Abstract
We demonstrate a technique to simultaneously stabilize the frequency and output power of a terahertz quantum-cascade laser (QCL). This technique exploits frequency and power variations upon near-infrared illumination of the QCL with a diode laser. It does not require an external terahertz optical modulator. By locking the frequency to a molecular absorption line, we obtain a long-term (one-hour) linewidth of 260 kHz (full width at half maximum) and a root-mean-square power stability below 0.03%. With respect to the free-running case, this stabilization scheme improves the frequency stability by nearly two orders of magnitude and the power stability by a factor of three.
Highlights
Terahertz (THz) quantum-cascade lasers (QCLs) are of great importance as local oscillators (LOs) for high-resolution heterodyne spectroscopy above 3 THz [1,2,3]
We demonstrate a technique to simultaneously stabilize the frequency and output power of a terahertz quantum-cascade laser (QCL)
A poly-4-methylpentene-1 (TPX) lens collimates the QCL beam, and a Mylar beam splitter divides the beam into two parts, where the reflected beam is used for power stabilization, while the transmitted beam passes through a methanol (CH3OH) gas cell for frequency locking to a molecular absorption line
Summary
Terahertz (THz) quantum-cascade lasers (QCLs) are of great importance as local oscillators (LOs) for high-resolution heterodyne spectroscopy above 3 THz [1,2,3]. Stabilizing the frequency and output power becomes even more challenging when the QCL is operated in a mechanical cryocooler, where vibration noise is intrinsically present. An alternative method, which has been well established for diode lasers, is the stabilization to an atomic or molecular absorption line to achieve absolute long-term stability of the laser frequency [18,19] This approach has been demonstrated for QCLs [20,21]. Amplitude stabilization of a 2.85-THz QCL has been achieved with a graphene-loaded, split-ring-resonator array acting as an external amplitude modulator. In this case, the transmittance of the modulator was controlled. We describe a method that allows for a simultaneous stabilization of the frequency and output power by taking advantage of the frequency and power regulation by near-infrared (NIR) excitation [24,25] in addition to frequency and power control by the modification of the driving current so that no external THz optical modulator is required
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