346
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
The laser is placed in a closed container equipped with electrical resistors and fans.
A small portion of the beam exiting the container passes through a photoacoustic
cell filled with 25 hPa of water vapor plus 100 hPa of nitrogen. This transmitted
signal is monitored and recorded for the verification of the spectral positioning.
The laser is driven by a dedicated electronics system that controls the power unit
and generates the control signals for flash lamp charge and trigger, Q switching,
and wavelength commutation with proper timing. The electronics system permits
the positioning of the servo-controlled filters and driving of the PZT spacers of the
thick etalon. It also checks the temperatures, the switch flows placed in the cooling
circuit, and the simmer current in the lamps. It generates alarms and stops laser
operation, if necessary, for material safety. The laser cavity and intracavity devices
are supported by a mechanical frame of high rigidity and stability based on a set
of three longitudinal Invar bars. After the mechanics are locked in the laboratory,
cavity realignment is usually unnecessary, even after handling, onboard installation,
and flight of the laser.
14.3.1.3 Receiver
The Ritchey–Chrétien telescope is used to collect the backscattered signal. Its aperis used to collect the backscattered signal. Its apercollect the backscattered signal. Its aperthe
aperture is 300 mm, and the total length is 0.80 m. The field of view of the telescope
can be adjusted from 1 to 9 mrad by a motorized diaphragm placed at the focus.
The coaxial configuration of the emitter and the receiver enables the geometric facreceiver enables the geometric facreceiver enables the geometric factor of the lidar to approach unity at ranges as short as 500 m. The receiving optics
includes a field lens, a collimating lens, an interference filter, and the detector. The
transmission central wavelength is selected by a three-cavity interference filter with
a spectral width of 1 nm and a peak transmission of 55%. It is mounted on a motorwidth of 1 nm and a peak transmission of 55%. It is mounted on a motorwidth of 1 nm and a peak transmission of 55%. It is mounted on a motor1 nm and a peak transmission of 55%. It is mounted on a motor1 nm and a peak transmission of 55%. It is mounted on a motornm and a peak transmission of 55%. It is mounted on a motornm and a peak transmission of 55%. It is mounted on a motor55%. It is mounted on a motor55%. It is mounted on a motor. It is mounted on a motormounted on a motorized wheel. The fi lter’s central wavelengths are chosen to match the selected absorpwheel. The fi lter’s central wavelengths are chosen to match the selected absorpwheel. The filter’s central wavelengths are chosen to match the selected absorpto match the selected absorpto match the selected absorption lines. Fine-tuning of the spectral band for optimum transmission at both λ on
and λ off is obtained by tilting the filter. The temperature of the receiving unit is kept
within ±1 K to ensure constant spectral positioning of the fi lters. The overall optito ensure constant spectral positioning of the fi lters. The overall optio ensure constant spectral positioning of the fi lters. The overall optiensure constant spectral positioning of the fi lters. The overall optiensure constant spectral positioning of the fi lters. The overall optiof the fi lters. The overall optiof the filters. The overall optical transmission of the receiver, from the steering mirror to the detector (including
the interference filter), is approximately 25%, which is measured with a tunable
continuous wave laser. A Thorn EMI 9658 photomultiplier is used as the detector.
The high voltage can be modifi ed from 950 V to 1250 V in four steps correspondmodifi ed from 950 V to 1250 V in four steps correspondmodified from 950 V to 1250 V in four steps correspondV to 1250 V in four steps correspondto
corresponding to gain levels from 10 5 to 10 6 . The detector is associated with a preamplifier and
an impedance adapter, and the resulting detection sensitivity ranges from 0.1 to 1
μW/V. The signal is digitized by a 12-bit analog-to-digital converter (ADC 600 from
Burr-Brown) with a 10-MHz sampling rate and a sensitivity of 0.8 mV/digit. The
fast 2048-gate memory permits a 200-ms signal recording. A second, synchronous
digitizing channel with the same characteristics is connected to a fast photodiode to
observe the emitted pulses. This channel provides the temporal reference for the sigthe emitted pulses. This channel provides the temporal reference for the sigthe emitted pulses. This channel provides the temporal reference for the sigthe temporal reference for the sigthe temporal reference for the signal emission and recording of the pulse temporal profile and energy. The digitized
signals are recorded on a shot-to-shot basis on an Exabyte tape recorder. Aircraft
navigation data, instrument parameters, and emitted wavelengths are also recorded
with the same device. A real-time Hewlett-Packard processor collects the data and
ensures the synchrony of the recordings.
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
The laser is placed in a closed container equipped with electrical resistors and fans.
A small portion of the beam exiting the container passes through a photoacoustic
cell filled with 25 hPa of water vapor plus 100 hPa of nitrogen. This transmitted
signal is monitored and recorded for the verification of the spectral positioning.
The laser is driven by a dedicated electronics system that controls the power unit
and generates the control signals for flash lamp charge and trigger, Q switching,
and wavelength commutation with proper timing. The electronics system permits
the positioning of the servo-controlled filters and driving of the PZT spacers of the
thick etalon. It also checks the temperatures, the switch flows placed in the cooling
circuit, and the simmer current in the lamps. It generates alarms and stops laser
operation, if necessary, for material safety. The laser cavity and intracavity devices
are supported by a mechanical frame of high rigidity and stability based on a set
of three longitudinal Invar bars. After the mechanics are locked in the laboratory,
cavity realignment is usually unnecessary, even after handling, onboard installation,
and flight of the laser.
14.3.1.3 Receiver
The Ritchey–Chrétien telescope is used to collect the backscattered signal. Its aperis used to collect the backscattered signal. Its apercollect the backscattered signal. Its aperthe
aperture is 300 mm, and the total length is 0.80 m. The field of view of the telescope
can be adjusted from 1 to 9 mrad by a motorized diaphragm placed at the focus.
The coaxial configuration of the emitter and the receiver enables the geometric facreceiver enables the geometric facreceiver enables the geometric factor of the lidar to approach unity at ranges as short as 500 m. The receiving optics
includes a field lens, a collimating lens, an interference filter, and the detector. The
transmission central wavelength is selected by a three-cavity interference filter with
a spectral width of 1 nm and a peak transmission of 55%. It is mounted on a motorwidth of 1 nm and a peak transmission of 55%. It is mounted on a motorwidth of 1 nm and a peak transmission of 55%. It is mounted on a motor1 nm and a peak transmission of 55%. It is mounted on a motor1 nm and a peak transmission of 55%. It is mounted on a motornm and a peak transmission of 55%. It is mounted on a motornm and a peak transmission of 55%. It is mounted on a motor55%. It is mounted on a motor55%. It is mounted on a motor. It is mounted on a motormounted on a motorized wheel. The fi lter’s central wavelengths are chosen to match the selected absorpwheel. The fi lter’s central wavelengths are chosen to match the selected absorpwheel. The filter’s central wavelengths are chosen to match the selected absorpto match the selected absorpto match the selected absorption lines. Fine-tuning of the spectral band for optimum transmission at both λ on
and λ off is obtained by tilting the filter. The temperature of the receiving unit is kept
within ±1 K to ensure constant spectral positioning of the fi lters. The overall optito ensure constant spectral positioning of the fi lters. The overall optio ensure constant spectral positioning of the fi lters. The overall optiensure constant spectral positioning of the fi lters. The overall optiensure constant spectral positioning of the fi lters. The overall optiof the fi lters. The overall optiof the filters. The overall optical transmission of the receiver, from the steering mirror to the detector (including
the interference filter), is approximately 25%, which is measured with a tunable
continuous wave laser. A Thorn EMI 9658 photomultiplier is used as the detector.
The high voltage can be modifi ed from 950 V to 1250 V in four steps correspondmodifi ed from 950 V to 1250 V in four steps correspondmodified from 950 V to 1250 V in four steps correspondV to 1250 V in four steps correspondto
corresponding to gain levels from 10 5 to 10 6 . The detector is associated with a preamplifier and
an impedance adapter, and the resulting detection sensitivity ranges from 0.1 to 1
μW/V. The signal is digitized by a 12-bit analog-to-digital converter (ADC 600 from
Burr-Brown) with a 10-MHz sampling rate and a sensitivity of 0.8 mV/digit. The
fast 2048-gate memory permits a 200-ms signal recording. A second, synchronous
digitizing channel with the same characteristics is connected to a fast photodiode to
observe the emitted pulses. This channel provides the temporal reference for the sigthe emitted pulses. This channel provides the temporal reference for the sigthe emitted pulses. This channel provides the temporal reference for the sigthe temporal reference for the sigthe temporal reference for the signal emission and recording of the pulse temporal profile and energy. The digitized
signals are recorded on a shot-to-shot basis on an Exabyte tape recorder. Aircraft
navigation data, instrument parameters, and emitted wavelengths are also recorded
with the same device. A real-time Hewlett-Packard processor collects the data and
ensures the synchrony of the recordings.