JPH0377498B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to an optical DA converter that converts digital optical data represented by a plurality of parallel input light beams into an analog quantity called optical power, and more specifically, an optical DA converter that converts digital optical data represented by a plurality of parallel input light beams into an analog quantity called optical power. Regarding DA converter. 2. Description of the Related Art In recent years, optical application technology has made remarkable progress, and various optical functional elements are required accordingly.
Among optical functional devices, optical DA conversion devices are used for optical communication,
It is possible to apply it to optical disks, optical sensors, etc., and there is a great need for it, but it has not yet been realized. An object of the present invention is to provide an optical DA conversion device that can be used for optical applications and that can effectively utilize an optical waveguide layer. The optical DA conversion device according to the present invention includes an optical waveguide layer formed of an acousto-optic material, and an optical waveguide layer for a group of light constituted by a set of a plurality of lights incident parallel to each other and weighted according to the propagation position. an optical switching means for switching the propagation optical path in one of two directions; an elastic device for generating a surface acoustic wave that sequentially diffracts each light of each light group propagating on each switched optical path, thereby deflecting the light from the optical path; It is equipped with a surface wave generator and a control circuit that controls the timing between switching of the optical switching means and generation of surface acoustic waves, and sequentially converts the digital quantities represented by each light group propagating on each switched optical path. , it is characterized by converting into an analog quantity represented by the power of the light propagated through the predetermined optical path and emitted. By condensing the light emitted from the optical waveguide layer, converting it into an electrical signal, and then integrating this, the optical signal converted into an analog quantity can be extracted as an electrical signal. As the light input/output means of the optical waveguide layer, a grating, a prism, or other optical coupling device can be used. The surface acoustic wave generator is preferably an interdigital transducer (IDT), but Gunn diodes or other elements may also be used. The optical switching means is constructed by arranging a large number of IDTs, and a DC voltage is applied to it.
In addition to the IDT array, there is a Y-shaped optical waveguide with two branches extending at twice the Bragg angle θ. Preferably, the Y-shaped optical waveguide is formed monolithically on the substrate on which the optical waveguide layer is formed, and its two branches are optically coupled to the optical waveguide layer. In the optical DA conversion device according to the present invention, most of it can be fabricated on one substrate, and the structure is very simple, so it can be easily mass-produced and provided at low cost. can. Furthermore, since it can be manufactured together with other optical functional elements on one chip, it is possible to integrate many elements on the same substrate. Especially in this invention,
Since it is equipped with a control circuit that controls the timing of switching the optical switching means and generation of surface acoustic waves, it is possible to switch the optical path of one group of light on the optical waveguide layer and perform optical DA conversion twice, or to convert the optical path of one group of light on the optical waveguide layer twice. Light group light DA
This makes it possible to effectively utilize the optical waveguide layer on the substrate and increase the degree of integration. Embodiments of the present invention will be described in detail below with reference to the drawings. Figures 1 and 2 illustrate the principle of this invention, that is, the deflection of light using the acousto-optic effect of surface acoustic waves, and when two parallel beams of light and the surface acoustic wave intersect, the surface acoustic wave This shows that the point in time at which the two beams of light are deflected by is different depending on the propagation position of the light by the time required for the surface acoustic wave to propagate through the mutual interval between the two beams. In FIG. 1, two beams of light P1 and P2 are incident in parallel on an optical waveguide layer made of an acousto-optic material. IDT1 is provided on this optical waveguide layer. By applying an ultrasonic voltage signal to the IDT 1, as shown by the parallel broken line,
Surface acoustic waves (hereinafter referred to as surface acoustic waves) are periodic refractive index fluctuations
SAW) is generated, and the SAW propagates on the surface of this waveguide layer at the propagation speed of sound. The SAW wavefront acts as a diffraction grating for light, so the SAW
Lights P1 and P2 incident at a small angle θ on the wavefront of
is completely reflected by the SAW when the following conditions are satisfied, and its traveling direction changes by 2θ. sinθ=λ/2・1/Λ...(1) where λ: wavelength of light Λ: period of SAW This is the deflection of light by Bragg diffraction using SAW. The time point at which the ultrasonic signal is printed on the IDT 1 is assumed to be t0. The light P2 is incident at a position closer to the IDT1 than the light P1. Let VS be the propagation speed of the SAW, and let l1 and l2 be the distances from the tip of the IDT 1 to the positions (diffraction points) where the lights P1 and P2 are deflected by the SAW, respectively. Light P2 begins to be deflected by the SAW at time t2, when l2/VS time has elapsed from time t0. The light P1 is slower than this, from time t0
At time t1, when the time l1/VS has elapsed, the beam begins to be deflected by the SAW. The points at which the lights P1 and P2 begin to be deflected are shifted by a time t1-t2 proportional to the mutual spacing l1-l2 of these lights in the SAW propagation direction. This situation is shown in FIG. 3 to 5 show embodiments of the invention. In FIG. 3, on the surface of a lithium niobate (LiNbO ₃ ) single crystal 11, which is an acousto-optic crystal,
By thermally diffusing titanium (Ti) at a temperature of approximately 1000°C, the refractive index is 3 to 5 times higher than that of the crystal substrate 11.
An optical waveguide layer 12 having a thickness of about 10 ^-3 and about several μm is formed. An optical coupling part 13 is formed at the input side end of this optical waveguide layer 12, and grating lenses 14 and 15 are formed at the output side end, so that a light group PA consisting of eight parallel beams P0 to P7 is formed. Light is introduced into the optical waveguide layer 12 from the coupling part 13 and subjected to optical DA conversion as described later while propagating within the optical waveguide layer 12, and is focused by lenses 14 and 15, respectively, and is transmitted through optical fibers 16, 17 and are respectively optically coupled and emitted. On the light incident side on the optical waveguide layer 12, each light P0
~IDT20-27 at the position intersecting with P7, respectively
The IDT array 30 is configured by arranging the IDTs in a line and connecting these IDTs to each other.
The electrodes of IDTs 20 to 27 are provided at intervals that satisfy the period Λ of equation (1), and are arranged at an angle θ with respect to the light beams P0 to P7.
It intersects with IDT row 3 by DC power supply 18
When a DC voltage is applied to the
This acts as a refraction grating for each light P0 to P7. The timing control circuit 29 is connected to the IDT column 30
Applying and stopping direct voltage to
It outputs signals Q1 and Q2 (see FIG. 4) that control the application and termination of the ultrasonic voltage signal to the IDT 40. When the signal Q1 is output at time t0 and a DC voltage is applied to the IDT array 30, the light group PA that was traveling straight is deflected by 2θ by Bragg diffraction as shown by the broken line, and the lens 14
The light is guided to the optical fiber 16 through the. On the light output side of the IDT array 30 on the optical waveguide layer 12, there is a SAW whose wavefront intersects the light group PA deflected by the IDT array 30 at an angle θ that satisfies Equation (1).
An IDT 40 is provided that generates a signal. This IDT
40, the ultrasonic voltage signal originating from the ultrasonic signal generator 19 is applied at time t0 by means of signal Q2. Let the distances between the SAW diffraction points of the lights P0 to P7 deflected by the IDT array 30 and the tip of the IDT 40 be l0, l1, . . . l6, l7, respectively. l0<l1<…<l6<l7. Referring to FIG. 4, at time t0, when a series voltage is applied to the IDT array 30 and an ultrasonic voltage signal is applied to the IDT 40, the lights P0 to P7 are first
deflected by column 30; At this point SAW
has not reached the optical path of each light, so each light P0~
P7 enters the optical fiber 16 through the lens 14. When time l0/VS has passed from time t0,
Since SAW reaches the diffraction point of light P0, light P0 becomes
It is diffracted by the SAW and no longer enters the lens 14 and optical fiber 16. The time during which the light P0 is incident on the optical fiber 16 is l0/VS. Similarly, P1...P6 are sequentially deflected by SAW, and finally, light P7 is deflected from time t0 to
Deflected by SAW when l7/VS time has elapsed. When the lights Pi (i=0-7) are deflected by the SAW, these lights no longer enter the lens 14 and the optical fiber 16. As is clear from the above distance relationship, l0/VS<l1/VS<
...<l6/VS<l7/VS. After the light Pi is deflected by the IDT array 30, the SAW
Since the time T until the light is deflected is T=li/VS as described above, the time T can be changed by changing the incident position of the light, that is, the distance li. If the power Pd of the light Pi is constant with respect to time, the time T is controlled by changing the incident position of the light, and the power Pd·li/VS of the light input to the optical fiber 16 via the lens 14 is controlled. be able to. In this way, as the distance li becomes smaller, the power of the light incident on the optical fiber 16 monotonically decreases. Therefore, the light P0 with the smallest distance li is assigned a weight of ²⁰ as the least significant bit LSB, and the light P7 with the largest distance li is assigned a weight of ²⁷ as the most significant bit MSB. Similarly, assign a weight of 2 ⁱ to other optical Pis. This weighting can be realized by setting the distance l0 for the LSB light P0 as a unit distance and setting the distance li for each light Pi to be a distance expressed by the following equation. li=2 ⁱ・l0 ...(2) For example, when distance l0 is 20 μm, each distance is as shown in the table below.

[Table] From the above, optical P0 is LSB, optical P7 is
Optical P0 expressed in 8-bit binary as MSB
It will be appreciated that the digital quantity by P7 is converted into an analog quantity of optical power incident on the optical fiber 16 via the lens 14. Fifth
The figure shows the optical signal waveform that enters the optical fiber 16 when the lights P0 to P7 representing the binary number 10110101 are input. Here, 1 means the presence of light and 0 means no light. The SAW generated from the IDT 40 propagates further on the optical waveguide layer 12 as time passes. Therefore, at an appropriate time t3 after the light P7 is deflected, the output of the signal Q1 is stopped, and the IDT
The application of DC voltage to the column 30 is stopped. SAW may continue to occur or may be stopped. Then, the light group PA travels straight again as shown by the broken line, passes through the lens 15, and enters the optical fiber 17. This straight-traveling light group PA also has a connection with the SAW.
(1) is satisfied. SAW of light P0 going straight there
Let (A) be a point at a distance l0 from the diffraction point of the IDT 40, and let ld be the distance between this point (A) and the tip of the IDT 40. Then, the above-mentioned time point t3 is changed to t
3-t0=ld/VS>l7/VS is satisfied. As a result, the lights P0 to P7 indicated by broken lines are
As in the above case, SAW is performed sequentially after time t3.
is deflected from the incident optical path to the lens 15. Therefore, the light that enters the optical fiber 17 via the lens 15 after time t3 is:
The waveform of the light that passes through the lens 14 and enters the optical fiber 16 is exactly the same, and the light group PA is subjected to optical DA conversion twice. If the light group PA is incident on the optical waveguide layer 12 before the time t0, the light before the time t0 will also be incident on the optical fiber 17 via the lens 15. By providing an optical switch, by making the incident timing of the light group PA coincide with t0, or by providing an optical switch in the optical fiber 17, it is easily possible to extract only the light that enters the optical fiber 17 after time t3. It is. This embodiment converts a group of optical digital signals into an optical DA.
This is particularly suitable for converting and extracting and using two groups of optical analog signals. For example, optical analog signals taken out from both optical fibers 16 and 17 can be compared to check optical DA conversion. In this embodiment, the optical signals P0 to P7 are weighted using binary numbers, but it goes without saying that arbitrary weighting can be performed depending on the incident position of the light.
Further, although the optical waveguide layer is formed by thermally diffusing Ti into LiNbO ₃ , it can also be realized using other acousto-optic materials and other methods. Furthermore, in the above embodiment, the first optical DA
In the conversion, the distance l0 is set to the distance corresponding to the weighted LSB, and the application of DC voltage to the IDT string 30 and the application of the ultrasonic signal to the IDT 40 are performed simultaneously. By slightly delaying the drive of , the distance l0 can be set longer, and vice versa. 6 and 7 show other embodiments. In FIG. 6, in addition to the above-mentioned light group PA, the optical waveguide layer 12 is provided with light beams P10 to 10, which are weighted according to the position and parallel to each other, in addition to the above-mentioned light group PA. The light group PB composed of P17 has an incident angle different from the light group PA by 2θ,
The light is also incident on the IDT array 30 so as to intersect with the light group PA. The DA-converted light is emitted via the optical coupling unit 28 and is connected to lenses 41 and 4, respectively.
4 and is received by photodetectors 42 and 45. The output electric signal of each photodetector 42, 45 is transmitted to an integrator 43,
46. Both detectors 42, 45 are controlled by signal Q2, detector 42 is controlled by signal Q
The detector 45 detects the incident light when the signal Q2 is at the H level, and the detector 45 detects the incident light when the signal Q2 is at the L level. The control signals Q1 and Q2 output from the timing control circuit 29 are different from those in the embodiments shown in FIGS. 3 to 5. Referring to FIG. 7, it is assumed that both optical groups PA and PB are input into the optical waveguide layer 12. At time t0, a DC voltage is applied to the IDT array 30, and an ultrasonic signal is applied to the IDT 40. The incident light groups PA and PB are each deflected by 2θ by the IDT array 30. Therefore, first, each light of the light group PA is sequentially deflected by the SAW generated from the IDT 40 to undergo optical DA conversion, and the light until it is deflected is detected by the detector 42. Then, at time t3 when the time ld/VS has elapsed from time t0, signal Q2 is inverted from H level to L level. At this point, SAW from IDT40
generation stops, but since the SAW generated between time points t0 and t3 continues to propagate, the light group PB is subsequently optically DA converted by this SAW,
The light is detected by the detector 45. At time t4 after the MSB light P17 in the light group PB is deflected, the signal Q1 is inverted to L level and the signal Q2 is inverted to H level. Therefore, since no DC voltage is applied to the IDT array 30 after time t4, both light groups PA and PB travel straight without being deflected by the IDT array 30. Therefore, first, the light group PB is the SAW generated from IDT40.
The light is converted into DA by being deflected by the
The light is received by the detector 42. ld/VS from time t4
At time t5 when time has elapsed, signal Q2 becomes L.
is reversed to the level, but has already occurred and is propagating
The light group PA is then optically DA converted by the SAW and detected by the detector 45. In this embodiment, as soon as the optical DA conversion by the SAW of one light group is completed, the signal Q2 is inverted to L level, and the generation of the SAW is stopped. And this already occurs and progresses
The other light group is subjected to optical DA conversion by SAW.
Therefore, when the optical DA conversion of the other optical group is completed, there is no longer any SAW on the optical path of both optical groups, and it is possible to immediately move on to the next optical DA conversion, and the optical The repetition period of DA conversion can be shortened.

[Brief explanation of drawings]

Figures 1 and 2 are for illustrating the principle of the invention, with Figure 1 being an explanatory diagram showing how light is deflected, and Figure 2 being a time chart showing the timing at which light is deflected. , Fig. 3 shows an embodiment of the present invention, and shows a configuration diagram of an optical DA converter, Fig. 4 is a time chart showing an optical signal output from the optical DA converter, and Fig. 5 is a diagram of an optical DA converter. FIG. 6 is a waveform diagram showing the output signal of the detector, FIG. 6 is a block diagram showing another embodiment, and FIG. 7 is a time chart showing the operation of the device shown in FIG. 1,20~27,40...IDT, 11...
LiNbO ₃ substrate, 12... Optical waveguide layer, 13, 28...
...Optical coupling unit, 14, 15, 41, 44... Lens, 16, 17... Optical fiber, 29... Timing control circuit, 30... IDT column, P0 to P7, P
10~P17...Light, PA, PB...Light group.

Claims (1)

[Claims] 1. An optical waveguide layer formed of an acousto-optic material; propagation in the optical waveguide layer of a group of light constituted by a set of a plurality of lights incident parallel to each other and weighted by propagation positions; Optical switching means that switches the optical path to either of two predetermined directions, and an elastic device that generates surface acoustic waves that deflect each light of each light group propagating on each switched optical path from the optical path by sequentially diffracting the light. It is equipped with a surface wave generator and a control circuit that controls the timing of switching the optical switching means and generation of surface acoustic waves, and sequentially converts the digital quantity represented by each light group propagating on each switched optical path. , an optical DA conversion device that converts the light propagated through the predetermined optical path and outputted into an analog quantity represented by the power. 2. The optical DA conversion device according to claim 1, wherein two light groups having different incident directions are sequentially subjected to optical DA conversion.