High-Dynamic-Range Laser Amplitude and Phase Noise - Next ...

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High-Dynamic-Range Laser Amplitude and Phase Noise - Next ...

646 IEEE JOURNAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 7, NO. 4, JULY/AUGUST 2001

is prudent to precede the analyzer with a good low-noise amplifier

(LNA). This will lower the system noise figure to several

decibels above and ensure shot-noise limited sensitivity. Our

system has a 40-dB gain amplifier with a noise figure of better

than 6 dB from 10 kHz to 40 MHz.

A typical RF spectrum analyzer has a useful frequency range

of 100 Hz–1 GHz, and it is usually desirable to augment this

with a lower frequency FFT analyzer with a range down to

0.01 Hz. The frequency boundary between the two instruments

will be determined by the noise performance of each analyzer,

its resolution bandwidth, and data acquisition speed. Modern

spectrum analyzers frequently come equipped with a “noise

measurement” function that calculates the noise power spectral

density, taking into account the shape factors of the RF and IF

filters in the analyzer. This feature should be exploited when

measuring broadband noise, as it is not accurate to simply

divide by the receiver’s 3 dB bandwidth. When encountering

coherent signals (spurs), however, they must be left in terms of

absolute power and not normalized to the receiver bandwidth

(this issue is frequently overlooked).

Fig. 4. Equivalent circuit of a transimpedance amplifier driven by a photodiode

including noise sources.

fore the output mean-squared noise voltage spectral density becomes

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V. PHOTORECEIVER DESIGN

A. Transimpedance Amplifiers

The measurement of shot noise on signals of large average

photocurrent presents a somewhat different set of requirements

from the usual optical signal detection. When we try to measure

very low power signals with high modulation index, we usually

use a transimpedance amplifier with very high feedback resistance

to overcome the noise in following stages. For detecting

high-speed optical data, we generally amplify the photodetector

with broadband 50- amplifiers. Neither of these approaches

is necessarily optimum for our problem (high average current),

and therefore it is useful to rethink the receiver design. The most

obvious solution is to use a transimpedance configuration with a

relatively low feedback resistance, which is required to keep the

dc output voltage within the power-supply constraints. Owing to

the prevalence of high-quality low-noise operational amplifiers,

we use them in our baseband receiver circuits. Since the input

resistance of the transimpedance configuration is extremely low,

the circuit speed will be determined by photodiode and slew-rate

limitations.

To optimize the photoreceiver design, we start by analyzing

the noise contributions in a standard transimpedance amplifier

and see how they vary as we adjust parameters such as feedback

resistance, average photocurrent, and circuit component noise.

The standard transimpedance amplifier shown as PR2 in Fig. 1

can be analyzed by taking account of all noise sources due to the

photodiode, the feedback resistance, and the equivalent op-amp

input voltage and current noise spectral densities [32], [33].

The equivalent circuit is shown in Fig. 4. The photodiode is

modeled as having two current sources: one due to shot noise

from the dark current and the other due to Johnson noise from

the series combination of dynamic and series resistance. At the

input of the op-amp are the input equivalent current and voltage

noise sources, and around the feedback resistor is the current

source due to Johnson noise. Since these noise sources are uncorrelated,

we must add them on a mean-squared basis. There-

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/Hz (28)

where

dark current;

op-amp input noise current spectral density;

op-amp input noise voltage spectral density;

photodiode dynamic resistance;

photodiode series resistance.

Op-amp (or any electronic amplifier) noise performance is

critical to achieving a low-noise photoreceiver. We used our

RF and FFT spectrum analyzers to verify the equivalent input

noise spectra of several op-amps [33] and chose the Burr–Brown

OPA-643 based on a combination of factors including noise performance,

speed, and availability.

Fig. 5 displays the variation in the mean-squared noise spectral

density terms in (28) as a function of feedback resistance

for the OPA-643. The sum of all noise sources is indicated by

the dashed line. The dark current noise and photodiode Johnson

noise are insignificant within the boundaries of this plot. The

noise current (2.5 pA/ ) and voltage spectral densities

(2.3 nV/ Hz) used were taken from the manufacturer’s data

sheet. Notice that this receiver will be shot-noise limited for

with mA. Also, for k , we reach a

point of diminishing returns since the shot-noise power/total-receiver-noise

power becomes constant at about 15 dB.

Another important issue in transimpedance amplifiers running

high average current is the maximum dc output voltage. To

keep this within the power-supply limits, one must balance the

needs of high average current and high feedback resistance. To

see this interaction, Fig. 5 includes constant dc output voltage

contours. For example, in order to maintain a 1-V dc output,

operation on this contour would allow a feedback resistance of

1k at 1-mA average photocurrent. As a reasonable compro-

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