Optics Lab

  The technique of chirped pulse amplification [1 - 4] was introduced by Prof. Mourou and his coworkers at the University of Rochester in the mid 1980s. Ti:Sapphire as a laser gain medium had not been introduced at that time and dyes like Rhodamine 6G were the main amplifier media. The principle of CPA is illustrated in the Figure.The basic idea is to stretch the pulse duration before amplification, thus significantly reducing its peak intensity. This low intensity optical pulse is then amplified, with a chance of significantly reduced gain damage [5]. Following amplification, the pulse is recompressed to its near original duration with high energy value per pulse. The essential criterion for obtaining a cleaner pulse with near initial pulse duration is a matched Stretcher – Compressor pair. This is done using a stretcher design proposed by Martinez44 and the compressor design proposed by Treacy [6].

 

[1] Strickland, D.; Mourou, G., Compression of amplified chirped optical pulses. Optics Communications 1985, 55, (6), 447-9.

[2] Maine, P.; Strickland, D.; Bado, P.; Pessot, M.; Mourou, G., Generation of ultrahigh peak power pulses by chirped pulse amplification. IEEE Journal of Quantum Electronics 1988, QE-24, (2), 398-403.

[3] Pessot, M.; Maine, P.; Mourou, G., 1000 Times expansion/compression of optical pulses for chirped pulse amplification. Optics Communications 1987, 62, (6), 419-21.

[4] Pessot, M.; Squier, J.; Mourou, G.; Harter, D. J., Chirped-pulse amplification of 100-fsec pulses. Optics Letters 1989, 14, (15), 797-9.

[5] Maine, P.; Strickland, D.; Bado, P.; Pessot, M.; Mourou, G., Generation of ultrahigh peak power pulses by chirped pulse amplification. IEEE Journal of Quantum Electronics 1988, QE-24, (2), 398-403.

[6] Pessot, M.; Maine, P.; Mourou, G., 1000 Times expansion/compression of optical pulses for chirped pulse amplification. Optics Communications 1987, 62, (6), 419-21.

 

  The femtosecond laser in lab is a commercial Clark – MXR CPA – 1000 system capable of producing 100 fs pulses of 800 nm light with 1mJ energy at variable repetition rate, though we usually run it at 1 kHz. The essential components of this laser system are

    (1) Millenia pump laser (Spectra Physics Inc.) - TBU

   (2) Mode-locked Ti:Sapphire oscillator (NJA-5, Clark MXR Inc.)

   (3) Pulse Stretcher

   (4) Q-switched intra-cavity doubled Nd:YAG pump laser (ORC-1000, Clark-MXR Inc.)

   (5) Ti:Sapphire based Regenerative amplifier and

   (6) Pulse compressor.

  This one also works based on the principle of Chirped pulse amplification. Millenia is a diode pumped, intra-cavity frequency-doubled Nd: YVO4 continuous wave (CW) laser that can produce up to 5.5 W of 532 nm light. The oscillator is pumped by Millenia CW laser output at 4.0 W, with active pointing stability (Point Master, ClarkMXR Inc.). The oscillator can be mode-locked to produce a train of femtosecond pulses (~100-150 fs) at 97 MHz repetition rate with 800 nm center wavelength and 300 mW average power. The center wavelength is also tunable in a small range from 780-836 nm without significantly affecting the pulse characteristics. Thus the oscillator is the most important part of the CPA-1000 system responsible for the generation of seed femtosecond pulses which are then amplified within the rest of the system. These 100 fs pulses are stretched out to roughly 300 ps in duration in the stretcher. A tiny fraction of the oscillator output is sent to a Pockels cell driver unit (DT-105, Clark-MXR Inc.) through a photodiode, which provides a trigger for synchronous firing of ORC-1000 Nd:YAG laser.

  The photodiode output is also used to monitor the mode-locked pulses and the stability of mode-locking on a daily basis. The stretched seed pulses are then injected into the regenerative – amplifier with the help of a time-gated polarization rotation device (Pockels cell), the timing of which is controlled by the Delay 1 setting of the DT-105 unit. The regenerative amplifier by itself is a lasing cavity with Ti:Sapphire gain medium pumped at 1 kHz by a Q-switched, intracavity frequency-doubled Nd:YAG laser with 8 W average power at center wavelength 532 nm and ~ 150 ns pulse duration. The seed beam, carefully overlapped with the amplifier cavity mode travels back and forth between the cavity end mirrors through the Ti:Sapphire crystal and gets amplified. The amplified seed pulse at the peak of its waveform is ejected out of the amplifier cavity, also known as cavity dumping, with the help of another time-gated polarization rotation, the timing of which is controlled by the Delay 2 setting of the DT-105 unit. A tiny fraction of the amplified output is sent to a photodiode for monitoring the waveform of the amplified seed pulse. The amplified pulses are then sent into the compressor which recompresses the pulse width back to its near original duration. The final output of the amplified pulse is ~ 0.8 mJ across the 785- 840 nm tunability range with 100-150 fs pulse duration and 1 kHz repetition rate.

 

Vendors/Suppliers of Ultrafast Laser System Components

Beam Splitter

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Harmonic Generator

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Delay Line (ultrafast Laser System)

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Mirrors

   https://www.thorlabs.com/navigation.cfm?guide_id=2042

   http://www.edmundoptics.com/optics/optical-mirrors/laser-mirrors/ultrafast-ti-sapphire-laser-mirrors/3024/

   http://search.newport.com/?i=1;q=Thin+film+processing;q1=Ultrafast+Mirrors;x1=chapter

Optical Power/Energy Meter

   http://search.newport.com/?x2=sku&q2=1936-R