THORLABS ULN15 Series, Руководство пользователя, Страница: 7 / 24

Ultra-Low Noise Laser Chapter 3: Description
Rev B, October 23, 2020 Page 5
3.2. Operating Characteristics
For any given choice of temperature setpoints, the ULN operates as a single mode laser over a number of ranges
of drive current. As the current is adjusted upwards from an initial operating point, the temperature of the gain chip
increases, which in turn changes the round-trip phase of the cavity mode. This causes the center wavelength of
the mode to increase. As the current is further increased, the lasing mode will shift far enough away from the
center of the FBG’s reflection peak that single mode operation is no longer possible. At this point, the laser enters
multimode operation. As the current is further increased, a new cavity mode will move into resonance with the
FBG reflection peak, and the laser will once again move into single mode operation. This power vs. current (LI)
behavior is shown in Figure 1
. The single mode regions are consistent and stable as long as the device’s
temperature is adequately controlled. The figure also plots the ULN’s LI behavior as the current is decreased.
With decreasing current, the device is able to stay in single mode operation within each particular single mode
range down to a lower current. This hysteresis is characteristic of the ULN.
Figure 1 Typical power vs. current (LI) plot. Only single mode power is plotted. This is a current-
tuning C-
configuration device operated at the optimal ΔT, which is the difference between T
FBG
and T
CHIP
.
The ULN laser
’s spectrum is much narrower than can be measured with a typical optical spectrum analyzer (20
pm resolution). However, side-mode suppression ratio (SMSR), center wavelength, and single mode behavior can
all be measured with such a tool. A typical single mode spectrum is shown in Figure 2. The SMSR is well over 70
dB.
0 100 200 300 400 500 600 700 800
0
20
40
60
80
100
120
140
160
180
Increasing Current
Decreasing Current
Single Mode Operating Regions
Power (mW)
Current (mA)