Frequency Combs

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Developing all-solid state mid-IR and THz frequency combs based on QCL gain media

This research line is dedicated to develop and study the physics of frequency combs based on broadband QCL gain media. Optical frequency combs have revolutionized many fields of research, by acting as a ruler in the frequency domain [1]. Their use in fundamental time metrology, frequency synthesis and spectroscopy has lead to impressive advances in those fields, to name a few. The fundamental significance and impact on science of frequency combs led to the Nobel Prize in Physics in 2005 for Theodor W. Hänsch and John L. Hall [2]. The development of optical frequency combs in the mid-IR and THz spectral regions is especially interesting since most light molecules have their fundamental roto- and vibrational absorption bands in this wavelength range. The absorption strength of most molecules is orders of magnitudes stronger compared with the overtone bands in the near-IR region, allowing a highly sensitive time-domain frequency-comb spectrometer [3].

Due to the unique gain dynamics of QCLs, frequency-comb generation in QCLs is different from fundamentally mode-locked lasers. In fact, the generation of ultra-short pulses in a QCL is hard to accomplish, because of the short upper state lifetimes (τ ~ 0.3 ps) compared with the time required for a pulse to finish one round-trip in a typical QCL cavity (τ ~64 ps). But the only requirement of an optical frequency comb is the equidistant spacing of its modes, and therefore a periodicity of the waveform is therefore sufficient. A phase and mode configuration, which leads to a constant output power, as is the case for a frequency modulated signal, would not be perturbed by the fast gain recovery time of the media. Fig. 1a illustrates a QCL frequency comb, which produces a nearly constant output power and at the same time has all the frequency modes equally spaced.

Frequency Comb principle  
Fig 1 a) Illustration of QCL frequency comb with nearly constant output power. Nevertheless the output is an optical frequency comb spectrum. b) Strong third order nonlinear four-wave-mixing processes generating equally distant modes.
The comb formation in QCLs occurs due to strong third-optical nonlinearities present in broadband continuous-wave QCL (see Fig. 2b) in combination with the fast gain recovery resulting in a fixed phase relationship. The generated spectrum is a comb of equidistant modes. The generated output resembles the one of a frequency-modulated signal.

Mid-IR QCL frequency combs

Nowadays, broadband mid-IR QCLs run in continuous-wave at room-temperature and feature very broadband emission without any spectral holes. Fig. 2 shows a measurement of the optical spectrum of such a QCL frequency comb operating in the mid-IR region. The laser operates in continuous-wave at room-temperature featuring an output power of 80 mW.

Comb Spectrum  
Fig 2 Broadband laser spectrum at room temperature in continuous-wave operation.

THz QCL frequency combs

In addition to mid-IR combs, QCLs can also be used to generate combs at THz frequencies, as the main driving mechanism is still four-wave mixing.
Our group has recently shown that it is possible to achieve octave-spanning lasers at THz frequencies using QCLs. This very wide frequency coverage is possible due to the gain engineering capability of QCLs coupled to the broadband nature of the THz waveguides (double-metal waveguide).

In usual QCLs (homogeneous), the active core embedded in the optical waveguide is composed of a repetition of units based on a single design. The homogeneous QCLs are characterized by homogeneous line-width broadening which means that the system suffers from gain narrowing due to mode-competition and that the envelope of the emitted spectrum is modulated by a Lorentzian function. Hence, in order to obtain a flat gain medium, a simple design optimization is not enough. The idea is to have a laser system that consists of many independent segments whose transition line-shapes are designed separately. The final gain spectrum of such a system would be the sum of the contributing transitions. This is achieved by stacking different active-region designs into a common waveguide, obtaining a so-called heterogeneous QCL (HQCL) [4]. The HQCL-concept was introduced already several years ago in the mid-infrared range but it has not been intensively developed in the THz.

Octave-spanning emission of a THz QCL is achieved by stacking three different active regions in the same waveguide. Its emission spans from 1.64 THz to 3.35 THz when operated in CW operation at a temperature of 25 Kelvin, as shown in Fig. 3b.
Similar to mid-IR QCL combs these broadband THz QCls also show comb operation over a limited driving current range. The characteristics of a THz QCL comb are displayed in Fig. 3 a. The shown comb has 1.55 mW of output power while spanning over a spectral bandwidth of 507 GHz.

THz Comb  
Fig 3 THz QCL comb operation. a) Light intensity-current-voltage curves of THz QCL comb. The shaded area indicates the current range comb operation is observed. The upper inset displays the intermode beatnote with a linewidth of 800 Hz at a driving current of 0.9 A. The lower inset shows the corresponding optical spectrum with a bandwidth of 507 GHz. b) Octave-spanning spectrum of the same laser measured at 25 Kelvin in continuous-wave operation (1.05A).

Applications: QCL-based dual-comb spectrometers

The combination of the high-output power of QCLs, the small size and the direct current injection scheme makes them the ideal candidate for small, portable, broadband and highly sensitive time-domain frequency-comb spectrometer in the mid-IR and THz regions. An extremely appealing application of frequency combs for high-precision spectroscopy is the so-called dual-comb spectroscopy, where multi-heterodyne detection is performed allowing Fourier transform spectroscopy with potentially high resolution, high sensitivity and no moving parts (see Fig. 4a and Fig. 4b) [3, 5]. As shown in Fig. 4a, one comb is used as a local oscillator while the other is used to interrogate a sample. We have demonstrated a proof-of-principle spectroscopy measurement of H2O with a QCL-based dual-comb spectrometer (see Fig. 4c). This line of research was the basis for the spin-off IRsweep of our group.

THz Comb  
Fig 4 QCL-based dual-comb spectrometer: a) principle of dual-comb spectroscopy. b) Multi-heterodyne beating created by two frequency combs with slightly different repetition frequencies. c) Transmission spectrum of H2O acquired with a QCL-based dual-comb spectrometer.

Research Vision:

In this research, we aim at developing the new generation of QCL comb devices.
- Understanding fully the physics behind comb formation and characterize the emitted light
- Explore the device boundaries for power, bandwith and wavelength coverage
- Generate new devices with self-referencing
- Leverage on the fact that QCLs are simultaneously non-linear devices and detectors to implement further functionalities at the chip level.

For more see: 
Mid-infrared frequency comb based on a quantum cascade laser, Nature (2012)
Octave-spanning semiconductor laser, Nature Photonics (2015)
Dual-comb spectroscopy based on quantum-cascade-laser frequency combs, Nature Communications (2014)
Intrinsic linewidth of quantum cascade laser frequency combs, Optica (2015)
Quantum cascade laser combs: effects of modulation and dispersion, Optics Express, (2015)  


[1] Udem, T., Holzwarth, R. & Hänsch, T. W. Optical frequency metrology. Nature 416, 233–237 (2002).
[2] the nobel prize in physics 2005 (2013),
[3] Keilmann, F., Gohle, C. & Holzwarth, R. Time-domain mid-infrared frequency-comb spectrometer. Opt. Lett. 29, 1542–1544 (2004).
[4] C. Gmachl, D. Sivco, R. Colombelli, F. Capasso, and A. Cho, Nature 415, 883 (2002).
[5] Coddington, I., Swann, W. C., and Newbury, N. R., Coherent multi-heterodyne spectroscopy using stabilized optical frequency combs, Physical Review Letters 100, 013902 (2008).

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