Balanced Receivers - Bookham - #1

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The Models 2007 and 2017 auto-balanced photoreceivers can cut the effects of laser noise by 50 dB, making even very small signals easy to see. In spectroscopy, ellipsometry, or heterodyne-detection experiments, these photoreceivers can yield shot-noise-limited performance without the need for lock-in amplifiers or high-frequency modulation.

The patented auto-balancing circuit uses a low-frequency feedback loop to maintain automatic DC balance between signal and reference arms. In effect, the circuit behaves as a variable-gain beamsplitter. This, in conjunction with the sub­traction node, cancels common-mode laser noise with greater than 50-dB rejection at frequencies less than 125 kHz. With the Nirvana photo-receiver, you won't have to operate in the RF regime to avoid unwanted laser noise. And you can adjust the loop bandwidth for your particular application.

This auto-balancing circuit, invented by Phil Hobbs of IBM, is shown schematically on the next page. It consists of two photodiodes, a cur­rent splitter, a current subtraction node, a trans­resistance amplifier, and a feedback amplifier.

The photoreceiver operates in three distinct modes—signal mode, balanced mode, and auto-balanced mode. The output of the photodetec-tor (A) can be expressed as A=(/_s-g*/R)*Rf. Here, /_s is the signal photodiode current, /r is the reference photodiode current, Rf is the value of the feedback resistor, and g is the cur­rent-splitting ratio, which describes how much of the reference current comes from the sub­traction node (/_sub) and how much comes from ground. In signal mode, g is zero and no refer­ence photocurrent comes from the subtraction node. Here, the output A is simply an amplified version of the signal current. In balanced mode, g is equal to 1, and all the reference photocur-rent comes from the subtraction node. In this mode, A=(/g-/R)«Rf, the photodetector behaves as an ordinary balanced photoreceiver, where laser noise is cancelled if the DC photocurrents are equal. In auto-balanced mode, g is electroni­cally controlled by a low-frequency feedback loop to maintain equal DC photocurrents cancelling laser noise regardless of the photocurrent.

An FC-receptacle design allows for FC-fiber as well as free-space input, giving you added flexibility. To ensure safe operation, we strongly recommend our Model 0901 power supply (page 138).

3.50" (88.9) LINEAR

8-32 (M4) THD-

reference

3.50" (88.9)

NBNC)

(BNC)

1.00" (25.4)

signal -

beam separation 2.00" (50.8)

beam height

1.00" (25.4)

power connector -

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C40B) 919-60B3