KR20030077089A - Method of up-conversion of all-optical signal through cross-gain modulation in semiconductor optical amplifier and apparatus thereof - Google Patents

Abstract

PURPOSE: A device for performing an up-conversion process for an all-optical signal using an XGM(Cross-Gain Modulation) of an SOA(Semiconductor Optical Amplifier) is provided to perform the XGM for an IF data signal and an LO(Local Oscillator) signal incident on the SOA, thereby compensating optical loss by an optical gain of the SOA. CONSTITUTION: A device generates an IF data signal having a using frequency within a frequency modulation bandwidth of an SOA, and makes the IF data signal incident on the SOA. The device generates an LO signal having a bigger frequency than the frequency modulation bandwidth of the SOA, and makes the LO signal incident on the SOA. The device performs an XGM process for the IF data signal and the LO signal incident on the SOA. The IF data signal is generated by directly modulating a DFB-LD(Distributed Feedback-Laser Diode).

Description

Method for up-conversion of all-optical signal through cross-gain modulation in semiconductor optical amplifier and apparatus information using semiconductor optical amplifier cross-gain modulation

The present invention relates to an all-optical signal up-conversion method using cross-gain modulation (XGM) of a semiconductor optical amplifier (hereinafter referred to as " SOA "). To the device.

As shown in Fig. 1, the radio signal transmission (Radio-on-fiber, hereinafter referred to as "RoF") technology over a light path is a central office (hereinafter referred to as "central office") with complex equipment capable of accommodating various communication services. It is attracting interest in broadband wireless subscriber systems because of its ability to focus on " CO " and to simply design multiple base stations (hereinafter referred to as " BS "). It is also an advantage that the system is flexible in network structure and the loss of wireless signal transmission is very small in optical fiber. In order to increase the overall data transmission capacity of the existing RoF system, it is required to introduce wavelength division multiplexing (WDM).

2 shows one example thereof. Millimeter wave signals that are simultaneously up-converted into a millimeter frequency band of multiple WDM signals of different wavelengths using a single Mach-Zenhder modulator (hereinafter referred to as "MZM"). Distribution techniques have been introduced (RA 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). Other variants of signal up-conversion to millimeter wave bands using MZM have been introduced, but signal up-conversion using MZM has the following problems.

First, the modulator operation characteristics are different according to the wavelength and polarization of the signal applied to the modulator. Second, the modulator has an insertion loss of about 5 dB or more. Third, there is a limit in the frequency band where signal up-conversion is possible depending on the limited modulation bandwidth of the modulator.

As an alternative for signal up-conversion, an intermediate frequency (IF) data signal is photodetected using an electrical local oscillator (LO) signal at each BS. Optoelectronic mixng techniques have been introduced, which utilize nonlinearities of three-terminal elements such as high electron mobility transistors (HEMTs) and heterobipolar transistors (HBTs). Doing. However, this method requires a high frequency band LO signal source for each BS, which complicates BS design.

To overcome the above problems, an all-optical method that uses optically different wavelengths for baseband or IF data signals and LO signals, and which can be up-converted by optical techniques, attracts attention. have.

As an example, as shown in Figure 3, the non-linear photo-detection characteristic of a high-speed photo-detector (hereinafter referred to as "PD") can be used to up-convert the IF data signal to the millimeter frequency band. All-optical techniques have been introduced (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). An all-optical approach (eg opticalPLL, injection locking, etc.) has already been announced that can generate high frequency LO signals using relatively low frequency electrical oscillators. The optical approach has the advantage of avoiding the need for a fast electrical oscillator and fast optical modulator for generating high frequency LO signals. In addition, since the optical LO signal can be generated from the CO and delivered to each BS, the problems associated with the electrical LO signal source needs at each BS raised in optoelectronic mixing techniques can also be overcome. In addition, the accessible signal up-conversion frequency band is limited only by the photodetector bandwidth, which is typically much wider than the optical modulator bandwidth. However, the method using the nonlinearity of the photodetector has a disadvantage in that the conversion efficiency is relatively low.

The present invention has been made to solve the problems in signal up-conversion techniques for increasing the data transmission capacity of the existing RoF system as described above.

Therefore, the object of the present invention does not have a problem of the incident light wavelength and polarization dependence of the existing optical modulator, it is possible to compensate the optical losses (optical losses) by the optical gain of the SOA, The present invention provides a method and apparatus for all-optical signal up-conversion using semiconductor optical amplifier cross-gain modulation capable of improving signal conversion efficiency.

1 is a view showing the configuration of a RoF system

2 is a view showing an example of applying the WDM technique to the RoF technology as a prior art

FIG. 3 illustrates a signal up-conversion technique using nonlinearity of a photodetector as another prior art. FIG.

4 is a conceptual diagram of the present invention

Figure 5 is a block diagram of an embodiment of the present invention

6A and 6B show light spectra before and after SOA passage.

7A and 7B show the RF-spectrum before and after SOA passage.

Summary of the Invention

The method according to the present invention comprises a first step of generating an IF data signal having a use frequency within a semiconductor optical amplifier frequency modulation bandwidth and incident into the semiconductor optical amplifier; Generating a LO signal having a frequency greater than the semiconductor optical amplifier frequency modulation bandwidth and injecting the LO signal into the semiconductor optical amplifier; And a third step of cross-gain modulating the IF data signal and the LO signal incident together into the semiconductor optical vapor.

The apparatus according to the invention comprises means for generating an IF data signal; Means for generating an LO signal; And a semiconductor optical amplifier configured to receive the IF data signal and the LO signal and cross-gain modulate. The frequency of the IF data signal is included in the semiconductor optical amplifier frequency modulation bandwidth, and the frequency of the LO signal is the semiconductor. The optical amplifier is characterized by greater than the frequency modulation bandwidth.

The present invention proposes an all-optical signal up-conversion method and apparatus using cross-gain modulation of a semiconductor optical amplifier. According to the present invention, there is no problem of incident light wavelength and polarization dependence of the conventional optical modulator, and the optical losses of the SOA can be compensated for by optical gain, and signal conversion is possible. Efficiency can be improved.

4 shows a conceptual diagram of the present invention. When the optical IF data signal and the LO signal go through the SOA together, they are optically up-converted by the cross gain modulation (XGM) effect of the SOA. In the present invention, optical IF signals and optical LO signals of different wavelengths pass through the SOA together. It is a feature of the invention that the IF data signal use frequency f _IF is within the SOA frequency modulation bandwidth, while the frequency f _LO of the LO signal is relatively higher than the SOA frequency modulation bandwidth.

IF data and LO light signals incident together into the SOA undergo XGM effects within the SOA. IF data signals within the SOA frequency modulation bandwidth modulate LO signals of different wavelengths, thereby projecting the IF data signal image onto the LO signal. do. On the contrary, the LO signal image can be projected onto the IF data signal by modulating the IF data signal, but in this case, since the LO signal frequency band is outside the SOA frequency modulation bandwidth, the modulation effect is extremely small. do. As a result, the IF data signal is projected onto the LO signal due to the XGM effect in the SOA, so that the IF data signal is up-converted to the LO signal band after photodetection in the photodetector.

Example

Hereinafter, an embodiment according to the present invention will be described in detail. 5 is a block diagram illustrating an embodiment for all-optical signal up-conversion with polarization-insensitive SOA.

In this embodiment, a MZM double-sideband suppressed optical carrier (MZM-DSB) modulation technique is used to generate an optical LO signal above the SOA modulation bandwidth. The IF signal was generated by directly modulating a distributed feedback-laser diode (DFB-LD) at 1 GHz. After combining the 1 GHz IF data signal and the 25 GHz LO signal having different wavelengths through a fiber coupler 57, the SOA 58 was passed together. At this time, the IF signal peak optical power was fixed to 0 dBm in front of the SOA 58 by a variable optical attenuator (VOA).

To generate the LO signal optically, the Mach-Zenhder Modulator (54, Mach-Zenhder Modulator) is set to Vπ and the DSB-SC modulation technique is employed. Irradiation of light by TLS (53, tunable laser source), which is a light source device that can control the wavelength of light, and can electrically modulate the Mach-Zender modulator 54 to 12.5 GHz to obtain a 25 GHz LO signal. (12.5 GHz) is determined by the limited available frequency bandwidth of the modulator. An er-doped fiber amplifier (EDFA) 55 is an optical amplifier using Er-doped fiber. The limiting component of the available LO signal frequency band can be overcome through optical heterodyne techniques. The optical heterodyne technique simultaneously inputs two optical carriers having a frequency difference corresponding to the frequency of the desired millimeter wave band to the photodetector PD to provide electrical power of a desired frequency component by mixing the two optical carriers. How to get a signal. The peak power of the +/- 1 sideband light of the modulated optical signal may be 9 dB greater than the optical carrier power by adjusting the electrical modulation power.

IF signals and LO signals are incident in the same direction as the SOA 58 and cross-modulate with each other within the SOA 58. The SOA 58 used in this example had a frequency bandwidth of about 3 GHz and used a 25 GHz LO signal that was much higher than the used SOA frequency modulation bandwidth. In Fig. 5, the PD 59 (photo detector) is an optical detector and the RF-SA 60 (RF-spectrum anlyzer) is a frequency analyzer of an RF signal.

6A and 6B are optical spectra of IF data and LO signals before and after SOA 58 pass. The IF and LO signal wavelengths are about 11 nm apart. In FIG. 6A, it can be seen that the peak power of the +/- 1 sideband in the LO signal light spectrum before SOA passage (dotted line) is greater than the optical carrier power. These two sidebands are 25 GHz apart, doubling the Mach-Zenhder modulator 54 driving frequency of 12.5 GHz.

6A and 6B, it can be seen that each optical power has an optical gain of about 16 dB while passing through the SOA 58. The side bands around λ _IF from FIG. 6B are generated by the XGM effect by the LO signal, but the modulation efficiency is very low (about -20 dB) because the LO signal frequency is higher than the SOA modulation bandwidth.

The +/- 1 sidebands of the LO signal λ _LO are also cross-modulated by the IF signal λ _IF , which is sufficient for signal up-conversion since it is within the SOA frequency modulation bandwidth. However, the XGM effects of these +/- 1 sidebands cannot be seen directly in the light spectrum due to the resolution limitations of the optical spectrum analyzer. The cross-gain modulated results are confirmed by RF-spectrum analyzer after photodetection as shown in FIGS. 7A and 7B.

In FIG. 7B, up-conversion occurs to 24 GHz lower sideband (LSB) and 26 GHz upper sideband by a 25 GHz optical LO signal after the IF signal (FIG. 7A) prior to signal up-conversion has passed through SOA 58. can confirm. As can be seen in FIGS. 7A and 7B, it can be seen that the up-converted LSB and USB signals (FIG. 7B) have a signal size about 10 dB larger than the IF signal (FIG. 7A) before conversion.

As a result, since the SOA gain is directly related to the signal up-conversion process, it means that it can have conversion gain, the efficiency of which is much greater than the efficiency in the prior art.

This property is considered to be very useful for RoF systems in which one optical LO signal is provided to a plurality of WDM IF (or baseband) signals.

The present invention has been described above with reference to specific embodiments thereof, but the technical scope of the present invention is limited to the above embodiments and should not be interpreted. The technical scope of the invention should be reasonably made by the interpretation of the claims.

The up-conversion technique proposed by the present invention is a structure that can be used in any LO frequency band. According to the present invention, the following effects can be obtained.

First, since one LO signal can be commonly used for the up-conversion process of multiple WDM signal sources, the cost associated with generating the LO signal is relatively low.

Second, by integrating WDM technology into existing RoF systems, it is possible not only to maximize data transmission capacity but also to maintain existing WDM networks.

Third, since the present invention can be applied to all services on the existing network, it is very flexible in the network structure design in that a separate operation is not necessary only for the structure of the present invention.

Fourth, since the LO signal is transmitted from the CO to the multiple BSs, the BS does not need a separate electric LO signal source and an electric mixer for signal up-conversion. Thus, the BS design is very simple.

Claims (8)

In the all-optical signal up-conversion method using a semiconductor optical amplifier (SOA), A first step of generating an IF data signal having a use frequency within the semiconductor optical amplifier frequency modulation bandwidth and incident into the semiconductor optical amplifier, A second step of generating an LO signal having a frequency greater than the semiconductor optical amplifier frequency modulation bandwidth and entering the semiconductor optical amplifier, And a third step of cross-gain modulation of the IF data signal and the LO signal that are incident together into the semiconductor optical amplifier. 2. The all-optical signal up-conversion method using the semiconductor optical amplifier cross-gain modulation. The method of claim 1, And the IF data signal is generated by directly modulating DFB-LD. The method of claim 1, And the LO signal is generated by a DSB-SC modulation method. The all-optical signal up-conversion method using semiconductor optical amplifier cross-gain modulation. The method of claim 1, And a step of combining the IF data signal and the LO signal through a fiber coupler between the second and third steps, wherein the all-optical signal up-conversion method using the semiconductor optical amplifier cross-gain modulation. Means for generating an IF data signal, Means for generating an LO signal, And a semiconductor optical amplifier configured to receive the IF data signal and the LO signal and cross-gain modulate. The frequency of the IF data signal is included in the semiconductor optical amplifier frequency modulation bandwidth, and the frequency of the LO signal is greater than the semiconductor optical amplifier frequency modulation bandwidth, all-optical signal up-up using the cross-gain modulation Conversion device. The method of claim 5, And said means for generating an IF data signal generates a signal by directly modulating a DFB-LD. 12. An all-optical signal up-conversion device using semiconductor optical amplifier cross-gain modulation. The method of claim 5, And the means for generating the LO signal generates a signal by a DSB-SC modulation method. The method of claim 5, An all-optical signal up-conversion device using semiconductor optical amplifier cross-gain modulation further comprising a fiber coupler for combining the IF data signal and the LO signal.