A radio-interferometric array usually consists of several spatially separated antennas observing the same celestial source and equipped with identical receivers. A high-resolution source image is produced by combining the receiver outputs, simulating a single larger antenna with an effective diameter set by the largest array element separation. Each element uses a heterodyne radio receiver, which produces a low-frequency replica of a radio signal by differencing it with a reference signal from a local oscillator (LO). The replica is easier to work with and carries all of the information of the original signal: frequency, phase, and amplitude. To effectively combine outputs from the array receivers and faithfully preserve signal information, each local oscillator must produce the required frequency with known phase.
For use at the submillimeter wavelengths observed with, for example, Smithsonian's Submillimeter Array (SMA),1 there are two main approaches for generating LO power sufficient to operate heterodyne receivers. One approach employs an InP Gunn oscillator operating at about 100GHz, followed by varactor diode frequency multiplier. The other approach uses a YIG (yttrium iron garnet) oscillator operating at about 20GHz, followed by a cascade of varactor multiplier and amplifier pairs. However, both approaches have shortcomings. The availability of suitable InP devices is limited, and amplifier diode frequency multiplier pairs suffer from high cost and large, broadband output power variations.
We describe here an approach based on beating two IR lasers in a photonic mixer. Previous attempts using this method have lacked the spectral purity and phase stability required to operate an interferometric array. At the Smithsonian Astrophysical Observatory, we have developed a 200–230GHz local oscillator, mostly using commercially available 1550nm laser communication components (see Figure 1).
Figure 1. Shown is a side view of the laser-based millimeter-wave local oscillator.
Figure 2 shows the basic configuration of our photonic LO. The optical path consists of a single 1550nm diode laser, a lithium niobate optical-phase modulator, a Mach-Zehnder interferometer (MZI) with a free spectral range of 75GHz and a 160—260GHz wavelength band, and a photomixer whose output is connected to a horn antenna. All of the optical devices and connections are polarization maintaining. The photomixer was designed and fabricated at the Central Laboratory of the Research Councils in the United Kingdom.2 The electrical path consists of a 15—20GHz YIG synthesizer, a frequency doubler, and a power amplifier connected to the RF port of the phase modulator.
Figure 2. Schematic of laser-based local oscillator showing optical and electrical paths. MZI: Mach-Zehnder interferometer. X2: Frequency doubler. YIG: Yttrium iron garnet synthesizer.
In our scheme, output from the laser is phase-modulated at twice the YIG frequency, then converted to an amplitude-modulated signal by the MZI. The photomixer, placed at one port of the MZI, generates a frequency comb spaced at the modulating frequency. Finally, output from the photomixer passes through a short waveguide section to cut off frequencies below 175GHz, and then to a pyramidal horn that couples radiation to the receiver.
The phase of the output radiation from the photonic LO is stabilized using a standard phase-lock system. A lower-frequency component of the radio comb, generated by a separate commercial photomixer placed at the second output port of the MZI, passes to a harmonic mixer, where it beats with the microwave reference of the interferometer. The output of the harmonic mixer is used to operate a phase-lock circuit that stabilizes the YIG oscillator's phase. Theoretical and experimental work show that the laser adds negligible phase noise to this photonic LO system, and that spectral purity and phase stability are similar to those based on Gunn oscillators.
Test observations at the SMA in Hawaii confirm that the photonic LO operates with adequate phase and frequency stability for radio interferometry. With the photonic LO incorporated into one element of a five-antenna subarray, we used the SMA to detect several quasars and image the ultracompact HII region G138.295+1.555.3 We believe that this was the first time that a photonic local oscillator has been successfully used for radio interferometry.