Light frequency change detecting method and apparatus

Claim 3

A method of detecting a frequency change according to claim 1, in which the filter is a low-pass filter and in which the modulation frequency f m at which the amplitude of the filter output is the largest is detected or in which the filter is a high-pass filter and in which the modulation frequency f m at which the amplitude of the filter output is the smallest is detected.

Claim 4

A method of detecting a frequency change according to claim 1 or 2, in which a DC component of light of frequency f o and a DC component of the frequency shifted light are detected by the photo-electric light receiving element to compensate for the influence on the filter output of the frequency characteristic of the modulator. of light, in which light of frequency f o is subject to a frequency change f d into light of a frequency (f o + fd), the apparatus comprising a light source (1) for generating light of frequency f o , a modulator driving circuit (9) for providing an oscillating signal at a modulation frequency f m which can be continuously swept, a modulator (8) for modulating the frequency shifted light of frequency (f o + f d ) by the modulation frequency f m to output light of frequency (f o + f d - f ), a reference light detector (15) for detecting the DC component of light of frequency f , a frequency shifted light DC component detecting circuit (17) for detecting DC component of the modulated light, a heterodyne detector circuit (10, 11) for receiving the light of frequency (f o + f d - f ) and heterodyne detecting the light using light of frequency f 0 to output an electrical signal of frequency (f d - f m ) , a low pass filter (12) with a predetermined band width for passing or blocking the electrical signal, an arithmetic circuit (18, 19, 20, 21) for eliminating the DC component of the output of the filter using the outputs from both the reference light detector and the frequency shifted light DC component detecting circuit and compensating for the influence on the output of the filter by the frequency charac - teristic of the modulator (8), and a detector (13) for detecting the modulation

Claim 7

An apparatus for detecting a frequency change according to claim 4, in which the filter is a low-pass filter (12) to detect the modulation frequency f m at which the amplitude of the filter output is the largest, or a high-pass filter (22) to detect the modulation frequency f m at which the amplitude of the filter output is the smallest.

Claim 10

An apparatus for detecting a frequency change according to claim 6, in which the arithmetic circuit includes a subtracting circuit (18) two square root computing circuits (19, 20) and a dividing circuit (21).

Claim 13

In a method for detecting a frequency change f d of a light, in which the light of a frequency f o is subject to a frequency change f d into a light signal of a frequency (f o + fd), said method being characterised in that the light signal of a frequency (f o + f d ) is shifted in frequency by modulation frequency f m by an modulator into a signal light of a frequency (f + f d - f ) said signal light and a reference light of a frequency f are received by a light receiving element and heterodyne detected to convert an electrical signal, said electrical signal is passed to a filter, DC component of a light of frequency f o and DC component of the shifted signal light are detected to compensate for the influence on a filter output by the frequency characteristic of said modulator, and the modulation frequency f m is continuously swept to find out the modulation frequency f m at which the amplitude of said filter output is the largest or the smallest, to thereby detect the frequency change f d .

Claim 14

In a method for detecting a frequency change f d of a light, in which the light of a frequency f o is subject to a frequency change f d into a light signal of a frequency (f o + fd), said method being characterised in that the light signal of a frequency (f o + f d ) is shifted in frequency by modulation frequency f m by an modulator into a signal light of a frequency (f + f d - f ) said signal light and a reference light of a frequency f are received by a light receiving element and heterodyne detected to convert an electrical signal, said electrical signal is passed to a filter, DC component of a light of frequency f o and DC component of the shifted signal light are detected to compensate for the influence on a filter output by the frequency characteristic of said modulator, and the modulation frequency f m is continuously swept to find out the modulation frequency f m at which the amplitude of said filter output is the largest or the smallest, to thereby detect the frequency change f d .