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We demonstrate a novel, self-starting optoelectronic oscillator based on an electro-absorption modulator in a fiber-extended cavity for generating an optical pulse stream with high-rate and ultra-low jitter capabilities. Optical pulses at 10GHz repetition rate are demonstrated with >90dBc/Hz side-mode suppression and the lowest timing jitter (42fs in the 100Hz–1MHz range) reported to date for a self-starting source. Along with the optical pulse stream, the oscillator also generates a 10GHz electrical signal with ultra-low phase noise.
©2003 Optical Society of America
1. Introduction
The ever increasing demand for higher bit rates in optically time-domain-multiplexed communication links, and in signal processing systems employing optical sampling, requires low-noise, high-repetition-rate, short-pulse sources. The performance of such systems is limited by the timing jitter of the pulse sources, which has led to an ongoing research effort to study and reduce this noise.
High repetition rate pulse sources are usually implemented by active mode locking of fiber or diode lasers [1–2] or by gating continuous-wave (CW) light with an electro-absorption modulator (EAM) [3], all of which require a microwave-driving source whose phase noise determines or limits the resultant jitter [4–6]. For example, by using an exceptionally low phase noise synthesizer (HP70428A), a 10GHz-rate fiber laser with a jitter lower than 10fs (in the 100Hz to 1MHz range) was demonstrated by Clark et al. [5]. Recently, Jiang et al. and Grein et al. reported on the observation of quantum-limited noise performance of mode-locked fiber and diode lasers by using a unique ultra-low-noise 9GHz-rate microwave source (“Poseidon Oscillator”) [7–8].
Passively mode-locked lasers, on the other hand, are self-starting, so an external microwave drive is not needed, but they tend to have more jitter than actively mode-locked lasers. A different approach for obtaining a self-starting pulse source is to extend the concept of an optoelectronic oscillator (OEO) suggested by Yao and Maleki [9] to include generation of an optical pulse train [10–11]. In this approach, in order to achieve low-jitter optical pulses, one drives a mode-locked diode laser, or a semiconductor optical amplifier in a ring cavity, by the low-phase-noise microwave signal obtained from the optoelectronic oscillator. This tandem approach makes the overall system very complicated.
In this paper, we report on a new type of compact, self-starting, OEO that incorporates an EAM, an optical-fiber delay line, and optical detection in a closed-loop resonating configuration. This hybrid source simultaneously generates, within the same optoelectronic cavity, an ultra-low-jitter stream of short optical pulses with high repetition-rate capabilities and the corresponding low-phase-noise microwave signal. We demonstrate the generation of 10GHz-rate, 20ps-wide, optical pulses with >90dBc/Hz side-mode suppression and with the lowest timing jitter reported to date for any self-starting configuration, i.e., 42fs over the 100Hz–1MHz frequency range and 19fs for the 1kHz to 1MHz range. The phase noise of the 10GHz electrical signal is ~-115 dBc/Hz at 10kHz offset from the carrier. The pulse-rate of this source can be easily scaled to 40GHz or higher by use of higher bandwidth EAM and other optoelectronic components in the cavity.
2. Principle of operation
A generic schematic of our OEO is shown in Fig. 1. It is similar to the scheme suggested in Ref. [9]; the main difference being the use of an EAM instead of an electro-optic Mach-Zehnder modulator. Our system utilizes the nonlinear transmission characteristics of the EAM (as shown in Fig. 1 inset) to convert CW light into a low-jitter stream of optical pulses, whose detection simultaneously creates a spectrally pure microwave signal. Light from a CW laser is injected into the EAM, whose output after passing through a long fiber is converted to an electrical signal by a photodetector. The output of the photodetector is amplified, filtered by a microwave bandpass filter, and fed back through a driver to the electrical input of the EAM. This configuration supports self-sustained oscillations at a frequency determined by the bandpass characteristics of the electrical filter, the bias setting of the EAM, and the length of the fiber. High stability and spectral purity are a consequence of the large energy stored in the optoelectronic cavity and the low round-trip loss due to propagation in the fiber. Therefore, the key to obtaining the lowest timing-jitter performance is to use the longest possible length of fiber in the loop. However, because the OEO’s longitudinal-mode spacing is inversely proportional to the cavity length, the ability of the electrical bandpass filter in selecting one longitudinal mode for oscillation is considerably enhanced by the sharp transmittance window of the EAM.
3. Experiment and results
CW light from a DFB laser at 1550nm is introduced into a 14GHz-bandwidth InGaAsP EAM having a 20dB extinction ratio and a bias voltage of -3V. The use of a high-Q electrical bandpass filter (Q ~ 1000) allows, together with the sharp rectangular-shaped transmittance window of the EAM, up to 3km of fiber to be used in the loop, yielding a 10GHz-rate microwave signal and optical pulse stream with >90dBc/Hz side-mode suppression. The oscillation condition is determined by the optoelectronic loop gain, which is controlled by the optical power and the electronic gain. Once the oscillation threshold is reached, the output becomes very stable.
The 10GHz-rate optical pulses are monitored at the output of the EAM and characterized in the time domain using a 45GHz-bandwidth photodetector/oscilloscope combination. Figure 2(a) shows the oscilloscope trace for the optical pulse stream, where the pulse width is measured to be 20ps. The optical spectrum of the clock pulses [Fig. 2(b)] is very symmetric with more than 20dB deep modulation of the 0.08nm-spaced peaks corresponding to the 10GHz pulse rate. The width of the modulation envelop Δν=0.18nm, which gives a time-bandwidth product of ΔνΔτ~0.45 that is almost transform limited for an assumed sech 2 pulse shape.
Fig. 2. (a) Time domain measurement of the optical pulses with a 45GHz bandwidth photodetector. (b) Optical spectrum with 0.01nm resolution.
Figure 3 shows the single-side-band (SSB) phase-noise spectrum of the 10GHz-detected signal monitored at the output of the electrical filter. The figure illustrates two important features: (a) The low-phase-noise behavior. For example, the phase noise at 1kHz and 10kHz offsets from the carrier is ~-100dBc/Hz and -115dBc/Hz, respectively. (b) The spectrum contains a series of peaks with ~67kHz separation that correspond to the non-oscillating side modes of the 3km-long OEO cavity. As shown, these side-modes are suppressed by >90dBc/Hz.
To evaluate the timing jitter, σJ, of the optical pulse stream, we integrate the spectral noise-power density over the offset frequency range of interest using the equation
where L(f) is the phase-noise spectral density, f R is the repetition-rate, and f min and f max are boundaries of the frequency range. Note that this value is an upper bound for the timing jitter, since the total integrated noise is due both to amplitude fluctuations as well as timing jitter [12]. We measured the timing jitter for five different lengths of standard single-mode fiber in the cavity: 400m, 1km, 1.7km, 2.4km, and 3km. Figure 4 shows the results for two different integration boundaries: 100Hz to 1MHz and 1kHz to 1MHz. One clearly observes improvement of the timing jitter with the cavity length, yielding inverse square-root length dependence, i.e., the phase-noise spectral density is inversely proportional to the cavity length. For the 3km-long fiber, the calculated RMS jitter from the measured spectrum was 42fs in the 100Hz to 1MHz frequency range and 19fs in the 1kHz to 1MHz frequency range. Note that the timing jitter could not be reduced further with longer lengths of fiber in the OEO cavity because of the limit imposed by random fiber-length fluctuations due to environmental effects. However, to the best of our knowledge, this is the lowest timing jitter obtained for a self-staring 10GHz-rate optical pulse source.
We would like to note that along with the various applications of such an ultra-low jitter optical-pulse and microwave-signal source, the EAM-based OEO could also act as a special voltage-controlled oscillator (VCO) with optical as well as electrical inputs and outputs. The ability to change the oscillation frequency by changing the bias voltage to the EAM, the bandpass characteristics of the electrical filter, or the length of the fiber, makes it possible to synchronize the OEO to an incoming optical, or electrical, signal via the injection locking process for clock recovery, demultiplexing, and other signal processing applications.
4. Conclusion
We have demonstrated the use of the inherent nonlinearity of an electro-absorption modulator in a self-starting optoelectronic oscillator to generate 20ps-wide optical pulses at 10GHz repetition rate with more than 90dBc/Hz side-mode suppression. The timing jitter of the pulse stream was characterized and measured to be ~40fs over a frequency range of 100Hz to 1MHz; this is the lowest jitter, to the best of our knowledge, reported for a self-starting 10GHz pulse source. A low phase-noise 10GHz electrical signal is simultaneously generated that can serve as a high quality synchronized reference for applications of this pulse source. The pulse-rate of this source can be easily scaled to 40GHz or higher by use of higher bandwidth EAM and other optoelectronic components in the cavity.
Acknowledgment
This work was supported by the National Science Foundation under Grants: ANI-0123495, ECS-0000241, and the IGERT DGE-9987577.
References and Links
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