H. Su

Dynamic properties of quantum dot distributed feedback lasers: high speed, linewidth and chirp

Semiconductor quantum dots (QDs) are nano-structures with three-dimensional spatial confinement of electrons and holes, representing the ultimate case of the application of the size quantization concept to semiconductor hetero-structures. The knowledge about the dynamic properties of QD semiconductor diode lasers is essential to improve the device performance and understand the physics of the QDs. In this dissertation, the dynamic properties of QD distributed feedback lasers (DFBs) are studied. The response function of QD DFBs under external modulation is characterized and the gain compression with photon density is identified to be the limiting factor of the modulation bandwidth. The enhancement of the gain compression by the gain saturation with the carrier density in QDs is analyzed for the first time with suggestions to improve the high speed performance of the devices by increasing the maximum gain of the QD medium. The linewidth of the QD DFBs are found to be more than one order of magnitude narrower than that of conventional quantum well (QW) DFBs at comparable output powers. The figure of merit for the narrow linewidth is identified by the comparison

Gain Compression and Above-Threshold Linewidth Enhancement Factor in 1.3-

Quantum-dot (QD) lasers exhibit many useful properties such as low threshold current, temperature and feedback insensitivity, chirpless behavior, and low linewidth enhancement factor (alphaH-factor). Although many breakthroughs have been demonstrated, the maximum modulation bandwidth remains limited in QD devices, and a strong damping of the modulation response is usually observed pointing out the role of gain compression. This paper investigates the influence of the gain compression in a 1.3-mum InAs-GaAs QD laser and its consequences on the above-threshold alphaH-factor. A model is used to explain the dependence of the alphaH-factor with the injected current and is compared with AM/FM experiments. Finally, it is shown that the higher the maximum gain, the lower the effects of gain compression and the lower the alphaH-factor. This analysis can be useful for designing chirpless QD lasers with improved modulation bandwidth as well as for isolator-free transmission under direct modulation.

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An alternative segmented-contact method for accurate measurement of the optical gain and absorption of quantum-dot and quantum-dash active materials with small optical gain is reported. The usual error from unguided spontaneous emission is reduced by subtracting signals acquired from three independently controlled sections as opposed to just two found in the conventional technique. The quantum-dot gain spectra are measured to a precision of less than 0.2 cm-1 at nominal gain values below 2 cm-1, and gain spectrum of quantum-dash sample is calculated with an error less than 0.3 cm-1 at a gain less than 1 cm-1. These accuracies are checked with a self-calibrating method. The internal optical mode loss measurement is also described

InAs–GaAs Quantum-Dot Lasers

External optical feedback effects on quantum dot (QD) laterally loss-coupled (LLC) distributed feedback (DFB) lasers are reported for the first time in this letter. The critical external feedback ratio that causes coherence collapse of the QD DFB is measured to be -14 dB. No spectral broadening at this feedback level is observed within the 0.06-nm resolution of the optical spectrum analyzer (OSA). Self-homodyne measurements also confirm that the rebroadened linewidth of the QD DFB under -14-dB feedback is still much smaller than the feedback-free linewidth. Under 2.5-Gb/s modulation, eye-diagram measurements show that the signal-to-noise ratio starts to degrade at a feedback ratio of -30 dB in the QD LLC-DFB, about 20 dB higher than a typical quantum-well DFB at the same output power and extinction ratio.

Optical gain and absorption of quantum dots measured using an alternative segmented contact method

Sub-picosecond timing jitter is demonstrated for 5 GHz, <10 ps optical pulses generated from monolithic passively mode-locked quantum dot lasers. Their low cost, compact size and DC-biased operation make them ideal for high speed optical interconnects.

High external feedback resistance of laterally loss-coupled distributed feedback quantum dot semiconductor lasers

We present wavelength conversion using nondegenerate four-wave mixing in loss-coupled distributed feedback lasers based on InAs quantum dots (QDs) grown on a GaAs substrate. The conversion efficiency is measured to be -15 to -30 dB for a signal-pump detuning range from 0.33 to 8 nm. The third-order optical susceptibility (/spl chi//sup (3)/) normalized to the optical linear gain in the QDs is estimated to be 3/spl times/10/sup -20/ m/sup 3//V/sup 2/ to 9/spl times/10/sup -20/ m/sup 3//V/sup 2/ for the signal-pump detuning range.

Low timing jitter, 5 GHz optical pulses from monolithic two-section passively mode-locked 1250/1310 nm quantum dot lasers for high-speed optical interconnects

The linewidth of laterally loss-coupled distributed feedback (DFB) lasers based on InAs quantum dots (QDs) embedded in an InGaAs quantum well (QW) is investigated. Narrow linewidth operation of QD devices is demonstrated. A linewidth-power product less than 1.2 MHz /spl middot/ mW is achieved in a device of 300-/spl mu/m cavity length for an output power up to 2 mW. Depending on the gain offset of the DFB modes from the QD ground state gain peak, linewidth rebroadening or a floor is observed at a cavity photon density of about 1.2-2.4/spl times/10/sup 15/ cm/sup -3/, which is much lower than in QW lasers. This phenomenon is attributed to the enhanced gain compression observed in QDs.

Nondegenerate four-wave mixing in quantum dot distributed feedback lasers

For understanding the fundamental processes in QDs and optimizing the design of QD optical devices, it is essential to obtain accurate optical gain and absorption spectra. An improved segmented-contact method is described that subtracts the unguided spontaneous emission that normally introduces error into the calculated gain and absorption. Using the technique a QD gain spectrum is measured to an accuracy of less than 0.2/cm at nominal gain values below 2/cm. This capability also enables precise measurement of waveguide internal loss, unamplified spontaneous emission spectra and Stark shift data.

Linewidth study of InAs-InGaAs quantum dot distributed feedback lasers

We measure, for the first time, the gain compression coefficient and above-threshold linewidth enhancement factor (alpha parameter) in quantum dot (QD) distributed feedback lasers (DFB) by time-resolved-chirp (TRC) characterization. The alpha parameter is measured to be 2.6 at threshold and increases to 8 when the output power of the QD DFB is increased to 3 mW. The dependence of the above-threshold alpha parameter on the optical power is found to be stronger than the optical gain compression effect alone can predict. The inhomogeneous gain broadening, gain saturation at the ground states and carrier filling in the excited states in QDs are proposed to explain the results.

Determination of optical gain and absorption of quantum dots with an improved segmented contact method

The correlations between the photoluminescence (PL) wavelength, integrated intensity, peak intensity, and FWHM with laser diode performance such as the maximum gain, injection efficiency, and transparency current density are studied in this work. The primary outcome is that the variation in PL intensity within a wafer originates primarily from differences in the radiative and non-radiative recombination rates and not from dot density variation. PL generated from 980 nm wavelength pumping appears to give more consistent data in assessing the optical quality of quantum dots that emit in the 1300 nm from the ground state.