Coherent light generators – Particular temperature control – Heat sink
Reexamination Certificate
2002-03-08
2004-07-06
Davie, James W. (Department: 2828)
Coherent light generators
Particular temperature control
Heat sink
Reexamination Certificate
active
06760352
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a semiconductor laser device and a semiconductor laser module provided with a semiconductor laser element for outputting a laser beam having a plurality of oscillation longitudinal modes.
BACKGROUND OF THE INVENTION
The recent and rapid spread of the Internet and sudden increase of connection between in-company LANs, has resulted in an increase of the number of communication calls and in an increase in data traffic. This increase in traffic has stressed current optical systems. To prevent the communication performance from deteriorating, the use wavelength division multiplexing (WDM) technologies has advanced and spread.
WDM systems support transmission volumes 100 times larger than the capacity of conventional fiber optic communications by superimposing a plurality of optical signals at different wavelengths on a single fiber. Current WDM systems are capable of long distance transmissions by performing optical amplification with an erbium-doped fiber amplifier (EDFA) or Raman amplifier. An EDFA is an optical fiber amplifier with erbium added. When light having a wavelength of a 1,550 nm band and serving as a transmission signal passes through the EDFA, an additional light emitted by an exciting laser with a wavelength of 1,480 nm or 980 nm is introduced to amplify the signal.
A Raman amplifier is an amplifier capable of directly amplifying signal light by using an already laid optical fiber as an amplifying medium and introducing amplifying light via using stimulated Raman scattering.
Typically for long distance optical transmission with a WDM system, the interval between repeaters must be small due to inefficient amplification. With more repeaters, costs increase. Therefore, to be able to increase the interval between repeaters, one can either increase the output of a semiconductor laser device used for a signal light source and/or improve the amplifying capacity of the repeater.
To meet the above requirements, a semiconductor laser element capable of outputting a laser beam of 250 mW or more is used for EDFA excitation. This higher power level requires high reliability from the semiconductor laser devices.
In the case of a WDM system it is especially important to maintain highly accurate oscillation control and high output operation, not only for the signal light source, but also for the exciting light source used for amplification. The heat of a semiconductor laser element produced due to current injection is known to be a large factor for degrading the oscillation control and high output operation. To compensate for this thermal degradation problem, various conventional approaches are used.
For example, in the case of a conventional semiconductor laser device, a thermistor for measuring the temperature of a semiconductor laser element is often set nearby the semiconductor laser element so that the temperature of the semiconductor laser element can be controlled by an electrothermal element such as a Peltier element.
FIG. 18
is a front view showing a schematic configuration of a conventional semiconductor laser device. In
FIG. 18
, a submount
102
formed by AlN having an insulating property and a high heat conductivity is set on a carrier
101
formed by CuW. A semiconductor laser element
103
for outputting a laser beam of a predetermined wavelength is set on the submount
102
. A submount
104
formed by AlN is set on the carrier
101
and, a thermistor
105
for measuring the temperature of the semiconductor laser element
103
is set on the submount
104
.
The semiconductor laser element
103
and the submount
102
are joined to each other through a metallic thin film
102
a
. The metallic thin film
102
a
contains layers of Ti, Pt, and Au at thicknesses of 60 nm, 200 nm, and 600 nm respectively. The semiconductor laser element
103
and submount
102
are joined on the metallic thin film
102
by a solder material such as AuSn. The thermistor
105
and submount
104
are also joined through a metallic thin film
104
a.
The face of the semiconductor laser element
103
to be joined with the submount
102
serves as a p-side electrode and the upper face serves as an n-side electrode. The semiconductor laser element
103
is set so that the active layer serving as a main heat generating source is present at the p-side electrode side and located nearby the submount
102
. The n-side electrode is connected to a negative electrode by an Au wire
106
a
. The p-side electrode is connected to the carrier
101
at the positive electrode side through the metallic thin film
102
a
and an Au wire
106
b.
Submount
102
secures the insulation of the semiconductor laser element
103
and functions as a heat sink of the semiconductor laser element
103
. In the case of the carrier
101
, as illustrated, the bottom is joined to a CuW base
110
by AuSn solder. The base
110
is set on a Peltier element
120
. The Peltier element
120
is controlled by a temperature control section (not illustrated) correspondingly to the temperature detected by the thermistor
105
. As a result, the temperature of the semiconductor laser element
103
is controlled by the thermistor
105
, Peltier element
120
, and the temperature control section.
The thermistor
105
is also insulated from the carrier
101
by the submount
104
to detect the temperature of the semiconductor laser element
103
through the submount
102
, carrier
101
, and submount
104
, each of which has a high heat conductivity.
The heat generated in the semiconductor laser element
103
is conducted to the thermistor
105
through the metallic thin film
102
a
, submount
102
, metallic thin film
102
b
, carrier
101
, metallic thin film
104
b
, submount
104
, and metallic thin film
104
a
in order. As recognized by the present inventors the heat conducting distance degrades overall operations as the actual temperature detection of the semiconductor laser element
103
is delayed. Moreover, the generated heat passes through the metallic thin films
102
a
,
102
b
,
104
b
, and
104
a
that are joined to each other by four AuSn solder joints. However, because the AuSn solder joints are used for the junction they respectively have a large heat joints resistance, and so the heat resistance of the above heat conducting path is increased. Moreover, because of the deterioration of the temperature detection accuracy, the temperature control accuracy is also deteriorated. Thus, in conventional WDM applications, the oscillation wavelength of the semiconductor laser element
103
is prone to becoming unstable due to heat generated at high powers, and the system optical output and service life are deteriorated.
Another limitation is present in conventional WDM applications. When supplying current to a semiconductor laser element in order to obtain a high optical output, a voltage drop of an Au thin film in the metallic thin film
102
a
occurs. Assuming the total resistance of the Au thin film as 0.12 &OHgr;, and an inter electrode voltage of a semiconductor laser element when a current of 1 A circulates through the semiconductor laser element equal to approximately 2V, the voltage drop of the semiconductor laser element in the resonator length direction becomes non-uniform by 0.12 V. This leads to the current injection to the semiconductor laser element to become non-uniform and the light density in the active layer also to become non-uniform. The present inventors have discovered this to accelerate deterioration of a device's optical output and service life.
All of the above described limitations are more pronounced in a semiconductor laser element that includes a diffraction grating. Examples of such laser elements include those disclosed in Japanese Patent Application No. 2000-323118, Japanese Patent Application No. 2001-134545, and Japanese Patent Application No. 2002-228669 filed on Oct. 28, 2000, May 1, 2001 and Jul. 27, 2001 respectively in the Japanese Patent Office, the entire contents of which being incorporated herein by reference. These types of laser
Tsukiji Naoki
Wakisaka Tsuyoshi
Yoshida Junji
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