WO2009107247A1

WO2009107247A1 - Method of and system for setting and controlling demodulator in optical communication system

Info

Publication number: WO2009107247A1
Application number: PCT/JP2008/053729
Authority: WO
Inventors: Taillandier De Gabory Emmanuel Le; Toshiharu Ito; Kiyoshi Fukuchi
Original assignee: Nec Corporation
Priority date: 2008-02-26
Filing date: 2008-02-26
Publication date: 2009-09-03
Also published as: JP2011517152A; JP5104963B2

Abstract

A method to set a demodulator used for an optical signal of a first frequency which is modulated by phase shift keying includes: passing a probe light having a second frequency, which is not equal to the first frequency, inside the demodulator; observing an output intensity of the probe light from the demodulator; and controlling the demodulator based on the observed output intensity to adapt the demodulator to the first frequency.

Description

DESCRIPTION

METHOD OF AND SYSTEM FOR SETTING AND CONTROLLING

DEMODULATOR IN OPTICAL COMMUNICATION SYSTEM Technical Field:

The present invention relates generally to optical communication technologies, and more particularly, to demodulation of received signals which has been modulated by differential phase shift keying (DPSK) technologies such as binary DPSK, differential quadrature phase shift keying (DQPSK) and the like. Background Art:

An optical sub-module is provided within an optical communication system and used for receiving light signals from a transmission medium. Such an optical sub-module generally comprises a receiver part and a control part.

The receiver part of the optical sub-module receives a light signal, and optically demodulates an optical carrier of the light signal, on which information has been imprinted. The receiver part comprises a demodulator, and a receiver converting the demodulated light signal to an electric signal. The control part includes the electronic circuitry necessary to the operation of the optical sub- module. The control part can but does not necessarily include a microprocessor to enable remote monitoring or enable remote functions, and/or a non-volatile memory unit to store operating parameters of the optical sub-unit. An electrical dispersion compensation part which compensates the distortion in the electric signal supplied from the receiver part may be provided at the output side of the receiver part. The electrical dispersion compensation part may be provided outside the optical sub-module as a dispersion compensation device.

In some cases, the optical sub-module can but does not necessarily comprise an emitter part so that the optical sub-module is provided with a function of transmitting light signals to the transmission medium. The emitter part emits a light signal modulated according to information transmitted to the optical sub- module, and the optical sub-module imprints the information on an optical carrier which is to be transmitted to the transmission medium. The emitter part includes a lightwave source and a modulator. An optical sub-module having an emitter part and transmitting and receiving a signal light to and from an opposing optical sub-module may be referred to as a transponder. An optical sub-module which has only a function of receiving a signal light from an opposing optical sub-module may be referred to as a receiver module.

In optical communications, the wavelength of the light carrying the information is specified inside a certain wavelength span. In the case of wavelength division multiplexing (WDM)₁ several light carriers transport information on the same transmission medium such as an optical fiber, therefore the specification on multiple wavelengths are more restrictive. Namely, the ITU (International Telecommunication Union) has codified usable wavelengths for several WDM configurations in the ITU standard G694 series documents. Therefore, in the case of an optical sub-module having an integrated light source, as the specified wavelengths are the same for emitter and receiver parts, there is a probability that the wavelength of the emitted signal and the wavelength of the received signal are nominally identical.

In the field of the optical telecommunication system and optical communication network, DPSK modulation schemes are widely used as the modulation and demodulation principle. One form of DPSK modulation is binary DPSK. In this form, the information is coded on two phases separated each from the other by π radians (i.e., 180 degrees). One other form of DPSK commonly employed is DQPSK₁ in which two binary bits of information are coded on four different phases, separated one from the closest other by half π radians (i.e., 90 degrees). DPSK demodulation and DQPSK demodulation can be optically performed by a one-bit delay interferometer (or one-symbol delay interferometer). Therefore, the receiver part of the optical sub-module is generally equipped with the one-bit delay interferometer. In the one-bit delay interferometer, the received optical signal is divided into two branches, one of the two branches is provided with one- bit delay, and the signals propagating on the two branches are then combined and interfered with each other. In this case, the accuracy of the interferometer delay has a direct impact on the demodulation quality, and therefore, on the bit error rate (BER) of the demodulated signal. As a result, setting accurately and controlling precisely 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 section to adjust the delay value has been proposed.

For higher bit rate applications in the optical communication, the delay value decreases, thus the delay setting tolerance becomes critical. Moreover, changes in external parameters such as ambient temperature or internal degradations such as aging of the interferometer tuning section have an influence on the delay accuracy, therefore on the BER of the signal.

For some applications, a fixed delay setting can be sufficient in terms of BER performance. However, for higher bit rates and for lifetime service operations, the degradation of the BER of the received signal is an issue that fixed delay setting cannot resolve. A method to set and control a demodulator using an optical delay interferometer in an optical sub-module is disclosed in JP-A-2005-080304 [1], which corresponds to US-2005/0047780-A1 [2]. In the method described in [1] or [2], information on quality criterion, such as BER, of the demodulated signal is fed back to the optical delay interferometer to adjust the relative delay at the optical delay interferometer. However, in the case of DQPSK or higher-order phase shift coding, there may be more than one tuning section on the demodulator. Having an independent feedback loop on each tuning section requires a BER monitor on each arm, i.e., in-phase and quadrature phase arms in the case of DQPSK, of the interferometer, which is not always available. In addition, BER monitoring may be used for feedback of other parts of an optical sub-module, such as the electrical dispersion compensation part or the compensation device. In that case, as both the demodulator and the compensation device have an influence on the BER of the received signal, the implementation of the feedback can be difficult.

The optical communication system in general consists of many parts and devices in addition to the one-bit delay interferometer. Since each of these parts and the devices may cause the degradation in the BER, it is difficult to independently adjust and set the delay amount in the interferometer based on the monitoring result of the BER to recover the BER. When the delay amount is adjusted by using the fed-back results of the BER monitoring of the received signal, the entire system may reach a local minimum state of the BER, not a global minimum state. An optical receiver of an optical transmission system in which DPSK-DD

(DPSK-direct detection) is used is disclosed in WO2005/088876 [3]. In the receiver of [3], a Mach-Zehnder interferometer is used for demodulating a received light signal and a light signal from one of the outputs of the interferometer is detected by a balanced photodetector. An example of methods for stabilizing the operation of a Mach-Zehnder interferometer is disclosed in WO2005/067189 [4]. In the system of [4], a control light having the same wavelength as a signal light is generated at the transmission side, sent to the reception side and applied to the interferometer which demodulates the signal light. The control light is then extracted from the output of the interferometer and then converted to an electric control signal, and a phase shift amount set to one branch path of the interferometer is controlled based on the control signal to optimize the operation of the interferometer. The control light and signal light are sent to the reception side in a time division manner. Disclosure of the Invention: Problems to be Solved by the Invention:

Thus, there is a need for a simple setting and control method and device for the demodulator on an optical sub-module. Further, there are advantages to the fact that these method and system do not rely on monitoring results of the BER of the received signal. The object of the present invention is to provide a setting method for a demodulator on an optical sub-module without using the result of BER monitoring of a received signal.

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

Means for Solving the Problem:

According to a first exemplary aspect of the present invention, a method to set a demodulator used for an optical signal of a first frequency which is modulated by phase shift keying includes: passing a probe light having a second frequency, which is not equal to the first frequency, inside the demodulator; observing an output intensity of the probe light from the demodulator; and controlling the demodulator based on the observed output intensity to adapt the demodulator to the first frequency.

According to a second exemplary aspect of the present invention, a method to set a demodulator used for an optical signal which is modulated by phase shift keying includes: passing a probe light having a frequency which is the same as a frequency of the optical signal inside the demodulator in an opposite direction to a direction in which the optical signal travels inside the demodulator; observing an output intensity of the probe light from the demodulator; and controlling the demodulator based on the observed output intensity to adapt the demodulator to the frequency of the optical signal. According to a third exemplary aspect of the present invention, a demodulator control system for controlling an optical demodulator which has a first signal port and a second signal port and is used for an optical signal of a first frequency includes: means for generating a probe light having a second frequency which is not equal to the first frequency; means for applying the probe light to the first signal port; means for extracting the probe light form the second signal port; means for observing intensity of the extracted probe light; and means for controlling a transmission characteristic of the demodulator based on the observed intensity to adapt the transmission characteristic to the first frequency.

According to a fourth exemplary aspect of the present invention, a demodulator control system for controlling an optical demodulator which has a first signal port and a second signal port and is used for an optical signal includes: means for generating a 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 form the second signal port; means for observing intensity of the extracted probe light; and means for controlling a transmission characteristic of the demodulator based on the observed intensity to adapt the transmission characteristic to the frequency of the optical signal, wherein the optical signal is applied to the second signal port and travels inside the demodulator from the second signal port to the first signal port.

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

According to a sixth exemplary aspect of the present invention, an optical transponder includes: a light source generating an emitted signal light; an optical modulator for modulating the emitted signal light to transmit the modulated signal light outside; and the demodulator control system described above, wherein the optical demodulator demodulates the optical signal received from the outside and the generating means generates the probe light from the emitted signal light.

In the exemplary aspects of the present invention, the demodulator can be set and adjusted without using the results of the BER monitoring of the received signal light. Further, since the received signal light and the probe light are simultaneously introduced to the demodulator, the demodulator can be adjusted during the normal operation of the demodulator in which the demodulator demodulates the received signal light.

Other principle features and advantages of the invention will become apparent to those skilled in the art by reviewing the following drawings, the detailed descriptions, and the appended claims.

Brief Description of the Drawings:

FIG. 1 is a schematic diagram showing an exemplary DPSK demodulator;

FIG. 2 is a graph representation of the spectral transmission of the DPSK demodulator shown in FIG. 1 ; FIG. 3 is a schematic diagram showing an exemplary DQPSK demodulator;

FIG. 4 is a graphical representation of the spectral transmission of the DQPSK demodulator shown in FIG. 3;

FIG. 5 is a block diagram showing a construction of an optical sub-module according to a first exemplary embodiment of the present invention;

FIG. 6 is a block diagram showing an example of an arrangement of a frequency separator for separating optical frequency, which can be used in various 29

exemplary embodiments of the present invention.

FIG. 7 is a block diagram showing a construction of an optical sub-module according to a second exemplary embodiment of the present invention;

FIG. 8 is a block diagram showing a construction of an optical sub-module of a modification of the second exemplary embodiment;

FIG. 9 is a block diagram showing a construction of an optical sub-module according to a third exemplary embodiment of the present invention;

FIG. 10 is a graph representation of the spectral transmission of the demodulator and the fiber Bragg grating (FBG) in an exemplary optical sub- module of the third exemplary embodiment;

FIG. 11 is a block diagram showing a construction of an optical sub- module according to a fourth exemplary embodiment of the present invention;

FIG. 12 is a block diagram showing a construction of an optical sub- module of a modification of the fourth exemplary embodiment; FIG. 13 is a block diagram showing an exemplary construction of a transponder;

FIG. 14 is a block diagram showing another exemplary construction of the transponder;

FIG. 15 is a block diagram showing a further exemplary construction of the transponder; and

FIG. 16 is a block diagram showing a still further exemplary construction of the transponder.

Detailed Description of Exemplary Embodiments:

In one exemplary embodiment of the present invention, a demodulator for differential phase shift keying (DPSK) modulation in an optical sub-module in an optical communication system is set and controlled. Wave light originates from an external source or a source implemented on the same optical sub-module as the demodulator is tapped to monitor the setting of the demodulator and a feedback loop to control and set the demodulator. The wavelength of the tapped light is shifted by an optical frequency shifter and the tapped light is used as a probe light for controlling and setting the demodulator. Alternatively, an independent light source for generating a probe light which has a shifted frequency from the received signal light is arranged and the probe light is coupled to the path of the received signal light. The difference in wavelength between received signal light and the probe light will be described later. The use of the optical frequency shifter has advantages that the separate light source for the probe light is not necessary and the adjustment of the wavelength difference is facilitated.

The probe light, i.e., the tapped light, passes through the demodulator so that the demodulator setting affects, by its transmission characteristics, the optical power of the probe light after passing the demodulator. The probe light is then separated from the received signal light and its intensity is converted to an electric signal by a photoelectric detector. The feedback loop compares the electric signal to a probe light corresponding to the ideal setting of the demodulator and tunes the demodulator to the ideal setting.

In one exemplary embodiment, modulation format is binary DPSK and the probe light and the received signal light are traveling in the same direction inside the demodulator. Coupling of the probe light on the received signal light is done with a coupler and separation of the probe light and the signal light is performed by an optical filter or by spatial separation with a dispersive element.

In another exemplary embodiment of the present invention, modulation format is binary DPSK and the probe light and the received signal light are traveling in the opposite directions inside the demodulator. Coupling of the probe light on the signal light is performed by an optical coupler and separation of the probe light and the signal light is performed by an optical circulator, or by an optical filter or by spatial separation with a dispersive element.

In a further exemplary embodiment, modulation format is differential quadrature phase shift keying (DQPSK) or higher-order DPSK, and the probe light and the received signal light are traveling in the same direction inside the demodulator. Coupling of the probe light and the received signal light is done with a coupler and separation of the probe light and the signal light is performed on each arm, i.e., in-phase (I) arm and quadrature (Q) phase arm, by an optical filter or by spatial separation with a dispersive element. Therefore, there are two separated monitoring light signal, one for each arm and the photoelectric detector and feedback loop are implemented for each the monitoring light signal. Here, the higher-order DPSK means N-ary DPSK or N-level DPSK, N being an integer greater than 4, such as differential 8-level phase shift keying (D8PSK) and differential 16-level phase shift keying (D16PSK). An example of the optical D8PSK demodulator is disclosed in 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) [5], and an example of D16PSK is disclosed in 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) [6]. In a further exemplary embodiment, modulation format is DQPSK or higher-order DPSK, and the probe light and the received signal light are traveling in the opposite directions inside the demodulator. Coupling of the probe light and the received signal light is done with a switching element and a coupler on each arm (i.e., I and Q arms) of the demodulator so that coupling is performed alternatively on each arm, one at a time. Separation of the probe light and the signal light is performed by an optical circulator or an optical filter or by spatial separation with a dispersive element. Therefore, there is only one separated monitoring light signal, one photoelectric detector and feedback loop are implemented. The feedback on both arms is performed alternatively, one arm at a time corresponding to alternation of the probe light on each arm of the demodulator. The reference electric signal used for feedback is provided alternatively, in respect to the arm on which feedback is enabled.

As a further exemplary embodiment, the continuous lightwave source can be tunable and tuned during operation of it. The optical frequency shifter shifts the optical frequency of the lightwave after it is tapped by an amount such that the shifted frequency can be separated enough from any optical frequency that the received signal light may have. As an example, if the usable optical frequencies for the received signal and the light source are located on a regular grid such as the grid defined by the ITU, the optical frequency shifting amount can be constant for any wavelength emitted by the light source. In this aspect, the separation means is tunable if it is an optical filter or a dispersive element and tuned to separate the tapped light with shifted optical frequency. The electrical signal reference is provided for each eventual feedback loop depending on the wavelength of the tunable lightwave source.

In accordance with some exemplary embodiments of the invention, when the electrical signal reference for the feedback loop is susceptible to have different values, depending on the wavelength of the probe light or the demodulator arm on which the probe light is coupled, the reference value is provided by a processing unit. The processing unit can be already implemented on the optical sub-module if one is already implemented or a different unit. As an aspect, the processing unit computes the reference value from the wavelength of the probe light, the power of the lightwave source, the optical spectral transmission properties of the demodulator, the spectral transmission of physical elements of the optical sub- module and a reference couple of calibrated data, which comprises the particular value of the electrical signal reference for a particular wavelength and the particular wavelength. As another aspect and for the concern of precision of optical elements, the processing unit provides the reference value from stored values depending on the probe light wavelength and the lightwave source emitting power. The list of the stored values is implemented at the calibration of the optical sub-module.

In a further exemplary embodiment of the invention, the reference value for feedback can be normalized to a chosen peak value in the spectral transmission property of the demodulator. In that aspect, a scanning of the spectral transmission property of the demodulator by tuning the demodulator around the chosen peak at the startup of the optical sub-module makes it possible to record the electrical voltage value at the peak and thus to calculate the reference value when the feedback is operated.

In the event of a change or drift in the demodulator setting, an exemplary embodiment detects the change or the drift. Moreover, the exemplary embodiment enables the demodulator to be tuned to cancel the effect of the change or the drift. The degradation in the BER that would occur is suppressed by the exemplary embodiments.

Tuning of and feedback on the demodulator setting according to the exemplary embodiments is possible when the demodulator is in the process of demodulating received signal light, and does not affect the process.

In the case where the received signal does not implement BER monitoring function or when this function is not implemented for all arms of the demodulator, an exemplary embodiment of the present invention enables to monitor the setting of each the arm of the demodulator and to tune each the arm independently.

Tuning of and feedback on the demodulator setting according to the exemplary embodiments is stable in spite of any BER spurious fluctuations due to any cause independent of the optical sub-module.

Tuning of and feedback on the demodulator setting according to the exemplary embodiments does not depend on other factors which affect the BER of a transmitted signal. Therefore, the implementation of feedback on active compensation devices, as for instance electric dispersion compensation device, using BER monitor is still trivial when the present invention is implemented.

According to the above exemplary embodiments, demodulator is accurately tuned and controlled independently of the wavelength of the received signal light. Even in the event of the received signal light and the probe light having the same wavelength, the demodulator is tuned and controlled without disturbing any demodulation process in progress at the same time.

The present invention does not generate any noise or disturbing signal at any wavelength that the received signal light is susceptible to have.

Before explaining the particular exemplary embodiments of the present invention, the configuration of a DPSK demodulator and a DQPSK modulator will be described.

FIG. 1 is schematic representation of an exemplary DPSK demodulator based on a one-bit delay interferometer. Such device is widely used. The DPSK demodulator 100 is constituted as a waveguide-type Mach-Zehnder interferometer, and has a tunable section 101 inserted in one optical path 103 of the interferometer and a one-bit delay section 102 inserted in the other optical path 104 of the interferometer. Each of optical paths 103, 104 is configured as a waveguide. A received signal light 110 is branched into the two optical paths 103, 104 by a beam splitter 120. A 3-dB coupler may be used instead of the beam splitter 120. The tunable section 101 generally includes an electronic heater which heats a portion of the optical path 103, and is provided for setting the demodulator 100 according to wavelength of the received signal light 110. When electric power applied to the heater of the tunable section is varied, the refractive index of the optical path 103 near the portion is also varied, and the phase of the lightwave propagating on the optical path 103 is thus varied in a range of several or several tens pieces of waves of the lightwave which is used as a carrier for the signal. Therefore, the delay of the light signal on the optical path 103 is controllable by varying the power applied to the tunable section 101. The one-bit delay section 102 includes a waveguide having a certain length which corresponds to the time duration of one bit of the signal. The light signal propagating on the optical path 104 delays for the time duration of one bit and indicates a signal state just before one bit.

The optical path 103, 104 are again joined at the directional coupler 105, and a constructive output 111 and a destructive output 112 are extracted from directional coupler 105. Since the light signal propagating on the optical path 104 delays for the time duration of one bit in comparison with the light signal propagating on the optical path 103, the output signal light from the directional coupler 105 gives the demodulation result of the received signal light which had been modulated by DPSK. The constructive and destructive outputs 111, 112 can be connected to a balanced photo-detector to detect received signal light 110 demodulated by the demodulator 101.

In order to precisely demodulate the received signal light and improve the BER, it is necessary to align the phases of the carrier lightwave which propagates on both optical paths 103, 104 to the directional coupler 105. This is the reason for inserting the tunable section 101 in the one optical path 103.

FIG. 2 shows the typical spectral transmission of the DPSK demodulator 100 for the constructive output 111. The demodulator 100 is set according to the optical frequency of the received light signal 110, which value is noted as f_r. FIG. 2 illustrates that the light intensity at the constructive output periodically changes in accordance with the change in the optical frequency of the received light signal 100. The goal of the present exemplary embodiment is to dispose the peak of the transmission at the frequency point of f_r. In FIG. 2, f_s indicates optical frequency of the probe light and f_t indicates optical frequency of light emitted from the emitter part of the optical sub-unit. These optical frequencies f_s and f_t will be described later. FIG. 3 is a schematic representation of an exemplary DQPSK demodulator based on a one-symbol delay interferometer. Such demodulator is widely used.

As shown in FIG. 3, the DQPSK demodulator 300 generally comprises two arms, i.e., an in-phase (I) arm and a quadrature phase (Q) arm. Each arm consists of a single interferometer with a one-symbol delay section and has a similar configuration as the binary DPSK demodulator 100 illustrated in FIG. 1. The received signal light 310 is distributed into I and Q arms by a beam splitter 320.

The I arm comprises a beam splitter 321, a tuning section 301 , a phase adjustment section 302, a one-symbol delay section 305, and a directional coupler 322. The tuning section 301 and the phase adjustment section 302 is provided on one optical path between the beam splitter 321 and the directional coupler 322 while the one-symbol delay section 305 is provided on the other optical path. Similarly, the Q arm comprises a beam splitter 323, a tuning section 303, a phase adjustment section 304, a one-symbol delay section 306, and a directional coupler 324. Each of the one-symbol delay sections 305, 306 includes a waveguide having a certain length which corresponds to the time duration of one symbol of the signal. The tuning sections 301, 303 are provided for tuning the I and Q arms, respectively, to the frequency of the received signal light 310. The phase adjustment sections 302, 304 are provided for giving a certain phase difference between the passed lights of the I and Q arms. In this case, delay amount which is set by the phase adjustment section 302 in the I arm can be π/4 radian, and the delay amount which is set by the phase adjustment section 304 in the Q arm can be -π/4 radian.

The I arm has a constructive output 311 and a destructive output 312, which are outputs of the directional coupler 322 and can be connected to a balanced photo-detector to receive I tributary of the signal 310 demodulated by demodulator 300. Similarly, the Q arm has a constructive output 313 and a destructive output 314 which are outputs of the directional coupler 324. The Q tributary of signal 310 demodulated by demodulator 300 can also be received at a balanced photo-detector connected to the outputs 313, 314. Configuration shown in FIG. 3 can be used for higher-order DPSK demodulator with an appropriate configuration of the phase of each arm.

FIG. 4 is the typical spectral transmission of the DQPSK demodulator 300. Curve 401 illustrates the spectral transmission for the constructive output 311 of the I arm and curve 402 for the constructive output 313 of the Q arm. Since the peaks of both curves 401 , 402 do not coincide with each other in principle, the demodulator 300 is set according to the optical frequency f_r of the received signal light 310 at which the transmission in the I and Q arms is relatively high and both transmission values are equal to each other.

FIG. 5 illustrates an optical sub-module according to a first exemplary embodiment of the present invention. The optical sub-module includes: a continuous lightwave source 501, a DPSK demodulator 504, a balanced photodetector (PD) 505, a divider 511 , an optical frequency shifter 512, a coupler 513, a notch filter 514, a frequency separator 515, a photodiode 516, a feedback circuit 517 and a processing unit 518. In this exemplary embodiment, the DPSK demodulator 504 has the same construction as the demodulator 100 shown in FIG. 1 and is set and controlled. The balanced photodetector 505 comprises a typical balanced photodiode (PD) and is used for converting the received signal light 503, having an optical frequency of f_r, after it has been demodulated by demodulator 504 into an electric signal. The continuous lightwave source 501 emits signal light 502 at optical frequency f_t. The light source 501 may be on the same optical sub-module as the demodulator 504 or remote. In some cases, the optical frequency f_t can be the same as the optical frequency f_r. This frequency configuration is useful for performing 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 for transmitting information to another optical sub-module (not shown). In this arrangement, a portion of light from the light source 501 is tapped or branched at the divider 511 , which is typically constructed as a directional coupler. As the optical frequency shifter 512, several structures of such a device have been reported. For example, a device disclosed in Shibuya ef a/., "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) can be used for the optical frequency shifter 512. The device of Shibuya et a/, is based on an electrooptic traveling phase grating which uses LiTaθ₃ crystal. However, other known devices which rely on acousto-optic devices, non-linear optical phenomenon or optical parametric generation can be used for the optical frequency shifter 512. The optical frequency shifter 512 shifts the frequency of the tapped light from f_t to f_s. The tapped light at the frequency f_s is used as the probe light and coupled with the received signal light 503 having frequency of f_r, by the coupler 513. Then both lightwaves travel inside demodulator 504 in the same direction.

The notch filter 514 is connected to one of the two outputs, i.e., constructive and deconstructive output ports 111, 112 shown in FIG. 1 , of the demodulator 504 and the frequency separator 515 is connected to the other of the output ports. The frequency separator 515 is provided for extracting light at frequency f_s. An exemplary structure of the frequency separator 515 is represented on FIG. 6, by reference numeral 900.

The frequency separator 900 shown in FIG. 6 is provided with the first to third ports A to C and has a well known structure based on a fiber Bragg grating (FBG) 912 which reflects the wave of frequency f_s. The separator 900 further includes an optical circulator 911 , of which the orientation is chosen to isolate f_s on the third port C. The frequency separator 515 is placed via the first port A thereof on one output port of demodulator 504, i.e., the constructive or destructive output, and it is oriented in order to separate the tapped signal of frequency f_s and to send the tapped signal to photodiode 516 via the third port C. The received signal of frequency f_r is isolated from f_s, and the component of f_r which passes through the FBG 912 is sent to the balanced photodetector 505 via the second port B. The notch filter 514 is provided for rejecting a component of frequency f_s and calibrated to have the same loss on frequency f_r as the frequency separator 515.

If a heater for varying temperature of the FBG 912 is provided, it is possible to make the frequency separator 900 tunable. The tunable frequency separator is useful when the light source 501 is tunable and frequency f_t is controlled based on a external command applied to the light source 501.

The photocurrent delivered by the photodiode 516 is proportional to the transmission of the demodulator 504 for the frequency f_s. In the case of a change of setting of the demodulator 504, the transmission curve shown in FIG. 2 shifts and the photocurrent changes. The photocurrent is supplied to the feedback circuit 517, such as a comparator, of which the reference photocurrent is delivered by the processing unit 518 and corresponds to the value corresponding to the transmission of the demodulator 504 ideally set for frequency f_r. In FIG. 2, transmission at frequency f_s which corresponds to the ideal transmission at frequency f_r is denoted by T₀. Therefore, the goal of the adjustment is to set the observed transmission for frequency f_s to T₀. It should be noted that transmission T₀ does not necessarily coincide with the peak transmission for frequency f_s. In order to accurately set the transmission peak of the demodulator 504 to the frequency f_r of the received signal light, it is preferable select the frequency f_s of the probe light such that the frequency f_s is located in the frequency region where the transmission curve has a large gradient. In many cases, the transmission curve is sinusoidal, and the frequency f_s is preferably set to a frequency corresponding to the middle value between the highest and lowest values of the transmission curve.

The feedback circuit 517 tunes the demodulator 504 in function of the error signal generated by the comparison of the photocurrent and the reference value. In the demodulator 504, tunable section 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 (i.e., the probe light) are simultaneously introduced to the demodulator 504. Therefore, the demodulator 504 can be adjusted during the normal operation of the demodulator 504 in which the demodulator 504 demodulates the received signal light. In this exemplary embodiment, the lightwave source 501 is tunable. The frequency shift (f_s-f_t) imprinted by the frequency shifter 512 is chosen so that for any frequency susceptible to be emitted by light source 501 , f_s is different from all frequency that received signal 503 is susceptible to have. In a case such as wavelength division multiplexing to which the ITU has codified usable wavelengths in the G694 series documents, where f_r and f_t are placed on the same grid of optical frequencies, a constant value of the frequency shift (f_s-f_t) can fulfill this condition. In addition, the frequency shift must be chosen so that the frequency separator 515 can resolve f_s and f_r. In the present embodiment, the processing unit 518 consists of a microprocessor, a digital analog converter to generate the reference signal to be transmitted to the feedback circuit 517, and a nonvolatile memory storage device. These components can be integrated or not, and used on components implemented on the optical sub-module as the demodulator 504 if they are implanted. The information of the frequency and optical power emitted by the lightsource 501 is available to the processing unit 518, and the processing unit 518 selects the corresponding reference signal value which is to be transmitted to the feedback circuit 517 according to this information. Rather than an absolute value, a relative value taken by normalizing by the photocurrent value at the local peak nearest to f_s is stored in the memory part of the processing unit 518. At the startup of the optical sub-module with the demodulator 504, a scan around the local peak nearest f_s is performed by tuning the demodulator 504 and by measuring the corresponding photocurrent values. The photocurrent value at the peak enables to retrieve the absolute value of the reference signal. The memory part stores the normalized reference values in a table manner, i.e., a two-dimensional array manner or a lookup table manner. The first column of the array withholds normalized reference values and the second column withholds the corresponding f_t values.

The frequency separator 515 and notch filter 514 must be tuned to frequency f_s, which is controlled by the processing unit 518. The processing unit 518 knows the information of frequency f_t and the frequency stored in the memory. FIG. 7 shows an optical sub-module according to the second exemplary embodiment. The optical sub-module shown in FIG. 7 has the similar construction as that shown in FIG. 5, but is different from the sub-module shown in FIG. 5 in the direction in which the probe light passes through the DPSK demodulator. The optical sub-module includes: a continuous lightwave source 601 , a DPSK demodulator 604, a balanced photodetector (PD) 605, a divider 611 , an optical frequency shifter 612, an optical circulators 613, 615, a filter 614, a photodiode 616, a feedback circuit 617 and a processing unit 618. In this embodiment, the DPSK demodulator 604 has the same construction as the demodulator 100 shown in FIG. 1 and is set and controlled. The continuous lightwave source 601 emits signal light 602 at optical frequency f_t. The light source 601 may be on the same optical sub-module as the demodulator 604 or remote.

In this arrangement, a portion of light from the light source 601 is tapped or branched at the divider 611 , which is typically constructed as a directional coupler. Tapped light at the frequency f_s is coupled on the path of the received signal light 603, at frequency f_r, on the constructive output by the optical circulator 613 which is disposed between one of outputs of DPSK demodulator 604 and photodetector 605. The filter 614 which is disposed between the other output of DPSK demodulator 604 and photodetector 605 is calibrated to have the same loss on frequency f_r as circulator 613. Since the signal component of frequency f_s does not pass through the filter 614 in this arrangement, 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. The signal light of frequency f_r and the tapped light of frequency f_s travel inside demodulator 604 in the opposite directions. An optical circulator 615 is provided at the input of the DPSK demodulator 604 in order to separate the tapped signal of frequency f_s and to send it to photodiode 616. The spectral range of each of optical circulators 613, 615 is wider than the range of frequencies allowed for f_s and f_t.

In this configuration, since the tapped light, i.e., the probe light, and the signal light travel inside the DPSK demodulator 604 in opposite directions, the frequency f_s of the tapped light can be the same as the frequency f_r of the signal light. Therefore, in one modification, light source 601 can emit the light signal 602 at frequency f_r which is then introduced to the circulator 613 without passing through an optical frequency shifter. FIG. 8 shows such an optical sub-module which is not equipped with an optical frequency shifter. In this modification, the feedback circuit 617 controls the demodulator 604 such that the detected value at the photodiode 616 is maximized. This modification is useful for performing bidirectional 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 the third exemplary embodiment which employs a DQPSK demodulator 704 instead of a DPSK demodulator. In this embodiment, the DQPSK demodulator 704 has the same construction as the demodulator 300 shown in FIG. 3 and is set and controlled. The optical sub-module shown in FIG. 9 has a configuration in which a combination of the balanced photodetector (PD) 505, notch filter 514, frequency separator 515, photodiode 516, and feedback circuit 517 shown in FIG. 5 is provided for each of the I and Q arms. More specifically, the optical sub-module shown in FIG. 9 includes: a continuous lightwave source 701, the DQPSK demodulator 704, balanced photodetectors 705, 706, a divider 711 , an optical frequency shifter 712, a coupler 713, notch filters 714, 715, frequency separators 716, 717, photodiodes 718, 719, feedback circuits 720, 721 and a processing unit 722. The continuous lightwave source 701 emits signal light 702 at optical frequency f_t. The light source 701 may be on the same optical sub-module as the demodulator 704 or remote. The balanced photodetector 705, 706 receive, respectively, the I and Q tributaries of the signal demodulated by demodulator 704. Feedback circuits 720, 721 tune the I and Q arms of demodulator 704, respectively, in the same way as the feedback circuit 517 with the demodulator 504 on FIG. 5. The reference signal for each feedback circuit is given by the same processing unit 722, which is the same as processing unit 518 but working in a dual way. Instead of one list of stored normalized reference values, there are two arrays of the same construction describe above, one for each arm of the demodulator 704.

The frequency separators 716, 717 and filters 714, 715 must be tuned to frequency f_s, which is controlled by the processing unit 722. The processing unit 722 has the information of frequency f_t and the frequency stored in the memory.

Next, an example of operation of the optical sub-module according to this exemplary embodiment will be explained. It is assumed that the frequency f_r of the signal light is compliant with the 100 GHz frequency grid and 100 GHz DQPSK modulation format is used.

The FSR (free spectral range) of the DQPSK demodulator is 50 MHz and the frequency f_r must be adjusted to a frequency shift of 12.5 GHz (i.e., π/4 ) from the modulator transmission peak, corresponding to the point of maximum slope of the linear transmission curve of the DQPSK demodulator 704. The linear transmission curve is illustrated by a solid line in FIG. 10. The horizontal axis of FIG. 10 represents a frequency difference Δ/ from one of the demodulator transmission peaks.

The worst case is that frequency f_t from the light source 701 equals to the frequency f_r of the received signal light, i.e., f_t=f_r. Therefore, the frequency f_s of the probe light must be chosen so that it is different from f_r, on a steep slope area of the transmission curve and outside any other possible values on the 100 GHz grid. The optimal value is f_s = f_t+25 GHz, corresponding to a frequency shift of 25 GHz. This frequency f_s is then on the point of maximum slope on the slope adjacent to the one of frequency f_r is located. The frequency separation is performed by the notch filter 716 composed of a fiber Bragg grating (FBG) 912 (see FIG. 6) and the circulator 911. The FBG can be of 25 GHz band pass, centered on the frequency middle of the frequency f_s and the next transmission peak. Since the frequencies of f_s and the next transition 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 reflection band of the FBG, and f_r and f_r+100 GHz are in the transmission bands. The linear transmission characteristics, as well as f_r and f_t are depicted on FIG. 10, where the reference is taken on the closest transmission curve from f_r.

If 4 dBm is a level of the tapped light from the source, in the case of the optical frequency shifter 712 with an efficiency of 50% and losses of 1 dB, the output light of f_s from the shifter has a power of -3 dBm in front of the demodulator 704. After passing a typical DQPSK demodulator with 6 dB losses and the notch filter (1 dB loss at the circulator), the power on the monitoring photo diode 718 for f_s is -10 dBm. Considering the a power of 12 dBm of the received signal at frequency f_r in front of the demodulator 704, the power at frequency f_r on the monitor photodiode 718 would be -20 dBm for a typical reflectance of -25 dB for the FBG. The power on the signal receiver 705 of the signal at frequency f_r is 6 dBm in that case, and the power of the control signal at f_s on the photodetector 705 is -39 dBm for a transmission of -30 dBm of the FBG 912 in the frequency separator 716.

FIG. 11 shows an optical sub-module according to the fourth exemplary embodiment. The optical sub-module shown in FIG. 11 has the similar construction as that shown in FIG. 9, but is different from the sub-module shown in FIG. 9 in the direction in which the probe light passes through the DQPSK demodulator. The optical sub-module includes: a continuous lightwave source 801, a DQPSK demodulator 804, balanced photodetectors 805, 806, a divider 811, an optical frequency shifter 812, an optical switch 813, filters 814, 815, optical circulators 816 to 818, a photodiode 819, a feedback circuit 820 and a processing unit 821. Since the signal component of frequency f_s does not pass through the filters 814, 815, a simple filter can be used as each of the filters 814, 815. In this embodiment, the DQPSK demodulator 804 has the same the construction as demodulator 300 shown in FIG. 3 and is set and controlled. The continuous lightwave source 801 emits signal light 802 at optical frequency f_t. The light source 801 may be on the same optical sub-module as the demodulator 804 or remote.

Optical switch 813 is controlled by the processing unit 821 to send the tapped light with the frequency f_s alternatively to circulators 816, 817, which are placed on the constructive outputs of the I and Q arms, respectively. The filters 814, 815 which may be simple filters and are placed on the deconstructive outputs of the I and Q arms, respectively, are calibrated to have the same loss on frequency f_r as circulators 816, 817. Tapped light with the frequency f_s and received signal light with the frequency f_r are traveling in the opposite directions inside the demodulator 804. The optical circulator 615 is provided at the input of the DQPSK demodulator 804 in order to separate the tapped signal of frequency f_s and to send it to photodiode 819. Photocurrent delivered by the photodiode 819 corresponds to light with the frequency f_s having passed through the I arm of demodulator 804 or through the Q arm depending on the position of the switch 813. The feedback circuit 820 works as feedback circuits 720, 721 shown in FIG. 9. The feedback circuit 820 is able to tune the respective arms of the demodulator 804 independently and alternately. The arm to be tuned and the reference value are decided by the processing unit 821 , which is similar to the processing unit 722, with the same double array. In addition, the processing unit 821 controls the switch 813 to coordinates the circulator 818 and the feedback circuit 820. In this configuration, since the tapped light, i.e., the probe light, and the signal light travel inside the DQPSK demodulator 704 in opposite directions, the frequency f_s of the tapped light can be the same as the frequency f_r of the signal light. Therefore, in one modification, the light source 801 can emit the light signal 802 at frequency f_r which is then alternately introduced to the circulators 816, 817 without passing through an optical frequency shifter. FIG. 12 shows such an optical sub-module which is not equipped with an optical frequency shifter. In this modification, the feedback circuit 820 controls the demodulator 804 such that the detected value at the photodiode 819 is maximized. This modification is useful for performing bidirectional communication between this optical sub-module and another optical sub-module using the same optical frequency

The optical sub-modules according to the third and fourth exemplary embodiments may applied to the cases in which a higher-order DPSK demodulator such as a D8PSK or D16PSK demodulator is used instead of the DQPSK demodulator. Han et al. [5] show two D8PSK receiver structures. One of these structures has four delay interferometers while the other has two delay interferometers. Each delay interferometer is similar to the one shown in FIG. 3, with different additional phase values. Including a tunable phase structure in those interferometers as shown in Han ef al. enables to use the current method, with the correct number of detection apparatuses.

In the same manner, Sambaraju ef a/. [6] shows a D16PSK (16-ary DPSK) detector based on six delay interferometers, each of which is similar to one shown in FIG. 3, but with different additional phases values. There again, such a structure with tunable phase structures can be used in combination with the current method and the correct number of detection apparatuses.

Each of the optical sub-modules in the above exemplary embodiments can be modified as a transponder which has a function of transmitting and receiving a signal light to and from an opposing optical sub-module. FIG. 13 illustrates a transponder in which a DPSK modulator 519 for modulating the emitted signal light 502 having the frequency f_t is added to the optical sub-module shown in FIG. 5. FIG. 14 illustrates a transponder in which a DPSK modulator 619 for modulating the emitted signal light 602 having the frequency f_t (=f_r) is added to the optical sub- module shown in FIG. 8. FIG. 15 illustrates a transponder in which a DQPSK modulator 723 for modulating the emitted signal light 702 having the frequency f_t is added to the optical sub-module shown in FIG. 9. FIG. 16 illustrates a transponder in which a DQPSK modulator 822 for modulating the emitted signal light 802 having the frequency f_t (=f_r) is added to the optical sub-module shown in FIG. 12.

All exemplary embodiments are operational for all values of f_t and f_r allowed. The information on f_t is known and transmitted to their respective processing unit. The information on f_r is not necessary. While exemplary embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.

Claims

1. A method to set a demodulator used for an optical signal of a first frequency which is modulated by phase shift keying, the method comprising: passing a probe light having a second frequency, which is not equal to the first frequency, inside the demodulator; observing an output intensity of the probe light from the demodulator; and controlling the demodulator based on the observed output intensity to adopt the demodulator to the first frequency.

2. The method according to claim 1 , wherein the probe light is introduced to the demodulator while the optical signal passes through the demodulator.

3. The method according to claim 1 or 2, wherein the optical signal and the probe light travel in the same direction inside the demodulator.

4. The method according to claim 1 or 2, wherein the optical signal and the probe light travel in opposite directions inside the demodulator.

5. The method according to any one of claims 1 to 4, wherein the demodulator is a differential quadrature phase shift keying demodulator or higher- order differential quadrature phase shift keying demodulator than the differential quadrature phase shift keying demodulator.

6. The method according to any one of claims 1 to 5, further comprising: generating an emitted signal light; tapping a portion of the emitted signal light; and shifting a frequency of the tapped portion of the emitted signal light to generate the probe light.

7. The method according to claim 6, wherein the frequency of the emitted signal light is equal to the first frequency.

8. A method to set a demodulator used for an optical signal which is modulated by phase shift keying, the method comprising: passing a probe light having a frequency which is the same as a frequency of the optical signal inside the demodulator in an opposite direction to a direction in which the optical signal travels inside the demodulator; observing an output intensity of the probe light from the demodulator; and controlling the demodulator based on the observed output intensity to adapt the demodulator to the frequency of the optical signal.

9. The method according to claim 8, wherein the demodulator is a differential quadrature phase shift keying demodulator or higher-order differential quadrature phase shift keying demodulator than the differential quadrature phase shift keying demodulator.

10. A demodulator control system for controlling an optical demodulator which has a first signal port and a second signal port and is used for an optical signal of a first frequency, comprising: means for generating a probe light having a second frequency which is not equal to the first frequency; means for applying the probe light to the first signal port; means for extracting the probe light form the second signal port; means for observing intensity of the extracted probe light; and means for controlling a transmission characteristic of the demodulator based on the observed intensity to adapt the transmission characteristic to the first frequency.

11. The system according to claim 10, wherein the optical signal is applied to the first signal port and travels inside the demodulator from the first input port to the second signal port.

12. The system according to claim 10, wherein the optical signal is applied to the second signal port and travels inside the demodulator from the second signal port to the first signal port.

13. The system according to any one of claims 10 to 12, wherein the demodulator is a differential quadrature phase shift keying demodulator or higher- order differential quadrature phase shift keying demodulator than the differential quadrature phase shift keying demodulator.

14. The system according to claim 12, wherein the demodulator is a differential quadrature phase shift keying demodulator, and the system further comprises a switch for alternately supplying the probe light to the first signal port for an in-phase arm of the demodulator and the first signal port for a quadrature arm of the demodulator.

15. The system according to any one of claims 10 to 14, wherein the generating means is a light source for generating emitted signal light which is used for transmitting a signal outside.

16. The system according to claim 15, further comprising means for shifting a frequency of the emitted signal light to generate the probe light.

17. The system according to claim 16, wherein the frequency of the emitted signal light is equal to the first frequency.

18. A demodulator control system for controlling an optical demodulator which has a first signal port and a second signal port and is used for an optical signal, comprising: means for generating a 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 form the second signal port; means for observing intensity of the extracted probe light; and means for controlling a transmission characteristic of the demodulator based on the observed intensity to adapt the transmission characteristic to the frequency of the optical signal, wherein the optical signal is applied to the second signal port and travels inside the demodulator from the second signal port to the first signal port.

19. The system according to claim 18, wherein the demodulator is a differential quadrature phase shift keying demodulator, and the system further comprising a switch for alternately supplying the probe light to the first signal port for an in-phase arm of the demodulator and the first signal port for a quadrature arm of the demodulator.

20. An optical receiver module comprising: an optical demodulator which has a first signal port and a second signal port and demodulates an optical signal of a first frequency; means for generating a probe light having a second frequency which is not equal to the first frequency; means for applying the probe light to the first signal port; means for extracting the probe light form the second signal port; means for observing intensity of the extracted probe light; and means for controlling a transmission characteristic of the demodulator based on the observed intensity to adapt the transmission characteristic to the first frequency.

21. An optical receiver module comprising: an optical demodulator which has a first signal port and a second signal port and demodulates an optical signal; means for generating a 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 form the second signal port; means for observing intensity of the extracted probe light; and means for controlling a transmission characteristic of the demodulator based on the observed intensity to adapt the transmission characteristic to the frequency of the optical signal, wherein the optical signal is applied to the second signal port and travels inside the demodulator from the second signal port to the first signal port.

22. An optical transponder, comprising: a light source generating an emitted signal light; an optical modulator for modulating the emitted signal light to transmit the modulated signal light outside; an optical demodulator which has a first signal port and a second signal port and demodulates an optical signal of a first frequency received from the outside; means for generating a probe light having a second frequency, which is not equal to the first frequency, from the emitted signal light; means for applying the probe light to the first signal port; means for extracting the probe light form the second signal port; means for observing intensity of the extracted probe light; and means for controlling a transmission characteristic of the demodulator based on the observed intensity to adapt the transmission characteristic to the first frequency.

23. An optical transponder, comprising: a light source generating an emitted signal light; an optical modulator for modulating the emitted signal light to transmit the modulated signal light outside; an optical demodulator which has a first signal port and a second signal port and demodulates an optical signal received from the outside; means for generating a probe light having the same frequency as the optical signal from the emitted signal light; means for applying the probe light to the first signal port; means for extracting the probe light form the second signal port; means for observing intensity of the extracted probe light; and means for controlling a transmission characteristic of the demodulator based on the observed intensity to adapt the transmission characteristic to the frequency of the optical signal, wherein the optical signal is applied to the second signal port and travels inside the demodulator from the second signal port to the first signal port.