Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1998-07-15
2002-02-12
Leung, Quyen (Department: 2881)
Coherent light generators
Particular active media
Semiconductor
C372S050121
Reexamination Certificate
active
06347107
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to the field of lasers, and specifically to laser diodes that contain an integral photodiode for monitoring and controlling laser intensity.
BACKGROUND OF THE INVENTION
Laser diodes have been manufactured for years and most contain a silicon photodiode inside a laser can. This photodiode (PD) is used to monitor the power of the laser and can be connected to a feedback circuit to coarsely control the laser intensity as the laser heats up.
FIG. 1
shows a semiconductor laser or laser diode
9
with a monitor photodiode
11
inside a sealed can defined by a cap
15
. Monitor photodiode
11
is attached to a stem
21
. A semiconductor laser element or laser chip
17
is mounted on a heat sink
19
, which is attached to stem
21
. This configuration is covered by a container or cap
15
, which has window
23
which allows passage of laser light.
As also shown in
FIG. 1
, semiconductor laser
9
includes a terminal
25
which provides a connection to a control circuit
27
. Terminal
25
includes a first line
25
a
which leads to laser element
17
, a second line
25
b
which leads to sensor or photodiode
11
, and a third line
25
c
which is a ground wire. Control circuit
27
energizes semiconductor laser element
17
in a known manner so as to emit a laser beam
100
with a light power output P
0
through window
23
. As further illustrated in
FIG. 1
, photodiode
11
is fixed to stem
21
with its light receiving surface
11
a
facing laser element
17
. Laser beam
100
with light power output P
0
is emitted from laser element
17
through window
23
and at the same time, a monitor beam
110
with a light power output of about 3% of P
0
is emitted from laser element
17
toward photodiode
11
. Monitor beam
110
incident on the monitor photodiode
11
generates a monitor signal. This monitor signal is fed back to control circuit
27
for driving laser element
17
so that the power output P
0
of the laser beam
100
emitted from the laser element
17
is maintained in a stable state. A typical example of a control circuit as described above is shown in, for example, U.S. Pat. No. 5,067,117.
In early designs, laser diodes were made in a wafer and laser elements
17
were scribed and cleaved to form front and back facets. These facets were not coated but had a reflectivity of 30% to 35% because of the high index of the semiconductor (GaAs, AlGaAs) material. This reflectivity was sufficient to attain a round trip gain of greater than unity to establish laser action. A problem with this design is that half of the light came out the front facet of the laser element, and the other half came out the back to strike the photodiode. An example of such a laser is the Hitachi 8312. In such a laser, the photodiode output appeared to track reasonably well with the laser output and a measurement of photodiode current vs. laser power was very linear.
In an effort to provide more power, laser manufacturers started to introduce lasers with dielectric mirrors on the back facet and modest anti-reflection (AR) coatings on the front facet. The mirror directed most of the light out the front of the laser, while allowing a small amount to leak through the back facet to the photodiode which was used to monitor and control laser power.
FIG. 2
shows a conventional laser element or laser chip design
32
with a multilayer mirror on the back facet. The structure of
FIG. 2
comprises a laser element
34
with a rear facet multilayer mirror
36
comprised of alternating amorphous silicon and aluminum oxide layers; and a front facet AR (aluminum oxide) coating
38
. A rated laser power P
0
(40), is emitted from the front facet, and a small fraction, P
0
/30 (42) is emitted from the rear facet to the photodiode.
The Hitachi 8314 laser has basically the same structure as the Hitachi 8312 laser, except that a multilayer mirror and an AR coating were applied as shown in FIG.
2
. The output power available was effectively doubled, but a measurement of photodiode current vs. laser power showed some non-linearity even if the laser stayed in a single longitudinal mode. This led to problems with respect to the photodiode current not being well correlated to the laser front facet power. At times it would be well correlated, and then behave erratically and unpredictably. In order to achieve a tight control of laser power, some laser arrangements included a complicated means of sampling the front facet beam with a prism or window to provide a well correlated signal for power control (see, for example, U.S. Pat. Nos. 5,067,117, 4,989,198, and 5,363,363).
Conventional multilayer periodic mirror systems are made of quarter wave thick layers of alternating high index and low index films. Versions of this structure are used by laser manufacturers to make high reflectivity mirrors for the back facet of the laser as shown in FIG.
2
. Such structures can be fairly uniform in reflectivity for a large range of wavelengths. However, it is the transmittance of coherent light in such structures that is the cause of the erratic behavior of the back facet photodiode output. When the laser heats up causing mode drift or mode hops, the wavelength changes, slowly and predictably in the first case and rapidly and erratically in the second case. When the laser is mode hopping, the correlation between the laser power and the photodiode current tends to be poor.
A back facet mirror made up of multiple quarter wavelength layers as described above can act as a Fabry-Perot etalon or an optical bandpass filter. The undesirable Fabry-Perot effect of the multilayer mirror can be better understood by examining the effect of a more typical Fabry-Perot etalon such as a glass plate.
FIG. 3
shows such an etalon which is basically a parallel plane glass plate
50
in a path of a laser beam
52
emitted by a laser
53
. Laser beam
52
passes through glass plate
50
and strikes a photodetector
54
. The power transmitted by glass plate
50
and measured by detector
54
is a function of the wavelength of laser
53
, due to constructive and destructive interference within the glass plate
50
from reflections from the two surfaces of glass plate
50
.
FIG. 4
illustrates a graph which shows the transmittance of glass plate
50
as a function of the wavelength and as a function of the reflectivity of the two surfaces of glass plate
50
, and is effectively the light level at detector
54
of FIG.
3
. As the reflectivity of the surfaces of glass plate
50
increases, the transmitted peaks narrow and the variation between the maximum and minimum level of light transmitted increases. In other words, the Fabry-Perot effect increases with reflectivity. Even a low reflectivity of about 4% has a noticeable effect. As the laser mode hops and changes wavelength, the light transmitted through the back facet mirror can vary even though the laser output from the front facet remains constant. There are three approaches to decrease the Fabry-Perot effect: (1) decrease the reflectivity of the surfaces of the glass plate, (2) add neutral density between the two surfaces of the glass plate to reduce the interference, and (3) tilt the glass plate in the beam such that the transmitted beam and the reflected beam do not overlap each other. In a laser diode, the last two approaches (2 and 3) are not easy to implement. The first approach (1) which may involve decreasing the reflectivity to as low as 4% will still not eliminate the Fabry-Perot effect, but it will inhibit the laser from lasing. Thus, the original cleaved, uncoated facet is preferable to a low reflectivity multilayer mirror.
In view of the above discussion, it can now be seen why the back facet diode current sometimes tracks the laser output very well and sometimes behaves very erratically. As the laser changes wavelength, the light transmitted to the back facet photodiode can vary substantially even though the laser output power has not changed.
For product designs where the back facet photodiode provides inadequate control, compli
Blish Nelson A.
Roddy James E.
Blish Nelson Adrian
Eastman Kodak Company
Leung Quyen
Novais David A.
LandOfFree
System and method of improving intensity control of laser... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with System and method of improving intensity control of laser..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and System and method of improving intensity control of laser... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2960966