WO2005022784A1

WO2005022784A1 - Optical receiver

Info

Publication number: WO2005022784A1
Application number: PCT/IB2004/051440
Authority: WO
Inventors: Michel W. A. Groenewegen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-08-29
Filing date: 2004-08-10
Publication date: 2005-03-10

Abstract

The present invention relates to an optical receiver having a first amplifier stage (204) to which photodiode (202) is coupled. Amplifier stage (204) has a low input impedance 'seen' by photo diode (202). This way a low RC constant is provided which increases the bandwidth of optical receiver (200). Cascaded amplifier stages (206) and (208) have variable resistors (210) to set the gain.

Description

Optical receiver

Present invention generally relates to the field of optical receivers, and more particularly without limitation to low-noise optical receivers having a wide bandwidth. Various optical receivers are known from the prior art. For example, US Patent No. 5,010,588 shows a low-noise optical receiver which comprises a photodiode connected to a high-impedance 3-stage HEMT preamplifier through an inductive/resistive modified T-network. The first amplifier stage provides signal gain and transforms the high input impedance to an intermediate value. The second amplifier stage provides additional gain at the intermediate impedance level and the third stage provides output impedance matching to a conventional 50 ohm system. An advanced optical receiver is commercially available from Philips

(component number CG0869). This optical receiver has a gain control circuit which is included at the input of the first amplifier stage. The amplifier itself is implemented by a characteristic impedance amplifier stage which is followed by a T-attenuator in order to get sufficiently low output return loss. Fig. 1 shows a corresponding block diagram. Prior art optical receiver 100 has photodiode 102 which is coupled to differential amplifier stage 104. Variable resistor 106 is coupled in parallel to photodiode 102 and to the input of amplifier stage 104. Amplifier stage 104 is a characteristic impedance amplifier, i.e. the input impedance of amplifier stage 104 Amplifier 104 is coupled to amplifier stage 108 which provides the output signal. Both amplifier stages 104 and 108 are fixed gain amplifiers; the gain control is performed by means of variable resistor 106. Optical receiver 100 is implemented by means of hybrid amplifier modules, composed of alumina substrates with integrated resistors, bi-polar silicon transistor dies and/or GaAs MESFET dies, SMD-components and other components. The present invention provides for an optical receiver having a first amplifier stage and at least a second cascaded amplifier stage. The first amplifier stage has a low input impedance This has the advantage that the capacitance C of the photodiode results in a low RC product and thus a high bandwidth. The gain of the optical receiver is adjusted by means of the second amplifier stage. For example the second amplifier stage has a resistor network with at least one variable resistor for setting the gain. In accordance with a further preferred embodiment of the invention the first amplifier stage has a fixed gain which provides for sufficient amplification to limit the noise independent of the gain level of the succeeding stages of the amplifier. Using a fixed gain low-noise amplifier as the first amplifier stage provides a constant equivalent input noise, independent of the gain. This provides a superior carrier to noise ratio performance as function of the optical input power, as the gain control implementation is no longer at the input of the amplifier but in the second amplifier stage. In accordance with a further preferred embodiment of the invention a current amplifier is used to provide the first amplifier stage with the low input impedance. In accordance with a further preferred embodiment of the invention the amplifier stages are implemented on a monolithic microwave integrated circuit (MMIC). A MMIC implementation has the advantage that an amplifier on a MMIC basis has much smaller distances between the active stages and has less self-inductance and parasitics within the amplifier. This leads to a considerably higher bandwidth.

In the following preferred embodiments of the invention will be described in greater detail by making reference to the drawings in which: Fig. 1 is a block diagram of a prior art optical receiver, Fig. 2 is a block diagram of an embodiment of an optical receiver in accordance with the present invention, Fig. 3 is a table comparing the performance of the prior art optical receiver of Fig. 1 and the optical receiver of Fig. 2.

Fig. 2 shows a block diagram of optical receiver 200. Optical receiver 200 has a photodiode 202 which is directly coupled to the input terminals of differential amplifier stage 204. The output of differential amplifier stage 204 is coupled to input of differential amplifier stage 206 which in turn is coupled to differential amplifier stage 208. One or more additional differential amplifier stages can be connected in a cascade following amplifier stage 208. The first amplifier stage, i.e. amplifier stage 204, has a low input impedance which is 'seen' from photodiode 202. This way a low RC constant of the input impedance of the first amplifier stage 204 and the photodiode 202 is provided. Due to the low RC constant optical receiver 200 has a high bandwidth. Amplifier stage 204 provides a fixed gain in order to provide constant equivalent input noise. This way the noise is kept at a satisfactory level independent of the gain of the succeeding stages. Amplifier stages 206 and 208 have variable resistors 210 for setting the gain of the respective amplifier stages. Preferably the amplifier stages 204, 206 and 208 are implemented in a monolithic microwave integrated circuit in order to further improve the bandwidth. Fig. 3 shows a table which compares the performance of the prior art optical receiver of Fig. 1 and an implementation of the optical receiver of the invention shown in Fig. 2. Column 300 shows the parameters frequency range, responsivity (maximum), responsivity (minimum), output return loss, equivalent input noise at 40 megahertz, electrical gain control range, equivalent input noise at 870 megahertz at maximum gain, equivalent input noise at 870 megahertz at typical gain and equivalent input noise at 870 megahertz at minimum gain for prior art optical receiver 100 whereas column 302 shows the same parameters for an implementation of optical receiver 200 of Fig. 2. As apparent from the table of Fig. 3 optical receiver 200 is superior to prior art optical receiver 100 in all respects. In particular optical receiver 200 has a relatively constant equivalent input noise independently from the gain. Further optical receiver 200 has a wide electrical gain control range. An even wider electrical gain control range can be implemented if one or more amplifier stages are added to the cascaded amplifiers. Further the bandwidth of optical receiver 200 is much wider than the bandwidth of optical receiver 300 due to the smaller RC constant which is a result of the low input impedance of the first amplifier stage. It is to be noted that usage of differential amplifiers is not an essential element of the present invention even though the above described preferred embodiment uses such differential amplifier stages. REFERENCE NUMERALS:

100 optical receiver

102 photo diode

104 amplifier stage

106 resistor

108 amplifier stage

200 optical receiver

202 photo diode

204 amplifier stage

206 amplifier stage

208 amplifier stage

210 resistor

300 column

302 column

Claims

CLAIMS:

1. An optical receiver comprising: a light receiving element (202) for converting a received optical signal to an electrical signal, a first amplifier stage (204) for amplification of the electrical signal, the first amplifier stage having an input, at least a second variable gain amplifier stage (206, 208) for amplification of an output signal of the first amplifier stage.

2. Optical receiver of claim 1 , the light receiving element being a photodiode (202).

3. Optical receiver of claim 1 or 2, the first amplifier stage having a fixed gain.

4. Optical receiver of claim 1, 2 or 3, the first amplifier stage being a current amplifier.

5. Optical receiver of any one of the preceding claims 1 to 4, the first amplifier stage and/or the second amplifier stage being differential amplifier stages.

6. Optical receiver of any one of the preceding claims 1 to 5, the first and second amplifier stages being implemented on a monolithic microwave integrated circuit.

7. Optical receiver of any one of the preceding claims 1 to 6, whereby the at least one second variable gain amplifier stage provides a gain control range of above 10 dB, preferably above 20 dB.