Theoretical Approach Why do we need ultra short technology?? INTRODUCTION: Generating ultrashort laser pulses that last a few femtoseconds is a highly active area of research that is finding applications ranging from micromachining to eye surgery. For optimum results it is often important to know a laser s wavelength, average power and pulse duration. While it is relatively easy to check the first two parameters with a spectrometer and a power meter respectively but measuring the latter is more complicated. The problem is that all conventional electronic photodetectors that convert a light signal into an electrical current respond too slowly to measure very short light pulses. The fact is, the most common
device used to determine the duration of less than picosecond pulses is an optical instrument called an autocorrelator - using an interferometer in combination with a nonlinear optical material. Other sophisticated pulse analysis equipment based on so-called FROG and SPIDER techniques can also measure the phase profile, amplitude and spectrum of a pulse. However, these really need their own article and will not be discussed here. PRINCIPLES: Scanning Autocorrelator work: The basic principle is a Michelson interferometer consisting of two arms ( about 50 mm long), one of which has its optical path length continually changed ( scanned ) to create a variable optical delay. An incoming laser pulse arriving at the autocorrelator is split into two replica pulses by a beam splitter. The two pulses then travel along the different arms of the interferometer before they are recombined by the beam splitter and focused to a common point in a nonlinear crystal. The crystal generates an upconverted light signal (often but not always a second harmonic) that is detected and displayed. The trace of the intensity of this mixing signal vs. the amount of the optical delay is known as an autocorrelation function (ACF). By assuming the shape of the pulse, its duration can be determined by multiplying the width of the ACF by a form factor.
Different types of autocorrelator introduce the optical time delay in different ways. The three most common techniques use a dispersive material such as special glass, a set of rotating mirrors or a stepper motor/scanner. The dispersive technique relies on slowing the propagation speed of the pulse by passing it through pieces of rotating glass. Another possibility is to have the two pulses travel different distances. This can be accomplished by using rotating mirrors that control the path length of reflected pulses. A final option is a variable-length arm. One of the arms is linearly extended by either a fast scanning system with limited scan range, or a stepper motor with slow movement but a wider scan range. PULSE DURATION RANGE: Pulses shorter than 100 fs are strongly affected by dispersion, so in this case the autocorrelator that uses a dispersive material is not suitable. When analysing such short pulses it is important that each optical element inside the autocorrelator (such as the crystal delay line and the beam splitter) has as little dispersion as possible. This can be done by using reflective instead of transmissive optics.
On the other hand, pulses longer than 5 ps place different requirements on the measuring system. In this case the dispersion effects are negligible, but scan ranges (time delays) need to be larger. A useful rule say that the maximum scan range equals four times the maximum pulse length. All types of scanning autocorrelator have an upper limit for their scan range. With rotating mirrors a delay of up to 300 ps can be achieved, while a spring-loaded scanner enables the measurement of pulses up to around 150 ps long. Stepper motor translators can measure up to several nanoseconds, but the size of such a device is considerable as a scan of 1 ns needs a physical scan range of about 15 cm. POWER & REPETITION RATE: These two parameters are interrelated and define the sensitivity requirements of the autocorrelator. For example, consider a train of identical 100 fs pulses with an average power of 100 mw. At a repetition rate of 1 khz (1000 pulses per second) each pulse reaches a peak power of 100 GW. However, at a higher repetition rate of 100 MHz the peak power is reduced to 1 MW. To take this into account the sensitivity of the autocorrelator is defined by the product of the peak power multiplied by the necessary average power. Highly sensitive autocorrelators reach a P PEAK xp AVERAGE of 10-7 W 2. This degree of sensitivity is required, for example, by a fibre laser with an average power of 0.1 mw at a repetition rate of 10 GHz and a pulse duration of 10 ps.
To provide such sensitivity, autocorrelators use highly reflective optics, efficient crystals, low-noise photomultiplier detectors and special noise-reducing data acquisition. For high-energy pulses at a low repetition rate, such as amplifier pulses, the priorities are entirely different. In this case, photomultipliers are not suitable and photodiodes are used as detectors instead. Depending on the scan frequency of the autocorrelator system it may be necessary to measure laser systems with a slow repetition rate in triggered mode so that only the pulse is measured and not the pause between two pulses. WAVELENGTH: Another important consideration is the wavelength of the pulses. To work well, the autocorrelator has to feature low-loss, low-dispersion optics suitable for the respective wavelength. It is vital that the nonlinear crystal can be phase matched at the wavelength in question so that it can generate an upconverted light signal. The detector must also be sensitive to the wavelength of the upconverted signal but insensitive to the fundamental wavelength, which should be filtered out.
SIGNAL GENERATION: All autocorrelators require a nonlinear crystal to convert the incoming pulses into an autocorrelator signal. Often the crystal performs second harmonic generation (SHG), which is also known as upconversion or frequency doubling. In this case it is necessary to phase match the crystal to the wavelength range by tilting it. An alternative design that is easier to operate but that sacrifices sensitivity is an autocorrelator that relies on two-photon absorption in a photodiode. This method is more cost efficient than SHG designs and does not require wavelength tuning, but it does have several disadvantages. First, the wavelength range is limited. Second, a background-free autocorrelation function is not available because the laser pulses cannot be filtered.