Optical wavelength monitor using blazed diffraction grating

• US 4,758,090 A
• Filed: 09/25/1986
• Issued: 07/19/1988
• Est. Priority Date: 09/25/1986
• Status: Expired due to Term

• Current Assignee: Alliedsignal Inc.
• Original Assignee: Alliedsignal Inc.
• Inventors: Schuma, Richard F.
• Primary Examiner(s): McGraw, Vincent P.
• Application Number:
  US06/911,562
• Time in Patent Office: 663 Days
• Field of Search: 356/350, 372/32, 350/162.22, 350/162.23, 350/320
• US Class Current: 356/483
• CPC Class Codes: G01C 19/721 Details
• CPC Class Codes: G01J 9/00 Measuring optical phase difference devices or arrangements for controlling the phase of light beams G02F1/01; Determining degree of coherence; Measuring optical wavelength spectrometry G01J3/00
• CPC Class Codes: Y10S 359/902 Holographic interferometer

Abstract

A wavelength monitor (25) uses a diffraction grating (35) and a beam splitter (31). Light from a superluminescent diode (SLD) is supplied to the beam splitter (31) and is projected onto the grating (35).

The grating is formed by holographically projecting a diffraction pattern onto a polished surface which is then photoetched, thereby creating a blazed diffraction grating corresponding to the projected pattern. The etched grating is used as a master, wherein the etched grating is coated with reflective material. The reflective material is then transferred to a prepared substrate that has an ultra-low thermal expansion coefficient to form a low cost replica diffraction grating. The grating (35) formed in this manner becomes a very stable and efficient dispersive element that forms a spectrum of the SLD output that is free of ghost images and accurately deproduces the spectral distribution. A pair of optical sensors (41, 42) detect an imbalance in reflected light, providing an indication of wavelength deviation.

Claims

1. Fiber optic gyroscope including, a broadband light source, a fiber optic coil, means to detect a Sagnac phase shift as a result of light passing through the fiber optic coil and a wavelength monitor to provide a wavelength deviation signal for controlling the light source, comprising:

(a) a blazed reflective diffraction grating;

(b) means to transmit light from the fiber optic coil to the blazed reflective diffraction grating, said means including light collimating means capable of protecting the light onto a significant surface area of the blazed refraction grating;

(c) a beam splitter receiving said light reflected from the refraction grating;

(d) the light supplied to a wavelength monitor being transmitted through the beam splitter to the collimating means;

(e) the light reflected into the beam splitter being transmitted through the collimating means in a manner which focuses the reflected light into the beam splitter;

(f) optical sensor means to detect an unbalanced state of the light entering the beam spliter; and

(g) the light reflected from the blazed diffraction grating being retroflected through the collimating means back to the beam splitter so that the optical sensor means detects an offaxis condition of the reflected light.

2. Fiber optic gyroscope as described in claim 1, further characterized by:

the collimating means being a collimating lens.

3. Fiber optic gyroscope as described in claim 1, further characterized by:

(a) the diffraction grating and the beam splitter each having means for mounting;

(b) a base upon which the diffraction grating and the beam splitter are mounted;

(c) the base and each means for mounting having mating surfaces, wherein the base and each means for mounting are mated; and

(d) the mating surfaces being polished sufficiently for each of said means for mounting adhering to the base by Van der Walls forces to secure each means for mounting to the base.

4. Fiber optic gyroscope as described in claim 3, further characterized by:

the mating surfaces further being secured by the use of a bonding material.

5. Fiber optic gyroscope as described in claim 1, further characterized by:

the broadband light source being a superluminescent diode.

6. Fiber optic gyroscope as described in claim 5, further characterized by:

means to effect a control signal in response to the detection of an unbalanced state of light entering the beam splitter, wherein the control signal is used to control the light frequency of the coherent light source.

7. Fiber optic gyroscope as described in claim 1, further characterized by the blazed diffraction grating being formed by:

(a) preparing a surface of a master grating;

(b) applying a photoresist material to the surface;

(c) holographically generating a pattern corresponding to a grating pattern to be etched into the master surface;

(d) projecting the holographically generated pattern onto photoresist so as to expose the photoresist;

(e) etching the master surface in order to contour the surface in accordance with the diffraction pattern as represented by the exposed photoresist;

(f) removing any remaining photoresist from the master surface;

(g) depositing a parting material on the master surface;

(h) coating of the master surface with a reflective material, the reflective material being coated over the parting material;

(i) preparing a surface of a new thermally stable substrate so as to permit the reflective material to be bonded to the new substrate;

(j) bringing the surface of the new substrate into contact with the coated surface of the master; and

(k) separating the surfaces in a manner which results in the reflective coating material remaining bonded to the new substrate and becoming separated from the master.

8. Interferometer including a coherent light source generating coherent light, a wavelength monitor, and means to conduct the coherent light to the wavelength monitor, characterized by the wavelength monitor including:

(a) a blazed reflective diffraction grating formed by preparing a master surface having a blazed diffraction grating pattern thereon, depositing of a parting material on the master, coating of the master with a reflective material, the reflective material being coated over the parting material, preparing a surface of a new substrate so as to permit the reflective material to be bonded to the new substrate, bringing the surface of the new substrate into contact with the coated surface of the master, and separating the surfaces in a manner which results in the reflective coating material remaining bonded to the new substrate and becoming separated from the master;

(b) means to transmit light to the blazed reflective diffraction grating, said means including light collimating means capable of protecting the light onto a significant surface area of the blazed diffraction grating;

(c) a beam splitter receiving said light reflected from the diffraction grating;

(d) the light supplied to a wavelength monitor being transmitted through the beam splitter to the collimating means;

(e) the light reflected into the beam splitter being transmitted through the collimating means in a manner which focuses the reflected light into the beam splitter;

(f) optical sensor means to detect an unbalanced state of the light entering the beam spliter; and

(g) the light reflected from the blaze diffraction grating being retroflected through the collimating means back to the beam splitter so that the optical sensor means detects an offaxis condition of the reflected light.

9. Interferometer as described in claim 8, further characterized by:

the collimating means being a collimating lens.

10. Interferometer as described in claim 8, further characterized by:

(a) the diffraction grating and the beam splitter each having means for mounting;

(b) a base upon which the diffraction grating and the beam splitter are mounted;

(c) the base and each means for mounting having mating surfaces, wherein the base and each means for mounting are mated; and

(d) the mating surfaces being polished sufficiently for each of said means for mounting adhering to the base by Van der Walls forces to secure each means for mounting to the base.

11. Interferometer as described in claim 10, further characterized by:

the mating surfaces further being secured by the use of a bonding material.

12. Interferometer as described in claim 8, further characterized by:

the coherent light source being a superluminescent diode.

13. Interferometer as described in claim 12, further characterized by:

means to effect a phase modulation in response to the detection of an unbalanced state of light entering the beam splitter, wherein the phase modulation is used to measure detected rate.