Locked Quantum Cascade Laser Research Articles

Coherent injection locking of quantum cascade laser frequency combs

Quantum cascade laser (QCL) frequency combs are a promising candidate for chemical sensing and biomedical diagnostics, requiring only milliseconds of acquisition time to record absorption spectra without any moving parts. They are electrically pumped and have a small footprint, making them an ideal platform for on-chip integration. Until now, optical feedback is fatal for frequency comb generation in QCLs and destroys intermodal coherence. This property imposes strict limits on the possible degree of integration. Here, we demonstrate coherent injection locking of the repetition frequency to a stabilized RF oscillator. For the first time, we prove that the spectrum of the injection locked QCL can be phase-locked, resulting in the generation of a frequency comb. We show that injection locking is not only a versatile tool for all-electrical frequency stabilization, but also mitigates the fatal effect of optical feedback on the frequency comb. A prototype self-detected dual-comb setup consisting only of an injection locked dual-comb chip, a lens and a mirror demonstrates the enormous potential for on-chip dual-comb spectroscopy. These results pave the way to miniaturized and all-solid-state mid-infrared spectrometers.

Injection locking of a terahertz quantum cascade laser to a telecommunications wavelength frequency comb

High-resolution spectroscopy not only can identify atoms and molecules but also can provide detailed information on their chemical and physical environment and relative motion. In the terahertz frequency region of the electromagnetic spectrum, where many molecules have fundamental vibrational modes, there is a lack of powerful sources with narrow linewidths that can be used for absorption measurements or as local oscillators in heterodyne detectors. The most promising solid-state source is the THz frequency quantum cascade laser (QCL), however, the linewidth of this compact semiconductor laser is typically too broad for many applications, and its frequency is not directly referenced to primary frequency standards. In this work, we injection lock a QCL operating at 2 THz to a compact fiber-based telecommunications wavelength frequency comb, where the comb line spacing is referenced to a microwave frequency reference. This results in the QCL frequency locking to an integer harmonic of the microwave reference, and the QCL linewidth reducing to the multiplied linewidth of the microwave reference, <100 Hz. Furthermore, we perform phase-resolved detection of the locked QCL and measure the phase noise of the locked system to be −75 dBc/Hz at 10 kHz offset from the 2 THz carrier.

Linewidth Enhancement Factor and Amplification of Angle-Modulated Optical Signals in Injection-Locked Quantum Cascade Lasers

AbstractThis paper presents a nonlinear analysis of the effect of linewidth enhancement factor (LEF) on the amplification of angle-modulated optical signals in injection-locked mid-infrared (IR) quantum cascade lasers (QCLs). A higher value of LEF tends to conserve the output angle modulation index of the amplified mid-IR signal particularly in the low-modulation frequency region. Further, a higher value of signal injection ratio produces a wider bandwidth of the locked QCL amplifier. The LEF introduces asymmetry in the lockband (LB) of the injection-locked QCL and this asymmetry increases with the increase in the value of LEF. Typically ratio of calculated lower-side LB to upper-side LB for an injection power level of – 20 dB and a LEF of unity is 1.67. The electron relaxation time in the uppermost subband lasing level in a three-level system has a profound effect on the LB asymmetry in a QCL.

A widely tunable 10-μm quantum cascade laser phase-locked to a state-of-the-art mid-infrared reference for precision molecular spectroscopy

We report the coherent phase-locking of a quantum cascade laser (QCL) at 10-μm to the secondary frequency standard of this spectral region, a CO2 laser stabilized on a saturated absorption line of OsO4. The stability and accuracy of the standard are transferred to the QCL resulting in a line width of the order of 10 Hz, and leading to the narrowest QCL to date. The locked QCL is then used to perform absorption spectroscopy spanning 6 GHz of NH3 and methyltrioxorhenium, two species of interest for applications in precision measurements.

Role of linewidth enhancement factor on the frequency response of the synchronized quantum cascade laser

This paper presents a closed-form analysis of the synchronized quantum cascade laser (QCL) and studies analytically the effect of linewidth enhancement factor (LEF) on the frequency response of the QCL. The analysis predicts that there can be two types of response curves of the QCL—one like series resonant circuit response and the other like antiresonant circuit response. The linewidth enhancement factor introduces asymmetry in the frequency response of the synchronized QCL. The nature of asymmetry in locked QCL is seen to be different from that reported for interband semiconductor lasers. The asymmetry in series resonant-like response is inclined towards the positive detuning side while that in antiresonant circuit-like response is inclined towards the negative detuning side. The degree of asymmetry in QCL response is not only determined by the LEF but also by the upper subband electron relaxation time and the operating wavelength of the QCL. Although the value of LEF in QCL is smaller than that for interband lasers, calculation shows that the LEF has a profound effect on the response of the locked QCL.

Higher order nonlinearity and synchronization of quantum cascade lasers

This paper presents a closed-form analysis of the synchronization phenomenon of the quantum cascade laser (QCL). The analysis has been made with considering higher order nonlinearity of the modal gain of the QCL. The frequency response characteristics of the synchronized QCL along with the stability of the locked state, the effect of nonlinearity on the lockband of the QCL and the amplitude limiting action of the locked QCL have been calculated. The analysis demonstrates the effect of higher order nonlinearity on the properties of the synchronized QCL.

Frequency and phase-lock control of a 3?THz quantum cascade laser

We have locked the frequency of a 3 THz quantum cascade laser (QCL) to that of a far-infrared gas laser with a tunable microwave offset frequency. The locked QCL line shape is essentially Gaussian, with linewidths of 65 and 141 kHz at the -3 and -10 dB levels, respectively. The lock condition can be maintained indefinitely, without requiring temperature or bias current regulation of the QCL other than that provided by the lock error signal. The result demonstrates that a terahertz QCL can be frequency controlled with 1-part-in-10(8) accuracy, which is a factor of 100 better than that needed for a local oscillator in a heterodyne receiver for atmospheric and astronomic spectroscopy.