A broadband, compact, all-electrically driven mid-infrared frequency comb based on a quantum cascade laser widens the scope of application of combs in this frequency range beyond that of sources which depend on a chain of optical components. Optical frequency combs are light sources that produce a comb-like spectrum, with sharp equidistant frequency modes, and have many uses in metrology and spectroscopy applications. The mid-infrared regime is particularly important for molecular fingerprinting, but so far the comb sources in this wavelength regime are bulky and rely on a chain of optical components. For wide practical applications, an electrically injected, compact scheme is desired. Andreas Hugi et al. now demonstrate a mid-infrared frequency comb generator based on a semiconductor device, a continuous-wave quantum cascade laser. Optical frequency combs1 act as rulers in the frequency domain and have opened new avenues in many fields such as fundamental time metrology, spectroscopy and frequency synthesis. In particular, spectroscopy by means of optical frequency combs has surpassed the precision and speed of Fourier spectrometers. Such a spectroscopy technique is especially relevant for the mid-infrared range, where the fundamental rotational–vibrational bands of most light molecules are found2. Most mid-infrared comb sources are based on down-conversion of near-infrared, mode-locked, ultrafast lasers using nonlinear crystals3. Their use in frequency comb spectroscopy applications has resulted in an unequalled combination of spectral coverage, resolution and sensitivity4,5,6,7. Another means of comb generation is pumping an ultrahigh-quality factor microresonator with a continuous-wave laser8,9,10. However, these combs depend on a chain of optical components, which limits their use. Therefore, to widen the spectroscopic applications of such mid-infrared combs, a more direct and compact generation scheme, using electrical injection, is preferable. Here we present a compact, broadband, semiconductor frequency comb generator that operates in the mid-infrared. We demonstrate that the modes of a continuous-wave, free-running, broadband quantum cascade laser11 are phase-locked. Combining mode proliferation based on four-wave mixing with gain provided by the quantum cascade laser leads to a phase relation similar to that of a frequency-modulated laser. The comb centre carrier wavelength is 7 micrometres. We identify a narrow drive current range with intermode beat linewidths narrower than 10 hertz. We find comb bandwidths of 4.4 per cent with an intermode stability of less than or equal to 200 hertz. The intermode beat can be varied over a frequency range of 65 kilohertz by radio-frequency injection. The large gain bandwidth and independent control over the carrier frequency offset and the mode spacing open the way to broadband, compact, all-solid-state mid-infrared spectrometers.

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Mid-infrared frequency comb based on a quantum cascade laser

Semiconductor lasers are convenient and compact sources of light, offering highly efficient operation, direct electrical control and integration opportunities. In this review we describe how semiconductor quantum-dot structures can provide an efficient means of amplifying and generating ultrafast (of the order of 100 fs), high-power and low-noise optical pulses, with the potential to boost the repetition rate of the pulses to beyond 1 THz. Such device designs are opening up new possibilities in ultrafast science and technology.

Mode-locked quantum-dot lasers

The processing of optical signals in the optical domain is an important issue resulting from the desire to take advantage of the full bandwidth of the optical fiber. In this paper, we present detailed investigations on a device, which utilizes a semiconductor laser amplifier in a loop mirror configuration (SLALOM). Different modes of operation are reported like nonlinear single pulse switching and two-pulse switching at different operation speeds (1-100 Gb/s). Furthermore, a number of applications of the SLALOM in photonic systems, like pulse shaping, decoding, retiming and time-division demultiplexing, are presented. In addition, the SLALOM can be used for an estimate of the linewidth enhancement factor /spl alpha/ and the carrier lifetime /spl tau//sub e/ in an SLA. >

SLALOM: semiconductor laser amplifier in a loop mirror

Progress in technology, theory and applications of high-frequency mode-locked diode lasers is reviewed. We present a systematic overview of a wealth of experimental and theoretical results and approaches reported recently in the field of physics, technology and applications of monolithic mode-locked laser diodes.

Monolithic and multi-gigahertz mode-locked semiconductor lasers: constructions, experiments, models and applications

Mode-locked semiconductor lasers which incorporate multiple contacting segments are found to give improved performance over single-segment designs. The functions of gain, saturable absorption, gain modulation, repetition rate tuning, wavelength tuning, and electrical pulse generation can be integrated on a single semiconductor chip. The optimization of the performance of mode-locked lasers in terms of material parameters, waveguiding parameters, electrical parasitics, and segment length is discussed experimentally and theoretically. >

/pdf/short-pulse-generation-using-multisegment-mode-locked-1y04xjn8ic.pdf

Short pulse generation using multisegment mode-locked semiconductor lasers

Carrier transport has a significant effect on the high-speed characteristics of semiconductor lasers. The authors show theoretically and experimentally that the low frequency rolloff and excess increase of damping due to carrier transport significantly limits the high-speed modulation bandwidth of long wavelength InGaAs(P)-InP quantum-well lasers. The inherent small conduction band offset and small hole diffusion constant of this material are responsible for the severe carrier transport effects. The authors also discuss the optimum design for the high speed modulation of the long wavelength lasers and the advantages of the InGaAlAs-InP material system. >

Long wavelength high-speed semiconductor lasers with carrier transport effects

The authors consider electrical and optical signal generation techniques using passively and hybridly mode-locked semiconductor lasers with intra-waveguide saturable absorbers. Microwave and millimeter-wave signals can be generated using mode-locked semiconductor lasers with the intra-waveguide saturable absorbers. Monolithic and external cavity devices optimized for electrical and optical signal generation are discussed along with measurements of amplitude noise, phase noise, output power, and repetition rate tunability. >

/pdf/microwave-and-millimeter-wave-signal-generation-using-mode-1ycb4qqbbd.pdf

Microwave and millimeter wave signal generation using mode-locked semiconductor lasers with intra-waveguide saturable absorbers

Semi-insulating (SI) planar buried heterostructure (SIPBH) lasers, designed to minimize device parasitics, have been fabricated for high-speed operation. The I-V characteristics of the Fe-doped SI InP employed for the current blocking layers were examined at elevated temperatures associated with high speed laser operation. It was found that the trap filled voltage, V/sub TF/, decreases linearly with increasing temperature. However, a SI InP layer with a V/sub TF/ at room temperature of 70 V demonstrated current blocking characteristics to temperatures in excess of 150 degrees C. >

http://ieeexplore.ieee.org/iel2/139/5236/00203009.pdf

J.G. Wasserbauer

Papers

Short pulse generation using multisegment mode-locked semiconductor lasers

Long wavelength high-speed semiconductor lasers with carrier transport effects

Microwave and millimeter wave signal generation using mode-locked semiconductor lasers with intra-waveguide saturable absorbers

High speed semi-insulating GaInAsP laser processing