WO2011133012A1

WO2011133012A1 - All optical multiband frequencies generation

Info

Publication number: WO2011133012A1
Application number: PCT/MY2010/000063
Authority: WO
Inventors: Mohd Adzir Mahdi; Yu Gang Shee; Norhakimah Md Samsuri; Mohamed Razman Yahya; Romli Mohamad; Muhammad Zamzuri Abdul Kadir; Abdullah Man; Syamsuri Yaakob
Original assignee: Telekom Malaysia Berhad
Priority date: 2010-04-23
Filing date: 2010-04-23
Publication date: 2011-10-27

Abstract

A device (10) is disclosed. The device (10) has an optical loop (12) configured to circulate light in a preferred direction (18) around the loop (12). The optical loop (12) has a Brillouin gain medium (14). The optical loop (12) also has an input (101) for introducing pump light (20) into the loop (12) such that the pump light (20) travels against the preferred direction (18) and through the Brillouin gain medium (14) causing the Brillouin gain medium (14) to provide a Brillouin gain. The device (10) also has an output (104) for outputting at least some of a light generated using the Brillouin gain.

Description

ALL OPTICAL MULTIBAND FREQUENCIES GENERATION

Field of the Invention The present invention generally relates to a device for generating light using Brillouin gain, and

particularly but not exclusively to a device for

generating electrical tones having GHz frequencies from light generated using Brillouin gain.

Background of the Invention

Stable millimeter wave and micrometer wave

(microwave) electrical sources are used in communications and many other applications. For example, in

communications applications an analogue electrical signal from a local oscillator may used as a carrier wave.

Alternatively, to recover a signal on the carrier

heterodyning may be required. Heterodyning produces sum and difference frequencies of the frequency of the local oscillator and frequency of the input signal of interest.

There is demand for higher data rates which is driving electrical frequencies upward. In some ·

circumstances it is difficult or too expensive to build the high frequency signals required to support higher data rates using standard techniques and apparatus, especially beyond a few GHz . Summary of Invention

According to one aspect of the invention there is provided a device comprising:

an optical loop configured to circulate light in a preferred direction around the loop, the optical loop comprising :

a Brillouin gain medium; an input for introducing pump light into the loop such that the pump light travels against the

preferred direction and through the Brillouin gain medium causing the Brillouin gain medium to provide a Brillouin gain; and

an output for outputting at least some of a light generated using the Brillouin gain.

In an embodiment, the output is further configured to selectively remove the generated light that is traveling against the preferred direction over that traveling in the preferred direction.

In an embodiment, the device comprises a photo- detector in optical communication with the output.

In an embodiment the device further comprises another optical loop also having the Brillouin gain medium, the other optical loop being configured to circulate light against the preferred direction. The other optical loop may comprise an optical amplifier. The other optical loop may comprise another input configured to introduce the pump light such that the pump light travels against the preferred direction. The other optical loop may comprise another output configured to remove at least some of the light generated using the Brillouin gain.

In an embodiment, the optical loop comprises a circulator which defines the preferred direction. The input and output may each comprise a respective port of the circulator. The input may comprise port n of the circulator. The output may comprise port n+3 of the circulator, n is a positive integer.

In an embodiment, the device further comprises an optical filter in optical communication with the photo- detector and the output. The optical filter may be configured to optically filter any light from the output before it is detected by the photo-detector. The filter may be configured to pass the pump light. The filter may be configured to block a portion of the light generated using the Brillouin gain. The filter may comprise an optical inter-leaver. The filter may comprise a Bragg grating .

In an embodiment, the loop comprises an optical fibre. The optical fibre may be adapted to provide significant Brillouin gain.

In an embodiment, the device comprises a light source configured to generate the pump light. The light source may comprise a laser source .

In an embodiment, the optical loop is configured to circulate light only in the preferred direction around the loop .

In an embodiment, the output is for outputting at least some of a light generated using the Brillouin gain from the optical loop.

In an embodiment; the device comprises a 20 GHz- spaced multi-wavelength fiber laser arranged for the circulation of a stokes signal through a 4 -port

circulator.

In an embodiment, the device comprises a wavelength de-multiplexer which produces dual -wavelength outputs for multiband radio- frequency (RF) generation.

According to a second aspect of the invention, there is provided a laser system comprising:

a Brillouin gain medium; and an optical source operationally coupled to the gain medium and configured to pump the Brillouin gain medium providing gain generating a light that predominantly comprises one or more even stokes lights.

In an embodiment, the laser system comprises first and second coupled resonators, the resonators having the Brillouin gain medium in common, and configured such that even stokes light resonates in the second resonator and one or more odd stokes lights resonate in the first resonator. One or both of the resonators may be a ring resonator.

In an embodiment, the second resonator comprises an output coupler from which the even stokes lights are output .

According to a third aspect of the invention there is provided an electrical signal generator comprising:

the laser system according to the second aspect of the invention; and

a photo-detector in optical communication with the laser .

Brief description of the Figures

In order to achieve a better understanding of the nature of the present invention embodiments will now be described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows a schematic diagram of one embodiment of a device according to one aspect of the present invention;

Figure 2 shows another embodiment of a device according to the one aspect of the present invention;

Figure 3 shows an example optical spectrum of the light leaving an output port of the device of figure 2. Figure 4 shows an example transmission spectrum for each of outputs 1 and 2 of a 50/100 GHz optical inter- leaver used in the embodiment in figure 2 ;

Figure 5 shows measured reflection and transmission spectrum of the example fibre Bragg gratings used in the embodiment in figure 2;

Figure 6 shows a measured transmission band of a wavelength de-multiplexer used in the embodiment in figure 2;

Figure 7 show example spectra of signals at the outputs of the wavelength de-multiplexer as amplified by an optional optical amplifier; and

Figures 8 and 9 show example spectra of respective electrical signals generated by the embodiment in figure 2.

Detailed Description of embodiments of the invention

Figure 1 shows a schematic diagram of one embodiment of a device according to one aspect of the present

invention generally indicated by the numeral 10. Device 10 is electro-optic, that is, has electrical and optical components, and is configured to generate electrical tones having GHz frequencies, for example around 20 GHz and integral multiples of 20GHz. This embodiment conveniently uses optical fibre and optical components that are

connected by lengths of optical fibre. However, it will be appreciated that other embodiments of the invention may comprise, instead of some or all of the optical fibre, one or more optical structures such as mirrors (for directing optical beams traveling though space) , crystals, pieces of glass and lenses.

The device 10 has an optical loop 12. The loop comprises an optical fibre 14 and an optical component 16 that has at least one preferred direction for the

propagation of light therethrough. The ends of the optical fibre 14 are in optical communication with the component 16.

In this embodiment, the component 16 is an optical circulator. The optical circulator of this embodiment has four ports: port 1 indicated by numeral 101; port 2 indicated by numeral 102; port 3 indicated by numeral 103; and port 4 indicated by numeral 104. Light entering one of the ports exits the next sequentially numbered port. For example, light entering port 1 will leave port 2. Light entering port n will leave port n + 1, where n is a positive integer. The exception to this may be for light entering the last port (4 in this case) which, for some circulators, is blocked. Other embodiments may use a circulator having more or less than 4 ports (such as 3 or 5 ports), or an optical isolator which has only 2 ports.

The optical fiber 14 is connected to ports two and three of the circulator 16. The circulator 16 defines a preferred direction, indicated by the arrow 18, that the light travels around the loop 12. Light traveling in the optical fibre 14 in the preferred direction 18 enters port 2 indicated by numeral 102, exits port 3 indicated by numeral 103, and is launched back into the optical fibre 14. Light can not travel in the other direction through the circulator, which directs light entering one of its ports to the next sequentially numbered port. Thus light traveling in the fibre 14 in the direction opposite the preferred direction enters port 3 and exits port 4, not port 2.

Light in the loop 12 travelling in the preferred direction repeatedly circulates around the optical loop 12. Light that enters port 102 of the circulator 16 goes around the loop 12 and comes back to port 102, and may then go around the loop again. The light moves through the loop and returns to the starting point, which may be arbitrarily taken to be port 102. The loop 12 may be described as a closed optical loop or optical circuit, or even an optical resonator. The optical loop 12 comprises a Brillouin gain medium, which in this embodiment is in the form of the optical fibre 14.

Stimulated Brillouin scattering, the process that gives Brillouin gain, is a phenomenon in which a photon is destroyed and another photon is generated, of slightly lower frequency, together with a phonon (which is an acoustic wave) . The process is stimulated by yet another photon of the same frequency as the generated photon. The process is only stimulated by a photon traveling in the direction opposite that for which the original photon is traveling. The process may occur in solid media such as glass - which constitutes an optical fibre - which may be described as a Brillouin gain medium.

In this embodiment the optical fibre 14 is adapted to provide significant Brillouin gain. That is, the fiber has certain parameters that are favorable for the provision of Brillouin gain. The optical fibre 14 is dispersion compensating fibre (DCF) which has, relative to standard fibre, a very small core area in which light is

concentrated and thus providing a strong interaction between the fiber and the light traveling through it.

That is, the parameter of the optical fibre core diameter is less than 5 micrometers. The fibre is also greater than one kilometer long, which is advantageous because Brillouin gain increases exponentially with fibre length. That is, the parameter of fibre length is more than lKm. This embodiment uses a 15 km length of DCF fibre so that significant Brillouin gain is supported at optical pump powers in the range of around 10-100 mW. This fibre is adapted to provide significant Brillouin gain at relatively low pump powers, eliminating the need for a powerful pump source .

In another embodiment, optical loop 12 comprises a Brillouin gain medium in the form of a Brillouin gain medium inserted into the optical fibre 14 (with the optical fibre not being especially adapted to provide significant optical gain) . In another embodiment, the Brillouin gain medium is only part of the optical fiber 14 and may be spliced to fiber pigtails that are attached, for example, to respective ports. The Brillouin gain medium may alternatively comprise a planer waveguide, or a crystal that may provide Brillouin gain. The gain medium in alternative embodiments may have fibre pigtails coupled at its input and output. Generally, any suitable

Brillouin gain medium may be used provided that the resulting Brillouin gain exceeds the optical losses of the loop 12. Device 10 also includes a laser source 20 and an optical fibre 24; optical fibre 24 couples the output of laser source 20 to port one of the circulator 16. Port one of the circulator thus functions as an input for introducing pump light generated by laser source 20 into the optical fibre 14 of loop 12. Port one may be

described as an input coupler. The pump light travels from the laser source 20 to the loop 12 along optical fiber 24 connecting the laser and port one . The laser source 20, in this embodiment, is an external cavity laser source which emits light having a frequency band of typically less than 1 MHz, usually around 150 kHz, and an optical power of around 1-10 mW. The output has a

wavelength in the C-band, for example, such as 1555nm. Any suitable alternative light source may be used such as a distributed feedback (DFB) laser, based around for example either a diode or an optical fibre doped with a rare earth such as erbium. An optical amplifier may be used in conjunction with the laser source 20 to increase the pump light power to 10-100 mW and thus increase the Brillouin gain. The pump light is launched into port one and leaves port two, traveling against the preferred direction through the optical fibre 14 causing the optical fibre 14 to provide a Brillouin gain. In this embodiment, the Brillouin gain peak is around 10 GHz below that of the pump light frequency. When the optical fibre 14 provides a Brillouin gain, there is spontaneous emission of a first stokes light at a centre frequency coincident with the peak Brillouin gain frequency. The first stokes light is emitted in the direction opposite to that of the pump in the preferred direction. The Brillouin gain for the first stokes light is in the preferred direction 18 only because Brillouin gain is inherently only for light travelling in the direction opposite the direction the pump is travelling, at least in an optical fibre. The first stokes light travels along the fibre 12 in the preferred direction 18, enters the circulator by port two indicated by numeral 102, leaves the circulator by port three indicated by the numeral 103 and is directed again along the fibre 14 in the preferred direction. That is, the first stokes light circulates in the preferred direction 18 around the loop 12. The first stokes light is amplified by the Brillouin gain each time it goes around the loop 12 in the preferred direction 18. The first stokes light is substantially confined to the loop 12.

Once the first stokes light becomes sufficiently intense it may itself act as a pump light. A second stokes light is thus generated in an analogous manner as for the first stokes, however it has a centre frequency approximately 10 GHz below that of the first stokes light or 20 GHz below that of the pump light. The second stokes light travels in the direction opposite to the preferred direction 18.

The optical device 10 is configured to selectively remove the light traveling against the preferred direction 18 over light that is traveling in the preferred direction 18. In this embodiment, the pump light from the laser source 20, having traveled through the fibre 12, and the second stokes do not circulate the loop 12. They pass from the fibre 14 into port three, and then out of port four. Port four is an output for the loop 12. It may be described as an output coupler.

The pump and second stokes light from the output 104 of the loop 12 travel along an optical fiber 26, which may be a patch cord, to a photo-detector 22. The photo detector comprises a photodiode fast enough to detect the beating between the pump and the second stokes

frequencies, at around 20 GHz, and convert this to a corresponding electrical signal at the beat frequency.

The photo detector may, in some embodiments, comprise an optical filter to filter the light leaving the output 104 of the loop. In another embodiment however, the second stokes is mixed with pump light from the laser source 20 that has not traveled through the loop 12, but rather pump light sourced from an optical tap or splitter placed after the laser source 20 but before the loop 12, for example.

Other sources of pump light may be employed as

appropriate, or even light from another laser.

Another embodiment of an optical device is shown in figure 2 and is generally indicated by numeral 40. Parts in figure 2 that are similar to parts in figure 1 are similarly numbered. In addition to the optical loop 12, the embodiment of figure 2 also has another optical loop 42 sharing the Brillouin gain medium 14, and circulator 16.

The other optical loop 42, however, is configured to circulate light in a direction opposite that of the preferred direction. Pump light from the laser source 20 is input into an input port 44 of an optical coupler 46. An example of a suitable optical coupler is the Thorlabs 10202A-90, a 2x2 single mode coupler for the 1550 nm band, having a 90:10 split ratio. In an alternative embodiment, a 99:1 split ratio is used. The light from the 90% port (or 99:1 in the alternative embodiment) then passes through an optical amplifier 48, such as, for example, a C-band erbium doped fiber amplifier (EDFA) 48 to amplify the pump light. In this embodiment the EDFA is pumped by a 1480 nm laser diode, although a 980 nm pump diode is also satisfactory in many situations. Any suitable optical amplifier may be used, such as a S or L band EDFA, semiconductor amplifier, Raman amplifier or even an amplifier operating in another optical communications window such as around wavelengths of 1.3 microns, provided the pump is amplified. The pump light then is input into port 1 of the circulator, out of port 2 and then around the optical fibre 14 which subsequently exhibits Brillouin gain. A net gain is experienced by a stimulating light provided that the Brillouin gain exceeds the optical losses around the loop 12. The pump light then enters port 3 of the circulator 16 and leaves port 4 of the circulator 16 and is coupled into another optical fibre 46 connected at its far end to another input port 50 of the optical coupler 46. Ten percent (or 99% in the alternative embodiment) of the pump light that entered the input port 50 then leaves the coupler output port 52. Having reached the output coupler port 52 for a second time the pump light has completed a circuit of the second loop, the pump light having traveled in the direction opposite that of the preferred direction 18. The light may then go around the other loop 42 again.

The first stokes light circulates around the first optical loop 12 in the preferred direction 18. It is substantially confined to the first optical loop 12. At least some of the second stokes light, however, circulates around the other optical loop 42 in the direction opposite the preferred direction. It is, at least in this

embodiment, amplified by the optical amplifier 48 each time it makes a circuit of the other optical loop 42. The second stokes light, may be intense enough to act as a pump for a third stokes light which may circulate around the first optical loop 12 in the preferred direction 18, especially if the second stokes is powerful enough to result in net gain for the third stokes . The third stokes light may in turn act as a pump for the fourth stokes light. The generation of even higher order stokes lights may occur if the lower order stokes lights are

sufficiently intense. The number of stokes can be

controlled by adjusting the pump power of the EDFA, the coupling ratios, the configuration of the optical fiber and other parameters, for example. The odd numbered stokes lights circulate in the first loop 12 and the even numbered stokes lights circulate in the other loop 42.

The optical coupler 46 also has an output port 54 which removes some of the pump and stokes light

circulating around the other optical loop 42.

The device 40 (or 10) may be described as a multi- wavelength Brillouin-Erbium fiber laser.

Figure 3 shows an example optical spectrum of the light leaving the output port 54 of the device 40. The four largest peaks are coherent and separated by around 20 GHz. These peaks represent the even stokes lights. The peaks intermediate the strong peaks represent odd stokes lights that have leaked out of the first loop 12 because of the optical components are not perfect. These peaks are suppressed by around 20 dB, however, relative to the even stokes peaks. This light is passed along an optical fibre 56 and detected at by a photo-detector, in this case comprising a fast optical photodiode, such as 58 or 60, which is configured to detect the beating between the pump light and the stokes light. An example of a suitable photo diode is New Focus model 1014, which has a 3dB bandwidth of 45 GHz, although any suitable photo-detector may be used .

The optical device 40 has optical filters such as 62, 64, and 66 in optical communication with respective photo- detectors and the output 54. The optical filters are configured to optically filter any light from the output before it is detected by the photo-detector. To generate a 60GHz millimeter wave and a 20 GHz microwave, four-wavelengths are generated by the optical device 40. Wavelength de-multiplexing is carried out by the filters 62,64,66 to select the desired optical signals for heterodyning. A wavelength de-multiplexer

incorporating a 50/100 GHz optical inter-leaver (OIL) 62 such as a step-phase Michelson interferometer (used in a reverse direction i.e. functioning as an optical de- interleaver) is used together with 2 fibre Bragg grating (FBG) filters 66 and 64. The transmission spectrum for each of outputs 1 and 2 of the 50/100 GHz optical

interleaver are shown in figure 4. The reflection and transmission spectrum of the fibre Bragg gratings are shown in figure 5. The transmission band of the

wavelength de-multiplexer when the OIL 62 and FBG filters are combined is shown in figure 6. The fiber Bragg grating filters are placed at the output of the interleaver to further suppress undesired wavelengths and clean up the optical signal for a cleaner electrical signal. An optical 20-GHz spaced dual-wavelength signal is obtained at output 1 of the inter-leaver 62 and it is further filtered by the FBG filter 66. A 3 -port circulator is used to output the reflected signals from the fiber grating. At output 2 of the inter-leaver 62, a 60 GHz spaced dual wavelength signal is filtered by FBG filter 64. An optical isolator is placed before the Bragg grating to block any reflected signals from going back into the inter-leaver 62. Figure 7 show example spectra of signals at the outputs of the wavelength de-multiplexer and amplified by an optional optical amplifier (not shown) before being heterodyned at the photodiodes. It may, in some circumstances, be difficult to filter a dense wavelength spacing of 10GHz signals because sharp and narrow-band filters having around 10 GHz bandwidth are not commercially available, to the best of the applicants knowledge. A 20-GHz optical signal (0.16 nm) is easier to filter, which may be a distinct advantage of this

embodiment 40 in some circumstances where price and practicality is important.

Figures 8 and 9 show example spectra of respective electrical signals around 20 GHz and 60 GHz generated by the two photodiodes by heterodyning the generated and filtered optical signals. The spectra were measured using electrical spectrum analyzers 68,70. Now that embodiments have been described, it will be appreciated that some embodiments have some of the

following advantages:

a high frequency (GHz) electrical signal generator is not required, which may be relatively very expensive or difficult or even impossible to achieve using conventional techniques ; microwave and millimeter wave electrical signals can be generated using the described opto-electronic

techniques more easily than pure electrical techniques; very fast optical signals, having bandwidths of multiples of 10 or 20 GHz can be generated, with the subsequent generation of respective electronic signals; direct modulation of an optical signal is not required;

a stable mm-wave source is provided, resulting from the Brillouin gain process;

Some variations on the specific embodiments include: the Brillouin gain medium may comprise a chalcogenide planar waveguide;

the Brillouin gain medium may comprise a planar waveguide ;

the Brillouin gain material may be chosen to give a particular stokes frequency shift that may not be around 10 GHz, depending on the application;

the circulator 16 may be replaced with an optical isolator placed between a pair of optical couplers;

the filters may comprise one or more thin-film filters ;

the EDFA may be replace by a praseodymium optical amplifier operating at wavelengths around 1.3 microns or a semiconductor amplifier operating at any suitable

wavelength;

the device may be configured to operate at another optical band;

the circulator 16 may have more than four ports; and the fiber 14 may be a highly nonlinear fiber.

It will be appreciated that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims

A device (10) comprising:

an optical loop (12) configured to circulate light in a preferred direction (18) around the loop (12), the optical loop (12) comprising:

a Brillouin gain medium (14);

an input (101) for introducing pump light into the loop (12) such that the pump light travels against the preferred direction and through the Brillouin gain medium (14) causing the Brillouin gain medium to provide a Brillouin gain; and

an output (104) for outputting at least some of a light generated using the Brillouin gain.

A device defined by claim 1 wherein the output (104) is further configured to selectively remove the generated light that is traveling against the preferred direction (18) over that traveling in the preferred direction.

A device defined by any one of the preceding claims comprising a photo-detector (22) in optical

communication with the output (104).

A device (10) defined by any one of the preceding claims further comprising another optical loop (42) also having the Brillouin gain medium (14), the other optical loop being configured to circulate light against the preferred direction.

A device defined by claim 4 wherein the other optical loop (42) comprises an optical amplifier.

6. A device defined by either one of the claims 4 or 5 wherein the other optical loop (42) comprises another input (44) configured to introduce the pump light such that the pump light travels against the

preferred direction.

7. A device defined by any one of the claims 4 to 6

wherein the other optical loop (42) comprises another output (54) configured to remove at least some of the light generated using the Brillouin gain.

8. A device defined by any one of the preceding claims wherein the optical loop (12) comprises a circulator (16) which defines the preferred direction.

9. A device defined by claim 8 wherein the input and

output each comprise a respective port of the

circulator (16)

10. A device defined by either one of claim 8 or claim 9, wherein the input comprises port n of the circulator (18) and the output comprises port n+3 of the

circulator (18) , where n is a positive integer.

11. A device defined by claim 3 further comprising an

optical filter (62) in optical communication with the photo-detector and the output, and configured to optically filter any light from the output before it is detected by the photo-detector.

12. A device defined by claim 11 wherein the filter (62) is configured to pass the pump light.

13. A device defined by claim 12 wherein the filter (62) is configured to block a portion of the light

generated using the Brillouin gain.

14. A device defined by any one of the previous claims wherein the loop (12) comprises an optical fibre (14) .

15. A device defined by claim 14 wherein the optical fibre (14) is adapted to provide significant

Brillouin gain.

16. A device defined by any one of the preceding claims comprising a light source (20) configured to generate the pump light.

17. A device defined by either one of the claim 11 or 12 wherein the filter (62) comprises an optical inter- leaver (62) .

18. A device defined by either one of the claims 11, 12 and 17 wherein the filter (62) comprises a Bragg grating (66) .

19. A device defined by any one of the previous claims wherein the optical loop (12) is configured to circulate light only in the preferred direction (18) around the loop.

20. A device defined by any one of the previous claims wherein the output (104) is for outputting at least some of a light generated using the Brillouin gain from the optical loop (12) .

21. A device defined by any one of the previous claims wherein the device (10) comprises a 20 GHz-spaced multi-wavelength fiber laser arranged for the

circulation of a stokes signal through a 4 -port circulator.

22. A device defined by any one of the previous claims wherein the device (10) comprises a wavelength de- multiplexer which produces dual-wavelength outputs for multiband radio-frequency (RF) generation.

23. A laser system (42) comprising:

a Brillouin gain medium (14) ; and

an optical source (20) operationally coupled to the gain medium and configured to pump the Brillouin gain medium providing gain generating a light that predominantly comprises one or more even stokes lights .

24. A laser system defined by claim 23 comprising first

(12) and second (46) coupled resonators, the

resonators having the Brillouin gain medium (14) in common, and configured such that even stokes light resonates in the second resonator (46) and one or more odd stokes lights resonate in the first

resonator (12) .

25. A laser system defined by claim 24 wherein the second resonator (46) comprises an output coupler (46) from which the even stokes lights are output.

26. An electrical signal generator (40) comprising:

the laser (42) defined by any one of claims 23 to 25; and

a photo-detector (60) in optical communication with the laser.

27. A device, laser or electrical signal generator

substantially as described herein with reference to the accompanying figures.