modulation, bit rate, lasers and diodes sources

Hello Forum,

I have a question about the bit rates offered by lasers and diode
sources.... 

A laser source and a diode source of the same central frequency are quite
different types of sources. The laser is more collimated, higher power
density, much smaller linewidth (bandwidth) which conveys the high
coherence properties that a laser has....

In telecom, it is often said that a lasers are used for high data rate
communication. Using external ON-OFF keying modulation, is it possible to
get a faster modulation with a laser than with a diode source?
Why?
Is there a limit on how fast we can chop (amplitude modulation) a diode
source? After a certain modulation speed, do the pulses generated with a
diode become worse than those generated with a laser?

I am not sure why lasers are said to provide higher data stream....unless
they can be modulated faster than other sources....



thanks
fisico32

fisico32 <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote:
(snip)

> A laser source and a diode source of the same central frequency are quite
> different types of sources. The laser is more collimated, higher power
> density, much smaller linewidth (bandwidth) which conveys the high
> coherence properties that a laser has....
 
> In telecom, it is often said that a lasers are used for high data rate
> communication. Using external ON-OFF keying modulation, is it possible to
> get a faster modulation with a laser than with a diode source?

The usual reason for using laser source is distance, but yes you
can modulate a laser faster.  The rate at which it can be switched
off depends on how long it takes the carriers to recombine. 
That is, in most cases, faster in a laser.  Still, it doesn't
seem to have been a problem yet.

-- glen

hello,

if distance and power density were not a problem for either a laser and a
diode, what does "how long it takes the carriers to recombine" exactly
mean? I am not clear on that...

In any communication system there is a carrier signal of central frequency
f and the modulating signal (message signal) that has a certain bandwidth.


A diode has a larger linewidth (bandwith) than a laser. Is there some
relation between the source linewidth and the bandwidth of the modulating
signal?


thanks












>fisico32 <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote:
>(snip)
>
>> A laser source and a diode source of the same central frequency are
quite
>> different types of sources. The laser is more collimated, higher power
>> density, much smaller linewidth (bandwidth) which conveys the high
>> coherence properties that a laser has....
> 
>> In telecom, it is often said that a lasers are used for high data rate
>> communication. Using external ON-OFF keying modulation, is it possible
to
>> get a faster modulation with a laser than with a diode source?
>
>The usual reason for using laser source is distance, but yes you
>can modulate a laser faster.  The rate at which it can be switched
>off depends on how long it takes the carriers to recombine. 
>That is, in most cases, faster in a laser.  Still, it doesn't
>seem to have been a problem yet.
>
>-- glen
>

On Jan 17, 7:38&#4294967295;am, "fisico32" <marcoscipioni1@n_o_s_p_a_m.gmail.com>
wrote:
> Hello Forum,
>
> I have a question about the bit rates offered by lasers and diode
> sources....
>
> A laser source and a diode source of the same central frequency are quite
> different types of sources. The laser is more collimated, higher power
> density, much smaller linewidth (bandwidth) which conveys the high
> coherence properties that a laser has....
>
> In telecom, it is often said that a lasers are used for high data rate
> communication. Using external ON-OFF keying modulation, is it possible to
> get a faster modulation with a laser than with a diode source?
> Why?
> Is there a limit on how fast we can chop (amplitude modulation) a diode
> source? After a certain modulation speed, do the pulses generated with a
> diode become worse than those generated with a laser?
>
> I am not sure why lasers are said to provide higher data stream....unless
> they can be modulated faster than other sources....
>
> thanks
> fisico32

Also recall that fibers tend to be dispersive so signals need to be
repeated often. In some cases a very high optical flux is employed to
exploit the non-linearity of the fiber and thereby convey information
by solitons. Its been a few years since I've looked at this in detail,
but the soliton process over existing fiber and using already in place
erbium doped amplifiers seemed to achieve something like a 4 to 10x
data rate improvement. I'm not sure what current state of the art
achieves.

Clay

On Mon, 17 Jan 2011 07:32:07 -0600, fisico32 wrote:
(top posting fixed)
>>fisico32 <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote: (snip)
>>
>>> A laser source and a diode source of the same central frequency are
> quite
>>> different types of sources. The laser is more collimated, higher power
>>> density, much smaller linewidth (bandwidth) which conveys the high
>>> coherence properties that a laser has....
>> 
>>> In telecom, it is often said that a lasers are used for high data rate
>>> communication. Using external ON-OFF keying modulation, is it possible
> to
>>> get a faster modulation with a laser than with a diode source?
>>
>>The usual reason for using laser source is distance, but yes you can
>>modulate a laser faster.  The rate at which it can be switched off
>>depends on how long it takes the carriers to recombine. That is, in most
>>cases, faster in a laser.  Still, it doesn't seem to have been a problem
>>yet.
>>
> hello,
> 
> if distance and power density were not a problem for either a laser and
> a diode, what does "how long it takes the carriers to recombine" exactly
> mean? I am not clear on that...

I gather from your artificial distinction between 'laser' and 'diode' 
that you do not realize that a laser diode and a light emitting diode are 
both diodes.  One emits non-coherent light, the other (with similar 
active material) exhibits laser action.  Both are semiconductor diodes.

When a diode conducts it does so because one or both sides of the 
junction are flooded with minority carriers (electrons in P-type 
semiconductor, holes in N-type semiconductor).  These carriers recombine 
with the local majority carriers.  The amount of time that it takes for 
these carriers recombine is a limit to the speed of the diode.

When a pair of carriers in a light emitting diode (regular or laser) 
recombine, they emit a photon.  So in such a diode, the carrier storage 
time is going to limit the speed that the source can be modulated.

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

> In any communication system there is a carrier signal of central
> frequency f and the modulating signal (message signal) that has a
> certain bandwidth.
> 
> A diode has a larger linewidth (bandwith) than a laser. Is there some
> relation between the source linewidth and the bandwidth of the
> modulating signal?

Theoretically there is.  I don't believe that the technology has advanced 
to the point where things are even close to this limit.


-- 
http://www.wescottdesign.com

fisico32 <marcoscipioni1@n_o_s_p_a_m.gmail.com> wrote:
 
> if distance and power density were not a problem for either a laser and a
> diode, what does "how long it takes the carriers to recombine" exactly
> mean? I am not clear on that...

Oops.  In semiconductor physics, carriers are electrons and holes.
After you turn off the current, it takes some time for them to be
used up in light generation.  That is faster in lasers.
 
> In any communication system there is a carrier signal of 
> central frequency f and the modulating signal (message signal) 
> that has a certain bandwidth.
 
> A diode has a larger linewidth (bandwith) than a laser. Is there some
> relation between the source linewidth and the bandwidth of the modulating
> signal?

Theoretically yes, but we are still a looong way from that.

Optical frequencies are in the 1e14Hz range, so over four orders
of magnitude from gigabit ethernet.

-- glen

On 01/17/2011 05:19 PM, glen herrmannsfeldt wrote:
> fisico32<marcoscipioni1@n_o_s_p_a_m.gmail.com>  wrote:
>
>> if distance and power density were not a problem for either a laser and a
>> diode, what does "how long it takes the carriers to recombine" exactly
>> mean? I am not clear on that...
>
> Oops.  In semiconductor physics, carriers are electrons and holes.
> After you turn off the current, it takes some time for them to be
> used up in light generation.  That is faster in lasers.
>
>> In any communication system there is a carrier signal of
>> central frequency f and the modulating signal (message signal)
>> that has a certain bandwidth.
>
>> A diode has a larger linewidth (bandwith) than a laser. Is there some
>> relation between the source linewidth and the bandwidth of the modulating
>> signal?
>
> Theoretically yes, but we are still a looong way from that.
>
> Optical frequencies are in the 1e14Hz range, so over four orders
> of magnitude from gigabit ethernet.

And if you talk about carrier bandwidth as a proportion of signal 
frequency, lasers work about as well as the spark-gap transmitters that 
were really super-duper high tech back in 1895.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott <tim@seemywebsite.com> wrote:
(snip)

> I gather from your artificial distinction between 'laser' and 'diode' 
> that you do not realize that a laser diode and a light emitting diode are 
> both diodes.  One emits non-coherent light, the other (with similar 
> active material) exhibits laser action.  Both are semiconductor diodes.

Yes, so a laser diode is a subset of LED, but it is not usual
to describe it that way.
 
> When a diode conducts it does so because one or both sides of the 
> junction are flooded with minority carriers (electrons in P-type 
> semiconductor, holes in N-type semiconductor).  These carriers recombine 
> with the local majority carriers.  The amount of time that it takes for 
> these carriers recombine is a limit to the speed of the diode.

I hadn't thought of the confusion from carrier modulation to
current carriers when I wrote that.

-- glen

Clay <clay@claysturner.com> wrote:
(snip)

> Also recall that fibers tend to be dispersive so signals need to be
> repeated often. In some cases a very high optical flux is employed to
> exploit the non-linearity of the fiber and thereby convey information
> by solitons. Its been a few years since I've looked at this in detail,
> but the soliton process over existing fiber and using already in place
> erbium doped amplifiers seemed to achieve something like a 4 to 10x
> data rate improvement. I'm not sure what current state of the art
> achieves.

Or put a phase conjugate mirror in the middle.  Then the second half
undoes the dispersion of the first half.

-- glen

On 01/17/2011 05:22 PM, glen herrmannsfeldt wrote:
> Tim Wescott<tim@seemywebsite.com>  wrote:
> (snip)
>
>> I gather from your artificial distinction between 'laser' and 'diode'
>> that you do not realize that a laser diode and a light emitting diode are
>> both diodes.  One emits non-coherent light, the other (with similar
>> active material) exhibits laser action.  Both are semiconductor diodes.
>
> Yes, so a laser diode is a subset of LED, but it is not usual
> to describe it that way.
>
>> When a diode conducts it does so because one or both sides of the
>> junction are flooded with minority carriers (electrons in P-type
>> semiconductor, holes in N-type semiconductor).  These carriers recombine
>> with the local majority carriers.  The amount of time that it takes for
>> these carriers recombine is a limit to the speed of the diode.
>
> I hadn't thought of the confusion from carrier modulation to
> current carriers when I wrote that.

Until you posted your revelation of the difference between (charge) 
carrier and (radio) carrier, I didn't either!

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html