JP2009027442A - Optical receiving circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform the synchronous detection of a QAM signal without using a pilot signal, in an optical receiving circuit. <P>SOLUTION: The optical receiving circuit performs heterodyne detection using local oscillation light having a wavelength nearly equal to the wavelength of a received signal, samples the detected output with a clock signal extracted from the received optical signal, selects a symbol having a prescribed amplitude from the sampling output, finds the phase difference between a carrier of the received optical signal and the local oscillation light on the basis of a sampling output corresponding to the selected symbol, and demodulates data from the phase difference and the sampling output. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

In the present invention, quadrature amplitude modulation (Quadrature Amplitude) is used as a modulation code.
The present invention relates to a multilevel optical transmission / reception technology using Modulation (QAM).

  In the backbone optical transmission system, a WDM transmission technique for multiplexing and transmitting a plurality of wavelengths in one optical fiber is applied, and economical and large-capacity information transmission is realized.

  As a modulation / demodulation method used in a conventional WDM transmission apparatus, conventionally, there is an IM-DD (Intensity Modulation Direct Detection) method in which binary intensity modulation is performed by turning on / off light intensity and detection is directly performed by a photodiode on the receiving side. It was general.

  In recent years, multilevel optical transmission / reception techniques have been studied in order to effectively use a limited optical transmission band and improve frequency utilization efficiency. Up to now, transmission using a 64-QAM modulation method with a symbol rate of 1 GSymbol / s has been reported (for example, see Non-Patent Document 1).

IEICE, "Proceedings of the 2007 IEICE General Conference", March 20-23, 2007, B-10-69, P408

  However, the conventional multilevel optical transmission / reception technology has the following problems. For the receiving side, synchronous detection is required. For example, in the technique described in Non-Patent Document 1, a pilot signal is transmitted by being superimposed on the main signal, and the local oscillation light source on the receiving side is phase-synchronized with the pilot signal. Therefore, heterodyne reception is performed.

  In such a transmission method for superimposing pilot signals, it is necessary to secure an extra band for the pilot signals, and there is a problem that a decrease in frequency utilization efficiency is inevitable.

  The present invention has been made in view of such a background, and an object of the present invention is to provide an optical receiving circuit that enables synchronous detection of a QAM signal without using a pilot signal.

  The present invention is an optical receiver circuit, and a feature of the present invention is that a local oscillation light source that outputs a local oscillation light having a wavelength substantially equal to the wavelength of the reception optical signal, a reception signal and the local oscillation light are mixed. Output optical 90 ° hybrid coupler, a first OE converter that converts an output signal from the optical 90 ° hybrid coupler from an optical signal to an electrical signal, and an electrical signal from the optical signal by the first OE converter. A data identification unit that determines the amplitude levels of the in-phase component and the quadrature component of the received signal converted into, converts each amplitude level into binary data and outputs the binary level, and is provided in front of the optical 90 ° hybrid coupler, An optical branching unit for branching the optical signal, a second OE conversion unit for converting one received optical signal branched by the optical branching unit into an electrical signal, and an optical signal generated by the second OE conversion unit. A clock extractor for extracting a clock signal equal to the symbol frequency of the received signal converted into an electrical signal; and means for sampling the received signal input to the data identifying unit by the output clock signal of the clock extractor It is characterized by that.

  According to this, since the synchronous detection of the QAM signal can be performed without using the pilot signal, it is possible to avoid a decrease in frequency use efficiency.

  In addition, an amplitude determination unit for obtaining the amplitude of the received signal input to the data identification unit, and only the symbols whose output from the amplitude determination unit is within a predetermined range are selected, and the sampling for the selected symbols is performed. A carrier phase regenerator that obtains a phase difference between the carrier phase and the local oscillation light source from the output of the means for performing the processing. As a result, it is possible to select symbols whose data can be demodulated normally.

  Furthermore, in order to extend the range of the amplitude at which data can be normally demodulated, a means for performing logic inversion or channel switching on the data signal output from the data identification unit can be provided.

  Further, the sampling means may include means for converting an analog signal received signal output from the first OE converter into a digital signal received signal and inputting the digital signal to the data identifying section.

  According to the present invention, since synchronous detection of a QAM signal can be performed without using a pilot signal, it is possible to avoid a decrease in frequency utilization efficiency.

  The configuration of the optical receiving circuit of this embodiment is shown in FIG. As shown in FIG. 1, the optical receiver circuit of this embodiment includes an optical branching unit 2 that branches a received optical signal into two paths, a local oscillation light source 1 that outputs continuous light having substantially the same wavelength as the received optical signal, An optical 90 ° hybrid coupler 3 that combines one of the branched optical signals and the optical signal of the local oscillation light source 1, and a balanced OE that receives the optical signal from the optical 90 ° hybrid coupler 3 and converts it into an electrical signal. OE converters 4-1 and 4-2 that are converters, and an analog / digital converter that samples analog electric signals from OE converters 4-1 and 4-2 into digital signals (hereinafter referred to as AD converters) 5-1 and 5-2, an OE converter 8 that squarely detects the other of the branched optical signals, and a clock frequency that extracts a clock frequency equal to the symbol rate from the electrical signal output from the OE converter 8. Output from the digital signal extraction unit 9, the AD conversion units 5-1 and 5-2, the amplitude determination unit 11 that determines the amplitude of the received signal, and the AD conversion units 5-1 and 5-2. A carrier phase recovery unit 12 that recovers the phase of the optical carrier from the digital signal to be output, calculates the amplitude values of the in-phase component and the quadrature component of the received signal from the output of the carrier phase recovery unit 12, and outputs the digital data by performing threshold determination And a channel switching / logic inverting unit 7 that performs logical inversion and channel switching of output data for binary data output from the data identifying unit 6.

  Here, the feature is that the clock signal extraction unit 9 is provided and the clock signal extracted from the reception signal is given to the AD conversion units 5-1 and 5-2, so that the received signal has the maximum eye opening. Sampling, selecting an amplitude determination unit 11 to select a symbol having a predetermined amplitude and reproducing the carrier phase, and providing a channel switching / logic inversion unit 7 to invert the logic of the output binary data and the channel It is in a place where switching is possible.

  Next, the operation of the present embodiment will be described with reference to FIG. 3 taking as an example the case where the received signal is a 16-QAM signal. FIG. 3 is a flowchart showing the operations of the data identification unit 6 and the channel switching / logic inversion unit 7 in this embodiment.

  In order to receive the 16-QAM signal, it is necessary to reproduce the carrier wave phase on the receiving side and discriminate the in-phase component and the quadrature component of the optical amplitude. In this embodiment, a local oscillation light source (L) 1 having a frequency substantially equal to the received signal (S) is mixed by an optical 90 ° hybrid coupler 3 (the optical 90 ° hybrid coupler 3 is a spatial optical system, a planar waveguide technology, etc. Is realized). Here, the optical frequency difference between the received signal and the local oscillation light needs to be sufficiently lower than the symbol rate.

The received signal and the local oscillation light are expressed as S (t) = E (t) e ^{jω0t + φ0 (t)}
L (t) = E _L0 e ^jω0t (Formula 1)
It expresses. Here, ω ₀ is the optical angular frequency of the carrier wave and the local oscillation light source 1, φ ₀ (t) is the phase difference between the carrier wave phase and the local oscillation light, and E (t) is the optical electric field modulated by the 16-QAM code. And E (t) = E _x (t) + jE _y (t), E _x (t), E _y (t) ∈ {−3E ₀ , −E ₀ , E ₀ , 3E ₀ } .
At the output of the optical 90 ° hybrid coupler 3,
E ₁ (t) = (S (t) + L (t)) / 2
E ₂ (t) = (S (t) −L (t)) / 2
E ₃ (t) = (S (t) + jL (t)) / 2
E ₄ (t) = (S (t) −jL (t)) / 2 (Formula 2)
It becomes. Here, E ₁ and E ₂ are detected by the OE conversion unit 4-1, and E ₃ and E ₄ are detected by the OE conversion unit 4-2. At this time, if the output currents I ₁ and I ₂ of the two OE conversion units 4-1 and 4-2 are I (t) = I ₁ (t) + jI ₂ (t), I (t) = 4RE _L0 E (T) exp {jφ ₀ (t)} (Formula 3)
It is expressed. Here, R is a coefficient representing the response of the OE conversion units 4-1 and 4-2. Therefore, if the phase difference φ ₀ (t) between the carrier light phase and the local oscillation light is known,
E (t) = (1/4 RE _L0 ) I (t) exp {−jφ ₀ (t)} (Formula 4)
From Equation 4, the received symbol E (t) can be demodulated.

The phase difference φ ₀ (t) between the carrier wave and the local oscillation light can be obtained as follows. As shown in FIG. 2, the amplitude and phase of the optical signal modulated by the 16-QAM code take various values, but the light intensity is 2E ₀ ² and 18E ₀ ² as shown by the solid lines in FIG. In this case, the phase shift is any one of π / 4, 3π / 4, 5π / 4, and 7π / 4. Here, the case where the phase shift takes any one of π / 4, 3π / 4, 5π / 4, and 7π / 4 is referred to as group A, and the others are referred to as group B.

First, with respect to the output signal AD-converted by the AD converters 5-1 and 5-2, the amplitude determination unit 11 determines whether the received symbol is group A (symbol whose light intensity is 2E ₀ ² and 18E ₀ ² ) B is determined (S1, S2). When the received symbol belongs to group A, if both sides of Equation 3 are raised to the fourth power, the phase term of E (t) becomes a constant.
φ ₀ (t _k ) = (1/4) arg [{I ₁ (t _a ) + jI ₂ (t _a )} ⁴ ] (Formula 5)
Thus, the phase difference φ ₀ (t) between the carrier wave phase and the local oscillation light can be obtained (S3). Here, t _a represents the sampling time of symbols belonging to group A.

Since the phase difference φ ₀ (t) includes the phase noise of the light source, the uncertainty due to the phase noise can be reduced by averaging over several symbols belonging to the group A.

For symbols of group B (symbols other than phase shifts of π / 4, 3π / 4, 5π / 4, 7π / 4), for example, φ ₀ (t) in the immediately preceding symbol belonging to group A Is used (S4). Using φ ₀ (t) obtained above, E (t) is demodulated by Equation 4 to obtain in-phase component and quadrature component data (S5). Φ ₀ (t) obtained from Equation 5 is −π / 4 <φ ₀ (t) ≦ π / 4.
Take a value in the range. If the actual phase difference is within this range, data can be demodulated normally. However, since the actual phase difference φ _0r (t) takes an arbitrary value in the range of 0 to 2π, when the actual phase difference is outside the above range, E (t) after demodulation is equal to the in-phase The component and the orthogonal component are interchanged, and the sign is reversed, so that normal data is not demodulated. In order to avoid this, a channel switching / logic inversion unit 7 is required at the output of the data identification unit 6. The channel switching / logic inversion unit 7 performs the demodulation process separately when φ _0r (t) takes the following four ranges.

π / 4 <φ _0r (t) ≦ π / 4
π / 4 <φ _0r (t) ≦ 3π / 4
3π / 4 <φ _0r (t) ≦ 5π / 4
5π / 4 <φ _0r (t) ≦ 7π / 4
First,
π / 4 <φ _0r (t) ≦ π / 4
If so, normal data is demodulated by E (t) calculated from Equation 4 (S6). When normal data is not demodulated, the in-phase component and the quadrature component are switched (S7).
5π / 4 <φ _0r (t) ≦ 7π / 4
If so, normal data is demodulated (S8). If normal data is not demodulated, only the in-phase component is logically inverted (S9).

π / 4 <φ _0r (t) ≦ 3π / 4
If so, normal data is demodulated (S10). When normal data is not demodulated, the in-phase component and the quadrature component are switched again, and only the in-phase component is logically inverted (S11).

3π / 4 <φ _0r (t) ≦ 5π / 4
If so, normal data is demodulated (S12). If it is not demodulated, the process ends abnormally.
The determination is based on whether the demodulated data is normal. Actually, the operation of the channel switching / logic inversion unit 7 may be set so that a known signal is received and correct data is demodulated.

  Further, in the case of 16-QAM, the channel switching / logic inversion unit 7 is a logic inversion of a 2-bit gray code, and an operation for inverting only the upper bits may be performed. Actually, the operation of the channel switching / logic inversion unit 7 is set so that a known signal is received and correct data is demodulated.

In the above description, the application example with respect to 16-QAM signal was demonstrated. Next, a case where the optical receiving circuit of this embodiment is applied to a 64-QAM signal will be described. FIG. 4 shows a constellation diagram of the 64-QAM signal. In the case of a 64-QAM signal, the phase difference between the carrier wave and the local oscillation light is obtained by the above-described calculation at the point where the phase shift amount is π / 4, 3π / 4, 5π / 4, 7π / 4, From the result, the QAM signal can be demodulated. However, as can be seen from FIG. 4, when the light intensity obtained from the sum of squares of I ₁ and I ₂ is 50E ₀ ² (indicated by a dotted line in the figure), π / 4, 3π / 4, 5π / 4, 7π / Since there are points other than 4, this level must be excluded.

In the above description, the OE conversion units 4-1 and 4-2 have been described as balanced OE converters. However, either E ₁ or E _{2 in} Formula 2 and either E ₃ or E ₄ are used. Similar demodulation is possible even if only one of them is received by a normal OE converter.

(Other examples)
In the optical receiver circuit of the above embodiment, the detection output is converted into a digital signal by the AD conversion units 5-1 and 5-2, and the subsequent processing is performed using this digital signal. A signal sampled with a clock signal extracted from the received optical signal may be used. The sampled signal may be an analog signal or a digital signal.

  Since the present invention can avoid a decrease in frequency use efficiency, it can be used to improve communication quality.

The block diagram of the optical receiver circuit of a present Example. The constellation diagram of a received optical signal (16-QAM). The flowchart which shows operation | movement of a data identification part and a channel switching and logic inversion part. The constellation diagram of a received optical signal (64-QAM).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Local oscillation light source 2 Optical branching part 3 Optical 90 degree hybrid coupler 4-1, 4-2 OE conversion part 5-1, 5-2 AD conversion part 6 Data identification part 7 Channel switching and logic inversion part 8 OE converter 9 Clock Extraction unit 10 Phase adjustment unit 11 Amplitude determination unit 12 Carrier phase recovery unit

Claims (4)

A local oscillation light source that outputs local oscillation light having a wavelength substantially equal to the wavelength of the received optical signal;
An optical 90 ° hybrid coupler that mixes and outputs a received signal and local oscillation light;
A first OE converter that converts an output signal from the optical 90 ° hybrid coupler from an optical signal to an electrical signal;
A data identification unit that determines the amplitude levels of the in-phase component and the quadrature component of the reception signal converted from the optical signal to the electrical signal by the first OE conversion unit, converts each amplitude level into binary data, and outputs the binary data;
An optical branching unit that is provided in a preceding stage of the optical 90 ° hybrid coupler and branches a received optical signal;
A second OE conversion unit that converts one received optical signal branched by the optical branching unit into an electrical signal;
A clock extraction unit for extracting a clock signal equal to the symbol frequency of the reception signal converted from the optical signal to the electric signal by the second OE conversion unit;
Means for sampling a reception signal input to the data identification unit by an output clock signal of the clock extraction unit. An amplitude determination unit for obtaining an amplitude of a reception signal input to the data identification unit;
Carrier phase recovery that selects only symbols whose output of the amplitude determination unit is within a predetermined range and obtains the phase difference between the carrier phase and the local oscillation light source from the output of the sampling means for the selected symbol The optical receiver circuit according to claim 1, further comprising:   3. The optical receiver circuit according to claim 1, further comprising means for performing logic inversion or channel switching on a data signal output from the data identification unit.   The sampling means includes means for converting an analog signal received signal output from the first OE converter into a digital signal received signal and inputting the digital signal to the data identification section. An optical receiver circuit according to 1.

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