JP3866082B2 - Method and apparatus for measuring characteristics of optical modulator - Google Patents

Description

【００１６】
次に、導波路２，３で変調を行った場合を考える。変調をかけていない場合と同様に、電界Ｅ_ｉｅｘｐ（ｊω_０ｔ）を有する光を入射させ、導波路２，３で変調を加え、更にＹ分岐２で合波した光の電界成分Ｅ（ｔ）は、次の式２で表せる。
【式２】

【００１７】
ここで、Ａ_１，Ａ_２は、変調の深さを表すパラメータ（変調指数）、ω_ｍは変調信号の角周波数、φ_１，φ_２は各導波路により異なる変調信号の位相、φ_Ｂ１，φ_Ｂ２は導波路の初期状態による位相を表す。
式２は、合波した光波が様々な周波数成分を有すること示している。ここでは、このような周波数成分の内、入射光と同じ周波数成分（式２中でｎ＝０の場合）と、＋１次の高次項（式２中でｎ＝１の場合）に着目する。
入射光と同じ周波数成分（ω_０）と＋１次の高次項（ω_０＋ω_ｍ）の光強度を測定する。図６に光スペクトラムアナライザの測定波形を示す。
これらの各成分の光強度は、次の式３（入射光と同じ周波数成分の光強度Ｐ_０）及び式４（＋１次の高次項の光強度Ｐ_１）で表せる。
【式３】

【式４】

【００１８】
また、ＤＣ電源により光変調器に印加するバイアス電圧を調整し、入射光と同じ周波数成分（ω_０）の出射光の光強度が最大となるように設定した時（図２のａ点にバイアス電圧が設定された状態）、入射光と同じ周波数成分（ω_０）の出射光の光強度値Ｐ_０ｂは、上記式３により、次の式５で表せる。
【式５】

【００１９】
また、バイアス電圧を調整し、入射光と同じ周波数成分（ω_０）の出射光の光強度が最小となるように設定した時（図２のｂ点にバイアス電圧が設定された状態）、＋１次の高次項（ω_０＋ω_ｍ。変調周波数に係る成分）の出射光の光強度値Ｐ_１ｃは、上記式４より、次の式６で表せる。
【式６】

【００２０】
ここで、変調をかけていない出射光の光強度値Ｐ_０ａで、上述したＰ_０ｂ，Ｐ_１ｃを規格化すると、次の式７及び８が得られる。
【式７】

【式８】

【００２１】
式７及び式８を用いて、変調の深さを表すパラメータＡ_１，Ａ_２を算出する。なお、式７及び式８は超越方程式になるが、Ａ_１，Ａ_２が小さい値を占めることから解の特定が可能となる。
そして、Ａ_１，Ａ_２を算出した後、光変調器の半波長電圧Ｖ_πは、次の式９により算出することが可能である。ただし、Ｖ_ｍは変調信号として印加した発振器からの電圧振幅を示す。
【式９】

【００２２】
上述したように、本発明の光変調器の特性測定方法によれば、光スペクトラムアナライザを用いて得られる光変調器の出射光のスペクトル分布から、上記の光強度値Ｐ_０ａ、Ｐ_０ｂ、Ｐ_１ｃを測定し、該光強度値に基づき光変調器の半波長電圧値が算出される。このため、従来のように出射光の光強度の波形を直接観察する必要が無く、高周波においても光変調器の特性を測定することが可能となる。
また、光変調器の半波長電圧を測定する際に、従来は光変調器に印加する変調信号の電圧振幅は半波長電圧値以上の範囲まで可変調整されていたが、式９により任意の値である変調信号の電圧振幅値Ｖｍを用いて半波長電圧を算出することが可能となるため、従来のような半波長電圧値まで変調信号を調整する必要がなく、しかも、変調信号の電圧振幅値を可変することも不要となる。
さらに、光スペクトラムアナライザにより高次の１次項のみを測定するため、アンプなどの高周波成分による測定への影響を受けないため、より正確な半波長電圧の測定が可能となる。
【００２３】
ＭＺ型光変調器では、図５に示す導波路２及び３に印加する変調電圧の違いによる変調の差異により、合波時には光の強度だけでなく位相も変調される。この現象をチャーピングと呼び、チャーピングの大きさは、次の式１０に表すαパラメータにより与えられる。
【式１０】

【００２４】
式１０の中で、ｄＩは光の強度変化量、ｄφは光の位相変化量を表し、また、Ａ１，Ａ２は、上述したように出射光のスペクトル分布より測定された特定の光強度値により算出されているため、αパラメータは算出可能である。
【００２５】
本発明は、上述した光変調器の特性測定方法に限るものではなく、例えば、上記特性測定方法で用いた、＋１次の高次項（周波数ω_０＋ω_ｍのスペクトル成分）を、−１次の高次項（周波数ω_０−ω_ｍのスペクトル成分）に置き換えて用い、同様な効果を得ることができるものなども包含する。
【００２６】
次に、本発明の光変調器の特性測定方法を用いた測定装置について説明する。
図７は、本発明の光変調器の特性を自動的に測定するための装置のブロック図である。
周波数ω_０を有するレーザ光は、偏波コントローラを通過して一定方向に偏波され、ＭＺ型光変調器に入射する。光変調器には、ＤＣ電源及び周波数ω_ｍの発振器とで形成される、特定のバイアス電圧を中心に周波数ω_ｍで振動する変調信号が印加される。
【００２７】
変調信号は光変調器に入力されると同時に、分岐回路を経てＲＦパワーメータにより変調信号の電圧振幅値Ｖｍ、バイアス電圧値などが検出・監視される。
ＲＦパワーメータにより測定された数値は、制御用コンピュータに入力され、後述する光変調器の特性測定を自動化するための検出信号の一つとして利用される。
【００２８】
光変調器に印加された変調信号に対応して、入射光は変調され、光変調器から出射光として出射される。
出射光は、光スペクトラムアナライザにより周波数に対する光強度分布であるスペクトル分布が測定される。
光スペクトラムアナライザの測定値、特に、レーザ光の周波数ω_０、＋１次の高次項（周波数ω_０＋ω_ｍ。または、−１次高次項であってもよい。）に係る光強度の測定値が、制御用コンピュータに入力される。
【００２９】
光変調器の特性測定を自動的に測定するプロセスについて説明する。
レーザ、偏波コントローラ、光スペクトラムアナライザなど測定に必要な各種機材を動作させた状態において、まず、制御用コンピュータからの指示により、ＤＣ電源及び発振器による光変調器への変調信号の印加を停止させ、無変調状態における周波数ω_０における光強度Ｐ_０ａを測定し、制御用コンピュータに該測定値を取り込む。
次に、制御用コンピュータからの指示により、光変調器に変調信号を印加させ、光スペクトラムアナライザからの周波数ω_０における光強度を測定し、制御用コンピュータに入力する。制御用コンピュータでは、周波数ω_０における光強度のモニタに合わせて、変調信号のバイアス電圧を規定するＤＣ電源の出力電圧を変化させ、該変化に応じてモニタする該光強度が最大となる光強度Ｐ_０ｂを決定する。
また、制御用コンピュータでは、周波数ω_０における光強度のモニタに合わせて、変調信号のバイアス電圧を規定するＤＣ電源の出力電圧を変化させ、該変化に応じてモニタする該光強度が最小となる状態を決定し、該状態時の＋１次の高次項（周波数ω_０＋ω_ｍ）の光強度Ｐ_１ｃを光スペクトラムアナライザにより測定し、その測定値を取り込む。
【００３０】
次に、制御用コンピュータに取り込まれた、各光強度値Ｐ_０ａ、Ｐ_０ｂ、Ｐ_１ｃに基づき、上記式７，８の方程式を解くことにより、Ａ１，Ａ２を決定し、さらに上記式９，１０を用いて、半波長電圧Ｖπ、チャープパラメータ（αパラメータ）を演算し、決定する。
なお、周波数ω_０や、周波数ω_ｍ及び電圧振幅値Ｖｍなどの各値については、光変調器の特性測定に際し使用するレーザや発振器などにより、予め設定されている値を制御用コンピュータに入力して用いても良いし、光スペクトラムアナライザやＲＦパワーメータにより測定された値を用いても良い。
また、上述した測定装置では、制御用コンピュータ上で、測定値の取り込みや各種演算も実行しているが、これらの情報処理については、制御用コンピュータとは別に設けられた記憶装置や演算装置を用いて実行しても良い。
【００３１】
図４の測定装置を用いて測定した結果を、図８，９に示す。
図８は、測定周波数１０ＧＨｚの変調信号を印加した場合の光変調器の特性測定結果である。ここでは、変調信号の電圧振幅値Ｖｍを２〜９Ｖの範囲で可変して、各電圧振幅値Ｖｍにおける半波長電圧値及びチャープパラメータを算出した。
図８を見ると、半波長電圧値及びチャープパラメータは、共に、電圧振幅によらずほぼ一定の値を示しており、本発明の測定方法及び装置が、光変調器の特性を適正に測定していることが理解できる。
なお、チャープパラメータについては、変調信号の電圧振幅値が半波長電圧値より大きい場合や、変調信号の電圧振幅値が小さい場合は、光変調器に係る特性の測定結果が若干変動している。これは、変調信号の電圧振幅値を、半波長電圧以上のように大きくすると、変調信号の波形に歪が発生し、光変調器からの出射光の光強度が、理論値から外れたりノイズを多く含むこととなり、正確な測定が困難となるためと考えられる。
また、変調信号の電圧振幅値が小さ過ぎると、＋１次の高次項など、変調を反映した光スペクトル分布が明確に形成されず、ノイズの影響を強く受けて正確な測定が困難となることが考えられる。
【００３２】
図９は、測定周波数１０〜４０ＧＨｚの変調信号を印加した場合の光変調器の特性測定結果を示す。
図９が示すように、本発明の測定方法及び装置を利用すれば、従来のように光変調器からの出射光の変動を直接観測すること無しに、高周波数の特性まで有効に測定できることが可能となる。
【００３３】
【発明の効果】
以上説明したように、本発明によれば、光変調器から出射する光信号のスペクトル分布を測定することにより、光変調器の特性を測定することが可能となるため、従来のような高周波数に対応した高精度な発振器やサンプリングオシロスコープなどを必要とせず、安価でかつ精度の高い測定が可能となる。
しかも、入射光と同じスペクトル成分における出射光の光強度が最大となる場合の該スペクトル成分の出射光の光強度値Ｐ_０ｂと、入射光と同じスペクトル成分における出射光の光強度が最小となる場合の変調周波数に係る成分の出射光の光強度値Ｐ_１ｃとを、変調電圧を印加しない場合の出射光の光強度値Ｐ_０ａにより規格化した値を用いることにより、光変調器に係る半波長電圧値やチャープパラメータを容易に算出することができる。
【図面の簡単な説明】
【図１】 ＭＺ型光変調器の概略図。
【図２】 ＭＺ型光変調器の印加電圧に対する出力光強の特性を示すグラフ。
【図３】 ＭＺ型光変調器の電気入力波形に対する光出力波形との関係を示すグラフ。
【図４】 本発明の光変調器の特性測定方法の一例を示すブロック図。
【図５】 ＭＺ型光変調器の光導波路の構成図。
【図６】 光変調器から出射する光信号のスペクトル分布を示すグラフ。
【図７】 本発明の光変調器の特性測定装置の一例を示す概略図。
【図８】 本発明を利用して測定した光変調器の各種特性値に対する、変調信号の入力電圧振幅の変化の影響を示すグラフ。
【図９】 本発明を利用して測定された、光変調器の各種特性と変調信号の周波数との関係を示すグラフ。
【符号の説明】
１０ ＭＺ型光変調器
２０ 光導波路
３０ 信号源[0001]
BACKGROUND OF THE INVENTION
The present invention divides incident light into two or more, such as a Mach-Zehnder interferometric optical modulator (hereinafter referred to as an MZ optical modulator) used for optical communication and optical measurement, and The present invention relates to a method and apparatus for measuring characteristics of an optical modulator that performs phase modulation by applying an electric signal to at least one and then generates an intensity-modulated optical signal by synthesizing the branched light. The present invention relates to a method and apparatus for measuring characteristics such as a half-wave voltage value and a chirp parameter value of the optical modulator during high frequency modulation.
[0002]
[Prior art]
With the recent increase in demand for communication of voice, data, etc., there is an increasing demand for an optical communication system that enables high-capacity data to be communicated at high speed.
In such a large-capacity and high-speed communication system, an MZ type optical modulator capable of high-speed modulation operation for data generation is used.
[0003]
The basic operation of the MZ type optical modulator will be described.
As shown in FIG. 1, the MZ type optical modulator includes an optical waveguide 20 for guiding a light wave on a substrate 10 having an electro-optic effect such as LiNbO _3, and a high-speed microwave band in the light wave. It is comprised by the electrode (not shown) etc. for applying a modulation signal. The operation principle of the MZ type optical modulator is that the light input from one end of the optical waveguide 20 is branched halfway and the refractive index changes depending on the magnitude of the voltage of the electric signal applied from the signal source 30. As the light passes through the substrate, a difference in speed occurs between the lights, and when the branched lights merge, a phase shift occurs, and the combined light output changes in intensity according to the electrical signal. Indicates.
[0004]
FIG. 2 is a graph showing the change in the optical output intensity with respect to the input voltage of the signal source 30 applied to the MZ type optical modulator. The point a in FIG. 2 represents an “On operation” state in which the light passing through the MZ type optical modulator is maximized, and the point b represents an “Off operation” state in which the light passing through is minimized. The voltage required for the switching operation between “operation” and “off operation” (the magnitude of the input voltage value between points a and b) is called a half-wave voltage (V _π ), and the characteristics of the optical modulator are It is one of the important parameters to evaluate.
[0005]
As a method for measuring the half-wave voltage, a voltage having a waveform shown as an “electrical input waveform” as shown in FIG. 3 is applied to the MZ type optical modulator from the signal source 30 of FIG. Observe the intensity waveform of the output light directly with a sampling oscilloscope. The light intensity waveform of the output light observed with the sampling oscilloscope is shown as “observation waveform of the optical modulator output” in FIG.
Then, by measuring the voltage applied to the optical modulator, that is, the input amplitude value of the “electrical input waveform” in FIG. 3 in a state where the amplitude value of the observation waveform is maximum as shown in FIG. The half-wave voltage of the modulator was measured.
[0006]
[Problems to be solved by the invention]
The characteristics of the optical modulator such as the half-wave voltage value change according to the frequency of the electrical signal applied to the optical modulator even in the same optical modulator. In addition, with the recent increase in the speed and capacity of optical communications, the driving frequency of the optical modulator is also increased, and it is required to accurately measure characteristics such as a half-wave voltage value even at a frequency of 10 GHz or higher. Yes.
However, when a measuring instrument for directly observing a waveform such as a sampling oscilloscope is used, if the frequency of the applied voltage increases, it becomes difficult to observe an accurate waveform due to the problem of the frequency characteristics of the light receiving system. In addition, it is more difficult to measure the characteristics of optical modulators such as accurate half-wave voltage values due to insufficient output from signal generators and amplifiers that drive optical modulators, and waveform distortion caused by harmonics. It was.
[0007]
An object of the present invention is to solve the above problems and provide a method and apparatus for accurately measuring characteristics such as a half-wave voltage value and a chirp parameter value of an optical modulator during high frequency modulation.
[0008]
[Means for Solving the Problems]
In order to solve the above problems, in the method for measuring characteristics of an optical modulator according to the present invention , phase modulation is performed by branching incident light into two or more and applying an electrical signal to at least one of the branched lights. Thereafter, a spectral distribution of the optical signal is measured for an optical modulator that generates an optical signal whose intensity is modulated by synthesizing the branched light, and the optical modulation is calculated from a measurement value related to the measured spectral distribution. A characteristic value related to intensity modulation of the vessel is calculated.
In the characteristic measuring method of the optical modulator according to claim 1, the characteristic value related to intensity modulation of the optical modulator is a half-wave voltage value of the optical modulator, and the characteristic measuring of the optical modulator according to claim 2 is performed. The method is characterized in that the characteristic value related to the intensity modulation of the optical modulator is a chirp parameter value of the optical modulator.
[0009]
In the optical modulator characteristic measuring method according to claim 3, in the optical modulator characteristic measuring method according to claim 1 or 2 , the electrical signal includes a bias voltage and a modulated signal, and the measured value is When the light intensity of the emitted light at the same spectral component as the incident light is maximized by variably adjusting the bias voltage, the light intensity value P _0b of the emitted light of the spectral component and the light intensity of the emitted light at the same spectral component as the incident light Is the light intensity value P _1c of the emitted light of the component related to the modulation frequency when the modulation signal is minimum, and the light intensity value P _0a of the emitted light when the modulation signal is not applied, and relates to the intensity modulation of the optical modulator In calculating the characteristic value, a value obtained by standardizing the light intensity value P _0b and the light intensity value P _1c with the light intensity value P _0a is used.
[0012]
According to a fourth aspect of the present invention, there is provided an optical modulator characteristic measuring apparatus using the optical modulator characteristic measuring method according to any one of the first to third aspects.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, although this invention is demonstrated in detail using a suitable example, the scope of the present invention is not limited to the said suitable example.
The measurement principle of the present invention will be described.
FIG. 4 is a schematic diagram of a measurement system used in the present invention. The light wave emitted from the laser is adjusted to a polarization suitable for modulation by the optical modulator by the polarization controller, and is incident on the optical modulator to be measured.
A bias voltage from a DC power source and an electrical signal from an oscillator are superimposed and applied to this optical modulator.
The optical modulator modulates incident light in response to the superimposed applied voltage. The outgoing light modulated by the optical modulator enters the optical spectrum analyzer, and the light intensity distribution according to the frequency component of the outgoing light is measured.
[0014]
FIG. 5 shows the configuration of the optical waveguide of the MZ type optical modulator to be measured.
When incident light is introduced into the waveguide 1, it is branched into two at the Y branch 1. The branched light propagates through the respective waveguides 2 and 3, and is combined by the Y branch 2, and finally the waveguide. 4 is emitted.
[0015]
Consider a case where light having an electric field E _i exp (jω ₀ t) is incident as incident light.
When the MZ optical modulator is not modulated, the emitted light can be assumed to be the same as the incident light, and thus the light intensity value P _0a of the emitted light can be _expressed by the following equation (1).
[Formula 1]

[0016]
Next, a case where modulation is performed by the waveguides 2 and 3 will be considered. Similarly to the case where no modulation is applied, light having an electric field E _i exp (jω ₀ t) is incident, modulated by the waveguides 2 and 3, and further, the electric field component E ( t) can be expressed by Equation 2 below.
[Formula 2]

[0017]
Here, A ₁ and A ₂ are parameters (modulation index) representing the depth of modulation, ω _m is the angular frequency of the modulation signal, φ ₁ and φ ₂ are the phases of the modulation signal that differ depending on each waveguide, φ _B1 , φ _B2 represents the phase according to the initial state of the waveguide.
Equation 2 shows that the combined light wave has various frequency components. Here, attention is paid to the same frequency component as that of the incident light (when n = 0 in Equation 2) and the + 1st order higher-order term (when n = 1 in Equation 2) among such frequency components.
The light intensity of the same frequency component (ω ₀ ) as that of the incident light and the + 1st order higher order term (ω ₀ + ω _m ) are measured. FIG. 6 shows the measurement waveform of the optical spectrum analyzer.
The light intensity of each of these components can be expressed by the following Expression 3 (the light intensity P ₀ having the same frequency component as the incident light) and Expression 4 (the light intensity P _{1 of the} + _1st order higher order term).
[Formula 3]

[Formula 4]

[0018]
Further, when the bias voltage applied to the optical modulator is adjusted by the DC power source and set so that the light intensity of the emitted light having the same frequency component (ω ₀ ) as that of the incident light is maximized (the bias is at point a in FIG. 2). In the state where the voltage is set), the light intensity value P _0b of the emitted light having the same frequency component (ω ₀ ) as the incident light can be expressed by the following Expression 5 using Expression 3 above.
[Formula 5]

[0019]
Further, when the bias voltage is adjusted and set so that the light intensity of the emitted light having the same frequency component (ω ₀ ) as that of the incident light is minimized (in the state where the bias voltage is set at the point b in FIG. 2), +1 The light intensity value P _1c of the emitted light of the next higher order term (ω ₀ + ω _{m, the} component related to the modulation frequency) can be expressed by the following expression 6 from the above expression 4.
[Formula 6]

[0020]
Here, when the above-described P _0b and P _1c are normalized with the light intensity value P _0a of the outgoing light that is not modulated, the following equations 7 and 8 are obtained.
[Formula 7]

[Formula 8]

[0021]
Using Equations 7 and 8, parameters A ₁ and A ₂ representing the modulation depth are calculated. In addition, although Formula 7 and Formula 8 become a transcendental equation, since A ₁ and A ₂ occupy small values, it is possible to specify a solution.
Then, after calculating A ₁ and A ₂ , the half-wave voltage V _π of the optical modulator can be calculated by the following Equation 9. However, V _m represents the voltage amplitude from the oscillator is applied as a modulation signal.
[Formula 9]

[0022]
As described above, according to the method for measuring the characteristics of the optical modulator of the present invention, the light intensity values P _0a , P _0b , P are calculated from the spectral distribution of the light emitted from the optical modulator obtained by using the optical spectrum analyzer. _1c is measured, and the half-wave voltage value of the optical modulator is calculated based on the light intensity value. For this reason, it is not necessary to directly observe the waveform of the light intensity of the emitted light as in the prior art, and the characteristics of the optical modulator can be measured even at high frequencies.
Further, when measuring the half-wave voltage of the optical modulator, conventionally, the voltage amplitude of the modulation signal applied to the optical modulator has been variably adjusted to a range equal to or greater than the half-wave voltage value. Since the half-wave voltage can be calculated using the voltage amplitude value Vm of the modulation signal, it is not necessary to adjust the modulation signal to the half-wave voltage value as in the prior art, and the voltage amplitude of the modulation signal It is not necessary to change the value.
Furthermore, since only the high-order first order term is measured by the optical spectrum analyzer, the measurement is not affected by high-frequency components such as an amplifier, so that more accurate half-wave voltage measurement is possible.
[0023]
In the MZ type optical modulator, not only the light intensity but also the phase is modulated at the time of multiplexing due to the difference in modulation due to the difference in modulation voltage applied to the waveguides 2 and 3 shown in FIG. This phenomenon is called chirping, and the size of chirping is given by the α parameter expressed by the following equation (10).
[Formula 10]

[0024]
In Expression 10, dI represents the light intensity change amount, dφ represents the light phase change amount, and A1 and A2 are determined by the specific light intensity values measured from the spectrum distribution of the emitted light as described above. Since it is calculated, the α parameter can be calculated.
[0025]
The present invention is not limited to the above-described characteristic measuring method of the optical modulator. For example, the + 1st order higher order term (spectral component of frequency ω ₀ + ω _m ) used in the above characteristic measuring method is changed to the −1st order. It also includes those that can be used in place of higher-order terms (spectral components of frequency ω ₀ -ω _m ) and that can obtain similar effects.
[0026]
Next, a measuring apparatus using the optical modulator characteristic measuring method of the present invention will be described.
FIG. 7 is a block diagram of an apparatus for automatically measuring the characteristics of the optical modulator of the present invention.
The laser light having the frequency ω ₀ passes through the polarization controller, is polarized in a certain direction, and enters the MZ type optical modulator. The optical modulator is formed by an oscillator of DC power and the frequency omega _m, the modulation signal oscillating at the frequency omega _m is applied around the specific bias voltage.
[0027]
The modulation signal is input to the optical modulator, and at the same time, the voltage amplitude value Vm and the bias voltage value of the modulation signal are detected and monitored by the RF power meter via the branch circuit.
The numerical value measured by the RF power meter is input to the control computer and used as one of detection signals for automating the optical modulator characteristic measurement described later.
[0028]
In response to the modulation signal applied to the optical modulator, the incident light is modulated and emitted from the optical modulator as outgoing light.
The emitted light is measured for a spectral distribution which is a light intensity distribution with respect to frequency by an optical spectrum analyzer.
The measured value of the optical spectrum analyzer, in particular, the measured value of the light intensity related to the frequency ω _{0 of the} laser beam, the + 1st order higher order term (frequency ω ₀ + ω _m, or may be the −1st order higher order term). To the control computer.
[0029]
A process for automatically measuring the optical modulator characteristic measurement will be described.
In the state that various equipment necessary for the measurement such as laser, polarization controller, optical spectrum analyzer, etc. is operated, first, the application of the modulation signal to the optical modulator by the DC power source and the oscillator is stopped by the instruction from the control computer. Then, the light intensity P _0a at the frequency ω ₀ in the non-modulation state is measured, and the measured value is taken into the control computer.
Next, in response to an instruction from the control computer, a modulation signal is applied to the optical modulator, and the light intensity at the frequency ω ₀ from the optical spectrum analyzer is measured and input to the control computer. In the control computer, the output voltage of the DC power source that defines the bias voltage of the modulation signal is changed in accordance with the monitoring of the light intensity at the frequency ω _{0, and} the light intensity at which the light intensity monitored is maximized according to the change. Determine P _0b .
Further, the control computer changes the output voltage of the DC power supply that defines the bias voltage of the modulation signal in accordance with the monitoring of the light intensity at the frequency ω _{0, and} the light intensity monitored according to the change is minimized. The state is determined, the light intensity P _1c of the + 1st order higher-order term (frequency ω ₀ + ω _m ) in the state is measured by the optical spectrum analyzer, and the measured value is captured.
[0030]
Next, A1 and A2 are determined by solving the equations 7 and 8 based on the light intensity values P _0a , P _0b and P _1c captured in the control computer. 10 is used to calculate and determine the half-wave voltage Vπ and the chirp parameter (α parameter).
For each value such as the frequency ω ₀ , the frequency ω _m, and the voltage amplitude value Vm, preset values are input to the control computer by a laser or an oscillator used for measuring the characteristics of the optical modulator. Alternatively, a value measured by an optical spectrum analyzer or an RF power meter may be used.
In the above-described measurement apparatus, measurement values are captured and various calculations are also executed on the control computer. For these information processing, a storage device and a calculation device provided separately from the control computer are used. May be used.
[0031]
The results of measurement using the measurement apparatus of FIG. 4 are shown in FIGS.
FIG. 8 shows the measurement results of the characteristics of the optical modulator when a modulation signal having a measurement frequency of 10 GHz is applied. Here, the voltage amplitude value Vm of the modulation signal was varied in the range of 2 to 9 V, and the half-wave voltage value and the chirp parameter at each voltage amplitude value Vm were calculated.
Referring to FIG. 8, both the half-wave voltage value and the chirp parameter show almost constant values regardless of the voltage amplitude, and the measurement method and apparatus of the present invention appropriately measures the characteristics of the optical modulator. I can understand that.
As for the chirp parameter, when the voltage amplitude value of the modulation signal is larger than the half-wave voltage value or when the voltage amplitude value of the modulation signal is small, the measurement result of the characteristics related to the optical modulator slightly varies. This is because when the voltage amplitude value of the modulation signal is increased to a half-wave voltage or more, the waveform of the modulation signal is distorted, and the light intensity of the light emitted from the optical modulator deviates from the theoretical value or causes noise. It is considered that accurate measurement is difficult due to the inclusion of a large amount.
Also, if the voltage amplitude value of the modulation signal is too small, the optical spectrum distribution reflecting the modulation, such as the + 1st order higher order term, is not clearly formed, and it is difficult to perform accurate measurement due to the strong influence of noise. Conceivable.
[0032]
FIG. 9 shows the result of measuring the characteristics of the optical modulator when a modulation signal having a measurement frequency of 10 to 40 GHz is applied.
As shown in FIG. 9, by using the measurement method and apparatus of the present invention, it is possible to effectively measure even high frequency characteristics without directly observing the fluctuation of the emitted light from the optical modulator as in the prior art. It becomes possible.
[0033]
【The invention's effect】
As described above, according to the present invention, the characteristics of the optical modulator can be measured by measuring the spectral distribution of the optical signal emitted from the optical modulator. This eliminates the need for high-accuracy oscillators and sampling oscilloscopes that support low-cost, and enables inexpensive and highly accurate measurement.
Moreover, the light intensity value P _0b of the outgoing light of the spectral component when the light intensity of the outgoing light at the same spectral component as the incident light is maximum, and the light intensity of the outgoing light at the same spectral component as the incident light are minimized. The light intensity value P _1c of the emitted light of the component related to the modulation frequency in this case is used as a value normalized by the light intensity value P _0a of the emitted light when no modulation voltage is applied. Wavelength voltage values and chirp parameters can be easily calculated.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of an MZ type optical modulator.
FIG. 2 is a graph showing a characteristic of output light intensity with respect to an applied voltage of an MZ type optical modulator.
FIG. 3 is a graph showing a relationship between an optical input waveform and an electric output waveform of an MZ type optical modulator.
FIG. 4 is a block diagram showing an example of a method for measuring characteristics of an optical modulator according to the present invention.
FIG. 5 is a configuration diagram of an optical waveguide of an MZ type optical modulator.
FIG. 6 is a graph showing a spectral distribution of an optical signal emitted from an optical modulator.
FIG. 7 is a schematic diagram showing an example of a characteristic measuring apparatus for an optical modulator according to the present invention.
FIG. 8 is a graph showing the influence of a change in the input voltage amplitude of the modulation signal on various characteristic values of the optical modulator measured using the present invention.
FIG. 9 is a graph showing the relationship between various characteristics of an optical modulator and the frequency of a modulation signal, measured using the present invention.
[Explanation of symbols]
10 MZ type optical modulator 20 Optical waveguide 30 Signal source

Claims (4)

Light that splits incident light into two or more, performs phase modulation by applying an electrical signal to at least one of the branched lights, and then generates an intensity-modulated optical signal by synthesizing the branched lights For the modulator,
Measuring a spectral distribution of the optical signal, and calculating a half-wave voltage value of the optical modulator, which is a characteristic value related to intensity modulation of the optical modulator, from a measured value related to the measured spectral distribution A method for measuring characteristics of an optical modulator.   Light that splits incident light into two or more, performs phase modulation by applying an electrical signal to at least one of the branched lights, and then generates an intensity-modulated optical signal by synthesizing the branched lights For the modulator,
  An optical modulator comprising: measuring a spectral distribution of the optical signal; and calculating a chirp parameter value of the optical modulator, which is a characteristic value related to intensity modulation of the optical modulator, from a measured value related to the measured spectral distribution Modulator characteristic measurement method. In the characteristic measuring method of the optical modulator according to claim 1 or 2 ,
The electrical signal consists of a bias voltage and a modulation signal,
The measured value is the light intensity value P _0b of the outgoing light of the spectral component when the light intensity of the outgoing light at the same spectral component as the incident light is maximized by the variable adjustment of the bias voltage, and the same spectral component as the incident light. The light intensity value P _1c of the emitted light of the component related to the modulation frequency when the light intensity of the emitted light at the minimum is, and the light intensity value P _0a of the emitted light when the modulation signal is not applied,
In calculating the characteristic value related to the intensity modulation of the optical modulator, a value obtained by standardizing the light intensity value P _0b and the light intensity value P _1c with the light intensity value P _0a is used. Measuring method of vessel characteristics. Characteristic measurement apparatus for an optical modulator, characterized in that using the characteristic measuring method for an optical modulator according to any one of claims 1 to 3.

Images