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Abstract
Photonic generation of linearly chirped microwave waveforms (LCMWs) using a monolithic integrated three-section laser is experimentally demonstrated in this work. All three sections of the laser cavity, including the front DFB section, phase section and rear DFB section, have the same active layer, which can avoid the butt-joint re-growth process. The gratings in both DFB sections are fabricated by the Reconstruction Equivalent Chirp technique, which can significantly decrease the difficulties in realizing precise grating structure. By adjusting the integrated three-section semiconductor laser to work in the period-one (P1) state and applying a sweeping signal to the front DFB section, the beating signal, i.e., an LCMW with a large time bandwidth product (TBWP), can be generated. In the current proof-of-concept experiment, an LCMW with a large TBWP up to 5.159 × 105 is generated, of which the bandwidth and the duration time are 6.7 GHz and 77 us respectively. The compressed pulse width is 150 ps. In addition, by adjusting the bias currents of the rear DFB section and front DFB section as well as the amplitude of the sweeping signals, LCMWs with tunable center frequency and tunable bandwidth can be achieved.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Linearly chirped microwave waveforms (LCMWs) are extensively applied in radar and sonar systems due to their unique characteristics in long distance detection, high degree of resolution and potential imaging recognition of the target [1]. Especially in the military field, LCMWs with a wide bandwidth and large time-bandwidth product (TBWP) are in more urgent demand for advancing the performance of radar and sonar systems [2]. In traditional electrical domain, LCMWs are usually generated using voltage-controlled oscillators [3], surface acoustic wave [4] or direct digital synthesizer technique [5]. However, all these methods usually suffer from a large degree of restrictions in the maximum of the central frequency, bandwidth and TBWP. To overcome the problems of the limitations in electrical domain, a large number of corresponding photonic methods have been proposed to generate LCMWs with a high central frequency and a large bandwidth in the optical domain [6–12]. Among these photonic techniques for LCMWs generation, one approach is named as spectral shaping and frequency-to-time mapping technique (SS-FTT) [6–8]. In this method, an ultra-short optical pulse shaped by an optical spectral shaper is mapped to the time domain by a dispersive intermediary to realize a chirped pulse. Generally, the optical spectral shapers contain Sagnac loop, Fourier transform pulse shaper and fiber Bragg grating. This method is attractive for its easy realization. However, apart from the poor pulse compression, LCMWs generated by this method have little tunability once the optical spectrum shapers are fixed. The second method is based on optical heterodyning technology [9, 10] by beating one sweeping optical signal with a continuous wave light, which is the most widely used approach to generating LCMWs now. Though the method has a good tunability in any central frequency and bandwidth, it often causes a large phase noise as a result of using non-coherent light sources.
Recently, a novel kind of scheme, employing the special characteristics of semiconductor lasers to generate LCMWs [11, 13], has drawn much attention. One method takes advantage of the period-one (P1) dynamics of optically injected semiconductor laser [11], while the other uses a single monolithically integrated amplified feedback laser (AFL) working on the dual-mode state [12, 13]. For the former one, the system is realized based on several discrete photonic components, which mainly consists of discrete semiconductor lasers, optical circulator, polarization controller (PC), and external intensity modulator. Therefore, the entire system suffers from bulky configuration, complexity and instability. The latter one represents a compact solution based on photonic integration. However, the monolithically integrated AFL needs complicated active-passive integration, and the generated LCMWs only have a small bandwidth of 3.3 GHz.
In this paper, a novel photonic approach based on a monolithically integrated three-section semiconductor laser to generating LCMWs is experimentally demonstrated. The integrated laser, which consists of three sections sharing the same active layer, can be fabricated by the Reconstruction Equivalent Chirp (REC) technique since the basic structures contained in both DFB sections are sampling gratings [14–16]. Therefore, the complicated active-passive integration process can be avoided and the sophisticated gratings in laser cavity can be formed by combining conventional holographic exposure with conventional um-photolithography benefitting from the REC technique. When the integrated three-section semiconductor laser operates in the P1 state and a sweeping signal is applied to the front DFB section or the rear DFB section, an LCMW with a large TBWP can be generated after optical-to-electrical conversion. In this work, the generated LCMW can reach up to a bandwidth of 6.7 GHz together with a temporal duration of 77 us. The corresponding TBWP achieves as high as 5.159 × 105. In addition, by adjusting the bias currents of the rear DFB section and front DFB section as well as the amplitude of the sweeping signal, the tunability of the central frequency and bandwidth is also achieved. To the best of our knowledge, it is the first time that using a monolithic integrated optically injected DFB semiconductor laser to generate LCMWs, which provides a simple and cost-effective method by making use of the advantage of photonic integration.
2. Operation principle
Figure 1(a) illustrates the experimental schematic of the proposed photonic LCMW generator. It is mainly composed of a monolithically integrated three-section laser and a wideband photodetector. In the system proposed, the key device is the single monolithically integrated three-section laser, which is similar to what has been described in [15, 16]. Figures 1(b)-1(d) show the schematic of laser structure, REC grating structure and photograph of the laser chip, respectively. The three-section laser consists of a rear DFB section, a phase section and a front DFB section, which are electrically isolated from each other. Both of the gratings in the two DFB sections are realized by the REC technique. The so-called REC technique refers to that the seed grating provides the basic DFB laser structure with uniform seed grating period Λ, while the complex DFB laser characteristics are determined by the longer sampling period Pr and Pf. Equivalent phase shift, equivalent chirp, multi-phase shift, CPM and other complex grating structures can be met with no doubt by changing the sampling pattern. Thus, a variety of laser structures can be produced flexibly on the same wafer at a time. Most importantly, the laser chip can be fabricated by conventional holographic exposure combined with conventional photolithography. In addition, in order to avoid interference and obtain a high electrical isolation, a small area of the contact layer is removed by etching between the adjacent sections. The three-section laser can work in numerous states from mutually injection locking, period-one oscillation, period-two oscillation, quasi-periodicity to chaos under different injection conditions [17, 18]. The radio frequency (RF) modulation is applied to the front DFB section. Adjusting appropriately injection currents, the laser can operate in the period-one state steadily to generate microwave signals we want.
Fig. 1 (a) Experimental setup using the laser module, (b) schematic of laser structure, (c) REC grating structure, (d) photograph of the laser chip.
Actually, in such a monolithically integrated laser, not only injection ratio but also lasing frequency is changed when tuning one of the input currents in rear DFB section, front DFB section or the RF port in our experiment, indicating frequency-modulated microwave signals can be generated if the related currents are periodically controlled when working in P1 state [19]. In this work, the bias currents of the rear section laser and the front section laser are labeled as IDC1 and IDC2. Adjusting IDC2 constantly when IDC1 is fixed can excite the P1 oscillation [19–21], where two dominant wavelengths are produced. As Fig. 2 shows, the red line represents the optical spectrum of the front section laser without any injection from the rear section laser. The black line represents the optical spectrum when the two section lasers are in P1 state. One mode is generated from rear DFB laser with frequency fr. The other is a red-shifted cavity mode of the front DFB laser with frequency ff’, which is changed with the variation of the current injected to the front section. Accordingly, the detuning frequency, which determines the frequency of beating signals, can be controlled by the injection current of the front laser section since both injection ratio and lasing frequency are changed. Detuning frequency here is defined as the frequency difference between the front DFB laser and the rear DFB laser. Finally, by heterodyning the two dominant optical modes on a photodetector, a chirped microwave signal can be generated when a sweeping signal is applied to the front DFB section. By inputting a sawtooth signal, the frequency of generated microwave signal will advance with the enhancement of input signal. In other words, an LCMW will be generated correspondingly. Therefore, it is of great possibility to generate an LCMW with a large TBWP by adopting this method.
Fig. 2 Optical spectrum of the free-running front DFB laser (red dashed curve) when IDC1 and IDC2 are set to be 0 mA and 80 mA separately and the three-section laser in P1 state (black solid curve), when IDC1 and IDC2 are set to be 75 mA and 80 mA separately.
3. Experimental demonstration
3.1 Experimental setup
A proof-of-concept experiment according to the setup in Fig. 1 is performed. The monolithically integrated three-section laser chip is packaged in butterfly housing as a module for measurement. IDC1 and IDC2 are provided by THORlabs (LDC205C) and ILXLightware (LDC-3724) respectively. Meanwhile, a sweeping signal generated from a 1.2 Gb/s arbitrary waveform generation (Tektronix AWG5014C) is applied to the RF port, which is connected to the front DFB section to realize frequency-modulated signals. Furthermore, for simplifying the entire system, the phase section is left unconnected in this experiment. The temperature of the laser module is kept at 25°C by a temperature controller (THORlabs 200C) during the entire process, ensuring a stable working environment. The output optical signals from the three-section laser are divided into two parts through a 50/50 optical splitter. One part of the optical signals are delivered into a high-speed photodetector (PD: U2T XPDV2120R) with a 50-GHz bandwidth to be converted into electric signals, which are then directly sent to an 80 GSa/s real-time oscilloscope (Lecroy SDA 830Zi-A) to observe the time-domain waveforms. Another part of the optical signals are transmitted to a high-resolution optical spectrum analyzer (Finisar WaveAnalyzer 1500s) to observe the working state of the three-section laser.
3.2 Static performance
First, the static characteristics of the three-section laser are measured without any RF input. The measured optical spectrum with IDC2 varied from 60 mA to 92 mA and IDC1 fixed at 77 mA is demonstrated in Fig. 3(a). As IDC2 increases, the oscillation wavelength of the front DFB section moves toward the longer wavelength side obviously and the wavelength of rear DFB section changes slightly. As a result, the frequency difference between the two DFB sections gets gradually larger. After optical-to-electrical conversion, the corresponding electrical spectrum showing the variation tendency of the beating signal is depicted in Fig. 3(b). The stronger the IDC2, the greater the output signal frequency while IDC1 is fixed. In addition, the generated frequency difference-current curves (f-IDC2 and f-IDC1) are also investigated, which is shown in Fig. 4. During the measurement, IDC1 is varied from 76 mA to 92 mA while IDC2 is adjusted from 60 mA to 95 mA. Different combinations of IDC1 and IDC2 can result in different frequency of beating signals. Meanwhile, the measured curves show that the beating frequency changes almost linearly with the variation of IDC2 under certain condition. Although the beginning of the curve has great curvature, it can be corrected mostly by the use of reverse fitting when optimizing the sweeping signals [11].
Fig. 3 (a) Measured optical spectrum with IDC2 varied from 60 to 92 mA while IDC1 being fixed at 77 mA, (b) measured electrical spectrum with IDC2 being varied from 60 to 92 mA while IDC1 being fixed at 77 mA.
3.3 Dynamic performance
Based on the static characteristics, the dynamic performances are measured by applying a periodic signal to the RF port and keeping IDC1 a constant. A 10-KHz sawtooth signal shown in Fig. 5(a) is injected into the three-section laser via the RF port to substitute the tunable current of the front DFB section, whose amplitude is 1.6 V and the currents of rear and front DFB section are fixed at 85 mA and 76 mA respectively. The corresponding waveform is shown in Fig. 5(b). The instantaneous frequency settled by calculating the output waveform for Hilbert transform is depicted in Fig. 5(c). The results show the bandwidth can be up to 9.8 GHz, but the instantaneous frequency changes nonlinearly, especially at the beginning of the time duration. In order to overcome this problem, the inverse fitting method is carried out to compensate for the bending tendency [11]. The optimized sweeping signal is shown in Fig. 6(a). The output waveform is shown in Fig. 6(b) and the corresponding instantaneous frequency is shown in Fig. 6(c). Compared with the chirped pulse before optimization, a better microwave waveform, i.e., an LCMW is created with a 77-us duration time suggesting 77% duty cycle and a 6.7-GHz bandwidth from 16.1 GHz to 22.8 GHz. The maximum TBWP realized finally is 5.159 × 105 which is considerably larger than previously reported in the domain of photonic integrated-based LCMW generators.
Fig. 5 (a) The injected sawtooth signal, (b) measured signal waveform in one period, (c) the calculated instantaneous frequency (the red dashed line is fitting curve).
Fig. 6 (a) The inverse fitting control signal, (b) measured LCMW in one period, (c) the calculated instantaneous frequency (the red dashed line is fitting curve).
The detection accuracy is fundamental to an excellent radar, which is often evaluated by the size of the autocorrelation. Figure 7(a) shows the result of the normalized autocorrelation of the generated pulse signal and Fig. 7(b) is the zoom-in view of the autocorrelation peak. The full width at half maximum (FWHM) of the compressed pulse is measured to be 150 ps, indicating a compression ratio of 5.133 × 105.
By adjusting IDC1, IDC2 as well as the amplitude of the sweeping signal, LCMWs with tunable central frequency and bandwidth are also realized as the Fig. 8 shows. When IDC1, IDC2 and the amplitude of the optimized control signal are fixed at 96.43 mA, 62.29 mA, 0.4 V respectively, the pulse time only has 24 us and the bandwidth is 1.5 GHz from 14.15 GHz to 15.65 GHz, leading to a TBWP of 3.6 × 104. If IDC1, IDC2 and the amplitude of the optimized control signal are set at 95.91 mA, 68.29 mA, 0.5 V, as the Fig. 8(b) shows, the corresponding pulse time is 16 us while the bandwidth is enlarged to 2.2 GHz, indicating a TBWP of 3.52 × 104. In a word, when these three variables are set in different values, we could get different LCMWs including changeable pulse time, central frequency and bandwidth just as Figs. 8(c)-8(f) show.
Fig. 8 Measured waveform and corresponding instantaneous frequency output from the three-section DFB laser with different IDC1, IDC2 and VRF. (a) IDC1 = 96.43 mA, IDC2 = 62.29 mA, VRF = 0.4 V, (b) IDC1 = 95.91 mA, IDC2 = 68.29 mA, VRF = 0.5 V, (c) IDC1 = 73.22 mA, IDC2 = 81.9 mA, VRF = 0.5 V, (d) IDC1 = 75.82 mA, IDC2 = 73.42 mA, VRF = 1.5 V, (e) IDC1 = 89.48 mA, IDC2 = 73.41 mA, VRF = 0.7 V, (f) IDC1 = 88.16 mA, IDC2 = 75.23 mA, VRF = 2 V (the red dashed line is fitting curve, CF: central frequency, BW: bandwidth).
In addition, the linewidth of the generated microwave signal is an important aspect that reflects the signal quality in P1 state. In this work, when IDC1 and IDC2 are adjusted to 102.26 mA and 60.58 mA, a relatively large 3-dB linewidth of 1.9 MHz is observed as the Fig. 9 shows. If IDC1 and IDC2 are set to be 101.26 mA and 72.17 mA, the corresponding 3-dB linewidth increases to 3.2 MHz. In our experiment, the linewidth is influenced by the intrinsic laser noise and the defect of accuracy and stability of the bias currents. However, the level of the linewidths we have achieved is comparable to that produced based on discrete devices [22, 23].
Fig. 9 Measured linewidths of the generated microwave signals with different IDC1, IDC2. (a) IDC1 = 102.26 mA, IDC2 = 60.58 mA, (b) IDC1 = 101.26 mA, IDC2 = 72.17 mA.
4. Discussion and conclusion
Several aspects of the experimental results demonstrated in this work can be further improved in the future work. Firstly, an additional method is required to narrow the linewidth (i.e., phase quality) of the generated microwave signals. For example, optical/electrical feedback can be employed to improve the signal quality of an LCMW generated using the P1 dynamics of a semiconductor laser [22, 23]. Secondly, the bandwidth of the LCMW needs to be further increased. One effective method can be the optimization of the structure of the integrated DFB semiconductor laser, which is expected to enhance the interaction between the integrated DFB sections. Thirdly, the maximum pulse duration time of the generated microwave signals is affected by two aspects. The one factor is the pulse repetition frequency (PRF) of the sweeping signal. The larger the repetition rate, the smaller the pulse repetition time and the smaller the corresponding microwave pulse time. The other factor is the voltage amplitude of the sweeping signal. By setting a higher input voltage with a fixed PRF, it is more likely to generate larger bandwidth. However, the integrated laser might work in other oscillation states from injection locking, period-two oscillation, quasi-periodicity to chaos rather than period-one state. No chirped microwave waveforms will be produced by heterodyning optical signals. As a result, we need to make a balance between chirp range and duration time inevitably. Last but not least, the linearity of the generated LCMW needs to be further improved. The optimization of the sweeping signal and/or appropriate working point of the bias currents should contribute to this improvement.
Now that the two DFB lasers are electrically isolated but not optically isolated, the two lasers only have interference in the light field and temperature. In the experiment, no matter which one DFB laser current is increased, the other cavity resonance will be red-shift simultaneously. The extra effect in power also happens between the two DFB lasers. In addition, due to the monolithic integrated structure, the two DFB lasers will interact with each other, resulting in mutual injection instead of unidirectional injection. The oscillation state in the monolithically integrated laser is more complex compared to that based on discrete devices. The other oscillation state from injection locking, period-two oscillation, quasi-periodicity to chaos will occur under different injection conditions, making the adjustments of bandwidth and duration time of the chirped microwave signal. As a result, throughout the experiment, precise control of the lasers’ working state is supposed to be guaranteed strictly.
To conclude, in this paper, we have proposed and demonstrated a novel photonic approach to generating LCMWs with a large TBWP based on a single monolithically integrated three-section semiconductor laser. By adjusting the integrated optically injected DFB lasers to operate in P1 state and applying sweeping signals to one of the DFB sections, LCMWs with a large TBWP have been obtained after optical-to-electrical conversion. The generated bandwidth in this experiment has reached up to 6.7 GHz and the corresponding sweeping frequency ranges from 16.1 GHz to 22.8 GHz. The pulse duration time is 77 us, leading to a TBWP and a compression ratio as large as 5.159 × 105 and 5.133 × 105, respectively. In addition, the tunability of the central frequency and bandwidth is also confirmed. Most importantly, the feasibility of such a novel method should attribute to the enabling device, i.e., the monolithically integrated three-section semiconductor laser based on the REC technique. In such an integrated semiconductor laser, both the identical wafer structure of the multi-section integration and the low-cost fabrication for sophisticated gratings guarantee a simple and cost-effective photonics integration approach to generating LCMWs.
Funding
National Natural Science Foundation of China for the Youth (61504170, 61504058); National Natural Science Foundation of China (61475193).
Acknowledgments
The authors would like to thank Dalian Canglong Optoelectronic Technology Co. Ltd., for their help in laser package and the Lecroy Technologies Co. Ltd., for the use of high-speed real-time oscilloscope.
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