How to terminate 50 ohm transmission line without drawing DC

Question

I am sending a 10 MB/sec digital data stream by amplitude moduating a infrared quantum cascade laser. The laser requires a hefty bias of about half an amp (which drops about 8.5 volts across the laser). I am mixing the digital signal with the bias using a broadband bias-tee (a Picosecond Pulse Labs 5546). The bias-tee expects a 50 ohm load on the AC+DC port. When I use a 50 ohm power resistor to terminate the line, all is well and the signal looks good on the scope. However, when I use a load that simulates the laser (a 5 watt 8.2 volt zener in series with a 1 ohm resistor) there is extreme ringing. This could be expected, since the dynamic impedance of this simulated load is about 2.5 ohms (the dynamic impedance of the laser is about 4 ohms at 500 ma).

I cannot just increase the load impedance with a series resistor, because at half an amp it would generate about 12 watts of heat inside the temperature-controlled laser enclosure. However, the modulation amplitude is only about 70 ma, which at 50 ohms generates only a quarter watt, which is acceptable. So what I need to do is present a 50 ohm load to just the RF and not the DC.

Is there an off-the-shelf device that will do this? If not, are there design examples of a passive network for this purpose?

Answer 1

After much experimentation in LTSpice, I devised a passive network that should work. R9, R10, R11 and C5 are internal to L3. R15, R16, R17 and C8 are internal to L4.

Here is the wideband AC analysis:

Answer 2

I'm just guessing here, but I'd guess it's some form of AC termination: basically a resistor and cap in series to GND. If your data stream is DC balanced then the cap is usually around 0.1 uF and the resistor is 50 ohms. If your data stream is not DC balanced then the cap is less than 800 pF, and the resistor is still around 50 ohms.

Note: This termination is ideal if your datastream is DC balanced. If it's not balanced, then you are really just trying different values until you find something that works-- it's not ideal, but it's close.

Answer 3

As I said in my comments, I don't see how this can be done. Having said that, I'm not sure why you want to add the signal and bias in this way. I only have a little history with communications lasers (and that was a long time ago) but I would expect a laser driver circuit which uses feedback from an integrated PIN diode to ensure that the minimum current (signal + bias) is just above the lasing threshold.

If you really need to add the signal to the bias remotely, the only way I can see is to separate the signal from the power and use a driver circuit to drive the laser.

Answer 4

The real problem here is not that the bias T wants a 50 ohm load, but that the transmission line between that and the laser demands an impedance match.

There are several potential ways to solve this

• connect the transmission line to the laser through the parallel combination of an inductor (to pass the bias) and a 47 ohm resistor (to raise the AC impedance to approximately that of the line)

• if the above resistor would convert too much power (from the AC modulation alone) to heat within the enclosure, the resistor and inductor can be moved to the source end. Measurements anywhere midway on the transmission line will show distorting reflections, but they will be mostly absorbed in the source resistor before they can reflect again to the load end and distort the applied modulation

• more efficient delivery of modulation power would use an impedance matching transformer to match the low laser impedance to the 50 ohm line impedance, the impedance ratio being the square of the turns ratio. An inductor bridging the transformer could couple the bias supply. However, a lossless (no resistor) matching network will not provide any place to safely absorb the consequences of any mismatch that does occur, and any reactance of the laser load will be presented back to the modulation source - so matching the modulation driver to the bias T/transmission line source end with a resistor would also be useful.

Answer 5

you can just use an impedance matched modulation source. it will swamp the reflected power and should eliminate the ringing. note that the signal will not look fine at the source, even when it is fine at the laser diode.