Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2000-02-25

2004-10-26

Vu, David (Department: 2828)

Coherent light generators

Particular resonant cavity

Distributed feedback

C372S045013, C372S075000, C372S072000, C372S099000

Reexamination Certificate

active

06810063

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device, and more specifically, to a semiconductor laser device adapted for use as a pumping light source for optical fiber amplifier, such as a laser device of the gain-waveguide type that oscillates with a wavelength of, e.g., 0.98 &mgr;m and requires high optical output of hundreds of mW, and capable of ensuring a linear current-optical output characteristic even during high-current operation.
2. Prior Art
In a semiconductor laser device that has an active layer (quantum region) formed of a quantum well structure, injected carriers are quantized toward quantum wells, and the state density of carrier energy is stepped. Accordingly, the gain coefficient suddenly rises in response to driving current, so that a laser beam can be oscillated even with use of a low threshold current density. The semiconductor laser device of this type delivers higher optical output than a semiconductor laser device that includes an active layer of a bulk semiconductor, so that it is being studied for practical use as a pumping light source for optical fiber amplifier.
The following semiconductor laser device that oscillates with a wavelength of 0.98 &mgr;m was an object of investigation by the inventors as a pumping light source for optical fiber amplifier. This device will now be described with reference to the accompanying drawings.
FIG. 1
is a side view showing the semiconductor laser device, and
FIG. 2
is a sectional view taken along line II—II of FIG.
1
.
The device has a layer structure of a semiconductor material, including a lower clad layer
2
of n-AlGaAs, an active layer
3
of a quantum-well structure made of InGaAs and GaAs, an upper clad layer
4
of p-AlGaAs, and a cap layer
5
of p-GaAs, which are stacked in layers on an n-GaAs substrate
1
. A part of the upper clad layer
4
and the cap layer
5
form a mesa structure, and a passivation film
6
of SiN is formed on the lateral of the mesa structure. Further, an upper electrode
7
of Ti/Pt/Au is formed on the cap layer
5
and the passivation film
6
, and a lower electrode
8
of AuGe/Ni/Au is formed on the back surface of the substrate
1
.
The device A is manufactured in the following manner. The aforesaid layer structure is formed on the n-GaAs substrate by, for example, the MOCVD method, and the upper and lower electrodes are formed on the upper and lower surfaces, respectively, of the layer structure. Thereafter, the resulting structure is cleft with a given cavity length L, a low-reflection film
9
of, e.g., SiN is formed on one end face (front facet) S
1
of the structure, and a high-reflection film
10
of, e.g., SiO
2
/Si is formed on the other end face (rear facet) S
2
.
In the case of the device A having this mesa structure, it is believed that high optical output can be effectively obtained by increasing the cavity length L. This is because if the cavity length L increases, the influence of heat can be lessened, so that high-optical output can be expected. If the cavity length is too long, however, the differential quantum efficiency of the device A lowers, so that higher current is required for high-optical output operation. Normally, therefore, the cavity length L of the device A with this construction is designed so that the cavity length L is not longer than 1,000 &mgr;m.
The inventors hereof examined the current-optical output characteristic for the case where the cavity length L of the device A with the layer structure shown in
FIGS. 1 and 2
was adjusted to 800 &mgr;m. Thereupon, the characteristic curve of FIG.
3
and the following new knowledge were obtained.
When a driving current (A
1
) of about 200 mA was injected, as seen from
FIG. 3
, a first kink (a
1
) was generated in the optical output, and the existing linear relation between the driving current and the optical output disappeared. If the driving current was further increased to a level (A
2
) of about 500 mA, a second kink (a
2
) was generated in the optical output. Thus, in the case of the device A, the two kinks a
1
and a
2
were generated in the current-optical output characteristic curve as the driving current was increased.
Accordingly, the inventors hereof first closely examined the oscillation spectrum of the device A. The following is a description of the results of the examination.
(1)
FIG. 4
shows an oscillation spectrum obtained when the injected current was at about 200 mA.
As seen from this oscillation spectrum, there is a small number of longitudinal modes which oscillate actually in a gain band g. The intensity of a central longitudinal oscillation mode B
0
is 5 dB or more higher than those of side modes B
1
and B
2
. As a whole, single longitudinal mode oscillation that is prescribed by the central longitudinal oscillation mode B
0
is dominant.
(2) An oscillation spectrum obtained when the first kink (a
1
) was generated indicates that the central longitudinal oscillation mode B
0
jumps to the side mode B
1
at a distance of about 0.4 nm therefrom when the gain band shifts to the longer wavelength side as the temperature of the device rises with the increase of the injected current.
The probability of generation of single longitudinal mode oscillation is related to a spontaneous emission factor (&bgr;sp) given by
&bgr;
sp=&Ggr;·&lgr;
4
·K/
4&pgr;
2
·n
3
·V·&dgr;&lgr;,
  (1)
where &Ggr; is the confinement coefficient of the active layer, &lgr; is an oscillation wavelength, K is a factor reflective of the complexity of the electric field for a transverse mode, n is an equivalent refractive index, V is the volume of the active layer, and &dgr;&lgr; is the half width of the spontaneous emission spectrum. It is believed that the smaller the value &bgr;sp, the higher the probability of generation of single longitudinal mode oscillation is.
In the case of the device A, therefore, the oscillation wavelength (&lgr;) is as short as 0.98 &mgr;m, so that &bgr;sp is lowered in proportion to the fourth power of &lgr;. Accordingly, the device A can be supposed to be able to cause single longitudinal mode oscillation with high probability.
The following problem will be aroused, however, if a module is constructed in a manner such that the device A that undergoes single longitudinal mode oscillation is connected to an optical fiber. A laser beam generated by single longitudinal mode oscillation has its noise properties lowered under the influence of return light from an end portion of the optical fiber. Further, the oscillation of the laser beam is made unstable by the return light. Accordingly, an optical output fetched from the module and monitor current are rendered unstable.
In order to use the device A as a reliable pumping light source for optical fiber amplifier, therefore, it is necessary to solve the above problem that is attributable to single longitudinal mode oscillation.
The result (2) implies the following situation. In consideration of gain differences caused between the longitudinal modes for single longitudinal mode oscillation for the aforesaid reason, the longitudinal mode hopping occur which causes substantial discontinuous fluctuations of the optical output when the gain band shifts to the longer wavelength side in response to temperature rise. When the injected current almost reaches the level A
1
, therefore, the current-optical output characteristic loses its linearity, so that the first kink (a
1
) is generated.
Then, the inventors hereof observed a far field pattern of the device A and obtained the findings shown in FIG.
5
.
In
FIG. 5
, curve C
1
represents a transverse oscillation mode for the case where the injected current is lower than A
2
, and curve C
2
represents a transverse oscillation mode for the case where the injected current is near A
2
(or where the second kink a
2
is generated).
If the injected current increases to A
2
, as seen from
FIG. 5
, unit-modal transverse oscillation modes shift horizontally from the center position of the device A (or undergo beam ste

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