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Synchronization of Lasers

Author: the photonics expert (RP)

Definition: the synchronization of light pulses generated with lasers

Categories: article belongs to category laser devices and laser physics laser devices and laser physics, article belongs to category fluctuations and noise fluctuations and noise, article belongs to category light pulses light pulses, article belongs to category methods methods

DOI: 10.61835/szm   Cite the article: BibTex plain textHTML   Link to this page   LinkedIn

The synchronization of lasers usually means that the temporal positions of generated laser pulses are adjusted such that the pulses from two lasers temporally coincide, or that the pulses from one laser coincide with some electronic signal. This is more clearly called timing synchronization. Some examples:

In some other cases, what is synchronized are actually the electric field oscillations of two continuous-wave lasers; this is more precisely called phase synchronization and is required e.g. for coherent beam combining. Both types of synchronization are treated in the following.

Synchronization of Pulses from Mode-locked Lasers

Mode-locked lasers emit very regular trains of ultrashort pulses. If two such lasers are operated with precisely the same pulse repetition rate, it can happen that each pulse emitted by one laser temporally coincides with one pulse of the other laser, and this over a long time.

If a laser is actively mode-locked, the emitted pulses are naturally in synchronism with the electronic drive signal. However, stable operation of such a laser often requires an active feedback system acting on the resonator length, since the optical modulator used for active mode locking has only a very limited power to “pull” the circulating pulses towards the desired temporal positions. In the case of passive mode locking, resonator length control (or a similar message for controlling the resonator round-trip time) is the only way to control the pulse timing.

The relative pulse timing can be disturbed by various effects. In most cases, it is dominantly affected by drifts of the length of the laser resonator and by mechanical vibrations. For example, consider a laser emitting 1-ps pulses with a pulse repetition rate of 100 MHz. If the resonator length changes only by 1 nm, this implies a timing error of ≈3.3 as (attoseconds) per round trip. Although that is not very much, within a million resonator round trips (i.e., within 0.01 seconds), the accumulated pulse timing error is already 3.3 ps, i.e., several times the pulse duration, and the temporal overlap of pulses is lost. Similarly, small temperature changes e.g. within a laser crystal can also affect the pulse timing. Fast vibrations are less of a problem, since the pulse timing cannot drift very far within a short oscillation period of such a vibration.

Due to the extremely high sensitivity of the pulse timing to various effects, long-term synchronized operation of such lasers requires the use of some automatic feedback system. It is quite easy to modify the resonator length e.g. with a piezo transducer mounted below one of the resonator mirrors with a range of e.g. a few micrometers; the greater challenge is to precisely measure the timing error to obtain a suitable error signal for driving the transducer.

The measurement precision achievable with ultrafast photodetectors and electronics is in many cases insufficient for that purpose. However, there are all-optical techniques – in particular, the use of an optical cross correlator. Here, the two pulses, the timing of which must be compared, are sent into a nonlinear crystal where some nonlinear interaction (e.g. sum frequency generation) can take place only if the two pulses meet within the crystal. A moderately fast photodetector, registering the nonlinear mixing product, delivers the signal, and this depends very sensitively on the relative timing – however, without telling the sign of the timing error, which is of course relevant for finding the required resonator length correction. In order to obtain information on that sign as well, one can use a balanced cross correlator, where one creates two such signals, where for one of these a small timing change is applied to one of the pulses, so that the maximum signal from both detectors is achieved for slightly different relative timings. The difference of the two signals then provides a suitable error signal, offering a very high timing discrimination (e.g. some volts per picosecond timing error or even more) together with the needed sign information.

If such an automatic feedback system works well and the lasers are well protected against vibrations, temperature changes, pump power fluctuations and the like, the remaining timing jitter can be extremely small: in some cases it has an r.m.s. value far below one femtosecond [6], i.e., a very small fraction of the pulse duration.

The control of the resonator length is most frequently used, but not the only possibility. For example, it has been shown that the timing of a passively mode-locked laser can also be influenced by control pulses hitting the saturable absorber used for mode locking [3]. Also, passive synchronization is possible if two lasers can interact by cross-phase modulation in a laser crystal, for example [2, 9].

It is also possible to synchronize the outputs of ultrafast amplifiers for high pulse energies. Such amplifiers often contain a regenerative amplifier, in which the amplified pulse does many resonator round trips. Nevertheless, the obtained pulse timing can be quite precise – not limited by the switching precision of a Pockels cell used for injecting and extracting the pulse. Before the amplifier system, one usually uses a pulse picker for reducing the pulse repetition rate by a certain factor. By changing that division factor, one can only achieve a course adjustment of the pulse frequency; for fine control, the pulse repetition rate of the seed laser needs to be controlled.

For the discussed kind of pulse synchronization, the optical frequencies are not relevant, but only the pulse envelopes; the optical frequencies may differ substantially between the two lasers. In some cases, however, one needs not only the synchronization of the pulse envelopes, but even phase synchronization of the oscillations of the electric fields (similar to the case of continuous-wave lasers, see below). For example, such kind of synchronization allows one to combine two ultrashort pulse lasers such that one obtains an overall wider bandwidth and shorter output pulses [4]. In that case, one requires two independent controls, acting on two different parameters of the laser emission: the pulse timing and the carrier–envelope offset. For example, one can use the combination of a piezo-actuated mirror and pump power control. See the article on carrier–envelope offset for more details.

Synchronization of Pulses from Q-switched Lasers

The technical details of the synchronization of actively Q-switched lasers are very different from those for mode-locked lasers:

  • The pulse durations are far longer – typically several nanoseconds or more. The required precision is often just some fraction (e.g. one tenth) of the pulse duration. This is sufficient, for example, if one does some kind of nonlinear frequency conversion such as sum or difference frequency generation.
  • The resonator length is not a critical parameter because the pulse timing is essentially determined by the gain dynamics, and small resonator length changes have a minor effect on the pulse duration.
  • The pulse emission in an actively Q-switched laser can simply be triggered via the used optical modulator (e.g., an acousto-optic modulator). However, the output pulse will always have some significant delay, amounting to a large multiple of the pulse duration. This means that it must be known ahead of time when a pulse is required.
  • In addition, the mentioned pulse delay can substantially fluctuate, e.g. if the energy stored in the laser crystal is not always the same: the more energy is stored, the higher the gain, and the smaller will be the pulse delay. If two different actively Q-switched lasers need to be synchronized, one of them may need the trigger signal at an earlier time than the other one, and both lasers must be optimized such that the pulse delay is quite reproducible.

The situation is again different for passively Q-switched lasers. Here, the pulse emission can be triggered by pumping the gain medium such that the optical gain becomes sufficiently high for pulse emission. Relatively precise timing is possible with pulsed pumping, but there is again a substantial delay between the pump pulse and the emitted pulse. In the case of continuous pumping, leading to the emission of a more or less regular pulse train, the pump power can be used to adjust the pulse repetition rate.

Phase Synchronization of Continuous-wave Lasers

Phase synchronization of continuous-wave lasers means that the electric field oscillations of their outputs are synchronized. Because the optical frequencies are very high, this implies an extremely high timing precision – well below one optical cycle, which may last a few femtoseconds only.

A relatively simple method is injection locking. Here, the output of one laser (the master laser) is injected into the other one (the slave laser); under suitable conditions, this will force the slave laser to emit phase-synchronously.

Instead, one can also use a method based on active feedback. The following is needed:

  • Both lasers need to be optimized for a very narrow emission linewidth, i.e., for very weak random phase changes of the output.
  • A suitable error signal must be generated. This can be easily obtained by interference between the two outputs e.g. on a beamsplitter. The difference of electronic signals from two photodetectors behind the beam splitter clearly tells what the phase difference is (within some limited range of phase differences).
  • The optical frequency of at least one of the lasers needs to be controlled accordingly, e.g. with piezo-actuated mirror.

In a modified scheme, it is also possible to stabilize lasers such that the difference of the optical emission frequencies exactly fits to some electronic signal; again, a kind of phase locking is possible.

Quantifying the Quality of Synchronization

For a single pair of pulses from to lasers, one can simply specify the timing error as the time difference between the pulse maxima (or possibly the “center of gravity” in the time domain). However, one often requires statistical information on the timing errors. One possibility is to specify the power spectral density of the timing error. In many cases, it is sufficient to know some r.m.s. (root mean square) value, which can be obtained by integrating the power spectral density over some frequency range (i.e., some range of noise frequencies) and taking the square root. The result can of course depend on the mentioned frequency range; therefore, the result is meaningful only in combination with these two values.

For phase synchronization, the same quantification is possible as with timing synchronization, just replacing the timing error with the phase error.

More to Learn

Encyclopedia articles:

Bibliography

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[3]N. H. Bonadeo et al., “Passive harmonic mode-locked soliton fiber laser stabilized by an optically pumped saturable Bragg reflector”, Opt. Lett. 25 (19), 1421 (2000); https://doi.org/10.1364/OL.25.001421
[4]R. K. Shelton et al. “Phase-coherent optical pulse synthesis from separate femtosecond lasers”, Science 293, 1286 (2001); https://doi.org/10.1126/science.1061754
[5]R. K. Shelton et al., “Subfemtosecond timing jitter between two independent, actively synchronized, mode-locked lasers”, Opt. Lett. 27 (5), 312 (2002); https://doi.org/10.1364/OL.27.000312
[6]T. R. Schibli et al., “Attosecond active synchronization of passively mode-locked lasers by balanced cross correlation”, Opt. Lett. 28 (11), 947 (2003); https://doi.org/10.1364/OL.28.000947
[7]Y. Kobayashi et al., “Phase-coherent multicolor femtosecond pulse generation”, Appl. Phys. Lett. 83 (5), 839 (2003); https://doi.org/10.1063/1.1598651
[8]W. Seitz et al., “All-optical synchronization and mode locking of solid-state lasers with nonlinear semiconductor Fabry-Pérot mirrors”, IEEE J. Sel. Topics Quantum Electron. 9, 1093 (2003); https://doi.org/10.1109/JSTQE.2003.819100
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[12]J. Janousek et al., “Investigation of passively synchronized dual-wavelength Q-switched lasers based on V:YAG saturable absorber”, Opt. Commun. 265 (1), 277 (2006); https://doi.org/10.1016/j.optcom.2006.03.002
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(Suggest additional literature!)

Suppliers

The RP Photonics Buyer's Guide contains 17 suppliers for laser synchronization devices. Among them:

Geola

laser synchronization devices

The ICMSG-100-5 digital pulse/delay generator is a four-channel device for synchronizing various types of equipment. It is primarily used in scientific and technological industries, such as laser photonics. It provides precise control over the timing of electronic signals, allowing for accurate synchronization of components in complex systems, such as pulsed laser systems, optics experiments, and other advanced technologies. The device is available in versions for laboratory use and for OEM use.

Menlo Systems

laser synchronization devices

Synchronize your pulsed laser sources with highest accuracy: Menlo Systems' RRE-SYNCRO provides state-of-the art phase lock electronics in a complete and user-friendly system. Initially developed for the optical frequency comb technology, the RRE-SYNCRO is the heart of numerous laser synchronization applications such as ASOPS and timing distribution. Hundreds of our systems serve in the field, from less complex applications, such as laser-to-RF and laser-to-laser synchronization, to complete solutions for the distribution of a master RF, an optical reference, or a PPS signal throughout a large-scale research facility, such as the geodetic observatory at Wettzell. With few fs achievable timing jitter, our synchronization technology is leading in terms of precision.

EKSPLA

SY4000 series pulse synchronization module with delay generator is designed to create up to 8 delayed output pulse sequences precisely synchronized to internal or external clock. The SY4000 module is a timing generator dedicated to the synchronization of laser components: AOM drivers, Pockels cell drivers, laser diode and flash lamp drivers, detectors, data acquisition systems, etc.

TOPTICA Photonics

laser synchronization devices

TOPTICA offers solutions for synchronization of ultrafast fiber laser and continuous-wave (cw) lasers. Synchronized femtosecond fiber lasers are a key element for applications like time-resolved microscopy and pump–probe spectroscopy. Phase and frequency locking of TOPTICA’s broad portfolio of tunable cw diode laser is supported by a range of versatile laser locking electronics.

Cycle

laser synchronization devices

The primary element to realize a high precision synchronization device is the timing detector because it dictates the smallest timing error that can be detected. Cycle offers the lowest noise timing detectors in the market based on its patented balanced optical cross-correlator (BOC) technology. Due to a balanced optical detection scheme, the BOC provides exceptionally high timing sensitivity, attosecond timing resolution, amplitude invariance and robustness against environmental fluctuations. The output of the BOC is a baseband voltage signal that is proportional to the timing jitter between the two optical pulse trains. This output is used in a phase-locked loop to synchronize the two optical sources.

We offer BOCs to synchronize lasers operating on 800 nm, 1030 nm and 1550 nm wavelengths. Additionally, we also offer two color BOCs (TCBOC) to synchronize lasers operating different center wavelengths (e.g., locking a Ti:Sapphire laser to a Er fiber oscillator).

EKSMA OPTICS

laser synchronization devices

EKSMA Optics offers a pulse synchronization module with the delay generator TG10 that is designed to create up to 8 delayed output pulse sequences precisely synchronized to an internal or external clock.

AeroDIODE

laser synchronization devices

SHIPS TODAY: the AeroDIODE TOMBAK pulse delay generator provides high frequency pulses, delays, and bursts. It is an ideal testing and timing control instrument for electronics, lasers, or camera setup. The adjustable detection threshold is as low as a few mV. This makes it an ideal tool for detecting ultra-short pulses through a photodiode. The pulse delay generator offers several operating modes including stand-alone generator, digital delay generator, frequency divider, burst generator, pulse picker, voltage level converter and arbitrary waveform generator (AWG). It is often used in a fiber modulation setup with EOMs, AOMs or SOAs. Applications include component testing, laser timing control, laser pulse picking, camera synchronization etc.

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