This paper reports generation of sub-5-ps Fourier-transform limited optical pulses from a 1.55-µm gain-switched single-mode distributed-feedback laser diode via nanosecond electric excitation and a simple spectral-filtering technique. Typical damped oscillations of the whole lasing spectrum were observed in the time-resolved waveform. Through a spectral-filtering technique, the initial relaxation oscillation pulse and the following components in the output pulse can be well separated, and the initial short pulse can be selectively extracted by filtering out the short-wavelength components in the spectrum. Short pulses generated by this simple method are expected to have wide potential applications comparable to mode-locking lasers.
© 2012 OSA
Short pulses generated from semiconductor laser diodes (LD) have wide applications as optical sources in not only optical communications and optical signal processing [1,2] but also multi-photon bio-imaging [3–5]. Most short pulses with duration ~ps from semiconductor laser diodes are generated by mode-locking . As a typical short-pulse generation method from semiconductor laser diodes, gain switching [1–4] can also be used to generate ps short pulses, and the pulse generation frequency can be tuned freely by tuning the frequency of the electrical pulse excitation. This advantage together with simplicity and low cost makes gain-switching of great important in real industrial applications of compact picosecond-pulse semiconductor lasers. Using gain switching, however, makes pulses with duration shorter than 10 ps very difficult to obtain. Various techniques have been developed in order to decrease the output pulse widths of gain-switched lasers, such as chirp compression technique with fiber Bragg grating [7–9], dispersion fiber [10,11] or nonlinear optical loop mirror , and spectral filtering technique with Fabry-Perot resonator , interference filter  or interferometer [15,16]. Compared with chirp compression, spectral filtering is of great advantages in simplicity, flexibility and feasibility for almost all semiconductor lasers, which make it very attractive in wide potential applications of compact gain-switched semiconductor short-pulse light sources. However, short pulse widths below 10 ps had been difficult to obtain only by spectral filtering until recently, while short pulse widths below 5 ps are highly desired for many applications.
Recently, 6-7-ps short pulses have been experimentally obtained by a 1.55-μm distributed feedback laser diode (DFB-LD) intensively excited by electric pulses having 100-ps duration and over 6-V amplitude, and the obtained short pulses were successfully used as optical sources in bio-imaging  and time-resolved spectroscopy  after amplification and wavelength conversion. Nevertheless, the 100-ps duration and the amplitude of over 6-V are the heavy-duty requirements for present electronic devices and systems, and the mechanism of the generation of picosecond optical pulses is still not clear.
In this paper, we demonstrate that through spectral filtering technique, optical pulses with pulse width as short as sub-5-ps can be obtained from a gain-switched 1.55-μm DFB-LD by electrical pulse excitations with nanosecond duration and 5-V amplitude that are very easy to be generated using general electronic devices. The main mechanism of the short pulse generation is considered to be the dynamical resonance wavelength shift during gain switching and the extraction of the short-pulse components by spectral filtering. The demonstration of picosecond-pulse generation from semiconductor lasers by a combination of these simple low-cost techniques of gain-switching, spectral filtering and ns electric pulse excitation enables wide potential applications of gain-switched semiconductor lasers.
2. Experimental setup
Figure 1 illustrates the experimental setup. A high-speed 1.55-μm InGaAsP multi-quantum-well DFB buried-heterostructure LD (NTT Electronics, NLK5C5EBKA, 18-GHz modulation bandwidth) maintained at 25 °C with a thermoelectric cooler was used for this study. A continuous-wave (cw) lasing threshold current of this device was 10 mA. A cw lasing spectrum at 15-mA injection current is shown in Fig. 2(a) , where single-mode lasing at 1548.9 nm is found.
In gain-switched operation, the DFB-LD was driven by rectangular electrical pulses with a 100-MHz repetition rate, 0.64-ns duration, and 5.0-V amplitude for 50-ohm load, which were generated by an electrical pulse generator (Agilent 81133A, 60-ps rise time, 15 MHz - 3.35 GHz repetition rate) with a homemade Radio Frequency (RF) amplifier, and a variable step electrical attenuator (HP 8494B, 0-11dB, DC-18 GHz). The pulse duration can be set to be longer than 1 ns in this experiment, but for the convenience in measuring the whole waveform including a long tail of the output pulse, we set the pulse duration to 0.64 ns. Since the duration of the picosecond optical pulses was measured by second-harmonic-generation (SHG) intensity correlation after spectral filtering, the optical pulses from the DFB-LD were led to an erbium-doped-fiber amplifier (EDFA) via a single-mode optical fiber to amplify the output power to increase the optical power to a proper level for the SHG intensity correlation. It should be noted that linear amplification (below 1 mW output power) and no change in spectral and temporal shapes were assured through this EDFA. For spectral filtering, a short-pass filter (SPF) and a tunable band-pass filter (BPF) with a bandwidth Δλ = 1 nm were inserted as shown in Fig. 1. Averaged optical powers, optical spectra, time-resolved waveforms, and autocorrelation traces of optical pulses were measured with an optical power meter, an optical spectrum analyzer (OSA) (ADVANTEST Q8384), a 40-GHz sampling oscilloscope (Agilent 86100C Infiniium DCA-J) with a 28-GHz optical detector, and an high-sensitivity autocorrelator with photon-counting detection, respectively. When necessary, a variable optical attenuator was inserted to protect the equipment.
3. Experiment results and discussions
Figure 2(b) shows optical-pulse spectra of the gain-switched DFB-LD driven by electrical pulsed 0.64-ns duration and 5.0-V amplitude (measured across a 50-ohm load), where lasing threshold was around 1.8 V. The broad peak from 1546.8 to 1549.5 nm with a sharp peak at 1548.7 nm is the main mode, and the peak at 1545.5 nm is a suppressed side mode of DFB-LD. From the comparison with Fig. 2(a), the sharp peak in the main mode is most possibly contributed by the quasi-steady-state-lasing component, and the broad short-wavelength side is then contributed by the pulse components. In the following, to extract the short pulse components in the short-wavelength side by spectral filtering, a SPF and a BPF were inserted as shown in Fig. 1. Therein, the SPF was specially used to cut the sharp peak contributed by the quasi-steady-state-lasing component, and the BPF was used to filter out the rest quasi-steady-state-lasing component and extract the short pulse components in the short-wavelength side. Narrower BPFs are helpful for the extraction, however if the BPF is too narrow, the pulses after the BPF will be broadened due to the effect of Fourier-transform limit.
Figure 3(a) shows the log-scaled spectra of the spectrally filtered short-wavelength side and long-wavelength side of the output pulses of the gain-switched DFB-LD driven at 5.0 V and amplified with the EDFA. Figure 3(b) shows the linear-scaled spectrum of the short wavelength side in Fig. 3(a). Note in the linear-scale spectrum in Fig. 3(b) that the short-wavelength part with the full-width at half maximum (FWHM) of 0.56 nm was successfully extracted.
Figure 4 shows the time-resolved waveforms of the EDFA-amplified pulses of the gain-switched DFB-LD driven without and with the spectral filtering, though the measured time response was limited mostly by the electric response of the detector and the oscilloscope (temporal resolution is ~19 ps). The waveform of the whole spectrum corresponds to the spectrum in Fig. 2(b), and those of the short-wavelength side components and long-wavelength side components after the spectral filtering correspond to the spectra in Fig. 3. The whole pulse waveform (without spectral filtering) consists of a fast pulse component of 25-ps duration and a following slow quasi-steady-state-lasing component, where the latter slow component occupies about 80% of the total pulse. It is noted that longer duration excitation pulse resulted in longer slow component. The enhanced intensity of the quasi-steady-state components relative to that of a fast pulse component in the waveform of the long-wavelength side demonstrates that the long-wavelength side of the spectrum is indeed dominated by the quasi-steady-state components. On the other hand, the pulse waveforms of the short-wavelength side after spectral filtering was drastically different where the long tail was completely removed and the pulse width was shortened to approximately 20 ps on the oscilloscope. This result indicates that the initial part of the pulse is indeed contributed by the short-wavelength side and shorter pulse is successfully obtained by extracting the initial part of the short pulse with spectral filtering. The sharp falling edge, the following undershoot and oscillations in the waveform of the pulse at short wavelength side after spectral filtering are caused by the electric response of the detection system (the photo diode and the oscilloscope); if the second oscillation is really generated, in general, there will be no undershoot between the two oscillations. These phenomena indicate that the pulse widths are much shorter than the time resolution of the detection system (19ps), and are too fast to be measureable accurately. Therefore, in order to measure the pulse widths accurately, we measured the intensity-autocorrelation trace of the pulses.
The SHG autocorrelation trace of the pulse after spectral filtering at short wavelength side is shown in Fig. 5 . From the sech2 fitting (dashed line) of the autocorrelation trace with FWHM of 7.5 ps, the pulse width was found to be 4.7 ps. Since the FWHM of pulse spectrum is 0.56 nm, the time-bandwidth product of the obtained short pulse is 0.33, which is very near the value of 0.32 for sech2 shaped pulse, demonstrating that the obtained short pulse at short-wavelength side is a Fourier-transform-limited pulse.
In addition, note that in Fig. 4, the pulse of the short wavelength side starts earlier than the long-wavelength side. This phenomenon is also a typical signature of the frequency chirping caused by the large carrier density fluctuation during gain switching, which is the main reason for the wavelength shift, and in turn makes short-pulse generation possible using a spectral filtering technique. The frequency chirping will be investigated in detail in later publications.
In conclusion, by using a simple spectral filtering technique to extract the short-wavelength component in pulse lasing spectrum, transform-limited sub-5-ps pulses were obtained from a gain-switched single-mode 1.55-μm DFB-LD by electric pulse excitation with nanosecond duration and 5-V amplitude; these electric characteristics are rather easy to obtain by the conventional high-speed electronic devices in the present days. In view of picosecond optical pulse generation, the present method will provide a simple and practical method. Combinations with optical amplification and nonlinear wavelength conversion can create many applications including multi-photon imaging and time-resolved spectroscopy as were already demonstrated in part. Furthermore, aside from the application aspects, detailed studies of the gain-switched semiconductor lasers under high-carrier-density excitation may bring new insights for ultrafast nonlinear dynamics producing picosecond optical pulses, which have not yet been sufficiently examined so far.
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