JPH02250041A - Demultiplexing device and beam deflecting device - Google Patents

Abstract

PURPOSE:To efficiently enable diffraction demultiplexing operation by crossing the traveling direction of a light beam to be demultiplexed by demultiplexing in an electrooptic material and the traveling direction of progressive wave phase gratings, and providing at least three cycles of phase gratings in section crossing the phase gratings. CONSTITUTION:A couple of electrodes 3 and 4 which extend in one direction are formed on the electrooptic material body 2 which varies in refractive index with an applied electric field and when a high-frequency voltage is applied between those electrodes, the progressive wave phase gratings are formed in the electrooptic material body 2. When an incident light beam is made to travel crossing an electric wave forming this phase gratings, the number of the phase gratings present in the section of the light beam increases substantially and the diffraction efficiency increases. Therefore, many subordinate beams which are frequency-modulated can be separated spatially. Consequently, the electrooptic material element is applied to the demultiplexer device for generating the frequency-modulated subordinate beams, so that the subordinate beams which are modulated at a higher frequency than an acoustooptical element can be generated.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a demultiplexing device that generates a plurality of frequency-modulated sub-beams from a single light beam using an electro-optic material element.

Furthermore, the present invention relates to a beam deflection device that generates a deflected beam by combining a plurality of frequency-modulated light beams.

(Prior art) As a light control element using diffraction by a conventional phase grating,
Acousto-optic deflectors and acousto-optic modulators that utilize traveling ultrasound waves are known. For example, when an acousto-optic deflector is used in the Ramannus region, an incident light beam is diffracted, producing multiple frequency-modulated light beams from a single light beam. When the frequency of the ultrasonic wave to be used is fm, the optical frequency of the n-th order diffracted light is frequency-shifted by nXfA with respect to the incident light beam, so the optical frequency of the incident light is ν. Then, the optical frequency shift of the n-th order diffracted light is given by the following equation.

νn −ν0 ±n fm ・・・・・・
(1) However, n is a positive integer. Therefore, by using this acousto-optic element, it is possible to generate a plurality of spatially separated light beams with a frequency shift of ±nfm relative to the incident light.

However, the transmission loss of acoustic waves increases as the frequency increases, so there is a limit to high frequency modulation, and with current technology, the frequency of acoustic waves used is several hundred.
MH2 is the limit. Therefore, the development of a beam generator capable of frequency modulation at higher frequencies is an important issue.

On the other hand, electro-optic modulators are known as devices that can control light at higher frequencies. This electro-optic modulator uses lithium tantalate (L) whose refractive index changes depending on the applied electric field.
iTaOl) and lithium niobe) (LINbOs)
Linearly polarized light is made perpendicularly incident on an electro-optical element made of the same material, and the amplitude is modulated according to the strength of the applied electric field.

(Problems to be Solved by the Invention) The present inventor considered the use of an electro-optic material element in an optical deflector in view of the high frequency characteristics of the electro-optic material element.

In other words, we will study an electro-optic demultiplexing device that generates traveling electric waves in an electro-optic material element by applying a high-frequency electric field to form a phase diffraction grating, and generates multiple frequency-modulated light beams by this phase grating. did. However, it has been found that this electro-optic demultiplexing device has the following problems.

(1) In order to diffract an incident beam by a phase grating, it is necessary to have phase gratings corresponding to multiple periods in the cross section of the incident beam. In this case, since the ultrasonic wave has a slow sound speed and a short wavelength (100MT (z, several tens of μll1),
A number of phase gratings are formed within the beam cross section. However, since the propagation speed of electromagnetic waves is fast, the period of the phase grating is long (on the order of several cm to several tens of cm for IGH2), and it is extremely difficult to form a large number of phase gratings within the cross section of the optical beam. It was practically impossible. Also,
A large driving power is required to obtain a large number of frequency-modulated lights with sufficient diffraction efficiency. For these reasons, a demultiplexing device using an electro-optic material element has not been put into practical use.

Therefore, the present invention solves the above-mentioned problems and provides an electro-optic demultiplexing device that can form a large number of phase gratings within the cross section of an incident light beam and can perform diffraction demultiplexing with high efficiency.

Furthermore, the present invention provides a beam deflection device that generates a high-resolution deflected beam by performing Fourier synthesis on a plurality of sub-beams generated by such a demultiplexing device.

(Means for Solving the Problems) A demultiplexing device according to the present invention includes an electro-optic material body whose refractive index changes depending on an applied electric field, and an electro-optic material body that extends in one direction across the electro-optic material body. an electro-optical element having a pair of electrodes; and a high-frequency power source connected to one end of the pair of electrodes and applying a high-frequency voltage to the pair of electrodes; forming a traveling wave phase grating by forming a refraction distribution along the direction in which the light beam is split, and making the traveling direction of the light beam to be demultiplexed in the electro-optic material obliquely intersect the traveling direction of the traveling wave phase grating; The present invention is characterized in that a cross section intersecting the phase grating of the light beam includes at least three periods of phase gratings.

Furthermore, the beam deflection device according to the present invention includes a demultiplexer that separates a light beam into a plurality of frequency-modulated and spatially separated sub-beams, a Fourier transform lens for Fourier decomposition of the separated sub-beams, and a Fourier transform lens for Fourier decomposition of the separated sub-beams, and a This system is characterized by comprising a phase compensator for making the phases of the sub-beams in-phase, and a Fourier transform lens for Fourier-synthesizing the sub-beams made in-phase.

(Function) A pair of electrodes extending to one side is formed on an electro-optic material body whose refractive index changes depending on an applied electric field, and when a high-frequency voltage is applied between these electrodes, a traveling wave phase grating is created inside the electro-optic material body. is formed. Promoting the incident light beam obliquely to the electrical waves forming the phase grating substantially increases the number of phase gratings present in the cross section of the light beam, increasing the diffraction efficiency. As a result, a large number of frequency-modulated sub-beams can be spatially separated. This allows the electro-optic material element to be applied to a demultiplexing device for generating frequency-modulated sub-beams, and therefore enables generation of sub-beams modulated at a higher frequency than an acousto-optic element. can.

Further, when a large number of frequency-modulated sub-beams are brought into phase by a phase compensator and the in-phase sub-beams are Fourier-combined, a combined light beam having sharp directivity is generated by interference. Since this combined light beam is generated periodically and moves along a fixed direction, it becomes a deflected beam in which the power is concentrated with good efficiency. As a result, a beam deflection device that does not require blanking can be realized.

(Example) Fig. 1 shows the structure of an example of a demultiplexing device according to the present invention, Fig. 1a is a diagrammatic sectional view showing the structure of an electro-optic material element, and Fig. and FIG. 1C is a plan view showing the intersecting state of the light beam and the traveling phase grating. The electro-optic material element 1 has an electro-optic material body 2 such as lithium tantalate or lithium niobate,
A coherent light beam is made incident on this electro-optic material body. Arrow a along which the electro-optic axis of the electro-optic material body 2 extends
First and second electrodes 3 and 4 are formed on both sides in a direction perpendicular to the direction, thereby configuring an electro-optic material element. These electrodes 3 and 4 can be constructed from strip lines. A high frequency power source 5 that generates a high frequency voltage of frequency fm is connected to one end of the electrodes 3 and 4 via a coaxial cable, and the other end of the electrode is short-circuited via a resistor R of the coaxial cable 6 to form a non-reflective termination. Since the refractive index of the electro-optic material body 2 changes depending on the applied electric field, a traveling high-frequency electric field propagates within the electro-optic material body 2 along the extending direction of the electrodes 3 and 4 due to the high-frequency voltage generated by the high-frequency power source 5. is formed, and this electric field forms a periodic refractive index distribution, thereby forming a traveling wave phase grating that travels in the direction of the arrow in the electro-optic material body 2. This traveling wave phase grating travels in the direction of the arrow within the electro-optic material at the same speed v1 as the high frequency electric field.

As shown in FIG. 1, a light beam (polarized in the direction of arrow C) emitted from a semiconductor laser 7 is converted into a flat parallel beam by a collimator 8 including a silitorical lens, and then enters an electro-optic material element 1. In the present invention, a light beam is made incident on the electro-optic material element 1 such that the incident light beam is oblique to the traveling direction of the phase grating. The incident light beam is diffracted by the phase grating traveling within the electro-optic material body 2, and 0th-order, 1st-order, etc. n-order diffracted lights are emitted as spatially separated sub-beams.

The optical frequency shift of this n-th sub-beam changes the optical frequency of the incident light to ν. Then, it is given by the following formula.

, lI = ν. ±nfs Here, fm is the frequency of the phase grating, that is, the frequency of the high frequency power source 5. These sub-beams are converted into parallel beams by a Fourier transform lens 8. As a result, 2n+1 frequency-modulated spatially separated sub-beams are formed by the electro-optic material element. As shown in FIG. 1C, in the present invention, the electrodes 3 and 4 are formed so that the incident light beam and the traveling direction of the phase grating cross obliquely. In this case, if the traveling direction of the incident light beam and the traveling direction of the phase grating are obliquely crossed, and the angle between the light beam and the phase grating is ψ, the effective intersection length between the light beam and the phase grating wave is approximately 1
/cosψ times, and as a result, the phase grating included in the beam cross section also increases by approximately 1/cosψ times. As a result, the phase grating included in the beam cross section also increases substantially by a factor of 1/cosψ. As a result, since the multi-periodic bone phase grating is included in the light beam cross section, the incident light beam can be diffracted with sufficiently good diffraction efficiency and a large number of frequency-modulated sub-beams can be generated. Note that in order to prevent reflections on the incident surface of the electro-optic material element, it is desirable to apply a non-reflective coating.

FIG. 2 is a plan view showing the configuration of a modified example of the electro-optic material element according to the present invention. In this example, a rectangular electro-optic material body 2 is used, and electrodes 3 and 4 (only electrode 3 is shown in the drawing) are formed obliquely to the longitudinal axis 2 of the body. and,
Longitudinal axis of electro-optic material body 2! A light beam is made perpendicularly incident on the end face 2a, which is orthogonal to the end face 2a. With this configuration, even if the light beam is incident perpendicularly to the electro-optic material element, the light beam and the phase grating wave can be obliquely crossed, and reflection loss at the incident surface can be eliminated. Note that the angle formed between the incident light beam and the phase grating wave can be effectively demultiplexed if there are phase gratings corresponding to at least three periods in the cross section of the light beam and the phase grating wave.

FIG. 3 is a plan view showing the configuration of another modification.

In this example, a rectangular electro-optic material body 2 is used, the light beam is made incident at a large angle of incidence with respect to the longitudinal axis 2, and the electrodes 3 are also formed obliquely to the longitudinal axis l. . With this configuration, the electro-optic material element 2
Since the cross-section of the light beam becomes flat inside, even if a light beam with a small cross-section is incident, the intersection length between the phase grating wave and the incident light beam can be made extremely large.

FIG. 4 is a schematic diagram showing an analytical model when an electro-optic material element constitutes a resonator. In the embodiment described above, the terminal ends of the electrodes 3 and 4 are short-circuited to provide a non-reflective terminal, but a resonator can also be constructed by opening the terminal ends of the electrodes to provide a reflective terminal.

When a resonator is constructed, the electric field becomes a standing wave within the electro-optic material element, and diffraction occurs due to the standing wave phase grating. In this case, since two standing waves whose traveling directions are opposite to each other are formed, the diffracted light is a combination of the diffracted lights by the traveling wave phase gratings whose traveling directions are opposite. Light diffracted in the same direction by phase gratings traveling in opposite directions has the opposite sign of its frequency shift, so
When a light beam is propagated perpendicular to the traveling direction of the phase grating, each of the sub-beams spatially separated by diffraction has a plurality of frequency components, and it is not possible to generate frequency-separated sub-beams.

On the other hand, when a light beam is incident obliquely to the direction of propagation of the standing wave, two traveling waves make up the incident light beam and the standing wave (the traveling wave from the power supply connection to the reflection terminal and the reflection terminal The intersecting angles with respect to the traveling waves (traveling waves traveling from the source to the power source connection side) are complementary angles to each other and are different from each other. On the other hand, the phase change induced in the incident light beam depends on the interaction length and crossing angle, so when the light beam is incident obliquely, there is a difference in the amount of phase change that the two traveling waves give to the light wave of the incident light beam. occurs. In FIG. 4, it is assumed that a light beam travels through the electro-optic material in the Z-axis direction, and that the traveling wave of the standing wave travels at a speed of 2 and at angles θ and π-θ with the Z-axis. Assuming that the traveling direction of the phase grating is the X'-axis direction, the change n(X') in the refractive index distribution of the electro-optic material in the interaction space with respect to the incident polarized wave is given by the following equation.

n(x')=n,+Δn5in(2K f m(t
-x'/V, )) Here, no is the refractive index of the electro-optic material with respect to the incident wave, X' is the distance in the traveling direction of the standing wave, The frequency Δn of the frequency electric field is the difference t between the maximum refractive index and the minimum refractive index due to the applied electric field, and the time Δ is the length of one period of the standing wave.

When an incident light beam propagates on a line where The formula holds true.

Δnj! (x, t) = (Δr+jri sin (z f van (1/ Vo
−cos θ/■, ]/[πf tml! (1/V
e cos θ/V, )Xsin (2πfm (
t-1/V, -cos/Vo)Xsinθ+φ)
--- (3) Here, ■. is the luminous flux φ in the electro-optic material, where cos θ=V, /V in the desired phase equation at X=O. corresponds to speed matching. Also, when r, 1 (1/ Vo cos θ/V11) = 1, the optical length change is length! While passing through, it is averaged for one period and becomes zero. The standing wave phase grating can be considered to be a combination of two traveling wave phase gratings whose intersection angles with the light beam are θ and π−θ, and the following equation holds true.

n(x')=no+Δ, sin (2zfmt)co
s(2πf@x'/Vm) ='A C(n0+Δn5in (2πfmI(t-x
'V+a)))+(n0+Δn5in (2πfj
t-x' Vm )))) Therefore, from equation (4), f ml (1/Vo+cos θ/V, ) =
1 ---(5) Make sure that formula (5) holds true! If and θ are selected, the light wave will be diffracted by only one of the two traveling waves that make up the standing wave, and the diffracted light will be spatially separated into frequency-modulated sub-beams. It turns out.

If a standing wave is formed in this manner, a phase grating is formed by the resonant standing wave, so that a seedling-raising effect can be achieved in which a phase grating with a large amplitude can be formed with a small driving power. In this case, if the electro-optic material is closed, T and O are used, the following values will be obtained as a design example.

■. = n. (C: speed of light in vacuum, n@: refractive index at 600 nm) (ε, s: relative dielectric constant of c-axis polarized microwave)
Here, l −1,0 cm, f 5 = 10 GHz
Then, θ=82.8” or 97.26.

Also, j! =4.0 cm, f5=10 G1
When 1z, θ=35.8@ or 144.2@.

FIG. 5 is a diagram showing the configuration of a modification of the standing wave excitation type demultiplexing device. Even if the optical beam to be demultiplexed and the traveling wave phase grating are obliquely crossed, there is a limit to the wavelength reduction. Therefore, in this example, a periodic phase grating is forcibly formed in the electro-optic material element using periodic electrodes, the electric field changes smoothly to form a phase grating, and a light beam is directed obliquely onto this phase grating. to generate frequency-modulated sub-beams. As shown in FIG. 5a, a rectangular electro-optic material element 10 having a longitudinal axis l is used, a grounded plate-shaped first electrode 11 is attached to the lower surface of the element 10, and the longitudinal axis is attached to the upper surface. 2
A first electrode group 12 and a second electrode group 13 extending in a direction perpendicular to the first electrode group 12 and the second electrode group 13 are periodically formed at a pitch Δ along the two longitudinal axis directions. The first electrode group 12 and the second electrode group 13 are arranged alternately, and the first electrode group 12 and the second electrode group 13 are connected to a high frequency power source 14. Note that the first electrode group 12 and the second electrode group 13
Voltages whose phases are shifted by π from each other are applied to . Then, the light beam is made incident at an angle θ with respect to the arrangement direction l of the electrode groups 12 and 13. When a high-frequency voltage is applied to the electrode group from the high-frequency power source 12, an electric field that periodically changes in the thickness direction of the electro-optic material element 10 is generated, and a smoothly changing refractive index distribution is generated inside the electro-optic material element 10. A phase grating is then formed. In this case, the intensity of the applied voltage within the cross section of the light beam is equivalent to a sine wave standing wave, and the refractive index change caused thereby can also be considered to be equivalent to a sine wave. Therefore, the following formula holds true for the refractive index change formula.

n(x”) = fi0+Δn5in(2πf+at)
cos(2gx'/Δ) On the other hand, since it can be considered that V, = fmI, equation (6) can be considered to be equivalent to equation (4). Therefore, if the incident angle θ and the interaction length 2 of the light beam are selected so as to satisfy equation (7), it is equivalent to the case where the light beam is diffracted by only one standing wave,
Spatially separated and frequency modulated sub-beams can be generated.

f (fm /V0 cos θ/△)=1・old・(
7) With this configuration, by reducing the electrode period (also the standing wave period), the traveling speed V,' of the phase grating can be equivalently slowed down, and the light beam with a narrow beam diameter can be reduced. It is possible to generate sub-beams with sufficient spatial separation. As shown in FIG. 6, electrode groups 12 and 13 are arranged at a pitch of Δ on one plane of the electro-optic material body.
It is also possible to form a phase grating with a refractive index distribution along the lateral direction by forming them alternately. The intersection angle θ in this case is determined as follows.

(1) △""2mL fm = 10GH2, fm2mm
Then, θ=14.2° or 165.8°.

(2) △ = 2mm, fm = 1.0 GHz,
When l=4 mm, θ=60.2° or 109.8°. FIG. 7 is a diagram showing the configuration of an example of the beam deflection device according to the present invention.

Interference fringes are created when two light beams of the same frequency interfere. In this case, when the optical frequencies of the two light beams differ by fm, the generated interference fringes move periodically with a movement period of 1/fm. Therefore, by interfering a plurality of beams with different optical frequencies, it is possible to generate a deflected beam that is deflected with a period fm. In this case, when a large number of light beams are emitted with the same phase from points lined up at equal intervals, beams with sharp directivity are formed with spatial periodicity, similar to an array tent antenna. In FIG. 6, the frequency ν. The optical beam 20 is incident on a demultiplexing device 21 according to the present invention. The incident light beam is passed through the demultiplexer 21
As mentioned above, the optical frequency ν・1−ν
. ±nfm sub-beams are generated spatially separated. These sub-beams are subjected to Fourier decomposition by the Fourier transform lens 22 and are made incident on the phase compensator 23 as parallel light beams. Since there are sub-beams that are optically shifted by half a wavelength in the Fourier-decomposed sub-beams, the phase compensator 2
3 makes all sub-beams in phase. Various types of phase compensators can be used as the phase compensator 23, and for example, a phase compensator that performs phase compensation using an electric field or a phase compensator that performs phase compensation by changing the thickness can be used.

As a result, the center frequency from the phase compensator 23 is ν. In±n
A number of in-phase sub-beams with a frequency shift of fm will be generated. These sub-beams are made incident on the Fourier transform lens 24 and subjected to Fourier synthesis. As a result, a large number of frequency-shifted and in-phase sub-beams interfere with each other to generate interference fringes (only three interference fringes are shown in the drawing to simplify the drawing).

Since these interference fringes move periodically from the top to the bottom of the drawing, if a through window 25 is provided with a spatial width of about one period, the interference pattern will be deflected in one direction toward the outside of the window 25. The light beam has a period f.

will occur periodically. Since this combined polarized beam has a finite directivity of the sub-beams and a finite beam deflection range, the interference fringes formed also become weaker as they move upward and downward from the center of the window. Therefore, the optical power of the polarized beam exiting through the window 25 is efficiently concentrated. As a result, a beam deflection device that does not require blanking can be realized. In this case, since the resolution of the generated deflected beam depends on the number of modulated sub-beams to be combined, the demultiplexer according to the present invention can be used as the modulated beam generator.

(Effects of the Invention) As explained above, according to the present invention, since the light beam to be demultiplexed and the traveling wave phase grating are configured to obliquely intersect, the number of phase gratings existing in the cross section of the light beam can be substantially reduced. can be increased, resulting in increased diffraction efficiency and generation of multiple frequency-shifted sub-beams.

Further, if the electro-optic material element constitutes a resonator and beam diffraction is performed only by the negative traveling wave phase grating, efficient diffraction can be achieved with low driving power.

Furthermore, in the present invention, since interference fringes are formed by compensating the phase of a large number of frequency-modulated sub-beams and combining a large number of in-phase sub-beams, a combined beam with sharp directivity can be generated. Since the cono composite beam is periodically deflected, it is possible to realize a beam deflection device that does not require blanking.

[Brief explanation of drawings]

Figures 1 a to c are diagrams showing the configuration of an example of the demultiplexing device according to the present invention, Figures 2 and 3 are diagrams showing the configuration of a modified example of the demultiplexing device, and Figure 4 is a diagram showing the configuration of a modified example of the demultiplexing device. Schematic diagram for analysis of excitation type demultiplexer Fig. 5a
and b are a perspective view and a sectional view showing the configuration of an example of a standing wave excitation type demultiplexing device, FIG. 6 is a perspective view showing the configuration of a modified example of the standing wave excitation type demultiplexing device, and FIG. 7 is a main 1 is a diagram showing the configuration of an example of a beam deflection device according to the invention; FIG. 1... Electro-optic material element 2... Electro-optic material body 3
．． 4... Electrode 5... High frequency power supply 21...
- Demultiplexing devices 22, 24...Fourier transform lens 23...Phase compensator 25...Wind No. 2
Figure patent applicant Osaka University President

Claims (1)

[Claims] 1. An electro-optic material element having an electro-optic material body whose refractive index changes depending on an applied electric field and a pair of electrodes extending in one direction across the electro-optic material body. and a high frequency power supply connected to one end of the pair of electrodes and applying a high frequency voltage to the pair of electrodes, the electro-optic material body has a refractive distribution along the extending direction of the electrodes. forming a traveling wave phase grating, the traveling direction of the light beam to be demultiplexed in the electro-optic material and the traveling direction of the traveling wave phase grating are obliquely intersected, and the traveling direction of the traveling wave phase grating intersects with the phase grating of the light beam. A demultiplexing device characterized in that it is configured such that a phase grating with at least three periods is included in a cross section. 2. The demultiplexing device according to claim 1, wherein the other ends of the pair of electrodes are short-circuited so that the end of the electro-optic material body is a non-reflection end. 3. A resonator is configured with the end of the electro-optic material body as a reflective end, and the power frequency of the high-frequency power source is f_m;
The interaction length of the traveling wave phase grating acting on the light beam is l, and the speed of light of the incident light beam in the electro-optic material body is V_
0, the intersection angle between the light beam and the traveling wave phase grating is θ, and the velocity of the electric wave forming the phase grating is V_m.The intersection angle θ and the interaction length l are set so that the following equation is satisfied.
The demultiplexing device according to claim 1, characterized in that f_ml(1/V_0■cosθ/V_m)=14, and one electrode of the pair of electrodes is a plate-shaped electrode extending in one direction. , the other electrode is composed of a first electrode group and a second electrode group arranged periodically and alternately at a predetermined pitch,
A phase grating is formed on the electro-optic material body by applying high frequency voltages of a frequency f_m whose phases are mutually shifted by π to the first electrode group and the second electrode group, and the traveling wave phase grating acts on the light beam. Let the interaction length be l, and the frequency of the applied voltage be f
_m, and the speed of light in the electro-optic material body of the light beam is V_
0, the intersection angle between the light beam and the phase grating is θ, and the inter-electrode pitch is ■, the intersection angle θ and the interaction length l are selected so that the following equation holds: 1. The demultiplexing device according to 1. l(f_m/V_0■cosθ/■)=1 5. The pair of electrodes is composed of a first electrode group and a second electrode group that are arranged periodically along one direction at a pitch of ■. 5. The demultiplexing device according to claim 4, wherein the high frequency power source is connected between the first electrode group and the second electrode group to form a phase grating in the electro-optic material body. 6. A demultiplexing device that separates a light beam into a plurality of frequency-modulated and spatially separated sub-beams, and a Fourier transform lens that performs Fourier decomposition of the separated sub-beams;
a phase compensator for making the phases of each sub-beam in phase;
A beam deflection device comprising: a Fourier transform lens for performing Fourier synthesis of in-phase sub-beams.