JP2718995B2 - Traveling-wave semiconductor laser amplifier - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a traveling-wave type semiconductor laser amplifier capable of directly amplifying an optical signal without exchanging an optical signal into an electric signal, and more particularly to a high-power, low-noise, and low operating current density. And a traveling-wave type semiconductor laser amplifier having a small dependence of the gain on the polarization plane.

[Related Art] A semiconductor laser amplifier is an element that coherently amplifies an incident signal light using stimulated emission in a semiconductor laser medium. For this reason, since an optical signal can be directly amplified without converting it into an electric signal, it is attracting attention as an important element for future ultrahigh-speed optical transmission systems and coherent optical transmission systems.
A traveling-wave semiconductor laser amplifier is an element that amplifies an optical signal by suppressing resonance due to reflection from both ends where light enters and exits. For this purpose, an anti-reflection film is formed on both ends [T. Saitoh et al. al., IEEE J.Quantum Electon., vol.QE−
23, pp. 1010-1020, 1987.), diagonal stripe structure (CE
Zah et al., Electron.Lett., Vol.23, pp.990-991,198
7.) and window edge structure [I. Cha et al., Electron Lett., Vol.
25, pp. 242-243, 1989].

FIG. 5 schematically shows the structure of a conventional traveling-wave type semiconductor laser amplifier, wherein 1 is an active layer formed of a heterojunction, 3 is a cladding layer, and 4 is incident from one end face of the active layer 1. An optical signal 5 is output light amplified and emitted from the other end face of the active layer 1, and I _OP is an operating current for injecting carriers into the active layer 1.

[Problems to be Solved by the Invention] By the way, if the power of the output light 5 when the gain of the traveling wave type semiconductor laser amplifier becomes half the value of the unsaturated gain is defined as the saturated output light power P _3dB , the saturated output light Power is Indicated by (1) In the equation, hv is the photon energy, A _g is the differential gain, tau _C carrier lifetime, d _a is the active layer thickness, w is the width of the active layer, gamma _TE is an optical mode confinement factor for the TE mode. FIG. 6 shows the calculation results of the dependence of d _a / Γ _TE and the confinement coefficient of each mode on the active layer thickness. The value of d _a / gamma _TE is minimized in the vicinity of the active layer thickness d _a = 0.2 [mu] m, with a small active layer thickness d _a region increases rapidly with decreasing thickness of the active layer d _a. Further, the carrier life sharply decreases as the injected carrier density increases due to the Auger effect. The mode confinement coefficient and the active layer length L depend on the carrier density N _e
And the following relationship.

In the equation (2), R is the end face reflectivity, _VTE is the Fabry-Perot mode ripple caused by residual end face reflection, and No is the carrier density when the medium becomes transparent.
When the active layer thickness da is reduced, the mode confinement decreases, the carrier density increases, and the carrier lifetime decreases. Therefore, the saturation output light power P _3dB increases with the decrease in the active layer thickness da. The noise figure F is given by F で 2n _sp (3). Where the population inversion parameter n _sp is It is represented by Since n _sp approaches 1 as the carrier density and the gain coefficient g increase, it can be seen that the noise characteristics can be improved by reducing the mode confinement coefficient as in the case of the saturated optical power.

On the other hand, the difference ΔG _TE / _TM between the signal gains of TE mode and TM mode is Indicated by In equation (5), Γ _TE and Γ _TM are TE,
The optical mode confinement coefficient for the TM mode, α is the mode absorption coefficient, L is the active layer length, and Gs is the single-pass gain.
Equation (5) shows that the polarization plane dependence of the signal gain depends on the ratio of the mode confinement coefficients. Normal active layer thickness
In da, the signal gain difference between modes becomes 5 dB or more [T. Saito
h et al., IEEE J. Quantum Electron., vol. QE-23, pp. 10
10-1020, 1987.]. In order to reduce the polarization dependence of this mode confinement, the active layer thickness da may be increased.
This is not only unfavorable in terms of the saturation and noise characteristics, but also has a problem that the current density of the operating current _Iop increases.

SUMMARY OF THE INVENTION The present invention has been made in order to solve the above problems, and provides a traveling-wave semiconductor laser amplifier having high output, low noise, low operating current density, and little dependence of gain on the polarization plane. With the goal.

[Means for Solving the Problems] To achieve the above object, a traveling wave semiconductor laser amplifier according to the present invention has an active layer composed of a double heterojunction, and has one end face of the active layer formed of an optical layer. The incident end face, the other end face as a light emitting end face, suppresses resonance due to reflection at these two end faces, and an optical signal having a wavelength substantially equal to the band gap is incident on the active layer from the light incident end. What is claimed is: 1. A traveling-wave semiconductor laser amplifier for injecting a current into an active layer and amplifying the optical signal by a stimulated emission effect, wherein at least one side of the active layer in a thickness increasing direction has a band gap larger than a band gap of the active layer. And a structure in which the active layer is a thin layer having a thickness of 0.11 μm or less.

[Operation] The present invention provides a traveling wave semiconductor laser amplifier having suppressed end face reflection, wherein an optical waveguide layer is provided on at least one side of the active layer, and by providing this waveguide layer, the thickness of the active layer can be increased. Decrease the polarization dependence of mode confinement. Further, by reducing the thickness of the active layer, even if the carrier density is increased, the current density does not need to be increased, and a low operating current density is maintained. Further, since the active layer is made thin, high output and low noise can be satisfied.

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view schematically showing the configuration of one embodiment of the present invention. In the drawing, 1 is an active layer having a layer thickness da, a layer length L, and a layer width W of a double hetero junction, 2, 2 are optical waveguide layers provided on both sides of the active layer 1, 3 is a cladding layer, and 4 is an active layer. optical signal incident from the light incident end face which is one end surface of the 1, 5 output light emitted is amplified from the light emitting end face which is the other end face of the active layer 1, I _op is injecting carriers into the active layer 1 Operating current. In the above description, the optical waveguide layers 2 and 2 and the cladding layer 3 are formed using a semiconductor laser medium, but in order to confine carriers and light in the active layer 1,
A medium having a band gap larger than the band gap of the active layer 1 is selected. Both end faces of the active layer 1, that is, the light incident end face of the optical signal 4 and the light emitting end face of the optical output 5 suppress the resonance due to the reflection at these two end faces and amplify the optical signal 4. Take measures and countermeasures such as forming a film. The waveguide layers 2, 2 have the same layer thickness dw / 2 on both sides of the active layer 1.
Are provided symmetrically. Even if the optical waveguide layer 2 is provided on one side of the active layer 1 with a layer thickness dw, the effects of the present embodiment described below can be obtained.

2A and 2B show a structure in which an optical waveguide layer is provided on an active layer (hereinafter referred to as SCH (Separate Confinement Heterostructur).
FIG. 3 e is a refractive index structure diagram of FIG.
As shown in the figure, a case of a symmetric double-sided SCH structure in which optical waveguide layers 2 are provided on both sides of an active layer 1 is shown, and FIG. In each figure, the horizontal axis indicates the thickness of each layer of the SCH structure, and the vertical axis indicates the refractive index of each layer. As shown in this figure, the refractive index of the optical waveguide layer 2 is smaller than that of the active layer 1. Thereby, light can be confined in the active layer 1.

Hereinafter, the operation of the embodiment configured as described above will be described.

In this embodiment, stimulated emission is caused by injecting a laser light signal having a wavelength substantially equal to the band gap from a light incident end face of the active layer 1 and flowing an operating current _IOP to inject carriers into the active layer 1. The optical signal is coherently amplified while suppressing resonance due to reflection, and output light is output from the light emitting end face. Here, the signal gain, the saturation output light power, the noise figure, and the operating current in this embodiment are
The dependence is shown not only on the thickness of the active layer 1 but also on the thickness of the optical waveguide layer 2.

FIGS. 3 (a), (b), (c), (d) and FIGS. 4 (a), (b), (c), (d) show symmetrical two-sided SCH,
FIG. 8 is a characteristic diagram showing the signal gain difference, the saturation output light power, the noise figure, and the operating current of the single-sided SCH structure with respect to TE / TM polarized light, depending on the active layer thickness and the optical waveguide layer thickness. Third
The figure shows the case of the symmetric double-sided SCH structure, and FIG. 4 shows the case of the single-sided SCH structure. The horizontal axis represents the active layer thickness da [μm], and the vertical axis represents the optical waveguide layer thickness dw [μm]. Is the signal gain difference ΔG _{TE / TM} [dB], and (b) is the saturated output optical power P _3dB [d
Bm], (c) shows the noise figure F [dB], and (d) shows the layer thickness dependence of the operating current I _OP [mA]. In each of the above figures, the end face reflectivity R, active layer length L, and signal gain of the active layer 1 in FIGS. 1 and 2 are 0.01%, 300 μm, and 20 d, respectively.
B, the case where the refractive index of the active layer 1, the optical waveguide layer 2 (bandgap wavelength 1.35 μm), and the refractive index of the cladding layer 3 at the wavelength of 1.55 μm are 3.524, 3.413, and 3.169, respectively.

From the above characteristic diagrams, the saturation and noise characteristics do not depend much on the thickness of the optical waveguide layer and can be improved as the thickness of the active layer is reduced, and the polarization plane dependence of the signal gain increases the thickness of the active layer or the optical waveguide layer. It can be seen that the better it can be improved. Therefore, if the thickness of the active layer is reduced and the thickness of the waveguide layer is increased, the polarization plane dependence of the signal gain can be reduced without changing the saturation and noise characteristics. That is,
Using an SCH structure in which an optical waveguide layer is provided on one or both sides of the active layer can satisfy high output and low noise and reduce the polarization plane dependence on the mode without changing the mode confinement coefficient. The present embodiment utilizes such an effect. In the above, in this embodiment, it is not necessary to increase the thickness of the active layer, so that even if the carrier density is increased, the operating current density does not need to be increased.

Table 1 shows the conventional structure with a normal active layer and one side of this embodiment.
6 is a table comparing characteristics of the SCH structure and the double-sided SCH structure, that is, a signal gain difference, a noise figure, and an operating current (density) of each structure with respect to TE / TM polarized light. Here, the active layer length L is
A comparison is made under the conditions of 300 μm and a saturation output light power P of _{3 dB} and a constant of 10 dBm.

From Table 1, it can be seen that both the single-sided SCH structure and the symmetrical SCH structure can improve the polarization dependence of the signal gain, and the operating current density is almost the same as the conventional example. In particular, in the case of a symmetric double-sided SCH structure, excellent characteristics can be expected.

[Effects of the Invention] As is apparent from the above description, according to the traveling wave type semiconductor laser amplifier of the present invention, the optical waveguide layers are provided on both sides of the active layer for suppressing the resonance due to the end face reflection and amplifying the laser light signal. By providing this, high output and low noise can be satisfied while maintaining low operating current density and low polarization plane dependency.

[Brief description of the drawings]

FIG. 1 is a perspective view schematically showing the structure of one embodiment of the present invention, and FIGS. 2 (a) and 2 (b) show a symmetrical double-sided SCH structure and a single-sided SCH.
Fig. 3 (a), (b), (c),
(D) is a characteristic diagram showing the dependence of the signal gain difference, the saturation output optical power, the noise figure, and the operating current on the active layer thickness and the optical waveguide layer thickness for TE / TM polarized light in the case of a symmetric double-sided SCH structure.
FIGS. 4 (a), (b), (c) and (d) show the signal gain difference with respect to TE / TM polarized light, the saturation output optical power, the noise figure and the operating current of the operating current in the case of a single-sided SCH structure. FIG. 5 is a characteristic diagram showing the dependency of the optical waveguide layer thickness, FIG. 5 is a schematic structural diagram of a conventional example, and FIG. 6 is a diagram showing a mode confinement coefficient with respect to each polarization plane and a relationship between the ratio and the active layer thickness. 1 ... active layer, 2 ... optical waveguide layer, 3 ... clad layer, 4
…… optical signal, 5 …… output light.

Claims (1)

(57) [Claims] 1. An active layer comprising a double heterojunction,
One end face of the active layer is used as a light incident end face, and the other end face is used as a light emitting end face to suppress resonance due to reflection at these two end faces, and to transmit an optical signal having a wavelength substantially equal to the band gap to the active layer. A traveling wave type semiconductor laser amplifier which is incident from the light incident end, injects a current into the active layer and amplifies the optical signal by a stimulated emission effect, wherein the active layer has at least one side in a thickness increasing direction. A traveling-wave semiconductor laser amplifier, comprising: an optical waveguide layer having a band gap larger than that of an active layer; and a structure in which the active layer is a thin layer having a thickness of 0.11 μm or less.