ABSTRACT
We investigate the high frequency modulation characteristics of mid-infrared surface-emitting ring and edge-emitting ridge quantum cascade lasers (QCLs). In particular, a detailed comparison between circular ring devices and ridge-QCLs from the same laser material, which have a linear waveguide in a "Fabry-Pérot (FP) type" cavity, reveals distinct similarities and differences. Both device types are single-mode emitting, based on either 2 nd- (ring-QCL) or 1 st-order (ridge-QCL) distributed feedback (DFB) gratings with an emission wavelength around 7.56 µm. Their modulation characteristics are investigated in the frequency-domain using an optical frequency-to-amplitude conversion technique based on the ro-vibrational absorptions of CH 4. We observe that the amplitude of frequency tuning Δf over intensity modulation index m as function of the modulation frequency behaves similarly for both types of devices, while the ring-QCLs typically show higher values. The frequency-to-intensity modulation (FM-IM) phase shift shows a decrease starting from â¼72 ∘ at a modulation frequency of 800 kHz to about 0 ∘ at 160 MHz. In addition, we also observe a quasi single-sideband (qSSB) regime for modulation frequencies above 100 MHz, which is identified by a vanishing -1 st-order sideband for both devices. This special FM-state can be observed in DFB QCLs and is in strong contrast to the behavior of regular DFB diode lasers, which do not achieve any significant sideband suppression. By analyzing these important high frequency characteristics of ring-QCLs and comparing them to ridge DFB-QCLs, it shows the potential of intersubband devices for applications in e.g. novel spectroscopic techniques and highly-integrated and high-bitrate free-space data communication. In addition, the obtained results close an existing gap in literature for high frequency modulation characteristics of QCLs.
ABSTRACT
An absolute-frequency terahertz (THz) dual-frequency comb spectrometer based on electro-optic modulators for tunable, high-resolution, and real-time rapid acquisition is presented. An optical line of a master frequency comb (filtered via optical injection locking) serves as the seed to electro-optically generate a pair of new frequency combs (probe and local oscillator). Photomixing both combs with another coherent line from the same original master comb generates a narrow linewidth THz dual-comb with teeth frequencies that can be referenced to a radio-frequency standard. The system is validated with a proof-of-principle measurement of a microwave filter in the W-band.
ABSTRACT
Electro-optic dual-comb spectrometers have proved to be a promising technology for sensitive, high-resolution and rapid spectral measurements. Electro-optic combs possess very attractive features like simplicity, reliability, bright optical teeth, and typically moderate but quickly tunable optical spans. Furthermore, in a dual-comb arrangement, narrowband electro-optic combs are generated with a level of mutual coherence that is sufficiently high to enable optical multiheterodyning without inter-comb stabilization or signal processing systems. However, this valuable tool still presents several limitations; for instance, on most systems, absolute frequency accuracy and long-term stability cannot be guaranteed; likewise, interferometer-induced phase noise restricts coherence time and limits the attainable signal-to-noise ratio. In this paper, we address these drawbacks and demonstrate a cost-efficient absolute electro-optic dual-comb instrument based on a frequency stabilization mechanism and a novel adaptive interferogram acquisition approach devised for electro-optic dual-combs capable of operating in real-time. The spectrometer, completely built from commercial components, provides sub-ppm frequency uncertainties and enables a signal-to-noise ratio of 10000 (intensity noise) in 30 seconds of integration time.
ABSTRACT
In this paper a new laser range finder approach is presented. It is based on a fan of beams of different optical frequencies that are projected to the target. The detected back-reflected beams contain frequency-encoded information about its reflection angles, which are used for the calculation of the range by means of triangulation methods. The fan of beams is generated from a fieldable electro-optic dual optical frequency comb generator and a diffractive optical system, without any moving parts. A proof-of-concept prototype of a range finder based on this approach was implemented and experimentally tested. An analysis of main sources of errors is also presented.
ABSTRACT
In this work, the generation of dual optical frequency combs based on gain-switching and optical injection locking is experimentally examined. The study reveals that an effective process of optical injection can lead to optimized RF combs in terms of span and signal-to-noise ratio. The system also minimizes the overlap of lines and reduces the number of optical components involved, eliminating the need for any external modulator (electro-optic, acousto-optic). The validation of the system was performed as a dual-comb spectrometer, which allowed for determination of the absorption and dispersion profiles of the molecular transition of H13CN at 1538.523 nm.
ABSTRACT
In this paper, a new approach to dual comb generation based on well-known optical techniques (Gain-Switching and Optical Injection Locking) is presented. The architecture can be implemented using virtually every kind of continuous-wave semiconductor laser source (DFB, VCSEL, QCL) and without the necessity of electro-optic modulators. This way, a frequency-agile and adaptive dual-comb architecture is provided with potential implementation capabilities from mid-infrared to near ultraviolet. With a RF comb comprising around 70 teeth, the system is validated in the 1.5 µm region measuring the absorption feature of H13CN at 1538.523 nm with a minimum integration time of 10 µs.
ABSTRACT
In this paper, a multiheterodyne architecture for molecular dispersion spectroscopy based on a coherent dual-comb source generated using a single continuous wave laser and electro-optic modulators is presented and validated. The phase-sensitive scheme greatly simplifies previous dual-comb implementations by the use of an electro-optic dual comb and by phase-locking all the signal generators of the setup eliminating, in this way, the necessity of any reference optical path currently mandatory in absorption-based instruments. The architecture is immune to the classical baseline and normalization problems of absorption-based analyzers and provides an output linearly dependent on the gas concentration. In addition, the simultaneous parallel multi-wavelength measurement approach has the ability to deliver an improved output bandwidth (measurement speed) over gas analyzers based on tunable lasers.
ABSTRACT
Dispersion-based spectroscopic techniques present many desirable features when compared with classical absorption spectroscopy implementations, such as the normalization-free operation and the extended dynamic range. In this Letter, we present a new sensor design based on direct optical processing for heterodyne conversion in tunable laser chirped dispersion spectroscopy that allows sensor implementations using low-speed photodetectors and low-cost FM demodulators. The performance of the new setup has been validated using as a target the ro-vibrational transition of methane at approximately 1650.96 nm.
