Quantum cascade lasers as metrological tools for space optics

We report a homogeneous quantum cascade laser (QCL) emitting at terahertz (THz) frequencies, with a total spectral emission of about 0.6 THz, centered around 3.3 THz, a current density dynamic range Jdr = 1.53, and a continuous wave output power of 7 mW. The analysis of the intermode beatnote unveils that the devised laser operates as an optical frequency comb (FC) synthesizer over the whole laser operational regime, with up to 36 optically active laser modes delivering ∼200 µW of optical power per optical mode, a power level unreached so far in any THz QCL FC. A stable and narrow single beatnote, reaching a minimum linewidth of about 500 Hz, is observed over a current density range of 240 A/cm2 and even across the negative differential resistance region. We further prove that the QCL FC can be injection locked with moderate radio frequency power at the intermode beatnote frequency, covering a locking range of 1.2 MHz. The demonstration of stable FC operation, in a QCL, over the full current density dynamic range, and without any external dispersion compensation mechanism, makes our proposed homogenous THz QCL an ideal tool for metrological applications requiring mode-hop electrical tunability and a tight control of the frequency and phase jitter.

Optical frequency comb synthesizers, operating in the harmonic regime, are metrological sources in which the emitted optical power is concentrated in a few modes, spaced by several multiples of the cavity free spectral range (FSR). This behavior reflects in a large correlation degree and, in principle, in an increased optical power per mode. In miniaturized quantum cascade lasers (QCLs), harmonic frequency combs (HFCs) are hence particularly attracting to explore quantum correlation effects between adjacent harmonic modes, enabled by the inherently large gain media third-order χ (3) Kerr nonlinearity, even if controlled generation of stable HFCs of predefined order, is typically demanding in such electrically pumped sources. Here, we demonstrate stable2nd order and 3rd order HFC emission in terahertz frequency QCLs by respectively patterning an individual or a couple of equally spaced distributed multilayer graphene absorbers on the top metallic waveguides.

The quantum cascade laser (QCL) is an important laser source in the mid-infrared and terahertz frequency range. The past twenty years have witnessed its tremendous development in power, wall plug efficiency, frequency coverage and tunability, beam quality, as well as various applications based on QCL technology. Nowadays, QCLs can deliver high continuous wave power output up to 5.1 W at room temperature, and cover a wide frequency range from 3 to 300 μm by simply varying the material components. Broadband heterogeneous QCLs with a broad spectral range from 3 to 12 μm, wavelength agile QCLs based on monolithic sampled grating design, and on-chip beam QCL combiner are being developed for the next generation tunable mid-infrared source for spectroscopy and sensing. Terahertz sources based on nonlinear generation in QCLs further extend the accessible wavelength into the terahertz range. Room temperature continuous wave operation, high terahertz power up to 1.9 mW, and wide frequency tunability form 1 to 5 THz makes this type of device suitable for many applications in terahertz spectroscopy, imaging, and communication.

Stable operation is one of the most important requirements for a laser source for high-precision applications. Many efforts have been made to improve the stability of lasers by employing various techniques, e.g., electrical and/or optical injection and phase locking. However, these techniques normally involve complex experimental facilities. Therefore, an easy implementation of the stability evaluation of a laser is still challenging, especially for lasers emitting in the terahertz (THz) frequency range because the broadband photodetectors and mature locking techniques are limited. In this work, we propose a simple method, i.e., relative phase locking, to quickly evaluate the stability of THz lasers without a need of a THz local oscillator. The THz laser system consists of a THz quantum cascade laser (QCL) frequency comb and a single-mode QCL. Using the single-mode laser as a fast detector, heterodyne signals resulting from the beating between the single-mode laser and the comb laser are obtained. One of the heterodyne beating signals is selected and sent to a phase-locked loop (PLL) for implementing the relative phase locking. Two kinds of locks are performed by feeding the output error signal of the PLL, either to the comb laser or to the single-mode laser. By analyzing the current change and the corresponding frequency change of the PLL-controlled QCL in each phase-locking condition, we, in principle, are able to experimentally compare the stability of the emission frequency of the single-mode QCL (fs) and the carrier envelope offset frequency (fCEO) of the QCL comb. The experimental results reveal that the QCL comb with the repetition frequency injection locked demonstrates much higher stability than the single-mode laser. The work provides a simple heterodyne scheme for understanding the stability of THz lasers, which paves the way for the further locking of the lasers and their high-precision applications in the THz frequency range.

Recent decade brought a tremendous progress laser-based gas detection techniques. This progress would not be possible without the relentless devotion of the scientific community in developing state-of-the-art sources capable of targeting strong absorption lines of gases localized in the mid-infrared (mid-IR) wavelength region. Although narrowband, semiconductor-based mid-IR lasers offer non-complex implementation their tuning range is usually limited to several cm−1. Therefore designing a sensor capable of simultaneous monitoring of several gas species, even with absorption profiles clustered nearby, requires incorporating individual expensive quantum cascade laser (QCL) or interband cascade laser (ICL) sources into a single platform. Recent development of reliable mid-IR comb sources, combined with clever methods of signal processing allowed for designing gas sensors capable of simultaneous monitoring of several gas analytes in fractions of a second [1]. Obtaining broadband pulsed radiation in the mid-IR region is usually accomplished by utilizing nonlinear conversion or in recently presented on-chip QCL combs [2]. Although QCL combs offer compactness and efficiency they do not allow straightforward control of the repetition frequency (f rep ) or the carrier-envelope offset frequency (f CEo ), which are critical in some particular cavity-enhanced comb-based sensing techniques, e.g. Vernier spectroscopy [3]. Here we present a fully-fiberized, f CEO -free mid-IR optical frequency comb source capable of covering 6–9 μm wavelength region with tunable f rep . The setup of the source is presented in Fig 1. The heart of the source is a ring-cavity graphene mode-locked laser generating sub-250 fs pulses at 1560 nm and f rep of 100 MHz. The pulses are split using a 50/50 coupler and directed to separate sections of the source. In the blue section the source the pulses are amplified to 80 mW in an all-fiber-based chirped pulse amplifier (CPA) and recompressed in a 8 m-long section of PM1550 fiber to ∼200 fs. In the red section a custom manufactured nonlinear fiber with tailored dispersion (PCF) is utilized to soliton-shift the 1.55 μm pulses into the 1.9–2 μm wavelength region. Subsequently the pulses are amplified in a CPA and recompressed to ∼200 fs. Combined 1.55 and 2 μm pulses are focused on a 3 mm-long orientation patterned gallium phosphide (OP-GaP) crystal to generate mid-IR radiation via differential frequency generation (DFG). Since both pulses originated from the same ML source, the generated mid-IR frequency comb is inherently fceo-free and can be used in laser-based gas sensors requiring stable carrier-envelope offset. The source is built entirely from PM single-mode fibers.

Feedback from a diode mixer integrated into a 2.8 THz quantum cascade laser (QCL) was used to phase lock the difference frequencies (DFs) among the Fabry-Perot (F-P) longitudinal modes of a QCL. Approximately 40% of the DF power was phase locked, consistent with feedback loop bandwidth of 10 kHz and phase noise bandwidth ~0.5 MHz. While the locked DF signal has ≤ 1 Hz linewidth and negligible drift over ~30 min, mixing measurements between two QCLs and between a QCL and molecular gas laser show that the common mode frequency stability is no better than a free-running QCL.

The need for highly performing and stable methods for mid-IR molecular sensing and metrology pushes towards the development of more and more compact and robust systems. Among the innovative solutions aimed at answering the need for stable mid-IR references are crystalline microresonators, which have recently shown excellent capabilities for frequency stabilization and linewidth narrowing of quantum cascade lasers with compact setups. In this work, we report on the first system for mid-IR high-resolution spectroscopy based on a quantum cascade laser locked to a CaF microresonator. Electronic locking narrows the laser linewidth by one order of magnitude and guarantees good stability over long timescales, allowing, at the same time, an easy way for finely tuning the laser frequency over the molecular absorption line. Improvements in terms of resolution and frequency stability of the source are demonstrated by direct sub-Doppler recording of a molecular line.

The single-step Rydberg excitation of cesium atoms requires a 319 nm ultraviolet laser with a narrow laser linewidth, high frequency stability, and high output power. To meet these requirements, in this work, we construct a high-power, single-frequency UV laser system at this wavelength. In this system, the frequency stabilization of the 1560.492 nm seed laser is critical to the performance of the ultraviolet laser. We employ nonlinear frequency conversion technology, the 1560.492 nm laser is frequency-doubled to 780.246 nm via a single pass through a PPLN crystal, and function integration is realized based on the modular parameter adjustment interface provided by the PyRPL software. Subsequently, the 1560.492 nm laser is stabilized to the D2 hyperfine transition line of Rb-87 atoms using polarization spectroscopy (PS) and radio-frequency-modulated saturation absorption spectroscopy (RF-SAS). A comparative study of these two techniques shows that RF-SAS achieves superior stabilization performance, with the residual frequency fluctuation of the frequency-doubled laser being 1.07 MHz over 30 min. According to frequency doubling theory, the actual residual frequency fluctuation of the 1560.492 nm fundamental-frequency laser can be calculated as 0.535 MHz. Compared with our earlier scheme that utilized an ultra-low-expansion (ULE) optical cavity as a frequency reference, the present scheme eliminates the long-term drift induced by environmental factors. In contrast to frequency stabilization relying on discrete instruments, this integrated scheme significantly reduces the cost, simplifies the system architecture, saves space, and greatly enhances the flexibility and controllability of the system. It therefore provides a reliable and cost-effective solution to ensure the portability and practicability of high-performance UV laser sources. This high-precision frequency stabilization scheme directly guarantees the performance of the 319 nm UV laser, suppressing its linewidth below 10 kHz. Thus, it fully meets the stringent laser linewidth and frequency stability requirements for the single-step Rydberg excitation of cesium atoms and provides a reliable light source foundation for subsequent precision spectroscopic measurements.

Because of an intrinsically low linewidth-enhancement factor, the quantum cascade laser (QCL) is especially favorable for patterning with a recently proposed 2-D photonic crystal (PC) lattice that substantially increases the device area over which optical coherence can be maintained. In this work, we use an original time-domain Fourier-transform (TDFT) algorithm to theoretically investigate the beam quality and spectral purity of gain-guided PC distributed-feedback (DFB) quantum cascade lasers. The conventional 1-D DFB laser and also the angled-grating DFB (/spl alpha/-DFB) laser are special cases of the PCDFB geometry. By searching the parameter space consisting of tilt angle, coupling coefficients, stripe width, and cavity length, we have theoretically optimized the PCDFB gratings for QCL gain regions. At a wavelength of 4.6 /spl mu/m, the simulations project single-mode emission from stripes as wide as 1.2 mm, and etendues of no more than three times the diffraction limit for 2-mm stripes. We also examine the tolerances required for single-mode and high-brightness operation. Comparisons are made to analogous simulations of a-DFB QCL lasers.

Recently, a novel type of molecular THz laser with ∼mW-level output power was demonstrated [1,2]. This laser is optically pumped by a mid-IR Quantum Cascade Laser (QCL), therefore resulting in a much more compact, frequency agile and deployable source compared to standard, CO 2 -laser pumped, far-infrared gas lasers. This achievement has revived the interest for molecular THz lasers, in particular for their exploitation as low-phase noise THz local oscillators (LOs) operating at room temperature. Compared to CO 2 -laser pumping, a QCL pump offers an unrivalled frequency agility that allows accessing an unprecedented number of molecular transitions. On the other hand, its lower frequency stability can potentially impact the frequency noise of the THz laser source [3–5]. Here we report the first measurement of the frequency noise power spectral density (PSD) of a 1.073THz ammonia laser pumped by a 10.3μm QCL. This is achieved by using a commercial harmonic mixer to generate a beating between the ammonia laser frequency and the 1080th harmonic of the 1GHz repetition rate of a custom-made, 1560nm frequency comb (FC). We then exploit a voltage-controlled oscillator to track the beat-note signal and realize the frequency to amplitude conversion [6]. The measured frequency noise PSD is shown in Fig.1. In the range ∼30–100kHz we find a frequency noise 2 -pumped, 2.5THz methanol laser, reaching 0.1Hz2/Hz (−110dBc) at 40kHz from the carrier, limited by the frequency noise of the FC harmonic. Finally, we will show that it is possible to actively phase-lock the QCL-pumped ammonia laser to the FC repetition rate harmonic by controlling the QCL drive current, demonstrating a sub-Hz linewidth. These results pave the way to the development of QCL-pumped molecular lasers as compact, low-phase noise THz sources operating a room temperature.

Despite the growing interest that quantum cascade lasers (QCLs) are gaining, they still present a few unclear aspects of their fundamental properties, such as spectral purity, that need to be deeply investigated when aiming to make these innovative laser sources suitable for high-resolution spectroscopy and metrology. This paper is a review of our efforts towards QCL-based high-resolution spectroscopy and of our experimental investigation of QCLs' frequency noise, aimed to discover the ultimate performances attainable by QCLs and to develop the exper- imental techniques required to achieve them. Our results, confirmed by several independent measurements, show that QCLs have a very small intrinsic linewidth buried under a large frequency-noise background. The development of appropriate frequency stabilization techniques will make QCLs well suited for high-resolution spectroscopy and metrology in the mid and far IR. C 2010 Society of Photo-Optical Instrumentation Engineers.

This study deals with the communication capabilities of two kinds of semiconductor lasers emitting in one of the atmosphere transparency windows, around 4 μm. One of these two lasers is a quantum cascade laser and the other one is an interband cascade laser. With the quantum cascade laser, a subsequent attenuation is added to the optical path in order to mimic the attenuation of free-space transmission of several kilometers. Direct electrical modulation is used to transmit the message and two-level formats, non-return-to-zero and return-to-zero, are used and compared in terms of maximum transmission data rate. The sensitivity to optical feedback is also analyzed, as well as the evolution of the error rate when reducing the optical power at the level of the detector. This work provides a novel insight into the development of future secure free-space optical communication links based on mid-infrared semiconductor lasers and sheds the light on improvements required to achieve multi-Gbits/s communication with off-the-shelf components.

The invention of quantum cascade laser (QCL) technology rejuvenates the spectroscopic monitoring of trace gas species providing an unprecedented access to the ‘molecular fingerprint region’ covering mid-infrared (mid-IR) spectral range of 3–25 μm. The recent technological advancements of QCL fulfil all the critical parameters as an ideal optical source of a portable and field-deployable mid-IR spectrometer for trace gas analysis in real time. Hence, QCL-based spectroscopic techniques have now widely been employed in various fields covering molecular spectroscopy, atmospheric and environmental trace gas sensing, medical diagnostics, chemical and explosive analysis, industrial process control, and many more. In this chapter, we first discuss various critical features of an optical source for trace gas sensing applications with subsequent brief mention of different mid-IR sources prior to the QCL. Thereafter, the importance of trace gas analysis in environmental and biomedical sciences has been presented in short, followed by a historic overview of QCL. Finally, we focus on the applications of QCL-based spectroscopic techniques in the field of atmospheric and biomedical sciences.

On-chip optical interconnection is a promising technology for wiring future large-scale integrated circuits, as a means to mitigate the considerable power dissipation of traditional wiring layers. Here, we fabricate an integrated optical link using a membrane distributed-feedback (DFB) laser and a p-i-n photodiode (PD) in a butt-jointed built-in coupling geometry. The optical link is formed on a Si substrate by benzocyclobutene bonding. The integrated DFB laser shows a low-threshold current of 0.48 mA. Light transmission between the DFB laser and the p-i-n PD is confirmed with static measurements of the optical link. The optical link has a 3-dB bandwidth of 11.3 GHz at a 2.73 mA DFB laser bias current and a -3 V p-i-n PD bias voltage. A data transmission experiment of the optical link is performed, using a nonreturn to zero, pseudorandom-bit-sequence with a word length of 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31</sup> -1 signals. With a DFB laser bias current of 2.5 mA, 10 Gbit/s data transmission with a bit-error-rate of 6 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> is successfully achieved.

Stable ultrashort light pulses and frequency combs generated by mode-locked lasers have many important applications including high-resolution spectroscopy, fast chemical detection and identification, studies of ultrafast processes, and laser metrology. While compact mode-locked lasers emitting in the visible and near infrared range have revolutionized photonic technologies, the systems operating in the mid-infrared range where most gases have their strong absorption lines, are bulky and expensive and rely on nonlinear frequency down-conversion. Quantum cascade lasers are the most powerful and versatile compact light sources in the mid-infrared range, yet achieving their mode-locked operation remains a challenge, despite dedicated effort. Here we report the demonstration of active mode locking of an external-cavity quantum cascade laser. The laser operates in the mode-locked regime at room temperature and over the full dynamic range of injection currents.