Generation and distribution a wide-band continuously tunable millimeter-wave signal with an optica

3090 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 10, OCTOBER 2005 Generation and Distribution of a Wide-Band Continuously Tunable Millimeter-Wave Signal With an Optical External Modulation Technique Guohua Qi, Jianping Yao, Senior Member, IEEE, Joe Seregelyi, Stéphane Paquet, and Claude Bélisle Abstract—A new technique to generate and distribute a wide- band continuously tunable millimeter-wave signal using an optical external modulator and a wavelength-fixed optical notch filter is proposed. The optical intensity modulator is biased to suppress the odd-order optical sidebands. The wavelength-fixed optical notch filter is then used to filter out the optical carrier. Two second-order optical sidebands are obtained at the output of the notch filter. A millimeter-wave signal that has four times the frequency of the microwave drive signal is generated by beating the two second- order optical sidebands at a photodetector. Since no tunable op- tical filter is used, the system is easy to implement. A system using an LiNbO 3 intensity modulator and a fiber Bragg grating filter is built. A stable and high spectral purity millimeter-wave signal tunable from 32 to 50 GHz is obtained by tuning the microwave drive signal from 8 to 12.5 GHz. The integrity of the generated mil- limeter-wave signal is maintained after transmission over a 25-km standard single-mode fiber. Theoretical analysis on the harmonic suppression with different modulation depths and filter attenua- tions is also discussed. Index Terms—Electrooptic modulator, fiber Bragg grating (FBG), microwave photonics, millimeter-wave generation, optical heterodye. I. INTRODUCTION GENERATION and transmission of microwave andmillimeter-wave signals over optical fibers are of great interest for applications such as broad-band wireless access net- works operating at millimeter-wave bands, antenna remoting, phased-array antennas, optical sensors, and radars [1]–[5]. Optical generation and transmission of electrical signals have been extensively investigated with most work to date focused on high-frequency signals, especially millimeter-wave signals. This is because there are fewer difficulties with conventional electronic techniques or optical techniques in generating and distributing lower-frequency electrical signals. Furthermore, the low-frequency modulated double-sideband (DSB) optical signal suffers less from the chromatic dispersion of the fiber than that of the high-frequency modulated signal when trans- mitting over standard single-mode fiber (SSMF) [6]–[8]. Manuscript received January 18, 2005. The work was supported by the Canadian Institute for Photonic Innovations. G. Qi and J. Yao are with the Microwave Photonics Research Laboratory, School of Information Technology and Engineering, University of Ottawa, Ottawa, ON, Canada K1N 6N5 (e-mail: jpyao@site.uottawa.ca). J. Seregelyi, S. Paquet, and C. Bélisle are with the Communications Research Centre, Ottawa, ON, Canada K2H 8S2. Digital Object Identifier 10.1109/TMTT.2005.855123 Therefore, the highest frequency of the signal produced by these DSB techniques is limited by the bandwidth of the laser or the external modulator and the fiber chromatic dispersion. To overcome these limitations, an optical generation scheme that uses narrow bandwidth optical components to generate high- frequency electrical signals becomes attractive. Optical microwave or millimeter-wave signal generation is usually based on heterodyne techniques by beating two optical carriers separated by the desired frequency in a square-law pho- todetector (PD). If the offset frequency of the two optical car- riers is stable and their phases are correlated, a high-quality elec- trical signal will be generated. However, after the transmission of the two optical carriers over an SSMF, the generated mi- crowave or millimeter-wave signal quality will be deteriorated because of the fiber chromatic dispersion [9]. Electrical signal generation based on optical heterodyning can be achieved by using either two stabilized lasers or one laser with an external optical modulator. The quality of the elec- trical signal produced by beating two free-running lasers rarely meets application specifications. Methods to further improve the signal quality, such as optical injection locking [10], [11] and optical phase-locked loop (OPLL) [12], [13], have been proposed. The OPLL techniques allow the suppression of the low-frequency components of the phase noise that deteriorate the generated signal. However, it is difficult to suppress the high- frequency components of the phase noise unless very narrow linewidth (in kilohertz) optical sources are used [14]. Methods using a laser with an external optical modulator, such as an optical intensity modulator or optical phase mod- ulator, have shown great potential for producing high-purity high-frequency millimeter-wave signals. These approaches are based on the inherent nonlinearity of the response of the optical modulator for generating high-order optical sidebands. Taking advantage of this property can dramatically lower the bandwidth requirements for the optical modulator and allows the use of a much lower frequency electrical drive signal. This can greatly reduce the cost of the system and makes it more practical to use. A method to generate millimeter-wave signals using an ex- ternal optical modulation technique was proposed by O’Reilly et al. in 1992 [15]. A frequency-doubled electrical signal was optically generated by biasing the Mach–Zehnder modulator (MZM) to suppress even-order optical sidebands. A 36-GHz millimeter-wave signal was generated when the MZM was driven by an 18-GHz microwave signal. Such a system was employed for a remote delivery of video services [16]. 0018-9480/$20.00 © 2005 IEEE QI et al.: GENERATION AND DISTRIBUTION OF WIDE-BAND CONTINUOUSLY TUNABLE MILLIMTER-WAVE SIGNAL 3091 In 1994, O’Reilly and Lane [17] proposed another method to generate a frequency-quadrupled electrical signal. Instead of biasing the MZM to suppress the even-order optical sidebands, the method [17] was based on the quadratic response of an optical intensity modulator. The optical carrier and the first- and third-order optical sidebands were suppressed by adjusting the drive signal level. A 60-GHz millimeter-wave signal was generated when a 15-GHz drive signal was applied to the MZM. However, to ensure a clean spectrum at the output of a PD, an imbalanced Mach–Zehnder filter with a free spectral range (FSR) equal to the spacing of the two second-order optical sidebands are used to suppress the unwanted optical components. Recently, an approach using an optical phase modulator to generate a frequency-quadrupled electrical signal was proposed [18]. In this approach, a Fabry–Perot filter was used to select the two second-order optical sidebands. An elec- trical signal that has four times the frequency of the electrical drive signal was generated by beating the two second-order sidebands at a PD. A key advantage of these approaches [17], [18] is that an optical modulator with a maximum operating frequency of 15 GHz can generate an millimeter-wave signal up to 60 GHz. However, since both approaches rely on the optical filter to select the two optical sidebands to generate tunable millimeter-wave signals, a tunable optical filter must be used, which significantly increases the complexity and the cost of the system. For system applications with frequency reconfigurability, such as a wide-band surveillance radar, spread-spectrum or software-defined radio (SDR), continuously tunable mil- limeter-wave signals are highly desired. The prospect of generating a wide-band continuously tunable single-frequency millimeter-wave signal using fixed optical filters and narrow bandwidth optical modulators becomes very attractive. In this paper, we propose a new approach that can optically generate a wide-band continuously tunable millimeter-wave signal without using a tunable optical filter. The system employs an optical intensity modulator, which is biased to suppress the odd-order optical sidebands. A fiber Bragg grating (FBG) serving as a wavelength fixed notch filter is then used to filter out the optical carrier. A stable low-phase noise millimeter-wave signal at four times the frequency of the electrical drive signal is generated at the output of a PD. A 32–50-GHz millimeter-wave signal is observed on an electrical spectrum analyzer (ESA) when the electrical drive signal is tuned from 8 to 12.5 GHz. The quality of the generated millimeter-wave signal is maintained after transmission over a 25-km SSMF. II. ANALYSIS A. Principle of the Proposed Approach The proposed millimeter-wave signal generation system is shown in Fig. 1. An electrical drive signal is applied to an MZM. The MZM is biased to suppress the odd-order optical sidebands. An optical notch filter is connected at the output of the MZM to remove the optical carrier. Two second-order optical sidebands are obtained at the output of the notch filter. A beat signal with four times the frequency of the electrical drive signal is gener- ated at a PD. Fig. 1. Diagram of the proposed microwave signal generation system. It is known that the electric field at the output of a lithium– niobate MZM, i.e., , can be approximately expressed by (1) where and are, respectively, the electric field amplitude and angular frequency of the input optical carrier, is the applied electrical drive voltage, and is the optical phase difference caused by between the two arms of the MZM. If the MZM is driven by a sinusoidal electrical signal and biased with a constant dc voltage, is expressed as (2) where is a constant phase shift determined by the constant dc-bias voltage, is the half-wave voltage at high frequency, and and are the amplitude and angular frequency of the electrical drive signal, respectively. Substituting (2) into (1), the electric field of the output optical signal can be written as (3) where is the Bessel function of the first kind of order and is the phase modulation index. When the MZM is driven by an electrical signal with adequate power, a large value of is obtained. In this case, (3) shows that the power in the input optical carrier will be spread out among the first-order, second-order, third-order, and higher order op- tical sidebands. The amplitude distribution of these sidebands is governed by the variation of Bessel functions parameterized by . Their amplitude is also affected by . If all these optical 3092 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 10, OCTOBER 2005 sidebands are fed to a square-law PD, harmonics of the elec- trical drive signal will be generated. The parameters and can be optimized, from the point-of-view of obtaining higher order electrical harmonics and maximizing its conversion effi- ciency. For example, is tuned to suppress all even-order op- tical sidebands so that the power of all even-order sidebands is transferred to the odd-order ones [15]. An efficiency-improved frequency-doubled electrical signal is then obtained. In our proposed method, the dc bias of the MZM is tuned to have . All the odd-order optical sidebands as- sociated with the term then vanish, as indicated in (3). Only the even-order optical sidebands are kept. The power in the odd-order optical sidebands is transferred to the even-order side- bands, improving the signal generation efficiency. If the elec- trical drive signal is applied to the MZM with an appropriate power level, optical sidebands up to the second order are gen- erated and all optical sidebands above the second order have an amplitude low enough to be ignored, but the optical carrier, rep- resented by the term with the zeroth-order Bessel function in (3), is still part of the spectrum. The optical signal can then be approximately expressed as (4) When this optical signal is fed to a PD, a strong frequency- doubled electrical signal and a weaker frequency-quadrupled electrical signal will be generated. However, when this optical signal that is composed of two second-order sidebands and one optical carrier is transmitted over a long span of optical fiber, the frequency-doubled electrical signal suffers from the chro- matic-dispersion-induced power penalty [6]–[8], which limits its applications. In addition, the presence of a frequency-dou- bled electrical signal will cause interference to the operation of the frequency-quadrupled electrical signal in a wide-band system application. To eliminate the frequency-doubled elec- trical signal, we propose the use of a wavelength-fixed optical notch filter to filter out the optical carrier, as shown in Fig. 1. The electric field of the optical signal at the output of the op- tical notch filter can then be approximately expressed as (5) Therefore, at the output of the optical notch filter, only two optical sidebands separated by four times the frequency of the drive signal are present. Applying this optical signal to a PD, an electrical signal that has four times the frequency of the elec- trical drive signal will be generated. The generated electrical signal can be written as (6) where is a constant that is related to the responsivity of the PD. Since the two optical sidebands originate from the same optical source, the frequency stability and phase noise of the generated signal are predominately determined by the electrical drive signal. Equation (6) also shows that the amplitude of the generated electrical signal can be maximized by optimizing the value . Fig. 2. Illustration of the optical spectrum at the output of the optical notch filter. Fig. 3. Illustration of the electrical spectrum at the output of a PD. It is important to note that the dc-bias level for which the odd-order optical sidebands are eliminated is not dependent on the frequency of the electrical drive signal. In addition, the op- tical carrier has a fixed wavelength; therefore, the optical notch filter does not need to be tunable. These characteristics ensure that the proposed approach can generate a frequency-tunable electrical signal by simply tuning the frequency of the electrical drive signal at a low-frequency band. B. Electrical Harmonic Suppression Analysis Usually in a wide-band electrical heterodyne system, espe- cially when the system operates over an octave bandwidth, an electrically tuned bandpass filter is inserted between the local oscillator and the mixer to suppress unwanted harmonics. This prevents potential in-band interference from corrupting the re- ceive signal. However, the use of an electrically tunable filter makes the system very complicated. A direct solution to this problem is to use a highly harmonic-suppressed local oscil- lator. In the following, we will analyze the harmonic-suppres- sion characteristics of the proposed approach. Assume that all odd-order optical sidebands generated by the modulation of the MZM by a sinusoidal signal can be com- pletely suppressed by using an appropriate dc-bias voltage. That means that the condition of is satisfied with a constant dc bias. Assume also that the attenuation of the optical notch filter at its center notch wavelength is in dB. Based on the above assumptions, from (3), the optical signal at the output of the optical notch filter can be written as (7) where is the optical electrical field attenuation factor, which is related to by . QI et al.: GENERATION AND DISTRIBUTION OF WIDE-BAND CONTINUOUSLY TUNABLE MILLIMTER-WAVE SIGNAL 3093 Fig. 4. Power intensity and harmonic suppressions versus modulation depth. (a) Power intensity I of the fourth-order harmonic. (b) Harmonic suppressions I =I and I =I . (Frequency of the electrical drive f = 12:5 GHz.) Usually, for a commercially available MZM, the maximum available phase modulation index is 2. When , Bessel functions for are all monotonically increasing with respect to and monotonically decreasing with respect to the order of Bessel function , and , , and . Thus, it is reasonable to ig- nore the optical sidebands with a Bessel coefficient higher than in our discussion. Therefore, (7) can be further simplified to (8) Equation (8) shows that the optical signal consists of an atten- uated optical carrier and four optical sidebands. The spectrum of this optical signal is illustrated as shown in Fig. 2. The arrow Fig. 5. Power variation of I versus frequency of the electrical drive signal. (Modulation depth � = 0:6.) Fig. 6. Harmonic suppressions versus frequency of the electrical drive signal. (a) Modulation depth � = 0:6. (b) Modulation depth � = 0:9. 3094 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 10, OCTOBER 2005 Fig. 7. Experimental setup for optical generation and transmission of millimeter-wave signals. (power amplifier: PA, optical spectrum analyzer: OSA). direction shows their initial phase with respect to the phase of the optical carrier before transmission. When the optical signal shown in Fig. 2 is transmitted over a single-mode fiber, the chromatic dispersion of the fiber will cause an extra phase shift to each optical sideband compared to the optical carrier. By expand the propagation constant of the fiber for each optical sideband to a Taylor series around the angular frequency of the optical carrier [19], i.e., (9) where and are the first- and second-order deriva- tive of the propagation constant at the angular frequency , respectively. The effect of higher order dispersion is ne- glected for the single-mode fiber at 1550-nm band [20], and can be expressed by the chromatic dispersion param- eter as (10) where is the speed of light in free space and is the frequency of the optical carrier. The electric field representing the optical signal at the end of the transmission over a single-mode fiber of length can be ob- tained by adding the transmission phase delay to the corresponding optical sideband shown in (8). Electrical harmonics will be generated by applying this optical signal to a PD. The output voltage of the generated high-frequency elec- trical signal is (11) where is the frequency of the electrical drive signal. The elec- trical spectrum of the generated signal expressed by (11) is il- lustrated as shown in Fig. 3. From (11), the power intensity of the fourth-order electrical harmonic is proportional to the coefficients of optical side- bands (12) The power intensities of the second- and sixth-order electrical harmonics , are (13) (14) For a distribution system that operates at 1550 nm with a transmission distance of 25 km over a standard single-mode fiber with ps nm km , the power intensity and harmonic suppressions of and versus the modula- tion depth are plotted in Fig. 4. Fig. 4(a) shows that the power intensity is monotonically increasing for , and Fig. 4(b) shows that the harmonic suppression is monotonically decreasing for and dB, and for and dB; the harmonic suppression is monotonically decreasing for , which is independent of the attenuation of the optical notch filter. With a large attenuation of the optical notch filter, a lower modulation depth corresponds to an improved har- monic suppression. As can be seen from Fig. 4(b), for and , the lower the modulation depth, the higher the harmonic suppression. However, lower modulation depth leads to a lower output power of the fourth-order electrical har- monic, as shown in Fig. 4(a). This problem can be solved at a low cost by using erbium-doped fiber amplifiers (EDFAs) in the 1550-nm band. Note that a lower modulation depth means a less power requirement for the electrical drive signal. Fig. 5 shows the power variation of the generated electrical signal , which is caused by the combined effects of the limited attenuation of the optical carrier and the chromatic dispersion of the fiber when tuning the frequency of the electrical drive signal QI et al.: GENERATION AND DIST