Terahertz wave phase difference measurement device

Patent metadata

Field	Value
Publication number	US-9835494-B2
Application number	US-201415116331-A
Country	US
Kind code	B2
Filing date	Sep 22, 2014
Priority date	Feb 7, 2014
Publication date	Dec 5, 2017
Grant date	Dec 5, 2017

Abstract

In order to provide a high-sensitivity terahertz wave phase difference measurement system having a high S/N ratio, terahertz interference waves are observed using a half mirror and a movable reference mirror, and the phase difference is calculated, by a terahertz wave generation/detection device that obtains a high S/N ratio by employing a terahertz wave generator for irradiating a non-linear optical crystal with angular phase-matched pump light and seed light, and a terahertz wave detector for irradiating a non-linear optical crystal with angular phase-matched pump light and terahertz waves. In order to match the optical path length of the pump light and the terahertz waves irrespective of the position of the movable reference mirror and the position of a measured object, a first optical delay device, and a second optical delay device that operates in conjunction with movement of a movable reference mirror of a Michelson interferometer, are introduced on the optical path of the pump light.

First claim

The invention claimed is:

1. A terahertz wave phase difference measuring system comprising:
a pulse laser light source which generates pump light;
a continuous wave laser light source which generates seed light;
a terahertz wave generator including a nonlinear optical crystal which launches the pump light and the seed light so as to meet a first angle phase matching condition for generating a terahertz wave and thereby generates a terahertz wave;
a terahertz wave detector including a nonlinear optical crystal which launches the terahertz wave and the pump light so as to meet a second angle phase mate matching condition for converting the terahertz wave into terahertz wave detection light and thereby generates the terahertz wave detection light;
a photodetector which detects the detection light;
a signal processor which converts an output signal of the photodetector into the intensity of a detected terahertz wave and records the intensity;
a beam splitter which bifurcates pump light from the pulse laser light source into two directions and guides a first bifurcated pump light to the terahertz wave generator and guides a second bifurcate pump light to a first optical delay device;
the first optical delay device and a second optical delay device which are placed on the optical path of the second pump light and guide the second pump light to the terahertz wave detector;
a terahertz wave interferometer including a half mirror which bifurcates a terahertz wave launched from the terahertz wave generator into two directions and combines terahertz waves respectively reflected from the two directions and guides the same to the terahertz wave detector, a terahertz wave optical system which applies a terahertz wave bifurcated into a first direction of the half mirror to a measured object and guides a reflected terahertz wave to the half mirror, and a movable reference mirror which reflects a terahertz wave bifurcated into a second direction of the half mirror with any optical path length and guides the same to the half mirror;
an interlocking mechanism for matching an amount of variation in optical path length of the second pump light of the second optical delay device with an amount of variation in optical path length of a terahertz wave of the movable reference mirror,
wherein a first optical path length which is a difference between the optical path length of first pump light from the beam splitter to the terahertz wave generator and the optical path length of second pump light going from the beam splitter, passing through the first optical delay device and the second optical delay device, and arriving at the terahertz wave detector and a second optical path length which is the optical path length of a terahertz wave going from the terahertz wave generator, being bifurcated at the half mirror, going in a second direction, being reflected at the movable mirror, passing through the half mirror again, and arriving at the terahertz detector are substantially matched with each other.

2. A terahertz wave phase difference measuring system according to claim 1 ,
wherein in the terahertz wave interferometer, a shutter which switches between letting through and blocking a terahertz wave is placed between the half mirror and the movable reference mirror,
wherein in a first measurement, a second bifurcated terahertz wave is blocked with the shutter, the first optical delay device is scanned, and a position where a peak of detection light is detected is taken as an initial position of the first optical delay device,
wherein in a second measurement, the shutter is opened to let through a first bifurcated terahertz wave, the first optical delay device and the second optical delay device are scanned such That the optical path length of pump light is constant, and a position where detection light is minimized is taken as an initial position of the second optical delay device, and
wherein in a third measurement, a first bifurcated terahertz wave is let through, the first optical delay device is fixed, the second optical delay device is scanned, and the intensity of detection light is observed to measure a phase difference between terahertz waves caused by a measured object.

3. A terahertz wave phase difference measuring system comprising:
a pulse laser light source which generates pump light;
a continuous wave laser light source which generates seed light;
a terahertz wave generator including a nonlinear optical crystal which launches the pump light and the seed light so as to meet a first angle phase matching condition for generating a terahertz wave and thereby generates a terahertz wave;
a terahertz wave detector including a nonlinear optical crystal which launches the terahertz wave and the pump light so as to meet a second angle phase matching condition for converting the terahertz wave into terahertz wave detection light and thereby generates the terahertz wave detection light;
a photodetector which detects the detection light;
a signal processor which converts an output signal the photodetector into the intensity of a detected terahertz wave and records the intensity;
the first optical delay device and a second optical delay device which guide pump light launched from the pulse laser light source and passing through the terahertz generator to the terahertz wave detector;
a terahertz wave interferometer including a half mirror which bifurcates a terahertz wave launched from the terahertz wave generator into two directions and combines terahertz waves respectively reflected from the two directions and guides the same to the terahertz wave detector, a terahertz wave optical system which applies a terahertz wave bifurcated into a first direction of the half mirror to a measured object and guides a reflected terahertz wave to the half mirror, and a movable reference mirror which reflects a terahertz wave bifurcated into a second direction of the half mirror with any optical path length and guides the same to the half mirror;
an interlocking mechanism for matching an amount of variation in optical, path length of the pump light of the second optical delay device with an amount of variation in optical path length of a terahertz wave of the movable reference mirror,
wherein a first optical path length which is the optical path length of pump light going from the terahertz wave generator, passing through the first optical delay device and the second optical delay device, and arriving at the terahertz wave detector and a second optical path length which is the optical path length of a terahertz wave going from the terahertz wave generator, being bifurcated at the half mirror, going in the second direction, being reflected at the movable mirror, passing through the half mirror again, and arriving at the terahertz detector are substantially matched with each other.

4. A terahertz wave phase difference measuring system according to claim 3 ,
wherein in the terahertz wave interferometer, a shutter which switches between letting through and blocking a terahertz wave is placed between the half mirror and the movable reference mirror,
wherein in a first measurement, a second bifurcated terahertz wave is blocked with the shutter, the first optical, delay device is scanned, and a position where a peak of detection light is detected is taken as initial position of the first optical delay device,
wherein in a second measurement, the shutter is opened to let through a first bifurcated terahertz wave, the first optical delay device and the second optical delay device are scanned such that the optical path length of pump light is constant, and a position where detection light is minimized is taken as an initial position of the second optical delay device, and
wherein in a third measurement, a first bifurcated terahertz wave is let through, the first optical

Assignees

• Hitachi High Tech Corp

Inventors

• Shiramizu Nobuhiro

Classifications

• G01B11/0675

  using interferometry · CPC title

• G01B9/02041

  characterised by particular imaging or detection techniques · CPC title

• G01J9/02 Primary

  by interferometric methods (using interferometers for measuring optically the linear dimensions of objects G01B9/02) · CPC title

• G01N21/3581

  using far infrared light; using Terahertz radiation · CPC title