 Ultra-Wideband Photonic Millimetre-Wave Synthesizers with Coaxial (DC-110GHz) and Rectangular Waveguide (69-112GHz) Output Ports
Introduction
A promising new concept for the generation of frequency-tunable cw-signals with low phase-noise is the photonic synthesizer based upon optical heterodyning. In comparison to conventional purely electric sources, a photonic synthesizer features ultra-wideband and high-frequency operation in the millimetre-wave (mmw) range. Because of these advantages, the photonic synthesizer concept can potentially become an alternative signal generation technology to existing all-electronic solutions for a number of applications including instrumentation, communications and sensing/radar. A current study launched by the European Space Agency (ESA) concluded that photonically based mmw or THz-frequency generation by using advanced photodetectors is even one of the most suitable candidates for the future [1]. In this paper, two different photonic mmw synthesizers are presented. The system allows tunability from DC to 110GHz with a flat frequency response and high output power levels of up to -3.23dBm. In addition, the synthesizer features a coaxial output (W1). Flatness and bandwidth are superior compared to commercially electronic mmw generators [2,3]. The second synthesizer is working in a frequency range exceeding the W-band with an improved flatness and an increased output power up to 0dBm. To the best of our knowledge, these presented synthesizers are the first photonic mmw synthesizers offering such wideband tunability.
Optical Heterodyning
In order to tune the output frequency of the investigated pin photodiode, an optical heterodyne laser setup was used to generate frequency-tunable and low phase noise cw signals. Here, a laser with a fixed output wavelength at 1.55µm and a tunable laser with attached polarisation control were employed. The output current of the heterodyne mmw signal generated by the photodiode is described as [4]
 ,
where fc and Dj denote the difference frequency and the difference phase of the two constituent optical input waves, respectively. The DC and RF responsivities of the photodiode are represented by s0 and sfc. The concept of optical heterodyning by using two single, independent lasers can be realized comparatively easy. A limiting factor of this arrangement is the achievable level of phase noise and phase stability which confines output power level as well as stability and line width of the generated mmw signal. Among using a phase-locked-loop within the setup, optical-injection locked lasers, mode-locked lasers or external modulators can be used to realize ultra-low phase noise and high phase stability, which is for instance investigated within the European IPHOBAC project [5].
Ultra-Wideband Photonic mmw-Synthesizer
The elements of the synthesizer are the above-named optical heterodyne setup and a high-power pin photodiode [6], which is further coupled to a coaxial W1 output (see Fig. 1). This enables a full frequency span from DC to 110GHz.
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Fig. 1: Schematic diagram of the photonic mmw synthesizer
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At first, the frequency behaviour within a frequency range from DC to 220GHz has been investigated (see Fig. 2). The measurement results from DC to 110GHz were achieved by using only calibrated devices. As can be seen, a flat and wideband frequency response was achieved with a total signal roll off of about 6dB and output power levels up to -3.23dBm. Considering a frequency range from 20 to 110GHz, total signal roll off is about 3dB. For frequencies larger than 110GHz, uncalibrated probes and mixers were used. Within a span of 110 to 220GHz, signal roll off is about 40dB. Another point of interest is the synthesizer’s output power versus applied bias voltage for different photocurrents at a fixed frequency of 97.8GHz (see Fig. 3). Providing the photodiode with a photocurrent of 10mA, the maximum output power is -4.88dBm, which would be already high enough for practical use. The generated RF-power can be controlled from -35 to -4.88dBm. The achieved results clearly outperform commercially available mmw sources regarding flatness and bandwidth which typically exhibit maximum output power variations of up to 12dB.
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Fig. 2: Wideband frequency response of the W1-coupled synthesizer at a reverse bias voltage of 3V and a photocurrent of 10mA |
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Fig. 3: Output power at 97.8GHz vs. reverse bias voltage at different photocurrent levels |
Photonic mmw Synthesizer for W-band Operation
Next, a limiting amplifier has been integrated in the synthesizer consisting of two stages and an interstage isolator. The mmw signal coupling is performed by WR10 rectangular waveguides (see Fig. 4).
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Fig. 4: Synthesizer with integrated amplifier and waveguide (WR10) coupling
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Corresponding to Fig. 5, the frequency response is extremely flat with a power fluctuation of less than 3dB within a frequency range from 69GHz to 112GHz.
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Fig. 5: Output power of the synthesizer vs. mmw frequency |
The total frequency tunability range is even larger than 50GHz. The power roll off for frequencies lower than 69GHz can be contributed to the lower cut-off frequency of the WR10 waveguide and the reduced gain of the amplifier. The roll off for frequencies above 112GHz is based on the reduced gain of the amplifier as well as the lower efficiency of the photodiode. Further point of interest is the output power tunability (see Fig. 6). For low photocurrents, the output power is approximately linear tunable within several magnitudes. Photocurrents in the µA-range furthermore allow a control over the entire output power range.
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Fig. 6: Output power of the synthesizer versus input power for different frequencies |
For the above presented results no phase-locking system had been employed. Thus the phase noise and phase stability of the generated signal is mainly determined by the stability of the wavelength-tunable laser in the two-mode laser set-up and it is not optimum yet. Experiments employing a new optical locking concept and advanced optical sources providing a much lower phase noise and a high frequency resolution are currently being carried out.
References
[1] B. Leone et al., "Optical Far-IR wave Generation - An ESA review study", Proceedings 14th International Symposium on Space Terahertz Technology, Tucson, USA, April 2003.
[2] See www.agilent.com
[3] See www.hp.com
[4] A. Stöhr et al., "Optical Generation of Millimeter-Wave Signals," in Microwave Photonics, S. Iezekiel (Ed.), John Wiley & Sons Ltd., ISBN: 0470848545, 2007.
[5] See www.ist-iphobac.org
[6] D. Trommer et al., "Above 100 GHz performance of waveguide integrated photodiodes measured by electro-optical sampling", Optical Fiber Communications Conference OFC 2003, 23-28 March 2003, pp. 343 – 345, vol.1, 2003
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