JP5123264B2 - Optical phase change measuring device - Google Patents

Description

  The present invention relates to an apparatus for measuring a phase change of an optical phase modulator. More specifically, the present invention relates to a phase change of a reflective optical phase modulator, an optical phase modulator, and an optical beam splitter disposed in front of the optical phase modulator. Relates to a device for measuring as a change in the reflection spectrum due to multiple interference between.

  With the recent increase in capacity and speed of optical communication networks, optical devices that control the phase of light have become important. An optical phase modulator is an optical device that changes the phase of input light in response to an external input signal such as a voltage and outputs it, and is known to be composed of materials or liquid crystals that exhibit an electro-optic effect. ing. When these optical phase modulators are actually used, it is necessary to accurately grasp the characteristics of how the phase of the output light changes with respect to the input signal.

  As a method of measuring the phase change of the optical phase modulator, in the case of a reflective optical phase modulator, an optical beam splitter is arranged in front of the optical phase modulator, and multiplexing between the optical beam modulator and the optical phase modulator is performed. There is a method of measuring as a change in the reflection spectrum due to interference. This corresponds to a high reflectivity of one side of a Fabry-Perot interferometer, and is called a Gires-Tournois interferometer (Non-patent Document 1). The Zeill-Turnova interferometer has features such as easy alignment of the optical axis and direct observation of a single polarization phase change since the optical system is arranged in a straight line. As described below, a dip due to multiple interference appears in the reflection spectrum of the Zeill-Turnova interferometer, so that the phase change of the optical phase modulator can be obtained by measuring the change of the center wavelength.

  FIG. 1 is a diagram showing a schematic configuration of an apparatus for measuring an optical phase change of a phase modulator using a Zeill-Turnova interferometer. In the apparatus shown in FIG. 1, the light from the broadband light source 5 is condensed by the optical collimator lens 1 and is incident on the optical phase modulator 2 to be measured as a light beam. An optical beam splitter 3 is disposed on the optical axis on the front surface of the optical phase modulator 2. The light beam splitter 3 reflects a part of the light beam to the optical collimator lens 1 and transmits a part of the light beam to the optical phase modulator 2. Then, the reflected light from the light beam splitter 3 and the optical phase modulator 2 is received by the optical collimator lens 1 again. Further, the reflected light output from the optical collimator lens 1 is guided to the optical spectrum analyzer 6 by the optical circulator 4 and observed.

  In the conventional measuring apparatus, the light beam splitter 3 and the optical phase modulator 2 are arranged so that the reflected light power to the optical collimator lens 1 is maximized. In other words, the normal lines of the reflecting surfaces of the light beam splitter 3 and the optical phase modulator 2 are usually adjusted to be perpendicular to the light beam. At this time, the reflection spectrum due to multiple interference is expressed by the following equation (1).

  Here, R1 and R2 are the reflectivities of the light beam splitter 3 and the optical phase modulator 2, respectively. In Expression (1), φ is the phase difference of the multiple interference light and is expressed by the following Expression (2).

  Here, L is the distance between the optical beam splitter 3 and the optical phase modulator 2, and ψ is the phase given by the optical phase modulator 2. In the conventional optical phase change measuring apparatus, when the light from the broadband light source 5 is detected by sweeping the wavelength and the reflected light spectrum is measured, when the phase is φ = (2m + 1) π, that is, the following equation (3) A dip having a minimum reflectance appears at a wavelength λm that satisfies the relational expression.

  Here, m is a non-negative integer. A plurality of dips appear at positions satisfying the expression (3). The wavelength interval Δλ between adjacent dip is expressed by the following equation (4).

  When an external signal is input to the phase modulator 2 and the phase of the reflected light to the optical beam splitter 3 is changed by δΨ, the position of the dip wavelength changes by δλm according to the following equation (5). To do.

  Therefore, by measuring how much the dip wavelength λm has changed with respect to the dip interval Δλ, the optical phase change δΨ of the phase modulator can be measured from the equation (5).

  The dip wavelength in the reflection spectrum can be measured with high accuracy using a combination of the broadband light source 5 and the optical spectrum analyzer 6 or a combination of a wavelength swept light source and an optical power detector.

F. Gires and P. Tournois, “Interf ▲ e ▼ rom ▲ e ▼ tre utilisable pour la compression d'impulsions lumineuses modul ▲ e ▼ es en fr ▲ e ▼ quence,” CR Acad. Sc. Paris, vol. 258, pp. 6112-6115, 1964.

  However, the conventional measuring apparatus has a problem that the reflectances of the light beam splitter 3 and the optical phase modulator 2 must be matched in order to accurately measure the wavelength position of the dip in the reflection spectrum. . This is because the depth of the dip is determined by the relationship between the reflectance R1 of the optical beam splitter and the reflectance R2 of the optical phase modulator. When the reflected light power at the dip is obtained from the equation (1), the following equation is obtained.

  From equation (6), when R1 and R2 are equal, the reflectivity at the dip becomes zero, but I (π) increases as the difference between R1 and R2 increases, so that the dip becomes shallow as shown in FIG. I understand. When the dip becomes shallow, it is difficult to distinguish the center wavelength, and it is difficult to accurately measure the wavelength position of the dip. For this reason, in the conventional optical phase change measuring device, it is necessary to prepare an optical beam splitter having a reflectivity that matches the reflectivity of the optical phase modulator.

  The present invention has been made in view of such problems, and the object of the present invention is to provide a dip wavelength in the reflection spectrum even when the reflectivity of the optical phase modulator and the optical beam splitter do not match. An object of the present invention is to provide an apparatus capable of accurately measuring a position and accurately measuring a phase change of an optical phase modulator.

  In order to achieve such an object, an optical phase change measuring apparatus according to claim 1 is an apparatus for measuring a phase change of a reflective optical phase modulator, and means for simultaneously outputting light having a wide wavelength range. Alternatively, a light source having means for sweeping and outputting the wavelength of light, and the output light from the light source is collected, incident on the optical phase modulator as a light beam, and reflected light from the optical phase modulator An optical collimator lens that receives and outputs the light, and is disposed on the optical axis of the light beam from the optical collimator lens to the optical phase modulator, and reflects a part of the light beam to the optical collimator lens An optical beam splitter that transmits to the optical phase modulator, an optical circulator that separates input light from the light source to the optical collimator lens and reflected light output from the optical collimator lens, and the optical collimator. Optical wavelength spectrum measuring means for measuring a reflection spectrum of the reflected light output from the lens, and by tilting the optical axis of the light beam with respect to the normal of the reflecting surface of the optical phase modulator to be measured A deep dip is obtained in the reflection spectrum.

  An optical phase change measuring apparatus according to claim 2 is an apparatus for measuring a phase change of a reflection type optical phase modulator, wherein the optical phase change measuring apparatus sweeps the wavelength of light or means for simultaneously outputting light of a wide-band wavelength. A light source having means for outputting, and an optical collimator that collects the output light from the light source, enters the optical phase modulator as a light beam, and receives and outputs the reflected light from the optical phase modulator A lens and an optical axis of a light beam from the optical collimator lens to the optical phase modulator, a part of the light beam is reflected by the optical collimator lens, and a part is transmitted to the optical phase modulator An optical beam splitter, an optical circulator for separating input light from the light source to the optical collimator lens and reflected light output from the optical collimator lens, and reflected light output from the optical collimator lens. And a light wavelength spectrum measuring means for measuring the reflection spectrum, and the normal of the reflection surface of the light beam splitter is tilted with respect to the optical axis of the light beam so that a deep dip can be obtained in the reflection spectrum. It is characterized by that.

  According to one aspect of the present invention, in an optical phase change measuring apparatus using a Zeal-Turnova interferometer, the equivalent reflectance of the optical beam splitter and the optical phase modulator can be adjusted. Even when the reflectivity of the optical phase modulator is different, a deep dip can be obtained in the reflection spectrum. As a result, the wavelength position of the dip can be accurately measured, and an apparatus for accurately measuring the phase change of the optical phase modulator can be realized.

It is a figure which shows the structure of the conventional optical phase change measuring apparatus. It is a figure which shows a reflection spectrum in case the difference of the reflectance of the optical beam splitter and optical phase modulator of the conventional optical phase change measuring apparatus is large. It is a figure which shows an example of a structure of the optical phase change measuring apparatus which concerns on this invention. It is a figure which shows the reflection spectrum measured with the optical phase change measuring apparatus of the 1st Embodiment of this invention. It is a figure which shows an example of a structure of another optical phase change measuring apparatus which concerns on this invention. It is a figure which shows the reflection spectrum measured with the optical phase change measuring apparatus of the 2nd Embodiment of this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 3 is a diagram illustrating an example of the configuration of the optical phase change measuring apparatus according to the first embodiment.
This optical phase change measuring apparatus includes an optical collimator lens 11, an optical beam splitter 13, an optical circulator 14, a broadband light source 15, and an optical spectrum analyzer 16.

  The broadband light source 15 outputs light having a broadband wavelength at the same time.

  The optical collimator lens 11 condenses the output light from the light source 15, enters the reflection-type optical phase modulator 12 to be measured as a light beam, and receives the reflected light from the optical phase modulator 12. Output to the optical circulator 14. The optical axis of the light beam is configured to be inclined with respect to the normal line of the reflecting surface of the optical phase modulator 12 to be measured. For example, the support (not shown) of the optical phase modulator 12 may be configured so that the optical phase modulator 12 is inclined with respect to the optical axis of the optical phase change measuring device.

  The light beam splitter 13 reflects a part of the light beam emitted from the optical collimator lens 11 to the collimator lens 11 and transmits a part thereof to the optical phase modulator 12.

  The optical circulator 14 inputs the light from the broadband light source 15 to the optical collimator lens 11 and guides the reflected light output from the optical collimator lens 11 to the optical spectrum analyzer 16. The optical spectrum analyzer 16 measures the reflection spectrum of the reflected light output from the optical collimator lens 11.

  When the broadband light source of FIG. 3 is non-polarized light, the polarization direction of light incident on the optical phase modulator can be defined by placing a polarizer or a polarization beam splitter.

  The apparatus configuration of the present embodiment is different from a normal Zeir-Turnova interferometer in that the optical axis of the light beam is configured to be inclined with respect to the normal line of the reflection surface of the optical phase modulator 12 to be measured. It is a feature. Here, the inclination of the optical axis of the light beam with respect to the reflection surface of the optical phase modulator 12 will be described. The relationship between the reflection spectrum caused by multiple interference, the reflectance R1 of the optical beam splitter 13 and the reflectance R2 of the optical phase modulator 12 in the optical phase change measuring apparatus of the present embodiment is expressed by the following equation (7).

  Α in Expression (7) is a factor (α ≦ 1) representing a change in reflected light power due to the configuration in which the normal line of the reflection surface of the optical phase modulator 12 is inclined with respect to the optical axis of the light beam. . The optical power ratio with respect to when the normal line of the reflecting surface of the optical phase modulator 12 is not inclined with respect to the optical axis of the light beam is indicated by α. That is, Expression (7) indicates that the configuration in which the optical phase modulator 12 is tilted corresponds to that the reflectance of the optical phase modulator 12 is equivalently reduced to α times. Therefore, in the optical phase change measuring apparatus of this embodiment, the optical axis of the beam is the method of the reflecting surface of the optical phase modulator 12 so that the term αR2 in the equation (7) matches the reflectance R1 of the optical beam splitter 13. It is adjusted so as to be inclined with respect to the line so that a deep dip can be obtained in the reflection spectrum. Preferably, the optical phase modulator 12 is adjusted to be inclined so that the depth of the dip is -10 dB.

  An optical phase change measurement method using the optical phase change measurement apparatus of FIG. 3 will be described. In the optical phase change measuring apparatus shown in FIG. 3, the inclination of the optical axis of the light beam with respect to the normal line of the reflecting surface of the optical phase modulator 12 is adjusted in advance. For example, while adjusting the inclination of the reflection surface of the optical phase modulator 12, the output light from the broadband light source 15 is incident on the optical beam splitter 13 and the optical phase modulator 12, and the spectrum of the reflected light is measured by the optical spectrum analyzer 16. taking measurement. When a deep dip as shown in FIG. 4 is obtained in the measured spectrum, the inclination of the optical axis of the light beam with respect to the normal of the reflecting surface of the optical phase modulator 12 is determined.

  First, in a state where the optical phase modulator 12 is not operating, the spectrum of the reflected light is measured by the optical phase change measuring device of FIG. 3, and the dip wavelength λm and the dip interval Δλ are specified. Further, in the state where the optical phase modulator 12 is operated, the spectrum of the reflected light is measured by the optical phase change measuring device of FIG. 3, and the dip wavelength λm ′ is specified.

  Next, when the dip wavelength change amount δλm = λm′−λm in the above two states is obtained, the phase change δΨ of the phase modulator can be determined based on the following equation (5).

Example 1
In the conventional optical phase change measuring apparatus and the optical phase change measuring apparatus of the present embodiment, the optical beam splitters 3 and 13 having a reflectance R1 = 0.5, and the optical phase modulator 2 having a reflectance R2 = 0.9, 12 were used to measure reflection spectra. Here, the distance between the light beam splitters 3 and 13 and the optical phase modulators 2 and 12 is L = 1.5 mm. The reflection spectrum was measured near a wavelength of 1.55 μm.

  First, as a comparative example, a reflection spectrum is measured in a conventional optical phase change measuring apparatus configured so that the reflected light power from the optical beam splitter 3 and the reflective optical phase modulator 2 to the optical collimator lens 1 is maximized. To do. As shown in FIG. 1, the conventional optical phase change measuring apparatus is adjusted so that the reflecting surfaces of the optical beam splitter 3 and the reflective optical phase modulator 2 are both perpendicular to the optical beam. In this case, a reflection spectrum as shown in FIG. 2 was obtained. In the reflection spectrum, a dip having a wavelength interval of approximately Δλ = 0.8 nm was observed. However, the dip depth in the reflection spectrum of FIG. 2 was only -2.7 dB. It was difficult to specify the dip wavelength.

  Next, as an example, the reflected light power from the reflection type optical phase modulator is 0.7 to the input light power ratio (-1.1 dB lower than R2 when the reflecting surface is vertical). The reflection spectrum is measured in the configured optical phase change measuring device of the present embodiment. As shown in FIG. 3, the optical phase change measuring apparatus of the present embodiment is adjusted by tilting the optical axis of the light beam with respect to the reflection surface of the optical phase modulator, so that a reflection spectrum as shown in FIG. 4 is obtained. As a result, the reflectivity at the dip was -10 dB, and a depth sufficient to accurately measure the wavelength position of the dip was obtained. It was easy to identify the dip wavelength from the reflection spectrum of FIG.

(Second Embodiment)
FIG. 5 is a diagram illustrating an example of the configuration of the optical phase change measuring apparatus according to the second embodiment. The optical phase change measuring apparatus according to the present embodiment is a first embodiment in which a deep dip is obtained in the reflection spectrum by adjusting the inclination of the optical axis of the light beam with respect to the normal line of the reflection surface of the optical phase modulator 12. Instead of this configuration, the inclination of the light beam splitter 13 with respect to the optical axis of the light beam is adjusted to obtain a deep dip in the reflection spectrum. In FIG. 5, the same components as those in FIG.

  Here, the inclination of the reflection surface of the light beam splitter 13 will be described. A reflection spectrum due to multiple interference in the optical phase change measuring apparatus of the present embodiment is expressed by the following equation (8).

  Β in Expression (8) is a factor (β ≦ 1) representing a change in reflected light power caused by tilting the reflecting surface of the light beam splitter 13. The optical power ratio with respect to when the normal line of the reflecting surface of the light beam splitter 13 is not inclined with respect to the optical axis of the light beam is indicated by β. In equation (8), tilting the reflection surface of the light beam splitter 13 reduces the reflectance of the light beam splitter 13 equivalently to β times, and the second time out of the multiple reflections from the optical phase modulator 12. It shows that the subsequent reflectance is equivalent to a decrease in proportion to the square of β. Therefore, as a result, in the optical phase change measuring apparatus of this embodiment, the change in the reflectance of the optical phase modulator 12 is larger, so that the reflectance of the optical beam splitter 13 and the reflectance of the optical phase modulator 12 are higher. In order to be substantially equivalent, the normal line of the reflection surface of the light beam splitter 13 is adjusted to be inclined with respect to the optical axis of the light beam so that a deep dip can be obtained in the reflection spectrum. Preferably, the optical beam splitter 13 is adjusted to be inclined so that the depth of the dip is -10 dB.

  An optical phase change measurement method using the optical phase change measurement apparatus of FIG. 5 will be described. In the optical phase change measuring apparatus shown in FIG. 5, the inclination of the normal line of the reflecting surface of the light beam splitter 13 with respect to the optical axis of the light beam is adjusted in advance. In order to adjust the inclination of the light beam splitter 13, for example, the output light from the broadband light source 15 is incident on the light beam splitter 13 and the optical phase modulator 12, and the spectrum of the reflected light is measured by the optical spectrum analyzer 16. When a deep dip as shown in FIG. 6 is obtained in the measured spectrum, the inclination of the normal line of the reflection surface of the light beam splitter 13 with respect to the optical axis of the light beam is determined.

  In the optical phase change measuring apparatus shown in FIG. 5 in which the inclination of the light beam splitter 13 is adjusted, the optical phase change is measured as in the first embodiment.

(Example 2)
In the conventional optical phase change measuring apparatus and the optical phase change measuring apparatus of the present embodiment, the optical beam splitters 3 and 13 having a reflectance R1 = 0.5, and the optical phase modulator 2 having a reflectance R2 = 0.9, 12 were used to measure reflection spectra.

  First, as a comparative example, a reflection spectrum is measured in a conventional optical phase change measuring apparatus configured so that the reflected light power from the optical beam splitter 3 and the reflective optical phase modulator 2 to the optical collimator lens 1 is maximized. To do. As shown in FIG. 1, the conventional optical phase change measuring apparatus is adjusted so that the reflecting surfaces of the optical beam splitter 3 and the reflective optical phase modulator 2 are both perpendicular to the optical beam. In this case, a reflection spectrum as shown in FIG. 2 was obtained. In the reflection spectrum of FIG. 2, the dip depth was only -2.7 dB. It was difficult to specify the dip wavelength.

  Next, as an embodiment, the reflected light power from the light beam splitter is configured to be 0.35 (-1.5 dB lower than R1 when the reflecting surface is vertical) in the ratio of the input light power. The reflection spectrum is measured in the optical phase change measuring apparatus of this embodiment. By tilting the reflection surface of the light beam splitter 13 in this way, the second and subsequent reflectances of the multiple reflections from the optical phase modulator 12 are equivalently reduced to 0.44. As shown in FIG. 5, the optical phase change measuring apparatus of the present embodiment is adjusted by tilting the reflection surface of the light beam splitter, so that a reflection spectrum as shown in FIG. 6 is obtained, and the reflectance at the dip is −10 dB. Thus, a depth sufficient to accurately measure the wavelength position of the dip was obtained.

  In the above embodiment, the case where the broadband light source 15 is used as the light source and the optical spectrum analyzer 16 is used as the optical wavelength spectrum measuring unit has been described as an example. However, instead of the broadband light source 15, the wavelength of light is swept. In this case, a wavelength swept light source to be output may be used, and an optical power detector may be used instead of the optical spectrum analyzer 16.

DESCRIPTION OF SYMBOLS 1,11 Optical collimator lens 2,12 Reflective optical phase modulator 3,13 Optical beam splitter 4,14 Optical circulator 5,15 Broadband light source 6,16 Optical spectrum analyzer

Claims (2)

An apparatus for measuring the phase change of a reflective optical phase modulator,
A light source having means for simultaneously outputting light of a wide wavelength range or means for sweeping and outputting light wavelength;
An optical collimator lens that collects output light from the light source, enters the optical phase modulator as a light beam, and receives and outputs reflected light from the optical phase modulator;
An optical beam splitter that is disposed on the optical axis of the light beam from the optical collimator lens to the optical phase modulator, reflects a part of the light beam to the optical collimator lens, and transmits a part to the optical phase modulator. When,
An optical circulator for separating the input light from the light source to the optical collimator lens and the reflected light output from the optical collimator lens;
A light wavelength spectrum measuring means for measuring a reflection spectrum of the reflected light output from the optical collimator lens,
An optical phase change measuring device characterized in that a deep dip is obtained in the reflection spectrum by tilting the optical axis of the light beam with respect to the normal line of the reflecting surface of the optical phase modulator to be measured. . An apparatus for measuring the phase change of a reflective optical phase modulator,
A light source having means for simultaneously outputting light of a wide wavelength range or means for sweeping and outputting light wavelength;
An optical collimator lens that collects output light from the light source, enters the optical phase modulator as a light beam, and receives and outputs reflected light from the optical phase modulator;
An optical beam splitter that is disposed on the optical axis of the light beam from the optical collimator lens to the optical phase modulator, reflects a part of the light beam to the optical collimator lens, and transmits a part to the optical phase modulator. When,
An optical circulator that separates input light from the light source to the optical collimator lens and reflected light output from the optical collimator lens;
A light wavelength spectrum measuring means for measuring a reflection spectrum of the reflected light output from the optical collimator lens,
An optical phase change measuring apparatus characterized in that a deep dip is obtained in the reflection spectrum by tilting the normal of the reflecting surface of the light beam splitter with respect to the optical axis of the light beam.

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