Research Index / Materials Science / Laser System

Our group uses one of the two 10 Hz laser systems at CUOS.  At 10 Hertz, a pulse occurs every 0.1 seconds and each pulse in our system lasts for 100 femtoseconds (1 fs = 0.000000000000001 seconds = 1 millionth of a billionth of a second).  The system is Ti:sapphire-based which means that the short pulses are created and amplified by sapphire crystal rods that are doped with titanium (Ti). So the rods are mostly sapphire, but have a small concentration (doping) of titanium throughout the rod.  Ti:sapphire rods can be excited (pumped) by green light and they will give off light (fluoresce) in the near-infrared -- a color between red and infrared.  They are used for short pulse systems because they can support a large color range (bandwidth) which is essential to produce the short pulses.  (There is an uncertainty relation in physics that says that for a laser pulse to be short in time it must be composed of lots of color and vice versa -- for a laser pulse to be monochromatic, composed of one color, it must be long in time or even continuous, not pulsed.)

Laser Wavelength: The center wavelength (color) of our system is 780 nanometers (nm) with a total spread (bandwidth) of 20 nm. Only a small portion of the bandwidth is in the visible range (red), with the rest being in the infrared (IR). Therefore, most of the beam is invisible to the eye. IR cards, IR viewers, and CCD cameras are used to follow the beam from place to place. CCD (charge-coupled device) cameras are sensitive to IR light so they can be used for alignment and photographs. (Most digital cameras use CCDs.)

Laser Components: Our 10 Hz system consists of the following:

Laser Output: The final output of this system is a 10 Hertz train of 100-fs pulses with an energy per pulse as high as 100 mJ and a contrast as high of 107 (peak-to-background ratio).

Chirped-Pulse Amplification: When short pulse lasers were first developed, there was a limit to the energy per pulse that could be achieved. This limit came from the fact that a short pulse in time has a high power (power is energy divided by time) or high intensity (intensity is power divided by area). The high power or high intensity of these lasers will damage the material used to amplify it. The method to avoid this damage was first developed by our CUOS director Gerard Mourou and his graduate student in 1985. The method, chirped-pulse amplification (CPA), involves stretching the pulse in time, then amplifying it, then re-compressing it to a short pulse. After stretching the pulse, the pulse has a power or intensity low enough to avoid damage. The trick is to stretch the pulse in time such that it can be re-compressed after amplification. This stretching method is to add a "chirp" to the pulse and is described below under Stretcher. A chirped pulse is a pulse which has a frequency or color that changes during the pulse time. Just as a bird's chirp changes in pitch during the bird's song (in time), the pulse's color changes during the pulse (in time). The compressor then removes this chirp to regain the short pulse, but now amplified.

Pump Lasers: An Argon laser operating at 5 Watts is used to pump the oscillator Ti:sapphire crystal. Four Nd:YAG lasers operating at a doubled-frequency of 532 nm (green) are used to pump the four amplifier Ti:sapphire crystals. (Nd:YAG is another crystal, Yttrium Aluminum Garnet, doped with Neodymium that is a common laser medium. It produces 1064-nm light which is infrared. If you use a doubling crystal to double the frequency or halve the wavelength, you get 532-nm light which is green.) The Nd:YAG lasers produce 10-ns pulses and are triggered to run at 10 Hz.  The laser system as a whole cannot be run faster than 10 Hz because 0.1 seconds is necessary for the Ti:sapphire crystals to cool down after being pumped by the Nd:YAG lasers. The properties of Ti:sapphire change with temperature. Thus, if the Ti:sapphire crystals were still warm when the next pulse came through, the optical properties would be different and the laser system would vary from pulse to pulse. So, to produce an amplified short pulse, we use a continuous Argon laser to create it and four "long-pulse" lasers to amplify it. (Each pump laser will be described with its respective amplifier below.)

Oscillator: The oscillator that creates the ultrashort pulse is a commercial laser from Clark-MXR. It contains a Ti:sapphire rod that is pumped by an Argon laser. The optics are set up in a V-shaped design to create the laser cavity. The laser can operate in continuous wave (CW) mode which means it produces continuous near-IR light with a narrow color range (bandwidth). More importantly, the laser can be mode-locked to produce 100-fs pulses with a bandwidth of 30 nm at a repetition rate of 100 MHz (one pulse every 10-millionth of a second).  Mode-locked means that the entire color range is amplified in the laser cavity, and thus, all the modes are "locked" in the laser cavity.  Normally, only a small range of colors can be amplified which gives the standard CW laser.  (I have a link to a detailed explanation of mode-locking which I will add here soon.)

Pre-Amplifier: For the cleanest pulses, the pre-amplifier is used. (It can be bypassed for simplicity when the cleanest pulses are not necessary.) The pre-amplifier is a multi-pass amplifier which allows 6 passes through the Ti:sapphire rod. It is pumped by a Nd:YAG laser which produces 10-ns pulses with 200 mJ of energy at 532 nm.  This amplification happens before the stretcher and regenerative amplifier (regen). The goal is to reduce the number of round-trips in the regen and reduce the background level to produce cleaner pulses (pulses with a higher peak-to-background contrast ratio). To avoid the damage problems mentioned above in the Chirped-Pulse Amplification section, the pre-amplifier is limited in the maximum level it can amplify the pulse.

Stretcher: The stretcher uses a grating setup to spread out the colors (bandwidth) of the short pulse in space. In the photo to the right, you can see the initial short pulse beam hit the grating. After it reflects off the grating, its color will be spread out in space. A mirror in the stretcher reflects the spread-out beam back to the grating, below the original beam. A flashlight is shown in the grating to the right to show that the grating does indeed spread the colors. (Since the flashlight is white light, it is made up of the rainbow of visible colors.)

Once the color of the short pulse is spread out in space, the design of the stretcher allows certain colors to travel a shorter path than others. A last bounce off the grating puts the colors back together in space, but now the colors at one end of the bandwidth arrive later than the colors at the other end. So now the short pulse has been stretched in time to a "long" pulse. With our design, the original 100-fs pulse is stretched in time by a factor of 10,000 to a final duration of 1 ns.  Since the different colors are spread across the pulse in time, the pulse is said to have a "chirp".  This is the basis for chirped-pulse amplification (CPA), as mentioned above.  Whether the pulse has a positive chirp or a negative chirp depends on whether the shorter or the longer wavelengths arrive first.

The stretcher, due to the way gratings work, can only transmit at best half of the energy that enters. So a 2-nJ short pulse coming from the oscillator leaves the stretcher as a 1-nJ long pulse.

Regenerative Amplifier: The regenerative amplifier (regen) is the first amplification stage after the stretcher and can amplify the pulse by almost a factor of one million. Its Ti:sapphire rod (shown above in the first photo on this page) is pumped by 5 mJ split off from a higher-energy Nd:YAG laser which produces a 10-ns pulse at 532 nm. The regen is actually a laser cavity and, as such, can produce near-IR laser pulses on its own -- so-called "free-running" laser pulses (pulses as long as the Nd:YAG laser: 10 ns).  However, if a near-IR pulse is injected into the regen at the right time, it can amplify itself by stimulating emission of its own radiation. (On the other hand, the free-running laser will stimulate emission of noise, creating so-called ASE: amplified spontaneous emission.) With each round trip in the regen cavity, the near-IR pulse is increasingly amplified. The amplification is very fast for the first 10-15 round trips in the cavity. After this, the amount of amplification begins to saturate. After saturation, the pulse energy actually decreases each round trip. However, the pulse energy is constant from pulse to pulse after saturation, whereas it can vary be as much as 50% before saturation. Thus, the best time to switch out the amplified pulse is one round trip after saturation. At this point, the pulse energy is both large in amplitude and stable from pulse to pulse. The final result is that a 1-nJ pulse from the stretcher can be amplified to a 0.5-mJ pulse.

2-Pass Amplifier: The 2-pass amplifier is the simplest of the multi-pass amplifiers. It consists of a Ti:sapphire rod pumped by green (532 nm) laser light, with the short pulse being amplified by 2 passes through the rod. The same Nd:YAG laser that pumps the regenerative amplifier is used to supply a 120-mJ, 10-ns pulse to the 2-pass amplifier. The near-IR pulse extracts enough energy from the crystal to amplify itself by a factor of 40 -- from 0.5 to 20 mJ.

4-Pass Amplifier: The 4-pass amplifier is very similar to the 2-pass amplifier. However, due to the larger energies involved, its Ti:sapphire crystal is pumped by two more powerful Nd:YAG lasers, one from each side of the crystal. Each Nd:YAG laser produces 10-ns, 600-mJ light at 532 nm. With four passes through the excited crystal, the near-IR pulse amplifies itself by a factor of 10 -- from 20 to 200 mJ.

Compressor: As the final step in the chirped-pulse amplification scheme, the amplified (chirped) pulse must be compressed to remove the chirp and regain the short pulse in time. The compressor consists of a pair of gratings that spread out the color range (bandwidth) of the amplified pulse. To undo the effect of the stretcher, the opposite end of the bandwidth (as the one in the stretcher) travels a shorter path than the other end of the bandwidth. All the colors in the bandwidth of the amplified pulse are once again traveling together to give a short, unchirped pulse. As in the stretcher, only half the energy can be transmitted through the compressor (due to the way gratings work). The entire compressor is placed in a vacuum chamber because a high-energy, short pulse will become distorted in air (at worst, it will spark causing most of the energy to be lost in the spark). The final output from this laser system leaves the compressor chamber through a vacuum pipe and enters a "turning box" which is uses a mirror to direct the laser beam to whatever experiment is desired. From the time the laser beam enters the compressor chamber, the laser beam is kept inside vacuum pipes to avoid distortions to the beam. In addition, any material through which the beam passes will distort it, so all optics must be reflective. For example, to focus the laser beam, a curved mirror must be used instead of a lens.

Single-Shot Auto-Correlator: This laser system produces electromagnetic pulses shorter than anything that can be produced electrically (light is an electromagnetic wave). As such, it is not possible to measure the pulse width (in time) by electrical means. Instead, the laser pulse is used to measure itself through a technique called an auto-correlation. An autocorrelation is a mathematical operation where a function is split into two pieces which are delayed with respect to each other, multiplied, and then integrated (summed) over time. This operation is implemented in the following manner (see the photo to the right): a) the pulse is split in two pieces by a beam-splitter; b) the pulses are delayed with respect to each other by using a translation stage to create a longer path for one of the pulses; c) the pulses are multiplied by overlapping them in a doubling crystal; and d) the multiplied pulses are integrated by looking at the total light on a detector.

The doubling crystal produces frequency-doubled light which means for near-IR, 780-nm light, the crystal produces blue, 390-nm light.  (Since frequency and wavelength are inversely related, double the frequency means half the wavelength.) For two pulses entering the crystal, the intensity of the blue light produced depends on the multiplication of the pulses in time and space. The detector looks only at the blue light and integrates (sums) it for 0.1 seconds (the time between laser pulses). Since the autocorrelation of a function of time is a function of delay time (between the pulses), the detected light must be measured for a range of delay times. To shorten the measurement time and simplify the setup, a single-shot auto-correlator is used. By using geometry, the two pulses are overlapped in the doubling crystal such that the delay time is mapped to the position on the detector. Thus, with a properly calibrated time-to-space mapping on the detector, the autocorrelation can be measured with a single pulse (single shot).

3rd-Order Auto-Correlator: A 3rd-order auto-correlator uses the tripled-frequency to measure pulse width, as compared to the doubled-frequency used by the single-shot auto-correlator (a 2nd-order auto-correlator). It is used to measure asymmetries in the pulse since a 2nd-order auto-correlator will always give a symmetric measurement in time. So, you can find out what is happening before and after your pulse. In many experiments and applications, what happens before the pulse is much more important than what happens afterwards. This is because a messy pre-pulse structure can modify things before the main pulse arrives, thereby changing the effect you're looking for. A messy post-pulse structure arrives after the main pulse arrives and is often too late to affect things. A 3rd-order autocorrelation will show you the asymmetries so you can clean up whatever is most important to you.

In addition, a 3rd-order autocorrelation can give a better measurement of the contrast of the laser pulse. The intensity contrast ratio (ICR) of a laser pulse is the ratio between the peak intensity and the background intensity. The background can be composed of multiple structures. So, the range (in time) over which the ICR is measured is important to note.

Research Index / Materials Science / Laser System

Copyright Center for Ultrafast Optical Science, University of Michigan

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