Wavelength-stabilized light source apparatus

• US 6,490,300 B1
• Filed: 07/03/2000
• Issued: 12/03/2002
• Est. Priority Date: 07/14/1999
• Status: Expired due to Fees

• Current Assignee: Mitutoyo Corporation
• Original Assignee: Mitutoyo Corporation
• Inventors: Okamoto, Kiyokazu, Nishioki, Nobuhisa
• Primary Examiner(s): Ip, Paul
• Assistant Examiner(s): ZAHN, JEFFREY N
• Application Number:
  US09/610,221
• Time in Patent Office: 883 Days
• Field of Search: 372/9, 372/20, 372/29.01, 372/29.011, 372/32, 372/38.02, 356/451
• US Class Current: 372/32
• CPC Class Codes: H01S 5/0687 Stabilising the frequency of the laser

Abstract

A part of the output light from a semiconductor laser (1), which has a wavelength controllably variable with an injection current, is split at a beam splitter (3) and introduced into a interference optical system (5). In order to vary an optical path length difference between two light beams in the interference optical system (5) to modulate interference fringes to be obtained, AOMs (55, 56) are arranged on the two split optical paths and an EOM (56) on one of them. The interference fringes obtained from the interference optical system (5) are received at a photoreceptive device (7), and from the received signal, the phase is detected by a phase detector (8) and the phase amplitude is detected by an amplitude extractor (9). In order to stabilize the wavelength, a controller (10) feedback controls the injection current to the semiconductor laser (1) so that the phase amplitude matches to a predetermined setting.

Claims

1. A wavelength-stabilized light source apparatus, comprising:

a coherent light source of which wavelength is variably controlled by a driver;

a beam splitter for splitting the output light from said coherent light source to obtain a part thereof as a control light;

a interference optical system for further splitting said control light in two, then giving a predetermined optical path length difference to the resultant two control lights and synthesizing them to generate interference fringes;

a modulator means for giving a variation to said optical path length difference in said interference optical system to modulate said interference fringes to be obtained;

a photoreceptive device for receiving said interference fringes obtained from said interference optical system;

a demodulator for detecting a degree of modulation of a received signal obtained from said photoreceptive device; and

a controller for feedback controlling a driving condition for said driver so that the output from said demodulator matches to a predetermined setting.

2. The wavelength-stabilized light source apparatus of claim 1, said modulator including:

two acoustic optical modulators arranged on respective optical paths of said two control lights in said interference optical system and driven with different angular frequencies from each other to generate an interfered beat signal with a difference between said angular frequencies, ω

c; and

an electrical optical modulator arranged on one of said optical paths of said two control lights in said interference optical system for phase-modulating said output light in the form of a sinusoidal wave with an angular frequency of ω

, said demodulator including;

a phase detector for FM-demodulating said received signal using a carrier signal with an angular frequency of ω

c to obtain the phase term of said received signal; and

an amplitude extractor for extracting an amplitude of said phase term detected by said phase detector, wherein

said controller feedback controls a driving condition for said coherent light source so that said amplitude of said phase term extracted by said amplitude extractor matches to a predetermined setting.

3. The wavelength-stabilized light source apparatus of claim 1, wherein said coherent light source comprises a semiconductor laser having a wavelength variably controllable with an injection current supplied from said driver.

4. A wavelength-stabilized light source apparatus, comprising:

a reference light source for emitting a coherent reference light with a constant wavelength;

a coherent light source of which wavelength is variably controlled by a driver;

a beam splitter for splitting the output light from said coherent light source to obtain a part thereof as a control light;

a interference optical system for receiving said control light split by said beam splitter and said reference light from said reference light source via different optical paths, splitting said control and reference lights respectively in two, then giving a predetermined optical path length difference respectively to the resultant two control and reference lights and synthesizing them to generate two interference fringes;

a modulator means for giving the same variation respectively to said optical path length differences of said reference and control lights in said interference optical system to modulate said two interference fringes obtained with respect to said reference and control lights;

a pair of photoreceptive devices for receiving said two interference fringes with respect to said reference and control lights from said interference optical system;

a pair of demodulators for detecting degrees of modulations of received signals obtained with respect to said reference and control lights from said pair of photoreceptive devices; and

a controller for feedback controlling a driving condition for said driver so that a ratio between the outputs from said pair of demodulators matches to a predetermined setting.

5. The wavelength-stabilized light source apparatus of claim 4, wherein said modulator imparts an additional or reductive displacement at a constant velocity both on said optical path length differences of said reference and control lights to amplitude-modulate said two interference fringes obtained with respect to said reference and control lights in accordance with wavelengths thereof respectively;

said pair of demodulators comprising a pair of frequency detectors for detecting frequencies, f1 and f2, of intensity variations of said amplitude-modulated received signals obtained with respect to said reference and control lights, and wherein said controller feedback controls said driver so that a ratio between said frequencies f1 and f2 detected by said frequency detectors matches to a predetermined setting.

6. The wavelength-stabilized light source apparatus of claim 5, said controller including:

a wavelength calculator for computing said wavelength of said coherent light source on the basis of a ratio between two input frequencies f1 and f2; and

a feedback controller for feedback controlling said driver so that a difference between said wavelength obtained by said wavelength calculator and a wavelength designated from the external matches to zero.

7. The wavelength-stabilized light source apparatus of claim 4, wherein said modulator imparts a vibration in the form of a sinusoidal wave with a predetermined amplitude of d and an angular frequency both on said optical path length differences of said reference and control lights to phase-modulate said received lights obtained with respect to said reference and control lights in accordance with wavelengths thereof respectively,

said pair of demodulators including:

a pair of phase detectors for PM-demodulating said phase-modulated received signals obtained with respect to said reference and control lights to detect the phase terms, ψ

1=(2π

d/λ

1)+φ

1 and ψ

2=(2π

d/λ

2)+φ

2 (where λ

1, λ

2 denote wavelengths of said reference and control lights, and φ

1, φ

2 initial phase magnitudes); and

a pair of amplitude extractors for extracting amplitudes of said phase terms, 2π

d/λ

1 and 2π

d/λ

2, obtained by said phase detectors, and wherein

said controller feedback controls said driver so that a ratio, λ

2/λ

1, between said amplitudes extracted by said amplitude extractors matches to a predetermined setting.

8. The wavelength-stabilized light source apparatus of claim 7, said controller including:

a wavelength calculator for computing said wavelength of said coherent light source on the basis of a ratio between two input amplitudes; and

a feedback controller for feedback controlling said driver so that a difference between said wavelength obtained by said wavelength calculator and a wavelength designated from the external matches to zero.

9. The wavelength-stabilized light source apparatus to claim 4, said modulator including:

a phase modulator for imparting a vibration in the form of a sinusoidal wave with a predetermined amplitude of d and an angular frequency of ω

both on said optical path length differences of said reference and control lights; and

acoustic optical modulators for frequency-modulating said received signals obtained with respect to said reference and control lights in accordance with wavelengths thereof by superimposing an interfered best signal with an angular frequency of ω

c both on said received signals of said reference and control lights;

said pair of demodulators including;

a pair of phase detectors for FM-demodulating said frequency modulated received signals obtained with respect to said reference and control lights using a carrier angular frequency of ω

c to detect the phase terms, ψ

1=(2π

d/λ

1)cosω

tφ

1 and ψ

2=(2π

d/λ

2)cosω

t+φ

2 (where λ

1, λ

2 denote wavelengths of said reference and control lights, and φ

1, φ

2 initial phase magnitudes); and

a pair of amplitude extractors for extracting amplitudes of said phase terms, 2π

d/λ

1 and 2π

d/λ

2, obtained by said phase detectors, and wherein said controller feedback controls said driver so that a ratio λ

2/λ

1, between said amplitudes obtained by said amplitude extractors matches to a predetermined setting.

10. The wavelength-stabilized light source apparatus of claim 9, said controller including:

a wavelength calculator for computing said wavelength of said coherent light source on the basis of said ratio between two inputs; and

a feedback controller for feedback controlling said driver so that a difference between said wavelength obtained by said wavelength calculator and a wavelength designated from the external matches to zero.

11. The wavelength-stabilized light source apparatus of claim 9, said phase modulator comprises an electric optical modulator.