This is a two-hour lab in which students will investigate the concepts of the Nominal Hazard Zone (NHZ), Optical Density (OD) as it applies to safety glasses, and safety concerns around green laser pointers which involve multiple wavelengths (the same concepts apply to a variety of solid-state laser systems).
BEFORE this lab ...
You must have successfully completed both the Laser Safety Quiz on Blackboard (certifying that you have seen the video and understand the basic safety procedures required in our lab) as well as the Safety Glasses Quiz in lab #0. Both quizzes should have been completed last week.
The prelab assignment is worth 10% of the total lab mark and is due at the beginning of the lab period. Late marks are not assigned if the prelab is not received at the beginning of the lab: you lose 10% of the total lab marks immediately with no recourse if it is not received upon entering the lab (extensions will NOT be given to "print it out" in the lab ... be prepared with the hardcopy already printed).
In this part of the lab students are introduced to safety concepts including the nominal hazard zone.
The laser employed in this part of the lab is class-IV. Safety glasses suitable for the wavelength range employed (i.e. 445nm) are required. Be careful with the position of the beam since it will start fires as well as burn flesh if carelessly positioned!
Since this lab involves alignment procedures, alignment glasses with a lower OD than normally employed will be used - this means that these glasses will protect against diffused reflections but will not protect against direct beam exposure - follow the guidelines given in the video (for example, by removing jewelry and watches), be sure to omit any shiny metal surfaces from the beam path, and do not bend-down or sit where the laser beam is at eye level.
There will undoubtedly be stray beams in the room from reflections off various optical components, for this reason ...
In a Class IV laser lab, use of safety glasses is not optional. Anyone found in the presence of an operating laser without safety glasses is subject to immediate expulsion!
The beam from a one watt 445nm laser is setup as per the diagram below in which the beam strikes a ceramic scatter plate producing a diffused reflection. A photodetector, measuring reflected power, is mounted such that it can be slid to any distance from the plate. A barrier prevents accidental contact with the beam (which will produce burns).
Ensure you are wearing safety glasses suitable for the 445nm radiation employed - for this lab alignment glasses will be used with a lower OD than normally used. Turn the main laser power on via the switch at the rear of the unit then turn the keyswitch on. Allow the beam power to stabilize for a minute then measure the output power with the Melles-Griot broadband power meter (NOT the OPHIR or Gentec meters which cannot handle powers in excess of 300mW).
Now, place the detector (the head of the OPHIR power meter, with the filter installed, or the GenTec meter) at a distance of 5cm from the ceramic plate, keeping it on the "non-beam" side of the barrier with the window of the detector facing the ceramic plate. Read the diffused power at this distance. Increase the distance between the ceramic plate and the detector in 1cm increments, recording the power each time, until a distance of 40cm is reached.
Plot two curves on the same graph: one, a theoretical curve showing how intensity should decrease according to the "R-squared" law, and the second the experimental results. For the theoretical graph, start with the power measured at the 5cm distance - the expected power at a distance of x is then P5cm*(52/x2) ... both curves must agree at 5cm. As usual ensure the graph has a title, axis labels with units, and gridlines (at least five in both axes).
Identify, using the experimental data, the distance where the NHZ is reached (based on current regulations).
Now, use the EasyHaz online laser safety calculator (http://lasersafetyu.kentek.com/easy-haz-laser-hazard-software-basic-web-version/) to predict the NHZ. Input the power from the experiment (as measured from the incident beam), the wavelength, and exposure time (assumed to be 0.25 seconds for visible exposure).
Print/Capture a screen-shot showing the results, which include the diffused reflection NHZ as well as the required OD of protective eyewear.
How does this value of NHZ compare with your observations?
In this part of the lab you will be evaluating the optical transmission characteristics of several laser safety glasses. Since all lasers employed are class IIIa, safety glasses are not required however common sense precautions as outlined in the video (e.g. removal of jewelry and watches) are required.
For each lens used in this lab (listed below), look-up the OD ratings (and, of course, corresponding wavelengths) as well as the VLT (Visible Light Transmission) from the manufacturer. The lenses used are from NOIR and are as follows:
You will be required to peruse the NOIR website to find data for each of these lenses. Note that some lenses are ANSI certified only and are being phased-out in favour of lenses with both ANSI and CE/EN207 (European) cerifications allowing worldwide usage. Start with the "filters" page to see all available filters and their related data.
The transmission of lenses will be evaluated at 950nm (IR), 632.8nm (Red), and 532nm (Green) and the stated OD (Optical Density) characteristics of these lenses will be verified.
Each laser used is class-3R or less (by ANSI standards <5mW) and so safety glasses are not required (although the operator must still protect against direct intrabeam viewing including reflections from shiny surfaces such as the detector). As per the prelab video, follow common safety precautions such as removal of jewelry and prohibit the entry of anything shiny into the beam path.
On most lenses, the wavelength range and OD at that range can be found (these are industrial blanks and so not all may be labelled). Write these down (or at least the lens identification on the package) as they will be required for the analysis later. Both European and North American standards can be found on many lenses - use the North American ANSI standards (marked as "OD"). Now, insert each lens into each laser beam, observing both the incident (with no lens inserted) and transmitted (lens inserted) powers - it will be necessary to change meter ranges). From these observations, calculate the percentage transmission and the Optical Density (OD) of each lens at each wavelength.
Each laser is already setup in a mount with all on the same post: do NOT realign these mounts - leave all lasers alone! The 532nm DPSS source is on the bottom, the 950nm diode laser in the middle, and the 633nm HeNe laser on the top. At the start of the lab, turn each ON and allow it to stabilize before use (you should be aware of the instabilities of most lasers from last year).
Begin with the HeNe laser (top). Do not move the HeNe laser mount. Move the detector (on a separate post with a translational mount) up and down to align it for maximum power: the detector mount has two knobs with the right-side being the adjustment and the left side being the lock. The incident power should read between 2mW and 4mW. On the second post (between the laser and the detector) are two filters - do not realign these filters. In front of the HeNe laser is a RED 630nm long-pass glass filter which will prevent spontaneous emission (the "blue glow") from the HeNe laser tube from reaching the detector.
This photo shows the filters in front of the HeNe laser (a red filter) as well as the 950nm diode source (an IR filter which blocks passage of all visible light).
Now, block the beam at the source between the laser and the filter (do NOT turn off the laser, and do not use your finger since it transmits IR radiation), ZERO the meter carefully, then unblock and read the incident power. Insert a lens into the beam path (between the filter and the detector), and measure the transmitted power at that specific wavelength. Switch the meter to a lower range and re-zero the meter as necessary to obtain as accurate a reading as possible: place the lens in front of the detector, block the laser beam and zero the meter (with the lens in place), and unblock the laser to measure the transmitted power through the lens. Do all lenses then move onto the next laser.
The photo shows a safety glass lens inserted into the HeNe beam. Did you notice something wrong here? You should have!
Use the following procedure for maximum accuracy:
You'll probably need two people to complete this - one to hold the lens and another to press buttons on the meter. While the incident power will remain constant, the transmitted power will vary for each lens - read all lenses for this laser. When done, select the next laser and be sure to set the meter for the wavelength in use. For the IR diode laser, align the detector again (raise/lower the detector head maximizing power). An IR detector card may help here. Repeat the same process as with the HeNe laser measuring the incident power and the transmitted power for all lenses at this specific wavelength.
Finally, repeat the same procedure with the 532nm source. A filter in unnecessary here since this source has an integral dielectric filter ensuring only 532nm radiation is emitted (without this filter, residual 808nm and 1064nm radiation would be expected).
Summary Table #1 (OD)
For each lens, produce a table with the following FIVE headings: Wavelength (nm), Incident power (mW), Transmitted power (mW or μW or nW), OD as calculated from the experiment, and OD stated on lens or in the documentation from the website for that specific lens (for that specific laser wavelength employed, if stated). If the lens does not state the OD for the specific wavelength in use, simply note this in the chart as "None Listed".
Explain when a lens is considered to meet the stated density and when it does not.
Following the prelab, the output of an inexpensive green laser pointer is examined.
First, don safety glasses suitable for 100mW of 1064nm radiation (although this seems odd right now, the reasons will become evident in a moment). If in doubt about the required OD specifications of the glasses, ASK!
Install a green laser pointer into the holder and secure it using the screw. Do NOT use the screw to hold the pointer in the "on" position as this will exceed the duty cycle of the pointer. Turn the pointer on by pressing the button and ensure it strikes the center of the power meter.
Set the wavelength to 532nm, turn on the laser pointer by pressing the button momentarily (it will stabilize after a second or two), and record the output power - this will be all three components combined. Now insert the 532nm (green) bandpass filter into the holder so it intercepts the output beam and record the power of the 532mW component alone. The holder is designed to accommodate filters of any thickness so try several slots until it fits properly and remains perpendicular to the beam. Wear a glove when handling filters (by the edges, regardless) only to avoid fingerprints.
Remove the 532nm filter and install an 808nm bandpass filter in the holder. Set the meter wavelength accordingly and repeat the measurement for residual 808nm pump diode radiation. Finally, remove the 808nm filter, install the 1064nm bandpass filter, reset the wavelength on the meter and measure the power of the 1064nm fundamental component.
Repeat the same four measurements with three other green laser pointers.
When analyzing the power of each component, be sure to account for the fact that the filters do not have 100% transmission at the target wavelength but rather attenuate the signal - this can be determined from the transmission curve for each filter provided here from the manufacturer. If, for example, the meter reads 5mW and the filter transmits 60% at the target wavelength the actual power is 5/0.6 or 8.33mW. In the lab report outline the measured optical power of each component, the transmission of the specific filter at that wavelength, and the actual power emitted from the laser as these will be required for the lab report.
A review of the key concepts from the lectures as employed in this lab ...
Here are the warning labels of various laser in the lab which you will use in this course. Key parameters such as wavelength and power (required to chose safety glasses properly) are found on these labels (which, by law, are required for all lasers). Some labels are from older lasers and so may feature different information for the current standard.
|Coherent Innova-90 Argon-ion laser operating at visible wavelengths. See the Fundamentals text by Csele for exact wavelengths.|
|Lee Lasers Q-Switched YAG laser. As a Q-Switched laser, the MPE is diferent than that for a CW laser and requires, for accuracy, knowledge of pulse duration and pulse rate (usually from the manufacturer or an SOP). The exact model is an LDP-50MQ - it may be converted to 532nm output as well however is currently operating only at 1064nm.|
|MPB IN-100 CO2 Laser.|
|Uniphase HeNe Laser.|
You'll also need a chart of MPE values to begin ... (from ANSI regulations)
For homework, compute, both manually (showing all calculations for required OD) and using EasyHaz software, the required OD for each of these lasers, producing a summary table as follows: Laser Type, Operating wavelength, Power Output, MPE (from OSHA regs), Exposure time (from OSHA regs), Required OD (manually calculated), Required OD (EasyHaz). This is required for the lab submission question 4c.
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.
Questions must be identified at the top of the page as QUESTION 1, QUESTION 2, etc. as shown to the left
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
For ALL CALCULATIONS, work must be shown! Answers without calculations will receive a mark of ZERO. Where a calculation is repeated many times (e.g. to complete a table of values) show ONE complete set of example calculations.