Knowing your frequency requirement is fairly simple if you are making laser heterodyning or optical modulation experiments. Your equipment should have a fairly flat response out to the highest frequency of interest, usually your highest modulation frequency. Oscilloscopes and other time domain equipment most commonly express their frequency bandwidth in terms of a 3-dB bandwidth (the frequency at which the power falls off to 50% of the value at DC). Amplifiers and spectrum analyzers, however, may express their frequency bandwidth in terms of a 1-dB bandwidth (the frequency at which the power falls off to 80% of the value at DC). In either case, to ensure that the equipment will have adequate bandwidth, determine your highest frequency, add a 20% safety margin, and make this your limiting bandwidth for determining your equipment.
For measuring pulses in the time domain, there really is no definite rule for knowing the required frequency bandwidth. Nonetheless, there is a good way to make an estimate. The photodiode converts photons to electrons. It responds to the intensity of the light; thus, if you relate the FWHM (full-width at half maximum) of the intensity in time to the FWHM of intensity in frequency, you can get some idea of the frequency requirements. For a Gaussian pulse shape with a FWHM of τ, the FWHM in frequency is simply 0.44/τ for a transform-limited pulse; for a sech2, it is 0.31/τ. Using this as your 3-dB bandwidth gives you a very high value, because the 3-dB point specified for electrical devices is where the power, not the voltage, falls to 50% of its value. Equivalently, this is where the voltage drops to √2 of its value. With the photodiode, you are interested in voltage measurements, therefore a more accurate bandwidth is 0.31/τ for the Gaussian or 0.22/τ for sech2. For safety, allow 20% more bandwidth, giving approximately 0.37/τ and 0.26/τ respectively. So on the average, a good rule of thumb is to have a frequency 3-dB bandwidth of 0.4/τ where τ is the FWHM.
Modeling the photodiode as an RC circuit is commonly known. There are basically three limiting factors to the speed of a photodetector: diffusion of carriers, drift transit time in the depletion region, and capacitance of the depletion region. The slowest of the three processes is the diffusion of carriers to the high-electric field depletion region from outside that region. To minimize this slow effect, carriers should be generated near or in the depletion region. The second process, transit time, is the time required for the carriers to drift across the depletion region and get swept out of the device. With sufficient reverse bias, these carriers will drift at their saturation velocities, on the order of 3x106 cm/s for GaAs. Lastly, the capacitance of the device will determine its RC time constant; R is the load resistance (usually 50 Ω). To maximize a photodiode response, the transit time is typically designed to be comparable to the RC time constant. For instance, given the saturation velocity for GaAs, a 1-ps transit time requires that the depletion layer not be thicker than 0.3 µm. For a comparable RC in a 50-Ω system, the capacitance must be <20 fF, Since C=εA/d (for GaAs ε=13) where the width, d=0.3 µm, and A is the area, your active photodiode area must be a maximum of 52.5 µm2, or a diameter of 8 µm.
Now that you know the minimum frequency bandwidth that is required to maintain the fidelity of your measurement, and you have chosen a photodiode with adequate bandwidth, every electrical component that follows the photodiode must be able to maintain this bandwidth. Let's first start with your cables. Your typical cable around the laboratory is usually RG-58 which is very lossy after 1–2 GHz. Microwave companies have cables that have acceptable losses up to 50 GHz. These cables have an acceptable loss but it is not negligible. Therefore keep all cable lengths to a minimum!
The next thing to keep in mind is that all the connectors must also be up to specifications. This includes the bias T which will allow you to bias the photodetector (note: bias Ts are not necessary with New Focus™ photodetectors). The frequency range of any connector is limited by the occurrence of the first circular waveguide mode in the coaxial structure. Decreasing the diameter of the outer conductor increases the highest usable frequency, while filling the air space with dielectric lowers the highest usable frequency. The BNC (Bayonet Navy) connectors most abundant in the lab are good up to 2 GHz. The SMA (sub-miniature A) connector is good to 24 GHz. The 3.5 mm which uses air as the insulator, can be mated with the SMA and is good up to 34 GHz. The 2.92 mm or Wiltron® K connector1 is good to 40 GHz and is compatible with APC-3.5 and SMA. The 2.4-mm connector is good up to 50 GHz, and the 1.85 mm or the Wiltron V connector is good to 65 GHz. Agilent makes 3.5 mm, 2.92 mm, 2.4 mm and 1.85 mm as well as the SMA, SMC (to 7 GHz), APC-7 (to 18 GHz) and the Type N 50-Ω (to 18 GHz) connectors. The performance of all connectors is affected by the quality of the interface for the mated pair. Great care must be taken with these connectors. A torque wrench which is permanently set to the correct torque value should be used to turn the male coupling nut while grasping the body of the connector firmly to keep it from rotating. As the male coupling nut becomes tighter, frictional forces will increase, and the nut and body will tend to lock up, which will cause the body to rotate. This wears away the plating and can score both the outer interface rim and the pin of both connectors. Once a connector has been over-torqued and damaged, it will damage to some extent each connector to which it is mated. This damage lowers its frequency performance. In addition, never hold a male connector coupling nut stationary while screwing the female counterpart into it. This destroys both connectors.
For your electrical instruments, Agilent and Tektronix1 manufacture digital oscilloscopes to 50 GHz and spectrum analyzers to 325 GHz. New Focus™ and other companies have amplifiers to 20 GHz. Remember that the fidelity of your measurement requires that the instrument's response be fairly flat over the frequency bandwidth of interest. This means that both the amplitude response must be flat and the phase response must be linear with frequency. If this is not true of your instrument, for example if your amplifier has a non-linear phase response, then it will distort your measurement. Your measured signal waveform will become slower.
Let's now look at three different frequency regimes, <1 GHz, DC to 25 GHz, and DC to 60 GHz. If the maximum frequency of interest is <1 GHz, or the pulse width is >400 ps, APDs may be adequate, and BNC connectors and RG-58 cables certainly are. For 0-25 GHz or pulse widths >16 ps, PINs or Schottky photodiodes are required as well as SMA connectors and high-performance flexible or semi-rigid cables. For the bias T, Agilent makes one good to 26.5 GHz as well as amplifiers good to 26.5 GHz. Agilent makes a scope good to 34 GHz, and their spectrum analyzers go to 22 GHz or 26.5 GHz. Tektronix also makes a digital oscilloscope good to 20 GHz, and spectrum analyzers good to 33 GHz. For 0-60 GHz or pulse width > 6.7 ps, you must use the highest quality equipment. This includes 1.85-mm or V connectors, ultralow-loss cables, and the best oscilloscopes from Agilent and Tektronix which both only go to 50 GHz. Wiltron has a bias T for 60-GHz operation. In order to extend to this frequency, the spectrum analyzers must use external mixers. Both Agilent and Tektronix have this capability.