Method for the generation of Ultra-short optical pulses

Abstract: Ultra-short transform-limited optical pulses, to be utilized for high bit-rate transmission systems, are obtained through direct modulation of a semiconductor laser (1) by means of pulses of such a duration as to excite only the first peak of the relaxation oscillations of the cavity of laser (1), thermally tuned to such a length that the pulse portions that overlap inside the cavity interfere constructively. The pulses emitted by the laser (1) are then made to pass in a fibre with high negative dispersion (8) to compensate for the phase effect due to the chirp.

Description: The invention described herein relates to optical fibre communications systems, and more particularly it concerns a method for the generation of ultra-short optical pulses to be utilized for high bit rate transmission systems. Optical communications systems at very high bit rates (typically from 10 to 100 Gbit/s and more) are currently being studied; in these systems, a plurality of channels are transmitted, and each of them conveys information represented by a succession of 0 and 1 pulses. The pulses of a channel are transmitted at relatively low bit rate (up to 10 Gbit/s) and between two successive pulses of a channel there are inserted, in a pre-determined sequence, the pulses relating to the other channels, transmitted at the same rate. This multiplexing method, well known from electronic signal technique, is named in the case at hand "Optical Time Division Multiplexing", commonly known with the acronym OTDM. It is evident that, to fully exploit the capacity of the transmission medium with the method described, it is desirable that the pulses be as narrow as possible in order to avoid interferences between the channels and to allow the correct demultiplexing at the receiving side. Actually, at those rates, demultiplexing must be performed completely optically. Some of the techniques proposed for this purpose, which exploit the so-called Four Wave Mixing (FWM) or the Kerr effect in optical fibre (Nonlinear Optical Loop Mirror, NOLM) and are based on the overlapping between the pulse of the channel to be extracted and a pump pulse within that fibre (which acts as a nonlinear medium), require that the two pulses remain overlapping as long as possible during the travel along the fibre. Furthermore the pulses must have such shape and band characteristics that the pulses themselves propagate as undistorted as possible. This requirement is generally expressed by saying that the pulses must be "transform limited", this expression meaning that the product between the duration or full width at half maximum (FWHM) Δt and the bandwidth Δv must have a certain value, corresponding to the theoretical minimum, which depends on the pulse shape: in particular, since the pulses that are most commonly used and that have yielded the best results in transmission are Gaussian and hyperbolic secant pulses, the term "transform limited" is used to indicate pulses where the product Δt·Δv takes a value that corresponds or is close to that of the Gaussian pulse or the hyperbolic secant pulse (0.441 and respectively 0.314). To generate pulses with these characteristics, it has been proposed to utilize the direct modulation of a semiconductor laser by means of pulses of such duration as to excite only the first peak of the laser relaxation oscillations (gain switching technique). In that condition the pulses emitted by the laser exhibit, because of the modulation, a high chirp and therefore, before being utilized, they are made to propagate in an optical fibre with such dispersion characteristics as to compensate the phase distortion produced by said chirp. This technique of generating ultrashort, transform-limited optical pulses is described for example by H.F. Liu et al. in "Generation of an extremely short singlemode pulse (2 ps) by fibre compression of a gain-switched pulse from a 1.3 µm distributed feedback laser diode", Applied Physics Letters 59 (11), 9 September 1991, by K.A. Ahmed et al. in "Nearly transform-limited (3-6 ps) generation from gain-switched 1.55 µm distributed feedback laser by using fibre compression technique", Electronics Letters, vol. 29, no. 1, 7 January 1993, or yet by J.T. Ong et al. in "Subpicosecond Soliton Compression of Gain-Switched Diode Laser Pulses Using an Erbium-Doped Fiber Amplifier", IEEE Journal of Quantum Electronics, vol. 29, no. 6, June 1993. The system described in the first of the above articles generates a pulse that yields a product Δt·Δν within the desired range, but utilizes a source whose wavelength (1.3 µm) does not coincide with the null-dispersion wavelength (∼1.55 µm) of the fibres utilized in the optical demultiplexing systems proposed until now; moreover, the pulse is strongly affected by noise and it has a shape (Lorentzian pulse) that is not normally exploited for information transmission. The methods described in the second and third articles utilize sources at a wavelength of 1.55 µm, as required for the subsequent demultiplexing, but originate pulses whose product Δt·Δν is very far from the desired interval. For not transform-limited pulses, optimizing the conditions for minimum laser pulse width in the order of 2 ps is described in International Journal of Electronics, vol. 60, no. 1, January 1986, pages 23-45. For this purpose among other parameters, also the temperature of the laser is varied, with the result that by increasing the temperature to a maximum of 25°C, for each intended pulse one spike only appears. According to the invention, a method is provided, which, thanks to the introduction of the temperature as an additional control parameter on the shape of the pulses, allows obtaining pulses at the wavelength required for the subsequent demultiplexing with a product Δt·Δν within the required range. The invention provides a method wherein, through direct modulation of a semiconductor laser, pulses corresponding to the first peak of the relaxation oscillations of the laser cavity are generated, which pulses have longer duration than their time of flight inside the cavity, so that in the latter there is an overlap between pulse portions that correspond to the time of flight; such pulses are made to pass inside a fibre with high negative dispersion to compensate the phase effect due to the chirp and the length of the laser cavity is thermally tuned to a value such that the pulse portions that overlap inside the cavity are associated to fields that interfere so as to enhance the central peak and minimize the influence of the queues. This length corresponds to a condition where there is a pure autocorrelation with minimum duration. The invention originated from the daily observation of the behaviour of the source, which has brought to the conviction that ambient temperature influenced its pulse emission characteristics. A more in-depth study has in fact demonstrated that the autocorrelation traces of the pulses coming out of the compensation fibre show a periodic behaviour as the laser temperature varies and in particular that pulses are periodically obtained whose characteristics of duration and shape are particularly satisfactory. Keeping into account the fact that the pulse duration is longer than the time of flight inside the laser cavity, so that the signal emitted by the laser results from the overlapping between different portions of a pulse, the inventors have deemed that this periodic behaviour was due to different interference conditions between the various pulse portions due to the variation of the length of the cavity with temperature, and thus temperature control has been exploited to bring the laser to operate in one of the conditions where the pulse has minimal duration, particularly in the condition closest to the normal optimal working conditions. The invention also relates to a method of operating a high bit-rate optical communications system in which a plurality of channels are transmitted, to each of them being associated an information item represented by a succession of the ultra-short optical pulses repeating themselves at relatively low bit rate with respect to the overall bit rate, and a completely optical demultiplexing is carried out, based on the overlapping between a pump pulse and the pulse of a channel to be extracted inside an optical fibre acting as a non linear medium, and in which both the ultra-short pulses to be transmitted and the pump pulse are generated with the method defined above. For further clarification, reference is made to the enclosed drawings, where: Figure 1 is a layout of an apparatus for the realization of the method according to the invention, Figure 2 shows the autocorrelation traces, obtained experimentally, of the pulses generated with the apparatus of Fig. 1 as the temperature varied; and Figure 3 shows the corresponding theoretical autocorrelation traces in the case of a Gaussian pulse. The apparatus shown in Fig. 1 is as described in the literature for the generation of ultrashort and transform limited pulses. A semiconductor laser 1 is brought to operate in gain-switching conditions by electrical pulses of suitable frequency (e.g. between about a hundred MHz and a few GHz) emitted by a comb generator 2 driven by a generator 3 of sinusoidal signals through an amplifier 4. The pulses are provided to the laser modulation input after having been attenuated by an attenuator 5 and added, in an LC circuit 6, to a bias current provided by a generator 7 and having such a value that laser 1 is kept well below the stimulated emission threshold. Advantageously, the laser is a distributed-feedback laser, because such a type of laser, in addition to having emission wavelengths within the range normally used for optical transmissions and in particular the wavelengths required by completely optical demultiplexing systems, has the best single-mode characteristics. Through a suitable optical system outlined by insulator 9 (which avoids reflections inside the cavity) and by lens 10, the pulses emitted by laser 1 are sent into a span of optical fibre 8 with high negative dispersion (for example a dispersion of between roughly -60 and -70 ps/nm/Km), of characteristics complementary to those due to the effect of phase distortion produced by the chirp, and of such a length as to compensate the aforementioned effect. The pulses leaving fibre 8 are sent to an optical amplifier 11 and then to the utilization devices (e.g. an electro-optical modulator if the pulses are utilized as transmission carrier, or the fibre of an optical demultiplexer, if the pulse is utilized as a pump signal for optical demultiplexing). As stated in the discussion of the prior art, the pulses obtained at the output of fibre 8 do not exhibit satisfactory characteristics in terms of product Δt·Δν. Therefore, according to the invention, a further correction of the width and shape of the pulse is performed, by acting upon the working temperature of the laser and thus on the optical length of the cavity. More particularly, in an equipment calibration phase, the working temperature is set to such a value that there are favourable conditions of interference between the various pulse portions corresponding to the time of flight inside the laser cavity, i.e. conditions in which the central peak is accentuated and the influence of the queues is minimized. The attainment of these conditions is detected by means of a self-correlator and a low frequency oscilloscope, both connected to the output of amplifier 11. For temperature adjustment, the conventional temperature control devices 12, with which all lasers are equipped, can be used. In this way, employing the invention does not require modifications of the source or the presence of additional equipment. A sufficiently fine adjustment (e.g. by steps of 1/10 of a degree or even less) can be obtained with a device comprising a Peltier-effect cell and a thermistor. In general, considering by way of example a Gaussian pulse, the optical field and intensity related to the pulse and to its first two foldings due to the reflections within the cavity, turn out to be respectively: E ( t )= E 0 {e - t 2 T 2 ·e i ω 0 t + k · e - ( t +τ) 2 T 2 · e i ω 0 ( t +τ) + k 2 · e - ( t +2τ) 2 T 2 · e i ω 0 ( t +2τ) } I ( t )= I 0 {e -2 t 2 T 2 + k 2 · e -2 ( t +τ) 2 T 2 + k 4 · e -2 ( t +2τ) 2 T 2 + 2 k [ e - t 2 +( t +τ) 2 T 2 + + k 2 · e - ( t +τ) 2 +( t +2τ) 2 T 2 ]cos(ω 0 τ)+2 k 2 · e - t 2 +( t +2τ) 2 T 2 ·cos(2ω 0 τ)} and the related autocorrelation function is given, as is well known, by In (1) and (2), ω o is the emission frequency of the laser, τ is the complete time of flight inside the cavity, T is the pulse duration and k is a phenomenological constant that takes into account the reflectivity and the residual gain of the cavity (as the pulse duration is longer than the time of flight inside the cavity, the cavity can be deemed to be still active when inside it the pulse undergoes the successive foldings spaced respectively by one and two times of flight). From relations (1) and (2), the skilled in the art is perfectly able to determine, given the duration of the pulses applied to laser 1 and the characteristics of the laser itself, the values of temperature where the most favourable interference between the various overlapping pulse portions is obtained, i.e. a pure autocorrelation profile condition with minimal duration. Among the various values that satisfy this condition, one will choose a value close to the working temperature considered optimal for the normal use of the laser, in particular the closest one. Figure 2 shows the autocorrelation traces obtained at the output of amplifier 11, as the temperature of the laser varies, in an experiment where: source 1 was a distributed feedback, single mode laser operating at 1548 nm and having a modulation band of 10 GHz; the pulses emitted by comb generator 2 had a duration of 70 ps and a nominal peak voltage of 15 V; fibre span 8, about 300 m long, had a dispersion, at 1548 nm, of -68 ps/nm/Km. When the temperature of laser 1 was made to vary from 8 °C to 35 °C, traces were obtained that repeated themselves with a periodicity essentially equal to 9°C. The traces are reported only for the temperature interval 17 °C - 26 °C, i.e. for the interval defined by the two consecutive pulses of optimal shape that were the closest to the normal working temperature (about 20 °C). In particular, the pulse at 17 °C displayed a duration of 3.46 ps and a product Δt·Δν of 0.39, thus rather close to the value required for Gaussian transform-limited pulses. The comparison with Figure 3, which illustrates the corresponding simulated autocorrelation traces obtained in the case of Gaussian pulse (i.e. according to the relations given above), by utilizing the same values of the parameters as utilized in the actual experiment, shows that the practical results are in good qualitative accord with the theoretical ones.

Claims: Method for the generation of ultrashort optical pulses, wherein, through direct modulation of a semiconductor laser (1), pulses corresponding to the first peak of the relaxation oscillations of the laser cavity are generated, which pulses have longer duration than their time of flight inside the cavity, so that in the latter there is an overlapping between different pulse portions, each corresponding to a time of flight, and such pulses are made to pass in a fibre (8) with high negative dispersion to compensate for the phase effect due to the chirp, characterized in that, in order to obtain transform limited pulses, the length of the laser cavity is thermally tuned to a value such that the overlapping pulse portions are associated to fields that interfere so as to enhance the central peak of the pulse and minimize the influence of the queues. Method as claimed in claim 1, characterized in that the optical length of the laser cavity is thermally tuned to the value that, among all values originating said interference conditions enhancing the central peak of the pulse and minimizing the influence of the queues, is the closest one to the optimal working temperature of laser (1). Method as claimed in claim 1 or 2, characterized in that the thermal tuning of the length of the cavity is obtained by means of the temperature control devices (12) of which the laser (1) is provided, in a calibration phase of a pulse generation equipment. Method of operating a high bit-rate optical communications system, wherein a plurality of channels are transmitted and to each of these channels is associated an information represented by a succession of ultra-short optical pulses that repeat themselves at a bit rate that is relatively low with respect to the overall bit rate, characterized in that a completely optical demultiplexing is performed, based on the overlapping between a pump pulse and the pulse of a channel to be extracted inside an optical fibre that acts as a non-linear medium, both the ultra-short pulses to be transmitted and the pump pulse being generated according to any of claims 1 to 3.

Inventors: CALVANI RICCARDO, CAPONI RENATO, CISTERNINO FRANCESCO, ROCCATO DIEGO, CALVANI, RICCARDO, CAPONI, RENATO, CISTERNINO, FRANCESCO, ROCCATO, DIEGO
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1995
Main classification: H01S3
IPC codes: H01S5, G02F1, H01S3