WO2002045303A1

WO2002045303A1 - Method and apparatus for removing non-linear distortion in optic transmitter

Info

Publication number: WO2002045303A1
Application number: PCT/KR2001/002070
Authority: WO
Inventors: Hyo Taik Lim
Original assignee: A & P Telecom Co., Ltd.
Priority date: 2000-11-30
Filing date: 2001-11-30
Publication date: 2002-06-06
Also published as: KR100350320B1; KR20020042322A; CN1167212C; CN1397114A

Abstract

An apparatus and method for removing nonlinear distortion of an optical transmitter are provided. Some of the power input to a laser diode (LD) are coupled. The coupled result is coupled with a signal containing nonlinear distortion components by a coupling photo diode (PD) within the LD. A signal obtained by shifting the phase of the coupled signal containing the nonlinear distortion components by 180E and maintaining the magnitude of the coupled signal is combined with the coupled signal, thereby removing input signal components and leaving only a distortion signal whose phase is opposite to an input signal. The magnitude and the phase of the distortion signal are adjusted to generate a signal having the same magnitude as and an opposite phase to the distortion signal, thereby removing nonlinear distortion in an optical transmitter. Here, the magnitude and the phase are the function of temperature, so magnitude and phase variation data according to a change in temperature is stored in a memory using a small microprocessor. By fine adjusting the magnitude and the phase according to a change in temperature, reliability against changes in temperature or circumstances can be secured. Accordingly, the apparatus and method are applied to optical transmitters needing high-efficiency and high-power laser diodes so that wide dynamic rage and satisfactory inter-modulation characteristics can be achieved.

Description

METHOD AND APPARATUS FOR REMOVING NON-LINEAR DISTORTION IN OPTIC TRANSMITTER

Technical Field The present invention relates to an apparatus and method for removing nonlinear distortion of an optical transmitter, and more particularly, to an apparatus and method for removing a distortion component of an optical transmitter due to a nonlinear characteristic using combination of a pre-distortion method and a feed-forward method and transmitting desirable information only through a laser diode.

Background Art

Unlike electrical transmission of signals, messages, or other types of information, in optical communication, information is transmitted using an electronic wave in an optical domain such as a laser beam. In optical communication, when audio or video information is converted into an electrical signal and sent to a transmitter, the transmitter converts the electrical signal into light and transmits the light to optical fiber. Here, a laser or a light emitting diode is used as an apparatus for converting an electrical signal into light. The light transmitted to the optical fiber travels at the speed of light while blinking several hundred million through several billion times per second. In order to prevent the intensity of the light from being weakened, repeaters for amplifying an optical signal are installed at intervals of 130-140 km so that the optical signal is maintained constant. Then, the optical signal is converted into an electrical signal by a receiver. Consequently original audio or video information can be received. Such optical communication is advantageous of clearly transmitting a large amount of audio and video at a high speed. Communication using copper lines or electric waves has far lower transmission speed and quality than optical communication.

In general wireless mobile communication, an optical transmitter restricts inter-modulation (IM) and dynamic rage of an optical repeater system because a laser diode, which is a main device for electrophotic conversion, has a poorer IM characteristic than a monolithic microwave integrated circuit (MMIC) or an amplifier having a general wireless frequency. Although the IM characteristic can be improved by using a high-efficiency and high-power laser diode, the high-efficiency and high-power laser diode is very expensive for its performance, so it is of no use. Moreover, there is a limitation in manufacturing a high-efficiency and high-power laser diode. Particularly, as used bands has increased due to integration between service companies, and as it has been required to transmit information far with high power, measures for overcoming the limitation are desired. In addition, with an increase in subscribers to mobile communication service, the number of frequency allocations for using repeaters also increases, so it is required that greater power is input to a conventional laser diode. Since an Input 3 order Interceptor Point (IIP3) of a usual high-efficiency and high-power laser diode is set to 28 dB, an IM characteristic is remarkably deteriorated as an input increases, which affects the entire system. Accordingly, at present, a laser diode having satisfactory efficiency (for example, electrophotic conversion efficiency) is used, and LC matching instead of resistance matching is used as input matching for a laser diode to increase efficiency or a high-power laser diode is used. However, it has been recognized that there is a limitation in applying the above methods to IS-95C and IMT-2000.

As shown in FIG. 1 , a conventional optical communication apparatus includes a transmitting unit 10, a transport unit 20, and a receiving unit 30. The transmitting unit 10 is provided for converting an electric signal RF_in to be transmitted into an optical signal. A light emitting diode (LED) and a laser diode (LD) can be used as a light emitting device in the optical communication apparatus. The LED performs natural emission using incoherent light having a disordered phase and is used for low-speed transmission (of, for example, no greater than 100 Mbps). The LED is cheap, operates at a temperature in a wide range, and has a long duration. The LD uses coherent light generated from induced emission of a semiconductor. The LD has a more complicated structure than the LED but can be used for high-speed data transmission (of, for example, Gbps). The transport unit 20 is provided for transporting an optical signal output from the transmitting unit 10 using an optical cable. The transport unit 20 is a wideband transport line in which a transport line is made of optical fiber and through which an optical signal is transported using the principle of waveguide. Since the transport unit 20 employs optical fiber using the principle of total reflection, an optical signal can be transported to a destination without being lost. The receiving unit 30 is provided for converting an optical signal received through the transport unit 20 into an original electrical signal. A PN photo diode, a PIN photo diode, an avalanche photo diode, or a photo transistor can be used as a light receiving device in the optical communication apparatus.

However, a signal containing a nonlinear distortion component is transmitted to the receiving unit 30 through the transport unit 20 as it is in such conventional optical communication apparatus, so the conventional optical communication apparatus is not suitable to most of optical communication networks requiring accuracy of information. To overcome this problem, a pre-distortion method through which a signal having the same magnitude as a distortion signal contained in a main signal and having an opposite phase to the distortion signal and a feed-forward method are used. A pre-distortion method and a feed-forward method are methods of designing an amplifier for reducing nonlinear distortion. An embodiment of a general pre-distortion method is shown in FIG. 1A, and an embodiment of a general feed-forward is shown in FIG. 1B. The above-described pre-distortion method and feed-forward method are widely known to those skilled in the art of this invention. Thus, detailed descriptions thereof will be omitted. In FIGS. 1 , 1A and 1 B, reference characters f_a and f_b denote main signal components, and reference characters C, D, E, and F denote distortion signal components.

However, such conventional optical communication apparatus has the following problems. First, since an optical communication apparatus converts an electrical signal into an optical signal, transports the optical signal over several tens of Km, and converts the optical signal into an electrical signal, a feed-forward method forming a closed loop cannot be applied to the optical communication apparatus. Second, in a pre-distortion method, distortion to occur in a main signal is predicted and measured, and a high frequency is voluntarily generated. However, an at least second-order high frequency is not controlled in the same manner as the main signal.

Therefore, a new technique for effectively removing a nonlinear distortion signal of an optical transmitter is desired.

Disclosure of the Invention

To solve the above-described problems, it is a first object of the invention to provide an apparatus and method for removing nonlinear distortion of an optical transmitter, in which a signal having the same magnitude as a nonlinear distortion signal and having an opposite phase to the nonlinear distortion is generated using combination of a pre-distortion method and a feed-forward method, thereby transmitting a main signal only from which the nonlinear distortion signal is removed. It is a second object of the invention to provide a an apparatus and method for removing nonlinear distortion of an optical transmitter, in which data about variation in the magnitude and phase of a signal according to a change in temperature is stored in a memory using a microprocessor to fine adjust the magnitude and phase of a signal according to a change in temperature, thereby reliably removing signal distortion arising from a change in temperature.

To achieve the above objects of the invention, there is provided an apparatus for removing nonlinear distortion of an optical transmitter. The apparatus includes an input matching unit for adjusting input resistance of a main signal to be transmitted; a first coupling unit for splitting the main signal; a variable signal generator for generating a signal having a magnitude and a phase which correspond to a constant ratio of the main signal split by the first coupling unit; a light emitter for converting the main signal into an optical signal and emitting optical energy; a photodetector optically connected to the light emitter, for coupling the optical signal emitted from the light emitter and a signal containing nonlinear distortion components; a temperature sensing unit optically connected to the light emitter, for measuring temperature around the light emitter; a microprocessor connected to the temperature sensing unit, for receiving a temperature value measured by the temperature sensing unit, extracting magnitude and phase variation data corresponding to the measure temperature value from the memory, and performing control to correct the coupled signal output from the photodetector; a first gain adjustor connected to the photodetector and the microprocessor, for receiving the coupled signal from the photodetector and the magnitude and phase variation data from the microprocessor, amplifying the coupled signal to adjust the gain of the coupled signal to have a predetermined amplitude, and shifting a phase of the gain adjusted signal by 180E; a third coupling unit connected to the variable signal generator and the first gain adjustor, for combining the signal output from the variable signal generator with a signal output from the first gain adjustor; a second gain adjustor connected to the third coupling unit, for fine adjusting a magnitude and a phase of a signal output from the third coupling unit so that the fine adjusted signal has a predetermined amplitude; and a second coupling unit connected to the input matching unit and the second gain adjustor, for combining a signal output from the input matching unit with a signal output from the second gain adjustor and applying the result of combination to the light emitter. There is also provided a method of removing nonlinear distortion of an optical transmitter. The method includes the steps of (a) matching input resistance of a main signal to be transmitted; (b) splitting the main signal and generating a signal having a magnitude and a phase which correspond to a constant ratio of the main signal; (c) coupling the main signal applied through a light emitter and a distortion signal to be generated; (d) measuring temperature around the light emitter and converting the measure temperature into a digital signal; (e) extract magnitude and phase variation data corresponding to a value of the temperature measured in step (d); (f) receiving a coupled signal generated in step (c) and the magnitude and phase variation data extracted in step (e), amplifying the coupled signal to adjust gain of the coupled signal, thereby generating a signal having a predetermined amplitude, and shifting the gain adjusted signal by 180E; and (g) combining the signal generated in step (b) with a signal generated in step (f), fine adjusting phase and magnitude of a signal resulting from the combination, combining the signal having fine adjusted phase and magnitude with a signal resulting from the matching in step (a), and applying the result of the latter combination to the light emitter.

The present invention can be effectively applied to optical communication apparatuses which convert an electrical signal into an optical signal for transmission and convert an optical signal into an electrical signal. Generally, in direct modulation using electrophotic conversion and photo-electric conversion, a signal modulated by applying an electrical signal to a laser diode (LD) and transmitted through optical fiber undergoes photo-electric conversion in a photo diode (PD). Here, due to the nonlinear characteristics of the LD, undesirable inter-modulation (IM) occurs. In all existing analog direct modulation, IM throughout the system is determined depending on the characteristics of an LD due to nonlinear distortion. Optical repeaters commercialized in wireless communication at present have such limitation. Moreover, with an increase in the number of subscribers to mobile phone service and an increase in the number of mobile phone service companies, a band to be served and a total input power increase. However, the characteristics of existing LDs cannot satisfy the above conditions. Accordingly, optical communication apparatuses which can be used for IS-95 and IMT-2000 require characteristics not involved with the limitation of the LD. To solve the above problems, the present invention couples some of the power input to an LD. The coupled result is coupled with a signal containing nonlinear distortion components by a coupling PD within the LD. A signal obtained by shifting the phase of the coupled signal containing the nonlinear distortion components by 180E and maintaining the magnitude of the coupled signal is combined with the coupled signal, thereby removing input signal components and leaving only a distortion signal whose phase is opposite to an input signal. The magnitude and the phase of the distortion signal are adjusted to generate a signal having the same magnitude as and an opposite phase to the distortion signal, thereby removing nonlinear distortion in an optical transmitter. Here, the magnitude and the phase are the function of temperature, so magnitude and phase variation data according to a change in temperature is stored in a memory using a small microprocessor. By fine adjusting the magnitude and the phase according to a change in temperature, reliability against changes in temperature or circumstances can be secured.

Brief Description of the Drawings FIG. 1 is a circuit diagram of a conventional optical communication apparatus.

FIG. 1A is a circuit diagram of an embodiment of a general pre-distortion method.

FIG. 1B is a circuit diagram of an embodiment of a general feed-forward method.

FIG. 2 is a block diagram of an apparatus for removing nonlinear distortion of an optical transmitter according to the present invention.

FIG. 3 is a circuit diagram of an embodiment of the apparatus shown in FIG. 2. FIG. 4 is a flowchart showing the operations of an apparatus for removing nonlinear distortion of an optical transmitter according to the present invention.

Best mode for carrying out the Invention Hereinafter, preferred embodiments of an apparatus and method for removing nonlinear distortion of an optical transmitter according to the present invention will be described in detail with reference to the attached drawings.

FIG. 2 is a block diagram of an apparatus for removing nonlinear distortion of an optical transmitter according to the present invention. The apparatus includes a first coupling unit 205, a variable signal generator 210, an input matching unit 215, a second coupling unit 220, a light emitter 225, a photodetector 230, a temperature sensing unit 235, a microprocessor 240, a memory 245, a first gain adjustor 250, a third coupling unit 255, a second gain adjustor 260, a transport unit 265, and a light receiver 270.

Each of the first through third coupling units 205, 220, and 255 used in the present invention is a passive device which splits an electrical signal or combines electrical signals. The first coupling unit 205 is realized as a splitting coupler. The second and third coupling units 220 and 255 are realized as combining couplers. The first coupling unit 205 splits a main signal into two signals. The variable signal generator 210 is connected to the first coupling unit 205 and generates a signal having a magnitude and a phase (for example, components M2 and P2) of a constant ratio (for example, -20 dB) in order to counterbalance a main signal which is detected by the photodetector 230. In the present invention, the variable signal generator 210 is realized as a variable coil so that the signal having the components M2 and P2 can be freely varied according to circumstances. The input matching unit 215 is connected to the first coupling unit 205 and employs LC matching of 50 Ω. The input matching unit 215 controls input resistance to match input resistance (of, for example, 50 Ω) with rated resistance (of, for example, 5~8 Ω) of a laser diode (LD) which will be described later. The second coupling unit 220 is connected to the input matching unit 215 and the second gain adjustor 260. The second coupling unit 220 combines a main signal transmitted through the input matching unit 215 and a signal having the same magnitude as a distortion signal adjusted by the second gain adjustor 260 and having an opposite phase to the distortion signal. The light emitter 225 is connected to the second coupling unit 220.

The light emitter 225 converts an electrical signal RFj_n to be transmitted into an optical signal, adds a distortion compensation signal, which is generated through combination performed by the second coupling unit

220 for compensating for a distortion signal, to the optical signal, and emits an optical signal approximate to a main signal. A light emitting diode (LED) or an LD is used as the light emitter 225 in an optical communication apparatus. The characteristics of an LED and LD are as follows.

In the present invention, it is preferable to use an LD which transmits information at a high speed and is suitable to long-distance communication.

The photodetector 230 is optically connected to the light emitter 225 and couples an optical signal emitted from the light emitter 225 with a signal containing a nonlinear distortion component. In the present invention, a photo diode is used as the photodetector 230. Photo diodes include a PN photo diode operating in a reverse bias mode, a PIN photo diode receiving an optical signal by artificially increasing a depletion layer, and an avalanche photo diode which has an excellent sensitivity and uses a method of generating new electrons from electrons generated from photons using an avalanche effect. In the present invention, one suitable to design specifications can be selected from among the above photo diodes and used as the photodetector 230.

The temperature sensing unit 235 is connected to the light emitter 225 and detects the temperature of the light emitter 225 to measure a detection value with respect to a change in temperature. In the present invention, a thermistor is used as the temperature sensing unit 235. Since an optical signal emitted from the light emitter 225 is a function of temperature, the optical signal is very sensitive to a change in temperature. Accordingly, it is necessary to vary a distortion compensation signal depending on a change in temperature. A thermistor is used based on a characteristic that its resistivity varies with a change in the temperature of the light emitter 225. The thermistor is connected to divided resistance in series and calculates temperature voltage using variation in its resistance and the voltage of divided resistance. In addition, the temperature sensing unit 235 may be an analog temperature sensing unit or a digital temperature sensing unit. When an analog temperature sensing unit (for example, a thermistor) is used, it is necessary to provide an analog-to-digital (A/D) converter to convert an analog value to digital value. In an embodiment of the present invention, a thermistor is used, so an A/D converter is needed.

The microprocessor 240 is connected to the temperature sensing unit 235. The microprocessor 240 receives a measured temperature from the temperature sensing unit 235, extracts magnitude and phase variation data corresponding to the measured temperature from the memory 245, and controls the first gain adjustor 250 to correct a coupled signal output from the photodetector 230. The memory 245 stores magnitude and phase variation data according to a change in temperature. A Read Only Memory (ROM) or an Electrically Erasable and Programmable ROM (EEPROM) can be used as the memory 245. The first gain adjustor 250 is connected to the photodetector 230 and the microprocessor 240. The first gain adjustor 250 receives a signal from the photodetector 230 and magnitude and phase variation data according to a change in temperature from the microprocessor 240, amplifies the received signal to control the gain so that the signal has a predetermined amplitude, and shifts the phase of the gain-controlled signal by 180E. The first gain adjustor 250 includes an amplifier for adjusting the gain of a signal and a delay device for shifting the phase of the signal. If signals having different amplitudes are determined to be turned ON/OFF using the same offset, a duty ratio changes resulting in modulation of a waveform. Accordingly, to prevent the waveform of a signal received from the photodetector 230, it is necessary to adjust an offset voltage or a gain coefficient using an amplifier. There is one method of adjusting an offset voltage or a gain coefficient using feedback. Specifically, a variation in the amplitude of a signal output from an amplifier is measured to adjust an offset voltage or a gain coefficient. There is another method of adjusting an offset voltage or a gain coefficient using feedforward to compensate for a limitation in the operating speed of an amplifier due to feedback time. Specifically, the amplitude of a signal output from a preamplifier is measured to adjust the offset voltage of the amplifier. In an embodiment of the present invention, the method using feedforward is used to adjust the offset voltage or gain coefficient of an amplifier.

The third coupling unit 255 is connected to the variable signal generator 210 and the first gain adjustor 250. The third coupling unit 255 combines a signal generated from the variable signal generator 210 with a gain-controlled and phase-shifted signal received from the first gain adjustor 250 to generate a signal having the same magnitude as a distortion signal generated from the photodetector 230 and having an opposite phase to the distortion signal. The second gain adjustor 260 is connected to the third coupling unit 255. The second gain adjustor 260 adjusts the magnitude and phase of a signal output from the third coupling unit 255 so that the signal has a predetermined amplitude and applies the adjusted signal to the second coupling unit 220. Like the first gain adjustor, the second gain adjustor 260 includes an amplifier for adjusting the gain of a signal and a delay device for shifting the phase of the signal.

The transport unit 265 is optically connected to the light emitter 225 and transports an optical signal emitted from the light emitter 225 to a particular destination through an optical fiber. An optical fiber cable is formed of glass which is thinner than a hair and is made of quartz (silicon oxide) having excellent transparency for transmission of light. The optical fiber cable includes a core formed of thin glass (having a large refractive index) at the center, a cladding (having a small refractive index) surrounding the core, and a jacket covering the core and cladding.

The diameter of the core is about 1/1000 mm. When a laser beam is radiated at one end of the optical fiber cable, the beam is continuously and totally reflected at the interface between the core and the cladding, so the beam travels through the optical fiber cable without being emitted outward. Dispersion characteristics are problematic transmission characteristics of optical fiber. The dispersion characteristics restrict the bandwidth of a transmission signal resulting waveform spreading, thereby restricting information transmission capacity. The dispersion characteristics include chromatic dispersion and mode dispersion. Chromatic dispersion indicates waveform spreading caused by a change in the refractive index of glass, i.e., a material of optical fiber, due to the wavelength of propagating light. Mode dispersion indicates waveform spreading caused by interference between modes due to difference in speed among different modes for propagation within optical fiber. Mode dispersion is a serious problem in a multi-mode, but mode dispersion can be minimized by using graded index-type optical fiber.

The light receiver 270 is optically connected to the transport unit 265. The light receiver 270 converts an optical signal received through the transport unit 265 into an original electrical signal and outputs a signal RF_0Ut which is the same as an original main signal. As described in FIG. 1 , a PN photo diode, a PIN photo diode, an avalanche photo diode, or a phototransistor can be used as a light receiving device used for the light receiver 270. In an embodiment of the present invention, a general PN photo diode is used. Hereinafter, the operations of an apparatus and method for removing nonlinear distortion of an optical transmitter according to the present invention will be described in more detail with reference to the attached drawings.

FIG. 3 is a circuit diagram of an embodiment of the apparatus shown in FIG. 2. FIG. 4 is a flowchart showing the operations of an apparatus for removing nonlinear distortion of an optical transmitter according to the present invention. In FIG. 3, reference characters © through ® denote output signals of main devices of the present invention. In FIGS. 2 and 3, the same reference numeral denotes the same member. An embodiment of an apparatus for removing nonlinear distortion of an optical transmitter according to the present invention is well illustrated in FIG. 3. Thus, a description thereof will not be repeated, but the devices shown in FIG. 3 will be cited in describing the operations of the present invention with reference to FIG. 4. A coupler CP1 splits a main signal © to be transmitted and applies the results of splitting to an input matching unit (for example, an LC matching circuit) and a delay device D1. The delay device D1 receives the split main signal © and generates a signal ® having a magnitude and a phase which correspond to a constant ratio (-20 dB) of the main signal © in step S405. The signal ® is used for removing main signal components f_a and fb from a signal to be corrected and leaving distortion components only. A photo diode PD1 generates a signal © by adding distortion components C and D to the main signal components f_a and f_b applied to an LD in step S410 Here, the signal © has the distortion component C=2f_a-fb and the distortion component D=2f_b-f_a as a magnitude and a phase, respectively An amplifier AMP1 adjusts an offset voltage and a gain coefficient with respect to the signal © generated by the photo diode PD1 in step S415.

Simultaneously, a thermistor Th measures temperature around the LD in step S420. Since the thermistor Th is connected to a divided resistance R in series, a voltage corresponding to a temperature is calculated using variation in the resistance of the thermistor Th and the voltage of the divided resistance R. The calculated voltage corresponding to a temperature is converted into a digital signal by an A/D converter 237 in step S425. The digital signal is applied to the microprocessor 240. The microprocessor 240 extracts magnitude and phase variation data corresponding to the measured temperature from the memory 245 in step S430 and applies the data to a delay device D2 through a digital-to-analog (D/A) converter 239 in step S435. The delay device D2 generates a signal ® by shifting the signal whose gain is controlled by the amplifier AMP1 by 180E based on the magnitude and phase variation data received through the D/A converter 239 in step S440. Here, the signal @ containing the undesirable distortion components C and D must be adjusted to have a magnitude, which is the same as that of the signal ® generated by the delay device D1 from the split main signal, and a phase which is opposite to that of the signal ®. A coupler CP3 combines the signals ® and the signal ® to generate a signal ©, and the phase and magnitude of the signal © is fine adjusted by an amplifier AMP2 and a delay device D3, in step S445. In other words, since the components f_a and f of the signal @ have the same magnitudes as and opposite phases to the components f_a and f_b of the signal ®, when the two signals ® and @ are combined, only the components C and D of the signal @ remain. The phase and magnitude of the signal © are fine adjusted by the amplifier AMP2 and the delay device D3 to convert the signal © into a signal ®. The signal © is converted into signal ® in order to more accurately and reliably remove distortion components. The signal © is combined with the main signal ® matched by a coupler CP2, thereby finally generating a signal ®. The components C and D of the signal ® have the same magnitudes as and opposite phases to the distortion components generated from the LD. Accordingly, when the signal ® is applied to the LD, the distortion components C and D are removed, and only the main signal components f_a and f_b to be transmitted remain. The LD converts the corrected signal into an optical signal and outputs the optical signal to an optical fiber cable in step S450 for transport. The optical signal transported through the optical fiber cable is converted into an electrical signal by a photo diode PD2, so an output signal ® which is the same as the original main signal is extracted, in step S455. The above-described invention is just an embodiment of the present invention. The present invention is not restricted to the above embodiment, and various modifications can be made thereto within the scope defined by the attached claims. For example, the shape and structure of each member specified in the embodiment can be changed.

Industrial Applicability

As described above, in an apparatus and method for removing nonlinear distortion of an optical transmitter according to the present invention, distortion components arising from a change in temperature and circumstances can be effectively removed. Therefore, the present invention can be applied to optical transmitters needing a high-efficiency and high-power laser diode so that a wide dynamic rage and satisfactory inter-modulation characteristics can be achieved.

In addition, the present invention can be effectively applied to optical transmitters for IS-95, integration type optical repeaters, and IMT-2000 in which service is limited due to an increase in total power arising from a broad band.

Claims

What is claimed is:

1. An apparatus for removing nonlinear distortion of an optical transmitter, the apparatus comprising: an input matching unit for adjusting input resistance of a main signal to be transmitted; a first coupling unit for splitting the main signal; a variable signal generator for generating a signal having a magnitude and a phase which correspond to a constant ratio of the main signal split by the first coupling unit; a light emitter for converting the main signal into an optical signal and emitting optical energy; a photodetector optically connected to the light emitter, for coupling the optical signal emitted from the light emitter and a signal containing nonlinear distortion components; a temperature sensing unit optically connected to the light emitter, for measuring temperature around the light emitter; a microprocessor connected to the temperature sensing unit, for receiving a temperature value measured by the temperature sensing unit, extracting magnitude and phase variation data corresponding to the measure temperature value from memory, and performing control to correct the coupled signal output from the photodetector; a first gain adjustor connected to the photodetector and the microprocessor, for receiving the coupled signal from the photodetector and the magnitude and phase variation data from the microprocessor, amplifying the coupled signal to adjust the gain of the coupled signal to have a predetermined amplitude, and shifting a phase of the gain adjusted signal by 180E; a third coupling unit connected to the variable signal generator and the first gain adjustor, for combining the signal output from the variable signal generator with a signal output from the first gain adjustor; a second gain adjustor connected to the third coupling unit, for fine adjusting a magnitude and a phase of a signal output from the third coupling unit so that the fine adjusted signal has a predetermined amplitude; and a second coupling unit connected to the input matching unit and the second gain adjustor, for combining a signal output from the input matching unit with a signal output from the second gain adjustor and applying the result of combination to the light emitter.

2. The apparatus of claim 1 , further comprising: a transport unit optically connected to the light emitter, for transporting the optical signal emitted from the light emitter to a destination using an optical fiber cable; and a light receiver optically connected to the transport unit, for converting the optical signal transported through the transport unit into an electrical signal.

3. The apparatus of claim 1 , wherein the input matching unit comprises an LC matching circuit.

4. The apparatus of claim 1, wherein the light emitter comprises a laser diode.

5. The apparatus of claim 1, wherein the temperature sensing unit comprises a thermistor and an analog-to-digital converter for converting an analog value of temperature measured by the thermistor into a digital signal.

6. The apparatus of claim 1, wherein each of the first and second gain adjustors comprises an amplifier for adjusting gain of an input signal and a delay device for shifting a phase of the input signal.

7. The apparatus of claim 1 , wherein the constant ratio is -20 dB of the main signal.

8. The apparatus of claim 1, wherein the photodetector comprises a photo diode.

9. The apparatus of claim 2, wherein the light receiver comprises a photo diode.

10. A method of removing nonlinear distortion of an optical transmitter, the method comprising the steps of:

(a) matching input resistance of a main signal to be transmitted; (b) splitting the main signal and generating a signal having a magnitude and a phase which correspond to a constant ratio of the main signal;

(c) coupling the main signal applied through a light emitter and a distortion signal to be generated; (d) measuring temperature around the light emitter and converting the measure temperature into a digital signal;

(e) extracting magnitude and phase variation data corresponding to a value of the temperature measured in step (d);

(f) receiving a coupled signal generated in step (c) and the magnitude and phase variation data extracted in step (e), amplifying the coupled signal to adjust gain of the coupled signal, thereby generating a signal having a predetermined amplitude, and shifting the gain adjusted signal by 180E; and

(g) combining the signal generated in step (b) with a signal generated in step (f), fine adjusting phase and magnitude of a signal resulting from the combination, combining the signal having fine adjusted phase and magnitude with a signal resulting from the matching in step (a), and applying the result of the latter combination to the light emitter.

11. The method of claim 10, further comprising the steps of: (h) transporting an optical signal emitted from the light emitter to a destination using an optical fiber cable; and

(i) converting the optical signal transported in step (h) into an electrical signal and extracting an output signal.

12. The method of claim 10, wherein the constant ratio in step (b) is -20 dB of the main signal.

13. The method of claim 10, wherein the signal generated in step (b) and a signal generated in step (f), which does not contain the distortion signal, have the same magnitude and opposite phases.