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All Fiber Doppler Velocity Interferometer Systems, a new kind of VISAR |
| Amplified Differential Optical Receivers to 10 Gb/s |
| International Order Policies |
Now with sensitivities to 60,000 V/W and speeds to 10 GHz.
Faster, more compact, and cheaper than our Award Winning Series 99 Photodetectors.
updated: March 2010
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Dimensions: 0.65" (16mm) diameter by 1" to 1.75" (25 to 40 mm), excluding connectors and optical fiber Optical Fiber and ConnectorWe 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, NTT-FC, SC, ST and SMA. The fiber end itself will be polished to any of the standard specifications: flat, PC, SPC or APC (8o). If you need something else, just specify so on the Order Form. Optional Light Collecting OpticsIf 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 Newport, Melles-Griot, ThorLabs and others and is "C" threaded for compatibility with Edmund Scientific's modular component line. Finally, large aperture catadioptric systems for collecting light from weak, distended or distant sources can be provided. Details available upon inquiry. Photodiode Selection
The two major considerations in selecting the photodiode for your application are spectral response and speed. Photodiodes based on different semiconductor materials have, in general, different ranges of light wavelength to which they are most sensitive. 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. Estimation of Signal LevelYou 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: dV [volts] = dI [watts] • Responsivity [amps/watt] • R [ohms] where dV is the change in output signal corresponding to a change of dI in the light power input and R is the load resistance, nominally 50ohms. Since the Responsivity is near unity, in general you will need on the order of 10mW in the optical input fiber in order to get output signals in the 100mV range. Photodiode SpecificationsSpecifications are listed for the usual photodiodes. Others are available, call or write for details.
Transimpedance Amplifier OptionsWhen 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 Darlington configurations. Expected signal levels can be estimated using the previous equation, but substituting the Transimpedance Gain from the table below in place of the load resistance. It should be noted that the gain figures in the specification below are approximate and that actual performance may differ by up to a factor of two depending on amplifier product variations, output loading and so on. Measurements of the noise spectrum have been made by independent labs on several of our systems. An InGaAs-2 Mod1 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 Mod2 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 differential digital data at 2.5Gb/s with light levels as low as a few microW peak.
Transimpedance Amplifier SpecificationsSpecifications are listed by the coupling available: both AC and DC or AC only. Other amplifiers are available, call or write for details. We have renamed the single stage detectors with a D for DC coupled and an A for AC coupled to simplfy specification and ordering. True DC Coupling - Single Stage
AC Coupling - Single Stage
M,F&A has added 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 Only - Two Stage
AC Coupling Only - 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 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 noise of an amplifier. Output Electrical ConnectorElectronic 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 OptionThe 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 will have an output offset bias of several volts when the photodiode is dark, due to transistor biasing. This offset is eliminated with the DC Offset Null option which uses a second matched reference amplifier to drive the ground side of the output connector. A multi-turn precision pot 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. Amplified versions are also available with AC output coupling through a capacitor. The higher gain versions use two stages of amplification and are currently available only as AC coupled units. TerminationThese 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 should 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. If you need to drive a 50 ohm; load with an amplified DC coupled module you will need the DC Offset Null option and should have external power.
Power ConsiderationsThe unamplified modules have very low current drain and the small internal batteries should last for a year or so of normal use. In the amplified modules, internal replaceable battery life is short. Use of the power cord configuration and an external power supply is recommended. Any power supply is a potential source of noise. The M, F & A External Power Supply provides noise free power to the photodiode and amplifiers from gel-cell batteries which are 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, and power is provided through a shielded cable with a connector that mates the module to the power supply. Larger versions powering more than one Series F Photodetector Modules are available. Series G Photodetector Module Summary and Prices
Series F Amplified Photodetector Module Summary and Prices
Please note that these modules contain technology at the leading edge of the state of the art. Prices and specs subject to change. How to OrderWe will build your Differential Optical Receiver to your specifications. The price of your detector is simply the addition of the prices of specified options with cost amounts listed. 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.
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Martin, Froeschner & Associates
tel: (+1) 925 989 4930
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