JP4199611B2 - Optical wavelength division multiplexing equipment - Google Patents

Description

【００２９】
波形２６がファイバ１０を歪みなく伝送されたものとすると、受信側の光学フィルタ１２に入力される図２の波形２７は波形２６と同じである。光学フィルタ２２のインパルスレスポンスｓ４（ｔ）である波形２８の全エネルギーの９０％のエネルギーを含む時間長をＴ４秒とすると、光学フィルタ１２の出力ポート２２における出力信号ｓ５（ｔ）の波形２９は波形２７と波形２８の畳み込み積分であり、数２で表される。
【００３０】
【数２】

【００３１】
その全エネルギーの９０％のエネルギーを含む時間長Ｔ５秒はおよそＴ３＋Ｔ４秒となっている。波形２９が検波器１４で検波されるとそのエンベロープ波形３０から１ビット波形３３が得られ、前後のビットの波形３４、３５を０と１の組み合わせで加算し、ジッタとノイズを載せると図４に示すアイパターンが得られる。
【００３２】
ここで、アイパターンが最大に開く条件、ナイキストの第２クリテリア、を求める。アイパターンの縦軸に関しては、アイパターンの最大開口ポイント３１と３２においてＣ／Ｎが最大になればよい。信号理論から明らかな様にノイズが重畳された系では、アイパターンの最大開口ポイントにおけるＣ／Ｎが最大になる条件は、波形２７と波形２８が数３に示す関係となっていれば良い。
【００３３】
【数３】

【００３４】
この関係は波形２７と波形２８の畳み込み積分が互いに共通の自己相関関数となることで、この場合、波形２７に対して光学フィルタ１２は「整合フィルタ」となる。理論的には、波形２７と波形２８が周波数領域で振幅が同じで位相が共役であればよいが、実際の受動素子でそうしたフィルタは実現できない。しかし、波形２７と波形２８がそれぞれピークに対して偶対称形であれば位相は考慮しなくて良いので、波形２７と波形２８がピークに対して偶対称形となりかつ同じ波形となれば良く、これに近い特性は現実に実現可能である。
【００３５】
次にアイパターンの横軸に対しては、アイパターンの最大開口ポイントの近辺において、前後のビットによる波形ができるだけ広い範囲で直交する、即ち０に近い値であればよい。このためには、波形２７、波形２８のピークから±Ｔ秒近辺でできるだけ０に収束していれば良く、エンベロープがオーバーシュート無く、急激に単調減衰する波形が望ましい。
【００３６】
信号理論から明らかな様に、時間領域あるいは周波数領域のいずれかの領域で、急激に矩形波の形で波形を減衰させるともう一方の領域ではオーバーシュートが出てまた信号の占有帯域幅あるいは信号長が広がる。このため両方の帯域を同時には制限できない。
【００３７】
たとえば、図５に示す周波数領域で矩形波的に選択性を持たせたバターワ−ス型（フラットトップ）フィルタでは、インパルスレスポンスにオーバーシュートが生じ長く尾を引く。このためこのフィルタを用いると隣接ビット信号のアイパターンの開口ポイント近まで残って尾を引いてノイズとして重畳されるので、符号間干渉の妨害信号となり信号品質が劣化する。また反対に図６に示すように時間領域で矩形波の１ビット信号波形を使用すると、スペクトラムが広がり、隣接チャネルの信号に重畳して隣接チャネル信号のＣ／Ｎを劣化させる。
【００３８】
両方の領域を同じように穏やかに制限する信号として、図７に示すガウス型波形がある。数４に両方の領域の関係を示す。
【００３９】
【数４】

【００４０】
この特性は両方の帯域を同様に穏やかに制限できるので、図１２に示した非特許文献１の例ではこのタイプのフィルタを採用している。しかし、実際のシステムでは、素子や信号の精度、必要とされる選択度、ノイズの性質、実際のフィルタの作りやすさ等から必ずしも最適ではない。たとえば、周波数領域で通過帯域内での反射損がやや急峻で希望信号のスペクトラムが削られる割には、阻止域の減衰が穏やかであり、妨害干渉の抑圧効果が小さい。
【００４１】
これに対し、周波数領域での通過帯域の反射損を少なくし、また阻止域の減衰を多くして特性を改善し、そして時間領域ではインパルスレスポンスにオーバーシュートを生じないフィルタとして、本発明では数５に示されるようなベッセル型のフィルタを用いる。
【００４２】
【数５】

【００４３】
また、ベッセル型フィルタの特性は図８に示される。数５の例は５次のベッセルフィルタの例であるが、他の次数のベッセルフィルタも同様にオーバーシュートが少ない性質を持ち、次数が高いほど周波数領域の選択特性が高くなる。
【００４４】
このようなベッセル型フィルタを用いることにより、特性を改善することができ、信号パワーの損失を抑えてＣ／Ｎを確保し、また、製造誤差による中心波長、あるいは中心周波数のずれに対し安定である。またアイパターンの横軸に関しては、ガウスタイプのフィルタの場合よりもアイの開口の幅が狭くなるが、実用上の差は殆ど無い。また、このようなベッセル型フィルタは、光学薄膜フィルタとして実現できる。
【００４５】
それ故、図１に示す実施例としては、先ず受信側の光学フィルタ７として、インパルスレスポンスの波形２８のエンベロープの半値幅が光パルス信号のハルス間隔Ｔ秒に近い光学ベッセルフィルタを用いる。５次のベッセルフィルタの場合、−３ｄＢ通過帯域幅はおよそ伝送レートＦビット／秒に近い値となる。つぎに送信信号の波形２６が波形２８に近くなるように送信側の光学フィルタ７と変調器出力の波形２４を決める。例えば、矩形波に近い光パルスの波形２４と−３ｄＢ帯域幅が受信側の光学フィルタ１２のそれよりもやや広いベッセルフィルタの光学フィルタとの組み合わせ、あるいはガウス波形に近いパルス波形２４と−３ｄＢ帯域幅が受信側の光学フィルタのそれよりも２倍以上広いバターワース特性の光学フィルタ７との組み合わせなどが可能であり、システムを構築し易い組み合わせとする。
【００４６】
【実施例】
本発明の実施例として図８に示す５次のベッセルフィルタを用いた場合と、比較例として図５に示す５次のバターワース型フィルタを用いた場合の比較を示す。
【００４７】
図３の入力信号が、９０％のエネルギーを含む時間幅が１ビット長T秒とする。両方のフィルタの−３ｄBの通過対域幅をともにFヘルツとする。基準としてフィルタの通過対域幅がともに十分広く、アイパターンに時間軸の擾乱であるジッタ、振幅の擾乱であるノイズが載っていなかったものとし、この時のアイパターンの開口の高さがともに１とする。
【００４８】
つぎに、それぞれのフィルタの−３ｄB通過帯域幅をFとし、アイパターンにパルス間隔Tの３％のジッタと、ノイズとしてアイパターンの開口１の１％のノイズがのるものとする。すると、アイパターンの開口の高さは、比較例であるバターワース型フィルタの場合は０．３０に減少するのに対し、本発明実施例であるベッセルフィルタでは０．５２となり、アイパターンの減少が抑えられた。これに伴い信号の誤り率の低下を抑えることができる。
【００４９】
以上より、本発明実施例によれば、Ｄ−ＷＤＭにおいて最良の信号品質の光波長多重伝送装置ができる。
【図面の簡単な説明】
【図１】本発明の光波長多重伝送装置を示す図である。
【図２】本発明の光波長多重伝送装置における送信側の信号波形を示す図である。
【図３】本発明の光波長多重伝送装置における受信側の信号波形を示す図である。
【図４】本発明の光波長多重伝送装置におけるアイパターンを示す図である。
【図５】バターワース型フィルタの特性を示す図である。
【図６】矩形波の信号を示す図である。
【図７】ガウス型フィルタの特性を示す図である。
【図８】本発明で用いるベッセルフィルタの特性を示す図である。
【図９】従来例の分散補償デバイスの例を示す図である。
【図１０】従来のＣＳ−ＲＺを示す図である。
【図１１】従来例のデュオバイナリ信号を示す図である。
【図１２】従来例の整合フィルタを用いたシステムを示す図である。
【符号の説明】
１、２ 光源
３、４ 変調器
５、６ 変調信号源
７、８、１２、１３ 光学フィルタ
９ 合波器
１０ ファイバ
１１ 分波器
１４ 検波器
１５ 光増幅器
１６、１７ 出力端子
１８、１９出力ポート
２０、２１、２２、２３ 出力ポート
２４ １ビット光パルス信号の波形
２５、２８ 光学フィルタのインパルスレスポンスの波形
２６、２９ 出力信号の波形
２７ 入力信号の波形
３０ エンベロープ波形
３１、３２ アイパターンサンプル点
３４、３５、３６ １ビット波形[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength division multiplexing (WDM) optical transmission apparatus that suppresses intersymbol interference between signals of 1-bit signals or 1-symbol signals, and suppresses interference interference between wavelength channels, thereby combining, transmitting, and separating. Relates to wave and detection techniques.
[0002]
[Prior art]
In a wavelength division multiplexing (WDM) system, assuming that the pulse interval of each bit signal is T (seconds) and the signal transmission rate is F (bits / second), the channel frequency interval is 40 Fhertz or more. However, for 2.5 Gbit / s, the channel wavelength interval is 100 GHz or more. In such a wavelength multiplexing system, interference between wavelengths and signal degradation caused by signal processing in wavelength multiplexing, transmission, splitting, and detection via a wavelength selection element have not been a problem.
[0003]
On the other hand, high-density wavelength division multiplexing (DWDM) (Dense WDM) has been studied in order to increase the use efficiency of wavelengths. In this wavelength multiplexing system, assuming that the pulse interval of each bit signal is T (seconds) and the signal transmission rate is F (bits / second), the channel frequency interval is 2.5 F Hertz, for example, the transmission rate F is 40 Gbits. The channel frequency interval is 100 GHz per second. In this case, interference between wavelengths and signal degradation caused by signal processing in wavelength multiplexing, transmission, splitting, and detection via the wavelength selection element are serious problems.
[0004]
For this reason, technologies for suppressing signal degradation, such as dispersion compensation and error correction in transmission systems, are being developed. However, it is not an accurate method in terms of signal transmission theory, but largely depends on empirical rules and usage techniques. The signal information theory is not a solution that quantifies the phenomenon from both the frequency domain and the time domain.
[0005]
Conventionally, as shown in FIG. 9, an apparatus device generally called dispersion compensation is an apparatus device that compensates for a simple slope of a group delay time of a fiber. In this dispersion compensation method, a complicated group delay time of a DWDM system is used. Deviation cannot be compensated, and signal degradation and communication quality degradation cannot be compensated.
[0006]
As a method for maintaining the communication quality by reducing the power of the occupied spectrum and reducing the inter-channel interference even if the 1-bit optical signal is shortened, a modulated signal such as CS-RZ (Carrier Suppressed Return to Zero) shown in FIG. However, the spectrum and optical pulse shape occupied by 1 bit are not different from the RZ (Return to Zero) signal.
[0007]
In addition, the duo binary method shown in FIG. 11 is a combination of coding and modulation methods, in which signal quality deterioration due to spectrum narrowing is dispersed on the time axis, and information deterioration is small in the entire system. However, this is not the optimum condition in terms of signal information theory.
[0008]
As is clear from the above, these conventional techniques do not always require optimum conditions in terms of signal information in both devices and modulation / demodulation systems.
[0009]
In recent research, a transmission method using a matched optical filter for a transmitter and a receiver as shown in FIG. 12 has been reported (Non-Patent Document 1). This study is still a theoretical stage, and instead of optical matched filters that cannot be realized in practice, the characteristics of a fictional super gaussian filter (Super Gaussian) are used as simulations, and normal RZ, CSRZ, and NRZ signals are compared. The signal quality is examined in combination.
[0010]
In this research, it is quite a technique that incorporates signal theory, but since a matched filter cannot be realized, it is still not always optimized in terms of signal theory in terms of combining super Gaussian filters and signals of ordinary modulation methods. There is room for improvement.
[0011]
[Non-Patent Document 1]
Bosco. G, et al., "On the use of NRZ, RZ, and CSRZ modulation at 40 Gb / s with narrow DWDM channel spacing" Lightwave Technology Sept. 2002
[0012]
[Problems to be solved by the invention]
As described above, in these conventional techniques, the optimum condition is not always obtained in terms of the signal information theory in both the device and the modulation / demodulation method.
[0013]
Therefore, the present invention provides an optical wavelength division multiplex transmission apparatus that suppresses intersymbol interference between signals of 1-bit signal or 1-symbol signal and suppresses interference interference between wavelength channels.
[0014]
[Means for Solving the Problems]
In the present invention, a 1-bit signal waveform and a wavelength selection filter for simultaneously suppressing intersymbol interference (Inter Symbol Interference) between bit signal sequences in each channel and interference interference between each channel are considered in the time domain and the frequency domain simultaneously. Thus, the optimum design is performed in terms of information signal theory. That is, in the present invention, the waveform of the 1-bit signal and the characteristics of the wavelength selection filter are grasped in both the time domain and the frequency domain, and a combination of characteristics having conditions as close as possible to the Nyquist criteria is obtained.
[0015]
Therefore, optical signals from a plurality of transmission units are multiplexed at the multiplexing unit, transmitted through one optical fiber, demultiplexed by the demultiplexing unit, guided to the corresponding receiving unit, and each transmitting unit is defined. An optical pulse generator having a channel wavelength and a transmission optical filter, and a corresponding receiving unit is an optical wavelength multiplex transmission apparatus having a reception optical filter, and the optical pulse is transmitted on the transmission side for each channel. The optical signal is received and detected on the receiving side through the optical filter. The pulse interval of each optical pulse train is expressed as T (seconds), and the signal transmission rate is expressed as F (bits / second). FT = 1, the channel frequency interval is between 1 and 3 times the transmission rate F, and at least the reception optical filter is a Bessel filter.
[0016]
In addition, the sum of the time length including 90% of the total energy of the impulse response of the optical filter of the transmission unit and the time length including 90% of the total energy of the optical pulse is 0. 0 of the pulse interval T. It is characterized by being 8 to 1.2 times.
[0017]
Furthermore, the -3 dB pass bandwidth of the optical filter of the receiving unit is 1 to 2 times the transmission rate F.
[0018]
[Action]
In particular, the present invention relates to a wavelength division multiplexing (WDM) optical transmission device by combining the optical filter of the present invention and a signal waveform, and particularly suppresses intersymbol interference in each channel of the multiplexed optical signal and interference interference between the channels simultaneously. Waves, transmission, demultiplexing and detection are possible, resulting in a high quality communication system.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings. It should be noted that the present invention is not limited to the following examples, and modifications and improvements can be made without departing from the spirit of the present invention.
[0020]
FIG. 1 shows an embodiment of the present invention. This is an example of two channels, but in practice it can be applied to a multi-channel optical multiplex transmission apparatus having a similar structure.
[0021]
On the transmission side, optical carrier signals of specified channel wavelengths are output from the light sources 1 and 2 and input to the modulators 3 and 4, respectively. Modulator signals from modulation signal sources 5 and 6 are input to the respective modulators, and modulated optical pulse signals are output from output ports 18 and 19 and input to optical filters 7 and 8. The signals of the respective filters are output from the output ports 20 and 21, combined by the multiplexer 9, passed through the fiber 10, and input to the receiving-side duplexer 11. The output from the duplexer 11 is input to the optical filters 12 and 13. The signals are output from the output ports 22 and 23 of each filter and input to the detector 14 and the optical amplifier 15. Detection signals and amplified optical signals are output from the respective output terminals 16 and 17.
[0022]
Here, in the present invention, the pulse interval of each optical pulse train is T (seconds), the signal transmission rate is F (bits / second), and FT = 1, and the channel frequency interval is 1 to 3 times the transmission rate F. In addition, a Bessel filter was used as at least the optical filters 12 and 13 on the receiving side.
[0023]
The sum of the time length including 90% of the total energy of the impulse response of the optical filters 7 and 8 on the transmitting side and the time length including 90% of the total energy of the optical pulse is the pulse interval T. It is preferable that it is 0.8 to 1.2 times.
[0024]
Further, the −3 dB pass bandwidth of the optical filters 12 and 13 on the receiving side is preferably 1 to 2 times the transmission rate F.
[0025]
Hereinafter, the present invention will be described in detail.
[0026]
In FIG. 1, attention is paid to a certain signal channel. For example, the light source 1, the modulator 3, the modulation signal source 5, the optical filter 7 and the optical filter 12, and the detector 14 are devices related to the signal channel. 2 and 3 show the time-domain waveforms of optical signals on the transmission side and the reception side, respectively.
[0027]
The time length including 90% of the total energy of the waveform 24 of the 1-bit optical pulse signal s1 (t) at the output port of the modulator 3 is T1 seconds, and 90% of the total energy of the impulse response s2 (t) of the optical filter. When the time length including the energy of T2 is T2 seconds, the waveform 26 of the output signal s3 (t) at the output port 20 of the optical filter 7 is a convolution integral of the waveform 25 and the waveform 26, and is expressed by Equation 1. The time length T3 seconds including 90% of the total energy is approximately T1 + T2 seconds.
[0028]
[Expression 1]

[0029]
Assuming that the waveform 26 is transmitted through the fiber 10 without distortion, the waveform 27 of FIG. 2 input to the optical filter 12 on the receiving side is the same as the waveform 26. When the time length including 90% of the total energy of the waveform 28 which is the impulse response s4 (t) of the optical filter 22 is T4 seconds, the waveform 29 of the output signal s5 (t) at the output port 22 of the optical filter 12 is This is a convolution integral of the waveform 27 and the waveform 28 and is expressed by the following equation (2).
[0030]
[Expression 2]

[0031]
The time length T5 seconds including 90% of the total energy is approximately T3 + T4 seconds. When the waveform 29 is detected by the detector 14, a 1-bit waveform 33 is obtained from the envelope waveform 30, and the waveforms 34 and 35 of the preceding and succeeding bits are added in a combination of 0 and 1, and jitter and noise are added. The eye pattern shown in FIG.
[0032]
Here, a condition for opening the eye pattern to the maximum, Nyquist's second criteria, is obtained. Regarding the vertical axis of the eye pattern, C / N may be maximized at the maximum opening points 31 and 32 of the eye pattern. As is clear from signal theory, in a system in which noise is superimposed, the condition that the C / N at the maximum aperture point of the eye pattern is maximized is that the waveform 27 and the waveform 28 have the relationship shown in Equation 3.
[0033]
[Equation 3]

[0034]
This relationship is that the convolution integral of the waveform 27 and the waveform 28 is a common autocorrelation function. In this case, the optical filter 12 is a “matched filter” with respect to the waveform 27. Theoretically, the waveform 27 and the waveform 28 may have the same amplitude and phase conjugate in the frequency domain, but such a filter cannot be realized with an actual passive element. However, if the waveform 27 and the waveform 28 are even-symmetrical with respect to the peak, the phase need not be taken into account, so that the waveform 27 and the waveform 28 are even-symmetrical with respect to the peak and have the same waveform. A characteristic close to this is actually realizable.
[0035]
Next, with respect to the horizontal axis of the eye pattern, in the vicinity of the maximum opening point of the eye pattern, the waveform by the preceding and succeeding bits may be orthogonal to the widest possible range, that is, close to zero. For this purpose, it is only necessary to converge to 0 as much as possible around ± T seconds from the peaks of the waveforms 27 and 28, and a waveform in which the envelope does not overshoot and abruptly monotonically attenuates is desirable.
[0036]
As is clear from signal theory, if the waveform is abruptly attenuated in the form of a rectangular wave in either the time domain or the frequency domain, an overshoot appears in the other domain and the occupied bandwidth of the signal or signal The length spreads. For this reason, both bands cannot be restricted simultaneously.
[0037]
For example, in a Butterworth type (flat top) filter having rectangular wave selectivity in the frequency domain shown in FIG. 5, an overshoot occurs in the impulse response, and the tail is long. For this reason, when this filter is used, it remains near the opening point of the eye pattern of the adjacent bit signal and is tailed and superimposed as noise, resulting in an interference signal of intersymbol interference and signal quality degradation. On the other hand, when a rectangular 1-bit signal waveform is used in the time domain as shown in FIG. 6, the spectrum is broadened and superimposed on the adjacent channel signal to degrade the C / N of the adjacent channel signal.
[0038]
A signal that gently and gently limits both regions is the Gaussian waveform shown in FIG. Equation 4 shows the relationship between both regions.
[0039]
[Expression 4]

[0040]
Since this characteristic can gently limit both bands similarly, this type of filter is employed in the example of Non-Patent Document 1 shown in FIG. However, in an actual system, the accuracy of elements and signals, the required selectivity, the nature of noise, the ease of making an actual filter, etc. are not necessarily optimal. For example, although the reflection loss in the passband is somewhat steep in the frequency domain and the spectrum of the desired signal is cut, the attenuation in the stopband is moderate, and the interference interference suppression effect is small.
[0041]
On the other hand, as a filter that reduces the reflection loss of the pass band in the frequency domain, improves the characteristics by increasing the attenuation of the stop band, and does not cause an overshoot in the impulse response in the time domain, the present invention has several A Bessel type filter as shown in FIG.
[0042]
[Equation 5]

[0043]
The characteristics of the Bessel type filter are shown in FIG. The example of Equation 5 is an example of a fifth-order Bessel filter, but the Bessel filters of other orders also have the property of less overshoot, and the higher the order, the higher the frequency domain selection characteristics.
[0044]
By using such a Bessel filter, characteristics can be improved, C / N can be secured by suppressing loss of signal power, and stable against a shift in center wavelength or center frequency due to manufacturing errors. is there. Regarding the horizontal axis of the eye pattern, the width of the eye opening is narrower than in the case of a Gaussian filter, but there is almost no practical difference. Moreover, such a Bessel type filter can be realized as an optical thin film filter.
[0045]
Therefore, in the embodiment shown in FIG. 1, first, as the optical filter 7 on the receiving side, an optical Bessel filter in which the half width of the envelope of the waveform 28 of the impulse response is close to the Halth interval T seconds of the optical pulse signal is used. In the case of a fifth-order Bessel filter, the −3 dB pass bandwidth is a value close to the transmission rate F bits / second. Next, the transmission-side optical filter 7 and the modulator output waveform 24 are determined so that the waveform 26 of the transmission signal is close to the waveform 28. For example, a combination of an optical pulse waveform 24 close to a rectangular wave and a Bessel filter optical filter whose -3 dB bandwidth is slightly wider than that of the optical filter 12 on the receiving side, or a pulse waveform 24 close to a Gaussian waveform and a -3 dB band A combination with the optical filter 7 having a Butterworth characteristic whose width is twice or more wider than that of the optical filter on the receiving side is possible, and the combination is easy to construct a system.
[0046]
【Example】
A comparison between the case where the fifth-order Bessel filter shown in FIG. 8 is used as an embodiment of the present invention and the case where a fifth-order Butterworth filter shown in FIG. 5 is used as a comparative example is shown.
[0047]
The input signal of FIG. 3 has a time width including 90% energy and a 1-bit length T seconds. Let the -3 dB pass-to-bandwidth of both filters be F hertz. As a reference, it is assumed that both the pass-band width of the filter is sufficiently wide, and that the jitter that is the disturbance of the time axis and the noise that is the disturbance of the amplitude are not on the eye pattern. Set to 1.
[0048]
Next, it is assumed that the -3 dB passband width of each filter is F, and that the jitter of 3% of the pulse interval T and the noise of 1% of the aperture 1 of the eye pattern appear on the eye pattern. Then, the height of the opening of the eye pattern decreases to 0.30 in the case of the Butterworth filter as a comparative example, whereas it becomes 0.52 in the Bessel filter as an example of the present invention, and the reduction of the eye pattern is reduced. It was suppressed. Accordingly, it is possible to suppress a decrease in signal error rate.
[0049]
As described above, according to the embodiment of the present invention, an optical wavelength division multiplexing transmission device having the best signal quality in D-WDM can be obtained.
[Brief description of the drawings]
FIG. 1 is a diagram showing an optical wavelength division multiplex apparatus of the present invention.
FIG. 2 is a diagram showing signal waveforms on the transmission side in the optical wavelength division multiplex transmission apparatus of the present invention.
FIG. 3 is a diagram showing signal waveforms on the receiving side in the optical wavelength division multiplex transmission apparatus of the present invention.
FIG. 4 is a diagram showing an eye pattern in the optical wavelength division multiplexing transmission apparatus of the present invention.
FIG. 5 is a diagram illustrating characteristics of a Butterworth filter.
FIG. 6 is a diagram illustrating a rectangular wave signal.
FIG. 7 is a diagram illustrating characteristics of a Gaussian filter.
FIG. 8 is a diagram showing characteristics of a Bessel filter used in the present invention.
FIG. 9 is a diagram illustrating an example of a conventional dispersion compensation device.
FIG. 10 is a diagram showing a conventional CS-RZ.
FIG. 11 is a diagram showing a conventional duobinary signal.
FIG. 12 is a diagram illustrating a system using a conventional matched filter.
[Explanation of symbols]
1, 2 Light source 3, 4 Modulator 5, 6 Modulation signal source 7, 8, 12, 13 Optical filter 9 Multiplexer 10 Fiber 11 Demultiplexer 14 Detector 15 Optical amplifier 16, 17 Output terminal 18, 19 Output port 20, 21, 22, 23 Output port 24 1-bit optical pulse signal waveform 25, 28 Optical filter impulse response waveform 26, 29 Output signal waveform 27 Input signal waveform 30 Envelope waveform 31, 32 Eye pattern sampling point 34 , 35, 36 1-bit waveform

Claims (3)

Optical signals from a plurality of transmitters are multiplexed by a multiplexer, transmitted via one optical fiber, demultiplexed by a demultiplexer, guided to a corresponding receiver, and each transmitter has a predetermined channel. An optical wavelength multiplex transmission apparatus having a wavelength optical pulse generator and a transmission optical filter, and a corresponding receiving unit having a reception optical filter, and transmitting optical pulses through the optical filter on the transmission side for each channel. The reception side receives and detects an optical signal through the optical filter, and represents the pulse interval of each optical pulse train as T (seconds) and the signal transmission rate as F (bits / second). FT = 1, the channel frequency interval is between 1 and 3 times the transmission rate F, and at least the receiving optical filter is a Bessel filter. The sum of the time length including 90% of the total energy of the impulse response of the optical filter of the transmitter and the time length including 90% of the total energy of the optical pulse is 0.8 to 0.8 of the pulse interval T. 2. The optical wavelength division multiplexing apparatus according to claim 1, wherein the optical wavelength division multiplexing apparatus is 1.2 times. The optical wavelength division multiplexing apparatus according to claim 1 or 2, wherein a -3 dB passband width of the optical filter of the receiving unit is 1 to 2 times a transmission rate F.

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