| Linear or logarithmic?
The question often arises as to which signal processing technique is better: an output linearly or logarithmicallly proportional to the input optical power. The use of logarithmic amplifiers for optical power monitoring results from the large dynamic range of relevant powers. For example, the OPM series can measure from 10mW to 30pW: more than 8 decades of optical power. This dynamic range is far beyond the capabilities of a linear amplifier in a single gain range. However, by breaking the measurement into range subgroups, linear amplifiers can easily cover the same dynamic range as a logarithmic amplifier. This method is slightly more complicated than using a logarithmic amplifier with a single gain, but it brings several advantages: 1. Linear amplifiers are more stable than logarithmic amplifiers 2. Linear amplifiers are faster than logarithmic amplifiers and settle more quickly following sudden input changes 3. Linear amplifiers are more
accurate than logarithmic amplifiers at higher outputs Consider an application example: a fibre optic power monitor is used to measure the power coupled into a device in an automated confectioning system. The device being confectioned may be a laser being pigtailed or fibre being connected to an AWG, for example. Now the automated positioning system takes the measured value of the coupled power (measured by the power monitor) and uses this value to control the motion stages positioning the fibre. The system searches for the position giving the maximum coupled optical power. Obviously, the accuracy of the positioning will depend on the accuracy of the measurement at the highest powers measured. Since the logarithm compresses data, the position dependance of a logarithmic amplifier is flatter than for the linear amplifier at higher power. Thus, the linear amplifier will allow the system to achieve better results in this application.
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Rise time from 10% to 90% of the final output value following a step input.
-3dB bandwidth.
Maximum gain.
Minimum gain.
Multiplicative spacing of the gains.
Maximum input current which can be measured linearly.
Output voltage range.
Noise equivalent input current at the highest gain range.
This value scales approximately inversely proportional to the gain setting.
Peak-peak values will be about 6x the RMS value.
G = OEM style compact box with gull wings for
mounting.
R = 19" rack system.
H = hand held unit with LCD display and keypad.
EU = 230V, 50Hz power supply.
US = 115V, 60Hz power supply.
Uni = 100-240V, 50-60Hz power supply.
Input (Single ended / Differential)
Single ended: the outer contact of the BNC input connector is at ground potential.
This configuration must be used if the current
source or sink is grounded within the application system.
Differential: both input contacts of the BR2 input connector are at floating
potentials. The outer shield is grounded.
This allows high common mode rejection and good shielding for low noise applications.
The differential input must NOT be used if the current source or sink is grounded within the application system.
BNC: coaxial connector with one inner signal lead and an outer braid used for shielding as well as the current return path in the signal input circuit.
The braid is grounded at the amplifier input.
BR2: connector with two inner signal leads and an outer braid used for shielding
only. The braid is grounded at the amplifier input.
Optical Power Monitor:
an amplifier which accepts an optical input and produces a voltage output
linearly proportional to the input power.
Gated integrating amplifier: an amplifier which accepts a current input (from a photodiode, for example) or
a voltage input and produces a voltage output linearly proportional to the input integrated over a period of time (gate period).
I: Current input.
V: Voltage input.
Maximum input power which can be measured linearly.
Noise equivalent input power at the highest gain
range.
This value scales approximately inversely proportional to the gain setting.
Peak-peak values will be about 6x the RMS value.
Diode material (UVS, S, IGA, G, xIGA)
UVS: Silicon photodiode trimmed for UV
applications. Wavelength range 200nm - 950nm.
S: Silicon photodiode. Wavelength range 450nm - 950nm.
IGA: InGaAs photodiode. Wavelength range 950nm - 1650nm.
G: Germanium photodiode. Wavelength range 950nm - 1500nm. Linear to 30mW input
power.
xIGA: Extended InGaAs photodiode. Wavelength range 950nm - 2200nm.
Input receptacle (FC, SMA, BM)
F: FC type fibre optic receptacle typically used
for single mode fibres.
S: SMA type fibre optic receptacle typically used for multi-mode fibres.
B: Free beam input directly onto the photodiode (no receptacle).
Other receptacles available upon request.
