GB2377502A

GB2377502A - Optical device

Info

Publication number: GB2377502A
Application number: GB0116992A
Authority: GB
Inventors: Andrew Alan House
Original assignee: Bookham Technology PLC
Current assignee: Lumentum Technology UK Ltd
Priority date: 2001-07-12
Filing date: 2001-07-12
Publication date: 2003-01-15
Also published as: WO2003007058A1; GB0116992D0

Abstract

An optical device comprises an optical signal attenuator (3) including attenuator control means, and an optical signal modulator (5) including modulator control means, the optical signal attenuator and the optical signal modulator optically coupled with each other in series by a silicon rib waveguide (1) and controlled by their respective control means such that a first optical signal may be attenuated by the attenuator and a second optical signal may be superimposed on the first signal by a modulation of the first signal by the modulator, the attenuation provided by the attenuator being substantially static in comparison to the modulation provided by the modulator. The optical device may be used in optical communication systems such as a wavelength division multiplexer (WDM).

Description

Optical Device The present invention relates to the modulation of optical signals, and in particular to superimposing optical signals on other optical signals, for example for identification purposes.

In optical communications, it is frequently desirable to add information to an optical signal, e. g. in order to identify a particular optical channel (i. e. an optical wavelength or range of wavelengths) from a plurality of optical channels. For example, when a wavelength division multiplexed optical signal is demultiplexed into a plurality of optical channels, it is generally desirable to be able to identify each demultiplexed channel individually, e. g. to enable monitoring of the power of the individual channels and/or to enable the directing of individual channels along separate communications routes.

Although the wavelength of each channel can itself be used to identify a particular channel, this requires expensive and complex wavelength-selective components, and there is therefore a need to provide a simpler method of identifying each individual channel. It is also frequently necessary to attenuate the optical signals of an individual channel independently of each other channel, for example in order to balance the optical power of each channel.

According to a first aspect, the present invention provides an optical device comprising an optical signal attenuator including attenuator control means, and an optical signal modulator including modulator control means, the optical signal attenuator and the optical signal modulator optically coupled with each other in series and controlled by their respective control means such that a first optical signal may be attenuated by the attenuator and a second optical signal may be superimposed on the first signal by a modulation of the first signal by the modulator, the attenuation provided by the attenuator being substantially static in comparison to the modulation provided by the modulator.

According to a second aspect, the invention provides the use of an optical device according to the first aspect of the invention, to attenuate a first optical signal by the optical signal attenuator, and to superimpose a second optical signal on the first optical signal by a modulation of the first signal by the modulator, the attenuation provided by the attenuator being substantially static in comparison to the modulation provided by the modulator.

According to a third aspect, the invention provides a method of superimposing a second optical signal on a first optical signal by means of a device according to the first aspect of the invention, the method comprising attenuating a first optical signal by the optical signal attenuator, and superimposing a second optical signal on the first optical signal by modulating the first signal by the modulator, the attenuation provided by the attenuator being substantially static in comparison to the modulation provided by the modulator.

The invention has the advantage that by providing an optical signal attenuator and an optical signal modulator optically coupled with each other in series, the modulation of the first optical signal is separated from the quasistatic attenuation of the signal. This is generally advantageous because any non-linearity in the performance of the attenuator does not, thereby, cause a non-linearity in the performance of the modulator (and vice versa).

Preferably, the optical device according to the invention further comprises a waveguide, the waveguide comprising the optical signal

attenuator and the optical signal modulator optically coupled with each other in series.

More preferably, the optical device comprises a plurality of waveguides, each of which comprises an optical signal attenuator and an optical signal modulator optically coupled with each other in series and arranged such that a

first optical signal propagated by a said waveguide may be attenuated by the attenuator of that waveguide and a second optical signal may be superimposed on the first signal by modulation of the first signal by the modulator of that waveguide.

The, or each, waveguide of the optical device preferably comprises a semiconductor waveguide, for example a silicon waveguide. The, or each, waveguide may, for example, comprise a channel waveguide, or a slab waveguide, or a ridge waveguide, or a rib waveguide, but rib waveguides are generally preferred. Most preferred are silicon rib waveguides, in which an elongate rib portion comprising an upper surface and two sides surfaces extends across a slab of silicon. Portions of the silicon slab on each side of the rib portion preferably form part of the waveguide, in addition to the rib portion. An optical confinement layer, preferably formed from silica, preferably provides a lowermost (i. e. lowermost with respect to the rib portion) extremity of the slab of silicon. Below the optical confinement layer there is preferably a substrate, preferably also formed from silicon.

Advantageously, the optical device according to the first aspect of the invention may be used in conjunction with, or may form part of, a wavelength division multiplexer and/or demultiplexer. The multiplexer and/or demultiplexer may comprise a diffraction grating, for example an arrayed waveguide grating, e. g. formed from an array of silicon rib waveguides.

Accordingly, a fourth aspect of the invention provides a wavelength division multiplexer and/or demultiplexer including one or more devices according to the first aspect of the invention.

A fifth aspect of the invention provides an optical signal blocker comprising a wavelength division demultiplexer, a device according to the first aspect of the invention, and a wavelength division multiplexer.

Where the device according to the invention forms part of a multiplexer and/or demultiplexer and/or blocker, it preferably comprises one or more input waveguides and/or one or more output waveguides optically coupled to an array of waveguides at opposite ends, respectively, of the array, the input and/or output waveguides comprising the optical signal attenuator and the optical signal modulator optically coupled with each other in series.

Preferably, the input and/or output waveguides are optically coupled to the array of waveguides by means of a free space region, such that each input and/or output waveguide is freely optically coupled to each waveguide of the array. The input and output waveguides preferably also comprise semiconductor waveguides, for example silicon rib waveguides. The input waveguide (s), arrayed waveguides and output waveguide (s) may be integrated on a single optical chip, for example.

The, or each, optical signal attenuator and/or the, or each, optical signal modulator of the device according to the invention preferably comprises a p-i-n diode. For example, the, or each, waveguide providing such an attenuator and modulator preferably further comprises a p-doped region on one side of the waveguide and an n-doped region on an opposite side of the waveguide, with a lightly doped or intrinsic region (the"i"region) between the n-and p-doped regions. For example, if the, or each waveguide providing such an attenuator and modulator comprises a rib waveguide, preferably the n-and p-doped regions are located on opposite sides of the rib portion of the waveguide. Preferably each p-i-n diode includes electrical conductors (e. g. metallic conductors) by which an electrical potential may be applied across the diode, thereby to inject free charge carriers into the region between the doped regions.

Advantageously, the, or each, p-i-n diode of the device according to the invention may comprise a diode as disclosed in United States Patent No.

5,908, 305 and/or UK Patent Application No. GB 0104384.3, the entire disclosures of which are incorporated herein by reference.

In designing the attenuator, the main design considerations are generally that it can achieve high attenuation for low current and power dissipation. In order to achieve this, the attenuator may comprise a plurality of p-i-n diodes configured in series, for example, two, three, four, five or more (but typically four) p-i-n diodes arranged in series. This generally allows the current to be lowered at the expense of increased drive voltage. Preferably, the attenuator is made as long as possible given the constraints of semiconductor chip dimensions. This is advantageous because the attenuator is more power efficient at lower free charge carrier concentrations, and the free carrier concentration required for a given attenuation is minimised by making the attenuator as long as possible. Often, the attenuator will have an active length of at least 10 mm, and even as long as 15 mm or longer if there is available space on the semiconductor chip.

The, or each, optical signal attenuator of the device controls the attenuation of an optical signal propagated through it, and preferably reduces the amplitude of the signal to a set level for an extended period of time. It was mentioned above that the attenuation provided by the attenuator is substantially static in comparison to the modulation provided by the modulator. This is not to say that the attenuation provided by the attenuator is necessarily completely static (although this is a possibility), merely that in comparison to the frequency of modulation provided by the modulator, the attenuation provided by the attenuator varies so infrequently as to be effectively substantially static.

For example, the attenuator may vary the attenuation of the optical signal every few seconds or minutes (or longer), as required. For example, for embodiments of the invention in which there is a plurality of waveguides including such attenuators, e. g. if the device comprises a waveguide array,

each attenuator may attenuate the optical signal propagated through it such that the optical signal power is balanced (or otherwise distributed) between the plurality of waveguides, and such power balancing may need to be carried out every few seconds or minutes, or longer.

The attenuator control means of the, or each, attenuator, preferably comprises control electronics. Preferably the, or each, attenuator is computer controlled, and the attenuator control means further comprises computer software.

The second optical signal which is superimposed on the first signal by modulation of the first signal preferably comprises a tone signal. The tone signal preferably has a frequency of at least 100 Hz, more preferably at least 1 kHz, even more preferably at least 50 kHz, especially at least 100 kHz.

Preferably the frequency of the tone signal is no greater than 200 MHz, more preferably no greater than 100 MHz, even more preferably no greater than 10 MHz, especially no greater than 1 MHz.

Preferably the second optical signal comprises a sine wave, a square wave, or substantially any other periodic waveform.

The modulator therefore is used preferably to superimpose a set frequency (preferably of 100 KHz or greater) amplitude modulation, preferably of fixed magnitude onto the optical signal. It is generally desirable for the modulation rate to be made as high as possible to allow greatest flexibility of application. In order to attain high modulation rates the modulation diode is normally best designed as a single diode device of short length. Device capacitance increases with device length and may become a speed limiting factor in very long devices. In order to achieve the highest modulation rates then it is generally necessary to reduce the separation of the p and n doped regions of the p-i-n diode as much as possible. The limit to the decrease of their separation is that the static doping of the p and n regions begins to

impinge upon the optical mode and leads to excess optical loss. If high modulation rate is essential then a certain amount of static loss from the doping might be tolerated to achieve fast operation. However, in this case the active length of the modulation diode would need to be kept relatively short so that the optical loss did not become unacceptably large.

Thus the modulation diode is preferably a single-stage device (i. e. a single p-i-n diode) with a length preferably of 500 um or less, in comparison to the greater than 10 mm length preferred for the attenuator. Also, the modulation diode preferably has a smaller dopant separation than the attenuator.

The control means of the, or each, optical signal modulator therefore controls the modulator preferably such that it superimposes a tone signal, preferably of a frequency as described above, on the optical signal propagated through that modulator. Advantageously, for embodiments of the invention comprising a plurality of waveguides, each of which comprises an attenuator and a modulator, each modulator may superimpose a tone signal on the optical signal propagated through it, that has a frequency which is different to the tone superimposed by the, or each, other modulator of the device. In this way, the tones applied to the signals propagated through the device may be used to identify the signal propagated through each waveguide individually, for example.

The modulator control means of the, or each, modulator, preferably comprises control electronics. Preferably the, or each, modulator is computer controlled, and the modulator control means further comprises computer software.

Preferably the, or each, optical attenuator is thermally isolated from the optical modulator with which it is optically coupled.

The invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram of an embodiment of the invention comprising a waveguide which comprises an optical signal attenuator and an optical signal modulator optically coupled with each other in series; Figure 2 (views a and b) are graphs showing the attenuation or modulation of an optical signal against the drive current for a p-i-n diode used as an attenuator or modulator in accordance with an embodiment of the invention; and Figure 3 is a schematic cross-sectional diagram of a p-I-n diode of an embodiment of the invention.

Figure 1 shows, schematically, an optical waveguide 1 (for example a silicon rib waveguide) comprising an optical signal attenuator 3 and an optical signal modulator 5 optically coupled with each other in series along the waveguide 1. The attenuator 3 and the modulator 5 are schematically represented by electrical conductors 7 which overlay doped regions indicated by reference numerals 9 and 11. The attenuator 3 comprises a series of four pairs of doped regions 9 arranged in series along the waveguide 1, the doped regions of each pair being arranged on opposite sides of the waveguide 1. The attenuator 3 may be substantially as disclosed in UK Patent Application No. GBOO19771. 5, the entire disclosure of which is incorporated herein by reference. The modulator 5 comprises a single pair of doped regions 11, arranged on opposite sides of the waveguide 1. The attenuator 3 is formed from a greater number of doped regions than is the modulator 5, since the attenuator is required to provide a greater degree of attenuation to an optical signal propagated by the waveguide 1 than is the modulator 5.

Figure 2a and Figure 2b are graphs showing the attenuation (in dB) of an optical signal against the drive current (in mA) for a p-i-n diode used as an attenuator in accordance with an embodiment of the invention. The graphs are intended to illustrate the fact that the relationship between the degree of attenuation of an optical signal provided by a p-i-n diode, and the electrical current supplied to the diode to drive the diode (i. e. to inject free charge carriers into the"i"region of the diode thereby to attenuate the optical signal) is generally not a linear relationship. Therefore, (unlike the present invention) if an optical attenuator which causes an attenuation to a first optical signal (i. e. a"background"attenuation, for example to balance the optical power between a plurality of waveguides) were required also to modulate the first optical signal in order to superimpose a second (tone) optical signal on it, the degree of modulation of the drive current required to provide a constant peak-to-peak amplitude modulation of the first optical signal would vary depending upon the background attenuation provided by the attenuator. If the tone signal were to be used to monitor optical power, for example, it would be necessary for it to have a constant peak-to-peak amplitude, and consequently it would be necessary to apply a constant peak-to-peak amplitude modulation to the first optical signal.

As shown by the graphs, for a particular change in the attenuation of the amplitude (or power) of an optical signal provided by the attenuator at a relatively low "background" attenuation, a relatively small modulation of the drive current is required, whereas at higher levels of background attenuation (and hence higher drive currents), in order to produce the same variation in the attenuation of the signal, a relatively large modulation of the drive current is required. For example, in Figure 2a, at a "background" attenuation level of approximately 4 dB (which requires a drive current of approximately 3 mA) a peak-to-peak amplitude modulation of 1 dB requires a modulation of the drive current of approximately 1.5 mA. However, at a background attenuation of approximately 32 dB (requiring a drive current of approximately 93 mA) a modulation of the drive current of approximately 1 dB requires a modulation of

the drive current of approximately 6 mA. (If the modulation of the drive current remained at 1.5 mA, the amplitude modulation of the optical signal-and hence the peak-to-peak amplitude of the applied tone signal-would drop to approximately 0.2 dB. ) It is therefore clear that if a single attenuator were used to provide both the background attenuation to a first optical signal and the supenmposition of a second optical signal on the first signal by providing a constant peak-to-peak amplitude modulation of the first signal, it would be necessary (according to the relationship demonstrated by Figure 2) to vary the peak-to-peak modulation of the drive current depending upon the degree of background attenuation. This would generally be difficult to achieve, and would generally require sophisticated control electronics and software. The present invention overcomes this problem by separating the application of the tone signal from the application of the background modulation.

A further advantage of the invention is that the modulator may be thermally decoupled from the attenuator. Since the attenuation-current characteristics of p-i-n diodes generally show a strong temperature dependence, changes in the background attenuation could introduce local temperature changes in the diode that could cause changes in the amplitude of a tone signal applied by the attenuator. By thermally decoupling the modulator from the attenuator, such temperature dependent modulation changes are substantially avoided.

Figure 3 is a schematic cross-sectional diagram of a p-i-n diode of an embodiment of the invention, which may be used as an optical attenuator or as an optical modulator. The p-i-n diode is formed laterally across a silicon rib waveguide by doping a silicon slab on each side of a rib portion 13 of the waveguide. The rib portion 13 comprises an upper surface 15 and opposite side surfaces 17, the rib portion extending above slab regions 19 of the waveguide on each side thereof. The rib portion 13 and slab regions 19 are

formed in an upper silicon layer 21 above a silica layer 23 which is itself supported on a substrate layer of silicon 25. On each side of the slab regions 19 of the waveguide are trenches 20 in the silicon, below which, are provided doped regions, a p-doped region 27 in one trench and an n-doped region 29 in the opposite trench. Metal conductors 31 lead to the doped regions 27 and 29, to enable a drive current to be applied to the p-i-n diode. A layer of silica 33 is formed over the rib portion 13 and slab regions 19 of the waveguide, and also under the metal conductors 31, in the latter case to provide electrical insulation.

Claims

Claims 1. An optical device comprising an optical signal attenuator including attenuator control means, and an optical signal modulator including modulator control means, the optical signal attenuator and the optical signal modulator optically coupled with each other in series and controlled by their respective control means such that a first optical signal may be attenuated by the attenuator and a second optical signal may be superimposed on the first signal by a modulation of the first signal by the modulator, the attenuation provided by the attenuator being substantially static in comparison to the modulation provided by the modulator.
2. The use of an optical device according to Claim 1, to attenuate a first optical signal by the optical signal attenuator, and to superimpose a second optical signal on the first optical signal by a modulation of the first signal by the modulator.
3. A method of superimposing a second optical signal on a first optical signal by means of an optical device according to Claim 1, the method comprising attenuating a first optical signal by the optical signal attenuator, and superimposing a second optical signal on the first optical signal by modulating the first signal by the modulator.
4. A device, use or method according to any preceding claim, in which the device further comprises a waveguide, the waveguide comprising the optical signal attenuator and the optical signal modulator optically coupled with each other in series.
5. A device, use or method according to Claim 4, in which the device comprises a plurality of waveguides, each of which comprises an optical signal attenuator and an optical signal modulator optically coupled with each other in series and arranged such that a first optical signal propagated by a

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said waveguide may be attenuated by the attenuator of that waveguide and a second optical signal may be superimposed on the first signal by modulation of the first signal by the modulator of that waveguide.
6. A wavelength division multiplexer and/or demultiplexer including one or more devices according to any preceding claim.
7. An optical signal blocker comprising a wavelength division demultiplexer, a device according to any preceding claim, and a wavelength division multiplexer.
8. A device, use or method according to any preceding claim, in which the second optical signal is a tone signal.
9. A device, use or method according to any preceding claim, in which the second optical signal has a frequency of at least 100 Hz, preferably at least 1 kHz.
10. A device, use or method according to any preceding claim, in which the second optical signal has a substantially constant peak-to-peak amplitude.
11. A device, use or method according to any preceding claim, in which the attenuator varies the attenuation of the optical signal no more frequently than once a second, preferably no more frequently than once a minute.
12. A device, use or method according to any preceding claim, in which the, or each, optical signal attenuator and the, or each, optical signal modulator each comprise p-i-n diodes.
13. A device, use or method according to Claim 12, in which the attenuator comprises a plurality of p-i-n diodes arranged in series.

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14. A device, use or method according to Claim 12 or Claim 13, in which the attenuator has a length of at least 10 mm.
15. A device, use or method according to any one of claims 12 to 14, in which the modulator comprises a single p-i-n diode.
16. A device, use or method according to any one of claims 12 to 15, in which the modulator has a length no greater than 500 Jlm.
17. A device, use or method according to Claim 4 or any claim dependent thereon, in which the, or each, waveguide comprises a silicon rib waveguide.