JP4009726B2 - Apparatus and method for measuring modulation frequency characteristic of photodetector - Google Patents

Description

  The present invention relates to a measuring apparatus and a measuring method for accurately measuring a modulation frequency characteristic of a photodetector used in optical communication and optical measurement.

  In optical communication and optical measurement, a signal placed on light in the form of intensity modulation is returned to an electric signal by an optical / electrical converter called a photodetector or a light receiver. What is important as a characteristic of the photodetector is a sensitivity characteristic with respect to the wavelength of light and a modulation frequency characteristic (hereinafter simply referred to as frequency characteristic).

  The sensitivity characteristic with respect to the wavelength is used to select an optical modulator that operates appropriately with respect to the wavelength of light to be used. On the other hand, the latter represents the ability to faithfully convert a signal placed on light into an electrical signal, and is an important parameter for constructing a highly reliable system in optical communication and optical measurement.

  A general flow of optical communication and optical measurement is shown in FIG. In optical communication, an electrical signal desired to be transmitted using an optical modulator is converted into an optical signal of light intensity and sent to a remote place through an optical fiber or the like. At the reception place, the electric signal carried on the optical signal is taken out by the photodetector. At this time, the degree of similarity between the waveform of the electrical signal before the optical transmission and the transmitted electrical signal largely depends on the frequency characteristic of the optical modulator and the frequency characteristic of the photodetector.

  On the other hand, in optical measurement, the original signal is a physical quantity to be measured, and is converted into an optical signal using some sensor (or measurement system). It is extracted as an electrical signal using a photodetector. Also in this optical measurement, the frequency characteristics of the sensor and the frequency characteristics of the photodetector are major factors that determine the reliability and accuracy of the measurement.

  Optical communication and optical measurement have various advantages compared to communication and measurement using only electrical signals by using signal transmission means called light. One of the important factors that determine system performance is It is a frequency characteristic.

  Conventional methods for measuring frequency characteristics of photodetectors are roughly divided into a method using a modulated light output as a reference and a method comparing with a reference photodetector.

FIG. 2 shows an example of a basic configuration for measuring the frequency characteristic of the photodetector. First, a light source 1 that can output intensity-modulated light is prepared. As a light source, a semiconductor laser 2 that can be directly modulated, or a combination of a laser 3 that does not have a modulation function and an external light modulator 4 can be considered. At this time, the light source 1 is required to have known modulation sensitivity and frequency characteristics of the modulator. An apparatus for accurately measuring the frequency characteristic of an external optical modulator is known (see Patent Document 1).
Japanese Patent Publication No. 8-20333

  In the configuration of FIG. 2, the intensity of the same frequency component as the modulation frequency included in the electrical output from the photodetector 6 is measured by the frequency analyzer 7 while changing the modulation frequency supplied by the oscillator 5. The frequency characteristic of the detector 6 can be measured.

  The conventional photodetector frequency characteristic method has the following problems. When an external optical modulator is used, it is necessary to measure the accurate frequency characteristics using a known device (see Patent Document 1) in advance, which increases labor.

  There is no means for accurately measuring the direct modulation characteristics of the semiconductor laser. For this reason, measurement is usually performed using a light source that can be modulated to a sufficiently high frequency and assuming that the frequency characteristics are flat in the target frequency range.

  On the other hand, when the modulation characteristic of the light source is unknown, a method using the reference photodetector 7 is used. First, a reference light detector 7 is connected instead of the measured light detector 6, and the output of the reference light detector 8 is measured while changing the modulation frequency of the light source 1.

  Next, the measured light detector 6 is replaced and the same measurement is repeated. If the frequency characteristic of the reference photodetector 7 is known, the frequency characteristic of the measured photodetector 6 can be found by comparison with the characteristic. Also in this case, since there is no means for accurately measuring the frequency characteristics of the reference photodetector, it is necessary to use a photodetector that has a sufficiently wide frequency range and that is expected to be flat in the target frequency range as a reference. It is customary.

  As described above, in order to measure the frequency characteristics of the photodetector, the modulation characteristics of the light source must be known, and in order to know the modulation characteristics of the light source, the characteristics of the photodetector must be known. There is a relationship.

  Further, even when an external modulator is used, the above-described disadvantage that the labor is increased causes an increase in measurement time and a decrease in measurement accuracy due to repeated measurement. Therefore, the measurement is often performed using a light source or a photodetector that is considered to have a flat modulation frequency characteristic as a reference. However, in this case, since the accuracy of the reference is unknown, the reliability of the measurement result is questionable.

  The present invention aims to solve the above-mentioned conventional problems, and realizes a method and an apparatus for accurately measuring the frequency characteristics of a measured photodetector without using a reference photodetector. The task is to do.

In order to solve the above-mentioned problems, the present invention provides an external optical modulator that intensity-modulates intensity-modulated light from a modulation semiconductor laser again at a modulation frequency different from the modulation frequency of the modulation semiconductor laser, and a frequency at which an electric signal is measured. An electric signal obtained by receiving again the intensity-modulated light by the photodetector that is the object to be measured, and converting the electric signal into the sum and difference of the two different modulation frequencies in the frequency analyzer. There is provided a modulation frequency characteristic measuring apparatus characterized in that characteristics of an arbitrary modulation frequency can be measured by taking out and measuring frequency components and changing the two modulation frequencies .

In order to solve the above-described problems, the present invention provides two external optical modulators that continuously and intensity-modulate laser light having no modulation function at different modulation frequencies, and frequency analysis that measures electrical signals. And an electric signal obtained by receiving the double intensity-modulated light by the photodetector to be measured and converting the light signal by the sum and difference of the two different modulation frequencies by the frequency analyzer. There is provided a modulation frequency characteristic measuring apparatus characterized in that a characteristic of an arbitrary modulation frequency can be measured by taking out and measuring a certain frequency component and changing the two modulation frequencies .

In order to solve the above-mentioned problems, the present invention performs intensity modulation on the intensity-modulated light from the modulation semiconductor laser again using an external optical modulator at a modulation frequency different from the modulation frequency of the modulation semiconductor laser, and the intensity modulation again. The received light is received by a light detector to be measured and converted into an electrical signal, and the electrical signal is measured by extracting a frequency component that is the sum and difference of the two different modulation frequencies with a frequency analyzer, A modulation frequency characteristic measuring method is provided, wherein characteristics of an arbitrary modulation frequency are measured by changing the two modulation frequencies .

In order to solve the above-described problems, the present invention continuously modulates the intensity of a laser beam having no modulation function by using two external optical modulators at two different modulation frequencies. The modulated light is received by a photodetector to be measured and converted into an electrical signal, and the electrical signal is measured by extracting a frequency component that is the sum and difference of the two different modulation frequencies with a frequency analyzer. The present invention provides a modulation frequency characteristic measuring method characterized by measuring characteristics of an arbitrary modulation frequency by changing the two modulation frequencies .

  According to the configuration of the present invention, for the purpose of accurately measuring the frequency characteristics of the photodetector, it is not necessary to know the characteristics of other devices used for the measurement accurately, and the measurement can be easily performed.

  In the conventional example, in order to measure the frequency characteristic of the optical detector, the frequency characteristic of the optical modulator must be known. Conversely, in order to measure the frequency characteristic of the optical modulator, the optical detector However, according to the present invention, the frequency characteristic of the photodetector can be accurately measured without knowing the frequency characteristic of the optical modulator. Can solve this problem.

  The best mode for carrying out the present invention will be described below with reference to the drawings based on the embodiments.

  FIG. 3 is a diagram showing the configuration of the first embodiment of the present invention. The intensity of the light from the light source 9 is modulated using the external modulator 10 at the frequency of the electrical signal supplied from the transmitter 12 to the external modulator 10. The intensity-modulated light using the external modulator 10 is intensity-modulated again using the external modulator 11 at the frequency of the electric signal supplied from the transmitter 13 to the external modulator 11. The measured light detector 14 receives light that has been doubly intensity-modulated by the external modulators 10 and 11 and outputs an electrical signal. Reference numeral 15 denotes a frequency analyzer that measures an electrical signal output from the measured photodetector 14.

  As the light source 9, a light source that can stably output light having a wavelength to be measured by the measured light detector 14 is used. The frequency analyzer measures the intensity of each frequency component contained in the electrical signal from the measured photodetector 14, and an example thereof is a spectrum analyzer. The frequency analyzer may be any device that can measure the signal intensity of the target frequency.

Further, the configuration of the first embodiment will be described focusing on the operation. First, the external modulator 10 is driven by a sine wave signal having an angular frequency ω 1 from the oscillator 12. If the optical angular frequency of the light source 9 is ω 0 , the output of the external modulator is expressed by the following formula 1.

  Here, mA is the modulation depth of the optical modulator 10 at the angular frequency ω1.

  Further, the external modulator 11 is driven at a sine wave frequency of the angular frequency (ω1 + ω2) from the oscillator 13. At this time, the output of the external modulator 11, that is, the input of the measured photodetector 14 is expressed by the following formula 2.

  Here, mB is the modulation depth of the external optical modulator 11 at the angular frequency (ω1 + ω2).

  When this output light is received by the photodetector 14, since ω0 has a very high frequency, the photodetector 14 outputs a signal in response to the average power. Therefore, the output of the photodetector 14 is expressed by the following Equation 3.

  Here, F (ω) is a function representing the frequency characteristic of the photodetector 14 at the angular frequency ω.

In this output signal, attention is focused on components of angular frequencies ω2 and 2ω1 + ω2 . The ratio of the sizes of these components is as follows, which is a value unrelated to mA and mB, which are characteristics of the optical modulators 12 and 13.

  Since the frequency analyzer can analyze the frequency component included in the input signal and measure the size of each component, the value of the following Equation 4 can be measured by the frequency analyzer 15.

  Here, ω2 is again set to ωL, and (2ω1 + ω2) is set to ωH. If the angular frequency of the oscillator 12 is ω1 = (ωH−ωL) / 2, the angular frequency of the oscillator 13 is ω1 + ω2 = (ωH−ωL) / 2, and ωH is sequentially changed while keeping ωL constant, From the ratio F (ωH) / F (ωL) of the magnitude of the component with the angular frequency measured by the frequency analyzer 15, the frequency characteristic of the optical modulator 14 when the characteristic at ωL is used as a reference is an unknown value. Therefore, it can be measured accurately without being affected by mA and mB. That is, the frequency characteristic of the optical modulator can be accurately obtained without knowing the frequency characteristic of the optical modulator.

  The frequency characteristic of the photodetector may be obtained based on a reference frequency (DC or an appropriate representative frequency). Strictly speaking, the method of the present invention cannot measure a value based on direct current, but if the measurement is performed with a small value of ωL, a frequency characteristic based on direct current can be obtained with sufficient accuracy.

  FIG. 4 is a diagram showing the configuration of the second embodiment of the present invention. The light source 9 and the external light modulator 11 in the configuration of FIG. 2 are replaced with a semiconductor laser 16 capable of direct modulation, and the operation is the same as in the first embodiment.

  According to the present invention, an accurate frequency characteristic of a photodetector can be easily measured, and thus can be applied to measurement of a photodetector in the optical communication field and the optical measurement field.

It is the schematic of the example of a measurement by an optical system. It is a general block diagram for measuring frequency characteristics of a photodetector. It is a figure explaining Example 1 of the present invention. It is a figure explaining Example 2 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 2 Direct modulation semiconductor laser 3 Laser 4 Optical modulator 5 Oscillator 6 Optical detector 7, 15, 21 Frequency analyzer 8 Reference optical detector 9 Laser 10 Optical modulator 11 Optical modulator 12 Oscillator 13 Oscillator 14 Optical detector 16 Direct Modulation Semiconductor Laser 17 Optical Modulator 18 Oscillator 19 Oscillator 20 Photodetector

Claims (4)

An external optical modulator that intensity-modulates the intensity-modulated light from the modulation semiconductor laser again at a modulation frequency different from the modulation frequency of the modulation semiconductor laser, and a frequency analyzer that measures an electrical signal,
An electrical signal obtained by receiving and converting the intensity-modulated light again into the light detector to be measured, and measuring the frequency component which is the sum and difference of the two different modulation frequencies by the frequency analyzer , A modulation frequency characteristic measuring apparatus capable of measuring characteristics of an arbitrary modulation frequency by changing the two modulation frequencies . Two external optical modulators that continuously and intensity-modulate laser light having no modulation function at different modulation frequencies, and a frequency analyzer that measures an electrical signal,
The frequency signal, which is the sum and difference of the two different modulation frequencies, is extracted from the electrical signal obtained by receiving and converting the double intensity-modulated light by the photodetector that is the object to be measured. And a modulation frequency characteristic measuring apparatus capable of measuring characteristics of an arbitrary modulation frequency by changing the two modulation frequencies . The intensity-modulated light from the modulation semiconductor laser is intensity-modulated again using an external optical modulator at a modulation frequency different from the modulation frequency of the modulation semiconductor laser, and the light whose intensity has been modulated again is detected by the light to be measured. The light signal is received and converted into an electric signal, and the electric signal is measured by extracting a frequency component which is the sum and difference of the two different modulation frequencies by a frequency analyzer , and changing the two modulation frequencies A modulation frequency characteristic measuring method , characterized by measuring a characteristic of a modulation frequency. Laser light having no modulation function is continuously intensity-modulated at two different modulation frequencies using two external light modulators, and the double intensity-modulated light is light to be measured. Light is received by a detector and converted into an electrical signal, and the electrical signal is measured by extracting a frequency component that is the sum and difference of the two different modulation frequencies with a frequency analyzer and changing the two modulation frequencies. A method for measuring a modulation frequency characteristic, comprising measuring a characteristic of a modulation frequency.

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