WO2003081811A1

WO2003081811A1 - Method of up-conversion of all-optical signal through cross-gain modulation in semiconductor optical amplifier and apparatus thereof

Info

Publication number: WO2003081811A1
Application number: PCT/KR2003/000519
Authority: WO
Inventors: Woo-Young Choi; Young-Kwang Seo; Chang-Soon Choi
Original assignee: Yonsei University
Priority date: 2002-03-25
Filing date: 2003-03-18
Publication date: 2003-10-02
Also published as: AU2003215937A1; KR20030077089A

Abstract

Disclosed herein is a method of up-conversion of all-optical signal through cross-gain modulation in a semiconductor optical amplifier and an apparatus thereof. The method according to the present invention comprises: a first step which generates an IF data signal used in a semiconductor optical amplifier's frequency modulation bandwidth, and incidents the IF data signal in the semiconductor optical amplifier; a second step which generates an LO signal having frequency higher than the semiconductor optical amplifier's frequency modulation bandwidth, and incidents the LO signal in the semiconductor optical amplifier; and a third step which performs cross-gain modulation (XGM) with the IF data signal and the LO signal.

Description

METHOD OF UP-CONVERSION OF ALL-OPTICAL SIGNAL

THROUGH CROSS-GAIN MODULATION IN SEMICONDUCTOR

OPTICAL AMPLIFIER AND APPARATUS THEREOF

Technical Field The present invention relates to a method and apparatus for up- conversion of an all-optical signal through Cross-Gain Modulation (hereinafter referred to as an "XGM") in a Semiconductor Optical Amplifier (hereinafter referred to as a "SOA").

Background Art As shown in Fig. 1, a Radio-on-Fiber (hereinafter referred to as an

"RoF") system becomes the object of interest in a Broadband Wireless Local Loop in that it concentrates complicated equipments which can accommodate various communication services on a Central Office (hereinafter referred to as a "CO"), and allows a plurality of Base Stations (hereinafter referred to as "BSs") to be designed in a simple manner. Additionally, the RoF system is advantageous in that it is a system applicable to various network structures and the transmission loss of an RoF signal is low. In order to increase overall data transmission capacity of the conventional RoF system, introduction of a Wavelength Division Multiplexing (hereinafter referred to as a "WDM") technology is required.

Fig. 2 illustrates an example of the WDM technology. There is introduced a millimeter wave signal distribution technology, through which a plurality of WDM signals having different wavelengths are simultaneously up- converted to a millimeter frequency band using a single Mach-Zenhder Modulator (hereinafter referred to as an "MZM") and then are transmitted through optical fibers, in a "R. A. Griffin et al. , Crosstalk Reduction in an Optical mm-Wave/DWDM Overlay for Radio-Over-Fibre Distribution, in Tech. Dig. MWP99. pp.131-134, 1999". Another example of signal up-conversion to a millimeter frequency band using the MZM has been introduced, but the signal up- conversion using the MZM has the following problems.

Firstly, operation properties of the modulator are different depending upon the wavelengths and polarizations of signals applied to the modulator.

Secondly, the modulator has insertion loss equal to or greater than about 5 dB.

Thirdly, the modulator has a limited modulation frequency bandwidth, so that it is limited to a narrow frequency band in which the signal up-conversion is possible.

For another method for signal up-conversion, there is introduced an optoelectronic mixing technique, in which an Intermediate Frequency (hereinafter referred to as an "IF") data signal photo-detected is mixed in each of BSs using an electrical Local Oscillator (hereinafter referred to as a "LO") signal. The method uses non-linearity of a three-terminal element, such as a High Electron Mobility Transistor (HEMT) or a HeteroBipolar Transistor (HBT). However, the method requires an LO signal source having a high frequency band in each of the BSs, so that the method has a problem that the design of the BSs is complicated.

To overcome the above problems, an all-optical method, which is capable of signal up-converting a baseband signal, an IF data signal or an LO signal using optically different wavelengths, becomes the object of interest.

For one example of the all-optical method, as in Fig. 3, there has been introduced an all-optical method, which is capable of signal up-converting an IF data signal to a millimeter wave frequency band using nonlinear photo-detection properties of a high speed Photo-Detector (hereinafter referred to as a "PD"), in M. Tsuchiya, et al, Nonlinear Photodetection Scheme and Its system

Applications to Fiber-Optic Millimeter-Wave Wireless Down-Links, IEEE Trans on Microwave Theory and Techniques, vol. 47, no. 7, pp. 1342-1350, 1999. There has been previously disclosed an optical heterodyne technique, such as an optical PLL and an injection locking, which generates a high frequency LO signal using an electrical oscillator generating a relatively low frequency. Accordingly, an all-optical approach has an advantage that it does not need a high speed electrical oscillator and a high speed optical modulator to generate a high frequency LO signal. Additionally, an optical LO signal is generated from a CO and transmitted to each of BSs, so that the problem raised in the optoelectronic mixing technique due to the need for the electrical LO signal source in each of the

BSs can be overcome. Further, an accessible signal up-conversion frequency band is restricted by the bandwidth of a PD, which is much broader than that of the optical modulator. However, the method of using the non-linearity of the PD has a problem that a signal conversion efficiency is relatively low.

Disclosure of the Invention

The present invention has been devised to overcome the problems of the methods for signal up-conversion for increasing the data transmission capacity of the conventional RoF system.

Accordingly, the object of the present invention is to provide a method and apparatus for up-conversion of an all-optical signal through an XGM in an

SOA. in which the problems of wavelength and polarization dependency of an input light of the conventional optical modulator are not generated, optical losses are compensated for by an optical gain of the SOA and a signal conversion efficiency can be improved. In order to accomplish the above object, the present invention provides a method for up-conversion of an all-optical signal through Cross-Gain Modulation in a semiconductor optical amplifier, comprising: the first step of generating an IF data signal having a working frequency in a range of a frequency modulation bandwidth of the semiconductor optical amplifier and allowing the IF data signal to be input to the semiconductor optical amplifier; the second step of generating an LO signal having a frequency higher than frequencies of the frequency modulation bandwidth of the semiconductor optical amplifier and allowing the LO signal to be input to the semiconductor optical amplifier; and the third step of cross-gain modulating the IF data signal and the LO signal having been input to the semiconductor optical amplifier.

In order to accomplish the above object, the present invention provides an apparatus for up-conversion of an all-optical signal through Cross-Gain Modulation in a semiconductor optical amplifier, comprising: a means for generating an IF data signal; a means for generating an LO signal; and the semiconductor optical amplifier for receiving the IF data signal and the LO signal and cross-gain modulating them; wherein a frequency of the IF data signal is in a range of a frequency modulation bandwidth of the semiconductor optical amplifier, and a frequency of the LO signal is higher than frequencies of the frequency modulation bandwidth of the semiconductor optical amplifier.

The present invention described above provides a method and an apparatus for up-conversion of an all-optical signal through an XGM in an SOA. According to the present invention, the problems of wavelength and polarization dependency of an input light of the conventional optical modulator are not generated, optical losses are compensated for by an optical gain of the SOA and a signal conversion efficiency can be improved.

Fig. 4 is a conceptual view according to the present invention. When an optical IF data signal and an LO signal are input to the SOA, the signals are up- converted by an XGM effect of the SOA in an optical manner. In the present invention, the optical IF signal and the optical LO signal having different wavelength pass through the SOA. A characteristic feature of the present invention is that the working frequency f_[F of the IF data signal is in a range of the frequency modulation bandwidth of the SOA, while the working frequency fLo of the LO signal is higher than the frequency modulation bandwidth of the SOA. The optical IF data signal and the optical LO signal input to the SOA mutually experience the XGM effect in the SOA. The IF data signal in the range of the frequency modulation bandwidth of the SOA modulates the LO signal having a different wavelength, so that an image of the IF data signal is reflected in the LO signal. Reversely, the LO signal modulates the IF data signal, so that an image of the LO signal may be reflected in the IF data signal. However, since the frequency band of the LO signal is out of the range of the frequency modulation bandwidth of the SOA, a limited modulation effect is achieved. Consequently, the IF data signal is reflected in the LO signal by the XGM effect in the SOA, is photo-detected in a PD, and is therefore up-converted in the frequency band of the LO signal.

Brief Description of the Drawings

Fig. 1 is a view showing a construction of an RoF system;

Fig. 2 is an example in which a WDM technique is applied to a RoF technique as a prior art; Fig. 3 is a view illustrating a method of signal up-conversion using non- linearity of an optical detector as another prior art;

Fig. 4 is a conceptual view according to the present invention;

Fig. 5 is a view showing a construction according to an embodiment of the present invention; Figs. 6a and 6b are views showing optical spectrums at the time before and after signals pass through an SOA, respectively; and

Figs. 7a and 7b are views showing RF spectrums at the time before and after signals pass through the SOA.

Best Mode for Carrying Out the Invention Hereinafter, an embodiment according to the present invention will be described with reference to the attached drawings. Fig. 5 is a block diagram according to the embodiment for up-conversion of an all-optical signal using a polarization-insensitive SOA.

In the embodiment, a MZM Double-Side Band Suppressed Optical Carrier (DSB-SC) technique is used to generate an optical LO signal having a frequency band equal to or higher than the modulation bandwidth of the SOA. An IF signal is generated by directly modulating a signal of a Distributed FeedBack-Laser Diode (DFB-LD) to a signal of 1 GHz. The IF data signal of 1 GHz and an LO signal of 25 GHz which have different wavelengths are added together by a fiber coupler 57, and then passes through the SOA 58. In this case, a peak optical power of the IF data signal is set to 0 dBm in a front end of the SOA 58 by a Variable Optical Attenuator (VOA) 52. In order to generate an LO signal in an optical manner, the operation voltage of an MZM 54 is set to Vπ and the DSB-SC modulation technique is employed. The LO signal of 25 GHz is obtained by irradiating a light generated by a Tunable Laser Source (TLS) 53 that is a light source which is capable of regulating a wavelength of a light, and of electrically modulating the light input to the MGM 54 to the LO signal of 25 GHz. The former frequency of 25 GHz is determined by a limited access frequency bandwidth of the MGM 54. An Er- Doped Fiber Amplifier (EDFA) 55 is an optical amplifier using an Er-doped fiber. A factor of limiting the access frequency band of the LO signal can be overcome by an optical heterodyne technique. The optical heterodyne technique is used to obtain an optical signal having a desired frequency by simultaneously inputting two optical carriers having frequencies, whose difference falls under a desired millimeter wave frequency band, to the PD 59 and mixing the two optical carriers. Of modulated optical signals, peak power of +/- 1 sidebands is regulated using an electrical modulation power, so that the peak power is increased by 9 dB or more, compared to optical carrier power.

The IF signal and the LO signal are input to the SOA 58 and then cross- modulated in the SOA 58. The frequency modulation bandwidth of the SOA 58 used in the embodiment is about 3 GHz, and the LO signal of 25 GHz much higher than the frequency modulation bandwidth of the SOA 58 is used. In Fig. 5, a PD 59 is a photo detector, and an RF-SA 60 is a RF-spectrum analyzer.

Figs. 6a and 6b are views showing optical spectrums of the IF data signal and the LO signal at the time before and after they pass through the SOA 58, respectively. The wavelengths of the IF signal and the LO signal are spaced apart by about 11 nm. In Fig. 6a, it is confirmed that the peak power of +/- 1 sidebands is greater than the optical carrier power in the optical spectrum of the LO signal (dotted line) at the time before the signal passes through the SOA 58 in Fig. 6a. The two sidebands are spaced apart from each other by 25 GHz, which is twice the driving frequency of 12.5 GHz of the MZM 54.

In Figs. 6a and 6b, it is confirmed that each optical carrier power obtains optical gain of about 16dB while the LO and IF signals pass through the SOA 58.

In Fig. 6b, surrounding sidebands of λικ are generated by XGM effect caused by the LO signal, but the frequency of the LO signal is higher than the modulation bandwidth of the SOA 58, so that a modulation efficiency is very low as much as -20 dB. +/- 1 sidebands of the LO signal having λ_Lo are cross-modulated by the

IF signal having λip, and the frequency of the IF signal is in the range of the frequency modulation bandwidth of the SOA 58, so that the IF signal is sufficiently up-converted. However, the XGM effect of the +/- 1 sidebands cannot be directly confirmed by an optical spectrum due to the limit of the resolution of an optical spectrum analyzer. Cross-gain modulated results are confirmed through an RF-spectrum analyzer after the signals are photo-detected as shown in Figs. 7a and 7b.

In Fig. 7b, it is confirmed that the IF signal (see Fig. 7a) is up-converted to Lower SideBand (LSB) of 24 GHz and Upper SideBand (USB) of 26 GHz after the IF signal at the time before it is up-converted passes through the SOA

58. As shown in Figs. 7a and 7b, each of the sizes of up-converted LSB and USB (see Fig. 7b) is greater than that of the IF signal at the time before up- conversion (see Fig. 7a) by about 10 dB.

Consequently, the gain of the SOA directly takes part in signal up- conversion, so that conversion gain is generated and a signal conversion efficiency is much greater that a prior art.

These properties is thought to be very useful to an RoF system which provides a single optical LO signal to a plurality of WDM IF signals or base signals. Although the present invention was described using a specific embodiment, the technical scope of the present invention should not be considered to be limited to the embodiment. The technical scope of the present invention should be rationally interpreted through the analysis of claims.

As described above, the up-conversion method proposed in the present invention can be used in any LO frequency band and obtain the following effects according to the present invention.

Firstly, a single LO signal can be used for up-conversion of a plurality of WDM signal sources, so that cost of generating the LO signal can be decreased.

Secondly, the WDM technique is applied to the conventional RoF system, so that data transmission capacity can be maximized, and, additionally, a conventional WDM network can be maintained as it is.

Thirdly, the present invention can be applied to all services on the conventional network, so that the present invention allows flexible design of a network structure because another manipulation is not needed to generate only the structure of the present invention.

Fourthly, since an LO signal is transmitted from a CO to a plurality of BSs, .another electrical LO signal source and an electrical mixer, for signal up- conversion, are not needed in each of the BSs. Accordingly, the design of the BSs becomes very simple.

Claims

1. A method for up-conversion of an all-optical signal through Cross- Gain Modulation in a semiconductor optical amplifier, comprising: the first step of generating an Intermediate Frequency (IF) data signal having a working frequency in a range of a frequency modulation bandwidth of the semiconductor optical amplifier and allowing the IF data signal to be input to the semiconductor optical amplifier; the second step of generating an Local Oscillator (LO) signal having a frequency higher than frequencies of the frequency modulation bandwidth of the semiconductor optical amplifier and allowing the LO signal to be input to the semiconductor optical amplifier; and the third step of cross-gain modulating the IF data signal and the LO signal having been input to the semiconductor optical amplifier.

2. The method according to claim 1, wherein the IF data signal is generated by directly modulating a Distributed FeedBack-Laser Diode (DFB- LD).

3. The method according to claim 1, wherein the LO signal is generated using a Double-Side Suppressed Optical Carrier (DSB-SC) modulation method.

4. The method according to claim 1, further comprising the step of adding the IF data signal and the LO signal through a fiber coupler after the second step and before the third step.

5. An apparatus for up-conversion of an all-optical signal through Cross- Gain Modulation in a semiconductor optical amplifier, comprising: a means for generating an IF data signal; a means for generating an LO signal; and the semiconductor optical amplifier for receiving the IF data signal and the LO signal and cross-gain modulating them; wherein a frequency of the IF data signal is in a range of a frequency modulation bandwidth of the semiconductor optical amplifier, and a frequency of the LO signal is higher than frequencies of the frequency modulation bandwidth of the semiconductor optical amplifier.

6. The apparatus according to claim 5, wherein the means for generating the IF data signal generates the IF data signal by directly modulating a Distributed FeedBack-Laser Diode (DFB-LD)

7. The apparatus according to claim 5, wherein the means for generating the LO signal generates the LO signal using a Double-Side Band Suppressed Optical Carrier (DSB-SC) modulation method.

8. The apparatus according to claim 5, further comprising a fiber coupler for adding the IF data signal and the LO signal.