Frequency and amplitude stabilized terahertz quantum cascade laser as local oscillator

Abstract

We demonstrate an experimental scheme to simultaneously stabilize the frequency and amplitude of a 3.5 THz third-order distributed feedback quantum cascade laser as a local oscillator. The frequency stabilization has been realized using a methanol absorption line, a power detector, and a proportional-integral-derivative (PID) loop. The amplitude stabilization of the incident power has been achieved using a swing-arm voice coil actuator as a fast optical attenuator, using the direct detection output of a superconducting mixer in combination with a 2nd PID loop. Improved Allan variance times of the entire receiver, as well as the heterodyne molecular spectra, are demonstrated. A terahertz (THz) quantum cascade laser (QCL) is the most promising solid-state source as the local oscillator (LO) for a high resolution heterodyne receiver operating at frequencies above 2 THz and, in particular, for a multi-pixel array receiver because of its high output power (typically mW). 1,2 Among different types of applications, a super-THz heterodyne receiver plays a vital role in astronomical observations, for instance to map a large number of fine structures and molecular lines associated with the formation of stars and planets in the Milky Way and nearby galaxies. 3 Those spectral lines at high frequencies are typically narrow ( a few MHz). Furthermore, the signals are, in general, very weak and deeply embedded within the noise. 3 The narrow spectral lines require either frequency or phase stabilization of a local oscillator. Since the intrinsic linewidth of a THz QCL is much narrower (<kHz) 4 than typical astronomical lines, frequency locking of a QCL will be sufficient. Until now, a considerable amount of progress has been made to achieve either phase locking or frequency locking to a THz QCL. [5][6][7] In essence, these experiments have established that the THz QCL, like other solid-state oscillators, is a voltage-controlled oscillator (VCO) whose frequency is determined by a control DC voltage.The weak signal lines require a long integration time during an observation and of course a high sensitivity of the mixer. The effective integration time is limited by the stability of the entire receiver, which is characterized by the Allan variance time for a given measurement bandwidth. 8 For a receiver based on a superconducting hot electron bolometer (HEB) mixer, the stability is dominated by the amplitude stability of the incident LO power. 9 It is known that the output power of a THz QCL is determined by both DC bias and operating temperature. Thus, any fluctuation or drift either in the bias or in the temperature may give rise to output power fluctuation. The power stability becomes problematic when the QCL is operated in a pulse tube cryocooler, where temperature variations and fluctuations are intrinsically present. Furthermore, the DC bias regulates the emission frequency of the laser at a given temperature. In order to stabilize the frequency and amplitude of the laser simultaneously, another means of tuning, besides fro...

Surface-emitting terahertz quantum cascade lasers with continuous-wave power in the tens of milliwatt range

“…The ability to operate terahertz frequency quantum cascade lasers 1 (THz QCLs) in continuous-wave (CW) is essential for many applications, including their use in local oscillator systems, 2 spectroscopy/trace-gas detection, [3][4][5][6] and direct bolometric imaging. 7 It is also often important to ensure that the lasers have low dissipated power.…”

Frequency Locking and Monitoring Based on Bi-directional Terahertz Radiation of a 3rd-Order Distributed Feedback Quantum Cascade Laser

“…We have not measured the absolute power of this particular laser since we are more interested in the ratio. However, based on the power measurement of a similar laser [5], we expect the maximal output power of the forward direction to be about 0.8 mW, while the other direction is 0.45 mW. Prior to frequency locking, we measure methanol absorption lines by sweeping the QCL bias voltage from 13.5 to 14.5 V, which tunes the frequency electrically, as confirmed by a separate FTS measurement.…”

“…In general, since the QCL is not inherently frequency stable, a system of frequency or phase locking [3,4] is required. So far, the radiation emitted from only one direction of the QCL has been used for both pumping a mixer and stabilizing the frequency of the source [5]. In this way, to achieve frequency locking, part of the beam power is unavailable for the mixer.…”

All-integrated terahertz modulators

“…topological insulator properties) [89]. It is worth mentioning the importance of this new class of device for amplitude and frequency stabilization of QCLs in metrological grade spectroscopy [90][91][92][93]. Naming all the possible applications of THz active devices in gas-and solid-state spectroscopy, from sensing to healthcare to investigation of novel state of matter and materials, could be per se the topic of another review, as the one reported recently in Ref.…”