JPH06303196A - Optical transmission system - Google Patents

Abstract

PURPOSE:To obtain an optical transmission system which is capable of determining both the setting conditions of the optical modulation degree for attaining a desired code error ratio and the number of frequency multiple channel and a technique setting the conditions automatically, in an optical transmission where a direct intensity modulation is performed by a frequency multiple digital signal. CONSTITUTION:In a transmission side, the signal source 10 of a frequency multiple digital signal, an adjusting part 11 adjusting an optical modulation degree, the number of frequency multiple channel or the both of them, and an optical transmission part 12 converting the signal into an optical signal and transmitting it to an optical reception part are provided. In a reception side, an optical reception part 13 and a detection part 14 detecting the instantaneous worst value of the distortion of a signal converted into an electrical signal and delivering the information to the transmission side are provided. Then in the reception side, the instantaneous worst value of the distortion of the reception signal is detected and the information is delivered to the transmission side. In the transmission side, the setting conditions of the optical modulation degree for attaining a desired code error ratio and the number of frequency multiple channel are determined based on the information, the conditions are realized in the adjusting part and they are outputted to the optical transmission part.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system for transmitting frequency-multiplexed digital signals such as those used in optical CATV and portable telephone signal optical transmission systems.

[0002]

2. Description of the Related Art Optical transmission of frequency-multiplexed signals is described in Tanabe et al.
"80-channel AM-FDM TV optical signal transmitter", National Technical Report Vol. 36 N
o. 6 Dec. 1990, C.I. J. Chung a
nd I.n. Jacobs, "TV channel c
apacity of lightwave mult
ichannel AM SCM systems a
s limited by laser thresh
old nonlinearity ”, in Opti
cal Communication Center
nce, March 1992, pp. It has very good strain characteristics as shown in 18-19. Due to such characteristics, in the frequency-multiplexed optical transmission system including the digital modulation signal, even if the sum of the optical modulation degrees of the signals of the respective channels is 1 or more, the I. M. I. Ha
bbab, "Multichannel M-QAM
For CATV Distribution ”, LE
OS Summer Topical Meeting
Digest, Wednesday, July 2
9, 1992 WB4 pp. It is considered that high quality transmission is possible as described in 21-22.

Digitally modulated signals can be transmitted with high quality even with a small carrier to noise ratio (hereinafter referred to as CNR). Therefore, even if the optical modulation degree is set to be small and optical transmission is performed, J. G. Proakis "Digital C
communications "Second Edi
section, McGraw-Hill Book Co
mpany, pp. It is considered that the transmission quality can be secured if the CNR in the relationship between the required CNR and the code error rate given by H.234-285 is obtained.

[0004]

On the other hand, a semiconductor laser (hereinafter referred to as LD) light is directly intensity-modulated by a frequency-multiplexed signal, and the amount of distortion in optical transmission is determined by the characteristics of the LD. In particular, instantaneous distortion occurs due to waveform clipping caused by the LD modulation current value of the frequency-multiplexed signal being equal to or less than the LD threshold current value. This waveform clipping is such that when the LD light is directly intensity-modulated and a signal is transmitted, the sum of the optical modulation degrees of each channel of the multiplexed signal (hereinafter referred to as the maximum optical modulation degree) is 1 or more (hereinafter referred to as overmodulation). If so, it is a phenomenon that occurs in principle and cannot be avoided. Distortion due to this waveform clipping is instantaneous, and therefore it is not a value that affects the code error rate when considered as an average value, but since it is instantaneous, its effect on the code error rate cannot be ignored. Conceivable.

Conventionally, such a distortion has hardly been considered, but it is experimentally observed as follows. That is, FIG. 5 shows the π / 4 shift Q of 33 channels.
It is a code error rate characteristic when a PSK signal is optically transmitted.
However, the other 32 channels except the one for measuring the bit error rate are unmodulated sine waves. The vertical axis represents the code error rate, and the horizontal axis represents the relative optical modulation index (hereinafter referred to as QPSK relative modulation index) to the other 32 channels of the π / 4 shift QPSK signal which is the central channel for measuring the code error rate. is there. The optical modulation degrees of the 32 channels are all equal, and the optical modulation degree m per channel is a parameter. 3 when QPSK relative modulation is 0 dB
All three channels have the same degree of optical modulation.

The respective channel arrangements are 822.8 to 829.
There are 33 channels at 200 MHz intervals at 2 MHz, and the frequency of the π / 4 shift QPSK signal of the central channel is 826 MHz. To be exact, π / 4 shift differential QP
SK modulation is performed with a root Nyquist filter characteristic and a pseudo-random signal of 15 stages of 42 kbps with a roll-off rate α = 0.5.

As can be seen from FIG. 5, when the modulation factor m per channel exceeds 4.2%, the code error rate characteristic deteriorates, and as the modulation factor m per channel increases, the characteristic deteriorates. I understand. Overmodulation does not occur until the modulation factor m per channel is approximately 3.3% (3.3% × 32 = 1.06), and the modulation factor m per channel does not exceed 4.2%. Even if modulation occurs, it is stochastically small and its influence hardly changes the code error rate characteristic, but beyond that, the code error rate characteristic deteriorates rapidly due to the effect of overmodulation. Under this condition, the CNR is sufficiently large, and it is clear that the deterioration of the code error rate characteristic is due to overmodulation.

Since it is a narrow band signal, it is CNR and CTB (composite triple beat) that mainly affect the code error rate characteristic, and the analog transmission characteristic is usually evaluated by measuring these. Intermodulation distortion can be treated in the same way as CTB, so here the distortion is CT
Consider only B. FIG. 6 shows the optical modulation index characteristics which are the measurement results of CNR and CTB in the experimental system in which the code error rate is measured.
Shown in. The vertical axis represents CNR and CTB, and the horizontal axis represents the modulation factor m per channel.
The / 4 shift QPSK signal was not input, and the CNR and CTB were measured at 826 MHz which is the frequency of this channel. However, both are values measured by time average using a spectrum analyzer. As can be seen from FIG. 5 described above, when there is no influence of overmodulation, the relationship between CNR and code error rate characteristics can be considered by the conventional theory shown in the above literature, but the influence of overmodulation appears. Once started, the code error rate characteristic deteriorates as the modulation degree m increases,
For example, the QPSK relative modulation factor (hereinafter referred to as m (QPSK)) for achieving the code error rate of 10 ^{-6 is} that the modulation factor m per channel increases from 4.2% to 7.3%. Therefore, from about -53 dB to about -16 dB, about 37
It is degraded by dB. However, the CTB shown in FIG. 6 does not show such a large deterioration. Naturally, the CNR is improved as the modulation degree m is increased.

As described above, the code error rate characteristic cannot be estimated from the CNR and CTB, and considering the relationship between the CNR and the code error rate characteristic in the conventional theory shown in the above-mentioned document has an influence of overmodulation. If there is, you can't. Therefore, it is not possible to evaluate the transmission performance except to install the code error rate measuring device on the transmitting side and the receiving side and directly measure the code error rate.
In addition, there is a problem that the optimum degree of optical modulation and the number of frequency-multiplexed channels cannot be set.

The present invention solves these problems, and in the optical transmission in which the semiconductor laser light is directly intensity-modulated with a frequency-multiplexed digital signal and transmitted, the optical signal of the frequency-multiplexed digital signal for achieving a desired code error rate is transmitted. An object of the present invention is to provide an optical transmission system capable of determining the setting conditions of the degree of modulation and the number of frequency-multiplexed channels from the instantaneous worst value of distortion even in the case of overmodulation, and a method for automatically performing the above operation.

[0011]

On the transmitting side, a signal source of a frequency-multiplexed digital signal, an adjusting unit for adjusting the optical modulation degree of the frequency-multiplexing digital signal or the number of frequency-multiplexed channels or both, and an output of the adjusting unit are provided. An optical transmission unit that converts the optical signal and transmits the optical signal to the optical reception unit is provided, and the optical reception unit on the reception side and the instantaneous worst value of the distortion of the signal converted into the electric signal by the optical reception unit are detected, A configuration is provided in which a detection unit that sends the information is provided on the transmission side.

[0012]

In optical transmission in which semiconductor laser light is directly intensity-modulated and transmitted by a frequency-multiplexed digital signal, the receiving side detects the instantaneous worst value of distortion of the received signal and sends the information to the transmitting side. On the transmitting side, the setting conditions of the optical modulation degree and the number of frequency-multiplexed channels of the frequency-multiplexed digital signal for achieving a desired code error rate are determined on the basis of the information, and the adjustment section realizes the conditions and realizes the optical Output to the transmitter. This realizes optical transmission with a desired code error rate characteristic.

When deciding the setting conditions of the optical modulation degree of the frequency-multiplexed digital signal and the number of frequency-multiplexed channels in accordance with the performance of the LD or the like when manufacturing the device, a spectrum analyzer measures the instantaneous worst value of distortion after optical transmission. , May be determined so that the desired bit error rate characteristic can be realized.

[0014]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an optical transmission device according to the first embodiment of the present invention.

In FIG. 1, 10 is a signal source of a frequency-multiplexed digital signal, 11 is an adjusting unit for adjusting the optical modulation degree of the frequency-multiplexing digital signal or the number of frequency-multiplexing channels, or both, and 12 is an optical signal for the output of the adjusting unit. An optical transmission unit for converting the light into an optical reception unit and transmitting the optical transmission to the optical reception unit.
Reference numeral 4 is a detection unit that detects an instantaneous worst value of distortion of a signal converted into an electric signal by the optical reception unit and sends the information to the transmission side, 15 is a detection unit output, and 16 is a transmission output.

The operation of the optical transmission apparatus of this embodiment having the above-mentioned structure will be described below.

The frequency-multiplexed digital signal from the signal source 10 is adjusted in its optical modulation degree, the number of frequency-multiplexed channels, or both by the adjusting section 11, and is input to the optical transmitting section 12. The optical transmitter 12 converts the optical signal into an optical signal, and after the transmission, the optical signal is converted into an electric signal in the optical receiver 13. Optical receiver 1
The output 3 is the transmission output 16 of the present optical transmission device. The detection unit 14 detects the instantaneous worst value of the distortion in the transmission output 16,
The information is sent to the adjustment unit 11 on the transmission side as the detection unit output 15. The transmission method of the detector output 15 does not depend on the transmission path such as an optical fiber or a coaxial cable. In order to achieve a desired bit error rate characteristic according to the detector output 15, the adjuster 11
The optical modulation degree of the frequency-multiplexed digital signal or the number of frequency-multiplexed channels or both are adjusted and output to the optical transmitter.
Details regarding the adjustment of the optical modulation degree and the number of frequency-multiplexed channels will be described below.

FIG. 6 shows the optical modulation index characteristics of CNR and CTB, both of which are values measured by time averaging using a spectrum analyzer. As described above, the deterioration of the code error rate characteristics is caused by these. It could not be estimated.
On the other hand, FIG. 2 shows the measurement result of the optical modulation degree characteristic of the instantaneous worst value [CTB _WORST ] of CTB. The instantaneous worst value is the value measured in the Max Hold mode of the spectrum analyzer TR4172 manufactured by Advantest, and the measurement conditions at this time are: span 100 kHz, sweep time 500 ms, resolution 3 kHz, video bandwidth 3
It is the worst value for 10 minutes at kHz. As can be seen from FIG. 2, the modulation factor m per channel is 4.2% to 7.3%.
The worst case value of CTB is about -65.
There is a deterioration of about 37 dB from dB to about -28 dB. This deterioration amount is the QP for achieving the above-mentioned code error rate of 10 ^-6.
It is equal to the deterioration amount of the SK relative modulation factor m (QPSK). Both m (QPSK) and the CTB instantaneous worst value are shown in FIG.
From this figure, it can be seen that both show almost the same deterioration tendency. For reference, the difference between the two is illustrated in FIG. 4 with respect to the optical modulation index m. Thus, the difference between the two is almost constant at about 12 dB when the modulation degree m per channel is 4.2% or more.

As described above, m (QPSK) and CT
Since the B instantaneous worst value has a substantially constant relationship, if the CTB instantaneous worst value is set to a certain value, the code error rate characteristic can be automatically set. For example, as a target specification, a code error rate of 10 ^-6 and a margin of 20 d
When trying to achieve with B, m (QPSK) is -20 dB
It should be so. The light modulation degree m for that is shown in FIG.
It can be seen that it is about 6.7%. At the same time C
It can be seen that the worst value for TB instant is about -33 dB. Therefore, while measuring the CTB instantaneous worst value,
If the light modulation degree m is set to 33 dB, the light modulation degree m automatically becomes 6.7%, and the target specification is achieved. The optical transmission device of FIG. 1 automatically performs this operation. If it suffices to find out that the target specification has been exceeded, the detection unit 14 monitors the CTB instantaneous worst value, and issues an alarm if a value worse than the CTB instantaneous worst value corresponding to the target specification is detected. You can do it like this.

The product of the optical modulation factor m of the frequency-multiplexed signal and the square root of the number of frequency-multiplexed channels is called the total modulation factor. If this is kept constant, the current amplitude distribution of the entire signal that modulates the LD is the same. Because of this, the conditions for modulating the LD are the same. Although the numerical conditions for adjusting the optical modulation index m are shown in this embodiment, considering the overall modulation index, it is sufficient to set the overall modulation index in order to achieve a desired code error rate characteristic. That is, the number of frequency-multiplexed channels may be changed instead of changing the optical modulation degree m. However, when changing the number of frequency-multiplexed channels, the square root is effective, so it is necessary to change at a ratio of the square to the optical modulation degree.

In the present embodiment, the detector 14 is provided on the receiving side to feed back information to the transmitting side. However, the detecting section 14 is provided on the transmitting side and a part of the optical output of the optical transmitting unit 12 is extracted. , Detect the instantaneous worst value of the distortion there,
The information may be fed back to the adjusting unit 11.

In this embodiment, the modulation method is QPS.
Although K is used, it is also effective for other digital modulation signals such as QAM (quadrature amplitude modulation).

[0024]

According to the present invention, the degree of optical modulation of the frequency-multiplexed digital signal for achieving a desired code error rate in optical transmission in which the semiconductor laser light is directly intensity-modulated and transmitted by the frequency-multiplexed digital signal. And the setting condition of the frequency multiplex channel number can be determined from the instantaneous worst value of distortion,
Moreover, the operation can be automatically realized, and its practical effect is great.

[Brief description of drawings]

FIG. 1 is a block diagram showing an optical transmission device according to a first embodiment of the present invention.

FIG. 2 is an optical modulation degree characteristic diagram of an instantaneous worst value [CTB _WORST ] of CTB.

FIG. 3 shows a QPSK relative modulation factor m (QPSK) and a CTB instantaneous worst value [CTB] for achieving a code error rate of 10 ^−6.
_WORST ]

FIG. 4 is an optical modulation degree characteristic diagram of a difference between a QPSK relative modulation degree m (QPSK) and a CTB instantaneous worst value [CTB _WORST ].

[Fig. 5] Bit error rate characteristic diagram

FIG. 6 is an optical modulation degree characteristic diagram of CNR and CTB.

[Explanation of symbols]

 10 signal source of frequency-multiplexed digital signal 11 adjusting unit 12 optical transmitting unit 13 optical receiving unit 14 detecting unit 16 transmission output

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location H04J 14/02 H04L 27/18 Z 9297-5K 9372-5K H04B 9/00 E (72) Inventor Hiroyuki Sasai 1006 Kadoma, Kadoma-shi, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (4)

[Claims] 1. In optical transmission in which a semiconductor laser light is directly intensity-modulated and transmitted by a frequency-multiplexed digital signal, the optical modulation degree and the number of frequency-multiplexed channels of the frequency-multiplexed digital signal for achieving a desired code error rate are set. An optical transmission system, wherein setting conditions are determined from an instantaneous worst value of distortion due to the frequency-multiplexed digital signal. 2. A frequency multiplexing digital signal modulation method is π.
The optical transmission system according to claim 1, wherein the optical transmission system is a / 4 shift QPSK (Quaternary Phase Modulation) signal. 3. In optical transmission in which a semiconductor laser light is directly intensity-modulated and transmitted by a frequency-multiplexed digital signal, an instantaneous worst value of distortion due to the frequency-multiplexed digital signal is detected, and the instantaneous worst value of the distortion is a predetermined value. If more than
An optical transmission method characterized by issuing a warning. 4. In optical transmission in which a semiconductor laser light is directly intensity-modulated and transmitted by a frequency-multiplexed digital signal, the receiving side detects an instantaneous worst value of distortion due to the frequency-multiplexed digital signal and sends the information to the transmitting side. The optical transmission system is characterized in that the transmitting side adjusts the optical modulation degree of the frequency-multiplexed digital signal or the number of frequency-multiplexed channels or both according to the information.