JP3999425B2

JP3999425B2 - Transmitter for optical fiber transmission of high frequency signals

Info

Publication number: JP3999425B2
Application number: JP33031199A
Authority: JP
Inventors: 幹夫前田; 浩之古田
Original assignee: Japan Broadcasting Corp
Current assignee: Japan Broadcasting Corp
Priority date: 1999-11-19
Filing date: 1999-11-19
Publication date: 2007-10-31
Anticipated expiration: 2019-11-19
Also published as: JP2001147408A

Description

【０００１】
【発明の属する技術分野】
本発明は、光ファイバの分散の影響を受けにくい高周波信号の光ファイバ伝送技術に係り、特に搬送波を抑圧した両側波帯変調による高周波信号の光ファイバ伝送用送信機に関する。
【０００２】
【従来の技術】
ミリ波長の高周波信号を光強度変調方式で光ファイバに伝送する場合、光ファイバに分散があると、上側波、搬送波、下側波に到着時間差が生じ、特定の伝送距離で受光すると光電力が大きくても干渉によりミリ波信号が得られないという問題がある。この問題を解決するために、干渉の原因となる側波帯の一方、あるいは搬送波を送信しない方法が考えられている。
【０００３】
前者は単一側波帯変調（ＳＳＢ：ＳｉｎｇｌｅＳｉｄｅＢａｎｄ）と呼ばれており、後者は両側波帯抑圧搬送波変調（ＤＳＢ−ＳＣ：ＤｏｕｂｌｅＳｉｄｅＢａｎｄ−ＳｕｐｐｒｅｓｓｅｄＣａｒｒｉｅｒ）と呼ばれている。
【０００４】
図２に示すようにＭＺ変調器１０中の２つの光位相変調器１１，１２に逆位相の正弦波を加え、その出力光を光位相差πで合成するとＤＳＢ−ＳＣ変調波が得られることはよく知られている。ＤＳＢ−ＳＣは分散の影響を受けにくい別の方式であるＳＳＢと比較すると、光変調器の動作周波数が半分でよいという長所がある（古田，前田，渋谷，小山田：“ＭＺ型光変調器を用いた高周波信号の光ファイバ伝送方式”信学技報、ＯＣＳ９９−５１、ｐｐ．５７−６２．）。
【０００５】
その反面、ＤＳＢ−ＳＣは光伝送系が２逓倍器として動作するため、適用できる高周波信号方式がＦＳＫ（高周波ＦＳＫ信号の光ファイバ伝送方式：特願平１０−２２６１６６号）、ＡＳＫ（ＡＳＫ高周波信号の光ファイバ伝送用光送信機：特願平１０−１８２４１１号）等に制限されるという短所がある。
【０００６】
ＤＳＢ−ＳＣにおいて上記変調方式の制限をなくした光ファイバ伝送用送信機の一例を図３に示す。図３において、１つの光源の出射光を光位相差をπに設定した２個のＭＺ（マッハツェンダ）変調器１０，２０により分配し、ＭＺ変調器１０，２０の対の光位相変調器１１，１２，２１，２２のそれぞれに、正弦波、すなわち、逆位相関係で、ＭＺ変調器１０，２０でπ／２の光位相差となる周波数ｆｍの正弦波（図３参照）を加え、それぞれの出射光を光位相差π／２で合成する。合成光は３台目のＭＺ変調器３０に導かれる。ＭＺ変調器３０の上の光導波路には下側波が、下の導波路には上側波が分離されるので、一方に設けた電極３１により，正弦波よりも十分低い搬送周波数ｆ_IFの変調信号で位相変調を行うと、不図示の受光器で２ｆｍ−ｆ_IFが得られることは上記のＤＳＢ−ＳＣ変調の原理から容易に類推が容易である。
【０００７】
【発明が解決しようとする課題】
図３の光回路により生成した光変調信号を受信機側で受光すると、不要な成分α（周波数２ｆｍ）やβ（搬送周波数２ｆｍ＋ｆ_IF）が発生する。αの正弦波は希望信号の近傍で、かつ、希望信号よりも強度が大きいため除去するには減衰特性の厳しいフィルタが必要となる。
【０００８】
図３の光回路において、変調信号の代わりにベースバンド信号で位相変調すれば不要な成分は発生しなくなる。しかしながら、この場合には、受光後に得られる高周波信号は無線系で一般に用いられている直交変調波とは異なる。
【０００９】
例えば、ＱＰＳＫ変調を行うには図３のＭＺ変調器３０の光位相変調器３１に加える信号は４値の振幅をとる必要がある。その振幅の値は信号が加えられていない光位相に対して−３π／４，−π／４，＋π／４，＋３π／４だけ移相するように選ばれる。位相の遷移は全て円周上で、電気の直交変調のように円の中心を通る遷移がない。結果として直交変調波に対して通常用いられるロールオフ特性の送受への均等（ルート）配分を適用とする、ビット誤り率特性が大きく劣化することが報告されている（山本他：“ＰＳＫ光波多重における光波間隔の検討”映情メ学会９８年次大会６−６）。
【００１０】
そこで、本発明の目的は、不要な成分がなく、入力変調信号と同じスペクトラムが受信側で得られる高周波ＦＳＫ信号の光ファイバ伝送用送信機を提供することにある。
【００１２】
【課題を解決するための手段】
このような目的を達成するために、請求項１の発明は、光ファイバに伝送するための送信高周波変調信号を送信する光ファイバ伝送用送信機であって、光源と、該光源からの出射光を分配する分配器と、前記分配器により分配された光をそれぞれ変調する第１のマッハツェンダ変調器および第２のマッハツェンダ変調器と、前記第１のマッハツェンダ変調器に設けられた第１の光位相変調器および第２の光位相変調器と、前記第２のマッハツェンダ変調器に設けられた第３の光位相変調器および第４の光位相変調器とを具え、前記送信高周波変調信号と高周波搬送波の周波数の正弦波が用意されており、前記第１の光位相変調器〜第４の光位相変調器に対して合成して加える前記送信高周波変調信号および高周波搬送波の正弦波の位相関係を
正弦波送信高周波変調信号
第１の光位相変調器００
第２の光位相変調器 π π
第３の光位相変調器 −０．５π ０．５π
第４の光位相変調器０．５π −０．５π
となし、前記第１の光位相変調器と前記第２の光位相変調器の出力光をπの位相差で合成し、前記第３の光位相変調器と前記第４の光位相変調器の出力光をπの位相差で合成し、前記第１のマッハツェンダ変調器と第２のマッハツェンダ変調器の出力光をπ／２の位相差で合成することを特徴とする。
【００１３】
【発明の実施の形態】
以下、図面を参照して本発明の実施形態を詳細に説明する。
【００１４】
（第１の実施形態）
本願発明者は、「ＢＰＳＫ変調信号を伝送しようとすると、光伝送系が２逓倍器として動作するため，受光後の高周波信号から変調成分が失われ、この原因が側波帯が両方とも変調されるためである」ことに気が付き、一方の側波帯を無変調とする本実施形態を発明した。
【００１５】
本発明第１の実施形態の回路構成を図１に示す。図１において、図２の従来例と同様の個所には同一の符号を付している。
【００１６】
図１において、光位相変調器１１には正弦波と不図示の高周波変調信号発生回路からの送信高周波変調信号を共に０（ゼロ）、光位相変調器１２には正弦波と高周波変調信号を共にπの位相関係となるように合成して加える。光位相変調器２１には正弦波を−π／２、高周波信号をπ／２の位相関係で合成し、光位相変調器２２には正弦波をπ／２、高周波信号を−π／２の位相関係で合成して加える。以上の正弦波と高周波変調信号の関係を下記に示す。
【００１７】

ＭＺ変調器１０および２０の出力光はπ／２の位相で合成される。これにより光出力のスペクトルから分るように受信機側では２ｆｍの変調波が得られる。
【００１８】
（第２の実施形態）
図１の電気移相器をハイブリッドで置き換えた回路構成を図４に示す。図４において、１０１は光周波数ｆｃの光源、１０２は光２分配器である。１０３は搬送波周波数がｆｍの高周波変調信号（発生器）、１０４は周波数ｆｍの正弦波，１０５は９０度のハイブリッドである。１０６は１８０度のハイブリッド、１０７はＭＺ変調器（図１のＭＺ変調器１）である。１０８は電極（図１の光位相変調器１１）、１０９は電極（図１の光位相変調器１２）である。１１０はＭＺ変調器（図１のＭＺ変調器２０）、１１１は電極（図１の光位相変調器２１）、１１２は電極（図１の光位相変調器２２）である。１１３は光位相差π／２の光２合成器、１１４は出力光である。なお、上記２つのＭＺ変調器の光合成位相差は上記電極に重畳する直流電圧を調整することで容易にπ／２に設定可能である。
【００１９】
回路動作は図１の回路と同じであり、詳細な説明を要しないであろう。
【００２０】
【発明の効果】
以上、説明したように、本発明によれば、送信する高周波の変調方式に依存しないＤＳＢ−ＳＣ信号を簡易な光回路で実現できる。また、従来発生していた不要な光成分も発生しない。
【図面の簡単な説明】
【図１】本発明実施例の回路構成を示す回路図である。
【図２】従来の回路構成を示す回路図である。
【図３】従来の他の回路を示す回路図である。
【図４】本発明実施形態の他の回路構成を示す回路図である。
【符号の説明】
１０、２０，３０ＭＺ変調器
１１，１２，２１，２２、３１光位相変調器[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-frequency signal optical fiber transmission technique that is not easily affected by optical fiber dispersion, and more particularly, to a high-frequency signal optical fiber transmission transmitter using double-sideband modulation with suppressed carrier waves.
[0002]
[Prior art]
When transmitting millimeter-wave high-frequency signals to an optical fiber using the optical intensity modulation method, if there is dispersion in the optical fiber, there will be a difference in arrival time between the upper wave, carrier wave, and lower wave. There is a problem that even if it is large, a millimeter wave signal cannot be obtained due to interference. In order to solve this problem, a method of not transmitting one of the sidebands causing interference or a carrier wave has been considered.
[0003]
The former is called single sideband modulation (SSB), and the latter is called double sideband suppressed carrier modulation (DSB-SC: Double Sideband Suppressed Carrier).
[0004]
As shown in FIG. 2, a DSB-SC modulated wave can be obtained by adding anti-phase sine waves to the two

optical phase modulators

11 and 12 in the MZ modulator 10 and synthesizing the output light with an optical phase difference π. Is well known. DSB-SC has the advantage that the operating frequency of the optical modulator may be half compared to SSB, which is another system that is not easily affected by dispersion (Furuta, Maeda, Shibuya, Koyamada: “MZ type optical modulator High-frequency signal optical fiber transmission system used "Science Technical Report, OCS 99-51, pp. 57-62.).
[0005]
On the other hand, since the optical transmission system operates as a doubler in DSB-SC, applicable high frequency signal systems are FSK (optical fiber transmission system for high frequency FSK signals: Japanese Patent Application No. 10-226166), ASK (ASK high frequency signal). The optical transmitter for optical fiber transmission: Japanese Patent Application No. 10-182411) and the like.
[0006]
FIG. 3 shows an example of an optical fiber transmission transmitter in which the above-described modulation scheme restriction is eliminated in DSB-SC. In FIG. 3, the light emitted from one light source is distributed by two MZ (Mach-Zehnder)

modulators

10 and 20 having an optical phase difference set to π, and a pair of

optical phase modulators

11 and 20 of the

MZ modulators

10 and 20 12, 21, and 22 are each added with a sine wave, that is, a sine wave having a frequency fm (see FIG. 3) having an optical phase difference of π / 2 in the

MZ modulators

10 and 20 in an antiphase relationship. The emitted light is synthesized with an optical phase difference of π / 2. The combined light is guided to the third MZ modulator 30. Since the lower side wave is separated from the optical waveguide above the MZ modulator 30 and the upper side wave is separated from the lower waveguide, the electrode 31 provided on one side modulates the carrier frequency f _IF sufficiently lower than the sine wave. When phase modulation is performed with a signal, it is easy to analogize from the principle of DSB-SC modulation that 2fm-f _IF can be obtained with a light receiver (not shown).
[0007]
[Problems to be solved by the invention]
When the optical modulation signal generated by the optical circuit of FIG. 3 is received on the receiver side, unnecessary components α (frequency 2fm) and β (carrier frequency 2fm + f _IF ) are generated. Since the α sine wave is in the vicinity of the desired signal and has a higher intensity than the desired signal, a filter with a strict attenuation characteristic is required to remove it.
[0008]
In the optical circuit of FIG. 3, unnecessary components are not generated if phase modulation is performed with a baseband signal instead of a modulation signal. However, in this case, the high-frequency signal obtained after light reception is different from the orthogonal modulation wave generally used in the radio system.
[0009]
For example, in order to perform QPSK modulation, a signal applied to the optical phase modulator 31 of the MZ modulator 30 in FIG. The value of the amplitude is selected so that the phase is shifted by −3π / 4, −π / 4, + π / 4, + 3π / 4 with respect to the optical phase to which no signal is added. All the phase transitions are on the circumference, and there is no transition through the center of the circle like the quadrature modulation of electricity. As a result, it has been reported that the bit error rate characteristic is greatly deteriorated by applying the equal distribution (root) distribution to the transmission and reception of the roll-off characteristic normally used for the orthogonal modulation wave (Yamamoto et al .: “PSK lightwave multiplexing”). Examination of Light Wave Intervals in the “The 98th Annual Conference 6-6)
[0010]
Accordingly, an object of the present invention is to provide a transmitter for transmitting an optical fiber of a high frequency FSK signal that has no unnecessary components and can obtain the same spectrum as an input modulation signal on the receiving side.
[0012]
[Means for Solving the Problems]
In order to achieve such an object, the invention of claim 1 is an optical fiber transmission transmitter for transmitting a transmission high-frequency modulation signal for transmission to an optical fiber, the light source and light emitted from the light source. a divider for dividing the said the first Mach-Zehnder modulator and the second Mach-Zehnder modulator for modulating the distributor by the distributed light, respectively, said first first optical phase provided in MZ modulator comprising a modulator and a second optical phase modulator, and a third optical phase modulator and the fourth optical phase modulator provided in said second Mach-Zehnder modulator, the transmission high-frequency modulation signal and high-frequency carrier A sine wave having a frequency of 1 is prepared, and the phase relationship between the transmission high frequency modulation signal and the sine wave of the high frequency carrier wave is synthesized and added to the first optical phase modulator to the fourth optical phase modulator.
Sine wave transmission high frequency modulation signal first optical phase modulator 0 0
Second optical phase modulator π π
Third optical phase modulator −0.5π 0.5π
Fourth optical phase modulator 0.5π−0.5π
The output light of the first optical phase modulator and the second optical phase modulator is synthesized with a phase difference of π, and the third optical phase modulator and the fourth optical phase modulator The output light is synthesized with a phase difference of π, and the output lights of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator are synthesized with a phase difference of π / 2.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0014]
(First embodiment)
The inventor of the present application stated, “When trying to transmit a BPSK modulated signal, the optical transmission system operates as a doubler, so that the modulation component is lost from the high-frequency signal after light reception, and this is because both sidebands are modulated. Therefore, the present embodiment has been invented in which one sideband is unmodulated.
[0015]
The circuit configuration of the first embodiment of the present invention is shown in FIG. In FIG. 1, the same parts as those in the conventional example of FIG.
[0016]
In FIG. 1, a sine wave and a transmission high-frequency modulation signal from a high-frequency modulation signal generation circuit (not shown) are both 0 (zero) in the optical phase modulator 11, and a sine wave and a high-frequency modulation signal are both in the optical phase modulator 12. It is synthesized and added so as to have a phase relationship of π. The optical phase modulator 21 combines a sine wave with a phase relationship of −π / 2 and a high frequency signal with a phase relationship of π / 2, and the optical phase modulator 22 has a sine wave of π / 2 and a high frequency signal of −π / 2. Add by phase relationship. The relationship between the above sine wave and the high frequency modulation signal is shown below.
[0017]

The output lights of the

MZ modulators

10 and 20 are combined with a phase of π / 2. As a result, a 2 fm modulated wave is obtained on the receiver side as can be seen from the spectrum of the optical output.
[0018]
(Second Embodiment)
FIG. 4 shows a circuit configuration in which the electric phase shifter of FIG. 1 is replaced with a hybrid. In FIG. 4, reference numeral 101 denotes a light source having an optical frequency fc, and reference numeral 102 denotes an optical two distributor. 103 is a high frequency modulation signal (generator) having a carrier frequency of fm, 104 is a sine wave of frequency fm, and 105 is a 90 degree hybrid. Reference numeral 106 denotes a 180-degree hybrid, and reference numeral 107 denotes an MZ modulator (MZ modulator 1 in FIG. 1). Reference numeral 108 denotes an electrode (optical phase modulator 11 in FIG. 1), and reference numeral 109 denotes an electrode (optical phase modulator 12 in FIG. 1). 110 is an MZ modulator (MZ modulator 20 in FIG. 1), 111 is an electrode (optical phase modulator 21 in FIG. 1), and 112 is an electrode (optical phase modulator 22 in FIG. 1). 113 is an optical 2 combiner having an optical phase difference of π / 2, and 114 is output light. Note that the photosynthesis phase difference between the two MZ modulators can be easily set to π / 2 by adjusting the DC voltage superimposed on the electrode.
[0019]
The circuit operation is the same as the circuit of FIG. 1 and will not require detailed description.
[0020]
【The invention's effect】
As described above, according to the present invention, a DSB-SC signal that does not depend on a high-frequency modulation scheme to be transmitted can be realized with a simple optical circuit. Further, unnecessary light components that have been generated conventionally are not generated.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a circuit configuration of an embodiment of the present invention.
FIG. 2 is a circuit diagram showing a conventional circuit configuration.
FIG. 3 is a circuit diagram showing another conventional circuit.
FIG. 4 is a circuit diagram showing another circuit configuration of the embodiment of the present invention.
[Explanation of symbols]
10, 20, 30

MZ modulator

11, 12, 21, 22, 31 Optical phase modulator

Claims

An optical fiber transmission transmitter for transmitting a transmission high-frequency modulation signal for transmission to an optical fiber,
A light source;
A distributor for distributing light emitted from the light source;
A first Mach-Zehnder modulator and a second Mach-Zehnder modulator for respectively modulating the light distributed by the distributor ;
A first optical phase modulator and a second optical phase modulator provided in the first Mach-Zehnder modulator;
A third optical phase modulator and a fourth optical phase modulator provided in the second Mach-Zehnder modulator,
Said transmission being high sinusoid of frequency of circumferential waves varying Choshin No. and high-frequency carrier is prepared, the transmission frequency modulation added synthesizing and relative to said first optical phase modulator to fourth optical phase modulators The phase relationship between the signal and the sine wave of the high-frequency carrier
Sine wave transmission high frequency modulation signal first optical phase modulator 0 0
Second optical phase modulator π π
Third optical phase modulator −0.5π 0.5π
Fourth optical phase modulator 0.5π−0.5π
And none,
The output lights of the first optical phase modulator and the second optical phase modulator are combined with a phase difference of π, and the output lights of the third optical phase modulator and the fourth optical phase modulator are combined. Combining with a phase difference of π,
An optical fiber transmission transmitter characterized by combining output lights of the first Mach-Zehnder modulator and the second Mach-Zehnder modulator with a phase difference of π / 2.