Types of Photoreceivers

The main types of photoreceivers discussed below are those that can detect optical signals with fast temporal responses or those containing high frequency components as well as detectors that are sensitive to low light levels or small differential changes in signals. Depending on the type of semiconductor used, the detectors can have spectral sensitivities anywhere from the UV region of the spectrum to the NIR. Furthermore, depending on the application being targeted, they can possess either free-space or fiber-coupled configurations. With the advancement of high-transmission-rate systems and short-pulse lasers, many applications now require high time-resolution or equivalently, high frequency-bandwidth optical detection. High-speed photoreceivers are critical for the measurement of the frequency and/or time response of optical systems. In the optical domain, this can include measuring the pulses of mode-locked laser systems, detecting the data stream of a frequency-multiplexed communication system, or providing increased resolution in dynamic, pump-probe spectroscopy. The minimum rise time for high-speed photoreceivers is less than 10 ps. Consequently, for optical signals with faster responses, optical gating techniques are required (see Laser Pulse Characterization). In the frequency domain, applications for high-speed photoreceivers include laser heterodyning experiments and millimeter-wave signal generation. The maximum frequency bandwidths for such detectors can exceed 50 GHz in well-designed devices.

Based on the discussion above, diffusion of carriers to the depletion region is a relatively slow process in reverse biased p-n junctions that could serve to limit the response time of a photoreceiver. To minimize this effect, a p-i-n photodiode is typically utilized where an un-doped intrinsic layer is sandwiched between the p and n layers in a p-n junction (see Figure 1). This structure effectively widens the depletion layer. This results in a greater proportion of the generated current being carried by the faster drift process instead of diffusion. The increased depletion width also allows for a reduction in the RC time constant (via a decreased junction capacitance) and increased area for capturing light. The p-i-n device structure is ubiquitous in high-speed photoreceivers and enables fast rise times and large bandwidths. However, the final measured optical signal will be as slow as the slowest component of a detection system even if a sufficiently fast photoreceiver is employed. Therefore, care should be taken when choosing connectors, cables, an oscilloscope, and a spectrum analyzer to measure a fast optical signal.

Balanced photodetection is a commonly used detection method to increase the SNR of a signal beyond simple amplification. It is particularly powerful in its ability to cancel laser noise, i.e., common mode noise, and to detect small signal fluctuations on a large DC signal. The simplest balanced detector uses two photoreceivers connected so their photocurrents cancel. The output of the detector pair is zero until there is some difference in the intensity of one of the beams, which causes the pair to become unbalanced and a net signal appears on the output. In practice, this is accomplished by using an auto-balancing circuit that includes a low-frequency feedback loop to maintain automatic DC balance between the signal and reference arms (see Figure 2). This balanced optical receiver permits detection of a small signal with a large background. One application of this balanced detection method is shown in Figure 2 where dynamic changes in a material’s optical properties can be measured. Another device that employs balanced detection is a quadrant-cell photoreceiver, which consists of four individual yet identical photodetectors positioned very close to each other. Signals from pairs of these detectors are used to generate differential signals in the horizontal and vertical directions while the sum of all quadrants is provided for normalization purposes. This photoreceiver is ideal for measuring deviations in the position of a laser beam such as that required in beam-stabilization systems.

Certain applications involving spectroscopic or fluorescence measurements require photoreceivers than can detect low-light-level signals. One approach involves utilizing photoreceivers with large conversion gains (up to 2x10¹¹ V/W). Such large gains are achieved through careful selection of the amplifier-resistor pair, where a large feedback resistor provides the high-gain values while an ultra-quiet amplifier keeps noise to a minimum. By using these photoreceivers in conjunction with an optical chopper and lock-in detection methods, sensitivity levels in the femtowatt range can be achieved. For experiments that require ultra-low light levels, e.g., those requiring only a few photons, a different type of photodiode known as an avalanche photodiode (APD) can be used. This photodiode has a junction so strongly reverse-biased that charge carriers generated in the junction acquire sufficient energy to excite new carriers by a process known as impact ionization. In this way, weak light can generate a current large enough to be detected by the accompanying circuit electronics. Another detector capable of detecting light at the single photon level is a PMT which is not a photoconductive detector like a photodiode, but rather a photoemissive detector.