Ultrafast Lasers

Ultrafast laser pulses are among the fastest man-made events. But there is more - ultrafast also means ultrapowerful. And, as we all know, with great power comes great tunability.

The high power within ultrafast laser pulses allows us to make practical use of nonlinear optical effects that are usually very weak. It is through such effects that the colour of the laser pulses can be tuned. The ultrafast lasers described below are based on the use of a Ti-doped sapphire crystal. Ti:Sapphire has very broad absorption and emission spectra. Just as Cr3+ ions do in ruby, Ti3+ ions replace the Al3+ ions of sapphire leading to strong (26% larger) distortion of the local electric fields. The energy levels of Ti are broadened by coupling to vibrational levels in sapphire. Also, Ti3+:Al2O3 has excellent thermal conductivity.

To many people, an ultrafast laser is a black box. Often, this is because it is from Coheren, as those from Newport - Spectra Physics usually come in a white box. In order to help me visualize what goes on inside the boxes, I made these animations and you might find them useful.    

The Tsunami laser from Newport Spectra-Physics

The RegA regenerative amplifier from Coherent

Fig. 1. Because Ti:Sapphire has such broad absorption and emission bands, lasing starts in many modes, which all compete for amplification. Because of cavity loss, self-focusing and acousto-optical modulation, the pulse with highest amplitude is eventually identified and amplified. Ti:Sapphire lasers typically emit light at 800 nm with a spectral range from 650 to 1000 nm. Pulses are 100 to 200 fs long and repetition rate is about 80 MHz.
     Although the laser is tunable, its spectral range does not extend to the visible frequencies, nor does it go very far in the infrared. Moreover, the power per pulse is not very high. Happily, ultrafast laser amplification brings solutions to all of these problems.   

Fig. 2. In this regenerative amplifier, the light pulses extract power from the Ti:Sapphire crystal over a total of 22 amplification passes. Too much power in the crystal can be a bad thing as it might damage the Ti:Sapphire. However, the pulses stretch in time due to dispersion in all the optical component along the cavity. This stretching brings their amplitude below the damage threshold. Once a pulse has taken all the available energy from the crystal, its amplification is maximal and it is then compressed to ultrashort again. The RegA produces 200 fs light pulses at 800 nm, with repetition rate 250 kHz. The output power is about 1 W. Still higher power per pulse can be achieved by lowering the repetition rate.  


The Spitfire regenerative amplifier from Newport - Spectra Physics

The visible Optical Parametric Amplifier from Coherent

Fig. 3. Rather than relying on dispersion in all the optical components along the cavity, this design from Newport - Spectra Physics begins by purposefully stretching the laser pulse. The amplitude of the pulse is then increased by consecutive passes through the Ti:Sapphire crystal. Finally, the pulse is recompressed to achieve very high peak powers and it exits the laser. The Spitfire can also produce 1 W of average output power but at the repetition rate of only 1 kHz, which means much more power per pulse. The problem with this amplification is that it is confined to a single frequency. Nevertheless, with so much power it is possible to make use of nonlinear optical effects and achieve wavelength tunable ultrafast laser pulses.  

Fig. 4. In an optical parametric amplifier (OPA), the parametric processes involve frequency (ω) mixing, such as ω1+ω2= ω3, or ω1ω2= ω3. These are known as sum frequency generation and difference frequency generation, respectively. The parametric processes allow the color change in the laser beam. In this OPA, the fundamental beam is split in two. One part is frequency doubled to 400 nm by passing into a BBO crystal. The other part is directed into a sapphire crystal where it produces a white light supercontinuum. Subsequently, the 400 nm and the white light are directed into another BBO crystal where the frequency mixing occurs. The resulting signal consists of ultrafast (about 200 fs) laser pulses that are tunable within the visible part of the spectrum: 480 to 700 nm range. On the same principles it is possible to build an OPA for tunable infrared light.    


The infrared Optical Parametric Amplifier from Newport - Spectra Physics


Fig. 5. In this schematic diagram, a Spitfire regenerative amplifier provides light pulses at the entrance of the OPA. Again the amplified pulses are split in two parts. One to create the white light supercontinuum and the other goes straight into a BBO crystal for frequency mixing with the white light. Here there is no frequency doubling to 400 nm, so the values for ω1+ω2= ω3, or ω1ω2= ω3 are different. Consequently, the resulting signal consists of ultrafast laser pulses that are tunable within the infrared part of the spectrum, i.e. between 1100 and 1600 nm.

     In a typical experiment, the pulses are guided through a variety of optical components to a particular sample where unusual light-matter interaction takes place. Subsequently, the light is directed to detectors, (ex. photodiodes or photo multiplier tubes). The detectors convert the light signal into an electric signal that can be measured with electronic instruments and recorded onto a computer.