We propose the simple pulse source, in which the pulses generated by large-signal directly modulated laser diode are phase-modulated, and then compressed into short pulses by an optimized length of DCF. On the one hand, phase modulator is used to enhance the negative chirp of large-signal directly modulated pulses. On the other hand, the large-signal directly modulated pulses are used to suppress the pedestal produced by the phase modulator. Using this technique, highly stable 10-GHz 5.5-ps optical pulses are obtained, which suppress pedestal up to 20dB and have a low timing jitter of 184fs.
© 2007 Optical Society of America
High-quality and reliable short pulses have many applications in OTDM system. Several techniques have been developed to produce short pulses [1–7]. Mode-locked laser is capable of generating very short pulses with a high repetition frequency, but it requires a complex equipment to realize stable operation. Gain-switching laser is a simple and reliable short pulse source, but the generated pulse has considerable timing jitter. The pulses generated by directly modulated laser have low timing jitter, but the pulse width is usually not short enough for OTDM system.
Recently, a simple optical pulse generator is reported  , in which continuous wave (CW) light is modulated by an electrooptic phase modulator and compressed into pulses by a group delay dispersive medium. However, the phase modulated CW light has both positive and negative chirp, so the compression typically results in pedestals after it passes through a group velocity dispersive medium, which makes it unsuitable for use in practical systems. Several techniques have been proposed to suppress pedestal, such as using intensity modulation and an optical bandpass filter .
In this paper, we report a simple and reliable optical short pulses source, in which large-signal directly modulated pulses are phase-modulated, and compressed into short pulses by an optimized length of dispersion compensation fiber (DCF). The pedestal produced by an electrooptic phase modulator can be suppressed by the large-signal directly modulated pulses. Using this technique, we obtain high-quality 5.5-ps pulses at a repetition rate of 10 GHz, with a high-pedestal suppression of up to 20dB, and 184-fs timing jitter determined by integrated phase noise over a frequency span of 100 Hz to 1 MHz.
2. Theoretical analysis
It is well established that modulation of the current supply to a semiconductor laser leads to modulation of both the optical intensity and the optical frequency  . Assuming that the pulse produced by the large-signal directly modulated laser diode is a Gaussian pulse, and the phase of the pulse changes with sinusoidal signal, the electrical field can be expressed as
where E0 is the amplitude of electrical field, ω0 is the angular frequency, fm is modulation frequency, Δθ1 is the equivalent phase modulation index of large-signal directly modulated laser and T0 is the pulsewidth which is defined as half width of optical pulse when amplitude decreases to 1/e of the peak. Initial phase π/2 indicate that the pulse has negative chirp.
where m is intensity modulation index, and α can be calculated by ,
where P 0 is the DC optical power, and ΔP is the peak-to-peak power excursion, Δν 1 is the optical frequency excursion, and fm is modulation frequency.
If the large-signal directly modulated pulses are then phase modulated by the same frequency fm, the modulated electrical field can be expressed as
where, Δθ2 is the modulation index of phase modulator, and Δφ is the phase difference. In this case, the instantaneous frequency of the light is given by
Optical frequency chirp can be described as
when Δφ=π/2, the chirp generated by the large-signal directly modulated laser has the same sign with the chirp generated by the phase modulator, and the optical frequency chirp (6) can be written as:
when t ε ((n-1/2) T, (n+1/2) T) (n is an integer), the optical frequency chirp is negative, and when t→n*T, the optical frequency chirp is almost linear, and the absolute value of the chirp reaches maximum.
A conceptual diagram is shown in Fig. 1. By appropriately adjusting delay to make Δφ=π/2, the negative chirp of large-signal directly modulated pulses can be enhanced, and the positive chirp of phase modulator can be prevented. The total optical frequency chirp is almost linear, with maximum negative chirp of:
Accordingly, an optimum length of DCF is used to compensate linear chirp, and the estimate optimum length of DCF can be given as:
where C is optical speed in vacuum, T0 is the FWHM of large-signal directly modulated pulse, D is dispersion parameter, λ is the central wavelength of optical pulse, and Δν is the FWHM of optical spectrum.
Substitute Δφ=π/2 into (4), and after Fourier Transform, we can get the spectral function:
From (10), the FWHM of optical spectrum with different phase modulation index and modulation frequency can be calculated assuming that the large-signal directly modulated pulse has a duty ratio of 25%, as shown in Fig. 2. So the FWHM of optical spectrum can be approximately shown as:
3. Experimental setup
Our short pulse source has a simple configuration, as shown in Fig. 3. 10GHz sinusoidal radio-frequency (RF) signals directly modulate a MQW-DFB laser (Irf=63mA, Ibias=50mA, Ithreshold=18mA), and produce 28-ps optical pulses at the wavelength of 1553 nm, as shown in Fig. 4. The optical spectrum is 0.494nm (about 62GHz in frequency), and the intensity modulation index is near 100%. The linewidth enhancement factor α can be calculated to about 6.2, so the phase modulation index Δθ1 is calculated to approximately 1.0π.
The large-signal directly modulated optical pulses pass through a variable optical delay line (ODL), and then are modulated by an integrated LiNbO3 phase modulator. The phase modulator is driven by the sinusoidal RF signals that are amplified by a RF amplifier. The RF power fed into the phase modulator is 23dBm, and the phase modulator index Δθ2 is approximately 0.9π. An adjustable optical delay line is used to allow temporal adjustment of the switching window created by the large-signal directly modulated pulses, which ensure the negative chirp of phase modulator to pass through and prevent the positive chirp. A polarization controller (PC) is used to align the polarization state of input light with the axis of phase modulator. The pulses with negative chirp that emerged from the phase modulator are then compressed into short pulses using a 147-m-long DCF (D=-142ps/nm/km) for chirp compensation.
The optical spectrum, eye diagram of sampling oscilloscope and autocorrelation trace are shown in Figs. 5(a) 5(b) and 5(c). The full-width at half-maximum (FWHM) of the optical spectrum of output short pulses is 0.912nm. The autocorrelation trace in the Fig. 5(c), measured by an intensity autocorrelation measurement with an assumption of gauss shape, shows a high quality 5.5-ps pulse with a pedestal suppression of about 20dB (limited by the measurement system resolution), which is suitable for OTDM system applications . The pulse has a time-bandwidth product of 0.63, and the pulse width is limited because DCF can only compensate for linear chirp and nonlinear chirp remains. The short pulses obtained in the present scheme are limited by the modulation index of the phase modulator. One way to obtain shorter pulse is to increase the modulation voltage, and another way is to serialize several phase modulators.
In contrast, we remove the directly modulated microwave signal, and use it as a CW light source. Besides, we use a 306-m-long DCF (D=-142ps/nm/km) to compensating for negative chirp. The optical spectrum and waveform are shown in Figs. 6(a) and 6(b). The FWHM of optical spectrum is only 0.454nm, because it doesn’t include the phase modulation produced by large-signal directly modulation. From Fig. 6(b), we can see that the pulses have a large pedestal, whose suppression ratio is only 5.6dB, because both negative chirp and positive chirp pass through a positive dispersion medium. The negative part of chirp can be compressed into the pulse and the positive part of chirp becomes scattered light.
Eye diagram accumulation of five minutes in Fig. 5(b) demonstrates the short pulses with long-term stability. Figure 7 shows the phase noise measurements with a microwave spectrum analyzer ranging from 100 Hz to 1 MHz offset frequencies. The phase noise corresponds to the RMS timing jitter of 165fs for HP 83711B signal generator, 184fs for the large-signal directly modulated pulses, 184fs for the 10 GHz short pulse source and 280fs for the phased modulated pulses without direct modulation. From Fig. 7, we can see that the phase noise of the 10 GHz short pulses source is the same as the phase noise of the large-signal directly modulated pulses, and close to the phase noise of HP signal generator. The phase noise of the phase modulated pulses without directly modulation is obviously increased relative to the 10 GHz short pulses source in the 100 KHz-1MHz regime, because the pulse produced by negative chirp can interfere with the scattered light produced by positive chirp, and beat noise is produced.
In summary, the 10GHz short pulse source with a directly modulated MQW-DFB laser and a phase modulator is very simple and reliable. The method has successfully provided high quality and stable 5.5ps pulses at 10GHz with low jitter of 184fs and high pedestal suppression of 20dB. This short pulse source is suitable for OTDM system, because of narrow pulsewidth, high pedestal suppression, low timing jitter, and long-term stability.
This work was partially supported by National Nature Science Foundation of China under Grant 60572013, Tianjin Nature Science Foundation under Grant 06YFJMJC01400, and the Program for New Century Excellent Talents in Ministry of Education of China.
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