FR2879053A1 - Optical signal regenerator device for e.g. dense wavelength division multiplexing network, has modulator with surface processed in antireflecting manner to limit interactions between converted signal and pumping signal parasitic reflection - Google Patents

Abstract

The device has an electro-absorption modulator (MEA) to receive pumping and sound signals (Sp, Ss) at respective identical optical wave lengths (lambda p, lambda s) via two surfaces (AR1, AR2) and to deliver a converted signal (Sc) via the surface (AR1). The surface (AR1) of the modulator is processed in an antireflecting manner to limit interactions between the converted signal and a parasitic reflection of the pumping signal. The modulator delivers the converted signal representing the modulations of the pumping signal at the optical wavelength (lambda s) and is polarized inversely and continuously.

Description

2R / 3R regenerative device all-optical based on a single

  The present invention relates to the field of optical transport networks, such as networks known as "DWDM" (for "Dense Wavelength Division Multiplexing").

  In this type of technique, optical fibers can carry information carried by a number of channels whose frequency and channel spacing are normalized (ITU standard standard wavelength grid).

  Because of the densification of the systems and the increase of the ranges, nonlinear effects in the fibers, such as the self-modulation of phase (so-called SPM), ls the mixture with four waves (said FWM) and the modulation cross-phase (so-called XPM) are very important. With the accumulated noise of the amplifiers (ASE), chromatic dispersion (CD) and polarization mode dispersion (PMD), they participate in the signal distortion, the increase of the temporal jitter and the degradation of the signal rate. 'extinction. A limitation of the scope is also induced.

  In the interest of restoring the integral information to the detection and to guarantee a high quality of transmission, an effective regeneration of the signal is sought. A regenerative device called "3R" allows "Re-amplify" by improving the extinction rate, "Reset" and "Re-synchronize" the signal.

  Currently the transport network uses an optoelectronic solution requiring conversion to electrical signal followed by electrical signal processing and optical conversion. An evolution towards all-optical networks (without optical / electrical / optical processing) is already proposed. With these so-called transparent networks, all-optical regenerators should increase the capacity and reduce the complexity of equipment.

  s To achieve all-optical regeneration, the non-linear effects of materials are often exploited.

  Among the current devices, the saturable absorber has the property of having a variable refractive index with the received optical power. Thus, at optical power less than that of saturation, the component is absorbent, otherwise it is transparent and the signal is transmitted. It thus makes it possible to re-amplify the signal and to eliminate the noise of the bits at "0" passively. To "clean" the bits at "1", a compression fiber and a bandpass filter are needed. Only 2R regeneration can be performed with this component.

  However, the saturable absorber has flow limitations because of the induced heating effects, which reduces the contrast. Furthermore, the device has to be combined with the compression fiber and a filter to perform regeneration 2R, and again to an additional device to perform 3R regeneration. The final system is extremely complex and very consumer. In addition, its spectral band is relatively narrow (7nm).

  Another device is based on fiber called "very non-linear" (or HNF). It allows a 2R regeneration through an SPM effect (self-phase modulation) and a narrowband filter. Indeed, the compression of the signal by the SPM induces a widening of the spectrum. The use of a narrow band filter placed after the HNF fiber eliminates noise in the "1" bits. However, to perform a 3R regeneration, an external device is required to synchronize the signal.

  The regenerator 2R based on non-linear fiber is interesting for its simplicity and low consumption. However, it is sensitive to a high spectral density that can induce unwanted nonlinear effects. In addition, it does not allow to re-synchronize the signal. To do this, it must be associated with another device, resulting in a complexity of implementation.

  A device based on electro-absorption modulators (called "MEA"), with two stages of wavelength conversion, is also used. Such a device is described in particular in the document: "Optical 3R regenerator using wavelength converters based on electroabsorption modulator for all-optical network applications", T Otani, Miyazaki T and Yamamoto S, IEEE Photonics Technology Letters, vol. 12, no. 4, April 2000.

  There is shown a device of this type, regenerator based on two-stage MEA modulators, in FIG. 1 relating to the prior art.

  A first stage ET-1 and a second stage ET-2 each comprise an electroabsorption modulator MEA and a circulator C. The input signal Si, of wavelength λp, corresponding to the pump signal of the first stage, is amplified ( AI) and applied via the circulator C to the MEA modulator in counterpropagating mode. A different wavelength probe signal λ is applied to the same modulator MEA and the converted signal of wavelength λ s is filtered (I3PF1) and amplified (A2) before being applied to the second stage ET-2. , as a pump signal. The modulator MEA of the second stage ET-2 thus receives the converted signal of the first stage, as well as a probe signal of wavelength λp (right-hand part of the figure), to finally deliver an output signal S0, filtered (BPF2 ) and wavelength λ.

  The MEA-based wavelength conversion is thus accompanied by a 2R regeneration. It is a cross-modulation of absorption (XAM). At the second stage of the device, a clock recovery resynchronizes the signal and fall back on the initial wavelength λp of the signal. A 3R regeneration is thus performed.

  The two-stage MEA-based wavelength conversion regenerator has shown interesting results at high rates and over a wide spectral band. However, the device is complicated and requires several power amplifiers increasing its consumption and cost.

  Of the solutions presented above, none are fully satisfactory. The present invention improves the situation.

  It proposes for this purpose a device regenerating an optical signal, in particular for telecommunication applications in networks in an all-optical configuration, comprising an electroabsorption modulator (MEA) comprising a first and a second face, for: - receiving by the first face, a pump signal, at a first optical wavelength; receiving by the second face, a probe signal, at a second optical wavelength, and delivering, by said first face, a converted signal, representing the modulations of the pump signal, substantially at said second optical wavelength.

  In the device within the meaning of the invention, the first and second wavelengths are identical and the at least one first face of the modulator comprises an antireflection treatment for limiting interactions between the converted signal and a parasitic reflection of the pump signal on the first signal. face.

  These interactions may typically be interference between the two signals.

  The regeneration device within the meaning of the invention, advantageously comprising a single modulator, is simple and relatively inexpensive compared to existing devices, requiring several amplifiers and tedious adjustments.

  The present invention is also aimed at the electroabsorption modulator of the above-mentioned device and then comprising an antireflection treatment at least on its first face receiving the pump signal and delivering the converted signal, so as to limit the interactions between the converted signal and a parasitic reflection of the pump signal on this first face.

  Other features and advantages of the invention will appear on examining the detailed description below, and the accompanying drawings in which, in addition to Figure 1 relating to the prior art: - Figure 2 shows schematically the device at sense of the invention; FIG. 3 illustrates the general operation of an MEA modulator; and FIG. 4 illustrates the influence of the crosstalk (or "cross-talk") between the reflected pump and the probe signals, on the Q factor of a device in the sense of the invention, in RZ modulation formats (curve in solid lines) and NRZ (dotted line curve).

  Referring to Figure 2 which shows schematically the device within the meaning of the invention.

  The device advantageously comprises a single MEA electroabsorption modulator in XAM (cross-modulation of absorption) configuration, receiving a pump signal Sp and a probe signal Ss for delivering a converted signal Sc. Typically, the so-called "pump" signal and the signal said "probe" can be distinguished in particular in that the pump signal is high power to saturate the electroabsorption modulator, while the probe signal can be of lower power.

  More specifically, with reference to FIG. 2, a circulator C receives the modulated pump signal Sp as the input signal Si of the device and routes it to the modulator MEA in the counterpropagating mode.

  The modulator MEA thus receives, by a first face referenced AR1 on ro Figure 2, the pump signal Sp. II naturally creates a spurious reflection Spr of this pump signal on the first face AR1 of the modulator MEA.

  Furthermore, the probe signal Ss is injected into the modulator MEA by a second face referenced AR2 in FIG. 2. The modulator MEA then delivers by its first face AR1 the converted signal Sc, which is therefore propagated in the same direction as the reflection parasite Spr of the pump signal Sp.

  To perform, in the sense of the invention, a regeneration of the pump signal Sp, the uncle length ss of the signal Ss signal and, therefore, the wavelength of the converted signal Sc is substantially the same as the length of wave Pp of the pump signal Sp, and its parasitic reflection Spr (λs = λp). Therefore, a cross-talk (or "cross-talk") is likely to intervene between the converted signal Sc and parasitic reflection Spr. It is recalled that the term "crosstalk" refers to a parasitic and untimely superposition of a signal on another signal giving rise to a mixture of the two. The term "crotch" is also commonly used to describe this phenomenon.

  The invention then proposes to perform a specific antireflection treatment of the first face AR1, at least, of the modulator MEA, so as to limit the intensity of parasitic reflection Spr of the pump signal Sp and hence recover the converted signal Sc ensuring negligible crosstalk.

  Finally, the circulator C receives the converted signal Sp and routes it to the output of the device, as an output signal S0. Preferably, an amplifier A of the pump signal Sp is provided upstream of the circulator C. Thus, the probe signal Ss and the pump signal Sp (modulated) are simultaneously injected into the inverse biased modulator MEA, in contra-propagative mode. With the circulator C, the probe signal, converted after passing through the MEA modulator, is recovered. Polarization controllers on either side of the MEA modulator (each diagrammed by two circles contiguous in FIG. 2) can be provided, in particular if the MEA modulator is sensitive to polarization. The MEA modulator is reverse biased continuously to increase contrast and reduce response time.

  Referring now to Figure 3, it is recalled that the general operation of a MEA modulator is defined as follows. The probe signal SS (continuous in the example shown but preferably comprising a clock signal for performing a recalibration within the framework of a regeneration 3R), at the wavelength and the pump signal Sp at the length of wavelength λ (modulated by a succession of bits at "0" or "1") are injected into the modulator MEA whose negative control voltage has been fixed so as to be able to saturate the absorption by the pump signal Si ,, for to deliver the converted signal Sc, modulated as the pump signal Sp, but at the wavelength λs.

  In the context of the present invention, in order to be able to carry out a regeneration, it is advantageous to convert the pump signal (X, s = p) at the same wavelength. In order to limit the effects of parasitic reflection of the pump signal on the first face AR1 of the MEA modulator, it is proposed to carry out a specific antireflection treatment at least on this first face AR1, making it possible to recover the converted signal Sp by making sure that a negligible cross-talk between the converted signal Sc and the parasitic reflection Spr of the pump signal Sp.

  Thus, the antireflection treatment on the face AR1 which receives the pump signal Sp advantageously has the effect of limiting spurious reflections Spr of the pump signal Sp, which are likely to interfere with the converted signal Sc (FIG. 2). For example, a 30dB extinction between the two signals is required. The fineness of the antireflection treatment depends on the values of the parameters of the MEA modulator, typically: - the saturation threshold of the structure, and the power of the probe at the applied bias voltage.

  Typically, for a structure that saturates to 15 dBm (current value), and with a probe power of 8 dBm, for example a reflection-light antireflection treatment of 10 -6 is chosen.

  Thus, to carry out a regeneration 2R, the probe signal Ss can be continuous and preferably at the same wavelength as the pump signal Sp. To perform a regeneration 3R after extraction, the probe signal Ss rather comprises a clock signal ( for example a sinusoid), preferably at the same clock frequency as the pump signal Sp and at the same wavelength. Re-synchronization (3R regeneration) is thus conducted.

  Referring now to FIG. 4, to show the low crosstalk effect on the converted signal Sc, the influence of crosstalk on the measurement of the bit error rate is shown. In particular, the degradation of the performance of a 10.709 Gb / s channel modulated in RZ (or NRZ) format is observed as a function of the (coherent) level of crosstalk. For this simulation, the signal-to-noise ratio of the reference channel is taken equal to 14 dB.

  The performance penalty is calculated as follows: AQ = 20 É log Q without Q crosstalk with crosstalk, the Q factor being itself related to the bit error rate by: TEB = exp (Q2 / 2) Q 2r For example, an MEA modulator may require pump and probe powers of 15 and 8 dBm, respectively, with insertion losses of 18 dB and anti-reflective treatment of 10 "6 (= 60 dB) In the case of NRZ regeneration , the probe signal level is -13 dBm (corresponding to 8 dBm 18 dB 3 dB) and the level of a residual pump signal is -45 dBm ro (corresponding to 15 dBm 60 dB) .The level of crosstalk is then 32 dB and the penalty on the Q factor is advantageously less than 0.2 dBQ.

  In general, an antireflection treatment is provided which confers a reflection coefficient of less than 10 -4 and preferentially close to 10 -6.

  One can provide to treat symmetrically the two faces AR1 and AR2 of the MEA modulator for the latter to operate symmetrically. However, it can be retained that in principle, it is sufficient that only the face receiving the pump signal is processed.

  Furthermore, tests have shown that the interference of the probe signal and the pump signal and / or its parasitic reflections, inside the MEA modulator, does not disturb the operation of the latter, in particular its transmission factor. the saturated state. Indeed, given the instantaneous saturation of the component and its recovery time absorption, the transmission factor is insensitive to local fluctuations in power in the component. io

  The unique MEA modulator device in the sense of the invention, highly antireflection treated, ensures a conversion on the same wavelength. The structure of the device in the sense of the invention is advantageously very simplified and its cost is reduced. This antireflection treatment is effective over the entire telephone band (called "C-band").

  Compared to the techniques of the prior art described in the introduction, a 2R or 3R regeneration is advantageously possible over a wide spectral band (typically greater than 15 nm). Thanks to its short response time and the absence of heating effects, the device can operate at high speeds. Finally, the device within the meaning of the invention is transparent to the modulation format RZ ("reset") or NRZ ("without reset").

Claims (12)

claims   1. Device for regenerating an optical signal, in particular for telecommunication applications in networks in an alloptic configuration, of the 1: ype comprising an electroabsorption modulator (MEA) comprising a first and a second face, for: - receiving by the first face (AR1), a pump signal (Sp) at a first optical wavelength (λp), - receive by the second face (AR2), a probe signal (ss) at a second optical wavelength (Ås) ), and deliver by said first face (AR1), a converted signal (Sc), representing the modulations of the pump signal (Sp), substantially to said second optical wavelength (λs), characterized in that the first ( Λp) and second (λs) wavelengths ts are identical and in that the at least one first face (AR1) of the modulator (MEA) has an antireflection treatment to limit interactions between the converted signal (Sc) and a parasitic reflection of the pump signal (Sp) on said pr front (AR1).   2. Device according to claim 1, characterized in that said modulator (MEA) is the only modulator of the device, and in that: the pump signal (Sp) is a modulated signal corresponding to an input signal (Si) of the device, - while the converted signal (Sc) corresponds to an output signal (Sc)) of the device.   3. Device according to claim 2, characterized in that the probe signal (Ss) comprises a clock signal for performing regeneration with resynchronization (3R) of the input signal (Si). he   4. Device according to one of the preceding claims, characterized in that the modulator (MEA) is mounted in the device for operating in cross modulation of the absorption.   5. Device according to one of the preceding claims, characterized in that the modulator (MEA) is reverse biased, continuously.   6. Device according to one of the preceding claims, characterized in that it comprises polarization controllers on either side of the modulator.   7. Device according to one of the preceding claims, characterized in that it comprises a circulator (C) receiving the pump signal (Sp) and conveying it to the modulator (MEA) in contra propagative mode and, in return, receiving the converted signal (Sc) and conveying it at the output of the device.   8. Device according to claim 7, characterized in that it further comprises an amplifier (A) of the pump signal (Sp) upstream of the circulator (C).   9. Device according to one of the preceding claims, characterized in that the first and second faces (AR1, AR2) of the modulator comprise antireflection treatment.   10. Device according to one of the preceding claims, characterized in that the antireflection treatment provides a reflection coefficient of the first lower face 10 "4 and preferably close to 10" 6.   11. Device according to claim 10, characterized in that the power of the probe signal (Sc) is close to 8 dBm and the power of the pump signal (Sp) is close to 15 dBm.   An electroabsorption modulator (MEA) comprising: a first face (AR1) for receiving a pump signal (Sp) modulated at a first optical wavelength (λp); a second face (AR2) for receiving a probe signal (Ss) at a second optical wavelength (λs), and adapted to deliver by the first face (AR1) a converted signal (Sc) representing the modulations of the pump signal (Sp), substantially at the second length d wave (Δs), characterized in that the first face (AR1) of at least the modulator (MEA) comprises a 1: anti-reflective treatment for limiting interactions between the signal to converted (Sc) and parasitic reflection of the pump signal (Sp). ) on said first face (AR1).