Optical pulse generator

Abstract: A short optical pulse generator is provided with a semiconductor laser (1) which oscillates continuously at a single wavelength, a semiconductor electro-absorption type optical modulator (2) which performs the intensity modulation of the output light from the laser (1), and a sinusoidal voltage generator (5) and a DC voltage generator (3) for driving the electro-absorption type optical modulator (2). A DC voltage is applied to the electro-absorption type optical modulator (2) so that the output light from the laser (1) is sufficiently extinguished. By applying a sinusoidal voltage to the optical modulator (2), short optical pulses are generated. In another embodiment two modulators are connected in series and a time delay is introduced between their respective modulating signals.

Description: The present invention relates to an optical soliton pulse generator for high-speed fiber optic communication. With the recent progress of optical amplification techniques, the fiber optic communication technology is now advancing toward ultra-long distance communication and it is becoming a reality to implement fiber optic communication across the Pacific Ocean without using any regenerative repeaters. With conventional transmission systems, however, an increase in the transmission rate allows the influence of deterioration of the transmission characteristic to grow which is based on the wavelength dispersion characteristic and nonlinear optical effect of the optical fiber, imposing limitations on the high-speed, large-capacity transmission. In recent years an optical soliton communication system has been in the limelight as a system which surmounts the limitations on the speeding up of transmission owing to the wavelength dispersion characteristic and the nonlinear optical effect. The optical soliton communication system is a system that makes positive use of wavelength dispersion characteristic and nonlinear optical effect of the optical fiber which are contributing factors to the deterioration of the transmission characteristics of prior art systems and that transmits optical short pulses intact while balancing the broadening of pulses by the wavelength dispersion in the optical fiber and the compression of pulses based on the nonlinear optical effect. A time multiplex and a wavelength multiplex system are also relatively easy to implement and are suitable for high-speed and large-capacity transmission. Heretofore, a pulse light source for the optical soliton communication has been implemented by a semiconductor mode locked laser with an external cavity or a gain switched semiconductor laser. To realize the soliton communication utilizing the optical fiber, an optical soliton pulse generating technique is of importance. The conditions for the optical pulse are that its temporal waveform is in the shape of a squared hyperbolic secant (sech 2 t, where t is time) and that its frequency spectrum does not excessively broaden and is expressed by a Fourier transformation of the temporal waveform (a Fourier transform limit). That is, it is necessary that the product of the full width at half maximum of the temporal waveform and the full width at half maximum of the frequency spectrum be 0.315. To suppress interference between adjacent optical pulses which occurs on the optical fiber transmission line, it is desirable that the pulse width be 20% of or less than the pulse interval. Incidentally, in an experimental optical soliton communication system at present, optical pulses are used when the product of the full width at half maximum of the temporal waveform and the full width at half maximum of the frequency spectrum is within 0.441 or so. A semiconductor mode locked laser with an external cavity, which is a typical example of conventional optical pulse generators, has a construction in which one end face of the laser diode is given an anti-reflection coating, a reflector is disposed in front of the said one end face and the other end face of the laser diode and the reflector constitute an optical resonator. Since the semiconductor laser is driven by a sinusoidaly modulated current signal synchronized with the light round trip time of the optical resonator, the modulation rate is fixed to the length of the optical resonator and the product of the full width at half maximum of the temporal waveform and the full width at half maximum of the frequency spectrum exceeds 0.5. Moreover, this method presents a problem in its long-term stability as it employs the optical resonator which is susceptible to changes in environmental conditions, for example, a temperature change or mechanical vibration. On the other hand, according to the gain switching method which drives the semiconductor laser directly with short current pulses, the modulation rate is not fixed in principle, but in practice, it is hard to change the modulation rate arbitrarily, because it is inevitable to use a resonator type microwave circuit such as a comb generator. With this method, the frequency spectrum excessively broadens more than in the case of employing the semiconductor mode locked laser, and the product of the full width at half maximum of the temporal waveform and the full width at half maximum of the frequency spectrum becomes greater than 1. Consequently, no soliton optical pulses can be obtained in this case; hence, it is necessary to use a new technique for suppressing the excess spectral broadening, such as a narrow-band optical filter. In either case, since the oscillation wavelength of the laser undergoes substantial changes with its direct modulation, the spectrum of pulses broadens excessively and they cannot be used intact as optical pulses for the soliton communication. Furthermore, it is difficult to implement an optical soliton pulse generator which is stable for a long period of time. The article "5-BG/S Performance of Integrated Light source consisting of λ/4 - shifted DFB and EA modulator with SI InP BN Structure" - which appeared at pages 1357-1362 in The Journal of Lightwave Technology, vol 8, No. 9 September 1990, describes an asymmetric λ/4 - shifted DFB laser for continuously oscillating an output light and a buffer layer loaded EA modulator for modulating the intensity of the output light from the laser by means of a sinusoidal voltage to provide optical pulses. This discloses the transmission of a 5-Gb/s NRZ (Non-Return to Zero) (1,0) pattern through 73-km conventional fiber with a dispersion of 1467 ps/mm. Thus, it does not provide for the generation of patterns of short optical pulses which are suitable for optical soliton communications. It is an object of the present invention to provide a short optical pulse generator for the soliton communication which obviates the above-mentioned defects of the prior art and which is variable in modulation rate and hence is capable of generating optical pulses free from excessive broadening of their spectra. According to the present invention there is provided an optical pulse generator for optical soliton communications comprising: a semiconductor laser for continuously oscillating an output light at a single wavelength; a sinusoidal voltage generator for generating a sinusoidal voltage; a semiconductor electro-absorption type optical modulator for modulating the intensity of the output light from said semiconductor laser by the sinusoidal voltage to provide optical short pulses, and a DC voltage source for applying a reverse DC bias voltage to said semiconductor electro-absorption type optical modulator, characterised in that the reverse DC voltage of the DC voltage source is set to a point where substantially no light passes through the optical modulator; and the amplitude of the modulation voltage of the sinusoidal voltage generator is set to a value which is at least twice that of the bias voltage. In embodiments of the invention the optical pulse generator applies semiconductor laser beams of a fixed intensity to the semiconductor electro-absorption type optical modulator and generates therefrom optical pulses through utilization of a nonlinear characteristic of the optical modulation so that its optical output shows a monotonous decrease substantially exponentially with an increase in the applied voltage. This characteristic is inherent in the semiconductor electro-absorption type optical modulator and is not found in a dielectric optical modulator whose optical output periodically varies with an increase in the applied voltage. By applying to the semiconductor electro-absorption type optical modulator a DC voltage biased to a point where its optical output is sufficiently attenuated and then by applying to the optical modulator a sinusoidal voltage of an amplitude about twice that of the DC voltage, the optical output pulse of the optical modulator automatically becomes shorter than the sinusoidal voltage by virtue of its nonlinear optical output characteristic mentioned above and the semiconductor electro-absorption type optical modulator can be operated so that it becomes transparent for a short period of time. Thus, short optical pulses can be generated. The temporal waveform of the optical pulse obtainable with this method is intermediate between squared hyperbolic secant and Gaussian waveforms. By changing the bias voltage from a value at which to obtain an extinction ratio of 10 dB to a value at which to obtain an extinction ratio of 50 dB and by setting the amplitude of each modulating voltage to twice that of the bias voltage, the full width at half maximum of the optical pulse obtainable in this instance changes from 25 to 10% of the period of the sinusoidal voltage. Moreover, since the semiconductor laser is caused to oscillate at a fixed wavelength and since the short optical pulses are produced by the semiconductor electro-absorption type optical modulator without directly modulating the semiconductor laser, the oscillation wavelength of the semiconductor laser is free from variations which are caused by the direct modulation of the laser, and hence it is possible to suppress excessive broadening of the spectrum, thus providing optical soliton pulses. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1A is a block diagram illustrating in block form a first embodiment of the optical pulse generator of the present invention which uses a semiconductor electro-absorption type optical modulator; Fig. 1B illustrates a characteristic diagram explanatory of the principle of the generation of short optical pulses by the semiconductor electro-absorption type optical modulator in this first embodiment; Fig. 2 is a sectional view illustrating the internal construction of a second embodiment of the present invention in which a semiconductor laser and an electro-absorption type optical modulator for generating pulses are integrated as a unitary structure; Fig. 3 is a sectional view showing the internal construction of a third embodiment in which two electro-absorption type optical modulators for generating pulses and information signals, respectively, are integrated as a unitary structure; Fig. 4 is a sectional view showing the internal construction of a fourth embodiment in which two electro-absorption type optical modulators for generating pulses and information signals, respectively, and a semiconductor laser are integrated as a unitary structure; and Fig. 5 is a block diagram illustrating an embodiment of the present invention from which a modulated light output is obtained. [Embodiment 1] A first embodiment is illustrated by Fig. 1A which consists of a block diagram of the optical pulse generator, and Fig. 1B is a graph for explaining the principle of generating short optical pulses by a semiconductor electro-absorption type optical modulator. A description will be given first of the principle of operation of the invention. In Fig. 1A, fixed output light of 1.55 µ m wavelength, which is emitted from an InGaAsP λ/4 shift distributed feedback (DFB) single-wavelength laser 1, is applied to an InGaAsP electro-absorption type optical modulator 2. A forbidden band wavelength of the InGaAsP modulation waveguide layer of the optical modulator 1 is set between 1.44 and 1.5 µ m. When supplied with no voltage, the optical modulator 1 is substantially transparent, but it absorbs light and does not transmit it therethrough as a reverse voltage applied thereto increases. Fig. 1B shows, on a log scale, the transmission characteristic (the extinction ratio) of the electro-absorption type optical modulator with respect to the reverse voltage applied thereto. The extinction ratio scaled in dB of the electro-absorption type optical modulator is substantially in proportion to the reverse voltage applied thereto. That is, the optical output is nonlinear with respect to the voltage. A bias voltage 4 is applied from a DC voltage source 3 to a point where substantially no light passes through the optical modulator, as shown in Fig. 1B. Then, the amplitude of a modulation voltage 6 which is the output of a sinusoidal voltage generator 5 is set to around twice that of the bias voltage 4 and the modulation voltage is applied to the optical modulator, by which short optical pulses 7 can be obtained. To confirm the above-noted effect, the present inventors tested the operation of this embodiment which employed an InGaAsP λ/4 shift DFB laser oscillating at a single wavelength of 1.55 µ m and an electro-absorption type optical modulator having an InGaAsP modulation waveguide layer of a 1.45 µ m forbidden band wavelength. In these tests, when the bias voltage was -3 V, the optical modulator output was reduced to 1/100 (an extinction ratio of 20 dB) that in the case of the bias voltage being zero. Furthermore, by applying a 5-GHz sinusoidal modulation voltage of a 6 V amplitude to the optical modulator, an ultrashort optical pulse was obtained which had a full width at half maximum of 40 picoseconds. When the bias voltage was changed from -2 V to -6 V and the amplitude of the bias voltage was set twice the bias voltage, the full width at half maximum of the optical pulse changed from 49 picoseconds to 22 picoseconds. Since the period of the sinusoidal voltage was 200 picoseconds, pulse widths in the range of 24.5 to 11% of the period were obtained. The present invention does not utilize such an optical resonator as in the case of the prior art example, and hence permits arbitrary setting of the modulation rate. In the tests of this embodiment, the frequency of the sinusoidal voltage was set to 15 and 20 GHz. When the bias voltage was -2 V and the amplitude of the sinusoidal voltage was 7.2 V, short optical pulses whose full widths at half maximum were 14 and 11 picoseconds were obtained when the frequency of the sinusoidal voltage was 15 and 20 GHz, respectively. Since the 3 dB bandwidth of the frequency characteristic of the optical modulator used was 7 GHz, the modulation efficiency at 15 and 20 GHz was 4 and 6 dB lower than that in the case of DC voltage. It was found, however, that the modulation rate could freely be selected, in principle, by increasing the modulating voltage in a manner to make up for such deterioration of the modulation efficiency. Next, measurements of light spectrums were made. The full widths at half maximum of the spectral envelope of temporal waveforms which has repetition frequencies of 5 and 15 GHz and full widths at half maximum of 22 and 14 picoseconds were 14 and 23 GHz, respectively. The product of the full width at half maximum of the temporal waveform and the full width at half maximum of the spectral envelope was 0.32 in either case. This value is very close to the product, 0.315, of the full width at half maximum of the temporal waveform and the full width at half maximum of the spectral envelope of the squared hyperbolic secant waveform which is the requirement for the soliton pulse. The optical pulse obtainable with the present invention substantially satisfies the soliton requirement and has a pulse width to be 20% of or smaller than the pulse interval, and hence it can be applied to the soliton fiber optic communication system. Then, a train of short pulses having a 60 psec full width at half maximum and a 2.48 GHz repetition frequency, produced by the pulse generator of this embodiment, was transmitted over an optical fiber 10,000 km long and pulse waveforms were observed. No remarkable deterioration was observed in the pulse waveforms even after the pulses were transmitted 10,000 km. This proves that the optical pulse generator of this embodiment can be used as a soliton light source for a long distance optical soliton communication. Incidentally, the light output characteristic in logarithmic scale or the extinction ratio scaled in dB of the electro-absorption type optical modulator need not always be linear to the applied voltage. In the case where the extinction ratio linearly increases in a low voltage region and the rate of its increase in a high voltage region slightly lowers, the resulting temporal waveform becomes closer to the squared hyperbolic secant waveform. This phenomenon was observed when the optical axes of an optical fiber and optical modulators disposed at its input and output ends with lenses or the like interposed there-between were slightly out of alignment. In the case where the extinction ratio scaled in dB linearly increases in the low voltage region and the rate of its increase slightly increases in the high voltage region, the resulting temporal waveform becomes very close to the Gaussian waveform. In either case, such a waveform variation is permissible in the case of the soliton light source. Moreover, when the wavelength of light incident to the optical modulator was changed from 1.52 to 1.57 µ m in this embodiment, short optical pulses of about the same waveform could be obtained. The pulse generator of this embodiment permits easy adjustment of the pulse width, and hence can be used not only to generate optical soliton pulses but also as an optical pulse generator which utilizes ordinary return-to-zero codes. The optical pulse generator of this embodiment does not employ an optical resonator, and hence is advantageous in that is unsusceptible to changes in temperature and other environmental conditions, that the modulation rate is variable, and that short optical pulses for the soliton communication, which is free from excessive spectral line broadening, can easily be generated by use of only the sinusoidal voltage without the necessity of using a special microwave voltage generator. It is postulated that a practical application of this embodiment will involve the use of a second optical modulator 14, as shown in Fig. 5, for generating an information signal which is added to the output light of the semiconductor electro-absorption type optical modulator 2. [Embodiment 2] Fig. 2 is a sectional view illustrating (as a part of a second embodiment of the present invention) a device or element in which the afore-mentioned semiconductor laser and the electro-absorption type optical modulator for generating short pulses are integrated. The InGaAsP λ/4 shift DFB single-wavelength laser 1 and the InGaAsP electro-absorption type optical modulator 2 used in the optical pulse generator of Embodiment 1 are integrated on an InP semiconductor substrate 11. Both elements are electrically isolated by a semi-insulating InP layer 12. The inventors made an integrated device in which the laser portion was 300 µ m long, the semi-insulating InP layer 12 was 50 µ m long and the modulator portion was 290 µ m long. Either end face of the integrated device is coated with an anti-reflection film to suppress variations in the oscillation wavelength which is caused by the reflection of light from the optical modulator to the laser. The electric isolation resistance between the laser and the optical modulator is 1 megohm and suppresses the electric interference between them. The isolation resistance of 1 megohm is enough large to suppress wavelength variations of the laser and to obtain the transform-limited short pulses. The forbidden band energy of an InGaAsP modulating waveguide layer is 1.45 µ m. When a 180 mA direct current was applied to the laser portion, it oscillated at a single wavelength of 1.55 µ m, and when a -2 V bias voltage was applied to the modulator portion, an extinction ratio of 25 dB was obtained. Moreover, when a sinusoidal voltage of a 5 GHz frequency and a 4 V amplitude was provided to the optical modulator portion, an optical pulse was obtained the full width at half maximum of which was 33 picoseconds. It was ascertained that the product of full width at half maximum of the temporal waveform and the full width of half maximum of the spectral envelope was 0.31, which is very close to the value of 0.315 for transform-limited sech 2 pulse and sufficient for the soliton light source. While this embodiment has been described to employ an InGaAsP λ/4 shift DFB laser as the single-wavelength laser, it may also be replaced by other single-wavelength lasers of any construction, such as a distributed Braggreflector laser and an ordinary DFB laser using a homogeneous or uniform diffraction grating, and a variable wavelength laser is also applicable. Moreover, this embodiment is not limited specifically to semiconductor materials of any particular series, and a quantum well structure may also be applied to the active layer of the laser and the modulating waveguide layer. A feature of the optical pulse generator according to this embodiment lies in that since the laser and the modulator are formed as a unitary structure, the optical coupling loss between them is substantially reduced in the one-chip semiconductor device and the DC voltage and the sinusoidal voltage are small because of high modulation efficiency. Fig. 3 is a sectional view illustrating (as a part of a third embodiment of the present invention) a device in which an electro-absorption type optical modulator for generating short pulses and an electro-absorption type optical modulator for generating information signals are integrated. In this embodiment, the InGaAsP electro-absorption type optical modulator 2 for generating short pulses, used in Embodiment 1, and an InGaAsP electro-absorption type optical modulator 14 for producing information signals are integrated on the InP semiconductor substrate 11, and both elements are electrically isolated from each other by the semi-insulating InP layer 12. An integrated optical modulator for optical pulse generator of this embodiment comprises a semiconductor substrate, a semiconductor electro-absorption type optical modulator for modulating an intensity of an input light, and an optical modulator for modulating the intensity of the output light from the first optical modulator, and moreover, the semiconductor electro-absorption type optical modulator and the optical modulator are integrated as a unitary structure on the semiconductor substrate. A feature of this embodiment resides in the additional provision of a signal generating modulator to Embodiment 1 without causing an increase in the optical loss. [Embodiment 4] Fig. 4 is a sectional view illustrating (as a part of a fourth embodiment of the present invention) a device in which two electro-absorption type optical modulators and a semiconductor laser are integrated. In this embodiment, the InGaAsP electro-absorption type optical modulator 2 for generating short pulses and the InGaAsP electro-absorption type optical modulator 14 for generating information signal in Embodiment 3 are integrated on the InP semiconductor substrate 11, together with the InGaAsP λ/4 shift DFB single-wavelength laser 1. The three elements are electrically isolated by the semi-insulating InP layer 12. An integrated optical pulse generator for optical soliton communications of this embodiment comprises the semiconductor laser, the first optical modulator and the second optical modulator are integrated as a unitary structure on the semiconductor substrate. While in the above the present invention has been described to employ InGaAsP electro-absorption type optical modulators, it is also possible to use absorption type optical modulators of a quantum well structure wherein the quantum well layer is formed by an InGaAsP or InGaAs layer and a quantum barrier layer is formed by an InP or InGaAsP layer which has a forbidden band energy larger than that of the quantum well layer. Furthermore, absorption type optical modulators of InGaAlAs/InAlAs and InGaAs/InAlAs quantum well structures can also be used in the present invention. The modulator for generating information signals can be formed by any kind of intensity modulator irrespective of construction and materials. Hence, this optical modulator for use in the present invention is not limited specifically to a semiconductor optical modulator but may also be dielectric optical modulator using LiNbO 3 or the like. As regards the integrated optical pulse generator, two or more arbitrary combinations of the laser and the modulators are possible and the above-mentioned quantum well structure can be used. Although in the above the pulse generators of the present invention have been described to be used for the soliton optical communication, they can also be used as optical pulse generators utilizing ordinary return-to-zero codes. Moreover, ultrashort optical pulses having a pulse width of 1 picosecond or less could be obtained by utilizing the nonlinearityy of an optical fiber or pulse compression effect by a saturable absorber after the generation of optical pulses by the present invention. Embodiments of the present invention having a construction such as described above produce the following effects. The optical pulse generator according to the present invention achieves the variable modulation rate and suppresses the excessive spectral line broadening, both impossible with the prior art, by a simple method which employs a semiconductor laser, an electro-absorption type optical modulator, a DC voltage source and a sinusoidal voltage generator, and the pulse generator is capable of generating ultrashort optical pulses free from the excessive spectral line broadening, and hence is very promising as an optical pulse generator for the soliton communication which is stable and reliable for a long period of time. The inventors believe that the optical pulse generator of the present invention is most promising from the viewpoints of easiness of handling of the device and its single-frequency operation.

Claims: An optical pulse generator for optical soliton communications comprising: a semiconductor laser (1) for continuously oscillating an output light at a single wavelength; a sinusoidal voltage generator (5) for generating a sinusoidal voltage; a semiconductor electro-absorption type optical modulator (2) for modulating the intensity of the output light from said semiconductor laser (1) by the sinusoidal voltage to provide optical short pulses, and a DC voltage source (3) for applying a reverse DC bias voltage to said semiconductor electro-absorption type optical modulator (2), characterised in that the reverse DC voltage (4) of the DC voltage source (3) is set to a point where substantially no light passes through the optical modulator (2); and the amplitude of the modulation voltage of the sinusoidal voltage generator (5) is set to a value which is at least twice that of the bias voltage (4). An optical pulse generator according to claim 1, further comprising: an optical modulator (14) for modulating the optical short pulses from said semiconductor electro-absorption type optical modulator (2) by an information signal. An optical pulse generator according to claim 1 or 2, wherein said semiconductor laser (1) and semiconductor electro-absorption type optical modulator (2) are integrated as a unitary structure on a semiconductor substrate (11). An optical pulse generator according to claim 2, wherein at least two of said semiconductor laser (1), said semiconductor electro-absorption type optical modulator (2) and said optical modulator (14) are integrated as a unitary structure on a semiconductor substrate (11). An optical pulse generator according to claim 1, wherein the temporal waveform of the optical output pulse is intermediate between squared hyperbolic secant and Gaussian waveforms, and the full width at half maximum of the optical output pulse is between 25% and 10% of the period of the sinusoidal voltage.

Inventors: MATSUSHIMA YUICHI, SUZUKI MASATOSHI, TANAKA HIDEAKI, MATSUSHIMA, YUICHI, SUZUKI, MASATOSHI, TANAKA, HIDEAKI
Applicants: KOKUSAI DENSHIN DENWA CO LTD, KOKUSAI DENSHIN DENWA KABUSHIKI KAISHA
Application year: 1993
Main classification: G02F1
IPC codes: H01S5, H04B10, G02F1