Expendable beam transmissometer

An apparatus for determining a beam attenuation coefficient of a medium, comprising:

• a signal source that generates a generated optical beam;
• a first arrangement that collimates the generated optical beam to farm a projected optical beam;
• a retroreflector that directs the projected optical beam;
• a second arrangement that receives the projected optical beam from the retroreflector and that focuses the projected optical beam to form a focused optical beam;
• a detector that detects a portion of the focused optical beam in order to produce an electrical signal so that the beam attenuation coefficient can be determined;
• a synchronizer that causes the signal source to generate the generated optical beam in a pulsed pattern and that causes the detector to detect the portion of the focused optical beam in concert with the pulsed pattern;
• a processor that receives the electrical signal from the detector, the processor configured to perform:
• (a) pulsing the signal source when generating the generated optical beam;
• and
• (b) synchronizing the detector with the generated optical beam;
• wherein (b) comprises:
• (i) amplifying a pulsed component of the electrical signal in order to reduce a noise component of the electrical signal;
• (ii) sampling the pulsed component in order to obtain a locked signal in concert with the generated optical beam;
• and
• (iii) converting the locked signal to a baseband signal.

A beam transmissometer for determining a beam attenuation coefficient of a medium, comprising:

• a signal source that generates an optical beam;
• a detector that detects a portion of the optical beam to obtain an electrical signal;
• and
• a synchronizer that causes the signal source to generate the optical beam in concert with a pulsed pattern and that causes the detector to detect a portion of the optical beam in concert with the pulsed pattern;
• wherein the synchronizer comprises:
• a timer that causes the signal source to generate the optical beam in concert with a pulse rate;
• a lock-in amplifier that processes the electrical signal in concert with the pulse rate;
• wherein the lock-in amplifier comprises:
• a bandpass filter that amplifies a pulsed component of the electrical signal and attenuates a noise component of the electrical signal;
• a mixer that samples the pulsed component of the electrical signal in order obtained a locked signal;
• and
• a low pass filter that converts the locked signal into a baseband signal.

A beam transmissometer for determining a beam attenuation coefficient in a water medium, comprising:

• a light emitting diode (LED) that generates a generated optical beam;
• a first arrangement that collimates the generated optical beam to form a projected optical beam;
• a retroreflector that directs the projected optical beam;
• a second arrangement that receives the projected optical beam from the retroreflector and that focuses the projected optical beam to form a focused optical beam;
• a photodiode that detects a portion of the focused optical beam in order to produce an electrical signal;
• a synchronizer that causes the LED to generate the generated optical beam in a pulsed pattern and that causes the photodiode to detect the portion of the focused optical beam in concert with the pulsed pattern, wherein the synchronizer comprises:
• a timer that causes the LED to generate the generated optical beam in concert with a pulse rate;
• a bandpass filter that amplifies a pulsed component of the electrical signal and attenuates a noise component of the electrical signal;
• a mixer that samples the pulsed component of the electrical signal in order obtained a locked signal;
• and
• a low pass filter that converts the locked signal into a baseband signal;
• and
• a transmitter that receives the baseband signal, determines information about the beam attenuation coefficient, and sends the information to a calculating unit.

A method for aligning optical components of a beam transmissometer, the method comprising the steps of:

• (a) constraining a housing support frame of the beam transmissometer to minimize movement of the housing support frame;
• (b) activating a signal source in order to produce an optical beam;
• (e) placing a retroreflector mounting into a far side of the housing support frame, wherein a corner cube has a first offset with respect to a centerline of the housing support frame;
• (d) positioning an electro-optics housing on a near side of the housing support frame, wherein the electro-optics housing comprises projecting and receiving apparatus for the optical beam;
• (e) verifying that the optical beam is centered in a receiving bore of the electro-optics housing;
• (f) in response to step (e), if the optical beam is not centered, rotating the retroreflector mounting;
• (g) in response to step (f), if the optical beam cannot be centered by rotating the retroreflector mounting, reconfiguring the retroreflector mounting, wherein the corner cube has a second offset;
• and
• (h) in response to step (g), repeating steps (e)–(g).