Characteristics of DPSS Lasers (2018F)

Small DPSS lasers producing 532nm green output are very common. These lasers feature a tiny amplifier of vanadate (Nd:YVO_{4}) 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.

- Browse section 7.6 in the
__Laser Modeling__text. Although the topic in this section is not relevant to the lab, the drawings and photographs in this section outline the basic structure of the DPSS laser you will be using. **The following prelab assignment (worth 15% of the total lab mark), is due in class before the lab period (the exact date is on the main course home page).**Late marks are not assigned if the prelab it is not received at the beginning of the class: you lose 15% of the total lab marks immediately with no recourse if it is not received upon*entering*the period in which it is due (extensions will NOT be given to "print it out" in the lab ... be prepared with the hardcopy already printed).In this prelab you will predict the output power of the HeNe laser from lab #4 using your determined values for both small-signal gain (g

_{0}) and threshold gain (g_{th}for the normal configuration, without the inserted glass slide).To do this, you must calculate the saturation intensity (in W/cm

^{2}) 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/e^{2}" 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 use the ULL lifetime figure from example 3.1 (pp 72) and not the one in the table in chapter 8. 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/cm

^{2}) or Power (W)? Was cross section in the correct units (cm^{2}). Finally, if the calculations were done correctly but the answer is still illogical, consider g_{0}- 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 g

_{th}and g_{0}from lab #4 as well as YOUR value for R_{OC}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

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
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.**no diode connected** - 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 counter-clock wise 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.
- 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)
- Place a white paper sheet in front of the power sensor
- While observing the paper for visible output, vary current from zero to the maximum current specified until green output is seen. This will verify DPSS operation.
- Turn the current to zero and remove the paper.
- 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.

VERIFY the maximum current on the ILX supply is set to 275mA BEFORE applying power to the diode! |

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. Plot the output power of the DPSS (in mW) versus pump diode current (in mA).

Determine, at the current at which green radiation appears from the DPSS, 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 section 3.7 (specifically equation 3.21, and you might refer back to section 2.7 as well which was required reading), section 7.6 from the prelab outlining the structure of the DPSS (and how, for example, it lacks an OC for 1064nm) as well as parameters for the intra-cavity KTP crystal, and section 8.4.3 which outlines key parameters for vanadate.

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*

**LAB SUBMISSION:**

*Note that questions #4 and 5 (the theoretical predictions) are worth more marks than other questions, aside from which these are new concepts and will be on the final test.*

- A graph of optical power-vs-current data (with current, in mA, on the x-axis) for the 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. Report it here in mW.
- 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). Now, compute I
_{sat}(in W/cm^{2}), then P_{sat}(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 and show how all parameters (such as I_{sat}and P_{sat}) were calculated. - Knowing g
_{0}of the 1064nm vanadate transition from the tables, and having calculated g_{th}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 g_{0}). A small portion of this will be converted to green radiation at 532nm (which you measured) which exits the OC. Finally, 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.