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17 Laser Focus World www.laserfocusworld.com January 2015
A 17 × 17 mm sensor is
mounted in a 50. 8 × 50. 8
mm meter head (MH). It
is shown here measuring
the power emitted from
a green laser pointer.
(Courtesy of Firebird
See page 20
SiC bolometer laser power sensor
has advanced specs
A laser-power meter
(LPM) that uses a volume
absorbing, single-crys-tal, silicon carbide (SiC)
bolometer and detects
wavelengths from 190
nm to 200 µm has been
developed at Firebird
Sensors (Reno, NV).
Attributes of SiC that
are important for LPM
sensing include wide
spectral range, high thermal conductivity ( 5. 1 W/cm·K), and
high binding energy ( 4. 67 eV). In addition, the SiC sensor lacks
mechanisms that cause drift: there is no diffusion of impurities
within the SiC up to temperatures >1200°C; there are no grain
boundaries; and the electrode structure is stable above 800°C.
In conjunction with heat-sink integration, these SiC attributes yield characteristics that include a dynamic range of 106 ( 1
m W to 1 k W), no fan or water-cooling requirement to measure
powers > 1 k W, exceptionally high damage threshold (
undamaged by peak pulse powers of 10 T W/cm2), high accuracy (±0.1%
at full scale), and short time between measurement cycles (~ 1 s).
The meter head (MH) includes a sensor and an aluminum heat
sink (see figure). A detachable handle located on top of the MH
is not shown. The so-called “MEGA” MH has two configurations of operation: sensor-sinked or sensor-isolated. The fast-response (FR) MH is in sinked configuration only.
The sinked configuration minimizes response time but requires
that a pulsed laser emit at a pulse rate of 10 Hz or more to give a
steady average power reading. The isolated configuration maximizes low-power sensitivity and can measure the average power
emitted from a pulsed laser at frequencies down to 2 Hz.
In the sinked configuration, heat is rapidly transferred from
the sensor to the heat sink, lowering the sensor’s natural 100%
response time to below 6 s. In the isolated configuration, the
sensor heat loss to the heat sink is minimized; this extends the
low frequency and low power
range by an order of magnitude,
but increases the natural 100%
response time to about 30 s.
In each configuration, the MEGA
and FR MHs can be used with
or without a concave focusing
mirror. The purpose of the mirror
is to further extend the low-power
range by increasing the amount
of laser radiation absorbed by the
sensor. The sensor’s absorption is
a function of wavelength and, to
some degree, of temperature.
At wavelength values below and
above 10. 6 µm, part of the laser
radiation passes through the sensor.
The sensor surfaces are roughened
to disperse reflected and transmit-
ted beams; however, a heat dump
should be located behind the sensor when high powers are
being measured or when a mirror is not used. At or near a 10. 6
µm wavelength, SiC exhibits the Restrahlen effect, where nearly
100% of the radiation is absorbed near the surface, then nearly
100% of the absorbed radiation is re-emitted from the surface.
In this case, the sensor surface should be misoriented from the
laser by 5° to 10°, and a heat dump should be placed in the
path of the reflected beam.
The sensor’s dynamic range of 106 is achieved by a combination of sensor configuration, adding the mirror when needed,
and maximizing the operating-temperature range by heat-sinking. The minimum power measurement capability is obtained
by isolating the sensor and using the mirror to maximize sensor
The maximum continuously measurable power at a given
wavelength is dictated by the operating-temperature range,
which depends on the sensor and heat-sink sizes—high power
can be measured in both sinked and isolated configurations. In
the sinked configuration, the maximum measurable power is