Integrated optical circuit for effecting stable injection locking of laser diode pairs used for microwave signal synthesis

#The present invention relates to the generation of microwave signals. More specifically, this invention relates to an integrated optical circuit that enables the stable injection locking of two distributed feedback (DFB) diode lasers, whose outputs can be converted into microwave signals.

#Frequency synthesis is used to generate 'signals at one or more precise frequencies. These signals may then be used to perform frequency conversion in radio frequency (RF) sensor and communication systems. Frequency synthesis may be provided by several different methods. Of concern in frequency synthesis, are the phase, frequency and amplitude stability of the generated signal. Since the generated signal may be used as a local oscillator signal for frequency up-conversion or down-conversion, instability in the signal results in decreased signal-to-noise performance.

#One method of frequency synthesis involves the generation of a multiple tone lightwave signal that can be converted into a RF carrier or local oscillator signal. In this method, optical heterodyning is used to create a sum or difference beat frequency from two optical wavelength tones. The sum or difference beat frequency is detected by a photodetector or similar apparatus to generate an RF carrier or local oscillator signal. However, the stability of the beat frequency signal is limited by the relative stability of each of the optical wavelength tones.

#FIG. 1 shows a photonic synthesizer according to the prior art;

#FIG. 2a shows an integrated optical circuit according to the present invention;

#FIG. 2b shows a photonic microwave synthesizer based on the integrated optical module according to the present invention;

#The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

#An apparatus for generating a microwave signal by optical heterodyning that has improved phase and amplitude stability according to a first embodiment of the present invention is described with reference to FIG. 2a, FIG. 2b, and FIG. 3, blocks 2-28. This apparatus comprises an integrated optical circuit, illustrated in FIG. 2a. Use of the integrated optical circuit to form a photonic synthesizer of microwave signals is illustrated in FIG. 2b. In this apparatus, an integrated optical circuit 201 couples optical energy from a master laser 203 to a first and second slave laser 216, 218. The integrated optical circuit 201 contains two ports 219, 221 at which the first and second slave lasers 216, 218 are butt coupled to the integrated optical circuit 201. The first and the second slave laser 216, 218 each receive the optical energy.

#The first slave laser 216 produces a first laser output based on the physical characteristics of the first slave laser 216 and the received optical energy. The second slave laser 218 produces a second laser output based on the physical characteristics of the second slave laser 218 and the received optical energy. The first and second laser outputs are then combined in a directional coupler 222 creating a primary and a secondary combined laser output. The primary combined laser output is sent to at least one primary output port 220 where the primary combined laser output can be used for heterodyning. The secondary combined laser output is preferably also sent to one or more secondary output ports 230 on the integrated optical circuit 201, where the secondary combined laser output can be used for monitoring or feedback control.