JP3576681B2 - Optical storage device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical semiconductor device, and more particularly, to an optical storage device that can hold data for a long time and can read and write at high speed.
[0002]
[Prior art]
In recent years, with the development of long-distance, large-capacity optical communication systems, large-capacity optical switching systems and optical information processing systems have become necessary. In addition, research on the realization of optical computers is also active. In such a system, not only an optical switch and an optical logic operation element but also an optical storage device are required. Techniques for storing and extracting light are difficult, and there are only studies on those that circulate in optical fibers, hole burning memories, photon echoes, and the like.
[0003]
However, this type of conventional optical storage device has the following problems. That is, when the optical fiber is circulated in the optical fiber, the device is large and the operation is unstable. Further, when a hole burning memory or a photon echo is used, there is a disadvantage that a special material such as a metal vapor, a dye, a rare earth element or the like must be used for the recording medium, or that the apparatus does not operate unless the temperature is low.
[0004]
[Problems to be solved by the invention]
As described above, in the conventional hole burning memory and the photon echo, there is a problem that the operation is not performed unless a special material is used or the temperature is low. In addition, the method of confining light by orbiting in an optical fiber requires a very large device and has poor stability.
[0005]
The present invention has been made in consideration of the above circumstances, and has as its object to realize a completely new optical system that can be realized with a practical material and with a simple configuration, and has excellent stability and high performance. A storage device is provided.
[0006]
[Means for Solving the Problems]
(Constitution)
In order to solve the above problems, the present invention employs the following configuration.
That is, according to the present invention, in an optical storage device utilizing light confinement and radiation, the number of light modes that can be coupled to a field in a two-level system or a quasi-two-level system is set to 0 (a state in which light cannot be emitted from the system). 1 (a state in which light can be emitted from the system); a means for irradiating the system with π / 2 pulse recording light once or plural times continuously; and a π pulse following the recording light on the system. Means for irradiating refresh light once or a plurality of times.
[0007]
Here, preferred embodiments of the present invention include the following.
(1) As a means for reducing the number of light modes to 0, a cavity is formed minutely on the order of the wavelength of light and the effect of the cavity QED (quantum electromagnetic effect) is used.
(2) As a means for setting the number of light modes to one, the system is irradiated with control light.
(3) A quantum well structure is sandwiched between Bragg reflectors to form a pseudo-two-level system between quantum well subbands.
(Action)
According to the present invention, there is provided means for changing the number of light modes that can be combined by the system in a two-level system or a quasi-two-level system between 0 and 1, and a π / 2 pulse recording light and a subsequent π / 2 pulse recording light are provided. Means for irradiating pulse refresh light are provided, which has an important meaning. This will be described below with reference to FIG.
[0008]
FIG. 4 shows u, v, and w, which are components of a Bloch vector, on a coordinate axis. u, v, and w mean the in-phase dipole moment, the quadrature-phase dipole moment, and the difference between the population of the upper level and the population of the lower level, respectively.
[0009]
Now, when a π / 2 pulse recording beam is incident on a system (two-level system or pseudo-two-level system) (a), the state of the system becomes a state in which the probability of being in the upper level is equal to the probability of being in the lower level. (B). Then, phase diffusion occurs with time. Here, when the π pulse is continuously incident, the vector is inverted as shown in the figure (c), and the vector that has been separated after a certain time completely matches (d).
[0010]
Usually at this time, emission of light occurs, known as an echo pulse or superradiance. Conventionally, even for materials that take a short time to emit this light or that take a long time to emit light in principle, the phase shift accumulates over time and the vectors completely match after inversion. In some cases, effective light was not emitted. For this reason, only a small portion of metal vapor or organic material can be used to retain the memory for a long time.
[0011]
In the present invention, the number of light modes that the system can couple in a two-level system or a quasi-two-level system is set to 0, that is, the light cannot be emitted from the system. This can be realized by using the effect of a so-called cavity QED in which the size of the resonator is reduced to the order of the wavelength of light. Therefore, when the system attempts to emit light, it is not actually emitted, and the vector undergoes phase diffusion again (e).
[0012]
Here, when the refresh light of the π pulse is continuously irradiated, the vector is inverted again (f), and the vector that has been separated after a certain time completely matches (g). By irradiating the π-pulse refresh light a plurality of times and repeating the inversion operation, not only can the light energy be stored in the system for a long time, but also the vector generated almost every time a slight phase shift occurs. It can be resolved by the pull-in phenomenon that occurs when they match. Therefore, a material having a short relaxation time such as a semiconductor can be used as a memory material.
[0013]
If the number of light modes that can be combined by the system at a desired time is changed from 0 to 1 by irradiation with control light, for example, light can be emitted to the outside (h).
As described above, according to the present invention, a means for changing the number of light modes that can be coupled to a field between 0 and 1 in a two-level system or a quasi-two-level system is provided. By providing a means for irradiating the system with the subsequent π pulse refresh light, a change in the system caused by π / 2 pulse irradiation can be maintained for a long time, and a stable storage operation can be performed.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments.
(1st Embodiment)
FIG. 1 is a sectional view showing a schematic structure of the optical storage device according to the first embodiment of the present invention.
[0015]
After forming an InAlAs buffer layer 11 on an InP substrate (not shown), a quantum well structure composed of an AlAs barrier layer (thickness 3 nm) 12 and an InGaAs well layer (thickness 1 nm) 13 is formed by metal organic chemical vapor deposition (MOCVD). Formed. The well layer 13 was doped with 2.0 × 10 ¹⁹ cm ^{−3 of} Si. Further, the InAlAs cap layer 14 was continuously grown.
[0016]
Next, after removing the InP substrate, Bragg reflectors 15 made of GaAs / AlAs were formed directly above and below the quantum well structure by a direct bonding method. Here, a quasi-two-level system is formed between the subbands of the quantum well.
[0017]
This sample was cooled to a low temperature of 4.2K and operated as an optical storage element. First, femtosecond π / 2 pulse recording light in a wavelength band of 1500 nm was generated using a NaCl color center laser, and the sample was irradiated with the recording light. Subsequently, femtosecond π refresh light was repeatedly applied at intervals of 1 picosecond. No light emission from the system was seen. This is understood to be because the system could not be coupled to the field due to the effect of the cavity QED.
[0018]
A few minutes later, light for controlling the cavity QED effect (for example, a sub-picosecond pulse in the 1300 nm wavelength band) was incident on the Bragg reflector portion, and light emission from the system was confirmed by the photodetector 16. This is presumably because the control light changed the refractive index of the Bragg reflecting mirror 15 to cause coupling between the system and the field. In this way, it was confirmed that the femtosecond π / 2 pulse recording light that first entered was actually held.
[0019]
FIG. 2 is an example of a pulse timing chart when storing a plurality of signals. After continuously irradiating the signal light (π / 2 pulse) twice so as to form a pattern of “1010”, the refresh light (π pulse) is irradiated at regular intervals. At this time, the frequencies of the two signal light pulses are slightly different from each other. By irradiating control light when reading is necessary, output light is obtained twice according to “1010”.
(Second embodiment)
FIG. 3 is a sectional view showing a schematic structure of an optical storage device according to the second embodiment of the present invention. The same parts as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0020]
In the present embodiment, a plurality of optical storage elements according to the first embodiment described above are integrated. That is, a plurality of the elements shown in FIG. 1 are integrated on an InP substrate 31, and a photodetector 16 is provided for each of them.
[0021]
With such a configuration, the individual elements hold the recording light by the irradiation of the refresh light subsequent to the irradiation of the recording light, and emit the recording light by the irradiation of the control light, as in the first embodiment. Therefore, when reading is performed, an output is obtained to the detector 16 only from the stored elements.
[0022]
Note that the present invention is not limited to the embodiment described above. In the embodiment, the element is formed by using a semiconductor material. However, even when a conventionally used atomic vapor or an organic material is used, a better memory operation than the conventional one can be obtained. In the embodiment, the operating wavelength is set at around 1500 nm, but may be in another wavelength region. In the embodiment, the pseudo-two-level system is formed between the subbands of the quantum well. However, the present invention is not limited to the pseudo-two-level system and can be applied to a structure having a two-level system. In addition, various modifications can be made without departing from the scope of the present invention.
[0023]
【The invention's effect】
As described in detail above, according to the present invention, means for changing the number of light modes that can be coupled to a field between 0 and 1 in a two-level system or a quasi-two-level system; By providing a means for irradiating π / 2 pulse recording light successively twice and a means for irradiating π pulse refresh light once or a plurality of times subsequent to the recording light in the system, a high-performance optical storage which could not be obtained conventionally. The element can be realized with a simple configuration. As a result, not only can the optical element used in the optical information processing system be obtained at low cost, but also its reliability is high, and the usefulness of the present invention is enormous.
[Brief description of the drawings]
FIG. 1 is a sectional view showing a schematic structure of an optical storage device according to a first embodiment.
FIG. 2 is a timing chart of pulses when storing, holding, and reading out signal light.
FIG. 3 is a sectional view showing a schematic structure of an optical storage device according to a second embodiment.
FIG. 4 is a diagram for explaining the operation principle of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... InAlAs buffer layer 12 ... AlAs barrier layer 13 ... InGaAs well layer 14 ... InAlAs cap layer 15 ... GaAs / AlAs Bragg reflector 16 ... photodetector 31 ... InP substrate

Claims (4)

Means for changing between 0 and 1 the number of modes of light that can be coupled to a field in a two-level system or a quasi-two-level system;
Means for irradiating the system with π / 2 pulse recording light that is continuous one or more times, means for irradiating the system with π pulse refresh light once or more times subsequent to the recording light,
An optical storage device comprising: 2. The method according to claim 1, wherein the means for reducing the number of light modes to zero uses a cavity QED effect (quantum electromagnetic effect) by forming a resonator minutely on the order of the wavelength of light. Optical storage device. 2. The optical storage device according to claim 1, wherein the system is irradiated with control light as means for setting the number of light modes to one. 2. The optical storage device according to claim 1, wherein the quantum well structure is sandwiched between Bragg reflectors to form a pseudo-two-level system between subbands of the quantum well.

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