PETRA-III Laser-wire

Fast Laser Description

The laser used for the new experiment at PETRA-III is a Master Oscillator Power Amplifier (MOPA).

The laser system is composed by a solid state mode-locked oscillator (Time-Bandwidth GE150), a 4-pass grating based stretcher and a 4 stages fiber laser amplifier.

A panoramic picture of the whole laser system is in the attachment "slide3" at the bottom of the page.

1- OSCILLATOR

The laser oscillator is a Nd:YVO4 solid state mode-locked oscillator emitting laser light at 1064 nm wavelength. The mode-locked pulses are generated by means of a semiconductor saturable absorber mirror (SeSAM) which produces Fourier-limited pulses with 10ps full width at half maximum (FHMW) and 0.2 nm bandwidth.

The repetition rate of the main oscillator is 62.45 MHz, which is the 8th subharmonic of the main PETRA RF clock 499.664 MHz. The oscillator pulses are synchronized to an external RF generator set @ 62.45 MHz (Rhode & Schwartz model SMB100A) by an electronic unit (Time-Bandwidth model CLX1100) that compares the phases between the laser pulses train and the external RF. The unit, through a feedback loop, controls a movable mirror within the laser cavity and by modifying the cavity length it changes the repetition rate of the laser until the locking with the RF source is achieved. The RF source is phase adjustable by +/-90 degrees (8ns range).

The final phase-locking between the main clock and the RF source is then achieved via a very low phase noise 10MHz reference running from PETRA RF control room, along the tunnel, to the laser room (detailed explanation in the "synchronization" section ......LINK.........).

A list of the laser oscillator parameters is summarized in the table below:

2- PULSE PICKER

The repetition rate of the laser pulses is controlled by an acousto-optic (AO) pulse picker. The device is inside the laser oscillator box and it operates via an AO deflector. The deflector allows only one every N pulses to go through to the output. The repetition frequency of the pulse picker can be selected in a range between 50 kHz (one every 1250 pulses) and 1.02 MHz (N=61). The transmission efficiency of the AO pulse picker is approximately 60%.

The repetition frequency of the pulse picker is set to 520.4 kHz, which correspond to a divider of 120 of the main oscillator frequency of 62.45 MHz. As the oscillator frequency is the 8th subharmonic of the main Petra III RF clock, the output laser repetition rate corresponds to its 480th subharmonic.

3- STRETCHER

The laser pulses coming out of the pulse picker are then sent to a 4-pass grating stretcher. The aim of the pulse stretcher is to introduce a positive dispersion which delays the spectral componets of the laser pulses and therefore increases the pulse lenght.

The laser beam hits the first grating at an angle of 18 Deg with respect to the grating surface. The spectral components of the pulses are reflected with a slightly different angle. When they reach the second grating they incide at different angles and are reflected at the same angle (the effect is the inverse of the first grating). The laser beam is then lifted by a periscope and reflected back onto the second grating at the same angle. The effect is that the spectral components are delayed transversally while travelling back to the first grating (the laser spot size looks like a line). Finally, the last reflection from the first grating transforms the transverse delay into longitudinal. In this strecher configuration, the path difference between the shorter and the longer wavelenght of the spectrum is given by:

Δx = 4D·Δθ·tanθ2

where D is the distance between the gratings, Δθ is the difference of diffraction angle of the two edge wavelengths and θ2 is the diffraction angle of the longer wavelenght. The diffraction angles of the shorter and longer wavelenghts depend on the gratings construction and the incidence angle and in our case are θ2=74.688 Deg and θ1=74.61 Deg.

The total distance between the gratings has been calculated to be 3m in order to obtain a final pulse duration of approximately 200ps.

The transmission efficiency of the 4-pass stretcher is approximately 50% (84% at each reflection).

The laser beam parameters at the output of the stretcher are summarised in the table below:

4- FIBER AMPLIFIERS

The laser amplifier is developed in 4 stages. The necessity of multiple stages amplification is due to the very low average power of the seed that is being amplified. In fact, the low power seed is capable of extracting only a limited amount of power stored in the fiber by the pump absorption. When the pump power increases beyond the maximum capabilities of the seed the seed pulse stops being amplified and the rest of the stored energy converts into amplified spontaneous emission (ASE). In addition to this there is another parasitic effect due to the inability of the seed to extract all the energy stored in the fiber. The amplified seed coming out of the fiber is partially reflected at the interface (by about 4%). This portion of seed travels back and forth and gets amplified by interacting with the residual energy stored in the fiber, generating a series of "echo" pulses.The presence of multiple amplification stages allows increasing the laser power gradually so those parasitic effects can be controlled and the amplfication process can be rendered more efficient.

The first two stages of amplification form the pre-amplifier and the last two the final amplifier. All the fiber amplifier stages work in a similar fashion. The seed laser coming from the previous stage is coupled into one end of the fiber after being reflected by a dichroic mirror (with high reflection for wavelenghts above 1000 nm and high transmission for wavelenghts below 1000 nm). The pump laser is coupled from the other end after being transmitted by a similar dichroic miror. The dichroic mirrors are necessary in order to ensure that only the amplified seed is present and all the pump light is dumped when propagating outside the fibers.

Between each fiber amplifier stage, there is a faraday isolator, which ensures elimination of any feedback laser light, a narrow band filter (3 nm around 1064 nm) to eliminate any residual pump light and parassitic laser emission and a 1/2 waveplate for adjusting the polarization into the next stage.

4.1- PRE-AMPLIFIER

Both stages of the pre-amplifier are made using a single mode Nd:doped fiber, pumped with a CW laser beam at 808 nm wavelenght. The pump for the two stages are obtained from the same pump laser diode by means of two beam sampler. The total power of the pump diode is 11.8 W.

The first beam sampler has a reflectivity of 5% and it's used to steer part of the pump (P1=600mW) to the first Nd:doped fiber, which is approximately 2m long. The output average power from the first pre-amplifier stage (after the Faraday isolator and the narrowband filter) is approximately 11 mW. For a laser beam of 200 ps and 520 kHz repetition rate, this corresponds to a peak power of 105 W.

The laser beam is then coupled into the second fiber, which is 4.5 m long. The residual part of the pump diode beam is again split into 30% (3 W ca), which is coupled into the second fiber, and 70% which is dumped. The output average power from the second pre-amplifier stage (after the Faraday isolator and the narrowband filter) is 90 mW, that corresponds to a peak power of 850 W.

The laser parameters after the pre-amplifier stages are summarised in the table below:

4.2- POWER AMPLIFIER

The amplifier is also divided into two stages. Both amplifier stages use a large mode area (LMA) Yb:doped fiber, pumped with a CW laser beam at 975 nm wavelenght. The pump for the two stages are obtained from the same pump laser diode by means of two beam sampler. The maximum power of the pump diode is 25 W but it's set to a power level of about 12 W.

The first beam sampler has a reflectivity of 30% and it's used to steer part of the pump (P1=3.6 W) to the first Yb:doped fiber, which is approximately 2.5m long and has a core size of 10 microns. At this pumping levels, the output average power from the first amplifier stage (after the Faraday isolator and the narrowband filter) is approximately 240 mW (peak power of 2.3 kW).

The laser beam is then coupled into the second fiber, which is 4.5 m long and has a 20 micron core diameter (which allowas high peak power handling capabilities). The residual part of the pump diode, namely 70% or approximately 9 W, is coupled into the second fiber for the final amplification boost. The output average power from the final amplifier stage (after the Faraday isolator and the narrowband filter) is 1.5 W, that corresponds to a peak power of 14 kW.

The laser parameters after chain of amplification stages are summarised in the table below: