48 • Principles of Operation
NEW FOCUS, Inc.
linewidth. The specular reflection or zero-order diffraction off the
grating serves as the output beam of the laser.
The angle between the grating and the end mirror determines the
lasing wavelength. Tuning is achieved by varying the angle using a
brushless DC motor to rotate the end reflector. Continuous (mode-hop
free) tuning requires selecting an appropriate rotation point.
Discontinuous tuning, characterized by periodic “mode-hops” results
from two competing wavelength-selection constraints, the mirror-
grating angle and the laser-cavity length. The laser-cavity length,
L
,
defines a discrete set of possible wavelengths or modes,
λ
N
, that can lase,
given by the equation
L
=
N
λ
N
/2, (
N
=
integer). The grating equation
insists that
λ = Λ(
sin
θ
i
+ sin
θ
d
)
, where
Λ
refers to the groove spacing
of the grating while
θ
i
and
θ
d
refer to the incident and diffracted angles
of the laser beam. Rotation of the end mirror causes parameters in
both equations to change. An appropriately selected point of rotation
synchronizes the two, such that the cavity length remains the same
number of half-wavelengths long as the mirror is being rotated. Thus
mode-hop free tuning is achieved. When this condition is not met, the
lasing wavelength will periodically hop from one mode to the next (e.g.
from
N
to
N
+1
) as the laser is tuned. The mechanical design of the
Vidia-Discrete laser provides truly mode-hop free tuning.
The laser controller provides current and temperature controls to the
laser cavity, as well as manual and computer controlled input/output
interfaces. The low-noise current supply drives the diode in the laser,
controlling the output power. The temperature control regulates the
laser-cavity temperature, providing a stable-output wavelength.
The laser wavelength is also affected by the current through the diode.
Changing the diode current affects the refractive index of the diode
lasers and therefore, the laser cavity length. The magnitude of the effect
is diode dependent, but is typically 25–150 MHz/mA.
64X7 rev E.fm Page 48 Tuesday, April 3, 2001 4:16 PM
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