Encased semiconductor laser device in contact with a fluid...

Details Encased semiconductor laser device in contact with a fluid... Encased semiconductor laser device in contact with a fluid...

2000-06-13

2002-05-28

Ip, Paul (Department: 2828)

Coherent light generators

Particular beam control device

Tuning

C372S035000, C372S028000, C372S034000, C372S032000, C356S329000

Reexamination Certificate

active

06396854

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a laser device and a method of producing a semiconductor device and, more particularly, to a laser device that allows adjustment of characteristic values of a semiconductor laser element after producing the laser, and a method of producing the laser device.
BACKGROUND ART
As the society becomes increasingly dependent on the information technologies, various communications apparatuses are being imposed with requirements to have multimedia capabilities. Thus processing speeds and data handling capacities of these apparatuses have been increased, and attempts have been made for the application of wavelength division multiplexing transmission technology to optical signal transmission over trunk lines such as submarine cables.
In the wavelength division multiplexing transmission, distributed feedback semiconductor laser devices (hereafter referred to as DFB-LD) is used as the light source with 20 to 100 DFB-LDs of different oscillation wavelengths being arranged in an array, each oscillating to emit light at a predetermined wavelength that is transmitted by an optical cable and combined with other light in a coupler. It is therefore important in achieving wavelength division multiplexing transmission to stabilize the oscillation wavelength of each DFB-LD and accurately determine the intervals between the oscillation wavelengths.
A band of wavelengths used in the wavelength division multiplexing transmission is defined according to a recommendation by ITU-T (International Telecommunications Union, Telecommunication Standardization Sector), with the interval between adjacent wavelengths being regulated to be 0.8 nm. Accordingly, it is recognized that the DFB-LD used as the light source preferably oscillates at a wavelength within ±0.1 nm of the defined wavelength.
However, the DFB-LD of the prior art has a problem that, since the accuracy of oscillation wavelength has been limited to about 1 nm because the processing margin is insufficient when producing an element while setting a particular wavelength in the processes of crystal growth and wafer processing, use of the DFB-LD as the light source for wavelength division multiplexing transmission has been impractical for the reason of the accuracy of oscillation wavelength.
Sudoh et al. (ELECTRONICS LETTERS; Jan. 30, 1997 Vol.33, No.3, p.216-p.217) have recently reported a DFB-LD that enabled it to tune the oscillation wavelength to a desired value by a method as follows: A part of an optical waveguide of a DFB-LD is provided with a film made of a material that changes refractive index depending on the heat generated by laser irradiation, and the refractive index of the film is changed by irradiating it with a laser beam while measuring the oscillation wavelength of the DFB-LD after the laser element has been made, thereby changing the effective refractive index of the waveguide and achieving the desired oscillation wavelength.
Although demands for laser elements having more accurate oscillation wavelengths are increasing, the demands cannot be satisfied in the production process only. Thus such techniques have been developed that achieve laser elements having the desired oscillation wavelengths by adjusting the oscillation wavelength after producing the laser element.
Adjustment of the characteristic values to be done after producing the laser element includes, in addition to the oscillation wavelength of the DFB-LD, adjustment of other characteristic values such as the photon density in an active layer of a ridge type semiconductor laser.
FIG. 10
is a sectional view showing the structure of a DFB-LD of the prior art.
In
FIG. 10
, reference numeral
1
denotes an n-InP substrate,
2
denotes an n-InP buffer layer,
3
denotes an n-InGaAsP light confinement layer,
4
denotes an MQW active layer,
5
denotes a p-InGaAsP light confinement layer,
6
denotes a diffraction grating layer,
7
denotes a p-InP first cladding layer,
8
denotes an Fe-doped InP embedding layer,
9
denotes an n-InP embedding layer,
10
denotes a p-InP second cladding layer,
11
denotes a p-InGaAs contact layer,
12
denotes an SiO2 insulating film,.
13
denotes a Cr/Au vapor deposited film and
14
denotes an anode of Au-plating layer.
15
denotes a metallic vapor deposited film and
16
denotes a cathode of Au-plating layer provided on the surface of the metallic vapor deposited film.
Oscillation wavelength &lgr; of the LD having the structure described above is given as follows assuming the effective refractive index neff of the optical waveguide and the interval &Lgr; of the diffraction grating.
&lgr;=2·neff·&Lgr;
When &Lgr; is 240 nm and neff is 3.23, for example, then &lgr; is 1550.4 nm.
Factors that determine the value of neff include the distribution of refractive index of the material in a region from which light leaks out that is a circular area about 2 &mgr;m in diameter, and particularly important are composition and film thickness of the n-InGaAsP light confinement layer
3
, the MQW active layer
4
and the p-InGaAs light confinement layer
5
that constitute the optical waveguide and the width of the optical waveguide.
However, the DFB-LD shown in
FIG. 10
is difficult to produce due to variations in the process, while maintaining uniform conditions for the composition and film thickness of the n-InGaAsP light confinement layer
3
, the MQW active layer
4
and the p-InGaAs light confinement layer
5
that constitute the optical waveguide and the width of the optical waveguide that are the factors which determine the value of neff, and therefore it has been difficult to produce the DFB-LD having oscillation wavelength of accuracy within ±0.1 nm due to the variation in neff.
One of the solutions for this problem is the wavelength tuning DFB-LD structure proposed by Sudoh et al. described previously.
FIG. 11
is a sectional view showing the structure of the wavelength tuning DFB-LD of the prior art.
In
FIG. 11
, reference numeral
21
denotes an n
+
-InP substrate,
22
denotes an n-InP layer,
23
denotes an active layer,
24
denotes a diffraction grating layer,
25
denotes a p-InP layer,
26
denotes a p
+
-InGaAs layer,
27
denotes Ti/Au electrode,
28
denotes a wavelength control film made of As
4
Se
5
Ge
1
and
29
denotes an Al
2
O
3
film.
When the wavelength control film
28
was irradiated with light emitted by a He-Ne laser of wavelength 632.8 nm with power density of 1.3 W/cm
2
, a wavelength shift of 0.14 nm was observed.
FIG. 12
shows an embedding type DFB-LD of wavelength tuning type of the prior art which is a modification of the wavelength tuning DFB-LD structure proposed by Sudoh et al. turned into an embedding type DFB-LD shown in FIG.
10
.
In
FIG. 12
, reference numerals identical with those used in FIG.
10
and
FIG. 11
denote the same or corresponding components.
Changes in the refractive index of As
4
Se
5
Ge
1
that constitutes the wavelength control film
28
due to the wavelength of Ar laser light are reported in the ELECTRONICS LETTERS mentioned previously by Sudoh et al, indicating that maximum change in the refractive index at wavelength 1.55 &mgr;m was 0.027.
Thus assuming that width w of the optical waveguide of
FIG. 11
is 1.3 &mgr;m, total width W of the optical waveguide including the embedding layers is 1.7 &mgr;m and thickness of the wavelength control film
28
is 0.5 &mgr;m, then the range of neff values of the adjustable effective refractive index determined upon computation of the light propagation mode is from 3.18716 to 3.18728. Consequently, when &Lgr; is 240 nm, adjustable range of the oscillation wavelengths is from 1529.84 nm to 1529.89 nm, giving a tunable band of 0.05 nm.
However, when the wavelength control film made of As
4
Se
5
Ge
1
is used as in the method described above, since the change in the refractive index of As
4
Se
5
Ge
1
caused by laser irradiation is an irreversible change, refractive index of the wavelength control film made of AS
4
Se
5
Ge
1
which has once decreased cannot be increased

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