GB2420036A

GB2420036A - Optical transceiver with a composite electrical input

Info

Publication number: GB2420036A
Application number: GB0424475A
Authority: GB
Inventors: Stephane Sicard
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2004-11-05
Filing date: 2004-11-05
Publication date: 2006-05-10
Also published as: GB0424475D0

Abstract

This invention relates to an improved optical transceiver. The invention provides an optical transceiver comprising a photodetector for receiving an optical signal and generating and electrical signal; an optical source for generating an optical signal in response to an electrical signal; and a modulator/demodulator arranged to receive a modulated electrical signal and to generate therefrom a plurality of transmission electrical signals; and receive a plurality of received electrical signals and to generate therefrom a modulated electrical signal; wherein the said plurality of transmission electrical signals are coupled to the optical source for transmission of corresponding optical signals; and the photodetector receives a plurality of optical signals and generates therefrom the said plurality of received electrical signals.

Description

S

- 1 - 2420036

OPTICAL TRANSCEIVER

This invention relates to an improved optical transceiver.

An optical transceiver typically receives a plurality of optical input channels and converts these to a plurality of corresponding electrical signals and also converts a plurality of received electrical signals to a plurality of optical signals for transmission.

A problem with known optical transceivers is that the each electrical signal is connected to the transceiver by a dedicated pin. The number of electrical signals which may be processed is thus limited by the number of pins available and hence the size of the transceiver package.

There is a requirement to increase the number of electrical input and outputs dealt with by a single transceiver, there is a corresponding conflicting requirement to reduce the package size. However, the size of the package limited by the number of pins rather than the number/size of the devices housed in the package.

The invention provides a new optical transceiver package, which allows both a reduction in size of the package together with an increase in the number of electrical signals, which may be accommodated.

According to the invention there is provided an optical transceiver comprising a photodetector for receiving an optical signal and generating and electrical signal; an optical source for generating an optical signal in response to an electrical signal; and a modulator/demodulator arranged to receive a modulated electrical signal and to generate therefrom a plurality of transmission electrical signals; and receive a plurality of received electrical signals and to generate therefrom a modulated electrical signal; wherein the said plurality of transmission electrical signals are coupled to the optical source for transmission of corresponding optical signals; and the photodetector receives a plurality of optical signals and generates therefrom the said plurality of received electrical signals.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 illustrates a known optical transceiver package; Figure 2 illustrates a photodetector; Figure 3 illustrates a semiconductor laser; Figure 4 illustrates an optical transceiver package in accordance with the present invention; and Figures 5a and 5b illustrates a modulation scheme.

Referring now to Figure 1, an optical transceiver package 1 comprises a housing 2 containing an optical transceiver 3.

An optical transceiver comprises a detector 5 for detecting optical energy and converting it to an electrical signal, together with an optical source 4 for generating an optical signal in response to an electrical signal.

A detector 4 is usually provided using a photodiode where light is incident on a semi-conductor material and because of an energy gap between valence and conduction bands the light energy is converted to electrical energy. One example is a photo diode (known as a PIN diode) as illustrated in Figure 2. The diode consists of a heavily doped p layer 6 and n layer 7 separated by an intrinsic layer 8. A reverse bias across the device sweeps carriers from the intrinsic layer leaving a depletion layer. Incident light 9 if ailing onto the depletion layer creates electron-hole pairs, and their carrier's drift across the region to terminal where they leave via a contact 10 as light induced current.

Another example of a detector is an avalanche photodiode (APD) in this device the applied voltage is much higher creating an electric field of up to V/cm. At such a high level photoinduced electrons are accelerated to a velocity at which they can lonise atoms in the intrinsic region and produce an avalanche effect. In this way the device has an internal gain, so it is much more sensitive than the PIN diode. However, the noise level is much higher.

An optical source 5 is generally provided by a semiconductor laser or by an electro-absorption modulator (EAM) (although light emitting diodes (LEDs) can be used where the distance is short and the bit-rate requirement is low).

An example of a semiconductor laser is illustrated in Figure 3. The laser comprises an active pn junction, with planar terminal plates parallel to the junction to which the supply voltage is applied. When the electric field is applied, carriers are injected from the majority to the minority side, where they create an excess of minority carriers, as they diffuse away from the junction recombination occurs and optical radiation is emitted. Two faces (known as facets), perpendicular to the junction plane are cut to provide reflecting surfaces, photons reflect backwards and forward between the facets stimulating release of further photons.

These facets allow the stimulated emission required to produces monochromatic light, when an electric field above a threshold is applied.

Referring back to Figure 1, the optical transceiver package 1 receives a plurality of optical signals via optical input channels 25-28 and sends a plurality of optical signals via optical output channels 29-32. The transceiver 3 converts each received optical signal to a corresponding electrical signal, which is then transmitted via electrical output channels 33-36. The transceiver 3 receives electrical signals via electrical input channels 37-40 and converts each received signal to a corresponding optical signal, which is then transmitted via optical output channels 29-32. Each input and output electrical channel 33-40 requires a dedicated pin, resulting in a total of eight pins in the

example illustrated.

Figure 4 illustrates an optical transceiver package according to the present invention. An optical transceiver package 1' comprises a housing 2' containing an optical transceiver 3' and a modulator/demodulator 50. The optical transceiver package 1' receives a plurality of optical signals via optical input channels 25'-28' and sends a plurality of optical signals via optical output channels 9'-12'. The transceiver 3' converts each received optical signal to a corresponding electrical signal. All the electrical signals are then passed through a modulator/demodulator 50 to convert the plurality of electrical signals to modulated electrical signals which may be sent using signal 51. The output of the modulator may also include an optional clock signal 52 to synchronize the modulated signal 51 Correspondingly the demodulator/demodulator 50 receives a signal from the electrical channel 53 and converts it into a plurality of electrical signals. The demodulator/demodulator may also receive an optional clock signal 54 to synchronize the modulated signal 53. The transceiver 3' receives said plurality of electrical signals via the modulator/demodulator 50 and converts each received signal to a corresponding optical signal which is then transmitted via optical output channels 29'-32'. Therefore less than one pin is required per optical signal to be transmitted/received.

Modulation is defined as the process of varying some characteristic of an electrical carrier wave in order to impose information onto it. In the prior art transceiver a binary modulation scheme is used such that each symbol represents a single bit (a 1 or a 0), for example see Figure 5a. In the transceiver of the present invention pulse amplitude modulation is used to provide two bits per symbol as shown in Figure Sb.

Other types of modulation, for example frequency modulation, phase modulation or combinations such as quadrature amplitude modulation could also be used.

It is to be recognised that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the scope of the present invention as defined in the following claims.

Claims

CLAIM

1. An optical transceiver comprising a photodetector for receiving an optical signal and generating and electrical signal; an optical source for generating an optical signal in response to an electrical signal; and a modulator/demodulator arranged to receive a modulated electrical signal and to generate therefrom a plurality of transmission electrical signals; and receive a plurality of received electrical signals and to generate therefrom a modulated electrical signal; wherein the said plurality of transmission electrical signals are coupled to the optical source for transmission of corresponding optical signals; and the photodetector receives a plurality of optical signals and generates therefrom the said plurality of received electrical signals.