KR101229803B1 - Apparatus For Up-converting Of All-optical Single Sideband Frequency - Google Patents

Abstract

An all-optical sideband frequency upconversion device is disclosed. The all-optical sideband frequency upconversion device generates an optical RF signal in the form of a single sideband as all-optical light. Therefore, it provides high conversion efficiency and high maximum RF frequency, and is strong in color dispersion of optical fiber to enable long-distance transmission of optical RF signal, and reuse of downlink carrier as uplink carrier to enable base station or remote base Facilitate installation, maintenance and repair. In particular, the power difference between the carrier and the sideband can be made small, so that a very good reception sensitivity can be obtained during signal transmission. The all-optical sideband frequency upconversion device can replace the frequency mixer (Mixer), which is an ultra-high frequency electronic device, in the optical-to-wireless system, and can be utilized to construct an economical wired / wireless integrated wireless communication system in the millimeter wave band.

Optical-to-Wireless Signal, Single Sideband, All-optical, Frequency Conversion

Description

Apparatus For Up-converting Of All-optical Single Sideband Frequency}

The present invention relates to an apparatus for generating an optical-wireless signal, and more particularly, to an apparatus for generating an all-optical single sideband frequency upconverter for generating an optical RF signal in the form of a single sideband.

Recently, research for high quality, high speed broadband multimedia traffic has been actively conducted in the field of wireless communication. For high capacity data transmission in broadband multimedia services, the frequency of wireless signals is increasing to the microwave or millimeter wave band. However, as the frequency of radio signals increases, electrical transmission lines generate high losses. Radio-over-Fiber (RoF) technology that can overcome this problem is a technology that transmits an optical-wireless signal containing a microwave or millimeter wave radio signal through the ultra-wideband, low-loss optical fiber. Using this technology, it is possible to build an economic wired / wireless integrated communication network by combining an optical communication-based wired communication network and a wireless communication network capable of communicating on the go.

1 shows a conceptual diagram of a general RoF system.

In general, a RoF system modulates data sent from a high-speed backbone network into an optical-wireless signal (or an optical RF signal) using a modulator at a central office (CO), and then modulates the signal in ultra-wideband, low-loss characteristics. It transmits to a base station (BS) through an optical fiber with a fiber, converts it into a wireless signal, and then wirelessly sends it to the user through an antenna.

The central base station of a RoF system typically generates an optical RF signal in the form of an optical double sideband (ODSB) using either direct modulation or external modulation in the simplest manner.

An optical RF signal in both sidebands has one carrier and two sidebands with data modulated. The optical RF signal sent to the base station is converted into an electrical RF signal using a photodetector. In this case, the converted signal has two bit components due to two sidebands. As the optical RF signal of both side bands is transmitted through the optical fiber, the two bit components have different phase shifts due to the color dispersion of the optical fiber. As a result, the magnitude of the RF signal detected by the photodetector at the base station may be rapidly attenuated due to the destructive interference of two bit components, which is called DICS (Dispersion-Induced Carrier Suppression). The DICS phenomenon degrades the performance of the RoF system and limits the distance of the link, which is a necessary task to be solved in the RoF system.

As a solution to solve the problems caused by the color dispersion of optical fiber in RoF system, there is a method of placing an all-optical frequency up-converter in the base station (BS) and a method of generating an optical RF signal of an optical single sideband (OSSB) in the central base station. There is this.

The method of placing an all-optical uplink converter in a base station (BS) places an all-optical up-converter in the base station because optical RF signals having a high carrier frequency are more sensitive to color dispersion. Separately, after transmitting to the base station to generate the optical RF signal using the all-optical frequency converter can reduce the effect of color dispersion. However, since the base station has an all-optical frequency up-converter, the structure of the base station becomes complicated, and it is relatively difficult to install, maintain, and repair as compared to the case of generating an optical RF signal from the central base station.

In the central base station, an external modulator may be used to generate a single sideband optical RF signal. The first method is to apply a dual-electrode Mach-zehnder modulator (DEMZM) to an electric RF signal with a phase difference of 90 degrees between the two electrodes while applying a quadrature bias. The second method is to remove one sideband using an optical filter after generating an optical RF signal of both sidebands using an external modulator such as a Mach-Zehnder modulator or an electro-absorption modulator (EAM). . Unlike the double sideband signal, the single sideband signal has only one bit component due to only one sideband, so that the DICS phenomenon due to the offset interference does not occur. However, the above two methods have the disadvantage that the power difference between the carrier and the sideband signal is very large, resulting in relatively low reception sensitivity when transmitting signals, and that the maximum possible RF frequency is limited by the frequency characteristics of the external modulator.

SUMMARY OF THE INVENTION An object of the present invention is to provide an all-optical frequency upconversion device capable of generating a single sideband optical RF signal.

An all-optical single sideband frequency up-conversion device according to an aspect of the present invention for achieving the above object of the present invention is an optical LO input signal having a wavelength of λ _LO consisting of two tones and a double sideband (DSB) ) And a demultiplexer (DEMUX) that separates two tones of the optical IF input signal having a wavelength of λ _IF and the optical LO input signal and sends them to paths I and path II, respectively. Optical amplifier to amplify the magnitude of the optical LO signal tone transmitted through DL and path I to correct the propagation delay time generated through II, and the output of the demultiplexer (DEMUX) and the optical IF input signal are received as inputs. Wavelength converter for converting the wavelength, the output of the optical amplifier and the output of the wavelength converter is combined MUX, select the single-side band optical RF signal from the output of the MUX It comprises an optical filter. The tone transmitted through path I of the optical LO input signal having the wavelength of λ _LO consisting of the two tones may be used as a carrier light source of an up-link frequency down converter. As an operation principle of the wavelength converter for converting the wavelength, cross gain modulation of SOA, cross phase modulation of SOA, or cross absorption modulation of EAM may be used.

In addition, the all-optical single-sideband frequency up-conversion device according to another aspect of the present invention for achieving the object of the present invention is n wavelength (λ _LOn (n = 1, 2, ..., n) generated in a multiple optical LO source )) Consists of multichannel optical LO input signals with two tones (λ _Cn and λ _Sn ) and double sideband (DSB) and n wavelengths (λ _IFn (n = 1, 2, n)), a demultiplexer that separates the wavelengths of λ _Cn and λ _Sn of the multichannel optical LO input signal and sends them to the optical amplifier and each wavelength converter. DEMUX), DL for correcting propagation delay time generated by multi-channel optical signals through respective paths, optical amplifier for amplifying the magnitude of the optical signal by inputting wavelengths of λ _Cn of the multi-channel optical LO input signal; Multi-channel output from the demultiplexer (DEMUX) N wavelength converters for converting wavelengths of λ _Sn of an optical LO input signal and a multi-channel optical IF input signal as inputs, and an output of the optical amplifier and an output of the wavelength converter are combined to provide a single side of n channels. And a MUX for generating the band optical RF signal.

According to the all-optical single-sideband frequency up-conversion method and apparatus of the present invention as described above, first, the optical single-sided band is affected by the DICS phenomenon caused by the color dispersion of the optical fiber in the millimeter wave band optical-wireless communication system. By using the frequency up-conversion method, the transmission distance of the system can be significantly increased, and the carrier of the single sideband optical RF signal can be reused as the carrier of the uplink, thereby reducing the complexity of the base station. You can save money. Second, it is possible to replace the frequency mixer, an electronic device used in the microwave or millimeter wave band at the central base station, and to easily generate optical signals over hundreds of GHz with high conversion efficiency and good sensitivity. -Can be used for wireless communication systems. Third, since it is an all-optical frequency up-conversion method, it can be easily applied to system capacity expansion using the WDM method.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the invention is not intended to be limited to the particular embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. Like reference numerals are used for like elements in describing each drawing.

The terms first, second, A, B, etc. may be used to describe various elements, but the elements should not be limited by the terms. The terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, and similarly, the second component may also be referred to as the first component. And / or < / RTI > includes any combination of a plurality of related listed items or any of a plurality of related listed items.

When a component is referred to as being "connected" or "connected" to another component, it may be directly connected to or connected to that other component, but it may be understood that other components may be present in between. Should be. On the other hand, when an element is referred to as being "directly connected" or "directly connected" to another element, it should be understood that there are no other elements in between.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "have" are intended to indicate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, and one or more other features. It is to be understood that the present invention does not exclude the possibility of the presence or the addition of numbers, steps, operations, components, components, or a combination thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art. Terms such as those defined in the commonly used dictionaries should be construed as having meanings consistent with the meanings in the context of the related art and shall not be construed in ideal or excessively formal meanings unless expressly defined in this application. Do not.

Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings.

2A and 2B are conceptual views illustrating the principle of the all-optical single sideband frequency up-conversion method according to an embodiment of the present invention, and FIG. 2A is a cross gain modulation (XGM) type wavelength converter and a demultiplexer. The structure of the all-optical single sideband frequency up-converter utilized is shown, and FIG. 2b shows the light spectrum at each position in the all-optical single-sideband frequency upconverter.

2A and 2B, an optical LO signal and an optical IF signal are input to an input of the all-optical sideband frequency up-converter. A shows an all-optical single-sideband frequency light LO signal and the IF signal f _IF optical frequency modulation of a two-light signal having a frequency difference of f _LO as the spectrum of the input portion of the up-converter.

The left part of the optical LO signal input to the demultiplexer is sent to the upper fiber (Path I) and the right part is the wavelength converter using cross gain modulation (XGM) with the optical IF signal. Is sent (Node C).

The wavelength converter is composed of SOA and uses the mutual gain modulation of SOA to copy the f _IF of the optical IF signal to the optical signal on the right side of the optical LO signal (node D). The multiplexer (MUX) combines the signals present in node B and node D. The light spectrum of the combined signals is shown at node E and the optical filter is used to select only the single sideband optical RF signal (node F).

Path I has a delay line (DL) and an optical amplifier such as SOA or EDFA. The role of DL is to compensate for the phase difference between two different optical LO signals that pass through Path I and Path II, and is essential to improve the phase noise characteristics of an all-optical sideband frequency up-converter. For example, when the path of Path II through which the optical LO signal passes is longer than the path of Path I, a delay line DL may be configured in Path I as shown in FIG. 2A. Alternatively, if the path of Path I through which the optical LO signal passes is longer than the path of Path II, a delay line DL may be configured in Path II.

Optical amplifiers such as SOA or EDFA compensate for the magnitude of the signals transmitted through Path I and Path II. Since the right part of the optical IF signal and the optical LO signal transmitted through the path II is amplified by the gain of the SOA used as the wavelength converter, the size is larger than the right part of the optical LO signal transmitted through the path I. If the two signals differ greatly in size, the reception sensitivity of the signal transmission through the optical fiber is very deteriorated. The optical amplifier in Path I amplifies the size of the left part of the optical LO signal to a level comparable to that of the optical IF signal and the right part of the optical LO signal transmitted through Path II.

3A to 3E are graphs showing characteristics of an all-optical single sideband frequency up-converter according to an embodiment of the present invention.

FIG. 3A shows an optical RF signal spectrum corresponding to the portion F of FIG. 2B made by using an implemented mutual gain modulated wavelength converter and an all-optical single sideband frequency up-converter utilizing a demultiplexer.

The optical RF signal generated by the all-optical single sideband frequency up-converter is transmitted to the base station through the optical fiber and then converted into an electrical RF signal by the photo detector. 3B shows the electrical RF signal detected at the base station.

FIG. 3C shows the conversion efficiency of the all-optical single sideband frequency up-converter according to the magnitude of the optical LO power with the current (gain) of the optical amplifier of Path I as a variable. As the optical LO power increases, the conversion efficiency also increases. However, due to the optical gain saturation of the semiconductor optical amplifier, the conversion efficiency saturates and decreases after -7 dBm optical LO power. As the applied current of the optical amplifier increases from 23 mA to 95 mA, it can be seen that the maximum conversion efficiency increases from 12 dB to 29 dB.

3D shows Carrier to Sideband Ratio (CSR) of the optical RF signal. As the applied current of the optical amplifier increases, the CSR increases to -3 dB due to the gain of the optical fiber amplifier. As a result, the power difference between the carrier and the side band is reduced, resulting in a relatively good reception sensitivity.

3E is an experimental result for viewing the DICS phenomenon due to the color dispersion of the optical fiber, and the electric RF power detected by the photodetector after passing the optical RF signal generated by using the all-optical single sideband frequency up-converter through the optical fiber of 46 km Shows the relationship between RF frequencies. In addition, the theoretical relationship between the electrical RF power and the RF frequency detected when the optical RF signal of the two side bands passes through the 46 km optical fiber for comparison with the two-side band modulation method is shown.

As shown in FIG. 3E, DICS phenomena appear in the optical RF signal of both side bands, but DICS phenomena are not found in the RF frequency range between 15 GHz and 42.5 GHz in the optical RF signal of the single side band. Although the maximum RF frequency is only up to 42.5 GHz due to the limitations of the configuration, an optical RF signal with an RF frequency of more than a few hundred GHz could be generated by using an appropriate demultiplexer.

4A to 4F illustrate the configuration and characteristics of a Wavelength Division Multiplexing (WDM) RoF system using a mutual gain modulated all-optical single sideband frequency up-converter according to an embodiment of the present invention.

In the system conceptual diagram shown in FIG. 4A, the WDM all-optical sideband frequency up-converter is a demultiplexer (DEMUX) for dividing a multi-channel optical LO signal for each channel, a plurality of semiconductor optical amplifiers (SOAn), and each for multi-channel wavelength conversion. It consists of a multiplexer (MUX) for combining the optical RF signals of the channel. An all-optical single-sideband frequency up-converter for WDM with n channels requires n wavelength converters and one optical amplifier.

As a mutual gain modulated wavelength converter and an optical amplifier, a semiconductor optical amplifier (SOA) or an optical fiber amplifier (EDFA) may be utilized. An arrayed waveguide grating (AWG) may be used as a multiplexer, and a demultiplexer may be implemented by combining an arrayed waveguide grating (AWG) and an optical interleaver. Here, a supercontinum light source, a mode-locked laser diode (MLLD), an optical frequency comb generator, or the like may be used as the multiple optical LO source.

In designing the WDM for WDM, the wavelength and channel spacing output from the optical LO source and whether the channels of the multiplexer and demultiplexer match each other should be considered. At the central base station, the multichannel optical RF signal generated by the all-optical single-sideband frequency up-converter for WDM is transmitted to a remote node through one optical fiber and distributed to each base station through each optical fiber through a demultiplexer.

FIG. 4B shows the light spectrum of the signal in each cross section shown in the system conceptual diagram of FIG. 4A.

Figure 4c shows an experimental configuration for the performance measurement of the bi-directional WDM RoF system using the all-optical SSB frequency upconverter for WDM. In the WDM RoF system, an all-optic single sideband frequency up-conversion technique is used in downlink, and is modulated in an optical RF signal composed of a single sideband in downlink in uplink. A wavelength reuse technique is used that uses an unused optical LO signal tone as an uplink carrier. In addition, in the all-optical SSB frequency upconverter of FIG. 4C, EDFA2 included in the above optical signal path is an optical amplifier made of an optical fiber, and thus the optical fiber has a long length. To correct the phase difference with the optical signal, a delay line (DL) is included in the optical signal path below.

In the downlink, a single sideband optical RF signal with a 60 GHz band 155 bps DPSK signal was generated using an all-optical sideband frequency up-converter, and it was detected by a photodetector (PD) at a base station (BS). The electrical spectrum of the RF signal is shown in Figure 4d.

In addition, we measured the BER characteristics according to the wavelength of the optical IF signal for WDM application and showed a power penalty of less than 2.5 dB in the wavelength range of 1530 ~ 1580 nm as shown in FIG.

FIG. 4f shows the BER characteristics of the downlink including 1.5m of air and the uplink using wavelength reuse, when transmitted through a 25 km optical fiber. As shown in FIG. 4F, the influence of the color dispersion of the optical fiber is hardly seen in both the downlink and the uplink.

The present invention is a type of all-optical signal processing, and the all-optical single sideband frequency up-conversion method is a method for generating an optical RF signal in the form of single sideband. It provides high conversion efficiency and high maximum RF frequency, and it is strong in color dispersion of optical fiber to enable long-distance transmission of optical RF signal and wavelength reuse technology that reuses downlink carrier as uplink carrier can be applied to system. It facilitates the installation, maintenance and repair of the remote base, and has the advantage of having a relatively good sensitivity because the power difference between the carrier and the sideband is small.

5 is a block diagram showing the configuration of an all-optical single-sideband frequency up-converter for WDM according to an embodiment of the present invention.

Referring to FIG. 5, the WDM all-optical sideband frequency upconverter according to an embodiment of the present invention has n wavelengths (λ _LOn (n = 1, 2, ..., n)) generated from a multiple optical LO source. Are composed of multichannel optical LO input signals with two tones (λ _Cn and λ _Sn ) and double sideband (DSB) and n wavelengths (λ _IFn (n = 1, 2 _,. , n)), a demultiplexer (DEMUX) that separates the wavelengths of λ _Cn and λ _Sn of the multichannel optical LO input signal and sends them to the optical amplifier and each wavelength converter. And an optical amplifier for amplifying the magnitude of the optical signal by inputting wavelengths of λ _Cn of the multi-channel optical LO input signal to correct the propagation delay time generated by the multi-channel optical signals passing through the respective paths, and the demultiplexer. Λ of the multichannel optical LO input signal output from (DEMUX) N wavelength converters converting wavelengths of _Sn and multi-channel optical IF input signals as inputs, and the output of the optical amplifier and the output of the wavelength converter combine to generate an n-channel single sideband optical RF signal Include MUX.

1 shows a conceptual diagram of a general RoF system.

2A and 2B are conceptual views illustrating the principle of an all-optical sideband frequency up-conversion method according to an embodiment of the present invention.

3A to 3E are graphs showing characteristics of an all-optical single sideband frequency up-converter according to an embodiment of the present invention.

4A to 4F illustrate the configuration and characteristics of a wavelength-multiplexed RoF system using a mutual gain modulated all-optical single sideband frequency up-converter according to an embodiment of the present invention.

5 is a block diagram showing the configuration of an all-optical single-sideband frequency up-converter for WDM according to an embodiment of the present invention.

Claims (5)

Receiving an optical local oscillation (LO) signal including a first optical signal and a second optical signal, demultiplexing the optical local oscillation signal to provide the first optical signal to a first path, and providing the second optical signal A demultiplexer providing the signal in a second path; A wavelength converter for copying an intermediate frequency f _IF of the input optical intermediate frequency IF signal to the second optical signal; A multiplexer for coupling the first optical signal provided in the first path with the optical signal output from the wavelength converter; And And an optical filter for selecting and outputting only a single sideband signal from the optical signal output from the multiplexer. The apparatus of claim 1, wherein the all-optical sideband frequency upconversion device is A delay line positioned on the first path and correcting a phase difference between the first optical signal and the second optical signal passing through the first path and the second path, respectively; And And an optical amplifier for amplifying the first optical signal to correspond to the magnitude of the second optical signal amplified by the wavelength converter. The optical amplifier of claim 2, wherein the optical amplifier  An all-optical sideband frequency up-conversion device, characterized in that it comprises a semiconductor optical amplifier (SOA) or an erbium-doped fiber amplifier (EDFA). The method of claim 1, wherein the wavelength converter Intermediate frequency of the optical intermediate frequency signal by using a mutual gain modulation phenomenon of a semiconductor amplifier, cross phase modulation of a Semiconductor Optical Amplifier Mach-Zehnder Interferometer (SOAMZI), or cross absorption modulation of EAM And an all-optical single sideband frequency up-conversion device, characterized in that it is radiated to the second optical signal. _{Demultiplexes} n optical local oscillation (LO) signals (λ _LOn ) having different wavelengths (n is a natural number of 1 or more), and thus, each optical local oscillation signal has a first optical signal (λ _Cn ) having a different wavelength. And a demultiplexer separating the second optical signal λ _Sn . A delay line correcting a propagation delay time of each of the plurality of first optical signals λ _Cn having different wavelengths provided from the demultiplexer; An optical amplifier amplifying the magnitude of each of the plurality of first optical signals whose propagation delay time is corrected through the delay line; Signal pairs λ _Sn and λ _IFn corresponding to each other among a plurality of second optical signals λ _Sn having different wavelengths and n optical intermediate frequency signals λ _IFn having different wavelengths are provided from the demultiplexer. N wavelength converters each provided and perform wavelength conversion on the received signal pairs; And And a multiplexer for combining the signals output from each of the n wavelength converters and the signals output from the optical amplifier to generate n single sideband optical RF signals.

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