JPS6184543A - Measuring method of wavelength dispersion of optical fiber - Google Patents
Measuring method of wavelength dispersion of optical fiberInfo
- Publication number
- JPS6184543A JPS6184543A JP20767784A JP20767784A JPS6184543A JP S6184543 A JPS6184543 A JP S6184543A JP 20767784 A JP20767784 A JP 20767784A JP 20767784 A JP20767784 A JP 20767784A JP S6184543 A JPS6184543 A JP S6184543A
- Authority
- JP
- Japan
- Prior art keywords
- optical
- measured
- optical fiber
- wavelength
- fiber
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Pending
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
- G01M11/33—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face
- G01M11/338—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face by measuring dispersion other than PMD, e.g. chromatic dispersion
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
- G01M11/33—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face
- G01M11/332—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face using discrete input signals
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
- G01M11/33—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face
- G01M11/335—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face using two or more input wavelengths
Landscapes
- Physics & Mathematics (AREA)
- Chemical & Material Sciences (AREA)
- Optics & Photonics (AREA)
- Analytical Chemistry (AREA)
- General Physics & Mathematics (AREA)
- Dispersion Chemistry (AREA)
- Light Guides In General And Applications Therefor (AREA)
Abstract
Description
【発明の詳細な説明】
Ｔ産業上の利用分野」
本発明は光ファイバの波長分散を測定する方法に関する
。DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for measuring chromatic dispersion of an optical fiber.
ｆ従来の技術」
周知の通り、光通信では光ファイバの優れた伝送特性が
要求されるが、これとともに光ファイへの伝送特性を正
確に測定することが重要となっている。f. Prior Art As is well known, optical communications require excellent transmission characteristics of optical fibers, and along with this, it has become important to accurately measure the transmission characteristics of optical fibers.
光ファイバの伝送特性は低損失性、広帯域性に分けられ
、波長分散に依存して定まる帯域の測定については既知
のパルス法、掃引変調法等が実施されている。The transmission characteristics of optical fibers are classified into low loss characteristics and broadband characteristics, and known pulse methods, sweep modulation methods, etc. are used to measure bands determined depending on chromatic dispersion.
上記におけるパルス法では被測定光ファイへ中に通した
光信号（光パルス）の伝搬時間差から波長分散を求める
ようにしており、掃引変調法では被測定光ファイバ中に
通した光信号の位相差により波長分散を求めるようにし
ているが、これら各法とも波長の異なる複数の光源を必
要とする点で共通性があり、この際の光源として一般に
ＬＤやＬＥＤが用いられている。In the above pulse method, chromatic dispersion is determined from the propagation time difference of the optical signal (optical pulse) passed through the optical fiber under test, and in the sweep modulation method, the phase difference of the optical signal passed through the optical fiber under test is determined. The chromatic dispersion is determined using the following method, but each of these methods has a common feature in that they require a plurality of light sources with different wavelengths, and LDs and LEDs are generally used as the light sources in this case.
ところが、これらの光源は発振波長が温度に依存してい
るため、測定時の温度条件が一定でないことにより発振
波長にバラツキが生じ、これが原因で精度の高い波長分
散測定が行なえなくなっている。However, since the oscillation wavelength of these light sources is dependent on temperature, variations in the oscillation wavelength occur due to uneven temperature conditions during measurement, making it impossible to perform highly accurate chromatic dispersion measurements.
Ｔ発明が解決しようとする問題点Ａ
本発明は測定時の温度条件に起因した問題点を解決して
光ファイバの波長分散が精度よく測定できる方法を提供
しようとするものである。Problem A to be Solved by the Invention The present invention aims to solve the problems caused by the temperature conditions during measurement and to provide a method for accurately measuring the wavelength dispersion of an optical fiber.
ｆ問題を解決するための手段ｊ
本発明は光発信系と波長分散測定系との間に被測定ファ
イバを介在させ、光発信系から被測定ファイバ、波長分
散測定系にわたってそれぞれ波長の異なる光信号を通し
、これら光信号相互の測定要素を波長分散測定系で検出
して光ファイバの波長分散を測定する方法において、使
用する光信号ごと、これらの波長を測定し、該各測定値
を基準にして各光信号相互の測定要素を検出することを
特徴としている。Means for Solving the f Problem j The present invention interposes a fiber to be measured between an optical transmission system and a chromatic dispersion measurement system, and transmits optical signals with different wavelengths from the optical transmission system to the fiber to be measured and the chromatic dispersion measurement system. In the method of measuring the chromatic dispersion of an optical fiber by detecting the mutual measurement elements of these optical signals with a chromatic dispersion measurement system, these wavelengths are measured for each optical signal used, and each measurement value is used as a reference. It is characterized by detecting the measurement elements of each optical signal.
・１作用」
本発明方法ではパルス法、掃引変調法等により所定の測
定要素（伝搬時間差、位相差など）を検出して光ファイ
バの波長分散を測定するとき、使用する光信号（光パル
ス）ごと、これらの波長を測定してその測定値を波長分
散測定系などへ入力し、該各測定値を基準にして各光信
号相互の測定要素を検出するから、使用時の波長が温度
により変化するとしてもその温度条件での使用波長が正
確に定まり、かかる波長に基づき波長一時間、波長−位
相差などの関係を正確に求めて光ファイバの波長分散を
高精度に測定し得る。・1 Effect” In the method of the present invention, when measuring the chromatic dispersion of an optical fiber by detecting predetermined measurement elements (propagation time difference, phase difference, etc.) using the pulse method, sweep modulation method, etc., the optical signal (optical pulse) used is Each time, these wavelengths are measured and the measured values are input to a chromatic dispersion measurement system, etc., and the measurement elements of each optical signal are detected based on the measured values, so the wavelength during use does not change depending on the temperature. Even so, the wavelength to be used under that temperature condition is accurately determined, and based on this wavelength, relationships such as wavelength-time, wavelength-phase difference, etc. can be accurately determined, and the chromatic dispersion of the optical fiber can be measured with high precision.
Ｔ実 施 例ｊ 以下本発明の実施例につき、図面を参照して説明する。T implementation example j Embodiments of the present invention will be described below with reference to the drawings.
第１図に示す本発明の実施例はパルス法による光ファイ
バの波長分＃！１．測定例である。The embodiment of the present invention shown in FIG. 1 uses the pulse method for the wavelength of an optical fiber #! 1. This is a measurement example.
同図において、パルス発生器ｌから出たパルスは遅延回
路２を通り、そのまま光源（レーザダイオード）３へ伝
搬され、当該光源３は所定の光信号（光パルス）を出射
する。In the figure, a pulse output from a pulse generator 1 passes through a delay circuit 2 and is propagated as it is to a light source (laser diode) 3, which emits a predetermined optical signal (light pulse).
光源３から出射された光信号はオン状態の光スィッチ４
から被測定ファイバ５へ入射され、さらに被測定ファイ
バ５から光検出器（ＡＰＤ）６、増巾器７を経てオシロ
スコープ８へ入力される。The optical signal emitted from the light source 3 is transmitted to the optical switch 4 in the on state.
The light is input from the fiber to be measured 5 to the optical detector (APD) 6 and the amplifier 7, and then to the oscilloscope 8.
上記において、光信号が被測定ファイバ５からオシロス
コープ８まで伝わるのに要した時間をＴとした場合、Ｔ
時間遅れたパルスがトリガ信号として遅延回路２からオ
シロスコープ日へ入力される。In the above, if the time required for the optical signal to travel from the fiber under test 5 to the oscilloscope 8 is T, then T
The time-delayed pulse is input as a trigger signal from the delay circuit 2 to the oscilloscope.
これによりオシロスコープ８の画面上では光信号が観測
でき、その光信号が被測定ファイバ５を伝搬するに要す
る時間が測定できる。As a result, the optical signal can be observed on the screen of the oscilloscope 8, and the time required for the optical signal to propagate through the fiber 5 to be measured can be measured.
かかる測定につき、発振波長の異なる光源３を用いてそ
れぞれの伝搬時間を測定することにより被測定ファイバ
５の波長分散が求められる。For such measurements, the chromatic dispersion of the fiber 5 to be measured is determined by measuring the propagation time of each light source 3 having different oscillation wavelengths.
この際の波長分散はオシロスコープ８だけでなく、既知
の波長分散測定機能を有するコンピュータ９により自動
測定かつ演算することによっても求められる。The chromatic dispersion at this time is determined not only by the oscilloscope 8 but also by automatic measurement and calculation by a computer 9 having a known chromatic dispersion measurement function.
本発明の上記実施例すなわちパルス法では、各測定ごと
に使用する光源３の発振波長を、光波長測定器ＩＯで測
定し、その測定値をオシロスコープ８、コンピュータθ
へそのまま入力し、さらに遅延回路２を経由してオシロ
スコープ８へ入力することにより波長一時間の関係を精
度よく測定し、これにより被測定ファイバ５の波長分散
を高い精度で測定する。In the above embodiment of the present invention, that is, in the pulse method, the oscillation wavelength of the light source 3 used for each measurement is measured by the optical wavelength measuring device IO, and the measured value is transferred to the oscilloscope 8 and the computer θ.
By inputting the signal as it is to the oscilloscope 8 via the delay circuit 2, the wavelength-time relationship can be measured with high accuracy, and thereby the chromatic dispersion of the fiber under test 5 can be measured with high accuracy.
なお、この実施例での光波長測定器１０は、１例として
回折格子を有するモノクロメータと受光部とからなるも
のが用いられ、これは最大光パワーを与える波長成分か
ら光の波長を求めるものである。The optical wavelength measuring device 10 in this embodiment is, for example, a monochromator having a diffraction grating and a light receiving section, which determines the wavelength of light from the wavelength component that gives the maximum optical power. It is.
パルス発生器１は１ＯＫＨｚのものが用いられる。The pulse generator 1 used is one with a frequency of 1 kHz.
遅延回路２はナノ秒単位から１００マイクロ秒程度のも
のが用いられ、トリ力信号として１００ナノ秒のパルス
を出力する。The delay circuit 2 is of the order of nanoseconds to about 100 microseconds, and outputs a pulse of 100 nanoseconds as a tri-force signal.
ざらに光源３としては、発振波長が０．９ ｐ−１１か
ら１．５５ｐｍまでのレーザダイオードか６個用いられ
、これらのうち、光パルスの半値幅は大きいもので１ナ
ノ秒、他は２００ピコ秒程度のものが用いられる。Roughly, as the light source 3, six laser diodes with oscillation wavelengths from 0.9 p-11 to 1.55 pm are used, and among these, the half width of the optical pulse is 1 nanosecond for the largest one, and 200 nanoseconds for the others. Time on the order of picoseconds is used.
第２図に示す本発明の実施例は掃引変調法による光ファ
イバの波長分散測定例である。The embodiment of the present invention shown in FIG. 2 is an example of chromatic dispersion measurement of an optical fiber using the sweep modulation method.
同図の実施例では、光源１１から連続に発振される光信
号（光パルス）にトラッキングジェネレータ１２を介し
て強度変調をかけ、同時に位相測定器１３にもその変調
光を参照信号として入力させる。In the embodiment shown in the figure, an optical signal (optical pulse) continuously emitted from a light source 11 is subjected to intensity modulation via a tracking generator 12, and at the same time, the modulated light is inputted to a phase measuring device 13 as a reference signal.
光源１１から出射された変調光信号はオン状態の光スィ
ッチ１５から被測定ファイバ１４へ入射され、さらに光
・電変換器１６で電気信号に変換されて位相測定器１３
へ入力され、当該位相測定器１３によりＥ記参照信号と
の位相差が測定される。The modulated optical signal emitted from the light source 11 enters the fiber under test 14 through the optical switch 15 in the on state, and is further converted into an electrical signal by the optical/electrical converter 16 and then sent to the phase measuring device 13.
The phase measuring device 13 measures the phase difference with the E reference signal.
この際、前記と同じく発振波長の異なる光源１１を用い
てそれぞれの位相差を測定すれば、波長−位相差の関係
により被測定ファイバ１４の波長分散が求められる。At this time, if the respective phase differences are measured using the light sources 11 having different oscillation wavelengths as described above, the chromatic dispersion of the fiber to be measured 14 can be determined from the relationship between wavelength and phase difference.
本発明の上記実施例すなわち掃引変調法では、各測定ご
とに使用する光源１１の発振波長を、光波長測定器１７
で測定し、その測定値をトラッキングジェネレータ１２
１位相測定器１３、コンピュータ１８へ入力することに
より波長−位相差の関係を精度よく測定し、これにより
被測定ファイバ１４の波長分散を高い精度で測定する。In the above embodiment of the present invention, that is, in the sweep modulation method, the oscillation wavelength of the light source 11 used for each measurement is determined by the optical wavelength measuring device 17.
and send the measured value to the tracking generator 12.
1 By inputting the data to the phase measuring device 13 and the computer 18, the relationship between wavelength and phase difference is measured with high precision, and thereby the chromatic dispersion of the fiber under test 14 is measured with high precision.
この際、被測定ファイバ１４の伝搬圧＃（長さ）と、光
源１１の光波長帯域と、その上下幅とによりトラッキグ
ジェネレータ１２の変調周波数を変える必要がある。At this time, it is necessary to change the modulation frequency of the tracking generator 12 depending on the propagation pressure # (length) of the fiber to be measured 14, the optical wavelength band of the light source 11, and its vertical width.
例えば光源（レーザダイオード）１１を１００ＭＨｚに
したとき１０ナノ秒が３６０°となり、したがってこの
場合は、レーザ光の波長による伝搬時間差が１０ナノ秒
程度となる伝搬距離（長さ）の被測定ファイバ１４まで
、変調周波数を変えることができる。For example, when the light source (laser diode) 11 is set to 100 MHz, 10 nanoseconds corresponds to 360°. Therefore, in this case, the measured fiber 14 has a propagation distance (length) such that the propagation time difference depending on the wavelength of the laser beam is about 10 nanoseconds. The modulation frequency can be changed up to
発振波長の１．２０〜１．３５ルｍの光源（レーザダイ
オード） １１を用いると、１Ｏｋｉ長の光ファイバで
は１．２０ｐｍと１．３５ｐｍとの笠が８ナノ秒程度と
なるため上記１００ＭＨｚで実用上十分といえる。If a light source (laser diode) 11 with an oscillation wavelength of 1.20 to 1.35 lm is used, the difference between 1.20 pm and 1.35 pm will be about 8 nanoseconds in an optical fiber of 1 ki length, so at the above 100 MHz, This can be said to be sufficient for practical use.
一般にレーザダイオードは０°Ｃ〜３０°Ｃにおいて発
振波長が１０３１１１変化する。Generally, the oscillation wavelength of a laser diode changes by 103111 points between 0°C and 30°C.
その結果、４０００ｍ長の単一モード光ファイ、＜では
分散値が０．９ｐｓ／ｎｍ／に＋ｓ変化し、これが誤差
となる。As a result, for a single mode optical fiber with a length of 4000 m, the dispersion value changes by +s to 0.9 ps/nm/, which causes an error.
この値は、必要とされる１、３ルｍ付近の分散値が３〜
４ｐｓ／ｎａｐ／ｋｍ以下であることを考えると大きい
が、前述した本発明の各実施例では上記の誤差０．９ｐ
ｓ／ｎｕ／ｌｖが０にできる。This value indicates that the required dispersion value around 1 and 3 m is 3~
This is large considering that it is less than 4 ps/nap/km, but in each of the embodiments of the present invention, the above error is 0.9 ps.
s/nu/lv can be set to 0.
したがって本発明では、元の誤差を合わせた値が１．０
ｐｓ／ｒｏｍ／ｋｍであるとき、これをＯ，Ｉｐｓ／ｎ
ｍ／ｋｍにまで低減することができ、光ファイバの分散
値をきわめて高精度に測定することができる。Therefore, in the present invention, the total value of the original errors is 1.0
ps/rom/km, then O, Ips/n
m/km, making it possible to measure the dispersion value of an optical fiber with extremely high precision.
？発明の効果」
以上説明した通り、本発明方法によるときは、光発信系
と波長分散測定系とを介して光ファイバの波長分散を測
定するとき、使用する各光信号の波長を測定し、該各測
定値を基準にして各光信号相互の測定要素を検出するか
ら、高精度の波長分散測定が実現できる。? Effects of the Invention As explained above, when using the method of the present invention, when measuring the chromatic dispersion of an optical fiber via the optical transmission system and the chromatic dispersion measurement system, the wavelength of each optical signal to be used is measured, and the wavelength of each optical signal to be used is measured. Since the measurement elements of each optical signal are detected based on each measurement value, highly accurate chromatic dispersion measurement can be realized.
第１図は本発明方法の１実施例を示すブロック回路図、
第２図は同上の他実施例を示すブロック回路図である。
１ ・・・パルス発生器（光発信系）
２ ・・書遅延回路（光発信系）
３ ・・・光源（光発信系）
４ ・・・光スィッチ（光発信系）
５ ・・・被測定ファイバ
６ ・・・光検出器（波長分散測定系）７ ・会・増巾
器（波長分散測定系）
８ ・・・オシロスコープ（波長分散測定系）９ ・・
・コンピュータ（波長分散測定系）ｌＯ・・−光波長測
定器
１１命・拳光源（光発信系）
１２・・・トラッキングジェネレータ
（光発信系）
１３・・・位相測定器（波長分散測定系）１４φ・・被
測定光ファイバ
１５・・・光スィッチ（光発信系〕
１Ｂ・・・光・電変換器（波長分散測定系）１７・・・
光波長測定器FIG. 1 is a block circuit diagram showing one embodiment of the method of the present invention;
FIG. 2 is a block circuit diagram showing another embodiment same as the above. 1...Pulse generator (light transmission system) 2...Written delay circuit (light transmission system) 3...Light source (light transmission system) 4...Optical switch (light transmission system) 5...Device to be measured Fiber 6 ... Photodetector (chromatic dispersion measurement system) 7 - Amplifier (chromatic dispersion measurement system) 8 ... Oscilloscope (chromatic dispersion measurement system) 9 ...
・Computer (chromatic dispersion measurement system) lO...- Optical wavelength measuring instrument 11 Life/Fist light source (light transmission system) 12... Tracking generator (light transmission system) 13... Phase measuring device (chromatic dispersion measurement system) 14φ... Optical fiber to be measured 15... Optical switch (optical transmission system) 1B... Optical/electrical converter (chromatic dispersion measurement system) 17...
Optical wavelength measuring device
Claims (1)
在させ、光発信系から被測定ファイバ、波長分散測定系
にわたってそれぞれ波長の異なる光信号を通し、これら
光信号相互の測定要素を波長分散測定系で検出して光フ
ァイバの波長分散を測定する方法において、使用する光
信号ごと、これらの波長を測定し、該各測定値を基準に
して各光信号相互の測定要素を検出することを特徴とす
る光ファイバの波長分散測定方法。A fiber to be measured is interposed between the optical transmission system and the chromatic dispersion measurement system, and optical signals with different wavelengths are passed from the optical transmission system to the fiber to be measured and the chromatic dispersion measurement system, and the measurement elements of these optical signals are In a method of measuring chromatic dispersion of an optical fiber by detecting it with a dispersion measurement system, the wavelengths are measured for each optical signal used, and the measurement elements of each optical signal are detected using each measured value as a reference. A method for measuring chromatic dispersion of an optical fiber, characterized by:
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP20767784A JPS6184543A (en) | 1984-10-03 | 1984-10-03 | Measuring method of wavelength dispersion of optical fiber |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP20767784A JPS6184543A (en) | 1984-10-03 | 1984-10-03 | Measuring method of wavelength dispersion of optical fiber |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| JPS6184543A true JPS6184543A (en) | 1986-04-30 |
Family
ID=16543738
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP20767784A Pending JPS6184543A (en) | 1984-10-03 | 1984-10-03 | Measuring method of wavelength dispersion of optical fiber |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JPS6184543A (en) |
-
1984
- 1984-10-03 JP JP20767784A patent/JPS6184543A/en active Pending
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US7110119B2 (en) | Determining an optical property by using superimposed delayed signals | |
| US6323950B1 (en) | Chromatic dispersion measurement for optical components | |
| CA1195138A (en) | Measuring chromatic dispersion of fibers | |
| US11300452B2 (en) | Spectral measurement method, spectral measurement system, and broadband pulsed light source unit | |
| EP1705471A1 (en) | Apparatus for measuring differential mode delay of multimode optical fiber | |
| US8734011B2 (en) | Distributed optical fiber temperature sensor based on optical fiber delay | |
| CA2403847C (en) | Method and apparatus for estimating chromatic dispersion in fibre bragg gratings | |
| GB2096762A (en) | Optical fibre sensor device | |
| KR100483147B1 (en) | System and method for measuring length of optical fiber | |
| US20010050767A1 (en) | Method and apparatus for measuring phase differences between intensity-modulated optical signals | |
| JP4599560B2 (en) | Reference signal light transmission system and reference signal light transmission method | |
| JPH0354292B2 (en) | ||
| JPH07294381A (en) | Method for decision of zero-dispersion wavelength of opticalwaveguide body | |
| JPH05248996A (en) | Wavelength dispersion measuring device for optical fiber | |
| JP2540670B2 (en) | Multi-type gas detector using optical fiber | |
| US7016023B2 (en) | Chromatic dispersion measurement | |
| JPS6184543A (en) | Measuring method of wavelength dispersion of optical fiber | |
| JPH04177141A (en) | Wave length dispersion measurement method of optical fiber | |
| JPH09133585A (en) | Optical pulse train measuring method | |
| JP2002509612A (en) | Wavelength measurement system | |
| JPS61105439A (en) | Method for measuring wavelength dispersion of optical fiber | |
| US6519028B2 (en) | Optical characteristic measuring apparatus, the method thereof and recording medium | |
| JPS61241630A (en) | Optical beat frequency measuring device | |
| JPH07218352A (en) | Measurement method by otdr | |
| Chen et al. | High-precision microwave photonic true time delay measurement based on multi-channel microwave interference |