720 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 13, NO. 7, JULY 2001Optoelectronic Synthesis of Milliwatt-LevelMulti-Octave Millimeter-Wave Signals Usingan Optical Frequency Comb Generator and aUnitraveling-Carrier PhotodiodeS. Fukushima, Member, IEEE, C. F. C. Silva, Student Member, IEEE, Y. Muramoto, and A. J. Seeds, Fellow, IEEEAbstract—Millimeter-wave signals were synthesized using anoptical frequency comb generator and a unitraveling-carrier pho-todiode. We obtained signals from 10 to 60 GHz with a maximumpower of 3 dBm at 60 GHz without the use of electrical amplifica-tion.Index Terms—Injection-locked lasers, photodetectors.I. INTRODUCTIONAHIGH-POWER tunable millimeter-wave synthesizer isof great interest for millimeter-wave applications such asmeasurements of newly developed devices and fiber-fed radiosystems. Optical frequency comb generators (OFCG) [1] arecandidates as sources for millimeter-wave fiber-fed wirelesssystems as well as dense wavelength-division-multiplexed(DWDM) systems. The amplified fiber loop OFCG can offera wide comb span with adjustable exact comb line spacing.Generation of more than 100 lines over a 1.8-THz bandwidthin the 1.55-m band has already been demonstrated [2]. An op-tical injection-locking scheme can be used to select individuallines from the comb. Using two injection-locking systems, twolines can be separated by an integral multiple of the originatingreference signal frequency. The generation of millimeter-wavesignals by this technique has been reported, though the outputpower was low (47 dBm) and the frequency limited to 40GHz by the characteristics of the photodetector [3], [4].With ahigh-speed high-saturation power photodiode,the mil-limeter-wave synthesizer described previously could expand thepower and frequency to find wider applications. An attractivedevice is the unitraveling-carrier photodiode (UTC-PD) [5]. Inthe UTC-PD, the light is absorbed in the p-region so that onlyfast electrons act as carriers but the generated holes remain inthe p-region as majority carriers. As a result, its response is veryManuscript received January 22, 2001; revised March 28, 2001. This workwas supported in part by the U.K. EPSRC PHOTON project and in part by theBrazilian Research Council CNPq.S. Fukushima is with the Department of Electronic and Electrical Engi-neering, University College London, London WC1E 7JE, U.K., on leave fromthe NTT Photonics Laboratories, Atsugi, Kanagawa 243-0124, Japan ([email protected]).C. F. C. Silva and A. J. Seeds are with the Department of Electronic andElectrical Engineering, University College London, London WC1E 7JE, U.K.Y. Muramoto is with the NTT Photonics Laboratories, Atsugi, Kanagawa243-0124, Japan.Publisher Item Identifier S 1041-1135(01)05579-3.Fig. 1. Block diagram of the millimeter-wave synthesis system.fast, with reported 3-dB bandwidth of 310 GHz [6], [7]. Due toits band structure, the saturation power is much higher than aconventional pin photodiode. An output voltage of 1.5 Vfora 50-load was reported, which is equivalent to a power of 7.5dBm [8].By applying selected comb lines from the OFCG to theUTC-PD through an erbium-doped fiber amplifier (EDFA), wesucceeded in experiments on multi-octave tunable, high-powerdirect optoelectronic millimeter-wave generation. The max-imum power we achieved was 3 dBm, at a frequency of 60GHz without any electrical amplification. Unlike conventionalelectrical oscillators, the UTC-PD can generate frequenciesover a multi-octave span with good power uniformity. A spanof 10–60 GHz with output power flatness of1.3 dB wasachieved in our experiment.II. PRINCIPLEThe configuration for millimeter-wave synthesis is shownin Fig. 1. It includes an OFCG, two injection-locked lasers aswavelength filters, an EDFA, and a UTC-PD. The OFCG emitsa comb with arbitrary line spacing equal to the microwavereference frequency,, driving the phase modulator in theamplified fiber loop. From more than 100 lines, each injec-tion-locked laser selects only one line. The two laser outputsare combined and amplified by the EDFA. The differencefrequency is an integral multiple of the microwave referencefrequency( : integer). These amplified laser signalsare incident on the UTC-PD and converted to the requiredmillimeter-wave signal of frequency. This systemenables milliwatt-level millimeter-wave generation withoutneeding an expensive broad-band millimeter-wave amplifiersince the signal amplification is performed optically.1041–1135/01$10.00 ©2001 IEEEFUKUSHIMA et al.: OPTOELECTRONIC SYNTHESIS OF mW-LEVEL MULTI-OCTAVE MILLIMETER-WAVE SIGNALS 721Fig. 2. (a) Experimental arrangement. (b) Typical spectra from the OFCG and the two lasers. (c) Spectrum example of the generated millimeter wave signal.As for the frequency limits, recently reported UTC-PDshave a 3-dB bandwidth of 310 GHz [7], sufficient for mostmillimeter-wave applications.III. EXPERIMENTSExperiments were carried out using the arrangement shownin Fig. 2(a). A tunable laser was employed as a reference laserwith wavelength 1555.9 nm. A 10-GHz reference signal wasapplied to the phase modulator and the OFCG generated morethan 40 lines with 10-GHz spacing and 400-GHz span. Weused a conventional DFB laser diode (DFB-LD) and a MarconiCaswell Limited sampled-grating DBR-LD (SG-DBR-LD)[9] as the wavelength filters. The DFB-LD could be lockedto any line in the OFCG output by adjusting the current andtemperature, but the current tuning range is not large. On theother hand, the SG-DBR-LD provides a current tuning rangewider than 60 nm for multi-octave millimeter-wave generationapplications. Typical spectra from the OFCG and combinedlaser outputs are shown in Fig. 2(b). The two selected linesare combined and then amplified by an EDFA. Finally theoutput light from the EDFA is input to the UTC-PD, where thehigh-power millimeter-wave signal is generated. The UTC-PDemployed in our experiments has an edge-coupling interface[8]. Its junction area wasm. It has the followingcharacteristics: 3-dB bandwidth ofGHz and responsivityof 0.4 A/W at a bias voltage of4V.The UTC-PD was not packaged;hence, the measurement wasdone as follows. The EDFA output was coupled to the side of theUTC-PD with a lensed-fiber with a coupling efficiency of 50%.The detected signal at 10-to-50 GHz was introduced to a spec-trum analyzer through a ground-signal-ground coplanar probe.In addition, a preselected mixer was employed for the 60-GHzmeasurement. The probe had losses of 0.28 dB