Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
2002-06-27
2004-10-19
Wong, Don (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S032000, C372S029011, C372S029015
Reexamination Certificate
active
06807199
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for inspecting conditions under which semiconductor laser diodes (hereinafter called LDs) output a fixed wavelength and a unit for the same.
2. Description of the Related Art
FIG. 1
shows the construction of a general distributed feedback LD. In the LD, when a current flows into the PN junction portion in an active layer
11
from the upper electrode
12
to the lower electrode
12
and the recombination of electrons and holes takes place in the vicinity of the junction, the spontaneous emission of light corresponding to the band gap of the semiconductor in the active layer takes place. When the light having certain wavelength in accord with an interval of groove
13
travels in LD, optical amplification takes place and LD oscillates at the wavelength. It is well known that the change of the temperature of the device or heat generated due to injected current generally makes occurrence of expansion or contraction of the semiconductor crystal such as the active layer and the grooves of LD, and that the optical characteristics of thus oscillating LD depends on the temperature of the device or heat generated due to injected current as above described, so, as a result, the oscillation wavelength is shifted. Furthermore, the amount of the shift of the oscillation wavelength of each LD may be somewhat varied in accordance with the accuracy of the interval of the groove, the value of the drive current, and the crystal structure of the active layer. Therefore, it is required to inspect precisely the temperature dependency of the oscillation wavelength of each of the LDs.
FIG. 2
shows a wavelength inspection unit.
In the drawing, an LD module
1
is shown, and, inside the module
1
, an LD, a laser mount for mounting the LD, a thermistor for monitoring the temperature of a heat sink portion to exhaust heat of the mount, and a Peltier element to cool and heat the laser mount (not shown) are mounted. A measurement jig
2
contains an electrical and thermal interface to the LD module
1
. A laser driving power source
4
, a temperature controller
5
, and an photo detection portion
6
are put together in a current-light output measurement unit (hereinafter called an I-L measuring unit)
3
. The photo detection portion
6
monitors the optical power of the LD through a fiber
9
. A wavelength measurement apparatus
7
counts the output wavelength of the LD. A computer
8
gives control data to each device, receives data obtained at each device, and processes the data and controls the system in accordance with the measurement procedures.
FIG. 3
shows a conventional example of a wavelength tuning process for tuning an LD to a fixed wavelength in a case in which the wavelength inspection unit is used and a wavelength varying item is a laser temperature. In this case, since the laser temperature cannot be directly measured, the temperature of the heat sink on which a LD is mounted is defined as the laser temperature.
For example, the initial temperature of the heat sink portion of the LD is set at 25° C. A current I, which is larger than a threshold current for laser oscillation, is applied to the LD, and a wavelength &lgr;
0
is measured. The ratio of the change of oscillation wavelength to the change of heat sink temperature is defined as a wavelength coefficient, and its initial value is defined as a basic wavelength coefficient A
0
. Therefore, a heat sink temperature T
1
is required to be (&lgr;
T
−&lgr;
0
)/A
0
+25° C. in order to obtain a target wavelength &lgr;
T
. A wavelength &lgr;
1
is measured at the temperature, and it is checked whether or not the difference between the wavelength &lgr;
1
and the target wavelength &lgr;
T
is in an allowable range. If it is out of specification, it is checked whether the difference is in another fine adjustment window (the range of wavelength is slightly wider than the allowable range). If the difference is within the another fine adjustment window, the wavelength &lgr;
1
can be broght to the target wavelength &lgr;
T
by changing the temperature a little. Then, a process for fine tuning, in which the temperature of the heat sink portion is changed by a certain temperature step and the measurement is continued until the oscillation wavelength reaches the target wavelength, is performed. On the other hand, if the difference is out of the fine adjustment window, the heat sink temperature T
2
is set to (&lgr;
T
&lgr;
1
)/A
0
+T
1
by using the target wavelength &lgr;
T
and the basic coefficient A
0
. Hereinafter, in the same way, an nth wavelength &lgr;
n
is measured, and then the wavelength &lgr;
n
is brought close to the target wavelength &lgr;
T
by continuing the operation until the difference comes within the allowable range or the fine adjustment window.
SUMMARY OF THE INVENTION
In the conventional method, it is possible to set the heat sink temperature for making an LD oscillated at a wavelength close to a target wavelength, but it is required to adjust the oscillation wavelength of the LD to the target wavelength in a further shorter time.
The initial basic wavelength coefficient used in the conventional method may be a theoretical value or an actual value, and it is a value which is characterized in that the amount of change of a wavelength is made to correspond to the amount of change of a wavelength varying item. However, the actual wavelength coefficient of each LD is different from each other. Therefore, when wavelength coefficients which have large differences from each other are used, the wavelength varying item is needed to be changed by large amount to result in a large difference in wavelength. Accordingly, a number of trial is required in order to reach the target wavelength.
FIG. 4
shows the difference between the conventional tuning method and a tuning method of the present invention. The solid line shows a characteristic of wavelength to temperature having a basic wavelength coefficient A
0
, and the dotted line shows the characteristic of wavelength to temperature of an LD to be tested. When the temperature of a heat sink portion is T
0
and an oscillation wavelength is &lgr;
0
at the beginning, the temperature T
1
at the crosspoint of the straight line having the basic wavelength coefficient A
0
and a straight line through a target wavelength &lgr;
T
is defined as a first setting temperature of the heat sink. An oscillation wavelength &lgr;
1
at this crosspoint is shown by point B. In the conventional method, a straight line having the same inclination as the basic wavelength coefficient A
0
is drawn from point B and the temperature is set to the temperature pointed at the crosspoint C
1
of the straight line and the straight line through the target wavelength &lgr;
T
to obtain the point D
1
for a second wavelength. On the other hand, in the present invention, an alternate long and short dash line having the inclination defined as the first wavelength coefficient A
1
from point B is drawn, and the crosspoint C
2
of the line and the straight line through the target wavelength &lgr;
T
is obtained. Since the wavelength when the temperature is set at this C
2
is a wavelength pointed by D
2
, it is clearly understood that the wavelength pointed by D
2
is closer to the target wavelength &lgr;
T
than the one pointed by D
1
.
In a wavelength inspection method of the present invention, tuning is performed such that a corrective wavelength coefficient in stead of the basic wavelength coefficient is obtained based on the amount of change of a wavelength varying item and the amount of change of a measured wavelength to the target wavelength, which is arisen from the above change of the wavelength varying item.
Furthermore, in a wavelength inspection method according to the present invention, a corrective wavelength coefficient is obtained in tuning of the first time and, after that, tuning is performed by using the so obtained corrective wavelength coefficient as a basic wavelength coefficient.
Furthe
Baba Isao
Ono Haruyoshi
Fujitsu Quantum Devices Limited
Menefee James
Westerman Hattori Daniels & Adrian LLP
Wong Don
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