JP2013258558A - Optical receiver and optical reception method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To implement a technique in which a distortion does not affect a wideband signal in an analog ROF technique which uses a phase modulation-direct detection system.SOLUTION: An optical receiver comprises: an optical differentiation circuit (210) which differentiates the phase modulation light optical-phase-modulated by an analog signal; a photoelectric converter (220) which converts output light from the optical differentiation circuit (210) into an electric signal; and an integration circuit (230) which integrates the electric signal from the photoelectric converter (220). An optical filter having frequency transfer characteristics equivalent to frequency transfer characteristics of the differentiation circuit is used as the optical differentiation circuit.

Description

  The present invention relates to an optical receiver and an optical reception method for receiving an optical phase modulation signal in which an analog signal is optical phase modulated.

  The radio over fiber (ROF) technology is a technology in which a radio apparatus is divided into a modulation / demodulation part and a radio transmission / reception part, and a radio modulation signal is transmitted between them in an optical fiber. By utilizing this ROF technology, the arrangement of wireless devices can be made flexible and integrated, and a highly functional wireless network can be realized.

  An analog ROF system that directly modulates light with a radio modulation signal is widely used as means for realizing ROF, and has a feature that the configuration of a remote station on which a radio transmission / reception part is mounted is simplified. In the analog ROF method, there are intensity modulation, frequency modulation and phase modulation as modulation methods. Recently, a method excellent in noise characteristics and distortion characteristics using a phase modulation-direct detection method has been proposed (for example, (Refer nonpatent literature 1.).

  In this phase modulation-direct detection method, as shown in FIG. 8, the optical transmitter 100 transmits phase-modulated light obtained by phase-modulating the light from the light source 110 with an analog signal, and the optical receiver 200 uses a Mach-Zehnder interferometer. The optical differential circuit 210 performs delay detection, and the photoelectric converter 220 converts it to an electrical signal to extract an analog signal. When delay detection is performed by the optical differentiation circuit 210 using a Mach-Zehnder interferometer, the intensity noise component of the light source 110 is largely canceled, so that a signal can be transmitted with high quality.

V. J. et al. Urick et al., “Phase Modulation With Interferometric Detection as an Alternative to Intensity Modulation with Detective for the Electric Link”. 55, no. 9, pp. 1978-1985, 2007.

  When the Mach-Zehnder interferometer is used, the demodulated waveform is distorted according to its frequency transfer characteristics. In the phase modulation-direct detection system studied so far, as shown in FIG. 10, the ratio band between the radio carrier frequency and the radio signal bandwidth is sufficiently large, so this point is not particularly taken into consideration. However, when the ratio band is small, as shown in FIG. 11, it is affected by distortion according to the frequency transfer characteristic.

  An object of the present invention is to realize a technique which is not affected by the above distortion even for a wideband signal in an analog ROF technique using a phase modulation-direct detection system.

  An optical receiver according to the present invention includes an optical differentiating circuit that differentiates phase-modulated light phase-modulated with an analog signal, a photoelectric converter that converts output light from the optical differentiating circuit into an electric signal, and the photoelectric converter. And an integration circuit for integrating the electrical signal from.

  In the optical receiver according to the present invention, the optical differentiation circuit may include an optical filter having a frequency transfer characteristic equivalent to that of the differentiation circuit.

  In the optical receiver according to the present invention, the optical filter may be an asymmetric Mach-Zehnder interferometer.

  In the optical receiver according to the present invention, the optical filter may be a Fabry-Perot interferometer.

  In the optical receiving method according to the present invention, the phase-modulated light obtained by optical-phase-modulating the analog signal is differentiated and converted into an electric signal, and the electric signal is integrated to extract the analog signal.

  In the optical receiving method according to the present invention, the phase-modulated light may be differentiated by passing through an optical filter having a frequency transfer characteristic equivalent to that of the differentiating circuit.

  In the optical receiving method according to the present invention, an asymmetric Mach-Zehnder interferometer may be used as the optical filter.

  In the optical receiving method according to the present invention, a Fabry-Perot interferometer may be used as the optical filter.

  In the present invention, the phase-modulated light is differentiated and converted into an electric signal, and then the electric signal is integrated, whereby distortion according to the frequency transfer characteristic of the Mach-Zehnder interferometer can be prevented. For this reason, according to the present invention, in the analog ROF technique using the phase modulation-direct detection method, the influence of the distortion can be prevented even for a wideband signal.

1 shows a first embodiment of the present invention. An example of a receiver configuration using a Mach-Zehnder interferometer is shown. An example of the constellation in 1st Embodiment is shown. The 1st example of an integration circuit is shown. The 2nd example of an integration circuit is shown. 2 shows a second embodiment of the present invention. An example of the transmission characteristic of the Fabry-Perot interferometer in the second embodiment is shown. The structural example of the analog ROF technique using a phase modulation-direct detection system is shown. A comparative example of constellation is shown. An example of frequency transfer characteristics when the ratio band is large is shown. An example of frequency transfer characteristics when the ratio band is small is shown.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(First embodiment)
FIG. 1 shows the configuration of the first embodiment of the present invention.
In the optical transmitter 100, the light output from the light source 110 is optically phase-modulated with an analog input signal in the optical phase modulator 120 and output. In the optical receiver 200, the phase-modulated light transmitted through the optical transmission line 300 is input to the optical differentiation circuit 210 and converted into an electrical signal by the photoelectric converter 220. The electrical signal is integrated by the integrating circuit 230 to restore the original analog signal. The optical differentiation circuit 210 is a circuit that differentiates the time waveform of the optical phase modulation signal with respect to time. The optical differentiation circuit 210 can use an optical filter having a frequency transfer characteristic equivalent to that of the differentiation circuit. Examples of such an optical filter include a Mach-Zehnder interferometer and a Fabry-Perot interferometer.

Details of the operation when the optical filter of the Mach-Zehnder interferometer 211 shown in FIG. 2 is used as the optical differentiation circuit 210 will be described below using mathematical expressions.
When the phase-modulated light S (t) is expressed by an equivalent low-pass expression,

Here, s (t) is an analog input signal normalized so that | s (t) | ≦ 1, and m is a modulation degree (m ≦ 1). When the delay time difference of the Mach-Zehnder interferometer 211 is τ and a phase difference π / 2 is added, outputs P ₁ (t) of the two photodiodes 221a and 221b of the balanced light receiving circuit 221 used as the photoelectric converter 220, P ₂ (t) is represented by the following equation, respectively.

となる。 The output P (t) of the balanced light receiving circuit 221 is

It becomes.

と近似される。 Here, if mπ (s (t) −s (t−τ)) ≈0,

Is approximated by

となる。 If 1 / τ is sufficiently larger than the signal bandwidth of s (t),

It becomes.

となり、もとのアナログ信号が復元される。 Therefore, by passing this signal through the integration circuit 230,

Thus, the original analog signal is restored.

  As a numerical simulation result for this embodiment, FIG. 3 shows a constellation after transmission at the time of QPSK signal transmission when the carrier frequency is 625 MHz, the transmission rate is 156.25 Mb / s, the delay time is 100 ps, and the modulation factor is 0.05. From the figure, it can be seen that the spread of the constellation is suppressed by this method compared with the conventional method. In comparison with the error vector amplitude (EVM), the conventional method is 30.4 dB, whereas in the present embodiment, it is improved to 34.7 dB.

  By using this embodiment, an analog signal can be transmitted without being affected by the characteristics of the optical filter in the phase modulation-direct detection system.

  As the integration circuit in this embodiment, use at a frequency higher than the cut-off frequency of a simple RC filter as shown in FIG. 4 or an integration circuit using an operational amplifier shown in FIG. 5 can be used. In addition, when this method is applied to the uplink side of a radio link, it is also possible to perform an integration operation by digital signal processing after AD conversion of the balanced reception output.

ここで、Ｒは反射率、Δｌは干渉計の長さ、ｃは光速である。 (Second Embodiment)
FIG. 6 shows a second embodiment of the present invention. The difference from the first embodiment is that an optical filter by a Fabry-Perot interferometer 212 is used as an optical differentiation circuit. The frequency transfer function T (f) of the Fabry-Perot interferometer 212 is given by the following equation.

Here, R is the reflectance, Δl is the length of the interferometer, and c is the speed of light.

  From Equation 7, the frequency transfer function as shown in FIG. 9 shows periodic transmission characteristics. At this time, the differential characteristic is obtained by setting the frequency of the phase-modulated light as a part of the upstream slope of the transmission characteristic.

  The present invention can be applied to the information communication industry.

100: Optical transmitter 110: Light source 120: Optical phase modulator 200: Optical receiver 210: Optical differential circuit 211: Mach-Zehnder interferometer 212: Fabry-Perot interferometer 220: Photoelectric converter 221: Balanced optical receiver circuits 221a and 221b : Photodiode 230: Integration circuit 300: Optical transmission line

Claims (8)

An optical differentiating circuit for differentiating phase-modulated light phase-modulated with an analog signal;
A photoelectric converter that converts output light from the photodifferential circuit into an electrical signal;
An integrating circuit for integrating the electrical signal from the photoelectric converter;
An optical receiver.   The optical receiver according to claim 1, wherein the optical differentiation circuit includes an optical filter having a frequency transfer characteristic equivalent to that of the differentiation circuit.   The optical receiver according to claim 2, wherein the optical filter is an asymmetric Mach-Zehnder interferometer.   The optical receiver according to claim 2, wherein the optical filter is a Fabry-Perot interferometer.   An optical receiving method in which phase-modulated light obtained by optical phase modulation of an analog signal is differentiated and converted into an electric signal, and the electric signal is extracted by integrating the electric signal.   6. The optical receiving method according to claim 5, wherein the phase-modulated light is differentiated by passing through an optical filter having a frequency transfer characteristic equivalent to that of the differentiating circuit.   The optical receiving method according to claim 6, wherein an asymmetric Mach-Zehnder interferometer is used as the optical filter.   The optical reception method according to claim 6, wherein a Fabry-Perot interferometer is used as the optical filter.

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