Synchronizing Lasers for Time-Resolved Microscopy and Spectroscopy

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Spectroscopy and microscopy enable the discovery and analysis of new phenomena in physics, chemistry, biology, and medicine, by using coherent light as the source for excitation and detection of the (electro-) optical response of the scanned sample. Pulsed lasers can be used to visualize dynamic processes with temporal resolutions of several femtoseconds and of microscopic spatial scale.

While titanium-sapphire lasers (Ti:sapph) have been used for many time-resolved or nonlinear spectroscopy and microscopy techniques in the past, recent advancements in laser technology have made fiber lasers equal alternatives. Their modularity and flexibility, in combination with powerful and reliable operation, make them robust tools. Several pulsed fiber laser outputs can be driven using only one mode-locked oscillator for multicolor or pump-probe measurements. This yields synchronized, phase-stable pulses between the individual laser outputs.

Moreover, commercial fiber lasers reliably produce pulses with excellent beam quality, best stability between consecutive pulses, and in durations below 30 fs. Thanks to the large surface of the amplifier fibers, high-energy conversion efficiency and very effective heat dissipation can be achieved. Saturable Absorber for passive mode-locking results in noise of very low intensity. This allows the development of highly efficient and cost-effective laser systems that are ideally appropriate for time-resolved and nonlinear microscopy.

Fiber Lasers for Spectroscopy and Microscopy

TOPTICA Photonics built a system made of several femtosecond fiber lasers that are asynchronously synchronized specifically for time-resolved spectroscopy and microscopy. The laser system presented in this article is an advanced pump-probe excitation source based on asynchronous optical sampling (ASOPS). Laser Quantum provided the synchronization electronics.

Figure 1. The full laser system (a and b) with its electrical (black) and optical (blue) connections: Each of the oscillators acts as Master or Slave for asynchronous optical pump-probe experiments. Both amplifiers (AMP) can be optically seeded by the Maser/Slave-system. They are spectrally tunable from 485 to 640 nm and from 830 to 1100 nm. The pump diodes for the main amplifiers are located in an external control unit for each laser for thermal decoupling. The ASOPS unit is connected to the Master/Slave lasers to enable synchronization of the pulse trains of both lasers.

Four individual fiber lasers are synchronized by the ASOPS electronics (Figure 1). Among these lasers, two are high-performance, fixed-frequency fiber lasers of TOPTICA’s FemtoFiber ultra product line. They provide laser pulses at a 780 nm central wavelength with 0.5 W average power and 150 fs, correspondingly 1050 nm with 5 W and <120 fs. A motorized compressor can be used to tune the pulse duration through the graphical user interface which enables compensation of the dispersion of the optical system (i.e. microscope objective).

The lasers are fitted with specialized oscillators that modulate the individual repetition rate by changing the resonator lengths with the help of piezo elements. This process enables them to be synchronized to one fixed repetition rate to achieve ASOPS. Laser Quantum’s TL-1000-ASOPS unit for pulse synchronization can be used to set the offset-frequency (ASOPS frequency).

Figure 2. Neither the ASOPS-synchronization nor output power scaling has an effect on the pulse duration of the Master/Slave lasers. The pulse shape was reconstructed using a numerical fit of the interferometric autocorrelation of the respective pulses.

The described changes in the resonator length (and hence the repetition rate) do not have any impact on the intensity of the laser pulses or the pulse duration (Figure 2). Additionally, the pulse duration of the FemtoFiber ultra lasers remains constant over the entire power range, that is, 0.5 W at 780 nm, and also 5 W at 1050 nm.

Noise of very low intensity in the observed frequency band is needed for high-resolution pump-probe experiments performed using synchronized lasers. This is provided by the patented SESAM-based oscillator technology of TOPTICA’s FemtoFiber lasers, as illustrated in Figure 3.

Figure 3. The relative intensity noise (RIN) of the FemtoFiber ultra 1050 fiber laser in the frequency region between 10 Hz and 10 MHz is compared to the detector noise (black curve).

The oscillator pulses of either the Femto Fiber ultra 1050 or 780 seed up two extra, tunable laser amplifier systems. These lasers are designed to cover a wide wavelength tuning range in the near-infrared spectral range (FemtoFiber pro TNIR, 930–1100 nm) and in the visible spectral range (FemtoFiber pro TVIS, 485–640 nm), as shown in Figure 4.

The Master and Slave oscillators can be freely selected for this modular laser system. Moreover, both the Master oscillator and the Slave oscillator can be used as seed independently for each of the two amplifiers. With this flexible setup, experiments with up to four synchronized excitation pulses are possible. On the other hand, two completely independent pump-probe experiments can be performed.

Figure 4. The amplifier systems support a relatively broad wavelength tuning from 485 to 640 nm (a), and 830 to 1100 nm (b), respectively

ASOPS

Laser Quantum’s synchronization electronics is based on patented DDS-technology (direct digital synthesis) and reaches temporal resolutions of less than 100 fs within a measurement interval of 1 ns. This was realized using two 1 GHz Ti:sapph oscillators. The temporal resolution using ASOPS is restricted by the repetition rate offset of pump and probe lasers following Equation 1:

Δτ = ΔfR/ΔfR2 (Eq. 1)

Hence, the temporal scan range is only restricted by the interpulse distance (i.e. 10 ns for 100 MHz repetition rate) of the emitted pulse trains without using an extra mechanical delay. The probing of the pulse is carried out at the sampling frequency of ΔfR. For data acquisition purposes, the intensity of the probe beam is digitally monitored and rescaled by multiplying the data with the factor ΔfR/fR (Figure 5). As a result, the ASOPS technology is not affected by electronic noise below ΔfR. And hence, most cases do not require lock-in detection.

Equation 1 illustrates that high repetition rates are advantageous for the sampling frequency and the temporal resolution using ASOPS. However, at repetition rates equal to or below the 100 MHz range, the temporal resolution is of the order of slightly above 100 fs (180–320 fs for 80 MHz system over the whole temporal delay of 12.5 ns, as shown in Figure 6). Such comparatively low repetition rates featuring interpulse distances of 10–20 ns are necessary for the visualization of longer decay times.

In order to completely characterize the employed ASOPS system, it is necessary to know the temporal resolution as a function of pulse delay, resulting in best performance over the entire scan range allowing for the detection of fast decay times far off the time zero (pulse overlap of pump and probe). This is only possible by using the DDS technology employed by Laser Quantum within their TL-1000-ASOPS unit which outperforms other technologies, such as the application of two independent reference signals to stabilize two lasers.

This concept is subjected to random phase noise while it was shown that when a single-sideband signal was used as a reference, the temporal resolution was considerably increased with increase in temporal delay.

Figure 5. Asynchronous optical sampling is performed using a combination of two lasers with an optically generated terahertz-pulse (1 GHz repetition rate, 10 kHz difference frequency). The temporal resolution τ increases linearly with the time. This way, the complete measurement region is sampled between two consecutive laser pulses.

The fiber lasers in this experiment are synchronized using the 125th harmonic of their repetition rate (80 MHz). The measurement of cross-correlation of a double-pulse with variable temporal delay of one of the two lasers is adopted to characterize the performance of the system. Thus, the achievable temporal resolution is assessed based on the position in the temporal measurement interval.

At small temporal delay, the FWHM of the cross-correlation response is around 180 fs, which corresponds to two convoluted Gauss-shaped pulses with 120 and 150 fs. For delay settings between 50 and 100 ps, the cross-correlation FWHM rises to about 320 fs. This is because of the uncompensated phase noise between two lasers in the frequency interval from 1 Hz to 10 kHz.

Figure 6. Full width at half maximum of the cross-correlation of both fiber lasers as a function of the position in the measurement region during asynchronous optical sampling (a), and phase noise between both lasers during stabilized operation (b).

Results

Kerr-rotations can be measured using the presented laser system. In these experiments, precession motion of the spin magnetization is induced in ferromagnetic materials by powerful pump pulses. This precession motion causes the emission of intense microwave radiation, which in turn causes the linear polarization of probe pulses to rotate. This polarization rotation is detected as a function of the temporal delay between the pump and probe pulses. Using these techniques, the temporal evolution of the spin precession motion is detected.

Earlier, it was possible to change the temporal delay only using mechanical delay stages with a relatively low accuracy of about 1 ps. As the mirrors of the delay stage have to be displaced to vary the temporal delay between pump and probe pulses, this method also needs long measurement intervals.

With an ASOPS setup, mechanical delay stages are not required. The detected signal is Fourier-transformed to obtain the temporal evolution of the spin precession motion. As the Fourier transformation requires only little acquisition times, an excellent signal-to-noise ratio is realized.

The presented laser system enables the manipulation of the microwave signal with extra laser pulses. As a result, the setup facilitates a unique flexibility for pump-probe measurements or multicolor excitation. It demonstrates the excellent modularity and variability of modern fiber lasers in time-resolved microscopy applications. Synchronized modular systems support time-resolved measurements as well as enable complex pump-probe methods such as ASOPS.

Fiber lasers not only offer the required tunability and flexibility but also the required pulse parameters. In addition, fiber lasers are very user-friendly: Their “push-button turnkey operation” ensures a simple initialization of the lasers without the need for alignment. Moreover, the advanced fiber lasers operate maintenance-free and they do not need expensive laboratory infrastructure such as high-voltage current or water cooling. This helps the user to fully concentrate on their experiment.

This information has been sourced, reviewed and adapted from materials provided by Toptica Photonics.

For more information on this source, please visit Toptica Photonics.

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