Abstract—A novel technique has been developed to generate ultra-stable millimeter-wave signal by optical heterodyning of the output from two slave laser (SL) sources injection-locked to the sidebands of a frequency modulated (FM) master laser (ML). Precise thermal tuning of the SL sources is required to lock the particular slave laser frequency to the desired FM sidebands of the ML. The output signals from the injection-locked SL when coherently heterodyned in a fast response photo detector like high electron mobility transistor (HEMT), extremely stable millimeter-wave signal having very narrow line width can be generated. The scheme may also be used to generate ultra-stable sub-millimeter-wave/terahertz signal. Keywords—FM sideband injection locking, Master-Slave injection locking, Millimetre-wave signal generation and Optical heterodyning. I. INTRODUCTION TRINGENT requirements from various emerging applications like broadband wireless access network for pico-cell mobile communication, software-defined radio, space-based phased array antenna etc. demand optical technique for generation of millimeter-wave/terahertz signals. The primary advantage of signal generation by optical technique, i.e. by optical heterodyning of laser signals, is the possibility of realizing low phase-noise (i.e. small time jitter) of the generated millimeter-wave/terahertz signal. In general, laser sources are coherent sources but two different laser sources will not be mutually coherent. Thus if two different laser sources are heterodyned then there is no phase correlation between the mixing waves, and hence the overall phase noise of the difference frequency signal will be the sum of the phase noise of the mixing waves. However, if we heterodyne two phase-coherent signals in a non-linear device then the effective phase-noise of the difference signal happens to be equal to the difference of the phase-noise of the two individual beating signals [1]. We have used this principle to generate ultra-stable i.e. low phase-noise millimeter-wave signal. The scheme consists of a mode-locked master laser and two slave lasers which are injection-locked to the desired FM sidebands of the frequency modulated master laser, as shown in Fig. 1. The signals from the two injection-locked slave Subal Kar, Soumik Das and Antara Saha are with the Institute of Radio Physics and Electronics, University of Calcutta, India (e-mail: subal.kar@fulbrightmail.org). Madhuja Ghosh is with the SAMEER Kolkata Centre, India (e-mail: madhuja89@gmail.com). lasers are then heterodyned in a fast response photo detector (PD), which in our case is a HEMT device, that is followed by a suitable filter (not shown in the figure) to select out the desired beat (i.e. the difference) frequency as millimeter-wave signal. As indicated in the figure, the scheme may as well be realized if the two FM sidebands of the modulated master laser after being processed through optical phase-lock loops (OPLL) are heterodyned in the photo detector to produce the millimeter-wave signal. Fig. 1 The scheme for generation of ultra-stable millimeter-wave signal II. ANALYTICAL BASIS OF THE SCHEME To accomplish FM sideband injection locking, the master laser (ML) is frequency modulated (FM) by a RF signal at a frequency, f m . The field amplitude E(t) of the FM modulated ML output is given by: )} ( ) 2 cos( 2 { 0 0 ) ( t t f t f j m e E t E ϕ π β π + + = (1) where φ(t) represents the random phase fluctuations of the optical carrier. β = f d /f m is the modulation index (f d being the maximum frequency deviation) and f 0 is the master laser frequency with no modulation. The optical frequency of slave laser I can be adjusted by varying its heat-sink temperature (thermal tuning) and DC drive current so that it coincides with the +2 sideband of the ML at (f 0 + 2f m ), while the output frequency of slave laser II is accordingly made to coincide with the – 2 sideband of the ML at (f 0 – 2f m ). Decomposing the FM portion of (1) into its Fourier components we have: ) ( ) ( 2 0 ) ( ) ( t j n n t nf f j n e e J t E m ϕ π β ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ∝ ∑ +∞ = −∞ = + (2) where Jn are the Bessel functions of first kind, of order n. The Optical Heterodyning of Injection-Locked Laser Sources — A Novel Technique for Millimeter-Wave Signal Generation Subal Kar, Madhuja Ghosh, Soumik Das, Antara Saha S World Academy of Science, Engineering and Technology International Journal of Electronics and Communication Engineering Vol:8, No:7, 2014 1048 International Scholarly and Scientific Research & Innovation 8(7) 2014 scholar.waset.org/1307-6892/9998841 International Science Index, Electronics and Communication Engineering Vol:8, No:7, 2014 waset.org/Publication/9998841