Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2002-03-27
2003-06-17
Lester, Evelyn (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S337000, C385S014000, C385S016000, C385S022000
Reexamination Certificate
active
06580544
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the priority benefit of Taiwan application Ser. No. 91100278, filed Jan. 11, 2002.
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a control technology and apparatus for bias correction on an optical modulator of optical emitter of external modulation type. More particularly, the present invention relates to a control method and apparatus used for a bias controller of automatic bias correction on an optical modulator of optical emitter of external modulation type.
2. Description of Related Art
In recent years, the optical fiber communication in the world has been well and greatly developed in flourish, wherein the internet and the cable TV have astonishing development. Under attraction of great business chance in the market with a large number of users, all dealers of the optical fiber communication have been developing for the goals of longer distance in fiber communication, wider bandwidth, and better quality. In
FIG. 1
, since the optical-fiber emitter
110
with a wavelength of 1559 nm can have low distortion and low insertion loss, and can implement with an optical amplifier
120
, an optical receiver
130
, so as to have the long distance network transmission with high quality. Therefore, it is greatly pushed and made to wide applications. In FIG.
2
A and
FIG. 2B
, the optical-fiber emitter can also be divided into a direct modulation type optical emitter and an external-modulation type optical emitter as shown in
FIGS. 2A and 2B
. The main difference of them is that the direct-modulation type optical emitter
240
is directly adding the transmission signal RF to the laser source
210
, whereby an optical form is exported through the optical amplifier. Also and, the external-modulation type optical emitter
280
is that when the output of the laser source
250
is input to the optical modulator
260
, the transmission signal
250
is externally added to the optical modulator
260
. Then, the optical amplifier
270
amplifies the signal and exports the optical output. The characteristic curve of the optical modulator is shown in FIG.
3
. In
FIG. 3
, it is observed that the characteristic of the optical modulation can be changed by using the external bias. In general, if it is applied on the high linear transmission system, such as the analog signals of the cable TV, the optical modulation bias can be adjusted to be located at the Quad point, so as to obtain the second order linearity with less distortion. If it is applied on the digital transmission system, the bias can be set at the peak or the null.
However when it is in the practical application, a phenomenon is found that the bias point of the optical modulator used in a high linear transmission system will shift along with the environmental temperature. If the bias point of the optical modulator has an error greater than +/−1 degree, it would cause the distortion of the second order non-linearity to be severely degraded. Therefore, for an optical emitter with external modulation type in commercial use, in order to obtain the high linear transmission with low distortion, it is necessary to set the bias voltage of the optical modulator to the bias point Quad. Also and, an offset is made on the shift of bias point Quad, which is caused by the temperature.
Currently, many solutions, such as the U.S. Pat. No. 5,812,297 and U.S. Pat. No. 5,343,324, are proposed on the bias voltage control of the optical modulator.
FIG. 4
is a conventional bias voltage controller of optical modulator of optical emitter for a external modulation type. The conventional bias voltage controller uses a 10.85 MHz signal generator
410
to generate a 10.85 MHz base signal, which is combined with a DC voltage and then sent to a bias voltage input terminal of the optical modulator. When a shift occurs at the best bias point of the optical modulator, that is the Quad point in
FIG. 3
, it will cause a distortion of the second order non-linearity in the transmission signal transmitted by the optical modulator. Then, a signal of 21.7 MHz is generated from the transmission signal exported by the optical modulator. This transmission signal goes through an optical coupler with 90:10 splitting and enters an optical receiver
420
, so as to convert the transmission signal into an electrical signal. The electrical signal is input to a wave detector
430
. The wave detector
430
can be a multiplying wave detector, a diode wave detector, an integrated circuit, or a discrete circuit. The wave detector
430
generates an error voltage, according to an amplitude and a phase of the signal of 21.7 MHz. The error voltage could be very small, and is necessary to be amplified by a DC-level amplifier
450
after passing the narrow-frequency low-pass filter
440
. In order to filter out the noise after amplification, it has to pass another narrow-frequency low-pass filter
460
. The error voltage is then enters a DC level shifter
470
for shifting the error voltage to a level, which is acceptable by the microprocessor
490
. An analog-to-digital converter (ADC)
480
further converts the error voltage and exports the output to the microprocessor
490
. According to the quantity of the error voltage, the microprocessor
490
controls a digital-to analog converter (DAC)
495
to export an offset voltage, so as to correct the bias voltage point of the optical modulator to the Quad point.
However, both the wave detector and the DC amplifier themselves have characteristic of producing compensation effect and zero-point shifting effect. In this situation, when the temperature changes, they will produce an extra voltage to the output of the bias voltage controller. This voltage would cause that the bias voltage controller cannot provide a proper offset voltage for correcting the bias voltage of the optical modulator to remain at the Quad point.
SUMMARY OF THE INVENTION
The invention provides a control method and an apparatus used for a bias controller of automatic bias correction on an optical modulator of external-modulation optical emitter. The correct offset voltage can be automatically measured and the correction is automatically made, so that the thermal effect can significantly reduced, and the output voltage can be adjusted.
The invention provides a control method for automatic bias correction on an optical modulator of external-modulation optical emitter. The bias controller provides a base signal, which is also combined with a DC voltage to form a bias voltage control signal. When the bias voltage control signal is input to the optical modulator, the bias voltage controller also receives the output of a first second-order harmonic component signal from the optical modulator. The bias voltage controller also takes a second second-order harmonic component signal generated from a base signal generator, which has the same frequency as the first second-order harmonic component signal. The bias voltage controller compares the first second-order harmonic component signal with the second second-order harmonic component signal, so as to produce a DC voltage for controlling the bias voltage of the optical modulator. The automatic correcting method includes dividing the method into a measuring cycle and a correcting cycle. When the bias voltage controller is at the measuring cycle, the receiving terminal is grounded. Then, the extra offset voltage and the zero shifting voltage from the bias voltage controller are measured. When the bias voltage controller is at the correcting cycle, the receiving terminal receives the first second-order harmonic component signal, and then compares the first second-order harmonic component signal with the second second-order harmonic component signal, so as to obtain a DC error voltage. The DC error voltage is subtracted from the extra offset voltage and the zero shifting voltage, so as to obtain a corrected DC error voltage. The corrected DC voltage is used to control the bias voltage of the optical modulator.
The invention pro
Hsu Yu-Hao
Kao Chih-Jung
Lin Ming-Chung
J.C. Patents
Lester Evelyn
New Elite Technologies, Inc.
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