Simulation of optically switchable and tunable ultrawideband monocycle generation using semiconductor optical amplifier and optical delay line

Future optical networks may require both wavelength conversion and bit shape regeneration in an all-optical domain. The possibility of pulse reshaping while providing wavelength conversion may support new demands over medium and large distances links (Kelly, 2001). Indeed, during propagation the optical data signal suffers deterioration due to the amplified spontaneous emission (ASE) from optical amplifiers, pulse distortion from intrinsic dispersion, crosstalk, and attenuation. All-optical regenerators may be important components for the restoration of these signals, providing complexity and cost reductions with the avoidance of optoelectronic conversions. The regeneration could be 2R (reamplification and reshaping) or 3R, which also provide retiming to solve jitter (Simon et al., 2008). Several 3R regenerators using the semiconductor optical amplifier (SOA) have been proposed, such as cascaded SOAs setups (Funabashi et al., 2006) or SOA based MachZehnder interferometers (MZI) (Fischer et al., 1999). However for small and medium distances systems, where the signal amplitude noise and distortions form the main problem and where jitter has fewer magnitudes, the simpler 2R processes can be adequate to keep signal quality (Simon et al., 1998). In addition, the SOA is a helpful device for both 2R-regeneration (Ohman et al., 2003) and wavelength conversion (Durhuus et al., 1996). Several techniques for 2R-regeneration based on SOAs have been proposed and tested, for example by using four-wave mixing (FWM) (Simos et al., 2004), cross-gain modulation (XGM) (Contestabile et al., 2005), integration within MZI (Wang et al., 2007), multimode interferometric SOA (Merlier et al., 2001), cross-phase modulation (XPM) with filtering (Chayet et al., 2004), and feed forward technique (Conforti et al., 1999). However, these techniques require complex designs and involve critical operation points, even the simplest ones based on XGM features. In addition, most of these techniques are not capable of wavelength conversion and regeneration simultaneously. Recently a regenerator based on cross-gain modulation was proposed using one SOA for wavelength conversion (in a counter-propagating mode) and another deeply saturated SOA (synchronized by an optical delay line) to achieve cross-gain compression (Contestabile 2005). This efficient approach has similarities with the all-optical feed-forward techniques. In addition, this regenerator could not done wavelength conversion if the wavelength of the

Highly stable and small size local oscillators is a very important component of any remote sensing, imaging, and telecommunication systems. The key factor influencing the frequency stability is quality factor of the resonance structure. Related close-in to carrier phase noise for offset frequencies above active device corner frequency reduces at least at a rate of 20 dB/decade while offset below corner frequency increases at rate of 30 dB/decade. A new approach in realization of the electrical oscillators is to augment electrical quality factor by optical energy storage components (e.g., fiber optic delay lines and ring resonators) to enhance effective quality factor (Q) to 10^4-10^6, well beyond available electrical Q of 2000 in dielectric resonator oscillators (DRO). Moreover, stability of resonance frequency has been impacted by the environmental induced (e.g., piezo and pyro properties of material) sensitivities. In addition, micro-scale design approaches are reported using multi-mode (both semiconductor and solid state) lasers for low phase noise intermodal oscillations. Finally, forced oscillation techniques of injection locking, phase locking, and mode-locking are attractive to further stabilize electrical and optical oscillation methods. In this thesis, development of broadband stable frequency sources is reported covering both X- and K-band using forced opto-electronic oscillator technique. The broadband frequency tuning is achieved using coarse electrical tuning of YIG bandpass filter combined with fine-tuned optical transversal filter using dispersive delay component and wavelength control of a fiber laser, as optical source. Forced opto-electronic oscillation techniques of self-injection locking (SIL), self-phase locked loop (SPLL), and self-injection locked phase locked loop (SILPLL) are optimized for further frequency stabilization of a frequency synthesizer. In SILPLL application, long delay line is crucial for substantial phase noise reduction; therefore, kilometer long optical fiber delay lines have been used to construct the optical feedback since their loss is extremely low compared to electrical delay lines. Meantime, open loop gain is another key parameter to achieve low phase noise in these feedback systems, as the phase noise reduction is proportional to phase locking loop gain. The phase noise and pull-in time can be greatly reduced in SILPLL as opposed to SIL or SPLL alone. A forced oscillation of fixed frequency DRO at 10 GHz system provide -137 dBc/Hz phase noise at 10kHz offset with under 10 fs aperture jitters, while OEO based X-band and K-band synthesizers provide phase noise of -137 dBc/Hz (<10 fs jitters) and -127 dBc/Hz (<12 fs jitters) at offset 10 kHz respectively in a computer controlled 19" rack-mountable OEO systems. After realization of the desired modular frequency synthesizer, development a compact opto-electronic oscillator system is demonstrated in my PhD study. Monolithic design and testing of multi-section quantum well lasers are reported on 5x2 mm2 InP chip using a commercial foundry service (i.e., SmartPhotonics). The designed lasers consist of 4 major sections of semiconductor optical amplifier (SOA), optical phase modulator (PM) and distributed Bragg reflector (DBR) mirrors. The DBR section provides wavelength selectivity while phase section and gain medium provide output wavelength tuning. An electro-absorption modulator (EAM) is prepared for mode number control using its wavelength sensitive absorption coefficient. The experiments supported predicted performance of this multi-mode laser structure and impact of combined forced oscillation of intermodal oscillations with mode locking method to realize a frequency stabilized and tunable RF signal source. Meantime, forced technique SILPLL and optimum mode locking techniques are applied in order to achieve improved phase noise performance. The free-running inter-modal oscillation frequencies suffers from poor phase noise of -5 dBc/Hz at 1 kHz and -32 dBc/Hz at 10kHz offset around 11.54 GHz, while under forced oscillation a -58 dBc/Hz at 1 kHz offset and -98 dBc/Hz at 10 kHz offset are achieved. A frequency tuning range of 800 MHz is also measured. In mode locked laser operation, the RF beat-notes forced with SILPLL topology provides a better performance. Improved phase noise of -79 dBc/Hz at 1kHz, -110 dBc/Hz at 10 kHz is measured with under 50 fs aperture jitters, while a broadband frequency tuning from 11 GHz to 13 GHz range is measured. The tuning range is potentially to be further improved, if the shared DBR laser pair symmetry could be improved. In summary, this thesis provides the solution for building RF synthesizer using forced opto-electronic oscillator and laser beat-notes oscillation output. The experimental results are corroborated with detailed analytical and numerical modeling to optimize oscillator phase noise performance. The modular design solution shows great stability and low phase noise, while the compact monolithic design solutions are quite attractive for a smaller size. Potential for full integration of multi-mode multi-section MQW laser with optical delay lines are also presented as a logical development path towards a compact frequency synthesizer.

Radio frequency (RF) transmission systems with high-precision phase stability are required by the next generation of particle colliders and light sources. An RF transmission system was designed to meet this requirement. In this system, RF signal generated at the sending end is modulated onto a continuous wave (CW) optical carrier, transmitted through an optical fiber, and demodulated at the receiving end. The phase drift is detected by a digital phase monitor with femtosecond-level accuracy and compensated by a motorized optical fiber delay line (ODL). The measurement results show that the long-term phase drifts can be stabilized to within 100 fs (pk–pk), 500 fs (pk–pk), and 1.8 ps (pk–pk) in a 400-meter-long optical fiber over 1 h, 24 h, and 10 days, respectively.

A novel microwave photonic notch filter structure is proposed and experimentally demonstrated. The structure is based on a recirculating delay line loop consisting of an optical variable delay line, a semiconductor optical amplifier (SOA) and a tunable narrowband optical filter. Negative tap is generated using wavelength conversion based on cross-gain modulation of an amplified spontaneous emission spectrum in the SOA. A narrow bandpass response with negative coefficients and a broadband all-pass response are combined to achieve a narrow notch response with flat passband which can eliminate interference with minimal impact on the expected signal. Experimental results show good agreement with theoretical analysis.

At a switching node in an optical packet switching (OPS) network, packet contention occurs whenever two or more packets try to reach the same output port of the node at the same wavelength, consequently it degrades network performance. Previous works on optical packet switching flourishingly adopted optical buffering with optical fibre delay line (FDL), wavelength conversion, or deflection routing to resolve packet contention. In this paper, we focus on the optical fibre delay line approach, especially the selection of proper length of FDL under asynchronous variable length packet traffic. As the length of fibre coils equipped in a FDL optical buffering system is impossible to be reconfigured frequently, a proper length of FDL should be determined during manufacture. The motivation is to observe the impact factors on FDL optical system performance in a WDM network scenario, and provide the empirical data as supports to select the proper length of FDL in an OPS node system.

We demonstrate an all-optical continuously tunable delay line system based on wavelength conversion in semiconductor optical amplifiers (SOAs), and group-velocity dispersion (GVD) in a dispersion-compensating fiber (DCF). The system operates, near 1550 nm, with a non-return-to-zero (NRZ) pattern at 10 Gb/s. A maximal optical delay up to 2700 ps is observed. The scheme achieves continuous control of a wide range of optical delays, wide signal bandwidth, nearly no pulse broadening, and very little spectral distortion.

A radio frequency (RF) photonic filter is experimentally demonstrated using an optical tapped delay line (TDL) based on an optical frequency comb and a periodically poled lithium niobate (PPLN) waveguide as multiplexer. The approach is used to implement RF filters with variable bandwidth, shape, and center-frequency.

In this study, an all-optical tunable time delay in optical fiber using wavelength conversion stage and dispersion stage is designed, simulated, evaluated and optimized. A noticeable optical tunable time delay is achieved at 40-Gbps bit rate system. An optimization process is carried out targeting a remarkable system performance based on bit error rate (BER) and Eye diagram analysis. This design successfully achieves 740-ps tunable optical time delay with pulse width (PW) of 2.5-ps at 40-Gbps delay system and a maximum relative delay (MRD) of 296. This system represents a step towards an integrated optical buffer to be used in communication systems.

A time-differential (TD) Brillouin optical correlation domain analysis (BOCDA) sensor system was applied to measure the Brillouin gain spectrum of a 1 km long sensing optical fiber. The optical delay line used in all BOCDA measurement systems was eliminated in the TD-BOCDA system by using a bit-delayed modulation relationship between the probe and pump lightwaves. These lightwaves were phase modulated using 216-1 pseudo-random binary sequence codes at 5 Gbps. A 2 cm dispersion-shifted fiber placed at the end of the 1 km optical fiber was distinctly identified by the Brillouin frequency extracted from the Brillouin gain spectrum measurement. To investigate the measurement stability of the TD-BOCDA system, experiments were conducted under two different pumping conditions. A semiconductor optical amplifier (SOA) and an intensity modulator (MOD) were compared for the pump chopper used in the TD-BOCDA system to detect the extinction ratio of the pump and the resulting noise in the Brillouin gain measurement. The stability of the Brillouin frequency measurement from the Brillouin gain spectrum in the TD-BOCDA system was investigated by increasing the average value of the measurement using either the SOA or MOD. The repeated-measurement deviation of the system with the SOA was only half of the deviation observed in the system with the MOD. The performance of TD-BOCDA is equivalent to or better than that of conventional BOCDAs in terms of measurement reliability. Moreover, TD-BOCDA is free from the drawbacks of traditional BOCDA, which uses time-delayed fibers and varies the bit rates.

This paper presents a MEMS switchable optical delay line, which works at terahertz (THz) region. The geometry of the unit cell is tunable by MEMS actuators [1]. In the experiment, it measures that the round trip time of the 5-µm silicon-metal cavity with the metamaterial slab is delayed by a factor of 100 when the gap of the metamaterial unit cell is shifted from 4 µm to 8 µm. Compared with the traditional tunable optical delay line [2], the metamaterial optical delay line has merits of fine tuning resolution, high tuning speed and feasible for high density integration.

The development of radio-frequency (RF) filters with bandwidth reconfigurability and frequency tunability in the MHz range is increasingly important to broadband wireless communication and radar systems. In this paper, we demonstrate a reconfigurable RF filter based on a silicon integrated optical true time delay line (OTTDL). The OTTDL is a four-path parallel structure with the reconfiguration enabled by thermo-optic Mach–Zehnder interferometer switches. By controlling the power splitting ratio of the switches, three different orders of finite impulse response filters with a variable bandwidth are realized. The bandwidth can be varied from 180.9 MHz to 662.0 MHz and the center frequency can be tuned in one free-spectral range up to 1 GHz.

In this paper, the optical fiber delay line is designed, in order to eliminate the phased array antenna aperture transition time and aperture effect. The input signal is translated from the electric domain to the optical domain by using the direct modulation mode. The optical fiber delay line is simulated by the simulation software OptiSystem. The results show that the characteristic of beam pointing invariance with changing frequency is realized by time delay compensation. This method can be applied to the phased array antenna. And it can eliminate the effect of the effective aperture phased array.

Compact optical delay line for long-range scanning

A 10 Gb/s all-optical clock extraction based on magnetically controllable fiber optical parametric oscillator (MC-FOPO) is demonstrated. The operation properties of the magnetic control unit, composed of a solenoid and a magneto-optic crystal of high Verdet constant, are experimentally investigated. By adjusting the drive current of the solenoid, the magneto-optic crystal unit may serve as a tunable optical fiber delay line with polarization control to some extent. The experimental results show that the MC-FOPO is capable of repetitively magnetic tunability desirable for all-optical clock recovery.

We analyze a silicon reconfigurable feed-forward optical delay line (ODL) composed of cascaded Mach-Zehnder interferometer (MZI) switches and waveguide delay pairs. Optical delay is step-tuned by switching among optical routes with an incremental length difference. The crosstalk caused by limited extinction ratios (ERs) of MZI switches and the waveguide loss restricts the ODL buffering capacity. A method to suppress the crosstalk and hence expand the buffering capacity is proposed by inserting variable optical attenuators (VOAs) between successive MZI switches. A design example shows that a seven-stage ODL without VOAs can delay 8-98 bits with a tuning step of 1 bit for a 40 Gbit/s non-return-to-zero (NRZ) 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> - 1 pseudo-random bit sequence (PRBS) optical signal. When VOAs with a 10 dB attenuation ratio are included, the signal-to-crosstalk ratio (SCR) of the output signal is increased by 17 dB on average, making the ODL capable of delaying 8-135 bits at its maximum.