CALTECH EE 243A - OPTICALLY-PUMPED THz LASER TECHNOLOGY

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OPTICALLY-PUMPED THz LASER TECHNOLOGY Eric R. Mueller Coherent - DEOS 1280 Blue Hills Ave. Bloomfield, CT 06002 (806) 243-9557 Abstract The recent myriad of advances in THz sources, detectors, and potential applications has brought this region of the spectrum to the attention of its widest audience to date. As applications are developed, the optimal source technology for each application will be requirement dependent. Modern optically-pumped THz laser technology may well be the source of choice for selected applications. The advent of reliable compact CO2 laser technology and its widespread commercial application, combined with recent developments in similarly reliable compact THz laser technology, has heralded a new era in optically-pumped THz lasers. This new generation of laser technology is being used on a long-duration space mission on NASA’s AURA satellite. Further, this technology has yielded easy-to-operate laboratory THz sources. The present paper will review the current state of optically-pumped THz laser technology and present some of the longer-term roadmaps available for this technology. I. Introduction During the past ten years the state-of-the-art in CO2 laser technology has advanced dramatically. These advances have resulted in substantial decrease in size, decrease in cost, and increase in reliability. This new generation of CO2 lasers is being used in a wide variety of commercial and scientific applications. Many of these applications require 24/7 operation without service. Many of the underlying technologies present in this new generation of C02 lasers are applicable to optically-pumped THz lasers (OPTL). Accordingly, modern OPTL's not only take advantage of the latest CO2 laser designs, but also incorporate many of the same design approaches. Also, as CO2 lasers are an integral component of OPTL's, the improvements in performance of CO2 lasers improve the performance of OPTL’s. This paper will review OPTL operation concepts, provide a comparison between past and modern OPTL technologies, provide examples of modern OPTL systems, examine a design for a compact OPTL, and review frequency agility technology for OPTL’s. The remainder of this paper will be organized in sections as follows: II – Overview of OPTL Operation; III – Comparison of Past vs Modern OPTL Technology; IV – Examples of Modern OPTL Systems; V – “Shoe Box” Concept OPTL; VI - Frequency Agility Addition to OPTL’s; VII – Conclusions, VIII – Acknowledgements, and IX - References. II. Overview of OPTL Operation A generalized schematic representation of an optically-pumped THz laser system is presented in Figure 1. The THz laser cell consists of: a vacuum envelope in which a molecular gas at low pressure is placed, some source of optical feedback (end mirrors), and a method of admitting IR pump radiation and emitting FIR radiation. A grating-tuned CO2 laser (emission in the 9 - 11 µm range) is typically used to pump the THz laser. This pump radiation is often admitted into the THz cavity through a small input-coupling-holeOptically-Pumped THz Laser THz Laser Cell Grating-Tuned CO 2 Laser THz Output Laser Gas @ ~ 200 mTorr Figure 1: Schematic diagram of a general OPTL system. in one end mirror. The THz radiation produced in the laser is then typically emitted through either an output-coupling-hole or some sort of uniform output coupler.1 To understand how this device produces THz radiation, one must examine the quantum-mechanical molecular processes, which take place. The present section provides only a very general overview of these processes; more thorough discussions are available in the literature.2 9.69 µm (31 THz)FIR118.8 µm (2.5 THz)CO 2{{Lowest Vibrational ManifoldFirst Excited Vibrational ManifoldMethanol (CH OH)3J = 16J = 15027 018nτK2.5 THz Transition Figure 2: Schematic energy diagram of 2.5 THz methanol laser. All optically-pumped THz lasers operate on molecular rotational transitions. For purposes of illustration we will consider a specific THz laser example for explanation. A representative diagram of the operation of an THz laser, operating on the 118.83 µm line in Methanol, is presented in Figure 2, an illustration of the origin of the methanol quantum numbers is presented in Figure 3, and a n - torsionalJ - rotational sublevelvibrational level (C - O stretch)K - projection of J in molecular frameτ - splitting of J due to oscillatory orbital motion Figure 3: - Illustration of methanol quantum numbers, the top “large” atom is oxygen, the bottom “large” atom is carbon, and the rest are hydrogen. physical diagram of the lasing process is presented in Figure 4. In the lasing process: 1) an infrared photon with an energy which very closely matches a Molecule loses C-O stretch energy after emitting FIR(predominately via collisions with waveguide walls)9.69 µm (31 THz)CO 2FIR118.8 µm (2.5 THz)J = 16J = 16J = 15Physical PictureMolecule loses C-O stretch energy after emitting FIR(predominately via collisions with waveguide walls)9.69 µm (31 THz)CO 2FIR118.8 µm (2.5 THz)J = 16J = 16J = 15Physical Picture9.69 µm (31 THz)CO 2FIR118.8 µm (2.5 THz)J = 16J = 16J = 15Physical Picture Figure 4: Physical representation of the 2.5 THz lasing process in methanol. transition from a particular rotational state in the ground vibrational manifold to a rotational state in an excited vibrational manifold is absorbed by a gas molecule, 2) if the conditions are correct this process causes a population inversion between rotational states,* 3) the inverted rotational transition lases and emits in the THz, 4) the molecule is left in the excited vibrational manifold and must return to the * either in the excited manifold due to the pumping, or in the lower manifold due to depletion of the lower stateground manifold before it can participate in a continuous-wave lasing process again. The lasing process of Figure 2 is further illustrated in Figure 4. In this example, the 9.69 µm infrared photon excites the C-O stretch mode. The molecule then lases between the J=16 and J=15 rotational levels, emitting a photon at 118.83 µm. With the large energy difference between the rotational and vibrational energy level separations, one might expect the lasing process to be quite inefficient. This is in fact the case. The majority of the pump radiation is simply converted to heat. The theoretical limit on


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CALTECH EE 243A - OPTICALLY-PUMPED THz LASER TECHNOLOGY

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