Optical carrier-suppressed single sideband (O-CS-SSB) Modulation using a hyperfine blocking filter based on a virtually imaged phased-array (VIPA)

Shijun Xiao, A.M. Weiner
2005 IEEE Photonics Technology Letters  
Using a hyperfine blocking filter based on a virtually imaged phased-array as the spectral disperser, we present a novel scheme of optical carrier-suppressed single sideband (SSB) modulation. Our experiments demonstrate sideband suppression of 28 dB at only a 2.9-GHz offset for single subcarrier amplitude modulated at 622 Mb/s. Furthermore, the hyperfine blocking filter can be tuned for simultaneous optical SSB and strong carrier suppression to 30 dB lower in optical power than the unsuppressed
more » ... sideband. Index Terms-Microwave photonics, single sideband (SSB) modulation, subcarrier multiplexing (SCM), virtually imaged phasedarray (VIPA). S UBCARRIER multiplexing (SCM) is a relatively new scheme in high-speed radio-fiber links [1], [2] . This technique gains advantages by the use of mature microwave signal processing techniques since microwave devices have several advantages over optical devices: the stability of oscillators, the frequency selectivity of filters, and the ease of implementing advanced modulation formats. In order to reduce power fading caused by fiber dispersion and increase spectral efficiency, optical single sideband (O-SSB) modulation is widely used in SCM. Optical carrier suppression is also important in order to improve modulation depths and thereby increase receiver sensitivity. Previous optical carrier-suppressed single sideband (O-CS-SSB) modulation experiments used relatively complex electronics and obtained sideband suppression limited to 15 dB (due to modulator nonlinearity) [1], [2] . Carrier suppression was obtained either via an optical Fabry-Pérot notch filter that must be precisely matched to the carrier frequency [1] or via tuning the modulator bias (in which case the carrier remained 8 dB higher than the unsuppressed sideband, again limited by modulator nonlinearity) [2] . Another O-CS-SSB modulation experiment reported in [3] has a similar design compared to that in [2], but the carrier suppression is dependent on very precise fiber coupler ratios and radio-frequency driving voltages. Recently, O-SSB modulation based on optical fiber Bragg grating (FBG) filtering in a tandem structure [4] was demonstrated with high sideband suppression ( 20 dB). This Fig. 1. (a) Setup for O-CS-SSB modulation; (b) setup for the hyperfine blocking filter. experiment was limited to subcarriers with 7-GHz frequency offset due to FBGs spectral resolution; neither data modulation nor carrier suppression were reported. O-SSB with strong carrier suppression has been the most efficient modulation format in radio-fiber links since the only power transmitted is the information sideband [5], [6]. Therefore, achieving simultaneous high sideband suppression and greater carrier suppression may allow increased dynamic range and spectral efficiency. In this letter, we introduce a hyperfine blocking filter for O-CS-SSB modulation, which allows us to demonstrate sideband suppression of 28 dB at only 2.9-GHz offset. Furthermore, the blocking filter can be tuned for simultaneous SSB and strong carrier suppression (down to 30 dB lower than the unsuppressed sideband). As a result, we demonstrate here, for the first time to our knowledge, simultaneous frequency translation (by 2.9 GHz) and data encoding and detection (622 Mb/s), which brings new prospects for high spectral efficiency in microwave photonics. Fig. 1(a) shows our experimental setup. We use a tunable laser source (Agilent 81 680 A) with an output wavelength at 1.55 m and a spectral linewidth below 0.1 pm (12.5 MHz) for the input (6 dBm in power) to a standard single electrodedrive Mach-Zehnder modulator (MZM) with an electrical 3-dB passband 30 GHz and a null transmission bias voltage V. The MZM is modulated by the output of a microwave mixer mixing a subcarrier at 2.9 GHz and a 622-Mb/s pseudorandom binary sequence in amplitude shift keying. The MZM's
doi:10.1109/lpt.2005.848564 fatcat:kb2yhuka55halfseab4y6j6lwm