JPH0448833A - Optical FM coherent optical transmission system and its equipment - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Industrial application field of the present invention
This invention relates to a coherent optical transmission system for transmitting such optical signals and its apparatus.

Conventional technology This type of optical transmission system (Japanese Patent Application No. 1-69702)
There is a coherent optical transmission method shown in No. This method will be explained below.The transmitting device electrically modulates the signal to be transmitted in advance, inputs this modulated signal to the semiconductor laser, and frequency modulates the output light of the semiconductor laser4.Then, the optical signal is sent to the receiving device. Transmit. The receiving device combines the optical signal from the local oscillation semiconductor laser with the optical signal from the transmitting device, converts both beat signals into electrical signals, and performs optical heterodyne detection. The obtained beat signal is FM demodulated, and the premodulated signal is further demodulated.

By the way, conventionally used FM demodulation circuits include, for example, "Easy to Understand FM Technology, written by Yuya Ito\Akira Fujii"
' (Electronic Science Series 26, Kosaido Sanpo Publishing) Ratio detection circuit or LPLL type FM demodulation circuit etc. 4 These conventional FM demodulation circuits (such as In the coherent optical transmission device described in the conventional example, when the beat signal after optical heterodyne detection is FM demodulated using a conventional FM demodulation circuit, the following problem is solved by the present invention: The spectral linewidth of semiconductor lasers used as light sources for transmitting equipment and local oscillation light sources for receiving equipment ranges from several MH2 to several tens of MHz.
z, the spectral linewidth of the beat signal after heterodyne detection is several tens of MHz. When such a beat signal is input to a conventional FM demodulation circuit, the broadening of the spectral linewidth appears as noise. In addition, in order to increase the signal-to-noise ratio, the present invention uses a special semiconductor laser with a spectral linewidth of 1 MHz or less, increases the modulation index, and widens the frequency band occupied by the modulation signal. For reference, the spectral linewidth is several MHz.
It is possible to use a semiconductor laser with a frequency of several tens of MHz from
An object of the present invention is to provide a coherent optical transmission system capable of good FM demodulation of N.

In order to solve the problem more than a means to solve the problem, we input the original signal, which has been electrically frequency-modulated or phase-modulated by the transmitting device, into a semiconductor laser and frequency-modulate the output light. After performing heterodyne detection in the receiving device, the product of the carrier component signal obtained by optical frequency modulation and the first side component signal is obtained, and the original signal is obtained through a band pass filter (BPF). Asahi Effect The effect when configured as above will be explained using equations.

The optical signal when the output light of the semiconductor laser is frequency-modulated with a signal of frequency fs is given by (yoE# = ACO5 (2πξ is 1βsin (2yrfs t))... (1) where LA is a constant and ξ is The frequency of the semiconductor laser light when not modulated, β is the modulation index, and the optical signal of the local oscillation semiconductor laser is EL'=B cos
(2yrζt) --- It is assumed that it is given by (2). However, LB is a constant, and ζ represents the frequency of light from the local oscillation semiconductor laser.

The beat signal after hetero gain detection is Es = Ccos (2yri = 1βs in
(2yrf* t)) ... (3) Tonamo However, LC is a constant, fs is the frequency of this difference from ξ, and when the modulation index β is β<1, the above equation can be approximated as E@saki C.J.
・cos(2πf@t) −CJ+ cos(2yrf@t−2yrfs
t)+CJI CO3(2πf・t+2πft)・・
・(4) is expressed as just L Js, J+ tt variables are Bessel functions of the first kind of 0th and 1st order of β, a The first term of equation (4) is the carrier component, and the second term is 1st
Even if the lower side wave component and the third term are the first upper side wave component, the frequency of the first side wave component of the second and third terms in equation (4) is
This is the difference or sum frequency between the frequency of the carrier wave component and the frequency of the modulation signal.

Taking the product of the carrier component and the first side component signal in equation (4), El = JI COS (2πfs t+ψ)x
(-J+ cos (2yrf@t-2yrfs
t) 10J+ cos (2πf@t+2πfst)
) = 2Js J+ cos (2yrfs 1ψ
)xsin(2πfs t)sin (2πfit)
=Js Jl (sin (ψ) −sin (4yrf @ t ten ψ) )
xsin (2yrf@t) = J−Jl sin (ψ) sin (2yr
fs t)−Js J+ /2x (cos (4
yrfa t-2yrig 1 ψ)-cos (
4πfs becomes 12πfs t+ψ)) (5). However, ψ is the delay phase difference between the carrier component signal and the first side component signal.

Finally, a band pass filter (BPF) extracts only the signal component of frequency f$ and removes harmonic components and noise in unnecessary frequency bands.

As mentioned above, the original signal is obtained by taking the difference between the frequency of the carrier wave and the frequency of the first side wave. Does this mean that the frequency fluctuations are the same?In the original signal obtained at the end, the frequency fluctuations are suppressed. Furthermore, the occupied frequency bandwidth of the optical frequency modulation signal is approximately twice the highest frequency of the signal manually input to the semiconductor laser, and the required bandwidth of the receiving device is also about the same.

Example An embodiment of the invention set forth in claim 1 is shown in FIG.

In FIG. 1, 500 is a transmitter, 1 is a semiconductor laser, 10 is an input signal, 11 is a transmission light emitted from the semiconductor laser l, 600 is a receiver, 2 is a local oscillation semiconductor laser f, and 12 is a local oscillation semiconductor laser 2. Outgoing light
3 is an optical coupler 4 is a light receiving port W! ,,5 is FM demodulation recovery
6 is a band pass filter (BPF), 90 is a receiving device 6
An output signal 20 from 00 is an input signal to the FM demodulation circuit 5. 30 indicates an output signal of the FM demodulation circuit 5. The operation will be explained below. In this example, assume that the original signal to be transmitted is a frequency modulated signal.

In the transmitting device 500, a frequency modulated signal 10 having a center frequency of fs is input to the semiconductor laser 1. In semiconductor laser l
When the signal current of the input signal 10 is slightly changed, the amplitude of the light emitted from the semiconductor laser 1 hardly changes, but the frequency shifts. In other words, frequency modulation is applied to the frequency of light. It is assumed that the optical spectrum at this time mainly consists of three components: a carrier wave with an optical frequency of ξ, a first lower side wave centered at ξ−fs, and a first upper side wave centered at ξ10fs.

This optical frequency modulated signal 11 propagates to receiving device 600.

The optical signal 11 transmitted to the receiving device 600 enters the optical coupler 3.The optical coupler 3 converts the frequency of the light into the optical signal 11 from the transmitting device 500 into the optical signal I2 of the local oscillation semiconductor laser 2. combine.

The optical signals of the two lasers combined by the optical coupler 3 enter the light receiving circuit 4, and the beat signals of both are detected.
Let the frequencies of the two lights ξ and this beat frequency be f.

That is, the plate f = ξ - ζ, and (y ζ - ξ), then the spectrum of the output signal 20 of the light receiving circuit 4 (y , three components of the first upper side wave centering on f/10 fs mainly appear, and harmonic components of the second side wave and higher are hidden in noise.

The FM demodulation circuit 5 multiplies the carrier wave component and the first side wave component. This signal 30 is passed through the BPF 6 to extract a signal having a center frequency of fs to obtain an output signal 90.

In the above example, the signal to be transmitted may be an FM signal, an FSK signal, a PM4M signal, or a PSK signal.Next, the invention according to claim 2 will be described in detail.

The invention set forth in claim 2 specifically shows the FM demodulation circuit 5 of the embodiment of the invention set forth in claim 1. The FM demodulation circuit is shown in FIG. In FIG. 2, 100 is a splitter, and 110 is a band-pass filter (B
PF), 120 indicates a mixer. The operation will be explained below. The overall operation of a coherent optical transmission device is
Since this is the same as the embodiment of the invention claimed in claim 1, here C'! .

The operation of only the FM demodulation circuit will be explained.

A carrier wave with one frequency, the first centered at f/-fs.
The signal 20 consists of three components: a lower side wave, and a first upper side wave centered on fs and fs.
The other side enters the mixer 120.4 The signal that enters the BPFIIO passes only the carrier wave component with frequency f.
Two components, the first lower side wave and the first upper side wave, are deleted. The mixer 120 multiplies the carrier component that has passed through iBPFlo and the other output signal of the splitter 100 and outputs it.The output signal 30 includes a signal with a center frequency of fs and a signal with a center frequency of 2fa±fs It contains noise in the low frequency region obtained by the product of carrier wave components.Other than the signal component whose center frequency is fs, BPF6 in Fig. 1 is included.
will be deleted.

Next, the invention according to claim 3 will be explained in detail.

The invention set forth in claim 3 specifically shows the FM demodulation circuit 5 of the embodiment of the invention set forth in claim 1. The FM @ tone circuit is shown in Figure 3. In FIG. 3, 130 is a directional coupler, and 40 and 50 are two output signals of the directional coupler 130. The operation will be explained below. Since the overall operation of the coherent optical transmission apparatus is the same as the embodiment of the invention set forth in claim 1, only the operation of the iLFM demodulation circuit will be explained here.

The first carrier wave whose frequency is f@, centering on fa-fs.
A signal 201 consisting of three components: a lower side wave, a first upper side wave centered at f@+fs enters the directional coupler 130.

By the way, the transfer characteristics and phase characteristics of the directional coupler 130 are as shown in FIGS. 4(1) and 4(2), respectively.

'Fh (2) It is the phase characteristic between PortO-2 when the phase between PortO-1 is taken as a reference.

The characteristics of this directional coupler (bandwidth is 500MH2~IG)
The figure shows a coupling degree of 3 dB at Hz.

As shown in Figure 4 (1), the transfer characteristics are as follows: The frequency is OHz.
, 1,5C:Hz, 3GHz, 4.5GHz
When hPortO-1 is shut off between FortO-2 and
There is a gap between them. Therefore, we changed the carrier frequency to 1.5G.
Hz, 3GHz.

4.5 GHz is a decisive number. fs is determined so that the frequencies of the first lower side wave and first upper side wave components are in a region where the loss of the transfer characteristic between Port O-2 is small. Figure 4 (
3) is f@=1.5GHz, fs=7
This is the spectral distribution of the carrier wave and the first side wave when the frequency is set to 50 MHz.

By arranging the signal frequencies as described above, the signal 40 shown in Figure 3 (Table
Side wave components are obtained. Then, the mixer 120 outputs the signal 40.
The product of the signal 50 and the signal 50 is taken as 1, and a signal with a center frequency f8 is obtained.

Advantages of the invention according to claim 3 over the invention according to claim 2: L(1) The number of parts is small, and (2) there is little loss due to branching.

Effects of the Invention The present invention (1) can suppress the frequency fluctuation of the light of the semiconductor laser; and (2) the band required for the receiver is the highest frequency of the frequency modulation signal or phase modulation signal manually applied to the semiconductor laser. (3) If multiple electrically frequency-modulated signals or phase-modulated signals with different center frequencies are added together and input to the semiconductor laser of the transmitting device, a multi-channel signal can be obtained. It has excellent effects, such as being able to multiplex and transmit

[Brief explanation of the drawing]

FIG. 1 shows a block diagram of a coherent optical transmission device according to a first embodiment of the present invention. FIG. 2 shows a block of an FM demodulation circuit in a coherent optical transmission device according to a second embodiment of the present invention. Figure 3 is a block diagram of an FM demodulation circuit in a coherent optical transmission device that is the third embodiment of the present invention. Figure 4 is a diagram showing the phase characteristics of a directional coupler and the spectrum distribution of a human signal. be. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 2... Semiconductor laser for local oscillation, 3... Optical coupler 4... Light receiving apposition 5...
FM demodulation circuit 6...Band pass filter (BP'F)
, 10... Human power signal 11 Transmission light emitted from semiconductor laser 1 12. Local oscillation light 20... Human power signal to FM demodulation circuit 5 30... Output signal of FM demodulation circuit 5, 40, 50...・Output signal 9 of directional coupler 130
0...Output signal from the receiving device 600, 100...
・Brancher 110...Band pass filter (BPF) with center frequency f., 120...Mixer, 130...
... Directional coupler 500 ... Transmission device 600 ...
Name of the receiving device agent: Patent attorney Shigetaka Awano

Claims (3)

[Claims] (1) The transmitting device performs optical frequency modulation on the output light of the semiconductor laser using an electrically frequency-modulated or phase-modulated original signal, and transmits the optical signal to the receiving device, and the receiving device performs local oscillation. The optical signal of the semiconductor laser for use is combined with the optical signal sent from the transmitting device, the beat signals of the two optical signals are converted into an electrical signal, and the signal obtained from the electrical signal is obtained by frequency modulation of the light. An optical FM coherent optical transmission system in which the product of the carrier wave component signal and the first side wave component signal is obtained, and the original signal is obtained using a band pass filter. (2) a transmitting device configured at least to frequency-modulate the output light of a semiconductor laser with an electrically frequency-modulated or phase-modulated original signal and transmit the optical signal to a receiving device; a local oscillation semiconductor laser; an optical coupler that couples the optical signal sent from the transmitting device with the optical signal of the local oscillation semiconductor laser; and an optical coupler that converts the output light of the optical coupler into an electrical signal and a light receiving circuit for obtaining beat signals of two optical signals; a splitter for branching the output of the light receiving circuit into two; and a first for extracting a carrier wave component of a frequency modulated optical signal from one output of the splitter. a mixer that takes the product of the carrier wave component obtained from the first band-pass filter and the other output signal of the splitter, and a second band-pass filter that extracts the original signal from the output signal of the mixer. An optical FM coherent optical transmission device comprising a receiving device comprising at least a bandpass filter. (3) a transmitting device configured at least to frequency-modulate the output light of a semiconductor laser with an electrically frequency-modulated or phase-modulated original signal and transmit the optical signal to a receiving device; a local oscillation semiconductor laser; an optical coupler that couples the optical signal sent from the transmitting device with the optical signal of the local oscillation semiconductor laser; and an optical coupler that converts the output light of the optical coupler into an electrical signal and a light receiving circuit that obtains beat signals of two optical signals; a directional coupler that branches the output of the light receiving circuit into two; a mixer that takes the product of the two output signals of the directional coupler; and an output of the mixer. The receiver comprises at least a bandpass filter that extracts the original signal from the signal, and frequency modulation of the light is performed in a frequency range in which the signal is not branched in the directional coupler and propagates only to one output end. An optical FM coherent optical transmission device in which the frequency of the light of the local oscillation semiconductor laser is controlled so that a carrier wave component of the signal is present.