JPH0277035A - Driving method for optical bistable element - Google Patents

Abstract

PURPOSE:To enable both setting and resetting operation with light pulse of a same shape by generating undershoot in outputted light and switching the outputted light from a high level to a low level. CONSTITUTION:An intensity P0 of outputted light is in the first stage at a low level L as compared to an intensity Pi of inputted light at a level PB of bias light, but the outputted light is switched by a setting light pulse to a higher level, and stabilized at a point H in a stationary state. When resetting light pulse having a same shape as the setting light pulse is impressed in this set state, the intensity P0 of the outputted light moves rightward transiently along a performance curve and the intensity is increased, but the intensity moves leftward as soon as the light pulse is eliminated, passing over the point H by the undershoot effect, passing further a switching point to a lower level. After it has fallen to a lower level, it is stabilized finally at a point L. Thus, both setting and resetting operations are executed with pulses having a same shape.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to an optical bistable device that bistablely switches output light into two levels, high and low, using a light pulse, similar to a flip-flop operation in an electronic circuit. This relates to a driving method.

[Summary of disclosure]

This invention is an optical bistable element that can have two output light levels, high and low, for a certain input light intensity, and the present invention is capable of switching the output light from a low level to a high level (set operation) or from a high level to a low level using a light pulse. This relates to a driving method for switching (reset operation) operation, and uses an optical bistable element in which two time constants act in series to utilize undershoot switching for switching from a high level to a low level. It has a function similar to the flip-flop operation in electronic circuits, and can be applied as an optical circuit in optical memory, optical computing, optical switching, etc.

[Conventional technology]

Optical bistable devices are attracting attention for their potential as high-speed, low-energy switching devices, optical devices that digitally process images as optical signals, or optical switching devices in optical communications.
It has been actively researched for the past year. Its basic characteristics are the second
As shown in the figure, in the input light vs. output light power characteristics,
Even if the initial manual optical power PI is increased, the output optical power 2
0 is at a low level with almost no increase, but it is switched to a high level at a certain critical input optical power PC, and on the other hand, when the output light is at a high level, decreasing the input light causes a critical value smaller than the critical value Pc. It is characterized by exhibiting hysteresis characteristics in that it is switched to a low level at PC'. For input optical power between these critical values pc and PC', the output light has two levels, high and low, so this optical device is called an optical bistable element, and information is transmitted at two levels. A wide variety of applications are expected, such as memory for storing, switching signals between two levels, threshold operation between human-powered light and output light, and optical logic operation between two human-powered optical signals. The most basic of these operations is 2.
switching between two levels.

The basic method for switching between two levels will now be explained with reference to FIG. input light to a critical value P
It is set as an optical signal with a bias light pa between C and Pc', and switching from low level to high level is performed by adding a light pulse (bias photon pulse light), and its peak value exceeds the upper critical value PC. On the other hand, switching from a high level to a low level is performed by reducing the manual optical power in a pulsed manner to below the lower critical value Pc' (dark pulse).

In other words, since there is no negative signal in light, in order to perform the operation shown in Figure 3, the power of the light input to the optical bistable element must be modulated in positive and negative directions in advance around the bias light level. There is a need.

In this case, during high-speed operation, an ultra-high-speed pulsed light modulator is required and it is inconvenient that separate light sources cannot be used for bias light and pulsed light. For this reason, methods have been searched for that can also switch from a high level to a low level by always adding a positive light pulse to the bias light. Below we discuss some methods that have been proposed to date for doing this.

FIG. 4(^) shows an optical flip-flop circuit constructed by connecting two sets of optical bistable elements in a cross-like configuration. This optical bistable element is made up of a combination of a light emitting diode LEDI and a photodetector PD, and further includes a second light emitting diode LED2 for performing an inverter operation. Rb
and R1 is a resistance. By combining two sets of such elements to create a configuration similar to a flip-flop circuit in an electronic circuit as shown in FIG. Light P0
As shown in the truth table in Figure 4(C),
set to one of two states. In the truth table shown in FIG. 4(C), S, R and Q represent PS, PR and PQ, respectively, and L and H represent low level and high level, respectively. Qn++ is S
In the outputs corresponding to and R, Q means the previous state, and shows that there is no change when both S and R are L. Moreover, X has an indefinite meaning. From this truth table and Fig. 4 (D), when a light pulse (high level) is input as the set human power P3, Po is set (high level), and when a light pulse (high level) is input as the reset input PR, It can be seen that Po is reset (low level); Pi means the inversion of PQ.

In Fig. 5 (^), in order to set and reset the output light using optical pulses, each output light from the electro-optic crystals El and E2 is detected by the photodetectors PDI and PO2, and each detected light is transferred to the amplifier AMPI. , two sets of electro-optic bistable elements are connected in series, and the output light P of the subsequent bistable element is applied to the electro-optic crystals El and E2 through AMP2.
. This is a conventional configuration in which a voltage proportional to the intensity of the MPI is fed back to the amplifier of the previous stage element via the delay element TD.

Here, P is a polarizer, C is an optical phase plate, A is an analyzer, and G
P is a polarizing beam splitter. The operating principle of this element is somewhat complicated, and details can be found at S, LImegak.
i, S, Tanaka and K, Inoue; IG
O (International Optical Conference)-13ConferenceDig
est, B5-9 (1984),
As shown in FIG. 5(B), in response to the occurrence of the input light P1, the output light P is generated. has been set and reset, and output light P0 with a pulse width of about 50 m5 has been obtained.

There is also an example in which a set/reset operation is performed using a configuration in which two time constants act in series using a pair of electro-optic bistable elements, and an example of this operation is shown in FIG.

In this case, the output light can be alternately set or reset by pulse-wise reducing the intensity of the input light having a certain bar-ass light level to zero level (Al
(called ternate Switching)
, the input optical pulses have a reverse relationship compared to the operating characteristics shown in FIG.

On the other hand, as an optical bistable element, there is an element in which a so-called nonlinear optical medium whose absorption coefficient or refractive index changes depending on the light intensity is inserted into an optical resonator consisting of opposing reflecting mirrors, that is, a Fabry-Bello resonator. . In devices using nonlinear absorption media, when the input light intensity is weak, the absorption is large and the output light intensity is small, but as the artificial light intensity increases, the absorption coefficient decreases, and at the same time it becomes transparent, the resonance effect of the optical resonator increases. When the input light intensity is mixed, the output light jumps to a high level all at once. Then, as the input light intensity is reduced, it suddenly drops to a low level at a certain manual light intensity,
It exhibits characteristics as shown in FIG.

In addition, in an element using a nonlinear refractive medium, if the resonator is initially kept in a non-resonant state with respect to the wavelength of the human-powered light, when the human-powered light intensity is weak, the output light intensity will be small corresponding to the non-resonant state. However, as the input light intensity increases, the refractive index of the medium changes, approaches a resonance state, and finally, at a certain input light intensity, the output light jumps to a high level. Then, when the manual light intensity is reduced, the input light intensity jumps to a low level at a certain input light intensity, and exhibits the characteristics shown in FIG.

By the way, as shown in Figure 3, bias light is input to the optical bistable device consisting of these Fabry-Bello resonators and a nonlinear optical medium, and a positive light pulse is added to this to perform set/reset operations. Attempts have also been made to make this possible. Its operating principle is that a resonant state or a non-resonant state is induced through a change in the absorption coefficient or refractive index of a nonlinear optical medium by inputting a light pulse, and a corresponding setting or reset is performed. In this case, the wavelengths of the optical pulses used for setting and resetting are generally different. Up to now, set/reset operations using optical pulses of different wavelengths have been confirmed in bistable semiconductor lasers configured by arranging a nonlinear refractive medium in a semiconductor laser resonator.

Finally, 5EE is an optical bistable element that has been attracting attention recently.
D (Self-Electrooptic Devi
ce), and its newest configuration is shown in FIGS. 7(A) and 7(B). The basics of this device are multiple quantum wells (MQW-Multiple Quantum Wells).
I-layer (1ntrinsic) in the form of mWell
) is a p-1-n photodiode structure. A quantum well is, for example, AlyGa+ -xAs/GaAs/
AlxGa+-xAs stacked structure, GaAs
GaAs and ^1 with a layer thickness of about 10nI11 or less
) This is a structure in which electrons, holes, and excitons are two-dimensionally confined in the GaAs layer due to the potential difference between BGa+ and xAs.

MQW is a multi-layered quantum well structure.

In the quantum well, the light absorption by excitons changes greatly depending on the external electric field, and a so-called light modulation effect becomes possible. The elements DI and D2 having a p-1-n photodiode structure are connected in series in a reverse bias voltage state. Here, one is used as an optical modulation element and the other as its load.If the operating state is such that optical absorption increases as the reverse bias voltage decreases, the input light increases - optical absorption increases - photocurrent As shown in the example of FIGS. 7(C) and 7(CD), bistable operation is obtained by the positive feedback a behavior of increase in voltage drop due to load, decrease in device voltage, and increase in optical absorption. In this example, the light intensity of the input light Pln2 input to the other element D2 is kept constant, and here, for the light intensity Plnl input to the one element DI, the two elements D1 and D2 The output light intensities of are in a complementary relationship with each other, that is, when P.utl is in the set state (high level), Pout2 is in the reset state (low level). 4(D), 5(B) and 6.
An example of the operation shown in the figure has not been reported.

[Problem to be solved by the invention]

Next, in the various conventional bistable elements mentioned above,
The reset operation will be analyzed, and its problems and the purpose of the present invention will be described.

First, regarding the operation method shown in Figure 3, as mentioned earlier, there is no negative optical pulse, so at least for the reset operation, a so-called dark pulse in the negative direction is created from the bias light level as input light. There is a need,
There are problems such as the need for an optical modulator for this purpose and the inability to use separate light sources to obtain bias light and dark pulse light. In particular, when the set and reset operations are performed one after another, it is desirable that the bias light and the set and reset lights be separate light sources even if they have the same wavelength. This is clear from the fact that the DC power supply voltage and signal voltage in electronic circuits are supplied independently.These problems also apply to the operation shown in Figure 6; The time width of the dark pulse is extremely limited; in this example, the pulse width is 12.5 ms, but this width is 1611 I
If it becomes more than S or less than Ions, such operation cannot be obtained. Next, in the configuration of FIG. 4 or 5, it is necessary to combine two sets of optical bistable elements, and in reality two sets of optical bistable elements are required, so the combination of these two sets of optical bistable elements is required. Unless the coupling is made more compact through integration, it is difficult to make it into a practical device. Furthermore, although set/reset operations in optical bistable devices and bistable semiconductor lasers consisting of an optical resonator and a nonlinear optical medium are quite practical, the wavelength of the pulsed light differs between setting and resetting, and There are problems such as the need to precisely control the wavelength.

Although there is a great possibility that the set/reset operation using the 5EED explained with reference to FIG. 7 will be possible, the implementation thereof has not yet been studied and its success or failure is not clear.

As can be seen from the above, by using an optical bistable device and applying a light pulse, the output light from the device can be adjusted to two levels:
There have been several proposals for methods of setting and resetting to the same level, but all of these methods require a dark pulse with a negative amplitude relative to the bias light level at the time of resetting. Optical modulation must be performed before optical input, it is difficult to perform continuous set/reset operations using many elements, and bias light and pulsed light must be transmitted from the same light source through the same path. In addition to the fact that two sets of optical bistable elements are required, there are also problems in that the method of connecting the pixel elements is impractical. Furthermore, in optical bistable devices that include a nonlinear optical medium in the optical resonator, the bias light, set light, and reset light each have different wavelengths, and these wavelengths must be precisely controlled, among other problems. was there.

Therefore, the purpose of the present invention is to use one optical bistable element,
Provided is a method for driving an optical bistable element that can perform both setting and resetting operations with an optical pulse of the same shape by using undershoot switching especially during reset with a configuration in which two time constants act in series. There is a particular thing.

(Means for Solving the Problems) In order to achieve such an object, the present invention provides an optical bistable element having two time constants and in which the two time constants act in series. By inputting a constant bias light, and in addition to the bias light, inputting optical pulses with almost the same pulse shape, the level of the output light from the optical bistable element can be sequentially changed between high and low levels. It is characterized in that when switching from a high level to a low level, an undershoot is caused in the output light to perform switching from a high level to a low level.

(Function) In the present invention, the device used may be one set of optical bistable devices, and a dark pulse is not required. Also, bias light,
The wavelength of the set and reset pulsed light may be any wavelength that can be used for the operation of each bistable element, and there is flexibility in that the same wavelength or different wavelengths can be selected.

Therefore, according to the present invention, separate light sources can be used for the bias light and the pulsed light for setting and resetting even if they have the same wavelength, and the power supply voltage and signal voltage in the electronic circuit can be separated or independent. This is extremely advantageous in terms of system configuration. Moreover, since the present invention is based on the universal principle of setting two time constants in an optical bistable element and utilizing undershoot switching, it is believed that it can be applied to various forms of optical bistable elements. The effect is great and it can be effectively used in various applications such as optical memory, optical computing, and optical switching.

(Example) Hereinafter, the present invention will be described in detail with reference to the drawings.

First, embodiments of the present invention will be described by taking an electro-optic bistable element as an example, and then the applicability of the present invention to other optical bistable elements will be described.

In the electro-optic bistable element, as shown in FIG. 8, incident light P1 is guided to an electro-optic light modulator 1, output light from this light modulator 1 is guided to a beam splitter 2, and the transmitted light is converted into output light. At the same time, the reflected light is guided to the photodetector 3. The detection output from the photodetector 3 is passed through the voltage amplifier 4 to the electro-optic optical modulator! to be applied. That is, a voltage V proportional to the output optical power Po of the electro-optic optical modulator m1 is fed back from the amplifier 4 to the optical modulator 1. Here, the time constant of the photodetector 3 to obtain the voltage V proportional to the output optical power PO is τ3
, the time constant giving the temporal response of the optical modulator 1 is τ2, then the temporal response of the electro-optic bistable element with such a configuration is z I (dV/dt) + V = a ・Po
(1) T 2 (dpo/dt)◆Po=T
(V) ・P t (2) is given. However, α is the proportion of the output light intensity that contributes to feedback, the feedback gain coefficient depends on the photoelectric conversion efficiency of the photodetector 3 and the amplification factor of the feedback amplifier 4, T (V) is the transmittance of the optical modulator l, and p , is the input optical power. When we integrate both equations, we get ? , τ, (d2V/dt2)+ (T , *T ,
) (dV/dt) tV-T(V)・α・h
(3) A second-order differential equation is obtained, and it can therefore be seen that overshoot and undershoot phenomena occur in the temporal response of the output light.

In the present invention, this undershoot is utilized particularly at the time of resetting, and the situation will be explained diagrammatically with reference to FIG. In FIG. 1, initially, with respect to the input light intensity Pl corresponding to the bias light level P6, the output light intensity P
0 corresponds to the L point and is at a low level, but it is switched to a high level by the setting light pulse and settles at the H point in a steady state.

This is a set operation. In this state, when a reset optical pulse that is the same as the set pulse is applied, the output optical intensity becomes P. moves transiently to the right of the H point along the operating curve, and the output light intensity PO increases, but as the optical pulse disappears, it rapidly moves to the left, passes the H point due to the undershoot effect, and reaches an even lower level. After passing through the switching point and dropping to a low level, it finally settles at point L.

That is, a reset operation is performed.

In order for both the set and reset operations to be performed perfectly in these operations, the time constant ratio I/2, the bias light level P6, the height and time width of the set and reset optical pulses, and the optical Setting the static operating point of bistable devices is important.

Setting of these parameters will be described below using an electro-optic bistable element as an example.

The transmission characteristic T(V) of the optical modulator 1 is given by the following equation.

T (V) −[1−kcos (θ+(m/Vyt)
V)]/2 (4) where k is a coefficient that gives the contrast ratio of the optical modulator 1 and is a constant determined by the optical modulator 1, and θ is a parameter that determines the static operating point of the element, and is a constant that determines the contrast ratio of the optical modulator 1. The bias voltage applied to the device 1 or the 8th
Optical modulator 1 and beam splitter 2 are shown by dotted lines in the figure.
It is set by an optical phase plate 5 placed between the two. Further, (2) is called a half-wavelength voltage, and is a voltage required to change the output light of the optical modulator 1 from the maximum or minimum value to the minimum or maximum value.

In order to perform the operation shown in Figure 1, that is, to perform both set and reset operations using a light pulse of the same shape, the following conditions must be met: when this element is in a low-level reset state, it is set to a high level by a light pulse. , and the conditions under which this element is reset to a low level by a light pulse when it is in a high level set state, and it is necessary to find a region that satisfies both conditions.

Figure 9 shows the peak value PP (lateral These are the results of numerical calculations using equations (1), (2), and (4) to determine the relationship between the bias light level Pa (vertical axis) and the bias light level Pa (vertical axis).

The area delimited by solid lines is a settable area, the area surrounded by broken lines is a resettable area, and the area that satisfies both (hatched area) is an area where both set and reset operations are possible. However, in FIG. 9, both P and Pa are standardized deviation amounts υp-(Pp-P
c)/Pc and V a'' (Pa-Pc) is expressed as Pc.

A calculation example of the set/reset operation is shown in FIG. 10, and an experimental example is shown in FIG. 11.

In Figure 1O, 2=0.5 at θ-270°, τI/
, Tp/τ+-5, Pp/Pc-0,955, Pa, /
Pc-0,349. In FIG. 11, 2 = 0-2 at τI/, and at Tp/.

=6.

In FIG. 9, the parameters θ, τl/2 and the optical pulse width Tp are set constant, but if these parameters are changed, the set and reset possible area also changes, and accordingly, in order to obtain the set/reset operation. conditions can be set more flexibly.

The present invention is not limited to the electro-optic bistable device of the above-described embodiment, but can also be applied to other optical bistable devices. The set/reset operations in that case will be described. Light emitting element LED shown in FIG. 4(A)
In the optical bistable device consisting of I, LED2 and photodetector PD, two time constants can be set by the time constant in the drive circuit of the light emitting device and the time constant of the photodetector, and by adjusting the value, τI/ A value of 2 can also be freely given. Furthermore, since the static operating point can also be controlled by adjusting the resistance nb in FIG. 4(A), the light emitting elements LEDI, LED2 and the photodetector PD can be The set/reset operation in the present invention is sufficiently possible with one set of optical bistable elements consisting of the following.

Furthermore, in an optical bistable device configured by arranging a nonlinear absorption medium or a nonlinear refraction medium within a Fabry-Bello optical resonator, two times are determined by the lifetime of photons within the optical resonator and the temporal response of the nonlinear optical medium. Since a constant can be obtained and the static operating point can also be controlled by adjusting the cavity length or the wavelength of the light, the differential equation for the photon and the differential equation for the medium (semiconductor medium Now, by analyzing the differential equations that give the time response of electrons, holes, or excitons, we can find a set-up method using the undershoot switching described in the present invention.
Reset operation is possible.

Finally, when using the 5EED shown in Figure 7, MQW
Two p-1-n photodiodes with an i-layer structure are used, one of which is used as an optical modulator and the other as a load, but if they are made to operate with different time constants, Two time constants can be set. Furthermore, since the static operating point can be adjusted by changing the reverse bias voltage, set/reset operations based on the present invention are possible.

〔Effect of the invention〕

As is clear from the above, in an optical bistable element configured such that two time constants act in series, in addition to a constant level of bias light that is input in advance, Both setting and resetting are performed manually using optical pulses, and at the time of resetting, switching is performed using the undershoot that occurs in the output light, so a single set of optical bistable elements is sufficient as the element used, and dark pulses are not required. Furthermore, the wavelength of the bias light and pulsed light for setting and resetting can be any wavelength that can be applied to the operation of each bistable element, and it is flexible, such as returning the same wavelength or different wavelengths. It has advantages such as being flexible. Therefore, according to the present invention, the bias light and the pulsed light for setting and resetting include:
Separate light sources can be used even for the same wavelength, which is extremely advantageous in terms of system configuration, in the same way that the power supply voltage and signal voltage in electronic circuits are supplied separately or independently. Moreover, since the present invention is based on the universal principle of setting two time constants in an optical bistable element and utilizing undershoot switching, it is believed that it can be applied to various forms of optical bistable elements. ,
Its effects are significant, including optical memory, optical computing,
It can be effectively used in various applications such as optical exchange.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of the principle of set/reset operation in the present invention. Figure 2 is an explanatory diagram of the basic operation of a general optical bistable element. Figure 3 is an illustration of the basic operation of an optical bistable element.・An explanatory diagram of the conventional method for resetting. Figures 4 (A> and (B)) are circuit diagrams each showing a configuration in which two sets of optical bistable elements consisting of a light emitting diode and a photodetector are coupled in a cross shape. and its logic circuit diagram, FIGS. 4(C) and (D), respectively, are shown in FIG. 4(A).
The truth table diagram and signal waveform diagram of an example of performing a set/reset operation using the configuration shown in Figure 5 (^) and (B) respectively show a set/reset operation using two sets of electro-optic bistable elements A configuration diagram of an example of an element that performs this and a signal waveform diagram showing an example of its operation. Figure 6 is an explanatory signal of a conventional example in which two time constants are set in an electro-optic bistable element and set/reset operations are performed using dark pulses. Waveform diagram, Figure 7 kt(8) and (B) are 5EED respectively.
(Self-Electrooptic Device
7 (C) and (D) are explanatory diagrams of its optical bistable operation. FIG. FIG. 9, a block diagram showing an example of the configuration, shows the peak value (νp-(P, -Pc)/p
normalized by c) and bias light level (ν5・(pa-
Fig. 10 is a signal waveform diagram showing an example of the set/reset operation obtained by calculation based on the present invention; Fig. 11 is an explanatory diagram of the range in which set/reset operations can be obtained for FIG. 2 is a signal waveform diagram showing experimental results regarding one embodiment of the present invention. ! ... Electro-optic light modulator, 2... Beam splitter, 3... Photodetector, 4... Voltage amplifier, 5... Optical phase plate.

Claims (1)

[Claims] 1) A constant bias light is input to an optical bistable element having two time constants, and the two time constants act in series. By inputting a light pulse, the level of the output light from the optical bistable element is sequentially switched between a high level and a low level, and when switching from the high level to the low level, an undervoltage is applied to the output light. A method for driving an optical bistable device, characterized in that switching from the high level to the low level is performed by generating a shoot.