1.

Introduction

Recently, a new technique area known as microwave photonics has been proposed to serve many microwave and optical applications.¹ Most investigations are focused on optical microwave signal generating,^{2, 3} filtering,^{4, 5, 6} frequency converting,^{7, 8, 9} etc. In fact, many advantages of optical signal processing, such as low loss and wide band operation, can be utilized in future microwave systems. In traditional microwave amplifiers, the signal gain decreases with increased bandwidth.¹⁰ In this paper, an optical amplification system is used to amplify the optically carried microwave signal. The original weak microwave signal is modulated on an optical carrier by a partly carrier-suppressed method and amplified by a commercial erbium-doped fiber amplifier (EDFA). The output microwave signal is obtained from a photodetector. With this method, a broad band covering $10\mathrm{MHz}$ to $10\mathrm{GHz}$ microwave signal amplification is achieved, the gain of signal is more than $10\mathrm{dB}$ , and the $3\text{-}\mathrm{dB}$ bandwidth is about $7\mathrm{GHz}$ , which is mainly limited by the bandwidth of the photodetector.

2.

Experiment

The experimental setup is shown in Fig. 1. An optical source in the wavelength of $1557\mathrm{nm}$ is fed into a Mach-Zehnder modulator (MZM) after a polarization controller. The MZM is biased with proper direct current and driven by the input radio frequency (RF) signal to generate a partly carrier-suppressed optical signal embedded with the microwave signal. This signal is sent into a commercial EDFA. An optical bandpass filter (OBPF) is connected after the EDFA to block amplified spontaneous emission (ASE) noise. The amplified signal can be obtained at the output of a $10\text{-}\mathrm{GHz}$ commercial photodetector (PD) and measured by an electrical spectrum analyzer (ESA; Agilent PSA E4446A). Thus, the RF signal from the source to the ESA experiences an optically carried and amplified process. If the original RF signal is a small one, it can be amplified by the EDFA and has a massive output after the PD. Because the gain spectrum of EDFA is wide enough to cover more than $40\mathrm{nm}$ and the small signal gain is more than $30\mathrm{dB}$ , this method can amplify the RF signal with ultrawide bandwidth.

Fig. 1

Experimental setup. TWL: tunable wavelength laser; PC: polarization controller; RF: radio frequency; MZM: Mach-Zehnder modulator; DC: direct current; EDFA: erbium-doped fiber amplifier; OBPF: optical bandpass filter; PD: photodetector; and ESA: electrical spectrum analyzer.

3.

Results and Discussion

In the traditional optical modulation scheme, the MZM is biased at the linear modulation point, $V_{DC}=V_{\pi }∕2$ , where $V_{\pi }$ is the half-wave voltage. In this case, the output of MZM is shown in Fig. 2a. The optical carrier holds most of the signal energy. If the driving RF signal is very small, nearly all the optical energy is focused on the carrier, which may cause the saturation of the EDFA and the photodetector. That means the carrier can be amplified except for the small RF signal. In order to improve the small RF signal gain, the MZM is biased at the partly carrier-suppressed point, which is between $V_{\pi }∕2$ and $V_{\pi }$ . The output of MZM is shown in Fig. 2b. The carrier is suppressed to have much lower power than the linear bias case. Thus, when the signal passes the EDFA, the carrier and sidebands will be both amplified with nearly the same gain. After the O/E converter, the amplified RF signal is achieved by the beat signal.

Fig. 2

Optical spectra out of the MZM. (a) DC biased at the linear modulation point, and (b) DC biased at the partly carrier-suppressed modulation point.

First, the amplification of the small signal at RF frequency of $10\mathrm{GHz}$ is measured. Figure 3a and 3b shows the wave form and spectrum of the source RF signal, respectively. The peak power at $10\mathrm{GHz}$ is about $-23\mathrm{dBm}$ and the noise floor is about $-90\mathrm{dBm}$ . Figure 3c and 3d shows the waveform and spectrum of amplified RF signal, respectively. The amplified RF signal has the peak power at $10\mathrm{GHz}$ is about $-13.8\mathrm{dBm}$ . However, the noise floor increases to near $-60\mathrm{dBm}$ , which mainly depends on the beat noise between the signal field and the ASE noise. In addition, the spectrum purity of the source and the amplified RF signal are almost the same.

Fig. 3

Comparison of source RF signal and amplified RF signal. (a) Source RF signal waveform; (b) source RF signal spectrum; (c) amplified RF signal waveform; and (d) amplified RF signal spectrum.

In order to measure the RF gain spectrum of the proposed method, we tuned the RF signal frequency from $10\mathrm{MHz}$ to $12\mathrm{GHz}$ . The RF signal power and signal-to-noise ratio (SNR) are measured with bandwidth of $10\mathrm{MHz}$ . Figure 4a shows the RF signal gain spectrum. In this figure, one can see that the small signal gain of this method is more than $20\mathrm{dB}$ . When the RF frequency is near $10\mathrm{GHz}$ , the signal gain is still larger than $10\mathrm{dB}$ . The $3\text{-}\mathrm{dB}$ bandwidth is about $7\mathrm{GHz}$ , which is mainly limited by the photodetector and modulator bandwidth. The gain curve and SNR of the source and amplified signal at $10\mathrm{GHz}$ are measured, respectively, as shown in Fig. 4b. In this figure, we can see that the gain is stable, with the RF source signal power variation from $-60\mathrm{dBm}$ to $-10\mathrm{dBm}$ . The SNR of the source and amplified signal both increase with the input power. However, the SNR penalty is about $10\mathrm{dB}$ , which is caused by the ASE noise of the EDFA and the noise in the photodetector.

Fig. 4

(a) Gain spectrum and (b) gain curve at frequency of $10\mathrm{GHz}$ and SNR comparison of source and amplified signal.

This method provides a simple optical technique to amplify ultrawide-band microwave signals. The system can be easily built with commercial optical devices, such as MZM modulators and optical amplifiers. We have tested $10\mathrm{MHz}$ to $10\mathrm{GHz}$ optically carried microwave signal amplification, and the gain is larger than $10\mathrm{dB}$ , notably $20\mathrm{dB}$ in low-frequency RF input signal cases. The noise character of this method is mainly limited by the EDFA and photodetector noise. The gain bandwidth and maximum output quantity are limited by the photodetector bandwidth and saturation output. If low-noise EDFA and high-quality PD, such as UTC-PD, are employed, the system performance can be improved greatly.

4.

Conclusion

An ultrawide-band microwave amplification method in the optical domain is proposed. This method utilized the large bandwidth capacity of optical devices to amplify optically carried microwave signals. The amplification range covers from $10\mathrm{MHz}$ to $10\mathrm{GHz}$ , with gain larger than $10\mathrm{dB}$ , and this method can work with an input RF signal less than $-60\mathrm{dBm}$ . The subsystem is made up of commercial devices and easily realized. The system performance can be improved greatly using high-quality devices.