A Discriminator Controlled Broad-band Optical Microwave Source

The difference frequency of two lasers is stabilised by an optical discriminator to provide a low noise source programmable from 27 to 40 GHz. A novel phase noise measurement technique is described using the optical discriminator itself. Introduction A paper' was presented at the 1996 lntemational Topical Meeting on Microwave Photonics which described a microwave source consisting of two lasers having their difference frequency stabilised by an optical discriminator. Measurements of phase noise, for an output frequency of GOOMHz, were included in that paper to demonstrate the phase noise improvement which could be obtained with the optical discriminator. The paper postulated that the phase noise spectrum is independent of output frequency and results confirming this were presented orally at the above meeting. This can be seen from the two laser signals: E1 = exp(jolt + a) (1 1 E2 = exp(jo2t + p) (2) The phase perturbations a and p give rise to phase noise sidebands. When the laser signals are combined and the difference frequency, (01 o2), is recovered using an optical detector, a and p will appear on the detector output, irrespective of the value of the difference frequency. This contrasts with quartz crystal derived microwave sources where the level of phase fluctuations and the concomitant sidebands rises rapidly with the order of frequency multiplication. The orally reported results extended to 20GHz, at which frequency the phase noise performance of the laser source was essentially the same as a Hewlett Packard 83711A Frequency Synthesiser. However these measurements were so close to the measurement floor of the instrumentation that they could not be regarded as absolute and simply confirmed the equivalence of the laser source to a high quality synthesiser. This problem is exacerbated at higher frequencies and a new measurement technique has therefore been devised and is described below. A two laser microwave source is now described which covers the frequency range of 27 to 40GHz, The actual frequency range evaluated being defined by the microwave components used in this particular experimental configuration and not the optics. The original phase noise measurements' were made using a Hewlett Packard 8565E Spectrum Analyser with an 85671A Phase Noise Measurement Utility. As this was unsuitable for measurements at frequencies up to 40GHz an alternative approach was used. Since the optical discriminator provides a reference for reducing phase noise sidebands it also provides a very convenient means for the measurement of these components. These appear at the discriminator output as the demodulated FM noise spectrum which can be readily converted to the phase noise spectrum of the two laser microwave source. This technique has the added advantage of yielding much higher sensitivities than the spectrum analyser approach. This has allowed much more accurate measurements to be made at offset frequencies above 1 OKHz, where the measurement sensitivity is greater than -1 40dBdHz compared with -1 1 OdBclHz for the spectrum analyser method. It has been checked that this system is consistent with the spectrum analyser at lower frequencies. This technique for phase noise measurement could readily be adapted for use with conventional low noise microwave sources. Svstem Description Figure 1 shows the elements of the two laser source. The outputs of Lasers 1 and 2 are combined in an optical coupler to produce an intensity modulation having a frequency equal to the laser difference frequency. A polarisation controller in series with the output of Laser 1 allows the index of this intensity modulation to be optimised. The output of the optical coupler is applied directly to Detector 2 and via a fibre-optic delay line to Detector 1 to recover the laser difference frequency. If the delay of the fibre-optic delay line is Td seconds, then the relative phase of the two detector outputs will vary at a rate equal to 2xTd rads/Hz as the difference frequency is varied. The phase sensitive detector, (PSD), senses this phase difference to produce a feedback signal which is applied to the control loop amplifier to produce an error voltage to correct the frequency of Laser 2. The control loop will therefore improve the stability of the laser difference frequency towards a value determined by the discriminator control characteristic. The degree of improvement is determined by the excess open loop gain in the control loop. The PSD output and hence the discriminator output will be a replica of the PSD transfer characteristic and will repeat every lmd in laser difference frequency. If the PSD gain equals Kp voltdradian and the static discriminator gain is KO volts/radian/sec, then, & = KpTd volts/rad/sec (3) This assumes the PSD has a sinusoidal characteristic and the inputs are in quadrature. Control LOOD Desian The open loop frequency response of the control loop, G(jw)H(jw), is given by: G(jo)H(jo) = KI&&e-l”TdGl(j~t) (4) Where: K1 = loop amplifier gain Laser 2 Gl(jo) = frequency response of loop filter. = tuning sensitivity of The following filter configuration was used for Glow) in the experimental work: Gl(jw) = (1 + joTI)(l + joT3) (5)