Hi,

I am trying to measure and model a comunication laser diode (TOSA) in
a TO can. In this case it is a 1550nm wavelength DFB laser intended
for 2.5Gbps. It will be used with a dual-loop laser driver, and we
are primarily interested in the monitor photodiode response rather
than the optical response.

Since I don't have an optical VNA, I am just doing electrical
measurements from the laser diode input to the photodiode output with
a standard 2-port VNA (8510C or 8753E). The TOSA has three
terminals: laser cathode, photodiode anode, common laser anode and
photodiode cathode. I am currently using a homemade fixture composed
of two SMA PCB connectors soldered end to end, with the center pins
shortened to allow the insertion of the TOSA. I am doing a coaxial
adapter removal calibration on the VNA, and I have created additional
fixtures in open, short, and thru configurations in order to develop
a model for the fixture.

I am having a couple of issues with this approach. First, the TOSA
is normally mounted on the edge of a PCB, with the laser and common
nodes connected on top of the PCB and the photodiode node connected
to the bottom of the PCB. This is obviously a different environment
than my fixture, so there will be some discrepancy. Second, in order
to connect the common lead I either have to bend the lead to reach
the fixture or add a short piece of wire. The inductance in the
ground lead significantly affects the results of the coupling between
input and output.

I am interested in obtaining a better fixture, but I don't know if
there is something commercially availible that would work well in
this situation. My other option is to design a PCB fixture myself
(with help from our PCB group). Please let me know if you have any
suggestions or comments.

Thanks,
Cory