From: email@example.com (Paul Mathews)
Subject: Re: laser diode operating range
Date: 19 Nov 2002 10:41:29 -0800
NNTP-Posting-Date: 19 Nov 2002 18:41:29 GMT
Winfield Hill wrote in message news:...
> Generally laser diodes are used with light-feedback servo-mode circuits.
> That is mandated because of the rather dramatic increase in light level
> that occurs above the "threshold" voltage, where true lasing begins and
> takes over the light emission from simple LED action.
> Consider applications where a variable range of light is required. Using
> laser diodes with light servos tempts one to use the servo mode to achieve
> the needed variable light intensity. However, often the laser diode's
> threshold is a substantial fraction of the maximum power, 25% or more.
> This can create possible unpredictable operation. If one servos close
> to the threshold where coherent narrow-wavelength emission ceases, one
> might as well have purchased an inexpensive LED. :-)
> So one question is, what are typical lasing thresholds as a fraction of
> the full laser power? How about for different laser diode wavelengths?
> - Win
I hope that you get a more complete answer than this one, but I will
venture this: It depends. While it is true that many laser diodes are
operated in servo mode, far more are simply driven with constant
current and/or current limiting. It depends on the process, the
construction, and the application. The real reason for servo control
is to achieve constant light output, as required in many
instrumentation and process applications for lasers. Concerning the
damage to threshold current ratio, this depends, also. These days,
there are devices with more like 25:1 ratios available, and some of
them have substantial incoherent output below threshold. My
experience with the low-cost Infineon/Osram SL495 and its descendents
indicates that threshold is in the neighborhood of 50mA, and that it
can be pulsed without degradation at currents up to about 1100mA.