1. Introduction

As the global mobile data traffic increases rapidly, broadband wireless communication systems that can support high data rates are actively being developed. A high data rate can be achieved by extending the carrier frequency to a millimeter-wave or terahertz frequency band. Radio-over-fiber (RoF) technology is a very attractive candidate for supporting such broadband wireless communication systems in millimeter-wave or terahertz frequency bands because it utilizes an optical fiber, featuring ultra-low transmission loss and ultra-wide bandwidth, to transmit radio frequency (RF) signals in the form of an optical signal. Despite its ultra-low loss and ultra-wide bandwidth characteristics, a single mode optical fiber has chromatic dispersion that affects the transmitted optical RF signal, which results in performance degradation of the overall RoF system [1]. In RoF systems, an optical RF signal is generated in a central office (CO) and transmitted over an optical fiber to base stations (BSs). The generated optical RF signal in the form of an optical double-sideband (ODSB) signal consists of one carrier and two sidebands. When the ODSB RF signal is transmitted, each spectral component undergoes different phase shifts due to the fiber chromatic dispersion. The transmitted optical RF signals are converted to electrical RF signals by a photodiode (PD) in the BSs. The electrical RF signal has two RF beating signals generated by the optical carrier and two optical sidebands that have a phase difference proportional to the frequency and fiber distance. As a result, the electrical RF signal power fluctuates as a function of the optical fiber distance and the electrical RF frequency, potentially causing serious performance degradation of RoF systems.

The power fluctuation problem can be alleviated by using an optical single-sideband (OSSB) RF signal consisting of one optical carrier and one optical sideband that produces an electrical RF signal with only one RF beating signal. Various schemes for OSSB RF signal generation utilizing external modulators [1,2], optical filters [3], and a semiconductor optical amplifier (SOA) with an optical interleaver [4] have been reported. However, the schemes using external modulators and optical filters had disadvantages, including a limited RF frequency bandwidth according to the external modulator bandwidth and low receiver sensitivity caused by the power difference between an optical carrier and an optical sideband [5]. The scheme using an SOA and an optical interleaver had a large maximum RF frequency bandwidth and similar optical carrier and optical sideband powers for improved receiver sensitivity using an additional optical amplifier, but it needed an additional delay line to reduce excessive phase noise caused by the imbalance between two different optical paths [6].

In this paper, a novel photonic single-sideband (SSB) frequency up-conversion technique based on the cross polarization modulation (XPolM) effect in an SOA is proposed and experimentally demonstrated. The XPolM effect in an SOA was previously used to generate an ODSB signal with a suppressed carrier in an all-optical frequency up-converter [7]. The proposed frequency up-conversion technique based on the XPolM effect in an SOA utilizes XPolM and cross gain modulation (XGM) effects to modulate two tones of the optical LO signal independently to generate an OSSB signal with an optical carrier and an optical sideband having a small power difference. The proposed photonic SSB frequency up-conversion scheme has a large maximum RF frequency bandwidth, a small power difference in the optical carrier and the optical sideband without an additional amplifier, negligible phase noise degradation during the up-conversion process, and a spurious free dynamic range (SFDR) suitable for microcellular personal communication systems.

2. Principle of the photonic SSB frequency up-conversion technique using the XPolM effect in an SOA

Figure 1 shows the principle of the photonic SSB frequency up-conversion technique utilizing the XPolM effect in an SOA. Figure 1(a) shows an RoF down-link system configured using the photonic SSB frequency up-converter based on the XPolM effect. The optical LO signal generator, the optical IF signal generator, and the photonic SSB frequency up-converter are located in the CO. An optical LO signal (λ_LO) that consists of two optical tones separated by an LO frequency (f_LO) is produced in the optical LO signal generator and directed to an SOA in the photonic SSB frequency up-converter. The spectrum of the optical LO signal (A) is shown in Fig. 1(b). A polarization controller (PC1) is used to keep the polarization state of the optical LO signal at approximately 45° to the SOA layers. An optical IF signal (λ_IF) in form of an ODSB signal with an IF frequency (f_IF), the optical spectrum of which is shown in B of Fig. 1(b), is generated in the optical IF signal generator and injected to the output port of the SOA using an optical circulator. Inside the SOA of the photonic SSB frequency up-converter, the injected optical LO signal is amplified and at the same time the polarization of the optical LO signal is modulated by the injected optical IF signal. The optical IF signal induces additional birefringence that causes different refractive index variation of the TE and TM mode optical LO signals. In other words, the additional birefringence induced by the optical IF signal injection causes the polarization rotation of the optical LO signal. The effect is called cross polarization modulation (XPolM) [8–11]. In addition to the XPolM effect, when the power of the injected optical IF signal gets high, a cross gain modulation (XGM) effect takes place in the SOA. Through the XGM effect the intensity of the optical LO signal is modulated by the optical IF signal. As a result, an optical LO signal with the polarization and intensity modulated by the optical IF signal is generated at the output of the SOA. The spectrum of the optical signal at the output of the SOA is shown in C of Fig. 1(b). This modulated optical LO signal is injected to an optical demultiplexer (DEMUX) through the optical circulator. The optical DEMUX splits two tones of the modulated optical LO signal and transmits the left tone to the upper optical fiber and the right tone to the lower optical fiber. The spectra of the signals in the upper optical fiber and the lower optical fiber are shown in D and E of Fig. 1(b), respectively. Two polarization controllers (PC2 in the upper optical fiber and PC3 in the lower optical fiber) change the polarization angles of the divided optical LO signal tones. The polarization-to-intensity converter converts the polarization-modulated optical LO signal to intensity-modulated optical LO signal. The XPolM effect in the SOA and the polarization-to-intensity converter along with PC2 and PC3 generate an inverted or a non-inverted mode modulated optical signal [10,11]. These modes are determined by the relative polarization angle of the optical LO signal to the orientation of the polarization-to-intensity converter that is controlled by PC2 and PC3. In the inverted mode the modulated signal and the input optical IF signal have opposite polarity, while in the non-inverted mode the modulated signal and the input optical IF signal have identical polarity. Meanwhile, the XGM effect generates an inverted mode modulated signal. At the output of the polarization-to-intensity converter an optical signal modulated through the XPolM and XGM effects is generated. An OSSB RF signal can be produced using a combination of the XPolM and XGM effects in the SOA and the polarization-to-intensity converter. In the photonic SSB frequency up-converter shown in Fig. 1(a), the right tone of the optical LO signal is directed to the lower optical fiber. PC3 in the lower optical fiber is adjusted to produce a non-inverted mode modulated optical signal through the XPolM effect that compensates for an inverted mode modulated optical signal through the XGM effect at an optimum optical IF signal power (P_{IF, opt}). The right tone of the optical LO signal modulated by the XGM effect in the inverted mode, the spectrum of which is shown in E of Fig. 1(b), is compensated by modulation through the XPolM effect in the non-inverted mode, producing a non-modulated right tone of the optical LO signal. The schematic transfer curve of the proposed photonic SSB frequency up-converter shown in Fig. 1(c) visualizes the modulation process of the right tone signal. The spectrum of the output non-modulated right tone of the optical LO signal is shown in F of Fig. 1(b), where only the right tone of the optical LO signal without sidebands from the IF signal exists. The left tone of the optical LO signal is directed to the upper optical fiber. PC2 in the upper optical fiber is also adjusted to generate the modulated left tone of the optical LO signal through the XPolM effect in the non-inverted mode. Note that PC2 is adjusted to produce a much larger non-inverting signal through the XPolM effect than the inverting signal generated through the XGM effect at the optimum optical IF signal power, which produces a non-inverted mode modulated optical signal at the optimum optical IF signal power (P_{IF, opt}). The modulation process of the left tone signal is also visualized in Fig. 1(c) using the schematic transfer curve of the photonic SSB frequency up-converter. The optical spectrum of the generated OSSB RF signal at the output of the polarization-to-intensity converter is shown in F of Fig. 1(b), which consists of the modulated left tone of the optical LO signal through the upper optical fiber and the non-modulated right tone of the optical LO signal through the lower optical fiber. The generated OSSB RF signal is then transmitted to the BS over a single mode optical fiber. In the BS, the transmitted OSSB RF signal is converted to an electrical RF signal by a PD. The electrical RF signal has no beating components at the RF frequencies of the lower sideband and the upper sideband (ƒ_{RF, LSB} = ƒ_LO – ƒ_IF and ƒ_{RF, USB} = ƒ_LO + ƒ_IF), and thus there will be no power fluctuation problem caused by fiber chromatic dispersion.

 

Fig. 1 Principle of a photonic SSB frequency up-conversion technique employing XPolM effect in an SOA. (a) RoF down-link configured using the photonic SSB frequency up-converter based on XPolM effect. (b) Optical signal spectra at various points of the OSSB frequency up-converter. (c) Schematic transfer function of the photonic SSB frequency up-converter showing the principle of generating an OSSB signal.

Download Full Size | PDF

3. Experiment and results

A schematic diagram of the experimental setup for the photonic SSB frequency up-converter based on the XPolM effect is shown in Fig. 2. In the optical LO signal generator, an optical signal with a wavelength (λ_LO) of 1556.15 nm and an electrical LO signal with a frequency of 30 GHz (ƒ_LO/2) were applied to a Mach-Zehnder modulator (MZM) biased at V_π for the optical carrier suppression (OCS) method. A fiber Bragg grating (FBG) placed at the output of the MZM was used to further suppress the optical carrier of the optical LO signal. The generated optical LO signal consists of two optical tones separated by 60 GHz (ƒ_LO). An optical isolator before the FBG is used to remove the reflected optical signals from the FBG. An erbium-doped fiber amplifier (EDFA1) was used to increase the power of the optical LO signal and PC1 was used to adjust the polarization angle of the optical LO signal. A variable optical attenuator (VOA1) at the output of PC1 was used to control the optical LO signal power. At the input of an SOA, an optical isolator was used to remove the optical signals from the SOA to the optical LO signal generator including the optical IF signal injected from the output of the SOA. In the optical IF signal generator, a double sideband optical IF signal (λ_IF = 1553.16 nm) with an IF frequency (ƒ_IF) of 5 GHz was generated using an electro-absorption modulator (EAM). PC2, EDFA2, and VOA2 were used to control the polarization and the intensity of the optical IF signal. The generated optical LO signal and the optical IF signal were directed to the input and output ports of the SOA (CIP SOA-XN-OEC-1550), respectively, so that the optical LO signal could be modulated by the optical IF signal through the XPolM and XGM effects. The SOA was biased at a current of 550 mA in this experiment. The optical interleaver, which serves as an optical DEXUM, separates the two tones of the optical LO signal and directs the left tone to the upper optical fiber path and the right tone to the lower optical fiber path. PC3 and PC4 were used to adjust the polarization angle of the left and right tones of the optical LO signal with respect to the orientation of a polarization beam splitter (PBS), so that the XPolM-based modulation mode could be controlled. The two tones of the modulated optical LO signal were combined by an optical coupler and then directed to the PBS. The PBS plays the role of a polarization-to-intensity converter and an OBPF was used to eliminate the ASE noise from the SOA. A PD was used to convert the output optical RF signal to an electrical RF signal. The optical and electrical signals were analyzed by an optical spectrum analyzer (OSA) and an electrical spectrum analyzer (ESA), respectively.

 

Fig. 2 Experimental setup for photonic SSB frequency up-conversion using the XPolM effect in an SOA.

Download Full Size | PDF

Figure 3 shows the measured powers of the two optical tones (left and right) of the optical RF signal as a function of the power of the optical IF signal tone (i.e., the single λ_IF tone without IF sidebands). For this measurement a pump/probe configuration, where the optical IF tone was used as a pump signal and the optical LO tones were used as a probe signal, was used. The optical LO signal power at the input of the SOA was set to −5 dBm. The two optical tones of the optical RF signal were measured at the output of the OBPF. These curves show the transfer curve of the photonic SSB frequency up-converter using the XPolM and XGM effects in an SOA and can be used for finding an optimum input optical IF signal power for OSSB signal generation. PC3 and PC4 are adjusted so that the XPolM-based modulation takes place in inverted mode when the power of the optical IF signal tone is very low and then it switches to a non-inverted mode when the power of the optical IF signal tone is increased beyond a certain value. When the power of the optical IF signal tone is below −8 dBm for the left tone and −4 dBm for the right tone, the power of the two tones of the optical RF signal decreases as the power of the optical IF signal tone increases since PC3 and PC4 are adjusted so that the XPolM-based modulation effect operates in an inverted mode in these ranges of the power of the optical IF signal tone. When the optical IF signal tone power increases, the XPolM effect is switched from an inverted mode to a non-inverted mode by the polarization angle change of the modulated optical LO signal. When the power of the optical IF signal tone is approximately −8 dBm for the left tone case and −4 dBm for the right tone case, the powers of the optical RF signal tones reach a minimum value since the non-inverted mode XPolM effect cancels the inverted mode XGM effect. When the power of the optical IF signal tone is increased beyond −8 dBm for the left tone case and −4 dBm for the right tone case, the powers of the optical RF signal tones increases since the non-inverted mode XPolM effect becomes larger than the inverted mode XGM effect. When the optical IF signal tone power is further increased beyond 0 dBm for the left tone case and 2 dBm for the right tone case, the powers of the optical RF signal tones decrease again since the inverted mode XGM effect becomes dominant. The different transfer curves of the photonic SSB frequency up-converter implemented by adjustment of PC3 and PC4 can be used for generation of the OSSB RF signal. When the optical IF signal tone power is set to −4 dBm, the transfer curve of the left tone case has a positive slope and that of the right tone case has a zero slope, which means that the left tone of the optical LO signal is modulated in the non-inverted mode while the right tone of the optical LO signal is not modulated at all. The schematic spectrum of the OSSB RF signal generated with the optical IF signal in ODSB format is shown in F of Fig. 1(b).

 

Fig. 3 Measured optical powers of the left tone and the right tone of the optical RF signal as a function of the power of the optical IF tone.

Download Full Size | PDF

Using the experimental setup of the photonic SSB frequency up-converter shown in Fig. 3, OSSB RF signals were generated. The optical spectra at various points of the experimental setup are shown in Fig. 4. The optical LO signal power was −5 dBm and the optimum optical IF signal power was −4 dBm. The LO frequency was 60 GHz and the IF frequency was 5 GHz. Figures 4(a)–4(e) show the spectra of the optical LO signal at the output of the LO signal generator, the optical IF signal at the output of the IF signal generator (VOA2), the modulated optical LO signal at the output of the SOA, the left tone of the modulated optical LO signal at the output of PC3, and the right tone of the modulated optical LO signal at the output of PC4. Figure 4(f) shows the spectrum of the optical RF signal at the output of the OBPF that has an IF-modulated left tone and a non-modulated right tone. The power of the optical carrier (the non-modulated right tone) and that of the sideband (the IF-modulated left tone) are similar, which is desirable for improved receiver sensitivity of the photonic SSB frequency up-converter. The spectrum of the electrical RF signal measured at the output of the PD is shown in Fig. 5. The lower sideband (LSB) and upper sideband (USB) of the electrical RF signal were generated at a frequency of 55 GHz (ƒ_LO - ƒ_IF) and 65 GHz (ƒ_LO + ƒ_IF), respectively. The conversion gain of the proposed photonic SSB frequency up-converter, which is defined as the ratio of the electrical RF signal power of the up-converted signal (LSB or USB) to the electrical IF signal power measured after VOA2, was −9 dB. This value is lower than other all-optical SSB frequency up-converter [4]. However, it can be easily increased by placing an additional SOA or EDFA after the PBS in the experimental setup.

 

Fig. 4 Measured optical spectra of the signal at (a) the output of the LO signal generator, (b) the output of the IF signal generator (VOA2), (c) the output of the SOA, (d) the output of PC3, (e) the output of PC4, (f) the output of the OBPF.

Download Full Size | PDF

 

Fig. 5 Spectrum of the electrical RF signal measured at the output of the PD.

Download Full Size | PDF

Figure 6(a) shows the normalized values of the measured electrical RF signal power (USB) as a function of the electrical RF frequency for transmission of an OSSB RF signal over 0 and 40 km single mode fiber (SMF) between the CO and the BS. The optical LO and IF signal powers were −5 dBm and −4 dBm, respectively. The IF frequency was set to 5 GHz and the LO frequency was scanned from 26 GHz to 70 GHz. It also shows the normalized value of the simulated electrical RF signal power (USB) as a function of the electrical RF frequency for transmission of an ODSB RF signal over a 40 km SMF. The simulation was done using the procedure shown in [12] with parameters including a center wavelength (λ₀) of 1550 nm, a fiber chromatic dispersion parameter (D) of 17 ps/(km-nm), and an SMF length of 40 km. As shown in Fig. 6(a), while the power of the electrical RF signal as a function of the electrical RF frequency for the transmission of the ODSB signal over a 40 km SMF had many power suppression points that for the transmission of the OSSB signal over a 40 km SMF had an almost flat response. Although the RF frequency was only investigated up to 75 GHz due to the limited bandwidth of the electrical spectrum analyzer and external mixer (HP 8565E and 11974V, respectively, bandwidth: < 75 GHz), the maximum RF frequency that can be generated by this OSSB frequency up-converter can be extended to more than several hundred gigahertz if a proper optical DEMUX is used.

 

Fig. 6 (a) Normalized value of the electrical RF signal power (USB) as a function of the RF signal frequency (OSSB signals transmitted over 0 km and 40 km SMF: measured, ODSB signal transmitted over 40 km SMF: simulated), (b) Normalized value of the electrical RF signal power (USB) as a function of the IF signal frequency

Download Full Size | PDF

The normalized value of the measured electrical RF signal power (USB) as a function of the electrical IF frequency is shown in Fig. 6(b). The LO frequency was fixed at 60 GHz and the optical LO and IF signal powers were −5 dBm and −4 dBm, respectively. At these conditions, the IF frequency bandwidth of the OSSB frequency up-converter is estimated to be a little more than 15 GHz. The IF frequency bandwidth is determined by the XPolM and XGM bandwidth of the SOA, which is limited by the carrier lifetime of the SOA [13].

Figure 7 shows the phase noise characteristics of the optical LO, IF, and up-converted RF signal as a function of the offset frequency. Because of the bandwidth limitation of the phase noise measurement equipment (phase noise measurement module in the electrical spectrum analyzer HP8565E, bandwidth: < 50 GHz), the up-converted RF frequency of 49 GHz was generated by mixing the LO frequency of 45 GHz and the IF frequency of 4 GHz. The optical LO and IF signal powers were −5 dBm and −4 dBm, respectively. As can be seen in Fig. 7, the phase noise of the up-converted RF signal (USB) closely follows that of the LO signal, indicating that the photonic SSB frequency up-converter has no serious phase noise degradation during frequency up-conversion. The phase noise of the up-converted RF signal at 49 GHz for the offset of 10 kHz was −93.17 dBc/Hz.

 

Fig. 7 Measured phase noises of the LO, IF, and up-converted RF signal (USB).

Download Full Size | PDF

The linearity of the photonics SSB frequency up-converter was investigated by a spurious free dynamic range (SFDR) measurement through a two-tone test. An additional electrical amplifier with a gain of 21 dB and a noise figure of 6 dB was used to amplify the electrical RF signal after the PD. The LO frequency (ƒ_LO) of 44 GHz and the two IF frequencies (ƒ_IF1 = 4.995 GHz and ƒ_IF2 = 5.005 GHz) were used for the two tone test. Figure 8(a) shows the electrical spectra of the SSB RF signal measured with a resolution bandwidth of 3 kHz. The optical LO power and the optical IF power were −5 dBm and −4 dBm, respectively. As shown in Fig. 8(a), the fundamental signals at the frequency of 48.995 GHz (ƒLO + ƒIF1) and 49.005 GHz (ƒLO + ƒIF2) and the IMD3 signals at the frequency 48.985 GHz (ƒLO + 2ƒIF1 - ƒIF2) and 49.015 GHz (ƒLO + 2ƒIF2 - ƒIF1) were generated. Figure 8(b) shows the measured electrical RF power of the fundamental and the IMD₃ signals as a function of the electrical IF signal power for the SFDR measurement. The noise floor measured with the 1 Hz resolution bandwidth was −130.5 dBm/Hz. The SFDR estimated from data shown in Fig. 8(b) was approximately 79.51 dB·Hz^2/3. The SFDR characteristic of this photonic SSB frequency up-converter was similar to those of other reported frequency up-converters [4,14,15].

 

Fig. 8 (a) Measured electrical spectra of the SSB RF signal for two tone test; (b) Electrical RF powers of the fundamental and the third-order inter-modulation distortion component of the SSB RF signal as a function of the electrical IF power

Download Full Size | PDF

4. Conclusions

A broadband photonic SSB frequency up-converter utilizing the XPolM effect in an SOA was proposed and experimentally demonstrated. The photonic frequency up-converter generated up-converted OSSB RF signals that had no power fluctuation problem caused by the fiber chromatic dispersion. The optical carrier and the optical sideband of the generated optical RF signal had similar power for high receiver sensitivity. The photonic SSB frequency up-converter had no serious phase noise degradation and a sufficient SFDR for microcellular personal communication systems. The results indicate the potential of the XPolM-based photonic SSB frequency up-converter for RoF systems in millimeter-wave or terahertz frequency bands.