JPH01179488A - Optical amplifier - Google Patents

Abstract

PURPOSE:To realize an optical amplifier large in gain band width by a method wherein an active layer comprising two or more quantum well structures different from each other in well thickness is provided to a semiconductor laser type optical amplifier. CONSTITUTION:A buffer layer 2 formed of an n-type A GaAs/AlGaAs multiple quantum well(MQW) layer 2, an n-type A GaAs clad layer 3, an MQW active layer 4, a p-type AlGaAs intermediate layer 5, a p-type AlGaAs clad layer 6, and a p-type A GaAs cap layer 7 are formed on an n-type GaAs substrate 1. The above MQW active layer 4 is composed of five periods of GaAs barriers and GaAs well layers different from each other in quantum well thickness. By these processes, a gain property constant over a wide frequency range can be obtained. And, a quantum well structure being employed, the efficiency of the carrier density toward an injecting current can be made high and an amplifier of this design can be operated with a low current.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical amplifier used to amplify an optical signal in an optical communication device or an optical switching device.

(Prior Art) So-called opto-electronic technology involves first converting an optical signal into an electrical signal, and then converting this electrical signal into an optical signal.

Optical amplifiers that perform amplification without going through electro-optic conversion are expected to improve the performance of optical fiber transmission systems and optical switching systems. As a means of realizing an optical amplifier, it is possible to use nonlinear scattering within an optical fiber such as Raman or Brillouin, but from the viewpoint of small size, low power consumption, and simple configuration, the gain a It is desirable to use an LD optical amplifier using

LD optical amplifiers can be roughly divided into Fabry-Perot (F-P)
), a resonant type that uses the DFB LD as it is in a bias state below the threshold value, and a traveling wave (TW) type that suppresses oscillation by reducing the end face reflectance of the FP type with an AR coating or the like. The traveling wave type has a wider gain wavelength band than the resonant type, can operate stably against temperature fluctuations, and can be expected to have excellent characteristics in terms of gain saturation and noise characteristics. For this reason, traveling wave semiconductor laser type optical amplifiers (TW-LD optical amplifiers) have been widely studied.

The structure of the active layer of an 'I'W-LD optical amplifier is often a normal double heterostructure (DH), but it is also possible to use a multiple quantum well (MQW) structure.
-LD has a larger proportion of gain coefficient increase with respect to current increase than DI-(-LD. Therefore, even if the end face is AR coated and used as a TW-LD optical amplifier, high gain can be obtained with very low current. It will be done.

Furthermore, since the active layer is thin and the optical confinement coefficient can be reduced, there is also the advantage that the saturation optical output intensity is improved.

(Problems to be Solved by the Invention) In the active layer of the MQW structure, energy states in the conduction band and forbidden band are quantized, and transitions occur between the levels. Therefore, the wavelength width of the gain spectrum is narrower than in the DH structure. In a TW-LD optical amplifier in which end face reflectance is sufficiently suppressed, the gain bandwidth is determined by the gain bandwidth of the active layer material. Therefore, the gain bandwidth of the TW-MQW-LD optical amplifier is considerably narrower than that of a TW-DH-LD optical amplifier having a normal DH structure active layer. This becomes a big problem when considering amplification of wavelength multiplexed (WDM) optical signals.

An object of the present invention is to provide an optical amplifier with a wide gain bandwidth that eliminates the above-mentioned problems.

(Means for Solving the Problems) An optical amplifier according to the first invention of the present application is a semiconductor laser type optical amplifier having an active layer made of a semiconductor material and an input/output end face for coupling input and output optical signals. The active layer is characterized in that it includes a plurality of quantum well structures having mutually different well thicknesses.

Further, an optical amplifier according to a second invention of the present application is a semiconductor laser type optical amplifier having an active layer made of a semiconductor material and an input/output end face for coupling input and output optical signals, wherein the active layer is a well layer or It is characterized by including a plurality of quantum well structures in which at least one of the barrier layers has a different composition.

(Function) The present invention utilizes the fact that the effective band gap of a quantum well structure can be controlled by the thickness of the well layer and the composition of the well layer or barrier layer. First of all, Ga As /
This point will be briefly explained using an A iJ Ga As quantum well structure as an example.

FIG. 3 is an energy band diagram of a GaAs/AρGaAs single quantum well (SQW) I structure.

It has a structure in which a GaAs well layer is sandwiched between AρK Ga+- and As barrier layers, and the barrier layer thickness is made infinite for simplicity. The energy of electrons and holes is quantized within this GaAs well to form discrete energy levels. This S
The effective bandgap of the QW structure is determined by the energy difference between the base units as shown in FIG.

Incidentally, the quantized energy levels of electrons and holes are determined by the potential depth and well thickness of the well.

In other words, the effective bandgap can be controlled by the composition of the well and barrier layer and the thickness of the well layer. Figure 4 shows GaAs/Aρ with GaAs as the well.
Well thickness and bandgap wavelength of KGal-IAs system SQW (wavelength of wave corresponding to bandgap energy)
It is calculated based on the relationship between By changing the well thickness and barrier composition, the bandgap wavelength can be controlled over 500 nm. L with such SQW in the active layer
The D optical amplifier has a gain spectrum peak near its bandgap wavelength.

The thickness of the well is usually about 150 or less because the quantum size effect does not become noticeable unless it is about the de Broglie wavelength or less. - The thickness of the active layer, which is also the waveguide layer at the tip of the force, is usually 1,000 to 2,000 layers. Therefore, an MQW-LD having multiple quantum wells in the active layer becomes possible. This MQ
In a W-LD, if the thickness of each well or the composition of the well or barrier is slightly different, each well will have a gain peak at a different wavelength, and the light guided through the active layer will have a gain effect in each well structure. You will receive each. M like this
When a QW-LD is used as an LD optical amplifier, the gain spectra in each well are superimposed, and an extremely flat gain spectrum can be obtained as a whole.

FIG. 5 schematically shows the gain spectrum of an MQW-LD optical amplifier whose active layer consists of quantum wells with four different thicknesses. By adopting such a structure, as shown in FIG. 5, a wide gain band that could not be obtained by using only one structure of quantum wells can be realized.

(Example) FIG. 1 is a perspective view showing the structure of an embodiment of the present invention.

Here, the quantum size effect appears most prominently in G a A
The case where all the s/G a AρAs-based materials are used will be explained.

First, the structure of the optical amplifier shown in FIG. 1 will be explained along with its manufacturing method. On the n-GaAs substrate 1, n AAa, < Gao, s As/ n − , which becomes a buffer layer, is formed.
A lot of GaAs! Ikodo (MQW)! 2゜n-AA
a, 40ao, * As clad/y d layer 3. MQW active layer 4. p-Aρ. , s Gao, s As intermediate layer 5゜p
A j! 0.4 Gao, aAs cladding layer6. P-A II o, + sG a o, a
, A S cap layer 7 is continuously grown by the MBE method.
Next is the photophosphorography method.

Using chemical etching, n-GaA is formed in stripes.
This stripe is formed by the zero-order LPE method, which performs etching that reaches the s-substrate 1. o,iaG ao
, 42A 8 layers 8. n Al o, 5aGao,
62AFs layer 9. In this case, due to the presence of the intermediate layer 5, the pn junction position by the buried layer 8.9 is automatically determined under the active layer 4. This structure is known as BGM treetop fin, and the details of this growth method are published in Proceedings of the 1984 National Conference of the Institute of Electronics and Communication Engineers, No. 1016 (1984).
).

The MQW active layer used here is a GaAs well layer, AAa,
It consists of 5 periods of 2 Gao, s As barriers.

The thickness of the GaAS well layer is 50. No, 100.

125 and 150 people, and the thickness of the barrier layer was 170 people.

All layers are non-doped.

Next, plP! SiO□ stripes 10 for current confinement are formed on the l, and electrodes 11 are formed on the n-side and p-side, respectively.
．． form 12. Input/output end face 1 formed by cleavage
SiN and AR coat (not shown in FIG. 1) films were formed on 3a and 13b by plasma CVD, respectively, to form a traveling wave type LD optical amplifier. The length of the device was approximately 250 g+n.

FIG. 2 is a diagram for explaining the operation of this embodiment,
2 is a cross-sectional view of the embodiment shown in FIG. 1 taken along the optical axis and perpendicular to the substrate. Figure 2 shows AR Coco-11.
A14a and 14b were shown. In this prototype sample,
The oscillation threshold of the AR coat image was >100 mA.

Spherical fibers 21a, 21b are used to couple the incident light into the active layer 4 and to extract the optical signal. When a forward bias is applied between the electrodes 11 and 12, the gain in the active layer 4 increases and an amplification function is obtained.

In this embodiment, since the active layer 4 includes five types of quantum wells having different thicknesses, flat gain characteristics can be obtained over a wide wavelength range of 810 to 850 nm. Moreover, since it has a quantum well structure, the efficiency of carrier density with respect to injection current is high, and it can operate with low current.

In this example, the well thickness was varied in each quantum well, but
A similar effect can be obtained by varying the well and barrier compositions. In addition, here we adopted a structure in which current is injected in a direction perpendicular to the well layer, but TJSi3fi
A structure in which carriers are injected laterally is also possible. In this case, there is an advantage that each well can be uniformly excited.

In this example, G a A s / G a A A
Although the explanation has been made using an s-thread material, it is clear that the present invention can be applied to any material system that can obtain the quantum size effect. There is no problem in adopting the device structure of a transverse mode control structure used in a normal LD instead of the BGM fin structure shown in the embodiment.

(Effects of the Invention) As explained in detail above, according to the present invention, an optical amplifier with a very wide gain bandwidth can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an embodiment of the optical amplifier according to the present invention, FIG. 2 is a cross-sectional view of the embodiment of FIG. 1 taken along the optical axis and perpendicular to the substrate, and FIG. An energy band diagram of a single quantum well (SQW) structure. Figure 4 is a characteristic diagram of well thickness and bandgap wavelength obtained by calculation for the SQW of Figure 3 with GaAs wells. Figure 5
The figure shows an MQ whose active layer consists of quantum wells with four different thicknesses.
FIG. 2 is a characteristic diagram schematically showing a gain spectrum of a WLD optical amplifier. 1...Substrate, 2...Multiple quantum well (MQW) NJ
, 3.6... cladding layer, 4... MQW active layer, 5
... Intermediate layer, 7... Cap layer, 8.9... Buried layer, 1 (L...S i O2 stripe, 11.1
2... Electrode, 13a. 13b...input/output end surface, 14a, 14b...AR
Coat film, 21a, 21b... tipped fiber.

Claims (1)

[Claims] 1. In a semiconductor laser type optical amplifier having an active layer made of a semiconductor material and an input/output end face for coupling input and output optical signals, the active layer has a plurality of wells having different well thicknesses. An optical amplifier characterized by including a quantum well structure. 2. In a semiconductor laser type optical amplifier having an active layer made of a semiconductor material and an input/output end face for coupling input/output optical signals, the active layer has a composition of at least one of a well layer and a barrier layer. An optical amplifier characterized by including a plurality of different quantum well structures.