JPS61127236A - Optical transmission system - Google Patents

Abstract

PURPOSE:To obtain a phase modulation system with excellent S/N by applying additive logical conversion to a modulation signal to attain phase modulation in a transmitter and using an optical system to demodulate in the lump a phase modulation and the additive logic conversion signal in a receiver to constitute simply the receiver. CONSTITUTION:An output optical signal of a phase modulator 14 propagates an optical transmission line 15 and reaches the receiver. A branch circuit 21 of the receiver branches an input optical signal to two optical paths A, B. The branching is executed so that the energy is shared even. A difference is given to the optical length in the optical paths A, B and the difference is set to a length corresponding to the time at m bits. The optical signal via the two optical paths is inputted to a branch and synthesis circuit 22. The optical signal in the optical path A in the circuit 22 is branched to two optical paths A1, A2, the optical signal of the optical path B is branched into two optical paths B1, B2, and the optical paths A1, B1 and the optical paths A2, B2 are constituted so that the optical paths are coincident respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention is utilized in digital optical communications. In particular, it relates to optical transmission systems using phase modulation.

[Conventional technology]

In conventional optical communications, a method of intensity modulating optical signals is widely used. Since the intensity modulation method has several drawbacks, a method using phase modulation is being considered as a method to improve this.

[Problem that the invention seeks to solve]

That is, the intensity modulation method has the disadvantage that high-speed modulation is not possible because there is a limit to the modulation speed of the laser and a limit to the high-speed operation of the switch through which a large current passes. Furthermore, there is a drawback that frequency fluctuation occurs in the laser output light, and when this output light is propagated through an optical fiber, waveform distortion occurs due to the influence of wavelength dispersion of the optical fiber. Furthermore, when intensity modulation is applied, the laser output is generally smaller than the continuous optical output.

On the other hand, the phase modulation method eliminates the above-mentioned drawbacks, but has the drawback that the configuration of the demodulator of the receiving device becomes complicated.

The present invention is an improvement on this, and aims to provide a phase modulation method in which a receiving device is simply constructed and has a good signal-to-noise ratio.

[Means for solving problems]

The present invention provides a transmitting device that transmits an optical signal modulated by a time-series digital input electrical signal, and a transmitting device that receives the transmitted output of the transmitting device via an optical transmission line, and outputs an electrical signal corresponding to the input electrical signal. In an optical transmission system that includes a receiving device that outputs a signal, the transmitting device includes a light source and an exclusive OR of an output logical value m bits (m is a natural number) before the input electrical signal. The receiving device includes a branching circuit that branches received input light into two optical paths, and a modulator that performs phase modulation on the output light of the light source using the output signal of this circuit. a branching/combining circuit that further branches each of the optical signals of the two optical paths into two optical signals, and combines the two optical signals so that each of the two optical signals coincides with the optical signals of the two optical paths, the branching circuit The optical path propagation time difference between the two optical paths from to the combining point of the branching and combining circuit is set to be the m-bit time, and two light receiving elements each receive the two combined output lights of the branching and combining circuit. and an amplifier which uses the output electric signals of these two light receiving elements as differential inputs.

[Effect]

The transmitting device performs summation logic conversion on the modulated signal to perform phase modulation, and the receiving device demodulates the phase modulation signal and the summation logic conversion signal all at once using an optical system.

〔Example〕

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention. One section in the figure shows a transmitting device, and the lower section shows a receiving device. In FIG. 1, solid lines indicate optical signal paths, and broken lines indicate electrical signal paths. A binary digital signal is input to the input terminal 11 in time series. This signal passes through a summation logic conversion circuit and is subjected to summation logic conversion. In this summation logic conversion circuit 12, if the logical value of the n-th bit of the input signal is Xn, the logical value Yn of the n-th output signal is Yn=Xn■Yn-m. However, m and n are natural numbers, and ■ indicates exclusive OR.

On the other hand, the laser light source 13 is a light source that emits light constantly, and its output is input to the phase modulator 14 together with the output of the summation logic conversion circuit 12, and the output light is subjected to phase modulation. In this phase modulator 14, if the logical value of the modulation signal input from the circuit 12 is "1", the r
In the case of OJ, the configuration is such that phase modulation is applied to the output light at the 0 phase.

This output optical signal propagates through the optical transmission line 15 and reaches the receiving device. In the receiving device, the input optical signal is branched into two optical paths A and B by a branching circuit 21. This branching is performed so that the energy is approximately equally divided. There is a difference in optical path length between these two optical paths A and B, and the difference is set to a length corresponding to the above-mentioned m-bit time. The optical signals that have passed through these two optical paths are input to the branching/combining circuit 22.

In this circuit 22, the optical signal on optical path A is branched into two optical paths A and Az, and the optical signal on optical path B is divided into two optical paths B1 and 2.
and B2, and the optical paths A1 and B are configured so that their optical paths coincide, and the optical paths A2 and B2 are configured so that their optical paths coincide.

The two output lights of this branching/combining circuit 22 are input to light receiving elements 23 and 24, respectively, and are converted into electrical signals. This output electrical signal is given to the differential input of one differential amplifier 25. The output of the differential amplifier 25 is
The code is input to the identification circuit 26 and code identification is performed. The identification output is sent to output terminal 27.

FIG. 2 shows a specific example of the configuration for optical paths A and B. That is, the branching/combining circuit 22 includes a lens 3 that converts an optical signal arriving at an optical transmission line 15 composed of an optical fiber into parallel light.
1, and a prism 32 that branches the light emitted from this lens 31 into two optical paths. The light reflected on the surface of the prism 32 is reflected on the mirrors 33 and 34 as an optical path A, and the light that has passed through the prism 32 is directly incident on the branching and combining circuit 22 as an optical path B.

This branching and combining circuit 22 includes a prism 35 and lenses 36 and 37. On the surface of this prism 35, the lights of the two optical paths A and B are combined.

Each of the prisms 32 and 37 has a right isosceles triangular cross section, and its reflectance and transmittance are each 50%. Mirrors 33 and 34 can change the relative lengths of optical path A and optical path B by changing the lengths of the optical paths indicated by symbols X and X' while maintaining the optical path shown.

In the device configured in this manner, the input signal at the terminal 11 is subjected to summation logic conversion. - For example, 011010
If 0010111... is the logical value of the signal at the terminal 11, the output of the summation logic conversion circuit 12 will be 11011000011010.... At this time, m is 1.

Phase modulation is performed by the modulator 14 using this signal as a modulation signal. A phase-modulated optical signal is transmitted to the optical transmission path 15, and is branched into two optical paths by a branching circuit 21 of the receiving device. Since there is a time difference of 1 bit between these two optical paths, when they are combined in the branching and combining circuit 22, the sum logic conversion is decoded, and the signal modulated to π phase and the signal modulated to 0 phase are The signals cancel each other out, and a signal 11011000011010... is obtained at the input of the light-receiving element 23, and a signal 00100111100101... whose logic is exactly the opposite of this signal is obtained at the input of the light-receiving element 24. can get. Since the output of the light-receiving element 23 is connected to the positive input of the differential amplifier 25, and the output of the light-receiving element 24 is connected to the negative input of the differential amplifier 25, the logical value equal to the positive input is obtained at the output. However, since the difference between both inputs is taken, the signal power is doubled. On the other hand, the noise on the two optical paths is generated by different light receiving elements and has no correlation with each other, so the noise power is F times as much. Therefore, compared to when the signal from one optical path is demodulated as is, can improve the signal-to-noise ratio by 3 dB.

The signal obtained at the output of the differential amplifier 25 is exactly equal to the logical value of the transmission signal applied to the input terminal 11 of the transmitter. This means that demodulation of summation logic conversion and demodulation of phase modulation are performed simultaneously. In this way, the receiving device can perform summation logic conversion and demodulation of the phase modulation signal with an extremely simple configuration.

In the above embodiment, the depth of phase modulation is set to O phase and π phase corresponding to the binary values of the digital signal.
The present invention can be implemented even when there is a phase shift. However, in this case, the signal-to-noise ratio deteriorates in accordance with the deviation.

Furthermore, the present invention can be implemented in the same manner even if the reflectance and transmittance of the prism are not exactly 50% as described above.

〔Effect of the invention〕

As explained above, according to the present invention, (1) the laser light source outputs continuous light, the spectral distribution of the output light can be extremely narrow; (2) the laser light source is not directly modulated; , the output light level can be increased, ■ Modulation is performed in the output optical path of the laser light source, so the speed of the modulated signal depends on the phase modulator and does not depend on the laser, ■ The receiver can be configured easily, ■ This has excellent effects such as improving the signal-to-noise ratio of the receiving device.

By implementing the present invention, it can be applied to optical fiber communications in the 1.5 μm band, which has been difficult to use for large-capacity communications due to the large chromatic dispersion of optical fibers, and can achieve greater capacity and longer lengths than conventional methods. It is possible to realize a relay-distance optical communication system.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention. FIG. 2 is a partial configuration diagram of the receiving device. 11... Input terminal, 12... Summation logic conversion circuit, 1
3... Shiza light source, 14... Phase modulator, 15...
- Optical transmission line, 21... Branch circuit, 22... Branch combining circuit, 23.24... Light receiving element, 25... Differential amplifier, 26... Identification circuit, 27... Output terminal .

Claims (2)

[Claims] (1) A transmitting device that transmits an optical signal modulated by a time-series digital input electrical signal, and an output electrical signal that receives the transmitted output of this transmitting device via an optical transmission line and corresponds to the input electrical signal. In an optical transmission system, the transmitting device includes: a light source; and a summator that performs an exclusive OR of the output logical value of m bits (m is a natural number) before the input electrical signal. The receiving device includes a logic conversion circuit and a modulator that performs phase modulation on the output light of the light source using the output signal of this circuit. a branching/combining circuit that further branches each of the optical signals on the optical path into two optical signals and combines the two optical signals so that each of the two optical signals coincides with the optical signals on the two optical paths; The optical path propagation time difference between the two optical paths to the combining point of this branching and combining circuit is set to be the time of the m bits, and furthermore, two light receiving elements each receiving the two combined output lights of the branching and combining circuit are arranged. , and an amplifier that receives the output electrical signals of these two light receiving elements as differential inputs. (2) The digital signal is a binary signal, and the modulator is configured to output π phase and 0 phase for this binary signal, and the branch circuit and branch synthesis circuit each have a binary signal with an energy level of approximately 50%. The optical transmission system according to claim (1), which is configured to perform branching.