NASA and NASA funded missions/instruments such as Aura (EOS CH-1)/MLS (Microwave Limb Sounder), SOFIA/GREAT and STO have demonstrated the need for local oscillator (LO) sources between 30 and 300 um (1 and 10 THz). For observations >2 THz, technologically mature microwave sources typically have microwatt power levels which are insufficient to act as LOs for a heterodyne receivers. LongWave Photonics is proposing to develop a compact, frequency agile, phase/frequency locked, power stabilized, single mode quantum cascade laser (QCL) system with > 2mW power output. The system includes distributed feedback grating (DFB) QCL arrays packed with multiple devices on a single semiconductor die with individual devices lasing at different frequencies. The source will be frequency agile over 150 GHz with center frequencies ranging from 2 to 5 THz range. The DFB QCL array will be packaged in a high-reliability Stirling cycle cooler. The source will be phase/frequency locked to a stable microwave reference synthesizer which allows continuous phase-locking ability over the THz laser tunable range with <100 kHz line width. The proposed system will be able to provide sufficient power for an LO at > 2 THz, with reduction of LO linewidth, and absolute frequency accuracy and with output power stabilized to reduce system noise. The whole system will be in a compact package which can be further reduced for a flight instrument.