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The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 μm range and operated in cw mode near room temperature (277 K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz(2)/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.

Abstract:We report on the measurement of the frequency noise power spectral density in a distributed feedback quantum cascade laser over a wide temperature range, from 128 K to 303 K. As a function of the device temperature, we show that the frequency noise behavior is characterized by two different regimes separated by a steep transition at ≈200 K. While the frequency noise is nearly unchanged above 200 K, it drastically increases at lower temperature with an exponential dependence. We also show that this increase is entirely induced by current noise intrinsic to the device. In contrast to earlier publications, a single laser is used here in a wide temperature range allowing the direct assessment of the temperature dependence of the frequency noise. Baillargeon, and A. Y. Cho, "Rapid-scan Doppler-limited absorption spectroscopy using mid-infrared quantum cascade lasers," Proc. SPIE 3758, 23-33 (1999). 4. T. L.

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We describe a radio-frequency (RF) discriminator, or frequency-to-voltage converter, based on a voltage-controlled oscillator phase-locked to the signal under test, which has been developed to analyze the frequency noise properties of an RF signal, e.g., a heterodyne optical beat signal between two lasers or between a laser and an optical frequency comb. We present a detailed characterization of the properties of this discriminator and we compare it to three other commercially available discriminators. Owing to its large linear frequency range of 7 MHz, its bandwidth of 200 kHz and its noise floor below 0.01 Hz 2 /Hz in a significant part of the spectrum, our frequency discriminator is able to fully characterize the frequency noise of a beat signal with a linewidth ranging from a couple of megahertz down to a few hertz. As an example of application, we present measurements of the frequency noise of the carrier envelope offset beat in a low-noise optical frequency comb.

We report on a technique for frequency noise reduction and linewidth-narrowing of a distributed-feedback mid-IR quantum cascade laser (QCL) that does not involve any optical frequency reference. The voltage fluctuations across the QCL are sensed, amplified and fed back to the temperature of the QCL at a fast rate using a near-IR laser illuminating the top of the QCL chip. A locking bandwidth of 300 kHz and a reduction of the frequency noise power spectral density by a factor of 10 with respect to the free-running laser are achieved. From 2 MHz for the free-running QCL, the linewidth is narrowed below 700 kHz (10 ms observatio The growing interest for high-resolution spectroscopy experiments has pushed scientists to investigate the ultimate limits that these devices can achieve in terms of frequency stability. Frequency noise and linewidth properties of free-running QCLs were investigated in various experimental setups in the mid-IR with distributed feedback (DFB) [4,5] and external cavity configurations [6], as well as in the terahertz domain [7]. Moreover, different active frequency-stabilization experiments for linewidth narrowing have been reported. Generally, a frequency-sensitive element is used to sense the fluctuations of the laser frequency and generate an error-signal that is usually fed back to the QCL injection current. Several methods were demonstrated using high-finesse optical cavities In this Letter, we present a different approach for frequency noise reduction and linewidth-narrowing of a 4.55 μm QCL that does not involve any optical frequency reference. Following the observation that frequency noise in DFB QCLs arises from electrical power fluctuations due to the electronic transport in the devices [13,14], in this work we assess and experimentally demonstrate linewidth narrowing using only the voltage fluctuations across the QCL as an error signal for a feedback loop. This error signal is generated without measuring the actual fluctuations of the QCL optical frequency. A similar approach aiming at using the voltage noise (VN) measured across a near-IR laser-diode in order to implement an electrical feedback to reduce the phase noise was proposed [15] but never demonstrated to the best of our knowledge. We show here a reduction of the frequency noise power spectral density (PSD) of one order of magnitude within the bandwidth of the feedback loop (>100 kHz).The laser used in our experiment is a 4.55 μm buriedheterostructure DFB QCL provided by Alpes Lasers. The stabilization scheme is shown in Fig. 1. The QCL is mounted on a Peltier-cooler operated at 20°C and an output power of 10 mW is obtained at an injection current of 260 mA (the threshold current is I th 220 mA). A lownoise current source is used to drive the QCL with a current noise lower than 1 nA∕ p Hz in order to avoid any linewidth broadening resulting from technical noise [16]. In these conditions, the contribution of the injection current noise to the frequency noise is negligible and the fluctuations of the QCL laser frequency are i...

We report on the wavelength tuning dynamics in continuous-wave distributed feedback quantum cascade lasers (QCLs). The wavelength tuning response for direct current modulation of two mid-IR QCLs from different suppliers was measured from 10 Hz up to several MHz using ro-vibrational molecular resonances as frequency-to-intensity converters. Unlike the output intensity, which can be modulated up to several gigahertz, the frequency-modulation bandwidth was found to be on the order of 200 kHz, limited by the laser thermal dynamics. A non-negligible roll-off and a significant phase shift are observed above a few hundred hertz already and explained by a thermal model.Since the first demonstration of quantum cascade lasers (QCLs) in 1994, 1 the number of promising application in the field of chemical sensing for biomedical and environmental sciences has been constantly rising during the last years. Indeed, thanks to the ability to tailor their emission wavelength and to target precisely selected ro-vibrational molecular transitions in the fingerprint region, QCLs were demonstrated to be very sensitive probes for a wide variety of molecules.2 Single-frequency QCLs are generally required for trace gas sensing and a common approach is to use a distributed feedback (DFB) grating etched at the surface of the QCL active region in order to force laser operation at a precise wavelength.3 Moreover, promising developments in the field of high-resolution spectroscopy can lead to the possibility of performing measurements with unprecedented precision, especially by linking spectrally narrow QCLs to optical frequency combs. 4,5 In order to push the limits of those highresolution experiments, narrow-linewidth sources of coherent light which can be achieved by active stabilization of DFB-QCLs to optical references with high-bandwidth servoloops are required. 6,7 For the most demanding applications in the field of high-resolution spectroscopy, frequency-noise analysis revealed that feedback loop bandwidths of several hundred of kHz are necessary for linewidth narrowing of DFB-QCLs. 7,8 While the picosecond carrier lifetime in QCLs allows a very fast modulation of the intensity above 10 GHz, 9,10 the modulation of the optical frequency-or wavelength-is limited by the thermal dynamics of the device. Indeed, unlike interband semiconductor laser diodes whose wavelength can be modulated at high speed through carrier density modulation, 11,12 the latter has no effect in QCLs because of the symmetric gain curve and associated independence of the refractive index at the gain peak (zero alpha parameter). 13 The wavelength tuning in DFB-QCLs is therefore mainly governed by the temperature dependence of the refractive index with a tuning rate of 1/k dk/dT % 7 Â 10 À5

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