Ultrafast Optics

Winter term 2022/2023, by Piotr Wasylczyk

The students are expected to:

1. Attend all live lectures in the classroom. During these meetings we will summarize the material from the on-line videos, discuss the home assignments and any questions that may have appeared.

2. Watch all on-line materials from the lecture web-page.

3. Complete all the home assignemnts and be prepared to present and discuss them in the classroom.

4. Take part in the lab tours.

Lista wykładów

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- How many millimetres light travels in vacuum in 1 ps.

- How the spectral bandwidth relates to the pulse duration and vice versa.

- How broad spectrum you need to generate a 100 fs pulse centered at 800 nm.

- Why it is easier to generate femtosecond laser pulses than picosecond ones.

- What is the order of magnitude of the temporal scale of molecular processes (e.g. vibrations period in simple diatomic molecules)?

- What is the peak intensity available with femtosecond pulses and how to estimate it from the average power, repetition rate and pulse duration.

- What is the fundamental concept behind nonlinear optical processes in transparent dielectric media.

  Lecture video (YT link)

  Lecture notes (.pdf)

  Home assignment:

    1. The instantaneous frequency tells us how the frequency changes as we scan across the pulse in time.
        The group delay tells us how different frequencies arrive at different times.
        Using the steps similar to these that were used to derive the formula for the instantaneous frequency, derive the formula for the group delay.

    2. Draw the electric field amplitude for a Gaussian pulse with positive linear chirp.
        Draw the pulse intensity and phase vs. time.
        Draw the pulse intensity and phase vs. frequency.

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- How a femtosecond pulse is described as the electric field amplitude vs. time.

- What is the concept of the pulse envelope, carrier frequency, instantaneous frequency and phase vs. time.

- What a Fourier transform limited pulse is,

- What a superposition of many monochromatic waves equally spaced in frequency, with their phases locked, is. What the relation between the number of these waves and their spacing and the resulting pulse width and spacing in time is.

- How a CW laser is built, what an amplification bandwidth is and what the cavity modes are and how their spacing relates to the cavity length.

- How to turn a CW laser into a pulsed one with passive mode-locking (nonlinear element + dispersion control).

  Home assignment:

    1. Find a commercial femtosecond laser (oscillator) specs and note the parameters: average power, pulse duration and repetition rate. From these three numbers, calculate:

    - the pulse energy,
    - the pulse instantaneous power,
    - the maximum possible pulse instantaneous intensity (assume reasonable focusing condition, with e.g. a single lens of moderate NA).

    2. Find at least three different approaches to passive mode-locking of fiber lasers (e.g. saturable absorbers deposited on fibre end/side, nonlinear polarization rotation, NOLM, NALM) . Make sure that you understand each technique at the basic (qualitative) level and that you can explain its fundamental concepts in a few sentences.

  Lecture video (YT link)

  Lecture notes (.pdf)

  Home assignment:

    1. Find a commercial femtosecond CPA amplifier (crystal of fiber) specs and note the parameters: average power, pulse duration and repetition rate. From these three numbers, calculate:

- the pulse energy,
- the pulse instantaneous power,
- the maximum possible pulse instantaneous intensity (assume reasonable focusing condition, with e.g. a single lens of moderate NA).

    Compare these numbers with the results for an ultrafast oscillator you got last week.

    2. Make sure you understand the fundamental differences between laser and parametric optical amplifiers and can list their pros and cons.

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- The concepts of laser and parametric amplifiers and the differences between them.

- The concept of the multipass and the regenerative (regen) laser amplifier.

- The fundamental ideas behind Chirped Pulse Amplification (CPA).

- What limits the pulse stretching in time in a typical grating streatcher.

- Why many high-power amplifiers have two stages.

- How vibrational molecular wavepackets are generated in molecules and how we can study their dynamics with pump-probe techniques.

  Home assignment:

    1. Explain the fundamental concept of the pump-probe ultrafast spectroscopy. Draw the schematic of a basic experimental setup. If you want to scan the delays in this experiment up to 10 ps with 10 fs resolution, what are the parameters of the translation stage you need to use in the delay line? Find the appropriate stage in one of the suppliers on-line.

    2. What are the two very efficient processes in biology that involve light absorption and why are the initial stages of these processes ultrafast?

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

- Concepts of the phase velocity, the group velocity and the group velocity dispersion in the context of femtosecond light pulse propagation.

- How the dispersion curve looks like for a typical glass in the visible and IR region.

- How the dispersion curve looks like for a typical optical fiber and what determines its shape.

- How to interpret the units of GVD and GDD.

  Home assignment:

   1. Check what is the maximum negative GDD (in fs2) offered by broadband chirped mirrors at 800 nm available on the market.

  Lecture video (YT link)

  Lecture notes (.pdf)

After the lecture check if you know/understand:

1. How femtosecond pulses can be shaped.

2. How a typical 4f shaper is built.

3. What modulators can be used in 4f shapers (LC, AOM, deformable mirrors), how they work and what are their pros and cons.

4. What limits the shaper performance - theoretical and technical limits.

  Home assignment:

    1. Draw a sketch of the 4f zero-dispersion stretcher with diffraction gratings and lenses. Mark the important distances and position of the Fourier plane.

    2. Explain how the liquid crystal spatial light modulator changes the phase and amplitude of the transmitted light.

   Lecture video (YT link)

   Lecture notes (.pdf)

   Home assignment:

    1. Explain how anharmonic potential leads to nonlinear polarization and thus to nonlinear optical response in transparent dielectrics (at a single atom level).

     2. Explain what the phase matching is and why it is important for macroscopic nonlinear response.

     3. Find out what are the maximum possible efficiencies available in the second harmonic generation with femtosecond laser pulses.

   Lecture video (YT link)

   Lecture notes (.pdf)

   Home assignment:

    1. Find at least five different crystals that are commonly used for SHG in the UV, VIS and NIR spectral range. Make sure you check their "commercial" names (such as "KDP") as well as their proper chemical names and the spectral tuning ranges.

    2. Find the characterists of a commercially available microstructured fiber for supercontinuum generation. What are the important parameters?

   Lecture video (YT link)

   Lecture notes (.pdf)

  Home assignment:

    1. Make sure you understand the basic concept of coherent control. How is it done? What can it achieve?

    2. Why are genetic algorithms used in coherent control experiments? How does a simple genetic algorithm work?

   Lecture video (YT link)

   Lecture notes (.pdf)

   Home assignment:

    1. Find a translation stage model that you would use to build a scanning interferometric autocorrelator for 100 fs pulses from a Ti:Sapphire oscillator. What are the most important parameters of the stage?

    2. Draw a schematic of a simple SHG FROG.

   Lecture video (YT link)

   Lecture notes (.pdf)

   Home assignment:

    1. Explain how it is possible that two pulses separated in time (they E-fields do not overlap) can interfere to produce fringes in the spectrum? Hint: look at how a short pulse looks like in time-space while it propagates in a prism compressor (Lecture 6).

    2. Draw a schematic of a SPIDER setup with Michelson-like interferometer and grating stretcher.

   Lecture video (YT link)

    Lecture notes (.pdf)

   Home assignment:

    1. What are the advantages of using short (femtosecond) laser pulses to machine various materials, compared to using long (or CW) laser sources?

    2. Find a few companies offering laser micromachining and check what they offer.

    3. Explain the basic concept of generating attosecond pulse by high harmonic generation in gases. Draw a schematic of a setup.

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