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Investigation and reduction of frequency noise in quantum cascade lasers

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Quantum cascade lasers (QCLs) are unipolar intersubband semiconductor lasers emitting in the mid-infrared spectral region. They have been the most widely used light sources for trace gas sensing and high resolution spectroscopy in the molecular fingerprint region for more than one decade owing to their remarkable properties, such as high optical power, room temperature operation or single-mode tunable emission using a distributed feedback (DFB) structure.

In our work with QCLs, we are particularly interested in the spectral and noise properties of these lasers. Low-noise narrow-linewidth laser sources are important in many applications such as very high resolution and accuracy spectroscopy or optical metrology. We are studying frequency noise in QCLs in order to better understand its origin as a first step, and to reduce it by different active stabilization methods in a second step.
 

Origin of frequency noise in QCLs

QCLs have the potential to be a narrow-linewidth light source with an intrinsic linewidth of a few hundred hertz only that results from their close-to-zero Henry’s linewidth enhancement factor. However, a narrow linewidth is generally not achieved in practice in free-running QCLs as a result of undesired noise that compromises their spectral properties.

A possible origin of excess frequency noise in QCLs is technical noise induced by the current source that drives the laser. DFB-QCLs operate at a relatively high current (up to 1000 mA) and high voltage (10-20 V) compared to their near-infrared laser diode counterparts. Achieving a low current noise in such conditions is thus more challenging than for standard laser diode drivers. With a typical current-tuning coefficient of several hundred MHz/mA in QCLs in the 4-5 µm spectral range, we showed that a current noise lower than 1 nA/Hz1/2 is generally required in order that the frequency noise inherent to the laser can be observed without additional degradation and associated linewidth broadening induced by the current driver [1]. For this purpose, we developed a home-made QCL control electronics (Figure 1), which encompasses a low-noise current source capable of delivering up to 1 A with a current noise of < 1 nA/Hz1/2, as well as a temperature controller that regulates the QCL temperature at the mK level.

Figure 1: Left: Calculated effect of white current noise of the laser driver on the linewidth of a QCL (based on typical parameters of a DFB-QCL at 4.5 µm). Right: Picture of a home-made QCL controller encompassing a low-noise current driver and a high stability temperature controller.

Even with the use of such a low-noise current source, the linewidth of DFB-QCLs is generally in the MHz or sub-MHz range [2]. The reason is the presence of frequency noise that is induced internally in the QCL structure. We have shown that frequency noise is produced by electrical fluctuations induced within the QCL structure by the electrons transport [3]. This electrical noise can be observed on the voltage across the QCL and we demonstrated that it is highly correlated with the frequency noise [6] as shown in Figure 2. Frequency noise is typically measured using the side of a molecular absorption line (of CO in the present case) as a frequency discriminator that converts the frequency fluctuations of the laser into intensity fluctuations that are measured with a photodiode. But studying the electrical noise is a simple and powerful mean to investigate noise processes in QCLs, e.g. by enabling measurements in regimes that are not accessible optically, such as sub-threshold or in conditions where the laser emission does not correspond to a suitable absorption line.

In collaboration with the QCL manufacturing company Alpes Lasers, we studied noise in a large set of DFB-QCLs in the 7 8 µm wavelength range, with different structures (e.g., ridge vs buried heterostructure - BH), different parameters (length, width, etc) and fabricated in six different processes, with the objective to identify some variables that can impact the noise generation. We showed a lower noise obtained in ridge waveguide QCLs then in buried heterostructure laser, despite the better overall performance of this latter type of QCLs, e.g. in term of output power and thermal dissipation.


Figure 2: Left: Schematic set-up used to measure the frequency noise of a QCL using a molecular absorption line as a frequency-to-intensity converter. Right: Correlation observed between voltage and frequency fluctuations in a QCL.

 

Frequency noise reduction in QCLs

Following the observation that frequency noise in QCLs arises mainly from electrical fluctuations, we assessed the possibility of using the voltage noise measured across the QCL as an error signal to reduce the frequency fluctuations and therefore narrow the laser emission linewidth using a feedback loop that does not involve any optical frequency reference for direct measurement of the optical frequency noise [6].

The voltage noise sensed across the QCL was used as a feedback signal to control the output power of a near-infrared laser diode that illuminates the top surface of the QCL (Figure 3). The resulting light absorption in the QCL structure enabled us to implement a fast control of the QCL active region temperature independently of the QCL drive current. With this feedback signal, the voltage fluctuations are strongly reduced as the QCL voltage varies with temperature. At the same time, a significant reduction of the optical frequency fluctuations was also observed (Figure 3, lower trace).

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 Figure 3:   Left: Schematic set-up of the new stabilization  method that uses the voltage noise across the QCL as an error signal to reduce the frequency noise. Right: Simultaneous recording of the QCL voltage and optical frequency. An important reduction of the voltage fluctuations, but also of the frequency noise, is achieved when the feedback loop is activated (t > 2 ms)

 
As a next step towards the frequency stabilization of QCLs and the realization of narrow linewidth laser sources in the mid-infrared, we are investigating frequency stabilization to a micro-resonator.
 

Relevant publications:

  1. L. Tombez, S. Schilt, J. Di Francesco, T. Führer, B. Rein, T. Walther, G. Di Domenico, D. Hofstetter, P. Thomann, Linewidth of a quantum cascade laser assessed from its frequency noise spectrum and impact of the current driver, Appl. Phys. B 109 (3), 407-414 (2012) PDF
  2. L. Tombez, J. Di Francesco, S. Schilt, G. Di Domenico, J. Faist, P. Thomann, D. Hofstetter, Frequency noise of free-running 4.6 μm distributed feedback quantum cascade lasers near room temperature, Opt. Letters 36(16), 3109-3111 (2011) PDF
  3. L. Tombez, S. Schilt, J. Di Francesco, P. Thomann, D. Hofstetter, Temperature dependence of the frequency noise in a mid-IR DFB quantum cascade laser from cryogenic to room temperature, Opt. Express 20 (7), 6851-6859, (2012) PDF
  4. L. Tombez, F. Cappelli, S. Schilt, G. Di Domenico, S. Bartalini, D. Hofstetter, Wavelength tuning and thermal dynamics of continuous-wave mid-IR distributed feedback quantum cascade laser, Appl. Phys. Lett. 103 (3), 031111-1 - 031111-5 (2013) PDF 
  5. S. Schilt, L. Tombez, G. Di Domenico, D. Hofstetter, Frequency Noise and Linewidth of Mid-infrared Continuous-Wave Quantum Cascade Lasers: An Overview, in The Wonders of Nanotechnology: Quantum and Optoelectronic Devices and Applications, M. Razeghi, L. Esaki, and K. von Klitzing, Eds., SPIE Press, Bellingham, WA, pp. 261-287 (2013). PDF
  6. L. Tombez, S. Schilt, D. Hofstetter, T. Südmeyer, Active linewidth-narrowing of a mid-IR quantum cascade laser without optical reference, Opt. Lett. 38 (23), 5079-5082 (2013) PDF
     

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