WO2006123163A1 - Method to generate and detect terahertz radiation - Google Patents

Method to generate and detect terahertz radiation Download PDF

Info

Publication number
WO2006123163A1
WO2006123163A1 PCT/GB2006/001848 GB2006001848W WO2006123163A1 WO 2006123163 A1 WO2006123163 A1 WO 2006123163A1 GB 2006001848 W GB2006001848 W GB 2006001848W WO 2006123163 A1 WO2006123163 A1 WO 2006123163A1
Authority
WO
WIPO (PCT)
Prior art keywords
laser
radiation
terahertz
light
frequency
Prior art date
Application number
PCT/GB2006/001848
Other languages
French (fr)
Inventor
David Graham Moodie
David William Smith
Original Assignee
The Centre For Integrated Photonics Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by The Centre For Integrated Photonics Limited filed Critical The Centre For Integrated Photonics Limited
Publication of WO2006123163A1 publication Critical patent/WO2006123163A1/en

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation

Definitions

  • This invention relates to the generation and/or detection of radiation in the terahertz and near-terahertz frequency spectrum, i.e. the range 80 GHz to 4THz.
  • this invention provides a system for generating and detecting terahertz radiation.
  • the system comprises a first laser and a second laser, each having a different frequency of laser light output, a radiation generating stage and a radiation detecting stage.
  • the radiation generating stage is arranged to generate terahertz radiation by beating the light output of the first laser with the light output of the second laser.
  • the radiation detecting stage is arranged to modulate light output from the first laser with received terahertz radiation.
  • a method for the generation and detection of RF and terahertz frequencies is disclosed.
  • the method proposed is particularly attractive for applications where the apparatus used to generate the radiation is near to the apparatus used to detect the radiation, as is often the case in certain types of measurement equipment.
  • the radiation generating stage may comprise a first optical coupler arranged to couple light from the first laser and light from the second laser to generate a beat frequency in the terahertz range.
  • the radiation generating stage may comprise a first photodetector coupled to a first antenna, the first photodetector being arranged to receive light from the optical coupler and generate radiation from the first antenna at the beat frequency.
  • RF, mm- wave or terahertz radiation may be generated by combining the output of two single frequency lasers and photomixing within a wideband photodetector.
  • the output of the photodetector is connected to an antenna designed for the required frequency band. Between the generator and receiver a sample whose propagation characteristics are to be measured can be placed.
  • the first and/or the second laser may be tuneable.
  • One of the lasers can be fixed frequency and one can be tuneable. Adjustment of the tuneable laser allows the beat frequency and hence frequency of the radiation to be set.
  • the frequency of the tuneable laser may be varied with time, in use, to generate a swept source of terahertz radiation.
  • the radiation detecting stage may comprise an optical modulator coupled to a second antenna and arranged to modulate light from the first laser with the signal received by the second antenna. This arrangement is different from the prior art in that an optical modulator is used to translate the received terahertz signal onto an optical carrier which can then be subsequently detected using an optical self-heterodyne scheme.
  • the receiver antenna may be connected to the electrodes of an optical modulator which is used to impose the received RF, mm-wave or terahertz signals onto an optical carrier derived from one of the lasers in the radiation generation apparatus
  • the radiation detecting stage may comprise a second optical coupler arranged to couple the modulated light with light from the second laser.
  • the radiation detecting stage may comprise a filter system for selecting particular frequency components of the output of the second optical coupler.
  • the filter system may be optical, but in the preferred embodiment is an electrical filter system.
  • the radiation detecting stage may comprise a second photodetector arranged to generate an electrical signal representative of at least some of the frequency components of the light from the optical coupler.
  • the radiation detecting stage may further comprise an electrical filter.
  • the combined optical output may be converted into an electrical • signal using a balanced photodiode receiver.
  • the output of the photodiode receiver may be connected through a filter of appropriate bandwidth to achieve the desired signal to noise ratio.
  • the radiation detecting stage may comprise a frequency shifter in the optical path between the first laser or the second laser and the second optical coupler.
  • the modulated optical signal may thus be combined with a frequency shifted version of the output from the second laser within the radiation generation apparatus.
  • the system may comprise a plurality of radiation generating stages and/or a plurality of radiation detecting stages, each supplied with light from the first and/or second lasers.
  • the invention extends to a radiation generating stage adapted for use in the system described and to a radiation detecting stage adapted for use in the system.
  • the invention provides a method for generating and detecting terahertz and near terahertz radiation using a shared pair of lasers or a laser with dual output frequencies to generate the beat frequencies required at the transmitter and within the receiver.
  • the beat signal between two lasers may be photo-detected and used to generate terahertz or near terahertz radiation.
  • the photodetector used to generate the terahertz or near terahertz signal may be a uni-travelling carrier photodiode.
  • One of the two lasers may also be used to provide an optical carrier which can be modulated by the received terahertz radiation.
  • the modulator used to impose the received terahertz or near terahertz on an optical carrier may be an electroabsorption modulator.
  • the modulator used to impose the received terahertz or near terahertz signal on an optical carrier may be a Lithium Niobate phase modulator or Mach Zehnder intensity modulator.
  • the optical output from the optical modulator connected to the antenna receiving terahertz or near terahertz radiation may be combined with light from the second of the two lasers prior to photodetection.
  • a frequency shifter may be placed between one of the lasers and the optical combiner prior to photodetection.
  • One or both lasers may be tuneable.
  • the lasers may be DFB or DBR semiconductor lasers.
  • the frequency of the tuneable laser may be varied with time to generate a swept source of terahertz or near terahertz radiation.
  • a single pair of lasers may be used to provide beat signals to multiple terahertz generators and/or detectors.
  • Optical amplifiers may also be included to increase the output power available from either laser or from both lasers. This arrangement would be especially desirable where an array of terahertz generators or receivers are required.
  • the optical amplifiers could be semiconductor optical amplifiers or doped erbium fibre laser amplifiers, for example.
  • the terahertz radiation generator can be switched using an external electrical signal or by gating one of the two optical inputs.
  • the apparatus may be configured as a terahertz or near terahertz radar or time domain reflectometer.
  • Figure 1 is a schematic view of an embodiment of the terahertz generation and detection scheme according to an embodiment of the invention.
  • Figure 2 shows the position of spectral components produced by the photomixing and modulation process according to Figure 1.
  • FIG. 1 is a schematic representation of the preferred embodiment of the terahertz generation and detection scheme according to the invention.
  • the system comprises a tuneable laser 1, typically a semiconductor DFB (distributed feedback) or DBR (distributed Bragg reflector) laser and a fixed frequency laser 2, typically a DFB or DBR laser.
  • Lasers 1 and 2 may both be located on the same chip. Light may be guided from one component to the next using optical fibre or optical waveguides.
  • An optical coupler 3 splits the light from laser 1 between the photomixer used for terahertz generation and the optical modulator 12.
  • An optical coupler 10 splits light from laser 2 between the photomixer and a frequency shifter 8.
  • An optical coupler 9 is used to combine signals derived from laser 1 and laser 2 for photomixing in a photodetector 4. Terahertz radiation having passed through a measurement sample 14 is coupled to electrodes on the optical modulator 12. The optical output from the optical modulator 12 is combined with the optical output from the frequency shifter 8 in an optical coupler 11. The output from the optical coupler 11 is connected to balanced photodiodes 5 and a differential amplifier 6. The electrical output from the differential amplifier 6 is filtered by a low pass electrical filter 7 to generate an electrical output at point 13 in Figure 1.
  • Figure 2 shows the position of spectral components produced by the photomixing and modulation process.
  • Figure 2a shows the position of spectral lines for the lasers 1 and 2 close to an optical carrier frequency of about 200 THz.
  • Figure 2b shows the terahertz signal (f2-fl) at the output of the photodetector 4.
  • Figure 2c shows the output of the optical modulator 12, indicating the optical carrier fl and sidebands.
  • Figure 2d shows the combined optical spectra at the output of one arm of optical coupler 11 which includes the addition of a frequency shifted spectral line f2 + f3.
  • the electrical output at 13 is shown in Figure 2e with a spectral line at f3 which is equal to the frequency offset produced by the frequency shifter 8.
  • Figure 1 shows the preferred embodiment for carrying out propagation measurements over a range of terahertz and near terahertz frequencies on a test sample 14.
  • a single frequency semiconductor laser 2, operating either within the visible spectrum or near infra-red spectrum is combined in optical coupler 9 with a signal from a tuneable laser 1 with approximately the same nominal wavelength.
  • Both laser 1 and 2 are typically either semiconductor distributed feedback lasers or distributed Bragg reflector lasers.
  • the combined signal is fed to a wide bandwidth photodetector 4, typically a uni-travelling carrier (UTC) photodiode.
  • UTC uni-travelling carrier
  • the optical spectrum at the input to the photodetector 4 is shown by the two spectral lines in Figure 2a centred around an optical frequency of 200 THz 5 corresponding to the typical wavelength chosen for the operation of the apparatus.
  • the electrical output signal from photodetector 4 is at the difference frequency (f2 - fl) between laser 1 and laser 2 and can be adjusted to produce the desired terahertz frequency by tuning the optical frequency fl of laser 1. This is shown by the spectral line in Figure 2b at terahertz or near terahertz frequency.
  • the output from photodetector 4 is coupled by a stripline or waveguide structure to an antenna 15.
  • the terahertz signal is then radiated at the test sample 14.
  • the signal collected from the sample 14 is collected on a receiver antenna which may be of similar construction to the transmitting antenna 15.
  • the signal received by the antenna may be the result of either transmission or scattering by the sample depending on the relative positioning of the two antennas with respect to the sample 14.
  • the output from the receiver antenna is coupled to a wideband optical modulator 12 by either stripline or electrical waveguides. With the appropriate type of modulator it is possible to modulate one or more characteristics of the optical carrier including the intensity, phase or polarisation.
  • the modulator device is an electro-absorption intensity modulator of a type known to have a good electrical frequency response.
  • Lithium Niobate-based optical phase or intensity modulators may be used.
  • the optical input into the optical modulator 12 is obtained by splitting of a proportion of the output from laser 1 through optical coupler 3.
  • the spectrum of the output of optical modulator 12 is shown in Figure 2c and consists of an optical carrier at the output frequency fl of laser 1 and a pair of sidebands produced by the received terahertz signal.
  • the amplitude and phase of the optical sidebands can be used to derive information about the terahertz propagation properties of the test sample 14.
  • the output of the modulator 12 is combined in coupler 11 with a frequency shifted version of the output from laser 2 derived from splitting a proportion of the light from laser 2 in coupler 10 and passing it through a frequency shifter 8.
  • the frequency shifter can be an acousto-optic device and will typically produce a frequency shift of the optical carrier in the range of IMHz to IGHz when driven by a suitable RF signal generator.
  • Figure 2d shows the spectral components at the output of one arm of coupler 11 and consist of lines for the carrier fl of laser 1, sidebands produced by the received terahertz signal and the frequency shifted output from laser 2 (£2 + f3).
  • the two outputs from coupler 11 are coupled to a balanced photodiode receiver 5 which helps reject common mode signals.
  • the output from the balanced receiver is filtered by a narrow band bandpass filter 7 centred at the frequency offset £ produced by frequency shifter 8.
  • the filtered electrical output signal at 13 is shown by spectral line in Figure 2e.
  • only the product resulting from the upper sideband fl+(f2-fl) beating with the frequency shifted spectral component £2+f3 can pass through the bandpass filter 7.
  • the frequency of the beat signal at 13 is independent of fluctuations in the absolute frequency of laser 1 or laser 2. This allows the sensitivity of the measurement to be improved by reducing the bandwidth of the electrical bandpass filter 7. If an electrical synchronous detection scheme follows the output 13 it is possible to further improve the sensitivity of the measurement. Synchronous detection would be possible if the same RF generator was used as a reference for both the frequency shifter 8 and the synchronous detector following output 13.
  • the method shown in the above embodiment allows terahertz propagation measurements to be made with good sensitivity and without limitations from laser phase noise. For some applications it is appreciated that tighter laser frequency stability will be required to ensure that the actual terahertz frequency to be generated by the beat signal can be set with precision. There are several well known locking techniques within the literature that could be used for this purpose and be combined with the present invention.
  • the description in Figure 1 describes a method for generating and detecting a single frequency in the terahertz or near terahertz spectrum. It would also be possible for the tuneable laser 1 within the preferred embodiment to be a swept frequency source so that the beat signal between laser 1 and laser 2 could be used to provide a source of terahertz or near terahertz frequency that varies with time. This arrangement would allow swept terahertz propagation measurements to be conveniently carried.
  • each generation or detection unit would be of a similar type to as described in the preferred embodiment but with all units driven by a shared laser pair. This configuration would allow arrays of detectors and possibly arrays of generators to be used for terahertz imaging with the advantage of improved sensitivity that the invention provides.
  • a system for generating and detecting terahertz radiation comprises a first laser 1 and a second laser 2, each having a different frequency of laser light output.
  • the system further comprises a radiation generating stage 4, 9 arranged to generate terahertz radiation by beating the light output of the first laser 1 with the light output of the second laser 1.
  • a radiation detecting stage 12 is arranged to modulate light output from the first laser 1 with received terahertz radiation.
  • this application discloses a method of generating and detecting high frequency radiation, particularly at terahertz frequencies, using combinations of optical modulation and photomixing.
  • a particular advantage of the disclosed method is that it allows a widely tuneable terahertz source to be achieved from compact semiconductor optoelectronic components without limitations from laser phase noise.
  • the method may reduce the effects of laser phase noise when generating and detecting terahertz and near terahertz signals.
  • Applications of the invention include spectroscopy, measurements, network and component analysis, security, terahertz imaging, radar and communications.

Abstract

A system for generating and detecting terahertz radiation comprises a first laser (1) and a second laser (2), each having a different frequency of laser light output. The system further comprises a radiation generating stage (4, 9) arranged to generate terahertz radiation by beating the light output of the first laser (1) with the light output of the second laser (2). A radiation detecting stage (12) is arranged to modulate light output from the first laser (1) with received terahertz radiation.

Description

METHOD TO GENERATE AND DETECT TERAHERTZ RADIATION
Field of the Invention
This invention relates to the generation and/or detection of radiation in the terahertz and near-terahertz frequency spectrum, i.e. the range 80 GHz to 4THz.
Background to the Invention
The beating of the output of two lasers to generate radio frequency (RF) signals is well known, see for example US patent 5,710,651 to Logan. This principle has also been used to generate terahertz frequencies. The generation of RF, mm- wave or terahertz signals by this method results in a signal whose linewidth is determined by the combined linewidth of the two lasers used. The linewidth of this beat signal can limit the achievable sensitivity of terahertz receivers which use down conversion because it is necessary to decrease the noise bandwidth of any post-mixing electrical filters.
Shared lasers in the generator and receiver can be used to avoid this problem. The possibility of shared lasers to generate a common reference for generator and receiver is recognised in US patent 6,348,683 to Verghese. However, Verghese requires a combined optical and electrical mixing process to be used for detecting the RF radiation, which depends crucially on the conversion characteristics of a photoconductive mixer and is not applicable to other types of photodetector which may have other desirable characteristics such as higher quantum efficiency or lower cost. Summary of the Invention Accordingly, this invention provides a system for generating and detecting terahertz radiation. The system comprises a first laser and a second laser, each having a different frequency of laser light output, a radiation generating stage and a radiation detecting stage. The radiation generating stage is arranged to generate terahertz radiation by beating the light output of the first laser with the light output of the second laser. The radiation detecting stage is arranged to modulate light output from the first laser with received terahertz radiation.
Thus, a method for the generation and detection of RF and terahertz frequencies is disclosed. The method proposed is particularly attractive for applications where the apparatus used to generate the radiation is near to the apparatus used to detect the radiation, as is often the case in certain types of measurement equipment.
The radiation generating stage may comprise a first optical coupler arranged to couple light from the first laser and light from the second laser to generate a beat frequency in the terahertz range. The radiation generating stage may comprise a first photodetector coupled to a first antenna, the first photodetector being arranged to receive light from the optical coupler and generate radiation from the first antenna at the beat frequency. Thus, RF, mm- wave or terahertz radiation may be generated by combining the output of two single frequency lasers and photomixing within a wideband photodetector. The output of the photodetector is connected to an antenna designed for the required frequency band. Between the generator and receiver a sample whose propagation characteristics are to be measured can be placed.
The first and/or the second laser may be tuneable. One of the lasers can be fixed frequency and one can be tuneable. Adjustment of the tuneable laser allows the beat frequency and hence frequency of the radiation to be set. The frequency of the tuneable laser may be varied with time, in use, to generate a swept source of terahertz radiation. The radiation detecting stage may comprise an optical modulator coupled to a second antenna and arranged to modulate light from the first laser with the signal received by the second antenna. This arrangement is different from the prior art in that an optical modulator is used to translate the received terahertz signal onto an optical carrier which can then be subsequently detected using an optical self-heterodyne scheme. This allows photodiodes with high conversion efficiency to be used in the detection scheme and avoids the limitation of laser phase noise of conventional heterodyne detection. The receiver antenna may be connected to the electrodes of an optical modulator which is used to impose the received RF, mm-wave or terahertz signals onto an optical carrier derived from one of the lasers in the radiation generation apparatus
The radiation detecting stage may comprise a second optical coupler arranged to couple the modulated light with light from the second laser. The radiation detecting stage may comprise a filter system for selecting particular frequency components of the output of the second optical coupler. The filter system may be optical, but in the preferred embodiment is an electrical filter system.
The radiation detecting stage may comprise a second photodetector arranged to generate an electrical signal representative of at least some of the frequency components of the light from the optical coupler. The radiation detecting stage may further comprise an electrical filter. Thus, the combined optical output may be converted into an electrical • signal using a balanced photodiode receiver. The output of the photodiode receiver may be connected through a filter of appropriate bandwidth to achieve the desired signal to noise ratio.
The radiation detecting stage may comprise a frequency shifter in the optical path between the first laser or the second laser and the second optical coupler. The modulated optical signal may thus be combined with a frequency shifted version of the output from the second laser within the radiation generation apparatus.
The system may comprise a plurality of radiation generating stages and/or a plurality of radiation detecting stages, each supplied with light from the first and/or second lasers. The invention extends to a radiation generating stage adapted for use in the system described and to a radiation detecting stage adapted for use in the system.
Viewed from a broad aspect, the invention provides a method for generating and detecting terahertz and near terahertz radiation using a shared pair of lasers or a laser with dual output frequencies to generate the beat frequencies required at the transmitter and within the receiver. The beat signal between two lasers may be photo-detected and used to generate terahertz or near terahertz radiation. The photodetector used to generate the terahertz or near terahertz signal may be a uni-travelling carrier photodiode. One of the two lasers may also be used to provide an optical carrier which can be modulated by the received terahertz radiation. The modulator used to impose the received terahertz or near terahertz on an optical carrier may be an electroabsorption modulator. The modulator used to impose the received terahertz or near terahertz signal on an optical carrier may be a Lithium Niobate phase modulator or Mach Zehnder intensity modulator.
The optical output from the optical modulator connected to the antenna receiving terahertz or near terahertz radiation may be combined with light from the second of the two lasers prior to photodetection. A frequency shifter may be placed between one of the lasers and the optical combiner prior to photodetection.
One or both lasers may be tuneable. The lasers may be DFB or DBR semiconductor lasers. The frequency of the tuneable laser may be varied with time to generate a swept source of terahertz or near terahertz radiation. A single pair of lasers may be used to provide beat signals to multiple terahertz generators and/or detectors.
Optical amplifiers may also be included to increase the output power available from either laser or from both lasers. This arrangement would be especially desirable where an array of terahertz generators or receivers are required. The optical amplifiers could be semiconductor optical amplifiers or doped erbium fibre laser amplifiers, for example. The terahertz radiation generator can be switched using an external electrical signal or by gating one of the two optical inputs. The apparatus may be configured as a terahertz or near terahertz radar or time domain reflectometer.
Brief Description of the Drawings
An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:
Figure 1 is a schematic view of an embodiment of the terahertz generation and detection scheme according to an embodiment of the invention; and
Figure 2 shows the position of spectral components produced by the photomixing and modulation process according to Figure 1.
In Figure 1, the following reference numerals are used:
Figure imgf000006_0001
Detailed Description of an Embodiment
Figure 1 is a schematic representation of the preferred embodiment of the terahertz generation and detection scheme according to the invention. The system comprises a tuneable laser 1, typically a semiconductor DFB (distributed feedback) or DBR (distributed Bragg reflector) laser and a fixed frequency laser 2, typically a DFB or DBR laser. Lasers 1 and 2 may both be located on the same chip. Light may be guided from one component to the next using optical fibre or optical waveguides. An optical coupler 3 splits the light from laser 1 between the photomixer used for terahertz generation and the optical modulator 12. An optical coupler 10 splits light from laser 2 between the photomixer and a frequency shifter 8. An optical coupler 9 is used to combine signals derived from laser 1 and laser 2 for photomixing in a photodetector 4. Terahertz radiation having passed through a measurement sample 14 is coupled to electrodes on the optical modulator 12. The optical output from the optical modulator 12 is combined with the optical output from the frequency shifter 8 in an optical coupler 11. The output from the optical coupler 11 is connected to balanced photodiodes 5 and a differential amplifier 6. The electrical output from the differential amplifier 6 is filtered by a low pass electrical filter 7 to generate an electrical output at point 13 in Figure 1.
Figure 2 shows the position of spectral components produced by the photomixing and modulation process. Figure 2a shows the position of spectral lines for the lasers 1 and 2 close to an optical carrier frequency of about 200 THz. Figure 2b shows the terahertz signal (f2-fl) at the output of the photodetector 4. Figure 2c shows the output of the optical modulator 12, indicating the optical carrier fl and sidebands. Figure 2d shows the combined optical spectra at the output of one arm of optical coupler 11 which includes the addition of a frequency shifted spectral line f2 + f3. The electrical output at 13 is shown in Figure 2e with a spectral line at f3 which is equal to the frequency offset produced by the frequency shifter 8.
As explained above, Figure 1 shows the preferred embodiment for carrying out propagation measurements over a range of terahertz and near terahertz frequencies on a test sample 14. A single frequency semiconductor laser 2, operating either within the visible spectrum or near infra-red spectrum is combined in optical coupler 9 with a signal from a tuneable laser 1 with approximately the same nominal wavelength. Both laser 1 and 2 are typically either semiconductor distributed feedback lasers or distributed Bragg reflector lasers. The combined signal is fed to a wide bandwidth photodetector 4, typically a uni-travelling carrier (UTC) photodiode. The optical spectrum at the input to the photodetector 4 is shown by the two spectral lines in Figure 2a centred around an optical frequency of 200 THz5 corresponding to the typical wavelength chosen for the operation of the apparatus. The electrical output signal from photodetector 4 is at the difference frequency (f2 - fl) between laser 1 and laser 2 and can be adjusted to produce the desired terahertz frequency by tuning the optical frequency fl of laser 1. This is shown by the spectral line in Figure 2b at terahertz or near terahertz frequency.
The output from photodetector 4 is coupled by a stripline or waveguide structure to an antenna 15. The terahertz signal is then radiated at the test sample 14. The signal collected from the sample 14 is collected on a receiver antenna which may be of similar construction to the transmitting antenna 15. The signal received by the antenna may be the result of either transmission or scattering by the sample depending on the relative positioning of the two antennas with respect to the sample 14. The output from the receiver antenna is coupled to a wideband optical modulator 12 by either stripline or electrical waveguides. With the appropriate type of modulator it is possible to modulate one or more characteristics of the optical carrier including the intensity, phase or polarisation. In the preferred embodiment, the modulator device is an electro-absorption intensity modulator of a type known to have a good electrical frequency response. Alternatively, for some applications Lithium Niobate-based optical phase or intensity modulators may be used. The optical input into the optical modulator 12 is obtained by splitting of a proportion of the output from laser 1 through optical coupler 3. The spectrum of the output of optical modulator 12 is shown in Figure 2c and consists of an optical carrier at the output frequency fl of laser 1 and a pair of sidebands produced by the received terahertz signal. The amplitude and phase of the optical sidebands can be used to derive information about the terahertz propagation properties of the test sample 14.
The output of the modulator 12 is combined in coupler 11 with a frequency shifted version of the output from laser 2 derived from splitting a proportion of the light from laser 2 in coupler 10 and passing it through a frequency shifter 8. The frequency shifter can be an acousto-optic device and will typically produce a frequency shift of the optical carrier in the range of IMHz to IGHz when driven by a suitable RF signal generator. Figure 2d shows the spectral components at the output of one arm of coupler 11 and consist of lines for the carrier fl of laser 1, sidebands produced by the received terahertz signal and the frequency shifted output from laser 2 (£2 + f3). The two outputs from coupler 11 are coupled to a balanced photodiode receiver 5 which helps reject common mode signals. For some realisations a single photodiode receiver may be acceptable where some overall performance can be sacrificed for simplicity. The output from the balanced receiver is filtered by a narrow band bandpass filter 7 centred at the frequency offset £3 produced by frequency shifter 8. The filtered electrical output signal at 13 is shown by spectral line in Figure 2e. In this embodiment, only the product resulting from the upper sideband fl+(f2-fl) beating with the frequency shifted spectral component £2+f3 can pass through the bandpass filter 7. The frequency of the beat signal at 13 is independent of fluctuations in the absolute frequency of laser 1 or laser 2. This allows the sensitivity of the measurement to be improved by reducing the bandwidth of the electrical bandpass filter 7. If an electrical synchronous detection scheme follows the output 13 it is possible to further improve the sensitivity of the measurement. Synchronous detection would be possible if the same RF generator was used as a reference for both the frequency shifter 8 and the synchronous detector following output 13.
The method shown in the above embodiment allows terahertz propagation measurements to be made with good sensitivity and without limitations from laser phase noise. For some applications it is appreciated that tighter laser frequency stability will be required to ensure that the actual terahertz frequency to be generated by the beat signal can be set with precision. There are several well known locking techniques within the literature that could be used for this purpose and be combined with the present invention.
The description in Figure 1 describes a method for generating and detecting a single frequency in the terahertz or near terahertz spectrum. It would also be possible for the tuneable laser 1 within the preferred embodiment to be a swept frequency source so that the beat signal between laser 1 and laser 2 could be used to provide a source of terahertz or near terahertz frequency that varies with time. This arrangement would allow swept terahertz propagation measurements to be conveniently carried.
In another configuration it could be possible to modulate the light from laser 1 between coupler 3 and coupler 9 to allow temporal properties of sample 14 to be obtained. For example, if the radiating and receiving antenna 15 are located close together it would be possible to carry out time domain reflectometry measurements of a test sample at terahertz frequencies.
In a further configuration it would be possible to have either multiple terahertz radiation generation units or multiple radiation terahertz detection units, or combinations of both. Each generation or detection unit would be of a similar type to as described in the preferred embodiment but with all units driven by a shared laser pair. This configuration would allow arrays of detectors and possibly arrays of generators to be used for terahertz imaging with the advantage of improved sensitivity that the invention provides.
In summary, a system for generating and detecting terahertz radiation comprises a first laser 1 and a second laser 2, each having a different frequency of laser light output. The system further comprises a radiation generating stage 4, 9 arranged to generate terahertz radiation by beating the light output of the first laser 1 with the light output of the second laser 1. A radiation detecting stage 12 is arranged to modulate light output from the first laser 1 with received terahertz radiation.
In general terms, this application discloses a method of generating and detecting high frequency radiation, particularly at terahertz frequencies, using combinations of optical modulation and photomixing. A particular advantage of the disclosed method is that it allows a widely tuneable terahertz source to be achieved from compact semiconductor optoelectronic components without limitations from laser phase noise. The method may reduce the effects of laser phase noise when generating and detecting terahertz and near terahertz signals. Applications of the invention include spectroscopy, measurements, network and component analysis, security, terahertz imaging, radar and communications.

Claims

Claims
1. A system for generating and detecting terahertz radiation, the system comprising: a first laser and a second laser, each having a different frequency of laser light output, a radiation generating stage; and a radiation detecting stage, wherein the radiation generating stage is arranged to generate terahertz radiation by beating the light output of the first laser with the light output of the second laser, and wherein the radiation detecting stage is arranged to modulate light output from the first laser with received terahertz radiation.
2. A system as claimed in claim 1, wherein the radiation generating stage comprises: a first optical coupler arranged to couple light from the first laser and light from the second laser to generate a beat frequency in the terahertz range; and a first photodetector coupled to a first antenna, the first photodetector being arranged to receive light from the optical coupler and generate radiation from the first antenna at the beat frequency.
3. A system as claimed in claim 1 or 2, wherein the radiation detecting stage comprises: an optical modulator coupled to a second antenna and arranged to modulate light from the first laser with the signal received by the second antenna.
4. A system as claimed in claim 3, wherein the radiation detecting stage comprises: a second optical coupler arranged to couple the modulated light with light from the second laser; and a filter system for selecting particular frequency components of the output of the second optical coupler.
5. A system as claimed in claim 4, wherein the radiation detecting stage comprises: a second photodetector arranged to generate an electrical signal representative of at least some of the frequency components of the light from the optical coupler; and an electrical filter.
6, A system as claimed in claim 4 or 5, wherein the radiation detecting stage comprises frequency shifter in the optical path between the first laser or the second laser and the second optical coupler.
7. A system as claimed in any preceding claim, wherein the first and/or the second laser is tuneable.
8. A system as claimed in claim 7, wherein the frequency of the tuneable laser is varied with time, in use, to generate a swept source of terahertz radiation.
9. A system as claimed in any preceding claim comprising a plurality of radiation generating stages and/or a plurality of radiation detecting stage, each supplied with light from the first and/or second lasers.
10. A radiation generating stage adapted for use in the system of any preceding claim.
11. A radiation detecting stage adapted for use in the system of any preceding claim.
PCT/GB2006/001848 2005-05-18 2006-05-18 Method to generate and detect terahertz radiation WO2006123163A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0510109A GB0510109D0 (en) 2005-05-18 2005-05-18 Method to generate and detect the radiation
GB0510109.2 2005-05-18

Publications (1)

Publication Number Publication Date
WO2006123163A1 true WO2006123163A1 (en) 2006-11-23

Family

ID=34708355

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB2006/001848 WO2006123163A1 (en) 2005-05-18 2006-05-18 Method to generate and detect terahertz radiation

Country Status (2)

Country Link
GB (1) GB0510109D0 (en)
WO (1) WO2006123163A1 (en)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2008067957A2 (en) * 2006-12-07 2008-06-12 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. ARRANGEMENT FOR THE ELECTRO-OPTICAL CONTROL AND FAST MODULATION OF THz TRANSMITTERS AND THz MEASURING SYSTEMS
DE102007044839A1 (en) * 2007-09-14 2009-05-20 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Method for generating and coherently detecting terahertz radiation
WO2009134523A1 (en) * 2008-04-29 2009-11-05 Textron Systems Corporation Heterodyne receiver using differential temperature control of laser sources
GB2469945A (en) * 2008-04-04 2010-11-03 Emcore Corp Terahertz frequency domain spectrometer with integrated dual laser module
US20120082199A1 (en) * 2010-10-05 2012-04-05 Sony Corporation Antenna measurement system and method
CN102544985A (en) * 2011-12-31 2012-07-04 湖南大学 Optical fiber type terahertz wave generation device and method based on modulation instability
CN105569658A (en) * 2015-12-12 2016-05-11 江苏师范大学 Coalcutter coal rock distribution recognition device and method adopting terahertz imaging technology
EP3144664A1 (en) * 2015-09-15 2017-03-22 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. System and method for determining characteristics of an object or a sample
CN113347054A (en) * 2021-04-27 2021-09-03 北京无线电计量测试研究所 Terahertz single-port space network reflection coefficient measuring device and method

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5710651A (en) * 1995-12-13 1998-01-20 California Institute Of Technology Remote millimeter-wave antenna fiber optic communication system using dual optical signal with millimeter-wave beat frequency
US6348683B1 (en) * 1998-05-04 2002-02-19 Massachusetts Institute Of Technology Quasi-optical transceiver having an antenna with time varying voltage
US20030178584A1 (en) * 2000-02-28 2003-09-25 Arnone Donald Dominic Imaging apparatus and method
US20040065831A1 (en) * 2002-05-31 2004-04-08 John Federici Terahertz imaging system and method
US20040130725A1 (en) * 2001-07-02 2004-07-08 Takashi Kido Propagation measuring apparatus and a propagation measuring method
FR2854504A1 (en) * 2003-04-30 2004-11-05 Thales Sa TERAHERTZ TRANSMISSION SOURCE AND OPTICAL SYSTEM COMPRISING SUCH A SOURCE

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5710651A (en) * 1995-12-13 1998-01-20 California Institute Of Technology Remote millimeter-wave antenna fiber optic communication system using dual optical signal with millimeter-wave beat frequency
US6348683B1 (en) * 1998-05-04 2002-02-19 Massachusetts Institute Of Technology Quasi-optical transceiver having an antenna with time varying voltage
US20030178584A1 (en) * 2000-02-28 2003-09-25 Arnone Donald Dominic Imaging apparatus and method
US20040130725A1 (en) * 2001-07-02 2004-07-08 Takashi Kido Propagation measuring apparatus and a propagation measuring method
US20040065831A1 (en) * 2002-05-31 2004-04-08 John Federici Terahertz imaging system and method
FR2854504A1 (en) * 2003-04-30 2004-11-05 Thales Sa TERAHERTZ TRANSMISSION SOURCE AND OPTICAL SYSTEM COMPRISING SUCH A SOURCE

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
LOFFLER T ET AL: "All-optoelectronic continuous-wave terahertz systems", PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON, SERIES A; MATHEMATICAL AND PHYSICAL SCIENCES, ROYAL SOCIETY, LONDON, GB, vol. 362, no. 1815, 15 February 2004 (2004-02-15), pages 263 - 281, XP002338368, ISSN: 0080-4614 *
MENDIS R ET AL: "COHERENT GENERATION AND DETECTION OF CONTINUOUS TERAHERTZ WAVES USING TWO PHOTOMIXERS DRIVEN BY LASER DIODES", INTERNATIONAL JOURNAL OF INFRARED AND MILLIMETER WAVES, SPRINGER, DORDRECHT, NL, vol. 26, no. 2, February 2005 (2005-02-01), pages 201 - 207, XP001226407, ISSN: 0195-9271 *

Cited By (21)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US8064740B2 (en) 2006-12-07 2011-11-22 Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V. Arrangement for the electro-optical control and fast modulation of THz transmitters and THz measuring systems
DE102006058395A1 (en) * 2006-12-07 2008-06-19 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Arrangement for the electrical control and fast modulation of THz transmitters and THz measuring systems
WO2008067957A3 (en) * 2006-12-07 2008-10-30 Fraunhofer Ges Forschung ARRANGEMENT FOR THE ELECTRO-OPTICAL CONTROL AND FAST MODULATION OF THz TRANSMITTERS AND THz MEASURING SYSTEMS
WO2008067957A2 (en) * 2006-12-07 2008-06-12 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. ARRANGEMENT FOR THE ELECTRO-OPTICAL CONTROL AND FAST MODULATION OF THz TRANSMITTERS AND THz MEASURING SYSTEMS
DE102006058395B4 (en) * 2006-12-07 2012-05-16 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Arrangement for the electrical control and fast modulation of THz transmitters and THz measuring systems
DE102007044839A1 (en) * 2007-09-14 2009-05-20 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Method for generating and coherently detecting terahertz radiation
US8901497B2 (en) 2007-09-14 2014-12-02 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Method for creating and coherently detecting terahertz radiation
GB2469945A (en) * 2008-04-04 2010-11-03 Emcore Corp Terahertz frequency domain spectrometer with integrated dual laser module
GB2469945B (en) * 2008-04-04 2011-05-25 Emcore Corp Terahertz frequency domain spectrometer with integrated dual laser module
WO2009134523A1 (en) * 2008-04-29 2009-11-05 Textron Systems Corporation Heterodyne receiver using differential temperature control of laser sources
US8073344B2 (en) 2008-04-29 2011-12-06 Textron Systems Corporation Heterodyne receiver using differential temperature control of laser sources
US20120082199A1 (en) * 2010-10-05 2012-04-05 Sony Corporation Antenna measurement system and method
CN102565549A (en) * 2010-10-05 2012-07-11 索尼公司 Antenna measurement system and method
US8565347B2 (en) 2010-10-05 2013-10-22 Sony Corporation Antenna measurement system and method
CN102544985A (en) * 2011-12-31 2012-07-04 湖南大学 Optical fiber type terahertz wave generation device and method based on modulation instability
EP3144664A1 (en) * 2015-09-15 2017-03-22 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. System and method for determining characteristics of an object or a sample
WO2017046278A1 (en) * 2015-09-15 2017-03-23 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. System and method for determining characteristics of an object or a sample
US10393650B2 (en) 2015-09-15 2019-08-27 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. System and method for determining characteristics of an object or a sample
CN105569658A (en) * 2015-12-12 2016-05-11 江苏师范大学 Coalcutter coal rock distribution recognition device and method adopting terahertz imaging technology
CN113347054A (en) * 2021-04-27 2021-09-03 北京无线电计量测试研究所 Terahertz single-port space network reflection coefficient measuring device and method
CN113347054B (en) * 2021-04-27 2022-07-29 北京无线电计量测试研究所 Terahertz single-port space network reflection coefficient measuring device and method

Also Published As

Publication number Publication date
GB0510109D0 (en) 2005-06-22

Similar Documents

Publication Publication Date Title
Pan et al. Photonics-based broadband microwave measurement
US8265488B2 (en) Electromagnetic transmission/reception system
Zou et al. Photonics for microwave measurements
EP3064956B1 (en) Fully optically controlled phased array radar transmitter
WO2006123163A1 (en) Method to generate and detect terahertz radiation
Ducournau et al. THz communications using photonics and electronic devices: the race to data-rate
US8604433B2 (en) Terahertz frequency domain spectrometer with frequency shifting of source laser beam
US10761126B2 (en) Electro-optic probe, electromagnetic wave measuring apparatus, and electromagnetic wave measuring method
US9400214B1 (en) Terahertz frequency domain spectrometer with a single photoconductive element for terahertz signal generation and detection
Burla et al. Integrated Photonic ${\rm K} _ {\rm u} $-Band Beamformer Chip With Continuous Amplitude and Delay Control
Che et al. Arrayed photomixers for THz beam-combining and beam-steering
US7382983B2 (en) Optical control type phased array antenna
Xu et al. True time-delay phased-array antenna feed system based on optical heterodyne techniques
CN209745811U (en) Broadband tunable terahertz detection device
US7068944B2 (en) Multi-function optical performance monitor
US20220397596A1 (en) System and method for multiplexing and demultiplexing rf signals using a plurality of rf-to-optical antennas
Taylor et al. Steering of an optically driven true-time delay phased-array antenna based on a broad-band coherent WDM architecture
Olvera et al. Architecture and component characterization of a high-resolution free-space vector network analyzer for the terahertz range
Biernacki et al. A two-channel optical downconverter for phase detection
Ayotte et al. Compact optical coherent receiver for avionics applications
García-Juárez et al. Optical heterodyne technique for generating and distributing microwave signals
US20230291167A1 (en) Pulsed rf generator, pulsed rf transmitter and related methods of operation
WO2023279330A1 (en) Coherent optical spectrum analysis
Huang et al. Dual-Functional Microwave Photonic System for Target Detection and Frequency Measurement
Esman 2.1. Microwave Functions Enabled by Photonics

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application
NENP Non-entry into the national phase

Ref country code: DE

WWW Wipo information: withdrawn in national office

Country of ref document: DE

NENP Non-entry into the national phase

Ref country code: RU

WWW Wipo information: withdrawn in national office

Country of ref document: RU

122 Ep: pct application non-entry in european phase

Ref document number: 06743941

Country of ref document: EP

Kind code of ref document: A1