Coherent light generators – Particular beam control device – Tuning
Reexamination Certificate
1999-02-16
2002-05-07
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Tuning
C372S020000, C372S096000
Reexamination Certificate
active
06385217
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to method and apparatus for providing a wavelength-locker arrangement which is wavelength independent and provides for the absolute wavelength stability of a laser diode.
BACKGROUND OF THE INVENTION
Absolute wavelength accuracy of a laser diode is of paramount significance for the successful deployment of a practical dense wavelength division multiplexed (WDM) transmission system. However, due to the aging of a laser and such phenomenon, laser wavelength shifts with time. This places an undue restriction on the remaining components of a WDM transmission system. Some of these laser phenomenon issues are alleviated by the use of a wave-locker, which monitors the laser wavelength and actively changes the temperature of the device which mounts the laser in order to compensate for any wavelength drifts. However, state of the art wave-lockers are constructed using micro-optic filters or etalons which add a significant cost and size to a laser diode device. In addition, such wave-lockers have high insertion loss which leads to a reduced signal-to-noise ratio due to the use of a prior optical tap in the system. Therefore, it is of significant advantage to construct an all-fiber device which is capable of limiting the wavelength drift in a laser diode without adding significant cost.
The concept of stabilizing the wavelength of a laser diode is based on the ability to calibrate absolute wavelength drifts into measurable absolute power changes. One way,.this is accomplished is by tapping a small fraction of the laser's optical output signal and sending the signal to a filter with a wavelength dependent response.
Referring now to
FIG. 1
, there is graphically shown an exemplary filter profile
10
in a transmission laser signal. A transmission value (dB) is shown along the vertical axis and wavelength in nanometers (nm) is shown along the horizontal axis. If a filter is designed such that the laser's center wavelength (laser line) is aligned along the edge of the filter in the manner shown in
FIG. 1
, it is possible to translate the wavelength information into power information. For example, it is seen in
FIG. 1
that as the wavelength of the laser increases, the power at the output of the filter will increase proportionally. However, such an arrangement by itself is not adequate to ensure absolute stability of the laser diode device because a change in the power of the laser diode can be misconstrued as a wavelength change. To overcome this problem, the response of the filter has to be monitored in reflection as well.
Referring now to
FIG. 2
, there is graphically shown an exemplary laser diode signal where a solid line
12
shows the laser transmission through a filter, and a dashed line
14
shows the laser transmission in reflection from the filter. As is shown, the reflection response
14
of the filter is complementary to the transmission response
12
of the laser diode so that an increasing wavelength will result with a decrease in the reflected power. Since a filter's transfer function is known apriori, the ratio of the transmitted to reflected power can be used to calibrate any power drifts. Finally, “y” tracking of the response of the filter, and changing the temperature of the laser, will hold the wavelength of the laser diode at a constant value. Two classes of wave-lockers have been commercially developed using the principle of operation described above. A first class is a filter-based wave-locker, and a second class is an etalon-based wave-locker.
Referring now to
FIG. 3
, there is shown a block diagram of an exemplary prior art filter-based wave-locker arrangement
20
(shown within a dashed line rectangle) illustrating the first class of wave-locker. The wave-locker arrangement
20
comprises a laser source (LASER)
22
, an optical power tap
24
, a wave-locking device
26
(shown within a dashed line rectangle) and a control unit (CONTROL)
28
. The optical output from the laser source
22
is received at an input of the power tap
24
. The power tap
24
divides the received laser signal into two portions, where a first portion of the laser signal is provided as an output of the wave-locker arrangement
20
, and a second portion of the laser signal is coupled to an input of the wave-locking device
26
. The control unit
28
receives first and second outputs from the wave-locking device
26
and generates therefrom appropriate control signals to the laser source
22
for maintaining its wavelength at a substantially constant value.
The wave-locking device
26
comprises a wide-band power splitter (PWR.SPLIT.)
30
, a wavelength discriminating filter (FILTER)
32
, a first photodetector (PHOTO DETECT)
34
, and a second photodetector (PHOTO DETECT)
36
. An exemplary optical response of the filter
32
is shown in
FIG. 4
where the vertical axis denotes transmission (dB) and the horizontal axis denotes wavelength (nm) in the manner of the graph of FIG.
1
. The second portion of the laser signal outputted from the power tap
24
is received at a first terminal of the wide-band power splitter
30
, a second terminal of the wide-band power splitter
30
is coupled to an input of the filter
32
, a third terminal of the wide-band power splitter
30
is coupled to an input of the second photodetector
36
, and a fourth terminal of the wide-band power splitter
30
is unused or pig-tailed. An output of the filter
32
is coupled to an input of the first photodetector
34
, and outputs from the first and second photodetectors
34
and
36
arc provided as electrical feedback signals to a control unit
28
which generates control signals to actively control the wavelength of the laser source
22
.
In operation, a fraction of light from a laser source
22
(typically 5%) is tapped off by the power tap
24
and sent to the wave-locking device
26
. In the wave-locking device
26
, the received fractional signal is split into two portions by the power splitter
30
. A first portion of the split signal is sent to the wavelength discriminating filter
32
, and a second portion of the split signal is effectively unused by being directed to the fourth terminal of the power splitter
30
. At the filter
32
, a part of the signal is reflected back through the power splitter
30
to the second photodetector
36
, and the remaining part of the received signal is transmitted to the first photodetector
34
. Each of the first and second photodetectors
34
and
36
generate electrical output signals corresponding to the received input signals. The two electrical signals from the first and second photodetectors
34
and
36
are provided as feedback signals to the control unit
28
. The control unit
28
uses the two feedback signals to generate control signals to the laser source
22
to monitor and control its wavelength.
The filter-based wave-locker arrangement
20
has two significant disadvantages. First, since the filter
32
is wavelength selective, it can only be used at a specific International Telecommunication Union (ITU) recommended wavelength. More particularly, the ITU has recommended a wavelength range of 1550-1576 nanometers (nm), which is called a 1550 window, within a grid or scale that has a reference frequency of 193.1 terahertz and 50 GHz intervals. For example, in a typical 50 GHz operation, in the 1550 nm window alone there are over 100 useable wavelengths, and such a device would require fabrication of over 100 different filters leading to cost and inventory issues. Second, due to the need for splitting the incoming power to the wave-locking device
26
in the power splitter
30
so as to be able to access both the reflective and transmitted signal, the actual signal reaching the photodetectors
34
and
36
is very small. Specifically, there is a loss of 3 dB for the transmitted signal and a 6 dB loss for the reflected signal, and the signal-to-noise ratio suffers significantly leading to errors in the control unit
28
.
Referring now to
FIG. 5
, there is shown a block diagram of an exemplar
Diner Fahri
Singh Harmeet
Flores Ruiz Delma R.
Hunton & Williams
Ip Paul
Qtera Corporation
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