In many precision sensing applications, the final detection sensitivity is tightly related to the intensity noise of the laser source, which might represent the ultimate limit to the sensor performance. In this framework, we present here the intensity noise characterization of three different mid-infrared semiconductor devices (two quantum cascade lasers and one interband cascade laser). A fast homemade balanced detection system is used to measure the intensity noise of the emitted radiation over a broad Fourier-frequency range, facilitating the observation of shot-noise-limited radiation under specific measurement conditions and detection efficiency. This study allows for a direct performance comparison of the most widespread laser sources in mid-infrared sensing systems.
A crucial element for the next generation of portable gas sensors for high-volume applications, especially involving chemical sensing of important greenhouse and pollutant gases, is the development of a low-cost, low-power consuming, single-frequency laser operating in the mid-infrared spectral range.
In this regard, we propose the implementation of a Quantum Cascade Surface Emitting Laser (QCSEL). Our design involves a linear microcavity with high reflectivity coated end-mirrors and a buried semiconductor diffraction grating to extract the light from the surface.
We present low threshold quantum cascade surface emitting lasers (QCSELs) emitting at wavelengths of 4.5 micrometers or 8 micrometers. To extract the light vertically from the InP-based buried heterostructure laser a second order InGaAs/InP grating is used. Both ridge facets are formed by dry-etching followed by coating a dielectric-metal film. Due to the high reflectivity of the facets, the cavity can be shortened well below 500 micrometers reducing the threshold power to several hundred milliwatts. The proposed device concept allows large-scale fabrication and wafer-level characterization. The results are an important step towards low-cost and low-power consuming quantum cascade lasers for portable MIR gas sensors.
We present strong radio-frequency current modulation close to their repetition frequency as a means to control the emitted state of quantum cascade laser frequency combs. In particular, more than doubling of the spectral bandwidth compared to free-running can be achieved throughout the dynamical range of the device. By changing the modulation frequency, the spectral bandwidth and center-frequency can be tuned and by fast switching between modulation frequencies we can multiplex spectral regions with negligible overlap from the same device. In the time-domain, we are able to transition from quasi-continuous to long-pulse output by injecting at high power.
In recent years, quantum cascade lasers have matured to become compact, powerful sources of coherent midinfrared light. Yet, the ultrafast carrier dynamics in these sources has so far restricted the formation of highintensity ultrashort pulses. In this work, we demonstrate the formation of ∼ 630 fs QCL pulses with a peak power of ∼ 4.3 W. We break the picosecond barrier in an approach similar to chirped pulse amplification, where we externally recompress the maximally chirped output of a quantum cascade laser frequency comb. Ultrashort pulse formation is confirmed with a novel asynchronous optical sampling technique. These results emphasise the potential of quantum cascade lasers also as sources for non-linear experiments in the mid-infrared.
WE WILL REVIEW RECENT ADVANCES IN THE REALIZATION OF WIDE BANDWIDTH, HIGH PERFORMANCE. FREQUENCY COMB SOURCES BASED ON QUANTUM CASCADE LASERS OPERATING BOTH IN THE THz AND MID-IR REGIONS OF THE E. M. Spectrum. In the Mid-IR, a grating compressor is employed to obtain pulses from a quantum cascade comb operating in CW. IN THE THz, COPPER-BASED DOUBLE METAL WAVE GUIDES ALLOW COMB OPERATION ABOVE LIQUID NITROGEN TEMPERATURE WITH RELATIVE Comb BANDWIDTHS OF 25%. Frequency COMB CONTROL BY MEANS of RF INJECTION AND COUPLED CAVITY SCHEMES WILL BE PRESENTED TOGETHER WITH NEW COMB characterization TECHNIQUES.
Quantum cascade laser-based frequency combs have attracted much attention as of late for applications in sensing and metrology, especially as sources for chip-scale spectroscopy at mid-infrared fingerprint wavelengths. A frequency comb is a light source whose lines are evenly-spaced, and only two frequencies are needed to describe the system—the offset and the repetition rate. Because chip-scale combs have large repetition rates, for many spectroscopic applications is important to be able to change both parameters independently, without substantially changing the comb spectrum or spectral structure. Although it is possible to modulate both the offset and the repetition rate of a comb by tuning the laser current and temperature, both properties affect the laser by changing its index of refraction, and both frequencies will be affected. Here, we show that by integrating a mirror onto a MEMS comb drive, the dispersion and group delay associated with a quantum cascade comb’s cavity can be modulated at kilohertz speeds. Because the MEMS mirror primarily affects the group delay of the cavity, it is able to adjust the comb’s repetition rate while leaving the offset frequency mostly unaffected. Since this adjustment is linearly independent from current adjustments and can be adjusted quickly, this provides an avenue for mutual stabilization of both parameters. In addition, we show that dynamic modulation of the comb drive is able to allow the laser to recover from comb-destroying feedback, making the resulting comb considerably more robust under realistic conditions.
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