Novel Dual-mode locking semiconductor laser for millimetre-wave generation P. Acedo 1, C. Roda 1, H. Lamela 1, G. Carpintero 1, J.P. Vilcot 2, S. Garidel 2 1 Grupo de Optoelectrónica y Tecnología Láser, Universidad Carlos III de Madrid, Avda de la Universidad 30, 28911 Leganes (Madrid) SPAIN 2 IEMN-CNRS. Lille, France pag@ing.uc3m.es ABSTRACT In this work we present a new mode-locked device that can be used for millimeter-wave photonic applications. Such device presents for certain bias conditions a dual-mode behavior we have investigated for millimeter wave generation. Through the small signal analysis of the device, we have identified a resonance at the frequency separation of the longitudinal modes that has allowed us to demonstrate signal transmission at 40 GHz. The millimeter wave signal generated in detection is studied in terms of phase noise and the noise intrinsic to the emitter. Keywords: Photonic millimetre wave generation, mode-locking semiconductor lasers, dual-mode lasers 1. INTRODUCTION The development of compact, low-cost, semiconductor optical sources for the millimeter-wave radio band in radioover-fiber systems is of great importance motivated to a large point by potential applications such as phased array antenna and mm-wave indoor personal communication systems [1,2]. However, direct modulation of semiconductor lasers is restricted to a maximum frequency of around 30 GHz imposed by the intrinsic diode laser relaxation oscillation frequency and damping [3]. In this sense the possibility of mode-locking a semiconductor laser for millimeter wave generation has attracted great interest during the last years [4]. In this sense, and recently, mode-locking of dual-mode lasers (dual-mode-locking) has proven to be an attractive technique to obtain more compact devices for mm-wave applications [7, 8]. Externally injection of a mm-wave signal allows synchronisation of both modes and a sinusoidal signal at the frequency separation between modes will be generated [8]. Active mode-locking by injecting a signal directly at the gain section of the semiconductor laser is an easy way to synchronise them. In this work we present a new monolithically integrated mode-locked laser source developed within the European Project MONOPLA [5] for the generation of optical pulses at rates of 40 GHz. This device, based on a MQW-DFB structure, consists of four sections: absorber, gain, phase (or extended cavity) and grating; that can be independently biased. Analysis of its optical spectrum has revealed that under certain bias conditions it presents stable dual-mode behaviour. Such two modes can be synchronised to generate mm-wave signals at the designed frequency at which the device was conceived, 39.5 GHz. Its performance as a signal generator can then be analysed and compared. This paper is organized as follows: first a description of the novel dual-mode semiconductor laser is presented in section 2. Section 3 is devoted to the experimental characterization of the device in terms of its static behavior: threshold current, optical spectrum and RIN. In this study, a zone of dual-mode operation of the device will be identified and so, in section 4, the study and experimental characterization of the device with this dual-mode behavior is carried out in the framework of millimeter-wave generation. We end the paper with the conclusions of our work.
2. DESCRIPTION OF THE NOVEL DUAL-MODE-LOCKED SEMICONDUCTOR LASER The mode-locked devices under study in this work have been fabricated within the European Project MONOPLA. The main objective of this project is the design and fabrication of a monolithic source of ultrafast, high repetition rate, optical pulses suitable for the next generation communication systems, compatible with both optical time division multiplexing (OTDM) and wavelength division multiplexing (WDM) [5]. Theses devices consist of GaInAsP/InP multiple quantum wells (MQW) structures, emitting at approximately 1550 nm, and are dimensioned (length of the cavity) to have a separation between longitudinal modes of 40 GHz [6]. As it is shown in figure 1, they consist of four sections grown on the same substrate : gain, extended cavity (or phase), grating, and absorber, where the active layer is all along the total length of the laser device. Each section, that can be independently biased, is active, and can participate on the light amplification. In the experiments that follow we have used the device labeled S04, which has a geometry of 300 µm gain section length, 370 µm phase section length, 200 µm grating section length, and of 150 µm absorber section length. Figure 1: Schematic structure of a monolithically integrated MQW mode-locked laser from MONOPLA project. In figure 2 we see a picture of one of the SO4 device used for this study. As we can see in this picture, the laser is mounted on a RF submount with two k-connectors for the bias and absorber sections. The rest of the pins are for biasing phase and grating sections and for the thermistor and TE cooler used for temperature stabilization. Figure 2: Picture of the first mode-locked laser from MONOPLA project.
3. CHARACTERIZATION OF THE NOVEL DUAL-MODE SEMICONDUCTOR LASER 3.1 P(I) and V(I) curves In order to determine the basic lasing characteristics of the device under study, an experimental characterization as a single section laser were performed to obtain its threshold current, quantum efficiency and series resistance. In a monoelectrode configuration [6], where all four sections are shorted, we measured its optical output power and driving voltage as function of the injected current. Experimental results for power and voltage versus injected current are shown in figure 3. A threshold current of I th = 60 ma, a quantum efficiency of η = 0.049 W A -1, and a series resistance of R s = 4.7 Ω were measured. Measurements biasing independently each section were also performed. For an applied voltage at the absorber of V abs = 0V, the multisection laser started to lase when the sum of the injected currents at the other three sections reached the threshold current for the monoelectrode configuration. Series resistance of the three sections were also calculated, obtaining a resistance for the gain section of R s g = 17.9 Ω, for the phase section R s ph = 18.7 Ω, and for the grating section R s gr = 18 Ω 1.5 1.5 Output Power (mw) 1 0.5 1 Voltage (V) 0.5 3.2 Optical spectrum 0 0 0 20 40 60 80 Current (ma) Figure 3: Output power and voltage as a function of current for the Novel Dual-mode laser in a monoelectrode configuration (see text). The optical spectrum of the device for different bias conditions was measured using an YVON-JOBIN TRIAX 550 monochromator with 0.05 nm of resolution. In figure 4 we show the measured spectra for several bias conditions. In all cases the absorber section was short-circuited (Vabs = 0V) while the others sections were biased independently. When the laser is above the threshold condition, several modes around 1565 nm wavelength appear. Separation between adjacent modes was calculated to be f sep = 39.5 GHz, that corresponds to the designed parameters for which the device was fabricated. In figures 4.b, 4.c, and 4.d, we can also appreciate two differentiated group of modes. Control of the number of modes oscillating, center frequency, and distribution can be achieved changing the bias currents of each section. A more detailed optical spectrum measurement was made using a Fabry-Perot interferometer with a free spectral range of 650 GHz. In this sense, in figure 5, we can see for certain bias conditions, the strong presence of only two modes with same amplitude, centered at 1565.5 nm, and separated 39.5 GHz. Others adjacent modes have much lower power levels. As mentioned in the introduction this later result it is important in the framework of photonic generation of millimeter wave. Dual-mode lasers with two longitudinal modes have been proposed for the generation of such signals through the synchronization of both modes with an external signal [7, 8], and several radio-over fiber applications described [9]. In this sense, in the next section we will continue the characterization of this device in a small signal regime to study the possibility of using the mode-locked laser developed within the project MONOPLA for millimeter wave generation when working in dual-mode configuration.
Intensity (a.u.) Intensity (a.u.) 1.56 1.562 1.564 1.566 1.568 1.57 Wavelength (um) 1.56 1.562 1.564 1.566 1.568 1.57 Wavelength (um) a) b) Intensity (a.u.) Intensity (a.u) 1.56 1.562 1.564 1.566 1.568 1.57 Wavelength (um) 1.56 1.562 1.564 1.566 1.568 1.57 Wavelength (um) c) d) Figure 4: Wavelength spectrum of the novel laser device. Biasing conditions are V abs = 0V, I gain = 30 ma, I phase = 30 ma, and a) I grating = 16 ma, b) I grating = 18 ma, c) I grating = 20 ma, d) I grating = 22 ma. 10 2.6 200 GHz 39.5 GHz 10 2.5 Log (a.u.) 10 2.4 10 2.3 10 2.2 Frecuency Figure 5: Detail optical spectrum and intermodal frequency separation measured with a Fabry- Perot interferometer. V abs = 0V, I gain = 30 ma, I phase = 30 ma, and I grating = 20 ma.
3.3 Relative intensity noise The last study we have carried out on the static behaviour of these new devices is the measurement of the laser intensity noise. It is important to note that the relative intensity noise (RIN) of a laser is frequency dependent and its frequency response gives also information of the dynamics of the laser as peaks at the resonant frequency in standard edge emitting lasers. In the case of mode-locked lasers, a new resonance should appear at the longitudinal mode intermodal frequency separation. RIN of the dual-mode laser is showed in figure 6, when biased for the stable dual-mode behavior and for frequencies higher than the intrinsic relaxation frequency. In such figure we can see how a peak appears at the predicted frequency of 39.5 GHz. Below 35 GHz the RIN is lower than the sensibility of our measurement system while above 35 GHz, is always lower than -105dBc/Hz. Such low value of RIN make it an excellent candidate for its use in actual lightwave communications systems, where RIN of less than -95dBc/Hz is a typical specification parameter. -105-110 39.5 GHz Relative Intensity Noise (db/hz) -115-120 -125-130 -135-140 -145-150 -155 shot noise limit -160 10 15 20 25 30 35 40 Frequency (GHz) Figure 6: RIN of the dual-mode laser. Bias conditions: V abs = 0V, I gain = 30 ma, I phase = 30 ma, and I grating = 20 ma. 4. MILLIMETRE-WAVE GENERATION USING NOVEL DUAL-MODE LOCKING SEMICONDUCTOR LASER In order to determine the possibility of using this device in dual-mode operation for millimeter wave generation we first introduced a tone at the longitudinal mode separation frequency in order to observe the output spectrum. For this reason we drive the gain section with a RF signal generated by a low phase noise CW generator (ANRITSU MG3695A). The output light was coupled to an optical fiber and detected with a high speed photodiode (u2t XPDV2020R). Due to reflections and high losses at fiber coupling an optical isolator and an EDFA were also used. The output signal was displayed using an electrical spectrum analyzer (ANRITSU MS2668C). The results are shown in Figure 7.
-40 RBW 10kHz VBW 10kHz Att 0dB -50 Power (dbm) -60-70 -80-90 37 37.5 38 38.5 39 39.5 40 Frequency (GHz) Figure 7: Received RF tone when modulating gain section with a RF signal of 39.4GHz. In this figure 7 we can clearly see that the introduced 39.5 GHz RF signal, well above the relaxation oscillation frequency of the intrinsic laser, is transmitted by the laser due to the synchronization of the two longitudinal modes. This result, nevertheless, has to be confirmed through the study of the small signal response of the laser to clearly demonstrate that this is a resonance due to the longitudinal modes that are locked by means of the external signal introduced in the gain section. 4.1 Small-signal modulation frequency response For this reason we have studied the small signal modulation response of the novel dual-mode laser that is shown in figure 8. The laser was biased at the stable dual-mode operation point and a RF signal of +6dBm was injected at the gain section. We can observe that a narrow peak appears at the same frequency than the intermodal frequency separation (39.5GHz). At least 20 db of gain was measured, proving generation of millimeter-wave frequencies by active modelocking. In figure 9 we show a detail of the small signal modulation response in the 39.5 GHz range. We can see that the 3 db bandwidth of the resonance peak has a value of around 600 MHz. The appearance of this resonant frequency with a 600 MHz bandwidth proves that this device is a very attractive candidate for narrow-band high-frequency application like millimeter lightwave communication systems.
-50 Modulation Response (db) -60-70 -80-90 -100 20 db -110 10 15 20 25 30 35 40 Frequency (GHz) Figure 8: Small-signal frequency response of the novel dual-mode locked laser. Bias conditions: V abs = 0V, I gain = 30 ma, I phase = 30 ma, and I grating = 20 ma (dual-mode operation). -54 Modulation Response (db) -55-56 -57-58 -59 3 db -60-400 -300-200 -100 0 100 200 300 400 Frequency Offset (MHz) Figure 9: Detail of the peak centered at 39.4 GHz showing a 600 MHz 3 db bandwidth.
4.2 Phase noise of the generated millimeter-wave signal In order to better characterize the behavior of this new dual-mode mode-locked laser for mm-wave applications we have studied the quality of the millimeter wave signal generated. For this reason we have measured the phase noise of the generated tone that is shown in figure 9 when modulating gain section at 39.4GHz and +8dBm. A phase noise of - 65dBc/Hz at a frequency offset of 1kHz and of -85dBc/Hz at 1MHz were measured. -40-50 Phase Noise (dbc/hz) -60-70 -80-90 -100 10 2 10 4 10 6 10 8 10 10 Frequency Offset (Hz) Figure 9: Phase noise of generated millimeter-wave signal at 39.4GHz using the novel dual-mode locked laser. 5. CONCLUSIONS In summary, we have demonstrated the possibility of using mode-locked semiconductor lasers for mm-wave generation at frequencies around 40 GHz. For this experiment we have used an experimental mode-locking semiconductor laser with a cavity length of 1mm and with 39.5 GHz intermodal frequency separation. Through the study of its optical spectrum we observed that under certain bias conditions it presents a dual-mode behaviour. Under this operation we have synchronized both modes by injecting at the gain section of the laser a signal at 39.5 GHz (dualmode-locking), obtaining at detection a pure tone at the injected frequency. The mm-wave generated was characterised to have a phase noise of -85dBc/Hz at 1MHz frequency offset. ACKNOWLEDGEMENTS This work has been carried out within the framework of the European Project MONOPLA (IST-2001-34642), founded by the European Commission under the Fifth Framework Programme, where the mode-locked laser devices have been designed, fabricated and tested.
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