Prashant P. Baveja

Self-Phase Modulation in Semiconductor Optical Amplifiers: Impact of Amplified Spontaneous Emission

This paper presents a detailed theoretical and experimental study of the impact of amplified spontaneous emission (ASE) on self-phase modulation in semiconductor optical amplifiers (SOAs). A theoretical model of pulse propagation in SOAs is developed that includes the ASE power and its effect on gain-saturation and gain-recovery. We study the impact of ASE on the nonlinear phase shift, frequency chirp, spectrum, and shape of amplified picosecond pulses at a range of drive currents. We verify our predictions experimentally by launching gain-switched picosecond pulses with 3-mW peak power into a commercial SOA exhibiting 9-ps gain-recovery time at a current of 500 mA. Understanding the impact of ASE on SOAs is important for applications that employs SOAs for all-optical signal processing and as data-network amplifiers.

Assessment of VCSEL thermal rollover mechanisms from measurements and empirical modeling

We use an empirical model together with experimental measurements for studying mechanisms contributing to thermal rollover in vertical-cavity surface-emitting lasers (VCSELs). The model is based on extraction of the temperature dependence of threshold current, internal quantum efficiency, internal optical loss, series resistance and thermal impedance from measurements of output power, voltage and lasing wavelength as a function of bias current over an ambient temperature range of 15-100 °C. We apply the model to an oxide-confined, 850-nm VCSEL, fabricated with a 9-μm inner-aperture diameter and optimized for high-speed operation, and show for this specific device that power dissipation due to linear power dissipation (sum total of optical absorption, carrier thermalization, carrier leakage and spontaneous carrier recombination) exceeds power dissipation across the series resistance (quadratic power dissipation) at any ambient temperature and bias current. We further show that the dominant contributors to self-heating for this particular VCSEL are quadratic power dissipation, internal optical loss, and carrier leakage. A rapid reduction of the internal quantum efficiency at high bias currents (resulting in high temperatures) is identified as being the major cause of thermal rollover. Our method is applicable to any VCSEL and is useful for identifying the mechanisms limiting the thermal performance of the device and to formulate design strategies to ameliorate them.

Interband Four-Wave Mixing in Semiconductor Optical Amplifiers With ASE-Enhanced Gain Recovery

We study, theoretically and experimentally, interband four-wave mixing in semiconductor optical amplifiers whose gain recovery is accelerated by amplified spontaneous emission (ASE). Across a broad range of wavelength shifts, we observe a considerable increase (over 20 dB) in the conversion efficiency, and a corresponding increase in the optical SNR (over 12 dB), as the device current is increased from 100 to 500 mA. For input pump powers below 1 mW, gain recovery in our device is dominated by internal ASE. Higher pump power levels reduce the conversion efficiency because of the pump-induced gain saturation near the output end. We show that wavelength shifts of up to 25 nm are possible, while maintaining a >;10% conversion efficiency and a high optical SNR (>;25 dB). A major advantage of our scheme is that the use of relatively low pump powers (<; 1 mW) reduces the electrical power consumption for such wavelength converters by more than a factor of 10. We discuss in detail the issue of optimum pump and signal powers. Our study is useful for realizing energy-efficient, modulation-format transparent, wavelength converters for optical networks.

Optimization of All-Optical 2R Regenerators Operating at 40 Gb/s: Role of Dispersion

We investigate numerically the interplay between dispersion and nonlinearity for optimizing the performance of an all-optical 2R regenerator based on self-phase modulation and spectral filtering at 40 Gb/s. By considering the extent of improvement in the Q factor (related to level of noise reduction), we show that the ratio of accumulated dispersion to the maximum nonlinear phase shift can be used to predict the performance of regenerators making use of fibers with very different lengths, dispersions, and nonlinear parameters. Our results show that fiber dispersion plays an important role and needs to be properly optimized. In general, fibers with larger dispersion perform better but require higher input powers. We also study the impact of fluctuations in dispersion from their nominal value and show that their impact is much less severe when fiber dispersion is relatively small.

Impact of Device Parameters on Thermal Performance of High-Speed Oxide-Confined 850-nm VCSELs

We study the impact of device parameters, such as inner-aperture diameter and cavity photon lifetime, on thermal rollover mechanisms in 850-nm, oxide-confined, vertical-cavity surface-emitting lasers (VCSELs) designed for high-speed operation. We perform measurements on four different VCSELs of different designs and use our empirical thermal model for calculating the power dissipated with increasing bias currents through various physical processes such as absorption within the cavity, carrier thermalization, carrier leakage, spontaneous carrier recombination, and Joule heating. When reducing the top mirror reflectivity to reduce internal optical absorption loss we find an increase of power dissipation due to carrier leakage. There is therefore a trade-off between the powers dissipated owing to optical absorption and carrier leakage in the sense that overcompensating for optical absorption enhances carrier leakage (and vice versa). We further find that carrier leakage places the ultimate limit on the thermal performance for this entire class of devices. Our analysis yields useful design optimization strategies for mitigating the impact of carrier leakage and should thereby prove useful for the performance enhancement of 850-nm, high-speed, oxide-confined VCSELs.

All-Optical Semiconductor Optical Amplifier-Based Wavelength Converters With Sub-mW Pumping

We experimentally study the wavelength conversion of 10-Gb/s return-to-zero signal using interband four-wave mixing inside a semiconductor optical amplifier with 10-ps gain-recovery time. Power of the converted signal exceeds that of the original signal for wavelength shifts of up to 8 nm. Our technique is energy efficient as the required input pump power is <; 1 mW . We discuss the observed performance of such a wavelength converter in terms of required pump power, conversion efficiency, and optical signal-to-noise ratio of the converted signal. For an 8-nm wavelength shift, the converted data signal exhibits no Q-factor degradation while having 2.7-dB power gain.

Pulse amplification in semiconductor optical amplifiers with ultrafast gain-recovery times

This paper presents detailed numerical and experimental study of SPM in semiconductor optical amplifiers (SOAs) with ultrafast gain-recovery times. These SOAs have a range of gain-recovery speed which is a function of drive current. At increased drive current, the amount of internal ASE in the SOA increases, which causes the small signal gain to saturate and reduces the gain-recovery time. Understanding pulse amplification in these SOAs is important for optimizing the performance of SOA-based optical regenerators and wavelength converters. Our study addresses the full range of gain-recovery times in commercial SOAs extending from less than 10 ps to >100 ps.

Spectral broadening in ultrafast semiconductor optical amplifiers induced by gain dynamics and self-phase modulation

We investigate experimentally the self-phase modulation (SPM) induced by gain dynamics on picosecond pulses in semiconductor optical amplifiers whose gain recovery is enhanced by amplified spontaneous emission (ASE). The observed pulse spectra are highly asymmetric at low drive currents but become more symmetric with increasing current, owing to the ASE-induced reduction in the gain-recovery time down to 9 ps. Furthermore, the amount of spectral broadening is shown to saturate with drive current. We show that a variety of spectral-lobe strengths is selectable, while maintaining a nearly constant small-signal gain, a feature desirable for all-optical signal processing applications.

High-speed tunable and fixed-wavelength VCSELs for short-reach optical links and interconnects

This paper presents a review of recent work on high speed tunable and fixed wavelength vertical cavity surface emitting lasers (VCSELs) at Chalmers University of Technology. All VCSELs are GaAs-based, employ an oxide aperture for current and/or optical confinement, and emit around 850 nm. With proper active region and cavity designs, and techniques for reducing capacitance and thermal impedance, our fixed wavelength VCSELs have reached a modulation bandwidth of 23 GHz, which has enabled error-free 40 Gbps back-to-back transmission and 35 Gbps transmission over 100 m of multimode fiber. A MEMS-technology for wafer scale integration of tunable high speed VCSELs has also been developed. A tuning range of 24 nm and a modulation bandwidth of 6 GHz have been achieved, enabling error-free back-to-back transmission at 5 Gbps.

Impact of photon lifetime on thermal rollover in 850-nm high-speed VCSELs

We present an empirical thermal model for VCSELs based on extraction of temperature dependence of macroscopic VCSEL parameters from CW measurements. We apply our model to two, oxide-confined, 850-nm VCSELs, fabricated with a 9-μm inner-aperture diameter and optimized for high-speed operation. We demonstrate that for both these devices, the power dissipation due to linear heat sources dominates the total self-heating. We further show that reducing photon lifetime down to 2 ps drastically reduces absorption heating and improves device static performance by delaying the onset of thermal rollover. The new thermal model can identify the mechanisms limiting the thermal performance and help in formulating the design strategies to ameliorate them.