KR20160050922A - Phase Modulation Apparatus based on Sagnac interferometer - Google Patents

Abstract

Disclosed is a phase modulation apparatus using a Sagnac interferometer and a Faraday cell. According to an aspect of the present embodiment, in the phase modulation apparatus, two circularly polarized beams traveling in opposite directions in a closed optical path of a Sagnac interferometer experience a differential phase shift by a magnetic field adjacent a modulation coil. The phase modulation apparatus comprises: a means for defining a closed optical path; a means for splitting an input beam into a first beam and a second beam moving in opposite directions; a means for combining the first beam and the second beam so as to form an output beam after the first beam and the second beam propagate the same distance along the optical path; a means, disposed on the optical path, for linearly polarizing the first beam and the second beam at a certain angle; a modulation means, disposed on the optical path, for inducing a phase shift based on Faraday effect by applying a magnetic field to the first beam and the second beam; and a means, disposed on the optical path, for circularly polarizing the first beam and the second beam into linearly polarized beams before the first beam and the second beam reach the modulation means and circularly polarizing the first beam and the second beam into linearly polarized beams after the first beam and the second beam pass through the modulation means.

Description

[0001] The present invention relates to a phase modulator based on Sagnac interferometer,

The present embodiment relates to a Sagnac interferometer based phase modulating apparatus. More specifically, it relates to a phase modulating device in which two circularly polarized beams traveling in opposite directions to a closed optical path of a sagnac interferometer undergo a differential phase shift by a magnetic field.

The contents described in this section merely provide background information on the present embodiment and do not constitute the prior art.

The Quantum Key Distribution System sends cryptographic key information to a single photon in a manner that regulates the polarization or phase of a single photon and sends it to the recipient. The receiver extracts cryptographic key information by applying a polarization receiving device, a phase modulating device, or the like. Such a single photon transmission is implemented based on optical communication technology, and a quantum cryptographic key distribution system for long distance transmission mainly uses a single mode optical fiber as a quantum channel. The transmission of polarization-modulated single photons to a single-mode fiber results in unstable polarization characteristics and poor transmission characteristics, so that cryptographic keys are distributed over phase modulation rather than polarization modulation.

In a quantum cryptographic key distribution system based on phase modulation, a time-division interference method is mainly used. For time-divisional optical interference, an asymmetric optical interferometer and a phase modulator are required. The asymmetric optical interferometer has a structure in which the lengths of two paths required for optical interference are different from each other. A single photon input to an asymmetric optical interferometer is divided into two distributions whose existence probability distributions have different coordinates in the time domain. The phase modulator modulates the phase of a single photon passing through one of the paths. The asymmetric optical interferometer of the receiver partitions the probability of presence into four coordinates in the time domain. If the path differences of the transmitting part and the asymmetric optical interferometer of the receiving part are equal to each other, two adjacent ones of the four single photon existence probabilities overlap each other and interference occurs. The receiver also has a phase modulator to modulate the phase of a single photon. If the sum of the phase modulation amounts added by the transmitter and the receiver is 2nπ (where n is an integer), the probability of existence of two overlapping single photons is maximized by constructive interference, and the sum of the phase modulation amounts is (2n + 1) π, the interference is minimized. Therefore, the optical interference performance affects the overall performance of the quantum cryptographic key transmission system.

For superior optical interference performance, the stability of the polarization and phase characteristics of the optical interferometer must be secured. The two single photons interfering in the receiver optical interferometer must have the same polarization and the phase by the entire optical path except for the phase modulation value added by the phase modulator should maintain a constant value. However, the asymmetric optical interferometer has a disadvantage in that it is susceptible to the influence of the surrounding environment such as temperature change, vibration, and sound because the two paths required for optical interference have different structures.

The main purpose of the present embodiment is to provide a Sagnac interferometer-based phase modulating device in which two circularly polarized beams that are traveling in opposite directions to each other close to each other in a Sagn optical interferometer are subjected to a differential phase shift by a magnetic field.

Another object of the present invention is to provide a quantum cryptographic key distribution system using a Sagnac interferometer-based phase modulating apparatus.

According to an aspect of this embodiment, there is provided a lithographic apparatus comprising: means defining a closed optical path; Means for dividing a first beam and a second beam traveling in opposite directions from each other in the optical path from an input beam; Means for coupling the first beam and the second beam to form an output beam after propagating the same distance along the optical path; Means for positioning the first beam and the second beam linearly polarized at a predetermined angle on the optical path; Modulation means for being located on the optical path and applying a magnetic field to the first beam and the second beam to induce a phase shift based on the Faraday effect; And means for positioning the first beam and the second beam on the optical path so as to circularly polarize the first beam and the second beam into a circularly polarized beam before arriving at the modulation means, And means for circularly polarizing the beam into a linearly polarized beam.

According to an aspect of this embodiment, there is provided a beam splitter for dividing an input beam into a first beam and a second beam; A pair of reflecting plates defining a closed optical path together with the beam splitter so that the first beam and the second beam form an output beam after propagating the same optical path in opposite directions; A pair of Faraday cells located on the optical path for linearly polarizing the first beam and the second beam at a predetermined angle; Modulation means for being located on the optical path and applying a magnetic field to the first beam and the second beam to induce a phase shift based on the Faraday effect; And means for positioning the first beam and the second beam on the optical path so as to circularly polarize the first beam and the second beam into a circularly polarized beam before arriving at the modulation means, There is provided a phase modulating apparatus comprising a pair of circular polarizing plates for circularly polarizing a linearly polarized beam.

As described above, according to the present embodiment, since it is basically constructed based on the Chanak interferometer, it is possible to provide a phase modulating device which is not affected by a change in temperature.

1 is an optical schematic diagram illustrating a basic configuration of a Sagnac interferometer.
FIG. 2 is a schematic diagram illustrating a sagac interferometer based phase modulating apparatus according to an embodiment of the present invention. Referring to FIG.
FIGS. 3A and 3B are diagrams illustrating states of polarization that are experienced while passing through optical elements of each optical pulse angle divided by the Sagnac interferometer-based phase modulator of FIG.
4 is a diagram showing the phase shift experienced by left-handed circularly polarized light and right-handed circularly polarized light passing through a magnetic field medium.

Hereinafter, some embodiments of the present invention will be described in detail with reference to exemplary drawings. It should be noted that, in adding reference numerals to the constituent elements of the drawings, the same constituent elements are denoted by the same reference symbols as possible even if they are shown in different drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

In describing the components of the present invention, terms such as first, second, A, B, (a), and (b) may be used. These terms are intended to distinguish the constituent elements from other constituent elements, and the terms do not limit the nature, order or order of the constituent elements. Throughout the specification, when an element is referred to as being "comprising" or "comprising", it means that it can include other elements as well, without excluding other elements unless specifically stated otherwise . In addition, '... Quot ;, " module ", and " module " refer to a unit that processes at least one function or operation, and may be implemented by hardware or software or a combination of hardware and software.

1 is an optical schematic diagram illustrating a basic configuration of a Sagnac interferometer.

As shown in FIG. 1, the optical path of the sagnac interferometer is composed of an input optical path, an optical ring formed by three reflection mirrors 130a, 130b, and 130c, and an output optical path. The incident beam 110 is incident on a beam splitter 120, which is a translucent plate, through an input optical path. The beam splitter 120 partially reflects and partially passes the incident beam 110. Thus, the incident beam 110 is divided into a reflected beam component 112a and a transmitted beam component 112b.

The reflected beam component 112a moves the optical ring through a series of reflectors 130a, 130b and 130c. The transmitted beam component 112b moves the optical ring through a series of reflectors 130c, 130b and 130a in a reverse order to the reflected beam component 112a. The beam 112a and beam 112b then reach the beam splitter 120. The beam 112a is partially transmitted in the direction of the input optical path of the interferometer, and part of the beam 112a is reflected in the direction of the output optical path. The beam 112b is also transmitted in the direction of the input optical path, and some of it is reflected in the direction of the output optical path. Some of these two beams interfere at the input and output paths. Destructive interference occurs at the output optical path, and compensatory interference occurs at the input optical path.

FIG. 2 is a schematic diagram illustrating a sagac interferometer based phase modulating apparatus according to an embodiment of the present invention. Referring to FIG.

The optical path of the sagnac interferometer according to an embodiment of the present invention is composed of an input optical path, an optical ring formed by two reflection mirrors 230a and 230b, and an output optical path. However, it will be apparent to those skilled in the art that, unlike the Sagnac interferometer illustrated in FIG. 2, Sagnac interferometers having various types of optical paths can be used. In the following description, it is assumed that the input optical pulse 210 input to the input optical path of the sagnac interferometer has a linear polarization state in the vertical direction.

A beam splitter 220 splits the input optical pulses 210 into optical pulses 212a and 212b that propagate in opposite directions of two substantially equal magnitudes. The divided optical pulses 212a and 212b are propagated to different optical paths. Faraday cells 240a and 240b are inserted into each path. Each Faraday cell 240a, 240b rotates the linear polarization of the split optical pulses 212a, 212b by -22.5 degrees. Accordingly, the divided optical pulses 212a and 212b have a linear polarization state of -45 degrees after passing through the two Faraday cells 240a and 240b. In addition, circular light plates 250a and 250b, that is, a quarter wave plate (QWP), are inserted into each path. The circular polarizer plates 250a and 250b convert the divided optical pulses into opposing circularly polarized optical pulses in opposite directions to each other. That is, one circular polarizer 250a makes the right circularly polarized light, and the other circular polarizer 250b makes left circularly polarized light. The optical pulses passing through the circular polarizers 250a and 250b are incident on the Faraday cells 260 in opposite directions to each other. As the light passes through the Faraday cell 260, the circularly polarized light pulse undergoes a phase shift (Faraday rotation) opposite to each other due to the current or magnetic field flowing in the magnetic field coil (modulation coil 261). Here, the direction of the phase shift is determined by the propagation direction of the circularly polarized light pulse with respect to the direction of the magnetic field. The current flowing in the magnetic field coil 260 is supplied by a modulation circuit (not shown). The circularly polarizing plates 250a and 250b convert the circularly polarized light pulses having passed through the Faraday cell 250 into linearly polarized light pulses, and the converted light pulses are recombined by the beam splitter 220, .

2, a pair of Faraday cells 240a and 240b, a pair of circular polarizers 250a and 250b, and a Faraday cell 260 are all shown to be located between two reflective mirrors 230a and 230b, At least some of the optical elements may be disposed at different locations on the optical path. Therefore, it will be understood by those skilled in the art that various embodiments employing the technical idea of the present invention can be implemented according to the arrangement of the optical paths of the Sagnac interferometer and / or the optical paths of the optical elements. 2 also shows a pair of Faraday cells 240a and 240b for rotating the linearly polarized light of the split optical pulses 212a and 212b by -22.5 ° respectively and a Faraday rotator 240a and 240b for providing Faraday rotation to the circularly polarized optical pulses Although the cell is described as being the same optical element, it is also possible to have different optical elements. For example, the Faraday cell 260 may be an Atomic Faraday cell.

FIGS. 3A and 3B are diagrams illustrating states of polarization that are experienced while passing through optical elements of each optical pulse angle divided by the Sagnac interferometer-based phase modulator of FIG.

3A, the optical pulse reflected by the beam splitter 220 has a linear polarization state in the vertical direction, and is incident on the Faraday cell 240a via the reflection plate 230a. The optical pulse passing through the Faraday cell 240a has a linear polarization state of -22.5 占 and is incident on the circularly polarizing plate 250a. The optical pulse passing through the circular polarizing plate 250a has a right circularly polarized state and passes through the Faraday cell 260 and undergoes phase shift (i.e., phase modulation) in the (+) direction. The optical pulse passing through the Faraday cell 260 still has a right circularly polarized state and is incident on another circularly polarizing plate 250b. The circularly polarizing plate 250b circularly polarizes the input optical pulses in the right circularly polarized state, and as a result, the optical pulses passing through the circularly polarizing plate 250b again have a -22.5 ° linearly polarized state. The light pulse passing through the circular polarizing plate 250b passes through another Faraday cell 240b, and as a result has a linear polarization state of -45 degrees. The light pulse is reflected at the reflector 230b and has a linear polarization state of 45 degrees and is reflected by the beam splitter 220 again to reach the output optical path with a linear polarization state of -45 degrees. As a result, the optical pulse having the linear polarization state of -45 DEG in the output optical path is phase-modulated in the (+) direction. In other words, among the input light pulses 210, the output light pulses corresponding to the components reflected by the beam splitter 220 have a linear polarization state of -45 degrees and are phase-modulated in the positive direction.

3B, the optical pulses transmitted from the beam splitter 220 have linear polarization states in the vertical direction, and are incident on the Faraday cell 240b via the reflection plate 230b. The optical pulse passing through the Faraday cell 240b has a linear polarization state of -22.5 占 and is incident on the circularly polarizing plate 250b. The light pulse passing through the circular polarizing plate 250b has a left circularly polarized state and passes through the Faraday cell 260 and undergoes phase shift (i.e., phase modulation) in the (-) direction. The light pulse passing through the Faraday cell 260 still has a left circularly polarized state and is incident on another circularly polarizing plate 250a. The circularly polarizing plate 250a causes right circularly polarized input optical pulses in the left circularly polarized state, so that the optical pulses passing through the circularly polarizing plate 250a again have a -22.5 ° linearly polarized state. The light pulse passing through the circular polarizing plate 250a passes through another Faraday cell 240a, and as a result has a linear polarization state of -45 degrees. The light pulse is reflected by the reflection plate 230a and has a linear polarization state of 45 °, and again passes through the beam splitter 220 to reach the output optical path. As a result, the optical pulse having the linear polarization state of 45 degrees in the output optical path is phase-modulated in the negative (-) direction. In other words, among the input light pulses 210, the output light pulses corresponding to the components transmitted by the beam splitter 220 have a linear polarization state of 45 degrees and are phase-modulated in the negative (-) direction.

The problem due to temperature-induced phase shifts is solved, according to embodiments of the present invention, by the fact that the divided optical pulses propagate in opposite directions to the same optical path. When the temperature of any part on the optical path rises, the phase velocities of both divided optical pulses rise equally. Therefore, even if the optical pulses recombine and interfere with each other at the output, their relative phase stays constant, and thus the combined amplitude is also unchanged. Thus, the optical pulse intensity of the phase modulating device is not affected by the change in temperature.

4 is a diagram showing the phase shift experienced by left-handed circularly polarized light and right-handed circularly polarized light passing through a magnetic field medium.

Figure 4 (a) illustrates the atomic transfer line of a circularly polarized photon and shows different Zeeman sublevels for left-circular polarization and right-circular polarization. . FIG. 4 (b) shows the phase change experienced by the circularly polarized transmitted light ray passing through the magnetic field medium. In the case of left-handed circularly polarized light and right-handed circularly polarized light,

The Sagnac interferometer-based phase modulating apparatus described above can be applied to a transmitting apparatus and a receiving apparatus of a quantum key distribution (QKD) system. That is, by adopting the Sagnac interferometer-based phase modulating device in the quantum cryptographic key distribution system designed on the basis of the phase modulation, it is possible to provide an advantage of being insensitive to the influence of the surrounding environment such as temperature change, vibration and sound. As a result, the optical interference performance insensitive to the influence of the surrounding environment can secure the entire performance of the quantum cryptographic key transmission system constantly despite the change of the surrounding environment.

The foregoing description is merely illustrative of the technical idea of the present embodiment, and various modifications and changes may be made to those skilled in the art without departing from the essential characteristics of the embodiments. Therefore, the present embodiments are to be construed as illustrative rather than restrictive, and the scope of the technical idea of the present embodiment is not limited by these embodiments. The scope of protection of the present embodiment should be construed according to the following claims, and all technical ideas within the scope of equivalents thereof should be construed as being included in the scope of the present invention.

110: input optical pulse 120: beam splitter
130a to 130c: reflection mirror 112a: reflected beam component
112b: transmitted beam component 210: input optical pulse
212a: reflected optical pulse 212b: transmitted optical pulse
214a to 214b: output optical pulse 220: beam splitter
230a to 230b: reflection mirrors 240a to 240b: Faraday cell
250a to 250b: circular polarizer 260: Faraday cell
261: magnetic field coil

Claims (8)

Means for defining a closed sight path;
Means for dividing a first beam and a second beam traveling in opposite directions from each other in the optical path from an input beam;
Means for coupling the first beam and the second beam to form an output beam after propagating the same distance along the optical path;
Means for positioning the first beam and the second beam linearly polarized at a predetermined angle on the optical path;
Modulation means for being located on the optical path and applying a magnetic field to the first beam and the second beam to induce a phase shift based on the Faraday effect; And
Wherein the first beam and the second beam are located on the optical path and circularly polarized into a circularly polarized beam before the first beam and the second beam arrive at the modulating means, and after the first beam and the second beam have passed through the modulating means Means for circularly polarizing a linearly polarized beam
/ RTI > The method according to claim 1,
Wherein the modulation means comprises:
And phase-modulates the first beam and the second beam differentially. The method according to claim 1,
Wherein the modulation means comprises:
A Faraday cell and a modulation coil for forming a magnetic field in the vicinity of the Faraday cell by an input modulation current. The method according to claim 1,
The means for circularly polarizing comprises:
A left-hand circularly polarizing plate and a right-handed circularly polarizing plate. The method according to claim 1,
The means for defining the closed optical path includes:
And a plurality of reflectors. The method according to claim 1,
Wherein said output beam comprises a first component having a linear polarization state of -45 DEG and a second component having a linear polarization state of 45 DEG. The method according to claim 6,
Wherein the first component is phase-modulated in the (+) direction and the second component is phase-modulated in the (-) direction. A beam splitter for splitting the input beam into a first beam and a second beam;
A pair of reflecting plates defining a closed optical path together with the beam splitter so that the first beam and the second beam form an output beam after propagating the same optical path in opposite directions;
A pair of Faraday cells located on the optical path for linearly polarizing the first beam and the second beam at a predetermined angle;
Modulation means for being located on the optical path and applying a magnetic field to the first beam and the second beam to induce a phase shift based on the Faraday effect; And
Wherein the first beam and the second beam are located on the optical path and circularly polarized into a circularly polarized beam before the first beam and the second beam arrive at the modulating means, and after the first beam and the second beam have passed through the modulating means A pair of circular polarizers for circularly polarizing the linearly polarized beam
/ RTI >

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