Techniques for increasing output power from mode-locked semiconductor lasers Page: 1 of 14
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Techniques for Increasing Output Power From Mode-Locked Semiconductor Lasers
Alan Mar and G. Allen Vawter
Sandia National Laboratories, Albuquerque, NM 87185 ,
FEB 1 2 L%3
Mode-locked semiconductor lasers have drawn considerable attention as compact, reliable, and rel te n pnsive
sources of short optical pulses. Advances in the design of such lasers have resulted in vast improvements in pu sewidth and
noise performance, at a very wide range of repetition rates. An attractive application for these lasers would be to serve as
alternatives for large benchtop laser systems such as dye lasers and solid-state lasers. However, mode-locked semiconductor
lasers have not yet approached the performance of such systems in terms of output power. Different techniques for
overcoming the problem of low output power from mode-locked semiconductor lasers will be discussed. Flared and arrayed
lasers have been used successfully to increase the pulse saturation energy limit by increasing the gain cross section. Further
improvements have been achieved by use of the MOPA configuration, which utilizes a flared semiconductor amplifier stage to
amplify pulses to energies of 120 pJ and peak powers of nearly 30W.
Keywords: laser diode, pulsed laser, mode-locked, high-power, MOPA, flared waveguide, tapered waveguide, laser array
Because of their compactness, reliability, efficiency, and relative low cost, mode-locked semiconductor lasers are
attractive sources of short optical pulses. Such pulse sources are of interest for use in physics measurements, for
instrumentation systems, and for telecommunications applications. Femtosecond pulsewidths have been achievedl,, which
is feasible due to the multi-terahertz optical gain bandwidths available from semiconductor media. With proper design, mode-
locked diode lasers have very low amplitude and timing jitter levels compared to other types of pulsed lasers. Femtosecond
rms timing jitter levels have been achieved, limited largely by phase noise in the driving electrical sources3. Terahertz
repetition rates have been demonstrated4, and devices are available over a wide range of wavelengths by utilizing
semiconductor bandgap engineering.
Typically, semiconductor lasers emit much less average power under mode-locked operation than they do under cw
conditions. This is fundamentally due to pulse broadening effects in semiconductor amplifiers arising from gain saturation
due to carrier depletion during the amplification process. This pulse distortion becomes particularly severe when the pulse
energies approach the saturation energy,
Esat= hv A
where A is the active region cross section, by the photon energy, F the confinement factor, and dg/dn the differential gain. In
passively (saturable absorber) mode-locked lasers, as the laser is driven to higher pulse energies, this broadening counteracts
the pulse shortening process in the saturable absorber or gain modulator, which may result in poor quality pulses or in the
cessation of mode-locked operation altogether. The saturation energy limit is about 2 pJ for a typical single-mode laser,
which, depending on the repetition rate, results in mode-locked average powers far below the laser's cw power capability. For
example, at a repetition rate of 1 GHz, this results in an internal average power of 2 mW within in the laser, which may
represent less than only 1 mW output power after output coupling losses.
An attractive application for these lasers would be to serve as alternatives for large benchtop laser systems such as
dye lasers and solid-state lasers. However, mode-locked semiconductor lasers have not yet approached the performance of such
systems in terms of output power. In this paper we will discuss various techniques for overcoming the problem of relatively
low output power. This will first include a brief discussion of pulse formation and amplification in diode laser amplifiers and
saturable absorbers. Then we will describe the use of laser arrays5, flared waveguide amplifier lasers6, and master oscillator
power amplifier (MOPA) lasers7 to achieve increased pulse energies and higher output powers. Average powers of 300 mW
with peak powers of nearly 30 W have now been obtained from a mode-locked MOPA laser.
IL. Modeling of Pulse Propagation in Laser Amplifiers
Because the optical pulses generated in mode-locked lasers are typically short compared to the transit time through
DISTRIBUTOR OF THiS DO IMI
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Mar, A. & Vawter, G.A. Techniques for increasing output power from mode-locked semiconductor lasers, article, February 1, 1996; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc665396/m1/1/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.