JPH0710334Y2 - Optical frequency shifter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Industrial application field" The present invention relates to an optical frequency shifting device that shifts the frequency of light as it is without converting the light wave into an electric signal.

"Prior Art" In the frequency shift device, as shown in FIG. 2, an incident light wave cos (ωt) having an angular frequency of ω is divided by a light wave distributor 11 into two light waves. One of the light waves is phase-shifted by 90 degrees in the π / 2 phase shifter 12. In this way, the first phase shifted 90 degrees from each other
1, the second light wave is obtained. The first light wave, which is the light wave directly emitted from the light wave distributor 11 in this example, enters the first amplitude modulator 13 and is amplitude-modulated by the cosine wave signal Acos (pt).
The outgoing light wave of the first amplitude modulator 13 can be expressed as Acos (pt) cos (ωt) (1). On the other hand, the second light wave, that is, the light wave emitted from the π / 2 phase shifter 12 in this example is represented by −sin (ωt), and this light wave is amplitude-modulated by the sine wave signal Asin (pt) by the second amplitude modulator 14. It The light wave emitted from the second amplitude modulator 14 can be expressed as −Asin (pt) sin (ωt) (2). Both outgoing light waves from the first and second amplitude modulators 13 and 14 are interference-coupled by a light wave combiner 15. As a result,
The light wave emitted from the light wave coupler 15 becomes Acos (pt) cos (ωt) −Asin (pt) sin (ωt) = Acos [(ω + p) t] (3), and the frequency is p with respect to the incident light wave cos (ωt). It will be changed by / 2π.

In FIG. 2, the optical elements are connected by parallel light or optical waveguides.

The conventional device used for the first and second amplitude modulators 13 and 14 has a structure as shown in FIG. The incident light wave is converted into a linearly polarized light wave by the polarizer 21 and is incident on the electro-optical element 22. The electro-optical element 22 passes through the analyzer 23 as it is when no electric field is applied to it. When an electric field is applied to the electro-optical element 22, the polarization plane of the linearly polarized light wave passing through the electro-optical element 22 is rotated according to the electric field strength. Therefore, the amount of light transmitted through the analyzer 23 is defined by the angle between the polarization plane of the incident light wave and the optical axis of the analyzer 23 being θ,
Let I be the amount of light incident on. Therefore, when an AC electric field is applied to the electro-optical element 22, an emitted light wave amplitude-modulated by the AC electric field is obtained from the analyzer 23.

[Problems to be Solved by the Invention] The substance of the electro-optical element 22 used in the conventional amplitude modulator is, for example, a crystal of lithium tantalate (TiTaO ₃ ). The half-wave voltage of this crystal, that is, the voltage for rotating the polarization plane by 90 ° is 2800 V, and the crystal length is 70 mm when the electrode interval is 1 mm. As described above, the conventional amplitude modulator requires a high voltage and becomes large in size, so that there is a problem that the optical frequency shifter using the same also becomes large in size and requires a high voltage.

[Means for Solving Problems] According to this invention, an incident light wave is distributed, a phase difference of 90 degrees is given between the two light waves, and the two light waves are further separated by the first and second amplitude modulators. (Pt), Asin (pt) amplitude modulation, the amplitude-modulated outgoing light is optically coupled to obtain an outgoing light wave whose frequency is shifted with respect to the incident light wave. As the second amplitude modulator, a liquid crystal element is used.

The amplitude modulator composed of liquid crystal elements can be constructed in a small size,
Moreover, it operates at that low voltage.

[Embodiment] An embodiment of the optical frequency shift device of the present invention will be described with reference to the drawings. The block configuration of this optical frequency shifting device is the second
It is the same as shown in the figure. In the present invention, liquid crystal elements are used as the first and second amplitude modulators 13 and 14. In the amplitude modulator composed of a liquid crystal element, for example, as shown in FIG. 1, a polarizer 32 and an analyzer 33 are arranged on both surfaces of a crystal cell 31, respectively. The incident light wave enters the polarizer 32, becomes linearly polarized light, and enters the liquid crystal cell 31. The liquid crystal cell 31 is filled with a nematic liquid crystal having a positive dielectric anisotropy, for example. It is assumed that the liquid crystal molecules are twisted in the major axis direction by 90 ° between the polarizer 32 side and the analyzer 33 side, and the twist is continuously changed between them. On the polarizer 32 side, the major axis direction of the liquid crystal molecules is the polarizer
It matches the 32 polarization directions.

The polarization direction of the linearly polarized light incident on the liquid crystal cell 31 via the polarizer 32 is rotated by 90 ° along the twist of the liquid crystal molecules while passing through the liquid crystal cell 31 (referred to as 90 ° optical rotation). Therefore, since the optical axes of the polarizer 32 and the analyzer 33 are orthogonal to each other, light is transmitted.

When a voltage is applied between opposing transparent electrodes (not shown) in the liquid crystal cell 31, the major axis of the liquid crystal molecules begins to tilt in the direction of the electric field when the voltage becomes equal to or higher than the threshold voltage, and the voltage of the liquid crystal molecule becomes greater than a certain voltage. The major axis is uniformly arranged in parallel with the electric field direction, and in this state, the 90 ° optical rotation disappears. Therefore, light is transmitted before the voltage is applied to the liquid crystal cell, but when the voltage is applied, the 90 ° optical rotation disappears and the light is not transmitted. That is, when the AC voltage Acos (pt) is applied to the liquid crystal cell 31, the amplitude of the emitted light wave changes by Acos (pt), and the amplitude-modulated emitted light wave is obtained.

If the thickness of the liquid crystal is about 10 μm, 90 ° optical rotation can be eliminated with a voltage of about 8V.

“Effect of device” As described above, in this device, since the first and second amplitude modulators 13 and 14 that are configured by the liquid crystal cell are used, the entire device can be made compact and the applied voltage can be reduced. Is also small. For example, the amplitude modulator formed of a liquid crystal cell can have a thickness of 5 mm square or less, and the size of the incident surface may be larger than the beam diameter of the incident light wave, and 5 mm square or less is sufficient. A sufficient modulation effect can be obtained when the applied voltage is 10 V or less.

[Brief description of drawings]

FIG. 1 is a perspective view showing an example of an amplitude modulator used in the optical frequency shifting device of the present invention, FIG. 2 is a block diagram showing the optical frequency shifting device, and FIG. 3 is used in a conventional optical frequency shifting device. It is a block diagram which shows an amplitude modulator.

Claims (1)

[Scope of utility model registration request] 1. An incident light wave is divided into two light waves by a light wave splitter, and these two divided light waves are made into first and second light waves with a phase shift of 90 degrees relative to each other by a π / 2 phase shifter. , The first light wave is amplitude-modulated by the signal Acos (pt) by the first amplitude modulator,
The second light wave is amplitude-modulated by the signal Asin (pt) by the second amplitude modulator, and the two outgoing lights of the first and second amplitude modulators are combined by the light wave combiner so that the frequency of the incident light wave is P /
An optical frequency shifting device for obtaining a (2π) shifted outgoing light wave, wherein each of the first and second amplitude modulators is composed of a liquid crystal element.