Spectroscopic applications and frequency locking of THz photomixing with distributed-Bragg-reflector diode lasers in low-temperature-grown GaAs

Abstract

A compact, narrow-linewidth, tunable source of THz radiation has been developed for spectroscopy and other high-resolution applications. Distributed-Bragg-reflector ͑DBR͒ diode lasers at 850 nm are used to pump a low-temperature-grown GaAs photomixer. Resonant optical feedback is employed to stabilize the center frequencies and narrow the linewidths of the DBR lasers. The heterodyne linewidth full-width at half-maximum of two optically locked DBR lasers is 50 kHz on the 20 ms time scale and 2 MHz over 10 s; free-running DBR lasers have linewidths of 40 and 90 MHz on such time scales. This instrument has been used to obtain rotational spectra of acetonitrile (CH 3 CN) at 313 GHz. Detection limits of 1ϫ10 Ϫ4 Hz 1/2 ͑noise/total power͒ have been achieved, with the noise floor dominated by the detector's noise equivalent power. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00538-X͔ Recent advances in the development of low-temperaturegrown ͑LTG͒ GaAs as a THz optical heterodyne photomixer, when combined with near-infrared semiconductor lasers, promise a coherent radiation source that is continuously tunable, compact, robust, and inexpensive.1-4 Such a frequency synthesis technique would benefit many THz applications including frequency metrology of near-infrared spectral lines, air/space-borne measurements for atmospheric and astrophysical studies, and high resolution molecular spectroscopy. A handful of groups 1-4 have developed LTG GaAs based THz sources. However, these sources suffer the disadvantages of having a cumbersome size, a limited tuning range, frequency instabilities, and/or expensive pump lasers. Through the use of resonant optical feedback [5][6][7] to lock the frequency of two distributed-Bragg-reflector ͑DBR͒ diode pump lasers, we have achieved a tunable source that is inexpensive, compact, narrow linewidth, and frequency stable. Figure 1 illustrates the experimental setup. The LTG GaAs photomixer is pumped by two optically locked DBR diode lasers. We use the same geometry developed by Hollberg and co-workers.5 A small fraction of each DBR laser's output is sent to a folding mirror ͑ mirror͒ and then to a confocal Fabry-Perot cavity. The DBR lasers ''see'' optical feedback only when they are on resonance with the FabryPerot cavities. Thus, narrowing of the linewidth via optical feedback is achieved in a controlled manner. Each cavity consists of two spherical mirrors glued onto quartz and piezoelectric translator ͑PZT͒ tubes. The PZT section allows one to tune the cavity-free spectral range ͓͑FSR͒, a͒ Electronic mail: pin@gps.caltech. edu FIG. 1. The experimental setup. Each of the two DBR lasers is optically locked to an independent confocal cavity. PZT translators tune the cavity lengths and the feedback path lengths ͑-mirror positions͒. The primary beams from the DBR lasers are combined by a beamsplitter. The combined beam goes through either A for heterodyne linewidth measurements, or B for submillimeter spectroscopy.

Nonlinear spectroscopy of exciton-polaritons in a GaAs-based microcavity

“…The detection of THz radiation is a challenging task, and typically bolometers at cryogenic temperatures are used [14,35]. Therefore, we suggest a precise measurement of the LP and 2p exciton energy levels as a mandatory preliminary experiment, which determines the exact THz frequency to be evidenced using a bolometer.…”

A tunable cavity-locked diode laser source for terahertz photomixing

“…The short-term linewidth of each laser is determined entirely by the optical feedback from the external cavity because the 3-kHz bandwidth of the locked loop circuit is much less than the laser linewidth. Short-term (50 ms) DBR linewidths of 50 kHz have been achieved by weak optical feedback from FP cavities [7]. The additional narrowing is due to a 180 phase shift over a narrow (60 MHz) cavity width compared to the same phase shift over 1.5 GHz in the scheme presented here.…”

Tunable, continuous-wave Terahertz photomixer sources and applications

“…8 Small biomolecules, like biotin, 170 were characterized. A very good example for extremely high-resolution CW THz spectroscopy with LTGaAs photomixers is given by Chen et al 171 Five resonances of acetonitrile were resolved at a pressure of 150 mTorr in a frequency window of only 96.1 MHz at 312.5 GHz. A relatively simple stabilization system for the lasers was implemented.…”