21ST INTERNATIONAL SYMPOSIUM ON SPACE TERAHERTZ TECHNOLOGY, OXFORD, 23-25 MARCH, 2010 Copyright 2010 California Institute of Technology. Government sponsorship acknowledged. x 3 x 3x 3 x 3Development of Local Oscillators for CASIMIR R. Lin, B. Thomas, J. Ward1, A. Maestrini2, E. Schlecht, G. Chattopadhyay, J. Gill, C. Lee, S. Sin, F. Maiwald, and I. Mehdi Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91101, USA. Email: [email protected] 1 now with Raytheon Company, Fort Wayne, Indiana 2 Observatoire de Paris, LERMA, University P&M Curie, Paris-VI. 77 Avenue Denfert-Rochereau 75014 Paris, France Abstract— We present the development of three local oscillator chains to be used on the CASIMIR (Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver) instrument onboard the SOFIA (Stratospheric Observatory for Infrared Astronomy) aircraft. All three chains use all solid-state GaAs-based components to amplify and multiply a ~1-3 mW input signal at W band. At room temperature, the 900 GHz source produces 50-100 !W of power from 800 to 930 GHz. The 1 THz source produces 50-120 !W of power from 960 to 1045 GHz. The 1.4 THz source produces 10-70 !W of power from 1320 to 1470 GHz. When cooled to 120 K, the 1.4 THz chain’s output power increases by approximately 3 dB with a peak power of 129 !W at 1395 GHz. Index Terms – CASIMIR, local oscillator, terahertz source, frequency multiplier, cascaded multipliers, GaAs Schottky diode, submillimeter wavelengths. I. INTRODUCTION The Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver (CASIMIR) is a multi-band, far infrared and submillimeter, high resolution, heterodyne spectrometer designed for high sensitivity observations of warm interstellar gas [1]. Multiple bands are being developed to study the transition lines of various molecular species. Of special interest are lines from H218O, H2D+, and N+ around 1 THz and 1.4 THz. The detectors for the receivers use advanced Superconductor-Insulator-Superconductor (SIS) mixers, pumped by solid-state local oscillator (LO) sources. We present here the development and characterization of three LO sources which cover 800-930 GHz, 970-1040 GHz, and 1320-1470 GHz, respectively. The first two sources will serve as local oscillators for the spectroscopic lines in the 800-1040 GHz range [2], while the 1.4 THz source will enable the study of the H2D+ line at 1.37 THz and the N+ line at 1.46 THz [3]. Each chain is composed of cascaded frequency multipliers driven by WR10 or WR8 power amplifiers. The multipliers are based on GaAs substrateless and membrane device technologies, which have been successfully demonstrated on the Herschel HIFI instrument [4],[5]. Measurements of SIS and HEB heterodyne mixers pumped by these LO chains prove that they are low noise and produce power at the correct frequency [6],[7]. II. 900 GHZ LO CHAIN The 900 GHz LO is driven by a few milliwatts of power in the 88-105 GHz range. Three cascaded power amplifiers amplify the input signal to ~100 mW. This is then followed by a wideband isolator, built by Millitech, and two stages of frequency triplers as shown in Fig 1. Fig.1: Schematic block diagram and photo of the 900 GHz LO chain, including power amplifiers, an isolator, two stages of frequency triplers, and a corrugated output horn. The first stage tripler provides output power in the 265-325 GHz range. Its circuit is based on a GaAs Monolithic Membrane Device (MoMeD) with beamleads for electrical connections and handling [8]. The tripler, previously described in [9], has an efficiency of about 7-9% across the band, with average output power ~7-9mW. The second stage tripler (M2) uses a 3 !m GaAs MoMeD with 4 anodes in a balanced configuration. Its design and characterization were previously reported in [10]. The output power of the complete chain was measured using an Erickson PM2 calorimeter by removing the corrugated output horn and attaching a custom-made 300x150 !m to WR10 waveguide transition. At fixed bias voltages for both tripler stages, there is a strong standing wave due to the lack of isolation between the two triplers. By tuning the bias voltages of the frequency multiplier diodes, we were able to obtain a smooth frequency vs. power sweep that still meets the required minimum 50 !W of output power across the full 800-930 GHz band. Figure 2 shows the results of the measurements, uncorrected for the loss due to the waveguide transition.21ST INTERNATIONAL SYMPOSIUM ON SPACE TERAHERTZ TECHNOLOGY, OXFORD, 23-25 MARCH, 2010 Copyright 2010 California Institute of Technology. Government sponsorship acknowledged. CASIMIR 1 THz Local Oscillator Chain020406080100120140950 960 970 980 990 1000 1010 1020 1030 1040 1050 1060 1070Output Frequency (GHz)Output Power (µW)RequirementOutput Power Fig.2: Performance of the 900 GHz LO Chain at room temperature. At fixed bias conditions, a standing wave pattern exists in the frequency sweep. By tuning the bias voltages properly, the nominal output power shown in dark blue is obtained. III. 1 THZ LO CHAIN The 1 THz LO is driven by a few milliwatts of power in the 107-117 GHz range. Four cascaded power amplifiers amplify the input signal to ~80-100 mW. This is then followed by two stages of frequency triplers as shown in Fig. 3. Due to the lack of a low-loss wide bandwidth isolator at WR8 frequencies, no isolation exists between the amplifier and the first stage tripler. The final stage tripler has a diagonal horn integrated into the block described in [11]. The CASIMIR horn is scaled such that the dimension of the square on the output flange is 1.86 mm. Fig.3: Schematic diagram and photo of the 1 THz LO Chain, including four power amplifier stages and two stages of frequency triplers. The first stage tripler gives output in the 325-350 GHz range. Its circuit is a scaled version of the 265-325 GHz tripler used in the 900 GHz LO chain. This tripler produces