the semiconductor laser element according to the invention. The output of such a frequency modulated laser light beam may further be applied to an optical system forming an interferometer with unequal arms and by demodulating it by a so-called self-beat method, the conventional adverse conditions inherent to the semiconductor laser oscillation output light such as the unsatisfactorily frequency stability and spreading of the spectrogram may be mitigated and an effect of stable frequency modulation and demodulation can be obtained.

Claims: A semiconductor laser light modulation and demodulation system comprising a semiconductor laser element (1) forming a laser resonator arranged to oscillate when a current exceeding a threshold value of the element is applied, characterized in that the frequency of oscillation of the output laser light beam of said semiconductor laser element (1) is modulated by a modulation signal by applying to an active region of said semiconductor laser element (1) a pressure wave or an electromagnetic wave which is modulated by said modulation signal so as to cause a variation in the effective length of said laser resonator formed by the semiconductor laser element (1), in that said modulated output laser light beam is split into two secondary beams which travel along two different light paths having different path length and are thereafter joined together to cause an interference therebetween, and in that the modulation signal is derived by demodulating the same frequency component as that of the pressure wave or the electromagnetic wave, said component corresponding to an intensity variation of said joined laser light secondary beams. A semiconductor laser light modulation system comprising a semiconductor laser element (1) forming a laser resonator arranged to produce oscillation when a current exceeding a threshold value of the element is applied, characterized in that the frequency of the oscillation of output laser light of said semiconductor laser element (1) is modulated by a modulation signal by applying at an active region of said semiconductor laser element (1) a pressure wave or an electromagnetic wave which is modulated by said modulation signal so as to cause a variation in the effective length of said laser resonator formed by said semiconductor laser element (1).

Inventors: OHBA, RYOJI
Applicants: HOKKAIDO UNIVERSITY
Application year: 1985
Main classification: H01S3
IPC codes: H04B10, H01S5, H01S3, G02F2