This is a one period (two-hour) lab in which students will investigate the use of a femtosecond laser including the conditions required for ultrafast pulse production.
First, obtain for reference the Tsunami modelocked laser user's manual part # 0000-232A. Available online, this manual describes the entire laser as well as tutorials on operational principles such as wavelength selection, group velocity delay, and pulse width measurement.
Second, read chapter 7 (specifically 7-1 to 7-6) of the above manual which describes the basic operations required to startup, optimize output power, and modelock the laser. You will note the procedure contains descriptions of "walking the mirrors", a procedure you used in PHTN1300. As a background, you might also look at sections 3-9 and 3-10 describing how the wavelength selection prisms and dispersion control work - this will give you a good idea of what you are actually affecting when adjusting the controls of the laser. Take a look, also, at the specifications in chapter 3 to have an idea of the powers and tuning range to expect when operating the laser.
Failure to read the manual ahead of time will result in an incredible waste of time in the lab as you attempt to determine what the controls do ... femtosecond lasers are complex machines and the basic background provided by the user's manual is essential as a start to understand how it works.
The laser employed in this lab is a class-IV. Safety glasses suitable for the wavelength range employed are required. There is a currently a debate in the laser safety community regarding the effectiveness of safety glasses as the femtosecond pulses can bleach dyes used in the lenses - PREVENT DIRECT EXPOSURE TO THE BEAM AND REFLECTIONS AT ALL COST for optimal safety!
Preheat both the laser and the 3955 modelocker. These require 45 minutes minimum to stabilize. When the laser is in use during the term, the power supply is always left in standby mode (i.e. with the key removed to prevent lasing, but with the main power switch on the pump laser left on) and the 3955 modelocker is also left ON but not enabled - this keeps the AOM modelocker heater "alive" stabilizing temperature.
Lab procedure updated 2016/03/01
First, start the laser in CW mode. Assuming the Millenia Vs pump laser is is STANDBY mode, ensure the water is turned on by checking for the green "flow" indicator behind the laser (since it is interlocked) and press the POWER button and wait for the diode temperatures to stabilize (about five minutes). Set the 407 power meter to the 0.3W range. When the pump laser is finally ready, press and hold the POWER button to start the pump laser: it will begin oscillating and will ramp the output power to 0.2W. Select the SETUP menu and OPEN the shutter. Now, from the MAIN menu press "P2" to ramp the pumnp power to 5.0 Watts
With the dispersion control set to 2, the wavelength control around 5.0, and the pump power at 5.0W CW output should be observed. Use the IST/REES laser spectrum analyzer (LSA) to view the output of the laser - the cursor wavelength will appear on the display as a blinking line. CW output should appear around 800nm as a large output around a single wavelength (you will likely need to adjust the wavelength on the LSA to see the output as a peak). Adjust the wavelength to 800nm as seen on the LSA (set the center wavelength to 800nm on the LSA first, then adjust the laser wavelength selector).
Optimizing the mirrors: As per the procedure you read as part of the prelab, optimize the output power by walking the OC and HR mirrors VERY gently and in VERY small increments of only a few angular degrees ... the laser requires very little adjustment. Adjust only M1 and M10 during this procedure, NEVER open the cover on the laser. As usual, during this procedure do NOT allow the laser to stop oscillating: be sure to optmize one direction (e.g. horizontal) before repeating with the other direction. You should achieve an output power as shown in figure 3.8 of the Tsunami manual (standard optics, Millenia Vs pump) and at least four LEDs of the PHOTODIODE signal on the 3955 modelocker should be lit (this is important - if the PHOTODIODE signal is too low it will never modelock).
Observing Mode Competition: At this point observe the tuning range of the laser under CW conditions. Set the wavelength control in approximately 5nm steps and record the output power of the laser. The tuning range might not be continuous. Observe, as well, competition between modes during tuning in which the output flips between two adjacent wavelengths. This is expected since the wavelength selector slit is set relatively wide open allowing many modes to simultaneously oscillate.
Achieving Modelocking: Set the wavelength of the laser to 800nm. Adjust the dispersion control for maximum CW output (record it). Ensure the photodiode signal is large on the 3955 modelocker then enable modelocking and set the phase for midpoint or slightly below (labelled "800" on the PHASE display). Modelocked output should now appear as a broad, but low-level, Gaussian output. You might have to adjust (i) the dispersion control and (ii) the phase control to achieve modelocking. Note the output power now. When modelocked output appears you might have to increase the gain of the LSA using the "high gain" button to see it properly but will know it is modelocked as the large central peak will disappear from the spectrum but the output power will not significantly change.
Observe the spectral width of the resulting pulse - the pulse length will be determined from this. The scope is capable of measuring FWHM using cursors.
The effects of Phase: Gradually increase the phase control to "900" to observe the chaotic behaviour and possible CW breakthrough (record all observations regarding what the output spectrum resembles and when a change occurs). Modelocking occurs only over a certain range. Starting at 900 (top), reduce the phase and record how the output looks as the laser achieves modelocking: the output pulse takes on several specific forms before actual modelocked output occurs. Ensure you know what "CW Breakthrough" looks like.
Self Modelocking: With modelocking achieved, DISABLE the modelocker. Usually, the output remains modelocked even without the modelocker active. In this state, perturb the laser by gently turning ONE mirror adjustment to detune the laser. When output stops, restore the mirror adjustment to optimal (output power) and observe the output now. Restore modelocking.
Dispersion Control: With the laser modelocked, vary the DISPERSION control and observe the effect on laser pulse output. You can optimize the pulse length in this manner (i.e. make the bandwidth as large as possible meaning the pulse is as fast as possible). If CW Breakthrough occurs, turn the dispersion control back to restore modelocked operation. Record the largest bandwidth possible.
While operating in modelocked mode, vary the wavelength control and note how the tuning range is continuous or discrete. Record the range over which modelocked output is possible.
A few things to note ...
An explanation of the wavelength selection system from the Tsunami User's Manual, pp. 3-9:
"The fs Tsunami laser is wavelength tuned using a prism sequence and a slit. This sequence provides a region in the cavity where the wavelengths are spatially spread. A variable slit is located in this dispersed beam. The output wavelength is tuned by changing the position of the horizontal slit in the vertical plane. The width of the slit can also be changed so that the bandwidth (and, hence, the temporal width) of the output pulse can be varied. This simple, straight-forward method covers the entire Ti:sapphire range for ultrashort pulses."
On the dispersion control:
From lecture notes, you should realize that the production of ultrafast pulses requires longitudinal modes to synchronize. Group Velocity Delay (GVD) is the effect whereby different wavelengths of light inside the cavity (i.e. different longitudinal modes) encounter different velocities. In any laser, optical elements induce positive delays (i.e. spreading of modes) which would foil modelocking. In our femtosecond laser GVD is controlled by inserting more, or less, of the prism glass into the intra-cavity beam path to compensate for this.
From the Tsunami User's Manual, pp. 3-9:
"The pulse width tuning characteristics of the Ti:sapphire laser are influenced by two factors: those inherent in the Ti:sapphire material itself and those from cavity parameters. While we cannot readily modify the Ti:sapphire material to change pulse width, we can modify the net group velocity dispersion (GVD) of the cavity. The optical components in the laser cavity introduce positive GVD and cause pulse spreading. Further pulse spreading is caused by self-phase modulation (SPM) in the Ti:sapphire rod, which results from the interaction of the short optical pulse with the nonlinear refractive index. In order to obtain stable, short output pulses, these effects must be compensated with negative GVD." "fs systems. Prism pairs are used to produce a net negative intracavity GVD in the fs system. Varying the amount and type of prism glass through which the intracavity beam travels changes the net intracavity GVD. Pulse widths from <35 to 150 fs can be obtained with various cavity configurations. Pulse width also depends on the operating wavelength."
On the slit control:
The prisms separate all intra-cavity wavelength spatially. A slit serves to select which of these dispersed wavelengths exists in the cavity and is used to tune the center wavelength of the laser. The width of the slit is adjusted only once: too wide and CW breakthrough occurs, too narrow and bandwidth is limited (and so pulses are broad). A narrow slit is desired when operating in purely CW mode (however using a femtosecond laser in this manner is akin to using a transport truck to drive to school each day: it will do it, but the overhead costs are high and there are easier ways to get CW tunable radiation).
You should READ APPENDICES A and B of the Tsunami manual which explains the modelocking technique, regenerating modelocking (i.e. the phase issue), GVD, etc.
The FIRST PAGE must be a title page containing nothing more than the title of the lab, the course, and the student's name and ID number
Answer each question as "1", "2", etc with each new question starting on a NEW PAGE so that question 2 starts on the top of a new page and question 3 starts at the top of a different page, etc. Where a question has multiple parts (e.g. 3a, 3b, 3c ...) answer each in a separate paragraph with a title identifying the question in the form "3a., 3b., 3c. ...". Do NOT answer an entire question (e.g. question 3) as a single paragraph.
This format will assist you in ensuring EACH and EVERY question is answered since marks cannot be given for work not completed, nor would it be expected that you could complete the TEST QUESTIONS which will most certainly be similar to those you see here! (Hint !)
The lab must be submitted in a report cover (preferably either a three-hole punched cover or one with a clamp on the left side, not a binder), and NEVER as a stapled mass of loose papers!
Failure to follow this simple format, used for all condensed labs in this course, will result in deduction of marks