THz investigation apparatus and method

Apparatus for measurement of a sample, comprising:

• means for generating electromagnetic radiation, arranged to generate an output signal comprising electromagnetic radiation and to transmit the output signal towards a sample space;
• detecting means for detecting a response signal, the detecting means comprising a photoconductive device and being arranged so that the sensitivity of detection of the response signal is modulated at a modulation frequency in dependence upon modulating radiation received by the photoconductive device;
• a first radiation source for the detecting means and a second radiation source for the detecting means, arranged such that the modulating radiation received by the photoconductive device comprises a mixture of radiation from the first radiation source for the detecting means and radiation from the second radiation source for the detecting means;
• control means for varying the modulation frequency at which the sensitivity of detection is modulated, the control means being adapted to vary the modulation frequency by varying the temperature of the first radiation source for the detecting means and/or the temperature of the second radiation source for the detecting means,
• wherein the control means is adapted to vary the temperature of the first radiation source within a first pre-determined range of temperature and to vary the temperature of the second radiation source within a second pre-determined range of temperature, wherein each of the first pre-determined range of temperature and the second pre-determined range of temperature is a range of temperature within which no mode hop occurs in the first radiation source or in the second radiation source respectively.

A method of measurement of a sample, comprising:

• generating an output signal comprising electromagnetic radiation by applying a mixture of radiation from a first radiation source and radiation from a second radiation source to a photoconductive device, and transmitting the output signal towards a sample space;
• varying the frequency of the electromagnetic radiation of the output signal by varying the temperature of the first radiation source and/or the temperature of the second radiation source;
• and
• detecting a response signal,
• wherein the temperature of the first radiation source is varied within a first pre-determined range of temperature and the temperature of the second radiation source is varied within a second pre-determined range of temperature, wherein each of the first pre-determined range of temperature and the second pre-determined range of temperature is a range of temperature within which no mode hop occurs in the first radiation source or in the second radiation source respectively.

A method of measurement of a sample, comprising:

• providing means for generating an output signal comprising electromagnetic radiation and transmitting the output signal towards a sample space;
• providing a detecting means for detecting a response signal;
• modulating the sensitivity of detection of the detecting means at a modulating frequency by applying a mixture of radiation from a first radiation source and from a second radiation source to a photoconductive device included in the detecting means;
• and
• varying the frequency at which the sensitivity of detection is modulated by varying the temperature of the first radiation source and/or the temperature of the second radiation source,
• wherein the temperature of the first radiation source is varied within a first pre-determined range of temperature and the temperature of the second radiation source is varied within a second pre-determined range of temperature, wherein each of the first pre-determined range of temperature and the second pre-determined range of temperature is a range of temperature within which no mode hop occurs in the first radiation source or in the second radiation source respectively.