© 2013 BaySpec, Inc. All rights reserved.
All BaySpec products are made in the USA
Go Further and Faster
with BaySpec’s dispersive Raman systems
BaySpec, Inc.
1101 McKay Drive | San Jose, California 95131
(408) 512-5928 | www.bayspec.com
Full Spectroscopy Solutions ProviderPervasive Spectral Sensing
r
Agility TM Dual-band Benchtop Raman
(532, 785, 1064 nm), Push-button
Operation
RamSpec TM - HR (4cm- 1) 1064
Dispersive Raman, Faster
Alternative to F T-Raman
Nomadic TM Multi-excitation
Confocal Raman Microscope
(532, 785, 1064 nm)
Fully Automatic, One-Shot Full
Range, Resolution up to 2 cm- 1
Forward current (IF) mA
Forward
voltage
(VF) V
VF
Back facet
detector
current
(IBD) A
IBD
Light
power
output
(L) m W
L
Kink test
(dL/dIF)
dL/dIF
49 Laser Focus World www.laserfocusworld.com September 2013
defined as the first maximum of the second derivative of the light output and is
a calculation based on L.
Back-facet monitor diode (BFMD) test.
This test verifies the response of the back-facet detector photodiode (also reverse-biased) to increase light output of the LD
as the drive current is increased. Typical
current measurement range is 0–100 mA,
and the required resolution is 0.1 mA.
Although this measurement is typically
performed with a picoammeter or electrometer, an SMU can be used instead if it
offers a low enough current measurement
range. Typically, a measurement range of
100 nA is more than adequate for low-powered optical devices. In this test, the
second SMU channel of a dual-channel
SMU can be used to bias the photodetector (if needed) and measure the photocurrent simultaneously. Or, a single-channel
SMU or a simple picoammeter can measure the back facet photocurrent.
Kink test/slope efficiency. This test
verifies the proportionality of the re-
lationship between the drive current
(IF) and L as depicted in Fig. 3. The
relationship between IF and L should
be linear about the nominal operating
range. If the relationship is truly linear
over the tested range, the first deriva-
tive of the curve will be a nearly hori-
zontal line. This is graphed as dL/dIF.
The first derivative will tend to amplify
any bumps or kinks in the light/current
(L-I) curve. If this curve has any signifi-
cant “kinks” (the curve does not appear
smooth), the laser is considered defec-
tive. If operated at the IF value corre-
sponding to the kink, the light output
will not be proportional. The maxi-
mum value of the second derivative of
the L vs. IF curve can be used to calcu-
late the threshold current, which is the
value of the drive current at which the
LD starts lasing. The kink and slope ef-
ficiency of a particular device are also
calculations based on the analysis of L.
Temperature testing. The LIV test is
often performed at multiple laser-diode
temperatures, such as at both the nominal temperature and the extremes of the
device specification. Another common
strategy is to test at several temperatures,
then analyze these families of LIV curves
to ensure the device meets the specification.
Test-system configuration
Figure 4 shows an LIV test system con-
figuration that includes a two-channel
SMU, a specialized 50 W autotuning
SMU instrument, and a computer (PC)
equipped with a GPIB interface card.
Each channel of the two-channel SMU
can source either a voltage or current and
simultaneously measure voltage and cur-
rent. This offers great convenience for
LIV testing by allowing one SMU chan-
nel to source current to the device under
test (DU T) and measure the voltage while
the other channel is monitoring the pho-
tocurrent of the detector near the DUT.
With many instruments, the PC con-
trols all aspects of the test. In each ele-
ment of a test sequence, the instruments
must be configured for each test, per-
form the desired action, and then return
the data to the controlling PC. The con-
trolling PC then must evaluate the pass/
fail criteria and perform the appropri-
ate action for binning the device under
test. Each command sent and executed
consumes precious production time and
FIGURE 3. A typical suite of LIV curves
includes a first-derivative curve (dL/dIF)
that contains a “kink,” showing a less-than-
optimal relationship of light output to current
input in the region of the kink.
