Underwater optical wireless communication system based on visible light

Rabee

Hi All;
I am a Ph.d student and part of my project (underwater optical wireless communication) is to design an optical wireless system.

I am using OPT101 in a reciver circuit based on FSK modulation. I got a problem with the effect of the strong ambient light. Even, I put an optical filter on the OPT101 but the problem still which is the strong ambient light affect the circuit from receiving the correct data. However, with the room light the circuit is working fine and reciving the data correctly but with the strong ambient light the circuit stop reciving data and sometimes just reciving rubbish data.
I am using XR2206 as FSK modulator at the transmitter and XR-2211 as FSK demodulator at the receiver. Also, I am using the green light at 520nm.

Any suugesstions and feedback will be appreciated.

Thanks all

Phil Pilgrim (PhiPi)

What is the optical bandwidth of your filter? The first thing that comes to mind is a very narrow-band optical filter centered at 520 nm, placed over the receiver's sensor. If you've already got a narrow-band filter, then I would suggest reducing the DC gain of your receiver and adding post-amplification tuned to your carrier frequency.

-Phil

Rabee

Thank you for your reply It is a band pass filter centered at (525nm +-10nm). I put this filter on the OPT101.I have an electronic band pass filter and amplifier after the OPT101 to filter the carrier frequencies and then to amplify the received signal. Actually, the problem is when the ambient light is strong for example if I put a table lamp close to the receiver then I cannot get the data or I get just rubbish.

Any suggestions?

Phil Pilgrim (PhiPi)

The ambient light is saturating your sensor, providing no headroom for the modulating signal to get through. To overcome this, you can either reduce the sensitivity of the sensor somehow or add layers of neutral-density optical filters to it until it comes out of saturation with the bright ambient light. You will then have to boost the tuned AC post-amplification to compensate.

There is also a chance that, if you're using a colored-glass filter (as opposed to an interference filter), that it will pass IR, in addition to the stated passband. Incandescent lamps are very rich in IR, and that could be what is saturating your sensor. A wide-band IR-blocking filter (e.g. Schott BG38) atop your color filter would help to eliminate interference from the IR. A quick check of the Schott (or other) catalog for your filter should help to ascertain its IR characteristics.

-Phil

Tracy Allen

Have you tried lowering the feedback gain of the OPT101 with a resistor from pin 5 to pin 2? The DC amplifier would saturate long before the photodiode itself. The gain can be made up later in the AC amplifier/filter.

Rabee

Thank you for your reply I tried to put 1kohm between pin 2 and 5 but the problem is I could not get the data until I put the transmitter so close to the receiver like 5cm. However, with the 1 M ohm I can get the data for more than 1m. I tried figure 1 below but it does not work! Then I used figure 2 and it works fine for more than 1m but still the problem with the strong ambient light!

Phil Pilgrim (PhiPi)

What is the make and part number of your green filter?

-Phil

Rabee

The filter type is COKIN P004 GREEN FILTER

I bought it from the website below

http://www.photocontinental.com.au/products/106010106000
Thank you

ElectricAye

Rabee wrote: »

The filter type is COKIN P004 GREEN FILTER

I bought it from the website below

http://www.photocontinental.com.au/products/106010106000
Thank you

Are you sure that's an interference filter? Just googling around a bit, it looks like a non-interference filter to me.

Maybe compare it to product number NT43-069 here:
http://www.edmundoptics.com/products/displayproduct.cfm?productID=3429&dc&PageNum=2#products

Tracy Allen

What FSK frequencies and data rate are you talking about? If you have an oscilloscope available, you should be able to probe what happens to the signal as ambient light increases. Adding 1k between pin 2 and pin 5 is extreme. Start with 100k, instead of the internal 1M. The AC post-amplification would have to be increased by a factor of 10 to make up for the lower front end gain. If it still saturates, try 10k and another factor of 10 in the post-amplification. The main thing in this situation is that most of the gain should be AC stages within the bandpass of your FSK signal.

The RCTIME circuit could not be expected to work for a communications scheme at any reasonable rate. That is a very crude approach for detecting abient light level based on the Stamp's RCTIME command.

Is there a definite direction involved in the underwater path, that would allow for narrowing the field of view by using a tube or lens or something of the sort?

There are tricks that can be played with two photodiodes and subraction of their front end currents, one detecting ambient+signal and the other only ambient, but that does get into some thorny issues of matching and circuitry.

Phil Pilgrim (PhiPi)

Cokin P-series filters are very wide bandpass, and they're most probably completely transparent to infrared. You really need a better filter than that to limit the effect of ambient light -- especially that from a nearby incandescent source. Looking at The Schott catalog, it appears that a BG38, combined with an OG515 might do the trick for you. Here's a link to Schott's website You can download an Excel worksheet, with which you can try various filter combinations to get your desired passband:

http://www.us.schott.com/advanced_optics/english/tools_downloads/download/index.html

I would stay away from interference filters. They have very narrow passbands, but their angle of acceptance is very restricted -- probably not what you want for the app at hand.

-Phil

ElectricAye

Rabee,

just so there's no confusion: when we talked about "optical interference filters", we're talking about a type of narrow-band filter that utilizes the optical phenomenon of light wave interference to narrow the light coming through to a very specific wavelength range, maybe +/- 10 to 50 nm. We're not talking about something analogous to electronic interference filtering, etc.

To Phil,
I know that interference filters are specified for a given entrance angle that is usually very narrow, but if I remember correctly, what happens at the larger angle is that the center wavelength that is allowed through the filter gets shifted a bit. I'm not sure by how much or at what range of angles, but I was just wondering if a broader angle of entrance would matter that much in a case like this.

EDIT: For example, here's an article where they tilted the filter 12 degrees just to shift the center of wavelength less than 3 nm:
http://spie.org/etop/2009/etop2009_2.18.37.pdf

Rabee

Thank you all for your replyI am using FSK to send data with 2.4kbps and the frequencies that I am using are f1=10 KHz and f2=12 KHz.I need the wide angle because the transmitter and the receiver will be moveable and there is a possibility for the transmitter and the receiver to be in air or in water. The application is swimming application. I attached a plot for the optical filter that I am using. I tested this filter and I got this plot.




Do you think it is better to use NT43-069 instead of the filter that I am using to solve the ambient light problem?

Thanks all for your reply and help

Rabee

I atached the plots for the measurments that I have done with and without the effect of the stronge ambient light. the distance between the transmitter and the receiver was more than 1m.

Phil Pilgrim (PhiPi)

If you need a wide acceptance angle, I would not use an interference filter. But it's very plain from your plot (and another I've seen) that the Cokin filter is hopeless. That's why I recommended the Schott filters. But you need to experiment with their Excel file to find the best combination. Mine was just a suggestion, based on past experience and a minimum of additional effort.

If the Schott filters are difficult to obtain in Oz (I take it that's where you're located), and if you can order from Edmund, they carry a wide variety of filters that could be combined to give you the passband you want. For example, they have both longpass and shortpass filters with sharp cutoffs that, when sandwiched together, would provide a fairly narrow passband. Just make sure that the short pass filters block well into the IR, or you will end up having the same problem with an incandescent ambient light source as you're having now.

-Phil

ElectricAye

Rabee wrote: »

...I need the wide angle because the transmitter and the receiver will be moveable and there is a possibility for the transmitter and the receiver to be in air or in water. The application is swimming application. ....

Okay. In that case definitely take Phil's advice. He's one of the gurus here, especially when it comes to electro-optics.

One thing to check if you're operating in water and air: if the light must pass through an air/water interface, you will want to watch out for situations in which total internal reflection could break your transmission path.

http://en.wikipedia.org/wiki/Total_internal_reflection

Rabee

Dear Phil,

Are you able to help me with ship this filter to Australia If I want to order the NT43-069 because I found that the shipping fee for this filter is $45 and the price of the filter is $50.

I thank this is a good narrow bandpass filter. I will also check with my supervisors.

Thanks

Phil Pilgrim (PhiPi)

The NT43-069 is an interference filter -- not recommended for your app. In any event, I'm not set up to expedite shipments to Australia.

I do notice that Edmund carries Schott filters. You might try their selection with the Schott Excel worksheet to find the best match.

-Phil