| Objective: ||Utilizes quantum dot mode-locked lasers for arbitrary waveform photonics,[1-2] improving pulse width, power, stability. Explore multisection cavity to reduce pulse width, increase peak power.
|| Description: ||Photonic systems have revolutionized telecommunications, computational capabilities, detection, and sensing systems. In this project, we want to understand the operating characteristics of short pulse, smaller than 5 ps, quantum dot mode locked lasers, and to determine the operating parameters of the device including the gain and absorption. It has been noted in the literature that the experimental challenge is to push the pulse width of passively mode-locked monolithic lasers down to about 0.2 ps, the bandwidth to about 50 nm, and the peak power to greater than 5 watts by using novel device geometries.
With the introduction of the quantum dot gain media to semiconductor lasers, the ability to tailor the active region to suit the mode-locked laser cavity and thereby produce a low threshold, low noise condition is leading to a reexamination of the passive mode-locked semiconductor laser as an ultra-short pulse optical source [1-2-3]. For the typical repetition rates of interest, 3 to 10 GHz, the laser cavity is on the order of 5 to 15 mm in length, which results in very low mirror and cavity losses that are unsuitable for quantum well gain regions. For the latter design, the bulk of the pump current is wasted on achieving optical transparency and producing unwanted spontaneous emission noise. On the other, hand in the quantum dot laser, the dot density can be easily modified to match the low cavity losses and thus minimize the transparency current density. Other advantages of quantum dots are the lower saturation intensity that is needed to bleach the absorber section and the ability to run the gain section under strong inversion for high peak pulsed powers. These features make monolithic quantum dot mode-locked lasers easier to lock and operate.
By virtue of their wide, flat, and inhomogeneously broadened gain spectrum, quantum dot lasers have been viewed as ideal candidates for generating ultrashort, ps pulses. Quantum well Fabry Perot lasers can only be driven slightly above threshold before mode-locking collapses due to the preference of single mode operation with increasing bias current. In principle the wide spectrum of the quantum dot Fabry Perot laser defeats the single mode tendency and allows for pulse width reduction by locking a broader spectrum of modes. In realizing this goal, however, some practical challenges are encountered. The inherently random distribution of sizes and shapes in the existing self-assembled quantum dot technology produces a graininess in the gain spectrum that can favor modes that are not contiguous in wavelength. This trend raises the issue of group velocity dispersion GVD in quantum dot lasers that could be limiting the narrowing of the pulse width. Although the variation of the index with carrier density is known to be very small in quantum dot lasers as evidenced by the small linewidth enhancement factor , little is known about the index dispersion with wavelength and its dependence on photon density, dot density, or dimensions. The objectives of this project are to map the group velocity dispersion as a function of these parameters and to work towards a more uniform, smoothly varying quantum dot gain envelope.
A passively mode-locked quantum dot laser source with an order of magnitude lower timing jitter (as low as 0.4 ps) than previously shown by the best quantum well devices and peak powers of 0.5 W for pulse widths of around 5 ps has been successfully demonstrated. The elimination of costly driver electronics in achieving these results is a significant step towards realizing low cost, compact, and ultrafast optical sources. Further improvements in this performance can be realized once there is a clearer understanding of the device operating parameters such as gain, absorption, and group velocity dispersion.
|| ||PHASE I: Investigate, model, and perform critical experiments on innovative nanophotonic material and process technologies. Accomplished through appropriate research and then design and analysis of the quantum dot device. Simulation of simple rate equation models. Report on technology demonstration.
|| || ||PHASE II: Further develop the proposed material and/or the relevant processes to fully demonstrate the properties of the quantum dot device through qualification tests, performed to validate the design and performance. Establish manufacturing processes for commercialization. Report on all technologies demonstrated, testing accomplished, data obtained, and analysis of the program with lessons learned.
|| ||DUAL USE COMMERCIALIZATION: Military application: This technology could lead to future military applications in unmanned combat vehicles, remoting of fixed receive and transmit capabilities, and in other new aerospace vehicles. Commercial application: Jet flight control, reusable launch vehicles, drive-by-light, industrial automation, cellular communications sites, fiber-to-the-home, cable TV, optical communications, ultrafast computer chip clock.
|| References: ||1. J. E. Malowicki, M.L. Fanto ML, M.J. Hayduk, et al., “Harmonically mode-locked glass waveguide laser with 21-fs timing jitter, ” IEEE Photonics Technology Letters Vol. 17 (1), pp. 40-42, January 2005.
2. T.A. Yilmaz, C.M. Depriest, A. Braun, J.H. Abeles, and P.J. Delfyett, “ Noise in fundamental and harmonic modelocked semiconductor lasers: Experiments and simulations,” IEEE Journal Of Quantum Electronics Vol. 39 (7), pp. 838-849, July 2003.
3. Y. K. Chen and M. C. Wu, "Monolithic colliding-pulse mode-locked quantum-well lasers," IEEE Journal of Quantum Electronics, Vol. 28, pp. 2176, 1992.
4. S. Sanders, L. Eng, J. Paslaski, and A. Yariv, "108-GHz passive-mode locking of a multiple quantum well semiconductor-laser with an intracavity absorber," Applied Physics Letters, Vol. 56, p. 310, 1990.
5. D.J. Derickson, R.J. Helkey, A. Mar, J.R. Karin, J.G. Wasserbauer, and J.E. Bowers, "Short pulse generation using multisegment mode-locked semiconductor-lasers," IEEE Journal of Quantum Electronics, Vol. 28, p. 2186, 1992.
|Keywords: ||semiconductor laser; quantum dot lasers; mode locking; arbitrary photonic waveform generation|