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Martin, Froeschner & Associates Series F Fast
Compact Photodetector Modules Now with sensitivities to over one million V/W and speeds to 10
GHz. A new line of inexpensive and compact fiber-coupled photodiode
modules simplify capturing fast transient light signals for analysis. These
devices are intended to approximate as closely as possible, a length of fiber
optic cable that can be connected directly onto the input jacks of your
oscilloscope or other electronic data acquisition device. Conversion of the
optical signal to electronic current and voltage occurs at the last possible
moment, within a mm or so of the input jacks of your scope. Co-ax cable and
the problems of reflections, termination loads, phase distortion and so forth
are totally eliminated. The devices are extremely compact and rugged, taking
up about as much room as a typical BNC terminator Dimensions: 0.65" (16mm) diameter by
1" to 1.75" (25 to 40 mm), excluding connectors and optical fiber Optical Fiber and Connector We will build your modules with whatever optical fiber, cable
and connectors you specify. We normally stock single-mode 9/125 µm and both
50/125 µm and 62.5/125 µm multimode fiber with 0.9mm polymer buffer and 3mm
Kevlar jacket. Other types such as 100/140 µm are available. The fiber to
photodiode connection is angle polished for low (-45dB) back reflection. The input end of the optical fiber will be fitted with whatever
type of optical fiber connector you desire. The range of normal options
include: none, i.e. a bare fiber pigtail, 2 mm ceramic ferrule only, Optional Light Collecting Optics If the signal you want to analyze is in the form of a beam in
free-space we can provide light collecting optics instead of or in addition
to the optical fiber connectors described above. A 6 mm diameter fixed focus
aspheric lens assembly is available. We also offer a micro-alignable fixture
with 18 mm aperture focusable achromatic lens. Precision fine thread
adjustments allow you to maximize the coupling of the incoming beam into the
fiber. The unit is also directly compatible with "micro-rail"
optical component systems made by
For example, Silicon based diodes are most sensitive in the
400nm (nearly UV) to 1000nm (near IR) spectral range and cover the visible
well. InGaAs is relatively insensitive to visible light but responds well to
the IR used in information systems. The intrinsic speed of the photodiode is also determined by the
semiconductor material. The higher electron mobility in InGaAs allows
response times which are simply not attainable with Si. Our latest GaAs model
offers unprecedented speed in the visible range. Other factors which might be
important to your needs, such as Dark Current and Capacitance are also listed
in the table on Photodiode Specifications. The photodiode in all our modules is reverse biased to minimize
intrinsic capacitance for maximally fast response. Specifications are listed for the usual photodiodes. Others are
available, call or write for details.
Transimpedance Amplifier Options When the light level gets below the mW regime, the expected
signal is uncomfortably near the noise floor of most systems. In order to get
acceptable signal levels from very low light levels the current from the
photodiode must be amplified. In the Series F we use hybrid bipolar
technology in Transimpedance Amplifier Specifications True DC Coupling with Offset Null - Single Stage
AC Coupling -
Single Stage
We
have recently greatly increased the amplification available in the AC coupled Photodetector
Modules. The new internal multi-stage AC coupled transimpedance amplifiers
provide overall conversion gains as high as 60,000 volts/watt in two stages
and over a million volts/watt in three stages, allowing detection of
sub-microwatt signals.
AC Coupling - Three
Stage
Note: Due to the relatively low output
power of the Mod 4 amplifier, those modules with an amplifier Model Number
ending in 4 will have Transimpedances about one-half of the value specified
above when driving a 50 ohm termination load. For example, the Mod 44A
delivers approximately 60,000 volts/amp into a high impedance termination and
about 30,000 volts/amp into 50 ohms. For all other models the specified
values are valid for either Hi-Z or 50 ohm termination. Keep in mind that the transimpedance amplifier is an option. If
your light levels are adequate, use the basic configuration Series G unamplified but biased
photodiode without the added current drain, cost and potential additional noise
of an amplifier. Estimation of Signal Level You can estimate the signal level (voltage) that you can expect
to see, for your light intensity, directly from the Responsivity as shown in
the graph above and in the table below, from: V [volts] = (I2 – I1 )[watts] • Responsivity
[amps/watt] • Z [ohms] where V is the output signal, I2 and I1
are the two input light intensities and Z is
the amplifier Transimpedance. Noise Specifications and Measurements Measurements of the noise spectrum have been made by independent
labs on several of our systems. In all measurements the results were
consistent with or better than the specifications shown. An InGaAs-2 Mod1D with DC Offset Null contributed about 1.5mV
rms over 20GHz with power on to both amplifiers and the photodiode and no
light input. (The scope by itself had an intrinsic noise floor of about 1.2mV
rms.) Calculations show this to be consistent with the Noise Figure specified
below. The equivalent optical noise density at the input is about 50pW Hz -1/2.
An AC coupled InGaAs-2 Mod2A system tested at one of the National
Laboratories showed a 0.3mV rms noise floor, also consistent with the Noise
Figure. The equivalent optical input noise density is about 30pW Hz-1/2.
This system reliably receives digital data at 2.5Gb/s with light levels as
low as a few microwatts. The noise from the photodiodes themselves is several orders of
magnitude below that of the amplifiers and may be ignored. In general, we have found that the noise contributed by our
amplified photodiode systems is below or at most comparable to the intrinsic
noise floor (about 1mV RMS) of most high speed oscilloscopes and digital data
acquisition systems Electronic output connector choices include BNC, TNC, SMA and
SMC in either gender. Wiltron-K, 7/16 and others are available at some extra
cost. Please note that BNC connectors are not recommended above 2 to 4 GHz.
If you need something really unusual, specify on your Request for Quotation
or call to discuss your needs. Output Coupling and DC Offset Null Option The output signal can be connected directly to the output
connector, DC coupling, or through a capacitor, AC output coupling. Amplified versions which are DC coupled would have an output
offset bias of several volts when the photodiode is dark, due to transistor
biasing. This offset is eliminated with the Offset Null configuration which
uses a second matched reference amplifier to drive the ground side of the
output connector. A multi-turn precision pot in the Power Supply controls the
input to this second amplifier so that any residual offset due to amplifier
mismatch, photodiode dark current or background light can be trimmed out. These modules are designed to be mounted directly onto the input
jack of your scope. When used this way there is no coax cable to cause
reflections so terminating with exactly 50 ohms is not necessary. The output
impedance of the amplified modules is already low so you may set the input
impedance of your scope to 1Mohm. The amplifiers will drive a 50 ohm load but
there may be a reduction in signal level for certain amplifier configurations
as explained above. The amplified Series F
modules are powered through a shielded multi-conductor cable. This cable is fitted with a connector which
is compatible with the M,F&A Power Supply. Any power supply is a potential source of
noise. The M,F&A Power Supply provides noise free power to the photodiode
and amplifiers from Nickel metal hydride batteries which are re-charged when
the photodetector module is not in use. When the photodetector module is
active, the system is completely disconnected from the external AC source. A
larger rack mount version capable of powering up to four Series F
Photodetector Modules is also available. Series F Amplified Photodetector Module
Summary and Prices
We will build your Photodetectors to your specifications. The
price of your detector is simply the sum of the prices of the chosen specified
options. Email us your specifications
from the list under Summary and Prices above. We will return a formal
quotation to confirm your specifications and expected delivery time. Quantity
discounts are available, send a Request for Quotation. Please note that these modules contain technology at the leading
edge of the state of the art. Prices and specifications are subject to
change. Martin, Froeschner
& Associates tel: (+1) 925 989
4930 Latest update June 2011 ©1999-2011 |
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