• 专利附录
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    • 专利摘要:
    A heterodyne receiver independent of polarization for optical signals modulated in phase and differentially encoded. The receiver has a simple architecture, which can be implemented with common components and low cost DFB lasers. It is based on an optical coupler (1) with three inputs and three outputs, and optical-electrical conversion with three photodiodes (4.1, 4.2, 4.3). The photodetected electrical signals are combined linearly (5) to cancel the terms of direct detection and common mode noise, emulating a balanced photodetection. The information contained in the optical phase is recovered by means of differential demodulation (7.1, 7.2). The receiver can operate independently of the polarization state thanks to the effect of the heterodyne detection which, together with the electric filtering (9), eliminates the random components that depend on the polarization state, without the need to duplicate the architecture of the receiver as in a receiver conventional with polarization diversity. (Machine-translation by Google Translate, not legally binding)
    公开号:ES2679403A1
    申请号:ES201730247
    申请日:2017-02-24
    公开日:2018-08-27
    发明作者:Jeison A. TABARES GIRALDO；Josep Prat Gomà
    申请人:Universitat Politecnica de Catalunya UPC；
    IPC主号:
    专利说明:

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    D E S C R I P C I Ó N
    INDEPENDENT HETERODINE POLARIZATION RECEIVER FOR PHASE MODULATED OPTICAL SIGNS AND DIFFERENTIALLY CODED
    SECTOR OF THE TECHNIQUE
    The present invention pertains to the optical communications sector, particularly to coherent detection optical systems applied to local fiber access networks.
    BACKGROUND OF THE INVENTION
    The application of coherent detection technologies to the new generations of ultra-dense wavelength multiplexing optical access networks will allow to increase the overall network performance in terms of maximum range, number of users and added capacity. Although there are already applications of coherent systems for metropolitan and long distance optical networks, the complexity and high cost of such systems has limited their use in access networks, where the simplicity and robustness of the implementation have the greatest relevance, since they translate into the final cost for the users. Thus, coherent receivers for access applications should be based on common low-cost components, both optical and electrical, and simple techniques that facilitate mass production and easy provisioning [1].
    The detection of the optical signal in a coherent receiver is performed by mixing said signal with another from the local laser, so that the difference in the polarization states of both signals can seriously degrade the performance of the receiver, in some cases preventing the Correct detection of information. In order to make coherent receivers immune to the polarization state of lasers, current commercial systems implement so-called polarized diversity receivers, which duplicate the receiver architecture to independently process both orthogonal components.
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    polarization, previously separated by a polarizing beam splitter.
    US2014050233 shows an example of the realization of a coherent receiver with polarization diversity based on heterodyne detection, in which the electrical signal obtained after the photodetection process consists of a high frequency carrier that carries the data signal. This intermediate frequency stage operates when the difference between the optical frequencies of the transmitting and local laser is greater than the bandwidth of the transmitted data signal.
    In said receiver, two polarizing beam splitters are connected to the optical input signal and the local laser to separate both orthogonal polarization components from each of the optical signals. The horizontal and vertical components are combined with each other respectively by means of optical couplers to obtain the coherent mixtures, and the four resulting optical signals are led to four photodetectors connected in pairs to make balanced photodetection. The result is two high frequency electrical signals, corresponding to the horizontal and vertical polarization states, which are subsequently filtered and electrically processed to extract the data signal. With this technique, a coherent independent detection of the polarization state is achieved, but the receiver becomes more expensive and its complexity and energy consumption are increased by requiring twice as many opto-electronic components.
    A second example of a heterodyne receptor independent of polarization is shown in [2], and is proposed as a simplification of the polarization diversity scheme mentioned above. In it, the optical input signal and that coming from the local laser are mixed in a multi-port optical coupler (i.e. at least two inputs and two outputs), and one of the outputs is connected to a polarizing beam splitter to separate the orthogonal polarization components from the coherent mixture, which are then photodetected independently by two photodiodes, one for each polarization state. Again, the result is two high frequency signals that will be processed electrically.
    This architecture requires half of the components (a polarizing beam splitter, an optical coupler and two photodiodes) than the conventional version of receiver with polarization diversity (two polarizing beam splitters, two couplers
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    optical and four photodiodes), but it has a couple of disadvantages to consider: first, only one branch of the coherent mixture between the input signal and the local laser is used causing a loss of power, which can be minimized if optical couplers are used with asymmetric coupling ratio (for example, an 80%: 20% ratio coupler); second, photodetection is not balanced, so that the receiver is influenced by the terms of direct detection and common mode noise, especially the intensity noise of the local laser that typically operates at high powers.
    Another alternative has recently been proposed for polarization independent receivers and consists of an architecture based on multi-port optical couplers, with three inputs and three outputs, as set out in WO2015079400. In this receiver, the received optical signal is connected to one of the three input ports of the optical coupler, while the local laser is connected to a polarizing beam splitter to separate its orthogonal components, which are then connected to the remaining two ports of the coupler. Subsequently, the three optical signals of the output of the coupler are photodetected by means of three photodiodes, and the three electrical signals obtained are properly processed in the electronic part of the receiver.
    It is important to mention that, although this type of receiver is not based on balanced photodetection, a linear combination of the three electrical signals obtained after photodetection can be performed to effectively cancel the terms of direct detection and noise components in common mode, emulating thus the behavior of a balanced receiver [3].
    Due to the distribution of the spectral components of the three photodetected electrical signals in the receiver mentioned above, it was demonstrated that by applying a considerable frequency difference between the transmitting laser and the local, the random components relative to the polarization state of the lasers and that they affect detection, are located beyond the bandwidth of the data signal and can be easily eliminated with electric filtering. In this way, the receiver operates in an intra-dine regime, that is, the intermediate frequency is less than the bandwidth of the data signal, and the detection process is independent of the polarization state, in this particular case for signals modulated in amplitude (ASK).
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    However, the previous intradine detection scheme cannot be applied to phase-modulated signals (PSK), since these are severely affected by frequency deviations, with a tolerance of less than 10% of the bit rate [ 4], while the intradino receiver mentioned above operates with frequency deviations greater than 65% of the bit rate.
    On the other hand, and in addition to the aforementioned inconvenience with the polarization states, the fact that in a coherent system two different lasers are used for transmission and reception causes differences in the optical phase, which fluctuates randomly, and these fluctuations are related directly with the spectral line width of lasers. An effective way to deal with this phase noise and enable the use of low-cost lasers (DFB lasers, for example) that typically have a considerable spectral line width, is to encode the information in the phase difference between consecutive symbols , instead of the absolute phase of each symbol. To implement this known differential technique, a simple encoder is needed in the transmitter, which can be implemented with logic gates; and a decoder or demodulator in the receiver based on the auto-correlation of the electrical signal that carries the information.
    References
    [1] J. Prat, I. Cano, M. Presi, I. Tomkos, D. Klonidis, G. Vall-llosera, R. Brenot, R. Pous, G. Papastergiou, A. Rafel, E. Ciaramella, “ Technologies for Cost-Effective udWDM-PONs, ”IEEE Journal of Lightwave Technology, 34 (2), 783-791,2016.
    [2] I. Cano, A. Lerín, V. Polo, J. Prat, “Simplified polarization diversity heterodyne receiver for 1.25 Gb / s cost-effective udWDM-PON.” In Optical Fiber Communication Conference (OFC2014), W4G-2 .
    [3] C. Xie, P. Winzer, G. Raybon, A. Gnauck, B. Zhu, T. Geisler, B. Edvold, “Colorless coherent receiver using 3 ^ 3 coupler hybrids and single-ended detection,” Optics express , vol. 20, no. 2, pp. 1164-1171,2012.
    [4] J. Tabares, V. Polo, I. Cano, and J. Prat, “Automatic A-control with offset compensation in DFB intradyne receiver for udWDM-PON,” IEEE Photonics Technology Letters, 27 (4), 443- 446, 2015
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    EXPLANATION OF THE INVENTION
    The object of the invention is a coherent optical receiver for the heterodyne detection of phase-modulated and differentially encoded optical signals, which can operate independently of the polarization state of the lasers. This receiver is based on a simple architecture, which can be implemented with common devices and low cost. Thus, with the receiver proposed in this invention, the use of the coherent receiver architecture based on three-input and three-output couplers is extended, to the reception of phase-modulated optical signals with heterodyne independent polarization detection.
    In the coherent transmission system, the data signal can be used to modulate both the amplitude and the phase of the optical signal generated by the transmitting laser. This modulation can be carried out by means of external modulators, or by means of direct laser modulation, an attractive method for access networks due to its low cost and greater optical emission power. In the case of direct phase modulation, the data signal must be pre-equalized according to the technique indicated in reference [1]. This equalization can be performed in analog or digital form, and the direct modulation of the laser precedes. In addition, in order to give robustness to the phase noise and prevent the propagation of errors, the data signal can also be differentially encoded in the transmitter, before the pre-equalization and subsequent direct modulation of the transmitter laser.
    In the optical part of the coherent receiver object of this invention, the signal from the optical transmission medium is connected to one of the input ports of a three-input and three-output optical coupler, to make a mixture consistent with the local laser. Said laser is connected to a polarizing beam splitter to separate its two orthogonal components, which are then connected to the two remaining ports of the three input and three output optical coupler. After the coherent mixing, the three resulting optical signals are photodetected by means of three photodiodes, obtaining three high-frequency electrical signals that carry the information contained in the optical field phase. The optical frequency of the local laser is adjusted in such a way that the receiver operates in a heterodyne regime, which implies that the intermediate frequency of the radiofrequency carrier after photodetection is greater than the bandwidth of the transmitted data signal.
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    Then, in the electronic part of this proposed receiver, a linear combination of the three photodetected electrical signals is made that allows to cancel the terms of direct detection and the components of noise in common mode, obtaining two electrical signals composed only by terms of detection consistent and containing the optical phase information. These two new electrical signals are filtered by two pass-band filters centered at the intermediate frequency, whose bandwidth adjusts to the bandwidth of the data signal. Then, the two signals are differentially demodulated to extract the information encoded in the phase difference between consecutive symbols, and then combined to cope with the effect of the polarization state of the lasers. An electric low-pass filter, adjusted to the bandwidth of the data signal, is finally responsible for suppressing unwanted components that depend on the state of polarization, in addition to reducing other noise components. The resulting signal is the variable on which bit recovery will be performed by means of a decision threshold.
    Thus, the coherent receiver object of the invention allows to detect phase-modulated optical signals, regardless of the state of polarization, using a low complexity architecture. As mentioned earlier, the intradine detection of PSK signals with this type of receiver based on three-input and three-output optical couplers completely closes the eye diagram in reception due to the large frequency deviation required. Consequently, the heterodyne detection method proposed in this invention, with an intermediate frequency properly selected to maximize the amplitude of decision in the recovery of bits, allows to eliminate unwanted polarization components by simple electrical filtering, without the need to duplicate the architecture of the receiver as is done in conventional polarization diversity systems.
    It should be noted that this invention provides a coherent receiver architecture for phase-modulated optical signals, based on heterodyne detection, in which the electrical signals obtained after photodetection are radio frequency carriers that carry the data signal. This implies that in the electrical part of the receiver where the signals are processed, there must be a mechanism that allows the data signal to be lowered from the intermediate frequency to the baseband. In this regard, the differential demodulation scheme fulfills the dual function of
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    extract the phase information and do frequency conversion. However, a frequency conversion stage can also be implemented, in which the photodetected signals are mixed with a radio frequency oscillator that oscillates at the intermediate frequency of the receiver. Thus, the electrical signals after frequency conversion are in baseband and the receiver can potentially detect multilevel modulation formats, such as those based purely on phase modulation (m-PSK), and those that combine phase modulation and amplitude (m- QAM).
    Another important feature of the invention is that it cancels the terms of direct detection and common mode noise components, emulating the balanced detection of the conventional heterodyne receiver with polarization diversity, but using only three photodiodes instead of four. Thanks to this, the complexity and energy consumption of the receiver is reduced, and its tolerance to the interfering power of other channels in a multi-user environment is increased.
    BRIEF DESCRIPTION OF THE DRAWINGS
    To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, a set of drawings is attached as an integral part of said description, where, for illustrative and non-limiting purposes, it has been represented the next:
    Figure 1 shows a block diagram of the coherent receiver embodiment according to the present invention.
    Figure 2 shows a block diagram of the differential demodulator embodiment.
    PREFERRED EMBODIMENT OF THE INVENTION
    The coherent optical receiver object of the invention, represented in Figure 1, in its preferred embodiment, implements an optical coupler (1) of three inputs and three outputs, with a 1: 1: 1 symmetrical coupling ratio, responsible for performing the mixing consistent between the optical input signal, coming from the transmission medium, and the local laser signal (2). Said laser can be of low cost, such as a DFB laser of those commonly proposed for access networks.
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    The local laser (2) is connected to a polarizing beam splitter (3) that separates its two orthogonal polarization components, which in turn are connected to the two remaining input ports of the optical coupler (1). It is important to mention that the local laser (2) should preferably emit linearly polarized light at 45 ° to ensure that both orthogonal components at the output of the polarizing beam splitter (3) have the same power, thus ensuring optimum receiver performance. For this reason, the optical connections between the local laser (2), the polarizing beam splitter (3) and the optical coupler (1) should preferably be made with fibers maintaining the polarization state.
    Another alternative embodiment of the receiver consists in connecting the polarizing beam splitter (3) to the received optical signal instead of the local laser (2), which is connected directly to the optical coupler (1). However, the drawback of this variation in the architecture is that the insertion losses of the polarizing beam splitter (3) cause a loss of power in the received signal, which typically already reaches the receiver with very low power levels. For this reason, the received optical signal should preferably be connected to an input port of the optical coupler (1), while the polarizing beam splitter (3) is preferably connected to the local laser (2), since the effect on the Emission power of said laser will be lower.
    The optical frequency of the local laser (2) must be adjusted to a value close to that of the transmitting laser, so that the receiver operates in a heterodyne regime, in which the electrical signals after photodetection pass through a higher intermediate frequency stage than the bandwidth of the data signal. The value of said intermediate frequency corresponds to the difference between the optical frequencies of the transmitting and local laser (2), and is preferably selected as an integer multiple of the bit rate, typically double or triple, to maximize the amplitude bit decision when phase modulated signals are differentially demodulated.
    At the output of the optical coupler (1), after coherent mixing, the three resulting optical signals are photodetected by three photodiodes (4.1, 4.2, 4.3). The three electrical signals obtained are combined linearly (5) according to the method described in the reference article [3] mentioned above, to cancel the
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    terms of direct detection and noise components in common mode, emulating the behavior of a balanced receiver. Said linear combination (5) is simple enough to be implemented by means of passive "hardware" if the receiver is analog, or in the signal processing stage after the analog-digital conversion, if the receiver is digital.
    The result of the linear combination (5) is two electrical signals, which are filtered by two pass-band filters (6.1, 6.2) centered at the intermediate frequency and adjusted in bandwidth to adapt to the bandwidth of the data signal . Since a coherent receiver has no selective optical filter at the input, these electrical filters are responsible for rejecting interference from adjacent channels, in addition to eliminating noise.
    The two filtered signals are differentially demodulated in their respective devices (7.1, 7.2) to extract the information encoded in the phase difference between consecutive symbols. It should be mentioned that this method of recovery of the optical phase requires an additional encoder in the transmitter. However, the simplicity of the demodulator and its robustness to phase noise make it an attractive method for access applications that require high performance and low complexity. Figure 2 shows the realization of a differential demodulator (7.1, 7.2), in which the electrical signal is multiplied by a version of itself that has been delayed a bit time (11). It should be remembered that each of the two electrical signals entering each differential demodulator (7.1, 7.2) consists of a high frequency carrier that carries the data signal. In this regard, differential demodulators (7.1, 7.2) are responsible for lowering the signals from the intermediate frequency to the baseband, extracting the phase information without the need for an extra frequency conversion stage.
    After demodulation, the two baseband signals are combined by means of an adder (8), and a resulting low-pass filtering (9) is applied to the resulting signal with a bandwidth close to 75% of the rate of bit transmission typically, to eliminate noise and remaining polarization components. Finally, the filtered signal is conducted to the decision part (10) of the receiver, in which the synchronism recovery is made and the bit decision is made by means of a comparison threshold.
    权利要求:
    Claims (9)
    [1]
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    1. A coherent receiver independent of polarization and based on heterodyne detection for phase-modulated and differentially encoded optical signals, characterized in that it comprises:
    - An optical coupler (1) with three inputs and three outputs to make the coherent mix between the optical input signal and that of the local laser (2).
    - A polarizing beam splitter (3) to separate the two orthogonal components from the state of polarization of the local laser (2).
    - Optical-electrical conversion of the coherent mixture by means of three photodetectors (4.1,4.2, 4.3).
    - A linear combination (5) of the three photodetected signals to cancel the terms of direct detection and noise components in common mode.
    - Two pass-band filters (6.1, 6.2) to cancel interference from other adjacent channels and eliminate noise.
    - Differential demodulation (7.1, 7.2) to retrieve the information contained in the optical carrier phase.
    - An adder (8) of the two demodulated signals to generate the signal that goes to the decision part of the receiver.
    - A low-pass filter (9) to eliminate random components that depend on the state of polarization and cancel noise.
    - A stage of recovery of synchronism and bit decision by means of a decision maker (10) based on comparison threshold.
    [2]
    2. The receiver according to claim 1 characterized in that it operates in a heterodyne regime with an intermediate frequency selected as an integer multiple of the bit rate to maximize the decision amplitude by differentially demodulating phase modulated signals.
    [3]
    3. The receiver according to claim 1 characterized in that the local laser (2) can be a low-cost DFB or other laser and preferably should emit linearly polarized light at 45 ° to optimize the detection process.
    [4]
    4. The receiver according to claim 1 characterized in that the polarizing beam splitter (3) can be connected to the optical signal received instead of the laser
    local (2).
    [5]
    5. The receiver according to claim 1 characterized in that the linear combination (5) of the photodetected signals can be performed by analog "hardware" or in a
    5 stage of digital signal processing.
    [6]
    6. The receiver according to claim 1 characterized in that the modulation of the optical phase in the transmitter can be performed with external modulators or by means of direct laser modulation.
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    [7]
    7. The receiver according to claim 1 characterized in that the signal may be differentially encoded in the transmitter.
    [8]
    8. The receiver according to claim 6 characterized in that the optical modulation in
    15 the transmitter can be multilevel, combining phase and amplitude modulation.
    [9]
    9. The receiver according to any one of claims 1 or 8, characterized in that it may include a frequency conversion step that allows the detection of multilevel modulation formats.
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    同族专利:
    公开号 | 公开日
    ES2679403B1|2019-05-10|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    EP0404054A2|1989-06-19|1990-12-27|Fujitsu Limited|Direct modulation phase-shift-keying system and method|
    WO2015079400A1|2013-11-29|2015-06-04|Scuola Superiore Di Studi Universitari E Di Perfezionamento Sant'anna|Polarisation-independent coherent optical receiver|
    法律状态:
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    ES201730247A|ES2679403B1|2017-02-24|2017-02-24|INDEPENDENT HETERODINO RECEIVER OF POLARIZATION FOR OPTICAL SIGNALS MODULATED IN PHASE AND DIFFERENTIALLY CODIFIED|ES201730247A| ES2679403B1|2017-02-24|2017-02-24|INDEPENDENT HETERODINO RECEIVER OF POLARIZATION FOR OPTICAL SIGNALS MODULATED IN PHASE AND DIFFERENTIALLY CODIFIED|
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