Optical receiver pdf

High speed signal output is supplied by 50 ohm cable with an SMA connector. Performance data represents typical characteristics as individual product performance may vary. No claims or warranties are made as to the application of Your most recent searches Delete. Frequent searches. Other sections. Optical Receiver 2 Pages Add to favorites. Catalog excerpts. Open the catalog to page 2. Neutronic [i] 2 Pages. Optical receivers 1. Examples of Photon Absorption 7.

This current generates two types of noise not multiplied by M Often thermal and quantum noise are the most significant. Noise Calculation Example In this case the SNR is called shot-noise or quantum noise limited. In this case the SNR is referred to as being thermal-noise limited. Typical SNR vs. Rise and fall times Photodiode has uneven rise and fall times depending on: 1.

Various pulse responses Pulse response is a complex function of absorption coefficient and junction capacitance Comparisons of pin Photodiodes NOTE: The values were derived from various vendor data sheets and from performance numbers reported in the literature. They are guidelines for comparison purposes. They are guidelines for comparison purposes only. Optical receiver Part B Signal Path through an Optical Link The SNR at which this transition occurs is called the threshold level.

The value of this minimum power level is called the receiver sensitivity. The current to voltage converter is perhaps the most important section of any optical receiver circuit. An improperly designed circuit will often suffer from excessive noise associated with ambient light focused onto the detector.

Many published magazine circuits and even many commercially made optical communications systems fall short of achievable goals from poorly designed front-end circuits. Many of these circuits are greatly influenced by ambient light and therefore suffer from poor sensitivity and shorter operating ranges when used in bright light conditions. To get the most from your optical through-the-air system you need to use the right front-end circuit.

TOP High Impedance Detector Circuit One method that is often shown in many published circuits, to convert the leakage current into a voltage, is illustrated in figure 6a. This simple "high impedance" technique uses a resistor to develop a voltage proportional to the light detector current. However, the circuit suffers from several weaknesses. If the resistance of the high impedance circuit is too high, the leakage current, caused by ambient light, could saturate the PIN diode, preventing the modulated signal from ever being detected.

Saturation occurs when the voltage drop across the resistor, from the photo diode leakage current, approaches the voltage used to bias the PIN device. To prevent saturation, the PIN must maintain a bias voltage of at least a few volts.

Consider the following example. Under certain bright background conditions a PIN photodiode leakage current of a few milliamps may be possible. If a 12v bias voltage were used, the detector resistance would have to be less than 10, ohms to avoid saturation.

With a 10K resistor, the conversion would then be about 10 millivolts for each microamp of PIN leakage current. But, to extract the weak signal of interest that may be a million times weaker than the ambient light level, the resistance should to be as high as possible to get the best current to voltage conversion. These two needs conflict with each other in the high impedance technique and will always yield a less than desirable compromise In addition to a low current to voltage conversion, there is also a frequency response penalty paid when using a simple high impedance detector circuit.

The capacitance of the PIN diode and the circuit wiring capacitance all tend to act as frequency filters and will cause the circuit to have a lower impedance when used with the high frequencies associated with light pulses. Furthermore, the high impedance technique also does not discriminate between low or high frequency light signals.

Flickering streetlights, lightning flashes or even reflections off distant car windshields could be picked up along with the weak signal of interest. The high impedance circuit is therefore not recommended for long-range optical communications. The resistor that converts the current to a voltage is connected from the output to the input of an inverting amplifier.

The amplifier acts as a buffer and produces an output voltage proportional to the photodiode current. The most important improvement the transimpedance amplifier has over the simple high impedance circuit is its canceling effect of the circuit wiring and diode capacitance. The effective lower capacitance allows the circuit to work at much higher frequencies.

However, as in the high impedance method, the circuit still uses a fixed resistor to convert the current to a voltage and is thus prone to saturation and interference from ambient light. Figure 6b. This technique is borrowed from similar circuits used in radio receivers. The circuit replaces the resistor with an inductor. A student in electronics may remember that an inductor will pass DC unaffected but will exhibit a resistance effect or reactance to AC signals.

The higher the frequency of the AC signals the higher the reactance. This reactance circuit is exactly what is needed to help extract the sometimes small modulated AC light signal from the large DC component caused by unmodulated ambient light. DC signals from ambient light will yield a low current to voltage conversion while high frequency AC signals will experience a high current to voltage conversion.

With the right circuit, an AC vs. DC conversion ratio of several million is possible. Such techniques are used throughout radio receiver circuits to process weak signals I n addition, as the Q increases so does the impedance of the LC circuit. Such high Q circuits can also be used in a transimpedance amplifier designed for optical communications. To obtain the highest possible overall impedance, the inductance value should be as large as possible and the capacitance should be as small as possible.

Since every inductor contains some finite parallel capacitance within its assembly, the highest practical impedance occurs when only the capacitance associated with the inductor assembly is used to form the LC network. Figure 6c In radio, connecting a capacitor in parallel with the inductor often produces high impedances and allowing the LC tuned circuit to resonant at a specific frequency. Such a circuit can be very frequency selective and can yield impedances of several mega ohms.

The degree of rejection to frequencies outside the center resonant frequency is defined as the "Q" of the circuit. As figure 6d depicts, a high Q will produce a narrower acceptance band of frequencies than lower Q circuits. TOP Figure 6d. Figure 6f lists the characteristics of some typical coils. TOP Transimpedance Amplifier Detector Circuit with Limited Q The use of a LC tuned circuit in a transimpedance amplifier circuit does improve the current to voltage conversion and does reject much of the signals associated with ambient light.

But, high Q circuits are prone to unwanted oscillations. As shown in figure 6g, to keep the circuit from misbehaving, a resistor should be wired in parallel with the inductor.

The effect of the resistor is to lower the circuit's Q. For pulse stream applications with low duty cycles short pulses with lots of time between pulses , it is best to keep the Q near 1. Figure 6g A Q of one exists when the reactance of the coil is equal to the parallel resistance at the desired frequency. If higher Qs were used, with low duty cycle pulse streams, the transimpedance amplifier would produce excessive ringing with each pulse and would be prone to self-oscillation. TOP Figure 6h and 6i illustrate what happens in a circuit with a low Q and high Q when processing single pulses.

In this kind of a combined re- Keywords—electro-optical modulator, microwave-photonic re- ceiver, so called MW photonic receiver, besides getting rid ceivers, wireless communication. Introduction [11]—[14]. The block-diagram of a MW photonic receiver is presented in Fig.

In the last decade microwave MW communication sys- tems expand rapidly the frequencies of their operation. The contem- porary communication systems are exploiting superhetero- dyne radio-frequency RF receivers as they best satisfy the requirements of modern communication systems.

These re- ceivers have higher selectivity and sensitivity compared to the other types of receivers [1].

However, with the increase of operating frequency the stray radiation of heterodyne local RF oscillator is increased.

This parasitic radiation Fig. Moreover, what is undesirable, it is possible to lo- nal Processing. While high Q-factor MW input circuitry is well established, the problem is finding a proper electro- optical modulator EOM , which would ensure strong in- teraction between electrical and optical waves. This is pos- sible only in optical resonant structure. The last permits to prolong electrical field interaction with optical wave con- fined within the resonator. From the known optical res- onators for microwave-photonic receivers Fabry-Perot F-P and disk or ring one are the most suitable.

The confinement of optical wave in F-P resonator depends on the reflectance of mirrors serving also for light input and output from the resonator. The confine- ment is proportional to the light survive time within the resonator. These two types of resonators are described by the same mathematics and can be characterized with the same parameters, and their application depends on the feasibility [15].

An initial real- ization of MW photonic receiver is relied on EOM based on high Q-factor microdisk resonator [3]—[7], [11]—[14]. Below the brief the receiver proposed for millimeter wave RF detection b. Electro-Optical Modulators of disk. For operation as MW resonator, the gold electrodes are located on the top and bottom of the microdisk. RF sig- Microwave Photonic Receivers nal from the microstrip line is applied to the metallic elec- trodes. For optical part operation, a single-mode laser in- 2.

The first circular optical modulator for a microwave-pho- The trapezoidal prism is used to input and output an op- tonic receiver was demonstrated in where a LiNbO3 tical radiation from the microdisk by means of evanescent microdisk cavity has been used [3]—[5].

EOM uses a z-cut waves. For this, the air gap between the microresonator and LiNbO3 disk resonator with optically polished curved side- the prism should be about the optical wavelength to fit the walls Fig. Evanescent prism-coupling is used to cou- optimal connection between the prism and the microdisk.

A metal electrode is 7. The resonant interaction of MW ra- structure fed by an RF signal is designed to overlap with diation with optical wave in the microresonator takes place the optical field. The frequency of the microwave carrier with an applied RF microwave field.

Schematics of the receiver proposed for mil- dius of disk and n is the refraction index of LiNbO3 in the limeter wave RF detection is presented in Fig. An electromagnetic wave received by a RF antenna feeds The semi-ring electrode is a standing-wave resonator with electrodes of the microphotonic modulator. The modula- open ends so its resonant frequency can be easily tuned by tor directly converts the RF signal to an optical carrier via changing its length.

This property has made the semi-ring the electro-optic effect. The phase- modulated optical sig- the preferred resonator in most microdisk modulator de- nal is internally converted to amplitude modulation through signs.