JP2830909B2 - Optical element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a broadband optical device such as a wavelength division multiplexed laser used in the fields of optical information processing and optical communication.

[0002]

2. Description of the Related Art Optical parallel processing utilizing the property of light that can be multiplexed in real space and wavelength space is effective for increasing the capacity and bandwidth of information processing systems and communication systems. As an optical element that can be used for the purpose of optical parallel processing using multiplexing in a wavelength space, an element having a wide band characteristic and a small wavelength dependent characteristic is desired. This is because it is desirable to operate as many optical modes having different wavelengths as possible and to obtain as uniform an optical power as possible between the modes. For example, in a laser device or an optical amplifier device, it is desirable that the gain medium of the gain medium is wide and the gain spectrum does not have a large wavelength-dependent characteristic, in other words, that the spectral characteristic is as flat as possible.
Further, when a gain medium having such a flat spectral characteristic is applied to a laser having a short cavity length such as a surface emitting laser, the optical output does not fluctuate even if the resonance wavelength changes due to a temperature change. It is also desirable as a gain medium for a short-cavity laser.

[0003] Conventionally, to realize such an optical element, for example, an I. Journal of Selective Topics in Quantum Electronics (IEEE Journal of Selecte Electronics) has been proposed.
d Topics in Quantum Elect
ronics), Vol. 1, No. 2, page 654 has proposed a technique as shown by Kajita et al. This is an example of realizing a gain medium having a wide band and a flat spectrum by changing the composition of each quantum well in a multiple quantum well often used as a gain medium of a semiconductor laser.
Even though the gain spectrum of each quantum well is narrow, each gain wavelength band is different, and as a result of overlapping the interest spectra of all quantum wells, a gain medium having a broadband and flat spectrum is realized. ing.

[0004]

However, the above-described laser and optical amplifier have the following disadvantages.

[0005] The kinetic energy of the carriers injected into each quantum well is distributed with thermal spread. At this time, a non-inverted dipole (electron-hole pair) exists on the wide energy side (short wavelength side) of the gain spectrum, and absorption due to this exists. This is because the quantum well has a continuous state density on the wide energy side. Thus, in the laser having the above configuration, a part of the gain generated in the quantum well having the gain band on the high energy side is canceled by absorption by the quantum well having the gain band on the low energy side, so that the efficiency is low and the flatness is low. It is difficult to obtain a spectrum. The width of the gain spectrum possessed by one quantum well is at most about 3 k _B T / 2 (k _B : Boltzmann constant, T: temperature) of thermal energy, and is about 25 meV at room temperature. Accordingly, a large number of quantum wells is required to obtain a wide band, which makes the above-mentioned absorption problem even more serious.

Further, in a surface emitting laser or the like having a short cavity length, when the number of layers of the multiple quantum well increases, it is difficult to place the quantum well at the antinode (the position where the intensity is highest) of the laser standing wave mode optical field. become. As a result, there is also a disadvantage that the optical coupling between the optical field and the medium is weakened and an apparent gain reduction occurs.

An object of the present invention is to overcome the above-mentioned drawbacks of the prior art, and to provide a gain medium having a flat spectrum over a wide band, an optical element such as a laser or an optical amplifier having the same characteristics using the gain medium, and a temperature. An object of the present invention is to provide an optical element such as a laser or an optical amplifier having excellent characteristics.

[0008]

According to the Summary of the invention of claim 1, wherein, in the formed optical element by a two-dimensional or three-dimensional carrier confinement structure semiconductor gain material having a size of about thermal de Broglie wavelength, the Career confinement
Variations in the confinement size of the structure and each size
Number of carrier confinement structures is inversely proportional to the square of size
Thus , an optical element characterized by controlling the carrier confinement structure is obtained.

According to a second aspect of the present invention, there is provided a semiconductor laser comprising a two-dimensional or three-dimensional carrier confinement structure having a size on the order of a thermal de Broglie wavelength, comprising a semiconductor gain material.
Carrier of each size with variation in the size
As the number of confinement structures is inversely proportional to the square of the size,
A semiconductor laser characterized by controlling the carrier confinement structure is obtained.

According to the third aspect of the present invention, in the optical amplifier made of a semiconductor gain material having a two-dimensional or three-dimensional carrier confinement structure having a size about the thermal de Broglie wavelength, the carrier confinement structure is confined.
Variations in size and closing of each size carrier
So that the number of confining structures is inversely proportional to the square of the size
An optical amplifier characterized by controlling the carrier confinement structure is obtained.

[0011]

Next, the basic operation of the present invention will be described.

In recent years, research on application of electronic devices and optical devices having a small-sized low-dimensional carrier confinement structure called a quantum wire or quantum dot has been actively conducted. These low-dimensional carrier confinement structures are characterized in that the confinement size is on the order of the thermal de Broglie wavelength λ _{T of the} carrier system (λ _T is obtained by the following equation (1)). It is expected that the so-called effect will be exhibited.

[0013]

(Equation 1) An advantage of these quantum mechanical size low-dimensional carrier confinement structures (hereinafter referred to as “quantum confinement structures”) for optical devices is that the density of states becomes steep. This means that physically speaking, dipoles (electron-hole pairs) that can exist in the medium can be concentrated in a certain discrete transition energy state. This is possible because the non-individuality of the carrier is lost by spatially confining the carrier, and as a result, the Pauli exclusion rule is relaxed. (If the Pauli exclusion rule does not work, , The degeneracy of the energy level cannot be solved because no exchange interaction occurs.)

On the other hand, experimentally, unfortunately, no steepening of the photoluminescence spectrum or the gain spectrum due to the steepening of the state density has been observed at present. This is because the size of the quantum confinement structure varies, and consequently the energy of the confinement level varies from structure to structure, and the transition energy varies. Have been. Experimentally, in the quantum dot system, a width wider than the photoluminescence spectrum width and the gain spectrum width of the quantum well may be observed.

The present invention intends to positively utilize the phenomenon that the unevenness of the size causes the uneven spread. In the conventional approach to the realization of the quantum confinement structure, it was intended to fabricate a quantum confinement structure of the same size as possible. This is reconsidered as a parameter in the quantum confinement structure whose confinement size can be artificially freely designed and controlled. When reconsidered in this way, the transition energy can be controlled by controlling the size of each quantum structure. Therefore, the shape of the state density when viewed as a whole system can be freely controlled. For example, R _min ≦ R ≦ R
_When a spherical quantum dot structure having a radius R of _max is manufactured such that the number N (R) of quantum dots having a radius R satisfies N (R) ∝R ⁻² , the corresponding state density of the entire system is obtained. Note that dN / dE is a shift ΔE∝R ⁻² of the transition energy due to the quantum effect, so that there is a relationship of dN / dE∝R ³ · dN / dR, and the corresponding transition energy E
A flat (independent of energy E) density of states can be created. Here, the transition energy E is obtained by the following equation (2). If the distance between adjacent quantum structures is sufficiently large, excitation transfer between quantum structures can be ignored, and carriers should be injected uniformly with equal probability into these quantum structures. A directly proportional gain spectrum should be obtained.

[0016]

(Equation 2) If a quantum confinement structure whose size distribution is artificially controlled is used as a gain medium, an optical device such as a laser or an optical amplifier having a flat gain spectrum over a wide band can be realized. In the two-dimensional quantum confinement structure (quantum wire) and three-dimensional quantum confinement structure (quantum dot), the state density of a simple substance does not exist or is small even on the high energy side. No absorption problems occur. Also, in contrast to the prior art in which the characteristics of the gain medium were controlled in the laser standing wave mode (longitudinal mode) direction,
In the present invention, since the size of the quantum confinement structure is controlled in a plane orthogonal to the laser standing wave mode (longitudinal mode), it is possible to concentrate all the media near the antinode of the laser mode unlike the conventional technique. .

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing an example in which the present invention is applied to a surface emitting laser on a GaAs substrate. First, GaAs / A is formed on the GaAs substrate 7 by the MBE method.
grown distributed Bragg reflector 4 made of multi-layer structure of the LAS, after the AlGaAs also GaAs to form an approximately 6nm thick In _0.2 Ga _0.8 As quantum well as a cladding layer, covering the surface of GaAs layer. After the growth up to this point, the active layer 3 including the quantum dot structure is formed by patterning the substrate by the method described below. First, a resist is applied to the surface and exposed with an electron beam to form a pattern having a diameter of 10 nm to 25 nm and a diameter of every 1 nm on the substrate surface at intervals of about 50 nm. Thereafter, etching is performed by reactive ion beam etching so that only the lower portion of the pattern remains. MBE growth is again performed on the surface of the thus formed quantum dot structures having different diameters.
Here, GaAs / AlAs is formed on the quantum dot active layer 3.
A distributed Bragg reflector 2 having a multilayer structure of
A GaAs layer for ohmic junction is grown on the surface.
Thereafter, electrodes 1 and 5 for injecting a current for determining an in-plane mode of the 10 μm surface emitting laser are formed by ordinary photolithography and wet etching.

In the embodiment of the present invention, an electron beam exposure pattern was prepared such that the number N (R) of quantum dots having a diameter R was N (R) ∝R ^-2 . The upper diagram of FIG. 2 shows the relationship between the size and the number of manufactured quantum dots. The corresponding state dN / dE of the entire system is as shown in the middle diagram of FIG. At this time, if a current is uniformly injected into each quantum dot and no excitation transfer occurs between the quantum dots, the gain obtained should be a broadband and flat gain spectrum as shown in the lower diagram of FIG. . In this example, _Rmin = 10 nm and _Rmax = 25 nm were selected. The corresponding gain spectrum band is E _min = 1.2
9eV (wavelength 964nm), E _max = 1.403eV
(Wavelength 886 nm).

Such a gain medium having a broad and flat gain spectrum is useful for a light source or an optical amplifier for wavelength multiplexing as described above, but at the same time, a temperature in a short-cavity surface emitting laser. It is effective in improving characteristics. In the case of the surface emitting laser manufactured according to the example of the present invention, it has been found that excellent characteristics such that laser oscillation is possible even when the element temperature is changed from -77 degrees Celsius to 180 degrees Celsius. Further, it was found that an excellent characteristic that the laser output hardly changed even when the element temperature was changed from 20 degrees Celsius to 80 degrees Celsius was obtained.

In the above embodiment, a semiconductor material is described as an example, but the present invention is not limited to this embodiment. In this embodiment, a laser using InGaAs as a quantum confinement structure and GaAs as a barrier layer has been described as an example, but InP, InGaAs, InGaAs, and the like are described.
It is also possible to apply to optical devices such as lasers and optical amplifiers using other semiconductor materials such as P. In this embodiment, an example using a surface emitting laser or a semiconductor reflecting mirror has been described. However, another structure such as an edge emitting laser, an optical amplifier, or a reflecting mirror using a dielectric multilayer film is used. Needless to say, the present invention can be applied to the case where there is.

[0022]

As described above, according to the present invention,
It is possible to realize a gain medium with a small wavelength dependence in a wide band, thereby realizing an optical element such as a semiconductor laser or an optical amplifier or an optical element having a good temperature characteristic such as a laser or an optical amplifier. .

[Brief description of the drawings]

FIG. 1 is a sectional view of a surface emitting laser according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating the relationship between the quantum dot size dependency of the number of quantum dots, the state density spectrum, and the gain spectrum in the embodiment shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cathode electrode 2 Upper distributed Bragg reflector 3 Active layer 4 Lower distributed Bragg reflector 5 Anode electrode 6 Ion implantation area 7 GaAs substrate

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-8-172217 (JP, A) JP-A-4-273492 (JP, A) JP-A-6-196819 (JP, A) JP-A-10- 112567 (JP, A) JP-A-9-148679 (JP, A) JP-A-9-162495 (JP, A) JP-A-8-236868 (JP, A) The 42nd Spring of 1995 Proceedings of the Society of Natural Sciences 30p-ZG-6 p. 1102 Proceedings of the 42nd Annual Meeting of the Society for Response Science, 1995, 30p-ZG-4 p. 1101 (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/18

Claims (3)

(57) [Claims] 1. An optical device comprising a semiconductor gain material having a two-dimensional or three-dimensional carrier confinement structure having a size on the order of a thermal de Broglie wavelength, wherein the confinement size of the carrier confinement structure varies.
And the number of carrier confinement structures of each size is
The carrier confinement is inversely proportional to the square of the
An optical element having a controlled structure . 2. A semiconductor laser comprising a two-dimensional or three-dimensional carrier confinement structure having a size on the order of thermal de Broglie wavelength semiconductor gain material, wherein the confinement size of the carrier confinement structure varies.
And the number of carrier confinement structures of each size is
The carrier confinement is inversely proportional to the square of the
A semiconductor laser having a controlled structure . 3. An optical amplifier comprising a semiconductor gain material having a two-dimensional or three-dimensional carrier confinement structure having a size on the order of a thermal de Broglie wavelength, wherein the confinement size of the carrier confinement structure varies.
And the number of carrier confinement structures of each size is
The carrier confinement is inversely proportional to the square of the
An optical amplifier whose structure is controlled .