Naota Akikusa

Theory of the Intrinsic Linewidth of Quantum-Cascade Lasers: Hidden Reason for the Narrow Linewidth and Line-Broadening by Thermal Photons

We have developed a theory of the intrinsic linewidths of laser output of single-mode quantum-cascade (QC) lasers in mid-infrared and terahertz (THz) ranges. In the theoretical treatment, the concept of an effective coupling efficiency of spontaneous emission, given by a fractional rate of spontaneous emission coupled into a lasing mode to total nonlasing relaxation, is introduced to clarify a hidden reason for the narrowness of the linewidths. A narrow linewidth (12-kHz) reported with a frequency-stabilized 8.5- distributed-feedback QC laser is successfully interpreted in terms of an extremely small effective coupling efficiency of spontaneous emission, caused by ultrafast nonradiative scatterings. The present theory predicts the presence of a minimum ldquolinewidth floorrdquo in a high-injection-current region and the independence of linewidth on detuning between gain-peak and emission wavelengths. The theoretical treatment is expanded to derive the further modified Schawlow-Townes formula including the line-broadening by black body radiation in a THz QC laser. The linewidth of a THz QC laser is predicted to be considerably broadened by black body radiation.

Measuring frequency noise and intrinsic linewidth of a room-temperature DFB quantum cascade laser

The frequency-noise power spectral density of a room-temperature distributed-feedback quantum cascade laser emitting at λ = 4.36 μm has been measured. An intrinsic linewidth value of 260 Hz is retrieved, in reasonable agreement with theoretical calculations. A noise reduction of about a factor 200 in most of the frequency interval is also found, with respect to a cryogenic laser at the same wavelength. A quantitative treatment shows that it can be explained by a temperature-dependent mechanism governing the transport processes in resonant tunnelling devices. This confirms the predominant effect of the heterostructure in determining shape and magnitude of the frequency noise spectrum in QCLs.

Intracavity quartz-enhanced photoacoustic sensor

We report on a spectroscopic technique named intracavity quartz-enhanced photoacoustic spectroscopy (I-QEPAS) employed for sensitive trace-gas detection in the mid-infrared spectral region. It is based on a combination of QEPAS with a buildup optical cavity. The sensor includes a distributed feedback quantum cascade laser emitting at 4.33 μm. We achieved a laser optical power buildup factor of ∼500, which corresponds to an intracavity laser power of ∼0.75 W. CO2 has been selected as the target molecule for the I-QEPAS demonstration. We achieved a detection sensitivity of 300 parts per trillion for 4 s integration time, corresponding to a noise equivalent absorption coefficient of 1.4 × 10−8 cm−1 and a normalized noise-equivalent absorption of 3.2 × 10−10 W cm−1 Hz−1/2.

Comb-assisted subkilohertz linewidth quantum cascade laser for high-precision mid-infrared spectroscopy

We report on the linewidth narrowing of a room-temperature mid-infrared quantum cascade laser by phase-locking to a difference-frequency-generated radiation referenced to an optical frequency comb synthesizer. A locking bandwidth of 250 kHz, with a residual rms phase-noise of 0.56 rad, has been achieved. The laser linewidth is narrowed by more than 2 orders of magnitude below 1 kHz, and its frequency is stabilized with an absolute traceability of 2×10−12. This source has allowed the measurement of the absolute frequency of a CO2 molecular transition with an uncertainty of about 1 kHz.

Subkilohertz linewidth room-temperature mid-infrared quantum cascade laser using a molecular sub-Doppler reference

We report on the narrowing of a room-temperature mid-IR quantum cascade laser by frequency locking it to a CO2 sub-Doppler transition obtained by polarization spectroscopy. A locking bandwidth of 250 kHz has been achieved. The laser linewidth is narrowed by more than two orders of magnitude below 1 kHz, and its absolute frequency is stabilized at the same level.

Room temperature, continuous-wave operation of quantum cascade lasers with single phonon resonance-continuum depopulation structures grown by metal organic vapor-phase epitaxy

We propose a new quantum-cascade laser structure with single phonon resonance-continuum depopulation and demonstrate room temperature, continuous-wave operation of the proposed 7.9μm quantum-cascade laser. The laser grown by metal organic vapor-phase epitaxy emits a cw output power of 36mW at 30°C, exhibiting a threshold current density of 2.23kA∕cm2. The cw operation is reported for higher temperatures up to slightly above 60°C. The proposed structure may lead to a successful commercial mass production of the lasers.

Mid-infrared frequency comb for broadband high precision and sensitivity molecular spectroscopy

We report on the experimental demonstration of the metrological and spectroscopic performances of a mid-infrared comb generated by a nonlinear downconversion process from a Ti:sapphire-based near-infrared comb. A quantum cascade laser at 4330 nm was phase-locked to a single tooth of this mid-infrared comb and its frequency-noise power spectral density was measured. The mid-infrared comb itself was also used as a multifrequency highly coherent source to perform ambient air direct comb spectroscopy with the Vernier technique, by demultiplexing it with a high-finesse Fabry-Perot cavity.

Detuning characteristics of the linewidth enhancement factor of a midinfrared quantum cascade laser

The linewidth enhancement factor of a 5.2-mum distributed-feedback quantum cascade laser above the threshold current caused by detuning from the gain peak was investigated. We clearly observed different values for the linewidth enhancement factor.

Spectroscopic detection of radiocarbon dioxide at parts-per-quadrillion sensitivity

A compact and simple laser spectroscopy apparatus at 4.5 μm, based on saturated-absorption cavity ring-down (SCAR), has approached the ultimate sensitivity of accelerator mass spectrometry, measuring radiocarbon dioxide concentration down to few parts per quadrillion.

Broadband Tuning of External Cavity Dual-Upper-State Quantum-Cascade Lasers in Continuous Wave Operation

A wide wavelength tuning of an external cavity quantum-cascade laser (QCL) based on the anticrossed dual-upper-state to multiple-lower-state design is demonstrated in continuous wave (cw) operation at room temperature. The tuning ranges of 321 cm-1 (Δλ/λ~22%) in pulsed operation and 248 cm-1 (Δλ/λ~17%) in cw operation are achieved, despite employment of the active region with translational symmetry. The present tuning range in cw operation substantially exceeds the values obtained with the QCLs based on conventional broadband active region designs. In addition, the continuous, single mode tuning is realized with its widely homogeneous gain spectrum.