1. Introduction

Various optical techniques have been of great interest on demonstrating tunable photonic microwave (MW), millimeter-wave (MMW), and submillimeter-wave (SMMW) signals in recent years owing to many potential applications in wireless communication systems [1–3], collision avoidance radars [4–6], biology and medicine [7, 8], astronomy [9], and sensors [10]. As compared to the traditional electronic method, optical photonic signal generators have prominent advantage in terms of large tunable frequency range, which are proposed by various research groups based on semiconductor lasers [11–15], optical fiber lasers [16–18], nonlinear photonic crystal waveguides [19], silicon material [20], optical amplifiers [21–23], and waveguides [24, 25]. Based on previous reports, semiconductor based multi-mode lasers which have broad spectral region, low cost, compatibility, small size, and low power consumption, are continually adopted to generate dual-wavelength resonance at the output. The dual-wavelength resonance is generated by means of external dual-beam injections which are used to obtain beat signals with various frequencies [26]. Recently, the quantum-dot-distributed semiconductor laser has been intensively researched and experimentally demonstrated to generate tunable photonic signals in which beat frequency is varied from several GHz to about one THz by injecting the external light into various residual modes, and judiciously adjusting the input power [11]. Undoubtedly, the proposed techniques for photonic signal generation have some competitive abilities and advantages of integration and size, but also face some limitations and challenges such as high power consumption, high bias current, narrow tunable range, and high cost in real time applications. Hence, we proposed and demonstrated tunable photonic signal generator using single-mode Fabry-Pérot laser diode (SMFP-LD) which provides prominent advantages of ultra-broadly tunable range, balanced peak power, and stable emission compared to other photonic signal generators. The beat frequency of our proposed scheme ranges from microwave, millimeter-wave to submillimeter-wave, which can be easily achieved by shifting the feedback wavelength. The SMFP-LD used in this experiment has built-in external cavity which has been used in wavelength conversion [27], switching [28], modulation [29], logic gates [30, 31], and flip-flop [32] over last decade, owing to its significant characteristics such as simple configuration, single-mode emission, high side-mode suppression ratio (SMSR), self-locking, low power consumption, low cost, low bias current, and broadly tunable range compared to other semiconductor lasers. Using these prominent properties of SMFP-LD, photonic generator is experimentally demonstrated in which an external feedback beam, from output spectrum, is again injected into the SMFP-LD by external feedback cavity. As a consequence, dual-wavelength resonance can be observed at the output, where the beat frequency between two resonance wavelengths can be easily extended from about 30 GHz to well over 3 THz by injecting proper wavelength in SMFP-LD through the external cavity.

In section 2, we discuss experimental setup and operation principle for the proposed scheme. In section 3, we present the experimental results and discussion for generated photonic signals with various beat frequencies. The final section concludes the proposed scheme of photonics signal generation.

2. Experimental setup and operation principle

The experimental setup of proposed scheme for generating photonic signals is illustrated in Fig. 1, in which various elements including SMFP-LD, optical spectrum analyzer (OSA), tunable filters (TF) with tunable wavelength range of 40 nm (1530 nm-1570 nm) and bandwidth of 0.5 nm ± 0.1 nm, Erbium-doped fiber amplifier (EDFA), polarization controller (PC), optical circulator, and 50/50 fiber coupler are used. The SMFP-LD used in the experiment has the single dominant mode under normal condition providing higher side mode suppression ratio. Single-mode Fabry-Pérot laser diode is obtained by modifying commercially available multi-mode Fabry Pérot laser diode (MMFP-LD) that has multi-mode spectrum output and free spectral range of 1.12 nm. The inclination of 6° present on optical coupling fiber in MMFP-LD is eliminated by cutting to zero degree, which forms an external cavity that provides only one single longitudinal mode with high amplitude, suppressing amplitude of other side modes present in MMFP-LD [33]. The dominant mode can be tuned to another wavelength by varying operating temperature and has the tunable range of about 10 nm. In Fig. 2, SMFP-LD is operated with the bias current of 25 mA and 22.7°C operation temperature. Under this condition, SMFP-LD has the dominant emission mode at the wavelength of 1539.8 nm with the SMSR of 32.1 dB.

 

Fig. 1 Experimental setup for photonic signal generation with SMFP-LD subject to an external cavity feedback. SMFP-LD: Single-mode Fabry-Pérot laser diode, OSA: Optical spectrum analyzer, EDFA: Erbium-doped fiber amplifier, PC: Polarization controller, TF: Tunable filter.

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Fig. 2 Output free-running spectrum of SMFP-LD.

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Based on the properties of free-running spectrum shown in Fig. 2, dual-mode emission with tunable frequency spacing can be achieved by properly selecting different feedback wavelength in the external cavity consisted of two tunable filters, EDFA, and PC as shown in Fig. 1, in which the output energy of SMFP-LD is equally divided by the fiber coupler. The half of output energy is extracted by OSA, and another half is fed back to SMFP-LD through external cavity. The power of feedback beam selected by TF₁ is effectively enhanced by EDFA and passes through PC and TF₂. The PC controls the optical polarization state to suppress the powers of other residual modes, which simultaneously suppresses the amplified noise and slightly tune the feedback power. TF₂ is used to select output wavelength from PC and further reduce the noise level. Finally, the feedback beam outputted by the external cavity is again fed back to the SMFP-LD to generate photonic signal. In order to achieve photonic signal with broad beat frequency, the dominant mode of SMFP-LD should be close to the edge of wavelength range of EDFA gain and filter. The feedback signal can be obtained by selecting any other wavelength except the dominant mode of SMFP-LD to generate high quality microwave, millimeter-wave, and submillimeter-wave signals by judiciously adjusting the filters, EDFA, and PC. The minimum frequency interval between two adjacent signals can reach about 10 GHz which corresponds to the wavelength difference of 0.08 nm.

3. Experimental results and discussion

Figure 3 illustrates photonic signals with different beat frequencies, which can be obtained by selecting proper system parameters of power and wavelength for the feedback beam. The resonating dual-wavelength exhibits nearly equal peak power level and high SMSR for all cases, and the parameters for SMFP-LD are same as those in Fig. 2. It is obvious that the feedback wavelength should be close to emission mode of SMFP-LD to obtain microwave signal that has minimum beat frequency of 30.4 GHz as illustrated in Fig. 3(a). To obtain the stable microwave signal, the feedback wavelength should be a little shorter than the dominant mode in the free-running spectrum so that other residual modes can be remarkably suppressed. As a result, the peak ratio between two resonance peaks is 0.36, and the corresponding SMSR ( = lowest peak power of two emission modes/highest peak of other residual modes) is as high as 36.6 dB for the case of 30.4 GHz beat frequency which is comparable to other research works [11, 34, 35], in which the corresponding SMSR ranges from around 20 dB to more than 40 dB. Obviously, based on the gain bandwidth of EDFA and filter bandwidth, the selected feedback wavelength should be tuned to longer wavelength to achieve high frequency microwave, millimeter-wave and submillimeter-wave signals which are, orderly, plotted in Figs. 3(b)-3(f). The peak ratio and SMSR are, respectively, less than 0.5 dB and more than 26 dB that should be attributed to judiciously adjusting the system parameters including power, and wavelength in the external feedback cavity. Compared to Fig. 3(a), it can be seen that the SMSR is less in other cases shown in Fig. 3 which is due to the residual modes of emission spectrum that are not thoroughly suppressed by the injected feedback beam.

 

Fig. 3 Output photonic signals with various beat frequencies. (a) 30.4 GHz, SMSR = 36.6 dB, (b) 40.4 GHz, SMSR = 32.1 dB, (c) 202.2 GHz, SMSR = 26.0 dB, (d) 1.52 THz, SMSR = 28.3 dB, (e) 3.18 THz, SMSR = 28.3 dB, (f) 3.40 THz, SMSR = 26.5 dB.

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In order to clearly display the peak ratio and SMSR of generated photonic signals, Figs. 4(a) and 4(b) plot the evolutions of peak ratio and SMSR as a function of beat frequencies. In measure, some feedback wavelengths are coarsely selected to achieve stable dual-mode resonance, whose frequency intervals are discontinuously varied from 30.4 GHz to 3.40 THz. As can be seen in Figs. 4(a) and 4(b), in all cases, the power ratio between two emission peaks is always less than 0.5 dB, and the SMSR is well over 25 dB. To measure the stability of achieved photonic signal, the peak ratio for arbitrary three cases with nearly equal peaks for photonic signals are measured for half an hour as shown in Fig. 5, in which one can see that the dual-mode resonance is very stable with time variation, and the corresponding peak fluctuation is always less than 0.19 dB that is remarkably lower than about 2 dB measured in previous result [36]. This verifies dual-wavelength emission with high SMSR and low peak difference can be achieved by selecting proper feedback beam. Here, it should be pointed out that the minimum beat frequency of achieved photonic signals is 30.4 GHz shown in Fig. 3(a).

 

Fig. 4 (a) Peak ratio and (b) SMSR with respect to beat frequency.

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Fig. 5 Peaks of dominant mode in red line with squares marks and feedback mode in blue line with triangle marks against the time for three cases of 30.4 GHz, 615.5 GHz, and 2.39 THz beat frequency.

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To measure the influence of feedback wavelength on the dual-wavelength resonance, beat frequency ranges from ~1.25 THz to ~1.50 THz are selected to observe the spectrum evolutions, which are plotted in Figs. 6(a), and 6(b) with bias current of 25 mA, and 22.7°C temperature, and bias current of 20 mA, and 24.3°C temperature, respectively. The peak ratio and SMSR under these operating conditions are illustrated in Fig. 6(c). The minimum wavelength variation in the spectrum evolution of Figs. 6(a) and 6(b) is 0.08 nm so that the peak wavelength spacing between two adjacent feedback beams is also equal to 0.08 nm which corresponds to the beat frequency of 10 GHz. Based on the Figs. 6(a) and 6(b), one can see that, as the beat frequency is increased from ~1.25 THz to ~1.50 THz, the minimum SMSR still is close to 20 dB. However, the peak ratio increases up to 5 dB in some cases, where the feedback wavelength is in the longer wavelength side of the adjacent residual mode as shown in Figs. 6(a) and 6(b). Hence, in order to achieve very low peak ratio fluctuation and high SMSR, these feedback wavelength should be ignored, making the proposed scheme quasi-continuous tuning for the very low peak ratio fluctuation and high SMSR requirements. In these regions of free-running spectrum, it is well-known that the mode competition between injected beam and lasing modes is very strong so that the locking behavior is easily achieved under the condition of low input power. Hence, to avoid the injection locking and strong mode competition, the feedback power is strictly limited. As a result, the corresponding peak ratio is comparatively larger compared with other cases. Even so, effective photonic signal can still be generated in these sensitive regions [35]. In addition, it is interesting that both the peak ratio and SMSR can be controlled to nearly 0 dB level and around 30 dB, respectively. The system parameters such as bias current and operation temperature in SMFP-LD have very strong influences on the output of the SMFP-LD. With bias current and temperature are, respectively, tuned to 20 mA and 24.3°C, the central wavelength of dominant mode shown in Fig. 6(b) is red-shifted compared to the case in Fig. 6(a) which is biased with the current of 25 mA and 22.7°C. However, as can be seen in Fig. 6(b), the feedback wavelength is also shifted to longer wavelength in order to match the beat frequency range of Fig. 6(a). As a result, the output characteristics of proposed photonics signal generator are not significantly changed.

 

Fig. 6 Evolution of optical spectrum, (a) with 25 mA bias current and 22.7°C temperature, (b) with 20 mA bias current and 24.3°C temperature, and (c) peak ratio (upper row) and SMSR (lower row) as a function of beat frequency where blue lines and green lines,respectively,correspond to the cases of Figs. 6(a) and (b).

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4. Conclusions

Photonics signal generation with beat frequency ranging from 30.4 GHz to 3.40 THz using Single-mode Fabry-Pérot laser diode subject to optical feedback using an external cavity is presented and experimentally demonstrated. By judiciously adjusting wavelength, and power of the feedback beam in external cavity, stable microwave, millimeter-wave and submillimeter-wave signals with nearly equal peak powers can be achieved, whose peak ratio is less than 0.5 dB, and the SMSR can overrun 25 dB. In some sensitive injection locking regions, although the peak difference between two oscillation peaks is up to 5 dB, an effective photonic signal can still be generated with the proposed scheme. In a word, ultra-broadly tunable photonic generation is realized using optical feedback injection on SMFP-LD, which can be used for communications, radars, and medical devices, and others.