Trident 10 Hz 10Hz Ultrafast Ti:Sapphire Amplifier

Transcription

1 Trident 10 Hz 10Hz Ultrafast Ti:Sapphire Amplifier User s Manual Tel. : 33 (0) Fax : 33 (0) rue du Bois Chaland CE 2926 LISSES EVRY Cedex, FRANCE

2

3 Table of contents 1. INTRODUCTION CHIRPED PULSE AMPLIFICATION FEMTOSECOND SYSTEM GENERAL LAYOUT OSCILLATOR STRETCHER REGENERATIVE AMPLIFIER AMPLIFIERS COMPRESSOR SAFETY LASER LIGHT SAFETY ELECTRICAL SAFETY SAFETY AND INFORMATION LABELS CONNECTION AND SYNCHRONIZATION WATER CONNECTIONS ELECTRICAL SUPPLY CONNECTIONS ELECTRO-OPTIC DRIVER (GENPULSE) Genpulse rear panel Genpulse front panel SYNCHRONIZATION Genpulse Internal synchronization principle START-UP AND SHUT DOWN PROCEDURES DAILY START-UP PROCEDURE Oscillator start-up Genpulse V2 electro-optic controller CFR 200 Start up Regenerative Amplifier Start-up DAILY SHUTDOWN PROCEDURE OPTICAL ALIGNMENT EQUIPMENT NEEDED FOR ALIGNMENT CLEANING THE OPTICS Tools Cleaning procedure TEST CONTROL PROCEDURE SHORT PULSE OSCILLATOR CHECK Output power measurement Pulse Train Monitoring Spectrum Control STRETCHER MODULE Optical Setup List of components Coarse alignment of the stretcher Fine alignment in the stretcher REGEN. AND PRE- AMPLIFIER MODULE Trident 10 Hz Table of content - 3

4 Optical Layout List of Components Regenerative amplifier cavity Optimization Regenerative amplifier complete alignment Regenerative Amplifier seeding Optimization Preamplifier Alignment COMPRESSOR MODULE General description List of components Hz compressor coarse alignment Complete alignment of the compressor Gratings parallelism adjustment Optimization of pulse duration TECHNICAL DATA SYSTEM PERFORMANCES COMPLETE LAYOUT SERIAL NUMBERS Trident 10 Hz Table of content - 4

5 1. Introduction The Trident 10 Hz laser system is a compact femtosecond laser source providing more than 20 mj pulse energy at a 10 Hz repetition rate. The pulse duration is about 40 fs. This laser source was designed by Amplitude Technologies. The system is a Titanium-Sapphire laser based on the so-called "Chirped Pulse Amplification" (CPA) technique. The system amplifies pulses from a Ti:Sapphire Oscillator and consists of a stretcher, a regenerative amplifier and a 10 Hz multi-pass amplifier with a joint pump laser, and a compressor Chirped pulse amplification The CPA technique consists of temporal stretching of the ultra short pulse delivered by an oscillator (by a factor of 1000 to 10000), in order to safely amplify this pulse in solid state materials. Stretching produces a chirped pulse. After amplification, the laser pulse is temporally compressed back to a duration as close as possible to its initial value. After the compression stage, one should obtain in principle a high intensity ultra short pulse free of chirp (see figure 1.1). Oscillator Stretcher Amplifier Compressor Figure 1.1: Chirped pulse amplification principle. Trident 10 Hz Introduction- 5

6 Stretching and compression are usually achieved using wavelength dispersive elements such as gratings or prisms. The idea is to create different optical paths for each wavelength of the spectrum. Figure 1.2 shows a typical stretcher design using two gratings and a telescope system (Note that this is not the Amplitude Technologies stretcher design, but its use is convenient for tutorial purposes). D<0 Grating 1 Imaging device Grating 1 G = -1 Grating 2 Mirror Figure 1.2: Stretcher principle. The stretching factor depends on the distance D between the two gratings. As shown in figure 1.2, the Blue path is longer than the Red one. Therefore, Blue wavelengths take more time to travel through the system than Red ones. Due to Fourier transform properties, a femtosecond pulse exhibits a broad spectrum (typically 30nm for a 35fs pulse). Since the bluer part of the spectrum is delayed compared to the redder part when travelling through the stretcher, the output pulse is stretched and looks like a temporal rainbow (red in the leading edge and blue in the trailing edge). The stretching factor depends on the spectral width of the input pulse and on the stretcher characteristics (grooves, density of the gratings, distance between the gratings, number of roundtrips in the stretcher, angle of incidence, etc...). For a given stretcher configuration, the stretched pulse duration is proportional to the input spectrum width. Trident 10 Hz Introduction- 6

7 Once stretched, the pulse can be amplified in several amplification stages, using regenerative and/or single-/multi-pass amplifiers. At the output of the amplifying system, the energy does not depend on the input pulse duration delivered by the oscillator. The limit comes from possible damage that could be caused to the amplifying material. Very few solid state materials can stand the high energy density that is needed to produce gain. This explains why ultra short pulses must be stretched prior to amplification. Safe operation of amplifiers requires effective intensities below 5 GW/cm 2. Obviously, the amplified pulse energy which can be reached without damage is higher if the stretched pulse is longer. Grating 1 Grating 2 Mirror Figure 1.3: Pulse compressor principle. After amplification the pulse must be compressed back to its initial duration. A compressor device based on a wavelength dispersion system similar to the stretcher (see figure 1.3) is commonly used. This compressor is theoretically able to compensate for any stretching introduced into the pulse, but the gratings need to be perfectly aligned. Particularly, the incident angle onto the compressor has to be finely adjusted in order to compensate for the stretcher and the dispersion effects through the amplifier. Trident 10 Hz Introduction- 7

8 1.2. Femtosecond system general layout The Trident 10 Hz is divided into two parts: the stretcher/compressor box and the regenerative amplifier and a 4-pass pre-amplifier box. The system layout is shown in the figure 1.4. Figure 1.4: System layout. The femtosecond system is installed on a 3.50 x 1.50m optical table. The different modules are built on bread-boards and a single cover protects the optics of the system from dust and air flow perturbation Oscillator The Oscillator is a commercial Micra manufactured by Coherent. (Refer to the manual for details). It is delivered with its own Verdi CW pump laser, as well as its own closed loop chiller (figure 1.5). Trident 10 Hz Introduction- 8

9 1.4. Stretcher Figure 1.5: Micra Oscillator The stretcher design consists of the all-reflective triplet combination of Öffner (see figure 1.6). The triplet combination is composed of two spherical concentric mirrors. The first mirror is concave and the second one is convex. This combination presents interesting properties for use in a pulse stretcher. It is characterized by perfect symmetry, so only the symmetrical aberration can appear (spherical aberration and astigmatism). This combination has no on-axes coma and exhibits no chromatic aberration because all the optical elements are mirrors (for more details, see ref: Aberration-free stretcher design for ultra short pulse amplification G. Cheriaux, F. Salin and al. OPTICS LETTERS March ). Figure 1.6: Aberration-free stretcher design. Trident 10 Hz Introduction- 9

10 1.5. Regenerative amplifier The first amplification stage consists of a regenerative amplifier producing around 1mJ stretched pulses at 10Hz. It includes two Pockels cells: one is used to seed the stretched pulse into the regenerative cavity and the other dumps out the pulse at the maximum energy level. The regenerative amplifier technique provides an excellent beam profile according to the TEMoo transverse mode of the resonator. An electronic module (Genpulse v2: see figure 1.7) is installed to synchronize and switch the different Pockels cells involved in the system. Figure 1.7: Synchronization and switching electronic module (Genpulse) Amplifiers The high power amplification delivered by the system is achieved using a multi-pass amplifier. The Nd:YAG Quantel CFR200 laser (up to 532nm) produces the gain in both the regenerative amplifier (pumped by 10mJ) and the multipass amplifier. Trident 10 Hz Introduction- 10

11 1.7. Compressor The amplified pulses are re-compressed to a duration as close as possible to the initial value using a classic compressor design (see figure 1.8). Two gratings with an optimized number of lines transmit the broad spectrum bandwidth with good efficiency. The geometry of the stretcher-compressor is designed to obtain the flattest phase dispersion in the overall system. Figure 1.8: Typical compressor design. For very high output power, the compressor is usually placed into a vacuum chamber to protect the laser from the non linear effects of air. Trident 10 Hz Introduction- 11

12 2. Safety 2.1. Laser light safety Several laser beams are involved in the femtosecond amplifier system. Because of their high intensity, the laser beams can cause serious injuries if safety precautions are not followed. The laser source is a potential hazard to eyes, not only from direct or secular reflection, but also from diffuse reflection. Damage to skin and fire hazards may also be caused by this kind of source. THIS EQUIPMENT USES VISIBLE AND INVISIBLE LASER RADIATION. EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION CAN CAUSE SERIOUS INJURIES. The following is a partial list of precautions to follow when using high power class IV pulsed lasers: o When the laser system is in operation, all people within the laser room must wear protective eye-wear adapted to the emitted radiation wavelength, Trident 10 Hz Safety - 12

13 o Never look directly into the laser beam. Even after secular or diffuse reflections a laser beam can cause serious injuries, o Set up experiments so that the laser beam is either well above or well below eye level, o Set up a controlled access area for laser operation, o Post clearly visible warning signs near the laser operation area, o o Block unused laser beams, o Work with strong ambient light whenever possible, o People working on the laser must avoid wearing reflective objects (wedding ring, watch, etc...), o The reflections of the laser beam are generally in the plane of incidence of the laser and it is strongly recommended not to have the eyes in this plane. o The interaction between a laser beam and certain classes of materials (flammable, explosives or volatile solvents) may be a source of fire. Do not use the laser in the presence of such materials Electrical safety Some components used in the femtosecond amplifier system are supplied with high voltage. These devices are protected with housings. Never remove the protective covers of elements using high voltages. Only an authorised and qualified person can manipulate these devices. WARNING: THE HIGH VOLTAGES USED BY THIS EQUIPMENT ARE SOURCES OF SERIOUS HAZARDS. THESE HIGH VOLTAGES ARE PRESENT EVEN WHEN THE LASER SYSTEM IS NOT BEING OPERATED. Trident 10 Hz Safety - 13

14 VISIBLE AND INVISIBLE LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION Wavelenght : nm Max Average Power : 3W CLASS IV LASER PRODUCT -IEC 825-1/2000 AMPLITUDE TECHNOLOGIES EVRY - Fr ance Tél : 33(0) Fax : 33(0) Type : PULSAR 100 Date : 07/2007 S/N : PUL User s manual system. Electrical safety label stickers are placed near sensitive components of the laser 2.3. Safety and information labels Safety labels are located near sensitive components of the laser system. Please make sure you have located these labels and rigorously follow the safety instructions. Label 1: Certification Label This label gives you information about the model type and serial number of the laser system. This label is located on the rear panel of the laser bench. Label 2 : Warning logotype The laser output label notes that this laser is a high power Class IV pulsed Laser. This label is located on the rear panel of the laser bench. It gives the stated energy output, the pulse duration and the wavelength. Ensure that adequate eye protection and beam handling precautions are observed. Trident 10 Hz Safety - 14

15 Label 3 : Aperture Label: These laser aperture warning labels are located near all laser outputs. Avoid eye or skin exposure to radiation emitted from these apertures. Label 4 : Laser Hazard: The laser is enclosed in protective housing that prevents the emission of visible and invisible radiation. Do not open or disconnect the cover interlock system in order to avoid eye or skin exposure to visible and invisible laser radiation. These labels are located on top covers of the protective housings. Trident 10 Hz Safety - 15

16 Cover safety interlocks Cover safety interlocks shut the laser emission off (pump lasers and internal electro-optic devices) when one of the protective housing lids is removed. The interlocks are located on each laser housing lid in order to ensure immediate inhibition of light emission even if the protective housing is slightly lifted and not totally removed. These interlocks can be deactivated, for optimization or service operation, by lifting the white sensor up. Cover safety interlock A green LED is on when the interlock defeats are secured Remote electrical connector Beam Shutter : A mechanical inter-cavity safety shutter is located inside the regenerative amplifier in order to mechanically stop the IR emission. Trident 10 Hz Safety - 16

17 3. Connection and synchronization 3.1. Water connections The modules that need to be cooled with water are: The base plate and the crystal of the oscillator, using a solid state water to air chiller. Chiller Oscillator The 10 Hz CFR200 pump laser is composed of a laser head and a power supply/cooling unit module. The cooling unit is based on a water to air heat exchanger. Refer to the Quantel CFR200 Nd:YAG laser user s manual for further information Electrical supply connections Table 3.1 summarizes the electrical requirements and maximum consumptions for each module delivered with the system: Module Voltage (phase nb) Maximum Power CFR 200 Power supply V 850 W Chiller * V 700 W OPSL CW laser * V 200 W Notebook computer V 200 W Trident 10 Hz Connection and Synchronization - 17

18 Electro-optic modules controller V 300 W Attenuator and compressor controller * V 80 W Supervision devices * V 300 W *Optional module Table 3.1: Electrical requirements 3.3. Electro-optic driver (Genpulse) The Genpulse module is an electronic device which controls all the Pockels cells involved in the amplifier system. It produces high voltage for the fast switching devices and the different Pockels cells synchronization signals. The Genpulse is also used to supply power to several photodiodes in the system and to drive the seeding beam and regenerative amplifier shutters. The Pockels cells assemblies, the photodiodes and the shutters should be connected as shown in figure 3.4. The Genpulse also sends trigger signals to the pump laser (to the flashlamps and Pockels cells within the Nd:YAG). The complete synchronization diagram of all devices and events will be developed in the next paragraph (see figure 3.9 for a synchronization overview). Synchromization and delay generator Genpulse 110V -240 V / 1 A Pockels cells Photodiodes Shutters Figure 3.4: Genpulse connection to the amplifier devices Genpulse rear panel The Genpulse connectors are located on the rear panel as shown in figure 3.5. Trident 10 Hz Connection and Synchronization - 18

19 RF signal in (fiber) Interlocks and shutters CFR pump laser flashes sync out Pockels cells (1 to 4) sync out Communication port Nd:YAG pump laser flashes sync out Oscilloscope trigger sync out Pockels cells (1 to 4) switch supply Pockels cell (1 to 4) HV supply Photodiodes power supplies Nd:YAG pump laser shutter control Nd:YAG pump laser QS sync out CFR pump laser QS sync out Figure 3.5: Genpulse rear panel connectors description. The main switch controls the main voltage to the module, RF in: Optical fibre connector. 80 MHz frequency range. Receives the signal from the oscillator pulse train, from a beam pick-off located in the stretcher, RF out: Optical fibre or BNC connector. This output is a sampled and amplified copy of RF in, it can be used to control the quality and the stability of the oscillator pulse train, FL (1 to 3): BNC connectors. TTL output. Synchronization output signals for the external control of the Nd:YAG pump lasers flashlamps, for the Nd:YLF acoustooptic modulator, or for the oscilloscope control. This signal, divided from the oscillator clock in normal operation, is generated internally even if the RF in signal is missing or unstable (lower frequency), QSW (1 to 3): BNC connectors. TTL output. Synchronization output signals for the external control of the Nd:YAG Pockels cell. This signal is stopped if the RF in signal is missing or unstable (lower frequency), Shutter (1 to 3): This signal is used to pilot the Nd:YAG pump laser shutter, Trident 10 Hz Connection and Synchronization - 19

20 HV (1 to 4): HV BNC connectors. Continuous High voltage supply to the different Pockels cell modules (see table 3.2), CH (1 to 4): BNC connectors. TTL output. Synchronization output for the corresponding high speed switch that sends HV to the linked Pockels cells, LV (1 to 4): Low voltage to run the high speed switches located in the different Pockels cell modules. Table 3.2 outlines the Pockels cell module voltage and function. 12V/200mA: these connectors are used to supply the photodiodes located in the system, Interlock: this connector is used for safety cover switches, Shutter: this connector is used for mechanical shutters, RS485: communication port to be used with supervision controller. Channel # Function Voltage 1 Pulse Picker * 6 kv 2 Regen. Input 3.2 kv 3 Regen. Output 3.2 kv 4 Pulse Cleaner * 6 kv *Optional module Table 3.2: Pockels cell modules function and Voltage (supplied by the Genpulse). Table 3.3 summarises the typical output signals delivered by the Genpulse. Output Connector Signal characteristics CH1 to 4 BNC TTL 6 µsec FL 1 to 3 BNC TTL 6 µsec QSW 1 to 3 BNC TTL 6 µsec Table 3.3: Synchronization signals characteristics Trident 10 Hz Connection and Synchronization - 20

21 Genpulse front panel The front panel of the Genpulse is shown in figure 3.6. The RF indicator is red when the oscillator pulse train is missing, or too low in power or frequency. When the fault is corrected, this button needs to be pressed to reset the Genpulse. The different Pockels cell switches also need to be restarted, The seed and regen switches correspond to the mechanical shutters located between the stretcher and the regenerative amplifiers, and also into the regenerative amplifier. These indicators are green when the shutters are open, The Display select knob is used to select a function in the display menu by turning the knob and pressing it to select, The CH1 to 4 knobs are used to activate the corresponding Pockels cell by pressing, and to adjust its delay by turning. The knob is surrounded by a green light when the channel is active, HV Ch1/Ch2 activates the high voltages of the Pockels cells (CH1 together with CH2, and CH3 with CH4). They are illuminated in green when HV is on, Figure 3.6: Genpulse front panel description. Trident 10 Hz Connection and Synchronization - 21

22 3.4. Synchronization Genpulse Internal synchronization principle The Genpulse is a digital device. It generates a 10Hz clock from the division of the oscillator pulse train. The triggers sent to the different amplifier Pockels cells and external devices (see figure 3.7) are synchronized with the oscillator, with adjustable delays. Some of the low frequency outputs are secured by the RF in sampler and will follow the internal low frequency clock if the RF in is missing or unstable. This design allows safer operation of the Nd:YAG flashlamps. Genpulse Oscillator Ch1 Ch3 Ch2 Ch4 Regen. Input Regen. Output Regenerative amplifier Optional module Pulse Cleaner Figure 3.7: Genpulse operation diagram The internal synchronization principle is shown in figure 3.8. All of the synchronization outputs (flashes, Q-Switches, AOM, Pockels cells) generated from Trident 10 Hz Connection and Synchronization - 22

23 the oscillator division are delayed with individual delay lines (AD#) that allows the synchronization of the pump pulses with the amplified pulse. Most of these delays are preset but can be adjusted using the delays menu. The Pockels cells delays are also adjustable using the Ch1-4 knobs. RF in RF out Digital Sampler Counter Divider 10Hz CH1 Delay RF fault switch CH2 Delay Low freq int Clock CH3 Delay FL 1 CFR Flashes FL 2 Nd:YAG Flashes FL1 Delay FL2 Delay CH4 Delay QSW 1 CFR QSwitch QSW1 Delay QSW 2 Nd:YAG QSwitch QSW2 Delay Figure 3.8: Genpulse synchronization principle. Trident 10 Hz Connection and Synchronization - 23

24 Oscillator pulse train Divided clock Flash1 Nd:YAG flash synchro QSW1 Nd:YAG QSW synchro. Regen. pump pulse CH2 delay HV Pockels 2 CH3 delay HV Pockels 3 Regen. pulse train Output amplified pulse HV Pockels 4 (cleaner) Figure 3.9: General Synchronization diagram of the system. Trident 10 Hz Connection and Synchronization - 24

25 4. Start-up and shut down procedures Before starting the laser, it is absolutely essential to check that safety requirements are satisfied. Make sure that all people within the laser area wear suitable (adapted to radiation wavelength emitted) protective goggles. All the interlock systems must be active (no shunts). The IR output beam must be blocked Daily start-up procedure Follow this step by step guide for the daily start-up procedure of the laser system. This procedure gives the manual operations. It does not include the computer controls and settings that can be made with the supervision software Oscillator start-up Cooling Unit First, verify that the cooling unit of the Oscillator is switched on. The coolant temperature must be +21 C. Figure 4-1: cooling unit Trident 10 Hz Start-up and shut down procedures - 25

26 Power supply of the Verdi pump laser The start-up procedure of this kind of oscillator is very simple Figure 4-2: oscillator's power supply o Turn the key to the On position. (2.1 on Figure 4-2) o Turn the button to set the power (2.2 on Figure 4-2) Wait about 5 minutes; corresponding to the warm-up time of the pump laser Mode locking Drive the oscillator into pulsed mode (Mode-locked) by pushing the button located at the back of the oscillator module. It is recommended to check the spectrum with a spectrometer. The spectrum must be broad without any narrow spikes corresponding to CW operation (see Figure 4-3). The best setting is case (c), a broad spectrum without any CW operation. Once the oscillator is in pulsed mode, the RF clock required by the Genpulse synchronization module is available. Trident 10 Hz Start-up and shut down procedures - 26

27 Intensity (a.u.) Wavelength (mn) Intensity (a.u.) Wavelength (mn) Intensity (a.u.) Wavelength (mn) Figure 4-3: Typical spectra obtained at the output of the oscillator. (a)-spectrum in CW regime. (b)-spectrum in mode-locking regime with a CW component, (c)-spectrum when the laser is perfectly mode-locked without any CW Genpulse V2 electro-optic controller Switch on the Genpulse electro-optic delay generator in order to create an external trigger for the devices requiring this (Pump lasers, pockels cells, ). Genpulse main switch Switch on the main switch at the rear panel of the Genpulse, Trident 10 Hz Start-up and shut down procedures - 27

28 «display select» button Figure 4-4: Genpulse front panel RF clock Validate the RF clock by pressing "RF default" button. If the default persists (button remains red), ensure the spectrum is ok, check the alignment in the optical fiber and that the oscillator is not working in "double pulse" mode. Once RF clock is validated, Qsw (Q-switch) and pockels cells synchronizations are available. Note that FL (Flashlamp) synchronizations are available as soon as the Genpulse is switched on. It may be useful for any device synchronization CFR 200 Start up Figure 4-5: CFR power supply and remote control Trident 10 Hz Start-up and shut down procedures - 28

29 o Turn the key on the front of the power supply on (The cooling unit pump within the power supply will start). o Switch on the Flashes using the remote control. o Switch on the pockels using the remote control (supposing Qsw synchronization is enabled). o Open the shutter (using +/- buttons). Pump laser synchronization via Genpulse display o Turn the "display select" button to select FL of pump 1 then push the button to activate the external synchronization. You may hear the flashes of the pump laser (Figure 4-6). o Turn the "display select" button to select Qsw of pump 1 then push the button to activate the external synchronization. Green laser emission should occur (If not, ensure the shutter of the laser is open). Figure 4-6: the Genpulse digital screen Trident 10 Hz Start-up and shut down procedures - 29

30 Wait about 20 minutes (corresponding to the warm-up time of the pump laser) before any adjustment of the Regenerative cavity Regenerative Amplifier Start-up Figure 4-7: Front panel of the Genpulse On the front panel of the Genpulse: o Activate the High Voltage Buttons ( HV2/HV3 and HV1/HV4) o Activate the Pockels cell CH1 (pulse picker) *Optional module o Activate the Pockels cell "CH2" (seed) o Open the Regen Shutter o Check on the oscilloscope that the build up time of the cavity is usual. If not, and if the warm up time of the laser is ended, then realign the cavity (see chapter 5.6.3) o Open the Seed shutter o Check on the oscilloscope that the build up time of the cavity is usual. If not, and if the warm up time of the laser is ended, then realign the cavity seeding (see chapter 5.6.5) o Activate the Pockels cell "CH3" (extraction) o Check the beam profile at the Regenerative amplifier exit Trident 10 Hz Start-up and shut down procedures - 30

31 o Activate the Pockels cell CH4 (pulse cleaner) *Optional module o Check the beam profile at the amplifier exit At this point, the usual build-up trace can be seen on the oscilloscope, as shown in Figure 4-8. Figure 4-8: Build up time If necessary, after 10 minutes of the warm-up time, the fine delays for the 4 Pockels cells can be adjusted using the corresponding knobs and simultaneously checking the monitoring photodiodes of the regenerative amplifier Daily Shutdown procedure Follow the procedure below to shut down the femtosecond system for a short period (overnight in the case of daily operation for example). Refer to the respective manual of each module if you want to shut down the system for a longer period. WARNING: NOTE THAT FOR DAILY USE, SOME MODULES, SUCH AS THE CHILLER OF THE OSCILLATOR OR OTHER ELECTRONICAL DEVICES MAY NEED TO STAY SWITCHED ON. Trident 10 Hz Start-up and shut down procedures - 31

32 Block all infrared laser beams o Deactivate the Pockels cell CH4 (pulse cleaner) *Optional module o Deactivate the Pockels cell CH3 (extraction) o Close the seed shutter using the front panel of the Genpulse. o Close the regen shutter using the front panel of the Genpulse. o Deactivate the Pockels cell CH2 (seeding) o Deactivate the Pockels cell CH1 (pulse picker) *Optional module o Deactivate the High Voltage Buttons ( HV2/HV3 and HV1/HV4) Stop the CFR200 pump lasers o Close the shutter via the remote control (Fig.4.6) o Stop the Pockels (Qsw) using the front panel of the Genpulse (Fig.4.5) o Stop the Flash (FL) using the front panel of the Genpulse o Stop the Pockels via the remote control o Stop the Flash via the remote control It is recommended to leave the cooling unit running for a few minutes after the flashlamps have been switched off. This allows gradual cooling of the laser rod. o Stop the CFR200 cooling unit. Stop the oscillator and its CW pump laser o Turn off the oscillator using the key (figure 4.1). Shut down the electro-optic controller (Genpulse), if desired : o Switch off the main power switch of the Genpulse controller. Other Power supplies. o Turn off, if desired, all power supplies attached to optional devices such as main computer, oscilloscope Trident 10 Hz Start-up and shut down procedures - 32

33 5. Optical Alignment The femtosecond system will first be installed and aligned by an Amplitude Technologies staff member. Nevertheless, a femtosecond system is quite complex and it is essential for researchers working with this system, to be able to handle it well and solve problems which may arise. This chapter describes how to control and realign the different parts of the system Equipment needed for alignment The alignment of the entire system requires the following equipment: Laser safety goggles to protect against the second Harmonic of the Nd:YAG laser (532 nm) and nm Ti:Sapphire laser emission. Powermeters able to measure 10 mw to 100 W of the second harmonic Nd:YAG wavelength as well as the Ti:Sapphire emission wavelength ( nm). An infrared viewer for the visualization of the IR beams, Fast photodiodes (with rise time better than 1 ns if possible), An oscilloscope with a bandwidth of 300 MHz or higher, Metric allen wrenches, A spectrometer (1 nm min. resolution in the range of nm) A Polaroid polarizer Trident 10 Hz Optical alignment - 33

34 5.2. Cleaning the optics In the laser, the optical elements are submitted to significant energy densities. Pollutants are capable of contaminating the optics and creating hot spots. These hot spots degrade the quality of the optical surfaces and coatings which may result in reduced laser efficiency. Cleaning the optics rarely needs to be done. This operation should only be performed if power loss or mode deterioration is observed Tools Dry neutral gas spray Optical cleaning paper (Kodak, Fisher ) Acetone Surgical tweezers Cotton swab Protective/latex gloves Cleaning procedure First clean the optics with the dry neutral gas spray. Do not put the spray into direct contact with the optics. If the optics are still dirty, proceed with the following procedure. 1. Correctly clean your hands or wear clean protective gloves. 2. Fold up the optical cleaning paper several times to obtain a little cushion of similar diameter to the optical element. Do not touch the cleaning surface of the optical paper 3. Humidify the cleaning paper with acetone. 4. Gently drag the paper over the surface to be cleaned. 5. If it is necessary to repeat the operation, take another piece of cleaning paper (never use the same cleaning paper twice). Trident 10 Hz Optical alignment - 34

35 WARNING: FOR THE STRETCHER AND COMPRESSOR MODULES, DO NOT TOUCH THE SURFACE OF THE GRATINGS. THEY ARE VERY FRAGILE AND EXPENSIVE AND CANNOT BE CLEANED Test Control Procedure Protective housing interlocks. Remove the protective housing cover/s and check that the laser stops. Replace the protective housing, and make sure that the key has to be set to the "STANDBY" position in order to be able to restart the laser. Defeat of safety interlocks. When the safety interlocks are defeated, make sure that the laser can operate, and that the green indicator is on. Remote interlock. Remove the connector and make sure that the laser stops. Replace the connector and make sure that the key has been set to the "STANDBY" position in order to be able to restart the laser. With a voltmeter, record the voltage between the pins of the remote interlock. Key actuator. Make sure that the key can not be removed in the "ON" position. Emission indicator. Make sure that the blue emission indicator is illuminated after the key actuator is switched on and laser light is emitted. Beam attenuator. Make sure that the attenuator slides properly. Ensure that the position is consistent with the labelling. Check with a sensitive screen that no light is emitted while the attenuator is closed. Manual reset. Make sure that the laser cannot be restarted after a safety fault or an interruption of mains power without setting the key to the "STANDBY" position first. Trident 10 Hz Optical alignment - 35

36 5.4. Short pulse Oscillator check The Coherent Micra oscillator is set on a breadboard which includes the CW 532nm laser. For more details about the oscillator and its pump laser, please refer to its manual Output power measurement Use a suitable Power meter (0-1W) for the measurement of the output of the oscillator. When a power meter is used the pulse energy can be obtained by dividing the average power by the pulse repetition rate. The pulse duration has no influence on the measurement on most power or energy meters. The specific performances of the short pulse oscillator are described in the Coherent user s manual. Note: the energy level of the short pulse oscillator should be in the range of 5nJ (about 400mW for an 80MHz frequency oscillator) Pulse Train Monitoring It is possible to monitor the pulse train of the oscillator with a fast photodiode and a high bandwidth oscilloscope. This visualization might be very useful during modelocking optimization of the oscillator. A typical pulse train record is shown in the Figure 5-1. Trident 10 Hz Optical alignment: oscillator - 36

37 Figure 5-1 : Typical pulse train out of the oscillator monitored by the photodiode Spectrum Control The temporal duration of the pulse is directly related to the width of the spectrum (at the output of the oscillator). It is easy to check the Full Width at Half Maximum of the spectrum at the output of the oscillator with a spectrometer. The spectrum should be centered on 800 nm with a FWHM of approximately nm. The Coherent oscillator is not a tunable system. Do not attempt to change the central wavelength or FWHM using the crystal position screw of the oscillator module. Typical spectrum records are shown in picture 4.3 ( Start-up and shut down procedure ). The presence of CW component in the laser is easily visible when a narrow spike appears in the spectrum. The mode-locking effect is activated by pushing the button on the rear panel of the Mantis. Trident 10 Hz Optical alignment: oscillator - 37

38 5.5. Stretcher Module The beam from the oscillator is reflected to the stretcher / compressor module using the folding mirrors and the periscope located on the optical table. The stretcher part of the module is represented in Optical Setup The stretcher module includes the Öffner stretcher. Erreur! Source du renvoi introuvable. shows the stretcher setup. All the optical elements are labelled in order to locate easily each component for the alignment procedure. Figure 5-2 : Setup of the stretcher module and index for each component The beam coming from the Oscillator hits the folding mirrors FM101 and goes into the stretcher through the periscope PR. Trident 10 Hz Optical alignment: stretcher - 38

39 The stretcher itself is made of grating G, concave mirror CVM, convex mirror CXM, corner cube CC and a prism P. The optical alignment of the stretcher will be described in the Optical Alignment part. The beam exits the stretcher via FM103, and passes through an isolator to stop optical feedback from the regenerative amplifier. It then passes through the polarizing cube P101, hits FM104 and goes into the Regenerative / Pre-Amplifier module passing through the Faraday Rotator and the Wave plate WP101. The beam then hits FM106 and finally passes through the second polarizing cube P102 of the isolator List of components Table 5-1 summarizes the different optical components of the stretcher module with Amplitude Technologies references. Stretcher legend description reference FM nm C FM103/ nm C FM nm C WD101 Mirror Rs=10% C Faraday Faraday Rotator C WP101 Half wave 800 nm C P101 Polarizing cube C P102 Polarizing cube C PR Periscope C CC Corner cube C CVM Concave Mirror C CXM Convex Mirror C P Prism C G Grating C Table 5-1: References of optical components included in the stretcher module. Trident 10 Hz Optical alignment: stretcher - 39

40 Coarse alignment of the stretcher The pointing of the input beam may change from day to day due to a realignment or to a temperature variation of the room. In this case the beam direction has to be re-adjusted in order to go through the stretcher cleanly. Use the following procedure to readjust the input beam in the stretcher: First, be sure that the oscillator is correctly aligned. Adjust the mirror FM101 looking at PH2 and at PH3 to send the beam properly in the stretcher as shown by Figure 5-3.a. Note that PH2 and PH3 are pinholes specially designed for the alignment of the stretcher as shown on Figure 5-6. Repeat these two adjustments until the beam passes through the two pinholes. Then, the beam will exit correctly the stretcher, which can be checked by looking the beam with a paper in front of the Faraday for example. PH2 or PH3 PH2 or PH3 a) Entrance b) Exit Figure 5-3: Beam alignment at the entrance (a) and at the exit (b) of the stretcher. On this scheme, pinholes are seen from the reflector CC. It is possible to check the spots hitting the grating of the stretcher, which must be as shown in Figure 5-4(remove pinholes PH2 and PH3 for this operation). You can also check that the beam exits correctly the stretcher by the pinholes PH2 and PH3 as shown on Figure 5-3.b. If you cannot manage to make the beam exit correctly, see next section for a complete alignment of the stretcher. Trident 10 Hz Optical alignment: stretcher - 40

41 Adjust the exit of the stretcher using FM103 to pass through PH5 and FM104 to pass through the output pinhole PH6. Figure 5-4 : Representation of the different beams hitting the stretcher grating when the oscillator is in CW operation (left) and in ML operation (right). For the best optimization of the oscillator beam alignment through the stretcher it might be necessary to repeat the operation several times. WARNING: FOR THE COARSE ADJUSTMENT, DO NOT TOUCH OTHER COMPONENTS THAN ONES MENTIONED IN THE PROCEDURE Fine alignment of the stretcher It may be necessary to re-align the stretcher completely. The setup of the stretcher and the different heights of the beam are summarized on following figures. Trident 10 Hz Optical alignment: stretcher - 41

42 Figure 5-5: Picture and ray-tracing of the stretcher. Figure 5-6: Beam height values into the stretcher and alignment tool used. WARNING: DO NOT TOUCH THE SURFACE OF THE GRATING. IT IS VERY FRAGILE AND EXPENSIVE AND CANNOT BE CLEANED. The alignment procedure is performed as follows: 1. Use the oscillator in CW mode, take the alignment tools provided with the system (see Figure 5-6) and place them in front of the grating G and in front of the corner cube C in their corresponding clamps PH2-PH3. In the following, the pictures of these pinholes represent them as they can be observed from the reflector CC. Trident 10 Hz Optical alignment: stretcher - 42

43 2. The direction of the beam between elements CC and G must follow a straight line materialized by the both pinholes holes at height 135 mm on the alignment tool (the red one on the tool sketch). You can make some walking tuning with mirrors FM101 and FM102 in order to help you. Use the mirror FM101 to centre the beam on the pinhole PH2. And adjust the mirror FM102 to centre the beam on the pinhole PH3. The beam must be located on the grating G as shown on Figure 5-7. PH2 and PH3 Figure 5-7: Position of the incoming beam on the grating. Repeat this step as long as the beam hitting the grating is not perfectly horizontal at 135 mm. 3. Use the pinhole PH4 to adjust the vertical tilt of the grating G, so that the reflected beam at the zero order is horizontal at 135 mm. 4. Place PH4 pinhole in front of the CVM convex mirror, and use the adjustment of the grating G mount to rotate the grating grooves. Thus optimize this adjustment to send the diffracted beam horizontally (it means at 135mm) on the concave mirror CVM. 5. Repeat the steps 3 and 4 as long as the both emergent beams (zero order and first order) from the grating are not perfectly at 135 mm. Trident 10 Hz Optical alignment: stretcher - 43

44 6. Adjust the orientation of the concave mirror CVM to send the beam to the centre of the convex mirror CVX. The beam height on the mirror CVX must be set at 125 mm. To help you, you can place the PH4 pinhole in front of the CVX mirror and make the beam enter this pinhole by the left hole at 135mm height (green point on the opposite figure, where PH4 is seen from CVM mirror), then use the spot on the center (see red point on the opposite figure) to adjust the CVM vertical and horizontal tilts. 7. Adjust the orientation of the convex mirror CVX so that the beam reflects vertically back to the concave mirror CVM at a height of 115 mm, and passes through both pinholes PH2 and PH3 properly. Note the position of the beam at this step on the grating on the Figure 5-8. PH2 & PH3 Figure 5-8: Position of the second pass on the grating. 8. Proceed to the adjustment of the reflector CC to reflect the beam back horizontally at a height of 145 mm. The height of the reflected beam can be adjusted by the vertical position of the upper mirror of the reflector. The horizontal adjustment of the beam can be controlled by the orientation of the reflector. It is possible to check the horizontality of the beam when the beam hits the convex mirror again at the same position. Rotate the reflector so that the spots on the convex mirror are well superimposed. PH2 & PH3 Trident 10 Hz Optical alignment: stretcher - 44

45 9. The beam must be reflected by the concave mirror for the fourth time at a height of 105 mm. It is then propagated through PH3 to the prism reflector P. PH3 10. Adjust the prism position and orientation so that the beam exits from the stretcher without being clipped. It exits at the same height as the input beam but with a lateral shift. PH2 & PH3 11. It is now necessary to check that there is no spatial chirp in the output beam of the stretcher. If the convex and concave mirrors are not in a perfect afocal position, the different wavelengths have different angles of propagation at the output of the stretcher. This can be checked by inserting a grazing incidence screen in the output beam and checking the spectrum at different locations of the beam (see Figure 5-9). If the spectrum center changes when you move the fiber into the beam, it means that there is some spatial chirp. Trident 10 Hz Optical alignment: stretcher - 45

46 Screen IR beam from the stretcher Spectrometer Fiber Figure 5-9: Check for spatial chirp from the stretcher. 12. It is then necessary to change the distance between the concave and the convex mirrors. To do this, translate the convex mirror a little (an adjustment screw is located on the CVX mount), and re-adjust the stretcher alignment: it should be enough to use only the concave mirror CVM to do this. Otherwise follow the procedure as described before. Then check if the spectrum is still changing. 13. A more precise method consists of observing the behavior of the far field output beam. At the output of the stretcher place a long focusing lens, f=1meter for example, and place a CCD camera at the focus point. Then place a strip of paper, about 2 cm wide, in front of the concave mirror. Move this stripe along the mirror and if somewhere you can see two focusing spots at the camera screen, there is some "spatial chirp" inside the stretcher. Trident 10 Hz Optical alignment: stretcher - 46

47 5.6. Regen. and pre- amplifier Module Optical Layout This module includes the regenerative amplifier and the first multi-pass amplifier. The optical layout is shown in Figure Figure 5-10: Regenerative and pre-amplifier module The seed beam coming from the stretcher is reflected by FM301, FM302 and FM303 and is then seeded into the regenerative amplifier with polarizers P302 and P303. A shutter, Sh301 allows or prevents the beam from seeding the regenerative amplifier (RGA). The RGA consists of two curved mirrors FM304 and FM305, two flat mirrors FM306 and FM307, two Pockels cells Pk302 and Pk303, and two polarizers P303 and P304. The beam is seeded by P303 and extracted by P304. The seeding Pockels cell is Pk302 and the extraction Pockels cell is Pk303. A shutter, Sh302, Trident 10 Hz Optical alignment: regen and pre-amplifier - 47

48 allows or prevents cavity lasing to occur. A Brewster cut Ti:Sapphire crystal C1 is located between the two curved mirrors FM304 and FM305. This module is pumped by a YAG CFR 200. The pump beam is split into two using the combination of a wave-plate WP302 and a polarizer P309. For the RGA, the pump beam successively hits FM325 before being focused by L309. The other part of the beam is reflected by P309 (vertical polarisation) and P310. The pump beam is separated into two parts by the beam splitter BS302 before being focused onto both sides of crystal C2 to pump the multi-pass amplifier. The short path is composed by FM327, FM328 and the lens L306. The long path is composed by FM329, FM330, FM331 and the lens L307. The leak through FM324 is used to monitor the power of the pump laser by sending it to the power measurement device D301. The beam exiting the regenerative amplifier by P304 and hits the folding mirror FM308. After FM309, the beam is seeded into the 5-pass multi-pass amplifier by FM310 and FM311. The multi-pass amplifier uses the butterfly configuration: the different passes overlap inside the Ti:Sapphire crystal C2. The beam path is as follows: the beam successively hits FM312, FM313, FM314, FM315, FM316, FM317, which is the exit mirror. The shutter Sh303 is used to work in single-shot regime. The leak through FM332 is used to measure the pre-amplifier power using detector D302. Camera Cam2 is used to check the pre-amplifier s spatial profile. A photodiode monitors the regenerative amplifier pulse train through mirror FM List of Components Table 5-2 summarizes the different optical components in the regenerative amplifier and the pre-amplifier module along with their Amplitude Technologies references. Trident 10 Hz Optical alignment: regen and pre-amplifier - 48

49 Regenerative Amplifier and Pre-Amplifier Legend description reference FM301 - FM nm C P301-P304 Polarizer@ 800 nm C P302 - P303 Polarizer@ 800 nm C FM304 - FM305 Curved 800 nm C Pk Pockels cell C FM306 - FM nm C C1 Brewster cut Ti:Sa Crystal C FM309 FM nm C W301 Wave-plate@ 800 nm C C2 Flat-Flat Ti:Sa Crystal C FM322-FM nm C WP302 Wave-plate@ 532 nm C P309 P310 Polarizer@ 532 nm C BS302 Beam 532 nm C L307 Spherical 800nm C L306 Spherical 800nm C L309 Spherical 800nm C Table 5-2: References of optical components included in the power amplifier module Regenerative amplifier cavity Optimization It is important in the regenerative amplifier alignment to de-correlate the behavior of the cavity itself from the seeding beam effect. Therefore, when the cavity is checked, first block the injection seeding of the regenerative amplifier. This also prevents any damage that can occur during the alignment optimization. The main criteria for the optimization of the regenerative cavity alignment are the pulse build up time (i.e. the delay between trigger of the second Pockels cell CH2 of the Genpulse - and the maximum of the pulse train) and the output beam profile, which must be as round as possible. Using these criteria, the following steps can be followed: Trident 10 Hz Optical alignment: regen and pre-amplifier - 49

50 1. Set the extraction Pockels cell delay (CH3 of the Genpulse) to a longer value than nominal in order to observe with the oscilloscope all of the nanosecond pulse train propagating in the cavity, or simply switch the CH3 trig off. 2. Reduce the pulse build up time as much as possible by slightly adjusting the pump beam mirror FM325 and the rear mirror FM306. Observe the beam profile at the output, in front of the polarizer P Switch on the second Pockels cell and adjust the delay in order to maximize the output power after polarizer P Proceed with seeding the cavity. It is possible that the new setting has changed the direction of the regenerative cavity, it is then necessary to proceed to the seeding optimization (see next chapter). 5. Optimize the output power of the seeded regenerative amplifier by adjusting the extraction Pockels cell delay (CH3 of the Genpulse). Be sure that only one pulse is dumped out (see Figure 5-11) (a) 4 (b) Intensity (a.u.) 3 2 Intensity (a.u.) Delay (nsec) Delay (nsec) Figure 5-11: Extraction Pockels cell delay optimization. a: No pulse after the maximum, the pulse is correctly dumped out. b: There is a pulse after the maximum, the delay of the Regen.out Pockels is wrong. Trident 10 Hz Optical alignment: regen and pre-amplifier - 50

51 Regenerative amplifier complete alignment WARNING: BECAUSE THE ALIGNMENT TO OBTAIN THE LASER EFFECT IS CRITICAL, PROCEED TO THE REGENERATIVE AMPLIFIER ALIGNMENT ONLY IF IT IS NOT POSSIBLE TO OBTAIN THE PEFORMANCES WITH THE COARSE ADJUSTMENT PROCEDURE OR IF THE LASING EFFECT FROM THE CAVITY IS LOST. First switch off the power supply of the Pockels cells in order to avoid any electrical risk. The alignment of the regenerative amplifier first uses the seeded beam from the stretcher without pumping the crystal. 1. Adjust mirror FM301 to send the beam into PH302, and use the mirror FM303 to send the beam into PH303. Once it is done, use FM303 to send the beam into PH304. The beam should be more or less centered in the crystal C1. It is possible to use a diffusing material (cleaning tissue for example) in front of the crystal to observe the transmitted light in order to centre the beam in the Ti:Sapphire rod. Adjust this setting with FM304 and then use FM305 to go through PH Adjust the orientation of the Pockels cell PK302. Place a plastic polarizer and a white screen after the Pockels cell and diffuse the input beam as shown in Figure Trident 10 Hz Optical alignment: regen and pre-amplifier - 51

52 Screen Diffusing element Pockels cell Polariser Figure 5-12: Setup for the Pockels cell orientation adjustment. 3. An image similar to the one shown in Figure 5-13 should appear. Adjust the Pockels cell to centre the direct beam in one quarter of the diffused rings. Scattered rings Direct beam Figure 5-13: Image of the transmitted light through the Pockels cell PC2. The correct setting is obtained when the direct beam is in one quarter of the diffused rings. 4. Remove the diffusing devices (screen, cleaning tissue and polarizer) and adjust mirror FM306 to reflect the beam back on itself. One part of the beam should be transmitted by the polarizers P303 and P Adjust mirror FM307 so that the beam is back-reflected on itself. Trident 10 Hz Optical alignment: regen and pre-amplifier - 52

53 6. Adjust the orientation of the Pockels cell PK303 in order to minimize the reflection on the polarizer P304. This Pockels cell can also be oriented with the same method explained in step 2-4, but this time the direct beam must be centered within the diffused rings (see Figure 5-14) Figure 5-14: Image of the transmitted light through the Pockels cells PC1, PC3 and PC4. The correct setting is obtained when the direct beam is centred in the diffused rings. 7. Remove the intra-cavity pinholes, block the beam coming from the stretcher and switch on the Pockels cells of the regenerative amplifier with a large delay between the two Pockels cells triggers. Send the pump beam into the crystal at the same location as the infrared beam. The laser effect will begin in the regenerative amplifier. 8. Optimize the cavity alignment with the rear mirror FM306. The pulse build-up time has to be as short as possible and the beam profile must be circular. 9. It is also possible to optimize the Pockels cell PK302 orientation by checking the spectrum and the pulse build-up time. The spectrum must be without any modulation and the pulse train occurring into the cavity must be as stable as possible. 10. The Pockels cell PK303 can be optimized by watching the signal at the output of the regenerative cavity (after polarizer P304) and, with a very long delay on Trident 10 Hz Optical alignment: regen and pre-amplifier - 53

54 Pockels cell PK303. The right orientation is obtained when the cavity leakage is as low as possible. 11. Now proceed to the cavity seeding following the procedure as shown in Figure 5-15(see next chapter). 12. The other Pockels cell (optional module), the pulse cleaner PK304, must be aligned as described in the steps 2 to 4 but with the direct beam centered on the scattering rings, like in step Regenerative Amplifier seeding Optimization In this chapter, the regenerative amplifier is assumed to be working properly. If this is not the case, refer to the previous chapter. The seeding into the regenerative amplifier may be optimized using the two adjustable reflectors which control the direction of the beam coming from the stretcher. In order to keep the system well aligned during a long period, it is strongly recommended to use the same settings for each adjustment. 1. First verify that the stretched beam is going through PH301. Note: this also corresponds to the regenerative amplifier cavity leak direction. 2. Adjust FM303 to seed the cavity. The main criterion for fine optimization is the pulse build up time in the regenerative amplifier cavity. Typical traces of the pulse evolution in the regenerative amplifier are shown in Figure The best adjustment is obtained when the pulse build up time is as short as possible and when the contrast ratio between the pulse seeded and the self-running laser effect in the cavity is the highest. 3. It is possible that no seeding occurs when adjusting the reflectors because the beam coming from the stretcher is completely misaligned compared to Trident 10 Hz Optical alignment: regen and pre-amplifier - 54

55 the regenerative amplifier direction. In this case it is recommended to observe the beam coming out of the regenerative amplifier before the mirror FM303 with a small white paper screen and to overlap the beam coming from the stretcher with the mirror FM301. Then place the screen in front of the mirror FM301 or at PH301 and overlap the two beams with help of the mirror FM303. Repeat the procedure two or three times and seeding must occur. It is then possible to follow the fine adjustment procedure described before with the help of an oscilloscope (a) 4 (b) 4 (c) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Delay (nsec) Delay (nsec) Delay (nsec) Figure 5-15: Optimization of the seeding into the regenerative amplifier: (a) ns regime, the cavity is not seeded, no spike appears, (b) the cavity is seeded but the contrast ratio is poor, (c) the cavity is seeded, the contrast ratio is high Preamplifier Alignment A lot of mirrors are used in the power amplifier but only a few of them have to be adjusted for power optimization. In this paragraph we will assume that the mirrors of the multi-pass amplifier do not need to be re-adjusted and only mirrors FM309 and FM310 will be used to inject the beam into the first amplifier. During the adjustment of the infrared beam, it is strongly recommended to stop the pump beam with a beam blocker, and to reduce the level of infrared light by changing the (CH3) delay. Trident 10 Hz Optical alignment: regen and pre-amplifier - 55

56 First slightly adjust mirror FM308 to send the beam through pinhole PH306 located before the mirror FM309. Adjust mirror FM309 in order to have the beam going through the amplifier passes, checking that the beam is not hitting the edges of the crystal or the edges of one the folding mirrors. The beam must travel without clipping and must be very close to the centre of Aperture PH307, even if the thermal lensing effect induced by the pump can change the beam direction a little. Unblock the pump beam at nominal power and re-optimize the delay of CH3. FM310 (injection) and FM326, FM329 (pump beams) can be slightly adjusted to get the best power and beam shape out of the amplifier. Trident 10 Hz Optical alignment: regen and pre-amplifier - 56

57 User s Manual 5.7. Compressor Module General description The compressor is used to re-compress the pulses after the amplification in the regenerative amplifier. As explained in the introduction, the compressor will compensate for the group delay dispersion (delay versus wavelength) introduced by the stretcher and the amplifiers. This group delay can be expended in a Taylor series: τ(ω) = τ 0 + A.(ω - ω 0 ) + B.(ω - ω 0 ) Note that the group delay is obtained by differentiation of the phase law φ(ω) = φ 0 + φ 1.(ω - ω 0 ) + φ 2.(ω - ω 0 ) 2 /2 + φ 3.(ω - ω 0 ) 3 /6+... and hence that A=φ 2 corresponds to the second order dispersion and B=φ 3/2 to the third order dispersion. In order to compensate for the group delay of the stretcher and amplifier, both the second and third order dispersion terms must be adjusted. These terms depend on the grating groove density, compressor length and the angle of incidence onto the grating. Since two conditions have to be fulfilled together, two free parameters are needed. The angle of incidence onto the grating and the compressor length are the two adjustable parameters of this system. The optical setup of the 10Hz compressor is shown in Figure It is based on the same design but the maximal aperture is adapted to the energy level. Note that it is necessary to keep all the information of the pulses and especially the entire spectrum to be able to re-compress correctly to short pulses. This means that it is very important to transmit all of the wavelengths of the spectrum through each part of the system. Trident 10 Hz Optical alignment: compressor - 57

58 User s Manual It is suggested to check the amplified pulse spectrum before any alignment of the compressor. Remember that the minimum spectral width to achieve 40fs is around 25nm. Figure 5-16: 10Hz compressor layout List of components Table 5-3 summarizes the different optical components of the compressor module with Amplitude Technologies references. 10Hz compressor legend description reference FM nm C G401 Grating C G402 Grating C R nm C L401 Spherical 800 nm C Table 5-3: 10Hz compressor components Trident 10 Hz Optical alignment: compressor - 58

59 User s Manual Hz compressor coarse alignment In this chapter, the compressor is assumed to be misaligned due to beam pointing changes from the amplifier, but the gratings angle or parallelism was not changed. For complete alignment, see next chapter. 1. Install the removable apertures PH401 and PH First adjust the last mirror located at the output of the main amplifier (FM321 on Figure 5-10) to center the beam on the FM401 mirror. 3. Check that the beam is centered on FM402 (the lower mirror of the periscope). Use FM401 to eventually adjust this setting. 4. Adjust FM402 to center the beam in the upper aperture of PH401 and FM403 to center the beam in the upper aperture of PH A compressed beam will appear close to the lower aperture of PH401. Repeat step 4 until this compressed beam is correctly centered. The compressor efficiency must be at least 67% Complete alignment of the compressor Alignment of the compressor can be achieved using the following procedure. The coarse alignment is assumed to have been done, so the beam is correctly centred in PH401 and PH First reduce the output energy by reducing the delay of the extraction Pockels cell or by using the attenuator. Trident 10 Hz Optical alignment: compressor - 59

60 User s Manual 2. Adjust the upper mirror FM403 of the periscope to direct the beam to the smallest grating of the compressor G401 at a beam height of H1 using PH402 (see Figure 5-17 for beam height values). The beam has to stay perfectly horizontal. Grating 2 Grating1 H2 H1 Figure 5-17: Beam height values in the 10Hz compressor. 3. Adjust the orientation of G401: put PH402 on the zero order reflection direction to check that the beam is reflected horizontally at the zero order reflection. 4. Adjust the orientation of the lines of the grating G401: put PH402 in front of G402 to check that the beam is reflected horizontally on the first order of diffraction. 5. Rotate grating G401 to center the beam on the grating G402 (the position of G402 must correspond to an angle of about 20 ). 6. Adjust the orientation of G402: put PH402 on the zero order reflection direction to check that the beam is reflected horizontally at the zero order reflection. 7. Rotate grating G402 in order to have an incident angle of the beam on the grating of about the same value as grating G401. The two gratings have to be as parallel as possible. Trident 10 Hz Optical alignment: compressor - 60

61 User s Manual 8. Adjust the lines orientation of grating G402: put PH402 in front of R401 to check that the beam is reflected horizontally on the first order of diffraction. 9. Check with an infrared viewer that the beam (which is dispersed) is well centred on grating G Set the reflector R401 on the beam diffracted by grating G Adjust the orientation of the reflector to reflect the beam back at elevation H2. Both mirrors of the reflector have to be moved to reflect the beam onto the grating by unclamping and moving the whole reflector mount angle. 12. The beams on the gratings must appear as shown in Figure Grating G401 Grating G402 H1=141mm H2=116mm Figure 5-18: Different beam position and distribution on the compressor gratings. 13. The compressed beam must leave the compressor through the lower hole of PH401. The compressor is now aligned but not optimized. Several parameters have to be adjusted for the optimization of the compressor: the grating parallelism, the angle of incidence on the gratings and the distance between the two gratings. Trident 10 Hz Optical alignment: compressor - 61