Underwater laser

[HVACctrl]

I am researching the possibility of building a device which utilizes an underwater laser. Can anyone give me advice on where to start? Are there any tricky considerations for underwater use? Green laser instead of red and why? Is refraction due to water temps a problem, etc...?

I would really appretiate any help anyone can provide! I'm new to lasers, but pretty much up to speed on control systems and a little on electronics.

Thanks,

Ed

 

[IRstuff]

There is a narrow window in the blue-green that is somewhat transmissive. This is the window that underwater mine detection system use.

However, the turbidity is the real killer in sea water. It effectively limits semi-narrow beam transmission to around 60 m.

TTFN

 

[Thorn3]

IRstuff is quite right. As a more simplistic explanation, you have to take into account that water absorbs different frequencies of light, with distance. As an example, while SCUBA diving, you will note that the deeper you go, the more light is absorbed by the water. As you get quite deep, you will note that everything looks kind of blueish. This is because blue is the last frequency of light to be absorbed. At depth, I have to use an alternate light source, if I want to see objects in their true color.
Thermalcline is another factor to take into account. This is a point where the water changes temperature and is seen as an actual layer or film. The thermalcline is also in motion, just as the surface of the water above. Thermal changes in the water, (convection current) such as water rising and falling will also cause deviation of the beam.
Salinity of the water, partical distribution in the water. There are many factors to take into account here, beam divergence etc, depending of course on your application.
Are you intending to bounce the laser light off an object and have it return to a collector? If so, make sure your target is at a point half way to maximum distance or less, that the light can travel before full obsorbtion occurs. Is the laser to be immersed completely traveling a horizontal plane? If it is to travel a vertical plane, it would help to immerse the business end of the laser below the surface of the water, to prevent reflection and refraction due to the surface itself.
Then, there is the more simplistic problem of deposition of ocean going particulates, on the optics of your system. Hmm.. I would like to hear of your results. I share your curiosity in this.

 

[IRstuff]

Note that some mine detection systems today use a doubled Nd:YAG at 532 nm. Note also that this requires a non-eyesafe energy.

You'll need to determine over what distance you wish your device to operate, how it's to be detected, etc. TTFN

 

[Thorn3]

I suppose some of your question is not answered. sorry, but your question inspired me to look into it further.
Ok, here it is:
Blue and green light, being a shorter wavelength than other colors in the visible spectrum, aside of course from purple, travels further in sea water than red, which is at the other end of the spectrum. Blue or green has what you would call an "optical window" in sea water.
Blue or green light, will maintain greater brightness and coherency over distance, versus red or other colors, in sea water.
Red light, will suffer intensity losses of 50%, over just 2 meters in sea water, where blue and green will have to travel 35 meters, to experience the same losses at the same power output.
If you use a green, 532nm, diode pumped YAG laser, at 200mW in sea water, it is possible to achieve 200-300 meters range. With the same type of laser, at 2000mW, the distance would then be approximately 2000-3000 meters.
I think this note is much closer to the information you were intitially looking for. I hope it helps you.
What is your application? The data given is if you wish to achieve underwater communications, holography etc. Underwater laser welding is another matter, due to the very short distances the light would travel. Losses would be quite minimal with whatever color you choose.
So, IRstuff was right on the mark, with his suggestion of a wavelength of 532nm. I hope the power ratings mentioned above, will be of some use to you in choosing how many milliwatts you will need to accomplish your goal.

 

[HVACctrl]

IRstuff and Thorn3,
First, thank you for the replies. I appretiate your input. I suppose it would be helpful for me to take some sort of crash course in optics.

My application is fairly simple: I am trying to build a device that is basically comprised of a laser source and a collector. The laser would be be used in no more than, say 4 or 5 feet below the water surface in a horizontal plane in either fresh or salt water. Fresh water is more the idea, saltwater could be possible also. I am hoping to be able to shoot underwater objects and determine their general distance- not a very great accuracy is needed.

Are you guys in some sort of a related buisiness? Do you have any resources to which you can point me? I've searched the web quite a bit, but haven't found very much. I also talked to a manufacturer of laser components, who put me in touch with a guy from the Dept. of Wildlife and Fisheries who does some underwater laser stuff. But, I haven't been able to reach him as of yet.

Thansk again for the help.

Regards,

Ed

 

[Thorn3]

Hello HVACctrl,
I have run into IRstuff here in the forum. I thouroughly respect his opinions and find his data to be quite accurate. However, I am not sure of his exact business, aside from his listing under Aerospace.
I am just a mechanical engineer in general. I started my own company a few years back. My work requires that I be at least familiar with many technologies, enough to make logical or practical decisions regarding their use.
It appears from your description, that you wish to develope some form of imaging device, such as could be compared to sonar. I will join you on the hunt for such material, and relay this information to you and any websites that I may find.

 

[HVACctrl]

Thorn3,
I really appretiate it- although its more than I would have ever expected. I, too, am a general mechanical engineer with some knowledge about many things, but maybe not alot of knowledge about any one thing (lasers for instance) .

You are right on with your comparison to sonar. My problem with sonar is that standard wide beam sonar (as used on depth finders, etc...) presents such a large beam that, in shallow water, reflection from the bottom causes interference. Narrow beam sonar would be a possibility, but I haven't gotten anywhere with that as of yet. The others thought is laser, which could, hopefully scan hoizontally just under the water's surface to "see" ahead.

This is sort of a personal project- not work related. But, I am hoping that with the right findings, I can potentially develop this into a nice device that can solve a problem and that has a definite market.

Regards,

Ed

 

[Thorn3]

Hello again HVACctrl,

Not a problem assisting with research. I think a lot more of us would accomplish many more goals, and more efficiently if everyone put forth enough effort.
From what I have gathered, you can use a laser effectively to scan an area to the depth you previously stated.
To start, you would need a laser diode assembly. Of course, it is preferable to use a diode pumped YAG laser, as you would achieve an output power that would place you into a good scan range effectively. But, initial costs to you would be somewhat higher.
The idea is to point the laser, as if you were holding a hose, and watering the lawn. You spray back and forth, about 60 degrees total travel lets say. You can do this by pointing the laser at a diagonal mirror, that is being rotated by a motor. For high speed scanning, you really should have the mirror spinning a full 360, at high rpm, to achieve greater density in your scan. You can modulate the lasers output, to coincide with the spinning of the mirror, so that you don't wind up with 360 deg., beam spread.
Then of course you would want to receive any bounce back of the laser light, via a CCD camera. Since we have already chosen the blue/green or 450nm-532nm range, this places the scanning laser within the visible spectrum. This means you can use any number of color CCD moduals.
An idea, maybe, due to the length and complexity of any further investigation..we should continue this through email. You can write me at thorn21@cableone.net. If you use Yahoo Messenger, I am alysixueh. In all fairness to anyone who may be tracking this conversation for their own education, or entertainment, whichever comes first, we can post follow-ups to our progress in a shorter, to the point way. Is this acceptable?

 

[IRstuff]

My occupation is Defense/Aerospace.

The only existing application as described are military mine detection systems. Assuming that both receiver and emitter are underwater eliminates the biggest headache in such systems, which is the surface reflectivity of the water. The #1 problem BY FAR, is the turbidity of the water. Turbidity is essentially suspended particles in the water such as animicules, dirt, etc. These tend to make dispersion effects that limit you ability to image meaningfully in most waters in the world. "Time of flight" is critical to most such systems. Single detector mine detection simply measures time from transmittal to a reflection from a submerged object. Imaging systems require time gating as well as intensification to achieve sufficient signal after time-gating. Time-gating is simply turning on the imager for an interval on the order of 10's of nanoseconds, which captures a slice of water. Multiple gates are required to develop a 3-D image.

Alternately, you may wish to re-consider sonar. Commercial imaging sonars are available and are essentially similar to the ultrasound systems in doctors' offices.

http://www.seascape.nl/sonar.htm

http://www.apl.washington.edu/programs/DIDSON/DIDSON.html

TTFN

 

[Thorn3]

Hello again,

I do agree with IRstuff regarding the use of sonar, but then again, this may be a matter of the heart as well. If you really want a laser imaging system, I have come across a wealth of data for you. Schematics, program codes, theory/physics etc for you. If you can hook up with a good programmer, I am sure you can come up with a decent filter program, that will enable you to filter out specific debri in the water, according to size.
The system was originaly designed for robotics use. It has a relatively small component count for what it does, surprisingly. I believe, that with some slight changes to the original system, you could achieve your goal of an underwater laser imaging system.
Note however, that your costs are going to be something that may make your wife cry. Either that, or simply beat you to a pulp in a variety of interesting ways. LOL. Really though, the materials that I have found for you will help you get where you want to go. I do believe, that with some experimentation, you will be successful. You will not have the complexity of a military application, but I do have to say, I am impressed with the material that I have seen. Do not fear the mainboard that is shown in some of the pictures. If you read carefully, the author explains that the circuits needed for the imaging system are only a very small portion of the board. The rest of the board is used for other applications.
If you need assistance generating a PCB from the schematic on the site, I will assist you with this.
IRstuff, I do believe you will find this material to be of great interest as well. The author has done all the work already and quite successfully.

Here is the site where you both can find the information that I am talking about:

http://www.seattlerobotics.org/encoder/200110/vision.htm

Hope this helps!

 

[IRstuff]

Thanks to Thorn3 for the cool link.

I should remind HVACctrl that the system described will work only in very clear water.

The range-gating is essential if the water is not visibly transparent. TTFN

 

[HVACctrl]

OK,
You guys have me feeling like a slacker. You find more and better data than I am able to find for my own project! I've just been extremely busy at work and home here lately.

I did go to the link, but I haven't had the time to sit down and absorb the document as of yet. Thank you for the help.

By the way, Thorn3, what is the nature of your company? Do you do PCB design, custom electronics or something similar? I figured your research capabilities and ability to pick up on otherwise unknown subjects may have evolved from having to do similar projects for clients. I'd be interested in knowing what you do.

I'll reply as soon as get some time to go back to the link and really take it all in.

Thanks again,

Regards,

Ed

 

[HVACctrl]

IRstuff,
Lake water with limited visibility- say 3'- is pretty much out of the question?

If this is the case, I suppose narrow beam sonar is the only answer.

Ed

 

[Thorn3]

Hello HVACctrl & IRstuff,
Yes, that link is a good one. I was quite pleased to find it. I spent roughly twelve hours finding a site that had all the details of laser ranging and imaging. Not an easy topic to find this sort of detail! Do not worry about others seemingly doing more work. It is the will to help, that gets people moving in the right direction. Always happy to help, as I have been in your situation, and still am really. I also have an interest in lasers and their applications.
As far as my personal info, here it is: I was adopted at 18 months old. By age ten, my parents thought I was so unusual, that they took me to Yale University to have my head examined and just about everything else you could imagine. My IQ turned out to be only 168, so I don't know what their problem was. Anyway, my engineering interest started when I was very young. I was awarded a scholorship to MIT when I was in the ninth grade, but my parents refused to allow it. Grrr.. I left them when I was 15.
Anyway, I went to school on my own budget, which greatly reduced my professional quality education. I studied mechanical and electronic drafting, mechanical engineering and in more recent years CAD. I did most of my studies on my own, on other subjects. I am the kind of person who goes to the store, then takes the product home, takes it apart, and modifies it so that it is better. I have always done this, even to my christmas toys as a kid. My friends hate that by the way. They always ask me "do you ever buy anything, and not take it apart the moment you walk in the door"? My answer is always "no". Engineers are not created in schools, they are simply born, then learn to be better.
My work history is varied. Over the years, I have worked my way into the engineering departments of every company I have worked for. It usually took about two weeks to accomplish the transfer from assembly, to engineering. I prefered this method of employment, as it offered me a true vision of what the company was all about, by starting in assembly. After this method of employment became burdensome, I just applied as an engineer, with my education to back me up.
I have always wanted to have my own prototyping company. It has been my goal to assemble people of different fields, to create a company that can develope products in a variety of fields. I met a man in Germany, who is an electronics and programming genius. I flew to Germany at his request, to become his company's mechanical engineer. His brother, the owner, was a drunk, drove the company in the ground, and now I am back in the states. I still have my friend out there though, and he is my first partner. The electronics and programming, to work with my mechanical. I have a female friend in NewYork, who is a business professional. She tracked me down and is now my business end person. Now, we are developing a project for ourselves. It is an audio chamber, requested by the medical community for research and treatment of patients from biological to mental problems. I am on my fourth prototype now. Really though, email is better for further discussion on that.
In order to develope the kinds of products that I am interested in, I do have to do a lot of research to find the answers/solutions that I need to complete the projects. I not only have to develope the chamber itself, but everything that goes into it. This covers quite a large selection of fields. Just one chamber has an earning potential of 200,000 dollars a year. So, we plan to make chambers, and set up salons for public use, as well as sell and lease them to doctors. Private persons can purchase them as well, but only if they can afford the 50,000 dollar price tag. It does everything from mental/biological health studies/therapy, to high end virtual networked gaming. Quite complex. Depends on the application really.
You and I are quite similar in our desires to explore the unknown and to develope a variety of products. I most recently lost the help of my electronics engineer in Germany, due to his need to get his brothers company back up and running. The company I last worked for, layed me off three times over the last few years. This is when I decided to devote full attention to my own company. It may be quite practical for us to further communication through email and chat, as indicated in an ealier note. Here is my email address again:

thorn21@cableone.net Yahoo Messenger: alysixueh

The company name is Thorn Design & Development. I am presently staying away from client based products, as my resources are greatly focused on the chamber project, and other smaller projects. My skills in PCB design originate from my drafting education. I am somewhat skilled in electronics, on a hobby level, but can design and fabricate PCB's using EagleCAD. I have the full version, courtesy of my friend in Germany. So, if people help me with more complex circuit design, I can make my own PCB's. All it takes is obtaining the schematic.

 

[Thorn3]

IRstuff,

Ok, in respect to range gating, in this application, would you suggest adjusting the attack and release time controls of the gate, or simply use abrupt gating?
If adjusting the gate on/off thresholds, would you suggest using a method such as hysteresis, to raise the gate open threshold, and lower the closing threshold?
Most importantly, couldn't this issue best be solved using clever programming?

 

[IRstuff]

The big problem with suspended particles is that they create a multipath environment, much like driving with high beams in heavy fog. Range-gating, as used in radar is a means of eliminating the ambiguity in time if arrival, by eliminating photons that took a more tortuous path. Bottomline is that if you can't see through it, it's unlikely that your laser will do any better.

The gates have to be quite small, considering that the speed of light in water is still close to 1 ft/ns. The range-gating simply allows you to separate the volume of water into slices, with some dispersion, but significantly less than if you took the entire return all at once. TTFN

 

[MEPDQ]

If you're only trying to measure distances of a few feet, why not try a Leica Disto? It's essentially a commercial-off-the-shelf hand-held laser interferometer that runs on (4) AAA batteries. It has a range in air of 100 meters and is accurate to 1/8". It reflects a red laser off a surface and reads out in whatever units you want. You could seal the thing into a heavy duty plastic bag and try it underwater. The red laser shouldn't lose much energy at the distances you're contemplating. I got mine for under $1000 and use it for quickly measuring building dimensions during energy studies. It's worked great for 2 years now. See Leica.com for more info. Now that you piqued my interest, I might be persuaded to grab a few zip-lock bags and take it to the swim club if you ask nicely. Good luck.

 

[HVACctrl]

MEPDQ,
THanks for the reply and suggestion. In effect, after further research, I have concluded that lasers will probably not be suitable for the application I have in mind. I need more distance and the water will be too dirty in many instances.

Thanks again,

Ed