Timing Jitter | previous | next | feedback |
Definition: fluctuations of the temporal positions of pulses
Pulse trains, generated e.g. in mode-locked lasers, exhibit some deviations of the temporal pulse positions from those in a perfectly periodic pulse train. This phenomenon is called timing jitter and is important for many applications, e.g. for long-range optical fiber communications or for optical sampling measurements. Another type of timing jitter occurs in Q-switched lasers, as also discussed below.
Timing Jitter of Mode-locked Lasers
The timing errors considered can be of different kinds:
- the deviations between the temporal pulse positions and those of perfectly regular clock ticks
- the deviations between the temporal pulse positions and those of the ticks of a real (noisy) oscillator (e.g. the electronic oscillator which drives the modulator of an actively mode-locked laser)
- the deviations of the pulse-to-pulse spacing from the average pulse period (pulse-to-pulse jitter or cycle jitter)
In telecom systems, the relevant jitter can be that between data-carrying pulses and a clock signal. The latter may have been extracted from the data stream itself, or transmitted separately. In the former case, the low-frequency jitter is transferred to the extracted clock signal, and often is not relevant for detection.
Timing errors may be quantified (→ noise specifications) in different ways:
- with an r.m.s. (root-mean-square) value for a certain measurement bandwidth
- as a power spectral density, either of the timing deviation or of the timing phase
Timing jitter is related to phase noise in the optical frequency components of the pulse train. In the absence of technical noise, the jitter of a mode-locked laser is limited by quantum noise, but in most cases it is dominated by vibrations and drifts of the laser resonator. Important theoretical results, based on analytical and numerical modeling, are discussed in Refs. [10, 12].
Figure 1: Theoretically calculated quantum-limited timing jitter spectrum of a 10-GHz Er:Yb:glass miniature laser. In reality, the low-frequency noise (e.g. below 10 kHz) is higher due to technical noise. To some extent, relaxation oscillations (at ∼ 230 kHz) are converted into timing jitter. With a feedback system, the long-term timing drift can be suppressed, so that the low-frequency jitter is strongly reduced.
The timing jitter of various kinds of mode-locked lasers (e.g. miniature bulk lasers, fiber lasers, or external-cavity diode lasers) can be very small (see Figure 1) – in some cases significantly smaller than that of high-quality electronic oscillators. This applies particularly to short time scales, where a laser can be used as a very precise timing reference (as a kind of flywheel). The long-term timing drifts can also be suppressed to extremely small levels using self-referenced frequency combs.
Measurement of Timing Jitter
There are a variety of different methods for measuring the timing jitter of mode-locked lasers:
- The popular von der Linde method [2] is based on the evaluation of RF spectra of photodiode signals. It is fairly simple, but rests on assumptions (small fluctuation amplitudes, no correlations between intensity and phase fluctuations, etc.) which are not always well satisfied, and is subjected to various technical limitations. In particular, phase noise of the local oscillator of the electronic spectrum analyzer can affect the results [11].
- The phase detector method (see e.g. [3]) is often applied to actively mode-locked lasers, where the laser timing is compared with that of the electronic driver signal (residual jitter). It is not easy to achieve good suppression of the influence of intensity noise, particularly due to the phenomenon of drifting mixer offsets. Refined methods are available to measure relative timing jitter between different lasers, which must, however, be synchronized.
- The relative timing jitter of free-running passively mode-locked lasers (but also of synchronized lasers) can be measured with a versatile method based on a beat note of photodiode signals, generated with appropriate RF techniques and numerical processing [11]. This method is very versatile, extremely sensitive, and relatively immune to a range of problems as frequently encountered with other methods. It is not affected by electronic oscillator noise and mixer offsets, and only very weakly by intensity noise. By comparison of different lasers, the absolute timing jitter can be obtained. For example, if one laser is known to have a lower jitter, the measured relative jitter is close to the absolute jitter of the other laser. If both lasers are very similar but uncorrelated, the recorded power spectral density at each noise frequency is twice that of a single laser.
Timing Jitter of Q-switched and Gain-switched Lasers
In an actively Q-switched laser, there is some time delay between the opening of the Q switch and the generated pulse. The magnitude of this time delay is subject to fluctuations, so that a timing jitter results even if the modulator signal is very regular. The origin of that jitter may be fluctuations of the pump power, but thermal effects, vibrations and other disturbances can also contribute. A variation of the pulse timing by more than the pulse duration (Figure 2) is not unusual in such lasers. The jitter can be reduced e.g. with injection seeding.
Figure 2: Simulated timing jitter of an actively Q-switched laser, caused by fluctuations of the pump power. The timing fluctuations are strongly correlated with those of peak power and pulse duration.
In passively Q-switched lasers, fluctuations of pump power may lead to a larger timing jitter, as a pulse is emitted as soon as the laser gain becomes high enough to overcome the losses. On the other hand, noise of the pulse energy is reduced.
If pulses from an actively Q-switched laser are used in a timing-critical setup, it can be a good approach to trigger that setup with a photodiode signal indicating the pulse arrival times, rather than with the modulator signal.
Gain-switched lasers also exhibit relatively large timing jitter, compared with mode-locked lasers.
Bibliography
| [1] | J. P. Gordon and H. A. Haus, “Random walk of coherently amplified solitons in optical fiber transmission”, Opt. Lett. 11 (10), 665 (1986) |
| [2] | D. von der Linde, “Characterization of the noise in continuously operating mode-locked lasers”, Appl. Phys. B 39, 201 (1986) |
| [3] | M. J. W. Rodwell et al., “Subpicosecond laser timing stabilization”, IEEE J. Quantum Electron. 25 (4), 817 (1989) |
| [4] | D. J. Derickson et al., “Residual and absolute timing jitter in actively mode-locked semiconductor lasers”, Electron. Lett. 26 (24), 2026 (1990) |
| [5] | H. A. Haus and A. Mecozzi, “Noise of mode-locked lasers”, IEEE J. Quantum Electron. 29 (3), 983 (1993) |
| [6] | L. A. Jiang et al., “Quantum-limited noise performance of a mode-locked laser diode”, Opt. Lett. 27 (1), 49 (2002) |
| [7] | M. C. Gross et al., “Spectral method for the simultaneous determination of uncorrelated and correlated amplitude and timing jitter”, Appl. Phys. Lett. 80 (20), 3694 (2002) |
| [8] | M. E. Grein et al., “Observation of quantum-limited timing jitter in an active, harmonically mode-locked fiber laser”, Opt. Lett. 27 (11), 957 (2002) |
| [9] | F. W. Helbing et al., “Carrier–envelope offset phase-locking with attosecond timing jitter”, IEEE J. Sel. Top. Quantum Electron. 9, 1030 (2003) |
| [10] | R. Paschotta, “Noise of mode-locked lasers. Part I: numerical model”, Appl. Phys. B 79, 153 (2004); R. Paschotta, “Noise of mode-locked lasers. Part II: timing jitter and other fluctuations”, Appl. Phys. B 79, 163 (2004) |
| [11] | R. Paschotta et al., “Relative timing jitter measurements with an indirect phase comparison method”, Appl. Phys. B 80 (2), 185 (2005) |
| [12] | R. Paschotta et al., “Optical phase noise and carrier–envelope offset noise of mode-locked lasers”, Appl. Phys. B 82 (2), 265 (2006) |
| [13] | F. Quinlan et al., “Ultralow-jitter and -amplitude-noise semiconductor-based actively mode-locked laser”, Opt. Lett. 31 (19), 2870 (2006) |
| [14] | S. Gee et al., “Self-stabilization of an actively mode-locked semiconductor-based fiber-ring laser for ultralow jitter”, IEEE Photon. Technol. Lett. 19 (7), 498 (2007) |
| [15] | O. Prochnow et al., “Quantum-limited noise performance of a femtosecond all-fiber ytterbium laser”, Opt. Express 17 (18), 15525 (2009) |
| [16] | R. Paschotta, “Timing jitter and phase noise of mode-locked fiber lasers”, Opt. Express 18 (5), 5041 (2010) |
| [17] | R. Paschotta, H. R. Telle, and U. Keller, “Noise of Solid State Lasers”, in Solid-State Lasers and Applications (ed. A. Sennaroglu), CRC Press, Boca Raton, FL (2007), Chapter 12, pp. 473–510 |
See also: noise specifications, timing phase, laser noise, phase noise, mode-locked lasers, frequency combs, Gordon–Haus jitter
Categories: fluctuations and noise, metrology
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