FR2722922A1 - Optical impulse generator e.g. for fibre optic transmission using solitons - Google Patents

Abstract

The generator includes a master laser (10) which injects light of a given wavelength into a slave laser (11) for gain switching. A bandstop filter allows the optical impulses generated by the slave laser to be filtered (12) with rejection of light of the given wavelength. An optical isolator (13) is positioned between the lasers. A control circuit (14) provides the slave laser with an injection current where variations as a function of time generate a gain switching function. The output (S) has an optical spectral limit which is near to the theoretical limit.

Description

"Optical pulse generator with low fluctuation
frequency "
DESCRIPTION
Technical area
The present invention relates to an optical pulse generator with a low "chirp", that is to say with a slight variation in the frequency of the optical carrier during a pulse. using a "master" laser and a "slave" laser
It applies in particular to optical telecommunications, and more particularly to digital transmissions by optical fibers, for example those which take advantage of solitons and wavelength division multiplexing.

Prior art
Currently. research in the field of optical pulse generation dedicated to soliton effect transmissions is relatively intense. There are at least four techniques for generating these semiconductor laser pulses with mode blocking, amplifying fiber ring laser with mode blocking, electro-absorption modulator operating in non-linear regime and semiconductor laser operating in gain switching . Each of these methods has advantages and disadvantages in terms of the quality of the time profiles, spectral quality, robustness, stability. repetition frequency adjustment and wavelength tunability.

 The present invention relates to a generator based on gain switching. A semiconductor laser operates in gain switching when the injection current of this laser is modulated quickly and strongly, with a clear passage of this injection current below the emission threshold of the laser. So. the relationship between the modulation signal and the modulated optical power is strongly non-linear. which makes it possible to obtain, under certain conditions, very brief optical pulses.

 The advantages of gain switching are simplicity, robustness and the possibility of continuously adjusting the repetition frequency over a wide range of frequencies. The major drawback of this gain switching technique lies in the presence of a strong "chirp". "(frequency fluctuation).

 To improve the spectral quality two techniques can be used. One uses a special optical fiber, the dispersion of which is negative and an optical filter. But the quality of the pulse although much improved often remains insufficient Indeed the temporal profile of the pulse often presents a foot too important The other consists in reinjecting a large part of the light produced by this laser after having previously filtered it .

 The present invention relates to a low "chirp" optical pulse generator using a master laser and a slave laser operating with gain switching.

Statement of the invention
The present invention provides an optical pulse generator with low frequency fluctuation. characterized in that it comprises a first laser. or master laser. injecting light at a given wavelength into a second laser. or slave laser, operating with gain switching, and a filter. whose bandwidth is offset with respect to said wavelength making it possible to filter the optical pulses emitted by this second laser and to reject this wavelength.

 Advantageously, this generator comprises an optical isolator mounted between the master laser and the slave laser so that light flows from the master laser to the slave laser, and a control circuit provided for sending an injection current to the slave laser, the variations of which i (t) as a function of time allow gain switching operation.

 The slave laser can be a semiconductor laser or laser diode. Advantageously, the master laser is a tunable laser.

 In a first embodiment, the slave laser is a single-mode laser, and the master laser emits continuous power at a wavelength close to the emission wavelength of the slave laser.

 In a second embodiment, the slave laser is a multimode laser, and the master laser emits continuous power at a wavelength close to one of the modes of the slave laser.

Advantageously, the slave laser is modulated directly by the injection current. the type of modulation being either a sinusoid for frequencies above 2 GHz. either pulses for frequencies
Less than 2 GHz. the rate of injection into the slave laser being at least one hundredth.

 In one embodiment, the slave laser is a laser diode which has only one access face. a coupler then being placed between the isolator and the slave laser.

Thus the generator of the invention has the following characteristics
- operation of the slave semiconductor laser in gain switching
- injection into this laser of light, at the wavelength Xrn, produced by the master laser
- filtering of the optical pulses emitted by the slave laser using the bandpass filter. whose bandwidth is offset from the wavelength r., and which rejects this wavelength.

 Through the injection of a high light rate and the use of a filter. The invention makes it possible to obtain optical pulses whose optical spectral width is close to the theonic limit corresponding to the Founer transform of the detected pulse. In addition, the use of the bandpass filter makes it possible to obtain a pulse whose carrier is pure (without "chirp") and to reduce its width since it selects the upper part of the pulse, and thereby eliminating the foot of the pulse which is generally too broad and asymmetrical. Finally, this filter eliminates the continuous level between the pulses.

 Apparently in the documents of the known art there is no mention of laser synchronization with a slave laser operating in gain switching. nor optical power emitted by a slave laser synchronized by a master laser and filtered by a bandpass filter which rejects the wavelength of the master laser.

Brief description of the drawings
- Figure 1 shows the block diagram of the device of the invention
- Figure 2 part a represents the vanation curve of the optical power PO of the slave laser as a function of time
- Figure 2 part b represents the curve of variation of the frequency fe of the slave laser as a function of time, in the absence of injection
- Figure 2 part c represents the curve of variation of the frequency fe of the slave laser as a function of time, in the presence of injection
- Figure 3 shows the diagram of an alternative embodiment of the device of the invention.

Detailed description of embodiments
The present invention relates to an optical pulse generator with a low "chirp", that is to say with a low vanation of the frequency of the optical carrier during a pulse. using a master laser 10 and a slave laser 11. The master laser 10 injects light at a given wavelength Am into the slave laser il which is a semiconductor laser or laser diode. operating with gain switching. A bandpass filter 12, the bandwidth of which is offset from said wavelength Xm? makes it possible to filter the optical pulses emitted by this second laser and to reject said wavelength im.

An optical isolator 13 is mounted so that light flows from the master laser 10 to the slave laser 11
A control circuit 14 is provided for sending an injection current to the slave laser 11 whose vanations i (t) as a function of time allow operation in gain switching.

This optical pulse generation device therefore has the following characteristics
- the slave laser 11 semiconductor is operated in gain switching
- Light is injected into this laser, at wavelength i, produced by the master laser 10
- the optical pulses emitted by the slave laser are filtered using the 12. bandpass filter. whose bandwidth is offset from; w. and who rejects i;,. The importance and how this filter works will be described below.

Through the injection of a high light rate and the use of a filter. The invention makes it possible to obtain optical pulses whose optical spectral width is close to the thonic limit corresponding to the Founer transform of the detected pulse (such pulses being called "transform limited optical pulses" in publications in English) ). Furthermore. as will be explained later, the temporal width of these
Pulses may be smaller than that obtained with gain switching operation without optical processing.

This invention corresponds to a laser (called "slave laser") synchronized by another laser (called "master laser"). The publications relating to the synchronization of an injection laser (in English "injection locking") almost always relate to slave lasers whose injection current is not modulated. However, we can mention the article by NA Olsson et al., Entitled "Chirp Free Transmission over 82.5 km of Single Mode Fibers at 2 Gbitls with
Injection Locked DFB Semiconductor lasers "published in Joumal of Lightwave
Technology (vol. LT-3. Pages 63-66. 1985) and more recently that of S.

Mohrdiek. H. Burkhard. and H. A'altier entitled "Chirp Reduction of Directly
Modulated Semiconductor Laser at 10 Gbis by Strong CW Light Injection "published in Journal of Lightwave Technolooy (vol. 12, pages 418-424, 1994) where the injection current of the slave laser is modulated but in a linear or almost linear fashion but not In gain switching, in fact in these two publications the temporal vanation of the optical power emitted by the slave laser must represent, as faithfully as possible, the vanation of the injection current, which is not at all the case in gain switching scheme.

 Apparently, laser synchronization has not been reported in the literature with a slave iaser operating in gain switching. Likewise, never. in the prior art, the optical power emitted by a slave laser synchronized by a master laser is only filtered by a bandpass filter which rejects the wavelength of the master laser.

 FIG. 1 therefore represents the block diagram of the device of the invention. The master laser 10 is a laser, preferably tunable, which emits continuous optical power (C ′ N. Laser) at the wavelength Xm close to the emission wavelength of the slave laser 11 when the latter is single-mode (generally of the Cl7 type), or close to one of the modes when the slave laser is multimode (generally of the Fabry-Perot type).

The slave laser 1 is modulated directly by the injection current. The type of modulation is either a sinusoid for frequencies above 2 GHz. either Impressions for frequencies below 2
GHz.

 The injection rate which is the ratio between the optical power of the master laser 10 injected into the slave laser 11 and the power into the slave laser. must high filter. that is to say at least the hundredth. The higher the rate. the larger the fastening and stability bands, and more. the residual "chirp" is further reduced.

 In the absence of injection. the slave laser 11 has, when it operates in gain switching, a frequency fe which varies practically linearly over a period of approximately trn centered at the top of the pulse (see FIG. 2 part b) trr = width at mid-amplitude of the optical pulse.

 In the presence of injection. and when there is locking, the frequency of the slave laser is Imposed by that of the master laser (fm), then the phase difference a between the injected field and the field of the slave laser is substantially proportional to:, - fe As fe varies over time as shown in Figure 2 part b. e also changes in time, which generates a frequency offset tf = (1: (2xi :)) x (d6idt) which is constant when the phase phase vanation as a function of time is linear as shown in Figure 2 part c . Thus the optical frequency of the top of the optical pulse is shifted. compared to that of the master laser over a width of approximately tm. To obtain a Pulse whose carrier is pure (without "chirp") it suffices therefore to filter the output signal by the optical bandpass filter 12 centered on fs = fm-Af.

This filter also makes it possible to reduce the width of the pulse since it selects the upper part of the pulse and thereby eliminates the foot of the pulse which is generally too wide and asymmetrical. Finally, this filter makes it possible to eliminate the continuous level between the pulses because it is at the frequency fm. For example, an optical pulse 15 ps wide is obtained experimentally in the absence of external injection. With a strong external injection we observe an offset Af of 80 GHz and, using a bandpass filter of 40 GHz wide, the optical pulse is reduced to 8 ps With a narrower filter it is possible to widen the impulse to bring it to the desired value while remaining free from "chirp"
The wavelength tuning of the optical pulses can be obtained by vanishing the wavelength of the master laser. The wavelengths available are those of the Fabry-Perot cavity of the laser diode.

However. by playing on the temperature of this laser diode, it is possible to move the lines of its Fabry-Perot cavity and then make this laser diode continuously tunable.

 Figure 3 shows a vanante to inject light when the slave laser is a laser diode which has only one access face.

A coupler 15 is used. The other elements already illustrated in FIG. 1 keeping the same reference.

Claims (6)

 1 Optical pulse generator with low frequency fluctuation, characterized in that it comprises a first laser (10), or master laser, injecting light at a given wavelength (m) into a second laser (11 ), or slave laser. operating with gain switching, and a filter (12), the bandwidth of which is offset from said wavelength ('.ru), making it possible to filter the optical pulses emitted by this second laser and to reject this length d 'wave I.,).  2. Generator according to claim 1, characterized in that it comprises an optical isolator (13) mounted between the master laser (10) and the slave laser (11) so that the light circulates from the master laser to the slave laser .  3. Generator according to claim 1, characterized in that it comprises a control circuit (14) provided for sending to the slave laser an injection current whose vanations i (t) as a function of time allow operation by switching of gain.  4. generator seion ia claim 1, characterized in that the slave laser (11) is a semi-constructive laser or laser diode.  5. Generator according to claim 1, characterized in that the master laser (10) is a tunable laser.  7. Generator according to claim 5, characterized in that the slave laser (11) is a multimode laser. and in that the master laser (10) emits continuous power at a wavelength (i.,) close to one of the modes of the slave laser (11).  6. Generator according to claim 5, characterized in that the slave laser (11) is a single mode laser. and in that the master laser (10) emits continuous power at a wavelength (Am) close to the emission wavelength of the slave laser (11)  8. Generator according to any one of the preceding claims, characterized in that the type of modulation is either a sinusoid for frequencies above 2 GHz. or pulses for frequencies below 2 GHz.  9. Generator according to any one of the preceding claims. characterized in that the rate of injection into the slave laser is at least one hundredth.  10. Generator according to claim 2, characterized in that the slave laser (11) is a laser diode which has only one access face and in that said generator comprises a coupler disposed between the insulator (13) and the slave laser (11).