The frequency where the photodetectors output electrical power has dropped 3 dB from a low-frequency reference. We specify the more conservative electrical 3 dB, rather than the larger optical 3 dB. (Optical 3 dB is equivalent to the electrical 6-dB frequency.)
Common-Mode Rejection Ratio (CMRR)
In our balanced photoreceivers, CMRR tells you how much noise rejection you can get by using the photoreceiver. The CMRR is deﬁned as
where VCM is the photodetectors output voltage (proportional to the laser power present on the reference and signal diodes) at a given frequency. VBAL is the balanced photoreceivers output voltage at the same frequency.
cw Saturation Power
The point at which the output of the photodetector becomes nonlinear. In our ≤1-GHz photoreceivers, this is limited by the ﬁrst-stage ampliﬁer. Unless otherwise noted, this is speciﬁed at the wavelength corresponding to peak responsivity.
In our ampliﬁers, this is the variation of the gain over the entire frequency bandwidth. For instance, the Model 1421 has a gain that varies from 7 dB to 10 dB. In terms of voltage gain, this is a variation from 2.2 to 3.2.
The width of the photodetectors output in response to a fast optical pulse, measured as a full width at half maximum (FWHM).
Maximum Conversion Gain
The transfer function of the photodetector or photoreceiver. Given in V/W, this tells you how much output voltage will result from a given optical input power.In a photoreceiver, conversion gain is the product of the photodetectors responsivity (R), the ampliﬁers gain (Ag), and the input impedance (Rin). For an unampliﬁed photodetector, the conversion gain is the product of the photodetectors responsivity and the load impedance (Rl).
Measured output voltage is the product of the conversion gain and the optical input power (Pin).
Maximum Optical Power
Damage threshold for the photodetector, speciﬁed at the wavelength corresponding to peak responsivity.
Maximum Power Out
In a 50-Ω system, 10 dBm is 1.0-V peak (for our ampliﬁers).
The weakest optical signal that can be detected. The noise-equivalent power (NEP) is the optical power that produces a signal-to-noise ratio of 1 in a 1-Hz bandwidth. The minimum optical power can be found using the relationship
where B is the entire measurement bandwidth. For photodetectors with no gain, the NEP is of limited usefulness, because the ampliﬁer or instrument that follows the photodetector will produce noise that far exceeds the noise produced by the photodetector. It is stated here solely for comparison purposes. Unless otherwise noted, this is speciﬁed at the wavelength corresponding to the peak responsivity.
The amount of excess noise above 174 dBm in a 1-Hz bandwidth at 290 K. In a 50-Ω system, 8 dB corresponds to an input noise current of 22 pA/.
We offer ﬁber-optic (FC) and free-space (FS) versions of most of our photodetectors and photoreceivers. The FC models have the ﬁber-optic connector aligned so that light from the ﬁber strikes the photosensitive region.
Here, the photodetector end of the ﬁber has been angle-polished to reduce optical back reﬂections to less than 35 dB. Single-mode ﬁbers are standard. In the multimode models, a GRIN lens focuses the
light onto the photodiode.
The impedance that the load sees. For our high-speed photodetectors, the output impedance is 50 Ω to provide a proper impedance match for most high-speed oscilloscopes and spectrum analyzers. In Model 1004, the output impedance is higher, resulting in higher sensitivity but lower bandwidth.
The diameter of the photodetectors active area. Overﬁlling may compromise the bandwidth of the device.
Most New Focus photoreceivers require a ±15-V power supply. We strongly recommend our Model 0901 power supply with its current-protection circuitry.
The fastest transition that can be measured. In general, it is the 1090% transition time and, in our frequency-domain-optimized photodetectors, is approximately related to the 3-dB frequency (f3-dB) by
The transfer function of the ampliﬁer in our photoreceivers. Given in V/A, it tells you what output voltage results from a given photocurrent.
Typical Maximum Responsivity
Responsivity (R) is the amount of photocurrent (Iphoto) that results from an optical input of 1 W. Use this number to calculate the photocurrent that will result from your experiments input power (Pin) using the formula
Responsivity is wavelength-dependent, and related to quantum efﬁciency (the number of electrons released per incident photon) by
where h is Plancks constant, n is the frequency of the incident radiation, and e is the electrons charge. The quantum efﬁciency of our high-speed photodetectors at 532 nm is about 47%. This is about twice that of a metal-semiconductor-metal (MSM) photodetector. We specify a typical value for the maximum responsivity versus wavelength. We cannot specify an absolute value, since responsivity varies slightly for every individual photodetector.
The voltage standing-wave ratio. It is the ratio of the maximum to the minimum amplitude of a standing wave that might occur because of impedance mismatch at the end of a transmission line. A VSWR of 1:1 indicates a perfectly matched termination.
For a reﬂection coefﬁcient R,
The range over which the photodetector operates. Below the short-wavelength cutoff, photons are absorbed outside the active region of the photodetector, which decreases the photodetectors bandwidth. Above the long-wavelength cutoff, the photons energy is less than the semiconductor bandgap and the photon is not absorbed by the photodetector.