JP2802366B2 - Ultrafast electro-optic deflector using oblique periodic polarization inversion - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses an electro-optic crystal and has a reversal period in the light traveling direction, but the inversion axis boundary is not orthogonal to the light wave traveling direction but obliquely oblique, resulting in a cross section of the light beam. The present invention relates to an ultra-high-speed electro-optical deflector using oblique periodic polarization reversal, which includes a plurality of inversion portions and causes a difference in a modulation effect in each portion of the beam cross section, and operates as a deflector without performing simple phase modulation. The technical field to which the present invention pertains is optoelectronics, optical information processing, optical communication, optical measurement, optical recording, and applicable products are deflectors, modulators, ultra-high speed optical disk reading / writing, high speed TV, optical pulse generator, An optical frequency shifter or the like.

[0002]

2. Description of the Related Art Conventional light beam deflection is mainly mechanical in a low speed range, and typically includes a rotating mirror, a vibrating mirror (galvano mirror), and a rotating polygon mirror (polygon) which are electrically and magnetically driven. It is something. An acousto-optic deflector (AOD) is most often used in a high-speed region of megahertz or higher where a mechanical method cannot be used. FIG. 1 shows an example of an acousto-optic deflector.

In FIG. 1, reference numeral 1 denotes an electroacoustic transducer, 2 denotes an incident light beam, 3 denotes an acousto-optic medium, 4 denotes a high-frequency power supply capable of sweeping the frequency, and 5 denotes a traveling-wave phase diffraction grating generated by ultrasonic waves. , 6 indicate a sound absorbing end,
The incident light beam 2 is separated into a non-diffracted beam 7 and a deflected light beam (diffraction beam) 8 by a traveling wave phase grating 5 generated by ultrasonic waves. The deflection direction of the deflected light beam changes when the frequency of the high frequency is changed, and the frequency shifts by the high frequency compared to the incident beam.

As shown in FIG. 1, an incident light beam 2 is applied to an acousto-optic medium 3 made of an optically transparent body via an electroacoustic transducer 1 to travel a traveling wave (compressive wave) of an acoustic wave (usually an ultrasonic wave). ) Is excited and transmitted, and a change in the refractive index that progresses in a sinusoidal waveform having a period of the acoustic wave wavelength is caused by the acousto-optic effect accompanying the traveling acoustic wave. It diffracts a light beam. The period of the phase diffraction grating 5 is changed by changing the frequency of the acoustic wave, thereby changing the diffraction angle to deflect the incident light beam 2. Since the optical frequency of the deflected light beam 8 is shifted from the frequency of the incident light beam by the acoustic frequency, the acousto-optic deflector is useful as a frequency shifter in addition to the deflector.

Since the speed of sound is about five orders of magnitude slower than the speed of light, the wavelength of the acoustic wave and the period of the grating are about five orders of magnitude shorter than the wavelength of the electromagnetic wave at the same frequency. The wavelength is about 30 micrometers, which is fine enough to diffract the light beam.
However, when the acoustic frequency exceeds about 1 GHz, the transmission loss of the sound wave in the acousto-optic medium increases, and its use becomes difficult. Further, at a frequency of several tens of GHz or more, the grating interval becomes equal to or less than a half wavelength of light, which is too fine to diffract and cannot be used.

FIG. 2 shows an example of a deflector using diffraction by a traveling wave phase grating generated by the electro-optic effect. In FIG. 2, 2 is an input light beam, 4 is a high-frequency drive power supply, 9 is an electro-optic medium, 10 is a traveling wave electric field, and 11 is accompanied by an electric traveling wave guiding phase grating. Reference numeral 12 denotes a high-frequency electric field transmission line, reference numeral 13 denotes a load, and reference numeral 14 denotes a light wave that has been subjected to Ramannas diffraction.

As shown in FIG. 2, an electro-optic medium 9 is used instead of an acousto-optic medium, and a traveling wave electric field 10
Is transmitted, a change in the refractive index occurs in proportion to the electric field, so that an optical deflector (electro-optical deflector EOD) similar to an acousto-optical deflector can be configured. In the 10 GHz band or higher, the wavelength of the radio wave in the electro-optic medium 9 is in the mm range, so that it is possible to include a plurality of gratings in the light beam, and the deflection operation of the light wave by the electric traveling wave guiding phase grating 11 becomes possible. . For this reason, a patent has been obtained as Patent No. 2053798 (Japanese Patent Publication No. 7-89180) (electro-optical demultiplexing device and beam deflecting device) according to the invention of the present inventors. This allows
The present inventors have succeeded in ultrafast light deflection of 1 spot / ps.

[0008]

However, 1 to 10 GHz
In the band, the grating interval (electric wavelength) becomes as large as about cm, and it becomes difficult to include a plurality of gratings in the light beam cross section. Therefore, in this frequency range, electro-optic beam deflection using an electric traveling wave phase grating cannot be used.

As described above, if a deflector and a frequency shifter operable in a high-speed range from gigahertz to several tens of GHz, which are the object of the present invention, can be obtained, the high-speed range in which a conventional acousto-optic deflector cannot be used is covered. 1-10 GHz that the inventors could not cover the electro-optical deflector previously invented
It is possible to cover a wide area, and it is expected to greatly contribute to the technological development of optoelectronics.

[0010]

According to the present invention, a crystal axis of an electro-optic crystal is periodically spatially synchronized with a cycle in which the phase of a high-frequency applied electric field observed by a light wave propagating through the electro-optic crystal is inverted. Means for inverting, and means for obliquely intersecting the crystal axis inversion boundary with the traveling direction of the light wave while quasi-matching the phases of the light wave and the high-frequency electric field. And a means for generating a high-frequency traveling-wave phase grating equivalently in the super-high-speed electro-optical deflector utilizing oblique periodic polarization reversal. The present invention further resides in an ultra-high-speed electro-optical deflector using the oblique periodic polarization reversal having a frequency shifter function.

[0011]

FIG. 3 shows a specific example of the present invention.
(A) and (b) will be described. FIG. 3 (a), (b)
, 15 is an incident light beam, 16 is a driving high frequency power supply, 17
Denotes an electro-optic crystal, 18 denotes an upper electrode, 19 denotes an output diffracted light beam, and 20 denotes a terminal load. Reference numeral 21 denotes a crystal axis inversion portion (portion with a small dot) of the electro-optic crystal.

The optical axis of the electro-optic crystal of the present invention is periodically inverted at regular intervals. The light beam obliquely intersects the lattice at a specific angle θ, and passes through the crystal so that the lattice is included in the beam cross section in a plurality of periods. In addition, a high-frequency transmission line is mounted on the electro-optic crystal surface so that a high-frequency electric field is applied to a portion through which the light beam passes so as to proceed. In the example of FIG. 3, a pair of electrodes are provided on the back and top surfaces of the crystal to form a strip line, but a coplanar electrode and other various microwave transmission lines can also be used. High-frequency drive power from a high-frequency power supply 16 is supplied from one end 23 of the high-frequency transmission line, and the other end is terminated by a non-reflection load 20.

In order for the traveling high-frequency electric field and the light beam to perform pseudo-speed matching, the light travels along with

(Equation 1) The crystal axis needs to be inverted every time. Here the group velocity of light [nu _g, the electrical signal phase velocity [nu _m in the light traveling direction, the frequency of the high frequency and f _m. Assuming that a crystal axis reversal interval measured in a direction perpendicular to the crystal axis reversal boundary is h, the traveling direction of light is inclined by θ = sin ⁻¹ (h / L _m ) (2) with respect to the crystal axis reversal boundary. Will intersect. In the case of FIG. 3, the crystal is inverted in the transverse direction in the beam cross section at every polarization inversion pitch d = h / cos θ = L _m tan θ (3), and L _m is determined by the equation (1). Also,
If θ is made small, d can be made as small as possible, and a plurality of gratings can be included in the beam cross section.

Before describing the operation and effect of the present invention, the operation principle of the known pseudo-speed matching optical phase deflector having the configuration shown in FIG. 4 will be described. This corresponds to θ = 90 ° in FIG.
= Infinity, and the crystal axis repeats inversion and non-inversion only in the traveling direction of light, but there is no periodic change in the beam cross section and in the lateral direction.

Now, since the group velocity of light is usually faster than the phase velocity of the electric signal, the light wave passes the electric signal don-don, and when passing the half-cycle, the electric signal has an opposite phase to the first electric signal. The refractive index change up to that point is reversed,
The phase modulation effect received so far begins to cancel, and is completely canceled when just overtaking by one cycle. Therefore, if the electro-optic crystal axis is inverted every time it passes by a half period, the inversion of the crystal axis overlaps the inversion of the electric signal, and the change in the refractive index due to the application of the electric field becomes in-phase, reducing the interaction length. No matter how long it is, the phase modulation effect is accumulated without being cancelled. Such a device is called pseudo-speed matching.

[0016] Now, the light waves were starting the z = 0 to t = t ₀ is the z = L arrive in _{t 1 = t 0 + L /} ν g. On the other hand, the electric signal that the light wave saw at the time of departure is t ₂ = t ₀ + L / ν at z = L.
_Since it reaches _m , at z = L, the light wave sees the time preceding the electric signal by t ₂ −t ₁ . When this time difference is just a half cycle of the electrical _{signal, L / ν m -L / ν} g = 1 / (2f m) (4) Thus,

(Equation 2) Is obtained. When the optical axis of the crystal is inverted at the length of L, the phase of the electric signal seen by the light is inverted, but the change in the refractive index becomes the same as that at z = 0, and the modulation effect is added. Equation (1 ′) is obtained when m = 0 in equation (1), that is, L ₀
Corresponding to Further, even if the phase is inverted for the first time at an odd multiple of L ₀ , the modulation effect of an even multiple thereof, that is, 2 mL ₀ , is canceled and wasted, but the last L ₀ remains and the same modulation is performed. The effect is obtained. Equation (1)
Is more comprehensive, including in this case.

As described above, the ideas up to this point have been already announced as pseudo-speed matched optical phase modulators. By adopting this structure, a high-efficiency electro-optic modulator using a long electro-optic crystal can be realized despite the difference between the speed of the electric signal and the speed of the light. , Currently has the largest modulation index in the world.

In the present invention, the traveling direction of the light wave has such a reversal period, but unlike FIG. 4, as shown in FIG. 3, the reversal boundary is not orthogonal to the traveling direction of the light wave but is sufficiently inclined and inclined. Are intersecting. As a result, the light beam cross section includes a plurality of inverting portions, and a difference appears in the modulation effect at each portion of the beam cross section, so that the light beam operates as a deflector instead of a simple phase modulator.

In the structure shown in FIG. 3, any part of the light beam cross section operates as a phase modulator having the same index. However, how the modulation is applied differs depending on the lateral position (x in the figure) of the beam cross section. FIGS. 5A and 5B show the induced refractive index change and the phase change in each section of the light beam incident at a certain timing when the interaction length is 2L ₀ . From this, it can be seen that as a result, the phase grating advances only in one lateral direction. The lattice spacing becomes the horizontal (x-direction) polarization reversal pitch, that is, d, and can be reduced regardless of the wavelength, so that it is difficult to realize a conventional acousto-optic deflector and electro-optic deflector, not to mention 1 to 10 GHz or more. A traveling-wave phase grating having an appropriate period that can be used even in a high-frequency band can be formed.

FIG. 5 shows that at least L ₀ is required for the operation to be possible.
It is understood that an interaction length of the length is sufficient. Further, it is also understood that if the inversion is continued for each L ₀ or L _m and the interaction length is set to an integral multiple of N times, the phase change becomes N times deep and the diffraction efficiency increases.

Now, the phase velocity ν in the z direction at the angular frequency ω _m
If the sinusoidal electric field E (z, t) when traveling at _p is given by E (z, t) = E _m sin [ω _m (t−z / v _p ) + φ] (5), FIG. (The total length is 2L ₀ ), t =
At t ₀ , the phase change received by the light incident at z = 0 is

(Equation 3) Is calculated. Here, Δk = ω _m (1 / ν _m −1 / ν ₀ ) = π / L ₀ (7), and (δn / δE) is a change in the refractive index of the electro-optical medium due to the unit applied electric field. This is a value specific to the optical medium.

Equation (6) indicates that while temporally sine-wave oscillating at an angular frequency ω, a spatial period of 2d and a velocity

(Equation 4) And the phase grating proceeds. That is, it can be seen that the structure of FIG. 3 or FIG. 5 has a function of diffracting a light beam as a sine wave phase grating traveling in the horizontal direction. In addition,
When reversal is repeated at L _m in the traveling direction, instead of equation (6),

(Equation 5) And the lateral spatial period of the traveling wave phase grating and the traveling speed in the lateral direction are both 1 / (2m + 1).

[0023]

As described above, when such a phase change occurs in the light beam cross section, the light beam is subjected to the Ramannas diffraction, similarly to the case of the Ramannas diffraction of the light beam by the traveling wave phase grating induced in the acousto-optic medium by the well-known acoustic wave. The light is diffracted, and the light is diffracted in a plurality of beams. nth order (n = 0, ± 1,
± 2, --- diffraction angle theta _n diffracted beam), the intensity I _n and the frequency shift amount with respect to the incident beam [nu _{Shift, n} is given by the following equation.

(Equation 6) Frequency shifter: Taking any one diffracted waves other than n = 0 results in a frequency shifter shift amount nf _m.

In order to make the device of the present invention a deflector, it is sufficient to change the high-frequency frequency for a small angle. On the other hand, when deflection is largely performed in one direction without a return line, the configuration shown in FIG. 6 may be used. In the conventional method, a traveling-wave phase grating is directly induced by a traveling-wave high-frequency electric field. However, the present invention has a novel feature in that a traveling-wave phase grating is equivalently generated using crystal axis inversion.

In FIG. 6, reference numeral 15 denotes an input optical signal. The input optical signal 15 is incident on the traveling wave phase grating 5 of the electro-optic crystal 17, where the crystal axis is inverted to generate an equivalent traveling wave phase grating. Then, the image is projected on the screen 27 via the next Fourier transform lens 24, the spatial modulator 25, and the Fourier transform lens 26.

In order to make the device of the present invention a modulator, the diffraction efficiency depends on the amount of phase change as can be seen from equation (10).
Therefore, by changing the driving high-frequency power and changing the applied electric field strength, diffracted light (n ≠ 0) and undiffracted light (n = 0)
Are modulated in intensity, and can be used as an optical modulator.

[Brief description of the drawings]

FIG. 1 is a diagram showing a state in which a traveling wave of an acoustic wave is excited and transmitted to an acousto-optic medium via a conventional electro-acoustic transducer, and a change in a refractive index that progresses in a sine wave shape having a period of the acoustic wave wavelength is obtained. It is explanatory drawing of the aspect which makes a light beam diffract by the traveling wave phase diffraction grating based on this.

FIG. 2 is a diagram showing an example of a conventional deflector using diffraction by a traveling wave phase grating generated by an electro-optic effect.

FIGS. 3A and 3B are diagrams for explaining the principle of an ultrahigh-speed electro-optical deflector using oblique periodic polarization inversion according to the present invention.

FIG. 4 is an explanatory diagram of the operation principle of a known pseudo velocity matching optical phase modulator.

FIGS. 5A and 5B are diagrams for explaining the operation principle of optical deflection according to the present invention.

FIG. 6 is a diagram for explaining the operation principle when the principle of the present invention is applied to an electro-optical deflector.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 electroacoustic transducer 2 incident light beam 3 acousto-optic medium 4 high-frequency power supply capable of sweeping frequency 5 traveling-wave phase grating generated by ultrasonic waves 6 sound absorption end 7 undiffracted beam 8 deflection beam (diffraction beam) 9 electricity Optical medium 10 Traveling wave electric field 11 Electric traveling wave induction phase grating 12 High frequency electric field transmission line 13 Load 14 Ramannas diffracted light wave 15 Incident light beam (incident light signal) 16 Driving high frequency power supply 17 Electro-optic crystal 18 Upper electrode 19 Outgoing diffracted light Beam 20 Termination load 21 Crystal axis reversal part of electro-optic crystal 22 Lower electrode 23 One end of high-frequency transmission line 24, 26 Fourier transform lens 25 Spatial modulator 27 Screen 28 Modulated light

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-60-212743 (JP, A) JP-A-9-146128 (JP, A) JP-A-6-265944 (JP, A) JP-A-3-3 284736 (JP, A) JP-A-2-250041 (JP, A) JP-A-6-95048 (JP, A) WO 95/4951 (WO, A) (58) Fields investigated (Int. Cl. ⁶⁾ G02F 1/00-1/313 G02F 2/00-2/02 JICST file (JOIS)

Claims (2)

(57) [Claims] A means for periodically inverting the crystal axis of the electro-optic crystal spatially in synchronization with a cycle in which the phase of a high-frequency applied electric field observed by a light wave propagating in the electro-optic crystal is inverted; While matching the phases of the light wave and the high-frequency electric field,
Means for obliquely intersecting the crystal axis inversion boundary with the light wave traveling direction,
The oblique periodic polarization is characterized in that the high frequency phase of the optical phase modulation received by the light wave beam changes linearly within the cross section of the light wave beam, and means for equivalently generating a high frequency traveling wave phase grating are provided. Ultra-high speed electro-optic deflector using inversion. 2. The ultra-high-speed electro-optical deflector utilizing oblique periodic polarization inversion according to claim 1, wherein said deflector has a frequency shifter function.