# Technical Note: Optical Detector Definitions of Characteristics

### 3-dB Bandwidth

The frequency where the photodetector’s 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 photodetector’s output voltage (proportional to the laser power present on the reference and signal diodes) at a given frequency. VBAL is the balanced photoreceiver’s 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.

### Gain Flatness

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.

### Impulse Response

The width of the photodetector’s 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 photodetector’s responsivity (R), the amplifier’s gain (Ag), and the input impedance (Rin). For an unamplified photodetector, the conversion gain is the product of the photodetector’s 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).

### Minimum NEP

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.

### Noise Figure

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/

### Optical Input

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.

### Output Impedance

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.

### Photodetector Diameter

The diameter of the photodetector’s active area. Overfilling may compromise the bandwidth of the device.

### Power Requirement

Most New Focus™ photoreceivers require a ±15-V power supply. We strongly recommend our Model 0901 power supply with its current-protection circuitry.

### Rise Time

The fastest transition that can be measured. In general, it is the 10–90% transition time and, in our frequency-domain-optimized photodetectors, is approximately related to the 3-dB frequency (f3-dB) by

### Transimpedance Gain

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 experiment’s input power (Pin) using the formula

Iphoto=RPin.

Responsivity is wavelength-dependent, and related to quantum efficiency (the number of electrons released per incident photon) by

Quantum Efficiency=R•hn/e,

where h is Planck’s constant, n is the frequency of the incident radiation, and e is the electron’s 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

### VSWR

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,

### Wavelength Range

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 photodetector’s bandwidth. Above the long-wavelength cutoff, the photon’s energy is less than the semiconductor bandgap and the photon is not absorbed by the photodetector.