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Application Note - High-Speed Asynchronous Optical Sampling | |
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Ultrafast time-domain spectroscopy Ultrafast optical time-domain spectroscopy with femtosecond lasers has been an important tool for researchers and industrial screening engineers for more than two decades. Applications range from investigations of fundamental excitations and relaxation processes in gases, bulk solids and nanostructures via picosecond ultrasound-based characterization of layer thicknesses, e.g. in chip fabrication processes, to security screening and life-science applications of terahertz radiation. The time-scales of interest lie between a few tens of picoseconds and a few femtoseconds. Thus, classical electronics are too slow, and optical correlation techniques are therefore the standard method of choice.  Conventional ultrafast time-domain spectroscopy is based on pump-probe schemes in which a single femtosecond (fs)-laser provides the pump and the probe pulses. A pump pulse excites a sample under investigation and creates a non-equilibrium state. The evolution of that state is interrogated using a time-delayed pulse that probes the sample response as function of time-delay versus the pump pulse.  The time-delay is usually adjusted by varying the distances that the pulses travel and thus their relative time-of-flight before arriving at the sample. Many data points are acquired at different time-delays to complete a full measurement. In most cases one pulse travels over a retroreflecting mirror whose position can be modified using a motorized translation stage or a vibrating membrane. One drawback of such techniques is that each time when the retroreflector is moved for time-delay adjustment, the measurement is halted and thus valuable measurement time is lost. Another enormous disadvantage of mechanical delay-scanning devices is that their scanning speed is limited by the weight of the optomechanical components. In most cases motorized stages require approximately 10 seconds to 1 minute to acquire one transient. This is unpractical for online signal monitoring and optimization. Also, slow drifts in laser power, technical noise and environmental variations are not averaged out efficiently. Especially if several tens of picoseconds are scanned and multiple transients are averaged for noise reduction, measurement times can approach several tens of minutes. High-speed asynchronous optical sampling (ASOPS) High-speed asynchronous optical sampling circumvents these problems by eliminating mechancial delay scanning devices from ultrafast time-domain spectroscopy systems. To this end two femtosecond lasers with repetition rates fR=1 GHz are employed that are stabilized at an offset of ΔfR=10 kHz (offsets between 2 kHz and 20 kHz are possible). The faster laser serves as the pump laser, the slower one as the probe laser. As result of the detuning, successive pairs of pump and probe pulses arrive at the sample with a delay that incrementally increases by 10 fs with each pulse pair. Thus, the delay between pump and probe pulses is linearly ramped from 0 to 1 ns. The ramp is reset to zero whenever the faster pump laser 'overtakes' the probe laser after exactly 100µs (the inverse of ΔfR) and a new measurement cycle starts. The time-delay τ as function of real-time t is given by a straight-forward linear relation: τ=(ΔfR/fR)×t. See Fig. 2 for an illustration of the high-speed ASOPS time-delaying principle.  Fig. 2.: ASOPS scheme – the time delay between pump and probe pulses is scanned by slightly differing pulse repetition rates. The difference frequency ΔfR determines the scan rate, while the temporal measurement window is given by the inverse of the pulse repetition rate 1/fR. In a high-speed ASOPS system, the pump and probe pulses are applied to the sample in exactly the same way as in standard setups, except that they originate from two seperate femtosecond lasers. The probe laser is detected using a fast photoreceiver and digitized with a fast A/D converter as a function of real-time. The real-time scale is then simply converted to a time-delay scale by applying a scaling factor ΔfR/fR. High-speed ASOPS permits a boost to ultrafast measurement techniques that is probably comparable to the step from an analog data plotter to a digital sampling oscilloscope. High-speed ASOPS permits to scan a 1 ns time-delay window at a rate between 2 kHz and 20 kHz, a performance that would be impossible with conventional techniques. The time-delay resolution can be as low as 45 fs (see discussion below). The high scan rate makes online signal monitoring and optimimization possible and permits to suppress technical and environmental noise very efficiently. It can also greatly enhance the data acquisition speed of imaging applications of ultrafast time-domain spectroscopy. Apart from a much accelerated data acquisition, inherent advantages of high-speed ASOPS are the absence of changes in optical spot size at the sample location and the absence of pointing instabilities originating from the scan unit. With indpendently tunable pump-and probe-lasers the realization of two-color experiments is straightforward, providing additional versatility. What determines the time-resolution? In mathematical terms, the pulse pair-to-pair increment is given by ΔfR/fR2, representing the basic limit to the time-delay resolution of a high-speed ASOPS experiment. It amounts to 10 fs for fR=1 GHz and ΔfR=10 kHz. Another limitation is given by the available bandwidth B for signal detection and digitization. Suitable A/D converters with at least 14-Bit resolution are available with approximately 100 MHz sampling rate. As result, in a system with ΔfR=10 kHz, there are 10000 data points per 1 ns time- delay window, or 1 datapoint each 100 fs. Similarly, in a system with ΔfR=2 kHz, there is 1 datapoint per each 20 fs. The time-delay resolution resulting from B can be written as Δτ=ΔfR/(fR×B). From the above relations it becomes clear that repetition rates in the range of 1 GHz are favorable if a sub-picosecond time-delay resolution is required and a multi-kHz scanning rate is to be maintained. The quality of the feedback loop that controls the offset frequency also crucially determines that time-delay resolution of high-speed ASOPS. Gigaoptics' repetition rate offset locking unit TL-1000-ASOPS is capable of maintaining a 45 fs time-delay resolution over the entire available 1 ns measurement window. Main building blocks for high-speed ASOPS Main building blocks of a high-speed ASOPS system are theGigajet TWINdual femtosecond oscillator and theTL-1000-ASOPSrepetition rate offset locking unit. This combination permits ultrafast pump-probe experiments with the following features: • no mechanical delay scanner • real-time ultrafast spectroscopy • 2-20 kHz scan rate • 1-2 ns time delay window • 45 fs typical time-delay resolution • 1 GHz spectral resolution • 6x10-7 t-1/2 signal resolution (5 mW probe beam. t is averaging time in seconds [2]. • two-color pump-probe measurements   Applications High-speed ASOPS can be applied whereever conventional ultrafast techniques are used. The most wide-spread application is ultrafast pump-probe spectroscopy. See Fig. 3 for a typical setup of a high-speed ASOPS pump-probe setup. Application examples are spectroscopy of ultrafast electronic and lattice dynamics in semiconductors [1,2], observation of structural dynamics in nanostructures [3], and investigations of multilayer-structures using picosecond ultrasound [4]. See Figs. 1,5 and 6 for signals acquired with high-speed ASOPS. High-speed ASOPS is also highly suited for THz time-domain spectroscopy, where the real-time capability is a great benefit for THz based material investigation and screening applications [2,5,6]. Specifically for THz time-domain spectroscopy, Gigaoptics has developed the transmission spectroscopy systemHASSP-THz.It provides a convenient all-in-one solution for customers who wish to purchase a ready-to-use high-speed ASOPS spectrometer rather than spending time to build their own device.    Screen shot of HASSP-THz data acquisition and analysis software. Left panels show full THz signal and zoom into first 100 ps. Right panels show FFT of time- domain data and a sample transmission spectrum (sample was a cell with water vapor at atmospheric pressure). Here, the acquisition time was set to 1 second, i.e. each of the windows is updated at 1 Hz rate. Satellites on time-domain data and modulations on spectrum are due to multiple reflection within THz emitter. References
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