By David Nugent
It’s ages since I last wrote about short pulse lasers. Fifteen years, four months and five days to be precise. That’s when my colleauges at Cambridge University and BT Research Laboratories and I published an Electronics Letter on the action of passive saturable absorbers (‘SAs’) embedded within the upper Bragg mirror of a VCSEL to create self-pulsations in the GHz region [1]. Within five months of publication, Honeywell filed a derivative patent whereby the carrier density within the SA region, and thereby self-pulsation rate, is controllable via electrical contracts.
Enough reminiscing from me! It’s great to see an entire CLEO session dedicated to the discussion of Short Pulse Lasers (Monday, 1.30pm to 3.15pm). Generally speaking the papers can be segrragated into three categories: quantum dot lasers, vertical cavity surface emitting lasers, and QW lasers. Here is a snapshot of each:
Vertical Cavity Surface Emitting
Keith Wilcox ( Univ. of Southampton) et al will kick off the Short Pulse Laser session with a presentation on modelocking mechanisms in VCSELs, featuring a 40-fold reduction in pulse shortening from picoseconds to femtoseconds.
Martin Hoffmann (ETH Zurich) et al will present the simulation of passively mode-locked electrically-pumped VCSELs. This shall be followed by experimental results papers by Benjamin Rudin (ETH Zurich) on modelocked integrated external-cavity surface emitting laser (MIXEL) with output power up to 660 mW and repetition rates of 10 GHz.
Quantum dot lasers
Mo Xia (Univ. of Cambridge, UK) et al will present work on superradiant pulses from tapered QD device. Emitted at rates approaching 5 MHz, the pulses have durations as short as 320 fs at a wavelength of 1270 nm.
Mingming Fen (NIST, USA) will discuss a bistable two-section QD laser with a switching time of 150 ps. Considering the single-pass amplification constraints of contemporary QD lasers, and thereby the mimimum device length required to achieve lasing, I wonder how much further the switching time could be reduced? Any ideas?
Quantum Well lasers
Adrian Quarterman (Univ. of Southampton, UK) will present work on gain-saturated optically-pumped mode-locked QW laser with sub-100 fs pulses . Admittedly I am not abreast of the QW literature, but is this the fastest mode-locked semiconductor laser ever reported? I am looking forward to seeing an electrically pumped version!
Mizuki Shirao (Tokyo Inst. of Technology, Japan) et al will finish off the session with a talk on large-signal analysis of a “laser transistor” based on AlGaInAs/InP. According to Shirao this device exhibits better eye diagrams over 40-Gb/s than a laser diode. I know nothing about laser transisters, so this is a particularly interesting presentation for me.
Clearly great strides are being made in the development of short-pulse lasers with pulse widths rapidly approaching the realm of comparatively bulky and expensive Ti-sapphire sources. The big question not obviously addressed by the CLEO abstracts is the question of pulse jitter and peak power. Whilst the average pulse energy can be boosted by semiconductor optical amplifiers, pulse jitter may prove a more obstinate barrier to adoption by end users. At least this is my thesis.
1: Nugent, D.G.H.; Plumb, R.G.S.; Fisher, M.A.; Davies, D.A.O.; , “Self-pulsations in vertical-cavity surface emitting lasers,” Electronics Letters , vol.31, no.1, pp.43-44, 5 Jan 1995
Dr David Nugent is Founder and CEO of Elucidare Limited, a boutique technology development and investment advisory business.
Posted: 11 May 2010 by
David Nugent
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