FR3049779A1 - DEVICE AND METHOD FOR ATTENUATING WAVELENGTH DERAPING IN A LASER TRANSMITTER - Google Patents

Abstract

The invention relates to a transmitting device (100) capable of transmitting in an optical fiber (F) an optical signal (S1) conveying data, comprising a laser (120) capable of generating said optical signal with a nominal optical power, a source electrical device (110) adapted to generate an electrical signal supplying the laser, the nominal optical power being reached from a power level of the electrical signal, said nominal level, a feature of the device being that, during a phase of ignition of the laser, the time taken by the electrical power to mount from a minimum electrical power below the nominal level, to a maximum electrical power greater than or equal to the nominal level, called the rise time, is greater than or equal to a threshold of determined rise time. The invention also relates to the associated method.

Description

Device and method for attenuating wavelength skidding in a laser transmitter 1. Field of the invention

The invention is in the field of passive optical networks (PONs) serving subscribers to electronic communications services, and more particularly in the case of direct or modulation modulation transmitters, used to transmit the data in the uplink direction, that is to say from the terminal equipment at the customers, called ONT (Optical Network Terminal), to a central equipment at the operator, called OLT (Optical Line Terminal). 2. State of the prior art

Widely used around the world, for example thanks to the deployment of G-PON technology, time division multiplexing (TDM) is based on the use of an optical "1 to N" coupler / combiner allowing downstream direction to send the optical signal from the exchange to the N customers.

In the upstream direction of the optical link, that is to say from the client to the central office, the physical architecture of the optical fiber network imposes operation not in TDM, but in TDMA (for Time Division Multiple Access). The principle of the TDMA is as follows: since there is only one receiver for each wavelength when there are as many transmitters as there are clients, the laser transmitter incorporated in the equipment The client must emit a very brief burst of data, typically a few microseconds or tens of microseconds, to then let the equipment of another client and so on.

Due to cost constraints, inexpensive laser emitters are generally preferred. The direct modulation of the laser, through the electric current applied to the laser, also reduces costs, avoiding the addition of an external optoelectronic modulator. However, the changes of electric current applied to the laser during the burst disrupt the equilibrium of the system, in particular the temperature of the laser, which has the effect of causing a modification of the index of the laser material and therefore a variation of the wavelength, called skidding, and called in English "wavelength drift of burst directiy mode modulated lasers", or "wavelength drift" succinctly.

This wavelength wander is not binding in the case of the G-PON, since the length range allocated to the technology is large in front of the extent of skidding. However, the increase in bandwidth requirements is forcing players in the optical access telecommunications market to increase the capacity of their systems. One of the solutions proposed, standardized to the UlT under the G.989 standard, consists of superimposing on time multiplexing a wavelength multiplexing so as to multiply the bit rate by the number of associated wavelengths: Time and Wavelength Division Multiplexing (TWDM) in the downstream direction, and Time and Wavelength Division Multiple Access (TWDMA) in the upstream direction. The proximity of wavelengths in the uplink direction, associated wavelength skidding can then have adverse consequences on the quality of transmissions. The optical signal of a transmitter will be briefly outside the range of wavelengths allocated to it, or even in the range of the neighboring channel, thus disrupting transmissions on the channels considered.

One of the aims of the invention is to overcome disadvantages of the state of the art. 3. DISCLOSURE OF THE INVENTION The invention improves the situation by means of an emitter device capable of transmitting in an optical fiber an optical signal carrying data, comprising a laser capable of generating said optical signal with an optical power. nominal, an electrical source capable of generating an electrical signal supplying the laser, the nominal optical power being reached from a power level of the electrical signal, said nominal level. The device is among others particular in that, during a phase of ignition of the laser, the time taken by the electrical power to rise from a minimum electrical power below the nominal level, up to a maximum electrical power greater than or equal to at the nominal level, said rise time, is greater than or equal to a determined rise time threshold.

In this way, the adiabatic frequency shift of the optical signal, or adiabatic skidding, which is positive and proportional to the optical power output of the transmitter, is progressive, instead of being sudden as in the prior art, where the The rise time of the power of the electrical signal must be less than a threshold of rise time imposed by the standard, for data coding optimization reasons.

The thermal frequency shift of the optical signal, or thermal skid, meanwhile is progressive, even with a very short rise time, but it is negative against, and the thermal skid is more progressive than the rise time is long.

It will be understood that, thanks to the invention, the thermal skid has more time to compensate for the adiabatic skidding, and that consequently the maximum value of the sum of the two skids is decreased during the ignition phase of the laser.

The rise time threshold is determined at least as a function of physical characteristics of the electrical source and / or the optical medium, so as to optimize the decrease of the resulting skid. The invention goes against the prejudices of the skilled person, who tends to minimize the rise time, on the one hand in order to be as close as possible successive bursts between them, and other in order to meet a standard such as NG-PON2 (G989.2) which imposes for example a rise time of less than 12.8 ns for 2.5 Gb / s or 10 Gb / s.

According to one aspect of the invention, the electrical source modulates the optical signal with the data, using the electrical signal during the generation of the optical signal by the laser.

Thanks to this aspect, the laser can be modulated directly by the electrical signal produced by the electrical source. In this mode, the modulating electric current has the form of the desired optical signal, that is to say, it is itself already modulated by the digital data to be transmitted by the transmitting device. In the case of NRZ modulation for example, the modulating electric current is composed of a series of high and low currents corresponding to the "1" and "0" components of the digital data to be modulated on the optical signal. Other formats than NRZ can of course be used. This so-called direct modulation has the advantage of being simple to implement, and inexpensive to use and in consumption, since it is not necessary to add an external component for the modulation and to supply it separately. in energy.

According to one aspect of the invention, the power source or other electrical source modulates the optical signal with the data, after its generation by the laser.

Thanks to this aspect, the envelope of the optical signal is first generated by laser thanks to the electrical source, then this source, or a separate electrical source internal or external to the device, is used to modulate the optical signal generated, for example by means of an electric shutter. In the case of an NRZ modulation for example, each shutter corresponding to a "0" of the digital data to be modulated on the optical signal. This so-called external modulation allows higher rates compared to the direct modulation, but requires a synchronization between the laser that produces an envelope of the optical signal, and the shutter that "cuts" the envelope.

According to one aspect of the invention, during a phase of extinction of the laser, the time taken by the electric power to descend from the maximum power to the minimum electrical power, called the descent time, is greater than or equal to a threshold of descent time determined.

Thanks to this aspect, the thermal skid has more time to compensate for the adiabatic skidding during the laser extinction phase, and consequently the maximum value of the sum of the two skids is decreased, as during the ignition phase. of the laser.

The descent time is determined at least according to the physical characteristics of the electrical source and / or the optical medium so as to optimize the decrease of the resulting skid.

According to one aspect of the invention, the minimum electrical power is substantially zero.

With this aspect, the power consumption is reduced between two bursts, that is to say between an extinction phase and a laser ignition phase.

According to one aspect of the invention, the minimum electrical power is substantially equal to a non-zero threshold from which an appearance of the optical signal is triggered, said threshold of the laser.

Thanks to this aspect, even if the power consumption is not reduced to the minimum between two bursts, the amplitude of the thermal fluctuations of the laser, on the other hand, is reduced between the moment of the emission of a burst and that of rest between two bursts, since the existence of a minimum continuous electrical power slows down the cooling of the optical medium. This has the advantage of reducing the thermal component of the overall skid, and make it more easily compensated by the adiabatic component.

According to one aspect of the invention, during the rise time or during the fall time, the increase or decrease of the power of the electrical signal is substantially linear.

With this aspect, the control of the electrical source is simple and inexpensive to implement.

According to one aspect of the invention, during the rise time or during the fall time, the increase or decrease in the power of the electrical signal is substantially instantaneous between the minimum power and a non-zero threshold from which an appearance of optical signal is triggered, called laser threshold, and is substantially linear between the threshold of the laser and the maximum power.

Thanks to this aspect, the control of the electrical source is not only simple and inexpensive to implement, but saves electrical power while optimizing the rise or fall times.

According to one aspect of the invention, the rise time or the descent time is at least 100 ns, for a maximum electrical power value between two and three times the threshold of the laser.

Experimental measurements with a laser threshold of about 18 mA, a maximum power of about 37 mA and a rise time of between about 200 and about 400 ns yielded about 8 GHz of total slip. This frequency slip is approximately 45% less than the skidding obtained with a substantially instantaneous increase in power, that is to say the progression of which is not controlled, and whose very short duration is for example approximately equal to at 0.8 ns.

The various aspects of the device which have just been described can be implemented independently of one another or in combination with one another. The invention also relates to an optical terminal of a passive optical network comprising a transmitting device such as that just described, capable of transmitting a rising optical signal, and a receiver device adapted to receive a downward optical signal.

This optical terminal is for example a ONT located at a customer, in a PON architecture based on TWDM technology. The invention finally relates to a method of transmitting in an optical fiber an optical signal conveying data, implemented by a device comprising a laser capable of generating said optical signal with a nominal optical power, an electrical source capable of generating an electrical signal supplying the laser, the nominal optical power being reached from a power level of the electrical signal, called the nominal level, the method being characterized in that it comprises, during a laser ignition phase , a step of progressively increasing the electric power from a minimum electrical power lower than the nominal level, to a maximum electrical power greater than or equal to the nominal level, the duration of the progressive increase, known as the rise time, being greater than or equal to a determined rise time threshold.

The device described above implements this method in all its embodiments. 4. PRESENTATION OF THE FIGURES Other advantages and characteristics of the invention will appear more clearly on reading the following description of particular embodiments of the invention, given as a simple illustrative and nonlimiting example, and the accompanying drawings. , among which: • Figure 1 shows an example of a structure of a transmitter device, according to a first embodiment of the invention, • Figure 2 shows the structure of a transmitter device, according to a second embodiment of the invention; FIG. 3A shows an electrical signal form produced according to the prior art and the resulting optical signal form; FIGS. 3B and 3C show forms of electrical signals produced according to two embodiments of the invention, and the resulting optical signal forms. FIG. 4 shows the performances in terms of wavelength skidding of the various forms of electrical signals illustrated above; FIG. 5 shows an example of implementation of the method of transmitting an optical signal in an optical fiber according to aspects of the invention. 5. Detailed description of at least one embodiment of the invention

In the remainder of the description, several embodiments of the invention are presented in the case of passive optical networks TWDM, but the invention is not limited to these application cases.

FIG. 1 shows an exemplary structure of a transmitting device, according to a first embodiment of the invention.

The transmitter device 100 comprises an electrical source 110 and a laser 120. The electrical source 110 supplies the laser 120 which generates an optical signal SI intended to be emitted in an optical fiber F. The electrical source 110 comprises an element 111 enabling control in the time the progression of the increase or decrease in current feeding the laser 120, so that this progression is not instantaneous. In this first embodiment, the electrical source 110 is modulated with the data to be transmitted in the fiber F, and the signal SI is a digital optical signal produced by direct modulation of the laser 120.

FIG. 2 shows the structure of a transmitting device according to a second embodiment of the invention.

The transmitter device 200 comprises an electrical source 210 and a laser 220. The electrical source 210 supplies the laser 220 which generates an optical signal S2 intended to be modulated before being emitted into an optical fiber F. As in the first embodiment , the electrical source 210 comprises an element 211 for controlling in time the progress of the increase or decrease of the current supplying the laser 220, so that this progression is not instantaneous. In this second embodiment, the electrical source 210 is not modulated with the data to be transmitted in the fiber F, and the signal S2 generated by the laser 220 is the envelope of the digital optical signal S3, which must be produced by external modulation. This external modulation is performed by a second electrical source 230 controlling a shutter 240, that is to say an external modulator modulating the signal S2 with digital data by blocking or releasing the passage of light. The shutter thus generates the signal S3 which is emitted in the fiber F.

From the point of view of the digital data carried in the fiber F, the signal S3 of this second embodiment is equivalent to the signal S1 of the first embodiment.

The elements 111 or 211, making it possible to control in time the progress of the increase or decrease in current supplying the laser 110 or 220, may be carried out by a person skilled in the art, for example by means of ordinary electronic components such as as transistors, resistors, capacitors, etc.

The main feature of the invention lies in the way of producing the optical signals SI or S2. For this reason, in the remainder of the document, the terms "optical signal" denote indifferently a modulated optical signal, such as the signal S1, or the envelope of an optical signal modulated or to be modulated, such as the signal S2. The same shortcut applies to "optical burst".

FIGS. 3A, 3B and 3C show the forms of electrical signals produced according to the prior art and according to several embodiments of the invention, and the resulting optical signal forms.

Each of FIGS. 3A, 3B and 3C comprises 2 graphs. In all the graphs of these figures, the time is on the abscissa and each gradation represents 0.5 ps. In the 3 upper graphs, the laser supply current is on the ordinate and each gradation represents 10 mA. The horizontal dotted line represents the laser threshold, which is slightly less than 20 mA. In the 3 lower graphs, the optical power of the laser, resulting from the feed current represented in the upper graphs, is on the ordinate and each gradation represents half of the average power of the laser.

In the context of the invention, whether in the first mode or the second embodiment described above, the optical signal considered is that consisting of a burst corresponding to a time window during which the device is authorized to transmit this. signal. The optical burst is generated by the laser using a corresponding electric burst. The power of the electric burst includes a rising phase and a falling phase, respectively corresponding to the beginning and the end of the optical burst.

According to the prior art, the rising phase of the electric burst is substantially instantaneous, as illustrated by the upper graph of FIG. 3A. The rising phase of the resulting optical burst is also substantially instantaneous, as illustrated by the lower graph of FIG. 3A.

According to the invention, the rising phase of the electric burst is not instantaneous and is progressive, of a duration greater than a threshold, as illustrated by the upper graphs of FIGS. 3B or 3C. In this case, the rising phase of the resulting optical burst is not instantaneous and is also progressive, as illustrated by the lower graphs of FIG. 3B or 3C.

Figure 4 shows the performances in terms of wavelength skidding of the various forms of electrical signals illustrated above.

In the graph of this figure, the time is on the abscissa and each gradation represents 0.2 ps. Wavelength skidding, also known as frequency skidding, is on the ordinate and each gradation represents 5 GHz.

Tests were carried out with the 3 forms of electric gusts illustrated by the graphs of FIGS. 3A, 3B and 3C. For these tests, the laser threshold is slightly less than 20 mA, and the electric current rises to about 40 mA.

In the instantaneous mode according to the prior art, illustrated in FIG. 3A, where the increase in power is not controlled, the current goes from 0 to 40 mA very rapidly, in approximately 0.8 ns.

In a first progressive mode according to a first variant of the invention, illustrated in FIG. 3B, the current goes from 0 to 40 mA, in a substantially linear manner, over a period of approximately 0.4 μs.

In a second progressive mode according to a second variant of the invention, illustrated in FIG. 3C, the current goes from a value which is slightly less than the threshold of the laser, to a value of 40 mA, in a substantially linear manner, in the space of about 0.2 ps.

Curve A illustrates the resulting skid for an optical signal generated according to the instantaneous mode of the prior art, as illustrated in FIG. 3A.

Curve B illustrates the resulting slip for an optical signal generated according to the first progressive mode of the invention, as illustrated in FIG. 3B.

Curve C illustrates the resulting skid for an optical signal generated according to the second progressive mode of the invention, as illustrated in FIG. 3C.

These tests have shown that during the first moments of the resulting optical signal, for a duration of about 0.1 ps in the case of these tests, the slip for a signal generated according to the invention is much lower than the slip generated according to the technique. earlier. It is during these first moments that the consequences of skidding at wavelength degrade the quality of the optical signal the most. After these first moments, the performances of the three different modes converge.

It was further observed that the performance of the second variant is greater than that of the first. From the point of view of skidding performance, it is therefore preferable that the lower value of the electric current during the climb is not zero. On the other hand, this second variant consumes more energy than the first.

To overcome this drawback, in a third non-illustrated variant, the electric current changes from the zero value to the value slightly lower than the threshold of the laser, in a substantially instantaneous manner, then progressively passes, like the previous variants, this value at the maximum value of the electric current. In this third variant, the energy consumption becomes even lower than that of the first variant, and the progressivity of the current rise is maintained during at least a portion of the rising phase, which ensures a progressive increase of the optical signal. generated and results in a skid lower than that of the instantaneous mode.

The embodiments of the invention which have just been described in relation to FIGS. 3B, 3C and 4 relate to the rising edge of the electric burst feeding the laser. The invention also applies, symmetrically, to the falling edge of the electric burst. The progressive descent of the current during at least part of the downward phase provides the same advantages as the progressive rise of the current during at least a portion of the rising phase.

FIG. 5 shows an exemplary implementation of the method of transmitting an optical signal in an optical fiber, according to aspects of the invention.

During a step E1, the electrical source 110 or 210 produces a burst of electric current for supplying the laser 120 or 220. The electrical source is configured so that at least part of the rising phase of the current is progressive. It can be configured so that moreover, at least part of the downward phase is progressive.

If the modulation is direct, the electric burst produced is modulated with data to be emitted, during a step El b. If the modulation is external, this step is skipped.

During a step E2, the laser is powered by the electric burst, modulated or not according to the modulation mode, direct or external, produced by the electrical source, and generated an optical burst.

If the modulation is external, the generated optical burst is modulated with data to be emitted, during a step E2b. This step can be implemented using a shutter 240, controlled by an electrical source 230 modulated with the data to be transmitted.

During a step E3, the modulated optical burst is emitted in an optical fiber.

The embodiments of the invention which have just been presented are only some of the possible embodiments. In particular, other forms of progression of the rise or fall of the current supplying the laser are possible within the scope of the invention. For example, the progressive rise, or gradual descent, of the laser power supply current in each burst may not be linear, but may have a sinusoidal shape to avoid any form of abrupt change in the laser power supply current.

These examples show that the invention makes it possible to reduce the wavelength wander induced by the emitters in the TWDM systems, thanks to the progressivity of the increase or decrease of the electric current supplying the laser.

Claims (11)

A transmitting device (100; 200) capable of transmitting in an optical fiber (F) an optical signal (S1, S3) conveying data, the device comprising a laser (120; 220) capable of generating said optical signal with a power nominal optical system, an electrical source (110; 210) adapted to generate an electrical signal supplying the laser, the nominal optical power being reached from a power level of the electrical signal, called the nominal level, the device being characterized in that during a laser ignition phase, the time taken by the electric power to rise from a minimum electrical power below the nominal level, to a maximum electrical power greater than or equal to the nominal level, called the rise time, is greater than or equal to a determined rise time threshold. Transmitting device according to claim 1, wherein the electrical source (110) modulates the optical signal (SI) with the data, using the electrical signal during the generation of the optical signal by the laser (120). Transmitting device according to claim 1, wherein the electrical source (210) or other electrical source (230) modulates the optical signal (S3) with the data, after its generation by the laser (220). 4. Transmitting device according to one of the preceding claims, wherein, during a phase of extinction of the laser (120; 220), the time taken by the electric power to descend from the maximum power to the minimum electrical power , said descent time, is greater than or equal to a determined descent time threshold. Transmitting device according to one of the preceding claims, wherein the minimum electrical power is substantially zero. 6. Transmitting device according to one of claims 1 to 4, wherein the minimum electrical power is substantially equal to a non-zero threshold from which an appearance of the optical signal (S1; S2) is triggered, said threshold of the laser (120; 220). 7. Transmitting device according to one of the preceding claims, wherein, during the rise time or during the descent time, the increase or decrease of the power of the electrical signal is substantially linear. 8. Transmitting device according to claim 5, wherein, during the rise time or during the descent time, the increase or decrease of the power of the electrical signal is substantially instantaneous between the minimum power and a non-zero threshold from which an appearance of the optical signal (S1, S2) is triggered, said threshold of the laser (120; 220), and is substantially linear between the threshold of the laser and the maximum power. Transmitting device according to one of the preceding claims, wherein the rise time or the descent time is at least 100 ns, for a maximum electrical power value between two and three times the threshold of the laser (120; 220). 10. Optical terminal of a passive optical network comprising a transmitter device (100; 200) according to one of claims 1 to 9, adapted to transmit a rising optical signal (SI; S3), and a receiver device adapted to receive a downlink optical signal. 11. A method of transmitting in an optical fiber (F) an optical signal (S1, S3) conveying data, implemented by a transmitting device (100; 200) comprising a laser (120; 220) capable of generating said optical signal with a nominal optical power, an electrical source (110; 210) adapted to generate an electrical signal supplying the laser, the nominal optical power being reached from a power level of the electrical signal, said nominal level, the method being characterized in that it comprises, during a phase of ignition of the laser, a step (El) of progressive increase of the electrical power from a minimum electrical power below the nominal level, up to an electric power greater than or equal to the nominal level, the duration of the progressive increase, known as the rise time, being greater than or equal to a determined rise time threshold.