sensasi: [LTspice] Re: Need help on Laser Model parameter

Jumat, 22 Februari 2008

[LTspice] Re: Need help on Laser Model parameter

Kimberley, Helmut, Hendrik,

When lasing, a laser diode don't behave like a normal silicon diode.

Rather than the forward voltage and dynamic resistance varying with
current and temperature, as is the case with a normal diode, the
laser's forward voltage is essentially constant.

This is because the colour of light created by the laser varies
little with conditions and the light is generated by the bandgap
voltage across the diode.

Once you take account of contact and material resistances and stray
capacitances, a typical laser model for single mode comms laser above
threshold current, could simply look like a 1V voltage source with ~
5 ohms series resistance and a few pF of capacitance.

For a comms laser model, the light output is generally considered to
be linearly proportional to the current above the lasing threshold
current.

The threshold current itself is temperature and age dependant, and
rises quite fast with temperature.

When switched at very high data rates the laser models can get far
more complex, especially if one starts to consider the laser's
transient response, as both the light colour and brightness become
time varying due to chirp etc.

Regards,
Dave Gee

P.S. And the photodiode essentially acts like a laser in reverse.
They are best modelled as a linear controlled current source
controlled by your "light" variable (typically modelled as a
voltage). Due to optical alignment issues the effective gain of the
photodiode can vary over quite a wide range. Typically due to the
small currents in the photodiodes they are quite heavily bandlimited
by their own capacitance, but they are fine for DC-KHz contol loops.

--- In LTspice@yahoogroups.com, "Helmut Sennewald"
<helmutsennewald@...> wrote:
>
> --- In LTspice@yahoogroups.com, "Kimberley" <peenkfrik@> wrote:
> >
> > Hi Hendrik Jan,
> >
> > Thanks for the help. I believe the mfgr should be able to provide
> > that parameter.
> >
> > It's a laser diode module so that includes a photodiode in the
> > package, probably why the subcircuit is a little more complicated
> > than the standard diode model.
> >
> > Best Regards,
> > Kimberley
>
> Hello Kimberley,
> There is no problem with this low "Is" in the simulation.
> The model parameters Is, N and Rs are just used to fit
> the forward voltage Vf(Idiode). The model parameter "Is" has
> practically no relation with the real leakage current.
> It's value is mostly many decades below the leakage current.
>
> Best regards,
> Helmut
>
> >
> > --- In LTspice@yahoogroups.com, "Hendrik Jan Zwerver"
> > <hjzwerver@> wrote:
> > >
> > > --- In LTspice@yahoogroups.com, "Kimberley" <peenkfrik@> wrote:
> > > >
> > > > Hi everyone,
> > > >
> > > > I've been trying to create a subcircuit for a laser diode
module
> > > > that I can use in LTSpice. Got this article from Cadence
about
> > > > modelling laser diodes and has the following sample
subcircuit
> > using
> > > > a 5W laser SLD1121VS from Sony.
> > > > ------------------------------------
> > > >
> > > > * LD = Laser diode cathode
> > > > * C = Common pin
> > > > * PD = Photodiode anode
> > > > .subckt SLD1121VS LD C PD
> > > > * Laser diode power vs. current
> > > > Eop op 0 TABLE {I(Vid)} (0,0) (37m,0.3m) (40m,0.5m) (47m,5m)
> > > > (100m,40m)
> > > > Rdummy op 0 1k
> > > > * Monitor diode power vs. current
> > > > Gopd C PD TABLE {V(op)} (0,0) (30m,1.5m)
> > > > * Laser diode forward current and breakdown
> > > > Dld C I dlaser
> > > > Vid I LD
> > > > .model dlaser D IS=5E-37 N=1 RS=2 BV=2 IBV=10u
> > > > *EG=2.8 XTI=3
> > > > * Detector capacitance and breakdown
> > > > Dpd PD C pdetec
> > > > .model pdetec D CJO=5p BV=15 IBV=10u
> > > > .ends
> > > >
> > > > ----------------------------------
> > > > Now I am not sure what IS or saturation current here means.
Is
> > it a
> > > > reverse bias parameter of the diode before it breaks down or
> > forward
> > > > current? 5E-37 seems to be a very small value. In the
datasheet,
> > > > Operating current for the Sony laser diode is 50mA TYP and
> > Threshold
> > > > current is 40mA TYP.
> > > > Please advise.
> > > >
> > > > Regards,
> > > > Kimberley
> > > >
> > >
> > > Hi Kimberly,
> > >
> > > The IS parameter can be measured in reverse bias but it is
actually
> > > most important for the forward characteristics:
> > > Vd=Rs*Id+N*k*T/q * LN(Id/IS+1)
> > > A lower value then GMIN for IS is for the reverse bias useless
> > because
> > > SPICE adds GMIN (minimum conductance) to the diode for
convergence
> > aid.
> > >
> > > I think it's a strange way to model a diode. The normal SPICE
diode
> > > should be sufficient to model the laser diode.
> > >
> > > Maybe you want to check out the diode modeling tool in the
files
> > section:
> > > files > util > model tools > diode.
> > > It comes with a help file too. Maybe that can help you explain
how
> > a
> > > diode works and what the basic parameters do.
> > >
> > > Regards,
> > >
> > > Hendrik Jan
> > >
> >
>