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5580 Vol. 55, No. 21 / July 20 2016 / Applied Optics
Research Article
Innovative high-gain optically pumped far-infrared laser
BIHE DENG,* KURT KNAPP, PING FENG, JOHN KINLEY, AND CURT WEIXEL
Tri Alpha Energy, Inc., P.O. Box 7010, Rancho Santa Margarita, California 92688, USA
*Corresponding author: bdeng@trialphaenergy.com
Received 19 April 2016; revised 22 June 2016; accepted 22 June 2016; posted 23 June 2016 (Doc. ID 263547); published 13 July 2016
A new optically pumped far-infrared (FIR) laser with separate pump beam reflector and FIR output coupler has been developed. The new design greatly simplifies the tuning of the laser and enables the optimization of the pump beam absorption without affecting the laser alignment. Working with formic acid (HCOOH) vapor pumped by a 30 W CO2 laser at the 9R20 line, a high laser gain of 3 dB∕m and power conversion efficiency of 16.4% of the Manley–Rowe limit are achieved for the FIR wavelength of 433 μm. © 2016 Optical Society of America
OCIS codes: (140.0140) Lasers and laser optics; (140.3070) Infrared and far-infrared lasers; (140.4130) Molecular gas lasers. http://dx.doi.org/10.1364/AO.55.005580
1. INTRODUCTION
Far-infrared (FIR) lasers are widely used in interferometers and polarimeters for plasma diagnostics. The long wavelength of these lasers means large phase signals are detected along with insensitiveness to mechanical vibrations [1,2]. Compact quan- tum cascade lasers in the submillimeter wave wavelengths are being developed, but the technology is not yet matured enough for practical applications [3]. Therefore, optically pumped FIR gas lasers are still the workforce for plasma interferometry and polarimetry [4–9]. High-gain and high FIR laser output power are required so that the laser beam can be split into multiple beams for simultaneous multichannel measurements [4–9]. A typical FIR laser resonator consists of a flat mirror, a dielectric waveguide, and an output coupler, as shown in Fig. 1. It is filled with molecular gas chosen based on the desired output laser wavelength. The molecules are excited to the higher vibrational energy level by absorbing infrared pump laser photons. The subsequent transition to a lower rotational energy sublevel within the same vibrational energy level branch results in the longer wavelength FIR laser output [10]. CO2 lasers are most frequently used as the pump source due to their high power and richness in wavelength, which can be tuned to match the resonant absorption of various working gas mole- cules. The flat mirror is coated to have near unity reflection coefficient and has a small hole (usually off center, 3–5 mm in diameter) for pump laser beam injection. It is sometimes referred to as the rear mirror, as it is at the opposite side of the output coupler, where the FIR laser beam exits. The output coupler has a finite transmission (coupling) coefficient so that laser power can be coupled out. This is the most critical com- ponent of FIR lasers, as it will determine the gain, output
power, and output laser beam quality of the lasers as well as how conveniently one can operate the laser. The laser described here is an innovative design that achieved the highest gain and laser output power when working with the formic acid (HCOOH) vapor at an FIR wavelength of 433 μm and, in the meantime, significantly improved the flexibility in terms of laser tuning and operation. It can be easily adapted to operate at other FIR laser wavelengths.
2. NEW FIR LASER DESIGN
For optically pumped FIR lasers, many works have been pub- lished on various output couplers, which are critical in deter- mining the laser gain, power, and output beam quality. The simplest output coupler is a mirror with a small clear hole at the center. The pump laser beam will leak through the hole leading to reduced FIR laser gain and power. The hybrid hole output coupler [Fig. 2(a)] prevents the pump beam leakage by coating the hole with dielectric layers, which have high reflec- tivity for the pump laser wavelength. The dielectric layer is thin compared with the FIR wavelengths, showing negligible reflec- tion. High-resistivity silicon or germanium is used as the sub- strate material for the dielectric and annular gold coatings. These hybrid hole couplers can achieve higher FIR laser power [10,11] but have the disadvantages of fast beam divergence and unstable transverse mode quality. The silicon substrate metal mesh-dielectric hybrid couplers (Fig. 2(b), [12]) yield much better output beam quality but lack the flexibility for tuning the coupling coefficient. The improved Fabry–Perot coupler [Fig. 2(c)] consisting of a quartz etalon and metal mesh can be optimized by tuning the spacing between the quartz plate
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