Coherent light generators – Particular temperature control
Reexamination Certificate
2000-02-11
2002-11-05
Ip, Paul (Department: 2828)
Coherent light generators
Particular temperature control
C372S029010, C372S029011, C372S029020, C372S038010, C372S038020, C372S031000
Reexamination Certificate
active
06477190
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention generally relates to optical modules, and more particularly to an optical module capable of stabilizing the wavelengths of light output signals.
A wavelength division multiplexing system (hereinafter referred to as a WDM system) using the wavelength division multiplexing technique has an increased transmission capacity as the number of wavelengths to be multiplexed increases. Thus, it is required to reduce the intervals of the wavelengths of the light signals in order to increase the channel capacity. However, it is necessary to improve the precision of the wavelengths of the light signals output from an optical module In order to reduce the intervals of the wavelengths of the light signals.
A conventional optical module has a module construction designed to suppress a time-based variation in the wavelength of a laser diode source or a variation therein due to the peripheral temperature and to lock the wavelength of the light signal to be output. An example of an optical module having such a module construction is an optical module having a wavelength locking function of suppressing a wavelength variation of the optical signal. The wavelength locking function is implemented by, for example, a wavelength detection module called wavelength locker.
A description will now be given, with reference to
FIGS. 1 and 2
, of an optical module which does not have a built-in locker.
FIG. 1
is a side view of such an optical module
1
, and
FIG. 2
is a top view thereof.
The optical module
1
includes a laser diode (LD) element
10
, an LD carrier
11
, a photodiode (PD) carrier
12
, a monitor photodiode (PD)
13
, a thermoelectric (TEC) element
14
, a first lens
15
, a thermistor (temperature sensing resistor)
16
, a mount carrier
17
, an optical isolator
18
, and a second lens
19
.
The LD element
10
, which is a light-emitting element, is mounted on the LD carrier
11
, and outputs light signals in the back and forth directions. The light signal emitted forward is collimated by the first lens
15
placed on the mount carrier
17
, and is supplied to the optical isolator
18
.
The optical isolator
18
allows the forward light signal supplied from the first lens
15
to pass therethrough, and shuts off the reflected light supplied from the second lens
19
in the backward direction. The light signal passing through the optical isolator
18
is focused by the second lens
19
, and is supplied to the optical fiber
20
.
The light signal output from the LD element
10
in the backward direction is monitored by the monitor PD
13
mounted on the PD carrier
12
, and is utilized to perform automatic power control (APC) directed to controlling the forward light signal output at a constant level.
The above-mentioned LD carrier
11
, the PD carrier
12
and the first lens
15
are mounted on the TEC element
14
via the mount carrier
17
. The thermistor
16
is mounted on the mount carrier
17
, and monitors the temperature in the vicinity of the LD element
10
.
A description will now be given, with reference to
FIGS. 3 and 4
, of an optical module having a built-in wavelength locker.
FIG. 3
is a side view of such an optical module
2
, and
FIG. 4
is a top view thereof. The optical module
2
is the same as the optical module
1
except for some parts, and parts that are the same as those shown in
FIGS. 1 and 2
are given the same reference numbers,
The optical module
2
includes the LD element
10
, the LD carrier
11
, the PD carrier
12
, the monitor PD
13
, the TEC element
14
, the first lens
15
, the mount carrier
17
, the optical isolator
18
, the second lens
19
, a back lens
21
, a PD carrier
22
, an optical filter
23
, a beam splitter (BS)
24
, and a monitor PD
25
.
The light signal backward emitted from the LD element
10
is focused by the back lens
21
and is supplied to the beam splitter
24
. Then, the beam splitter
24
reflects part of the received light signal, and allows the remaining part thereof to pass therethrough. Thus, the backward light signal is split into two lights. One of the two light beams thus split is monitored by the monitor PD
25
mounted on the PD carrier
22
, and is utilized to perform the APC control directed to controlling the light signal output emitted forward to a constant level. The other of the two split-light signals is supplied, via the optical filter
23
, to the monitor PD
13
mounted on the PD carrier
12
.
The optical filter
23
used in the optical module
2
has a transmittance characteristic which is inclined to the wavelength of the light signal. For example, the optical filter
23
is an etalon filter (Fabry-Perot etalon filter), a lowpass filter, highpass filter, or a bandpass filter. A wavelength fixing control method of locking the wavelength of the light signal output from the LD element
10
is implemented using the outputs of the monitor PD
13
and the monitor PD
25
.
FIG. 5
is a block diagram illustrating an example of the wavelength fixing control. The light signal backward emitted from the LD element
10
is partially reflected by a beam splatter
24
-
1
, and is then supplied to the monitor PD
25
. Of the light signal backward emitted from the LD element
10
, the light signal passing through the beam splitter
24
-
1
is reflected by a beam splitter
24
-
2
, and is supplied to the monitor PD
13
via a bandpass filter used as the optical filter
23
. The monitor PDs
13
and
25
supply monitor currents as shown in
FIG. 6
to a divider circuit
26
, which will be described later.
FIG. 6
is a graph of a monitor current vs. oscillation wavelength characteristic.
The monitor current output from the monitor PD
25
has a flat characteristic which does not depend on the frequency. The monitor current output from the monitor PD
13
indicates the performance of the optical filter
23
because the monitor PD
13
is supplied with the light signal via the optical filter
23
.
For example, if it is wished to lock the oscillation wavelength at a wavelength &lgr;1 shown in
FIG. 6
, the oscillation wavelength of the LD element is set to &lgr;1 by utilizing the situation in which the oscillation wavelength of the LD element
10
varies due to the operation temperature thereof. Then, the divider circuit
26
is supplied with the monitor currents output from the PDs
13
and
25
.
The divider circuit
26
performs a dividing operation on the values of the supplied monitor currents, and produces a resultant output signal as shown in
FIG. 7
, which shows an example of the output signal of the divider circuit
26
.
As shown in
FIG. 7
, the output value of the divider circuit
26
increases or decreases when the oscillation wavelength deviates from &lgr;1. A temperature control circuit
27
shown in
FIG. 5
controls the TEC element
14
in accordance with the value supplied form the divider circuit
26
, and controls the temperature of the periphery of the LD element
10
, whereby the oscillation wavelength of the LD element
10
can be adjusted.
However, the conventional optical module as shown in
FIGS. 3 and 4
needs a large area for mounting the beam splitter
24
which splits the incident light into the two light components. Hence, the distances between the LD element
10
and the monitor PDs
13
and
25
increase, and the back lens
21
is required to compensate for the long distances. Thus, a large number of components is used to form the conventional optical module, which increases the cost.
Also, there is an increased number of points at which an optical adjustment such as an optical axis alignment is carried out. This increases the number of assembly steps.
Further, in order to increase the transmission or channel capacity, it is required to use a tunable LD element which can be tuned to a plurality of oscillation wavelengths in a single optical module and to improve the precision of the wavelengths of the light signals emitted from the tunable LD element.
SUMMARY OF THE INVENTION
It is a general object of the present inv
Komiyama Manabu
Kunikane Tatsuro
Kuribayashi Masaki
Sakai Yoshimitsu
Fujitsu Limited
Ip Paul
Rodriguez Armando
Staas & Halsey , LLP
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