JP5104963B2 - Method and system for setting and controlling a demodulator in an optical communication system - Google Patents

Description

  The present invention relates generally to optical communication technologies, and more particularly to differential phase shift keying (DPSK), such as binary DPSK, differential quadrature phase shift keying (DQPSK), and the like. (Differential phase shift keying) The present invention relates to demodulation of a received signal modulated by a technique.

  An optical sub-module is provided in the optical communication system and is used to receive an optical signal from a transmission medium. Such an optical submodule generally includes a receiver unit and a control unit.

  The receiver section of the optical submodule receives the optical signal and optically demodulates the optical carrier wave of the optical signal to which information is applied. The receiver unit includes a demodulator and a receiver that converts the demodulated optical signal into an electrical signal. The control unit includes electronic circuits necessary for the operation of the optical submodule. The control unit may comprise a microprocessor that enables remote monitoring or remote operation and / or a non-volatile memory unit that stores the operating parameters of the optical subunit, but these are not essential. Absent. An electrical dispersion compensation unit that compensates for distortion in the electrical signal supplied from the receiver unit may be provided on the output side of the receiver unit. The electric dispersion compensation unit may be provided outside the optical submodule as a dispersion compensation device.

  In some cases, the optical submodule may include a radiating unit so that the optical submodule has a function of transmitting an optical signal to the transmission medium, but this is not essential. The radiating unit emits an optical signal modulated according to the information sent to the optical submodule, and the optical submodule engraves the information on an optical carrier to be transmitted to the transmission medium. The radiating unit includes a light wave source and a modulator.

  An optical submodule having a radiating portion, which transmits signal light to the opposing optical submodule and receives signal light from the opposing optical submodule, can be called a transponder. An optical submodule having only a function of receiving signal light from an opposing optical submodule can be referred to as a receiver module.

  In optical communications, the wavelength of light carrying information is specified within a certain wavelength range. In wavelength division multiplexing (WDM), several optical carriers carry information on the same transmission medium, such as an optical fiber. Therefore, the specifications for multiple wavelengths are more restrictive. In other words, the ITU (International Telecommunications Union) defines a plurality of wavelengths that can be used for some WDM configurations in the ITU standard G694 series of documents. Thus, in the case of an optical sub-module having an integrated light source, the wavelength of the emitted signal and the wavelength of the received signal are assumed to be the same for the radiating portion and the receiver portion as the specified wavelengths. May be nominally the same.

  In the field of optical telecommunications systems and optical communication networks, the DPSK modulation method is widely used as the principle of modulation and demodulation. One form of DPSK modulation is two-phase DPSK. In this form, information is encoded on two phases that are separated by π radians (ie, 180 °) from each other. Another form of DPSK that is commonly used is that two bits of information are stored in binary on four different phases that are π / 2 radians (ie, 90 °) from each other. DQPSK to be encoded.

  DPSK demodulation and DQPSK demodulation can be performed optically by a 1-bit delay interferometer (or 1-symbol delay interferometer). Therefore, the receiver section of the optical submodule generally includes a 1-bit delay interferometer. In a 1-bit delay interferometer, a received optical signal (received optical signal) is divided into two paths (branches), and one of the two paths includes a 1-bit delay (1-bit delay element). The signals that have propagated along one path are then superimposed and interfered with each other. In this case, the accuracy of the delay in the interferometer has a direct influence on the demodulation quality, and therefore also has a direct influence on the bit error rate (BER) of the demodulated signal. As a result, accurately setting and accurately controlling the delay value of the interferometer improves the BER of the demodulated signal. In order to adaptively set the delay value of the interferometer, a controllable delay interferometer having a tuning unit for adjusting the delay value has been proposed.

  For applications with higher bit transfer rates in optical communications, the delay value has become smaller and tolerances in delay settings have become severe. Furthermore, changes in external parameters such as ambient temperature and internal degradation such as aging in the interferometer adjuster have an effect on the accuracy of the delay, ie on the BER of the signal.

  For some applications, a fixed delay setting is sufficient in terms of BER performance. However, for operation over a high bit rate and lifetime, degradation of the BER of the received signal becomes a problem that cannot be solved with a fixed delay setting.

  A method of setting and controlling a demodulator using an optical delay interferometer in an optical submodule is disclosed in Japanese Patent Application Laid-Open No. 2005-080304 (Patent Document 1), which is disclosed in US Patent Publication 2005/0047780. This corresponds to (Patent Document 2). In the method described in Patent Document 1 or Patent Document 2, information on the quality evaluation standard of the demodulated signal such as BER is fed back to the optical delay interferometer, and the relative delay in the optical delay interferometer is adjusted. Is done. However, in DQPSK or higher-order phase shift encoding, there may be two or more adjustment units in the demodulator. Having an independent feedback loop for each adjuster means that for each arm of the interferometer, i.e. for each arm of the in-phase and quadrature phase arms in the case of DQPSK, Although it is necessary to monitor the BER, this is not always possible. Furthermore, BER monitoring may be used for feedback of other parts in the optical sub-module, such as an electrical dispersion compensator or a compensation device. In such a case, both the demodulator and the compensator have an effect on the BER of the received signal, which can make it difficult to implement feedback.

  In general, an optical communication system includes a number of parts and devices in addition to a 1-bit delay interferometer. Each of these parts and devices can cause degradation in the BER, so it is difficult to recover the BER by independently adjusting and setting the amount of delay in the interferometer based on the BER monitoring results. When the delay amount is adjusted using the feedback result of the BER monitoring of the received signal, the BER may fall into the BER minimum value state instead of the minimum value state as a whole system.

  An optical receiver of an optical transmission system using DPSK-DD (DPSK-direct detection) is disclosed in International Publication WO 2005/088876 (Patent Document 3). In the receiver of Patent Document 3, a Mach-Zehnder interferometer is used to demodulate a received optical signal, and an optical signal from one of the outputs of the interferometer is received by a balanced photodetector. It is detected.

  An example of a method for stabilizing the operation of the Mach-Zehnder interferometer is disclosed in International Publication WO 2005/067189 (Patent Document 4). In the system of Patent Document 4, control light having the same wavelength as signal light is generated on the transmission side, sent to the reception side, and supplied to an interferometer that demodulates the signal light. Next, the control light is extracted from the output of the interferometer and then converted into an electrical control signal, and the amount of phase shift set for one branch path of the interferometer is controlled based on this control signal. To optimize the operation of the interferometer. The control light and the signal light are sent to the receiving side in a time division form.

JP 2005-080304 A US Patent Publication No. 2005/0047780 International Publication WO2005 / 088886 International Publication WO2005 / 067189

Y. Han et al., "Simplified receiver implementation for optical differential 8-level phase-shift keying," Electronics Letters, Vol. 14, No. 21, PP. 1372-1373 (October 2004) R. Sambaraju et al., "16-level differential phase shift keying (D16PSK) in direct detection optical communication systems," Optics Express, Vol. 14, No. 22, pp. 10239-10244 (October 2006) Shibuya et al., "10-GHz-order optical frequency shifter using Bragg-diffraction-type electrooptic traveling phase grating," IEEE Conference on Lasers and Electro-Optics (CLEO) 2004, vol. 2, pp. 2 (May 2004)

  Accordingly, there is a need for a simple setup and control method and apparatus for a demodulator on an optical submodule. Furthermore, there is an advantage that these methods and systems do not depend on the monitoring result of the BER in the received signal.

  An object of the present invention is to provide a setting method that does not use a result of BER monitoring in a received signal for a demodulator on an optical submodule.

  Another object of the present invention is to provide a setting and control method for a demodulator on an optical submodule without using the result of BER monitoring in a received signal.

  According to the first exemplary embodiment of the present invention, the method of setting the demodulator used for the optical signal of the first frequency modulated by the phase shift keying is not equal to the first frequency. Passing the probe light having a frequency of 2 through the demodulator, observing the output intensity of the probe light from the demodulator, and the observed output intensity so that the demodulator adapts to the first frequency. Controlling the demodulator based on

  According to the second exemplary embodiment of the present invention, the method of setting the demodulator used for the optical signal modulated by phase shift keying is opposite to the direction in which the optical signal propagates in the demodulator. The probe light having the same frequency as the optical signal in the direction is passed through the demodulator, the output intensity of the probe light from the demodulator is observed, and the modulator is adapted to the optical signal frequency. And controlling the demodulator based on the observed output intensity.

  According to a third exemplary embodiment of the present invention, a demodulator control for controlling an optical demodulator having a first signal port and a second signal port and used for an optical signal of a first frequency. The system extracts means for generating probe light having a second frequency not equal to the first frequency, means for applying probe light to the first signal port, and probe light from the second signal port. Means, means for observing the intensity of the extracted probe light, means for controlling the transmission characteristic of the demodulator based on the observed intensity so that the transmission characteristic of the demodulator is adapted to the first frequency, and .

  According to a fourth exemplary embodiment of the present invention, a demodulator control system for controlling an optical demodulator having a first signal port and a second signal port and used for an optical signal comprises: Means for generating probe light having the same frequency as the signal, means for applying probe light to the first signal port, means for extracting probe light from the second signal port, and intensity of the extracted probe light Means for observing and means for controlling the transmission characteristics of the demodulator based on the observed intensity so that the transmission characteristics of the demodulator are adapted to the frequency of the optical signal, the optical signal comprising: Applied to the port and propagates within the demodulator from the second signal port to the first signal port.

  According to a fifth exemplary embodiment of the present invention, the optical receiver module comprises a demodulator and the demodulator control system described above.

  According to a sixth exemplary embodiment of the present invention, an optical transponder includes a light source that generates radiated signal light, an optical modulator that modulates the radiated signal light, and transmits the modulated signal light to the outside. The optical demodulator demodulates and generates an optical signal received from the outside, and the means for generating generates probe light from the radiated signal light.

  In an exemplary embodiment of the invention, the demodulator can be set and adjusted without using the result of BER monitoring of the received signal light (received signal light). Further, since the received signal light and the probe light are simultaneously introduced into the demodulator, the demodulator can be adjusted during the normal operation of the demodulator where the demodulator is demodulating the received signal light.

  Other principal features and advantages of the present invention will become apparent to those skilled in the art by reference to the following drawings, detailed description, and appended claims.

FIG. 2 is a schematic diagram illustrating an exemplary DPSK demodulator. 2 is a graphical representation of spectral transmission characteristics of the DPSK demodulator shown in FIG. FIG. 2 is a schematic diagram illustrating an exemplary DQPSK demodulator. 4 is a graphical representation of spectral transmission characteristics of the DQPSK demodulator shown in FIG. It is a block diagram which shows the structure of the optical submodule based on the 1st exemplary embodiment of this invention. FIG. 6 is a block diagram illustrating an example of a configuration of a frequency separator for separating optical frequencies that can be used in various exemplary embodiments of the present invention. It is a block diagram which shows the structure of the optical submodule based on the 2nd example embodiment of this invention. It is a block diagram which shows the structure of the optical submodule of the modification of 2nd exemplary embodiment. It is a block diagram which shows the structure of the optical submodule based on the 3rd illustrative embodiment of this invention. 7 is a graphical representation of spectral transmission characteristics of a demodulator and a fiber Bragg grating (FBG) in an exemplary optical sub-module of a third exemplary embodiment. It is a block diagram which shows the structure of the optical submodule based on the 4th exemplary embodiment of this invention. It is a block diagram which shows the structure of the optical submodule of the modification of 4th exemplary embodiment. It is a block diagram which shows the example structure of a transponder. FIG. 6 is a block diagram illustrating another exemplary configuration of a transponder. It is a block diagram which shows another example structure of a transponder. FIG. 10 is a block diagram illustrating yet another exemplary configuration of a transponder.

  In an exemplary embodiment of the invention, a demodulator for differential phase shift keying (DPSK) in an optical sub-module is set and controlled in an optical communication system. Light waves from an external light source, or a light source built into the same optical submodule as the demodulator, can be used to monitor the demodulator settings and monitor the feedback loop for controlling and setting the demodulator. ) (Part is taken out). The wavelength of the tapped light is shifted by the optical frequency shifter, and the tapped light is then used as probe light to control and set the demodulator. Alternatively, an independent light source that generates probe light having a frequency shifted from the received signal light is disposed, and the probe light is coupled (coupled) to the path of the received signal light. The difference in wavelength between the received signal light and the probe light will be described later. There is an advantage in using the optical frequency shifter in that it is not necessary to provide a separate light source for the probe light and the frequency difference can be easily adjusted.

  The probe light, that is, the tapped light passes through the demodulator, and the setting of the demodulator affects the optical power of the probe light after passing through the demodulator because of the transmission characteristics of the demodulator. Next, the probe light is separated from the received signal light, and its intensity is converted into an electric signal by a photoelectric detector. The feedback loop compares the electrical signal with the probe light corresponding to the ideal setting of the demodulator and adjusts the demodulator towards that ideal setting.

  In one exemplary embodiment, the modulation scheme is two-phase DPSK, and the probe light and the received signal light propagate in the demodulator in the same direction. The probe light is coupled to the received signal light by a coupler (coupler), and the probe light and the signal light are separated by an optical filter or by spatial separation using a dispersive element.

  In another exemplary embodiment, the modulation scheme is two-phase DPSK, and the probe light and the received signal light propagate in opposite directions in the demodulator. The probe light is coupled to the received signal light by an optical coupler, and the probe light and the signal light are separated by an optical circulator, an optical filter, or by spatial separation using a dispersive element.

  In yet another exemplary embodiment, the modulation scheme is differential quadrature phase shift keying (DQPSK) or higher order DPSK, and the probe light and the received signal light propagate in the demodulator in the same direction. . The probe light is coupled to the received signal light by a coupler, and the probe light and the signal light are separated from each arm, that is, an in-phase (I) arm and a quadrature (Q) phase. In each of the arms, it is performed by an optical filter or by spatial separation using a dispersive element. Thus, there are two independent monitoring light signals, one for each arm, and a photoelectric detector and feedback loop are implemented for each monitoring light signal.

  Here, higher order DPSK means that N is an integer greater than 4, and N-ary DPSK or N value such as differential 8-level phase shift keying (D8PSK) or differential 16-level phase shift keying (D16PSK). It means DPSK. An example of an optical D8PSK demodulator is described by Y. Han et al., "Simplified receiver implementation for optical differential 8-level phase-shift keying," Electronics Letters, Vol. 14, No. 21, PP. 1372-1373 (October 2004 (Non-Patent Document 1), an example of D16PSK is described in R. Sambaraju et al., “16-level differential phase shift keying (D16PSK) in direct detection optical communication systems,” Optics Express, Vol. No. 22, pp. 10239-10244 (October 2006) (non-patent document 2).

  In yet another exemplary embodiment, the modulation scheme is DQPSK or higher order DPSK, and the probe light and the received signal light propagate in opposite directions in the demodulator. The coupling between the probe light and the received signal light is performed on the switch element and each arm so that the coupling is performed alternately on each arm of the demodulator (that is, the I and Q arms) so as to be one arm at the same time. Implemented by a combiner. The separation of the probe light and the signal light is performed by an optical circulator, an optical filter, or a spatial separation using a dispersive element. Therefore, there is only a single separated monitoring light signal, and one photoelectric detector and feedback loop are implemented. The feedback on both arms is alternately performed so as to be one arm at the same time, corresponding to the replacement of the probe light on each arm of the demodulator. The reference electrical signal used for feedback is given alternately for the arm that is capable of feedback.

  As yet another exemplary embodiment, the continuous light source may be tunable and tuned during operation. The optical frequency shifter shifts the optical frequency of the optical wave after the optical wave is tapped so that the shifted frequency can be sufficiently separated from any optical frequency that the received signal light may have. As an example, assuming that a plurality of frequencies that can be used for the received signal and the light source are arranged in a regular grid such as a grid defined by the ITU, the shift amount of the optical frequency is the light source. It can be constant for any wavelength emitted from. In this aspect, if the separating means is an optical filter or dispersive element and is tuned to separate tapped light having a shifted optical frequency, the separating means is tunable. Depending on the wavelength of the tunable light source, an electrical signal reference value is provided for each resulting feedback loop.

  In accordance with some exemplary embodiments of the present invention, when the electrical signal reference value to the feedback loop tends to have different values depending on the wavelength of the probe light or the demodulator arm to which the probe light is coupled. The reference value is given by the processing unit. If one processing unit is already mounted, the processing unit that gives the reference value may be already mounted in the optical subunit. Alternatively, the processing unit may be a different unit. As one aspect, the processing unit includes: a wavelength of the probe light; a power of the light wave source; a light spectral transmission characteristic of the demodulator; a spectral transmission characteristic of the physical element of the optical subunit; A reference value is calculated from a reference pair of calibration data consisting of a specific value of the reference value. As another aspect, in connection with the precision of the optical element, the processing unit provides a reference value from the stored value depending on the wavelength of the probe light and the radiation power of the light source. The list of stored values is incorporated when the optical submodule is calibrated.

  In yet another exemplary embodiment of the present invention, the feedback reference value may be normalized to a selected peak value in the spectral transmission characteristics of the demodulator. In that aspect, scanning the demodulator's spectral transmission characteristics by adjusting the demodulator around the selected peak at the start of the optical sub-module records the voltage value at the peak, thereby operating the feedback It is possible to calculate the reference value when doing.

  Upon a change or drift in the demodulator settings, the exemplary embodiment detects the change or drift. Furthermore, this exemplary embodiment allows the demodulator to be adjusted to offset the effect of that change or drift. Deterioration in BER that may have occurred is suppressed by this exemplary embodiment.

  Adjustment of the demodulator setting and feedback to this setting according to this exemplary embodiment is possible even when the demodulator is performing a process of demodulating the received signal light, and this feedback does not affect the process. .

  Even if the received signal does not include a BER monitoring function, or even if this function is not implemented for all arms of the demodulator, the exemplary embodiment of the present invention allows the setting of each arm of the demodulator. Allows to monitor and adjust each arm independently.

  Adjustment of the demodulator setting and feedback to this setting according to this exemplary embodiment is stable despite any BER spurious variation due to any cause independent of the optical sub-module.

  Adjustment of the demodulator setting and feedback to this setting according to this exemplary embodiment does not depend on other factors affecting the BER of the transmitted signal. Therefore, even if the present invention is implemented, the implementation of feedback to an active compensator using BER monitoring, such as an electrical dispersion compensator, remains trivial.

  According to the above exemplary embodiment, the demodulator is adjusted and controlled with high accuracy regardless of the wavelength of the received signal light. Even when the received signal light and the probe light have the same wavelength, the demodulator is adjusted and controlled without interfering with the ongoing demodulation process.

  The present invention does not generate noise or generate interference signals at any wavelength that is likely to be the wavelength of received signal light.

  Before describing specific exemplary embodiments of the present invention, a DPSK demodulator and a DQPSK demodulator will be described.

  FIG. 1 schematically represents an exemplary DPSK demodulator based on a 1-bit delay interferometer. Such devices are widely used. The DPSK demodulator 100 is configured as a waveguide type Mach-Zehnder interferometer, and an adjustment unit 101 inserted in one optical path 103 of the interferometer and a 1-bit delay inserted in the other optical path 104 of the interferometer. Part 102. Each of the optical paths 103 and 104 is configured as a waveguide. The received signal light 110 is branched and introduced into the two optical paths 103 and 104 by the beam splitter 120. A 3 dB coupler (3 dB coupler) may be used instead of the beam splitter 120.

  The adjustment unit 101 generally includes an electric heater that heats a part of the optical path 103, and is provided to set the demodulator 100 in accordance with the wavelength of the received signal light 110. When the power applied to the heater of the adjustment unit changes, the refractive index of the optical path 103 near that part also changes, and as a result, the phase of the light wave propagating through the optical path 103 is also a light wave used as a signal carrier wave. Changes within the range of several cycles or tens of cycles. Therefore, the delay of the optical signal on the optical path 103 can be controlled by changing the power applied to the adjustment unit 101.

  The 1-bit delay unit 102 includes a length of waveguide corresponding to the 1-bit duration of the signal. The optical signal propagating in the optical path 104 is delayed by one bit time, and indicates the signal state just one bit before.

  The optical paths 103 and 104 become one again in the directional coupler 105, and a constructive output 111 and a deconstructive output 112 are drawn from the directional coupler 105. Since the optical signal propagating in the optical path 104 is delayed by one bit time compared to the optical signal propagating in the optical path 103, the output signal light from the directional coupler 105 is modulated by DPSK. The demodulation result of the received signal light is given. The constitutive and non-structural outputs 111 and 112 can be connected to a balanced photodetector to detect the received signal light 110 demodulated by the demodulator 101.

  In order to accurately demodulate the received signal light and improve the BER, it is necessary to align the phase of the carrier wave propagating through both optical paths 103 and 104 to the directionality detector 105. This is the reason why the adjustment unit 101 is inserted into one optical path 103.

FIG. 2 shows typical spectral transmission characteristics of the DPSK demodulator 100 with respect to the constitutive output 111. Demodulator 100 is set according to the optical frequency of the received optical signal 110, the optical frequency value is represented by f _r. FIG. 2 shows that the light intensity at the constitutive output changes periodically as the optical frequency of the received optical signal 100 changes. The goal of this exemplary embodiment is that to bring the peak of the transmittance to the frequency point f _r. In FIG. 2, f _s represents the optical frequency of the probe light, f _t represents the optical frequency of the light emitted from the emitting portion of the optical subunits. It will be described later these light frequencies f _s and f _t.

  FIG. 3 schematically represents an exemplary DQPSK demodulator based on a one symbol delay interferometer. Such a demodulator is widely used.

  As shown in FIG. 3, the DQPSK demodulator 300 generally has two arms: an in-phase (I) arm and a quadrature (Q) arm. Each arm is composed of a single interferometer having a one-symbol delay unit, and has the same configuration as that of the two-phase DPSK demodulator 100 shown in FIG. The received signal light 310 is distributed to the I and Q arms by the beam splitter 320.

  The I arm includes a beam splitter 321, an adjustment unit 301, a phase adjustment unit 302, a symbol delay unit 305, and a directional coupler 322. The adjustment unit 301 and the phase adjustment unit 302 are provided on one optical path between the beam splitter 321 and the directional coupler 322, while the one symbol delay unit 305 is provided on the other optical path. ing. Similarly, the Q arm includes a beam splitter 323, an adjustment unit 303, a phase adjustment unit 304, a 1 symbol delay unit 306, and a directional coupler 324. Each of the 1-symbol delay units 305 and 306 includes a certain length of waveguide corresponding to the duration of one symbol of the signal. Adjustment units 301 and 303 are provided to tune the I and Q arms to the wavelength of received signal light 310, respectively. The phase adjustment units 302 and 304 are provided to give a phase difference between the light beams that have passed through the I and Q arms. In this case, the delay amount set by the I-arm phase adjustment unit 302 may be π / 4 radians, and the delay amount set by the Q-arm phase adjustment unit 304 may be −π / 4 radians.

  The I arm has a constitutive output 311 and a non-constitutive output 312, which are the outputs of the directional coupler 322 for receiving the I branch of the signal 310 demodulated by the demodulator 300. Can be connected to a balanced photodetector. Similarly, the Q arm has a constitutive output 313 that is the output of the directional coupler 324 and a non-constitutive output 314. The Q branch of the signal 310 demodulated by the demodulator 300 can also be received by a balanced photodetector connected to the outputs 313 and 314. The configuration shown in FIG. 3 can also be used for higher order DPSK demodulators with appropriate phase configurations for each arm.

FIG. 4 shows typical spectral transmission characteristics of the DQPSK demodulator 300. A curve 401 indicates a spectral transmission characteristic with respect to the I-arm constituent output 311, and a curve 402 indicates a spectral transmission characteristic with respect to the Q-arm constituent output 313. Since the peaks of both curves 401, 402 do not coincide in principle, the demodulator 300 is such that the transmission of the I and Q arms is relatively high and both transmission values are equal to each other. the frequency is set according to the optical frequency f _r of the received signal light 310.

FIG. 5 shows an optical sub-module according to the first exemplary embodiment of the present invention. The optical sub-module includes a continuous light source 501, a DPSK demodulator 504, a balanced photodetector (PD) 505, a distributor (divider) 511, an optical frequency shifter 512, a coupler (coupler) 513, a notch filter 514, and frequency separation. Device 515, photodiode 516, feedback circuit 517, and processing unit 518. In this exemplary embodiment, the DPSK demodulator 504 has the same configuration as the demodulator 100 shown in FIG. 1 and is set and controlled. Balanced optical detector 505 is provided with a typical balanced photodiode (PD), the received optical signal 503 having the optical frequency of f _r, the after the optical signal is demodulated by a demodulator 504, an electric signal Used to convert to The continuous light source 501 emits signal light 502 at the optical frequency f _t . The light source 501 may have the demodulator 504 on the same optical submodule, or may be provided separately. In some cases, the optical frequency f _t can be the same as the optical frequency f _r. This frequency arrangement is useful for bidirectional communication between this optical sub-module and another optical sub-module using the same optical frequency. Alternatively, the optical frequency f _t may be different from the optical frequency f _r. The signal light 502 can be used to transmit signals to other optical submodules (not shown).

In this configuration, a part of the light from the light source 501 is tapped or branched in a distributor 511 which is typically a directional coupler. Several structures of such devices have already been reported as optical frequency shifters 512. For example, Shibuya et al., "10-GHz-order optical frequency shifter using Bragg-diffraction-type electrooptic traveling phase grating," IEEE Conference on Lasers and Electro-Optics (CLEO) 2004, vol. 2, pp. 2 (May 2004) (Non-Patent Document 3) can be used for the optical frequency shifter 512. Shibuya et al.'S device is based on an electrooptic traveling phase grating called LiTaO ₃ crystal. However, other known devices that rely on acousto-optic devices, nonlinear optical phenomena, or optical parametric oscillation can be used as the optical frequency shifter 512. The optical frequency shifter 512 shifts the frequency of the tapped light from f _t to f _s . Tapped optical frequency f _s is used as the probe light, the coupler 513, is coupled to receive the signal light 503 having a frequency of f _r. Both light waves propagate in demodulator 504 in the same direction.

The notch filter 514 is connected to one of the two outputs of the demodulator 504, ie, one of the constituent and non-constitutive output ports 111, 112 shown in FIG. It is connected to the other of these output ports. The frequency separator 515 is provided for extracting light having the frequency f _s . An exemplary configuration of frequency separator 515 is indicated in FIG.

A frequency separator 900 shown in FIG. 6 includes first to third ports A to C, and has a well-known structure based on a fiber Bragg grating (FBG) that reflects a wave of frequency f _s . Separator 900 further includes an optical circulator 911 whose orientation is selected to separate f _s on the third port. The frequency separator 515 is placed on one output port of the demodulator 504 via its first port A, ie, a configurable or non-configurable port, and separates the tapped signal of frequency f _s , The direction is set so that the tapped signal is sent to the photodiode 516 via the third port C. Received signal in the frequency f _r is separated from f _s, component of f _r passing through the FBG912 is sent to the balanced optical detector 505 through the second port B. Notch filter 514 is provided to remove the component of the frequency f _s, it is calibrated to have the same losses as the frequency separator 515 with respect to the frequency f _r.

If a heater that changes the temperature of the FBG 912 is provided, the frequency separator 900 can be tuned. The tunable frequency separator is useful when the light source 501 is tunable and the frequency _ft is controlled based on an external command applied to the light source 501.

The photocurrent output by the photodiode 516 is proportional to the transmittance of the demodulator 504 with respect to the frequency f _s . When the setting of the demodulator 504 changes, the transmittance curve shown in FIG. 2 shifts and the photocurrent changes. The light current is fed to the feedback circuit 517, such as a comparator, to the feedback circuit 517, corresponding to the ideal set transmission of the demodulator 504 with respect to the frequency f _r processing unit 518 The reference photocurrent output from is also supplied. 2, corresponding to the ideal transmission at a frequency f _r, the transmittance at the frequency f _s is represented by T _o. Therefore regulatory goal is to set the transmittance to be observed for the frequency f _s to T _o. The transmittance T _o is not necessarily consistent with the transmittance at the peak with respect to the frequency f _s. To precisely set the peak of the transmittance of the demodulator 504 with respect to the frequency f _r of the received signal light, as the transmission curve is located, the frequency f _s of the probe light in the frequency region having a large gradient, This frequency f _s is preferably selected. In many cases, the transmission curve is sinusoidal and the frequency f _s is preferably set to a frequency corresponding to an intermediate value between the highest and lowest values in the transmission curve.

  The feedback circuit 517 adjusts the demodulator 504 according to the function of the error signal generated by the comparison between the photocurrent and the reference value. In the demodulator 504, the adjustment unit 101 (see FIG. 1) is controlled by the output of the feedback circuit 517.

  In this configuration, the received signal light and the tapped light (that is, the probe light) are simultaneously introduced into the demodulator 504. Therefore, the demodulator 504 can adjust the received signal light even during the normal operation of the demodulator 504 in which the demodulator 504 is demodulating.

In this exemplary embodiment, the wavelength of the light source 501 can be adjusted. For any frequency which is easily emitted by the light source 501, from all frequencies received signal 503 is likely to have so f _s are different, frequency shifts caused by frequency shifter 512 (f _s -f _t) is selected. If the ITU is arranged in the same grid of f _r and f _t Hikari Toga frequency as wavelength division multiplexing which defines the wavelength available in G694 series documents, frequency shift of a fixed value ( f _s −f _t ) can satisfy this condition. Further, the frequency shift must be selected so that the frequency separator 515 can separate f _s and f _t .

In this exemplary embodiment, processing unit 518 comprises a microprocessor, a digital / analog converter that generates a reference signal to be transmitted to feedback circuit 517, and a non-volatile memory storage device. These components may or may not be integrated. If they are embedded, the aforementioned components may be used on components implemented as demodulator 504 on the optical submodule. The processing unit 518 can use the frequency and optical power information emitted by the light source 501, and based on this information, the processing unit 518 determines the corresponding reference signal value to be transmitted to the feedback circuit 517. select. Relative values obtained by normalizing with photocurrent values at the local (local) peak closest to f _s , rather than absolute values, are stored in the memory portion of processing unit 518. At start-up of the optical sub-module comprising the demodulator 504, a scan around the local peak closest to f _s is performed by adjusting the demodulator 504 and measuring the corresponding photocurrent value. The photocurrent value at the peak makes it possible to restore the absolute value of the reference signal.

The memory unit stores the normalized reference values in a table format, that is, in a two-dimensional array format or a lookup table format. The first column of the array holds the normalized reference value and the second column holds the corresponding _ft value.

The frequency separator 515 and notch filter 514 need to be tuned for the frequency f _s controlled by the processing unit 518. Processing unit 518 knows the information of the frequencies stored in the frequency f _t and memory.

FIG. 7 shows an optical sub-module according to the second exemplary embodiment. The optical submodule shown in FIG. 7 has the same configuration as that shown in FIG. 5, but is different from the submodule shown in FIG. 5 in that probe light passes through DPSK. 7 includes a continuous light source 601, a DPSK demodulator 604, a balanced photodetector (PD) 605, a divider 611, an optical frequency shifter 612, optical circulators 613 and 615, a filter 614, a photo A diode 616, a feedback circuit 617, and a processing unit 618 are provided. In this embodiment, the DPSK demodulator 604 has the same configuration as the demodulator 100 shown in FIG. 1, and is set and controlled. The continuous light source 601 emits signal light 602 at the optical frequency f _t . The light source 601 may be on the same optical submodule as the demodulator 604 or may be provided separately.

In this configuration, a portion of the light from the light source 601 is tapped (ie, branched) in a distributor 611, typically configured as a directional coupler. Touch light at the frequency f _s is the constitutive output, by the circulator 613, which is disposed between one photodetector 605 of the output of the DPSK demodulator 604, the optical path of the reception signal light 603 of frequency f _r Combined with Filter 614 disposed between the other output and the photodetector 605 of the DPSK demodulator 604 is calibrated to have the same losses as circulator 613 with respect to the frequency f _r. In this arrangement, since the signal component of the frequency f _s does not pass through the filter 614, it is not necessary to provide a notch filter having a steep frequency characteristic as the filter 614. A simple filter can be used as the filter 614. A tapped optical frequency f _r signal light and the frequency f _s of the propagate in opposite directions inside the demodulator 604. An optical circulator 615 is provided at the input of the DPSK demodulator 604 to separate the tapped signal of frequency f _s and send it to the photodiode 616. Each spectral range of the optical circulator 613, 615 is wider than the range of frequencies that are allowed to f _s and f _t.

In this configuration, since the tapped light, that is, the probe light and the signal light propagate in the DPSK demodulator 604 in the reverse direction, the frequency f _s of the tapped light is the same as the frequency f _{r of the} signal light. Can be. Thus, as one variation, the light source 601 can emit a light signal 602 of frequency f _r, then the optical signal, without passing through the optical frequency shifter, it is introduced into the circulator 613. FIG. 8 shows such an optical submodule without an optical frequency shifter. In this modification, the feedback circuit 617 controls the demodulator 604 so that the detection value at the photodiode 616 becomes the maximum value. This variation is useful for realizing bi-directional communication between this optical sub-module and another optical sub-module using the same optical frequency.

  FIG. 9 shows an optical sub-module according to a third exemplary embodiment that uses a DQPSK demodulator 704 instead of a DPSK demodulator. In this embodiment, the DQPSK demodulator 704 has the same structure as the demodulator 300 shown in FIG. 3, and is set and controlled.

The optical submodule shown in FIG. 9 has the combination shown in FIG. 5 consisting of a balanced photodetector (PD) 505, a notch filter 514, a frequency separator 515, a photodiode 516, and a feedback circuit 517 as I and It has the structure provided with respect to each of Q arm. More specifically, the optical submodule shown in FIG. 9 includes a continuous light source 701, a DQPSK demodulator 704, balanced photodetectors 705 and 706, a distributor 711, an optical frequency shifter 712, a combiner 713, and a notch filter. 714, 715, frequency separators 716, 717, photodiodes 718, 719, feedback circuits 720, 721, and a processing unit 722 are included. The continuous light source 701 emits signal light 702 at an optical frequency f _t . The light source 701 may be on the same optical submodule as the demodulator 704 or may be provided separately. Balanced photodetectors 705 and 706 receive the I and Q branches of the signal demodulated by demodulator 704, respectively.

  Feedback circuits 720 and 721 adjust the I and Q arms of demodulator 704, respectively, in the same manner as feedback circuit 517 with demodulator 504 of FIG. The reference signal for each feedback circuit is provided by the same processing unit 722, which is the same as the processing unit 518 but operates in a duplex manner. Instead of a single list of stored normalization reference values, there are two arrays of the same configuration as described above, each corresponding to each arm of the demodulator 704.

The frequency separators 716, 717 and filters 714, 715 must be tuned (tuned) to the frequency f _s controlled by the processing unit 722. The processing unit 722 has information of the frequency stored in the frequency f _t and memory.

Next, an example of the operation of the optical submodule of this exemplary embodiment will be described. Frequency f _r of the signal light conforming to 100GHz frequency grid, it is assumed that 100GHz DQPSK modulation system is used.

DPQSK demodulator FSR (free spectral range; free spectral range) is 50 MHz, the frequency f _r must be adjusted from the peak of the transmittance of the modulator to frequency shift of 12.5 GHz (i.e. [pi / 4) . Where this frequency shift is DQPSK
Corresponds to the point of maximum slope in the transmission curve in the linear representation of the demodulator. The linear transmission curve is shown by the solid line in FIG. The horizontal axis of FIG. 10 represents the frequency difference Δf from one of the transmittance peaks of the demodulator.

The worst case is when the frequency f _t of the light source 701 is equal to the frequency f _r of the received signal light, i.e., the case of f _t = f _r. Therefore, the frequency f _{s of the} probe light is on a steep slope in the transmission curve, different from f _r , and outside any other possible value on the 100 GHz grid, Need to be selected. The optimal value is f _s = f _t +25 GHz, which corresponds to a frequency shift of 25 GHz. Also, the frequency f _s, one of the frequencies f _r is the maximum slope point on the inclined closer to where located.

Frequency separation is performed by a notch filter 716 consisting of a fiber Bragg grating (FBG) 912 (see FIG. 6) and a circulator 911. The FBG has a 25 GHz pass band, and the center of the band can be located at a frequency intermediate between the frequency f _s and the next transmittance peak. Since the frequencies of f _s and the next transmittance peak are f _t +37.5 GHz and f _t +25 GHz, respectively, the center frequency of the FBG is set to f _t +31.25 GHz. As a result, f _s is in the FBG reflection band and f _r and f _r +100 GHz are in the transmission band. The linear transmission characteristics along with f _r and f _t are shown in FIG. 10 where the reference is taken on the transmittance curve closest to f _r .

In the case of an optical frequency shifter 712 having 50% efficiency and 1 dB loss, if the tapped light level from the light source is 4 dBm, the output light of f _s from the frequency shifter is input to the demodulator 704. And has a power of -3 dBm. After passing through a typical DQPSK demodulator with 6 dB loss and a notch filter (1 dB loss in the circulator), the power at the monitoring photodiode for f _s is -10 dBm. Considering the power of 12dBm of the reception signal of the frequency f _r at the inlet to the demodulator 704, the power of the frequency f _r in the monitoring photodiode 718 to -25dB typical reflectance associated FBG, will become -20dBm . Power in the signal receiver 705 of the signal at frequency f _r is in this case 6 dBm, control in the f _s of the optical detector 705 (Control) signal power of -30dBm in FBG912 in the frequency separator 716 For transmission, it is -39 dBm.

FIG. 11 shows an optical submodule of the fourth exemplary embodiment. The optical submodule shown in FIG. 11 has the same structure as that shown in FIG. 9, but differs from the submodule shown in FIG. 9 in the direction of the probe light passing through the DQPSK demodulator. Yes. 11 includes a continuous light source 801, a DQPSK demodulator 804, balanced photodetectors 805 and 806, a splitter 811, an optical frequency shifter 812, an optical switch 813, filters 814 and 815, and optical circulators 816 to 818. , A photodiode 819, a feedback circuit 820, and a processing unit 821. Since the signal component of the frequency f _s does not pass through the filters 814 and 815, a simple filter can be used for each of the filters 814 and 815. In this embodiment, the DQPSK demodulator 804 has the same configuration as the demodulator 300 shown in FIG. 3, and is set and controlled. The continuous wave light source 801 emits signal light 802 at an optical frequency f _t . The light source 801 may be provided on the same optical submodule as the demodulator 804 or may be provided separately.

The optical switch 813 is controlled by the processing unit 821 and alternately sends tapped light having the frequency f _s to the circulators 816 and 817 provided at the constituent outputs of the I and Q arms, respectively. And a simple filter is provided to the non-constitutive output well, respectively I and Q arms filters 814 and 815 in the frequency f _r, is calibrated to have the same losses as a circulator 816, 817. And the received signal light having an optical frequency f _r which is tapped with a frequency f _s is propagated in opposite directions to each other inside the demodulator 804. An optical circulator 815 is provided at the input of the DQPSK demodulator 804 to separate the tapped light of frequency f _s and send it to the photodiode 819. The photocurrent output from the photodiode 819 corresponds to light having the frequency f _s that has passed through the I arm or the Q arm of the demodulator 804 depending on the position of the switch 813. The feedback circuit 820 operates in the same manner as the feedback circuits 720 and 721 shown in FIG. The feedback circuit 820 can adjust each arm of the demodulator independently and alternately. The arm to be adjusted and the reference value are determined by the processing unit 821, which is similar to the processing unit 722 and operates in a dual manner. Further, the processing unit 821 controls the switch 813 to cause the circulator 818 and the feedback circuit 820 to cooperate.

In this configuration, the tapped light, that is, the probe light and the signal light propagate in the reverse direction in the DQPSK demodulator 704, so that the frequency f _s of the tapped light is the same as the frequency f _{r of the} signal light. Can be. Thus, in one variant, the light source 801 can emit light signals 802 of the frequency f _r, the optical signal is then without passing through the optical frequency shifter, introduced alternately to the circulator 816 and 817 Is done. FIG. 12 shows such an optical submodule without an optical frequency shifter. In this modification, the feedback circuit 820 controls the demodulator 804 so that the value detected by the photodiode 819 becomes the maximum value. This variation is useful for realizing bi-directional communication between this optical sub-module and another optical sub-module using the same optical frequency.

  The optical submodules of the third and fourth exemplary embodiments can also be applied when a high-order DPSK demodulator such as D8PSK or D16PSK is used instead of the DQPSK demodulator.

  Han et al. Show two D8PSK receiver structures (Non-Patent Document 1). One of these structures has four delay interferometers and the other has two delay interferometers. Each delay interferometer is similar to the delay interferometer shown in FIG. 3, although it has a different additional phase value. The inclusion of a tunable phase structure in these interferometers as shown in Han et al. Makes it possible to utilize the method according to the invention by using the correct number of detectors.

  Similarly, Sambaraju et al. Show a D16PSK (16-element DPSK) detector based on six delay interferometers (Non-Patent Document 2), although each delay interferometer has a different additional phase value. This is the same as the delay interferometer shown in FIG. Therefore, such a structure with a tunable phase structure can also be used in combination with the method according to the invention and the correct number of detection devices.

Each of the optical submodules in the above exemplary embodiment can be modified as a transponder having a function of transmitting signal light to the opposing submodule and receiving signal light from the opposing submodule. FIG. 13 shows a transponder in which a DPSK modulator 519 that modulates emitted signal light (radiated signal light) 502 having a frequency f _t is added to the optical submodule shown in FIG. FIG. 14 shows a transponder in which a DPSK modulator 619 that modulates a radiation signal light 602 having a frequency f _t (= f _r ) is added to the optical submodule shown in FIG. FIG. 15 shows a transponder in which a DQPSK modulator 723 that modulates the radiation signal light 702 having the frequency f _t is added to the optical submodule shown in FIG. FIG. 16 shows a transponder in which a DPSK modulator 622 for modulating the radiation signal light 802 having the frequency f _t (= f _r ) is added to the optical submodule shown in FIG.

All exemplified embodiments can be used for all values of acceptable f _t and f _r. Information about f _t is known to their respective processing units, and transmitted. Information about _fr is not necessary.

  While certain terminology has been used to describe the preferred embodiment of the invention, such description is for illustrative purposes only, and modifications and variations are possible without departing from the spirit and scope of the appended claims. It should be understood that

Claims (23)

A method of setting a demodulator used for an optical signal of a first frequency modulated by phase shift keying, comprising:
Allowing probe light having a second frequency not equal to the first frequency to pass through the demodulator;
Observing the output intensity of the probe light from the demodulator;
Controlling the demodulator based on the observed output intensity so that the demodulator adapts to the first frequency ;
Having a method. While passing said optical signal to said demodulator, for introducing the probe light to the demodulator, the method according to claim 1.   The method according to claim 1 or 2, wherein the optical signal and the probe light travel in the same direction in the demodulator.   The method according to claim 1 or 2, wherein the optical signal and the probe light travel in opposite directions in the demodulator. The demodulator differential quadrature phase shift keying, or, are configured to demodulate the light modulated by the difference Doi phase shift keying by four-phase remote order, to claim 1 5. The method according to any one of 4 above. Generating signal light using an external light source ;
And to tap a portion of the pre-relaxin issue light,
And to generate the probe beam by shifting the frequency of the tapped portion of the front relaxin No. light,
The method according to any one of claims 1 to 5, further comprising: Frequency before venlafaxine No. light is equal to the first frequency, the method according to claim 6. A method of setting a demodulator used for an optical signal modulated by phase shift keying, comprising:
Passing probe light having the same frequency as the optical signal in the direction opposite to the direction in which the optical signal propagates in the demodulator;
Observing the output intensity of the probe light from the demodulator;
Controlling the demodulator based on the observed output intensity so that the modulator adapts to the frequency of the optical signal;
Having a method. The demodulator differential quadrature phase shift keying, or, are configured to demodulate the light modulated by the difference Doi phase shift keying by four-phase remote order, to claim 8 The method described. A demodulator control system for controlling an optical demodulator having a first signal port and a second signal port and used for an optical signal of a first frequency,
Means for generating probe light having a second frequency not equal to the first frequency;
Means for applying the probe light to the first signal port;
Means for extracting the probe light from the second signal port;
Means for observing the intensity of the extracted probe light;
Means for controlling the transmission characteristics of the demodulator based on the observed intensity so that the transmission characteristics of the demodulator are adapted to the first frequency;
A system comprising:   The system of claim 10, wherein the optical signal is applied to the first signal port and propagates within the demodulator from the first signal port to the second signal port.   The system of claim 10, wherein the optical signal is applied to the second signal port and propagates within the demodulator from the second signal port to the first signal port. The demodulator differential quadrature phase shift keying, or, are configured to demodulate the light modulated by the difference Doi phase shift keying by four-phase remote higher, 10 to claim 13. The system according to any one of items 12.   The demodulator is a differential quadrature phase shift demodulator, and the system includes the first signal port for the in-phase arm of the demodulator and the first signal port for the quadrature arm of the demodulator. The system according to claim 12, further comprising a switch that alternately supplies the probe light to each other. The means for generating is a light source for generating a radiation signal beam used to send signals, according to any one of claims 10 to 14 systems.   The system according to claim 15, further comprising means for generating the probe light by shifting a frequency of the radiation signal light.   The system of claim 16, wherein the frequency of the emitted signal light is equal to the first frequency. A demodulator control system that has a first signal port and a second signal port to control an optical demodulator used for an optical signal,
Means for generating probe light having the same frequency as the optical signal;
Means for applying the probe light to the first signal port;
Means for extracting the probe light from the second signal port;
Means for observing the intensity of the extracted probe light;
Means for controlling the transmission characteristics of the demodulator based on the observed intensity so that the transmission characteristics of the demodulator are adapted to the frequency of the optical signal;
With
The optical signal is applied to the second port and propagates through the demodulator from the second signal port to the first signal port.   The demodulator is a differential quadrature phase shift demodulator, and the system includes the first signal port for the in-phase arm of the demodulator and the first signal port for the quadrature arm of the demodulator. The system according to claim 18, further comprising a switch that alternately supplies the probe light to each other. An optical demodulator having a first signal port and a second signal port for demodulating an optical signal of a first frequency;
Means for generating probe light having a second frequency not equal to the first frequency;
Means for applying the probe light to the first signal port;
Means for extracting the probe light from the second signal port;
Means for observing the intensity of the extracted probe light;
Means for controlling the transmission characteristics of the demodulator based on the observed intensity so that the transmission characteristics of the demodulator are adapted to the first frequency;
An optical receiver module comprising: An optical demodulator having a first signal port and a second signal port for demodulating an optical signal;
Means for generating probe light having a frequency equal to the frequency of the optical signal;
Means for applying the probe light to the first signal port;
Means for extracting the probe light from the second signal port;
Means for observing the intensity of the extracted probe light;
Means for controlling the transmission characteristics of the demodulator based on the observed intensity so that the transmission characteristics of the demodulator are adapted to the frequency of the optical signal;
With
The optical receiver module, wherein the optical signal is applied to the second port and propagates in the demodulator from the second signal port to the first signal port. A light source that generates radiation signal light;
An optical modulator for modulating the radiation signal light, to send a modulated signal light,
An optical demodulator having a first signal port and a second signal port, receiving an optical signal of a first frequency and demodulating the received optical signal ;
Means for generating, from the radiated signal light, probe light having a second frequency not equal to the first frequency;
Means for applying the probe light to the first signal port;
Means for extracting the probe light from the second signal port;
Means for observing the intensity of the extracted probe light;
Means for controlling the transmission characteristics of the demodulator based on the observed intensity so that the transmission characteristics of the demodulator are adapted to the first frequency;
An optical transponder comprising. A light source that generates radiation signal light;
An optical modulator for modulating the radiation signal light, to send a modulated signal light,
An optical demodulator having a first signal port and a second signal port for receiving and demodulating an optical signal;
Means for generating probe light having the same frequency as the optical signal from the radiation signal light;
Means for applying the probe light to the first signal port;
Means for extracting the probe light from the second signal port;
Means for observing the intensity of the extracted probe light;
Means for controlling the transmission characteristics of the demodulator based on the observed intensity so that the transmission characteristics of the demodulator are adapted to the frequency of the optical signal;
With
The optical signal is applied to the second port, and propagates in the demodulator from the second signal port to the first signal port.

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