Time-resolved Chirp Research Articles

Time-resolved frequency chirp control of semiconductor lasers using digital signal processing

The theoretical possibility of precisely controlling the time-resolved chirp in a directly modulated semiconductor laser using a single drive digital-to-analog converter (DAC) controlled via a novel digital signal processing (DSP) algorithm is investigated. A semi-analytical expression for the requisite drive current is obtained through algebraic back-calculation of the large signal laser rate equations, in tandem with a first-order numerical approximation of the optical power and chirp functional relationship. A 25-GHz peak-to-peak frequency modulation (FM) at a bit-rate of 28-Gb/s is demonstrated at a 6 dB extinction ratio, while completely suppressing the transient chirp contribution. Furthermore, a bipolar frequency chirp signal resulting in a differential phase shift keying (DPSK) modulation format at a bit-rate of 28-Gb/s was achieved with an error vector magnitude (EVM) below 1%. A 5-bit look-up table (LUT) was found to be adequate for capturing the requisite DSP drive current needed to fully suppress the transient chirp as demonstrated by preliminary optical link simulations at 28-Gb/s.

Characterization of a Low-Cost, Monolithically Integrated, Tunable 10G Transmitter for Wavelength Agile PONs

Dynamically reconfigurable passive optical networks (PONs) using time-division multiplexing and dense wavelength division multiplexing will require low-cost, high-performance customer premises equipment to be economically viable. In particular, substantial cost savings can be achieved through the use of efficient re-growth free, foundry-compatible fabrication techniques. Using this strategy, this paper presents the first detailed characterization of a monolithically integrated transmitter comprised of a discretely tunable slotted Fabry–Perot ridge waveguide laser, an absorptive modulator and a semiconductor optical amplifier (SOA) produced using a standard off-the-shelf AlInGaAs/InP multiple quantum well epitaxial structure. This first generation device demonstrates a discrete single-mode tuning range of approximately 12 nm between 1551nm and 1563 nm with a side-mode suppression ratio ≥30 dB. Moreover, the integrated modulator section is shown to support transmission at 10 Gb/s using non-return to zero on-off keying with an extinction ratio in excess of 8 dB. Furthermore, using a time-resolved chirp measurement technique to examine dynamic deviations in the set carrier frequency, the modulator section exhibits a chirp contribution of $\mu \text{s}$ which are faster than the thermal response time of the transmitter material.

Time-resolved chirp properties of semiconductor optical amplifiers in high-speed all-optical switches

The chirp properties of semiconductor optical amplifiers in all-optical switches are numerically investigated using a field propagation model. The chirp dynamics in the blue-shift and red-shift sideband are analyzed under the injection of random optical pump pulses. We also analyze the impact of the blue-detuned filtering scheme that is used to eliminate the pattern effect and enhance the operating speed of the optical switching. The reason for overshoots in eye diagrams in the blue-detuned filtering scheme is explained. We find that overshoots result from the ultrafast blue chirp induced by carrier heating and two-phonon absorption. These results are very useful for semiconductor optical amplifier-based ultrafast all-optical signal processing.

Time-resolved chirp measurement for 100GBaud test systems using an ideal frequency discriminator

In this paper we present multi-channel chirp measurements of wide-band sources, using a programmable Fourier-domain optical processor (FDOP) as a near-perfect linear frequency discriminator element followed by a fast photodiode and electrical sampling oscilloscope. The electric field of a 10.7Gbit/s phase-encoded data source and a directly modulated laser diode are simultaneously interrogated with this measurement system. The constellation diagram of the phase-encoded data source is demonstrated, and a comparison with another phase-sensitive measurement technique is performed. Additionally, an extension to this technique is demonstrated in which the time-resolved chirp of a picosecond-duration mode-locked laser diode with a 260GHz spectral bandwidth is characterised using the FDOP and a high-bandwidth optical sampling oscilloscope. This measurement ensemble has sufficient temporal resolution to characterise random or repetitive data signals up to 100GBaud.

Spectral filtering of multiple directly modulated channels for WDM access networks by using an FP etalon

Spectral and waveform reshaping schemes can enhance the transmission distance of fiber links that use directly modulated lasers as transmitters. We prove the feasibility of using a simple Fabry-Perot (FP) etalon as the spectral reshaper for applications in wavelength division multiplexing (WDM) access networks. The transient chirp and adiabatic chirp of a directly modulated laser are analyzed in detail by using the time-resolved chirp measurement. The effects of the original extinction ratio and the adiabatic chirp on the spectral reshaping are clarified to obtain the optimal operation conditions. It is shown that placing a single-cavity FP etalon filter after multiple 10 Gbits/s directly modulated lasers can extend their transmission distances from 50 km in the 1.55 μm wavelength window. Due to the limited filtering capability of the etalon, the choice of the original extinction ratio and finesse of the etalon is discussed in detail from the experiments and simulation.

Time-Resolved Chirp Properties of SOAs Measured With an Optical Bandpass Filter

Chirp properties induced by semiconductor optical amplifiers (SOAs) are investigated by a simple chirp measurement method using an optical bandpass filter. We measure the chirp properties of signals generated by an SOA-based wavelength converter at noninverted and inverted logic operations. These results demonstrate well the features of the chirp properties induced by SOA.

Time-resolved chirp in an InAs∕InP quantum-dash optical amplifier operating with 10Gbit∕s data

We describe time-resolved chirp measurements in InAs∕InP quantum-dash optical amplifiers operating at 1550 nm. We highlight the roles of gain saturation and of the saturating pulse duration relative to the gain recovery time. Using 10Gbit∕s data, we demonstrate a low transient α parameter of less than one which causes negative chirp at the leading edge and positive chirp during the trailing edge of the input pulse.

Correlation between dispersion penalty and time-resolved chirp for an integrated widely tunable electroabsorption-modulated SGDBR laser across the EDFA gain bandwidth

We report, for the first time, the experimental relationship between time-resolved chirp and dispersion penalty across the 30-nm tuning range of a sampled-grating distributed Bragg reflector semiconductor-optical-amplifier electroabsorption-modulator laser. The extinction ratio, time-resolved chirp, and dispersion power penalties at transmission distances of 200, 275, and 350 km are presented for various values of effective alpha. Finally, a suitable method of computing the effective alpha from time-resolved spectroscopy is also presented.

Multiple-quantum-well Mach-Zehnder modulators: Comparison of calculated and measured results for modulator properties and system performance

A measurement-based model of a π phase-shift multiple-quantum-well Mach-Zehnder modulator with asymmetric Y-branch waveguides is presented. Using this model, calculated and measured results for the transmission contours, α-parameter, intensity modulation, and time-resolved chirp are compared. The suitability of the model for system performance evaluation is demonstrated by examining the implications of the modulator chirp on 10-Gb/s transmission over nondispersion shifted fiber, and the combined effect of the modulator chirp and group delay ripple of a dispersion compensating fiber Bragg grating on 10-Gb/s dispersion compensated transmission. Calculated and measured results for the system performance are compared.

Measurement-based method for characterizing the intensity and phase modulation properties of SOA-MZI wavelength converters

A novel model is described for an integrated all-active semiconductor optical amplified Mach Zehnder interferometric (SOA-MZI) wavelength converter that is based on the static dependence of the power- and /spl alpha/-parameter for the wavelength-converted signal on the input power, the intensity modulation frequency response, and the phase modulation frequency response. This provides a consistent procedure for the device characterization and modeling in which both the intensity and phase of the wavelength-converted signal are directly related to the intensity of the input signal. Good agreement is demonstrated between calculated and measured results for the time-resolved chirp and intensity.

Penalty-free 10 Gb/s NRZ transmission over 100 km of standard fiber at 1.55 μm with a blue-chirp modulator integrated DFB laser

We present transmission beyond the dispersion limit with a blue-chirp modulator integrated DFB laser for the first time. By suppressing the hole pile-up in the modulator, stable modulation characteristics were realized in negative /spl alpha/ regime. Blue-chirp operation was clearly observed by time-resolved dynamic chirp measurement. Penalty-free 10 Gb/s transmission over 100 km of standard fiber at 1.55 /spl mu/m was demonstrated with the device. This modulator integrated DFB laser is promising for use as a high-power blue-chirp transmitter in multigigabit transmission systems.

Chirp and system performance of integrated laser modulators

Time-resolved chirp measurements and laser-threshold measurements are used to understand the chirp performance of integrated laser electroabsorption modulators. The different effects of the intrinsic modulator chirp component and chirp caused by optical feedback, from reflection at the output facet into the laser, are observed. A figure of merit is introduced to assess the chirp performance, which shows good correlation with dispersion penalty measurements. The effect of reflection-induced chirp is considerably reduced by operating the modulator with negative chirp. An optimized device is reported, which delivers purely negative chirp with reasonable optical power. >

Controlling the chirp in electroabsorption modulatorsunder digital modulation

Time-resolved chirp measurements are presented for a multiquantum well electroabsorption modulator which are in good agreement with Kramers-Kronig calculations. The device delivers negative chirp by control of both the operating wavelength and electrical drive.

Time-resolved chirp in mode-locked semiconductor lasers

Numerical simulations using the transmission-line laser model show that the instantaneous frequency of a 10 ps mode-locked pulse undergoes a rapid red-shift of 60 GHz during the first half of the pulse followed by a slow recovery of 10 GHz. Self-phase modulation is thought to cause this shift.

Time-resolved chirp evaluations of Gbit/s NRZ and gain-switched DFB laser pulses using narrowband Fabry-Perot spectrometer

A compact, easy-to-use, low-cost spectrometer for resolving the chirped spectrum of DFB lasers is demonstrated using narrowband Fabry-Perot etalons with resolutions comparable to grating spectrometers. Time-resolved chirp measurements of Gbit/s NRZ and gain-switched DFB laser pulses are presented.