Laser Frequency Comb Research Articles

Near-Full Current Dynamic Range Thz Quantum Cascade Laser Frequency Comb

Near-Full Current Dynamic Range Thz Quantum Cascade Laser Frequency Comb

Field-enhancing tapered planarized waveguides for THz quantum cascade laser frequency combs

We present a new planarized waveguide geometry for THz quantum cascade laser frequency combs with improved waveguide losses, RF and thermal dissipation properties. Ridge devices display broadband free-running comb states, and the THz emission can be further broadened by RF injection. Tapered waveguide devices feature a strong field-enhancement effect, which results in an improved comb performance. This includes free-running comb states with strong single beatnotes up to nearly -30 dBm at 90 K, almost three orders of magnitude stronger than for ridge devices. Improved comb operation is maintained also for high operating temperatures, up to 115 K.

Underwater Arbitrary Distance Measurement with Laser Frequency Comb for Real-Time Correction of Underwater Refractive Index

Underwater Arbitrary Distance Measurement with Laser Frequency Comb for Real-Time Correction of Underwater Refractive Index

Smart and agile 88 W Yb-fiber frequency comb laser

Smart and agile 88 W Yb-fiber frequency comb laser

Laser Frequency Combs with Fast Gain Recovery: Physics and Applications

AbstractThe dynamics of semiconductor lasers with fast gain is generally considered to suppress amplitude modulation, resulting in a frequency‐modulated output. Since their first demonstration of frequency comb operation, quantum cascade lasers—ideal representatives of this class of lasers with fast gain dynamics—were indeed shown to emit frequency‐modulated combs. The understanding of these frequency comb regimes has progressively evolved, being refined from simpler arguments to more advanced theories, considering the role of Kerr nonlinearity and linewidth enhancement factor. By now, the traditional frequency‐modulated combs are rather well understood in quantum cascade lasers, but new types of frequency combs have emerged, reopening interesting physics questions in these lasers. Recent advancements on two open frontiers will be discussed: harmonic frequency combs, characterized by mode skipping of many free spectral ranges, and spatiotemporal structures with solitonic features, recently reported in unidirectional ring cavities. These new types of frequency combs open technological and fundamental perspectives, sprouting new interest in quantum cascade lasers.

Octave-spanning low-loss mid-IR waveguides based on semiconductor-loaded plasmonics

Plasmonic waveguides are crucial building blocks for integrated on-chip mid-infrared (mid-IR) sensors, which have recently attracted great interest as a sensing platform to target enhanced molecular sensing. However, while hosting a wide range of applications from spectroscopy to telecommunication, the mid-IR lacks suitable broadband solutions that provide monolithic integration with III-V materials. This work reports a novel concept based on hybrid semiconductor-metal surface plasmon polariton waveguides, which result in experimentally demonstrated low loss and broadband devices. Composed of a thin germanium slab on top of a gold layer, the waveguiding properties can be directly controlled by changing the geometrical parameters. The measured losses of our devices are as low as 6.73 dB/mm at 9.12 µm and remain <15 dB/mm in the mid-IR range of 5.6–11.2 µm. The octave-spanning capability of the waveguides makes them ideal candidates for combination with broadband mid-IR quantum cascade laser frequency combs and integrated spectroscopic sensors.

Toward robust and practical interband cascade laser frequency combs: A perspective

Multimode emission from a semiconductor laser can sometimes take the form of a low-noise frequency comb with equidistant separation between the modes. Two general types of “passive” comb operation have been explored experimentally: (1) the periodic short-pulse mode achieved by intracavity mode locking, usually obtained by incorporating a saturable absorber section into the cavity, and (2) the so-called frequency-modulated (FM) mode in which the output intensity can vary within wide bounds but does not completely turn off between pulses, and the instantaneous frequency is linearly chirped over a round trip. The FM mode sometimes manifests as a “sparse” harmonic state, with individual modes spaced by multiples of the cavity free spectral range. This Perspective reviews the current understanding of these modes, along with the conditions under which they may arise in practical devices. We then consider in detail the case of type-II mid-infrared interband cascade laser (ICL) frequency combs. Our simulations clarify the roles of carrier dynamics and group velocity dispersion and identify design modifications that may substantially improve the device performance. We find no fundamental roadblocks to the development of practical mode-locked ICL frequency combs that emit short pulses with broad spectral bandwidth for dual comb spectroscopy and other applications, alongside the FM combs that have already been demonstrated experimentally.

Controlling Quantum Cascade Laser Optical Frequency Combs through Microwave Injection (Laser Photonics Rev. 15(12)/2021)

Quantum Cascade lasers In article number 2100242, Barbara Schneider and co-workers present strong microwave injection in quantum cascade laser frequency combs as a means for controlling the lasing state. Varying modulation frequency and power enables us to broaden and shift the spectral output as well as modify the phase profile. Moreover, the fast gain dynamics enable rapid switching between separate spectral regions with just a single device.

Controlling Quantum Cascade Laser Optical Frequency Combs through Microwave Injection

AbstractIn this work, control over the precise state emitted by quantum cascade laser frequency combs through strong radio‐frequency current modulation close to their repetition frequency is demonstrated. In particular, broadening of the spectrum from about 20 cm−1 to an average bandwidth of 60 cm−1 can be achieved throughout most of the current dynamical range while preserving the coherence, as measured by shifted wave interference Fourier transform spectroscopy (SWIFTS). The required modulation frequency to achieve this broadening is red‐shifted compared to the free‐running beatnote frequency at increasing modulation powers starting from 25 dBm, whereas the range where it occurs narrows. Outside of this maximum‐bandwidth range, the spectral bandwidth of the laser output is gradually reduced and the new center frequency is red‐ or blue‐shifted, directly dependent on the detuning of the modulation frequency. By switching between two modulation frequencies detuned symmetrically with respect to the free‐running beatnote, two multiplexed spectral regions with negligible overlap from the same device at rates of at least 20 kHz can be generated. In the time‐domain, with both SWIFTS and interferometric autocorrelation (IAC) measurements a transition from quasi‐continuous output to long‐pulsed ( ps) output is shown by ramping up the injection power to 35 dBm.

Spectrally resolved linewidth enhancement factor of a semiconductor frequency comb

The linewidth enhancement factor (LEF) has recently moved into the spotlight of research on frequency comb generation in semiconductor lasers. Here we present a novel modulation experiment that enables direct measurement of the spectrally resolved LEF in a laser frequency comb. By utilizing a phase-sensitive technique, we are able to extract the LEF for each individual comb mode in any laser type. We first investigate and verify this universally applicable technique using Maxwell–Bloch simulations. Following, we present the experimental demonstration on a quantum cascade laser frequency comb, confirming the predicted key role of the LEF in frequency comb dynamics.

In-Situ Evaluation of the Pitch of a Reflective-Type Scale Grating by Using a Mode-Locked Femtosecond Laser

Major modifications are made to the setup and signal processing of the method of in-situ measurement of the pitch of a diffraction grating based on the angles of diffraction of the diffracted optical frequency comb laser emanated from the grating. In the method, the improvement of the uncertainty of in-situ pitch measurement can be expected since every mode in the diffracted optical frequency comb laser can be utilized. Instead of employing a Fabry-Pérot etalon for the separation of the neighboring modes in the group of the diffracted laser beams, the weight-of-mass method is introduced in the method to detect the light wavelength in the Littrow configuration. An attempt is also made to reduce the influence of the non-uniform spectrum of the optical comb laser employed in the setup through normalization operation. In addition, an optical alignment technique with the employment of a retroreflector is introduced for the precise alignment of optical components in the setup. Furthermore, a mathematical model of the pitch measurement by the proposed method is established, and theoretical analysis on the uncertainty of pitch measurement is carried out based on the guide to the expression of uncertainty in measurement (GUM).

Engineering the spectral bandwidth of quantum cascade laser frequency combs.

Quantum cascade lasers (QCLs) facilitate compact optical frequency comb sources that operate in the mid-infrared and terahertz spectral regions, where many molecules have their fundamental absorption lines. Enhancing the optical bandwidth of these chip-sized lasers is of paramount importance to address their application in broadband high-precision spectroscopy. In this work, we provide a numerical and experimental investigation of the comb spectral width and show how it can be optimized to obtain its maximum value defined by the laser gain bandwidth. The interplay of nonoptimal values of the resonant Kerr nonlinearity and cavity dispersion can lead to significant narrowing of the comb spectrum and reveals the best approach for dispersion compensation. The implementation of high mirror losses is shown to be favorable and results in proliferation of the comb sidemodes. Ultimately, injection locking of QCLs by modulating the laser bias around the round trip frequency provides a stable external knob to control the frequency-modulated comb state and recover the maximum spectral width of the unlocked laser state.

Laser soliton microcombs heterogeneously integrated on silicon.

Silicon photonics enables wafer-scale integration of optical functionalities on chip. Silicon-based laser frequency combs can provide integrated sources of mutually coherent laser lines for terabit-per-second transceivers, parallel coherent light detection and ranging, or photonics-assisted signal processing. We report heterogeneously integrated laser soliton microcombs combining both indium phospide/silicon (InP/Si) semiconductor lasers and ultralow-loss silicon nitride (Si3N4) microresonators on a monolithic silicon substrate. Thousands of devices can be produced from a single wafer by using complementary metal-oxide-semiconductor-compatible techniques. With on-chip electrical control of the laser-microresonator relative optical phase, these devices can output single-soliton microcombs with a 100-gigahertz repetition rate. Furthermore, we observe laser frequency noise reduction due to self-injection locking of the InP/Si laser to the Si3N4 microresonator. Our approach provides a route for large-volume, low-cost manufacturing of narrow-linewidth, chip-based frequency combs for next-generation high-capacity transceivers, data centers, space and mobile platforms.

Resolving nonuniform temperature distributions with single-beam absorption spectroscopy. Part II: Implementation from broadband spectra

Several past studies have described how absorption spectroscopy can be used to determine spatial temperature variations along the optical path by measuring the unique, nonlinear response to temperature of many molecular absorption transitions and performing an inversion. New laser absorption spectroscopy techniques are well-suited to this nonuniformity measurement, yet present analysis approaches use only isolated features rather than a full broadband spectral measurement. In this work, we develop a constrained spectral fitting technique called E″-binning to fit an absorption spectrum arising from a nonuniform environment. The information extracted from E″-binning is then input to the inversion approach from the previous paper in this series (Malarich and Rieker, JQSRT 107455 [1]) to determine the temperature distribution. We demonstrate this approach by using dual frequency comb laser measurements to resolve convection cells in a tube furnace. The recovered temperature distributions at each measurement height agree with an existing natural convection model. Finally, we show that for real-world measurements with noise and absorption model error, increasing the bandwidth and the number of measured absorption transitions may improve the temperature distribution precision. We make the fitting code publicly available for use with any broadband absorption measurement.

Coherently-averaged dual comb spectrometer at 7.7 µm with master and follower quantum cascade lasers.

We demonstrate coherent averaging of the multi-heterodyne beat signal between two quantum cascade laser frequency combs in a master-follower configuration. The two combs are mutually locked by acting on the drive current to control their relative offset frequency and by radio-frequency extraction and injection locking of their intermode beat signal to stabilize their mode spacing difference. By implementing an analog common-noise subtraction scheme, a reduction of the linewidth of all heterodyne beat notes by five orders of magnitude is achieved compared to the free-running lasers. We compare stabilization and post-processing corrections in terms of amplitude noise. While they give similar performances in terms of signal-to-noise ratio, real-time processing of the stabilized signal is less demanding in terms of computational power. Lastly, a proof-of-principle spectroscopic measurement was performed, showing the possibility to reduce the amount of data to be processed by three orders of magnitude, compared to the free-running system.

Spectral envelope control for a flat frequency comb spectrum

Laser frequency combs have properties that make them promising spectrograph calibration light sources. One drawback for this application is the high dynamic range in the supercontinuum spectra of some frequency combs. We aim to flatten the spectrum of a Ti:sapphire laser frequency comb to improve the calibration performance for a Fourier transform spectrograph. For this, we develop a compact Fourier transform optical pulse shaping setup, which enables control of the spectral envelope via dispersion of the light onto a spatial light modulator (SLM). We demonstrate that this setup allows us to flatten the comb spectrum in the wavelength range 780 to 980 nm. For 90% of the wavelength range, the dynamic range is below 3 dB. In the flattened spectra 3% more comb lines can be detected. The line center fit precision, however, is reduced due to noise caused by the SLM.

Quantum Cascade Lasers: Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning (Laser Photonics Rev. 15(6)/2021)

In article number 2000417, Luigi Consolino and co-workers demonstrate a novel degree of freedom for controlling the emission of quantum cascade lasers (QCLs). It is based on small optical frequency tuning (SOFT) by means of a white LED. The slightly perturbative regime accessible in these conditions allows tweaking the semiconductor complex refractive index. The resulting additional tuning knob is capable of phase stabilizing the modes spacing of a QCL comb.

IGRINS RV: A Precision Radial Velocity Pipeline for IGRINS Using Modified Forward Modeling in the Near-infrared*

Abstract Application of the radial velocity (RV) technique in the near-infrared is valuable because of the diminished impact of stellar activity at longer wavelengths, making it particularly advantageous for the study of late-type stars but also for solar-type objects. In this paper, we present the IGRINS RV open-source python pipeline for computing infrared RV measurements from reduced spectra taken with IGRINS, an R ≡ λ/Δλ ∼ 45,000 spectrograph with simultaneous coverage of the H band (1.49–1.80 μm) and K band (1.96–2.46 μm). Using a modified forward-modeling technique, we construct high-resolution telluric templates from A0 standard observations on a nightly basis to provide a source of common-path wavelength calibration while mitigating the need to mask or correct for telluric absorption. Telluric standard observations are also used to model the variations in instrumental resolution across the detector, including a yearlong period when the K band was defocused. Without any additional instrument hardware, such as a gas cell or laser frequency comb, we are able to achieve precisions of 26.8 m s−1 in the K band and 31.1 m s−1 in the H band for narrow-line hosts. These precisions are empirically determined by a monitoring campaign of two RV standard stars, as well as the successful retrieval of planet-induced RV signals for both HD 189733 and τ Boo A; furthermore, our results affirm the presence of the Rossiter–McLaughlin effect for HD 189733. The IGRINS RV pipeline extends another important science capability to IGRINS, with publicly available software designed for widespread use.

Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning

AbstractFull phase control of terahertz (THz)‐emitting quantum cascade laser (QCL) combs has recently been demonstrated, opening new perspectives for even the most demanding applications. In this framework, simplifying the set‐ups for control of these devices will help to accelerate their spreading in many fields. This study reports a new way to control the emission frequencies of a THz QCL comb by small optical frequency tuning (SOFT), using a very simple experimental setup, exploiting the incoherent emission of an ordinary white light‐emitting diode. The slightly perturbative regime accessible in these conditions allows tweaking the complex refractive index of the semiconductor without destabilizing the broadband laser gain. The SOFT actuator is characterized and compared to another actuator, the QCL driving current. The suitability of this additional degree of freedom for frequency and phase stabilization of a THz QCL comb is shown and perspectives are discussed.

Frequency microcomb stabilization via dual-microwave control

Optical frequency comb technology has been the cornerstone for scientific breakthroughs in precision metrology. In particular, the unique phase-coherent link between microwave and optical frequencies solves the long-standing puzzle of precision optical frequency synthesis. While the current bulk mode-locked laser frequency comb has had great success in extending the scientific frontier, its use in real-world applications beyond the laboratory setting remains an unsolved challenge due to the relatively large size, weight and power consumption. Recently microresonator-based frequency combs have emerged as a candidate solution with chip-scale implementation and scalability. The wider-system precision control and stabilization approaches for frequency microcombs, however, requires external nonlinear processes and multiple peripherals which constrain their application space. Here we demonstrate an internal phase-stabilized frequency microcomb that does not require nonlinear second-third harmonic generation nor optical external frequency references. We demonstrate that the optical frequency can be stabilized by control of two internally accessible parameters: an intrinsic comb offset ξ and the comb spacing frep. Both parameters are phase-locked to microwave references, with phase noise residuals of 55 and 20 mrad respectively, and the resulting comb-to-comb optical frequency uncertainty is 80 mHz or less. Out-of-loop measurements confirm good coherence and stability across the comb, with measured optical frequency instability of 2 × 10−11 at 20-second gate time. Our measurements are supported by analytical theory including the cavity-induced modulation instability. We further describe an application of our technique in the generation of low noise microwaves and demonstrate noise suppression of the repetition rate below the microwave stabilization limit achieved.