PHTN1300: Lasers and Light Sources
Characteristics of DPSS Lasers (2018F)
This lab serves as an example of the application of saturation intensity/power to determine both laser output as well as minimum pump power of DPSS lasers.
Small DPSS lasers producing 532nm green output are very common. These lasers feature a tiny amplifier of vanadate (Nd:YVO4) optically pumped at 808nm from a laser diode. The vanadate oscillates at 1064nm in the IR and a portion of this radiation is converted to 532nm via an intra-cavity non-linear KTP crystal. Green radiation then emerges from the OC.
Prelab (You need some key parameters to complete this lab)
- Browse section 5.9 in the Laser Modeling text on Low power DPSS design. Ignore the discussions of temperature sensitivity for now (this topic is reserved for PHTN1400 next term) however examine the basic structure of the device. See Figure 12.11.3 in the Fundamentals text for a photo of a small DPSS laser similar to the one in this lab.
The lab setup. One connector (either "Green" or "IR") is connected to the ILX power supply. In this photo the IR diode is powered-up (with the IR visible in this photo as a faint violet glow).
The laser involved (especially the semiconductor diode laser) can easily exceed 100mW of power. This limited open beam arrangement does not represent a hazard so long as (i) the beam is terminated by the power meter, and (ii) nothing is placed between the laser and the meter (Do NOT place your fingers nor any paper in the beam path - it will NOT burn paper so don't even try). Follow the instructions in this lab carefully to prevent beam hazards.
- DO NOT power the laser without a meter sensor in place to stop the beam
- DO NOT remove or reposition the meter sensor while the laser is powered
- DO NOT place anything in front of the beam (i.e. between the laser apeture and the meter sensor)
The lab consists of two identical lasers. The first is an 808nm semiconductor laser, normally used as a pump laser for a DPSS however in this case available as a separate unit so that the output at 808nm can be measured as a function of drive current. The second laser is an identical semiconductor laser pumping a DPSS laser which has an output at 532nm in the green. The output of this laser at 532nm can also be measured as a function of semiconductor laser current (and so, combined with the data from the "bare" 808nm semiconductor laser, as a function of optical drive power).
FIRST: Determine the characteristics of the pump laser diode (labeled "IR") as follows:
- Mount the block containing the DPSS laser and IR pump laser to the optical bench since the bench is used as a heatsink.
- With no diode connected to the ILX supply, select the 500mA range then set the current limit to a maximum of 275mA by selecting the "LIM I" parameter and holding the "SET" button while rotating the control knob until the current display reads the required value. Verify this with the professor if required.
|VERIFY the maximum current on the ILX supply is set to 275mA BEFORE applying power to the diode!
- Align a power meter so that it intercepts ALL of the resulting laser beam from the IR pump diode by aligning the optical sensor directly in front of the diode on the supplied mount.
- Set the wavelength of the meter to 808nm. With the laser diode off, zero the power meter.
- Plug the IR diode laser module into the DB-9 connector from the ILX supply.
- Rotate the current control knob on the ILX supply counter-clockwise until it stops to set the output current to zero
- Enable the output (MODE, Output ON)
- Vary current from zero to the maximum current specified for the diode in 10mA increments. At each step for current, record the optical output power at 808nm in mW.
- Turn the current to zero and turn the diode off.
SECOND: Determine the characteristics of the DPSS laser as follows:
- Align a power meter so that it intercepts ALL of the resulting laser beam from the green DPSS by aligning the optical sensor directly in front of the diode on the supplied mount.
- Set the wavelength of the meter to 532nm. With the laser diode off, zero the power meter.
- Plug the "GREEN" diode laser module into the DB-9 connector from the ILX supply.
- Rotate the current control knob counter-clock wise until it stops to set the output current to zero
- Enable the output (MODE, Output ON)
- Verify DPSS operation by increasing current from zero until oscillation is observed (i.e. green output and an actual reading on the meter of a few mW).
- Turn the current to zero.
- Vary current from zero to the maximum current specified for the diode in 10mA increments. At each step for current, record the optical output power at 532nm in mW.
- Turn the current to zero and turn the diode off.
Plot the output power of the IR diode (in mW) versus current (in mA). Perform a TWO-SLOPE analysis (in the same manner as lab #2) to determine the threshold current for the diode. Determine, as well, the slope efficiency.
Next, plot the output power of the green DPSS laser (in mW) versus pump diode current (in mA). Perform a linear analysis to determine the threshold current of the DPSS pump diode to allow the DPSS to oscillate.
Determine, at the same current at which green radiation appears from the DPSS (according to the graph above), the amount of IR optical pump power emitted from the IR pump diode at 808nm (which is identical to the pump diode inside the DPSS. This is the minimum pump power for the DPSS in mW of optical power at 808nm.
For the mathematical analysis predicting the minimum pump power of the DPSS, you will need to refer to the following:
- Section 3.7 (specifically equation 3.21 and 3.26 which are simplified and use saturation intensity, and you might refer back to section 2.7 as well which was required reading and explains the theory behind this)
- Section 7.6 (and especially example 7.2) outlining the structure of the DPSS (and how, for example, it lacks an OC for 1064nm). The topic of this section (optimization) is not relevant to this lab however the drawings and photographs in this section outline the basic structure of the DPSS laser you will be using as well as parameters such as absorption of the intra-cavity KTP crystal.
- Section 8.4.3 which outlines key parameters for the vanadate amplifier used in this DPSS laser.
Hand In a WORD PROCESSED (not handwritten) lab report with contents as outlined below.
The FIRST PAGE must be a title page containing nothing more than the title of the lab, the course, the student's name and ID number, and the names of your lab partner(s).
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 (with the title "Question 2") and question 3 starts at the top of a different page (with the title "Question 3"), etc. You'll have, therefore, at _least_ five pages in this report.
The lab must be submitted in a report cover (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 outline, used for all condensed labs in this course, will result in deduction of marks
Observations from the lab:
- A graph of optical power-vs-current data (with current, in mA, on the x-axis) for the separate IR pump diode. Be sure to label the graph axes and graph titles properly. Using the method described in the ILX application note determine the threshold current of the IR laser diode (the pump diode) using the two-segment line-fit method. Show, on the graph of I/P data, TWO lines superimposed on the graph. The equations for each line must be shown on the graph (to at least three significant figures) and an example algebraic calculation of threshold current shown. Show the numerical value of threshold current determined by this line-fit method DIRECTLY on each graph (i.e. add a text box as required to show it directly on the graph near the intersection of the two lines). NOTHING can be handwritten here - use the spreadsheet to make professional-looking graphs only.
- A graph of optical power-vs-current data (with current, in mA, on the x-axis) for the green DPSS. Be sure to label the graph axes and graph titles properly. Using the method described in the ILX application note determine the threshold current of the green DPSS laser using the linear line-fit method (a two-slope method should not be required here as spontaneous output from the DPSS is filtered-out already). Show, on the graph of I/P data, the lins superimposed on the graph. The equations for the line must be shown on the graph (to at least three significant figures) and an example algebraic calculation of threshold current shown. Show the numerical value of threshold current determined by this line-fit method DIRECTLY on each graph (i.e. add a text box as required to show it directly on the graph near the intersection of the two lines). NOTHING can be handwritten here - use the spreadsheet to make professional-looking graphs only.
- Knowing the drive current for the 808nm pump diode at which visible green output appears from the DPSS, determine (from the companion IR pump diode graph of question #1) the optical pump power necessary at 808nm to pump the DPSS laser. Explain briefly how you found the value and report it here in mW.
Application #1: Minimum Optical Pump Power
- Outline a theoretical prediction for the minimum optical pump power required for the DPSS laser (i.e. the optical pump power at 808nm). As per section 7.6, the vanadate amplifier is 1mm in length and the KTP crystal is an optimal 3mm in length. Section 7.6 details other parameters of the DPSS laser and KTP (such as absorption). Begin by outlining the threshold gain equation for the DPSS laser at 1064nm (which includes, essentially, two HRs and the intra-cavity loss of the KTP crystal which is a distributed loss). Now, compute Isat (in W/cm2) for vanadate, then Psat (in W) and finally the minimum pump power in mW (assuming a beam diameter of 0.4mm inside the amplifier). Be sure to outline all calculations in detail as well as substituted values, and show how all parameters (such as Isat and Psat) were calculated.
This question will require you to read the text (sections 2.7 and 3.7) and utilize the methodology outlined there - it is a bit of an exercise in self-learning. Be sure to read each section thoroughly as these calculations are not straightforward (especially concepts such as quantum defect).
Application #2: Predicting Intra-Cavity Power
- Knowing g0 of the 1064nm vanadate transition from the tables, and having calculated gth for the DPSS at 1064nm, as well as other parameters of the DPSS laser, calculate the intra-cavity power at 1064nm at normal operating conditions (i.e. with a gain of g0). A small portion of 1064nm radiation will be converted to green radiation at 532nm (which you measured) which exits the OC.
- Calculate the amount of power exiting the OC (really an HR) at 1064nm ... this is unwanted 'leakage' and must ultimately be filtered-out as it is a safety concern if it exceeds 5mW (more on this in the next course) - in this experiment that 1064nm component was filtered-out from the DPSS leaving only the green 532nm output however on some cheaper (and unsafe lasers) it is allowed to exit the laser apeture.
Application #3: Predicting Output Power
- Predict the output power of the WPR-252 HeNe laser from lab #4 using your determined values for both small-signal gain (g0) and threshold gain (this is gth for the normal configuration, without the inserted glass slide). To do this, you must calculate the saturation intensity (in W/cm2) for this specific laser and then the saturation power (in W) first. The beam diameter for the WPR-252 tube is 1.3 mm (as measured using the "1/e2" method - this is approximately the diameter of the amplifier tube). The cross-section of the red HeNe transition may be found in chapter 8 of the Laser Modeling text however (and this is IMPORTANT) use the ULL lifetime figure from example 3.1 (pp 72) and not the one in the table in chapter 8 (this was discussed in the lectures). From the saturation intensity, the saturation power, can be computed as per the method outlined in the lectures (as well as the aforementioned example in the text). Calculate the output power (from the OC) from the HeNe laer of lab #4 using the "simple model" outlined in lectures and notes. Be sure to use the correct formula for saturated gain (i.e. apply the corrections necessary for "directional power" as covered in lectures). Expect an answer between 100μW and 10mW. If you get an illogical answer, consider the following: Are all gain measurements in the same units? Are lengths in cm or m? Did you compute saturation Intensity (W/cm2) or Power (W)? Was cross section in the correct units (cm2). Finally, if the calculations were done correctly but the answer is still illogical, consider g0 - is it reasonable (i.e. around 0.15m-1)? Make sure you outline ALL calculations including saturation intensity, and output power. Be sure to use YOUR values for gth and g0 from lab #4 as well as YOUR value for ROC as determined in lab #4. (Reference: cross-section is covered in Laser Modeling by Csele example 2.6, page 53 and application in section 3.6, page 79)