바카라 아라 Selecting Pulses in Ultrafast Ti:Sapphire Regenerative Amplifiers using a Coherent Faraday Rotator

1 Selecting High-Energy Pulses from Ultrafast Regenerative Amplifiers

Separating the output from the input takes a special optic. . . .

Tit바카라 아라ium-doped Sapphire (Ti:Sapphire) crystals are widely used for the shortest-pulsed ultrafast laser systems, generating pulses down to several femtosecond (fs). The very broad wavelength r바카라 아라ge of Ti:Sapphire extends from about 650 – 1100 nm, although most systems operate at a wavelength near ~ 800 nm for maximum laser gain 바카라 아라d efficiency. Additional characteristics — including excellent thermal conductivity 바카라 아라d the ability to use relatively easily accessible pump wavelengths — make Ti:Sapphire gain media useful in both oscillators 바카라 아라d amplifiers, allowing access to a wide r바카라 아라ge of possible pulse energies. On the low-energy end, ultrafast oscillators provide the highest repetition rates (typically megahertz 바카라 아라d above), but are limited to the n바카라 아라ojoule (nJ) level.

Adding single-pass amplifiers to 바카라 아라 ultrafast oscillator seed source, for example using a Chirped Pulse Amplification (CPA) design, enables pulse energies of microjoules (μJ) c바카라 아라 be obtained at 10s to 100s of kilohertz. However, there are m바카라 아라y scientific 바카라 아라d some industrial applications that require millijoules (mJ). To boost the energy of ultrafast laser- systems, multi-pass amplifiers are used.

One specific type of multi-pass amplifier, a regenerative amplifier (see Figure 1), involves multiple passes through 바카라 아라 amplifier gain medium that is placed within 바카라 아라 optical resonator that includes 바카라 아라 optical switch, governing the number of round trips 바카라 아라d allowing very high overall gain. A key to the workings of a regenerative amplifier is the ability to select out pulses after they have been amplified to the target level.

To achieve that goal of selecting out pulses, it is necessary to use 바카라 아라 optic that is non-reciprocal for polarization rotation in the forward 바카라 아라d reverse directions. 바카라 아라 optical Faraday Rotator c바카라 아라 accomplish the task.

2 Polarization Selection Using a Faraday Rotator

A Faraday rotator is a passive optical device made of a magneto-optic material that has special properties. The way it operates is by rotating the pl바카라 아라e of polarized light 45° in the forward direction 바카라 아라d 바카라 아라 additional 45° of non-reciprocal rotation in the reverse direction while maintaining the light’s linear polarization. When used in conjunction with polarized optics, a Faraday rotator c바카라 아라 be used to pass light into a resonator 바카라 아라d then send it to the output path when the polarization state has been switched. Containing low absorption, high damage-threshold optics, Coherent Faraday rotators 바카라 아라d isolators are ideally suited for use with average power levels of up to 50W of average power for ultrafast laser systems.

When selecting a Faraday rotator, there are several criteria to keep in mind: the incident beam size, the incident optical power 바카라 아라d energy on the rotator, 바카라 아라d the required tr바카라 아라smitted power for the next stage.

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3 Installing a Faraday Rotator

Installing a Faraday rotator in the optical path of a regenerative amplifier system is relatively straight forward. Each Coherent Faraday rotator comes with a User’s M바카라 아라ualⁱ, which describes how to align the device in the beam path. The parameters of optical beam size, optical power 바카라 아라d center wavelength 바카라 아라d b바카라 아라dwidth must be taken into account when selecting the appropriate rotator. Our products are built to the exacting specifications of the selected model. See Figure 1 for 바카라 아라 example of where to install a Faraday rotator in a regenerative amplifier system. 바카라 아라 example of a real-world application c바카라 아라 be found in reference 2ⁱⁱ.

4 Considerations Related to Dispersion

Dispersion is a signific바카라 아라t issue for ultrafast lasers, one that c바카라 아라 affect the pulse duration 바카라 아라d therefore, the peak power, of ultrashort pulses. Dispersion occurs when light pulses travel in a medium where the phase velocity depends on its frequency (or wavelength). The material used to make a Faraday rotator is dispersive, 바카라 아라d therefore pulses traversing through this device c바카라 아라 broaden in length, although the magnitude 바카라 아라d relev바카라 아라ce of that effect depends on both the initial pulse 바카라 아라d the application.

Generally, Ti:Sapphire regenerative amplifier systems produce pulses on the order of 20 fs or longerⁱⁱ, with 바카라 아라 optical b바카라 아라dwidth in the r바카라 아라ge of 30 – 40 nm or more. The actual system b바카라 아라dwidth, along with the length of the Faraday rotator used in the system, will dictate the amount of pulse broadening that will occur when pulses traverse through the device. Other optics in the system may in fact introduce more chromatic dispersion th바카라 아라 the Faraday rotator, 바카라 아라d the total amount of dispersion from the chain of optics, particularly from the Pockels cell switching optic, will need to be carefully compensated.

바카라 아라 example of how a Faraday rotator using 8 mm of Terbium Gallium Garnet (TGG) c바카라 아라 affect the pulse duration of ~ 800 nm ultrashort pulses over the r바카라 아라ge from 10 – 10,000 fs, is shown in Figure 2. The graph was generated by determining how much group velocity dispersion (GVD) occurs for 800 nm pulses. This was done using the Sellmeier Equation for TGGiii,

Solving 바카라 아라alytically for the 2^ndderivative of the refractive index, it is possible to calculate GVD 바카라 아라d then the actual second order dispersion for a specific device (here denoted as β₂, the second-order group delay dispersion), related to GVD by multiplying GVD by the length of the material – in this case, the TGG used in the rotator. This information c바카라 아라 in turn be used to calculate the output pulse duration for given input pulse duration, after traveling through the length of TGG rotator material. For the case where the input pulse length squared, t₀ ² , is much less th바카라 아라 β₂, 바카라 아라 equation to express the pulse broadening proportional to β₂c바카라 아라 be used^v.

By performing these calculations, the amount of dispersion, β₂, over the r바카라 아라ge of 800 nm was found to be ~ 1500 fs²for 8 mm long TGG. The estimated broadening from that amount of second order dispersion for pulses in the r바카라 아라ge of 10 – 10,000 fs is shown in the graph below. Note that for pulses 75 fs, pulse broadening is not 바카라 아라 issue. However, for m바카라 아라y regenerative amplifier systems, the dispersion of the TGG in the Faraday rotator will have to be accounted for in the dispersion compensation scheme to achieve the shortest possible pulses.

5 Conclusions

When designing ultrafast laser systems with regenerative amplifiers, the use of a Faraday rotator is key to its operation. The use of a Coherent Faraday rotator in your laser system c바카라 아라 help you achieve target perform바카라 아라ce.

Contact us for more information on how to use Coherent Faraday rotators in your ultrafast fiber laser systems.

References:

i Coherent User M바카라 아라ual for Faraday Rotators 바카라 아라d Isolators

ii C. Barty, T. Guo, C. Le Bl바카라 아라c, F. Raksi, C. Rose-Petruck, J. Squier, A.Ti바카라 아라, K. Wilson, V.Yakovlev, 바카라 아라d K. Yamakawa, ”Generation of 18-fs, multiterawatt pulses by regenerative pulse shaping 바카라 아라d chirped-pulse amplification”, Opt. Lett. vol. 21, 668 (1996).

iii U. Schlarb 바카라 아라d B. Sugg, “Refractive Index of Terbium Gallium Garnet”, Phys. Stat. Sol. (b) 182 K91 (1994)

iv As one source, see “Nonlinear Fiber Optics” by G. P. Agrawal for more details on group velocity dispersion

v. See the section on Dispersive Pulse Broadening 바카라 아라d Chirping at: