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    • 专利摘要:
    The present invention relates to a method of direct modulation of the optical phase of a laser by means of a coded pulse signal and variable duty cycle of maximum one bit in duration. The technique consists of generating a signal of three or more levels that indicate the phase changes by positive or negative pulses and keep at logical level "0" when the phase is constant. This is achieved by means of an encoder. The electric pulse signal is amplified to obtain the desired phase changes in the optical signal and directly modulates a laser. The optical signal will have the information in the phase of it signaling the phase changes that can of two or more values and that will be detected by a receiver preferably coherent with a differential decoder.
    公开号:ES2659692A1
    申请号:ES201631207
    申请日:2016-09-16
    公开日:2018-03-16
    发明作者:Josep Prat Gomà；Ivan CANO VALADEZ
    申请人:Universitat Politecnica de Catalunya UPC；
    IPC主号:
    专利说明:

    DIRECT MODULATION METHOD OF THE LASER OPTICAL PHASEMEANS OF A SIGNAL OF CODED PULSATIONS AND WORK CYCLE
    VARIABLE
    SECTOR OF THE TECHNIQUE
    Optical communications with emphasis on local access networks
    BACKGROUND OF THE INVENTION
    Ultra-dense wavelength multiplexing networks have been
    studied for several years as an alternative to improve the performance of optical access networks. Although there are wavelength multiplexing applications in metropolitan and long optical communications
    distance (> 100km), its use in access networks has been limited by the cost of the devices [1]. In particular, the transmitter generally modulates the phase
    optics and requires an external modulator that makes users' terminals more expensive. On the other hand, the receiver is usually of a coherent type in this type of networks, either homodyne or heterodyne. These receivers are characterized by using a laser as an optical local oscillator that mixes with the signal of
    data.
    Because the optical signal is generated with one laser and detected with another,
    there are fluctuations in their relative phases. This causes an electrical phase noise that is related to the spectral line width of
    Lasers Therefore, high quality lasers are required [2]. In order to reduce the effect of this noise, the information is sent in the phase difference
    in consecutive symbols instead of in the absolute phase directly. It is based on the realistic premise that the speed of the symbols is greater than the variations of the phases. The additional requirement to do this is a
    encoder in the transmitter and a decoder in the receiver [2].
    In order to simplify the transmitter, lasers with managed chirp (CML) based on interierometers or feedback filters have been proposed
    distributed [1, 3]. 3 and 4 level signals have been demonstrated that do not require
    of differential coding. However, the CML requires a configurator of
    optical spectrum coupled to the laser [1, 3, 4]. The transmission of a bipolar differential signal has also been recently demonstrated by directly modulating a laser whose phase response was previously equalized [5].
    The present innovation uses a multilevel signal consisting of pulses with a duty cycle of a maximum bit time to directly modulate a laser and generate a phase-modulated optical signal without requiring a phase modulator external to the laser as usual. Making use of coding
    differential is effected by the phase noise and the pulse signals are
    obtained from another encoder. The electrical pulse signal is conditioned to obtain the desired levels in the optical phase and is preferably detected with a coherent receiver and a decoder
    differential. Unlike the CPFSK and MSK modulations, in this innovation
    the direct current is zero, the pulses can have variable duty cycles up to a bit time and it is possible to obtain signals of more than two levels.
    Bibliography[1) W. Jia, et al., "Generation and transmission of 10-Gbaud optical o /.- RZDQPSK signals using a chirp-managed DBR laser, "in IEEE J LightwaveTechnol , vol. 30, no. 21, pp. 3299-3304, November 2012.[2) K.P. Ha, Phase modulated optical communication systems, New York:Springer, 2005.[3) F. Fan Zhencan, D. Mahgerefteh, "Chirp managed lasers: a new technologyfor 10 Gbps optical transmitters, "Telekom Photonics Optik & Photonik, no. 4,pp. 39-41, December 2007.[4) Y. Matsui, et al., "Chirp-managed directly modulated laser (CML)," in IEEEPhoton Technol Let /., Vol. 18, no. 2, pp. 385-387, January 2006.[5) I.N. Cano, et al., "Direct phase modulation DFB for cost-effective ONUtransmitter in udWDM-PONs, "in IEEE Photon. Technol. Lett., vol. 26, no. 10,pp. 973-975, May 2014.
    [6J J. Proakis, Digital communication systems, 2nd ed. , New Jersey: PrenticeHall, 2002.
    EXPLANATION OF THE INVENTION
    The present invention provides a method for generating a phase-modulated optical signal from the direct modulation of a laser by means of an electrical data signal consisting of pulses of three or more levels.
    and with a duty cycle of maximum one bit time. In the transmitter the data signal will first be differentially encoded (Fig. 1) And then it will pass through another encoder (Fig. 2) that will produce the multilevel pulse signal (Fig. 3), where
    pulses other than the central value will represent changes in the optical phase. In particular, in the case of three levels illustrated in Fig. 3, the values
    Positives (greater than the central value) will produce optical phase changes of +8 at
    end of time T; negative values (less than the central value), optical phase changes of -8, and the central ("O" logical) values will not produce changes in the optical phase. This multilevel pulse signal will be amplified so that, when directly modulating a laser, it produces the desired changes in the optical phase thanks to the "chirp" phenomenon of the semiconductor laser. These levels
    Optics will be detected in a receiver preferably of a coherent type with a differential decoder.
    The object of the invention is an optical communication system (Fig. 4). In
    this, the data signal is differentially encoded (401) and in pulses (402) and then conditioned (403) to modulate the light produced by a laser (404) directly. The optical signal is then sent by means of optical fiber or some other means to the receiver (405) based on coherent optical detection or
    Direct detection and transforms it into electrical. The electrical signal is then
    decode in (406) to recover the original signal. In short, the system of
    optical communication consists of:
    a) A transmitter based on a laser whose phase is directly modulated
    (Fig. 5). The transmitter includes a pre-encoder (501) that transforms the original data signal into an encoded multilevel pulse signal.
    differentially. This signal is conditioned (502) and subsequently
    modulates the phase of a laser (503). An optical filter (504) can be
    used to limit the signal bandwidth. b) An optical transmission link
    c) A receiver that can be based on direct detection or
    preferably in detection consistent with a differential demodulator or decoder (Fig. 6) and dispersion compensator (when required). The electrical signal, after being
    detected by photodiodes, it goes through a decoder (602) and an equalizer (603). It can also include a radio frequency stage
    to lower the signal to baseband and one or several stages of equalization and filtering to reduce signal noise.
    The transmitter is the main object of the present invention. For networks of
    Optical access, transmitters based on the direct modulation of lasers are popular for their low cost. They are usually modulated in
    optical intensity producing a data signal and an optical carrier. Without
    However, they can also be frequency modulated and, as a consequence,
    with the proposed processing, in phase. In order to directly modulate the laser optical phase reliably, the present invention generates a
    multilevel pulse signal that indicates changes in the optical phase when the values are different from the central value; in case it is the core value the phase is maintained. Changes in the optical phase of the modulated signal
    they depend on the electrical level of the pulse signal, so it is conditioned depending on the type of modulation. The present invention proposes a precoder (Fig. 7) to produce an encoded pulse signal differentially consisting of:
    one. A differential encoder (Fig. 1) that is formed by an XNOR logic gate (101), or equivalent, whose inputs are the original data signal and the feedback signal fed back with a one-bit delay (102).
    2. An encoder that produces the multilevel pulse signal and which consists
    in a difference between the current bit and the previous bit delayed (Fig. 2). He
    Delay defines the pulse duty cycle, which will be a maximum of one bit time (201). It is important to highlight that a three-level signal has been explained by its
    simplicity This corresponds to the minimum number of binary phase levels (BPSK). The technique can be extended to more than three
    levels. In particular, for a signal with 5 levels a quaternary phase modulated signal (QPSK) 10 would be generated and, in general for 2 * m + 1 levels an m-PSK phase signal, where m is an even value greater than 4. It also can
    also use the duobinary coding that uses a low-pass filter. In addition, in order to reduce the total bandwidth of the signal, you can conform the spectrum of the signal by means of equalizers and filters.
    The receiver, meanwhile, will be based primarily on consistent detection Optical phase with a differential decoder.
    BRIEF DESCRIPTION OF THE DRAWINGS
    In order 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: Fig. 1 shows a block diagram of the differential encoder.
    Fig. 2 shows the block diagram of the encoder to generate the multi-level pulse signal.
    30 Fig. 3 illustrates a three-level pulse signal with a bit-time duty cycle
    Fig. 4 shows the block diagram of an optical communication system_
    Fig. 5 presents a diagram of a directly modulated optical transmitter.
    Fig. 6 presents a block diagram of an optical reception system.
    Fig. 7 represents the complete encoder of the transmitter and consisting of a
    differential encoder (701) And a pulse encoder (702).
    Fig. 8 represents a diagram of the optical system with the encoder (C) in the
    transmitter (Tx), medium (M) and receiver (Rx).
    PREFERRED EMBODIMENT OF THE INVENTION
    The preferred implementation consists of an encoder in the transmitter that generates a multi-level pulse signal that directly modulates a laser to obtain a phase-modulated optical signal. The receiver will preferably be consistent and may have a decoder to correctly detect the signal. No modifications are necessary to the fiber distribution plant
    Optics or some other means.
    The encoder can be digital or analog, and will precede the modulation of the
    optical phase It is composed of the following elements:
    • A differential encoder that is formed by an XNOR logic gate whose inputs will be:
    o The data signal without zero return
    o The feedback signal from the XNOR output delayed a bit time.
    • A pulse encoder that will generate a multilevel signal of
    pulsations where the latter will have a duty cycle of maximum one bit time. This encoder is a subtractor with the following entries:
    o The data signal without zero return (may or may not be encoded
    differentially)
    o The data signal without zero return (may or may not be encoded
    differentially) with a delay that can be variable and that
    it will be a maximum of one bit time and that will define the cycle of
    Pulse work.
    This will produce a three-level pulse signal that will indicate changes in the optical phase of the signal. The central level of this pulse signal will indicate that no
    there is change in the optical phase, while the positive and negative values
    (taking as a reference the central value) will produce positive and negative changes respectively in the optical phase of the modulated signal. The pulse signal will be conditioned, that is, it will be amplified and can be equalized to adjust its bandwidth. The phase change values produced in
    The optical signal will depend on the positive and negative levels of the amplified pulse signal. The pulse signal will preferably modulate directly
    a laser producing a phase modulated optical signal.
    It should be noted that the description in the previous paragraph refers to a signal of
    three levels corresponding to a binary phase modulation, BPSK. It has been
    described this modulation for being the simplest. However, the method to generate signals of more levels can be extended, in particular with 5
    levels a quaternary phase modulated signal (QPSK) would be obtained and for
    2 * m + 1 levels an m-PSK signal, where m is an even value greater than 4. Other
    Modulation that can be used is the duobine type that uses another encoder and a low-pass filter. Finally, in order to reduce the total bandwidth of the signal, equalizers and filters can be used to shape the spectrum of the signal.
    The receiver will preferably be of a coherent type and may carry a differential decoder in case the coding has been done in the
    transmitter. This decoder can be digital or analog and will be in charge of comparing the difference in the phases of two consecutive symbols and determine the corresponding bit. The receiver may include other elements such as equalizers and filters that reduce the effects of the channel on the received signal.
    权利要求:
    Claims (10)
    [1]
    1. A method of direct modulation of the optical phase of a laser by means of
    a coded pulse signal and variable duty cycle characterized by a transmitter based on the direct modulation of a laser by means of a digital electrical signal consisting of pulsations
    of three or more levels and with a maximum duty cycle one bit period at levels other than "O" (central value) and that produces an optical signal
    whose optical phase is modulated in such a way that the "O" state represents that there is no change in the optical phase while the remaining levels indicate positive or negative changes in the optical phase at values that depend on
    the amplitude of the pulsation.
    [2]
    2. The method of claim 1 wherein the data pulse signal may have three levels for BPSK, five for QPSK and at least seven levels for m-PSK.
    [3]
    3. The method of claim 1 wherein the pulse signal may be of the duobinary type.
    [4]
    4. The method of claim 1 wherein the signal may be differentially encoded in the transceiver.
    [5]
    5. The method of claim 1 wherein the pulses in the transceiver can be generated in analog or dig ital form.
    [6]
    6. The method of claim 1 wherein the shape of the pulses in the
    transceiver can be shaped by analog equalizers or
    digital
    [7]
    7. The method of claim 1 characterized in that the transceiver transmitter can include an optical filter.
    [8]
    8. The method of claim 1 characterized in that the receiver of the
    Transceiver is preferably based on consistent detection.
    [9]
    9. The method of claim 1 which can include a transceiver characterized in that the receiver can also be of the direct detection type.
    [10]
    10. The method of claim 1 characterized in that the receiver of the
    Transceiver includes a differential decoder.
    The method of claim 1 characterized in that the receiver of the
    Transceiver includes equalization and filtering functions.
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