Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
1998-12-02
2001-11-13
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S239000, C359S199200, C385S002000
Reexamination Certificate
active
06317247
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bias voltage stabilizing method for an electricity-optical modulator based on an off-level sampling, and in particular to an improved bias voltage stabilizing method for an electricity-optical modulator based on an OFF-level sampling which is capable of stabilizing an output characteristic of a Mach-Zehnder (MZ) interference type optical modulator formed of a LiNbO
3
or electrical-optical polymer.
2. Description of the Conventional Art
Generally, the operation level of an electricity-optical modulator is different in accordance with an application system. For this, a DC voltage may be applied to a signal electrode or an additional bias voltage electrode may be provided for controlling an operation level of a modulator. However, a conventional electricity-optical modulator has a DC drift phenomenon with respect to the DC power. In addition, the operation level may be changed due to a surrounding temperature and moisture, stress, etc. This operation level variation may result in a distortion of a high speed modulated optical signal and a degradation of an extinction ratio.
FIGS. 1A and 1B
illustrate an output type based on an initial operation level of an optical modulator when the identical digital signals are inputted. As shown in
FIG. 1A
, a digital signal is inputted as indicated by “1” and is converted into an optical signal by the optical modulator having a modulator output characteristic as indicated by “2”, and is outputted as indicated by “3”. At this time, the initial bias voltage level “a” which is the initial level of an output characteristic curve of a modulator is coincided with the bias voltage level of an electrical signal. In addition, in this case, it means that the electrical signal is well converted into an optical signal.
However, as shown in
FIG. 1B
, the operation level which is the minimum level of a modulator output characteristic curve is shifted from the initial bias voltage level. Therefore, when the same is shifted to the level position of the electrical signal “1”, the optical signal as indicated by “3” is distorted, so that it is impossible to identify a high bit or low bit. When the same is shifted away from the initial bias voltage level, the output signal level of the converted optical signal may be changed.
Therefore, it is needed to control the operation level. For this, an additional bias voltage is installed at the optical modulator for thereby controlling the operation level. However, an initial optical modulator operation level determined based on a DC bias voltage applied to an optical wave guide layer may be changed due to a photo refractive effect of a medium, a DC drifting phenomenon based on an accumulation of a space charge, a surrounding temperature variation, etc.
In order to continuously maintain an initial state of an operation level, the bias voltage is changed in accordance with an output state of an optical modulator, an output light stabilizing method is needed.
One of the most used methods is a stabilizing method using a dithering signal as shown in FIG.
2
.
As shown in
FIG. 2
, a second harmonic signal is extracted from a dithering signal
1
using a band pass filter
2
, a frequency doubler
3
, and a phase controller
4
. An optical signal from the optical modulator
5
is detected by a photo detector
6
and passes through a low noise amplifier
7
. The optical signal outputted from the low noise amplifier
7
is controlled by a lock-in amplifier
8
in accordance with a second harmonic signal of the dithering signal outputted from the phase controller
4
. The output signal from the lock-in amplifier
8
is inputted into an adder
12
through a low band pass filter
9
and an integrator
10
. A fundamental wave signal of the dithering signal
1
passes through the band pass filter
2
and a DC bias voltage signal inputted from the DC bias voltage unit
11
are added with the signal from the integrator
10
by the adder
12
for thereby controlling the optical modulator
5
.
A fundamental wave of a dithering signal modulated by applying a 1~100 kHz small electrical envelop dithering signal to the input light or a second harmonic signal is extracted for thereby obtaining an error signal, and then the bias voltage is controlled based on a feed-back control using the thusly obtained signal.
If the second harmonic signal of the dithering signal is extracted, and an error signal is used, the error signal value becomes 0(zero) at the level in which the linearity is best, and at this level, the code of the level of the error signal is changed. Therefore, it is possible to stabilize the position of the operation level of the optical modulator
5
at the level in which the linearity is best by controlling the DC bias voltage
11
, namely, a reference voltage inputted into the adder
12
in accordance with the level of the output signal of the lock-in amplifier
8
.
However, when forming the feed-back circuit using a fundamental wave of a dithering signal of the optical modulator or a second harmonic wave as an error signal, the values of the error signal to be checked in accordance with the traffic character is changed. Therefore, the lock-in detection method based on the known dithering signal is not proper for the optical gate application of the optical modulator.
FIG. 3
illustrates a circuit for controlling the bias voltage for providing the identical traffic characteristic compared to a valid DC component of an electrical input signal and an optical output signal based on the conventional method.
An input signal is inputted into an optical modulation driver
24
as a modulation signal
23
, and an optical modulator
22
receives a source light of a laser diode
21
and changes a modulation signal
23
into an optical signal in accordance with a driving of an optical modulation driver
24
. At this time, the signal passed through the low pass filter (LPF)
25
and the reference voltage
27
are differentially amplified by a first differential amplifier
26
with respect to the modulation signal
23
, and an optical signal outputted from the optical modulator
22
is detected by the photo detector (PD)
29
and passes through the low pass filter
30
and then an optical signal is detected based on a pre-amplifier
31
, and an output signal from the pre-amplifier
31
and an output signal from the first differential amplifier
26
are differentially amplified by a second differential amplifier
32
and then the thusly amplified signal is inputted into a PI (Proportional Integrated) control circuit
33
, so that the PI control circuit
33
controls the output characteristic of the optical modulator
22
.
The modulation signal
23
which is an electrical signal inputted into the optical modulator
22
is filtered using a low band pass filter
25
for thereby obtaining a reference voltage proportional to the amount of the traffic of the signal. In addition, an optical output from the optical modulator
22
is measured and then is filtered using the low pass filter(LPF)
30
for thereby obtaining a DC voltage proportional to the traffic signal of the output light. If the output signal from the optical modulator
22
is modulated identically to the electrical signal, the DC voltage (namely, the output voltage of the pre-amplifier
31
) of the output of the optical modulator
22
and the value of the electrical reference voltage (namely, the output voltage of the first differential amplifier
26
) should be identical. Therefore, the voltage difference between the reference voltage of the electrical signal and the optical output is obtained by the second differential amplifier
32
, so that the optical modulator
22
is modulated based on the DC control circuit
33
in accordance with the error signal. Therefore, it is possible to obtain an optical output modulated identically to the electrical signal inputted.
The above-described method may be adapted to the burst traffic characteristic, but uses an error signal proportional to the absolut
Han Sun Gyu
Hong Hyun Hah
Hwang Wol Yon
Park Heuk
Yang Choong Reul
Ben Loha
Cohen & Pontani, Lieberman & Pavane
Electronics and Telecommunications Research Institute
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