JP2642174B2 - Optical heterodyne detection circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical heterodyne detection circuit, and more particularly to an optical heterodyne detection circuit capable of efficiently receiving a high-speed signal.

[Prior Art] Conventionally, an optical heterodyne detection circuit that detects information such as the phase difference, intensity, and frequency of both lights by using interference generated by mixing an optical carrier frequency and a signal output from an optical local oscillator has been developed. Provided.

As is well known, the optical heterodyne detection method generally has improved light receiving sensitivity (reception sensitivity) compared to the direct detection method, and can equalize (delay equalization) the chromatic dispersion of an optical fiber by an intermediate frequency circuit. Promising as a transmission method with a long relay interval.

[Problems to be Solved by the Invention] However, a conventional light receiving circuit of a wide band using the optical heterodyne detection method requires a light receiving circuit having a center frequency twice the bit rate and a double bandwidth. High speed was difficult. As described above, it is difficult to perform optical heterodyne detection in a high-speed region where the influence of chromatic dispersion of an optical fiber appears, so that dispersion of an optical fiber, which is a characteristic of optical heterodyne detection, cannot be equalized by an electric circuit.

On the other hand, in the phase diversity detection method using a baseband circuit, the difference between the signal light frequency and the local oscillation light frequency is set to about 1/10 of the bit rate, so that the bandwidth of the light receiving circuit is also DC.
(DC) to a bit rate, so that the load on an electric circuit can be reduced as compared with optical heterodyne detection. However, the phase diversity detection method has a problem in that delay information cannot be equalized in an electric circuit unlike optical heterodyne detection because information on the phase of light is lost when compensating for chromatic dispersion.

An object of the present invention is to provide an optical heterodyne detection circuit capable of delay-equalizing an optical fiber dispersion by an electric circuit using a phase diversity detection method in view of the above-mentioned problems.

[Means for Solving the Problems] To achieve the above object, an optical heterodyne detection circuit according to the present invention provides a light for mixing input signal light and local light oscillation light.
A 90 ° hybrid, a light receiving unit for receiving the two mixed lights output from the light 90 ° hybrid and converting them into two electrical received signals,
A frequency converter for converting a frequency of the two received signals into a higher frequency by multiplying each of the two received signals by a sine wave, and outputting two 90 ° hybrid signals from two outputs of the frequency converter. A hybrid, a delay equalizer for delay equalizing the two 90 ° hybrid signals, and a demodulator for demodulating an output signal of the delay equalizer,
A combining unit for combining the two demodulated signals demodulated by the demodulation unit into one signal.

[Operation] The present invention converts the signal frequency received once using the phase diversity detection method to a higher frequency, reproduces the received signal including the phase, and performs delay equalization. In the same manner as in the optical heterodyne detection method, the delay (wavelength fraction) of the optical fiber can be compensated (delay equalization) by an electric circuit.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

 FIG. 1 shows a circuit configuration of one embodiment of the present invention.

In the figure, 1 is a local oscillation light source, 2 is a light 90 ° hybrid that combines the signal light and the local oscillation light of the local oscillation light source 1, 4 and 5 are light receiving elements that receive the combined light flux, 6 , 7 are amplifiers, 8 is a local oscillator, and 9 and 10 are mixers. Numeral 100 denotes a 90 ° hybrid for combining electric signals received and photoelectrically converted, and includes π / 2 phase shifters 11 and 12 and a combiner 1
Consists of 3,14. 15 and 16 are delay equalizers, 17 and 18 are demodulators, and 19 is a synthesizer.

First, the operation of the circuit of this embodiment will be briefly described.

Two light multiplexed by the optical 90 ° hybrid 2 beat occurs corresponding to the difference between the angular frequency omega _L of the angular frequency of the signal light omega _S and the local oscillator light (FIG. 2 (A), (B )reference).
The beat has the same beat frequency at point Y with reference to point X, for example, but has a phase difference of ± π / 2 according to the magnitude relationship between the frequencies of the signal light and the local oscillation light.

When these two lights are photoelectrically converted by the light receiving elements 4 and 5 and the electric signal amplified by the amplifier 6 is multiplexed with the local oscillation signal of the local oscillator 8 by using the mixers 9 and 10 to be converted to a high frequency. When the transformed signal angular frequency angular frequency of the local oscillator 8 and _{ω LO, ω LO ± (ω} S -ω L)
(See FIGS. 2C and 2D). At point a of the output line of the mixer 9, the two signals have the same phase difference (see FIG. 2 (C)). However, at point b of the output line of the mixer 10, the two signals have a phase difference of ± π / 2 compared to the point a (see FIG. 2 (D)). When these signals having a phase difference are combined using a 90 ° hybrid 100,
出力 The output from the hybrid 100 is 0 at the point c of the output line because the ω _LO − (ω _S −ω _L ) component is canceled and becomes only the ω _LO + (ω _S −ω _L ) component (second). FIG. (E). On the other hand, at point d on the output line of the 90 ° hybrid 100, the ω _LO + (ω _S −ω _L ) component of the output is canceled to become 0, and only the ω _LO − (ω _S −ω _L ) component is provided. (See FIG. 2 (F)). Therefore, these two signals sent to the points c and d reproduce the state of the light spectrum including the phase, and are the same as the detection performed by the conventional optical heterodyne detection. Therefore, these signals are delayed and equalized by the delay equalizers 15 and 16 and the demodulators 17 and
If the demodulation is performed at 18 and the synthesis is performed at the synthesizer 19, the chromatic dispersion of the optical fiber can be compensated.

Next, the operation of the present embodiment described above will be described in more detail using the following equations. Here, the signal light that has been frequency-modulated as the signal light will be described as an example. The waveform S of the signal light at this time is And Here, P _S is the power of the signal light, Jn (X) is the n-th order Bessel function of the first kind, beta is the modulation index, omega _S is the angular frequency of the signal light, omega _m is the modulation angular frequency, k _n is n-th propagation constant sideband, L is the fiber length of the optical fiber, is [Phi _S is the phase (arbitrary value).

On the other hand, the waveform L of the local oscillation light is And Here, P _L is the power of the local oscillation light, ω _L is the angular frequency of the local oscillation light, and φ _L is the phase (arbitrary value).

The signal light and the local oscillation light are mixed by the light 90-degree hybrid 2, and the two signal lights are incident on the light receiving elements 4 and 5, respectively.

The received waveform _Rx by the light receiving element 4 and the amplifier 6 is given by ignoring the DC component. Becomes Here, η ₁ is the quantum efficiency of the light receiving element 4, e is the electron charge, h is Planck's constant, ν is the frequency of light (ω _S =
2πν), G ₁ represents the gain of the amplifier 6. In the following, for simplicity, Then, R _{x in the} above equation (3) is Becomes

Further, the reception waveform R by the light receiving element 5 and the amplifier 7
_y is the same as above Becomes The signals R _x and R _{y of the} above equation (3) ′ or (4) and the output T of the local oscillator 8 Is multiplied by mixers 9 and 10, the signal Ra at point a becomes Becomes Similarly, the signal R _{b at the} point _b is Becomes The signals Ra and Rb in the above equations (6) and (7) are ± (ω) around ω _LO as shown in FIGS. 2 (C) and 2 (D).
_A spectrum exists at the frequency of _S− ω _L ). At this time,
Assuming that Φ _S −Φ _LO of the signal Ra at the point a is 0, the above equation (6),
According to (7), it can be seen that the two frequency components are in phase and the signal at point b has a phase difference of ± π / 2.

The signal R _c at the point _c is half of the signal at the point a. And a signal obtained by delaying half of the signal R _b 'at the point _b by π / 2 And the sum of (See FIG. 2 (E)).

The signal of point d R _d is a signal delayed half a [pi / 2 by the phase of the signal R _a of a point And half of the signal R _b at point _b And the sum of (See FIG. 2 (F)).

Thus, the signal R _c of point c, optical heterodyne becomes detection and equalization, the signal R _d of d points local oscillation light source when the optical frequency of the local oscillator light L of the local oscillation light source 1 has the omega _L - [omega] _LO 1 is equivalent to optical heterodyne detection when the optical frequency of the local oscillation light L is ω _L + ω _LO . When the frequency of the signal light is substantially equal to the optical frequency of the local oscillation light, the above equation (8)
(9) (assuming ω _S = ω _L ), Becomes

That is, the chromatic dispersion of the optical fiber becomes the same sign at point c and the opposite sign at d, so that the delay equalizers 15, 1
Can be compensated for by inserting Further, when a delay equalizer having an inverse characteristic cannot be inserted, it is possible to compensate for the chromatic dispersion of the optical fiber by inserting a delay equalizer into only one side and performing demodulation.

[Effects of the Invention] As described above, the delay of an optical fiber cannot be compensated by an electric circuit by the phase diversity detection method in the related art. However, according to the present invention, the signal frequency is once converted to a higher frequency. Thus, since the received signal of the signal light is reproduced including the phase, the effect of performing the delay equalization can be obtained similarly to the heterodyne detection.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a circuit configuration of an embodiment of the present invention, and FIGS. 2 (A) to (F) each show a spectrum of an output signal at each point in FIG. It is a characteristic diagram. 1 ... local oscillation light source, 2 ... light 90 ° hybrid, 4,5 ... light receiving element, 6,7 ... amplifier, 8 ... local oscillator, 9,10 ... mixer, 11,12 ... π / 2-phase shifter, 13,14 ... combiner, 15,16 ... delay equalizer, 17,18 ... demodulator, 100 ... 90 ゜ hybrid.

Claims (1)

(57) [Claims] 1. A light for mixing an input signal light and a local oscillation light.
A 90 ° hybrid; a light receiving unit for receiving the two mixed lights output from the light 90 ° hybrid and converting them into two electrical reception signals; and a light receiving unit for outputting the two reception signals output from the light receiving unit. A frequency converter for converting the frequency of the two received signals to a higher frequency by multiplying a sine wave; a 90 ° hybrid for outputting two 90 ° hybrid signals from two outputs of the frequency converter; A delay equalizer for delay equalizing each of the 90 ° hybrid signals, a demodulator for demodulating an output signal of the delay equalizer, and a synthesizer for synthesizing two demodulated signals demodulated by the demodulator into one signal. An optical heterodyne detection circuit, comprising: