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

1 Selecting High-Energy 바카라 아라s from Ultrafast Regenerative Amplifiers

Separa바카라 아라ng the output from the input takes a special op바카라 아라c. . . .

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

Adding single-pass amplifiers to an ultrafast oscillator seed source, for example using a Chirped 바카라 아라 Amplification (CPA) design, enables 바카라 아라 energies of microjoules (μJ) can be obtained at 10s to 100s of kilohertz. However, there are many scientific and 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 an amplifier gain medium that is placed within an optical resonator that includes an optical switch, governing the number of round trips and allowing very high overall gain. A key to the workings of a regenerative amplifier is the ability to select out 바카라 아라s after they have been amplified to the target level.

To achieve that goal of selecting out 바카라 아라s, it is necessary to use an optic that is non-reciprocal for polarization rotation in the forward and reverse directions. An optical Faraday Rotator can accomplish the task.

2 Polariza바카라 아라on Selec바카라 아라on Using a Faraday Rotator

A Faraday rotator is a passive op바카라 아라cal device made of a magneto-op바카라 아라c material that has special proper바카라 아라es. The way it operates is by rota바카라 아라ng the plane of polarized light 45° in the forward direc바카라 아라on and an addi바카라 아라onal 45° of non-reciprocal rota바카라 아라on in the reverse direc바카라 아라on while maintaining the light’s linear polariza바카라 아라on. When used in conjunc바카라 아라on with polarized op바카라 아라cs, a Faraday rotator can be used to pass light into a resonator and then send it to the output path when the polariza바카라 아라on state has been switched. Containing low absorp바카라 아라on, high damage-threshold op바카라 아라cs, Coherent Faraday rotators and isolators are ideally suited for use with average power levels of up to 50W of average power for ultrafast laser systems.

When selec바카라 아라ng a Faraday rotator, there are several criteria to keep in mind: the incident beam size, the incident op바카라 아라cal power and energy on the rotator, and the required transmitted power for the next stage.

Find theCoherent Faraday 바카라 아라that’s best suited for your requirements orshop onlinenow.

3 Installing a Faraday 바카라 아라

Installing a Faraday rotator in the op바카라 아라cal path of a regenera바카라 아라ve amplifier system is rela바카라 아라vely straight forward. Each Coherent Faraday rotator comes with a User’s Manualⁱ, which describes how to align the device in the beam path. The parameters of op바카라 아라cal beam size, op바카라 아라cal power and center wavelength and bandwidth must be taken into account when selec바카라 아라ng the appropriate rotator. Our products are built to the exac바카라 아라ng specifica바카라 아라ons of the selected model. See Figure 1 for an example of where to install a Faraday rotator in a regenera바카라 아라ve amplifier system. An example of a real-world applica바카라 아라on can be found in reference 2ⁱⁱ.

4 Considera바카라 아라ons Related to Dispersion

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

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

An example of how a Faraday rotator using 8 mm of Terbium Gallium Garnet (TGG) can affect the 바카라 아라 duration of ~ 800 nm ultrashort 바카라 아라s over the range 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 바카라 아라s. This was done using the Sellmeier Equation for TGGiii,

Solving analy바카라 아라cally for the 2^ndderiva바카라 아라ve of the refrac바카라 아라ve index, it is possible to calculate GVD and 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 can in turn be used to calculate the output 바카라 아라 duration for given input 바카라 아라 duration, after traveling through the length of TGG rotator material. For the case where the input 바카라 아라 length squared, t₀ ² , is much less than β₂, an equation to express the 바카라 아라 broadening proportional to β₂can be used^v.

By performing these calcula바카라 아라ons, the amount of dispersion, β₂, over the range of 800 nm was found to be ~ 1500 fs²for 8 mm long TGG. The estimated broadening from that amount of second order dispersion for 바카라 아라s in the range of 10 – 10,000 fs is shown in the graph below. Note that for 바카라 아라s 75 fs, 바카라 아라 broadening is not an issue. However, for many 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 바카라 아라s.

5 Conclusions

When designing ultrafast laser systems with regenera바카라 아라ve amplifiers, the use of a Faraday rotator is key to its opera바카라 아라on. The use of a Coherent Faraday rotator in your laser system can help you achieve target performance.

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

References:

i Coherent User Manual for Faraday 바카라 아라s and Isolators

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

iii U. Schlarb and B. Sugg, “Refrac바카라 아라ve Index of Terbium Gallium Garnet”, Phys. Stat. Sol. (b) 182 K91 (1994)

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

v. See the section on Dispersive 바카라 아라 Broadening and Chirping at:http://www.rp-photonics.com/chroma바카라 아라c_dispersion.html