KR20220086300A - Measurement device for optical phase modulator using sagnac interferometer - Google Patents

Abstract

An optical phase modulator measuring apparatus using a Sagnac interferometer includes a Sagnac interferometer, a laser source emitting an optical signal to the Sagnac interferometer, and a polarizer for converting the optical signal into 90 degree polarized light, the Sagnac interferometer, the optical signal A splitting coupler, an optical fiber coil, a phase modulator connected on one path between the coupler and the optical fiber coil, and a π/2 phase bias connected on the other path between the coupler and the optical fiber coil It includes an irreversible phase element with

Description

Optical phase modulator measuring device using Sagnac interferometer

The present invention relates to an optical phase modulator measuring device, and more particularly, to an optical phase modulator measuring device using a Sagnac interferometer.

A phase modulator is a passive element that modulates the phase of an optical signal using an applied radio frequency (RF) signal. The phase modulator is an essential element for the realization of optical systems, optical communication, and optical microwave technology, and is used in various photonic systems. Phase modulators are mainly used in optical fiber sensors, gyroscopes, integrated optical sensors, and high-performance optical integrated circuits. Recently, photonic based microwave technology (Microwave photonics) has been applied to overcome the technical limitations of radar systems using electronic devices.

Since the RF signal and the optical signal are mixed and used in the phase modulator, it is essential to measure the frequency response characteristic of the phase modulator to use the phase modulator in a high frequency band of several tens of GHz or more. As a method for measuring the frequency response characteristics of the phase modulator, the interference signal can be observed by inserting the phase modulator into one path of the Mach-Zehnder interferometer. In this case, it is possible to measure while changing the modulation amplitude and frequency of the interference signal, but the condition that the phase difference (phase bias) of the interferometer should be stable at π/2 is required. When measuring the phase modulator using optical fiber, since the optical path is several meters, it is difficult to avoid disturbance of the phase bias due to external temperature and perturbation, thus limiting stable and repeatable measurement.

In a method that does not use an interferometer, it is possible to extract the optical modulation amplitude and frequency response by measuring and analyzing the side wave of the optical output of the device under test (DUT) with a spectrometer. However, in this case, it is necessary to use a spectrometer with very high resolution, for example, a Fabry-Perot filter. there is As another method, there is a method of measuring the phase modulation characteristic by using the polarization modulation characteristic in the chip state of the phase modulator, but the application is limited to the module type phase modulator to which an optical fiber is attached.

The technical problem to be solved by the present invention is to provide a phase modulator measuring device capable of accurately measuring the frequency response characteristics of a phase modulator using a structure in which an irreversible phase element is inserted into a Sagnac interferometer.

An apparatus for measuring an optical phase modulator using a Sagnac interferometer according to an embodiment of the present invention includes a Sagnac interferometer, a laser source emitting an optical signal to the Sagnac interferometer, and a polarizer for converting the optical signal into 90 degree polarization, the The Sagnac interferometer includes a coupler for splitting the optical signal, an optical fiber coil, a phase modulator connected on one path between the coupler and the optical fiber coil, and another path between the coupler and the optical fiber coil. and includes an irreversible phase element with a π/2 phase bias.

The irreversible phase element includes a first polarization rotator connected to the coupler through an optical fiber and rotating a polarized signal by 45 degrees, a second polarization rotator connected to the optical fiber coil and rotating the polarized signal by 45 degrees, and the second polarization rotator is connected to the optical fiber coil and rotates the polarized signal by 45 degrees; A 0/4 wave plate positioned between the first polarization rotator and the second polarization rotator may be included.

At least one of the first polarization rotator and the second polarization rotator may be a Faraday rotator.

The optical phase modulator measuring device using the Sagnac interferometer is a photodetector that converts the interference signal formed by the Sagnac interferometer into an electrical signal, and a network analyzer that compares the optical signal and the interference signal to analyze the frequency response characteristics of the phase modulator , and a circulator for applying the path of the optical signal to the coupler or the photodetector.

The laser source may be an ASE source emitting an Amplified Spontaneous Emission (ASE) optical signal.

An optical phase modulator measuring apparatus using a Sagnac interferometer according to another embodiment of the present invention includes a laser source emitting an optical signal, a polarizer for converting the optical signal into 90 degree polarization, and an irreversible phase element having a π/2 phase bias and a Sagnac interferometer that receives the optical signal and forms an interference signal, a photodetector that converts the interference signal formed by the Sagnac interferometer into an electrical signal, and compares the optical signal with the interference signal to set the frequency of the phase modulator It includes a network analyzer that analyzes response characteristics.

The Sagnac interferometer includes a coupler for splitting the optical signal, an optical fiber coil, and a phase modulator connected on one path between the coupler and the optical fiber coil, and on the other path between the coupler and the optical fiber coil. It may include the irreversible phase element connected to.

The irreversible phase element includes a first polarization rotator connected to the coupler through an optical fiber and rotating a polarized signal by 45 degrees, a second polarization rotator connected to the optical fiber coil and rotating the polarized signal by 45 degrees, and the second polarization rotator is connected to the optical fiber coil and rotates the polarized signal by 45 degrees; A 0/4 wave plate positioned between the first polarization rotator and the second polarization rotator may be included.

At least one of the first polarization rotator and the second polarization rotator may be a Faraday rotator.

The laser source may be an ASE source emitting an Amplified Spontaneous Emission (ASE) optical signal.

The apparatus for measuring an optical phase modulator using a Sagnac interferometer according to an embodiment of the present invention can accurately measure the polarization state and frequency response characteristics of a phase modulator widely used in various photonic systems and photon-based microwave technology.

In addition, the optical phase modulator measuring apparatus according to the embodiment of the present invention has a structure in which the irreversible phase element is inserted into the Sagnac interferometer, and thus the use of measuring equipment can be minimized.

The optical phase modulator measuring apparatus according to an embodiment of the present invention can be widely used in a photonic system using a phase modulator, a photon-based RF system, and the like in the future.

1 is a block diagram illustrating an optical phase modulator measuring apparatus using a Sagnac interferometer according to an embodiment of the present invention.
2 is a conceptual diagram illustrating an irreversible phase bias device according to an embodiment of the present invention.
3 is an exemplary diagram illustrating equipment that actually implements an optical phase modulator measuring apparatus using a Sagnac interferometer according to an embodiment of the present invention.
4 is a graph showing the frequency response characteristics of the phase modulator measured using an optical phase modulator measuring apparatus using a Sagnac interferometer according to an embodiment of the present invention and the frequency response characteristics of the phase modulator measured using a conventional measuring device; to be.

Hereinafter, with reference to the accompanying drawings, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. The present invention may be embodied in several different forms and is not limited to the embodiments described herein.

In order to clearly explain the present invention, parts irrelevant to the description are omitted, and the same reference numerals are assigned to the same or similar elements throughout the specification.

In addition, throughout the specification, when a part "includes" a certain component, this means that other components may be further included, rather than excluding other components, unless otherwise stated.

Hereinafter, an optical phase modulator measuring apparatus using a Sagnac interferometer according to an embodiment of the present invention will be described with reference to FIGS. 1 to 3 .

1 is a block diagram illustrating an optical phase modulator measuring apparatus using a Sagnac interferometer according to an embodiment of the present invention. 2 is a conceptual diagram illustrating an irreversible phase bias device according to an embodiment of the present invention. 3 is an exemplary diagram illustrating equipment that actually implements an optical phase modulator measuring apparatus using a Sagnac interferometer according to an embodiment of the present invention.

1 to 3, an optical phase modulator measuring device 10 using a Sagnac interferometer is a laser source 110, a polarizer 120, a circulator 130, a 50:50 coupler ( 140 ), a phase modulator 150 , an irreversible phase element 160 , an optical fiber coil 170 , a photodetector 180 and a network analyzer 190 .

The laser source 110 is an optical source that emits an optical signal to the Sagnac interferometer. The laser source 110 may be a 1550 nm tunable laser. The laser source 110 may be an ASE source emitting an Amplified Spontaneous Emission (ASE) optical signal.

The polarizer 120 transmits the uniaxial polarization of the optical signal emitted from the laser source 110 to the circulator 130 . The polarizer 120 may serve as a polarization filter for converting and fixing the optical signal emitted from the laser source 110 to 90 degree polarization.

According to an embodiment, the laser source 110 and the polarizer 120 may be replaced with a narrow line width laser source having a wavelength of several nm, and in this case, the cost and insertion loss of the device are reduced. have.

The circulator 130 serves to select a path of an optical signal. That is, the circulator 130 may serve to apply the path of the input optical signal to the 50:50 coupler 140 or the photodetector 180 . A polarization-controlled optical signal through the polarizer 120 is applied to the Sagnac interferometer through the circulator 130 .

The Sagnac interferometer includes a 50:50 coupler 140 , a phase modulator 150 , an irreversible phase element 160 , and an optical fiber coil 170 .

The 50:50 coupler 140 splits the optical signal incident through the circulator 130 into two polarized signals of substantially equal intensity. The two divided polarized signals travel the same optical path in opposite directions. According to an embodiment, the circulator 130 and the 50:50 coupler 140 may be replaced with a 2×2 coupler, and in this case, there is an advantage in that the cost and insertion loss of the device are reduced.

The phase modulator 150 is connected on one path between the 50:50 coupler 140 and the optical fiber coil 170 , and the irreversible phase element 160 is between the 50:50 coupler 140 and the optical fiber coil 170 . The optical fiber coil 170 is connected between the phase modulator 150 and the irreversible phase element 160 . The length of the optical fiber coil 170 may be manufactured/changed from 1 m to 20 m.

The first polarized signal divided by the 50:50 coupler 140 is transmitted to the irreversible phase element 160 side. The first polarization signal is transmitted in the order of the irreversible phase element 160 , the optical fiber coil 170 , and the phase modulator 150 , and returns to the 50:50 coupler 140 .

The second polarized signal divided by the 50:50 coupler 140 is transmitted to the phase modulator 150 side. The second polarized signal is transmitted to the phase modulator 150 , the optical fiber coil 170 , and the irreversible phase element 160 in this order and returns to the 50:50 coupler 140 .

In this case, the irreversible phase element 160 rotates the polarization axes of the first polarization signal and the second polarization signal by 90 degrees, and the phase modulator 150 modulates the phase of the optical signal. The irreversible phase element 160 has a π/2 phase bias, and sets a π/2 phase bias to the Sagnac interferometer.

As illustrated in FIG. 2 , the irreversible phase element 160 includes two polarization rotators 161 and 162 and a zero quarter waveplate 163 positioned therebetween. At least one of the two polarization rotators 161 and 162 may be a Faraday rotator. The Faraday rotator is a polarization rotator based on the Faraday effect and the magneto-optic effect. The first polarization rotator 161 may be connected to the 50:50 coupler 140 through an optical fiber, and the second polarization rotator 162 may be connected to the optical fiber of the optical fiber coil 170 . The first polarization rotator 161 and the second polarization rotator 162 rotate the polarization signal by 45 degrees. The 0/4 wave plate 163 does not rotate the polarized signal, but has a low dependence on temperature and high stability to the external environment, so it can serve to create an accurate phase delay.

The first polarized signal and the second polarized signal returned to the 50:50 coupler 140 via the phase modulator 150 are combined by the 50:50 coupler 140 to form an interference signal. The interference signal formed by the Sagnac interferometer is incident on the photodetector 180 through the circulator 130 .

The photodetector 180 converts the interference signal into an electrical signal. The network analyzer 190 analyzes the electro-optical characteristic (frequency response characteristic) of the phase modulator 150 by comparing the optical signal applied to the photodetector 180 and the interference signal.

The optical path through which the optical signal illustrated in FIG. 1 is transmitted may be formed of a polarization maintain optical fiber that maintains the optical signal as one polarization.

In this way, the Sagnac interferometer is configured as a polarization-maintaining interferometer, and a Device Under Test (DUT), that is, a phase modulator 150 is inserted into the Sagnac interferometer. At this time, the phase bias becomes 0 due to the reversible characteristic of the Sagnac interferometer, π In order to set the bias to /2, the irreversible phase element 160 is inserted into the Sagnac interferometer. Through this structure, the influence of external perturbation can be minimized, and π is stably π for a long time without the need to adjust the bias with a polarization controller each time. Since the /2 phase bias can be maintained, the frequency response of the phase modulator 150 can be measured without distortion.

If, instead of inserting the irreversible phase element 160 in the Sagnac interferometer, a polarization controller that adjusts the polarization of the optical signal incident from the laser source 110 to the 50:50 coupler 140 is used, real-time phase bias adjustment And distortion of the frequency response according to the polarization state may occur.

However, as in the embodiment of the present invention, if the Sagnac interferometer is configured as a polarization-maintaining interferometer and the irreversible phase element 160 is inserted into the Sagnac interferometer, real-time phase bias adjustment and distortion of the frequency response according to the polarization state can be prevented in advance. have.

Equation 1 represents the magnitude of the interference signal of the Sagnac interferometer before applying the irreversible phase element 160 to the Sagnac interferometer.

는 광학 바이어스 크기의 초기값을 의미하며,

는 광학 바이어스의 위상값을 의미한다. 광학 바이어스

가 충분히 작은 경우 수학식 1은 아래의 수학식 2와 같이 표현될 수 있다.here,

is the initial value of the optical bias size,

is the phase value of the optical bias. optical bias

When is sufficiently small, Equation 1 can be expressed as Equation 2 below.

When Equation 2 is developed and solved, since it is expressed in the form of cos(2ωt), the phase value ω of the phase modulator 150 cannot be directly calculated with respect to the applied radio frequency signal.

To solve this, when the irreversible phase element 160 (π/2 phase bias element) is applied as in the embodiment of the present invention, the magnitude E ₀ of the interference signal by the irreversible phase element 160 is expressed as Equation 3 can

As shown in Equation 4, it is possible to ensure linearity between the magnitude of the interference signal and the applied radio frequency signal, and the phase value ω of the phase modulator 150 can be directly calculated.

As described above, the optical phase modulator measuring device according to the embodiment of the present invention has a structure in which an irreversible phase element is inserted into the Sagnac interferometer, and thus the use of measuring equipment can be minimized, and the polarization state and frequency response of the phase modulator 150 . characteristics can be accurately measured.

4 is a graph showing the frequency response characteristics of the phase modulator measured using an optical phase modulator measuring apparatus using a Sagnac interferometer according to an embodiment of the present invention and the frequency response characteristics of the phase modulator measured using a conventional measuring device; to be.

Referring to FIG. 4 , a Keysight N43743D device was used as a conventional measurement device.

Analysis of the electro-optical characteristics (frequency response characteristics) of the phase modulator using the optical phase modulator measuring device using the Sagnac interferometer according to an embodiment of the present invention and the electro-optical characteristics of the phase modulator using conventional measuring equipment It can be seen that the results are similar.

The drawings and the detailed description of the described invention referenced so far are merely exemplary of the present invention, which are only used for the purpose of explaining the present invention, and are used to limit the meaning or the scope of the present invention described in the claims. it is not Therefore, those skilled in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Accordingly, the true technical protection scope of the present invention should be defined by the technical spirit of the appended claims.

10: optical phase modulator measuring device
110: laser source
120: polarizer
130: circulator
140: 50:50 coupler
150: phase modulator
160: irreversible phase element
170: optical fiber coil
180: photodetector
190: network analyzer

Claims (10)

sagnac interferometer;
a laser source emitting an optical signal to the Sagnac interferometer; and
and a polarizer that converts the optical signal into 90 degree polarized light,
The sagnac interferometer is
a coupler splitting the optical signal;
fiber optic coil;
a phase modulator connected on one path between the coupler and the optical fiber coil; and
An optical phase modulator measuring apparatus using a Sagnac interferometer comprising an irreversible phase element connected on the other path between the coupler and the optical fiber coil and having a π/2 phase bias. According to claim 1,
The irreversible phase element,
a first polarization rotator connected to the coupler through an optical fiber and rotating the polarized signal by 45 degrees;
a second polarization rotator connected to the optical fiber coil and rotating the polarized signal by 45 degrees; and
An optical phase modulator measuring apparatus using a Sagnac interferometer comprising a 0/4 wave plate positioned between the first polarization rotator and the second polarization rotator. 3. The method of claim 2,
An optical phase modulator measuring apparatus using a Sagnac interferometer, wherein at least one of the first polarization rotator and the second polarization rotator is a Faraday rotator. According to claim 1,
a photodetector for converting the interference signal formed by the Sagnac interferometer into an electrical signal;
a network analyzer for analyzing a frequency response characteristic of the phase modulator by comparing the optical signal and the interference signal; and
The optical phase modulator measuring apparatus using a Sagnac interferometer further comprising a circulator for applying the path of the optical signal to the coupler or the photodetector. According to claim 1,
The laser source is an ASE (Amplified Spontaneous Emission) optical phase modulator measuring device using an ASE source that emits an optical signal Sagnac interferometer. a laser source emitting an optical signal;
a polarizer for converting the optical signal into 90 degree polarization;
a Sagnac interferometer comprising an irreversible phase element having a π/2 phase bias and receiving the optical signal to form an interference signal;
a photodetector for converting the interference signal formed by the Sagnac interferometer into an electrical signal; and
and a network analyzer for analyzing a frequency response characteristic of the phase modulator by comparing the optical signal with the interference signal. 7. The method of claim 6,
The sagnac interferometer is
a coupler splitting the optical signal;
fiber optic coil; and
a phase modulator connected on one path between the coupler and the optical fiber coil; and
An optical phase modulator measuring apparatus using a Sagnac interferometer including the irreversible phase element connected on the other path between the coupler and the optical fiber coil. 8. The method of claim 7,
The irreversible phase element,
a first polarization rotator connected to the coupler through an optical fiber and rotating the polarized signal by 45 degrees;
a second polarization rotator connected to the optical fiber coil and rotating the polarized signal by 45 degrees; and
An optical phase modulator measuring apparatus using a Sagnac interferometer comprising a 0/4 wave plate positioned between the first polarization rotator and the second polarization rotator. 9. The method of claim 8,
An optical phase modulator measuring apparatus using a Sagnac interferometer, wherein at least one of the first polarization rotator and the second polarization rotator is a Faraday rotator. 7. The method of claim 6,
The laser source is an ASE (Amplified Spontaneous Emission) optical phase modulator measuring device using an ASE source that emits an optical signal Sagnac interferometer.

Images